32- and 28-Macrocyclic polyammonium cations assisted pseudo two-dimensional assembly of citrate stabilized gold nanoparticles

Department of Chemistry, National Institute of Technology, Agartala-799 055, Tripura, India E-mail : t_k_misra@yahoo.com EYES Testing Laboratory, Yung Shin Pharma. Ind. Co. Ltd., Taichung, Taiwan Manuscript received online 16 July 2013, revised 26 September 2013, accepted 01 November 2014 Macrocyclic polyammonium cations (MCPAC), [32]ane-(NH2 +)8 .8Cl– (32-MCPAC), and [28]ane-(NH2 +)6O2 .6Cl– (28-MCPAC), which are highly water soluble, are well reputed for their anion recognition and metal nanoparticles (MNPs) stabilization properties. First time, we have employed them as cross-linkers for organizing negatively charged citratestabilized gold nanoparticles (Au-NPs) into a pseudo two-dimensional (2-D) array. Assembled Au-NPs were characterized by UV-Vis spectroscopy and Transmission Electronic Microscopy (TEM). The UV-Vis spectrum of citrate-stabilized AuNPs so prepared of diameter ~12 nm shows Surface Plasmon Resonance (SRP) at ca. 520 nm, indicating discrete nature of Au-NPs. The UV-Vis spectra of Au-NPs in the presence of 32-/28-MCPAC show red-shifted and broadened SRP maxima centered at ca. 580 nm for 32-MCPAC and at ca. 605 nm for 28-MCPAC which are attributed to the inter-particle plasmon absorbance, indicating large compact isotropic assembly of Au-NPs. The concrete evidence was obtained from the TEM images which reveal that the particles are arranged pseudo 2-D without any severe agglomeration. Photographs of all the solutions were taken and indicate how the color of almost discrete Au-NPs (wine-red color) changes to get assembled Au-NPs (bluish). The study of pH effect on assembling Au-NPs reveals that the electrostatic attraction followed by hydrogen bonding interaction between MCPAC and citrate ions leads to the formation of 2-D array of Au-NPs. In addition, at pH 12.30 when the cross-linking property of MCPACs was nullified, Au-NPs were formed rod-like aggregation. Assembled Au-NPs would be futuristic promising materials, especially for optoelectronic devices