I have synthesized silver nanoparticles from leaf extract of a plant (green synthesized silver nanoparticles). I have tested its antimicrobial activity for E.coli. But, zone of inhibition was not recorded. What could be the reason ?
Richa Das , assuming that you confirmed the formation of AgNPs by the UV-Vis (simplest method), several factors affecting the antibacterial activity can be considered:
- Nanoparticle size
What is the size of your green synthesized AgNPs?
Usually, the larger the particle, the lower the (re)activity.
Did you use AgNPs as synthesized, or did you centrifuge and/or dry them?
If you centrifuge and/or dry NPs, then consider proper resuspension before the assessment of antibacterial activity. Ultrasonication may be required to break down large aggregates since aggregated particles will demonstrate lower (or neutral) activity.
- Concentration
Maybe the applied concentration was too low, consider trying higher concentrations of AgNPs.
- Antibacterial activity assessment method
Which method did you use for the zone inhibition - well diffusion, disk diffusion, or a different method?
For example, in the case of the disk diffusion method, freshly prepared disks are preferred, since drying and long-term storage of NP-coated/loaded disks can reduce/neutralize the activity of NPs.
As for the well diffusion method, if you have larger or precipitating NPs, their penetration efficiency, therefore the activity, will be lower.
- Culture media composition
Medium components can influence the activity of NPs. Try another composition of agar medium but consider that selected bacterial strains also show normal growth on that medium without NPs.
From my experience, LB-agar is good for green synthesized AgNPs, but it may also depend on the origin and composition of NPs (e.g., plant extract, bacteria, fungi, etc).
- Bacterial strain
E. coli is a well-known model microorganism to start antibacterial activity tests. In your case, resistance is less likely, but for comparison, you may try to check antibacterial activity using another bacterial strain.
- Plant extract versus AgNPs
If you haven't done yet, check, if plant extract demonstrates antibacterial activity or not. It can serve as a reducing agent for the synthesis of AgNPs, but the coating density of AgNPs or the nature of coating agents also may affect the activity of NPs.
Hope the abovementioned will be helpful. Good luck!
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