Silver-modified nitrogen-doped graphene quantum dots as luminescent temperature sensor and a performance comparison to a benzothiadiazole-derivative with aggregation-induced enhanced emission

Photoluminescence characteristics of silver-modified nitrogen-doped graphene quantum dots (N-GQDs-Ag) were studied as a function of temperature variation, and results were compared with those of the benzothiadiazole-based fluorescent compound 4-(4-methoxyphenoxy)-7-styrylbenzo[c][1,2,5]thiadiazole. Graphene quantum dots (GQDs), a subclass of carbon quantum dots, are known for their intense, stable, and environmentally responsive luminescence, as well as their biocompatibility and ease of surface modification, which makes them promising candidates for luminescent temperature sensing. In this study, temperature sensitivity was evaluated between 20 and 50 °C in both liquid medium and immobilized states (on PVA or PMMA polymer matrices), with this range being particularly relevant for biological applications (in vivo and in vitro). N-GQDs-Ag showed superior sensitivity, with a temperature coefficient of 1.9 % °C⁻¹ in liquid medium and 2.9 % °C⁻¹ in polymer matrices. Signal intensity decreased by 35 % in the liquid medium and by 57 % in the polymer matrix at 40 °C. The luminescence was fully reversible upon cooling, showing a stable response over multiple cycles. The temperature dependence exhibited a strong linear correlation (R²= 0.990 in the liquid medium, R²= 0.999 in the polymer matrices). Toxicity assays in S. cerevisiae revealed no inhibition of cell growth at N-GQDs-Ag concentrations up to 1 % v/v, suggesting promising biocompatibility. These findings support the potential of N-GQDs-Ag as nanothermometers for use in biological assays, opening up opportunities for further testing in other cellular and in vivo systems.