Our primary research interests lie at the intersection of nanotechnology, colloids, solid-liquid, surfactants, bio-molecules interfaces, especially to investigate the mechanism of the growth of nanoparticles and their surface modifications to tailor their shape and size for a particular application. We recently demonstrated the shape control in gold nanoparticles and formation mechanism of Janus nanoparticles using molecular dynamic simulations and experiments. 

Shape Control in Gold Nanoparticles

Shape modulation of nanoparticles is crucial for their tailored applications; however, it depends on surfactants, ions, reactants, and other additives present in the growth solution. Here we dissect the role of surfactants, their counterions (halide ions), silver ions, and gold reactant in gold nanoparticle anisotropic growth using polarizable surfaces and nanoseed molecular dynamics simulation models. Our planar surface models predict a 14%–16% increment in cetyltrimethylammonium bromide (CTAB) coverage on Au(111) and Au(100) due to the surface polarization effect. The CTAB micelle adsorbs compactly similar to that observed on non-polarizable surfaces. The cetyltrimethylammonium chloride (CTAC) micelle remains in solution leaving the polarizable gold surfaces unprotected, similar to that observed with the non-polarizable surfaces, which favors isotropic growth. The cetyltrimethylammonium iodide (CTAI) micelle adsorbs with higher surface densities than CTAB on all the surfaces. The surface polarizable penta-twinned nanoseed model predicts the total surface coverage of the cetyltrimethylammonium cation (CTA+), Br− and Ag+ to be around two times higher on the side as compared to the tip of the nanoseed, leading to a 2.6 times higher initial rate of adsorption of AuCl2− on the tip than on the side. Predicted CTA+ surface densities on the tip and the side of the nanoseed are consistent with experimental results. Our simulations explain the growth mechanism of anisotropic nanoparticles and the microscopic origin of their controlled shapes

The implication of adsorption preferences of ions and surfactants on the shape control of gold nanoparticles: a microscopic, atomistic perspective, Nanoscale, 2021, 13, 19549-19560

We elucidate the crucial role of the cetyl trimethylammonium bromide (CTAB) surfactant in the anisotropic growth mechanism of gold nano-bipyramids, nano-objects with remarkable optical properties and high tunability. Our simulations predict different surface coverages of the CTAB (positively charged) heads and their (bromide) counterions as function of the gold exposed surfaces. High concentration of CTAB surfactant promotes formation of gold nanograins in solution that work as precursors for the smooth anisotropic growth of more elongated nano-bipyramidal objects. Nanobipyramids feature higher index facets with respect to nanorods, allowing higher CTAB coverages that stabilize their formation and leading to narrower inter-micelles channels that smooth down their anisotropic growth. Absorption spectroscopy and scanning electron microscopy confirmed the formation of nanograins and demonstrated the importance of surfactant concentration on driving the growth towards nano-bipyramids rather than nanorods. The outcome explains the formation of the monodisperse bipyramidal nano-objects, the origin of their controlled shapes and sizes along with their remarkable stability.

On the origin of controlled anisotropic growth of monodisperse gold nanobipyramids, Nanoscale, 2021, 13, 15292-15300

Directly manipulating and controlling the size and shape of metal nanoparticles is a key step for their tailored applications. we combined molecular dynamics simulations and experimental studies to understand the microscopic origin of the asymmetric growth mechanism in gold nanorods. We dissected the role of the different component of the solution such as CTAB surfactant, the geometry of the seed particles, halide and silver ions on the growth of nanorods.

From gold nanoseeds to nanorods: The microscopic origin of the anisotropic growth, Angew. Chem. Int. Ed., 2016, 55, 11960-11964

The role of halide ions in the anisotropic growth of gold nanoparticles: a microscopic, atomistic perspective, Phys. Chem. Chem. Phys., 2016, 18, 13246-13254

Understanding the microscopic origin of gold nanoparticle anisotropic growth from molecular dynamics simulations, Langmuir, 2013, 29, 48, 14954–14961

Janus nanoparticles formation mechanism

Our simulations explained the mechanism behind the Janus nanoparticles formation and suggested the formation of domains rich in acid-terminated chains, on one hand, and of domains rich in amide-functionalized ethylene glycol oligomers, on the other hand. In particular, within the amide-ethylene glycol oligomers region, a key role is played by the formation of interchain hydrogen bonds. The predicted phase segregation is experimentally confirmed by the synthesis of 35 and 15 nm gold nanoparticles functionalized with several binary mixtures of ligands. 

                                             Meena et al., ACS Nano, 2017, 11(7), 7371-7381

Nanophase segregation of self-assembled monolayers on gold nanoparticles, ACS Nano, 2017, 11, 7, 7371–7381