Abstract:
A nanosensor and methods to manufacture are disclosed. For example, a detection system for detecting the presence of a target substance can include a nanosensor that includes a sensing layer, and a plurality of sockets embedded within the body of the sensing layer, each socket having a physical profile matching a shape of the target substance such that, when target substances occupy the sockets, at least one measurable physical characteristic of the sensing layer changes.

Description:
INCORPORATION BY REFERENCE 
     This application claims the benefit of U.S. Provisional Application No. 61/808,466 entitled “CHEMICAL AND BIOLOGICAL SENSOR FABRICATION USING NANOIMPRINT LITHOGRAPHY AND SOCKET CREATION” filed on Apr. 4, 2013, the content of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Generally, chemical sensors need both selectivity, i.e., the ability to accurately discern one substance from another, and sensitivity. Nanotechnology provides the possibility of producing compact, highly sensitive, compound-specific chemical sensors. However, producing a nanosensor that incorporates the requisite selectivity and sensitivity is potentially very challenging because, as a general rule, the physical traits of a sensor that promote selectivity and sensitivity tend to work against each other. 
     SUMMARY 
     Various aspects and embodiments of the invention are described in further detail below. 
     In an embodiment, a detection system for detecting the presence of a target substance includes a nanosensor that includes a sensing layer, and a plurality of sockets embedded within the body of the sensing layer, wherein each socket includes a plurality of socket members and each socket has a physical profile matching a size and shape of at least a portion of the target substance, and polarity if appropriate for the selected target substance, such that, when target substances occupy the sockets, at least one measurable physical characteristic of the sensing layer changes. 
     In another embodiment, a method for manufacturing a detection system for detecting the presence of a target substance includes forming a nanosensor that includes a sensing layer, and forming a plurality of sockets incorporated within and/or on a surface of the body of the sensing layer, wherein each socket includes a plurality of socket members and each socket has a physical profile matching a size and shape of at least a portion of the target substance, and polarity if appropriate for the selected target substance, such that, when target substances occupy the sockets, at least one measurable physical characteristic of the sensing layer changes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein: 
         FIG. 1  is a side view of an example nanosensor. 
         FIGS. 2A-2D  are plan views depicting the manufacture of sockets for the example nanosensor of  FIG. 1 . 
         FIG. 3  depicts an example nanosensor and reference device used in a sensing system. 
         FIG. 4  is a flowchart outlining an example set of operations usable to produce and use the nanosensor of  FIGS. 1-3  above. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The disclosed methods and systems below may be described generally, as well as in terms of specific examples and/or specific embodiments. For instances where references are made to detailed examples and/or embodiments, it is noted that any of the underlying principles described are not to be limited to a single embodiment, but may be expanded for use with any of the other methods and systems described herein as will be understood by one of ordinary skill in the art unless otherwise stated specifically. 
       FIG. 1  is a side view of an example nanosensor  100 . As shown in  FIG. 1 , the nanosensor  100  includes a substrate  110 , an optional intermediate layer  120  and a sensing layer  130 . 
     The substrate  110  can be any of a large variety of non-conductive or semi-conductive materials, such as silicon, diamond, sapphire, germanium, and silicon carbide. Alternatively, the substrate  110  can be any of a large variety of metals, such as copper, silver and gold of a single metal crystal of a known crystal orientation. The nature of the substrate  110  may lend itself to manufacturing ease, for sensing functionality and/or for mechanical support. Depending on the circumstances and design goals, the substrate  110  may not be necessary. 
     The intermediate layer  120  for the example of  FIG. 1  is boron nitride heated and annealed so as to form a hexagonal matrix, which can provide an electrically insulating layer with a similar lattice structure for graphene to lay upon thus better exploiting graphene&#39;s inherent properties. However, in other embodiments some other material(s) may be applied depending on the nature and composition of the sensing layer  130 . Further, in other embodiments, the intermediate layer  120  may be eliminated. For example, if the substrate  110  is made from silica and the sensing layer  130  is graphene, the intermediate layer  120  may be eliminated. Similarly, if the sensing layer  130  is made from another suitable material other than graphene, the intermediate layer  120  may be eliminated. 
     The composition of the intermediate layer  120  may also change dependent on a type of sensing used. By way of example, if the sensing is done using other than electrical characteristics, such as x-ray fluorescence, infrared, Raman or Terehertz sensors, a conducting substrate such as copper can be used. By way of another example, the intermediate layer  120  may be composed of copper if surface waves were employed to do the sensing, but not if overall resistance change is to be measured. Normally, an insulating substrate is needed if a change in resistance of the sensor layer  130  is used to detect the target substance. 
     The sensing layer  130  for the example of  FIG. 1  is made of a graphene sheet having a thickness of a single molecule. However, in other embodiments the sensing layer  130  may use multiple sheets of molecule-thick graphene, or a graphene composite being more than a single molecule thick. Still further, it is to be appreciated that, in a variety of embodiments, substances other than graphene may be used depending on the desired physical traits of the sensing layer  130 . That is, the type of material and thickness of the sensing layer  130  will depend on the type of substance/molecule sensed (hereinafter, the “target substance”) and the method for sensing the presence of targeted molecules/substances, such as measuring changes in conductivity, measuring changes in optical transmission properties, measuring changes in resonance (e.g., resonant frequency and/or system Q), and so on. 
     For the purposes of this disclosure there are two major divisions of “socket” production, one for small molecules/substances, and one for large molecules/substances. 
       FIGS. 2A-2B  depict plan (top-down) views of an exemplary sensing layer portion  130 A of the sensing layer  130  of  FIG. 1  as it is populated with small-molecule sockets. As shown in  FIG. 2A , the sensing layer portion  130 A, which in this embodiment consists of a molecule-thick layer of graphene, has a hexagonal molecular structure. While not shown in  FIG. 2A , an ultraviolet interference pattern can be projected onto the sensing layer portion  130 A to make some areas relatively hot and other areas relatively cool. It is in the cool areas where sockets formation will occur. Accordingly, fine placement control of individual sockets may be achieved according to a grid pattern. 
     For large target molecules/substances, individual segments of a socket member may attach to graphene more strongly than does the target substance as a whole. Accordingly, one can place first (partial) socket members PS in a grid placement on the (graphene) sensing layer portion  130 A by selecting conditions such that the socket members can only adsorb onto the sensing layer at predetermined locations. As an example, an interference pattern of ultraviolet light could produce a grid of hot and cold spots on the sensor surface, allowing the socket member to adsorb onto the sensor surface at only the cold spots. 
     Sockets may be formed within the sensing layer  130  or atop the sensing layer  130 . Typically, sockets will have at least two distinct members that comprise the sockets.  FIG. 2B  depicts a first of two socket members PS placed according to a grid pattern. The first socket members PS are exposed to the target substance, while maintaining conditions that prevent the adsorption of the target substance on the background sensor surface, but do allow attachment to the partial socket members. The attachment is accomplished by a combination of shape and van der Waals attraction of atoms within the partial socket and target substance that have complimentary partial electrical charges. 
     Once all excess socket member materials are removed from the area of the sensing layer portion  130 A, a second socket portion can be formed onto the sensing layer portion  130 A as will be further discussed below. The second socket members are designed so that they attach to the free side of the target substance, but do not ‘sit down’ on the sensing layer portion  130 A (to avoid placement anywhere except at the half-socket/target substance locations). 
       FIG. 2C  depicts a plan view of socket members {PS, PS 2 } that are embedded within the sensing layer portion  130 A with a target substance TS represented between the socket members {PS, PS 2 }.  FIG. 2D  depicts an example where the sockets {PS, PS 2 } are formed atop the sensing layer portion  130 A with a target substance TS represented between the socket members {PS, PS 2 }. In the example of  FIG. 2D , the socket members {PS, PS 2 } are depicted as being askew/not aligned with the hexagonal matrix of the (graphene) sensing layer portion  130 A. This is because energetically it is unlikely that the atoms of a partial socket would align directly atop of the atoms of a graphene layer. 
     For both the examples of  FIG. 2C  and  FIG. 2D , the target substance TS is attracted to the socket members {PS, PS 2 } by van der Waals forces, which as known in the relevant arts are not hard covalent bonds (such as those that bind the socket molecules together), but more distant associations that hold the socket parts to the graphene, and which hold the target substance in a socket. 
     In addition to the use of interferometric methods for location placement, magnetic fields and/or chemical tethers may be applied to the substrate and/or to the partial sockets to assist with molecular location and orientation. For the purpose of this disclosure, a “chemical tether” is any chemical compound or element that can attract or attach itself to a particular portion of a molecule to assist in orienting and placing the partial socket or target substance upon a sensing layer. Typically, each socket will employ one or more (usually two) separate chemical tethers. For example, the chlorine atom in the target substance chlorobenzene can use a hetero-atom, such as nitrogen in the socket, as a tether because the partial positive charge of the nitrogen attracts chlorine&#39;s partial negative charge. In some embodiments, the spacing and/or orientation of individual sockets may be random. However, in other embodiments it is envisioned that the spacing of individual sockets may be uniform and/or the orientation of sockets be made common to the extent practical, necessary, or otherwise desirable. In still other embodiments, however, it may be advantageous that socket orientation occur such that angular orientation differs according to physical position. For example, it may be desirable to orient sockets according to a sine wave as a function of distance. Other orientations may take the form of a Guassian function, a radial orientation, and so on. That is, socket orientation can be determined on a case-by-case basis according to desirable design parameters, and is not limited to any example disclosed herein. In another scenario, where specificity would be sufficiently satisfied through shape and size then nano-imprinting or nano-stamping can be used for the fabrication, and a chemical tether as mentioned above would not be required. 
     Returning to  FIG. 2D , once the loosely-held second socket member PS 2  is in place, chemical reactions are implemented that remove the functional groups that prevented the second socket member PS 2  from attaching to the surface of the sensing layer portion  130 A, and allow the second socket member PS 2  to ‘sit down’ on the sensing layer portion  130 A and be held by van der Waals forces, completing the socket. 
     Then the sensing layer portion  130 A is heated sufficiently to release the target substance from the sockets  210 , and the sockets  210  are ready to be used. 
     There need be no tradeoff between sensitivity and selectivity given a combination of the size, shape and locations of the polar elements on the socket produce selectivity. 
     For large target substances, the target substance may be attracted more strongly to a graphene (or other material) surface than any one partial socket. In this case, there are several approaches to the creation of sockets. 
     In a first non-limiting approach, the target substances are adsorbed in the grid formation onto a graphene surface, and the socket members added, one by one, in the same manner as the second socket member in the case of small molecules. However, from the second socket member on, the socket members will attach not only to the target substance, but also to the other socket member based upon complementary functional groups, forming a strongly interlinked socket that as a whole adsorbs to graphene more strongly than does the target substances as a whole. In another non-limiting approach, a socket can be manufactured around a target substance on a substrate other than graphene, desorbed as a whole and then adsorbed onto graphene as a combination socket-target. Then the target substance can be desorbed from the graphene-socket surface in preparation for using the sensor. 
     Because different chemical compounds will have different physical and electrical field shapes, the sockets  210  are formed such that the target substances (or specific portions thereof) should precisely fit within the sockets, matching the required size, shape and electrical field conformation. For example, the sockets  210  can be made into the shape and size of a chlorobenzene molecule, an odorizing taggant chemical (or other volatile residue), drugs or drug metabolites (e.g., benzoylecgonine), and so on. In addition to sensing a single compound, the sockets  210  can be manufactured so as to receive a class of compounds. 
     By way of further example in the case where a biological detector is desired, the sockets  210  can be made in the form of a target virus or in the shape of volatile waste products, which can be an indication of food contamination, infections or other issues. For instance, botulism bacteria produce waste products that could be sensed using the disclosed methods and systems. 
       FIG. 3  depicts an example nanosensor-based sensing system  300 . As shown in  FIG. 3 , the sensing system  300  includes a nanosensor  100 , an optional reference device  310  and a processing system  320  having a set of interface electronics  322  usable to monitor the sensor  100  and reference device  310 . 
     Generally, the reference device  310  can take the form of the sensor  100  with or without sockets. Assuming that similar processing is used on both the sensor  100  and the reference device  310 , the reference device  310  should react in similar ways to the sensor  100  with respect to temperature, aging and other environmental conditions. If the reference device  310  incorporates sockets, such sockets should be sealed such that contamination by target substances is prohibited. 
     Assuming that the sensor  100  is configured to change resistance upon exposure to a target substance, the sensor  100  and the reference device  310  can be, for example, configured in the form of a resistor bridge such that the processing device  320  can measure changes in output voltage between the sensor  100  and the reference device  310 . 
     Assuming that the sensor  100  is manufactured so as to be well characterized, the reference device  310  can be eliminated. Similarly, if the processing device  320  can be calibrated to individual sensors, the reference device  310  can be eliminated. 
     Other forms of sensing may require more sophisticated interface electronics and processing. By way of example, if the resonant characteristics of the sensor  100  change upon exposure to a target substance, the interface circuitry  322  can take the form of a pair of oscillator driving circuits using the sensor  100  and reference device  310  to determine frequency and/or system quality Q, where after the processing circuitry  322  can measure differences in frequency output. Other prospective physical changes, such as capacitance and optical transmission qualities, will require an appropriately configured interface circuitry  322 . 
       FIG. 4  is a flowchart outlining an example set of operations usable to produce and use the nanosensor of  FIGS. 1-3  above. While the below-described steps are described as occurring in a particular sequence for convenience, it is noted that the order of various operations may be changed from embodiment to embodiment. It is further noted that various operations may occur simultaneously or may be made to occur in an overlapping fashion. 
     The process starts in S 402  where a substrate, such as a silicon wafer or a metallic sheet is prepared, i.e., formed, cleaned and so on. In S 404 , an intermediate layer, such as the hexagonal boron nitride layer discussed above with respect to  FIG. 1 , is placed upon the substrate of S 402 . In S 406 , a sensing layer, such as one or more layers of molecule-thick graphene, can be directly or indirectly placed atop the substrate of S 402 . 
     In varying examples, one could use copper with a (1,1,1) crystal orientation to deposit a hexagonal boron nitride layer in a correct, single crystal orientation, upon which the graphene is deposited in the optimum crystal structure. The copper then can be used as part of a capacitance circuit, with the HBN as the insulator. Such a configuration would be useful if capacitance were used as a sensing mechanism. 
     In other varying examples when graphene is to be use to detect resistance change, graphene (which is generally highly conductive) can be placed on an insulating material. Silicon dioxide is a viable insulating material for the intermediate layer  120 , but given that graphene and hexagonal boron nitride have a common hexagonal structure, hexagonal boron nitride tends to be a more suitable material when graphene is used as a sensing layer. Also, given that there is an extra resistance at each crystal boundary, a single crystalline orientation can produce better results than non-single crystalline embodiments. Therefore, when resistance is used as a detection mechanism, the best results may be attained if a metallic base of a single crystal orientation is used upon which to deposit the other materials. Copper and gold can be purchased in single crystals of predetermined orientation. The process continues to S 408 . 
     Steps/operations S 408 -S 418  describe one method of forming sockets in a sensing layer appropriate to target substances. Steps/operations S 408 -S 418  are meant to convey a single non-limiting example that may be replaced by other approaches, such as micro-stamping, depending on any number of factors, such as economics, reliability, the type of target substance, and so on. 
     In S 408  any of various grid placement and orientation steps may be applied in conjunction with molecular placement to the sensing layer. Among which is the imposition of a rectangular interference grid of ultraviolet light on the graphene/sensing surface. The spots of positive interference are relatively thermally hot, and as a result target substances will not adsorb at such locations. Where there is negative interference, however, the graphene/sensing surface stays relatively cool, and target substance adsorb onto the graphene/sensing surface. A magnetic field and/or molecular tethers may optionally be applied to orient individual target substances, if common orientation is desired. Control continues to step S 410 . 
     In S 410  first socket members tailored to the target substance(s) are placed and absorbed into or onto the sensing layer such that sockets can be built around the target substances without encountering other sockets or target substances, with the excess then removed. Next, in S 412 , the target substance(s) are deposited onto the sensing layer, with the excess then removed. Control continues to S 414 . 
     In S 414  chemical precursors to second socket member is deposited onto the sensing layer, with the excess removed, and in S 416  chemical reactions are preformed on the precursors to complete socket creation. Control continues to S 418 . 
     In S 418 , once the sockets are completed, the graphene/sensing surface is heated sufficiently to release the target substances from the sockets, but not enough to release the socket members from the graphene/sensing surface. Van der Waals forces between sockets and target substances act as tethers. It is important that socket members adhere to the graphene/sensing surface, and to each other, more strongly than they adhere to the target substance, so the target substances can be released from the sockets without destroying the sockets. 
     In S 420 , an appropriate set of post-processing steps can be performed, such as other cleaning, drying and cutting. In S 422 , individual sensors can be paired with appropriate reference devices, which as stated above can take the form of a device formed according to  FIG. 3  (without sockets), or an identical sensor with its sockets sealed to prevent exposure to target substances. Finally, in S 424  an appropriate set of signal processing steps can be performed using the sensor and reference device to determine whether or not the sensor has been exposed to the target substance. 
     While the invention has been described in conjunction with the specific embodiments thereof that are proposed as examples, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, embodiments of the invention as set forth herein are intended to be illustrative, not limiting. There are changes that may be made without departing from the scope of the invention.