Probing the emission dynamics in nitrogen-doped carbon dots by reversible capping with mercury(ii) through surface chemistry

Abstract

In this study, the mechanistic insight and emission dynamics have been explored for size-dependent nitrogen-doped carbon quantum dots (namely 3A, 3B & 3C) with toxic metal Hg²⁺ ions via a capping and uncapping mechanism. The excitation and pH-dependent emission profile of N-CQDs is assigned to multiple centers involving higher energy aromatic core states (π–π*) and lower energy oxygen- and nitrogen-based functional groups (nO₂p–π* and nN₂p–π*). From experimental and theoretical validation, the highly negatively charged surface of 3A is found to be mainly due to the high abundance of –COOH and NH₂ groups promoting weak (COO⁻)₂–Hg²⁺, NH₂–Hg²⁺ bond formation with the reduction of the sp² carbon content with different concentrations of Hg²⁺ ions. Thus, the combined effect shrinks the P.L. signals (area and intensity) associated with the π–π* and n–π* transitions through the static and dynamic quenching pathway (LOD ∼ 0.209 μM). Furthermore, the restoration of PLE and P.L. signals is carried out by uncapping Hg²⁺ ions. The high recovery percentage (∼maximum −96%) is mainly governed by COOH and NH₂ groups rather than the deeply buried core state. Thus, the prominent reversible quenching associated with surface states is well supported by changes in zeta potential measurements. Our investigation demonstrates the direct dependence of surface chemistry on the proposed emission dynamics of carbon dots.