An electrically conductive, flow-through, fixed bed adsorption membrane (FCME) made of carbon nanotubes (CNT) installed in an electrochemical flow cell, was applied for the highly efficient adsorption and detection of organic pollutants. Three analytes with different chemical nature, i.e., parathion ethyl, tartrazine and diquat, were chosen as model systems to demonstrate the capabilities of the system. Adsorptive stripping voltammetry (AdSV) performed by the FCME provided directly the amount of analyte adsorbed; in contrast to an adsorption column, that monitors the effluent concentration. The adsorption capacity and kinetic constants were obtained by AdSV and were comparable with those predicted by the Thomas model. The FCME enabled the detection of nanomolar levels of tartrazine and parathion, and submicromolar levels of diquat with a linear range of three orders of magnitude. In addition to being a very sensitive analytical tool, FCME is an adsorption membrane that enables its simple electrochemical regeneration.
We studied the direct electron transfer (DET) between glucose and electrogenerated AuCl4- catalysed by glucose oxidase (GOx) and gold nanoparticles (AuNPs) by scanning electrochemical microscopy (SECM). Well-defined AuNPs were prepared and attached onto an insulating surface. Studying the current transients of a gold microelectrode held within a few microns from the surface revealed that the AuNPs interacted with the GOx and were crucial for the DET from the glucose. We investigated very carefully the effect of pH and the size of the AuNPs on electron transfer. AuNPs of 16, 40 and 80 nm diameter were applied. The kinetics of electron transfer was analysed by the Michael-Menten kinetic mechanism. Interestingly, we found that the fastest DET was exhibited by the 40 nm AuNPs at pH 3 and 5. At higher pH, electron transfer was better catalysed by the 80 nm AuNPs. This was rationalized by the effect of the pH on the enzymatic structure and the charge of the AuNPs.