A Review on Recent Trends in Green Synthesis of Gold Nanoparticles for Tuberculosis

Tuberculosis (TB) is a contagious disease that has affected mankind. The anti-TB treatment has been used from ancient times to control symptoms of this disease but these medications produced some serious side effects. Herbal products have been successfully used for the treatment of TB. Gold is the most biocompatible metal among all available for biomedical purposes so Gold nanoparticles (GNPs) have sought attention as an attractive biosynthesized drug to be studied in recent years for bioscience research. GNPs are used as better catalysts and due to unique small size, physical resemblance to physiological molecules, biocompatibility and non-cytotoxicity extensively used for various applications including drug and gene delivery. Greenly synthesized GNPs have much more potential in different fields because phytoconstituents used in GNP synthesis itself act as reducing and capping agents and produced more stabilized GNPs. This review is devoted to a discussion on GNPs synthesis with herbs for TB. The main focus is on the role of the natural plant bio-molecules involved in the bioreduction of metal salts during the GNPs synthesis with phytoconstituents used as antitubercular agents.


Introduction
Tuberculosis (TB) is a bacterial infectious disease caused by Mycobacterium tuberculosis, one of the oldest bacterial diseases. TB is still affecting and posing major health, social and economic burdens at the global level. However, low and middle-income countries are mainly affected. If the disease would not be managed efficiently then TB will be resurged due to some other diseases like HIV infection as well as multiple drug-resistant tuberculosis (MDR-TB) by considering these facts in 1993, the World Health Organization (WHO) took an unprecedented step and declared TB a global emergency. 1,2 Synthetic anti-TB drugs are a two-edged sword while they destroy pathogenic M. tuberculosis they also select for drug-resistant bacteria against which those drugs are then ineffective1. TB either kills the infected individual or renders him/her incapable of assuming normal functions. Upon gaining entry into a new host, M. tuberculosis may result in an active infection or remain latent. 3 TB is spread via various sources like infectious aerosols from an infected person. TB infections and their development are represented in Figure 1.
Wide ranges of phytoconstituents having the desired pharmacological effect on the body were responsible for anti-tubercular activity includes alkaloids 4-6 glycosides 7-9 glycoterpenoids, 10 diterpenoids glycosides, 11 tannins, 12 phenolics and amides 13-18 xanthones 19-23 quinones, 24 sterol 25-28 triterpenoids. [29][30][31][32][33][34][35][36][37] Terpenoids are scope for compounds that can be developed as future antimycobacterial drugs. It has been reported that ursolic and oleanolic acids are not so toxic and possess antimicrobial activity against some multi-resistant bacteria. 34,[38][39][40][41] Various antimycobacterial chemical compounds have also been isolated from plants, including ellagitannin punicalagin, allicin, and these compounds offered various clues for effective management of the disease to lessen the global burden of TB and drug-resistant M. tuberculosis strains. 42 In this review, the author has emphasized the green synthesis of gold nanoparticles (GNPs) with herbs for TB (Antimicrobial and antibacterial activity). The main focus is on the role of the natural plant biomolecules involved in the bioreduction of metal salts during the GNPs synthesis with phytoconstituents used as antitubercular agents. The plants having phytoconstituents acting as antitubercular agents discussed in Table 1.

Green synthesis of gold nanoparticle
In the late 1990s, the development of non-toxic methods has embraced the principles of green chemistry. 155 Green synthesis of metal nanoparticles has received widespread attention in the past decade due to its ability to meet environmental and economic goals simultaneously without using the chemical and cost-effective too. Green synthesis common approaches for GNPs have been shown in Figure 3. For the green synthesis of GNPs, the antioxidant components of the studied plant extracts are responsible for the reduction of metal salts, leading to the growth and stabilization of the GNPs. 156 Medicinal herbs having phytochemicals like as alcohols, phenols, proteins, terpenes, alkaloids, saponins, etc which can act as reducing as well as capping agents in the GNPs biosynthesis. 157,158
Terpenoids play a crucial role in the reduction process of metal ions into nanoparticles, like eugenol the main terpenoid present in many plants 111 . GNPs of Schinus molle L extract contain phenol, which shows that the differences in transmittance. Purified phenolics like gallic and protocatechuic acid act as reducing and capping agents in GNP synthesis because of the involvement of functional groups present in this phenolic compounds. [161][162][163] These findings can help to determine the mechanism of metal nanoparticles by using crude extracts formation and stabilization. Cinnamomum verum extract contains polyols like (flavones and terpenoids) and polysaccharides, both contents act as reducing agent in metal ion synthesis. 164 Flavonoids belong to the group of polyphenolic compounds that comprise several subgroups: anthocyanins, isoflavonoids, flavonols, chalcones, flavones, and flavanones, which can actively participate in the reduction and chelation of metal ions into nanoparticles. Literature established that reactive hydrogen atom release from tautomeric transformations of flavonoids from the enol-form to the keto-form can reduce metal ions to form nanoparticles. For example, flavonoids luteolin and rosmarinic acid present in Ocimum basilicum extracts it is the transform from the enol-to the keto-form. 111 Apiin glycoside obtained from Lawsonia inermis used for the synthesis of anisotropic gold and quasi-spherical silver nanoparticle. 165 The oxygen atoms belonging to 3-hydroxy and 4-oxo, and the 5-hydroxy and 4-oxo groups, are the preferred potential sites for chelation on quercetin. 166 Many proteins contain active sites for metal ion accumulation and reduction where GNPs can form and be stabilized. In the process of nanoparticles formation, Protein donates electrons to react with metal ions and their subsequent stabilization that leads to the formation of nanoparticles. 167 Some low molecular weight protein bands present in the soya bean extract, this may have been used up in biosynthesis of GNPs. 168 The compounds present in the extracts can act as reducing as well as stabilizing agents and render more biocompatibility to the green synthesis of GNPs. 169 High cost, use of environmentally hazardous chemicals, nonavailability and presence of toxic capping agents limit the use of various physical and chemical methods. [170][171] These limitations contributed the need for the development of new methods and materials for the production of nanoparticles based on the principles of ''Green synthesis'' . The emphasis in this approach is on the synthesis and application of the nanoparticles for a maximum societal benefit, with minimal impact on the ecosystem. 172 In Table 3 some part of plants which have been exploited by researchers for making AuNPs from the last decades have been summarised.

Role of microorganisms for the green synthesis of GNPs
A variety of microorganisms are interacted with inorganic metals like gold, zinc, and silver and are known to use in bioleaching of minerals. 213 Microbial cells treated with gold nanostructures synthesize by gold salts which are then isolated and purified using various techniques to obtain GNPs. Table 4 reflects a variety of microbes along with their genus which was used to make GNPs of different size ranges.

Role of biomolecules for the green synthesis of GNPs
Biomolecules produced by living organisms to catalyze biological functions, such as nucleic acids, amino acids, lipids, and carbohydrates, possess hydroxyl and carbonyl functional groups in their structure which can reduce Au3+ ions to Au0 neutral atoms. These Au0 neutral atoms are then capped to form stabilized GNPs. 234 This method can use for the biosafety of the reactants in GNPs synthesis. In Table 5 various biomolecules with type and size have been discussed.

Bioreactors for green synthesis of gold nanoparticles
Phytomining is the approach through which plants can reduce metal ions both on their surface and in various organs and tissues remote from the ion penetration site. The metals like copper, gold, silver, platinum, iron, and many others accumulated by the plants can be recovered after harvesting methods. For example, Brassica juncea and Medicago sativa, both the plant accumulate 50 nm silver nanoparticles (13.6% of their weight) when grown on silver nitrate as a substrate whereas M. sativa accumulate 4 nm gold icosahedra, 250 and Iris pseudacorus (yellow iris) accumulate 2 nm semi-spherical copper particles when grown on substrates containing salts of the respective metals. Few approaches have been demonstrated in which different varieties of plant extracts have been used in combination with different varieties of acids and salts of metals. 170,251 Factors affecting the formation of metal nanoparticles in plants Various limitations of nanoparticle synthesis by phytoconstituents are observed and it needed to be resolved carefully before industrial manufacture. The prime limitation is the intricacy in the identification of the phytoconstituents present in plants responsible for the NPs synthesis and therapeutic activity. The amount of reducing agent needs to be controlled because it hampers the reduction rate which results in the formation of large aggregated nanoparticles. Simultaneously the process parameter like thermal heating must be under controlled  because during synthesis it can damage and denature various active molecules like sugars, and proteins resulting in the loss of activity. The reaction rate can be optimized by controlling the reduction reaction by varying the concentration of phytochemicals carefully. All the factors affecting the green synthesis of metal nanoparticles are presented in Figure 4. To improve the efficacy, size and morphology of nanoparticles synthesized from biological sources by microorganisms several parameters need to be monitored like microorganism type, growth medium, growth stage (phase), synthesis conditions, reaction mixture pH, substrate concentrations, size, shape, incubation temperature and reaction time. The reduction process and stability of the biologically synthesized nanoparticles have a major concern and have to be controlled to improve the efficacy of the biologically synthesized nanoparticles. Major limitations in biologically synthesized nanoparticles are, the reduction process is quite slow and stable due to the decomposition of microorganisms over time. 111,157,[252][253][254] Nanoparticle aggregation is high at highly acidic pH over the reduction process and nucleation of reduced atoms. This may be related to the fact that a larger number of functional groups that bind and nucleate tetra-chloroauric acid ions become accessible at acidic pH. 115,[255][256][257] Efficiency and reaction rate of metal nanoparticle synthesis increases as an increase in the temperature. High temperatures required for crystal particle formation (nucleation rate is higher as increases the temperature). Interaction of phytochemicals with the nanoparticle surface may alter by elevated temperatures. [258][259][260][261][262][263] Morphological diversity of the nanoparticles: triangles, hexagons, pentagons, cubes, spheres, ellipsoids, nanowires, and nanorods may occur due to the variation in concentration and composition of bioactive compounds present in plants. 252,264 Green synthesis of gold nanoparticles for tuberculosis Apart from diversified biomedical applications, GNPs have been reported for antimicrobial activity against food and agriculture pathogens. 199 Inherent property of antibacterial and antimicrobial 265 activity of GNPs along with the entrapped plant extract, facilitate the early recovery from TB. The proposed mechanism for antibacterial activity of GNPs is that it increases gene expression in the redox process which leads to the death of bacteria and fungi. The nano size, surface area and photo thermic nature of GNPs directly influenced the antimicrobial activity. 266 Another excepted mechanism is that intracellularly GNPs attached to the sulfur base present in cells in the form of thiol group in enzymes which leads the disturbance of respiratory chain suddenly by the generation of a large number of free radicals leading to death. On the contrary, the GNPs decrease ATP activities wherein they reduce the t RNA and ribosomal interaction. GNPs also block the transmembrane hydrogen efflux however lesser concentration of GNPs can inhibit bacterial growth about 250-fold. Due to the smaller size of GNPs then bacterial cells, they stick on the cell wall of pathogens and delay cell process, causing death. Some report shows a different mechanism when compared to other metal nanoparticles. GNPs due to the charge difference on the cell wall and nanoparticle surfaces it attracts bacterial DNA. On the other side, GNPs show the varied activity of gram-positive and gram-negative bacteria, which are classified based on the thick layer called peptidoglycan. Peptidoglycan generally consists of two joined amino sugars, N-acetylglucosamine and N-acetylmuramic acid (NAM), with a pentapeptide coming off the NAM forming an inflexible structure to diffuse the GNPs. Therefore, the peptidoglycan is very strong in gram-positive bacteria that penetrate GNPs across cell wall whereas gram-negative bacteria contain a thin layer which easily undergoes cell death. The anti-microbial activity also assisted by the concentration of capping agents and purification methods apart from the size and peptidoglycan thickness. In green synthesized GNPs the antimicrobial activity may be due to the synergistic effects of GNPS with plant extracts. 267 The biophysical interactions between bacteria and nanoparticle occur through aggregation biosorption and cellular uptake that can damage the membrane and produce toxicity. 268 The mechanism of antibacterial activity of the GNPs is attributed to the generation of reactive oxygen species that causes an increase of the oxidative stress of microbial cells and the release of intracellular lactate dehydrogenase enzyme into extracellular medium in form of vacuole formation as an indication of potent activity. [269][270][271] Such effect was enhanced and exaggerated by photothermal degeneration in a combined approach, GNPs-laser, which causes quick loss of cell membrane integrity. 272 GNPs have advantages over other metal nanoparticles because they are chemically inert, biocompatible nature and not producing cytotoxicity. Gold is used internally in humans for the last 50 years. 273 Physical properties of the nanoparticle may differ from their corresponding parent materials by decreasing the size of nanoparticles and this relation offered many opportunities for many scientific breakthroughs. GNPs produced good antibacterial activity. It had been shown their best result when particles aggregation is not observed at high levels. GNPs with the same shape and size exhibited different inhibitory effects by changing surface modifications agents. 265 It can also use in targeted molecular imaging in living subjects. 274 Recentely Gupta et al reported that the GNPs of ethanolic and hydroalcoholic exhibited anti-tubercular activity only at MIC 2.5 µg/mL and 20 µg/mL, respectively while ethanolic and hydroalcoholic extracts showed activity at much higher concentrations 50 µg/mL and 75 µg/mL, respectively. 275 GNPs from Nigella arvensis (NA-GNPs) leaf extract were evaluated for antibacterial, antioxidant, cytotoxicity and catalytic activities and Chahardodli et al observed that NA-GNPs showed excellent cytotoxicity effects against H1299 and MCF-7 cancer cell lines with an IC50 value of 10 and 25 μg/mL, respectively and catalytic activity of NA-GNPs against methylene blue was 44%. 276 Cheng et al synthesize GNPs using Chenopodium formosanum shell extract and concluded that GNPs possessed potent antibacterial activity against Escherichia coli and Staphylococcus aureus. 277 Sunderam et al 278 reported that green synthesized GNPs of Anacardium occidentale leaves extract, data presents good antibacterial effect against Escherichia coli and Bacillus subtilis and exhibited 74.47% viability on PBMC and 23.56% viability on MCF-7 cell lines at a maximum concentration of 100 µg/mL. 278 Katas et al 279 reported that the concentration of chitosan needed to synthesize antibacterial chitosan-GNPs with Lignosus rhinocerotis (LRE) was lower than those without LRE, suggesting that the addition of LRE as reducing agent resulted in higher antibacterial activity. Thus, chitosan as a stabilizing or capping agent and LRE as a reducing agent for the production of GNPs improved antibacterial activity of their resultant nanoparticles. [276][277][278][279] Veena et al 280 developed the green synthesis of Vitex negundo GNPs from leaf extracts and results exhibited strong antibacterial activity against gram-negative strains and moderate activity against gram-positive strains. 280 The overview of the review is presented in Figure 5.

Conclusion
The study of green synthesis of GNPs is a quickly evolving field in nanotechnology for TB. The present review summarises exhaustive literature for plants containing phytoconstituents having antitubercular activity along with the understanding of the synthesis of GNPs not only using plant extracts but biomolecules, microorganism, and various bioreactors. A detailed study is needed to give a lucid mechanism of biosynthesis of GNPs using biomolecules; microorganism present in different plant extracts which will be valuable to improve the properties of GNPs for TB treatment. With green chemical syntheses of these nanomaterials, researchers will able to conduct in-depth studies investigating biomedical applications without further biocompatibility preparations. In the coming years, the green chemistry procedure which utilizes plants their constituents, microorganisms, and biomolecules for nanoparticle preparation for TB has used as an alternative to conventional physicochemical methods since it is facile, rapid, cost-effective, and ecofriendly.

Ethical Issues
Not applicable.

Conflict of Interest
Authors declare no conflict of interest in this study.