Silver nanoparticles as dual sensors for selective detection of hexavalent chromium and cyanide ions

Silver nanoparticles stabilized in starch (AgNPs@starch), were synthesized via chemical reduction, yielding spherical nanostructures with an average diameter of (5 ± 1) nm. The surface plasmon resonance (SPR) band at 400 nm served as the analytical signal for the colorimetric detection of hexavalent chromium [Cr(VI)] and cyanide (CN^-) ions under various aqueous conditions. Cr(VI) detection was conducted using AgNPs@starch at pH 2, 4 and acetate buffer at pH 4.75 (ABS pH 4.75), in the presence of ethylenediaminetetraacetic acid (EDTA) as a stabilizer. Although slightly higher sensitivity was observed in unbuffered systems at pH 4, the buffered environment ensured improved nanoparticle stability and extended the detection range (0-100 μM) without compromising the limit of detection (LOD) of 0. 2 μM. The use of the pH 4.75 buffer reduced oxidative degradation of AgNPs, preserving optical performance and reducing the rate of absorbance decrease upon Cr(VI) addition compared to unbuffered media. Analysis of real water samples (coming from Suquía River in Córdoba, Argentina) and common interferents confirmed the selectivity and sensitivity of the nanosensor under both buffered and unbuffered conditions. Cyanide was also selectively and rapidly detected using AgNPs@starch in borate buffer at pH 8.4 (BBS pH 8.4), achieving an LOD of 0.24 μM over a concentration range 0-85 μM. The AgNPs@starch sensor maintained excellent analytical performance in the presence of common interferents (Ca²⁺, Mg²⁺, halides) and in real water samples, with calibration curves exhibiting relative errors below 5% compared to pure systems.