An APTES-modified thiophene-based Schiff base microelectromechanical sensor for the analysis of Cu2+ ions in water: Counterion and pH effects: Experimental and DFT study

• Quartz tuning forks with thiophene Schiff base-modified APTES sensing layers. • LODs for CuSO 4 , CuCl 2 and Cu(NO 3 ) 2 were 13.9 fM, 295 fM, and 758 fM, respectively. • Resonance frequency shifts were: CuSO 4 (125 Hz) >CuCl 2 (113 Hz) >Cu(NO 3 ) 2 (103 Hz). • The presence of counterions directly influences the resonance frequency shifts of MQTFs. • Solution pH values directly impact the resonance frequency shifts of MQTFs. • The sensing layer structure on the quartz surface of the QTF was confirmed by XPS. Microelectrochemical quartz tuning fork (MQTF) sensors functionalized with 3-aminopropyl triethoxysilane-modified thiophene Schiff base (APTES-SB) sensing monolayers were evaluated for the sensitive detection of Cu 2+ ions in water. X-ray photoelectron spectroscopy (XPS) confirmed the formation of the sensing layers. Using minute sample volumes of dilute aqueous solutions of three different copper salts under various pH conditions their frequency shifts were measured with APTES-SB-MQTFs. The frequency shifts were found to be sensitive to both the pH and the Cu 2+ counterions and were in the order: CuSO 4 > CuCl 2 > Cu(NO 3 ) 2 , with corresponding limits of detection of 185, 296, and 758 fM. Since water or food contamination by heavy metals including Cu 2+ , in general, poses serious environmental and health risks sensitive and rapid methodologies to detect and quantify copper in water the study was extended in a preliminary study to detect Cu 2+ using real water samples relative to standards in deionized water. The sensing probe can respond to CuSO 4 in the presence of other cations (Ca 2+ , Mg 2+ , Na + and K + ) in a commercial bottled water sample. DFT calculations using B3LYP/LANL2DZ were conducted to elucidate the hypothetical host–guest sensing mechanisms of the individual metal ions with the ATPES-SB sensing molecules. The DFT-computed ΔIE int (kJ mole -1 ) trends are consistent with the experimental findings, indicating the same relative interaction magnitude order between the three tested analytes. The experimental data confirmed the sensitivity of the analytical methodology for the trace detection of Cu 2+ as a representative heavy metal ion in water. Variables such as pH and the counterions used in the test solutions were evaluated to determine the potential limitations of the methodology.