Time-resolved fluorescence properties of quenched fluorescein sodium, including self-quenching and collisional quenching by iodide, have been studied by using a picosecond time-correlated single-photon counting (TCSPC) apparatus, together with an upconversion spectrophotofluorometer with a time resolution better than 300 fs. The steady-state fluorescence intensity of fluorescein sodium reached the maximum when its concentration was 510 μM with pH > 9. Both the fluorescence intensity and lifetime decreased with increasing concentrations of NaI quencher. When the NaI concentration was 12.2 M, a monoexponential decay with a lifetime as short as 17 ps was exactly determined for the first time using the femtosecond-resolved upconversion system. Picosecond time-resolved fluorescence measurements of circular permuted green and yellow fluorescent proteins (cpGFP and cpYFP) were reported, demonstrating that the fluorescence decay of quenched fluorescein sodium is a better approximation of the instrument response function (IRF) needed for the accurate deconvolution of fluorescence lifetime data, particularly for detectors used in the visible spectral region. We believe that this picosecond lifetime standard will find wide applications in fluorescence lifetime imaging microscopy (FLIM).

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Microwave approach has been widely used for the synthesis of fluorescent carbon dots (CDs) due to its advantages of short reaction time and homogeneous heating. Currently, it is still difficult to synthesize red light-emitting CDs (R-CDs) via the microwave-assisted method at atmospheric pressure. Herein, we report a solvent-controlled synthesis of R-CDs using domestic microwave. As a result, for an ammonium citrate tribasic, formamide and glycerol blend reaction solvent, the R-CDs with emission peak of 622 nm were achieved and exhibited a photoluminescence (PL) quantum yield of 37.4% and excitation-independent PL emission spectrum. During the formation process of R-CDs, glycerol and formamide not only promote the carbonization of ammonium citrate tribasic, but can also enhance the crystalline nature of CDs. Finally, the warm white light-emitting diode (WLED) with a high colour rendering index (CRI) of 90.9 and Commission Internationale de L'Eclairage (CIE) coordinates of (0.344, 0.333) was realized by combining conventional yellow YAG:Ce phosphor and R-CDs/mesoporous silica (MPS) composites with a blue chip. This result demonstrates a low-cost R-CD synthetic method for potential substitution for red phosphor materials in solid-state lighting applications.