1. Introduction
Natural dyes refer to colorants derived from organic materials including plants and animals. They have been used since ancient times to impart color to textiles, leather, paper, food, and other materials [
1]. In contrast to synthetic dyes, which are chemically synthesized from petrochemical sources, natural dyes are obtained from renewable resources making them more sustainable and eco-friendly [
2]. The earliest evidence of textile dyeing dates back to civilizations like Egypt, China, India, and Greece that developed expertise in producing colors from diverse natural sources [
3]. Despite the dominance of synthetic dyes since the late 19th century due to their cost, consistency, and fastness properties, natural dyes have witnessed a resurgence in recent years. Growing awareness about the toxicity and environmental impact of synthetic dyes has led to increased demand for sustainable natural colorants [
4]. Consumers are also seeking uniqueness, artisanal quality, and cultural connections associated with natural dyes. Their non-toxic, non-allergic, and soothing qualities appeal to environmentally and health-conscious buyers [
5]. Natural dyes can be derived from two main sources:
Plant Dyes: Obtained from various parts of plants such as leaves, flowers, fruit, bark, and roots. Key examples include indigo (blue) from
Indigofera tinctoria L., madder (red) from
Rubia tinctorum L., turmeric (yellow) from
Curcuma longa L., and henna (orange) from
Lawsonia inermis L. [
6].
Animal Dyes: Derived from insects and shellfish, e.g., cochineal (red) from cochineal insects and Tyrian purple (violet) from sea snails. They produce luxury colors but limited supply [
7,
8].
Compared to synthetics, the limitations of natural dyes include lower color consistency, weaker colorfastness properties, and higher cost. However, renewable sourcing, biodegradability, and lack of harmful effluents make them an eco-friendly and sustainable choice [
9]. Natural dyeing techniques like fermentation, botanical prints, and natural mordanting allow the creation of novel shades in an environmentally low-impact manner. There has been growing research interest in discovering new alternative sources of natural dyes as well as improving extraction and application techniques [
10] Natural dyes from different botanical sources have been examined for their potential in textile coloration as well as imparting functional properties to fabrics [
11]. Plant-based dyes containing bioactive compounds [
12,
13] can provide fabrics with antibacterial, UV protective, antioxidant, and other beneficial effects in addition to coloration [
5]. This study focuses on the extraction of natural dyes from the leaves of the
Conocarpus erectus plant and their application to wool and silk textiles.
Conocarpus erectus is a genus of flowering plants and shrubs belonging to the
Combretaceae family. It has two main species—
Conocarpus erectus (buttonwood) and
Conocarpus lancifolius (swamp tea) that grow across tropical and subtropical regions [
14]. Previous research has shown the presence of phenolic compounds like flavonoids, tannins, gallic acid, and anthocyanins in
Conocarpus erectus leaves (as shown in
Figure 1), which can serve as natural dyes [
15]. These phytochemicals are also known to have antimicrobial, antioxidant, and anti-inflammatory bioactivities [
16]. Hence,
Conocarpus erectus leaf [
17] dyes have the potential to impart functional properties, in addition to an eco-friendly coloration for textiles.
Various solvents like water, ethanol, and methanol can be used for extracting dyes from plant materials based on the solubility of their colorant compounds [
18]. For water-soluble dyes, aqueous extraction is a common green technique that avoids the use of organic solvents. Alkali like sodium hydroxide is sometimes added to aid dissolution in water [
19]. The dye extract can then be applied to fabrics by techniques including dyebaths, printing, or surface coating. Mordants help fix dyes to textile fibers and enhance fastness. Metal salts like potassium aluminum sulfate (alum), ferrous sulfate, stannous chloride, and copper sulfate are commonly used [
20]. Ultrasonication has provided rapid and uniform distribution of dyes onto textile fibers while eliminating the need for high temperatures or other harsh chemicals. The cavitation effects of ultrasound waves enhanced dye diffusion by breaking down aggregated dye particles [
21]. Sonication also avoided the need for constant mechanical stirring [
22]. Sonication dyeing produced more uniform coloration and faster dye penetration into the fabrics compared to conventional dyebath methods.
This research investigates the aqueous extraction of dyes from Conocarpus erectus leaves and their application on wool and silk fabrics via pre-mordanting and sonication techniques. The effects of pH on the color strength and functional properties of dyed fabrics are examined. The fastness, antibacterial activity, UV protection, and mosquito-repellency properties of the dyed textiles are tested. This study provides insights into the suitability of Conocarpus erectus leaf dyes as renewable colorants for eco-friendly multifunctional textiles. The key novelty of this work is the use of Conocarpus erectus leaf extracts as renewable and sustainable colorants for dyeing wool and silk textiles. To the best of our knowledge, this is the first scientific report on the application of Conocarpus erectus dyes for the functionalization of high-value protein fabrics.
3. Result and Discussion
3.1. Extraction and Dyeing
An aqueous alkaline extraction was employed using sodium hydroxide (NaOH) to aid the dissolution of colorant compounds from the
Conocarpus erectus leaves. The alkaline medium promotes ionization of phenolic groups, enhancing their solubility in water [
29]. The impact of extraction pH on the stability of anthocyanin dyes has been reported [
30,
31]. Maintaining pH stability above 5 during extraction and dyeing preserves the flavylium cation form of anthocyanins responsible for vibrant colors [
31,
32].
Aqueous extraction of
Conocarpus erectus leaves yielded a brown-colored dye solution. The dye uptake and depth of shade increased on the wool and silk fabrics at higher pH (acidic), e.g., anthocyanin stability and absorption at acidic pH [
30]. This can be attributed to a greater extent of dye dissolution and increased swelling of fibers facilitating the diffusion of dye molecules into the fabric structure [
33]. Wool fabrics showed better dye absorption and deeper hues compared to silk. The amino acids present in the wool fiber structure likely formed strong coordination complexes with the metal mordant and dye compounds.
3.2. Color Strength K/S
The color strength of dyed wool and silk samples was determined in terms of K/S and CIELAB coordinates (L*, a*, b*, C*, h°), which are given in
Table 1 and
Table 2, respectively.
Likewise, if b* is negative, it is the representation of blue color; on the other hand, the positive b* values indicate yellowness. The L* value ranges from 0 to 100, the lower values indicate the sample is darker than the control sample. The higher K/S value was observed at pH 3 in the case of both wool and silk. The most acidic treatment (pH 3) produced the deepest shade with the highest K/S (2.5709) and lowest lightness (L* = 58.66), along with the highest red-yellow color component (a* = 4.55; b* = 15.35), chroma (C* = 16.01), and lowest hue angle (h° = 73.48). As the pH increased, the depth of shade and chroma progressively decreased, resulting in a lighter and less saturated coloration. At pH 5, the lightest shade among the dyed samples was obtained (K/S = 1.7833; L* = 64.17) with color values closer to the undyed wool. The findings clearly demonstrate the pH-sensitivity of wool dyed under acidic versus neutral conditions, with acidic pH promoting deeper dye uptake and higher chromaticity.
Similarly, the depth shade of silk was maximized (
Table 2) at pH 3 dyeing (K/S = 2.4712) along with marginally lower lightness than pH 2 (L* = 56.57). This indicates the most effective dye uptake under slightly acidic versus more acidic or neutral conditions, resulting in the darkest-colored silk with the highest chroma (C* = 15.03).In addition, the acidic dyeing conditions (lower pH) resulted in silk with hue angles shifted towards the red and yellow end of the visible spectrum compared to undyed silk and silk dyed at the higher, neutral pH of 5. The acidic environment enhanced uptake of the red and yellow dyes leading to increased red-yellow coloration in the silk fibers. While pH 3 gave the optimal combination of depth, chroma, and red-yellow hue components, pH 2 and pH 5 dyeing also greatly increased the vibrancy and chromaticity relative to undyed silk. In summary, a slightly acidic pH of 3 promotes superior dye uptake and intense coloration of silk compared to a more acidic or neutral pH.
3.3. Antibacterial Activity
The dyed wool fabrics exhibited antibacterial activity against both
S. aureus and
E. coli bacteria, as seen from the zones of inhibition in the agar diffusion tests as shown in
Figure 3a,b. Wool fabric dyed at pH 5 showed a clear zone of inhibition against both pathogens (
S. aureus and
E. coli). Silk fabrics displayed lower antibacterial effects against both pathogenic microorganisms. However, both fabrics were active in inhibiting bacterial growth, which was validated by the quantitative test (
Table 3). It was observed that dyed silk fabric rendered 88.51% and 87.93% reduction in bacterial colonies for
S. aureus and
E. coli, respectively, while wool fabric showed 91.22 and 91.98% reduction for
S. aureus and
E. coli, respectively.
The antimicrobial property can be attributed to the bioactive phytochemicals extracted from the
Conocarpus erectus leaves and imparted onto the dyed fabrics. Phenolic compounds including flavonoids and tannins are known to have antibacterial effects by disrupting bacterial cell membranes [
34]. Previous studies have demonstrated the antibacterial properties of
Conocarpus erectus extracts attributed to bioactive phytochemicals like flavonoids and tannins [
35,
36,
37,
38]. These compounds are able to disrupt bacterial cell membranes through interactions with surface proteins and lipids [
38]. Our results indicate dyeing confers antibacterial functionality to both silk and wool fabrics, with a higher efficacy seen for wool.
The cationic nature of wool proteins at acidic pH may facilitate electrostatic attractions with negatively charged functional groups of dye molecules [
39]. This could enable the binding of dye compounds containing antimicrobial phenolics to cationic sites on wool through charge–charge interactions. Further studies are required to characterize the specific bioactive constituents and their mechanisms of attachment to wool and silk fibers. (
Table 3) [
40]. Overall, the natural
Conocarpus erectus dyeing process imparts antibacterial effects to both protein-based fabrics, with an enhanced inhibition activity observed for wool over silk. This demonstrates the potential of sustainable botanical colorants to develop functional textiles preventing microbial growth for medical and hygiene applications.
3.4. Impact of Dye Concentrations on Antibacterial Activity
The antimicrobial efficacy of natural dyes can vary based on the extraction concentrations from plant materials. Higher dye content typically enhances antibacterial effects by increasing the availability of bioactive phytochemicals to disrupt microbial cells [
41,
42].
In this work, a fixed 1 g/L NaOH extraction adjuvant concentration was employed, which provided sufficient dye uptake for visible coloration. Further optimization of extraction parameters could maximize yields of potent antibacterial fractions.
The dyebath concentration influences the antimicrobial performance of dyed fabrics [
43]. Dye concentrations ranging from 2 to 10% o.w.f. were assessed against both Gram-positive and Gram-negative bacteria. Maximum bacterial growth inhibition of 92% for
S. aureus and 94% for
E. coli was achieved at 10% dyebath concentration. Antibacterial activity exhibited a positive correlation with increasing dye content due to greater availability of active compounds. The bacterial reduction values are reported along with standard error margins in
Table 3 based on triplicate experimental measurements. The low standard error range of ±1.2–2.8 suggests good reliability and repeatability of the antimicrobial testing method.
3.5. Fastness Properties
The dyed wool and silk fabrics exhibited good fastness levels during washing and rubbing, as shown in
Table 4 and
Table 5. The color fastness to washing was rated between 3 and 4 on the grey scale for both changes in shade and staining on the multi-fiber fabric. A rating of 4 indicates negligible change in color. The color fastness to rubbing also ranged from 3 to 4 for dry and wet rubbing with minimal to negligible staining on the white test cloth. Fastness ratings of 3 and above are considered commercially acceptable [
44]. The mordant played a key role in improving the fastness by enhancing the binding between the natural dye and fabric [
7]. As seen in
Table 6, the light fastness of the dyed protein fabrics ranged from moderate to good, with blue wool scale ratings between 5 to 6. Wool dyed at a higher pH of 2, 3, and 5 showed the best light fastness of 5–6. The light fastness of silk was slightly lower compared to wool, with ratings from 4 to 5 under different dyeing conditions. Dyed silk and wool displayed enhanced lightfastness compared to the undyed control fabrics, which faded significantly upon light exposure.
The improved light fastness can be attributed to the binding between fiber amino groups and dye compounds, especially in the presence of the copper mordant. Metal mordants help form coordination complexes leading to superior wash and light fastness. The results indicate the suitability of the sonicated natural dyeing method using Conocarpus erectus for producing wool and silk fabrics with good functional properties, in addition to acceptable light fastness.
3.6. UV Protection
The ultraviolet protection factor (UPF) analysis of wool and silk fabrics dyed with
Conocarpus erectus extract under different pH conditions is presented in
Table 7. Undyed silk had a UPF of 14 while undyed wool had a higher UPF of 16, indicating “good” ultraviolet protection (
Figure 4). After dyeing, the UPF increased for both fabrics under all pH conditions tested [
7]. The highest UPF of 30 (Very Good category) was achieved for wool dyed at pH 3.
For silk, the UPF ranged from 18 at pH 2 to 24 at pH 3. Silk showed enhancement of UPF after dyeing, with values ranging from 18 at pH 2 up to 27 at pH 5. The highest UPF of 27 for silk dyed at pH 5 indicates good ultraviolet-blocking ability. Natural colorants like flavonoids and tannins are able to absorb UV radiation thereby improving the ultraviolet protection capacity of textiles [
45].
Conocarpus erectus leaves contain phenolic compounds which likely imparted UV absorbing characteristics to the dyed wool and silk fabrics in addition to visible coloration. The results indicate the suitability of
Conocarpus erectus dyes for developing protective clothing and accessories. In addition to dye chemistry, changes in fabric structure during processing can influence UPF. Wet processing often causes fiber swelling and fabric shrinkage, increasing fabric thickness and density [
46]. This shrinkage effect reduces porosity and enhances opacity to incident light. While the exact shrinkage was not quantified in this study, some thickening and densification likely occurred for dyed silk and wool. This would contribute to increased ultraviolet reflection and absorption.
Future systematic investigation is required to decouple the individual effects of fiber structural changes from UV-absorbing dye compounds in improving the ultraviolet protection capacity of fabrics.
3.7. Mosquito Repellency
The cage test results shown in
Table 8 moderate repellent activity for wool and silk fabrics dyed with
Conocarpus erectus extracts against
Aedes aegypti mosquitoes. The cage test experiments for evaluating mosquito repellency of dyed fabrics were performed in triplicate using 25 Aedes aegypti mosquitoes per test. The treated fabric gloves were introduced for a standard 5 min duration for each replicate measurement. The dyed wool and silk samples showed 63–75% protection against mosquito bites based on the percent repellency calculation compared to 31–71% for the undyed fabrics. The repellency was lower than synthetic compounds, but the dye conferred noticeable insect-repelling properties. Plant-derived dyes contain volatile components like terpenoids which are known to have insecticidal effects [
47]. Dyeing with botanical colorants like
Conocarpus erectus offers an alternative eco-friendly approach to developing textiles with mosquito-repelling functionality. While the cage test provides rapid screening, actual wear trials are better indicators of repellent durability over extended durations. Prior studies using plant extracts report effective protection times ranging from 3–6 h after topical application [
48]. The current dyeing method produced durable coloration with moderate fastness. Hence, the mosquito repellent effects may potentially persist for several hours of fabric wear, requiring confirmation in future investigations, and further long-term testing will be needed to establish the full duration over which the
Conocarpus-dyed fabrics can maintain their mosquito repellent activity when exposed to washing, light, and other environmental conditions during usage. But, prior studies on plant-based dyes suggest the Insect-repelling effects persist through multiple wear cycles [
49].
3.8. SEM Investigations of Silk and Wool and Mechanism of Dye Attachment with Wool
The surface morphology of silk and wool fabrics was examined with scanning electron microscopy (SEM) after dyeing the fabrics with natural dyes extracted from
Conocarpus erectus leaves. SEM micrographs (
Figure 5a,b) revealed an even distribution of dye particles adhered to the surfaces of both silk and wool fibers. Qualitative analysis of the images showed stronger adherence of dye particles to silk fibers compared to wool fibers. The SEM images demonstrated distinct differences in dye particle morphology and surface coverage between silk and wool fibers. On silk, platelet-like dye particles were observed fully coating the fibrils in flaky layers. Wool-dyed areas appeared smoother with more localized dye aggregations. The differential binding and aggregation patterns are likely attributed to inherent variances in the chemical makeup and physical structure of the protein-based fibers.
Compared to wool, the natural dye showed a notably higher affinity for silk. Overall, SEM imaging enabled clear visualization of post-dyeing interactions and surface morphologies, serving as an effective materials characterization technique in this study of natural dye binding mechanics with luxury protein fibers.
A mechanism is proposed for dyeing wool with anthocyanin from Conocarpus erectus leaves in the presence of Cu2+ mordant. The overall process is supported by water H2O. Due to the strong complex formation between dye and wool fiber, the colorfastness rating is better with mordants such as Al3+, Fe2+, and Cu2+.
3.9. Conclusions
This study demonstrated the extraction of natural dyes from Conocarpus erectus leaves and their application to wool and silk fabrics via an ultrasonic-assisted dyeing method. Aqueous alkaline extraction provided a green and sustainable approach to obtaining plant-based dyes. The effects of dyeing process parameters such as pH were examined. Acidic pH was found to be optimal for superior dye uptake and depth of shade on the protein-based fabrics. Sonication at 40 kHz enhanced the dye absorption, diffusion, and uniformity compared to conventional dyeing. In addition to effective coloration, the dyed fabrics displayed functional properties like antibacterial activity against S. aureus and E. coli, good washing and rubbing fastness, ultraviolet protection, and mosquito repellency. SEM analysis revealed differences in surface morphology and dye aggregation on silk versus wool. The Conocarpus erectus dyes displayed a stronger affinity for the silk fibers. Overall, the ultrasonic dyeing method using renewable Conocarpus erectus leaf extracts enables sustainable production of colored wool and silk textiles with multiple functionalities. Further characterization of the phytochemical compounds in the dyes can provide directions for enhancement. The results demonstrate the promise of plant-based dyes coupled with ultrasonic techniques for eco-friendly multifunctional textiles.