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Engineering Silver Nanoparticle Impregnated Activated Carbon for Water Purification

A research proposal submitted to Science and Engineering Research Council


Rajdip Bandyopadhyaya

Department of Chemical Engineering

Indian Institute of Technology Bombay

Powai, Mumbai 400076





NEW DELHI - 110 016

December 2011


(To be filled by applicant)

(Sections 101 to 192 to be on separate sheet(s))
101. Project Title: Engineering Silver Nanoparticle Impregnated Activated Carbon for Water Purification
102. Broad Subject: Engineering Sciences
103. Sub Area: Chemical Engineering
104. Duration in months: 36 months

105. Total cost: Rs. 49,44,000/- + Rs. 9,88,800 (20% overhead) = Rs. 59,32,800/-

106. FE Component: 47,774 US$ and 3,257 Euro

107. Project Category: Basic Research

111. Principal Investigator: Rajdip Bandyopadhyaya

112. Designation: Associate Professor

113. Department: Chemical engineering

114. Institute Name: Indian Institute of Technology Bombay

115. Address: Chemical Engineering Dept.

Indian Institute of Technology Bombay

Powai, Mumbai 400076
116. Date of Birth: 24-th November, 1969

117. Telephone: (022) 2576 7209

Fax: (022) 2572 6895


118. Co-Investigator: K. V. Venkatesh

119. Designation: Professor

120. Department: Chemical engineering

121. Institute Name: Indian Institute of Technology Bombay

122. Address: Chemical Engineering Dept.

Indian Institute of Technology Bombay

Powai, Mumbai 400 076
123. Date of Birth: 7-th June, 1967

124. Telephone: (022) 2576 7223

Fax: (022) 2572 6895


Registration No.: (to be filled by DST)

Project Title:………………………………………………..
Principal Investigator……………………………….. Institution i)……………………ii)…………………


191. Project summary

Removal of microorganisms from drinking water to obtain potable water is a critical requirement in various societies across the world, and particularly in India. Activated carbon (AC) has been the adsorbent of choice for removal of colour, odour and other impurities from water due to its extremely high specific surface area. However, carbon on its own does not remove or kill microorganisms. On the other hand, silver (Ag) is known to possess excellent antimicrobial properties and almost no toxic effect for humans. We therefore want to investigate the efficacy of silver embedded activated carbon (Ag-AC) as a composite material for disinfection and purification of water. The proposed project would look at both bulk solution and structured templates, for synthesis of nanoparticles of controlled size, shape and composition. Therefore, it would address the very important mechanism of assembly (nucleation and growth of clusters and molecules) and interactions (particle growth by diffusion and reaction) at nano and mesoscopic length scales. The second aspect that will be explored is differential functionalization of a porous carbon surface, so as to make some regions of the material more interactive towards metal deposition. This will help us engineer a more favourable biotic-abiotic interaction for removal of microorganisms in water, such as, Escherichia Coli, Bacillus Subtilis, fungal systems etc. The insight gained via combined experimental and modeling work will help identify optimum synthesis and impregnation techniques for making Ag-AC composites to achieve the best antibacterial properties in water treatment.

192. Key words (maximum 6): Water purification, activated carbon, nanoparticle, silver,

Impregnation, antibacterial property

200. Technical details

210. Introduction

211. Origin of the proposal

Removal of microorganisms from drinking water to obtain potable water is a critical requirement in various societies across the world, and particularly in India. Activated carbon (AC) has been the adsorbent of choice for removal of colour, odour and other impurities from water. This is due to the extremely high specific surface area (in the range of 1000 – 1500 m2/g) of AC, owing to the presence of a large number of pores in the granules of AC (Figs. 1a and 1b),. However, carbon on its own does not remove or kill microorganisms. On the other hand, silver (Ag) is known to possess excellent antimicrobial properties and almost no toxic effect for humans. We therefore want to investigate the efficacy of silver embedded activated carbon (Ag-AC) as a composite material for disinfection and purification of water.

Presently, AC (without Ag) is commercially employed as a cylindrical filter-bed (in the form of a replaceable filter-candle in domestic or other water purification systems), consisting of millimeter or few hundreds of micrometer sized individual porous granules of cabon (Fig. 1a). The carbon granules are bonded with a polymeric adhesive to form the filter-bed. The intraparticle pores in each of the granules form a randomly connected network of pores. Diameter of this porous-channel network typically ranges in the order of a few micrometers down to nanometers (Figs. 1b – 1d). Thus, there are several length scales starting from the larger individual granules to the very small intraparticle porous network, giving rise to a very high specific surface area for adsorption and physicochemical purification of water.

However, in order to impart antimicrobial properties to such a module, it is necessary to establish the maximum possible contact area between Ag with any microorganisms present in the water flowing through the filter-bed. We expect this to be achievable, if Ag is embedded as very small nanoparticles - either on the external surface of the granules of AC, or inside the intraparticle pores of AC (Fig. 1d). Morphology and location of Ag nanoparticles (in solid AC granules) will determine this contact area, and will hence dictate the optimum performance of such a system. In the current proposed research, we will address these two parameters – morphology and location of nanoparticles on a porous solid adsorbent. More specifically, we will investigate how to achieve the maximum contact, by optimizing these two parameters, given a fixed amount of Ag available for embedding in AC.

212. Definition of the problem

Based on the above discussion, three aspects therefore would be fundamentally important for this application: (i) controlled method of synthesizing Ag nanoparticles – leading to a specific particle morphology in terms of size, shape and composition; (ii) superior technique of embedding/impregnation of Ag in AC – leading to a favoured, engineered location of particles on the surface or pores of AC; (iii) continuous column-mode studies to interpret kinetics of disinfection (e. g. removal of E. coli from water) using Ag-AC composite.

The first of these parameters, namely morphology, is given in terms of size, shape and composition of nanoparticles. Understanding the mechanism of formation of nanoparticles and to thereby control its size, shape and composition is therefore critical in order to derive the maximum benefit. In this regard, it is most important to have monodisperse particles – since the structural, chemical and other properties of interest are a function of particle size in the 1-100 nm size range. Choice of a proper synthesis step for nanoparticles is the first step to address this.

Commonly employed bulk colloidal routes to inorganic nanoparticle formation involve precipitation or reduction of an aqueous salt solution (for synthesizing metal, metal oxide, semiconductor etc.) or sol-gel reaction of suitable precursors in water (for metal oxide). However, these methods do not provide adequate control over mixing and diffusion of reactants as reaction occurs in the bulk solvent (water) phase. This results in spatially non-uniform nucleation, growth and agglomeration processes during particle formation. As a result, the product is often polydisperse, with large variation in size and shape. In addition, for multicomponent particulate products (consisting of two chemical species in a single particle), there can be spatial segregation of the two species, implying compositional differences and therefore variability of the properties on a micro to mesoscopic length scale. However, by using stabilizing agents like citrates, one can have a somewhat better control, wherein citrate stabilized Ag nanoparticle dispersions in water can be made by the citrate reduction method.

In contrast to the above bulk solvent routes, one can also use polymeric templates - like dendrimer (Fig. 2a) molecules dissolved in water – via which the mixing and relevant reaction can be confined within a pre-defined zone of the dendrimer molecule – leading to synthesis of a more controlled nanoparticulate product. Dendrimers are few nanometers in diameter, e.g. a 4.5 generation poly-amidoamine (PAMAM) dendrimer molecule has a diameter of 5 nm (Fig. 2b). In addition, by tailoring the surface functional group on the outer surface of the dendrimer molecule, one can have different states of hydrophilicity and alter the location of nanoparticle formation on to the outer surface. Furthermore, controlled increase in the size of the dendrimer template (by increasing the generation number) offers a relatively easy route to control the nanoparticle size. It is hence challenging to explain the mechanism of formation of nanoparticles by observing the variation in particle size in response to changes in template size, reactant concentration and other controlling parameters affecting reaction, nucleation and growth rates.

It is important to note here that both citrate and PAMAM dendrimers are bio-compatible and non-toxic, and can therefore be present in the final Ag-AC product, which comes in direct contact with the treated water.

Therefore, one of the focus will be that of understanding the mechanism of formation of nanoparticles via templates (like dendrimers, emulsions etc.) in the aqueous medium (Fig. 3a). This may give a better size, shape and composition control, applicable for different classes of nanoparticles in general and metals like Ag in particular. Stabilized dispersion of Ag nanoparticles in water has shown promise as an antimicrobial agent in various contexts. Thus the basic understanding gained in nanoparticle formation mechanisms will be utilized to prepare and test various kinds of Ag dispersions in water (either templated or as citrate stabilized). These dispersions impregnated in AC can be potential candidates for complete disinfection of water.

The second parameter to be addressed is the location of Ag nanoparticles in the porous carbon granule of AC. To this end, we will synthesize silver nanoparticles in different ways – either in-situ within the pores, or as an externally prepared citrate or PAMAM stabilized dispersion, impregnating the dispersion onto AC. The aim will be to achieve the maximum disinfection of drinking water for a given minimum amount of Ag.

The understanding achieved in this part of the research will help us gain insight in the interactions of nanoparticles impregnated on a disordered porous adsorbent with a flowing water phase having bacterial cells like Escherichia Coli (E. coli). E. coli is a water-borne bacterial pathogen and is a model system for disinfection studies. It will eventually help us come up with strategies of engineering a porous adsorbent differentially – for example to make the external surface of the adsorbent more hydrophilic, compared to the internal surface of the intraparticle pores - thereby promoting more effective contact between the flowing water phase with the Ag nanoparticles. We will utilize dry processes like plasma or UV treatment of AC to achieve such differential wetting characteristics between external surface and hidden pores inside. This will lead to establishment of a mechanism of favoured locations of impregnating metal nanoparticles from a dispersion onto a porous adsorbent, so as to engender a fruitful and better contact of the nanoparticles with the target pathogens in a flowing water stream.

213. Objectives

Accordingly, as per the problem definition, our research in this area will first look at three different aspects of forming nanoparticles of controlled morphology – size, shape and composition – each of which may impact antibacterial properties of Ag.

The first aim is to determine the effect of different length scales of the templating moiety (dendrimer or emulsion) in which the nanoparticle is formed. This will also bring out the role of confining the processes of reaction, nucleation and growth in a cavity, whose size (in the range of few nanometers) is systematically varied by using dendrimers of different generations or emulsions of different average drop size, ultimately culminating in a bulk aqueous medium, where the relevant eddy-mixing length scales are of the order of a few micrometers – enabling a study of mixing length-scale effect on nanoparticle morphology.

As the available specific surface area increases with decreasing Ag nanoparticle size, the rate of formation of reactive oxygen species is expected to increase, so a higher antibacterial activity may be observed for smaller nanoparticles. Presently, we can synthesize Ag nanoparticles in the size range of 10 – 50 nm. We will aim synthesizing smaller size nanoparticles with a narrower size distribution, so as to achieve an enhanced effect with a smaller amount of Ag in the Ag-AC composite.

Next objective is to understand the effect of shape. Synthesis of particles in the porous channels of activated carbon (AC) can be tuned to make spherical or cylindrical nanoparticles or nanorods of Ag, and determine parameters controlling shape and its transition from sphere to other anisotropic forms. One literature report suggests that triangular Ag nanoparticles have better activity. This will be systematically tested against different shapes synthesized by us.

Afterwards, concentration of reactants and the overall reaction scheme will be decided for multi-component systems (like Ag-Cu or Ag-Fe nanoparticles, since Cu and Fe are also known to have some desirable antimicrobial properties like Ag) to obtain either core-shell morphology or homogeneous composition of the two metals present in a single nanoparticle. This will enable composition control in each single particle rather than achieving an overall particle ensemble characteristics – leveraging best biocidal properties.

In conjunction to the above experiments, we would develop models and simulate nanoparticle formation in dendrimers, emulsions and citrate stabilized systems. The primary interest would be in predicting particle size distribution obtained as a function of experimental conditions, and thereby provide theoretical understanding towards precise control of particle size, shape and composition for maximum benefit in water disinfection.

On the application front, the above theoretical and experimental insights will be synchronised to design the ideal Ag impregnation system onto AC. Research findings in optimum surface treatment of AC and preferential location of Ag nanoparticles on AC will result in an optimum Ag-AC composite filter for pathogen-free potable water. Simultaneously, insights gained from continuous flow-studies of an E. coli laden water stream against an Ag embedded carbon surface/column will show how optimum contact between metal, carbon and bacteria is established for possible contact-kill mechanism of E. coli. Fluorescence studies and imaging will elucidate the dynamics of flow and biotic-abiotic contact here.
Finally, we will address issues on how to engineer a very good adhesion of metal nanoparticles to a disordered carbon surface during the impregnation process, so that the metal does not leach out from the AC surface over time. That way, the antibacterial properties of an Ag-GC composite filter will remain effective for long-term water disinfection needs of a given unit, which is typically one year for a domestic set-up. One has to here apply principles of Chemical Engineering, so as not to have a very high pressure drop due to the possibility of excess Ag blocking the pores of AC, yet have sufficient quantity of strongly bonded (high adhesive force) Ag for complete removal of E. coli within the time period of contact, the latter being only a few seconds in a typical domestic filter unit.

Thus, the overall research objective would to study the synthesis, structure, dynamics and properties of nanoparticles and their impregnation and interaction with disordered porous adsorbents, so that one can engineer the required amount and location of Ag in an AC filter unit, thereby obtaining pathogen-free water over a sustained period of time, without any loss of activity of Ag.

With this viewpoint, the work proposed research will involve both modeling and experimental investigations.
220. Review of status of Research and Development in the subject
221. International status
The efficacy of Ag-GC in achieving antimicrobial properties will largely depend on making controlled size, shape and composition of Ag nanoparticles, either in bulk or in templates like emulsions or dendrimers. To understand these aspects, one has to therefore have a complete particle formation mechanism, leading to controlled nanoparticle properties in general; which more specifically will apply in Ag synthesis and its impregnation in AC.
However, the challenge of controlling nanoparticle size, shape and composition is only partly resolved till date. Pileni’s pioneering experimental work on metal particle synthesis using functionalized surfactants in the formation of reverse micelle (leading to water-in-oil microemulsions) gives some insight into both size and shape control. Many subsequent experiments1,2 consistently showed that particle size can be controlled by the microemulsion drop size. Her group also demonstrated3 that nanoparticles of various shapes could be produced by two techniques. Firstly,4 to use templates having a shape similar to that expected of the particles. However, only a fraction of the particles followed the shape of the template. Secondly,5 shape control was attempted by adding a salt during the particle formation process. Both routes independently showed significant effect on particle shape. Use of mixed surfactants6 in making reverse micelles also showed anisotropy in final particle shape. It turns out that all the methods to produce anisotropic particles depend on one single conclusion; particle growth is preferential in certain directions than others. The reason would be that adsorption of ions, surfactants, polymers or impurities on some of the preferred crystal faces either slowed down or stopped particle growth perpendicular to that crystal face. Apart from such a qualitative argument, no model could explain, quantitatively, anisotropy in particle shape.
In a seminal work by Bawendi’s group, a precise size control of II-VI semiconductor nanoparticles was achieved by a method called TOPO synthesis.7 In this method, two reactants are taken in the form of organometallic precursors and decomposed at a high temperature (300 °C) in a bath of a surfactant, [trioctyl phosphine oxide (TOPO)]. The process is in thermodynamic equilibrium, and by adjusting temperature and hence particle growth rate, one can control particle size. Alivisatos’ group, following TOPO synthesis, showed that shape control8 could be achieved by adding HPA (hexyl phosphonic acid) as an additive in the reaction mixture. His group synthesized9 rods, tetrapods and arrow shaped nanoparticles following this formulation. However neither Pileni’s nor Alivisatos’ methods are useful for our purpose, as the former involves an oil medium as the bulk solvent, whereas, the latter uses organometallic precursors and phosphorous-based additives, none of which can be used for processing potable water.
On the modeling front, both Bawendi’s and Alivisatos’ groups have embarked on explaining the anisotropic growth through first principle theoretical models10, 11 of free energy calculations. Rabani’s molecular dynamic simulation12 shows the preferential growth in c-axis of CdSe crystal, which gives us a possible first explanation for the nanorod formation. Sundmacher’s group have followed BaSO4 nanoparticle formation by Monte Carlo (MC) simulations,13, 14 which are based on the MC methods of Bandyopadhyaya et al.15 Hirai’s group synthesized various single component16, 17 and core-shell18 semiconductor nanoparticles, by the reverse micellar route. Their work also included some preliminary population balance equation (PBE) based modeling.16, 19
222. National status

Pradeep and co-workers have focussed on the synthesis of silver and gold nanoparticles, metal oxide coated silver and gold core-shell nanoparticles etc., by aqueous phase salt reduction method. In this, a salt precursor is reduced by a suitable reducing agent in an aqueous medium, in the presence of a stabilizer, which can protect the nanoparticles from self aggregation. Although particle size and shape control are achieved by varying the concentration ratio of salt to stabilizer, such a control remains merely empirical. They further studied self assembly of these particles, characterization and applications in antibacterial water treatment.20 However, they employed Ag nanoparticles in a polyurethane foam; the latter is not used in water purification studies, and hence one requires studies using activated carbon, widely used and standardized in water purification. Pal and co-workers have worked extensively on single component and core-shell nanoparticles by various routes including reverse micelle, seed mediated growth and aqueous phase reduction methods. Gold21 nanoparticles of various shapes have also been synthesized by this group. Some evidences to the effect of space confinement on particle size are due to this group. However, a quantitative explanation for size and shape control is yet to be proposed. Research in Sastry’s group aims to synthesize nanomaterials of various morphologies using various colloidal routes and biological templates.22 He showed that use of biological templates significantly controls size and shape of nanoparticles. Nevertheless, these biological templates are suitable only for making a specific material, and the role of template on particle shape is not completely understood.

There have been a good amount of PBE based modeling and MC based simulation work for developing nanoparticle formation mechanism in water-in-oil microemulsion systems. Although microemulsions are not applicable for the present application involving water purification, the modeling and simulation framework already developed for these systems are useful for our present research and are summarized below.
In this regard, with significant modeling and analysis - Kumar, Gandhi, Bandyopadhyaya and others presented some of the initial mechanisms of nanoparticle formation,15, 23, 24 with further improvement of simulation techniques later.25 Combining experiments and modeling, other have made important contributions to the understanding of nanoparticle formation. In a series of experimental papers, they have addressed the effect of oil medium on the particle size of silver chloride,26 silver27 and calcium carbonate.28 Population balance29, 30 and Monte Carlo31, 32, 33 simulation have been performed for the explanation of the dependence of particle size on various parameters like reactant concentration, molar ratio of reactants, water drop size etc. Accounting for realistic particle nucleation and reactant exchange rates, models from Bandyopadhyaya’s group34, 35 show excellent comparison with their own and others’ experimental data. They have also developed a comprehensive Monte Carlo simulation method for nanoparticle formation by mixing two reactants in two solutions.34
To summarize, only very few researchers have attempted to explore the underlying principles in the nanoparticle formation processes. Although various research groups across the world synthesize nanoparticles of various materials in different sizes and shapes, the precise control of size, shape and composition over a broad regime of experimental and process conditions is not yet achieved.
One solution is to use biocompatible polymeric templates like dendrimers However, it should be remembered that template size and shape are not the only controlling parameters. There are other factors like, surface functional groups of the templates, chemical structural differences inside the template (as in case of a dendrimer). These will play a crucial role in particle formation. Our research goal is to fill this void.
Therefore, a comprehensive particle formation mechanism based on simultaneous experimental and modeling work will enable us to a-priori specify experimental conditions for the synthesis of nanoparticles in general, and Ag in particular; with desired properties, like stability, biocompatibility and antimicrobial characteristics.
Our current work
Modeling and Simulation:

In the past, PI and his research group have published models (Fig. 3b) and simulation of spherical,36 core-shell 37,38 and anisotropic nanoparticles,39 of different semiconductor materials (CdS, CdS-ZnS, PbS-ZnS, ZnO, ZnS) and validated these with experimental data of both their own and others’. Thus the PI has a theoretical framework, elucidating the simultaneous interplay of species-transport, chemical reaction, nucleation, particle growth and coagulation, leading to a nanoparticle dispersion in a liquid phase. This will be further generalized to tackle the special issues of intermediate metal cluster formation, finally translating to metal nanoparticle formation.


On the application front, we have successfully made Ag particle embedded AC for the first time in the recent past (Figs. 1c and 1d), to assess its ability in inhibiting the growth of E. coli.40 Ag-AC was made by impregnating AC with AgNO3 and then reducing it to metallic Ag. Plate assay showed slight inhibition of E. coli, even with Ag-AC prepared from 0.005 M AgNO3, but this and shake flask tests showed a conspicuous effect only for higher concentrations of 0.1 M – 1 M AgNO3 (Fig. 4) . Flow tests further indicated that Ag-AC made from 1.0 M AgNO3 caused a desirable 3 orders of reduction in E. coli number concentration in less than 30 seconds. Based on these preliminary results one can conclude that, about 9 - 10.5 wt.% of embedded Ag in the final Ag-AC product is necessary for the requisite complete inhibition of E. coli, killing bacteria in the contact-mode for up to 350 liters of flowing water. These results have shown us that Ag-AC possesses antibacterial property and can be used for disinfection to produce potable quality water. We have started Ag nanoparticle synthesis by citrate reduction too (Fig. 5).

Our results, therefore, establishes the role of Ag used directly in solid AC granules, facilitating potential adoption of already used AC,41 as a value added Ag-AC product for potable water production. This is in contrast to the previous work in the literature focusing only on the role of colloidal solutions of metallic Ag particles,42,43 or Ag ions in some cases,44 in mitigating bacterial growth.
Next we want to study surface modification of AC and other methods of Ag synthesis and impregnation (discussed in detail in the section on “Methodology”) for their disinfection capabilities. So these are the directions (emanating from recent work in our group) in which further investigation will be carried out in the proposed research project.

(a) (b)

(c) (d)
Figure 1. Scanning electron microscope (SEM) images of: (a) pure activated carbon (AC) granules, showing different size, shape and surface texture of each granule, scale bar: 400 micron. (b) a pure AC granule showing channel-like interconnected cylindrical pores in it, scale bar: 20 micron. (c) Ag impregnated AC (Ag-AC) granule, showing Ag particles and clusters (as bright white spots), made by in-situ impregnation and chemical reduction to Ag, scale bar: 50 micron. (d) higher magnification image of Ag-AC granule of Fig. 1(c), showing Ag particles both inside the pore and on the external surface of the granule, scale bar: 10 micron.



Figure 2. (a) Schematic of a single dendrimer molecule; highly branched monomers leading to tree-like generational structure. (b) Schematic of carboxyl-terminated 4.5 generation PAMAM [ploy (amidoamine)] dendrimer. Black spherical dot is a representative nanoparticle location, which can be varied to be either inside, or on the carboxyl-terminated surface functional groups.


Figure 3. (a) Schematic of nucleation (of a cluster of molecules) and growth (of nuclei) processes leading to nanoparticle formation. (b) A representative form of a population balance equation (PBE) capturing the nanoparticle formation mechanism from our model.

Figure 4. Plate test with E. coli, showing 3 inhibition zones formed by Ag-GC samples.

  1. (b)

Figure 5. (a) Schematic of the citrate reduction method for synthesizing Ag nanoparticle. (b) Photograph showing Ag nanoparticle dispersion synthesized by us via citrate reduction. (c) TEM image Ag nanoparticles from above, typical particle size is 20-25 nm.


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  1. Panáček, A.; Kvítek, L; Prucek, R; Kolář, M; Večeřová, R; Pizúrová, N; Sharma, V. K.; Nevěčná, T.; Zbořil, R. Silver colloid nanoparticles: Synthesis, characterization, and their antibacterial activity. J. Phys. Chem. B 2006, 110, 16248.

  2. Feng, Q. L.; Wu, J.; Chen, G. Q.; Cui, F. Z.; Kim, T. N.; Kim, J. O. A mechanistic study of the antibacterial effect of silver ions on Escherichia coli and Staphylococcus aureus. J. Biomed. Mater. Res. 2000, 52, 662.

224. Review of expertise available with proposed investigating group/institution in the subject of the project

The principal investigator, Rajdip Bandyopadhyaya, has considerable experience in experiments and modeling of colloids and nanomaterials. Several PhD and M. Tech. students are working with the PI on various problems in the area of nanoparticles, porous materials, thin films, composites, modeling and simulation.

In addition, other analytical facilities for measurement and characterization are already available in the IIT Bombay campus which would be utilized on a prior booking basis. Some of these are: TEM, SEM, BET, XRD, FTIR, Laser Raman spectrometer etc.

Co-investigator, Prof. K. V. Venkatesh, is an expert in Biosystems Engg. and leads a large research group. Their in-house developed methylene blue reduction test (MBRT) would be utilized to assess the kinetics of E. coli death by quantifying the death rate through spectroscopic measurements. In addition, the usual plate assay and shake flask tests would also give complementary information on the antibacterial properties of Ag-GC.

Dr. V. Shankar (Filtrex Technologies Pvt. Ltd., Bangalore) has extensive experience in working with activated carbon (AC) based water filters and purification systems. In fact, the PI has already published some of their joint research results.40 Further innovation and optimization in both nanoparticle synthesis and its impregnation is required in order to improve the performance and make Ag-GC composite a viable alternative.

225. Patent details (domestic and international) No patents on Ag-GC composites in water treatment.
230. Work plan
231. Methodology

  1. Nanoparticles would be first formed in a bulk medium like aqueous solution, without the use of any template. For this, we will synthesize Ag nanoparticles in an aqueous medium by reducing AgNO3 with tri-sodium citrate at near boiling temperature or by reduction via UV-visible radiation. Subsequently, we would prepare Ag embedded AC samples by adsorption of these Ag nanoparticles from its aqueous dispersion. By varying the reactant concentrations, time and reaction temperature one can change the particle size.

  1. Next, we will use carboxylate terminated PAMAM dendrimers and emulsions to synthesize Ag nanoparticles in an aqueous medium. This Ag dispersion will also be used to prepare Ag-AC composite. This will be compared in its properties and applications with the citrate stabilized Ag nanoparticle dispersion.

  1. In contrast to methods 1 and 2, where externally prepared Ag dispersion is adsorbed into AC granules, in this step, we will generate Ag nanoparticles in-situ in AC. For this, we will impregnate the AC pores with aqueous AgNO3 salt solutions by soaking the powder overnight. This will be followed by in-situ reduction (either thermally or by chemically, via NaBH4) to Ag nanoparticles in the granule itself. Thermal reduction of AgNO3 will be done by heating the soaked AC at 300 C, whereas, for in-situ chemical reduction, aqueous NaBH4 solution will be added at room temperature.

We will employ variation of pore size, reactant concentration, rate of addition of reactants to vary relative rates of reaction, nucleation, diffusion in pores and particle growth rate, to obtain either spherical nanoparticles or nanorods of Ag in the pores. For in-situ Ag formation, the reactant solutions have to diffuse inside the pores and capillary forces would play a determining role. Conditions of shape transition and shape control will be elucidated in this part of the research.

  1. Next, we will compare the antibacterial properties of externally embedded (methods 1 and 2) and in-situ impregnated (method 3) Ag-AC composites.

  1. In another direction, we will perform surface modification of AC by each of the following techniques separately – by plasma treatment or exposure to UV-visible light. Subsequently, we will embed Ag into these surface modified AC samples by each of the methods 1, 2 and 3, and assess improvement of antibacterial properties.

  1. Force of adhesion of nanoparticles to the carbon surface, as a function of time will be evaluated, while being exposed to a continuous flow of water, as is the situation during regular use of the Ag-AC composite as a water filter for disinfection purposes.

  1. One also needs to establish the mechanism of antibacterial properties; is it a contact-kill or leach-kill scenario?

  1. Evaluation of the loss in activity of Ag particles over time and development of ways to prevent it will also be an important step.

Composition control is also of interest in this research project. This will be accomplished by studying multicomponent particles (two materials in a single particle) in either core-shell or homogeneous form. For the former, spherical Ag nanoparticles will be formed as usual. Subsequently aqueous Cu salt solution will be added, which will react with excess reducing agent to form a metallic Cu layer on Ag nanoparticle, forming a core-shell structure. Shell thickness will be estimated from DLS measurements, which can be controlled by the method of addition and relative rates of reaction. In contrast, starting with the pre-dissolved metal salt solutions in water will finally result in a homogeneous (Ag-Cu) nanoparticle. Such compositional heterogeneity can therefore be controlled at a very precise nanometer length scale.

In all experiments, the nanoparticle size distribution will be obtained from dynamic light scattering (DLS) as a function of time. Final particle size and shape will also be obtained from transmission electron microscopy (TEM) with composition from energy dispersive X-Ray (EDAX). UV-Visible spectrophotometer will be used to monitor Ag formation by its absorption peak at 420 nm. Pore size distribution of the AC granules and its specific surface area and specific pore volume would be obtained from BET and BJH gas adsorption experiments.
The experimental results will be compared with modeling of particle size in bulk solution, emulsion and dendrimers. Population balance equation and Monte Carlo simulation would be used as the modeling framework. Existing models and codes of the PI will be extended further and modified for taking account of bigger dendrimer templates, particle shape anisotropy, core-shell morphology for multicomponent systems and concentration based diffusion mechanisms inside a dendrimer. Location of nanoparticle formation – either inside or on the external surface of a dendrimer molecule, achievable by manipulation of surface groups of the dendrimer – is also a key variable of interest, and worthy of investigation. Mechanistic understanding gained from modeling particle formation would help us in selecting and optimizing experimental conditions.
Such general strategies will form guidelines for optimum synthesis strategies for Ag nanoparticles and Ag-AC composite formulations. These will be used for antibacterial tests by plate test, shake flask test and flow tests. A case-control based study will then be carried out to find the potential of the nanoparticle impregnated AC in producing pathogen-free water, compared to the currently used AC filter, having no nanoparticles.

232. Organization of work elements

Figure 6. Fundamental scientific issues related to controlled nanoparticle synthesis and impregnation in porous solid adsorbent.

233. Time schedule of activities giving milestones (bar diagram in Section 410)
1. (0-6 months)

Literature review, obtaining quotations, purchase and installation of equipments, recruitment of personnel to work in the project.

2. (4-9 months)

In-situ synthesis of Ag nanoparticle in activated carbon (AC) by chemical and thermal reduction of impregnated AgNO3. Testing of antimicrobial properties of Ag embedded AC (Ag-AC).

3. (4-12 months)

Preparation of citrate stabilized Ag nanoparticle dispersion - both with and without UV-visible light - its impregnation in activated carbon (AC) and testing of antimicrobial properties.

4. (7- 15 months)

Preparation of dendrimer and emulsion templated Ag nanoparticle dispersion, different impregnation strategies in activated carbon (AC) and testing of antimicrobial properties.

5. (10-24 months)

Plasma and/or UV exposure of AC for surface modification, with subsequent Ag impregnation (in surface modified AC) by all forms of coated Ag particles. Investigation of enhancement of E. coli removal due to surface modification.

6. (4-24 months)

Modeling and simulation of particle size distribution. Comparison of model predictions with experimental measurement of size, shape and composition for nanoparticles.

7. (25-30 months)

Further tuning and optimization of experimental conditions to be undertaken for different Ag-AC composites by comparison with model predictions. Simultaneous testing to be done for biocompatibility, minimization of leaching-loss of Ag by enhancing adhesion with carbon bed, extent of water disinfection and elucidation of the bacteria-kill mechanism.

8. (25- 33 months)

Implementation of the optimized (obtained by synergistic experimental and modeling studies) routes of Ag-AC formation in a small water disinfection unit, for testing and standardization of engineering issues – like minimizing pressure drop and sustaining high filtration rate of potable water from filter-bed, maximum continuous usage of Ag without drop in disinfection activity.

9. (31 – 36 months)

Exploring the scientific insight gained in biotic-abiotic interaction of the three-way metal, bacteria, carbon contacting mechanism in doing exploratory studies in other systems of interest, wherever living and non-living matter interact.

234. Suggested plan of action for utilization of research outcome expected from the project
Strengthening of basic research in India by this project

The proposed project would look at both bulk solution and structured templates, like emulsions and dendrimers, for synthesis of nanoparticles of controlled size, shape and composition. Therefore, it would address the very important mechanism of assembly (nucleation and growth of clusters and molecules) and interactions (particle growth by diffusion and reaction) at nano and mesoscopic length scales. It would also provide us with a handle to synthesize materials at nanoscale by experimental parameters specified a-priori.

The second aspect it will explore is differential functionalization of a porous carbon surface so as to make some regions of the material more interactive towards metal deposition, and thereby engineer a more favourable biotic-abiotic interaction with E. coli bacteria.

This work will also be able to address various fundamental scientific issues which are key to understanding interfacial and system size effects in nanomaterials and have implications beyond the systems investigated presently. Firstly, what is the precise role of restricting mixing and reaction in a confined space of the dendrimer, when these confined reaction-pockets occasionally communicate by Brownian collision, in comparison to a bulk situation, where communication is by turbulent mixing at micrometer length scales. Also the precise role of different surface groups on controlling molecular interaction will bear out by their direct influence on controlling the location of the nanoparticle – which can be either on the external surface of AC or in the internal pores of AC. Secondly, what is the possibility of heterogeneous nucleation of silver nanoparticles inside the pores? Thirdly, how is diffusion and adsorption of small molecules and nanoparticles in a porous structure dependent on the pore geometry and pore size? Finally, how does a nanoparticle or guest molecule impregnate itself into the pores? This will be controlled by capillary forces and wetting characteristics of the aqueous solution on the walls of the internal pores.

Future outlook and utilization of research outcome
Presently, world over, safe drinking water is made by disinfecting either chemically or by UV treatment and occasionally by heating/boiling. Except chemical treatment, other methods require electricity, and therefore these methods deprive people of safe water in areas where there is no electricity/fuel. Chemical treatments are easy to perform and cheaper. Unfortunately, all chemicals produce byproducts which prove harmful to the consumer in the long run. Therefore there is an urgent need for alternatives. Silver impregnation is very effective in overcoming the problems of the above routes. It has been in use since time immemorial, and has no known side effects. Therefore, silver disinfection is a potential alternative relevant for all.

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