Regular ArticleMolybdenum disulfide (MoS2) nanosheets-based hydrogels with light-triggered self-healing property for flexible sensors
Graphical abstract
Introduction
Recently, the flexible electronic devices have attracted extensive interests due to their potential applications in a wide range of fields, such as energy storage devices [1], [2], [3], wearable electronics [4], [5], [6], memory devices [7], [8] and soft robotics [9], [10], [11]. Among them, the mechanical sensors, capable of sensing the physical pressures and deformation, are the most common ones to mimic the human skin [12], [13], [14], [15]. Apart from the mechanical sensors involving contact sensing, the noncontact sensing to realize touchless perceptions is also quite important for fabrication of superior skin-like electronics [16], [17]. Therefore, it is of great significance to design the flexible electronics with both touch-based and touchless sensing abilities.
As it known to all, the electronic sensors inevitably become damaged in practical applications. However, the in situ repairing for electronics is quite challenging. Imparting the electronic sensors with self-healing property provides a promising approach to realize in situ repairing and prolong the service life. As for the spontaneous self-healing process without external stimuli, it usually requires long healing time and leads to reduced mechanical properties after healing [18], [19]. In contrast, the healing processes under external stimuli cost short healing time with high healing efficiency [20], [21]. Among various stimuli, heating is one of the most common strategies to activate the healing behaviors [22], [23], [24]. However, temperature-activated self-healing systems are greatly influenced by the geometries and accessibilities of materials in the direct heating process, which is not suitable for in situ repairing of electronic sensors [25]. Furthermore, the use of direct heating towards the whole materials may cause adverse effect for the non-damaged areas of the electronics [23], [24]. Hence, the remotely external stimuli, instead of direct heating manner, are quite preferred to induce the healing process for the electronic sensors.
An efficient approach for designing of remotely thermal-induced self-healing materials is to convert light into local heat due to its simple operation [26], [27]. It can induce the “on-demand” healing process by manipulation of the excitation light over a long distance without affecting the other non-damaged areas [25], [27]. In comparison to the ultraviolet light, the near infrared light (NIR) is more friendly and energy efficient [28], [29]. In recent years, molybdenum disulfide (MoS2) nanosheets have emerged as excellent NIR photothermal conversion agent as well as conductors with unusual electronic properties [30], [31], [32]. They have higher molar extinction coefficient of 29.2 L·g−1·cm−1 in the near infrared region in comparison to the gold nanorods (13.9 L·g−1·cm−1) and reduced graphene (24.6 L·g−1·cm−1) [31]. Various methods have been developed to prepare MoS2 nanomaterials, including physical vapor deposition (PVD) [33], [34], chemical vapor deposition (CVD) [35], [36], and solution chemical process [37], [38]. Among them, the chemically exfoliated MoS2 (ce-MoS2) nanosheets can be synthesized in scalable quantities with great colloidal stability in aqueous media [38]. More importantly, ce-MoS2 nanosheets can be facilely functionalized through the conjugation of the defects in their internal and perimeter edges with thiol ligands [39], [40]. Thus, it opens the door for the construction of stable and non-leaking MoS2-based flexible electronics. Clearly, ce-MoS2 nanosheets possess many desirable traits as both NIR photothermal conversion agent and conductive materials in the fabrication of remotely thermal-induced self-healing electronics. However, the MoS2-based flexible sensors with self-healing property have been rarely studied by now.
Herein, a flexible electronic sensor with NIR light-triggered self-healing property (Gel-PEG-MoS2, GPM hydrogel), employing the MoS2 nanosheets, gelatin (Gel) and poly(ethylene glycol) (PEG) cross-linker, was fabricated. In order to obtain a stable and non-leaking hydrogel sensor, the MoS2 nanosheets, gelatin and poly(ethylene glycol) were chemically conjugated through thiol-MoS2 coordination bonding and thiol-ene click reaction. After damaged, the GPM hydrogel exhibited fast self-healing abilities under NIR irradiation. Moreover, the hybrid hydrogel with contact and noncontact sensing abilities exhibited promising applications as soft mechanical sensor and light-sensitive electronics.
Section snippets
Materials
Gelatin (from porcine skin), 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irg 2959, 98%) and trinitrobenzenesulfonic acid (TNBS, 5% (w/v) in H2O) were purchased from Sigma-aldrich (St. Louis, MO, USA). Methacrylic anhydride (MA, 94%) was purchased from Aladdin Industrial Corporation (Shanghai, China). Dialysis tubing (12–14 kDa cutoff) was purchased from Viskase (USA). 4-Arm ploy(ethylene glycol) thiol (4-Arm PEG-SH, Mn = 5 k) was purchased from Sinopeg Biotech Corporation (Xiamen,
Fabrication of the GPM hydrogel
A variety of nanocomposite hydrogels have been developed as flexible electronic sensors. Herein, a MoS2-based hydrogel sensor with self-healing property was first fabricated via simple thiol-ene chemistry. As shown in Fig. 1a, ce-MoS2 nanosheets were firstly surface-functionalized by the 4-arm poly(ethylene glycol)-thiol (PEG-SH) through ligand conjugation between the nanosheets and thiol terminal in PEG-SH. It has been reported that the atomic defects located at the edge and basal plane of
Conclusions
In summary, the ce-MoS2 nanosheets were firstly employed as effective photothermal conversion agents for fabrication of NIR light-induced self-healing hydrogel sensors. The prepared GPM hydrogel self-healed rapidly under NIR light irradiation in only 90 s. The healing efficiencies increased and reached up to 91.3% with very small amount of MoS2 loading (from 0.04 wt‰ to 0.72 wt‰) and irradiation time from 90 s to 240 s. Further experimental results indicated that the well dispersed MoS2
CRediT authorship contribution statement
Xu, Wenya: Methodology, Data curation, Formal analysis, Writing-original draft; Wang, Wen: Methodology, Data curation, Formal analysis, Writing-original draft; Chen, Simou: Data curation, Software; Zhang, Rui: Validation, Formal analysis; Wang, Yuxin: Data curation, Visualization; Zhang, Qi: Validation, Visualization; Yuwen, Lihui: Project administration, Funding acquisition; Yang, Wen Jing: Investigation, Funding acquisition, Writing-review & editing; Wang, Lianhui: Conceptualization, Funding
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors would like to acknowledge the financial support of this study from National Key Research and Development Program of China (2017YFA0205302), Natural Science Foundation of Jiangsu Province (BK20180089), National Natural Science Foundation of China (51503101), Key Research and Development Program of Jiangsu (BE2018732), Natural Science Key Fund for Colleges and Universities in Jiangsu Province (17KJA430011) and NUPTSF (NY218029).
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These authors contributed equally to this work.