Surface state-regulated redox carbon nanodots for plasmonic morphology-dependent ratiometric sensing

Abstract

Controlled-synthesis of carbon nanodots (CNDs) with tunable properties has always been challenging. Here, novel CNDs (blue-emitting CNDs (b-CNDs) and green-emitting CNDs (g-CNDs)) with tunable redox abilities and emission are synthesized via one-pot hydrothermal method. The redox ability of CNDs is regulated by surface state tailoring. The abundant reductive groups on the surfaces endow the b-CNDs appealing reducibility, whereas g-CNDs with fewer surface functional groups are deficient in reducibility. Using HAuCl4 indicator, the distinct reducibility of CNDs is visually identified. Gold salts are reduced to Au nanobelts under reduction and stabilization of b-CNDs, causing plasmons-induced color changes, whereas g-CNDs have no reduction effect on Au3+. The size-evolution process of Au nano-species can guide not only plasmonic absorbance alteration but also fluorescence (FL) and second-order scattering (SOS) changes. Therefore, based on the plasmonic morphology-induced SOS increasement and the Förster resonance energy transfer-induced FL switch off, the b-CNDs contribute to a ratiometric sensor coupled with FL and SOS for Au3+ detection. The ratiometric strategy displays a better sensitivity and selectivity and a broader linear range than single-signal SOS or FL method. The regulation of redox CNDs and ratiometric assay of Au3+ provide a new prospect for the preparation and application of versatile CNDs.