A heating treatment strategy for inducing size and shape change of composite nanoparticles in solutions is described. The composite nanoparticles are ∼2 nm gold cores encapsulated with alkanethiolate monolayers. The development of abilities in size and shape controls constitutes the motivation of this work. We demonstrated a remarkable evolution of the preformed particles in solutions toward monodispersed larger core sizes with well-defined and highly faceted morphologies. The particles thus evolved were encapsulated with the thiolate shells, and exhibited striking propensities of forming long-range ordered arrays. The morphological and structural evolutions were characterized using transmission electron microscopy, X-ray diffraction, UV−vis and infrared spectroscopies. Although temperature-driven crystal growth is known for nonencapsulated particles, the evolution of the thiolate-encapsulated nanoparticles in solutions into well-defined morphologies represents an intriguing example of temperature manipulations in size monodispersity and shape control.

This paper describes the characterizations of structural and electrochemical properties of nanoparticle thin films derived by a one-step exchange-cross-linking-precipitation route. While there exists a stepwise layer-by-layer construction method, our motivation stems from seeking an alternative and simpler pathway to prepare such thin films as electrode nanomaterials. The model system consisting of thiolate-encapsulated gold nanocrystals and r,ω-alkanedithiol cross-linkers was studied. The mixing of these two components in solutions allowed sequential exchanging, cross-linking, and eventual precipitation of the dithiol-cross-linked nanocrystals as thin films on almost any substrates. A series of comparative microscopic, spectroscopic, and electrochemical analyses were performed on thin films derived from nanocrystals of 2and 5-nm core sizes. The 5-nm particles were fabricated by size and shape evolution of preformed 2-nm particles. The films were specularly reflecting, electronically continuous, and remarkably comparable with stepwisederived thin films in structural, electronic, and electrochemical properties. The electrochemical data were discussed in terms of thiolate binding and barrier properties of the core-shell structures, which may have potential chemical recognition applications.We report in this paper the characterizations of nanoparticle thin films prepared via a one-step exchange-cross-linkingprecipitation route from gold nanoparticles encapsulated with thiolate monolayers. This class of core-shell nanomaterials is interesting because there are considerable potential technological applications in the areas of microelectronics, optic devices, magnetic materials, catalysis, and chemical recognition. [1][2][3][4][5] Since the first report of the two-phase synthesis protocol, 6 the stable and soluble thiolate-encapsulated nanoparticles have attracted enormous interests. 1 One area is to use such nanoparticles as building blocks toward nanostructured thin films, exploring individual or collective nanoparticle properties. Among several * To whom correspondence should be addressed: (phone)

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This paper presents findings of the creation of biomimetic ion-gating properties with core-shell nanoparticle network architectures. The architectures were formed by hydrogen-bonding linkages via an exchange-cross-linking-precipitation reaction pathway using gold nanoparticles capped with thiolate shell and alkylthiols terminated with carboxylic groups as model building blocks. Such network assemblies have open frameworks in which void space is in the form of a channel or chamber with the nanometer-sized cores defining its size, the geometric arrangement defining its shape, and the shell structures defining its chemical specificity. The formation of the network linkages via head-to-head hydrogen-bonded carboxylic terminals and the reversible pH-tuned structural properties between neutral and ionic states were characterized using infrared reflectance spectroscopic technique. The biomimetic ion-gating properties were demonstrated by measuring the pH-tuned network "open-close" responses to charged redox probes. Such redox responses were shown to depend on the degree of protonation-deprotonation of carboxylic groups at the interparticle linkages, core sizes of the nanoparticles, and charges of the redox probes. Differences in structural networking, pH-tuning, and electrochemical gating properties were identified between the network films derived from nanoparticles of two different core sizes (2 and 5 nm). The mechanistic correlation of these structural properties was discussed. These findings have added a new pathway to the current approaches to biomimetic molecular recognition via design of core-shell nanoparticle architectures at both nanocrystal and molecular scales.

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