Reversible Photoisomerization of Spiropyran on the Surfaces of Au25 Nanoclusters.

Au25 nanoclusters functionalized with a spiropyran molecular switch are synthesized via a ligand-exchange reaction at low temperature. The resulting nanoclusters are characterized by optical and NMR spectroscopies as well as by mass spectrometry. Spiropyran bound to nanoclusters isomerizes in a reversible fashion when exposed to UV and visible light, and its properties are similar to those of free spiropyran molecules in solution. The reversible photoisomerization entails the modulation of fluorescence as well as the light-controlled self-assembly of nanoclusters.

Reversible trapping and reaction acceleration within dynamically self-assembling nanoflasks.

The chemical behaviour of molecules can be significantly modified by confinement to volumes comparable to the dimensions of the molecules. Although such confined spaces can be found in various nanostructured materials, such as zeolites, nanoporous organic frameworks and colloidal nanocrystal assemblies, the slow diffusion of molecules in and out of these materials has greatly hampered studying the effect of confinement on their physicochemical properties. Here, we show that this diffusion limitation can be overcome by reversibly creating and destroying confined environments by means of ultraviolet and visible light irradiation. We use colloidal nanocrystals functionalized with light-responsive ligands that readily self-assemble and trap various molecules from the surrounding bulk solution. Once trapped, these molecules can undergo chemical reactions with increased rates and with stereoselectivities significantly different from those in bulk solution. Illumination with visible light disassembles these nanoflasks, releasing the product in solution and thereby establishes a catalytic cycle. These dynamic nanoflasks can be useful for studying chemical reactivities in confined environments and for synthesizing molecules that are otherwise hard to achieve in bulk solution.

Magnetic field-induced self-assembly of iron oxide nanocubes.

Self-assembly of inorganic nanoparticles has been studied extensively for particles having different sizes and compositions. However, relatively little attention has been devoted to how the shape and surface chemistry of magnetic nanoparticles affects their self-assembly properties. Here, we undertook a combined experiment-theory study aimed at better understanding of the self-assembly of cubic magnetite (Fe3O4) particles. We demonstrated that, depending on the experimental parameters, such as the direction of the magnetic field and nanoparticle density, a variety of superstructures can be obtained, including one-dimensional filaments and helices, as well as C-shaped assemblies described here for the first time. Furthermore, we functionalized the surfaces of the magnetic nanocubes with light-sensitive ligands. Using these modified nanoparticles, we were able to achieve orthogonal control of self-assembly using a magnetic field and light.

Light-controlled self-assembly of non-photoresponsive nanoparticles.

The ability to guide the assembly of nanosized objects reversibly with external stimuli, in particular light, is of fundamental importance, and it contributes to the development of applications as diverse as nanofabrication and controlled drug delivery. However, all the systems described to date are based on nanoparticles (NPs) that are inherently photoresponsive, which makes their preparation cumbersome and can markedly hamper their performance. Here we describe a conceptually new methodology to assemble NPs reversibly using light that does not require the particles to be functionalized with light-responsive ligands. Our strategy is based on the use of a photoswitchable medium that responds to light in such a way that it modulates the interparticle interactions. NP assembly proceeds quantitatively and without apparent fatigue, both in solution and in gels. Exposing the gels to light in a spatially controlled manner allowed us to draw images that spontaneously disappeared after a specific period of time.

New Protocols for the Synthesis of Stable Ag and Au Nanocluster Molecules.

"Catching" metals in the nonmetallic form in solution, as they grow to bulk, is one of the most exciting areas of contemporary materials research. A new kind of stabilization to catch the nonmetallic form of noble metals with small thiols has evolved as an exciting area of synthesis during the past decade. Gold clusters stay in the frontline of this research, yielding new "molecules" composed of a few to several hundreds of atoms. By taking guidelines from gold cluster research, various new protocols for silver nanoclusters were developed. In this Perspective, we highlight the recent advances on the synthesis of atomically precise silver, gold, and their alloy clusters with a special emphasis on silver. As a result of intense efforts of the recent past, clusters such as Ag7,8(SR)7,8, Ag7(-S-R-S-)4, Ag9(SR)7, Ag32(SR)19, Ag44(SR)30, Ag140(SR)53, Ag280(SR)140, and Ag152(SR)60 (SR and S-R-S refer to thiolate and dithiolate ligands, respectively) were added to the literature. Moreover, "silver-covered" and "gold-covered" alloy clusters have also been synthesized. Early reports of the crystallization of such clusters are available. Several of these clusters are shown to act as sensors, catalysts, and pesticide degradation agents, which suggests that these materials may find applications in daily life in the foreseeable future.

Thiolate-protected Ag32 clusters: mass spectral studies of composition and insights into the Ag-thiolate structure from NMR.

Clusters composed of a 32 silver atom core, protected with thiolates of glutathione (GSH) and N-(2-mercaptopropionyl)glycine (MPGH), were synthesized by a solid-state route in milligram scale. They do not exhibit surface plasmon resonance unlike their larger sized nanoparticle analogues but show molecule-like features in absorption and luminescence spectra, falling in the visible window. The compositions Ag32SG19 (SG: thiolate of glutathione) and Ag32MPG19 (MPG: thiolate of MPGH) were identified from electrospray ionization mass spectrometry (ESI MS). Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) was not successful for -SG protected clusters as reported before, but for Ag32MPG19 a peak at 6.1 kDa was seen at a threshold laser intensity. This peak shifted to low mass region with increasing laser intensity due to systematic losses of Ag2S. Further confirmation of the composition Ag32SG19 was made using various studies such as XPS and EDAX. One-dimensional (1D) and two-dimensional (2D) NMR spectroscopic investigations of Ag32SG19 provided interesting spectral features which indicated the dominant -[SR-Ag-SR]- structural motif. This structural motif as the predominant entity is found for the first time in silver clusters.

Sunlight mediated synthesis and antibacterial properties of monolayer protected silver clusters.

Glutathione protected, silver clusters were synthesized within gel cavities, using sunlight. Compared to the conventional chemical reduction process, this method is cheaper and environmentally friendly as it involves the use of natural resources. The as-synthesized silver quantum clusters in aqueous medium show a distinct step-like behavior in their absorption profile. They have been characterized with various spectroscopic and microscopic techniques such as UV/Vis Spectroscopy, Luminescence Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), High Resolution Transmission Electron Microscopy (HRTEM), and X-ray Photoelectron Spectroscopy (XPS). Polyacrylamide gel cavities seemingly control the growth of the particles. The cluster synthesis is scalable by increasing the amount of reagents yielding hundreds of milligrams in a single step. The antibacterial properties of the as-synthesized Ag clusters were studied against a Gram negative and Gram positive organism, Escherichia coli and Staphylococcus aureus, respectively.

Luminescent sub-nanometer clusters for metal ion sensing: a new direction in nanosensors.

We describe the application of a recently discovered family of materials called quantum clusters, which are sub-nanometer particles composed of a few atoms with well-defined molecular formulae, exhibiting intense absorption and emission in the visible region in metal ion sensing, taking Ag(25) as an example. The changes in the optical properties of the cluster, in both absorption and emission upon exposure to various metal ions in aqueous medium are explored. The cluster can detect Hg(2+) down to ppb levels. It can also detect 5d block ions (Pt(2+), Au(3+) and Hg(2+)) down to ppm limits. Hg(2+) interacts with the metal core as well as the functional groups of the capping agents and the interaction is concentration-dependent. To understand the mechanism behind this type of specific interaction, we have used spectroscopic and microscopic techniques such as UV-vis spectroscopy, luminescence spectroscopy, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). Specific reasons responsible for the interaction of Hg(2+) have been proposed.

Conversion of double layer charge-stabilized Ag@citrate colloids to thiol passivated luminescent quantum clusters.

A red luminescent silver cluster was synthesized in milligram quantities by the direct core reduction of the most widely studied class of large silver nanoparticles, namely silver@citrate of tens of nanometres diameter. No byproducts such as thiolates were detected, unlike in the case of typical methods of making such clusters. The route provides nearly pure clusters. The possibility to make diverse clusters from large nanoparticles expands the scope of cluster research.