Nanotoxicology, June 2011; 5(2): 244–253 Antibacterial efficacy of silver nanoparticles of different sizes, surface conditions and synthesis methods MEGHAN E. SAMBERG 1 , PAUL E. ORNDORFF 2 , & NANCY A. MONTEIRO-RIVIERE 1 1 Clinical Sciences, Center for Chemical Toxicology Research and Pharmacokinetics, North Carolina State University, Raleigh, NC, and the Joint Department of Biomedical Engineering at North Carolina State University and the University of North Carolina-Chapel Hill, and 2 Population Health and Pathobiology, North Carolina State University, Raleigh, NC, USA (Received 2 June 2010; accepted 16 September 2010) Abstract Silver nanoparticles (Ag-nps) are used as a natural biocide to prevent undesired bacterial growth in clothing and cosmetics. The objective of this study was to assess the antibacterial efficacy of Ag-nps of different sizes, surface conditions, and synthesis methods against Escherichia coli, Ag-resistant E. coli, Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), and Salmonella sp. Ag-nps samples were synthesized by: Base reduction with unmodified surfaces and used as synthesized (‘unwashed’; 20, 50 and 80 nm) or after 20 phosphate buffer washes (‘washed’; 20, 50 and 80 nm), or synthesized by laser ablation with carbon-stabilized surfaces (‘carbon-coated’; 25 and 35 nm). Unwashed Ag-nps were toxic to all bacterial strains at concentrations between 3.0–8.0 mg/ml. The washed Ag-nps and carbon-coated Ag-nps were toxic to all bacterial strains except Ag-resistant E. coli at concentrations between 64.0–1024.0 mg/ml. Ag-resistant E. coli died only when treated with unwashed Ag-nps or its supernatant, both of which contained formaldehyde. Keywords: Silver, nanoparticles, nanotechnology, nanotoxicity, microbiology Introduction Silver (Ag) is a natural biocide and compared to titanium, zinc, and copper, Ag nanoparticles (Ag- nps) have shown the greatest antimicrobial efficacy against bacteria, viruses and other eukaryotic micro- organisms (Gong et al. 2007). Ag sulfadiazine is the standard care for the prevention of widespread bac- terial growth on the skin of burn patients (Moyer et al. 1965). Nanomaterials have specific physicochemical characteristics that may differ from their bulk form due to their increased surface area to volume ratio that results in heightened reactivity (Fubini et al. 2007). The development of new synthesis methods may result in high yield concentrations and stable disper- sions of Ag-nps, thereby increasing antibacterial applications of Ag-nps, which is currently the most common used nanomaterial of all engineered pro- ducts in the world (www.nanotechproject.org 2009). Recent investigations have shown similar effects for low concentrations of Ag-nps and Ag ions having effective biocidal concentrations in the nanomolar and micromolar ranges (Lok et al. 2006; Pal et al. 2007). Ag-nps have been shown to inactivate bacteria and inhibit cellular functions such as growth, perme- ability, regulation of enzymatic activity, and respira- tory processes hypothesized by a preferential attachment to the phosphate and sulfur groups of the cell membrane (Sondi and Salopek-Sondi 2004; Baker et al. 2005; Morones et al. 2005; Lok et al. 2006; Li et al. 2010). Studies have related various physicochemical property of Ag-nps to their antibac- terial effect: Concentration (Sondi and Salopek- Sondi 2004; Pal et al. 2007), bacterial type (Kim et al. 2007), bacterial strain (Ruparelia et al. 2008), Ag-nps structure (Kim et al. 2007; Pal et al. 2007), Ag-nps size (Sondi and Salopek-Sondi 2004; Baker et al. 2005; Morones et al. 2005; Paná cek et al. 2006), and addition of surfactants or polymers (Dror-Ehre et al. 2009). Despite the rapidly increasing number of silver- containing products, there exist uncertainties Correspondence: Dr Nancy A. Monteiro-Riviere, PhD, Fellow ATS, ACT, Professor of Investigative Dermatology and Toxicology, North Carolina State University, Center for Chemical Toxicology Research and Pharmacokinetics, 4700 Hillsborough Street, Raleigh, NC 27606, USA. Tel: +1 919 513 6426. Fax: +1 919 513 6358. E-mail: nancy_monteiro@ncsu.edu ISSN 1743-5390 print/ISSN 1743-5404 online Ó 2011 Informa UK, Ltd. DOI: 10.3109/17435390.2010.525669