Silver nanoparticles via olive pomace extract: a sustainable approach for cadmium detection in mineral water

Objective Heavy metals, such as cadmium, are among the most concerning pollutants due to their persistence, bioaccumulation, and toxicity, posing severe risks to human health and ecosystems; as such there is a need to develop sensitive and cost-efficient methods to detect these contaminants in water. In recent years, nanotechnology has emerged as a promising platform for developing innovative sensors for environmental monitoring. With metal nanoparticles, particularly silver nanoparticles (AgNPs), is possible to perform colorimetric detection of analytes through visible color changes, thanks to the surface plasmon resonance (SPR) phoenomenon. In this study, is described a novel colorimetric approach for detecting Cd2+ ions in aqueous environments, utilizing AgNPs synthesized through an eco-friendly method. Methods Dehydrated olive pomace was extracted using an ethanol:water (80:20, v/v) solvent mixture. The AgNPs’ synthesis was optimized evaluating the effect of pH, temperature, and extract concentration. The AgNPs were characterized by scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDX), Dynamic light scattering (DLS) and Fourier-transform infrared (FT–IR). For the application as colorimetric sensors various metal ion solutions were tested, to which only Cd2+ was detectable. The colorimetric response to Cd²⁺ was evaluated by adding 1 mL of ultrapure water and 1 mL of AgNPs@PE suspension to a cuvette, followed by incremental additions of Cd²⁺ solution. The sensor was tested in natural drinking water samples spiked with 3 μM Cd²⁺. The ionic composition of the water samples was analysed using ICP–MS. Results Optimal experimental conditions for the AgNPs’ synthesis were found at 70°C, with the addition of 20 μL of 1N NaOH, 20μL of pomace extract diluted 1:100 and 1.4mL of AgNO3 for 20 mL of water solution. The formation of the nanoparticles was observed after 25 minutes by the typical SPR band of AgNPs at around 410nm in the UV-Vis spectra. The STEM analysis revealed the presence of predominantly spherical nanoparticles; at higher magnifications the nanoparticles appear surrounded by diffuse halos, which are likely attributable to the functionalization by phytochemicals from the extract. EDX confirms the presence of Ag, additional signals, corresponding to C and O, are ascribed to organic capping agents derived from the extract. The zeta potential and FT-IR spectrum of AgNPs@PE gave an ulterior confirmation of the functionalization. DLS analysis of AgNPs confirmed the nanoparticle size, with a mean diameter of 37.5 nm. For the application as colorimetric sensor 100μL of various metal ions solutions were added to a cuvette containing 1mL of AgNPs solution and 1mL of ultrapure water, from the spectra collected at various times only cadmium showed the formation of a secondary band at 530nm. The linearity was determined between 1.00 and 2.90 μM with linear regression coefficient of 0.994. Two mineral water samples were collected and spiked with Cd²⁺ ions, the recovery was estimated between 110.35% and 98.97%, demonstrating a good reliability for the proposed method. Conclusions In this study AgNPs were synthesized using pomace extract as both capping and stabilizing agent; these nanoparticles were then fully characterized via STEM, DLS, zeta potential and FT-IR. In addition, the AgNPs were successfully tested as colorimetric sensors for the selective detection of Cd2+ ions in drinking water samples.