A simple hydrothermal method was applied to prepare carbon nanodots (C dots) from o-phenylenediamine (OPD). The C dots exhibit photoluminescence at 567 nm when excited at 420 nm. In the presence of Cu²⁺ ions, the colour of C dots changes from yellow to orange, with an increased PL intensity as a result of the formation of Cu(OPD)₂ complexes on the surfaces of C dots. The D-band to G-band ratios of C dots in the absence and presence of 80 nM Cu²⁺ ions are 1.31 and 4.75, respectively. The C dots allow the detection of Cu²⁺ ions with linearity over a concentration range of 2–80 nM, with a limit of detection of 1.8 nM at a signal-to-noise ratio of 3. The cell viability values of A549, MCF-10A, and MDA-MB-231 cells treated with 3 μg mL⁻¹ of C dots are all greater than 99%, showing their great biocompatibility. Having great water dispersibility, photostability, chemical stability (against NaCl up to 0.5 M), great selectivity, and biocompatibility, the C dots have been employed for the localization of Cu²⁺ ions in the cancer cells (A549 cells) treated with 10 μM Cu²⁺ ions.

An electrochemical approach has been employed for the preparation of photoluminescent carbon nanodots (C-dots) from glycine under alkaline conditions. The formation of C-dots is through electro-oxidation, electro-polymerization, carbonization, and passivation. The as-prepared C-dots possess excitation-wavelength-dependence and pH sensitive photoluminescence (PL) properties and are stable in solution containing high salt concentration (up to 500 mM NaCl). Detection of hemoglobin using C-dots has been demonstrated through a fluorescence resonance energy transfer (FRET) process. The PL intensity (excitation/emission wavelengths 365/440 nm) of C-dots is inversely proportional to the concentration of hemoglobin over a range of 0.05–250 nM (r = 0.99), with a limit of detection (signal-to-noise ratio 3) of 30 pM. We have validated this assay by determining the concentrations of hemoglobin in five representative diluted blood samples, with results being in good agreement with that obtained by using a commercial hemoglobin meter. The water-dispersible and photostable C-dots have been applied to obtain bloodstained images and fingerprints.