Cheap, Flexible, and Thermal-Sensitive Paper Sensor through Writing

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A Cheap, Flexible and Thermal-sensitive Paper Sensor through Writing with Ionic Liquids Containing Pencil Leads Saijun Sun, Zhilong Duan, Xun Wang, Gan Lai, Xinyue Zhang, Hao Wei, Lianhe Liu, and Ning Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08737 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

A Cheap, Flexible and Thermal-sensitive Paper Sensor

through

Writing

with

Ionic

Liquids

Containing Pencil Leads Saijun Sun,1 Zhilong Duan, 1 Xun Wang, 1 Gan Lai, 2 Xinyue Zhang, 1,* Hao Wei, 1 Lianhe Liu1 and Ning Ma 1,* 1

Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education,

College of Materials Science and Chemical Engineering, Harbin Engineering University, China E-mail: [email protected]; [email protected] 2

College of Underwater Acoustic Engineering, Harbin Engineering University, China

KEYWORDS. ionic liquid pencil leads, paper chips, thermal sensor, near-infrared light, writable ABSTRACT. The flexible and portable paper-based sensors have a broad potential application in electronic detection and devices. In this work, a flexible thermo-responsive paper sensor was reported by written on A4 paper with composite pencil leads which contain thermo-responsive pyrene-based ionic liquid [Pyrmim]+[Br]-. The [Pyrmim]+[Br]- was transferred onto A4 paper surface with graphite by pencil writing for the facile preparation of thermal-sensitive paper chips. The as-prepared paper sensor was much sensitive to the NIR irradiation and warm objects. What is more, the pliable paper chip also had the regular responses along with the varication of the folding angles, which could be employed for the angle goniometer of electronic robots.

ACS Paragon Plus Environment

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1. INTRODUCTION With the construction of intelligent society, flexible and portable sensors as the vital part have been applied to various areas, such as electronic displays,1-4 bionic robots,5,6 and health monitoring.7-9 More and more efforts are devoted to improving sensitivity and stability of the devices with lower cost and energy consumption, as well as easier processing and preparation. Among the multifarious substrates for the flexible sensor constitution, the simple and facile available paper holds the most promising future owing to its plentiful virtues,

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for instance,

porous structures for carrying much more responsive matter, abundant hydroxyl content in favor of well surface modification, and even less expensive compared to other substrates. Nowadays, paper chips based on carbon materials,12-15 semi-conductors,16-18 polymers19-22 as well as environmental sensitive compounds have been widely used for the commercial real-time detection and in-situ analysis of disease, water quality or poisonous gas. For example, Wang and coworkers reported a very simple method to prepare polypyrrole arrays for paper sensors via direct pen-writing and sequent chemical oxidation.20 Moreover, various techniques such as printing,23-25 photolithography26-28 and spin-coating29 have also been exploited for the functional paper-chip production. Such approaches have greatly reduced the cost and simplified the processes for the manufacture of flexible sensors. However, the necessity of special inks, instruments or templates has restricted their promotion to some degree. In spites of the high threshold for the above mentioned, directly writing sensing elements on paper via pencils is a high cost-effective method without any critical terms. 30-32 Graphite and clay are common elements for commercial pencil lead, and the proportion of graphite determines the blackness as well as conductivity of pencils. The calcination process under high temperature


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is essential as to the fabrication of pencil lead. Nevertheless, a majority of sensing materials would present instability or even thermolysis during the calcination, which has posed an obstacle to the promotion of pencil sensors. Among the sensing species, ionic liquids (ILs), as charming and green materials, have attracted numerous attentions, owing to their extraordinary properties33-35 as fluidity in wide temperature range, high conductivity with a wide electrochemical window, less volatility and certain stability even under high temperature. Moreover, recent researches have proved that ILs also hold the ultra-thermal-sensitive nature and been applied to thermal sensors.36,37 The fluent ILs are the optimal matters for the flexible device fabrication, while the fluidity would inevitably engender troubles like liquid leakage and swelling the holders as well. To combine ILs as thermal-sensitive elements with the mixture of graphite and clay for the pencil leads preparation, it would take three advantages at least: (1) The thermal-stability and less volatility of ILs ensure them free from decomposition or denaturation during the calcination process, which would be the ideal sensing substances for pencil leads. (2) The high temperature and pressure processes could enhance the interaction between ILs and other fillers, which would firmly conserve ILs in the matrix and effectively prevent liquid leakage. (3) More importantly, graphite as a black material possesses the ability to convert light over a wide wavelength range into heat that could directly induce the variation of ILs resistance for thermal sensing. In this contribution, we have improved the ingredients of pencil leads via doping a thermal-sensitive ionic liquid and named ionic liquid-pencil lead (ILPL). The specific ionic liquid contains the conjugated pyrenyl group that would show a strong interaction with graphite, and ensure the transfer stability of heat or electrons between the ionic liquid and graphite.38 Then the soft thermal-sensors are fabricated on A4 paper by simply writing sensing arrays in virtue of the


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customized pencils. Through regulating the contents of ionic liquid, 40 mg ionic liquid mixed in the pencil leads with a certain mass has presented the optimal responsive ability, and the paper chips could remain thermal-responsibility after more than 1000 cycles of near infrared (NIR) light irradiation. To connect the paper sensor with an electronic display, the real-time and precise temperature could be read, which almost agreed well with the measurement by an infrared camera. What is more, the flexible pencil-writing paper chip could keep considerable conductivity even under drastically folding, indicating its good stability as flexible sensors.

2. EXPERIMENT SECTION Materials. All of the commercial agents were used here without any purification. The pyrenebutyric acid and 1,6-dibromohexane were purchased from J&K Chemical Co., Ltd. N-methylimidazole was supplied by Acros. Graphite was offered by Tianshengda Co., Ltd. (Qingdao, China). All other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. Instruments. The 1H NMR spectrum was detected by the Bruker AV500M NMR Spectrometer. The TGA curve was obtained on a TA Instruments Q50 thermal gravimetric analyzer. SEM images were captured by the QUANTA-200 microscope with an acceleration voltage of 15.0 kV. UV-Vis spectra were recorded on a Persee TU-1901 spectrometer. Photographs were taken by a digital camera. Hardness measurement was occupied in the Shore Durometer (Guangzhou Landtek Instrument Co., Ltd). The NIR testing was operating on the CHI660 electrochemical workstation (Shanghai CH instrument Co., Ltd, China) with under the irradiation of an 808 nm laser source


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(HTOE, Beijing, China). A thermography camera (Fluke TiX560 9Hz Thermal Imager) was employed to detect the object surface temperature. Synthesis of pyrene-containing ionic liquid [Pyrmim]+[Br]-. A mixture of 1.44 g pyrenebutyric acid, 4.6 mL 1,6-dibromohexane and 1.50 g K2CO3 as a catalyst was dissolved in 40 mL acetone. Then the solution was stirred at 70oC overnight under refluxing. The intermediate was purified by column chromatography with CH2Cl2 as eluent. The purified intermediate (2.26 g) mixed with 1.25 mL 1-methylimidazole was added to 40 mL acetone. Ionic liquid [Pyrmim]+[Br]- was obtained after the quaternization reaction at 70oC overnight. The product of ionic liquid precipitated from the solution and was collected by filtration and washing with acetone for several times. Preparation of ionic liquid pencil leads. Firstly, different weight of [Pyrmim]+[Br]- (20 mg, 30 mg, 40 mg, 50 mg, 60 mg) was separately well dispersed in the distilled water (1.50 g) via ultrasound treatment for 30 min, and then blended with graphite (0.60 g), clay (0.50 g), carbon black (0.20 g), glycerol (0.10 g) and dimethyl silicone (0.10 g). Next, the dehydration process of the homogeneous mixture was occupied in a muffle furnace under 190oC for 3 h. Finally, a high pressure about 20 MPa was applied to the desiccated powder for molding the pencil lead in desired shape, as well as compressing the graphite and ionic liquid for commendable conductivity. Fabrication of the ILPL-based paper chips. Two conductive Ag electrodes were compactly smeared on the A4 paper with a fixed space about 5 mm. Just followed by drawing a straight line about 20 rounds between the formed electrodes under a steady force, the ILPL-based paper chip was readily prepared for the thermal detection.


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Thermal sensitive tests through the ILPL-based paper chips. The paper chip was connected with an electrochemical workstation for offering a steady direct-current power of 1.0 V. The I-t curve was collected via switching the applied NIR laser as setting the time interval as 60 s, while the thermography camera recorded the surface temperature of our paper chips.

3. RESULTS AND DISCUSSION To realize a stable and sensitive IL-based sensing, it is important and necessary to design and prepare a stable composite with graphite and ionic liquid components. As we know, polycyclic aromatic hydrocarbons (PAHs) possess the delocalized π-bond, which is just a comfortable match with graphite. The coexisting π-conjugated structure would reinforce the interaction force between PAHs and graphite to form a stable supramolecular composite. Herein, pyrenebutyric acid, as one of the typical compounds which could form strong π-π interaction with graphite, was applied to the preparation of PAHs based ionic liquids. A reasonable mass ratio of pyrenebutyric acid and 1,6-dibromohexane firstly reacted in acetone in the presence of basic carbonate at 70oC for an overnight refluxing. After the nucleophilic substitution reaction, the purified bromo-tailored intermediate was mixed and reacted with 1-methylimidazole to prepare the pyrenebutyric ester based imidazolium ionic liquid ([Pyrmim]+[Br]-) via a quaternization reaction (Figure 1a). Both the reactions were accomplished in a mild condition with a high yield. The 1H NMR spectrum of [Pyrmim]+[Br]- was shown in Figure 1b, the chemical shifts for imidazole and pyrene group are located at 6.8-7.2 ppm and 7.8-8.4 ppm, respectively. In the Figure 1b, the signal at 4.22 assigned to -OOC-CH2-CH2- adjacent to imidazole is observed,


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indicating that the formation of the structure of ILs after the reaction. The as-prepared ionic liquid was a yellowish powder at room temperature (the inset of Figure 1c), as well as presented a high thermal stability with the decomposition temperature at around 300oC, as measured by the TGA studies (Figure 1c).

Figure 1. The synthetic route (a), the 1H NMR spectrum (b), and the TGA curve (c) for [Pyrmim]+[Br]-.

An available and facile approach to fabricating ILPL was to mix a suitable amount of water, clay, graphite with the synthetic [Pyrmim]+[Br]-, followed by the molding press process under high pressure with a pelleting equipment and 190oC calcining for 3 h (Figure 2a). The asprepared pencil leads about 1 cm in diameter show an appearance of dark grey tablets with slight metallic luster (Figure 2b). The proportion of ionic liquid had a certain effect on the hardness, morphology and mechanical strength for the formed pencil leads. As shown in Figure S1, the hardness presented slight rising trends with the increase of ionic liquid contents (Figure S1a), while the different ILPLs showed similar morphologies under scanning electronic microscopy (SEM) observation (Figure S1b-f). The layered structure of graphite was free from any


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destruction when composited with [Pyrmim]+[Br]- ionic liquid (Figure 2e, 2f), which would guarantee the conductivity of graphite. Porous structure of A4 paper made a convenience for loading and supporting [Pyrmim]+[Br]- and graphite from the pencil leads, which can form a continuous, firm and well conductive film on the paper substrates (Figure 2c). What is more, this strong absorbability ensured that any arbitrary pencil sketches like a cute panda could be easily painted on A4 paper (Figure 2d).


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Figure 2. The fabrication and characterization of ILPLs. (a) The schematic procedure for the preparation of ILPL and ILPL-based paper chip. The photograph for the as-prepared pencil leads with different ionic liquid content (b), and a panda drawn on the paper (d). The scanning electron microscope (SEM) images for the A4 paper with (left) and without (right) pencil-trace (c), the morphology of ILPL without any ionic liquid (e) and containing 40 mg ionic liquid (f). Employing an ionic liquid pencil lead, to scrawl a line on paper between two Ag electrodes with a fixed distance of about 5 mm, was an effortless and effective approach for preparing thermal-sensitive ILPL-based paper chips (Figure S3a). To ensure that a certain residual mass of the pencil trace was transferred onto the A4 paper surface, lines on paper were written in the similar strength and same repeat counts during the writing preparation. The absorption spectra of pyrene-containing ionic liquid and ILPL are shown in Figure S2. It can be seen that the [Pyrmim]+[Br]- ionic liquid does not show any absorption to near-infrared (NIR) light, while the ILPL film exhibits a wide absorption to the range of visible and NIR lights, including NIR light of 808 nm. The black graphite is skilled in absorbing NIR light, then quickly transferring the absorbed NIR to heat. The supplementary heat would accelerate migration of the doping ionic liquid and arouse the resistance alteration for ILPL-based paper chips, sequentially. Therefore, for the non-contact and quantitative control of heat release, the thermal-sensing ILPL-based paper chip could be easily triggered and make a sensitive response to a NIR irradiation of 808 nm. The photographs of thermal-sensitive ILPL-based paper chip and the NIR testing installation in this experiment were shown in Figure S3. As studied by the electrical resistance measurements, the formed thermal-sensing paper chips could make a sharp current increasing upon the NIR laser irradiating due to the sensitive thermoresponsiveness of [Pyrmim]+[Br]- ionic liquid. The ionic liquid contents embedded in the pencil lead, was one key factor for the thermal responsive behaviors. As shown in Figure 3a, under the same NIR power, paper sensor with the pencil lead containing 40 mg [Pyrmim]+[Br]- alone appeared a prominent current increasing, which was 4-5 times higher than those with ionic liquid


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contents of 20 mg, 30 mg, 50 mg and 60 mg. This phenomenon could be attributed to the following reasons. The thermo-responsiveness of the paper chips is an integrated result, which involves conductivity and quality of the chips. With the increasing [Pyrmim]+[Br]- ionic liquid contents, the integrated thermal responsiveness was firstly enhanced due to the increased content of responsive ionic liquid. When the content was further increased to such as 50 mg and 60 mg, the responsiveness was weakened for the formation of defects and non-continuous microstructure with more ionic liquids during writing, which greatly influenced the electric conductivity and thermal conductivity. For this reason, the chips written by the pencil leads containing 40 mg [Pyrmim]+[Br]- ionic liquids were mainly investigated due to the optimal conductivity and responsiveness. It should be noticed that curves in Figure 3a are normalized for better comparison of the thermal responsiveness of these pencil leads. As a contrast, the graphite paper chip with a common pencil lead without any ionic liquid, expressed a low signal noise ratio of the thermal response (Figure S5). The occurrence of graphite could prompt the transformation from the NIR light into heat, enhancing the conductivity of graphite desultorily. On the account of the regular temperature-dependent for the ionic liquid conductivity, as well as the strong π-π interaction between [Pyrmim]+[Br]- and graphite, the mixture would have a positive improving on the signal resolution. Due to the applied NIR power determining on the heat import, it was another influence on the current magnitude of the IL-based paper sensors. A higher NIR power would work up a stronger thermal response. Followed by the NIR power increasing from 0.20 W to 0.40 W, the surface temperatures of the paper chips detected by the infrared thermal camera were increasingly altered from 39.1oC to 75.5oC. At the same time, the thermal responses of the ILPL-based paper chip containing 40 mg [Pyrmim]+[Br]-, almost exhibited a linear growth from 1.38% to 7.17%


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with increasing NIR power (Figure 3b). What is more, the average response of this ILPL-based paper chip under 0.30 W irradiation, remained stability approximate 3.5% between the on-off state of the NIR laser for about 100 cycles, showing a potential application for the practical thermometer (Figure 3c). In addition, additional experiment was carried out to investigate the performance of our paperbased sensor at temperatures below the room temperature. In the follow-up experiment, we choose 0oC as the experiment temperature. The paper chip experienced several cycles between 0oC and 35oC (the temperature was detected by IR camera) to study the responsive behavior in a cooling condition, instead of heating or NIR irradiation. Since the cooling process is relatively slower than heating, a longer cycle time of 3 min was chosen. As shown in Figure S4, although the cyclic curve exist some small floating due to our cooling experiment conditions, however, the sensing behavior of our paper chip proves that this kind of paper chip can work at lower temperatures. The current decreased during cooling process and increased upon heating, which agree very well with the results under the condition of heating or NIR irradiation.


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Figure 3. The responsive behavior of the ILPL-based paper chips. NIR responses for the paper chips with different ionic liquid contents (20 mg, 30 mg, 40 mg, 50 mg and 60 mg), under the same irradiation power of 0.30 W (a). NIR responses for the ILPL-based paper chip containing 40 mg ionic liquid, under different irradiation power (0.20 W, 0.25 W, 0.30 W, 0.35 W and 0.40 W) (b). NIR sensing of the ILPL-based paper chip irradiation containing 40 mg ionic liquid for 100 cycles under the 0.30 W irradiation (c). The explication for the nature of ionic liquid conductivity is still in doubt. One typical theory is the hopping-mechanism39 which defines holes acting as charge carriers are derived from the relative thermal motion of ionic groups at room temperature. The size and location of vacancies spread randomly for the irregular ionic thermal migration. In quick succession, both cations and anions tend to hop towards the vacancies without a dominant direction (the top of Scheme 1). When ionic groups are placed in an electric field, the equilibrium of electric neutrality would be disturbed. As to maintain the neutrality of thus ionic liquid structure, cations would directionally hop to the cathode, while anions were inclined to hop to the anode. The generated current from the orientable hopping would transmit in the direction of the extra electric field, which is the essence of the ionic liquid conductivity (the middle of Scheme 1). As to this ionic liquid-graphite composite, the [Pyrmim]+[Br]- groups fit closely with the graphite substrate due to the strong π-


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π interaction between the two components. Triggered by the NIR laser, the black graphite could rapidly transfer the absorbing light to heat, which would elevate its surface temperature. The internal motion of ionic liquid would be accelerated by the accessorial heat, thereby leading to the magnification of apparent current, which was the NIR-sensing mechanism of our ILPL-based paper chips (the bottom of Scheme 1).

Scheme 1. A schematic diagram for the thermal-sensitive mechanism of the ILPL-based paper chip. In order to achieve the portable application to our daily life, an electronic display that could directly read the practical temperature in a real-time way, was connected with the ILPL-based paper chip to study the thermal responsiveness as a thermometric sensor. Surface temperature of the paper chip with 40 mg [Pyrmim]+[Br]- doping was proportional to the applied NIR power, and especially, appeared well linear fitting when the laser power ranged from 0.20 W to 0.45 W (Figure S6). Herein, the stability fitting relationship between the response current of ILPL-based


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paper chip and practical temperature of the chip surface, is the basic guarantee for programming the linked electronic display. Ionic liquid holding the excellent capacity of rapid response with thermal variation, real-time temperature detection of the ILPL-based paper chip could be achieved along with altering the NIR power (Movie S1). The infrared thermal camera could precisely and promptly record the object surface temperature via a sensitive thermal detector. However, the exorbitant price of thus infrared thermal camera has set a prodigious jam for its promotion. Fortunately, under three laser power, each temperature reading on the linked display was 16oC, 52oC and 70oC, respectively (Figure 4a, 4c and 4e), and the simultaneous monitoring by the infrared thermal camera was just 17.9oC, 51.8oC and 69.8oC, respectively (Figure 4b, 4d and 4f). The estimated value from such two sensors acted much approximating to each other. Hence, our inexpensive ILPL-based paper chip could be suitably qualified to the accurately thermal measurement. Furthermore, the flexible nature of the paper chip endowed the ability to well contact with non-flat objects, such as a curving surface, like a beaker filled with warm water. The surface temperature was 31oC tested by the ILPL-based paper chip (Figure 4g), which gave a minor error to the result about 31.6oC from the infrared thermal camera (Figure 4h).


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Figure 4. Thermal detection of the ILPL-based paper chips with 40 mg [Pyrmim]+[Br]- doping. The surface temperature read by ILPL-based paper chips connected with an electronic display (a, c, e), and an infrared thermal camera (b, d, f) with increasing the NIR power, respectively. The surface temperature of a beaker filled with water detected by our ILPL-based paper chip (g), and the infrared camera (h), respectively. The ILPL-based paper chip also exhibited the high sensitivity to folding processes, which could be potentially used as an electronic goniometer. An ILPL-based paper chip was well fixed on a relative stiff paper, followed by folding the paper under different angles (Figure 5a). In this experiment, angles of 180o and 0o meant the open state and the close state, respectively. The ILPL-based paper chip was firstly closed in a step-by-step way with included angles of 180o,


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120o, 90o, 60o and 10o (blue area), and then the chip was reversely opened with included angles of 10o, 60o, 90o, 120o and 180o (red area). The current for this ILPL-based paper chip showed a slight increase accompanied with the closing process and the current kept stable when the folding angle was fixed at a certain value (Figure 5b). Because each angle corresponded to a certain current value, the electronic goniometer could be readily acquired by set up the relationship between folding angle and sensing current. Furthermore, the formed ionic liquid containing pencil leads chip also had an intense adhesion on the porous paper. It can be found that the luminance of a LED light linked to our paper chip was well maintained when the paper chip experienced from the open state (Figure 5c) to the folding state (Figure 5d).

Figure 5. The folding sensing with the flexible ILPL-based paper chip. The photograph (a) and the current curve of a paper chip under varied folding degree (b). The images of a LED light linked to the paper chip in the open state (c) and the folding state (d), the voltage applied on the LED is 2.2 V.


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The responsiveness to folding of the ILPL paper chip can be attributed to following reasons. On one hand, during the folding process, graphite slice at the folding point separated from the paper substrate and experienced squeezing by the folding process. This squeezing enlarged the contact area of the graphite components and resulted in the decrease of resistance during the folding process (included angle from 180o to 0o). On the other hand, upon unfolding, the graphite components were no longer squeezed and there were lots of defects and cracks formed in the pencil trace film, which lead to an increased resistance of the paper chip, as shown in Figure S7b and S7c, while the pencil trace film show a uniform surface before folding (Figure S7a). For this reason, the current value decrease with the increase of the included angle during the unfolding process.

4. CONCLUSION In conclusion, we have successfully developed a pencil-writing method to conveniently fabricate the thermal-sensitive ILPL-based paper chip. The synthetic [Pyrmim]+[Br]- had an intense interaction with the graphite substrate, due to the π-π conjugated force between the pyrene group and the graphite layer. The as-prepared ionic liquid pencil leads could scrawl on paper smoothly, which has opened a cheap, facile and adjustable approach to the flexible as well as portable paper sensors. The ILPL-based paper chip showed rapid sensitivity of the non-contact NIR light. An electronic display was also linked to the paper chip for directly and precisely reading the surface temperature of a warm object. By folded the paper chip gradually, the current of thus chip varied regularly along with the folding angle, which would have a possible potential application for the angle goniometer. The improvement or modification of the used ionic liquid


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would prompt this pencil-writing method for wider application in the sensing field. Moreover, by means of the printing technique, the sensors would become more controllable in production, more complicated in patterning and more accurate in detection.

ASSOCIATED CONTENT Supporting Information. The basic characterization of ILPLs (hardness, SEM observation, UVVis absorption), and supplementary studies on the ILPL-based paper chip (photograph for NIR test installation, responsive behavior under a cooling condition, photo-thermal conversion and SEM observation of the chips before and after folding). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: E-mail address: [email protected] (X. Zhang). E-mail address: [email protected] (N. Ma). Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS


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This work was supported by the National Natural Science Foundation of China (21374009) and the Fundamental Research Funds of the Central University (HEUCFJ171003).

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Cheap, Flexible, and Thermal-Sensitive Paper Sensor through Writing

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