Sustainable green synthesis of Hedychium coronarium leaf extract-stabilized silver nanoparticles and their applications: colorimetric sensing of Sn2+ and Hg2+ and antifungal and antimicrobial properties†

Hedychium coronarium (Hc) (commonly known as Gulbakawali) leaf extract was used for the stable and sustainable green synthesis of silver nanoparticles (Hc-AgNPs), which were biodegradable and non-toxic. Hedychium coronarium leaf extract was used as a reducing agent to stabilize the Hc-AgNPs by converting Ag⁺ to Ag⁰ without adding any capping agent. It demonstrated stability for up to six months, and no agglomeration was observed. The Hc-AgNPs were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), ultraviolet-visible spectrophotometry, and fluorescence spectroscopy analysis. The UV-visible spectrum supported the formation of stable Hc-AgNPs by displaying a strong surface plasmon resonance (SPR) peak at 440 nm. FT-IR spectra showed the functional groups present in the leaf extract of Hedychium coronarium, which was the primary source of secondary metabolites attached to Ag⁰. XRD analysis revealed a distinct 2θ peak of Hc-AgNPs at 38.15°, indicating a face-centred cubic structure with a crystallite size of 22.6 ± 1 nm at the (111) plane. Moreover, TEM demonstrated the spherical morphology of the Hc-AgNPs with an average particle size of 22.42 ± 1 nm. The photophysical characteristics of the Hc-AgNPs, as highlighted by their UV-vis and fluorescence characteristics, revealed their semiconducting nature with an impressive band gap (E_g) value of 3.78 eV. Fascinatingly, the fluorescence activity of Hc-AgNPs at 504 nm showed a broad emission band corresponding to the absorption band at 251 nm. We performed the selective colorimetric sensing of Sn²⁺ metal ions using Hc-AgNPs, which demonstrated a detection limit of 10⁻³ M, suggesting their potential as very good solid biosensors. Interestingly, the Hc-AgNPs showed antifungal activity, which has not been reported before. Specifically, the results showed that the Hc-AgNPs had a higher fungitoxicity effect against Aspergillus flavus (59.58 ± 3.68) than against Fusarium oxysporum (57.93 ± 4.18). The antibacterial activity of the Hc-AgNPs was evaluated against three Gram-negative phytopathogenic bacteria: Xanthomonas oryzae (X. oryzae), Ralstonia solanacearum (R. solanacearum), and Erwinia carotovora (E. carotovora), showing effective inhibition zones of 16.33 ± 0.57, 15.33 ± 0.57, and 14.33 ± 0.57 mm, respectively. These results indicate that the Hc-AgNPs could inhibit these phytopathogenic bacteria with varying degrees of effectiveness in the order of X. oryzae > R. solanacearum > E. carotovora.