The Effect of Counterions on the Detection of Cu2+ Ions in Aqueous Solutions Using Quartz Tuning Fork (QTF) Sensors Modified with L-Cysteine Self-Assembled Monolayers: Experimental and Quantum Chemical DFT Study

In this study, a sensing device employing gold-coated quartz tuning fork (QTF) modified with self-assembled monolayer (SAM) of L-cysteine was evaluated for the sensitive detection Cu2+ ions in aqueous solutions. Three copper (II) salts, CuSO4, CuCl2, and Cu(NO3)2, at four different concentrations (10−12, 10−10, 10−8, 10−6 M) small (100 μL) water sample amounts were each used as analytes to investigate influence their counterions ions. It found that, among counterions, sulfate anion had largest effect upon water, following order: SO42− > Cl− NO3−. The lower limit detected 10−12 M range. frequency shifts measured QTFs relative deionized inversely proportional concentration/mass analytes. Density functional theory calculations conducted understand on respective electronic interaction energies apparent host–guest binding gold surface-bound molecules. Gas phase (both uncorrected BSSE) solution (ΔIE) calculated B3LYP/LANL2DZ ωB97XD levels showed that stability complexes [L-cysteine]⊃[CuSO4] [L-cysteine]⊃[CuCl2] [L-cysteine]⊃[Cu(NO3)2], which supports our experimental findings, they same order experimentally observed salts tested: CuSO4 CuCl2 Cu(NO3)2.