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

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 μL) 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 (ΔIE) calculated at the B3LYP/LANL2DZ and ωB97XD levels of theory showed that the stability for the complexes were in the following order: [L-cysteine]⊃[CuSO4] > [L-cysteine]⊃[CuCl2] > [L-cysteine]⊃[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.

Optimized Design of Lithium Niobate Tuning Forks for the Measurement of Fluid Characteristic Parameters

“…The dimensions of the LNTF can be optimized based on the energy loss as being represented by the reciprocal of the quality factor; the greater the quality factor, the less energy is lost. The total energy dissipation of the LNTF in the fluid is 1 Q tot . Material support loss 1 Q s , fluid viscosity loss 1 Q v , and acoustic radiation loss 1 Q a are the three basic factors that contribute to the total energy dissipation [18].…”

“…The total energy dissipation of the LNTF in the fluid is 1 Q tot . Material support loss 1 Q s , fluid viscosity loss 1 Q v , and acoustic radiation loss 1 Q a are the three basic factors that contribute to the total energy dissipation [18].…”

“…Equation (1) gives the resonant frequency of the LNTF in the air. Figure 2 gives the relationship between the resonant frequency of LNTF and the length and width of its arm (the identity lg on the vertical coordinate of the graph is a log function with base 10 that is monotonically increasing from 0 to ∞).…”

“…Measuring the density and viscosity of fluid is widely needed in the industrial field and useful for guiding production steps. For example, in the production or synthesis of various chemicals, monitoring the change of their density and viscosity under high temperature and pressure is very important because the changing trend can greatly affect the quality of the chemicals [1]. Moreover, the measurement is also useful in other fields, including transformer oil quality monitoring [2], the start-up vibration characterization of space shuttles [3], wine brewing monitoring, bioconcentration detection in the biomanufacturing business [4], and so on.…”

Label Free Detection of Programmed Death Ligand 1 Protein Biomarker by Quartz Tuning Fork-Based Biosensor

“…Biosensors based on quartz tuning forks (QTF) offer an effective alternative while overcoming various issues with traditional sensors. [14][15][16][17] In comparison, the QTF-based biosensor is low cost, simple, rapid, highly sensitive and inexpensive. 17,18 QTF is an acoustic resonator constructed of two forks made of piezoelectric quartz crystal.…”