Quantum-Sized Gold Clusters as Efficient Two-Photon Absorbers

Gold Quantum Boxes: On the Periodicities and the Quantum Confinement in the Au₂₈, Au₃₆, Au₄₄ , and Au₅₂ Magic Series

Revealing the size-dependent periodicities (including formula, growth pattern, and property evolution) is an important task in metal nanocluster research. However, investigation on this major issue has been complicated, as the size change is often accompanied by a structural change. Herein, with the successful determination of the Au44(TBBT)28 structure, where TBBT = 4-tert-butylbenzenethiolate, the missing size in the family of Au28(TBBT)20, Au36(TBBT)24, and Au52(TBBT)32 nanoclusters is filled, and a neat "magic series" with a unified formula of Au8n+4(TBBT)4n+8 (n = 3-6) is identified. Such a periodicity in magic numbers is a reflection of the uniform anisotropic growth patterns in this magic series, and the n value is correlated with the number of (001) layers in the face-centered cubic lattice. The size-dependent quantum confinement nature of this magic series is further understood by empirical scaling law, classical "particle in a box" model, and the density functional theory calculations.

Linear and Nonlinear Optical Properties of Monolayer-Protected Gold Nanocluster Films

Gold nanoclusters have been extensively studied in solution for their unique optical properties. However, many applications of nanoclusters involve the use of the material in the solid state such as films. Au25(SR)18 in polymeric hosts was used as the model for studying the optical properties of nanocluster films. Different film-processing conditions as well as types of polymers were explored to produce a good-quality film that is suitable for optical measurements. The best optical film was made using Au25(C6S)18 and polystyrene. The formation of nanocluster films drastically reduces the intercluster distances to a few nanometers, which were estimated and characterized by optical absorption. The steady-state absorption and emission properties of the nanocluster film maintained their molecular characteristics. The emissions from the nanocluster films are found to be strongly enhanced at 730 nm with a smaller enhancement at 820 nm when the intercluster distance is below 8 nm. The emission enhancement can be attributed to the energy transfer between clusters due to the small intercluster distance. Two-photon Z scan revealed that the two-photon absorption cross sections are in the order of 10(6) GM, which is an order of magnitude higher than it is in solution. The two-photon absorption enhancement is correlated with strong dipole coupling. These results show that metal nanoclusters can be made into optical quality films, which increase the interaction between clusters and enhances their linear and nonlinear optical responses.

Hybrid polymeric nano-capsules loaded with gold nanoclusters and indocyanine green for dual-modal imaging and photothermal therapy

Currently there is a great need to design nano-carriers which enable multi-modal imaging of tumors and administration of therapeutics with high efficacy simultaneously. Herein we report a facile and robust approach to fabricate multifunctional nano-capsules based on double emulsions. The hybrid nano-capsules were loaded with BSA capped gold nanoclusters (AuNCs) and indocyanine green (ICG) for dual-modal imaging and effective photothermal therapy. RGD peptides were conjugated onto the surface of the hybrid nano-capsules to target cells overexpressing integrin α_νβ₃. We demonstrated that the as-prepared nano-capsules can be used for both one-photon and two-photon fluorescence imaging of tumor cells, and subsequent photothermal ablation with high effectiveness. The background noise of tissue autofluorescence was dramatically suppressed in the two-photon imaging mode. Therefore, it assisted a great deal to acquire images with a high signal-to-noise ratio using the hybrid nano-capsules. Due to a unique combination of the one-photon/two-photon fluorescence imaging and highly-effective photothermal properties, the hybrid nano-capsules may find broad biomedical applications as attractive theranostic nanomaterials.

Two-photon absorption of ligand-protected Ag15 nanoclusters. Towards a new class of nonlinear optics nanomaterials

Theoretical and experimental two-photon absorption cross sections of thiolated small silver cluster Ag₁₅L₁₁ exhibiting extraordinary large TPA.

High-resolution separation of thiolate-protected gold clusters by reversed-phase high-performance liquid chromatography

This perspective summarizes our work on high-resolution separation of thiolate-protected gold clusters using reversed-phase high-performance liquid chromatography, new findings obtained by those separation, and future prospects for this field.

DFT studies on the influence of ligation on optical and redox properties of bimetallic [Au4M2] clusters

Computational studies have been employed to understand the lowest energy conformers and the properties of bare [Au₄M₂] and ligated [Au₄M₂(SCH₃)₆], [Au₄M₂(PH₃)₆]²⁺ clusters in both gas phase and implicit solvent media.

A closer look at two-photon absorption, absorption saturation and nonlinear refraction in gold nanoclusters

We report both the dispersion of nonlinear refraction and two-photon absorption of Au nanoclusters in a wide range of wavelengths.

Grouping and aggregation of ligand protected Au9 clusters on TiO2 nanosheets

Au₉ clusters forming groups of clusters on titania nanosheets at least partially consist of individual clusters both before and after annealing. Au₉ clusters also can attach as individual clusters.

Understanding Ligand-Exchange Reactions on Thiolate-Protected Gold Clusters by Probing Isomer Distributions Using Reversed-Phase High-Performance Liquid Chromatography

Thiolate-protected gold clusters (Aun(SR)m) have attracted considerable attention as functional nanomaterials in a wide range of fields. A ligand-exchange reaction has long been used to functionalize these clusters. In this study, we separated products from a ligand-exchange reaction of phenylethanethiolate-protected Au24Pd clusters (Au24Pd(SC2H4Ph)18), in which Au25(SR)18 is doped with palladium, into each coordination isomer with high resolution by reversed-phase high-performance liquid chromatography. This success has enabled isomer distributions of the products to be quantitatively evaluated. We evaluated quantitatively the isomer distributions of products obtained by the reaction of Au24Pd(SC2H4Ph)18 with thiol, disulfide, or diselenide. The results revealed that the exchange reaction starts to occur preferentially at thiolates that are bound directly to the metal core (thiolates of a core site) in all reactions. Further study on the isomer-separated Au24Pd(SC2H4Ph)17(SC12H25) revealed that clusters vary the coordination isomer distribution in solution by the ligand-exchange reaction between clusters and that control of the coordination isomer distribution of the starting clusters enables control of the coordination isomer distribution of the products generated by ligand-exchange reactions between clusters. Au24Pd(SC2H4Ph)18 used in this study has a similar framework structure to Au25(SR)18, which is one of the most studied compounds in the Aun(SR)m clusters. Knowledge gained in this study is expected to enable further understanding of ligand-exchange reactions on Au25(SR)18 and other Aun(SR)m clusters.

Two Photon Absorption in II-VI Semiconductors: The Influence of Dimensionality and Size

We report a comprehensive study on the two-photon absorption cross sections of colloidal CdSe nanoplatelets, -rods, and -dots of different sizes by the means of z-scan and two-photon excitation spectroscopy. Platelets combine large particle volumes with ultra strong confinement. In contrast to weakly confined nanocrystals, the TPA cross sections of CdSe nanoplatelets scale superlinearly with volume (V(∼2)) and show ten times more efficient two-photon absorption than nanorods or dots. This unexpectedly strong shape dependence goes well beyond the effect of local fields. The larger the particles' aspect ratio, the greater is the confinement related electronic contribution to the increased two-photon absorption. Both electronic confinement and local field effects favor the platelets and make them unique two-photon absorbers with outstanding cross sections of up to 10(7) GM, the largest ever reported for (colloidal) semiconductor nanocrystals and ideally suited for two-photon imaging and nonlinear optoelectronics. The obtained results are confirmed by two independent techniques as well as a new self-referencing method.

The intrinsic luminescence of individual plasmonic nanostructures in aqueous suspension by photon time-of-flight spectroscopy

We have studied the intrinsic one-photon excited luminescence of freely diffusing gold nanoparticles of different shapes in aqueous suspension. Gold nanospheres were used as a reference, since their luminescence has been investigated previously and their light absorption and scattering properties are described analytically by Mie theory. We then studied gold nanobipyramids and nanostars that have recently gained interest as building blocks for new plasmonic nanosensors. The aim of our study is to determine whether the luminescence of gold nanoparticles of complex shape (bipyramids and nanostars) is a plasmon-assisted process, in line with the conclusions of recent spectroscopic studies on spheres and nanorods. Our study has been performed on particles in suspension in order to avoid any artefact from the heterogeneous environment created when particles are deposited on a substrate. We employ a recently developed photon time-of-flight method in combination with correlation spectroscopy of the light scattered by the particles to probe the luminescent properties of individual particles based on a particle-by-particle spectral analysis. Furthermore, we have performed resonant light scattering spectroscopic measurements on the same samples. Our work demonstrates the power of our time-of flight method for uncovering the plasmonic signatures of individual bipyramids and nanostars during their brief passage in the focal volume of a confocal set-up. These spectral features of individual particles remain hidden in macroscopic measurements. We find that the intrinsic photoluminescence emission of gold bipyramids and gold nanostars is mediated by their localized surface plasmons.

Largely enhanced saturable absorption of a complex of plasmonic and molecular-like au nanocrystals

A saturable absorber is a nonlinear functional material widely used in laser and photonic nanodevices. Metallic nanostructures have prominent saturable absorption (SA) at the plasmon resonance frequency owing to largely enhanced ground state absorption. However, the SA of plasmonic metal nanostructures is hampered by excited-state absorption processes at very high excitation power, which usually leads to a changeover from SA to reversed SA (SA→RSA). Here, we demonstrate tunable nonlinear absorption behaviours of a nanocomplex of plasmonic and molecular-like Au nanocrystals. The SA→RSA process is efficiently suppressed, and the stepwise SA→SA process is fulfilled owing to energy transfer in the nanocomplex. Our observations offer a strategy for preparation of the saturable absorber complex and have prospective applications in liquid lasers as well as one-photon nonlinear nanodevices.

Atomically precise metal nanoclusters: stable sizes and optical properties

Controlling nanoparticles with atomic precision has long been a major dream of nanochemists. Breakthroughs have been made in the case of gold nanoparticles, at least for nanoparticles smaller than ∼3 nm in diameter. Such ultrasmall gold nanoparticles indeed exhibit fundamentally different properties from those of the plasmonic counterparts owing to the quantum size effects as well as the extremely high surface-to-volume ratio. These unique nanoparticles are often called nanoclusters to distinguish them from conventional plasmonic nanoparticles. Intense work carried out in the last few years has generated a library of stable sizes (or stable stoichiometries) of atomically precise gold nanoclusters, which are opening up new exciting opportunities for both fundamental research and technological applications. In this review, we have summarized the recent progress in the research of thiolate (SR)-protected gold nanoclusters with a focus on the reported stable sizes and their optical absorption spectra. The crystallization of nanoclusters still remains challenging; nevertheless, a few more structures have been achieved since the earlier successes in Au102(SR)44, Au25(SR)18 and Au38(SR)24 nanoclusters, and the newly reported structures include Au20(SR)16, Au24(SR)20, Au28(SR)20, Au30S(SR)18, and Au36(SR)24. Phosphine-protected gold and thiolate-protected silver nanoclusters are also briefly discussed in this review. The reported gold nanocluster sizes serve as the basis for investigating their size dependent properties as well as the development of applications in catalysis, sensing, biological labelling, optics, etc. Future efforts will continue to address what stable sizes are existent, and more importantly, what factors determine their stability. Structural determination and theoretical simulations will help to gain deep insight into the structure-property relationships.

Investigation of a naked Ag7 cluster: configurations and spectral characteristics

The possible configurations of experimental products are Ag₇-2 and Ag₇-4 molecules, though the Ag₇-1 cluster is the most stable molecule.

Disulfide-gated mesoporous silica nanoparticles designed for two-photon-triggered drug release and imaging

We report two-photon cancer cell killing through mesoporous silica nanogates.

Recent Progress in the Functionalization Methods of Thiolate-Protected Gold Clusters

Nanomaterials that exhibit both stability and functionality are currently considered to hold great promise as components of nanotechnology devices. Thiolate-protected gold clusters (Aun(SR)m) have long attracted attention as functional nanomaterials. Magic Aun(SR)m clusters are an especially stable group of thiolate-protected clusters that have particularly high potential as functional materials. Although numerous application experiments have been conducted for magic Aun(SR)m clusters, it is important that functionalization methods are also established to allow for effective utilization of these materials. The results of recent research on heteroatom doping and the use of other chalcogenide ligands strongly suggest that these strategies are promising as functionalization methods of magic Aun(SR)m clusters. In this Perspective, we focus on studies relating to three representative types of magic clusters-Au25(SR)18, Au38(SR)24, and Au144(SR)60-and discuss the recent progress and future issues.

Non-linear optical properties of gold quantum clusters. The smaller the better

By developing a new method for synthesizing atomically monodisperse Au15 nanoclusters stabilized with glutathione molecules and using the current state-of-the-art methods for synthesizing monodisperse protected Au25 nanoclusters, we investigated their nonlinear optical (NLO) properties after two-photon absorption. The two-photon emission spectra and the first hyperpolarizabilities of these particles were obtained using, in particular, a hyper-Rayleigh scattering technique. The influence on NLO of the excitation wavelength, the size as well as the nature of the ligands is also explored and discussed. Au15, the smallest stable thiolated gold nanocluster, presents remarkable nonlinear properties with respect to two-photon processes. The two-photon absorption cross-section at 780 nm for Au15 is ∼65,700 GM. This experimental cross-section value points to a quantum yield for two-photon emission of about 3 × 10(-7) at 475 nm for Au15. The first hyperpolarizability β for Au15 clusters (509 × 10(-30) esu), as compared to Au25 clusters (128 × 10(-30) esu), is larger considering the difference in the number of gold atoms. Also, 10(30) β per atom values reported for Au15 and Au25 clusters are more than two orders of magnitude larger than the values reported for Au NPs in the size range 10-50 nm, outlining the quantum cluster regime.

Enhancing near IR luminescence of thiolate Au nanoclusters by thermo treatments and heterogeneous subcellular distributions

A five-to-ten fold enhancement, up to ca. 5-10% quantum efficiency, of near IR luminescence from monothiolate protected gold nanoclusters was achieved by heating in the presence of excess ligand thiols. An emission maximum in the 700-900 nm range makes these Au nanoclusters superior for bioimaging applications over other emissions centered below 650 nm due to reduced background interference, albeit visible emissions could have higher quantum efficiency. The heating procedure is shown to be effective to improve the luminescence of Au nanoclusters synthesized under a variety of conditions using two types of monothiols: mercaptosuccinic acid and tiopronin. Therefore, this heating method is believed to be a generalizable approach to improve the near IR luminescence of aqueous soluble Au nanoclusters, which enables better bioimaging applications. The high quantum yield is found relatively stable over a wide pH range. PEGylation of the Au nanoclusters reduces their quantum efficiency but improves their permeation into the cytoplasm. Interestingly, z-stack confocal analysis clearly reveals the presence of Au nanoclusters inside the cell nucleus in single cell imaging. The finding addresses controversial literature reports and demonstrates the internalization and heterogeneous subcellular distributions, particularly inside the nucleus. The high luminescence intensity, small overall dimension, cell and nuclear distribution, chemical stability and low-to-non toxicity make these Au nanoclusters promising probes for broad cell dynamics and imaging applications.

Supramolecular functionalization and concomitant enhancement in properties of Au(25) clusters

We present a versatile approach for tuning the surface functionality of an atomically precise 25 atom gold cluster using specific host-guest interactions between β-cyclodextrin (CD) and the ligand anchored on the cluster. The supramolecular interaction between the Au25 cluster protected by 4-(t-butyl)benzyl mercaptan, labeled Au25SBB18, and CD yielding Au25SBB18∩CDn (n = 1, 2, 3, and 4) has been probed experimentally using various spectroscopic techniques and was further analyzed by density functional theory calculations and molecular modeling. The viability of our method in modifying the properties of differently functionalized Au25 clusters is demonstrated. Besides modifying their optoelectronic properties, the CD moieties present on the cluster surface provide enhanced stability and optical responses which are crucial in view of the potential applications of these systems. Here, the CD molecules act as an umbrella which protects the fragile cluster core from the direct interaction with many destabilizing agents such as metal ions, ligands, and so on. Apart from the inherent biocompatibility of the CD-protected Au clusters, additional capabilities acquired by the supramolecular functionalization make such modified clusters preferred materials for applications, including those in biology.

Poly β-cyclodextrin inclusion-induced formation of two-photon fluorescent nanomicelles for biomedical imaging

A novel two-photon absorption (TPA) nanomicelle through the “host–guest” chemistry has been developed and successfully applied in tumor tissue imaging in this work.

Ultrafast relaxation dynamics of phosphine-protected, rod-shaped Au20 clusters: interplay between solvation and surface trapping

Excited-state intramolecular charge transfer dynanmics and coherent oscillation of ligand-protected rod shaped Au₂₀ clusters were modulated through the competition between solvation and surface trapping.

A DFT study of a new class of gold nanocluster-photochrome multi-functional switches

The structural and electronic properties of dithienylethene photochromic molecules grafted onto a Au₂₅ nanocluster are reviewed and electron/energy transfers are discussed with the help of (TD-)DFT calculations.

Luminescent gold nanoparticles: A new class of nanoprobes for biomedical imaging

Our fundamental understanding of cell biology and early diagnosis of human diseases have been greatly benefited from the development of fluorescent probes. Over the past decade, luminescent gold nanoparticles (AuNPs) with diverse structural parameters including particle size, surface ligands, valence state and grain size have been synthesized and have begun to emerge as a new class of fluorescent probes for bioimaging because of their great biocompatibility, robust photophysical properties and tunable emissions from the visible range to the near infrared region. In this minireview, we summarize the recent progress in applications of different-sized luminescent AuNPs as imaging probes for both in vitro and in vivo levels.

Gold cluster-nanoparticle diad systems for plasmonic enhancement of photosensitization

Quantum-sized gold clusters are deposited on TiO2 both as a photosensitizer and catalyst, and coupled to plasmonic gold nanoparticles as a light harvesting antenna. Photocurrent enhancement was observed for Au25(SG)18 and Au38(SG)24 but not for Au102(SG)44 (SG = glutathione). The maximum enhancement factor of ~9 is reached at 900 nm.

An ultrafast look at Au nanoclusters

In the past 20 years, researchers studying nanomaterials have uncovered many new and interesting properties not found in bulk materials. Extensive research has focused on metal nanoparticles (>3 nm) because of their potential applications, such as in molecular electronics, image markers, and catalysts. In particular, the discovery of metal nanoclusters (<3 nm) has greatly expanded the horizon of nanomaterial research. These nanosystems exhibit molecular-like characteristics as their size approaches the Fermi-wavelength of an electron. The relationships between size and physical properties for nanomaterials are intriguing, because for metal nanosystems in this size regime both size and shape determine electronic properties. Remarkably, changes in the optical properties of nanomaterials have provided tremendous insight into the electronic structure of nanoclusters. The success of synthesizing monolayer protected clusters (MPCs) in the condensed phase has allowed scientists to probe the metal core directly. Au MPCs have become the "gold" standard in nanocluster science, thanks to the rigorous structural characterization already accomplished. The use of ultrafast laser spectroscopy on MPCs in solution provides the benefit of directly studying the chemical dynamics of metal nanoclusters (core), and their nonlinear optical properties. In this Account, we investigate the optical properties of MPCs in the visible region using ultrafast spectroscopy. Based on fluorescence up-conversion spectroscopy, we propose an emission mechanism for these nanoclusters. These clusters behave differently from nanoparticles in terms of emission lifetimes as well as two-photon cross sections. Through further investigation of the transient (excited state) absorption, we have found many unique phenomena of nanoclusters, such as quantum confinement effects and vibrational breathing modes. In summary, based on the differences in the optical properties, the distinction between nanoclusters and nanoparticles appears at a size near 2.2 nm. This is consistent with simulations from a free-electron model proposed for MPCs. The use of ultrafast techniques on these nanoclusters can answer many of the fundamental questions about the nature of these exciting nanomaterials and their applications.

Precursor engineering and controlled conversion for the synthesis of monodisperse thiolate-protected metal nanoclusters

In very recent years, thiolate-protected metal nanoclusters (or thiolated MNCs) with core sizes smaller than 2 nm have emerged as a new direction in nanoparticle research due to their discrete and size dependent electronic structures and molecular-like properties, such as HOMO-LUMO transitions in optical absorptions, quantized charging, and strong luminescence. Synthesis of monodisperse thiolated MNCs in sufficiently large quantities (up to several hundred micrograms) is necessary for establishing reliable size-property relationships and exploring potential applications. This Feature Article reviews recent progress in the development of synthetic strategies for the production of monodisperse thiolated MNCs. The preparation of monodisperse thiolated MNCs is viewed as an engineerable process where both the precursors (input) and their conversion chemistry (processing) may be rationally designed to achieve the desired outcome - monodisperse thiolated MNCs (output). Several strategies for tailoring the precursor and the conversion process are analyzed to arrive at a unifying understanding of the processes involved.