Microwave-assisted synthesis of Fe-based single-atom nanozyme: a colorimetric approach to detect Cr(VI)

Chromium(VI) (Cr(VI)), a toxic metallic element, is commonly present in water sources and exhibits high toxicity. There is an urgent need to establish an efficient and selective Cr(VI) detection method. Nanozyme-based colorimetric detection is characterized by simplicity, intuitiveness, and high sensitivity, making it advantageous for rapid and visible Cr(VI) detection. We synthesized a colorimetric approach to detect Cr(VI) via coupling iron-nitrogen-doped carbon (Fe-N-C) with 8-hydroxyquinoline (8-HQ) as the specific recognition component in the current study. Fe-N-C was generated through microwave heating within 5 min, a significantly shorter duration compared to traditional electric heating methods. Cr(VI) detection was achieved by utilizing the excellent peroxidase-mimicking activity of Fe-N-C, selecting 3,3',5,5'-tetramethylbenzidine (TMB) as the chromogenic substrate and 8-HQ as the recognition element. The method is based on the specific affinity between 8-HQ and Cr(VI), which helps to reproduce the blue features of oxidized TMB (oxTMB). Furthermore, the sensing mechanism of the sensor was systematically studied. It was shown that 8-HQ, as a radical scavenger, competed with TMB for radicals and thus inhibited TMB oxidation. The developed sensor has high sensitivity (detection limit of 0.13 μM) and a broad linear range (1 to 130 μM). It selectively detects Cr(VI) over other cations and works well in Yangtze River water samples.