ABSTRACT
Many of the currently used bioaffinity assays are surface-based assays, including enzyme-linked immunosorbent assay (ELISA), bead-based assay, and various new sensing concepts using optical, electrical, acoustic, and mechanical approaches to translate the biointeraction events into physically detectable signals. A capable biosensor not only requires a robust detection method but also requires a good surface matrix to capture analyte and transducer signals. The basic requirements for the surface matrix are: 1) the ligand needs to remain active, accessible, and in good orientation; 2) the matrix needs to be resist to nonspecific binding (NSB); and 3) the matrix needs to be physically and chemically stable and able to
withstand moderate environmental change. Oftentimes, the surface matrix also needs to be optimized for a specific physical detection method. Up to now, most of the surface biosensors are based on planar functional surfaces with the ligands attached in a twodimensional (2D) arrangement and distributed in a rather narrow distance range from the sensing surface. Because of the growing demand for sensitivity, researchers started to focus on three-dimensional (3D) matrices for biosensors, where the ligand molecules are distributed spatially in a broader distance range from the sensor surface, from dozens of nanometers to submillimeters, depending on the detection physics. The 3D matrix can be constructed physically. For example, Sailor used a porous silicon matrix to effectively increase the sensitivity of white-light interferometry sensors; Qian used the Opal matrix with an enormous surface-area-to-volume ratio for photonic crystal-based biosensors; porous metal has been widely used in electrochemical biosensors because of its high area-to-volume ratio; and Jonah and Yu recently developed an surface plasmon resonance (SPR) sensor based on porous gold. The 3D matrix can also be fabricated chemically: the mostly famous and commercially successful example is the dextranbased chip series from Biacore. There are also hydrogel-based matrices that emerge to show great potential. In additional to the above, layer-by-layer approaches have also been investigated to increase the surface ligand density, which, however, only provides quasi-3D matrices. The 3D matrices offer a much larger ligand density per area, which means more analyte molecules can be captured, obviously benefiting the detection sensitivity. The enhanced surface-to-volume ratio also facilitates analyte-binding efficiency. Some of the chemically based 3D matrices even offer a nearly homogenous binding environment between the analyte solution and the surface-immobilized ligand in contrast to the heterogeneous binding scenario of a 2D sensor surface, where the analyte has to diffuse from the bulk phase to the interface. In this contribution, we illustrate the benefit of a 3D matrix based on a dextran polymer brush on surface plasmon field-enhanced fluorescence spectroscopy (SPFS). The detection method, SPFS, is based on SPR, which is probably the most used physical principle for surface biosensing. SPR happens at the metal/dielectric interface, where the metal electrons are resonantly excited by external light.
