Sensitive and Fast Humidity Sensor Based on A Redox Conducting

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Sensitive and Fast Humidity Sensor based on a Redox Conducting Supramolecular Ionic Material for Respiration Monitoring Hailong Yan, Li Zhang, Ping Yu, and Lanqun Mao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04350 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016

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Analytical Chemistry

Sensitive and Fast Humidity Sensor Based on A Redox Conducting Supramolecular Ionic Material for Respiration Monitoring Hailong Yan,†,‡ Li Zhang,†,‡ Ping Yu,†,‡,* and Lanqun Mao†,‡ †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical

Chemistry for Living Biosystems, Institute of Chemistry, the Chinese Academy of Sciences (CAS), Beijing 100190, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

*

Corresponding Author. E-mail: [email protected], Fax: +86-10-62559373. 1

ACS Paragon Plus Environment


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ABSTRACT Real-time monitoring of respiratory rate (RR) is highly important for human health, clinical diagnosis and fundamental scientific research. Exhaled humidity-based RR monitoring has recently attracted increased attention because of its accuracy and portability. Here, we report a new design of an exhaled humidity sensor for the real-time monitoring of the RR based on a synthetic redox conducting supramolecular ionic material (SIM). The humidity-dependent conducting SIM is prepared by ionic self-assembly in aqueous solutions of

electroactive

2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic

acid)

(ABTS)

and

1,10-bis(3-methylimidazolium-1-yl) decane (C10(mim)2). By taking full advantage of the high hygroscopicity and water stability arising from the ionic and hydrophobic interactions between two building blocks (i.e., ABTS and C10(mim)2), the SIM-based humidity sensor exhibits both high sensitivity (less than 0.1% relative humidity) and fast response time (ca. 37 ms). These excellent properties allow this humidity sensor to non-invasively monitor the RRs of not only humans but also rats that have a much faster RR and much smaller tidal volume than humans. Moreover, this sensor could also be efficiently used for the real-time monitoring of the recovery process of rats from anaesthesia.

2


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INTRODUCTION As one of the four most important human vital signs (i.e., pulse rate, blood pressure, respiratory rate [RR] and body temperature), respiration is an intrinsically essential life event in which gases are exchanged with the external environment and the body’s normal activities maintained. In this context, the development of new devices capable of the real-time monitoring of the RR is highly important for human health and clinical diagnosis.1-3 To date, the continuous monitoring of respiratory function has been mainly restricted to haemoglobin-oxygen saturation (SpO2) data derived from pulse oximetry, thermistor and surface acoustic waves, and piezoelectric transducers.4-7 However, measuring SpO2 is inadequate when supplemental oxygen is used, and the need for expensive instrumentation limits the use of the other methods mentioned. In this context, monitoring the humidity of exhaled air has recently been introduced as a new strategy to determine the RR because of its accuracy, portability and reliability.8 To this end, a humidity sensor with a fast response time and high sensitivity is very essential. Among the transductions employed as humidity sensors, electrical humidity sensors have attracted increasing interest because of their high sensitivity and selectivity and relative simplicity. Although the transduction mechanisms of different materials vary, the material’s hygroscopicity is a key property and is directly related to the sensitivity of the resulting humidity sensors.9-13 Thus, the sensitivity of such humidity sensors can be improved by increasing the surface area of the materials (i.e., increasing the hygroscopy) by preparing microporous, one-dimensional (1D) or 2D materials.14-26 However, the relatively slow sorption/desorption rate of water molecules (i.e., humidity) results in slow responses, ranging from seconds to minutes. Although the response time of these humidity sensors could be improved using graphene oxide20 or supramolecular nanofibres21-22 as sensing materials, designing a new type of material with both high hygroscopicity and fast 3



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sorption/desorption rate of water molecules is highly desirable for sensitive and fast humidity sensing, and further for efficient exhaled air-based RR monitoring. Ionic compounds normally exhibit high hygroscopicity because of their strong polarization.27-32 However, traditional ionic salts (e.g., calcium chloride, magnesium chloride, and zinc chloride) readily become deliquescent because of their strong affinity for moisture, which invalidates their application as humidity sensors.33 To avoid this issue, some types of polymers (typically, Nafion) have been used to develop humidity sensors with a fast response time and high sensitivity because of their hygroscopicity and poor deliquescence. However, the relatively large driving potential (e.g., 5 V) results in poor stability because of the oxidation of the electrode (e.g., Au).34 In this case, a new type of water-stable ionic material able to conduct carriers (ions/electrons) at a low bias potential is urgently needed for RR monitoring. Motivated by this need, we are interested in preparing water-stable supramolecular ionic materials by ionic self-assembly from an aqueous medium.35-40 Our previous attempts have revealed that water-stable supramolecular ionic materials (SIMs) based on an imidazolium-based dication (e.g., 1,10-bis(3-methylimidazolium-1-yl) decane, C10(mim)2) and electroactive dianionic (e.g., 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), ABTS) with good water stability, adaptive encapsulation capability39 and solvent stimuli-responsive properties can be prepared.40 Interestingly, herein, we find that solid-state supramolecular materials created via ionic self-assembly show humidity-dependent conductivity. A SIM with this property, combined with its water stability, would constitute a strong basis for constructing ultrasensitive and ultrafast humidity sensors for RR monitoring at a relatively low polarized potential (i.e., 0.5 V). These excellent properties would support the application of the presented SIM-based humidity sensors, which should be particularly useful for monitoring the RR of both humans and living rats. To the best of our knowledge, 4


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this study is the first to real-time investigate RR monitoring based on redox conducting supramolecular materials and should open a new pathway for the development of gas sensors.

EXPERIMENTAL SECTION Chemicals. diammonium

2,

salt

2’-Azino-bis was

purchased

(3-ethylbenzothiazoline-6-sulfonic from

Sigma-Aldrich

and

used

acid)

(ABTS)

as

supplied.

1-Methylimidazole, 1,10-dibromodecane, KPF6, LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, CuCl2, NaCl, KCl, KNO3 and K2SO4 were all obtained from Aladdin Reagent Co. (China) and used without further purification. Preparation of Materials. According to our earlier work,39 the SIM was synthesized by mixing an aqueous solution of the hydroxide-exchanged form of C10(mim)2 (2 mM) with an equal amount of ABTS (2 mM) under stirring for 2 h at room temperature to produce a white precipitate. The precipitate was washed several times with doubly distilled water, collected by centrifugation, and dried in a vacuum drying oven. The ionic material C10(mim)2-PF6 as a control was obtained by mixing an aqueous solution of C10(mim)2 (2 mM) with an equal amount of KPF6 (2 mM) under stirring for 2 h to produce a colorless precipitate, then washed several times with doubly distilled water Characterization. The morphology and size of the materials were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800) at an accelerating voltage of 15 kV. The PXRD patterns were measured on a D/max2500 with a Cu Kα source (λ = 1.541 Å). Electrical measurements were obtained using a CH Instruments model 660B electrochemical analyser (Shanghai, China). Thermal gravimetric analysis (TGA) was performed on a Netzsch STA 409 PC/PG thermal analyzer. The SIM powder was deposited in an alumina crucible and heated in a continuous flow of N2 with a ramp rate of 10 °C/min 5



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from room temperature to 800 °C. All electrical data were acquired in a relatively quiet environment to avoid unnecessary interference in the tests. Fabrication of the Humidity-Sensing Device. First, 5 mg of the SIM was dispersed in 2 mL of H2O to form a homogenous dispersion. Then, 0.1 mL of the dispersion was drop-coated onto an Au interdigital electrode (IDE), which was first dried in air and then annealed at 60 °C for 4 h under vacuum, cooled in air. The dimensions of the Au IDE, which was purchased from Jihua Co. (Siping, China), were as follows: 300-µm electrode width and 100-µm electrode separation. Twelve pairs of electrodes were printed onto a ceramic substrate (15 mm × 10 mm × 1 mm). Prior to the modification, the IDE was clean in piranha solution followed by washing in distilled water several times, and then dried by nitrogen flow. The humidity was controlled using different solutions saturated with LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, CuCl2, NaCl, KCl, KNO3 and K2SO4 and was simultaneously monitored by a commercial hygrometer (DT-615, Shenzhen, China). The gas sensing characteristics of the humidity-sensing devices were determined via the relevant i-t curve measurements conducted at different RH condition under room temperature. The sensitivity (S) was calculated by ∆I/I0, where ∆I was the amount of current change induced after exposure of the device to the humidity atmosphere and I0 was the initial current under nitrogen atmosphere. The fabricated devices were stable over ten months when stored under ambient conditions. The pellet was prepared by the mechanical compression of 18 mg SIM solid at the pressure of 20000 psi, and then peeled off, and copper wires were then used as contacting electrodes with conductive silver paste. Real-time RR Recording of SD Rats. Adult male SD rats were purchased from Vital River Co (Beijing, China), weighing 350 to 400 g. The animals were housed on a 12:12 h light-dark schedule with food and water ad libitum. We performed all experiments during the light cycle. The animals were anaesthetized with chloral hydrate (dissolved in normal saline 6


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solution with a concentration of 10% by mass) via intraperitoneal administration (400 mg/kg, i.p.). After the loss of righting reflex, the animals were carefully lied on a heated platform (37 °C), and positioned onto a stereotaxic frame finally. Then a bare or SIM-modified device was kept a fixed distance of 1 cm from the anaesthetized rats’ mouths to record the RR. The online recording of amperometry was made without any more artificial stimulation and obstacle to rat’s respiratory behavior during anesthesia. The sampling interval and applied voltage was set to 0.1 s and 0.5 V, respectively. In addition, anaesthetized rats were observed for at least one additional hour to record any abnormal behavior until recovery, along with counting the number of thoracic movements for 1 minute as respiratory rate every 5 minutes. Appearance of righting reflex was defined as the time when the animals first righted themselves. When the recovery signs came, the recording for anaesthetized rats would stop. The awakened rats were then put back into the cage and had access to water and food through the follow-up rehabilitation process. All procedures were approved by the Beijing Association on Laboratory Animal Care and the Association for Assessment and Accreditation of Laboratory Animal Care and performed according to their guidelines.

RESULTS AND DISCUSSION Structure of the SIM. To prepare a SIM by ionic self-assembly, ABTS dianion, a reversible and fast redox probe used in electrochemical and biochemical reactions,41 was chosen as the anion building block, and the germinal imidazolium dication C10(mim)2 was selected as the cation building block because of its relatively large size and long hydrophobic carbon chain (Figure 1a). As shown in Figure 1b, the self-assembled supramolecular material exhibits a typical hexagonal microstructure, and its size ranges from several to tens of micrometres, suggesting the formation of a SIM with a 2D microstructure. Such a structure would be particularly useful for the development of highly sensitive humidity sensors.19 The 7



b

a ABTS N -

O3 S

N

S

S

N N

SO 3-

C10(mim)2 N

1 µm

N

d

Ι (4.55%)

100 80

N

85%

60 40

95%

c

Intensity

N

Weight / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ΙΙ

75% 63% 55%

20

24% 0 5

100 200 300 400 500 600 700 800

10

15

20

25

30

2θ

T / ˚C

Figure 1. . (a) Chemical structures of the dianion ABTS and the dication C10(mim)2. (b) SEM image of the self-assembled hexagonal SIM. (c) TGA curve of the SIM powder. Process Ι, dehydration of structural water; and 4.55% means the percentage change of structural water in mass of SIM. Process ΙΙ, degradation under high temperature. It can be thermally stable up to 350 °C. (d) PXRD patterns of the SIM under different RH levels, as indicated in the figure.

involvement of the hydrophobic interaction imbues the as-prepared SIM with stability in aqueous solution, which is a challenging and vital property for humidity sensing based on ionic compounds. Moreover, thermal gravimetric analysis (TGA) curve for dry SIM show its excellent thermostability up to 350 °C (Figures 1c). In detail, dehydration of structural water in SIM start at 100 °C, and 4.55% means the percentage change of structural water in mass of SIM, which is in agreement with the empirical formula (C10(mim)2·ABTS·2H2O). This result suggests a strong interaction between the SIM and the water molecule, indicating that the SIM is hygroscopic. Therefore, the adsorption property of SIM for water molecules provides the basis for the resulting material’s humidity-dependent conductivity, vide infra. In 8


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addition, the powder X-ray diffraction (PXRD) data collected at different humidity levels shows no obvious shift, suggesting the structural stability of the SIM over a range of humidity levels (Figure 1d). This material’s good water stability, 2D microstructure, strong interaction with water molecules (i.e., hygroscopicity) and structural stability make it a strong candidate for RR monitoring via sensitive and fast humidity sensing, as demonstrated below. Humidity-Dependent Electrical Conductivity of SIM Pellet. Before fabricating the SIM-based humidity-sensing device for RR monitoring, this material’s humidity-dependent conductivity was explored. The SIM’s solid-state electrical properties were investigated using a sandwich method, as shown in Figure 2a. Typically, the SIM pellet was prepared by mechanically compressing the sample into pellets with a length of 5 mm, a width of 2 mm and a thickness of 1 mm. Both sides of each pellet were attached to copper wire (1 mm in diameter) with silver conductive paste, creating electronic conductors on the surfaces of the pellet. Figure 2b shows typical I-V curves of a SIM pellet under dry N2 (black curve), 11% RH (red curve) and 23% RH (blue curve) humidity atmospheres. Under a N2 atmosphere, the SIM exhibits a very low conductivity (i.e., 1.03×10-13 S/cm), which is similar to that of an insulator. This results is understandable since the SIM was lack of localized π electrons, and the relatively large size of C10(mim)2 is difficult to accommodate as counterion for the electron self-exchange of ABTS in the solid state. When the SIM was exposed to a humidified atmosphere (e.g., 11% or 23%), its electronic conductivity sharply increased. This conductivity increase was resulted from ionization of the adsorbed water molecules, and further assistance the electron self-exchange of ABTS. Moreover, another ionic material (i.e., C10(mim)2(PF6)2) without electroactive building block shows the relatively weak dependence of the electronic conductivity on the humidity level (Figure S1), essentially supports the redox-based mechanism proposed here. This humidity responsive property 9



could be further used to development of the exhaled-based RR monitoring as demonstrated below.

a

60

Silver Paste 2mm

5mm

b

30

1mm

I / pA

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Copper Wire

0

N2

-30

11% 23%

-60 -0.10

-0.05

0.00

0.05

0.10

E/V

Figure 2. . (a) Schematic illustration (upper panel) and picture (lower panel) of the cuboid pellet with a length of 5 mm, a width of 2 mm and a thickness of 1 mm, which was prepared by the mechanical compression of 18 mg SIM solid at the pressure of 20000 psi. (b) I-V curves of the SIM pellet under different atmospheres at room temperature: N2 (black curve), 11% RH (red curve), 23% RH (blue curve). Scan rate, 1 mV s-1.

Humidity Sensing. In order to systematically explore the performance of SIM, gold interdigital electrode was utilized here (Figure S2). To test the humidity-sensing device, a glassy humidity chamber was designed to accurately control the atmospheric humidity with two switches for nitrogen flow and a tiny fan motor to accelerate the dispersion of the water vapour (Figure 3a). Before the experiments, the device was purged with nitrogen for 30 min to remove any traces of moisture impurity in the system. A commercial sensor was used as a benchmark for the RH response. When the thin film device was exposed to low-moisture air (11% RH) for 12 cycles, relative to nitrogen at a bias voltage of 0.5 V, the current through the sensor promptly increased and then gradually reached a fairly stable value. When the sensor was switched to nitrogen again, the current abruptly decreased and rapidly reached a relatively stable value, indicating that the SIM-based humidity sensor exhibits excellent reproducibility and reversibility (Figure 3b). On the other hand, the bare IDE device does not 10


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show any obvious current change because there is no active site for H2O binding.

60

a

b

SIM Device

RH Inlet

━ SIM

45

I / pA

N2 Inlet

Outlet

━ Bare

30

15

0 0

600

1200

1800

2400

3000

t/s

50 24000

40

10 0

12000

6000

4

Log R

20

d 18000

R ( I/IN2)

30

c

I / nA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 2 1

20

40

60

80

RH / % 0

400

800

1200

1600

2000

20

t/s

40

60

80

RH / %

Figure 3. (a) Digital photograph of the home-made set-up for in vitro humidity detection. (b) Current response at 0.5 V of bare (black curve) and SIM-modified (red curve) IDE devices to dynamic switching (n = 12) between N2 and 11% RH at room temperature, respectively. (c) Current response obtained using the SIM-modified device during continuous switching between dried N2 and various RH levels of 11% (wine curve), 23% (navy curve), 33% (yellow curve), 43% (magenta curve), 52% (cyan curve), 67% (blue curve), 75% (red curve), and 86% (black curve). (d) Plot of the current change as a function of the RH. The red line is the fitted curve on the natural exponential scale. Inset, linear calibration curve of the current change as a function of the RH.

Moreover, as the RH increased, the current increased exponentially (Figures 3c). The ratio of the current increase (R) was thus used to quantify the humidity by the following equation: R = (IRH-IN2)/IN2 = ∆IRH/IN2 where IN2 is the initial current response recorded in the N2 atmosphere, and IRH is the current response at each different RH value. As the RH increased, the logarithm of the ratio of the

11



current increase (R) increased linearly, with a dynamic linear concentration range from 11% to 86% (lg(R) = 0.0426RH + 0.7085, r2 = 0.997) over which the current changed by more than 4 orders of magnitude (Figure 3d). The detection limit (S/N = 3) was lower than 0.1%, confirming that the SIM-based humidity sensor had a high sensitivity and a low detection limit, which present the opportunity for trace moisture measurement in complex systems.

1.25

a

0.75

1

37 ms

0.50

152.7

152.9

153.0

20 10 0

0.01

152.8

30

0.1

0.25 0.00

40

b I / nA

1.00

I / µA

Normalized Current

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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450 500 550 600 650

t/s

━ Fresh Preparation ━ After 10 months 0

200

400

600

t/s

t/s

Figure 4. (a) Typical current response of the present humidity-sensing device at bias potentials of 0.5 V. (b) Typical amperometric response at 75% RH obtained for a freshly prepared device (blue curve) and after the device was kept at room temperature for 10 months (red curve).

In addition, as one of the important factors for evaluating the performance of humidity sensors, the response time was measured under a pulsed flow of humid air as the time taken to reach 90% of the peak value starting at a value of 10% (Figure S3). The results portrayed in Figure 4a show a response rate of about 37 ms with a good recovery time and an ultimate 100% reversibility. Compared with previously reported humidity sensors based on other materials or techniques (Table S1), the present sensor exhibits a high sensitivity and a fast response time at a relatively low operation voltage (i.e., 0.5 V). In addition, after storing the SIM-based humidity sensor at room temperature for 10 months, no obvious change in the current responses was observed, confirming its high stability (Figure 4b). We have evaluated its accuracy and practical functionality for humidity detection (Figure S4), demonstrating its 12


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applicability in many cases. The high sensitivity, fast response time and good reproducibility and stability verify that the SIM-based humidity sensor is a good candidate to efficiently monitor the RR of both humans and anaesthetized rats.

18

a

15

b

RH 65% N 2

I / nA

12

N 2

9

CO

O

2

2

6 3

A

+

–

0 0

500 1000 1500 2000 2500

t/s

120

100

c

d 80

90

RH (%)

RH (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

60

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30 0

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30

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t/s

0

10

20

30

40

50

t/s

Figure 5. .(a) Schematic illustration of a volunteer breathing trial. (b) Amperometric responses of the present device to the common gases in breath, as indicated in the figure. (c, d) Current responses to the RH of the breath exhaled from the mouth cavity (c) and nasal cavity (d) over 10 breaths.

Monitoring Human Breathing. Because of the fast response and high sensitivity of the SIM-based humidity sensor, its applicability for monitoring human RR was first explored based on exhaled humidity (Figure 5a). Before the breathing trial, the selectivity was determined by exposing the sensor to the main components of respiratory gas, such as nitrogen, oxygen, carbon dioxide and moist air. Among these components, only moist air resulted in an increased current response; under the same conditions, the other vapours caused no response (Figure 5b), validating the SIM-based humidity sensors’ appropriateness

13



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for RR monitoring. The sensor successfully recorded ten current peaks induced by respiratory moisture from a health adult male when the sensor was held at a distance of 4 cm from the mouth cavity. As shown in Figure 5c, the RH sensor’s signal sharply rose during exhaling and then dropped to its ambient value during inhaling, consistent with normal breathing cycles. The SIM-based humidity sensor could also be used for monitoring the RR based on the gas exhaled from the volunteers’ nasal cavity, as confirmed by recording 10 breaths and observing one peak corresponding to one exhale (Figures 5d). These results demonstrate the ability of the SIM-based sensor to monitor the RR of humans, which could be useful for patient health care monitoring. Real-time RR Monitoring of Anaesthetized Rats. To further study the applicability of the present SIM-based humidity sensor, the real-time RR monitoring of anaesthetized rats was investigated. The RR of rats (i.e., 80~200 min-1) is almost ten times faster than that of humans,49 and the humidity change is very small. Moreover, real-time RR monitoring is very important to not only reflect the degree of anaesthesia but also monitor the animal’s physiological condition. Figure 6 shows the typical current change during the real-time RR monitoring of anaesthetized Sprague-Dawley (SD) rats with the SIM-based humidity sensor. After being treated with chloral hydrate dissolved in saline solution at a dose of 400 mg/kg, the rats gradually lost their independent consciousness and were positioned on a stereotaxic frame without any unnecessary constraint. The sensor was held at a distance of 1 cm from the anaesthetized rats’ nasal cavities. Compared to the bare IDE (Figure 6a, black curve), the SIM-based sensor exhibited a noticeable and repetitive current signal corresponding to the respiratory pattern under narcotism (Figure 6a, red curve). The typical current-time curve continuously recorded the RR of an anaesthetized rat, clearly demonstrating that the SIM-based sensor could be used for the real-time and non-invasive RR monitoring of anaesthetized rats. 14


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160

a

b

━ Modified

Air

rat’s breathing

5s

140

RR / min-1

An anesthetized

40 pA

━ Bare

120

100

━ Signal ━ Counting

80 0

15

30

45

60

75

t / min

c

0.4%

42

1%

3s

39

2%

3s

36

3s 3%

RH (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3s

33

30

0

1000

2000

3000

4000

t/s

Figure 6. . (a) Respiratory signals obtained at bare (black curve) and SIM-modified (red curve) IDEs for an anaesthetized SD rat. Inset, digital photograph of an anaesthetized rat. (b) Comparison of the RRs estimated by counting the number of thoracic movements per minute (red curve) and recording the signal using the SIM device (black curve) from the anaesthetized stage to revival. Bias voltage, 0.5 V. (c) Real-time monitoring of the revival process of an anaesthetized rat. Inset, the amplified responses in four different states (indicated by arrows) over 30 s.

To investigate the capability of this SIM-based sensor for real-time RR monitoring during different physiological processes, the RRs during different revival processes from anaesthesia in SD rats were monitored and recorded. As shown in Figure 6b, the RRs of anaesthetized rats were continuously recorded until the animals woke up. Anaesthetized rats exhibit different physiological states during their revival processes, as demonstrated previously.42-44 The different peak heights and frequencies corresponded to the different

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physiological states that occur during the general metabolism of the anaesthetic agent (Figure 6b), confirming that the sensor reported here can successfully record the changes in the physiological states resulting from treatment with an anaesthetic agent in real time, even for small tidal volumes and fast RRs. Moreover, the recorded RRs clearly revealed four different stages in the revival of the anaesthetized rats, as indicated by the arrows in the figure: 76 min-1, 92 min-1, 112 min-1, and 144 min-1. By counting the number of thoracic movements at 5-min intervals, we explored the real-time signal accuracy of the RR based on the SIM-based sensors. The time-dependent plots of the RRs revealed good agreement between the current signal and the counted thoracic movements throughout the recording period (n = 3, Figure 6c), confirming the efficiency of the SIM-humidity sensors for RR monitoring of anaesthetized rats. These results indicate that the SIM-based humidity sensor is particularly useful for non-invasively monitoring the RR without restraining or hindering access to the anaesthetized rats during narcosis. In addition to its miniaturized size, the SIM-based sensor is a promising new platform for ultrahigh-sensitivity real-time monitoring of a physiological indicator of respiration with high temporal resolution for patients in clinical, surgical, and postoperative contexts.

CONCLUSIONS The SIM-based humidity sensor developed here exhibits fast and sensitive responses and could be used for real-time RR monitoring of both human and anaesthetized rats. This rational design of the SIM structure constitutes a clear basis for the device’s humidity-sensing property, which makes the resulting sensors particularly useful for RR monitoring. We strongly believe that the present SIM-based sensors could be useful for the real-time monitoring of a physiological indicator of respiration with high temporal resolution in patients in clinical, surgical, and postoperative contexts by combining flexible substrate 16


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materials and a wireless technique. This study not only opens a new pathway for the design of novel types of gas sensors based on solid-state redox conducting materials but also provides a new concept for the development of organic solid conducting materials.

ACKNOWLEDGEMENTS This work is financially supported by NSF of China (Grant No. 21621062, 21435007, and 21210007 for L. Mao, 21322503 and 21475138 for P. Yu), and the National Basic Research Program of China (2013CB933704 and 2016YFA0200104), and the Chinese Academy of Sciences.

ASSOCIATED CONTENT Supporting Information. Additional experimental data including the electronic behaviour of C10(mim)2PF6, the image of the IDE, and the comparison with reported humidity sensors. This material is available free of charge via the Internet at http://pub.acs.org.

AUTHOR INFORMATION Corresponding Author *

[email protected]

Notes The authors declare no competing financial interests.

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Sensitive and Fast Humidity Sensor Based on A Redox Conducting

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