Combustion synthesis is a green, energy-saving approach that permits an easy scale-up and continuous technologies. This process allows for synthesizing various nanoscale materials, including oxides, nitrides, sulfides, metals, and alloys.
Magnetic nanoparticles (MNPs) have gained significant attention recently, particularly in the biomedical field 
. These nanoparticles exhibit high efficiency due to their large surface-area-to-volume ratio, chemical stability, and easy surface functionalization. The biocompatibility, rapid magnetic targeting, and high loading capacity of MNPs make them versatile in delivering therapeutic agents, improved multimodal imaging, and in magnetic hyperthermia 
. In combination with other NPs, they also permit other bio-applications, such as serving as anti-microbial agents.
MNPs have demonstrated potential in targeting drug delivery to specific locations by precisely tracking drug distribution, bio-distribution, and metabolism. These materials offer a promising strategy for site-specific drug delivery for cancerous tissues, inflammation, and cardiovascular diseases. The particles show excellent potential in controlling the treatment of the disease while minimizing toxicity to healthy cells. Furthermore, magnetic nanoparticles offer an efficient separation of biomolecules and cells. The magnetic properties of these nanoparticles allow them to immobilize biomolecules or cells on a magnetic surface 
. MNPs can also be used for imaging by exploiting their magnetic properties, offering improved image contrast and resolution due to their magnetization behavior 
. In hyperthermia for cancer treatment, the goal is to increase the temperature of the target tissue from 42° to 44°. This effect can be achieved using magnetic nanoparticles, known as magnetic hyperthermia 
. The main challenges of this cancer therapy are improving the heating power of such nanoparticles and controlling the local temperature near the tumor.
The synthesis of MNPs involves various methods, such as chemical precipitation, co-precipitation, sol–gel, microemulsion, hydrothermal, and combustion synthesis, to achieve the desired size, morphology, and magnetic properties 
. The chemical precipitation method consists of precipitating metal ions from a salt solution by adding a reducing agent and a surfactant to control the particle size and morphology. Although this method produces MNPs with small dimensions and narrow size distribution, it requires high-temperature conditions and toxic reagents 
. In the co-precipitation method, by changing the pH and reaction time, the size and magnetic properties of the MNPs can be controlled and produced at a lower cost with good magnetic properties 
. The sol–gel method includes the hydrolysis and condensation of metal alkoxides in a sol–gel matrix 
. This method produces large quantities of nanoparticles with good magnetic properties but is time-consuming and requires high-temperature conditions.
The micro-emulsion comprises systems consisting of two immiscible phases (e.g., oil and water) and a surfactant 
. Metal ions are reduced in the nanoscale droplets, resulting in MNPs with a narrow particle size distribution. This process is relatively simple and fast, producing MNPs with a narrow distribution of particle size. Finally, hydrothermal method involves reacting metal salts in an autoclave under high temperature and pressure to create MNPs 
. The size and shape of the nanoparticles can be controlled by modifying the reaction time, temperature, and precursor concentration. Since this technique involves high-temperature conditions, it yields MNPs with excellent crystallinity.
Solution combustion synthesis (SCS) is a green, energy-saving approach that allows for an easy scale-up and continuous technologies 
. SCS contains self-sustained exothermic reactions along an aqueous or sol−gel media. This process permits synthesizing various nanoscale materials, including oxides, nitrides, sulfides, metals, and alloys.
2. Solution Combustion Synthesis: Fundamentals
The solution combustion synthesis (SCS) method is a highly versatile approach and relies on the occurrence of non-catalytic, self-sustained exothermic reactions in solutions or gels 
. Such reactive solutions contain oxidizers and fuels dissolved in a solvent.
For instance, in the iron-nitrate-hydrates–HMTA system in an argon atmosphere, a self-sustained reaction in an aqueous solution of the metal nitrite and the fuel, Fe(NO3
O + C6
, results in the formation of various magnetic phases 
. The reaction is highly exothermic, and the adiabatic combustion temperature (Tad
) exceeds 2500 K. The Tad
can be controlled by adjusting the amount of water (n) and fuel-to-oxidizer molar ratio ϕ = C6
. It can be seen that at ϕ = 1 and n = 0, Tad
= 2630 K, while at n = 4 and ϕ = 4, it is below 1000 K. The phase composition of the solid-state product changes correspondingly. The wide specter of magnetic phases, including Fe, Fe3
C, and Fe3
N, can be synthesized in the same system by optimizing combustion parameters (n, ϕ). Close to stoichiometry (ϕ = 1), the Fe3
phase is in equilibrium, while at large ϕ, the Fe3
C prevails. The narrow parametric n-ϕ region corresponds to the formation of the pure iron phase.
SCS can be achieved through different reaction modes 
, including volume SCS, self-propagating high-temperature synthesis (SHS), impregnated SCS, cellulose-assisted SCS, templated SCS, spray SCS, and SCS of thin films. Each mode has unique features and can be applied to fabricate various materials, including oxides, metals, alloys, nitrides, and carbides, with high specific surface areas and narrow size distributions 
The liquid state of the reaction media permits different types of impregnated SCS modes. The solution can be impregnated into the inert porous media, followed by reaction initiation. In this case, the maximum reaction temperature is lower than the SHS mode because of the inert dilution of the system. This approach was used to prepare high specific surface area supported catalysts 
. The solution can also be impregnated into active porous media, e.g., cellulose 
. Cellulose-impregnated SCS is typically used for low-exothermic systems because the combustion of cellulose contributes to the total exothermicity of the system. For different systems, the burning of cellulose may precede or follow the reaction in the solution. The catalytic or non-catalytic high specific surface area materials can be prepared using these modes 
The last version of the impregnated mode is templated-SCS. A solution is impregnated into the media with the desired pore size distribution, e.g., silica nanotubes. After a self-sustained reaction, the template is removed by dissolution in an appropriate solvent, while synthesized particles have a narrow size distribution corresponding to the nanotubes’ diameter 
3. Solution Combustion Synthesis of Magnetic Compounds
3.1. Iron-Oxide-Based Materials
In recent work, the iron oxide magnetic particles were fabricated by SCS using citric acid as a fuel 
. The aqueous solution of ferric nitrate nonahydrate, Fe(NO3
O, and citric acid, C6
, mixed in a molar ratio of 6:5 (ϕ = 0.8) was utilized. The reaction can be represented as follows:
Fe(NO3)3 + (ϕ-1/6)·C6H8O7+9/2(ϕ-1)O2 → 1/2·Fe2O3(s) + 6(ϕ-1/6)CO2(g) + 4(ϕ-1/6)H2O(g)+ 3/2 N2(g)
Heating sources such as a heating mantle with a temperature of 450 °C and a microwave oven (245 GHz, 800 W) were used in the VSCS process, but exact synthesis conditions were not reported.
The XRD analysis revealed that the phase composition of the produced powders was influenced by the heating method used. In the case of heating in the mantle, the product (P1) involved primarily (97%) magnetite (Fe3O4), while in the case of microwave heating, the product (P2) was mostly (85%) maghemite (γ-Fe2O3).
The medium particle size was 15 and 11 nm for P1 and P2, respectively. Correspondingly, the BET-specific surface area was 42 and 71 m2/g, respectively. Both powders displayed superparamagnetic behavior with a saturation magnetization of 67(P1) and 42(P2) emu/g, respectively.
Overall results demonstrate that iron oxide NPs with the desired phase composition can be successfully fabricated by SCS in the range size from 4 to 100 nm. The obtained materials were shown to be biocompatible and allowed the preparation of a stable colloidal suspension.
3.2. Nickel Oxide and Ni-Based Alloys
The SCS process of nickel oxide (NiO) is a process that has been widely used to investigate the intrinsic mechanism of self-sustained chemical reactions in homogeneous solutions (cf., 
). Several fuels have been utilized to fabricate NiO nanoparticles. The first system to produce pure Ni in the combustion wave was the nickel-nitrite–glycine system. However, a recent publication suggested the exotic fuel, i.e., Areca catechu leaf extract, with the following elemental composition: carbon (55 wt%), oxygen (36 wt.%), potassium (4 wt.%), chlorine (2.5 wt%), silicon (1 wt%), and calcium (0.7 wt%)). The SCS was accomplished in VSCS mode with a furnace temperature of 500 °C. The well-crystalline FCC NiO phase possesses a crystalline size of ~5.6 nm.
Furthermore, the synthesized NiO NPs’ effectiveness in inhibiting pancreatic α-amylase and their anticancer activity was compared with the standard drug Metformin. Metformin showed inhibitory effects on α-amylase activity with a concentration of test drug needed to inhibit cell growth by 50% (IC50) of 232 µg/mL. The IC50 value of the extract was found to be 268 µg/mL. The biological assay results revealed that NiO NPs exhibited significant cytotoxicity against human lung cancer cells, showing considerable cell viability.
In another recent publication 
, the authors report the SCS of CoCuNi alloy, which possesses the morphology of hollow spheres, using the spray SCS mode to produce this unique structure. The CoCuNi shell’s thickness ranged from 10 to 30 nm, and the spheres' outer diameter was 0.2 to 2 µm. The magnetic properties of the alloy were studied at room temperature, revealing high saturation magnetization in the range of 65–75 emu/g, making it an excellent candidate for drug delivery applications.
3.3. Complex Oxides
The Solution Combustion Synthesis (SCS) technique effectively produces complex oxides with desirable magnetic properties. One example is synthesizing sub-micrometric (250–300 nm) BiFeO3
particles using the volume combustion synthesized mode 
. Bismuth nitrate-pentahydrate, iron nitrate nonahydrate, nitric acid, and glycine were used. The balanced chemical reaction can be written as follows:
Bi(NO3)3 + Fe(NO3)3 + 2pHNO3+4nH2NCH2COOH = (9n-2.5p-7.5)O2 → BiFeO3(s) + (10n+p) H2O (g) +8nCO2(g) + (2n+p+3)N2(g)
The n and p coefficients represent the number of glycine and nitric acid moles used per mole of Bi(NO3)3·5H2O, respectively. It is worth noting that all synthesized powders were post-annealed at 350 °C for 2 h and then at 500 °C for 2 h. Finally, the powders were heat-treated at 600 °C for 3 h in air. The XRD analysis of all powders indicates the formation of a rhombohedral BiFeO3 phase with a size of the crystallite range of 35 to 60 nm.
Another example is the SCS of nano-scale ferrites, where CoFe2O4, NiFe2O4, and Co0.5Ni0.5Fe2O4 ferrite NPs were produced using cobalt, nickel, and iron nitrates precursors as the oxidizing agent and glycine as the fuel. The average size of the particles in the CoFe2O4, NiFe2O4, and Co0.5Ni 0.5Fe2O4 samples were 40 ± 10 nm, 26 ± 8 nm, and 32 ± 7 nm, respectively. The hysteresis loops of CoFe2O4 NPs showed typical characteristics of hard ferrimagnetic material, whereas the magnetization curves of NiFe2O4 NPs corresponded to soft ferromagnetic material.
The effects of phase purity and stoichiometry on the catalytic behavior of the metal ferrites were studied by evaluating their reduction efficiency of 4-nitrophenol, a common organic contaminant in wastewater. The phase-impure NiFe2O4 sample, which featured segregated metallic Ni clusters on its surface, exhibited exceptional performance in reducing 4-nitrophenol.
3.4. Zinc Oxide
Zinc oxide (ZnO) is a safe inorganic antimicrobial agent recognized by the US FDA for use in humans. The available literature suggests that ZnO particles of varying morphology and size can be produced using SCS 
. Synthesis typically involves using zinc nitrate hexahydrate with different fuels.
Zinc oxide nanomaterials doped with Ag and Au were synthesized by using urea as a fuel, and silver nitrate (AgNO3
, 98.9%) and tetra chloroauric-III-acid hydrate (HAuCl4
O, 98%) for doping 
. The VSCS mode was used with a furnace temperature of 500 °C. The result was sub-micron (100–500 nm) flower-like 3D particles with uniformly distributed Au and Ag on the surface, with an average cluster size of 3–10 nm.
The antimicrobial study of such NPs was performed on Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus, while antifungal tests were carried out with the yeast Eremothecium ashbyii. The results showed that the Ag-doped ZnO nanomaterial had a degradation efficiency of 45% against methylene blue. Antimicrobial and antifungal activity studies have shown that pure ZnO is more effective against Escherichia coli and Staphylococcus aureus, and Ag-doped ZnO is more effective against Eremothecium ashbyii.
Overall, intensive research shows that SCS allows the synthesis of ZnO nanoparticles in a wide range (12–500 nm) of sizes. It also allows easy doping of such particles with the desired elements
3.5. Calcium-Based Compounds
Calcium phosphates (CaPs) are ceramics being explored for biomedical and orthopedic applications 
. However, their poor biodegradability remains challenging. Two approaches to enhance the ceramics’ properties are to (i) dope and (ii) use a new synthesis method 
. The SCS method, in particular, is an attractive pathway for doping.
For example, the CaP nanostructures were synthesized using calcium nitrate tetrahydrate Ca(NO3
O, di-ammonium hydrogen phosphate (NH4
, glycine, and nitric acid 
. The reactive aqueous solution was heated to a temperature of 60 °C for total evaporation of the solvent. Subsequently, the temperature was increased to 140 °C until the initiation of the combustion reaction. The obtained ash was calcined at 800 °C for 2 h. XRD analysis revealed the formation of hydroxyapatite and beta-tricalcium phosphate phases, with traces of calcium pyrophosphate. The later phase disappeared after calcination.
The synthesized CaP powder was tested as a potential support for photoactive drugs using hypericin (HY). It was demonstrated that 54 μg/mL is the amount of CaP particles loaded with HY in the presence of light needed for reducing the parasites’ population by half. This value is low compared to the amount of free HY (about 458 μg/mL) needed to exhibit an EC50 effect.
Si-doped CaP ceramics were also fabricated using calcium nitrate tetrahydrate (Ca(NO3
O), ammonium dihydrogen phosphate (NH4
), and tetraethyl orthosilicate (TEOS) (Si(OC2
) as precursors 
. The glycine and citric acid were employed as fuels. The TEOS was hydrolyzed in the presence of HNO3
and deionized water. The solutions were sequentially supplemented with calcium nitrate, ammonium dihydrogen phosphate, and fuel and heated up to about 330°C using a digital hot plate. XRD patterns showed the presence of primary hydroxyapatite (HA) and some amount of beta-tricalcium phosphate (βTCP) in all synthesized samples. Crystallite sizes of HA and βTCP in the powders synthesized using glycine and citric acid were 15 nm and 43 nm, respectively.
Bioactivity, cell viability, bone-like nodule formation ability, biodegradability, and cell attachment studies were conducted using Simulated Body Fluid (SBF) media. It was shown that the bioactivity of the Si-doped sample with 0.1 mole Si4+ synthesized using glycine was 255%, i.e., 2.5 times higher than those of undoped samples.
The SCS method can synthesize CaPs-based nanoceramics with attractive biomedical properties, and doping can enhance their properties further.