Polydopamine-based synthesis of an In(OH)₃–PDMS sponge for ammonia detection by switching surface wettability

Abstract

An In(OH)₃–PDMS sponge has been synthesized by covalent modification of PDA. The sponge can switch surface wettability reversibly between superhydrophobicity and superhydrophilicity when detecting ammonia. The water contact angle changes from 141.5° to 0° after storage in an ammonia atmosphere, which means an ammonia responsive surface wettability switch. The minimum NH₃·H₂O concentration that the In(OH)₃–PDMS can detected is 5% in 2 h. This kind of quick and sensitive ammonia detector is promising simply by virtue of the surface wettability conversion.

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Introduction

In recent years, the design and synthesis of metal–organic coordination polymers has drawn much interest because of one of the potential applications in gas separation and sensors in detecting CO₂,^1,2 NH₃,³ CH₄,⁴ NO₂ (ref. 5) and H₂.⁶ Surface wettability switches are sensitive for the outside atmosphere, however, they have rarely been investigated for gas detection.^7,8 Indium hydroxides and oxides are important semiconductor materials, which have drawn much interest due to their special semiconducting and optical properties.^9–14 Besides the superior optical and semiconductive properties, In(OH)₃ shows subacidity in aqueous atmosphere,¹⁵ which makes it easy to interact weakly with NH₃·H₂O molecules. Zhu et al. fabricated In(OH)₃ films for NH₃ detection.¹⁶ However, the films have a limited maximum sensitivity, weak mechanical properties and need a long time to response. To date no In(OH)₃ micro- and nanoparticles–organic coordination polymer with high sensitive and efficient NH₃ detection performance has been reported.

Here, this study is undertaken to synthesize In(OH)₃ micro- and nanoparticles by covalent modification of the porous polydimethylsiloxane (PDMS) sponge surface using polydopamine (PDA). Chemists found out that polydopamine (PDA) can form durable coating on a variety of materials, such as metals, glass, and polymers.^17–21 Besides, the PDA can be used as a platform for secondary reactions, including bringing nanoparticles, grafting amino-/thiol-containing molecules, etc.²² PDMS is the most commonly used elastomeric matrix in metal–organic coordination polymers.^23–26 In our previous study,²⁷ we successfully fabricated three-dimensional interconnected microporous PDMS sponges which possess flexible skeleton, high compressibility and thermal stability, thus providing an ideal substrate for combining In(OH)₃. It can be expected, therefore, that In(OH)₃–PDMS coordination polymer prepared on PDA-functionalized supports will exhibit sensitive NH₃ detection performances and excellent reproducibility of switchable surface wettability between superhydrophobicity and superhydrophilicity.

Experimental

Materials

SYLGARD® 184 Silicone Elastomer prepolymer (Sylgard 184A, M_w ≈ 22 000 g mol⁻¹) and the thermal curing agent (Sylgard 184B, M_w ≈ 15 000 g mol⁻¹) were purchased from Dow Corning. Sanding sugar, S_A (400–500 μm); coarsely granulated sugar, S_C (1400–1600 μm) were purchased from Wal-Mart supermarket. InCl₃, urea ((NH₂)₂CO), NH₃·H₂O were brought from Aladdin. Dopamine and tris(hydroxymethyl)aminomethane (Tris) were obtained from Aladdin.

Preparation of In(OH)₃ micro- and nanoparticles

The In(OH)₃ micro- and nanoparticles were fabricated by low temperature hydrothermal synthesis in a well closed glass bottle containing an aqueous solution of InCl₃ and (NH₂)₂CO. In order to prepare In(OH)₃ with different micro- and nanostructures, an aqueous solution consisting of 0.8 mmol InCl₃ and 1 mmol, 2 mmol, 4 mmol, 8 mmol (NH₂)₂CO has been used for the reaction, respectively. After the solution was heated at 95 °C for 24 h, the obtained In(OH)₃ rinsed with deionized water and ethanol in turn to remove any possible contamination from residual InCl₃ and (NH₂)₂CO. Then the sample was dried in an oven at 50 °C for 6 h.

Preparation of PDMS sponges

The preparation of PDMS sponge was done according to a previous procedure.²⁶ PDMS prepolymer with a curing agent ratio of 10 : 1 was diluted with a decent amount of cyclohexane in a vessel. The solvent casting process was prepared as two steps: (1) sugar particles were subjected to 85% humidity for periods from 0 to 24 h to achieve fusion of sugar prior to solvent casting; (2) samples were dried in a desiccator for 4 h at 70 °C before adding to the prepared PDMS solution. Constant temperature and humidity test chamber held at 40 °C were used to create a 85% humidity environment for fusion of sugar. The vessel was left for 1 h to become saturated with PDMS. After curing in an oven at 50 °C for 12 h, the blend was put in a beaker of 70 °C water to dissolve sugar and then in ethanol to remove cyclohexane. Approximately 24 hours were allowed for this process to occur. Consequently, the PDMS sponges were dried in an oven at 50 °C.

Polydopamine (PDA) modification of the PDMS sponge surface

The obtained In(OH)₃ was dispersed in Tris–HCl (pH = 8.5) buffer with 0.2 mg mL⁻¹ dopaminehydrochloride and a piece of clean PDMS sponge (2 cm × 2 cm × 2 cm) was soaked to the as-prepared mixture by continuous stirring for 24 h at room temperature. After it was rinsed with ethanol and deionized water and dried at 40 °C for 24 h, the In(OH)₃–PDMS sponge was acquired.

Characterization

The obtained In(OH)₃ and In(OH)₃–PDMS sponge were coated with a layer of gold using a S-4800 Field Emission Scanning Electron Microscope (SEM) operating at 15 kV. Energy dispersive X-ray spectroscopy (EDS) analysis was done with the auxiliary equipment to SEM. The morphology and detailed structure of the In(OH)₃ were determined by transmission electron microscopy (TEM, JEM-2100). The X-ray diffraction (XRD) was performed on D/Max B. The water CA measurement was performed to investigate the surface wettability of In(OH)₃–PDMS sponge with JC2000D3. Average CA values were obtained by measuring the sample at five different positions.

Result and discussion

The In(OH)₃–PDMS sponges were fabricated by polydopamine (PDA) modification (Fig. 1). SEM was used for clarifying the morphological evolution of In(OH)₃ at various (NH₂)₂CO concentrations. Fig. 2a–d were the SEM images of In(OH)₃ prepared by an aqueous solution consisting of 0.8 mmol InCl₃ and 1 mmol, 2 mmol, 4 mmol, 8 mmol (NH₂)₂CO, respectively. Fig. 2a showed microcubes with sharp edges and corners in the size of 1–3 μm at low concentrations. However, a few strip-like cubes have also been synthesized possibly due to the anisotropic growth of microcubes. By increasing (NH₂)₂CO concentration to 2 mmol, so few nanorods with size of 500–800 nm immerged in microcubes (Fig. 2b). The nanorods became denser with the (NH₂)₂CO concentration increasing to 4 mmol (Fig. 2c). For the In(OH)₃ prepared via 8 mmol (NH₂)₂CO, the multiple structures consisting of microcubes and nanorods have been fabricated (Fig. 2d). These microcubes and nanorods are well crystallized, which can be corroborated by the SAED pattern of the sample (Fig. 2e). The TEM image further indicated the multiple structures consisting of microcubes with size of 1–3 μm and nanorods with size of 500–800 nm (Fig. 2f). The microcubes and the nanorods stacked together to form In(OH)₃ micro- and nanoparticles.

Fig. 1 Schematic illustration of chemical structures and preparation for In(OH)₃–PDMS sponge.

Fig. 2 Morphology of the In(OH)₃ prepared by controlling the reaction conditions. (a–d) In(OH)₃ prepared by an aqueous solution consisting of 0.8 mmol InCl₃ and 1 mmol, 2 mmol, 4 mmol, 8 mmol (NH₂)₂CO, respectively; (e) SAED pattern of the sample; (f) multiple structures consisting of microcubes and nanorods.

Fig. 3a displayed an X-ray diffraction pattern, in which all the diffraction peaks can be easily indexed to a body centered cubic (bcc) phase of In(OH)₃ (JCPDS 76-1463). The strong and sharp diffraction peaks revealed that a well-crystallized sample. The chemical composition of In(OH)₃ was further characterized by EDS (Fig. 3b). In addition to the elements Au and C from conductive rubber, the In/O atomic ratio was about 1 : 3, which was in good agreement with the theoretical In/O atomic ratio of In(OH)₃.

Fig. 3 XRD and EDS patterns of In(OH)₃. (a) XRD patterns of In(OH)₃ can be indexed to a body centered cubic (bcc) phase of In(OH)₃ (JCPDS 76-1463); (b) spectrum of elements shows that the film is mainly composed of indium and oxygen.

SEM images indicated that the In(OH)₃ micro- and nanoparticles were firmly fixed on PDMS substrate. Fig. 4a showed a smooth surface with a three-dimensional porous structure of pristine PDMS sponge. After PDA modification, numerous In(OH)₃ micro- and nanoparticles were stick on the surface and holes of sponge (Fig. 4b and c). The TEM image proved the core–shell structure of PDA modification In(OH)₃ micro- and nanoparticles (Fig. 4d). It was very interesting to observe that several In(OH)₃ micro- and nanoparticles random aggregated together in the PDA capsule.

Fig. 4 Morphology of the In(OH)₃–PDMS sponge modified by PDA. (a) Smooth surface with a three-dimensional porous structure of pristine PDMS sponge; (b and c) In(OH)₃ micro- and nanoparticles were stick on the surface and holes of sponge; (d) core–shell structure of PDA@In(OH)₃.

The surface wettability of the In(OH)₃–PDMS sponge was measured by the water contact angle measurement. Fig. 5a–d were the contact angle (CA) with different In(OH)₃ structure. Clearly, the sponge containing In(OH)₃ microcubes were easy to be wetted by water droplets when adding 1 mmol and 2 mmol (NH₂)₂CO. The water contact angle was measured to be 100.7° and 110.8°, respectively (Fig. 5a and b). When using 4 mmol (NH₂)₂CO, the CA slightly increased to 121.6°, and the sliding angle was measured to be 5.5° (Fig. 5c). Remarkable enhancement of hydrophobicity can be observed on micro- and nanoparticles. Due to the In(OH)₃ consisting of both microcubes and nanorods, the CA was as high as 141.5°, the sliding angle was 4.7° (Fig. 5d). The water contact angle measurement exhibited that the surface wettability can be tuned by the In(OH)₃ structure.

Fig. 5 Surface wettability of the In(OH)₃–PDMS sponge. (a) Shape of a 2 μL water droplet on the surface of the sponge containing In(OH)₃ microcube in the air with the water contact angle of 100.7°; (b and c) In(OH)₃ microcube and little nanorod in the air with a water contact angle of 110.8° and 121.6°; (d) In(OH)₃ microcube and nanorod multiple in the air with a water contact angle of 141.5°; (e) smart surface wettability switched between superhydrophobicity and superhydrophilicity.

The surface wettability of In(OH)₃ was decided by the surface free energy and surface roughness. The In(OH)₃ exhibited hydrophobicity because symmetrical and well bonded of indium ions and hydroxyl groups have the lowest surface free energy, which greatly lowered the surface free energy of In(OH)₃. Another reason was the strong combination between the hydroxyls forming a relatively saturated network of hydrogen bonds, thereby decreasing the polarity of the surface hydroxyls greatly and further reduced the surface free energy. The In(OH)₃ consisting of both microcubes and nanorods increased the surface roughness by creating surface structure, as describe by Cassie and Baxter's equation.²⁸ The Cassie–Baxter equation is: cos θ_c = f₁ cos θ₁ + f₂ cos θ₂, where f₁ is the solid fraction in contact with the droplet and f₂ is the air fraction. For the In(OH)₃ microcube, the large voids made it easy for water droplets to penetrate in the gaps between microcubes and replace the trapped air, which greatly decreased the air fraction f₂ and thereby causing a relatively lower water contact angle (Fig. 6a). For the multiple structured that consist of microcubes and randomly arrayed nanorods, a large volume of air can be trapped in the micro- and nanogaps. Since the air in the nanogaps was difficult to be discharged, the air would be trapped by water droplets. Therefore, the air fraction f₂ was improved remarkable, and the hydrophobicity was enhanced according to the equation. Moreover, after modification with PDA, several In(OH)₃ micro- and nanoparticles random aggregated together in the PDA capsule, which made In(OH)₃ arrange more tightness to trap air. The water droplets are not easy to contact the gap bottom and thus formed the Cassie–Baxter state (Fig. 6b). Therefore, In(OH)₃–PDMS sponge showed superhydrophobicity.

Fig. 6 Wetting theory of In(OH)₃–PDMS sponge. (a) Water droplets are easy to penetrate in the microcube In(OH)₃; (b) an air pocket is formed at the interface between the water droplet and the surface of the sponge because of the increase of air fraction.

To investigate the ammonia sensitive behavior, the In(OH)₃–PDMS sponge was exposed to an ammonia hydroxide atmosphere for 2 h, the superhydrophobic sponge turned superhydrophilic with a water contact angle of 0°. Interestingly, the liquid repellency of sponge renewed when the In(OH)₃–PDMS sponge treated was heated at 90 °C for 24 h. Furthermore, the ammonia sensitive properties still maintained after 10 times detection and heat treatment (Fig. 5e). Since In(OH)₃ showed subacidity in aqueous atmosphere, the hydrogen atoms easy to be deprived when the In(OH)₃–PDMS sponge were exposed to NH₃·H₂O. The hydroxyl group in the polar NH₃·H₂O molecules tended to capture the hydrogen atoms on the surface of In(OH)₃ that were formerly bound by the lattice hydroxyls inside the In(OH)₃ because of the stronger basicity of NH₃·H₂O. Thus, the hydrogen atoms formed a relatively strong combination with NH₃·H₂O molecules (Fig. 7). The In(OH)₃ micro- and nanoparticles randomly dispersed in the hierarchical porous structure of PDMS, which dramatically increased the direct contact area between NH₃ and In(OH)₃. Consequently, the NH₃·H₂O molecules would fix on the In(OH)₃ and form a layer of ammonia hydroxide on the In(OH)₃ micro- and nanoparticles, which greatly increased the surface free energy of the In(OH)₃ and finally obtained superhydrophilic with a water contact angle of 0°.

Fig. 7 Reversible reaction between NH₃·H₂O and In(OH)₃.

In order to detect a maximum sensitivity, the experiment was carried out in a well closed glass device with various NH₃·H₂O concentrations for 2 h at room temperature. From the Fig. 8, the water contact angle decreased with the NH₃·H₂O concentration increasing. The sponge still exhibited superhydrophobicity when the concentration was 0 and 100%. This indicated that NH₃·H₂O molecules play the main role in improving the hydrophilicity of the sponge. Dry NH₃ and water vapor had no effect to change water contact angle. When the concentration of the NH₃·H₂O was less than 4%, the energy was not enough to break the hydrogen bonds of the In(OH)₃. Increasing the NH₃·H₂O concentration, the hydrogen bonds broke partly. When the NH₃·H₂O concentration increased to 5%, the hydrogen bonds broke totally, and the hydrogen atoms of the In(OH)₃ formed a relatively strong combination with NH₃·H₂O molecules. Consequently, the NH₃·H₂O molecules would fix on the In(OH)₃ and form a layer of ammonia hydroxide, which greatly increased the surface free energy of the In(OH)₃ and finally obtained superhydrophilic with a water contact angle of 0°. The minimum NH₃·H₂O concentration that the In(OH)₃–PDMS can detected was 5%, which indicated the high sensitivity to NH₃ and could be used in air humidity. It ascribed to the In(OH)₃ micro- and nanoparticles densely aggregating in the holes and surface of the sponge. The hydrophilicity of the sponge was intensified with the increase of NH₃·H₂O concentration. The result further confirmed that not the NH₃ but the NH₃·H₂O molecules react with In(OH)₃. The pH of the sponge surface changed from 6.0 to 8.0 after being stored in water and ammonia hydroxide atmosphere respectively. The liquid repellency of In(OH)₃–PDMS sponge renewed when the sponge was heated at 90 °C for 24 h because the NH₃·H₂O molecules decomposed and escaped from the surface of In(OH)₃.

Fig. 8 Plots of water contact angles varying with the different NH₃·H₂O concentration.

Conclusions

Overall, we have successfully synthesized In(OH)₃ micro- and nanoparticles by covalent modification of the porous PDMS sponge surface using PDA. The superhydrophobic sponge can be tuned to superhydrophilic ones at ammonia hydroxide atmosphere. The liquid repellency of In(OH)₃–PDMS sponge renewed when the sponge was heated at 90 °C for 24 h. More importantly, the minimum NH₃·H₂O concentration that the In(OH)₃–PDMS can detected was 5% in 2 h, which exhibits high sensitive and efficiency. Because of the ecofriendly, economic and simplicity fabrication, the In(OH)₃–PDMS sponge could be a promising material in developing new types of NH₃·H₂O detectors by virtue of the surface wettability change.

Acknowledgements

The authors sincerely acknowledge “The agricultural science and technology achievements transformation projects (2013GB236006 56)”, “the international cooperation project of Jiangsu Province (BZ2013010)”, and “the Shanghai production-study-research cooperation projects (CXY-2014-023)”.

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