Flexible and Reusable Non-woven Fabric Photodetector Based on Polypyrrole/Crystal Violate Lactone for NIR Light Detection and Writing
Rapid NIR light detection and/or writing has drawn much attention, but their practical applications have been limited by obtaining such NIR photodetectors. To address this problem, we have developed a simple and versatile strategy to prepare a non-woven fabric photodetector. The blue non-woven fabric photodetector has been prepared by coating photo-thermochromic ink (including crystal violet lactone (CVL) as the thermo-sensitive dye, polypyrrole (PPy) nanospheres as the photothermal component and hydroxyethyl cellulose (HEC) as the polymer matrix) on white non-woven fabric. When the blue fabric photodetector is irradiated by NIR (808-nm as model, 0.75 W cm−2) laser, the discoloration occurs in 35 s, and higher laser intensity confers more rapid discoloration. This discoloration results from the photothermal effect of PPy which confers the elevation of temperature (> 50 °C) and then converts CVL to its leuco form (colorless). When the laser is turned off, the temperature drops to below the transition temperature (< 43 °C), and then CVL reverts to its initial blue color. Moreover, different figures and images can be easily printed on the fabric photodetector by 808 nm laser, and then they can be erased automatically under ambient conditions, with excellent cycling stability. Therefore, this fabric photodetector may act as a new platform for rapid NIR light detection and writing.
KeywordsNon-woven fabric Photodetector Photothermal effects NIR light detection
Photodetectors are the devices that convert the optical signals into electrical signals or pixels, and they have been widely used in various applications such as optical communication, biomedical science, target acquisition and night vision [1, 2, 3, 4]. Currently, different photodetectors have been developed, and they respond to different light including ultraviolet (UV), visible (Vis) and near-infrared (NIR). Among them, NIR photodetectors have attracted increasing interests, since NIR light is abundant in the solar spectrum and importantly it is not easy to be perceived by the human eye. Conventional NIR photodetectors can be categorized into bulk semiconductors (such as CuSbS2 , InAsSb , and GaSb ) and nanomaterial detectors (such as 0D quantum dots , 1D nanowires , and 2D sheets ). These NIR photodetectors exhibit high responsivity, speed, efficiency, and broad detection wavelength. However, they suffer from some limitations, such as excessive costs, complex preparation processes and complicated equipment that lead to high energy consumption . Therefore, it is still necessary to develop new NIR photodetectors that are low-cost and are easy to be directly observed by eyes.
Smart fabrics with rewritable features are an emerging field, and they have drawn significant interests owing to their potential benefits such as resource sustainability and mainly performing additional functions that conventional fabrics cannot. Several smart fabrics with color response have been prepared, including SiO2-naphthopyran fabrics , epoxy-modified thermochromic fabric  and reversible thermochromic fibers . Nevertheless, they suffer from some drawbacks, including photoinstability, the need for direct heating, and lack of printing/erasing features. To solve these problems, photoreversible color switching systems (PCSS) have been proposed, and they are generally composed of metal oxides as photocatalytic agents, redox dyes as reversible color indicators, and polymers as the matrix. When the PCSS are irradiated by UV/visible light, metal oxide nanomaterials in the PCSS produce photogenerated holes which are captured by the sacrificial electron donors (SEDs), whereas the remaining electrons rapidly reduce the dyes to their leuco form. The recoloration process is predominantly triggered by O2 in the presence of heat or visible/NIR light in some cases. Several color switching media have been reported such as TiO2-based [15, 16], SnO2–x-based , Au-based  and WO3-based [19, 20]. However, the successful application of PCSS has been mostly in rewritable papers and smart inks. In addition, most of these systems rely on ultraviolet or visible light for writing and direct heating for erasing which is detrimental to the polymer substrate . To improve the performance, our group has prepared the smart fabric by coating TiO2–x/Dye/HEC (smart ink) on cotton fabric, which can remotely and rapidly respond to 365 nm for writing and 808-nm for erasing . However, this system still requires a light source for recoloration. In some cases, there is a need to come up with new systems that can re-color automatically without the need for a light source. If such systems are conveyed on a non-woven fabric to prepare a photodetector, they can be applied in many fields, such as visual sensors, wearable displays and secure communication.
It is well known that thermo-sensitive dyes can change color through several mechanisms including reversible interconversion of isomers, molecular rearrangement, molecular orientations and changes in the crystal packing [22, 23]. Usually, the color of thermo-sensitive dyes can disappear when heated above the transition temperature, and re-appear when the temperature is below the transition temperature. For example, crystal violet lactone (CVL) is a typical three-component thermochromic dye, including color former, color developer and solvent . The color transition temperature of CVL can be adjusted in the range of 30–70 °C by the solvent used and/or molar ratios of components [24, 25, 26]. In addition, our group has demonstrated that some NIR photothermal nanomaterials (such as CuS [27, 28], W18O49  and Polypyrrole (PPy) ) can efficiently convert NIR laser into heat for many applications (such as the phototherapy of tumors, NIR-shielding film). Among them, PPy is very attractive due to their advantages including broad NIR photoabsorption range, high stability and photothermal effect [31, 32]. Inspired by thermo-sensitive dyes and NIR photothermal effects of PPy nanomaterials, herein we have proposed a prototype of flexible and reusable non-woven fabric photodetector. With CVL as a model of dye, PPy nanomaterials as photothermal agent and hydroxyethyl cellulose (HEC) as the polymer matrix, we have developed a photo-thermochromic ink which is used to coat on non-woven fabric. The blue PPy/CVL/HEC-coated non-woven fabric photodetector exhibits excellent and rapid NIR (808-nm as model) light detection abilities. Upon irradiation of 808-nm laser (0.75 W cm−2), it changes color from blue to colorless in 35 s. When 808-nm laser is turned off, it reverts to the original color under ambient conditions in 60 s. Especially, figures/images can be remotely printed on the surface of the fabric by 808-nm laser and then erased automatically under ambient conditions.
Iron chloride hexahydrate (FeCl3·6H2O, 99%), pyrrole monomer (98%), cetyl alcohol, bisphenol A and sodium dodecyl sulfate (SDS) were received from Sino pharm Chemical Regent Co., Ltd (China). Polyvinyl alcohol (PVA) and hydroxyethyl cellulose (HEC) was purchased from Aladdin (Shanghai) Co., Ltd. Crystal violet lactone (CVL) was purchased from Anhui Kuer Biological Engineering Co., Ltd (China). The non-woven fabrics were purchased from Nantong Jikang Non-woven fabric Co., Ltd (China). Other chemicals are commercially available and were used without further purification.
Synthesis of PPy Nanospheres and CVL
PPy nanospheres were prepared by a modified one-step method . In a typical process, FeCl3 (2.3 mmol, 0.622 g) and PVA (0.8 g) were added into 30 mL deionized water, and the solution was stirred for 1 h in an ice bath (~ 5 °C). Then, pyrrole monomer (1 mmol, 70 μL) was added into the above solution to react for 4 h. Then, PPy sample was centrifuged and washed for three times with distilled water.
Crystal violet lactone (CVL) was synthesized by a typical method . Briefly, cetyl alcohol (20 g) was heated at 50 °C to melting state, and then leuco crystal violet lactone (colorless, 0.2 g) and bisphenol A (3 g) were added into the above solution. The resulting CVL solution was continuously stirred at 90 °C for 1 h. After cooling down to room temperature, CVL powder was obtained, and it exhibited an intense blue color.
Preparation of PPy/CVL/H2O ink
To investigate the photodetection properties in liquid form, PPy/CVL/H2O ink was prepared in a two step method. The first step was to prepare the CVL solution by adding blue CVL powder (0.5 g) and SDS (0.01 g) into distilled water (20 mL), and then the solution was heated to 55 °C for 1 h under magnetic stirring at 2000 rpm. In the second step, an aqueous solution (1 mL) of PPy dispersion (60 µg mL−1) was then added into the above CVL solution (5 mL). Then, the resulting solution was stirred for 20 min, and 3 mL solution was transferred into a glass cuvette for subsequent tests. To discolor, the PPy/CVL/H2O ink was irradiated by a NIR light (808-nm as the model, 0.75 W cm−2) for 0–12 s. To recolor, the glass cuvette was allowed to cool in ambient conditions. The absorption spectra of solution were recorded during the discoloration and recoloration process by UV–vis-NIR spectrophotometer (Shimadzu UV-2550).
Construction and Color Switching of Non-woven Fabric Photodetector
Non-woven fabrics were tailored to have the sizes of 6 × 6 cm2. To prepare the viscous ink, as-prepared PPy/CVL/H2O ink (6 mL) was mixed homogeneously with HEC solution (4 mL, 33.33 mg mL−1) under magnetic stirring for 15 min. Then the PPy/CVL/HEC ink was coated on the surface of the non-woven fabrics by doctor’s blade method. The resultant fabric was dried at room temperature for 12 h, forming a smooth, flexible and dark blue colored fabric.
To investigate the photodetection properties, the non-woven fabric was irradiated with 808-nm laser (0.75 W cm−2), and its photoabsorption spectra were recorded. To monitor the reverse color transition, the fabric was allowed to recolor under ambient conditions. To investigate the photothermal effects, the non-woven fabric photodetector was irradiated with 808-nm laser with various intensities (0.25, 0.50, 0.75 and 1.00 W cm−2), and their corresponding temperatures were real-time recorded by using a thermographic camera (FLIR-A300, FLIR Systems Inc.). To study the rewritable features of the non-woven fabric, figures were printed by irradiating the fabric with 808-nm laser through photomasks with various designs.
Characterization and Measurement
The sizes and morphologies were examined by scanning electron microscope (SEM, Hitachi S-4800) and/or transmission electron microscopy (TEM, FEI Talos F200S). Fourier transform infrared (FT-IR) spectrum was performed by IRPrestige-21 spectrometer (Shimadzu). The hydrodynamic diameter of PPy nanospheres was measured by a Zetasizer Nano ZS (Malvern). The thermal analysis of CVL was measured by differential scanning calorimeter (DSC) (Mettler-Toledo, Switzerland). Photoabsorption spectra of PPy suspension and PPy/CVL/H2O ink were recorded on a UV–vis-NIR spectrophotometer (Shimadzu UV-2550). The diffuse spectra of the non-woven fabric were measured by a UV–vis-NIR spectrophotometer (Shimadzu UV-3600) and a white standard of BaSO4 was used as a reference.
Results and Discussion
Blue non-woven fabric photodetector has been prepared by coating PPy/CVL/HEC ink on white non-woven fabric. Under the irradiation of 808-nm laser (0.75 W cm−2), the blue PPy/CVL/HEC-coated non-woven fabric rapidly changes to a greyish color in 35 s due to the photothermal effect of PPy. This effect confers the rapid elevation of temperature (> 50 °C) and then converts CVL to its leuco form (colorless). When the laser is turned off, the temperature drops to below the transition temperature (< 43 °C), and then CVL reverts to its initial blue color. Moreover, various figures and images can be easily printed on the fabric photodetector by 808-nm laser, and then erased automatically under ambient conditions, with excellent cycling stability. Therefore, the present non-woven fabric with rewritable features has great potential to be applied in areas such as anti-counterfeit technology, visual sensors and secure communications.
This work was financially supported by the National Natural Science Foundation of China (51773036 and 51972056), Shanghai Shuguang Program (18SG29), Natural Science Foundation of Shanghai (18ZR1401700), Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-03-E00055), the Fundamental Research Funds for the Central Universities, and DHU Distinguished Young Professor Program.
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