Highly sensitive fluorescence detection of copper ion based on its catalytic oxidation to cysteine indicated by fluorescein isothiocyanate functionalized gold nanoparticles

An innovative fluorescence method for sensitive detection of copper ion (Cu 2+ ) was developed based on fluorescein isothiocyanate functionalized gold nanoparticles (FITC-AuNPs). Due to the stronger binding affinity of isothiocyanate functional group to gold, FITC molecules could adsorb on the surface of AuNPs, forming a simple fluorescence resonance energy transfer (FRET) system, and the fluorescence intensity of FITC was remarkably quenched. Upon adding cysteine, FITC could be displaced from the surface of AuNPs because the formation constant ( K f ) of Au–S linkage ( K f (AuS − )   =   4   ×   10 35 ) was much higher than Au–SCN linkage ( K f (Au(SCN) 2− )   =   10 23 ), leading to the recovery of fluorescence intensity. However, Cu 2+ could catalyze O 2 oxidation of cysteine, and the generated disulfide cystine could not remove FITC from AuNPs’ surface. Therefore, the recovery of fluorescence intensity was much weaker when compared with that of in the absence of Cu 2+ . And on the basis of this principle the concentration of Cu 2+ could be detected quantitatively. Under optimal conditions, our method exhibited high selectivity toward Cu 2+ and provided a good linear relationship in the range of 1.0–17.0   nM with the detection limit of 0.37   nM calculated by 3 σ / S . Furthermore, complicated synthetic procedures and poor water solubility could be ignored in this proposed fluorescent sensor.