US 20070108465 A1
A preferred embodiment biosensor is a multi-layer micro-porous thin film structure. Pores in a top layer of the micro-porous thin film structure are sized to accept a first molecule of interest. Pores in a second layer of the micro-porous thin film structure are smaller than the pores in the top layer and are sized to accept a second molecule of interest that is smaller than the first molecule of interest. The pores in the second layer are too small to accept the first molecule of interest. The pores in the top layer and the pores in the second layer are sized and arranged such that light reflected from the multi-layer micro-porous thin film structure produces multiple superimposed interference patterns that can be resolved. In preferred embodiments, the multi-layer micro-porous thin film structure is a porous silicon thin film multi-layer structure formed on a silicon substrate, such as a silicon wafer. Specific and nonspecific binding can be detected with biosensors of the invention. The position of peaks in the Fourier transform of the reflection spectrum and the shift in peak amplitudes can be used to determine the presence and quantity of targeted biological molecules of interest.
1. A biosensor, comprising:
a multi-layer micro-porous thin film structure;
pores in a top layer of the micro-porous thin film structure being sized to accept a first molecule of interest;
pores in a second layer of the micro-porous thin film structure being smaller than said pores in the top layer and being sized to accept a second molecule of interest that is smaller than said first molecule of interest, said pores in the second layer being too small to accept the first molecule of interest;
wherein said pores in the top layer and said pores in said second layer are sized and arranged such that light reflected from the multi-layer micro-porous thin film structure produces multiple superimposed interference patterns that can be resolved.
2. The biosensor of
3. The biosensor of
4. The biosensor of
5. The biosensor of
6. The biosensor of
7. A method for biosensing, the method comprising:
exposing a biological analyte to a multi-layer micro-porous thin film structure that has a top layer with larger pores than a second layer, the pores being sized to produce multiple superimposed interference patterns that can be resolved in the reflectivity spectrum;
exposing the multi-layer micro-porous thin film structure to light to produce a reflectivity spectrum;
sensing the reflectivity spectrum to obtain reflectivity data;
extracting optical parameters from the reflectivity data;
determining, from the optical parameters, whether at least one biomolecule of interest is present in said biological analyte.
8. The method of
9. The method of
10. The method of
This application claims priority under 35 U.S.C. §119 to provisional application Ser. No. 60/660,421 filed on Mar. 10, 2005 and entitled Porous Microstructure Multi Layer Spectroscopy and Biosensing.
The invention was made with Government assistance from the National Cancer Institute of the National Institutes of Health Contract No. N01-C0-37117 and the Air Force Office of Scientific Research Grant No. F49620-02-1-0288. The Government has certain rights in the invention.
A field of the invention is biosensing. Particular example applications for porous microstructures of the invention include label-free biomolecule sensing.
Porous microstructures have been demonstrated to produce characteristic spectral interference patterns. Porous silicon has been used to produce characteristic spectral interference patterns, and other semiconductors and insulators can be etched to produce characteristic spectral interference patterns. Introduction of analyte into such a porous microstructure produces a shift in the spectral interference pattern. The interference based sensing is largely impervious to less complex optical sensing and detection methods.
As an example, chemical or biomolecule detection can be based on changes in the spectral interference pattern that results from the reflection of white light at the interfaces above (air or solution) and below a porous silicon layer formed in a silicon wafer. Spectral positions of the Fabry-Pérot fringes shift as a function of the refractive index of the material filling the pores. Biomolecule penetration into the pores of porous Si layers, driven either by nonspecific adsorption or by specific binding (to an antibody, for instance) is observed as a shift of the Fabry-Pérot fringes to longer wavelengths. This corresponds to an increase in refractive index of the film as protein displaces aqueous solution in the pores.
The tuning of pore sizes, patterns and distributions in layers, such as porous silicon has been demonstrated. Pores can be tuned to trap a particular analyte, for example based upon pore size or by preparing the pores with a material to bind analyte. Pore size is controlled by an appropriate choice of electrochemical etching conditions. A biosensor can be formed, for example, by tuning pore size to accommodate a biomolecule of interest while keeping the pores small enough to avoid light scattering effects. Etching conditions affect pore size. Conditions that can affect pore size include, for example, the type of material being etched, doping levels (if any), resistivity, etching current density, etc. A wide range of pore sizes and morphologies can be obtained.
Single layer porous microstructures that are based on changes in the spectral interference pattern have been used. Others have used multi-layer porous silicon to achieve biosensors based on optical transduction methods other than wavelength shifts of the interference pattern. For example, Martin-Palma et al. detected binding of polyclonal mouse antibodies to an amine-modified porous Si multilayer by observing a reduction of the intensity of reflected light. Martin-Palma, et al. Microelectronics Journal, 2004, Vol. 35, pp. 45-48. Additionally, Chan et al. formed porous Si multilayer structures such as Bragg mirrors and microcavity resonators and used modulation of the photoluminescence spectra from these structures to distinguish between Gram(−) and Gram(+)bacteria. Chan et al., Journal of the American Chemical Society, 2001, Vol. 123, pp. 11797-11798. The non-interference based sensing methods have difficulty in noisy environments, require more sensitive equipment to achieve comparable sensitivity, and fail to account for common measurement conditions, e.g., signal drift due to thermal fluctuation, changes in sample composition, or degradation of the sample matrix.
Porous silicon films with a distribution of pore diameters in the x-y plane (parallel to the surface of the wafer) have been demonstrated as size-exclusion matrices to perform an on-chip determination of macromolecule dimensions. Karlsson, et al, H. J. Colloid Interface Sci., 2003, 266, 40-47. These films were generated by electrochemically etching Si in aqueous ethanolic HF using an asymmetric electrode configuration. Biomolecules penetrate the film and are detected only in regions where the pores are large enough. A disadvantage of this approach is that determination of protein size requires optical sampling over a relatively large area of the porous Si film.
A preferred embodiment biosensor is a multi-layer micro-porous thin film structure. Pores in a top layer of the micro-porous thin film structure are sized to accept a first molecule of interest. Pores in a second layer of the micro-porous thin film structure are smaller than the pores in the top layer and are sized to accept a second molecule of interest that is smaller than the first molecule of interest. The pores in the second layer are too small to accept the first molecule of interest. The pores in the top layer and the pores in the second layer are sized and arranged such that light reflected from the multi-layer micro-porous thin film structure produces multiple superimposed interference patterns that can be resolved. In preferred embodiments, the multi-layer micro-porous thin film structure is a porous silicon thin film multi-layer structure formed on a silicon substrate, such as a silicon wafer. Specific and nonspecific binding can be detected with biosensors of the invention. A shift in the position of peaks in the Fourier transform of the reflection spectrum and/or a shift in the intensity of the peak amplitudes can be used to determine the presence and quantity of targeted biological molecules of interest.
A preferred embodiment of the invention is a biosensor. A sensor of the invention include a multi-layer micro-porous thin film structure, where the layers have pores of different sizes. The pores are sized and arranged in the multi-layer structure to produce optical interference patterns. In a preferred biosensor, pores in a top layer of the micro-porous thin film structure are sized to accept a first molecule of interest. Pores in a second layer of the micro-porous thin film structure are smaller than the pores in the top layer and are sized to accept a second molecule of interest that is smaller than the first molecule of interest. Pores in the second layer are too small to accept the first molecule of interest. Pores in the top layer and said pores in said second layer are sized and arranged such that light reflected from the multi-layer micro-porous thin film structure produces multiple superimposed interference patterns that can be resolved.
An example embodiment biosensor demonstrates an optical interferometer that incorporates a reference channel into the optical response of the structure. In an example embodiment, the pore sizes in different layers also provide for separation and detection of the analyte of interest. In additional embodiments, the pores in separate layers are also biologically prepared, e.g. with at least one layer among layers having differently sized pores contains an antibody.
Referring now to the drawings, a preferred biosensor 10 is shown in
Size discrimination occurs in the z direction (perpendicular to the plane of the wafer surface, at an air or solution interface). The layer 12 includes pores sized to accept a first molecule of interest. Pores in the second layer 14 of the micro-porous thin film structure are smaller than pores in the top layer 12 and are sized to accept a second molecule of interest that is smaller than the first molecule of interest. Light λ reflected from the layers 12, 14, 16 contains three superimposed interference patterns a, b, and c, which can be resolved by fast Fourier transform (FFT) of the reflectivity spectrum. Interference of beams a and b occurs from reflections at the interfaces bordering layer 1, interference of beams b and c originates from layer 2, and interference of beams a and c originates from layer 3. Analyte-containing solution is introduced at the top of the structure. The three peaks in the FFT provide an indication of the relative partitioning of the large and small molecules in the two layers of this size-selective membrane. Additional layers would produce additional superimposed interference patterns.
The ability of the pores in the layers 12 and 14 to discriminate among biomolecules of interest has been verified experimentally. Experiments were conducted with bovine serum albumin (BSA) and sucrose as the large and small probe molecules, respectively. Penetration of BSA exclusively into the top (large pore) layer and of sucrose into both layers is detected by optically allowing the detection of the larger BSA in the presence of a 100-fold (by mass) excess of sucrose. Artisans will recognize that the general applicability to other biomolecules, as well, and will appreciate additional details of preferred embodiments as well as broader aspects of the invention from the following experimental results. The work here demonstrates a simple and powerful means to discriminate and detect molecules based on size and to correct for drift in composition and other matrix effects with a designed nanostructure.
1. Fabrication and Stabilization of p-Type Porous Si Double-Layers
Porous silicon (Si) double-layers with a layer of larger pores on top of a layer with smaller pores can be prepared using an electrochemical etch consisting of a short period of high applied current followed by a longer period at low current. In the experiments, the current density profile consisted of 11 s at 500 mA/cm2 followed by 55 s at 167 mA/cm2. This waveform was applied to a highly doped (10−3Ω-cm) p-type (100)-oriented single crystal Si wafer in ethanolic HF solution. For comparison, individual single-layer structures were also prepared. “Single-layer 1” was prepared using the first part of the double-layer waveform (11 s at 500 mA/cm2), and “single-layer 2” was prepared using the second part of the double-layer waveform (55 s at 167 mA/cm2). The resulting freshly etched porous Si samples are hydride-terminated and slowly degrade in air or water by oxidative or hydrolytic corrosion. To prevent corrosion in the aqueous solutions used in experiments, siloxy-terminated porous Si surfaces were prepared by thermal oxidation. Oxidation increases the hydrophilicity of porous Si, allowing water to effectively infiltrate the pores.
2. Determination of Porosity and Thickness of Porous Si Layers
The porosities and thicknesses of the individual porous Si layers were independently determined by gravimetry, SEM (thickness only) and by optical measurements. The average values obtained from at least three measurements are summarized in Table 1. Gravimetric measurements were performed by weighing the sample before etch, after etch, and after chemical dissolution of the porous layer. Cross-sectional scanning electron microscopy (SEM) reveals that the conditions used to prepare “single-layer 1” (11 s at 500 mA/cm2) produce a ˜2600 nm-thick porous Si film with cylindrical pores possessing diameters of ˜50 to 100 nm. The conditions used to prepare “single-layer 2” (55 s at 167 mA/cm2) produce a film of ˜5400 nm in thickness. The pores in this layer are smaller, with diameters that are too small to be reliably resolved in the SEM images (<20 nm).
The results are confirmed by an approximately exponential dependence of the pore diameter on the current density for highly-doped p-type samples, a fact that has been previously recognized. The porous Si double-layer, produced by applying the “single-layer 1” etching conditions followed immediately by the “single-layer 2” conditions, possesses a combination of the two single layers one on top of the other. An SEM image obtained of the sample displayed the porous structure; layer thicknesses (˜2900 nm and ˜5600 nm for layers 1 and 2, respectively), and pore sizes were consistent with the images obtained from the single-layers.
Thickness and porosity of the layers were also determined by optical measurements. The Bruggeman theory, one of a number of effective medium approximations, has been shown to predict the porosity of porous Si in reasonable agreement with gravimetric determinations. In this technique, the porous Si white-light reflection spectrum is measured with the film held in air and in the series of solvents ethanol, acetone, and hexane, having refractive indices 1.360, 1.357, and 1.375, respectively. The difference between the spectra can be attributed to the changes in optical thickness as the medium in the pores changes, with the assumption that all the void spaces in the film are filled equally (i.e., no remaining air bubbles). The value of the product 2 nL, where n and L are the refractive index and the thickness of the film, respectively, are obtained from the reflectivity spectra. For the Bruggeman calculation, only the product nL is used. Data obtained from a given sample in air and in the three liquids is then fit to the two-component Bruggeman approximation, yielding an over-determined solution for both the porosity and the thickness of the sample. The value of refractive index for oxidized porous Si used in the Bruggeman fit that provided the most self-consistent results was 2.1.
The calculated porosities of the porous Si single-layers (Table 1) agree with the results obtained by gravimetry. However, the film thickness values obtained from the Bruggeman calculations and the SEM measurements deviate somewhat from the gravimetric results. This can be explained since the gravimetric measurements do not account for the increase in mass and in volume that occurs when the porous Si film is oxidized. The gravimetry instead measures of the amount of Si lost in etching. Thus, in the case of oxidized porous Si films, gravimetry provides an under-estimation of film thickness. The SEM and optical measurements are more accurate measures of film thickness.
3. Interpretation of Interferometric Reflectance Spectra
The spectra, obtained using a spectrometer and a white light (tungsten) illumination source in a 90° backscatter configuration, display a series of interference fringes. These fringe patterns result from Fabry-Pérot interference of light reflected from the various interfaces present in the structures. The fringe maxima are described by the Fabry-Pérot relationship given in eq. 1
where m is an integer, L is the thickness of the porous Si layer, n is the average refractive index, and λ is the wavelength of incident light. The factor of 2 derives from the 90° backscatter configuration of the illumination source and detector. The term 2 nL is thus the optical path, referred to as the effective optical thickness (EOT) in this application. The pore dimensions in these structures are too small to effectively scatter light, and each porous layer is treated as a single medium with a single value for refractive index.
As expected from the relationship of eq. 1, the series of Fabry-Pérot fringes observed in the “single-layer 1” and the “single-layer 2” samples are spaced evenly in frequency (
where δi represents the phase relationship of layer i:
Here ni represents the refractive index of layer i with thickness Li. The terms ρa, ρb, and ρc in eq. 2 represent the index contrast at each of the interfaces a, b, or c (see
Where nair, n1, n2, and nsi represent the refractive index of air (or of the solution), layer 1, layer 2, and bulk Si, respectively. The quantities n1 and n2 represent the total refractive index of the layer and everything it contains (silicon, SiO2, solution, air, biomolecule, etc.). A least-squares fit of the reflectivity spectrum of the double layer to eq. 2 is shown in
4. Fast Fourier Transform (FFT) of Reflectance Spectra
A FFT of the spectrum provides a computationally simple and reliable technique for extracting the optical parameters. More complex models can also be used, such as models incorporating the frequency dispersion of refractive index, the effect of multiple reflections, and the instrumental response function. Such models provide a more rigorous description of the optical properties of the porous Si films. However, the FFT is a faster and more convenient method of extracting the optical parameters. The FFT can provide more reliable data for complicated optical structures, in particular when the optical constants are changing due to analyte admission into the pores. For biosensing, the relative change in these optical constants is the important parameter.
The Fourier transforms of the optical interference spectra of
The value of 2 nL can be obtained directly as the position of the peak in the FFT. Thus the “single-layer 1” film in air displays a spectral interference pattern (
The sum of the values of 2 nL for layer 1 and 2 is predicted by eq. 2 to be equal to the value of 2 nL for layer 3, and there is agreement within the error of the measurement (20,150 nm vs 20,100 nm, respectively). As with the least-squares fits of the spectra to eqs. 1 and 2, the FFT analysis yields a 2 nL value for each individual layer of the double-layer that is smaller than the value obtained from the corresponding single-layer, consistent with the SEM measurements (Table 1).
The information contained in both the intensity and in the position of the peaks in the Fourier transform can be related to the optical constants presented in the double-layer interference model of eq. 2. As discussed above, the position of a peak in the FFT is equal to the EOT, or 2 nL of a porous Si layer. It is related to the phase relationship of eq. 3 by:
Eq. 2 represents a sum of 3 cosine terms corresponding to the 3 optical layers represented in
where A1, A2 and A3 are the amplitudes of the FFT peaks corresponding to layer 1, 2, and 3 of
5. Changes in the Optical Parameters Due to Infiltration of Molecules
Infiltration of a liquid into the porous structures leads to predictable changes in both the position and in the intensity of the peaks in the FFT spectrum. The intensity changes correspond to changes in the relative reflectivity of the various interfaces in the structure. As seen in
The decrease in intensity of the FFT peaks that occurs when the sample is immersed in liquid is most noticeable for samples that have been extensively oxidized, where the refractive index of the solid component of the porous film is closer to pure SiO2 (n=1.5) than to Si (n=3.8). Ozone-oxidation is milder, producing less SiO2 in the film. For a given porosity, ozone-oxidized films have a larger refractive index than thermally oxidized films, and the peak corresponding to “layer 1” is more discernable in the FFT when such samples are immersed in liquids. However, the ozone-oxidized films were found to be insufficiently stable in the aqueous solutions, and so the thermally oxidized films are preferred for introduction of molecules via an aqueous solution.
Shifts in the position of the peaks in the FFT spectrum indicate a change in average refractive index (EOT or 2 nL) of the film. Infiltration of ethanol into the pores leads to an increase in the value of EOT for either the single- or the double-layer films. The “single-layer 1” film displays a spectrum with a peak in the Fourier transform at 6950 nm in air, increasing to 9080 nm in ethanol. Similarly, the FFT peak from the “single-layer 2” film shifts from 14,400 nm to 17,000 nm when the film is placed in the liquid.
The peaks in the FFT spectrum of the double-layer also shift upon infiltration of ethanol, in line with the single-layer results. As mentioned above, the peak corresponding to layer 1 of the double-layer structure often become too weak to observe upon immersion in ethanol due to the loss in index contrast, but the peaks corresponding to layer 2 and to the combination of both layer 1 and layer 2 (layer 3 of
6. Sensing of Bovine Serum Albumin and Sucrose in Double Layers
The penetration of biomolecules into a porous Si Fabry-Pérot layer leads to an increase in EOT resulting from partial replacement of aqueous buffer by molecules with a larger index of refraction. Bovine serum albumin (BSA) and sucrose were used to test the ability of the porous Si double-layer to discriminate biomolecules of different sizes. BSA is a heart-shaped protein with dimensions roughly of the order of 3×8×8 nm (at pH 4) and a molecular weight of 68 kDa. Sucrose is a small molecule with dimensions less than 2×2×2 nm, and a molecular weight of 342 g/mol. Because of the difference in pore dimensions between the two layers of the double-layer, both molecules are expected to be admitted into layer 1 and only sucrose is expected to enter into layer 2.
Aqueous buffer solutions of the test molecules were introduced to the porous Si films using a transparent flow-cell such that real-time reflectivity spectra could be obtained. The flow-cell was flushed with either pure pH 4 buffer or pure pH 7 buffer followed by pure pH 4 buffer in between samples. The purpose of the pH 7 rinse is to more efficiently remove the BSA molecule from the oxidized porous Si surface by changing the net charge on the molecule. The isoelectric point of BSA is 4.8, so at pH 4 it has a net positive charge while oxidized porous Si is negative, leading to strong non-specific binding. At pH 7 both BSA and oxidized porous Si are negatively charged, and electrostatic forces aid in expulsion of the protein from the pores.
Protein adsorption to silica is an extremely complex process that depends on pH, temperature, ionic strength, and solvent properties. Others have shown attachment of poly(ethylene glycol) or large biomolecules to the inner pore surfaces minimizes non-specific adsorption to porous Si.
In the experiments, the surfactant Triton X-100 was added to the solutions in order to enhance diffusion of BSA out of the pores of thermally oxidized porous Si. It was found that even the addition of detergent could not displace all of the BSA from the surface, and the initial introduction of BSA to the porous Si double-layer forms a thin coating that is not removed with subsequent rinsing steps. Thus, all samples in the biomolecule binding studies were pre-treated with BSA solution and then rinsed with pH 7 buffer followed by pH 4 buffer. After this “pre-treatment” which led to a new baseline, both sucrose and BSA were found to readily diffuse in and out of the pores and to provide reproducible binding curves during the flow cell experiments. Sucrose is a neutral molecule, and did not display any such enhanced binding effects with the samples used in the present study.
7. Shifts in FFT Peak Positions (EOT) on Exposure to BSA and Sucrose
The response of the layers can be probed using the FFT methods, as described above. As described above, the interference spectrum of a double-layer film displays a complicated pattern of maxima and minima that corresponds to a combination of the Fabry-Pérot interference spectra from the three layers defined in
When the double-layer sample is exposed to a solution containing 1 mg/mL of BSA and 50 mg/mL sucrose, the FFT peaks corresponding to layers 2 and 3 shift to larger values, corresponding to an increase in EOT as the molecules enter the pores (
When only BSA is added (
8. Detection of Biomolecules in Double-Layers by Differential EOT Measurement
The main difference in composition between layer 1 and layer 2 is that layer 1 contains the protein BSA and layer 2, because of its smaller pores, does not. In addition, because they have different porosities, the relative amount of SiO2 and solution are different in each layer. If this latter difference can be accounted for, then a technique to measure the binding of protein in the film is provided. Binding of protein can be measured as the difference in index between layer 1 (n1) and layer 2 (n2). Since the total refractive index of a layer is related to its EOT by eq. 5, the difference can be expressed as:
Here the term γ is a weighting factor that accounts for the differences in porosity and thickness between the two layers. It assumes that the index of a layer is proportional to its fractional composition, which is not strictly valid if the Bruggeman effective medium approximation applies. However, for small changes in n as encountered in the present biosensing application it is approximately correct. If γ=2 and L1=L2, the left side of eq. 7 reduces to (n1-n2)L, as expected. The quantity γ indicates how the relative responses of the two layers scale with each other, and can be determined empirically from the sucrose data by application of eq. 8:
Where EOTi(sucrose) is the EOT measured for layer i infused with buffer containing sucrose, and EOTi(buffer) is the EOT measured for layer i infused with buffer only. For the data in
9. Dependence of Differential EOT Measurement on Concentration of Matrix Interferents
The effect of a changing sample matrix on the differential EOT measurement is shown in
In this experiment, a constant amount of BSA (1 mg/mL) is tested in the presence of sucrose concentrations varying from 0 to 100 mg/mL. The effective optical thickness (EOT, or 2 nL) of layers 2 and 3 and the weighted difference of the two values are plotted as in
10. Detection of Biomolecules in Double-Layers by Measuring Shifts in FFT Peak Intensity
The amplitude of a peak Ai in the FFT spectrum is related to the reflectivity of the two interfaces bordering layer i by eq. 6. The reflectivity at a given interface is related to the index contrast by eq. 4. When protein infuses into either layer 1 or layer 2, it exerts an effect on the magnitude of Ai for both layers, since they share a common interface. Binding of protein can be measured as the ratio of peak amplitudes A1 to A2. This ratio is related to the index contrast at two of the three interfaces present in the double-layer (eq. 4 and 6) by eq. 9:
Here, there is no weighting factor to account for differences in porosity and thickness between the two layers as was introduced in eq. 7. In the present case the data deriving from layer 1 is used instead of layer 3. In some instances the amplitude A1 in the FFT spectrum can be too small to measure reliably, especially when using fully oxidized porous Si (where the index contrast of layer 1 is small relative to its neighboring layers). In such cases it is also possible to isolate the response to BSA by plotting the ratio A3/A2.
The amplitude of the FFT peaks corresponding to layer 1 and layer 2 of the double-layer (A1 and A2), and the ratio of A1 to A2 are presented in
As can be seen, the plots of A1 and A2 vs time are very noisy. The noise in the measurement relates to fluctuations in lamp intensity, bubbles in the flow cell, cell temperature, and other undetermined experimental variables during the course of the measurement. The measurement of EOT (FFT peak position) is less sensitive to such variations. However, the errors apparent in the plots of A1 and A2 are correlated, and they are effectively eliminated in the ratio of A1 to A2, as shown in
11. Experimental Procedures
All reagents were used as received. Potassium phosphate pH 7 buffer was obtained from Fisher, Inc. as a 0.05 M potassium phosphate Certified Buffer Solution (Cat. No. SB108-20). The potassium biphthalate pH 4 buffer was obtained from Fisher, Inc. as a 0.05 M potassium biphthalate Certified Buffer Solution (Cat. No. SB98-500). Bovine Serum Albumin (BSA), lyophilized powder, 1×crystallized, >97% purity, was obtained from Sigma-Aldrich (Cat. No. A 4378). Sucrose was obtained from EM Science (Cat. No. SX 1075-1). Aqueous HF (48%) and ethanol (99.9%) were supplied by Fisher Scientific and AAper, respectively. Porous Si samples were prepared from single-crystalline highly-doped p-type Si (0.0008-0.0012Ω-cm resistivity, <100> oriented, B-doped, from Siltronix Corp.)
Porous Si samples were prepared by anodization of the degenerately doped p-type Si wafers in ethanolic HF solution (3:1 v/v 48% aqueous HF:ethanol) in a two-electrode configuration using a platinum mesh counter-electrode. Si wafers with an exposed area of 1.2 cm2 were contacted on the back side with a strip of aluminium foil and mounted in a Teflon etching cell. Galvanostatic anodization was performed in the dark. Single-layers were etched by applying 500 mA/cm2 for 11 s (“single-layer 1”) or 167 mA/cm2 for 55 s (“single-layer 2”). Double-layer samples were prepared by application of 500 mA/cm2 for 11 s followed immediately by application of 167 mA/cm2 for 55 s. After etching, the samples were rinsed thoroughly with ethanol and then dried under a stream of nitrogen.
Scanning electron microscopy (SEM) images were obtained with a Cambridge EM-360 electron microscope using an accelerating voltage of 20 keV. In order to avoid sample charging anomalies, the porous Si samples were sputter-coated with a thin layer of gold (20 nm) prior to the SEM analysis.
Interferometric reflectance spectra of porous Si were collected by using an Ocean Optics CCD S-2000 spectrometer fitted with a microscope objective lens coupled to a bifurcated fiber optic cable. A tungsten light source was focused onto the center of a porous Si surface with a spot size of approximately 1-2 mm2. Reflectivity data were recorded with a CCD detector in the wavelength range of 400-1000 nm, with a spectral acquisition time of 100 ms. Typically 10 spectral scans (1 s total integration time) were averaged before FFT processing. Both the illumination of the surface and the detection of the reflected light were performed along an axis coincident with the surface normal.
Porous Si samples were thermally oxidized by heat treatment in a tube furnace (Fisher Blue M). The samples were heated at 600° C. for 1 h in ambient air, then allowed to cool to room temperature.
Three porous Si samples were weighed on a laboratory balance with a resolution of 10 μg before (m1) and after etching at mA/cm2, respectively. Each sample was thermally oxidized and weighed again (m2). The oxidized porous Si layer was then dissolved in 48% aqueous HF:ethanol (3:1, v:v), and the wafer remaining was weighed (m3). The porosity P was calculated using the following equation:24
The thickness W of the porous Si layer was determined by applying the equation:
Biomolecule penetration experiments were carried out in a custom-made flow cell. Briefly, the flow cell was constructed of plexiglass and connected via an outlet and inlet to a peristaltic pump. The light beam was focused on the surface of the sample through the plexiglass cover and interference spectra were recorded.
The wavelength axis of the spectrum from the Ocean Optics spectrometer was calibrated using a least-squares fit of five spectral lines observed from a neon lamp, at 585.3, 614.3, 640.2, 703.2, and 811.5 nm. The data spacing is approximately 0.4 nm. The x-axis was inverted and a linear interpolation was applied such that the data were spaced evenly in units of nm−1. A Hanning window was applied to the spectrum, it was redimensioned to 4096 data points and the zero padded to the power of two. A discrete Fourier transform using a multidimensional fast prime factor decomposition algorithm from the Wavemetrics, inc IGOR program library (FFT) was applied.
12. Fabrication of Two Layer Biosensor
The example biosensors for this experiment were generally consistent with
13. Results Summary
Spectra were taken to determine the response of the biosensor to Protein A (
The response of the protein A-modified sensor to IgG (ratio change ca. 0.27 for 0.05 mg/mL) is approximately ten times greater than the change observed for strongly-adsorbed Protein A (ca. 0.029). Part of this difference is due to the differences in molecular weight (Protein A: 42 kDa, Rabbit IgG ca. 150 kDa), which can account for ˜3.6-fold difference in response. The additional difference may be due to Protein A binding two IgG molecules under these conditions as has been reported for some solution studies or to differences in the refractive index of IgG relative to Protein A. The sensor response here is proportional to a change in the interfacial refractive index contrast rather than just a change in magnitude of the index.
The double-layer biosensor provides a quantitative measurement of equilibrium binding constants. The thermodynamic equilibrium binding constant can be obtained from the A3/A2 signal at equilibrium for various analyte concentrations by application of a Langmuir fit to the binding isotherm. A fit of the average values of three trials yields a calculated equilibrium dissociation constant KD of 3×10-7 mol/L for rabbit IgG binding to protein A, consistent with the published values. The results demonstrate that you can use a change in intensity of the peaks to correct for background.
The approach should also be amenable to other label-free transduction modalities that utilize refractive index changes, such as surface plasmon resonance or microcavity resonance. The built-in reference channel and Fourier method of analysis provides a general means to compensate for changes in sample matrix, non-specific binding, temperature, and other experimental variables.
An advantage of the interferometric biosensor optical structures of the invention compared to other optical transduction methods is that the sensitivity of the measurement can be improved. Additionally, compared to other optical transduction methods, the equipment needed to monitor binding events can be simplified. The interferometric biosensor of preferred embodiments provides the basis to incorporating sophisticated detection functions, such as correction for drifts due to thermal fluctuation, changes in sample composition, or degradation of the sample matrix.
While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.
Various features of the invention are set forth in the following claims.