Manzhou Zhu

Quantum Sized Gold Nanoclusters with Atomic Precision

Gold nanoparticles typically have a metallic core, and the electronic conduction band consists of quasicontinuous energy levels (i.e. spacing δ ≪ k(B)T, where k(B)T is the thermal energy at temperature T (typically room temperature) and k(B) is the Boltzmann constant). Electrons in the conduction band roam throughout the metal core, and light can collectively excite these electrons to give rise to plasmonic responses. This plasmon resonance accounts for the beautiful ruby-red color of colloidal gold first observed by Faraday back in 1857. On the other hand, when gold nanoparticles become extremely small (<2 nm in diameter), significant quantization occurs to the conduction band. These quantum-sized nanoparticles constitute a new class of nanomaterial and have received much attention in recent years. To differentiate quantum-sized nanoparticles from conventional plasmonic gold nanoparticles, researchers often refer to the ultrasmall nanoparticles as nanoclusters. In this Account, we chose several typical sizes of gold nanoclusters, including Au(25)(SR)(18), Au(38)(SR)(24), Au(102)(SR)(44), and Au(144)(SR)(60), to illustrate the novel properties of metal nanoclusters imparted by quantum size effects. In the nanocluster size regime, many of the physical and chemical properties of gold nanoparticles are fundamentally altered. Gold nanoclusters have discrete electronic energy levels as opposed to the continuous band in plasmonic nanoparticles. Quantum-sized nanoparticles also show multiple optical absorption peaks in the optical spectrum versus a single surface plasmon resonance (SPR) peak at 520 nm for spherical gold nanocrystals. Although larger nanocrystals show an fcc structure, nanoclusters often have non-fcc atomic packing structures. Nanoclusters also have unique fluorescent, chiral, and magnetic properties. Due to the strong quantum confinement effect, adding or removing one gold atom significantly changes the structure and the electronic and optical properties of the nanocluster. Therefore, precise atomic control of nanoclusters is critically important: the nanometer precision typical of conventional nanoparticles is not sufficient. Atomically precise nanoclusters are represented by molecular formulas (e.g. Au(n)(SR)(m) for thiolate-protected ones, where n and m denote the respective number of gold atoms and ligands). Recently, major advances in the synthesis and structural characterization of molecular purity gold nanoclusters have made in-depth investigations of the size evolution of metal nanoclusters possible. Metal nanoclusters lie in the intermediate regime between localized atomic states and delocalized band structure in terms of electronic properties. We anticipate that future research on quantum-sized nanoclusters will stimulate broad scientific and technological interests in this special type of metal nanomaterial.

Kinetically controlled, high-yield synthesis of Au25 clusters.

A facile, low-temperature method has been developed for synthesizing Au25 clusters in high yield. It was discovered that by controlling the formation kinetics of the Au(I) intermediate species, exclusive formation of one-sized clusters (Au25) can be achieved, which represents an important advance in the synthesis of monodisperse gold clusters.

A 200-fold Quantum Yield Boost in the Photoluminescence of Silver-Doped AgxAu25−xNanoclusters: The 13 th Silver Atom Matters

The rod-shaped Au25 nanocluster possesses a low photoluminescence quantum yield (QY=0.1%) and hence is not of practical use in bioimaging and related applications. Herein, we show that substituting silver atoms for gold in the 25-atom matrix can drastically enhance the photoluminescence. The obtained Ag(x)Au(25-x) (x=1-13) nanoclusters exhibit high quantum yield (QY=40.1%), which is in striking contrast with the normally weakly luminescent Ag(x)Au(25-x) species (x=1-12, QY=0.21%). X-ray crystallography further determines the substitution sites of Ag atoms in the Ag(x)Au(25-x) cluster through partial occupancy analysis, which provides further insight into the mechanism of photoluminescence enhancement.

Facile, large-scale synthesis of dodecanethiol-stabilized Au38 clusters.

It has long been a major challenge to achieve synthetic control over size and monodispersity of gold thiolate nanoclusters. Among the reported Aun thiolate clusters, Au38 has been shown to be particularly stable but was only obtained as a minor product in previous syntheses. In this work, we report a bulk solution synthetic method that permits large-scale, facile synthesis of truly monodisperse Au38 nanoclusters. This new method explores a two-phase ligand exchange process utilizing glutathione-capped Aun clusters as the starting material. The ligand exchange process with neat dodecanethiols causes gold core etching and secondary growth of clusters, and eventually leads to monodisperse Au38 clusters in high purity, which eliminates nontrivial postsynthetic separation steps. This method can be readily scaled up to synthesize Au38(SC12H25)24 in large quantities and thus makes the approach and Au38 nanoclusters of broad utility.

Chiral Au25 Nanospheres and Nanorods: Synthesis and Insight into the Origin of Chirality

Chirality in nanoparticles is an intriguing phenomenon. Herein, we have devised a well-defined gold nanoparticle system for investigating the origin of chirality in nanoparticles. We have designed chiral thiols (R- and S-isomers) and synthesized chiral gold nanoparticles composed of 25 gold atoms and 18 ligands, referred to as Au(25)(pet)(18), where pet represents chirally modified phenylethylthiolate -SCH(2)CH(CH(3))Ph at the 2-position. These optically active nanoparticles are close analogues of the optically nonactive phenylethylthioalte-capped Au(25)(pet)(18) nanoparticles, and the latters crystal structure is known. On the basis of the atomic and electronic structures of these well-defined Au(25) nanoparticles, we have explicitly revealed that the ligands and surface gold atoms of Au(25)(pet)(18) play a critical role in effecting the circular dichroism responses from the nanoparticles. Similar effects are also observed in chiral Au(25) rods. The mixing of electronic states of ligands with those of surface gold atoms constitutes the fundamental origin of chirality in such nanoparticles.

Crystal Structure of Selenolate-Protected Au24(SeR)20 Nanocluster

We report the X-ray structure of a selenolate-capped Au24(SeR)20 nanocluster (R = C6H5). It exhibits a prolate Au8 kernel, which can be viewed as two tetrahedral Au4 units cross-joined together without sharing any Au atoms. The kernel is protected by two trimeric Au3(SeR)4 staple-like motifs as well as two pentameric Au5(SeR)6 staple motifs. Compared to the reported gold-thiolate nanocluster structures, the features of the Au8 kernel and pentameric Au5(SeR)6 staple motif are unprecedented and provide a structural basis for understanding the gold-selenolate nanoclusters.

One-Pot Synthesis of Robust Core/Shell Gold Nanoparticles

A one-pot synthesis of thermally stable core/shell gold nanoparticles (Au-NPs) was developed via surface-initiated atom transfer radical polymerization (ATRP) of n-butyl acrylate (BA) and a dimethacrylate-based cross-linker. The higher reactivity of the cross-linker enabled the formation of a thin cross-linked polymer shell around the surface of the Au-NP before the growth of linear polymer chains from the shell. The cross-linked polymer shell served as a robust protective layer, prevented the dissociation of linear polymer brushes from the surfaces of Au-NPs, and provided the Au-NPs excellent thermal stability at elevated temperature (e.g., 110 degrees C for 24 h). This synthetic method could be easily expanded for preparation of other types of inorganic/polymer nanocomposites with significantly improved stability.

6-Substituted quinoline-based ratiometric two-photon fluorescent probes for biological Zn2+ detection

New ratiometric two-photon fluorescent probes are developed from 6-substituted quinolines for biological Zn(2+) detection. They show large red shifts and good ratiometric responses upon Zn(2+) binding. They also exhibit high ion selectivities and large two-photon absorption cross sections at nearly 720 nm. Because the new probes are cell-permeable, they can be used to detect intracellular zinc flux under two-photon excitation.

Metal Exchange Method Using Au25 Nanoclusters as Templates for Alloy Nanoclusters with Atomic Precision

A metal exchange method based upon atomically precise gold nanoclusters (NCs) as templates is devised to obtain alloy NCs including CuxAu25-x(SR)18, AgxAu25-x(SR)18, Cd1Au24(SR)18, and Hg1Au24(SR)18 via reaction of the template with metal thiolate complexes of Cu(II), Ag(I), Cd(II), and Hg(II) (as opposed to common salt precursors such as CuCl2, AgNO3, etc.). Experimental results imply that the exchange between gold atoms in NCs and those of the second metal in the thiolated complex does not necessarily follow the order of metal activity (i.e., galvanic sequence). In addition, the crystal structure of the exchange product (Cd1Au24(SR)18) is successfully determined, indicating that the Cd is in the center of the 13-atom icosahedral core. This metal exchange method is expected to become a versatile new approach for synthesizing alloy NCs that contain both high- and low-activity metal atoms.

The Structure and Optical Properties of the [Au18(SR)14] Nanocluster

Decreasing the core size is one of the best ways to study the evolution from Au(I) complexes into Au nanoclusters. Toward this goal, we successfully synthesized the [Au18(SC6H11)14] nanocluster using the [Au18(SG)14] (SG=L-glutathione) nanocluster as the starting material to react with cyclohexylthiol, and determined the X-ray structure of the cyclohexylthiol-protected [Au18(C6H11S)14] nanocluster. The [Au18(SR)14] cluster has a Au9 bi-octahedral kernel (or inner core). This Au9 inner core is built by two octahedral Au6 cores sharing one triangular face. One transitional gold atom is found in the Au9 core, which can also be considered as part of the Au4(SR)5 staple motif. These findings offer new insight in terms of understanding the evolution from [Au(I)(SR)] complexes into Au nanoclusters.