Synthesis of ZIF-8. By mixing Zn(NO3)2 methanol solution and an excess of 2-methylimidazole methanol solution, white ZIF-8 crystals were obtained with a yield of about ~34%. 2-Methylimidazoles, an organic bridging ligands, are partially ionized in methanol solution to form 2-methylimidazolate ions, which combine with zinc ions to form a three-dimensional network structure, forming a porous material similar to zeolite.21
SEM image of ZIF-8 on the wood specimen. Fig. 1 shows SEM images of (a) ZIF-8 and (b) ZIF-8 on the wood surface. As shown in Fig. 1(a), the morphology of ZIF-8 analyzed by HR-SEM is a rhombic dodecahedral shape with sharp edges and smooth faces. In a dried solid, the individual particles with a size of about 200-400 nm are separated and do not stick to each other.
The morphology of ZIF-8 on wood (Fig. 1(b)) is somewhat different from that of ZIF-8, and shows an almost spherical shape and reduced sizes. Moreover, ZIF-8 particles are agglomerated to form large clusters, unlike ZIF-8 particles. When ZIF-8 is dispersed in water, 2-methylimidazolate ions bonded to Zn2+ ions react with water and are converted to 2-methylimidazole, released into an aqueous solution, and part of the framework collapses. ZIF-8 becomes a positively charged particle and its counter ion is OH-. At this time, the liberated 2-methylimidazole is absorbed into the wood specimen. During the drying process, these particles agglomerate with each other to form lumps.19,20,25
Amount of treatment material remaining on the wood specimen. Since the concentration of the treatment chemicals used in this study was very dilute, the mass change before and after treatment was very small, making it difficult to measure in the range of 0.001 g.
Termite test results of wood specimens treated with 2-methylimidazole. Fig. 2 shows the results of termite experiments using 2-MeIm-treated wood specimens.
The mortality data measured after breeding for 28 days are shown in Table 1, and the wood consumption data per gram of termites are shown in Table 2.
Table 1. Mortality data of bred termites in lumber immersed in 2-MeIm aqueous solution and ZIF-8 aqueous dispersion.
Table 2. Wood consumption data by bred termites from lumber immersed in 2-MeIm aqueous solution and ZIF-8 aqueous dispersion.
Mortality according to concentration change of 2-MeIm. In the case of untreated wood specimens, a mortality rate of 16.0% was observed as the average value of the four conditions. In the case of wood treated with 2-MeIm, a slightly higher mortality rate (29.7%) was observed in the no-choice test when treated with 0.01% aqueous solution compared to the control. In the case of treatment with an aqueous solution of 0.05% or more, all termites died before 28 days in all three conditions, and a mortality rate of 100% was observed. In the absence of food, it was observed that all termites died after breeding them for 11 days. The average survival period of termites was 25 days in lumber treated with 0.05% aqueous solution, 22 days in lumber treated with 0.1% aqueous solution, and 10 days in lumber treated with 0.3% aqueous solution, and all termites died of starvation within 28 days. This is because termites recognize 2-MeIm and avoid eating wood specimens treated with 2-MeIm aqueous solution. In the choice test in which untreated and treated wood were added together, an average mortality rate of 17.8% was observed in the four conditions, which is very similar to the mortality rate (16.0%) measured in the control. It is believed that these measurement results were obtained because untreated food was provided. Therefore, it is believed that 2-MeIm is not toxic to termites, but that termites recognize 2-MeIm and avoid it.
Wood consumption according to concentration of 2-MeIm. In the case of untreated wood (control), a consumption of 1.101 g of wood per gram of termites was observed as an average value for the four conditions. For wood treated with 2-MeIm, wood specimens treated with 0.01% aqueous solution, and the no-choice test result showed a slightly lower wood consumption (0.971 g) than the control. This was the result of 70.3% of the termites surviving and consuming the wood until day 28 because the wood did not contain sufficient amounts of 2-MeIm. Wood consumption of 0.073 g was observed for wood specimens treated with 0.05% aqueous solution, 0.041 g for wood specimens treated with 0.1% aqueous solution, and 0.009 g for wood specimens treated with 0.3% aqueous solution. It can be seen that the higher the concentration of the treatment solution, the more 2-MeIm is contained in the wood, preventing the termites from consuming the wood.
The results of the choice tests clearly show the selectivity of termites to wood. In the case of wood specimens treated with 0.01% aqueous solution, both treated and untreated wood were consumed, and the sum was 1.125 g, which is very similar to that of the control (1.150 g). However, termites selectively consumed untreated wood (treated : untreated = 87.3% : 12.7%). In the case of wood specimens treated with more than 0.05% aqueous solution, the amount of wood consumed in all three conditions (1.082 g, 1.062 g, 1.081 g) showed values very similar to those of the control (1.150 g). The consumption rates of untreated wood were 98.7%, 98.0%, and 98.7%, respectively, and the termites selectively consumed only untreated wood. Again, 2-MeIm is not toxic to termites, the effect is the result of termites recognizing and avoiding 2-MeIm.
AWPA E1 rating values. Fig. 2 shows photographs of dried wood from the experiments with different concentrations of 2-MeIm. The AWPA E1 rating values of the no-choice test results were 6 (severe attack, 30-50% of cross sectional area affected) for wood treated with 0.01% aqueous solution and 9 (slight attack, up to 3% of cross sectional area affected) for wood treated with 0.05% aqueous solution, 9.5 (trace, surface nibbles permitted) for wood treated with 0.1% aqueous solution, and 10 (sound) for wood treated with 0.3% aqueous solution, respectively.
The phenomenon of covering wood with sand in the choice experiment. An unusual feature was observed in the choice test results of wood treated with 2-MeIm. In experiments using wood treated with aqueous solutions above a certain concentration, it was observed that termites covered the treated wood with sand and excrement over time. It seems that the termites, which initially moved randomly, recognized the wood treated with 2-MeIm, covered it with sand and excrement, and secreted repellent pheromones to prevent the access of other termites.
Termite test results of wood specimens treated with ZIF-8. Solid state ZIF-8 is known to be a very stable material. After synthesis in methanol media, it is stable enough to be separated by a small centrifugal force. The dried ZIF-8 has a regular shape as confirmed in the SEM image. Mixing ZIF-8 with water gives a very stable colloidal dispersion. This means that ZIF-8 particles present in water have a very large charge. ZIF-8 dispersed in water (containing natural amount of CO2) undergoes a slow hydrolysis reaction. In this process, 2-MeIm is released and ZIF-8 acquires a large positive charge. The released 2-MeIm is absorbed into the wood specimen and ZIF-8 is coated in a solid state on the surface of wood treated with the ZIF-8. Since 2-MeIm has a very high solubility in water (29 g/100 mL of water), 2-MeIm absorbed in wood may be rapidly eluted when it rains or in high humidity, so it is expected that the durability of the treatment effect will be low in those conditions. However, since ZIF-8 is a solid material, it is expected to exhibit a continuous termite repellent effect by releasing 2-MeIm through slow hydrolysis.26 Fig. 3 shows the results of termite experiments using ZIF-8 treated wood specimens.
Mortality according to concentration of ZIF-8. For the untreated wood specimens, a mortality rate of 14.3% was observed as the average value of the 3 conditions. In the case of wood treated with ZIF-8, in the no-choice test, all termites died before 28 days in the three conditions, and a mortality rate of 100% was observed. The average survival period of termites was 14 days in lumber treated with 0.1% aqueous dispersion, 16 days in lumber treated with 0.2% aqueous dispersion, and 11 days in lumber treated with 0.3% aqueous dispersion and all termites died of starvation within 28 days. This is because termites recognize and avoid 2-MeIm released from wood and 2-MeIm released from hydrolysis of solid ZIF-8 adsorbed on the surface, and die of starvation without eating. In the choice test, an average mortality rate of 15.0% was observed in the three conditions, which is very similar to the mortality rate (14.3%) measured in the control. It is believed that these measurement results were obtained because untreated food was provided.
Wood consumption according to concentration of ZIF-8. For the untreated wood (control), consumption of 1.194 g of wood per gram of termites was observed as an average value for the three conditions. For wood treated with ZIF-8, in the case of wood specimens treated with 0.01% aqueous dispersion, the no-choice test result showed a slightly lower wood consumption (0.971 g) than the control. In the case of wood treated with ZIF-8, the no-choice test showed wood consumption of 0.026 g for wood specimens treated with 0.1% aqueous dispersion, 0.014 g for wood specimens treated with 0.2% aqueous dispersion, and 0.018 g for wood specimens treated with 0.3% aqueous dispersion. It can be seen that wood was not consumed. The results of the choice tests clearly show the selectivity of termites to wood. The amount of wood consumed in all three conditions (1.203 g, 1.219 g, 1.241 g) showed values very similar to those of the control (1.194 g). The consumption rates of untreated wood were 99.3%, 98.8%, and 99.0%, respectively, and termites selectively consumed only untreated wood.
AWPA E1 rating values. Fig. 3 shows photographs of dried wood from the experiments with different concentrations of ZIF-8. The AWPA E1 rating values of the no-choice test results were 9.5 (trace, surface nibbles permitted) for wood treated with 0.1% aqueous dispersion, 10 (sound) for wood treated with 0.2% aqueous dispersion, and 10 (sound) for wood treated with 0.3% aqueous dispersion, respectively.
The phenomenon of covering wood with sand in the choice experiment. In the choice test with wood treated with ZIF-8, as in the result of 2-MeIm, it was observed that termites covered the treated wood with sand and excrement over time. As mentioned earlier, it appears that termites recognize wood treated with ZIF-8, cover it with sand and excrement, and secrete repellent pheromones to prevent access by other termites.
Termite test results on wood specimens sprayed with ZIF-8 dispersion. To investigate the applicability of ZIF-8 dispersion, wood and sand were directly treated with ZIF-8 dispersion. Fig. 4 shows the results of termite experiments using ZIF-8 treated wood specimens. The mortality and the wood consumption data are shown in Table 3.
Table 3. Mortality and wood consumption data of bred termites in wood sprayed with ZIF-8 aqueous dispersion on wood placed on sand in a container.
Mortality according to concentration of ZIF-8. For the untreated wood specimens, a mortality rate of 16.8% was observed as the average value. In the case of wood treated with ZIF-8, all termites died before 28 days in the three conditions, and a mortality rate of 100% was observed. The average survival period of termites was 25 days in lumber treated with 0.1% aqueous dispersion, 22 days in lumber treated with 0.2% aqueous dispersion, and 10 days in lumber treated with 0.3% aqueous dispersion and all termites died of starvation within 28 days. A consumption of 1.233 g of wood per gram of termites was observed for untreated wood.
For wood treated with ZIF-8, a wood consumption of 0.108 g was observed for wood treated with 0.1% dispersion, 0.034 g for wood treated with 0.2% dispersion, and 0.023 g for wood treated with 0.3% dispersion. It can be seen that by releasing sufficient amounts of 2-MeIm, the termites consumed very little wood.
Based on these results, spraying wood with a ZIF-8 dispersion of 0.3% or more can prevent termite access. The AWPA E1 rating values were 8 (moderate attack, 3-10% of cross sectional area affected) for wood treated with 0.1% aqueous dispersion, 9.5 (trace, surface nibbles permitted) for wood treated with 0.2% aqueous dispersion, and 10 (sound) for wood treated with 0.3% aqueous dispersion, respectively.