CDs microcapsules polyurethane film with improved light environment, self-healing and high strength for plant greenhouses to promote plant growth

2.1 Morphology and chemical composition of CDs and CDs microcapsules

Firstly, the morphology and chemical composition of the CDs and CDs microcapsules were investigated. Fig. 1a shows a high angle annular dark field scanning transmission electron microscope image of CDs, which shows good dispersion of CDs, and the inset of Fig. 1a shows the particle size distribution of the CDs, which are mostly concentrated at 5 nm. Transmission electron micrographs of the CDs show them to be spherical (Fig. 1b). The inset of Fig. 1b shows a high-resolution transmission electron micrograph of the CDs, and the lattice spacing of the CDs was calculated to be 0.23 nm, corresponding to the (1 0 0) crystal plane of the graphene carbon atoms. (Jeong et al., 2023) Fig. 1c shows an SEM image of a CDs microcapsule showing a core–shell structure with a much larger particle size compared to CDs, which according to the inset of Fig. 1c is about 90 um. Fig. 1d shows photoelectron diffractograms (XRD) of CDs and CDs microcapsules, both with diffraction peaks appearing at 18°, corresponding to the (1 0 0) crystallographic plane of the graphene carbon atom, (Xie et al., 2018) indicating that the non-qualitative structure of the CDs was not altered by encapsulation with PVA. The surface chemistry of CDs and CDs microcapsules was explored using FT-IR spectroscopy, as shown in Fig. 1e, both CDs and CDs microcapsules have absorption peaks at 3430 cm⁻¹, 2890 cm⁻¹, 1705 cm⁻¹ and 1610 cm⁻¹ corresponding to the functional groups C-OH, (Dai et al., 2022) C–H, (Li et al., 2022) C = O (Guo et al., 2022) and C = C (Zhang et al., 2019) respectively. The CDs showed an asymmetric stretching vibrational absorption peak of C-NH-C at 1080 cm⁻¹; (Xu et al., 2022; Wang et al., 2021) while no vibrational absorption peak of C-NH-C was detected in the CDs microcapsules. The surface elemental and functional group composition of the CDs and CDs microcapsules were further analysed by X-ray photoelectron spectroscopy (XPS) and the XPS spectra showed (Fig. 1f) that the CDs had a distinct O signal at 531 eV, an N signal at 400 eV and a C signal at 284 eV. (Zhu et al., 2021) In contrast, the CDs microcapsules only show an O signal at 531 eV and a C signal at 284 eV. Fig. S1a-c show the high resolution energy spectra of the CDs and Fig. S2a-b show the high resolution energy spectra of the CDs microcapsules. (Park et al., 2021) the XPS and FT-IR results are consistent due to the fact that the CDs are heavily diluted after preparation into CDs microcapsules, resulting in almost no detection of the N element, and the inability of the FT-IR and XPS to detect the internal elements as the CDs are encapsulated by PVA. In summary, it was demonstrated that the PVA did not react chemically with the CDs and effectively encapsulated them internally and was not an ordinary physical blending.

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Fig. 1

(a) HAADF-STEM of CDs [the illustration is shown the particle size of CDs]; (b) TEM image of CDs [the illustration is shown the HRTEM of CDs]; (c) SEM of CDs microcapsule [the illustration is shown the particle size of CDs microcapsules]; (d) XRD spectra of CDs and CDs microcapsules; (e) FT-IR spectrum of CDs and CDs microcapsules; (f) XPS spectrum of CDs and CDs microcapsules.

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2.2 Slow release behaviour and adhesion properties of CDs microcapsules

Electron leap in the UV is the reason why CDs can fluoresce. (Liang et al., 2022) The UV spectrophotometer was used to probe the electron leap of CDs. Fig. S3a shows the UV absorption spectra of CDs. The UV absorption peak of CDs is located around 350 nm and corresponds to the n → π* electron jump of the oxygen-containing functional group. (Feng et al., 2017) Fig. S3b shows the fluorescence spectra of the CDs solution under the excitation of different wavelengths, and its emission peak is located at 460 nm, which indicates that the CDs can absorb light from 360 to 430 nm wavelengths, and it can convert UV light into blue light with good light conversion effect. Fig. 2a shows the fluorescence spectra of CDs and CDs microcapsules under wavelength excitation at 365 nm, both of which show very similar fluorescence emission behaviour, with their emission peaks located at 460 nm. The inset of Fig. 2a shows that the CDs solution and the CDs microcapsules can emit similar intense blue light under UV irradiation, suggesting that the fluorescence emission centre of the CDs microcapsules originates from the CDs. In order to further prove that the fluorescence of the CDs microcapsules comes from the CDs, the fluorescence lifetimes of the two microcapsules were tested. Fig. 2b shows the fluorescence decay curves of the CDs with a fluorescence lifetime of 0.46 ns, and Fig. 2d shows the fluorescence decay curves of the CDs microcapsules with a fluorescence lifetime of 0.10 ns. The fluorescence decay curves of the two microcapsules are similar, and both of them show the double-exponential function, which proves that the fluorescence of the two microcapsules is from the same. The two fluorescence decay curves are similar and both show double exponential function, which proves that the fluorescence of them come from the same luminous centre. The quantum yield of CDs is 14.7 %. The fluorescence intensity of CDs was explored at different concentrations, and as shown in Fig. S3d, the fluorescence intensity reached its strongest at a concentration of 0.05 mg/mL. However, since the CDs solution concentration will be diluted by other reagents during the preparation of CDs microcapsules, the optimal fluorescence intensity for the preparation of CDs microcapsules corresponds to CDs concentration of 0.5 mg/mL (Fig. S3e). The above results demonstrated that the CDs microcapsules can overcome the fluorescence quenching and have good light conversion performance in the solid state, which is the basis for their application in the regulation of plant light environment.

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Fig. 2

(a) The PL of CDs and CDs microcapsules at the excitation wavelength of 365 nm [the illustration is CDs and CDs microcapsules under UV light]; (b) The fluorescence decay curve of CDs and CDs microcapsules; (c) Fluorescence intensity of CDs and CDs microcapsule over time; (d) TG of CDs and CDs microcapsules [the illustration is shown the SEM of CDs microcapsules]; (e) Leaf surface under microscope: (1) Sprayed with CDs microcapsules; (2) The first simulated rain wash; (3) The second simulated rain wash; (4) The third simulated rain wash.

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CDs microcapsules can be used on plant foliage as light-converting materials to promote plant growth, and must also have good slow-release and adhesion properties, so the slow-release and adhesion properties of CDs microcapsules were verified. Fig. 2c shows the fluorescence intensity changes of CDs and CDs microcapsules over 7 days. The fluorescence intensity of CDs fluctuates slightly in the first five days, but due to the fluorescence quenching and photobleaching properties of CDs, its fluorescence intensity decreases greatly on the sixth day, and its fluorescence intensity decreases by 76.86 % compared with that on the fifth day. The PVA-coated CDs microcapsules could not only overcome the solid-state fluorescence quenching of CDs, but also had a good slow-release effect, and the fluorescence intensity on the seventh day decreased by only 25.87 % compared with the first day, and still showed fluorescence performance even after 15 days, which was conducive to long-term photoconversion and proved that the CDs microcapsules had a good slow-release performance. The slow-release performance of CDs microcapsules is closely related to their surface pores. The inset of Fig. 2d shows the SEM image of a single CDs microcapsule, from which it can be seen that the surface of the CDs microcapsule has a lot of pores with different sizes, and these pores provide excellent conditions for the slow release of CDs. Fig. 2d shows the thermal weight loss analysis of CDs and CDs microcapsules. CDs started to degrade substantially at 250 °C, whereas the temperature reached 400 °C before substantial degradation after using PVA-coated CDs, which indicates the enhanced thermal stability of CDs microcapsules. All these results indicate that the CDs microcapsules have good slow release properties.

To verify the adhesion of CDs microcapsules, CDs and CDs microcapsules were sprayed on plant foliage after simulating rainwater rinsing, and they were placed under a microscope for observation. Fig. 2e shows pictures of foliage after spraying CDs microcapsules under microscope, and its inset shows pictures of the same area under UV irradiation using a microscope (1, 2, 3, and 4 are freshly sprayed with carbon spot microcapsules, simulated rain wash for the first time, simulated rain wash for the second time, and simulated rain wash for the third time, respectively). When the CDs microcapsules were sprayed onto the leaves, a large number of CDs microcapsules were evenly dispersed and adhered to the leaf surface. After the first simulated rain washing, there were still a large number of CDs microcapsules adhering to the leaf surface; when simulated rain washing for the second time, some CDs microcapsules could still be seen adhering to the leaf surface; and when simulated rain washing was performed for the third time, there were almost no CDs microcapsules left on the leaf surface. Fig. S4a shows the picture of the leaf surface after spraying CDs under the microscope, when CDs were just sprayed onto the leaf surface, the stems along the leaf surface showed the phenomenon of agglomeration, and the CDs could be washed off after only simulating the rainwater rinsing once, which indicated that the adhesion of the CDs were greatly enhanced after the CDs were prepared into CDs microcapsules, which was attributed to the presence of a large number of –OH on the surface of PVA, (Aslam et al., 2018) which could enhance the surface bonding of the CDs microcapsules in the leaf surface of the plant.

Previously, the low toxicity of CDs has been reported in several literatures, and we verified its biocompatibility. CDs microcapsules were configured into solutions of different concentrations, and oocysts were selected as experimental objects to observe the survival quantity of oocysts under different concentrations of CDs microcapsules solutions. At high concentrations, the survival rate of oocysts was still greater than 80 % (Fig. S5), indicating low toxicity of CDs microcapsules.

Combined with the above data and analyses, the CDs microcapsules have good slow-release properties and adhesion, and have the potential to promote plant growth. Fig. 2f shows the schematic diagram of CDs microcapsules for promoting plant growth, spraying CDs microcapsules on the plant leaf surface, due to the good adhesion of CDs microcapsules, it can be well adhered to the plant leaf surface, the CDs microcapsules absorb UV rays from sunlight and convert them into blue fluorescence, improving the light environment of the plant growth, and achieving the purpose of promoting the growth of plants.

2.3 Preparation and characterization of CDs microcapsules polyurethane film

Polyurethanes prepared by integrating physical cross-linking due to high-density hydrogen bonding in isophthalic dihydrate and dynamic covalent cross-linking by D-A bonding into the same polymer structure. And the synergistic enhancement effect produced by the introduced dual-cross-linking network made the macromolecule a self-healing and tough copolymerised polyurethane, and finally CMPF was prepared using polyurethane-coated CDs microcapsules.

Fourier Infrared spectroscopy as well as Raman tests were first carried out to prove the successful preparation of the polyurethane. Fig. 3a shows the infrared spectra, the stretching vibration peak of –NH- at 3323 cm⁻¹ and the out-of-plane bending vibration peak at 1543 cm⁻¹, and the stretching vibration peak of –CN- at 1333 cm⁻¹, and the stretching vibration peak of –CN- at 1660 cm⁻¹ attributed to the stretching vibration peak of C = O in the urethane ester, which is attributed to the influence of amino group and the formation of hydrogen bond between hydroxyl and carbonyl, which increases the polarization strength of the chemical bond, and shifts the original absorption band of C = O group to the low frequency region. O group shifted to the low-frequency region due to the influence of the amino group and the formation of hydrogen bond between the hydroxyl group and the carbonyl group, which increased the polarization strength of the chemical bond and shifted the original absorption band of the C = O group to the low-frequency region, which indicated that the –NCO group in the raw material, isophorone diisocyanate (IPDI), had reacted with –OH completely to form the urethane group (–NH(CO)O-). In addition, the stretching vibration peaks of –NH- all appeared near 3323 cm⁻¹ instead of 3440 cm⁻¹, indicating that the –NH- groups are almost always conjugated with the carbonyl groups in the polymer to form hydrogen bonds, forming the microcrystalline regions of the hard segments. The characteristic peak of -C = C- at 1510 cm⁻¹ indicated that the cross-linker 4,4-diphenylmethane bismaleamide (BMI) was successfully introduced into the prepolymer capped by aromatic isophthalic acid dihydrazide/2,5-furandimethanol (ID/BHMF). In addition, there was a small fluctuation in the infrared spectrum at 1760 cm⁻¹ after curing of the material and a weak vibration peak of the furan group of the raw material at 775 cm⁻¹, which indicated that the furan ring group was consumed, and the bismaleimide and the linear urethane containing the furan ring generated a D-A bond through a cycloaddition reaction, and the self-repairing urethane with a bis-crosslinked network structure was successfully prepared. To further demonstrate the successful generation of D-A bonds, tests were carried out using Raman spectroscopy, and the characteristic absorption peak of the D-A bond located at 1760 cm⁻¹ can be seen in the Raman spectrum in Fig. 3b. The XRD graph of the polyurethane film was tested (Fig. S6) and a clear diffraction peak located at 20° was seen, proving that the material has an amorphous structure.

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Fig. 3

(a) FT-IR of polyurethane; (b) Raman of polyurethane; (c) Plot of typical stress–strain curves of polyurethane and polyurethane after self-healing; (d) Visible light transmittance of polyurethane.

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The mechanical properties of the polyurethane film were tested. The stress–strain curves are shown in Fig. 3d, and there was no yielding, indicating that the synthesised self-repairing polyurethane material is a typical elastomer. With a tensile strength of up to 72.5 MPa and an elongation at break of 1241.0 %, the final toughness of 235.712 MJ/m³ was calculated. When deformation occurs during stretching, the hydrogen bond is sacrificed to dissipate energy before the covalent bond (D-A bond) breaks, and the broken –NH- bond also has the opportunity to recombine with other broken –NH- bonds in the neighbouring chain and form a weak temporary hydrogen bond, which increases the intermolecular chain force, and at the same time, the presence of the D-A covalent cross-linking increases the stability of the polymer network, which ensures the intermolecular force, and in addition the effective recombination of the intermolecular hydrogen bonding further improves the material's toughness, which indicates that the material achieves a unity of the mechanical properties and elasticity. Typical stress–strain tests were carried out on the samples after the self-repair treatment (Fig. 3c), and the tensile strength was as high as 60.7 MPa, and the elongation at break was 1064.6 %, which was calculated to be 85 % for the stress repair rate and 86 % for the strain repair rate. These results demonstrate that the polyurethane films continue to exhibit excellent mechanical properties even after self-repair. Finally, the visible light transmittance was tested and showed excellent transparency with a transmittance of 89.5 %. These results show that polyurethane films have great potential for use as greenhouse films for agricultural applications.

2.4 CDs microcapsules polyurethane film for greenhouse film to promote vegetable growth

CMPF can be used as greenhouse film for vegetable cultivation to promote the growth of vegetables, and the mechanism diagram is shown in Fig. 4a. When ultraviolet light is irradiated on the greenhouse film, CDs microencapsulated on polyurethane loaded on the polyurethane will convert the ultraviolet light into blue light, which can promote the growth of plants. The effect of CDs microencapsulated polyurethane films prepared by adding different CDs microcapsules to polyurethane as greenhouse films on lettuce growing was investigated. Four groups of polyurethane films with different contents of CDs microencapsulated polyurethane films (0 mg/L, 2000 mg/L, 4000 mg/L, 6000 mg/L) were used for growing lettuce in greenhouse films, and the post-growth lettuce was collected after 45 days and tested. As shown in Fig. 4b, lettuce grown with CMPF grew more vigorously than the blank group, and the effect was most pronounced for lettuce grown with greenhouse film prepared with CMPF at a concentration of 4000 mg/L. Fresh weights of lettuce from all four groups were weighed, and the lettuce grown with CMPF at a concentration of 4000 mg/L had a 177 % increase in fresh weight compared to that of the blank group (Fig. 4c), and the water content is shown in Fig. S7. The dry weights of four groups of lettuce were tested (Fig. 4d), and the experimental groups were generally heavier than the blank group, and the dry weight of lettuce grown with CMPF at a concentration of 4000 mg/L was 243.5 % of that of the blank group, which indicates that the accumulation of organic matter in lettuce was increased after the use of CDs microcapsules. Photosynthesis is essential for the accumulation of organic matter in plants. (Wang et al., 2017) Therefore, the content of photosynthetic pigments in lettuce was tested. Fig. 4c shows the pigment content of the lettuce with an increase in chlorophyll a, chlorophyll b and carotenoids. Lettuce with 4000 mg/L CMPF showed a 14.5 % increase in chlorophyll a, 188.5 % increase in chlorophyll b, and 43.3 % increase in carotenoids compared to the blank group. In the growth of lettuce, the main role of chlorophyll b is to absorb blue light. (Li et al., 2018) Blue light has an impact on chlorophyll synthesis, and light can participate in the regulation of chlorophyll synthesis. Microcapsules convert ultraviolet light into blue light, regulate the growth environment of lettuce, and promote the synthesis of chlorophyll. Secondly, the absorption peak of chlorophyll a is mainly 430 nm and 660 nm, and the absorption peak of chlorophyll b is mainly 456.9 nm and 643.8 nm. (Liu and Iersel, 2021) The fluorescence emission peak of the capsule is located at 460 nm, which is very close to the absorption peak of chlorophyll b, which is the reason why the content of chlorophyll b is greatly increased. The increase in blue light leads to the production of more chlorophyll b in the body of the lettuce, which accelerates photosynthesis and achieves an increase in the accumulation of organic matter.

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Fig. 4

(a) The diagram of CDs microcapsules polyurethane film as greenhouse film to promote lettuce growth; Effect of CMPF prepared using microcapsules with different levels of CDs on the growth of lettuce: (b) Length and morphology; (c) Fresh weigh; (d) Dry weigh; (e) Pigment Content; (f) Protein content.

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Finally, four groups of lettuce were tested for soluble protein content (Fig. 4f). The soluble protein of lettuce also increased after the application of CMPF, and the soluble protein content of lettuce at 4000 mg/L increased by 17.9 % compared with the blank group. All these results indicated that the CMPF had a significant effect in promoting the growth of lettuce, and the effect was optimal at the concentration of 4000 mg/L.