Easily Fabricated Carbon Nanotube Electrodes for Low-Ppb-Level Chromium(VI) Detection

Abstract

Due to its high toxicity, the detection of trace levels of hexavalent chromium, Cr(VI), has attracted much interest in analytical chemistry. Several sensitive techniques have been developed for the determination of Cr(VI), including spectroscopic and electrochemical techniques. For electrochemical detection, mercury electrodes have been demonstrated to be very sensitive for the detection of Cr(VI). The potential toxicity of mercury electrodes, however, limits its practical application. As another good option, gold has become more and more popular, and has also demonstrated to exhibit more electrochemical reversibility than carbon electrodes. However, considering the price and fabrication feasibility for gold, carbon electrodes are still of interest due to the lower price and easy assembly. Recently, Hallam, et al. 1 developed graphite screen printed electrodes for the electrochemical detection of Cr(VI). However, the sensitivity was only ~0.8 nA/ppb and the detection range was 100 to 1000 ppb with a detection limit of 19 ppb, which is much less than those for gold electrodes.2,3 We believe that using nanostructured carbon electrodes can potentially improve these characteristics to match the performance of gold electrodes, resulting in a sub-ppb detection method for Cr(VI). Here we describe a facile and printable technique to fabricate the electrodes with carbon nanotubes (CNT) for the electrochemical detection of Cr(VI). The electrodes showed currents as high as 500 nA/ppb Cr(VI) for large CNT/filter paper flexible electrodes and ~5 nA/pbb Cr(VI) for CNT/printed electrodes. The limit of detection was shown as low as 5 ppb Cr(VI). The electrode could be deposited on multiple substrates, including FTO glass, flexible gold coated plastic, commercial printed electrodes. We also developed a flow detection system with CNT/printed electrodes, which could detect Cr(VI) continuously and easily similar to a microfluidic system.