Flame-retardant, highly sensitive strain sensors enabled by renewable phytic acid-doped biotemplate synthesis and spirally structure design
Introduction
Flexible and highly sensitive sensors with long-term structural and functional safety have become a focal point for many new applications, including soft robotics [1], environmental monitoring [2], and wearable electronics [3], [4], [5], [6], [7], [8], [9]. In particular, resistive-strain sensors play an important role in the development of human motion detection systems. These sensors rely on the transduction of mechanical excitation into a change of electrical resistance, allowing for the high sensitivity to monitor different physiological signals [10], [11], [12], [13], [14]. However, widespread applications of the highly sensitive flexible sensors are limited by sophisticated micro-nano-structure design [15], the complicated fabrication processes [16], and expensive conductive fillers (grapheme [17], gold nanowires [18]). Therefore, green and sustainable materials or technologies to prepare highly sensitive flexible sensors with functional safety are highly desired.
In recent decades, conductive polymers, including polyaniline (PANI), polypyrrole, polythiophene and polyacetylene, have been widely used to construct functional sensors [19], [20], [21], [22], [23], [24], [25]. In particular, PANI has attracted considerable attention due to its unique properties: low-cost synthesis, high environmental stability, excellent electrical properties, and especially doping/dedoping chemistry [26], [27], [28]. During the synthesis process, doping acid plays a key role in the conductivity of PANI composites. Generally, the doping acids previously employed include both organic doping acids [29] (p-toluenesulfonic acid and sulfosalicylic acid) and inorganic doping acids [30] (sulfuric acid, hydrochloric acid, and phosphoric acid). Although these doping acids exhibit promising properties, most of them are harmful to human health due to the corrosivity and toxicity, which restricts the applications of the resultant composites in skin-touchable sensors. In addition, most acid-doping PANI-based conductors are flammable, and the utilization of electric power may cause fire accidents when sensors suffer from short circuit, overload, overheating, and so on [31]. To tackle this issue, a straightforward method is to add mass flame-retardant additives to the flexible substrate to achieve ideal flame retardancy. However, a considerably high loading and toxicity of flame retardants would restrict the construction of conductive networks and deteriorate the processing features of the composites [32], [33].
As a nontoxic and biological organic acid, natural-derived phytic acid (PA) is an eco-friendly material which has already been widely used as a cross-linker [34], a gelator [35] and in food system [36], etc [37]. PA can react with PANI by protonating the nitrogen groups on PANI, which endow PANI with excellent conductivity and phosphorus [38]. Recently, Xu and colleagues presented a PA-doped PANI-based flexible sensor with great self-healing ability and high stretchability for sensing applications [10]. Despite that, the high sensitivity with unique structure design and the flame retardancy performance of PA-doped PANI-based flexible sensors have yet been investigated. Therefore, an effective strategy to realize high sensitivity and good flame-retardant of PA-doped PANI-based flexible sensors is highly demanded.
In this work, we prepared a highly sensitive, flame-retardant and cost-efficient flexible sensor via a facile and eco-friendly PA-doped bio-template synthesis as well as spirally structure design. As a phosphorous and sustainable organic acid, PA dopes nanohybrids of PANI and cellulose nanocrystals (CNC), which serve as excellent conductive and flame-retardant nanofillers to fabricate the flexible sensor. We confirm the high sensitivity of this spirally structured sensor in a wide range of strain caused by liquid droplets with different densities and heights as well as fingers bending. In addition, this flexible sensor can self-extinguish in 20 s after ignition, showing excellent flame retardancy. This sustainable and green strategy can be extended to prepare highly sensitive and flame-retardant electronic devices, which opens up a wide range of new technological possibilities.
Section snippets
Material design
The fabrication process of the spirally structured highly sensitive sensor is illustrated in Fig. 1. As a nontoxic bio-derived organic acid with high phosphorus content (28 wt%), PA can deliver more active flame-retardant atoms per molecule. Besides, a PA molecule can provide six strongly dissociated protons, resulting in sufficient electrical conductivity of PANI [39], [40]. CNC is a natural cellulose derivative with good environmental sustainability. The high specific surface area, high
Conclusions
In summary, we demonstrated a facile, eco-friendly, and sustainable strategy to fabricate a highly sensitive and flame-retardant sensor. Benefitting from ingenious spirally structure and renewable PA doped synthesis, the resulted flexible sensor exhibits excellent sensitivity with the minimum strain detection limit of 0.05%, which can distinguish the strain caused by liquid droplets with different densities and at different heights. In addition, the sensor is able to detect various human
Acknowledgments
The authors thank the National Natural Science Foundation of China (51673121 and 51873123) and the Foundation of State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (Grant No. FW1804) for financial support.
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