Thiol-engineered borophene photonanozyme for selective Hg(II) sensing: experimental and DFT insights into electronic structure regulated catalysis

The development of efficient noble-metal-free platforms for selective Hg(II) detection remains highly desirable for cost-effective and sustainable Hg(II) sensing. In this context, thiol engineering is one of the most widely explored sensing strategies for Hg(II) ions. However, the development of efficient 2D thiol-based photonanozymes for the selective detection of Hg(II) remains relatively underexplored. To address this, we have developed L-cysteine-functionalized borophene nanosheets (L-Cys@BNS) as a nanozyme for the selective colorimetric detection of Hg(II). The L-Cys@BNS acts as an efficient light-driven peroxidase-mimic nanozyme for the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H₂O₂). Upon interaction with Hg(II), the selective thiol-binding of L-cysteine results in a concentration-dependent change in TMB oxidation, yielding a visible colorimetric response. The developed sensor exhibits good linearity (R² = 0.9890) within the range of 0-50 ppb. The limit of detection was 2.45 ppb, and the limit of quantification (LOQ) was 7.42 ppb, along with satisfactory recovery in real water (89.29-102.84%). The spin-polarized Density Functional Theory (DFT) reveals that the borophene nanosheets (BNS) after functionalization with L-cysteine provide accessible thiol sites for mercury binding. DFT calculations further revealed that Hg(II) coordinates strongly to the thiolate sulfur of L-Cys@BNS, forming an S-Hg-Cl complex with a calculated S-Hg bond length of 2.36 Å. This coordination was accompanied by Hg-Cl bond elongation from 2.29 to 2.43 Å, while electronic-structure analysis supported S-Hg interactions and indicated that electronic states remained available at the Fermi level. Therefore, this work demonstrated that L-Cys@BNS provides an effective photonanozyme-based platform for selective Hg(II) detection, with the underlying sensing mechanism supported by its electronic-structure characteristics.