Suspended hybrid films assembled from thiol-capped gold nanoparticles
© Haltas et al.; licensee Springer. 2012
Received: 11 May 2012
Accepted: 6 June 2012
Published: 6 June 2012
In this work, we explored the formation processes of suspended hybrid thin films of thiol-capped Au nanoparticles (AuNPs) inside metal oxide tubular structures. We found that a balance between in-film interactions of the AuNPs and boundary interactions with metal oxides is a key in making these special organic–inorganic thin films. The hybrid films process many processing advantages and flexibilities, such as controllable film thickness, interfacial shape and inter-AuNPs distance, tuning of particle sizes, thiol population, chain lengths, and other new properties by introducing functional groups to thiol chains. Among their many unique features, the assembly-disassembly property may be useful for future on-off or store-release applications.
KeywordsNanoparticles interfaces thin films hybrid self-assembly disassembly
Ligands (e.g., alkanethiolate, phosphines, amines, peptides, DNA, and polymers) assisted organization of Au nanoparticles (AuNPs) has received great research attention over the past two decades [1–13]. As starting building units, surface-passivated AuNPs have been synthesized into various shapes and sizes [1–16]. Because of the presence of surface ligands, these AuNPs can self-assemble into larger constructs via van der Waals interactions existing among the ligand molecules. For example, superlattices, supported films, supracrystals, and sponges have been constructed with thiol-capped AuNPs [1, 2, 7, 12–15]. In their two-dimensional arrays/assemblies, in particular, monolayer or multilayers of AuNPs have been prepared on solid supports (e.g., typically on polymeric films of transmission electron microscopy (TEM) copper grids) [1, 2, 7, 12, 13]. Although a significant research progress has been made, to the best of our knowledge, suspended hybrid films organized from thiol-capped AuNPs have not been reported. In view of their hybrid nature (i.e., organic thiols plus inorganic AuNPs), which can be finely tuned by chemical methods, the self-assembled thin films may provide desirable functionality and porosity for future applications.
In this work, as part of our recent effort in this area, we have prepared suspended hybrid thin films composed of thiol-capped AuNPs. Our findings reveal that inter-particulate interaction among the thiol-stabilized AuNPs plays an important role in film formation, while a certain degree of interaction with boundary materials is also essential in making these organic–inorganic thin films.
In our synthesis, the AuNPs were synthesized according to a modified two-phase protocol [14–16], in which thiol molecules [1-dodecanethiol (DT) or 3-mercaptopropionic acid (MPA)] were used as surface capping agents for AuNPs (see supporting information (SI)-1 in Additional file 1). In a typical film preparation, anodic aluminum oxide (AAO) membrane templates were placed in a glass petri dish, and a drop of AuNP suspension (ca 0.02 mL; standard solution, see SI-1 in Additional file 1) was added onto each membrane, followed by a natural drying process. The same procedure was repeated several times in order to attain a desired film thickness. Afterwards, the AAO membranes were immersed in a plastic petri dish that contained an aqueous TiF4 solution (0.04 M) for a selected period of time (30 to 120 min). After the deposition of TiO2 nanotubes on their channels, the AAO membranes were washed with deionized water and air-dried at room temperature. The AAO templates were then removed by immersion in 1 M NaOH solution overnight. The solid products were collected and washed with deionized water and ethanol, followed by vacuum-drying. The product structure, morphology, and composition were investigated by TEM (JEM-2010, JEOL Ltd., Akishima, Tokyo, Japan), energy dispersive X-ray spectroscopy (EDX), and selected area electron diffraction.
Results and discussion
Concerning their applications, we note that this type of hybrid films (or hybrid membranes) possesses many structural advantages and processing flexibilities: (1) AuNPs (or other metallic NPs) can be tuned with different sizes (e.g., monodisperse AuNPs, 2.1 ± 0.3 nm and 6.2 ± 0.6 nm; see SI-8 in Additional file 1); (2) inter-AuNP distance depends on the thiol population and chain lengths (e.g., 3.8 ± 0.4 nm and 8.6 ± 0.5 nm; see SI-8 in Additional file 1); (3) film thickness and interfacial shape can be engineered; and (4) new properties can be obtained by introducing functional groups to thiol chains. All these control factors will determine the final porosity and functionality of the suspended hybrid films in future applications (e.g., gas separation). For instance, these films can be used as a catalyst or a sealant for the enclosure of nano test tubes [18–20]. At a relatively low temperature, the AuNPs in these films are readily fused into single metallic gold plugs at 150°C to 250°C, as reported in Figure 4 g,h. On the other hand, while they are very stable in aqueous solution, the suspended hybrid films of AuNPs are soluble (and thus removable) in common organic solvents such as toluene and cyclohexane. The assembly-disassembly property that these films possess may be useful for on-off or store-release operations.
In summary, we have explored the formation processes of suspended hybrid thin films of thiol-capped AuNPs inside metal oxide tubular structures. It has been found that a balance between the in-film interactions of the AuNPs and boundary interactions with metal oxides is a key in making these organic–inorganic thin films. Among many unique features, the assembly-disassembly property may be useful for on-off or store-release applications.
Anodic aluminum oxide
Energy dispersive X-ray spectroscopy
Transmission electron microscopy.
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51104194), Doctoral Fund of Ministry of Education of China (20110191120014), No.43 Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Natural Science Foundation Project of CQ CSTC (CSTS2010BB4058). Dr. Zhang gratefully acknowledges Prof. Zeng Hua Chun for his kind discussions and the National University of Singapore for their technical supports.
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