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Hydrogel films have been used extensively in the preparationofbiosensors and biomedical devices. The characteristics of the aqueousinterface of the polymer layer are significant for the biosensor ordevice function; likewise, the changing mechanical properties of thermoresponsivepolymers are an important feature that affects the polymer behavior.Atomic force microscopy was used here to characterize both the surfaceand the mechanical properties of polymeric hydrogel films preparedfrom a thermoresponsive terpolymer of N-isopropylacrylamideand acrylic acid with benzophenonemethacrylate as a photoreactivecross-linker comonomer. The force–distance curves thus obtainedwere analyzed to assess both the surface forces and the mechanicalresponse that were associated with the hydrogel. These propertieswere investigated as a function of temperature, in water and in Trisbuffer, for different degrees of polymer cross-linking. For samplesin water, the distance over which the surface forces were effectivewas found to remain constant as the temperature was increased from26 to 42 °C, even though the mechanical response indicated thatthe samples had been heated past the lower critical solution temperature,or LCST. The bulk of the polymer becomes less soluble above the LCST,although this does not seem to affect the surface properties. Thismay be due to the segregation of the acrylic acid-rich polymer segmentsnear the gel surface, which is in agreement with reports for relatedsystems. [ABSTRACT FROM AUTHOR]

Hydrogel materials consisting of water-swollen polymer networks exhibit a large number of specific properties highly attractive for a variety of optical biosensor applications. This properties profile embraces the aqueous swelling medium as the basis of biocompatibility, non-fouling behavior, and being not cell toxic, while providing high optical quality and transparency. The present review focuses on some of the most interesting aspects of surface-attached hydrogel films as active binding matrices in optical biosensors based on surface plasmon resonance and optical waveguide mode spectroscopy. In particular, the chemical nature, specific properties, and applications of such hydrogel surface architectures for highly sensitive affinity biosensors based on evanescent wave optics are discussed. The specific class of responsive hydrogel systems, which can change their physical state in response to externally applied stimuli, have found large interest as sophisticated materials that provide a complex behavior to hydrogel-based sensing devices. [ABSTRACT FROM AUTHOR]

Abstract: Rapid detection of hormones at sub-ng/ml concentrations is of tremendous importance for diagnostic purposes, quality control, and environmental monitoring. In this respect, we report a novel label-free biosensor based on hydrogel optical waveguide spectroscopy (HOWS) for the sensitive detection of 17β-estradiol (E2). This approach was implemented by using a thin hydrogel layer of a carboxylated poly(N-isoproprylacrylamide) (PNIPAAm) terpolymer that was attached to a metallic sensor surface in order to simultaneously serve as a binding matrix and an optical waveguide. Refractive index changes that are accompanied with the specific capture of biomolecules from an aqueous sample on the sensor surface were probed by resonantly excited hydrogel optical waveguide modes. To optically excite and interrogate these waves, an optical setup based on Kretschmann configuration of attenuated total reflection (ATR) method that is compatible with surface plasmon resonance (SPR) was used. We demonstrate that HOWS offers a higher binding capacity, good anti-fouling properties, improved figure of merit, and E2 detection limit of 50pg/ml which is seven times better than that obtained by a regular surface plasmon resonance (SPR) biosensor. [Copyright &y& Elsevier]

Abstract: A new approach to signal amplification in fluorescence-based assays for sensitive detection of molecular analytes is reported. It relies on a sensor chip carrying a one-dimensional photonic crystal (1DPC) composed of two piled up segments which are designed to increase simultaneously the excitation rate and the collection efficiency of fluorescence light. The top segment supports Bloch surface waves (BSWs) at the excitation wavelength and the bottom segment serves as a Bragg mirror for the emission wavelength of used fluorophore labels. The enhancement of the excitation rate on the sensor surface is achieved through the resonant coupling to BSWs that is associated with strong increase of the field intensity. The increasing of collection efficiency of fluorescence light emitted from the sensor surface is pursued by using the Bragg mirror that minimizes its leakage into a substrate and provides its beaming toward a detector. In order to exploit the whole evanescent field of BSW, extended three-dimensional hydrogel-based binding matrix that is functionalized with catcher molecules is attached to 1DPC for capturing of target analyte from a sample. Simulations supported by experiments are presented to illustrate the design and determined the performance characteristics of BSW-enhanced fluorescence spectroscopy. A model immunoassay experiment demonstrates that the reported approach enables increasing signal to noise ratio, resulting in about one order of magnitude improved limit of detection (LOD) with respect to regular total internal reflection fluorescence (TIRF) configuration. [Copyright &y& Elsevier]