The generation of superoxide and hydrogen peroxide by exposure of fluorescent whitening agents to UVA radiation and its relevance to the rapid photoyellowing of whitened wool

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Abstract

Various fluorescent whitening agents (FWAs), chosen from the three major classes used commercially on textiles (stilbenes, pyrazolines and coumarins), produced hydrogen peroxide and superoxide radical anions when irradiated in aqueous solution with UVA light at 366 nm, near their absorption maximum. In contrast, none of these FWAs produced singlet oxygen on irradiation under similar aqueous conditions. The formation of superoxide, rather than 1O2, suggests a mechanism where the excited singlet state of the FWA may undergo ionization to produce either an FWA radical cation and a free electron which is accepted by molecular oxygen, or an electron transfer reaction via formation of semi-reduced and semi-oxidized FWA radicals. Aqueous tryptophan also generates hydrogen peroxide and superoxide when irradiated at 366 nm, but the rate of H2O2 production increases significantly in the presence of an FWA. When wet FWA-treated wool fabrics are irradiated with simulated sunlight, they produce significantly more H2O2 (by a factor of four) than peroxide-bleached wool. Photogeneration of H2O2 and O2radical dot by electron transfer reactions from the excited state of the FWA, rather than energy transfer to 1O2, probably contribute significantly to the rapid photoyellowing of wet FWA-treated wool and silk fabrics which remains a serious commercial shortcoming of these fibres.

Introduction

Fluorescent whitening agents (FWAs) are important additives which improve the appearance of various commercial products, particularly in the paper, plastics, detergent and textile industries. In the absence of FWAs these products usually absorb in the blue spectral range of natural sunlight, causing them to appear off-white or yellow. FWAs absorb UVA radiation in the range 360–380 nm and reemit visible blue or violet light as fluorescence. Small quantities of FWAs dispersed into appropriate products can compensate for the unwanted yellow appearance, making them appear whiter and brighter.

There are three major chemical classes of commercial FWAs, based on the stilbene, coumarin and pyrazoline structures, of which the most widely used are the stilbenes, including distyryl biphenyls, (DSBP). The photochemistry of FWAs has been extensively studied, both in solution and in the solid phase, as stability to sunlight is important in maintaining a white product with good lightfastness. The presence of oxygen and moisture during irradiation eventually leads to oxidation of the FWA which is then rendered less effective, causing the substrate ultimately to revert to a yellow appearance. Photoisomerisation of the fluorescent trans-isomer to the non-fluorescent cis-isomer is also important for stilbene whiteners, especially in solution [1]. However this gradual loss of effectiveness of the whitener through life does not explain the very rapid photoyellowing observed when FWAs are applied to the proteinaceous fibres wool and silk, especially when wet [2], [3]. The severe lack of photostability for whitened wool products is an ongoing commercial shortcoming, and a problem that would clearly benefit from a better understanding of the photochemistry. Of particular relevance to the photoyellowing of FWA-treated wool by sunlight is the fact that no yellowing occurs in the absence of atmospheric oxygen [4].

It is well known that irradiation of certain dyes in dilute solution can produce singlet oxygen (1O2) via a Type II or energy transfer process between the excited triplet state of the dye and molecular oxygen in its ground triplet state (Scheme 1, where D represents the dye molecule). Common examples of singlet oxygen sensiters are Rose Bengal, Methylene Blue and Eosin Y.

In the presence of an electron donor (E) some dyes generate H2O2 and O2 via an electron transfer mechanism shown in Scheme 2 [5]. At dye concentrations >10−5 M, the ground state of the dye itself can sometimes act as the electron donor, resulting in semi-reduced and semi-oxidised dye radicals (Equation (6)) [6].

Some dyes have the ability to act as both singlet oxygen sensitisers and H2O2/O2radical dot generators, depending upon the particular conditions [6]. It was of interest to consider which of these mechanisms is dominant when FWA-treated textiles are exposed to sunlight, particularly for wool. The theory that singlet oxygen is the primary reactive oxygen species involved in the photoyellowing of untreated wool was first proposed in 1976 by Nicholls and Pailthorpe [7], and extended to include bleached wool and wool treated with a fluorescent whitening agent (FWA) in a subsequent publication by Nicholls in 1980 [8]. Nicholls suggested that interaction of the triplet FWA molecule with oxygen to produce 1O2 (Scheme 1) was the more likely mechanism for the photoyellowing of FWA-treated wool than an electron transfer process [8].

In recent years significant doubt has been cast on the singlet oxygen mechanism of wool photoyellowing. One major criticism is that the photoyellowing of wool is far more rapid when the wool is wet, but the lifetime of singlet oxygen in water is 4.2 μs, compared with 14 ms in the gas phase [9]. Recent work by Millington and Kirschenbaum [10] has shown that the rate of photoyellowing of wet wool by simulated sunlight in water and in D2O is very similar. The lifetime of 1O2 in D2O (67.8 μs) is significantly higher than in water, and reactions which involve 1O2 are usually greatly enhanced in D2O [9]. Millington and Kirschenbaum also showed that hydroxyl radicals are produced when wet wool is irradiated with both UVA (366 nm) and blue (425 nm) light using a fluorescent probe [10]. The amounts of radical dotOH produced on irradiation of wool are the same in solutions of the probe in H2O and D2O, showing that the formation of radical dotOH does not occur via 1O2. Although 1O2 has been detected in irradiated wool by Smith [11], he suggested that singlet oxygen is involved in the photobleaching of wool. He found that when wool is irradiated at 265 and 350 nm, 1O2 was detected at the higher wavelength only. He also postulated that photoyellowing of wool by sunlight is much faster in the wet state because any 1O2 generated by visible wavelengths, which would lead to concurrent photobleaching in the dry state, is rapidly quenched by water [11].

It is also relevant that in earlier work on FWA-treated wool fabrics exposed to simulated sunlight when wet, hydrogen peroxide was detected during the rapid photoyellowing of the wool [12]. When FWA-treated wool was doped with H2O2 before irradiation it yellows rapidly. However in the presence of reducing agents such as sodium bisulphite or thiourea dioxide, far less yellowing was observed. It was suggested that the generation of H2O2 during exposure to sunlight could be responsible for the increased rate of yellowing of FWA-treated wool, especially when wet [12].

It was not clear from earlier work whether the wool protein, the FWA or both were involved in the photochemical mechanism resulting in H2O2 generation, although only wet FWA-treated wool produced measurable amounts of H2O2 after irradiation with simulated sunlight [12]. Certain proteins and their photoproducts, particularly those found in eye lenses such as β-crystallin, have been shown to generate H2O2 and superoxide when exposed to UVA radiation [13], [14], [15]. Previous work also has shown that irradiation of pigment dispersions such as cadmium sulphide [16], zinc oxide [17] and metal-free phthalocyanine [18] can also generate superoxide and H2O2.

In this study aqueous solutions of FWAs chosen from the three major classes used commercially on textiles (stilbenes, pyrazolines and coumarins) were irradiated using a UVA source in the presence of atmospheric oxygen and analyzed for H2O2, superoxide radical anion and singlet oxygen. The assay methods used in this study have recently been applied to the study of reactive oxygen species produced in human eye lenses during cataract formation [13], [15], [19].

Some further studies were also carried out on FWA-treated wool fabrics and, together with data from previous published studies, the probable photochemical mechanisms involved in the yellowing of FWA-treated wool are discussed.

Section snippets

Materials

l-Tryptophan, imidazole, methylene blue (MB), xylenol orange (XO), sorbitol, iron (II) ammonium sulphate and N,N-dimethylnitrosoaniline (RNO) were obtained from Sigma Aldrich. The enzymes catalase (ex bovine liver) and superoxide dismutase (SOD) were obtained from Bohringer Mannheim (Castle Hill, NSW). The fluorescent whitening agents Uvitex NFW, CF and WGS solids (Ciba) and Leucophor PAT, Hostalux PN and Hostalulx N2R liquids (Clariant) were kindly supplied by the manufacturers. Of these only

Detection of H2O2 and O2radical dot in FWA and tryptophan solutions

The three major classes of FWA that can be applied to textiles, stilbenes, pyrazolines and coumarins, have different structures and chemistries, but it has been shown previously that all three types of FWA vastly increase the rate of photoyellowing of wool [3]. Since a previous preliminary study [21] on two stilbene FWAs in solution also showed that both produced H2O2 when irradiated with UV light near their absorption maxima at 366 nm, it was of some interest to see if H2O2 production was

Discussion

The almost linear increase in H2O2 formation with time during UVA irradiation of stilbene and pyrazoline FWAs and the increased levels of H2O2 formed in the presence of SOD (Fig. 3) suggest that the H2O2 is produced by dismutation of the superoxide anion as shown in Scheme 3. The fact that no 1O2 is produced by irradiated FWA solutions is further evidence for an electron transfer rather than an energy transfer mechanism. The reactions shown in Scheme 2 are certainly one possibility, but a

Conclusions

It has been demonstrated that aqueous solutions of several commercial FWAs from the three main classes (stilbenes, pyrazolines and coumarins) produce both the superoxide anion and H2O2 and not singlet oxygen on exposure to UVA radiation. The formation of superoxide suggests that an electron transfer process from the FWA to oxygen is the dominant mechanism. This probably occurs either via formation of semi-reduced and semi-oxidized dye radicals as shown in Scheme 2, or via a direct Type I

Acknowledgements

Funding for this project was provided by Australian wool growers and the Australian Government through Australian Wool Innovation Limited.

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