Influence of Silver Nanoparticle Synthesis Route on the Antibacterial Performance and Optical and Mechanical Properties of Paper

Abstract

Background and Objective:
This study aimed to compare two chemical reducing agents for the synthesis of silver nanoparticles, namely reduction with sodium borohydride (Ag/NaBH₄) and trisodium citrate (Ag/TSC), and to evaluate their effects on nanoparticle morphology, antibacterial performance, and the optical and mechanical properties of paper. The main focus was to elucidate the role of the reducing agent in controlling particle size, colloidal stability, nanoparticle deposition pattern on fiber surfaces, and the final performance of the modified paper.

Materials and Methods:
Silver nanoparticles were synthesized from silver nitrate precursor by chemical reduction using trisodium citrate and sodium borohydride as reducing agents. In the Ag/TSC route, the silver nitrate solution was heated to boiling, followed by dropwise addition of trisodium citrate solution. In the Ag/NaBH₄ route, sodium borohydride solution was reacted with silver nitrate solution after cooling in an ice bath, and the resulting solution was freeze-dried and redispersed in distilled water to obtain a final concentration of 90 ppm. Handsheets with a basis weight of 60 g/m² were prepared and surface-modified by immersion in silver nanoparticle solutions for 10 s, followed by drying at ambient temperature. The antibacterial activity of the samples was evaluated using the inhibition-zone method against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Staphylococcus epidermidis. Nanoparticle formation was examined by ultraviolet–visible spectroscopy (UV–Vis), and nanoparticle morphology and deposition on fiber surfaces were studied using field-emission scanning electron microscopy (FESEM). Optical properties, including brightness and opacity, and mechanical properties, including tensile strength and tear strength, were measured. Data were analyzed using one-way analysis of variance.

Results:
UV–Vis spectroscopy showed that both nanoparticle systems exhibited a distinct surface plasmon resonance band near 400 nm, confirming successful formation of silver nanoparticles. FESEM images revealed that the average particle sizes of the Ag/NaBH₄ and Ag/TSC systems were approximately 20 and 22 nm, respectively. Despite the close particle sizes, Ag/TSC showed a more uniform surface distribution, whereas Ag/NaBH₄ exhibited more localized aggregation. Antibacterial tests of both nanoparticle solutions and modified papers demonstrated that both systems had notable inhibitory effects against the tested bacteria; however, Ag/NaBH₄ generally produced larger inhibition zones. After nanoparticle immobilization on the paper surface, antibacterial activity was maintained, while the control paper showed no considerable inhibitory effect. The effects of treatment type, bacterial strain, and their interaction were significant at the 1% probability level. The highest sensitivity was observed for S. epidermidis, whereas P. aeruginosa showed the lowest sensitivity. Modification of paper with silver nanoparticles significantly affected the optical and strength properties at the 99% confidence level. Both treatments increased opacity and reduced brightness, with a more pronounced brightness reduction in the Ag/TSC-treated paper. The tensile index decreased in both treatments, particularly in Ag/TSC, whereas tear index showed no significant difference compared with the control.

Conclusion:
Both chemical synthesis routes were capable of producing silver nanoparticles effective for antibacterial modification of paper. However, the Ag/NaBH₄ system showed stronger inhibitory performance, particularly against Gram-positive bacteria, which may be associated with its slightly smaller particle size and greater bioavailability of active silver species. In contrast, Ag/TSC exhibited a stronger plasmonic response and a more uniform surface distribution, but caused greater reductions in paper brightness and tensile strength. Therefore, selection of an appropriate synthesis route for producing silver nanoparticle-based antibacterial papers should be based on a balance among antibacterial activity, nanoparticle distribution uniformity, and preservation of the optical and mechanical properties of paper.