Ultrasensitive electrochemical sensors based on Cu and Cu@Ag nanorods for simultaneous heavy metal detection

This work reports the development of ultrasensitive miniaturized electrochemical device for heavy metal sensing. A laser engraver based patterning of fluorine-doped tin oxide (FTO) sheet was done to draw an etched pattern forming a miniaturized 3-electrode configuration. A layer of Ag/AgCl ink served as pseudo-reference electrode. The sensing electrode was coated using low-cost Cu nanorods (CuNRs) grown radially along the {110} surface with aspect ratio of 8.0 and Cu@Ag core-shell nanorods (Cu@AgNRs) formed via galvanic displacement for simultaneous electrocatalytic detection of heavy metal ions (Pb(II), Cd(II), Hg(II), and Zn(II)) present in water. The electroactive surface area of the prepared devices is 0.026, 0.093 and 0.125 cm2 for bare FTO, CuNRs/FTO and Cu@AgNRs/FTO, respectively. Bimetal Cu@AgNRs/FTO sensor exhibited the lowest limit of detection of 1, 2, 5 and 6 nM, respectively, detecting Cd(II), Pb(II), Zn(II), and Hg(II) ions, and it was 2, 2, 3 and 4 nM, respectively, for simultaneous detection of Zn(II), Pb(II), Cd(II) and Hg(II). The Cu@AgNRs/FTO based device showed distinct peak-to-peak separation by 0.40, 0.25 and 0.51 V for Zn(II)-Cd(II), Cd(II)-Pb(II) and Pb(II)-Hg(II), respectively. This device was highly sensitive (583.6–1261.8 μA․μM−1․cm−2) for heavy metal detection over CuNRs/FTO (15.9–107.4 μA․μM−1․cm−2). The Cu@AgNRs/FTO based sensors demonstrated good reproducibility (relative standard deviation ≤ 5%) with recovery (>90%) in the case of all target heavy metals simultaneously present in environmental water samples. Hence, the Cu nanorods based miniaturized sensing platforms developed in the present study for simultaneous heavy metal detection are potential low-cost alternatives providing a repeatability of upto 4 cycles unlike the commercial screen-printed electrodes.