Self-assembled monolayers of L-cysteine on gold and DGT devices for the determination of potentially bioavailable-copper and total copper in natural waters by square-wave voltammetry

An electrochemical sensor for the detection of potentially bioavailable-copper and total copper in natural waters was prepared by adsorbing a self-assembled monolayer (SAM) of L-cysteine (1 mm in 0.05 M phosphate buffer, pH5.5) on the surface of a gold mini-electrode for 30 min. The modified electrode surface was stable for at least 2 days.Preconcentration of Cu(II) in phosphate buffer was held at open-circuit potential for 5 min under stirred conditions, forming a copper-cysteine complex on the electrode. Square-wave voltammetry (SWV) was performed from +600 to -200 mV at 75mV s-1 in quiescent solution after 30 s rest time; the electrochemical reaction which provided the analytical current signal was the reversible reduction of Cu(II) to Cu(I). The gold-SAM L-cysteine electrode showed good sensitivity, selectivity, precision, recovery and accuracy for copper in the presence of a large amount of other metals and potential interferences. The electrochemical response was linear (R2 = 0.970) between zero (blank) to 0.4 mg L-1 Cu(II); the limit of detection and limit of quantitation were 2 and 6 μg L-1, respectively.The performance of the electrode was tested in real samples. Diffusive gradient in thin film (DGT) devices (which were used to simulate the lipid bilayer in biological tissues) contained a filter membrane (to remove particles), a hydrogel diffusion layer and a sorption (Chelex-100) layer, which accumulated metal ions that represented the potentially-bioavailable metal fraction [1]. DGT devices were deployed about 15-20 cm below the water surface for 24-30 hours at two sites (8 km apart) along the Dee River (Mount Morgan, Queensland, Australia): one located upstream (at Dam No. 7) and the other downstream (at Kenbula Weir) of the decommissioned Mount Morgan Copper & Gold Mine. The accumulated copper were eluted in 1 M HNO3 and analysed on a gold-SAM L-cysteine electrode using the developed SWV procedure. Total copper content of grab water samples from the same sites were determined by SWV and by atomic absorption spectroscopy (AAS) or inductively coupled plasma- emission spectrometry (ICP-ES) for comparison.Dam No. 7 water samples contained 6.0±0.9 μgL-1 while Kenbula Weir 14800±1200 μgL-1 potentially-bioavailable Cu. These represented 80% and 40% of their total copper content (7.5 ±0.7 and 36400±280 μgL-1, respectively). The Kenbula Weir is highly impacted by acid-mine drainage with extremely elevated copper levels relative to the Australian water quality trigger value of 8.0 μg L-1 [2].The portable laboratory instrumentation used in the study was successfully tested at the field, powered by a sealed lead-acid battery, providing attractive in-situ monitoring for immediate yet reliable analytical data.References[1] H. Zhang and W. Davison (1995). Performance characteristics of diffusion gradients in thin films for the in-situ measurement of trace metals in aqueous solution. Anal. Chem. 67:3391-3400.[2] Australian and New Zealand Environment and Conservation Council (ANZECC) (2000). National Water Quality Management Strategy – Australian Water Quality Guidelines for Fresh and Marine Waters.AcknowledgmentsA grant from the CQU Faculty of Sciences, Engineering & Health is gratefully acknowledged.