Green synthesis of metal and metal oxide nanoparticles: a pathway to sustainable energy and sensing applications

3.1 Role of biomolecules in synthesis and stabilization

The synthesis of MNPs and MONPs through biological or green methods involves complex processes influenced by various factors, including reaction conditions, specific biomolecules, and precursor choices. These techniques not only synthesize but also stabilize NPs using biomolecules such as proteins, polysaccharides, and other organic substances [95]. Recent studies have revised early biosynthesis theories, highlighting the roles of proteins, reducing sugars, and polysaccharides in the synthesis and stability of NPs [96], [97], [98]. Functional groups such as carboxyl, amide, sulfhydryl, thiol, and hydroxyl are crucial for binding and reducing metallic ions. Additionally, negatively charged groups or atoms are necessary for the adsorption of capping agents and NP stabilization. Each biological material’s unique chemical composition and metabolic processes influence its interaction with metal ions, thereby controlling NP synthesis. Green NP synthesis typically occurs extracellularly or intracellularly from living cells (Figure 6), biomolecules extracted from living cells, or cell-free supernatants [99], 100]. Although there are similarities in the biosynthesis of different MNPs, slight variations lead to distinct characteristics. Variations in element concentrations or reaction conditions can result in different types of NPs, such as metal chloride or MONPs, instead of pure MNPs. Numerous factors affect the green synthesis of MNPs and MONPs, respectively, including the types of plants, microorganisms, and biomolecules involved, as well as reaction conditions such as pH and working temperature. Plants and microorganisms contain a variety of biological compounds that act as reducing or stabilizing agents during NP formation. The process involves initiation, growth, stabilization, and termination phases, each affected by the nature and concentration of these biomolecules.

Figure 6:

Schematic representation of microorganism-mediated metallic NP biosynthesis.

3.2 Influence of reaction conditions on nanoparticle formation

Optimizing reaction parameters such as the concentration of plant extracts and metal precursors, pH, temperature, and reaction time is essential for controlling the shape, size, and production rate of NPs. Numerous studies have shown that longer reaction times typically increase NP production. For example, Gou et al. found that with a cell-free extract of Lysinibacillus sphaericus MR-1, silver NPs could be produced. The concentration of NPs tripled when the reaction time was extended from 30 to 60 min [101]. Similarly, Hutchinson et al. observed the same trend for gold NPs, where the concentration of produced NPs increased with longer reaction times [102].

The pH level plays a crucial role in determining the size and stability of NPs. In acidic environments, NPs tend to be larger, while alkaline conditions favor the formation of smaller, more stable NPs. Smaller NPs offer a higher surface-area-to-volume ratio, providing more active sites for electrochemical reactions, which enhances the charge storage capacity of batteries and supercapacitors, allowing them to store more energy [103]. A study by Hanafy et al. [104] investigated the green synthesis of titanium dioxide NPs using A. vera extract as a reducing agent and titanium tetrachloride (TiCl₄) as a precursor across different pH levels (acidic, neutral, and basic). The study revealed that TiO₂ NPs were spherical, with particle sizes decreasing from acidic to basic conditions. The smallest NPs (13.3 nm), synthesized under basic conditions, exhibited the pure anatase crystalline phase.

Similarly, research by Fernando and Zhou [105] conducted a study that highlighted how pH impacts the dynamics of silver NPs’ (AgNPs) aggregation, dissolution, and stability. Their findings revealed that pH levels affect the oxidative dissolution and surface charge of AgNPs, resulting in different behaviors under acidic and alkaline conditions. Specifically, higher pH levels are associated with a decrease in particle size over time, which, in turn, reduces aggregation and enhances stability [105]. Similarly, research by Velgosová et al. [106] focused on the effect of pH on the synthesis of silver NPs using the green microalgae Parachlorella kessleri. Their research showed that pH has a crucial influence on the formation of polyhedral AgNPs, with sizes between 5 and 60 nm (Figure 7). Ultraviolet–visible (UV–vis) analysis and transmission electron microscopy (TEM) observations showed that higher pH levels (8 and 10) produced polyhedral, finely structured NPs with a narrow size distribution, maintaining an average size of 15 nm and remaining stable over time. Conversely, NPs synthesized at lower pH levels (2, 4, and 6) exhibited reduced stability by the tenth day of the experiment, with a broader size distribution [106].

Figure 7:

UV–vis spectra of AgNPs on the 14th day of the experiment. The inset shows the TEM image and the size distribution of AgNPs. Reprinted from ref. [106], copyright 2024, with permission from Elsevier.

Temperature is a critical factor in the synthesis of NPs, influencing their size, shape, and production rate. Higher temperatures generally speed up the reduction of metal ions, resulting in the creation of smaller NPs; however, excessively high temperatures can degrade phytoconstituents, compromising NP stability. Therefore, the optimal temperature for NP synthesis varies depending on the plant extract and metal involved. Researchers discovered that higher temperatures encourage both nucleation and growth in a recent study on the biological synthesis of AgNPs using Cinnamomum camphora leaf extract. This was demonstrated by an increase in the growth rate constant (k2) and the nucleation rate constant (k1). The constants k1 and k2 are key kinetic parameters related to the nucleation and growth processes in the formation of AgNPs during the reduction of the silver precursor (AgNO₃) by a reductant. k1 represents the nucleation rate, indicating how quickly new NP nuclei are formed, with higher values of k1 leading to faster nucleation. k2 is the growth rate constant, reflecting the speed at which these nuclei grow by accumulating silver atoms, where a higher k2 means faster particle growth. These constants are integral to the redox-crystallization model, which describes the formation of NPs through simultaneous reduction and crystallization. Whereas k2 grew roughly linearly with temperature, k1 increased marginally between 70 and 80 °C and substantially above that point. The AgNPs’ size grew somewhat between 70 and 80 °C (Figure 8) and then quickly fell between 80 and 90 °C, indicating that the sharply increased k1 rather than the dramatically decreased k2 was responsible for the size reduction at higher temperatures [107].

Figure 8:

TEM images and corresponding size distribution of the AgNPs synthesized under insufficient precursors at different temperatures: (a) 70 °C, (b) 75 °C, (c) 80 °C, (d) 85 °C, and (e) 90 °C. Reprinted from ref. [107], copyright 2024, with permission from Elsevier.

Another study using hawthorn fruit water extracts to synthesize AgNPs and CuNPs found that NP size decreased with rising temperatures. The researchers also noted that, at 60 °C, the size of metal-based NPs shrank across all systems, regardless of the precursor salt used, and that the rate of NP formation increased as the temperature rose [108].

The morphology of MNPs/MONPs is highly influenced by synthesis temperature, often resulting in diverse shapes such as spherical, hexagonal, triangular, rod-like, and irregular forms. In a study conducted by Sneha et al. [109], the impact of temperature on the morphology of gold NPs synthesized using Piper betel leaf extract was investigated. At 20 °C, triangular NPs were predominantly observed, while temperatures between 30 and 40 °C led to the formation of octahedral NPs with sizes ranging from 5 to 500 nm. As the temperature increased further to 50–60 °C, the resulting NPs exhibited more uniform sizes and predominantly spherical shapes.

4.1 Structural characterization

XRD techniques are extensively used to study synthesized materials, providing insights into phase identification, phase fractions, impurity phases, crystal structure, atomic distribution, and crystallite size. XRD is a non-destructive technique valuable for examining changes in interlayer spacing and the crystalline properties of materials [110], [111], [112]. The Scherrer equation, a mathematical tool utilized in XRD analysis, estimates the size of crystalline grains. When X-rays interact with a crystalline substance, they are diffracted, creating a peak pattern on a detector. The broadening of these peaks indicates the size of the crystallites (grains). The Scherrer equation typically estimates grain size based on the broadening of the most prominent XRD peak [113], 114].

XRD is particularly useful for powder samples, providing volume-averaged results that are statistically representative. Through the comparison of XRD peak positions and intensities with reference patterns provided by the International Centre for Diffraction Data (ICDD), the composition of NPs can be determined. Research indicates that crystallite sizes derived from XRD can vary from 9 to 53 nm, with peak broadening affected by aspects such as particle size and lattice strain. XRD often estimates a larger size than the magnetic size due to multiple domains within a particle. For very large particles, the true size might be larger than the XRD-derived value due to undetected crystal boundaries [115]. For instance, in Figure 9, the XRD pattern of TiO₂ NPs synthesized using soaked Bengal gram bean extract shows that the powder is a mixture of anatase (85.1 %) and rutile (14.9 %) phases, as determined by Rietveld refinement [68]. The anatase phase corresponds to the space group I4₁/amd, while the rutile phase belongs to P4₂/mnm. The Bio-TiO₂ displays high crystallinity, with prominent peaks for anatase at 2θ values, such as 25.28°, 37.81°, and others, assigned to specific crystal planes (JCPDS 83-2243). Peaks for rutile were observed at 27.42°, 36.11°, and additional values (JCPDS 77-0441). Using Scherrer’s equation, the crystallite sizes were estimated to be approximately 13 nm for anatase and 24 nm for rutile, with the calcined Bio-TiO₂ showing a mean crystallite size of around 14 nm.

Figure 9:

XRD of biosynthesized TiO₂ NPs. Reprinted from ref. [68], copyright 2024, with permission from Elsevier.

To provide a comprehensive comparison of the XRD patterns for various MNPs and MONPs, multiple XRD graphs from different studies were compiled into a single figure. This allows for a side-by-side visual comparison of the crystallinity, phase identification, and peak positions corresponding to different NP systems synthesized using green methods. For instance, the XRD pattern of LiCoO₂ (Figure 10a) [57] NPs displays sharp peaks indexed to a rhombohedral structure, indicating high crystallinity, while the spinel LTO NPs and green lithium titanate (GSLTO) NPs exhibit strong peaks corresponding to cubic spinel planes (Figure 10b), along with minor peaks attributed to impurities [33]. Likewise, pure and Co-doped TiO₂ NPs also exhibit anatase phase peaks in the XRD pattern (Figure 10c) [116], but the Co-doped samples possess additional peaks typical of Co₃O₄. For example, XRD spectra of Fe₃O₄ NPs made by A. sativum showed temperature-induced phase transformation of Fe₃O₄ to γ-Fe₂O₃ as the annealing temperature increased (Figure 10d) [81]. CuONPs, on the other hand, showed the goniometric determinator of monoclinic morphology of copper oxide (Figure 10e) [117]. From the XRD pattern of AgNPs synthesized with the help of the A. vera extract, the peaks characteristic of the Ag standard can be easily located and several peaks suggest the existence of undesired organic matter too (Figure 9f) [118].

Figure 10:

XRD pattern of (a) green synthesized LiCoO₂ NPs [57], (b) LTO and GSLTO [33], (c) biosynthesized pure TiO₂ and Co–doped TiO₂ [116]. Schematic view of (d) the inverse spinel crystal structure of magnetite (Fe₃O₄) [81], (e) CuONPs synthesized from the leaf extract of Calotropis procera [117], and (f) drop-cast AgNPs obtained from A. vera leaf [118]. Copyright 2024, with permission from Elsevier.

4.2 Microscopic techniques

4.2.1 Scanning electron microscopy

Microscopic techniques such as SEM are frequently used to analyze the morphology of crystal structures. These methods reveal the structural complexities and size variations of MNPs and MONPs depending on their synthesis. SEM provides detailed surface images, examining structures up to a few microns deep [87]. This technique is especially valuable for studying the size and surface characteristics of MNPs and MONPs. For example, Figure 11a shows the SEM images of CuONPs, revealing a narrow size distribution with an average particle size of 40 nm. Figure 11b presents the morphological characteristics of the synthesized CuONPs, examined through SEM, displaying a distinct rod-like morphology with an average size of 88 nm.

Figure 11:

SEM images of CuONPs synthesized from the leaf extract of (a) Calotropis procera [117] and (b) Eucalyptus globulus [119]. Copyright 2024, with permission from Elsevier.

4.2.2 Transmission electron microscopy

TEM provides insights into the internal structures of NPs, offering high-resolution images of their morphology. TEM is used to investigate the size, shape, and aggregation state of NPs [120]. In the green synthesis of TiO₂ NPs, various techniques were used to investigate their morphological properties. Figure 12 shows TEM images that illustrate a uniform size distribution of spherical particles with minimal aggregation. The particle size, as observed from TEM, is quantified using a particle size distribution histogram, which reveals an average crystallite size of approximately 14 nm, as shown in Figure 12b. Additionally, selected area electron diffraction (SAED) patterns provide complementary information by revealing the crystallographic structure of the NPs. The SAED technique produces a diffraction pattern from a selected area of the sample, indicating whether the NPs are single crystalline, polycrystalline, or amorphous. For the biosynthesized TiO₂, a diffused ring pattern in the SAED (Figure 12c) results typically suggest a polycrystalline structure, confirming the presence of multiple crystallites oriented in various directions and offering a visual representation of the NP’s crystalline arrangement.

Figure 12:

Representative (a) TEM of biosynthesized TiO₂ NPs. (b) Histogram showing the particle size distribution of TiO₂ NPs. (c) SAED pattern of biosynthesized TiO₂ NPs. Reprinted from ref. [68], copyright 2024, with permission from Elsevier.

4.2.3 Atomic force microscopy

The atomic force microscope, a high-resolution scanning probe technique, measures local properties such as friction, magnetism, and height with nanometer precision. AFM excels in force measurement, imaging, and manipulation, allowing detailed topographical mapping and mechanical property analysis. AFM uses a pointed probe to interact with the surface of the sample to collect three-dimensional data at the nanoscale. The benefit of AFM is that it can see individual NPs, giving important information about their distribution, size, shape, and surface morphology. Furthermore, AFM makes it possible to study dynamic processes such as the development, aggregation, and surface contacts of NPs as well as biological structures and biomolecules, all of which contribute to a thorough understanding of how these entities behave in various environmental conditions [121], 122]. A small volume of TiO₂ NPs synthesized using Aspergillus flavus was analyzed using AFM in the tapping mode (Figure 13) [123]. The AFM images revealed aggregated structures with significant surface roughness and an average particle height of about 10 nm. Porosity, roughness, and fractal dimensions were evaluated through image processing. AFM analysis also showed that increased TiO₂ concentration led to smoother layers, though no linear trend in roughness was observed. The presence of redox enzymes from A. flavus contributed to an increase in the surface area of the synthesized TiO₂ NPs.

Figure 13:

AFM images of (a) topography of the surface of synthesized TiO₂ NPs in cross-sectional view and (b) top view of the synthesized TiO₂ NPs. Reprinted from ref. [123], copyright 2024, with permission from Elsevier.

4.3 Spectroscopic techniques

4.3.1 Ultraviolet–visible spectroscopy

Spectroscopic methods provide detailed information for determining the chemical composition of substances. One widely used technique is UV–vis, a powerful, simple, and rapid method that measures the absorption of ultraviolet and visible light, or the optical density, of a chemical substance in solution. UV–vis is particularly effective for characterizing NPs, as it offers valuable insights into the electronic structure and properties of materials. This technique is essential in fields such as chemistry, biochemistry, and materials science because it highlights electronic transitions, helping to identify functional groups, to determine the concentration of absorbing species, and to study chemical reactions. UV–vis spectroscopy is especially useful for analyzing chromophores, which absorb light due to the presence of conjugated double bonds or specific functional groups [124], 125]. A study by Sikder et al. [126] presents an accessible approach for detecting, measuring, and analyzing the PVP-coated silver NPs (PVP-AgNPs) dissolution through the observation of their optical characteristics using UV–vis spectroscopy (Figure 14). Researchers explored the relationship between NP size and both the extinction coefficient (ɛ) and the maximum absorbance wavelength (λ _max) of PVP-AgNPs. The study monitored the size, concentration, and extinction spectra of these NPs, as they dissolved in synthetic seawater over a 96-h period. Silver NP concentrations were measured using inductively coupled plasma-mass spectroscopy (ICP-MS), while size variations were tracked using AFM. The research developed formulas to calculate the size and concentration of NPs, as well as the dissolved silver. The results indicate that UV–vis spectroscopy is a quick and efficient method for detecting and quantifying PVP-AgNPs, monitoring their dissolution, and tracking changes in NP size [126].

Figure 14:

Time-dependent evolution of the optical properties of PVP-stabilized silver nanoparticles in seawater. (a) UV–vis spectra of 97.9 ± 4.5 μg-Ag L−1 PVP-AgNPs in seawater as a function of time and (b) percentage loss of UV–vis absorbance at λmax. Reprinted from ref. [126], copyright 2024, with permission from Elsevier.

4.3.2 Fourier transform infrared

Another useful spectroscopic technique is FTIR, which can offer characteristic vibrations of chemical bonds, providing insights into chemical functions. FTIR spectroscopy compares the wavenumber frequency and band transmittance (which ranges from 100 to 0 %) of a substance [127], 128]. For instance, the FTIR spectrum of LiCoO₂ NPs (Figure 15a) shows an absorption band at 597 cm⁻¹, corresponding to the asymmetric stretching vibration of Co–O in CoO₆ octahedral structures. Additionally, bands at 1,023–1,453 cm⁻¹ indicate the presence of C–O and carboxylate groups, while peaks at 2,921–3,420 cm⁻¹ are attributed to C–H stretching vibrations and water absorption on the NP surface, confirming the interaction of these groups during synthesis. In another study on spinel Li₂TiO₃ LTO and GSLTO NPs (Figure 15b), the vibrational bands observed at 1,437–1,502 cm⁻¹ and 1,442–1,507 cm⁻¹ are linked to the asymmetric stretching of Li₂CO₃. Additionally, weak bands in the 600 cm⁻¹ range represent spinel lattice vibrations, with other bands around 3,452–3,456 cm⁻¹ corresponding to O–H stretching, indicating surface hydroxyl groups and water interactions.

Figure 15:

FTIR spectra of (a) green as-synthesized LiCoO₂ NPs [57] and (b) LTO and GSLTO nano anodes [33]. Copyright 2024, with permission from Elsevier.

The major differences in the peaks of MNPs and MONPs in UV–vis and FTIR spectroscopy arise from their distinct electronic and vibrational properties.

In UV–vis spectroscopy, MNPs such as gold (Au), silver (Ag), and copper (Cu) exhibit surface plasmon resonance (SPR) bands due to the collective oscillation of conduction electrons when irradiated with light. These SPR peaks are usually sharp and intense, appearing in the visible region – for example, around 520 nm for gold, 400–450 nm for silver, and 560–600 nm for copper, depending on the particle size, shape, and surrounding medium [129]. In contrast, MONPs, such as ZnO, TiO₂, or Fe₂O₃, do not exhibit SPR because they are semiconducting or insulating in nature. Instead, they display broad absorption bands in the UV or visible region, corresponding to bandgap transitions. For example, ZnO and TiO₂ absorb strongly below 400 nm, while Fe₂O₃ absorbs across the visible range due to its narrow bandgap [130].

In FTIR spectroscopy, MNPs typically show weak or no absorption in the IR region because metals do not have permanent dipole moments to interact with IR radiation. Any peaks observed are generally attributed to capping or stabilizing agents adsorbed on the NP surface, such as carboxylate or hydroxyl groups. On the other hand, MONPs show distinct and strong IR absorption bands, especially due to metal–oxygen (M–O) stretching vibrations, which occur typically in the 400–700 cm⁻¹ region. For example, Zn–O bonds absorb around 450–500 cm⁻¹, and Ti–O–Ti stretching modes appear near 500–700 cm⁻¹ [131]. Additionally, hydroxyl groups from adsorbed water can appear as broad peaks around 3,400 cm⁻¹ (O–H stretch) and ∼1,600 cm⁻¹ (H–O–H bending) in metal oxides.

Thus, while MNPs are characterized by SPR bands in UV–vis and weak FTIR signals, MONPs show bandgap-related absorption in UV–vis and strong M–O vibrational peaks in FTIR, reflecting their fundamental differences in electronic and bonding structure.

4.3.4 Raman spectroscopy

Raman spectroscopy is a non-destructive, label-free technique used to characterize the chemical composition of materials. When a sample is exposed to monochromatic light, inelastic scattering occurs, transferring energy between the light and the sample. This energy transfer, which varies with different chemical bonds, generates peaks in a Raman spectrum that can be used to identify chemical species and determine their concentration. By combining Raman spectroscopy with optical trapping, which uses a laser beam to hold a single particle, it is possible to analyze individual NPs and assess their biochemical composition and variability [135], 136]. For LTO and green-synthesized LTO (Figure 16a), Raman spectroscopy identifies Ti–O and Li–O bond vibrations, with key bands observed at 659 cm⁻¹ for Ti–O and others related to Li–O bonds. In Co-doped TiO₂ (Figure 16b), the Raman spectra confirm the anatase phase and reveal additional peaks linked to Co₃O₄ formation at higher doping levels.

Figure 16:

Raman spectrum of (a) LTO and GSLTO nano anodes [33] and (b) biosynthesized pure TiO₂ and Co-doped TiO₂ [116]. Copyright 2024, with permission from Elsevier.

5.1 Use of green synthesized metal and metal oxide nanoparticles in supercapacitors and batteries

Batteries, supercapacitors, and fuel cells are examples of energy conversion and storage devices that are essential for efficiently converting electrical and chemical energies. Supercapacitors, in particular, are designed to store and release energy rapidly. They typically consist of two electrodes, each made of porous electroactive materials with a high surface area and are separated by a porous membrane. These electrodes are connected by an electrolyte, which facilitates the movement of ions between them. The performance of supercapacitors, as well as other energy devices, is influenced by various electrochemical properties of both the electrodes and the electrolyte. These properties are typically evaluated using techniques such as galvanostatic charge–discharge (GCD) measurements, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) [137], 138].

Supercapacitors store energy through electrochemical processes, and they exhibit different mechanisms for storing charge. For instance, non-Faradaic double-layer capacitance, which stores charge through rapid ion adsorption/desorption processes, is the basis for electric double layer capacitors (EDLCs) [139]. This results in characteristic rectangular CV curves and triangular-symmetric GCD graphs. On the other hand, pseudocapacitors, which are composed of inorganic semiconducting materials and conducting polymers, use Faradaic capacitive processes for storing charge. Reversible redox reactions that take place at or close to the electrode material’s surface are involved in these processes. Pseudocapacitors have fast charge storage kinetics that are comparable to EDLCs, despite the Faradaic nature of charge storage [140].

New ideas, such as intercalation pseudocapacitance, redox pseudocapacitance, and underpotential deposition pseudocapacitance, have been brought to light by advancements in nanoscale engineering of electrode materials [141]. Achieving a delicate equilibrium in the composition, morphology, and surface characteristics of these materials emerges as a critical factor in enhancing their energy storage capabilities. Hybrid supercapacitors represent a fusion of the advantageous features of batteries and supercapacitors within an asymmetric structural framework. Precise adjustments in the ratios of electrode thicknesses wield significant influence over parameters such as charge–discharge time, the average state of charge, and temperature distribution, thereby playing a pivotal role in bolstering overall performance and efficiency.

Metals and metal oxides emerge as promising candidates for pseudocapacitive electrode materials in supercapacitors, with their structural and electrochemical properties molded by the specific metal and oxide compound synthesis techniques used. Nanoscaling of these materials amplifies their surface area, specific capacitance, and energy density, making them highly coveted for supercapacitor and battery applications. Nonetheless, hurdles such as inadequate electrical conductivity, cycling capabilities, surface area, and power density demand careful attention. Transition metal oxides (TMOs) must demonstrate robust electronic conductivity, diverse oxidation states, and swift ion intercalation/deintercalation during redox reactions to qualify as suitable candidates for supercapacitor electrodes. Optimal electrode materials for supercapacitors should feature a high surface area, excellent electronic conductivity, porosity, and strong electrochemical stability. Transition metal compounds exhibit structural and performance advantages that position them as promising contenders for enhancing supercapacitor functionality.

TMOs have been extensively studied for their potential uses in supercapacitors, with an increasing emphasis on environmentally friendly synthesis methods. CuO nanostructures are particularly notable because of their stability and outstanding electrochemical characteristics. In a study conducted by Kumar et al., a straightforward template-free green synthesis method was demonstrated to produce CuO nanoporous material. This method utilized copper acetate as the precursor and dried fruit extract of Indian black pepper used as the reducing agent under microwave irradiation [142]. The resultant CuO was identified as a single monoclinic crystalline phase with pore diameters ranging from 3 to 8 nm, a specific surface area of 81.23 m²/g, and a nanoporous morphology. With a specific capacitance of 238 F/g at 5 mV/s, nanoporous CuO demonstrated exceptional electrochemical energy storage performance, surpassing the specific capacitance of commercially available CuO (75 F/g). Additionally, it showed efficient desalination performance in capacitive deionization systems. Matinise et al. conducted a noteworthy investigation on the environmentally friendly production of cobalt (II, III) oxide (Co₃O₄) NPs through the use of Moringa oleifera plant extract [143]. These spinel Co₃O₄ NPs on a nickel foam electrode demonstrated pseudocapacitive behavior that was examined by means of EIS, GCD, and CV in a 3 M KOH solution. Pairs of redox peaks were visible in the CV curves, which suggested that the Ni/Co₃O₄ electrode was pseudocapacitive. The Warburg impedance and tiny semicircle seen in the EIS data suggest that the electrochemical reactions occurring on the electrode surface were both diffusion-controlled and kinetically controlled. A specific capacitance of roughly 1,060 F/g at a discharge current density of 2 A/g was found by the charge–discharge testing. CuONPs were created in a different work utilizing the seed extract from Tribulus terrestris. With a specific capacitance of 369 F/g and an astounding capacitance retention of 96.4 % after 6,000 cycles, these CuONPs showed promise for use in supercapacitors. Furthermore, the environmentally friendly synthesis of zinc-doped cerium oxide NPs was studied, using tulsi leaf extract as a reducing agent. The maximum specific capacitance was obtained at an 11 mol% concentration at a reduced scan rate, according to electrochemical studies. Zinc-doped cerium oxide NPs have shown great promise for supercapacitor applications, according to CV, GCD, and EIS investigations [144]. Furthermore, the synthesis of ZnMn₂O₄ NPs using Corallocarpus epigaeus extract was successfully demonstrated. Electrochemical studies revealed a capacitance of 380 F/g in an aqueous electrolyte solution, underscoring the material’s efficiency and potential for supercapacitor applications [145]. Similarly, manganese oxide (Mn₃O₄) synthesized from olive leaf extract has shown a specific capacitance of 583.7 F/g [146]. Ruthenium oxide (RuO₂) is another promising supercapacitor electrode material due to its highly theoretical specific capacitance and corrosion resistance. A study reported the green synthesis of RuO₂ NPs using Aspalathus linearis extract as a reducing, oxidizing, and capping agent. The synthesized RuO₂ NPs were deposited onto nickel foam (NiF) to form a NiF/RuO₂ electrode material. Electrochemical analysis revealed a specific capacitance of 750 F g⁻¹ at a current density of 10 A g⁻¹, with a retention rate of 97.5 % under the same conditions [147]. In comparison, RuO₂ NPs, synthesized from Anacyclus pyrethrum, have demonstrated specific capacitances of 209 F/g [148].

Recent studies have also explored various plant-based synthesis methods for supercapacitor electrode materials (Table 2). For instance, LiZnVO₄ NPs synthesized from Hibiscus rosa-sinensis leaves and flower-like TiONPs synthesized from Calotropis gigantea plant leaves have demonstrated specific capacitances of 88.7 F/g and 804 F/g, respectively [149], 150]. Additionally, iron adorned with carbon hydrochar extracted from bamboo that has been KHCO₃-activated has shown specific capacitances of 467 F/g [151]. Composite materials like CuFeS₂ synthesized from mimosa leaf extract have also shown promising specific capacitances of 501 F/g [152]. However, after being synthesized using leaf extract from Solanum nigrum, the gold NPs achieved only 80 F/g, an unsatisfactory specific capacitance [153].

In a study by Sumantha et al. [159], an extract from dried Tamarindus indica seeds was used to synthesize nickel manganese oxide NPs (NiMnO₃ NPs). The structure of the produced NiMnO₃ NPs was confirmed to be platelet-like by FESEM and HR-TEM. The EDAX profile showed that the particles were composed of only oxygen, manganese, and nickel. A graphite sheet coated with NiMnO₃ NPs was used as the working electrode in the electrochemical studies, which used a three-electrode setup. The electrolyte was 1.0 M KOH. EDLCs are characterized by their rectangular forms and the lack of redox peaks in the CV plots at 5 and 100 mV/s (Figure 17). Compared to the other samples, sample ‘e’ showed better electrochemical performance because of its narrower bandgap. Non-Faradaic charge transfer with a symmetrical triangular structure and longer discharge duration at lower current density was further validated by GCD studies. Extraordinary redox peaks were seen at higher scan rates, suggesting high-capacity fractions and strong reversibility. At 1 A/g, sample ‘e’ displayed a maximum specific capacitance of 84.15 F/g; as current density increased, capacitance decreased. These findings suggest that the green synthesized NiMnO₃ NPs may have potential applications as anode materials in LIBs.

Figure 17:

Electrochemical studies of the NiMnO₃ NPs: (i) CV plot at 5 mV/s, (ii) CV plot at 100 mV/s, and (iii) CV plot of sample ‘e’ at different scan rates. GCD at a current density of (iv) 1 A/g, (v) 2 A/g, and (vi) 5 A/g. (vii) GCD of ‘e’ at current densities of 1, 2, and 5 A/g. (viii) Plot of the specific capacitance vs current densities. Reprinted from ref. [159], copyright 2024, with permission from Elsevier.

Using green synthesis techniques, MNPs and MONPs have also been successfully used to create electrode materials for battery applications (Table 2). For sodium-ion batteries (SIBs), zinc oxide NPs (ZnONPs) show great promise as anode materials. The green synthesis of ZnONPs utilizing plant-based techniques has been the subject of numerous investigations. In a recent work, ZnONPs were produced at calcination temperatures of 700, 800, and 900 °C using leaf extract from Mariposa Christia vespertilionis [35]. The examination of nitrogen gas adsorption revealed that when the temperature of calcination increased, the size and volume of the pores decreased. With an initial discharge capacity of 591 mAh/g at 0.1 C over 100 cycles, the ZnONPs calcined at 700 °C showed the best rate capability and might be used as anode materials for SIBs. Another study used Chromolaena odorata leaf extract to synthesize ZnONPs at 800 and 900 °C [156]. Electrochemical performance studies revealed that ZnONPs prepared with C. odorata extract and calcined at 800 °C exhibited better recyclability than those calcined at 900 °C, making them viable anode materials for SIBs due to their improved recyclability. A ZnAl₂O₄-coated LiFePO4 (ZnAl₂O₄@LFP) electrode was created using a polypropylene glycol-assisted sol–gel technique, and it was examined as a potential cathode material for LIBs [160]. Higher charge and discharge capacities of about 122 and 95 mAh/g, respectively, were shown by the ZnAl₂O₄@LFP electrode. The ZnAl₂O₄@LFP electrode’s Coulombic efficiency increased dramatically over the course of the cycle, rising from 80 % in the first cycle to 99.8 % in the eighth cycle, showing outstanding stability. The LFP cathode’s structural stability and electrochemical performance were significantly improved by the ZnAl₂O₄ coating. Additionally, green methods have been used to synthesize other promising electrode materials, such as TiO₂ NPs. A study by Pakseresht et al. demonstrated the synthesis of TiO₂ NPs using Matricaria chamomilla flower extract [161]. After the TiO₂ cathodes were made on nickel foam, their electrochemical properties were assessed using CV, GCD tests, and electrochemical impedance measurements in an ECC-Air test cell. As a result, the Bio-TiO₂ air cathode demonstrated a specific capacity of 500 mAh/g and a complete discharge capacity of 2000 mAh/g over 30 steady cycles. Another study by Pillai et al. demonstrated the biosynthesis of TiO₂ NPs with an average particle size of 12 nm using beetroot extract [162]. The synthesized TiO₂ NPs were examined as anode material in LIBs, with the Li/TiO₂ half-cell analyzed in the 1–3 V potential range at a C/10 rate. The results from cycling stability and rate capability tests indicated that this material outperformed earlier TiO₂ NP reports. Remarkably, it delivered a discharge capacity of 149 mAh/g even at a 20 C rate.

Kheradmandfard et al. introduced a novel ultrafast green microwave-assisted method for synthesizing high-entropy oxide (HEO) NPs, consisting of Mg, Cu, Ni, Co, and Zn in a single-phase rock salt structure [163]. The NPs exhibited a uniform particle-size distribution of 20–70 nm, with an average size of 44 nm. The HEO NPs demonstrated a remarkable stability over 1,000 cycles at 1 A/g and demonstrated exceptional lithium storage capabilities when used as anode materials for Li-ion batteries. The nanocrystalline HEO electrode’s discharge capacity was assessed at a range of current densities from 0.1 to 5 A/g (Figure 18a). An irreversible solid–electrolyte interphase (SEI) film and Li₂O discharge product formed in the second cycle, resulting in a drop in discharge capacity from 686 mAh/g at 0.1 A/g to 400 mAh/g. The specific capacity stabilized at about 250 mAh/g at 5 A/g. The discharge capacity surpassed the initial value when the current density was restored to 0.1 A/g, indicating the superior structural stability of the HEO NPs for reversible Li⁺ uptake at high current densities. Additionally, without experiencing any appreciable capacity fading over 1,000 cycles, the electrode demonstrated exceptional cycling performance at 1 A/g (Figure 18b). Because of their distinct chemistry and entropy stabilization, HEOs have a stable and reversible lithiation performance that allows them to tolerate volumetric changes throughout repeated cycles of charge and discharge.

Figure 18:

Electrochemical performance of the electrode material. (a) Rate performance at different current densities ranging from 0.1 to 5 A/g and (b) cycling performances at 1 A/g for 1000 cycles. Reprinted from ref. [164], Copyright 2024, with permission from Elsevier.

5.2 Use of green synthesized metal and metal oxide nanoparticles in hydrogen production

NPs have become integral to various fields, including energy harvesting, storage, biofuel production, and biosensors. In particular, NPs play a significant role in enhancing microbial metabolism for hydrogen production by facilitating more efficient electron transfer, even under challenging conditions. This advancement has yielded promising outcomes in biohydrogen production using various MNPs and MONPs. These NPs not only improve the rate of hydrogen production but also contribute to optimizing the stability and efficiency of the process.

NPs promote biohydrogen production, thanks to their surface properties and quantum effects. The small size of NPs increases their specific surface area, thus enhancing their electron adsorption capacity. Moreover, the quantum effect is directly linked to the rate of electron exchange between NPs and catalytic enzymes, such as hydrogenase, which plays a crucial role in the transformation of hydrogen into protons and vice versa, thus facilitating energy transfer [165]. Various green synthesized MNPs and MONPs are used in bio-hydrogen production (Table 3).

According to a study by Yildirim et al., dark fermentative bio-hydrogen production was influenced by green-synthesized silver oxide NPs derived from a microalgae strain [164]. The biosynthesized silver oxide NPs produced with Chlorella sp. green microalgae exhibited a uniform structure and an average particle size of 85 nm. The addition of these NPs led to increased hydrogen production by fermentative bacteria. At a concentration of 400 μg/L, the highest hydrogen yield of 299 mL, equivalent to 2.44 mol H₂/mol glucose, was achieved, representing a 17 % increase compared to the control. The study also demonstrated that commercial silver NPs and green-synthesized silver oxide NPs were equally effective. Dogmaz and Cavas explored the use of the biomass of marine green seaweed Ulva lactuca, considered biological waste in the Mediterranean ecosystem, for biohydrogen production via green synthesized silver NPs under various conditions [170]. They studied the effects of temperature, agitation rate, pH, and sodium borohydride (NaBH₄) concentration on biohydrogen production. The optimal biohydrogen yield of 58.2 mL was achieved at 50 °C and pH 11. Higher agitation rates and NaBH₄ concentrations generally increased hydrogen production, while acidic pH levels yielded better results than basic conditions. A comparative analysis of the effects of silver NPs on Clostridium beijerinckii’s fermentative biohydrogen (bio-H₂) production was conducted in another study [171]. In a batch process, green-synthesized silver NPs were created using DU-C2 actinomycete and henna (Lawsonia inermis). The results indicated that henna-mediated AgNP synthesis increased the rate of hydrogen production, achieving 1.71 mol H₂/mol glucose after 72 h of silver NP incubation with C. beijerinckii. In contrast, the acidic pH and antibacterial properties of AgNPs produced by DU-C2 inhibited H₂ production.

Copper NPs were shown to have a negative impact on hydrogen generation by Clostridium acetobutylicum and Enterobacter cloacae strains [172]. Hydrogen yields were found to be 3.5 %–2.9 % lower at a concentration of 2.5 mg/L compared to the controls, with yields of approximately 1.74 and 1.44 mol H₂/mol hexose, respectively. At a concentration of 12.5 mg/L, a significant reduction in hydrogen output of 56.9 %–72.2 % was observed, which was attributed to the inhibitory effects of CuNPs. Gold NPs are known to enhance enzyme activity and immobilization. Zhu et al. demonstrated photosynthesis-mediated biomineralization of AuNPs inside Chlorella cells, where the photosynthesis-driven reduction of Au₃ ⁺ to Au⁰ allowed AuNPs to localize around the thylakoid membrane [173]. This augmented hydrogen production under sunlight by enhancing hypoxic photosynthesis and transferring photoelectrons to hydrogenase, resulting in an 8.3-fold increase in hydrogen production under monochromatic 560 nm light irradiation. Palladium NPs (PdNPs) and iron NPs (FeNPs) also improve biohydrogen yields. Aygun et al. demonstrated the use of Nigella sativa seed extract on the biosynthesis of palladium–silver NPs (Pd–AgNPs), showing high catalytic efficiency for hydrogen production [174]. Another study used pomegranate peel extract to biosynthesize iron nanoformulations with different Fe₂O₃ morphologies, finding promising results for photoelectrochemical water splitting and solar hydrogen production [175]. Nickel oxide NPs (NiONPs) are also effective in enhancing hydrogen production. Studies have shown that green-synthesized NiONPs from Eichhornia crassipes extract significantly improved fermentative hydrogen production, achieving a 47.29 % increase in yield and enhancing hydrogenase activity by 623 % [167]. Additionally, bimetallic nickel ferrite NPs (NiFe₂O₄ NPs) synthesized using the Eichhornia crassipes extract improved hydrogen production by 112.32 % when added to lignocellulosic hydrolyzate fermentation, enhancing key enzyme activities and conversion efficiencies of glucose, xylose, and overall substrate to hydrogen gas [176].

Apart from the improvements achieved in green-synthesized NPs, research in advanced catalyst design strategy, specifically those that utilize high-entropy alloys (HEAs) and laser-assisted synthesis, is now becoming more promising for sustainable hydrogen production. HEAs are complex systems whereby there are several principle components in nearly equiatomic ratios which provide tunable electronic architectures and a great flexibility in compositions. As such, these characteristics allow constructing strong and synergistic active sites, hence making HEAs desirable for electrocatalytic water splitting applications. A good example is the dendritic thin-film CuCrFeNiCoP HEA, which demonstrated outstanding bifunctional catalytic activity toward the hydrogen evolution reaction (HER) and toward the oxygen evolution reaction (OER). The addition of phosphorus into the alloy improved the HER kinetics as a proton acceptor without destroying the face-centered cubic lattice of HEA. This material showed excellent electrochemical stability at even high currents such as 100 mA cm⁻², which proves its applicability to prolonged hydrogen production at the industrial level of conditions [177]. In a parallel approach, a high-entropy single-atom catalysts containing multiple metals was synthesized on graphene oxide using pulsed laser irradiation in liquid, achieving low overpotentials for HER (49 mV) and OER (398 mV), outperforming commercial Pt/C. The enhanced activity stemmed from rapid photoreduction and optimized surface composition [178]. Additionally, a laser-assisted method enabled the rapid and scalable synthesis of HEA NPs with up to 20 elements, showing excellent HER activity and durability across various electrolytes [179]. A pulsed laser shock technique was also developed for gas-free synthesis of transition metal phosphides from MOFs, producing stable catalysts with low water-splitting voltages and potential for broader applications such as energy storage and biosensing [180].

5.3 Use of green synthesized metal oxide nanoparticles in thermal energy storage

While solar energy is an important renewable resource, its availability is limited to daylight hours and is reduced on cloudy days. To address this, phase change materials (PCMs) are used in latent heat thermal energy storage (LHTES) systems, which store and release energy through melting and solidification. LHTES systems provide a more stable and compact alternative compared to sensible heat storage (SHS) and thermochemical storage (TCS). They range from basic PCM containers to sophisticated systems with advanced features. Despite higher initial costs and leakage risks, LHTES systems provide higher thermal storage density and nearly isothermal behavior, requiring only one sealed container and operating across a wide temperature range. LHTES systems have diverse applications, including electronic cooling, air conditioning, LIB cooling, building heating, waste heat recovery, and solar food drying. They are also used in solar power storage systems, like concentrated solar power (CSP), and medical applications, like smart textiles [181], 182].

PCMs are categorized by their melting temperatures into low, middle, and high ranges. They can be organic or inorganic, with organic PCMs such as paraffin and fatty acids being non-toxic and stable, but having low thermal conductivity. Inorganic PCMs such as salt hydrates and metallic PCMs offer latent heat of fusion and higher thermal conductivity but can suffer from supercooling and corrosion. Eutectic PCMs, a blend of multiple PCMs, also provide latent heat of fusion and high thermal conductivity, although they are costly [183]. Various studies have shown the effectiveness of different eutectic mixtures for thermal energy storage. Enhancements for LHTES systems focus on improving thermal conductivity to reduce melting and solidification times. Among the methods are fins and heat pipes embedded, using conductive porous materials, applying multiple PCMs, and dispersing NPs [184]. Heat pipes and fins, either alone or together, significantly enhance thermal performance by increasing the contact area and reducing thermal resistance [185], [186], [187]. Highly conductive porous materials, including carbon foams and metal, impregnated with PCMs improve heat transfer [188]. Studies have shown that these materials, especially metal foams, lead to more uniform temperature distribution and faster phase change processes. The use of multiple PCMs can optimize melting and solidification processes by placing PCMs with decreasing melting points along the direction of heat transfer fluid (HTF), reducing charging and discharging times. Nano-encapsulation prevents interaction between enhancement agents and PCMs, improving stability and performance. Nano-enhanced PCMs (NePCMs) incorporate NPs to enhance thermal conductivity without significantly reducing PCM volume. This method can be used alone or in combination with other enhancements. Studies have explored various NP types and shapes, PCM types, and additional enhancements to improve LHTES systems’ thermal performance [189].

MNPs and MONPs significantly enhance thermal energy storage systems by improving the thermophysical properties of PCMs. Their unique characteristics make PCMs more efficient for energy storage applications. Studies have shown that incorporating NPs such as zinc oxide (ZnO), titanium dioxide (TiO₂), ferric oxide (Fe₂O₃), and silicon dioxide (SiO₂) into PCMs enhances thermal conductivity and stability, reduces phase change temperature ranges, and eliminates supercooling issues.

For instance, a study on magnesium nitrate hexahydrate PCM revealed that adding 0.5 wt% of TiO₂, ZnO, Fe₂O₃, and SiO₂ NPs improved thermal conductivity by 147.5 %, 62.5 %, 55 %, and 45 %, respectively [190]. This enhancement not only improved the heat transfer characteristics but also increased the charging and discharging rates by 33 % and 77.5 %, respectively, for TiO₂ NPs, making it a potential material for instant solar water heating systems. Another study demonstrated the development of nano-enhanced PCMs using capric acid (CA) and manganese dioxide (α-MnO₂) NPs synthesized via a green method using Ficus retusa leaves [191]. The resulting nanocomposites, tested at 1 %, 2 %, and 3 % weight fractions, showed excellent thermal energy storage capabilities, with DSC analysis indicating a thermal energy storage range of 145–164 kJ/kg. These PCMs demonstrated outstanding thermal reliability over 500 thermal cycles and maintained stability within the working temperature range, as confirmed by TGA results. Silver NPs (AgNPs) have also shown remarkable potential in enhancing TES systems. A study focused on medium-temperature organic PCMs embedded with AgNPs (20–40 nm) at concentrations from 0.2 % to 1.0 % found that 0.8 % AgNPs enhanced thermal conductivity from 0.218 W/mK to 0.44 W/mK, an improvement of 101 % [192]. The composite maintained thermal stability up to 220 °C and exhibited only a 1 % variation in latent heat from the base PCM. Paraffin wax as a PCM doped with 1 %–2 % AgNPs showed a 1.25-fold increase in thermal conductivity. DSC results indicated a melting temperature of 57.3 °C with 2 % AgNPs, higher than pure paraffin wax. Experiments on a stepped basin solar still (SS) using AgNP-based paraffin wax demonstrated significant improvements in water temperature, evaporation rate, and potable water generation, with outputs of 7.98 kg/m² compared to 6.73 kg/m² for pure paraffin wax and 3.61 kg/m² without thermal energy storage [193]. In another study, salt hydrate PCMs modified with cellulose nanofibril (CNF) and AgNPs tackled the supercooling challenge. The composite showed a supercooling degree of 1.2 °C and a 31.6 % increase in thermal conductivity compared to pure sodium acetate trihydrate (SAT). The composite maintained thermal reliability with only a 2 % enthalpy reduction after 100 melting/freezing cycles [194].

Green synthesis of MNPs and MONPs has shown promising results comparable to chemically synthesized ones. For instance, a study developed an environment-friendly method to produce anatase nano-TiO₂ on graphene sheets, resulting in a highly crystalline product with particle sizes ranging from 9 nm to 25 nm and demonstrating superior photoelectrochemical properties, indicating its potential use in TES systems [195]. Similarly, green synthesis using plant extracts has been explored for producing MONPs. Iron oxide NPs synthesized using extracts from Fan palm, Dombeya wallichii, and Pyrus comminis showed efficient dye adsorption properties for water remediation. These green synthesized NPs were eco-friendly, scalable, non-toxic, and stable, highlighting their potential for broader applications in nanotechnology and TES [196]. Despite these promising results, publications on the use of green synthesized MNPs and MONPs in TES are still limited. However, studies have demonstrated that green synthesis can produce NPs with features similar to those synthesized chemically. For example, AgNPs synthesized using the Mussaenda frondosa leaf extract showed a remarkable antioxidant activity and long-term antimicrobial properties, comparable to chemically synthesized AgNPs [197]. Given the proven efficacy of chemically synthesized MNPs and MONPs in improving thermal energy storage, green synthesized NPs hold great potential. Their eco-friendly synthesis and comparable performance suggest that they could play a significant role in developing sustainable and efficient TES systems. As research advances, the application of green synthesized NPs in TES could lead to more environmentally friendly and cost-effective energy storage solutions, leveraging their improved thermal properties and stability.

5.4 Use of green synthesized metal and metal oxide nanoparticles in sensing

In recent years, sensors have become increasingly vital in the energy sector due to their ability to provide real-time data collection and transmission, leading to significant advancements in cost savings and energy efficiency. The implementation of advanced sensors enhances the durability, robustness, and reliability of energy processes, facilitating self-powering capabilities, efficient wireless transmissions, and non-invasive installations. These improvements have broad applications, including integrated performance indicators, analytics, decision-making, diagnostics, and optimization procedures.

Sensors are essential across all stages of energy production, whether the source is renewable or non-renewable. For example, sensors monitor oil and gas wells and pipelines, track the position of wind turbine blades, and measure absolute motion and direction in power plants. In renewable energy systems, sensors optimize the performance of solar panels and wind turbines by continuously adjusting to environmental conditions to maximize energy capture [198], [199], [200].

The use of nanomaterial-based signal amplification has brought significant advancements to sensor technology within the energy field. MNPs, such as gold and silver, exhibit unique optical properties through localized SPR. This phenomenon results in noticeable color changes based on the NPs’ dispersion and aggregation states, allowing for the detection of ultra-low concentrations of target analytes. These properties enhance the sensitivity and specificity of sensors in various applications, including environmental monitoring and energy management [201], 202]. Furthermore, green-synthesized NPs support sustainable practices by being eco-friendly and offer tailored characteristics that improve the efficiency of energy systems (Table 4).

Humidity sensors are critical in various energy applications, as they help monitor and control environmental conditions that can affect the performance and longevity of energy systems. High humidity levels can lead to moisture ingress, corrosion, and degradation of components, negatively impacting the efficiency and reliability of energy storage devices, fuel cells, and other energy technologies.

A recent study demonstrated the synthesis of LiZnVO₄ NPs using a green synthesis approach with Hibiscus rosa-sinensis leaves as a novel fuel [149]. The synthesized LiZnVO₄ NPs exhibited diverse properties and applications, such as strong antioxidant activity, excellent performance in supercapacitors, and significant potential as sensors for sodium nitrite and humidity. The LiZnVO₄ NPs were found to be excellent materials for humidity sensing, exhibiting a sensitivity factor of 28.0, a sensitivity of 0.3 MΩ/%RH, and a detection limit of 5 % RH. These properties highlight the potential of LiZnVO₄ NPs in developing efficient and reliable humidity sensors for energy applications.

Another study focused on the synthesis of CuONPs using green tea extract as fuel and their subsequent application in humidity sensing [209]. The NPs were characterized by various techniques and were found to be spherical with an average particle size of 64.5 nm. The humidity sensing performance of the CuONPs was thoroughly investigated, revealing significant variation in resistance with changes in humidity, indicating excellent sensing capabilities. The CuONPs achieved a remarkable sensing response of 99 % within a relative humidity (RH) range of 7–97 % RH. Additionally, the response time was measured at 22 s and the recovery time was measured at 31 s, demonstrating the rapid response and recovery characteristics of the sensors. Importantly, the CuONPs exhibited minimal hysteresis and maintained good stability over a period of two months.

Another recent study highlighted the humidity sensing capabilities of magnesium oxide NPs (MgONPs) synthesized using green tea extract as a fuel. The MgONPs were synthesized through a green synthesis method and characterized by various techniques. Structural and functional characterizations confirmed the successful formation of MgONPs, while morphological studies revealed irregular clusters of particles, which increased surface area and porosity. Using spin coating equipment, the sample was coated on a glass plate to create a sensor film of the MgONPs for use in humidity sensing measurements. At room temperature, the MgONPs showed exceptional humidity sensing capabilities, with a sensitivity of 99.84 %. The response time was rapid at 14 s, with a recovery time of 26 s. These results indicate that MgONPs can quickly and accurately respond to changes in humidity. Additionally, the MgONPs exhibited minimal hysteresis, excellent linearity, and stable performance over time. The low limit of detection (LOD) further emphasizes their efficiency as humidity sensors. The sensing mechanism is based on the formation of chemisorption and physisorption layers, which alter the electrical properties of the MgONPs in response to humidity changes [211].

Finally, a recent study showcases the creation of a low-cost and adjustable ambient temperature humidity sensor utilizing green-synthesized AgNPs [212]. A straightforward green method was used to create the AgNPs, utilizing extracts from Pistia stratiotes as a reducing agent for AgNO₃ under light conditions. AgNP generation was greatly impacted by the synthesis parameters, which included AgNO₃ concentration, reaction time, pH value, and light irradiation. The AgNPs displayed spherical forms with different sizes depending on the pH levels. AgNPs were deposited onto a transparent polyethylene substrate with pre-patterned Ag interdigitated electrodes using a drop coating technique to create the humidity sensor. At room temperature, the resulting flexible sensor exhibited excellent repeatability and stability, along with high sensitivity to RH. The sensor’s electrical resistance and response displayed linear correlations with RH in the range of 20–85 %, indicating outstanding performance in humidity detection. Additionally, the sensor demonstrated fast recovery and response times of 10 and 11 s, respectively.

Gas sensors are also crucial in the energy field, ensuring safety, efficiency, and environmental compliance. They monitor emissions such as CO₂, SO₂, NO _x , and CO in power plants, helping to meet environmental regulations. In infrastructure, gas sensors detect leaks in natural gas pipelines and hydrogen storage, preventing hazards and ensuring safe operation. They also optimize fuel cell performance by monitoring hydrogen and oxygen concentrations. In biogas and biomass systems, gas sensors manage methane and CO₂ levels to enhance efficiency and quality. Additionally, they are used in combustion control to improve efficiency and reduce emissions. Gas sensors also monitor air quality in energy facilities, ensuring a safe working environment and minimizing environmental impact. Overall, these sensors are vital for managing modern energy systems and advancing sustainable energy solutions [213], 214].

Methane (CH₄) is extensively utilized for generating electricity, producing hydrogen and ethylene, and for residential heating. Its high volatility poses significant explosion risks, especially in confined spaces when mixed with air. Early detection of methane is crucial for preventing potential explosions in both industrial and domestic settings. Gas sensors are commonly used for this purpose, with semiconducting metal oxide sensors being particularly favored due to their cost effectiveness, ease of production, and operational simplicity. Research has explored the use of peroxidase enzymes from Euphorbia amygdaloides and Ficus carica leaves to synthesize γ-Fe₂O₃ NPs [203]. These NPs were evaluated for their features related to structure, morphology, and methane gas sensing. γ-Fe₂O₃ NPs derived from F. carica exhibited superior response, selectivity, and shorter response/recovery times compared to those from E. amygdaloides, which only showed a 15 % response at 150 °C to 1 ppm methane gas. Impedance spectroscopy revealed that resistance due to grain boundaries was a key factor influencing gas sensing performance. The F. carica-derived γ-Fe₂O₃ NPs showed promising potential for industrial applications.

Another study concentrated on the biological synthesis of zinc oxide NPs (ZnONPs) using Camellia sinensis (green tea) powder. This method, which used co-precipitation, ensures an environmentally friendly production process. The ZnONPs sensor showed promise for advanced sensing applications due to its remarkable sensitivity and selectivity to a number of volatile organic compounds (VOCs), including acetone, ammonia, formaldehyde, and benzene [215]. Using Ixora coccinea leaf extract, more research examined the gas-sensing capabilities of biosynthesized ZnONPs. There was less aggregated ZnO powder as a result of the extract’s capping action. In the investigation, ZnO samples with and without I. coccinea were subjected to ethanol vapor sensing at temperatures ranging from 40 to 800 ppm of test gas. Both samples showed optimal sensing at 285 °C, with the sensor response increasing linearly with ethanol concentration. Differences in gas response were attributed to changes in NP morphology, with sensors exhibiting varying response and recovery times [216]. A separate study developed Ag-coated ZnONPs through green synthesis using the T. terrestris leaf extract. These NPs, measuring 6–10 nm in diameter, demonstrated enhanced ethanol sensing properties at ambient temperature. The resistance values of Ag-coated ZnO sensors decreased more significantly compared to pure ZnO sensors, indicating improved performance [217]. Additionally, ZnONPs were synthesized using both green and chemical methods and their performance in detecting liquefied petroleum gas (LPG) was compared. Green synthesized ZnONPs, derived from A. vera extract and zinc nitrate, exhibited a random spherical morphology, while chemically synthesized ZnO showed a more uniform spherical shape with particles ranging from 50 to 60 nm in size. Both types of ZnO sensors displayed maximum sensitivity at 250 °C for 1,000 ppm LPG, with chemically synthesized ZnO showing slightly better sensitivity than the biosynthesized variant [218].

The latest developments have also related to the development of PLA (polylactic acid) nanocomposites as flexible, biodegradable matrices for sensors that provide an integrated environmental sustainability with a high sensitivity and mechanical ruggedness for wearable/structural applications. PLA nanocomposites are light in weight, eco-friendly, cheap, and have superior biocompatibility. They are usually produced by the addition of nano-fillers which include the likes of carbon nanotube, quantum dots, nanoclay, nanofiber, graphene and other polymers into the PLA matrix. These nanocomposites have improved electrical properties, high thermal conductivity, better dielectric constants, improved thermal stability, and biodegradability as compared to pure PLA [219].Therefore, PLA nanocomposites have been applied broadly in different sensing applications. The reviews that were recently published summed up the standard approaches such as the chemical/physical modifications and electrospinning to fabricate the PLA nanocomposites and provided their performance in the sensors for moisture, piezo/strain, chemical/bio, and thermal [219], [220], [221].

With these developments in morphology-controlled nanomaterials for sensing, recently it was introduced by a rational electrochemical design of complex cuprous oxide (Cu₂O) microarchitectures through a step-by-step electrodeposition method [222]. It was possible to change the overpotential between thermodynamic and kinetic control and control the growth of Cu₂O in real time. This adherence of Cl⁻- ions selectively to the {100} crystal facets regulated the rates of growth of the facets. Also, using polyvinyl alcohol (PVA) ensured that the microarchitectures in the electrode surface had uniform evolution, and the structures were perfected to noble metal microstructures that retained their shape and proved to be excellent substrates for surface-enhanced Raman scattering (SERS). This strategy creates a new electrochemical route to create advanced inorganic architectures with a high sensitivity and reproducibility in SERS-based sensors. This is complemented by another study which revealed an artificially controllable electrodeposition technique to tune the morphology and electronic functionality of Cu₂O as working for ReRAM gadgets [223]. A variety of the Cu₂O morphologies, e.g. (200)-oriented quadrangular pyramids and (111)-oriented triangular pyramids, were selectively obtained due to the selective provision of specific Cu₂O surface facets by the use of Pb and Sb additives, which play the oppositional roles in the OH⁻ The coexistence of Sb and Pb led to nanostructured films comprising crystallized NPs in an amorphous matrix. What is important, the treating of forming-free ReRAM devices using the obtained Cu₂O films led to the high on/off resistance ratios (∼1.2 × 10⁴) and the outstanding electrical/thermal stability. In a related study, the effects of manufacturing parameters on carbon nanofiber (CNF)/polylactic acid (PLA) filaments for fused filament fabrication (FFF) additive manufacturing (AM)-produced strain sensors were investigated. By using a design of experiments (DOE) approach, the impact of CNF weight fraction, extrusion temperature, and number of extrusions on sensor performance was explored. It was found that extruding the CNF/PLA material at 185 °C for two total extrusions significantly improved the electrical properties compared to the unmodified material. The optimal manufacturing procedure allowed the creation of piezoresistive dog-bone shaped sensors with varying CNF content (5.0, 7.5, and 10.0 wt %) and different sizes. These sensors exhibited strong and repeatable behavior under monotonic and cyclic loading, with the 7.5 wt% CNF/PLA sensors showing greater strain sensitivity. However, 10.0 wt% CNF/PLA sensors demonstrated better repeatability in their piezoresistive response, and sensor size had less impact on their strain sensing performance. This study is an important step toward the realization of low-cost, easily produced, and highly customizable strain sensors using FFF technology.