Bio-inspired colouration on various textile materials using a novel catechol colorant

Abstract

Bio-inspired by the adhesive properties of marine mussels, a novel method was developed for colouration on various kinds of textile materials. Through the introduction of a catechol group as an adhesive ‘anchor’ into commercially available colorants, various textile materials were easily coloured at room temperature via a simple dip-coating procedure. The UV-visible results indicated that the introduction of the catechol group did not obviously change the absorption of the parent colorants. The coloured fabrics were evaluated by colorimetric analysis, scanning electron microscopy, optical microscopic analysis and colouration fastness tests. Compared to the control colouration, the obtained results showed that this novel method gave textile materials a better colouration, which was verified by the obviously deeper colour appearance and higher K/S values. This colouration method was especially suitable for those textile materials difficult to be coloured by traditional methods, such as polylactic acid and polypropylene. Compared to traditional methods, this method was more environmentally friendly due to reduced energy consumption and colouration auxiliaries needed. It is an innovative direction of colouration on textile materials.

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Introduction

In the traditional colouration process, the application of dyes has a high dependence on the nature of the fibres due to the specific interaction between dye molecules and fibres. It lacks the efficacy to dye various fibres with one dye. For example, reactive dyes are suitable for cellulosic fibres, while acidic dyes are usually used for protein fibres. In order to improve the dyeability, a high temperature is often needed in the colouration process, which implies high energy consumption. At the same time, various auxiliaries including salt, dispersing agent, levelling agent, penetrating agent and accelerating agent are also used to improve the dyeability. At the end of colouration, these auxiliaries remain in the colouration bath and are discharged as effluent, resulting in serious adverse impacts to the environment.^1,2

In order to solve these problems, we are able to resort to nature. In nature, marine mussels possess a remarkable adhesive ability to all types of solid surfaces. This adhesive can harden to form a permanently water-resistant plaque within a few seconds after contacting with a substrate.^3–9 Recently, biomimetic research on marine mussel adhesive proteins (MAPs) has attracted much attention due to its versatile adhesion. So far, it has been found that all MAPs contain 3,4-dihydroxy-L-phenylalanine (DOPA), an amino acid formed by post-translational hydroxylation of tyrosine residues. It is believed that DOPA is responsible for the versatile adhesive of MAPs through cross-linking and different physicochemical interactions between DOPA and solid surface^10–15 including coordination bonds, π electron interactions, hydrogen bonds, ionic bonds and covalent bonds.¹⁶ Inspired by this, researchers have developed bio-based innovative polymer materials^17–22 and functional surface coatings using catechol-containing molecules.^23–25 These studies have succeeded to some extent in reproducing the versatile features of mussel adhesion. However, none of the work targets the biomimetic adhesive colouration on various textile materials using a catechol-containing colorant.

Similar to DOPA, dopamine also has the main functional groups of catechol and ethylamine, and it also easily polymerizes in the presence of oxidants or alkali under room temperature.^13,26 Thus, dopamine is an ideal molecule to biomimic DOPA. In our recent work,²⁷ dopamine was successfully applied to adhesive surface colouration in a simple way. This method worked well on various materials, especially those resistant to colouration. It is a promising alternative to the traditional colouration techniques. But, the colour gamut is not so rich. Well-known, the existed commercially available colorants display rich colour gamut. If dopamine is covalently bound to these available colorants as a ‘molecular anchor’ to fasten on the fibres, not only the colour gamut would largely enrich, but also the obtained catechol colorants would display good dyeability on textile materials through the multiple physicochemical interactions of dopamine. In this case, high energy consumption and the addition of various auxiliaries perhaps would be unnecessary, which is more environmentally friendly.

Continuing our interests, herein, we report a versatile colouration method on various textile materials using a catechol colorant, as shown in Fig. 1. In this method, a commercially available colorant of Acid Yellow 11 (AY) was used as a model of chromophore. After the modification, dopamine moiety in colorant D1 would act as a colouration ‘anchor’ to fasten on the fibres. Also, a control colorant D2 with similar structure but without catechol group was also synthesized for colouration evaluation.

Fig. 1 Colouration process with catechol colorant and traditional colorants.

Experimental

Materials and instruments

Six textile materials were used in the study including woven fabrics of wool (255 g m⁻²), polyethylene terephthalate (PET, 123 g m⁻²), polylactic acid (PLA, 118 g m⁻²), polybenzimidazole (PBI, 145 g m⁻²), silk (101 g m⁻²) and knitted polypropylene (PP, 79 g m⁻²). They were purchased from China Dyeing Holding, Ltd. Before use, these fabrics were washed with 5 g L⁻¹ sodium dodecyl sulfate solution at 60 °C for 20 min to remove possible surface impurities. The fabrics were then rinsed thoroughly in deionized water and dried in air. All chemicals were purchased from Sigma-Aldrich and used as received.

High resolution mass spectrometry (HRMS) was recorded using a Micromass Q-TOF 2 mass spectrometer. Nuclear magnetic resonance spectroscopy (¹H NMR and ¹³C NMR) were recorded on a Varian 400 spectrometer using TMS as an internal standard. The absorption spectra were recorded on a Perkin Elmer Lambda 18 UV-vis spectrophotometer.

Colorants synthesis

Colorants were synthesized in a one-pot reaction. Protected dopamine (TBDMS–dopamine) was obtained using t-butyldimethylsilyl (TBDMS) chloride as protection agent.²⁸ The AY chloride was obtained using AY as starting material by the reaction with thionyl chloride.²⁹ AY chloride (0.075 g, 0.02 mmol), TBDMS–dopamine (0.076 g, 0.2 mmol) and triethylamine (0.024 g, 0.24 mmol) were added into 30 mL acetone. The mixture was stirred at room temperature for 5 h. After the solvent was removed under vacuum, the residual was added into 2 mL tetrahydrofuran. Under stirring, tetra-n-butylammonium fluoride (TBAF) (0.41 mmol) solution in THF was added and the mixture was further stirred for another 30 minutes at room temperature. Then, THF was removed under reduced pressure and the residual was purified with thin layer chromatography using DCE–EA (v/v, 3 : 1) as eluent to obtain 0.060 g of dye D1 as a yellow powder. Yield: 60.8%. ¹H NMR (DMSO-d₆, Fig. S1†): δ 13.24 (br, 1H), 8.76 (s, 1H), 8.68 (s, 1H), 8.17 (d, J = 8, 2H), 7.88 (d, J = 8, 2H), 7.66 (d, 3H), 7.48 (t, 2H), 7.26 (t, 1H), 6.61 (d, J = 8, 1H), 6.53 (s, 1H), 6.39 (d, J = 8, 1H), 2.89 (q, 2H), 2.48 (t, 2H), 2.35 (s, 3H); ¹³C NMR (DMSO-d₆, Fig. S2†): δ 187.16, 145.00, 143.64, 129.55, 129.33, 127.86, 127.80, 125.90, 125.85, 119.18, 117.15, 117.06, 116.48, 116.42, 116.38, 115.92, 115.45, 44.59, 34.77, 11.75; HRMS (TOF, ES⁺, Fig. S3†) m/z = 494.1514 (M + H)⁺, calcd for C₂₄H₂₄N₅O₅S = 494.1498.

The control colorant D2 was synthesized in a similar way. After the solvent was removed under vacuum, the residual was purified by crystallization from chloroform to obtain D2. Yield: 93.1%. ¹H NMR (CDCl₃, Fig. S4†): δ 13.51 (br, 1H), 8.14 (d, 2H) 7.83 (d, 2H), 7.48 (m, 4H), 7.29 (m, 3H), 7.17 (d, 1H), 7.10 (d, 2H), 6.57 (br, 1H), 2.40 (s, 3H); ¹³C NMR (CDCl₃, Fig. S5†): δ 174.29, 158.30, 149.81, 141.83, 140.87, 136.30, 134.37, 129.82, 129.43, 128.50, 126.39, 125.73, 122.10, 117.73, 116.06, 11.88; HRMS (TOF, ES⁺, Fig. S6†) m/z = 434.1302 (M + H)⁺, calcd for C₂₂H₂₀N₅O₃S = 434.1287.

Colouration procedure

All fabrics were immersed in 0.05% w/v colorant solution in methanol (liquor ratio 20 : 1) at 25 °C for 24 hours. Then, the fabrics were taken out of the solution, and treated with 1% potassium periodate solution for one minute. After drying, the fabrics were washed in 0.5% detergent at 60 °C for 10 minutes to remove the unfasten colorant, and then air dried.

Colouration measurements

Colorimetric analyses of the fabrics were conducted using a Macbeth colour-eye 7000A spectrophotometer. For each measurement, the average of four repeated measurements was used.

The CIE (International Commission on Illumination) system (CIE L*a*b*, using CIE D65/10 degree illuminant/observer condition) was used to define the colours of the fabrics. L* is a scale of lightness with a value of 0–100 for perfect black to white. Scale a* indicates a red-green character of colour, in which positive values indicate the redness and negative values indicate greenness. Scale b* indicates a yellow-blue character of colour, in which positive values indicate yellowness and negative values indicate blueness. The total colour difference was calculated based on ΔL*, Δa* and Δb* between two colours using the following equations:³⁰

ΔE = [(ΔL*)² + (Δa*)² + (Δb*)²]^1/2

From the reflectance values of the coloured materials, the colour depth (K/S) was calculated using the Kubelka–Munk equation.³¹ The maximum K/S values at 400 nm of coloured materials were reported. Wash fastness was assessed using grey scales according to AATCC Test Method 61-2010 3A at 71 °C for 45 min in the presence of 0.15% power detergent and 100 steel balls.³² Crocking fastness was assessed using Crockmeter method according to AATCC Test Method 8-2007. Light fastness was obtained according to the standard method: ISO 105 B02-2013 (xenon-arc lamp).

The cross-section images of the coloured fibres were investigated using an optical microscope (microscope: Nikon Optiphot-POL from Germany, software: high-performance Leica LAS software). During sample preparation, the light blue nylon yarns were used to support the sample yarns. The morphology of the fibres was investigated by scanning electron microscope (SEM) on a TEM 3000 Tabletop Microscope (Hitachi, Japan).

Results and discussion

Synthesis and characterization

AY chloride was obtained using thionyl chloride as chlorination agent because its reactivity is moderate and the final by-products were gases, which were easy to be after-treated. The tert-butyldimethylsilyl chloride was used for the catecholic group protection because it had a moderate stability and can be removed in the presence of TBAF under mild condition. The structure of pure colorants was confirmed by their HRMS and NMR spectra.

It is interesting to note, a ¹H brand singlet at upfield of about δ 13 was observed in their ¹H-NMR spectra. It was attributed to the protons in NH group and indicated that the azo bond existed in the tautomer of hydrazone, as shown in Fig. 2. These results are consistent with the reported literatures.^29,33,34

Fig. 2 The hydrazine tautomer of dye D1.

Absorption spectra

The absorption spectra of the catechol colorant D1 as well as two control colorants were investigated in various solvents. Fig. 3 gives an example of their absorption spectra in methanol. It showed that their maximum absorptions (λ_max) located at ca. 390 nm. Also, their λ_max were tested in the other solvents, as listed in Table 1. Compared to the λ_max of its parent colorant AY and control colorant D2, D1 had a similar λ_max in these solvents. It was expected because their chromophores were unchanged after the chemical reaction. In more polar solvents, such as methanol, ethanol and acetic acid, three colorants had absorptions at ca. 387–391 nm, which was a little bit lower than those in less polar solvents, such as toluene and DMF. Their absorptions showed a slight dependence on solvent polarity.

Fig. 3 The absorption spectra of the colorants in methanol.

Table 1 The maximum absorption wavelength of the colorants in different solvents^a

Dye	Toluene	DMF	Acetone	EA	Methanol	Ethanol	Acetic acid
a DMF, N,N-dimethylformamide; EA, ethyl acetate.
AY	395	392	388	387	389	386	387
D1	391	402	388	385	391	387	386
D2	390	397	390	386	388	387	386

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Colorimetric analysis

Fabrics coloured with AY, D1 and D2 were evaluated by colorimetric analysis. As well-known, PLA fabrics are difficult to be coloured because traditional colorants have low affinity for PLA fabrics. Until now, only a limited number of colorants have been found to have good adsorption on PLA fibre at the appropriate colouration conditions.^35–37 Similarly, PP and PBI are also very difficult to be coloured with the traditional colorants due to their hydrophobicity and high crystallinity.

In this study, it is expected that the catechol colorant D1 would show the ability of adhesive adsorption to various textile materials.^{19–21,24,25} Once colorant molecules contacted with fibres, the existence of catechol functional group could increase the exhaustion of colorant molecules on fibre surfaces. In this case, the colorants were not easily to be removed away, even in the washing process.

On the various fabrics of PLA, PP, PET, silk and wool, the obtained colour depth with catechol colorant D1 were obviously better than those with control colorants AY and D2, as shown in Fig. 4. On wool fabrics, colorant AY gave the better colour depth. This was expected because AY is a typical acid colorant for wool fabrics.³⁸ It was still observed that catechol colorant D1 gave better results than traditional disperse colorant D2. The coloration difference could be further shown by their K/S values, listed in Table 2. Colorants AY and D2 has the smaller molecular volumes than that of catechol D1, indicating their stronger colour strength. However, they gave the coloured fabrics lower K/S values than catechol D1 under the same conditions. These results indicated that catechol D1 remained more on the fabrics after soap washing and showed better dyeability on various fabrics due to the existence of a ‘catechol anchor’.

Fig. 4 The photographic images of the dyeing results for the fabrics difficult to be dyed with traditional dyes. (A) Blank fabric, (B) dyed with AY, (C) dyed with control dye D2, (D) dyed with catechol dye D1.

Table 2 Colorimetric analysis of the dyed fabrics

Dyed fabrics	Dyes	L*	a*	b*	ΔE^a	K/S
a The coloured samples compared to the corresponding blank fabrics.
PLA	Blank	90.0	0.7	8.8	—	—
	AY	90.0	−1.48	15.9	7.5	0.3
	D2	88.4	−3.1	27.1	18.8	0.7
	D1	86.7	−0.9	36.2	27.6	1.4
PP	Blank	92.3	−0.1	0.8	—	—
	AY	92.2	−0.6	3.0	2.3	0.1
	D2	89.2	−4.2	25.9	25.6	0.5
	D1	87.1	2.2	49.3	48.8	1.6
PET	Blank	92.3	−0.7	1.5	—	—
	AY	91.8	−2.4	6.6	5.5	0.1
	D2	90.1	−5.7	26.4	25.5	0.6
	D1	88.9	−4.4	35.2	34.2	1.0
PBI	Blank	68.5	3.4	20.3	—	—
	AY	68.0	2.5	33.0	12.5	4.4
	D2	68.2	2.2	31.6	10.7	3.9
	D1	66.1	2.8	32.9	12.9	4.8
Wool	Blank	89.0	0.6	2.2	—	—
	AY	83.4	0.1	68.3	66.4	13.0
	D2	85.5	−3.5	42.5	40.7	5.3
	D1	84.7	−0.7	50.8	48.9	7.1
Silk	Blank	90.1	3.7	−1.5	—	—
	AY	87.3	−2.1	24.9	27.2	1.5
	D2	85.4	−4.7	47.4	49.8	3.3
	D1	85.5	−3.6	43.3	46.2	3.6

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Table 2 also listed the colour coordinates CIE L*, a*, b* and the total colour difference ΔE of the dyed fabrics. Except wool fabrics coloured with AY, all fabrics coloured with catechol D1 had lower L* values of lightness, which indicated higher colour depth when compared to those coloured by control AY and D2. These results correlated to the results shown by the photographic images in Fig. 4. Due to their close a* values, the values of total colour difference ΔE of the coloured fabrics were mainly attributable to their great difference in b* values. Except individual cases, fabrics coloured with catechol D1 showed higher b* values, indicating that these fabrics showed a yellower colour compared to those fabrics coloured with AY and D2. This further confirmed that catechol D1 gave better colour depth on various fabrics, because the colorants used in this study are yellow colour. These obtained results were consistent with their photographic images shown in Fig. 4 and K/S results in Table 2.

Microscopic analysis

The coloured fibres were also investigated with the aid of microscopic analysis. Their surface morphology before and after colouration was first measured, as shown in Fig. 5. It can be seen that blank fibres (left, Fig. 5) have smoother surfaces. After colouration with catechol colorant D1, their surfaces roughness was clearly changed. On the fibre surfaces, it was observed some micro-scaled particles, which was obviously from the colorants. This clear contrast in their surface morphology confirmed the successful colouration on various fabrics with the catechol colorant.

Fig. 5 The surface morphology of various fibres before (left) and after (right) colouration with catechol colorant D1. Bar = 50 μm.

Their cross-sections before and after colouration were further investigated with optical microscope, as shown in Fig. 6. It can be seen that the cross-sections of blank fibres (left, Fig. 6) were pale brownish, showing their original colours. After colouration with catechol colorant D1, the cross-sections of the fibres were clearly changed, which gave a yellower colour. This change also confirmed the successful colouration on various fabrics with the catechol colorant.

Fig. 6 The microscopic images of the cross-section view of various fibres before (left) and after (after) colouration.

Colour fastness test

The colour fastness of the fabrics coloured with the catechol colorant D1 was evaluated, as listed in Table 3. The results showed that the coloured fabrics had good laundering fastness to all fabrics. The ratings of colour changes were in the range of 4–4/5. This showed that only few colorants were washed away during the fastness test, indicating the good fasten of the catechol colorant D1 molecules on the fibres. For the colour staining evaluations, all the cases gave a highest rating of 5, except for the silk fabric which have a slightly lower rating of 4/5, these results indicated that there were nearly no staining on the neighbouring multi-fibre fabrics. For the crocking fastness, the catechol colorant D1 achieved 4/5 ratings of dry crocking fastness on PLA, PET and silk fabrics and 3 and 4 on PP and wool fabrics, respectively. The coloured fabrics had wet crocking fastness ratings in the range of 4–4/5. Concerning the light fastness listed in Table 3, the coloured fabrics of wool and silk had better ratings of 4, while other fabrics had lower ratings, which would be perhaps improved via the increase of their obtained colour depth.

Table 3 Fastness of the coloured fabrics^a

Fabric	Washing fastness							Crocking fastness		Light fastness
	Colour change	Staining
		W	Ay	P	N	C	Ae	Dry	Wet
a W, wool; Ay, acylic; P, polyester; N, nylon; C, cotton; Ae, acetate.
PLA	4	5	5	5	5	5	5	4–5	4	2–3
PET	4–5	4	5	5	5	5	5	4–5	4	1–2
PP	4–5	5	5	5	5	5	5	3	4	1–2
Wool	4–5	5	5	5	5	5	5	4	4–5	4
Silk	4–5	4–5	5	5	5	5	5	4–5	4–5	4

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Conclusions

In conclusion, we had developed a simple method for colouration of various textile materials using a catechol colorant. Through the adhesive adsorption of a catechol group, this approach could successfully colour various textile materials, including natural fibres and synthetic fibres, even the fibres difficult to colouration with traditional colorants. Furthermore, this method showed the advantages of substrate-independence and room temperature colouration, which indicated the convenience to application and low energy consumption. It is interesting and worthy to be further studied in future.

Acknowledgements

The authors acknowledge the funding of the GRF project (no. PolyU 5316/10E) by the Research Grants Council of the Hong Kong SAR Government and the National Natural Science Foundation of China (no. 21376197), and the studentship by the Hong Kong Polytechnic University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06004k

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