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Despite tremendous progress in the field of fluorescence-based anticounterfeiting, the advanced anticounterfeiting techniques are still posing challenges all over the world due to their cost and reliability. Recently, light-emitting atomically precise nanoclusters have emerged as attractive building blocks because of their well-defined structure, function, and stable photoluminescence. Herein, we report the room temperature fabrication of a stable, flexible, nontoxic, and low-cost precision nanocluster-based luminescent ink for the stencil printing of an optically unclonable security label. Nanocluster-based printing ink shows brilliant photoluminescence owing to its extended C-H···π/π···π interactions. Spectroscopic and microscopic investigations show that intercalated nanoclusters in the printed security labels are highly stable as their optical features and molecular compositions are unaffected. The exceptional mechanical, thermal, photo, and aqueous stabilities of the printed security labels endorse to demonstrate the printing and smartphone-based electronic reading of the quick response code on a currency. Finally, confidential information protection and decryption under a precise window of light have been achieved by adopting the optical contrast illusion. The overall cost of the security label is found to be approximately 0.013 USD per stamp.

Monolayer thiol-protected noble metal nanoclusters are attractive nanoscale building blocks for well-defined colloidal superstructures. However, achieving facile reversible self-assembly of nanoclusters using external stimuli is still in its infancy. Herein, we report the synthesis and photon-assisted reversible self-assembly of thiolated azobenzene-stapled Au25 nanoclusters. Photoactivation of functionalized nanoclusters in dichloromethane by irradiating ultraviolet light at 345 nm results in a visual change and formation of disc-like colloidal superstructures (d ∼100-1000 nm). The superstructures readily disassemble into individual nanoclusters upon irradiating with visible light at 435 nm. Systematic changes in both the electronic absorption bands and nuclear magnetic resonance spectra of chromophores in solution suggest that the photoisomerization of surface ligands drives the self-assembly. High-resolution transmission electron microscopy, electron tomographic reconstruction, dynamic light scattering, and small-angle X-ray powder diffraction show that the disc-like superstructures contain densely packed nanoclusters. Long-range self-assembly and disassembly under ultraviolet and visible light, respectively, demonstrate reversible photoswitching in nanoclusters.

Ligand protected noble metal nanoparticles are excellent building blocks for colloidal self-assembly. Metal nanoparticle self-assembly offers routes for a wide range of multifunctional nanomaterials with enhanced optoelectronic properties. The emergence of atomically precise monolayer thiol-protected noble metal nanoclusters has overcome numerous challenges such as uncontrolled aggregation, polydispersity, and directionalities faced in plasmonic nanoparticle self-assemblies. Because of their well-defined molecular compositions, enhanced stability, and diverse surface functionalities, nanoclusters offer an excellent platform for developing colloidal superstructures via the self-assembly driven by surface ligands and metal cores. More importantly, recent reports have also revealed the hierarchical structural complexity of several nanoclusters. In this review, the formulation and periodic self-assembly of different noble metal nanoclusters are focused upon. Further, self-assembly induced amplification of physicochemical properties, and their potential applications in molecular recognition, sensing, gas storage, device fabrication, bioimaging, therapeutics, and catalysis are discussed. The topics covered in this review are extensively associated with state-of-the-art achievements in the field of precision noble metal nanoclusters.

Multimodal and multifunctional contrast agents receive enormous attention in the biomedical imaging field. Such contrast agents are routinely prepared by the incorporation of organic molecules and inorganic nanoparticles (NPs) into host materials such as gold NPs, silica NPs, polymer NPs, and liposomes. Despite their non-cytotoxic nature, the large size of these NPs limits the in vivo distribution and clearance and inflames complex pharmacokinetics, which hinder the regulatory approval for clinical applications. Herein, we report a unique method that combines magnetic resonance imaging (MRI) and fluorescence imaging modalities together in nanoscale entities by the simple, direct and stable conjugation of novel biotinylated coordination complexes of gadolinium(III) to CdSe/ZnS quantum dots (QD) and terbium(III) to super paramagnetic iron oxide NPs (SPION) but without any host material. Subsequently, we evaluate the potentials of such lanthanide-speckled fluorescent-magnetic NPs for bioimaging at single-molecule, cell and in vivo levels. The simple preparation and small size make such fluorescent-magnetic NPs promising contrast agents for biomedical imaging.

International audience; Over the last decade, single-molecule optical microscopy has become the gold-standard approach to decipher complex molecular processes in cellular environments. [1-3] Single-molecule fluorescence microscopy has several advantages such as ease of application, high sensitivity, low invasiveness and versatility due the large number of available fluorescent probes. It bears however some drawbacks related to the poor photostability of organic dye molecules [4] and auto-fluorescent proteins [5-7] and and to the relatively large size of semiconductor nanoparticles in the context of live cell applications. [4,8,9] The overall size of the functional biomarkers is a general issue for any imaging approach because of steric hindrance effects in confined cell regions. Small red-shifted nano-emitters that are highly photostable are not currently available, while they would combine the best physical and optical penetration properties in biological tissues. Although single-molecule absorption microscopy was early used to detect single-molecules [10] at cryogenic temperatures, it is only with the advent of photothermal microscopy [11,12] that practical applications of absorption microscopy were developed in single-molecule research. Photothermal imaging (PhI)

This thematic article is written with an intention to provide an overview of our recent research on the modulation of quantum dots photoluminescence and the applications of quantum dots to bioimaging. In particular, we summarize the physiochemical factors that affect the photoluminescence of CdSe or CdSe/ZnS quantum dots at ensemble and single-molecule levels, blinking and blinking suppression of quantum dots, bioconjugation of quantum dots, applications of bioconjugated quantum dots for analyzing some selected biophysical processes at single-molecule levels, and intracellular delivery of quantum dot bioconjugates. These research subjects are correlated with the state of the art accomplishments in the quantum dot field.

Nanomaterials with well-defined size, shape, composition, and surface functionalities offer multimodal and multifunctional platforms for various bioanalytical, bioimaging, and therapeutic applications. In this review, we focus on the different theranostic formulations of nanomaterials based on gold, silver, silica, semiconductor quantum dots, upconversion lanthanides, oxide magnets, polymers, liposomes, carbon nanotubes, graphene and carbon nanohorns, and their applications in photothermal and photodynamic therapy of cancer.

Quantum dots-based electron donor-acceptor systems play a rising role in the design of renewable and carbon-free energy harvesting technologies. In this article, we discuss the photofabrication of fullerene-shelled quantum dots supramolecular nanoparticles, in which the fullerene shell acts as not only a well-defined electron acceptor but also a robust protecting layer against the photocorrosion of the quantum dot core. We evaluate the ensemble and single-molecule electron transfer from the core to the shell in the nanoparticles and the photocurrent response of a photoelectrochemical cell constructed using the nanoparticles. The supramolecular nanoparticle has been prepared by the covalent tethering of a fullerene-thiol monolayer to the quantum dot followed by the photochemical reactions of free fullerene-thiol to the tethered monolayer. The nanoparticles are characterized using scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Correlated single-photon emission and the two-state ON-OFF photoluminescence show that single quantum dots are included in the supramolecular nanoparticles. The fullerene-shells suppress the blinking of single quantum dots by acting as well-defined electron traps, without allowing the transfer of Auger electrons to unknown traps. Electron transfer from the quantum dot-core to the fullerene-shell is apparent from the short ON and OFF durations in the photoluminescence intensity trajectories of single quantum dots, quenching of the photoluminescence intensity and lifetime of quantum dots at the ensemble level, and the characteristic transient absorption band of the anion radical of fullerene. We next construct a photoelectrochemical cell using the supramolecular nanoparticles, and the transferred electron is externally driven in the cell to generate ∼400 μA/cm(2) photocurrent. Electron transfer from the highly stable quantum dots to the protecting fullerene-shells places the supramolecular nanoparticles among the most promising antenna systems for the construction of cost-effective and stable next generation solar energy harvesting systems.

A highly luminescent freestanding film composed of the quantum cluster, Au 15, was prepared. We studied the utility of the material for specific metal ion detection. The sensitivity of the red emission of the cluster in the composite to Cu 2+ has been used to make a freestanding metal ion sensor, similar to pH paper. The luminescence of the film was stable when exposed to several other metal ions such as Hg 2+, As 3+, and As 5+. The composite film exhibited visual sensitivity to Cu 2+ up to 1 ppm, which is below the permissible limit (1.3 ppm) in drinking water set by the U.S. environmental protection agency (EPA). The specificity of the film for Cu 2+ sensing may be due to the reduction of Cu 2+ to Cu 1+/Cu 0 by the glutathione ligand or the Au 15 core. Extended stability of the luminescence of the film makes it useful for practical applications. � 2012 American Chemical Society.

We have prepared Au 15 quantum clusters anchored to ?, ?, and ?cyclodextrin (CD) cavities. The synthesis process involves the core etching of larger clusters and the simultaneous trapping of the clusters formed inside the CD cavities. The clusters were characterized by various tools, such as optical absorption and luminescence spectroscopies, electrospray ionization-mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS), circular dichroism spectroscopy, and two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy. Trapping of the cluster in the CD cavity was proven by circular dichroism and also by rotational Overhauser effect spectroscopy (ROESY), in terms of the distinct cross peak between proton "e" of the glutathione (-SG) ligand and the ? 3 "e" proton of CD. Dynamic light scattering (DLS) studies showed a hydrodynamic diameter of ?3-4 nm, indicating one CD molecule per cluster with an extension of one water of hydration. The clusters are intensely luminescent, with major lifetime components of 28, 71, and 24 ps for Au 15@ ?CD, Au 15at?CD, and Au 15at?CD, respectively. The clusters also are strongly luminescent in the solid state. Both in the solution and in the solid state, the luminescence is sensitive to solvents/vapors. The clusters adhere to glass plates, and the solvent dependency of luminescence was used to create patterns that are erased upon gradual evaporation of the solvent. This self-erasing property was further demonstrated with clusters supported on a thin layer chromatography (TLC) plate. Selective detection of metal ions using the luminescence of the clusters is reported. Evaporation of the cluster solutions leads to luminescent gel-like materials. � 2011 American Chemical Society.

We synthesized fluorescent, porphyrin-anchored, Au 22 clusters in a single step, starting from well-characterized Au 25 clusters protected with glutathione (-SG) by a combined core reduction/ligand exchange protocol, at a liquid-liquid interface. The prepared cluster was characterized by UV/vis, photoluminescence, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, elemental analysis, and matrix-assisted laser desorption ionization mass spectrometry. The absence of a 672 nm intraband transition of Au 25 and the simultaneous emergence of new characteristic peaks at 520 and 635 nm indicate the formation of the Au 22 core. An increase in the binding energy of 0.4 eV in Au 4f core-level peaks confirmed the presence of a reduced core size. Quantitative XPS confirmed the Au/S ratio. The presence of a free base, tetraphenylporphyrin (H 2TPPOAS-), on the Au 22 core was confirmed by fluorimetric titrations with Cu 2+ and Zn 2+ ions. From all of these, the composition of the cluster was determined to be Au 22[(-SG) 15(-SAOPPTH 2) 2], which was supported by mass spectrometry and elemental analysis. We utilized the fluorescence nature of these water-soluble clusters for the fabrication of fluorescent patterns by soft lithography. The patterns were studied using tapping-mode atomic force microscopy and confocal fluorescence imaging. � 2009 American Chemical Society.

Effect of ligand exchange on the optical and photoluminescence properties of Au 25 SG 18 ( SG -glutathione thiolate) clusters was investigated. A fluorescein-based dye, 5-((2-(and-3)-S-(acetylmercapto)succinoyl)amino)-fluorescein (SAMSA) was anchored on water soluble Au 25 SG 18 clusters by the place exchange reaction and the solid-state emission of the exchange product was studied. Inherent fluorescence image of these clusters was mapped. Organic soluble Au 25( SC 2 H 4 Ph 18 clusters were synthesized as a model system which also showed temperature-dependent solid-state emission, in good agreement with the results from water soluble Au 25 clusters.

Fluorescent gold nanoparicles were prepared and fluorescent 3D superlattices were made with them at an air/water interface. Fluorescein-labeled Au@MSA, denoted Au@MSA/SAMSA was prepared by ligand exchange reaction. The emission peak of Au@MSA/SAMSA is found to appear at 675 nm, greatly red-shifted compared to the SAMSA solution sample or an isolated Au@MSA/SAMSA particle. It is also noted that the parent fluorophore, fluorescein isothiocyanate (FITC), shows a red shift of ca. 105 nm in the solid state compared to its emission spectrum in solution. Particles of core diameter 3.43 nm exhibit hexagonal assembly extending over several micrometers. The superlattices are fluorescent even though SAMSA is close to the metallic core and strong quenching is expected.

Ligand exchange offers an effective way to modify the properties of the recently prepared quantum clusters of gold. To tune optical and photoluminescence properties of one of the most stable quantum clusters of gold, AU25SG18 (SG-glutathione thiolate), we functionalized it by the exchange of - SG with functionalized -SG and with an altogether different ligand, namely, 3-mercapto-2-butanol (MB). The products were characterized by various techniques such as optical absorption (UV - vis), Fourier-transform infrared (FT - IR), nuclear magnetic resonance (NMR), X-ray photoelectron (XPS), and luminescence spectroscopies, mass spectrometry, and thermogravimetry (TG). Analyses of the TG data helped to establish the molecular composition of the products. Ligand exchange reaction was monitored by NMR spectroscopy, and it was found that the exchange reaction follows a first order kinetics. The XPS study showed that after the exchange reaction there was no change in the chemical nature of the metal core and binding energy values of Au 4f7/2 and 4f5/2, which are similar in both the parent and the exchanged products. Photoluminescence studies of these clusters, done in the aerated conditions, showed that the excitation spectrum of the MB-exchanged product is entirely different from the acetyl- and formyl-glutathione exchanged products. The inherent fluorescence and solid-state emission of these clusters were observed. This intense emission allows optical imaging of the material in the solid state. The emission is strongly temperature dependent. The synthesis of a diverse variety of clusters and their chemical stability and intense luminescence offer numerous applications in areas such as energy transfer, sensors, biolabeling, and drug delivery. � 2008 American Chemical Society.

Optimal single particle tracking experiments in live cells requires small and photostable probes, which do not modify the behavior of the molecule of interest. Current fluorescence-based microscopy of single molecules and nanoparticles is often limited by bleaching and blinking or by the probe size. As an alternative, we present in this chapter the synthesis of a small and highly specific gold nanoprobe whose detection is based on its absorption properties. We first present a protocol to synthesize 5-nm-diameter gold nanoparticles and functionalize them with a nanobody, a single-domain antibody from camelid, targeting the widespread green fluorescent protein (GFP)-tagged proteins with a high affinity. Then we describe how to detect and track these individual gold nanoparticles in live cell using photothermal imaging microscopy. The combination of a probe with small size, perfect photostability, high specificity, and versatility through the vast existing library of GFP-proteins, with a highly sensitive detection technique enables long-term tracking of proteins with minimal hindrance in confined and crowded environments such as intracellular space.

Multimodal and multifunctional nanomaterials are promising candidates for bioimaging and therapeutic applications in the nanomedicine settings. Here we report the preparation of photouncaging nanoparticles with fluorescence and magnetic modalities and evaluation of their potentials for in vitro and in vivo bioimaging. Photoactivation of such bimodal nanoparticles prepared using photouncaging ligands, CdSe/ZnS quantum dots, and super paramagnetic iron oxide nanoparticles results in the systematic uncaging of the particles, which is correlated with continuous changes in the absorption, mass and NMR spectra of the ligands. Fluorescence and magnetic components of the bimodal nanoparticles are characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and elemental analyses using energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). Bioconjugation of the nanoparticles with peptide hormones renders them with biocompatibility and efficient intracellular transport as seen in the fluorescence and MRI images of mouse melanoma cells (B16) or human lung epithelial adenocarcinoma cells (H1650). Biocompatibility of the nanoparticles is evaluated using MTT cytotoxicity assays, which show cell viability over 90%. Further, we combine MRI and NIR fluorescence imaging in C57BL/6 (B6) mice subcutaneously or intravenously injected with the photouncaging nanoparticles and follow the in vivo fate of the nanoparticles. Interestingly, the intravenously injected nanoparticles initially accumulate in the liver within 30 min post injection and subsequently clear by the renal excretion within 48 h as seen in the time-dependent MRI and fluorescence images of the liver, urinary bladder, and urine samples. Photouncaging ligands such as the ones reported in this article are promising candidates for not only the site-specific delivery of nanomaterials-based contrast agents and drugs but also the systematic uncaging and renal clearance of nanomaterials after the desired in vivo application.

Renewable resource of carbon-free energy is crucial in the current scenario of global warming and our growing energy needs. Besides, the significance of safer energy resources has been underscored by many nuclear mishaps, including the recently crippled Fukushima Nuclear power plant in Japan. Sunlight is the most promising alternative to nuclear fuels and greenhouse-gas-emitting fossil and alcohol fuels. (Published: 11 June 2013) Citation: Nano Reviews 2013, 4 : 21079 - http://dx.doi.org/10.3402/nano.v4i0.21079

Nanoprobes based on quantum clusters (QC) with near-infrared fluorescence, magnetic-resonance-imaging contrast, and singlet-oxygen-sensitized intracellular fluorescence are studied. The generation of singlet oxygen and singlet-oxygen-sensitized fluorescence uncaging by magnetic and NIR-emitting nanoparticles are exploited for multimodal bioimaging in vitro.

We show that the third order optical nonlinearity of 15-atom gold clusters is significantly enhanced when in contact with indium tin oxide (ITO) conducting film. Open and close aperture z-scan experiments together with non-degenerate pump-probe differential transmission experiments were done using 80 fs laser pulses centered at 395 nm and 790 nm on gold clusters encased inside cyclodextrin cavities. We show that two photon absorption coefficient is enhanced by an order of magnitude as compared to that when the clusters are on pristine glass plate. The enhancement for the nonlinear optical refraction coefficient is ~3 times. The photo-induced excited state absorption using pump-probe experiments at pump wavelength of 395 nm and probe at 790 nm also show an enhancement by an order of magnitude. These results attributed to the excited state energy transfer in the coupled gold cluster-ITO system are different from the enhancement seen so far in charge donor-acceptor complexes and nanoparticle-conjugate polymer composites., To appear in Optics Express (2013); http://dx.doi.org/10.1364/OE.21.008483

Mercaptosuccinic acid protected gold nanoparticles (Au@MSA) self assemble to form superlattice (SL) crystals at the air-water interface. These have been used for gas adsorption. The current-voltage (I-V) characteristics of the SL film with embedded SL crystals, obtained by four probe measurements, show Ohmic conduction. The conductance observed was proportional to the polarizability of the adsorbed gases. The current through the SL decreases on adsorption of the gas along with decrease in the SERS intensity of a probe molecule from the crystals. We rationalise our observation of the linear dependence of the conductance on the polarizability of the adsorbed gas using a simple model calculation. Variation of the conductance may be useful in designing electrical switches operating at the nanometre length scales. � 2011 The Royal Society of Chemistry.

We developed a cheap and rapid method for the fabrication of 3D nanoparticle superlattices (SLs) of Au@SGAN and Au@MSA (N-acetyl glutathione (SGAN) and mercaptosuccinic acid (MSA) protected gold nanoparticles, respectively) in gram scale, at a liquid-liquid interfaces under flowing nitrogen gas. While available methods take several weeks to make crystalline SLs, the present route makes them in a day. Morphology of these crystals was examined with scanning electron microscopy (SEM), and their structures were probed using transmission electron microscopy (TEM). The surface enhanced Raman scattering (SERS) studies of these crystals were done using crystal violet (CV) molecules as the analyte which exhibited a detection limit of 10 -8 M. The SERS spectrum was used to map the Raman images of the superlattce crystals. SERS from the edges of the crystal showed more enhancement than from the flat surfaces, which is in good agreement with theoretical reports of such anisotropic structures. The sides of the crystals are not sharp, and they show corrugations at the nanometer scale. This helps to produce more "hot spots" at the edges, which result in larger electric field enhancement from these locations. The enhancement factors (EF) for Au@MSA and Au@SGAN SLs were calculated to be around 1.47 x 10 6 and 3.60 x 10 5, respectively. More enhancements from Au@MSA SL compared to that of Au@SGAN could be attributed to the smaller chain length of the MSA molecule, which allows closer analyte approach to the nanoparticle surface. � 2009 American Chemical Society.

Fluorescent three-dimensional (3-D) superlattices of dansyl glutathione protected gold nanoparticles, with potential applications in molecular detection, have been synthesized at an air/water interface by controlling the pH of the nanoparticle suspension. The number of fluorophores per nanoparticle was calculated to be ?127. Morphologies of the superlattice crystals were examined using scanning electron microscopy (SEM). Most of the crystals observed were triangular in shape. High-resolution transmission electron microscopy (HRTEM) and small angle X-ray scattering (SAXS) were used to study the packing of nanoparticles in these crystals. Both these studies showed that the nanoparticles were arranged in a face-centered cubic (fcc) pattern with a particle-particle distance (center-center) of ?10.5 nm. Evolution of the crystal morphologies with time was also examined. The fluorescence properties of these triangles were studied using confocal fluorescence imaging and confocal Raman mapping, which were in good agreement with the morphologies observed by SEM. The superlattice exhibits near-infrared (NIR) absorption in the range 1100-2500 nm. Easy synthesis of such functional nanoparticle-based solids makes it possible to use them in novel applications. We utilized the fluorescence of dansyl glutathione gold superlattice crystals for the selective detection of bovine serum albumin (BSA), the major protein constituent of blood plasma, based on the selective binding of the naphthalene ring of the dansyl moiety with site I of BSA. � 2009 Tsinghua University Press and Springer-Verlag.

Photoinduced electron transfer in donor-acceptor systems composed of quantum dots (QDs) and electron donors or acceptors is a subject of considerable recent research interest due to the potential applications of such systems in both solar energy harvesting and degradation of organic pollutants. Herein, we employed single-molecule imaging and spectroscopy techniques for the detection of photochemical reactions between 1,4-diaminobutane (DAB) and CdSe/ZnS single QDs. We investigated the reactions by analyzing photoluminescence (PL) intensity and lifetime of QDs at ensemble and single-molecule levels. While DAB was applied to single QDs tethered on a cover slip or QDs dispersed in a solution, PL intensity of QD continuously decreased with a concomitant increase in the PL lifetime. Interestingly, these changes in the PL properties of QD were predominant under high-intensity photoactivation. We hypothesize that the above changes in the PL properties surface due to the transfer of an electron from DAB to Auger-ionized QD followed by elimination of a proton from DAB and the formation of a QD-DAB adduct. Thus, a continuous decrease in the PL intensity of QDs under high-intensity photoactivation is attributed to continuous photochemical reactions of DAB with single QDs and the formation of QD-(DAB)n adducts. We believe that detection and analysis of such photochemical reactions of single QDs with amines will be of considerable broad interest due to the significant impact of photoinduced electron transfer reactions in energy management and environmental remediation. Keywords: quantum dots; single-molecule; electron transfer; Auger ionization; CdSe/ZnS; photoluminescence; photochemical reaction (Published: 13 October 2011) Citation: Nano Reviews 2011, 2 : 6366 - DOI: 10.3402/nano.v2i0.6366

Glutathione protected Au25 quantum clusters, exhibiting characteristic fluorescence, have been uniformly coated inside and outside of ?-Ala-L-Ile dipeptide nanotubes. These coated structures have been imaged using the inherent fluorescence of Au25. Upon exposure to an electron beam, in a transmission electron microscope, the quantum clusters gradually transform to gold nanoparticles, of the metallic size regime. The nanoparticles grow to a size of 4.5 nm and thereafter the particle size is unaffected by electron beam exposure. The nanotubes are intact and this template is shown to control the uniformity of the size of the nanoparticles grown. The quantum clusters can be loaded selectively inside the tubes using capillarity of the nanotubes. The sizes of the nanoparticles grown are tuned using electron beam exposure. � 2009 The Royal Society of Chemistry.