pH Alteration in Plant-Mediated Green Synthesis and Its Resultant Impact on Antimicrobial Properties of Silver Nanoparticles (AgNPs)

Abstract

Plant-mediated green synthesis is a cost-effective and eco-friendly process used to synthesize metallic nanoparticles. Experimental pH is of interest due to its ability to influence nanoparticle size and shape; however, little has been explored in comparison to the influence of this parameter on the therapeutic potential of resultant metallic nanoparticles. Our work investigated the influence of pH alternation on antimicrobial properties of plant-mediated green synthesized (using Spinacia oleracea leaf extract) silver nanoparticles. We further investigated if the antimicrobial activity was sustained at 8 weeks (after initial green synthesis). Antimicrobial properties were evaluated against Escherichia coli, Staphylococcus aureus, and Candida albicans. Our work confirmed that experimental pH in plant-mediated green synthesis of silver nanoparticles influenced their resultant antimicrobial properties. Silver nanoparticles generated at experimental pH 4,5, and nine showed activity against E. coli which was sustained at various levels over 8 weeks. No antimicrobial activity was observed against S. aureus, and weak antimicrobial activity against C. albicans. These interesting findings highlight the importance of experimental pH. Further understanding of the role experimental pH plays on resultant metallic nanoparticle properties as it relates to biological and therapeutic impact is required, which will have an impact on wider applications beyond antimicrobial activity.

Keywords: antimicrobial efficacy; green synthesis; pH; silver nanoparticle.

Figure 1

The visual observation of the reaction solution affected by pH was observed after (a) 0 h, (b) 2 h, and (c) 3 h of synthesis [N/A refers to sample without pH adjustment].

Figure 2

UV-Vis spectra of samples observed after 3 h of synthesis affected by pH: (a) AgNP-6, (b) AgNP-8, and (c) AgNP-9.

Figure 3

FTIR spectra of samples after 3 h of synthesis affected by pH: (a) Spinacia oleracea leaf extract, (b) AgNP −4, (c) AgNP−5, (d) AgNP−6, (e) AgNP−N/A, (f) AgNP−8, and (g) AgNP−9.

Figure 4

SEM images samples (a) AgNP-4; (b) AgNP-5; (c) AgNP-6; (d) AgNP-N/A; (e) AgNP-8; (f) AgNP-9.

Figure 5

TEM images of nanoparticle (a) AgNP-N/A show a mixture of spherical and rod shapes with an average length of around 90 nm, and longer needle-like fibre shapes can also be observed with their length reaching over 400–500 nm; (b) AgNP-8 nanoparticles are mostly in spherical particle shapes sized below 100 nm.

Figure 6

Plates after 24 h incubation based on resazurin assay with the test organism were: (a) E. coli; (b) S. aureus; (c) C. albicans [pink colour refers to microorganism growth and purple indicates the antimicrobial activity; “New”, newly synthesized nanoparticles; “Old”, nanoparticles that had been kept for ± 8 weeks; AgNP-4-9, a nanoparticle sample prepared in various pH with replicates].

Figure 7

Control of plate after 24 h incubation based on resazurin assay for the test organism [pink colour refers to microorganism growth, and purple indicates the antimicrobial activity; spinach extract at pH 4–9; negative control with antibiotic gentamicin (10 ¼g/mL) for bacteria and antifungal agent clotrimazole (10 ¼g/mL) for fungi, and positive control with just microbes and broth].].