Abstract
In this study, a sensing device employing a gold-coated quartz tuning fork (QTF) modified with a self-assembled monolayer (SAM) of L-cysteine was evaluated for the sensitive detection of 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), and 10(-6) M) in small (100 mu L) water sample amounts were each used as analytes to investigate the influence of their counterions in the detection of the Cu2+ ions. It was found that, among the counterions, the sulfate anion had the largest effect upon the detection of Cu2+ in water, in the following order: SO42- > Cl- > NO3-. The lower limit of detection of the Cu2+ ions detected was in the 10(-12) M range. The frequency shifts measured with the QTFs relative to deionized water were inversely proportional to the concentration/mass of the analytes. Density functional theory calculations were conducted to understand the effect of the counterions on the respective electronic interaction energies for the apparent host-guest binding of the analytes with L-cysteine and with gold surface-bound L-cysteine molecules. Gas phase (both with and uncorrected BSSE) and solution phase interaction energies (Delta IE) calculated at the B3LYP/LANL2DZ and omega B97XD levels of theory showed that the stability for the complexes were in the following order: [L-cysteine]superset of[CuSO4] > [L-cysteine]superset of[CuCl2] > [L-cysteine]superset of[Cu(NO3)(2)], which supports our experimental findings, as they were in the same order as the experimentally observed order for the copper salts tested: CuSO4 > CuCl2 > Cu(NO3)(2).