The biosensor working principle is linked to three main components: a biological recognition system (bioreceptor), a physical-chemical transducer, and an electronic system that processes and displays the signal [46,155]. The first recognition interface guarantees different selectivity and accuracy according to the detection method/biological element (e.g., enzymes, antibodies, DNA, microorganisms, receptors, cells) [44,49,50,55]. The bioreceptor interacts with the target analyte, resulting in a biochemical reaction. The transducer is responsible for converting this reaction into a demonstrable signal, which is associated with the concentration of the analyte and can be quantified by phenomena based on optics, acoustics, mechanics, calorimetry, electronics, or electrochemistry [156]. These devices provide small dimensions, low-cost fabrication, and real-time detection, making them an attractive dispositive for quantitative and semiquantitative analyses [157]. 

Among the different transducers available to convert biochemical signals into measurable signals, electrochemical systems have frequently been used as the detection mode in commercial biosensors. Their quantification involves detecting a redox reaction in the transducer when the bioreceptor interacts selectively with the analyte in the solution, which generates an electrochemical signal. This signal can induce amperometric and potentiometric responses, field-effect transistors (FET), and conductometric responses [52,56]. The advances in biosensing analyses require understanding the charge transport and the electron transfer that occurs in the electrode/electrolyte interface, which will lead to an efficient bond between the bioreceptor and the target analyte, and will immobilize the biomolecule in the transducer [43,47,158].

The physicochemical characteristics of the transducer interface can be adapted and improved to increase the quantitative accuracy, selectivity, and reactivity of the techniques. Constructing a modified layer is highly desirable and will allow a specific biological function to be better retained on the transducer’s surface. [128]. One approach to optimizing the electron transfer in the interface electrode/electrolyte is to design a suitable surface (support) depending on the properties and the stability of the biomolecule to guarantee its immobilization onto the electrode. Improving the biomolecule and electrode surface integration will allow a high biosensor sensitivity [159,160,161,162].

The BNC has formidable properties that make it a superb candidate to support biomolecule immobilization onto the transducer surface. Its high specific surface area and nanoporous structure, for example, facilitate the penetration of the biomolecules, resulting in higher sensitivity and faster biosensor response time [29,61,163,164]. Furthermore, this nanomaterial possesses high tensile strength, crystallinity, hydrophilicity, and great water-holding capacity [60]. However, it lacks conductivity [60,103,163,164], a drawback to its applications in electrochemical biosensing. A strategy to overcome this issue is to prepare a BNC nanocomposite that contains conductive materials, such as graphene oxide, carbon nanotubes, conductive polymers, and gold nanoparticles.

The methods of manufacturing the BNC-based materials can be carried out through the nanofabrication of materials incorporated in the BNC nanofibrils or by a polymer-based approach. BNC-based nanocomposites can have high catalytic selectivity because of their interaction with biomolecules. They can achieve dimensions between 2 and 20 nm, similar to nanoparticles (NPs), and introducing NPs increases the biosensors’ electronic and optical transduction characteristics [43]. The interaction of biomolecules and NPs on the polymer matrices, such as BNC, is possible by applying physical, chemical, or biological methods [43,46]. Carbon nanotubes (CNT) have also attracted interest in manufacturing BNC-based materials due to their high surface area, electrical conductivity, and good chemical and thermal stability. Furthermore, it has been shown that the elastic modulus and the ultimate strength of polymer composites increase even with the incorporation of small amounts of CNTs, which enhance these matrices’ mechanical and thermal properties [115].

The polymer-based approach integrates the fundamental properties of biosensing applications. Conducting polymers, such as polyaniline (PANI) [97,165] and polypyrrole (PPy) [114], have frequently been used to prepare BNC-based materials. The conjunction of the conducting characteristics of the conjugated polymers with the porous structure of nanocellulose material results in excellent sensing characteristics and elevates the BNC’s capacity to immobilize biomolecules. This happens due to BNC’s good water-holding capacity and hydrophilic properties, which enhance the conducting polymers’ physical and structural properties [98,103,163]. These polymers also enhance the transduced analytical signal generated by interacting these immobilized biomolecules with the target analyte [51,166] and, consequently, have been employed in biosensing analyses [51,98,164]. However, due to their low solubility in common organic solvents and poor mechanical properties, their application has been restricted to some electronic devices [167,168]. The following subsection demonstrates these nanocomposites’ applicability in constructing biosensor devices.

Even though it is challenging to compare different manufacturing strategies, Table 1 presents a brief summary of the performance parameters of biosensors that contain a layout with BNC and alternative polymers (CS, Alg, and synthetic nonconductive polymers) as immobilizing substrates. The alternatives show the unique advantages and drawbacks of BNC in biosensing applications [169,170]. Nonetheless, BNC-based biosensors have performance in the same range or improved linear range and detection limits. However, when measured, their stability (preservation of functionality over time assay), repeatability (preservation of functionality over multiple tests assay), and reproducibility (preservation of functionality over the manufacture of different electrodes), present mixed positive (lower relative standard deviation (RSD)) and negative (higher RSD) results in relation to the other alternatives.

These biosensing applications do not involve any special rules or general ways to theoretically predict the biosensor performance to detect components from selecting immobilizing polymers. Nevertheless, in addition to target analytes and selected biorecognition entities, the differences in the detection achievements depend on polymer features, synergic effects through functionalization materials and methods, and performance test protocols [171,172,173,174,175]. There are a lot of combinations and facets that are yet to be explored in biosensor research with bacterial nanocellulose [175]. Given this context, the next sections are not limited to discussing BNC advantages and drawbacks but to addressing the BNC-based nanocomposites’ potential in constructing straightforward and sustainable advanced biosensor devices.

The application of BNC nanocomposites using gold nanoparticles (AuNP) was first described in the paper by Zhang and colleagues [104]. The glassy carbon electrode was used as the transducer and, to provide a suitable surface for the horseradish peroxidase (HRP) enzyme immobilization, the nanocomposites were applied onto its surface. For the BNC-nanocomposite synthesis, the authors used a polyethylenimine (PEI) solution that acted as a reducing agent and a linking molecule. The PEI solution was mixed with BNC nanofibers, and HAuCl₄ was added. The effective reduction of HAuCl₄ and, consequently, the formation of the AuNPs were demonstrated when the BNC nanofibers gradually turned purple. The formation mechanism of the nanocomposites was studied, and it was found to involve three steps. Firstly, hydrogen bonds bonded the BNC’s hydroxyl groups with the PEI amine groups. Subsequently, the free amine groups of PEI protonated and conjugated with the ion AuCl₄^-. Lastly, the ion AuCl₄^- was reduced by PEI and nucleated on the BNC surface. To confirm that the network structure of the BNC-Au nanocomposites effectively helped entrap the biomolecule HRP and to check its biocatalytic activity, the constructed biosensor was used to determine hydrogen peroxide. For that, hydroquinone was used as an electron mediator. The electrochemical response of the biosensor with and without the BNC-Au nanocomposites was compared. For the HRP/BNC-Au/GCE biosensor, the catalytic effect of HRP was successfully observed since the hydroquinone reduction current increased significantly in the presence of H₂O₂. The same was not observed for the HRP/BNC/GCE and HRP/Au/GCE biosensors, demonstrating much smaller electrocatalytic reduction peak currents toward H₂O₂. The results revealed that the combined effects of BNC and AuNPs, such as BNC’s biocompatibility network structure and AuNPs’ high conductivity, make the BNC-Au nanocomposites a suitable matrix for enzyme immobilization.

In order to polish up their latest work [104], Wang et al. [126] used BNC-Au nanocomposites to create a glucose biosensor. The preparation of the modified electrode included BNC-Au deposition onto the GCE surface, followed by the immobilization of the catalytic enzymes glucose oxidase (GOx) and HRP. The point of using both enzymes was to minimize the interference resulting from oxidations and the reduction process of other compounds at the working potential. To investigate if the nanocomposites improved the biosensor response, the electrocatalytic activities of the enzyme toward glucose were compared in the biosensors with and without the BNC-Au. The amperometric response of the biosensor without the nanocomposites indicated that its sensitivity was not high enough to detect low concentrations of glucose. The authors attributed this result to the absence of a biocompatible environment for the enzymes, which led to the low biocatalytic activity of these biomolecules. In contrast, the biosensor containing the BNC-Au nanocomposites exhibited an excellent amperometric response even at low concentrations of glucose, confirming that, due to the BNC networking, the enzymes were effectively retained onto the transductor surface and their biocatalytic activity was preserved. Furthermore, the AuNPs increased electrical conductivity on the electrode surface, obtaining a limit of detection (LOD) of 2.3 µM.

Wang and colleagues [128] investigated whether the BNC-Au nanocomposites were suitable to immobilize proteins of different sizes, such as hemoglobin (HB) and myoglobin (MB). The transducer chosen was the glassy carbon electrode, which was modified with BNC-Au nanocomposites. The cyclic voltammetry (CV) curve of BNC-Au/GCE showed that the resistivity of Au-BNC was significantly reduced compared to BC itself, indicating that AuNPs were efficiently attached to the BNC nanostructure. Subsequently, the proteins were immobilized onto the modified electrode surface, and their biocatalytic activity was investigated by detecting H₂O₂ in hydroquinone (HQ) as an electron mediator. The Hb- and Mb-based biosensors presented biocatalytic activity and rapid amperometric response toward H₂O₂ (linear response ranged from 10 µM to 1000 µM and 10 µM to 100 µM; detection limits were 3.9 µM and 5.1 µM, respectively), proving that the BNC-Au nanocomposites were suitable to immobilize different proteins sizes.

BNC-Au nanocomposite was applied in the work of Li et al. [54] to facilitate laccase (Lac) enzyme immobilization on the surface transducer (GCE) and create a biosensor for hydroquinone detection. Therefore, vacuum filtration was applied to deposit AuNPs on the bacterial cellulose. After that, the BNC-Au was adhered to the GCE, followed by the Lac immobilization onto the transducer surface. The CV showed a satisfactory electrochemical response to the GCE/BNC-Au/Lac biosensor, which confirmed that there was direct electron transfer between the electrode surface and the electroactive center of the immobilized enzyme. The number of electroactive species present on the electrode surface was also calculated. The calculation showed a more significant concentration of electroactive species on the biosensor that used BNC along with AuNPs for the enzyme’s immobilization [194]. These results indicated that a significant amount of Lac was immobilized on the BNC’s surface, which confirmed that its substantial surface area and nanostructure help biomolecule immobilization. To finish, the response of the biosensor toward hydroquinone was assessed by direct electron transfer (DET). The immobilized enzyme showed a great biological electrocatalyst, with the linear response of the hydroquinone ranging from 30 nm to 100 nM and a detection limit of 5.71 nM.

Kim and colleagues applied carbon nanotubes in their study [179] to promote a direct electron transfer between the biomolecule and the electrode surface. Its excellent properties, such as outstanding electrical conductivity, were mixed with the BNC’s good biocompatibility and ultrafine network to promote the enzyme’s catalytic activity. To prepare the BNC/CNT nanocomposites, the suspension of CNTs was vacuum filtered through the BNC hydrogel, and subsequently, the BNC containing the CNTs was vacuum dried. A thin BNC/CNT composite film was obtained at the end of the process. A GOx solution was prepared in a phosphate buffer (pH 7.00) and dropped on the dried BNC/CNT film to immobilize the biomolecule. The authors did not use supporting electrodes to measure the electrochemical performance of the BNC/CNT composite film; instead, the film itself was applied as the electrode, and a silver epoxy tape was attached to the edge of the film to make an electrical contact. The CVs obtained using the BNC/CNT/GOx electrode showed peaks referring to the reduction and oxidation reaction of the redox center of GOx immobilized. This result proved there was an efficient electron transfer between redox enzymes and the BNC/CNT electrodes, and the immobilized GOx retained its catalytic ability.

Looking to develop a self-powered biosensor, Lv et al. [180] applied BNC, AuNPs, and carboxylic multiwalled carbon nanotubes (c-MWCNTs) as nanocomposites to fabricate the dispositive. A c-MWCNTs solution was first ultrasonically ground along with a BNC solution, forming a homogeneous suspension. This suspension was filtered and dried, forming a BNC/c-MWCNTs film. Subsequently, the obtained film was immersed in a solution containing PEI and HAuCl₄ to promote AuNP formation on its surface. The carboxyl groups on the BNC/c-MWCNTs surfaces served as anchor sites for AuNP nucleation, an interaction that prevented aggregation of AuNPs during reduction. For the fabrication of the self-powered biosensor, the researchers applied two enzymes: glucose oxidase (GOx) acting as bioanode and a Lac-based biocathode, wherein both immobilized onto the BNC/c-MWCNTs/AuNPs solution by electrostatic attraction. The electrochemical behavior of both bioanode and biocathode was investigated by CV. GOx-modified BNC/c-MWCNTs/AuNPs exhibited a pair of redox peaks that were attributed to the redox reaction of the GOx immobilized. In the same way, Lac-modified BNC/c-MWCNTs/AuNPs were investigated, and the biocathode exhibited a pair of well-defined reduction and oxidation peaks. These results confirmed that there was direct electron transfer between the electrode surface and the electroactive center of the enzymes, implying that the enzyme’s electron transfer could be conducted through AuNPs and c-MWCNTs on the BNC. This further indicated a good coupling between the enzymes and BNC/c-MWCNTs/AuNPs electrodes, which was associated with the BNC’s nanofiber network and biocompatibility.

Developing an electrochemical biosensor for bacterial detection requires an adequate substrate for the bacteriophages’ immobilization. In addition to being biocompatible and having a surface area that allows the immobilization of the phage particles, the bioprobe demands an ambient that preserves its tail’s ability to infect the host bacterium. As BNC can meet such requirements and offer a nontoxic environment, Farooq and colleagues [186] applied it to create a biosensor for detecting S. aureus. As BNC lacks conductivity, carboxylated multiwalled carbon nanotubes (c-MWCNTs) were attached to the BNC matrix to impart electrical conductivity. Phase immobilization is frequently done by the electrostatic approach, which requires an interaction between the phage capsid proteins, which have a negative charge, and the substrate. Here, PEI was added to provide a positive charge on the surface of BNC/c-MWCNTs nanocomposites. DPV analyzed the biosensor electrochemical response, and the results showed a current increase along with the bacteria concentration, which determined the S. aureus density. These results validated the hypothesis that the BNC nanocomposites are a suitable environment for bacteriophage immobilization.

Polypyrrole is a highly conductive polymer that enhances the electrochemical response in sensing analyses. Ghasemi and colleagues [187] demonstrated using this polymer associated with the BNC nanostructure and TiO₂-Ag nanoparticles to monitor the growth of pathogenic bacteria in food. The BNC/PPy/TiO₂-Ag nanocomposite was synthesized by chemical polymerization. In this approach, the BNC film is the transducer itself, and there is no need for a support electrode. The sensor was connected to a multimeter, and the film’s resistance change was measured. Both gram-positive and gram-negative bacteria were used to evaluate the sensor’s response to the bacteria. Centrifuged suspensions of the bacteria were added to the film in different concentrations, and the relative electrical resistance difference (RRD) was recorded. The researchers also investigated the sensor’s response by applying different PPy concentrations to the BNC nanocomposite’s fabrication. The results showed that the sensor’s sensitivity was increased by increasing the amount of PPy until it reached a maximum value that started lowering the sensitivity. These data showed the value of applying the right amount of conductive polymers and demonstrated how it would help to achieve sensitive electrodes.

The increasing use of nanocellulose in recently published articles on biosensors shows tremendous results on a laboratory scale [16,195]. However, successful materials for biosensing commercial solutions present a main challenge: industrial scale [16]. In previous sections, we emphasized the BNC potential for several electrochemical biosensing applications. Bacterial nanocellulose fits the new paradigm for sensing applications that consist of sustainable and robust frameworks [16]. This nanocellulose resource joins the growing field of cellulose in bioelectronics showing modular modifications during its production and suitable properties for advanced nanoscale composites, such as for flexible and miniaturized devices.

To ensure cost-effectiveness, the BNC production scale-up can be optimized in different steps, from feedstock to functionalization [195]. As demonstrated by Abol-Fotouh and colleagues, expensive substrates for a bacteria culture medium might be replaced by renewable feedstocks such as sugar cane bagasse, wood processing residues, or agro-industrial waste [30]. Beyond bioprocess technologies, straightforward techniques have been developed from advances in bioengineered and synthetic biology studies, offering new strategies related to biosynthetic and genetic modifications through the cellulose synthesis pathway [195,196]. These genetic approaches have been showing promising results in increased cellulose production [197], as well as in rationalizing the BNC functionalization design in molecular [129] and 3D levels [198].

In the next few years, bacterial nanocellulose and its synthesized machinery might be engineered at the DNA level to achieve in vivo functionalization, discarding the need for chemical and physical functionalization steps [195]. Indeed, synthetic biology approaches have already exploited fibrous amyloid protein polymer production, which might suffer modifications through rational engineering with new protein modules and bacterial cellulose fibers [199,200]. Although advances on the genetic scale are still in the early stages, Gao and colleagues [129] demonstrated that modifications on sugar substrate could turn into BNC with new properties and morphology without any modification in the bacterial fermentation pathway.

In addition to genetic modifications of nanocellulose synthesis machinery, BNC has been applied as a sustainable and modulated scaffold for engineered living material, in which an engineered living cell could be embedded into a nanocellulose membrane (or another biomaterial) or cocultivated with a nanocellulose-producing bacteria. These materials are dynamic and responsive, with programmable properties, and might play diverse roles in wound healing, tissue engineering, antibacterial treatment, or biosensing [198,201]. In a proof of concept, Long and colleagues [202]. built a cell-based sensor platform to test the ability of nanocellulose to preserve cell viability and bioactivity in a highly efficient adhesion strategy. An engineered Escherichia coli with a recombinant surface-exposed CBM2a (Carbohydrate-binding module-2a) and an L-arabinose biosensing genetic system was embedded into BNC carriers. The bacteria bound tightly to BNC carriers without any substrate modification and could optically report the presence of L-arabinose in water and soil samples. One year earlier, Farooq and colleagues had taken a similar approach [186] without genetic modification with phages for pathogen detection.

Furthermore, the increased interest in wearable electronic devices has required flexible biosensors [203]. The substrate for fabricating this wearable sensing platform requires improved mechanical flexibility, chemical and thermal stability, biocompatibility, and conformal contact with the skin [191,204,205,206]. Paper-based biosensors, such as dipstick, lateral flow assay, and microfluidic paper-based analytical devices, have received significant attention as they allow for low-cost, portable, and disposable platforms. Nanopaper that is made entirely of BNC has all the advantageous features exhibited by conventional paper, such as versatility, abundance, transparency, flexibility, and cost. Nanopaper also obviates the earlier drawbacks by offering much lower thermal expansion and much higher chemical, mechanical, and thermal stability [40].

In the study by Naghdi and colleagues [207], the team explored optical transparency, high flexibility, porosity, biodegradability, and printability to develop a BNC-based optical sensor. The device aimed at visually sensing human serum albumin (HSA) in human blood serums via curcumin embedded in bacterial cellulose nanopaper. The authors developed a “lab-on-nanopaper” device that was entirely ecofriendly owing to their use of curcumin and nanopaper as safe, nontoxic, and containing green materials, with a lack of need for sophisticated instrumentation and using the minimum required sample volume (~5 µL) for HSA detection.

Gomes and colleagues [191] developed an electrochemical biosensor made on BNC substrate for lactate detection in artificial sweat. The strategy of enzymatic immobilization was based on the direct covalent binding of biomolecules with the functionalized bacterial cellulose substrate instead of immobilization onto the electrode surface. The mechanical tests showed that BNC had a remarkable capacity to stretch and could be used as a substrate in wearable devices. Although bacterial and vegetal cellulose have similar chemical compositions, BNC possesses a greater surface area and exhibits superior mechanical properties. When comparing it with previously reported wearable lactate biosensors, they found that the researchers’ proposed sensor exhibited a similar linear range, and their approach offered significant advantages regarding fabrication strategies. Furthermore, they proposed a biocompatible substrate and superior flexible properties.