The effects of L-histidine on the light and perspiration stability of 4,4’-diamino-stilbene-2,2’-disulfonic acid-based fluorescent whiteness agents on cotton fabrics

Abstract

The influence of L-histidine on the light and perspiration stability of 4,4’-diamino-stilbene-2,2’-disulfonic acid-based fluorescent whiteness agents (DSD-based FWAs) for cotton fabrics was investigated in this paper. The results indicated that the single component L-histidine accelerated the photo-discoloration of DSD-based FWAs. In the absence of L-histidine, DSD-based FWAs tended to have poor light stability in the strongly acidic condition (pH < 4), whereas they had relatively high and consistent stability under the opposite circumstance (pH > 4). However, in the presence of L-histidine solution, the light stability of DSD-based FWAs was poorer than their counterparts without L-histidine, and was enhanced linearly with the growth of pH value. Therefore, absorption actions caused by L-histidine, oxidization-related promotion for FWAs and photo-decomposition of L-histidine in the acidic condition have been proposed to analyze the comprehensive effects of L-histidine in perspiration on the light stability of DSD-based FWAs on cotton fabrics.

Keywords

L-histidine fluorescent whiteness agents light and perspiration stability absorption effect chemical impact

The fluorescent whiteness agent (FWA), which is generally employed to enhance the whiteness and brightness characteristics of textiles, is a commercially important category of dyes that can absorb light in the near ultraviolet region and re-emit light in the visible region.^1–3 4,4’-diamino-stilbene-2,2’-disulfonic acid (DSD) is widely used as a chemical intermediate for most FWAs in the textile field because of its great conjugated system containing double carbon bonds.^4,5 With various remarkable advantages, including great affinity for fibers, high whitening efficiency and superior water solubility, DSD-based FWAs are the most popular for cotton fabrics.^6–8

However, previous investigations demonstrated that DSD-based FWAs on cotton fabrics tended to have poor light fastness resulting from the trans-cis photo-isomerization during the irradiation, and consequently had negative effects on the visual quality of cotton fabrics and even on the value of the whole garments.^6,9–11 Moreover, Zhuang et al.¹² disclosed that most dyed cotton fabrics, under the real situation, had more chance of being simultaneously exposed to human perspiration and sunlight during hot summer days, leading to the different light stabilities and photo-reaction mechanisms. Later, Li et al.¹³ also validated some toxic substances (aromatic amines), which could threaten human health due to direct contact with skin, when analyzing the photochemical degradation products of selected dyes mixed with artificial perspiration. Hence, the light and perspiration stability of DSD-based FWA on cotton fabrics is an urgent issue with far-reaching practical significance.

It is generally acknowledged that human sweat consists of numerous components, such as moisture (the major part), sodium chloride, lactic acid and L-histidine.^13–15 According to the conclusions drawn by Chen et al.,¹⁶ the light and perspiration stability of DSD-based FWA was apt to be influenced by both sodium chloride and lactic acid. As one of the crucial elements, L-histidine has been ignored regarding its contribution to the light and perspiration stability of FWAs on cotton fabrics. Furthermore, Okada and Kawanishi¹⁷ and Okada et al.^18,19 demonstrated that L-histidine played a significant role in the photo-fading of Cu-complex azo dyes on cellulose by different forms. In contrast, Millington²⁰ suggested that L-histidine caused the fluorescent quenching of FWAs and even the yellowing of their cotton substances via an accelerating effect on generation of active oxygen species (AOS) when exposed to simulating radiation.

The specific reasons for the above trends and the mechanisms of their processes still remain unclear today. Therefore, the objective of this research was to study the possible impacts of L-histidine on the light and perspiration stability of DSD-based FWAs on the cotton fabrics and analyze the interactions of these influences under simultaneous exposure to simulating irradiation and artificial sweat.

Experimental details

Materials and instruments

Scoured and bleached plain woven cotton fabric and two kinds of DSD-based FWAs (Figure 1) were used. The artificial perspiration (Table 1) was prepared according to the American Association of Textile Chemists and Colorists test standard (AATCC 15-2009). All the reagents used throughout this research were of analytical purity.

Figure 1.

The structures of 4,4’-diamino-stilbene-2,2’-disulfonic acid-based fluorescent whiteness agents (DSD-based FWAs).

Table 1.

Compositions of artificial perspiration of the American Association of Textile Chemists and Colorists (AATCC) standard^a

Components	L-histidine monohydrochloride monohydrate	Disodium hydrogen phosphate	Sodium chloride	Lactic acid (85%)
Concentration (g/L)	0.25	1.0	10.0	1.0

The pH value of perspiration should be controlled at 4.3 ± 0.2.

Application of DSD-based FWAs

A dye bath (200 mL) was prepared at room temperature (20℃) in a dark environment with FWAs (0.2% o.w.f.) and sodium sulfate (8% o.w.f.). The cotton fabrics (5 g) were boiled in distilled water for 30 minutes and dried in a vacuum oven at 60℃ before being immersed in the dye bath. Subsequently, the dye bath temperature was raised to 50℃ in 30 minutes, and then sodium sulfate (12% o.w.f.) was evenly added every 10 minutes in two installments. After being dyed at 50℃ with continuous stirring for 30 minutes, the fabrics were removed, rinsed thoroughly with distilled water and finally dried naturally. The pH value of the dye liquor was controlled by sodium bicarbonate between 7 and 9 and all of the operations were completed in the dark circumstance.

Irradiation procedure

The artificial perspiration of AATCC standard (105% o.w.f.) or the solution of single component L-histidine monohydrochloride (105% o.w.f.) was evenly dripped on the surface of whitened cotton fabrics. Then, the treated fabrics were promptly placed in the XENOTEST150S+ light fastness test instrument. The fabrics exposing to the simulated daylight with the distilled water (105% o.w.f.) were set as the control group. The tested samples were exposed to simulated sunlight induced by a 1500 W xenon-arc lamp for 20 h, with the temperature and the relative humidity controlled at 35℃ and 40%, respectively.

Measurements of the whiteness

The whiteness degree of brightened cotton fabric after irradiation was detected using the Datacolour SF650 and was evaluated according to the International Commission on Illumination (CIE). For each whitened cotton fabric, an identical experiment was carried out three times, and thus all the whiteness results reported in this study are an average of the values obtained from the three same samples, with all errors marked on the figures.

Results and discussion

The whiteness degree of cotton fabrics treated with DSD-based FWAs after being exposing to imitating irradiation for 20 h in different conditions viz. artificial perspiration of AATCC standard, distilled water and L-histidine solution is shown in Figure 2. It indicates that the sequence of order of the whiteness is the following for the two selected DSD-based FWAs: artificial perspiration of AATCC standard > Control > L-histidine, which suggests that despite the slight influence of the distilled water in solution, the L-histidine itself accelerates the photo-discoloration of DSD-based FWAs on the cotton fabrics, compared with the artificial perspiration of the AATCC standard. In order to make the effects caused by the L-histidine contained in artificial perspiration clearer, the studied DSD-based FWAs were separated in order to expose them to the simulating light under different pH value-based environments with or without L-histidine.

Figure 2.

The whiteness of brightened cotton fabrics after irradiating for 20 h in different conditions.

Influence of pH values on the light and perspiration stability of DSD-based FWAs

Figures 3 and 4 illustrate that the pH value of the illuminating environment plays a dominant role in the light stability of DSD-based FWAs on cotton fabric. Specifically, DSD-based FWAs in the absence of L-histidine tend to have poor light stability under the strongly acidic conditions (pH < 4), while relatively high and consistent stability can be obtained with the opposite circumstance (pH > 4).

Figure 3.

The whiteness of cotton fabrics treated with FWA-DMS after exposure to imitating irradiation for 20 h with or without L-histidine in different pH conditions.

Figure 4.

The whiteness of cotton fabrics treated with FWA-VBU after exposure to imitating irradiation for 20 h with or without L-histidine in different pH conditions.

The pH-related protonation of the DSD-based FWA resulting from its nitrogen atom (N) is the main contributor to this trend. In the strongly acidic conditions, the combination of the amide group (-NH-) contained in the DSD-based FWA and the hydrogen proton (H⁺) in the acidic solution promotes the forming of an imide ion (-NH²⁺) with strong electron absorption property, thus changing the chromophore of the FWA and weakening its intramolecular charge transfer ability. Consequently, the fluorescent property and the whitening performance of the DSD-based FWA are reduced significantly.^21–23 In addition, due to its rigid plane structure induced by the stilbene-structural conjugate system, the FWA is apt to aggregate and trans-cis photo-isomerize in the acidic condition, facilitating the decrease of the light stability.

Absorption effect of L-histidine on the light stability of DSD-based FWAs

It is particularly noticeable that the light stability of the DSD-based FWA in the absence of L-histidine is considerable higher than its counterpart with L-histidine, although they are exposed to the solar-simulated radiation under the identical pH condition for the same time. Interestingly, unlike the pure DSD-based FWA, whose whiteness variation has an inflection point when the pH is around 4, the DSD-based FWA with L-histidine increases its light fastness linearly with the growth of pH value.

It is generally accepted that five different forms of L-histidine (shown in Scheme 1), which have different adsorption performances on cotton fibers, exist in its solution and may vary with the pH value of the environment.¹⁸ The neutral L-histidine is represented by His. When the pH is lower than its isoelectric point (7.5), the L-histidine tends to possess positive charge ( ${HisH}_{2}^{2 +}$ or HisH⁺) and this trend may strengthen with the drop of the pH value, which means that the lower the pH value is, the more L-histidine with a positive charge will occur. In this case, mutually adsorptive competition for the cellulose with negative charge may be introduced between the FWA separated from fabrics and the L-histidine. The main force supporting the absorption of DSD-based FWAs with cotton fibers is the relatively weak physical interaction composed of Van der Waal’s force and hydrogen bond.²⁴ As a consequence of the low pH value, the L-histidine has much easier access to the surface of cotton fibers than its FWA counterpart. Moreover, some FWAs with a negative charge on cellulose can also be absorbed by the L-histidine and subsequently dissolved in the water of perspiration. After that, the intramolecular space steric hindrance may be reduced dramatically, and the photo-isomerization becomes easier, which may affect the fluorescent property negatively. Meanwhile, with the continuous evaporation of the moisture, the concentration of sweat gradually increases, leading to the variation of the pH value and consequently aggravation of the absorption of L-histidine on fibers. Once the surface of cotton is occupied by L-histidine, the desorbed DSD-based FWA would be more likely to expose to the light and to reduce its light stability. Therefore, when it comes to the perspiration of AATCC standard, whose pH value is controlled at 4.3 ± 0.2, the L-histidine plays a positive role in accelerating photo-induced discoloration of the DSD-based FWA on the cotton fabrics.

Scheme 1.

Five forms of L-histidine in different conditions.

Although the gap of whiteness between the whitened fabrics with and without L-histidine is narrowed to some extent with the rise of pH value, the light stability of the DSD-based FWA with L-histidine is still poorer than its counterpart under the condition that the pH value is greater than 7.5. Nevertheless, the combined effects of the mutually adsorptive competition for the cellulose and the absorption of the DSD-based FWA on cotton fabrics may be inhibited by the presence of L-histidine with negative charge (His^– or His^2–). Hence, it demonstrates that apart from the physical impact, the L-histidine probably has the chemical influence on the light stability of the DSD-based FWA.

Chemical impact of L-histidine on the light stability of DSD-based FWAs

Based on our previous researches in terms of photo-oxidation of FWAs,¹⁶ the following mechanism has been suggested for the chemical behaviors of L-histidine with FWAs during the exposure to the light.

Photo-oxidation of FWA:

FWA + h ν \to 1 FWA * \to 3 FWA *

(1)

3 FWA * + FWA \to [FWA] 2 * \to [FWA + - FWA -]

(2)

[FWA + - FWA -] + O 2 \to FWA + • + FWA + O_{2}^{- •}

(3)

1 FWA * \to FWA + • + e -

(4)

e - + O 2 \to O_{2}^{- •}

(5)

FWA + • + O_{2}^{- •} or O 2 \to OxidizedFWA

(6)

After being excited by the light energy absorbed by the stilbene-structural conjugated system, the single state (¹FWA*) and the triple state (³FWA*) are produced respectively. The ³FWA* may integrate its nearby FWA to form the ion pair encounter complex

[FWA + - FWA -]

(Equation (2)), and subsequently be oxidized to generate the semi-oxidized FWA radical anion (FWA^+•) and the superoxide radical anion (

O_{2}^{- •}

), as displayed in Equation (3). The ¹FWA* tends to be ionized by the light (Equation (4)), and the produced electron (e^–) is a critically important contributor to the formation of

O_{2}^{- •}

, which is also the crucial factor to the oxidization of DSD-based FWA (Equation (6)).

Promotion of L-histidine:^20,25–27

His + h ν \to 1 His * \to His + • + e -

(7)

His + O 2 + h ν \to His + • + O_{2}^{- •}

(8)

His + • + [FWA + - FWA -] \to [His + - FWA -] + FWA + •

(9)

The absorbed light and the surplus energy transferred from the nearby FWAs can excite the L-histidine from the ground state (His) to the single state (¹His*), which tend to be easily ionized, generating the semi-oxidized histidine radical anion (His^+•) and the e^– (Equation (7)). However, the created e^– can promote the ionization of oxygen (O₂), thus facilitating the producing of

O_{2}^{- •}

(Equation (5)). Simultaneously, the histidine may be oxidized by O₂ under exposure to the solar-simulated radiation and increase the amount of

O_{2}^{- •}

(Equation (8)). Compared with the O₂, the

O_{2}^{- •}

tends to react with FWA much more readily. In addition, the His^+• can disassemble the

[FWA + - FWA -]

to form the new ion pair encounter complex

[His + - FWA -]

and the FWA^+• (Equation (9)). The generation of

[His + - FWA -]

leads to the desorption of FWA on cotton fabrics, and the other product (FWA^+•) drives the photo- degradation of FWA (Equation (6)). In consequence, the presence of L-histidine accelerates the photo-oxidization of DSD-based FWA.

Reaction in the acidic condition:^28–30

O_{2}^{- •} + H + \to HOO • \to 1 O 2

(10)

His + HOO • \to His + • + HOO -

(11)

HOO - + H + \to H 2 O 2 + hv \to 2 HO •

(12)

H 2 O 2 + O_{2}^{- •} \to HO • + OH - + 1 O 2

(13)

OH - + H + = H 2 O

(14)

In the acidic condition, such as the artificial perspiration of the AATCC standard, the singlet oxygen (¹O₂) or the hydroperoxy radical (HOO^•) is produced from the reaction between the

O_{2}^{- •}

and the hydrogen proton (H⁺), displayed in Equation (10). The histidine may be reacted with the HOO^•, generating the hydroperoxy anion (HOO^–) and the His^+• (Equation (11)). After that, the hydrogen peroxide (H₂O₂), which is produced by the combination of the H⁺ and the HOO^–, can be further decomposed by light to form the hydroxyl radical (HO^•) on one hand, and be oxidized by the

O_{2}^{- •}

to gain HO^•, OH^– and ¹O₂ on the other hand (Equations (12) and (13)). Once the ¹O₂ has been produced, the L-histidine will be attacked immediately during irradiation via two processes (presented in Scheme 2). The first way is that the oxohistidine may be obtained firstly and further react with the O₂ induced by the adsorbed light, with the L-histidine degraded to the aspartic acid and many other products finally. The other approach is the initially light-induced dioxetane-histidine formation and the subsequent photo-cleavage and photo-oxidation, leading to the degradation products. Therefore, the ¹O₂, which can be generated more easily in the acidic condition with more H⁺, promotes the photo-degradation of L-histidine, which is to say that the presence of acid has an inhibiting effect on the photo-reaction of DSD-based FWAs on cotton fabrics. This is also the reasonable explanation for the phenomenon that there is no inflection point (pH is nearly 4) existing in the whiteness variation of the DSD-based FWA with L-histidine when exposed to the imitating irradiation in different pH conditions.

Scheme 2.

The photo-degradation reaction between L-histidine and singlet oxygen.

Conclusion

In the absence of L-histidine, the pH-related protonation of the DSD-based FWA and its rigid plane structure induced by the stilbene-structural conjugate system lead to the significant decrease of the light stability of FWA on cotton fabrics in the strongly acidic condition (pH < 4). However, these behaviors can be suppressed in the opposite environment (pH > 4), which means that the DSD-based FWA tends to have relatively high and consistent stability under this circumstance.

In the presence of L-histidine, five species with different adsorption performances depend on the pH value of the environment. In the acidic condition, the L-histidine with positive charge ( ${HisH}_{2}^{2 +}$ or HisH⁺) tends to adsorb on the surface of cellulose and the DSD-based FWAs on cotton fabrics. However, the adsorption effects of L-histidine can be inhibited by its counterpart with negative charge (His^– or His^2–) in the alkaline environment. On the other hand, the L-histidine may accelerate the photo-oxidization of DSD-based FWAs via generating the semi-oxidized FWA radical anion (FWA^+•) and the superoxide radical anion ( $O_{2}^{- •}$ ). Interestingly, the hydrogen proton (H⁺) in the strongly acidic solution promotes the formation of singlet oxygen (¹O₂), which can facilitate the photo-degradation of L-histidine, thus inhibiting the photo-reaction of DSD-based FWAs on cotton fabrics.

Regarding the artificial perspiration of the AATCC standard, whose pH value is controlled at around 4.3, the L-histidine is more likely to possess a positive charge and consequently have the adsorption effects on both of the cellulose and the DSD-based FWAs on cotton fabrics; hence, the accelerating influence of L-histidine would occur reasonably. However, the acidity of the artificial perspiration of the AATCC standard may restrain the chemical effects of L-histidine on the DSD-based FWA on cotton fabrics.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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