Silver Nanoparticles for Colorimetric Dual Ion Sensing and Development of Paper Sensors: A Strategy Towards Waste Valorisation and Sustainability

Iron and copper are essential for all living organisms, and their balance is crucial as both deficiency and excess can cause health problems. Therefore, this study presents a colorimetric method for detecting Fe³⁺ and Cu²⁺ ions in aqueous samples using silver nanoparticles (AgNPs) synthesised from Ocimum sanctum (Tulasi) leaf extract (TLE). It is observed that AgNPs show optimum plasmonic properties at a precursor-leaf extract ratio of 1:5, reaction temperature of 60 °C and reaction time of 2 h. The AgNPs exhibit the face-centred cubic (fcc) structure and show a surface plasmon resonance peak at 413 nm, hydrodynamic size of 18 ± 5 nm, zeta potential of − 25.5 mV and particle size of 57 nm. FTIR spectra confirm the stabilisation of AgNPs. It is worthy to note that, AgNPs exhibit selective detection of Fe³⁺ and Cu²⁺ over other metal ions and the detection mechanism is proposed based on the reduction potential values. The quantitative detection range for Fe³⁺ and Cu²⁺ are found to be 0–800 μM and 0–600 μM, with the detection limits of 9.1 µM and 19.5 µM, respectively. Additionally, AgNP-based paper sensors for Cu²⁺ detection show qualitative and quantitative colorimetric performance with a detection limit of 23.1 µM. These findings suggest that both AgNPs solution and AgNP-based paper sensors are the potential candidates for metal ion detection.