Subject Number : S-19-MS-0012
Support Type : Collaborative research
Proposal Title (English) : Solvation and Interface Dynamics of Silver Nanoparticles Surrounded by Polymer Matrix and Small Organic Molecules
Username (English) : U. Deva Priyakumar
Affiliation (English) : International Institute of Information Technology, Hyderabad, India

1. Summary
Metal nanoparticles have extensive applications in various fields of research including catalysis and bio-nanosensor development.1 Properties of metal nanoparticles are dependent on their size and shape, which are in turn determined by several factors.2 One of the important factors is the nature and concentration of the stabilizing agents. Stabilizing agents are employed to prevent nanoparticle flocculation and increase the stability of nanoparticles by lowering their energy through different mechanisms that can be broadly classified into three categories, steric, electrostatic and electrosteric. In steric stabilization, flocculation is inhibited by a protective layer formed by the adsorption of bulky organic molecules such as polymers and surfactants at the particle surface. In this study, we performed atomic level molecular dynamics to understand the effect of concentration of the monomeric (N-ethylpyrrolidone, EP) and polymeric forms of polyvinylpyrrolidone (PVP).
2. Computational method
Au55 nanoparticles having icosahedral shape were chosen to model the coalescence behavior of metal nanoparticles. The molecular dynamics simulations were performed making use of the Lennard-Jones (LJ) potentials for metals obtained by Heinz et al.,3 CHARMM force field for EP/PVP reported by Fichthorn and coworkers4 along with CHARMM general purpose force field (CGenFF)5 and the TIP3P water model. Umbrella sampling simulations were performed by using the center of mass distance between the two Au55 nanoparticles as the reaction coordinate for calculating the free energy profiles for the flocculation process of two nanoclusters. These calculations were performed for multiple concentrations of EP and PVP in aqueous solution.
3. Results ad Discussion
Free energy profiles for the respective systems were determined by finding the potential of mean force for coalescence using the center of mass distance between the nanoparticles as the reaction coordinates (Figure 1). The associated state corresponds to the distance of about 12 Å, and any minima found lower than this distance indicate that the two NPs have started coalescing (for eg. 3M EP and 4M EP systems). The free energy profiles obtained for the NPs in simple aqueous environment indicates that it is only diffusion limited with no barrier to cross. This agrees with experimental observations that an external support system is essential for the realization of metals in nanoscale. In the presence of EP as the stabilizing agent, significant changes in the free energy profiles are observed. The free energy minimum corresponding to the associate state is found to be higher than that in the aqueous simulations. However a distinct barrier for the association process is not observed in the 1M EP system. Our previous study to study the structure and dynamics of EP adsorbed Au-Pd NPs showed that at lower concentrations whole surface of the NP is not covered by the stabilizing agent.6 The barriers for the polymeric PVP are much higher than that corresponding to monomeric EP. The MD trajectories were further analyzed to understand the structural and energetic factors that control the dynamics of polymer dispersed nanoparticle and the flocculation process. Currently, we are in the process of summarizing this work and is being planned for publication.

Figure 1. Potential of mean force (kcal/mol) profiles corresponding to the flocculation of two Au55 nanoclusters in aqueous solution and multiple concentrations of EP and PVP.
4. References
[1] Torres Galvis, H. M.; Bitter, J. H.; Khare, C. B.; Ruitenbeek, M.; Dugulan, A. I.; de Jong, K. P. Science 2012, 335, 835–838.
[2] Narayanan, R.; El-Sayed, M. A. Catalysis with J. Phys. Chem. B 2005, 109, 12663–12676.
[3] Heinz, H.; Vaia, R. A.; Farmer, B. L.; Naik, R. R. J. Phys. Chem. C 2008, 112, 17281–17290.
[4] Zhou, Y.; Saidi, W. A.; Fichthorn, K. A. J. Phys. Chem. C 2014, 118, 3366–3374.
[5] Vanommeslaeghe, K. et al. J. Comput. Chem. 2010, 31, 671-690.
[6] Gupta, A.; Boekfa, B.; Sakurai, H.; Ehara, M.; Priyakumar, U. D. J. Phys. Chem. C 2016, 120, 17454-17464.
5. Publication/Presentation
(1) H. Yoshida, M. Ehara, U. Deva Priyakumar, T. Kawai, T. Nakashima, Chem. Sci. 11, 2394-2400 (2020).
6. Patent N/A