Utilization of Nanotechnology and Nanomaterials in Biodiesel Production and Property Enhancement

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Introduction
Developing molecular-scale machines and devices that are a few nanometers (10 −9 m) wide, much smaller than a cell, can be broadly characterized as the field of nanotechnology. The effect of nanoparticles on the creation and manufacturing of biofuels/biodiesels is currently estimated using a variety of nanomaterials, including nanofibers, nanotubes, and nanometals [1].
The use of nanotechnology and nanomaterials in biodiesel research has emerged as a viable instrument for delivering efficient methods to raise production quality at a reasonable cost. Due to their tiny size, particular characteristics, and traits, including a high surface area to volume ratio, considerable crystallinity, catalytic activity, adsorption capacity, and stability, nanoparticles (NPs) have several benefits over biodiesel production [2]. With additional features that support high potential recovery, carbon nanotubes, and metal oxide nanoparticles are often utilized as nanocatalysts to manufacture biofuel and biodiesel [3,4]. The use of nanotechnology in the manufacture and improvement of biodiesel is critically examined in this paper, along with the main obstacles and promising future developments.
Microalgae are a promising feedstock for the manufacture of biodiesel. Diverse nanoparticles may quickly enhance the effectiveness of the microalgae harvesting process. Additionally, the reuse of nanomaterials and the incorporation of cell harvesting, disruption, and extraction also help to lower costs. Additionally, a variety of nanocatalysts have the potential to improve biodiesel conversion efficiency [5].
However, increasing combustion efficiency and lowering harmful emissions have become popular research areas in the engine industry and related fields. According to various research, nanoadditives to diesel-biodiesel fuel blends have shown notable outcomes. Numerous investigations and research findings on nanoparticles have shown the critical function that nanoadditives play in enhancing the efficiency of internal combustion engines and lowering the emission of hazardous pollutants [6].
Many researchers have extensively investigated the applications of nanotechnology and nanomaterials assisted biodiesel production and its efficiency enhancement which will be summarized in this review paper [7][8][9]. Hence, this review examines the use of nanotechnology in several biodiesel manufacturing process phases, including microbial culture, lipid extraction, hydrocarbon purification from oil, and transesterification. The use of nanoparticles as fuel additives for diesel-biodiesel fuel blends will also be covered in this paper. This review also discusses the benefits and drawbacks of using nanotechnology at different process stages.

Overview of Nanotechnology
The core components of the burgeoning nanotechnology research industry are known as nanoparticles. They come in various forms, including cylindrical, spherical, flat, conical, and tubular, and varying diameters between 1 and 100 nm. Nanoparticles may be amorphous or crystalline, packed tightly or loosely, and comprised of single or many crystal solids. They can also exist in all four dimensions as zero, one, two, and three dimensions [10].
Various classifications of nanoparticles based on surface, material, size, and structure are shown in Figure 1. Based on where they come from and the fundamental chemical elements that make up their structure, nanoparticles may be categorized as carbon-based, organic, inorganic, or composite based. Long chains of carbon atoms joined together in unique configurations, such as spherical in fullerenes, honeycomb lattice in carbon nanotubes, or conical in carbon nanofibers, make up the whole of carbon-based nanoparticles. On the other hand, organic nanoparticles are biodegradable, nontoxic, and mostly used in the medical industry as a drug delivery system, antioxidant, etc. [11].
The researchers have discovered the potential of nanoparticles as an addition because of the significant impact nanoparticles have on the base fuel's ignition and combustion characteristics. Concerning the field of CI engines, metal and metal oxide nanoparticles, which fall under the category of inorganic nanoparticles, have far more advantageous properties to offer, such as their capacity to boost reactivity, speed up evaporation rates by serving as an oxygen buffer, and enhance the calorific values, thermal conductivity, and viscosity of the base fuel. Because they may improve the thermophysical characteristics of the fuel, nanoparticles are used as an additive for CI engines. Nanoparticles have a more excellent surface-to-volume ratio due to their nanostructure, a desired feature for mixes used in CI engines as they give a bigger reactive surface for the chemical reaction and combustion [12].

Methods of Nanomaterial Synthesis.
A nanofluid is a fluid created by evenly scattering nanoparticles in a liquid fluid. The synthesis and characterization of nanofluids are crucial because different nanomaterials significantly impact the dispersion and stability of these fluids. The use of nanoparticles has been the subject of many studies in recent years. They have progressed in manipulating nanoparticle size, shape, and porosity and their physical and chemical characteristics. Therefore, choosing an appropriate preparation technique is crucial for nanofluids. Nanofluids are typically synthesized in one or two steps, while newer procedures have also been developed. Numerous physical, chemical, biological, and hybrid strategies may be used to create nanoparticles ( Figure 2) [13,14].
2.1.1. One-Step Preparation Method. Nanoparticles and the base solution are mixed in the one-step process. Compared to other approaches, the one-step method's key benefits include:  Journal of Nanomaterials (1) lower production costs due to the method's simplicity and lack of need for drying, storage, or dispersion, (2) due to the minimal aggregation of nanoparticles, the nanofluid created by the one-step technique may remain stable for an extended period. Laser ablation, direct evaporation, vapor deposition, and submerged arc welding nanoparticle synthesis technologies are now the principal approaches for the one-step synthesis of nanofluids. Vacuum evaporation was initially used by Akoh et al. [15] to produce 0.25 nm ferromagnetic metal superparticles in the one-step approach. According to Tran and Soong [16], nanoparticles with a diameter of 9-21 nm were created by laser ablation without dispersants or surface chemicals. Using the gas coalescence concept, Lo et al. [17] constructed a submerged arc nanosynthesis system where copper aerosols rapidly amalgamated into nanoparticles in the presence of dielectric liquid. As a result, metallic nanofluids were formed from the nanoparticles. This process is typically utilized to manufacture copper, copper oxide, cuprous oxide, and copper phase nanoparticles to create a metal nanofluid.

Two-
Step Preparation Method. First, nanoparticles are made and then blended with the base fluid in a two-step procedure. In terms of dispersion efficiency and stability, two-step nanofluids are the most often employed approach. Bottom-up and top-down nanomaterials synthesis methods are now the most widely used [10].
The bottom-up approach involves accumulating components, such as atoms, aggregates, and nanoparticles. Chemical vapor deposition, sol-gel, pyrolysis, and biosynthesis are the most often utilized techniques. Researchers currently choose the sol-gel approach because of its straightforward synthesis, scalability, and controllability. An environmentally friendly process called biosynthesis uses bacteria, plant extracts, fungus, and precursors to create nontoxic, biodegradable nanoparticles [16].
Reducing the bigger size of materials to nanoscale particles is the top-down method. Laser ablation, nanolithography, mechanical grinding, and thermal breakdown are frequently employed techniques. Mechanical grinding is a physical technique for producing nanoparticles, which involves plastically deforming bulk materials into particle forms. Using improved photolithography, nanolithography can shrink massive materials from microns to <10 nm. Nanolithography may be done using various techniques, including electron beam, optical, nanoimprinting, multiphoton, and scanning probe lithography. Laser solution ablation is a trustworthy top-down technique, typically more dependable than traditional chemical reduction techniques for synthesizing precious metal nanoparticles [19].

Factors
Affecting the Synthesis of Nanoparticles. The production, characterization, and use of nanoparticles are all affected by various circumstances. According to several studies, produced nanoparticles may alter in nature depending on the adsorbate and catalyst activity utilized in the synthesis process. In some instances, the dynamic character of the produced nanoparticles has been described, with distinct Journal of Nanomaterials kinds of symptoms and consequences changing with time and the environment. Furthermore, several other critical variables might impact how nanoparticles are synthesized. These include pH levels, temperatures, concentrations of extracts, concentrations of raw materials, sizes, and processes. Some dominant factors that affect nanoparticle synthesis are described in the following sections [18].
2.2.1. Particular Method or Technique. Chemical or biological processes, including a variety of organic or inorganic compounds and living organisms, as well as physical approaches employing mechanical operations, may all be used to synthesize nanoparticles. The advantages and disadvantages of each technique are unique. Nanoparticles may be made using biological processes, which employ nontoxic and ecologically benign ingredients in combination with green technology, making them more environmentally friendly and acceptable than previous methods [19].

pH of the Solution.
The production of nanoparticles is influenced by the pH of the solution. The pH of the solution medium affects the size and texture of produced nanoparticles. Thus, the pH of the solution medium may be altered to adjust the size of nanoparticles [19].

Synthesis Temperature and Pressure.
For nanoparticle production, temperature and pressure are crucial. The nature (size and form) of the nanoparticle produced depends on the temperature of the medium in which the reaction takes place. Nanoparticles may be synthesized at different temperatures using any of the three processes. Temperatures >350°C are required for the physical approach, whereas temperatures <350°C are required for the chemical method. In most situations, temperatures as low as 100°C or ambient temperature are required to produce nanoparticles [20].

Synthesis
Time. The amount of time of the reaction medium is incubated significantly impacts the kind and quality of nanoparticle that is generated. The synthesis procedure, exposure to light, storage conditions, and other factors significantly impacted how the generated nanoparticles' features changed over time. Particles may aggregate as a result of longterm storage, contract or expand over that time, have a shelf life, and so on, all of which impact their potential [20].

Particle Shape and
Size. The characteristics of nanoparticles are significantly influenced by particle size. For instance, it has been shown that the melting point of nanoparticles drops as they approach the nanoscale range. Because nanoparticles in various configurations contain equivalent amounts of energy, changing their shape is simple. The nanoparticle's form changes due to the typical energy type utilized during nanoparticle analysis. The chemical characteristics of produced nanoparticles are significantly influenced by their dynamic nature and form [20].
2.2.6. Pore Size. The porosity of the produced nanoparticles significantly impacts their quality and use. To boost their usage in the drug delivery and medicinal fields, biomolecules have been successfully immobilized onto nanoparticles [20].

Various Nanoparticles Used for Production of Biodiesel
Biodiesel is produced by the transesterification process in which the triglyceride is reacted with alcohol in presence of a catalyst to produce biodiesel and glycerol. Various nanomaterials and nano-sized particles can be used as biodiesel production catalysts [21].
3.1. Magnetic Nanoparticles. In general, magnetic nanoparticles (MNPs) are categorized as a subclass of nanoparticles that may be manipulated or controlled by magnetic fields. This particular form of nanomaterial often consists of two parts: a chemical component with functionality and a magnetic component (typically Fe, Ni, or Co). Due to their tiny size, high surface-to-volume ratio, and exceptional quantum characteristics, MNPs are of tremendous interest to scientists and researchers. Magnetic nanostructure materials of various types are now being researched for use as effective catalysts. Typically, these materials consist of Fe, Co, Ni, platinum alloys, and other metal oxides.

Carbon Nanotubes (CNTs).
CNTs are a kind of nanomaterial that stands out above others in terms of their mechanical, thermal, structural, and biocompatible qualities. They are widely employed in biodiesel applications. When employed correctly, multiwalled carbon nanotubes (MWCNT) nanomaterial improves both the diesel engine's efficiency and the blending efficiency of biodiesel-diesel.
3.3. Graphene and Graphene-Derived Nanomaterial. Graphene oxide (GO), graphene, and reduced graphene oxide (RGO) are all used as catalysts in biodiesel production. Although graphene and materials based on it are effective catalysts for manufacturing biodiesel, researchers have always had difficulty developing high-quality graphene and its derivatives.

Other Nanoparticles Applied in Heterogeneous Catalysis.
Because of the significant characteristics, such as tiny pore size, specific surface area, acidic quantity, and acidic strength, inorganic nanoparticles are sometimes utilized as heterogeneous catalysts in a variety of applications. Biodiesel may be made using a variety of nanomaterials, including H-form zeolites, cation-exchange resins, transition-metal oxides, solid acids, heteropoly compounds, and solid carbonaceous acids [21].

Transesterification of Oil into Biodiesel in the Presence of
Nanocatalysts. Transesterification process is the most promising process which is a reversible reaction as shown in Figure 3. It is carried out by reacting fatty acids (oils) with alcohol in the presence of a suitable catalyst to obtain FAMEs (biodiesel) and glycerol. A strong base like sodium hydroxide, sodium methoxide, and potassium hydroxide or a strong acid such as hydrochloric acid and sulfuric acid can be used as a catalyst [22]. The catalyst is usually used to start the reaction and acts as an alcohol solubilizer; it is essentially required because alcohol is sparingly soluble in the oil phase, and noncatalyzed reactions are extremely slow. The catalyst helps in alcohol solubility and ultimately facilitates the reaction to proceed at a reasonable rate [23]. The separation of catalyst from the reaction mixture is the most difficult. After the completion of reaction, the catalyst needs to be neutralized or removed with a large amount of hot water, which generates a large amount of industrial wastewater and makes the process expensive [24]. Figure 4 shows the schematic overview of the process of biodiesel production. The catalyst's specific surface area is one of the most essential variables in the generation of biodiesel. A more excellent surface/volume ratio in a nanocatalyst improves the catalyst's performance because there are more active sites for reactions to take place on the catalyst's surfaces. The high specific surface area of nanocatalysts creates favorable circumstances for the transesterification reaction. Calcium oxide is a common nanocatalyst for biodiesel production due to its inexpensive cost and strong catalytic activity. CaO can thus be utilized either alone or in conjunction with other materials to produce biodiesel. MgO, SrO, and ZnO are other nanocatalysts utilized in the transesterification procedure. Again, Fe 3 O 4 , CuFe 2 O 4 , activated carbon, and other materials can be used to enhance these catalysts' characteristics [24][25][26][27][28][29][30][31].
The ideal biodiesel production feedstock includes edible oil sources such as palm, sunflower, soybean, olive, maize, rapeseed, safflower, peanut, and coconut oil [26]. Table 1 provides an overview of the use of nanocatalysts for manufacturing biodiesel from vegetable oil, waste cooking oil (WCO) extracted from animal fat, and algal oil for different operation conditions. Table 1 displays that nanocatalysts can significantly increase biodiesel yields, most of the biodiesel yield from these nanocatalysts is above 90%.
The reusability and recovery of nanocatalysts in the biodiesel manufacturing process are the most significant advantages of heterogeneous nanocatalysts. In these procedures, the nanocatalyst is collected and used again after each cycle in the manufacture of biodiesel. Nanocatalyst recovery is frequently accomplished using chemical means. The main product and byproduct from the reaction mixture can be recovered from the reaction mixture quickly and easily with the use of heterogeneous catalysts. There is no need for a washing step with this kind of catalyst. Nanocatalysts proposed numerous benefits for the esterification process, including faster reaction times, reduced reaction time, and quick and easy separation from the reaction mixture. According to previous studies, the catalyst can be easily recovered utilizing an external magnetic field and recovered multiple times without significantly losing its catalytic activity and biodiesel yield [32][33][34][35][36][37][38].

4.2.
Biodiesel Production using Nanoimmobilized Lipase. In recent years, various nanomaterials have garnered much interest as potential carriers for immobilizing enzymes [44][45][46]. Compared to other materials for enzyme immobilization, nanomaterials have much larger surface areas, one of the numerous benefits they provide as immobilization carriers. Utilizing nanoimmobilized enzyme systems may help increase enzymes' activity, stability, and reusability, leading to a considerable reduction in the costs associated with their usage [47][48][49]. Several researchers have immobilized lipases on functional nanomaterials, and there is reason to be optimistic about their potential uses ( Table 2).

Nanotechnology for Algal Biodiesel
Microalgae are a promising feedstock for the manufacture of biodiesel ( Figure 5). Nanomaterials were added to an algal culture system to promote microalgal growth and induce lipid buildup. Furthermore, the efficiency of lipid extraction may be improved by using nanomaterials. The following sections discuss the current and noteworthy nanotechnology uses in algal biodiesel generation. Nanotechnology's applicability in algal culture, lipid buildup, extraction, and transesterification has been critically appraised in the following sections [55][56][57].

Algal Cultivation and Lipid
Induction. Illumination is one of the prerequisites for growing algae. Low sunlight exposure, inappropriate for growth, may occur in high-density microalgae cultures. Thus, the issue of soft light illumination has been solved by using metallic NPs with localized surface plasmon resonances (LSPR) technology [58]. The coupled oscillations of free electrons at a metallic dielectric contact are the foundation of LSPR technology [59]. Light at a certain   Journal of Nanomaterials   wavelength is selectively absorbed and scattered to resonant interactions between photons and surface plasmons. Strong blue light backscattering from a solution of silver nanoparticles was used in an experiment with Chlamydomonas reinhardtii and Cyanothece 51142 in PBR to measure a 30% increase in cell growth [59]. In a different research, spheroidal silver nanoparticles and gold nanorods were used to boost the chlorophyll and carotenoid pigment accumulation in Chlorella vulgaris [60].
To promote microalgae growth and lipid accumulation, nanomaterials are also employed as nutritional supplements for photosynthetic microalgae cells in a culture medium. It is said that although microalgae's typical growing medium contains specific nutrients and minerals, it is insufficient for the optimum level of development. The absence of essential nutrients may be supplemented by metallic nanoparticles found in particular wastewater. It is conceivable to add nanoparticles to microalgae growing medium to positively impact lipid accumulation since these nanomaterials may increase microbial activity [61].
The hybrid nanoparticles' functionalized groups can accelerate the uptake and absorption of the CO 2 present in the culture medium. They provide the microalgae's photosynthetic system with a sufficient supply of CO 2 . In contrast to the suppression of carbon nanotubes (CNTs), nano-Fe 2 O 3 , and nano-MgO, He et al. [62] demonstrated that only nano-Fe 2 O 3 increased cell growth. Intriguingly, the NPs mentioned aboveimproved lipid production and total lipid content by raising their concentrations. MgSO 4 NPs may cause microalgae to flocculate, reducing their access to light and, as a result, increasing their chlorophyll content to stabilize their photosynthetic activity. This is likely a defensive mechanism reaction. In a report, Metzler et al. [63] made similar claims. San et al. [64] also noted that Chlorella vulgaris, green microalgae, grew more quickly when cultured with silica nanoparticles ranging in size from 38 to 190 nm.

Microalgae
Harvesting. More than a quarter of the total production costs of the process from culture to the manufacture of biofuels are incurred during the algal biomass harvesting from the aqueous phase. One of the most practical and affordable technologies for collecting algae is flocculation, which is used in conjunction with other processes, including centrifugation, magnetophoretic separation, membrane-based filtering, and electrolysis [65].
Additionally, apart from flocculation, collecting smaller algal cells in size range of 2-4 µm is challenging and continues to be a significant obstacle for effective harvesting technology [66]. One of the most effective methods for collecting microalgae recently has been magnetic flocculation. Magnetic nanoparticle harvesting is a rapid, low-cost, and energy-efficient procedure. Target cells are coated with magnetic nanoparticles during this process, which is influenced by an external magnetic field. Separation occurs, and the microalgal biomass is detached from the magnetic nanoflocculant for recycling. Fishponds and freshwater bodies are using magnetic iron oxide nanoparticles to remove algae.
The algal harvesting process in Figure 6 uses Fe 3 O 4 nanoparticles, and the magnetic flocculant with the maximum flocculation was created using iron oxide and 0.1 mg/mL cationic polyacrylamide (CPAM). Chlorella sp. was incubated with CTAB-decorated Fe 3 O 4 nanoparticles to evaluate the harvesting effectiveness; as a result, 96.6% of the microalgae were harvested at a dosage of 0.46 g particle/g cell [67].

Lipid Extraction.
The lipid extraction process is one of the primary contributors to the overall cost of producing biodiesel from microalgae. Because algae's cell walls are made of complex polysaccharides and glycoproteins, they have a broad spectrum of chemical resistance and high mechanical strength [58]. Using various solvents to extract lipids from algae is the most used method (chloroform, methanol, hexane, etc.).
They are used in various ratios, either alone or in combination. With the use of different solvent combinations, it has been shown that the lipid production of algae dramatically changes. The extraction conditions may also impact the total lipid yield, including homogenization, ultrasonication, and irradiation. All of these elements should be considered to create an optimum extraction methodology with a greater lipid yield, which may increase the total cost of downstream processing for biodiesel manufacturing [68].
Recent advances in nanotechnology have provided trustworthy, secure, and economical alternatives. Nanomaterials, unlike solvents, provide nontoxic methods of lipid extraction in addition to their greater percentage of oil extraction effectiveness. Additionally, the time-consuming and expensive solvent-lipid separation phase in the standard extraction procedure may be removed by using nanomaterials [68].

Transesterification of Algal
Oil using Nanocatalysts. The utilization of nanomaterials has several advantages, one of which is their high surface-to-volume ratio, which provides a large surface area for the immobilization of lipase. In addition, the tiny pore size of nanomaterials allows for a high reactant diffusion rate to be supplied at the active regions of the lipase enzyme. Because they are so simple to recover from reaction mixtures and have improved thermostability, magnetic nanoparticles can provide an excellent alternative that is both convenient and economical [53]. In research, the covalent attachment of T. lanuginosus lipase to aminofunctionalized magnetic nanoparticles resulted in a conversion efficiency of 90% for biodiesel [54]. This was done by using magnetic nanoparticles. Using P. cepacia lipase (PCL) immobilized on Fe 3 O 4 magnetic nanoparticles, Huang et al. [55] reported a 100% success rate in converting oil into biodiesel.

Reusability and Recovery of Nanocatalysts
The reusability and recovery of nanocatalysts in the biodiesel manufacturing process are the most significant advantages of using heterogeneous nanocatalysts. The nanocatalyst is employed in these procedures in multiple cycles to produce biodiesel, and it is recovered and reused at each stage. Nanocatalyst recovery is frequently accomplished using chemical means. The main product and byproduct from the reaction mixture can be recovered from the reaction mixture quickly and easily with the use of heterogeneous catalysts. There is no need for a washing step with this kind of catalyst. Numerous benefits were proposed for the esterification process using nanocatalysts, including quick and easy separation from the reaction mixture and faster mixing of the reactants with the catalyst.
An external magnetic field allows for easy catalyst recovery, and previous studies have shown that it may be recovered multiple times without a noticeable decrease in catalytic activity or biodiesel yield. Table 3 summarizes the various nanocatalysts utilized in the manufacture of biodiesel, as reported by Esmaeili et al. [69]. Also included the data on nanocatalysts' reusability and the number of cycles at which they were employed. These results reveal that the nanocatalysts used in this study can be reused multiple times without suffering a noticeable drop in biodiesel output.

Nanoadditives-Blended Biodiesel in Diesel Engines
Biodiesel has several drawbacks in CI engines, including a 10% drop in fuel efficiency on an energy basis, slightly greater density, poor fuel atomization, piston ring sticking, lower cloud and pour points, cold starting issues, and significant NO x emissions. These drawbacks may be solved by using a few relatively recent techniques, such as adding gasoline additives and utilizing hybrid fuel, which improve engine performance and reduce exhaust emissions. Nanoparticles have emerged as a unique and potential addition to current additives used in biodiesel and diesel fuels, resulting in lower emissions and improved engine performance. Many researchers have concentrated on nanoadditives fuel modification approaches to increase combustion efficiency. The pollutants emitted by CI engines are subject to severe emission regulations worldwide. Gasoline additives may change a variety of fuel qualities, including density, volatility, and sulfur content, all of which impact emissions [70].
Incorporating the nanoparticle into liquid fuels as an additional energy carrier has enhanced combustion properties. Thus, the researchers investigated whether these modified fuels might be used in diesel engines. Metal oxides of Fe, Al, B, Pt, Ce, Cu, Ti, and Co have long been utilized as additives in biodiesel and diesel fuel mixtures. Carbon nanotubes (CNTs), graphene, and graphene oxide (GO) nanoparticles have recently become popular as additions in biodiesel-diesel blends [71][72][73].
7.1. Fuel Properties Enhancement. Nanoparticles offer unique features (such as a greater surface area/volume ratio, a higher combustion rate, and so on) that make them ideal for various Journal of Nanomaterials engineering purposes. In addition, nanometric materials may achieve the necessary chemical and thermal properties standard. Combining different nanoparticles with biodiesel has been used in several studies to increase engine performance and reduce pollution. Figure 7 shows the microexplosion process of the diesel-metal additive. It has been found that mixing NPs in BD improves atomization [74,75]. The physical and thermal parameters (such as radiative heat transfer, thermal conductivity, and mass diffusivity) of diesel fuel were greatly enhanced by adding nanoparticles to it. Adding nanoadditives to biodiesel also increases the thermochemical parameters (calorific value, density, viscosity, flashes point, and fire point).

Engine Performance and Emission Characteristics.
Adding nanoparticles to diesel-biodiesel fuel blends can improve their radiation, heat, and mass transfer performance to obtain fuller combustion and higher thermal efficiency. Diesel engine performance parameters, in particular brake-specific fuel consumption (BSFC) and brake thermal efficiency (BTE), could be improved significantly due to nanofluid additives. The BSFC is the ratio of the engine's fuel consumption ratio to its power output over a specified period. Because the engine needs more gasoline to run at a similar level, a lower BSFC number is anticipated. The comparison with engine load is crucial because BSFC typically declines as load increases. The viscosity, density, volumetric fuel injection, and calorific value are the performance factors that control the BSFC. On the other hand, brake thermal efficiency refers to the ratio between the engine's actual brake power and the energy transmitted to the engine. BTE can be used to explore how different fuels and fuel blends affect an engine's performance. Due to superior radiative and heat mass transfer capabilities, the inclusion of nanoparticles to diesel-biodiesel fuel emulsions promotes rapid and thorough combustion, which significantly increases combustion efficiency [76].
Many researchers have found that when engines run on biodiesel-diesel, the high amount of oxygen in the biodiesel's structure leads to complete combustion, resulting in lower HC and CO emissions [66][67][68][69][70]. The nanoparticles would improve the oxidation process during combustion, leading to increased NO x emissions and lower HC and CO emissions. In addition, some researchers found that nanoadditives could reduce NO x emissions [72,73]. Table 4 displays the impact of different nanofluid additive dosage levels on performance metrics under full load. It is observed that nanoadditives could significantly improve BTE and reduce BSFC and noxious emissions.

Human and Environmental Safety
Concerns of Nanotechnology-Based Large-Scale Production The impacts of nanoparticle exposure on people and the environment are anticipated to increase their widespread use in several areas substantially. In light of this, a new area called nanotoxicity has developed recently, with a primary emphasis on the risks that nanoparticles pose to human health and the environment. Because nanoparticle surfaces may operate as nanocatalysts and are tiny enough to reach human cells, these reactions can have various harmful consequences on human tissues. In addition, most nanoparticles are made of metals, which are already well-known to be highly hazardous to particular organs and readily bioaccumulated by humans [85].
Humans are most exposed to nanoparticles by ingestion, absorption, or cutaneous contact. Through any of the methods mentioned above, nanoparticles may enter the circulation and go to any organ in the body, which can have harmful effects that are greatly influenced by the organ's activity level, size, and concentration. The kidney, lungs, and liver have high  Journal of Nanomaterials metabolic rates, which puts them at a greater risk of interacting with any substance the body takes in, including nanoparticles [86].
Therefore, research on the potentially harmful effects of nanoparticles on these organs in animal models is essential. Multiwall carbon nanotubes, for example, have been shown to induce inflammatory and fibrotic responses in the lungs of Sprague-Dawley rats when injected intratracheally at doses of 0.5, 2, or 5 mg [87]. Furthermore, according to scientists, CNTs might aggregate into the tissues of the lungs and the surrounding areas to create collagen-rich granulomas that protrude into the bronchial lumen and cause alveolitis.
Wistar rats were exposed to subacute and intraperitoneal doses of TiO 2 nanoparticles, which led to pathological alterations in the liver, increased platelet count, and moderate inflammation. Additionally, it was shown that rats exposed to titanium had bioaccumulated titanium in their liver, lungs, and brain, adversely affecting the animals' neurobehavioral development and their organs [88]. Kwon et al. [89] described the exposure of a freshwater invertebrate to these particles,   demonstrating their ability to permeate into its digestive system, concerning environmental concerns over TiO 2 nanoparticles. These particles sometimes significantly damaged the gut epithelial cells and microvilli, which changed the test subject's shape. Although the environmental toxicity of nanoparticles is not well known, natural populations of aquatic, terrestrial, and atmospheric creatures may experience some long-term consequences, as shown in Figure 8. The primary issue stems from the fact that these particles are tiny enough to interact with biomolecules including proteins, lipids, and DNA, which may readily be collected by creatures in the food chain and result in significant physiological damage [90]. Further research on the ecotoxicity of nanoparticles in animal models is required to understand better the specific harm these particles may inflict and the genuine hazard of particle buildup. These researches will be crucial in stopping or lessening the hazardous effects of the nanoparticles on both exposed persons and the environment as a whole.
When using these incredibly beneficial particles in largescale operations like biodiesel manufacturing, there should be worries about their positive and negative consequences. Understanding these negative impacts will aid researchers in developing innovative solutions to lessen them, preserving a balance between their use and the security of any environments that could be exposed to these nanoparticles.

Future Prospects
In order to implement the reaction process on a broad scale, technological development is required to reduce mass-transfer resistance, reduce energy consumption, and maximize the use of byproducts. The scope for preserving optimum reaction conditions grows, allowing for more biodiesel to be produced using nanotechnology. High-quality biodiesel, discounted labor and energy costs, and decreased water and chemical usage have all contributed to the rising popularity of the data-driven machine learning (ML) technique in the biodiesel systems [91]. Hence, the optimum concentration and size of nanocatalysts could be optimized efficiently using the ML approach.
In addition, there has only been a limited amount of research done on employing NPs as fuel additives to this point, and new methods are needed to address issues such nanoparticle aggregation, erosion, and settling. Trapping the unburned nanoparticles in the exhaust has become more difficult due to the addition of nanoadditions to biodiesel. As a result, efforts are being made to develop a filter that can remove unburned nanoadditives from the exhaust of diesel vehicles. It is important to study what happens to the air quality after the nanoparticle additives are burned in the engine. Several methods have been used to investigate the toxicity of nanoparticles, the majority of which involve in vitro examination of nanotoxicity [92]. However, in vivo interaction should be explored in detail, with a special emphasis on nanoparticles utilized in biodiesel.

Conclusion
The scientific community is paying close attention to biodiesel since it has several advantages over fossil fuels. This type of renewable bioenergy has the potential to revolutionize the transportation industry and offer a suitable replacement for traditional diesel. The creation of nanocatalysts using various nanomaterials, which can overcome most of the drawbacks associated with conventional homogeneous and heterogeneous catalysts, has been discovered to play a crucial role in the biodiesel industries. Due to their exceptional and distinctive qualities, such as high reactivity, improved catalytic performance, selectivity, economic feasibility, and ecofriendliness, nanocatalysts are becoming more and more critical in manufacturing biodiesel. Therefore, various nanocatalysts, including those based on metals, carbon, magnetic materials, nanoferrites, nanoclays, etc., can be employed to increase biodiesel production from a variety of feedstocks.
This review has broached interests in the progress and recent applications of the utilization of nanotechnology and nanomaterials in biodiesel production and property enhancement. When enzymes are immobilized, their activity and stability are increased, which increase biodiesel output. Nanocatalysts are essential for biodiesel generation because they aid in the transesterification process. Heterotrophic algae cultivation might benefit from nanotechnology-enabled lipid accumulation. Disruption of cells and extraction of lipids from algae are two further applications for nanotechnology.
To enhance the efficiency of diesel engines, nanofluids may be added to diesel-biodiesel blends to increase evaporation, minimize ignition delay, increase flame temperatures, and maintain flames for extended periods. Nanoadditive mixed combinations of biodiesel may be used to address other issues, such as carbon monoxide emissions, hydrocarbon emissions, NO x emissions, combustion, and evaporation.
However, research in the use of NPs as fuel-additives and the in vivo toxicity of NPs remains sparse, leaving a number of technical gaps in the field of nanotechnology-based biodiesel synthesis. Last but not least, life cycle analysis (LCA), a cost-benefit analysis (CBA), and a safety assessment (SA) of using nanomaterials in biodiesel production are all crucial for illuminating current issues and outlining potential avenues for future study.

Conflicts of Interest
The author declares that there is no conflicts of interest.