Gold Nanoparticles in Bioanalytical Assays and Sensors

In this review we report two major applications of gold nanoparticles in the field of bioassay and sensing. The first application is a unique, sensitive, and highly specific immunoassay system for antibodies using gold nanoparticles. The assay is based on the aggregation of gold nanoparticles that are coated with protein antigens in the presence of their corresponding antibodies. Aggregation of the gold nanoparticles results in an absorption change at 620 nm that is used to calibrate the amount of antibodies. The effects of pH, temperature, and the concentration of protein A-coated gold nanoparticles on the sensitivity of the assay were investigated. A dynamic range of two orders of magnitude and a limit of detection of 1 μg/mL of anti-protein A were observed.

The second application of nanoparticles is luminescence nanosensors, which have their potential use as site-specific probes in samples of limited dimensions. Novel methods of nanosensor fabrication to obtain nanosensors with improved analytical properties are reported. A new approach for controlled synthesis of fluorescence nanosensors for pH measurements is also presented. Gold nanoparticles were used as a supportive matrix for the sensing component. Polymer layers that include the active sensing element were deposited on the gold nanoparticles surface using an electrostatic-based layer by layer deposition method. Polymer layers of alternating charges were deposited on the particle surface through attractive electrostatic interactions. Such method enabled a more precise control of the size, size distribution and density of fluorophores on each particle. The study shows that this is an effective way to fabricate particle-based fluorescent nanosensors that are stable and effective in measuring the pH in aqueous media.