Silver nanoclusters

Attention has been given to the synthesis of bimetallic silver-gold clusters [71] due to their effective catalytic properties, resistance to poisoning, and selectivity [72]. Recently molecular materials with gold and silver nanoclusters and nanowires have been synthesized. These materials are considered to be good candidates for electronic nanodevices and biosensors [73]. 

A specific example where heterogeneous supports provide nanoparticle size-control is the immobilization of homogeneous silver nanoparticles on polystyrene [366]. This work was extended later to the development of a one-pot method for the size-selective precipitation of silver nanoparticles on PVP-protected thiol-functionalized silica. During the immobilization of very small silver nanoclusters both the size of the silver nanoclusters and the thickness of the silver layer on the support could be controlled directly by the reaction parameters applied (Fi re 16) [367]. 

Figure 26. Constant current mode STM image of isolated (A), self-organized in close-packed hexagonal network (C) and in fee structure (E) of silver nanoclusters deposited on Au(l 11) substrate (scan size (A) 17.1 x 17.1 nm, f/t=—IV, /t=ltiA, (C) 136 X 136 nm, f/t = — 2.5 V, /t = 0.8 tiA, (E) 143 x 143 nm, = —2.2 V, /, = 0.72 nA). I U) curves and their derivatives in the inserts of isolated (B), self-organized in close-packed hexagonal network (D) and in fee structure (F) of silver nanoclusters deposited on Au(l 11) substrate. (Reprinted with permission from Ref. [58], 2000, Wiley-VCH.)...

The broad emission band displayed by these silver/dendrimer constructs actually was found to consist of 5 overlapping fluorescent peaks caused by individual silver/dendrimer complexes. Each of these complexes evidently contained a uniquely sized silver nanocluster, which resulted in an individual emission peak. Therefore, all the silver/dendrimer complexes together in solution presented a combined average of these 5 discrete emission peaks, and thus displayed the broad emission band covering nearly 200 nm in width across the spectrum. 

Lesniak et al. (2005) also describe the preparation and use of similar dendrimer/silver nanoclusters using G-5 PAMAM dendrimers terminated with either amino, hydroxyl, or... 

In a more simple and cheap way, silver clusters can be prepared in aqueous solutions of commercially available polyelectrolytes, such as poly(methacrylic acid) (PM A A) by photo activation using visible light [20] or UV light [29]. Ras et al. found that photoactivation with visible light results in fluorescent silver cluster solutions without any noticeable silver nanoparticle impurities, as seen in electron microscopy and from the absence of plasmon absorption bands near 400 nm (F = 5-6%). It was seen that using PMAA in its acidic form, different ratios Ag+ MAA (0.15 1-3 1) lead to different emission bands, as discussed in the next section (Fig. 12) [20]. When solutions of PMAA in its sodium form and silver salt were reduced with UV light (365 nm, 8 W), silver nanoclusters were obtained with emission band centered at 620 nm and [Pg.322]

Peyser LA, Vinson AE, Bartko AP, Dickson RM (2001) Photoactivated fluorescence from individual silver nanoclusters. Science 291 103-106... 

Xu H, Suslick KS (2010) Water-soluble fluorescent silver nanoclusters. Adv Mater 22 1078-1082... 

Diez I, Pusa M, Kulmala S, Jiang H, Walther A, Goldmann AS, Muller AHE, Ikkala O, Ras RHA (2009) Color tunability and electrochemiluminescence of silver nanoclusters. Angew Chem Int Ed 48 2122-2125... 

Shang L, Dong S (2008) Facile preparation of water-soluble fluorescent silver nanoclusters using a poly electrolyte template. Chem Commun 9 1088-1090... 

Gwinn EG, O Neill P, Guerrero AJ, Bouwmeester D, Fygenson DK (2008) Sequence-depen-dent fluorescence of DNA-hosted silver nanoclusters. Adv Mater 20 279-283... 

Yu J, Patel Sandeep A, Dickson RM (2007) In vitro and intracellular production of peptide-encapsulated fluorescent silver nanoclusters. Angew Chem Int Ed 46 2028-2030... 

Zhang J, Xu S, Kumacheva E (2005) Photogeneration of fluorescent silver nanoclusters in polymer microgels. Adv Mater 17 2336-2340... 

Shang L, Dong SJ (2008) Silver nanocluster-based fluorescent sensors for sensitive detection of Cu(II). J Mater Chem 18 4636 1640... 

Lan G-Y, Fluang C-C, Chang FI-T (2010) Silver nanoclusters as fluorescent probes for selective and sensitive detection of copper ions. Chem Commun 46 1257-1259... 

Guo W, Yuan J, Dong Q, Wang E (2010) Highly sequence-dependent formation of fluorescent silver nanoclusters in hybridized DNA duplexes for single nucleotide mutation identification. J Am Chem Soc 132 932-934... 

Kamat PV, Flumiani M, Hartland GV (1998) Picosecond dynamics of silver nanoclusters. Photoejection of electrons and fragmentation. J Phys Chem B 102 3123-3128... 

The unique electrocatalytic role of benzoic acid-protected silver nanoclusters in the Wolff rearrangement of a-diazo ketones has been disclosed. The presence of a Agn°/Agn+ redox couple facilitates a non-classical electron-transfer process, involving chemical reactions interposed between two electron-transfer steps occurring in opposite directions.32... 

Mechanistic studies suggested that the in situ reduction of silver salts to silver nanoclusters (Ag ) produces the true reagent in the Wolff rearrangement.7 As a result, such reactions have been improved by directly using silver nanoclusters.8... 

Chavan and coworkers provide evidence that the Wolff rearrangement is facilitated by the formation of silver nanoclusters, which initiate electron transfer to the diazo compound providing 8. While the precise fate of this species remains to be firmly established, they suggest a multicycle process involving the intermediacy of a silver carbene 10 (Scheme 8.2).10 12 Decomposition of the silver carbene to the free carbene 14 precedes rearrangement to ketene 13, which is then trapped with water to provide the carboxylic acid 15 (Scheme 8.2). 

Due to its high efficiency and regioselectivity, the Wolff rearrangement was extensively studied and reviewed, with copper or silver being the most efficient metals for this transformation (121). Similar reactivity can be achieved via ultraviolet (UV) light, sonication, or microwave irradiation (122). A free carbene-based pathway was proposed as a general mechanism. Basic additives are usually required when silver is used as the catalyst Silver nanoclusters are known to form under such conditions, which may serve as the real catalyst (Fig. 24) (123,124). 

Rand B. P., Peumans, P. and Forrest, S. R. (2004) Long-range absorption enhancement in organic tandem thin-film solar cells containing silver nanoclusters. J. Appl Phys., 96 7519-7526. 

Fleger Y, Mastai Y, Rosenbluh M, Dressier DH (2009) Surface enhanced Raman spectroscopy of aromatic compounds on silver nanoclusters. Surf Sci 603 788-793... 

Muniz-Miranda M, Ottaviani ME (2004) Silver nanoclusters in mesoporous silica, as obtained by visible-laser irradiation. Laser Phys 14 1533-1538... 

P. V. Kamat M. Flumiani G. Hartiand, Picosecond dynamics of Silver nanoclusters. Photoejection of electrons and fragmentation. /. Phys. Chem. B 1998, 302, 3123-3128. 

Figure 3 Radiation-induced metal clusters, (a) Silver nanoclusters stabilized by PVA (10 nm). (b) STM imaging ofa single duster of the blue sol of silver oligomers Agd formed by y irradiation (n = 4). (c) Clusters ofAg, partially reduced by irradiation and then chemically developed by EDTA. (100 nm large and 15 nm thick), (d) TEM bright-held Image ofNi , PVA clusters (5 nm). (e) Two-dimensional self-assembled array of gold dusters (PVA) on mica with remarkable homodisperse size (5 nm). (f) Monocrystalline Pt nanotubes with CPCI (10 nm diameter and a few 100 nm long), (g) Pt nanorods with CTAB (3-4 nm thick and 20-40 nm long).

Silver nanoclusters showing Cotton effects have also been prepared by reduction of Ag(I) salts complexed by It seems that the formation of optically-active... 

Using a zeolite support rather than a non-microporous soUd like alumina can be very fruitful, and the same silver nanoclusters on alumina and H-MFI have very different activity. On zeolite, they catalyse NOx SCR by methane, but they lead to combustion of methane at temperatures as low as 300 °C [28]. 

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