Abstract:
A biomolecular array includes a substrate across which is distributed an array of discrete regions of a porous substance formed from a porogen-containing organosilicate material. The porous substance is designed to bind chemical targets useful in biotechnology applications, such as gene expression, protein, antibody, and antigen experiments. The regions are preferably optically isolated from each other and may be shaped to enhance detection of optical radiation emanating from the porous substance, e.g., as a result of irradiation of the regions with ultraviolet light. The discrete regions may be configured as microscopic wells within the substrate, or they may reside on top of the substrate in the form of microscopic mesas.

Description:
This application is a continuation of Applicants&#39; application Ser. No. 11/184,702 filed Jul. 19, 2005 and titled “Discrete nano-textured structures in biomolecular arrays, and method of use”, which is a divisional of Applicants&#39; application Ser. No. 10/214,951 filed Aug. 7, 2002 and also titled “Discrete nano-textured structures in biomolecular arrays, and method of use” (now U.S. Pat. No. 6,962,822), both of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention is in the field of biomolecular arrays, and more particularly, the invention relates to biomolecular arrays having high areal density. 
     BACKGROUND 
     Biomolecular arrays have quickly developed into an important tool in life science research. Microarrays, or densely-packed, ordered arrangements of miniature reaction sites on a suitable substrate, enable the rapid evaluation of complex biomolecular interactions. Because of their high-throughput characteristics and low-volume reagent and sample requirements, microarrays are now commonly used in gene expression studies, and they are finding their way into significant emerging areas such as proteomics and diagnostics. 
     The reaction sites of the array can be produced by transferring, to the substrate, droplets containing biological or biochemical material. A variety of techniques can be used, including contact spotting, non-contact spotting, and dispensing. With contact spotting, a fluid bearing pin leaves a drop on the surface when the pin is forced to contact the substrate. With non-contact spotting, a drop is pulled from its source when the drop touches the substrate. With dispensing, a drop is delivered to the substrate from a distance. Reaction sites on the array can also be produced by photolithographic techniques (such as those employed by Affymetrix or NimbleGen, for example). 
     The quality of the reaction sites directly affects the reliability of the resultant data. Ideally, each site would have a consistent and uniform morphology and would be non-interacting with adjacent sites, so that when a reaction occurred at a given site, a clear and detectable response would emanate from only that one site, and not from neighboring sites or from the substrate. To reduce the overall size of an array while maximizing the number of reaction sites and minimizing the required reagent and sample volumes, the sites on the array should have the highest possible areal density. 
     With the present microarray technology, which is dominated by the use of flat substrates (often glass microscope slides), areal density is limited. To increase the signal from a given reaction site, the interaction area between the fluid and the substrate should be maximized. One way to do this is by using a surface that promotes wetting. A flat surface that promotes wetting, however, can lead to spots (and thus sites) having irregular shapes and compositions. A flat wetting surface can also lead to the spreading of fluid from its intended site into neighboring sites. Thus, flat surfaces are intrinsically limited by fluid-surface interactions that force a tradeoff between the desired properties of the reaction sites. 
     To make the sites more uniform, the surface can be made non-wetting. Unfortunately, this reduces the interaction area between the fluid and the surface and therefore reduces the signal that would otherwise be obtainable. In addition, since droplets do not adhere well to a flat non-wetting surface, deposition volumes can vary from site to site, and droplets can slide away from their intended place. 
     There is still a need for an improved biomolecular microarray apparatus that has a high areal density of sites and that permits the collection of data with good signal/noise ratio. Such an apparatus would ideally have sites of consistent and uniform spot morphology. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the invention are directed to chemical and/or biochemical applications, and include a substrate having an array of discrete regions of a porous substance designed to bind chemical targets. These regions are preferably optically isolated from each other and may be shaped to enhance detection of optical radiation emanating from the porous substance, e.g., fluorescence as a result of irradiation of the regions with ultraviolet, visible, or infrared light. For example, these regions may have a parabolic or hemispherical contour. The discrete regions themselves may be microscopic wells formed in the substrate, or they may reside on top of the substrate as microscopic mesas. 
     One preferred embodiment is a device that includes a substrate across which is distributed an array of discrete regions of porous material to which are bound respective chemical targets, in which the porous material is formed from a porogen-containing organosilicate material. These regions may be advantageously optically isolated from each other and may have respective boundaries contoured to enhance detection of optical radiation emanating from the discrete regions. They may have, for example, a characteristic transverse dimension that is between 1 and 200 microns, between 1 and 100 microns, or between 1 and 50 microns. Furthermore, these regions may be adjoined by an optical coating designed to enhance optical emission from them, in which case the optical coating and the porous material may be located on opposite sides of boundaries of the regions. The regions may further include a hydrophobic coating. 
     The array may comprise discrete wells formed in a surface of the substrate. The wells may have respective volumes that are at least 25% filled by the porous material, at least 50% filled, or at least 75% filled. The wells may further include non-porous vertically oriented members that are in contact with the substrate and pass through the wells. Alternatively, the array may include discrete mesas over a surface of the substrate. The mesas may include non-porous vertically oriented members that are in contact with the substrate and pass through the mesas. 
     Another embodiment of the invention is a device that includes a substrate across which is distributed a two-dimensional array of discrete regions of porous material to which chemical targets can be attached, in which the porous material is formed from a porogen-containing organosilicate material. These regions may have a characteristic transverse dimension that is between, for example, 1 and 200 microns, 1 and 100 microns, or 1 and 50 microns. The regions may be wells formed in a surface of the substrate, and these wells may advantageously have respective volumes that are at least 25% filled by the porous material, and in addition, they may be optically isolated from each other. Also, the array may include discrete mesas over a surface of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a biochip having an array of microscopic wells (“microwells”) that contain porous material, in accordance with a preferred implementation of the invention; 
         FIGS. 2A and 2B  are cross sectional side views of biochips that have microwells of different shapes; 
         FIG. 3  is an enlarged view of a microwell that includes optical and hydrophobic coatings; 
         FIG. 4  is a cross sectional side view of a biochip in which an optical coating has been applied to the underside of the substrate; 
         FIG. 5  shows porous material within the microwells of the biochip; 
         FIGS. 6A and 6B  show cross sectional and plan views, respectively, of a biochip in which the microwells include vertical members that facilitate introduction of biochemical material into the microwells; 
         FIGS. 7A-7G  illustrate a series of steps used to form a biochip having porous material arranged in the form of microscopic mesas (“micromesas”); and 
         FIG. 8  shows a biochip that includes an array of micromesas having respective vertical members therein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the invention are now described with reference to the accompanying figures, in which like numerals refer to like parts.  FIG. 1  shows a plan view of a biochip  10  that includes a substrate  16  into which a number of small wells  22  or microwells have been formed. As discussed below, the microwells  22  have porous material therein (see  FIG. 5 , for example), resulting in a significant increase in effective surface area, and thereby permitting more sensitive detection measurements to be made. The microwells  22  (and the micromesas discussed below) may have a characteristic transverse (lateral) dimension of about 1-500 microns, preferably 1-200 microns, more preferably 1-100 microns, still more preferably 1-50 microns, or most preferably 1-10 microns (e.g., if the microwells have a circular cross section, their diameters may be about 1 micron; for a square cross section, the corresponding square may be about 1 micron×1 micron). The depth of each microwell  22  (or height of the micromesas discussed below) may be about 1-50 microns or more preferably 0.5-50 microns. The width of the substrate  16  that separates adjacent microwells  22  (or the distance separating the micromesas discussed below) is preferably sufficient to optically isolate one microwell  22  from adjacent microwells, e.g., 0.1-10 microns. The material separating adjacent microwells  22  is preferably optically opaque; if this material is not intrinsically optically opaque, the microwells may have roughened surfaces so that light is scattered, or these surfaces may be coated (as discussed below). 
     The substrate  16  may be either an organic or an inorganic material. For example, the substrate may be silicon or glass which has undergone a (dry or wet) mask/etch process (e.g., those used in the field of semiconductor processing) to form the microwells  22 . An isotropic process can be used to form wells  22   a  having a contoured shape, e.g., a parabolic shape, such as that depicted in  FIG. 2A . (Other preferred shapes include prismatic, cylindrical, and hemispherical.) An anisotropic process (such as is attainable in some dry etch processes) is more appropriate for the formation of microwells  22   b  that are cylindrically shaped, like those shown in  FIG. 2B . 
     Alternatively, the substrate  16  may be plastic. In this case, an embossing technique known to those skilled in the art may be used, in which protrusions in the embossing master penetrate the plastic to form the microwells  22   a  or  22   b . Likewise, an injection molding process may used to form the substrate  16 /microwells  22  assembly. In general, whether the substrate  16  is plastic, glass, silicon, or another material, wells  22  having a contoured shape (like the wells  22   a  of  FIG. 2A ) are preferred, as they permit more sensitive optical detection, as discussed below. As used herein, the term optical includes ultraviolet, visible, and infrared electromagnetic radiation. 
     The microwells  22  preferably have walls  30  that are hydrophobic to reduce unwanted spreading of aqueous reagents added to the porous material  54 . If plastic is used as the substrate  16 , this property may be intrinsic to the plastic, as is the case with such thermally stable materials like polycarbonates, polyesters, polyimides, polyazoles, and polyolefins. Alternatively, one may coat or otherwise treat the walls  30  of the wells  22   a  or  22   b  so that they are hydrophobic, e.g., a hydrophobic coating  36  such as an organic wax or surface active reagent (such as hexamethyldisilazane) may be applied to the walls  30 . In the event that an optical detection arrangement is used (in which optical radiation at an input wavelength is directed towards chemical or biochemical material bound to porous material in the wells  22 , and optical radiation at an output wavelength emanates away from the wells, as discussed in greater detail below), an optical coating  42  may be first applied to the walls  30 , followed by the application of the hydrophobic coating  36  (see  FIG. 3 ). The hydrophobic coating  36  is preferably not absorbing at either the input or the output wavelength, whereas the optical coating  42  is preferably both non-absorbing and reflective at both of these wavelengths. If a thin-walled embossed plastic is used as a substrate  16   a , then a reflective optical coating  48  such as silver or aluminum may be applied to the underside of the substrate, as shown in  FIG. 4 . The microwells  22  are preferably at least 25% filled (in the volumetric sense) with a porous material  54 , still more preferably at least 50% filled with porous material, and most preferably substantially filled with porous material (e.g., 75-90% or more, as illustrated in  FIG. 5 ). One preferred nanoporous material is formed using an organosilicate material (such as methylsilsesquioxane, or MSSQ) that has been mixed with a sacrificial porogen in a solvent. (See, for example, U.S. Pat. No. 5,895,263 to Carter et al. issued Apr. 20, 1999 and titled “Process for manufacture of integrated circuit device”, which is hereby incorporated by reference.) The solvent containing the porogen and MSSQ is applied over the substrate  16  by spraying, spin coating, or doctor blading (or another technique known to those skilled in the art), so that the wells  22  are filled with the solvent/MSSQ/porogen mixture. Excess solvent on the substrate  16  may be wiped away, and the solvent is allowed to evaporate. As the remaining MSSQ/porogen mixture is then heated (or exposed to an oxygen plasma), the porogen decomposes within the MSSQ host material, leaving tiny voids therein. In this manner, porous material  54  is formed in the microwells  22 . (Note that the particular method used for producing the porous material  54  may impact the hydrophilicity of the porous material.) This process may be repeated as desired, until, for example, the microwells are substantially filled with porous material  54 . If multiple coatings are applied, each coating may be cured to 250° C. in an inert atmosphere to produce a layered nanohybrid which is subsequently cured to &gt;400° C. to produce porosity. The substrate  16  may be polished or etched as needed to remove any excess material that remains on the top of the substrate between the microwells  22 . Organosilicate materials that may be used in combination with porogens include inorganic materials such as sol-gel silica, silica, and spin-on glasses, and inorganic-like materials such as substituted silsesquioxanes (SSQs) (such as methyl SSQ, hydrido SSQ, alkyl SSQ, aryl SSQ), as well as copolymers of the foregoing. 
     Alternatively, controlled pore glass may be used. Controlled pore glass is made starting with a borosilicate material that is heated, resulting in separation of the borates and the silicates within the borosilicate material. After then leaching out the borates, one is left with a glass having pores of substantially uniform size. One commercially available source of controlled pore glass is Controlled Pore Glass, Inc., Lincoln Park, N.J. A slurry made from solvent and microscopic particles of controlled pore glass may be made (or silica aerogel particles can also be used, either alone or in a matrix of sol-gel silica, silica, spin-on glasses, substituted silsesquioxanes (SSQs) (such as MSSQ, hydrido SSQ, alkyl SSQ, aryl SSQ), and copolymers thereof) and passed over the substrate  16 . After the solvent has evaporated, any excess pore glass on the substrate  16  may be polished or scraped off, and the remaining pore glass may be sintered in situ so that the pore glass is bound within the microwells  22 , i.e., to the walls  30  of the microwells. If necessary, the substrate  16  may then be polished back to ensure that the pore glass resides only within the microwells  22 , and not on top of the substrate  16 . (Alternatively, one can vapor deposit borosilicate glass into the microwells, polish, leach out the borates, and anneal.) A more elaborate method for adding pore glass particles to the microwells involves the use of patterned electric and/or magnetic fields. The particles can be drawn into the wells  22  electrokinetically, or if controlled pore glass particles having magnetic impurities therein are used, by a magnetic field. The pore glass particles can then be manipulated by introducing, underneath the substrate  16 , a patterned electric and/or magnetic field having high field gradients and/or strengths, so that the pore glass particles are drawn into the microwells  22 . To this end, one can position a plate having a patterned array of metal protrusions underneath the substrate  16 , with the protrusions being aligned with respective microwells. 
     The apparatuses disclosed herein can be used with a variety of tagged detection methods, and are well suited for use with detection methods that employ optical detection techniques. The porous material  54  in each of the microwells  22  within the biochip  10  (or the porous material in the “micromesas”, see below) is individually prepared with a chemical or biochemical target or material. For example, in a common gene expression experiment, each microwell  22  may contain a different oligonucleotide or DNA fragment attached to the porous material  54  using the same chemical derivatization procedure known to those in the art for planar substrates. (See, for example M. C. Pirrung,  Angew. Chem. Int. Ed ., vol. 41, 2002, pp. 1276-1289.) RNA may be extracted from cells treated with a drug of interest, and DNA copies from this RNA may then be constructed which are then “tagged” with dyes that fluoresce (e.g., in the visible region of the spectrum) when exposed to input radiation (e.g., ultraviolet, or even visible or infrared). A solution containing this “tagged” DNA may then be washed over the biochip  10 , so that the tagged DNA binds itself to any complementary DNA that has been previously attached to the porous material  54  in the microwells  22 . Emission from an input electromagnetic radiation source (e.g., ultraviolet, visible or infrared) may then be directed onto the biochip  10 , and emission from the fluorescent dyes identifies those particular microwells  22  (and thus those DNA strands) that complement the RNA extracted from the cells. More sensitive detection is possible if the microwells have boundaries that are appropriately contoured, e.g., if they have a parabolic shape, since a greater fraction of the light from the fluorescent dyes can then be collected. Even greater detection sensitivity is possible if reflecting layers are incorporated. A sensor or array of sensors can be used to detect the fluorescent emission, and the data can be processed by a computer. (For additional details regarding biochip technology, see, for example, “Making chips to probe genes”, Samuel K. Moore,  IEEE Spectrum , vol. 38, March 2001, pp. 54-60.) Analogous detection methods may be used in connection with complementary RNA and DNA strands, antibody-antigen, ligand-receptor, agonist-receptor, antagonist-receptor, enzyme-substrate, and enzyme-inhibitor combinations. 
     As discussed above, each of the microwells  22  in the biochip  10  may be individually prepared with chemical or biochemical material, i.e., biochemical material may be bioconjugated or “bioattached” to the porous material  54  in each microwell  22 . This bioattachment is generally intrinsic to the porous material, i.e., independent of its size and/or shape. Linkers (i.e., primers) may be utilized for attaching the biochemical material to the porous material. Such a linker may advantageously include a substrate binder (e.g., (EtO) 3 Si), a spacer (such as linear alkyl, aryl alkyl, alkylene ether), and a group having reactive functionality (e.g., NH 2 , OH, COOH). Further details regarding the chemistry of linkers can be found in the article by M. C. Pirrung, supra. 
     Biochemical material may be attached to the porous material  54  through a primer or directly via one of a variety of techniques, such as contact spotting, non-contact spotting, or dispensing. If one of these techniques is used with the embodiment shown in  FIG. 5 , for example, a fluid bearing pin or drop of fluid is brought into direct contact with the porous material  54  in the microwells  22 . To reduce the possibility that the porous material  54  might be damaged as a result of this fluid transfer process, one can use the alternative microwell construction illustrated in  FIGS. 6A  (a cross sectional, side view of a substrate  16   c ) and  6 B (a partial plan view). The substrate  16   c  has a number of microwells  22   c  therein, each of which includes a mechanically robust vertical member  60  that passes through the porous material  54  and is preferably located at or near the center of the microwell  22   c . The vertical member  60  is shown as being an integrated part of the non-porous substrate  16   c , and may be formed along with the microwells  22   c  as a result of an etch/mask process like one of those described herein. With the embodiment of  FIGS. 6A and 6B , the fluid bearing pin or drop of fluid that contains the biochemical material may be brought into contact with the vertical member  60 , thereby allowing the fluid to dissipate away from the vertical member into the porous material  54 . In this way, the structural integrity of the porous material  54  is protected. The boundaries of the wells  22   c  may have one of a number of shapes, e.g., they may be contoured as in  FIG. 2A  or cylindrically-shaped as shown in  FIG. 2B . 
       FIGS. 7A-7E  correspond to steps in a lithographic process leading to the “micromesa” structure shown in  FIGS. 7F and 7G , which is an alternative to the microwell apparatuses discussed above.  FIG. 7A  shows a substrate  70  onto which a porogen-containing organosilicate  76  (dissolved in solvent) such as MSSQ has been deposited. As shown in  FIG. 7B , the organosilicate  76  is converted to porous material  80  upon exposure to heat or an oxygen plasma.  FIG. 7C  illustrates how a photoresist  86  is then laid over the porous material  80 . The photoresist is exposed to UV light and developed, leading to the structure shown in  FIG. 7D . Etching of the porous material  80  results in the structure illustrated in  FIG. 7E . As shown in  FIG. 7F , the remaining photoresist is then removed, leaving the porous material  80  in the form of micromesas  90  that reside on the substrate  70 . The micromesas  90  may advantageously be in the form of cylinders or rectangular parallelepipeds.  FIG. 7G  shows how an absorptive coating  92  may be introduced between the micromesas  90 , in order to provide better optical isolation between them. The coating  92  used between the micromesas  90  may be advantageously hydrophobic. As an alternative to the fabrication process outlined above, the organosilicate  76  may be converted to porous material  80  at a later stage in the process, e.g., after the remaining photoresist shown in  FIG. 7E  has been removed. 
     Alternatively, the micromesas  90  can be made in a non-lithographic process by contact molding a film (of MSSQ and porogen) from an embossing master. The film can be heated to 250° C. in an inert atmosphere to generate the nanohybrid. The embossing master can be removed, and porosity may be generated by heat or chemical treatment. Thereafter, a plasma descum process can be used to remove any thin film left in the patterned depressions of the film on the substrate. 
     These micromesa fabrication processes may be modified so that the micromesas include a vertical member that passes through the porous material.  FIG. 8  shows a substrate  100  that includes vertical members  106  surrounded by porous material  80 , with each micromesa/vertical member unit  104  optionally separated by a coating  110 . In this case, extra processing steps are required to form the vertical members  106  out of the substrate  100 . The vertical members  106  can then be used when biochemical material is brought into contact with the porous material  80  to guard against the possibility of damaging the porous material, as discussed previously in connection with the microwell embodiments. During lithography, the walls of the mesas and also the surfaces between the mesas may be coated with a material opaque to the emitted fluorescence to improve optical site isolation. In addition, a reflecting layer (not shown) can be included below the mesas to reflect fluorescence emission, thereby enhancing the system&#39;s sensitivity. 
     The system sensitivity for preferred embodiments disclosed herein is at least a factor of 100 greater than that which can be achieved on a dense planar surface, and at least a factor of 10 greater than that which can be achieved on a non-porous surface that has been roughened, e.g., via an etching process. 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than the foregoing description. All changes within the meaning and range of equivalency of the claims are to be embraced within that scope.