Highly sensitive optical chip immunoassays in human serum

Energy Transfer-Based Recognition of Membrane Cholesterol by Controlling Intradistance of Linker

Gold nanoparticles (AuNPs) are good candidates for donor material in energy transfer systems and can easily be functionalized with various ligands on the surface with Au-S bonding. Cyclodextrin (CD) forms inclusion complexes with fluorophores due to its unique structure for host-guest interaction. In this study, we fabricated βCD-functionalized AuNPs using different lengths of thiol ligands and recognized cholesterol to confirm the energy-transfer-based turn-on fluorescence mechanism. AuNP-βCD conjugated with various thiol ligands and quenched the fluorescein (Fl) dye, forming βCD-Fl inclusion complexes. As the distance between AuNPs and βCD decreased, the quenching efficiency became higher. The quenched fluorescence was recovered when the cholesterol replaced the Fl because of the stronger binding affinity of the cholesterol with βCD. The efficiency of cholesterol recognition was also affected by the energy transfer effect because the shorter βCD ligand had a higher fluorescence recovery. Furthermore, we fabricated a liposome with cholesterol embedded in the lipid bilayer membrane to mimic the cholesterol coexisting with lipids in human serum. These cellular cholesterols accelerated the replacement of the Fl molecules, resulting in a fluorescence recovery higher than that of pure lipid. These discoveries are expected to give guidance towards cholesterol sensors or energy-transfer-based biosensors using AuNPs.

Optical biosensors: an exhaustive and comprehensive review

Optical biosensors have exhibited worthwhile performance in detecting biological systems and promoting significant advances in clinical diagnostics, drug discovery, food process control, and environmental monitoring. Without complexity in their pretreatment and probable influence on the nature of target molecules, these biosensors have additional advantages such as high sensitivity, robustness, reliability, and potential to be integrated on a single chip. In this review, the state of the art optical biosensor technologies, including those based on surface plasmon resonance (SPR), optical waveguides, optical resonators, photonic crystals, and optical fibers, are presented. The principles for each type of biosensor are concisely introduced and particular emphasis has been placed on recent achievements. The strengths and weaknesses of each type of biosensor have been outlined as well. Concluding remarks regarding the perspectives of future developments are discussed.

On the reasons for α-lactalbumin adsorption on a charged surface: a study by Monte Carlo simulation

This work studies α-lactalbumin adsorption on a charged substrate using Monte Carlo simulation. The protein is represented by a coarse-grained model with enough components as to reproduce the complex behavior of α-lactalbumin on electrically-charged substrates. The simulation results in particular can reproduce protein adsorption when both the protein and the substrate are negatively charged. The energetic and entropic contributions to the free energy of the adsorption process are estimated and analyzed. The effects of the charge regulation mechanism, the localization of titratable groups in α-lactalbumin as well as the distribution of small ions around the interface are studied in detail. Both the asymmetrical distribution of the charged groups of the protein and the counterion distribution play predominant roles in α-lactalbumin adsorption on a substrate with the same sign of electrical charge.

Poly(l-glutamic acid)-g-poly(ethylene glycol) external layer in polyelectrolyte multilayer films: Characterization and resistance to serum protein adsorption

Formation of protein-resistant surfaces is a major challenge in the design of novel biomaterials and an important strategy to prevent protein adsorption is the formation of protein-resistant coatings. It can be achieved by proper modification of surfaces, e.g., by immobilization of hydrophilic polymers such as poly(ethylene glycol) (PEG). An appropriate method to immobilize PEG at charged surfaces is the adsorption of copolymers with PEG chains grafted onto polyelectrolyte backbone. The growing interest in the use of polyelectrolyte multilayer coatings in biomedical applications to improve biocompatibility and/or to prepare coating with antiadhesive properties has been the main reason for these studies. Therefore the aim was to produce protein resistant polyelectrolyte multilayer films. They were formed via the layer-by-layer approach, while their pegylation by the deposition of pegylated polyanion, PGA-g-PEG, as an external layer. The influence of PEG chain length and grafting density of PGA-g-PEG copolymers on the protein antiadhesive properties of pegylated polyelectrolyte multilayer films was investigated. To monitor the formation of pegylated and non-pegylated multilayer films, adsorption of the following proteins: HSA, Fibrinogen, and FBS were measured by quartz crystal microbalance (QCM - D). We found that protein adsorption onto all pegylated polyelectrolyte multilayers was significantly reduced in comparison to non-pegylated ones. Long-term performance tests confirmed the stability and the durability of the protein resistant properties of the pegylated multilayers. Antiadhesive properties of tested surfaces pegylated by PGA-g-PEG were compared to the available data for pegylated polycation PLL-g-PEG.

Point-of-Need bioanalytics based on planar optical interferometry

This review brings about a comprehensive presentation of the research on interferometric transducers, which have emerged as extremely promising candidates for viable, truly-marketable solutions for PoN applications due to the attested performance that has reached down to 10(-8) in term of effective refractive index changes. The review explores the operation of the various interferometric architectures along with their design, fabrication, and analytical performance aspects. The issues of biosensor functionalization and immobilization of receptors are also addressed. As a conclusion, the comparison among them is attempted in order to delve into and acknowledge their current limitations, and define the future trends.

Synthesis and Characterization of New Functional Photo Cross-Linkable Smart Polymers Containing Vanillin Derivatives

The synthesis of new functional monomers based on vanillin is reported. The monomers further were used in the synthesis of different temperature-responsive photo cross-linkable polymers via free radical polymerization with N-isopropyl acrylamide and a maleimide photo cross-linker. These polymers were characterized by NMR, FTIR and UV spectroscopy, as well as gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). Critical solution temperatures were determined by UV spectroscopy. Hydrogel thin films were formed by spin coating of a polymer solution over gold with adhesion promotor followed by cross-linking by UV irradiation. The swelling properties were determined by surface plasmon resonance coupled with optical waveguide spectroscopy. The swelling behavior of the hydrogel films was determined as a function of temperature. The incorporation of a dialkyl amino group compensated the hydrophobic effect of the vanillin monomer. Transition temperatures in the physiological range could be obtained.

Point of care diagnostics for sexually transmitted infections: perspectives and advances

Accurate and inexpensive point-of-care (POC) tests are urgently needed to control sexually transmitted infection epidemics, so that patients can receive immediate diagnoses and treatment. Current POC assays for Chlamydia trachomatis and Neisseria gonorrhoeae perform inadequately and require better assays. Diagnostics for Trichomonas vaginalis rely on wet preparation, with some notable advances. Serological POC assays for syphilis can impact resource-poor settings, with many assays available, but only one available in the U.S. HIV POC diagnostics demonstrate the best performance, with excellent assays available. There is a rapid assay for HSV lesion detection; but no POC serological assays are available. Despite the inadequacy of POC assays for treatable bacterial infections, application of technological advances offers the promise of advancing POC diagnostics for all sexually transmitted infections.

Investigation of non-specific signals in nanoporous flow-through and flow-over based sensors

Porous materials are ideal hosts to fabricate high sensitivity devices. Their large specific area and the possibility to modify the type and the strength of the matrix-analyte interactions allow the realization of sensors with finely tailored characteristics. In this article, we investigate how mass transport across the nanoporous structure influences the response due to the non-specific signal by comparing flow-through versus flow-over geometries. We observed a systematic overestimation of the sensitivity for porous substrate devices made of closed-ended pores compared with open-ended pore ones. Our analysis shows that such an effect is due to (unbound) analytes or contaminants that remain trapped within the pores and are not removed by rinsing of the sample. This result was verified by measuring similar samples in both flow through and flow over configurations, as well as their residual response after blockage of all their active sites. We also notice that sensors based on free-standing membranes show similar results independent of the fact that mass transport is induced by either an external pressure source or simply by Brownian motions.

Biosensor Technology and the Clinical Biochemistry Laboratory – Issue of Signal Interference from the Biological Matrix

This chapter discusses the potential use of biosensor technology in the clinical biochemistry laboratory. Various relevant key aspects of biosensor technology are introduced such as the chemistry of attachment of probes to device surfaces and a summary of the main categories of sensors based on electrochemistry, acoustic-wave physics and optical science. Important performance characteristics of typical clinical measurements are appraised with examples being presented. Following this discussion, the relevant issues of device selectivity, sensitivity, dynamic range and calibration with respect to target concentration, and possibility for label-free operation are evaluated. A critical issue for potential clinical measurement is the mandatory requirement for devices to function in biological fluids and matrices, with avoidance of signal interference caused by nonspecific surface adoption. Solutions for the latter problem are summarized. The chapter closes with a look at the possible features of biosensor technology that could be employed in the clinical biochemistry laboratory.

Chemical surface modifications for the development of silicon-based label-free integrated optical (IO) biosensors: a review

Increasing interest has been paid to label-free biosensors in recent years. Among them, refractive index (RI) optical biosensors enable high density and the chip-scale integration of optical components. This makes them more appealing to help develop lab-on-a-chip devices. Today, many RI integrated optical (IO) devices are made using silicon-based materials. A key issue in their development is the biofunctionalization of sensing surfaces because they provide a specific, sensitive response to the analyte of interest. This review critically discusses the biofunctionalization procedures, assay formats and characterization techniques employed in setting up IO biosensors. In addition, it provides the most relevant results obtained from using these devices for real sample biosensing. Finally, an overview of the most promising future developments in the fields of chemical surface modification and capture agent attachment for IO biosensors follows.

Rapid, multiparameter profiling of cellular secretion using silicon photonic microring resonator arrays

We have developed a silicon photonic biosensing chip capable of multiplexed protein measurements in a biomolecularly complex cell culture matrix. Using this multiplexed platform combined with fast one-step sandwich immunoassays, we perform a variety of T cell cytokine secretion studies with excellent time-to-result. Using 32-element arrays of silicon photonic microring resonators, the cytokines interleukin-2 (IL-2), interleukin-4 (IL-4), interleukin-5 (IL-5), and tumor necrosis factor alpha (TNFα) were simultaneously quantified with high accuracy in serum-containing cell media. Utilizing this cytokine panel, secretion profiles were obtained for primary human Th0, Th1, and Th2 subsets differentiated from naïve CD4+ T cells, and we show the ability to discriminate between lineage commitments at early stages of culture differentiation. We also utilize this approach to probe the temporal secretion patterns of each T cell type using real-time binding analyses for direct cytokine quantitation down to ∼100 pM with just a 5 min-analysis.

Zwitteration as an alternative to PEGylation

A direct, head-to-head comparison of the efficacy of a zwitterionic versus a poly(ethylene glycol), PEG, coating in preventing protein adsorption to silica and aggregation of silica nanoparticles is presented. The same siloxane coupling chemistry was employed to yield surfaces with similar coverages of both types of ligand. Nanoparticle and planar surfaces were challenged with salt, serum, lysozyme, and serum albumin at 25 and 37 °C. While both types of surface modification are highly effective in preventing protein adsorption and nanoparticle aggregation, the zwitterion provided monolayer-type coverage with minimal thickness, whereas the PEG appeared to yield a more three-dimensional coating. The mechanism for adsorption resistance is thought to be based on preventing ion pairing between protein and surface charges, which releases counterions and water molecules, an entropic driving force enough to overcome a disfavored enthalpy of adsorption.

Multi-step surface functionalization of polyimide based evanescent wave photonic biosensors and application for DNA hybridization by Mach-Zehnder interferometer

The process of surface functionalization involving silanization, biotinylation and streptavidin bonding as platform for biospecific ligand immobilization was optimized for thin film polyimide spin-coated silicon wafers, of which the polyimide film serves as a wave guiding layer in evanescent wave photonic biosensors. This type of optical sensors make great demands on the materials involved as well as on the layer properties, such as the optical quality, the layer thickness and the surface roughness. In this work we realized the binding of a 3-mercaptopropyl trimethoxysilane on an oxygen plasma activated polyimide surface followed by subsequent derivatization of the reactive thiol groups with maleimide-PEG(2)-biotin and immobilization of streptavidin. The progress of the functionalization was monitored by using different fluorescence labels for optimization of the chemical derivatization steps. Further, X-ray photoelectron spectroscopy and atomic force microscopy were utilized for the characterization of the modified surface. These established analytical methods allowed to derive information like chemical composition of the surface, surface coverage with immobilized streptavidin, as well as parameters of the surface roughness. The proposed functionalization protocol furnished a surface density of 144 fmol mm(-2) streptavidin with good reproducibility (13.9% RSD, n=10) and without inflicted damage to the surface. This surface modification was applied to polyimide based Mach-Zehnder interferometer sensors to realize a real-time measurement of streptavidin binding validating the functionality of the MZI biosensor. Subsequently, this streptavidin surface was employed to immobilize biotinylated single-stranded DNA and utilized for monitoring of selective DNA hybridization. These proved the usability of polyimide based evanescent photonic devices for biosensing application.

Waveguide-based biosensors for pathogen detection

Optical phenomena such as fluorescence, phosphorescence, polarization, interference and non-linearity have been extensively used for biosensing applications. Optical waveguides (both planar and fiber-optic) are comprised of a material with high permittivity/high refractive index surrounded on all sides by materials with lower refractive indices, such as a substrate and the media to be sensed. This arrangement allows coupled light to propagate through the high refractive index waveguide by total internal reflection and generates an electromagnetic wave-the evanescent field-whose amplitude decreases exponentially as the distance from the surface increases. Excitation of fluorophores within the evanescent wave allows for sensitive detection while minimizing background fluorescence from complex, "dirty" biological samples. In this review, we will describe the basic principles, advantages and disadvantages of planar optical waveguide-based biodetection technologies. This discussion will include already commercialized technologies (e.g., Corning's EPIC(®) Ô, SRU Biosystems' BIND(™), Zeptosense(®), etc.) and new technologies that are under research and development. We will also review differing assay approaches for the detection of various biomolecules, as well as the thin-film coatings that are often required for waveguide functionalization and effective detection. Finally, we will discuss reverse-symmetry waveguides, resonant waveguide grating sensors and metal-clad leaky waveguides as alternative signal transducers in optical biosensing.

Fabrication of carbohydrate surfaces by using nonderivatised oligosaccharides, and their application to measuring the assembly of sugar-protein complexes

This way up. Dual polarisation interferometry was used to design and characterise a surface on which the orientation and density of immobilised carbohydrates was suitable for studying their interactions with proteins. Lactoferrin was shown to adopt two orientations: "end-on" or "side-on", while for FGF-2 a single monolayer of protein was observed. The new surface can be used to elucidate the binding of proteins to carbohydrates and the geometry of the complexes, a frequently controversial area. Surface-based tools, such as microarrays and optical biosensors, are being increasingly applied to the analysis of carbohydrate-protein interactions. A key to these developments is the presentation of the carbohydrate to the protein target. Dual polarisation interferometry (DPI) is a surface-based technique that permits the real-time measurement of the changes in thickness, refractive index and mass of adsorbates 100 nm thick or less on the surface of a functionalised waveguide. DPI has been used to design and characterise a surface on which the orientation and density of the immobilised carbohydrates is suitable for studying their interactions with proteins and where nonspecific binding is reduced to less than 5 % of total binding. A thiol-functionalised surface was derivatised with a heterobifunctional crosslinker to yield a hydrazide surface. This was treated with oligosaccharides, derived from keratan sulfate (KS) chondroitin sulfate (CS) and heparin, that possess a reducing end. To block the unreacted hydrazide groups, the surface was treated with an aldehyde-functionalised PEG. The heparin DP-10 surfaces were then used to determine the performance of the immobilised DP-10 with respect to binding of two well-characterised proteins, lactoferrin (Lf) and fibroblast growth factor-2. The results show that Lf could adopt two different orientations, at high protein loadings the protein layer thickness corresponded to an "end-on" orientation of Lf, whilst rinsing with buffer saw the Lf molecules adopt a "side-on" configuration. In the case of FGF-2, a single monolayer of protein bound to DP-10 was observed. These results demonstrate that the new surface can be used to resolve key questions relating to the binding of proteins to carbohydrates, including, when used in DPI, the resolution of the geometry of complexes, an area that is frequently controversial.

Sensitive optical biosensors for unlabeled targets: A review

This article reviews the recent progress in optical biosensors that use the label-free detection protocol, in which biomolecules are unlabeled or unmodified, and are detected in their natural forms. In particular, it will focus on the optical biosensors that utilize the refractive index change as the sensing transduction signal. Various optical label-free biosensing platforms will be introduced, including, but not limited to, surface plasmon resonance, interferometers, waveguides, fiber gratings, ring resonators, and photonic crystals. Emphasis will be given to the description of optical structures and their respective sensing mechanisms. Examples of detecting various types of biomolecules will be presented. Wherever possible, the sensing performance of each optical structure will be evaluated and compared in terms of sensitivity and detection limit.

Rapid Assay of Trace Immunoglobulin M by a New Immunonanogold Resonance Scattering Spectral Probe

Nanogold, 8 nm in size, was used to label goat antihuman immunoglobulin M (GIgM) to obtain a new immunonanogold resonance scattering (RS) probe (Au-GIgG) for quantitation of trace immunoglobulin M (IgM). The Au-GIgG combined with IgM to form nanogold-labeled immunocomplex causes the RS intensity at 580 nm to be enhanced, in pH 4.49 KH2PO 4-Na2HPO4 buffer and in the presence of polyethylene glycol 6000. The enhanced RS intensity at 580 nm (ΔI580 nm) is proportional to the IgM concentration in the range of 1.5 to 2000 ng/mL, with a lower detection limit of 0.98 ng/mL. The immunonanogold RS assay was used to assay IgM in serum samples, with sensitivity, selectivity, and simplicity. ( Journal of Biomolecular Screening 2008:302-308)

Horseradish peroxidase-functionalized gold nanoparticle label for amplified immunoanalysis based on gold nanoparticles/carbon nanotubes hybrids modified biosensor

This paper describes the combination of electrochemical immunosensor using gold nanoparticles (GNPs)/carbon nanotubes (CNTs) hybrids platform with horseradish peroxidase (HRP)-functionalized gold nanoparticle label for the sensitive detection of human IgG (HIgG) as a model protein. The GNPs/CNTs nanohybrids covered on the glass carbon electrode (GCE) constructed an effective antibody immobilization matrix and made the immobilized biomolecules hold high stability and bioactivity. Enhanced sensitivity was obtained by using bioconjugates featuring HRP labels and secondary antibodies (Ab(2)) linked to GNPs at high HRP/Ab(2) molar ratio. The approach provided a linear response range between 0.125 and 80ng/mL with a detection limit of 40pg/mL. The immunosensor showed good precision, acceptable stability and reproducibility and could be used for the detection of HIgG in real samples, which provided a potential alternative tool for the detection of protein in clinical laboratory.

Evanescent field Sensors Based on Tantalum Pentoxide Waveguides - A Review

Evanescent field sensors based on waveguide surfaces play an important rolewhere high sensitivity is required. Particularly tantalum pentoxide (Ta₂O₅) is a suitablematerial for thin-film waveguides due to its high refractive index and low attenuation.Many label-free biosensor systems such as grating couplers and interferometric sensors aswell as fluorescence-based systems benefit from this waveguide material leading toextremely high sensitivity. Some biosensor systems based on Ta₂O₅ waveguides alreadytook the step into commercialization. This report reviews the various detection systems interms of limit of detection, the applications, and the suitable surface chemistry.

A label-free electrochemical immunoassay for carcinoembryonic antigen (CEA) based on gold nanoparticles (AuNPs) and nonconductive polymer film

A simple and sensitive label-free electrochemical immunoassay electrode for detection of carcinoembryonic antigen (CEA) has been developed. CEA antibody (CEAAb) was covalently attached on glutathione (GSH) monolayer-modified gold nanoparticle (AuNP) and the resulting CEAAb-AuNP bioconjugates were immobilized on Au electrode by electro-copolymerization with o-aminophenol (OAP). Electrochemical impedance spectroscopy and cyclic voltammetry studies demonstrate that the formation of CEA antibody-antigen complexes increases the electron transfer resistance of [Fe(CN)(6)](3-/4-) redox pair at the poly-OAP/CEAAb-AuNP/Au electrode. The use of CEA antibody-AuNP bioconjugates and poly-OAP film could enhance the sensitivity and anti-nonspecific binding of the resulting immunoassay electrode. The preliminary application of poly-OAP/CEAAb-AuNP/Au electrode for detection of CEA was also evaluated.

Nanocarriers: Promising Vehicle for Bioactive Drugs

Development of new delivery systems that deliver the potential drug specifically to the target site in order to meet the therapeutic needs of the patients at the required time and level remains the key challenge in the field of pharmaceutical biotechnology. Developments in this context to achieve desired goal has led to the evolution of the multidisciplinary field nanobiotechnology which involves the combination of two most promising technologies of 21st century--biotechnology and nanotechnology. Nanobiotechnology encompasses a wide array of different techniques to improve the delivery of biotech drugs, and nanoparticles offer the most suitable form whose properties can be tailored by chemical methods. This review highlights the different types of nanoparticulate delivery systems employed for biotech drugs in the field of molecular medicine with a short overlook at its applications and the probable associated drawbacks.

Application of europium(III) chelate-dyed nanoparticle labels in a competitive atrazine fluoroimmunoassay on an ITO waveguide

We have demonstrated the use of an optical indium tin oxide (ITO) (quartz) waveguide as a new platform for immunosensors with fluorescent europium(III) chelate nanoparticle labels (Seradyn) in a competitive atrazine immunoassay. ITO as a solid surface facilitated the successful use of particulate labels in a competitive assay format. The limit of detection in the new nanoparticle assay was similar to a conventional ELISA. The effect of particle size on bioconjugate binding kinetics was studied using three sizes of bioconjugated particle labels (107, 304, and 396nm) and a rabbit IgG/anti-IgG system in a 96-well plate. A decrease in particle size resulted in faster binding but did not increase the assay sensitivity. Flux calculations based on the particle diffusivity prove that faster binding of the small particles in this study was primarily due to diffusion kinetics and not necessarily to a higher density of antibodies on the particle surface. The results suggest that ITO could make a good platform for an optical immunosensor using fluorescent nanoparticle labels in a competitive assay format for small molecule detection. However, when used in combination with fluorescent particulate labels, a highly sensitive excitation/detection system needs to be developed to fully utilize the kinetic advantage from small particle size. Different regeneration methods tested in this study showed that repeated washings with 0.1 M glycine-HCl facilitated the reuse of the ITO waveguide.

An Isothermal-Isobaric Langevin Thermostat for Simulating Nanoparticles under Pressure: Application to Au Clusters

We present a method for simulating clusters or molecules subjected to an external pressure, which is exerted by a pressure-transmitting medium. It is based on the canonical Langevin thermostat, but extended in such a way that the Brownian forces are allowed to operate only from the region exterior to the cluster. We show that the frictional force of the Langevin thermostat is linked to the pressure of the reservoir in a unique way, and that this property manifests itself when the particle it acts upon is not pointlike but has finite dimensions. By choosing appropriately the strength of the random forces and the friction coefficient, both temperature and pressure can be controlled independently. We illustrate the capabilities of this new method by calculating the compressibility of small gold clusters under pressure.

One-step synthesis of amino-dextran-protected gold and silver nanoparticles and its application in biosensors

A sensitive method for the detection of the lectin protein concanavalin A (Con A) was developed using amino-dextran (AD)-protected gold (AD-Au) and silver nanoparticles (AD-Ag) as sensitive optical probes. The AD-Au and AD-Ag nanoparticles were synthesized by directly applying amino-dextran as a reductive and protective reagent. The size of the nanoparticles could be altered by changing the molar ratio of AD to the metal salt. The amino-dextran bound to Con A by forming a 4:1 Au-Con A complex at neutral pH, and the nanoparticles were induced to aggregate by Con A. The absorption intensity of the nanoparticles decreased linearly with as the Con A concentration was increased from 3.85 x 10(-8) to 6.15 x 10(-7) M. The Au-Con A complex was dissociated by the disaccharide isomaltose, which has a higher affinities for Con A than Au; this competitive strategy could also be used to detect similar types of saccharides.

Nanoparticle labels in immunosensing using optical detection methods

Efforts to improve the performance of immunoassays and immunosensors by incorporating different kinds of nanostructures have gained considerable momentum over the last decade. Apart from liposomes, which will not be discussed here, most groups focus on artificial, particulate marker systems, both organic and inorganic. The underlying detection procedures may be based either on electro-magnetical or optical techniques. This review will be confined to the latter only, comprising nanoparticle applications generating signals as diverse as static and time-resolved luminescence, one- and two-photon absorption, Raman and Rayleigh scattering as well as surface plasmon resonance and others. In general, all endeavors cited are geared to achieve one or more of the following goals: lowering of detection limits (if possible, down to single-molecule level), parallel integration of multiple signals (multiplexing), signal amplification by several orders of magnitude and prevention of photobleaching effects with concomitant maintenance of antigen binding specificity and sensitivity. Inorganic nanoparticle labels based on noble metals, semiconductor quantum dots and nanoshells appear to be the most versatile systems for these bioanalytical applications of nanophotonics.

Detection of subpicomolar concentrations of human matrix metalloproteinase-2 by an optical biosensor

We describe in this paper the development of a one-step sandwich assay for the highly sensitive and fast detection of human matrix metalloproteinase (MMP)-2 (EC 3.4.24.24), using surface plasmon resonance (SPR). For the assay, two ligands were selected: monoclonal anti-MMP-2 antibody Ab-2 and the tissue inhibitor of metalloproteinases (TIMP)-2. They were chosen on the basis of (1) their affinities to MMP-2, (2) the efficiency of immobilization to the sensor chip, (3) the efficiency of adsorption to colloidal gold, and (4) the stability of these protein-coated gold particles. The assay included mixing of MMP-2 with antibody Ab-2 adsorbed to colloidal gold with a diameter of about 20nm and injection into the flowcell of the SPR instrument containing immobilized TIMP-2. By using colloidal gold particles an amplification factor of 114 and a detection limit of 0.5pM for MMP-2 were obtained. The precision of the assay was high even at low analyte concentrations, the standard deviation being 8.3% for five determinations of 1pM MMP-2. No significant binding was observed with the structurally related MMP-9. The assay is far more sensitive and faster than commonly used methods for MMP-2 detection. As TIMP-bound MMP-2 is not detected by this method, the assay can be applied for measuring free MMP-2, reflecting the imbalance of free and inhibitor-bound enzyme in various pathological situations.

A new quantitative optical biosensor for protein characterisation

A new optical biosensor is described based on a dual waveguide interferometric technique. By addressing the waveguide structure with alternate polarisations the optogeometrical properties (density and thickness) of adsorbed protein layers at the sensor (solid)-liquid interface have been determined. Differences in the waveguide mode dispersion between the transverse electric (TE) and transverse magnetic (TM) modes allow unique solutions for adlayer thickness and refractive index to be determined at all stages during the formation process. The technique has been verified using standard protein systems and by comparing the data with published work using X-ray crystallography and neutron reflection techniques. The data obtained was found to be in excellent agreement with previously reported X-ray experiments given that typical film thicknesses for streptavidin layers were in the range 5.5-6.5 nm compared with the short axis crystal structure of between 4.8 and 5.6 nm. The precision of the measurements taken was of the order of 40 pm with respect to adsorbed adlayer thicknesses. This biosensor approach provides measurements of both thickness and density of adlayers to a high precision, simultaneously and in real time enabling detail of the structure and function of proteins to be elucidated. From such data it is possible to obtain information on the orientation, distortion and efficiency of immobilisation procedures as well as the interaction event of interest. The technique is expected to find utility with those interested in protein structure and function. This is an area of growing importance within the life sciences as the demand for quantitative analytical techniques increases with the growth in "proteomics".

Biochips beyond DNA: technologies and applications

Technological advances in miniaturization have found a niche in biology and signal the beginning of a new revolution. Most of the attention and advances have been made with DNA chips yet a lot of progress is being made in the use of other biomolecules and cells. A variety of reviews have covered only different aspects and technologies but leading to the shared terminology of "biochips." This review provides a basic introduction and an in-depth survey of the different technologies and applications involving the use of non-DNA molecules such as proteins and cells. The review focuses on microarrays and microfluidics, but also describes some cellular systems (studies involving patterning and sensor chips) and nanotechnology. The principles of each technology including parameters involved in biochip design and operation are outlined. A discussion of the different biological and biomedical applications illustrates the significance of biochips in biotechnology.

Optimisation of glass surfaces for optical immunosensors

The surfaces of glass sensor chips were modified with dextran to generate a layer protecting the sensor surface from unspecific protein binding and also serving as a matrix for covalent protein immobilisation. Dextran was coupled to the glass surface in different concentrations either covalently on amino-functionalised glass chips or via biotin-avidin binding. Unspecific binding of BSA was monitored with the grating coupler system, and was increasingly suppressed with increasing dextran concentrations. Using a solution with 100 mg/ml carboxymethylated dextran decreased the signals to approximately 2% of those obtained at an untreated glass chip. Antibodies were successfully immobilised in the dextran and binding to the corresponding Cy5-labelled antigen was repeatedly monitored using a fluorescence sensor system (total internal reflection fluorescence (TIRF)).

Biomedical applications of protein chips

The development of microchips involving proteins has accelerated within the past few years. Although DNA chip technologies formed the precedent, many different strategies and technologies have been used because proteins are inherently a more complex type of molecule. This review covers the various biomedical applications of protein chips in diagnostics, drug screening and testing, disease monitoring, drug discovery (proteomics), and medical research. The proteomics and drug discovery section is further subdivided to cover drug discovery tools (on-chip separations, expression profiling, and antibody arrays), molecular interactions and signaling pathways, the identification of protein function, and the identification of novel therapeutic compounds. Although largely focused on protein chips, this review includes chips involving cells and tissues as a logical extension of the type of data that can be generated from these microchips.

Immunosensors--principles and applications to clinical chemistry

INTRODUCTION

Immunosensors are affinity ligand-based biosensor solid-state devices in which the immunochemical reaction is coupled to a transducer. The fundamental basis of all immunosensors is the specificity of the molecular recognition of antigens by antibodies to form a stable complex. This is similar to the immunoassay methodology. Immunosensors can be categorized based on the detection principle applied. The main developments are electrochemical, optical, and microgravimetric immunosensors. In contrast to immunoassay, modern transducer technology enables the label-free detection and quantification of the immune complex.

METHODS

The analysis of trace substances in environmental science, pharmaceutical and food industries is a challenge since many of these applications demand a continuous monitoring mode. The use of immunosensors in these applications is most appropriate. Similarly, a series of clinical problems may be solved by continuous monitoring of certain analytes.

CONCLUSIONS

Clinical chemists should take advantage of immunosensors in clinical diagnostics. There are many recent developments in the immunosensor field which have potential impacts. The future role of this technique in intralaboratory, as well as bedside testing, will become even more important as the clinical laboratory is faced with increasing pressure to contain costs.

Optical chip immunoassay for hCG in human whole blood

We report on the development of an integrated optic chip sensor for performing rapid and sensitive immunoassays with human whole blood using human chorionic gonadotropin (hCG) as the model system. The optical chip is based on the Hartman interferometer, which uses a single planar lightbeam to address multiple interferometers, each comprising a signal/reference pair of sensing regions. The binding of antigen to specific capture antibodies on the signal sensing region causes a change in the refractive index of the surface layer, which is detectable by its effect on the evanescent field of the guided lightbeam. The reference-sensing region is coated with an irrelevant antibody, which optically cancels a large fraction of the non-specific adsorption that occurs on the specific-sensing region when the sensor is tested with clinical specimens. This work extends previous experiments with buffer and human serum to measurements in undiluted whole human blood. Optical chips were stored dry after surface functionalization, and rehydrated with blood. Colloidal gold nanoparticles conjugated to a second anti-hCG monoclonal antibody were used to provide signal amplification, thereby enhancing assay sensitivity, in a one-step procedure with the gold conjugate added to the test sample immediately prior to measurement. Background signals due to non-specific binding (NSB) in blood were found to be higher than those previously reported with human serum. In addition, a high level of background signal was found with the gold conjugate, which had not been observed in experiments with either buffer or serum. Nevertheless, hCG could be detected at 0.5 ng/ml within 10 min of sample application. The sensor response was linear over the concentration range 0.5-5 ng/ml hCG, as compared with the clinically-relevant range 0.3-1.5 ng/ml. Detection at higher concentrations was affected by scattering from large amounts of bound gold nanoparticles. However, initial binding rate measurements could be used to maintain assay quantitation.

Recent trends in protein biochip technology

This article presents current trends and advances in protein biochip technologies that rely upon extraction and retention of target proteins from liquid media. Analytical strengths as well as technical challenges for these evolving platforms are presented with particular emphasis on selectivity, sensitivity, throughput and utility in the post-genome era. A general review of protein biochip technology is provided, which delineates approaches for protein biochip format and operation, as well as protein detection. A focused discussion of three protein biochip technologies, Biomolecular Interaction Analysis (Biacore, Uppsala, Sweden), Surface Enhanced Laser Desorption/Ionisation (SELDI) ProteinChip Arrays (Ciphergen Biosystems, Fremont, CA, USA) and Fluorescent Planar Wave Guide (Zeptosens, Witterswil, Switzerland), follows along with examples of relevant applications.