Femtosecond dynamics of solvated oxygen anions. II. Nature of dissociation and caging in finite-sized clusters

Ultrafast dissociation and recombination dynamics of (O_2)^−_n, n=3–10 was studied using femtosecond,time-resolvedphotoelectron spectroscopy. The observed transients of nascent fragment anions, following 800 nm fs pulse excitation, exhibit a biexponential rise with two distinct time constants. The time constants, which vary with the number of solvent O_2 molecules, clearly show the solvation effect in two different dissociation pathways. Consistent with the bifurcation picture in the preceding paper, the direct subpicosecond dissociation(τ_1=110-620 fs, depending on n) is governed by electron recombination and kinematics of the half-collision. The second pathway is indirect (τ_2=0.7–8.0 ps, for O^−_6 to O^−_(20)) and controlled by intramolecular vibrational-energy redistribution. In the solvent cage, only O^−_(16), O^−_(18), and O^−_(20) show the reformation of the bond, with the caging time constant decreasing from 4 ps for the first two to 2 ps for the latter. This caging through ion-induced dipole interaction is then followed by vibrational relaxation on the time scale of 12 to 3 ps, for O^−_(16) to O^−_(20). The time scale for the initial direct caging is two to five times slower than that previously observed for diatoms, neutral, or ionic, in van der Waals clusters. We suggest that this initial slower caging is due to the reorientation of O^−_2 and O_2 to acquire a proper geometry for O^−_4 bond reformation. In these finite-sized homogeneous clusters, we compare theory with experiment. We also found a correlation between the vertical detachment energy and n^(−1/3), for n in the range of 2–10, which allow for a connection between the mesoscopic structures and a bulk-type dielectric continuum, with an effective dielectric constant

Femtosecond dynamics of solvated oxygen anions. I. Bifurcated electron transfer dynamics probed by photoelectron spectroscopy

The ultrafast dissociationdynamics of O−_6⋅X (X=O_2, N_2, Xe, or N_2O) was investigated by femtosecondphotoelectron spectroscopy. The transients, monitoring nascent O−_2, exhibit biexponential rises with two distinct time constants—the fast component (τ_1∼200 fs) corresponds to the joint rate constant for electron recombination and direct dissociation of the O−_4 core perturbed by solvent molecules, whereas the slow component (τ_2=2.0–7.7 ps, depending on the solvent) corresponds to the process for the liberation of O−_2, which is governed by vibrational predissociation and intramolecular vibrational-energy redistribution. These observations are consistent with the mechanism proposed in the earlier communication of this work [Paik et al., J. Chem. Phys. 115, 612 (2001)]. The wave packet bifurcates via two separate dissociation pathways: electron transfer followed by electron recombination, and electron transfer followed by vibrational predissociation. Unlike all other solvents, the anomalous behavior observed for O−_6⋅N_2O—a threefold increase in τ_2 value, compared to the other solvents, and a factor of 10 increase for τ_2, compared to that of O−_6—reflects the more effective energy dissipation via solute–solvent vibration-to-vibration and rotational couplings. Moreover, for all solvents, the ratio of the slow-rise contribution to the total signal can be correlated with the degree of cooling, supporting the concept of bifurcation in the two channels

Femtochemistry of mass-selected negative-ion clusters of dioxygen: Charge-transfer and solvation dynamics

Femtosecond, time-resolved photoelectron spectroscopy is used to investigate the dissociation dynamics of mass-selected anionic molecular-oxygen clusters. The observed transient photoelectron signal for the clusters (O_2)^−_n  (n = 3–5) shows the O^−_2 production; for n = 1 and 2, we observe no time-dependence at this wavelength of 800 nm. The observed transients are bi-exponential in form with two distinct time constants, but with clear trends, for all investigated cluster sizes. These striking observations describe the reaction pathways of the solvated core and we elucidate two primary processes: Charge transfer with concomitant nuclear motion, and direct dissociation of the O^−_4 core-ion via electron recombination; the former takes 700–2700 fs, while the latter is on a shorter time scale, 110–420 fs. Both rates decrease differently upon increasing cluster size, indicating the critical role of step-wise solvation

A comparative study of polyurethane nanofibers with different patterns and its analogous nanofibers containing MWCNTs

Tissue engineering is a multidisciplinary field that has evolved in various dimensions in recent years. One of the main aspects in this field is the proper adjustment and final compatibility of implants at the target site of surgery. For this purpose, it is desired to have the materials fabricated at the nanometer scale, since these dimensions will ultimately accelerate the fixation of implants at the cellular level. In this study, electrospun polyurethane nanofibers and their analogous nanofibers containing MWCNTs are introduced for tissue engineering applications. Since MWCNTs agglomerate to form bundles, a high intensity sonication procedure was used to disperse them, followed by electrospinning the polymer solutions that contained these previously dispersed MWCNTs. Characterization of the produced nanofibers has confirmed production of different non-woven mats, which include random, semi-aligned and mostly aligned patterns. A simultaneous and comparative study was conducted on the nanofibers with respect to their thermal stability, mechanical properties and biocompatibility. Results indicate that the mostly aligned nanofibers pattern presents higher thermal stability, mechanical properties, and biocompatibility. Furthermore, incorporation of MWCNTs among the different arrangements significantly improved the mechanical properties and cell alignment along the nanofibers

Femtosecond dynamics of solvated oxygen anions. I. Bifurcated electron transfer dynamics probed by photoelectron spectroscopy

The ultrafast dissociationdynamics of O−_6⋅X (X=O_2, N_2, Xe, or N_2O) was investigated by femtosecondphotoelectron spectroscopy. The transients, monitoring nascent O−_2, exhibit biexponential rises with two distinct time constants—the fast component (τ_1∼200 fs) corresponds to the joint rate constant for electron recombination and direct dissociation of the O−_4 core perturbed by solvent molecules, whereas the slow component (τ_2=2.0–7.7 ps, depending on the solvent) corresponds to the process for the liberation of O−_2, which is governed by vibrational predissociation and intramolecular vibrational-energy redistribution. These observations are consistent with the mechanism proposed in the earlier communication of this work [Paik et al., J. Chem. Phys. 115, 612 (2001)]. The wave packet bifurcates via two separate dissociation pathways: electron transfer followed by electron recombination, and electron transfer followed by vibrational predissociation. Unlike all other solvents, the anomalous behavior observed for O−_6⋅N_2O—a threefold increase in τ_2 value, compared to the other solvents, and a factor of 10 increase for τ_2, compared to that of O−_6—reflects the more effective energy dissipation via solute–solvent vibration-to-vibration and rotational couplings. Moreover, for all solvents, the ratio of the slow-rise contribution to the total signal can be correlated with the degree of cooling, supporting the concept of bifurcation in the two channels

Topical application of epidermal growth factor accelerates wound healing by myofibroblast proliferation and collagen synthesis in rat

Recombinant human epidermal growth factor (rhEGF) stimulates the proliferation and migration of epithelial cells in human cell culture systems and animal models of partial-thickness skin wounds. This study investigated the effect of a topical rhEGF ointment on the rate of wound healing and skin re-epithelialization in a rat full thickness wound model, and verified whether or not the rhEGF treatment affected both myofibroblast proliferation and collagen synthesis in the dermis. When rhEGF (10 µg/g ointment) was applied topically twice a day for 14 days, there was significantly enhanced wound closure from the 5th to the 12th day compared with the control (ointment base treatment) group. A histological examination at the postoperative 7th day revealed that the rhEGF treatment increased the number of proliferating nuclear antigen immunoreactive cells in the epidermis layer. In addition, the immunoreactive area of alpha-smooth muscle actin and the expression of prolyl 4-hydroxylase were significantly higher than those of the control group. Overall, a topical treatment of rhEGF ointment promotes wound healing by increasing the rate of epidermal proliferation and accelerating the level of wound contraction related to myofibroblast proliferation and collagen deposition

Epigallocatechin-3-gallate protects toluene diisocyanate-induced airway inflammation in a murine model of asthma

AbstractEpigallocatechin-3-gallate (EGCG), a major form of tea catechin, has anti-allergic properties. To elucidate the anti-allergic mechanisms of EGCG, we investigated its regulation of matrix metalloproteinase (MMP-9) expression in toluene diisocyanate (TDI)-inhalation lung tissues as well as TNF-α and Th2 cytokine (IL-5) production in BAL fluid. Compared with untreated asthmatic mice those administrated with EGCG had significantly reduced asthmatic reaction. Also, increased reactive oxygen species (ROS) generation by TDI inhalation was diminished by administration of EGCG in BAL fluid. These results suggest that EGCG regulates inflammatory cell migration possibly by suppressing MMP-9 production and ROS generation, and indicate that EGCG may be useful as an adjuvant therapy for bronchial asthma

Femtochemistry of mass-selected negative-ion clusters of dioxygen: Charge-transfer and solvation dynamics

Femtosecond, time-resolved photoelectron spectroscopy is used to investigate the dissociation dynamics of mass-selected anionic molecular-oxygen clusters. The observed transient photoelectron signal for the clusters (O_2)^−_n  (n = 3–5) shows the O^−_2 production; for n = 1 and 2, we observe no time-dependence at this wavelength of 800 nm. The observed transients are bi-exponential in form with two distinct time constants, but with clear trends, for all investigated cluster sizes. These striking observations describe the reaction pathways of the solvated core and we elucidate two primary processes: Charge transfer with concomitant nuclear motion, and direct dissociation of the O^−_4 core-ion via electron recombination; the former takes 700–2700 fs, while the latter is on a shorter time scale, 110–420 fs. Both rates decrease differently upon increasing cluster size, indicating the critical role of step-wise solvation

Imaging, Spectroscopy, Mechanical, Alignment and Biocompatibility Studies of Electrospun Medical Grade Polyurethane (Carbothane™ 3575A) Nanofibers and Composite Nanofibers Containing Multiwalled Carbon Nanotubes

In the present study, we discuss the electrospinning of medical grade polyurethane (Carbothane™ 3575A) nanofibers containing multi-walled-carbon-nanotubes (MWCNTs). A simple method that does not depend on additional foreign chemicals has been employed to disperse MWCNTs through high intensity sonication. Typically, a polymer solution consisting of polymer/MWCNTs has been electrospun to form nanofibers. Physiochemical aspects of prepared nanofibers were evaluated by SEM, TEM, FT-IR and Raman spectroscopy, confirming nanofibers containing MWCNTs. The biocompatibility and cell attachment of the produced nanofiber mats were investigated while culturing them in the presence of NIH 3T3 fibroblasts. The results from these tests indicated non-toxic behavior of the prepared nanofiber mats and had a significant attachment of cells towards nanofibers. The incorporation of MWCNTs into polymeric nanofibers led to an improvement in tensile stress from 11.40 ± 0.9 to 51.25 ± 5.5 MPa. Furthermore, complete alignment of the nanofibers resulted in an enhancement on tensile stress to 72.78 ± 5.5 MPa. Displaying these attributes of high mechanical properties and non-toxic nature of nanofibers are recommended for an ideal candidate for future tendon and ligament grafts