Stable Polyelectrolyte Humidity Sensors: Preparation and Application
Authors: Fei Teng , Dai Jianxun , Zhao Hongran , Zhang Tong 
Published: May 2020

Introduction

Respiration monitoring is important for evaluating human health [1]. The human exhaled gas is always accompanied by high humidity, so humidity sensors are promising for respiration monitoring [2]. Importantly, a fast response to humidity change and good stability under high humidities must be guaranteed for the utilized humidity sensor. In order to achieve fast response and good stability, humidity sensors based on stable polyelectrolytes were prepared via in-situ thiol-ene click polymerization. This method provides a novel way for preparing high quality humidity sensors.

Method

The polyelectrolytes were prepared by thiol-ene click polymerization (Fig. 1). Mercaptopropyl polyhedral oligomeric silsesquioxane (50.0 mg), 1,4-divinylbenzene (30.0 mg), sodium p-styrene sulfonate hydrate (5.6 mg), and benzoin dimethyl ether (4.0 mg) were dissolved in mixed solvents of methanol (0.75 mL) and THF (0.5 mL). Then, the obtained mixture was drop-cast onto the ceramic plate interdigitated carbon electrodes. The substrates covered with precursor solution were irradiated under UV light (365 nm, 0.12 J/cm2) for 30 min. The cross-linked humidity sensors were then rinsed with the mixed solvents of methanol and THF to remove unreacted monomers and the catalyst. The thickness of the sensing film is ∼30 μm.

Results and Conclusions

The impedance results of PMDS and PPDS sensors in a batch were tested at different relative humidities (RHs) (Fig. 2a). There is only tiny response for these two sensors in the humidity range of 11-54% RH. At high humidity range, PPDS sensors show an incredibly small variation, while PMDS sensors show an impedance change of nearly 2 orders of magnitude. PMDS sensor’s response/recovery time between 33 and 95% RH is 0.29/0.47 s (Fig. 2b), and that of PPDS sensor is 0.75/0.48 s (Fig. 2c). In addition, the PMDS sensor shows good long-term response/recovery time stability and impedance stability. The PMDS sensor could distinguish the variation of breath rate and depth among a normal breath, a fast breath, a slow breath, a deep breath, a random breath, and a paused breath (Fig. 3). It is worthy to note that almost every cycle behaves inhaled and exhaled platforms. The signal equilibrium value is very important and beneficial to recognize the single inspiration and expiration.
The polyelectrolytes were in situ cross-linked on the substrates with interdigitated electrodes successfully by thiol-ene click polymerization. The obtained optimum sensor shows high response and recovery speeds to humidity change and good stability. Different respiration patterns can be distinguished, and the breath rate/depth of detection subjects can also be determined by the sensor., which is meaningful in future diagnostic breath analysis.

References

Q.Trung, N.-E.Lee, Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human-Activity Monitoring and Personal Healthcare. Adv. Mater. 28 (2016) 4338-4372. doi: 10.1002/adma.201504244.

Kano, K.Kim, M.Fujii, Fast-Response and Flexible Nanocrystal-Based Humidity Sensor for Monitoring Human Respiration and Water Evaporation on Skin. ACS Sens. 2 (2017) 828−833. doi: 10.1021/acssensors.7b00199.

Figure 1