Production of MCC from laboratory waste cotton
The naturally obtained cellulose material is composed with glucose units connected by β-1,4 glycosidic bonds (Figs. 1a and b). To prepare MCC from the laboratory cotton waste, the samples were first treated with NaOH for alkaline treatment and then H2SO4 for acid hydrolysis (Figs. 1ci-iii). Next, the treated samples were dried at 80°C (24 h) to obtain the MCC powder (Figs. 1iv and v). The synthesized MCC was further characterized by morphological analyses to examine the size, surface, and surface structure to confirm the chemical composition, functional group, thermal stability, particle size distribution, and stability of the MCC.
Characterization of MCC: Morphology analysis
A morphological analysis of the MCC sample was done utilizing FESEM and TEM. The FESEM results revealed a uniform particle size distribution and rod-like structure particles (Figs. 2a and b). Further, the shape of the particles increased the surface area and increased the reactivity of the fiber. Natural MCC was used to determine the surface morphology of the prepared MCC to get the uniform size of particle, and a slightly rough formation was observed because of the chemical treatment during the delignification process. The surface morphology of the prepared MCC was further examined using FESEM with accelerated electrons under 15 kV of energy. Note that MCC is produced by breaking the fibers via acid hydrolysis (Kale et al. 2017). The results showed a particle size range of 22.1 µm to 37.2 µm with an average size of 29.4 µm. FETEM was also used to study morphology at different magnification scales. The results revealed irregularly shaped particles with individual molecules in different ranges (Figs. 2c and d). Moreover, at a higher magnification, the cotton-based MCC shape was different from the FESEM image
Surface chemical bonds analysis: FTIR
The FTIR spectra of the MCC sample were examined in the range of 500–4000 cm− 1 (Fig. 3a). According to the infrared spectra of the MCC, O − H stretching was observed at 3446.94 cm− 1 and C = O carbonyl ring stretching occurred at 1649.50 cm− 1. Thus, the alkali treatment decreased the hydrogen bonding of cellulose by removing hydroxyl groups (–OH) when reacting with NaOH, whereas the C − H and C − O bonds of cellulose exist in the polysaccharide aromatic rings (Theivasanthi et al. 2017). C − H stretching in the cellulose structure was noticed at 2891.11 cm− 1, whereas C − O stretching was noted in the range of 1050–1150 cm− 1. Moreover, a C − O−C pyranose ring in cellulose occurred at 1061.19 cm− 1. Further, the peak at 1381.10 cm− 1 confirms the presence of cellulose in the MCC sample. Moreover, the peak at 900 cm− 1 occurred because of the glycosidic linkage between the sugar units (Trilokesh and Uppuluri 2019).
Elemental analysis: EDX
EDX equipped with FESEM was employed to find the chemical composition of the MCC. The EDX spectra peaks of MCC correspond to the energy levels of C and O, as shown in Fig. 3b, wherein C and O are major peaks at 0.3 KeV and 0.6 KeV, respectively, indicating the adequate synthesis of MCC from the laboratory cotton waste. The percentages of O and C were 57.47% and 42.53%, respectively, in the MCC.
Surface Elemental Composition and Chemical State: XPS
XPS analysis was used to find the chemical composition of the MCC surface (Figs. 4a-c). The results clearly show that O and C are the dominant peaks at binding energies of 531.74 eV and 284.68 eV, respectively. This confirms that the synthesized MCC mainly consisted of O and C, which are the major elements in cellulose.
Crystallographic Structure Analysis: XRD
XRD was used to elucidate the crystallinity of the MCC. Generally, the polymer material of MCC is semi-crystalline because it still contains an amorphous part in addition to the dominant crystalline parts (Alavudeen et al. 2017). Crystalline polymers produce sharp peaks, whereas amorphous polymers tend to produce blunt or widened peaks (Fig. 5a). The MCC created from laboratory cotton exhibited the typical diffraction peaks of a crystalline cellulose II structure at 12.2°, 20°, and 22.03°, which represent the diffraction planes of 1–10, 110, and 020, respectively (French 2014). After alkaline treatment and acid hydrolysis on cotton, the cellulose I peak 22.03° (020) was split into two peaks 20° (110) and 22.03° (020) indicating the formation of cellulose II (French 2014; Trilikesh and Uppuluri 2019). The CrI of MCC was reported as the percentage of crystallinity calculated according to the diffraction patterns. Based on the diffraction patterns, the CrI of the prepared MCC was decreased as 78.7% after alkaline treatment and acid hydrolysis, as compared with that of cotton (81.2%). This decrease in CrI is caused by the conversion of the structure from cellulose I to cellulose II, which is associated with the destruction of the cellulose I structure in cotton by molecular chain cleavage and the subsequent reformation of the crystalline structure to cellulose II (Yue et al. 2013).
Thermal Stability: TGA
The thermal stability of MCC was confirmed using a TGA. The TGA curves of the synthesized MCCs are shown in Fig. 5b. The initial weight loss of the materials began at 50°C – 100°C because of the evaporation of moisture on the sample surface (Haque et al. 2015; Nasution et al. 2017). Meanwhile, the thermal decomposition of MCC began at 250°C to 370°C, accounting for a total weight loss of 70%. These results indicate that the MCC created from laboratory cotton has high thermal stability.
Stability and Size Distribution: Zeta potential and particle size analysis
A zeta potential analysis was used to elucidate the stability of the particles in the MCC sample by measuring the surface charge of the particles in the solution. The zeta potential for the MCC was − 76.51 mV, which indicates the stability of the particles (Fig. 6a). This value is consistent with that reported by Mahajan et al. (2014). Thus, MCC is suitable for the encapsulation of enzymes and other molecules. In addition, a particle size analyser was used to examine the MCC particle size. When MCC is dispersed in a solution, it exhibits Brownian motion, in which smaller particles exhibit faster motions. The scattered light from the particles displays fluctuations corresponding to individual particles when the laser light illuminates the particles under the effect of Brownian motion. This fluctuation is detected using the pinhole-type photon detection method and can determine the particle size and particle size distribution. The particle size of MCC had a diameter of 37.80 µm, as shown in Fig. 6b, with a polydispersity index (PDI) of 0.304. The low PDI value of the prepared MCC indicates a narrow particle size distribution, suggesting an even particle size. Therefore, this study confirmed that laboratory cotton could be used as a potential raw material for isolating MCC for use as a green material in many industrial applications.