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Responsive Photonic Hydrogel-Based Colorimetric Sensors for Detection of Aldehydes in Aqueous Solution Xiaolu Jia, Tian Zhang, Jianying Wang, Haiying Tan, Yuandu Hu, Lianbin Zhang, and Jintao Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00186 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Responsive Photonic Hydrogel-Based Colorimetric Sensors for Detection of Aldehydes in Aqueous Solution Xiaolu Jia,1,† Tian Zhang,1,† Jianying Wang,2 Ke Wang,1 Haiying Tan,1 Yuandu Hu,1 Lianbin Zhang,1,* and Jintao Zhu1,3,* 1

Key Laboratory of Materials Chemistry for Energy Conversion and Storage (HUST),

the Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China 2

Hubei Key Laboratory of Polymer Materials, School of Materials Science and

Engineering, Hubei University, Wuhan 430062, China 3



Shenzhen Research Institute of HUST, Shenzhen 51800, China

These authors contribute equally to this work

*Corresponding authors E-mail: [email protected] (L. Z.); [email protected] (J. Z.) Tel: 86-27-87793240; Fax: 86-27-87543632

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ABSTRACT: In this work, we present a fast and efficient strategy for the preparation of responsive photonic hydrogels for aldehyde sensing by combining the self-assembly of monodisperse

carbon-encapsulated

Fe3O4

nanoparticles

(NPs)

and

in-situ

photopolymerization of polyacrylamide (PAM) hydrogels. The responsive photonic hydrogels exhibit structural color variation after being treated with formaldehyde aqueous solution, which can be attributed to the chemical reaction between the amide groups in the hydrogels and the formaldehyde. We have also shown that the photonic hydrogels can be used to determine the concentration of formaldehyde and to differentiate aldehydes through a facile reflection spectra shift and color change. This study provides a facile strategy for the visualized determination of aldehyde in aqueous solution. KEYWORDS: Responsive photonic crystals, Hydrogels, Self-assembly, Aldehyde sensor, Structural color

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1. INTRODUCTION Formaldehyde is an important industrial chemical and is widely used in many fields, including wood fixatives, textile, cosmetics and pharmaceutical processes.1 However, it is also a kind of best-known pollutants and poses great threat to the health of human beings and animals because of its carcinogenic and mutagenic properties.2-4 Various detective strategies and analytical methods have been developed to determine formaldehyde, such as fluorimetry,5,

6

infrared spectroscopy,7 photoacoustic

spectroscopy,8 chromatography,9, 10 electrochemistry,11, 12 field effect sensors,13 and quartz crystal microbalances.14 Although these methods can effectively detect the concentration of formaldehyde, they were mainly based on the spectroscopic analysis or sensors, and generally required the use of specific instruments and suffered from the high cost and complicated processing. Nowadays, it is still highly desired to develop facile, low-cost and visual methods for rapid on-site formaldehyde determination. Photonic crystals (PCs) are periodic optical nanostructures that can exhibit structural colors. Responsive PCs (RPCs) can alter their structures upon external stimuli and show obvious variation of structural color.15-27 Therefore, RPCs can be used as colorimetric sensors with a facile readout capability, allowing for a visual detection. These RPCs have been used for the visualized determination of solvent,15-20 pressure,21 pH,22-24 humidity,25 glucose,26 and ions27 with satisfactory sensitivity and responsiveness. In our previous report, we have developed a responsive photonic hydrogel by

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self-assembly of monodispersed carbon-encapsulated Fe3O4 nanoparticles (NPs) and in-situ photopolymerization of cross-linked hydrogels.21, 22 The responsive photonic hydrogels are highly sensitive to mechanical compression and organic solvents. Herein, by optimizing the composition and structure of the photonic hydrogels, we have successfully applied the responsive photonic hydrogels for the visualized determination of aldehyde in aqueous solution. Compared with traditional aldehydes sensors,5-14 the responsive photonic hydrogels reported in the current study have the advantages of rapid preparation, visual readout, low cost, and fast detection. The entire preparation process for the current responsive photonic hydrogels merely takes less than 30 min. Furthermore, concentration of aldehydes can be determined by directly observing the structural color of photonic hydrogels, greatly facilitating the sensing process.

2. EXPERIMENTAL SECTION 2.1. Materials. Ferrocene (purity > 98%), ethylene glycol (purity > 99%), hydrogen peroxide solution (H2O2, 30% solution in water), acetone (purity > 99%), formaldehyde solution (40% solution in water), acetaldehyde solution (40% solution in water), propionaldehyde solution (40% solution in water), and acrylamide (AM, purity

>

98%)

were

purchased

from

Sinopharm.

2-Hydroxy-4'-(2-hydroxyethoxy)-2-methyl-propiophe (photoinitiator 2959, purity > 98%) and N,N-methylenebisacrylamide (BIS, purity > 98%) were purchased from Aldrich. All the chemicals were used as received without further purification. Deionized water was used for all the experiments. -4ACS Paragon Plus Environment

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2.2. Methods. Synthesis of carbon-encapsulated Fe3O4 NPs Fe3O4 NPs were prepared through a previously reported procedure.28 Typically, ferrocene (0.45 g) was dissolved in acetone (40 mL) to form a brown solution. Then, H2O2 solution (1.5 mL) was added slowly to the solution, and the mixture was stirred for 30 min. Subsequently, the mixture was transferred into a Teflon-lined autoclave and heated at 180 °C for 70 h, followed by cooling down to room temperature. The obtained precipitates were washed with acetone three times and were dried at 60 °C for 6 h under vacuum. Preparation of responsive photonic hydrogels Typically, AM (0.5 g), BIS (0.02 g), and photoinitiator 2959 (0.01 g) were dissolved in 2 mL of ethylene glycol, and the mixture was sonicated for 5 min to obtain a transparent solution. Then, 0.008 g of the as-prepared Fe3O4 NPs was added to the solution and the mixture was sonicated for 10 min, with the concentration of the Fe3O4 NPs in the suspension being 4 mg/mL. The suspension was sandwiched between two clean glass slides to form a thin film with a thickness of 300 µm under a magnetic field of 800 GS, under which the Fe3O4 NPs self-assembled into one-dimensional (1D) chain-like structure within 30 s. Finally, polymerization was carried out under UV irradiation for ~ 15 min, resulting in a cross-linking density of 4% (mass ratio between the cross-linker and monomer). The ethylene glycol was replaced with water by immersing the film in deionized water.

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2.3. Characterization. Transmission electron microscopy (TEM) investigation was performed on an FEI Tecnai G220 TEM with a CCD camera (Gatan USC4000, Gatan) operated at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) investigation was performed on a Sirion 200 FEI SEM at an acceleration voltage of 10 kV. Reflection spectra characterizations of the photonic hydrogels were carried out on a fiber optic spectrometer (USB4000, Ocean Optics). Digital photographs of the photonic hydrogels films were obtained with a Canon IXUS 105 digital camera. IR spectra were recorded on the infrared spectrometer of Bruker Equinox55.

3. RESULTS AND DISCUSSION 3.1. Fabrication of Photonic Hydrogels Monodispersed Fe3O4 NPs were readily obtained via a one-step solvothermal method.28 Figure 1a shows the morphologies of the as-obtained superparamagnetic Fe3O4 NPs, which was actually covered with a layer of carbon with carboxyl groups on the surface. The spherical NPs had an average diameter of 125 ± 6 nm. The presence of the negatively charged carboxyl groups on the surface of carbon greatly improved the stability in the dispersed media and prevented the aggregation of Fe3O4 NPs.29-31 We have previously shown that when the electrostatic repulsive and magnetic forces between Fe3O4 NPs reached a balance, the NPs can form 1D chain structure under external magnetic field.21, 22 The 1D ordered chain structure of Fe3O4 NPs can be fixed in the PAM hydrogels after the photo-polymerization of AM. In the preparation of the photonic hydrogel, an optimized concentration of 4 mg/mL for the -6ACS Paragon Plus Environment

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Fe3O4 NPs and cross-linking density of 4% was chosen in this study (see the Supporting Information (SI), Figure S1 and S2). Morphology of the photonic hydrogels was characterized by the SEM investigation (Figure 1b and Figure S3), and the results indicated that the Fe3O4 NPs in the hydrogels maintained their 1D ordered chain structure. Due to the periodically ordered chain structure of NPs in the hydrogels, brilliant structural color can be observed with naked eyes (Figure 1c). The reflection spectra of photonic hydrogels were recorded and shown in Figure 1d. The position of the maximum reflection peak of the photonic hydrogels follows the Bragg’s law: λmax=2 neff d

(1)

Where λ is the diffraction wavelength, neff is the effective refractive index of the material, d is the center-to-center space of colloidal particles. Thus, the diffraction wavelength and resulting structural color can be tuned by manipulating the lattice spacing of the NPs in the photonic hydrogels.17, 18 3.2. Photonic Hydrogels for aldehyde Sensing Hydrogels are flexible polymer networks in aqueous solution and the volume of hydrogels can be changed under external stimuli. If PCs are combined with hydrogels, the volume changes of hydrogels network can vary the center-to-center space of colloidal particles, leading to a variation of structural color. Such a color change of RPCs under external stimuli can be used for visual detection application. In the current study, the as-prepared RPCs consist of PAM, whose amide groups can effectively undergo hydroxymethylation reaction with aldehyde in an aqueous

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solution of Na2CO3,32 leading to a chemically structural variation of the hydrogels and thus a corresponding color change of the RPCs. To this end, we immersed the responsive photonic hydrogels in an aqueous solution (30 mL) of 0.3 mol/L formaldehyde and 0.06 mol/L Na2CO3 for 2 h at 5 oC. After being treated, the photonic hydrogels was then placed in deionized water for the measurement of the reflection spectra. The structural color of the hydrogels film changed from blue to green gradually as indicated in Figure 2a and 2f, and the reflection peak showed an obvious red-shift in 15 min and ultimately shifted to 547 nm within 2 h (Figure 3). The responsive mechanism of the photonic hydrogels can be ascribed to the variation of Flory-Huggins interaction parameter between the polymer and solvent.33-35 As illustrated in Figure 4, with the hydroxymethylation reaction between amide groups and formaldehyde taking place, the polar hydroxyl group was produced in the hydrogels, leading to the increase in the polarity of the hydrogels and increase of polymer-water interaction parameter.33-35 The enhanced interaction between the polymer and water resulted in the expanding of the photonic hydrogels and the increase of the lattice spacing, which leads to color change. Figure 5 compares infrared spectra of the hydrogels before and after being reacted with formaldehyde. The appearance of the C-O stretching vibration at 1018 cm-1 in the hydrogels after the treatment with formaldehyde demonstrated the successful hydroxymethylation reaction. The shift of the N-H bending vibration to 1539 cm-1 after the treatment with formaldehyde demonstrated that the primary amine was turned into a secondary amine.32, 36 As a control experiment, the photonic hydrogels

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were also placed in the aqueous solution of Na2CO3 without formaldehyde for 2 h and then were immersed in the deionized water. The reflection peak of the photonic hydrogels shows no clear shift (Figure 6), indicating that the reaction between amide groups and formaldehyde was the main reason for the structural color variation. Furthermore, we found that as the concentration of formaldehyde solution decreased from 0.3 mol/L to 0.03 mol/L, the reflection peak of photonic hydrogels shifted from 501 nm to 518 nm (Figure 2). While for the formaldehyde solution with a concentration of 0.003 mol/L, a red shift of 7 nm of the reflection peak was observed (Figure 2). By utilizing the spectra shift and color change of the hydrogel, we can readily determine the concentration of the formaldehyde solution. Moreover, the RPCs developed in this study can be used for the determination of the other aldehydes (Figure 7). For example, after being immersed in acetaldehyde (0.3 mol/L) solution for 2 h, the reflection peak of photonic hydrogels shifted from 501 nm to 539 nm. Furthermore, the reflection peak of photonic hydrogels shifted from 501 nm to 521 nm after reaction with propionaldehyde (0.3 mol/L). After reaction with formaldehyde, acetaldehyde, and propionaldehyde, respectively, the polarity of photonic hydrogels increased differently, which made the volume of photonic films and structural color changed differently. Due to the maximal polarity increase of photonic hydrogels after being reacted with formaldehyde, the reflection peak of photonic hydrogels had a maximal red-shift. Therefore, the current responsive photonic hydrogels can be used to differentiate aldehydes by a simple color change. 3.3. Selectivity of the Photonic Hydrogels

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For an effective aldehydes sensor, the selectivity is one of the most important aspects for practical applications. In order to demonstrate the selectivity of the photonic hydrogels toward aldehydes, the photonic hydrogels were immersed in the aqueous solution of 0.06 mol/L Na2CO3 containing different interfering chemicals, such as acetone, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), glucose, sodium acetate, urea, polyethylene glycol, lysine, pyridine, and their mixtures. As shown in Figure 8, the reflection peak of photonic hydrogels did not change after 2 h immersion in the presence of these possible interferences, demonstrating that the photonic hydrogels had better selectivity toward aldehydes.

4. CONCLUSIONS In summary, we have demonstrated a fast and efficient strategy for the preparation of responsive photonic hydrogels for aldehyde sensing through the rapid self-assembly of magnetic NPs and in-situ photo-polymerization. The photonic hydrogels can be used to selectively detect aldehyde in aqueous solution by the change of structural color, and also can be used for distinguishing different aldehydes. This work provides a facile strategy for the preparation of visualized chemosensors for aldehydes sensing.

ASSOCIATED CONTENT Supporting Information available: Additional figures showing the NPs suspension with different concentrations, reflection spectra for hydrogels with varied cross-linking density, and surface morphology of the dried photonic hydrogel. This materials is available free of charge via the internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (L. Z.); [email protected] (J. Z.) Notes The authors declare no competing financial interest.

Acknowledgements We gratefully acknowledge funding for this work provided by the National Natural Science Foundation of China (51525302), Natural Science Foundation of Hubei Scientific Committee (2016CFA001), Open Research Fund of State Key Lab of Polymer Physics & Chemistry, CIAC, CAS (2017-27) and Shenzhen Science and Technology Project (JCYJ20150630155150194). We also thank HUST Analytical and Testing Center for allowing us to use its facilities.

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Figures:

Figure 1. (a) SEM image of monodisperse carbon-encapsulated Fe3O4 NPs. Inset in (a): TEM image of carbon-encapsulated Fe3O4 NPs with core–shell structure. (b) SEM image of the linearly-ordered Fe3O4 NPs with 1D structure in PAM hydrogels. Inset in (b): the magnified SEM image of the 1D Fe3O4 NPs chain. (c) Photograph of the photonic hydrogels. (d) Reflection spectra of the as-obtained photonic hydrogels.

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Figure 2. (a-f) Photographs and (g) corresponding reflection spectra of the photonic hydrogels treated in the aqueous solution containing 0.06 mol/L Na2CO3 (pH = 11.3) and formaldehyde of (a) 0 mol/L, (b) 0.003 mol/L, (c) 0.015 mol/L (d) 0.030 mol/L, (e) 0.150 mol/L, (f) 0. 3 mol/L for 2 h, then placed in deionized water (pH = 6.8). The scale bar in (a) applies to (b-f). (h) The dependency of reflection peak on the concentration formaldehyde.

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Figure 3. Reflection spectra of the photonic hydrogels treated in the aqueous solution containing 0.06 mol/L Na2CO3 and 0.3 mol/L formaldehyde for different durations.

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Figure 4. Schematic illustration showing the variation of lattice spacing and reaction in the photonic hydrogels immersing in the aqueous solution containing formaldehyde. Volume of photonic hydrogels expands after being treated with formaldehyde, resulting in the increase of lattice space of the NPs and corresponding red-shift of the reflection peak.

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Figure 5. Infrared spectra of the responsive photonic hydrogels before and after being treated with the aqueous solution containing 0.3 mol/L formaldehyde.

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Figure 6. Reflection spectra of the photonic hydrogels in the aqueous solution containing Na2CO3 (0.06 mol/L) for 2 h.

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Figure 7. (a-d) Photographs and (e) corresponding reflection spectra of the photonic hydrogels treated in the aqueous solution containing 0.06 mol/L Na2CO3 (pH = 11.3) and (a) 0 mol/L formaldehyde, (b) 0.3 mol/L formaldehyde, (c) 0.3 mol/L acetaldehyde. The scale bar in (a) applies to (b) and (c). (d) 0.3 mol/L propionaldehyde for 2 h, then placed in deionized water (pH = 6.8).

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Figure 8. Reflection spectra of the photonic hydrogels treated with the Na2CO3 (0.06 mol/L) aqueous solution containing 0.3 mol/L glucose, sodium acetate, urea, polyethylene glycol, lysine, acetone, DMF, DMSO, and a mixture of glucose, urea, and lysine, for 2 h, respectively.

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For the table of contents use only: Title: Responsive Photonic Hydrogel-Based Colorimetric Sensors for Detection of Aldehydes in Aqueous Solution Authors: Xiaolu Jia, Tian Zhang, Jianying Wang, Ke Wang, Haiying Tan, Yuandu Hu, Lianbin Zhang, and Jintao Zhu TOC figure:

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