Achieving high sensitivity is a major goal in biological assays. Highly sensitive methods that allow detection of disease markers present at low levels during the early stages of disease progression could potentially increase the chance to treat various diseases.
Nanoparticles and enzymes have been employed as amplification tags for sensitive detection of DNA, proteins and pathogens. An electrochemical (EC) transducer has been used to convert the signal readout, owing to its fast, simple and low cost characteristics.
Analogous to the unique optical properties of quantum dots (QDs)-sized tunable fluorescent emission, the EC signatures of QDs provide the capabilities as EC tags for quantitation and multiplexing of biological species. The well-resolved peaks with different potentials and height reflect the identity and quantity of the target analytes respectively.
Colloidal gold nanoparticles were employed as labels in an immunoassay of mouse Immunoglobulin (IgG). The signal amplification was achieved through the nanoparticle-promoted precipitation of silver onto the gold nanoparticle tags. The silver layer was dissoluted and potentiometrically measured, whose potential output was related to the concentration of mouse IgG in the sample solution.
Prussian blue nanoparticle-encapsulated polystyrene microbeads were first used as labels for sensitive EC detection of DNA. Each single bead is capable of carrying a large number of reporters. Multiple EC tags, instead of one single tag, were involved in a single DNA hybridization event, resulting in a dramatically amplified signal readout.
A novel strategy coupled with the DNA polymerase chain reaction (PCR) technique was developed for highly sensitive detection of protein. The target protein was sandwiched between two DNA aptamers and the guanine rich aptamer was then released and amplified by the PCR. The target protein was measured down to 5.4 fM by monitoring the sensitive chronopotentiometric stripping response of guanine from the PCR amplified product.
The surface chemistry optimization for rapid, sensitive EC detection of pathogen is also discussed. Factors that affect the performance of this bioassay, including washing, surface blocking and different compositions of self-assembled mixed layers were systematically evaluated.