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M. Franckevicius / Medical Physics in the Baltic States 7 (2009) 34 - 38 applications [12, 14]. Suppose dendrimers and silver nanoparticles can compose fluorescent, biocompatible and highly photostable dendrimer nanocomposite that may be used for cell labelling [15]. In this work optical properties of liquid crystalline (LC) poly(propylene-imine) (PPI) dendrimers of fourth generation with encapsulated silver nanoparticles were investigated for medical applications. 2. Experiment Liquid crystalline PPI dendrimer of fourth generation with –OC5H11 terminal mesogenics units and Ag silver nanoparticles were used. Synthesis of liquid crystalline PPI dendrimers were described in detail in ref. [16] by condensation of 4-(40-ethoxybenzoyloxy) salicylaldehyde with the terminal amino groups of the G = 4 of poly(propylene imine) (PPI-(NH2)32) using a method previously described [17]. For small silver nanoparticles preparation briefly 200 mL 0.1 mM AgNO3 solution was reduced with 2 mL 0.5 mM NaBH4 in the presence of tri-sodium citrate (0.3 mM) under gentle stirring. Immediately after injection of the reducing agent, 2 mL of a 5 wt% aqueous solution of PVP was added [18]. Incorporation of silver nanoparticles in dendrimers was involved by using concentrations of the same molar ratios of both materials in chloroform solution. Firstly liquid crystalline dendrimers were dissolved in chlorophorm solution. Aqueous solution of silver nanoparticles was evaporated and then nanoparticles were mixed with dendrimer chloroform solution. Encapsulated LC PPI dendrimers with silver nanoparticles were characterized by optical absorption spectroscopy. Optical spectra were recorded by doublebeam spectrophotometer Perkin Elmer Lambda 19. More information of purity of dendrimers was described in ref. [16]. Structural and optical characteristics of used silver nanoparticles were described by Bastys et all. [18]. 3. Results Structure of liquid crystalline PPI dendrimers of fourth generation is presented in Fig. 1. Also silver nanoparticles a few nanometer in size in the liquid crystalline PPI dendrimer were encapsulated. 35 Fig.1. Fourth-generation PPI-[C2]32 liquid crystalline dendrimer Figure 2 shows modeled liquid crystalline PPI dendrimer like-structure consisting of four terminal flexible chains on periphery were demonstrating empty spaces inside branching units. In this empty spaces could be hosted silver nanoparticles. Calculations of the same dendrimers structures composed of six and eight terminal chains show occupation of empty spaces are 1 to 3nm 2 in square. Fig.2. The calculated periphery of the PPI dendrimer structures composed of 4 terminal flexible chains displaying empty spaces between chains These empty spaces can be used as containers or holders for silver nanoparticles or drugs. According calculations size of encapsulated silver nanoparticles in to dendrimer couldn’t be bigger than these calculated sizes. Figure 3 shows transmission electron microscope images (TEM) of silver nanoparticles. Background of the image shows initial nanoparticles, whose are used for incorporation into liquid crystalline PPI dendrimer.
M. Franckevicius / Medical Physics in the Baltic States 7 (2009) 34 - 38 Fig.3. TEM image of silver nanoparticles in size ~2-3nm and ~100nm. The steady state normalized absorption spectrum of pure LC PPI G4 dendrimer in chloroform solution presented in fig. 4, where three absorption bands at 260 nm, 315 nm and 375 nm were recorded. Absorbance 2.5 2.0 1.5 1.0 0.5 0.0 LC PPI G4 250 300 350 400 450 500 Wavelength, nm Fig.4. Absorption spectra of liquid crystalline PPI G4 dendrimer First two absorption bands situated in lower wavelengths interval are related with dendrimer central core and interior. Therefore low intensity absorption band near 375nm is related with dendrimers periphery 36 More accurate analysis of these LC dendrimers of various generations in different concentrations in chloroform was described in our previous work [19]. Absorption band dislocated at longer wavelengths expect major advertence, because flexible liquid crystalline periphery of dendrimer could interact with silver nanoparticles. Bands maximum position of liquid crystalline dendrimers depends on generation and corresponded functional units. By increasing dendrimer size, extinction maximum position moves to longer wavelengths, while for first two bands they are almost stationar in the same wavelength position in UV region. Band shift also depends and from length of used mesogenics units on periphery. When mesogens are bigger, shift dependence from generation is wider. Therefore for these studies we have used family of dendrimers with bigger mesigenic units. Optical spectra changes corresponded with terminal units and could tell us about possible interactions between dendrimer and nanoparticle. In fig. 5a and fig. 5b there are shown of silver nanoparticles plasmon resonance band in aqueous and chloroform solutions, and absorption spectra of liquid crystalline PPI dendrimer encapsulated with silver nanoparticles in fig. 5c. It’s known, that the dipole absorption maximum of the silver nanoparticle rapidly shifts to longer wavelengths by increasing particle size [9] and indistinctly by the used environment. Also absorption bands markedly changes depending on environment. In our case intensity of absorption bands of these Ag nanoparticles differs for two different media. In chloroform solution, absorption maximum of silver nanoparticles decreases in about 10 times comparing with that in aqueous solution. Therefore position of bands for these different solvents was almost nonchanged. Similar results of silver nanoparticles were obtained and in distilled water, acetone and ethanol solutions by Tilaki et. all. [10]. In figure 5c plotted normalized absorption spectra of liquid crystalline dendrimer interacting with silver nanoparticles. In comparison with below plotted spectra, absorption maximum is shifted to towards longer wavelength region about 20 nm. This shift of absorption band emerges from dendrimers interaction with silver nanoparticle.
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