Though creation and characterization of water soluble luminescent silver nanodots were achieved only in the past decade, a large variety of emitters in diverse scaffolds have been reported. Photophysical properties approach those of semiconductor quantum dots, but relatively small sizes are retained. Because of these properties, silver nanodots are finding ever-expanding roles as probes and biolabels. In this critical review we revisit the studies on silver nanodots in inert environments and in aqueous solutions. The recent advances detailing their chemical and physical properties of silver nanodots are highlighted with an effort to decipher the relations between their chemical/photophysical properties and their structures. The primary results about their biological applications are discussed here as well, especially relating to their chemical and photophysical behaviours in biological environments (216 references).

Developing luminescent silver nanodots for biological applications

A combined experimental and theoretical study of small gold cluster anions is performed. The experimental effort consists of ion mobilitymeasurements that lead to the assignment of the collision cross sections for the different cluster sizes at room temperature. The theoretical study is based on ab initiomolecular dynamics calculations with the goal to find energetically favorable candidate structures. By comparison of the theoretical results with the measured collision cross sections as well as vertical detachment energies (VDEs) from the literature, we assign structures for the small Au n − ions (n<13) and locate the transition from planar to three-dimensional structures. While a unique assignment based on the observed VDEs alone is generally not possible, the collision cross sections provide a direct and rather sensitive measure of the cluster structure. In contrast to what was expected from other metal clusters and previous theoretical studies, the structural transition occurs at an unusually large cluster size of twelve atoms.

The structures of small gold cluster anions as determined by a combination of ion mobility measurements and density functional calculations

We present a systematic study of the electronic properties and the geometric structure of noble metal clusters ${X}_{n}^{\ensuremath{\nu}}$ ($X=\mathrm{Cu}$, Ag, Au; $\ensuremath{\nu}=\ensuremath{-}1,0,+1$; $n\ensuremath{\leqslant}13$ and $n=20$), obtained from first-principles generalized gradient approximation density functional calculations based on norm-conserving pseudopotentials and numerical atomic basis sets. We obtain planar structures for the ground state of anionic $(\ensuremath{\nu}=\ensuremath{-}1)$, neutral $(\ensuremath{\nu}=0)$, and cationic $(\ensuremath{\nu}=1)$ species of gold clusters with up to 12, 11, and 7 atoms, respectively. In contrast, the maximum size of planar clusters with $\ensuremath{\nu}=\ensuremath{-}1,0,+1$ are $n=(5,6,5)$ for silver and (5,6,4) for copper. For ${X}_{20}$ we find a ${T}_{d}$ symmetry for gold and a compact ${C}_{s}$ structure for silver and copper. Our results for the cluster geometries agree partially with previous first-principles calculations, and they are in good agreement with recent experimental results for anionic and cationic gold clusters. The tendency to planarity of gold clusters, which is much larger than in copper and silver, is strongly favored by relativistic effects, which decrease the $s\text{\ensuremath{-}}d$ promotion energy and lead to hybridization of the half-filled $6s$ orbital with the fully occupied $5{d}_{{z}^{2}}$ orbital. That picture is substantiated by analyzing our calculated density matrix for planar and three-dimensional clusters of gold and copper. The trends for the cohesive energy, ionization potentials, electron affinities, and highest accupied and lowest unoccupied molecular orbital gap, as the cluster size increases, are studied in detail for each noble metal and rationalized in terms of two- and three-dimensional electronic shell models. The most probable fragmentation channels for ${X}_{n}^{\ensuremath{\nu}}$ clusters are in very good agreement with available experiments.

Trends in the structure and bonding of noble metal clusters

We have investigated the lowest-energy structures and electronic properties of the ${\mathrm{Au}}_{n}(n=2--20)$ clusters based on density-functional theory with local density approximation. The small ${\mathrm{Au}}_{n}$ clusters adopt planar structures up to $n=6.$ Flat cage structures are preferred in the range of $n=10--14$ and a structural transition from flat-cage-like structure to compact near-spherical structure is found around $n=15.$ The most stable configurations obtained for ${\mathrm{Au}}_{13}$ and ${\mathrm{Au}}_{19}$ clusters are amorphous instead of icosahedral or fcc like, while the electronic density of states sensitively depends on the cluster geometry. Dramatic odd-even alternative behaviors are obtained in the relative stability, highest occupied and lowest unoccupied molecular orbit gaps, and ionization potentials of gold clusters. The size evolution of the electronic properties is discussed and the theoretical ionization potentials of ${\mathrm{Au}}_{n}$ clusters compare well with experiments.

Density-functional study of Au n ( n = 2 – 2 0 ) clusters: Lowest-energy structures and electronic properties

This review focuses on noncovalent metal ion-ligand complexes and measurements of the bond energies of such species. The method utilized in this work is threshold collision-induced dissociation (CID), as achieved using a guided ion beam tandem mass spectrometer. Accurate determination of bond energies requires attention to many details of the experiments and data analysis. These details are discussed thoroughly and compared to other methods. A comprehensive listing of metal-ligand bond dissociation energies determined by threshold CID is provided. This list includes a variety of metals (alkalis, magnesium, aluminum, and first and second row transition metals), many different types of ligands, and variations in the number of ligands. The trends in these values are discussed, and we elucidate the importance of ion-dipole and ion-induced dipole interactions, chelation, different conformers and tautomers, steric interactions, solvation phenomena, and electronic effects such as hybridization and promotion.

Noncovalent metal–ligand bond energies as studied by threshold collision‐induced dissociation

The threshold collision-induced dissociation method is applied to study the fragmentation patterns and to measure the dissociation energies of small anionic copper clusters (Cun−, n=2–8) and their monocarbonyls (CunCO−, n=3–7). For the bare clusters, the main reaction channels are loss of an atom and loss of a dimer. For the copper cluster monocarbonyls, the main channel is loss of CO. Dissociation energies for the loss of an atom from bare copper cluster anions, D0(Cun−1−–Cu), show even–odd alternation. The species with the highest dissociation energy, Cu7−, and the highest carbonyl desorption energy, Cu5CO−, have eight valence electrons, consistent with closed shells in the jellium model. Bond energies are compared with theoretical models.

Threshold collision-induced dissociation of anionic copper clusters and copper cluster monocarbonyls

The energy-resolved collision-induced dissociation method is applied to measure the fragmentation patterns, cross sections, and dissociation energies of small anionic silver clusters (Agn−,n=2–11). The main reaction channels are found to be loss of atom and loss of dimer, with dimer loss favored for odd n values. The dissociation energies for the loss of atom, D0(Agn−1−–Ag), show strong even–odd alternation. Threshold models that account for collisional activation efficiency, kinetic shifts, and competitive shifts are employed to obtain dissociation energies. A critical examination of the models is performed using a thermochemical cycle comparing sequential atom loss with dimer loss.

Measurement of the dissociation energies of anionic silver clusters (Agn−, n=2–11) by collision-induced dissociation

The photodecomposition kinetics of silver cluster anions, Agn− (n=7–11), has been investigated. The time-resolved intensities of Agn− parent ions and Agn−1− and Agn−2− photofragment product ions are measured following excitation with visible laser radiation, 415–750 nm. The atom-loss and dimer-loss product yields and reactant cluster ion depletion are compared to elucidate the decomposition kinetics of photoexcited silver cluster anions, including electron loss channels. Both prompt, direct electron photodetachment and delayed, statistical electron emission are observed in competition with cluster fragmentation product channels for some clusters. Dissociation threshold energies are determined by fitting the measured time profiles for fragmentation products using a statistical unimolecular dissociation model. The photodissociation lifetime method for measuring cluster dissociation energies is compared with previous energy-resolved collision-induced dissociation experiments on silver cluster anions.

Competitive fragmentation and electron loss kinetics of photoactivated silver cluster anions: Dissociation energies of Agn− (n=7–11)

Time-resolved photodissociation and threshold collision-induced dissociation of anionic gold clusters

The bond dissociation energies of palladium trimer anion, Pd3−, and its carbonyls, Pd3(CO)n− (n=1–6), are measured in the gas phase by the energy-resolved collision-induced dissociation method The values obtained are D0(Pd2−−Pd)=226±036 eV for the bare cluster and D0(Pd3(CO)n−1−−CO)=178±032 eV, 174±022 eV, 147±022 eV, 113±015 eV, 111±015 eV, and 114±017 eV for n=1–6, respectively, for the carbonyls The results show a general decrease of the bond energy with an increasing number of carbonyls, with two relatively stable structures, Pd3(CO)2− and Pd3(CO)6− A symmetric Pd3(CO)2− structure with two three-fold bridged carbonyls is postulated

Vassil A. Spasov

Papers

Threshold collision-induced dissociation of anionic copper clusters and copper cluster monocarbonyls

Measurement of the dissociation energies of anionic silver clusters (Agn−, n=2–11) by collision-induced dissociation

Competitive fragmentation and electron loss kinetics of photoactivated silver cluster anions: Dissociation energies of Agn− (n=7–11)

Time-resolved photodissociation and threshold collision-induced dissociation of anionic gold clusters

Binding energies of palladium carbonyl cluster anions: Collision-induced dissociation of Pd3(CO)n− (n=0–6)