Simple and selective sensing of cysteine using gold nanoparticles conjugated with a thermoresponsive copolymer having carboxyl groups.

We describe a simple, yet selective cysteine sensor based on gold nanoparticles (AuNPs) conjugated with thermoresponsive copolymers, the carboxyl groups of which are incorporated. Copolymer-conjugated AuNPs, used as the cysteine sensor, in a solution form sediment when cysteine is added. Heating followed by cooling the solution induces phase transitions of the thermoresponsive copolymers, resulting in an acceleration of sedimentation of the copolymer-conjugated AuNPs. The absorbance of supernatants at 520 nm, which are ascribed to a surface plasmon band of discrete AuNPs, decays with increasing concentration of cysteine. Sedimentation of the copolymer-conjugated AuNPs is specific to cysteine. The addition of other popular amino acids, or ascorbic acid, causes no sedimentation of the AuNPs. The relationship between the absorbance of the supernatant at 520 nm and the cysteine concentration provides a sigmoidal profile at a concentration range between 1 x 10(-6) to 6 x 10(-6) mol dm(-3). The determination of cysteine in a supplement is achieved using an inflection point on the sigmoidal profiles.