Chemiluminescence character of ZnS quantum dots with bisulphite-hydrogen peroxide system in acidic medium

In this work, ZnS quantum dots (QDs) capped with mercaptopropanoic acid were applied to the hydrogen peroxide-hydrogen sulfite chemiluminescence (CL) system. The CL intensity of the system was significantly enhanced by ZnS QDs in an acidic medium. The reactive oxygen species like superoxide ion (•O₂^-), sulfite (•SO₃^-), sulfate (•SO₄^-), and hydroxyl (•OH) radicals were generated in the CL reaction. It is of worth to know that the order of addition of reagents sturdily influences CL intensity, indicating different free radical generation in response to the different orders. Interestingly, the addition of water to an optimum concentration ensued a further increase in CL signal, which may be due to hydrolysis reaction at higher concentration of NaHSO₃. The enhanced CL was induced by excited ZnS QDs, which could be produced from the combination of hole (QDs (h⁺)) and electron (QDs (e^-)) injected QDs as well as by chemical resonance energy transfer from ¹O₂ and SO₂∗ to ZnS QDs. Four emitters such as ¹O₂, (O₂)₂∗, SO₂∗, and ZnS QDs∗ were detected in the CL system. Mechanistic investigation indicated that QDs acted as a catalyst, first decomposing H₂O₂ to generate free radicals and second prompting CL by energy transfer and electron-transfer annihilation effects. Distinct from most QDs CL reactions which are classically attained in basic conditions, this system operates in an acidic medium. This may intrigue an avenue for investigating the CL property of QDs in an acidic medium and promote its application in various fields.