Guanidine-Functionalized Fluorescent sp 2 Carbon-Conjugated Covalent Organic Framework for Sensing and Capture of Pd(II) and Cr(VI) Ions.

Palladium is a key element in fuel cells, electronic industries, and organic catalysis. At the same time, chromium is essential in leather, electroplating, and metallurgical industries. However, their unpremeditated leakage into aquatic systems has caused human health and environmental apprehensions. Herein, we reported the development of an sp 2 carbon-conjugated fluorescent covalent organic framework with a guanidine moiety (sp 2 c-gCOF) that showed excellent thermal and chemical stability. The sp 2 c-gCOF showed effective sensing, capture, and recovery/removal of Pd(II) and Cr(VI) ions, which could be due to the highly accessible pore walls decorated with guanidine moieties. The fluorescent sp 2 c-gCOF showed higher selectivity for Pd(II) and Cr(VI) ions, with an ultra-low detection limit of 2.7 and 3.2 nM, respectively. The analysis of the adsorption properties with a pseudo-second-order kinetic model showed that sp 2 c-gCOF could successfully and selectively remove both Pd(II) and Cr(VI) ions from aqueous solutions. The polymer also showed excellent capture efficacy even after seven consecutive adsorption-desorption cycles. Hence, this study reveals the potential of fluorescent sp 2 c-gCOF for detecting, removing, and recovering valuable metals and hazardous ions from wastewater, which would be useful for economic benefit, environmental safety, human health, and sustainability. The post-synthetic modification of sp 2 c-COF with suitable functionalities could also be useful for sensing and extracting other water pollutants and valuable materials from an aqueous system.