Polyvinylpyrrolidone

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Iridescent Chiral Nematic Cellulose Nanocrystal/Polyvinylpyrrolidone Nanocomposite Films for Distinguishing Similar Organic Solvents Yunlong Gao, and Zhaoxia Jin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04899 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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ACS Sustainable Chemistry & Engineering

Iridescent Chiral Nematic Cellulose Nanocrystal/Polyvinylpyrrolidone Nanocomposite Films for Distinguishing Similar Organic Solvents Yunlong Gao and Zhaoxia Jin* Department of Chemistry, Renmin University of China, Beijing 100872, China. KEYWORDS: cellulose nanocrystal; polyvinylpyrrolidone; structural color; chiral nematic liquid crystalline phase; organic solvents detection;

Mailing Address: Department of Chemistry, Renmin University of China, 59 Zhongguancun Avenue, Haidian District, Beijing 100872, PR China. E-mail Address: [email protected]

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ABSTRACT

Fast and easily distinguishable color change is the simplest sensor signal recognized by naked eyes. Rapid color change has been done via filling or removing liquid inside channels constructed by multilayered reflectors in biological species for camouflage or signaling. By mimicking it, a test paper for fast detection of similar organic solvents or water-content in ethanol has been designed based on nanocomposite films composed of cellulose nanocrystals (CNC) and polyvinylpyrrolidone (PVP) with chiral nematic structure. The chiral nematic architecture and structural color of CNC can be kept in CNC/PVP nanocomposites in a wide range of PVP content, up to 70 wt%. Moreover，we have observed that the nanocomposite containing higher weight percentage of PVP showed more distinguishable color difference while dipping in similar solvents. Owing to the wide solubility of PVP in organic solvents and the magnifying effect via increasing content of PVP, CNC/PVP nanocomposite film can work as discrimination sensor by presenting apparent color change while dipping in several groups of similar

organic

solvents,

such

as

homologues

(methanol/ethanol),

skeletal

isomers

(1-propanol/2-propanol), halogenated hydrocarbons (choloroform/dichloromethane), and even ethanol containing small amount of water. The fast color change of CNC/PVP has been induced by the sensitive response of its structural color to volume fraction change of CNC in swollen state.


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INTRODUCTION Changing color rapidly is an efficient method for camouflage, signalling and communication of some biological species.1 Bream fish Pentapodus paradiseus2 and tortoise beetle Charidotella egregia3 are able to change their colors in response to surrounding environment. The conversion times are generally within seconds3 or minutes.2 Detailed investigations have revealed that such fast a color-change was achieved through bringing or removing fluid inside channels composed of multilayer reflectors in elytron of Charidotella Egregia or paradise whiptail. Structural color based on periodically multilayer reflectors plays dominated role in this process. The environment-responsive structural color in biological species inspires scientists to develop new photonic materials with responsive colors based on self-assembled photonic structures.1, 4-12 Cellulose nanocrystals (CNCs) prepared through sulfuric acid hydrolysis of native cellulose, usually have anisotropic rod-like shapes (approximately 5-20 nm in diameter and 100-300 nm in length)13 and negatively charged surfaces (sulfate ester on the surface),14 which cause them to form left-hand chiral nematic liquid crystalline phases at relatively low concentration.15 Upon evaporation-induced self-assembly (EISA), CNCs retain their left-hand chiral nematic organization as they self-assemble into strong iridescent solid film, which is capable of reflecting left-handed circularly polarized light.16-17 When additives were dropped into CNC suspension, they organized together with CNCs, and the chiral nematic structure can be captured in these CNC-based

nanocomposites.18-19

Various

additives,

such

as

Si(OEt)4,

Si(OMe)4,

1,2-bis(trimethoxysilyl)ethane, 1,2-bis(triethoxysilyl)ethane,20-21 monomers like acrylamide, N-isopropylacrylamide, acrylic acid, 2-hydroxyethylmethacrylate,22-23 water-soluble polymers as polyvinyl alcohol and polyethylene glycol,24-27 and gold nanorods, silver nanowires and



fluorescent latex nanoparticles

28-30

have been intercalated into the chiral nematic structure of

CNCs. The reflected light wavelength of CNC-based nanocomposites can be tuned within visible light spectrum (500 ~ 800 nm) that offers a possibility of preparing sensitive sensor recognized by naked eyes. Compared with visual sensors based on inverse opal films with highly ordered nanopores,

6-8, 10-11, 31-32

the CNC self-assembled nanostructure has good compatibility with

diverse materials, making it easy for regulating surface property of sensors, thus CNC nanocomposites are promising candidate as new structural-color-based platform for sensor applications. Polymer additives in self-assembled CNC enlarge the helical pitch of CNC, resulting in the redshift of reflective light wavelength, but there is scarce investigation demonstrating the broad and fast responsibility of CNC-based nanocomposites to environmental stimuli, except humidity.27, 33-34 Exploring the broad application of CNC nanoarchitecture as visual sensor with high sensitivity is still challenging, but very attractive. In this work, flexible nanocomposite films composed of chiral-nematicly organized CNCs and amorphous polyvinylpyrrolidone (PVP) have been explored through comprehensive characterizations such as scanning electron microscopy (SEM), polarized optical microscopy (POM), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy and visible-near infrared spectroscopy (vis-NIR). We noticed that PVP as additive in the CNC self-assembled architecture presented three important features. The first is PVP is a neutral and amorphous polymer, and it cannot be cross-linked in EISA process. Therefore, its addition has little disturbance to the CNC self-assembled nanostructure, even in the composite with PVP content up to 70 wt%. The second feature is that PVP is soluble in wide range of organic solvents that extends the adaptability of CNC-based composite films. The third is that increasing PVP content in CNC/PVP composites will enlarge their color difference while dipping in similar organic


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solvents. For the convenience of naked-eye’s recognition, we have particularly focused on the iridescent color-change from visible to near infrared region of CNC/PVP nanocomposite films (from colored to pale) while immersing in organic solvents. Owing to their responsiveness to multiple solvents selectively, these CNC/PVP nanocomposite films have shown distinguishable color responses to similar solvents in wetting, such as homologues (methanol/ethanol), skeletal isomers (1-propanol/2-propanol) and halogenated hydrocarbons (chloroform/dichloromethane (DCM)). Moreover, CNC/PVP composite films also work for water detection in ethanol via dip-in. To the best of our knowledge, these films are the first-reported CNC-based sensors for distinguishing homologues, skeletal isomers, halogenated hydrocarbons and small amount of water in organic solvents.

Our investigation provided an operable, convenient, green and

cost-efficient pathway to develop sensors for identifying different organic solvents, which may have broad application as gauge sensors for different chemicals or solvent-responsible reflective filters.

EXPERIMENTAL SECTION

Materials. Filter paper (ashless) used to exact CNCs was obtained from Whatman Ltd. Polyvinylpyrrolidone (PVP, average Mw 58000) was purchased from Alfa Aesar Inc. Sulfuric acid was purchased from Sinopharm Chemical Reagent Co., Ltd. Methanol, ethanol (anhydrous ethanol),

acetone,

tetrahydrofuran

(THF),

1-propanol,

2-propanol,

chloroform

and

dichloromethane (DCM) were all purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. All were of reagent grade and used without further purification. Deionized water (Millipore



Q > 18 MΩ cm) was used for all experiments. Microporous membrane with 0.45 µm pore size was purchased from Jinteng Laboratory Equipment Co., Ltd. Preparation of Cellulose Nanocrystals. The extraction of CNCs from Whatman ashless filter paper was conducted following previously reported references.20, 35Filter paper was first cut into pieces and then hydrolyzed in sulfuric acid (64 wt%) at 45 oC under vigorous stirring for 45 min. 10 mL of sulfuric acid solution was used for 1g filter paper. The cellulose suspension was then diluted with cold Millipore water (about ten times the volume of the acid solution used) to stop the hydrolysis and allowed to settle about 1 week. The clear top layer was removed and the remaining cloudy part was then dialyzed inside dialysis membrane tubes with a 3500 molecular weight cut-off against slow running de-ionized water for 1-2 weeks. This final suspension was filtered through 0.45 µm membrane and concentrated by evaporation to reach a concentration of 3 wt%. The pH value of final CNC suspension was about 3-4. Preparation of CNC/PVP Nanocomposite Films. In a typical procedure, about 4-5 mL of aqueous CNC suspension (3 wt%) was sonicated for 10 min and subsequently various amounts of PVP aqueous solution (3 wt%) were slowly added to produce CNC/PVP mixtures with target ratios of CNC/PVP = 90/10-30/70 w/w. The mixed suspension was then stirred for about 2-3 days. Then homogeneous mixture was poured into a Petri dish and air-dried for 3-4 days. Free-standing CNC/PVP nanocomposite films were obtained after slow evaporation at room temperature and were named as CP90/10~CP30/70 (shown in Table S1). The thickness of these films was about 20~25 µm. Pure CNC or PVP films were prepared by casting aqueous CNC suspension (3 wt%) or PVP solution (3 wt%) directly onto Petri dish and evaporated at the same condition.


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Characterizations. Scanning electron microscopy (SEM) was performed on SU8010 electron microscope (Hitachi) at an accelerating voltage of 4 kV. The samples were coated with a thin layer of gold before characterization. Energy dispersive X-ray (EDX) mapping was also carried out on the SEM at an accelerating voltage of 15 kV without gold plating. Polarized optical microscopy (POM) was conducted on a Scope 1 microscope (Axio Zeiss) in transmission mode with a 10× eyepiece and a 10× objective. All images were taken with the polarizers in a perpendicular (crossed) arrangement. Infrared spectra were collected on a Nicolet iN10 MX FT-IR infrared spectrometer (Thermo Scientific) equipped with an attenuated total reflection (ATR) accessory. All the spectra were recorded at a resolution of 4 cm-1 and 32 scans were co-added. Raman spectra were performed on XploRA PLUS (Horiba Scientific) using a laser (532 nm, 25 mW) and the backscattering configuration. Spectra were calibrated using the 520 cm-1 line of a silicon wafer and the laser beam was focused by an optical microscope with a 100× objective and 100 µm slit. Infrared and Raman spectra were neither baseline-corrected nor smoothed. Visible-Near infrared spectra were measured by an R1 angle-resolved spectroscopy system (Idea Optics) in the 400 ~ 1000 nm range with air as reference. Transmission spectra were collected with 0o rotating arms so that the surfaces of the films were perpendicular to the beam path. Pieces of the each sample were tested and the maximum transmitted peak was set to 100%. Solvent Identification and Water Content Detection. The pieces (about 0.5-2 cm2) of CNC/PVP nanocomposite films, taken from the center of original films in Petri dish, were dipped into different kinds of anhydrous organic solvents or solvents with different content of water. Vis-NIR spectra of these films were measured after dipping for 2 min because their colors have no change after about 2-5 min. In order to measure accurately, 10 pieces of each sample



were tested for collecting maximum transmitted wavelengths. Digital pictures were also taken for every films before and after dipping.

RESULTS AND DISCUSSION

Morphological Characteristics. PVP is an amorphous polymer and may be dissolved in both polar and nonpolar solvents.36-37 The addition of PVP in CNC offers interesting and potentially useful features for CNC/PVP nanocomposites. First of all, large amount of PVP can be added into the chiral nematic phase of CNCs. As shown in Table S1, the weight percentage of PVP in these nanocomposites is range from 10 wt% to 70 wt%, which are named as CP90/10 or CP30/70, respectively. The presence of chiral nematic structure in the CNC/PVP nanocomposite films was verified by SEM images on their cross-sections (Figure 1 and Figure S1), which demonstrated the twisting but layered structure in CNC/PVP composite films with various ratios, even in CP30/70 cases (Figure 1c). In the high resolution images (Figure 1d-f and Figure S1f-j), the fan-like appearance of a left-handed helicoidal arrangement of CNCs can be clearly detected. Both the twisting layered structure and the left-handed helicoidal arrangement indicated the formation of chiral nematic structure.38 In composite films (Figure 1e and 1f), CNC rods stuck to each other and the boundaries of CNCs became slightly blurry compared with those in pure CNC film (Figure 1d), showing PVP homogeneously intercalated into the chiral nematic structure of original CNCs. The even distribution of nitrogen throughout the cross-section of both CP90/10 and CP50/50 films in EDX mapping (Figure S2) has supported the covering of PVP on CNC as well.


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Figure 1. SEM images on cross-section of pure CNC and the CNC/PVP nanocomposite films with different weight ratios: (a) pure CNC, (b) CP60/40, (c) CP30/70. Corresponding SEM images with high magnification: (d) pure CNC, (e) CP60/40, (f) CP30/70.

The sophisticated helicoidal nanostructure of CNC/PVP nanocomposites has shown identical chiral nematic phase in POM characterization (Figure 2 and Figure S3-S4). When the pure CNC film was viewed in transmission mode between crossed polarizers in a microscope, the film clearly showed the multi-domain structure (Figure S3a), where the direction of the chiral nematic axis changed with location in the film. The appearance of fingerprint texture (Figure 2 and Figure S4) in domains, when films were viewed orthogonally to the chiral nematic axis, confirmed the formation of chiral nematic structure.38 The addition of PVP has little influence on the chiral nematic self-assembled structure of CNCs, fingerprint texture could be obtained in CP30/70 nanocomposites (Figure 2d). If the addition amount of PVP is over 70 wt% in nanocomposite film, fingerprint texture will be hardly found, indicating chiral nematic structure has been destroyed. The order-disorder transition point in CNC/PVP nanocomposite film is as



high as 70 wt% of PVP that is the top value in CNC/polymer composites to our knowledge, whereas it is 40 wt% in CNC/PVA24 and 50 wt% in CNC/PEG25 systems, respectively. This considerably high transition point is ascribed to the amorphous and neutral nature of PVP. The neutral PVP has weak influence to the surface charge of CNC that is important for maintaining its self-assembly. In case of semi-crystalline PVA and PEG, the nucleation and growth of their crystalline domains accompanied with the self-organization of CNC in EISA, resulting in their phase separation and destruction of self-assembled CNC architecture when their contents were relatively high in the composites (> 40 wt%). Besides, PVP cannot be cross-linked in EISA that also helps it miscible well with CNC. Therefore, CNC/PVP composites have highest transition point compared with other reported CNC/polymer systems.

Figure 2. Optical microscope images (cross-polarized light) with high magnification of pure CNC and the CNC/PVP nanocomposite films with different weight ratios: (a) pure CNC, (b) CP90/10, (c) CP60/40 and (d) CP30/70. 10 Environment ACS Paragon Plus

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Structural Analysis. The tolerance of CNC self-assembled nanostructure to large amount of PVP is also due to the strong interaction between CNC and PVP. Since the abundant hydroxyl groups on the surface of CNCs and amide groups in PVP, hydrogen bonding is a possible interaction between them that can be proven by FT-IR characterizations (Figure 3 and Figure S5). The assignment of all FT-IR peaks based on literatures39-40 was summarized in Table S2. FT-IR spectra of CNCs, PVP and CNC/PVP nanocomposite films in the 3700-3000 cm-1 region are shown in Figure 3a. Pure CNC film exhibited two main bands at 3274 cm-1 and 3332 cm-1, which can be assigned to the O-H stretching mode of the hydroxyl groups on CNCs. When the content of PVP increased continuously, the band at 3332 cm-1 exhibited a 7 cm-1 higher-wavenumber-shift to 3339 cm-1 in CP30/70 film, indicating that inherent hydrogen bonds of CNCs became weaker due to the new coming interaction between amide groups of PVP and hydroxyl groups in composites. Meanwhile, the band at 3274 cm-1 decreased gradually and completely disappeared in CP30/70 film, while a shoulder at higher wavenumber side, 3405 cm-1, arose and became stronger in CP30/70 film, both demonstrating the weakening of hydrogen bonds of CNCs with the addition of PVP as well. In addition, the band at 1109 cm-1 in FTIR spectra can be assigned to the C-O-C pyranose ring skeletal vibration of CNCs (Figure 3b). In pure CNC cases, the C-O-C group may form hydrogen bonds with hydroxyl groups nearby. As the content of PVP increases, more hydroxyl groups on the surface of CNCs will interact with amide groups, leading to the C-O-C vibration shifting to 1111cm-1. The original hydrogen bonds in pure CNC film were interrupted due to the formation of hydrogen bond between amide groups of PVP and hydroxyl groups on CNCs.



Moreover, the interaction between amide groups of PVP and hydroxyl groups on the surface of CNCs can also be identified by using Raman spectroscopy. Raman spectra of CNCs, PVP and CNC/PVP nanocomposite films were shown in Figure 3c, Figure S6 and Table S2, in which the assignment of all peaks has been conducted based on literatures.40-41 The amide I band of pure PVP can be observed at 1663 cm-1. When PVP was added in CNCs, the amide I band underwent red shift, showing 1654 cm-1 in CP90/10 film due to the interaction of amide and hydroxyl groups of CNC. Hydroxyl groups have a strong electron-withdrawing effect on the oxygen atom of carbonyl group. As a result, the bond strength of the carbonyl groups decreased, leading to the lower wavenumber shift

of amide I band in comparison with pure PVP. This is

consistent with the discussion of O-H stretching of the CNC hydroxyl groups on FT-IR spectra. Optical Characterizations. Given that the chiral nematic organization of CNCs was still preserved in the CNC/PVP nanocomposite films, vis-NIR transmittance spectra were employed to describe the variations of optical property of these chiral nematic structures. As shown in Figure 4a, films with increasing amounts of PVP from CP90/10 to CP60/40 were discussed. The peak wavelength of pure CNC film was at 558 nm. When PVP was added, the peak red-shifted from 558 nm to 597 nm in CP90/10 film, and finally to 939 nm in CP65/35 film. The spectra showed a gradual red shift in the peak wavelength with the increasing amount of PVP, which is common in many CNC-based composites, such as CNC/silica,20 CNC/resin23 and CNC/surfactant42 composites. The iridescence of composite films shown in Figure 4b, changed from green (pure CNC) to yellow (CP90/10) and red (CP80/20), and finally became colorless (CP70/30). However, two issues should be emphasized (shown in Figure 4c and d): First of all, vis-NIR spectra was transmitted wavelength but optical images represented reflected color; Secondly, the incident light was perpendicular to the film surface in vis-NIR measurements, but


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in order to exhibit iridescence of films clearly, there was deviation angle (about 70o) between the plane surface of films and camera when optical images were taken, so there would be slight deviation between peak wavelength in vis-NIR spectra and iridescence in optical images.

Figure 3. FT-IR and Raman characterizations of pure CNC, pure PVP and the CNC/PVP nanocomposite films with different weight ratios: (a) FT-IR spectra in the region of 3700-3000 cm-1, (b) FT-IR spectra in the region of 1140-1080 cm-1, and (c) Raman spectra in the region of 1600-1720 cm-1.



Figure 4. Vis-NIR spectra (a) and corresponding optical images (b) of pure CNC and the CNC/PVP nanocomposite films with different weight ratios. The diameter of each petri dish was 5.5 cm in (b). Schematic illustration of the light path in measurement of vis-NIR spectra (c) and in taking optical images (d).

Color Changes in Different Organic Solvents. To investigate their potential as solvent-test-paper, vis-NIR spectra of pure CNC, CP90/10, CP85/15, CP80/20 and CP75/25 films immersed in different solvents were studied (Figure 5, Figure S7-S12 and Table 1). Pure CNC and CP90/10 films immersed in ethanol, acetone, chloroform and tetrahydrofuran (THF) were taken as examples, respectively. According to the solvation of CNC/PVP in these solvents, these four solvents belong to different types (Figure 5a): (i) soluble for both CNCs and PVP such as ethanol (Figure 5b). The redshift value of peak wavelength for pure CNC was 51 nm (from 558 nm to 609 nm), and that for CP90/10 film was 114 nm (from 599 nm to 713 nm), showing ethanol is a good solvent for both CNC and PVP. (ii) acetone (Figure S7), producing solvation


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for CNCs only. The redshift values for pure CNC and CP90/10 film were 17 nm (from 558 nm to 575 nm) and 26 nm (from 599 nm to 625 nm), respectively. Since PVP cannot be dissolved in acetone, we suppose that the 26 nm redshift for CP90/10 film should be attributed to the solvation for CNC. (iii) chloroform (Figure S8), producing solvation for PVP only. The redshift value was -5 nm for pure CNC film (from 558 nm to 553 nm). The slight decrease may be because of the little difference on the average refractive index of chloroform and CNC (Table 2). For CP90/10 film, the peak wavelength redshifted from 599 nm to 622 nm (shift value 23 nm). (iv) THF (Figure S9), producing solvation for neither CNCs nor PVP. The difference values of peak wavelength were -5 nm (558 nm and 553 nm) for CNC film, and +4 nm for CP90/10 film (from 599 nm to 603 nm). Without solvation for PVP or CNCs, the change of peak wavelength of CNC/PVP composite films is limited in units, which is not observable by naked eyes. The iridescence for CP90/10 film in soluble solvents (ethanol) changed from yellow color to red color (Figure 5b), but the change from iridescence to colorless (as CP80/20 in 1-propanol shown in Figure 5c, and in chloroform shown in Figure S8) is much easily distinguishable.

Table 1 Maximum Transmitted Wavelengths (nm) on Vis-NIR Spectra of Pure CNC and the CNC/PVP Nanocomposite Films Before and After Dipping in Different Organic Solvents. Maximum transmitted

Samples

CNC

CP90/10 CP85/15 CP80/20 CP75/25

wavelength (nm) Solvents None (Before dipping)

558±9

599±6

660±8


717±6

795±10


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Methanol

711±11

-

-

-

-

Ethanol

609±7

713±10

-

-

-

Acetone

575±5

625±8

673±9

744±10

843±10

Tetrahydrofuran (THF)

553±4

603±4

656±4

720±5

797±6

1-Propanol

583±6

681±7

808±8

929±12

-

2-Propanol

575±5

661±7

754±9

868±9

-

Chloroform

553±3

622±5

710±6

820±11

943±11

Dichloromethane (DCM)

552±4

617±5

690±8

783±9

902±12

Table 2 Refractive Indexes of CNC, PVP and Different Organic Solvents.24, 44-46 Material

Refractive Index

CNC

1.54

PVP

1.53

Methanol

1.3276

Ethanol

1.3610

Acetone

1.3620

Tetrahydrofuran (THF)

1.4050

1-propanol

1.3850


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2-propanol

1.3776

Chloroform

1.4467

Dichloromethane (DCM)

1.4237

In preparing test-paper, unambiguous change improves the distinguishing sensitivity. Therefore, we concentrated on the systems with color change from iridescent to colorless while they were dipped in solvents, in which the peak wavelength in vis-NIR spectra shifted from ~700 nm to above 800 nm. Just take CP80/20 film in 1-propanol as an example (Figure 5c). When the film was dipped in 1-propanol, the color changed from red to colorless or transparency with its transmitted peak signal red-shifting from 717 nm (red in Vis region) to 929 nm (colorless in NIR region), which could be clearly discovered by naked eyes. This distinguishable change shows CP80/20 film can work as ideal indicator for 1-propanol.



Figure 5. (a) The change in maximum transmitted wavelength on vis-NIR spectra of pure CNC and the CNC/PVP nanocomposite films with different weight ratios before and after dipped in ethanol, acetone, chloroform and tetrahydrofuran. And vis-NIR spectra and corresponding optical images of pure CNC and the CNC/PVP nanocomposite films before (dot line) and after (solid line) dipped in ethanol (b) and 1-propanol (c). These images of composite films before dipping are presented below for comparison.

Similar Organic Solvents Discrimination. In organic solvents, homologues, skeletal isomers and differently halogenated hydrocarbons are hardly distinguished by visual method. Thus we have tried CNC/PVP composite films to test their color-related response to these similar 18 Environment ACS Paragon Plus

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solvents. Methanol and ethanol are a couple of homologue. When dropping CP90/10 film into ethanol, the transmitted peak was 713 nm, exhibiting the red color (Figure 6b and 6c). However, when CP90/10 film was dropped into methanol, the transmitted peak was red-shifted out of 1000 nm (beyond the measure range of the vis-NIR instrument we used) showing white color that may be caused by light scattering (Figure 6a). This distinguished color change of CP90/10 film in ethanol and methanol makes it a selective sensor for methanol. Likewise, CP85/15 film can identify 1-propanol and 2-propanol by showing colorless or red color, separately, resulting in a test paper for the couple of skeletal isomer as well (Figure 6d and 6e). Their transmitted peaks were 808 nm (colorless) for 1-propanol and 754 nm (red color) for 2-propanol (Figure 6f), respectively. Similarly, thanks to the peculiar solvation of different halogenated hydrocarbons for PVP, the CP80/20 film can also be used to identify chloroform and dichloromethane (Figure 6g and 6h), the transmitted peaks were 820 nm and 783 nm (Figure 6i), respectively. Because of the wide-range miscibility of CNC and PVP keeping chiral nematic phase in their nanocomposites, the nanocomposite of CNC and PVP with a proper composition suitable for testing particular organic solvents can be easily found.



Figure 6. Optical images of CP90/10 films dipped in methanol (a) and ethanol (b), CP85/15 film dipped in 1-propanol (d) and 2-propanol, CP80/20 film in chloroform (g) and DCM (h). The magnified images were located at right-down corner for each sample. And vis-NIR spectra of CP90/10 films dipped in ethanol (c), CP85/15 film dipped in 1-propanol and 2-propanol (f), and CP80/20 film dipped in chloroform and DCM (i).


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Small Amount of Water Discrimination. The small amount of water in organic solvents is hardly detected via visual pathway. To date, the responsiveness of the CNC-based chiral nematic films to relative humidity, in which water absorption is thought as a determinate factor, has been reported in several articles.27, 34 However, there is no research on water-content-detection in organic solvents based on CNC-based nanocomposite films. So we have also explored the application of CNC/PVP composite films as test paper for detection of small amount of water in organic solvents. CP90/10 film was tested in THF containing different amounts of water (Figure 7a). In anhydrous THF, the peak wavelength of this film in vis-NIR spectrum was 607 nm. In THF containing 2% (v/v) water, the peak wavelength red-shifted to 628 nm. It red-shifted continuously with increasing water content, to 783 nm (red color) and 840 nm (colorless) in 8% or 10% (v/v) water contents of THF, respectively. In particular, ethanol is widely used in chemical laboratory, but it is very easy to form an azeotrope (about 95% (v/v)) with water while stored in humid environment. We noticed that CNC/PVP composite film could be used as a test paper for the water-content detection of ethanol. The peak wavelengths of CP90/10 film in anhydrous ethanol and ethanol with 5% (v/v) water were 717 nm (red color) and 897 nm (colorless) (Figure 7b), respectively. Such a color change was easily identified by naked eyes.



Figure 7. Vis-NIR spectra and optical images (below) of CP90/10 film dipped in (a) THF and (b) ethanol with different contents of water.

Response Time and Cyclic Use Test. Slow response of photonic films can be regarded as major limitation to their development in broad applications. Immersing CNC/PVP nanocomposite films in solvents causes an immediate redshift taking about 120-180 s to reach equilibrium. For example, for CP90/10 film in ethanol, the peak wavelength was red-shifted from 610 nm to 718 nm in 120 s (Figure 8a). In addition, these CNC/PVP nanocomposite films as test paper can be recycled 3-5 times, either by drying swollen paper (Figure 8b) or dipping it in nonsolvent of CNC (Figure 8c). Cyclic use of CNC/PVP composite film efficiently decreases their cost as test paper for distinguishing organic solvents.


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Figure 8. (a) The change of peak wavelength of CP90/10 film dipped in ethanol with different time; (b) The cycled change of peak wavelength of CP85/15 film repeatedly dipping in acetone and drying; (c) The peak wavelength changes of CP90/10 film while alternately dipping in chloroform and ethanol; and (d) ∆λmax with the weight percentages of PVP in CNC/PVP nanocomposite films while dipping in similar solvents, 1-propanol and 2-propanol (black line), and chloroform and DCM (red line).

Discussion. The relationship between the peak wavelength and the helical pitch of the chiral nematic structure of CNC/PVP nanocomposite films can be used to understand the responsive color change of CNC/PVP nanocomposite-based test paper. The peak wavelength (λmax) depends



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on the average refractive index (navg), the helical pitch (P) and the angle of incident light and sample (θ), according to the following equation:43

λ m ax = n avg × P × sin(θ )

(1)

The refractive indexes of pure CNC and pure PVP are 1.5424 and 1.53,44 respectively. The refractive indexes of all solvents used are listed in Table 2.45-46 The organic solvents generally show lower refractive indexes (1.3~1.5) compared with pure CNC and PVP, thus the filling of organic solvents in CNC/PVP nanocomposites will decrease λmax of solvent-swollen CNC/PVP system slightly. The decrease of λmax of pure CNC in chloroform and THF was due to this effect. While keeping the incident light perpendicular to the film surface in the characterization of vis-NIR spectroscopy, the change of λmax only relates with the helical pitch (P). The redshift of the peak wavelength for CNC/PVP composite films indicated that the helical pitch of the chiral nematic structure increased. Shütz et al. have investigated the packing of CNC in aqueous suspension from isotropic to fully liquid crystalline phase.15 They observed that the average pitch increased with the decrease of volume fraction of CNC. CNC/PVP composites film dipped in solvents, no matter the solvent can dissolve CNC or PVP, will have a decrease of volume fraction of CNC in swollen system, thus inducing the enlargement of helical pitch of chiral nematic CNC (Scheme 1a), and in turn the redshift of iridescence of CNC/PVP films. For the kind of solvents in which both CNC and PVP are soluble (ethanol, methanol), the solvent amount in swollen CNC/PVP nanocomposites was large, inducing the severe decrease of volume fraction of CNC and significant increase of its helical pitch; as for the solvent only soluble for CNC or PVP (acetone, chloroform), moderate amount of solvent was taken in CNC/PVP nanocomposites, leading to slight expansion of helical pitch of swollen films. The ability of CNC/PVP nanocomposites for distinguishing similar organic solvents is interesting. PVP has


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marginal difference in solubility in these similar organic solvents, but the small difference can be magnified to a significant change in iridescent color of CNC/PVP films with increasing content of PVP (Scheme 1b). We have calculated the ∆λmax of CNC/PVP nanocomposite films with different ratios of PVP while dipping in similar solvents (Figure 8d). Although CNC is insoluble in chloroform and dichloromethane, and PVP has little difference in solubility in chloroform and dichloromethane (based on the peak values listed in Table 1), their ∆λmax value was magnified via increasing the weight percentage of PVP in CNC/PVP nanocomposites, from 5 nm in CP90/10 to 37 nm in CP80/20, which is visually distinguished (Figure 6g and 6h). The visual identifying of 1-propanol and 2-propanol is also based on the same mechanism. Small difference of PVP in solubility in similar organic solvents can be magnified to visually-distinguishable change in CNC/PVP nanocomposites by increasing amount of PVP, which is superior to other CNC-based systems because their response to organic solvents can only be identified by Vis-IR spectroscopy.

21-22, 47

The magnifying effect makes a distinct feature for CNC/PVP in reported

CNC-based solvent-responsible sensors. The structural color change in CNC/PVP system fulfilled the requirement for sensitive sensors. CNC/PVP composites with chiral nematic nanostructure provide simple, cheap and efficient platform to develop test paper for different organic solvents.



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Scheme 1. Schematic illustration of discriminating similar organic solvents, such as homologues, skeletal isomers, halogenated hydrocarbons and even ethanol containing small amount of water by CNC/PVP nanocomposite films. (a) The helical pitch of swollen films is increased. (b) ∆λmax, the difference value of maximum transmitted wavelengths for CNC/PVP films

dipping

in

similar

organic

solvents

(for

example

1-propanol/2-propanol), enlarges with the increasing PVP content.

CONCLUSION


chloroform/DCM

and

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In summary, we have systematically investigated CNC/PVP nanocomposite films with assembled chiral nematic structure and corresponding structural color. The structural periodicity and thereby the photonic band gap can be persisted in these composites even at 70% weight percentage of PVP, which is the highest value of reported additives. The amorphous nature of PVP and interactions between CNC and PVP contributed to this highest addition value together. The structural color change of CNC/PVP composite films in response to different organic solvents via wetting has been explored in detail. The addition of PVP in CNC not only extended the solvent range to response, but also improved visual distinguishability for similar solvents by magnifying color difference of dipped films via increasing PVP content in CNC/PVP nanocomposites. Thus CNC/PVP composite films have shown their ability to discriminate homologues, skeletal isomers, and halogenated hydrocarbons, even small amount of water in ethanol, by providing fast and clear visual signal in dip-in. The structural color change in CNC/PVP systems fulfilled the requirement for solvent-responsible sensor. CNC/PVP nanocomposite films supply a simple but effective platform to develop green and cheap test paper for detection of organic solvents.

ASSOCIATED CONTENT

Supporting Information. Additional tables as well as figures (such as SEM, EDX mapping, POM, FT-IR spectra, Raman spectra and vis−NIR spectra) are demonstrated in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION



Corresponding Author *E-mail:[email protected] Funding Sources The National Natural Science Foundation of China (Grant 21374132，51673210).

ACKNOWLEDGMENT

We would like to express our thanks to Prof. Yapei Wang in Renmin University of China for his help with Polarized optical microscopy measurements. We highly appreciated the kind help from Prof. Yanlin Song in Institute of Chemistry Chinese Academy of Sciences, in the characterization of visible-Near infrared spectroscopy.

ABBREVIATIONS

CNC, cellulose nanocrystal; PVP, polyvinylpyrrolidone; EISA, evaporation-induced self-assembly, THF, tetrahydrofuran; DCM, dichloromethane; SEM, scanning electron microscopy; POM, polarized optical microscopy; FT-IR, Fourier-transform infrared; vis-NIR, visible-near infrared; EDX, Energy dispersive X-ray; ATR, attenuated total reflection; P, pitch; UV-vis, ultraviolet- visible.

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TABLE OF CONTENTS

Iridescent

nanocomposite

films

composed

of

natural

cellulose

polyvinylpyrrolidone can distinguish similar organic solvents via dip-in.


nanocrystal

/

Polyvinylpyrrolidone

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