PhD candidate in Chemistry (OleMiss) working on next generation atomically precise nanomaterials.
Science and technology is ever growing, evolving, and modernizing to provide the world with optimal and efficient products. In this growth, between 1850-1950s the importance of size of matter was realized and the field of Nanoscience and Nanotechnology was born. In the field of Nanoscience, gold nanoparticles (Au NPs) is one the most widely studied to transform their exquisite properties into viable applications in drug delivery, catalysis, energy conversion and storage, etc. Researchers have been facing the challenge of obtaining highly monodisperse Au NPs for several decades to understand their unique properties and their dependence with size and structure. In Au NPs, there is a special class of thiolate (-SR, R-hydrocarbon tail) ligand protected Au NPs which are extremely stable with precise number of Au atoms and ligand (-SR) group like any typical chemical compounds with a molecular formula [(Au)n(SR)m] in the size range of 1-3 nm. Hence these atomically precise Au NP compounds are termed as gold nanomolecules (Au NMs, also known as nanoclusters and nanocrystals). As the size increases more than 3 nm, a certain degree of polydispersity in size is commonly observed. The discovery of Au NMs field of research started shining light on the distinct changes in properties with size evolution (quantum confinement, plasmonics) with atomic precision and fill the void in our understanding of various phenomena in nanoscale regime. Structure determines property and understanding that correlation enables us to manipulate matter at atomic level. The molecular nature of Au NMs offer the opportunity to study these nanoscale structures at subatomic level by growing quality single crystals for single crystal X-ray crystallography (sc-XRD). Au NMs with metallic core and hydrocarbon ligands are assembled into single crystals by vapor diffusion of non-solvent into solution containing those molecularly pure NMs. The crystallographic structure models are used to study their properties theoretically and correlate to experimental data (optical, electrochemical etc). However, as the size increases >2 nm the chances of growing single crystals become increasingly difficult. In our field of research, we synthesize Au NMs using simple wet chemical techniques, characterize them (mass spectrometry, optical, crystallography), and develop applications.
Abstract: Crystal structure determination has revolutionized modern science in biology, chemistry, and physics. However, the difficulty in obtaining periodic crystal lattices which are needed for X-ray crystal analysis has hindered the determination of atomic structure in nanomaterials, known as the "nanostructure problem". Here, by using rigid and bulky ligands, we have overcome this limitation and successfully solved the X-ray crystallographic structure of the largest reported thiolated gold nanomolecule, Au133S52. The total composition, Au133(SPh-tBu)52, was verified using high resolution electrospray ionization mass spectrometry (ESI-MS). The experimental and simulated optical spectra show an emergent surface plasmon resonance that is more pronounced than in the slightly larger Au144(SCH2CH2Ph)60. Theoretical analysis indicates that the presence of rigid and bulky ligands is the key to the successful crystal formation.
Pub.: 04 Apr '15, Pinned: 15 Mar '18
Abstract: Structural information on nanometer-sized gold particles has been limited, due in part to the problem of preparing homogeneous material. Here we report the crystallization and x-ray structure determination of a p-mercaptobenzoic acid (p-MBA)-protected gold nanoparticle, which comprises 102 gold atoms and 44 p-MBAs. The central gold atoms are packed in a Marks decahedron, surrounded by additional layers of gold atoms in unanticipated geometries. The p-MBAs interact not only with the gold but also with one another, forming a rigid surface layer. The particles are chiral, with the two enantiomers alternating in the crystal lattice. The discrete nature of the particle may be explained by the closing of a 58-electron shell.
Pub.: 20 Oct '07, Pinned: 15 Mar '18
Abstract: We report a detailed study on the optical properties of Au279(SR)84 using steady-state and transient absorption measurements to probe its metallic nature, time-dependent density functional theory (TDDFT) studies to correlate the optical spectra, and density of states (DOS) to reveal the factors governing the origin of the collective surface plasmon resonance (SPR) oscillation. Au279 is the smallest identified gold nanocrystal to exhibit SPR. Its optical absorption exhibits SPR at 510 nm. Power-dependent bleach recovery kinetics of Au279 suggests that electron dynamics dominates its relaxation and it can support plasmon oscillations. Interestingly, TDDFT and DOS studies with different tail group residues (−CH3 and −Ph) revealed the important role played by the tail groups of ligands in collective oscillation. Also, steady-state and time-resolved absorption for Au36, Au44, and Au133 were studied to reveal the molecule-to-metal evolution of aromatic AuNMs. The optical gap and transient decay lifetimes decrease as the size increases.
Pub.: 01 Mar '18, Pinned: 02 Mar '18
Abstract: Colloidal nanoparticles are being intensely pursued in current nanoscience research. Nanochemists are often frustrated by the well-known fact that no two nanoparticles are the same, which precludes the deep understanding of many fundamental properties of colloidal nanoparticles in which the total structures (core plus surface) must be known. Therefore, controlling nanoparticles with atomic precision and solving their total structures have long been major dreams for nanochemists. Recently, these goals are partially fulfilled in the case of gold nanoparticles, at least in the ultrasmall size regime (1-3 nm in diameter, often called nanoclusters). This review summarizes the major progress in the field, including the principles that permit atomically precise synthesis, new types of atomic structures, and unique physical and chemical properties of atomically precise nanoparticles, as well as exciting opportunities for nanochemists to understand very fundamental science of colloidal nanoparticles (such as the stability, metal-ligand interfacial bonding, ligand assembly on particle surfaces, aesthetic structural patterns, periodicities, and emergence of the metallic state) and to develop a range of potential applications such as in catalysis, biomedicine, sensing, imaging, optics, and energy conversion. Although most of the research activity currently focuses on thiolate-protected gold nanoclusters, important progress has also been achieved in other ligand-protected gold, silver, and bimetal (or alloy) nanoclusters. All of these types of unique nanoparticles will bring unprecedented opportunities, not only in understanding the fundamental questions of nanoparticles but also in opening up new horizons for scientific studies of nanoparticles.
Pub.: 02 Sep '16, Pinned: 06 Feb '18
Abstract: We report the discovery of an unprecedentedly large, 2.2 nm diameter, thiolate protected gold nanocrystal characterized by single crystal X-ray crystallography (sc-XRD), Au279(SPh-tBu)84 named Faradaurate-279 (F-279) in honor of Michael Faraday's (1857) pioneering work on nanoparticles. F-279 nanocrystal has a core-shell structure containing a truncated octahedral core with bulk face-centered cubic-like arrangement, yet a nanomolecule with a precise number of metal atoms and thiolate ligands. The Au279S84 geometry was established from a low-temperature 120 K sc-XRD study at 0.90 Å resolution. The atom counts in core-shell structure of Au279 follows the mathematical formula for magic number shells: Au@Au12@Au42@Au92@Au54, which is further protected by a final shell of Au48. Au249 core is protected by three types of staple motifs, namely: 30 bridging, 18 monomeric, and 6 dimeric staple motifs. Despite the presence of such diverse staple motifs, Au279S84 structure has a chiral pseudo-D3 symmetry. The core-shell structure can be viewed as nested, concentric polyhedra, containing a total of five forms of Archimedean solids. A comparison between the Au279 and Au309 cuboctahedral superatom model in shell-wise growth is illustrated. F-279 can be synthesized and isolated in high purity in milligram quantities using size exclusion chromatography, as evidenced by mass spectrometry. Electrospray ionization-mass spectrometry independently verifies the X-ray diffraction study based heavy atoms formula, Au279S84, and establishes the molecular formula with the complete ligands, namely, Au279(SPh-tBu)84. It is also the smallest gold nanocrystal to exhibit metallic behavior, with a surface plasmon resonance band around 510 nm.
Pub.: 11 Oct '17, Pinned: 06 Feb '18