Upon Ultraviolet Irradiation Research Articles

Stretchable photochromic hydrous TiO2‐embedded glycerol‐poly(dimethylsiloxane) film for rewritable information storage

AbstractPhotoreversible, color‐changing films have great potential in various optical applications. In this study, we developed a photochromic film by embedding droplets of metatitanic acid (hydrous TiO2) and glycerol in commercially available poly(dimethylsiloxane) (PDMS) silicone elastomers. Benefiting from the electron–hole pair scavenging abilities of glycerol, the resulting film exhibited rapid photoresponse, high photoreversible stability, excellent antiaging capability, and remarkable stretchability. The degree of color changes upon ultraviolet (UV) irradiation was effectively controlled by adjusting both the concentration of hydrous TiO2 (1–20 wt%) and UV irradiation time (5–30 min), thereby increasing total color‐changing ability (ΔE). High‐resolution patterns could be repetitively printed, erased, and rewritten (≥20 times) with no significant loss of clarity. After a 30‐day exposure to ambient air, there were no significant aging effects in the photochromic ability of the films, thus demonstrating promising potential as rewritable films for information storage.

Nanotextured surfaces with iron oxide and titania for antibacterial and water purification applications via supersonic spraying

Microbial infections pose a significant threat to public health worldwide. Antibacterial treatments are required to prevent bacterial contamination and protect public health. Herein we supersonically sprayed iron and titania NPs which are nontoxic earth-abundant and low-cost antibacterial materials to fabricate Fe2O3/TiO2-based antibacterial hybrid films capable of inhibiting E. coli by up to 100% upon ultraviolet (UV) light irradiation. The supersonically sprayed iron and titania NPs were annealed to prepare Fe2O3/TiO2-based metal oxide nanosheets. The photomobilized electrons and holes generated superoxide anions and hydroxyl radicals respectively via reduction and oxidation. The bare Fe2O3-based and hybrid Fe2O3/TiO2-based films were photoactivated via visible and UV lights and exhibited E. coli inhibition rates of 42.1% and 99.9% respectively. In addition the hybrid Fe2O3/TiO2-based films were superhydrophilic which promoted the strong adhesion of bacteria-contaminated aerosols between the surrounding atmosphere and films. The water-purification capability of the films was confirmed by the efficient photodegradation of a methylene blue solution upon UV irradiation.

Effects of surface hydroxyl ions on the color of sintered Al2O3 characterized by X-ray photoelectron and infrared spectroscopy

Motivated by the different chromaticity present in the sintered Al2O3 prepared from various same-grade powders, this study examined the probable factor that affects the color by surface chemistry. Only the surface of the sintered Al2O3 changed to a darker ivory color upon ultraviolet (UV) irradiation, which recovered its initial color after a couple of weeks, implying possible surface interaction. Deconvoluting the O 1s XPS peak into the O-1 and O-2 components showed that the amount of surface hydroxyl ions varies according to the powder type and decreases by UV irradiation. In addition, hydroxyl ions adsorbed on the surface of sintered Al2O3 were distinguished by Fourier transform infrared (FT-IR) spectroscopy. Based on the correlation between the XPS O-2 area and the intensity of each FT-IR peak, the hydroxyl ions attached to Al3+ in the tetrahedral interstitial sites most likely affect the color of sintered Al2O3.

Identification of Salmonella Serovars before and after Ultraviolet Light Irradiation by Fourier Transform Infrared (FT-IR) Spectroscopy and Chemometrics

Vibrational properties and structural behavior upon ultraviolet (UV) irradiation are reported here for DNAs isolated from three serovars of Salmonella bacteria, respectively (S. enteritidis, S. infantis and S. typhimurium) by using Fourier transform infrared spectroscopy (FT-IR). The FT-IR absorbance spectra were recorded for bacterial DNAs, control and UV irradiated samples (30 min, 1 h, 1 h 30 min), respectively, and are presented in the wavenumber ranges from 800 to 1800 cm−1 and from 3000 to 4000 cm−1, respectively. Molecular specific structural information has been found based on DNA marker bands, which were monitored primarily in terms of intensity changes. As a general remark, a serovar’s dependent nonlinear behavior has been detected for some FT-IR band intensities corresponding to Salmonella DNAs, as a function of irradiation time. Particularly, alterations in DNA functional groups and molecular geometry have been observed. Chemometric analysis such as principal component analysis (PCA) and hierarchical clustering analysis (HCA) were successfully applied for the identification of inter- and intra- serovar differences between DNA spectra, before and after UV irradiation. The elucidation of the main spectral features characteristic to pathogenic bacterial DNAs offers further opportunity for bacteria identification and for establishing the mechanisms of infections, leading to the development of innovative diagnosis tools and more effective therapies. Particularly, DNA-damaging agents are most effective in cancer therapy.

Cyclodextrin-bearing telechelic poly(2-isopropyl-2-oxazoline): Extremely large shifts of phase transition temperature by photo-responsive guest inclusion

As a dual-stimuli-responsive polymer, α−cyclodextrin-bearing telechelic poly(2-isopropyl-2-oxazoline) (CD-PiPrOx-CD) was prepared by living cationic ring opening polymerization followed by click reaction (copper-catalyzed alkyne-azide cycloaddition). The phase transition temperature (TCP) of CD-PiPrOx-CD (0.33 mM) in 10 mM PBS solution (150 mM NaCl, pH 7.4) was 42.9 °C, and dramatically changed to 8.5 °C upon addition of the azobenzene-bearing penta(ethylene glycol) (Az-5EG) (3.3 mM). A change in the TCP was again observed for the mixture solution of CD-PiPrOx-CD and Az-5EG upon ultra-violet (UV) irradiation at 365 nm. Under 365-nm UV irradiation for 30 min, the TCP of CD-PiPrOx-CD changed to 41.3 °C, indicating the release of Az-5EG from the host-guest complex. During alternate irradiation at 365 and 254 nm, the TCP changed in a fully reversible manner.

Photocontrollable ambipolar transistors with π-conjugated diarylethene photochromic channels

Photocontrollable ambipolar transistors were developed by using diarylethene (DAE) layers with dibenzo[b,d]thiophene substituents as the transistor channels. An important feature is that both electron transport and hole transport occurred in the transistors upon ultraviolet (UV) light irradiation. The result indicates that the closed-ring isomer in the DAE molecules served as an ambipolar channel. Alternating UV and visible light irradiation controlled both the hole and electron currents, this was accompanied by photoisomerization from the closed-ring (semiconductor) to open-ring isomers (insulator). The light-induced on/off ratios in the drain currents were in the 95–225 range for the hole transport and in the 6–22 range for the electron transport. We found that the energy level alignment between the work function of the electrode and the molecular orbitals of the closed-ring isomer was responsible for the ambipolar transport. Our finding therefore offers a new strategy for developing optoelectronic organic transistors.

Highly efficient photocontrol of mitotic kinesin Eg5 ATPase activity using a novel photochromic compound composed of two azobenzene derivatives

Mitotic kinesin Eg5 plays an important physiological role in cell division. Several small-molecule inhibitors of Eg5 are the focus of cancer therapies. Azobenzene is a photochromic compound exhibiting cis-trans isomerization upon ultraviolet (UV) and visible (VIS) light irradiation. Photochromic compounds of azobenzene derivatives, mimicking Eg5-specific inhibitors of STLC, indicated photoreversible inhibitory effects on Eg5 ATPase activity; however, the photoreversible switching efficiency was not significant. This study presents a novel synthesized photochromic Eg5 inhibitor 2, 3-bis[(2,5-dioxo-1-{4-[(E)-2-phenyldiazen-1-yl]phenyl}pyrrolidin-3-yl)sulfanyl] butanedioic acid (BDPSB), which is composed of two azobenzenes. BDPSB exhibited cis-trans isomerization with UV and VIS light irradiation. The trans form of BDPSB significantly inhibited microtubule-dependent ATPase activity of Eg5, with an IC50 of 74 μM. Cis BDPSB showed weak effects on the microtubule-dependent ATPase activity. The results suggest that the novel photochromic Eg5 inhibitor BDPSB, which exhibits highly efficient photoswitching, shows a switch 'ON' and 'OFF' behaviour with VIS and UV light irradiation.

Optical Activity and Optical Anisotropy in Photomechanical Crystals of Chiral Salicylidenephenylethylamines.

Introducing chirality into photomechanical crystals is beneficial for the diversification of mechanical motion. Measurement of the chiroptical and optical anisotropic properties of chiral crystals is indispensable for evaluating photomechanical crystals. The platelike crystals of S- and R-enantiomers of photochromic N-3,5-di-tert-butylsalicylidene-1-phenylethylamine in enol form (enol-(S)-1 and enol-(R)-1) caused bending motion with twisting upon ultraviolet (UV) light irradiation, due to shrinkage along the length and width directions of the irradiated surface, based on the optimized crystal structure of the photoisomerized trans-keto-(S)-1. By employing the generalized high-accuracy universal polarimeter (G-HAUP), optical anisotropic (linear birefringence, LB; linear dichroism, LD) as well as chiroptical (circular birefringence, CB; circular dichroism, CD) spectra of both the enantiomeric crystals on the (001) face were simultaneously measured before and under continuous UV irradiation. The LD peak was observed at 330 nm in the negative sign, derived from the π-π* transition of the intramolecularly hydrogen-bonded salicylidenimino moiety. The CD spectra of the S and R crystals revealed the negative and positive Cotton effect at 330 nm, respectively, and new peaks appeared at 460 nm under UV light irradiation due to photoisomerization to the S and R trans-keto isomers at around 10% conversion. The CB and CD spectra evaluated by the HAUP measurement were opposite to those measured in the hexane solution, as well as those simulated by quantum chemical calculation. The dissymmetry parameter, g, of the enol-(S)-1 crystal along the c axis (0.013) was approximately 10 times larger than the g values in the solution (0.0010) and by calculation (0.0016).

The Epidermal Growth Factor Receptor Increases Cytokine Production and Cutaneous Inflammation in Response to Ultraviolet Irradiation

The epidermal growth factor receptor (EGFR) is activated in cutaneous keratinocytes upon ultraviolet (UV) exposure and has been implicated in ultraviolet-(UV-)induced inflammation and skin tumorigenesis. Egfr mutant mice and EGFR inhibitors were used to investigate the hypothesis that EGFR activation augments inflammation following UV irradiation. Topical treatment of mouse skin with the EGFR inhibitor AG1478 before UV exposure suppressed UV-induced erythema, edema, mast cell infiltration, and neutrophil infiltration. Genetic ablation of Egfr and EGFR inhibition by AG1478 also suppressed the increase in the proinflammatory cytokines tumor necrosis factor α (TNF-α), interleukin-1α, KC (murine IL-8), and cyclooxygenase-2 (COX-2) after UV exposure of cultured keratinocytes. Finally, genetic ablation of inhibition of EGFR in cultured keratinocytes decreased p38 activation after UV, while inhibition of p38 kinase reduced COX-2 expression after UV. These data demonstrate that EGFR regulates multiple aspects of UV-induced inflammation and suggest activation of p38 kinase leading to increased COX-2 and cytokine expression as one mechanism through which it acts.

Photocontrol of ATPase Avtivity of Kinesin Kif18A using Various Existing Azoboenzene Derivatives

The shape and function of the mitotic spindle depends on the cooperation action of various kinesins. Recently, it was demonstrated that the kinesin Kif18A is a central component for the correct alignment of chromosomes at the spindle equator. BTB-1 inhibits ATPase activity of kif18A but not of other mitotic kinesins. BTB-1 is a potent inhibitor for kinesin Kif18A. In mitosis cell, inhibition of Kif18A by BTB-1 occur disorder chromosome congression and cell lead to apotosis. In vitro, BTB-1 inhibits the motility of Kif18A in a reversible manner. BTB-1 inhibits Kif18A ATPase activity in an ATP competitive manner but microtubule. BTB-1 is composed of chloro group and the nitro group and the two aromatic rings. Azobenzen is one of the photochromic molecules that change their shapes and properities drastically upon ultra-violet (UV) and visible (VIS) light irradiation. Previously, we have incorporated azobenzen derivative into functional region of ATP driven motor protein and succeeded to control microtubule dependent ATPase activity reversibly by UV-VIS light irradiation. Interestingly, BTB-1 shows structural similarity to azobenzen derivatives. In this study, we examined photo-reversible inhibitory effects of various existing azobenzen derivatives for the ATPase activity of kif18A. Cis isomer of 4-aminoazoboenzene-4-sulfonic acid induced by UV irradiation inhibited ATPase activity of Kif18A more effectively than its trans isomer. Other azobenzen derivate were also examined.This work was supported by Grant-in-Aid for Challenging Exploratory Research(23650276)from Japan Society for the Promotion of Science (JSPS).

Photocontrol of Motor Proteins using Photo-Responsive Calmodulin Dimer

Calmodulin (CaM) is a physiologically important Ca2+-binding protein that participates in numerous cellular regulatory processes. CaM undergoes a conformational change upon binding to Ca2+, which enables it to bind to target proteins for specific responses. For example, Ca2+/CaM regulates function of myosin V, myosin light chain kinase (MLCK) etc. Previously, we succeeded to photocontrol CaM function using photochromic molecule N- (4-phenylazophenyl) maleimide (PAM), which reversibly undergoes cis-trans isomerization upon ultraviolet (UV) and visible (VIS) light irradiation. The CaM mutants, which have a single cysteine in the functional region of CaM, were prepared and modified with PAM. The binding of PAM-CaM to target peptide M13 was controlled reversibly upon UV and VIS light irradiation. In this study, we prepared the photo responsive dimer CaM by cross-linking of CaM mutants with bifunctional photochromic compound, 4,4′-azobenzene-dimaleimide (ABDM) in order to apply to photo control of motor proteins. For the CaM dimer, it is expected that the relative special configuration of each of CaM cross-linked with ABDM changes by UV-VIS light irradiation. Subsequently, we prepared the fusion protein, K355-M13 composed of kinesin motor domain and M13 peptide. The monomeric K355-M13 formed dimer configuration by CaM dimer. In the presence of Ca2+, two K355-M13 bound to the each site of CaM dimer resulted in formation of kinesin dimer linked by CaM-ABDM-CaM. The phtocontrol of the motor activity and microtubule dependent ATPase activity of the photochromic kinesin dimer was studied. Application of CaM-ABDM-CaM to MLCK and myosin was also performed.

Collagen VI protects against neuronal apoptosis elicited by ultraviolet irradiation via an Akt/Phosphatidylinositol 3-kinase signaling pathway

Collagen VI, one of the extracellular matrix proteins, has been implicated in regulating cell proliferation and reducing apoptosis in several different systems. However, the role of collagen VI in the central nervous system remains unclear. In this manuscript, we demonstrated that upon ultraviolet (UV) irradiation, mouse primary hippocampal neurons specifically up-regulate the expression of Col6a1, Col6a2, and Col6a3 mRNA and secreted collagen VI protein. Augmentation of collagen VI mRNA and protein after UV irradiation may have a neuroprotective role as suggested by the fact that extracellular supplying soluble collagen VI protein, but not other collagen proteins, reduced UV induced DNA damage, mitochondria dysfunction, and neurite shrinkage. We also tried to determine the signaling molecules that mediate the protective effect of collagen VI via Western blot and inhibitor analysis. After collagen VI treatment, UV irradiated neurons increased phosphorylation of Akt and decreased phosphorylation of JNK. Inhibiting Akt/Phosphatidylinositol 3-kinases (PI3K) pathway diminished the protective effect of collagen VI. Our study suggested a potential protective mechanism by which neurons up-regulate collagen VI production under stress conditions to activate Akt/PI3K anti-apoptotic signaling pathway.

Characterization of spectral responses of humic substances upon UV irradiation using two-dimensional correlation spectroscopy

The spectral responses of a leaf litter derived humic substance (LLHS) and Suwannee River fulvic acid (SRFA) upon ultraviolet (UV) A irradiation were characterized using two-dimensional correlation spectroscopy (2D-COS) based on the absorption and the synchronous fluorescence spectra at different irradiation times. A 12 day irradiation on the humic substances (HS) resulted in higher reduction of the absorbance relative to the dissolved organic carbon concentration, suggesting that aromatic chromophores were preferentially oxidized and/or non UV-absorbing compounds were generated by the photobleaching. Synchronous fluorescence spectra revealed the preferential removal of fulvic-like and humic-like fluorophores and delayed response of protein-like fluorescence upon the irradiation. The spectral features at long wavelengths (>430 nm) appear to be affected by intra-molecular interactions of the individual chromophores associated with shorter wavelengths. Absorption-based 2D-COS demonstrated that there are three types of absorption bands for the two HS, which changed sequentially in the order of 290–400 nm → 200–250 nm → 250–290 nm. In addition, two or three distinctive fluorescence bands in response to the irradiation were identified from 2D-COS. The sequential orders and the associated wavelength bands were possibly explained by the irradiation wavelengths and the differences between direct and indirect photochemical reactions. The interpretation of the 2D-COS results was very consistent with the kinetic rate constants individually calculated at several discrete wavelengths. Our study demonstrated that 2D-COS could be used as a powerful tool in identifying distinctive bands of HS that have dissimilar behavior and the associated sequential orders by visualizing the spectral changes at continuous wavelengths.

Fabrication and propagation characterization of As_2S_8 chalcogenide channel waveguide made by UV irradiation annealing

Changes in the refractive index of amorphous chalcogenide As2S8 films upon ultraviolet (UV) exposure and annealing at different temperatures are investigated in detail, indicating an index contrast of the order of 10(-2) in the As2S8 channel waveguide. An As2S8 channel waveguide is fabricated using UV well irradiation and then annealing near the glass transition temperature and shows a low propagation loss of 0.76 dB/cm and good propagation characterization at the 1310 nm guided mode.

Structural characteristics of UV‐irradiated polyurethane elastomers extended with α,ω‐alkane diols

AbstractA series of polyurethane (PU) elastomers were prepared by the reaction of poly (ε‐caprolactone) (PCL) and 4, 4′‐diphenylmethane diisocyanate (MDI), extended with a series of chain extenders (CEs) having 2‐10 methylene units in their structure. The synthesized samples were irradiated for 50, 100, and 200 h as such the spectral distribution of the light is good match for terrestrial solar radiation. Fourier transform infrared (FTIR) spectroscopy was used to study the chemistry of monomers and final PU samples. The structural changes in the PU samples upon ultraviolet (UV) irradiation were characterized using FTIR spectroscopic technique. In comparison with non irradiated samples, there was a continuous decrease of the bands upon long‐term UV irradiation. The results revealed that final PU sample extended with 1,6‐hexane diol (1,6‐HDO) to 1, 10 decane diol (1,10‐DDO) have been fully degraded at 200 h irradiation. Comparing PU sample extended with 1, 6‐HDO and 1, 10‐DDO, it was found that the degradation rate of later is higher than former. It was concluded that CEs length has considerable effect on thermal degradation of PU and by increasing CEs length thermal degradation upon UV irradiation shifted to lower temperature. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Photodegradation of organic light-emitting devices observed in nitrogen-filled environment

Degradation of organic light-emitting devices (OLEDs) upon ultraviolet (UV) irradiation has been studied by measuring luminance–voltage (L–V) and current–voltage (I–V) characteristics of the devices in a nitrogen-filled glove-box. Photo-oxidation or reaction is no longer the main origin of the degradation for the devices protected by nitrogen. Conventional double-layer OLEDs with tris(8-hydroxyquinoline) aluminum (Alq 3) as the electron transport material and single-layer devices containing Alq 3 as the only organic material exhibit different degradation behaviors: both L–V and I–V characteristics degrade severely for the irradiated double-layer devices, whereas whether I–V degrades or not in a single-layer device is closely related to the species of the charge carriers flowing in the device. By comparing electroluminescent and photoluminescent degradation behaviors of the single-Alq 3-layer devices, we conclude that lowered fluorescent quantum efficiency and hole current after UV irradiation are two origins of the degraded characteristics of the devices isolated from the moist environment.

Identification of a Single Tryptophan Residue as Critical for Binding Activity in a Humanized Monoclonal Antibody against Respiratory Syncytial Virus

We have identified a single tryptophan (Trp) residue responsible for loss of binding and biological activity upon ultraviolet (UV) light irradiation in MEDI-493, a humanized monoclonal antibody (MAb) against respiratory syncytial virus (RSV). This finding provides a better understanding of structure-function relationship in a 150-kDa protein. Irradiation of MEDI-493 with UV light resulted in spectral changes typical of Trp photoproducts and in a progressive loss of MEDI-493 binding and biological activity as measured by ELISA, Biacore, and cell-based assays. Mass spectrometric characterization of the proteolytic peptides generated from the UV irradiated MEDI-493 confirmed that most methionine (Met) and a few Trp residues were oxidized to various extents upon exposure to UV light. Among Trp residues, only Trp-105, containing the most solvent-exposed indole moiety in MEDI-493 and residing in a complementary-determining region (CDR) of the heavy chain, was significantly oxidized. When bound to a synthetic antigenic peptide, MEDI-493 showed significant resistance toward binding activity loss during UV irradiation. A second MAb (MEDI-524) with Trp-105 replaced by phenylalanine (Phe) showed a similar pattern of Met oxidation, but no loss of binding and biological activity following irradiation. Treatment of both MAbs with Met- and Trp-specific oxidizing reagents showed that oxidation of Trp-105 correlated with the activity loss, whereas Met oxidation did not affect the activity. These results demonstrate that Trp-105 in MEDI-493 is responsible for the UV light-induced effects.

Photocatalytic degradation of methylene blue on nanocrystalline TiO 2: Surface mass spectrometry of reaction intermediates

Photocatalytic degradation reactions of methylene blue on nanocrystalline TiO 2 (nc-TiO 2) films were studied in situ by surface mass spectrometric techniques including secondary ion mass spectrometry, laser desorption direct ion mass spectrometry, and laser desorption/laser photoion mass spectrometry. The parent ion of methylene blue and/or its reduced form leucomethylene blue was observed with little fragmentation by all methods prior to ultraviolet exposure. The surface composition changed upon ultraviolet (UV) irradiation in air, an observation ascribed to photocatalytic reactions induced by UV photons: the parent molecule signal diminished and intermediate reaction products such as sulfoxides and sulfones were detected from the TiO 2 surface. After prolonged UV irradiation these species also vanished and the methylene blue appeared to be almost completely mineralized.

Sol–gel preparation and luminescent properties of CeO 2:Ln (Ln = Eu 3+ and Sm 3+) thin films

New transparent CeO 2:Ln (Ln = Eu 3+ or Sm 3+) thin-film phosphors have been fabricated by a sol–gel method. The CeO 2:Ln films with the Ln concentrations of 1 at.% showed the strongest photoluminescence (PL) upon ultraviolet (UV) light excitation. At higher Ln concentrations, the PL intensity decreased because of the nonradiative energy transfer. For Eu 3+ ions doped in the CeO 2 lattice with inversion symmetry, an orangey red emission due to a magnetic-dipole 5 D 0 → 7 F 1 transition (591 nm) was dominant when excited at a wavelength of 340 nm. In contrast, much stronger orange emissions (573 nm) due to 4 G 5/2 → 6 H 5/2 were observed for Sm 3+ ions. Apparently, the Eu 3+ and the Sm 3+ ions had different efficiencies of the excitation through the energy transfer from the “Ce 4+–O 2−” charge transfer state induced by the UV irradiation.

Surface dilatational rheological study of spread polymethacrylate monolayers modified by E/Z photoisomerisation

Abstract Photoinduced changes of the surface behavior of spread monolayers of three 4′ substituted phenyl-azo-phenoxy-alkyl-methacrylate polymers were characterized by using π / A isotherms, surface dilatational elasticities, and viscosities. The behavior of two homopolymers is compared, which differ by variation of the p -substitution from p OCH 3 (polymer II ) to OCF 3 (polymer I ) and a copolymer with OCF 3 and 50% of the hydroxyethylmethacrylate (HEMA) moiety (polymer III ). Based on the photokinetics in solution and spin-coated films the properties of the monolayers in two steady states are characterized, which differ in the content of the Z isomeric form. It is much higher upon ultra violet (UV) irradiation at 360 nm as in the case of the steady state established by exposure at 450 nm. On the molecular level the thermal Z / E isomerisation of the polymer I is significantly slower compared with that of the polymer II . In contrast to the copolymer III UV irradiation causes brittle monolayers of polymers I and II , which are not able to absorb higher surface pressure than about 10 mN m −1 . The surface dilatational rheological behavior of the polymers was compared only for E isomer dominated monolayers, as the Z rich layers of polymer I and II could not be deformed without mechanical destruction. In contrast polymer III could bear deformation because of the monolayer softening effect of the HEMA group. The surface rheological behavior of all polymers is visco-elastic, with the order of the values of both, dilatational elasticities and viscosities, polymer II > I > III , and whereas polymer III could be compressed to higher surface coverage.