Field of the invention The present invention relates to the provision of UV sensitive compositions and sensors comprising the compositions. The sensors can find use in a wide range of applications including as dosimeters for indicating the amount of exposure to UV light of a person or an article.

Background of the Invention

Ultraviolet radiation (UVR) is the term used to describe the section of the electromagnetic spectrum, which lies between x-rays and visible light. The greatest natural source of UV radiation is the sun, although many artificial sources also exist including: black- lights, halogen lights, fluorescent and incandescent lamps, welding arcs and certain types of lasers. UVA, i.e. electromagnetic radiation spanning the wavelength range 315-400 nm, is the most common, naturally-encountered form of UVR, as only a small portion of solar UVA is absorbed by atmospheric ozone while the reverse is true for solar UVB i.e. 280-315 nm a large portion of which is blocked by the ozone layer.

On a typical summer' s day approximately 6% of terrestrial UV light is UVB but this contributes 80% towards the harmful effects associated with the sun, while the remaining 94% UVA contributes to the other 20%. ^λ Both UVA and UVB radiation have been shown to increase biological melanin production and hence pigmentation of human skin, consequently both are used in tanning booths. ² The higher energy UVR UVC light, (λ<280 nm) is not observed in nature at significant levels as it is absorbed by the earth' s atmosphere. Applications of artificial UVC sources include the use of germicidal lamps, i.e. Hg fluorescent tubes without phosphor coating, to destroy bacteria, ³ ' ⁴

Over-exposure to UVR can be hazardous to human health. The severity of the damage will depend on the source of UV, its wavelength and intensity, the duration of exposure and an individual' s sensitivity. Short term exposure can cause acute effects such as erythema i.e. the reddening of the skin or sunburn, and enhanced melanogensis, the process that causes an individual to develop a suntan. While the latter still remains a largely socially-desired side effect, the former is not. UVR can also damage DNA under the skin resulting in local immune suppression. ⁵ Cumulative DNA damage can encourage the development of skin cancers such as melanomas. ^6"11 In the UK, approximately 8,000 malignant melanoma cases are diagnosed and around 1,800 die from the condition ¹² each year

The amount of solar UV radiation absorbed by the skin at any time is known as the erythemal dose. In quantifying an individual' s personal exposure to UVR, the term the

^minimum erythemal dose' (MED) is useful, where the MED is defined as the minimum amount of radiation likely to cause erythema. The MED for an individual is largely dependent on their skin type of which 6 types, I - VI, have been defined ¹ where a skin type of I is associated with skin with little natural melanin (red-heads) and so likely to burn quickly and type VI, is associated with skin with very high levels of natural melanin (very dark skinned individuals) and is therefore much more resistant to erythema.

As a consequence, there is an increasing need for an inexpensive, disposable personal UV dosimeter, which would provide an individual with a continuous measure of their total UV exposure during the day. There are many- electronic UV dosimeters available, unfortunately most are rather bulky and relatively expensive for personal use. ¹⁵

A number of promising UV-activated photochemical reactions have been identified as possible routes to generate an effective dosimeter, and some have been commercialised. One such commercial product is the solar wrist band developed by Solar Safe ^®17 for people with skin type II. It comprises a polymer matrix in which a structurally photochromic material is dispersed. The wristbands are personal dosimeters, which change colour upon exposure to UVR and are designed to indicate when to apply sunscreen and when it is time to get out of the sun.

SunHealth Solutions LCC has produced a dosimeter called a SunSignal UV Sensor. ¹⁸ These respond specifically to UVB radiation and are comprised of a radiation sensitive material including an organic halogen such as hexachloroethane, which is capable of producing at least one acidic product, such as HCl, upon UV exposure, and a pH indicator, such as methyl orange, capable of producing a colour change in response to the UV generation of the acidic product. ¹⁹

In a previous application (WO03021252) we disclosed a colorimetric UV sensor that utilises a semiconductor photocatalyst . For example the UV sensor can comprise a hydroxy ethyl cellulose film containing: a redox dye, for example methylene blue (MB) , a sacrificial electron donor such as triethanolamine (TEOA) and as photocatalyst, titania nano particles (TiO ₂ ) . When used without an oxygen (O ₂ ) barrier i.e. naked, the film acts as a UV indicator in which the TiO ₂ nanoparticles absorb the UV light enabling it -A- to photo-oxidise the TEOA and simultaneously reduce the redox dye from its highly coloured (blue) oxidised form, (MB) to its colourless form, leuco methylene blue (LMB) .

Oxygen present in air reacts rapidly with the LMB, re- oxidising it to MB. As a result the degree of bleachedness exhibited by a naked MB film of this type is directly dependent upon the level of UVR, due to a dynamic equilibrium between the photoreduction process (MB→LMB) and the dark re-oxidation process (LMB + O ₂ →MB) . Therefore in an atmosphere of essentially constant oxygen content (the air) the colour of the film will be dependent on the currently incident UV radiation and will not show a measure of the cumulative UV dose.

When covered with an O ₂ impermeable barrier, such as glass or regenerated cellulose, the re-oxidation step is not possible and the covered MB film is able to act as a UV dosimeter. As the oxygen barrier prevents return of the dye towards the reduced state the colour of the film shows the dose of UV absorbed.

It is an object of the present invention to provide alternative UV sensors that can find use as dosimeters for measuring an individual' s exposure to UV radiation and in other applications where an indication of or measurement of exposure to UV light is desired.

Summary of the Invention

According to a first aspect the present invention provides a composition for detecting ultraviolet light comprising: at least one redox sensitive dye which displays different visible properties in the oxidised and reduced forms and is reactive to oxygen when in its reduced state,- at least one electron donor,- at least one semiconductor material specifically sensitive to light of about 200-400nm; wherein upon irradiation of said semiconductor material by light of about 200-400nm an electron is donated by the electron donor to the semiconductor material which in turn provides an electron to the redox sensitive dye causing the redox sensitive dye to be reduced; and wherein the pH of the composition is at a neutral or sufficiently acid pH so as to render the reduced form of the dye insensitive to oxidation by oxygen.

The redox sensitive dyes used in the invention display a high sensitivity to oxygen when in their reduced (leuco) form. They are usually rapidly oxidised back from the reduced state to the oxidised state in the presence of atmospheric oxygen, for example. The redox sensitive dyes may be azine, oxazine, thiazine or indophenol dyes. Other redox sensitive dyes may be used. It will be understood that the term redox sensitive dye refers to a substance that exhibits colour in the visible light region or fluorescence .

Surprisingly it has been found that at a suitable pH, neutral or a selected acid pH, depending on the on the dye and composition used, the compositions of the invention can show a different behaviour. The normal oxidation of the reduced form of the dyestuff by oxygen in air can be significantly slowed and even stopped. This effect allows the use of readily available and inexpensive, but normally air sensitive when reduced, dyes, in sensors of the invention.

As mentioned above, the semi conductor material is specifically sensitive to light of about 200-400nm (ie. UV light) and this is understood to mean that the semi conductor material is substantially insensitive to light outside ^• the range of about 200-400nm.

Thus, when irradiated by UV of the appropriate wavelength (i.e. light of energy greater than or equal to its bandgap) the semiconductor material becomes electronically excited, i.e. activated under UV irradiation. The electronically excited state of the semiconductor material is a better oxidising agent than its non-excited, ground-state, form. As a consequence the excited semiconductor material is able to oxidize the electron donor present in the sensor formulation.

The electron donor is chosen so that this process is irreversible, i.e. the electron donor is sacrificed. The key products of the above photoinduced electron transfer reaction are the irreversibly oxidised form of the sacrificial electron donor and the reduced form of the semiconductor material. The latter then reduces the redox- sensitive dye from (typically) its highly-coloured form, to its less-coloured, reduced form. The semiconductor material is also able to store, or pool, reduction potential on its surface and/or reduce other species present so that they can act as an electron pool to reduce the redox- sensitive dye. The depth of this electron pool will depend directly upon the duration of the irradiation; the longer the deeper. The overall process enables the semiconductor material to return to its original state, ready to absorb another photon of light and begin the process of electron transfer from the electron donor to the redox-sensitive dye again. As a consequence, the semiconductor material is usually referred to as a photocatalyst, or photosensitiser, i.e. a material that absorbs light and then effects a change but, is itself left unchanged at the end of the process.

The above describes in general terms the principles behind the light-activation step that turns the redox sensitive dye present in the composition from its initial usually coloured (and/or non-fluorescent) form to its reduced

(usually non-coloured and/or fluorescent) form.

For the necessary reactions and the colour and/or fluorimetric change to occur the redox sensitive dye and the other components of the composition must be brought into intimate contact. Thus, the composition may be in the form of a tablet or pellet in which the components are, for example, pressed together, or as a plastic film in which the components are encapsulated in some medium, such as a polymer. Alternatively, the composition may be in the form of an ink which may be printed to form a label, logo or text i.e. writing.

The intimate contact of the various components allows the composition to undergo a reduction reaction wherein there is a transfer of electrons from the photogenerated reduced form of the semiconductor material to the redox sensitive dye .

Azine, thiazine, oxazine and indophenol dyes may be used in the invention. Examples of azine dyes include but are not limited to: methylene violet 3RAX or safranine O, examples oxazine dyes include but are not limited to: resazurin and cresyl violet acetate, examples of thiazine dyes include but are not limited to: methylene blue and thionin. Preferred dyes include dichloroindophenol (DCIP) and methylene blue (MB) . A mixture of dyes may be used. It will be understood that the selected dye or dyes may be used in the manufacture of a composition of the invention in their usual, commercially available, form. For example methylene blue may be used as the chloride salt and dichloroindophenol as the sodium salt. Other forms of a chosen dye may be used.

Typically the pH of the composition is neutral (7) or lower as discussed below with reference to specific examples. Preferably the pH is pH 2 or less for methylene blue and pH 6 or less for dichloroindophenol. The desired pH, where the selected dye or dyes is not sensitive to oxidation is readily determined by simple experiment. Some compositions may not require pH adjustment to effect the removal of the sensitivity to oxidation.

If pH adjustment is required then the composition comprises an added acid component . Preferred acids that may be employed include hydrochloric, nitric, perchloric and sulfuric acids. Organic acids may also be used. The pH of the composition may be buffered to a selected pH by use of a buffering agent or agents if desired.

A composition for detecting ultraviolet light where the oxidation of the reduced form of the dye does not occur at a significant rate has a number of advantages over previously known compositions, especially when used in a UV sensor as discussed with respect to another aspect of the invention described hereafter. -S- The semiconductor material has the ability to form an excited electronic state that is sufficiently oxidising to oxidize the sacrificial electron donor and has a reduced form that is able to reduce the redox sensitive material.

By semiconductor material is meant materials which are usually solids which have an electronic structure comprising a nearly filled valence band and a nearly empty conductance band. The difference in energy between these two levels is called the bandgap of the semiconductor. A semiconductor material has a bandgap that typically lies in the range of 0.1-4eV and exhibits a degree of conductivity that is usually less than that of metals which have bandgaps less than 0. IeV but greater that that of insulators which have bandgaps greater than 4eV. The conductivity of the semiconductor material increases with increasing temperature. The semiconductor material may be classed as a photosensitiser or a photocatalyst i.e. a material that is able to promote a process through the creation of an electronically exited state generated by the absorption of a photon of light. The energy of the light is usually greater than or equal to the bandgap. The initial excitation of the system is followed by subsequent electron transfer and/or energy transfer, which results in overall photsensitised or photocatalysed reactions. In a photosensitised or photocatalysed reaction the photocatalyst or photosensitiser remains chemically unchanged at the end of the overall reaction.

The semiconductor material may be an oxide of titanium (such as titanium (IV) oxide; TiO ₂ or strontium titanate; SrTiO ₃ ), tin (such as tin(IV) oxide ,-SnO ₂ ) , cerium (CeO ₂ ) , niobium (Nb ₂ O ₅ ) , tantalum (Ta ₂ O ₅ ) , tungsten (such as tungsten(VI) oxide ,-WO ₃ ) or zinc (such as zinc (II) oxide ;

ZnO) and mixtures thereof.

As noted earlier, it is mostly the UVB component of solar UV that is responsible for solar- induced biological damage, such as sunburn. Therefore a semiconductor material that is more effective at absorbing UVB light than UVA light can provide a sensor that can is a dosimeter for measuring absorbance of the damaging UVB light. Titania has a band gap of only 3.2 eV and is effective at absorbing both UVA and UVB light. Accordingly a semiconductor material that selectively absorbs UVB light (280-315 nm equivalent to a band gap of 4.4 to 3.9 eV) is preferred, for example in applications where biological damage is measured. For other applications a semiconductor material that absorbs UV light over another selected range may be employed. In some applications a mixture of two or more semiconductor materials may be used in a composition of the invention, for example to broaden the range of sensitivity to UV light.

More preferably, for UVB selectivity the semiconductor material has a band gap of between 3.4 and 4.4 eV. Even more preferably the semiconductor material has a band gap that lies in the range 3.6 to 4.1 eV. Most preferably the semiconductor material has a band gap of 3.6 eV. Preferably the semiconductor material is tin (IV) oxide (SnO ₂ ) , most preferably nanocrystalline tin (IV) oxide (SnO ₂ ). SnO ₂ has the highly preferred band gap of ca. 3.6 eV and can be used to prepare highly effective compositions that are selective for UVB as discussed hereafter with reference to specific embodiments. The electron donor has the ability to donate electrons, preferably irreversibly. Typically, the electron donor is a mild reducing agent, at the pH selected to render the dye insensitive to oxidation. The electron donor may, for example, be an amine for example ethylenediaminetetraacetic acid disodiutn salt or triethanolamine (Na ₂ EDTA or TEOA) , reducing saccharide (such as glucose and fructose) , readily oxidisable polymer (such as polyvinyl alcohol) , and other general anti-oxidant (such as ascorbic and citric acid) or easily oxidizable material (such as a polyol, such as glycerol) and/or mixtures thereof.

It will be appreciated that certain reducing agents are basic, for example TEOA, and therefore act to increase the pH environment of the dye in a composition. Where a basic component such as TEOA is used in a composition of the invention adjustment of pH with acid may be required to achieve the required inactivation of the oxidation of the dye. Glycerol is a preferred electron donor.

The compositions of the invention may further comprise a binder which binds all the components together. The binder may be a polymeric material such as gelatin, hydroxyethyl cellulose (HEC) , polyvinyl alcohol (PVA) , ethyl cellulose (EC) , cellulose acetate (CEA) , polyvinylpyrrolidone (PVP) , polyvinylbutral (PVB), polyethylene oxide, and polymethylmethacrylate (PMMA) .

The oxidation pathway for the conversion of the reduced form of the dye to the oxidised form in compositions of the invention has been switched off by the choice of an appropriate pH. The composition can therefore be used in a sensor, which can be a reliable dosimeter for measuring exposure to ultraviolet light. Thus according to a second aspect the present invention provides a sensor for detecting UV light including a composition comprising: at least one redox sensitive dye which displays different visible properties in the oxidised and reduced forms and is reactive to oxygen when in its reduced state; at least one electron donor; at least one semiconductor material specifically sensitive to light of about 200-400nm; wherein upon irradiation of said semiconductor material by light of about 200-400nm an electron is donated by the electron donor to the semiconductor material which in turn provides an electron to the redox sensitive dye causing the redox sensitive dye to be reduced; and wherein the pH of the composition is at a neutral or sufficiently acid pH so as to render the reduced form of the dye insensitive to oxidation by oxygen.

The compositions may be supported on at least one inert material, such as glass, paper, fabric, ceramic and metal.

The composition may be in the form of an ink and printed on a substrate to form a sensor.

Starting with a sensor where the composition has its dye component in its fully oxidised (normal) state, exposure to ultraviolet light of wavelengths that the semiconductor absorbs results in reduction of the dye by the mechanism discussed above for the prior art sensors of WO03021252. As the corresponding reverse reaction, oxidation by air, does not occur, or is greatly reduced, the colour change of the dye is effectively irreversible and provides an indication of the total, i.e. time integrated, exposure to ultraviolet light. The behaviour of the sensor can be calibrated to provide a measure of the actual exposure to the ultraviolet light that is independent of any exposure to oxygen (typically from the air) .

Thus, according to a third aspect the invention provides a dosimeter for measuring a quantity of UV light comprising a sensor according to the second aspect of the invention.

Use of a sensor of the invention as a dosimeter is advantageous in comparison to the dosimeters of WO03021252 where the oxidation reaction is prevented by placing an oxygen (O ₂ ) impermeable barrier over the redox system. Such a barrier may have limited effectiveness and if it does leak oxygen then the oxidation reaction will occur leading to an inaccurate indication of UV absorbed.

For example the sensor is used as a disposable device formed to be worn by an individual as a UV dosimeter to show when too much exposure to sunlight occurs. For example the device may take the form of a plastic wristband that includes an area coated with or impregnated with the UV sensitive composition. The sensor may be used in label form with the composition printed on the label as an ink, for example. The sensor may be in the form of a transfer applied to the skin or a composition in the form of an ink may be used to mark directly on to skin. In use such sensors, and prior art sensors with O ₂ sensitive dye, can be coated with sunscreen to provide a measure of the UV dose absorbed through the sunscreen if desired.

These sensors may be subject to damage especially if being worn during sporting activities. In such circumstances if the composition used is of the, prior art, air (oxygen) sensitive type measurement of UV absorbed could be invalid.

Even if fitted with an oxygen barrier the reverse (oxidation) reaction could occur as a result of leakage through a damaged oxygen barrier leading to false, low results. Making use of a composition of the present invention can avoid this problem.

A composition of the present invention the sensor will provide correct results. The operation of the dosimeter will not be compromised by oxygen ingress through a damaged barrier layer .

Furthermore by selection of the appropriate amounts and type of each component of the photo sensitive composition a sensor can be constructed where the end point of reaction (i.e. the point where all of the dye in the composition has been converted to the reduced form) indicates that a selected quantity of ultraviolet light has been absorbed. For example where methylene blue (MB) is utilised the reduced form leuco methylene blue (LMB) is colourless. A sensor can be prepared where the change to colourless indicates absorption of a chosen quantity of ultraviolet light.

A sensor may comprise an array of two or more compositions each with different end points of reaction, for example of decreasing sensitivity, allowing indication of the absorption of increasing amounts of UV. For example a sensor using methylene blue may include a row of adjacent bands or areas having compositions of decreasing sensitivity. As each band becomes colourless in succession the increasing dose of UV absorbed over time is particularly easily seen. A sensor may have two or more compositions with sensitivity to different wavelengths of UV light. For example a sensor may be made with an area having a composition sensitive to UVA and another area a composition sensitive to UVB. For example, by selection of the semiconductor material as discussed above. Such a sensor can find application in tanning salons where UV light is used to tan an individual' s skin. The sensor can show the dose of UVA administered. At the same time the sensor indicates if a level of the, more damaging, UVB that is considered unsafe has been administered.

Alternatively using a composition sensitive to both UVA and UVB (200-400nm) and another only sensitive to UVB can also be useful. The UVB sensitive composition will warn of overexposure to the more damaging UVB light whilst the UVA and UVB sensitive composition gives a measure of overall exposure to UV (dose of tanning radiation) .

Sensors may include permanently coloured areas, whose colour corresponds to that of the UV sensitive composition when a particular amount of UV has been absorbed. Visual comparison between the composition and a coloured area or coloured areas allows estimation of the UV light absorbed.

The above methods are particularly useful, for example where a personal dosimeter to determine exposure of an individual to sunlight is desired. Alternative means of assessing the amount of UV absorption may be employed, especially in other applications. For example the colour

(or fluorescence) of the UV sensitive composition may be measured using a spectrophotometer. This approach can be useful when an automated measurement of UV dose is desired.

For example, for a large number of sensors or where particularly accurate results are required. Sensors of the invention may be used in tags attached to large numbers of items being subject to a UV sterilisation procedure. The sensors provide a measure of the UV exposure that can be rapidly confirmed by the results from the spectrophotometer and collated automatically for analysis in a computer.

Other means can be employed to determine a dosage of UV exposure. For example, if the composition is formed in a suitably thin layer then the layer will be relatively transparent when a coloured dye is reduced by UV absorption. A light sensitive layer (for example a visible light sensitive photovoltaic layer) may be provided under the composition. The light sensitive layer detects light passing through the composition when the colour of the dye is reduced and leads to an alarm signal being generated.

Typically, the compositions of the invention are stable with the dye in its oxidised state for many months provided they are protected from UV light. Sensors of the invention may therefore be provided with a removable UV impermeable barrier. For example a sheet of UV impermeable plastics material such as polymer containing a polyoxyalkylene, provided with a layer of a peelable pressure sensitive adhesive that is attached to the sensor to cover and protect the UV sensitive composition from UV light until it is removed at the point of use .

Brief description of the Drawings

Fig.l shows Absorption spectra of a sensor of the invention which includes a composition of the invention,- Fig.2 a and b show photographs of a sensor before and after UV irradiation; Fig.3 is a schematic illustrating reaction processes for a typical UV dosimeter;

Fig.4 shows response to UV light over time of a preferred embodiment of the sensor of the invention; Fig.5 shows absorption change with time of sensors of the invention following irradiation with different strengths of

UVB;

Fig.6 shows absorption change with time of sensors of the invention containing different dye concentrations; Fig.7 shows absorption change with time of sensors of the invention containing different amounts of photocatalyst ; and

Fig.8 shows absorption change with time of sensors of the invention containing different amounts of electron transfer agent

Detailed Description of the Invention with reference to the preparation of compositions of the invention and results of tests

Materials

All chemicals were purchased from Aldrich Chemicals unless otherwise specified. The water used to produce inks was double distilled and deionised, and the polymer used to produce the films was hydroxy ethyl cellulose (HEC) (medium viscosity) which was purchased from Fluka. 2,6- dichloroindophenol sodium salt hydrate, 98% dry weight, was purchased from Alfa Aesar. The tin (IV) oxide used was a nanopowder, (<100 nm particle size (BET) ) also purchased from Aldrich Chemicals.

Methods

UV visible spectra for sample films were recorded using a Lambda 35 UV Visible spectrophotometer (Perkin Elmer, UK) . The DCIP (dichloroindophenol) UV dosimeter films were irradiated for 300 seconds with spectra recorded at varying intervals .

UV irradiation of samples was carried out using UVA or UVB light provided by two 8 W fluorescence tubes (Vilber Lourmat) , with the appropriate emission spectra maximum peak in these regions i.e. at 365 and 315 nm respectively) . The irradiance (i.e. radiant power per unit area) for each lamp was measured as 3 mW cm ^"2 using a Multi-sense 100 UV light meter fitted with the appropriate UVA and UVB sensors .

The UV solar simulator used in this work comprised a 180 W xenon arc lamp, with UG5 and the WG20 filters placed inline as described previously by Diffey ² . The former allows transmission at UV wavelengths and absorbs in the visible region, while the latter absorbs in the short wavelength UVC region. The UVI of the UV solar simulated light was measure using a SafeSun™ solar meter ¹⁷

UV sensitive ink compositions and preparation of dosimeters A typical UV dosimeter casting ink, was prepared in the absence of significant levels of UV light (i.e. ambient room light) by dissolving 5 g of HEC in 95 ml water at room temperature, followed by stirring for 24 hours. 5 mg of DCIP, 100 mg SnO ₂ and 100 mg glycerol were then added to 2 g of the HEC polymer solution. The suspension was well stirred to ensure dissolution of the dye and dispersion of the SnO ₂ . The blue-coloured casting solution contained 5 phr of DCIP i.e. 5 parts per hundred resin (or 5 g of DCIP for 100 g polymer) . The pH of the composition was 6. The pH of ink compositions using the same components and prepared as above, is typically around pH 5.7. Films were cast on quartz discs, 25 mm in diameter and 1 mm, thick using a spin coater. Thus, a few drops of casting solution were deposited on the surface of the disc, which was then spun at 2400 rpm for 15 seconds. The final product was then dried for 2-3 minutes at 70 ⁰ C and cooled to room temperature (5 minutes) before use. The final UV dosimeter film product was a blue coloured, ca. 3.9 μm thick (as measured using a scanning electron microscope) on a quartz disc, referred to forthwith as a standard DCIP film. This product constitutes a sensor of the invention with the film being a composition of the invention.

Optical characteristics of a DCIP film

A series of casting solutions were prepared comprising the standard DCIP UV sensitive ink formulation with various components omitted, with the exception of the encapsulating polymer HEC and solvent, water. These solutions were used to cast the following films on quartz discs: HEC, Glycerol/HEC, DCIP/HEC and the typical dosimeter itself DCIP/Sn0 ₂ /Glycerol/HEC. The UV/Visible absorption spectra of these films were recorded and the results are illustrated in Figure 1. Line A shows the absorption spectrum of an HEC film and also of a HEC/glycerol film. Line B shows absorption spectrum of an HEC/DCIP film and line C shows the absorption spectrum of the DCIP/Sn0 ₂ /Glycerol/HEC composition prior to radiation.

From Figure 1 it is clear that the polymer (HEC) , the electron donor (glycerol) and the redox dye, DCIP, do not absorb to any great extent in the UVA and UVB regions, whereas SnO ₂ does absorb significantly, especially in the

UVB region. SnO ₂ is an n-type, wide band-gap semiconductor (Eg = 3.5 eV i.e. absorption threshold 354 nm) ²⁰ and is responsible for the broad absorption shoulder between 200 and 290 nm seen in the UV visible absorption spectrum of the standard ink film (see Figure 1) The DCIP/HEC spectrum has a maximum absorption at 636 nm which gives the film its blue colour.

When irradiated with UVB light (λ _max (emission) = 315 nm) a typical DCIP film changed colour from blue to white/colourless as seen in Figure 2, where Figure 2a shows the film before irradiation and figure 2b shows the film after irradiation. This process was monitored spectrophotometrically via the disappearance of the absorption band due to the DCIP at 636 nm. The colour change observed arises as a result of photogenerated holes in the SnO ₂ reacting with the glycerol present, a sacrificial electron donor, to yield glyceraldehyde which can then be oxidised further to glyceric acid. The photogenerated electrons are then able to reduce DCIP to its leuco-form, which is stable in air when in the composition. These major processes are illustrated in Figure 3.

A typical set of UV visible absorption spectra recorded for a DCIP film as a function of UVB irradiation time are illustrated in Figure 4. UVB light of 3mWcm ^"2 was employed. Spectra were recorded every 10 seconds for the first 90 seconds and then every 30 seconds thereafter for a total of 300 seconds. The reduction in absorbance with time is clearly shown. Using this data, and those from the same experiment conducted using UVA light instead (also 3mWcm ^"2 ) , it was possible to plot the variation in change in absorbance at λmax i.e. ΔAbs _636/ as a function of irradiation time illustrated in the inset diagram in Figure 4. These results show that the photo- induced decolouration of the standard DCIP film was not observed when the film was irradiated with 3 mW cm ^"2 UVA light rather than UVB indicating that the light emitted by the UVA lamp (a black light lamp, λ _max (emission) = 365 nm) does not contain photons of sufficiently high energy to create the necessary electron- hole pairs to effect the photoreaction of DCIP. Other work showed that even when UVB light was used no decolouration occurred for a DCIP film in which the SnO ₂ had been omitted i.e. SnO ₂ semiconductor sensitizer is essential for the DCIP film to work as a dosimeter.

Recovery of films of a DCIP composition of the invention

In the reaction scheme of Figure 3 it is assumed that the photocatalysed bleaching of DCIP by glycerol, sensitised by SnO ₂ , is irreversible i.e. the reduced form of DCIP, colourless leuco DCIP, is not readily oxidised by ambient oxygen in the polymer/glycerol film environment. This feature is necessary for the DCIP composition to function as a dosimeter. In order to determine the post irradiation stability of a photobleached standard film, such a film was fully converted in to its bleached form by irradiating for 10 minutes under 3 mW cm ^"2 UVB light and the absorbance at 636 nm monitoring over the following 12 hours. These results revealed that over this time period the film only regained little (8%) of its original colour and for the purposes of a UV dosimeter, the photobleaching of DCIP can be considered as irreversible. Kinetics of photobleaching of a DCIP film

In one set of experiments, a number of standard DCIP films were exposed to different UVB irradiances,- specifically 1, 2, 3, 4, 5 and 6 mW cm ^"2 of UVB light for a total of 300 seconds. The absorbances of the DCIP films (at λmax) under test were measured spectrophotometrically and the initial rate, ri of decolouration for each film was determined from the plot of the change in absorbance at λmax (636) as a function for irradiation time. A plot of the change in ΔAbs ₆₃₆ versus irradiation time arising from this work is illustrated in Figure 5 along with the associated plot of r _± as a function of UV irradiance, shown in the inserted graph. Lines A to E on the main graph show respectively the application of 1, 2, 3, 4, 5 mW cm ^"2 of UVB light.

The observed direct dependence of r _± upon irradiance is not unusual in semiconductor photocatalysis and, in this case, is indicative of very effective trapping of the photogenerated holes by the glycerol that is present . The DCIP concentration in a standard DCIP film is 0.07 M (≡ 5 phr) and film thickness is 0.00039 cm, which allows the number of moles of DCIP per cm ^"2 i.e. n _o to be determined as 6.99xlO ^"8 moles cm ^"2 . Using this information, the line of best fit to the data in the inset in Figure 5

(0.0018 Abs units s ^"1 mW ^"1 cm ² ) and the initial ΔAbs ₆ 36 of a DCIP film (0.4055), the initial number of DCIP molecules that are photobleached per cm ² per second, Ri ^* was determined as 7.19xlO ¹³ molecules cm ^"2 s ^"1 . Since the UBV lamp used in this work emitted 1.59xlO ¹⁵ photons s ^"1 cm ^"2 , and the fraction of UVB light (i.e. light at 315 nm) absorbed by the DCIP film f (= ca.0.845), a quantum efficiency for the photobleaching process of 5.4% was calculated, which is not unusual in a semiconductor process involving an efficient sacrificial electron donor.

The effect of dye concentration on the kinetics of the system was investigated by preparing a series of films with different levels of DCIP. A set of casting inks were prepared containing 1 to 20 phr DCIP, which were cast on to quartz discs and irradiated with 3 mW cm ^"2 UVB light. The r _± of decolouration for each film was determined from the plot of AAbS ₆₁₂ against irradiation time illustrated in Figure 6 , and was then plotted against [DCIP] , as shown in the insert diagram in Figure 6. Lines A to E on the main graph show respectively the results for films containing 20,15,10,5 and 1 phr DCIP.

It can be seen that the r _± does not change significantly with [DCIP] suggesting the kinetics of DCIP photobleaching are zero order. Such zero order kinetics are not unusual in semiconductor photocatalysis and indicates that DCIP molecules occupy all the photocatalytic sites. This is hardly surprising as the standard film has an effective

[DCIP] of approximately 0.07 M in the film.

The sensitivity of the standard DCIP UV composition towards UVB light can be readily varied by changing the amount of UV absorbing semiconductor (SnO ₂ ) present. Thus, a series of Sn0 ₂ /DCIP/glycerol/HEC casting inks were prepared containing 10 to 200 phr SnO ₂ and used to spin films on quartz discs. Using these films a series of ΔAbs ₆ χ ₂ versus irradiation time profiles were generated with 3 mW cm ^"2 UVB source and the results are illustrated in Figure 7. The films contained 5 phr DCIP, 100 phr glycerol and the amount of SnO ₂ was 0,10,30,50,100 and 200 phr for lines A to F respectively. The ri of decolouration for each film was determined and found to be directly proportional to the level of SnO ₂ present over the range studied. As it is the UVB activation of the semiconductor which initiates the reduction reaction, it was expected that an increase in

[SnO ₂ ] would in turn cause an increase in the rate of reduction.

The effect of the level of glycerol present was also investigated. Glycerol acts as a sacrificial electron donor, trapping the photogenerated holes and thus promoting the reduction of the DCIP molecules by the photogenerated electrons. A series of casting inks were prepared, containing 0 to 300 phr glycerol, and were used to produce films which were then irradiated with 3 mW cm ^"2 UVB light. A series of ΔAbs ₆ i2 versus irradiation time profiles were generated as shown in Figure 8 and used to determine r _± of decolouration, which were then plotted against [glycerol] as seen in insert of Figure 8. The films contained 5 phr DCIP, 100 phr SnO ₂ and the amount of glycerol was 0,30,50,100,200 and 300 phr for lines A to E respectively.

These results show that above 100 phr ^'' the initial rate of photobleaching of DCIP is not improved, suggesting that hole-trapping is very efficient above 100 phr. More notable is the observation that photobleaching of the DCIP is effected in the absence of glycerol although at a much slower rate (line A) . This implies that SnO ₂ was able to utilise a source of electrons other than glycerol to effect the photoreduction of DCIP, most likely the polymer, HEC. Solar UV work using a DCIP film composition

In a final set of experiments UV solar simulated light (UVI 5) was used to irradiate a standard DCIP film. The observed variation of the absorbance of this film at

636 nm, i.e. ΔAbs ₆₃₆ , as a function of MED {^minimum erythemal dose') is illustrated in Figure 9. Line A shows the results for a standard HEC film of 5 phr DCIP, 100 phr glycerol and 100 phr SnO ₂ . Line B shows the results for A

HEC film of 5 phr DCIP, 100 phr glycerol and only 45 phr

SnO ₂ .

These results show that a standard DCIP ink is a little too UV sensitive for use as an indicator of impending sunburn to someone with skin type II at the point at which MED=I in Figure 9, as it is fully bleached by MED = 0.4 (line A) . However, previous work had demonstrated that the UV sensitivity of a DCIP/Sn0 ₂ /glycerol/HEC film can be moderated by using less SnO ₂ . (see Figure 7) Thus, in a separate experiment the level of SnO ₂ in the DCIP film formulation was dropped from 100 to 45 phr and its photobleaching monitored as a function of MED, the results of which are also illustrated in Figure 9 (line B) . These findings show that the latter film is bleached by an MED = 1 and so is an appropriate warning indicator of a solar UV dosage sufficient to cause sunburn in a person with skin type II. By varying the levels of SnO ₂ it is possible to make UV dosimeters that would indicate when sunburn was likely to be imminent for most other skin types. References

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