Voltammetric detection of organohalides by redox catalysis: Improved sensitivity by immobilisation within a cubic phase liquid crystal

Two combined strategies are reported for improving the sensitivity of organohalide detection by redox catalysis. These are, improvement of the second order rate constant (k) for catalytic reduction of the organohalide, and improvement of the rate of substrate diffusion. Values of k are calculated for both alkyl and aryl halides, from slow scan rate cyclic voltammograms in homogeneous solution. It is shown that a Zn(II) porphyrin exhibits higher catalytic rates than the previously used Co(II) porphyrin or Co(I) salen. Amperometric and rotating disk electrode studies of electropolymerised films of the Zn(II) porphyrin, reveal that at optimum thickness, mediator-substrate reaction and substrate diffusion are the rate limiting steps. Hence, immobilisation of the Zn(II) porphyrin within the more open structure of a cubic phase liquid crystal produces an increase in sensitivity of approx. 10 times, and lowers the limit of detection by one order of magnitude. The optimised sensor responds linearly to seven organohalides in the range 0.1 μM to 1.0 μM with a sensitivity of 6.95 A M^-1 cm^-2. Chronoamperometric experiments with a microdisk electrode show that the rate of charge transport in the cubic phase films (apparent diffusion coefficient, D_E = 5.65 × 10^-10 ± 0.11 × 10 ^-10 cm² s^-1) is faster than in the electropolymerised films (D_E = 3.64 × 10^-11 ± 0.02 × 10^-11 cm² s^-1). The variation of D_E with the concentration of Zn(II) in the cubic phase suggests that diffusion of charge is predominantly by electron self-exchange, rather than by physical movement.