Zinc Complex Derived from ZnCl₂-Urea Ionic Liquid for Improving Mildew Property of Bamboo

Abstract

The nanometer zinc complex, formed in chloride-urea ionic liquid (IL), was studied with the objective of enhancing the mildew resistance of bamboo. The nano-Zinc complex layer was coated on the bamboo surface by a simple and mild heating process. The SEM analysis revealed that the morphology of the nanometer Zinc complex layer on the bamboo surface varied with the reaction time of bamboo in zinc chloride (ZnCl₂)/urea ionic liquid. The result of EDS and FTIR analysis showed that zinc and chlorine were successfully coated on the surface of bamboo. In this study, it was found that the optimum condition was 2 h of reaction with 1:2 molar ratio of zinc chloride to urea, where the nano-Zinc complex layer on the bamboo surface was the most uniform and dense to present the bamboo with the strongest mildew resistance. The infection value of Trichoderma viride, Aspergillus niger V. Tiegh, and Penicillium citrinum Thom after 28 day was 0.

1. Introduction

In recent decades, sustainable development has remained a vital component of modern-day society. The utilization of biomass materials can relieve environmental pressure to a great extent, which is a positive response to sustainable development [1,2]. As the second most abundant natural resource, bamboo has great potential to be an alternative for wood, and it now remains an indispensable ingredient of modern life because of its attractive advantages, such as rapid growth, grace figure, biodegradability, excellent integrative performance including good mechanical properties, high strength, excellent toughness, and good rigidity [3,4,5]. Bamboo has been applied to various fields, such as furniture, artwork, construction, and decoration [6]. However, in common with other biological materials, bamboo products’ main problem is their susceptibility to fungi, because of its high content of protein, sugar, and starch [7], which seriously affects the utilization of bamboo products and results in massive bamboo resources and economic losses. In addition, the suitable pH value of most molds is weak acid; bamboo itself is acidic, especially suitable for mold growth. Therefore, mildew-proof treatment is of great importance in the efficient application of bamboo.

Numerous methods have been used to improve the mildew resistance of bamboo so as to improve the service life and extend the service range of bamboo. Conventionally, these anti-mildew treatments can be divided into physical and chemical methods. The physical treatments include thermal [8,9,10] and Gamma radiation treatment [11]. Chemical treatments perform better in mildew proofing. Chemical mold-resisting agents, such as chitosan–copper complex (CCC) [7], chromated copper arsenate (CCA) [12], camphor leaf extract [13], and alkaline copper quaternary compounds (ACQ) [14] have been used to improve the mildew resistance of bamboo. However, many traditional preservatives are restricted or banned from being used due to their potential toxicity of arsenic, chromium, and halogens, which may be toxic to human bodies and biological systems. In recent years, metal oxides in nanoscale particles, such as TiO₂ [15,16], ZnO [17,18,19,20], CeO₂ [21], and AgO [22], have been studied for bamboo mildew prevention because of their environmental non-toxic and economic advantages.

Li et al. [15] prepared anatase crystal TiO₂ on bamboo surface by the room temperature synthesis method. The results showed that the crystallinity increased with the longer reaction time at room temperature. Compared with the original bamboo material, anatase crystal TiO₂-coated bamboo material has better anti-fungal ability under natural climate conditions. Song et al. [17] Prepared ZnO nanostructures on the bamboo surface by using zinc nitrate (Zn(NO₃)₂) and hexamethylenetetramine (C₆H₁₂N₄) as raw materials. In laboratory tests, mold fungi including Aspergillus niger, Penicillium citrinum, and Trichoderma viride were chosen as the target fungi, and the results indicated that the ZnO-coated bamboo had a better resistance against A. niger and P. citrinum, but not good against T. viride. Among the tested mold fungi, T. viride was the most tolerant one. Liu et al. [21] concluded that CeO₂ nanoparticles could be used as a very promising coating material to modify silk for UV-protection and antibacterial applications. Pandoli et al. [22] prepared bamboo with a homemade nano-silver (Ag-NPS) colloidal solution from bamboo to enhance its anti-fungal properties. The lower the diameter, the better the antibacterial effect of Ag-NPS. Self-made nanoparticles (NP-01), with a particle size of about (14.3 ± 3.6) nm and particle concentration of 1.25 × 10¹¹ particles/mL, can inhibit the growth of Aspergillus Niger by 53% at a concentration of 2.00 mg·L⁻¹. Tests of untreated bamboo and engineered biocomposites Ag-NPS/bamboo exposed to air and ambient humidity (about 70–80%) in summer showed that all untreated samples showed fungal colony formation, while Ag-NPS treated samples were well preserved and were not affected by fungal degradation.

Among the mentioned nanomaterials, nano-ZnO is particularly attractive because of its excellent performance in antibacterials [23,24], UV-resistance [25], and thermal stability [26,27]. Most of the current studies on mildew prevention of bamboo using zinc oxide are based on or combined with the hydrothermal method. However, the hydrothermal process is cumbersome and requires high operating conditions, which need to be carried out in a closed environment.

Here, we develop a simple new method to fabricate a nano-Zinc complex-coated bamboo surface by immersing bamboo in zinc chloride (ZnCl₂)/urea ionic liquid (IL) for some time. The IL was prepared by heating mixtures of zinc chloride and urea in a water bath with different concentrations. Compared with the traditional approaches, this technique is simple enough and easy to operate. Moreover, the resulting nano-Zinc complex-coated bamboo surface exhibited excellent anti-mildew property.

2. Materials and Methods

2.1. Materials

Bamboo was obtained from China Resources Bamboo Co., Ltd., Zhejiang, China, bamboo aged four years. The exterior and interior layers of the bamboo were removed, and then they were processed into blocks with the dimensions of (50 ± 1) mm (length) × (20 ± 0.5) mm (width) × (5 ± 0.5) mm (thickness). All blocks were ultrasonically washed three times with deionized water and 99.7% ethanol to remove surface contamination and dried at 80 °C for 24 h.

Zinc chloride and urea were bought from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, and Tianjin Zhiyuan Chemical Reagent Co. Ltd., Tianjin, China, respectively.

Trichoderma viride Pers. ex Fr., Aspergillus niger V. Tiegh, and Penicillum citrinum Thom were bought from the Academy of Sciences Quality Inspection Biotechnology Co. Ltd. (Beijing, China).

2.2. Fabrication of Nano–Zinc Complex Layer on the Bamboo Surface

Clear ILs were obtained by heating a mixture of zinc chloride and urea in a beaker at a temperature of 90 °C for one hour in a water bath and then adding twice the amount of deionized (DI) water relative to urea when the temperature dropped to 60 °C (Figure 1). Then, the bamboo block was taken out, and excess IL washed with ultrapure water three times. Finally, the modified bamboo was dried at 80 °C for 48 h. The mass ratio of IL was n(ZnCl₂)/n(urea) = 1:2. The IL was synthesized with ZnCl₂/urea molar ratios [n(ZnCl₂)/n(urea) = 1:2]—from here on referred to as U2. Samples with different reaction times under the U2 condition were labeled U2-tx—for example, U2-t2 means 2 h of reaction at U2 concentration.

2.3. Anti-Mildew Property Test

The anti-mildew property was evaluated according to the Chinese standard GB/T18261-2013 [28]. Six dimensioned bamboo samples were tested with Trichoderma viride, Aspergillus niger V. Tiegh, and Penicillium citrinum. After 4 weeks of culture, the area of infection was measured, and the infection value (IV) of bamboo material was recorded. The description of the surface infection value of the sample was shown in

Table 1. The description of infection value.

IV	Area of Infection
0	No hypha or mold
1	Surface infection area less than 1/4
2	Surface infection area between 1/4 and 1/2
3	Surface infection area between 1/2 and 3/4
4	Surface infection area over 3/4

. The lower the infection value, the better the anti-mildew effect.

3. Characterization

The specimens’ microstructures were analyzed by scanning electron microscopy (SEM, Zeiss, Supra55, Berlin, Germany) with an X-ray energy dispersive spectrometer (EDS, Genisis XM, Oxford, London, UK). Before SEM examination, all of the samples were sputtered with a thin layer of gold film to improve their conductivity. The chemical compositions of the samples were determined by energy-dispersive spectroscopy (EDS, attached to the SEM). The reaction mechanisms of samples were carried out by a Fourier transform infrared spectroscopy (FT-IR, Nicolet 380, Thermo Electron Instruments, Waltham, MA, USA).

4. Results and Discussion

4.1. Morphology of Zinc Complex Coatings on Bamboo Surface

The appearance of the bamboo before and after treatment is shown in Figure 2, its natural color has not changed much. As shown in Figure 2b, the reaction time was 1 h of bamboo block, and its surface color was almost no difference compared with the original bamboo block. In Figure 2c, more white zinc complexes were attached to the surface of the bamboo block with a reaction time of 2 h. As shown in Figure 2d, with the increase in reaction time, the attachment to the surface of bamboo block increased and became more uniform. A scanning electron microscope (SEM) was utilized to detect the morphologies of bamboo surface (Figure 3). The surfaces of immersed samples with different treatment conditions were used for SEM analysis. Figure 3 showed the surface morphology of the original bamboo (OB) and the modified bamboo treated under varying conditions. The surface morphology of bamboo with different reaction time was different. As shown in Figure 3a, the microstructure of the pristine bamboo was clear, and the bamboo fibers were approximately 20–30 micron in diameter. The surface grain holes of original bamboo were clearly visible. There was no covering on the surface of the original bamboo, which provided a rough surface. As Figure 3 shows, the Zinc complex layer’s morphologies on the bamboo surface changed with reaction time. As shown in Figure 3b, when the processing time was 1 h, a relatively smooth layer of waxy Zinc complex was observed on the surface of sample U2-t1. The amount of zinc complex was relatively small, and the morphology of bamboo surface was still clearly visible. As seen in Figure 3c, when the processing time was increased to 2 h, the surface of sample U2-t2 was completely covered with nano-Zinc complex. From the high-power SEM image, it can be clearly observed that the surface of bamboo has a flower-like distribution of nano-Zinc complex. The surface of the zinc complex prepared under U2-t2 was rougher than that of the zinc complex prepared under the condition of U1-t1. When the processing time was extended to 3 h, the nano-Zinc complex layer became more uniform, as shown in Figure 3d. The nano-Zinc complex coated on the surface of modified bamboo became more uniform with increasing reaction time. The surface area of the nano-Zinc complex reached its maximum when the treatment time was 2 h. The optimal coverage of the nano-Zinc complex layer on the bamboo surface appeared under the treatment condition of U2-t2.

4.2. Chemical Structure of the Bamboo Surface

EDS analyzed the main chemical elements on the surface of bamboo before and after modification. As shown in Figure 4, the elements carbon and oxygen could obviously be detected from the spectra of the pristine sample. Signals from Zn and Cl could also be observed in the spectrum of the ZnCl₂/urea-IL-treated bamboo samples. The surface element content of bamboo surface before and after treated is shown in

Table 2. Surface element content of bamboo surface before and after treated.

Element	OB		U2-t1		U2-t2		U2-t3
	Wt.%	At.%	Wt.%	At.%	Wt.%	At.%	Wt.%	At.%
C	58.00	65	53.71	62.91	39.98	56.63	40.71	51.68
O	41.36	34.78	40.21	35.35	32.17	34.10	31.72	30.23
Cl	-	-	2.02	0.80	8.98	4.31	7.63	3.28
K	0.64	0.22	0.44	0.16	0.34	0.15	0.37	0.15
Zn	-	-	3.62	0.78	18.53	4.82	7.76	1.81
N	-	-	-	-	-	-	11.81	12.85

. The atomic fractions of sample U2-t1 and U2-t3 were Zn = 0.78% and Zn = 1.81%, respectively. On the surface of sample U2-t2, the Zn element accounted for 4.82%. Meanwhile, the ratio Zn/Cl was 1.12 at U2-t2. The ratio Zn/Cl of sample U2-t1 and U2-t3 was 0.97 and 0.55, respectively. This indicated that the maximum available zinc complex loaded on the surface of sample U2-t2. N from urea was only found at U2-t3, which may be due to urea inhibited the loading of nano-Zinc complex on bamboo.

Figure 5 showed the FTIR spectrum of the original bamboo and the bamboo modified by IL. The stretching vibration frequency of carbonyl in the infrared spectrum moves towards a low wavenumber when carbonyl oxygen coordinates with metal [29]. Compared to the original bamboo, the wavenumber of the absorption peak of the carbonyl group decreased from 1685 to 1650 cm⁻¹, which corresponds to urea [30]. This means that the carbonyl oxygen in the urea molecule forms a coordination bond with the central ion, Zn²⁺. The new absorption peak that appeared at 2240 cm⁻¹ could also verify that zinc oxide [20], which was synthesized in the ionic liquid, was attached to bamboo’s surface. Compared to OB, the band strength of the modified samples at 2931 and 2849 cm⁻¹ (R–C–H) was higher, indicating that Zinc salt was loaded on the bamboo surface under the reaction condition [11]. The absorption peak at 3424 cm⁻¹ corresponded to the intramolecular hydroxyl group of bamboo, and the modified samples showed contracted and strengthen peak. The strengthening of hydroxyl group bond energy indicates that the ionic liquid bonds with bamboo by a physical hydrogen bond.

4.3. Anti-Mildew Properties

Weak mildew resistance greatly limited the application of bamboo in practical production, the content of sugar and protein in bamboo tissue is high, and mildew is easy to occur under proper temperature and humidity. Therefore, it is of great significance to improve the mildew resistance for the practical application of bamboo. Figure 6 shows the antibacterial and anti-mold properties of the control sample (OB) and the treaed sample (U2-t1, U2-t2, U2-t3) that were qualitatively investigated by Trichoderma viride, Aspergillus niger V. Tiegh, and Penicillium citrinum Thom for 4 weeks. The infection value (IV) that was used to evaluate the mildew resistance of bamboo was shown in Figure 7. The lower the IV, the better the anti-mildew effect. It could be seen that Trichoderma viride, Aspergillus niger V. Tiegh, and Penicillium citrinum Thom grew successfully on the OB surface, and consequently, the IV of bamboo blocks tested by three fungi was 4, suggesting that the original bamboo had no resistance against mildews. For sample U2-t1, it was first infected with Trichoderma viride, Aspergillus niger V. Tiegh, and Penicillium citrinum Thom on day 12, day 7, and day 15, respectively. It reached the maximum IV of 2, 3, and 2 for Trichoderma viride, Aspergillus niger V. Tiegh, and Penicillium citrinum, respectively, within 4 weeks. Sample U2-t2 showed excellent resistance to all three molds. All three strains failed to infect the samples during the four-week trial. For sample U2-t3, the first Trichoderma viride, Aspergillus niger V. Tiegh, and Penicillium citrinum Thom infections occurred on day 15 and day 20, respectively. Within 28 days, the IV of three kinds of mold on sample U2-t3 reached the highest level 1 on day 15 and day 20. Therefore, IL treatment delayed the time of fungus infection and reduced the harm of fungus to bamboo. This may be due to the nano-Zinc complex destructive effect on fungal cell walls and membranes. The zinc ions in nanometer zinc oxide will gradually free, when in contact with the fungus, then the zinc ions will bind to the fungal active protease in the body and make it inactive, thus killing the fungus [23,24]. Therefore, it can be concluded that the nano-Zinc complex formed in the ZnCl₂/urea IL was closely attached to the surface of the bamboo, which endowed bamboo with excellent mildew resistance to various molds, including Trichoderma viride, Aspergillus niger V. Tiegh, and Penicillium citrinum Thom.

5. Conclusions

In this paper, the influence of reaction time on the morphology and mildew proof effect of zinc salt containing zinc oxide on the surface load of bamboo was investigated. The bamboo before and after modification was characterized by SEM-EDS, FTIR, and XRD, and the mildew proof performance of bamboo under different conditions was investigated. An effective and simple technology was successfully developed for the improvement of the anti-mildew property of bamboo. The bamboo surface was modified in ZnCl₂/urea-IL; thus, the nano-Zinc complex layer, which was fabricated by coordination reaction between zinc chloride and urea, could combine with bamboo through hydrogen bonding. When the reaction time was 2 h, the mildew proof effect of the modified bamboo sample was the best, and the IV of the three kinds of Fungi after 28 day was 0. The optimized parameter showed excellent anti-mildew performance. This work provided a novel and effective approach to prolong the service life of bamboo. It can be used in architecture, decoration, furniture, and other fields.