Biosurfactants in biocorrosion and corrosion mitigation of metals: An overview

2.1 Glycolipids

Glycolipids are compounds that contain fatty acids and carbohydrate moieties. They correspond to the group of compounds based on the nature of lipids and carbohydrate components [23]. According to the nature of lipids, they are categorized as rhamnolipids (RH), trehalose lipids, and sophorose lipids. They contain a hydrophobic lipid tail and one or more hydrophilic sugar groups linked by a glycosidic bond. The main structure of the glycolipids consists of a mono- or oligosaccharide group attached to a sphingolipid or glycerol group with one or two fatty acids. They are commonly found in Gram-positive microorganisms. Glycolipids are compounds containing fatty acids and carbohydrate moieties. They correspond to the group of compounds based on the nature of lipids and carbohydrate moieties [23]. Based on the nature of lipids, they are categorized as RHs, trehalose lipids, and sophorose lipids.

The other groups of compounds, like RHs, are derived from Pseudomonas aeruginosa and trehalose lipids and are found in various Gram-positive bacteria belonging to the mycolates group [24]. The sophorose lipids are derived from non-pathogenic yeast [25].

2.2 Lipopeptides and lipoproteins

Lipopeptides contain linear or cyclic oligopeptides acylated with fatty acids. They are commonly synthesized from Bacillus and Pseudomonas [26]. They are fatty acids linked to a peptide chain with a hydrophilic head and are linked to the lactone ring in the case of cyclic peptides. As lipopeptides are highly stable, biodegradable, and non-toxic, they play a vital role in various fields like cosmetics, healthcare, biomedical, pharmaceutical, and food industries [27]. Lipoproteins are surface-active biosurfactants produced from Bacillus subtilis. They are complex particles that have a hydrophobic core at the centre of non-polar lipids. They are surrounded by a hydrophilic membrane surrounded by an amphipathic coat of protein, phospholipid, and cholesterol [28].

2.3 Surfactants

Surfactants are cyclic lipopeptides produced mainly by various strains of Gram-positive bacteria, specifically from the genus B. subtilis, and they are the most efficient biosurfactants used in various industries. The composition consists of hydroxy fatty acid and seven amino acid peptide rings [29]. These seven amino acids contain l-asparagine, glutamic acid, l-leucine, l-violin, and two d-leucines with a long hydrophobic fatty acid chain. Surfactants are found to possess many biomedical properties, including antiviral, antifungal, anti-mycoplasma, and anti-bacterial activities [30,31,32].

2.4 Lichensysin

Lichensysin is a highly effective anionic cyclic lipoheptapeptide that is produced by Bacillus licheniformis using glucose as its main source. It produces several biosurfactants that act efficiently in the presence of some other biosurfactants, and they are very stable at higher temperatures and pH and under alkaline environments [33].

2.5 Fatty acids and phospholipids

Phospholipids contain hydrophobic fatty acid chains with hydrophilic moieties. In nature, phospholipids occur as several classes with different polar moieties. Because of their amphiphilic nature, they can be used as emulsifiers. The growth of n-alkanes results in the production of large quantities of phospholipid fatty acid surfactants by several bacteria and yeast. The amphiphilic nature of these lipids depends on the length of the hydrocarbon chain in their structure [34].

5.1 Coco monoethanolamide (CMEA)

CMEA was investigated as a corrosion inhibitor for the protection of mild steel using 1 M HCl [47]. The inhibition efficiency of CMEA at 0.6163 mM and 60°C was found to be 99.01%. It performs as a mixed-type inhibitor, and its adsorption at the metal–acid interface follows the Langmuir isotherm. Theoretical findings from density functional theory, Monte Carlo, and molecular dynamics simulations were used to demonstrate their experimental results, and the potentiodynamic polarization studies revealed that the Tafel slopes at different inhibitor concentrations follow the same trend with reduced corrosion current density (j _corr). The inhibition was mainly cathodic and inhibited the hydrogen evolution reaction, as evidenced by the shift towards the cathodic direction. This shows that solution resistance was greater for the solution containing inhibitor, and corrosion resistance was also increased when the inhibitor dosage increased, which shows that the inhibitors form a protective layer on the surface of the metal. Further, they suggest that CMEA develops a defensive layer on the surface of the metal through a physiochemisorption mode. The functional groups like >C═O, –OH, and >NH present in the CMEA get protonated in an acidic solution and form cationic molecules in the medium, as well as the counter ions (anions) induce the metallic surface to behave as negative. Thus, the cations formed by CMEA and anions on the metal surface form a defending layer on the metal surface. They reported that the CMEA bio-surfactant can act as a potent inhibitor for mild steel.

5.2 Asparagine and arginine

The two biosurfactants, viz. sodium N-dodecyl asparagine, and sodium N-dodecyl arginine, were employed as green corrosion inhibitors on the dissolution of mild steel (MS37-2) in aqueous sodium chloride solutions [48]. Electrochemical impedance measurements showed that sodium N-dodecyl asparagine and sodium N-dodecyl arginine surfactants were effective in inhibiting MS37-2 corrosion in NaCl solution. The polarization measurements revealed that the biosurfactants added to the sodium chloride solution altered both the anodic and cathodic branches of the polarization curves towards lower current densities. The anodic and cathodic reactions were slowed down, preventing corrosion on MS-37-2.

Electrochemical impedance studies showed that the increase in corrosion resistance and decrease in the double-layer capacitance was due to the continuous replacement of the water molecule by the adsorption of the biosurfactants, which reduces the interfacial tension and forms a protective film on the surface of MS-37-2. The study concluded that the inhibition efficiency depends on the concentration, nature, and type of the surfactant. With increasing the concentration of biosurfactants, the interfacial tension on the metal surface is minimized, thus protecting it from corrosion. The microbial-influenced corrosion by the Pseudomonas species on carbon steel ST37 was investigated using biosurfactants derived from the Bacillus species [49]. They determined the least inhibitory concentration and minimum biofilm inhibitory concentration using the broth microdilution technique, and the corrosion of carbon steel was determined by weight loss measurements. The corrosion rates were determined by antibiofilm assays and they revealed that the biofilm on the metal surface formed by aerobic microorganism Pseudomonas species induces the corrosion by utilizing the oxygen present in the biofilm and results in the formation of anodic sites on the surface. A thin layer of biofilm was found on carbon steel treated with a biosurfactant, and a few bacteria cells were attached to the surface of the layer. Thus, the added biosurfactant eradicated the biofilm and reduced the microbial corrosion of carbon steel.

5.3 RH (Pseudomonas species)

The corrosion inhibition of aluminium alloy in acidic rainwater by RH biosurfactant was investigated [50]. They synthesized RH biosurfactants from the strains of Pseudomonas species and carried out the corrosion test in Aluminium alloy D16T A by electrochemical studies at room temperature. They showed that the aluminium alloy in acidic rainwater was protected when it was immersed in the RH biosurfactant. After 3 h of exposure to acid rain and the addition of biosurfactants at concentrations of 0.1, 0.25, and 0.5 g L⁻¹, the impedance magnitude increases by 0.1 Hz. A similar effect was also observed after 24 h exposure to the metal alloy with a biosurfactant in rainwater. The increase in impedance magnitude represents the formation of a barrier film on the aluminium surface by the biosurfactants. As RHs contain hydroxyl, carboxyl, and carbonyl functional groups, they exist as carboxylate anions in the solution. These anions are bonded to the outer regions of the oxide film through the physical mode of adsorption. In addition, the added RHs form a low soluble complex with the aluminium alloys and are deposited at the anodic sites. They concluded that environmentally safe corrosion inhibitors could be synthesized for aluminium alloy corrosion by using sustainable surfactants.

5.4 Glycolipid (Pseudomonas mosselii F01)

Parthipan et al. investigated the microbial corrosion inhibition using the glycolipid biosurfactant against bacterial strains of Streptomyces parvus B7, B. subtilis A1, Acinetobacter baumannii MN3, and Pseudomonas stutzeri NA3 on carbon steel. The glycolipid-based biosurfactant was synthesized from Pseudomonas mosselii F01 [51]. The rate of corrosion was monitored using weight loss and electrochemical studies. They reported 87% corrosion inhibition using weight loss, decreased corrosion current and potential, and high resistance capacity. The highest level of corrosion inhibition was due to the high energy interaction between the added glycolipid and carbon steel. They proved that glycolipid-based biosurfactants can act both as corrosion inhibitors and biocide. The reason for inhibition was explained by the mechanism of inhibition, as biosurfactants possess anti-bacterial activity: the active microbes present in the biosurfactant absorb onto the corrosive bacterial cell and restrict the biological reaction to take place. It was also attributed to the fact that biosurfactant molecules’ adsorption on the metal surface is that their interaction energy with the metal surface is higher than that of water molecules with the metal surface. Metals in different corrosive environments protected by several microorganisms are provided in Table 2.

5.5 Marino bacter salsuginis

The Bacillus marine bacteria, Marino bacter salsuginis, was employed to study the corrosion behaviour of X80 pipeline steel in marine environments [52]. Electrochemical impedance spectroscopy and polarization studies were carried out for the corrosion monitoring process. Due to the formation of the biofilm, the pipeline steel has higher polarization resistance, charge transfer resistance, and reduced corrosion current density. Bacillus subtilis and Bacillus licheniformis were employed as pitting corrosion inhibitors for aluminium 2024 by electrochemical impedance spectroscopy [53]. They reported that the secretion of 20 amino acids reduced the corrosion rate of the aluminium alloy at pH 6.5. They found that many uniform pits were found on the surface of the alloy in the uninhibited medium, whereas reduced pits in the presence of microorganisms due to the formation of biofilm and the capacitive nature of the impedance spectra were observed using electron microscopes. The presence of carboxylic acid groups decreases the corrosion rates due to the production of polyaspartate and γ-polyglutamate by B. licheniformis.

5.6 RH (P. aeruginosa)

The mechanism of inhibition was explained by the chelation of aluminium ions and carboxylic acid through Coulombic interaction, hydrogen bonds, and dipole–dipole interaction. RH produced by P. aeruginosa was investigated as an effective corrosion inhibitor for X70 carbon steel in seawater [54]. According to the weight loss measurements, they reported that the corrosion rate of carbon steel decreased from 18 to 0.05 mm/year with the addition of 0.1% (m/V). The increase in inhibition rate was attributed to the interaction of hydroxide molecules of RH on the metal surface.

5.7 RH

The corrosion protection efficiency of RH was studied on X65 carbon steel in CO₂-Saturated Oilfield-Produced Water [55] using the gravimetric method and electrochemical measurements. They found an inhibition effect of 90% on increasing the concentration of biosurfactant. From the polarization bipolar branch curves, the cathodic and anodic branches represent only one reaction process but predominate along the anodic side. They also reported that the addition of RHs decreased the corrosion current density, and they act as a mixed type of inhibitor by forming a thick protective layer on the surface of the metal. The interaction of the RHs and metal surface occurs between the rhamnopyranose ring and C═O groups and the vacant d orbitals of carbon steel.

RHs extracted from P. aeruginosa were used against corrosive strains of B. subtilis A1, S. parvus B7, P. stutzeri NA3, and A. baumannii MN3 on concrete rebars in 0.5 N NaCl using electrochemical studies [51]. According to their report, the biosurfactant system included in mixed consortia demonstrated an inhibition efficiency of approximately 87% in the presence of naturally occurring corrosive bacterial consortia. Also, the XRD and experiments on weight loss and electrochemical analysis validate the function of biosurfactants in the corrosion inhibition process and the biosurfactant’s efficacy as a microbial corrosion inhibitor. The electrochemical studies reported that a good corrosion inhibitory effect was observed for carbon steel, which contains RHs and increased corrosion current and charge transfer resistance. Further, the interaction of RHs onto the metal surface was confirmed by the formation of a protective layer with the help of microscopic studies.

Zin et al. [56] investigated the corrosion inhibition efficiency of aluminium alloy by RH bio-complex synthesized from Pseudomonas sp. PS-17 strain using Tafel plots. It was found that the aluminium alloy in the presence of RH showed a pattern of frequency dependence phase angle, which indicates the possible formation of a barrier film on the surface of the metals. Finally, they concluded that an increase in the rate of re-passivation of the aluminium alloy on newly formed surfaces indicates that the RH biocomplex can act as an effective corrosion inhibitor.

5.8 Phospholipids

Phospholipids extracted from vegetable oil waste were used to prevent corrosion on the carbon steel surface in 15% hydrochloric acid. Various chemical surfactants like acetone C.70 (C12-C14 alkyltrimethylammonium chloride), cationic SAA, catapav (C16-C18 alkyldimethylbenzylammonium chloride, cationic surfactant), oxypav (tertiary amine oxides, nonionic surfactant), CP-20 (condensation products of mono- and dialkylphenols with ethylene oxide, nonionic surfactant), and oleylamidopropyldimethylamine (a mixture of zwitterionic surfactant, oleylamidopropyldimethylamine, and sodium chloride) were added as additives with the phospholipids [57]. The corrosion rate was determined using a weight loss method, and its electrochemical corrosion rate was estimated based on the corrosion current density. They reported that phospholipids were found to possess 86.5% inhibition efficiency. The combined use of surfactants and phospholipids tends to show a synergistic effect on the corrosion inhibition efficiency. Phospholipids, when combined with surfactants, tend to show a high level of corrosion protection, up to 95%.

5.9 Penicillium citrinum and Trichoderma

The biosurfactant produced from P. citrinum and Trichoderma, which were isolated from municipal dumpsite, was used as a corrosion inhibitor for mild steel bar in NaCl solution [58]. Corrosion inhibition efficiency was tested using weight loss measurements at different concentrations of the fungi. Mild steel bars were immersed in different concentrations (v/v) of 5, 7.5, and 10% of inhibitors for immersion times of 14, 28, and 100 days. To compare its efficiency, parallelly, Tween 80, a non-organic surfactant, was also used to study the corrosion inhibition property of the mild steel bar in NaCl solution. It was found that the biosurfactant with 10% concentration showed the highest inhibition efficiency of 58.01%, and also, the biosurfactant of Penicillium citrinum was found to be more effective in corrosion protection than the Trichoderma and non-organic surfactant. In addition, they reported that when mild steel bars were immersed in the corrosive media with and without inhibitors, the colour of the solution changed from clear to brownish, and some sludge formation was observed. However, the solution containing biosurfactants does not produce sludge and does not contain any colour. According to the study, biosurfactants not only prevent corrosion but are also used for the bioremediation process [59].

5.10 Pseudomonas fluorescens

The corrosion behaviour of AISI 304 stainless steel was investigated using Gram-negative bacteria Pseudomonas fluorescens in NaCl medium by electrochemical studies [60]. Hiemenz and Rajagopalan determined the surface activity and critical micelle dilution by surface tension measurements using the Wilhelmy plate method [61]. According to them, when the samples are immersed in biosurfactants, the formed oxide layer on the steel surface is more compact, which decreases the density of pits to a higher anodic/cathodic surface area. When the corrosive medium is too aggressive, the corrosion will increase at a faster rate; the formed oxide layer does not nucleate before the biosurfactant gets adsorbed on the metal surface. Hence, they concluded that for better protection efficiency, the surface of the metal must be pre-passivated before the biosurfactant addition.

5.11 Bacillus sp.

Biosurfactant produced by indigenous oil reservoir bacteria Bacillus sp. was aimed to determine the corrosion caused by Pseudomonas sp. 1 and Pseudomonas sp. 2 on Carbon Steel ST37 using the weight loss method [62]. The authors used 15% of glutaraldehyde as a positive control. They revealed that biosurfactant decreased the corrosion rates due to the formation of biofilms, which reduces the microbial-induced corrosion, and the added Bacillus Sp. as an inhibitor able to inhibit the film formation of Pseudomonas sp. 1 and Pseudomonas sp. 2. The intervention effects given by their studies are summarized as follows:

1. Minimum inhibitory concentration of the biosurfactant: 62.5 μg ml⁻¹

2. Minimum biofilm inhibitory concentration (MBIC) of the biosurfactant: 31.25 μg ml⁻¹

3. Minimum biofilm eradication concentration for 50% eradication (MBEC50) of the biosurfactant: 500 μg ml⁻¹

4. Biosurfactant is able to inhibit the attachment of Pseudomonas sp. 1 and Pseudomonas sp. 2 to Carbon Steel ST37 surface

5. Biosurfactant is able to eradicate the preformed biofilm on the steel surface

6. Reduction of corrosion rate in Carbon Steel ST37: from 4.56 × 10⁻⁴ to 3.31 × 10⁻⁴ mm year⁻¹ based on the MBIC value and from 5.18 × 10⁻⁵ mm year⁻¹ to 2.7 × 10⁻⁵ after biofilm eradication at the MBEC value.

The mechanism of inhibition was explained as follows: the biosurfactant prevented certain bacteria from adhering to the steel surface and removed the existing biofilm, which lowered the rate at which Carbon Steel ST37 corroded. This type of biosurfactant can be used as a good anti-corrosion agent in the oil industry.

5.12 Bio-MOF-1

Feng et al. [63] encapsulated benzimidazole in the micropores of bio-MOF-1 and investigated its anti-corrosion durability on mild steel using electrochemical studies. As benzimidazole has hetero atoms with lone pair of electrons, both benzimidazole and bio-MOF-1 showed synergistic corrosion protection. They reported that the impedance arc value decreased when the immersion time increased and showed the highest inhibition efficiency of 85.36% recorded for the encapsulated inhibitor.

5.13 Surfactins and quorum quenching lactonase

MIC on steel coupons was investigated by Huang et al. using some biologically active compounds, such as surfactin, capsaicin, gramicidin, and an enzyme quorum quenching lactonase [64]. They evaluated the effectiveness of different coating additives in reducing the formation of corrosion tubercles on steel surfaces. They found that the coating additives, surfactin and the quorum quenching lactonase, significantly reduced the formation of corrosion tubercles by 31, 36, and 50%, respectively. They demonstrated the potential of highly stable quorum quenching lactonases to provide a reliable, cost-effective method to treat steel structures and prevent biocorrosion. The main intervention effects given by them are summarized as follows: surfactin: reduced the number and coverage of tubercles on steel coupon surfaces by 31 and 37%, respectively. Quorum quenching lactonase reduced the formation of corrosion tubercles by 50%. Bacterial community composition: significant differences were observed in the bacterial communities on experimental and control coupons for all treatments (p < 0.05). The mechanism of inhibition was attributed to the use of a silica gel matrix to encapsulate anti-corrosion biochemicals and enzymes, allowing their activity to last for several months, even after all cells were dead. The enzymatic activity of the silica gel coating, including the lactonase enzyme, was greatly reduced after 42 days of exposure to water. The decrease in the observed corrosion compared to the silica gel coating control was statistically significant for surfactin and the lactonase enzyme. These findings suggest that the encapsulated additives, particularly the lactonase enzyme, play a role in inhibiting corrosion on steel surfaces.

5.14 Pseudomonas fragi

The corrosion inhibition effect of some aerobic bacterium Pseudomonas fragi, facultative anaerobe Escherichia coli DH5α on SAE 1018 carbon steel was investigated by Jayaraman et al. [65]. They reported that the presence of P. fragi or E. coli DH5α resulted in 2- to 10-fold lower mass loss compared to sterile controls. Increasing the temperature from 23 to 30°C resulted in a 2- to 5-fold decrease in corrosion inhibition with P. fragi, whereas the same shift in temperature resulted in a 2-fold increase in corrosion inhibition with E. coli DH5α. Also, in a few steel coupons, corrosion observed with non-biofilm-forming S. lividans TK24 was observed. The mechanism of inhibition was explained as follows: the live cells produce a protective biofilm that inhibits corrosion, resulting in a significant reduction in mass loss when compared to sterile controls. The biofilm, which is made up of both living and dead cells, is embedded in a sparse glycocalyx matrix and is essential for inhibiting corrosion.

6.1 Bioremediation

Bioremediation is a process that uses living organisms to remove or neutralize pollutants from a contaminated environment. Biosurfactants, which are surface-active compounds produced by microorganisms, play a crucial role in enhancing bioremediation and biodeterioration efforts [66,67,68]. Pollutants such as air, soil, and water are universally present in the environment. Many researchers reported the carcinogenic effects of organic pollutants on human health [69,70]. The microbial species take up an efficient role in the eradication of pollution and are involved in the formation of intermediate compounds like glycolipids. These glycolipids play an active role in the bioremediation of pollutants [71,72,73].

It was reported that biosurfactants play a crucial role in dispersing hydrocarbons in the medium [74]. Pseudomonas species produce RH with very low surface tension and alter their cell wall. Man-made activities like manufacturing, mining, and usage of electronic devices have led to the liberation of metals like Ba, Cd, Hg, Ni, Pb, and Sr in the environment [75]. These toxic metals become a major contaminant when it is mixed with water and soil. The contaminated soil sites may be converted into landfill sites in the future. In such cases, this may cause a severe impact on the environment and affect the food chain.

Recently, biosurfactants were employed to remove these pollutants from the environment in order to resolve this issue [77]. When the biosurfactants are added to the environment, they interact with the pollutants through any one of the possible reactions like electrostatic interaction, ion exchange, counter-ion binding, or precipitation dissolution [78]. The added biosurfactants chelate with the metal ion surface and form a micelle [79]. This micelle helps to remove the metals from the contaminated environment [80].

Cadmium metal was removed from the environment by an RH-based biosurfactant [81], and it was found that it was very effective in the removal of cadmium by forming a complex between cadmium(ii) and the added RH, thus reducing its toxicity. Similarly, strontium was removed by adding very low concentration (80 ppm) of RH [82]. It was reported that the biosurfactant lipopeptide was synthesized from C. lipolytica and was employed to remove Cu and Zn. The study found that the added lipopeptide exhibited 96% efficiency in the removal of metals [83]. Singh et al. also found that lipopeptides were also effective in reducing heavy metals like CD, Fe, and Pb [84].

6.2 Pharmaceutical industries

The various functional groups of biosurfactants confer them to possess many potential applications in various fields. The hydrophilic and hydrophobic nature of biosurfactants is supposed to act like detergents on the cell walls of many bacteria and fungi and tend to possess antimicrobial activity [85]. Several biosurfactants have strong anti-bacterial, antifungal, and antiviral activity.

Das et al. reported that the biosurfactants extracted from marine Bacillus circulans exhibit excellent antimicrobial activity against Gram-positive and Gram-negative pathogens [86]. Fernandes et al. [87] investigated the antimicrobial properties of Bacillus subtilis R14 against various bacterial strains. They reported that lipopeptides were very effective against the bacterial strains. Also, it was reported that the biosurfactants were found to control the adhesion of microorganisms on the infected sites of the wound [88]. Thus, the biosurfactants were employed on the wounded surface before the application of medicine in order to prevent the colonization or grouping of microorganisms (Rivardo et al., [89]).

6.3 Nanoparticle synthesis

Nanotechnology is a wide area and attracts the attention of many researchers as it converts matter into the near-atomic scale to produce new structures. It provides many scientific advancements in various fields, such as energy, materials, electronics, coatings, cosmetics, manufacturing, and many other daily activities. Nanomaterials synthesized through nanotechnology are unique due to their physical, chemical, and mechanical properties. Many researchers are focusing on the synthesis of the nanomaterial through various methods, like chemical reduction, co-precipitation, hydrothermal, sol–gel, and micro-emulsion. These methods involve high temperature, pressure, and energy during the synthesis, and they also generate many biologically toxic byproducts as they pollute the environment [90]. Moreover, it faces a main disadvantage of the use of chemicals and self-agglomeration in aqueous solutions [91]. Hence, the bio-fabrication of nanomaterial is a more sustainable, cheap, and eco-friendly method. Biosynthesis of nanoparticles involves the usage of natural plant extracts, polysaccharides, gum exudates, algae, fungi, yeast, etc. [92,93,94]. More recently, researchers have focused on the synthesis of nanomaterials using biosurfactants as they reduce environmental impacts, reduce waste, and improve the stabilization of nanomaterials using sustainable substrates [95]. Biosurfactants are surface-active compounds that possess both hydrophilic and hydrophobic properties. They are also called surface-active biomolecules, which are mainly synthesized from microorganisms, and they tend to show exceptional properties like low toxicity, specificity, and relative ease of synthesis [96]. Also, when compared with chemical surfactants, biosurfactants have several advantages, such as environmental cleaning by biodegradation and depollution of industrial effluents [97,98,99].

Silver nanoparticles were synthesized using a biosurfactant produced by Bacillus cereus UCP [100]. The synthesized nanoparticles were characterized using UV-VIs spectroscopy, scanning electron microscopy, and zeta potential methods. The study reported that the average size of their obtained nanoparticles was 20 nm. And it was found that the synthesized nanoparticles were found to possess 85% inhibition against P. fellutanum and A. niger.

Nickel oxide nanorods were synthesized using commercial RH as a stabilizer. The synthesized nanorods were found to possess a size of 22 nm in diameter [101]. Silver nanoparticles were characterized by UV-vis absorption spectroscopy. The morphology was found to be spherical by TEM and SEM.

Water-soluble RHs from P. aeruginosa were employed to synthesize ZnS spherical nanoparticles [102]. They obtained the nanoparticle in the size of 4.5 nm.

Polyaniline nanofibres and nanotubes were synthesized using RHs [103]. They reported that when comparing the synthesis of PAN with other additives, the PAN synthesized using RH showed uniform morphology and size.