A Porous Silicon Optical Biosensor: Detection of Reversible Binding of IgG to a Protein A-Modified Surface

Label-free optical detection of aptamer-protein interactions using gold-capped oxide nanostructures

Optical biosensors based on noble nanostructures currently receive attention due to their highly efficient, simultaneous analysis of a number of important biomolecules from proteomics to genomics. In this study, the combination of localized surface plasmon resonance (LSPR) with interferometry in the relative reflected intensity (RRI) spectrum of the gold-capped oxide nanostructure was thoroughly exploited for label-free detection of aptamer-protein interactions. The fabrication of gold-capped oxide nanostructure involved the deposition of gold on the surface of porous anodic alumina (PAA) layer chip. This novel nanomaterial enabled us to simultaneously monitor the changes in both LSPR and interferometric characteristics since the biomolecular interactions occur. After immobilizing the aptamer I on the chip surface, our sensor could be easily applied for specific detection of thrombin and aptamer II with a limit of detection of 1 nM thrombin in the sample. Our optical biosensing device connecting with the gold-capped oxide nanostructure has a high potential for highly sensitive monitoring of the other biomolecular interactions such as protein-protein interactions, DNA-protein interactions, DNA-DNA hybridizations, and ligand-receptor interactions with a massively parallel detection capability in a high-throughput system.

pH-triggered release of vancomycin from protein-capped porous silicon films

Objective: An in vitro model system for pH-triggered release of the antibiotic vancomycin from porous Si films is studied. Method: Vancomycin is infused into a mesoporous Si film from a mixed aqueous/acetonitrile solution and trapped by a capping layer containing the protein bovine serum albumin (BSA). The protein effectively traps vancomycin in the porous nanostructure at pH 4.0; the protein dissolves and vancomycin is released into solution when the pH increases to 7.4. The surface chemistry of porous Si exerts a substantial effect on the efficacy of drug loading. The amount of drug loading is larger in freshly-etched (hydrophobic, hydrogen-terminated) porous Si and smaller in methyl-modified, undecylenic acid-modified and thermally oxidized samples. The quantity of drug loaded in a freshly etched porous Si chip is proportional to the thickness of the porous layer, which exhibits a constant volume loading efficiency of 31% (v/v). Flow-cell experiments designed to mimic the transition from pH 4 to 7 that occurs when material moves from the stomach to the upper intestinal tract were performed on the freshly etched films and vancomycin- and BSA-release rates were quantified from the effluent of the flow cell by high-pressure liquid chromatography analysis. Results & conclusion: There is a small, constant rate of vancomycin release at pH 4 that is independent of the amount of drug loaded in the pores. This is attributed to diffusion of vancomycin from the BSA-capping layer. The release rate increases five- to tenfold when the pH of the solution in the flow cell increases to 7.4; 100% of the drug is released within 3 h of this increase.

Microsystems technology and biosensing

This review addresses the recent developments in miniaturized microsystems or lab-on-a-chip devices for biosensing of different biomolecules: DNA, proteins, small molecules, and cells, especially at the single-molecule and single-cell level. In order to sense these biomolecules with sensitivity we have fabricated chip devices with respect to the biomolecule to be analyzed. The details of the fabrication are also dealt with in this review. We mainly developed microarray and microfluidic chip devices for DNA, protein, and cell analyses. In addition, we have introduced the porous anodic alumina layer chip with nanometer scale and gold nanoparticles for label-free sensing of DNA and protein interactions. We also describe the use of microarray and microfluidic chip devices for cell-based assays and single-cell analysis in drug discovery research.

Interfacing the nanostructured biosilica microshells of the marine diatom Coscinodiscus wailesii with biological matter

Biosilicified nanostructured microshells from the marine diatom Coscinodiscus wailesii have been properly functionalised to bind a molecular probe which specifically recognises a target analyte. The chemical modification process has been characterised by Fourier transformed infrared spectroscopy. Fluorescence measurements demonstrate that the antibodies we used, even if linked to the amorphous silica surface of C. wailesii microshells, still efficiently recognise their antigens. These low cost and largely available natural materials can be thus used as transducers elements for optical biosensors or as targeting microcapsules for drug delivery.

Color me sensitive: amplification and discrimination in photonic silicon nanostructures

A new paper in this issue discusses some of the considerations for fabrication of biosensor devices in the porous silicon material system. The results focus on optical detection of protease activity on specific substrates that have been immobilized in a porous silicon matrix. Solutions to important limitations of porous microsensors are presented: stabilizing the sensor against corrosion-induced zero point drift, minimizing the effects of nonspecific protein binding, and enhancing the optical response by incorporation of a catalytic reaction in the sensing scheme.

Whole blood optical biosensor

The future of rapid point-of-care diagnostics depends on the development of cheap, noncomplex, and easily integrated systems to analyze biological samples directly from the patient (e.g. blood, urine, and saliva). A key concern in diagnostic biosensing is signal differentiation between non-specifically bound material and the specific capture of target molecules. This is a particular challenge for optical detection devices in analyzing complex biological samples. Here we demonstrate a porous silicon (PSi) label-free optical biosensor that has intrinsic size-exclusion filtering capabilities which enhances signal differentiation. We present the first demonstration of highly repeatable, specific detection of immunoglobulin G (IgG) in serum and whole blood samples over a typical physiological range using the PSi material as both a biosensor substrate and filter.

Continuous planar phospholipid bilayer supported on porous silicon thin film reflector

Reconstituting artificial membranes for in vitro studies of cell barrier mechanisms and properties is of major interest in biology. Here, artificial membranes supported on porous silicon photonic crystal reflectors are prepared and investigated. The materials are of interest for label-free probing of supported membrane events such as protein binding, molecular recognition, and transport. The porous silicon substrates are prepared as multilayered films consisting of a periodically varying porosity, with pore dimensions of a few nanometers in size. Planar phospholipid bilayers are deposited on the topmost surface of the oxidized hydrophilic mesoporous silicon films. Atomic force microscopy provides evidence of continuous bilayer deposition at the surface, and optical measurements indicate that the lipids do not significantly infiltrate the porous region. The presence of the supported bilayer does not obstruct the optical spectrum from the porous silicon layer, suggesting that the composite structures can act as effective optical biosensors.

Steric crowding effects on target detection in an affinity biosensor

This work quantifies the impact of steric crowding on whole antibody (Ab) receptor immobilization and target Ab detection and also demonstrates how the versatile biotin/streptavidin receptor immobilization system must be tuned to optimize target detection in designing biosensors. Results are demonstrated on a label-free optical biosensor fabricated from n-type macroporous porous silicon (PSi) with approximately 88-107 nm diameter pores. We employ a sandwich assay scheme comprising a linking chemistry (biotin/streptavidin) to attach biotinylated anti-rabbit IgG (receptor) to detect rabbit IgG (target). A "bottom-up" approach was taken to investigate each layer of the sandwich assay to optimize target binding. Steric crowding was observed to hinder subsequent layer binding for each layer in the sandwich (biotin, streptavidin, and receptor). Our results give definitive evidence that the onset of steric crowding within the biotin layer occurs at a surface coverage of 57%, which is much higher compared to that from published work on well-ordered self-assembled biotin monolayers on planar gold surfaces. This difference is attributed to the topographical heterogeneity of the PSi substrate. Streptavidin (SA) binding to surface-linked biotin was altered by preblocking the streptavidin binding sites with biotin. Through consistent trends in data, preblocking SA was shown to reduce steric crowding within the SA layer, which translated into increased receptor immobilization. The final detection range of rabbit IgG was 0.07-3 mg mL(-1) (0.4-17 ng mm(-2)), and binding specificity was demonstrated by employing an anti-chicken IgG control receptor. This study underlines the importance of considering binding avidity and surface topography in optimizing chip-based biosensors.

A simplified biomolecule attachment strategy for biosensing using a porous Si oxide interferometer

A simple strategy for linking biomolecules to porous Si surfaces and detecting peptide/drug binding is described. Porous Si is prepared using an electrochemical etch and then thermally oxidized by heating in ambient atmosphere. Bovine serum albumin (BSA) is then non-covalently adsorbed to the inner pore walls of the porous Si oxide (PSiO(2)) matrix. The BSA layer is used as a linker for covalent attachment of the peptide Ac-L-Lysine-D-Alanine-D-Alanine (KAA) using published bioconjugation chemistry. BSA-coated surfaces functionalized with KAA display specificity for the glycopeptide vancomycin while resisting adsorption of non-specific reagents. While the biomolecule attachment strategy reported here is used to bind peptides, the scheme can be generalized to the linking of any primary amine-containing molecule to PSiO(2) surfaces.

Submicrometer dimple array based interference color field displays and sensors

We report a technique for producing bright color fields over extended surfaces, via optical interference, with the capability of producing arbitrary visible colors in areas as small as 100 microm2. Periodic arrays of submicrometer dimples are fabricated on reflective silicon surfaces, and diffraction-induced mutual interference of light reflected from the upper and lower levels of the dimpled surfaces generates color depending on wavelength scaled dimple depth and periodicity. Colors of the entire visible spectrum can be generated by dimple arrays with different dimple depths. The topological permeability of such an open surface readily allows infusion of liquids, with different refractive indices, for color switching and detection. These easy to fabricate, scalable, robust devices, on solid as well as flexible supports, could find a wide range of applications such as cheap high-resolution printable dye/pigment-free displays, reliable index-of-refraction sensors with color readout for liquids, and lab-on-chip liquid flow monitors.

Preparation of a biochip on porous silicon and application for label-free detection of small molecule-protein interactions

A new approach for the preparation of a biochip on porous silicon and the application of the biochip for detection of small molecule-protein interactions with desorption/ionization on porous silicon (DIOS) was demonstrated. The galvanostatically etched porous silicon substrates were chemically modified firstly to yield carboxylic acid terminated surfaces, and then the protein was covalently attached to the surface through amide bonding. By applying a solution of candidate chemicals to the surface and a subsequent wash step, the masses of captured compounds could be analyzed by DIOS. DIOS has advantages of being a direct detection tool compared to the classic fluorescence or chemiluminescence methods, because the process of labeling molecules employed in the fluorescence or chemiluminescence methods can sometimes alert the properties of the labeled molecule. The recognition between proteins and their binding partners is efficient and selective. A good tolerance to disturbance and high enrichment factor of the biochip to the analytes was observed. As an on-chip-based approach, the demonstrated method has a potential to perform in a high-throughput format.

Hybrid lipid bilayers in nanostructured silicon: a biomimetic mesoporous scaffold for optical detection of cholera toxin

Cholera toxin levels are optically detected by affinity capture within hybrid lipid bilayer membranes formed in the nanostructures of porous silicon photonic crystals.

The smart Petri dish: a nanostructured photonic crystal for real-time monitoring of living cells

The intensity of light scattered from a porous Si photonic crystal is used to monitor physiological changes in primary rat hepatocytes. The cells are seeded on the surface of a porous Si photonic crystal that has been filled with polystyrene and treated with an O2 plasma. Light resonant with the photonic crystal is scattered by the cell layer and detected as an optical peak with a charge-coupled-device spectrometer. It is demonstrated that exposure of hepatocytes to the toxins cadmium chloride or acetaminophen leads to morphology changes that cause a measurable increase in scattered intensity. The increase in signal occurs before traditional assays are able to detect a decrease in viability, demonstrating the potential of the technique as a complementary tool for cell viability studies. The scattering method presented here is noninvasive and can be performed in real time, representing a significant advantage compared to other techniques for in vitro monitoring of cell morphology.

Delivery of nanogram payloads using magnetic porous silicon microcarriers

A transport and delivery system for nanogram quantities of molecular species that does not use microfluidic channels, pumps, or valves is described. Microparticles consisting of magnetic porous silicon are prepared, and loading and delivery of an enzymatic payload are demonstrated. The high porosity (60%) porous Si host particles are made magnetic by infusion of superparamagnetic iron oxide nanoparticles (30 nm-diameter magnetite, Fe(3)O(4)) under oxidative conditions. After magnetite incorporation, the porous microparticle is still empty enough to accommodate nanogram quantities of a molecular payload; the enzymes horseradish peroxidase or pronase E are used in the present work. The assembly can be transported to a microliter water droplet containing the enzyme substrate with the aid of an external magnetic field. The enzyme is released into the droplet upon contact. The particles can be transported through air or a hydrocarbon liquid without loss in enzymatic activity of the payload.

Porous silicon-based optical microsensor for the detection of l-glutamine

The molecular binding between the glutamine-binding protein (GlnBP) from Escherichia coli and L-glutamine (Gln) is optically transduced by means of a biosensor based on porous silicon nano-technology. The sensor operates by the measurement of the interferometric fringes in the reflectivity spectrum of a porous silicon Fabry-Perot layer. The binding event is revealed as a shift in wavelength of the fringes. Due to the hydrophobic interaction with the Si-H terminated surface of the porous silicon, the GlnBP protein, which acts as a molecular probe for Gln, penetrates and links into the pores of the porous silicon matrix. We can thus avoid any preliminary functionalization process of the porous layer surface, which is also prevented from oxidation, at least for few cycles of wet measurements. The binding of Gln to GlnBP has also been investigated at different concentration of GlnBP.

"Smart dust": nanostructured devices in a grain of sand

The term "smart dust" originally referred to miniature wireless semiconductor devices made using fabrication techniques derived from the microelectronics industry. These devices incorporate sensing, computing and communications in a centimetre-sized package. This article discusses the construction of much smaller silicon-based systems, using the tools of nanotechnology. The synthesis of millimetre- to micron-sized functional photonic crystals made from porous silicon is described. It is shown how the various optical, chemical, and mechanical properties can be harnessed to perform sensing, signal processing, communication and motive functions.

Hydrolysis of acetylcholinesterase inhibitors--organophosphorus acid anhydrolase enzyme immobilization on photoluminescent porous silicon platforms

We report on the immobilization of an OPAA enzyme on luminescent porous silicon devices, and on the utilization of this new platform to hydrolyze p-nitrophenyl-soman.

Syntheses of alkenylated carbohydrate derivatives toward the preparation of monolayers on silicon surfaces

This note describes the synthesis of different alkenylated carbohydrate derivatives suitable for direct attachment to hydrogen-terminated silicon surfaces. The derivatives were alkenylated at the C-1 position, while the remaining hydroxyl groups were protected. The development of such new carbohydrate-based sensing elements opens the access to new classes of biosensors.

Functionalized silicon membranes for selective bio-organism capture

Membranes with various pore size, length, morphology and density have been synthesized from diverse materials for size-exclusion-based separation. An example is the sterilization of intravenous lines by exclusion of bacteria and viruses using polyvinylidene fluoride membranes with 0.1-microm-diameter pores. Chemically specific filtration has recently been addressed for small molecules. Nevertheless, specific bio-organism immobilization and detection remains a great technical challenge in many biomedical applications, such as decontamination or analysis of air and liquids such as drinking water and body fluids. To achieve this goal, materials with controlled pore diameter, length and surface chemistry are required. In this letter, we present the first functionalized silicon membranes and demonstrate their ability to selectively capture simulated bio-organisms. These extremely versatile and rigid devices open the door to a new class of materials that are able to recognize the external fingerprints of bio-organisms-such as size and outer membrane proteins-for specific capture and detection applications.

Biomolecular screening with encoded porous-silicon photonic crystals

Strategies to encode or label small particles or beads for use in high-throughput screening and bioassay applications focus on either spatially differentiated, on-chip arrays or random distributions of encoded beads. Attempts to encode large numbers of polymeric, metallic or glass beads in random arrays or in fluid suspension have used a variety of entities to provide coded elements (bits)--fluorescent molecules, molecules with specific vibrational signatures, quantum dots, or discrete metallic layers. Here we report a method for optically encoding micrometre-sized nanostructured particles of porous silicon. We generate multilayered porous films in crystalline silicon using a periodic electrochemical etch. This results in photonic crystals with well-resolved and narrow optical reflectivity features, whose wavelengths are determined by the etching parameters. Millions of possible codes can be prepared this way. Micrometre-sized particles are then produced by ultrasonic fracture, mechanical grinding or by lithographic means. A simple antibody-based bioassay using fluorescently tagged proteins demonstrates the encoding strategy in biologically relevant media.

A streptavidin surface on planar glass substrates for the detection of biomolecular interaction

Based on the requirements of biomolecular interaction analysis on direct optical transducers, a streptavidin surface is examined. A general protocol was developed allowing the immobilization of biotinylated compounds using the rife biotin-streptavidin system. This type of surface modification can be applied to all biosensors using glass surfaces as sensor devices. Reflectometric interference spectroscopy (RIfS), a label-free, direct optical method was used to demonstrate the quality of the transducer surfaces. The surface modification is based on an aminofunctionalized polyethylene glycol layer covalently bound to the silica surface of the transducer and shows very little nonspecific binding. Biotin molecules can be easily coupled on such layers. Streptavidin followed by a biotinylated estrone derivative was immobilized by incubation of the biotinylated transducer surface. For the streptavidin layer we obtained interference signals corresponding to a protein monolayer. Finally, using a surface prepared as described above, biomolecular interaction experiments with an antibody against estrone were carried out to show the quality of the transducer surface. With RIfS all of the affinity-based surface modifications can be detected online and time resolved.

Affinity biosensor for avidin using a double functionalized dendrimer monolayer on a gold electrode

We have developed an affinity biosensor system based on avidin-biotin interaction on a gold electrode. As the building block of an affinity-sensing monolayer, a fourth-generation (G4) poly(amidoamine) dendrimer having partial ferrocenyl-tethered surface groups was prepared and used. The unmodified surface amine groups from dendrimers were functionalized with biotinamidocaproate, and the biotinylated and electroactive dendritic monolayer was constructed on a gold electrode for the affinity-sensing surface interacting with avidin. An electrochemical signal from the affinity biosensor was generated by free glucose oxidase in electrolyte, depending on the degree of coverage of the sensing surface with avidin. The sensor signal decreased correlatively with increasing avidin concentration and approached a minimum level when the sensing surface was fully covered with avidin. The detection limit of avidin was about 4.5 pM, and the sensor signal was linear ranging from 1.5 pM to 10 nM under optimized conditions. From the kinetic analysis using the biotinylated glucose oxidase, an active enzyme coverage of 2.5 x 10(-12) mol/cm(2) on the avidin-pretreated surface was registered, which demonstrates the formation of a spatially ordered and compact protein layer on the derivatized electrode surface.