Abstract:
A microfluidically-controlled transmission mode nanoscal surface plasmonics sensor device comprises one or more arrays of aligned nanochannels in fluid communication with inflowing and outflowing fluid handling manifolds that control the flow of fluid through the array(s). Fluid comprising a sample for analysis is moved from an inlet manifold, through the nanochannel array, and out through an exit manifold. The fluid may also contain a reagent used to modify the interior surfaces of the nanochannels, and/or a reagent required for the detection of an analyte.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The Federal Government has certain rights pertaining to the present invention pursuant to Contract No.: NNX-08-CD-36 awarded by the National Aeronautics and Space Administration. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a nanoscale surface plasmonics sensor comprising a fluid control system for delivering fluids to the sensor, and a quantitative protein assay method. 
     2. Description of Related Art 
     Conventional quantitative protein assays of bodily fluids typically involve multiple steps to obtain desired measurements. Such methods are not well suited for fast and accurate assay measurements in austere environments such as spaceflight and in the aftermath of disasters. Consequently, there is a need for a protein assay technology capable of routinely monitoring proteins in austere environments. For example, there is an immediate need for a urine protein assay to assess astronaut renal health during spaceflight. The disclosed nanoscale surface plasmonics sensor provides a core detection method that can be integrated to a lab-on-chip device that satisfies the unmet need for such a protein assay technology. 
     Assays based upon combinations of nanoholes, nanorings, and nanoslits with transmission surface plasmon resonance (SPR) are used for assays requiring extreme sensitivity and are capable of detecting specific analytes at concentrations as low as 10 −14  M in well controlled environments. Existing SPR-based sensors, however, do not lend themselves to repetitive assays of biological fluids because they are not compatible with fluidic control systems, sample handling, and washing between samples. 
     The present SPR sensor with nanofluidic control overcomes the aforementioned limitations associated with existing protein assays. The SPR-based sensor provides for a protein sensor and assay method that may also be used for the detection and quantitation of a wide variety of analytes from a wide variety of sources. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention incorporates transmission mode nanoplasmonics and nanofluidics into a single, microfluidically-controlled device. The device comprises one or more arrays of aligned nanochannels that are in fluid communication with inflowing and outflowing fluid handling manifolds that control the flow of fluid through the array(s). The array acts as an aperture in a plasmonic sensor. Fluid, in the form of a liquid or a gas and comprising a sample for analysis is moved from an inlet manifold, through the nanochannel array, and out through an exit manifold. The fluid may also contain a reagent used to modify the interior surfaces of the nanochannels, and/or a reagent required for the detection of an analyte. 
     The device operates in a transmission mode configuration in which light is directed at one planar surface of the array, which functions as an optical aperture. The incident light induces surface plasmon light transmission from the opposite surface of the array. The presence of a target analyte is detected by changes in the spectrum of light transmitted by the array when a target analyte induces a change in the refractive index of the fluid within the nanochannels. 
     This occurs, for example, when a target analyte binds to a receptor fixed to the walls of the nanochannels in the array. Independent fluid handling capability for individual nanoarrays on a nanofluidic chip containing a plurality of nanochannel arrays allows each array to be used to sense a different target analyte and/or for paired arrays to simultaneously analyze control and test samples in parallel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an end-on cross section drawing showing the construction of a nanochannel array according to the present invention. 
         FIG. 2  is an end-on cross-section of a nanochannel coated on its metal surface with an analyte-binding material. 
         FIG. 3  is an illustration of a nanochannel array with microfluidic fluid control. 
         FIG. 4  is a side cross-section view along a nanochannel of a nanofluidic wafer. 
         FIG. 5  illustrates a urine protein analysis device comprising three nanochannel plasmonic sensor arrays. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Nanofluidics, as used herein, refers to the behavior, manipulation, and control of fluids that are confined inside flow channel structures in which the cross-sectional dimensions are between 10 and 800 nanometers. 
     A “nanochannel,” as used herein, is a tubular structure having a rectangular cross-sectional shape. The dimensions of a channel are described by length, depth, and width, wherein the depth is measured perpendicular to the plane of a nanofluidic chip containing the nanochannel and length and width are measured in directions lying in the plane of a wafer containing a nanochannel array. Maximum depth and width, when used to describe a nanochannel having a rectangular cross-section, refer to a channel having a constant width and depth. 
     The sensor comprises a flat, transparent dielectric substrate  1  upon which a 30 nm to 500 nm thickness metal film  2  is formed ( FIG. 1 ). The metal film  2  may be formed directly on the substrate  1  or the substrate  1  may be coated with a 1-10 nm layer of another metal such as chromium or titanium to promote adhesion of the metal layer  2  to the substrate  1 . The metal film  2 , in turn, is covered with a 1-50 nm thickness of a transparent dielectric layer  3 . Aligned, uniform nanoslits having a width of between 10 nm and 800 nm, preferably 30 nm to 300 nm, are milled all the way through the transparent dielectric layer  3  and the metal film  2  with a regular periodicity ranging from 100 nm to 800 nm to form a nanoslit array. The nanoslits may be milled, for example, by means of a dual beam scanning electron microscope/focused ion beam. A transparent top layer  4  covers the transparent dielectric layer and seals the tops of the nanoslits to form nanochannels  5  which, in turn, form a nanochannel array  6 . The number of nanochannels per array may range from 5 to 5000 and preferably from 20 to 100. The metal film  2  may be made of any suitable metal and preferably a metal selected from Au, Ag, Cu, Pt, or combinations thereof. The transparent dielectric substrate  1 , dielectric layer  3 , and transparent layer  4  may be made, for example of PDMS, PMMA, quartz, SiOx, or a glass. In preferred embodiments, the substrate is made of quartz or a glass, the metal layer is made of gold or silver, the dielectric layer is made from SiOx or a glass, and the transparent top layer is made of PDMS or PMMA. 
     EXAMPLE 
     Array Fabrication 
     A quartz microscope slide is cleaned with a piranha solution (3:1H 2 SO 4 /H 2 O 2 ) at 80° C. for at least 10 minutes, rinsed with deionized water, and dried under nitrogen. A 1-3 nm Ti layer is deposited on the quartz surface using an e-beam evaporator. A 100 nm-200 nm Au film is deposited on the Ti layer. Nanoslits are milled with a focused ion beam system. For a typical nanoslit array, sets of 40 individual nanoslits are fabricated with a spacing defined by the array&#39;s periodicity. For transmission measurements, a reference window is milled into the same Au film that contains the nanoslit arrays. Normal beam conditions for the reference window are 30 kV and 30 pA. 
     All or a portion of the luminal surfaces of the nanochannels in an array may be modified to control their binding and or light transmission characteristics such as nonspecific binding and refractive index. To facilitate selective detection of particular target analytes, all or a portion of the lumenal surfaces of the nanochannels may be coated with substances that selectively bind to one or more analytes. For example, a self-assembling monolayer  2   a  of molecules capable of cross-linking or associating with target analyte specific binding agents may be formed on the lumenal surfaces of the metal layers  2  of the nanochannels  5  ( FIG. 2 ). To selectively detect serum albumin, for example, in urine or other sample fluids, gold surfaces in nanochannels may be coated with a monolayer of N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP) with a cross-linking group for anti-albumin antibody immobilized to the nanoslit surface, followed by coupling of the SPDP with anti-albumin antibody. To selectively detect IgG, protein A may be immobilized to gold nanoslit surfaces via SPDP. 
     To move fluids, including samples and reagents through the nanochannels in an array, the nanochannels are in fluid communication with inlet and outlet manifolds and means for moving fluid. The nanochannels may be formed in such a way as to have open ends that communicate directly with manifolds that overlap the nanochannels in the wafer containing the array. Such an arrangement can be formed, for example, by etching inlet and outlet manifolds into the substrate, metal, and dielectric layers before applying the transparent top layer of the wafer. Alternatively, the nanochannels may be formed as sealed tubes and communicate with manifolds located in a plane above or below the plane of the nanochannel array.  FIG. 3  is a scanning electron microscope (SEM) image of a wafer  13  comprising 3 μm deep inlet  10  and outlet  11  manifolds formed in a transparent top layer overlapping the sealed ends  14  of 125 nm deep nanochannels in a nanochannel array  6 .  FIG. 4  is a side cross-section view (not to scale) showing the relative positions of the wafer components including nanochannel inlet  10   a  and outlet  11   a  as well as the relative positions of a light source and detector in a sensor device comprising the wafer. Arrows within the wafer indicate the path of fluid flow, while arrows outside the wafer indicate the direction of light directed toward and light transmitted from the wafer. 
     A protein SPR sensor device comprising three nanochannel arrays  6  is illustrated in  FIG. 5 . The apparatus consists of a light source  46 , an optical detection system  41 , a data acquisition unit  42 , and a microfluidic-based urine protein assay cartridge  43  comprising three nanochannel plasmonic sensor arrays  6 . The surfaces of the nanochannels in the plasmonic sensor arrays  6  are functionalized with an ultrathin film of receptors that may be nonspecific for binding to protein generally or may specific for binding target proteins to be detected in the urine. The sensor integrates to a microfluidic network  45  and pumping means  12  configured for reagent and fluid flow handling. Nanoslit array transmission spectra of light from a white light source  46  incident upon the top of the arrays  6  are captured by an optical detection system  41  comprising a fiber optical array, mini-spectrometer or CCD, for example, and processed and stored in a data acquisition unit  42 . 
     Fluid communication between a nanochannel array  6  and fluid handling manifolds allows fluid to be moved through the nanochannel array  6  using a pumping means  12  configured to move fluid through the nanochannel array  6 . The pumping means  12  includes, for example, electrokinetic, electrothermal, and peristaltic pumps and may be incorporated into the cartridge  43  or may be a separate unit as shown in  FIG. 5 . The fluid handling capability of an individual nanoarray may be incorporated into nanofluidic chips containing a plurality of nanochannel arrays with each array being used to sense a different target analyte, for example, or to assay test and control samples simultaneously. 
     Devices of this type may also be used to detect a wide variety of analytes including proteins in biological samples such as urine, blood, saliva, as well as samples of non-biological origin. 
     Reference to particular embodiments of the present invention have been made for the purpose of describing a nanoscale surface plasmonics sensor with nanofluidic control. It is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.