Redox voltammetry of sub-parts per billion levels of Cu2+ at polyaspartate-modified gold electrodes

An electrochemical sensor for the detection of Cu2+ is reported which incor porates poly-l-aspartic acid (PLAsp) with 32-96 aspartate units as a select ive ligand for the metal ion, PLAsp is covalently attached to a gold electr ode modified with a monolayer of 3-mercaptopropionic acid using carbodiimid e coupling via an N-hydroxysuccinimide (NHS) ester intermediate. The acid s ide groups and deprotonated peptide nitrogens on two aspartate moieties are thought to be primarily responsible for chelation of Cu2+, which remains b ound when reduced to Cu+. A consequence of the multiple binding points that are available with a polypeptide is the low detection limit. The lowest co ncentration detected was 3 nM (0.2 ppb) achieved with Osteryoung square wav e voltammetry. This detection limit compares favourably with that of ICP-OE S and previously reported cysteine-modified electrodes. Analysis of tap and lake water samples using the PLAsp-modified electrode agreed well with ICP -OES analysis of the same samples.