A label-free fluorescent peptide probe for sensitive and selective determination of copper and sulfide ions in aqueous systems

Abstract

A label free fluorescent peptide probe (HDSGWEVHH) was used for Cu²⁺ and S²⁻ determination in aqueous solution. Our results demonstrated that HDSGWEVHH is highly selective and sensitive for monitoring free Cu²⁺ concentration via quenching of the probe fluorescence upon Cu²⁺ binding. The mechanism of the complexation is investigated with Cyclic Voltammetry (CV), ¹H nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) spectroscopy and computational techniques. Theoretical calculation results indicated the binding ratio of the probe to Cu²⁺ is 2 : 1 and the binding constant was obtained as 1.72 × 10 ⁸ M⁻¹. Cu²⁺ concentration can be detected with the detection limit of 16 nM. Free Cu²⁺ concentration released from the metallothionein–Cu complex at different pH values was detected. Cu²⁺ concentration in real water and tea samples was also detected, and the results were consistent with the ones monitored by atomic absorption spectrometer. Because of the exceedingly small K_sp value of CuS (1.27 × 10⁻³⁶), S²⁻ can sequester Cu²⁺ from HDSGWEVHH to restore the tryptophan (W) fluorescence. Thus the HDSGWEVHH–Cu²⁺ complex can also be used for S²⁻ detection. The S²⁻ concentrations can be monitored with a detection limit of 19 nM. The assay is also amenable to measurement of S²⁻ concentration in pure water samples. Thus the probe designed herein is sensitive, label free, low cost, and environmentally friendly for Cu²⁺ and S²⁻ determination in aqueous solutions.