Liquid crystal sensor for Cr(III)-citrate detection via interfacial coagulation

Trivalent chromium (Cr(III)) and its highly soluble carboxyl complexes, often discharged into the environment by industries such as electroplating, leather tanning, and textile manufacturing, present severe risks to human health and ecosystems due to their high toxicity. These compounds are notoriously difficult to detect and remove during wastewater treatment, as they can persist in aqueous environments. Consequently, there is a pressing need for the development of simple, cost-effective, and reliable methods for their detection, which can improve monitoring, facilitate timely interventions, and enhance environmental protection efforts.

In this study, we developed a liquid crystal (LC)-based sensor for detecting Cr(III)-citrate in aqueous environments. The sensor utilizes the amphiphilic ligand tributylhexadecylphosphonium bromide (THPB), which is strategically doped into the LC matrix. When subjected to polarized optical microscopy, the THPB-doped LC displayed a transition from a dark to a bright optical state specifically in the presence of Cr(III)-citrate, demonstrating high selectivity over other metal ions, anions, chelating groups and metal-citrate complexes. Comprehensive analyses at both bulk and molecular levels demonstrated that this notable optical transition is facilitated by strong electrostatic interactions between THPB and Cr(III)-citrate, resulting in interfacial coagulation at the LC/aqueous interface. Impressively, the sensor achieved a detection limit as low as 5 μM for Cr(III)-citrate in water, significantly below the regulatory limits set for industrial discharge.

This work addresses the urgent need for simple, effective analytical techniques to detect and monitor Cr(III)-citrate in aqueous environments without relying on complex instrumentation, making it ideal for a promising tool for on-site environmental monitoring and assessment. It also highlights the broader potential of LC-based sensors for efficient environmental monitoring of other metal ion complexes.