Investigating Cellulose Nanocrystal and Polyvinyl Alcohol Composite Film in Moisture Sensing Application

: This study focused on utilizing cellulose nanocrystal (CNC)–polyvinyl alcohol (PVA) composite in optical sensor applications to detect high humidity conditions and determine water concentration in ethanol. We focused on the composite’s effectiveness in moisture absorption to demonstrate visual color change. We demonstrated that the different molecular weights of PVA significantly affect CNC’s chiral nematic structure and moisture absorption capability. PVA with molecular weight 88 k–97 k exhibited the disintegration of its chiral nematic structure at 30 wt%, whereas low molecular weight PVA (n~1750) showed no structural disintegration even at 100 wt% concentration when analyzed through UV-Vis spectroscopy. Further, the thermal crosslinking of the CNC-PVA composite showed no significant loss of moisture sensitivity for all molecular weights of the PVA. We observed that the addition of PVA to the sulfated CNC obtained from sulfuric acid hydrolysis did not facilitate moisture absorption significantly. A CNC-PVA sensor was developed which can detect high humidity with 2 h. of exposure time. 2,2,6,6-tetramethylpiperidin-1-piperidinyloxy oxidized CNC (TEMPO-CNC) having carboxylic functionality was also used to prepare the CNC-PVA composite films for comparing the effect of functional groups on moisture sensitivity. Finally, we demonstrated a facile method for utilizing the composite as an optical sensor to detect water concentration in ethanol efficiently; thus, it can be used in polar organic solvent dehydration applications.


Introduction
Optical sensors can have numerous advantages in industries like food, semiconductors, beverages, packaging, etc. Easy-to-maintain, wireless, and low-cost colorimetric optical sensors could be highly beneficial in detecting humidity or the presence of moisture content in the system [1].Optical sensors are made by utilizing building blocks with unique structural colors generated due to light interferences.The bird's feathers, fish scales, or insect cuticles are some examples of natural materials available [2].Currently, the colorimetric humidity sensors used for such applications contain cobalt chloride, which adversely affects human health [3,4].Moreover, finite fossil fuel resources pose additional challenges as the photonic materials used to develop optical sensors are mostly from polymers, polystyrene spheres, graphene oxide, etc. [5][6][7].Therefore, exploring biobased photonic material to develop a green, efficient, durable, low-cost, and nontoxic sensor is important for the betterment of the environment.
Cellulose nanocrystal (CNC) is a biodegradable and renewable nanomaterial produced from biomass or agricultural waste, which is abundantly present in nature and thus has inexpensive and abundant raw material [8][9][10].In addition to many lucrative properties [10][11][12][13][14][15][16][17][18][19], CNC offers unique structural colors which can be utilized as a photonic material to develop optical sensors, ink, displays, etc. [2,20,21].Commercially, a rod-shaped CNC is extracted from cellulose by degrading the amorphous region through sulfuric acid hydrolysis [22,23].This also incorporates sulfate ester content on the surface of CNC, which facilitates this material to exhibit liquid crystal properties at higher concentrations [10,24].CNC's liquid crystal structure exhibits chiral nematic organization, which is a helical arrangement of nematic layers around the cholesteric axis [10,25].A complete rotation of the structure around the axis is defined as a cholesteric pitch.This chiral nematic structure is restored in the solid, self-assembled CNC film and exhibits structural color or iridescence.A typical reflectance or transmittance peak is observed for self-assembled CNC film in the range between 200 nm and 800 nm.The reflected wavelength can be tuned by altering the structure's chiral nematic pitch.This tuning can be obtained by the addition of external stimuli such as moisture, electrolytes, polymers, or other stimuli.The change in the reflected wavelength directly alters the color of the material surface, thus creating an optical sensing capability for different stimuli conditions.It has been observed that the increment in pitch length causes the wavelength peak to shift to a higher side, thus exhibiting a redshift, and vice versa.
The iridescence property of CNC film has been investigated for use in optical sensor applications, which has been widely reported by many [2,3,26,27], depicted in Figure 1.The re-dissolution of CNC in water creates poor cycling stability.Thus, CNC-based optical sensors are prepared by adding stimuli-responsive materials to CNC to prepare a composite that can offer stability as well as efficiency [1].CNC/polyethylene glycol (PEG) composite was reported to detect different humidity conditions [2].Similarly, composite with glucose, glycerol was also used for humidity sensing [28,29].The presence of abundant hydroxyl groups on CNC coupled with these hydrophilic materials further increases the moisture intercalation in the structure to cause a redshift.However, higher humidity propels higher moisture absorption in the structure, which weakens the adhesion in the matrix, therefore softening the structure and impacting the structural stability and durability of the material [30].CNC was also utilized with other materials, such as polyacrylamide, polyacrylic acid, polyvinyl pyrrolidone, and silver nanoparticles, etc., to detect organic solvents and their concentrations, pHs, acid vapors, etc. [21,26,27,31,32].Although multiple studies were carried out to develop CNC-based optical sensors, factors like being wireless and easy to operate, multifunctionality, durability, and being biodegradable still need to be addressed more significantly.
rides 2024, 5, FOR PEER REVIEW 3 PVA composite's applicability in humidity sensing.Thermal crosslinking was performed on the sensor to provide structural integrity during the application.Therefore, its effect on the optical property was also determined.A detailed sensitivity analysis was provided for the CNC-PVA composites using PVA with three different (low, medium, and high) molecular weights.We further replaced the sulfated CNC with TEMPO-CNC to observe the sensor's moisture sensitivity.In the second part, a smart photonic material was developed that can broadly detect ethanol-water concentrations.The insoluble nature of PVA in polar organic solvent resisted structural disintegration when submerged in the solution.Further, PVA served here as a matrix to provide stability and durability to the composite.To the author's knowledge, this is the first time a detailed study has been presented to show the effect of PVA molecular weight, thermal crosslinking, and CNC functionality on designing CNC-PVA composite for moisture sensing application.

Materials and Methods
Sodium from cellulose nanocrystal (CNC) aqueous gel was purchased from the University of Maine, Orono, ME, USA, with an 11 wt% concentration and 1.1 wt% sulfur.A Polyvinyl alcohol (PVA) is a non-toxic, biodegradable polymer containing abundant hydroxyl groups in its structure [33].Several articles reported that PVA exhibits high mechanical strength and enhanced moisture barrier properties when CNC is added [34,35].PVA applications have also been used in developing optical fiber sensors for humidity sensing applications [36][37][38][39] or in developing self-healable electronic skin [40].However, the utilization of PVA to facilitate moisture-intercalation in the structure to develop a CNC-based photonic material is limited.Moreover, the utilization of PVA with CNC for preparing a colorimetric sensor or indicator is limited as well.Yu et al. [28] prepared a smart, humidity-responsive photonic material with the addition of CNC/glycerol/glucose/PVA.In this study, glycerol and glucose both contributed to moisture absorption, whereas the PVA addition was also focused on the reduction in the film's brittleness.An excellent solvent, acid, and alkali-resistant optical coating was developed by Zhang et al. [41] using CNC-PVA composite preparation by glutaraldehyde crosslinking.Overall, the investigation of the CNC-PVA composite as a colorimetric sensor in a moisture sensing application is still under evaluation and therefore requires a thorough investigation.
We conducted the research in two parts.First, we investigated the responsiveness of the composite to moisture with respect to the following factors: PVA molecular weight, concentration, thermal crosslinking, and the functionality of CNC.The addition of PVA to CNC causes reduced moisture sensitivity despite PVA's hydrophilic nature.In this study, we solely used PVA of varying molecular weight to investigate its effect on moisture sensitivity and prepared a complete green optical sensor with CNC and PVA that can be responsive to high humidity conditions.We provided a thorough study on the CNC-PVA composite's applicability in humidity sensing.Thermal crosslinking was performed on the sensor to provide structural integrity during the application.Therefore, its effect on the optical property was also determined.A detailed sensitivity analysis was provided for the CNC-PVA composites using PVA with three different (low, medium, and high) molecular weights.We further replaced the sulfated CNC with TEMPO-CNC to observe the sensor's moisture sensitivity.In the second part, a smart photonic material was developed that can broadly detect ethanol-water concentrations.The insoluble nature of PVA in polar organic solvent resisted structural disintegration when submerged in the solution.Further, PVA served here as a matrix to provide stability and durability to the composite.To the author's knowledge, this is the first time a detailed study has been presented to show the effect of PVA molecular weight, thermal crosslinking, and CNC functionality on designing CNC-PVA composite for moisture sensing application.

Composite Preparation
A self-assembled pure CNC and a TEMPO-CNC were prepared by diluting the initial stock material with deionized (DI) water to 1.66 wt%.A CNC suspension of 0.4 g was mixed with various weight percentages of PVA w.r.t the initial amount of CNC, i.e., 0% to 50% and 100%.The detailed composition is listed in Table 1.A total suspension of 20 g was prepared and magnetically stirred for 2 h at room temperature.The films were developed by pouring the suspension into the polystyrene Petri dish and allowing evaporation to take place at room temperature.The solid film was taken out after 3 days.The films were kept at 50% relative humidity (RH) with a controlled room temperature.Thermal crosslinking was only performed on the pure CNC-PVA composite films, which were kept in the oven at 150 • C for 30 min.After this, the films were kept in a controlled environment for at least 24 h before further experimentation.The final sensor preparation was carried out for ethanol-water concentration detection by sonicating the suspension for 1 min with 25% amplitude to have uniform particle dispersion.Longer sonication time was avoided so that the CNC structure would not be affected.A test paper method of detection was used to determine the color change for the organic solvent concentration detection.The sensor was cut into small rectangular strips, submerged in the solution for 4 min, and then the result was detected.A cyclic test was performed to determine the reusability of the film.A 50 wt% ethanol-water solution was selected, and transmittance minima were recorded for 10 iterations.At first, the dry initial state transmittance was determined, and then the film was dipped in the solution for 4 min for data collection.Around 7 to 10 min were required for the composite to return to its initial dry state.

Zeta Potential
The Zeta potential of the pure CNC and TEMPO-CNC was determined using a Zeta sizer Nano ZS Malvern instrument, United Kingdom, listed in Table 2.The concentration used for the measurement was 0.83 wt%.A minimum of three measurements were performed to report the average value with the standard deviation.
Table 2. Zeta potential of pure CNC and TEMPO-CNC; the concentration used 0.83 wt%.

Scanning Electron Microscopy
A field-emission scanning electron microscope, JEOL JSM-7000F (JEOL USA Inc., Peabody, MA, USA), was used to take the cross-sectional images of pure CNC, TEMPO-CNC, and CNC-PVA composite films.The films were exposed to liquid nitrogen before the fracture and gold sputtered before imaging.

Cross-Polarized Microscopy
The cross-polarized microscopy was performed using a Nikon (Melville, NY, USA) Eclipse Ni microscope with a Nikon DS-RI2 camera and a 20×/0.45LU Plan Fluor to observe the birefringence property of the CNC-PVA composite.Two different angles, 0 • and 45 • , were used to capture the image of the composite.

UV-Vis Spectroscopy
A Thermo Scientific Genesys 10S UV−vis spectrometer, Waltham, MA, USA, with a wavelength range of 200−800 nm was used to determine the transmittance profiles of the samples.The minimum transmittance peak was used to indirectly correlate the pitch length and to detect the optical changes under external stimuli conditions.

Fourier Transform Infrared Spectroscopy (FTIR)
FTIR was performed to measure the transmission spectra of the pure CNC and TEMPO-CNC solid film, using a Nicolet 6700 FTIR spectrometer, Thermo Scientific, Waltham, MA, USA, for the frequency range of 500-4000 cm −1 with 64 cumulative scans.

Sensitivity Analysis
A relative humidity (RH) glass box was used to detect the transmittance peak shift when exposed to higher humidity conditions.Various saturated salt solutions produce different humidity conditions within a closed container.This study used a 50% RH (controlled RH chamber) and a 95% RH (humidity box) to conduct the following experiments.

Tensile Strength Determination
The tensile strengths of the films were determined by using Instron, Model No. 5982, Norwood, MA, USA.Rectangular strips were cut to perform the test at 50% RH.The crosshead speed used for all the experiments was 1.5 mm/min.

Film Thickness
The thickness of the films was determined by Testing Machine Inc, New Castle, DE, USA, Model 49-71 (0-1.25 mm range).Primarily, the thickness varied from 50-70 microns for the pure CNC and the composite films.

Effects of Different MW PVA on Optical Response of Composite Films
The typical fingerprint or chiral nematic structure of the self-assembled CNC film depicted in Figure 2A became more uniformly distributed by the addition of PVA, as shown in Figure 2B,C.The addition of the PVA caused the redshift in the transmittance peak minima, λ min ; therefore, a slight greenish appearance was observed with the higher PVA concentration, as depicted in Figure 2. The intercalation of a polymer chain in the CNC's helical structure expanded the pitch, which resulted in a redshift depicted in Figure 3. Wang et al. [42] reported the increase in pitch length of the CNC structure with the increasing PVA concentration and observed the loss of ordered structure beyond 55 wt% of the PVA concentration.We observed that this concentration was dependent on the molecular weight of PVA.The cholesteric pitch is correlated with Bragg's diffraction.Therefore, an indirect correlation can be drawn through the transmittance peak minima, λ min .The absence of minima in the transmittance profile was considered to be due to the disintegration of the chiral nematic structure, also reported and discussed by Wang et al. [34], and Parit et al. [43].For LMW, the 50 wt% concentration did not show any disintegration of the chiral nematic structure; however, at 100 wt% concentration, λ min disappeared, indicating the loss of ordered structure as shown in Figure S1a from the Supplementary Materials.For HMW PVA, the 30 wt% concentration depicted a drop in the peak wavelength, as shown in Figure 4a, indicating that the chiral structure was in the process of disintegration (Figure S1b).Contrastingly, an increase of around ~200 nm in λ min was observed for the LLMW PVA addition from 10 wt% to 100 wt%, as shown in Figures 3 and 4b, and no structural disintegration was observed even for 100 wt%.Because higher molecular weight represents longer polymeric chains with higher stiffness, when introduced into the CNC structure, it restricts proper movement during film formation, thus incorporating instability and disintegrating the chiral nematic structure [44].Moreover, Yu et al. also reported the relationship between the threshold concentration and the molecular weight [28].

Moisture Sensitivity Analysis of Composite Films
The composite films were exposed to 95% RH for 2 days to be fully saturated, and the λmin was recorded to observe the increment from 50% RH.The sensitivity analysis was

Moisture Sensitivity Analysis of Composite Films
The composite films were exposed to 95% RH for 2 days to be fully saturated, and the λmin was recorded to observe the increment from 50% RH.The sensitivity analysis was

Moisture Sensitivity Analysis of Composite Films
The composite films were exposed to 95% RH for 2 days to be fully saturated, and the λmin was recorded to observe the increment from 50% RH.The sensitivity analysis was

Moisture Sensitivity Analysis of Composite Films
The composite films were exposed to 95% RH for 2 days to be fully saturated, and the λ min was recorded to observe the increment from 50% RH.The sensitivity analysis was performed to determine the composite's moisture absorption capacity at various PVA concentrations for different molecular weights, as shown in Figure 5.In the literature, the CNC/PVA composite was reported to have higher mechanical properties with a low moisture uptake.Thus, the sensitivity analysis was important to determine the optimal molecular weight and concentration requirement.Nguyen et al. [45] and Silvério et al. [46] observed an increase in the tensile strength and a decrease in water vapor permeability for neat PVA with the addition of CNC.For all molecular weights, the increase in the moisture sensitivity was found to be quite low as ~(10-15) nm.The addition of PVA to the CNC matrix creates hydrogen interactions between two components; thus, it reduces the accessible hydroxyl group for moisture intercalation.Yu et al. [28] observed a drop in responsiveness to moisture with the increasing molecular weight of PVA.Henceforth, this creates a challenge to utilize PVA with CNC as a moisture-intercalating agent for humidity detection.The upper limit of the higher molecular weight PVA to prepare a stable composite further poses a challenge.Therefore, LLMW PVA was selected for high humidity detection.
olysaccharides 2024, 5, FOR PEER REVIEW performed to determine the composite's moisture absorption capacity at various PVA centrations for different molecular weights, as shown in Figure 5.In the literatur CNC/PVA composite was reported to have higher mechanical properties with a low ture uptake.Thus, the sensitivity analysis was important to determine the optimal m ular weight and concentration requirement.Nguyen et al. [45] and Silvério et al. [4 served an increase in the tensile strength and a decrease in water vapor permeabili neat PVA with the addition of CNC.For all molecular weights, the increase in the mo sensitivity was found to be quite low as ~(10-15) nm.The addition of PVA to the matrix creates hydrogen interactions between two components; thus, it reduces the a sible hydroxyl group for moisture intercalation.Yu et al. [28] observed a drop in re siveness to moisture with the increasing molecular weight of PVA.Henceforth, this cr a challenge to utilize PVA with CNC as a moisture-intercalating agent for humidity d tion.The upper limit of the higher molecular weight PVA to prepare a stable comp further poses a challenge.Therefore, LLMW PVA was selected for high humidity d tion.

Effect of Thermal Crosslinking on the Transmittance Peak Minima
The higher humidity exposure on the composite film causes a swollen structur to moisture intercalation.This makes the composite more susceptible to structural age and can affect the durability of the sensor.Thermal crosslinking causes the form of hydrogen bonds by evaporating moisture from the structure.An increase in hyd bonds through thermal crosslinking was reported by Tsioptsias et al. [47].Although mal crosslinking can provide structural integrity [41,45], this can lead to the loss of able hydroxyl groups, thus reducing the responsiveness to moisture.The effect of th crosslinking is depicted in Figure 6.A slight drop of transmittance peak minima wa served post-thermal crosslinking due to the formation of more H-bonds, which facil shorter pitch.Around a ~25 nm drop in λmin was observed for 40 wt% LLMW PVA thermal crosslinking.Because the λmin drop was not significantly high, a thermally linked CNC-PVA sensor can be considered for further experimentation.

Effect of Thermal Crosslinking on the Transmittance Peak Minima
The higher humidity exposure on the composite film causes a swollen structure due to moisture intercalation.This makes the composite more susceptible to structural damage and can affect the durability of the sensor.Thermal crosslinking causes the formation of hydrogen bonds by evaporating moisture from the structure.An increase in hydrogen bonds through thermal crosslinking was reported by Tsioptsias et al. [47].Although thermal crosslinking can provide structural integrity [41,45], this can lead to the loss of available hydroxyl groups, thus reducing the responsiveness to moisture.The effect of thermal crosslinking is depicted in Figure 6.A slight drop of transmittance peak minima was observed post-thermal crosslinking due to the formation of more H-bonds, which facilitated shorter pitch.Around a ~25 nm drop in λ min was observed for 40 wt% LLMW PVA after thermal crosslinking.Because the λ min drop was not significantly high, a thermally crosslinked CNC-PVA sensor can be considered for further experimentation.

Humidity Detection
The high humidity detection was carried out by CNC-80 wt% LLMW composite, and the result is depicted in Figure 7.The presence of birefringence in the film, depicted in Figure S2, confirms the presence of the chiral nematic structure at 80 wt% LLMW PVA addition.Adding PVA restricts moisture intercalation in the structure and allows a ~15 nm peak shift.Thus, a higher amount of PVA was required to ensure enough moisture penetration in the structure to observe the visual color change of the composite.LLMW PVA was selected as it can be used up to 100 wt% in the composite preparation.Thermal crosslinking was also performed to identify the composite performance.The transmittance peak minima shift obtained around 80 nm with and without thermal crosslinking.Visually, the color change was identifiable and similar in both cases.The increase in the transmittance peak due to high humidity exposure was caused by swelling the chiral nematic structure, thus increasing the cholesteric pitch.This observation is in line with the previously reported literature and that reported by Yao et al. [2] for CNC/Polyethylene glycol, Lu et al. [1] for CNC/Polyacrylamide, etc. [3,21].Lu et al. [1] observed a redshift of around 92 nm for the relative humidity that increased to 97% RH.Again, the redshift was found to be quite significant for the CNC-PEG composite.The composite changed from green to orange when the relative humidity increased from 30% RH to 90% RH [2].The composite's detection time was also experimentally determined and depicted in Figure 7a.The experiment continued for 6 h; however, the maximum peak shift was observed at 2 h and remained almost the same for the rest of the time.To increase moisture sensitivity

Humidity Detection
The high humidity detection was carried out by CNC-80 wt% LLMW composite, and the result is depicted in Figure 7.The presence of birefringence in the film, depicted in Figure S2, confirms the presence of the chiral nematic structure at 80 wt% LLMW PVA addition.Adding PVA restricts moisture intercalation in the structure and allows a ~15 nm peak shift.Thus, a higher amount of PVA was required to ensure enough moisture penetration in the structure to observe the visual color change of the composite.LLMW PVA was selected as it can be used up to 100 wt% in the composite preparation.Thermal crosslinking was also performed to identify the composite performance.The transmittance peak minima shift obtained around 80 nm with and without thermal crosslinking.Visually, the color change was identifiable and similar in both cases.The increase in the transmittance peak due to high humidity exposure was caused by swelling the chiral nematic structure, thus increasing the cholesteric pitch.This observation is in line with the previously reported literature and that reported by Yao et al. [2] for CNC/Polyethylene glycol, Lu et al. [1] for CNC/Polyacrylamide, etc. [3,21].Lu et al. [1] observed a redshift of around 92 nm for the relative humidity that increased to 97% RH.Again, the redshift was found to be quite significant for the CNC-PEG composite.The composite changed from green to orange when the relative humidity increased from 30% RH to 90% RH [2].The composite's detection time was also experimentally determined and depicted in Figure 7a.The experiment continued for 6 h; however, the maximum peak shift was observed at 2 h and remained almost the same for the rest of the time.To increase moisture sensitivity and response time, we further investigated the TEMPO-CNC-PVA composite for the same.
Polysaccharides 2024, 5, FOR PEER REVIEW 9 and response time, we further investigated the TEMPO-CNC-PVA composite for the same.

Effect of Polyvinyl Alcohol on Carboxylated CNC
The addition of the PVA to the CNC matrix caused limited sensitivity to moisture and showed prominent color change only at high humidity conditions.The moisture uptake was found to be dependent on the functionalization of cellulose.Previous studies showed a higher moisture absorption for TEMPO-oxidized cellulose nanofibrils compared to the sulfated ones [30,48].Therefore, we incorporated carboxylated CNC in the composite to determine the composite's moisture sensitivity at 95% RH.Previously, glucose or glucan was used by Meng et al. [49] to develop a based humidity sensor.Additionally, Yu et al. [28] also prepared a CNC/glucose/glycerol/PVA composite for a similar application.The chiral nematic structure of pure TEMPO-CNC is indicated in Figure 8A, where the typical fingerprint structure can be observed.Compared to the pure CNC, TEMPO-CNC showed a peak shift from 1637 to 1602 cm −1 .Hop et al. also noticed this typical peak shift post-TEMPO oxidation [50].Furthermore, a broad and higher intensity peak from ~1602 to 1637 cm −1 was noticed, indicating a higher amount of the carboxylic acid group in the structure qualitatively, as depicted in Figure 8B.Carboxylated CNC was used to prepare a composite with PVA and moisture intercalation, and the time was recorded.Pure TEMPO-CNC exhibited transmittance minima around 440 nm, which redshifted ~520 nm for the 30 wt% LLMW PVA addition and completely disappeared for 50 wt% PVA, as shown in Figure 8C.Visually, the structural iridescence was weaker than the pure CNC, as depicted in Figures 2D and E. This limits the range of the PVA addition for preparing the composite.TEMPO-CNC was also used to further evaluate the increase in sensitivity and only a slight increment in λmin ~60 nm for the 30wt% LLMW PVA addition was observed.Although we observed increased moisture absorption through the λmin shift by introducing carboxylic acid groups, the upper limit to add LLMW PVA was lower than the pure CNC, thus limiting its application in humidity detection.

Effect of Polyvinyl Alcohol on Carboxylated CNC
The addition of the PVA to the CNC matrix caused limited sensitivity to moisture and showed prominent color change only at high humidity conditions.The moisture uptake was found to be dependent on the functionalization of cellulose.Previous studies showed a higher moisture absorption for TEMPO-oxidized cellulose nanofibrils compared to the sulfated ones [30,48].Therefore, we incorporated carboxylated CNC in the composite to determine the composite's moisture sensitivity at 95% RH.Previously, glucose or glucan was used by Meng et al. [49] to develop a based humidity sensor.Additionally, Yu et al. [28] also prepared a CNC/glucose/glycerol/PVA composite for a similar application.The chiral nematic structure of pure TEMPO-CNC is indicated in Figure 8A, where the typical fingerprint structure can be observed.Compared to the pure CNC, TEMPO-CNC showed a peak shift from 1637 to 1602 cm −1 .Hop et al. also noticed this typical peak shift post-TEMPO oxidation [50].Furthermore, a broad and higher intensity peak from ~1602 to 1637 cm −1 was noticed, indicating a higher amount of the carboxylic acid group in the structure qualitatively, as depicted in Figure 8B.Carboxylated CNC was used to prepare a composite with PVA and moisture intercalation, and the time was recorded.Pure TEMPO-CNC exhibited transmittance minima around 440 nm, which redshifted ~520 nm for the 30 wt% LLMW PVA addition and completely disappeared for 50 wt% PVA, as shown in Figure 8C.Visually, the structural iridescence was weaker than the pure CNC, as depicted in Figure 2D,E.This limits the range of the PVA addition for preparing the composite.TEMPO-CNC was also used to further evaluate the increase in sensitivity and only a slight increment in λ min ~60 nm for the 30 wt% LLMW PVA addition was observed.Although we observed increased moisture absorption through the λ min shift by introducing carboxylic acid groups, the upper limit to add LLMW PVA was lower than the pure CNC, thus limiting its application in humidity detection.

Morphology and Mechanical Property
The cross-sectional image of pure CNC shows the typical fingerprint texture, as depicted in Figure 9A.The CNC-PVA cross-sectional image shows the presence of the CNC structure between the PVA matrix.The tensile strength was determined for 10 to 30 wt% LLMW CNC-PVA composite to demonstrate the structural integrity of the composite.PVA is a thermoplastic polymer; thus, the elongation at the break increased from ~1.5% to 3.8% for the 30 wt% PVA concentration with the increase in its concentration.The addition of soft PVA in the CNC matrix provided more energy dissipation while stretching, displaying prominent plastic deformation, whereas the pure CNC showed a sharp fracture, as depicted in Figure 9a (bottom row) [41].Similarly, Zhang et al. [41] also reported observing plastic deformation for the CNC and PVA composite as well.We observed an increase in tensile strength from ~40 MPa to ~70 MPa by the addition of 30 wt% LLMW PVA.The hydrogen interaction between CNC and PVA created a stronger structure, thus increasing the tensile strength.A similar observation was reported by Zhou et al. [51], where the tensile strength and elongation at the break increased up to 3 wt% for the CNC addition in neat PVA.

Morphology and Mechanical Property
The cross-sectional image of pure CNC shows the typical fingerprint texture, as depicted in Figure 9A.The CNC-PVA cross-sectional image shows the presence of the CNC structure between the PVA matrix.The tensile strength was determined for 10 to 30 wt% LLMW CNC-PVA composite to demonstrate the structural integrity of the composite.PVA is a thermoplastic polymer; thus, the elongation at the break increased from ~1.5% to 3.8% for the 30 wt% PVA concentration with the increase in its concentration.The addition of soft PVA in the CNC matrix provided more energy dissipation while stretching, displaying prominent plastic deformation, whereas the pure CNC showed a sharp fracture, as depicted in Figure 9a (bottom row) [41].Similarly, Zhang et al. [41] also reported observing plastic deformation for the CNC and PVA composite as well.We observed an increase in tensile strength from ~40 MPa to ~70 MPa by the addition of 30 wt% LLMW PVA.The hydrogen interaction between CNC and PVA created a stronger structure, thus increasing the tensile strength.A similar observation was reported by Zhou et al. [51], where the tensile strength and elongation at the break increased up to 3 wt% for the CNC addition in neat PVA.
The physical mixing of CNC and PVA only through magnetic stirring did not create a uniform matrix.Therefore, to prepare the final sensor for ethanol-water concentration detection, sonication of the suspension was performed, which produced a more uniform visual texture in the matrix, as shown in Figure 10A,B.The colorful texture, shown in Figure 10C,D, confirms signature birefringence, thus confirming the available photonic property of the composite.We utilized 40 wt% LLMW PVA to prepare the sensor, which can deliver sufficient resilience to the sensor and provide the required performance.The physical mixing of CNC and PVA only through magnetic stirring did not crea a uniform matrix.Therefore, to prepare the final sensor for ethanol-water concentrati detection, sonication of the suspension was performed, which produced a more unifor visual texture in the matrix, as shown in Figures 10A and B. The colorful texture, show in Figures 10C and D, confirms signature birefringence, thus confirming the available ph tonic property of the composite.We utilized 40 wt% LLMW PVA to prepare the sens which can deliver sufficient resilience to the sensor and provide the required performan

Ethanol-Water Concentration Detection
The reported literature on organic solvent concentration detection through CNC-based optical sensors is limited.Gao et al. [26] developed a CNC/polyvinylpyrrolidone composite for determining different organic solvents, including methanol, ethanol, chloroform, etc.Similarly, Bai et al. [20] and Verma et al. [7] prepared a CNC/citric acid composite to detect different organic solvents.Kelly et al. [21] demonstrated CNC/polyacrylamide-based photonic hydrogel for water concentration detection in ethanol and broadly depicted the hydrogel's color change through polarized optical microscopy.
The insoluble nature of PVA in polar organic solvent was utilized here to develop a biodegradable, easy-to-maintain, low-cost, and durable sensor that can efficiently detect water concentration in ethanol content.The insoluble nature of PVA in polar organic solvent allowed the composite not to disintegrate or swell when submerged for a day, as depicted in Figure S4.Adding PVA to CNC allowed the composite to remain stable in the ethanol-water solution at different concentrations.An easy and facile method, i.e., dipping the film in the solution, was developed to record the λ min shift at different ethanol-water concentrations, as depicted in Figure 11.This allowed the composite to remain intact post-experimentation.

Ethanol-Water Concentration Detection
The reported literature on organic solvent concentration detection through CNCbased optical sensors is limited.Gao et al. [26] developed a CNC/polyvinylpyrrolidone composite for determining different organic solvents, including methanol, ethanol, chloroform, etc.Similarly, Bai et al. [20] and Verma et al. [7] prepared a CNC/citric acid composite to detect different organic solvents.Kelly et al. [21] demonstrated CNC/polyacrylamide-based photonic hydrogel for water concentration detection in ethanol and broadly depicted the hydrogel's color change through polarized optical microscopy.
The insoluble nature of PVA in polar organic solvent was utilized here to develop a biodegradable, easy-to-maintain, low-cost, and durable sensor that can efficiently detect water concentration in ethanol content.The insoluble nature of PVA in polar organic solvent allowed the composite not to disintegrate or swell when submerged for a day, as depicted in Figure S4.Adding PVA to CNC allowed the composite to remain stable in the ethanol-water solution at different concentrations.An easy and facile method, i.e., dipping the film in the solution, was developed to record the λmin shift at different ethanolwater concentrations, as depicted in Figure 11.This allowed the composite to remain intact post-experimentation.
With increasing water concentrations, the redshift in the transmittance profile was observed for the composite, which is shown in Figure 12.The exposure time for all concentrations was maintained at 4 min.Therefore, the cholesteric pitch increment occurred due to the increased number of available water molecules in the solution, as illustrated in Figure 11.The addition of 20 wt% water shifted the λmin to around 55 nm.The more significant color change began from the 30 wt% water content, resulting in olive green with a peak shift of ~116 nm.A complete orange color was obtained for the 50 wt% water content, and significant transmittance minima shifts occurred at ~230 nm.Though the 10 wt% With increasing water concentrations, the redshift in the transmittance profile was observed for the composite, which is shown in Figure 12.The exposure time for all concentrations was maintained at 4 min.Therefore, the cholesteric pitch increment occurred due to the increased number of available water molecules in the solution, as illustrated in Figure 11.The addition of 20 wt% water shifted the λ min to around 55 nm.The more significant color change began from the 30 wt% water content, resulting in olive green with a peak shift of ~116 nm.A complete orange color was obtained for the 50 wt% water content, and significant transmittance minima shifts occurred at ~230 nm.Though the 10 wt% water content did not show significant color change, a slight peak shift occurred, as depicted in Figure 12.Interestingly, the highest peak in the transmittance profile also showed an increase in the percent of transmittance and a small redshift in wavelength occurred as well.A slope ratio was calculated by considering the slope before and after the peak (explained in Figure S5), which showed a consistent behavior with increasing water content in the ethanol-water solution.The ratio of this slope exhibited a decreasing pattern with the increasing water concentration, as shown in Figure 13a.
Finally, the cyclic test was performed for 10 iterations with the 50 wt% water concentration in the solution to conduct a performance check.The presence of a higher amount of water can affect the film's performance; therefore, we opted for a 50 wt% ethanol-water solution, which demonstrated repeatable data throughout all the iterations, as depicted in Figure 13b.Therefore, the water concentration in ethanol can be detected via this composite through its transmittance profile and a color change up to the 10 wt% water concentration.This optical sensor can broadly be utilized in organic solvent dehydration applications.
well.A slope ratio was calculated by considering the slope before and after the peak (explained in Figure S5), which showed a consistent behavior with increasing water content in the ethanol-water solution.The ratio of this slope exhibited a decreasing pattern with the increasing water concentration, as shown in Figure 13a.Finally, the cyclic test was performed for 10 iterations with the 50 wt% water concentration in the solution to conduct a performance check.The presence of a higher amount of water can affect the film's performance; therefore, we opted for a 50 wt% ethanol-water solution, which demonstrated repeatable data throughout all the iterations, as depict Figure 13b.Therefore, the water concentration in ethanol can be detected via this com site through its transmittance profile and a color change up to the 10 wt% water con tration.This optical sensor can broadly be utilized in organic solvent dehydration a cations.

Conclusions
The CNC-PVA composite showed excellent optical sensing properties when detecting the water concentration in ethanol-water solutions.PVA was used as the matrix for CNC to provide structural integrity to the composite, which allowed the sensor to be more stable and durable while sensing.The significant visual color change of the composite was observed with distinct transmittance minima increments.We also observed that the slope ratio of the curve changed and exhibited a decreasing pattern with the increased water concentrations.Consistent results were found when the composite was subjected to 10 iterations.The detailed study on the moisture sensitivity of the CNC-PVA composite exhibited limited applicability in humidity sensing.The addition of PVA to CNC forms hydrogen interactions that reduce the availability of hydroxyl groups for moisture capture.A similar observation was also found for TEMPO-CNC as well.We observed a slightly increased moisture sensitivity for TEMPO-CNC; however, adding 50 wt% of the lowest molecular weight PVA (n =1750) showed structural disintegration, which again imposed limitations on the formulation and moisture sensitivity.Our study demonstrated that, with the increasing molecular weight of PVA, CNC's structural disintegration occurs with a reduced amount of PVA addition.Further, the moisture sensitivity did not differ much with the thermal crosslinking of the composite.Therefore, this composite can be useful for high humidity sensing and ethanol-water concentration detection.

Figure 1 .
Figure 1.An overall schematic of the application area of the CNC-based optical sensors.

Figure 1 .
Figure 1.An overall schematic of the application area of the CNC-based optical sensors.

Figure 3 .
Figure 3. Transmission spectra of the CNC-LLMW PVA composite films at various concentrations at 50% RH (without thermal curing).

Figure 3 .
Figure 3. Transmission spectra of the CNC-LLMW PVA composite films at various concentrations at 50% RH (without thermal curing).

Figure 3 .
Figure 3. Transmission spectra of the CNC-LLMW PVA composite films at various concentrations at 50% RH (without thermal curing).

Figure 12 .
Figure 12.The pure CNC-40 wt% LLMW PVA film color variation at time = 4 min with different water concentrations in ethanol and transmittance profiles at different ethanol-water concentrations.