https://orcid.org/0000-0001-8329-5526
Abstract
Introduction: This review article gives an overview of the biogenic synthesis of metal nanoparticles (mNPs) while using Litchi chinensis extract as a reducing and stabilizing agent. The subtropical fruit tree i.e lychee contains phytochemicals such as flavonoids, terpenoids, and polyphenolic compounds which act as reducing agents and convert the metal ions into metal atoms that coagulate to form mNPs. Methodology: Different methodologies adopted for the synthesis of lychee extract and its use in the fabrication of mNPs under different reaction conditions such as solvent, extract amount, temperature, and pH of the medium have also been discussed critically in detail. Techniques: Different techniques such as FTIR, UV–visible, XRD, SEM, EDX, and TEM adopted for the analysis of biogenic synthesis of mNPs have also been discussed in detail.
Applications of mNPs: Applications of these prepared mNPs in various fields due to their antimicrobial, antiinflammatory, anticancer, and catalytic activities have also been described in detail.
Introduction
Metal nanoparticles (mNPs) have gained great importance because of their vast applications in many fields such as electronics, catalysis, 1 medicine, wastewater, environmental science, and biomedical sensing,2 etc. In pharmaceuticals, these particles act as therapeutic agents for the treatment of cancer and various other diseases. Removal of contaminants from water and its purification by mNPs via adsorption, catalysis, and antimicrobial agents falls in environmental sustainability. Nowadays, due to their vast applications, they are gaining the focus of scientists in technological advancements. These applications are based on their distinct characteristics such as structure, size in the range of 1–100 nm, with enhanced physical, chemical, and biological properties.1
mNPs can be synthesized by physical, chemical, and biological methods. However, these physical and chemical methods have some drawbacks such as slow process, less effective, costly,3 and use of a large number of chemicals along with harsh reaction conditions. Some solvents used in these methods are hazardous to human health and the environment. So, these methods are not eco-friendly. The solution to this problem is the green synthesis method4,5 which is solvent-free and eco-friendly. In green synthesis, biological processes are adopted to synthesize mNPs while using biological extract as a reducing and stabilizing agent.
Different biological processes include the use of extract obtained from algae, fungi, and plants or the use of different enzymes and bacteria etc. as reducing agents to convert metal ions into metal atoms. These are chemical-free and environmentally friendly methods for the fabrication of mNPs. Plants are important among reported biological sources and have different parts such as leaves, seeds, roots, stems and fruit peels, etc which are used to prepare plant extract. The most valuable biological method is using biological waste such as fruit peels to prepare extract due to their easy availability and use for beneficial purposes. Fruit peels of lychee, a subtropical fruit tree are discarded/dumped into the soil without any use. Lychee fruit peels consist of flavonoids, terpenoids, and polyphenolic compounds. These are important phytochemicals enriched with hydroxyl groups. These hydroxyl groups act as reducing agents and convert the metal ions to the metal atoms that coagulate to form mNPs. Their long polymer chains provide the additional support to cadge mNPs and prolong their life span. Therefore, the use of different parts of lychee such as fruit peel,6 seed,7 and leaf extract1 for the fabrication of mNPs has been reported and gained much attention due to easy availability.
Shende et al. reported the synthesis of AgNPs and AuNPs by using lychee fruit peel extract as a reducing/stabilizing agent while AgNO3 and HAuCl4 as sources of Ag+ and Au+3 ions, respectively.6 Water was used as a solvent during the preparation of the peel extract. They observed that the SPR band associated with Ag and AuNPs was red-shifted and increased in intensity with the increase in reaction time. HRTEM analysis confirmed the synthesis of spherical shaped mNPs.
Singh and Gupta also reported the successful synthesis of AgNPs from lychee fruit peel extract.8 The extract was prepared in water as a solvent while AgNO3 was used as a source of Ag+ ions to produce silver NPs. They observed that microwave irradiation of silver nitrate solution and peel extract for 1 min converted the solution to a dark brown color indicating the successful fabrication of AgNPs. Surface Plasmon resonance (SPR) band appeared at 371 nm in UV–visible spectra further confirming the formation of AgNPs. They also observed that prepared AgNPs were found more efficient antibacterial agents against Escherichia coli and Bacillus subtilis as investigated by the disc diffusion method.
Here, we cover the overview of the synthesis of different types of metal nanoparticles by using lychee extract as a reducing/stabilizing agent via different methods. Section 1 covers the brief introduction of mNPs and their synthesis via different methods along with the use of lychee as a source of phytochemicals for this purpose. Section 2 covers the different methods for the preparation of lychee extract using different solvents. Section 3 describes the synthesis of mNPs by using lychee extract as a reducing/stabilizing agent along with their characterization by different techniques. Section 4 covers their use as adsorbents and catalysts in wastewater treatment along with their uses in biomedical field.
Preparation of lychee extract and its use in the preparation of metal nanoparticles
Lychee is rich source of phytochemicals such as flavonoids condensed tannins, luteolin, anthocyanins, proanthocyanidins, terpenoids, ketones, carboxylic acids, erucic acid, geranyl isovelarate, 2-hexadecanol, phenolics, reducing sugars, starch, ascorbic acid, citric acid and aldehydes which do not only reduce the metal ions but also contribute towards the functional properties and stabilization of mNPs.9,10 The hydroxyl groups in phenolics, tannins, flavonoids, and ascorbic acid can act as reducing agents by donating electrons to metal ions and converting them into metal atoms which further coagulate to form mNPs. The amine, carboxyl, sulfhydryl, and hydroxyl groups in reducing sugars, starch, and proanthocyanidins can bind to metal surfaces via covalent or electrostatic interactions and stabilize them by forming a protective layer around mNPs.6 So, different parts of lychee such as fruit peel,11 and leaves1 are reported for the preparation of extract which is alternatively used for the synthesis of different types of mNPs.
The preparation of lychee extract is a key step in fabricating the metal nanoparticles. In this step, extractions of bioactive compounds from lychee peel are required. Researchers reported the presence of thirty-two different major compounds in peel extract. These compounds include erucic acid, geranyl isovalerate, 2-hexadecanol, ɑ-acrenol tetra decane etc.6 These compounds can act as reducing agents to reduce metal ions. Different solvents such as ethanol, methanol, and water in different ratios have been reported for the extraction of bioactive compounds present in lychee.8,12,13 Thus, the choice of extraction solvent, temperature, and pH of the medium are crucial factors for the preparation of extract enriched with bioactive compounds. Polar compounds are extracted by using water and the less polar compounds are extracted by using organic solvents like ethanol or methanol etc. However, a broad mixture of solvents is required to improve the variety of extracted phytochemicals. Thus, the polarity of solvent is also a crucial factor that plays a vital role in their use for the synthesis of mNPs. An ideal solvent should be chosen which don’t have any interactions with biomolecules during plant extract preparation.
For the preparation of L. chinensis fruit peel extract, peels are washed with tap water first and then with deionized water to remove any dust particles. These leaves are dried at room temperature to make them suitable for grinding. After that leaves were pulverized, ground into powder, added in distilled water,14 and boiled to prepare extract. The solution turns pink with the passage of time. Then the prepared extract is filtered through Whatman no. 1 filter paper. Sometimes, the extract is centrifuged to remove the heavier biomacromolecules.8 The prepared plant extract is stored at ambient temperature or sometimes at low temperature 4 °C to keep it active for further use. Schematic representation for the preparation of lychee extract is shown in Fig. 1.
Diagrammatic representation for the preparation of lychee extract and its use in the synthesis of metal nanoparticles.
Various mono metal nanoparticles such as AgNPs, AuNPs, SeNPs, ZnONPs, CoONPs, Fe3O4NPs15 NPs and bimetallic NPs such as Ag-Au,14,16 Ag-SeNPs7 have been prepared by using their salts as a source of metal ions and lychee peel extract as reducing and stabilizing agents as reported by different researchers.6,9,11 These mNPs can be prepared at room temperature, in the dark, heated at elevated temperature, or under microwave irradiation.1,9,10 The preparation of mNPs is performed in the dark because some compounds with hydroxyl functional groups, responsible for the reduction of metal ions present in extract are photosensitive such as flavonoids. These compounds can decompose when exposed to sunlight and result in decreased reducing/stabilizing activities.
Fabrication reactions performed in the dark also prevent the undesired photochemical reactions that could change the properties of mNPs such as their shape, size, and stability. Moreover, the absence of light and performing the reaction at room temperature slow down the reaction rate providing more suitable environment and sufficient time to obtain mNPs with desired properties. The mNPs obtained in the dark at room temperature are uniformed-size which may lead to improved properties such as catalysis and antimicrobial activities. mNPs are synthesized by heating the lychee peel extract and the aqueous solution of the metal precursor. The formation of mNPs is indicated by a visible color change in the reaction mixture.6 The reaction mixture is then centrifuged and pellets of mNPs are collected from any uncoordinated/unwanted biological materials.1 Different parameters such as reaction time, temperature, the concentration of metal precursor, pH, and incubation time in the dark can control the properties of mNPs.13 Thus, these mNPs have shown useful applications due to their distinct physicochemical characteristics.
Shende et al. reported the synthesis of AgNPs and AuNPs by using AgNO3 and HAuCl3 as metal precursors respectively and lychee peel extract.6 Peel extract was prepared by using dried powder of lychee peels along with water under heating until the mixture was turned pink in color. After that, the mixture was filtered and centrifuged to remove unwanted moieties. mNPs were prepared by shaking the reaction mixture containing peel extract and precursor salt at 40 °C for 5 h in a shaker incubator. Then the reduction of respective metal ions and formation of mNPs was confirmed due to the color change and gradual appearance of peak at 430 and 540 nm for Ag and AuNPs, respectively as monitored by UV–visible spectroscopy. They also studied the effect of reaction conditions such as temperature and salt concentration on the synthesis of mNPs. Temperature was changed from 4 to 50 °C while salt concentration was varied from 0.3 to 5.0 mM. They observed that pale yellow coloured AgNPs were obtained when the reaction proceeded for 5 h while using 1 mM solution of AgNO3 at 40 °C. AuNPs were also successfully fabricated under the same reaction conditions with the colour of reaction mixture turned ruby red from pale red and fabrication of NPs was completed in 2 h.
They observed no sharp difference in rate of fabrication of mNPs in the first 30 min, at different temperatures. However, the rate of formation of mNPs was enhanced abruptly with the rise of temperature up to 50 °C both in the case of Ag and AuNPs. Similarly, a good amount of Ag and AuNPs was obtained when 5 mM of their respective salts were used. They also performed phytochemical estimation of the extract by different analytical assays and confirmed the presence of phytochemicals as shown in Fig. 2.
Phytochemical composition of lychee peel extract.
The reduced mNPs were characterized using various techniques such as HRTEM, EDS, DLS, XRD and FTIR. They evaluated the formation of AgNPs induced by lychee peel extract via UV–visible spectra. The peak was obtained at 430 nm which showed the successful formation of silver NPs and the peak at 540 nm showed the successful formation of gold nanoparticles. They observed that AgNPs were monodispersed and spherical while AuNPs were exotic in shape as estimated by HRTEM analysis. Particles were discrete and well dispersed without any agglomeration. XRD showed cubic structure and DLS demonstrated that the size of the particles was in agreement with HRTEM results. EDX analysis confirmed the presence of Ag and Au elements in the prepared samples. They also performed FTIR analysis to recognize functional groups of different biomolecules responsible for the stabilization of mNPs. The broad peak at 3,350 cm−1 showed the hydroxyl (-OH) group of phenols/alcohol. The peaks at 1,610 cm−1 did not show any prominent shift which attributed to the C=C group. The peak of the L. chinensis peel extract (LCPE) at 2,923 cm−1 (C-H stretch) was seen to be diminished while new peaks appeared at 1,739 cm−1.
Murad et al.12 and Kaur et al.14,16 also reported the preparation of Ag and AuNPs by using lychee peel extract as a reducing and stabilizing agent and investigated their applications in different fields. Lychee extract used for the fabrication of different mNPs and their uses in different fields are given in detail in Table 1.
Use of lychee extract for synthesis of different mNPs, their characterization, and applications.
| Metal nanoparticles | Characterization | Applications | References |
|---|---|---|---|
| AgNPs | Uv–visible analysis confirmed the formation of mNPs by the appearance of the SPR band at 450 nm. XRD analysis confirmed the crystalline nature of the nanoparticles. TEM analysis indicated that nanoparticles were found in the range of 9–50 nm. FTIR analysis confirmed the presence of N-H bond as part of amines and O-H group as part of alcohols, phenols, and carbohydrates. | Photocatalytic potential of AgNPs was investigated against degradation of methylene blue (MB) dye. 47.80% dye was degraded under sunlight for 24 h in the presence of AgNPs catalyst. The degradation rate was increased with the increase of irradiation time. AgNPs were also found effective against gram positive bacteria such as S. aureus and Bacillus subtilis and gram-negative bacteria, Salmonella typhi. | 1 |
| AgNPs | UV–vis analysis SPR peak at 422 nm and confirmed the formation of AgNPs. FTIR spectra confirmed the presence of biological molecules involved in AgNPs fabrication such as O-H group of phenolic compounds = C-H group of carbohydrates, N-H group of proteins, C=O, C=C and C-N functional group. TEM analysis showed that AgNPs were found in spherical shape in range of about 5 to 15 nm. | The bactericidal activity of mNPs was investigated against B. subtilis by the well diffusion method. The results revealed significant sporicidal activity in a range of 4.46–22.32% and 4.46–61.6% at 100 and 200 μg/mL of concentration of AgNPs during the exposure time of 0–8 h. | 10 |
| AgNPs | UV-analysis showed the shift in peak from 402 to 419, 404, 428 nm when antibiotics were added in NPs suspension. It shows their conjugation with antibiotics (amoxicillin, cefixim and streptomycin). FTIR analysis demonstrated the slight change in peak position on interaction of antibiotics with mNPs. XRD spectrum showed mean crystalline size of the silver nanoparticles found to be 3.9 ± 0.848 nm and their conjugates showed mean size of 9.9 ± 5.400 nm (amoxicillin), 7.15 ± 2.757 nm (cefixim) and 5.8 ± 3.091 nm (streptomycin), respectively. TEM analysis indicated the formation of spherical-shaped AgNPs. | Effective anti-bacterial properties against both gram-positive and gram-negative bacteria but they showed more effect on conjugation with selected antibiotics against gram-negative type bacteria. | 17 |
| ZnO NPs | UV–vis analysis showed that maximum absorption peak for ZnO NPs was obtained at 322 nm. XRD analysis revealed peaks with 2θ values of 31.9°, 34.8°, 36.6°, 48.5°, 56.9°, 66.6° and 69.1° which indicated the crystalline ZnO NPs. XRD also confirmed the position of Zn and O in hexagonal structure. FTIR analysis confirmed the presence of OH group vibrations due to moisture, C-OH stretching vibrations, C-H stretching, primary and secondary amines of proteins or enzymes and vibrational stretching of Zn-O between 460 cm−1. | Anti-bacterial performance against both gram-positive and gram-negative bacteria and as an adsorbent for removal of Congo red (CR) dye from wastewater. | 11 |
| AgNPs | UV–vis spectroscopy confirmed the synthesis of AgNPs by showing SPR band at 453 nm. XRD confirmed the face-centered cubic crystalline property of AgNPs at 2θ values ranging from 20°–70°. EDX showed strong signal energy peaks for silver atoms in the range of 3 keV. Selected area electron diffraction (SAED) analysis showed three bright circular rings which confirmed the (111), (200), (220) Bragg’s reflection planes of XRD. Size, shape and morphology of AgNPs was determined by HRTEM in the range of 4–8 nm. FTIR analysis confirmed various functional groups responsible for capping and stabilization of AgNPs such as –OH group, sp2 C-H vibration, C-C=C symmetric stretching vibration of aromatic rings, and C-O stretch. | Photocatalytic, antioxidant, and antimicrobial activities. The biosynthesized AgNPs showed efficient antimicrobial activities against Staphylococcus aureus, Bacilllus subtilis, and Escherichia coli. While commercial AgNPs showed moderate activity. The antioxidant activity of AgNPs was assessed by DPPH scavenging assay and the biosynthesized AgNPs showed effective free radical inhibition while commercial AgNPs showed moderate activity. Similary biogenic synthesized AgNPs showed superior photocatalytic activity in reducing MB compared to commercial AgNPs due to surface coated biomolecules in biogenic AgNPs which behaved as reservoir of electrons that kept the metal in reduced form. | 18 |
| Metal nanoparticles | Characterization | Applications | References |
|---|---|---|---|
| AgNPs | Uv–visible analysis confirmed the formation of mNPs by the appearance of the SPR band at 450 nm. XRD analysis confirmed the crystalline nature of the nanoparticles. TEM analysis indicated that nanoparticles were found in the range of 9–50 nm. FTIR analysis confirmed the presence of N-H bond as part of amines and O-H group as part of alcohols, phenols, and carbohydrates. | Photocatalytic potential of AgNPs was investigated against degradation of methylene blue (MB) dye. 47.80% dye was degraded under sunlight for 24 h in the presence of AgNPs catalyst. The degradation rate was increased with the increase of irradiation time. AgNPs were also found effective against gram positive bacteria such as S. aureus and Bacillus subtilis and gram-negative bacteria, Salmonella typhi. | 1 |
| AgNPs | UV–vis analysis SPR peak at 422 nm and confirmed the formation of AgNPs. FTIR spectra confirmed the presence of biological molecules involved in AgNPs fabrication such as O-H group of phenolic compounds = C-H group of carbohydrates, N-H group of proteins, C=O, C=C and C-N functional group. TEM analysis showed that AgNPs were found in spherical shape in range of about 5 to 15 nm. | The bactericidal activity of mNPs was investigated against B. subtilis by the well diffusion method. The results revealed significant sporicidal activity in a range of 4.46–22.32% and 4.46–61.6% at 100 and 200 μg/mL of concentration of AgNPs during the exposure time of 0–8 h. | 10 |
| AgNPs | UV-analysis showed the shift in peak from 402 to 419, 404, 428 nm when antibiotics were added in NPs suspension. It shows their conjugation with antibiotics (amoxicillin, cefixim and streptomycin). FTIR analysis demonstrated the slight change in peak position on interaction of antibiotics with mNPs. XRD spectrum showed mean crystalline size of the silver nanoparticles found to be 3.9 ± 0.848 nm and their conjugates showed mean size of 9.9 ± 5.400 nm (amoxicillin), 7.15 ± 2.757 nm (cefixim) and 5.8 ± 3.091 nm (streptomycin), respectively. TEM analysis indicated the formation of spherical-shaped AgNPs. | Effective anti-bacterial properties against both gram-positive and gram-negative bacteria but they showed more effect on conjugation with selected antibiotics against gram-negative type bacteria. | 17 |
| ZnO NPs | UV–vis analysis showed that maximum absorption peak for ZnO NPs was obtained at 322 nm. XRD analysis revealed peaks with 2θ values of 31.9°, 34.8°, 36.6°, 48.5°, 56.9°, 66.6° and 69.1° which indicated the crystalline ZnO NPs. XRD also confirmed the position of Zn and O in hexagonal structure. FTIR analysis confirmed the presence of OH group vibrations due to moisture, C-OH stretching vibrations, C-H stretching, primary and secondary amines of proteins or enzymes and vibrational stretching of Zn-O between 460 cm−1. | Anti-bacterial performance against both gram-positive and gram-negative bacteria and as an adsorbent for removal of Congo red (CR) dye from wastewater. | 11 |
| AgNPs | UV–vis spectroscopy confirmed the synthesis of AgNPs by showing SPR band at 453 nm. XRD confirmed the face-centered cubic crystalline property of AgNPs at 2θ values ranging from 20°–70°. EDX showed strong signal energy peaks for silver atoms in the range of 3 keV. Selected area electron diffraction (SAED) analysis showed three bright circular rings which confirmed the (111), (200), (220) Bragg’s reflection planes of XRD. Size, shape and morphology of AgNPs was determined by HRTEM in the range of 4–8 nm. FTIR analysis confirmed various functional groups responsible for capping and stabilization of AgNPs such as –OH group, sp2 C-H vibration, C-C=C symmetric stretching vibration of aromatic rings, and C-O stretch. | Photocatalytic, antioxidant, and antimicrobial activities. The biosynthesized AgNPs showed efficient antimicrobial activities against Staphylococcus aureus, Bacilllus subtilis, and Escherichia coli. While commercial AgNPs showed moderate activity. The antioxidant activity of AgNPs was assessed by DPPH scavenging assay and the biosynthesized AgNPs showed effective free radical inhibition while commercial AgNPs showed moderate activity. Similary biogenic synthesized AgNPs showed superior photocatalytic activity in reducing MB compared to commercial AgNPs due to surface coated biomolecules in biogenic AgNPs which behaved as reservoir of electrons that kept the metal in reduced form. | 18 |
Use of lychee extract for synthesis of different mNPs, their characterization, and applications.
| Metal nanoparticles | Characterization | Applications | References |
|---|---|---|---|
| AgNPs | Uv–visible analysis confirmed the formation of mNPs by the appearance of the SPR band at 450 nm. XRD analysis confirmed the crystalline nature of the nanoparticles. TEM analysis indicated that nanoparticles were found in the range of 9–50 nm. FTIR analysis confirmed the presence of N-H bond as part of amines and O-H group as part of alcohols, phenols, and carbohydrates. | Photocatalytic potential of AgNPs was investigated against degradation of methylene blue (MB) dye. 47.80% dye was degraded under sunlight for 24 h in the presence of AgNPs catalyst. The degradation rate was increased with the increase of irradiation time. AgNPs were also found effective against gram positive bacteria such as S. aureus and Bacillus subtilis and gram-negative bacteria, Salmonella typhi. | 1 |
| AgNPs | UV–vis analysis SPR peak at 422 nm and confirmed the formation of AgNPs. FTIR spectra confirmed the presence of biological molecules involved in AgNPs fabrication such as O-H group of phenolic compounds = C-H group of carbohydrates, N-H group of proteins, C=O, C=C and C-N functional group. TEM analysis showed that AgNPs were found in spherical shape in range of about 5 to 15 nm. | The bactericidal activity of mNPs was investigated against B. subtilis by the well diffusion method. The results revealed significant sporicidal activity in a range of 4.46–22.32% and 4.46–61.6% at 100 and 200 μg/mL of concentration of AgNPs during the exposure time of 0–8 h. | 10 |
| AgNPs | UV-analysis showed the shift in peak from 402 to 419, 404, 428 nm when antibiotics were added in NPs suspension. It shows their conjugation with antibiotics (amoxicillin, cefixim and streptomycin). FTIR analysis demonstrated the slight change in peak position on interaction of antibiotics with mNPs. XRD spectrum showed mean crystalline size of the silver nanoparticles found to be 3.9 ± 0.848 nm and their conjugates showed mean size of 9.9 ± 5.400 nm (amoxicillin), 7.15 ± 2.757 nm (cefixim) and 5.8 ± 3.091 nm (streptomycin), respectively. TEM analysis indicated the formation of spherical-shaped AgNPs. | Effective anti-bacterial properties against both gram-positive and gram-negative bacteria but they showed more effect on conjugation with selected antibiotics against gram-negative type bacteria. | 17 |
| ZnO NPs | UV–vis analysis showed that maximum absorption peak for ZnO NPs was obtained at 322 nm. XRD analysis revealed peaks with 2θ values of 31.9°, 34.8°, 36.6°, 48.5°, 56.9°, 66.6° and 69.1° which indicated the crystalline ZnO NPs. XRD also confirmed the position of Zn and O in hexagonal structure. FTIR analysis confirmed the presence of OH group vibrations due to moisture, C-OH stretching vibrations, C-H stretching, primary and secondary amines of proteins or enzymes and vibrational stretching of Zn-O between 460 cm−1. | Anti-bacterial performance against both gram-positive and gram-negative bacteria and as an adsorbent for removal of Congo red (CR) dye from wastewater. | 11 |
| AgNPs | UV–vis spectroscopy confirmed the synthesis of AgNPs by showing SPR band at 453 nm. XRD confirmed the face-centered cubic crystalline property of AgNPs at 2θ values ranging from 20°–70°. EDX showed strong signal energy peaks for silver atoms in the range of 3 keV. Selected area electron diffraction (SAED) analysis showed three bright circular rings which confirmed the (111), (200), (220) Bragg’s reflection planes of XRD. Size, shape and morphology of AgNPs was determined by HRTEM in the range of 4–8 nm. FTIR analysis confirmed various functional groups responsible for capping and stabilization of AgNPs such as –OH group, sp2 C-H vibration, C-C=C symmetric stretching vibration of aromatic rings, and C-O stretch. | Photocatalytic, antioxidant, and antimicrobial activities. The biosynthesized AgNPs showed efficient antimicrobial activities against Staphylococcus aureus, Bacilllus subtilis, and Escherichia coli. While commercial AgNPs showed moderate activity. The antioxidant activity of AgNPs was assessed by DPPH scavenging assay and the biosynthesized AgNPs showed effective free radical inhibition while commercial AgNPs showed moderate activity. Similary biogenic synthesized AgNPs showed superior photocatalytic activity in reducing MB compared to commercial AgNPs due to surface coated biomolecules in biogenic AgNPs which behaved as reservoir of electrons that kept the metal in reduced form. | 18 |
| Metal nanoparticles | Characterization | Applications | References |
|---|---|---|---|
| AgNPs | Uv–visible analysis confirmed the formation of mNPs by the appearance of the SPR band at 450 nm. XRD analysis confirmed the crystalline nature of the nanoparticles. TEM analysis indicated that nanoparticles were found in the range of 9–50 nm. FTIR analysis confirmed the presence of N-H bond as part of amines and O-H group as part of alcohols, phenols, and carbohydrates. | Photocatalytic potential of AgNPs was investigated against degradation of methylene blue (MB) dye. 47.80% dye was degraded under sunlight for 24 h in the presence of AgNPs catalyst. The degradation rate was increased with the increase of irradiation time. AgNPs were also found effective against gram positive bacteria such as S. aureus and Bacillus subtilis and gram-negative bacteria, Salmonella typhi. | 1 |
| AgNPs | UV–vis analysis SPR peak at 422 nm and confirmed the formation of AgNPs. FTIR spectra confirmed the presence of biological molecules involved in AgNPs fabrication such as O-H group of phenolic compounds = C-H group of carbohydrates, N-H group of proteins, C=O, C=C and C-N functional group. TEM analysis showed that AgNPs were found in spherical shape in range of about 5 to 15 nm. | The bactericidal activity of mNPs was investigated against B. subtilis by the well diffusion method. The results revealed significant sporicidal activity in a range of 4.46–22.32% and 4.46–61.6% at 100 and 200 μg/mL of concentration of AgNPs during the exposure time of 0–8 h. | 10 |
| AgNPs | UV-analysis showed the shift in peak from 402 to 419, 404, 428 nm when antibiotics were added in NPs suspension. It shows their conjugation with antibiotics (amoxicillin, cefixim and streptomycin). FTIR analysis demonstrated the slight change in peak position on interaction of antibiotics with mNPs. XRD spectrum showed mean crystalline size of the silver nanoparticles found to be 3.9 ± 0.848 nm and their conjugates showed mean size of 9.9 ± 5.400 nm (amoxicillin), 7.15 ± 2.757 nm (cefixim) and 5.8 ± 3.091 nm (streptomycin), respectively. TEM analysis indicated the formation of spherical-shaped AgNPs. | Effective anti-bacterial properties against both gram-positive and gram-negative bacteria but they showed more effect on conjugation with selected antibiotics against gram-negative type bacteria. | 17 |
| ZnO NPs | UV–vis analysis showed that maximum absorption peak for ZnO NPs was obtained at 322 nm. XRD analysis revealed peaks with 2θ values of 31.9°, 34.8°, 36.6°, 48.5°, 56.9°, 66.6° and 69.1° which indicated the crystalline ZnO NPs. XRD also confirmed the position of Zn and O in hexagonal structure. FTIR analysis confirmed the presence of OH group vibrations due to moisture, C-OH stretching vibrations, C-H stretching, primary and secondary amines of proteins or enzymes and vibrational stretching of Zn-O between 460 cm−1. | Anti-bacterial performance against both gram-positive and gram-negative bacteria and as an adsorbent for removal of Congo red (CR) dye from wastewater. | 11 |
| AgNPs | UV–vis spectroscopy confirmed the synthesis of AgNPs by showing SPR band at 453 nm. XRD confirmed the face-centered cubic crystalline property of AgNPs at 2θ values ranging from 20°–70°. EDX showed strong signal energy peaks for silver atoms in the range of 3 keV. Selected area electron diffraction (SAED) analysis showed three bright circular rings which confirmed the (111), (200), (220) Bragg’s reflection planes of XRD. Size, shape and morphology of AgNPs was determined by HRTEM in the range of 4–8 nm. FTIR analysis confirmed various functional groups responsible for capping and stabilization of AgNPs such as –OH group, sp2 C-H vibration, C-C=C symmetric stretching vibration of aromatic rings, and C-O stretch. | Photocatalytic, antioxidant, and antimicrobial activities. The biosynthesized AgNPs showed efficient antimicrobial activities against Staphylococcus aureus, Bacilllus subtilis, and Escherichia coli. While commercial AgNPs showed moderate activity. The antioxidant activity of AgNPs was assessed by DPPH scavenging assay and the biosynthesized AgNPs showed effective free radical inhibition while commercial AgNPs showed moderate activity. Similary biogenic synthesized AgNPs showed superior photocatalytic activity in reducing MB compared to commercial AgNPs due to surface coated biomolecules in biogenic AgNPs which behaved as reservoir of electrons that kept the metal in reduced form. | 18 |
Hussain reported the synthesis of AgNPs by using lychee peel extract and AgNO3 solution.13 Two solutions were prepared and the effect of different parameters such as temperature (25 °C to 55 °C), incubation time (1–72 h), pH (4,7,8), silver nitrate concentration (0.1 mM to 2.0 mM) on reduction of silver ions to AgNPs was investigated. The contact time of silver nitrate salt and lychee extract directly controls the stability and uniformity of AgNPs. It was observed that increased incubation time offers more time for the nucleation of mNPs and to stabilize them which results in increased size of nanoparticles. In optimized conditions, excessive long time may aggregate nanoparticles and alleviate their uniformity. Temperature impacts the speed of reduction. At lower temperature, the reaction rate was slow, and at high temperature, reaction rate was good and promoting the growth of NPs speedily but too high temperature may cause the agglomeration of mNPs. Temperature variation from 35 °C to 45 °C favours the good amount of mNPs along with the controlled size and morphology. pH range of 4.5–7 was found to be favourable for synthesis of mNPs. The concentration of AgNO3 from 0.1 mM-1.0 mM offers enough silver ions for the formation of a good quantity of AgNPs.
Onwudiwe reported the synthesis of CoO NPs by using lychee peel extract via two methods i.e. boiling process and microwave irradiation.9 In the boiling method, L. chinensis extract and cobalt acetate solution were mixed, stirred, and heated at 80° C until the mixture was turned into a visible dark green color. The change in colour confirmed the successful preparation of Co(OH)2 NPs formation. While, in microwave irradiation, extract and cobalt acetate solution were ultra-sonicated to homogenized solution. The reaction mixture was then put in Teflon-lined microwave reactor vessel and microwaved which also indicated the color change and cobalt hydroxide NPs were confirmed. In both methods, calcination was done to obtain CoO NPs from Co(OH)2 NPs. So, the obtained Co(OH)2 was calcined at 200 °C, resulting into its decomposition and loss of water to obtain Co3O4 NPs. These nanoparticles were characterized by XRD, FTIR SEM, and TEM techniques. XRD showed that the crystalline size of synthesized NPs obtained from boiling method was ≈24.25 nm and the crystalline size of NPs obtained from microwave irradiation was ≈30.28 nm which was higher than boiling method. The functional biomolecules present in fruit peel extract were confirmed by FTIR analysis between 4,000 and 400 cm−1. FTIR spectrum showed broad peaks around 3,331 cm−1 which corresponded to O–H stretching vibration, while the strong peaks around 1,552 and 1,583 cm−1 corresponded to the O–H bending vibration. The only bands found around 654 and 551 cm−1 were due to the Co–O vibration of the oxidized metal which confirmed the dehydration of the cobalt hydroxide and the decomposition of the peel extract to form the oxide. They monitored that TEM confirmed the spherical morphologies and monodispersity of cobalt hydroxide nanoparticles and cobalt oxide nanoparticles showed an elongated rod-like morphology.
Different other researchers also reported the preparation of different mNPs by using lychee extract as a reducing and stabilizing agent and investigated their applications in different fields.19,20
Applications
Wastewater treatment
The most valuable asset for human beings and other living things on earth is water. The major threat to this valuable asset is industrial wastes including heavy metal ions, and dyes.21 Dyes are used by many industries such as food, plastic, textile, leather, and pharmaceutical.22 Dyes are produced yearly at large scale and 15% of these dyes are thrown in water bodies during the dyeing process and cause serious environmental and health issues.23 Compounds containing nitrogen such as nitrobenzene, aniline are major cause of water pollution.24 Industrial wastes contain most of these compounds. These compounds are deteriorating the water gradually due to the agricultural and human activities.25 Industrial and domestic waste also contain toxic substances such as dyes, nitrogen containing compounds and other pollutants. They have adverse effects on animals, human beings and aquatic life.26 They can cause skin irritation, nausea, respiratory tract problem, cancer, dermatitis, ulceration of skin, diarrhoea and painful colic when uptake by living organisms.27,28 Different methods have been developed to remove these toxic pollutants from aquatic environments such as adsorption, biodegradation, flocculation, photocatalytic degradation, oxidation and catalytic reduction but they have some drawbacks due to high cost and less efficiency.21,29 Among these methods, adsorption and catalytic methods have gained much attention for the removal of toxic pollutants in aquatic medium. Best way to get rid of these pollutants from aquatic environment is to reduce them catalytically into eco-friendly products in the presence of reducing agent such as NaBH4 or irradiation under light.1,6 The degradation of toxic pollutants by the catalytic reduction method using some chemicals as reducing agents is a fast and less time-consuming process. But this method consumes some chemicals during reaction that can have some drawbacks. Photocatalytic reduction is a slow process but it is environmentally friendly because no chemical moieties are required, only sunlight is utilized for degradation purpose. The adsorption method can also be used for the degradation of these toxic pollutants into less toxic substances in which a suitable adsorbent is required to capture these pollutants from wastewater. Yet, the adsorption process transfers the pollutants from one medium to another and doesn’t find any permanent treatment process. Lychee extract-induced synthesized bio-inorganic nanoparticles find applications as catalysts and adsorbent for the treatment of different types of pollutants present in wastewater.
Use as an adsorbent
In recent years, adsorption process has shown effective results in wastewater treatment in different industries. A number of mNPs or metal oxide NPs are used as adsorbents to treat contaminants present in an aqueous medium.1,11 Biogenic synthesized mNPs are used as adsorbent for the removal of different pollutants from wastewater due to various factors such as low cost of preparation of mNPs, low material consumption, reusability of mNPs adsorbent and excellent properties of adsorbent (high surface area and high adsorbent capacity).30 High surface properties of mNPs are crucial factor for their use as an adsorbent. Various factors affect the adsorption capacity of mNPs such as pH, temperature, concentration of pollutants and mNPs concentration. The adsorption capacity and interaction between adsorbent and pollutants can be determined using adsorption isotherms. Different isotherms models such as Langmuir, Freundlich and Tempkin isotherm model are considered to study adsorption process of mNPs for various pollutants. In this adsorption process, pollutants first get adsorbed over the surface of the mNPs via physical or chemical interaction and the surface of the mNPs becomes saturated with pollutants at equilibrium. Pollutants can be desorbed, making the active sites available for the adsorption of the upcoming pollutant molecules. Adsorption kinetics is very important to study adsorption dynamics at a constant rate. Different kinetics models such as pseudo-first order, pseudo-second order, Elovich and intra-particle diffusion models are used to analyse adsorption rate. Researchers reported the use of lychee-induced synthesized mNPs as an adsorbent for the removal of dyes such as MB and other pollutants such as phosphates and nitrates from aqueous medium.20
Sachin et al. reported the synthesis of ZnO NPs by using lychee fruit peel extract and zinc acetate dihydrate to capture Congo red (CR) dye by adsorption process.11 The adsorption process was examined by the contact of CR with the surface of adsorbent ZnONPs. Initially, adsorptive removal of CR was high due to the availability of more active sites of adsorbent and strong attractive forces between CR molecules and ZnO NPs. But after 30–120 min, it was decreased to a level of equilibrium due to saturation of the number of active sites. They observed that the removal of CR molecules was increased with the increase of their contact time with ZnONPs. The adsorbent dosage of ZnO NPs increased the removal percentage of CR dye. pH also played crucial role in removal of CR dye. The results showed that removal efficiency remained consistent over the pH range of 2–4 but decreased above pH 4. At below pH 7, the surface of the adsorbent is enriched with the positive charge which favors the attraction of negatively charged dye molecules. But at above pH 4, repulsion between dye molecules and hydroxyl ions on ZnO NPs decreased the removal efficiency of the dyes. Pseudo-first-order model and pseudo second order model was used to study the adsorption of CR dye on ZnO NPs. Pseudo-first order model has less correlation coefficient (R2 = 0.91) as compared to the pseudo-second order model with a high correlation coefficient (R2 = 0.99). Thus, the results prove that pseudo-second order explains the adsorption process appropriately. Different isotherm models such as Langmuir, Freundlich and Temkin isotherm models were used to determine adsorption capacity and interactions between CR-ZnO NPs. These models were used to understand the adsorption process in batch experiments. Langmuir model explained the pressure dependence of molecules and assumed that a saturated monolayer of dye is formed on the surface of ZnONPs. The value of the correlation coefficient calculated was R2 = 0.99. Hence, CR adsorption on ZnO NPs was found favourable. Freundlich isotherm model revealed that variation in adsorption has a direct relation with pressure and the value of correlation coefficient was calculated to be R2 = 0.94. Hence, this model also favored the CR adsorption on ZnO NPs. Tempkin isotherm model analyzed indirect CR-ZnO interactions in the adsorption process. They have observed that heat of adsorption decreased linearly by increase in coverage due to interactions between CR molecules and ZnO NPs and it came out to be 6.52 J/mol. Hence, results showed that green synthesized ZnO NPs showed excellent catalytic activity and removed around 98% Congo red dye from the wastewater. This adsorbent also showed potential for the removal of interference ions such as Na+, Mg+, Cl−, SO4−2 and PO4−2 from toxic dyes in industrial applications.
Le et al. reported the synthesis of lychee peels-derived biochar-supported calcium ferrite magnetic (LP-BC@CaFe2O4) nanocomposite.20 Firstly, CaFe2O4NPs were synthesized by sol gel method and then biochar-supported CaFe2O4 via wet impregnation method. These prepared nanocomposites were characterized by different techniques such as X-ray diffraction (XRD), Scanning electron microscopy-Energy dispersive X-ray (SEM–EDX), Fourier transform infrared (FTIR), and vibrating sample magnetometer (VSM). They observed that these mNPs acted as a reusable adsorbent for the removal of phosphate (PO4−3) and nitrate (NO3−) from wastewater. This removal of PO4−3 and NO3− was done by adsorption mechanisms such as ligand exchange, electrostatic interaction and surface complexation. They observed that the adsorption behaviour of PO4−3 and NO3− depended on 5.0% loading ratio of CaFe2O4 on pristine biochar (LP-BC). pH of the medium also affected the adsorptive removal of PO4−3 and NO3− ions by biochar-supported CaFe2O4 nanocomposite. NO3− showed maximum adsorptive removal at pH 6 while PO4−3 ions showed maximum adsorption at pH 7. An increase in pH of the solution up to 11 showed a negative impact on the adsorptive removal of PO4−3 and NO3− ions from the aqueous medium. The reason is that in acidic conditions, the surface of biochar gets protonated and enriched with the positive charge which attracts the negative charges of PO4−3 and NO3− ions by electrostatic interaction. The surface groups such as Ca-OH and Fe-OH present in biochar-supported CaFe2O4 were also protonated to generate positively charged groups under the same condition and were used to adsorb PO4−3 and NO3− ions from wastewater. They also observed that with an increase in adsorbent dosage, the adsorption capacities of biochar to uptake PO4−3 and NO3− ions were decreased. They studied the effect of co-existing ions such as chlorine, phosphate, sulfate, nitrate, bicarbonate, sodium, and potassium which competed with PO4−3 and NO3− for active adsorption sites on biochar’s surface. Among them chlorine (Cl−) and bicarbonate (HCO3−) showed the highest competition and dropped the adsorption capacity of both PO4−3 and NO3−. It was due to the more affinity of –OH2+ groups on biochar’s surface towards Cl− and HCO3− and due to the competition in ligand exchange between Cl−, HCO3− and NO3−, HPO4− due to the effect of similar valence between Cl− and HCO3− with NO3− and HPO4− and radius of hydration ions in aqueous solution in acidic medium. Effect of contact time was also studied on the adsorptive removal of PO4−3 and NO3− ions. Both PO4−3 and NO3− showed superior efficiency on modified biochar (LP-BC@CaFe2O4) than pristine biochar (LP-BC) due to more oxygen-containing surface functional groups on LP-BC@CaFe2O4. Their kinetic studies revealed Elovich and pseudo-first order reactions. The isotherm studies were done by using Freundlich, Langmuir, Sips, and Redlich-peterson models. The adsorption isotherm data showed an increase at low adsorption equilibrium concentrations of PO4−3 and NO3− on biochar and became constant when there was a further increase in adsorption equilibrium concentration. The isotherm studies fitted best with elovich and sips model and suggest that the adsorption process for the removal of PO4−3 and NO3− ions by LP-BC@CaFe2O4 nanocomposite was chemisorption in nature with on heterogeneous surface properties of the adsorbent. The results showed that LP-BC@CaFe2O4 nanocomposite showed significant adsorption capacity to remove PO4−3 and NO3− ions from wastewater in both single and binary systems.
Use as catalyst
Metal nanoparticles possess high surface area and surface to volume ratio due to which they are commonly used in heterogeneous catalysis and speed up different organic reactions.31 mNPs have shown excellent potential in various reactions including dehalogenation and dehydrogenation. Small-sized mNPs generally show good catalytic activity as compared to the large sized mNPs.32 Lychee extract-induced biogenic synthesized mNPs have been reported as catalyst for the degradation of various toxic dyes such as methylene blue (MB)1 and Congo red (CR)11 present in aqueous medium. These nanoparticles can be used as efficient catalysts in chemical degradation reactions or in photo-induced degradation reactions. Different reducing agents such as hydrazine, NaBH4, and N, N- dimethylformamide (DMF) are reported to perform the chemical reduction reactions in the presence of a catalyst. While in photo-catalysis, the mixture of pollutants with nano-catalysts is exposed to light irradiation. mNPs have resistance to self-poisoning due to which they act as an efficient catalyst for the degradation of pollutants present in the wastewater. During degradation process, the percentage degradation of dyes can be estimated by the following relation.33
In qu. (1) Co is the initial concentration of dye solution and C is the concentration of dye after completion of catalytic degradation reaction. Different factors such as dye concentration, catalyst amount and concentration of reducing agent, irradiation time, and intensity of radiations also affect the percentage degradation values of the dyes. In photo-catalysis, by increasing the exposure time of dye and mNPs complex mixture to sunlight, percentage degradation of dyes increases. But increase in concentration of dye decreases the degradation value of dyes which is due to the reason that more number of molecules of dyes adsorb over the surface of catalyst resulted in less absorption of light by catalyst and decreased degradation value of dyes. Catalysts provide active sites for reduction reaction.34 The more is the amount of catalyst more will be the active sites available for reaction and increase the rate of reaction. Higher is the amount of reducing agent more is the percentage degradation of reduction of dyes. Radiations increase the activity of catalysts. Thus, higher intensity radiations also increase the percentage reduction of dyes. Different researchers have reported the use of lychee induced mNPs as catalyst to accelerate the rate of various organic reactions in order to degrade pollutants and to get useful products.1,6,11 Chemical degradation and photo-catalytic degradation of dyes follows the pseudo-first order kinetics as the rate of reaction depends upon the concentration of dye in both cases as given below6
Where Ct is available concentration of dye at any time. Pseudo-first-order can be expressed in concentration terms as follows
By applying initial conditions such as t = 0, Ct = Co in Eq (6).
By putting values of C from eq. (7) to eq. (6).
Here, Co is the concentration of dye when the reaction is not started and Ct is the concentration of dye at any time. Equation (11) can be expressed in terms of absorbance as shown:
The value of kapp can be calculated from the slope of the plot of ln[At/ Ao] vs time while using eq. (12).
Shende et al. reported the synthesis of Ag and AuNPs by using lychee fruit peel extract as a reducing and stabilizing agent.6 They observed the change of color indicates the successful fabrication of AgNPs. UV–visible spectra indicate the appearance of the SPR band at 430 and 540 nm in spectra of Ag and AuNPs, respectively. HRTEM analysis illustrates the synthesis of monodispersed and spherical-shaped mNPs. XRD analysis indicates the synthesis of crystalline-shaped Ag and AuNPs. The reaction conditions such as salt concentration, agitation time, and temperature were changed to get the maximum amount of Ag, and AuNPs. 5 mM salt solution and 50 °C temperature was found favorable to get a good amount of Ag and AuNPs. Ag and AuNPs showed significant catalytic activity for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as monitored by UV–visible spectroscopy at 25 °C in quartz cuvette by using freshly prepared NaBH4 solution as reducing agent. The 4-NP peak at 317 nm shifted to 400 nm due to the formation of phenolate ions on the addition of sodium borohydride. The peak at 400 nm was decreased with the appearance of another peak at 300 nm associated with 4-aminophenolate ion. The change in color from light yellow to colorless indicates the successful conversion of 4-NP into 4-AP. They observed that reactions followed pseudo-first-order rate kinetics. They observed that the value of kapp for the reduction of 4-NP was found as 2.436 × 10−4 and 1.36 × 10−4 min−1 in the presence of Ag and AuNPs, respectively.
Different other researchers reported the biosynthesis of mNPs using lychee peel extract as a source of reducing agent/stabilizing agent and their use as a catalyst for the reduction of toxic dyes and nitro aromatic compounds.14,18
Medical activities
Green synthesized noble mNPs exhibit promising nano-medicine and biomedical applications.35 Bimetallic nanoparticles as compared to mono-metallic nanoparticles have a more promising approach towards biomedical applications due to their synergistic properties. The mNPs are employed in a variety of applications such as antimicrobial agents, anticancer activity, drug loading, photo-thermal therapy,36 gene delivery, biological imaging, magnetic fluid hyperthermia (MFH), magnetic resonance imaging (MRI), and antimicrobial activity. Among them, the following applications are described below.
Antimicrobial activity
Treatment of microbial infections has become more challenging due to the evolution of microbial resistance towards conventional antibiotics. The improper use of antibiotics is one of the major factors that has raised the development of antibiotic resistance in bacterial strains. As a result, there is a requirement for the quick development of novel bactericidal agents. Metal nanoparticles have shown remarkable antibacterial efficacy against many bacterial species, serving as a possible alternative to antibiotics. Wide applications of mNPs are based on their nano size and high surface-to-volume ratio. Due to their high surface properties, mNPs interact with microbial membranes easily.6,37 These mNPs affect the permeability of cell membrane, and metal ions released from mNPs leach into the microbial cells, interact with various cellular organelles and biomolecules such as enzymes, ribosomes, DNA, proteins,18 lysosomes, and disturb the gene expression, protein activation and enzyme activity.38 Researchers reported high antimicrobial properties against a number of bacterial species such as gram-positive Staphylococcus aureus and B. subtilis and gram-negative bacteria Salmonella typhi.1 It was also investigated that AgNPs show more antimicrobial activity against gram-positive and gram-negative bacteria than AuNPs.39 High antimicrobial activity of AgNPs against gram-negative bacteria is due to the presence of thin layer of peptidoglycan in their cell wall which can be easily ruptured as compared to gram-positive bacteria which have thick walled surrounding the cells. The proposed mechanism of bacterial cell death caused by AgNPs involves the generation and attack of reactive oxygen species (ROS). Molecular oxygen captures electrons to generate superoxide anions, which further react with H2O2 to generate hydroxyl radicals. Electrons are then absorbed from water and hydroxyl radicals which cause mineralization of bacterial cell. ROS production resulted in rupture of cell membrane, protein activation or denaturation, DNA and mitochondrial damage and ultimately cell death.40 Sometimes silver ions enter into extracellular matrix from surface of mNPs and interact with peptidoglycan present in cell wall and plasma membrane leading to damage of cell wall structure. AgNPs can also interact with thiol groups present in proteins and cause prevention of bacterial DNA replication. The antimicrobial activity of mNPs also depends on size, shape and stability. By altering size, shape and morphology of mNPs, their antimicrobial activity can also be altered.41
Different researchers reported the synthesis of mNPs and studied their antimicrobial activities against different bacterial species present in wastewater. Badola and Negi reported the synthesis of AgNPs by using L. chinensis peel extract as a reducing/capping agent.1 They investigated the antimicrobial potential of synthesized AgNPs against several bacterial strains by disk diffusion method as shown in Fig. 3. It was observed that AgNPs showed no activity against E. coli while they were proved effective against S. aureus, B. subtilis and S. typhi. Although, E. coli is gram-negative bacteria like S. typhi. However, its outer wall is quite complex with a thick lipid bilayer with lipopolysaccharides. Thus, its outer membrane structure and composition are different from S. typhi, show a barrier for the mNPs, and restrict their access to the inner part of the bacterial cells. Thus no zone of inhibition was observed in the case of E. coli as compared to the S. typhi.
Antimicrobial activity of AgNPs against gram’s positive and gram’s negative bacteria.
Sachin et al. reported the synthesis of ZnO NPs by using lychee fruit peel extract as a reducing/stabilizing agent while zinc acetate dihydrate was used as a source of metal ions.11 The crystalline nature of ZnO NPs was confirmed by XRD analysis. FTIR analysis gave the stretching vibration of the metal-oxygen bond (Zn-O) at 460 cm−1 which further indicates the successful fabrication of ZnO NPs. SEM analysis indicates the formation of roughly spherical and poly-dispersed mNPs. They used ZnO NPs as an adsorbent for the removal of CR, a toxic dye from an aqueous medium by batch method. They also tested the antimicrobial behaviour of ZnO nanoparticles against different kinds of bacteria such as S. aureus, Pseudomonas aeruginosa, B. subtilis, and E. coli by well diffusion assay. These bacteria were incubated in nutrient broth for 24 h at 37 °C. Then saline solution and varying concentrations of nanoparticles were added and incubated for 16 h at 37 °C. The inhibitory zone was found as 23, 22, 17, and 15 mM against E. coli, P. aeruginosa, B. subtilis, and S. aureus, respectively while using 100 μg/mL of ZnO NPs. The antibacterial performance of ZnO NPs was found to be more for gram-positive than gram-negative bacteria due to the difference in thickness of cell wall. This enhanced antimicrobial behavior of ZnO NPs was also due to their higher surface-volume ratio which facilitates the adsorption of ZnO NPs at the contact point with the bacterium.
Anticancer activity
Among non-infectious diseases, the major cause of mortality in the world is cancer.38 mNPs have shown potential against cancer treatment due to their unique properties. These mNPs can be functionalized with targeting ligands to selectively accumulate in cancer cells and tissues, allowing early detection and imaging. In cancer therapy, alkylating agents and antimetabolites are used commonly but they have toxic side effects. Hence, there is a need for non-toxic agents for cancer treatment. mNPs have unique physicochemical properties that are known as intrinsic antitumor effects. Bimetallic NPs synthesized using green method possess distinct properties like small size, large surface area, and high charge density which enable them to penetrate cancer cells and induce cell death.16 These distinct properties can facilitate their anticancer activity either through their intrinsic properties or external stimuli such as infrared rays or magnetic fields. The external stimuli can generate ROS which causes cancer cell death as shown in Fig. 4.
Process of cancerous cell death by using green synthesized metal nanoparticles.
ROS is a non-polar molecule and can easily diffuse into cancer cells. mNPs have been monitored to cause cellular mutations, inhibit enzymatic actions, and cause morphological changes in cells resulting in cell death. A broad spectrum anticancer drug 5-FU is pyrimidine analog which is also used in breast cancer treatment and other malignancies but it has some limitations due to its adverse effects.7 So, its efficiency can be enhanced by combinational approach by using non-toxic plant derived components. Kaur et al. reported the synthesis of silver core (SCNPs) and gold core (GCNPs) by using lychee peel extract.16 This study revealed the anticancer activities of SCNPs and GCNPs against MCF-7 breast cancer cells and emphasized cell cycle regulation, production of ROS and cancer cell proliferation. These mNPs interfered with DNA replication and cell division by inducing cell cycle arrest in G2M and It was observed that SCNPs showed more potential for inhibiting MCF-7 cell growth over an extended duration. Both NPs produced large amount of ROS which may contribute to their anticancer and anti-proliferative effects and significantly induce cell death. The mechanism of ROS involved the generation of free radicals and activate intracellular signalling pathways toward apoptosis.
Iqbal et al. reported the synthesis of AgNPs using lychee peel extract.42 The extract was prepared in water as a solvent while AgNO3 was used as a source of Ag ions to prepare silver NPs. Successful fabrication of these NPs was confirmed by UV–visible analysis as a peak appeared at 417 nm associated with AgNPs. XRD analysis confirmed the crystalline nature of AgNPs. SEM analysis shows that well-dispersed and spherical-shaped AgNPs were synthesized. These biosynthesized AgNPs were also used with four dilutions for anticancer activity against human epithelial type-2 cancer cells (Hep-2) and human breast adenocarcinoma cell lines (MCF-7) using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazoliumbromide) calorimetric technique. The AgNPs showed significant cytotoxicity. The maximum cell death rate was obtained with 16 ppm AgNPs solution to stop cancerous cell growth.
Hussain reported the synthesis of silver (Ag), selenium (Se), silver selenium (AgSe), and 5-fluorouracil (5-FU) AgSe NPs by using lychee seed extract.7 They also studied their cytotoxicity response in combination with Plasmonic photo Thermal Therapy (PTT). Successful fabrication of these mNPs was confirmed by various analytical techniques such as UV–visible, SEM, EDX, and thermogravimetric analysis (TGA). They observed the cytotoxic effect of these NPs with and without photothermal effects by CCK8 antineoplastic activity. These mNPs showed significant anti-cancer activity. The apoptotic inducing ability of these mNPs in human pancreatic cancer cells (PANC-1) cells was studied by propidium iodide (PI) staining, apoptotic (Bax) / antiapoptotic (Bcl-2) gene expression by using RT-PCR and western blotting. They monitored the results of PI staining which showed higher necrosis of cancer cells in 5-FUAg-Se > AgSe > as compared to Ag and Se NPs. They also observed that Bax showed increased expression for induction of mitochondrial alteration and the decreased Bcl-2 level showed inhibition of antiapoptotic gene expression in PANC-1 cells. Final results showed that Ag NPs revealed high number of cell death with and without laser irradiation due to reason that PTT prompted cell death depending upon laser dosage, type, irradiation time period and subcellular NPs localization.
Antioxidant activity
Metal nanoparticles have attained much importance for their antioxidant activity due to their unique physicochemical properties and have proved significantly efficient in scavenging of free radicals and ROS, and as catalysts in antioxidant reactions. mNPs have high surface to volume ratio and high surface energy which allows them to scavenge the radicals such as 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical,43 hydroxyl radicals. Various mNPs can catalyze the reaction that converts ROS into less harmful substances. It has been investigated that mNPs can suppress the overproduction of ROS and protects the proteins, lipids, and mitochondrial membranes from ROS attack thus, protect against oxidative damage (Fig. 4). Free radicals can cause damage to mutating cells. Antioxidants neutralize these harmful free radicals and prevent this damage. mNPs can interact with free radicals, donate electrons to them and neutralize their activity.
Antioxidant activity can be determined by 1,1-diphenyl-2-picrylhydrazyl (DPPH) due to its ability to reduce free radicals. The antioxidant activity of the solution is measured by a DPPH radical scavenging assay and ABTS+ radical scavenging assay methods. However, there are some limitations of using DPPH radical scavenging assay because it focuses on a single type of radicals. So, complementary assays targeting different types of radicals are required to use which enhance the overall antioxidant capacity of lychee peel extract.15 The value of radical scavenging ability (RSA) can be calculated by following the equation
Where A is the sample absorbance and Ao shows the standard absorbance. Abdullah et al. reported the synthesis of lychee peel extract-based magnetic Fe3O4 NPs (LCPEx-MIONPs).15 Fe3O4NPs showed antioxidant activity and showed significant potential in grape preservation. The antioxidant activity was evaluated by the disc diffusion method against DPPH free radicals. The complete colour change was recorded at 100% inhibition. This high percentage inhibition coupled with complete colour change caused LCPEx to neutralize DPPH radicals. It was also observed that inherent magnetic properties in LCPEx-MIONPs may facilitate the electron transfer reactions which took part in the reduction of free radicals and subsequently increased their antioxidant activity.
Khan et al. reported the synthesis of AgNPs by using aqueous lychee peel extract and AgNO3 solution in dark to minimize the photoactivation of AgNO3 for complete mixing at room temperature.18 The prepared AgNPs were characterized by using various analytical techniques such as UV–visible, XRD, EDX, HRTEM, FTIR, and selected area electron diffraction (SAED). These mNPs showed significant antimicrobial properties against certain types of bacteria. They monitored that the cytotoxic assay confirmed that biosynthesized AgNPs were non-toxic to normal healthy RBCs. These mNPs also showed photocatalytic degradation of methylene blue dye. They showed significant antimicrobial and photocatalytic activities due to their small size, spherical morphology, and high dispersion. These mNPs also showed antioxidant activity which was assessed by DPPH scavenging assay by using vitamin C as standard. They observed effective free radical inhibition by biosynthesized AgNPs while moderate activity was observed in commercial AgNPs. The high activity of biosynthesized mNPs was due to the capping molecules on the surface of NPs, their well-dispersed suspension, and smaller size than the commercial AgNPs.
Miscellaneous applications
mNPs have shown wide range of miscellaneous applications such as sensors, optics, food preservation and packaging,44 agriculture, cosmetics and personal care,45 lubricants, construction materials and energy storage and conversion.19 mNPs are used in various batteries such as lithium-ion batteries and significantly enhance their performance by improving their stability, capacity, conductivity, and safety. This is due to the unique properties of mNPs, high surface potential, and improved mechanical strength. The mNPs such as Si, Sn, Ge, Fe, Ni, Co, Mn, and metal oxide NPs are used in lithium-ion batteries. Among them, some are used as anode and some as cathode leading to high power density, cycling stability, and faster charging/discharging rates. However, they undergo volume expansion and contraction during lithiation and delithiation process leading to capacity fading. To address this problem, carbon-coated mNPs are used to accommodate these volume changes. The carbon matrix can be in the form of carbon nanotubes or graphene. During lithiation, Li+ ions move from the cathode through the electrolyte and are inserted into the metal oxide structure at the anode. In the beginning, metal oxide is reduced to metal atoms and lithium oxide leading to high capacity of batteries. While in delithiation, Li+ ions are extracted from the anode and move back again to cathode as shown in Fig. 4. This reverse reaction is important for the cycling stability of the battery. The carbon-coated mNPs increase the electrical conductivity, surface area, capacity and structural integrity of electrodes.
Xie et al. reported the fabrication of Fe3O4/carbon composites with a unique hierarchical structure including heterogeneous carbonaceous skeleton, compositions, and functionalities.19 They synthesized Fe3O4/carbon composites by extracting polyphenols from waste lychee seeds and coordinated it with metal ions to assemble an inorganic–organic hybrid coated on carbon nanotube substrate under high-temperature carbonization and inert N2 flow. These prepared composites were characterized by various analytical techniques such as FTIR, X-ray photoelectron spectroscopy (XPS), XRD, SEM, TEM and EDS. The prepared Fe3O4/carbon composites were selected as a proof-of-concept product and showed high potential for the development of lithium-ion batteries with stable charge/discharge capability. When used as an anode material of lithium-ion batteries, the Fe3O4/carbon composites showed an efficient electrochemical performance with high rates, high energy storage and cycling stability. The electrochemical performance was evaluated by using 2,032 coin-type cells with Li foil as negative electrode (reference electrode) and prepared Fe3O4/carbon composite as positive electrode. The electrochemical properties of Fe3O4/carbon composite anode such as oxidation/reduction reaction process during cycling and reversibility were further investigated by CV tests. The equation for redox reaction
They monitored the excellent rate performance of Fe3O4/carbon composite anode by varying current densities between 0.1 and 5A g−1. It was seen that capacity retention is as high as 45% for a 50-fold increase in current density. The reversible discharge specific capacity of Fe3O4/carbon composite anode was restored to 524 mAh g−1 when current density was restored to 0.1A g−1 showing efficient rate performance. The role of mNPs in lithium-ion batteries is shown diagrammatically in Fig. 5.
Working of lithium-ions batteries.
Thus, green synthesized mNPs can be used in wastewater treatment as catalysts/adsorbents along with their potential to kill microbes present in water media. A diagrammatic representation of the applications of lychee extract-induced green synthesized mNPs in different fields such as catalysis, antimicrobial, antioxidant, and anticancer agents is shown in Fig. 6.
Diagrammatic representation of different applications of lychee extract-induced biogenic synthesized mNPs.
Conclusion and future directions
This study provides a comprehensive review of the synthesis of metal nanoparticles using lychee plant extract as a reducing and stabilizing agent. Plant extracts can reduce metal ions into metal atoms which coagulate to form metal nanoparticles. They also can stabilize mNPs for a long time. Therefore, plant extract-based mNPs and metal oxide NPs are considered eco-friendly, simple, and cheap. These benefits make the plant extract-based method for preparation of mNPs more attractive for large-scale production and their applications in various fields. The energy content in mNPs is high which makes them produce ROS. ROS proves harmful to the DNA and proteins of microorganisms ultimately causing their death. So, mNPs have found applications as catalysts, antimicrobial, anticancer, antioxidant activities, and various other miscellaneous applications.
Consent to publish
All authors declare to publish the work.
Author contributions
Khalida Naseem worked as the main supervisor and contributed the majority of the writing of the review article. Sana Asghr wrote the first draft of the paper. Kiky Corneliasari Sembiring and Mohammad Ehtisham Khan contributed towards the major corrections in the final manuscript. Asima Hameed and Shazma Massey proofread the paper for final submission and contributed to the revision process.
Conflict of interest statement. Authors have no relevant financial/non-financial conflict to disclose.
