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    Green Synthesis of Gold Nanoparticles (AuNPs) from Plants

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    (This entry belongs to Entry Collection "Biopharmaceuticals Technology ")

    Definition

    Gold nanoparticles (AuNPs) are becoming promising cancer therapeutic and diagnostic metal NPs that attract researchers due to their unique physiochemical properties such as stability, biocompatibility, high thermal activity, optical, electrical, high surface area to volume ratio surface chemistry, and multifunctionalization, etc. By fine tuning the components and concentrations, AuNPs can be easily manufactured into various forms and sizes. AuNPs have also shown significant advancement in treating inflammatory diseases and bacterial infections.

    1. Properties of Gold Nanoparticles

    To synthesize stable nanoparticles (NPs), gold (Au) is regarded as suitable metal. Most of the physical properties of inorganic nanoparticles are found to be dependent on the size and shape of the NPs. The gold nanoparticles (AuNPs) have wide applications in different fields due to their specific optical and physical properties. AuNPs possess significant properties, such as (a) small size (1–100 nm), (b) physical and chemical properties based on size, shape, and composition, (c) excellent robustness, (d) qualitative and quantitative target binding ability, etc. [1].

    1.1. Shape and Size

    There are various methods for the synthesis of AuNPs having different shapes and sizes. The size and shape of AuNPs are the two primary parameters that control the chemical, physical and electrocatalytic properties of gold nanoparticles. Metal nanoparticles in the size range of 1–10 nm have size-dependent properties compared to bulk materials [2]. There are very few methods for the production of AuNPs with uniform sizes. Michael Faraday introduced the two-phase system to prepare AuNPs by reducing gold salt by phosphorous in carbon disulphide for the first time. Researchers have developed a popular method for synthesising smaller AuNPs. They reduced gold salt by sodium borohydride in the presence of the capping agent dodecanethiol [3]. They confirmed the size of the NPs in the range 1–3 nm using HRTEM (High-Resolution Transmission Electron Microscopy). It is possible to control the size of NPs by varying the concentration of thiols. G. Frens agreed with Turkevich’s data that reduction of gold chloride salt using sodium citrate solution is an efficient method to prepare monodispersed gold nanoparticles of different diameters. By changing the ratio of reactants, independent nucleation and growth of metal nanoparticles with different diameters can be achieved [4][5]. The size of AuNPs are tuneable through the alteration of gold precursor and reducing the power of reducing agents. It is observed that strong reducing agents, such as NaBH4 offer the synthesis of small size AuNPs and weaker reducing agents, such as citrate, resulted in comparatively larger NPs. Researchers have investigated the effect of chloride ion concentration on the size of AuNPs via citrate reduction. They find that size of gold NPs are caused by the aggregation of gold NPs induced by chloride ions [6]. In a typical synthesis, chloroauric acid (HAuCl4) is reduced by 5% sodium citrate solution at room temperature. The chloride ion concentration is varied using different concentrations of NaCl solutions. They performed the same experiment using NaBH4 as a reductant, and the interesting results are placed in Table 1. The table includes the data for the effect of different concentrations of NaCl on the size of AuNPs in the presence of different reactants such as sodium citrate and sodium borohydride, respectively. With an increase in NaCl concentration from 1 to 20 mM, the UV-visible absorption maxima (max) are also shifted towards higher wavelengths that indicate an increased size of AuNPs.
    Table 1. The effect of Cl ion concentration and reductant on AuNPs size.
    HAuCl4 (0.25 mM) + 5%
    Sodium citrate
    (Both the reactants are in 5:1 ratio)
    Concentration of NaCl (mM) UV-Visible Absorption Maxima [λmax (nm)] Size Ranges from TEM
    (nm)
    1 517 19 nm (±7)
    5 520 25 nm (±11)
    10 525 38 nm (±21)
    15 528 40 nm (±31)
    20 531 47 nm (±36)
    HAuCl4 (0.25 mM) + Sodium borohydride (NaBH4) 0 490 3
    20 520 12
    Solvents used in the synthesis of NPs play an important role as either interaction between nanoparticle surface and solvent molecule or solvent and ligand molecules direct its final size and morphology [7][8]. Scholars have challenged the two-phase synthesis of NPs by Brust and co-workers and developed a single-phase synthesis of monodispersed gold nanoparticles using borane complexes as a reductant in organic solvents. In a typical process, AuPPh3Cl (0.25 mmol) and dodecanethiol (0.125 mmol) is mixed with benzene, and 2.5 mmol solution of the tert-butylamine-borane complex is added as reductant that forms 6.2 nm size gold nanoparticles [9]. In a modified Stucky’s method, Song and coworkers have prepared mono dispersed AuNPs from the reaction of AuPPh3Cl with an amine borane complex tert-butylamine borane (TBAB) having thiol ligand. In the absence of thiol, the AuNPs are found to have polydispersity, while in the presence of thiol, the synthesized NPs are monodispersed, having a size of 5.0 nm ± 0.4 nm [10]. To synthesize large size AuNPs, the seeded growth method is preferred. In this method, small size gold nanoparticles have been synthesized that act initially as seeds for the growth of larger gold nanoparticles. Then, separation of nucleation and growth of the NPs, we may increase the size of AuNPs up to 300 nm [2][11]. Stanglmair and co-workers have reported synthesizing monodispersed AuNPs with 20 nm average diameter in size via the seeded growth method. They synthesized gold nanoparticles of 9 nm size, in toluene as solvent and oleylamine as reductant cum stabilizing agent, were further used as a seed to produce AuNPs of 20 nm average size. Researchers have reported the synthesis of spherical shape AuNPs from H[AuCl4]·3H2O as a precursor, ascorbic acid as reductant and sodium citrate as a stabilizer. In the first step, they were able to synthesize NPs with 30 nm size, which then acts as a seed for the production of AuNPs, having 69 nm and 118 nm sizes and even further growth [12]. The report shows that AuNPs with different sizes and shapes, such as long nanorods, short nanorods, cubes and spheres can be prepared via reversible flocculate formation surfactant micelle-induced depletion interaction. To obtain different shapes of NPs, the tuning of surfactant concentration and extraction of flocculates from the sediment are important steps [13]. Jianhui Zhang, with his co-workers, has investigated the shape-selective synthesis of AuNPs with controlled size and different shapes, such as hexagons, belts, rods, triangles, octahedrons and dumbbells. In the process, water molecules are attached with poly(vinylpyrrolidone) (PVP) and n-pentanol to form a two-phase system of water/PVP/n-pentanol (WPN). PVP can act as a reductant and stabilizer where the presence of water can modify the reducing ability of PVP. However, they utilized PVP as a capping agent rather than a reductant. They have observed the region-selective distribution of water and PVP in the WPN system, which offers kinetically controlled growth of novel AuNPs nanostructures [14]. Researchers have studied the effect of temperature on the size of AuNPs by synthesizing gold nanoparticles varying temperature using tetraoctylammonium bromide (TOAB) as a stabilizer. The room temperature synthesis confirms AuNPs with an average size of 5.2 nm having a spherical shape. When the annealing of the AuNPs synthesis was performed at 100 °C for 30 min, a drastic change in the shapes of the gold nanoparticles was observed. The sizes of nanoparticles change from 5.2 nm to 6 nm, bearing shapes such as hexagons, pentagons, and squares under HRTEM observation, whose corresponding three-dimensional shapes are cuboctahedron, icosahedron, and a cube, respectively. On annealing, at 200 °C, the morphology, as well as size of AuNPs, changed drastically. The HRTEM shows the average size of the NPs to be 15 nm with different shapes, such as hexagon, triangle and pentagons. Similarly, nanoparticles and nanocubes are obtained when annealing is performed at 300 °C [15]. An interesting process, called the dewetting process, has attracted the researcher in the synthesis of nanoporous AuNPs. In this method, Au/Ag bilayer alloy film is initially produced, where AuNPs are much smaller in comparison to silver nanoparticles (AgNPs). Then, AgNPs are removed by treating the Au/Ag bilayer in 65 wt% HNO3 solution at 21 °C, called dealloying. After dealloying, Au (5 nm)/Ag (20 nm) bilayer AuNPs with 274 nm are found, while Au (10 nm)/Ag (20 nm) bilayer-formed AuNPs have a diameter of 307 nm [16]. Researchers have reported the synthesis of cap-shaped AuNPs with 110 nm size by evaporating gold adsorbed on polystyrene [17]. The atom-transfer radical polymerisation (ATRP) is a technique used by scholars to prepare monodispersed nanoparticles that might be useful for synthesizing NPs of other precursors [18]. Researchers have prepared gold nanoclusters in the size range 7–20 nm having positive and negative charges in presence of polyamidoamine dendrimers (PAMAM) or sodium citrate [19]. Luca and co-workers have synthesized gold nanostar (AuNS) from HAuCl4 as a precursor using hydroxylamine as a reductant above pH 11 maintained by HaOH solution. AuNS are formed in the pH range 12–12.5 where below pH 11, no reduction occurs to Au (III) species. Thus, pH plays an important role in the size and morphology determination of AuNPs [20]. The graphical abstract presented in Figure 2 tries to include varieties of available shapes for AuNPs.
    Figure 2. Different shapes available for gold nanoparticles.

    1.2. Optical Properties

    Nanoparticles possess excellent optical properties that are different from individual molecules and bulk metals. The optical properties of AuNPs related to surface plasmon resonance (SPR) are one of the reasons behind the vast success of AuNPs in nanoscience and technology [21]. As NPs are exposed to light, the oscillating electromagnetic field of light automatically induces collective coherent oscillation in the free electrons present in the conduction band of NPs. This eventually results in the charge separation that forms a dipole oscillation in the electric field of light. The amplitude of this oscillation reaches the zenith of maximum at a particular frequency known as surface plasmon resonance (SPR). The extent of SPR can be measured using a UV-visible spectrophotometer as the SPR absorbance for nanomaterials is much stronger than other metals. As per Mie theory, the SPR band intensity and wavelength depend upon factors, such as metal type, size, shape and structure of NPs, composition, and dielectric constant of the medium [22]. For gold nanorods, the PA spectra are found to split into two modes, namely transverse and longitudinal. It is interesting to note that gold Nanospheres having size ~20 nm in diameter have a characteristic strong PA absorption band centred at ~522 nm while for nanorods, there was observed a two-band centred at ~522 nm and ~698 nm for transverse and longitudinal SPR. The gold nanoparticles of size < 2 nm in diameter do not show such absorption [23]. The longitudinal SPR (LSPR) of such branched nanostructure is well understood with the help of the plasmon hybridisation model (PH). This method calculates the LPSR of complex structures, assuming it to be the result of LPSR of simpler structures [24]. Optical transmission spectroscopy can be employed to study surface plasmon excitation for two identical interacting spherical AuNPs. Researchers have studied SPR for three pairs of AuNPs with sizes 450 nm, 300 nm and 150 nm in interaction. It is found that, with a decrease in the inter-particle distance, red shifts in SPR are observed, while blue shift is found for orthogonal polarisation [25]. AuNPs can enhance the Raman signal from 106 to 1015 when exposed p monochromatic light. This phenomenon is called the surface-enhanced Raman scattering (SERS) technique that can be applied to distinguish tumour cells, mark tumour cells, or monitor tumour metabolism. AuNPs contain radioactive atoms that help in achieving desired radioactivity for treatment.

    1.3. Electrical Properties

    The semiconductor industries believe that complementary metal-oxide semiconductors (CMOS) will reach their functional limits within 10–15 years. Then, nanomaterials or molecular assemblies on the nanometer level will occupy the space. Promising concepts developed in recent years include single-electron devices that retain their scalability up to molecular level. Individual charge carriers can be handled by exploiting Coulombic effects in metallic single-electron devices with tunnel junctions with micrometer size. Such a field is termed single electronics (SE). The AuNPs have attracted the research’s attention in the approach to bridge the gap between CMOS and true atomic scale in the future [26]. In the case of nanoelectronics, monodispersed nanoparticles have a potential lot. Metal nanoparticles having a diameter < 2 nm are required for such devices to achieve the Coulomb blockade effect at room temperature [27]. Researchers have synthesized AuNPs through a green synthetic procedure using Solanum nigrum, Ricinus communis and Morus nigra, etc., extract as reducing agents. They performed experiments to evaluate the effect of adding AuNPs in the DC electrical conductivity and found that, with the increased addition of AuNPs, the DC electrical gradually increases [28]. Researchers have studied the size dependency of electronic properties of AuNPs nanoclusters up to 14 atoms through density functional theory and agree that the energetic and electronic properties of AuNPs nanoclusters depend on the size structures NPs [29].

    2. Green Synthesis of Gold Nanoparticles (AuNPs) from Plants

    Different physical and chemical synthesis protocols have been well known for the biosynthesis of AuNPs. However, most of those protocols were not well accepted due to toxic chemicals and elevated temperature in the synthesis process. They may be harmful to human beings and the environment [30][31]. The most common biosynthetic method is the extracellular nanoparticle production method [32]. The green synthesis of gold nanoparticles has been reported using plant tissues, bacteria, fungi, actinomycetes, etc. (Figure 3) [33]. However, the green synthesis of AuNPs from the plant is an eco-friendly approach. In the biosynthesis of AuNPs from the plant, different plant parts (leaf, bark, stem, root, etc.) are used as sources chopped into small pieces and boiled in distilled water to obtain the extract. By filtration and centrifugation, the extract can be purified. For metal salt solution HAuCl4, AgNO3 generally is mixed with plant extract at room temperature [34][33]. Plant extracts contain various metabolites or organic compounds (alkaloids, flavonoids, proteins, polysaccharides, cellulose, and phenolic compounds) and secondary metabolites, which are utilized for nanoparticle synthesis [35]. These can involve the bio reduction of metallic ions to NPs and act as stabilizing agents [36]. Plant extracts contain proteins that have functionalized amino groups (–NH2) that can actively participate in the reduction reaction of AuNPs [29]. The functional groups (such as –C–O–C–, –C–O–, –C=C–, and –C=O–) present in phytochemicals, such as flavones, alkaloids, phenols, and anthracenes involve the generation of AuNPs. In this phenomenon, no external stabilising/capping agents are used because different phytochemicals act as reducing and stabilising/capping agents for the extracellular biosynthesis of AuNP, replacing the toxicity of chemicals such as sodium borohydride (NaBH4) [37]. The bio reduction mechanism involves reducing metal ions from their mono or divalent oxidation state to a zero-valent state. After that, the nucleation of the reduced metal atoms takes place [38]. Ultimately, the metallic salt solution containing extract is reduced into Au3+ to Au0, and the synthesis of AuNP proceeds within minutes to hours using a one-pot, single-step and eco-friendly method [39]. Due to the presence of a variety of phytochemicals in plant extract, no particular mechanism for this synthesis process is reported. The variation in composition and concentration of reducing agents in plant extracts is responsible for different sizes, shapes, and morphological nanoparticle synthesis [40]. Researchers have reported that the size and morphology of nanoparticles can be expected to be different by changing the synthesis parameters, including pH, metal salt, pH, temperature and reaction time [41].
    Figure 3. Green synthesis of AuNPs from a plant. Plant extract and metal salt solution HAuCl4 were mixed. After that, the resultant solution is centrifuged, which results in the bio reduction of metallic ions to AuNPs. Phytochemicals act as reducing, as well as stabilizing/capping, agents in this process. The resultant AuNPs are characterized by using SEM, TEM, FTIR, XRD, etc.
    Synthesized AuNPs were initially identified in the change in reaction colour (formation of red colour) through UV-vis spectrophotometer analysis. DLS, XRD and SAED confirmed the crystalline structure of gold nanoparticles, and the size, shape and distribution of nanoparticles were visualized by TEM image. Based on FTIR analysis, it can be confirmed that functional groups such as –C–O–C–, –C–O–, –C=C–, and –C=O are the capping ligands of the nanoparticles [42]. Different plant parts are used as a source for AuNP biosynthesis. Some green synthesized AuNPs from various plant parts are listed in Table 2.
    Table 2. Green synthesis of Gold Nanoparticles (AuNPs) from different plants.
    Plant Plant Part Reactive Compound Salt Solution Size (nm) Shape Characterization References
    Artemisia vulgaris
    (Mugwort)
    Leaves Polyphenols, flavonoids, terpenoids HAuCl4 50–100 Spherical, triangular,
    hexagonal
    UV-vis Spectroscopy, XRD, FT-IR, DLS, ZP, TEM and EDX. [43]
    Clitoria ternatea
    (Asian
    pigeonwings)
    Leaves Alcoholic, amine groups, halo compounds HAuCl4 100 Rod UV-vis spectroscopy, FTIR, XRD, TEM, EDX [44]
    Murraya koenigii Spreng (Curry leaves) Leaves Polyphenols, quercetin, quercetin-3-glucoside, flavonoids HAucl4 20–40 Spherical XRD, EDX, FT-IR, HPLC, TEM, UV-vis spectra, Fluorescence microscopy. [45]
    Artocarpus
    hirsutus (Wild jack)
    Leaves Polyphenols, flavonoids, terpenoids HAuCl4 5–40 Spherical XRD, UV-visible spectra, FT-IR and TEM [46]
    Justicia glauca (Thaasi murungai) Leaves Lignans[(+) pinoresinol, (+)-medioresinol], alkaloids, flavonoids,
    steroids (sitosterol-3-0-glucoside), terpenoids
    HAuCl·3H2O 32 Hexagonl, spherical UV-vis spectral analysis. X-ray, XRD, TEM, FTIR, EDX, CV, DPV. [47]
    Terminalia arjuna (Arjun tree) Leaves Arjunetin, leucoanthoc-yanidins, hydrolyzable tannins HAuCl4 20–25 Spherical UV-visible spectra, FT-IR, XRD, AFM and TEM [48]
    Memecylon umbellatum Leaves Protein, saponins, polyphenols, carbohydrate HAuCl4, AgNO3 15–25 Spherical, triangular,
    hexagonal
    UV-visible spectra, FTIR, energy-dispersive x-ray spectroscopy, TEM, [49]
    Mangifera indica Leaves Terpenoids, flavonoids, thiamine HAuCl4·3H2O 17–20 Spherical UV-vis, TEM and XRD. [50]
    Olive Leaves Proteins, oleoropein, apigenin-7-glucoside, luteolin-7-glucoside HAuCl4·3H2O 50–100 Triangular, spherical,
    hexagonal
    UV-vis spectroscopy, photoluminescence, TEM, XRD, FTIR and thermogravimetric analysis. [51]
    Coreopsis
    lanceolate
    Leaves Antioxidants like sugars, flavonoids HAuCl4, 20–30 Spherical, quasi
    spherical
    UV-vis spectroscopy, TEM and spectro fluorimetry [52]
    Cassia auriculata
    (Matura tea tree)
    Leaves Polysaccharides, flavonoids AuCl3,
    Au2Cl6
    15–25 Spherical, triangular,
    hexagonal
    X-ray diffraction, TEM, SEM-EDAX, FT-IR and visible absorption spectroscopy. [53]
    Lonicera Japonica
    (Japanese
    honeysuckle)
    Flower Amino acids AgNO3, HAuCl4 8 Triangular tetrahedral UV-vis spectrophotometer, FTIR, XRD, EDX, SEM and HRTEM. [54]
    Nyctanthes
    arbortristis (Night flowering
    jasmine)
    Flower alkaloids, flavonoids HAuCl4 15–25 Spherical UV-vis spectro photometer, TEM, XRD, FTIR, NMR. [55]
    Guazuma
    ulmifolia (Bay
    cedar)
    Bark Tannins, proanthocya-nidins, precocene, catechins. HAuCl4·3H2O, AgNO3 20–25 Spherical UV-vis spectroscopy, FT-IR, XRD, AFM and HR-TEM analyses [56]
    Salix
    (Willow tree)
    Bark Tannins, alkanoids, flavonoids, alkaloids. AuCl4H9O4 15 Spherical UV-vis spectroscopy, XRD, TEM, and HR-TEM. [57]
    Acacia nilotica
    (Gum Arabic
    tree)
    Bark Protein, phenols, tannins, terpenoids,
    saponins
    HAuCl4·3H2O 10-15 Unshaped, quasispherical UV-vis spectroscopy, XRD, EDX, TEM, FTIR, DPV. [58]
    Musa paradisiaca
    (Banana)
    Peel Phenolic compounds, gallocatechin,
    dopamine
    HAuCl4 50 Spherical UV-vis spectroscopy, FTIR, XRD, TEM, Zeta potential analysis and EDX. [59]
    Mangifera indica
    Linn (Mango)
    Peel Phenols, carboxylic acids HAuCl4 3.26–21.68 Quasi-spherical UV-vis spectrum, XRD, TEM, and FTIR. [60]
    Terminalia arjuna
    (Arjun tree)
    Peel Polyphenols HAuCl4 60 Triangular hexagonal
    pentagonl
    UV spectroscopy, HRTEM, XRD, FESEM, EDX, DLS, and zeta potential analyses. [61]
    Lantana camara
    (Wild sage)
    Fruit Ursolic acid, iridoid glycosides, monoand
    sesquiterpe-nes, flavonoids
    HAuCl4 150–300 Triangular UV-vis-NIR, TEM, SAED, DLS, and XRD techniques. [62]
    Citrus (Lemon,
    tangerine, orange)
    Fruit Citric acid, proteins HAuCl4 32.3, 43.4,
    56.7
    Spherical, triangular UV-visible spectra. TEM XRD, SEAD, DLS. [63]
    Citrus maxima
    (Pomelo)
    Fruit Polypeptides/proteins, terpene, ascorbic acid HAuCl4·4H2O 15–35 Spherical UV-vis spectroscopy, SEM, XRD and FTIR. [64]
    Pear Fruit Sugars, amino acids, proteins HAuCl4 20–400 Triangular hexagonalpolyhedral UV-vis spectroscopy, TEM, AFM, XRD, XPS, EDAX. [65]
    Sterculia
    acuminate (Pola
    plant)
    Fruit Phenolic compounds HAuCl4 9.37–38.12 Spherical TEM, X-ray diffraction, UV-vis spectroscopy and FTIR, and X-ray photoelectron spectrometry. [66]
    Pistacia
    integerrima
    (Zebra wood)
    Galls Monoterpenes, triterpenoids, sterols,
    dihydromal—valic acid.
    HAuCl4·3H2O 20–200 Grain-like UV-vis spectroscopy, FTIR and SEM. [67]
    Abelmoschus
    esculentus (Okra)
    Seed Proteins, polysaccharides, glycoprotein HAuCl4 45–75 Spherical, uneven shape UV-visible spectroscopy, XRD, FTIR, AFM, FESEM and EDX. [68]
    Theobromo
    cacao
    (Cocoa)
    Seed Polyphenols HAuCl4 150–200 Spherical UV-vis-NIR
    spectrophotometer, SPR, TEM, XRD, FTIR, XPS
    [69]
    Hevea
    brasiliensis
    (Para rubber
    tree)
    Latex isoprene, proteins AuCl3, Au2Cl6 50 Spherical, triangular UV-vis spectroscopy, SEM, TG/FT-IR, XED. [70]
    Zingiber officinale
    (Ginger)
    Rhizome Oxalic acid, ascorbic acid, Phenylpropanoids, zingerone. HAuCl4, AgNO3 10–20 Spherical, triangular,
    truncated triangular,
    hexagonal
    UV-visible spectroscopy, SEM-EDS, TEM, XRD, FTIR. [71]
    Curcuma longa
    (Turmeric)
    Rhizome Phenolic (curcumin), triterpenoids, alkaloid, sterols. HAuCl4,
    AgNO3
    5–60 Oblong spherical UV-vis spectroscopy, XRD, TEM, HR-TEM, thermogravimetric analysis. [72]
    Panax ginseng C.A. Meyer (Korean red ginseng) Rhizome Saponin glycoside (ginsenoside),
    polysaccha-rides, flavones, peptide
    glycans
    HAuCl4·3H2O 2–40 Spherical UV-visible spectra, TEM, FTIR. [73]
    Acorus calamus
    (Sweet flag)
    Rhizome Asarone, caryophyllene, isoasarone, methyl isoeugenol, safrole. HAuCl4·3H2O 10 Spherical UV-visible spectral analysis, XRD and FT-IR, SPR, HR-TEM, SEM with EDAX. [74][75]
    Acacia nilotica
    (Gum Arabic
    tree)
    Bark Protein, phenols, tannins, terpenoids,
    Saponins.
    HAuCl4·3H2O 10–50 Unshaped, quasispherical. UV-vis spectroscopy, XRD, TEM, EDX, DPV and FTIR. [58]
    Guazuma
    ulmifolia (Bay
    cedar)
    Bark Tannins, proanthocya-nidins, precocene, catechins. HAuCl4·3H2O and AgNO3 20–25 Spherical UV-vis spectroscopy, FT-IR, XRD, AFM and HR-TEM analyses. [56]
    Areca catechu
    (Pinang palm)
    Nut Polyphenols, fats, proteins, carbohydrate, flavonoids. HAuCl4 13.70 Spherical UV-visible, TEM, XRD, and FTIR. [76]
    Momordica
    Cochinchinensis.
    Biomass Proteins HAuCl4 10–80 Spherical, oval, triangular UV-visible, FT-IR, XRD, TEM, SPR and EDX [77]
    Palm oil
    mill effluent
    Palm oil Proteins, flavonoids, reducing sugars, alkaloids HAuCl4·3H2O 13–25 Spherical UV-vis spectroscopy, TEM, XRD, and FTIR. [78]
    Macrotylomauniflorum (Horse
    gram)
    Whole
    plant
    Proteins, carbohydrate, antioxidant HAuCl4·3H2O 14–17 Spherical UV-visible spectroscopy, TEM, XRD and FTIR analysis. [79]

    2.1. Advantages and Limitations of the Synthesis Methods

    Chemical methods for the synthesis of AuNPs have many limitations, which include environmental and biocompatibility concerns. Some of the chemicals used in the synthesis of gold nanoparticles during chemical synthesis can affect our environment and are the cause of risks for administering them into living organisms, thus limiting the biological applications of such AuNPs [80]. Therefore, various biological methods have been devised for the synthesis of AuNPs to limit these concerns. The green synthesis of AuNPs is a simple, safe, dynamic and facile process as its protocol follows a moderate environment without extreme temperatures or pressures. It is a cost-effective, rapid, environmentally benign, and biocompatible process, thus safe for clinical research. AuNPs are being synthesized through different physicochemical methods [81]. However, biogenic reduction of the gold salt to synthesize AuNPs is an inexpensive, eco-friendly and safe process. Neither toxic chemicals, such as sodium borohydride NaBH4, are used, nor are any contaminants or harmful/dangerous by-products produced in this process. Moreover, a considerable number of AuNPs of controlled size and morphology can be easily synthesized. Their stability and reduction potential are attributed to bioactive molecules present in these biological resources. Green synthesized AuNPs application improves the diagnosis and treatment of many human diseases [37]. Out of many biological resources, plant extracts are reported to be a more beneficial resource. Various plant metabolites, such as alkaloids, polyphenols (catechin, flavones, taxifolin, catechin and epicatechin, and phenolic acids), alcoholic compounds, glutathiones, polysaccharides, antioxidants, organic acids (ascorbic, oxalic, malic, tartaric, and protocatechuic acid), quinones, proteins, and amino acids are involved in the formation of NPs by the reduction of metal ions. FT-IR and HPLC tests were used to indicate the presence of these capping agents in the synthesized NPs [35]. Therefore, in this prospect, using plant sources for Au NPs synthesis can open new horizons in future. The primary goal of green nanotechnology is to curtail forthcoming environmental and human health risks associated with the use of nanotechnology products and inspire the substitution of existing products with a more environmentally friendly nano-product. AuNPs synthesis through this green method can contribute to other fields such as green photocatalyst, drug delivery, anti-microorganism, adsorbent, detector, and green separation science and technology [36]. The green synthesis of AuNPs from bacteria is a slow process, so the synthesis process can take a long time, comprising hours and even days. Green synthesis from fungi is better than the previous one, as fungi produce a large number of proteins and reactive compounds. As a result, the reaction process can be scaled up using fungi as a source [75][82]. Although green synthesis of AuNPs from the plant has many advantages, the limitation of using a plant as a source for the synthesis of AuNPs is that the identification of reactive components is difficult as plant biomass comprises a large number of organic components [83][84]. Biomolecules in the plant source contain various functional groups, which can play an essential role in synthesizing AuNPs, but different biomaterials show different reducing abilities. So, it is crucial to first determine their reducing ability before using them in the synthesis reaction [85][86].

    2.2. Plant-Based Synthesized Gold Nanoparticles as Anticancer Agents

    Increasing nanotechnology applications have gained broad attention in various sectors in recent years, but not restricted to medical, cosmetics, medical devices, electrical and electronic, drugs, food and packaging [87]. The most promising approach in nanotechnology is to develop nanomaterials for use in healthcare. In recent years, it has been observed that nanomaterials, such as gold nanoparticles (AuNPs), are of great interest to humans due to their wide range of uses in agriculture, remediation, medicine, health aspects, industry, pharmaceuticals, etc. [88]. Preliminary studies have shown that green synthesized AuNPs have various biological functions, such as antimicrobial, antiviral, anti-inflammatory, antioxidant and anticancer activity. In recent years, the use of plant-derived AuNPs has brought significant advances in cancer diagnosis and treatment, although some work in this area began mainly a few decades ago [87]. Notably, studies have demonstrated the usefulness of AuNPs as anticancer agents, in addition to photothermal agents, contrast agents and drug carriers. However, there are no previous literature reports on the molecular mechanism of tumour inhibition mediated by plant AuNPs. A recent resurgence of the anticancer effects of AuNPs from plant extracts has taken great strides so far. Despite these encouraging advances, more research is needed to understand the molecular consequences in cancer therapy, such as cellular toxicity, mitochondrial toxicity, apoptosis, necrosis and the production of reactive oxygen species (ROS). Several studies and reviews have been undertaken to investigate the anti-cancer potential of green synthesized AuNPs from different plant species. Scholars have reported on the green synthesis of AuNPs from several important plants and their applicability in various biomedical applications [89]; in this context, other authors have also proposed the implication of biosynthesized AuNPs in various applications. Researchers have reported on the aqueous and ethanolic extract of Taxus baccata synthesized nanostructure AuNPs. They were characterized by different techniques, such as UV–Vis spectroscopy, TEM, SEM and FT-IR. The MTT assay was performed to examine the anticancer activity of colloidal AuNPs on cell lines, such as Caov-4, MCF-7 and HeLa. In addition, an in vitro experiment on cell exposure to T. baccata-mediated AuNPs confirms the caspase-independent death program as an anti-cancer mechanism with increased efficacy for cancer therapy. This issue has been explored using flow-cytometry and real-time PCR [90]. Many plants (Camellia sinensis, Coriandrum sativum, Mentha arvensis, Phyl-lanthus amarus, Artabotrys hexapetalus, Mimusops elengi, Syzygium aromaticum) were described by Priya and Iyer, which showed that the green synthesized AuNPs have anticancer activity against the human breast cancer cell line, i.e., MCF7 and found that AuNPs at a minimum concentration of 2 μg/mL for cancer therapy are as effective as standard drugs. Moreover, the increase in the nanoparticle concentration is directly proportional to the effectiveness against cancer [91]. The increasing demand for biosynthesized gold nanoparticles has been greatly facilitated in medical applications, particularly in targeted drug delivery, one of the most recent advances in nanotechnology. Further studies have shown that the use of the Dysosma pleiantha rhizome can improve cancer therapy, which has been proven experimentally by tracking the biosynthesized AuNPs using an aqueous extract. The morphological characteristics of AuNPs are spherical with an average size of 127 nm, characterized by various techniques, such as UV-Vis spectroscopy, FT-IR, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, they also suggested the promising role of biosynthesized AuNPs with enhanced activity against cell proliferation. Finally, they concluded that the D. pleiantha rhizome has antimetastatic potential by interfering with the microtubule polymerization in the human fibrosarcoma cell line HT-1080 [92]. It is clear from the previous research that green synthesized AuNPs holds better choice over other methods because of their cost-effectiveness, non-toxicity and feasibility in cancer therapy. However, this theory is backed up by the evidence in Virmani et al., from which the author concludes that biosynthesized nanoparticles have potential antitumor activity compared to chemically synthesized nanoparticles. They reviewed available methods that could be used to predict anticancer activity against many cancerous (HeLa, MCF-7, A549 and H1299) and normal (HEK293) cell lines. The extract derived from Ocimum tenuiflorum in the cell viability assay illustrates that biosynthesized AuNPs at lower concentrations were more pronounced and non-toxic, compared to HEK293, which can effectively inhibit the growth of various cancer cell lines with an IC50 value of 200 μg/mL. In contrast, the analysis shows that chemically synthesized nanoparticles have contributed negatively to anti-cancer properties even at high concentrations. The results conclude that the use of the chemically synthesized nanoparticles method for cancer therapy offers no obvious advantage [93]. Researchers have reported in their study that the internalization of AuNPs using the extract of Allium cepa was non-toxic to cells. Allium cepa has various pharmacological properties, including anti-cancer activity; however, over the past year, Allium cepa-derived nanoparticles have been of utmost importance in healthcare [94]. Moreover, some important applications of green synthesized AuNPs as an anti-cancer agent are summarized in Table 1. Mostly, the chemically synthesized gold nanoparticles (AuNPs) have been extensively exploited to date; only a few studies have been reported for plant-based green synthesized AuNP in vivo therapy, toxicity, and biodistribution. A study reported that green synthesized gold nanoparticles using leaf extract of Peltophorum pterocarpum (PP) for doxorubicin delivery both in vitro and in vivo in the C57BL6/J female mice. Administration of biosynthesized doxorubicin-loaded (b-Au-PP-Dox) drug delivery system displayed the significant inhibition of cancer cell growth (A549, B16F10) in vitro as well as inhibition of tumour growth in vivo model compared to free doxorubicin and untreated one [95]. Similarly, leaf extracts of Mentha piperita-generated AuNPs were tested against MDA-MB-231 and A549, and normal 3T3-L1 cell lines in vitro, as well as the anti-inflammatory and analgesic activities, were studied on a Wistar rat model. AuNPs showed significant anticancer activities in vitro. However, the in vivo analysis gave positive results for both the activities with less potency as compared to the standard drugs, which suggests that AuNPs might be used in combination with standard drugs to enhance their efficacy [96]. These aforementioned novel in vivo studies have set a new frontier for the potential use of plant-based AuNPs for therapy and drug delivery systems as a cost-effective and eco-friendly approach in the near future. Multifunctionality is the key factor of nanovectors in cancer-specific therapy. Combinatorial therapy with phytoconstituents in cancer therapy has been thoroughly investigated and well documented in the present scenario. More recently, a re-evaluation of this concept has led to the use of a combination of phytochemicals which have been under constant investigation and are particularly used as potent natural anti-cancer agents. This was introduced to overcome some inherent limitations on toxicity, specificity, hazardous and reduced action. The anticancer activity on various cancer cell lines by AuNPs synthesized by using plant extracts are depicted in Table 3.
    Table 3. Showing anticancer activity of gold nanoparticles using plant extracts and characterization technique.
    Sl No. Plant Extract Used Anticancer Activity Type Characterization Shape Size References
    1 Brazilian Red Propolis Hydroethanolic extract Bladder (T24) and prostate (PC-3) cancer cell line SPR, UV-Vis spectroscopy, NTA, TEM, EDXS, SAED, FTIR, TGA, Spherical 8–15 nm [97]
    2 Abies spectabilis Aqueous extract Bladder cancer T24 cell line TEM, SAED, UV-visible spectroscopy, EDX, FTIR, AFM, XRD Spherical 20–200 nm [98]
    3 Benincasa hispida Aqueous extract HeLa cells and normal osteoblasts cell line UV-Visible Spectroscopy, DLS, Zeta sizer, TEM, FTIR Spherical 22.18 ± 2 nm [99]
    4 Butea monosperma Aqueous water extract Normal endothelial cells (HUVEC, ECV 304) and cancer cell lines (B16F10, MCF-7, HNGC2 and A549) UV-visible spectroscopy, XRD, TEM, FTIR, DLS, XPS Spherical, rod, triangular, hexagonal 30 nm [100]
    5 orchid Orchid plant extract(whole) Breast cancer AMJ 13 cell lines UV-Vis spectroscopy, TEM, AFM, FTIR Spherical 14–50 nm [101]
    6 Taxus baccata Ethanolic extract Breast (MCF7), cervical (HeLa), ovarian (Caov-4) cancer cell line UV-Vis spectroscopy, TEM, Zetasizer, FTIR, EDX, AFM Spherical, semispherical, hexagonal, triangular 20 nm [90]
    7 Marsdenia tenacissima Leaf extract A549 lung cell line UV-vis, spectroscopy, AFM, EDS, TEM, FTIR, XRD, SAED Spherical, anisotropic 50 nm [102]
    8 Argemone mexicanaL. Aqueous extract Human colon cancer cell line, HCT-15 TEM, XRD, FTIR Hexagonal 20–40 nm [103]
    9 Couroupita guianensis Aqueous extract Leukemia cell line UV-vis spectroscopy, FTIR, XRD, SEM, TEM Spherical, triangular, tetragonal, pentagonal 7–48 nm [104]
    10 Lycium chinense Fruit extract Human breast cancer MCF7 cell line and non-diseased RAW264.7 (murine macrophage) cells UV-vis spectroscopy, FTIR, XRD, FETEM, EDX, SAED Poydispersed, agglomerated 20–100 nm [105]
    11 Tabebuia argentiea Aqueous, flower extracts Hepatic cells (Hep G2) cell line EDX, SEM Spherical 56 nm [106]
    12 Dendropanax morbifera Aqueous, leaf extract A549 lung cancer cell line and human keratinocyte cell line UV-Vis spectroscopy, EDX, FETEM, XRD, DLS Polygonal, hexagonal 5–10 nm [107]
    13 Halymenia dilatata Aqueous extract Human colorectal adenocarcinoma cells (HT-29) UV-Vis spectrophotometry, FTIR, XRD, FESEM, HRTEM, EDX, Zetasizer, DLS Triangular, spherical 16 nm [108]
    14 Dracocephalum kotschyi Leaf extract Cervical cancer (HeLa), leukemia (K562) cell lines UV-Vis spectrophotometry, TEM-SAED, SEM-EDAX, XRD, Zeta potential, DLS, FTIR Spherical 11 nm [109]
    15 Sargassum glaucescens Water extract (seaweed) Cervical (HeLa), liver (HepG2), breast (MDA-MB-231), leukemia (CEM-ss) cell lines UV-Vis spectroscopy, SEM, TEM, EDX Spherical 3.65 ± 1.69 nm [110]
    16 Trachyspermum ammi Seed extract HepG2 cancer cell line UV-Vis spectroscopy, XRD, TEM, DLS, FTIR Spherical 16.63 nm [111]
    17 Musa acuminata colla Aqueous, Flower extract MCF-7, normal Vero cell line UV-Vis, FTIR, XRD, SEM, EDAX Spherical 10.1–15.6 nm [112]
    18 aegle marmelos, eugenia jambolana and soursop Fruit extract Human breast cancer cell line MCF-7 UV-Vis spectroscopy, TEM, FTIR, Zeta potentiometer Spherical 18.28,16 nm [113]
    19 Muntingia calabura Aqueous, fruit extract Hep2 cells line UV-Visible spectroscopy, DLS, FTIR, TEM Spherical, oval 27 nm [114]
    20 Nigella sativa Ethanolic leaf extract Hep-G2 liver cancer cell line TEM, XRD, EDS, FTIR, UV-Vis spectroscopy Anisotropic 13–78 nm [115]
    21 Marsilea quadrifolia L. Aqueous Leaf extract PA-1 and A549 cell line TEM, XRD, EDX, FTIR, UV-Vis spectroscopy Spherical 10–40 nm [116]
    22 Ocimum sanctum leaf extract Dalton’s lymphoma UV-Vis spectroscopy, XRD, SEM, TEM, FTIR Spherical 12–20 nm [117]
    23 Bauhinia tomentosa Linn Aqueous, leaf extract A549, HEp-2, MCF-7 cell line FESEM, HRTEM, FTIR, EDX, XRD, TGA, UV-Vis spectroscopy Spherical 11.5–40 nm [118]
    24 Shorea tumbuggaia Bark extract Thyroid cancer (SW579) cell lines XRD, HRTEM, SAED, DLS, zeta potential, FTIR, UV-Vis spectroscopy Spherical 20 nm [119]
    25 walnut green Shell extract MCF7 cells UV-Vis spectroscopy, XRD, TEM, Spherical, triangular 10–50 nm [120]
    26 Cassia tora Leaf extract Colon cancer cells UV-Visible spectroscopy, FTIR, TEM, zeta potential, dark field microscopy Spherical 57 nm [121]
    27 Abutilon indicum Water extract of leaves HT-29 cells UV-Vis spectroscopy, SPR, FTIR, DLS, EDAX, TEM, zeta potential, XRD, TGA Spherical 1–20 nm [122][123]
    28 Catharanthus roseus Water extract of Leaves MCF7 and HepG2 cell line UV-Vis spectroscopy, HRTEM, XRD, TEM Spherical and triangular 15–28 nm [122][124]
    29 Gymnema sylvestre Water extract of leaves HT29 cell line UV-Vis spectroscopy, SEM, EDAX, XRD, FTIR Spherical 72.8 nm [122][125]
    30 Hibiscus sabdariffa Water extract of leaves U87 cell line UV-vis spectroscopy, XRD, FTIR, XPS, TEM Spherical 10–60 nm [126][127]
    31 Hygrophila spinosa Water extract of leaf HeLa cell line XRD, SEM, EDAX, DLS, FTIR and UV-Vis spectroscopy. Triangular and spherical 50–80 nm [122][128]
    32 Moringa oleifera Water extract of leaves A549 and SNO cells DLS, TEM, UV-Vis spectroscopy, zeta potential Spherical and polyhedral 10–20 nm [122][129]
    33 Podophyllum hexandrum L. Water extract of leaves HeLa cell line UV-Vis spectroscopy, TEM, XRD, FTIR Spherical 5–35 nm [122][130]
    34 Elettaria cardamomum Seed, aqueous extract HeLa cancer cell line UV-Vis spectrophotometer, SAED, FTIR, XRD Spherical 15.2 nm [131]
    35 Coleous forskohlii Root extract HEPG2 liver cancer cell line UV-Vis spectrophotometer, HRTEM, FTIR, XRD, PSA Spherical 10–30 nm [132]
    36 Scutellaria barbata Aqueous extract Pancreatic cancer cell lines (PANC-1) UV-visible spectroscopy, TEM, SAED, AFM, FTIR, DLS, EDX Spherical 0.4 μm–1 μm [133]
    37 Sargassum incisifolium Aqueous extract HT-29, MCF-7 cancer cell line, MCF-12a non cancer cell line TEM, XRD, UV-Vis spectroscopy, zeta potential, FTIR, EDX, DLS, ICP-AES Spherical 12.38 nm [134]
    38 Panax notoginseng Leaf extract PANC-1 cell line UV-Vis spectroscopy, TEM, DLS, FTIR, AFM, SAED Hexagonal, spherical, oval, triangular 80–12 nm [133]
    39 Antigonon leptopus Leaf extract Human adenocarcinoma breast cancer (MCF-7) cells UV-Vis spectroscopy, XRD, SAED, FTIR, HRTEM, EDX Spherical 13–28 nm [135]
    40 Mukia Maderaspatna Aqueous, leaf extract MCF 7 breast cancer cell line UV-Vis spectroscopy, EDAX, SEM, TEM, FTIR Spherical, circular, triangular 20–50 nm [136]
    41 Hevea brasiliensis Latex extract CHO-K1 cell line UV-Vis spectroscopy, XRD, TEM, FTIR spherical 9 nm [137]
    42 Lonicera 4 japonica Flower extract Cervical cancer (HeLa) cell line UV-Vis spectroscopy, EDX, XRD, GCMS, TEM, FTIR Polydisperse (spherical, triangular, hexagonal) 10–40 nm [138]
    43 Anacardium occidentale Leaf extract MCF-7 cell line UV-Vis spectroscopy, TEM, XRD, FTIR Spherical 10–30 nm [139]
    44 Sasa borealis Aqueous, leaf extract AGS (Gastric adenocarcinoma) cell line UV-Vis spectroscopy, TEM, EDX, XRD, FTIR, GCMS Oval, spherical 10–30 nm [140]
    45 Alternanthera Sessilis Aqueous, leaf extract Cervical cancer (HeLa) cell line UV-Vis spectroscopy, HRTEM, EDX, SAED, AFM, FTIR Spherical 20–40 nm [141]
    46 Bauhinia purpurea Aqueous, leaf extract Lung carcinoma cell line (A549) UV-Vis spectroscopy, HRTEM, SAED, XRD, EDX, FTIR Spherical, polygonal 20–100 nm [142]
    47 Crassocephalum rubens Aqueous, leaf extract MCF-7 and Caco-2 cells UV-Vis spectroscopy, TEM, FTIR Spherical 20 ± 5 nm [143]
    48 Backhousia citriodora Aqueous, leaf extract MCF-7 breast cancer cell line and the HepG2 liver cancer cell line UV-Vis spectroscopy, TEM, zeta potential, XRD, FTIR Spherical 8.40 ± 0.084 nm [144]
    49 Petroselinum crispum Aqueous, leaf extract Human cancerous colorectal cell line UV-Vis spectroscopy, TEM, EDX, FTIR.XRD Spherical, semi-rod aggregates, flower-shaped nanoparticles 20–80 nm [145]
    50 Indigofera tinctoria Aqueous, leaf extract lung cancer cell line A549 UV-Vis. spectroscopy, FTIR, XRD, TEM, EDX, AFM Spherical, triangular, hexagonal 6–29 nm [146]

    This entry is adapted from 10.3390/molecules26216389

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