Mohammad Siddiq

A shape memory hydrogel induced by the interactions between metal ions and phosphate

A novel ferric-phosphate induced shape memory (SM) hydrogel is prepared by the one-step copolymerization of isopropenyl phosphonic acid (IPPA) and acrylamide (AM) in the presence of a crosslinker polyethylene glycol diacrylate (PEGDA). Different from the traditional SM hydrogels, our SM hydrogel can be processed into various shapes as needed and recovers to its original form in ‘multiconditions’ such as in the presence of a reducing agent or in the presence of a competitive complexing agent. This unique feature is attributed to the fact that the oxidized ferric ions show a high complexation ability with phosphate groups of IPPA, which acts as a physical crosslinker to form the secondary networks within the hydrogels to induce the shape memory effect. The memory behavior was totally reversible, owing to Fe3+ that can be reduced to Fe2+ and extracted by the complexing agent. Particularly, the SM hydrogels exhibit controllable and good mechanical characteristics by introduction of the ferric ions, i.e., the elastic modulus can increase from 2 kPa to 70 kPa dramatically. Learning from biological systems, phosphate-metal ion based hydrogels could become an attractive candidate for various biomedical and environmental applications.

Silver nanoparticles containing hybrid polymer microgels with tunable surface plasmon resonance and catalytic activity

Multi-responsive poly(N-isoprpylacrlamide-methacrylic acid-acrylamide) [P(NIPAM-MAA-AAm)] copolymer microgel was prepared by free radical emulsion polymerization. Silver nanoparticles were fabricated inside the microgel network by in-situ reduction of silver nitrate. Swelling and deswelling behavior of the pure microgels was studied under various conditions of pH and temperature using dynamic light scattering. A red shift was observed in surface plasmon resonance wavelength of Ag nanoparticles with pH induced swelling of hybrid microgel. The catalytic activity of the hybrid system was investigated by monitoring the reduction of p-nitrophenol under different conditions of temperature and amount of catalysts. For this catalytic reaction a time delay of 8 to 10min was observed at room temperature, which was reduced to 2 min at high temperature due to swelling of microgels, which facilitated diffusion of reactants to catalyst surface and increased rate of reaction.

Synthesis, Characterization, and Silver Nanoparticles Fabrication in N-isopropylacrylamide-Based Polymer Microgels for Rapid Degradation of p-Nitrophenol

Multiresponsive poly(N-isopropylacrylamide-co-methacrylic acid) microgels were synthesized by precipitation polymerization in aqueous medium. Then silver-poly(N-isopropylacrylamide-co-methacrylic acid) hybrid microgels were prepared by in-situ reduction of silver ions. Formation of microgels was confirmed by Fourier transform infrared spectroscopic analysis. pH and temperature sensitivity of microgel was studied by dynamic light scattering. Hydrodynamic radius of microgels decreases with increase in temperature at pH 8.20 and show volume phase transition temperature around 45°C. At pH 2.65, hydrodynamic radius decreases with increase in temperatures upto 35°C but further increase in temperature causes aggregation and microgel becomes unstable due to increase of hydrophobicity. With increase in pH of medium, the hydrodynamic radius of microgels increases sigmoidally. Formation of silver nanoparticles inside microgel and pH dependence of surface plasmon resonance wavelength of the hybrid microgels were investigated by ultraviolet-visible spectroscopy. The value of surface plasmon resonance band and absorbance associated with surface plasmon resonance band increases with increases in pH of the medium. The apparent rate constant of reduction of p-nitrophenol was found to be linearly dependent on volume of hybrid microgels used as catalyst. The system has a potential to be used as effective catalyst for rapid degradation of industrial pollutant.

Effect of crosslinker feed content on catalaytic activity of silver nanoparticles fabricated in multiresponsive microgels

We investigated the effect of crosslinking density of poly(N-isopropyl acrylamide-co-acrylic acid) microgels on catalytic activity of silver nanoparticles fabricated hybrid microgels. Multiresponsive poly(N-isopropyl acrylamide-co-acrylic acid) microgels with 2, 4, 6 and 8 mole percentage of N,N-methylene-bis-acrylamide were synthesized by emulsion polymerization. These microgels were characterized by dynamic light scattering and were used as microreactors to synthesize silver nanoparticles. Hybrid system was characterized by ultraviolet-visible spectroscopy. The catalytic activity of hybrid microgels with different crosslinker content was compared by studying the reduction of pnitrophenol as a model reaction. Kinetics of reaction was monitored by spectrophotometry. The value of the apparent rate constant decreases from 0.568 to 0.313min−1, when content of crosslinker are increased from 2 to 8 mole percentage respectively. This decreases in value of apparent rate constant is due to increase in diffusional barrier offered by high crosslinking of polymer network at high mole percentages of N,N-methylene-bis-acrylamide.

Ce-/S-codoped TiO2/Sulfonated graphene for photocatalytic degradation of organic dyes

TiO2 is an abundant and environmentally benign material, but has a wide band gap, which greatly confines its applications in photocatalysis. Doping and modifying the material composition are both generally used to change and control the photocatalytic activity of semiconductors. Herein, we describe a method and resulting activity of depositing Ce-/S-codoped TiO2 nanoparticles (NPs) on water-soluble sulfonated graphene (SGE) sheets, which guarantees a direct contact and satisfactory electron transfer between the semiconductor and graphene. The Ce/S–TiO2 NPs are homogeneously fixed on the surface of SGE sheets with an average particle size of ∼7 nm. The resulting composite showed noticeable activity in photodegrading methyl orange (κ = 0.425 h−1). This improved performance can be attributed to the synergistic effects of Ce- and S-codoping toward TiO2 and the composite action between TiO2 NPs and SGE. This type of novel composite is expected to stimulate the development of doped and graphene-involved photocatalysts for addressing environmental problems.

Effect of molecular structure on the hydration of structurally related antidepressant drugs

Apparent molal volumes and adiabatic compressibilities of aqueous solutions of the amphiphilic cationic antidepressant drugs butriptyline and doxepin hydrochlorides have been determined from density and ultrasound velocity measurements in the temperature range 20–50°C. Critical concentrations for aggregation of these drugs were obtained from ultrasound velocity measurements. Negative deviations from the Debye–Hückel limiting law of the apparent molal volume were obtained from both drugs in all temperature ranges, except for doxepin at 50°C, which provides evidence of no pre-association at concentrations below the critical concentration. Apparent molal adiabatic compressibilities of the aggregates formed by these drugs were typical of those corresponding for an aggregate formed by a stacking process.

Synthesis, characterization and modification of Gum Arabic microgels for hemocompatibility and antimicrobial studies.

Gum Arabic (GA) microgels were successfully prepared via reverse micellization method with high yield (78.5±5.0%) in 5-100μm size range using divinyl sulfone (DVS) as a crosslinker. The GA microgels were degraded hydrolytically 22.8±3.5% at pH 1 in 20days, whereas no degradation was observed at pH 7.4 and pH 9 at 37°C. By using diethylenetriamine (DETA), and taurine (TA) as chemical modifying agents, GA microgels were chemically modified as GA-DETA and GA-TA, and the zeta potential values of 5.2±4.1 and -24.8±1.3mV were measured, respectively in comparison to -27.3±4.2mV for GA. Moreover, blood compatibility of GA, GA-TA, and GA-DETA microgels was tested via in vitro protein adsorption, % hemolysis ratio, and blood clotting index. All the microgels were hemocompatible with% hemolysis ratio between 0.23 to 2.05, and the GA microgels were found to be highly compatible with a blood clotting index of 81±40. The biocompatibility of GA, GA-DETA and GA-Taurine microgels against L929 fibroblast cells also revealed 84.4, 89.1, and 67.0% cell viability, respectively, at 25.0μg/mL concentration, suggesting great potential in vivo biomedical applications up to this concentration. In addition, 5 and 10mg/mL minimum inhibition concentrations of protonated GA-DETA microgels (GA-DETA-HCl) were determined against E. coli and S. aureus, respectively.

Probing the micellar properties of Quinacrine 2HCl and its binding with surfactants and Human Serum Albumin

This manuscript reports physicochemical behavior of an antimalarial drug Quinacrine 2HCl (QUN) drug as well as its interaction with surfactant and Human Serum Albumin (HSA). Surface tension and specific conductivity were employed to detect the critical micelle concentration (CMC) and thus its surface and thermodynamic parameters were calculated. Solublization of this drug within micelles of anionic surfactant sodium dodecyl sulfate (SDS) has also been studied. UV/Visible spectroscopy was used to calculate partition coefficient (Kx), free energy of partition and number of drug molecules per micelle. The complexation of drug with HSA at physiological conditions (pH 7.4) has been analyzed by using UV/Visible and fluorescence spectroscopy. In this way the values of drug-protein binding constant, number of binding sites and free energy of binding were calculated.

Temperature-Responsive Poly(N-Isopropylacrylamide-Acrylamide-Phenylboronic Acid) Microgels for Stabilization of Silver Nanoparticles

Poly(N-isopropylacrylamide-acrylamide-phenylboronic acid) [P(NIPAM-AAm-PBA)] microgels of uniform size were prepared by the chemical reaction of 3-aminophenylboronic acid with poly(N-isopropylacrylamide-acrylamide-acrylic acid) [P(NIPAM-AAm-AA)] microgels in aqueous medium in the presence of N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride catalyst via carbodiimide coupling. Silver (Ag) nanoparticles were prepared using seed-mediated growth method and stabilized in P(NIPAM-AAm-PBA)] microgels. Ag nanoparticles and hybrid microgels were characterized by transmission electron microscopy, UV–visible, and dynamic light scattering techniques. The temperature-responsive behavior of hybrid microgels was found to be similar to that of the pure microgels. The value of volume transition temperature of hybrid microgels was found to be slightly higher than that of pure microgels due to shielding effect of Ag nanoparticles present on the surface of microgel particle. The decrease in the size of hybrid microgels as compared to that of pure microgels in swollen state is due to physical cross-linking by Ag nanoparticles inside the network of microgels. The stable hybrid polymer microgel system has a potential to be used for different applications.

Intermolecular Interactions of Polymer/Surfactants Mixture in Aqueous Solution Investigated by Various Techniques

The interaction between diblock copolymer with ionic surfactants in aqueous solution were studied employing surface tensiometery, electrical conductivity, steady-state fluorescence spectroscopy and dynamic laser light scattering at 303 K. Surface tension measurements were used to determine critical micelle concentration (CMC) and, thereby, its free energy of micellization (ΔGm), free energy of adsorption (ΔGads), surface excess concentration (Γ), and minimum area per molecule (A). The negative values of (ΔGm) both in case of SDS and CTAB confirmed the spontaneity of the processes. Conductivity measurements were used to determine critical micelle concentration (CMC), critical aggregation concentration (CAC), polymer saturation point (PSP), degree of ionization (α), and counterion binding (β). Addition of polymers could effectively reduce the CMC value of surfactants and, thus increase the detergency. Dynamic light scattering experiments were performed to check the changes in physiochemical properties of the block copolymer micelles take place due to the interactions of diblock copolymer with ionic surfactants. The ratio of (I1/I3) indicates the polarity of the pyrene micro environment and confirms the detection of micelle as well as polymer-surfactant interactions. Aggregation number (N), number of binding sites (n), and free energy of binding (ΔGb) for pure surfactants as well as for polymer-surfactant mixed micellar systems were determined by fluorescence quenching method.