Microwaves in advanced oxidation processes for environmental applications. A brief review

This review article focuses, albeit non-exhaustively, on the influence of microwave radiation on photoassisted processes often referred to as Advanced Oxidation Processes. In particular, we describe and illustrate the possible advantages of microwaves in TiO₂-assisted photodegradations and photomineralizations of various organic pollutants such as herbicides and endocrine disruptors, among others. Described are also various reactor configurations involving UV/visible radiation and microwaves, with the former being supplied either by traditional Hg lamps or alternatively by electrodeless lamps activated by microwaves. To place the use of microwaves on processes occurring in aqueous TiO₂ dispersions in perspective regarding environmental applications, we first introduce the various sources of pollutants and subsequently describe in brief the various advanced oxidation processes such as UV/peroxidation, UV/ozonation and the photo-Fenton process(es) in addition to direct photolysis either by sunlight or by artificial light sources.     

Oxygen Inhibition and Coating Thickness Effects on Uv Radiation Curing of Weatherfast Clearcoats Studied by Photo-DSC

Radiation curing is an environmentally-friendly technology. Furthermore, radiation curing is a faster, energy saving and more efficient industrial process than the heat-curable process. One of the most important requirements for the widespread application of UV curable coatings in the coating industry is that they are stable vs. atmospheric degradation. Today's state of the art in oxidative drying and thermosetting coatings is the use of light stabilizers to protect polymers vs. the damage of outdoor exposure. Oxygen has a detrimental effect on the cure response of free radical systems, especially in thin-film coatings. Differential photocalorimetry (photo-DSC) was used to investigate the oxygen effect and the use of light stabilizers on UV curing of photocurable formulations based on acrylate materials. Coating thickness influence was also considered. This revised version was published online in August 2006 with corrections to the Cover Date.     

Visible-to-UV Photon Upconversion: Recent Progress in New Materials and Applications

Ultraviolet (UV, λ<400 nm) light is essential for various photochemical reactions, but its intensity in the solar spectrum is very low, and light sources that artificially generate high-energy UV light are inefficient and environmentally unfriendly. A solution to this problem is photon upconversion (UC) from visible (vis, λ>400 nm) light to UV light. Among several mechanisms, UC based on triplet-triplet annihilation (TTA-UC) in particular has made remarkable progress in recent years. The development of new chromophores has enabled highly efficient conversion of low-intensity visible light into UV light. In this review, we summarize the recent development of visible-to-UV TTA-UC, from the development of chromophores and their production into films to their application in various photochemical processes such as catalysis, bond activation and polymerization. Finally, challenges and opportunities in future material development and applications will be discussed.     

Effect of light intensity and wavelengths on photodegradation reactions of riboflavin in aqueous solution

A study of the effect of light intensity and wavelengths on photodegradation reactions of riboflavin (RF) solutions in the presence of phosphate buffer using three UV and visible radiation sources has been made. The rates and magnitude of the two major photodegradation reactions of riboflavin in phosphate buffer (i.e., photoaddition and photoreduction) depend on light intensity as well as the wavelengths of irradiation. Photoaddition is facilitated by UV radiation and yields cyclodehydroriboflavin (CDRF) whereas photoreduction results from normal photolysis yielding lumichrome (LC) and lumiflavin (LF). The ratios of the photoproducts of the two reactions at 2.0 M phosphate concentration, CDRF/RF (0.09-0.22) and CDRF/LC (0.54-1.75), vary with the radiation source and are higher with UV radiation than those of the visible radiation. On the contrary, the ratios of LF/LC (0.15-0.25) increase on changing the radiation source from UV to visible. The rate is much faster with UV radiation causing 25% degradation of a 10(-5) M riboflavin solution in 7.5 min compared to that of visible radiations in 150-330 min.     

UV-induced changes in pigment content and light penetration in the fruticose lichen Cladonia arbuscula ssp. mitis

The response of the lichen, Cladonia arbuscula (Wallr.) Flot. ssp. mitis (Sandst.) Ruoss to enhanced UV-B (280-315 nm) radiation was investigated with respect to: (a) changes in phenolic content; (b) differential pigment accumulation under visible and UV radiation with increasing distance from thallus apices; and (c) the internal distribution of UV-B radiation within the thallus measured with quartz optical fibres. In a short-term experiment, lichens were exposed for 7 days in a growth chamber to visible light with or without additional UV-B radiation. For a longer term experiment, lichens were grown outdoors under both natural UV radiation, and supplemental UV-A (315-400 nm)+UV-B provided by lamps. Controls were placed under filters that removed the radiation below 290 nm from the natural sunlight. The concentration of total phenolic compounds was measured spectrophotometrically at the termination of the experiments, in different parts of the lichen podetia. UV-exposed lichens showed increased accumulation of phenolics compared to those not grown under UV. At the termination of the long-term experiment, fibre optic measurements of the penetration of radiation into lichen thallus reflected the influence of growth under UV radiation, whereby UV was more strongly attenuated as compared to that in lichens not exposed to enhanced levels of UV-B radiation. Results indicated that in Cladonia, UV-B radiation induces the accumulation of phenolic compounds that may have a protective role. In addition, the morphological distribution of phenolic compounds was different under visible and supplemental UV-B radiation. Internal radiation measurements served to visualise the attenuation of radiation with thallus depth for different wavelengths in the UV-B waveband.     

Pristine and BN doped graphyne derivatives for UV light protection

This report aims to explore the possibility of using graphyne derivatives as UV‐light protector. Boron (B) and nitrogen (N) atoms are systematically substituted into the structures, and we find that BN‐substituted analogs exhibit distinct characteristics compared with their parent two‐dimensional structure. Due to the presence of BN at different sites, the optical band gap is tuned from infrared to UV via visible region depending on substitution sites. These findings will lead the way to utilize these BN doped structures in various optoelectronic applications such as in hybrid solar cell, electroluminescence cell, light emitting cell, and as selective electromagnetic radiation absorber. The origin of this tunable optical response and band gap is explained in the light of partial density of states analysis and electron density distribution. The presence of strong absorption peak in UV region indicates that these materials may be used as an excellent candidate for UV light protection. © 2015 Wiley Periodicals, Inc.     

Scratch resistance and weatherfastness of uv-curable clearcoats

UV-cuing has found an increasing number of industry allocations over the past decade due to its unique benefits, e.g. solvent free formulations, high cure speed and low temperature processing. In addition to these benefits two additional properties of uv-cured coatings are of today's interest, especially in the automotive industry: scratch resistance and resistance against chemicals. One of the most important requirements for a broad use of uv-curable coatings in the coating industry is that coatings are stable against degradation caused by atmospheric influences since coatings for outdoor use are subject to especially harsh weathering conditions, e.g. uv-light, oxygen, moisture and air pollutants. This weathering leads to a degradation of the polymeric binder. Clearcoats containing photoinitiators based on bis-acylphosphinoxide (“BAPO”) and a combination of hydroxyphenyl-s-triazine uv-absorber and a sterically hindered amine as a light stabilizer package show a very good curing behavior as well as an improved weatherfastness over a long period of time and a good scratch and chemical resistance.     

Photobiology of ocular melanocytes and melanoma

There are two different types of ocular melanocytes and melanomas. Conjunctival melanocytes are located on the surface of the eye and are exposed to visible light and UV radiation. Recently, epidemiological studies demonstrated that sunlight plays a definite role in the occurrence of conjunctival melanoma, as it does in cutaneous melanoma. Uveal melanocytes consist of the iridal melanocytes, which are exposed to visible light and UV radiation; and the ciliary body melanocytes and choroidal melanocytes, which are not exposed to light radiation. Epidemiological studies demonstrated that sunlight may play a role in the occurrence of iridal melanoma, but may not be a major factor in the etiology of ciliary body and choroidal melanomas. Uveal melanocytes differ from epidermal melanocytes in that epidermal melanocytes respond to UV radiation and skin color becomes darker after exposure to sunlight; but uveal melanocytes do not respond to UV radiation and the iris color remains stable after exposure to sunlight. Recently, in vitro studies indicate that this phenomenon is determined both by cellular factors and environmental factors.     

The Angular Distribution of Solar Ultraviolet, Visible and Near-Infrared Radiation from Cloudless Skies

Abstract— The amount of solar radiation intercepted by an object depends on the orientation of the object with respect to the sun and the angular distribution of the diffuse component of solar radiation, which is commonly considered to be approximately isotropic. The angular distribution of the diffuse UV, visible and near-infrared insolation was measured at several solar zenith angles between 32° and 68° under cloudless skies at Lauder, New Zealand (45S), and shown to be anisotropic. The diffuse solar UV radiation increases markedly with solar elevation and is a large proportion of the total UV irradiance. The diffuse visible light and infrared radiation are small components of the total irradiance and almost independent of solar elevation. The angular distribution of erythemal UV radiation was tabulated and is available on request.     

The role of melanin as protector against free radicals in skin and its role as free radical indicator in hair

Throughout the body, melanin is a homogenous biological polymer containing a population of intrinsic, semiquinone-like radicals. Additional extrinsic free radicals are reversibly photo-generated by UV and visible light. Melanin photochemistry, particularly the formation and decay of extrinsic radicals, has been the subject of numerous electron spin resonance (ESR) spectroscopy studies. Several melanin monomers exist, and the predominant monomer in a melanin polymer depends on its location within an organism. In skin and hair, melanin differs in content of eumelanin or pheomelanin. Its bioradical character and its susceptibility to UV irradiation makes melanin an excellent indicator for UV-related processes in both skin and hair. The existence of melanin in skin is strongly correlated with the prevention against free radicals/ROS generated by UV radiation. Especially in the skin melanin (mainly eumelanin) ensures the only natural UV protection by eliminating the generated free radicals/ROS. Melanin in hair can be used as a free radical detector for evaluating the efficacy of hair care products. The aim of this study was to investigate the suitability of melanin as protector of skin against UV generated free radicals and as free radical indicator in hair.     

UV index forecasts and measurements of health-effective radiation

While erythemal irradiance as a potentially damaging effect to the skin has been extensively studied and short-term forecasts have been issued to the public to reduce detrimental immediate and long-term effects such as sunburn and skin cancer by overexposure, beneficial effects to human health such as vitamin D(3) production by UV radiation and melatonin suppression by blue visible light have attained more and more attention, though both of them have not become part of forecasting yet. Using 4years of solar radiation data measured at the mid-latitude site Lindenberg (52°N), and forecast daily maximum UV index values, an overall good correspondence has been found. The data base of solar UV radiation and illuminance has also been used to analyze effects of clouds and aerosols on the effective irradiance. Optically thick clouds can strongly modify the ratios between erythemal and vitamin D(3) effective irradiance such that direct radiative transfer modeling of the latter in future UV forecasts should be preferably used. If parameterizations of vitamin D(3) effective irradiance from erythemal irradiance are used instead, the optical cloud depth would have to be taken into account to avoid an overestimation of vitamin D(3) with parameterizations neglecting cloud optical depth. Particular emphasis for the beneficial effects has been laid in our study on low exposure. Daily doses of solar irradiation for both vitamin D(3) and melatonin suppression do not reach minimum threshold doses even with clear sky and unobstructed horizon during the winter months.     

Near-Infrared (>1000 nm) Light-Harvesters: Design,Synthesis and Applications

Organic molecules can absorb or emit light in UV, visible and infra-red (IR) region of solar radiation. Fifty percent of energy of solar radiation lies in the IR region of solar spectrum and extended π-conjugated molecules containing low optical band gap can absorb NIR radiations. Recently IR molecules have grabbed the attention of synthetic chemists. Although only few molecules have been reported so far such as derivative of BODIPY, naphthalimide, porphyrins, perylene, BBT etc., they have shown highest absorbing capacity towards greater than 1100 nm. These compounds have potential applications in different fields, such as for biomedical and optoelectronic applications. In this review, we present different classes of light-harvesters with harvesting range above 1000 nm.     

Accompanying of parameters of color,gloss and hardness on polymeric films coated with pigmented inks cured by different radiation doses of ultraviolet light

In the search for alternatives to traditional paint systems solvent-based, the curing process of polymer coatings by ultraviolet light (UV) has been widely studied and discussed, especially because of their high content of solids and null emission of VOC. In UV-curing technology, organic solvents are replaced by reactive diluents, such as monomers. This paper aims to investigate variations on color, gloss and hardness of print inks cured by different UV radiation doses. The ratio pigment/clear coating was kept constant. The clear coating presented higher average values for König hardness than pigmented ones, indicating that UV-light absorption has been reduced by the presence of pigments. Besides, they have indicated a slight variation in function of cure degree for the studied radiation doses range. The gloss loss related to UV light exposition allows inferring that some degradation occurred at the surface of print ink films.     

Radiation curable materials – principles and new perspectives

Increasing environmental concerns and the ensuing legislation to cut emissions of volatile organic compounds (VOCs) have been major driving forces behind the development of radiation cured coatings over the past 25 years. Today radiation cured coatings are known for their good overall performance and their excellent resistance against chemical and physical surface damages. Advanced photoinitiator systems allow the light stabilisation of UV‐curable formulations and the outdoor application of the coating. The rapid curing, combined with the possibility of immediate processing of the coated objects opens the way for radiation curing – in 100%‐, water based‐, and dual cure systems as well as for radiation curable powder coatings – for a wide variety of application.     

Discovery of Key TIPS‐Naphthalene for Efficient Visible‐to‐UV Photon Upconversion under Sunlight and Room Light**

While many studies have been done on triplet–triplet annihilation‐based photon upconversion (TTA‐UC) to produce visible light with high efficiency, the efficient TTA‐UC from visible to UV light, despite its importance for a variety of solar and indoor applications, remains a challenging task. Here, we report the highest visible‐to‐UV TTA‐UC efficiency of 20.5 % based on the discovery of an excellent UV emitter, 1,4‐bis((triisopropylsilyl)ethynyl)naphthalene (TIPS‐Nph). TIPS‐Nph is an acceptor with desirable features of high fluorescence quantum yield and high singlet generation efficiency by TTA. TIPS‐Nph has a low enough triplet energy level to be sensitized by Ir(C6)₂(acac), a superior donor that does not quench UV emission. The combination of TIPS‐Nph and Ir(C6)₂(acac) realizes the efficient UV light production even with weak light sources such as an AM 1.5 solar simulator and room LEDs.     

Isomerizations between nitrosyl halides X-N=O and isonitrosyl halides X-O-N: a matrix-spectroscopic study

Irradiation of nitrosyl bromide BrNO (4) with light of the wavelength lambda = 248 nm and nitrosyl chloride CINO (6) with lambda = 193 nm in an argon matrix at 10 K leads to the corresponding isomers isonitrosyl bromide BrON (5) and isonitrosyl chloride CION (7). Both new compounds 5 and 7 have been identified by comparison of the experimental and calculated (BLYP/6-311 + G*) IR spectra. Nitrosyl fluoride FNO (8) could not be transferred into isonitrosyl fluoride FON (9). The back reactions 5-->4 and 7-->6 can be initiated by UV (lambda > 310 nm), visible or IR light. The retransformation also occurs spontaneously in the matrix at 10 K under exclusion of any UV/Vis or IR radiation. Surprisingly, the reaction rates of these spontaneous back reactions are temperature independent between 8.5 and 25 K. The mechanism of these processes is discussed.     

Non-coherent visible and infrared radiation increase survival to UV (254 nm) in Escherichia coli K12

Interactions between visible or infrared (IR) and ultraviolet (UV, 254 nm) radiation have been studied in E. coli. Pre-illumination with non-coherent monochromatic 446, 466, 570 and 685 nm radiation, as well as with polychromatic red and IR radiation at room temperature, leads to increased cell survival after a subsequent irradiation with UV light. In the thermic range of the spectrum (red and IR), IR but not red light pre-treatment is able to increase cell survival to a subsequent lethal heat (51 degrees C) challenge, suggesting that increased UV survival may be due to IR-induced heat-shock response. On the other hand, visible-light-induced resistance may be due to a different mechanism, possibly involved with unknown bacterial light receptors.     

New chiral photochromic menthone-containing homopolymers and copolymers - synthesis, phase behaviour and photo-optical properties

A new approach for the preparation of photosensitive materials for coloured data recording and storage is advanced. This approach involves the synthesis of copolymers containing nematogenic and combined (-)-arylidene-p-menthan-3-one chiral photochromic fragments in one monomer unit. The conditions for the formation of an LC phase in the chiral photochromic homopolymers as a function of the structure of the side groups have been identified; for a new series of copolymers, the effect of their composition on the phase behaviour and photooptical properties is considered. Planarly oriented films of the copolymers show selective light reflection in the UV, visible, and near IR spectral regions. The photochemical behaviour of the homopolymers and copolymers in dilute solution and as films has been studied. In these systems, under UV radiation, the E-Z isomerization of the (-)-arylidene-p-menthan-3-one fragment is the dominating process. The kinetic features of the photoprocess are revealed, and the effective quantum yields calculated. The isomerization process leads to dramatic changes in the anisometry of the chiral side groups, and the helical twisting power decreases. Therefore, as a result of UV radiation of films of the copolymers, the selective light reflection peak is shifted to a longer wavelength spectral region. This means that such polymeric films can be considered as promising materials for colour data recording and as storage media.     

Cu-grafted TiO2 photocatalysts: effect of Cu on the action spectrum of composite materials

This paper describes a simple technique for the modification of titania with copper to enhance its photocatalytic performance. In addition to the absorption of UV light resulted in band-to-band excitation of electrons, TiO₂ grafted with copper species absorbs radiation in the visible region of spectrum, and it is able to completely oxidize volatile organic pollutants both under UV and visible light. The action spectra of pristine and Cu-grafted TiO₂ photocatalysts are measured and discussed to elucidate the reasons for appearance of the activity under visible light.     

Polymer Nanoparticles for Controlled Release Stimulated by Visible Light and pH

Polymer nanoparticles are prepared by self‐assembly of visible light and pH sensitive perylene‐functionalized copolymers which are synthesized by quaternization between 1‐(bromomethyl)perylene and the dimethylaminoethyl units of poly(dimethylaminoethyl methacrylate) (PDMAEMA). The perylene‐containing polymethacrylate segments afford the system visible light responsiveness and the unquaternized PDMAEMA segments afford the system pH responsiveness. The self‐assembled nanoparticles exhibit a unique dual stimuli response. They can be photocleaved under visible light irradiation, shrunken to smaller nanoparticles at high pH, and swollen at low pH. The structural change endows the nanoparticle with great potential as a sensitive nanocarrier for controlled release of Nile Red and lysozyme under this stimulation. The visible light responsiveness and synergistic effect on the release of loaded molecules with the dual stimulation may obviate the need for harsh conditions such as UV light or extreme pH stimulation, rendering the system more applicable under mild conditions.