(603c) A Facile Nanoparticle Synthesis/Extraction Strategy to Target Pt Nanoparticle Microarrays and Superlattices

The creation of nanometer-scale particles as stable building blocks for the assembly of supramolecular structures such as microarrays has been an area of significant interest. Much of the recent interest in the assembly of nanoparticle arrays has been driven by their prospective applications including electronic devices, optical materials, sensors, molecular catalysts, and others. The availability of stable spherical nanoparticle building blocks with extremely narrow size distribution is a prerequisite to the assembly of these microarrays. A phase-transfer approach has been widely employed in the preparation of thiol-protected metal nanoparticles since it was first reported by Brust et al. This Brust-Schiffrin method utilizes a phase transfer catalyst to introduce metal ions into an organic solution where the nucleation and growth of the particles and encapsulation of the thiol molecules occur simultaneously. However, this approach requires the use of a significant amount of expensive phase-transfer catalyst and organic solvent as well as elaborate and time-consuming postsynthesis processing to narrow the size distribution of the product. Also, it has been reported that the transfer of the unprotected Pt particles from aqueous phase to organic phase via ligand exchange could be facilitated by the addition of concentrated HCl. In this case, the particles' agglomeration could have occurred before or during the transfer, resulting in their wide size distribution and, after evaporating the organic solvent, their random arrangement on the surface of the substrate (such as a TEM copper grid). In order to achieve the monodisperse nanoparticles (to act as building blocks for the assembly of the microarray), it is essential to introduce some water-soluble capping agent to effectively passivate the surface of the particles and suppress the growth of the particles in aqueous phase before the extraction. Two ?green? and inexpensive carbohydrates, β-D glucose and Sodium Carboxymethyl Cellulose (Na CMC), that possess different functional groups such as ?OH and ?COO, were employed in the current study to meet this requirement. Our FT-IR results clearly indicate that β-D glucose interacts with Pt nanoparticles via the ?OH group and that Na CMC bonds on the surface of the Pt nanoparticles via both the ?OH and ?COO groups, implying that Na CMC caps the particles better than β-D glucose. As per the FT-IR information and considering the interaction between these capping agents and particles, it is reasonable to assume that the thiol-protected particles yielded upon Na CMC-assisted extraction should have a more narrow size distribution compared to that obtained with β-D glucose-assisted extraction. TEM studies were designed to clarify the above mentioned assumption. As expected, the Na CMC capped Pt nanoparticles had a narrower size distribution than the β-D glucose capped nanoparticles. Before extraction the sizes of the β-D glucose and Na CMC capped Pt nanoparticles were 4.2 nm (SD: 0.94nm) and 3.9 nm (SD: 0.5 nm), respectively. After extraction, the sizes of the dodecanethiol capped Pt nanoparticles achieved upon β-D glucose-assisted and Na CMC-assisted extraction were 4.3 nm (SD: 0.9 nm) and 4.1 nm (SD: 0.47 nm), respectively, while the absence of any capping agent before extraction resulted in the widest size distribution (average size: 4.7 nm and SD: 1.3 nm) of the dodecanethiol-capped Pt nanoparticles. Clearly, these data from TEM imaging further confirm our assumption stemming from the FT-IR spectra. It is important to understand the effect of the particle size distribution on the supramolecular assembly of nanoparticle arrays. Three dispersions of Pt nanoparticles in hexane solvent were produced using the three methods described above. These dodecanethiol capped Pt nanoparticles of three distinct size distributions (SD = 1.3 nm; SD = 0.90, and SD = 0.47 nm) were used to examine the ability to form microarrays by simply evaporating the organic solvent. The Pt particles with SD of 1.3 nm (particle size: 4.7 nm) could only be assembled into random arrangement architectures, while local order appears in the assemblies of the particles with SD of 0.90 nm (particle size: 4.3 nm). Interestingly, the Pt nanoparticles with SD of 0.47 nm (particle size: 3.9 nm), were organized into long range, structurally ordered arrays and superlattices. Thus, we have developed and systematically studied a straightforward synthesis/extraction approach whereby monodisperse Pt nanoparticles are achieved by selecting a suitable capping agent, such as Na CMC, which has relatively strong interactions with the particles in the aqueous phase before extraction into an organic phase through ligand exchange. These Pt nanoparticles were successfully assembled into structurally ordered nanoparticle microarrays due to the extremely narrow size distribution of the Pt nanoparticle building blocks produced in this manner.