Unveiling the role of α- and β-CDs in gold nanoparticle gel-based sensors for Fe³⁺ colorimetric detection

The detection of Fe³⁺ in water samples is critical for mitigating potential harm to environmental and human health. This study presents a comparative investigation of α-cyclodextrin (α-CDs) and β-cyclodextrin (β-CDs) as stabilizing agents in the development of gold nanoparticle (AuNPs)-based gel sensors for Fe³⁺ colorimetric sensing. The AuNPs were synthesized using ortho-hydroxybenzoic acid (o-HBA) as a reducing agent. The red-wine colloidal solutions of AuNPs-o-αCDs and AuNPs-o-βCDs were measured by a UV–Vis spectrophotometer, showing a surface plasmon resonance (SPR) band at 517 nm for both. Field emission scanning electron microscopy – energy dispersive X-ray (FESEM-EDX) analysis revealed well-dispersed AuNPs within the gel matrix. Particle size analysis (PSA) and zeta potential measurements further indicated a smaller particle size and superior colloidal stability in the AuNPs-o-βCDs. The analytical performance of both sensors was evaluated through precision, accuracy (%Recovery), and comparison with atomic absorption spectroscopy (AAS) via t-test assessments. Both sensors demonstrated acceptable precision and accuracy, showing no statistically significant difference from AAS results (p > 0.05). Notably, the gel-AuNPs-o-βCDs sensor exhibited enhanced sensitivity (0.20 mg/L) and faster detection, achieving stable readings within 15 min. In contrast, the gel-AuNPs-o-αCDs had a LoD of 0.57 mg/L and required 20 min for stable readout. Additionally, the gel-AuNPs-o-βCDs demonstrated a higher linear correlation with Fe³⁺ concentration (R² = 0.9999) than the gel-AuNPs-o-αCDs (R² = 0.9994). These findings suggested that β-CDs provide superior stabilization and enhanced performance in the gel-based AuNPs sensors, establishing them as a highly promising candidate for rapid, reliable, and accurate Fe³⁺ detection in tap water.