Fluorescence Intensity and Lifetime Cell Imaging with Luminescent Gold Nanoclusters

Enhanced Plasmon Based Ag and Au Nanosystems and Their Improved Biomedical Impacts

Numerous specialists and academics have backed the improved physicochemical characteristics of metal substrate (Ag, Au) based composite nanoparticles for a number of applications, including pharmaceuticals, optoelectronics, and environmental impact. Insights of Ag and Au NPs-based nanomaterials will be discussed, as well as important production, physicochemical, and biotechnological characteristics. The plasmon capacities of Ag and Au NPs, along with their customisable form, scale, and surface modification could be described by specified geometries and constituent contents. It was revealed that interaction dynamics of Ag and Au implanted nanomaterials with dopants/defects ratios seem to be more effective in stimulating pathogens by interrupting biochemical reactions. As a result, we focus on defect science in Ag and Au-based nanoscale materials, taking into account surface morphology, ionic packing, and chemical phase assessment. This chapter will cover the important optical, geometrical, and physicochemical features of Ag and Au nanomaterials, and their pharmacological significance.

Gold nanoclusters: An ultrasmall platform for multifaceted applications

Gold nanoclusters (Au NCs) with a core size below 2 nm form an exciting class of functional nano-materials with characteristic physical and chemical properties. The properties of Au NCs are more prominent and extremely different from their bulk counterparts. The synthesis of Au NCs is generally assisted by template or ligand, which impart excellent cluster stability and high quantum yield. The tunable and sensitive physicochemical properties of Au NCs open horizons for their advanced applications in various interdisciplinary fields. In this review, we briefly summarize the solution phase synthesis and origin of the characteristic properties of Au NCs. A vast review of recent research work introducing biosensors based on Au NCs has been presented along with their specifications and detection limits. This review also highlights recent progress in the use of Au NCs as bio-imaging probe, enzyme mimic, temperature sensing probe and catalysts. A speculation on present challenges and certain future prospects have also been provided to enlighten the path for advancement of multifaceted applications of Au NCs.

Synthesis of biocompatible, BSA capped fluorescent CaCO3 pre-nucleation nanoclusters for cell imaging applications

Novel, photostable, multicolour fluorescent, highly biocompatible, water soluble, BSA capped pre-nucleation CaCO3 nanoclusters (FCPN) (∼1.3 nm) are developed using a facile biomineralization process. M. oleifera leaf extract and BSA protein are used as sources of ascorbic acid and capping agent, respectively. The developed FCPN shows fluorescence in the blue, green, and yellow/red region with an average life time of 1.05, 6.23 and 30.60 ns, respectively. The MALDI-MS measurements reveal that these nanoclusters are 16, 50, 73, 222 and 936 molecules big. These FCPN, when incubated (up to 7 days) with MG-63 cells, demonstrate an increase in cell viability percentage with time period as compared to their control samples. Furthermore, these incubated cells were investigated using confocal microscopy to estimate the FCPN diffusion penetration depth using CTCF analysis. It has been observed that blue and green emitting FCPN penetrated 6 μm, whereas red emitting FCPN traversed only 4 μm. The relative quantum yield (Rhodamine 6G = 0.92) of FCPN for green emission was found to be 0.0175 in water. The prepared nanoclusters displayed four months shelf-life. These FCPN were prepared using an environmentally benign, inexpensive, green synthetic route without using toxic reducing agents. Furthermore, the current report discusses the detailed results, obtained from X-ray photoelectron spectroscopy, MALDI-MS, Fourier transform infrared spectroscopy, UV-visible, fluorescence spectroscopy, lifetime measurements, electron microscopy, fluorescence microscopy and confocal microscopy.

Aggregation-Enhanced Photoluminescence and Photoacoustics of Atomically Precise Gold Nanoclusters in Lipid Nanodiscs (NANO2)

The authors designed a structurally stable nano-in-nano (NANO2) system highly capable of bioimaging via an aggregation-enhanced NIR excited emission and photoacoustic response achieved based on atomically precise gold nanoclusters protected by linear thiolated ligands [Au25(SC n H2n+1)18, n = 4-16] encapsulated in discoidal phospholipid bicelles through a one-pot synthesis. The detailed morphological characterization of NANO2 is conducted using cryogenic transmission electron microscopy, small/wide angle X-ray scattering with the support of molecular dynamics simulations, providing information on the location of Au nanoclusters in NANO2. The photoluminescence observed for NANO2 is 20-60 times more intense than that of the free Au nanoclusters, with both excitation and emission wavelengths in the near-infrared range, and the photoacoustic signal is more than tripled. The authors attribute this newly discovered aggregation-enhanced photoluminescence and photoacoustic signals to the restriction of intramolecular motion of the clusters' ligands. With the advantages of biocompatibility and high cellular uptake, NANO2 is potentially applicable for both in vitro and in vivo imaging, as the authors demonstrate with NIR excited emission from in vitro A549 human lung and the KB human cervical cancer cells.

Molecular reactivity of thiolate-protected noble metal nanoclusters: synthesis, self-assembly, and applications

Thiolate-protected noble metal (e.g., Au and Ag) nanoclusters (NCs) are ultra-small particles with a core size of less than 3 nm. Due to the strong quantum confinement effects and diverse atomic packing modes in this ultra-small size regime, noble metal NCs exhibit numerous molecule-like optical, magnetic, and electronic properties, making them an emerging family of "metallic molecules". Based on such molecule-like structures and properties, an individual noble metal NC behaves as a molecular entity in many chemical reactions, and exhibits structurally sensitive molecular reactivity to various ions, molecules, and other metal NCs. Although this molecular reactivity determines the application of NCs in various fields such as sensors, biomedicine, and catalysis, there is still a lack of systematic summary of the molecular interaction/reaction fundamentals of noble metal NCs at the molecular and atomic levels in the current literature. Here, we discuss the latest progress in understanding and exploiting the molecular interactions/reactions of noble metal NCs in their synthesis, self-assembly and application scenarios, based on the typical M(0)@M(i)-SR core-shell structure scheme, where M and SR are the metal atom and thiolate ligand, respectively. In particular, the continuous development of synthesis and characterization techniques has enabled noble metal NCs to be produced with molecular purity and atomically precise structural resolution. Such molecular purity and atomically precise structure, coupled with the great help of theoretical calculations, have revealed the active sites in various structural hierarchies of noble metal NCs (e.g., M(0) core, M-S interface, and SR ligand) for their molecular interactions/reactions. The anatomy of such molecular interactions/reactions of noble metal NCs in synthesis, self-assembly, and applications (e.g., sensors, biomedicine, and catalysis) constitutes another center of our discussion. The basis and practicality of the molecular interactions/reactions of noble metal NCs exemplified in this Review may increase the acceptance of metal NCs in various fields.

Ultrasmall Au and Ag Nanoclusters for Biomedical Applications: A Review

Noble metal (e.g., Au, Ag, Pt, Pd, and their alloys) nanoclusters (NCs) have emerged as a new type of functional nanomaterial in nanoscience and nanotechnology. Owing to their unique properties, such as their ultrasmall dimension, enhanced photoluminescence, low toxicity, and excellent biocompatibility, noble metal NCs-especially Au and Ag NCs-have found various applications in biomedical regimes. This review summarizes the recent advances made in employing ultrasmall Au and Ag NCs for biomedical applications, with particular emphasis on bioimaging and biosensing, anti-microbial applications, and tumor targeting and cancer treatment. Challenges, including the shared and specific challenges for Au and Ag NC toward biomedical applications, and future directions are briefly discussed at the end.

An overview on the current understanding of the photophysical properties of metal nanoclusters and their potential applications

Photophysics of atomically precise metal nanoclusters (MNCs) is an emerging area of research due to their potential applications in optoelectronics, photovoltaics, sensing, bio-imaging and catalysis. An overview of the recent advances in the photophysical properties of MNCs is presented in this review. To begin with, we illustrate general synthesis methodologies of MNCs using direct reduction, chemical etching, ligand exchange, metal exchange and intercluster reaction. Due to strong quantum confinement, the NCs possess unique electronic properties such as discrete optical absorption, intense photoluminescence (PL), molecular-like electron dynamics and non-linear optical behavior. Discussions have also been carried out to unveil the influence of the core size, nature of ligands, heteroatom doping, and surrounding environments on the optical absorption and photophysical properties of metal clusters. Recent findings reveal that the excited-state dynamics, nonlinear optical properties and aggregation induced emission of metal clusters offer exciting opportunities for potential applications. We discuss briefly about their versatile applications in optoelectronics, sensing, catalysis and bio-imaging. Finally, the future perspective of this research field is given.

Ag(I)-Thiolate-Protected Silver Nanoclusters for Solar Cells: Electrochemical and Spectroscopic Look into the Photoelectrode/Electrolyte Interface

Intrinsic low stability and short excited lifetimes associated with Ag nanoclusters (NCs) are major hurdles that have prevented the full utilization of the many advantages of Ag NCs over their longtime contender, Au NCs, in light energy conversion systems. In this report, we diagnosed the problems of conventional thiolated Ag NCs used for solar cell applications and developed a new synthesis route to form aggregation-induced emission (AIE)-type Ag NCs that can significantly overcome these limitations. A series of Ag(0)/Ag(I)-thiolate core/shell-structured NCs with different core sizes were explored for photoelectrodes, and the nature of the two important interfacial events occurring in Ag NC-sensitized solar cells (photoinduced electron transfer and charge recombination) were unveiled by in-depth spectroscopic and electrochemical analyses. This work reveals that the subtle interplay between the light absorbing capability, charge separation dynamics, and charge recombination kinetics in the photoelectrode dictates the solar cell performance. In addition, we demonstrate significant improvement in the photocurrent stability and light conversion efficiency that have not been achieved previously. Our comprehensive understanding of the critical parameters that limit the light conversion efficiency lays a foundation on which new principles for designing Ag NCs for efficient light energy conversion can be built.

Iridium Nanoparticles for Multichannel Luminescence Lifetime Imaging, Mapping Localization in Live Cancer Cells

The development of long-lived luminescent nanoparticles for lifetime imaging is of wide interest as luminescence lifetime is environmentally sensitive detection independent of probe concentration. We report novel iridium-coated gold nanoparticles as probes for multiphoton lifetime imaging with characteristic long luminescent lifetimes based on iridium luminescence in the range of hundreds of nanoseconds and a short signal on the scale of picoseconds based on gold allowing multichannel detection. The tailor-made IrC₆ complex forms stable, water-soluble gold nanoparticles (AuNPs) of 13, 25, and 100 nm, bearing 1400, 3200, and 22 000 IrC₆ complexes per AuNP, respectively. The sensitivity of the iridium signal on the environment of the cell is evidenced with an observed variation of lifetimes. Clusters of iridium nanoparticles show lifetimes from 450 to 590 ns while lifetimes of 660 and 740 ns are an average of different points in the cytoplasm and nucleus. Independent luminescence lifetime studies of the nanoparticles in different media and under aggregation conditions postulate that the unusual long lifetimes observed can be attributed to interaction with proteins rather than nanoparticle aggregation. Total internal reflection fluorescence microscopy (TIRF), confocal microscopy studies and 3D luminescence lifetime stacks confirm the presence of bright, nonaggregated nanoparticles inside the cell. Inductively coupled plasma mass spectrometry (ICPMS) analysis further supports the presence of the nanoparticles in cells. The iridium-coated nanoparticles provide new nanoprobes for lifetime detection with dual channel monitoring. The combination of the sensitivity of the iridium signal to the cell environment together with the nanoscaffold to guide delivery offer opportunities for iridium nanoparticles for targeting and tracking in in vivo models.

Fluorescent metal quantum clusters: an updated overview of the synthesis, properties, and biological applications

A brief history of metal quantum clusters, their synthesis methods, physical properties, and an updated overview of their applications is provided.

Luminescent Gold Nanoparticles with Size-Independent Emission

Size-independent emission has been widely observed for ultrasmall thiolated gold nanoparticles (AuNPs) but our understanding of the photoluminescence mechanisms of noble metals on the nanoscale has remained limited. Herein, we report how the emission wavelength of a AuNP and the local binding geometry of a thiolate ligand (glutathione) on the AuNP are correlated, as these AuNPs emit at different wavelengths in spite of their identical size (ca. 2.5 nm). By using circular dichroism, X-ray absorption, and fluorescence spectroscopy, we found that a high Au-S coordination number (CN) and a high surface coverage resulted in strong Au(I) -ligand charge transfer, a chiral conformation, and 600 nm emission, whereas a low Au-S CN and a low surface coverage led to weak charge transfer, an achiral conformation, and 810 nm emission. These two size-independent emissions can be integrated into one single 2.5 nm AuNP by fine-tuning of the surface coverage; a ratiometric pH response was then observed owing to strong energy transfer between two emission centers, opening up new possibilities for the design of ultrasmall ratiometric pH nanoindicators.

Surface-confined fluorescence enhancement of Au nanoclusters anchoring to a two-dimensional ultrathin nanosheet toward bioimaging

Gold nanoclusters (Au NCs) as ultrasmall fluorescent nanomaterials possess discrete electronic energy and unique physicochemical properties, but suffer from relatively low quantum yield (QY) which severely affects their application in displays and imaging. To solve this conundrum and obtain highly-efficient fluorescent emission, 2D exfoliated layered double hydroxide (ELDH) nanosheets were employed to localize Au NCs with a density as high as 5.44 × 10(13) cm(-2), by virtue of the surface confinement effect of ELDH. Both experimental studies and computational simulations testify that the excited electrons of Au NCs are strongly confined by MgAl-ELDH nanosheets, which results in a largely promoted QY as well as prolonged fluorescence lifetime (both ∼7 times enhancement). In addition, the as-fabricated Au NC/ELDH hybrid material exhibits excellent imaging properties with good stability and biocompatibility in the intracellular environment. Therefore, this work provides a facile strategy to achieve highly luminescent Au NCs via surface-confined emission enhancement imposed by ultrathin inorganic nanosheets, which can be potentially used in bio-imaging and cell labelling.

Nanoporous Metal Papers for Scalable Hierarchical Electrode

Nanoporous metals similar to paper in form are developed using Japanese washi paper as a template to create hierarchical porous electrodes. This method is used to create a trimodal -nanoporous Au electrode, as a well as a hierarchical NiMn electrode that achieves high electrochemical capacitance and a rapid rate of oxygen evolution.

Aqueous synthesis of near-infrared highly fluorescent platinum nanoclusters

A one-step synthesis of near infrared fluorescent platinum nanoclusters (PtNCs) in aqueous medium is described. The proposed optimized procedure for PtNC synthesis is rather simple, fast and it is based on the direct metal reduction with NaBH4. Bidentated thiol ligands (lipoic acid) were selected as nanoclusters stabilizers in water media. The structural characterization revealed attractive features of the PtNCs, including small size, high water solubility, near-infrared luminescence centered at 680 nm, long-term stability and the highest quantum yield in water reported so far (47%) for PtNCs. Moreover, their stability in different pH media and an ionic strength of 0.2 M NaCl was studied and no significant changes in fluorescence emission were detected. In brief, they offer a new type of fluorescent noble metal nanoprobe with a great potential to be applied in several fields, including biolabeling and imaging experiments.

Single Probe for Imaging and Biosensing of pH, Cu(2+) Ions, and pH/Cu(2+) in Live Cells with Ratiometric Fluorescence Signals

It is very essential to disentangle the complicated inter-relationship between pH and Cu in the signal transduction and homeostasis. To this end, reporters that can display distinct signals to pH and Cu are highly valuable. Unfortunately, there is still no report on the development of biosensors that can simultaneously respond to pH and Cu(2+), to the best of our knowledge. In this work, we developed a single fluorescent probe, AuNC@FITC@DEAC (AuNC, gold cluster; FITC, fluorescein isothiocyanate; DEAC, 7-diethylaminocoumarin-3-carboxylic acid), for biosensing of pH, Cu(2+), and pH/Cu(2+) with different ratiometric fluorescent signals. First, 2,2',2″-(2,2',2″-nitrilotris(ethane-2,1-diyl)tris((pyridin-2-yl-methyl)azanediyl))triethanethiol (TPAASH) was designed for specific recognition of Cu(2+), as well as for organic ligand to synthesize fluorescent AuNCs. Then, pH-sensitive molecule, FITC emitting at 518 nm, and inner reference molecule, DEAC with emission peak at 472 nm, were simultaneously conjugated on the surface of AuNCs emitting at 722 nm, thus, constructing a single fluorescent probe, AuNC@FITC@DEAC, to sensing pH, Cu(2+), and pH/Cu(2+) excited by 405 nm light. The developed probe exhibited high selectivity and accuracy for independent determination of pH and Cu(2+) against reactive oxygen species (ROS), other metal ions, amino acids, and even copper-containing proteins. The AuNC-based inorganic-organic probe with good cell-permeability and high biocompatibility was eventually applied in monitoring both pH and Cu(2+) and in understanding the interplaying roles of Cu(2+) and pH in live cells by ratiometric multicolor fluorescent imaging.

Microwave-assisted synthesis of water-soluble, fluorescent gold nanoclusters capped with small organic molecules and a revealing fluorescence and X-ray absorption study

Colourless solutions of blue light-emitting, water-soluble gold nanoclusters (AuNC) were synthesized from gold colloids under microwave irradiation using small organic molecules as ligands. Stabilized by 1,3,5-triaza-7-phosphaadamantane (TPA) or L-glutamine (GLU), fluorescence quantum yields up to 5% were obtained. AuNC are considered to be very promising for biological labelling, optoelectronic devices and light-emitting materials but the structure-property relationships have still not been fully clarified. To expand the knowledge about the AuNC apart from their fluorescent properties they were studied by X-ray absorption spectroscopy elucidating the oxidation state of the nanoclusters' gold atoms. Based on curve fitting of the XANES spectra in comparison to several gold references, optically transparent fluorescent AuNC are predicted to be ligand-stabilized Au5(+) species. Additionally, their near edge structure compared with analogous results of polynuclear clusters known from the literature discloses an increasing intensity of the feature close to the absorption edge with decreasing cluster size. As a result, a linear relationship between the cluster size and the X-ray absorption coefficient can be established for the first time.

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.

Surface-chemistry effect on cellular response of luminescent plasmonic silver nanoparticles

Cellular response of inorganic nanoparticles (NPs) is strongly dependent on their surface chemistry. By taking advantage of robust single-particle fluorescence and giant Raman enhancements of unique polycrystalline silver NPs (AgNPs), we quantitatively investigated effects of two well-known surface chemistries, passive PEGylation and active c-RGD peptide conjugation, on in vitro behaviors of AgNPs at high temporal and spatial resolution as well as chemical level using fluorescence and Raman microscopy. The results show that specific c-RGD peptide−α_vβ₃ integrin interactions not only induced endosome formation more rapidly, enhanced constrained diffusion, but also minimized nonspecific chemical interactions between the NPs and intracellular biomolecules than passive PEGylation chemistry; as a result, surface enhanced Raman scattering (SERS) signals of c-RGD peptides were well resolved inside endosomes in the live cells, while Raman signals of PEGylated AgNPs remained unresolvable due to interference of surrounding biomolecules, opening up an opportunity to investigate specific ligand–receptor interactions in real time at the chemical level.

Simultaneous RGB emitting Au nanoclusters in chitosan nanoparticles for anticancer gene theranostics

Advanced theranostic materials hold promise for targeted delivery of drugs, with the ability to follow the transport as well as its consequences. This should, ideally, be possible with minimum invasive surgery and having no or minimum cytotoxicity of the materials. It requires development of newer materials whose physical properties would allow for easy probe, which could carry the therapeutic molecules, which will be stable under physiological conditions, and at the same time would be able to permeate barriers to the target. We report the development of a composite consisting of highly fluorescent Au nanoclusters and the biopolymer chitosan, which could easily be converted into nanoparticles and would form a stable polyplex with suicide gene for induction of apoptosis in cervical cancer cells. The simultaneous red, green, and blue fluorescence from the nanoclusters provided convenient optical imaging and flow cytometry probes, without having to use additional dyes. Moreover, the colloidal nanocluster-polymer composite could be converted into solid film and be stored with the retention of optical properties. The pH tunable optical properties in the medium were also intact in the films that quickly dissolved in water with retention of properties.

Copper clusters as novel fluorescent probes for the detection and photocatalytic elimination of lead ions

A new homogeneous assay for a fast, selective and sensitive detection and elimination of lead ions has been developed using copper clusters as novel fluorescent probes in aqueous solutions.