Article pubs.acs.org/acssensors
Ultrafast Paper Thermometers Based on a Green Sensing Ink Xinglei Tao, Hanyu Jia, Yonglin He, Shenglong Liao, and Yapei Wang* Department of Chemistry, Renmin University of China, Beijing 100872, China S Supporting Information *
ABSTRACT: With the use of an ionic liquid as the ultrathermosensitive fluid, a paper thermometer is successfully developed with intrinsic ability of ultrafast response and high stability upon temperature change. The fluidic nature allows the ionic liquid to be easily deposited on paper by pen writing or inkjet printing, affording great promise for large-scale fabrication of low-cost paper sensors. Owing to the advantages of nonvolatilization, excellent continuity and deformability, the thermosensitive ink trapped within the cellulose fibers of paper matrix has no leakage or evaporation at open states, ensuring the excellent stability and repeatability of thermal sensing against arbitrary bending and folding operation. By shortening the heat exchange distance between ionic liquid and samples, it takes only 8 s for the thermometer to reach an electrical equilibrium at a given temperature. Moreover, the paper thermometer can be applied to remotely monitor temperature change with the combination of a wireless communication technology. KEYWORDS: thermometer, ionic liquid, paper sensor, foldable, thermal imaging
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of solvents. Ionic liquids have recently been exploited as a new family of thermal-sensitive fluids for flexible and self-healing electronics.15−19 With proper selection of cation and anion species, ionic liquids could be given with tunable electrical conductivity, viscosity, freezing point, and wettability, offering great promise for facile deposition of sensing circuits on paper matrix by fluent writing or printing technique.20 Additionally, negligible volatilization of ionic liquids allows them to be stable in atmosphere for long-term use.21 To prevent unfavorable leakage from functional devices, many efforts were devoted to encapsulating ionic liquids in microchannels or microchambers for sensing applications. However, complicated procedures involved in the design of microchannels or microchambers give rise to high cost. On the other hand, these external packages impede thermal exchange between ionic liquids and external environment, significantly prolonging the response time and impairing the sensitivity during measurement process.16,17 Herein, an ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][Tf2N]), was transferred to regular A4 paper by means of pen writing or inkjet printing, generating a low-cost and paper-based thermometer with ultrafast response. Notably, the ionic liquid can fully penetrate into the paper substrate and shows no leakage during bending and folding processes relying on the capillary effect. The nonvolatile and hydrophobic nature of the ionic liquid ensures the high stability of paper thermometers during longterm use in air. The inherent advantages of paper, including high flexibility and foldability, afford great promise for applications in multidimensional thermal imaging and practical
aper-based electronics are attracting great attention in research fields of personal health monitor systems, energy storage, sensors, flexible displays, and field effect transistors, owing to their intrinsic advantages of low cost, light weight, and high flexibility.1−6 Paper sensors, as one of the most extensively investigated elements, lay a firm foundation on collecting and magnifying task-specific information from ambient environment. With increasing efforts on the development of paper sensors, various sensing materials serving as active layer of paper sensors have been exploited, including inorganic semiconductors,7 conducting polymers,8,9 and nanocarbon materials.10−12 These solid sensing materials have been fully developed due to their advantages of rapid fabrication, facile modification, diverse combination, and scalable production. However, solid sensing materials deposited on paper substrates meet problems of electrical continuity damage and device failure led by material fracture and detachment, which result from intensive friction, repeating folding, or over-threshold bending.13 Deposition of active materials on paper by means of writing or printing to prepare low-cost electronics is also challenging. Nowadays, growing interest is given to liquid sensing materials on paper substrates, benefiting from not only excellent deformability at arbitrary bending configuration, but also unique adhesion due to the capillary affinity by tremendous cellulose fibers in paper.4,14 In this regard, liquid sensing materials are ideal alternatives for the development of paperbased sensors with high throughput if they are combined with easy deposition techniques. To choose suitable liquid materials as sensing inks on paperbased matrix, four factors including moderate viscosity, poor volatilization, good stability, and high sensitivity to specific stimulation should be taken into consideration. So far, few materials can fulfill these requirements without the participation © 2017 American Chemical Society
Received: February 1, 2017 Accepted: March 2, 2017 Published: March 2, 2017 449
DOI: 10.1021/acssensors.7b00060 ACS Sens. 2017, 2, 449−454
Article
ACS Sensors
Figure 1. Fabrication and thermal sensing property of pen-written ionic liquid paper chip. (a) Schematic illustration of the fabrication process of pen-written ionic liquid paper chip. (b) Optical images of ionic liquid based paper chip. The distance between two Au electrodes is fixed as 0.5 cm. The yellow dotted box region marked in (b) is magnified as an SEM image in (c). (d) Molecular structures of ionic liquid [EMIm][Tf2N] serving as ink. (e) On−off cycles of thermal response of paper chip operated between 45 °C and room temperature (25 °C). The diameter of the pen tip is selected as 0.5 mm.
Figure 2. Optimization of thermal sensing properties and ultrafast thermal response of ionic liquid paper chip. (a) Time-dependent diffusion ranges of ionic liquid trace written on A4 paper with different pen tip diameters. The diameters of pen tip are chosen as 0.3, 0.5, 0.7, and 1.0 mm. (b) Timedependent thermal responses of ionic liquid paper chip written with four different pen tip diameters. Testing temperatures were fixed as 45 and 25 °C. The space between Au electrodes is fixed as 0.5 cm. Inset SEM image shows the pen tip of rollerball pen with diameter of 0.5 mm. (c) Timedependent thermal responses of paper chips with four different spaces of 0.25, 0.50, 0.75, and 1.00 cm. Testing temperatures were fixed as 45 and 25 °C. The diameter of pen tip is 0.5 mm. (d) On−off cycles of thermal response of paper chip operated between 45 °C and room temperature (25 °C). Red dotted box in (d) is enlarged in (e) and blue dotted box is enlarged in (f). (g) Thermal response time of ionic liquid paper chip for different testing temperatures difference. (h) Relationship between thermal responses and temperature differences of ionic liquid paper chip. Theoretical equation of fitting curve is marked with a blue box. (i) Long-term thermal sensing stability of ionic liquid paper chip.
use on mobile handsets. By means of inkjet printing, paper thermometer arrays could be scaled to realize real-time
monitoring of temperature and potential thermal imaging on a smart phone through wireless communication. 450
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gold electrodes shows no influence on the thermal response of paper thermometers, as shown in Figure 2c. In this regard, the width and the length of ionic liquid lines are not involved in determining the thermal sensing properties of paper thermometers, which only relies on the intrinsic thermal sensitivity of ionic liquid. To further evaluate the thermal response of paper thermometers, the response time and the correlation of temperature with conductivity change were quantified. As seen in Figure 2d, choosing the diameter of pen tip as 0.5 mm and the space between two electrodes as 0.5 cm, the sensor is excellently stable and repeatable under periodical heating and cooling. The conductivity change remains at 60.31% ± 0.02% for more than 14 cycles between 45 °C and room temperature (25 °C). As magnified in Figure 2e and f, the conductivity increases once the sensor is exposed to high temperature and it rapidly reaches equilibrium. Accordingly, the response time against thermal heating is about 8 s, regardless of the difference of conductivity change at different testing temperatures (Figure 2g). This pen-written paper thermometer is much faster for thermal sensing than the previous ionic liquid-based sensors which take a few minutes to reach thermal equilibrium. Such a rapid thermal response is attributed to the decrease of heat exchange distance by direct contact between ionic liquid and samples. The change of conductivity as a function of temperature was perfectly established to fit the Vogel− Tamman−Fulcher (VTF) equation.24
In terms of convenient processing and cost saving, pen writing is the preferred choice to deposit ionic liquid on paper directly. As illustrated in Figure 1a, two gold electrodes (5 mm wide, 1.5 cm long) were deposited on A4 paper through magnetic sputtering evaporation setup and pure [EMIm][Tf2N] was refilled into an ink-free ball pen. A straight line of ionic liquid was written between two gold electrodes on a piece of A4 paper. The cellulose fibers in the region written with ionic liquid form a denser network, strongly distinguishing from the adjacent area without added ionic liquid (Figure 1c). [EMIm][Tf2N] was chosen as the ink with respect to its low viscosity and hydrophobic property in comparison with other ionic liquids (Figure 1d), which facilitates the writing process while resists the hydration by moisture in atmosphere.22 Connecting two electrodes with an external power supply, the ionic liquid line exhibits a better electrical conductivity and shows ultrasensitive thermal response upon externally environmental temperature change. It should be noted that the conducting performance is only contributed by ionic liquid line because natural paper is electrically insulated with a sheet resistance of over 1015 Ω sq−1.4,23 To quantify the sensing ability of this ionic liquid-based paper thermometer against temperature change, the thermal response was represented by the relative change of conductivity ΔG/G0 (%) as in eq 1: ⎛ I − I0 ⎞ ΔG /G0 = ⎜ ⎟ × 100 ⎝ I0 ⎠
(1)
σ(T ) = σ∞ exp(−B /(T − Tv ))
where G0 and I0 are the initial conductivity and current at room temperature, respectively, and I refers to the real-time current in measurement process. As shown in Figure 1e, a typical thermometer consisting of paper, gold electrodes, and ionic liquid line (Figure 1b) was tested for repeated thermal sensing under alternating heating and cooling. Remarkably, thermal response of paper thermometer reaches 60.5% by increasing the ambient temperature from room temperature (25 °C) to 45 °C and the result is reliable as it keeps unchanged after multiple cycle of heating and cooling (Figure 1e). Two factors including the diameter of pen tip and the space of two gold electrodes were evaluated to confirm their impact on the resistance and the sensing ability of ionic liquid lines. To fully comprehend the likely influence caused by fluid, the diffusion behaviors of ionic liquid on the paper has to be investigated, which may also interfere electrical conductivity and thermal sensing property. Four pen tips with different diameters, including 0.3, 0.5, 0.7, and 1.0 mm (Figure S3), were selected to study diffusion behaviors and thermal response of ionic liquid ink. As shown in Figures 2a and S1a, with diameters of 0.5, 0.7, and 1.0 mm, the diffusion distance in the lateral direction and the resistance of ionic liquid lines kept increasing over time while thermal response remained almost unchanged (Figure 2b). Notably, the thermal response of paper thermometers is not relevant to the diameter of pen tip. However, the paper thermometer written by the pen with a tip diameter of 0.3 mm exhibits poorer sensing ability in comparison with other paper thermometers written by pen with larger tip diameter. Moreover, the thermal response of paper thermometer written with pen tip of 0.3 mm drops even to zero with increasing the exposure time in ambient environment. We assume that the ionic liquid ink written on paper by the pen with tip diameter of 0.3 mm becomes less and less continuous to keep enough electrical continuity owing to the diffusion behavior. Additionally, the space between two
(2)
where σ(T) is the electrical conductivity of IL at a certain temperature T, and factors including σ∞, B, and Tv are constants. Based on the VTF equation, the conductivity change referring to thermal response can be expressed by temperature change, σ (T ) ΔG = −1 G0 σ0 B ⎡ ΔG ⎢ (T0 − Tv)2 + 1 = exp⎢ 1 1 G0 ⎣ T0 − Tv + ΔT
(3)
⎤ ⎥ ⎥ ⎦
(4)
As shown in Figure 2h, a fitting curve between the thermal response and temperature essentially satisfies with the eq 4, which is useful to accurately estimate the temperature in the presence of a specific conductivity change. It should be noted that no decomposition of the paper thermometer happens and the thermal response is maintained within a steady range at a given temperature difference for over one month and even longer (Figure 2i), which promises long-term service in open systems. The intrinsic fluidic nature of ionic liquid ensures that the paper thermometers can be operated under mechanical deformation without destroying their conducting and sensing performance, including bending or folding operation. As shown in Figure 3a, the paper thermometer is flexible to be bent with different angles. There is less than 8.00% increase of the resistance when two Au electrodes are moved close to each other (distance between two electrodes ∼0 cm) from originally stretching state. We suppose that the little resistance change is attributed to the weakened electrical contact among cellulose fibers loaded with ionic liquid at bending state. Notably, the paper thermometer can endure bending for many times that its 451
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liquid were manually written on the other side and ensured to penetrate through the paper to connect with gold electrodes. Lastly, another eight parallel gold electrodes were deposited on top of ionic liquid pixels, crossing over previous eight electrodes on the opposite side. When a heated metal rod is placed close to the sensing array at a specific position, the sensor is able to output a thermal mapping of temperature distribution (Figure 4d), which well matches the vis-IR photos captured with a digital camera (Figure 4b) and an infrared camera (Figure 4c). Superior to infrared cameras which can only read surface temperature in one direction, the paper thermometer with sensing arrays can simultaneously detect the temperature of curved surfaces. For example, surface temperature of a handlike model and a dice with six faces was measured by using the infrared camera; however, only one specific side of these samples could be measured. Although other sides could be measured by turning over the object or changing the view direction of camera, it is still impossible if the samples are placed in narrow spaces or special areas (Figures S5 and S6). The ease of bending and folding allows the paper thermometer to surround the object, which is more convenient to read temperatures from all directions. As shown in Figure 4e, a temperature thermometer with sensing array was bent and attached on the hand model. More than reading the temperature of palm, the back of the hand was also spatially monitored in real time. Additionally, folding the thermometer to match the shape of the dice, several dice faces could be monitored simultaneously (Figure 4j). It is believed that the high flexibility of paper sensors is also adaptable to more complex shapes via desirable folding manipulation. To fully develop the potential of scalable production of paper thermometers, inkjet printing technique is adopted in the fabrication of large-scale sensing arrays on paper substrate depending on its inherent advantages including noncontact generation of task-specific patterns on large areas, free pattern design on a personal computer, and labor-saving production.29−32 As shown in Figure 5a, a word “Sensors” was successfully printed by ionic liquid ink, which was doped with Rhodamine B to highlight the printed pattern on the paper substrate. Achievement of inkjet printing of ionic liquid on paper makes it possible to prepare larger integrated sensing arrays. Moreover, urgent demands on real-time and remote monitoring system push the combination of paper thermometer arrays with wireless communication.33−35 As shown in Figure 5b, a mobile phone was connected with a paper sensing array through wireless communication. With converting the real-time resistance changes into digital signals, the calculated temperature of sensor could be displayed on the mobile phone over a wireless Bluetooth communication link. Notably, owing to the ultrafast sensing ability of paper thermometers, the subtle temperature change could be detected with ultrahigh sensitivity when a finger was close to the paper thermometers (Movie S1). Ultrafast thermal response and wireless communication give the paper thermometer the ability of ultrasensitive and real-time thermal imaging on smart phone. In conclusion, we have successfully developed a kind of ultrasensitive paper thermometer with the use of ionic liquid as a sensing ink through facile pen-writing or inkjet printing technique. Ionic liquid based paper thermometer exhibits ultrafast and stable thermal response to multiple heating− cooling processes, showing no dependence on diffusion range and resistance of ionic liquid on the paper matrix. Moreover, the paper thermometer not only possesses long-term stability
Figure 3. Thermal sensing properties of ionic liquid paper chip at bending or folding state. (a) Cyclic test of resistance variations of ionic liquid paper chip at bending or originally flat state. Inset is a schematic illustration of bending state of ionic liquid paper chip. The original distance between two electrodes is 1.0 cm. (b) Resistance variations of ionic liquid paper chip with different fold number, including original flat state, 1 fold, 3 folds and infinite folds. (c) On−off thermal responses of ionic liquid paper chip between bending and original state. (d) On−off thermal responses of ionic liquid paper chip with different fold numbers. Insets show ionic liquid paper chips with infinite folds. Testing temperatures of both tests were fixed as 45 °C and room temperature (25 °C).
resistance remains unchanged after alternative bending and stretching for 110 cycles. In addition to bending, the paper thermometer can also be folded without leaching out any ionic liquid. As shown in Figure 3b, the resistance of paper thermometer only increased 1.92% and 6.69% corresponding with folding numbers of 1 and 3, respectively. Even seriously crumpling up the sensor with generating infinite folding number, the increase of resistance is still below a reasonable value (20.50%, as seen in Figure 3b). It is assumed that folding operation may cause further diffusion of ionic liquid in the lateral direction at the crease places and thus lead to the increase of resistance. The high flexibility endows paper thermometers with the capability of testing temperature change of subjects with nonplanar shapes. As shown in Figure 3c, with periodical heating and cooling, thermal responses of the paper thermometer for the same temperature show nearly the same value, ignoring its bending or flat state. As a practical example, a cylindrical metal rod with high surface temperature was successfully monitored by wrapping a paper thermometer on it. Moreover, the thermal sensing performance is also independent of the folding operation. As illustrated in Figure 3d, thermal responses of paper thermometers with different folding numbers are consistent, promising a long-term utilization without worrying about possible damages by nonprofessional manipulation. Temperature sensing array is a significant component for facilitating its spatially resolved applications in two-dimensional monitoring of temperature field, medical imaging, and human skin.25−28 As a principle demonstration, we fabricated an 8 × 8 temperature sensing array on an 8 × 8 cm2 paper substrate, with each pixel size controlled as 25 mm2. The fabrication process of pen-writing paper thermometers with ionic liquid arrays is shown in Figure 4a. First, eight parallel gold electrodes were deposited on a piece of A4 paper. Then, 64 pixels of ionic 452
DOI: 10.1021/acssensors.7b00060 ACS Sens. 2017, 2, 449−454
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Figure 4. Fabrication and multidimensional thermal imaging of ionic liquid paper chip with 8 × 8 pixel array. (a) Schematic illustration of fabrication process of ionic liquid paper chip with 8 × 8 pixel array. Optical images of a paper chip with 8 × 8 pixel array contacted with a heated metal rod (b), a simulated human hand (e), or a partially heated dice (h). IR images recorded by an infrared camera exhibit surface temperature of paper chip arrays after contacted with a heated metal rod (c), a simulated human hand (f), and a heated dice (i). Two dimensional thermal imaging of paper chip array after contact with a heated metal rod (d), a simulated hand (g), and a heated dice (j).
tions in multidimensional thermal imaging and temperature sensing matrix. Printing technique could be also employed in the preparation of paper sensors, indicating possibility of largescale production. It should be emphasized that our device exhibits great potential to the applications of soft robotics, artificial skins, and medical implants, where flexible temperature sensors are highly desired.
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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/acssensors.7b00060.
Figure 5. (a) Photographs of printed patterns on an A4 paper with use of ionic liquid. The blue dotted box marked in the left photo is magnified as printed word of “Sensors”. (b) Photograph of mobile phone running the temperature measuring software, continuously displaying the incoming data stream from the paper thermometer.
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Experimental details; time dependent resistance plots; TEM images; cyclic and temperature variations; IR images (PDF) Video showing real-time and wireless temperature change monitoring with paper thermometer (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
but also offers an attractive solution to eliminate the influence of bending and folding operation. The prominent sensing performance and flexibility allow for their successful applica-
ORCID
Yapei Wang: 0000-0001-5420-0364 453
DOI: 10.1021/acssensors.7b00060 ACS Sens. 2017, 2, 449−454
Article
ACS Sensors Author Contributions
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The manuscript was written with the contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21422407, 51373197, and 21674127) and the Open Fund of the State Key Laboratory on Integrated Optoelectronics (Grant No. IOSKL2015KF23).
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