Analysis of Electrochemically Elusive Trace Metals with Carbon Fiber Microelectrodes

There is great interest in rapidly monitoring metals of biological and environmental interest. Electrochemistry is traditionally a powerful tool for metal analysis but can be limited by its scope and low temporal resolution. The scope is limited by the potential window of the working electrode and rapid analysis is limited, in part, by the need for nucleation/growth for preconcentration. In prior work, we showed that a rapid equilibrium mediated preconcentration process facilitated fast scan cyclic voltammetry (FSCV) responses to Cu(II) and Pb(II) at carbon fiber microelectrodes (CFMs). In this manuscript, we apply this same principle to Ca(II), Al(III), Mg(II), and Zn(II), metal ions that are traditionally difficult to electroanalyze. We demonstrate FSCV reduction peaks for these four metals whose positions and amplitudes are dependent on scan rate. The adsorption profiles of these ions onto CFMs follow Langmuir's theory for monolayer coverage. We calculate the thermodynamic equilibrium constant of metal adsorption onto CFMs and find that these constants follow the same order as those previously reported by other groups on other activated carbon materials. Finally, a real-time complexation study is performed with ligands that have preference for divalent or multivalent ions to probe the selectivity of the real-time signal. We observe a linear relationship between formation constant ( k_f) and % change in the FSCV signal and use this correlation to, for the first time, report the k_f of an Al(III)-complex. This work demonstrates the versatility of FSCV as a method with capacity to extend the scope of rapid electroanalysis.