Optical biosensors. Monitoring studies of glycopeptide antibiotic fermentation using white light interference

Monolithically integrated broad-band Mach-Zehnder interferometers for highly sensitive label-free detection of biomolecules through dual polarization optics

Protein detection and characterization based on Broad-band Mach-Zehnder Interferometry is analytically outlined and demonstrated through a monolithic silicon microphotonic transducer. Arrays of silicon light emitting diodes and monomodal silicon nitride waveguides forming Mach-Zehnder interferometers were integrated on a silicon chip. Broad-band light enters the interferometers and exits sinusoidally modulated with two distinct spectral frequencies characteristic of the two polarizations. Deconvolution in the Fourier transform domain makes possible the separation of the two polarizations and the simultaneous monitoring of the TE and the TM signals. The dual polarization analysis over a broad spectral band makes possible the refractive index calculation of the binding adlayers as well as the distinction of effective medium changes into cover medium or adlayer ones. At the same time, multi-analyte detection at concentrations in the pM range is demonstrated.

Label-free quantitation of a cancer biomarker in complex media using silicon photonic microring resonators

Recent advances in label-free biosensing techniques have shown the potential to simplify clinical analyses. With this motivation in mind, this paper demonstrates for the first time the use of silicon-on-insulator microring optical resonator arrays for the robust and label-free detection of a clinically important protein biomarker in undiluted serum, using carcinoembryonic antigen (CEA) as the test case. We utilize an initial-slope-based quantitation method to sensitively detect CEA at clinically relevant levels and to determine the CEA concentrations of unknown samples in both buffer and undiluted fetal bovine serum. Comparison with a commercial enzyme-linked immunosorbent assay (ELISA) kit reveals that the label-free microring sensor platform has a comparable limit of detection (2 ng/mL) and superior accuracy in the measurement of CEA concentration across a 3 order of magnitude dynamic range. Notably, we report the lowest limit of detection to date for a microring resonator sensor applied to a clinically relevant cancer biomarker. Although this report describes the robust biosensing capabilities of silicon photonic microring resonator arrays for a single parameter assay, future work will focus on utilizing the platform for highly multiplexed, label-free bioanalysis.

A possibility of detection of the non-charge based analytes using ultra-thin body field-effect transistors

Ultra-thin body of p-type field-effect transistors were developed as transducer for biosensors. Changes of conductance resulted from the changes of the surface potentials of ultra-thin body field-effect transistors (UTB-FETs) due to surface chemical modifications were demonstrated. The channel surface of UTB-FETs were modified with N-[3-(trimethoxysilyl)propyl]ethylenediamine (AEAPTMS) and then gold nanoparticles (AuNPs) to immobilize the bio-component, the genetically engineered Delta(5)-3-ketosteroid isomerase (Art_KSI) or the Art_KSI conjugated with charged reporter (Art_KSI_mA51). The binding of charge-based molecules or nanoparticles has been demonstrated to strongly affect the conductivity of UTB-FETs; the increase or decrease of the conductance depends on the polarity of the immobilized molecules or nanoparticles. A new protocol involving the detection of a non-charged analyte relied on the competitive binding of analyte (19-norandrostendione) and a charged reporter (mA51) with KSI. When exposed to a 19-norandrostendione solution (10 microM), the conductance of Art_KSI_mA51-modified UTB-FET increased by 265 nS ( approximately 12%). On the other hand, conductance of Art_KSI-modified UTB-FET showed no distinct change under the same detection conditions.

Strategies for label-free optical detection

A large number of methods using direct detection with label-free systems are known. They compete with the well-introduced fluorescence-based methods. However, recent applications take advantage of label-free detection in protein-protein interactions, high-throughput screening, and high-content screening. These new applications require new strategies for biosensors. It becomes more and more obvious that neither the transduction principle nor the recognition elements for the biomolecular interaction process alone determine the quality of the biosensor. Accordingly, the biosensor system has to be considered as a whole. This chapter focuses on strategies to optimize the detection platform and the biomolecular recognition layer. It concentrates on direct detection methods, with special focus on optical transduction. Since even this restriction still leaves a large number of methods, only microrefractometric and microreflectometric methods using planar transducers have been selected for a detailed description and a listing of applications. However, since many review articles on the physical principles exist, the description is kept short. Other methods are just mentioned in brief and for comparison. The outlook and the applications demonstrate the future perspectives of direct optical detection in bioanalytics.

Potential of label-free detection in high-content-screening applications

The classical approach of high-content screening (HCS) is based on multiplexed, functional cell-based screening and combines several analytical technologies that have been used before separately to achieve a better level of automation (scale-up) and higher throughput. New HCS methods will help to overcome the bottlenecks, e.g. in the present development chain for lead structures for the pharmaceutical industry or during the identification and validation process of new biomarkers. In addition, there is a strong need in analytical and bioanalytical chemistry for functional high-content assays which can be provided by different hyphenated techniques. This review discusses the potential of a label-free optical biosensor based on reflectometric interference spectroscopy (RIfS) as a bridging technology for different HCS approaches. Technical requirements of RIfS are critically assessed by means of selected applications and compared to the performance characteristics of surface plasmon resonance (SPR) which is currently the leading technology in the area of label-free optical biosensors.

Multivalence and spot heterogeneity in microarray-based measurement of binding constants

Microarray technology is increasingly used for a miniaturised and parallel measurement of binding constants. In microarray experiments heterogeneous functionalization of surfaces with capture molecules is a problem commonly encountered. For multivalent ligands, especially, however, binding is strongly affected by receptor density. Here we show that high-resolution imaging of microarrays followed by image segmentation and separate analysis of bright and dark parts provides valuable information about ligand binding. Binding titrations were conducted with monovalent and bivalent fluorescent ligand peptides for the model receptor vancomycin. Microarrays were scanned with a confocal microscope and inhomogeneous spots were evaluated either as a whole or after segmentation into bright and dark areas. Whereas the binding constant for the monovalent ligand was hardly affected by spot heterogeneity, for the bivalent ligand affinity was higher for the parts of the spots with a greater density of receptors. This information was lost if the spots were analysed as a whole. These results reveal that imaging resolution may be a key factor in miniaturised binding assays, emphasising the importance of high-resolution images and image segmentation for new techniques, for example SPR imaging.

A proteomic biosensor for enteropathogenic E. coli

The study of proteins and the molecules with which they interact on an organismwide scale is critical to understanding basic biology, and understanding and improving human health. New platform technologies allowing label-free, quantitative array-based analysis of proteins are particularly desirable. We have developed an analytical technology, reflective interferometry (RI), which provides specific, rapid, and label-free optical detection of biomolecules in complex mixtures. In order to evaluate the suitability of RI for proteomics, we have prepared a series of arrays bearing the extracellular domain of the secreted enteropathogenic Escherichia coli (EPEC) protein Translocated Intimin Receptor (Tir). These arrays are able to selectively detect the extracellular domain of the protein Intimin, Tir's natural binding partner. Furthermore, we demonstrate the use of RI and Tir-functionalized arrays for the selective detection of EPEC directly from culture.

Reflectometric interference spectroscopy combined with MALDI-TOF mass spectrometry to determine quantitative and qualitative binding of mixtures of vancomycin derivatives

This paper describes the combination of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with label free bio-interaction analysis based on reflectometric interference spectroscopy (RIfS). The potential of this concerted approach is demonstrated by measuring the binding properties of different vancomycin-type glycopeptide antibiotic mixtures. Although RIfS is sensitive and does not require use of a label, it cannot determine which components of a mixture have bound to the surface after incubation. Fortunately, each bound species has a unique mass that can, afterwards, be determined by mass spectrometry. Thus, the screening capability of RIfS is combined with the identification capability of mass spectrometry.

Direct optical sensors: principles and selected applications

In the field of bio and chemosensors a large number of detection principles has been published within the last decade. These detection principles are based either on the observation of fluorescence-labelled systems or on direct optical detection in the heterogeneous phase. Direct optical detection can be measured by remission (absorption of reflected radiation, opt(r)odes), by measuring micro-refractivity, or measuring interference. In the last case either Mach-Zehnder interferometers or measurement of changes in the physical thickness of the layer (measuring micro-reflectivity) caused, e.g., by swelling effects in polymers (due to interaction with analytes) or in bioassays (due to affinity reactions) also play an important role. Here, an overview of methods of microrefractometric and microreflectometric principles is given and benefits and drawbacks of the various approaches are demonstrated using samples from the chemo and biosensor field. The quality of sensors does not just depend on transduction principles but on the total sensor system defined by this transduction, the sensitive layer, data acquisition electronics, and evaluation software. The intention of this article is, therefore, to demonstrate the essentials of the interaction of these parts within the system, and the focus is on optical sensing using planar transducers, because fibre optical sensors have been reviewed in this journal only recently. Lack of selectivity of chemosensors can be compensated either by the use of sensor arrays or by evaluating time-resolved measurements of analyte/sensitive layer interaction. In both cases chemometrics enables the quantification of analyte mixtures. These data-processing methods have also been successfully applied to antibody/antigen interactions even using cross-reactive antibodies. Because miniaturisation and parallelisation are essential approaches in recent years, some aspects and current trends, especially for bio-applications, will be discussed. Miniaturisation is especially well covered in the literature.

Determination of Binding Constants on Microarrays with Confocal Fluorescence Detection

Confocal laser scanning microscopy was employed for the determination of binding constants of receptor-ligand interactions in a microarray format. Protocols for a localized immobilization of amine containing substances on glass via GOPTS (3-glycidyloxypropyl)trimethoxysilane) were optimized with respect to the detection of ligand binding by fluorescence. Compatibility with miniaturization by nanopipetting devices was ensured during all steps. The interaction of the tripeptide L-Lys-D-Ala-D-Ala with vancomycin immobilized on glass served as a model. To minimize consumption of ligand, binding constants were determined by stepwise titration of binding sites. The binding constant of the unlabeled ligand was determined by competitive titration with a fluorescently labeled analogue. The determined binding constants agreed well with those determined by other techniques, previously. Labeled ligand bound stronger than the unlabeled one. This difference was dye-dependent. Still, binding was specific for the tripeptide moiety confirming that ligand and fluorescent analogue competed for the same binding sites these results validate the determination of binding constants by competitive titration. The protocols established for confocal fluorescence detection are applicable to axially resolved detection modalities and screening for unlabeled ligands by competitive titration in general.

Online monitoring of solid-phase peptide syntheses on glass-type surfaces using white light interference

Label-free optical detection methods are of particular value for the investigation of biomolecular interactions. A label-free method based on reflectometric interference spectroscopy is described which enables both the on-line monitoring of solid phase peptide syntheses and subsequently the determination of antibody binding to these peptides without cleavage from the support. The method uses SiO(2) transducers that were modified with diaminopolyethylene glycol. The stepwise coupling of different amino acids to the transducer surface was investigated and the complete monitoring of the synthesis of a viral epitope was performed. The success of the synthesis was proven via binding of a specific monoclonal antibody to the transducer-bound product. In order to demonstrate specificity the binding was inhibited with the same peptide epitope added in solution. The approach is attractive especially in the field of high throughput screening since both the synthesis and the interaction with the biological receptor can be monitored using the same technique.