LIGHT-CONTROLLED ELECTROKINETIC ASSEMBLY OF PARTICLES NEAR SURFACES

Abstract: A method and apparatus for the manipulation of colloidal particulates and biomolecules at the interface between an insulating electrode such as silicon oxide and an electrolyte solution. Light-controlled electrokinetic assembly of particles near surfaces relies on the combination of three functional elements: the AC electric field-induced assembly of planar aggregates; the patterning of the electrolyte/silicon oxide/silicon interface to exert spatial control over the assembly process; and the real-time control of the assembly process via external illumination. The present invention provides a set of fundamental operations enabling interactive control over the creation and placement of planar arrays of several types of particles and biomolecules and the manipulation of array shape and size. The present invention enables sample preparation and handling for diagnostic assays and biochemical analysis in an array format, and the functional integration of these operations. In addition, the present invention provides a procedure for the creation of material surfaces with desired properties and for the fabrication of surface-mounted optical components.

Description: Light-controlled Electrokinetic Assembly of Particles Near Surfaces Field of the Invention: The present invention generally relates to the field of mateπals science and analytical chemistry. The present invention specifically relates to the realization of a complete, functionally integrated system for the implementation of biochemical analysis in a planar, miniaturized format on the surface of a conductive and/or photoconductive substrate, with applications in pharmaceutical and agricultural drug discovery and in in-vitro or genomic diagnostics. In addition, the method and apparatus of the present invention may be used to create material surfaces exhibiting desirable topographical relief and chemical function¬ ality, and to fabricate surface-mounted optical elements such as lens arrays. Background of the Invention I - Ions, Electric Fields and Fluid Flow: Field-induced Formation of Planar Bead Arrays Electrokinesis refers to a class of phenomena elicited by the action of an electric field on the mobile ions surrounding charged objects in an electrolyte solution. When an object of given surface charge is immersed in a solution containing ions, a diffuse ion cloud forms to screen the object's surface charge. This arrangement of a layer of (immobile) charges associated with an immersed object and the screening cloud of (mobile) counterions in solution is referred to as a "double layer". In this region of small but finite thickness, the fluid is not electroneutral. Consequently, electric fields acting on this region will set in motion ions in the diffuse layer, and these will in turn entrain the surrounding fluid. The resulting flow fields reflect the spatial distribution of ionic current in the fluid. Electroosmosis represents the simplest example of an electrokinetic phenomenon. It arises when an electric field is applied parallel to the surface of a sample container or electrode exhibiting fixed surface charges, as in the case of a silicon oxide electrode (in the range of neutral pH). As counterions in the electrode double layer are accelerated by the electric field, they drag along solvent molecules and set up bulk fluid flow. This effect can be very substantial in narrow capillaries and may be used to advantage to devise fluid pumping systems. Electrophoresis is a related phenomenon which refers to the field-induced transport of charged particles immersed in an electrolyte. As with electroosmosis, an electric field accelerates mobile ions in the double layer of the particle. If, in contrast to the earlier case, the particle itself is mobile, it will compensate for this field-induced motion of ions (and the resulting ionic current) by moving in the opposite direction. Electrophoresis plays an important role in industrial coating processes and, along with electroosmosis, it is of particular interest in connection with the development of capillary electrophoresis into a mainstay of modern bioanalytical separation technology. In confined geometries, such as that of a shallow experimental chamber in the form of a "sandwich" of two planar electrodes, the surface charge distribution and topography of the bounding electrode surfaces play a particularly important role in deteπnining the nature and spatial structure of electroosmotic flow. Such a "sandwich" electrochemical cell may be formed by a pair of electrodes separated by a shallow gap. Typically, the bottom electrode will be formed by an oxide-capped silicon wafer, while the other electrode is formed by optically transparent, conducting indium tin oxide (TTO). The silicon (Si) wafer represents a thin slice of a single crystal of silicon which is doped to attain suitable levels of electrical conductivity and insulated from the electrolyte solution by a thin layer of silicon oxide (SiOx). The reversible aggregation of beads into planar aggregates adjacent to an electrode surface may be induced by a (DC or AC) electric field that is applied normal . to the electrode surface. While the phenomenon has been previously observed in a cell formed by a pair of conductive ITO electrodes (Richetti, Prost and Barois, J. Physique Lettr. 45, L-1137 through L-1143 (1984)), the contents of which are incorporated herein by reference, it has been only recently demonstrated that the underlying attractive interaction between beads is mediated by electrokinetic flow (Yeh, Seul and Shraiman, "Assembly of Ordered Colloidal Aggregates by Electric Field Induced Fluid Flow", Nature 386, 57-59 (1997), the contents of which are incorporated herein by reference. This flow reflects the action of lateral non-uniformities in the spatial distribution of the current in the vicinity of the electrode. In the simplest case, such non-uniformities are introduced by the very presence of a colloidal bead near the electrode as a result of the fact that each bead interferes with the motion of ions in the electrolyte. Thus, it has been observed that an individual bead, when placed near the electrode surface, generates a toroidal flow of fluid centered on the bead. Spatial non-uniformities in the properties of the electrode can also be introduced deliberately by several methods to produce lateral fluid flow toward regions of low impedance. These methods are described in subsequent sections below. Particles embedded in the electrokinetic flow are advected regardless of their specific chemical or biological nature, while simultaneously altering the flow field. As a result, the electric field-induced assembly of planar aggregates and arrays applies to such diverse particles as: colloidal polymer lattices ("latex beads"), lipid vesicles, whole chromosomes, cells and biomolecules including proteins and DNA, as well as metal or semiconductor colloids and clusters. Important for the applications to be described is the fact that the flow- mediated attractive interaction between beads extends to distances far exceeding the characteristic bead dimension. Planar aggregates are formed in response to an externally applied electric field and disassemble when the field is removed. The strength of the applied field determines the strength of the attractive interaction that underlies the array assembly process and thereby selects the specific arrangement adopted by the beads within the array. That is, as a function of increasing applied voltage, beads first form planar aggregates in which particles are mobile and loosely packed, then assume a tighter packing, and finally exhibit a spatial arrangement in the form of a crystalline, or ordered, array resembling a raft of bubbles. The sequence of transitions between states of increasing internal order is reversible, including complete disassembly of planar aggregates when the applied voltage is removed. In another arrangement, at low initial concentration, beads form small clusters which in turn assume positions within an ordered "superstructure". π - Patterning of Silicon Oxide Electrode Surfaces Electrode patterning in accordance with a predetermined design facilitates the quasi-permanent modification of the electrical impedance of the EIS (Electrolyte- Insulator-Semiconductor) structure of interest here. By spatially modulating the EIS impedance, electrode-patterning determines the ionic current in the vicinity of the electrode. Depending on the frequency of the applied electric field, beads either seek out, or avoid, regions of high ionic current. Spatial patterning therefore conveys explicit external control over the placement and shape of bead arrays. While patterning may be achieved in many ways, two procedures offer particular advantages. First, UV-mediated re-growth of a thin oxide layer on a properly prepared silicon surface is a convenient methodology that avoids photolithographic resist patterning and etching. In the presence of oxygen, UV illumination mediates the conversion of exposed silicon into oxide. Specifically, the thickness of the oxide layer depends on the exposure time and may thus be spatially modulated by placing patterned masks into the UV illumination path. This modulation in thickness, with typical variations of approximately 10 Angstroms, translates into spatial modulations in the impedance of the Si/SiOx interface while leaving a flat and chemically homogeneous top surface exposed to the electrolyte solution. Second, spatial modulations in the distribution of the electrode surface charge may be produced by UV-mediated photochemical oxidation of a suitable chemical species that is first deposited as a monolayer film on the SiOx surface. This method permits fine control over local features of the electrode double layer and thus over the electrokinetic flow. A variation of this photochemical modulation is the creation of lateral gradients in the EIS impedance and hence in the current generated in response to the applied electric field. For example, this is readily accomplished by controlling the UV exposure so as to introduce a slow lateral variation in the oxide thickness or in the surface charge density. As discussed below, control over lateral gradients serves to induce lateral bead transport and facilitates the implementation of such fundamental operations as capturing and channeling of beads to a predetermined destination along conduits in the form of impedance features embedded in the Si/SiOx interface. Photochemical patterning of functionalized chemical overlayers also applies to other types of electrode surfaces including ITO. HI - Light-controlled Modulation of the Interfacial Impedance The spatial and temporal modulation of the FJS-impedance in accordance with a pattern of external iUumination provides the basis to control the electrokinetic forces that mediate bead aggregation. The light-modulated electrokinetic assembly of planar colloidal arrays facilitates remote interactive control over the formation, placement and rearrangement of bead arrays in response to corresponding illumination patterns and thereby offers a wide range of interactive manipulations of colloidal beads and biomolecules. To understand the principle of this methodology, it will be helpful to briefly review pertinent photoelectric properties of semiconductors, or more specifically, those of the EIS structure formed by the Electrolyte solution (E), the Insulating SiOx layer (I) and the Semiconductor (S). The photoelectric characteristics of this structure are closely related to those of a standard Metal-Insulator-Semiconductor (MIS) or Metal-Oxide-Semiconductor (MOS) devices which are described in S.M. Sze, "The Physics of Semiconductors", 2nd Edition, Chapt. 7 (Wiley Interscience 1981), the contents of which are incorporated herein by reference. The interface between the semiconductor and the insulating oxide layer deserves special attention. Crucial to the understanding of the electrical response of the MOS structure to light is the concept of a space charge region of small but finite thickness that forms at the Si/SiOx interface in the presence of a bias potential. In the case of the EIS structure, an effective bias, in the form of a junction potential, is present under all but very special conditions. The space charge region forms in response to the distortion of the semiconductor's valence and conduction bands ("band bending") in the vicinity of the interface. This condition in turn reflects the fact that, while there is a bias potential across the interface, there is ideally no charge transfer in the presence of the insulating oxide. That is, in electrochemical language, the EIS structure eliminates Faradaic effects. Instead, charges of opposite sign accumulate on either side of the insulating oxide layer and generate a finite polarization. In the presence of a reverse bias, the valence and conduction band edges , of an n-doped semiconductor bend upward near the Si/SiOx interface and electrons flow out of the interfacial region in response to the corresponding potential gradient. As a result, a majority carrier depletion layer is formed in the vicinity of the Si/SiOx interface. Light absoφtion in the semiconductor provides a mechanism to create electron-hole pairs within this region. Provided that they do not instantaneously recombine, electron-hole pairs are split by the locally acting electric field, and a corresponding photocurrent flows. It is this latter effect that affords control over the electrokinetic assembly of beads in the electrolyte solution. To understand in more detail the pertinent frequency dependence of the light-induced modulation of the EIS impedance, two aspects of the equivalent circuit representing the EIS structure are noteworthy. First, there are close analogies between the detailed electrical characteristics of the electric double layer at the electrolyte-oxide interface, and the depletion layer at the interface between the semiconductor and the insulator. As with the double layer, the depletion layer exhibits electrical characteristics similar to those of a capacitor with a voltage-dependent capacitance. As discussed, illumination serves to lower the impedance of the depletion layer. Second, given its capacitive electrical response, the oxide layer will pass current only above a characteristic ("threshold") frequency. Consequently, provided that the frequency of the applied voltage exceeds the threshold, illumination can lower the effective impedance of the entire EIS structure. This effective reduction of the EIS impedance also depends on the light intensity which determines the rate of generation of electron-hole pairs. In the absence of significant recombination, the majority of photogenerated electrons flow out of the depletion region and contribute to the photocurrent. The remaining hole charge accumulates near the Si/SiOx interface and screens the electric field acting in the depletion region. As a result, the rate of recombination increases, and the efficiency of electron-hole separation, and hence the photocurrent, decreases. For given values of frequency and amplitude of the applied voltage, one therefore expects that as the illumination intensity increases, the current initially increases to a maximum level and then decreases. Similarly, the impedance initially decreases to a minimum value (at maximum current) and then decreases. This intensity dependence may be used to advantage to induce the lateral displacement of beads between fully exposed and partially masked regions of the interface.. As the illumination intensity is increased, the fully exposed regions will correspond to the regions of interface of lowest impedance, and hence of highest current, and beads will be drawn into these regions. As the fully exposed regions reach the state of decreasing photocurrent, the effective FJS impedance in those regions may exceed that of partially masked regions, with a resulting inversion of the lateral gradient in current. Beads will then be drawn out of the fully exposed regions. Additionally, time- varying changes in the illumination pattern may be used to effect bead motion. IV - Integration of Biochemical Analysis in a Miniaturized, Planar Format The implementation of assays in a planar array format, particularly in the context of biomolecular screening and medical diagnostics, has the advantage of a high degree of parallelity and automation so as to realize high throughput in complex, multi-step analytical protocols. Miniaturization will result in a decrease in pertinent mixing times reflecting the small spatial scale, as well as in a reduction of requisite sample and reagent volumes as well as power requirements. The integration of biochemical analytical techniques into a miniaturized system on the surface of a planar substrate ("chip") would yield substantial improvements in the performance, and reduction in cost, of analytical and diagnostic procedures. Within the context of DNA mampulation and analysis, initial steps have been taken in this direction (i.e., miniaturization) by combining on a glass substrate, the restriction enzyme treatment of DNA and the subsequent separation of enzyme digests by capillary electrophoresis, see, for example, Ramsey, PCT Publication No. WO 96/04547, the contents of which are incorporated herein by reference, or the amplification of DNA sequences by application of the polymerase chain reaction (PCR) with subsequent electrophoretic separation, see, for example, U.S. Patent Nos. 5,498,392 and 5,587,128 to Wilding et al. , the contents of which are incorporated herein by reference. While these standard laboratory processes have been demonstrated in a miniaturized format, they have not been used to form a complete system. A complete system will require additional manipulation such as front-end sample processing, binding and functional assays and the detection of small signals followed by information processing. The true challenge is that of complete functional integration because it is here that system architecture and design constraints on individual components will manifest themselves. For example, a fluidic process is required to concatenate analytical steps that . require the spatial separation, and subsequent transport to new locations, of sets of analyte. Several possibilities have been considered including electroosmotic pumping and transport of droplets by temperature-induced gradients in local surface tension. While feasible in demonstration experiments, these techniques place rather severe requirements on the overall systems lay-out to handle the very considerable DC voltages required for efficient electroosmotic mixing or to restrict substrate heating when generating thermally generated surface tension gradients so as to avoid adverse effects on protein and other samples. Summary of the Invention The present invention combines three separate functional elements to provide a method and apparatus facilitating the real-time, interactive spatial manipulation of colloidal particles ("beads") and molecules at an interface between a light sensitive electrode and an electrolyte solution. The three functional elements are: the electric field-induced assembly of planar particle arrays at an interface between an insulating or a conductive electrode and an electrolyte solution; the spatial modulation of the interfacial impedance by means of UV-mediated oxide regrowth or surface- chemical patterning; and, finally, the real-time, interactive control over the state of the interfacial impedance by light. The capabilities of the present invention originate in the fact that the spatial distribution of ionic currents, and thus the fluid flow mediating the array assembly, may be adjusted by external intervention. Of particular interest is the introduction of spatial non-uniformities in the properties of the pertinent EIS structure. As described herein, such inhomogeneities, either permanent or temporary in nature, may be produced by taking advantage of the physical and chemical properties of the EIS structure. The invention relates to the realization of a complete, functionally integrated system for the implementation of biochemical analysis in a planar, miniaturized format on the surface of a silicon wafer or similar substrate. In addition, the method and apparatus of the present invention may be used to create material surfaces exhibiting desirable topographical relief and chemical functionality, and to fabricate surface-mounted optical elements such as lens arrays. The combination of three functional elements endows the present invention with a set of operational capabilities to manipulate beads and bead arrays in a planar geometry to allow the implementation of biochemical analytical techniques. These, fundamental operations apply to aggregates and arrays of particles such as: colloidal polymer lattices, vesicles, whole chromosomes, cells and biomolecules including proteins and DNA, as well as metal or semiconductor colloids and clusters. Sets of colloidal particles may be captured, and arrays may be formed in designated areas on the electrode surface (Figs, la, lb and Figs. 2a-d). Particles, and the arrays they form in response to the applied field, may be channeled along conduits of any configuration that are either embedded in the Si/SiOx interface by UV-oxide patterning or delineated by an external pattern of illumination. This channeling (Figs, lc, Id, le, Figs. 3c, 3d), in a direction normal to that of the applied electric field, relies on lateral gradients in the impedance of the FJS structure and hence in the field-induced current. As discussed herein, such gradients may be introduced by appropriate patterns of illumina¬ tion, and this provides the means to implement a gated version of translocation (Fig. le). The electrokinetic flow mediating the array assembly process may also be exploited for the alignment of elongated particles, such as DNA, near the surface of the electrode. In addition, the present invention permits the realization of methods to sort and separate particles. Arrays of colloidal particles may be placed in designated areas and confined there until released or disassembled. The overall shape of the array may be delineated by UV-oxide patterning or, in real time, by shaping the pattern of illumination. This capability enables the definition of functionally distinct compartments, permanent or temporary, on the electrode surface. Arrays may be subjected to changes of shape imposed in real time, and they may be merged with other arrays (Fig. If) or split into two or more subarrays or clusters (Fig. lg, Figs. 4a, 4b). In addition, the local state of order of the array as well as the lateral particle density may be reversibly adjusted by way of the external electric field or modified by addition of a second, chemically inert bead component. The present invention also allows for the combination of fundamental operations to develop increasingly complex products and processes. Examples given herein describe the implementation of analytical procedures essential to a wide range of problems in materials science, pharmaceutical drug discovery, genomic mapping and sequencing technology. Important to the integration of these and other functionalities in a planar geometry is the capability, provided by the present invention, to impose tempo- rary or permanent compartmentalization in order to spatially isolate concurrent processes, or sequential steps in a protocol and the ability to manipulate sets of particles in a manner permitting the concatenation of analytical procedures that are performed in different designated areas on the substrate surfaces. Brief Description of Drawings Other objects, features and advantages of the invention discussed in the above brief explanation will be more clearly understood when taken together with the following detailed description of an embodiment which will be understood as being illustrative only, and the accompanying drawings reflecting aspects of that embodiment, in which: Figs, la-h are illustrations of the fundamental operations for bead mampulation; Figs. 2a and 2b are photographs illustrating the process of capturing particles in designated areas on the substrate surface; Figs. 2c and 2d are photographs illustrating the process of excluding particles from designated areas on the substrate surface; Figs. 3a and 3b are illustrations of the oxide profile of an Si/SiOx electrode; Figs. 3c and 3d are photographs of the channeling of particles along conduits; Figs. 4a and 4b are photographs of the splitting of an existing aggregate into small clusters; Fig. 5 is a photograph of the lensing action of individual colloidal beads; Figs. 6a-c are side view illustrations of a layout-preserving transfer process from a microtiter plate to a planar cell; Fig. 7 is a photograph of the inclusion of spacer particles within bead clusters; Fig. 8 is an illustration of binding assay variations; Figs. 9a and 9b are illustrations of two mechanisms of particle sorting; Fig. 10 is an illustration of a planar array of bead-anchored probe-target complexes; and Fig. 11 is an illustration of DNA stretching in accordance with the present invention. Detailed Description of the Preferred Embodiments The three functional elements of the present invention may be combined so as to provide a set of fundamental operations for the interactive spatial manipulation of colloidal particles and molecules, assembled into planar aggregates adjacent to an electrode surface. In the following description, fundamental operations in this "toolset" are described in order of increasing complexity. Specifically, it is useful to adopt a classification scheme based on the total number of inputs and outputs, or "terminals", involved in a given operation. For example, the merging of two separate arrays, or sets of particles, into one would be a "three-terminal" operation, involving two inputs and one output. The converse three-terminal operation, involving one input and two outputs, is the splitting of a given array into two subarrays. Experimental conditions yielding the phenomena depicted in the various photographs included herein are as follows. An electrochemical cell is formed by a pair of planar ITO electrodes, composed of an ITO layer deposited on a glass substrate, or by a Si/SiOx electrode on the bottom and an ITO electrode on the top, separated by a typical gap of 50 microns or less. Given its dependence on the photoelectric properties of the Si/SiOx interface, light control is predicated on the use of a Si/SiOx electrode. Leads, in the form of platinum wires, are attached to the ITO and to the silicon electrode, which is first etched to remove the insulating oxide in the contact region, by means of silver epoxy. The cell is first assembled and then filled, relying on capillary action, with a suspension of colloidal beads, 1 or 2 microns in diameter, at a typical concentration of 0.1 % solids in O.lmM azide solution, corresponding to approximately 2xlO Λ 8 particles per milliliter. The number is chosen so as to yield between Vi and 1 full monolayer of particles on the electrode surface. Anionic (e.g., carboxylated polystyrene, silica), cationic (e.g. , aminated polystyrene) or nominally neutral (e.g. , polystyrene) have all been used to demonstrate the basic phenomena underlying the three functional elements of the present invention. The silicon electrode was fabricated from a 1 inch-square portion of a Si (100) wafer, typically 200-250 microns thick, n-doped to typically 0.01 Ohm cm resistivity, and capped with a thin oxide of typically 30-40 Angstroms thickness. A thick oxide layer of typically 6000-8000 Angstrom thickness, grown under standard conditions in a furnace at 950 degrees C, may be etched by standard photolithography to define the structures of interest. Alternatively, a thin oxide layer may be regrown on a previously, stripped surface of (100)-orientation under UV illumination. Given its ease of implementation and execution, UV-mediated oxide regrowth is the preferable technique: it provides the means to pattern the surface by placing a quartz mask representing the desired pattern in the path of UV illumination and it leaves a chemically homogeneous, topographically flat top surface. To avoid non-specific particle adsoφtion to the electrode surface, stringent conditions of cleanliness should be followed, such as those set forth in the General Experimental Conditions below. The fundamental one-terminal operation is a "capture-and-hold" operation (Fig. la) which forms an array of particles in a designated area of arbitrary outline on the surface that is delineated by UV-mediated oxide patterning or by a corresponding pattern of illumination projected on an otherwise uniform Si/SiOx substrate surface. Figs. 2a and 2b illustrate bead capture on a surface characterized by a very thin oxide region 22 (approximately 20-30 Angstroms in thickness) and correspondingly low impedance, while the remaining surface is covered with the original, thick oxide with correspondingly high impedance. In Fig. 2a, there is no applied field, and hence, no bead capture. In contrast, in Fig. 2b, an electric field is applied (lOVp-p source, 1 kHz) and bead capture occurs within the thin oxide region 22. Under these conditions, an array starts to grow within less than a second and continues to grow over the next approximately 10 seconds as beads arrive from increasingly larger distances to add to the outward growing perimeter of region 22. Growth stops when the array approaches the outer limit of the delineated target area, i.e. , the area defined by the thin oxide having a low impedance. The internal state of order of the captured aggregate of beads is determined by the strength of the applied voltage, higher values favoring increasingly denser packing of beads and the eventual formation of ordered arrays displaying a hexagonally crystalline configuration in the form of a bubble raft. The array remains in place as long as the applied voltage is present. Removal of the applied voltage results in the disassembly of the array. The "capture-and-hold" operation may also be implemented under illumination with visible or infrared light, for example by simply projecting a mask patterned with the desired layout onto the Si/SiOx electrode. A regular 100W quartz microscope illuminator has been used for this purpose on a Zeiss UEM microscope, with apertures or masks inserted in the intermediate image plane to provide the required shape in the plane of the electrode (when focused properly under conditions of Koehler illumination). Alternatively, an IR laser diode with output of 3 mW at 650 - 680nm also has been used. The use of external illumination rather than oxide patterning for the spatial confinement of particles allows the confinement pattern to be easily modified. Related to "capture-and-hold" is the one-terminal operation of "exclude- and-hold" (Fig. lb) which clears particles from a designated area on the surface. Increasing the frequency of the applied voltage to approximately 100kHz leads to an inversion in the preference of particles which assemble in the thin-oxide portion of the surface (e.g., region 22, Fig. 2b) and instead form structures decorating the outside of the target area perimeter. Rather than relying on this effect, the exclusion of particles from the desired areas is also accomplished, in analogy to the original "capture-and-hold" operations, by simply embedding the corresponding structure in the Si/SiOx interface by UV- mediated oxide regrowth. In the example of Figs. 2c and 2d, this is achieved, under conditions otherwise identical to those described above, with respect to Figs. 2a and 2b, by applying 20V (pp) at 10kHz. While the oxide thickness in the non designated areas 24 is approximately 30 Angstroms, the value in the designated square areas 26 is approximately 40 Angstroms, implying a correspondingly higher impedance at the applied frequency. The "capture-and-hold" operation enables the spatial compartmentalization of the substrate surface into functionally distinct regions. For example, particles of distinct chemical type, introduced into the electrochemical cell at different times or injected in different locations, can be kept in spatially isolated locations by utilizing this operation. The fundamental two-terminal operation is translocation (Fig. lc), or the controlled transport of a set of particles from location O to location F on the surface; here, O and F are target areas to which the above-described one-terminal operations may be applied. The one-dimensional, lateral bead transport used in translocation is achieved by imposing a lateral current along a conduit connecting areas O and F, as shown in Figs. 3a and 3b or by projecting a corresponding linear pattern of illumination. In this channeling operation, beads move in the direction of lower impedance in the direction of the arrow shown in Figs. 3a and 3b, in accordance with the underlying electrokinetic flow. Oxide patterning may be utilized in two ways to create a lateral current along the Si/SiOx interface. The simplest method is depicted in Fig. 3c and shows a large open holding area 32 fed by three narrow conduits 34 defined by etching a thermal oxide. Beads move to the holding area 32 along the narrow conduits 34 to form a bead array. Fig. 3d is a large scale view of the array of Fig. 3c. The principle invoked in creating transport is that of changing the aspect ratio (narrow conduit connected to wide holding area) of the embedded pattern with constant values of thin oxide thickness inside and thick oxide outside, as illustrated in Fig. 3a. In Figs. 3c and 3d, the applied voltage was 10V (pp) at 10kHz. An alternative approach for creating bead transport, enabled by UV-mediated oxide regrowth, is to vary the oxide thickness along the conduit in a controlled fashion. This is readily accomplished by UV exposure through a graduated filter. Differences in the oxide thickness between O and F of as little as 5-10 Angstroms suffice to effect lateral transport. In this situation, the aspect ratio of the conduit and holding areas need not be altered. This is illustrated in Fig. 3b. The use of external illumination to define conduits, by varying the illumination intensity along the conduit to create the requisite impedance gradient, has the advantage that the conduit is only a temporary structure, and that the direction of motion may be modified or reversed if so desired. The present invention provides for mechanisms of light-mediated active linear transport of planar aggregates of beads under interactive control. This is achieved by adjusting an external pattern of illumination in real time, either by moving the pattern across the substrate surface in such a way as to entrain the illuminated bead array or by electronically modulating the shape of the pattern to induce motion of particles. Two modes of light-mediated, active transport are: i) Direct Translocation ("tractor beam") which is a method of translocating arrays and of delineating their overall shape by adjusting parameters so as to favor particle assembly within illuminated areas of the surface, as described herein. Arrays simply follow the imposed pattern. The rate of motion is limited by the mobility of particles in the fluid and thus depends on particle diameter and fluid viscosity. ii) Transverse Array Constriction is a bead transport mechanism related to peristaltic pumping of fluids through flexible tubing. The light-control component of the present invention may be used for a simple implementation of this very general concept. A multi-component planar aggregate of beads is confined to a rectangular channel, by UV-patterning if so desired, or simply by light. Beads are free to move along the channel by diffusion (in either direction). An illumination pattern matching the W 15 transverse channel dimension is set up and is then varied in time so as to produce a. transverse constriction wave that travels in one direction along the channel. Such a constriction wave may be set up in several ways. A conceptually simple method is to project a constricting mask onto the sample and move the projected mask pattern in the desired fashion. This method also may be implemented electronically by controlling the illumination pattern of a suitable array of light sources, thus obviating the need for moving parts in the optical train. The control of lateral bead transport by changing or moving patterns of illumination has the advantage that it may be applied whenever and wherever (on a given substrate surface) required, without the need to impose gradients in impedance by predefined UV patterning. On the other hand, a predefined impedance pattern can provide additional capabilities in conjunction with light-control. For example, it may be desirable to transport beads against a substrate-embedded impedance gradient to separate beads on the basis of mobility. Conduits connecting O and F need not be straight: as with tracks directing the motion of trains, conduits may be shaped in any desirable fashion (Fig. Id). A gated version of translocation (Fig. le) permits the transport of particles from O to F only after the conduit is opened (or formed in real time) by a gating signal. This operation utilizes UV oxide patterning to establish two holding areas, O and F, and also light control to temporarily establish a conduit connecting O and F. An alternative implementation is based on an oxide embedded impedance gradient. A zone along the conduit is illuminated with sufficiently high intensity to keep out particles, thereby blocking the passage. Removal (or reduction in intensity) of the illumination opens the conduit. In the former case, light enables the transport of beads, while in the latter case, light prevents the transport of beads. The fundamental three-terminal operations are the merging and splitting of sets or arrays of beads (Figs, lf and lg). The merging of two arrays (Fig. lf) involves the previous two fundamental operations of "capture-and-hold", applied to two spatially isolated sets of beads in locations Ol and 02, and their respective channeling along merging conduits into a common target area, and their eventual channeling, subsequent to mixing, or a chemical reaction, into the final destination, a third holding area, F. This is accomplished, under the conditions stated above, by invoking one-terminal and gated two-terminal operations. The splitting of an array into two subarrays (Fig. lg) is a special case of. a generally more complex sorting operation. Sorting involves the classification of beads in a given set or array into one of two subsets, for example according to their fluores¬ cence intensity. In the simpler special case, a given array, held in area O, is to be split into two subarrays along a demarcation line, and subarrays are to be moved to target areas FI and F2. Under the conditions stated above, this is accomplished by applying the "capture-and-hold" operation to the array in O. Conduits connect O to FI and F2. High intensity illumination along a narrowly focused line serves to divide the array in a defined fashion, again relying on gated translocation to control transport along conduits away from the holding area O. An even simpler version, termed indiscriminate splitting, randomly assigns particles into FI and F2 by gated translocation of the array in O into FI and F2 after conduits are opened as described above. Figs. 4a and 4b show a variant in which beads in region O (Fig. 4a) are split into multiple regions FI, F2, ... Fn (Fig. 4b). This reversible splitting of an aggregate or array into n subarrays, or clusters, is accomplished, for carboxylated polystyrene spheres of 2 micron diameter at a concentration corresponding to an electrode coverage of a small fraction of a monolayer, at a frequency of 500Hz, by raising the applied voltage from typically 5V (pp) to 20V (pp). This fragmentation of an array into smaller clusters reflects the effect of a field-induced particle polarization. The splitting is useful to distribute particles in an array over a wider area of substrate for presentation to possible analytes in solution, and for subsequent scanning of the individual clusters with analytical instruments to make individual readings. The three functional elements of the present invention described herein may be also combined to yield additional fundamental operations to control the orientation of anisotropic objects embedded in the electroosmotic flow created by the applied electric field at the electrode surface. The direction of the flow, in the plane of the substrate, is controlled by gradients in the impedance that are shaped in the manner described in connection with the channeling operation. This is used to controllably align anisotropic objects as illustrated in Fig. lh, and may be applied to stretch out and align biomolecules, such as DNA. An additional fundamental operation that complements the previous set is that of permanently anchoring an array to the substrate. This is best accomplished by invoking anchoring chemistries analogous to those relying on heterobifunctional cross-linking agents invoked to anchor proteins via amide bond formation. Molecular . recognition, for example between biotinylated particles and surface-anchored streptavidin, provides another class of coupling chemistries for permanent anchoring. General Experimental Conditions The functional elements, namely the electric-field induced assembly of planar particle arrays, the spatial modulation of the interfacial impedance by means of UV-mediated oxide or surface-chemical patterning and finally, the control over the state of the interfacial impedance by light which are used in the present invention, have been demonstrated in experimental studies. These studies employed n-doped silicon wafers (resistivities in the range of 0.01 Ohm cm), capped with either thermally grown oxide layers of several thousand Angstrom thickness, or with thin oxide layers, regrown after removal of the original "native" oxide in HF, under UV illumination from a deuterium source in the presence of oxygen to typical thicknesses between 10 and 50 Angstroms. Lithographic patterning of thermally grown oxide employed standard procedures implemented on a bench top (rather than a clean room) to produce features in the range of several microns. Surfaces were carefully cleaned in adherence with industry standard RCA and Piranha cleaning protocols. Substrates were stored in water produced by a Millipore cleaning system prior to use. Surfaces were characterized by measuring the contact angle exhibited by a 20 microliter droplet of water placed on the surface and viewed (from the side) through a telescope. The contact angle is defined as the angle subtended by the surface and the tangent to the droplet contour (in side view) at the point of contact with the surface. For example, a perfectly hemispherical droplet shape would correspond to a contact angle of 90 degrees. Surface chemical derivatization with mercapto-propyl-trimethoxysilane (2% in dry toluene) produced surfaces giving typical contact angles of 70 degrees. Oxidation of the terminal thiol functionality under UV irradiation in the presence of oxygen reduced the contact angle to zero in less than 10 min of exposure to UV from the deuterium source. Other silane reagents were used in a similar manner to produce hydrophobic surfaces, characterized by contact angles in excess of 110 degrees. Simple "sandwich" electrochemical cells were constructed by employing kapton film as a spacer between Si/SiOx and conductive indium tin oxide (ITO), deposited on a thin glass substrate. Contacts to platinum leads were made with silver epoxy directly, to the top of the ITO electrode and to the (oxide-stripped) backside of the Si electrode. In this two-electrode configuration, AC fields were produced by a function generator, with applied voltages ranging up to 20V and frequencies varying from DC to 1 MHz, high frequencies favoring the formation of particle chains connecting the electrodes. Currents were monitored with a potentiostat and displayed on an oscilloscope. For convenience, epi-fluorescence as well as reflection differential interference contrast microscopy employed laser illumination. Light-induced modulations in EIS impedance were also produced with a simple 100W microscope illuminator as well as with a 3mW laser diode emitting light at 650-680 nm. Colloidal beads, both anionic and cationic as well as nominally neutral, with a diameter in the range from several hundred Angstroms to 20 microns, stored in a NaN 2 solution, were employed. Close attention was paid to colloidal stability to avoid non-specific interactions between particles and between particles and the electrode surface. Bacterial contamination of colloidal suspensions was scrupulously avoided. Typical operating conditions producing, unless otherwise indicated, most of the results described herein, were: 0.2 mM NaN 2 (sodium azide) solutions, containing particles at a concentration so as to produce not more than a complete monolayer of particles when deposited on the electrode; applied DC potentials in the range of 1 -4V, and AC potentials in the range of 1-10V (peak-to-peak) and 500Hz - 10kHz, with an electrode gap of 50 microns; anionic (carboxylated polystyrene) beads of 2 micron diameter, as well as (nominally neutral) polystyrene beads of 2-20 micron diameter. The method and apparatus of the present invention may be used in several different areas, examples of which are discussed in detail. Each example includes background information followed by the application of the present invention to that particular application. Example I - Fabrication of Surfaces and Coatings with Designed Properties The present invention may be used to fabricate planar surfaces and coatings with designed properties. Specifically, the functional elements of the present invention enable the formation of arrays composed of particles of a wide range of sizes (approximately 100 Angstrom to 10 microns) and chemical composition or surface functionality in response to AC or DC electric fields. These arrays may be placed and. delineated in designated areas of the substrate, and the inteφarticle spacing and internal state of order within the array may be controlled by adjusting the applied field prior to anchoring the array to the substrate. The newly formed surfaces display pre-designed mechanical, optical and chemical characteristics, and they may be subjected to further modification by subsequent treatment such as chemical cross-linking. The mechanical and/or chemical modification of surfaces and coatings principally determines the interaction between materials in a wide range of applications that depend on low adhesion (e.g., the familiar "non-stick" surfaces important in housewares) or low friction (e.g. , to reduce wear in computer hard disks), hydrophobicity (the tendency to repel water, e.g., of certain fabrics), catalytic activity or specific chemical functionality to either suppress molecular interactions with surfaces or to promote them. The latter area is of particular importance to the development of reliable and durable biosensors and bioelectronic devices. Finally, a large number of applications depend on surfaces of defined topography and/or chemical functionality to act as templates controlling the growth moφhology of deposited materials or as "command surfaces" directing the alignment of optically active molecules in deposited thin organic films, as in liquid crystal display applications. Extensive research has been devoted to the formation of surfaces by adsoφtion of thin organic films of known composition from the liquid or gas phase by several methods. Notwithstanding their seeming simplicity and wide-spread use, these methods can be difficult to handle in producing reliable and reproducible results. In addition, molecular films are not well suited to produce surfaces displaying a regular topography. An alternative approach to the problem is the modification of conductive surfaces by electrophoretic deposition of suspended particulates. This is a widely used technique in industrial settings to produce paint coatings of metal parts, and to deposit phosphor for display screens. The active deposition process significantly enhances the kinetics of formation (in contrast to passive adsoφtion of organic films from solution), an important consideration in practical applications. Electrophoretic deposition requires high DC electric fields and produces layers in which particles are permanently adsorbed to the surface. While particles in so-deposited monolayers are usually randomly distributed, the formation of polycrystalline monolayers of small (150 Angstrom) gold colloids on carbon-coated copper grids is also known. However, the use of carbon-coated copper grids as substrates is not desirable in most applications. Prior art methods have been described for the formation of ordered planar arrays of particles under certain conditions. For example, the formation of ordered colloidal arrays in response to AC electric fields on conductive indium tin oxide (ITO) electrodes is known. However, the resulting layers were composed of small patches of ordered arrays, randomly distributed over the surface of the otherwise bare ITO substrate. Arrays of monodisperse colloidal beads and globular proteins also have been previously fabricated by using convective flow and capillary forces. However, this latter process has the disadvantage of leaving deposited particle arrays immobilized and exposed to air, making it difficult to modify arrays by subsequent liquid phase chemistry. The present invention provides a method of forming planar arrays with precise control over the mechanical, optical and chemical properties of the newly created layer. This method has several distinct advantages over the prior art. These result from the combination of AC electric field-induced array formation on insulating electrodes (Si/SiOx) that are patterned by UV-mediated oxide regrowth. The process of the present invention enables the formation of ordered planar arrays from the liquid phase (in which particles are originally suspended) in designated positions, and in accordance with a given overall outline. This eliminates the above-stated disadvantages of the prior art, i.e., dry state, irregular or no topography, random placement within an aggregate, immobilization of particles and uncontrolled, random placement of ordered patches on the substrate. An advantage of the present invention is that arrays are maintained by the applied electric field in a liquid environment. The process leaves the array in a state that may be readily disassembled, subjected to further chemical modification such as cross-linking, or made permanent by chemical anchoring to the substrate. Furthermore, the liquid environment is favorable to ensure the proper functioning of many proteins and protein supramolecular assemblies of which arrays may be composed. It also facilitates the subsequent liquid-phase deposition of additional layers of molecules (by chemical binding to beads or proteins in the deposited layer), the cycling of arrays between states of different density and internal order (including complete disassembly of the array) in response to electric fields and the chemical cross-linking of particles into two-dimensionally connected layers, or gels, formed, for example, of chemically functionalized silica spheres. The present invention can be practiced on insulating electrodes such as oxide-capped silicon, to minimize Faradaic processes that might adversely affect chemical reactions involved in the gelation process or in anchoring the array to the substrate. The use of Si/SiOx electrodes also enables the control of array placement by external illumination. The formation of colloidal arrays composed of small particles in accordance with the present invention provides a route to the fabrication of surfaces with relief structure on the scale of the particle diameter. Aside from their optical properties, such "micro-rough" surfaces are of interest as substrates for the deposition of DNA in such a way as to alleviate steric constraints and thus to facilitate enzyme access. Particles to which the invention applies include silica spheres, polymer colloids, lipid vesicles (and related assemblies) containing membrane proteins such as bacteriorhodopsin (bR) " a light-driven proton pump that can be extracted in the form of membrane patches and disks or vesicles. Structured and functionalized surfaces composed of photoactive pigments are of interest in the context of providing elements of planar optical devices for the development of innovative display and memory technology. Other areas of potential impact of topographically structured and chemically functionalized surfaces are the fabrication of template surfaces for the controlled nucleation of deposited layer growth and command surfaces for liquid crystal alignment. The present invention also enables the fabrication of randomly heterogeneous composite surfaces. For example, the formation of arrays composed of a mixture of hydrophobic and hydrophilic beads of the same size creates a surface whose wetting and lubrication characteristics may be controlled by the composition of the deposited mixed bead array. In this way, the location of the individual beads is random, but the relative proportion of each type of bead within the array is controllable. Example π - Assembly of Lens Arrays and Optical Diffraction Elements The present invention can be used to fabricate lens arrays and other surface-mounted optical elements such as diffraction gratings. The functional elements of the present invention enable the placement and delineation of these elements on ITO, facilitating integration with existing planar display technology, and on Si/SiOx, facilitating integration with existing silicon-based device technology. Silica or other oxide particles, polymer latex beads or other objects of high refractive index suspended in an aqueous solution, will refract light. Ordered planar arrays of beads also diffract visible light, generating a characteristic diffraction pattern of shaφ spots. This effect forms the basis of holographic techniques in optical information processing applications. A. - The present invention provides for the use of arrays of refractive colloidal beads as light collection elements in planar array formats in conjunction with low light level detection and CCD imaging. CCD and related area detection schemes will benefit from the enhanced light collection efficiency in solid-phase fluorescence or luminescence binding assays. This assay format relies on the detection of a fluorescence signal indicating the binding of probes to bead-anchored targets in the vicinity of the detector. To maximize through-put, it is desirable to monitor simultaneously as many binding events as possible. It is here that array formation by the methods of the present invention is particularly valuable because it facilitates the placement and tight packing of beads in the target area monitored by the CCD detector, while simultaneously providing for the additional benefit of lensing action and the resulting increase in light collection efficiency. Increased collection efficiency has been demonstrated in experiments employing individual, large (10 micron diameter) polystyrene beads as lensing elements to image small (1 micron diameter) fluorescent polystyrene beads. Under the experimental conditions set forth above an applied voltage of 5V (pp) at 300 Hz induced the collection of small particles under individual large beads within a second. This is shown in Fig. 5, where small beads alone, e.g., 52, appear dim, whereas small beads, e.g. , 54, gathered under a large bead 56 appear brighter and magnified. The small beads redisperse when the voltage is turned off. B. - The use of colloidal bead arrays as diffraction gratings and thus as holographic elements is known. Diffraction gratings have the property of diffracting light over a narrow range of wavelengths so that, for given angle of incidence and wavelength of the illuminating light, the array will pass only a specific wavelength (or a narrow band of wavelengths centered on the nominal value) that is determined by the inter-particle spacing. Widely discussed applications of diffraction gratings range from simple wavelength filtering to the more demanding realization of spatial filters and related holographic elements that are essential in optical information processing. The present invention provides for a rapid and well controlled process of forming planar arrays in a state of crystalline order which will function as. surface-mounted optical diffraction elements. In addition, the resulting surfaces may be designed to display topographical relief to enhance wave-length selective reflectivity. These arrays may be formed in designated areas on a substrate surface. In contrast to the slow and cumbersome prior art method of fabricating such arrays by way of forming equilibrium crystals in aqueous solutions of low salt content, the present invention provides a novel approach to rapidly and reliably fabricate particle arrays at a solid-liquid interface. This approach relies on field-induced formation of arrays to trigger the process, and on UV-mediated patterning or light control to position and shape the arrays. In addition, the inter-particle distance, and internal state of order, and hence the diffraction characteristics of the array, may be fine-tuned by adjusting the applied electric field. For example, a field-induced, reversible order-disorder transition in the array will alter the diffraction pattern from one composed of shaφ spots to one composed of a diffuse ring. The assembly of such arrays on the surface of silicon wafers, as described herein, provides a direct method of integration into existing microelectronic designs. Arrays may be locked in place by chemical coupling to the substrate surface, or by relying on van der Waals attraction between beads and substrate. Example LI - A Novel Mechanism for the Realization of a Particle-Based Display The present invention provides the elements to implement lateral particle motion as a novel approach to the realization of a particle-based display. The elements of the present invention provide for the control of the lateral motion of small particles in the presence of a pre-formed lens array composed of large, refractive particles. Colloidal particulates have been previously employed in flat-panel display technology. The operating principle of these designs is based on electrophoretic motion of pigments in a colored fluid confined between two planar electrodes. In the OFF (dark) state, pigments are suspended in the fluid, and the color of the fluid defines the appearance of the display in that state. To attain the ON (bright) state, particles are assembled near the front (transparent) electrode under the action of an electric field. In this latter state, incident light is reflected by the layer of particles assembled near the electrode, and the display appears bright. Prototype displays employing small reflective particles in accordance with this design are known. However, these displays suffered from a number of serious problems including: electrochemical degradation and lack of colloidal stability as a result of prolonged exposure to the high DC electric fields required to achieve acceptable switching speeds; and non-uniformities introduced by particle migration in response to field gradients inherent in the design of the addressing scheme. The present invention provides a novel mechanism for the design of a particle-based display which takes advantage of electric field-induced array formation as well as controlled, field-induced lateral particle displacements. First, a lens array composed of colloidal beads is formed. This lens array also serves as a spacer array to maintain a well-defined gap between the bottom electrode and the top electrode that may now be placed over the (pre-formed) array. This facilitates fabrication of uniform flat panel displays with a narrow gap that is determined by the particle diameter. Next, small colloidal particles are added to the electrolyte solution in the gap. These may be fluorescent, or may be reflecting incident white light. Under the action of an AC electric field of appropriate frequency, these small particles can be moved laterally to assemble preferentially within the footprint of a larger bead. When viewed through a larger bead, small fluorescent beads assembled under a large bead appear bright as a result of the increased light collection efficiency provided by the lensing action of the large bead; this is the ON state (Fig. 5). When moved outside the footprint of the larger bead, particles appear dim, and may be made entirely invisible by appropriate masking; this is the OFF state. The requisite lateral particle motion may be induced by a change in the applied voltage or a change in light intensity. Each large or lensing bead introduces a lateral nonuniformity in the current distribution within the electrolyte because the current is perturbed by the presence of each lensing bead. In contrast to the prior art displays, the present invention employs AC, not DC fields, and insulating (rather than conductive) electrodes, thereby minimizing electrochemical degradation. The lateral non-uniformity introduced by the lens array is desirable because it introduces lateral gradients in the current distribution within the display cell. These gradients mediate the lateral motion of small beads over short characteristic distances set by the diameter of the large lensing beads, to effect a switching between ON and OFF states. Thus, the present invention readily accommodates existing technology for active matrix addressing. Example IV - Layout-Preserving Transfer of Bead Suspensions from Microtiter Plate to Planar Cell The present invention provides a method to transfer suspensions of beads, or biomolecules to the electrode surface in such a way as to preserve the spatial encoding in the original arrangement of reservoirs, most commonly the conventional 8x12 arrangement of wells in a microtiter plate. Such a fluid transfer scheme is of significant practical importance given that compound libraries are commonly handled and shipped in 8x12 wells. The present invention utilizes chemical patterning to define individual compartments for each of MxN sets of beads and confine them accordingly. In the present instance, patterning is achieved by UV-mediated photochemical oxidation of a monolayer of thiol-terminated alkylsilane that is chemisorbed to the Si/SiOx substrate. Partial oxidation of thiol moities produces sulfonate moities and renders the exposed surface charged and hydrophilic. The hydrophilic portions of the surface, in the form of a grid of squares or circles, will serve as holding areas. In accordance with the present invention, the first function of surface-chemical patterning into hydrophilic sections surrounded by hydrophobic portions is to ensure that droplets, dispensed from different wells, will not fuse once they are in contact with the substrate. Consequently, respective bead suspensions will remain spatially isolated and preserve the lay-out of the original MxN well plate. The second role of the surface chemical patterning of the present invention is to impose a surface charge distribution, in the form of the MxN grid pattern, which ensures that individual bead arrays will remain confined to their respective holding areas even as the liquid phase becomes contiguous. The transfer procedure involves the steps illustrated in Figs. 6a-c. First, as shown in sideview in Fig. 6a, the MxN plate of wells 62 is registered with the pattern 64 on the planar substrate surface. Well bottoms 62, are pierced to allow for the formation of pendant drops of suspension or, preferably, the process is facilitated by a fixture (not shown) providing MxN effective funnels to match the geometric dimensions of the MxN plate on the top and reduce the size of the dispensing end. Such a dispensing fixture will also ensure the precise control of droplet volumes, adjusted so as to slightly overfill the target holding area on the patterned substrate surface. The set of MxN drops is then deposited by bringing them in contact with the hydrophilic holding areas of the pre-patterned substrate and relying on capillary action. Next, the plate is retracted, and the top electrode is carefully lowered to form the electrochemical cell, first making contact as shown in Fig. 6b, with individual, liquid-filled holding areas on the substrate to which suspensions are confined. Overfilling ensures that contact is made with individual suspensions. The electric field is now turned on to induce array formation in the MxN holding areas and to ensure the preservation of the overall configuration of the MxN sets of beads while the gap is closed further (or filled with additional buffer) to eventually fuse individual droplets of suspension into a contiguous Uquid phase as shown in Fig. 6c. In the fully assembled cell of Fig. 6c, while the droplets are fused together, the beads from each droplet are maintained in and isolated in their respective positions, reflecting the original MxN arrangement of wells. The present invention thus provides for the operations required in this implementation of a layout-preserving transfer procedure to load planar electrochemical cells. Example V - Preparation of Heterogeneous Panels of Particles The present invention provides a method to produce a heterogeneous panel of beads and potentially of biomolecules for presentation to analytes in an adjacent liquid. A heterogeneous panel contains particles or biomolecules which differ in the nature of the chemical or biochemical binding sites they offer to analytes in solution. In the event of binding, the analyte is identified by the coordinates of the bead, or cluster of beads, scoring positive. The present method relies on the functional elements of the invention to assemble a planar array of a multi-component mixture of beads which carry chemical labels in the form of tag molecules and may be so identified subsequent to performing the assay. Diagnostic assays are frequently implemented in a planar format of a heterogeneous panel, composed of simple ligands, proteins and other biomolecular targets. For example, in a diagnostic test kit, a heterogeneous panel facilitates the rapid testing of a given analyte, added in solution, against an entire set of targets. Heterogeneous panels of proteins are of great current interest in connection with the emerging field of proteome research. The objective of this research is to identify, by scanning the panel with sensitive analytical techniques such as mass spectrometry, each protein in a multi-component mixture extracted from a cell and separated by two-dimensional gel electrophoresis. Ideally, the location of each spot uniquely corresponds to one particular protein. This analysis would permit, for example, the direct monitoring of gene expression levels in a cell during a particular point in its cycle or at a given stage during embryonic development. The fabrication of an array of heterogeneous targets is central to recently proposed strategies of drug screening and DNA mutation analysis in a planar format. The placement of ligands in a specific configuration on the surface of a planar substrate serves to maintain a key to the identity of any one in a large set of targets presented simulta¬ neously to an analyte in solution for binding or hybridization. In an assay relying on fluorescence, binding to a specific target will create bright spots on the substrate whose spatial coordinates directly indicate the identity of the target. Three principal strategies have been previously employed to fabricate heterogeneous panels. First, protein panels may be created by two-dimensional gel electrophoresis, relying on a DC electric field to separate proteins first by charge and then by size (or molecular weight). Even after many years of refinement, this technique yields results of poor reproducibility which are generally attributed to the poorly defined properties of the gel matrix. Second, individual droplets, drawn from a set of reservoirs containing solutions of the different targets, may be dispensed either by hand or by employing one of several methods of automated dispensing (or "printing" ; see e.g. , Schena et al. , Science 270, 467-470 (1995), the contents of which are incorporated herein by reference). Printing has been applied to create panels of oligonucleotides intended for screening assays based on hybridization. Printing leaves a dried sample and may thus not be suitable for proteins that would denature under such conditions. In addition, the attendant fluid handling problems inherent in maintaining, and drawing samples from a large number of reservoirs are formidable. Third, target ligands may be created by invoking a variant of solid phase synthesis based on a combinatorial strategy of photochemically activated elongation reactions. This approach has been limited by very formidable technical problems in the chemical synthesis of even the simplest, linear oligomers. The synthesis of non-linear compounds in this planar geometry is extremely difficult. The present invention of forming heterogeneous panels requires the chemical attachment of target ligands to beads. Ligands may be coupled to beads "off-line" by a variety of well established coupling reactions. For present puφoses, the bead identity must be chemically encoded so it may be determined as needed. Several methods of encoding, or binary encoding, of beads are available. For example, short oligonucleotides may serve the purpose of identifying a bead via their sequence which may. be determined by microscale sequencing techniques. Alternatively, chemically inert molecular tags may be employed that are readily identified by standard analytical techniques. In contrast to all prior art methods, the present invention provides a novel method to create heterogeneous panels by in-situ, reversible formation of a planar array of "encoded" beads in solution adjacent to an electrode. The array may be random with respect to chemical identity but is ordered with respect to spatial position. This procedure offers several advantages. First, it is reversible so that the panel may be disassembled following the binding assay to discard beads scoring negative. Positive beads may be subjected to additional analysis without the need for intermediate steps of sample retrieval, purification or transfer between containers. Second, the panel is formed when needed, that is, either prior to performing the actual binding assay, or subsequent to performing the assay on the surface of individual beads in suspension. The latter mode minimizes potential adverse effects that can arise when probes bind to planar target surfaces with a high concentration of target sites. Third, to accommodate scanning probe analysis of individual beads, inteφarticle distances within the array may be adjusted by field-induced polarization or by the addition of inert spacer particles that differ in size from the encoded beads. Fig. 7 shows the use of small spacer beads 72 for separating encoded beads 74. As shown, the spacing of beads 74 is greater than the spacing of comparable beads in Fig. 4b. Finally, UV-mediated oxide regrowth, as provided by the present invention, readily facilitates the embedding of a grid pattern of selected dimension into the substrate to ensure the formation of small, layout-preserving subarrays in the low-impedance fields of the grid. To create the panel, a multi-component mixture of beads carrying, for example, compounds produced by bead-based combinatorial chemistry, is placed between electrodes. Each type of bead may be present in multiple copies. Arrays are formed in response to an external field in a designated area of the electrode surface. This novel approach of in-situ assembly of panels relies on beads that carry a unique chemical label, or code, to permit their identification subsequent to the completion of a binding assay. This invention facilitates on-line tagging of beads by way of a photochemical bead- coloring method. Selected beads in an array are individually illuminated by a focused light source to trigger a coloring reaction on the bead surface or in the bead interior to indicate a positive assay score. Beads so marked can be subsequently separated from unmarked beads by a light-activated sorting method described herein. Numerous UV- activated reactions are available to implement this bead-coloring method. The present invention provides for several methods of discarding beads with negative scores, typically the vast majority, while retaining those with positive scores. This method take advantage of the fact that, in contrast to all prior art methods, the array represents a temporary configuration of particles that is maintained by the applied electric field and may be rearranged or disassembled at will. This capability, along with the fact that biomolecules are never exposed to air (as in the prior art method of printing) facilitates the in-situ concatenation of analytical procedures that require the heterogeneous panel in conjunction with subsequent, "downstream" analysis. First, if positive beads are clustered in a subsection of the array, the light-controlled array splitting operation of the present invention may be invoked to dissect the array so as to discard negative portions of the array (or recycle them for subsequent use). Second, if positive and negative beads are randomly interspersed, a fluorescence-activated sorting method, implemented on the basis of the present invention in a planar format, as described herein, may be invoked. In the case of fluorescence- activated sorting, positive and negative beads may be identified as bright and dark objects, respectively. In the special case that only a few positive beads stand out, these may be removed from the array by locking onto them with optical tweezers, a tool to trap and/or manipulate individual refractive particles under illumination, and disassembling the array by removing the field, or subjecting the entire array to lateral displacement by the fundamental operations of the present invention. The typical task in screening a large set of compounds is one of looking for a very small number of positive events in a vast number of tests. The set of discarded beads will typically involve the majority at each stage in the assay. The procedure of the present invention therefore minimizes the effort invested in negative events, such as the challenging in-situ synthesis of target ligands irrespective of whether or not they will prove to be of interest by binding a probe offered in solution. The method of forming a heterogeneous panel according to the present invention contains beads of each type in generally random assembly. The creation of a heterogeneous panel with each position in the panel containing a cluster of beads of the same type, that is, beads originating in the same reservoir (Fig. 6a), may be desirable so as to ensure a sufficiently large number of positive events to facilitate detection. A practical solution follows from the application of the layout-preserving fluidic transfer scheme described herein. In this procedure, beads from an MxN well plate are transferred layout-preservingly onto a chemically patterned substrate in such a way as to preserve the spatial encoding of bead identities. Example VI - Binding and Functional Assays in Planar Bead Array Format The present invention can be used to implement mixed-phase binding assays as well as certain functional assays in a planar array format. Several combinations are possible reflecting the presence of probe or target in solution, on the surface of colloidal beads, or on the electrode surface. The methods of the present invention facilitate the formation of a planar array to present targets to probes in solution prior to performing the binding assay ("preformed" array; Fig. 8). Alternatively, a planar array of beads may be formed in front of a detector surface subsequent to performing the binding assay in suspension ("postformed" array; Fig. 8). The present invention also provides the methods to implement functional assays by enabling the assembly of certain cell types adjacent to a planar detector or sensor surface to monitor the effects of exposure of the cells to small molecule drugs in solution. Binding assays, particularly those involving proteins such as enzymes and antibodies, represent a principal tool of medical diagnostics. They are based on the specific biochemical interaction between a probe, such as a small molecule, and a target, such as a protein. Assays facilitate the rapid detection of small quantities of an analyte in solution with high molecular specificity. Many procedures have been designed to produce signals to indicate binding, either yielding a qualitative answer (binding or no binding) or quantitative results in the form of binding or association constants. For example, when an enzyme binds an analyte, the resulting catalytic reaction may be used to generate a simple color change to indicate binding, or it may be coupled to other processes to produce chemical or electrical signals from which binding constants are determined. Monoclonal antibodies, raised from a single common precursor, may be prepared to recognize virtually any given target, and immunoassays, based on antibody-antigen recognition and binding, have developed into an important diagnostic tool. As with enzyme binding, antibody binding of an antigenic analyte may be detected by a variety of techniques including the classic method of enzyme-linked immunoassays (ELISA) in which the reaction of an antibody-coupled enzyme is exploited as an indicator.. A common and conceptually simple scheme ensures the detection of antibody binding to a target analyte by supplying a fluorescently labeled second antibody that recognizes the first (or primary) antibody. Binding assays involving soluble globular proteins are often performed in solution to ensure unbiased interactions between protein and target. Such liquid phase assays, especially when performed at low concentrations of target or probe, minimize potential difficulties that may arise when either target or probe are present in abundance or in close proximity. By the same token, the kinetics tend to be slow. Cooperative effects, such as crowding, arising from the close proximity of probes must be carefully controlled when either probe or target is chemically anchored to a solid substrate. Nonetheless, this latter solid phase format of binding assays is also very commonly employed whenever the situation demands it. For example, the presence of a protein on the surface of a cell may be exploited in "panning" for the cells that express this protein in the presence of many other cells in a culture that do not: desired cells attach themselves to the surface of a container that is pre-coated with a layer of a secondary antibody directed against a primary antibody decorating the desired cell-surface protein. Similarly, certain phages may be genetically manipulated to display proteins on their surface, and these may be identified by a binding assay involving a small molecule probe such as an antigen if the protein displayed is an antibody (Watson et al. , "Recombinant DNA", 2nd Edition (Scientific American Books, W.H. Freeman and Co. , New York , NY, 1983), the contents of which are incorporated herein by reference). In addition, the planar geometry accommodates a variety of optical and electrical detection schemes implemented in transducers and sensors. A combination of liquid phase and solid phase assay may be developed by using beads that are decorated with either probe or target, as in procedures that employ decorated magnetic beads for sample preparation or purification by isolating binding from non-binding molecules in a given multi-component mixture. Recent examples of the use of these beads include the purification of templates for DNA sequencing applications or the extraction of mRNAs from (lysed) cells by hybridization to beads that are decorated with poly-adenine (poly A) residues. Functional assays involving suitable types of cells are employed to monitor extracellular effects of small molecule drugs on cell metabolism. Cells are placed in the immediate vicinity of a planar sensor to maximize the local concentration of agents , released by the cell or to monitor the local pH. The present invention provides the means to implement mixed phase binding assays in a planar geometry with a degree of flexibility and control that is not available by prior art methods. Thus, it offers the flexibility of forming, in-situ, reversibly and under external spatial control, either a planar panel of target sites for binding of analyte present in an adjacent liquid phase, or a planar array of probe-target complexes subsequent to performing a binding assay in solution. Binding may take place at the surface of individual beads suspended in solution, at the surface of beads pre-assembled into arrays adjacent to the electrode surface, or at the electrode surface itself. Either the target or probe molecule must be located on a bead to allow for a bead-based assay according to the present invention. As shown in Fig. 8, if the probe molecule P is located on a bead, then the target molecule T may be either in solution, on a bead or on the electrode surface. The converse is also true. For example, the methods of the present invention may be used to implement panning, practiced to clone cell surface receptors, in a far more expeditious and controlled manner than is possible by the prior art method. Given a substrate that has been coated with a layer of antibody directed against the sought-after cell surface protein, the present invention facilitates the rapid assembly of a planar array of cells or decorated beads in proximity to the layer of antibodies and the subsequent disassembly of the array to leave behind only those cells or beads capable of forming a complex with the surface-bound antibody. A further example of interest in this category pertains to phage displays. This technique may be employed to present a layer of protein targets to bead-anchored probes. Bead arrays may now be employed to identify a protein of interest. That is, beads are decorated with small molecule probes and an array is formed adjacent to the phage display. Binding will result in a probe-target complex that retains beads while others are removed when the electric field is turned off, or when light-control is applied to remove beads from the phage display. If beads are encoded, many binding tests may be carried out in parallel because retained beads may be individually identified subsequent to binding. The methods of the present invention readily facilitate competitive binding assays. For example, subsequent to binding of a fluorescent probe to a target-decorated bead in solution and the formation of a planar bead array adjacent to the electrode, fluorescent areas within the array indicate the position of positive targets, and these may be further probed by subjecting them to competitive binding. That is, while monitoring the fluorescence of a selected section of the planar array, an inhibitor (for enzyme assays) or other antagonist (of known binding constant) is added to the electrochemical cell, and the decrease in fluorescence originating from the region of interest is measured as a function of antagonist concentration to determine a binding constant for the original probe. This is an example of a concatenation of analytical steps that is enabled by the methods of the present invention. The fact that a probe-target complex is fixed to a colloidal bead, as in the methods of the present invention, conveys practical advantages because this facilitates separation of positive from negative events. Particularly when solid phase assays are performed on a planar substrate, an additional advantage of planar bead arrays is the enhancement of light collection efficiency provided by the beads, as discussed herein. If desired, beads may serve strictly as delivery vehicles for small molecule probes. That is, an array of probe-decorated beads is formed adjacent to a target- decorated surface in accordance with the methods of the present invention. UV-activated cleavage of the probe from the bead support will ensure that the probe is released in close proximity to the target layer, thereby enhancing speed and efficiency of the assay. The identity of the particular probe interacting with the target may be ascertained from the positional location of the bead delivering the probe. The methods of the present invention apply not only to colloidal beads of a wide variety (that need no special preparative procedures to make them magnetic, for example), but also to lipid vesicles and cells that are decorated with, or contain embedded in their outer wall, either probe or target. The methods of the present invention may therefore be applied not only to bead-anchored soluble proteins but potentially to integral membrane receptors or to cell surface receptors. In particular, the rapid assembly of cells in a designated area of the substrate surface facilitates the implementation of highly parallel cell-based functional assays. The present invention makes it possible to expose cells to small molecule drug candidates in solution and rapidly assemble them in the vicinity of a sensor embedded in the electrode surface, or to expose pre-assembled cells to such agents that are released into the adjacent Uquid phase. In the simplest case, all cells wiU be of the same type, and agents wiU be administered sequentiaUy. Even in this sequential version, electrokinetic . mixing wiU enhance through-put. However, as described herein, the methods of the present invention also enable the paraUel version of binding assays and thus of functional assays in a planar format by encoding the identity of different ceUs by a "Layout- Preserving Transfer" process from an 8x12 weU plate, as discussed herein, and to isolate ceUs scoring positive by providing feed-back from a spatially resolved imaging or sensing process to target a specific location in the array of ceUs. Example VII - Separation and Sorting of Beads and Particles The present invention can be used to implement several procedures for the separation and sorting of coUoidal particles and biomolecules in a planar geometry. SpecificaUy, these include techniques of lateral separation of beads in mixtures. Individual beads may be removed from an array formed in response to an electric field by the appUcation of optical tweezers. The separation of components in a given mixture of chemical compounds is a fundamental task of analytical chemistry. SimUarly, biochemical analysis frequently caUs for the separation of biomolecules, beads or ceUs according to size and/or surface charge by electrophoretic techniques, whUe the sorting (most commonly into just two sub-classes) of suspended ceUs or whole chromosomes according to optical properties such as fluorescence emission is usually performed using field-flow fractionation including flow cytometry and fluorescence-activated ceU sorting. In a planar geometry, bead mixtures undergoing diffusion have been previously separated according to mobiUty by appUcation of an AC electric field in conjunction with Uthographic patterning of the electrode surface designed to promote directional drift. EssentiaUy, the AC or pulsing electric field is used to move smaU beads in a particular direction over a period of time. CapiUary electrophoresis has been implemented in a planar geometry, see e.g., B.B. Haab and R.A. Mathies, Anal. Chem 67, 3253-3260 (1995), the contents of which are incorporated herein by reference. The methods of the present invention may be appUed in several ways to implement the task of separation, sorting or isolation in a planar geometry. In contrast to the prior art approaches, the present invention provides a significant degree of flexibili¬ ty in selecting from among several available procedures, the one best suited to the particular task at hand. In some cases, more than one separation technique may be appUed, and this provides the basis for the implementation of two-dimensional separation. . That is, beads may be separated according to two different physical-chemical characteristics. For example, beads may first be separated by size and subsequently, by raising the appUed frequency to induce chain formation, by polarizability. This flexibiUty offers particular advantages in the context of integrating analytical functionaUties in a planar geometry. Several techniques wiU now be described. i) The present invention may be used to implement " sieving" in lateral, electric field-induced flow on surfaces patterned by UV-mediated oxide regrowth to sort beads in a mixture by size. The fundamental operations of the invention are invoked to set up directed lateral particle motion along conduits laid out by UV-mediated oxide regrowth. Conduits are designed to contain successively narrower constrictions through which particles must pass. Successively finer stages aUow only successively smaUer particles to pass in this "sieving" mechanism (Fig. 9a). As shown in Fig. 9a, the primary particle flow is in the direction left to right, while a transverse flow is estabhshed in the top to bottom direction utilizing an oxide profile as shown. Additionally, rows of barriers 92 made from thick oxide are positioned along the conduit with the spacing between the barriers in each row decreasing in the transverse direction. As the particles move along the conduit, the rows of barriers act to separate out smaUer particles in the transverse direction. In contrast to previous methods based on electrophoretic separation, large DC electric fields, and the attendant potential problem of electrolysis and interference from electroosmotic flow in a direction opposite to the field-directed particle transport, the present invention uses AC electric fields and lateral gradients in interfacial impedance to produce transport. The present method has the advantage of avoiding electrolysis and it takes expUcit advantage of electroosmotic flow to produce and control particle transport. In addition, the use of Si/SiOx electrodes enables the use of the Ught-control component of the present invention to modify lateral transport of beads in real time. For example, external iUumination may be employed to locaUy neutralize the lateral impedance gradient induced by UV-mediated oxide regrowth. Particles in these neutral "zones" would no longer experience any net force and come to rest. This principle may be used as a basis for the implementation of a scheme to locaUy concentrate particles into shaφ bands and thereby to improve resolution in subsequent separation. ii) The present invention may be used to implement "zone refining", a process of excluding minority components of a mixture by size or shape from a growing crystalline array of majority component. This process expUcitly depends on the . capabiUties of the present invention to induce directional crystallization. The process of zone refining is employed with great success in producing large single crystals of siUcon of very high purity by excluding impurities from the host lattice. The concept is famiUar from the standard chemical procedure of purification by recrystaUization in which atoms or molecules that are sufficiently different in size, shape or charge from the host species so as not to fit into the forming host crystal lattice as a substitutional impurity, are ejected into solution. By enabling the growth of planar arrays, in a given direction and at a controUed rate, the present invention faciUtates the implementation of an analogous zone refining process for planar arrays. The most basic geometry is the linear geometry. A multi-component mixture of beads of different sizes and/or shapes is first captured in a rectangular holding area on the surface, laid out by UV-patterning. Next, crystallization is initiated at one end of the holding area by iUumination and aUowed to slowly advance across the entire holding area in response to an advancing pattern of iUumination. In general, differences of approximately 10% in bead radius trigger ejection. iii) The present invention may be used to implement fractionation in a transverse flow in a manner that separates particles according to mobility. Field-flow fractionation refers to an entire class of techniques that are in wide use for the separation of molecules or suspended particles. The principle is to separate particles subjected to fluid flow in a field acting transverse to the flow. A category of such techniques is subsumed under the heading of electric-field flow fractionation of which free-flow electrophoresis is a pertinent example because it is compatible with a planar geometry. Free-flow electrophoresis employs the continuous flow of a replenished buffer between two narrowly spaced plates in the presence of a DC electric field that is appUed in the plane of the bounding plates transverse to the direction of fluid flow. As they traverse the electric field, charged particles are deflected in proportion to their electrophoretic mobility and coUected in separate outlets for subsequent analysis. In contrast to conventional electrophoresis, free-flow electrophoresis is a continuous process with high throughput and it requires no supporting medium such as a gel. The present invention enables the implementation of field-flow fractionation in a planar geometry. As previously discussed herein, impedance gradients imposed by UV-oxide profiling serve to mediate particle motion along the electrode surface in response to the external electric field. In a ceU with a narrow gap, the resulting electrokinetic flow has a "plug" profile and this has the advantage of exposing aU particles to identical values of the flow velocity field, thereby minimizing band distortions introduced by the paraboUc velocity profile of the laminar flow typicaUy employed in free-flow electrophoresis. A second flow field, transverse to the primary flow direction, may be employed to mediate particle separation. This deflecting flow may be generated in response to a second impedance gradient. A convenient method of imposing this second gradient is to take advantage of UV-oxide patterning to design appropriate flow fields. Both longitudinal and transverse flow would be recirculating and thus permit continuous operation even in a closed ceU, in contrast to any related prior art technique. Additional flexibUity is afforded by invoking the Ught-control component of the present invention to illuminate the substrate with a stationary pattern whose intensity profile in the direction transverse to the primary fluid flow is designed to induce the desired impedance gradient and hence produce a transverse fluid flow. (Fig. 9b). This has the significant advantage of permitting selective activation of the transverse flow in response to the detection of a fluorescent bead crossing a monitoring window upstream. Non-fluorescent beads would not activate the transverse flow and would not be deflected. This procedure represents a planar analog of flow cytometry, or fluorescence-activated ceU sorting. iv) The invention may be used to induce the formation of particle chains in the direction normal to the plane of the electrode. The chains represent conduits for current transport between the electrodes and their formation may reflect a field-induced polarization. Chains are much less mobUe in transverse flow than are individual particles so that this effect may be used to separate particles according to the surface properties that contribute to the net polarization. The effect of reversible chain formation has been demonstrated under the experimental conditions stated herein. For example, the reversible formation of chains occurs, for carboxylated polystyrene beads of 1 micron diameter, at a voltage of 15 V (pp) at frequencies in excess of 1MHz. v) The invention may be used to isolate individual beads from a planar array. Fluorescence binding assays in a planar array format, as described herein, may produce singular, bright beads within a large array, indicating particularly strong binding. To isolate and retrieve the corresponding beads, optical tweezers in the form of a shaφly focused laser spot, may be employed to lock onto an individual bead of interest. The Ught-control component of the present invention may be used in conjunction with the optical tweezers to retrieve such an individual bead by moving the array relative to the bead, or vice versa, or by disassembling the array and retaining only the marked bead. This is a rather unique capability that will be particularly useful in the context of isolating beads in certain binding assays. Commercial instrumentation is avaUable to position optical tweezers in the field of a microscope. Larger scale motion is facilitated by translocating the array in-situ or simply by moving the external sample fixture. This process lends itself to automation in conjunction with the use of peak-finding image analysis software and feedback control. vi) The invention may be used to implement a Ught-induced array sectioning ("shearing") operation to separate fluorescent, or otherwise deUneated portions of an array from the remainder. This operation makes it possible to segment a given array and to isolate the corresponding beads for downstream analysis. The basis for the implementation of this array segmentation is the Ught-control component of the present invention, in the mode of driving particles from an area of a Si/SiOx interface that is illuminated with high intensity. It is emphasized here that this effect is completely unrelated to the Ught-induced force on beads that underUes the action of optical tweezers. The present effect which operates on large sets of particles, was demonstrated under the experimental conditions stated herein using a 100W illuminator on a Zeiss UEM microscope operated in epi-iUuminauon. A simple implementation is to superimpose, on the uniform iUumination pattern appUed to the entire array, a line-focussed beam that is positioned by manipulation of beam steering elements external to the microscope. Beads are driven out of the illuminated linear portion. Other implementations take advantage of two separately controUed beams that are partiaUy superimposed. The linear sectioning can be repeated in different relative orientations of shear and array. Example VHI - Screening for Drug Discovery in Planar Geometry The functional elements of the present invention may be combined to implement procedures for handling and screening of compound and combinatorial Ubraries in a planar format. The principal requisite elements of this task are: sample and reagent . deUvery from the set of original sample reservoirs, commonly in a format of 8x12 weUs in a microtiter plate, into a planar ceU; fabrication of planar arrays of targets or of probe-target complexes adjacent to the planar electrode surface prior to or subsequent to performing a binding assay; evaluation of the binding assay by imaging the spatial distribution of marker fluorescence or radioactivity, optionaUy foUowed by quantitative pharmacokinetic measurements of affinity or binding constants; isolation of beads scoring positive, and removal from further processing of other beads; and coUection of specific beads for additional downstream analysis. The present invention relates to aU of these elements, and the fundamental operations of the invention provide the means to concatenate these procedures in a planar format. A central issue in the implementation of cost-effective strategies for modern therapeutic drug discovery is the design and implementation of screening assays in a manner facilitating high throughput while providing pharmacokinetic data as a basis to select promising drug leads from a typicaUy vast Ubrary of compounds. That is, molecular specificity for the target, characterized by a binding constant, is an important factor in the evaluation of a new compound as a potential therapeutic agent. Common targets include enzymes and receptors as weU as nucleic acid Ugands displaying characteristic secondary structure. The emerging paradigm for lead discovery in pharmaceutical and related industries such as agricultural biotechnology, is the assembly of novel synthetic compound Ubraries by a broad variety of new methods of soUd state "combinatorial" synthesis. Combinatorial chemistry refers to a category of strategies for the paraUel synthesis and testing of multiple compounds or compound mixtures in solution or on solid supports. For example, a combinatorial synthesis of a linear oUgopeptide containing n amino acids would simultaneously create aU compounds representing the possible sequence permutations of n amino acids. The most commonly employed implementation of combinatorial synthesis reUes on coUoidal bead supports to encode reaction steps and thus the identity of each compound. Beads preferred in current practice tend to be large (up to 500 microns in diameter) and porous to maximize their compound storage capacity, and they must be encoded to preserve the identity of the compound they carry. Several methods of encoding, or binary encoding, of beads are available. Two examples are as foUows. First, beads may be labeled with short oUgonucleotides such as the 17-mers typicaUy employed in hybridization experiments. The sequence of . such short probes may be determined by microscale sequencing techniques such as direct Maxam-GUbert sequencing or mass spectrometry. This encoding scheme is suitable when the task caUs for screening of Ubraries of nucleic acid Ugands or oUgopeptides. Second, members of a combinatorial Ubrary may be associated with chemicaUy inert molecular tags. In contrast to the previous case, these tag molecules are not sequentiaUy linked. Instead, the sequence of reaction steps is encoded by the formal assignment of a binary code to individual tag molecules and their mixtures that are attached to the bead in each successive reaction step. The tags are readUy identified by standard analytical techniques such as gas chromatography. This general encoding strategy is currently employed in the synthesis of combinatorial Ubraries on coUoidal beads. Commercial compound Ubraries are large, given that even for the aforementioned 17-mer, the number of sequence permutations is 4 A 17, or approximately 10 A 10. However, the high specificity of typical biological substrate-target interactions impUes that the vast majority of compounds in the coUection wiU be inactive for any one particular target. The task of screening is to select from this large set the few potential lead compounds displaying activity in binding or in functional assays. The principal drug discovery strategy widely appUed to natural compound Ubraries in the pharmaceutical industry is to select individual compounds from the Ubrary at random and subject them to a series of tests. Systematic screening procedures are thus required to implement the rapid screening and scoring of an entire Ubrary of synthetic compounds, in practice often containing on the order of 10 A 7 items. In current practice, compounds are first cleaved and eluted from their soUd supports and are stored in microtiter plates. Further sample handling in the course of screening reUes primarily on robotic pipetting and transfer between different containers, typicaUy weUs in microtiter plates. WhUe robotic workstations represent a step in the direction of automating the process, they rely on the traditional format of microtiter plates containing 8x12 weUs and sample handling by pipetting and thus represent merely an incremental operational improvement. A significant additional consideration is the need to conserve reagent and sample by reducing the spatial scale of the analytical procedures. The present invention provides a set of operations to realize integrated sample handling and screening procedures for bead-based compound Ubraries in a planar format. This wiU significantly reduce time and cost due to reagent and sample volumes. The principal advantage of the methods of the present invention is that they provide a . large set of fundamental operations to manipulate sets of beads in a planar format, permitting the handling of beads between stations in a multi-step analytical procedure. In particular, as previously described herein, the methods of the present invention faciUtate the implementation of the foUowing pertinent procedures: transfer of samples from microtiter plates to a planar electrochemical ceU; formation of heterogeneous panels of target sites adjacent to the substrate surface; soUd phase binding assays; and isolation of specific beads from an array. In addition, the fundamental operations of the present invention provide the means to concatenate these procedures on the surface of a planar electrode. As described herein for hybridization assays, several variants are possible. That is, binding assays may be performed by aUowing protein targets such as enzymes to bind to compounds on the surface of a bead, either in suspension or arranged in a planar array. The common practice of combinatorial chemistry based on large porous carrier beads accommodates the concurrent handling of smaUer beads to whose outer surface compounds are anchored via inert chemical spacers. Such smaU beads (up to 10 microns in diameter) are readUy manipulated by the methods of the present invention. Large beads are used as labeled compound storage containers. Alternatively, binding between target and a radioactively or otherwise labeUed probe may occur in solution, within microtiter plate weUs, if compounds have already been cleaved from their synthesis support. In that case, probe-target complexes may be captured by complexation to encoded beads in each weU, for example via the secondary antibody method of coupling the protein target to a bead-anchored antibody. Bead-captured probe-target complexes are then transferred to the planar ceU for proximity analysis and further processing as iUustrated in Fig. 10. As shown in Fig. 10, probe- target complexes 102 are aUowed to form in solution. Antibody coated beads 104 are added to the solution, resulting in a bead anchored complex 106. The bead anchored complexes 106 are deposited onto electrode 108 from weUs 110, and a planar array of bead anchored complexes is formed. When fluorescent probes 114 are used, these impart fluorescence to the bead anchored complex, faciUtating detection. The methods and apparatus of the present invention are weU suited to the task of identifying a smaU number of positive events in a large set. The imaging of an entire array of probe-target complexes is further enhanced by proximity to an area detector, and by bead lensing action. The isolation of a smaU number of positive scores from the array is readUy achieved, for example by applying optical tweezers, as described herein. The large remainder of the array may then be discarded. This in turn considerably reduces the complexity of applying more stringent tests, such as the determination of binding constants, because these may be restricted to the few retained beads. These tests may be directly appUed, without the need for additional sample transfer to new containers, to the samples surviving the first screening pass. Example DC - Hybridization Assays in Planar Array Format The present invention can be used to implement soUd phase hybridization assays in a planar array format in a configuration related to that of a protein binding assay in which target molecules are chemicaUy attached to colloidal beads. The methods of the present invention faciUtate the formation of a planar array of different target oUgonucleotides for presentation to a mixture of strands in solution. Alternatively, the array may be formed subsequent to hybridization in solution to facilitate detection and analysis of the spatial distribution of fluorescence or radioactivity in the array. Considerable research and development is presently being invested in an effort to develop miniaturized instrumentation for DNA sample extraction and preparation including amplification, transcription, labeling and fragmentation, with subsequent analysis based on hybridization assays as weU as electrophoretic separation. Hybridization assays in planar array format are being developed as a diagnostic tool for the rapid detection of specific single base pair mutations in a known segment of DNA, and for the determination of expression levels of ceUular genes via analysis of the levels of corresponding mRNAs or cDNAs. Hybridization of two complementary single strands of DNA involves molecular recognition and subsequent hydrogen bond formation between corresponding nucleobases in the two opposing strands according to the rules A-T and G-C; here A, T, G and C respectively represent the four nucleobases Adenine, Thymine, Guanosine and Cytosine found in DNA; in RNA, Thymine is replaced by UracU. The formation of double-strand, or duplex, DNA requires the pairing of two highly negatively charged strands of DNA, and the ionic strength of the buffer, along with temperature, plays a decisive role. As previously discussed herein, two principal methods to prepare heterogeneous arrays of target strands on the surface of a planar substrate are micro-dispensing ("printing") and in-situ, spatiaUy encoded synthesis of oUgonucleotides representing aU possible sequence permutations for a given total length of strand. In this context, hybridization must necessarily occur in close proximity to a planar substrate surface and this condition requires care if compu ations from steric hindrance and from non-specific binding of strands to the substrate are to be avoided. Non-specific adsoφtion can be a serious problem, especiaUy in the presence of DC electric fields employed in current commercial designs that rely on electrophoretic deposition to accelerate the kinetics of hybridization on the surface. In addition, there are the technical difficulties, previously discussed herein, resulting from steric hindrance and from coUective effects reflecting the crowding of probe strands near the surface. In the context of DNA analysis, coUoidal (magnetic) beads are commonly used. For example, they are employed to capture DNA in a widely used screening procedure to select cDNAs from clone Ubraries. SpecificaUy, cDNAs are aUowed to hybridize to sequences within long genomic DNA that is subsequently anchored to magnetic beads to extract the hybridized cDNA from the mixture. The present invention facilitates the formation of planar arrays of oUgonucleotide-decorated coUoidal beads, either prior to or subsequent to hybridization of a fluorescence probe strand to the bead-anchored target strand or subsequent to hybrid¬ ization in free solution and bead capture of the end-functionalized target strand. In contrast to prior art methods, the present invention does not require hybridization to occur in the vicinity of planar substrate surface, although this is an option if bead-anchored probe strands are to be deUvered to substrate-anchored target strands. The abiUty to perform hybridization either in solution, on the surface of individual beads, or at the substrate surface provides an unprecedented degree of flexibihty. In addition, the advantages of bead arrays, as described herein, make it feasible to select and isolate individual beads, or groups of beads, from a larger array on the basis of the score in a hybridization assay. This isolation facilitates the implementation of subsequent assays on the strands of interest. The fact that beads remain mobUe also means that beads of interest may be coUected in designated holding areas for microsequencing, or may be moved to an area of substrate designated for PCR amplification. The methods of the present invention may be used to implement a hybridization assay in a planar array format in one of two principal variations. AU 44 involve the presence of the entire repertoire of beads in the planar array or panel formed . adjacent to the electrode surface for parallel read-out. As with heterogeneous panels in general, the arrangement of beads within the array is either random (with respect to chemical identity), and the identity of beads scoring high in the binding assay must be determined subsequently, or it is spatiaUy encoded by invoking the "Layout-Preserving Transfer" method of sample loading described herein. The former variant is readUy implemented and accommodates array formation either prior to or subsequent to performing the binding assay. For example, binding may be performed in suspension before beads are assembled into the array. As with the aforementioned cDNA selection procedure, the method of the present invention also accommodates the use of beads as capture elements for end-functionalized target DNA, for example, via biotin-streptavidin complexation. In this latter case, beads serve as a deUvery vehicle to coUect aU probe-target complexes to the electrode surface where they are assembled into an array for ease of analysis. In particular, proximity CCD detection of beads on electrodes wiU benefit from the lensing action of the beads in the array. This version of the assay is preferably used if only a smaU number of positive scores are expected. Hybridization to a pre-formed bead array can take advantage of a variant of the assay which preserves spatial encoding. An array of bead clusters is formed by the "Layout-Preserving Transfer" method previously described herein, and exposed to a mixture of cDNAs. The resulting spatial distribution of fluorescence intensity or radioactivity reflects the relative abundance of cDNAs in the mixture. This procedure reUes on the detection of a characteristic fluorescence or other signal from the probe-target complex on the surface of a single bead. Given the fact that the array is readUy held stationary by the methods of the present invention, image acquisition may be extended to attain robust signal-to-noise for detection of low level signals. For example, a signal generated by a bead of 10 micron diameter with at most 10 Λ 8 probe-target complexes on the surface of the bead may be detected. Bead lensing action also aids in detection. As with the implementation of drug screening, the functional elements of the present invention may be combined to perform multiple preparative and analytical procedures on DNA. Example X - AUgnment and Stretching of DNA in Electric Field-Induced Flow The present invention can be used to position high-molecular weight DNA in its coUed configuration by invoking the fundamental operations as they apply to other coUoidal particles. However, in addition, the electrokinetic flow induced by an electric field at a patterned electrode surface may be employed to stretch out the DNA into a linear configuration in the direction of the flow. Procedures have been recently introduced which rely on optical imaging to construct a map of cleavage sites for restriction enzymes along the contour of an elongated DNA molecule. This is generaUy known as a "restriction map". These procedures, which facilitate the study of the interaction of these and other proteins with DNA and may also lead to the development of techniques of DNA sequencing, depend on the abiUty to stretch and aUgn DNA on a planar substrate. For individual DNA molecules, this has been previously achieved by subjecting the molecule to elongational forces such as those exerted by fluid flow, magnetic fields acting on DNA-anchored magnetic beads or capiUary forces. For example, DNA "combs" have been produced by simply placing DNA molecules into an evaporating droplet of electrolyte. If provisions are made to promote the chemical attachment of one end of the molecule to the surface, the DNA chain is stretched out as the receding line of contact between the shrinking droplet and the surface passes over the tethered molecules. This leaves behind dry DNA molecules that are attached in random positions within the substrate area initiaUy covered by the droplet, stretched out to varying degrees and generaUy aUgned in a pattern of radial symmetry reflecting the droplet shape. Linear "brushes", composed of a set of DNA molecules chemicaUy tethered by one end to a common line of anchoring points, have also been previously made by aUgning and stretching DNA molecules by dielectrophoresis in AC electric fields appUed between two metal electrodes previously evaporated onto the substrate. The present invention invokes electrokinetic flow adjacent to an electrode patterned by UV-mediated regrowth of oxide to provide a novel approach to the placement of DNA molecules in a predetermined arrangement on a planar electrode surface, and to the stretching of the molecules from their native coU configuration into a stretched, linear configuration that is aUgned in a pre-determined direction. This process is shown in Fig. 11 and is accompUshed by creating controUed gradients in the flow vicinity across the dimension of the DNA coU. The velocity gradient causes different portions of the coU to move at different velocities thereby stretching out the coU. By maintaining a stagnation point at zero velocity, the stretched coU wiU be fixed in position. This method has several advantages over the prior art approaches. First, DNA molecules in their coUed state are subjected to Ught control to form arrays of desired shape in any position on the surface. This is possible because large DNA from cosmids or YACs forms coUs with a radius in the range of one micron, and thus acts in a manner analogous to coUoidal beads. A set of DNA molecules may thus be steered into a desired initial arrangement. Second, UV-patterning ensures that the elongational force created by the electrokinetic flow is directed in a predetermined direction. The presence of metal electrodes in contact with the sample, a disadvantage of the dielectrophoretic prior art method, is avoided by eliminating this source of contamination that is difficult to control especiaUy in the presence of an electric field. On patterned Si/SiOx electrodes, flow velocities in the range of several microns/second have been generated, as required for the elongation of single DNA molecules in flow. Thus, gradients in the flow field determines both the fractional elongation and the orientation of the emerging linear configuration. Third, the present invention faciUtates direct, real-time control of the velocity of the electric field-induced flow, and this in turn conveys expUcit control over the fractional elongation. WhUe the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skiUed in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims: CLAIMS l. A method for manipulating particles suspended at an interface between an electrode and an electrolyte solution, the method comprising the following steps: generating an electric field at an interface between an electrode and an electrolyte solution; patterning either the surface or interior of said electrode to modify its electrochemical properties; and illuminating said surface with a predetermined light pattern to control the movement of said particles in accordance with said predetermined light pattern and the electrochemical properties of said electrode. 2. The method of claim 1, wherein said electric field is at least one of a constant and a time varying electric field. 3. The mediod of claim 1 , wherein said patterning step is performed using at least one of UV-mediated oxide regrowth, surface chemical patterning and surface charge density profiling. 4. The method of claim 1, wherein said electrode is a light sensitive electrode. 5. The method of claim 1, wherein said illuminating step is performed using at least a single, spatially modulated light source. 6. The memod of claim 1 , wherein the illuminating step comprises the further step of: illuminating a selected area of said electrode to cause the particles to move into said selected area. 7. The method of claim 1, wherein the illuminating step comprises the further step of: illuminating a selected area of said electrode surface with a high intensity light pattern so as to cause the particles to move out of said selected area. 8. The mediod of claim 1 , wherein said illuminating step is time varying. 9. The method of claim 8 , wherein a traveling transverse constriction wave is established along said electrode. 10. The method of claim 1, wherein the patterning step comprises the further step of: creating a selected area of low impedance on said electrode to cause the particles to move into said selected area. 11. The method of claim 10, wherein the frequency of said electric field is adjusted in order to place particles at tht boundary delineating said area of low impedance. 12. The method of claim 1, wherein the patterning step comprises the further step of: providing said surface with low impedance except for a selected area to cause the particles to move out of said selected area. 13. The method of claim 1, wherein said patterning step is used to create first and second areas of low impedance on said electrode, and said illuminating step is used to selectively connect said first and second areas to cause said particles to selectively move between said first area and said second area. 14. The method of claim 1, wherein said patterning step is used to create first, second and third areas of low impedance on said surface, and said illuminating step is used to selectively cause said colloidal particles to move from said first and second areas into said third area. 15. The method of claim 1 , wherein said patterning step is used to create first, second and third areas of low impedance on said surface, and said illuminating step is used to selectively cause said particles to separate and move from said first area into said second and third areas. 16. The method of claim 1, further comprising the following steps: defining a pattern on said electrode, said pattern including a narrow conduit connected to a wide area; and forming a substantially constant, reduced thickness oxide on said electrode, said reduced thickness oxide substantially corresponding to said narrow conduit and said wide area. 17. The method of claim 1, further comprising the following steps: defining a pattern on said electrode, said pattern including a narrow conduit; and forming a variable thickness oxide on said electrode, said variable thickness oxide corresponding substantially to said narrow conduit and increasing from a first thickness at a first end of said narrow conduit to a second thickness at a second end of said narrow conduit. 18. The method of claim 17 , wherein said variable thickness oxide increases substantially linearly from said first thickness to said second thickness. 19. A method of transverse electrokinetic movement of particles at an interface between an electrode and an electrolyte solution, the method comprising the following steps: providing a light-sensitive electrode and an electrolyte solution; generating an electric field at an interface between said electrode bU and said electrolyte solution; and illuminating said electrode with a predetermined light pattern to form lateral gradients in the electrochemical properties of said electrode to control the movement of said particles in accordance with said iUumination pattern in a direction substantially orthogonal to the direction of said electric field. 20. A method of transverse electrokinetic movement of particles at an interface between an electrode and an electrolyte solution, the method comprising the following steps: providing an electrode and an electrolyte solution; generating an electric field at an interface between said electrode and said electrolyte solution; and patterning said electrode to form lateral gradients in the electrochemical properties of said electrode to control the movement of said particles in accordance with said pattern created in said electrode in a direction substantially orthogonal to the direction of said electric field. 21. A sorting apparatus for implementing the differential lateral displacement of particles suspended at an interface between an electrode and an electrolyte solution, said apparatus comprising: an electric field generator which generates an electric field at said interface; an electrode; an electrolyte solution having a substantially continuous flow which effects the displacement of said particles in a direction substantially parallel to said interface; said electrode being patterned and having its electrochemical properties modified; an illumination source which illuminates said electrode widi an adjustable, predetermined light pattern; and bl a plurality of particles located in said electrolyte solution, said particles being acted upon by a combination of forces arising from said substantially continuous electrolyte flow and from said electric field in accordance with said predetermined light pattern and said electrode electrochemical properties, said particles being displaced in accordance with variations in the physical and chemical properties which determine the mobility of said particles. 22. The sorting apparatus of claim 21, wherein said patterning includes a plurality of rows of intermittently spaced barrier areas of high impedance, the intermittent spacing of the barriers decreasing from one row to the next, said rows being positioned transversely across said electrode; said electrode being further patterned to include an impedance profile characterized in that said impedance profile decreases in the direction across said electrode, said impedance profile having a high value at one side of said electrode corresponding to the row of barriers having the largest intermittent spacing, and a low value at an opposite side of said electrode corresponding to the row of barriers having the smallest intermittent spacing; said electric field causing said particles to move in a direction substantially transverse to said electrolyte flow in accordance with said variation in impedance between said first and second sides of said electrode, said rows of intermittently spaced barriers acting to separate particles by size in accordance with the intermittent spacing of said rows of barriers. 23. The sorting apparatus of claim 21, wherein said electrode is a light sensitive electrode. 24. The sorting apparatus of claim 21, wherein said impedance profile is created by a predetermined illumination pattern. 25. The sorting apparatus of claim 21, wherein: said electrode patterning includes an area of low impedance bordered by an area of high impedance, said low impedance area including a narrow conduit in communication with a wide conduit, both said conduits being oriented parallel to the direction of said continuous flow of said electrolyte; said wide conduit including a row of intermittently spaced areas of high impedance barriers traversing the width of said wide conduit; a portion of said plurality of particles being optically distinguishable from the remaining particles; a detector for visually inspecting said particles traversing the length of said narrow conduit in response to said continuous flow of electrolyte; said illumination pattern being substantially in the shape of a rectangle having a longer dimension adjusted to be substantially equal to the width of said wide conduit, said rectangle having a smaller dimension which is adjusted to be substantially equivalent to the diameter of said particles, said pattern being located in front of said barriers, and said illumination pattern conforming to an intensity profile placing a maximal value of intensity in the center of said wide conduit and decreasing symmetrically to lower values of intensity at the two sides of said wide conduit; and a delay activation circuit which activates said illumination profile in response to a signal derived from said visual inspection of said particles so as to cause an illuminated particle to be displaced from regions of maximum intensity to regions of lower intensity of said intensity profile and to be deflected into the intermittent spaces between said barriers. 26. A method of dynamically assembling and disassembling an aπay of particles at an interface between an electrode and an electrolyte solution, the method comprising the following steps: providing an electrode, an electrolyte solution and an interface therebetween; providing a plurality of particles located in said electrolyte solution; oό _ patterning said electrode to include at least one area of modified electrochemical properties; illuminating said electrode with a predetermined light pattern; generating an electric field at said interface to cause the assembly of an array of particles in accordance with the predetermined light pattern and the electrochemical properties of said electrode; and removing said electric field to cause the disassembly of said array of particles. 27. The method of claim 26, wherein said predetermined light pattern is adjusted to reconfigure said particle array in accordance with said predetermined light pattern. 28. The method of claim 26, wherein said particle array is compositionally random and said particles are chemically encoded to include chemically or physically distinguishable characteristics. 29. A method of forming a spatially encoded array including multiple types of particles suspended at an interface between an electrode and an electrolyte solution, said method comprising the following steps: providing an electrode and an electrolyte solution; providing multiple types of particles, each type being stored in accordance with chemically or physically distinguishable particle characteristics in one of a plurality of reservoirs, each reservoir containing a plurality of like-type particles suspended in said electrolyte solution; providing said reservoirs in the form of an MxN grid arrangement; patterning said electrode to define MxN compartments corresponding to said MxN grid of reservoirs; depositing MxN droplets from said MxN reservoirs onto said corresponding MxN compartments, each said droplet originating from one of said reservoirs and remaimng confined to one of said MxN compartments and each said droplet containing at least one particle; positioning a top electrode above said droplets so as to simultaneously contact each said droplet; generating an electric field between said top electrode and said MxN droplets; using said electric field to form a particle array in each of said MxN compartments, each said particle array remaining spatially confined to one of said MxN droplets; illuminating said MxN compartments on said patterned electrode with a predetermined light pattern to maintain the position of said particle arrays in accordance with said predetermined light pattern and the pattern of MxN compartments; and positioning said top electrode closer to said electrode thereby fusing said MxN droplets into a continuous liquid phase, while maintaining each of said MxN particle arrays in one of the corresponding MxN compartments. 30. The method of claim 29, wherein said compartments are hydrophilic and the remainder of said electrode surface is hydrophobic. 31. A method of forming an optical lens array including particles suspended at an interface between an electrode and an electrolyte solution, said method comprising the following steps: providing an electrode and an electrolyte solution having an interface therebetween; providing a plurality of particles located in said electrolyte solution; patterning said electrode to include at least one area of modified impedance; and generating an electric field at said interface to cause said particles to assemble into an ordered array within said area of modified impedance. 32. The method of claim 31 , wherein said lens array is used to enhance light collection from said electrode surface. 33. The method of claim 31, wherein said lens array acts to magnify the size and intensity of small particles underneath said lens array. 34. The method of claim 31, wherein said lens array comprises large particles on the order of 10 microns in diameter and said small particles have a diameter on the order of 1 micron. 35. A diffraction grating formed using particles suspended at an interface between an electrode and an electrolyte solution, said grating comprising: an electrode and an electrolyte solution having an interface therebetween; a plurality of particles located in said electrolyte solution, said particles including large particles and small particles; said electrode being patterned to include at least one area of modified impedance; and an electric field generator which generates an electric field at said interface causing said large and small particles to assemble into an array, said small particles being located between said large particles to create a large particle separation distance corresponding substantially to the size of said small particles, said large particle separation distance also determining the spatial frequency characteristics of said diffraction grating, said frequency characteristics being adjustable in dependence on the size of said small particles. 36. An optical display formed using particles suspended at an interface between an electrolyte solution and an electrode, said display comprising: an electrode and an electrolyte solution having an interface therebetween; a plurality of particles located in said electrolyte solution, said particles including large particles and small particles; said electrode being patterned to include at least one area of modified impedance; an electric field generator which generates an electric field at said interface causing said large particles to assemble in accordance with the impedance of said patterned electrode, said electric field generator generating a time varying electric field causing said small particles to selectively move under said large particles to form an on-pixel, and also causing said small particles to selectively move away from said large particles to form an off-pixel in accordance with a selected frequency of said time varying electric field. 37. A bioanalytical assay implemented using at least one array of particles, said particles being suspended at an interface between an electrode and an electrolyte solution, said assay comprising: an electrode and an electrolyte solution therebetween; a plurality of molecules located in said electrolyte, said molecules including a first type of molecule and a second type of molecule; a biochemical protocol implementation unit which effects a biochemical interaction between said first and second types of molecules, said interaction resulting in the formation of paired entities, and said implementation unit operating to detect the formation of said paired entity; a plurality of particles located in said electrolyte solution; an electric field generator which generates an electric field at said interface; said electrode being patterned to include at least one area of modified electrochemical properties; and an illumination source positioned to illuminate said surface with a predetermined light pattern to control the movement of said particles in accordance with said predetermined light pattern and the electrochemical properties of said electrode. 38. A method of implementing a bioanalytical assay using at least one array of particles, said particles being suspended at an interface between an electrode and an electrolyte solution, said method comprising the following steps: providing an electrode and an electrolyte solution therebetween; providing a plurality of molecules, said molecules including a first type of molecule and a second type of molecule; performing a biochemical protocol to effect the biochemical interaction between said first and second types of molecules, said interaction resulting in the formation of paired entities, said protocol including the additional step of detecting the formation of said paired entities; providing a plurality of particles in said electrolyte solution; generating an electric field at said interface; patterning said electrode to include at least one area of modified electrochemical properties; and illuminating said surface with a predetermined light pattern to control the movement of said particles in accordance with said predetermined light pattern and the electrochemical properties of said electrode. 39. The method of claim 38, further comprising the step of marking individual distinguishable particles within said particle array by initiating a photochemical color-reaction in response to targeting said particles with a focused illumination source. 40. The method of claim 38, further comprising the step of re-configuring said particle using interactive adjustments of said predetermined illumination pattern to isolate distinguishable particles within said array. 41. The method of claim 38, further comprising the following steps: providing a plurality of particles, each portion of said plurality having a plurality of a distinct type of molecule; forming an array of said plurality of particles having said types of molecules, in accordance with said predetermined light pattern and the electrochemical properties of said electrode; admitting a plurality of an additional type of molecule into said electrolyte solution under conditions favoring formation of paired entities with molecules on the surface of particles in said particle array; and detecting the formation of paired entities on surfaces of a subset of particles assembled into said particle array, said paired entities thereby becoming distinguishable. 42. The method of claim 38, further comprising the foUowing steps: providing a particle array including a plurality of types of particles, each type of particle having a plurality of a distinct type of molecule, said types of molecules being potentially capable of interacting with said molecules in solution to form paired entities; providing said particles with a plurality of one type of molecule in said electrolyte solution under conditions favoring formation of paired entities with molecules on the surface of said particles; forming an array of said particles in accordance with said predetermined light pattern and the electrochemical properties of said electrode, said particles having either an unpaired type of molecule or a paired entity, said paired entities rendering particular types of particles distinguishable; and detecting said paired entities on surfaces of a portion of said particles assembled into said particle array. 43. The method of claim 38, further comprising the following steps: attaching a plurality of types of molecules to the surface of said particles, each said particle having a plurality of molecules of one type; attaching a single distinct type of molecule to said electrode surface; introducing a plurality of said particles having a plurality of types of molecules into said electrolyte solution; forming an array of said particles in accordance with said predetermined light pattern and the electrochemical properties of said electrode surface under conditions favoring the biochemical interaction and formation of a paired entity between molecules on said particles and molecules on said electrode surface; and disassembling said particle array, retaining and thereby selecting from the plurality of types of molecules initially introduced only those particles having specific types of molecules of demonstrated biochemical affinity for molecules on said electrode surface. 44. The method of claim 43, wherein said particles include antibody-producing cells. 45. The method of claim 38, further comprising the following steps: attaching a plurality of types of molecules to the surface of said particles, each said particle having a plurality of molecules of one type; attaching a single distinct type of biological target to said electrode surface; introducing a plurality of said particles having a plurality of types of molecules into said electrolyte solution; forming an array of said particles in accordance with said predetermined light pattern and the electrochemical properties of said electrode surface; releasing molecules from said particles at a predetermined location by separating said molecules from said particles in response to a chemical or photochemical stimulus under conditions favoring the biochemical interaction and formation of paired entities between molecules released from said particles and biological targets displayed on said electrode surface; and marking for identification such types of said particles having molecules which when released cause a detectable response in proximal biological targets. 46. The method of claim 45, wherein said biological targets include cells grown in culture on said electrode, said cells being modified in a detectable way by exposure to said first type of molecules released from said particle array. 47. The method of claim 38, further comprising the foUowing steps: placing the first and the second of said types of molecule in said electrolyte solution under conditions favoring the biochemical interaction and formation of paired entities of said molecules; providing a plurality of said particles to capture said paired entities in solution; and forming an aπay of said particles displaying said paired entity in accordance with said predetermined light pattern and the electrochemical properties of said electrode surface. 48. The method of claim 47, wherein said particle capture is foUowed by the formation of a spatially encoded aπay of a plurality of types of distinct paired entities initially stored in a set of MxN distinct reservoirs of solution. 49. The method of claim 38, wherein said paired entities include at least one of receptor-ligand, antibody-antigen and enzyme-substrate. 50. The method of claim 38, wherein said paired entities include matching strands of oligonucleotides or strands of DNA or RNA, and formation of said paired entities involves hybridization. 51. A method for performing multiple chemical and biochemical analytical procedures using at least one particle array, said method comprising the following steps: providing an electrode and an electrolyte solution having an interface therebetween; generating an electric field at an interface between an electrode and an electrolyte solution; patterning said electrode to modify the electrochemical properties of said electrode; illuminating said surface with a predetermined light pattern to control the movement of said particles in accordance with said predetermined light pattern and the electrochemical properties of said electrode; performing a first procedure on a portion of said particles to produce a first reaction set of particles; isolating said first reaction set of particles in accordance with said predetermined light pattern; and performing a second procedure on said first reaction set of particles to produce a second reaction set of particles. 52. The method of claim 51, wherein said performing and isolating steps are interactively controlled in real time by way of an adjustable illumination pattern. 53. The method of claim 51, wherein said performing and isolating steps are dynamically reconfigurable. 54. A method of manipulating nucleic acid, including DNA or RNA, comprising the following steps: providing an electrode, an electrolyte solution and an interface therebetween; providing a plurality of nucleic acid molecules in said electrolyte solution, said nucleic acid molecules being in a coiled configuration; generating an electric field at said interface to cause the movement of said particles; patterning said electrode to include areas of modified electrochemical properties which in conjunction with said electric field create controlled gradients in the flow velocity across the nucleic acid, said velocity gradient causing different portions of the nucleic acid to move at different velocities such that the nucleic acid is stretched in the direction of the local velocity gradient; and maintaining a stagnation point of zero velocity such that the nucleic acid is substantially fixed in position. 55. The method of claim 54, wherein said electrode is a light-sensitive electrode. 56. The method of claim 54, wherein said velocity gradient and said stagnation point are created by a predetermined pattern of illumination. 57. An apparatus for the manipulation of particles suspended at an interface between an electrode and an electrolyte solution, said apparatus comprising: an electrode and an electrolyte solution; an electric field generator for generating an electric field at an interface between said electrode and said electrolyte solution; said surface or interior of said electrode being patterned to modify its electrochemical properties; and an illumination source which illuminates said surface with a predetermined light pattern to control the movement of said particles in accordance with said predetermined light pattern and the electrochemical properties of said electrode. 58. An apparatus for the transverse electrokinetic movement of particles at an interface between an electrode and an electrolyte solution, said apparatus comprising: a light-sensitive electrode and an electrolyte solution; an electric field generator which generates an electric field at an interface between said electrode and said electrolyte solution; and an illumination source which illuminates said electrode with a predetermined light pattern to form lateral gradients in the electrochemical properties of said electrode to control the movement of said particles in accordance with said illumination pattern in a direction substantially orthogonal to the direction of said electric field. 59. A sorting method for implementing the differential lateral displacement of particles suspended at an interface between an electrode and an electrolyte solution, said method comprising the following steps: providing an electrode; providing an electrolyte solution having a substantially continuous flow which effects the displacement of said particles in a direction substantially parallel to said interface; generating an electric field at said interface; patterning said electrode in order to modify its electrochemical properties; illuminating said electrode with an adjustable, predetermined light pattern; and providing a plurality of particles located in said electrolyte solution, said particles being acted upon by a combination of forces arising from said substantially continuous electrolyte flow and from said electric field in accordance with said predetermined light pattern and said electrode electrochemical properties, said particles being displaced in accordance with variations in the physical and chemical properties which determine the mobility of said particles. 60. An apparatus for dynamically assembling and disassembling an array of particles at an interface between an electrode and an electrolyte solution, said apparatus comprising: an electrode, an electrolyte solution and an interface therebetween; a plurality of particles located in said electrolyte solution; said electrode being patterned to include at least one area of modified electrochemical properties; an illumination source which illuminates said electrode with a predetermined light pattern; an electric field generator which generates an electric field at said interface to cause the assembly of an aπay of particles in accordance with the predetermined light pattern and the electrochemical properties of said electrode; and an electric field removal unit which removes said electric field to cause the disassembly of said aπay of particles. 61. An apparatus for forming a spatially encoded array including multiple types of particles suspended at an interface between an electrode and an electrolyte solution, said apparatus comprising: an electrode and an electrolyte solution; multiple types of particles, each type being stored in accordance with chemically or physically distinguishable particle characteristics in one of a plurality of reservoirs, each reservoir containing a plurality of like-type particles suspended in said electrolyte solution; said reservoirs being ananged in the foπn of an MxN grid aπangement; said electrode being patterned to define MxN compartments coπesponding to said MxN grid of reservoirs; MxN droplets which are deposited from said MxN reservoirs onto said corresponding MxN compartments, each said droplet originating from one of said reservoirs and remaining confined to one of said MxN compartments and each said droplet containing at least one particle; a top electrode positioned above said droplets so as to simultaneously contact each said droplet; an electric field generator which generates an electric field between said top electrode and said MxN droplets; said electric field being used to form a particle aπay in each of said MxN compartments, each said particle aπay remaining spatially confined to one of said MxN droplets; O 65 an illumination source which illuminates said MxN compartments on said patterned electrode with a predetermined light pattern to maintain the position of said particle aπays in accordance with said predetermined light pattern and the pattern of MxN compartments; and said top electrode being positioned closer to said electrode thereby fusing said MxN droplets into a continuous liquid phase, while mamtaining each of said MxN particle aπays in one of the corresponding MxN compartments. 62. An optical lens aπay including particles suspended at an interface between an electrode and an electrolyte solution, said lens array comprising: an electrode and an electrolyte solution having an interface therebetween; a plurality of particles located in said electrolyte solution; said electrode being patterned to include at least one area of modified impedance; and an electric field generator which generates an electric field at said interface to cause said particles to assemble into an ordered aπay within said area of modified impedance. 63. A method for forming a diffraction grating using particles suspended at an interface between an electrode and an electrolyte solution, said method comprising the following steps: providing an electrode and an electrolyte solution having an interface therebetween; providing a plurality of particles located in said electrolyte solution, said particles including large particles and small particles; patterning said electrode to include at least one area of modified impedance; and generating an electric field at said interface to cause said large and small particles to assemble into an array, said small particles being located between said large particles to create a large particle separation distance coπesponding substantially to the size of said small particles, said large particle separation distance also determining the spatial frequency characteristics of said diffraction grating, said frequency characteristics being adjustable in dependence on the size of said small particles. 64. A method for forming an optical display using particles suspended at an interface between an electrolyte solution and an electrode, said method comprising the following steps: providing an electrode and an electrolyte solution having an interface therebetween; providing a plurality of particles located in said electrolyte solution, said particles including large particles and small particles; patterning said electrode to include at least one area of modified impedance; generating an electric field at said interface causing said large particles to assemble in accordance with the impedance of said patterned electrode, said electric field generator generating a time varying electric field causing said small particles to selectively move under said large particles to form an on-pixel, and also causing said small particles to selectively move away from said large particles to form an off-pixel in accordance with a selected frequency of said time varying electric field. 65. An apparatus for performing multiple chemical and biochemical analytical procedures using at least one particle aπay, said apparatus comprising: an electrode and an electrolyte solution having an interface therebetween; an electric field generator which generates an electric field at an interface between an electrode and an electrolyte solution; said electrode being patterned to modify the electrochemical properties of said electrode; an illuminating source which illuminates said surface with a predetermined light pattern to control the movement of said particles in accordance with said predetermined light pattern and the electrochemical properties of said electrode; means for performing a first procedure on a portion of said particles to produce a first reaction set of particles; means for isolating said first reaction set of particles in accordance with said predetermined light pattern; and means for performing a second procedure on said first reaction set of particles to produce a second reaction set of particles. 66. An apparatus for manipulating nucleic acid, including DNA or RNA, said apparatus comprising: an electrode, an electrolyte solution and an interface therebetween; a plurality of nucleic acid molecules in said electrolyte solution, said nucleic acid molecules being in a coiled configuration; an electric field generator which generates an electric field at said interface to cause the movement of said particles; and said electrode being patterned to include areas of modified electrochemical properties which in conjunction with said electric field create controlled gradients in the flow velocity across the nucleic acid, said velocity gradient causing different portions of the nucleic acid to move at different velocities such that the nucleic acid is stretched in the direction of the local velocity gradient, wherein a stagnation point of zero velocity is maintained such that the nucleic acid is substantially fixed in position.

Inventors: SEUL MICHAEL, SEUL, MICHAEL
Applicants: SEUL MICHAEL, SEUL, MICHAEL
Application year: 1997
Main classification: G01N33
IPC codes: B01J19, B01L3, C07B61, C12M1, C12N15, C12Q1, G01N15, G01N27, G01N33, G02B3, G02B5, G09F9, G21K1, H05H3