Cysteine-Modified Cellulose-Based ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Citric Acid/Cysteine-Modified Cellulose-Based Materials: Green Preparation and Their Applications in Anticounterfeiting, Chemical Sensing, and UV Shielding Heng Chen,† Xiaohui Yan,‡ Qian Feng,† Pengchao Zhao,† Xiayi Xu,† Dickon H. L. Ng,§ and Liming Bian*,†,‡,§,∥,⊥,# †

Department of Biomedical Engineering, The Chinese University of Hong Kong, Shatin, New Territories, 999077 Hong Kong, P. R. China ‡ Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, 999077 Hong Kong, P. R. China § Shun Hing Institute of Advanced Engineering, The Chinese University of Hong Kong, Shatin, New Territories, 999077 Hong Kong, P. R. China ∥ Shenzhen Research Institute, The Chinese University of Hong Kong, CUHK Shenzhen Research Institute Building, No.10, second Yuexing Road, Nanshan District, Shenzhen 518057, P. R. China ⊥ China Orthopedic Regenerative Medicine Group (CORMed), Hangzhou 310058, P. R. China # Centre for Novel Biomaterials, The Chinese University of Hong Kong, Shatin, New Territories, 999077 Hong Kong, P. R. China S Supporting Information *

ABSTRACT: Functionalized cellulose-based materials are in high demand for many applications. In this work, we report a green approach to fabricate a type of versatile cellulose-based material through facile citric acid/cysteine treatment. For its preparation, cellulose-based materials were conjugated with citric acid/cysteinebased fluorophores (CCFs) by simply soaking them in a concentrated citric acid/cysteine aqueous solution followed by drying above 80 °C. Chemical modification occurred, which was completed in the swollen state of the cellulose, and the highest conjugating ratio reached 1.6 wt %. It was noted that the treatment had no effect on the crystallinity of the cellulose while the structural morphology of various cellulose-based components in the material was maintained. We also found that the CCF-modified cellulosebased products had remarkable fluorescence, a selective quenching ability toward chloride ions, and excellent UV absorption capacity. Thus, they could have new applications in anticounterfeiting, chemical sensing, and UV shielding. Furthermore, our developed route to fabricate these CCF-modified cellulose-based products was environmentally friendly since water was the only solvent, and no organic solvent was involved throughout the procedures. KEYWORDS: Cellulose, Citric acid, Cysteine, Anticounterfeiting, Chloride sensing, UV shielding

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INTRODUCTION Cellulose is a renewable and abundant natural polymeric material that can be derived from plants, algae, bacteria, and marine animals. In addition, cellulose is an environmentally friendly raw material due to its microorganism-mediated biodegradation.1,2 As the shortage of nonrenewable resources and deterioration of the environment intensify, the development of cellulose-based materials has received increasing attention in the past two decades. Because of their multihydroxyl structure, high stability, and excellent biocompatibility,3,4 cellulose-based materials are ideal polymeric carriers for conjugation of various functional groups. The obtained functionalized cellulose-based materials have been used for chemical sensing,5−10 bioimaging,11−13 anticounterfeiting,14,15 and UV shielding.16−18 However, chemical modification of © 2017 American Chemical Society

cellulose-based materials with functional groups is still plagued by a limited choice of solvents for cellulose. Cellulose is insoluble in water and normal organic solvents because of its high polarity, strong intermolecular hydrogen bonding, and hydrophobic interactions within cellulose.19 Chemical modifications of cellulose are typically conducted in special solvents like N,N-dimethylacetamide/LiCl, dimethyl sulfoxide (DMSO)/tetrabutylammonium fluoride trihydrate (TABF), N-methylmorpholine-N-oxide, ionic liquids, etc.20,21 The utilization and disposal of these solvents are of high cost, and have potentially negative effects on the environment. In Received: July 21, 2017 Revised: September 20, 2017 Published: October 31, 2017 11387

DOI: 10.1021/acssuschemeng.7b02473 ACS Sustainable Chem. Eng. 2017, 5, 11387−11394

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the one-pot preparation of CCF-modified cellulose-based materials. used as received without further purification. Deionized (DI) water was obtained from an Aqua Solutions purification system (Aqua Solutions Inc., Jasper, GA). Characterization. Nuclear magnetic resonance (NMR) spectra were obtained using a Bruker AV400 spectrometer. Solid-state carbon13 cross-polarization/magic angle spinning (13C CP/MAS) NMR spectra were recorded on a Bruker Avance III 600 MHz wide bore spectrometer operating at 14.1 T. A zirconium oxide rotor was used. Acquisition was performed with contact time for CP of 3 ms, MAS speed of 12.5 kH, and a recovery delay of 3 s. Tetramethylsilane was used as a reference for the calibration of chemical shift. The Fourier transform infrared (FTIR) spectra were recorded using a Nicolet iS10 IR spectrometer by using the potassium bromide method. Electrospray ionization mass spectrometry (ESI-MS) was carried out on an AB SCIEX Triple TOF 4600 mass spectrometer. The optical properties of cellulose-based materials were studied by using ultraviolet−visible (UV−vis) absorption spectroscopy and fluorescence spectroscopy. The UV−vis absorption spectra were recorded using a UV−vis-NIR spectrophotometer (Cary 5000, Varian), and the fluorescence spectra were recorded on a FluoroLog-3 spectrofluorometer (Horiba Jobin Yvon) at room temperature. The morphology and atomic species of the cellulose-based materials were investigated by using field emission scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS, JSM-7800F). AFM measurements were performed using a Nanoscope III AFM system. The X-ray diffraction (XRD) patterns were recorded on a Rigaku XRD-6000 diffractometer using Cu Kα radiation (λ = 0.154 nm) at 40 kV and 30 mA. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Kα spectrometer (Axis Ultra, Kratos Analytical Ltd.) with an Al Kα excitation source. Tensile tests of cellulose films were carried out and measured by using an MTS QT/1L tensile testing machine. Modification of Various Cellulose-Based Materials with CCF. Using cellulose powders as an example, a typical modification procedure is presented as follows: A 1 g portion of cellulose powders was added to the mixed aqueous solution containing citric acid (2 M) and cysteine (2 M). After being soaked for 30 min, the cellulose powders were separated by centrifugation and further dried in an oven above 80 °C for a predetermined time period. Then, the dried cellulose powders were washed with excessive hot water thoroughly until no fluorescence was detected in the residual water and, finally, were dried at room temperature. The modification procedure of cellulose fibers is the same as that of cellulose powders. For cellulose paper and film, the modification procedures are much simpler because centrifugation separation is not required.

addition, the use of toxic chemical agents for the modification of cellulose adds complexity to manufacturing operations and the burden of waste discharge treatment. Therefore, the development of aqueous-phase reactions along with minimizing the use of toxic chemical agents are highly desirable for the chemical modification of cellulose-based materials. Citric acid and cysteine are small molecules naturally involved in metabolism. Recently, it was confirmed that they generate highly fluorescent conjugated compounds through a multidehydration reaction under high temperature.22 Citric acid/cysteine-based fluorophores (CCFs) exhibit high quantum yield, good biocompatibility, and relatively low cost. A variety of CCF-modified fluorescent materials have been developed for bioimaging,23,24 drug delivery,25 and chemical sensing.26 Herein, we report a facile and green approach to chemically modify cellulose-based materials with CCF. Upon soaking in a concentrated citric acid/cysteine aqueous solution followed by drying above 80 °C, various cellulose-based materials can be modified with CCF. In this approach, the modification is conducted in the swollen state of cellulose, and water is the only solvent used for the reaction and postprocessing, significantly reducing the fabrication cost and minimizing the negative environmental impact. In addition, this modification strategy does not require dissolution of cellulose and enables conjugation of fluorophores throughout the entire volume of the cellulose-based materials without altering their physical structures. The obtained CCF-modified cellulose-based materials can be used in various applications, such as anticounterfeiting, chemical sensing, and UV shielding.

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EXPERIMENTAL SECTION

Materials. Citric acid (99.5%), L-cysteine (99%), and microcrystalline celluloses (average particle size, 50 μm) were purchased from J&K Chemical Ltd. (Shanghai, China). Microcrystalline celluloses, degreasing cottons (Sinopharm Chemical Regent Co., Ltd., Shanghai, China), qualitative filter papers (Fushun City Civil Affairs filter paper factory), and regenerated cellulose dialysis membrane (thickness, 50 μm; Spectrum Laboratories Inc.) were selected as cellulose-based materials with fixed shapes consisting of powders, fibers, papers, and films, respectively. The cellulose films were boiled for 10 min in distilled water and extensively washed before use, and other materials were 11388


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Figure 2. (a) 13C CP/MAS NMR spectrum of CCF-modified cellulose film. (b) FTIR spectra of neat cellulose powders (red curve) and CCFmodified cellulose powders (blue curve). (c) XPS wide-scan spectra of neat cellulose film (red curve) and CCF-modified cellulose film (blue curve). (d) High-resolution XPS C 1s spectra of neat cellulose film (red curve) and CCF-modified cellulose film (blue curve).

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RESULTS AND DISCUSSION Structural Characterization. The strong fluorescence of CCF has been carefully studied by Kasprzyk et al., and they found that the main fluorophore was a conjugated compound (i.e., thiazolo pyridine carboxylic acid, TPA) that is derived from the complicated multidehydration reaction between citric acid and cysteine (Figure S1).22 TPA contains two carboxyl groups, and it still has the potential to react with the hydroxyl groups of cellulose through esterification. Although cellulose is insoluble in water and common organic solvents, the amorphous domains of cellulose still allow the penetration of solvent molecules (e.g., water molecules), which results in the swelling of cellulose. The amorphous domains of cellulose are also accessible to dissolved reagent molecules, and this unique characteristic facilitates the chemical modification of cellulose in the swollen state without cellulose dissolution.3 In this study, the formation of TPA and its conjugation with cellulose-based materials were achieved in the one-pot approach. As shown in Figure 1, various cellulose-based materials, including powders, fibers, papers, and films, were first soaked in a concentrated aqueous solution of citric acid and cysteine to allow the penetration of citric acid and cysteine molecules through the amorphous domains of cellulose. After that, soaked cellulosebased materials were easily separated from the aqueous solution through centrifugation or filtration. In a subsequent drying process, multidehydration reactions between citric acid and cysteine and esterification between TPA and cellulose occurred facilitated by the evaporation of water. The concentrated aqueous solution of citric acid (2 M) and cysteine (2 M) was dried at 120 °C for 24 h. The product was analyzed by NMR and ESI-MS (Figures S2−S5), and it was confirmed that the

main product was TPA. TPA exhibited high solubility in hot water (90 °C). Therefore, the unconjugated TPA products along with the unreacted citric acid/cysteine can be removed from modified cellulose materials by washing with hot water. In the control groups, cellulose-based materials were soaked with an aqueous solution of citric acid (2 M), dried at 80 °C for 24 h, washed with hot water, soaked with an aqueous solution of cysteine (2 M), dried at 80 °C for 24 h, and washed with hot water again. No fluorescence was observed in these cellulosebased materials, and the result suggested that the conjugation occurred after the formation of TPA. The physical adsorption of TPA on cellulose-based materials was also investigated. Cellulose-based materials were soaked in a TPA aqueous solution (3 mg/mL) for 1 h and dried at room temperature overnight. After washing with excessive water, no fluorescence was observed in these cellulose-based materials. Thus, the physical adsorption of TPA on cellulose-based materials was negligible after washing with water. For an investigation into the structure change after modification, CCF-modified cellulose-based materials were characterized by 13C CP/MAS NMR, XPS, FTIR, SEM, AFM, and XRD. The 13C CP/MAS NMR spectrum of the CCF-modified cellulose film with drying treatment at 120 °C for 24 h is shown in Figure 2a. The resonance signals of 97.7, 114.5, and 140.5−164.6 ppm are attributed to carbon atoms of the pyridone moiety of TPA. Meanwhile, the resonance signals in the region 35.3−50.8 ppm belong to carbon atoms of citric acid, and these signals indicate that the esterification between citric acid and cellulose accompanies the conjugation of TPA and cellulose. FTIR analysis also reveals the sign of esterification in CCF-modified cellulose powders as made 11389


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Figure 3. Digital photographs of CCF-modified cellulose powders (a, e), fibers (b, f), paper (c, g), and film (d, h) under visible and UV light (λex = 365 nm) irradiation, respectively.

evident by a new peak of the carbonyl group at 1735 cm−1 in its FTIR spectrum (Figure 2b). Furthermore, in the XPS spectra of the CCF-modified cellulose film (Figure 2c), three new peaks appear at 163, 227, and 399 eV, and they are attributed to S 2p, S 2s, and N 1s, respectively. These new elements of sulfur and nitrogen belong to conjugated TPA. The C 1s peak (Figure 2d) is deconvoluted into 6 parts at 284.8, 285.6, 286.6, 288.1, 288.7, and 289.2 eV. Compared to those of neat cellulose, the new peaks at 285.6, 288.7, and 289.2 eV indicate the presence of CN, NCO, and OCO bonds, respectively. Moreover, under irradiation from a hand-held UV lamp (λex = 365 nm), bright blue emissions were observed for all CCFmodified cellulose-based materials (Figures 3). These results confirm that TPA has been conjugated to cellulose successfully. SEM and AFM analyses were performed to study the surface morphology of these cellulose films. As shown in Figure 4, there is no obvious difference between the surfaces of the CCF-

modified cellulose film and neat cellulose films, and this result suggests that the modification does not alter the morphology of the cellulose film. Furthermore, XRD analysis was employed to investigate the crystal structure of these cellulose films. The CCF-modified film exhibits an XRD pattern that is similar to that of the neat film, and the three typical diffractions at 2θ = 12.0°, 20.2°, and 21.8° are assigned to the crystal structure of cellulose II (Figure 4e).28 This result reveals that the chemical modification has no significant influence on the crystal structure of the cellulose films, and this indicates that the CCF modification largely occurred in the amorphous domains of cellulose. These results confirm the successful conjugation of CCF to these cellulose-based materials, and the modification has little influence on the original crystal structure and morphology of these cellulose-based materials. Furthermore, during the entire procedure, no organic solvents are used, and water, which is the only solvent used, is a truly green solvent.29 CCF has been confirmed to exhibit good biodegradability.26 Therefore, these CCF-modified cellulose-based materials are also environmentally friendly materials with minimal adverse environmental impacts after degradation. Optical Property. For convenience, the cellulose films were chosen for optical property characterizations. The UV−vis spectrum of the CCF-modified cellulose film exhibits a new absorption peak at 351 nm, and it is not observed in the spectrum of the neat cellulose film (Figure 5a). The new peak is due to the n → π* transition of conjugated TPA.30,31 The influence of the drying temperature and time on the extent of modification was examined on the basis of the intensity of the absorption peak (Figure 5c,d). The results indicated that the absorption intensity increased as the drying temperature and time increased. Therefore, the extent of CCF modification of cellulose can be controlled by adjusting the drying temperature and time. As shown in the fluorescence spectra (Figure 5b), the modified cellulose film exhibits a maximum emission under excitation at 360 nm, and the maximum emission wavelength remains at 435 nm independent of the excitation wavelength. The fluorescence may be derived from the conjugated structure of TPA where π−π* electronic excitation leads to emission from the lowest energy band.32 In comparison to the pure solid form of TPA, the modified cellulose powders exhibit a much brighter blue emission under UV light (λex = 365 nm) irradiation (Figure S6). This result suggests that cellulose provides the chemical basis for CCF conjugation and disperses CCF to avoid solid-state quenching of CCF via its physical structure. In addition, with the neat cellulose film in the TPA

Figure 4. SEM images of neat cellulose film (a) and CCF-modified cellulose film (b). AFM images of neat cellulose film (c) and CCFmodified cellulose film (d). (e) XRD patterns of neat cellulose film (red curve) and CCF-modified cellulose film (blue curve). 11390


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Figure 5. (a) UV−vis spectra of neat cellulose film and CCF-modified cellulose film. (b) Fluorescence spectra of CCF-modified cellulose film. UV− vis spectra of cellulose films with different drying times at 80 °C (c), and those of cellulose films with different drying temperatures for 24 h (d).

solution as the reference, the conjugating ratio of TPA on cellulose was evaluated according to the intensity of the characteristic peak at 351 nm (Figures S7−S9). For the cellulose film with drying treatment of 120 °C for 24 h, the conjugating ratio of TPA on cellulose is about 1.6 wt %. The high conjugating ratio indicates that the conjugation of TPA with cellulose is in-depth modification, rather than only the surface modification. Tensile Properties. To investigate the influence of modification on the mechanical properties of cellulose-based materials, tensile properties of CCF-modified cellulose films were measured. As shown in Table 1, the tensile strengths of modified cellulose films are all higher than that of the neat cellulose film, and it increases with increasing drying temperature. The enhanced tensile stiffness should be attributed to the cross-linking action of TPA and citric acid. Meanwhile, modified cellulose films are more brittle than the neat film with reduced break and yield strain, and this is likely because of

the restricted mobility of cellulose chains by TPA and citric acid cross-linking. Thus, CCF modification enhances the tensile stiffness of cellulose-based materials but decreases its stretchability. Applications. Facilitated by the remarkable fluorescence property, new applications of these cellulose-based materials have been developed. Currently, cellulose-based materials have been widely used in commodity packaging, and the anticounterfeit packaging is a key strategy to safeguard authentic products. The concentrated citric acid/cysteine aqueous solution can be used as an invisible ink for anticounterfeit printing. After printing and drying, the concealed information can be permanently marked on cellulose packaging. For example, the pattern “MADE IN CUHK” was printed on a piece of printer paper using a seal with the concentrated citric acid/cysteine aqueous solution ink followed by drying in an oven at 80 °C for 24 h or carefully heating for less than 1 min by using a commercial hot air gun (see Video S1 in the Supporting Information). The pattern was clearly seen under irradiation by a hand-held UV lamp (λex = 365 nm; Figure 6a) but was invisible under daylight conditions (Figure 6b). Furthermore, the pattern remained on the paper despite repeated rinsing with water, and this antirinsing property may be due to the strong chemical conjugation between the TPA and cellulose in the paper. In addition, doping with fluorescent fibers is an important anticounterfeiting strategy for the manufacture of banknotes (Figure S10), and the CCF-modified cellulose fibers may be an alternative to the current doping fibers. Recently, citrate-based fluorescent materials have been reported by Yang et al.26,27 for chloride sensing in the diagnosis of cystic fibrosis based on fluorescence quenching of citratebased fluorescent materials by chloride ions in acidic conditions. Herein, the chloride sensing capacity of the modified cellulose film was investigated in a simplified model

Table 1. Tensile Propertiesa of Various Cellulose Films sample break stress (MPa) break strain (%) yield stress (MPa) yield strain (%) tangent modulus (GPa)

CFneat

CF80

CF100

CF120

60.5 ± 3.5

93.7 ± 0.9

104.3 ± 3.1

125.1 ± 2.6

11.7 ± 0.8

7.7 ± 0.7

4.4 ± 0.3

2.9 ± 0.3

68.5 ± 2.1

94.3 ± 1.2

104.6 ± 3.1

125.6 ± 2.8

10.1 ± 0.2

7.6 ± 0.7

4.4 ± 0.2

2.9 ± 0.3

4.5 ± 0.3

8.2 ± 0.2

10.7 ± 0.5

13.8 ± 0.5

a CFneat, CF80, CF100, and CF120 refer to neat cellulose and modified cellulose with drying temperatures of 80, 100, and 120 °C for 24 h, respectively.

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Figure 6. Digital photographs of marked printer paper under UV (a) and visible light (b) irradiation. (c) Emission spectra of CCF-modified cellulose film in HCl solutions of different concentrations. (d) Five consecutive rinse/recovery cycles of fluorescence intensity of modified cellulose film. (e) Transmittance spectra of neat cellulose film and CCF-modified cellulose film fabricated at different drying temperatures for 24 h. (f) Digital photographs of neat cellulose film (top) and modified cellulose film fabricated with drying temperature of 120 °C (bottom) under natural daylight.

Excessive UV exposure is known to cause serious harm to humans and materials. This type of exposure causes DNA damage, immune suppression, and skin diseases like skin cancer. In addition, UV exposure is responsible for the degradation of materials and loss of original mechanical properties.33,34 The UV shielding film is an effective tool to block UV light and avoid the detrimental effects of UV irradiation. Cellulose film is an outstanding film material due to its high transparency, excellent stability, and good mechanical property. However, it normally lacks adequate absorption capacity for UV light, especially UVA (320−400 nm), which is the major component of solar UV irradiation. For improvement of the UV absorption capacity, some UV absorbers have been introduced into cellulose films using various complicated methods.28,35,36 Herein, our CCF-modified cellulose film exhibited excellent UV absorption capacity. The UV−vis transmittance spectra (200−800 nm) of the modified cellulose films with different drying temperatures and a drying time of 24 h are shown in Figure 6e. The modified cellulose films exhibit two strong block bands against UV transmittance that are located below 270 nm and at approximately 355 nm. These bands correspond to the absorption peaks of CCF,31 which suggests that the conjugated CCF contributed to the excellent absorption capacity of the modified cellulose film for UV light, especially for UVA and UVC (200−275 nm). Furthermore, the UV transmittance of the modified cellulose film decreased as

using HCl solutions. For detection of chloride ions at concentrations less than 1 M, the modified cellulose film with low CCF conjugation was selected (soaked cellulose film was dried at 80 °C for 1 h). The fluorescence intensities of the film in HCl, HNO 3 , and H 2 SO 4 solutions with different concentrations were determined under irradiation with a 360 nm UV light (Figure 6c, and Figure S11). The HCl solution exhibited maximum quenching efficiency, and a good linear correlation was observed between the quenching ratio (I0/I−1) and CHCl via the following equation: I0/I − 1 = 1.178 × C HCl

(R2 = 0.998)

(1)

where I0 and I are the fluorescence intensities of the modified cellulose film in DI water and the HCl solution, respectively, and CHCl is the HCl concentration. Furthermore, the quenched fluorescence of the film rapidly recovered after rinsing with DI water. The rinsed film exhibited the same quenching performance when immersed in the HCl solution again, and the quenching ratio of the fluorescence intensities of the modified film exhibited little change even after 5 consecutive rinse/ recovery cycles (Figure 6d). These results demonstrate that the CCF-modified cellulose film is a promising recyclable fluorescent probe for the selective sensing of chloride ions under acidic conditions. 11392


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research is also supported by project BME-p3-15 of the Shun Hing Institute of Advanced Engineering, The Chinese University of Hong Kong. This work is supported by the Health and Medical Research Fund, the Food and Health Bureau, the Government of the Hong Kong Special Administrative Region (reference 03140056). This research is supported by the Chow Yuk Ho Technology Centre for Innovative Medicine, The Chinese University of Hong Kong.

the drying temperature that was used during the modification increased. The modified cellulose film obtained at a drying temperature of 120 °C exhibited the best UV absorption performance, and the average UV transmittance through the film decreased to less than 5%. However, the film maintained a high average visible light transmittance of approximately 75%. The visual appearance of the film exhibited a transparency that was comparable to that of neat cellulose film (Figure 6f). On the basis of the facile modification and excellent UV absorption performance, the CCF-modified cellulose film is expected to be an ideal alternative to commercial UV shielding film. In addition, the modified cellulose fibers are expected to be used for manufacturing UV-protective clothing.

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CONCLUSIONS In summary, we have developed a green and facile approach to modifying various cellulose-based materials with CCF. In the swollen state of cellulose, the formation of CFFs and their conjugation with cellulose were successfully achieved using a one-pot approach. This approach allows for the modification of cellulose-based materials without cellulose dissolution and helps to maintain the original morphology of the cellulosebased materials. In addition, water is the only solvent used during the reaction and postprocessing, which eliminates pollution problems arising from the use of organic solvents and effectively reduces the cost of manufacturing operations. Furthermore, with the acquired fluorescent property, these modified cellulose-based materials have the potential for new applications, such as anticounterfeit marking, sensing chloride ions, and UV shielding. The facile and green modification strategy developed in this study together with the versatile performance of the resulting modified cellulose-based materials will greatly facilitate the utilization of cellulose in novel applications and promote sustainable development.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02473. Schematic illustrations, NMR spectra, ESI-MS spectra, digital photographs, UV−vis spectra, calibration curve, and emission spectra (PDF) Video S1: Seal of concentrated citric acid/cysteine aqueous solution is printed on printer paper, dried, and observed under UV light, while remaining invisible under visible light (MPG)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liming Bian: 0000-0003-4739-0918 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Project 31570979 is supported by the National Natural Science Foundation of China. The work described in this paper is supported by a General Research Fund grant from the Research Grants Council of Hong Kong (Project 14220716). This 11393


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