Fluorescent carbon dot embedded silica nanocomposites as tracers for hydrogeological investigations: a sustainable approach†

The injected tracer technique using nanoparticles has evoked a lot of research interest in hydrogeological research as it encompasses a broad spectrum of applications in water resource management. The present work deals with developing carbon dot embedded silica-based nanocomposites using a microwave-assisted co-polycondensation method. The synthesized carbon dot-embedded silica nanocomposites have been characterized for their structural and functional characteristics using UV-visible spectroscopy, photoluminescence spectroscopy (PL), lifetime analysis, Raman spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier Transform Infrared spectroscopy (FTIR) and X-ray Diffractometry (XRD). The results obtained showed that carbon dots having a size of less than 5 nm had been successfully embedded into the silica structure, and the nanocomposite as such shows interesting optical properties. Laboratory scale column experimental studies were further conducted to ascertain the applications of the synthesized carbon dot-embedded silica nanocomposite for hydrological studies. Experiments were performed by varying the filling materials (sand/soil) in the column during which different concentrations of the nanotracer were injected under the continuous flow of water at a constant flow rate of 5 ml min⁻¹ followed by monitoring the detection of carbon dots for a definite time. The developed nanocomposite was found to exhibit satisfactory results in terms of the detection and recovery of carbon dots when injected as a tracer in an experimental hydrological study. About 99% of the nano tracer could be regained when ∼0.5 g of the CD-SiO₂ nanotracer is injected into the column and the detection was much faster with a peak detection time of 6 minutes. The better traceability and retention of the original optical properties of the developed tracer under different experimental conditions could be attributed to the optimal size of the nanocomposite system. Thus, the current challenges faced in groundwater flow analysis such as huge time consumption/expenses can be resolved to a significant extent considering the better traceability of the developed nanotracer.