Morphology and adhesion of biomolecules on silicon based surfaces

Sub-terahertz silicon-based on-chip absorption spectroscopy using thin-film model for biological applications

Spectroscopy in the sub-terahertz (sub-THz) range of frequencies has been utilized to study the picosecond dynamics and interaction of biomolecules. However, widely used free-space THz spectrometers are typically limited in their functionality due to low signal-to-noise ratio and complex setup. On-chip spectrometers can revolutionize THz spectroscopy allowing integration, compactness, and low-cost fabrication. In this paper, a low-loss silicon-based platform is proposed for on-chip sub-THz spectroscopy. Through functionalization of silicon chip and immobilization of bio-particles, we demonstrate the ability to characterize low-loss nano-scale biomolecules across the G-band (0.14-0.22 THz). We also introduce an electromagnetic thin-film model to account for the loading effect of the immobilized biomolecules, i.e. dehydrated streptavidin and immunoglobulin antibody, as two key molecules in the biosensing discipline. The proposed platform was fabricated using a single mask micro-fabrication process, and then measured by a vector network analyzer (VNA), which offers high dynamic range and high spectral resolution measurements. The proposed planar platform is general and paves the way towards low-loss, cost-effective and integrated sub-THz biosensors for the detection and characterization of biomolecules.

Frontiers in nanotribology: Magnetic storage, bio/nanotechnology, cosmetics, and bioinspiration

The word "nanotribology" was introduced for the first time in the title of a paper and a book in 1995. This field encompasses fundamental studies of surface characterization, adhesion, friction, scratching, wear, and lubrication at the atomic scale. At most solid-solid interfaces of technological relevance, contact occurs at numerous asperities. It is of importance to investigate a single asperity contact in the fundamental tribological studies. A nanoprobe sliding on a surface in probe-based microscopies, including atomic force microscopy (AFM) at ultralow loads, simulates one such contact. AFMs and depth-sensing nanoindentation techniques are also used for nanomechanical characterization. The field is referred to as nanomechanics. AFMs can also be used for nanoelectrical characterization which includes electrical resistance, surface potential, and capacitance mapping. Research in the field of nanotribology and nanomechanics was initiated by or for the magnetic storage industry in the late 1980s. Later in the mid-1990s, nanotribology and nanomechanics research became important in bio/nanotechnology devices which involve relative motion, as well as ultrathin films. Adhesion, friction and wear issues in bio/nanotechnology devices led to the development of the field of bio/nanotribology. Research in ultrathin films used in the cosmetic industry, including hair, hair conditioner, skin, and skin cream, led to development of the field of nanotribology in cosmetics. Biologically inspired design, adaptation, or derivation from nature, referred to as biomimetics or bioinspiration, can guide us to initiate and produce nanomaterials, nanodevices, and processes in a sustainable and environmentally friendly manner. So called, green nanotribology research is important in this field. This perspective article presents an overview of fundamental understanding of nanotribology and nanomechanics and their applications in various fields ranging from magnetic storage, bio/nanotechnology, hair and hair conditioner, skin and skin cream, and bioinspiration (green nanotribology).

Label-free biosensing with a multi-box sub-wavelength phase-shifted Bragg grating waveguide

Sub-wavelength grating (SWG) metamaterials have been considered to provide promising solutions in the development of next-generation photonic integrated circuits. In recent years, increasied interest has been paid to silicon photonic planar biosensors based on SWG geometries for performance enhancement. In this work, we demonstrate a highly sensitive label-free phase-shifted Bragg grating (PSBG) sensing configuration, which consists of sub-wavelength block arrays in both propagation and transverse directions. By introducing salt serial dilutions and electrostatic polymers assays, bulk and surface sensitivities of the proposed sensor are characterized, obtaining measured results up to 579.2 nm/RIU and 1914 pm/nm, respectively. Moreover, the proposed multi-box PSBG sensor presents an improved quality factor as high as ∼ 8000 , roughly 3-fold of the microring-based counterpart, which further improves the detection limit. At last, by employing a biotin-streptavidin affinity assay, the capability for small molecule monitoring is exemplified with a minimum detectable concentration of biotin down to 2.28 × 10 - 8 M .

Refractive index sensing in the visible/NIR spectrum using silicon nanopillar arrays

Si nanopillar (NP) arrays are investigated as refractive index sensors in the visible/NIR wavelength range, suitable for Si photodetector responsivity. The NP arrays are fabricated by nanoimprint lithography and dry etching, and coated with thin dielectric layers. The reflectivity peaks obtained by finite-difference time-domain (FDTD) simulations show a linear shift with coating layer thickness. At 730 nm wavelength, sensitivities of ~0.3 and ~0.9 nm/nm of SiO₂ and Si₃N₄, respectively, are obtained; and the optical thicknesses of the deposited surface coatings are determined by comparing the experimental and simulated data. The results show that NP arrays can be used for sensing surface bio-layers. The proposed method could be useful to determine the optical thickness of surface coatings, conformal and non-conformal, in NP-based optical devices.

Efficiency optimisation of proteins on a chip

This study elucidates that the protein reorientation on a chip can be changed by an external electric field (EEF) and optimised for achieving strong effective binding between proteins. Protein A and its binding protein immunoglobulin G (IgG) were used as an example, in addition to an anticancer peptide (CB1a) and its antibody (anti-CB1a). The binding forces (BFs) were measured by atomic force microscopy (AFM) with EEFs applied at different angles (EEF°). The optimal angle (OA) of the EEF (OAEEF°) corresponding to the maximum binding force (BFmax) was obtained. The results showed that the BFmax values between IgG/Protein A and anti-CB1a/CB1a were 6424.2 ± 195.3 pN (OAEEF° = 45°) and 729.1 ± 33.2 pN (OAEEF° = 22.5°), respectively. Without an EEF, the BF values were only 730.0 ± 113.9 pN and 337.3 ± 35.0 pN, respectively. Based on these observations, we concluded that the efficient optimisation of protein-protein interaction on a chip is essential. This finding is applicable to the industrial fabrication of all protein chips.

Bacteria repelling on highly-ordered alumina-nanopore structures

Bacteria introduce diseases and infections to humans by their adherence to biomaterials, such as implants and surgical tools. Cell desorption is an effective step to reduce such damage. Here, we report mechanisms of bacteria desorption. An alumina nanopore structure (ANS) with pore size of 35 nm, 55 nm, 70 nm, and 80 nm was used as substrate to grow Escherichia coli (E. coli) cells. A bacteria repelling experimental method was developed to quantitatively evaluate the area percentage of adherent bacterial cells that represent the nature of cell adhesion as well as desorption. Results showed that there were two crucial parameters: contact angle and contact area that affect the adhesion/desorption. The cells were found to be more easily repelled when the contact angle increased. The area percentage of adherent bacterial cells decreased with the decrease in the contact area of a cell on ANS. This means that cell accessibility on ANS depends on the contact area. This research reveals the effectiveness of the nanopored structures in repelling cells.

Elucidating molecular mass and shape of a neurotoxic Aβ oligomer

Alzheimer's disease (AD), the most prevalent type of dementia, has been associated with the accumulation of amyloid β oligomers (AβOs) in the central nervous system. AβOs vary widely in size, ranging from dimers to larger than 100 kDa. Evidence indicates that not all oligomers are toxic, and there is yet no consensus on the size of the actual toxic oligomer. Here we used NU4, a conformation-dependent anti-AβO monoclonal antibody, to investigate size and shape of a toxic AβO assembly. By using size-exclusion chromatography and immuno-based detection, we isolated an AβO-NU4 complex amenable for biochemical and morphological studies. The apparent molecular mass of the NU4-targeted oligomer was 80 kDa. Atomic force microscopy imaging of the AβO-NU4 complex showed a size distribution centered at 5.37 nm, an increment of 1.5 nm compared to the size of AβOs (3.85 nm). This increment was compatible with the size of NU4 (1.3 nm), suggesting a 1:1 oligomer to NU4 ratio. NU4-reactive oligomers extracted from AD human brain concentrated in a molecular mass range similar to that found for in vitro prepared oligomers, supporting the relevance of the species herein studied. These results represent an important step toward understanding the connection between AβO size and toxicity.

Electrical biomolecule detection using nanopatterned silicon via block copolymer lithography

An electrical biosensor exploiting a nanostructured semiconductor is a promising technology for the highly sensitive, label-free detection of biomolecules via a straightforward electronic signal. The facile and scalable production of a nanopatterned electrical silicon biosensor by block copolymer (BCP) nano-lithography is reported. A cost-effective and large-area nanofabrication, based on BCP self-assembly and single-step dry etching, is developed for the hexagonal nanohole patterning of thin silicon films. The resultant nanopatterned electrical channel modified with biotin molecules successfully detects the two proteins, streptavidin and avidin, down to nanoscale molarities (≈1 nm). The nanoscale pattern comparable to the Debye screening length and the large surface area of the three-dimensional silicon nanochannel enable excellent sensitivity and stability. A device simulation confirms that the nanopatterned structure used in this work is effective for biomolecule detection. This approach relying on the scalable self-assembly principle offers a high-throughput manufacturing process for clinical lab-on-a-chip diagnoses and relevant biomolecular studies.

Impact of polymer shell on the formation and time evolution of nanoparticle-protein corona

The study of protein corona formation on nanoparticles (NPs) represents an actual main issue in colloidal, biomedical and toxicological sciences. However, little is known about the influence of polymer shells on the formation and time evolution of protein corona onto functionalized NPs. Therefore, silica-poly(ethylene glycol) core-shell nanohybrids (SNPs@PEG) with different polymer molecular weights (MW) were synthesized and exhaustively characterized. Bovine serum albumin (BSA) at different concentrations (0.1-6 wt%) was used as model protein to study protein corona formation and time evolution. For pristine SNPs and SNPs@PEG (MW=350 g/mol), zeta potential at different incubation times show a dynamical evolution of the nanoparticle-protein corona. Oppositely, for SNPs@PEG with MW≥2000 g/mol a significant suppression of corona formation and time evolution was observed. Furthermore, AFM investigations suggest a different orientation (side-chain or perpendicular) and penetration depth of BSA toward PEGylated surfaces depending on the polymer length which may explain differences in protein corona evolution.

Designing nanostructured block copolymer surfaces to control protein adhesion

The profile and conformation of proteins that are adsorbed onto a polymeric biomaterial surface have a profound effect on its in vivo performance. Cells and tissue recognize the protein layer rather than directly interact with the surface. The chemistry and morphology of a polymer surface will govern the protein behaviour. So, by controlling the polymer surface, the biocompatibility can be regulated. Nanoscale surface features are known to affect the protein behaviour, and in this overview the nanostructure of self-assembled block copolymers will be harnessed to control protein behaviour. The nanostructure of a block copolymer can be controlled by manipulating the chemistry and arrangement of the blocks. Random, A-B and A-B-A block copolymers composed of methyl methacrylate copolymerized with either acrylic acid or 2-hydroxyethyl methacrylate will be explored. Using atomic force microscopy (AFM), the surface morphology of these block copolymers will be characterized. Further, AFM tips functionalized with proteins will measure the adhesion of that particular protein to polymer surfaces. In this manner, the influence of block copolymer morphology on protein adhesion can be measured. AFM tips functionalized with antibodies to fibronectin will determine how the surfaces will affect the conformation of fibronectin, an important parameter in evaluating surface biocompatibility.

Bioadhesion: a review of concepts and applications

Bioadhesion refers to the phenomenon where natural and synthetic materials adhere to biological surfaces. An understanding of the fundamental mechanisms that govern bioadhesion is of great interest for various researchers who aim to develop new biomaterials, therapies and technological applications such as biosensors. This review paper will first describe various examples of the manifestation of bioadhesion along with the underlying mechanisms. This will be followed by a discussion of some of the methods for the optimization of bioadhesion. Finally, nanoscale and macroscale characterization techniques for the efficacy of bioadhesion and the analysis of failure surfaces are described.

Theory, fabrication and applications of microfluidic and nanofluidic biosensors

Biosensors are a broad array of devices that detect the type and amount of a biological species or biomolecule. Several different types of biosensors have been developed that rely on changes to mechanical, chemical or electrical properties of the transduction or sensing element to induce a measurable signal. Often, a biosensor will integrate several functions or unit operations such as sample extraction, manipulation and detection on a single platform. This review begins with an overview of the current state-of-the-art biosensor field. Next, the review delves into a special class of biosensors that rely on microfluidics and nanofluidics by presenting the underlying theory, fabrication and several examples and applications of microfluidic and nanofluidic sensors.

Chip-based protein-protein interaction studied by atomic force microscopy

In this article, a technique for accurate direct measurement of protein-to-protein interactions before and after the introduction of a drug candidate is developed using atomic force microscopy (AFM). The method is applied to known immunosuppressant drug candidate Echinacea purpurea derived cynarin. T-cell/CD28 is on-chip immobilized and B-cell/CD80 is immobilized on an AFM tip. The difference in unbinding force between these two proteins before and after the introduction of cynarin is measured. The method is described in detail including determination of the loading rates, maximum probability of bindings, and average unbinding forces. At an AFM loading rate of 1.44 × 10(4) pN/s, binding events were largely reduced from 61 ± 5% to 47 ± 6% after cynarin introduction. Similarly, maximum probability of bindings reduced from 70% to 35% with a blocking effect of about 35% for a fixed contact time of 0.5 s or greater. Furthermore, average unbinding forces were reduced from 61.4 to 38.9 pN with a blocking effect of ≈ 37% as compared with ≈ 9% by SPR. AFM, which can provide accurate quantitative measures, is shown to be a good method for drug screening. The method could be applied to a wider variety of drug candidates with advances in bio-chip technology and a more comprehensive AFM database of protein-to-protein interactions.

Elastic properties of a protein-polymer-grafted surface

Surfaces grafted with poly(methyl methacrylate) (PMMA) and streptavidin were synthesized through click chemistry to investigate the role of surface stiffness on protein adsorption as the hydrophilic and hydrophobic surface coverage of the substituents vary. Surface topographies coupled with the nanoindentation results indicated that, with the appropriate selections of polymer coverage and chain length, the extent of non-specific protein adhesion could be controlled by the hydrophobic interactions between PMMA, biotin, and streptavidin. It was shown that, when the molecular weight and stiffness of PMMA was close to that of streptavidin, patchy PMMA morphologies were obtained, which help inhibit the non-specific adsorption of streptavidin.

Protein conformation changes on block copolymer surfaces detected by antibody-functionalized atomic force microscope tips

Conformational changes of fibronectin (Fn) deposited on poly(methyl methacrylate) and poly(acrylic acid) block copolymers with identical chemical compositions were detected using an antibody-functionalized atomic force microscope (AFM) tip. Based on the antibody-protein adhesive force maps and phase imaging, it was found that the nanomorphology of the triblock copolymer is conducive to the exposure of the arginine-glycine-aspartic acid (RGD) groups in Fn. For the first time, X-ray photoelectron spectroscopy was used to elucidate surface chemical composition and confirm AFM results. The findings demonstrate that block copolymer nanomorphology can be used to regulate protein conformation and potentially cellular response.

Transition mode long period grating biosensor with functional multilayer coatings

We report our latest research results concerning the development of a platform for label-free biosensing based on overlayered Long Period Gratings (LPGs) working in transition mode. The main novelty of this work lies in a multilayer design that allows to decouple the problem of an efficient surface functionalization from that of the tuning in transition region of the cladding modes. An innovative solvent/nonsolvent strategy for the dip-coating technique was developed in order to deposit on the LPG multiple layers of transparent polymers. In particular, a primary coating of atactic polystyrene was used as high refractive index layer to tune the working point of the device in the so-called transition region. In this way, state-of-the-art-competitive sensitivity to surrounding medium refractive index changes was achieved. An extremely thin secondary functional layer of poly(methyl methacrylate-co-methacrylic acid) was deposited onto the primary coating by means of an original identification of selective solvents. This approach allowed to obtain desired functional groups (carboxyls) on the surface of the device for a stable covalent attachment of bioreceptors and minimal perturbation of the optical design. Standard 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide / N-hydrosuccinimide (EDC / NHS) coupling chemistry was used to link streptavidin on the surface of the coated LPG. Highly sensitive real-time monitoring of multiple affinity assays between streptavidin and biotinylated bovine serum albumin was performed by following the shift of the LPGs attenuation bands.

Bioadhesion of various proteins on random, diblock and triblock copolymer surfaces and the effect of pH conditions

The adhesive interactions of block copolymers composed of poly(methyl methacrylate) (PMMA)/poly(acrylic acid) (PAA) and poly(methyl methacrylate)/poly(2-hydroxyethyl methacrylate) (PHEMA) with the proteins fibronectin, bovine serum albumin and collagen were studied by atomic force microscopy. Adhesion experiments were performed both at physiological pH and at a slightly more acidic condition (pH 6.2) to model polymer-protein interactions under inflammatory or infectious conditions. The PMMA/PAA block copolymers were found to be more sensitive to the buffer environment than PMMA/PHEMA owing to electrostatic interactions between the ionized acrylate groups and the proteins. It was found that random, diblock and triblock copolymers exhibit distinct adhesion profiles although their chemical compositions are identical. This implies that biomaterial nanomorphology can be used to control protein-polymer interactions and potentially cell adhesion.

On the adsorption behavior of biotin-binding proteins on gold and silica

Streptavidin (SAv), avidin (Av), and neutravidin (NAv) have become widely used molecular tools in biotechnology thanks to their remarkable affinity for biotin. Their tetravalency renders these molecules particularly interesting for the functionalization of solid-liquid interfaces. Using the quartz crystal microbalance with dissipation monitoring, we systematically investigate the deposition of biotin-binding proteins to two surfaces that are popular in biotechnology: gold and silica. We find that simple physisorption of biotin-binding proteins is a viable method to confer biotin-binding functionality to gold surfaces. Both SAv and Av form dense, stable protein monolayers that retain biotin-binding activity and are largely inert to the unspecific binding of bovine serum albumin. Furthermore, we report that SAv resists adsorption to silica over a wide range of pH and ionic strength. The contrast in the binding behavior of SAv on silica and on gold suggests a simple strategy for the selective biofunctionalization of nano- or microstructured surfaces.

Long period grating working in transition mode as promising technological platform for label-free biosensing

We present the development of a platform for label-free biosensing based on overlayered Long Period Gratings (LPGs) working in transition mode. Nano-scale layers of Polystyrene (PS) with different thicknesses were deposited onto the same LPG to test the performances of the device in different working points of its modified sensitivity characteristic. Adsorption dynamic of biotinylated bovine serum albumin (BBSA) onto the PS overlays was on-line monitored as well as a subsequent streptavidin (SA) binding dynamic on the biotinylated sites of the protein ad-layer. Experimental results show that overlayered LPGs are among the most sensitive refractive index transducers to be employed in label-free biochemical detection and that wide margins of further optimization exist.

Nanoscale adhesion, friction and wear studies of biomolecules on silane polymer-coated silica and alumina-based surfaces

Proteins on biomicroelectromechanical systems (BioMEMS) confer specific molecular functionalities. In planar FET sensors (field-effect transistors, a class of devices whose protein-sensing capabilities we demonstrated in physiological buffers), interfacial proteins are analyte receptors, determining sensor molecular recognition specificity. Receptors are bound to the FET through a polymeric interface, and gross disruption of interfaces that removes a large percentage of receptors or inactivates large fractions of them diminishes sensor sensitivity. Sensitivity is also determined by the distance between the bound analyte and the semiconductor. Consequently, differential properties of surface polymers are design parameters for FET sensors. We compare thickness, surface roughness, adhesion, friction and wear properties of silane polymer layers bound to oxides (SiO(2) and Al(2)O(3), as on AlGaN HFETs). We compare those properties of the film-substrate pairs after an additional deposition of biotin and streptavidin. Adhesion between protein and device and interfacial friction properties affect FET reliability because these parameters affect wear resistance of interfaces to abrasive insult in vivo. Adhesion/friction determines the extent of stickage between the interface and tissue and interfacial resistance to mechanical damage. We document systematic, consistent differences in thickness and wear resistance of silane films that can be correlated with film chemistry and deposition procedures, providing guidance for rational interfacial design for planar AlGaN HFET sensors.

Detection of clinically relevant levels of protein analyte under physiologic buffer using planar field effect transistors

Electrochemical detection of protein binding at physiological salt concentration by planar field effect transistor platforms has yet to be documented convincingly. Here we report detection of streptavidin and clinically relevant levels of biotinylated monokine induced by interferon gamma (MIG) at physiological salt concentrations with AlGaN heterojunction field effect transistors (HFETs). The AlGaN HFETs are functionalized with a silane linker and analyte-specific affinity elements. Polarity of sensor responses is as expected from n-type HFETs to negatively and positively charged analytes. Sensitivity of the HFET sensors increases when salt concentration decreases, and the devices also exhibit dose-dependent responses to analyte. Detection of clinically relevant MIG concentrations at physiological salt levels demonstrates the potential for AlGaN devices to be used in development of in vivo biosensors.

Nanotribology and nanomechanics in nano/biotechnology

Owing to larger surface area in micro/nanoelectromechanical systems (MEMS/NEMS), surface forces such as adhesion, friction, and meniscus and viscous drag forces become large when compared with inertial and electromagnetic forces. There is a need to develop lubricants and identify lubrication methods that are suitable for MEMS/NEMS. For BioMEMS/BioNEMS, adhesion between biological molecular layers and the substrate, and friction and wear of biological layers may be important, and methods to enhance adhesion between biomolecules and the device surface need to be developed. There is a need for development of a fundamental understanding of adhesion, friction/stiction, wear, the role of surface contamination and environment, and lubrication. MEMS/NEMS materials need to exhibit good mechanical and tribological properties on the micro/nanoscale. Most mechanical properties are known to be scale dependent. Therefore, the properties of nanoscale structures need to be measured. Component-level studies are required to provide a better understanding of the tribological phenomena occurring in MEMS/NEMS. The emergence of micro/nanotribology and atomic force microscopy-based techniques has provided researchers with a viable approach to address these problems. This paper presents an overview of micro/nanoscale adhesion, friction, and wear studies of materials and lubrication studies for MEMS/NEMS and BioMEMS/BioNEMS. It also presents a review of scale-dependent mechanical properties, and stress and deformation analysis of nanostructures.

XPS and AFM characterization of the enzyme glucose oxidase immobilized on SiO(2) surfaces

A process to immobilize the enzyme glucose oxidase on SiO2 surfaces for the realization of integrated microbiosensors was developed. The sample characterization was performed by monitoring, step by step, oxide activation, silanization, linker molecule (glutaraldehyde) deposition, and enzyme immobilization by means of XPS, AFM, and contact angle measurements. The control of the environment during the procedure, to prevent silane polymerization, and the use of oxide activation to obtain a uniform enzyme layer are issues of crucial importance. The correct protocol application gives a uniform layer of the linker molecule and the maximum sample surface coverage. This result is fundamental for maximizing the enzyme bonding sites on the sample surface and achieving the maximum surface coverage. Thin SiO2 layers thermally grown on a Si substrate were used. The XPS Si 2p signal of the substrate was monitored during immobilization. Such a signal is not completely shielded by the thin oxide layer and it is fully suppressed after the completion of the whole protocol. A power spectral density analysis on the AFM measurements showed the crucial role of both the oxide activation and the intermediate steps (silanization and linker molecule deposition) to obtain uniform immobilized enzyme coverage. Finally, enzymatic activity measurements confirmed the suitability of the optimized protocol.

Engineering functional protein interfaces for immunologically modified field effect transistor (ImmunoFET) by molecular genetic means

The attachment and interactions of analyte receptor biomolecules at solid-liquid interfaces are critical to development of hybrid biological-synthetic sensor devices across all size regimes. We use protein engineering approaches to engineer the sensing interface of biochemically modified field effect transistor sensors (BioFET). To date, we have deposited analyte receptor proteins on FET sensing channels by direct adsorption, used self-assembled monolayers to tether receptor proteins to planar FET SiO2 sensing gates and demonstrated interface biochemical function and electrical function of the corresponding sensors. We have also used phage display to identify short peptides that recognize thermally grown SiO2. Our interest in these peptides is as affinity domains that can be inserted as translational fusions into receptor proteins (antibody fragments or other molecules) to drive oriented interaction with FET sensing surfaces. We have also identified single-chain fragment variables (scFvs, antibody fragments) that recognize an analyte of interest as potential sensor receptors. In addition, we have developed a protein engineering technology (scanning circular permutagenesis) that allows us to alter protein topography to manipulate the position of functional domains of the protein relative to the BioFET sensing surface.

Physicochemically modified silicon as a substrate for protein microarrays

Reverse phase protein microarrays (RPMA) enable high throughput screening of posttranslational modifications of important signaling proteins within diseased cells. One limitation of protein-based molecular profiling is the lack of a PCR-like intrinsic amplification system for proteins. Enhancement of protein microarray sensitivities is an important goal, especially because many molecular targets within patient tissues are of low abundance. The ideal array substrate will have a high protein-binding affinity and low intrinsic signal. To date, nitrocellulose-coated glass has provided an effective substrate for protein binding in the microarray format when using chromogenic detection systems. As fluorescent systems, such as quantum dots, are explored as potential reporter agents, the intrinsic fluorescent properties of nitrocellulose-coated glass slides limit the ability to image microarrays for extended periods of time where increases in net sensitivity can be attained. Silicon, with low intrinsic autofluorescence, is being explored as a potential microarray surface. Native silicon has low binding potential. Through titrated reactive ion etching (RIE), varying surface areas have been created on silicon in order to enhance protein binding. Further, via chemical modification, reactive groups have been added to the surfaces for comparison of relative protein binding. Using this combinatorial method of surface roughening and surface coating, 3-aminopropyltriethoxysilane (APTES) and mercaptopropyltrimethoxysilane (MPTMS) treatments were shown to transform native silicon into a protein-binding substrate comparable to nitrocellulose.

Nanoscale adhesion, friction and wear studies of biomolecules on silicon based surfaces

Protein layers are deployed over the surfaces of microdevices such as biological microelectromechanical systems (bioMEMS) and bioimplants as functional layers that confer specific molecular recognition or binding properties or to facilitate biocompatibility with biological tissue. When a microdevice comes in contact with any exterior environment, like tissues and/or fluids with a variable pH, the biomolecules on its surface may get abraded. Silicon based bioMEMS are an important class of devices. Adhesion, friction and wear properties of biomolecules (e.g., proteins) on silicon based surfaces are therefore important. Adhesion was studied between streptavidin and a thermally grown silica substrate in a phosphate buffered saline (PBS) solution with various pH values as a function of the concentration of biomolecules in the solution. Friction and wear properties of streptavidin (protein) biomolecules coated on silica by direct physical adsorption and a chemical linker method were studied in PBS using the tapping mode atomic force microscopy at a range of free amplitude voltages. Fluorescence microscopy was used to study the detailed wear mechanism of the biomolecules. Based on this study, adhesion, friction and wear mechanisms of biomolecules on silicon based surfaces are discussed.