Linear and Nonlinear Optical Response in Silver Nanoclusters: Insight from a Computational Investigation

A comprehensive review of atomically precise metal nanoclusters with emergent photophysical properties towards diverse applications

Atomically precise metal nanoclusters (MNCs) composed of a few to hundreds of metal atoms represent an emerging class of nanomaterials with a precise composition. With the size approaching the Fermi wavelength of electrons, their energy levels are well-separated, leading to molecule-like properties, like discrete single electronic transitions, tunable photoluminescence (PL), inherent structural anisotropy, and distinct redox behavior. Extensive synthetic efforts and electronic structure revelation have expanded applicability of MNCs in catalysis, optoelectronics, and biology. This review highlights the intriguing photophysical and electrochemical behaviors of MNCs and their regulatory parameters and applications. Initially, we present a brief discussion on the evolution of MNCs from gas-phase naked metal clusters to monolayer ligand-protected MNCs along with representative studies on their electronic structure. Due to their quantized molecular orbitals, they often exhibit PL, which can be regulated based on their capping ligands, number of atoms, crystal packing, presence of heterometal, and surrounding environment. Apart from PL, the relaxation pathways of MNCs on an ultrafast time scale have been extensively studied, which significantly differ from that of plasmonic metal nanoparticles. Moreover, their interaction with high-intensity light results in unique non-linear optical properties. The synergy between MNCs in a hierarchical self-assembled structure has been exploited to enhance their PL by precisely tuning their non-covalent interactions. Moreover, several NC-based hybrids have been designed to exhibit efficient electron or energy transfer in the photoexcited state. In the next section, we briefly focus on the redox behavior of NCs and facile electron transfer to suitable substrates, which result in enzyme-like catalytic activity. Utilizing these photophysical and electrochemical behaviors, NCs are widely employed in catalysis, optical sensing, and light-harvesting applications, which are also discussed in this review. In the final section, conclusions and open questions for the NC research community are included. This review will provide a comprehensive view of the emerging physicochemical properties of MNCs, thereby enabling an understanding for their precise modulation in future.

Recent Advances in Engineered Noble Metal Nanomaterials as a Surface-Enhanced Raman Scattering Active Platform for Cancer Diagnostics

Recently, noble metal nanomaterials have been extensively studied in the fields of biosensing, environmental catalysis, and cancer diagnosis and treatment, due to their excellent electrical conductivity, high surface area, and individual physical and optical properties. Early research on the surface-enhanced Raman scattering (SERS) effect was focused on the cognition of the SERS phenomenon and enhancing its sensitivity for single-molecule detection. With the development of nanomaterials and nanotechnology, the advances and applications based on SERS substrates have been accelerated. Among them, noble metal nanomaterials are mainly used as SERS-active substrates to enhance SERS signals owing to their compelling surface plasmon resonance (SPR) properties. This review provides recent advances, perspectives, and challenges in SERS assays based on engineered noble metal nanomaterials for early cancer diagnosis.

Sequential Growth as a Mechanism of Silver-Glutathione Monolayer-Protected Cluster Formation

Silver monolayer-protected clusters (MPCs) are an important new class of small metal nanoparticles with discrete sizes and unique properties that are eminently tunable; however, a fundamental understanding of the mechanisms of MPC formation is still lacking. Here, the basic mechanism by which silver-glutathione MPCs form is established by using real-time in situ optical measurements and ex situ solution-phase analyses to track MPC populations in the reaction mixture. These measurements identify that MPCs grow systematically, increasing in size sequentially as they transform from one known species to another, in contrast to existing models. In the new sequential growth model of MPC formation, the relative stability of each species in the series results in thermodynamic preferences for certain species as well as kinetic barriers to transformations between stable sizes. This model is shown to correctly predict the outcome of silver MPC synthetic reactions. Simple analytic expressions and simulations of rate equations are used to further validate the model and study its nature. The sequential growth model provides insights into how reactions may be directed, based on the interplay between relative MPC stabilities and reaction kinetics, providing tools for the synthesis of particular MPCs in high yield.

Theoretical Prediction of Optical Absorption and Emission in Thiolated Gold Clusters

Although the photoluminescence of gold clusters has been extensively studied so far, there are still questions on the origin of the emission in these materials. In this work, we report time-dependent density functional theory calculations on the absorption and emission spectra of the well-studied Au₂₅(SR)₁₈^- cluster, the lowest energy isomer of the Au₃₈(SR)₂₄ cluster, and five isomers of the Au₂₂(SR)₁₈ cluster. Good agreement between the calculated and measured absorption spectra, as well as with the lowest-energy emission values for these clusters, was demonstrated, verifying the accuracy of the theoretical methods employed. Our results for Au₂₅(SR)₁₈^- explain a newly observed feature in the absorption peak, also rationalizing the optical response in terms of the superatom model. The analysis of the absorption and emission characteristics of the Au₂₅(SR)₁₈^- and Au₃₈(SR)₂₄ clusters provides an estimate of the spectral regions, where fluorescence or phosphorescence is predicted to occur. Interestingly, we find that for Au₂₂(SR)₁₈, one of the five proposed structures could be present at a significant concentration in the sample, even though it is not the lowest in energy structure, which can be explained, in part, by solvent effects.

Enhanced two-photon absorption of ligated silver and gold nanoclusters: theoretical and experimental assessments

Ligated silver and gold nanoclusters belonging to a non-scalable size regime with molecular-like discrete electronic states represent an emerging class of extremely interesting optical materials. Nonlinear optical (NLO) characteristics of such quantum clusters have revealed remarkable features. The two-photon absorption (TPA) cross section of ligated noble metal nanoclusters is several orders of magnitude larger than that of commercially-available dyes. Several such case studies on NLO properties of ligated silver and gold nanoclusters have been reported, making them promising candidates for various bio-imaging techniques such as multiphoton-excited fluorescence microscopy. However, the structure-property relationship is of great importance and needs to be properly addressed in order to design new nonlinear optical materials. Using small ligated silver nanoclusters as test systems, we illustrate how theoretical approaches together with experimental findings can contribute to the understanding of structure-property relationships that might ultimately guide nanocluster synthesis.

Green Synthesized Plasmonic Silver Systems for Potential Non-Linear Optical Applications: Optical Limiting and Dual Beam Mode Matched Thermal Lensing

Bioactive compound functionalized plasmonic systems are evolving as a promising branch of nanotechnology. In this communication the synthesis of bioactive compound mimosine-based silver nanoparticles (AgNPs) and their non-linear optical and thermo-optic properties are presented. UV-Visible spectroscopy, optical bandgap measurement, fluorescence spectroscopy, and high-resolution transmission electron microscopy (HRTEM) techniques were used to characterize the synthesized AgNPs. An open aperture z-scan technique was used to determine the non-linear optical parameters. A very strong reverse saturable absorption (RSA) and low optical limiting threshold were observed for the present mimosine decorated AgNP system. The thermo-optic property of the present system was evaluated using a highly sensitive dual beam mode matched thermal lensing spectroscopic technique. A comparison of the low limiting threshold (242MWcm−2) and thermo-optic property (thermal diffusivity, D=1.13×10−7m2s−1) with similar systems proves its capability for non-linear optical and thermo-optic applications.

Two-photon absorption and photoluminescence of colloidal gold nanoparticles and nanoclusters

This review provides a comprehensive description of nonlinear optical (NLO) properties of gold nanoparticles, which can be used in biological applications. The main focus is placed on two-photon absorption (2PA) and two-photon excited photoluminescence (2PEL) - the processes crucial for multiphoton microscopy, which allows deeper imaging of the material and causes less damage to the biological samples in comparison to conventional (one-photon) microscopy. We present the basics of 2PA measurement techniques and a summary of recent achievements in the understanding of multiphoton excitation and the resulting photoluminescence in gold nanoparticles, both plasmonic ones and small nanoclusters with molecule-like properties. The examples of 2PA applications in bioimaging are also presented, with a comment on future challenges and applications.

Nanocluster superstructures or nanoparticles? The self-consuming scaffold decides

We show that using the same reaction procedure, by hindering or allowing the formation of a reaction intermediate, the Ag+dodecanethiolate polymeric complex, it is possible to selectively obtain Ag dodecanethiolate nanoparticles or Ag dodecanethiolate nanoclusters in the size range 4-2 nm. Moreover, the Ag dodecanethiolate nanoclusters display a lamellar superstructure templated from the precursor Ag+dodecanethiolate polymeric complex. A plausible formation mechanism is illustrated where, starting from the precursor and scaffold lamellar Ag+ thiolate polymeric complex, first the nanocluster Agn0 core is formed by reduction of isoplanar Ag+ ions, followed by Ag+ thiolate units that build protection, the nanocluster shell, around the core. The nanoclusters are characterized by elemental analyses, XRD, ATR-FTIR, XPS, XAS, MALDI, ESI, UV-Vis and fluorescence measurements. The luminescent Ag15(dodecanethiolate)11·2H2O nanocluster is achieved in good yield after 4 hours of reaction whereas after 2 hours, the luminescent Ag35(dodecanethiolate)16 is isolated. Both Ag nanoclusters present emission bands in the range 330-450 nm, the shifting depending on the excitation wavelength. This phenomenon is attributed to a possible dipolar state causing distribution in energies due to variability of dipole-dipole interactions. Moreover, both nanoclusters further present a NIR emission at about 700 nm independent from the excitation wavelength. Thanks to their optical and structural properties, the synthesized nanoclusters, perfect molecular/nanoparticle hybrids, have great potentiality for new applications in nanotechnologies.

Theoretical Analysis of Optical Absorption and Emission in Mixed Noble Metal Nanoclusters

In this work, we studied theoretically two hybrid gold-silver clusters, which were reported to have dual-band emission, using density functional theory (DFT) and linear and quadratic response time-dependent DFT (TDDFT). Hybrid functionals were found to successfully predict absorption and emission, although explanation of the NIR emission from the larger cluster (cluster 1) requires significant vibrational excitation in the final state. For the smaller cluster (cluster 2), the Δ H(0-0) value calculated for the T1 → S0 transition, using the PBE0 functional, is in good agreement with the measured NIR emission, and the calculated T2 → S0 value is in fair agreement with the measured visible emission. The calculated T1 → S0 phosphorescence Δ H(0-0) for cluster 1 is close to the measured visible emission energy. In order for the calculated phosphorescence for cluster 1 to agree with the intense NIR emission reported experimentally, the vibrational energy of the final state (S0) is required to be about 0.7 eV greater than the zero-point vibrational energy.

Importance of double-resonance effects in two-photon absorption properties of Au25(SR)18. Chem Sci 2017. [PMID: 28626569 PMCID: PMC5471455 DOI: 10.1039/c7sc00968b]

We show that double-resonance effects for Au₂₅(SR)₁₈^– are less pronounced and do not lead to significantly enhanced two-photon absorption cross-sections.

The two-photon absorption (TPA) cross-sections of small thiolate-protected gold clusters have been shown to be much larger than typical small organic molecules. In comparison with larger nanoparticles, their TPA cross-sections per gold atom are also found to be larger. Theoretical simulations have suggested that the large enhancement of these TPA cross-sections comes from a one-photon double-resonance mechanism. However, it remains difficult to simulate TPA cross-sections of thiolate-protected gold clusters due to their large system size and a high density of states. In this work, we report a time-dependent density functional theory (TDDFT) study of the TPA spectra of the Au₂₅(SR)₁₈^– cluster based on a damped response theory formalism. Damped response theory enables a consistent treatment of on- and off-resonance molecular properties even for molecules with a high density of states, and thus is well-suited for studying the TPA properties of gold clusters. Our results indicate that the one- and two-photon double-resonance effect is much smaller than previously found, and thus is unlikely to be the main cause of the large TPA cross-sections found experimentally. The effect of symmetry breaking of the Au₂₅(SR)₁₈^– cluster due to the ligands on the TPA cross-sections has been studied and was found to only slightly increase the cross-section. Furthermore, by comparing with larger nanoparticles we find that the TPA cross-section per gold atom scales linearly with the diameter of the particles, and that the Kerr non-linear response of the Au₂₅(SR)₁₈^– cluster is on the same order as that of bulk gold films.

A Theoretical Investigation of the Structure and Optical Properties of a Silver Cluster in Solid Form and in Solution

Using density functional theory (DFT) and linear and quadratic response time-dependent DFT, we investigated the structure and optical properties of a silver sulfide cluster with the interesting property of dual emission that was observed when in crystal form but not in solution. Since the dual fluorescence is observed only in the crystal, a supposition of stabilization of a higher-energy excited state by an excimer-like complex was analyzed by calculations for a cluster dimer, formed through π-stacking of aromatic groups bonded to the sulfur atoms. However, because of the complexity of the system, a simple one-dimensional method for dimer optimization, which works moderately well in predicting the red-shifted fluorescence compared to its absorption in a naphthalene dimer, predicts only partially the red shift for the emission energy. Interestingly, calculations of the two-photon absorption (TPA) cross-section on the optimized isolated cluster as well as the crystal structure geometry indicate significant off-resonance TPA. While some materials have significantly larger TPA cross-sections, such a TPA cross-section off-resonance could be useful. The high density of states in the dimer system results in a higher probability for significant resonance enhancement and thus much larger TPA cross-sections.

Rational Design of Biomolecular Templates for Synthesizing Multifunctional Noble Metal Nanoclusters toward Personalized Theranostic Applications

Biomolecule-templated or biotemplated metal nanoclusters (NCs) are ultrasmall (<2 nm) metal (Au, Ag) particles stabilized by a certain type of biomolecular template (e.g., peptides, proteins, and DNA). Due to their unique physiochemical properties, biotemplated metal NCs have been widely used in sensing, imaging, delivery and therapy. The overwhelming applications in these individual areas imply the great promise of harnessing biotemplated metal NCs in more advanced biomedical aspects such as theranostics. Although applications of biotemplated metal NCs as theranostic agents are trending, the rational design of biomolecular templates suitable for the synthesis of multifunctional metal NCs for theranostics is comparatively underexplored. This progress report first identifies the essential attributes of biotemplated metal NCs for theranostics by reviewing the state-of-art applications in each of the four modalities of theranostics, namely sensing, imaging, delivery and therapy. To achieve high efficacy in these modalities, we elucidate the design principles underlying the use of biomolecules (proteins, peptides and nucleic acids) to control the NC size, emission color and surface chemistries for post-functionalization of therapeutic moieties. We then propose a unified strategy to engineer biomolecular templates that combine all these modalities to produce multifunctional biotemplated metal NCs that can serve as the next-generation personalized theranostic agents.

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.