Stephan T. Dubas

Green synthesis of silver nanoparticles for ammonia sensing.

Silver nanoparticles synthesized by a reagent less method involving only UV radiation have been used in colorimetric assay for the detection of ammonia in solution. The silver nanoparticles were synthesized by the exposure of a silver nitrate solution to a low-power UV source in the presence of poly(methacrylic acid) (PMA), which acted both as reducing and capping agent. The synthesis of the silver nanoparticles was studied by monitoring the changes in position and amplitude of the localized plasmon resonance (LSPR) band using UV-vis spectroscopy. The morphology of the particles was studied using transmission electron microscopy which confirmed the formation of spherical particles with an average particle size around 8 nm. Interestingly, the silver nanoparticles solution was found to display a strong color shift from purple to yellow upon mixing with increasing concentration of ammonia ranging from 5 to 100 ppm. Hence, the nanoparticles prepared with this method could be used as colorimetric assay for sensing applications of ammonia in water.

Increased dispersion and solubility of carbon nanotubes noncovalently modified by the polysaccharide biopolymer, chitosan: MD simulations

Abstract In order to explain the solubility of carbon nanotubes (CNT), including single walled CNTs, wrapped with chitosan of a 60% degree of deacetylation, MD simulations were applied to represent three chitosan concentrations, using two pristine CNTs (pCNT–pCNT), and one and two CNTs wrapped (pCNT–cwCNT and cwCNT–cwCNT). The CNT aggregation was observed in pCNT–pCNT and pCNT–cwCNT due to van der Waals interactions between tube–tube aromatic rings, and inter-CNT bridging by chitosan, respectively. At higher chitosan concentrations, such that most to all of CNTs were wrapped with chitosan, charge–charge repulsion was found to separate robustly the cwCNTs and lead to a well dispersed solution.

Deposition of Polyelectrolyte Multilayers to Improve the Color Fastness of Silk

The color fastness to washing of silk fibers dyed with scarlet dye was improved by the deposition of a thin polymer film which prevented desorption of the dye in standard soap solution. Dyed silk fibers were coated with polyelectrolytes multilayers (PEM) by following the so-called layer-by-layer deposition method in which up to 30 layers of cationic poly(diallyldimethylammonium chloride) and anionic poly(sodium 4-styrene sulfonate) were deposited. The growth of the film was confirmed using attenuated total reflectance Fourier transform infrared spectrophotometry and the dye exhaustion from the silk fiber was monitored using UV-Vis spectroscopy. The ability of the PEM to act as an electrostatic barrier against the release of the negatively charged dye was investigated as a function of the terminating top layer. The results showed that the negative layer-terminated PEM is more efficient at preventing the release of the dye than the positively terminated PEM.

Loading of curcumin in polyelectrolyte multilayers.

Polyelectrolyte multilayer (PEM) thin films prepared using the layer-by-layer technique are proposed as a matrix for the immobilization of 1,7-bis-(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-2,5-dione (curcumin), a lipophilic model drug. The PEM assembly was based on the layer-by-layer deposition of cationic poly(diallyldimethyl-ammonium chloride) (PDADMAC) and anionic poly(4-styrene sulfonate, sodium salt) (PSS) onto a quartz slide. Curcumin was loaded by dipping the PEM film into a dilute solution of curcumin dispersed in an 80/20% v/v water/ethanol solution. Within a few minutes, the film turned bright yellow as a result of the curcumin loading. The effect of the solvent composition, curcumin concentration and film thickness on the final concentration of curcumin in the PEM films was measured by UV-vis spectroscopy. The loading of curcumin was driven by its partitioning in the PEM film, and its partitioning coefficient between the 80/20 solvent and the PEM thin film was found to have a value of 2.07 x 10(5). The extinction coefficient of curcumin loaded into PEM was calculated to 64,000 M(-1) cm(-1). Results show that the loading of curcumin into the PEM films increased with the number of deposited layers, implying that curcumin partitioned into the bulk of the thin film. The maximum curcumin dose in the PEM film was measured by exposing films of various thicknesses to a high concentration (0.01% w/v) of curcumin and recording the maximum absorbance after saturation. The films thicknesses were controlled by the number of deposited PDADMAC/PSS layers (10, 20, 30, 40, 50, and 60). Results show that increasing amounts of curcumin could be loaded into the film with an increasing number of layers and up to 8 microg/cm(2) of curcumin could be loaded into a 20-layer film. These results demonstrate that the loading of lipophilic curcumin in PEM thin films is done through a partitioning mechanism and that the PDADMAC/PSS film can be used as a loading matrix for lipophilic drugs.

Surface modification of polypropylene non-woven fibers with TiO2 nanoparticles via layer-by-layer self assembly method: Preparation and photocatalytic activity.

Polypropylene (PP) meltblown fibers were coated with titanium dioxide (TiO2) nanoparticles using layer-by-layer (LbL) deposition technique. The fibers were first modified with 3 layers of poly(4-styrenesulfonic acid) (PSS) and poly(diallyl-dimethylammonium chloride) (PDADMAC) to improve the anchoring of the TiO2 nanoparticle clusters. PDADMAC, which is positively charged, was then used as counter polyelectrolyte in tandem with anionic TiO2 nanoparticles to construct TiO2/PDADMAC bilayer in the LbL fashion. The number of deposited TiO2/PDADMAC layers was varied from 1 to 7 bilayer, and could be used to adjust TiO2 loading. The LbL technique showed higher TiO2 loading efficiency than the impregnation approach. The modified fibers were tested for their photocatalytic activity against a model dye, Methylene Blue (MB). Results showed that the TiO2 modified fibers exhibited excellent photocatalytic activity efficiency similar to that of TiO2 powder dispersed in solution. The deposition of TiO2 3 bilayer on the PP substrate was sufficient to produce nanocomposite fibers that could bleach the MB solution in less than 4hr. TiO2-LbL constructions also preserved TiO2 adhesion on substrate surface after 1cycle of photocatalytic test. Successive photocatalytic test showed decline in MB reduction rate with loss of TiO2 particles from the substrate outer surface. However, even in the third cycle, the TiO2 modified fibers are still moderately effective as it could remove more than 95% of MB after 8hr of treatment.

Layer-by-Layer Surface Modification of Fruits with Edible Nano-Coatings

A new method for the surface modification of the surface of fruits based on the Layer-by-Layer (LbL) deposition of polyelectrolyte multilayers (PEM) is presented. Mangoes fruits were coated by sequential dipping in solutions of either Poly(diallyldimethylammonium chloride), (PDADMAC) or Poly(styrene sulfonate sodium salt) (PSS). After the deposition of only few polyelectrolyte layers, the skin of the fruits was changed from hydrophobic to hydrophilic as shown by contact angles measurements decreasing from 90 to 10. Fourier transform infrared spectroscopy (FTIR) was used to confirm the deposition of the PEM coating on the fruit which was then used as a matrix to load curcumin as model compound. Ultraviolet Visible (UV-Vis) spectroscopy was used to evaluate the amount of a curcumin loaded on the fruit surface as a function of the thickness of the film by indirect leaching in ethanol. This coating method provides a new approach to dope active compound on the fruit surface such as anti-oxidant, fragrance, color and other nutriment which could increase the market value of fruits.

Dispersion of Multiwalled Carbon Nanotubes with Water-Soluble Polyaniline Blend Poly(Sodium 4-Styrenesulfonate)

Water-soluble polyaniline blend poly(sodium 4-styrenesulfonate), (PANI.PSS) was used to disperse multiwalled carbon nanotubes (MWCNTs) by noncovalent surface modification. The anionic of PANI.PSS solution was prepared by interfacial polymerization of aniline monomer in the presence of PSS as the blending reagent to provide water solubility. The optimal conditions to prepare stable modified MWCNTs/PANI.PSS aqueous dispersions are presented. The interaction of MWCNTs and PANI.PSS have been investigated and explained according to the result of UV-Vis spectroscopy and zeta potential analysis. The MWCNTs/PANI.PSS complex solution was further confirmed more evidence by Transmission electron microscopy (TEM). Results showed that the MWCNTs dispersed with PANI.PSS are highly dispersible in water, which open up the new possibilities for the fabrication of composite films.

Enhanced CO2 capturing over ultra-microporous carbon with nitrogen-active species prepared using one-step carbonization of polybenzoxazine for a sustainable environment

Nitrogen-enriched porous carbon has been a promising material for CO2 capture in the recent decades. To enhance the performance of CO2 adsorption, both an N-active site and the textural properties are crucial determinants. Herein, ultra-microporous carbon with N-active species was prepared using two synthesis procedures: 1) one-step carbonization of a polybenzoxazine (PBZ) precursor at 800 °C, and 2) the CO2 activation process at 900 °C. The activated porous carbon had the higher specific surface area (943 m2/g) and a total pore volume (0.51 cm3/g) compared to un-activated porous carbon (335 m2/g and 0.19 cm3/g, respectively). In addition, the presence of N-active species such as pyridine-N, secondary-N, pyridone-N, and oxide-N in the carbon structures could be clearly observed in the high-resolution XPS spectra. The CO2 adsorption measurement was performed at 30 and 50 °C under a wide range of pressures (1-7 bar). The maximum amount of CO2 uptake was ca. 3.59 mmol/g for the activated porous carbon operated at 30 °C and a CO2 pressure of 7 bar, which was due to the high specific surface area and the large micropore volume. Specifically, carbon with a 3D interconnected pore structure, derived from the sol-gel process of the PBZ precursor, exhibited good structural stability and consequently led to better absorption capability under the high atmospheric pressure of CO2. The enhanced CO2 adsorption capability for the as-prepared porous carbon was based on two mechanisms: physisorption as a result of textural properties and chemisorption as a result of the acid-base interaction between the basic N functionality and the acidic CO2 gas. All results suggested that ultra-microporous carbon with N-active species prepared from polybenzoxazine is a promising adsorbent for CO2 capture and storage, which can be used at a wide range of pressures and in many applications e.g. flue gas adsorption and natural gas production.

Tunable Silver Leaching from Polyelectrolyte Multilayers Thin Films

Silver nanoparticles were deposited on glass slides and surgical suture as antibacterial agent. The silver nanoparticles were prepared by chemical reduction with sodium borohydride and using a synthetic polyelectrolyte as capping agent. Poly (4-styrenesulfonic acid-co-maleic acid) sodium salt PSSMA was used to stabilize the silver nanoparticles and provide an anionic surface charge which then allowed the layer-by-layer deposition method with poly (dially dimethyl ammonium chloride) PDADMAC. Various concentration of capping agent were used to prepare the silver nanoparticles which were then deposited on glass slide and surgical suture. The layer-by-layer deposition of the nanoparticles was studied using UV-Vis spectroscopy by monitoring the intensity of the characteristic Plasmon band of the nanoparticles at 400nm. The leaching of the silver nanoparticles in buffered solutions of pH 3,7 and 9 was monitored by recording the decrease in absorbance of silver nanoparticles film as a function of time for each pH solutions and each capping concentrations. Finally, suture material coated with silver nanoparticles were tested for their antibacterial activity against Staphylococcus aureus and results showed that all coated sutures had more than 99% bacterial reduction. So these suture material could be applied to use in medical products for promoted wound healing and decreased bacterial colony leading to relieve inflammation of patient.

Carbaryl Detection by Gold Nanoparticles Electrodes for Fruit Application Model

A new method has been developed for the determination of carbaryl pesticides, , in water and fruit samples using gold nanoparticles electrode conductivity detection. The effects of pH on the binding between carbaryl and humic acid (HA) and number of layer coated electrode were investigated and optimized. HA modified the polyelectrolyte multilayer thin films (PEMs) coating electrode was used to detect carbaryl. The current was decreased 4.75% after dipping in HA for 10 minutes. The current was decreased around 7.28% from the initial value after HA binding with carbaryl. The results demonstrated that the newly developed method was an efficient procedure for the detection of carbamate pesticides in water.