Rapid capture and visual detection of copper ions in aqueous solutions and biofluids using a novel cellulose-Schiff base

In this work, a novel colorimetric sensor was prepared via a cellulose-polyethylenimine-based Schiff base (cellulose-Schiff base) for capturing and detecting copper ions (Cu²⁺) in aquatic environments and life systems. The cellulose-Schiff base displayed continuous color changes from inherent pale yellow to deep green due to the formation of a cellulose-Schiff base-Cu²⁺ charge transfer complex within 30 s, indicating excellent “naked-eye” detection capability for Cu²⁺ ions in aqueous solution. The limit of detection (LOD) was 1.054 ppm, lower than the U.S. EPA guidelines for drinking water of 1.3 ppm. Moreover, the cellulose-Schiff base colorimetric sensor was successfully used to capture and detect exogenous Cu²⁺ ions in biofluids (simulated urine and bovine serum). The proposed sensing mechanism of the cellulose-Schiff base for Cu²⁺ ions is that N atoms (C=N and N–H) in the sensor capture and concentrate Cu²⁺ ions to form sensor-Cu²⁺ complexes, then intramolecular charge transfer occurs as a consequence. In addition, test strips based on cellulose-Schiff base were developed, and provide a convenient, cheap, sensitive, and reliable monitoring platform for on-site semi-quantitative analysis of Cu²⁺ ions in real samples.