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Doped fluorescent carbon dots (CDs) have drawn widespread attention because of their diverse applications and attractive properties. The present report focusses on the origin of photoluminescence in nitrogen-doped CDs (NCDs), which is unraveled by the interaction with Cu(2+) ions. Detailed spectroscopic and microscopic studies reveal that the broad steady-state photoluminescence emission of the NCDs originates from the direct recombination of excitons (high energy) and the involvement of defect states (low energy). In addition, highly selective detection of Cu(2+) is achieved, with a detection limit of 10 μm and a dynamic range of 10 μm-0.4 mm. The feasibility of the present sensor for the detection of Cu(2+) in real water samples is also presented.

We present a Förster resonance energy transfer (FRET)-based fluorescence detection of vitamin C [ascorbic acid (AA)] using cadmium sulphide quantum dots (CdS QDs) and diphenylcarbazide (DPC). Initially, DPC was converted to diphenylcarbadiazone (DPCD) in the presence of CdS QDs to form QD-DPCD. This enabled excited-state energy transfer from the QDs to DPCD, which led to the fluorescence quenching of QDs. The QD-DPCD solution was used as the sensor solution. In the presence of AA, DPCD was converted back to DPC, resulting in the fluorescence recovery of CdS QDs. This fluorescence recovery can be used to detect and quantify AA. Dynamic range and detection limit of this sensing system were found to be 60-300 nM and 2 nM, respectively. We also performed fluorescence lifetime analyses to confirm existence of FRET. Finally, the sensor responded with equal accuracy to actual samples such as orange juice and vitamin C tablets. Graphical abstract Schematic showing the FRET based fluorescence detection of ascorbic acid.

Although carbon dots (CDs) are an attractive alternative to the toxic semiconductor based luminescent materials, the plethora of carboxyl group present in its surface bestows a weak solubility in non‐polar media. Here, we demonstrated the synthesis of CDs having high solubility in non‐polar environment referred as ‘organophilic CDs’ (OCDs) via hydrothermal reaction of cyclohexane. Cyclohexane acts as a carbon source as well as dispersion media. The OCDs was well characterized using spectroscopic and microscopic methods. Further, we utilized OCDs for the selective sensing of 2,4,6‐trinitrophenol (TNP) over other structurally similar nitroaromatics (NAs), with a limit of detection of 887 nM. Detailed photophysical studies have been conducted to understand the photoluminescence (PL) quenching mechanism. The quenching was due to primary inner filter effect (IFE), which happens by the absorption of NAs at the excitation wavelength of OCDs and not due to commonly observed electron transfer mechanisms in these class of compounds.

Here we report a single step, rapid synthetic strategy for white light emitting nitrogen rich carbon dots (NCDs) under a range of excitation wavelengths by carbonizing ethylenediamine using P2O5 and water.