Experimental study of quantum dot and dash lasers

Quantum dashes are elongated quantum dots. Polarized edge-photovoltage and spontaneous emission spectroscopy are used to study the anisotropy of optical properties in 1.5μm InGaAsP and AlGaInAs-based quantum dash lasers. Strain, which causes TM-polarized transitions to be suppressed at the band edge, coupled with carrier confinement and dash shape leads to an enhancement of the optical properties for light polarized along the dash long axis, in excellent agreement with theoretical results. An analysis of the integrated facet and spontaneous emission rate with total current and temperature reveals that, in both undoped and p-doped InGaAsP-based quantum dash lasers at room temperature, the threshold current and its temperature dependence remain dominated by Auger recombination. We also identify two processes which can limit the output power and propose that the effects of the dopant in p-doped InGaAsP-based lasers dominate at low temperature but decrease with increasing temperature. A high threshold current density in undoped AlGaInAs-based quantum dash laser samples studied, which degrade rapidly at low temperature, is not due to intrinsic carrier recombination processes. 1.3μm GaAs-based quantum dots lasers have been widely studied, but there remains issues as to the nature of the electronic structure. Polarized edge-photovoltage spectroscopy is used to investigate the energy distribution and nature of the energy states in InAs/GaAs quantum dot material. A non-negligible TM-polarized transition, which is often neglected in calculations and analyses, is measured close to the main TE-polarized ground state transition. Theory is in very good agreement with the experimental results and indicates that the measured low-energy TM-polarized transition is due to the strong spatial overlap between the ground state electron and the light-hole component of a low-lying excited hole state. Further calculations suggest that the TM-polarized transition reduces at the band edge as the quantum dot aspect ratio decreases.

Design of a silicone reservoir intravaginal ring for the delivery of oxybutynin.

Oxybutynin, a drug of choice in the treatment of urinary incontinence, has low oral bioavailability due to extensive first-pass metabolism. A toxic metabolite, N-desethyloxybutynin, has been linked to adverse reactions to oral oxybutynin. This study, therefore, reports on the design of an oxybutynin intravaginal ring (IVR) of reservoir design, comprising an oxybutynin silicone elastomer core encased in a non-medicated silicone sheath, manufactured by reaction injection moulding at 50oC. An unusually high initial burst release of oxybutynin (42.7 mg in 24 h) was observed in vitro with a full length core (100 mg drug loading), with subsequent non-zero order drug release. Use of fractional segment cores substantially reduced the burst effect, yielding linear cumulative drug release versus time plots from days 2 to 14. Thus, a 1/8 fractional segment core gave a 24 h burst of 11.28 mg oxybutynin and, thereafter, zero order release at the target dose of 5 mg/day over 14 days. Two oxybutynin cores, each 1/16 of full length, gave a greater release than a single 1/8 core, due to core segment end effects resulting in an increased surface area for release. The burst release was investigated by determining drug solubilities in the propan-1-ol product of elastomer condensation cure (390 mg/ml) and in the elastomer itself (13.9-20.21 mg/ml, by direct extraction and indirect thermal methods). These high oxybutynin solubilities were considered the major contributors to the burst effect. It was concluded that use of a fractional segment core would allow development of a suitable oxybutynin reservoir IVR.

Intravaginal ring delivery of the reverse transcriptase inhibitor TMC 120 as an HIV microbicide

TMC 120 (Dapivirine) is a potent non-nucleoside reverse transcriptase inhibitor that is presently being developed as a vaginal HIV microbicide. To date, most vaginal microbicides under clinical investigation have been formulated as single-dose semi-solid gels, designed for application to the vagina before each act of intercourse. However, a clear rationale exists for providing long-term, controlled release of vaginal microbicides in order to afford continuous protection against heterosexually transmitted HIV infection and to improve user compliance. In this study we report on the incorporation of various pharmaceutical excipients into TMC 120 silicone, reservoir-type intravaginal rings (IVRs) in order to modify the controlled release characteristics of the microbicide. The results demonstrate that TMC 120 is released in zero-order fashion from the rings over a 28-day period and that release parameters could be modified by the inclusion of release-modifying excipients in the IVR. The hydrophobic liquid excipient isopropyl myristate had little effect on steady-state daily release rates, but did increase the magnitude and duration of burst release in proportion to excipient loading in the IVR. By comparison, the hydrophobic liquid poly(dimethylsiloxane) had little effect on TMC 120 release parameters. A hydrophilic excipient, lactose, had the surprising effect of decreasing TMC 120 burst release while increasing the apparent steady-state daily release in a concentration-dependent manner. Based on previous cell culture data and vaginal physiology, TMC120 is released from the various ring formulations in amounts potentially capable of maintaining a protective vaginal concentration. It is further predicted that the observed release rates may be maintained for at least a period of 1 year from a single ring device. TMC 120 release profiles and the mechanical properties of rings could be modified by the physicochemical nature of hydrophobic and hydrophilic excipients incorporated into the IVRs.

Steric effects in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol using an Ir/C catalyst

The liquid phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol has been carried out over a graphite-supported iridium catalyst. The effect of reaction parameters such as temperature, pressure, concentration of reactant, the effect of addition of product to the feed and pre-reduction of the catalyst were studied. In situ pre-reduction of the catalyst with hydrogen had a very significant enhancing effect on the conversion of cinnamaldehyde and selectivity of the catalyst to cinnamyl alcohol. Kinetic analysis of the pre-reduced catalyst showed that the reaction is zero order with respect to cinnamaldehyde and first order with respect to hydrogen. The reaction follows an Arrhenius behaviour with an activation energy of 37 kJ mol(-1). Detailed analysis of the reaction showed that hydrogenation of the C=C double bond to give hydrocinnamaldehyde predominantly occurred at low conversions of cinnamaldehyde (

Target Detection and Tracking With Heterogeneous Sensors

We present a multimodal detection and tracking algorithm for sensors composed of a camera mounted between two microphones. Target localization is performed on color-based change detection in the video modality and on time difference of arrival (TDOA) estimation between the two microphones in the audio modality. The TDOA is computed by multiband generalized cross correlation (GCC) analysis. The estimated directions of arrival are then postprocessed using a Riccati Kalman filter. The visual and audio estimates are finally integrated, at the likelihood level, into a particle filter (PF) that uses a zero-order motion model, and a weighted probabilistic data association (WPDA) scheme. We demonstrate that the Kalman filtering (KF) improves the accuracy of the audio source localization and that the WPDA helps to enhance the tracking performance of sensor fusion in reverberant scenarios. The combination of multiband GCC, KF, and WPDA within the particle filtering framework improves the performance of the algorithm in noisy scenarios. We also show how the proposed audiovisual tracker summarizes the observed scene by generating metadata that can be transmitted to other network nodes instead of transmitting the raw images and can be used for very low bit rate communication. Moreover, the generated metadata can also be used to detect and monitor events of interest.

Incorporation of novel 1-alkylcarbonyloxymethyl prodrugs of 5-fluorouracil into poly(lactide-co-glycolide) nanoparticles

Incorporation of 1-alkylcarbonyloxymethylprodrugs of 5FU into poly(lactide-co-glycolide) nanoparticles using nanoprecipitation methods gave increased loading efficiencies over that obtained using the parent drug substance. SEM studies revealed spherical nanoparticles of around 200 nm in diameter, corresponding well with measurements made using photon correlation spectroscopy. The C-7 prodrug gave the best mean loading of 47.23%, which compared favourably to 3.68% loading achieved with 5FU. Loading efficiency was seen to follow the hydrophilic-lipophilic balance in the homologue series, where increases in lipophilicities alone were not good predictors of loading. Drug release, in terms of resultant 5FU concentration, was monitored using a flow-through dissolution apparatus. Cumulative drug release from nanoparticles loaded with the C-5 prodrug was linear over 6h, with approximately 14% of the total available 5FU dose released and with no evidence of a burst effect. The flux profile of the C-5-loaded nanoparticles showed an initial peak in flux in the first sampling interval, but became linear for the remainder of the release phase. C-7-loaded nanoparticles released considerably less (4% in 6 h) with a similar flux pattern to that seen with the C-5 prodrug. The C-9-loaded nanoparticles released less than 1% of the available 5FU over 6 h, with a similar zero-order profile. The C7 prodrug was deemed to be the prodrug of choice, achieving the highest loadings and releasing 5FU, following hydrolysis, in a zero-order fashion over a period of at least 6 h. Given the lack of burst effect and steady-state flux conditions, this nanoparticulate formulation offers a better dosing strategy for sustained intravenous use when compared to that arising from nanoparticles made by direct incorporation of 5FU. (c) 2007 Elsevier B.V. All rights reserved.

Adsorption and photocatalysed destruction of cationic and anionic dyes on mesoporous titania films: Reactions at the air-solid interface

Cationic dyes, such as methylene blue (MB), Thionine (TH) and Basic Fuschin (BF), but not anionic dyes, such as Acid Orange 7 (AO7), Acid Blue 9 (AB9) and Acid Fuschin (AF), are readily adsorbed onto mesoporous titania films at high pH (pH 11), i.e. well above the pzc of titania (pH 6.5), due to electrostatic forces of attraction and repulsion, respectively. The same anionic dyes, but not the cationic dyes, are readily adsorbed on the same titania films at low pH (pH 3), i.e. well below titania's pzc. MB appears to adsorb on mesoporous titania films at pH 11 as the trimer (lambda(max) = 570 nm) but, upon drying, although the trimer still dominates, there is an absorption peak at 665 nm, especially notable at low [MB], which may be due to the monomer, but more likely MB J-aggregates. In contrast, the absorption spectrum of AO7 adsorbed onto the mesoporous titania film at low pH is very similar to the dye monomer. For both MB and AO7 the kinetics of adsorption are first order and yield high rate constants (3.71 and 1.481 g(-1) min(-1)), indicative of a strong adsorption process. Indeed, both MB and AO7 stained films retained much of their colour when left overnight in dye-free pH 11 and 3 solutions, respectively, indicating the strong nature of the adsorption. The kinetics of the photocatalytic bleaching of the MB-titania films at high pH are complex and not well-described by the Julson-Ollis kinetic model [A.J. Julson, D.F. Ollis, Appl. Catal. B. 65 (2006) 315]. Instead, there appears to be an initial fast but not simple demethylation step, followed by a zero-order bleaching and further demethylation steps. In contrast, the kinetics of photocatalytic bleaching of the AO7-titania film give a good fit to the Julson-Ollis kinetic model, yielding values for the various fitting parameters not too dissimilar to those reported for AO7 adsorbed on P25 titania powder. (C) 2008 Elsevier B.V. All rights reserved.

A kinetic study of the liquid-phase hydrogenation of citral on Au/TiO2 and Pt-Sn/TiO2 thin films in capillary microreactors

The kinetics of the liquid-phase hydrogenation of citral (3,7-dimethyl-2,6-octadienal) on Au/TiO2 and Pt-Sn/TiO2 thin films was studied in the temperature range 313-353 K and citral concentrations of 0.25-10.0 mol m(-3). The thin films were deposited onto the inner walls of silica capillaries with internal diameter of 250 mu m. First-order dependence on hydrogen pressure and near zero order dependence on citral concentration were observed for the initial rate of citral hydrogenation over the Pt-Sn/TiO2 and Au/TiO2 thin films. The Au/TiO2 catalyst prevents citronellal formation. The highest yield of unsaturated alcohols was obtained on the Pt-Sn/TiO2 film at a reaction temperature of 343 K, liquid residence time of 30 min and a citral conversion of 99%. (C) 2011 Elsevier B.V. All rights reserved.

The Kinetics of Semiconductor Photocatalysis in Activity-Indicator Films

The kinetics of dye reduction, in photocatalyst indicator ink films on self cleaning glass, is studied with respect to dye concentration. The water-based, photocatalyst indicator inks comprised a redox dye, D-ox, a sacrificial electron donor (glycerol) and a polymer, hydroxyethyl cellulose. The dyes used were: Resazurin (Rz), dichloroindo-phenol (DCIP) and methylene blue (MB), although the latter required acidification of the ink (0.01M HCl) to make it work effectively under ambient conditions. Under anaerobic conditions, the photoreduction of each of the dyes, in an otherwise identical ink formulation, on Activ (TM) self-cleaning glass is zero-order with respect to [D-ox]. Seven commercial samples of Rz, each in a typical ink formulation, were tested on the same piece of self-cleaning glass under aerobic conditions and produced a striking range (over 280%) of different apparent activities for the glass, when there should have been none. The underlying cause of this variation in assessed activity is shown to be due to the combination of a variation in the purity of the commercial samples and the zero-order nature of the kinetics of indicator dye reduction. The relevance of this work and the latter observation, in particular to future use of these films for the rapid assessment of the activities of new and established photocatalytic films, is briefly discussed.

Thick titanium dioxide films for semiconductor photocatalysis

Thick paste TiO2 films are prepared and tested for photocatalytic and photoinduced superhydrophilic (PSH) activity. The films are effective photocatalysts for the destruction of stearic acid using near or far UV and all the sol-gel films tested exhibited a quantum yield for this process of typically 0.15 %. These quantum yields are significantly greater (4-8-fold) than those for titania films produced by an APCVD technique, including the commercial self-cleaning glass product Activ(TM). The films are mechanically robust and optically clear and, as photocatalysts for stearic acid removal, are photochemically stable and reproducible. The kinetics of stearic acid photomineralisation are zero order with an activation energy of ca. 2.5 Kj mol(-1). All titania films tested, including those produced by APCVD, exhibit PSH. The light-induced fall, and dark recovery, in the water droplet contact angle made with titania paste films are similar in profile shape to those described by others for thin titania films produced by a traditional sol-gel route. (C) 2003 Elsevier Science B.V. All rights reserved.

Characterisation of the photocatalyst Pilkington Activ (TM): a reference film photocatalyst?

Pilkington Glass Activ(TM) represents a possible suitable successor to P25 TiO2, especially as a benchmark photocatalyst film for comparing other photocatalyst or PSH self-cleaning films. Activ(TM) is a glass product with a clear, colourless, effectively invisible, photocatalytic coating of titania that also exhibits PSH. Although not as active as a film of P25 TiO2, Activ(TM) vastly superior mechanical stability, very reproducible activity and widespread commercial availability makes it highly attractive as a reference photocatalytic film. The photocatalytic and photo-induced superhydrophilitic (PSH) properties of Activ(TM) are studied in some detail and the results reported. Thus, the kinetics of stearic acid destruction (a 104 electron process) are zero order over the stearic acid range 4-129 monolayers and exhibit formal quantum efficiencies (FQE) of 0.7 X 10(-5) and 10.2 x 10(-5) molecules per photon when irradiated with light of 365 +/- 20 and 254 nm, respectively; the latter appears also to be the quantum yield for Activ(TM) at 254 nm. The kinetics of stearic acid destruction exhibit Langmuir-Hinshelwood-like saturation type kinetics as a function of oxygen partial pressure, with no destruction occurring in the absence of oxygen and the rate of destruction appearing the same in air and oxygen atmospheres. Further kinetic work revealed a Langmuir adsorption type constant for oxygen of 0.45 +/- 0.16 kPa(-1) and an activation energy of 19 +/- 1 Kj mol(-1). A study of the PSH properties of Activ(TM) reveals a high water contact angle (67) before ultra-bandgap irradiation reduced to 0degrees after prolonged irradiation. The kinetics of PSH are similar to those reported by others for sol-gel films using a low level of UV light. The kinetics of contact angle recovery in the dark appear monophasic and different to the biphasic kinetics reported recently by others for sol-gel films [J. Phys. Chem. B 107 (2003) 1028]. Overall, Activ(TM) appears a very suitable reference material for semiconductor film photocatalysis. (C) 2003 Elsevier Science B.V All rights reserved.

KINETIC-STUDY OF PHOTOINDUCED ELECTRON-TRANSFER REACTIONS SENSITIZED BY COLLOIDAL CDS

The kinetics of the recovery of the photoinduced transient bleaching of colloidal CdS in the presence of different electron acceptors are examined. In the presence of the zwitterionic viologen, N,N'-dipropyl-2,2'-bipyridinium disulphonate, excitation of colloidal CdS at different flash intensities generates a series of decay profiles which are superimposed when normalized. The shape of the decay curves are as predicted by a first-order activation-controlled model for a log-normal distribution of particles sizes. In contrast, the variation in flash intensity in the presence of a second viologen, N,N'-dipropyl-4,4'-bipyridinium sulphonate, generates normalized decay traces which broaden with increasing flash intensity. This behaviour is predicted by a zero-order diffusion-controlled model for a log-normal distribution of particle radii. The photoreduction of a number of other oxidants sensitized by colloidal CdS is examined and the shape of the decay kinetics interpreted via either the first- or zero-order kinetics models. The rate constants and activation energies derived using these models are consistent with the values expected for an activation- or diffusion-controlled reaction.

Infection-Responsive Drug Delivery from Urinary Biomaterials Controlled by a Novel Kinetic and Thermodynamic Approach

Purpose. The pH-dependent physicochemical properties of the antimicrobial quinolone, nalidixic acid, were exploited to achieve ‘intelligent’ drug release from a potential urinary catheter coating, poly(2-hydroxyethylmethacrylate) (p(HEMA)), in direct response to the elevated pH which occurs at the onset of catheter infection.
Methods. p(HEMA) hydrogels, and reduced-hydrophilicity copolymers incorporating methyl methacrylate, were loaded with nalidixic acid by a novel, surface particulate localization method, and characterized in terms of pH-dependent drug release and microbiological activity against the common urease-producing urinary pathogen Proteus mirabilis.
Results. The pH-dependent release kinetics of surface-localized nalidixic acid were 50- and 10-fold faster at pH 9, representing the alkaline conditions induced by urease-producing urinary pathogens, compared to release at pH 5 and pH 7 respectively. Furthermore, microbiological activity against P. mirabilis was significantly enhanced after loading surface particulate nalidixic acid in comparison to p(HEMA) hydrogels conventionally loaded with dispersed drug. The more hydrophobic methyl methacrylate-containing copolymers also demonstrated this pH responsive behavior, but additionally exhibited a sustained period of zero-order release.
Conclusions. The paradigm presented here provides a system with latent, immediate infection-responsive drug release followed by prolonged zero-order antimicrobial delivery, and represents an ‘intelligent’, infection-responsive, self-sterilizing biomaterial.

Phenol degradation by powdered metal ion modified titanium dioxide photocatalysts

Conventional water purification and disinfection generally involve potentially hazardous substances, some of which known to be carcinogenic in nature. Titanium dioxide photocatalytic processes provide an effective route to destroy hazardous organic contaminants. This present work explores the possibility of the removal of organic pollutants (phenol) by the application of TiO2 based photocatalysts. The production of series of metal ions doped or undoped TiO2 were carried out via a sol–gel method and a wet impregnation method. Undoped TiO2 and Cu doped TiO2 showed considerable phenol degradation. The efficiency of photocatalytic reaction largely depends on the photocatalysts and the methods of preparation the photocatalysts. The doping of Fe, Mn, and humic acid at 1.0 M% via sol–gel methods were detrimental for phenol degradation. The inhibitory effect of initial phenol concentration on initial phenol degradation rate reveals that photocatalytic decomposition of phenol follows pseudo zero order reaction kinetics. A concentration of > 1 g/L TiO2 and Cu doped TiO2 is required for the effective degradation of 50 mg/L of phenol at neutral pH. The rise in OH- at a higher pH values provides more hydroxyl radicals which are beneficial of phenol degradation. However, the competition among phenoxide ion, Cl- and OH- for the limited number of reactive sites on TiO2 will be a negative influence in the generation of hydroxyl radical. The dependence of phenol degradation rate on the light intensity was observed, which also implies that direct sunlight can be a substitute for the UV lamps and that photocatalytic treatment of organic pollutants using this technique shows some promise.

Kinetics of methylene blue (MB) photocatalyzed reduction and dark regeneration in a colorimetric oxygen sensor

Performance data for a dye based, regenerable oxygen sensor (Mills and Lawrie [1], Mills et al. [2]) are analyzed to develop useful kinetic models for sensor photoactivation (dye reduction) and dark, oxygen detection (dye oxidation). The titania loaded, thin film sensor exhibits an apparent first order photoactivation of the dye, which we demonstrate (Section 3.2 and Fig. 4) is due to a kinetic disguise of a zero order photoreaction occurring through a non-uniformly illuminated sensor film. The observed zero order, slow recovery due to dye oxidation by dioxygen (O2 detection) appears best rationalized by a model assuming a near O2-impermeable skin developing on the sensor surface as solvent is evaporatively removed following sensor film casting and curing.