Reconfigurable materials with multi-functionality and reversibility1,2 are crucial for the development of sensors3, actuators4,5, wearable devices6,7, and soft robotics8–10. A number of such materials with specific functionality or responsiveness were obtained via the hybridization of polymer matrices with inorganic inclusions such as nano-sized carbons11–13, metals14,15, metal oxides16, and others17–19. Versatility of the composite can be engineered via a multi-scale design at different levels, ranging from the constituting polymer chain20–22 to microstructured features 23,24. In designing them, a critical issue would be how to achieve hierarchical interactions between the multiscale constituents so that they can address synergistically complex tasks25. This is challenging, given that the bespoke nano-inclusions usually maintain good physicochemical stability with limited reversibility or reconfigurability in the polymer support.
Solution processable perovskites may provide new opportunities for the hierarchical design of responsive polymer composites with complex functions. Perovskites are widely used in photovoltaics26,27, light-emitting diodes28–31, display32, and photodetectors33 with long carrier lifetime, high carrier mobility, and high fluorescence quantum yield. However, as ionic solids, they are highly sensitive to water, i.e., water can induce (ir)reversible phase change34–36 or even dissociation of the perovskite crystal37–40. Tremendous efforts have been devoted to improving their stability against water, for example by using hydrophobic passivation41,42 and encapsulation43–45. Here, we ask ourselves if this inherent instability can be utilized and manipulated with a hydrophilic and hydratable polymer network, leading to novel responsive polymer composites with hierarchical synergies and exotic functionalities capable of complex device operations.
We first designed the structure and fabrication scheme of a multi-layered and perovskite-embedded composite film (Fig. 1a, also see details in “Methods” and Figure S1). A polymeric three-layer film was constructed via stepwise spin-coating (Fig. 1a, left). The top layer is a water-absorbing polyvinyl alcohol (PVA) film that was cross-linked with sodium benzoate under light irradiation (Figure S2). This layer can be arbitrarily patterned by using masks (see Figure S3 for arraying process); the middle layer is a compliant polydimethylsiloxane (PDMS) film; and the under layer is a transparent supporting substrate based on polyethylene terephthalate (PET), polymethyl methacrylate (PMMA) or glass. A ligand-free inorganic cesium lead halide (Cs-Pb-Br) perovskite was next introduced into the cross-linked PVA layer by swelling it with the precursor solution (Fig. 1a, middle), and the perovskite content was regulated by tuning the solution concentration (Figure S4). After subsequent drying and post-treatment, we obtained the desired composite film (Fig. 1a, right). Cross-sectional scanning electron microscope (SEM) images (Fig. 1b and Figure S5) and corresponding energy dispersive spectroscopy (EDS) results (Figure S6) of the as-prepared film show that perovskite nanoparticles (NPs) with sizes of 200–400 nm are evenly distributed in PVA (also see fluorescence micrograph in Figure S7 and transmission electron microscope images in Figure S8).
The prepared composite film is transparent under natural light and fluorescent under ultraviolet (UV) light (Fig. 1c, left). After wetting with an ultrasonic humidifier (Figure S9), the film becomes translucent and non-fluorescent (Fig. 1c, right). Meanwhile, the patterned film with arrays of circular disks (Fig. 1d, left and middle) showed a similar change of the fluorescence and transparency upon water spraying (Fig. 1d, right and Figure S10). In particular, photoluminescence (PL) spectroscopy showed that the emission peak of the film in the dry state locates at around 510 nm, with a full width at half maximum (FWHM) of about 20 nm. After wetting, the emission peak completely disappeared (Fig. 1e). This on/off switching of fluorescence is reversible and reproducible (see Video S1) with a consistent peak position and intensity for at least fifteen cycles (Fig. 2f). We also found that the transmittance of the film in the visible light range was above 90 % in the dry state and below 10 % in the wet state (Fig. 1g). This is caused by the wrinkle formation in the swollen PVA layer that can significantly scatter incident light46 (bottom inset in Fig. 1g, and see the 3D microscope image in Figure S11). The changes in transmittances also demonstrated good reversibility and repeatability (see Video S2) in a fifteen-cycle study (Fig. 1h), as the PVA layer becomes flat again after de-swelling (top inset in Fig. 1g).
To understand the reversible fluorescence change, we conducted X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis of the composite film. In the dry (original) state. XPS results (Figure S12 and Table S1) show that the embedded perovskite has a stoichiometric composition of CsPb2Br5. The XRD study presents characteristic peaks at 11.7°, 23.4°, 35.4° and 47.8°, fitting well with the crystal planes (002), (210), (312) and (420) of the tetragonal CsPb2Br5 phase47 (PDF#25–0211), respectively (Fig. 2a and Figure S13). These peaks disappeared after water treatment and recovered after drying (Fig. 2a). We thus assume that perovskite NPs have dissociated into free ions (Cs+, Pb2+, Br− or PbBr64−) that are dissolved in the swollen PVA network in the wet state, and the ions can recrystallize into NPs again after water evaporation (Fig. 2b). To verify this, electrical measurements on the same film sample were performed (Fig. 2c and Figure S14). The conductivity of the film indeed increases from 10− 4 S·m− 1 to 10− 1 S·m− 1 after wetting and reduces back to 10− 4 S·m− 1 after drying. The conductivity in the wet state is at least one order of magnitude higher than that of the reference sample containing no perovskite. These results further explain the fluorescence quenching in the wet state and subsequent recovery in the dry state.
The reversible dissociation and recrystallization of perovskite species above is highly intriguing in connection to fact that perovskites are highly susceptible to water molecules, i.e. water would usually induce irreversible dissociation of these ionic crystals, strongly affecting their opto-electric propertiese48–50. Such a reversible ion-to-crystal transformation (RICT) can be attributed to the cross-linked PVA network. Further control experiments show that perovskite NPs cannot grow uniformly in PVA film without cross-linking (Figure S15). Likewise, their dissociation becomes irreversible without cross-linking. Notably, PVA hydrogels have been used as solar steam generators in seawater desalination for restricting ion diffusion while allowing water evaporation51,52. Their networks can also be used as templates to regulate the growth of metal NPs such as Fe, Co, Ni, Cu, etc.53,54 Thus, we postulate that the PVA network serve as semi-enclosed “cages” (Fig. 2b) that not only absorb a sufficient amount of water to dissociate perovskite NPs but at the same time confines the migration of ions (Cs+, Pb2+, Br− or PbBr64−) and subsequently nano-spatially nucleates their recrystallization for realizing RICT. It is worth noting that such RICT also works for a lead-free double perovskite Cs2AgBiBr6 (Figure S16), i.e., its composite film also shows good reversibility and reproducibility of the fluorescence signal in at least four cycles (Figure S17).
We further studied the sequence of wrinkle formation and transformation of perovskite NPs and their eventual synergy during wetting. Our real-time monitoring of water spraying on the composite film shows that the transparency declined (1–2 s, Video S2) before the fluorescence quenching takes place (4–5 s, Video S1). This result suggests that water molecules first permeate and swell the PVA matrix to form wrinkles and subsequently react with the embedded perovskites NPs to dissolve them into the constituting ions. We further prepared different composite film with different amounts of perovskite (2.5–11.5 vol.%) with a fixed PVA layer thickness of 3.6 µm and measured the water absorption amount per unit area of these films. We found that the amount of water required to completely quench the fluorescence increases from 3.2 to 4.5 mg/cm2 as function of increasing perovskite concentration, while the amount of water needed to form wrinkles remains almost constant at a value of ~ 0.6 mg/cm2 (Fig. 2d). These results confirm that water first triggers wrinkles and then quenches the fluorescence.
Interestingly, we could quantitatively evaluate the amount of water absorption based on the light scattering properties of the swollen films. In particular, we built a measuring setup by combining a camera and a precision balance to synchronously record the surface brightness (grayscale) and weight of a film sample in real time. (see details in “Methods” and Figure S18). We could thus obtain time-varying curves of the average grayscale value and water absorption (Fig. 2e) and quantitatively establish their relation after normalization with a reference sample (Fig. 2f and Figure S19). For the film sample with PVA thickness of 1.8 µm (6 wt.% cross-linker), a linear dependency in the range of 0–0.3 mg/cm2 can be observed, with good reproducibility in repeating runs, even at different illuminances (15 and 120 lux). Moreover, this linear response range can be precisely engineered by tuning the film thickness (Figure S20 and S21). Specifically, increasing the PVA layer thickness from 1.8 to 11.6 µm linearly increases the saturated absorption of water from 0.3 to 2.4 mg/cm2 (Fig. 2g), respectively, approaching the fluorescent sensing range of bespoke perovskite NPs.
Based on the above results, we anticipate the prospect to regulate the water absorption capability (or sensing threshold) and linear response range of the composite films by tuning the perovskite content and PVA thickness, respectively. Notably, the on/off switching of fluorescence is expected not to interfere with the gradual change of transmittance or grayscale of the film, and these two responses can thus be manipulated independently as the fluorescence switching of perovskite requires a large amount of water qualitatively, while wrinkle formation is a gradual physical process sensitive to a lower amount of water. This markedly different response of the different constituents of the polymer composite will be critical for practical applications in sensing and other fields.
As a demonstration, we studied the application of the composite film as a sensor in the real-time detection of water spraying. Rapid, low-cost, and non-disposable detection of the amount and distribution of sprayed droplets at given locations, e.g., on the leaves or in the field, is urgently needed in precision agriculture but yet to be realized. Currently, water sensitive paper (WSP) is widely used for the agricultural spray detection55–58, which is neither reusable nor suited for real-time monitoring. Moreover, it follows a routine of “field sampling first and lab analysis later” that severely limits its application in today’s unmanned agricultural practices.
We first conducted a droplet impact experiment and found that our composite film with hydrophilic surface facilitates the droplet deposition with little rebounding or splashing (Figure S22). Ideally, we can obtain real-time spraying data by recording the distribution of grayscale values or fluorescence at different positions of the composite film. In practice, however, we found that water droplets could spread laterally on the surface layer and overlap with each other, resulting in a reduced spatial resolution (Fig. 3a, left). To address this issue, we used the patterned composite film containing arrays of circular disks (~ 300 µm) with the assumption that the isolated cells can work independently to localize the diffusion of water (Fig. 1d). We found the arrayed film provided a much higher resolution of the droplet distribution under the same spraying conditions (Fig. 3a, right), i.e., the response of the spots remained small in size and was almost free of lateral extension (also see Figure S23). We further took a series of micrographs from a typical spray-recovery process (Fig. 3b, also see Video S3). As expected, the hydrophobic PDMS patches in between cells prevents water from spreading, and the distribution of the wrinkled cells reflects the distribution of water droplets. We thus can obtain the distribution of the sprayed droplets by using the patterned film sensor.
Moreover, we found that the patterned structure also significantly improves the overall flexibility of the film, enabling it to be arbitrarily folded or twisted with little effect on its sensing capability (Fig. 3c and Figure S24). A bending test under a microscope showed that at the same bending radius of 2.5 mm (Figure S25), the continuous film without array presented unfavorable wrinkles and even cracks (Fig. 3d, left), while the curved film with arrays retained the original intact morphology (Fig. 3d, right). The arrays break the continuity of the surface layer, which reduces the shear stress transfer between layers during bending and thus minimizes mechanical strain and wrinkles59–62. These results highlight the application potential of our composite film in wearable detection.
We next studied the sensing performance of our composite film in different application scenarios. We first demonstrated a bandage-like device that can be attached onto plant leaves (Fig. 4a). In particular, we peeled off the pre-prepared composite film (~ 5 vol.% perovskite in 3.6 µm PVA on PDMS, size ~ 1×1 cm) from its original substrate (glass) and transferred it to leaves of a plant (Schefflera octophylla, for example). The flexible film can be readily fitted to the leaf surface (Fig. 4b). Its fluorescence disappears after spraying and recovers after drying, ready for the next test (Fig. 4c). Therefore, we consider that the composite film can be used as a wearable sensor in practice, i.e., by placing it at different positions of the crop (leaves, stems, fruits, etc.) for the on/off detection of spraying and its distribution. Notably, the response range of our composite film to water (0–4.5 mg/cm2, Fig. 2d) is consistent with the actual spraying densities of pesticides63,64 (e.g., insecticides, herbicides, fungicides, etc.) ranging from 0.5 to 3.0 mg/cm2 (or 50–300 L/ha) in agriculture. Moreover, fluorescence-based detection, enabled by our composite film, can be performed under low ambient light or even at night.
We further designed a portable device (Fig. 4d) with the composite film sitting on the top of a sealed home-made box for spraying detection in the field. A micro-camera with Wi-Fi transmitter was installed at the bottom center of the box for monitoring the film and transmitting real-time signals wirelessly. A water spraying test with this device was first performed in a laboratory environment, acquiring the result of one spray-recovery circle (Fig. 4e and Figure S26), where the white spots were attributed to the local wrinkles triggered by water droplets. After the test, our device could recover spontaneously and be reused. We further performed an outdoor test in a field environment under natural, ambient light (Figure S27) and obtained results after background subtracting (Fig. 4f). A database of droplets could be established based on image recognition, and we could thus statistically analyze and acquire the desired spraying information quantitatively including the number of droplets (N), their average size (s), the average spacing between droplets (d), the total water volume (V), and the coverage ratio (C) (see the analysis and a typical list of results in Figure S28). Moreover, because the sensor devices are portable and recoverable, they can be placed at different positions (p1, p2, ...pn) and record a series of data at different moments (t1, t2, ...tn), respectively. All these data obtained from our composite film sensors are acquired in real time, and thus they can provide timely feedback and guidance on field operations, e.g. spraying operation by unmanned aerial vehicle (UAV). In this way, we can achieve precise spraying control of pesticides in an automated and intelligent manner for precision agriculture (Fig. 4g).
In summary, we have developed reconfigurable polymer composites presenting water-switchable transmittance and fluorescence with good reversibility and independent controllability. The top hydratable layer of the composite can form surface wrinkles reversibly, triggered by water, while perovskites inside the layer undergo reversible ion-to-crystal transformations because of nanoconfinement in the voids of the crosslinked hydrogel. The two responses can work cooperatively, permitting an unrivaled sensing application of the composite film in precision agriculture, i.e., a rapid and reusable on-site detection of water spraying in terms of quantity, threshold, and distribution. Our findings suggest that multiple functionalities can emerge from the hierarchical interactions between the multiscale constituents, which would give access to perovskite-based smart composites and flexible devices with novel applications.