Liviu Sacarescu

Green synthesis of the silver nanoparticles mediated by pullulan and 6-carboxypullulan

Unoxidized and carboxylated pullulan (obtained by pullulan oxidation using TEMPO-sodium hypochlorite-sodium bromide system) have been used as mediators for the silver nanoparticles formation (AgNPs), under environment-friendly conditions: using aqueous solutions, room temperature and notably, by using both mediators as reducing and stabilizing agents. The formation of AgNPs was first screened by measuring the surface plasmon resonance peak in the range of 380-440 nm using UV-vis spectroscopy. The morphology of the synthesized silver nanoparticles was determined by TEM, which indicated that the AgNPs differ on shape and thickness of the polymer shell by varying the silver nitrate concentration, different size and shape of AgNPs was achieved. The presence of elemental silver and the crystalline structure of the AgNPs were confirmed by EDX and XRD analyses. Moreover, the possible functional groups of pullulan (oxidized pullulan) responsible for the reduction and stabilization of AgNPs were evaluated using FT-IR. The results showed that both, pullulan and 6-carboxypullulan could be successfully used as reducing as well as capping agents for the AgNPs synthesis which shows potential antimicrobial activity against Gram positive and Gram negative bacteria.

Si-H functional polysilanes via a homogeneous reductive coupling reaction

A polysilane copolymer with reactive Si-H side groups was obtained through a homogeeous coupling reaction of dichlorodiphenylsilane with dichloromethylsilane. The reaction was carried out in a tetrahydrofuran (THF) solution of a sodium-potasium alloy complex with 18-crown-6 with a well defined composition of alkali metal ion pairs (Mt + /crown ether, Mt ― ) at -75°C. The product was characterized using 1 H NMR, 13 C NMR, FT-IR and UV/Visible spectroscopies and gel permeation chromatography. The result were compared with those obtained by the heterogeneous coupling reaction of the same monomers.

Synthesis, characterization and solution behaviour of oxidized pullulan

Various amounts of carboxyl groups were introduced at C-6 of the non-ionic, water soluble polysaccharide, i.e. pullulan, by applying the well-established TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), sodium hypochlorite/sodium bromide oxidation protocol, varying the reaction time. The oxidized products, more water-soluble than pullulan, were further characterized by FT-IR, (1)H NMR and (13)C NMR techniques, in order to assess the degree of oxidation. The absolute molecular weight measurements performed using a multiangle laser light scattering molecular weight analyzer, reveals a sharp drop of the molecular weight of the samples oxidized for longer reaction times. The second virial coefficients (A2), increased from unoxidized pullulan to the oxidized samples. Dynamic light scattering (DLS) measurements provide zeta potentials and hydrodynamic radius for all studied samples. The viscosity of the initial and oxidized pullulan dilute aqueous solutions was studied in detail. All oxidized samples except the highest oxidized pullulan sample (OxPu8) showed strong polyelectrolyte behaviour, whereas this effect is less pronounced for OxPu8 due to the high degradation of the chains. The intrinsic viscosity and the interaction parameter were determined at 25 °C as a function of solvent ionic strength according to Wolf approach. The dependence of these parameters on the salt concentration follows Boltzmann sigmoid model.

Synthesis of polysilane–bis(salicyliden)ethylenediamine Ni(II) complex

Abstract This paper describes the synthesis of a new coordination polymer prepared through the polycondensation reaction of α,ω-bis(chloromethyl)-polymethylphenylsilane with the Ni(II) complex of bis(salicylidene)ethylenediamine (salen). For this purpose, a new preparation method of the chloromethylated polysilane was presented along with the synthesis and characterisation of the polymer–metal complex. IR, 1 H-NMR and UV–Vis spectral analyses as well as GPC and TGA were used to investigate the new chemical structures.

Nanosized spinel ferrites synthesized by sol-gel autocombustion for optimized removal of azo dye from aqueous solution

Nanosized spinel ferrites MFe2O4 (M = Ni, Co, and Zn) have been prepared by sol-gel autocombustion method using citric acid as a fuel agent. The materials are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). The spinel ferrites have been applied for Congo-Red (CR) dye adsorption using batch technique. Different kinetic and equilibrium models have been fitted by nonlinear regression to analyze the adsorption data. In accordance with Langmuir isotherm, the maximum adsorption capacity at 293 K is 14.06mg/g for CoFe2O4 and 17.13 mg/g for NiFe2O4. The values of mean free energy determined from Dubinin-Radushkevich isotherm are higher than 8 (kJ mol-1), indicating a chemisorption mechanism. Based on the calculated thermodynamic parameters (free energy, enthalpy, and entropy) the adsorption of CR onto ferrites is a spontaneous and endothermic process. Response surface methodology has been applied to construct the multiple regression models for prediction of the adsorption capacity and removal efficiency. The model-based optimization has been performed using genetic algorithms and desirability function approach. The single-objective optimization has yielded a maximum value of color removal efficiency of 98.995%, using NiFe2O4 adsorbent. The multiobjective optimization has resulted in the improvement of both removal efficiency and adsorption capacity.

Study of the nanostructuration capacity of some azopolymers with rigid or flexible chains

Azobenzene polymers have been the subject of intensive research due to their unique and unexpected properties that allow various applications triggered by light. One of their attractive features is the possibility of changing the orientation of azobenzene chain through trans-cis-trans photoisomerization cycles to photoinduce birefringerence and linear dichroism in thin polymer films. The photochromic behavior and surface structuring capacity of rigid or flexible polymers having side azobenzene units have been investigated. The polyimides with side azobenzene units have rigid structure, while polysiloxane chains modified with azobenzene groups and nucleobases are considered materials with flexible structure. During the photochromic studies of the investigated polymers, a mechanism concerning the possibility to generate a fluid phase under UV/VIS irradiation was proposed. The photoisomerisation kinetic by UV-Vis irradiation in the solid state compared with the solution and the cis-trans azobenzene relaxation phenomena were presented. The surface structuring capacity of these polymers was studied using a Nd:YAG laser at 355 nm, at different incident fluencies and pulse numbers.

Hybrid Nanostructures Containing Sulfadiazine Modified Chitosan as Antimicrobial Drug Carriers

Chitosan (CH) nanofibrous structures containing sulfadiazine (SDZ) or sulfadiazine modified chitosan (SCH) in the form of functional nanoparticles attached to nanofibers (hybrid nanostructures) were obtained by mono-axial and coaxial electrospinning. The mono-axial design consisted of a SDZ/CH mixture solution fed through a single nozzle while the coaxial design consisted of SCH and CH solutions separately supplied to the inner and outer nozzle (or in reverse order). The CH ability to form nanofibers assured the formation of a nanofiber mesh, while SDZ and SCH, both in form of suspensions in the electrospun solution, assured the formation of active nanoparticles which remained attached to the CH nanofiber mesh after the electrospinning process. The obtained nanostructures were morphologically characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SDZ release profiles and kinetics were analyzed. The SDZ or SCH nanoparticles loosely attached at the surface of the nanofibers, provide a burst release in the first 20 min, which is important to stop the possible initial infection in a wound, while the SDZ and SCH from the nanoparticles which are better confined (or even encapsulated) into the CH nanofibers would be slowly released with the erosion/disruption of the CH nanofiber mesh.

Polyhydrosilanes I. Synthesis

Abstract Using the well known Wurtz coupling technique, polyhydrosilanes copolymers with diphenyl - and methyl(phenyl) - sequences, were synthesised. This work describes the possibility to maintain stable Si-H group and to obtain in good yields, soluble, high molecular, functional polysilanes. IR, 1-NMR, 13C, UV spectral data, quantitative elemental Si and H (from Si-H) analyses and indirect molecular weight investigation sustain the molecular structure of the copolysilanes.

Photoreductive generation of amorphous bismuth nanoparticles using polysaccharides – Bismuth–cellulose nanocomposites

A simple and highly reproducible synthesis of amorphous bismuth nanoparticles incorporated into a polysaccharide matrix using a photoreduction process is presented. As precursor for the generation of the Bi nanoparticles, organosoluble triphenylbismuth is used. The precursor is dissolved in toluene and mixed with a hydrophobic organosoluble polysaccharide, namely trimethylsilyl cellulose (TMSC) with high DSSi. The solution is subjected to UV exposure, which induces the homolytic cleavage of the bismuth-carbon bond in BiPh3 resulting in the formation of Bi(0) and phenyl radicals. The aggregation of the Bi atoms can be controlled in the TMSC matrix and yields nanoparticles of around 20 nm size as proven by TEM. The phenyl radicals undergo recombination to form small organic molecules like benzene and biphenyl, which can be removed from the nanocomposite after lyophilization and exposure to high vacuum. Finally, the TMSC matrix is converted to cellulose after exposure to HCl vapors, which remove the trimethylsilyl groups from the TMSC derivative. Although TMSC is converted to cellulose, the formed TMS-OH is not leaving the nanocomposite but reacts instead with surface oxide layer of the Bi nanoparticles to form silylated Bi nanoparticles as proven by TEM/EDX.

Photosensitive crown ether-siloxane copolymers bearing azobenzene chromophores

This paper presents the synthesis procedure of new photosensitive siloxane–crown ether copolymers using the polycondensation reaction of an oligosiloxane with COCl end groups and bisazo derivatives of dibenzo-18-crown-6 polyether. The azodiphenol compounds were prepared via the coupling reaction of 4,4 0 -diamine-DB18C6 diazonium salt with phenol or 1-naphthol. The polymers were characterized by IR absorption and 1 H-NMR spectra as well as by thermal analysis and GPC. A study of the photoisomerization process as well as the thermal relaxation of azoaromatic chromophores in the polysiloxane–crown ether copolymer in dimethyl sulfoxide solution and of the corresponding complexes with KSCN was also presented. 2002 Elsevier Science Ltd. All rights reserved.