Polymeric membrane-modified voltammetric sensors for lipophilic analytes with nanomolar detection limit: Key parameters influencing the response characteristics

Highlights: • The partition coefficient of the analyte between the aqueous sample and the organic membrane. • The membrane volume to sample volume ratio. • The binding constant of constituents in the sample that bind the analyte. • Understanding of the influence of the sensor design parameters on the overall sensor response. Thin plasticized PVC membrane-coated glassy carbon working electrodes have been used for the voltammetric measurement of highly lipophilic, electroactive drugs. Compared to conventional working electrodes, these membrane-coated electrodes exhibit remarkable detection limit and selectivity and are less prone to electrode fouling. The unique performance characteristics of these sensors are related to the large partition coefficient of the analyte in the membrane coating where it is oxidized in a non-aqueous membrane phase. To analyze the influence of the key parameters of the response of membrane-coated sensors, we derived theoretical expressions on the voltammetric response of the sensors. In our analysis we considered 1) the partition coefficient ($P_{mw}$) of the analyte between the aqueous sample and the organic membrane, 2) the membrane volume to sample volume ratio ($V_m/V_w$), and 3) the binding constant of constituents in the sample that bind the analyte ($K$). The results of our theoretical analysis have been tested through voltammetric measurement of highly lipophilic analytes with $log{P}_{ow}$ values (logarithm of the partition coefficient between octanol and water) ranging between 0.3 and 7.5. By understanding of the influence of the sensor design parameters on the overall sensor response, these parameters can be tuned for optimized response slope, detection limit, etc., for solving specific analytical tasks.