Drying of Polyacrylamide Composite Gels Formed with Various Kappa- Carrageenan Content

Drying of polyacrylamide (PAAm)-κ-carrageenan (κC) composite gels were monitored by using steady-state fluorescence technique. Disc shaped gels were formed from acrylamide (AAm) and N, N′- methylenebisacrylamide(Bis) with various κ- carrageenan (κC) contents by free radical crosslinking copolymerization in water. Pyranine (P) was doped as a fluorescence probe, and scattered light, I _sc , and fluorescence intensities, I, were monitored during drying of these gels. It is observed that fluorescence intensity of pyranine increased as drying time is increased for all samples. The increase in I was modeled using Stern- Volmer equation and diffusion with moving boundary. It is found that desorption coefficient, D decreased as κC contents were increased. Supporting gravimetrical and volumetric experiments were also carried out during drying of PAAm- κC composite gels.     

Swelling Kinetics of PAAm–κ-Carrageenan Composites: A Fluorescence Technique

The steady-state fluorescence (SSF) technique was introduced for studying swelling of disc-shaped polyacrylamide (PAAm) gels containing various amount of κ?carrageenan (κC). They were prepared by free-radical cross-linking copolymerization. N,N^′-methylenebisacrylamide (BIS) and ammonium persulfate (APS) were added as a cross-linker and an initiator, respectively. Composite gels were prepared at 80°C with pyranine as a fluorescence probe. After drying of these gels, swelling kinetics were performed in water at 60°C by real-time monitoring of the pyranine fluorescence intensity, I, which decreased as swelling proceeded. The Li–Tanaka equation was used to determine the swelling time constants, τ ₁, and cooperative diffusion coefficients, D ₀, from fluorescence intensity, weight, and volume variations of the gels during the swelling processes in all cases. It was observed that τ ₁ decreased and D ₀ increased as the κC concentrations in the composites were increased indicating that high κC gels swell faster than low κC gels.     

Small Molecule Sorption and Desorption in and Out of Iota‐Carrageenan Gels

Small molecule sorption and desorption in and out of Iota‐Carrageenan was studied by using steady‐state fluorescence (SSF) technique. Pyranine dissolved in water used as fluorescence probe. Fluorescence emission intensity, I _p from pyranine was monitored for studying sorption and desorption processes at various temperatures. The Fickian model was applied to produce sorption, D_s, early desorption, D_ed, and desorption, D_d, coefficients. Corresponding activation energies were obtained and found to be 20.5 kJ mol^?1, 7.0 kJ mol^?1 and 34.9 kJ mol^?1, respectively. The observed D_ed value is an order of magnitude smaller than the D_s and D_d coefficients. On the other hand, sorption processes were shown to be twice as fast as desorption processes.     

Effect of κ-carrageenan on volume phase transition for polyacrylamide (PAAm) hydrogel using the fluorescence technique

Steady-state fluorescence (SSF) technique was employed for studying swelling of polyacrylamide (PAAm) gels with various content of κ-carrageenan (κC). Disc shaped composite hydrogels were prepared by free-radical crosslinking copolymerization of acrylamide (AAm) with various amounts κC. N,N′-methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator, respectively. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine was monitored during in situ swelling processes of composite gels. It was observed that fluorescence intensity values decreased as swelling is proceeded. Li–Tanaka equation was used to determine the swelling time constants, τ and cooperative diffusion coefficients, D from intensity variations during the swelling processes. It was shown that swelling time constants, τ decreased and diffusion coefficients, D increased as the κC content in the composites are increased.     

Drying of heterogels swollen in organic vapor

Fast transient fluorescence technique (FTRF), which uses a Strobe Master System (SMS), is used to study swelling and drying of disc shape heterogels. Disc shape heterogels are prepared by free radical copolymerization (FCC) of methyl (methacrylate) (MMA) and styrene (S) with ethylene glycol dimethacrylate (EGDM). Pyrene (P_y) is introduced as a fluorescence probe during polymerization and lifetimes, τ, of P_y are measured during the in-situ swelling process. Chloroform was used as an organic vapor agent to induce gel swelling. It is observed that τ values decrease as swelling proceeds. The Li-Tanaka equation is used to determine the time constant, τ_c, and cooperative diffusion coefficients, D _c, for the swelling processes. Lifetimes of pyrene increase during drying. An empirical equation is introduced to determine the desorption coefficient, D, for drying. Heterogels with high S content swell and dry much more slowly than heterogels with low S content.     

Steady state fluorescence technique for studying phase transitions in PAAm–PNIPA mixture

The steady state fluorescence (SSF) technique was used to study the sol-gel phase transition during free radical crosslinking copolymerisation of various amounts of acrylamide (AAm) and N-isopropylacrylamide (NIPA) mixtures. N,N′-methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and an initiator, respectively. Pyranine (8-hydroxypyrene-1, 3, 6-trisulfonic acid, trisodium salt, HPTS) was added as a fluoroprobe for monitoring the polymerisation. It was observed that pyranine molecules bind to AAm and NIPA chains upon the initiation of the polymerisation, thus the fluorescence spectra of the bonded pyranines shift to the shorter wavelengths. Fluorescence spectra from the bonded pyranines allowed us to monitor the sol-gel phase transition, without disturbing the system mechanically, and to test the universality of the sol-gel transition as a function of polymer concentration ratios. Observations around the gel point of PAAm–PNIPA mixtures show that the gel fraction exponent β obeyed the percolation result.     

Film formation from PS latex doped PNIPAM hydrogels at various heating and cooling rates

Film formation from polystrene (PS) latex doped poly(N-isopropylacrylamide) (PNIPAM) hydrogels was studied by using photon transmission technique. The transmitted light intensity, I _tr was monitored during film formation process. Films were prepared by annealing, 10 wt% PS doped PNIPAM particles at five different heating and cooling rates at temperatures ranging from 10 to 100°C. I _tr presented a hysteresis loops during heating–cooling cycles, which were explained by void closure and void reconstruction processes. The corresponding activation energies were measured during reversible film formation process. Void closure and void reconstruction models were introduced to produce the activation energies.     

Cation Effect on Slow Release from Alginate Beads: A Fluorescence Study

In this study, spherical alginate beads containing pyranine (P _y ) as a fluorescence probe were prepared by ionotropic gelation of a sodium alginate solution. The steady state fluorescence technique was used to study pyranine release from the alginate beads crosslinked with calcium, barium and aluminum ions, respectively. The slow release of P _y was observed with the time drive mode of the spectrophotometer at 512 nm. Fluorescence emission intensity (I _p ) from P _y was monitored during the release process, and the encapsulation efficiency (EE) of pyranine from the alginate beads was calculated. The Fickian Diffusion model was used to measure the release coefficients, D _sl . It was seen that the slow release coefficients of pyranine from the alginate beads crosslinked with Ca²⁺, Ba²⁺, and Al³⁺ ions increased in the following order: D _sl (Al³⁺)> D _sl (Ca²⁺)> D _sl (Ba²⁺). In contrast, the initial amount of pyranine and EE into the beads showed the reverse behavior.     

The Role of Pyranine in Characterization of PAAm-κC Composites by Using Fluorescence Technique

Polyacrylamide (PAAm) doped by κ-carrageenan (κC) gels were prepared with various amounts of κC varying in the range between 0 wt.% and 3 wt.%. Steady-state fluorescence (SSF) technique was employed for studying sol-gel transition and swelling of PAAm-κC composite gels which were prepared by free-radical crosslinking copolymerization. Pyranine was introduced as a fluorescence probe. Pyranine molecules start to bind to acrylamide polymer chains upon the initiation of the polymerization, thus the spectra of the bonded pyranines shift to the shorter wavelengths. Fluorescence spectra from the bonded pyranines allow one to monitor the sol-gel transition and to test the universality of the sol-gel transition as a function of some kinetic parameters like polymer concentration. Observations around the gel point, t _c for PAAm-κC composite gels showed that the gel fraction exponent β obeyed the percolation result for low κC (<2.0 wt. %) however classical results were produced at higher κC (>2.0 wt.%). On the other hand, fluorescence intensity of pyranine was measured during in situ swelling process at various amounts of κC and it was observed that fluorescence intensity values decreased as swelling is proceeded. Li-Tanaka equation was used to determine the swelling time constants, τ and cooperative diffusion coefficients, D.     

Universality of sol--gel phase transition of κ-carrageenan in various salts: a steady state fluorescence study

Thermal phase transitions of κ-carrageenan in NaCl, KCl and CaCl₂ solutions were studied using steady state fluorescence (SSF) technique. Pyranine was introduced as a fluorescence probe for studying sol--gel phase transitions. Scattered light, I _sc and fluorescence intensity, I was monitored against temperature to determine the sol--gel (T _sg) phase transition temperatures. It was observed that T _sg values are strongly correlated to NaCl, KCl and CaCl₂ contents. The weight average degree of polymerization, DP _w and gel fraction G, exponents (γ and β) were measured and found to be in accord with the classical Flory--Stockmayer model, i.e., γ and β were found to be close to 1.0, independent of salt content.     

Monitoring the gelation of polyacrylamide–sodium alginate composite by fluorescence technique

Polyacrylamide (PAAm)–sodium alginate (SA) composite was prepared with different amounts of SA varying in the range between 0.06% and 2% (w/v). The PAAm–SA composite was characterized by the steady-state fluorescence technique. Pyranine was added as a fluoroprobe for monitoring the polymerization. It was observed that pyranine molecules bind to AAm and SA chains upon the initiation of the polymerization. Thus, the fluorescence spectra of the bonded pyranines shift to the shorter wavelengths. Fluorescence spectra from the bonded pyranines allowed us to monitor the sol–gel phase transition, and to test the universality of the sol–gel transition as a function of SA contents. Observations around the critical point show that the gel fraction exponent, β, and the weight average degree of polymerization exponent, γ, agreed with the percolation result for (<0.25% (w/v)) SA contents. However, classical results were produced at (<2% (w/v)) SA contents.     

Ethenyl indoles as neutral hydrophobic fluorescence probes

3‐(4‐Nitrophenylethenyl‐E)‐NH‐indole ( 1 ), 3‐(4‐nitrophenyl ethenyl‐E)‐N‐acetylindole ( 2 ), and 3‐(4‐nitrophenylethenyl‐E)‐N‐benzenesulfonylindole ( 3 ) are relatively less fluorescent in organic solvents, with fluorescence quantum yield (Φ_f) in the range of 0.002 to 0.066 depending on the solvent polarity. However, in bovine serum albumin (BSA)‐phosphate buffer, the fluorescence of these compounds gets drastically enhanced with Φ_f in the range of 0.21 to 0.67, depending on the substituent on the indolic nitrogen atom. Additionally, linear increase in the fluorescence intensity of 2 and 3 occurs on increasing the BSA concentration. These fluorescence properties together with the neutral, hydrophobic nature of these compounds make these fluorophores good fluorescence probe for studying the micropolarity of proteins like BSA and in general the ligand–protein interactions. Copyright © 2007 John Wiley & Sons, Ltd.     

TICT excited states of o‐ and p‐N,N‐dimethylamino analogues of GFP chromophore

Even though all the p‐N,N‐dimethylaminobenzonitrile (p‐DMABN), cis‐o‐DMABDI, and cis‐p‐DMABDI (the N,N‐dimethylamino analogues of green fluorescence protein chromophore) have the same electron‐donating N,N‐dimethylamino group, unlike the dual fluorescence of p‐DMABN, both cis‐o‐DMABDI and cis‐p‐DMABDI display single fluorescence. To figure out the interesting phenomena, the CAM‐TD‐B3LYP method and the cc‐pVDZ basis set were used to explore geometries, molecular orbitals, electronic transition, dipole moment, and potential energy surfaces of the S₁ excited states of cis‐o‐DMABDI and cis‐p‐DMABDI. We found that the S₁ excited states of cis‐o‐DMABDI and cis‐p‐DMABDI are ¹(π, π*) charge transfer excited states with twisted structures, where the N,N‐dimethylaminobenzene moiety functions as an electron donor, the methyleneimidazolone moiety serves as an electron acceptor, and the electron donor is linked with the electron acceptor by the C─C single bond (P‐bond). The fluorescent emissions of cis‐o‐DMABDI and cis‐p‐DMABDI predicted by the CAM‐TD‐B3LYP/cc‐pVDZ level are quite consistent with the experimental results. For the cis‐o‐DMABDI and cis‐p‐DMABDI, the S₁ locally excited state is less stable than the S₁ twisted intramolecular charge transfer state, and the S₁ LE state is not a stationary point (global minimum). That is why both cis‐o‐DMABDI and cis‐p‐DMABDI display single fluorescence.     

Turn-off Fluorescence Chemosensor for Iron with Bis(2-aminoethyl)-2-(9-fluorenyl)malonamide Functionlized SBA-15

An bis(2-aminoethyl)-2-(9-fluorenyl)malonamide as fluorophore ligand was immobilized onto mesoporous silica type SBA-15 via post synthesis grafting. The obtained material was characterized by small and wide angle X-ray diffraction, N₂ adsorption–desorption, Fourier transform infrared spectroscopy, Raman spectroscopy and thermogravimetric analysis that indicate the successful immobilization of the ligand on the surface of mesoporous silica. The sensing ability of the obtained material was studied by addition of the cations Fe³⁺, Mg²⁺, Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺ and Zn²⁺ to water suspensions of the assayed solid. Of all the cations tested addition of Fe³⁺ ion to a suspension of this material resulted in the largest decrease in the fluorescence intensity. Turn-off photoluminescence of this material was remarkably observed for iron ions in comparing of the other cations. A good linearity between the fluorescence intensity of this material and the concentration of Fe³⁺ ion is constructed, which enables it as a fluorescence chemosensor for detecting the Fe³⁺ ion with a suitable detection limit of 1.35?×?10^?5. It can be introduced as a novel fluorescent sensor in aqueous solution for a lot of practical applications in chemical, environmental and biological systems.     

Electroluminescence enhancement in ultraviolet organic light‐emitting diode with graded hole‐injection and ‐transporting structure

Electroluminescent intensity and external quantum efficiency (EQE) in ultraviolet organic light‐emitting diodes (UV OLEDs) have been remarkably enhanced by using a graded hole‐injection and ‐transporting (HIT) structure of MoO₃/N,N ′‐bis(naphthalen‐1‐yl)‐N,N ′‐bis(phenyl)‐benzidine/MoO₃/4,4′‐bis(carbazol‐9‐yl)biphenyl (CBP). The graded‐HIT based UV OLED shows superior short‐wavelength emis‐ sion with spectral peak of ～410 nm, maximum electroluminescent intensity of 2.2 mW/cm² at 215 mA/cm² and an EQE of 0.72% at 5.5 mA/cm². Impedance spectroscopy is employed to clarify the enhanced hole‐injection and ‐transporting capacity of the graded‐HIT structure. Our results provide a simple and effective approach for constructing efficient UV OLEDs. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Vibrational Excitation of N2(B,C) and N+2(B) States in N2 and N2 – H2 Flowing Microwave Discharges

The ro‐vibrational spectra of N₂ microwave discharges have been analysed by emission spectroscopy. It is deduced the rotational and vibrational temperatures of N₂ states. The characteristic of vibrational temperature Θ₁ of the N₂ (X, v) ground state has been specifically determined.It has been found that the N₂ (C, B, v') and N⁺₂ (B, v') radiative states are directly excited by electron collisions on the N₂ (X,v) ground state at a N₂ gas pressure of 0.1 Torr (discharge tube of 5 mm I.D, microwave power 100 Watt) with a Θ₁ value near 10⁴ K. At higher gas pressure up to 5 Torr, the N₂ (C, v') states remain alone to be mainly excited by electron collisions on N₂ (X, v). It is considered the excitations of the N₂ (B, v') states by collisions of electrons and N₂ (X,v > 4) vibrational molecules on the N₂ (A) metastable states.With x < 9% H₂ into N₂, it is observed an increase of N₂, 2^nd pos intensity, resulting of an increase of high energy electrons. Inversely, the N₂, 1^st pos intensity decreased, partly following the decrease of low energy electrons (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and Characterization of a Fluorescent Water-Borne Polyurethane Based on a Novel Naphthalimide Dye

A waterborne-polyurethane (WPU) dye, based on the fluorescent dye 4-diamino propane-N-allyl-1,8- naphthalimide (WPU-DAN), was synthesized by attaching 4-diamino propane-N-allyl-1,8- naphthalimide (DAN) onto both ends of the polyurethane (PU) chains according to a prepolymer?ionomer process. The synthetic process was confirmed by Fourier transform infrared spectroscopy (FTIR) and UV-visible absorption. The glass transition temperature, molecular weight and average particle sizes were measured. The glass transition temperatures of WPU and WPU-DAN were 46.6 and 49.8°C, respectively. In addition, the particle size distributions of WPU-DAN and WPU were 140 and 134 nm, respectively. The thermal behaviour of WPU-DAN showed improvement compared to WPU. The fluorescence intensity of WPU-DAN was enhanced more than DAN due to the naphthalimide groups attached to the chains, and the fluorescence intensity of WPU-DAN and DAN were increased by increasing temperature. Moreover, the fluorescence intensity of WPU-DAN emulsion was stable during 30 days and no loss of fluorescence intensity occurred for these days.     

Stilbene-Like Molecules as Fluorescent Probes Applied for Monitoring of Polymerization Process

2-(p-N,N-dimethylaminostyryl)benzoxazole (OS), 2-(p-N,N-dimethylaminostyryl)-benzothiazole (SS) and 2-(p-N,N-dimethylaminostyryl)naphtiazole (PS) were prepared and their absorption and fluorescence spectra were measured in various solvents at room temperature. On the basis of the solvatochromic behavior the ground state (μ_g) and excited state (μ_e) dipole moments of these pN,N-dimethylaminostyryl derivatives were evaluated. The dipole moments (μ_g and μ_e) were estimated from solvatochromic shifts of absorption and fluorescence spectra as function of the dielectric constant (ɛ) and refractive index (n) of applied solvents. The absorption spectra only slightly are affected by the solvent polarity in contrast to the fluorescence spectra that are highly solvatochromic and display a large Stokes shift. The analysis of the solvatochromic behavior of the fluorescence spectra as function of Δf (ɛ, n) revealed that the emission occurs from a high polarity excited state. The large dipole moment change along with the strongly red-shifted fluorescence, as the solvent polarity is increased, demonstrate the formation of an intramolecular charge transfer state (ICT). Compounds under the study were used as fluorescence probes for monitoring the kinetics of polymerization. The study on the changes in fluorescence intensity and spectroscopic shifts of studied compounds were carried out during thermally initiated polymerization of methyl methacrylate (MMA) and during photoinitiated polymerization of 2-ethyl-2-(hydroxymethyl)propane-1,3-diol triacrylate (TMPTA).     

Effect of Calcium Ion Concentration on Small Molecule Desorption from Alginate Beads

Spherical alginate beads were prepared by ionotropic gelation of sodium alginate through the use of calcium ions. Pyranine (Py) was added to the alginate solution as a small molecule probe for fluorescence studies. Desorption of Py in water from the alginate beads cross-linked with calcium ions was studied by using the steady state fluorescence technique. The fluorescence emission intensity (I) from Py was monitored during the desorption process at 512 nm using the time drive mode of the spectrofluorometer. The increase in I was attributed to Py release from the beads. The Fickian diffusion model was used to calculate the desorption coefficients, D_, which were found to be increased up to 3% (w/v) CaCl₂ concentration in the beads, and then decreased with a further increase of CaCl₂ content. On the other hand, the encapsulation efficiency of Py in the calcium alginate beads presented the reverse behavior compared to D. It was observed that, when the content of CaCl₂ was increased, the incubation time, t_0, for the start of desorption increased.     

Correlation of the rates of solvolysis of diphenylphosphinyl chloride using an extended form of the Grunwald–Winstein equation

The specific rates of solvolysis of diphenylphosphinyl chloride ( 1 ) have been measured at 25.0 °C in 30 solvents. For six representative solvents, studies were made at several temperatures and activation parameters determined. These were used to calculate a value at 25.0 °C in 100% 2,2,2‐trifluoroethanol (TFE) from values at higher temperatures. The 31 solvents gave a reasonable extended Grunwald–Winstein plot, correlation coefficient (R) of 0.920, which improved to 0.956 when the four TFE–ethanol points were excluded. The sensitivities (l and m) were similar to those obtained for dimethyl phosphorochloridate and phosphorochloridothionate and for N,N,N′,N′‐ tetramethyldiamidophosphorochloridate. As with the three previously studied solvolyses, an S_N2 pathway is proposed for the solvolyses of 1 . Copyright © 2007 John Wiley & Sons, Ltd.