Cu–Ethanolamine Nanozymes Promote Urushiol Oxidation of Lacquer

Abstract

In order to control the production cost of lacquer products, Cu–ethanolamine nanozymes were synthesized to simulate laccase to catalyze the oxidation and polymerization of urushiol. First-principles calculation results indicate that the D-band center of Cu center in the nanozymes was closer to the Fermi level than that of laccase, so Cu–ethanolamine was more conducive to the adsorption of substrate. The activation energy of Cu-ethanolamine catalyzed the oxidation of urushiol was significantly lower than that of laccase. Therefore, we inferred that the synthesized Cu–ethanolamine had a better catalytic effect on urushiol and was more conducive to paint film drying. By comprehensive comparison, the drying characteristics of the Cu–ethanolamine and raw lacquer with a 1:20 ratio are found to be closest to those of the raw lacquer, and the drying time is significantly shortened. The reaction results of the drying process performance test on the sample indicate that the composite lacquer can achieve the market-desired effect and performance requirements of the paint process.

1. Introduction

Raw lacquer is a kind of natural coating polymer material, which is often used to coat the surfaces of objects as a protective layer [1,2,3]. The raw paint is collected from the bast of sumac trees, which survive for about ten years. The color gradually increases as it oxidizes in the air. Raw lacquer was originally used for repairing porcelain, gluing wood and other uses, because it can be cured over time into a film and applied to utensils or wooden tools with good adhesion [4,5]. With the deepening of the study of raw paint, more properties of raw paint have been further revealed. For example, raw paint has excellent heat resistance and insulation, dark color, high gloss and hardness and good chemical resistance and corrosion resistance [6,7]. Modified raw lacquer can be used in many fields, such as ships, decoration and protection, high-grade wood furniture and textiles and inner walls of oil pipelines in the petroleum industry [8,9,10]. However, raw lacquer also has some shortcomings, such as high requirements for dry conditions regarding a specific temperature and humidity environment [11]. Regarding the process of lacquer self-drying, questions of how to improve the film formation of lacquer and accelerate the oxidation of laccase have become the research content of more and more researchers.

The film-forming process of lacquer drying is essentially a process of film-forming polymerization of urushiol catalyzed by laccase in lacquer [12,13]. The copper ions in laccase play a catalytic role [14]. In order to stabilize the activity of laccase, the drying and forming of raw lacquer film should be carried out at a temperature of 20–30 °C and a relative humidity of 80–90% [15]. Under the catalytic action of laccase, the urushiol group reacts rapidly to form urushiol Kun and reacts with other urushiol molecules at the same time. Due to its high activity and instability, urushiol reacts with urushiol further with phenol-group and hydrocarbon-group side-chain reactions [16,17]. Laccase is a polyphenol oxidase containing copper ions, soluble in urushiol. Although the total content of laccase in raw lacquer is very low, their role is particularly important [18,19,20,21,22,23,24]. In the drying process of raw paint, the copper ions in laccase catalyze the drying of paint films by transferring electrons. The activity of laccase has an important effect on the drying of raw paint. Changes in pH, temperature and humidity affect the drying rate of paint [25,26]. The drying process of laccase-catalyzed oxidation of urushiol for film-forming polymerization reactions takes a long time, which is limited by external humidity and temperature condition, and raw lacquer without laccase activity cannot dry itself, resulting in a waste of raw lacquer materials [27].

With the development of nanotechnology, nanozyme catalysts provide the possibility to catalyze high-efficiency reactions [28,29,30]. Nanozyme catalysts are combine the power of natural and artificial catalysis. The use of nanozyme catalysts can be designed according to specific needs, and the cost is lower than that of biological catalysis. Nanozymes also have high stability, less storage conditions and good adaptation to environmental conditions [31,32,33,34,35]. They have applications in green synthesis, new energy, medicine and other fields [36,37,38]. Currently, the preparation methods of nanozyme catalysts mainly include coprecipitation methods, chemical combination methods, in situ polymerization methods, fixation methods, etc. [39,40]. At present, nanozyme catalysts are widely used in new catalytic expansion. For example, Asuri et al. [41] used biological nanozymes to achieve liquid–liquid, bidirectional biological conversion, and the rate of the enzyme interface was also increased by three times. In the process of studying the drying performance of raw lacquer, Yang Jianhong et al. [32] found that the drying performance of raw lacquer film can be improved by adding a small amount of active maleic semi-ester surfactant to the lacquer liquid. Gao Renjin et al. [42] used hexamethylenetetramine (HMTA) to decompose formaldehyde and amine in a high-temperature environment to shorten the drying time of raw lacquer and improve the alkali resistance and gloss of raw lacquer. Many previous works have reported that complexes of metal ions and amines can be used as simulated enzyme catalysts [43,44].

In this study, Cu–ethanolamine nanozymes were synthesized to simulate laccase to catalyze the oxidation and polymerization of urushiol. Natural lacquer and Cu–ethanolamine nanozymes were fully mixed and stirred at constant temperature and humidity. The curing film-forming time and drying performance of the composite quick-drying raw lacquer agent and the effects of different ratios of nanozymes on the drying film of raw lacquer were repeatedly observed and studied. The purpose of this study was to catalyze the oxidation polymerization of a raw lacquer phenol by nanozymes and shorten the drying time of raw lacquer, so as to realize the actual efficiency of the production of processed lacquer products and control the production cost of processed lacquer products to a certain extent.

2. Method

2.1. Theoretical Calculation

All calculations were performed using DMol3 code based on density functional theory (DFT). The code implementing Dmol3 was provided by Material Studio. In order to test the effect of the dispersion force on the system, we tested the DFT and DFT-D methods and the results are shown in Tables S1 and S2. The results show that the dispersion force does not affect our qualitative analysis of the catalytic effects of Cu–ethanolamine nanase and laccase. So, we chose the conventional DFT method. The effective nu-91 clear potential was adopted as well as the effective Core pseudo-potential (ECP) to mimic relativity or accommodate scalar relativistic effects, and the atomic wave function was adopted as well as the double numerical basis group (DNP) with a p-orbital polarization function. The PBE (Perdew–Burke–Ernzerhof) functional has the characteristics of high precision prediction, wide applicability, high efficiency calculation, and good stability. Therefore, under generalized gradient approximation (GGA), PBE is chosen as the form of electron exchange-related potential. Geometric optimization was performed before energy calculation, and the convergence criterion was set as energy deviation less than 1.0 × 10⁻⁵ a.u. The transition state search was then performed using the LST/QST method provided in the Dmol3 program. It combines the advantages of both LST and QST models and can more accurately describe the energy change and configuration change in the reaction process. Specifically, a single reaction path is introduced between products and reactants by the LST method to obtain an intermediate state of the highest energy state. Then, the Climbing Image Nudged Elastic Band (CI-NEB) method developed by Henkelman et al. was adopted to obtain the minimum energy path (MEP) of its reaction, and the energy state and configuration of TS were accurately determined by QST.

2.2. Materials and Reagent Preparation

The main materials and reagents used in the experiment: lacquer, copper chloride solid particles, ethanolamine solvent, alcohol, glass negatives, hairbrush, liquid tube, scoop, measuring cup, stirring rod, grinding rod, hairbrush, lacquer plate lignin tire, paper tape.

2.3. Preparation of Cu–Ethanolamine

Cu–ethanolamine was synthesized directly used solid copper chloride particles without pure water to react with ethanolamine solvent. Firstly, ethanolamine solution and solid copper chloride particles were weighed according to GB/T3186-2006 [45]. Then, the ethanolamine solution was dripped into the solid copper chloride particles with a drip tube. At this time, the color of the solvent became dark blue, accompanied by heating, and there were particle impurity precipitates in the solvent. According to the observation, there was a small amount of agglomeration after mixing in this group, so the Cu–ethanolamine was obtained by fully grinding it with a grinding rod by stirring and compounding. Based on previous work in the literature [44], the effect of a mixture of CuCl₂ and ethanolamine with a ratio of 1:1 on the catalytic oxidation of raw lacquer was specifically studied.

2.4. Preparation of Raw Lacquer and Cu–Ethanolamine Composite Film

Using pure raw lacquer as raw material, since the raw lacquer was to be layered under relatively long-standing conditions, the raw lacquer was fully stirred before preparing the sample to make its composition more uniform, aiming to achieve better experimental results. After stirring evenly, a small amount of sample raw lacquer was taken out with a sampling tube (small needle tube). Secondly, the transparent glass plate was fully cleaned with ethanol reagent and then repeatedly wiped with white dry toilet paper. Finally, some samples of raw lacquer were dipped with a hairbrush and coated on the treated glass plate. Using pure raw lacquer and Cu–ethanolamine as raw materials, the raw lacquer and Cu–ethanolamine were compounded according to the ratios of Cu–ethanolamine/raw lacquer = 1:30, 1:20, 1:10, 1:5, 1:1.

2.5. Drying Process Performance Test

Firstly, the paper tape was used to define the sample area smeared on the surface of the plain tire. Secondly, the refined raw lacquer after blending was taken, and the sample was evenly smeared on the left side of the defined area of the plain tire with a hairbrush. The paint complex was taken and the sample was evenly applied to the right side of the defined area of the plain tire with a hair brush. After the coating is completed, the paint tray was sent to a shaded dry room for drying observation.

3. Results and Discussion

3.1. Construction of Cu Nanozyme Active Centers

In the oxidation process of urushiol catalyzed by laccase, an electron is transferred from the urushiol to oxidizing laccase En–Cu²⁺, which generates semi-quinone radicals and reduces to laccase En–Cu⁺. Urushiol and urushiol quinone are formed by disproportionation of semi-quinone radicals. The reduced laccase En–Cu⁺ has a high affinity to oxygen, and is immediately oxidized to oxidizing laccase En–Cu²⁺, and then participates in the subsequent oxidation of urushiol, thus forming a cyclic system [46]. Therefore, we believe that accelerating the formation of semi-quinone radicals can accelerate the whole oxidation process of urushiol.

By carefully studying the structure of laccase, it can be found that laccase has a relatively conserved structural composition of four active centers of copper ions, in which T1 copper is the substrate binding site, resulting in electron transfer [47,48,49]. Therefore, in order to simplify the calculation, T1 copper was selected as the prototype model, one copper atom was used as the active site, the structure of three histidine coordination structures was used as the model of the active center of laccase, and ethanolamine was used to replace histidine to construct the Cu–ethanolamine nanozymes model, as shown in Figure 1.

In order to further study the internal mechanism of laccase and Cu–ethanolamine catalyzing urushiol, we calculated the density of state (DOS) for laccase and Cu–ethanolamine to study their electronic structure. Previous studies have shown that the catalytic center of laccase is copper, so we focused on analyzing the density of state for the D band in the laccase and Cu–ethanolamine and drew the projected DOS diagram as shown in Figure 2. According to the traditional D-band center theory, the difference in D-band center (ε_d) relative to Fermi level of transition metal catalysts can well reflect the substrate adsorption energy on the catalyst center. From Figure 2, it can be qualitatively observed that due to the coordination between pyrrole nitrogen and Cu atoms, the D-band electronic structure of laccase has a lower degree of localization, and the D-band center of the complex is far from the Fermi level. Due to the coordination between amino nitrogen and Cu atoms, the D-band localized electronic structure of Cu–ethanolamine is relatively high, and the D-band center of the catalyst center is closer to the Fermi level. The results showed that the D-band center of Cu–ethanolamine (−8.868 eV) was closer to the Fermi level value, (0 eV) than that of laccase (−11.249 eV), so Cu–ethanolamine was more conducive to the adsorption of substrate, that is, it had a potential catalytic effect on the formation of urushiol quinone, and accelerated the drying process of paint film.

3.2. Kinetic Study on the Oxidation of Urushiol Catalyzed by Two Different Active Centers

The early stage of paint film drying is the oxidation of urushiol catalyzed by laccase to form urushiol quinone. The specific reaction process is the transfer of an electron from urushiol to oxidizing laccase En–Cu²⁺, which produces semi-quinone radicals and reducing laccase En–Cu⁺. Urushiol and urushiol quinone were formed by dismutation of semi-quinone radicals, and Cu–ethanolamine simulated the process of oxidation of urushiol catalyzed by laccase [46]. We calculated the transition state of this reaction process. For this reason, we constructed two models of laccase-catalyzed urushiol and Cu–ethanolamine-catalyzed urushiol and conducted a transition state search for the reaction process. According to transition state theory, reactant molecules are not simply formed directly through simple collisions but must pass through a transition state to form a high-energy activated complex and reach a certain amount of activation energy required for this transition state and then convert into products. We calculated the activation energies from the initial state to the transition state of the reaction of Cu–ethanolamine and laccase with urushiol, respectively. As shown in Figure 3, the activation energy of the Cu–ethanolamine to the transition state (1.19 eV) was significantly lower than that of the laccase to the transition state (3.36 eV). Compared with laccase, Cu–ethanolamine nanozymes reduce the activation energy required for the conversion process of urushiol quinone, accelerate the formation of semi-quinone radical and, thus, accelerate the whole oxidation process of urushiol. Therefore, we can infer that the catalytic effect of synthesized Cu–ethanolamine on urushiol is better than that of laccase, which is more conducive to the drying of paint films.

3.3. Paint Film Drying Tests with Different Cu–Ethanolamine/Lacquer Ratios

Firstly, we used a pipette to extract 0.1 g of ammonium complex and a syringe to measure 3.0 g of raw lacquer according to GB/T3186-2006 [45]. At this point, the 1:30 composite sample underwent a slight reaction, causing the color of the raw lacquer to oxidize to a dark gray and the transparency to decrease. The viscosity of the lacquer composite was moderate and there were a few particles of impurities. Using a brush, we applied the lacquer sample onto a glass plate for film coating and marked the sample as B1. Due to concerns about impurities in the composite, it was filtered and processed. The filtered sample was applied onto a glass plate for film coating using a brush. The filtered sample was marked as B2. We designated the raw lacquer blank group B1,and B2 as sample group R1, which was sent to the drying room for observation, as shown in Figure 4.

Following the above method, we added 0.1 g of Cu–ethanolamine dropwise into 2.0 g of raw lacquer for composite. The complex reaction is mainly characterized by the color changing to black, and the color reaction is more obvious than 1:30. Thus, the increase in oxidation rate is deduced. There is still a small amount of particle impurities. It can also be seen that the viscosity increases and the transparency decreases. Some of the composites were coated on the glass plate with a hairbrush, and the sample was named as B3. After the impurities were filtered, the samples were coated with a brush again, and the labeled sample was named B4. The raw lacquer blank groups B3 and B4 were recorded as the sample group R2 and sent to a shaded dry room for drying observation.

Then, 0.1 g of Cu–ethanolamine was dropped into 1.0 g of raw lacquer for compounding. The composite reaction was mainly manifested by the accelerated oxidation rate. After full compounding, the color of the raw lacquer became black. The particle impurities increased, the viscosity increased and tended to solidify, and the transparency decreased. Some of the composites were coated on the glass plate with a hairbrush, and the sample was recorded as B5. After the impurities were filtered, the samples were coated with a brush again, and the labeled sample was named B6. The raw lacquer blank groups B5 and B6 were recorded as the sample group R3 and sent to a shaded dry room for drying observation.

An amount of 0.2 g of Cu–ethanolamine was dropped into 1.0 g of raw lacquer for compounding. The composite reaction was mainly manifested by the instantaneous oxidation of the raw lacquer color into black and the decrease in transparency after full compounding. The viscosity increased, the curing reaction occurred, the thickness of the paint layer increased compared with the first three groups, and there were still particle impurities. Some of the composites were coated on the glass plate with a hairbrush, and the sample was named B7. After the impurities were filtered, the samples were coated with a brush again, and the labeled sample was named B8. The raw lacquer blank groups B7 and B8 were recorded as the sample group R4 and sent to a shaded dry room for drying observation.

Last, 0.5 g of Cu–ethanolamine was dropped into 0.5 g of raw lacquer for compounding, and the composite was agglomerated and the paint film was destroyed.

The Nuclear Magnetic Resonance and Fourier Transform Infrared Spectroscopy data before and after sample filtration are shown in Figures S1 and S2. It can be seen that there is no significant difference in NMR and FTIR data before and after sample filtration. Unfortunately, due to the complex composition of the raw lacquer sample, we did not obtain a single sample signal with sufficient high resolution. Due to the preparation process of raw lacquer and laccase, we cannot obtain high-purity and single-component raw lacquer and laccase in the actual production process. Based on the complexity of the composition of lacquer, further structural analysis will be conducted in future work after improvement of filtration and purification technology.

3.4. Drying Status Comparison of Cu–Ethanolamine Composite Coatings at Different Times

For the R1 sample group, the viscosity of the paint film decreased during about 8 h of drying, but the surface of the paint film still had slight viscosity, as shown in Figure 5. For the R2 sample group, during the approximately 8 h drying process, the adhesion of the paint film decreased, and the film achieved full drying at around the eighth hour. For the R3 sample group, during the approximately 8 h drying process, the viscosity of the paint film decreased. By the end of the second hour, the paint film began to approach surface dryness, and by the eighth hour, the paint film was completely dry. For the R4 sample group, the drying process lasted for about 8 h. The viscosity of the paint film decreased over time, and the paint film began to approach surface dryness at around the end of the second hour. The paint film was fully dry at the end of the eighth hour.

Through the drying test of the combination of Cu–ethanolamine nanozymes and raw lacquer, it can be concluded that the copper chloride particles are directly combined with ethanolamine and raw paint under the optimal drying environment of raw paint (T = 25 °C; R = 80% relative humidity) [50] has a good drying effect in a short time. And the effect of the composite paint film of Cu–ethanolamine nanozymes and raw lacquer in different proportions is also different. During the 8 h drying observation process, it was found that the composite coating generated by compounding the Cu–ethanolamine nanozymes with the lacquer in a ratio of 1:30 exhibited poor drying performance in the eighth hour. Additionally, there was no significant change in the drying condition observed hourly. However, both sample 1:20 and sample 1:10 reached the requirements of GB/T1728-2020 [51] for surface drying and solid drying of paint film at the eighth hour and had a good drying effect. In addition to temperature and humidity, PH, storage time, and type of solvent all affect the activity of the nanozymes and, thus, the drying time [52]. In following work, we will further explore the influence of these factors on Cu–ethanolamine nanozymes, in order to guide the actual production of paint.

The drying rate of sample 1:5 is the fastest and the drying reaction is the most obvious. In the third hour, the surface drying effect under the requirements of the finger touch method was achieved. Therefore, it is concluded that different contents of desiccant have an effect on the drying effect. By comprehensive comparison, the drying characteristics of the sample with 1:20 ratio are found to be closest to those of the raw lacquer, and the reaction results of the drying process performance test on the sample with a 1:20 ratio indicate that the composite lacquer can achieve the effect and performance requirements of the paint process. Because its dry oxidation color is black, it is suitable for the surface treatment of primer or melanin painting technology, which can effectively save costs, shorten the drying time of raw paint, and reduce the time and cost of the production cycle of lacquer works.

In addition, we also constructed an iron-active center of Fe–ethanolamine nanozymes according to the structure of the Cu–ethanolamine nanozymes [53] and calculated the adsorption energy of the three substances on urushiol and urushiol quinone. The structure is shown in Figure S3. According to the calculation results, it can be seen that Cu–ethanolamine nanozymes have stronger adsorption energy for urushiol and urushiol quinone than that of Cu amino acids and Fe–ethanolamine (Table S1), which may have higher urushiol oxidation ability and faster drying times.

We further tested the catalytic activity of Cu–ethanolamine nanozymes on ABTs, as shown in Figure S4. It can be seen that there is no significant change in color before and after adding ABTs. Therefore, we speculate that Cu–ethanolamine nanozymes do not exhibit significant catalytic activity towards ABTs, thus proving its good selectivity towards urushiol. Due to the complexity of the composition of lacquer, further NMR and FTIR analysis will be conducted in future work.

3.5. Application of Cu–Ethanolamine Nanozymes in Practical Imaging Effects

During the drying process of the paint, due to the initial increase in the drying rate of the substrate, the overall progress in sample production has been improved. It is only when the paint composite has been drawn that the raw paint meets the drying standard. Compared with the drying rate of raw paint, the paint composite greatly reduces the production time of raw paint products and saves production costs, as shown in Figure 6.

4. Conclusions

In this work, we referred to the active center structure of laccase and designed a Cu–ethanolamine nanozyme structure with higher activity. The theoretical calculation results indicate that Cu–ethanolamine nanozymes have higher D-band active electrons compared to laccase, promoting the occurrence of urushiol oxidation kinetics. Further experiments have shown that in different mixing ratios, 1:20 has better drying and film forming performance. The combination of theoretical calculation and experimental research of the above systems will greatly shorten the time required for the drying of raw lacquer, effectively reducing the cost of surface coating with raw lacquer. Due to the preparation process of raw lacquer and laccase, we cannot obtain high-purity and single-component raw lacquer and laccase in the actual production process. We believe that with scientific and systematic research on traditional Chinese lacquer technology, more and more related studies can provide clearer comparative experiments, making traditional lacquer art more instructive.