Rapid and scalable synthesis of sulfur quantum dots through ozone etching: photoluminescence and FRET-mediated Co2+ sensing

Abstract

Sulfur quantum dots (SQDs), an emerging class of zero-dimensional luminescent nanomaterials, are envisioned to excel over traditional compound quantum dots in various practical applications. Herein, we introduce a green and facile ozone-assisted top-down approach for the rapid synthesis of luminescent SQDs from inexpensive and abundant bulk sulfur powder. The formation of SQDs involves the dissolution of bulk sulfur powder into small particles in an alkaline environment, followed by the oxidation of polysulfide ions (S_x²⁻) into zero-valent sulfur (S⁰) by ozone. Benefitting from the strong oxidizing potential of ozone, the synthesis time has been notably reduced to 4 h. The as-synthesized Oz-SQDs exhibit a nearly monodisperse size (2.5–6 nm), tunable emission, colloidal stability, good thermal stability, and excellent photostability with a reasonable photoluminescence quantum yield of 9.26%. The etching of surface polysulfide species by a powerful oxidizing agent such as ozone leads to the passivation of trap states, thereby enhancing the emission properties to a greater extent. Fluorescence sensing experiments point to the feasibility of using Oz-SQDs as a fluorescent probe for the sensitive and selective detection of Co²⁺ based on the Förster resonance energy transfer (FRET) mechanism. The fluorescence intensity of Oz-SQDs exhibits a linear relationship with the Co²⁺ concentration in the range of 19.6 to 56.6 μM, and the detection limit is calculated to be 2.44 μM. Bestowed with favorable luminescence properties and good aqueous dispersibility, it is believed that SQDs realized in this work may emerge as a promising candidate for a wide range of potential applications.