Introduction:
Nanobiotechnology serves as an imperative technique in the development of clean, nontoxic, and eco-friendly procedures for the synthesis and congregation of metal NPs having the intrinsic ability to reduce metals by specific metabolic pathways. Silver nanoparticles are particularly interesting nanoscale systems due to the ease with which they can be synthesised and modified chemically. Synthesis of silver nanoparticles has been widely researched due to their interesting size and shape-dependent properties [26]. These novel properties can be tailored by controlling their size, shape, composition, structure and crystallinity. Due to their size features and advantages over available anti-microbial properties, chemical imaging drug agents and drugs, silver nanoparticles particles have been examined as potential tools for medical imaging as well as for treating diseases [27].
Surface modification of nanoparticles may generate novel and interesting opportunities to develop efficient antimicrobial agent. Although Ag nanoparticles exhibit significant antibacterial properties, they also possess some disadvantages. Studies have found that nanoparticles have demonstrated toxic action on mammalian cells [28]. However, recent studies have demonstrated that combining Ag nanoparticles with antibiotics may reduce the toxicity of both agents towards mammalian cells and synergistically enhance their antimicrobial activity. Moreover, combination of nanoparticles and antibiotics lower the amount of both agents in the dosage which also increases antimicrobial properties. Due to this conjugation, the concentrations of antibiotics were increased at the site of antibiotic to microbe contact and thus accelerated the binding between microbes and antibiotics [28]. Another alternative to overcome the toxicity towards mammalian cells is the use of peptide linkages to Silver nanoparticles [28].
The interaction of metal ions with biomolecular targets i.e. amino acids, peptides or proteins is known to play a fundamental role in several biological routes including electron transfer reactions, metal transport and storage. Additionally, the inhibitions of enzymes by metal complexes with ligands are well known. It functions by ligand exchange reactions where the ligand present in the administered drug is substituted by the targeted enzyme which is a primary application in many metal-based drugs.
Therefore, it is one of the most significant processes in bioinorganic chemistry [29] Surface modification of Ag nanoparticles can also help in tuning their properties to suit various applications and determine the interaction of the components as such chemical functionalisation of the surface of Ag nanoparticles has attracted much attention [30]. Functionalised nanoparticles can be applied in various areas including engineering, medical and biological applications. The main aspect of functionalisation is to optimise the active sites on the nanoparticle surface i.e. hydrophilic, hydrophobic, conductive etc [30].
The core-shell nano bio structure enhances the thermal and chemical stability of the nanoparticles, improves solubility, makes them less cytotoxic and allows conjugation of other molecules to these particles [32].
Amino acids are natural molecules characterized by a chiral carbon that makes bond with a carboxylic acid group, an amine group, a hydrogen atom, and a side chain that is specific to each amino acid. As a quaternary compound, amino acids are a combination (primarily) of carbon, oxygen, hydrogen, and nitrogen. Amino acids have been shown to be useful in the synthesis of silver nanoparticles and silver nanoparticles have been synthesised and characterised by the reduction of amino acids [34].
Silver nanoparticles synthesized in water and subsequently capped with amino acids can contribute immensely in various applications such as drug delivery and gene transfer. Recently, it has been shown that stable gold and silver nanoparticles that were surface-functionalized with either tyrosine or tryptophan residues were strategically synthesised and examined their potential to inhibit the amyloid aggregation of insulin [35]. This result offers significant opportunities for developing nanoparticle-based therapeutics against diseases related to protein aggregation.
The major focus of this chapter is to address theissues by developing Chiral tryptophan and tyrosine silvernanoparticles ina highly facile and controllable manner using amino acids as reducing andfunctionalizing agents. Although, amino acid based synthesis of nanomaterials is available in literature, [36] their application at thenano-bio interface has not been explored widely. Moreover, their capabilityto produce chiral amino acid silvernanoparticles provides an opportunity toinvestigate their biological applications with different composition andsurface functionality.The amino acids were chosen over conventional reducing agents formetal nanoparticles synthesis because amino acids-mediated synthesis ofnanoparticles offers advantages of making nanoparticles of differentcompositions in a single step without using any additional stabilizers or toxic chemical reducing agents in contrast to using other reducing agents [37] and this is a major step towards greener synthesis of nanomaterials [38].
Further, amino acids such as tryptophan andtyrosine have the potential to reduce silver ions into theirnanoparticulate forms through their indole and phenol groups, respectively, [36] while their amine and carboxylic acid groups remain intact and mayessentially behave the similar manner as amino acids in a protein. Therefore, the availability of amine and carboxyl groups of theamino acids on the nanoparticle surface may provide further opportunitiesand biological identity to anchor amino acid-functionalized metalnanoparticles to biological surfaces through polyvalent interactions, like that in enzymatic interaction with any substrate [35]. Moreover, amino acids are zwitterionic in nature and therefore surface charge of amino acidfunctionalisednanoparticles can be easily tuned by varying the solution pH. These properties of amino acids, when combined with the nanoparticlecomposition in a cooperative manner, amino acid-functionalizednanomaterials have significant potential to be used as generic materials forenzyme-like or antibacterial applications [35].
In this chapter, synthesis and characterisation of silver nanoparticles using Chiral Tryptophan and Tyrosine amino acids is described. Tryptophan (Trp or W) and tyrosine (Tyr or Y)amino acids were selected as reducing agent to produce different surfacefunctionalization on nanoparticulate systems due to their similar isoelectricpoints (tryptophan: 5.89, tyrosine: 5.66) and hydropathy indexes(tryptophan: -0.9, tyrosine: -1.3,), however both the amino acids havedifferent side functional groups (tyrosine: phenol group, tryptophan: indolering) and these side groups are responsible for the reduction of metal ions. In addition, this chapter also describes about the importance of Chiral Tryptophan and Tyrosine amino acids on the synthesis of silver nanoparticles.
1.2 Materials and Methods:
1.2.1 Synthesize of D, L- Tryptophan Stabilized Silver Nanoparticles
An advanced procedure illustrated by Daima et al., (2015) has been utilized in order to develop D, L-tryptophan as well as D; L-tyrosine reduced silver nanoparticles (Ag NPs). In the general procedure of the synthesis of D or L-tryptophan reduced silver nanoparticles, a conical flask having a capacity 2000ml was used. It was filled with a mixture having deionized water (DI) (Milli-Q), 0.1 M KOH, and 0.01 M tryptophan having a ration (v/v) 780 mL, 20 mL and100 mL, respectively. The ingredients were stirred by using a magnetic stirrer and at the end, 100 mL of 1 M Ag2SO4 was added into the flask. Then it was heated for 4 minutes and the material in the flask was continuously stirred. The solution was kept in the heat that was turned into yellow color after approximately 15 minutes, but the stirring was not ceased. Subsequently, the heating was stopped, and the solution was stirred for additional 10 minutes. After this procedure, the paraffin seal was utilized in order to seal the flask, and it was left to be stabilized for the duration of one day (24 hours). In order to concentrate this sample, the solution under the reduced pressure of 70˚C was evaporated while rotating so that the anticipated concentration could be achieved.
1.2.2 Production of D, L-Tyrosine Stabilized Silver Nanoparticles
The procedure adopted for the production of D, L-Tyrosine Stabilized Silver Nanoparticles is almost similar to the preparation of Tryptophan Silver Nanoparticles. In the typical procedure of synthesis of D, L-Tyrosine Stabilized Silver Nanoparticles a conical flask having capacity 2000 mL was taken, and it was filled with a mixture of DI water, 0.1 M KOH, and 0.01 tyrosine in a ratio (v/v) 780 mL, 20 mL, and 100 mL, respectively. The ingredients were stirred using a magnetic stirrer, and at the end, 100 mL of 1 M Ag2SO4 was added into the flask. Then, it was heated for 4 minutes and the material in the flask was continuously stirred. The solution was kept in the heat, and after approximately 15 minutes, it was turned into yellow, but stirring was not ceased again. Ultimately, the heating was stopped, and the solution was stirred for additional 10 minutes. After this procedure, the paraffin seal was utilized in order to seal the flask, and it was left to be stabilized for the duration of one day (24 hours). In order to concentrate this sample, the solution under the reduced pressure of0˚C was evaporated while rotating so that the required concentration can be achieved.
For the procedure of dialysis, tubing (Sigma D9527) having the molecular weight of 14,000 was utilized for the purification of AgNPs and the elimination of unreacted chemicals was conducted. Before utilizing the dialysis tubing, it was properly rinsed in deionized (DI) water that was subsequently boiled in DI water for minimum 5 minutes. After this, the tube was rinsed again, and it was filled with the solution of Nanoparticles and ends were clamped. Afterward, the sealed dialysis bag was adjusted in the 2 L beaker that was filled with DI water (Milli-Q) and kept for dialysis for one day (24 hours) on a magnetic stirrer. During this procedure, at the 3rd and the 10th hour, water changes were conducted. Utilizing the 12 kDa dialysis membrane, the free metal ions, and the unbound amino acids were removed from the nanoparticles, and the solution was kept for dialysis overnight. The nanoparticle solutions remained stabilized after the procedure of concentration and dialysis, and it showed that nanoparticles are strongly covered with the amino acids. Under the standardized laboratory conditions, the solutions prepared were observed to be stable at room temperature for the duration of 6 months. Subsequently, they were utilized for the biological studies.
For the preparation of samples of SEM and TEM, approximately 3 uL solutions of the concentrated nanoparticles were drop cast on lacey carbon support grids for TEM and silicon chips for SEM, and it was allowed to be dried before imaging process. For the sample preparation for Zeta potential measurements, UV-Visible absorption spectroscopy, DLS, the AgNP solutions were diluted utilizing DI water (Milli-Q) in order to attain the absorption peak of ≅ 1 optical density before the procedure of reading. In order to prepare the sample for TGA and FTIR analysis, the solutions of concentrated Nanoparticles was dropped cast on a silver substance, but they were dried before analysis.
1.3. Results and Discussion
Tyrosine and tryptophan were utilized as reducing agents in order to stabilize silver nanoparticles preparation in the aqueous medium. In this case, the phenol group of tyrosine and the indole group of tryptophan participated in the reduction of the metallic ions [39] nanoparticles that were synthesized utilizing the aforementioned procedure displayed stability for one day. In order to evaluate their stability for next six months, further analysis was required. The silver nanoparticles once produced, they were characterized for their specific structure, charge, shape, size, and stability by utilizing the aforementioned mentioned technique and the results for each method have been shown below.
Transmission Electron Microscopy (TEM)Studies of Chiral Tyrosine and Tryptophan Silver Nanoparticles
The images of D and L-tryptophan silver Nanoparticles that has been taken through TEM have been displayed in the Figure 2.3.1. Figure 2.3.2 displays the images of D and L-Tyrosine AgNP taken through the similar method.
It has given a comprehensive representation to each sampling.
Figure 2.3.1. TEM images of D tryptophan (A) and L tryptophan (B) reduced silver nanoparticles. Scale bars correspond to 50 nm.
Figure 2.3.2. TEM images of D tyrosine (A) and L tyrosine (B) reduced silver nanoparticles. Scale bars correspond to 50 nm.
Once the samples are dried, they appear not to be aggregated. It has been confirmed through the TEM analysis that the silver nanoparticles are pseudo spherical and they are often referred as quasi-spherical in shape. Figure 1.3.1 and 1.3.2 has shown the morphology of the silver nanoparticles that is in good agreement with the results that have been previously published in the literature [35]. On the other hand, biological activities are affected by the size and shape of the nanoparticles [40]. The images obtained from TEM have displayed that the isoforms of tryptophan and tyrosine silver nanoparticles look similar in shape, but they have a different size. The excessive absorption of tryptophan and tyrosine on the surface of silver nanoparticles might lead to aggregation of nanoparticles [37]. The nanoparticles that have been synthesized with amino acids have a quasi-spherical or spherical shape having an approximate size of 10 - 40 nm.
The silver nanoparticles and the gold nanoparticles have a difference in size, and it has been noticed that gold nanoparticles have a size ranging from 5 to 10 nm whereas silver nanoparticles have a size ranging from 10 to 40 nm. However, the shape is quasi-spherical or spherical in the context of silver and gold nanoparticles [35].
Scanning electron microscopy (SEM):
Figure 2.3.3 presents the SEM images of D and L-Tryptophan AgNP whereas Figure 2.3.4 represents the images for D and L-Tyrosine AgNP. SEM has a limited resolution due to which the difference in shape and size obtained for the Nanoparticles is not very clear. (Repeat the experiment)
Figure 2.3.3. SEM images of D tryptophan (A) and L tryptophan (B) reduced silver nanoparticles. Scale bars correspond to 5 um.
Figure 2.3.4. SEM images of D tyrosine (A) and L tyrosine (B) reduced silver nanoparticles. Scale bars correspond to 5 um.
Dynamic Light Scattering Measurements of Chemically Distinct Spherical silver
Nanoparticles
Dynamic light scattering (DLS) was further utilized to characterize the silver nanoparticles. It was used to determine the size of the nanoparticles under consideration. The particle size distribution of the silver Nanoparticles (Chiral forms of Tryptophan and Tyrosine) has been displayed in the Table 2.3.2 and Table 2.3.3, respectively. D, L-Tryptophan, and tyrosine silver nanoparticles have the average hydrodynamic diameter of 41.9 nm. It was noticed that the values obtained from DLS were larger compared to TEM imaging. DLS facilitates in obtaining the size of the respective particles that are apparently moving in the solution and on the other hand, TEM provides the physical size of the nanoparticles when they have been dried [41]. In this context, hydrodynamic size can be regarded as the size of a hypothetical hard sphere that will diffuse at an equivalent rate as the particles are being characterized. It can be measured by using the Stokes – Einstein relation (Pecora, 2000).
Furthermore, the TEM gives the physical size of the particles while the size that is measured by DLS is linked to the surface of the particles. Consequently, the data obtained by DLS are always larger than those obtained by TEM [41]. In addition, DLS signal are controlled by the polydispersity of the sample that depends on particles size and larger particles potentially scatter extra light than smaller particles. DLS and TEM have provided such size and shape of the silver nanoparticles that are in good agreement with the previously published literature [42].
Measuring Zeta Potential of Tyrosine Silver Nanoparticles and Chiral Tryptophan
Zeta potential is a physical property, and this specific term is employed for the electro kinetic potential in colloidal systems such as bio-nanocolloidal systems. It facilitates in the provision of essential and valuable information about the stability of the colloidal systems [42]. The ζ-potential measurements for the chiral formation of tyrosine and tryptophan-based AgNPs are presented in Table 2.3.4 and Table 2.3.5, respectively. L-Tyrosine AgNPs provided negative value of -17mV for zeta potential, whereas D-Tyrosine AgNPs showed slight higher negative value of -20mV showing that D, L-Tyrosine AgNPs possess the lowest level of stability than D, L-tryptophan AgNPs and it has been displayed in Table 2.3.4and 2.3.5 The stabilizing agent on the AgNP surface can be further confirmed by the measuring the charge at the surface of the nanoparticle.
In fact, the mark for the stability of the particle is the magnitude of the zeta potential [35]. Because of the presence of the tryptophan and tyrosine amino acids on the surface, the pH of the nanoparticles can specify the possible charges. The surface charge of the nanoparticles will be negative if the pH of the solution become higher than the isoelectric point that is 5.66 or 5.89 [35].
On another hand, the nanoparticles will have a positive charge if the pH is lower than the isoelectric point. At the physical pH, the nanoparticles of tyrosine and tryptophan were negatively charged. The higher value of the negative zeta potential was the indication of the stability of the nanoparticles and their aggregation in the media. This is because that aggregation is induced by the nutrients and the inorganic salts. The surface charge associated to biological activities are supposed to be excluded because all the nanoparticles have the same surface charge. Due to the zwitter nature of the tryptophan and tyrosine, the charge on the surface of the amino acids can be simply changed by altering the pH of the solution.
The gold nanoparticles had the similar negative charge of zeta potential as that of the D, L-tryptophan, and tyrosine silver nanoparticle. Tryptophan and Tyrosine of AuNPs. D-Tyrosine AuNPs produced a highest negative value which is -27mV, whereas D-Tryptophan AuNPs displayed lowest negative value (-18.1mV). On the other hand, L-Tryptophan AgNPs generated highest negative value (-26.9mV). Subsequently, it indicatedthat D and L form of Tyrosine are coated with AuNPs possess the highest degree of stability than tyrosine coated with AgNPs.
Tyrosine Silver Nanoparticles and Chiral Tryptophan's: UV-Visible Spectroscopy
Surface Plasmon Resonance (SPR) dominates the optical absorption spectra of metal nanoparticles that shift to longer wavelengths due to increasing particle size. Plasmon absorption of silver nanocluster's shape and position extremely depends on the particle shape, size, surface adsorbed species and dielectric environment. According to Mie’s theory, in the absorption of spherical nanoparticles, just a single SPR band is anticipated to be present while based on the shape of particles anisotropic particles may produce two or more SPR bands [43]. Because of transverse oscillations, a single peak is expected exclusively for AgNPs, though shifting of the peak or broadening can be caused by a reduction in the symmetry of the particles such as quasi-spherical particles [44]. Additionally, numerous other factors including the geometry, size, aggregation state and surface characteristics of the particles have effects on the position of this peak along with a refractive index of the surrounding medium [45].
The UV-Visible spectra of the Chiral Tryptophan and Tyrosine AgNPs are presented in Figure 2.3.5 and Figure 2.3.6, respectively.
A peak having maximum value shown by all spectra lies between 410-420 nm that is in good agreement with expected values reported in the literature [46].
Figure 2.3.5: UV-visible absorption spectra of stabilised D Tryptophan AgNPs (A) and L Tryptophan AgNPs (B).
Figure 2.3.6: UV-visible absorption spectra of stabilised D Tyrosine AgNPs (A) and L Tyrosine AgNPs (B).
A maximum absorbance having a value of 410 nm with the particular broader peak is generated by D and L-Tryptophan AgNPs. This is because the broadening is usually associated with a polydispersity (wider size distribution) or aggregation within the sample, although the minor redshift noticed (in another words, a shift toward higher wavelengths) is usually due to a small increase in size [47]. With maximum absorption for both particles at 415-425 nm, both D and L-tyrosine showed stabilized AgNPs with distinctive absorbance spectra [35].
According to Kelly et al. (2003), the occurrence of a band at app. 410 nm, (SPR) shows the formation of spherical AgNPs for the absorption spectra. On another hand, Mie theory describes that size of the nanoparticle and the maximum absorption wavelength is interrelated to each other, the nanoparticle with a smaller diameter will have the smaller wavelength [30] .
For the UV-Vis spectral analysis, the main parameters were the intensity of the band that shows the formation of huge quantities of nanoparticles and the full-width at half maximum (abbreviated as FWHM). Subsequently, the size distribution of the colloidal dispersion is showed by FWHM of the UV-Vis spectral bands [30]. When the FWHM is smaller, the polydispersity will be lesser keeping the homogeneous size of the nanoparticle. Changes in the external environment of the nanoparticle effects SPR where redshift of the maximum absorbance is caused by an upsurge in the refractive index of the surrounding medium. Particularly tryptophan has the highest refractive index at the value of 1.485 [48] compared to the tyrosine having refractive index about1.47 [49], which additionally confirm its maximum absorption towards higher wavelengths. When the amino acid-reduced metal nanoparticles showed no sign of aggregation by keeping it in deionized water for more than six months under standard laboratory conditions, it reveals the stability of these particles. Therefore, preparation of highly stable, free metal ion, alloy nanoparticles and monometallic are the significant benefit of amino acids-mediated preparation of these respective nanomaterials.
Atomic Absorption Spectroscopy: Chiral Tryptophan's and Tyrosine Silver Nanoparticles
In each sample, after nanoparticles preparation, atomic absorption spectroscopy (AAS) was employed in order to measure the metal content because all prepared metal nanoparticles may not be reduced the reaction. So, AAS studies were carried out on aqua regia (a corrosive mixture containing 1HNO3 + 3HCl) digested nanoparticle solutions after nanoparticle formation. The obtained data for AAS regarding Tyrosine AgNPs samples and Chiral Tryptophan showing varying tendency is presented in Table 2.3.6. The D forms Tryptophan and tyrosine AgNPs has higher molar ratios compared to the L forms Tryptophan and tyrosine AgNPs representing that the silver metal was reduced by L-amino acids was better compared to D-amino acids.
Fourier Transform Infrared Spectroscopy Analysis (FTIR) of Chiral Tryptophan and Tyrosine silver Nanoparticles:
Tryptophan and Tyrosine silver nanoparticle's FTIR spectroscopic analysis, found by amino acids was conducted to evaluate interaction mechanism of functional groups with the metal nanoparticles present in tryptophan or tyrosine amino acids interact.Moreover, FTIR analysis facilitates in information regarding the surface chemistry of nanoparticle [50].
According to Iosin et al. (2010), spherical silver nanoparticles that are frequently regarded as biocompatible are prepared by Tryptophan (TRP) which is one of the important amino acids. Similar to all amino acids it consists of an amino and carboxyl groups, and furthermore, it includes an indole ring present on the side chain. Numerous specific properties of molecule and tryptophan are developed due to the presence of indole ring shows hydrophilic nature. Among D and L forms of Tryptophan. D- Form is less available while later (L- form) is usually constitute a part of most of the proteins.
Figure 2.3.7. FTIR spectral analysis of (A) D-tryptophan (black curve) and D-tryptophan silver nanoparticles (red curve) and (B) L-tryptophan (black curve) and L-tryptophan silver nanoparticles (red curve).
Figure 2.3.7 are illustrating FTIR spectra of D and L-tryptophan and D and L-tryptophan AgNPs. The decrease in chloroauric acid during nanoparticle preparation is associated to indole group, and the tryptophan molecules were possible to be attached to the surface of the nanoparticle through coordination complex with the primary and secondary (pyrole) amine groups [35]. The typical images of the primary amine and tryptophan carbonyl groups is indicated by D and L-Tryptophan showed by diverse peak pattern in a range of 1600 cm-1 to 3800cm-1.On another hand, the shift of the carbonyl stretching frequency of D-tryptophan from 1634 cm-1 to 1977 cm-1 (displayed as black curve and red curve, respectively), can be ascribed to the formation of carboxylate ions during the process of reaction [47]. Moreover, Figure 2.3.12b presents the FTIR spectra of L-tryptophan prepared silver nanoparticles, while the shift of the carbonyl stretching frequency of D-tryptophan from 1765 cm-1 to 1965 cm-1 (black curve and red curve, respectively) that can be ascribed to the development of carboxylate ions during the respective reaction. In different samples, the corresponding difference in the value of shifts is associated to the changeable degree of carboxylate ion formation.
Figure 2.3.8. FTIR spectral analysis of (A) D-tyrosine (black curve) and D-tyrosine silver nanoparticles (red curve) and (B) L-tyrosine (black curve) and L-tyrosine silver nanoparticles (red curve).
Based on FTIR analysis of D, L – Tyrosine silver nanoparticles and D, L-Tyrosine, D-tyrosine has a carbonyl stretching frequency with a value about 1609 cm-1 (Figure 2.3.8) that shifted to 1834 cm-1 (Figure 2.3.8) in the case of D-tyrosine silver nanoparticles after preparation of nanoparticles due to the carboxylate ion formation. Likewise, L-tyrosine has a carbonyl stretching frequency around 1609 cm-1 (Figure 1.3.18b represented by black curve) that shifted to 1862 cm-1 (Figure 1.3.14a represented by red curve) in the case of LD-tyrosine silver nanoparticles after preparation of nanoparticles due to the formation of the carboxylate ion.
Furthermore, the peak associated to ν(OH) is absent regarding tyrosine AgNPs, and NH3+ peak becomes prominent and sharp that supports the oxidization process of tyrosine’s phenolic group during the reduction process of chloroauric acid [35]. Moreover, after oxidation of the phenolic group happens during nanoparticle preparation, a quinone shaped structure emerges that consequently binds to the surface of the tyrosine AgNPs [35]. FTIR spectroscopy provided the evidence that nanoparticles contain surface carboxylate groups, Moreover, the negative surface charge is also supported by the presence of surface carboxylate groups observed by values of zeta potential. Furthermore, by comparing FTIR spectra of pristine amino acids before and after reduction of metal ions, the mechanism of reduction of silver ions by tyrosine or tryptophan molecules is evidently observed. Indole and the Heterocyclic amine such as pyrrole usually undergo oxidative polymerization in the presence of an oxidizing agent. In this context, the role of pyrrole to reduce silver ions in order to produce Ag nanoparticles is reported in many studies [35]. In addition, many studies reported electrochemical methods for oxidative polymerization of indole by using its secondary amine or the alpha-C of the secondary amine [35]. As amino acid tryptophan has a benzopyrrole (indole) group, so it can undergo oxidative polymerization using similar pathway to reduce silver ions in order to create Ag nanoparticles. The absence of this respective peak after formulation of nanoparticles shows the deprotonation of amine groups in the reduction process of metal ions. Subsequently, nanoparticles having similar amino acid shells on the surface proposed by the similar vibrational features of all these nanoparticles and these amino acids were held on the surface of the metal in a structured pattern as indicated by the symmetry in the vibrational bands.
ThermoGravimetric Analysis (TGA) of Chiral Tryptophan and Tyrosine silver Nanoparticles:
Thermo Gravimetric Analysis (TGA) of Tyrosine and Chiral Tryptophan Silver Nanoparticles
The procedure of thermal analysis in which the change in chemical and physical properties is measured as the function of time and enhancing temperature is termed as Thermogravimetric analysis which is abbreviated as TGA [51]. With TGA, the information regarding the physical phenomenon like the second order phase transition that includes sublimation, absorption, vaporization, desorption and adsorption can be evaluated. TGA has been utilized to study the Chiral tryptophan and tyrosine amino acid-coated silver nanoparticles. The D, L-tryptophan (displayed as a red curve) and D, L- Tryptophan coated silver nanoparticles (displayed as a blue curve) are shown in Figure 2.3.9a and Figure 2.3.9b, respectively.
Figure 2.3.9: TGA analysis of (A) D-tryptophan (red curve) and D, Tryptophan coated silver nanoparticles (blue curve) and (B) L-tryptophan (red curve) and L- Tryptophan coated silver nanoparticles (blue curve).
About 50% monatomic weight loss was observed at 183˚ C for both D, L-tryptophan silver nanoparticle as obvious in Figure 2.3.9 that indicated amino acid molecules and trapped water that have been bonded by the non-covalent interactions like hydrogen bonding to coated-AgNPs. On another hand, for L-Tryptophan silver nanoparticles there has been an enhancement in weight at 610oC, but reduction at 640oC which is an indication of amino acids binding to the silver nanoparticles at this temperature, [37].
Figure 2.3.10: TGA analysis of (A) D-tyrosine (red curve) and D tyrosine coated silver nanoparticles (blue curve) and (B) L-tyrosine (red curve) and L- tyrosine coated silver nanoparticles (blue curve).
In addition, it has been indicated in Figure 2.3.10, that there was fixed loss of 20% in monotonic weight at 75˚ C and another 50% weight loss at 180˚ C for D- tyrosine silver nanoparticles, while for L-tyrosine silver nanoparticles there was constant loss of 50% monotonic weight at 180˚ C, which shows that there is amino acids molecules and trapped water that have been bounded by non-covalent interactions like hydrogen bonding to coated-AgNPs [37]. The presence of carboxylic acid groups facilitated the nanoparticles produced by tryptophan and tyrosine to conduct the reaction with proteins. Many of amine groups in proteins can conduct the reaction with carboxylic acids at the surface of AgNPs and subsequent attachment with them. This shows that the silver proteins can be stabilized because of the attachment of functional nanoparticles. Moreover, TGA confirmed the binding of amino acids to the surface of nanoparticles prepared in this study.
Stability of silver nanoparticles: Experiments needs to be done
Cyclic Voltametric analysis of silver nanoparticles: Experiments needs to be done
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