Fluorescence anisotropy of protein - Gold nanoclusters
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Red-emitting gold nanoclusters (BSA-Au 25 ) have been synthesized in Bovine Serum albumin and found to have a characteristic fluorescence lifetime of 1.25μs. Fluorescence anisotropy measurements of BSA-Au25 reveal a dramatic increase in protein size as the pH is reduced from 7 to 3.1, consistent with the change of BSA from N form to E form. This study suggests the possibility of exploiting fluorescent protein-gold nanoclusters to probe protein conformational changes in response to the local environment.Keywords:
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A fundamental understanding of the structural growth of thiolate-protected gold nanoclusters not only benefits experimental synthesis but also will advance the methodology for structural predictions and for rational design of highly stable nanoclusters. Herein, we report numerous new structures (11 total) of thiolate-protected gold nanoclusters predicted from theoretical modulation of the double-helical cores of experimentally determined nanoclusters. Among these newly predicted structures, Au32(SR)22, Au40(SR)26, and Au48(SR)30 are obtained by adding a defective layer containing 4 gold atoms on a structural sequence of experimentally crystallized nanoclusters, namely, Au28(SR)20, Au36(SR)24, and Au44(SR)28. The generic growth pattern underlying this sequence of nanoclusters can be viewed as adding the highly stable tetrahedral Au4 unit on the double-helical cores. Likewise, the other eight newly predicted structures, including two groups of isomeric structures corresponding to the sequence of experimentally determined Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32 nanoclusters, are successfully predicted. Density functional theory calculations show that these 11 newly predicted nanoclusters exhibit large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps and all-positive harmonic vibrational frequencies, suggesting their high chemical stabilities. Additional analyses on the structures and properties suggest that these newly predicted nanoclusters are very likely to be synthesized in the laboratory. Confirmation by experiments would validate the new strategy for structural prediction of thiolate-protected gold nanoclusters by taking advantage of a large structure database of crystallized ligand-protected gold nanoclusters with a variety of gold cores.
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We report a simple and environmentally-benign method for the synthesis of Pd nanoclusters through a continuous etching process. With the help of L-methionine, newly-formed Pd nanoparticles (larger than 50 nm) will gradually reduce their size, finally resulting in the formation of tiny nanoclusters with a diameter around 1.4 nm. A following sol–gel process can encapsulate these highly-active nanoclusters into a silica matrix. In this way, Pd nanoclusters can be stabilized and can sustain a high temperature treatment up to 600 °C. Moreover, these Pd nanoclusters are proved to be promising for a catalytic Suzuki reaction. Due to the composite structure of nanoclusters and silica, the Pd@SiO2 catalysts show integrated merits including good catalytic activity, high stability and notable cyclability.
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The anisotropy decay of a fluorescently-labelled macromolecule provides information on the internal and global dynamics of the macromolecule. Weber was a pioneer of fluorescent probes, polarization and polarized phase-modulation methods and revealed the power of combining or comparing these methods to disentangle complex modes of emission depolarization. In this paper we take a similar course and show that when measurements of dynamic depolarization are combined with steady-state anisotropy, complex anisotropy decays can be deduced from measurements at a single modulation frequency. Specifically, a double exponential anisotropy decay can be resolved by combining one of the polarized emission phasors with the steady-state anisotropy. The key is the polarized phasor ellipse plot which provides a convenient visualisation aid and reduces the dimensionality of the minimisation problem from three variables to one variable. We illustrate these concepts with an experimental measurement of the anisotropy decay of a small cytoplasmic fluorescent probe in live cells.
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Recently, we have shown that metal ions free in solution may be determined at low levels by fluorescence anisotropy (polarization) measurements. Anisotropy measurements enjoy the advantages of wavelength ratiometric techniques for determining metal ions such as calcium, because anisotropy measurements are ratiometric as well. Furthermore, fluorescence anisotropy may be imaged in the microscope. An advantage of anisotropy not demonstrated for wavelength ratiometric approaches using indicators such as Fura-2 and Indo-1 is that under favorable circumstances anisotropy-based determinations exhibit a much broader dynamic range in metal ion concentration. Determinations of free Zn(II) in the picomolar range are demonstrated.
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Recent advances in fluorescent noble metal nanoclusters, e.g. Au and Ag, have prompted the development of fluorescent film sensors. These metal nanoclusters possess the unique and intriguing physical and chemical properties including strong photoluminescence and excellent photostability. Up to date, a wide variety of organic ligands have been used for the synthesis of the nanoclusters. These organic ligands not only provide versatile connecting antennae for facile immobilization of the nanoclusters for fabricating fluorescent film sensors, but also facilitate the interaction of the encapsulated nanoclusters with the analytes, leading to improved sensor performance. The present review shows the recent progress in the fabrication and applications of fluorescent film sensors based on Au and Ag nanoclusters. Keywords: Fluorescent film sensors, gold nanoclusters, silver nanoclusters, protein-protected nanoclusters.
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Atomically precise noble metal nanoclusters are ultrasmall particles that are typically composed of tens to hundreds of metal atoms in the core (equivalent sizes 1–3 nm). This new class of nanomaterials is unique in that they are atomically precise and possess uniform structures, high stability, and attractive properties. Built on the significant success of Au nanoclusters, Ag nanoclusters have recently received increasing attention. The majority of reported silver nanocluster sizes exhibit molecular-like properties, whereas larger ones exhibit plasmons characteristic of metallic state (as opposed to molecular state in smaller sizes). Both molecular (i.e., nonmetallic) and metallic nanoclusters hold promise in a wide range of applications. To deepen the understanding of their physical and chemical properties, precise control over size and determination of the crystal structure are the top priorities. In recent developments, dozens of silver nanoclusters with definite formulas have been prepared through various strategies, albeit the structural determination still lags behind. In this short Review, we summarize the recent progress in ligand-protected silver nanoclusters, including the size-focusing synthetic methods, new sizes, structures, and properties.
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