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  Chemical Sensors 2011, 1: 10
  Research Article
Application of 1, 4-benzenedimethanethiol self-assembled monolayer on gold piezoelectrode as a sensor for determination of Hg(II)
  Abdunnaser M. Etorki, Elfituri Elakkari, Elhamassi Ali  
Department of Chemistry, University of Al-Fateh, PO-Box-13203, Tripoli, Libya
  We describe the development of a combined acoustic wave and electrochemical sensor for the detection of mercury ions from aqueous solutions based on the formation of self-assembled monolayer of 1,4-benzenedimethanethiol (BDMT) films. Gold electrodes on 10 MHz AT-cut piezoelectric quartz crystal resonators were modified with 1,4-benzenedimethanethiol. Measurements of resonant frequency for bare and modified electrodes, both immersed and emersed, were used to determine the extent of the ligand immobilization; since the films are extremely thin, the frequency response can be interpreted in purely gravimetric terms using the Sauerbrey equation. The quality and integrity of the BDMT films were assessed by cyclic voltammetric measurements of [Fe(CN)6]-3,-4 redox couple chemistry. BDMT-functionalised electrodes were exposed to solutions of mercury ions in order to assess the ability of the surface-bound ligand to complex the Hg(II) ions. The co-exiting metal ions such as Pb2+, Cd2+, Cu2+, Zn2+, Ni2+ and Ag+ were examined with the same ligand, single Hg(II) ion solutions were used and the extent of Hg(II) complexation was determined (via the QCM frequency change) as a function of concentration. Factors associated with solvent and solvation were explored via parallel sets of measurements under immersed and emersed conditions. The variation of surface concentrations with solution concentration was evaluated using a range of isotherms-Langmuir, Temkin, Freundlich, Frumkin, El-Awady and Flory-Huggins- based on different physical models. Selectivity for different metal ions was assessed by comparing the binding constants derived from the isotherms for single Hg(II) ion solutions and by competitive binding experiments using mixed metal ion solutions. The extent of Hg(II) uptake was also evaluated coulometrically by integration of voltammetric responses of electrodes exposed to Hg(II) ion solution and transferred to metal free background electrolyte. Interestingly, the extent to which the Hg(II) ion species are retained on the surface during redox cycling varies for different metals. The combination of (BDMT) SAM (self-assembled monolayer) selectivity and QCM sensitivity provides a potentially useful metal ion sensor in the nanomolar range. Supporting XPS measurements provide unequivocal evidence of the presence of surface- bound metal(s), together with metal speciation (following various electrochemical manipulations), metal:ligand stiochiometry, and selectivity between metal ions in competitive binding experiments.
  Gold piezoelectrode; Mercury determination; 1, 4-benzenedimethanethiol; Self-assembled monolayer  

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