Silver shows the highest antimicrobial activities amongst all metals. It is better than many first line antibiotics. The antimicrobial properties of silver can be tuned by altering its physical and surface properties. Researchers have demonstrated enhancement in the antibacterial properties of silver with decreasing particle size from bulk to nano. In the present article, we study the effect of particle size of silver at nanoscale on their antimicrobial properties. Two samples of silver nanoparticles (SNPs) of same physical size (≈8 nm) but different hydrodynamic size (59 and 83 nm) are prepared by chemical reduction of AgNO3 with oleylamine followed by phase transfer with triblock copolymer Pluronic F-127. Their antimicrobial properties are investigated by microdilution method against clinically important strains of gram positive (S. aureus and B. megaterium) and gram negative (P. vulgaris and S. sonnei) bacteria. Nearly 38-50% enhancement in the antibacterial action of SNPs was observed when their hydrodynamic size was reduced to 59 nm from 83 nm. It has been observed that the antibacterial action of SNPs was governed by their hydrodynamic size and not by their crystallite and physical size. The phenomenological model was also proposed which makes an attempt to explain the microscopic mechanism responsible for the size dependent antibacterial activities of silver.

No abstract

Visible light-responsive photocatalysts are the most promising candidates for green bioremediation processes that will degrade toxic organic industrial waste into harmless compounds. Among the photocatalysts, TiO 2 is best suited for large-scale photo-induced bioremediation processes mainly because of low cost and abundance. The major obstacle in its utilization as photocatalyst is its poor response to sunlight due to its wide energy band gap. This article reports sol-gel synthesis of pristine and cobaltdoped TiO 2 nanoparticles (TNPs). Titanium (IV) isopropoxide is hydrolyzed and condensed into amorphous titanium dioxide gel by water/ethanol under acidic conditions. Irrespective of the Co concentration, TNPs always crystallize into anatase phase when calcine at 500°C. No signature of other isomorphous phases, i.e., rutile or brookite, is detected. The optical band gap of pristine (0 % Co doped) TNPs is 3.03 eV (k = 409 nm), which decreases up to 1.93 eV (k = 642 nm) when Co concentration in TiO 2 matrix increases from 0 to 2 %. Co(?2) substitution at Ti(?4) site generates additional oxygen vacancies in the TiO 2 unit cell, which introduces extra energy levels in the forbidden band that reduces the indirect energy band gap of TNPs. Co doping in TNPs makes them sensitive to visible radiation, and hence, their photoresponse is expected to be better under sunlight than pristine bulk titania, which is active only in the UV region of the electromagnetic spectrum. Graphical Abstract

Silver nanoparticles have a huge share in nanotechnology based products used in clinical and hygiene products. Silver nanoparticles leaching from these medical and domestic products will eventually enter terrestrial ecosystems and will interact with the microbes present in the land and water. These interactions could be a threat to biorecycling microbes present in the Earth's crust. The antimicrobial action towards biorecycling microbes by leached silver nanoparticles from medical waste could be many times greater compared to that of silver nanoparticles leached from other domestic products, since medical products may contain traditional antibiotics along with silver nanoparticles. In the present article, we have evaluated the antimicrobial activities of as-synthesized silver nanoparticles, antibiotics - tetracycline and kanamycin, and antibiotic-adsorbed silver nanoparticles. The antimicrobial action of silver nanoparticles with adsorbed antibiotics is 33-100% more profound against the biorecycling microbes B. subtilis and Pseudomonas compared to the antibacterial action of silver nanoparticles of the same concentration. This study indicates that there is an immediate and urgent need for well-defined protocols for environmental exposure to silver nanoparticles, as the use of silver nanoparticles in nanotechnology based products is poorly restricted.