Colloids and Surfaces B: Biointerfaces 104 (2013) 133–139 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces B: Biointerfaces jou rn al h om epage: www.elsevier.com/locate/colsurfb Bactericidal activity of Ag-doped multi-walled carbon nanotubes and the effects of extracellular polymeric substances and natural organic matter Rina Su a , Yinjia Jin a , Yang Liu b , Meiping Tong a,∗ , Hyunjung Kim c,∗∗ a The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China b Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta T6G 2W2, Canada c Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea a r t i c l e i n f o Article history: Received 20 May 2012 Received in revised form 25 November 2012 Accepted 5 December 2012 Available online 13 December 2012 Keywords: Ag 0 /MWCNTs Bacteria disinfection Reactive species EPS NOM a b s t r a c t The objective of this study was to determine the bactericidal mechanisms of Ag-doped multi-walled carbon nanotube (MWCNT) nanoparticles (Ag 0 /MWCNTs) to Escherichia coli DH5. The contributions of silver ion dissolution, reactive species, and direct contact on bacteria inactivation were systematically determined. The relatively higher survival rate of bacteria exposed to 0.02 mg L -1 Ag + ions (the maximum concentration of Ag + ions dissolved from Ag 0 /MWCNTs) suggested that the antibacterial property of Ag 0 /MWCNTs was not caused by silver ion dissolution. The effects of each reactive species ( • OH, H 2 O 2 , • O 2 - , h + , and e - ) on the disinfection process were investigated by using multiple scavengers, and the results showed that • OH b , • OH s , and h + play important roles in bactericidal actions. The significance of • OH b , • OH s , and h + in the disinfection process was further confirmed in the partition systems combined with scavenger. The antibacterial effects of these reactive species mainly arose through direct contact of the nanocomposites with the bacteria. The effects of extracellular polymeric substances (EPS) and natural organic matter (NOM) on the inactivation of bacteria were also investigated. The lower antibacterial effect observed for EPS-rich bacteria relative to EPS-poor bacteria demonstrated the protective effects of EPS in the disinfection system. The decreased bacterial toxicity effect acquired by the addition of humic acid (as the model NOM) in the disinfection system demonstrated the influence of NOM on the bacterial toxicity of nanocomposites, where the sorption of NOM onto the surface of the nanocomposites contributed to the decreased antibacterial effects. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Due to their excellent antimicrobial properties, Ag nanopar- ticles (Ag NPs) have recently attracted considerable attention in a variety of fields, including within the cosmetics industry [1,2], medical institutions [3,4], food production [5,6], and water treat- ment plants [7,8]. However, because of their high surface energy, Ag NPs tend to be oxidized and to aggregate into large particles after being dispersed in water. The wide application of Ag NPs as large- area antimicrobial agents in water treatment processes is therefore greatly inhibited [9,10], and intensive research efforts have recently focused on stabilizing Ag NPs [11–22]. This focus has yielded the alternative method of depositing Ag NPs on host nanomaterials, rather than anchoring organic molecules onto the surfaces of Ag ∗ Corresponding author at: School of Environment and Energy, Shenzhen Gradu- ate School of Peking University, Shenzhen 518055, PR China. Tel.: +86 10 62756491; fax: +86 10 62756526. ∗∗ Corresponding author. Tel.: +82 63 2702370; fax: +82 63 2702366. E-mail addresses: tongmeiping@iee.pku.edu.cn (M. Tong), kshjkim@jbnu.ac.kr (H. Kim). NPs to prevent their aggregation and oxidation [13–22]. Different materials, including SiO 2 , zeolite, TiO 2 , activated carbon, and car- bon nanotubes (CNTs) have been used as host materials for Ag NPs [13–22]. In particular, CNTs have been recognized as a superior host material to support Ag NPs, due to their unique structure and bacte- ricidal properties [18–22]. For example, by ultrasonic irradiation of dimethylformamide and silver acetate precursors in the presence of CNTs, Rangari et al. [20] recently synthesized Ag/CNTs hybrid NPs, and found that the nanocomposites demonstrated stronger antimicrobial activity toward four types of bacteria relative to the commercially available Ag NPs and pristine CNTs. By utilizing the acid-treated multi-walled CNTs (MWCNTs) as the supporter, Li et al. [21] very recently synthesized Ag/MWCNT nanocomposites, find- ing that the hybrid nanomaterials processed excellent bactericidal activity. Although the antibacterial properties of Ag/CNTs have been pre- viously demonstrated [18–22], far less attention has been directed toward understanding the mechanisms governing the antimi- crobial activity of Ag/CNTs hybrid NPs and these mechanisms, therefore, require further investigation. Moreover, previous studies have shown that extracellular polymeric substances (EPS), biopoly- mers located on or around cell surfaces, have protective effects for 0927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfb.2012.12.002