Incorporation of an N-methyl Amino Acid into a Template Peptide Alters Anisotropy in the Crystal Growth of Gold Nanoparticles Synthesized by the Peptide Template Method

BACKGROUND

Gold nanocrystals have unique physicochemical and biocompatible properties, and hold promise for use as catalysts and in the fields of electronics, photonic and/or plasmonic devices, sensing and/or imaging systems, targeted drug delivery, and photothermal therapies. A variety of organic templates have been used to control the size, shape, and structure of gold nanocrystals, and to modify their surfaces. For the control of the shape of gold nanocrystals, we previously designed and synthesized a β-sheet-forming nonapeptide (RU006: Ac- AIAKAXKIA-NH2, X = L-2-naphthylalanine, Nal). A mixture of RU006 and HAuCl4 in water produced ultrathin gold nanoribbons with 50-100 nm wide, several nanometers high, and microns long.

OBJECTIVES

The main objective of this study is the control of the nanoribbon crystal growth by designing and synthesizing RU006 analogs containing an N-methyl-L-alanine residue.

METHODS

We report (i) the design and synthesis of four RU006 analogs in which an L-alanine (Ala) at four positions in the RU006 sequence (N-methylated RU006 analogs) is replaced with an N-methyl alanine, (ii) conformational and morphological analyses of the self-assembled Nmethylated RU006 analogs, (iii) gold nanocrystal synthesis by the peptide templating method with N-methylated RU006 analogs, and (iv) the roles of peptide self-assembly in anisotropic gold crystal growth.

RESULTS

RU006 with an N-methyl moiety at the center position resulted in flattened/platelet gold nanocrystals. It was also found that decreasing the mole fraction of RU006 in mixtures with Nmethylated RU006 analogs afforded significantly different absorption spectra compared to that obtained using RU006 alone under gold nanocrystal synthesis conditions.

CONCLUSION

We found that morphology of gdd nanocrystals is significantly affected by electron transfer from the naphthalene rings to HAuCl4, accompanied by cross-linking reactions between spatially adjacent naphthalene rings within the hydrophobic cavity of a template assembly.

Anisotropic Growth of Hydroxyapatite in Stretched Double Network Hydrogel

Bone tissues possess excellent mechanical properties such as compatibility between strength and flexibility and load bearing owing to the hybridization of organic/inorganic matters with anisotropic structure. To synthetically mimic such an anisotropic structure of natural organic/inorganic hybrid materials, we carried out hydroxyapatite (HAp) mineralization in stretched tough double network (DN) hydrogels. Anisotropic mineralization of HAp took place in stretched hydrogels, as revealed by high brightness synchrotron X-ray scattering and transmission electron microscopic observation. The c-axis of mineralized HAp aligned along the stretching direction, and the orientation degree S calculated from scattering profiles increased with increasing in the elongation ratio λ of the DN gel, and S at λ = 4 became comparable to that of rabbit tibial bones. The morphology of HAp polycrystal gradually changed from spherical to unidirectional rod-like shape with increased elongation ratio. A possible mechanism for the anisotropic mineralization is proposed, which would be one of the keys to develop mechanically anisotropic organic/inorganic hybrid materials.

Controlling Shape Anisotropy of ZnS-AgInS2 Solid Solution Nanoparticles for Improving Photocatalytic Activity

Independently controlling the shape anisotropy and chemical composition of multinary semiconductor particles is important for preparing highly efficient photocatalysts. In this study, we prepared ZnS-AgInS₂ solid solution ((AgIn)_xZn_2(1-x)S₂, ZAIS) nanoparticles with well-controlled anisotropic shapes, rod and rice shapes, by reacting corresponding metal acetates with a mixture of sulfur compounds with different reactivities, elemental sulfur, and 1,3-dibutylthiourea, via a two-step heating-up process. The chemical composition predominantly determined the energy gap of ZAIS particles: the fraction of Zn²⁺ in rod-shaped particles was tuned by the ratio of metal precursors used in the nanocrystal formation, while postpreparative Zn²⁺ doping was necessary to increase the Zn²⁺ fraction in the rice-shaped particles. The photocatalytic H₂ evolution rate with irradiation to ZAIS particles dispersed in an aqueous solution was significantly dependent on the chemical composition in the case of using photocatalyst particles with a constant morphology. In contrast, photocatalytic activity at the optimum ZAIS composition, x of 0.35-0.45, increased with particle morphology in the order of rice (size: ca. 9 × ca. 16 nm) < sphere (diameter: ca. 5.5 nm) < rod (size: 4.6 × 27 nm). The highest apparent quantum yield for photocatalytic H₂ evolution was 5.9% for rod-shaped ZAIS particles, being about two times larger than that obtained with spherical particles.