Nanoporous Anodic Alumina Rugate Filters for Sensing of Ionic Mercury: Toward Environmental Point-of-Analysis Systems

Herein, we present an ultrasensitive, cost-competitive, and portable optical sensing system for detecting ionic mercury in environmental water. This analytical system combines structurally engineered and chemically modified nanoporous anodic alumina rugate filters (NAA-RFs) with reflection spectroscopy (RfS). The sensing performance of the proposed system is assessed through several tests, establishing its sensing performance (i.e., linear working range from 1 to 100 μM of Hg²⁺, low limit of detection 1 μM of Hg²⁺ ions (i.e., 200 ppb), and sensitivity of 0.072 nm μM^–1), chemical selectivity (i.e., exposure to different metal ions Co²⁺, Mg²⁺, Ni²⁺, Cu²⁺, Pb²⁺, Fe³⁺, Ca²⁺, Cr⁶⁺, and Ag⁺) and metal ions binding mechanism (i.e., fitting to Langmuir and Freundlich isotherm models). Furthermore, the detection of Hg²⁺ ions in tap and environmental water (River Torrens) is successfully carried out, demonstrating the suitability of this system for developing environmental point-of-analysis systems.