Environmental Copper Sensor Based on Polyethylenimine-Functionalized Nanoporous Anodic Alumina Interferometers.

Anthropogenic copper pollution of environmental waters from sources such as acid mine drainage, antifouling paints, and industrial waste discharge is a major threat to our environment and human health. This study presents an optical sensing system that combines self-assembled glutaraldehyde-cross-linked double-layered polyethylenimine (PEI-GA-PEI)-modified nanoporous anodic alumina (NAA) interferometers with reflectometric interference spectroscopy (RIfS) for label-free, selective monitoring of ionic copper in environmental waters. Calibration of the sensing system with analytical solutions of copper shows a linear working range between 1 and 100 mg L-1, and a low limit of detection of 0.007 ± 0.001 mg L-1 (i.e., ∼0.007 ppm). Changes in the effective optical thickness (ΔOTeff) of PEI-GA-PEI-functionalized NAA interferometers are monitored in real-time by RIfS, and correlated with the amount of ionic copper present in aqueous solutions. The system performance is validated through X-ray photoelectron spectroscopy (XPS) and the spatial distribution of copper within the nanoporous films is characterized by time-of-flight-secondary ion mass spectroscopy (TOF-SIMS). The specificity and chemical selectivity of the PEI-GA-PEI-NAA sensor to Cu2+ ions is verified by screening six different metal ion solutions containing potentially interfering ions such as Al3+, Cd2+, Fe3+, Pb2+, Ni2+, and Zn2+. Finally, the performance of the PEI-GA-PEI-NAA sensor for real-life applications is demonstrated using legacy acid mine drainage liquid and tap water for qualitative and quantitative detection of copper ions. This study provides new opportunities to develop portable, cost-competitive, and ultrasensitive sensing systems for real-life environmental applications.