Formats for submission of projects

Indian Institute of Technology Bombay

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

117. Telephone: [phone]

Fax: [phone]

Email: [e-mail], [e-mail]

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: [phone]

Fax: [phone]

Email: [e-mail]

Registration No.: (to be filled by DST)

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

iii)……………………………

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 experiments^1,2 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 models^{10, 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 component^{16, 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 balance^{29, 30} and Monte Carlo^{31, 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 group^{34, 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.