Highly sensitive and selective colorimetric and plasmonic sensor of Fe based on gold nanostructures biosynthesized with Opuntia joconostle fruit peel.

Biogenic gold nanostructures have been obtained, for the first time, by bio-reduction of Au ions with an aqueous extract of Opuntia joconostle fruit peel (Oj-AuNPs). This particular methodology is completely green, since room temperature was used, and no infusion preparation was required. The Oj-AuNPs exhibit a broad surface plasmon resonance (SPR) signal, with a maximum at 556 nm. SEM and TEM observations show Oj-AuNPs with mean size around 80 nm and raspberry-like morphologies, mainly, made of smaller nanoparticles glued by the biomass. When freshly prepared Oj-AuNPs solution is placed in contact with Fe, the solution changes from brown to a green-grayish color, being the only metal ion changing the Oj-AuNPs solution color among the other twelve metal ions probed, including Fe. Once the Fe ions are detected, the SPR of the Oj-AuNPs becomes broader with a considerable red shift. Furthermore, smaller Au nanostructures, with better defined morphologies than those in the original Oj-AuNPs, are observed by TEM. Therefore, the conceivable mechanism of the naked eye and plasmonic detection of Fe by Oj-AuNPs involves the disaggregation of the original larger gold nanostructures. Sensitivity studies of the Oj-AuNPs detection of Fe were performed from 200 ppm to 0.1 ppb. The limit of detection (LOD) and limit of quantification (LOQ) for Fe are 0.023 and 0.079 ppb, respectively. Moreover, the Oj-AuNPs colorimetric sensor was effectively tested for highly sensitive detection of Fe in tap water.