Chromium has long been recognized as a toxic, mutagenic and carcinogenic metal. It is toxic to microorganism, plants, animals and humans. Chromium

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Cuprous oxide nanoparticles (Cu2O NPs) were fabricated in reverse micellar templates by using lipopeptidal biosurfactant as a stabilizing agent. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectrum (EDX) and UV–Vis analysis were carried out to investigate the morphology, size, composition and stability of the nanoparticles synthesized. The antibacterial activity of the as-synthesized Cu2O NPs was evaluated against Gram-positive B. subtilis CN2 and Gram-negative P. aeruginosa CB1 strains, based on cell viability, zone of inhibition and minimal inhibitory concentration (MIC) indices. The lipopeptide stabilized Cu2O NPs with an ultra-small size of 30 ± 2 nm diameter exhibited potent antimicrobial activity against both Gram-positive and Gram-negative bacteria with a minimum inhibitory concentration of 62.5 µg/mL at pH5. MTT cell viability assay displayed a median inhibition concentration (IC50) of 21.21 μg/L and 18.65 μg/mL for P. aeruginosa and B. subtilis strains respectively. Flow cytometric quantification of intracellular reactive oxygen species (ROS) using 2,7-dichlorodihydrofluorescein diacetate staining revealed a significant ROS generation up to 2.6 to 3.2-fold increase in the cells treated with 62.5 µg/mL Cu2O NPs compared to the untreated controls, demonstrating robust antibacterial activity. The results suggest that lipopeptide biosurfactant stabilized Cu2O NPs could have promising potential for biocompatible bactericidal and therapeutic applications.

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BiocatalysisBacillus sp. IntroductionHexavalent chromium [Cr(VI)] compounds are used in a wide variety of commercial processes and unregulated disposal of chromium containing effluents has led to the contamination of soil, aquatic sediments, and surface and groundwater environments. Chromium, a steel-grey, lustrous, hard and brittle metal, occurs in nature in the bound form that constitutes 0.1-0.3 mg/kg of the Earth's crust. It has several oxidation states ranging from (ÀII) to (þVI), the trivalent and hexavalent states being the most stable. A maximum acceptable concentration of 50 mg/L for Cr(VI) in drinking water has been established on the basis of health considerations (Kiilunen, 1994). In some American states, the exposure limit for Cr(VI) is as low as 15 mg/L for humans and 10 mg/L for aquatic organisms (Levitskaia et al., 2008) which is below the detection limit for most low cost colorimetric methods. Cr(VI) concentrations above the allowable limit cause cancer in humans and aquatic fauna, and is acutely toxic at much higher concentrations (U.S. EPA, 1978; Federal Register, 2004 A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / w a t r e s ARTICLE IN PRESS0043-1354/$ -see front matter ª