In direct detection assays, AuNPs are chemically bound to the unclad optical fiber surface. Receptor ligands are subsequently functionalized to the AuNPs. During detection, target analytes bind directly to the receptor ligands. Formation of analyte-receptor complexes induces a local change of refractive index near the sensing surface, producing an optical response proportional to the mass concentration of the bound target analytes.
Enhancement assays require the use of two binding partners. Capture receptors (RC) are fixed onto the unclad optical fiber while detection receptors (RD) are bound to the AuNPs. During a sensing event, RD-functionalized AuNPs are introduced to the RC-functionalized sensing surface with the target analytes (A). AuNPs are brought to the sensing surface through formation of sandwich-like RD-A-RC complexes. Presence of AuNPs at the sensing surface results in a dramatic optical response.
Competition assays are suitable for the detection of small molecules. In this detection scheme, the AuNPs-functionalized sensing surface is modified with target analytes. In a sensing event, a sample of free-flowing analytes are premixed with corresponding bioreceptors followed by introduction to the sensing surface. Free-flowing analytes and bound analytes then "compete" for binding with the bioreceptor. Binding of bioreceptor to the bound analyte results in an optical response.
The rate and strength of interactions between molecular species are important in biochemical research. These properties are measured through the kinetic and affinity constants, respectively, of the analyte of interest. The label-free and real-time monitoring of interactions between molecular species through FOPPRTM produces binding kinetic curves unique to the specific analytes. Kinetic and affinity information can be calculated through post-process data deconvolution and curve fitting.