A resistive sensor for humidity detection based on cellulose/polyaniline

Abstract

Ambient humidity is an important parameter that affects the manufacturing and storage of several industrial and agricultural goods. In the view of the Internet of Things (IoT), single sensors could be associated with an object for smart monitoring enabling optimum conditions to be maintained. Nevertheless, the production of cost-effective humidity sensors for indoor and outdoor environmental monitoring currently represents the main bottleneck in the development of this technology. Herein we report the results obtained with sensors exclusively made of cellulose and polyaniline (cell/PANI) under strictly controlled relative humidity (30-50 RH%) and temperature (21 ± 1 °C) achieved with a climatic chamber that simulates the conditions of indoor air humidity, and at different RH% in a lab test chamber set-up. Cell/PANI sensors, prepared with a simple, inexpensive, and easily scalable industrial paper process, show a linear trend with a slope of 1.41 μA RH%^-1 and a percentage of sensitivity of 13%. Response time as well as percentage of sensitivity results are similar to those of a commercial digital-output relative humidity and temperature sensor (DHT22) employed in parallel for comparison. The commercial sensor DHT22 has a sensitivity of 14%. This low-cost sensor has potential applications in agriculture, food monitoring, and medical and industrial environments as a disposable sensor for humidity detection.

Scheme 1. (A) Cell/PANI fibers preparation; (B) cell/PANI sheet preparation; (C) cell/PANI sensor: front and back.

Fig. 1. Schematic diagram of the lab test chamber set-up: chamber volume 500 cm³, RH% stabilized by saturated salt solutions in N₂; alternatively dry or wet N₂ has been used.

Fig. 2. Schematic diagram of the climatic chamber set-up, chamber volume 111 000 cm³, air flow.

Fig. 3. Response and recovery times of the cell/PANI under a pulse stimulus obtained with different RH% value under N₂ atmosphere: (A and B) at long cycling time from 2 to 44 RH% and from 22 to 44 RH%, respectively; (C) at short cycling time from 2 to 44 RH%. (D) Sensor response during repeated changes in humidity between 22 and 44%. All tests were conducted under bias voltage of 0.100 V at stable 21 ± 1 °C temperature in a 500 cm³ chamber and repeated three times.

Fig. 4. (A) Response current vs. time of the cell/PANI under pulse stimuli obtained by switching between 30–50 RH% (up and down) in the climate chamber. The test was conducted by applying 0.100 V at 21 ± 1 °C under air atmosphere. For comparison the response in RH% vs. time of a commercial DHT22 sensor is reported; (B) current versus RH% response curves characteristic for cell/PANI. Calibration function: y = 1.41x +156.18 (R² = 0.999); (C) ΔS/S₀%) response vs. RH% obtained from three different devices, the error bars represent the standard deviations. Calibration function: y = 3.36x − 103.13 (R² = 0.998).

Fig. 5. Comparison between the response of the cell/PANI and the commercial DHT22 under pulse stimuli obtained by switching between 30–50 RH% in a climate chamber at 21 ± 1 °C under air atmosphere. The measurements with cell/PANI were conducted under an applied voltage of 0.100 V.

Fig. 6. Response vs. RH relationship of cell/PANI and commercial DHT22 sensor (calibration functions: y = 3.34 x − 102.10 and y = 3.44 x − 103.31 respectively). The applied voltage was 0.100 V (T = 21 ± 1 °C) and the measurements were repeated three times. The experiments were carried out in air.

Fig. 7. Humidity hysteresis characteristic curve of cell/PANI evaluated in the climatic chamber (a = increase of RH%, b = decrease of RH%). The experiments (n = 3) were carried out in a climate chamber 111 000 cm³ volume, in air, by applying a bias voltage of 0.100 V at stable 21 ± 1 °C temperature.