Carbon dots (CDs) are promising luminescent nanomaterials, valued for their tunable photophysical properties and ease of surface modification, making them ideal for sensing applications1. Existing pH sensors based on CDs often have a limited operational range and rely on intensity changes rather than wavelength shifts. Furthermore, typical syntheses require complex or toxic reagents2. We present a rapid, green, and cost-effective method for synthesising CDs with dual environmental sensitivity that overcomes these challenges.
CDs were synthesised using a simple combustion method, employing non-toxic and low-cost precursors: citric acid, urea and sodium hydroxide. The material was subjected to combustion at 260°C. Structural and optical characterisation was performed using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy with ATR attachment (IR-ATR), and UV-Vis and photoluminescence (PL) spectroscopy. PL studies included time-dependent emission measurements in water and alcohols, as well as in solutions with varying pH.
The obtained CDs exhibited strong, tunable photoluminescence, high photostability, and excellent dispersibility in aqueous media. XRD analysis confirmed a partially amorphous structure with a graphene-like core and polymeric surface functionalities. CDs demonstrated continuous emission shifts across a broad pH range (1–14); an increase in pH in water caused a blue shift, while red shifts were observed in alcohols. IR spectroscopy revealed dynamic surface interactions during solvent evaporation, with faster changes in methanol than in ethanol. Quantum yield (QY) values were 15% (water), 11% (methanol), 22% (ethanol), and 13.5% (pH 13).
Ultimately, the sustainable and scalable synthesis yielded multifunctional carbon dots. Their unique sensitivity to both pH and hydroxyl-rich environments, including the ability to differentiate between methanol and ethanol, makes them promising materials for low-cost, selective environmental sensors and biomedical diagnostics. Time-dependent emission changes suggest dynamic equilibrium processes at the CD surface, highlighting their potential as intelligent sensors.
