Interface analysis in biosensor design

Hybrid bilayer membranes as platforms for biomimicry and catalysis

Hybrid bilayer membrane (HBM) platforms represent an emerging nanoscale bio-inspired interface that has broad implications in energy catalysis and smart molecular devices. An HBM contains multiple modular components that include an underlying inorganic surface with a biological layer appended on top. The inorganic interface serves as a support with robust mechanical properties that can also be decorated with functional moieties, sensing units and catalytic active sites. The biological layer contains lipids and membrane-bound entities that facilitate or alter the activity and selectivity of the embedded functional motifs. With their structural complexity and functional flexibility, HBMs have been demonstrated to enhance catalytic turnover frequency and regulate product selectivity of the O₂ and CO₂ reduction reactions, which have applications in fuel cells and electrolysers. HBMs can also steer the mechanistic pathways of proton-coupled electron transfer (PCET) reactions of quinones and metal complexes by tuning electron and proton delivery rates. Beyond energy catalysis, HBMs have been equipped with enzyme mimics and membrane-bound redox agents to recapitulate natural energy transport chains. With channels and carriers incorporated, HBM sensors can quantify transmembrane events. This Review serves to summarize the major accomplishments achieved using HBMs in the past decade.

Recent Progress on Semiconductor-Interface Facing Clinical Biosensing

Semiconductor (SC)-based field-effect transistors (FETs) have been demonstrated as amazing enhancer gadgets due to their delicate interface towards surface adsorption. This leads to their application as sensors and biosensors. Additionally, the semiconductor material has enormous recognizable fixation extends, high affectability, high consistency for solid detecting, and the ability to coordinate with other microfluidic gatherings. This review focused on current progress on the semiconductor-interfaced FET biosensor through the fundamental interface structure of sensor design, including inorganic semiconductor/aqueous interface, photoelectrochemical interface, nano-optical interface, and metal-assisted interface. The works that also point to a further advancement for the trademark properties mentioned have been reviewed here. The emergence of research on the organic semiconductor interface, integrated biosensors with Complementary metal–oxide–semiconductor (CMOS)-compatible, metal-organic frameworks, has accelerated the practical application of biosensors. Through a solid request for research along with sensor application, it will have the option to move forward the innovative sensor with the extraordinary semiconductor interface structure.

Inorganic iron-sulfur clusters enhance electron transport when used for wiring the NAD-glucose dehydrogenase based redox system

Wiring the active site of an enzyme directly to an electrode is the key to ensuring efficient electron transfer for the proper performance of enzyme-based bioelectronic systems. Iron-sulfur complexes, the first link between proteins and mediating molecules in the biological electron transport chain(s), possess an intrinsic electron transport capability. The authors demonstrate the application of inorganic iron-sulfur clusters (Fe-S) viz. FeS, FeS₂, Fe₂S_3, and Fe₃S₄, as molecular wires to mediate electron transport between a glucose-selective redox enzyme and the gold electrode. It is shown that Fe-S can emulate the functionality of the natural electron transport chain. Voltammetric studies indicate a significant improvement in electron transport, surface coverage, and resilience achieved by the Fe-S-based glucose anodes when compared to a conventional pyrroloquinoline quinone (PQQ)-based electrode. The Fe-S-based glucose anodes showed glucose oxidation at a potential of +0.5 V vs. Ag/AgCl with Tris-HCl buffer (pH 8) acting as a carrier. The current densities positively correlated with the concentrations of glucose in the range 0.1–100 mM displaying detection limits of 0.77 mM (FeS), 1.22 mM (FeS₂), 2.95 mM (Fe₂S₃)_, and 14.57 mM (Fe₃S₄). The metal-anchorable sulfur atom, the strong π-coordinating iron atom, the favorable redox properties, low cost, and natural abundance make Fe-S an excellent electron-mediating relay capable of wiring redox active sites to electrode surfaces.

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Aryl Diazonium Chemistry for the Surface Functionalization of Glassy Biosensors

Nanostring resonator and fiber-optics-based biosensors are of interest as they offer high sensitivity, real-time measurements and the ability to integrate with electronics. However, these devices are somewhat impaired by issues related to surface modification. Both nanostring resonators and photonic sensors employ glassy materials, which are incompatible with electrochemistry. A surface chemistry approach providing strong and stable adhesion to glassy surfaces is thus required. In this work, a diazonium salt induced aryl film grafting process is employed to modify a novel SiCN glassy material. Sandwich rabbit IgG binding assays are performed on the diazonium treated SiCN surfaces. Fluorescently labelled anti-rabbit IgG and anti-rabbit IgG conjugated gold nanoparticles were used as markers to demonstrate the absorption of anti-rabbit IgG and therefore verify the successful grafting of the aryl film. The results of the experiments support the effectiveness of diazonium chemistry for the surface functionalization of SiCN surfaces. This method is applicable to other types of glassy materials and potentially can be expanded to various nanomechanical and optical biosensors.

Diazonium Chemistry for the Bio-Functionalization of Glassy Nanostring Resonator Arrays

Resonant glassy nanostrings have been employed for the detection of biomolecules. These devices offer high sensitivity and amenability to large array integration and multiplexed assays. Such a concept has however been impaired by the lack of stable and biocompatible linker chemistries. Diazonium salt reduction-induced aryl grafting is an aqueous-based process providing strong chemical adhesion. In this work, diazonium-based linker chemistry was performed for the first time on glassy nanostrings, which enabled the bio-functionalization of such devices. Large arrays of nanostrings with ultra-narrow widths down to 10 nm were fabricated employing electron beam lithography. Diazonium modification was first developed on SiCN surfaces and validated by X-ray photoelectron spectroscopy. Similarly modified nanostrings were then covalently functionalized with anti-rabbit IgG as a molecular probe. Specific enumeration of rabbit IgG was successfully performed through observation of downshifts of resonant frequencies. The specificity of this enumeration was confirmed through proper negative control experiments. Helium ion microscopy further verified the successful functionalization of nanostrings.

A methodological combined framework for roadmapping biosensor research: a fault tree analysis approach within a strategic technology evaluation frame

Biosensor technology began in the 1960s to revolutionize instrumentation and measurement. Despite the glucose sensor market success that revolutionized medical diagnostics, and artificial pancreas promise currently the approval stage, the industry is reluctant to capitalize on other relevant university-produced knowledge and innovation. On the other hand, the scientific literature is extensive and persisting, while the number of university-hosted biosensor groups is growing. Considering the limited marketability of biosensors compared to the available research output, the biosensor field has been used by the present authors as a suitable paradigm for developing a methodological combined framework for "roadmapping" university research output in this discipline. This framework adopts the basic principles of the Analytic Hierarchy Process (AHP), replacing the lower level of technology alternatives with internal barriers (drawbacks, limitations, disadvantages), modeled through fault tree analysis (FTA) relying on fuzzy reasoning to count for uncertainty. The proposed methodology is validated retrospectively using ion selective field effect transistor (ISFET) - based biosensors as a case example, and then implemented prospectively membrane biosensors, putting an emphasis on the manufacturability issues. The analysis performed the trajectory of membrane platforms differently than the available market roadmaps that, considering the vast industrial experience in tailoring and handling crystallic forms, suggest the technology path of biomimetic and synthetic materials. The results presented herein indicate that future trajectories lie along with nanotechnology, and especially nanofabrication and nano-bioinformatics, and focused, more on the science-path, that is, on controlling the natural process of self-assembly and the thermodynamics of bioelement-lipid interaction. This retained the nature-derived sensitivity of the biosensor platform, pointing out the differences between the scope of academic research and the market viewpoint.

Bio-organic materials in the atmosphere and snow: measurement and characterization

Bio-organic chemicals are ubiquitous in the Earth's atmosphere and at air-snow interfaces, as well as in aerosols and in clouds. It has been known for centuries that airborne biological matter plays various roles in the transmission of disease in humans and in ecosystems. The implication of chemical compounds of biological origins in cloud condensation and in ice nucleation processes has also been studied during the last few decades, and implications have been suggested in the reduction of visibility, in the influence on oxidative potential of the atmosphere and transformation of compounds in the atmosphere, in the formation of haze, change of snow-ice albedo, in agricultural processes, and bio-hazards and bio-terrorism. In this review we critically examine existing observation data on bio-organic compounds in the atmosphere and in snow. We also review both conventional and cutting-edge analytical techniques and methods for measurement and characterisation of bio-organic compounds and specifically for microbial communities, in the atmosphere and snow. We also explore the link between biological compounds and nucleation processes. Due to increased interest in decreasing emissions of carbon-containing compounds, we also briefly review (in an Appendix) methods and techniques that are currently deployed for bio-organic remediation.

An electrode array for electrochemical immuno-sensing using the example of impedimetric tenascin C detection

Electrochemical biosensors allow simple, fast and sensitive analyte detection for various analytical problems. Especially immunosensors are favourable due to specificity and affinity of antigen recognition by the associated antibody. We present a novel electrode array qualified for parallel analysis and increased sample throughput. The chip has nine independent sample chambers. Each chamber contains a circular gold working electrode with a diameter of 1.9 mm that is surrounded by a ring-shaped auxiliary electrode with a platinum surface. The corresponding silver/silver chloride reference electrodes are embedded in a sealing lid. The chip is open to the full range of electrochemical real-time detection methods. Among these techniques, impedance spectroscopy is an attractive tool to detect fast and label-free interfacial changes originating from the biorecognition event at the electrode surface. The capabilities of the novel electrode array are demonstrated using the example of tumour marker tenascin C detection. This glycoprotein of the extracellular matrix is expressed in cancerous tissues, especially in solid tumours such as glioma or breast carcinoma. Electrodes covered with specific antibodies were exposed to tenascin C containing samples. Non-occupied binding sites were identified using a secondary peroxidase-conjugated antibody that generated an insoluble precipitate on the electrode in a subsequent amplification procedure. The charge transfer resistance obtained from impedimetric analysis of ferri-/ferrocyanide conversion at the electrode served as analytic parameter. This assay detected 14 ng (48 fmol) tenascin C that is sufficient for clinical diagnostics. The electrode surface could be regenerated at least 20-fold without loss of its analytical performance.

Probing fundamental film parameters of immobilized enzymes--towards enhanced biosensor performance. Part I--QCM-D mass and rheological measurements

Enzyme immobilization is an ever-growing research-area for both analytical and industrial applications. Of critical importance in this area are the effects of immobilization procedures upon the functionality of the immobilized biomolecules. Both beneficial and detrimental effects can be conferred through the selection and tuning of the immobilization procedure. Quartz-crystal microbalance with dissipation (QCM-D) has been previously used to great effect in tracking alterations to thin films of biomolecules immobilized onto quartz transducers. In this study, we investigate the ability of QCM-D to track and monitor film parameters of a monolayer of laccase immobilized on a series of self-assembled monolayers (SAMs), differing in lateral density of binding residues on the SAM and height of the SAM from the quartz surface. Both mass gains and rheological parameters for these varying surfaces were measured and trends later compared to the apparent enzyme kinetics of the immobilized laccase films, assessed electroanalytically (Paper II in this two part study). For covalent attachment of proteins, both shear and viscosity were increased relative to physically adsorbed proteins. An increase in lateral density of protein-binding surface of the SAM components was shown to increase the shear/viscosity of the resultant film while an increase in distance from the electrode (through incorporation of lysine linkers) was shown to decrease the shear/viscosity while simultaneously increasing the wet mass gain of the films. Shear and viscosity may be indicative of both enzyme denaturation and increased lateral protein packing within the film structure hence it is assumed that less distortion occurs with the inclusion of linkers which allow for more optimal protein immobilization.

Biosensing technologies for Mycobacterium tuberculosis detection: status and new developments

Biosensing technologies promise to improve Mycobacterium tuberculosis (M. tuberculosis) detection and management in clinical diagnosis, food analysis, bioprocess, and environmental monitoring. A variety of portable, rapid, and sensitive biosensors with immediate "on-the-spot" interpretation have been developed for M. tuberculosis detection based on different biological elements recognition systems and basic signal transducer principles. Here, we present a synopsis of current developments of biosensing technologies for M. tuberculosis detection, which are classified on the basis of basic signal transducer principles, including piezoelectric quartz crystal biosensors, electrochemical biosensors, and magnetoelastic biosensors. Special attention is paid to the methods for improving the framework and analytical parameters of the biosensors, including sensitivity and analysis time as well as automation of analysis procedures. Challenges and perspectives of biosensing technologies development for M. tuberculosis detection are also discussed in the final part of this paper.

Reactive surface coatings based on polysilsesquioxanes: controlled functionalization for specific protein immobilization

The key designing in reliable biosensors is the preparation of thin films in which biomolecular functions may be immobilized and addressed in a controlled and reproducible manner. This requires the controlled preparation of specific binding sites on planar surfaces. Poly(methylsilsesquioxane)-poly(pentafluorophenyl acrylates) (PMSSQ-PFPA) are promising materials to produce stable and adherent thin reactive coatings on various substrates. Those reactive surface coatings could be applied onto various materials, for example, gold, polycarbonate (PC), poly(tetrafluoroethylene) (PTFE), and glass. By dipping those substrates in a solution of a desired amine, specific binding sites for protein adsorption could be immobilized on the surface. The versatile strategy allowed the attachment of various linkers, for example, biotin, l-thyroxine, and folic acid. The adsorption processes of streptavidin, pre-albumin, and folate-binding protein were monitored using surface plasmon resonance (SPR), Fourier transform infrared (FTIR) spectroscopy, fluorescence spectroscopy, and atomic force microscopy (AFM). The presented protein immobilization strategy, consisting of four steps (a) spin-coating of PMSSQ-PFPA hybrid polymer from tetrahydrofuran (THF) solution, (b) annealing at 130 degrees C for 2 h to induce thermal cross-linking of the PMSSQ part, (c) surface analogues reaction with different amino-functionalized specific binding sites for proteins, and (d) controlled assembly of proteins on the surface, may find various applications in future biosensor design.

Fundamental studies of the cytochrome c immobilization by the potential cycling method on nanometer-scale nickel oxide surfaces

This work describes the performance of cytochrome c/nickel oxide nanoparticles/glassy carbon electrode, prepared by the electrochemical deposition of the nickel oxide nanoparticles (NiO NPs) on the glassy carbon (GC) electrode surface and the cytochrome c immobilization on the nickel oxide nanoparticle surfaces. An extensive sample examination with the help of the SEM and AFM presented the existence of different geometrical shapes of the nickel oxide particles. These geometrical structures could lead to the better immobilization of proteins on their surfaces. The resulting electrode displayed an excellent behavior for the redox of the cytochrome c. Also, the resulting heme protein exhibited a direct electrical contact with the electrode because of the structural alignment of the heme protein on the nanometer-scale nickel oxide surfaces. This method could be suitable for applications to nanofabricated devices. In the end, it was concluded that the cytochrome c could be tethered to the nanometer-scale nickel oxide surfaces.

Comparison of random and oriented immobilisation of antibody fragments on mixed self-assembled monolayers

The sensitivity of immunosensors is strongly dependent on the amount of immobilised antibodies and their remaining antigen binding properties. The use of smaller and well-oriented antibody fragments as bioreceptor molecules influences the final immunosensor signal. The aim of this study was to compare the immunosensor responses of different immobilised antibody fragments, such as F(ab')2 and Fab', with their parental IgG. In addition, we evaluated the oriented versus the random covalent immobilisation method of the Fab' fragments. First, an optimisation of cleavage protocol to generate these F(ab')2 and Fab' fragments was performed. Subsequently, we pursued a study with limited denaturation effects during immobilisation of the bioreceptor molecules and with reduced steric hindrance during antigen binding using mixed self-assembled monolayers (SAM) of thiols as the chemical linking layer. The Surface Plasmon Resonance technique was used to evaluate the degree of immobilisation of the antibody fragments and their parental IgGs on the mixed SAMs and the binding signals of their specific antigens. In this study, we demonstrate that for a particular antibody/antigen system (anti-hIgG/hIgG), the optimised fragmentation protocol in combination with an oriented immobilisation of Fab' fragments on mixed SAMs leads to a >2-fold increase of the antigen binding signals compared to randomly covalent immobilised full-length antibodies.

Atomic force microscopy study of Escherichia coli lactose permease proteolipid sheets

Proteolipid sheets (PLSs) obtained using the vesicle fusion technique on a convenient surface are the base to obtain transmembrane protein biosensors. In this preliminary work, we have screened several physicochemical conditions to optimize the visualization of proteolipid sheets formed between different phospholipid matrices and the membrane protein lactose permease (LacP) by atomic force microscopy (AFM). When LacP was reconstituted in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes, the proteolipid sheets were densely packed with an upper layer that protruded from a background layer. Several lipid protein molar ratios (LPR) were screened. High resolution analysis of the upper layer revealed a quasi-crystalline arrangement formed by small entities that could be attributed to the protein. The approach described here may be suitable for the rational design of biosensors based in other transmembrane proteins.

Wetting films of lipids in the development of sensitive interfaces. An electrochemical approach

Solid-supported thin liquid films of lipids, contacting an electrolyte phase, turned out to be a dependable system in the field of bosensors. The investigations of these objects during the past decade reveal some of their intriguing features enabling the application in constructions of receptor part with 'two-dimensional' arrangement. As a model system, complementary to the other artificial analogs of biomembranes, the wetting films of lipids offer certain advantages concerning the compromise between the stability and flexibility of the molecular structures involved in sensing. The basic principles underlaying the techniques of formation, as well as the conditions of the films stability are emphasized in the present consized review. While no limitations are imposed by the way of preparation, the films seem especially profitable in conjunction with methods for electrochemical signal transduction. In this regard some prominent examples are discussed.

A single-channel sensor based on gramicidin controlled by molecular recognition at bilayer lipid membranes containing receptor

A novel ion-channel sensor based on a membrane bound receptor and a single gramicidin channel is described, in which the binding of an analyte to the membrane bound receptor modulates the single-channel activity of gramicidin. The sensor is composed of a planar bilayer lipid membrane (BLM) containing biotin-labeled phosphatidylethanolamine as receptor for avidin and gramicidin as signal transducer. When the receptor catches an analyte (avidin or ferritin-labeled avidin (FA)) at the membrane surface, the bilayer structure is locally distorted and the gramicidin monomer/dimer kinetics is modulated in a manner that the fraction of channel opening with a short lifetime ( < or = 100 ms) to the total opening events increases. The fraction was found to increase with the concentration of avidin from 1.0 x 10(-9) to 1.0 x 10(-6) M and of FA from 1.0 x 10(-9) to 1.0 x 10(-8) M. With dinitrophenyl-labeled PE embedded as receptor in the BLM for monoclonal anti-dinitrophenyl antibody (anti-DNP), the fraction of channel openings ( < or = 100 ms) increased with the concentration of anti-DNP from 2.0 x 10(-9) to 2.0 x 10(-7) g/ml. Bovine serum albumin (BSA) and anti-BSA antibody caused no changes in the channel opening. The possible mechanism of analyte-induced modulation of single-channel activity of gramicidin is also discussed.

Improved procedures for immobilisation of oligonucleotides on gold-coated piezoelectric quartz crystals

The high sensitivity and specificity of DNA hybridisation techniques makes them powerful tools for environmental or clinical analysis. This work describes the development of a DNA piezoelectric biosensor for the detection of the hybridisation reaction. Attention was focused on the choice of the coating chemistry that could be used for the immobilisation of oligonucleotides onto the gold surface of the quartz crystal. Four immobilisation procedures were tested and compared considering the amount of immobilised probe, the extent of the hybridisation reaction, the possibility of regeneration and the absence of non-specific adsorption. All the experiments were performed with oligonucleotides of 25 bases (probe, target and non-complementary oligonucleotide). The four coating methods were all based on the use of self-assembled monolayers (SAM). Three of them employed the interaction between streptavidin and biotin for the immobilisation of a biotinylated probe. Results indicated that immobilisation of a biotinylated probe on streptavidin linked to a layer of carboxylated dextran provides higher sensitivity for the detection of the hybridisation reaction, absence of non-specific adsorption and a higher stability with respect to the regeneration step.

Identifying bacteria in human urine: current practice and the potential for rapid, near-patient diagnosis by sensing volatile organic compounds

Urinary tract infection (UTI) represents a significant burden for the National Health Service. Extensive research has been directed towards rapid detection of UTI in the last thirty years. A wide range of microbiological and chemical techniques are now available to identify and quantify bacteria in urine. However, there is a clear and present need for near, rapid, sensitive, reliable analytical methods, preferably with low-running costs, that could allow early detection of UTI and other diseases in urine. Here we review the "state of the art" of current practice for the detection of bacteria in urine and describe the advantages of the recent "e-nose" technology as a potential tool for rapid, near-patient diagnosis of UTI, by sensing volatile organic compounds (VOCs).

Biomaterials integrated with electronic elements: en route to bioelectronics

Bioelectronics is a progressing interdisciplinary research field that involves the integration of biomaterials with electronic transducers, such as electrodes, field-effect-transistors or piezoelectric crystals. Surface engineering of biomaterials, such as enzymes, antigen-antibodies or DNA on the electronic supports, controls the electrical properties of the biomaterial-transducer interface and enables the electronic transduction of biorecognition events, or biocatalyzed transformation, on the transducers. Bioelectronic sensing devices, biosensors of tailored sensitivities and specificities, are being developed.

Effects of nonionic surfactants on electrochemical behavior of ubiquinone and menaquinone incorporated in a carbon paste electrode

A carbon paste electrode, in which the carbon particles were coated with a thin layer of a nonionic surfactant (NIS), was constructed with a pasting liquid containing ubiquinone (UQ) or menaquinone (MQ). It has revealed that the layer acts not only as a diffusion barrier but also as a matrix for the redox reaction of quinones at electrode surface, and its effects on the electrochemical behavior of quinones depend on both the physico-chemical structure of a surfactant and the kind of quinones. Further, such a modification was applied to the preparation of an enzyme electrode in which the quinone molecule act as a redox mediator and the influences on the sensitivity of the glucose biosensor was demonstrated.

Micropreparative ligand fishing with a cuvette-based optical mirror resonance biosensor

We have previously demonstrated the role of an optical biosensor (BIAcore 2000) as a specific detector to monitor chromatographic fractions during the purification and characterisation of ligands for orphan biomolecules. We have now extended this application to perform micropreparative ligand fishing directly on the sensor surface using an automated cuvette-based optical biosensor (Iasys Auto+) equipped with a high-capacity carboxymethyldextran surface (surface area 16 mm2). Using a F(ab)2' fragment of the A33 monoclonal antibody as bait, we have recovered microgram quantities of essentially homogeneous A33 ligand from the sensor surface in a form suitable for subsequent sensitive and specific down stream analysis (micropreparative HPLC, sodium dodecyl sulphate-polyacrylamide gel electrophoresis and Western blotting). The design of the cuvette-based system facilitates recovery of desorbed material from the constrained workspace in small volumes at high concentration. The use of on-surface detection allows the surface viability to be continuously monitored and permits direct quantitation of both bound and recovered material.

Instrumental biosensors: new perspectives for the analysis of biomolecular interactions

The use of instrumental biosensors in basic research to measure biomolecular interactions in real time is increasing exponentially. Applications include protein-protein, protein-peptide, DNA-protein, DNA-DNA, and lipid-protein interactions. Such techniques have been applied to, for example, antibody-antigen, receptor-ligand, signal transduction, and nuclear receptor studies. This review outlines the principles of two of the most commonly used instruments and highlights specific operating parameters that will assist in optimising experimental design, data generation, and analysis.

Modification of glassy carbon surfaces with synthetic laminin-derived peptides for nerve cell attachment and neurite growth

Interactions between cultured nerve cells and surfaces are of importance for the implantation of biocompatible electrode materials such as glassy carbon (GC). Since implants serve as recording sensors in prosthetic neuroscience, we investigated whether coating electrodes with certain laminin derivatives containing the peptide sequences SIKVAV, CDPGYIGSR, PDSGR, YFQRYLI, and RNIAEIIKDA influences neuronal adhesion and neurite outgrowth in vitro. The coating of GC was performed by electrochemical polymerization and, for comparison, by adsorption or covalent coupling. Electrochemical polymerization is suitable for the coupling of peptides to GC, as shown by amino acid analysis and sequencing. Embryonic chicken retinal ganglion cells and brain cells (days E7 or E17) were used for both attachment and growth studies. Surfaces made by electrochemical polymerization of peptides were more efficient than those made by adsorption or covalent coupling of peptides. Synthetic cyclic peptide derivatives of CDPGYIGSR and 18-mer SIKVAV were found to be more efficient than the linear peptides. Competitive effects that resulted in a decreased cell attachment could be found upon application of soluble peptides. Nevertheless, irrespective of the method of coating, peptides were less efficient compared with the whole laminin molecule, as expected from its multiple adhesion sites. When small GC pins were implanted into the brain of E17 chicken after coating with the 18-mer SIKVAV peptide, nerve cell attachment was observed in vivo. The results suggest that chronically implantable materials may exert a higher neurocompatibility when coated with synthetic peptides.

Bioelectronic noses: a status report. Part II

The present state of the art to record or to mimic electronically the human senses of olfaction and taste is characterized. In this part II, strategies are outlined to utilize chemical and biological structures with their different complexities which serve as sensor elements in (bio-) electronic noses. Finally a survey is given on the computer-science aspects of odor recognition based on these elements.

Surface modifications for the development of piezoimmunosensors

Four different techniques for the immobilisation of proteins onto the gold electrode of a piezoelectric quartz crystal were investigated. The examined techniques were adsorption, avidin-biotin binding and two different types of covalent binding on self-assembled monolayers (SAM), dithiobis(succinimidylpropionate) (DSP) and a dextran modified thiol monolayer. The reaction of the immobilised proteins (bovine serum albumin (BSA) and anti-human IgG) with their specific antibodies, anti-BSA and hIgG (50 and 200 micrograms/ml) were studied using a quartz crystal microbalance and then compared. Many cycles of measurements were performed on the same crystal regenerating the gold surface with a solution of glycine.HCl, 100 mM, pH 2.1. The interactions of the immobilised reagents with non-specific antibodies were also studied. The adsorption protocol was the quickest, but did not allow regeneration with glycine.HCl. Thiol-dextran coated surfaces did not show any detectable response to non-specific reagents, but needed a very long and complicated protocol. DSP and avidin-biotin coating procedures were easy and not too long. They seemed to have the best characteristics of reproducibility among different crystals and possibility of regeneration of the coated surface, but the percentage of non-specific binding was high.

Bioelectronic noses: a status report. Part I

The present state of the art to record or mimic electronically the human senses of olfaction and taste is characterized. In this part I, an introduction to our present understanding in the development of electronic and bioelectronic noses is given. Finally the natural olfactory system is described in detail.

Quartz crystal microbalances for quantitative biosensing and characterizing protein multilayers

The use of quartz crystal microbalances (QCMs) for quantitative biosensing and characterization of protein multilayers is demonstrated in three case studies. Monolayers of QCM-based affinity biosensors were investigated first. Layers of a thiol-containing synthetic peptide constituting an epitope of the foot-and-mouse-disease virus were formed on gold electrodes via self-assembly. The binding of specific antibodies to epitope-modified gold electrodes was detected for different concentrations of antibody solutions. Oligolayers were studied in a second set of experiments. Dextran hydrogels were modified by thrombin inhibitors. The QCM response was used in a competitive binding assay to identify inhibitors for thrombin at different concentrations. Multilayers of proteins formed by self-assembly of a biotin-conjugate and streptavidin were investigated next. The QCM frequency response was monitored as a function of layer thickness up to 20 protein layers. A linear frequency decay was observed with increasing thickness. The decay per layer remained constant, thus indicating perfect mass coupling to the substrate. Frequency changes a factor of four higher were obtained in buffer solution as compared to measurements in dry air. This indicates a significant incorporation of water (75% weight) in the protein layers. This water behaves like a solid concerning the shear mode coupling to the substrate. The outlook discusses briefly the need for controlled molecular engineering of overlayers for subsequent QCM analysis, and the importance of an additional multiparameter analysis with other transducer principles and with additional techniques of interface analysis to characterize the mechanical coupling of overlayers as biosensor coatings. A promising trend concerns the use of QCM-arrays for screening experiments.