Nanocluster
Nanoclusters are atomically precise, crystalline materials most often existing on the 0–2 nanometer scale. They are often considered kinetically stable intermediates that form during the synthesis of comparatively larger materials such as semiconductor and metallic nanocrystals. The majority of research conducted to study nanoclusters has focused on characterizing their crystal structures and understanding their role in the nucleation and growth mechanisms of larger materials.
Materials can be categorized into three different regimes, namely bulk, nanoparticles and nanoclusters. Bulk metals are electrical conductors and good optical reflectors and metal nanoparticles display intense colors due to surface plasmon resonance. However, when the size of metal nanoclusters is further reduced to form a nanocluster, the band structure becomes discontinuous and breaks down into discrete energy levels, somewhat similar to the energy levels of molecules. This gives nanoclusters similar qualities as a singular molecule and does not exhibit plasmonic behavior; nanoclusters are known as the bridging link between atoms and nanoparticles. Nanoclusters may also be referred to as molecular nanoparticles.
History of nanoclusters
The formation of stable nanoclusters such as Buckminsterfullerene has been suggested to have occurred during the early universe.In retrospect, the first nanoclustered ions discovered were the Zintl phases, intermetallics studied in the 1930s.
The first set of experiments to consciously form nanoclusters can be traced back to 1950s and 1960s. During this period, nanoclusters were produced from intense molecular beams at low temperature by supersonic expansion. The development of laser vaporization technique made it possible to create nanoclusters of a clear majority of the elements in the periodic table. Since 1980s, there has been tremendous work on nanoclusters of semiconductor elements, compound clusters and transition metal nanoclusters.
Subnanometric metal clusters typically contain fewer than 10 atoms and measure less than one nanometer in size.
Size and number of atoms in metal nanoclusters
According to the Japanese mathematical physicist Ryogo Kubo, the spacing of energy levels can be predicted bywhere EF is Fermi energy and N is the number of atoms. For quantum confinement ? can be estimated to be equal to the thermal energy, where k is the Boltzmann constant and T is temperature.
Stability
Not all the clusters are stable. The stability of nanoclusters depends on the number of atoms in the nanocluster, valence electron counts and encapsulating scaffolds. In the 1990s, Heer and his coworkers used supersonic expansion of an atomic cluster source into a vacuum in the presence of an inert gas and produced atomic cluster beams. Heer's team and Brack et al. discovered that certain masses of formed metal nanoclusters were stable and were like magic clusters. The number of atoms or size of the core of these magic clusters corresponds to the closing of atomic shells. Certain thiolated clusters such as Au2518,, and also showed magic number stability. Häkkinen et al explained this stability with a theory that a nanocluster is stable if the number of valence electrons corresponds to the shell closure of atomic orbitals as.Synthesis and stabilization
Solid state medium
s can be used to create nanocluster beams of virtually any element. They can be synthesized in high vacuum by with molecular beam techniques combined with a mass spectrometer for mass selection, separation and analysis. And finally detected with detectors.Mass Analyzer
Wein filter In Wien filter mass separation is done with crossed homogeneous electric and magnetic fields perpendicular to ionized cluster beam. The net force on a charged cluster with mass M, charge Q, and velocity v vanishes if E = Bv/''c. The cluster ions are accelerated by a voltage V'' to an energy QV. Passing through the filter, clusters with M/''Q = 2V''/ are not deflected. These cluster ions that are not deflected are selected with appropriately positioned collimators.Quadrupole mass filter The quadrupole mass filter operates on the principle that ion trajectories in a two-dimensional quadrupole field are stable if the field has an AC component superimposed on a DC component with appropriate amplitudes and frequencies. It is responsible for filtering sample ions based on their mass-to-charge ratio.
Time of flight mass spectroscopy Time-of-flight spectroscopy consists of an ion gun, a field-free drift space and an ion cluster source. The neutral clusters are ionized, typically using pulsed laser or an electron beam. The ion gun accelerates the ions that pass through the field-free drift space and ultimately impinge on an ion detector. Usually an oscilloscope records the arrival time of the ions. The mass is calculated from the measured time of flight.
Molecular beam chromatography In this method, cluster ions produced in a laser vaporized cluster source are mass selected and introduced in a long inert-gas-filled drift tube with an entrance and exit aperture. Since cluster mobility depends upon the collision rate with the inert gas, they are sensitive to the cluster shape and size.
Aqueous medium
In general, metal nanoclusters in an aqueous medium are synthesized in two steps: reduction of metal ions to zero-valent state and stabilization of nanoclusters. Without stabilization, metal nanoclusters would strongly interact with each other and aggregate irreversibly to form larger particles.Reduction
There are several methods reported to reduce silver ion into zero-valent silver atoms:- Chemical Reduction Chemical reductants can reduce silver ions into silver nanoclusters. Some examples of chemical reductants are sodium borohydride and sodium hypophosphite. For instance, Dickson and his research team have synthesized silver nanoclusters in DNA using sodium borohydride.
- Electrochemical Reduction Silver nanoclusters can also be reduced electrochemically using reductants in the presence of stabilizing agents such as and tetrabutylammonium.
- Photoreduction Silver nanoclusters can be produced using ultraviolet light, visible or infrared light. The photoreduction process has several advantages such as avoiding the introduction of impurities, fast synthesis, and controlled reduction. For example Diaz and his co-workers have used visible light to reduce silver ions into nanoclusters in the presence of a PMAA polymer. Kunwar et al produced silver nanoclusters using infrared light.
- Other reduction methods Silver nanoclusters are also formed by reducing silver ions with gamma rays, microwaves, or ultrasound. For example silver nanoclusters formed by gamma reduction technique in aqueous solutions that contain sodium polyacrylate or partly carboxylated polyacrylamide or glutaric acids. By irradiating microwaves Linja Li prepared fluorescent silver nanoclusters in PMAA, which typically possess a red color emission. Similarly Suslick et al. have synthesized silver nanoclusters using high ultrasound in the presence of PMAA polymer.
Stabilization
Thiols Thiol-containing small molecules are the most commonly adopted stabilizers in metal nanoparticle synthesis owing to the strong interaction between thiols and gold and silver. Glutathione has been shown to be an excellent stabilizer for synthesizing gold nanoclusters with visible luminescence by reducing Au3+ in the presence of glutathione with sodium borohydride. Also other thiols such as tiopronin, 2-phenylethanethiol, thiolated α-cyclodextrin and 3-mercaptopropionic acid and bidentate dihydrolipoic acid are other thiolated compounds currently being used in the synthesis of metal nanoclusters. The size as well as the luminescence efficiency of the nanocluster depends sensitively on the thiol-to-metal molar ratio. The higher the ratio, the smaller the nanoclusters. The thiol-stabilized nanoclusters can be produced using strong as well as mild reductants. Thioled metal nanoclusters are mostly produced using the strong reductant sodium borohydride. Gold nanocluster synthesis can also be achieved using a mild reducant tetrakisphosphonium. Here a zwitterionic thiolate ligand, D-penicillamine, is used as the stabilizer. Furthermore, nanoclusters can be produced by etching larger nanoparticles with thiols. Thiols can be used to etch larger nanoparticles stabilized by other capping agents.
Dendrimers Dendrimers are used as templates to synthesize nanoclusters. Gold nanoclusters embedded in poly dendrimer have been successfully synthesized. PAMAM is repeatedly branched molecules with different generations. The fluorescence properties of the nanoclusters are sensitively dependent on the types of dendrimers used as template for the synthesis. Metal nanoclusters embedded in different templates show maximum emission at different wavelengths. The change in fluorescence property is mainly due to surface modification by the capping agents. Although gold nanoclusters embedded in PAMAM are blue-emitting the spectrum can be tuned from the ultraviolet to the near-infrared region and the relative PAMAM/gold concentration and the dendrimer generation can be varied. The green-emitting gold nanoclusters can be synthesized by adding mercaptoundecanoic acid into the prepared small gold nanoparticle solution. The addition of freshly reduced lipoic acid gold nanoclusters become red-emitting fluorophores.
Polymers Polymers with abundant carboxylic acid groups were identified as promising templates for synthesizing highly fluorescent, water-soluble silver nanoclusters. Fluorescent silver nanoclusters have been successfully synthesized on poly, microgels of poly polyglycerol-block-poly copolymers polyelectrolyte, poly etc. Gold nanoclusters have been synthesized with polyethylenimine and poly templates. The linear polyacrylates, poly, act as an excellent scaffold for the preparation of silver nanoclusters in water solution by photoreduction. Poly-stabilized nanoclusters have an excellent high quantum yield and can be transferred to other scaffolds or solvents and can sense the local environment.
DNA, proteins and peptides DNA oligonucleotides are good templates for synthesizing metal nanoclusters. Silver ions possess a high affinity to cytosine bases in single-stranded DNA which makes DNA a promising candidate for synthesizing small silver nanoclusters. The number of cytosines in the loop could tune the stability and fluorescence of Ag NCs. Biological macromolecules such as peptides and proteins have also been utilized as templates for synthesizing highly fluorescent metal nanoclusters. Compared with short peptides, large and complicated proteins possess abundant binding sites that can potentially bind and further reduce metal ions, thus offering better scaffolds for template-driven formation of small metal nanoclusters. Also the catalytic function of enzymes can be combined with the fluorescence property of metal nanoclusters in a single cluster to make it possible to construct multi-functional nanoprobes.
Inorganic scaffolds Inorganic materials like glass and zeolite are also used to synthesize the metal nanoclusters. Stabilization is mainly by immobilization of the clusters and thus preventing their tendency to aggregate to form larger nanoparticles. First metal ions doped glasses are prepared and later the metal ion doped glass is activated to form fluorescent nanoclusters by laser irradiation. In zeolites, the pores which are in the ångström size range can be loaded with metal ions and later activated either by heat treatment, UV light excitation, or two-photon excitation. During the activation, the silver ions combine to form the nanoclusters that can grow only to oligomeric size due to the limited cage dimensions.