Recyclable luminescent carbon dots nanopaper for flexible electronics

The use of sustainable materials in high-tech devices is one way to decrease the carbon footprint and tackle global climate change. We first synthesized blue-emissive carbon dots (CDs) from biocompatible onion inner epidermal cells using solvothermal method. Then, cellulose nanofiber was prepared by TEMPO oxidization, followed by homogenization from soft wood source. Finally, the blue emissive CDs-cellulose nanofibers-based nanopaper was fabricated by simple roller-coating approach, and its optical and morphological properties were investigated by transmittance, photoluminescence, fourier-transform infrared (FTIR) and scanning electron microscopy techniques. The results indicate that nanopapers have a high light emission, and that their transparency may be easily adjusted by varying the proportion of CDs content. These nanopapers can be incorporated into flexible and stretchable electronics and optical sensor platforms.


Introduction
Cellulose is the most sustainable and biocompatible polymeric material in the world, which can be extracted from wood or plants [1]. Various nanocellulose (NCs) derivatives can be obtained from different processing approaches [2]. Nanocrystalline Cellulose (NCC) and nanofibers cellulose (NFC) are the most common NCs. Depending on the chemical processes used during or after synthesis, the surface of NCs can be functionalized with a variety of functional groups, including -OH, -COOH, and others [2]. Its abundant surface chemistry makes it simple to synthesize hybrid materials based on NCs that conduct electricity when combined with other materials like graphene, C60, conducting polymers, and other electrically conducting inorganic nanomaterials. Due to their distinct chemical makeup and superior mechanical qualities, NCs have also attracted significant interest in a number of other fields, such as food packaging, wastewater treatment, biomedical applications, sensors, flexible substrates, batteries, catalysis, and printed electronic devices [2,3]. NFC's high stiffness and strength make them a popular choice for composite reinforcement [4], and NFC-based composites or nanopapers can be made transparent, lightweight and flexible [5]. NCs containing conducting polymer [6], 2D materials [7,8], carbon materials [9] and metal nanowires (e.g. Ag and Cu) [10,11], can be used as an active component in flexible electronic and photonic devices.
Luminescent materials (LMs) are an important component of many optoelectronic devices, including light emitting diodes, solar cells, laser devices, photocatalysts, optical multiplexing devices, imaging devices, and sensors, to mention a few. Due to their unique opto-electronic capabilities [12,13] and extensive surface chemistry [2], carbon dots (CDs) made from green materials have recently drawn attention as LMs due to their potential use in bioimaging [14], water treatment [15], sensors [2], solar cells [12,13], supercapacitors [13], and catalysis [13]. CDs can be synthesized from biomass and their waste, plant extracts, amino acids, protein and others [13]. Owing to their low cost, high quantum yield, high stability, and low toxicity, CDs are potential candidates to replace phosphors and cadmium in white light-emitting diodes [12,13], solar cells [12,13], and water treatment [15]. Fluorescent CDs are also receiving increasing attention in various biomedical and chemical sensing applications [14].
The development of light emissive NCs-based nanopaper is potentially important for numerous applications including intelligent packaging [16], sensors [2], printed and flexible optoelectronic devices [16]. Regarding this, experiments on emissive and CD-contained NFC films have been conducted, mostly employing pressurized extrusion processes [17] and centrifugations [18]. To progress and prepare NFC-based nanopaper for industrial scale applications in printed and flexible optoelectronic devices, a sustainable roll-to-roll (R2R) compatible production method needs to be pursued. In this work, we investigate the synthesis of CDs from onion inner epidermal cell (OEC). Then, we demonstrate a R2R compatible simple roller-coating approach to fabricate CDs-containing blue emissive nanopaper. Our findings could potentially accelerate the advancement of green flexible and stretchable electronics, optical sensing, smart packaging, multifunctional transparent substrates and barrier layers [19].

NFC hydrogel preparation
NFC was prepared using TEMPO oxidation [20] and subsequent homogenization. First, 5 g Southern bleached softwood kraft from International Paper was stirred for 30 min in 500 ml deionized water. Then 0.08 g TEMPO, 0.5 g NaBr and 35 ml sodium hypochlorite (12.5 wt%) were added at room temperature. The mixture was then stirred for 72 h. The pH was maintained at 10.5 through frequent monitoring and the addition of 1 mol l −1 sodium hydroxide solution as needed. After the TEMPO oxidation treatment, the obtained aqueous suspension was passed twice through a high pressure (10 kpsi) nano homogenizer (Nano DeBEE) with a channel dimension of 65 µm. The homogenized NFC aqueous suspension was centrifuged for 20-30 min at 5000 rpm until assynthesized NFC was sedimented, and then the supernatant was decanted. The obtained NFC hydrogel was washed several times until the pH of the supernatant reached 8.

Synthesis of CDs
The CDs were synthesized by a solvothermal reaction. OEC were removed from an onion and 5 g were pulverized in a blender. The crushed OEC suspension was poured into a 100 ml autoclave, and 60 ml of dionized (DI) water was added to the suspension. An oven was used to bring the autoclave up to 180 • C, where it stayed for 8 h. The synthesis procedure is illustrated in figure 1. After it had cooled, the solution was filtered and placed in the refrigerator to be used later without being diluted or concentrated.

Preparation of CDs-NFC composite-based nanopaper
A variety of OEC-CDs solutions (2/5/10 and 15 ml) were prepared using the same stock solution without any dilution. Consequently, each OEC-CDs solution was mixed with 9 g NFC hydrogel, followed by stirring the mix solution for 10 min using a shaker. A relatively dried composite paste was obtained by vacuum filtration of the mixed suspension of OEC-CDs-NFC using 0.2 µm PTFE (Polytetrafluoroethylene) hydrophilic filter for few hours. Finally, the resulting paste was roller coated on the PET (Polyethylene Terephthalate) and glass substrate to obtained OEC-CDs-NFC nanopaper. The fabrication process of OEC-CDs-NFC composite pastes and nanopaper is shown in figure 2.

Characterization
The absorption and transmittance measurement were carried out with Cary 7000 UV-vis spectrophotometer in the 200-800 nm wavelength range at room temperature. The photoluminescence (PL) measurements were carried out using Horiba fluorescence spectrometer (QM-8450-22-C-Spectrofluorometer). A diode laser (340 nm) was used as the excitation source, with the laser beam directed at an angle of 30 • with respect to the detection plane. The CDs were characterized using FEI Tecnai G2 F20 field emission Transmission electron microscope (FE-TEM) at 120 KV accelerating voltage. scanning electron microscopy (SEM), Zeiss Gemini 500, was performed to analyze the morphology of the nanopaper.

TEM and optical characterization of CDs
The procedures for synthesis of OEC-CDs were shown in figure 1, and the detailed synthesis process are described in experimental section. The TEM image (figure 3(A)) shows OEC-CDs having spherical shape, with an average particle size of ∼15.7 nm. The UV-Vis absorption spectrum ( figure 3(B)) of OEC-CDs shows a miniature peak around 360 nm, a sharp peak at 280 nm, and a shoulder peak around 225 nm in accordance with previous report [21]. The PL spectrum (figure 1(C)) of OEC-CDs was recorded at an excitation wavelength of 360 nm. A strong blue emissive peak is observed at 410 nm as shown in figure 3(B). Figure 4 shows images of pure NFC and OEC-CDs-NFC-based nanopapers on glass substrates under room lighting and UV exposure. Figure 5 depicts the as-fabricated nanopapers on PET and free standing nanopaper.     The optical properties of free-standing nanopapers were investigated in order to determine light transmittance and emissivity. The transmittance spectra (figures 6(A and (B)) of pure and CDs-contained NFC nanopapers show a decreasing light transmittance with increasing concentration of the CDs. The transmittance was significantly reduced, especially below 400 nm, where CDs absorb light strongly. These findings suggest that the transmittance of CDs-NFC nanopaper can be tuned by varying the concentration of CDs based on a given application need. Other Studies [22,23] revealed that the transparency of the NFC-based nanopaper can also be tuned by homogenizing NFC at different pressures. Higher pressure causes cellulose fibers to become smaller, which decreases optical scattering and enhances the transparency of NFC nanopapers. One peak for the pure NFC-based nanopaper can be seen around 430 nm, with a shoulder peak appearing around 502 nm (figure 6(C)). The 430 nm peak corresponds to intrinsic emissive behavior of NFC [24]; while the peak ∼502 nm is due to the fluorescence properties of NFC (range of 500-600 nm) caused by structural property of the material and are independent of the presence of lignin [25].

Optical characterizations of pure NFC-and OEC-CDs-NFC based nanopaper
Within the sample, glycosides linkages can randomly generate unsaturated bonds, which could explain the NFC fluorescence. Fourier-transform infrared (FTIR) (figure 6(D)) was used to validate this bond in our NFC system. The broad regions 3600-3200 cm −1 corresponding to the vibration of -OH, and the band at 2900 cm −1 is caused by stretching vibration of −CH 2 and −CH 2 OH. The absorption peak at 1031 cm −1 is attributed to CO stretching at the C3 position in cellulose [25]. The minor peak at 895 cm −1 is due to the ß-glycosides linkages of glucose ring of Nano cellulose [25].

SEM characterizations of as-fabricated nanopaper
The surface and cross-sectional morphology of the nanopaper were investigated by SEM as shown in figure 7. Figure 7(A) shows SEM surface images of pure NFC and CDs-NFC nanopaper. The stacked cellulose fibrils are shown in the images in agreement with previous literature [20]. The cross-sectional SEM images ( figure 5(B)) indicate that the thickness of pure NFC nanopaper is ca. 100 µm and slightly less than that of CD-containing nanopaper. This slight increase in the thickness of the CD-NFC nanopaper over pure NFC paper is due to the higher viscosity of CD-NFC paste. Although the sample size used in the demonstration above is small, our approach is amenable to low-cost R2R large-scale production of emissive NFC nanopaper for a variety of flexible electronics and photonic applications [26].

Spray coating fabrication of pure NFC and OEC-CDs-NFC based nanopaper on rigid and flexible substrates
We extended our approach to fabricate NFC-CDs nanopaper film on glass and PET substrates using household spraying bottle. NFC-CDs water-based ink was prepared and multilayers were spray coated onto the substrates. Digital images of the fabricated films are presented in figure 8 thus demonstrating the versatility and simplicity of our approach. This approach will enable us to fabricate biocompatible luminescent nanopaper with minimal waste and fabricated over large area with low-cost coating approaches.

Conclusion
In brief, we successfully synthesized CDs from sustainable OECs and biocompatible NFC from softwood pulp. The absorption and emission spectra, as well as TEM analysis of as-synthesized OEC-CDs were carried. The experimental results indicate that CDs have absorption peaks at 230 nm, 280 nm, 360 nm, and an emission peak at 410 nm, with an average particle size of about 15 nm. Moreover, the nanopaper's transmittance and PL were measured. The surface and cross-sectional images of the nanopaper were examined using SEM. An average thickness of 100 µm was measured. Our work demonstrates a potentially scalable and simple method for producing light-emitting nanopaper based on biocompatible NFC and CDs, which could have an impact on the advancement of green materials-based smart packaging, sensing, and flexible electronic technologies in the future.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).