*2.3. Preparation of CTA-NCC*/*CGQD Nanocomposite Thin Film*

Glass cover slips (24 mm × 24 mm × 0.1 mm) were used as the substrates. The glass slip was first sputtered with gold (SC7640 sputter coater machine) for 67 seconds to obtain 50 nm of gold thin film [46–48]. Then, spin coating technique was used to deposit the CTA-NCC/CGQD solution homogenously on the gold surface. About 1000 µL of CTA-NCC/CGQD solution was added on a gold coated glass slip and was spun at 3000 rev/min for 30 seconds using spin coater P-6708D to obtain around 12–15 nm thickness of the CTA-NCC/CGQD layer. The summarized flow chart for the preparation of CTA-NCC/CGQD is shown in Figure 1.

of glucose [43–45].

25].

**2. Materials and Methods** 

*2.1. Reagent and Materials* 

*2.2. Preparation of Chemicals* 

purchased from R&M Marketing (Essex, UK).

*2.3. Preparation of CTA-NCC/CGQD Nanocomposite Thin Film* 

To enhance the NCC properties, the hydroxyl functional group in NCC can be modified by using several methods [9–12]. In this present work, NCC has been cationically modified using hexadecyltrimethylammonium bromide (CTA). CTA can enhance the absorption by improving hyperchromicity and sensitization of NCC [13–17]. To further increase the performance, this modified NCC was chosen as a matrix for carboxyl functionalized graphene quantum dots (CGQD) as CGQD has beneficial and unique properties including hydrophilicity, strong photoluminescence and photo-stability [18–20]. Due to its outstanding properties, CGQD provides unprecedented opportunities for different fields of application such as optical sensing, catalysis and bioimaging [21–

As far as we know, the optical properties of the CTA cationically modified NCC/CGQD (CTA-NCC/CGQD) nanocomposite thin film and its potential application for detection of glucose using surface plasmon resonance technique (SPR) have yet to be reported. SPR is known as a simple optical method for surface studies of thin films and can act as a very sensitive spectroscopy for detection of a variety of targets [26–42]. Hence in this study, the fabrication of the CTA-NCC/CGQD nanocomposite thin film, its characterization and potential sensing application were explored.

Hexadecyltrimethyl ammonium bromide (CTA) and nanocrystalline cellulose (NCC) were purchased from Sigma Aldrich (St. Louis, MO, USA). Carboxyl-graphene quantum dots (CGQD) solution (0.1 wt%) was purchased from ACS Material (Pasadena, CA, USA) and glucose was

To prepare NCC solution, 1 g of NCC was diluted in 100 mL deionized water. Then, 0.2 g of CTA was diluted in 20 mL of deionized water to obtain CTA solution. NCC solution was then dropped into CTA solution drop by drop while heat stirred for 24 hours. The CTA-NCC solution was centrifuged at 3000 rpm for 15 minutes. Then, CTA-NCC/CGQD solution (0.05 wt%) was obtained by dispersing 1 mL of CGQD into 1 mL of CTA-NCC. The glucose solution was prepared by dissolving 9.91 mg of glucose with 100 mL of deionized water to produce 10 μM of glucose solution. To prepare glucose solution with various concentration, the 10 μM of glucose solution was diluted with deionized water based on the formula M1V1 = M2V2 to obtain 0.005, 0.01, 0.03, 0.05 and 0.1 μM

Glass cover slips (24 mm × 24 mm × 0.1 mm) were used as the substrates. The glass slip was first sputtered with gold (SC7640 sputter coater machine) for 67 seconds to obtain 50 nm of gold thin film [46–48]. Then, spin coating technique was used to deposit the CTA-NCC/CGQD solution homogenously on the gold surface. About 1000 μL of CTA-NCC/CGQD solution was added on a gold coated glass slip and was spun at 3000 rev/min for 30 seconds using spin coater P-6708D to

**Figure 1.** Preparation of hexadecyltrimethylammonium bromide (CTA)-nanocrystalline cellulose **Figure 1.** Preparation of hexadecyltrimethylammonium bromide (CTA)-nanocrystalline cellulose (NCC)/carboxyl-functionalized graphene quantum dots (CGQD) thin film. *Crystals* **2020**, *10*, x FOR PEER REVIEW 3 of 12

#### (NCC)/carboxyl-functionalized graphene quantum dots (CGQD) thin film. *2.4. Characterization Instrument 2.4. Characterization Instrument*

The Fourier transform infrared (FTIR) spectrum of CGQD, CTA-NCC and CTA-NCC/CGQD solutions were analyzed using the Fourier Transform Infrared Spectrometer model spectrum 100 (PerkinElmer, Waltham, MA, USA), with wavelength set from 400 to 4000 cm−<sup>1</sup> which is to determine the functional groups and the chemical interaction of the composites. Other than that, the purity of the compound can be obtained from the collection of the absorption band from the spectrum. For optical properties, the absorption of all samples with wavelength range from 220 nm to 500 nm was investigated using UV–Vis-NIR spectrometer (UV-3600 Shimadzu, Kyoto, Japan). The absorbance coated thin film was measured at room temperature. The energy band gap was determined by analyzing the graph of absorption peak against wavelength obtained using UV–Vis spectrometer. The Fourier transform infrared (FTIR) spectrum of CGQD, CTA-NCC and CTA-NCC/CGQD solutions were analyzed using the Fourier Transform Infrared Spectrometer model spectrum 100 (PerkinElmer, Waltham, MA, USA), with wavelength set from 400 to 4000 cm−1 which is to determine the functional groups and the chemical interaction of the composites. Other than that, the purity of the compound can be obtained from the collection of the absorption band from the spectrum. For optical properties, the absorption of all samples with wavelength range from 220 nm to 500 nm was investigated using UV–Vis-NIR spectrometer (UV-3600 Shimadzu, Kyoto, Japan). The absorbance coated thin film was measured at room temperature. The energy band gap was determined by analyzing the graph of absorption peak against wavelength obtained using UV–Vis spectrometer.

#### *2.5. Surface Plasmon Resonance 2.5. Surface Plasmon Resonance*

**3. Results** 

*3.1. FTIR Analysis* 

Surface plasmon resonance (SPR) is used to identify the potential of CTA-NCC/CGQD nanocomposite thin film for glucose detection. SPR is an optical process in which light satisfying resonance conditions excite a charge-density wave propagating along the interface between a dielectric material and metal by p-polarized and monochromatic light beam [49]. The reflected light intensity is reduced at a specific incident angle producing a sharp shadow due to the resonance occurs between surface plasmon wave and incident beam [50]. The SPR measurement was carried out by determining the reflected He-Ne laser beam (532.8 nm, 5 mW) [51]. Figure 2 shows the setup of SPR sensor. The SPR setup consisted of an He-Ne laser, a light attenuator, a polarizer and optical chopper (SR 540) and an optical stage driven by a stepper motor MM 3000 with a resolution of 0.001◦ (Newport, CA, USA). The reflected beam was detected by photodiode and then processed by the lock-in-amplifier (SR530) [52–55]. Surface plasmon resonance (SPR) is used to identify the potential of CTA-NCC/CGQD nanocomposite thin film for glucose detection. SPR is an optical process in which light satisfying resonance conditions excite a charge-density wave propagating along the interface between a dielectric material and metal by p-polarized and monochromatic light beam [49]. The reflected light intensity is reduced at a specific incident angle producing a sharp shadow due to the resonance occurs between surface plasmon wave and incident beam [50]. The SPR measurement was carried out by determining the reflected He-Ne laser beam (532.8 nm, 5 mW) [51]. Figure 2 shows the setup of SPR sensor. The SPR setup consisted of an He-Ne laser, a light attenuator, a polarizer and optical chopper (SR 540) and an optical stage driven by a stepper motor MM 3000 with a resolution of 0.001° (Newport, CA, USA). The reflected beam was detected by photodiode and then processed by the lock-in-amplifier (SR530) [52–55].

**Figure 2.** Experimental setup of surface plasmon resonance (SPR) sensor. **Figure 2.** Experimental setup of surface plasmon resonance (SPR) sensor.

quantum dots and the peak at 1037 cm−1 represented the C–O stretching [18].

The FTIR spectrum of CGQD, CTA-NCC and CTA-NCC/CGQD solutions are shown in Figure 3. From the spectrum of the CGQD solution, the peak present at 3310 cm−1 represented O–H stretching. The peak at 2891 cm−1 was attributed to the C-H stretching. The characteristic band appearing at 1625 cm−1 corresponded to the stretching of C=O of the carboxylic group in graphene

#### **3. Results** stretching vibration. The peak at 2885 cm−1 corresponded to C–H stretching. The peak at 1637 cm−<sup>1</sup>

#### *3.1. FTIR Analysis* can be assigned to C=O stretching and is similar to the peak for both spectrums of CGQD and CTA-NCC. The characteristics band that appeared at 1032 cm−1 corresponded to the stretching of C–O.

The FTIR spectrum of CGQD, CTA-NCC and CTA-NCC/CGQD solutions are shown in Figure 3. From the spectrum of the CGQD solution, the peak present at 3310 cm−<sup>1</sup> represented O–H stretching. The peak at 2891 cm−<sup>1</sup> was attributed to the C–H stretching. The characteristic band appearing at 1625 cm−<sup>1</sup> corresponded to the stretching of C=O of the carboxylic group in graphene quantum dots and the peak at 1037 cm−<sup>1</sup> represented the C–O stretching [18]. From the results, it is successfully confirmed that the functional groups of O–H, C–H, C=O and C–O existed in the composite solution. CGQD that are rich with oxygen-containing groups might interact with the hydroxyl groups and oxygen atoms in CTA-NCC through the hydrogen bonding. Furthermore, the possible structure of the composite is also presented in Figure 4.

*Crystals* **2020**, *10*, x FOR PEER REVIEW 4 of 12

NCC thin film where there was a broad absorption peak at 3277 cm−1 that was attributed to the O-H

Next, in the spectrum of CTA-NCC solution, the peak at 3332 cm−1 corresponded to the O-H stretching. The peak at 1617 cm−1 was attributed to the stretching vibration of C–O and the peak at

**Figure 3.** FTIR spectrum of CGQD, CTA-NCC and CTA-NCC/CGQD solutions.

**Figure 3.** FTIR spectrum of CGQD, CTA-NCC and CTA-NCC/CGQD solutions.

Next, in the spectrum of CTA-NCC solution, the peak at 3332 cm−<sup>1</sup> corresponded to the O–H stretching. The peak at 1617 cm−<sup>1</sup> was attributed to the stretching vibration of C–O and the peak at 1056 cm−<sup>1</sup> corresponded to C–O stretching [56].

The spectra of CTA-NCC/CGQD solution displayed the properties similar to CGQD and CTA-NCC thin film where there was a broad absorption peak at 3277 cm−<sup>1</sup> that was attributed to the O–H stretching vibration. The peak at 2885 cm−<sup>1</sup> corresponded to C–H stretching. The peak at 1637 cm−<sup>1</sup> can be assigned to C=O stretching and is similar to the peak for both spectrums of CGQD and CTA-NCC. The characteristics band that appeared at 1032 cm−<sup>1</sup> corresponded to the stretching of C–O. From the results, it is successfully confirmed that the functional groups of O–H, C–H, C=O and C–O existed in the composite solution. CGQD that are rich with oxygen-containing groups might interact with the hydroxyl groups and oxygen atoms in CTA-NCC through the hydrogen bonding. Furthermore, the possible structure of the composite is also presented in Figure 4.

**Figure 4.** The possible structure of CTA-NCC/CGQD nanocomposite.

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

Next, in the spectrum of CTA-NCC solution, the peak at 3332 cm−1 corresponded to the O-H stretching. The peak at 1617 cm−1 was attributed to the stretching vibration of C–O and the peak at

The spectra of CTA-NCC/CGQD solution displayed the properties similar to CGQD and CTA-NCC thin film where there was a broad absorption peak at 3277 cm−1 that was attributed to the O-H stretching vibration. The peak at 2885 cm−1 corresponded to C–H stretching. The peak at 1637 cm−<sup>1</sup> can be assigned to C=O stretching and is similar to the peak for both spectrums of CGQD and CTA-NCC. The characteristics band that appeared at 1032 cm−1 corresponded to the stretching of C–O. From the results, it is successfully confirmed that the functional groups of O–H, C–H, C=O and C–O existed in the composite solution. CGQD that are rich with oxygen-containing groups might interact with the hydroxyl groups and oxygen atoms in CTA-NCC through the hydrogen bonding.

Furthermore, the possible structure of the composite is also presented in Figure 4.

1056 cm−1 corresponded to C–O stretching [56].

0

100

CGQD

CTA-NCC

CTA-NCC/CGQD

Transmittance (a.u.)

200

300

**Figure 4.** The possible structure of CTA-NCC/CGQD nanocomposite. **Figure 4.** The possible structure of CTA-NCC/CGQD nanocomposite. *3.2. Optical Studies* 

#### *3.2. Optical Studies* The optical properties were analyzed using the absorbance spectrum of the thin film with

The optical properties were analyzed using the absorbance spectrum of the thin film with wavelength range 220–500 nm. The absorbance curves for CGQD, CTA-NCC and CTA-NCC/CGQD thin film are presented in Figure 5. wavelength range 220–500 nm. The absorbance curves for CGQD, CTA-NCC and CTA-NCC/CGQD thin film are presented in Figure 5.

**Figure 5.** Absorbance spectrum of CGQD, CTA-NCC and CTA-NCC/CGQD thin film.

**Figure 5.** Absorbance spectrum of CGQD, CTA-NCC and CTA-NCC/CGQD thin film. As shown in Figure 5, the absorbance curve for the thin film is different. From the figure, the CTA-NCC/CGQD thin film shows the highest absorption spectra with the absorption peaks at 222.9 nm, 225.9 nm and 264.9 nm. The highest absorption was contributed by the modification of CGQD with CTA-NCC, which confirmed the presence plasmon resonance in carbonaceous material [57]. On As shown in Figure 5, the absorbance curve for the thin film is different. From the figure, the CTA-NCC/CGQD thin film shows the highest absorption spectra with the absorption peaks at 222.9 nm, 225.9 nm and 264.9 nm. The highest absorption was contributed by the modification of CGQD with CTA-NCC, which confirmed the presence plasmon resonance in carbonaceous material [57]. On the other hand, the lowest absorbance belongs to CGQD thin film. The characteristic peaks that

= logଵ

= 2.303

peaks for sulfur doped graphene quantum dots which are at 216–464 nm [59].

with the presence of the sample *It*.

expressed as

the other hand, the lowest absorbance belongs to CGQD thin film. The characteristic peaks that

of the carbonyl groups [58]. In addition, it can be observed that the maximum absorption length can be determined from 263.04 nm to 266.63 nm. The results obtained are in the range of the absorption

To proceed with the determination of the optical band gap, the relationship between the absorbance and the intensities of the monochromatic light was used [60]. The absorbance, *A* of samples can be related with the ratio of the initial light intensity on the detector *I*0 to the light intensity

The absorbance coefficient is another quantity that can be measured. It is a very useful quantity which is used to compare samples of a varying thickness. The absorbance coefficient, *α* can be

௧ (1)

(2)

appeared in the nanocomposite thin film can be attributed to the presence of π → π\* bond transitions of the carbonyl groups [58]. In addition, it can be observed that the maximum absorption length can be determined from 263.04 nm to 266.63 nm. The results obtained are in the range of the absorption peaks for sulfur doped graphene quantum dots which are at 216–464 nm [59].

To proceed with the determination of the optical band gap, the relationship between the absorbance and the intensities of the monochromatic light was used [60]. The absorbance, *A* of samples can be related with the ratio of the initial light intensity on the detector *I*<sup>0</sup> to the light intensity with the presence of the sample *I<sup>t</sup>* .

$$A = \log\_{10} \frac{I\_0}{I\_t} \tag{1}$$

The absorbance coefficient is another quantity that can be measured. It is a very useful quantity which is used to compare samples of a varying thickness. The absorbance coefficient, α can be expressed as *Crystals* **2020**, *10*, x FOR PEER REVIEW 6 of 12

$$a = 2.303 \frac{A}{t} \tag{2}$$

where *t* is the thin film thickness in meters and the α is in units of m−<sup>1</sup> . The energy band gap of these composites has been figured out with the help of the absorption coefficient. To obtain the optical band gap from the absorption spectra, the Tauc relation is used: composites has been figured out with the help of the absorption coefficient. To obtain the optical band gap from the absorption spectra, the Tauc relation is used: = (ℎ − )

$$a = \frac{k\left(hv - E\_{\mathcal{S}}\right)^{\text{tr}}}{hv} \tag{3}$$

where *k* is a constant, *h* is the Plank's constant, *v* is the frequency of the incident photon, the multiplication of *h* and *v*, *hv* represents the incident photon energy, *E<sup>g</sup>* is the optical band gap and *n* is the state of transition. In this study we use *n* = 1/2 for direct transitions. Rearranging Equation (3) gives multiplication of *h* and *v*, *hv* represents the incident photon energy, *Eg* is the optical band gap and *n*  is the state of transition. In this study we use *n =* 1/2 for direct transitions. Rearranging Equation (3) gives (ℎ)ଶ = (ℎ − ) (4)

$$\left(alw\right)^{2} = \left(k\left(l\nu - E\_{\mathcal{S}}\right)\right) \tag{4}$$

Based on Equation (4), a graph of *(*α*hv)*<sup>2</sup> against *hv* can be plotted using linear fitting techniques and the optical band gap of the thin films can be determined [61–63]. According to Abdulla and Abbo (2012), the intersection of the straight line on the x-axis is taken as the value of the optical band gap [64]. The graph of (α*hv*) <sup>2</sup> versus *hv* for CGQD thin film, CTA-NCC thin film and CTA-NCC/CGQD thin film are shown in Figures 6–8, respectively. and the optical band gap of the thin films can be determined [61–63]. According to Abdulla and Abbo (2012), the intersection of the straight line on the x-axis is taken as the value of the optical band gap [64]. The graph of (*αhv*)2 versus *hv* for CGQD thin film, CTA-NCC thin film and CTA-NCC/CGQD thin film are shown in Figures 6–8, respectively.

**Figure 6.** Optical band gap for CGQD thin film.

**Figure 6.** Optical band gap for CGQD thin film.

3.50 3.75 4.00 4.25

Eg = 4.143 eV

hv(eV)

0.0

0.2

(αhv)(×1012cm-1eV)2

0.4

0.6

gives

3.50 3.75 4.00 4.25

Eg = 3.867 eV

hv(eV)

**Figure 6.** Optical band gap for CGQD thin film.

where *t* is the thin film thickness in meters and the *α* is in units of m−1. The energy band gap of these composites has been figured out with the help of the absorption coefficient. To obtain the optical

(ℎ − )

where *k* is a constant, *h* is the Plank's constant, *v* is the frequency of the incident photon, the multiplication of *h* and *v*, *hv* represents the incident photon energy, *Eg* is the optical band gap and *n*  is the state of transition. In this study we use *n =* 1/2 for direct transitions. Rearranging Equation (3)

Based on Equation (4), a graph of *(αhv)*2 against *hv* can be plotted using linear fitting techniques and the optical band gap of the thin films can be determined [61–63]. According to Abdulla and Abbo (2012), the intersection of the straight line on the x-axis is taken as the value of the optical band gap [64]. The graph of (*αhv*)2 versus *hv* for CGQD thin film, CTA-NCC thin film and CTA-NCC/CGQD

ℎ (3)

(ℎ)ଶ = (ℎ − ) (4)

band gap from the absorption spectra, the Tauc relation is used:

thin film are shown in Figures 6–8, respectively.

0.0

0.2

(αhv)(×1012cm-1eV)2

0.4

0.6

=

**Figure 7.** Optical band gap for CTA-NCC thin film.

**Figure 8.** Optical band gap for CTA-NCC/CGQD thin film.

**Figure 8.** Optical band gap for CTA-NCC/CGQD thin film. As can be seen from the figures, the intersection of the linear fitted line on the x-axis gives the value of the optical band gap. The term band gap is denoting the energy difference between the top of the valence band to the bottom of the conduction band where electrons can jump from one band to another. It necessitates a specific minimum extent of energy for the transition to permit an electron to jump from a valence band to a conduction band and this energy is called as the band gap energy. The optical band gap energies of CGQD, CTA-NCC and CTA-NCC/CGQD are 3.867 eV, 4.143 eV and 4.127 eV, respectively. Based on the result, CTA-NCC had the highest band gap energy among energy band gap results of all three thin films. The variations of optical band gap for the composite thin films, probably due to the presence of CTA-NCC solution as the band gap of CTA-NCC/CGQD, is higher than CGQD. The CTA-NCC/CGQD has a higher energy band gap as compared to CGQD thin film, which is in good agreement with the work reported by Daniyal et al. (2018), i.e., CTA-NCC increased As can be seen from the figures, the intersection of the linear fitted line on the x-axis gives the value of the optical band gap. The term band gap is denoting the energy difference between the top of the valence band to the bottom of the conduction band where electrons can jump from one band to another. It necessitates a specific minimum extent of energy for the transition to permit an electron to jump from a valence band to a conduction band and this energy is called as the band gap energy. The optical band gap energies of CGQD, CTA-NCC and CTA-NCC/CGQD are 3.867 eV, 4.143 eV and 4.127 eV, respectively. Based on the result, CTA-NCC had the highest band gap energy among energy band gap results of all three thin films. The variations of optical band gap for the composite thin films, probably due to the presence of CTA-NCC solution as the band gap of CTA-NCC/CGQD, is higher than CGQD. The CTA-NCC/CGQD has a higher energy band gap as compared to CGQD thin film, which is in good agreement with the work reported by Daniyal et al. (2018), i.e., CTA-NCC increased the optical band gap of the composite [56].

#### the optical band gap of the composite [56]. *3.3. Potential Sensing Analysis*

0.03, 0.05 and 0.1 μM.

*3.3. Potential Sensing Analysis*  The SPR experiment was first conducted using gold CTA-NCC/CGQD nanocomposite thin film The SPR experiment was first conducted using gold CTA-NCC/CGQD nanocomposite thin film in contact with deionized water (or 0 µM of glucose). The resonance angle for the first part

in contact with deionized water (or 0 μM of glucose). The resonance angle for the first part of this

different concentrations of glucose solution. The SPR experiment was then continued for different concentrations of glucose solution that ranged from 0.005 μM to 0.1 μM. The glucose solution was injected into the cell one after another [65]. The reflectance as a function of incident angle of CTA-NCC/CGQD thin film, in contact with different concentrations of glucose solutions is shown in Figure 7. From the curves, the resonance angle can be obtained for the glucose concentration of 0.005, 0.01,

From Figure 9, it can be observed that the resonance angle was shifted to the right and it further increased with the increase of glucose concentration [66,67]. The resonance angles obtained for 0.005, 0.01, 0.03, 0.05 and 0.1 μM were 54.571°, 54.732°, 54.755°, 54.767° and 54.769°, respectively. The shift of the SPR curves and resonance angle change demonstrated that the CTA-NCC/CGQD of this experiment was obtained as 54.400◦ , where this value was used to compare the resonance angle for different concentrations of glucose solution. The SPR experiment was then continued for different concentrations of glucose solution that ranged from 0.005 µM to 0.1 µM. The glucose solution was injected into the cell one after another [65]. The reflectance as a function of incident angle of CTA-NCC/CGQD thin film, in contact with different concentrations of glucose solutions is shown in Figure 7. From the curves, the resonance angle can be obtained for the glucose concentration of 0.005, 0.01, 0.03, 0.05 and 0.1 µM.

From Figure 9, it can be observed that the resonance angle was shifted to the right and it further increased with the increase of glucose concentration [66,67]. The resonance angles obtained for 0.005, 0.01, 0.03, 0.05 and 0.1 µM were 54.571◦ , 54.732◦ , 54.755◦ , 54.767◦ and 54.769◦ , respectively. The shift of the SPR curves and resonance angle change demonstrated that the CTA-NCC/CGQD nanocomposite thin film has affinity towards glucose, where its incorporation with SPR can be a potential sensor for glucose. *Crystals* **2020**, *10*, x FOR PEER REVIEW 8 of 12 nanocomposite thin film has affinity towards glucose, where its incorporation with SPR can be a potential sensor for glucose.

**Figure 9.** SPR curves for CTA-NCC/CGQD nanocomposite thin film for glucose solution with **Figure 9.** SPR curves for CTA-NCC/CGQD nanocomposite thin film for glucose solution with different concentrations (0.0–0.1 µM).

#### different concentrations (0.0–0.1 μM). **4. Conclusions**

**References** 

**2017**, *162*, 115–120.

**4. Conclusions**  In this study, the CTA cationically modified NCC/CGQD nanocomposite thin film has been successfully fabricated. The functional groups that existed in the thin film were confirmed from the FTIR results. The absorbance value of CTA-NCC/CGQD was the highest with energy band gap of 4.127 eV. The studies of the CTA-NCC/CGQD nanocomposite thin film using the SPR technique have successfully shown that the novel thin film can detect various concentrations of glucose with the lowest detection of 5 nM. This study gives an important idea that the CTA-NCC/CGQD nanocomposite thin film has high potential as an application in sensing glucose when incorporated In this study, the CTA cationically modified NCC/CGQD nanocomposite thin film has been successfully fabricated. The functional groups that existed in the thin film were confirmed from the FTIR results. The absorbance value of CTA-NCC/CGQD was the highest with energy band gap of 4.127 eV. The studies of the CTA-NCC/CGQD nanocomposite thin film using the SPR technique have successfully shown that the novel thin film can detect various concentrations of glucose with the lowest detection of 5 nM. This study gives an important idea that the CTA-NCC/CGQD nanocomposite thin film has high potential as an application in sensing glucose when incorporated with the SPR technique and can be further investigated in future studies.

**Author Contributions:** Conceptualization, methodology, writing—original draft preparation, N.N.M.R.; validation, supervision, writing—review and editing, funding acquisition, Y.W.F.; investigation, formal analysis, N.A.A.A.; software, resources, N.A.S.O.; visualization, N.S.M.R. and W.M.E.M.M.D. All authors have **Author Contributions:** Conceptualization, methodology, writing—original draft preparation, N.N.M.R.; validation, supervision, writing—review and editing, funding acquisition, Y.W.F.; investigation, formal analysis, N.A.A.A.; software, resources, N.A.S.O.; visualization, N.S.M.R. and W.M.E.M.M.D. All authors have read and agreed to the published version of the manuscript.

with the SPR technique and can be further investigated in future studies.

read and agreed to the published version of the manuscript. **Funding:** This research was funded and supported by the Ministry of Education Malaysia through the **Funding:** This research was funded and supported by the Ministry of Education Malaysia through the Fundamental Research Grant Scheme (FRGS) (FRGS/1/2019/STG02/UPM/02/1) and Putra Grant Universiti Putra Malaysia.

Fundamental Research Grant Scheme (FRGS) (FRGS/1/2019/STG02/UPM/02/1) and Putra Grant Universiti Putra Malaysia. **Acknowledgments:** The authors acknowledged the laboratory facilities provided by the Institute of Advanced Technology, Department of Physics, and Department of Chemistry, Universiti Putra Malaysia.

**Acknowledgments:** The authors acknowledged the laboratory facilities provided by the Institute of Advanced **Conflicts of Interest:** The authors declare no conflict of interest.

1. Azrinaa, Z.A.Z.; Beg, M.D.H.; Rosli, M.Y.; Ramli, R.; Junadi, N.; Alam, A.K.M.M. Spherical nanocrystalline cellulose (NCC) from oil palm empty fruitbunch pulp via ultrasound assisted hydrolysis. *Carbohydr. Polym.* 

Technology, Department of Physics, and Department of Chemistry, Universiti Putra Malaysia.
