ABSTRACT
Hydrogels have become widely used materials in numerous important fields such as drug delivery and tissue engineering and have allowed great improvements in the performance of analytical and bioanalytical technologies for detection of chemical and biological species. Their highly open structure makes them attractive to serve as a binding matrix that can accommodate large amounts of target analyte, provide a natural microenvironment for biomolecular recognition elements, and enable design of surfaces that can resist nonspecific adsorption from complex samples for highly specific biosensor devices. In addition, a special class of “smart” hydrogels responsive to external stimuli (e.g., through temperature or pH changes) holds potential for development of biosensors with enhanced sensitivity and implementing of new biosensor schemes for sensitive analysis of molecular analytes. This chapter provides an overview of recent advances in our and other laboratories, as well as
practical leads for design, characterization, and implementation of hydrogel surface architectures for ultrasensitive affinity biosensors based on evanescent wave optics. 9.1 IntroductionHydrogels are insoluble, cross-linked, water-swollen polymer networks of hydrophilic homopolymers or copolymers. Owing to their highly open structure and large inner surface, hydrogels can accommodate large amounts of molecules with specific functions. Hydrogels become irreplaceable materials in numerous important areas ranging from pharmaceutical applications (e.g., drug delivery and tissue engineering) [1-6] to biosensor technologies for detection of chemical or biological analytes [7-12]. In biosensor applications, hydrogel materials are employed at an interface between an analyzed sample and a transducer. Typically, hydrogels are modified with biomolecular recognition elements (BREs) such as antibodies, enzymes, or biomimetic moieties based on molecular imprinting in order to specifically recognize target analytes present in a liquid sample (Fig. 9.1). Compared with other types of biointerfaces (e.g., based on self-assembled monolayers [SAMs]), hydrogels can accommodate orders-of-magnitude larger amounts of recognition elements, provide increased stability of incorporated biomolecules, and offer routes for implementing additional functionalities (e.g., separation of target analyte from other molecules in a sample, simplified methods for readout). In addition, the class of “smart” gels that can respond to external stimuli become of interest for development of entirely new biosensor schemes. For instance, miniature hydrogel sensor elements were integrated in a contact lens for the analysis of glucose in tear fluid. The hydrogel element served both as a host for recognition elements and as an optical transducer for detection of glucose [13]. In this chapter, we focus on biosensors in which hydrogel-based binding matrices with thicknesses up to micrometers are probed by an evanescent field of surface plasmon (SP) and optical waveguide waves. The optical detection of analyte-binding events is carried out through monitoring of binding-induced refractive index changes or by fluorescence spectroscopy. We discuss key performance characteristics of hydrogel-based binding matrices and
their implementation in biosensor devices. Let us note that detailed information on the synthesis and characterization of hydrogels and their application in other areas can be found in chapter 8. Thinner polymer brushes developed for biosensor applications are discussed in chapter 11.
