L-Cysteine functionalized graphene oxide nanoarchitectonics: A metal-free Hg2+ nanosensor with peroxidase-like activity boosted by competitive adsorption

Developing non-noble metal, even metal-free chemical sensors for the detection of toxic heavy metal ions is significantly desirable for economically and environmentally sustainable application but has heretofore remained elusive. Herein, a L-cysteine functionalized graphene oxide nanosheet (CGO) nanoarchitectonics, greenly synthesized by a very simple method at room temperature, was utilized to realize the simultaneous enrichment and colorimetric detection of trace mercury ions (Hg2+). It was discovered that CGO, as a nanozyme mimic exhibited greatly enhanced peroxidase-like catalytic activity than the pristine graphene oxide. By exploring the interactions of CGO nanozyme with colorimetric substrate, 3,3',5,5'-tetramethylbenzidine (TMB) and target Hg2+ ions, we found that the sensing principle was based mainly on the competitive adsorption between Hg2+ ions and TMB over CGO. The pre-capture of Hg2+ ions hindered the TMB binding on CGO, resulting in the promoted oxidation of TMB by H2O2 to produce more colored oxidation products, from which the colorimetric sensing of Hg2+ was realized with a good detection effect on 5 μg L-1 solution. As an enrichment-sensing integration platform, this metal-free sensor is cost-effective and sensitive, and presents considerable anti-interference ability over other metal ions. Overall, this work not only expands the application of graphene-based materials in colorimetric detection but also provides a general sensing principle to construct highly sensitive sensors.