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Optical biosensing is an application technology to detect the chemical and biological components in system, which is based on the selective expression of the optical signal from the system. Optical biosensors consist of sensing devices based on spectral chemistry, optical waveguide spectroscopy and measurement techniques, and targets are analyzed using the spectral informations from them based on the absorption, reflection, fluorescence or chemiluminescence, refraction, scattering and other optical properties. Since its rapid, sensitive, accurate and high selectivity, optical biosensors are widely used. Gold nanoparticles（AuNPs） are the focus of nanomaterials in recent years, in addition to surface effect, quantum size effect and macroscopic quantum tunneling effect and other characteristic properties of nanomaterials, its excellent optical properties, electrical properties and good biocompatibility have made AuNPs been widely used in the biochemical analysis, its applications in gene regulation, drug screening, small molecules and protein detection have been studied successfully. In this article, we construct optical biosensors based on AuNPs to achieve the simple, rapid and accurate detection of analyte.1. In this chapter, we utilized the color change of AuNPs system caused by target protein to establish a colorimetric method for the detection of the target protein. Specific binding would occur between specific binding protein and its recognition sites, we chose TATA-binding protein as a target protein, and labeled the two single-stranded DNA on the AuNPs, and TATA binding protein recognition sites existed in the complementary sequence from the two single strands. A red-to- purple color change would occur followed the hybrid of DNA tethered on AuNPs, and color remained red with the enzyme digestion by Exo III, but when the target protein existed, which could prevent AuNPs from enzyme digestion by Exo III due to specific binding with recognition sites in the hybrid parts, the system was still purple, so we could achieve the detection of TATA-binding protein according to this phenomenon. The results showed that the sensor can achieve the sensitivity detection of TATA binding protein, and had a linear detection range from 0 to 120 nM, with a detection limit of 10 nM.2. This chapter was based on that the existence differences of AuNPs would cause different effects on surface enhanced Raman scattering （SERS）. Single-stranded DNA （ssDNA） could be adsorbed on AuNPs surface easily, so it would protect AuNPs from salt-induced aggregation, but double-stranded DNA （dsDNA） had no this ability. When the targets existed, due to the reaction with the ssDNA, ssDNA became dsDNA or hairpin formation and released from AuNPs surface, then AuNPs aggregated without the protection of ssDNA in the presence of salt. AuNPs in different states had different SERS enhancement effects, which was the basis of detection for DNA and Hg2+. Experimental results showed that the method could be used to achieve the detection of DNA, Hg2+. For DNA system, the detection range was from 50 to 300 nM, and the detection limit was 75 nM; for Hg2+ system, the detection range was from 0 to 3.0μM, and the detection limit was 8 nM.3. AuNPs could quench the fluorescence from the fluorophore near AuNPs surface, and the presence of targets would eliminate this quenching, we could achieve the detection of targets based on the change of fluorescence intensity. We labeled a peptide chain with a fluorophore in the end on the AuNPs, and the peptide contained the reaction sequence of Caspase-3 protease. Because of the quenching effect of AuNPs, the fluorescence was very small, but in the presence of Caspase-3, due to the reaction with peptide chain, the peptide chain terminal portion with the fluorophore was away from AuNPs surface, and fluorescence recovery occurred, so we could detect Caspase-3 according to the fluorescence change. The results showed that the sensor could achieve sensitive detection of Caspase-3, the linear detection range was from 0 to 0.6μg /μL, and the detection limit was 0.015μg /μL. We applied this method to living cells for the detection of Caspase-3, and the results showed the good feasibility.
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