If the materials are basically the same type, wouldn't it mean that the size has also an influence? But I think it depends also on the type of nano materials: nano particles or nanotubes. For nano particles, it might be that bactericidal property is also particle size dependent. For nanotubes, cell interactions are size-dependent and bacteria interactions are also influenced.

Thanks Anca and Dr. Iva. I am asking about the same material (nanoparticle) with similar chemical composition having different surface area. recent studies have shown that shape of the nanoparticle also affects its antibacterial property, but finally is it the surface area of a nanoparticle only is going to play an important role in its bactericidal activity?

In your case for the same material, surface area is not the only parameter for the bacterial activity. Particle size, and the dimensionality also plays an important role in the field of materials science. Spheres (zero dimensional) would show different property, while rods-like (one dimensional) will show different and the sheets (two dimensional) will yet show some different activity.

Thank Mr. Muhammad. Could you kindly give few references regarding this ?

Prashanth! if you compare a surface active materials of similar chemical built but different surface areas, the material (nanoparticlse in you case) with higher surface area would certainly have higher numbers of surface active site available to exhibit bactericidal property. But this would be true if the said action proceed only through surface reactions which has to be investigated first. To be sure, you must test them under identical conditions and find out the responsible factors for particular bactericidal action. If surface action is responsible then surface active sites/radicals/components on both the materials may be quantified for comparison. I do agree with Muhammad Ehsan that morphology of nano-particles also matter towards activity, however, shape of particle is likely to change while going down to nano scale from micron level.

Metal nanoparticles as such shown different properties from bulk material. the change in the properties depends on size, shape, composition and stability of nanoparticles. For antibacterial activity of heavy metals their ions are most toxic than bulk material. and it is proposed that most of the metal act at their molecular level to inhibit bacterial growth. So for antimicrobial activity a metal particle have to produce or have maximum active site at molecular level and maximum exposure to bacterial cell to interact with cellular components. Now if size of particle is big it will have less surface area and less penetrability into the cell and will have lesser chances of interaction. While nanoparticles with smaller size will have broader surface area, better penetration, and maximum chance for interaction. So higher the surface area, smaller the size and better stability will have higher the antimicrobial activity.

Order of antimicrobial activity for molecular active metal- ION>Nanoparticles>Bulk material.

Thank you very much Mr. Mradul and Mr. Navneet for your valuable opinion.. In my case a nano metal oxide particle having lower surface area is showing better bactericidal property than the same metal oxide having higher surface area, but both the particles have been prepared by different methods and both have different shapes. Antibacterial studies have been carried out under identical conditions. How to justify this..Could you kindly explain?

Size doesn't always matter... perhaps answer lies in the anisotropy, surface charge & surface chemistry...

Size of a metal oxide can alter several physico-chemical parameters of the oxide interaction with the microorganism. A direct effect on phagocytosis may be expected and this typically has an impact in the biocidal action. This point has been noted and discussed previously here. The second issue, also mentioned, is related to the point of zero charge (PZC also called isoelectric point) of the oxide which may be modified depending on the oxide nature. Modification of PZC may affect surface interaction with the microorganism by electrostatic effects and also potential (cation) lixivitation effects depending on the experimental conditions of your experiments and the specific oxide used.

Thanks Mr. Sridhar & Mr. Marcos.. Could you please tell how do I justify my observations? ... With which experimental part/analysis of my samples I will be able to justify this behaviour?

Antibacterial activity depends on various parameters. To explain it, it is needed to what conditions you consider to be identical for the activity. Like, 1) Had used Macfarland culture. 2) which method you have used: Tube dilution or Disk Diffusion. 3) Had tested a standard antibiotic along with test. 4) Had tested for solvent or vehicle control. 5) Had compared Standard antibiotic and vehicle control along with the both drug. If you have answer for these you can explain it in a better way.

Dear Prashanth,

as a starting point you have to clarify the mechanism of the bactericidal action.

In lot of cases like nanosilver the action is through dissolved ions. Metal oxide - which one? Try to measure concentration of metal ions in solution and compare for your different metal oxide dispersions. Did you determined the size distribution in your aqueous dispersions?

Dear Dr. Titus,

My samples are ZnO nanopowders. size distribution means DLS you mean to say sir?

Would assume that e.g. DLS can give you the info about state of aggregation/agglomeration. Nevertheless, additional info on dissolution rate of Zn-species might be necessary to clarify the mechanisms.

How to carry out it sir? Could you please explain ?

You have to search for available analytical methods for determination of Zn species as function of contact time with water. Maybe ion selective electrodes can be used.

Ok. Thank you very much sir