Colorimetric recognition and sensing of thiocyanate with a gold nanoparticle probe and its application to the determination of thiocyanate in human urine samples

A colorimetric method for the determination of thiocyanate (SCN−) ion with a cystamine-modified gold nanoparticle (Au NP) probe is presented. In this method, recognition is based on electrostatic attraction and directional hydrogen bonding between thiocyanate and cystamine on the surface of an Au NP. In phosphate buffer solution (PBS, pH 5.2, 10 mM), the cystamine-modified Au NPs readily aggregated upon incubation with N,N-dimethyl-1-naphthylamine (denoted “2N”), and a visible change in the color of the solution from red to blue was observed. When present, thiocyanate interacted with the gold nanoparticle probe more prominently than 2N, thereby protecting the gold nanoparticles and attenuating the degree of aggregation. The solution was observed (by the naked eye) to change in color from blue to purple and then back to red as a function of thiocyanate concentration (<10 μM). Iodide was noted to be a significant interferent; however, the optical absorption spectrum in the presence of iodide was fortunately easily distinguished from that for thiocyanate, thereby making it possible to discriminate iodide from thiocyanate. It was possible to determine thiocyanate in human urine samples using this method. This colorimetric method opens up a new avenue for assaying thiocyanate considering its rapid readout and simple implementation, which makes it convenient to determine thiocyanate in biological samples, especially at levels below 100 μM.