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A Multi-Walled Carbon Nanotube-based Biosensor for Monitoring Microcystin-LR in Sources of Drinking Water Supplies
Citation:
Han, C., A. Doepke, W. Cho, V. Likodimos, A. Delacruz, T. Back, W. Heineman, H. Halsall, V. Shanov, M. Schulz, P. Falaras, AND D. Dionysiou. A Multi-Walled Carbon Nanotube-based Biosensor for Monitoring Microcystin-LR in Sources of Drinking Water Supplies. Advanced Functional Materials: Americas . Wiley Interscience, Malden, MA, 23(14):1807-1816, (2013).
Impact/Purpose:
A multi-walled carbon nanotube-based electrochemical biosensor is developed for monitoring microcystin-LR (MC-LR), a toxic cyanobacterial toxin, in sources of drinking water supplies. An extremely sensitive Faradaic electrochemical impedance biosensor for monitoring microcystin-LR in sources of drinking water supplies was developed using mm-long MWCNT arrays grown by water-assisted chemical vapor deposition with vertical alignment on catalytically patterned Si substrates. SEM and HR-TEM analysis showed vertically well-oriented MWCNTs and well-defined, multi-walled CNTs in the array. High temperature thermal treatment (2500oC) in an inert environment (Ar atmosphere) was exploited to anneal inherent structural defects and enhance the crystallinity of the pristine MWCNTs arrays. The MWCNT electrodes were functionalized by electrochemical oxidation in NaOH solution in order to produce oxygenated surface groups. A high degree of oxygen functionalization was achieved according to systematic cyclic voltammetry, micro-Raman spectroscopy, X-ray photoelectron spectroscopy, optical microscopy, and Faradaic EIS measurements. Specifically, cyclic voltammetry revealed well-defined redox peaks in the presence of redox species, while Faradaic EIS measurements showed a marked decrease of the Faradaic electrochemical impedance for the functionalized MWCNT arrays due to their improved wettability, directly evidenced by optical microscopy. Analysis of the Raman spectra showed the presence of oxygen-containing functional groups on the functionalized MWCNT electrodes after high temperature treatment by enhancing distinct defect-activated Raman bands as a function of the NaOH concentration. A significant increase of oxygen up to 25% atomic concentration was determined by XPS analysis after the MWCNT electrodes were functionalized. The oxygenated surface groups provided the sites for linking agents and subsequent attachment of specific antibodies and toxin molecules. A two-step linking procedure enabled the immobilization of MC-LR onto the functionalized MWCNT array electrodes, and conjugation of monoclonal antibodies specific to MC-LR in the incubation solutions that provided the required specificity for detecting toxin. Validation and performance evaluation of the MWCNT array biosensor used Faradaic electrochemical impedance spectroscopy, which showed a clear increase of the electron-transfer resistance after MC-LR and antibody conjugation. A linear sensing response was thereby established over a wide MC-LR concentration range (0.05 to 20 μg•L-1) that allowed toxin detection well below the WHO provisional guideline limit of 1 μg L-1 for MC-LR in drinking water.
Description:
A multi-walled carbon nanotube-based electrochemical biosensor is developed for monitoring microcystin-LR (MC-LR), a toxic cyanobacterial toxin, in sources of drinking water supplies. The biosensor electrodes are fabricated using dense, mm-long multi-walled CNT (MWCNT) arrays grown on patterned Si substrates by water-assisted chemical vapor deposition. Scanning electron microscopy and high-resolution transmission microscopy show that the electrodes consist of vertically well-aligned MWCNTs arrays with a narrow size distribution. High temperature thermal treatment (2500 oC) in an Ar atmosphere (an inert environment) is used to enhance the crystallinity of the pristine materials. After the heat treatment, electrochemical functionalization of the MWCNT electrodes in alkaline solution is performed to produce oxygen-containing functional groups on the MWCNT surface. Micro-Raman spectroscopy, including polarized Raman measurements on the aligned MWCNT arrays, together with X-ray photoelectron spectroscopy, cyclic voltammetry, optical microscopy and Faradaic electrochemical impedance spectroscopy (EIS) confirm the creation of abundant oxygenated surface groups on the functionalized MWCNTs. These oxygen functionalities provide the anchoring sites for linking molecules that allow the immobilization of MC-LR onto the MWCNT array electrodes. For the determination of MC-LR, an indirect measurement approach is employed. Addition of the monoclonal antibodies specific to MC-LR in the incubation solutions provides the required sensor specificity for toxin detection. The performance of the MWCNT array biosensor is evaluated by Faradaic EIS measurements that demonstrate a marked increase of the electron-transfer resistance upon antibody conjugation. A linear dependence of the electron-transfer resistance on the MC-LR concentration is observed in the range of 0.05 to 20 μg•L-1, which enables cyanotoxin monitoring well below the World Health Organization (WHO) provisional concentration limit of 1 μg/L for MC-LR in drinking water.
