Research on bioenergy generation has been growing at a relentless pace owing to the concerns that arose from dwindling fossil fuel reserves and the environmental pollution associated with the exploitation of these resources. Studies have reported that emission of greenhouse gases such as carbon dioxide (CO₂) from the burning of fossil fuels has led to climate change issues [1,2]. Since the transportation sector accounts to 60% of the estimated oil demand for the year 2030 (116 million barrels per day), it is essential to replace fossil-fuel-based energy sources in vehicles with renewable energy sources [3,4,5]. In an attempt to gradually replace petroleum and coal resources, various renewable energy sources such as solar, wind, hydrothermal and biomass have been explored to date, with an increased interest in the latter, since lignocellulosic biomass is available abundantly from agriculture and forestry, with an estimated annual production of 2 × 10¹¹ tonnes [6]. It was also reported that in 2011, 38 million tonnes of biomass were used for biofuels in the EU, out of 1.2 billion tonnes of biomass that were generated from various crops [7].

Despite the availability of lignocellulosic biomass in abundance, the technologies for biomass processing remain limited. For instance, the biomasses are recalcitrant in nature due to their cellulose crystallinity and non-reactive lignin, thus requiring pre-treatment techniques to allow the cellulose to be amenable for an enzymatic hydrolysis process [8]. This is required for the extraction of fermentable sugar for subsequent biofuel generation processes [9,10]. Figure 1 shows the cellulose and hemicellulose that accommodate 6 and 5 carbon sugars rigidly bounded with the lignin, where cellulose forms structure of the cell walls, while hemicellulose assists in the cross-linking between the non-cellulosic and cellulosic polymer via covalent bonding [11]. It is noteworthy that biomass resources currently deployed as feedstock in human or non-human food chains should not be used for chemical processing to ensure sustainability. For instance, wheat straw is a common fodder for raising animals and therefore is available in short supply due to increase in meat consumption; wheat straw should not be considered as a viable option for bioenergy generation [12].

Generally, biochemical or thermochemical routes can be employed for the conversion of lignocellulosic raw materials into bioenergy. The biochemical route uses microorganisms and various enzymes to convert components of the feedstock into sugars, followed by fermentation to produce ethanol [13]. Since the biochemical route uses various enzymes to convert feedstock to biofuel, the efficacy of the enzyme is important to improving the conversion process. On the other hand, the thermochemical route involves gasification technologies to produce simple sugars that are fermentable for biofuel production. In a study by Daystar et al. [14], it was highlighted that thermochemical conversion can use a wider range of feedstock compared to biochemical conversion and produces considerably higher alcohol yields. Additionally, unlike the biochemical process, the thermochemical process is not affected by the lignin in the biomass, although presence of moisture content can affect alcohol yield as well emissions in the thermochemical process. A general finding from most studies indicates that high cost and limited availability of existing infrastructure have posed limitations on obtaining high quality and yield of bioenergy.

However, with the emergence of nanotechnology, the limitations can be addressed efficiently since nanotechnology involves nanomaterials, which exhibit unprecedented characteristics and properties, all of which are highly useful in areas of bioenergy generation.

Enzyme immobilization on support materials is useful to enhance enzyme characteristics such as high activity at most pH values, reusability and selectivity and reduce their inhibition. Some studies have reported that immobilization of cellulase enzyme makes it more resistant to structural alterations which may be caused by increase in the temperature [29]. An excellent enzyme immobilization on solid support ensures good distribution of the catalyst with minimal agglomeration. For instance, covalent binding between the support and enzyme have been reported to increase the enzyme activity [53,69]. Other than covalent binding, approaches such as adsorption, ionic bonding, entrapment and encapsulation have been reported for enzyme immobilization. In comparison to other NPs, MNPs have gained significant interest as promising support carriers in enzyme immobilization due to factors highlighted in Section 2.1 of this work. Most importantly, in enzyme immobilization, the large surface-area-to-volume ratio of MNPs is highly advantageous to loading a large number of enzymes on their surface, which in turn increases the catalytic activity of the enzymes. Several techniques can be used for immobilization of enzymes on the MNP support. For example, MNP functionalization for cellulase immobilization can be on silica-functionalized MNP, amino-functionalized MNPs, composite-functionalized MNPs, chitosan-functionalized MNPs or carrier-free functionalized MNPs.

For instance, sulfonated magnetic carbonaceous nanoparticles, which were used for the hydrolysis of various lignocellulosic biomass, showed significant levels of glucose yields of jatropha, bagasse and plukenetia hulls of 35.6%, 58.3% and 35.8%, respectively, with the capability of recycling the catalyst at least seven times with a high catalyst recovery rate of 92.8% [70]. In another study by Goh et al., enzyme immobilization was performed on magnetic single-walled carbon nanotubes for application in biofuel production. It was reported that the enzyme was recyclable, which reduced the cost of biofuel production while also retaining their activity for a month in acetate buffer at 4°C [71]. In another study, 93% binding efficiency and 50% activity retainment after 16th cycle was achieved with a nanobiocatalyst system for biofuel production using functionalized magnetic nanoparticles immobilized with β-glucosidase isolated from fungus [72]. In biodiesel production, immobilization of lipase on polydopamine-coated magnetic nanoparticles enhanced the pH and thermal stability while achieving 73.9% binding efficiency, more than 70% of activity even after 21 repeated cycles and easy recoverability from reaction mixture [73]. Similar findings were also reported in a study by Hu et al., where immobilization on magnetic nanoparticles increased biodiesel production by 95% when a reaction was performed at 65 °C for a duration of 3 h. Additionally, the catalyst was reusable 14 times while allowing recoverability of more than 90% [74]. To date, various MNPs have been explored as support carriers in enzyme immobilization. A summary of the various MNPS that have been used as the support carriers in enzyme immobilization and their effect on bioenergy production is summarized in Table 1.

In the biogas production process, nanoparticles have proven to show promising results in the anaerobic processes (biochemical processes to convert various organic materials into biogas and its constituents), in particular as electron donors, acceptors and cofactors of key enzymes such as [Fe]- and [Ni-Fe]-hydrogenase [84]. For example, nanoparticles increase the hydrolysis of organic matter due to their large surface-area-to-volume ratio, which enables microorganisms to bind onto the active sites of the molecule. This subsequently stimulates the biochemical process for the activity of hydrogenase enzymes and ferredoxins. Various nanoparticles have been used in the anaerobic digestion process such as zero-valent metals, metal oxides and carbon-based nanomaterials.

Generally, the anaerobic digestion process involves four important steps: (i) hydrolysis, (ii) acidogenesis, (iii) acetogenesis and (iv) methanogenesis. Nanoparticles such as carbon-based materials have been reported to be useful in all the four stages. For example, in a study by Velimirovic et al. [85], carbon-based nanomaterials in the hydrolysis stage, when available as electron donors, can lower the oxygen reduction potential of the water environment, which is highly beneficial in the hydrolysis reaction. In the acidogenesis and acetogenesis processes, studies have reported that the addition of 12.5 g/L biochar enhanced the biohydrogen production from 22.6 to 96.3 mL/g [86], the addition of 8.3 g/L of biochar increased the biohydrogen yield from 750.4 to 944.5 mL/L from food waste [87], and the addition of 600 mg/L of sawdust biochar improved biohydrogen production from 31.5 to 36.5 mL [88]. In all the aforementioned studies, it was highlighted that addition of carbon-based nanomaterials enhanced the performance of acid formation processes, which resulted in improved yield of hydrogen. Similar enhancement in biogas production were also reported in other studies using zero-valent ions (ZVI); i.e., 0.1 wt.% ZVI using waste-activated sludge enhanced biogas production by 30.4% [89], whereas the highest methane formation rate (0.310 mmol CH₄ formed/mol Fe⁰. day) was reported using the finest grade iron ZVI [90].

Studies have also been performed to study the effect of nanoparticles on microbial communities in the anaerobic digestion process. For example, Wang et al. used four types of nanoparticles, i.e., ZVI, Ag, Fe₂O₃ and MgO, to investigate their effect on biogas production from waste-activated sludge, and it was reported that ZVI (10 mg/g) and Fe₂O₃ (100 mg/g TSS) increased biogas production by 120% and 117%, respectively, which suggests that ZVI and Fe₂O₃ nanoparticles improve the activity of the methanogenic bacteria [91]. This was also supported in another study by Yang et al., which reported that addition of ZVI nanoparticles increases the population of methanogens in an anaerobic digester [92].

Biohydrogen production is performed by anaerobic bacteria via metabolic routes to generate molecular hydrogen. It has been reported that the activity of these microorganisms can be enhanced with the use of nanoparticles to increase electron transfers and kinetics in metabolic processes to produce biohydrogen due to their capability to react faster with electron donors. The role of nanoparticles in various biohydrogen process is as described below.

Nanoparticles have been reported to improve the dark fermentation process. For instance, 5 nm gold nanoparticles improved substrate utilization and biohydrogen yield by 56% and 46%, respectively, owing to the large surface-area-to-volume ratio of gold nanoparticles, which provided a stimulatory effect to produce biohydrogen [93]. It has also been highlighted in other studies that gold nanoparticles enhance the activity of biohydrogen-producing enzymes, which mediate the transfer of electrons such as [Fe-Fe]- and [Ni-Fe]-hydrogenases and ferredoxins [94,95]. In other studies, zero-valent iron (Fe⁰) nanoparticles were used for dark fermentation of grass, and it was shown that Fe⁰ nanoparticles stimulated activity of a hydrogenase enzyme to produce a higher yield of biohydrogen (i.e., maximum hydrogen yield of 64.7 mL/g dry grass; 73% higher than control experiments) [96]. Other studies have also reported the use of metallic nanoparticles such as Pb, Ag and Cu along with FeO nanoparticles immobilized on porous silica (SiO₂), and highest yield and production rate was achieved using FeO nanoparticles, i.e., 38% and 58%, respectively, in comparison to control experiments [97]. Ni-graphene-based nanoparticles have also been reported to enhance dark fermentative biohydrogen production, in a study where maximum yield of 41.3 H₂/g COD with 105% increase in H₂ yield was obtained [98].

Nanoparticles have also drawn significant interest as means to improve the biomass growth, photosynthetic activity, nitrogen metabolism and protein level of microalgal species to produce biohydrogen. The key roles of nanoparticles in photo-fermentative biohydrogen production are as summarized below:

• Nanoparticles behave as the catalytic agents to generate metabolic pathways to promote synthesis of chlorophyll a, chlorophyll b, carotenoids and anthocyanin, lipid production and nitrogen metabolism [99,100].
• Nanoparticles enhance production of carbohydrates, which results in increased growth of algal cells. For example, silica nanoparticles enhanced growth of microalgal cells (measured from chlorophyll concentration), because silica nanoparticles scattered light within a reactor to ensure uniform light distribution during the photosynthetic process, which in turn promoted growth of microalgal cells [101]. Similar findings were also reported using zero-valent iron (Fe⁰) [102] and TiO₂ nanoparticles, which increased chlorophyll and carotenoid pigments [103].
• Nanoparticles enhance activity of key enzymes for metabolism of microalgal species such as glutamate dehydrogenase, glutamate–pyruvate transaminase, glutamine synthase and nitrate reductase [104,105]. The capability of nanoparticles to maintain the pH of a medium and to promote the activity of hydrogenase enzymes and substrate hydrolysis may promote higher biohydrogen yield by enhancing biohydrogen-producing metabolic pathways such as acetate and butyrate reactions [32,103,106].

Photocatalytic biohydrogen production involves the breaking of water molecules into H₂ and O₂ in the presence of an illuminating source. A photocatalyst such as TiO₂ has been commonly preferred owing to its high photocatalytic performance, non-toxicity, chemical stability and cost effectiveness [107,108]. For instance, nanoparticles such as Pt-TiO₂-activated carbon generated a high hydrogen production rate of 7490 µmol/h/g photocatalyst, which is 75 times higher than conventional Aeroxide TiO₂ P25 catalyst [107]. Other studies have reported the combination of TiO₂-graphene has higher light absorption efficiency than TiO₂ alone, which resulted in higher charge separation efficiency to yield higher hydrogen [109]. The effect of various nanoparticles on the hydrogen yield rate is summarized in Table 2.

Generally, lignocellulosic materials are processed for bioethanol production via three major operations, which include (i) pretreatment for delignification to liberate the cellulose and hemicellulose, (ii) hydrolysis to produce fermentable sugars such as glucose, xylose, arabinose, galactose or mannose and (iii) fermentation of reducing sugars. In a very recent study, Saeed et al. [124] used graphitic carbon nitride (g-C₃N₄) nanomaterials (Figure 2A) and laser irradiation (Figure 2B) to increase bioethanol production from potato waste. Authors reported that the control sample without laser irradiation or g-C₃N₄ showed only 4% yield of bioethanol, while the addition of g-C₃N₄ coupled with laser irradiation increased bioethanol yield to 56.8% (Figure 2D).

In another study by Gupta et al. [125], zinc oxide nanoparticles were used to enhance bioethanol production from rice straw, and a maximum ethanol yield of 0.0359 g/g dry weight-based plant biomass was attained at 200 mg/L concentration of ZnO nanoparticles (Figure 2C). Additionally, the possibility for reusability and recovery of the nanoparticles makes the entire process more economical. Similarly, in another study by Ivanova et al. [126], it was reported that bioethanol fermentation was enhanced with the use of alginate magnetic nanoparticles entrapped with yeast cells, with the productivity rate reaching up to 264 g/Lh at 70% particle loading. It was also interesting to note that the magnetic particles with fixed yeast cells were stable for more than a month at 4 °C in saline condition. In a similar vein, Kim et al. [127] reported enhanced bioethanol production in syngas fermentation using methyl-functionalized silica nanoparticles. After 9 h of cultivation time, it was reported that ethanol concentrations without and with nanoparticles were 0.1150 and 0.3060 g/L,w respectively, which indicates that ethanol production was enhanced by 166.1% by the use of nanoparticles. When producing liquid fuels through fermentation, it is expected that nanomaterials will influence the biochemical conversion process by affecting the enzymatic activity or the mass transfer rate.