Self-Assembled Nanostructures Formation in Hybrid Metal-Mesogenic Systems

This chapter reviews the methods of supramolecular chemistry and cryochemistry to study the formation, morphological and structural properties, and possible applications of hybrid nanostructures and nanosized aggregates comprising plasmonic metals (silver and copper) and several mesogenic compounds, which exhibit different liquid crystalline mesophases — nematics, smectics, and cholesterics. Low-temperature vacuum co-condensation of reagent vapors on cooled surfaces of quartz, KBr, CaF 2 , or polished copper was used to synthesize hybrid nanosystems including silver and copper and long-chain mesogenic derivatives of alkylcyanobiphenyls under molecular beam condi- tions. Controlled thermal treatment of the samples allowed the directed formation of metal nanoparticles of definite size from 2 up to 100 nm. It was shown that the procedures of temperature treatments and molecular self-organization of different liquid crystalline phases controlled the size and morphology of nanoparticles and their aggregates, which were formed in the system. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) data of the samples show the formation of orientationally ordered structures in nematic mesophases. Formation of flat 2D aggregates was found in layered smectic mesophases. Optical absorbance spectra of silver/4-pentyl-4-cyanobiphenyl (Ag/ 5 СВ ) and copper/4-pentyl-4-cyanobiphenyl (Cu/5CB) samples encapsulated in polymer poly-para-xylylene at 300 K contained characteristic bands of plasmonic absorbance of metal nanoparticles at 420 – 440 nm and 560 – 600 nm. Rising metal concentration in the sample led to the performable growth of rod-like metal particles with anisometric ratio l/d > 20 and intensive absorbance at higher wavelengths ( λ ≥ 650 nm). New hybrid nanosystems based on biomolecules of cholesterol or its heteroatomic analog thiocholesterol including nanosized silver particles of d = 5.0 (cid:1) 0.5 and d = 2.5 (cid:1) 0.5 nm, respectively, were obtained. Highly ordered 1D-, 2D-, and 3D structures containing silver nanoparticles were formed from concentrated organic sols at different support surfaces by removing the inert solvent from triple metal/ligand/solvent system and by cooling the binary metal/ligand system from isotropic state to the cholesteric liquid crystalline mesophase. The microstructure and composition of the hybrid nanosystems were characterized by FTIR, UV-Vis spectro- scopy, TEM, and selected area electron diffraction (SAED). It was shown that hybrid nanosystems based on silver nanoparticles covered by a stabilizing layer of mesogenic molecules of thiocholesterol display selectivity in adsorption of optical isomers of the selected compounds. definite morphology with biological molecules and blocks opens new perspectives for obtaining new bio-compatible instruments for imaging and sensing biological objects in living organisms and targeted medical therapy. The hybrid metal-mesogenic nanosystems obtained were also used as liquid crystalline component for modification of silica gel nanoparticles and production of chiral matrices for thin-film chromatography. The conditions that allowed the production of silica gel plates modified by chiral hybrid metal-mesogenic nanosystems from toluene sols were developed. The new chiral matrices for thin-film chromatography possessed enantioselectivity related to optical isomers of 2,2 0 -diamino-1,1 0 -binaphtol (DABN) and trifluoro antrenylethanol (TFAE). We have succeeded in selecting optical isomers of TFAE with selecting factor equal to 1.56. In the future, the hybrid silver-thiochlesterol systems obtained in this work can be used as chiral stationary phase also for high-performance liquid chromatography (HPLC). One can expect the higher values of selectivity effects due to the increasing pathway for selecting compounds in comparison with thin layer chromatography technique.