Cysteamine-decorated gold nanoparticles for plasmon-based colorimetric on-site sensors for detecting cyanide ions using the smart-phone color ratio and for catalytic reduction of 4-nitrophenol

In this paper, we have reported the cyanide ions (CN-) sensing in environmental water samples using cysteamine-capped gold nanoparticles (Cyst-AuNPs) by spectrophotometric, colorimetric, and smartphone-based RGB color detection. The surface plasmon resonance shift at around 525 nm for the Cyst-AuNPs could be used to detect quantitatively the amounts of CN- with concomitant alteration of their color from wine red to purple visualized by the naked eye. For the first time, the Cyst-AuNPs-based visual sensing of CN- was performed using smartphone-based detection with its detection limit of 159 × 10-9 M, ten times lower than that of the highest tolerance level (2 × 10-6 M) permitted by the world health organization. The Cyst-AuNPs displayed excellent specificity for detecting the concentration of 30 × 10-6 M even amid the presence of other interfering inorganic anions with their concentrations about five times higher than it. Environmental real water samples were used to arrange the three different CN- concentrations for plasmon-based colorimetric detection and smartphone-based method. Additionally, the catalytic performance of Cyst-AuNPs was demonstrated for the fast catalytic conversion of hazardous 4-nitrophenol (selected environmental contaminant) to the analogous amino aromatic compounds. A chemical kinetic study showed the conversion rate to be estimated as 1.65 × 10-2 s-1. Cyst-AuNPs can find an application in colorimetric sensing of CN- while being able to be utilized as a catalytic nanomaterial for ecological remedies associated with health care.