Nanocluster

<h2 id="history-of-nanoclusters">History of nanoclusters</h2>
The formation of stable nanoclusters such as <a href="/facts/Buckminsterfullerene/F0VebtPK">Buckminsterfullerene</a> (C60) has been suggested to have occurred during the early universe.<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a><a class="footnote-ref" id="fnref:21" href="#fn:21">21</a> 
In retrospect, the first nanoclustered ions discovered were the <a href="/facts/Zintl_phases/RdV0yDXf">Zintl phases</a>, intermetallics studied in the 1930s. 
The first set of experiments to consciously form nanoclusters can be traced back to 1950s and 1960s.<a class="footnote-ref" id="fnref:22" href="#fn:22">22</a> During this period, nanoclusters were produced from intense molecular beams at low temperature by supersonic expansion. The development of <a href="/facts/Laser_vaporization/VmXJnyLU">laser vaporization</a> 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 <a href="/facts/Semiconductor/zFf3mGTE">semiconductor</a> elements, compound clusters and <a href="/facts/Transition_metal/0slGWoLQ">transition metal</a> nanoclusters.<a class="footnote-ref" id="fnref:23" href="#fn:23">23</a>
Subnanometric metal clusters typically contain fewer than 10 atoms and measure less than one nanometer in size.<a class="footnote-ref" id="fnref:24" href="#fn:24">24</a><a class="footnote-ref" id="fnref:25" href="#fn:25">25</a><a class="footnote-ref" id="fnref:26" href="#fn:26">26</a><a class="footnote-ref" id="fnref:27" href="#fn:27">27</a><a class="footnote-ref" id="fnref:28" href="#fn:28">28</a>

<h2 id="size-and-number-of-atoms-in-metal-nanoclusters">Size and number of atoms in metal nanoclusters</h2>
According to the Japanese mathematical physicist <a href="/facts/Ryogo_Kubo/hAHo0S85">Ryogo Kubo</a>, the spacing of energy levels can be predicted by


 
 
 
 δ
 =
 
 
 
 E
 
 
 F
 
 
 
 N
 
 
 
 
 {\displaystyle \delta ={\frac {E_{\rm {F}}}{N}}}
 
 
where EF is <a href="/facts/Fermi_energy/S4Q8ShGR">Fermi energy</a> and N is the number of atoms. For <a href="/facts/Potential_well/EMQxyqDT">quantum confinement</a> 𝛿 can be estimated to be equal to the <a href="/facts/Thermal_energy/eVaKAC7K">thermal energy</a> (δ = kT), where k is the <a href="/facts/Boltzmann_constant/5DIejyR3">Boltzmann constant</a> and T is temperature.<a class="footnote-ref" id="fnref:29" href="#fn:29">29</a><a class="footnote-ref" id="fnref:30" href="#fn:30">30</a>

<h2 id="stability">Stability</h2>
Not all the clusters are stable. The stability of nanoclusters depends on the number of <a href="/facts/Atom/FoQ4kGN5">atoms</a> in the nanocluster, <a href="/facts/Valence_(chemistry)/ck7cHByj">valence</a> <a href="/facts/Electron/L0KBo5cW">electron</a> counts and encapsulating scaffolds.<a class="footnote-ref" id="fnref:31" href="#fn:31">31</a> In the 1990s, Heer and his coworkers used <a href="/facts/Supersonic_gas_separation/MAHjbUmy">supersonic expansion</a> of an atomic cluster source into a <a href="/facts/Vacuum/BTk3goyV">vacuum</a> in the presence of an <a href="/facts/Inert_gas/3EkBEoYh">inert gas</a> and produced atomic cluster beams.<a class="footnote-ref" id="fnref:32" href="#fn:32">32</a> Heer's team and Brack et al. discovered that certain masses of formed metal nanoclusters were stable and were like magic clusters.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a> 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 Au25(SR)18, Au38(SR)24, Au102(SR)44 and Au144(SR)60 also showed <a href="/facts/Magic_number_(physics)/v9PwXFzQ">magic number</a> stability.<a class="footnote-ref" id="fnref:34" href="#fn:34">34</a> 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 <a href="/facts/Atomic_orbital/tKhgl6mL">atomic orbitals</a> as (1S2, 1P6, 1D10, 2S2 1F14, 2P6 1G18, 2D10 3S2 1H22.......).<a class="footnote-ref" id="fnref:35" href="#fn:35">35</a><a class="footnote-ref" id="fnref:36" href="#fn:36">36</a>

<h2 id="synthesis-and-stabilization">Synthesis and stabilization</h2>
<h3>Solid state medium</h3>
<a href="/facts/Molecular_beam/4TppLCGP">Molecular beams</a> can be used to create nanocluster beams of virtually any element. They can be synthesized in high <a href="/facts/Vacuum/BTk3goyV">vacuum</a> by with molecular beam techniques combined with a mass spectrometer for mass selection, separation and analysis. And finally detected with detectors.<a class="footnote-ref" id="fnref:37" href="#fn:37">37</a>

<h4>Cluster Sources</h4>
Seeded supersonic nozzle Seeded supersonic nozzles are mostly used to create clusters of low-<a href="/facts/Boiling_point/E2BK9QoS">boiling-point</a> metal. In this source method metal is vaporized in a hot oven. The metal vapor is mixed with (seeded in) inert carrier gas. The vapor mixture is ejected into a vacuum chamber via a small hole, producing a supersonic <a href="/facts/Molecular_beam/4TppLCGP">molecular beam</a>. The expansion into vacuum proceeds <a href="/facts/Adiabatic_process/rJj42U5h">adiabatically</a> cooling the vapor. The cooled metal vapor becomes <a href="/facts/Supersaturation/YaTN0D1L">supersaturated</a>, condensing in cluster form.
Gas aggregation Gas aggregation is mostly used to synthesize large clusters of nanoparticles. Metal is vaporized and introduced in a flow of cold inert gas, which causes the vapor to become highly supersaturated. Due to the low temperature of the inert gas, cluster production proceeds primarily by successive single-atom addition.
Laser vaporization Laser vaporization source can be used to create clusters of various size and polarity. <a href="/facts/Pulsed_laser/PIcW85dx">Pulse laser</a> is used to vaporize the target metal rod and the rod is moved in a spiral so that a fresh area can be evaporated every time. The evaporated metal vapor is cooled by using cold <a href="/facts/Helium/yRul93TG">helium</a> gas, which causes the cluster formation.
Pulsed arc cluster ion This is similar to laser vaporization, but an intense electric discharge is used to evaporate the target metal.
Ion sputtering Ion sputtering source produces an intense continuous beam of small singly ionized cluster of metals. Cluster ion beams are produced by bombarding the surface with high energetic inert gas (<a href="/facts/Krypton/llkNlCke">krypton</a> and <a href="/facts/Xenon/9E3XR69R">xenon</a>) ions. The cluster production process is still not fully understood.
Liquid-metal ion In liquid-metal ion source a needle is wetted with the metal to be investigated. The metal is heated above the melting point and a potential difference is applied. A very high electric field at the tip of the needle causes a spray of small droplets to be emitted from the tip. Initially very hot and often multiply ionized droplets undergo evaporative cooling and fission to smaller clusters.

<h4>Mass Analyzer</h4>
Wein filter In <a href="/facts/Wien_filter/Hs3UxCus">Wien filter</a> mass separation is done with crossed homogeneous electric and magnetic fields perpendicular to ionized cluster beam. The net force on a charged cluster with <a href="/facts/Mass/HHdc8XmF">mass</a> M, charge Q, and <a href="/facts/Velocity/Q3o1LVDe">velocity</a> v vanishes if E = Bv/c . The cluster ions are accelerated by a <a href="/facts/Voltage/EN2po6N2">voltage</a> V to an energy QV. Passing through the filter, clusters with M/Q = 2V/(Ec/B) are not deflected. These cluster ions that are not deflected are selected with appropriately positioned <a href="/facts/Collimator/UAsMlkeh">collimators</a>.
Quadrupole mass filter The <a href="/facts/Quadrupole_mass_analyzer/44BwKrPM">quadrupole mass filter</a> operates on the principle that ion <a href="/facts/Trajectory/STNN33FK">trajectories</a> in a two-dimensional quadrupole field are stable if the field has an AC component superimposed on a DC component with appropriate <a href="/facts/Amplitude/aEckuXJM">amplitudes</a> and <a href="/facts/Frequency/QUSbPaW8">frequencies</a>. It is responsible for filtering sample ions based on their <a href="/facts/Mass-to-charge_ratio/QaKpLt6B">mass-to-charge ratio</a>.
Time of flight mass spectroscopy <a href="/facts/Time-of-flight_mass_spectrometry/6bj9bgGF">Time-of-flight spectroscopy</a> consists of an <a href="/facts/Ion_gun/lQkpBsby">ion gun</a>, a field-free <a href="/facts/Wire_chamber/BKstxtE9">drift space</a> and an ion cluster source. The neutral clusters are ionized, typically using pulsed laser or an <a href="/facts/Electron_beam_processing/dNlumRKv">electron beam</a>. The ion gun accelerates the ions that pass through the field-free drift space (flight tube) and ultimately impinge on an ion detector. Usually an <a href="/facts/Oscilloscope/HMdD8XVk">oscilloscope</a> records the arrival time of the ions. The mass is calculated from the measured <a href="/facts/Time_of_flight/DSINDYyn">time of flight</a>.
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 <a href="/facts/Collision_theory/65DaYrTP">collision rate</a> with the <a href="/facts/Inert_gas/3EkBEoYh">inert gas</a>, they are sensitive to the cluster shape and size.

<h3>Aqueous medium</h3>
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.

<h4>Reduction</h4>
There are several methods reported to reduce silver ion into zero-valent silver atoms:

<ul><li>Chemical Reduction Chemical reductants can reduce silver ions into silver nanoclusters. Some examples of chemical reductants are <a href="/facts/Sodium_borohydride/D2Hwts5x">sodium borohydride</a> (NaBH4) and <a href="/facts/Sodium_hypophosphite/RewpxDzG">sodium hypophosphite</a> (NaPO2H2.H2O). For instance, Dickson and his research team have synthesized silver nanoclusters in DNA using sodium borohydride.<a class="footnote-ref" id="fnref:38" href="#fn:38">38</a><a class="footnote-ref" id="fnref:39" href="#fn:39">39</a></li>
<li>Electrochemical Reduction Silver nanoclusters can also be reduced <a href="/facts/Electrochemistry/C0RUWpBH">electrochemically</a> using reductants in the presence of stabilizing agents such as dodecanethiol [de] and <a href="/facts/Tetrabutylammonium/thxACdb6">tetrabutylammonium</a>.<a class="footnote-ref" id="fnref:40" href="#fn:40">40</a></li>
<li>Photoreduction Silver nanoclusters can be produced using ultraviolet light, visible or infrared light. The <a href="/facts/Photoreduction/kBnR8Lcp">photoreduction</a> 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 <a href="/facts/Infrared/44XdJNcc">infrared</a> light.<a class="footnote-ref" id="fnref:41" href="#fn:41">41</a><a class="footnote-ref" id="fnref:42" href="#fn:42">42</a></li>
<li>Other reduction methods Silver nanoclusters are also formed by reducing silver ions with <a href="/facts/Gamma_rays/5Drf913J">gamma rays</a>, <a href="/facts/Microwave/HmVSN31Y">microwaves</a>, or <a href="/facts/Ultrasound/31Pa5IH9">ultrasound</a>. For example silver nanoclusters formed by gamma reduction technique in aqueous solutions that contain <a href="/facts/Sodium_polyacrylate/GJKm1or9">sodium polyacrylate</a> or partly carboxylated <a href="/facts/Polyacrylamide/vuKfsKhf">polyacrylamide</a> or <a href="/facts/Glutaric_acid/yDCNYN6s">glutaric acids</a>. 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.<a class="footnote-ref" id="fnref:43" href="#fn:43">43</a><a class="footnote-ref" id="fnref:44" href="#fn:44">44</a></li></ul>
<h4>Stabilization</h4>
Cryogenic gas molecules are used as scaffolds for nanocluster synthesis in solid state.<a class="footnote-ref" id="fnref:45" href="#fn:45">45</a> In aqueous medium there are two common methods for stabilizing nanoclusters: <a href="/facts/Electrostatics/bxkuW6s4">electrostatic</a> (charge, or inorganic) stabilization and <a href="/facts/Steric_effects/TnX5c8HG">steric</a> (organic) stabilization. Electrostatic stabilization occurs by the adsorption of <a href="/facts/Ion/3bePpDna">ions</a> to the often-<a href="/facts/Electrophile/1AHFg1zm">electrophilic</a> metal surface, which creates an <a href="/facts/Electrical_double_layer/QhTTkyVs">electrical double layer</a>. Thus, this <a href="/facts/Coulomb_repulsion/V0lR5NKv">Coulomb repulsion</a> force between individual particles will not allow them to flow freely without agglomeration. Whereas on the other hand in steric stabilization,the metal center is surrounded by layers of sterically bulk material. These large <a href="/facts/Adsorbate/ARH9peYM">adsorbates</a> provide a steric barrier which prevents close contact of the metal particle centers.<a class="footnote-ref" id="fnref:46" href="#fn:46">46</a>
Thiols <a href="/facts/Thiol/VemKEDP9">Thiol</a>-containing small molecules are the most commonly adopted stabilizers in metal nanoparticle synthesis owing to the strong interaction between thiols and gold and silver. <a href="/facts/Glutathione/5bSxjusL">Glutathione</a> has been shown to be an excellent stabilizer for synthesizing gold nanoclusters with visible <a href="/facts/Luminescence/1QkIWyUj">luminescence</a> by reducing Au3+ in the presence of glutathione with <a href="/facts/Sodium_borohydride/D2Hwts5x">sodium borohydride</a> (NaBH4). Also other thiols such as <a href="/facts/Tiopronin/8M55EqlQ">tiopronin</a>, 2-phenylethanethiol, <a href="/facts/Cyclodextrin/Ek2deqhM">thiolated α-cyclodextrin</a> and <a href="/facts/3-mercaptopropionic_acid/qD5XcM3j">3-mercaptopropionic acid</a> and bidentate <a href="/facts/Dihydrolipoic_acid/XlAy2v4b">dihydrolipoic acid</a> 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 <a href="/facts/Mole_fraction/nW4EepmJ">molar ratio</a>. 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 (NaBH4). Gold nanocluster synthesis can also be achieved using a mild reducant <a href="/facts/Tetrakis(hydroxymethyl)phosphonium_chloride/FVmM87ST">tetrakis(hydroxymethyl)phosphonium</a> (THPC). Here a <a href="/facts/Zwitterion/P6PzHHcS">zwitterionic</a> thiolate <a href="/facts/Ligand/6etB3qeW">ligand</a>, D-<a href="/facts/Penicillamine/1N0ErBRB">penicillamine</a> (DPA), 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.

Look up mercaptoundecanoic acid or microgel in Wiktionary, the free dictionary.

Dendrimers <a href="/facts/Dendrimer/3aXUKBej">Dendrimers</a> are used as templates to synthesize nanoclusters. Gold nanoclusters embedded in <a href="/facts/Poly(amidoamine)/rbKqhRB8">poly(amidoamine)</a> dendrimer (PAMAM) 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 <a href="/facts/Wavelength/kcJU0ymz">wavelengths</a>. The change in fluorescence property is mainly due to surface modification by the <a href="/facts/Colloidal_gold/AvLN368l">capping agents</a>. Although gold nanoclusters embedded in PAMAM are blue-emitting the <a href="/facts/Spectrum/wPUjkpqI">spectrum</a> can be tuned from the <a href="/facts/Ultraviolet/kncBUqn5">ultraviolet</a> to the <a href="/facts/Near-infrared_spectroscopy/gWCvHgYb">near-infrared</a> (NIR) 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 (MUA) into the prepared small gold nanoparticle solution. The addition of freshly reduced <a href="/facts/Lipoic_acid/9wTHHWBf">lipoic acid</a> (DHLA) gold nanoclusters (AuNC@DHLA) become red-emitting <a href="/facts/Fluorophores/wDWBmYhn">fluorophores</a>.<a class="footnote-ref" id="fnref:47" href="#fn:47">47</a><a class="footnote-ref" id="fnref:48" href="#fn:48">48</a>
Polymers <a href="/facts/Polymer/eF32E50K">Polymers</a> with abundant <a href="/facts/Carboxylic_acid/U4xuMosy">carboxylic acid</a> groups were identified as promising templates for synthesizing highly fluorescent, water-soluble silver nanoclusters. Fluorescent silver nanoclusters have been successfully synthesized on <a href="/facts/Poly(methacrylic_acid)/J0USQsg5">poly(methacrylic acid)</a>, microgels of poly(N-isopropylacrylamide-acrylic acid-2-hydroxyethyl acrylate) polyglycerol-block-poly(<a href="/facts/Acrylic_acid/x2l3lrI2">acrylic acid</a>) <a href="/facts/Copolymer/35D97DuV">copolymers</a> <a href="/facts/Polyelectrolyte/3A3jLXkh">polyelectrolyte</a>, poly(methacrylic acid) (PMAA) etc.<a class="footnote-ref" id="fnref:49" href="#fn:49">49</a> Gold nanoclusters have been synthesized with <a href="/facts/Polyethylenimine/BXhSol9h">polyethylenimine</a> (PEI) and <a href="/facts/Polyvinylpyrrolidone/lacTojoU">poly(N-vinylpyrrolidone)</a> (PVP) templates. The linear <a href="/facts/Polyacrylate/3z6dXElL">polyacrylates</a>, poly(methacrylic acid), act as an excellent scaffold for the preparation of silver nanoclusters in water solution by <a href="/facts/Photoreduction/kBnR8Lcp">photoreduction</a>. Poly(methacrylic acid)-stabilized nanoclusters have an excellent high <a href="/facts/Quantum_yield/XPWtrzvL">quantum yield</a> and can be transferred to other scaffolds or solvents and can sense the local environment.<a class="footnote-ref" id="fnref:50" href="#fn:50">50</a><a class="footnote-ref" id="fnref:51" href="#fn:51">51</a><a class="footnote-ref" id="fnref:52" href="#fn:52">52</a><a class="footnote-ref" id="fnref:53" href="#fn:53">53</a><a class="footnote-ref" id="fnref:54" href="#fn:54">54</a><a class="footnote-ref" id="fnref:55" href="#fn:55">55</a><a class="footnote-ref" id="fnref:56" href="#fn:56">56</a>
DNA, proteins and peptides DNA <a href="/facts/Oligonucleotide/Gc99fPnC">oligonucleotides</a> are good templates for synthesizing metal nanoclusters. Silver ions possess a high affinity to <a href="/facts/Cytosine/JW20j33u">cytosine</a> 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 <a href="/facts/Macromolecule/039ULSLg">macromolecules</a> such as <a href="/facts/Peptide/C4ltc7UG">peptides</a> and <a href="/facts/Protein/Jxmagu3E">proteins</a> have also been utilized as templates for synthesizing highly fluorescent metal nanoclusters. Compared with short <a href="/facts/Peptide/C4ltc7UG">peptides</a>, large and complicated proteins possess abundant binding sites that can potentially bind and further reduce metal <a href="/facts/Ion/3bePpDna">ions</a>, thus offering better scaffolds for template-driven formation of small metal nanoclusters. Also the catalytic function of <a href="/facts/Enzyme/DSI1Fh7y">enzymes</a> can be combined with the fluorescence property of metal nanoclusters in a single cluster to make it possible to construct multi-functional nanoprobes.<a class="footnote-ref" id="fnref:57" href="#fn:57">57</a><a class="footnote-ref" id="fnref:58" href="#fn:58">58</a><a class="footnote-ref" id="fnref:59" href="#fn:59">59</a><a class="footnote-ref" id="fnref:60" href="#fn:60">60</a><a class="footnote-ref" id="fnref:61" href="#fn:61">61</a>
Inorganic scaffolds Inorganic materials like glass and <a href="/facts/Zeolite/SNrXy4v2">zeolite</a> 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 <a href="/facts/Angstrom/T5mjdd97">Ångström</a> 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.<a class="footnote-ref" id="fnref:62" href="#fn:62">62</a><a class="footnote-ref" id="fnref:63" href="#fn:63">63</a>

<h2 id="properties">Properties</h2>
<h3>Magnetic properties</h3>
Most atoms in a nanocluster are surface atoms. Thus, it is expected that the <a href="/facts/Magnetic_moment/VpAQKJtr">magnetic moment</a> of an atom in a cluster will be larger than that of one in a bulk material. Lower coordination, lower dimensionality, and increasing interatomic distance in metal clusters contribute to enhancement of the magnetic moment in nanoclusters. Metal nanoclusters also show change in magnetic properties. For example, <a href="/facts/Vanadium/UvKkKTSm">vanadium</a> and <a href="/facts/Rhodium/xmBd9aFG">rhodium</a> are <a href="/facts/Paramagnetism/vj1SMbbp">paramagnetic</a> in bulk but become <a href="/facts/Ferromagnetism/fH3HFnSl">ferromagnetic</a> in nanoclusters. Also, <a href="/facts/Manganese/j9ELd1TL">manganese</a> is antiferromagnetic in bulk but ferromagnetic in nanoclusters. A small nanocluster is a <a href="/facts/Nanomagnet/FdLmruTk">nanomagnet</a>, which can be made nonmagnetic simply by changing its structure. So they can form the basis of a nanomagnetic switch.<a class="footnote-ref" id="fnref:64" href="#fn:64">64</a><a class="footnote-ref" id="fnref:65" href="#fn:65">65</a>

<h3>Reactivity properties</h3>
Large surface-to-volume ratios and low coordination of surface atoms are primary reasons for the unique <a href="/facts/Reactivity_(chemistry)/PuJ2pmRT">reactivity</a> of nanoclusters. Thus, nanoclusters are widely used as catalysts.<a class="footnote-ref" id="fnref:66" href="#fn:66">66</a> Gold nanocluster is an excellent example of a <a href="/facts/Catalyst/LPBS7TQC">catalyst</a>. While bulk gold is chemically <a href="/facts/Chemically_inert/oglbxzgk">inert</a>, it becomes highly reactive when scaled down to nanometer scale. One of the properties that govern cluster reactivity is <a href="/facts/Electron_affinity/1mf4A396">electron affinity</a>. <a href="/facts/Chlorine/y27UwYBM">Chlorine</a> has highest electron affinity of any material in the <a href="/facts/Periodic_table/umdGIfUb">periodic table</a>. Clusters can have high electron affinity and nanoclusters with high electron affinity are classified as super halogens. Super halogens are metal atoms at the core surrounded by <a href="/facts/Halogen/YeFgWHeA">halogen</a> atoms.<a class="footnote-ref" id="fnref:67" href="#fn:67">67</a><a class="footnote-ref" id="fnref:68" href="#fn:68">68</a>

<h3>Optical properties</h3>
The optical properties of materials are determined by their electronic structure and <a href="/facts/Band_gap/IjIeh95l">band gap</a>. The energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital (<a href="/facts/HOMO/LUMO/XyofpH0p">HOMO/LUMO</a>) varies with the size and composition of a nanocluster. Thus, the optical properties of nanoclusters change. Furthermore, the gaps can be modified by coating the nanoclusters with different ligands or <a href="/facts/Surfactant/zgVX6g8o">surfactants</a>. It is also possible to design nanoclusters with tailored band gaps and thus tailor optical properties by simply tuning the size and coating layer of the nanocluster.<a class="footnote-ref" id="fnref:69" href="#fn:69">69</a><a class="footnote-ref" id="fnref:70" href="#fn:70">70</a><a class="footnote-ref" id="fnref:71" href="#fn:71">71</a><a class="footnote-ref" id="fnref:72" href="#fn:72">72</a>

<h2 id="applications">Applications</h2>
Nanoclusters potentially have many areas of application as they have unique optical, electrical, magnetic and reactivity properties. Nanoclusters are <a href="/facts/Biocompatibility/UbGXs4XW">biocompatible</a>, ultrasmall, and exhibit bright emission, hence promising candidates for fluorescence bio imaging or cellular labeling. Nanoclusters along with fluorophores are widely used for staining cells for study both in vitro and in vivo. Furthermore, nanoclusters can be used for sensing and detection applications.<a class="footnote-ref" id="fnref:73" href="#fn:73">73</a> They are able to detect <a href="/facts/Copper/I8PEE8oX">copper</a> and <a href="/facts/Mercury_(element)/WlxfoHva">mercury</a> and silions in an aqueous solution based on fluorescence quenching. Also many small molecules, biological entities such as <a href="/facts/Biomolecules/DGvFzN7R">biomolecules</a>, proteins, <a href="/facts/DNA/nB89Iauz">DNA</a>, and <a href="/facts/RNA/HxPeNfNj">RNA</a> can be detected using nanoclusters. The unique <a href="/facts/Reactivity_(chemistry)/PuJ2pmRT">reactivity</a> properties and the ability to control the size and number of atoms in nanoclusters have proven to be a valuable method for increasing activity and tuning the selectivity in a catalytic process. Also since nanoparticles are magnetic materials and can be embedded in glass these nanoclusters can be used in optical data storage that can be used for many years without any loss of data.<a class="footnote-ref" id="fnref:74" href="#fn:74">74</a><a class="footnote-ref" id="fnref:75" href="#fn:75">75</a><a class="footnote-ref" id="fnref:76" href="#fn:76">76</a><a class="footnote-ref" id="fnref:77" href="#fn:77">77</a><a class="footnote-ref" id="fnref:78" href="#fn:78">78</a>

<h2 id="further-reading-reviews">Further reading (reviews)</h2>
<ul><li>"Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles" by Chakraborty and Pradeep <a class="footnote-ref" id="fnref:79" href="#fn:79">79</a></li></ul>
<h2 id="further-reading-primary-references">Further reading (primary references)</h2>
<ul><li>Gary, Dylan C.; Flowers, Sarah E.; Kaminsky, Werner; Petrone, Alessio; Li, Xiaosong; Cossairt, Brandi M. (2016-02-10). <a href="https://doi.org/10.1021/jacs.5b13214">"Single-Crystal and Electronic Structure of a 1.3 nm Indium Phosphide Nanocluster"</a>. Journal of the American Chemical Society. 138 (5): 1510–1513. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2016JAChS.138.1510G">2016JAChS.138.1510G</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1021%2Fjacs.5b13214">10.1021/jacs.5b13214</a>. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0002-7863">0002-7863</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/26784649">26784649</a>.</li>
<li>Kunwar, P; Hassinen, J; Bautista, G; Ras, R. H. A.; Toivonen, J (2016). <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4820741">"Sub-micron scale patterning of fluorescent silver nanoclusters using low-power laser"</a>. Scientific Reports. 6: 23998. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2016NatSR...623998K">2016NatSR...623998K</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1038%2Fsrep23998">10.1038/srep23998</a>. <a href="/facts/PMC_(identifier)/dX1zMt71">PMC</a> <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4820741">4820741</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/27045598">27045598</a>.</li>
<li>Kunwar, P; Hassinen, J; Bautista, G; Ras, R. H. A.; Toivonen, J (2014). <a href="https://aaltodoc.aalto.fi/handle/123456789/26539">"Direct Laser Writing of Photostable Fluorescent Silver Nanoclusters in Polymer Films"</a>. ACS Nano. 8 (11): 11165–11171. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1021%2Fnn5059503">10.1021/nn5059503</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/25347726">25347726</a>.</li>
<li>Beecher, Alexander N.; Yang, Xiaohao; Palmer, Joshua H.; LaGrassa, Alexandra L.; Juhas, Pavol; Billinge, Simon J. L.; Owen, Jonathan S. (2014-07-30). <a href="https://doi.org/10.1021/ja503590h">"Atomic Structures and Gram Scale Synthesis of Three Tetrahedral Quantum Dots"</a>. Journal of the American Chemical Society. 136 (30): 10645–10653. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2014JAChS.13610645B">2014JAChS.13610645B</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1021%2Fja503590h">10.1021/ja503590h</a>. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0002-7863">0002-7863</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/25003618">25003618</a>.</li>
<li>Gary, Dylan C.; Terban, Maxwell W.; Billinge, Simon J. L.; Cossairt, Brandi M. (2015-02-24). <a href="https://doi.org/10.1021/acs.chemmater.5b00286">"Two-Step Nucleation and Growth of InP Quantum Dots via Magic-Sized Cluster Intermediates"</a>. Chemistry of Materials. 27 (4): 1432–1441. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1021%2Facs.chemmater.5b00286">10.1021/acs.chemmater.5b00286</a>. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0897-4756">0897-4756</a>. <a href="/facts/OSTI_(identifier)/9Q4WHJRV">OSTI</a> <a href="https://www.osti.gov/biblio/1182518">1182518</a>.</li>
<li>Tanaka S. I, Miyazaki J, Tiwari D. K., Jin T, Inouye Y. (2011). "Fluorescent Platinum Nanoclusters: Synthesis, Purification, Characterization, and Application to Bioimaging". Angewandte Chemie International Edition. 50 (2): 431–435. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1002%2Fanie.201004907">10.1002/anie.201004907</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/21154543">21154543</a>.{{cite journal}}: CS1 maint: multiple names: authors list (link)</li>
<li>Karimi N, Kunwar P, Hassinen J, Ras R. H. A, Toivonen J (2016). <a href="https://aaltodoc.aalto.fi/handle/123456789/32811">"Micropatterning of silver nanoclusters embedded in polyvinyl alcohol films"</a>. Optics Letters. 41 (15): 3627–3630. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2016OptL...41.3627K">2016OptL...41.3627K</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1364%2Fol.41.003627">10.1364/ol.41.003627</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/27472635">27472635</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:3477288">3477288</a>.{{cite journal}}: CS1 maint: multiple names: authors list (link)</li></ul>

<h2 id="references">References</h2>

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<li id="fn:15">Chakraborty, I; Govindarajan, A; Erusappan, J; Ghosh, A; Pradeep, T; Yoon, B; Whetten, R. L.; Landman, U. (2012). "The Superstable 25 kDa Monolayer Protected Silver Nanoparticle: Measurements and Interpretation as an Icosahedral Ag152(SCH2CH2Ph)60 Cluster". Nano Letters. 12 (11): 5861–5866. Bibcode:2012NanoL..12.5861C. CiteSeerX 10.1.1.720.7249. doi:10.1021/nl303220x. PMID 23094944. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Petty, J. T.; Story, S. P.; Hsiang, J. C.; Dickson, R. (2013). "DNA-Templated Molecular Silver Fluorophores". Journal of Physical Chemistry Letters. 4 (7): 1148–1155. doi:10.1021/jz4000142. PMC 3670773. PMID 23745165. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3670773" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3670773</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
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<li id="fn:39">Chakraborty, I; Govindarajan, A; Erusappan, J; Ghosh, A; Pradeep, T; Yoon, B; Whetten, R. L.; Landman, U. (2012). "The Superstable 25 kDa Monolayer Protected Silver Nanoparticle: Measurements and Interpretation as an Icosahedral Ag152(SCH2CH2Ph)60 Cluster". Nano Letters. 12 (11): 5861–5866. Bibcode:2012NanoL..12.5861C. CiteSeerX 10.1.1.720.7249. doi:10.1021/nl303220x. PMID 23094944. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:39" class="footnote-back-ref">↩</a></li>
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</ol>

Nanocluster open-in-new

Nanocluster