Study on the Synergistic Effect of Plant Dyes and Mordants on the Dyeing and Anti-Mold Effect of Moso Bamboo

Abstract

Bamboo’s single color and susceptibility to mold substantially limit its wide application. Therefore, dyeing and mold prevention have become pivotal technologies for the high-value-added utilization of bamboo. This study selected the extracts of three plants (Caesalpinia sappan L. (Cs), Rubia cordifolia L. (Rc), and Carthamus tinctorius L. (Ct)) for dyeing and mold prevention experiments. The results showed that the three extracts had good dyeing effects on bamboo, with total color differences (ΔE*) of 31.69, 21.61, and 32.29 compared to untreated bamboo, respectively. Additionally, these had a moderate inhibitory effect on mold. The introduction of metal mordants effectively enhances the dyeing effect of plant dyes and the effectiveness of mold inhibition. Through the joint modification of Cs and Cu, the color fixation rate increased from 3.12% to 9.20% compared with the Cs extract. A Cu 1 g:300 mL Cs extract impregnation of bamboo can completely inhibit the growth of Aspergillus niger, and a 1 g:1100 mL ratio can completely inhibit the growth of Trichoderma viride. This study provides a new concept for applying plant dyes in the dyeing and mold prevention treatment of bamboo. The dual-effect treatment of dyeing and mold prevention enhances the visual characteristics of bamboo while imparting it with good mold prevention performance.

1. Introduction

Synthetic dyes are mostly used for bamboo dyeing. With the pursuit of healthy and environmentally friendly outcomes, researchers are turning their attention to natural dyes [1,2,3]. Natural dyes include plant, animal, mineral, and microbial dyes [4]. Compared with other natural dyes, plant dyes have the advantages of complete chromatography, environmental protection, sustainable regeneration, large production, and certain antibacterial effects [5,6]. Studies have shown that Eucalyptus wood extract can be used as a natural dye for textiles, offering good wash and light fastness [7]. Pomegranate peel dye has been optimized for dyeing bamboo pulp fibers, meeting fabric color fastness requirements [8]. Processing residues and invasive plants can also be utilized as dye materials [9,10,11]. Additionally, plant-derived essential oils have been used to enhance the anti-mold properties of wood-based panels [12]. However, these have the disadvantages of low up-dyeing and low color fixation rates. Plant-source organic mold inhibitors and inorganic nanomold inhibitors, as new types of mold inhibitors, are widely used in bamboo mold treatment. Plant-derived organic mold inhibitors contain alkaloids, flavonoids, phenols, and other components because the plants display antibacterial properties. The study determined that plant mold inhibitors have a better inhibitory effect on bacteria such as Staphylococcus aureus and Escherichia coli. However, the inhibition of mold is less effective [13,14,15]. Research on essential oils has demonstrated their antimicrobial effects [16,17]. Inorganic nanomold inhibitors such as Ag, Zn, and Cu have better mold inhibition performance and a broad spectrum [18,19]. For example, ZnO nanocrystals prepared on bamboo surfaces via low-temperature hydrothermal methods have shown significant anti-mold effects against Aspergillus niger and Penicillium citrinum [20]. Nano-silver can inhibit Aspergillus niger [21]. However, these display shortcomings such as high cost, straightforward loss, and low toxicity [22,23].

Plant dyes have both dyeing and fungistatic effects. The dyeing and color-fixing effects are improved by adding metal-based mordants in the textile industry [24]. Metal mordants, such as ferrous chloride and sulfate, have been found to improve the thermal stability and crystallinity of fabrics dyed with natural dyes [25]. To enhance the impregnation of dyes and mordants, bamboo needs to be pretreated to improve its permeability [26,27,28]. This indicates that these are the prerequisites for producing a joint effect from dyeing and mold prevention processes and properties. In this study, we considered bamboo as the research object. We selected three red plant dyes (Caesalpinia sappan L. (Cs), Rubia cordifolia L. (Rc), and Carthamus tinctorius L. (Ct)) and three metal mordants (CuSO₄·5H₂O (Cu), (NH₄)₂Fe(SO₄)₂·6H₂O (Fe), and KAl(SO₄)₂·12H₂O (Al)) for dyeing and anti-mold treatment of bamboo. We studied the joint effect of the plant dyes and metal mordants on the dyeing and anti-mold treatment of bamboo to provide a new concept and theoretical basis for investigating the green, high-efficiency, and inexpensive dyeing and anti-mold treatment process and technology of bamboo.

2. Materials and Methods

2.1. Chemicals and Materials

Moso bamboo (Phyllostachys heterocycla var. pubescens), with the outer and inner bamboo skins removed and free of nodes, had dimensions of 50 × 20 × 5 mm.

Cs stems (Kunming, Yunnan, China), Rc roots (Taiyuan, Shanxi, China), and Ct flowers (Urumqi, Xinjiang, China) were dried at 50 °C for 48 h, crushed in a pulverizer, passed through a 100-mesh sieve, and sealed for storage. All plant materials were purchased as dried medicinal herbs from Guilin Zhongyao Pharmacy Chain Co., Ltd. (Liuzhou, China) in October 2024.

Cu, Fe, and Al were purchased from Sinopharm Group Chemical Reagents Co., Ltd. (Beijing, China). Potato Dextrose Agar (PDA) and Potato Dextrose Broth (PDB) were purchased from Guangdong Huankai Microbial Technology Co., Ltd. (Guangzhou, China). Aspergillus niger V. Tiegh. (A. niger) and Trichoderma viride Pers. ex Fr. (T. viride) were obtained from Guangdong Microbial Culture Collection Center (Guangzhou, China).

2.2. Experimental Methods

2.2.1. Preparation of Dye Solution

Plant dyes and pure water in a ratio of 1 g:250 mL were extracted at 70 °C for 60 min using a constant-temperature water bath (Zhiborui HH-8, Zhiborui, Changzhou, China). The extract was then filtered through a glass funnel with qualitative filter paper (medium-speed; LeiGu, China) to obtain the Cs dyeing solution. This method was used to prepare the Rc and Ct dyeing solutions as well.

2.2.2. Dyeing and Color Measurement of Bamboo Wood

Dyeing Treatment of Bamboo Wood

Four 250 mL Erlenmeyer flasks (ShuNiu GG-17, ShuNiu, Chengdu, China) each received 250 mL of Cs dyeing solution. The three metal mordants (1.5 g each) were placed into 1 of the flasks. Each flask was fed with three dry-treated bamboo slices to ensure that the bamboo sheet was immersed completely in the dyeing solution in a constant-temperature water bath at 70 °C for the 90 min dyeing. The dye-treated bamboo piece was immersed in 500 mL of water to rinse the floating color and dried in the drying box at 60 °C for 12 h. Four groups of Cs dyeing bamboo slices (Cs, CsAl, CsCu, and CsFe) were prepared. The Ct groups (Ct, CtAl, CtCu, and CtFe) and Rc groups (Rc, RcAl, RcCu, and RcFe) were prepared in a similar manner. An additional group of boiled control was also prepared.

Color Measurements

The CIE 1976 Lab uniform color space system was used to calculate the color difference values. In the light source box (3nh, China), the D65 light source was selected. A color difference meter (Minolta CR-10 Plus, Japan) was used to measure the dyed bamboo slices, and the characteristic values of color were recorded. In each piece of bamboo, three points were considered on one side of the bamboo bark near the inner side and one side of the bamboo bark near the outer side. The average of the measurement results of each group of bamboo slices (three slices in each group) was considered as the color value of the dyed bamboo slices [29]. The color difference value was calculated according to the following formula. The sensory degree of color difference was assessed according to

Table 1. NBS units and degree of color difference.

NBS Unit	Level of Chromatic Aberration
0–0.5	Minute color difference (imperceptible)
0.5–1.5	Small color difference (barely perceptible)
1.5–3	Perceivable change (noticeable in controlled light)
3–6	Marked change (distinctly visible)
>6	Extremely marked change (different color)

Note: The NBS unit classification is based on the U.S. National Bureau of Standards Monograph 104 (1968). The value of ∆E* is divided by the constant K to obtain the NBS unit color difference value, and the value of K is taken as 1 in CIELab space. The perceived color difference levels are adapted from Judd’s criteria (1939).

[30].

Lightness difference:

$$∆{\mathrm{L}}^{*}={{\mathrm{L}}_1}^{*}-{{\mathrm{L}}_2}^{*}$$

(1)

Chromaticity difference:

$$∆{\mathrm{a}}^{*}={{\mathrm{a}}_1}^{*}{{-\mathrm{a}}_2}^{*}$$

(2)

$$∆{\mathrm{b}}^{*}={{\mathrm{b}}_1}^{*}-{{\mathrm{b}}_2}^{*}$$

(3)

Total color difference:

$$∆{\mathrm{E}}^{*}=\sqrt{{(∆{\mathrm{L}}^{*})}^2+{(∆{\mathrm{a}}^{*})}^2+{(∆{\mathrm{b}}^{*})}^2}$$

(4)

In the CIE 1976 Lab uniform color space system: L* represents lightness, ranging from 0 (black) to 100 (white); a* indicates the red–green axis, with values ranging from −128 (greenness) to +127 (redness); b* indicates the yellow–blue axis, with values ranging from −128 (blueness) to +127 (yellowness).

Here L₂*, a₂*, and b₂* represent the lightness value, red–green index, and yellow–blue index of the bamboo piece before dyeing, respectively; L₁*, a₁*, and b₁* are the indexes of the bamboo piece after dyeing; and ΔE* is the total color difference of the test piece after dyeing.

2.2.3. Mold Inhibition of Plant Dyes and Mordants

Mold Inhibition by Plant Dyes

A. niger and T. viride were inoculated in a flat Petri dish and placed in an incubator at 28 °C for 7 d. With the dye solution preparation method described in Section 2.2.1, three types of plant dye solution were prepared as the test solution for mold inhibition. These were diluted five and ten times. Five milliliters of differently diluted plant dye solutions were taken in a 20 mL glass bottle. Five milliliters of two times the concentration of PDB was added. Subsequently, 0.1 mL of spore suspension was aspirated (approximately (1–2) × 10⁶/mL). It was then placed in an incubator at 28 °C for four weeks.

Mold Inhibition of Metal Mordants

The metal mordant and pure water (1 g:5 mL) were mixed as a test solution for mold inhibition. This was in accordance with the dilution multiplicity range [10–50] (10 times the interval). The original solution was diluted similarly as in the plant dye mold inhibitory test method of inoculation and culture for four weeks. Subsequently, the results were observed. If complete mold inhibition was not achieved within the initial dilution range, the test solution was serially diluted in incremental multiples until the maximum dilution factor (at which no mold growth occurred) was identified.

2.2.4. Experiment on the Joint Effect of Dyeing and Mold Prevention

Dyeing and Color-Fixing Treatment of Bamboo Wood

The Cs dyeing solution was filtered and divided into four Erlenmeyer flasks. These were installed with distillation columns. Each portion was volume-determined to 250 mL. Two of these were organized into a group. Herein, one group had 1.5 g of Cu added. A flask in each group was added with three pieces of dry-treated bamboo slices. These were reacted for 90 min in a constant-temperature water bath at 70 °C. The solution in this flask was the residual dye solution. The solution in the flask into which the bamboo slices had not been added was used as the control dye solution. The dyeing bamboo was removed, and the surface of the dye solution was drained. Then, it was placed in a 250 mL pure water beaker, and the water wash was stirred for 15 min. Then, the water wash solution and an equal amount of residual dye solution were mixed to obtain the diluted residual solution.

Absorbance Photometric Measurement of Dyeing and Color Fixation Rates

A UV–visible spectrophotometer (Shimadzu UV-2600, Shimadzu, Kyoto, Japan) was used to test the maximum absorption wavelengths of the Cs and Cs-Cu dyeing solutions and the absorbance values of each stain [31]. The dye uptake rate E was calculated according to Equation (5), and the dye fixation rate F was calculated according to Equation (6):

$$\mathrm{E}=(1-{\displaystyle \frac{{\mathrm{A}}_1{\mathrm{n}}_1}{{\mathrm{A}}_0{\mathrm{n}}_0}})\times 100\%$$

(5)

$$\mathrm{F}=(1-{\displaystyle \frac{{\mathrm{A}}_2{\mathrm{n}}_2}{{\mathrm{A}}_0{\mathrm{n}}_0}})\times 100\%$$

(6)

where E is the dye uptake rate, A₀ is the absorbance value of the control dye solution, n₀ is the dilution factor of the control dye solution, A₁ is the absorbance value of the residual dye solution, n₁ is the dilution factor of the residual dye solution, F is the dye fixation rate, A₂ is the absorbance value of the diluted residual solution, and n₂ is the dilution factor of the diluted residual solution.

Impregnated Bamboo Wood

The mold inhibition test in the solution state has fewer interfering factors than the mold inhibition test involving impregnation into the bamboo material. The mold inhibition experiment in the liquid state was conducted first. Subsequently, the mold inhibition experiment in the impregnated bamboo material was conducted. According to the ratio of Cs and pure water (1 g:50 mL), the solution was extracted in a constant-temperature water bath at 70 °C for 90 min, and the Cs solution was filtered. Cu was compounded with the Cs solution and pure water according to the range of dilution [100–240] at 20 time intervals and inoculated with A. niger, and according to the range of dilution [300–440] at 20 time intervals and inoculated with T. viride.

Considering that the immersion of the bamboo material into the solution would have reduced the content of mold-inhibiting substances, Cs solution extract and pure water were used as the dispersing medium. The medium was fixed in a 250 mL triangular flask, and Cu was added to the dispersing medium at ratios of 1 g:300 mL, 1 g:500 mL, 1 g:700 mL, 1 g:900 mL, 1 g:1100 mL, 1 g:1300 mL, 1 g:1500 mL, 1 g:1700 mL, and 1 g:1900 mL; the samples were named according to these ratios as follows: CsCu1:300, WCu1:300, CsCu1:500, WCu1:500, and so on. A group of Cs solution and a group of pure water were prepared as the control. Five bamboo slices were added and covered with a rubber stopper. Subsequently, the sterilization treatment was completed in the autoclave at 121 °C for 15 min. Subsequently, the autoclave was cooled naturally for 10 h to complete the dyeing treatment.

Determination of the Mold Resistance of Impregnated Bamboo Wood

The experiment on the effectiveness of mold prevention was conducted according to the Chinese National Standard (GB/T 18261-2013, the Effectiveness of Anti-fungal Agent for the Prevention and Control of Mold and Discoloration Bacteria on Wood [32]). The grade of the infection value of the specimen was determined according to

Table 2. Grading of the surface infection values of specimens.

Infectious Value	Specimen Infected Area
0	No mycelium or mold on the surface
1	Infected area < 1/4 of the surface
2	Infected area < 1/4–1/2 of the surface
3	Infected area < 1/2–3/4 of the surface
4	Infected area > 3/4 of the surface

. The mold growth was observed and recorded every 7 d, with the data at 28 d as the final result.

The anti-mold effectiveness is calculated according to Equation (7):

$$\mathrm{G}=(1-{\displaystyle \frac{{\mathrm{D}}_1}{{\mathrm{D}}_0}})\times 100\%$$

(7)

where G is the anti-mold efficacy, D₀ is the average infection value of the control specimen, and D₁ is the average infection value of the treated specimen.

2.2.5. Analysis of Dyeing Anti-Mold Effect

Fourier-Transform Infrared Spectroscopy Analysis

The specimen was scraped to powder. Subsequently, it was baked to complete dryness in the drying oven. After cooling, the specimen powder and potassium bromide were mixed in a ratio of 1:100. The mixture was ground uniformly and tested by an infrared spectrometer (Shimadzu, IRAffinity-1S, Japan). The scanning wave number range was set as 400–4000 cm⁻¹, the number of scans was 20 times, and the resolution was 4.

Scanning Electron Microscope Observation

The longitudinal sections of the bamboo slices were cut, dried in a drying oven at 60 °C to absolute dryness, and sprayed with gold using an ion sputtering apparatus (Beijing Kyky, SBC-12, China). Subsequently, the microscopic morphology of the samples was observed using a scanning electron microscope (SEM; Beijing Kyky, EM6200, China), with the accelerating voltage set at 17 kV.

Determination of Copper Content and Its Effect on the Color of Bamboo Wood

The specimen (CsCu1:300, WCu1:300, CsCu1:900, WCu1:900) was taken in a 50 mL polytetrafluoroethylene ablation tube. Subsequently, 10 mL of concentrated nitric acid and 1 mL of hydrofluoric acid were added. The tube was placed in the stainless steel reactor for ablation. Subsequently, it was placed in the drying oven at 190 °C for approximately 600 min. Subsequently, the heating was stopped, and the specimen was cooled. After being diluted 10 times with pure water, the solution was placed in a 25 mL plastic volumetric flask. Finally, the calibration curve of the standard solution was made according to the concentration points of the curve at 0, 0.5, 1.0, 2.0, 5.0 mg/L, respectively, to make a good calibration curve of the standard solution. Subsequently, the samples were tested for their copper content using an inductively coupled plasma emission spectrometer (ICPOES; Agilent730, Agilent Tehnologies, Tokyo, Japan) in sequence. After dry treatment of the impregnated bamboo, the color value of the treated bamboo was determined.

3. Results and Analysis

3.1. Surface Color Values of Dyed Bamboo Wood

The experimental results revealed that the total color difference values (ΔE*) of the bamboo materials after dyeing exceeded 6 (NBS unit), indicating a large color difference compared to untreated bamboo (

Table 3. L*a*b* values and color differences of bamboo wood before and after dyeing.

Sample	L2*	a2*	b2*	ΔL*	Δa*	Δb*	ΔE*	Trial Color
Boil	80.83	6.92	28.32	2.50	1.17	5.12	5.81
Cs	59.83	21.48	47.63	−20.45	14.72	19.22	31.69
CsAl	55.92	38.70	30.82	−23.92	31.93	3.00	40.01
CsCu	45.58	31.83	19.60	−35.22	25.05	−8.67	44.08
CsFe	34.93	8.68	4.23	−45.02	1.82	−22.58	50.40
Ct	67.88	20.73	40.20	−11.96	13.74	11.63	21.61
CtAl	70.38	14.73	36.35	−11.17	7.82	7.88	15.75
CtCu	67.27	14.65	36.47	−13.83	7.33	8.20	17.67
CtFe	57.47	10.55	27.25	−21.87	3.68	−1.07	22.20
Rc	57.30	27.65	32.10	−24.28	20.93	3.88	32.29
RcAl	64.87	20.80	36.65	−16.97	14.03	8.33	23.54
RcCu	69.12	10.45	35.65	−11.72	3.33	7.43	14.27
RcFe	59.27	10.10	30.58	−21.62	3.23	2.32	21.98

). The color difference was perceived strongly, which implied that the dye had a better effect on the surface dyeing of bamboo materials. The total color difference values in descending order were CsFe (50.40) > CsCu (44.08) > CsAl (44.01) > Rc (32.29) > Cs (32.19). The total color difference values of these groups were over 30. In terms of brightness, the brightness of post-dyed bamboo wood was lower than that of pre-dyed bamboo wood. The descending order of brightness difference was CsFe (−45.02) > CsCu (−35.22) > Rc (−24.28) > CsAl (−23.92) > CtFe (−21.87). In terms of the red–green indices, all the post-dyed bamboo slices had a positive value and showed more reddish characteristics. The descending order of values was CsAl (31.93) > CsCu (25.05) > Rc (20.93) > Cs (14.72) > RcAl (14.03). In terms of the yellow–blue indices, the post-dyed bamboo slices showed varying degrees of yellowish or bluish tendencies. The descending order of color difference was CsFe (−22.58) > Cs (19.22) > Ct (11.63) > CsCu (−8.67) > RcAl (8.33). The experimental results revealed that CsFe had a significant effect on the total color difference, brightness, and yellow–blue index [33]. It had a smaller effect on the red–green value, with a difference of 1.82. CsCu had a higher effect on the total color difference, brightness, and red–green and yellow–blue indices. In addition, CsAl and RcAl had higher effects than the other groups. Therefore, based on the dyeing effect of bamboo and the purpose of studying the combined effect of dyes and mordants, the combinations of CsFe, CsCu, CsAl, and RcAl were selected as the focus of the subsequent study.

3.2. Results of Mold Inhibition of Plant Dyes

As shown in Figure 1, the three plant stains showed mold growth at different dilutions. Based on the observation of the liquid surface of the inoculated glass vials, the liquid surface of the Ct dye solution at different dilutions was covered with A. niger. Meanwhile, the growth of A. niger and T. viride on the liquid surface of the other dye solutions showed that the higher the dilution, the better the growth of molds. Therefore, in addition to Ct (which did not inhibit A. niger), the other dye solutions showed a certain degree of inhibition of A. niger and T. viride. However, the effectiveness of the inhibition of molds was exceptionally low.

3.3. Results of Mold Inhibition of Metal Mordants

Al has no inhibitory effect on A. niger. The maximum dilution time for T. viride is one. Fe has maximum dilution times of 20 and 40 for A. niger and T. viride, respectively. Cu has maximum dilution times of 100 and 300 for A. niger and T. viride, respectively. The anti-mold effect of Cu was significantly better than those of the other two metal mordants. Combined with the experimental results of the dyeing effect of the aforementioned plant dyes on bamboo, CsCu was selected from the combination of CsFe, CsCu, CsAl, and RcAl for the experiments on the combined effect of dyeing and mold prevention.

3.4. Dyeing and Color Fixation Rates Under Combined Effects

The maximum absorption peak wavelength of the Cs dyeing solution was 446 nm, and that of the CsCu dyeing solution was 498 nm. The absorbance curves of the Cs and CsCu dyeing solutions are shown in Figure 2. The staining rate of the Cs dye uptake rate was 9.63%, and the dye fixation rate was 3.12%, while those of the CsCu dyeing solution were 10.62% and 9.20%, respectively (

Table 4. Uptake and fixation rates of the two types of dyeing solutions.

Sample	λmax	A0n0	A1n1	A2n2	E/%	F/%
Cs	446	0.737	0.666	0.714	9.63	3.12
CsCu	498	0.913	0.816	0.829	10.62	9.20

Note: λmax: Maximum absorption peak wavelength; A₀: absorbance of control solution; n₀: dilution factor of control solution; A₁: absorbance of residual solution; n₁: dilution factor of residual solution, A₂: absorbance of diluted residual solution; n₂: dilution factor of diluted residual solution; E: dye uptake rate; F: dye fixation rate.

). The uptake rate and fixation rate for the CsCu dyeing of bamboo were higher than that for the Cs dyeing solution. The dyeing effect of plant-dyed bamboo wood was enhanced by the combined effect of the CsCu mordant.

3.5. Mold Inhibition of Impregnated Bamboo Under Combined Effect

In the liquid state (Figure 3), both CsCu (Cs extract–Cu) and WCu (water–Cu) solutions showed similar maximum inhibitory effects against A. niger at a dilution factor of 1:140. For T. viride, complete inhibition was achieved at a higher dilution factor of 1:300. Notably, CsCu did not exhibit superior mold inhibition compared to WCu at these dilution levels. However, at higher dilutions (1:160 for A. niger; 1:320 for T. viride), the CsCu solution demonstrated significantly reduced fungal growth compared to WCu. A powdery precipitate was observed in the CsCu solution, likely due to complexation between the plant extract and copper ions. This reaction may have partially reduced the bioavailability of anti-mold agents in the liquid phase. Similarly, if complexation occurs in the bamboo, the mold inhibition effect of CsCu in bamboo is expected to be enhanced compared to WCu.

As can be observed in

Table 5. Mold inhibition effect of CsCu and WCu on bamboo materials.

Mold	Sample	Area Infected (CsCu, Days)			Infectious Value	Resistance/%	Area Infected (WCu, Days)			Infectious Value	Resistance/%
		14	21	28			14	21	28
A. niger	ctrl	9/10	1	1	4	-	1	1	1	4	-
	300	0	0	0	0	100	0	0	0	0	100
	500	0	0	1/30	1	75	0	1/4	1/3	2	50
	700	0	¹⁄₆	1/4	2	50	1/40	1/4	1/3	2	50
	900	1/30	¹⁄₆	⅔	3	25	1/2	5/6	9/10	4	0
T. viride	ctrl	1	1	1	4	-	1	1	1	4	-
	1100	0	0	0	0	100	0	0	0	0	100
	1300	0	0	1/30	1	75	0	1/5	1/4	1	75
	1500	0	1/10	1/8	1	75	0	1/4	1/3	3	25
	1700	0	1/4	1/3	3	25	1/30	1/3	2/3	3	25

, the following occurred on the 14th day of the mold inhibition experiment: inoculation of A. niger, Cs extract treatment of bamboo, A. niger displaying an infected area of 9/10, complete infection of the pure-water control group of bamboo, CsCu1:700 preventing the growth of A. niger, CsCu1:900 exhibiting an infected area of 1/30, WCu1:700 treatment of bamboo mold infecting 1/40 of the surface, WCu1:900 infected area attaining 1/2 inoculation of T. viride, the control group being infected completely, the CsCu-treated bamboo not displaying green wood mold growth, and the WCu1:1700 group displaying a mold infection area of 1/30. The following could be observed on the 21st day: CsCu1:500 A. niger did not grow, WCu1:500 A. niger exhibited an infected area of 1/4, the CsCu1:1300 group did not display T. viride infection, and the WCu1:1300 group displayed a T. viride infection area of 1/5. On the 28th day (final experimental results, Figure 4), the following were observed: for Cs extract and pure water as the dispersing medium, the 1:300 group was not infected with A. niger, and the 1:1100 group was not infected with T. viride. Under an equal proportion of Cu, the area of mold infection of CsCu was smaller than that of WCu, and the effectiveness of mold inhibition of CsCu was higher. This may have been due to the low proportion of copper sulfate added to bamboo. Cs displayed its mold inhibition effect. During the experiment, the mold onset time of CsCu was later than that of WCu. The final mold inhibition results showed a higher mold inhibition efficacy. Therefore, the combined effect of CsCu enhanced the mold inhibition efficacy of modified bamboo.

3.6. Fourier-Transform (FTIR) Analysis of Bamboo

As shown in Figure 5, Cs dyeing-solution-treated bamboo (curve c) and CsCu dyeing-solution-treated bamboo (curve d) showed enhanced absorption peaks near 2400 cm⁻¹ compared with bamboo (curve a). This may be related to the phenolic compounds in Cs. The 1600–1700 cm⁻¹ region is usually associated with the C=C stretching vibration of the aromatic ring. Cs (curve b) shows a distinct absorption peak in this region, thereby indicating the presence of aromatic compounds. Bamboo treated with Cs or CsCu (curves c and d) also shows absorption peaks in this region, although with an intensity lower than that of Cs (curve b). This indicates that the aromatic compounds in Cs may have been partially transferred to bamboo. In the region of 800–1000 cm⁻¹, which is usually associated with the C-H bending vibration of aromatic compounds, Cs (curve b) shows multiple absorption peaks. Treated bamboo (curves c and d) also shows absorption peaks in this region. However, their intensities and locations differ from those of Cs. It can be concluded that Cs or CsCu sulfate may have acted on the bamboo.

3.7. SEM Analysis of Bamboo

SEM images of bamboo before and after dyeing are shown in Figure 6 [34,35,36]. The surface of the vessel wall of untreated bamboo (Figure 6a) was smooth. Furthermore, the pores on the vessel walls were clear, and starch granules were present in the vessels. In the bamboo treated with the Cs dye solution (Figure 6b), the surface of the vessel walls was rougher than that of the untreated material. In addition, the starch granules disappeared, a few of the pores of the vessel walls were filled with the Cs dye solution, and the grayscale on the surface of the vessels was of different shades. The surface of bamboo treated with CsCu dye (Figure 6c) was also smoother; the starch particles disappeared; and the dye uniformly covered and filled the pores of vessel walls, with a uniform and darker gray surface. The CsCu dyeing solution is better than the Cs dyeing solution for dipping bamboo owing to its dye filling effect in the vessels, surface covering status, and color uniformity. Therefore, it showed a better dyeing rate and dyeing effect in the dyeing experiment, and higher anti-mold efficacy in mold inhibition experiments. CsCu1:900 specimens, in addition to the mold inhibition effect of copper sulfate itself, also limit the growth of mold by the dyeing solution sufficiently occluding the vessel pits. The combined effect of Cs-Cu laid the foundation for the dual synergistic treatment of dyeing and anti-mold.

3.8. Results of the Determination of Copper Content and Its Effect on the Color of Bamboo Wood

From the results of copper sulfate content determination in

Table 6. Determined values of the copper content of the treated bamboo materials.

Sample	Sampling Mass/g	Instrument Readings	Unit	Conversion Content	Unit	Mass Fraction %
CsCu1:300	0.1966	2.5861	mg/L	3288.5	mg/kg	0.3288
WCu1:300	0.2012	2.7929	mg/L	3470.4	mg/kg	0.3470
CsCu1:1100	0.2009	1.1434	mg/L	1422.9	mg/kg	0.1423
WCu1:1100	0.2019	1.1945	mg/L	1479.1	mg/kg	0.1479

, the copper content in the bamboo treated with Cs extract and pure water as the dispersing medium was mainly determined by the amount of copper sulfate added. In the case of an equal proportion of added copper sulfate, the copper content of the copper sulfate bamboo in Cs was lower than that of copper sulfate bamboo in pure water. However, the copper sulfate bamboo with Cs had better mold inhibition effectiveness. Therefore, the bamboo treated with plant dyes as the dispersing medium displayed higher mold inhibition effectiveness at a lower copper sulfate content.

The color value results in

Table 7. L*a*b* values of treated bamboo.

Sample	L*	a*	b*	Trial Color	Test Piece	L*	a*	b*	Trial Color
CsCu1:300	39.7	26.4	19.2		CsCu1:1100	39.4	27.4	18.2
CsCu1:500	45.1	27.2	23.1		CsCu1:1300	43.6	28.7	22
CsCu1:700	41.8	27.5	20.5		CsCu1:1500	42.5	27.9	19.5
CsCu1:900	40.5	27.9	20.2		CsCu1:1700	45.8	28.2	23.2

clearly show that under the condition of extraction concentration in accordance with the Cs/pure water ratio of 1 g:50 mL, the addition amount of copper sulfate does not have a significant influence on the color difference value of the dyed bamboo. Moreover, it does not imply a correlation between the addition amount and color difference. Therefore, in the double-effect treatment of CsCu achieving both dyeing and mold prevention, the amount of copper sulfate added mainly determines the mold prevention effect of impregnated bamboo. After the dyeing achieves the expected effect, the amount of copper sulfate added should be primarily determined according to the mold prevention effect.

4. Discussion

This study presents a green, dual-functional approach using plant-based dyes (Cs, Rc, and Ct) combined with metal mordants to simultaneously enhance the aesthetics and mold resistance of bamboo. Unlike traditional methods that separately address dyeing and anti-mold treatments—often relying on synthetic chemicals—this integrated protocol fulfills both objectives, responding to the urgent need for eco-friendly bamboo modification. This approach aligns with recent initiatives to substitute toxic dyes and preservatives with plant-based alternatives [37,38].

The combination of Cs and Cu demonstrated a significant synergistic effect. The dye fixation rate increased from 3.12% (Cs alone) to 9.20% (CsCu), and similar interactions between metal ions and plant dyes have been documented to enhance dye stability in textile applications [25,39]. However, this study uniquely demonstrates this synergy’s applicability to bamboo substrates. CsCu achieved complete inhibition of A. niger and T. viride, outperforming pure copper sulfate (WCu) at equivalent concentrations. This finding is consistent with studies indicating that plant-derived ligands can improve the bioavailability and antifungal efficacy of metal ions, suggesting a potential reduction in the required dosage of metal-based preservatives.

This study supports the viability of plant–metal synergies for sustainable material science. The stability of plant–metal complexes under UV exposure or humidity cycling is untested. Future research will incorporate advanced statistical models (e.g., multivariate regression) to further explore the interactions between dye concentration, mordant ratios, and bamboo wood, as well as evaluate long-term performance under environmental stressors.

5. Conclusions

This study demonstrates that Cs, Rc, and Ct, dyed individually or in combination with metal mordant, could significantly alter the surface color characteristics of bamboo. Plant dye–metal mordant synergies provide a dual-functional solution for bamboo valorization, simultaneously enhancing aesthetic appeal (dye uptake rate from 9.63% to 10.62%; dye fixation rate from 3.12% to 9.20%) and anti-mold performance. The integrated approach reduces reliance on synthetic chemicals while aligning with circular economy principles.