Constructing ultra-stable photothermal plastics assisted by carbon dots with photocaged reactivity

Abstract

•Photothermal P-CDs were achieved•P-CDs were employed for fabricating high-performance photothermal plastics•The photothermal plastics were applicable for solar-thermal-electricity conversion Photothermal materials, especially photothermal plastics, are crucial building blocks in energy science, agriculture, and bio-medical applications. Covalently immobilizing the photothermal carbon dots in plastic matrix is a promising method for producing efficient photothermal plastics. However, the reactive moieties of photothermal carbon dots are often destroyed because of harsh preparation conditions, preventing their covalent interaction with plastic matrix. This work addresses the challenge by developing carbon dots (P-CDs) with photocaged reactivity. The P-CDs maintained stable moieties in the harsh preparation process, while reactive moieties, which can form covalent bond with plastic matrix, were in situ generated upon light irradiation. In such a manner, efficient and robust photothermal P-CD plastics were obtained. More interestingly, P-CD plastics were successfully employed for constructing a solar-driven thermoelectric generator (TEG) for energy generation. Photothermal materials, especially photothermal plastics, are crucial building blocks for functional devices. Covalently immobilizing the photothermal carbon dots in plastic matrix is a promising method for producing efficient photothermal plastics. However, the reactive moieties of photothermal carbon dots are often destroyed because of harsh preparation conditions, preventing their covalent interaction with plastic matrix. Here, we conceptualized carbon dots with photocaged reactivity (P-CDs) for producing ultra-stable photothermal plastics. During the formation of P-CDs, hydroxyl moieties were maintained in the preparation environment. Upon UV irradiation, hydroxyl moieties of P-CDs were in situ converted into aldehyde groups and reacted with amino groups in the polysaccharide matrix, producing P-CD plastics. As-obtained P-CD plastics showed strong stability against solution immersion and UV aging. In particular, the P-CD plastics showed a high photothermal conversion efficiency of 46.6%. Such efficient and robust photothermal P-CD plastics were further applied to prepare a solar-driven thermoelectric generator (TEG) for energy generation. Photothermal materials, especially photothermal plastics, are crucial building blocks for functional devices. Covalently immobilizing the photothermal carbon dots in plastic matrix is a promising method for producing efficient photothermal plastics. However, the reactive moieties of photothermal carbon dots are often destroyed because of harsh preparation conditions, preventing their covalent interaction with plastic matrix. Here, we conceptualized carbon dots with photocaged reactivity (P-CDs) for producing ultra-stable photothermal plastics. During the formation of P-CDs, hydroxyl moieties were maintained in the preparation environment. Upon UV irradiation, hydroxyl moieties of P-CDs were in situ converted into aldehyde groups and reacted with amino groups in the polysaccharide matrix, producing P-CD plastics. As-obtained P-CD plastics showed strong stability against solution immersion and UV aging. In particular, the P-CD plastics showed a high photothermal conversion efficiency of 46.6%. Such efficient and robust photothermal P-CD plastics were further applied to prepare a solar-driven thermoelectric generator (TEG) for energy generation. IntroductionPhotothermal plastics, i.e., plastics that convert photons into thermal energy, show huge potential for self-healing materials, smart devices, and actuators.1Huang J. Kaner R.B. Flash welding of conducting polymer nanofibres.Nat. 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Mater. 2017; 27: 1701138https://doi.org/10.1002/adfm.201701138Crossref Scopus (86) Google Scholar Photothermal plastics are typically prepared by physically mixing photothermal reagents, such as organic dyes and nanoparticles, in a plastic matrix.3Launay V. Caron A. Noirbent G. Gigmes D. Dumur F. Lalevée J. NIR organic dyes as innovative tools for reprocessing/recycling of plastics: benefits of the photothermal activation in the near-infrared range.Adv. Funct. Mater. 2021; 31: 2006324https://doi.org/10.1002/adfm.202006324Crossref Scopus (27) Google Scholar,5Han M. Kim B. Lim H. Jang H. Kim E. Transparent photothermal heaters from a soluble NIR-absorbing diimmonium salt.Adv. Mater. 2020; 32: 1905096https://doi.org/10.1002/adma.201905096Crossref PubMed Scopus (22) Google Scholar Although substantial progress has been achieved in this field, several challenges remain. As most photothermal plastics are produced by physical mixing, they suffer from the problem of the leakage of photothermal reagents from the matrix. In addition, organic photothermal plastics are generally prone to photobleaching. These challenges hinder their practical applications.Carbon dots (CDs) are optical carbon nanomaterials that have sizes below 10 nm.6Yuan F. Yuan T. Sui L. Wang Z. Xi Z. Li Y. Li X. Fan L. Tan Z. Chen A. et al.Engineering triangular carbon quantum dots with unprecedented narrow bandwidth emission for multicolored LEDs.Nat. Commun. 2018; 9: 2249-2259https://doi.org/10.1038/s41467-018-04635-5Crossref PubMed Scopus (485) Google Scholar,7Zhao B. Tan Z. Fluorescent carbon dots: fantastic electroluminescent materials for light-emitting diodes.Adv. Sci. 2021; 8: 2001977https://doi.org/10.1002/advs.202001977Crossref Scopus (62) Google Scholar CDs were first discovered serendipitously in 2004 as an impurity in the synthesis of single-walled carbon nanotubes (CNTs) and were purified by preparative electrophoresis.8Xu X. Ray R. Gu Y. Ploehn H.J. Gearheart L. Raker K. Scrivens W.A. Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments.J. Am. Chem. Soc. 2004; 126: 12736-12737https://doi.org/10.1021/ja040082hCrossref PubMed Scopus (2874) Google Scholar In 2006, Sun et al. reported the preparation of luminescent CDs using laser ablation and surface passivation.9Sun Y.P. Zhou B. Lin Y. Wang W. Fernando K.A.S. Pathak P. Meziani M.J. Harruff B.A. Wang X. Wang H. et al.Quantum-sized carbon dots for bright and colorful photoluminescence.J. Am. Chem. Soc. 2006; 128: 7756-7757https://doi.org/10.1021/ja062677dCrossref PubMed Scopus (3513) Google Scholar Since then, rapidly increasing numbers of studies on CDs have been carried out, as CDs can be easily prepared from readily available substances using simple synthetic routes and they have excellent optical properties and biocompatibility.10Hu C. Li M. Qiu J. Sun Y.P. Design and fabrication of carbon dots for energy conversion and storage.Chem. Soc. Rev. 2019; 48: 2315-2337https://doi.org/10.1039/c8cs00750kCrossref PubMed Scopus (385) Google Scholar, 11Jin L. Li J. Liu L. Wang Z. Zhang X. Facile synthesis of carbon dots with superior sensing ability.Appl. Nanosci. 2018; 8: 1189-1196https://doi.org/10.1007/s13204-018-0755-3Crossref Scopus (21) Google Scholar, 12Wang B. Lu S. The light of carbon dots: from mechanism to applications.Matter. 2022; 5: 110-149https://doi.org/10.1016/j.matt.2021.10.016Abstract Full Text Full Text PDF Scopus (85) Google Scholar, 13Zhang X. Jiang M. Niu N. Chen Z. Li S. Liu S. Li J. Natural-product derived carbon dots: from natural products to functional materials.ChemSusChem. 2018; 11: 11-24https://doi.org/10.1002/cssc.201701847Crossref PubMed Scopus (211) Google Scholar Moreover, CDs also showed stable and efficient photothermal conversion properties.14Li D. Han D. Liu L. Jing P.T. Zhou D. Ji W.Y. Wang X.Y. Zhang T.F. Shen D.Z. Supra-(carbon nanodots) with a strong visible to near-infrared absorption band and efficient photothermal conversion.Light Sci. Appl. 2016; 5: e16120-e16127https://doi.org/10.1038/lsa.2016.120Crossref PubMed Scopus (200) Google Scholar,15Su Y. Chang Q. Xue C. Yang J. Hu S. Solar-irradiated carbon dots as high-density hot spots in sponge for high-efficiency cleanup of viscous crude oil spill.J. Mater. Chem. A. 2022; 10: 585-592https://doi.org/10.1039/d1ta08670gCrossref Scopus (7) Google Scholar Ideally, covalently immobilized CDs in plastic matrix would be an effective method for fabricating ultra-stable, highly efficient, and robust photothermal plastics, addressing the above-mentioned challenges. Nevertheless, realization of the goal was hindered by the lower reactivity of photothermal CDs. This is because most of photothermal CDs were prepared under conditions with high temperature and high pressure,13Zhang X. Jiang M. Niu N. Chen Z. Li S. Liu S. Li J. Natural-product derived carbon dots: from natural products to functional materials.ChemSusChem. 2018; 11: 11-24https://doi.org/10.1002/cssc.201701847Crossref PubMed Scopus (211) Google Scholar where reactive moieties would be easily destroyed. To this end, attention was paid to photocages. Photocages are general tools in biological chemistry. The activity of functional molecules can be “caged” by photosensitive molecules (photocages).16Griffin D.R. Kasko A.M. Photodegradable macromers and hydrogels for live cell encapsulation and release.J. Am. Chem. Soc. 2012; 134: 13103-13107https://doi.org/10.1021/ja309004cCrossref PubMed Scopus (16) Google Scholar,17Kaplan J.H. Forbush B. Hoffman J.F. Rapid photolytic release of adenosine 5′-triphosphate from a protected analog: utilization by the sodium: potassium pump of human red blood cell ghosts.Biochem. 1978; 17: 1929-1935https://doi.org/10.1021/bi00603a020Crossref PubMed Scopus (624) Google Scholar Under UV irradiation, the photocage releases the target substance and activates its function. Here, we used vanillin to prepare 4-(4-(hydroxymethyl)-2-methoxy-5-nitrophenoxy) butanoic acid and further produce carbon dots with photocaged reactivity (P-CDs). As-prepared P-CDs showed photocaged reactivity (Figure 1A ). Specifically, P-CDs were stable with surface hydroxyl moieties that can be transformed into reactive aldehyde moieties upon UV irradiation. Ultra-stable robust P-CD plastics were prepared by irradiating a mixture of P-CDs and amino-functionalized hyaluronic acid (HA-CDH) with UV light, which triggered an “aldehyde-amine” crosslinking reaction between P-CDs and the matrix (Figure 1B). The obtained P-CD plastics were extremely robust, and no leakage of P-CDs was observed when the plastics were immersed in aqueous solution. The P-CD plastics demonstrate high photothermal efficiency, which remained stable after long-time storage, UV aging, and treatments in solutions. As a demonstration of potential applications, as-developed photothermal P-CD plastics were successfully used to construct a solar-driven TEG (Figure 1C).Results and discussionCharacterization of P-CDsThe precursor molecule, 4-(4-(hydroxymethyl)-2-methoxy-5-nitrophenoxy) butanoic acid, was synthesized from vanillin as described in previous reports.16Griffin D.R. Kasko A.M. Photodegradable macromers and hydrogels for live cell encapsulation and release.J. Am. Chem. Soc. 2012; 134: 13103-13107https://doi.org/10.1021/ja309004cCrossref PubMed Scopus (16) Google Scholar,18Hong Y. Zhou F. Hua Y. Zhang X. Ni C. Pan D. Zhang Y. Jiang D. Yang L. Lin Q. et al.A strongly adhesive hemostatic hydrogel for the repair of arterial and heart bleeds.Nat. Commun. 2019; 10: 2060-2070https://doi.org/10.1038/s41467-019-10004-7Crossref PubMed Scopus (313) Google Scholar The precursor molecule and 1,10-diaminodecane were then dissolved in ethanol and transferred to a high-pressure reactor. After the reaction, the system was cooled to room temperature to allow the collection of the resulting P-CDs (Figure 1A). The microstructure of the P-CDs was characterized using X-ray diffraction (XRD). A characteristic diffraction peak was observed at around 23° (Figure S1), which is attributed to the (002) facet of graphite by comparison with the standard JCPDS card (no. 41-1487). This sharp peak represents a high degree of graphitization of the P-CDs. A broad peak was also observed at 20°–26°, which suggests the presence of some amorphous P-CDs. The crystal distance of the as-prepared P-CDs was measured to be around 0.12 nm via transmission electron microscopy (TEM) (Figure 2A ). Dynamic light scattering analysis showed that P-CDs had an average size of ∼3.7 nm (Figure S2). To ensure the reproducibility, P-CDs were prepared in five batches under equal conditions and all of them showed similar sizes (Figure S2).Figure 2The characterization, photothermal conversion performance, and schematic mechanism of P-CDsShow full caption(A) HR-TEM images of P-CDs.(B) XPS survey spectrum of P-CDs.(C) Time-dependent changes in absorbance spectra of P-CDs (5 μg/mL aqueous solution) under UV light irradiation.(D) FT-IR spectra of P-CDs before and after 20 min UV irradiation.(E) The photo-kinetic plots of the in situ hydroxyl-to-aldehyde conversion of P-CDs. The content of aldehyde groups (ALD %) was used to investigate conversion efficiency.(F) Temperature changes of P-CDs before and after UV irradiation upon artificial solar irradiation (100 mW cm−2).(G) Fluorescence of P-CDs before and after 20 min UV irradiation.(H) Schematic illustration of mechanism of solar-to-thermal effect of P-CDs and the increase of photothermal conversion efficiency after UV irradiation (G, ground state; S, excited state). See also Figures S1–S11 and Schemes S1 and S2.View Large Image Figure ViewerDownload Hi-res image Download (PPT)According to X-ray photoelectron spectroscopy (XPS), the P-CDs contain C (∼75.17%), N (∼2.07%), and O (∼22.76%) (Figure 2B). The abundant oxygen contents should mainly originate because:19Zhu Z. Zhai Y. Li Z. Zhu P. Mao S. Zhu C. Du D. Belfiore L.A. Tang J. Lin Y. Red carbon dots: optical property regulations and applications.Mater. Today. 2019; 30: 52-79https://doi.org/10.1016/j.mattod.2019.05.003Crossref Scopus (137) Google Scholar,20Arcudi F. Đorđević L. Prato M. Design, synthesis, and functionalization strategies of tailored carbon nanodots.Acc. Chem. Res. 2019; 52: 2070-2079https://doi.org/10.1021/acs.accounts.9b00249Crossref PubMed Scopus (103) Google Scholar (1) P-CDs were prepared in ethanol as solvent at high temperature and pressure, indicating that the surface would be easily oxidized and generate lots of oxygen atoms. (2) The precursor molecules contained hydroxyl moieties and P-CDs inherited these moieties from the precursor molecules. High-resolution XPS measurements on elemental C, O, and N showed the presence of C=O, C–O, and O–H moieties. The C1s spectrum showed four peaks corresponding to C–C (284.8 eV), C–O (286.3 eV), C–N/C–N–C (285.1eV), and C=O (289.1 eV) linkages (Figure S3A). The O1s spectrum could be deconvoluted into three doublets corresponding to C–O/O–H (531.7 eV) and O=C–O (532.9 eV) linkages (Figure S3B). The C–N–C signal in the N1s spectrum suggests the formation of amide moieties (Figure S3C) by reaction of carboxylic acid and amine groups during the solvothermal preparation of P-CDs. In agreement with the XPS analysis, the Fourier transform infrared (FT-IR) spectrum also demonstrated the presence of –NH2 (3,528 cm−1), –OH (3,269 cm−1), and –O-C=O (1,710 cm−1) moieties in the P-CDs (Figure S4).Having determined the morphology and structure of the P-CDs, we next investigated their photosensitivity. The absorbance peak of the P-CDs at 350 nm was red shifted to 375 nm upon UV irradiation for 80 s (Figure 2C), which is similar to the shift observed for 4-(4-(hydroxymethyl)-2-methoxy-5-nitrophenoxy) butanoic acid, the precursor molecule (Figure S5). Generally, the hydroxyl moieties of such precursor molecule converted to aldehyde moieties upon UV irradiation (Scheme S1).21Chen Z. Sun W. Butt H.J. Wu S. Upconverting-nanoparticle-assisted photochemistry induced by low-intensity near-infrared light: How low can we go?.Chem. Eur. J. 2015; 21: 9165-9170https://doi.org/10.1002/chem.201500108Crossref PubMed Scopus (69) Google Scholar As a result, we speculated that a photoreaction similar to that of the photocage should occur in P-CDs. A comparison of 1H NMR spectra between P-CDs without/with UV irradiation also suggests that aldehyde moieties (signal at 9.65 ppm) appeared after UV irradiation, which was similar to the precursor molecule (Figure S6). We further used the FT-IR spectra of P-CDs before and after UV irradiation to verify the generation of aldehyde moieties. A pronounced signal at 1,701 cm−1, attributed to aldehyde groups,22Liu J. Li S. Aslam N.A. Zheng F. Yang B. Cheng R. Wang N. Rozovsky S. Wang P.G. Wang Q. Wang L. Genetically encoding photocaged quinone methide to multitarget protein residues covalently in vivo.J. Am. Chem. Soc. 2019; 141: 9458-9462https://doi.org/10.1021/jacs.9b01738Crossref PubMed Scopus (38) Google Scholar appeared in the FT-IR spectrum after UV irradiation (Figure 2D), confirming the in situ generation of aldehyde moieties upon UV irradiation of P-CDs. According to the photo-kinetic curve of P-CDs (Figure 2E), no hydroxyl-to-aldehyde conversion was observed without UV irradiation. However, the in situ generation of aldehyde groups increased upon increased UV irradiation. The aldehyde content (ALD %) of P-CDs went up to 89.3% upon UV irradiation for 20 min (Figure 2E). The P-CDs also showed highly increased absorbance upon irradiation over several cycles, with no change in absorbance when the UV light source was removed (Figure S7). The in situ aldehyde-amine coupling reaction of P-CDs with carbohydrazide (CDH) was also confirmed by 1H NMR spectroscopy (Scheme S2; Figure S8). Therefore, P-CDs demonstrate excellent photo controllability and aldehyde groups can be generated as required.Photothermal conversion of the P-CDs was further investigated using simulated solar irradiation (100 mW cm−2, 20 min). The temperature of irradiated P-CDs increased from 32.2°C to 57.1°C with a photothermal conversion efficiency of 33.2%, whereas the temperature of un-irradiated P-CDs only increased from 31.1°C to 38.5°C with a photothermal conversion efficiency of 9.8% (Figure 2F). More interestingly, different exposure time of P-CDs to UV sources resulted in different photothermal conversion. Specifically, P-CDs demonstrated a temperature increase from ∼7.4°C to ∼24.9°C when the exposure to UV light went up from 0 to 20 min (Figure S9). This difference is partially attributed to the enhanced absorbance of P-CDs after UV irradiation, as shown in Figure S10. Furthermore, the fluorescence emission of P-CDs after UV irradiation was much lower than that before UV irradiation (Figure 2G), suggesting that radiative migration of excited state P-CDs after UV irradiation was greatly suppressed. In addition, the suppressed radiative migration was also confirmed by decreased fluorescence lifetime and quantum yield of P-CDs after UV irradiation (Figure S11). As a result, the non-radiative migration of excited states of P-CDs was enhanced after UV irradiation (Figure 2H). Taking all these results together, the enhanced photothermal conversion of P-CDs was as follows: the absorbance range of P-CD powders in the visible region was strongly widened after hydroxyl-to-aldehyde conversion, indicating that P-CDs can absorb and utilize solar photons more efficiently. After P-CDs absorbed the solar photons, the energy was consumed in non-radiative migration and boosted photothermal conversion. All of these results demonstrate the readily high in situ reactivity and excellent photothermal conversion of P-CDs under UV irradiation. Such P-CDs, with the in situ formation of aldehyde groups, can thus be promising for the preparation of robust photothermal plastics.Preparation of P-CD plasticsP-CD plastics were further prepared by mixing P-CDs with HA-CDH. Specifically, P-CDs were dispersed in an aqueous solution of HA-CDH, where HA-CDH was prepared as described previously (Scheme S3).22Liu J. Li S. Aslam N.A. Zheng F. Yang B. Cheng R. Wang N. Rozovsky S. Wang P.G. Wang Q. Wang L. Genetically encoding photocaged quinone methide to multitarget protein residues covalently in vivo.J. Am. Chem. Soc. 2019; 141: 9458-9462https://doi.org/10.1021/jacs.9b01738Crossref PubMed Scopus (38) Google Scholar The mixture was then exposed to 375 nm UV light to trigger the in situ formation of aldehyde groups on the P-CDs, which then reacted with the amine groups in HA-CDH. A new peak at 1,559 cm−1 assigned to C=N moieties appeared in the FT-IR spectra after 20 min irradiation (Figures S12A and S12B), verifying the expected covalent crosslinking reaction. P-CD plastics were obtained by drying the crosslinked mixture at room temperature (Figure 3A ). The whole process was reproducible since P-CD plastic made in five batches showed similar FT-IR spectra (Figure S12B). According to atomic force microscopy (AFM), the P-CD plastics showed a rough surface morphology demonstrating small aggregates of P-CDs with a length of 3–3.2 μm and a width of 1–2 μm (Figure 3B). Such rough morphology could be caused by P-CDs interacting with HA-CDH chains during the photo-triggered crosslinking of aldehyde and amine groups. Moreover, such rough surfaces are beneficial for light management and positively contribute to the ensuing photothermal conversion.23Yang L. Yan Z. Yang L. Yang J. Jin M. Xing X. Zhou G. Shui L. Photothermal conversion of SiO2@Au nanoparticles mediated by surface morphology of gold cluster layer.RSC Adv. 2020; 10: 33119-33128https://doi.org/10.1039/d0ra06278bCrossref PubMed Scopus (10) Google ScholarFigure 3Characteristics of P-CD plasticsShow full caption(A, C, and D) Digital images of (A) P-CD plastics, (C) non-crosslinked P-CD plastics, and (D) control plastics and the corresponding schematic microstructures.(B) AFM topography of a typical P-CD plastic.(E–H) Mechanical properties of control plastics, non-crosslinked P-CD plastics, and P-CD plastics. (E) Stress-strain curves, (F) tensile strength, strain to failure, (G) toughness, and (H) Young’s moduli.(I) Tensile strengths and Young’s moduli of P-CD plastic compared with currently widely used plastics.(J) UV-vis spectra showing the absorbance of P-CDs after immersing P-CD plastics (2 cm × 1 cm × 60 μm), non-crosslinked P-CD plastics (2 cm × 1 cm × 60 μm), GO-HA plastic (2 cm × 1 cm × 60 μm), and CNT-HA plastic (2 cm × 1 cm × 60 μm) in water/ethanol (v/v = 1/1). See also Figures S12–S14, Schemes S3 and S4, and Table S1.View Large Image Figure ViewerDownload Hi-res image Download (PPT)To investigate the mechanical performance of P-CD plastics, two plastics (non-crosslinked P-CD plastic and control plastic as shown in Figures 3C and 3D, respectively) were also prepared for comparison. Stress-strain measurements of P-CD plastics showed that the composition of the P-CDs and photo-triggered covalent crosslinking greatly affected their mechanical performances. Non-doped HA-CDH plastic (control plastic) showed an average tensile strength of ∼28.4 MPa and an average strain to failure of ∼3.0% (Figures 3E and 3F). Non-crosslinked P-CD plastic with the sole addition of P-CDs in the plastics without UV irradiation showed an average tensile strength of ∼59.3 MPa and a strain to failure of ∼3.8% (Figures 3E and 3F). These results demonstrate that the simple addition of P-CDs can enhance the non-covalent interactions in the matrix and thus improve its tensile strength. Density functional theory (DFT) calculations were used to investigate this effect (Scheme S4; Figure S13). The models showed that many more hydrogen bonds were formed between added P-CDs and HA-CDH chains, with a ratio of HA-CDH chains/P-CDs of 2:1 (of about 10 hydrogen bonds within the simulation model), compared with only about 3 hydrogen bonds between HA-CDH chains within the simulation model (Figure S13). The increase of the number of P-CDs led to more hydrogen bonds (∼15) with a ratio of HA-CDH chains/P-CDs of 1:2. The simulated binding energy between HA-CDH chains and P-CDs (∼−4.3 eV) was also higher than that between HA-CDH chains (∼−1.2 eV) (Table S1).After the photo-crosslinking of P-CDs with the plastic matrix, the tensile strength and strain to failure of the P-CD plastics further increased to ∼124 MPa and ∼10.1%, respectively (Figures 3E and 3F). P-CD plastics also showed higher toughness (7.0 MJ cm−3) than the control plastic (0.3 MJ cm−3) and non-crosslinked P-CD plastic (1.2 MJ cm−3) (Figure 3G). In addition, P-CD plastics showed a Young’s modulus of ∼1,900 MPa, which was doubled in value compared with control plastic (Figure 3H). Notably, P-CD plastics possessed superior tensile strengths and Young’s moduli in comparison with other widely used plastics (Figure 3I).24Wang J. Emmerich L. Wu J. Vana P. Zhang K. Hydroplastic polymers as eco-friendly hydrosetting plastics.Nat. Sustain. 2021; 4: 877-883https://doi.org/10.1038/s41893-021-00743-1Crossref Scopus (23) Google Scholar The curve of dissipation factor (tan δ) of the non-crosslinked P-CD plastic showed a linear increase between 40°C and 114°C with a glass transition temperature of 113.4°C, which indicates steady softening of the composite plastics (Figure S14). In comparison, crosslinked P-CD plastics showed softening behaviors in the range of 160°C–190°C with a glass transition temperature of 187.4°C (Figure S14). Therefore, the photo-crosslinking reaction induced by UV irradiation strongly increased both the mechanical strength and the thermal stability of P-CD plastics.At the same time, the P-CDs crosslinked in the plastic matrix can effectively prevent the leakage problem of photothermal agent. According to UV-vis spectroscopy, no absorbance signals of P-CDs was observed in the aqueous ethanol after treating the crosslinked plastics over 70 h, indicating no detectable leakage of P-CDs (Figure 3J). In contrast, the aqueous ethanol after treating non-crosslinked P-CD plastics over 70 h showed obvious absorbance signals for P-CDs, indicating the leakage of P-CDs from the matrix of non-crosslinked P-CD plastic (Figure 3J). Furthermore, photothermal plastics were also made via mixing the most general photothermal reagents, including graphene oxide (GO) and CNTs, with the HA-CDH matrix. The resultant GO-HA plastic and CNT-HA plastic showed obvious leakage after immersion in water (Figure 3J). All these results demonstrate the success of our rational strategy via UV light irradiation during the preparation process to increase both the mechanical performance and thermal stability of the polysaccharide-based plastics, as well as to prevent the leakage of the photothermal reagents from the plastics matrix.Photothermal conversion of P-CD plasticPrevious results showed that aldehyde moieties were crucial for the photothermal conversion of P-CDs. However, aldehyde moieties of P-CDs changed to –C=N moieties when P-CD plastics were formed. The effect of such moieties on the photothermal performance should be studied first. To this end, P-CDs powders with different ratios of –CHO and –C=N were prepared. Excitingly, the temperature increase of P-CD powders was even slightly enhanced (from ∼24.9°C to 28.1°C) with increasing ratio between –C=N and –CHO moieties, indicating improved photothermal efficiency (Figure S15). This might be attributed to the further promoted non-radiative migration caused by –C=N moieties.25Liu Y. Li L. Yue M. Yang L. Sun F. Xu G. Fu Y. Ye F. A switch-on fluorescent probe for detection of mesotrione based on the straightforward cleavage of carbon-nitrogen double bond of Schiff base.Chem. Eng. J. 2022; 430: 132758https://doi.org/10.1016/j.cej.2021.132758Crossref Scopus (12) Google Scholar,26Deng H.H. Huang K.Y. He S.B. Xue L.P. Peng H.P. Zha D.J. Sun W.M. Xia X.H. Chen W. Rational design of high-performance donor-linker-acceptor hybrids using a Schiff base for enabling photoinduced electron transfer.Anal. Chem. 2020; 92: 2019-2026https://doi.org/10.1021/acs.analchem.9b04434Crossref PubMed Scopus (34) Google Scholar This result encouraged us to further investigate the photothermal conversion of P-CD plastics. The absorbance spectra were first measured as this is a crucial parameter for photothermal conversion (Figure 4A ). P-CD plastic showed strong absorbance over a wide range of wavelengths (200–2,600 nm), which overlapped well with the solar spectrum. This result indicated the potential of these plastics as a solar absorber for photothermal conversion (Figure 4A). Notably, P-CD plastics showed higher absorbance than non-crosslinked P-CD plastic, which is consistent with the observed UV-triggered absorbance enhancement of P-CDs in aqueous solutions. Photothermal conversion by the P-CD plastics was investigated by exposing the plastics to standard 1 sun (100 mW cm−2) irradiation. The temperature of the P-CD plastics increased from ∼24.3°C to ∼59.5°C after 20