The membranes of CNCs are studied extensively for their wide range of applications in industries, biomedical, and energy storage devices. Membranes derived from nanocellulose retain the properties of cellulose and mainly act as a substrate upon which functionalization can occur. The surfaces can be modified depending on the function to be performed (Sardjono et al. 2019). They are cost-effective, easy to fabricate and rarely release hazardous chemical effluents compared to other methods. A few of those applications are enlisted below.
Sensors are devices used to detect analytes or stimuli that can be used for biomedical and industrial applications. Sensors have to be stable, ensure reproducibility, and be sensitive to measure over a range of concentrations. The hydroxyl groups present in cellulose make it suitable for sensing applications as it provides mechanical stability and flexibility (Ansari et al. 2021). The presence of extensive intrachain and interchain hydrogen bonds, hydroxyl groups, electrostatic interactions and tunable cellulose polarization make an array of stimuli-responsive cellulose nanomaterials (CNM) (Zhu et al. 2020).
Nanocellulose can be incorporated into sensor systems in different ways, and they can act as a template, a reducing agent, a dispersant, a reinforcing agent etc. (Dai et al. 2020). Nanocellulose -based membranes and nanopapers are 2D nanocomposites lightweight, flexible, thermally and chemically stable with microporous structure (Dai et al. 2020). These composites exhibit exceptional sensing capabilities by the introduction of carbon nanotubes (CNTs), carbon quantum dots (CQDs), organic dyes, metal nanoparticles etc. (Fan et al. 2020). Sensors based on films are rampant, while nanocellulose membrane-based sensors are still in the initial stages of development (Abbasi-Moayed et al. 2018)(Morales-Narváez et al. 2015).
Biosensors use a biological molecule as a recognition element. CNMs that are biocompatible and show low cytotoxicity are suitable for biological applications. Biomedical applications include using CNM based glucose monitoring methods, urea detection, drug delivery, tissue engineering etc.
The detection of glucose is vital for the control of diabetes mellitus. For instance, a hybrid cellulose nanocrystal/ magnetite biosensor was recently developed for the detection of glucose in sweat and saliva (Tracey et al. 2020). Regular strips have glucose oxidase (GOx) affixed, converting the glucose to gluconic acid with hydrogen peroxide as a by-product. The hybrid cellulose nanocrystal/ magnetite biosensor strip has magnetite nanoparticles that reduce H2O2 and oxidize ABTS (2,2’-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid), resulting in a colour change, making it a self-indicating reaction. These films can be placed on the skin or the tongue to detect glucose. A novel method for the detection of small molecules and cells was developed, where gold nanorods were incorporated with cellulose nanofiber (CNF) to form a versatile membrane by Surface-Enhanced Raman Scattering (SERS) (Zhang et al. 2018). The CNF matrix, due to its nanoporous morphology, helps retain the analyte for competent SERS detection. Other examples of optical biosensing techniques include fluorescent approaches, colorimetric methods, and bioluminescence (Golmohammadi et al. 2017). A self-assembling BC-AuNP (Bacterial cellulose- gold nanoparticle) nanocomposite recently developed, leads to the generation of a colorimetric mechanoplasmonic bioanalytical sensor, that facilitates optical detection by mechanical stimuli (Eskilson et al. 2020).
4.1.2 Gas sensors
Detection of gases like NO2, ammonia and ethylene are essential for industries. Colorimetric and electrochemical sensors have been used for gas detection alone or combined with other techniques. Mixing amine-based polymers with micro fibrillated cellulose (MFC) creates a hybrid transport membrane based on nanocellulose. For instance, a study demonstrated CO2 capture from N2 and CH4. A polyvinyl amine product, Lupamin 9095, was added to enhance the CO2 flux across the selective layer. The improvement in the CO2 adsorption rate is indicative of the increased affinity of the material with the acid component on amine functionalization (Ansaloni et al. 2017). Likewise, gas separation membranes derived from nanocellulose could successfully separate CO2 through a mobile carrier mechanism (Venturi et al. 2019). The amino acid L-arginine blended with carboxymethylated nanofibrillated cellulose (CMC-NFC) enhanced the permeability and exhibited absolute selectivity. Zhang et al. (2017) developed a functionalized cellulose/ graphene oxide (GO) proton-conducting membrane to detect ethanol (Zhang et al. 2017). GO has a low permeation for gases like methanol, ethanol and methane, but when it covalently links with cellulose fibres, it becomes an ideal material for electrochemical gas sensing applications. A very recent study using nanocellulose/graphene oxide (GONC) hybrid membrane attached to SnO2 nanosheets (NS) looked into selective H2 detection (Fig. 5) (Jung and Jang 2021). The pore size of 0.3 nm led to the effective removal of large gas molecules like H2S, CO and selectively allowed the diffusion of H2 molecules. The SnO2 NS@GONC sensor showed greater selectivity for H2 than the SnO2 NS and SnO2 NS@GO sensors (Fig. 6).
4.1.3 Chemical sensor
Cholesterol measurement is a routine test employed to check cardiovascular diseases. Chemical methods, enzymatic assays, colorimetric assays used are often tedious processes. Electrochemical sensing methods have gained prominence due to their selectivity, reproducibility, fast response and stability. Such a sensor was developed for cholesterol detection by combining both electrochemical sensing and molecular imprinting techniques (Anirudhan et al. 2018). Using glassy carbon electrode surface (GCE) with silyated graphene oxide-grafted chemically modified nanocellulose (Si-GO-g-CMNC), both cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studies were conducted. CV showed a detection range of 5.18–25.9 micro mol/L and DPV, 0.6475–10.360 mmol/L. Cholesterol recovery within the scope of 98.0% indicated the reliability of the sensor. Some chemicals on prolonged exposure can cause serious health problems, for instance formaldehyde which is widely used in industries, is a dangerous air pollutant. A formaldehyde sensor based on nanofibrous polyethyeleimine (PEI)/ bacterial cellulose membranes coated with quartz crystal microbalance could detect formaldehyde in the range of 1-100 ppm at room temperature (Hu et al. 2011). Figure7 portrays that the amount of PEI in the membrane was key to the sensing properties. The presence of large number of hydroxyl groups in BC and amine groups in PEI provides the fibers with a large number of strong hydrogen bonds. The interaction between PEI and BC prevents the aggregation of PEI and ensures a uniform coating on the surface of BC nanofibers, which can then interact with formaldehyde molecules. This is illustrated in Fig.8.
4.2 Energy applications
Nanocellulose is a suitable material for energy storage applications due to its unique properties based on its intrinsic structure. They have a high Youngs modulus of 1.38 GPa and a strength of 2–3 GPa. Due to extensive hydroxyl groups, chemical modifications on the surface and integration with other active materials are possible. Flexible energy storage devices are made with nanocellulose fibres as they are used to fabricate films/aerogel substrates. Direct methanol fuel cells (DMFC) are becoming popular as their use also extends to portable devices, other than having a compact design, high power density, reliability etc. Nafion membranes, which are polymeric electrolyte membranes, are currently in use but are disadvantageous. They have a high methanol crossover, leading to Pt catalyst poisoning, reducing the electrical performance and affecting cell efficiency. Research later led to polyvinyl alcohol, a non-toxic and biodegradable polymer whose methanol barrier property was better than Nafion but had lower proton conductivity. A study used sulfosuccinate acid (SSA) to modify cellulose to form films (Seo et al. 2009). The presence of hydroxyl groups on cellulose makes it easy to modify. The increase in SSA content could add to the cellulose membranes' ion exchange capacity and proton conductivity. A study using nanostructured Bacterial cellulose-Poly(4-styrene sulfonic acid) (PSSA) composite membranes displayed through-plane proton conductivity higher than 0.1 S cm− 1 at 940 C and 98% relative humidity (RH), which decreased to 0.0042 S cm− 1 at 60% RH (Gadim et al. 2014). The composite membranes were produced by in situ free radical polymerization of sodium 4-styrenesulfonate using poly (ethylene glycol) diacrylate (PEGDA), as cross-linker. The high proton conductivity can be attributed to the presence of a large number of sulfonic acid groups in PSSA. The proton conductivity increases with increase in PEGDA content, as seen in Fig. 9. Also, very high temperatures tend to make Nafion and pure PSSA membranes soft, but the presence of BC gives these membranes ample mechanical strength.
Later, a simple impregnating nanocellulose membrane with SSA, which showed improved methanol barrier property and lower proton conductivity than Nafion117 was developed (Sriruangrungkamol and Chonkaew 2021). However, sulfonated-modified nanocellulose membranes have high prospects to emerge as eco-friendly polymer electrolyte membranes in future DMFC applications.
4.3 Electronics- related applications
Nanocellulose can be considered a highly flexible and ultra-thin substrate, supporting electronic components for various applications. Biodegradable membranes from CNC, CNF and BNC can be used in fuel cell applications (Bayer et al. 2016). The fabricated cellulose nanopapers exhibited proton conductivity dependent on relative humidity, method of preparation and temperature. The CNC paper membrane was more conductive than the CNF paper membrane, primarily due to the increased charge carriers and the hydrophilicity/ acidity of the sulfuric acid groups added during the synthesis process.
Recently, a study used nanocellulose membrane to fabricate renewable flexible electronics (Mao et al. 2021). They used evaporation induced transfer printing technology (Fig. 10) to create nanocellulose-based liquid metal (NC-LM) printed circuit as liquid metal ink cannot be directly applied onto the nanocellulose membrane. As shown in Fig. 11, the NC-LM circuits can be used as wrist bands or attached to the nails and can also be used as multi-layer circuits, layered on top of each other, as they are extremely thin. NC-LM based wearable electronics research is in its infancy and has tremendous potential as they are both economical and environmentally friendly. Supercapacitors are a significant application of nanocellulose-based electronic devices with other applications like electric vehicles, fuel cells, smart consumer electronic devices etc. (Hsu and Zhong 2019).
4.4 Desalination applications
Desalination and water treatment strategies are the need of the hour, especially in developing countries where real-life applications can be the difference between life and death. Nanocellulose membranes are suitable for water treatment applications due to their non-toxicity, recyclability, inert nature, mechanical performance, sustainability and energy efficiency. They can be used as conventional paper filters while controlling their pore sizes. The membranes synthesized by Liu et al. (2019) showed superior dye rejection performance to Nylon 66 membranes with similar pore sizes (Liu et al. 2019). These CNF membranes fabricated with an ultrathin Graphene oxide (GO) barrier layer are easy to create. They are superior to the membranes currently used for water purification and food industries.
There have been numerous reviews on the use of membranes for water treatment (Cruz-Tato et al. 2017; Tan et al. 2020; Mautner 2020). Many carboxyl and hydroxyl groups on the membrane surface aids in water treatment applications, acting as adsorbents. TEMPO-oxidized cellulose nanofibers (TOCN) that used lysozymes (LYS), a natural protein as an adhesive, showed excellent results for the removal of heavy metal ions oil droplets and molecules (> 3 nm). The carboxyl groups of TOCN and the amine groups of LYS have an electrostatic interaction that enables them to form stable membranes. The amyloid-like oligomers from LYS helped the TOCN to stick together. Due to the presence of various functional groups like hydroxyl, carboxyl, amino and thiol groups, it was an efficient water purifier, especially in the removal of boron, a toxic pollutant (Huang et al. 2021).
One of the challenges of using membranes is the risk of fouling. Earlier, fouling was treated using chemical methods, which were not very efficient as it provided only temporary relief and led to increasing costs and wastage of chemicals. The self-cleaning and anti-fouling properties of nanocellulose-enabled thin-film nanofibrous composite (TFNC) ultrafiltration membranes were studied for their ultrafiltration of BSA protein solution and wastewater. The adhesion of biomolecules to the membrane surface was hindered by surface charge, which can be controlled by the degree of oxidation of CNF (Yang et al. 2021). A novel technique by Jiang et al. (2019) uses membranes created from bacterial nanocellulose (BNC) incorporated with reduced activated carbon oxide (RGO) during its growth and inhibit biofouling, “the Achilles heel of membrane processes”, also the RGO/BNC membranes showed bactericidal activity upon illumination with light (Flemming et al. 1997; Jiang et al. 2019). Forward osmosis (FO) is a technique that can be effective against fouling, as it creates a lower transmembrane pressure when compared to other conventional water treatment methods. A study used this principle of FO to deal with wastewater treatment (Cruz-Tato et al. 2017). Silver and platinum nanoparticles were added to these nanocellulose based composite membranes to enhance their properties of the membranes. Interestingly, fouling continued in the membranes to an extent, but an improvement in solute rejection and permeation of the membrane was observed. Self-cleaning membranes are important as membranes under constant use tend to have very low flux and are unsuitable for practical applications. UV assisted self-cleaning TiO2/TCNC membranes by Zhan et al. (2018) is a sustainable and straightforward technique for removing oil from oil/water emulsions (> 99.5%) (Zhan et al. 2018). Tunicate cellulose nanocrystals (TCNC) due to their pore size and special wettability are apt for oil/water separations. On UV-light irradiation, the membranes showed improved underwater oil contact angles and water fluxes. Multifunctional materials take care of a range of problems as they present a range of properties. Yet, processing poisonous cations, anions, and oil from water have many challenges. The oils that are also a pollutant can affect the adsorption of the membranes, affecting the removal of cations and anions directly. Since cellulose shows outstanding acid, alkali, and salt tolerances, they are suitable for synthesizing separation materials. Hydrogels have excellent water-retaining and absorbing abilities, leading researchers to combine both of their properties to fabricate a nanocellulose hydrogel coated titanate-bismuth oxide membrane for the treatment of cations, anions and oil contained in polluted water (Xiong et al. 2018). The cations were trapped in TNF’s layers (titanate nanofibers). After the exchange with Na+ ions, the anions formed irreversible compounds, like Bi4O5I2, by occupying the oxygen holes of delta-Bi2O3, providing more stability. The removal efficiency of cations and anions was also observed due to the membranes' electrostatic adsorption and hydrogen bond interaction. A nanocomposite membrane that showed tremendous potential in adsorbing anionic and cationic organic dyes was developed by researchers (Vilela et al. 2019). When combined, zwitterionic poly (2-methacrylolyloxyethyl phosphorylcholine) (PMPC) and bacterial nanocellulose give optically transparent nanocomposites that inhibit the growth of pathogenic bacteria. The dye removal capacity for a nanocomposite membrane containing 79 wt.% of PMPC was estimated to be 4.44 ± 0.32 mg g− 1 for methylene blue (cationic dye) and 4.56 ± 0.43 mg g− 1 for methyl orange (anionic dye) (Fig. 12).
The active area of water decontamination has a new entrant in bacterial nanocellulose aerogel membranes (Leitch et al. 2016), which was first studied for membrane distillation (MD). Ferreira-Neto et al. (2020) created aerogel membranes of bacterial nanocellulose and molybdenum disulphide (MoS2) nanosheets, which made it quite efficient in the removal of dyes and heavy metals (Ferreira-Neto et al. 2020). It is bifunctional, acting as both a photocatalyst and an adsorbent, unlike earlier membrane technologies that look into photocatalytic properties for in-flow water purification. The FT-IR and XRD analysis confirmed that the use of MoS2 did not compromise the structural integrity of the BC membrane up to six photocatalytic cycles. Even though a surface passivation layer of MoO3 was observed, it did not affect the overall performance of the membrane. The EDS and mass analysis showed only about < 0.3% of total Mo leached into the solution, indicating photostability of the membrane. One of the primary challenges of MD is sustainability as they require high input of energy, while photo-thermal membrane distillation (PMD) is efficient in terms of energy. A bilayer membrane made of environmentally sustainable materials, polydopamine (PDA) and bacterial nanocellulose (BNC) was developed for PMD, which exhibited a permeate flux of 1.0 kg m− 2 h− 1 under one sun irradiation (Wu et al. 2021). The membrane, fluoro-silanized using (tridecafluoro-1,1,2,2-tetrahydrooctyl)-trichlorosilane (FTCS), gives high salt rejection (> 99.9%) and only permits vapor transport. High porosity (~ 93%) and excellent optical activity (~ 98%) enhance the PMD performance. These solar-driven membranes possess bactericidal properties, minimizing bio-film formation and microbial aggregation, thereby increasing the longevity of the membrane. A state-of-the-art solar-driven sweater desalination technique was developed, which combined the two-dimensional membrane and three-dimensional foam into a single product (Zhang et al. 2020). The membrane's mechanical strength combined with the foam's high porosity makes it different from conventional nanocellulose membranes. Functionalities are introduced to the nanocellulose foam membrane (AGM) by adding carbon nanotubes (CNT). The water evaporation rate of AGM-CNT (1.67 kg m− 2 h− 1) was higher than the normal CNF-CNT (1.1 kg m− 2 h− 1) membranes, revealing the combined effect of the porous and hydrophilic nature of the AGM-CNT’s.
4.5 Applications in dermatology and cosmetics
Research in dermatology and cosmetics is on the rise, especially the search for natural ingredients which are much more compatible with the skin. BC has been extensively commercialized for dermal requirements due to its high-water retention capacity and the ability to hydrate the skin (Almeida et al. 2014). A group of researchers could successfully study the effect of dry BC: CMC (Bacterial cellulose and Carboxymethyl Cellulose) incorporated into generic cosmetic creams, replacing the traditionally used surfactants (Martins et al. 2021). Both viscosity and texture of the creams could be replicated with the replacement of 5.5% surfactants with only 0.75% BC: CMC. More ingenious methods are being created to incorporate BC and other cellulosic derivatives into uses other than topical applications. More recently, researchers developed an innovative patch for dermo-cosmetic applications based on two biopolymers, hyaluronic acid (HA) and BC (Fonseca et al. 2021). The microneedle (MN) application method is durable to the human skin revealing its cutaneous compatibility. The effectiveness of the system was demonstrated using Rutin, a natural antioxidant. The peculiar porous structure, water retention ability, moldability and good mechanical properties make BC a good biopolymer for cosmetic purposes. The regenerative properties of HA and the capacity of BC to release bioactive molecules makes it suitable for skin applications. Transdermal drug delivery is another application of nanocellulose membranes. This method is now being preferred over the traditional ones as there is hardly any interaction with a systematic circulation.
A recent study demonstrated the transdermal delivery of crocin, an active agent of saffron (Crocus sativus), loaded into BNC membranes (Abba et al. 2019). A gradual release of crocin over 7 hours and cumulative release of 215 µg/cm2 was observed, which can be associated with the unique morphology of BNC membranes having an entangled nanoscale structure and 3D network. Further research in this area can help include other water-soluble active compounds in transdermal delivery applications.