Individually Addressable Gel-Integrated Voltammetric Microelectrode Array for High-Resolution Measurement of Concentration Profiles at Interfaces

Abstract

The application of a novel voltammetric probe, based on an individually addressable gel-integrated microelectrode array (IA-GIME), for real-time, high-spatial resolution concentration profile measurements at interfaces is described. Reliability and validity of steep metal concentration gradients obtained with this novel system have been demonstrated by performing systematic tests at well-controlled liquid-liquid and liquid-solid interfaces. The liquid-liquid interface was formed by two layers of aqueous solutions with different components; only one layer contained trace metal ions (Pb(II) and Cd(II)); the individually addressable microelectrode array was placed at the interface of the liquid-liquid system; the concentration profiles were recorded as function of time; and the effective diffusion coefficients were calculated. The liquid-"solid" interface was formed from an aqueous solution layer overlying a bed of silica particles saturated with an aqueous solution. The sensor array has been used to monitor the diffusion processes of Tl(I) or Pb(II) from the liquid phase to the "solid" phase. The influences of porosity, geometry of the porous media, and complexation between metal ion and silica, on the diffusion processes, have been studied. All these results show that correct diffusion profiles of metal ions at interfaces can be obtained with 200-microm resolution with the IA-GIME. They also demonstrate that, for measurements in "solid" phase, the aforementioned factors must be considered carefully for correct calibration of any electrodes and the gel-integrated microelectrodes are unique tools to enable calibration of the sensors with synthetic solutions.

Real-Time Electrochemical Monitoring: Toward Green Analytical Chemistry

“…14 Buffle's group has also made pioneering contributions to in situ measurements of trace metals 15 and recently described a submersible and robust voltammetric in situ profiling (VIP) system (based on advanced microprocessor and telemetry technology) 16 and a novel voltammetric microelectrode array for real-time monitoring of the fluxes of metals across the sediment-water interface (in connection to an effective protective agar coating). 17 Such surface protection is crucial for achieving the system robustness (essential for a long-term in situ monitoring) in the presence of coexisting surface-active constituents. Metal speciation studies in non-saline inland water can greatly benefit from the minimization of resistance effect at ultramicroelectrodes (that obviates the need for an external electrolyte).…”

Application of Sphere Cap Mercury Microelectrodes and Scanning Electrochemical Microscopy (SECM) for Heavy Metal Monitoring at Solid/Solution Interfaces

“…If the volume of the pool is small, as compared with the depletion zone, which develops during the preconcentration step, hindered and restricted diffusion may occur [43], both because of the soil particles packing around the microelectrode, and soil physical characteristics such as porosity and tortuosity of pores [4,5,10,14,44,45]. These facts would cause a sensible decrease of the effective diffusion coefficients of the metal ions within the soil, ultimately leading to a decrease of the stripping current, even if the metal ion concentration were effectively larger.…”

“…On average, the current recorded in the soils was about 0.34 (AE 0.3) times that of the aqueous phase. Since, under the latter pH condition, complexation reactions of metal ions with soil particles or organic matter, should be negligible [14,20], the current decrease should reflect, essentially, the physical characteristics of the soil or hindered diffusion in the mass transport during the preconcentration step.…”

“…In situ investigation of these phenomena, therefore, requires microanalytical systems that possess spatial resolution, other than sensitivity, selectivity and accuracy [14].…”

“…In addition, the use of microelectrodes reduces the requirement of a well-defined cell geometry, thus leading to the simplification of the electrochemical apparatus [22]. A twoterminal system, in place of the classical and bulky three electrode cell apparatuses, is only needed, while the rapid achievement of steady-state conditions, typical of very small electrodes [22], provides a means to avoid forced convection when preconcentration steps are required for increasing the sensitivity of trace element analysis [14,23,25].…”