Effect of Morphology and Concentration on Crossover between Antioxidant and Pro-oxidant Activity of MgO Nanostructures

Abstract

The toxicity of nanomaterials can sometimes be attributed to photogenerated reactive oxygen species (ROS), but these ROS can also be scavenged by nanomaterials, yielding opportunities for crossover between the properties. The morphology of nanomaterials also influences such features due to defect-induced properties. Here we report morphology-induced crossover between pro-oxidant activity (ROS generation) and antioxidant activity (ROS scavenging) of MgO. To study this process in detail, we prepared three different nanostructures of MgO (nanoparticles, nanoplates, and nanorods) and characterized them by HRTEM. These three nanostructures effectively generate superoxide anions (O2 •–) and hydroxyl radicals (•OH) at higher concentrations (>500 μg/mL) but scavenge O2 •– at lower concentrations (40 μg/mL) with successful crossover at 200 μg/mL. Nanorods of MgO generate the highest levels of O2 •–, whereas nanoparticles scavenge O2 •– to the highest extent (60%). Photoluminescence studies reveal that such crossover is based on the suppression of F2+ and the evolution of F+, F2 +, and F2 3+ defect centers. The evolution of these defect centers reflects the antibacterial activity of MgO nanostructures which is initiated at 200 μg/mL against Gram-positive S. aureus ATCC 29737 and among different bacterial strains including Gram-positive B. subtilis ATCC 6633 and M. luteus ATCC 10240 and Gram-negative E. coli ATCC K88 and K. pneumoniae ATCC 10031. Nanoparticles exhibited the highest antibacterial (92%) and antibiofilm activity (17%) against B. subtilis ATCC 6633 in the dark. Interestingly, the nitrogen-centered free radical DPPH is scavenged (100%) by nanoplates due to its large surface area (342.2 m2/g) and the presence of the F2 + defect state. The concentration-dependent interaction with an antioxidant defense system (ascorbic acid (AA)) highlights nanoparticles as potent scavengers of O2 •– in the dark. Thus, our findings establish guidelines for the selection of MgO nanostructures for diverse therapeutic applications.

Bioactive silver phosphate/polyindole nanocomposites

“…Biolm inhibition of S. aureus and E. coli was quantied by a colorimetric detection process. 6,16,58 Specically, fresh inoculums (1 Â 10 6 CFU mL À1 , 200 mL) was pipette out white 96 wellplate onto which the nanocomposite solutions (1g L À1 ) were added and incubated at 37 C for 4 days. Control wells were marked without the nanomaterials.…”

“…2A and B) which in advance quanties the amount of extracellular polymeric substances, generated by biolms and indirectly monitors biolm biomasses. 6,16,58 The respective inhibition efficiency of these nanocomposites is calculated according to the method suggested by Jaiswal et al 73 It is noteworthy that the Ag 3 PO 4 /Pln nanocomposites inactivate the microbial biolms to various extents. The highest biolm inhibition efficiency against E. coli was observed for Pln2, followed by Pln1 and Pln3 ( Fig.…”

“…particle size, morphology and crystal defects). [4][5][6] Understanding the mechanism of the antibacterial activity of these nanomaterials is important to control their dosing in vivo and any potential environmental impact. 7 The ability of nanomaterials to generate reactive oxygen species (ROS) including superoxide anions (O 2 c À ), 8 hydroxyl radicals ($OH), 8 singlet oxygen ( 1 O 2 ) 9,10 and secondary oxygen centered species such as hydrogen peroxide (H 2 O 2 ) which is developed during the disproportionation of O 2 c À which further transforms into $OH and 1 O 2 , 9,10 is of particular interest in regards to toxicity due to the ability of such oxygen centered reactive species to oxidize various cellular constituents.…”

“…14 Various nanomaterials display antibacterial activity: e.g. ZnO, 15,16 TiO 2 , 17,18 MgO, 6 ZnO/TiO 2 nanohybrids, 19 ZnO/ZnFe 2 O 4 , 20 CuO, 21 Ag 3 PO 4 (ref. 22) etc.…”

“…22) etc. In vitro studies detecting ROS derivation from nanomaterials enables the establishment of the underlying mechanism of antibacterial activity to some extent, as the production of different ROS (O 2 c À , $OH and 1 O 2 ) are dependent on the size, [23][24][25] shape, 24,26,27 and heterostructures of the nanomaterials, 6 but there remains some ambiguity in their mechanism of function.…”