Highly sensitive photometric determination of cyanide based on selective etching of gold nanorods

The authors show that gold nanorods (AuNRs) are viable optical nanoprobes for the determination of cyanide ions. The method is based on the measurement of localized surface plasmon resonance (SPR) absorption which strongly depends on the width-to-height ratio of AuNRs. AuNRs are selectively etched by cyanide at the transverse faces, this leading to a decrease in the AuNR aspect ratio and resulting in a blue-shifted SPR absorption and color change from peacock blue to pink. The mechanism of selective etching of the AuNR tips was studied, and optimum conditions for etching were established. The absorption ratio of transverse to longitudinal SPR absorptions (at 538 and 640 nm, respectively) was proportional to the concentration of cyanide in the range from 1.65 nM to 0.5 mM, with a 0.5 nM detection limit. This colorimetric assay is selective, sensitive, and has been successfully applied to quantify cyanide in spiked tap, pond, and waste water.