Substrate-dependent thermal sensing using silver nanoparticles synthesized via laser ablation in solution.

Silver nanoparticles (Ag NPs) are highly valued for their stability and unique properties in sensing applications. In this study, we report a novel observation that Ag NPs deposited on fibrous paper showed an unexpected negative Temperature Coefficient of Resistance (NTCR), unlike the usual positive TCR seen on glass or other substrates. This substrate-dependent electrical response opens new opportunities for flexible temperature sensing technologies. The Ag NPs were synthesized using Laser Ablation Synthesis in Solution (LASiS), a rapid and eco-friendly method of producing Ag NPs. However, LASiS often results in broad size distributions due to post-ablation accumulation or target surface fragmentation, which limits their applications requiring tailored properties. We optimized laser parameters (5W, 1064 nm DPSS laser) to synthesize spherical Ag NPs in deionized (DI) water, with a narrow size distribution of 10-110 nm, as confirmed by FESEM and TEM, with a high concentration of ~ 128 mg/L. These Ag NPs deposited on glass and fibrous paper exhibited contrasting temperature-resistance behaviours. We hypothesize that repeated heat treatments and interactions with the fibrous substrate caused substantial oxidation in the paper-based sample, forming Ag₂O, as confirmed by TGA analysis. Ag₂O likely contributed to the observed NTCR effect as it acts as a p-type semiconductor. The findings highlight controlled Ag NP synthesis with a narrow size distribution and the critical influence of substrates on functional properties of NP-based systems.