Coronavirus disease 2019 (COVID-19) highlights the importance of rapid and reliable diagnostic assays for the management of virus transmission. Here, we developed a one-pot hydrothermal method to prepare Si-FITC nanoparticles (NPs) for the fluorescent immunoassay of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid protein (N protein). The synthesis of Si-FITC NPs did not need post-modification, which addressed the issue of quantum yield reduction during the coupling reaction. Si-FITC NPs showed two distinct peaks, Si fluorescence at λ_em = 385 nm and FITC fluorescence at λ_em = 490 nm. In the presence of KMnO₄, Si fluorescence was decreased and FITC fluorescence was enhanced. Briefly, in the presence of N protein, catalase (CAT)-linked secondary antibody/reporter antibody/N protein/capture antibody immunocomplexes were formed on microplates. Subsequently, hydrogen peroxide (H₂O₂) and Si-FITC NPs/KMnO₄ were injected into the microplate together. The decomposition of H₂O₂ by CAT resulted in remaining of KMnO₄, which changed the fluorescence intensity ratio of Si-FITC NPs. The fluorescence intensity ratio correlated significantly with the N protein concentration ranging from 0.02 to 50.00 ng/mL, and the detection limit was 0.003 ng/mL, which was more sensitive than the commercial ELISA kit with a detection limit of 0.057 ng/mL. The N protein concentration can be accurately determined in human serum. Furthermore, the COVID-19 and non-COVID-19 patients were distinguishable by this method. Therefore, the ratiometric fluorescent immunoassay can be used for SARS-CoV-2 infection diagnosis with a high sensitivity and selectivity.

Carbon dots (CDs) have gradually become a widely favored type of fluorescent nanomaterials with unlimited and promising applications. This work reports a means of breaking the shackle of method to develop new red and white CDs by introducing free radicals. The resulting white emissive CDs in this strategy are employed to demonstrate electroluminescent white-light-emitting diodes (WLEDs) and achieve a record-high external quantum efficiency (0.95%) of one-step-produced white CDs (WCD)-LEDs, which dramatically simplifies the whole fabrication processes of WLEDs. With additional passivation treatment, the red CDs (RCD2) with excellent properties such as N and S co-doping, bright (quantum yield = 49%) and stable photoluminescence (PL), large positive zeta potential (+20.5 mV), and two-photon fluorescence are obtained. Such RCD2 are used for rapid staining (5 min) of yeast cells. The two CDs synthesized via this method have outstanding performance in different aspects, which provides new promise of CDs for further functionalization and applications.