The use of mixed self-assembled monolayers as a strategy to improve the efficiency of carbamate detection in environmental monitoring

We report a simple strategy to obtain an efficient enzymatic bioelectrochemical device, in which the enzyme acetylcholinesterase (AChE) was immobilized on gold electrodes functionalized with mixed self-assembled monolayer (SAM(mix)) of 11-mercaptoundecanoic acid (11-MUA) and 2-mercaptoethanol (C2OH). Development of the modified electrodes included the chemical adsorption of SAM(mix) on gold surface followed by immersion in AChE solution, resulting in the final Au/SAM(mix)/AChE configuration. For comparison, the electrochemical performance of Au/11-MUA/AChE and Au/C2OH/AChE electrodes were also investigated. The performance of the modified electrodes toward acetylthiocholine hydrolysis was investigated via cyclic voltammetry and chronoamperometric measurements, revealing a fast increase in anodic current with a well-defined peak upon addition of acetylthiocholine iodide to the electrolytic solution. The anodic currents for Au/SAM(mix)/AChE electrodes were significantly higher than those for Au/11-MUNAChE and Au/C2OH/AChE electrodes. The Au/SAM(mix)/AChE architecture provided by the SAM(mix) surface promoted a high oxidation current of thiocholine at 0.31 V without the need of electron mediators. The chronoamperometric biosensor developed here provided a linear response to carbaryl in the concentration range of 0 to 1.75 mu M. The detection limit and quantification values for carbaryl were found to be 3.45 x 10(-10) M and 1.15 x 10(-9) M, respectively. Michaelis-Menten kinetics, K-M(app), of 0.46 mM was obtained, indicating that the electrode architecture employed is advantageous for fabrication of enzymatic devices via physical adsorption process with improvement of the biocatalytic properties. (C) 2012 Elsevier Ltd. All rights reserved.