Molecular Weight Identification of Compounds Involved in the Fungal Synthesis of AgNPs: Effect on Antimicrobial and Photocatalytic Activity.

The biological synthesis of silver nanoparticles (AgNPs) for medical, environmental, and industrial applications is considered an alternative to chemical synthesis methods. Additionally, the reducing, capping, and stabilizing molecules produced by the organisms can play a key role in the further activity of AgNPs. In this work, we evaluated the synthesis of AgNPs by four molecular weight fractions (S1: <10 kDa, S2: 10 to 30 kDa, S3: 30 to 50 kDa, and S4: >50 kDa) of mycelia-free aqueous extract produced by the white-rot fungus Stereum hirsutum and their effect on the antimicrobial activity against Pseudomonas syringae and photocatalytic decolorization of nine synthetic dyes exposed to sunlight radiation. All synthesis assay fractions showed the characteristic surface plasmon resonance (SPR) with 403 to 421 nm peaks. TEM analysis of synthesized AgNPs showed different sizes: the whole mycelia-free extracts S0 (13.8 nm), S1 (9.06 nm), S2 (10.47 nm), S3 (22.48 nm), and S4 (16.92 nm) fractions. The results of disk diffusion assays showed an inverse relation between antimicrobial activity and the molecular weight of compounds present in the mycelia-free aqueous extract used to synthesize AgNPs. The AgNPs synthesized by S0 (14.3 mm) and S1(14.2 mm) generated the highest inhibition diameter of P. syringae growth. By contrast, in the photocatalytic assays, the AgNPs synthesized by the S2 fraction showed the highest discoloration in all the dyes tested, reaching 100% of the discoloration of basic dyes after 2 h of sunlight exposure. The maximum discoloration observed in reactive and acid dyes was 53.2% and 65.3%, respectively. This differentiation in the antimicrobial and photocatalytic activity of AgNPs could be attributed to the capping effect of the molecules present in the extract fractions. Therefore, the molecular separation of synthesis extract enables the specific activities of the AgNPs to be enhanced.