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Nonessential heavy metals are toxic to human health. In this study, mercury, a hazardous metal, was detected by colorimetric analysis using Murraya koenigii. The eco-friendliness of this method was also emphasized. UV spectrum is a broad peak observed at 200–250 nm in P. guajava leaf extracts. The UV spectrum of green synthesized P. guajava exhibited an absorption band of 418 nm, which confirms the nanoparticle synthesis. FTIR analysis of the vibrational peak around 3307 cm−1 is assigned to ν(O–H) stretching that could possibly emanate from carbohydrates or phenolics. The peaks found around 2917 and 2849 cm−1 are ascribed to the –C-H stretch of the alkyl group, and the peak around 1625 cm−1 is due to the enolic β-diketones or -C = O stretch of carboxylic acids, while the corresponding -C-O stretch is observed around 1375 and 1029 cm−1. The assignment of peaks is similar. It is clear from the SEM image that the constituent parts were non-uniform sphere-shaped, agglomerated, and of an average size of 30.9 nm. XRD analysis was utilized to determine the structural characteristics and crystalline nature of P. guajava. The observed intensity peaks at 32.35°, 36.69°, 39.24°, 44.76°, 59.42°, and 67.35° represent the 2θ values for P. guajava in the diffraction pattern, aligning with the values in the standard database. The synthesized AgNPs tested antibacterial properties against various strains of microorganisms, including Escherichia coli, 25 μg/mL 6.02 ± 0.17 and 100 μg/mL 7.3 ± 0.05, Staphylococcus aureus, 25 μg/mL 05.02 ± 0.07 and 100 μg/mL 11.3 ± 1.12, Streptococcus mutans, 25 μg/mL 04.02 ± 0.19 and 100 μg/mL 11.1 ± 0.11, Enterococcus faecalis, 25 µg/mL 0.8.05 ± 0.11 and 100 µg/mL 11.7 ± 0.02. The short novelty of Psidium guajava (guava) lies in its potential relevance to human health, as it has been found to possess bioactive compounds with various medicinal properties, such as antimicrobial, antioxidant, and anti-inflammatory activities, making it a promising natural resource for therapeutic applications.