Layer Assemblies

METAL COLLOID STRUCTURES THROUGH LAYER-BY-LAYER ASSEMBLY 

All these features make LBL an ideal technique for the assembly of core-shell and anisotropic nanoparticles (among others), since the nanoparticles can be completely synthesized prior to the assembly process, and the desired properties can be designed in the first synthetic stage. We only describe here two examples of the versatility of this technique for the assembly of metal nanoparticles, which allows to obtain structures with special optical features. 

The interest of the assembly of silica-coated gold nanoparticles relies on the possibility of controlling the particle volume fraction by means of the variation of the silica shell thickness, provided that the assembled films are close-packed. In closepacking conditions, the separation between metal cores is just twice the thickness of the coating shell, which can be controlled during the synthesis of the colloids. This system has been recently studied in detail by Ung et al., and described here are the main results on the influence of metal nanoparticles volume fraction (through interparticle interactions) on the optical properties of the films. 

In refs. 11 and 168, experimental and calculated absorbance and reflectance spectra are compared for thin films with different metal contents (through variation of silica shell thickness). This comparison shows that, as the gold volume fraction increases (the separation between nanoparticles decreases), there is a red-shift of the plasmon resonance, as well as a broadening of the band. This effect originates in the dipole-dipole interactions between neighboring nanoparticles, and a separation of just 15 nm is 

both the transmission and reflection properties of thin gold films can be easily controlled by means of an adjustment of the thickness of the silica shell surroimding each nanoparticle, so that dipole-dipole interparticle interactions are effectively screened. Detailed studies using different metal core sizes have not been carried out yet, but it is expected that the distance at which interactions are effectively screened will scale up with particle size, being of the order of one particle diameter. 

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In an alternative procedure designed to deal with minute volumes of liquid, Walter38 set up a layer cell based upon the technique employed in instant colour photographic films, Such a cell designed to determine potassium ions made use of two layer assemblies terminating in valinomycin electrodes, so that with a standard potassium chloride solution added to one assembly, and the... 

Since the diameters of both kinds of particles are about 200 mn, the thickness of a 10-layer assembly should be about 1.8 p,m. However, the cross section of a sample subjected to 10-fold dipping indicates a thickness of only 1.2 jim (Fig. 21). Similar observations were made by others [94,97] and can be ascribed to the low substrate coverage reached in each of the dipping steps. Serizawa et al. [97] used the second approach and determined the adsorption using QCM and SEM. An SEM image after two adsorption steps indicated a very irregular, amorphous structure, in agreement with Fulda and coworkers [93]. 

Bioinspired Porous Hybrid Materials via Layer-by-Layer Assembly... 

Kommireddy, D., Sriram, S., Lvov, Y. and Mill, D. (2006) Layer-by-layer assembled nanopartide thin films — a new surface modification approach for stem cell attachment. Biomaterials, 27, 4296—4303. 

Encapsulation via the layer-by-layer assembly of multilayered polyelectrolyte (PE) or PE/nanoparticle nanocomposite thin shells of catalase in bimodal mesoporous silica spheres is also described by Wang and Caruso [198]. The use of a bimodal mesoporous structure allows faster immobilization rates and greater enzyme immobilization capacity (20-40 wt%) in comparison with a monomodal structure. The activity of the encapsulated catalase was retained (70 % after 25 successive batch reactions) and its stability enhanced. 

G. Decher, M. Eckle, J. Schmitt, and B. Struth, Layer-by-layer assembled multicomposite films. Curr. Opin. Coll. Interface Sci. 3, 32—39 (1998). 

FIGURE 15.8 Schematic description of layer-by-layer assembly of CNTs with PDDA on the GC surface. 

M. Zhang, Y. Yan, K. Gong, L. Mao, Z. Guo, and Y. Chen, Electrostatic layer-by-layer assembled carbon canotube multilayer film and its electrocatalytic activity for 02 reduction. Langmuir 20, 8781-8785 (2004). 

M. Zhang, K. Gong, H. Zhang, and L. Mao, Layer-by-layer assembled carbon nanotubes for selective determination of dopamine in the presence of ascorbic acid. Biosens. Bioelectron. 20, 1270-1276 (2005). 

Y.L. Zhou, Z. Li, N.F. Hu, Y.H. Zeng, and J.F. Rusling, Layer-by-layer assembly of ultrathin films of haemoglobin and clay nanoparticles with electrochemical and catalytic activity. Langmuir 18, 8573-8579 (2002). 

In the first chapter, on electrochemical atomic layer epitaxy, Stickney provides a review of experimental methodology and current accomplishments in the electrodeposition of compound semiconductors. The experimental procedures and detailed fundamental background associated with layer-by-layer assembly are summarized for various compounds. The surface chemistry associated with the electrochemical reactions that are used to form the layers is discussed, along with challenges and issues associated with device formation by this method. 

Javey, A. Nam, S. Friedman, R. S. Yan, H. Lieber, C. M. 2007. Layer-by-layer assembly of nanowires for three-dimensional, multifunctional electronics. Nano Lett. 7 773-777. 

Rogach, A. L. Koktysh, D. S. Harrison, M. Kotov, N. A., Layer by layer assembled films of HgTe nanocrystals with strong infrared emission, Chem. Mater. 2000, 12, 1526 1528... 

Figure 1. Schematic representation of the artificial photosynthetic reaction center by a monolayer assembly by A-S-D triad and antenna molecules for light harvesting (H), lateral energy migration and energy transfer, and charge separation across the membrane via multistep electron transfer (a) Side view of mono-layer assembly, (b) top view of a triad surrounded by H molecules, and (c) energy diagram for photo-electric conversion in a monolayer assembly.

Jan E, Kotov NA (2007) Successful differentiation of mouse neural stem cells on layer-by-layer assembled single-walled carbon nanotube composite. Nano Lett 7 1123-1128. 

Du, N., et al., Homogeneous coating ofAu andSn02 nanocrystals on carbon nanotubes via layer-by-layer assembly a new ternary hybrid for a room-temperature CO gas sensor. Chemical Communications, 2008(46) p. 6182-6184. 

Kong, B.-S., J. Geng, and H.-T. Jung, Layer-by-layer assembly of graphene and gold nanoparticles by vacuum filtration and spontaneous reduction of gold ions. Chemical Communications, 2009(16) p. 2174-2176. 

Sheng, K. Bai, H. Sun, Y. Li, C. Shi, G., Layer-by-layer assembly of graphene/polyaniline multilayer films and their application for electrochromic devices. Polymer 2011. 

The major part of the reports discussed above provides only a qualitative description of the catalytic response, but the LbL method provides a unique opportunity to quantify this response in terms of enzyme kinetics and electron-hopping diffusion models. For example, Hodak et al. [77[ demonstrated that only a fraction of the enzymes are wired by the polymer. A study comprising films with only one GOx and one PAH-Os layer assembled in different order on cysteamine, MPS and MPS/PAH substrates [184[ has shown a maximum fraction of wired enzymes of 30% for the maximum ratio of mediator-to-enzyme, [Os[/[GOx[ fs 100, while the bimolecular FADH2 oxidation rate constant remained almost the same, about 5-8 x 10 s ... 

Kotov, N.A. (2003) in Multilayer Thin Films Layer-by-Layer Assembly of Nanoparticles and Nanocolloids Intermolecular Interactions, Structure and Materials Perspectives (eds G. Decher and B.J. SchlenofF), Wiley-VCH, Weinheim. 

The films formed in these covalent layer-by-layer assembly procedures are similar to the ionic assemblies prepared by other layer-by-layer self assembly procedures. Unlike the more defined Langmuir-Blodgett multilayer assemblies that have well defined layers, the layers of electrophilic polymer and nucleophihc polymer are intermixed to an extent. Similar effects are seen in ionic layer-by-layer assembhes. 

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