Impact of density of coating agent on antibacterial activity of silver nanoparticle impregnated plasma treated activated carbon
Graphical abstract
Introduction
Silver nanoparticles (Ag-NPs) are well established as one of the most efficient water disinfecting agents (Biswas and Bandyopadhyaya, 2016a, Biswas and Bandyopadhyaya, 2016b, Sondi and Salopek-Sondi, 2004). The particles are impregnated in a host matrix to prevent their aggregation and to also avoid their leaching into the environment. However, in most cases, antibacterial performance of only free NPs are tested in the batch mode (Li, 2012), whereas for water disinfection applications, it is more appropriate and essential to assess the performance in a continuous flow system.
Conventionally, different coating agents have been used for stabilizing and controlling the growth of Ag-NPs during nanoparticle synthesis and its impregnation. The effect of different coating agents like, sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), polysorbate 80 (Tween 80), polyethylene glycol (PEG), citrate etc. in the aggregation kinetics and stability of NPs was already reported in previous works (Kvítek et al., 2008, Tejamaya et al., 2012). Tri-sodium citrate was extensively used as both a reducing and a coating agent during Ag-NP synthesis, since it is non-toxic and considered safe for drinking water applications (Biswas and Bandyopadhyaya, 2016a, Srinivasan et al., 2013). To the best of our knowledge, the effect of coating density of citrate on Ag-NPs, on the antibacterial activity has not been extensively investigated.
Toxicity of Ag-NPs to bacterial cells is primarily because of the release of Ag+ ion, via the dissolution of Ag-NPs (Jung et al., 2008, Li, 2012, McShan et al., 2014). Now, coating agent is used primarily for stabilizing the NPs and controlling their size during the Ag-NP synthesis. However, the coating agent also affects the surface chemistry and thereby the ion release kinetics, during cell-killing.
Therefore, in the present work, antibacterial activity of citrate coated Ag-NPs of same particle size and of same loading percentage in activated carbon (AC) (as Ag/AC), but having different coating densities of citrate have been assessed in a continuous flow-column. The performance has been evaluated in terms of both release of Ag+ ion and detachment of Ag-NPs from the AC surface, for assessing the effect of coating density on the water disinfection performance during long term water disinfection.
Section snippets
Synthesis of Ag-NPs with different coating density
For synthesizing Ag-NPs, silver nitrate (AgNO3, Qualigens, India) was used as a precursor and tri-sodium citrate (Na3C6H5O7, 2H2O, Qualigens, India) as a reducing as well as capping agent. In the first method, typically, 70 mL, 0.01 mol/L AgNO3 was mixed with 7 mL, 0.01 mol/L trisodium citrate and the reaction mixture was placed in a ultraviolet (UV) chamber (365 nm wavelength) for 12 hr, to form Ag-NPs (Appendix A. Fig. S1a). In the second method, 100 mL, 7 mmol/L tri-sodium citrate of pH 11.1 was
Synthesis and characterization of Ag-NPs with low and high citrate coating density
Fig. S6a and b (in Appendix A) shows FEG-TEM images of well dispersed spherical Ag-NPs, having low and high citrate coating density, respectively. From these and similar images, particle size distributions were generated (Fig. 1a and b), using the ImageJ software for particle size measurement. A total of 500–600 particles were measured from three independent syntheses, by measuring 150–200 particles from each synthesis run. Ag-NPs with low and high citrate coating density show a mean particle
Conclusions
Effect of coating density of citrate on silver nanoparticle (Ag-NP) was assessed for water disinfection. For this, Ag-NPs with different coating densities (12.05 and 46.17 molecules/nm2 of Ag-NP surface area, termed as Ag-NPs with low and high coating density, respectively) were synthesized. These were separately impregnated in plasma treated, activated carbon (AC), and designated as Ag/AC, i.e., AC containing Ag-NP. In spite of having similar NP size (mean diameter of 29 and 27 nm) and similar
Acknowledgments
We thank the Department of Chemical Engineering, Indian Institute of Technology (IIT Bombay) for providing FEG-SEM and TGA-DSC facilities. We also thank sophisticated analytical instrumental facility (SAIF), IIT Bombay for FEG-TEM, FEG-SEM and ICP-AES.
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