Silver (Ag) and copper (Cu) are well known antibacterial materials. Nanostructured materials containing Ag and Cu nanoparticles (NPs) are also antibacterial thanks to the small size and large surface area of the metal particles, which allow them to penetrate bacterial cell walls and membranes. As a result, the antimicrobial effect is directly dependent on the nanoparticles’ size and shape. We have prepared silicon nanowires decorated with Ag NPs or Cu NPs and examined their antibacterial properties. We have found that the Ag NPs-Si nanowires (SiNWs) nanostructures are biocompatible with human lung adenocarcinoma epithelial cells while possessing strong antibacterial properties against E. coli bacteria.
We examined the antibacterial activity of Ag NP and Cu NP-modified SiNWs against Gram-negative bacterial strain E. coli (MTCC 726) obtained from Microbial Type Culture Collection, IMTECH, Chandigargh, India. The bactericidal effect was studied in nutrient agar (NA) medium and 60 µL of bacterial culture (around 108 colony forming unit [CFU]) through agar well diffusion assay. We investigated the activity of the modified SiNWs using a visible bacterial colony count approach, growth kinetics and leakage of sugars and proteins. Bacterial culture with unmodified SiNWs was used as a control.
We found that 94 ± 5% of bacteria on SiNWs decorated with Ag NPs were destroyed. This number was 86 ± 7% for bacteria on SiNWs decorated with Cu NPs. The amount of sugars and proteins leaked by E. coli interacting with Ag NP- or Cu NP-SiNWs was measured and compared with unmodified SiNWs. At the beginning of the experiment (t=0), the amount of sugars leaked for all the substrates is below 5 µg/mg, but this amount increases over time to reach 60.17, 101.12 and 86.5 µg/mg for unmodified SiNWs and SiNWs decorated with Ag NPs and Cu NPs, respectively, after eight hours contact with E. coli. Similarly, the SiNWs decorated with Ag or Cu NPs also accelerated protein leakage through the E. coli membrane.
As for protein leakage, the amount initially leaked from bacteria in the presence of unmodified SiNWs was found to be 1.02 µg/mg, while that from bacteria in contact with Ag NP or Cu NP-SiNWs substrates was 3.75 and 2.04 µg/mg, respectively. These amounts increased to 42.24 and 25.43 µg/mg for E. coli in contact during eight hours with Ag NPs or Cu NP-SiNWs substrates, respectively.
The SiNWs decorated with Ag and Cu NPs possess strong antibacterial activity against E. coli in the following order: SiNWs < Cu NP-SiNWs < Ag NP-SiNWs.
The substrates investigated in this work are easy to produce and also display good photocatalytic properties when irradiated with visible light. Combining the photocatalytic activity and antibacterial properties of these substrates thus holds great promise for designing very efficient antibacterial substrates.
More information can be found in the journal Nanotechnology (in press).
About the author
This interdisciplinary research work was conducted by four research teams at the: (1) Institut de Recherche Interdisciplinaire (IRI, USR CNRS-3078), Université Lille1, France; (2) Unité de Développement de la Technologie du Silicium (UDTS), Algiers, Algeria; (3) Laboratoire de Physique Quantique et Systèmes Dynamiques, Université de Sétif, Algeria; (4) CSIR-North East Institute of Science & Technology, Assam, India. The SiNWs were prepared and characterized by the French team (Ouarda Fellahi, Dr Yannick Coffinier, Dr Lionel Marcon and Dr Rabah Boukherroub) and the Algerian team (Dr Toufik Hadjersi, Prof. Mustapha Maamache). The antibacterial part of the work was carried out in collaboration with the CSIR-North East Institute of Science and Technology, Jorhat, India. Dr Manash R Das, Dr Ratul Saikia and Rupak Sarma from the CSIR-NEIST contributed to the colony count, growth kinetics, and sugar and protein leakage in this study.