Combined experimental and computational study of the photoabsorption of the monodoped and nondoped nanoclusters Au24Pt(SR)18, Ag24Pt(SR)18, and Ag25(SR)18.

Assessing the accuracy of first-principles computational approaches is instrumental to predict electronic excitations in metal nanoclusters with quantitative confidence. Here we describe a validation study on the optical response of a set of monolayer-protected clusters (MPC). The photoabsorption spectra of Ag25(DMBT)18-, Ag24Pt(DMBT)182- and Au24Pt(SC4H9)18, where DMBT is 2,4-dimethylbenzenethiolate and SC4H9 is n-butylthiolate, have been obtained at low temperature and compared with accurate TDDFT calculations. An excellent match between theory and experiment, with typical deviations of less than 0.1 eV, was obtained, thereby validating the accuracy and reliability of the proposed computational framework. Moreover, an analysis of the TDDFT simulations allowed us to ascribe all relevant spectral features to specific transitions between occupied/virtual orbital pairs. The doping effect of Pt on the optical response of these ultrasmall MPC systems was identified and discussed.

Glutathionylation primes soluble glyceraldehyde-3-phosphate dehydrogenase for late collapse into insoluble aggregates.

Protein aggregation is a complex physiological process, primarily determined by stress-related factors revealing the hidden aggregation propensity of proteins that otherwise are fully soluble. Here we report a mechanism by which glycolytic glyceraldehyde-3-phosphate dehydrogenase of Arabidopsis thaliana (AtGAPC1) is primed to form insoluble aggregates by the glutathionylation of its catalytic cysteine (Cys149). Following a lag phase, glutathionylated AtGAPC1 initiates a self-aggregation process resulting in the formation of branched chains of globular particles made of partially misfolded and totally inactive proteins. GSH molecules within AtGAPC1 active sites are suggested to provide the initial destabilizing signal. The following removal of glutathione by the formation of an intramolecular disulfide bond between Cys149 and Cys153 reinforces the aggregation process. Physiological reductases, thioredoxins and glutaredoxins, could not dissolve AtGAPC1 aggregates but could efficiently contrast their growth. Besides acting as a protective mechanism against overoxidation, S-glutathionylation of AtGAPC1 triggers an unexpected aggregation pathway with completely different and still unexplored physiological implications.

Atomically Precise Metal Nanoclusters: Novel Building Blocks for Hierarchical Structures.

Atomically precise ligand-protected nanoclusters (NCs) constitute an important class of compounds that exhibit well-defined structures and, when sufficiently small, evident molecular properties. NCs provide versatile building blocks to fabricate hierarchical superstructures. The assembly of NCs indeed offers opportunities to devise new materials with given structures and able to carry out specific functions. In this Concept article, we highlight the possibilities offered by NCs in which the physicochemical properties are controlled by the introduction of foreign metal atoms and/or modification of the composition of the capping monolayer with functional ligands. Different approaches to assemble NCs into dimers and higher hierarchy structures and the corresponding changes in physicochemical properties are also described.

Predictive optical photoabsorption of Ag24Au(DMBT)18 - via efficient TDDFT simulations.

We report a computational study via time-dependent density-functional theory (TDDFT) methods of the photo-absorption spectrum of an atomically precise monolayer-protected cluster (MPC), the Ag24Au(DMBT)18 single negative anion, where DMBT is the 2,4-dimethylbenzenethiolate ligand. The use of efficient simulation algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, allows us to employ a variety of exchange-correlation (xc-) functionals at an affordable computational cost. We are thus able to show, first, how the optical response of this prototypical compound, especially but not exclusively in the absorption threshold (low-energy) region, is sensitive to (1) the choice of the xc-functionals employed in the Kohn-Sham equations and the TDDFT kernel and (2) the choice of the MPC geometry. By comparing simulated spectra with precise experimental photoabsorption data obtained from room temperature down to low temperatures, we then demonstrate how a hybrid xc-functional in both the Kohn-Sham equations and the diagonal TDDFT kernel at the crystallographically determined experimental geometry is able to provide a consistent agreement between simulated and measured spectra across the entire optical region. Single-particle decomposition analysis tools finally allow us to understand the physical reason for the failure of non-hybrid approaches.

X-Ray-Induced Growth Dynamics of Luminescent Silver Clusters in Zeolites.

Herein, AlKα X-rays are used to drive the growth of luminescent silver clusters in zeolites. The growth of the silver species is tracked using Auger spectroscopy and fluorescence microscopy, by monitoring the evolution from their ions to luminescent clusters and then metallic, dark nanoparticles. It is shown that the growth rate in different zeolites is determined by the mobility of the silver ions in the framework and that the growth dynamics in calcined samples obeys the Hill-Langmuir equation for noncooperative binding. Comparison of the optical properties of X-ray-grown silver clusters with silver clusters formed by standard heat treatment indicates that the latter have a higher specificity toward the formation of luminescent clusters of a specific (small) nuclearity, whereas the former produce a wide distribution of cluster species as well as larger nanoparticles.

Dual-Mode, Anisotropy-Encoded, Ratiometric Fluorescent Nanosensors: Towards Multiplexed Detection.

A nanosensor with dual-mode fluorescence response to pH and an encoded identification signal, was developed by exploiting excitation energy transfer and tailored control of molecular organization in core-shell nanoparticles. Multiple signals were acquired in a simple single-excitation dual-emission channels set-up.

Tuning the energetics and tailoring the optical properties of silver clusters confined in zeolites.

The integration of metal atoms and clusters in well-defined dielectric cavities is a powerful strategy to impart new properties to them that depend on the size and geometry of the confined space as well as on metal-host electrostatic interactions. Here, we unravel the dependence of the electronic properties of metal clusters on space confinement by studying the ionization potential of silver clusters embedded in four different zeolite environments over a range of silver concentrations. Extensive characterization reveals a strong influence of silver loading and host environment on the cluster ionization potential, which is also correlated to the cluster's optical and structural properties. Through fine-tuning of the zeolite host environment, we demonstrate photoluminescence quantum yields approaching unity. This work extends our understanding of structure-property relationships of small metal clusters and applies this understanding to develop highly photoluminescent materials with potential applications in optoelectronics and bioimaging.

Variable Doping Induces Mechanism Swapping in Electrogenerated Chemiluminescence of Ru(bpy)32+ Core-Shell Silica Nanoparticles.

The impact of nanotechnology on analytical science is hardly overlooked. In the search for ever-increasing sensitivity in biomedical sensors, nanoparticles have been playing a unique role as, for instance, ultrabright labels, and unravelling the intimate mechanisms which govern their functioning is mandatory for the design of ultrasentitive devices. Herein, we investigated the mechanism of electrogenerated chemiluminescence (ECL) in a family of core-shell silica-PEG nanoparticles (DDSNs), variously doped with a Ru(bpy)32+ triethoxysilane derivative, and displaying homogeneous morphological, hydrodynamic, and photophysical properties. ECL experiments, performed in the presence of 2-(dibutylamino)ethanol (DBAE) as coreactant, showed two parallel mechanisms of ECL generation: one mechanism (I) which involves exclusively the radicals deriving from the coreactant oxidation and a second one (II) involving also the direct anodic oxidation of the Ru(II) moieties. The latter mechanism includes electron (hole) hopping between neighboring redox centers as evidenced in our previous studies and supported by a theoretical model we have recently proposed. Quite unexpectedly, however, we found that the efficiency of the two mechanisms varies in opposite directions within the DDSNs series, with mechanism I or mechanism II prevailing at low and high doping levels, respectively. Since mechanism II has an intrinsically lower efficiency, the ECL emission intensity was also found to grow linearly with doping only at relatively low doping levels while it deviates negatively at higher ones. As the ζ-potential of DDSNs increases with the doping level from negative to slightly positive values, as a likely consequence of the accumulating cationic charge within the silica core, we attributed the observed change in the ECL generation mechanism along the DDSN series to a modulation of the electrostatic and hydrophobic/hydrophilic interactions between the DDSNs and the radical cationic species involved in the ECL generation. The results we report therefore show that the ECL intensity of a nanosized system cannot be merely incremented acting on doping, since other parameters come into play. We think that these results could serve as valuable indications to design more efficient ECL nano- and microsized labels for ultrasensitive bioanalysis.

Photoswitchable NIR-Emitting Gold Nanoparticles.

Photo-switching of the NIR emission of gold nanoparticles (GNP) upon photo-isomerization of azobenzene ligands, bound to the surface, is demonstrated. Photophysical results confirm the occurrence of an excitation energy transfer process from the ligands to the GNP that produces sensitized NIR emission. Because of this process, the excitation efficiency of the gold core, upon excitation of the ligands, is much higher for the trans form than for the cis one, and t→c photo-isomerization causes a relevant decrease of the GNP NIR emission. As a consequence, photo-isomerization can be monitored by ratiometric detection of the NIR emission upon dual excitation. The photo-isomerization process was followed in real-time through the simultaneous detection of absorbance and luminescence changes using a dedicated setup. Surprisingly, the photo-isomerization rate of the ligands, bound to the GNP surface, was the same as measured for the chromophores in solution. This outcome demonstrated that excitation energy transfer to gold assists photo-isomerization, rather than competing with it. These results pave the road to the development of new, NIR-emitting, stimuli-responsive nanomaterials for theranostics.

Nanopatterning of Surfaces with Monometallic and Heterobimetallic 1D Coordination Polymers: A Molecular Tectonics Approach at the Solid/Liquid Interface.

The self-assembly of multiple molecular components into complex supramolecular architectures is ubiquitous in nature and constitutes one of the most powerful strategies to fabricate multifunctional nanomaterials making use of the bottom-up approach. When spatial confinement in two dimensions on a solid substrate is employed, this approach can be exploited to generate periodically ordered structures from suitably designed molecular tectons. In this study we demonstrate that physisorbed directional periodic arrays of monometallic or heterobimetallic coordination polymers can be generated on a highly oriented pyrolitic graphite surface by combinations of a suitably designed directional organic tecton or metallatecton based on a porphyrin or nickel(II) metalloporphyrin backbone bearing both a pyridyl unit and a terpyridyl unit acting as coordinating sites for CoCl2. The periodic architectures were visualized at the solid/liquid interface with a submolecular resolution by scanning tunneling microscopy and corroborated by combined density functional and time-dependent density functional theory calculations. The capacity to nanopattern the surface for the first time with two distinct metallic centers exhibiting different electronic and optical properties is a key step toward the bottom-up construction of robust multicomponent and, thus, multifunctional molecular nanostructures and nanodevices.

Surface-induced selection during in†situ photoswitching at the solid/liquid interface.

Here we report for the first time a submolecularly resolved scanning tunneling microscopy (STM) study at the solid/liquid interface of the in situ reversible interconversion between two isomers of a diarylethene photoswitch, that is, open and closed form, self-assembled on a graphite surface. Prolonged irradiation with UV light led to the in situ irreversible formation of another isomer as by-product of the reaction, which due to its preferential physisorption accumulates at the surface. By making use of a simple yet powerful thermodynamic model we provide a quantitative description for the observed surface-induced selection of one isomeric form.

Drug-loaded fluorescent cubosomes: versatile nanoparticles for potential theranostic applications.

In this work, monoolein-based cubosomes were doped with two fluorescent probes, namely, fluorescein and dansyl, properly modified with a hydrocarbon chain to increase their encapsulation efficiency within the monoolein palisade. The same nanocarriers were also loaded with quercetin, a hydrophobic molecule with potential anticancer activity. Particularly, the cubosomes doped with the modified fluorescein probe were successfully exploited for single living cell imaging. The physicochemical and photophysical characterizations reported here, along with the well-known ability of cubosomes in hosting molecules with pharmaceutical interest, strongly encourage the use of these innovative fluorescent nanocarriers for theranostic purposes.

Understanding the photophysical properties of coumarin-based Pluronic-silica (PluS) nanoparticles by means of time-resolved emission spectroscopy and accurate TDDFT/stochastic calculations.

The photophysical properties of two 7-aminocoumarin molecules with flexible and rigid alkyl moieties at the 7-nitrogen atom have been investigated in ethanol and in Pluronic-silica nanoparticles (PluS NPs) by means of time-resolved emission spectroscopy (TRES) and time-dependent density functional theory (TDDFT). Although the two coumarin derivatives have very different photophysical properties in solution, they show quite similar photophysical behaviour when embedded into the NPs, where an increase in the fluorescence quantum yield of about 10 times was observed for the more flexible molecule. TDDFT calculations employing long-range corrected functionals and with proper account of environmental effects reveal that the formation of an accessible twisted-intramolecular charge transfer state (TICT) is possible for 7-aminocoumarin molecules with flexible alkyl groups in fluid solution, where a conical intersection between the S1 and S0 states is observed at a dihedral angle of about 80°. The excited state dynamics of the population density of this reaction coordinate in ethanol and in silica NPs investigated through the resolution of a generalized Smoulochowsky equation shows that this deactivation mechanism is drastically hampered in a silica matrix, in good agreement with experimental evidence. Steady state and time resolved measurements also suggest that at high concentration for both the dyes intermolecular interactions into the silica matrix lead to fluorescence quenching. TDDFT/PCM calculations clearly indicate that the strong quenching and red shift observed is imputable to the formation of excimers with CT character after absorption of the monomeric species.

Prevention of self-quenching in fluorescent silica nanoparticles by efficient energy transfer.

Stars that shine bright: A high local dye concentration in doped silica-based core­shell nanoparticles causes self-quenching and spectral broadening (top images). This phenomenon jeopardizes the potential advantages of heavily doped systems. Förster resonance energy transfer (FRET) to an acceptor co-included in the silica led to ultrabright nanoparticles (bottom images) with a preselected narrow-band emission and a pseudo-Stokes shift of 129 nm.

Copper and silver nanowires for CO2 electroreduction.

Copper and silver nanowires have been extensively investigated as the next generation of transparent conductive electrodes (TCEs) due to their ability to form percolating networks. Recently, they have been exploited as electrocatalysts for CO2 reduction. In this review, we present the most recent advances in this field summarizing different strategies used for the synthesis and functionalization/activation of copper and silver nanowires, as well as, the state of the art of their electrochemical performance with particular emphasis on the effect of the nanowire morphology. Novel perspectives for the development of highly efficient, selective, and stable electrocatalysts for CO2 reduction arise from the translation of NW-based TCEs in this challenging field.

High performance multi-purpose nanostructured thin films by inkjet printing: Au micro-electrodes and SERS substrates.

Nanostructured thin metal films are exploited in a wide range of applications, spanning from electrical to optical transducers and sensors. Inkjet printing has become a compliant technique for sustainable, solution-processed, and cost-effective thin films fabrication. Inspired by the principles of green chemistry, here we show two novel formulations of Au nanoparticle-based inks for manufacturing nanostructured and conductive thin films by using inkjet printing. This approach showed the feasibility to minimize the use of two limiting factors, namely stabilizers and sintering. The extensive morphological and structural characterization provides pieces of evidence about how the nanotextures lead to high electrical and optical performances. Our conductive films (sheet resistance equal to 10.8 ± 4.1 Ω per square) are a few hundred nanometres thick and feature remarkable optical properties in terms of SERS activity with enhancement factors as high as 107 averaged on the mm2 scale. Our proof-of-concept succeeded in simultaneously combining electrochemistry and SERS by means of real-time tracking of the specific signal of mercaptobenzoic acid cast on our nanostructured electrode.

Structural changes in a protein fragment from abalone shell during the precipitation of calcium carbonate.

Mineralized tissues grow through biologically controlled processes in which specific macromolecules are involved. Some of these molecules, which are present in very low concentrations and are difficult to localize and characterize, become entrapped inside the mineralized tissue. Herein, a protein fragment, GP, which was obtained by the alkaline digestion of the green sheet of the abalone shell, is used as a probe to study the changes in molecular structure that occur during the precipitation of calcium carbonate. This important goal was achieved by exploiting a fluorescent tag in GP. The experimental results that were obtained by using spectroscopic-, chromatographic-, and microscopic techniques indicate that GP controls the precipitation kinetics and morphology of calcium carbonate crystals, and that it only undergoes structural reorganization when entrapped inside calcium carbonate crystals. To the best of our knowledge, this report represents one of the first studies on the conformational changes of a protein fragment that is involved in biomineralization processes on moving from the solution phase into the mineral phase.

Luminescent chemosensors based on silica nanoparticles.

The field of nanoparticles is amazingly many-sided and consequently their applications range between many different areas from industry to bio-analysis and catalysis. In particular, luminescent nanoparticles attract close attention in the areas of biology, medical diagnosis and therapy, where they already find many applications. In this so fascinating and wide framework we have focussed our attention on luminescent silica nanoparticles able to act as sensing materials. We highlight here the importance, especially with the aim of sensing, of gaining precise knowledge and control of their structures; the performance of a chemosensor is, in fact, totally dependent on its design. We then briefly present the state of the art and the progress both in the synthetic protocols and in the application of luminescent silica nanoparticles as chemosensors. We present many recent examples, organized into two main sections, the first dealing with systems presenting the signalling units on the surface (dye coated silica nanoparticles, DCSNs) and the second with systems entrapping the dyes inside the silica matrix (dye doped silica nanoparticles, DDSNs).

Green and blue electrochemically generated chemiluminescence from click chemistry--customizable iridium complexes.

Cationic cyclometalated iridium complexes containing two anionic phenylpyridine (ppy) ligands and the neutral bidentate triazole-pyridine ligand, 2-(1-substituted-1H-1,2,3-triazol-4-yl)pyridine (pytl), were investigated. The complexes display a rich and reversible electrochemical behavior, upon investigations by cyclic voltammetry in strictly aprotic conditions, that couples with excellent emission quantum yields and long lifetimes of the excited states. Therefore, in organic media, all complexes have generated intense green electrochemiluminescence (ECL) through the so-called annihilation procedure and, importantly, a modulation of the emission energy (to blue) has been easily obtained by simple fluorination of the ppy ligand. Finally, taking advantage of their remarkable solubility in water, intense ECL was also obtained from aqueous buffer solutions using the co-reactant method, thus making all the investigated complexes highly promising for their effective use as ECL labels in bioanalytical applications.

Luminescent silica nanoparticles: extending the frontiers of brightness.

Silica nanoparticles are versatile platforms with many intrinsic features, such as low toxicity. Proper design and derivatization yields particularly stable colloids, even in physiological conditions, and provides them with multiple functions. A suitable choice of dyes and synthetic strategy may, in particular, yield a very bright nanosystem. Silica nanoparticles thus offer unique potential in the nanotechnology arena, and further improvement and optimization could substantially increase their application in fields of high social and economic impact, such as medical diagnostics and therapy, environmental and food analysis, and security. This paper describes silica-based, multicomponent nanosystems with intrinsic directional energy- and electron-transfer processes, on which highly valued functions like light harvesting and signal amplification are based.