Impact of density of coating agent on antibacterial activity of silver nanoparticle impregnated plasma treated activated carbon

https://doi.org/10.1016/j.jes.2017.08.008Get rights and content

Abstract

To use stabilized nanoparticles (NPs) in water as disinfectants over a very long period, the amount of coating agent (for NP stabilization) needs to be optimized. To this end, silver nanoparticles (Ag-NPs) with two different coating densities of tri-sodium citrate (12.05 and 46.17 molecules/nm2, respectively), yet of very similar particle size (29 and 27 nm, respectively) were synthesized. Both sets of citrate capped NPs were then separately impregnated on plasma treated activated carbon (AC), with similar Ag loading of 0.8 and 0.82 wt.%, respectively. On passing contaminated water (containing 104 CFU Escherichia coli/mL of water) through a continuous flow-column packed with Ag/AC, zero cell concentration was achieved in 22 and 39 min, with Ag-NPs (impregnated on AC, named as Ag/AC) having lower and higher coating density, respectively. Therefore, even on ensuring similar Ag-NP size and loading, there is a significant difference in antibacterial performance based on citrate coating density in Ag/AC. This is observed in lower coating density case, due to both: (i) higher Ag+ ion release from Ag-NP and (ii) stronger binding of individual Ag-NPs on AC. The latter ensures that, Ag-NP does not detach from the AC surface for a long duration. TGA-DSC shows that Ag-NPs with a low coating density bind to AC with 4.55 times higher adsorption energy, compared to Ag/AC with a high coating density, implying stronger binding. Therefore, coating density is an important parameter for achieving higher antibacterial efficacy, translating into a faster decontamination rate in experiments, over a long period of flow-column operation.

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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