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Cellulose Nanocrystals/Polyacrylamide Composites of High Sensitivity and Cycling Performance To Gauge Humidity Tao Lu,† Hui Pan,† Jun Ma,‡ Yao Li,† Syeda Wishal Bokhari,† Xueliang Jiang,§ Shenmin Zhu,*,†,∥ and Di Zhang† †

State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China ‡ School of Advanced Manufacturing and Mechanical Engineering, University of South Australia, Adelaide, South Australia 5095, Australia § School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430205, China ∥ National Engineering Research Center for Nanotechnology, Shanghai, P. R. China S Supporting Information *

ABSTRACT: Cellulose nanocrystals (CNCs) have attracted much interest due to their unique optical property, rich resource, environment friendliness, and templating potentials. CNCs have been reported as novel photonic humidity sensors, which are unfortunately limited by the dissolution and unideal moisture absorption of CNCs. We, in this study, developed a high-performance photonic humidity composite sensor that consisted of CNCs and polyacrylamide; chemical bonding was induced between the two components by using glutaraldehyde as a bridging agent. The composites inherited the chiral nematic structure of CNCs and maintained it well through a cycling test. A distinct color change was observed for these composites used as a humidity indicator; the change was caused by polyacrylamide swelling with water and thus enlarging the helical pitch of the chiral nematic structure. The composites showed no degradation of the sensing performance through cycling. The excellent cycling stability was attributed to the bonding between polyacrylamide and CNCs. This composite strategy can extend to the development of other photonic indicators. KEYWORDS: cellulose nanocrystals, chiral nematic liquid crystals, humidity response, sensor, bioinspired, photonic materials

1. INTRODUCTION Cellulose nanocrystals (CNCs) are generally prepared by treating cellulosic biomass with sulfuric acid, which results in sulfate ester groups on the surface; hence, CNCs form stable colloidal suspension, and at micrometer scale they can selfassemble into chiral nematic liquid crystalline.1 CNCs have drawn great attention due to their unique optical property, rich resource, environment friendliness, and templating potentials. More importantly, the chiral nematic structure remains even when the water in the suspension has evaporated, leading to a unique film of iridescence which has attracted many applications such as coloration, reflection, and security.1−6 Recently, the functionality of the CNCs has been extended and tuned by responsive polymers, to be used as responsive composite indicators.7−13 MacLachlan’s group has extensively researched photonic hydrogels comprising CNCs and responsive polymers. It is the first report to prepare flexible iridescent photonic composites consisting of CNCs and melamine−urea−formaldehyde, with the composite color tuned by external pressure.10 In further © 2017 American Chemical Society

studies, CNCs were assembled with a number of hydrogels polyacrylamide, poly(N-isopropylacrylamide), poly(acrylic acid), poly(2-hydroxyethyl methacrylate), and poly(ethylene glycol methacrylate)to create responsive CNC/polymer nanocomposites. The color of these nanocomposites could be tuned by external stimuli. For example, for a CNC/ polyacrylamide hydrogel soaked in a mixture of water and ethanol, it was found that increasing the ethanol ratio proved effective to shift the color from dark red to yellow.11 These studies established a solid foundation for the development of responsive CNC photonic hydrogels toward novel sensors and other chiral optoelectronic devices. Humidity sensing is of great importance to many applications, such as biomedical science, food industry, and agriculture.14−17 The market size of humidity sensors was estimated to reach over US $2 billion in 2020.18 Mark P. Received: March 31, 2017 Accepted: May 10, 2017 Published: May 10, 2017 18231

DOI: 10.1021/acsami.7b04590 ACS Appl. Mater. Interfaces 2017, 9, 18231−18237

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic of Synthesis of Cellulose Nanocrystals (CNCs)/Polyacrylamide Compositesa

a

The rectangle at the bottom shows how glutaraldehyde bridges CNCs with polyacrylamide. The square reveals the structural change of iridescent films as a function of humidity.

Andrews reported that a neat CNC film could be used to gauge humidity through a reflectance spectrum, since water vapor proved to expand the pitches of the CNCs’ chiral nematic structure.6 But it is far from real applications unless the following challenges are addressed. First, CNCs are inherently susceptible to redissolution in water, leading to poor cycling stability; they must be stabilized without compromising the chiral nematic phase. Second, neat CNC film has a narrow sensing range for humidity due to its limited moisture absorption. Elastomers upon chemical linking demonstrate exceptional cycling performance,19 so the first challenge should be addressed by chemical linking CNCs from our perspectives. According to the Bragg diffraction principle, increasing the sensing range for the second challenge can be obtained with a larger volume change through adding into CNCs high water vapor absorption polymers from our knowledge, e.g., poly(ionic liquid), poly(ethylene glycol) acrylate, and polystyrene-bquaternized poly(2-vinylpyridine) block copolymers. Of these, polyacrylamide is the most promising because of its exceptional moisture absorption and the associated enormous volume change. Therefore, hypotheses made herein include (i) using glutaraldehyde to chemical linking CNCs would improve the cycling stability and (ii) compounding polyacrylamide with CNCs would improve the sensing range. Herein we report the fabrication of a highly stable humidity indicator, which will comprise chiral nematic cellulose nanocrystals (CNCs) and polyacrylamide. When exposed to variety humidity, the volume of the polymer polyacrylamide should change by water vapor uptake or dehydration, leading to a great change of the CNCs’ helical pitch. As a result, an enhanced color change will be observed. A key strategy is that glutaraldehyde will be introduced into the system as the double aldehyde group of glutaraldehyde could react with both amino groups of polyacrylamide and hydroxyl groups of CNCs. That is polyacrylamide will be chemically bonded onto the CNC surface to improve the device stability.

2.2. Synthesis of Cellulose Nanocrystals (CNCs). Commercial cotton was first immersed into alkali solution to degrease. The degreased cotton was hydrolyzed in sulfuric acid (64 wt %, 10 mL of sulfuric acid solution per gram of cotton) at 65 °C with vigorous stirring for 1 h. The cellulose suspension was then diluted with cold deionized water (∼10 times the volume of the acid solution used) to end the hydrolysis and allowed to settle overnight. The clear top layer was decanted, and the remaining cloudy layer was centrifuged. The supernatant was decanted, and the resulting thick white suspension was washed 3 times with the water to remove all soluble cellulosic materials. The thick white suspension was placed inside dialysis membrane tubes (12 000−14 000 molecular weight cutoff) and dialyzed against deionized water at a low speed for 1−4 days to reach a constant pH. The suspension was dispersed by ultrasound treatment in a Fisher Sonic Dismembrator (Fisher Scientific) for 10 min at 60% power and then diluted to a desired concentration (3 wt %). The final CNC suspension was measured to have a zeta potential of −49.4 mV with a pH value of ∼3. 2.3. Synthesis of the Photonic Composites. Acrylamide (100 mg), potassium persulfate (3 mg), and N,N′-methylene bis(acrylamide) (3 mg) were dissolved in deionized water (10 mL) to form precursor solution. After that, glutaraldehyde (25 wt %, 60 μL) was added; then three batches of CNC suspension each 5 mL were respectively mixed with the solution of different volume 0.8, 1.6, and 3 mL to produce a series of composites at 5, 10, and 15 wt %. These mixtures were sealed in bottles with N2 atmosphere for polymerization to occur at 60 °C for 6 h. Afterward, the mixtures were cast onto polystyrene Petri dishes, followed by evaporation for several days to obtain iridescent composite films; these films were denoted CNC-P5 (containing 5 wt % polyacrylamide), CNC-P10 (10 wt %), and CNCP15 (15 wt %). A control sample (CNC-Control) was prepared with the same procedure as per the composites but with no glutaraldehyde in the precursor. 2.4. Characterization. Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4800 field emission microscope at an accelerating voltage of 10 kV. FT-IR results were recorded on KBr pellets with a Thermo Fisher Nicolet 6700 spectrophotometer. Atomic force microscopy (AFM) images were taken on a multimode NanoScope IIIa system (Digital Instruments). Zeta-potential measurements were made using a Malvern Zetasizer at 25 °C. For humidity responsive measurements, saturated salt solutions of LiCl, MaCl2, Mg(NO3)2, NaCl, and K2SO4 were used to provide accurate relative humidity at 11, 33, 54, 75, and 97%, respectively. The saturated vapor pressure of the different salt solutions is different, leading to the different relative humidities, and the relative humidity of each solution was measured by using an Omega HH314A humidity temperature meter. Spectra were recorded on ARM61 microscopic angle-resolved spectrometer by Shanghai Ideaoptics Corporation, and optical images were taken by a digital optical microscope (Keyence VHX-1000). All films were pasted onto quartz plates and sealed in a chamber with saturated salt solutions inside. The detailed information

2. EXPERIMENTAL SECTION 2.1. Materials. Acrylamide, potassium persulfate, glutaraldehyde, NaOH, sulfuric acid, LiCl, MaCl2, Mg(NO3)2, NaCl, and K2SO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. N,N′Methylene bis(acrylamide) was purchased from Sigma-Aldrich Co. LLC. Acrylamide was purified before use. Deionized water was used as solvent. 18232


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ACS Applied Materials & Interfaces about the experimental setup can be found in Scheme S1. Polarized optical images were obtained by an optical microscope with crossed polarizers (Zeiss Axio Scope A1Microscope) at 90° azimuth.

The analysis of FT-IR spectra (Figure 2) evidenced the chemical bonding formed between polyacrylamide and CNCs

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of CNC/Polyacrylamide Composites. Scheme 1 reveals the synthesis process. First, cellulose nanocrystals (CNCs) suspension was obtained by the hydrolysis of natural cotton using sulfuric acid. The AFM image showed a needle-like morphology for the CNCs (Figure S1) with a diameter of less than 20 nm and a length of 300−400 nm. The CNCs were negatively charged with a zeta potential of −49.4 mV (Figure S2) with obvious fingerprint texture in the polarized optical image, indicating strong chiral nematic characteristics (Figure S3). Then, monomer acrylamide, initiator potassium persulfate, and bridging agent glutaraldehyde were added into the suspension, leading to a homogeneous mixture. The polymerization occurred at 60 °C in the mixture, during which glutaraldehyde bridged CNCs with polyacrylamide through chemical bonding (reaction mechanism shown in Scheme S2). The product was cast onto several polystyrene Petri dishes, followed by natural evaporation for several days to obtain iridescent composite films. The content of polyacrylamide could be readily tuned by adjusting the mixture composition. For convenience, the composites containing 5, 10, and 15 wt % of polyacrylamide were respectively denoted CNC-P5, CNC-P10, and CNC-P15. A control sample was also prepared by keeping the same process except removing glutaraldehyde, which is denoted CNC-Control. The multilayered structures of a dried CNC film and its nanocomposites are investigated by scanning electron microscopy (SEM). The left-handed twisting rod morphology of a chiral nematic phase is obvious for all the samples in their crosssection images.20 The nanocomposite containing 5 wt % polyacrylamide (CNC-P5) and 10 wt % polyacrylamide (CNCP10) has similar structural regularity to the neat CNC film (Figures 1b and 1c). But the regularity is slightly lowered for CNC-P15, as shown in Figure 1d; CNCs appear to be coated by the polymer, blurring the multilayered structure.

Figure 2. FT-IR spectra of neat CNC film, CNC-P10, the control sample (CNC-Control; see details in Experimental Section), and neat polyacrylamide.

for all the composites. For neat polyacrylamide, the peak at 1662 cm−1 represents −CO stretching (amide I), and this peak still exists in CNC-P10, indicating the existence of polyacrylamide. In CNC-P10 where glutaraldehyde is used, a new peak appeared at 1646 cm−1 characteristic of imine stretching vibrations,21 which is attributed to the reaction between the amino groups of polyacrylamide and the aldehyde groups of glutaraldehyde.22−24 In terms of neat CNC film, the peak at 1637 cm−1 represents the bending vibration of the −OH groups in cellulose.25 As is well-known, the aldehyde groups of glutaraldehyde can react with hydroxyl groups easily to form −C−O bonds.26−28 Thus, glutaraldehyde herein acts as the bridging agent to form chemical bonds between the CNCs and polyacrylamide (as shown in Scheme 1 and Scheme S2). By contrast, this phenomenon is not observed for the control sample which has no glutaraldehyde. 3.2. Photonic Properties of CNC/Polyacrylamide Composites. Strong birefringence is observed in all images by polarized optical microscope in Figure S4. This means the formation of a fingerprint texture after water evaporation, indicating that chiral nematic phase can be established during the drying in the presence of polyacrylamide. A red-shift is observed with increase in the polymer content, which agrees with the following reflectance spectra in Figure 3. The reflectance investigation unlocks the evolution of iridescent color of the samples. In Figure 3, neat CNC film has a reflectance peak at ∼500 nm with blue-green color. A redshift in the reflectance spectra is seen with increase in the

Figure 1. Cross-section SEM images of (a) pure CNC film and (b−d) CNC/polyacrylamide composites respectively containing 5, 10, and 15 wt % polymer.

Figure 3. Reflectance spectra of neat CNC film and its polyacrylamide composites. The reflectance peak value depends on the polymer content. 18233


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Figure 4. Humidity sensing CNC/polyacrylamide composites: reflectance peaks (a) under different humidity; reflectance spectra (b), (c), and (d) respectively for CNC-P5, CNC-P10, and CNC-P15; and optical microscope images below (c) and (d).

sufficient to swell the composite. For CNC-P10 described in Figure 4c, the reflectance peak shifts from 568 nm at relative humidity 11% to higher wavelength of 576, 593, 621, and 650 nm respectively corresponding to humidity 33%, 54%, 75%, and 97%; these relate to color change from green to red, as indicated in the optical microscope images below Figure 4c. In Figure 4d, CNC-P15 has reflectance peak values of 590, 598, 621, 663, and 708 nm respectively at 11%, 33%, 54%, 75%, and 97% humidity, which correspond to color changes as shown in the images below Figure 4d. CNC-P15 has larger wavelength shifts than CNC-P10. It is worth noting that the reflectance peak wavelength of CNC-P15 at 97% humidity shifts to a nearinfrared region at 710 nm, resulting in faded red. To detect the response time, all three samples are first placed under 11% humidity and then exposed to 97%, recording the reflectance spectra every 30 s (as shown in Figure S5). The reflectance peak made a quick red-shift before 1 min, and the whole response process needs 2−3 min to get equilibrium. Polarized optical images also show the robustness of the chiral nematic structure under various humidity. The fingerprint texture of CNC-P10 can be observed at each of the testing humidity. A strong birefringence phenomenon is seen in the dotted box in Figure S6. The color of the composite film evolves from green to reda red shiftwith the humidity increase from 11 to 97%. This was consistent with the reflectance spectra and optical microscopic images in Figure 4. The reflectance peak caused a red shift of ∼50 nm when a dry CNC film was exposed to 95% relative humidity.6 This work is respectable, but the 50 nm shift may not sufficient for colorimetric sensing. The limited shift is due to the low moisture absorption of CNCs. By contrast, our CNC/ polyacrylamide composites have shown far better sensitivity in humidity sensing. A red shift of 92 nm was found for all of the composites. The enhancement in sensitivity is due to the introduction of polyacrylamide. Since polyacrylamide absorbs more moisture than CNCs and it can expand significantly, the introduction of polyacrylamide into CNCs would greatly

polymer content, and this phenomenon can be explained by the following analysis. It is known that chiral nematic structures are actually onedimensional photonic crystals. These structures are able to selectively reflect circularly polarized light with a handedness which matches the following equation:

λmax = navg P sin θ

(1)

where navg is the average refractive index, P is the helical pitch, and θ is the angle between the liquid crystal surface and the incident light (herein light perpendicular to the sample surface, so sin θ = 1).29 The reflectance peak λmax is related to the average refractive index and helical pitch of the composites. As CNCs and polyacrylamide have similar refractive indexes (1.54 and 1.50), navg can be treated as a constant in our system. Therefore, the red-shift in the reflectance peak was determined only by the helical pitch. The helical pitch increases with the polymer content, leading to the red-shift. However, the reflectance intensity reduces with increase in the content. This is due to the structural collapse at a high polymer content, as revealed by the SEM in Figure 1. 3.3. Humidity Responsive Sensitivity of the CNC/ Polyacrylamide Composites. It is known that polyacrylamide is highly sensitive to humidity, and it has been widely used as humidity sensors.30−32 The combination of the unique optical properties of CNCs with that of polyacrylamide is expected to create composites of high-sensing performance. The performance was first investigated by detecting the reflectance change at relative humidity from 11 to 97%. Figure 4 shows the reflectance spectra and colors of the composites. All three samples show red-shift when the humidity rises, which means that the responses can be tailored by the composite composition. For the composite CNC-P5 which contains 5 wt % polyacrylamide (Figure 4b), only a small reflectance shift from 540 to 570 nm is seen when relative humidity rises from 11 to 97%. This suggests that the 5 wt % polymer is not 18234


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Figure 5. Cycling stability of (a) and (b) for CNC-P10 and (c) and (d) for the control sample (CNC-Control). The two samples were soaked in water and then dried for ten cycles in total.

expand the CNCs’ helical pitch, which should lead to a larger red shift in reflectance spectra. Besides, the responsive property can be readily tuned by varying the polymer fraction in the composites. 3.4. Cycling Stability of the CNC/Polyacrylamide Composites. As a humidity indicator, the cycling stability is another key aspect. The stability of CNC-P10 (a composite containing 10 wt % polyacrylamide) was measured by immersing the samples into water for 10 min to reach equilibrium, followed by drying afterward. The results are shown in Figures 5a,b. When it was immersed into water, the sample had an obvious red shift to the near-infrared region (∼760 nm). This red shift is much larger than the aforementioned humidity sensing process (Figure 4c) because in this experiment liquid water rather than vapor directly contacted the sample. In Figures 5a,b, no compromise in the sensing performance is observed after 10 cycles. The control sample CNC-Control comprises CNCs and polyacrylamide with no chemical linking, different from other composites which were bridged by glutaraldehyde; that is, CNCs and polyacrylamide were physically blended. Figures 5c and 5d contain the stability measurement of CNC-Control. Both the intensity and position of the reflectance peak evolve with the cycling, and the peak intensity fades obviously with the cycles. The reflectance intensity of CNC-Control at the first cycle takes 48% of that of the original state, and it reduces to 34% and 17% respectively at the fifth and tenth cycle. Furthermore, the peak position was found to shift. In specific, the peak at the first cycle red shifts from 580 to 760 nm, and after 10 cycles it was at 720 nm. A detailed explanation is given in the next paragraph. The poor stability of CNC-Control may relate to its structure destruction through the cycling. In order to find out the reason, the cross-section morphologies of CNC-P10 and CNC-Control were examined by SEM before and after the cycle test. In Figure 6, the chiral nematic structure of CNC-P10 after the test (Figure 6b) remains intact similar to the original state (Figure 6a). CNC-Control has chiral nematic structures before the cycling (Figure 6c), but this structure is largely destroyed after

Figure 6. Cross-section SEM images of CNC-P10 (a composite containing 10 wt % polyacrylamide) before (a) and after (b) 10 cycles and the control sample before (c) and after (d) 10 cycles.

the test (Figure 6d) due to the aggregation of polyacrylamide. The disappearance of the chiral nematic structure would explain the intensity fade in reflectance peak, and the polymer aggregation may result in the wavelength shift. It is now clear that the excellent stability of CNC-P10 is contributed by the chemical linking from glutaraldehyde. Each glutaraldehyde molecule has double aldehyde groups, each of which can react with the amide groups of polyacrylamide and the hydroxyl groups of CNCs. As shown in Scheme 1 and the FT-IR results in Figure 2, −C−O bonding would form between glutaraldehyde and CNCs and −CN bonding would form between glutaraldehyde and polyacrylamide. Although water can significantly swell CNC-P10, the polymer and CNCs would not separate from each other due to these chemical bondings, leading to excellent cycling performance.

4. CONCLUSION In summary, a humidity indicator of high sensitivity and stability was developed by using bioinspired, photonic 18235


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composites which comprised chiral nematic cellulose nanocrystals, polyacrylamide, and glutaraldehyde. The composite color shifted from green to red as relative humidity increased from 11% to 97%, and the whole color change corresponded to a wavelength range of 92 nm. Such a high sensitivity was caused by polyacrylamide swelling with water and thus enlarging the helical pitch of the chiral nematic structure of CNCs. The composites showed no degradation of the sensing performance through cycling, which was attributed to the bonding between polyacrylamide and CNCs. This nanocomposite approach is extendable to the development of various photonic indicators, which are useful to applications such as sensors, decorations, and anti-counterfeiting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04590. Experimental setup, reaction mechanism, zeta-potential result of CNC dispersions; AFM image, POM images and the response time of CNCs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.Z.). ORCID

Jun Ma: 0000-0002-6676-7779 Shenmin Zhu: 0000-0003-3145-4446 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of National Key R&D Program of China (2016YFA0202900), the National Science Foundation of China (51672173), Shanghai Science and Technology Committee (16520710900,14JC1403300, 14DZ2261204, 14520710100), and Science and Technology Planning Project of Guangdong Province (2016A010103018).

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REFERENCES

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