Environmentally Friendly Controlled Synthesis of Gold Nanostars with Collagen by One-Step Reduction Method

Abstract

In this study, gold nanostars (AuNSs) were prepared by a facile and environmentally friendly method through the one-step reduction process with collagen as the stabilizing agent. The use of collagen, a highly biocompatible protein with many functional amines groups, can facilitate the simultaneous controlled synthesis and surface protecting of gold nanoparticles in one step. This synthetic process was operated in the aqueous solution of tetrachloroauric acid (HAuCl₄) at room temperature, in which ascorbic acid serves as a reductive agent. The influence of collagen concentration (0.02-0.06 mM) on the morphology of AuNSs was carefully studied to clarify its dual roles as stabilizing and controlling agents for the growth of the particles. Besides that, by simply adjusting reaction components such as the molar ratio of ascorbic acid to HAuCl₄ and pH value, the length of the AuNS tips was also controlled. This study could offer a novel modified approach in the controlled synthesis process of AuNSs with the biomolecules collagen. The resulting AuNSs were then characterized by ultraviolet-visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), zeta potential, Fourier transform-infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and circular dichroism (CD), as well as selected area electron diffraction (SAED). UV-Vis spectroscopy showed the formation of AuNSs with the maximum surface plasmon resonance peak at 600-639 nm. TEM results revealed that the average particle size of the AuNSs stabilized by the collagen ranged from 27.39 nm to 41.55 nm, depending on the experimental composition and the pH values. HRTEM, EDS, and SAED results prove a more precise insight into the formation of pure gold nanocrystals. Analysis of the current results may also help better understand the growth mechanism of AuNSs during the synthesis process in the presence of collagen. The Au concentration quantified by the inductively coupled plasma mass spectrometry (ICP-MS) technique after separating and decomposing with microwave-assisted digestion exhibits that the synthesis of AuNSs has a high yield of 88.62%. Additionally, the colloidal stability of AuNS-collagen against different NaCl concentrations, pH, temperatures, and storage time was also examined through UV-Vis spectroscopy. The investigation results reveal that AuNS-collagen remains stable in NaCl 2.0% (w/v), from mildly acidic to neutral pH (4-7), below the temperature of 40°C, and within 21 days postsynthesis. The AuNS synthesized by this eco-friendly method is promising for many potential applications in biomedical field.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

In recent years, significant efforts have been made to develop different synthetic strategies to prepare exceptionally anisotropic gold nanostructures for various applications because of their intrinsic optical [1] and chemical properties [2]. Especially in biomedical fields, controlling the size and shape of gold nanoparticles is necessary, as these properties have many effects on pharmacokinetics. Among the various anisotropic morphologies of gold nanoparticles, star-shaped nanostructure has attracted significant interest due to its unique optical properties [3], known as high localization of surface plasmon resonance [4, 5]. Controlled synthesis of gold nanoparticles generally involves fine-tuning the experimental parameters, such as reagent composition [2], surfactant concentration [6], pH conditions [7], and the reaction temperature [2, 8] to obtain particles with desirable morphology. To date, many groups have developed various synthetic approaches of shape control through a two-step process, together with using surfactant as the template for directing nanoparticle growth [6, 9]. Vijayaraghavan and coworkers showed the well-defined and high-yield multibranched gold nanoparticles using a gemini cationic surfactant N,N,N,N${}^{\prime }$-tetramethyl-N,N-ditetradecylethane-1,2-diammonium bromide (C₁₄C₂C₁₄Br₂) [10]. Baginsky et al. used cetyltrimethylammonium bromide (CTAB) to tune the length of branches and modified the surfaces of particles with chitosan [11]. Phan and coworkers can obtain a well-defined star-shaped structure in chitosan solution through selective pH levels of 1 and 3 [12]. Zhang and coworkers reported that by changing the ratios of cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC), multibranched gold nanoparticles could be prepared via a convenient seed-mediated method [13]. Despite abundant reports on the successful synthesis of branch-shaped gold nanoparticles, most synthetic strategies miss the mark of safety for biomedical applications. Furthermore, to our knowledge, attempts at synthesizing star-liked gold nanostructure through a one-step reduction approach using biopolymer collagen are severely lacking.

Collagen is the most abundant extracellular matrix protein and the main component of connective tissue [14, 15]. Due to advantageous properties such as nontoxicity, good biodegradability, and biocompatibility [16], the use of collagen as the natural polymer in the field of metal nanoparticle preparation has been intensively growing in recent years. This fibrous protein has a unique structure of triple-helical chains held together by hydrogen bonds and Vander Waals forces [14]. In the controlled synthesis of gold nanoparticles, there are several key advantages to use collagen as the stabilizing agent. When changing the environmental conditions such as pH and temperature, these interactions could be disrupted and lead to the transformation of the triple helix to the denatured state, in which collagen exists as random coil conformation [16]. These random coils could be deposited effectively onto the specific crystal facets of particles through their abundant functional groups and then slow down the growth of nanoparticles along with these facets. Thus, they can induce preferential growth along other directions to form the multibranched nanostructure. In addition, utilizing collagen can improve the size-controlled synthesis of gold nanoparticles based on the inner limit pore size of the polymeric matrixes, which can act as the template for the growth of nanoparticles [17].

Herein, we report an efficiently facile one-step procedure to synthesize star-liked shape and high-yield nanoparticles using natural protein collagen with ascorbic acid as a reducing agent (Scheme 1). We have utilized a modified method by replacing the conventional surfactant CTAB with collagen molecules and performed a systematic study by governing reaction parameters to prepare the star-shaped gold nanostructure. Besides, to elucidate the protecting role of collagen in the controlled synthesis of AuNSs, a low concentration of collagen was used in this experiment.

[figure omitted; refer to PDF]

The TEM results of the colloidal sample confirm that the particles are primarily star-liked in shape, and the size of AuNSs can be governed by adjusting the collagen concentration. In the synthesis with low concentrations of collagen at 0.02 and 0.03 mM (Figures 4(a)–4(d)), the average size of particles ranges from 32.27 nm to 27.39 nm, respectively. It is also noticed that the tips of the star-liked gold nanoparticles in these samples are blunt. With the increase in collagen concentration from 0.04 mM to 0.05 mM (Figures 4(e)–4(h)), the average size of particles increases from 29.79 nm to 41.55 nm, and the tips of nanoparticles become more extended and sharper. Additionally, the particles in the samples synthesized with the higher concentration of 0.04 mM and 0.05 mM (Figures 4(e) and 4(g)) were prone to form more separately and discretely in colloidal systems than particles prepared with lower collagen concentration (0.02 mM and 0.03 mM) (Figures 4(a) and 4(c)). However, the gold nanoparticle synthesized with a higher collagen concentration of 0.06 mM was unstable for a short time and tended to self-agglomerate when analyzed with the TEM technique. TEM results proved that collagen concentration could be the crucial factor in controlling the length of the nanostar tips. It was noticed that collagen molecules could also bind to AuNS particles through cysteine residues, stabilizing the particles through the electrostatic interaction. Previous studies have shown that collagen molecules bind to gold nanoparticles through electrostatic interaction because they possess many charge moieties to attract the gold nanoparticles with polarity [27, 28]. Significantly, collagen can tightly bind to gold nanoparticles with small size on its matrix, but in collapsed collagen structures, and inability in uncollapsed collagen structures [28]. Thus, it is necessary to combine appropriate pH conditions with suitable collagen concentrations to stabilize collagen in the controlled synthesis of AuNSs. Based on the diffusion-limited growth mechanism [29], as increased collagen concentration at the allowable threshold, the diffusion of the Au atom through the colloidal solution effectively decreased due to the higher viscosity of collagen. Thus, the possibility of Au atom to the crystal surface is strongly restricted and resulted in the formation of diffusion-controlled morphologies [29]. Additionally, the addition of AgNO₃ to the reaction solution provides an amount of Ag⁺ ions, which is predicted to play an essential role in the growth mechanism of AuNSs. The explanation for the role of the Ag⁺ ions was proposed based on the underpotential deposition of silver on the various facets of a gold crystal, further restricting the growth of initial particles and resulting in symmetry-breaking [2, 30]. It was found that metallic silver strongly binds to the Au {110} surface, slowing down the growth rate of Au on these facets [2]. The other crystal facets could grow faster, leading to the formation of AuNSs. Additionally, the AuNSs synthesized with collagen are relatively small compared to previous studies with seed-mediated and one-step reduction methods (Table 3).

[figures omitted; refer to PDF]

Table 3

Summary of the gold nanoparticles synthesized with different stabilizing agents.

Concentration of HAuCl4/volume in water	Reaction condition	Stabilizing agent	Method	AuNP size (nm)	AuNP shape	Reference
0.31 g HAuCl4, 0.39 g N-(2-mercaptopropionyl)glycine	NaBH4	Collagen	Cross-linking via EDC	$$2.4\pm 0.7$$	Spherical	Castaneda et al. [37]
0.01 M/1.0 mL	Acid ascorbic	Chitosan	One-step	$$111.3\pm 56.2$$	Nanostar	Phan et al. [12]
0.04 mM/10 mL	CTAB 0.1 M, ascorbic acid 10 mM	Chitosan	Seed-mediated	70-100	Nanostars	Baginskiy et al. [11]
500 mM/5.0 $\mu$L	10 μl of NH2OH·HCl, HAuCl4 (1.0 ml, 2.5 mM)	Lysin	Seed mediated	$$97\pm 22$$	Nanostars	Avila-Alejo et al. [38]
0.1 M/250 $\mu$L	CTAB (10 mL/0.1 M) AgNO3 (50 $\mu$l, 0.1 M)		Seed-mediated	55-105	Nanostars	Khan et al. [39]
25 mM	AgNO3 10 mM, pH 4.0, and ascorbic acid 5.0 mM	Collagen type I	One-step	27-41	Nanostars	This work
5 mM	Albumin solution (BSA, 80 $\mu$L, 10 mg/mL), ascorbic acid 100 mM	Albumin	One-step	150	Nanostars	Sasidharan et al. [40]
0.5 mM	PVP ($M_w=\mathrm{10,000}$ g.mol-1; 10 mM) in dimethyl formamide (DMF)	PVP	Polymer base	40	Nanostars	Vanhecke et al. [41]
0.25 mM	Sodium citrate	Gelatin type A	One-step	$$18\pm 2$$	Spherical	Suarasan et al. [42]
0.254 mM	Sodium citrate 25.4 mM	Collagen in PBS	One-step	$$10.2\pm 2.7$$	Spherical	Unser et al. [43]
10 mM/0.1 mL	10 mM glucosamine (sugar)/(HAuCl4) from 1 to 60	Glucosamine	One-step	55-72	Nanostars	Moukarzel et al. [44]
0.2% (w/w)	Luminol in 0.1 mol/L NaOH	Chitosan-luminol	One-step	90	Nanoflower	Wang et al. [45]

To analyze the colloidal solution’s purity, the sample of AuNS synthesized with the collagen concentration of 0.05 mM was determined by energy-dispersive X-ray spectroscopy (EDS). The peak at 2.1 keV was observed on the EDS elemental spectrum assigned to the Au signal. The EDS result presented in Figure 5 showed that AuNSs are composed of gold (95 atomic %), and only a trace amount of chloride is observed (2 atomic %), confirming the formation of the pure gold nanocrystal. The high-resolution transmission electron microscopy (HRTEM) study offers insight into the atomic structures of gold nanoparticles. Characteristic lattice fringes are observed in Figure 6 with a $d$-spacing of 2.37 Å, which is assigned to {111} lattice space of metallic gold [7], indicating the large exposed area of the corresponding facet.

[figure omitted; refer to PDF]

The obtained fractions were subsequently decomposed by the microwave-assisted digestion method before quantifying the gold content with the ICP-MS technique. Because the AuNSs were synthesized in collagen with complex matrices, the microwave-assisted acid decomposition procedure was suitable for digestion and accurate determination of Au contents [19, 60]. Table 7 exhibits the gold quantification results of three fractions determined by ICP-MS. The gold concentration of AuNS total is 19.67 mg L^-1, which could be involved nanoparticles or/and ionic forms of unreacted precursors. In the AuNS-resuspended fractions, the gold content measured by ICP-MS is approximately 17.46 mg L^-1, mainly attributed to the formation of AuNSs in the colloidal sample. Compared to the Au content determined in AuNS total, the obtained concentration of gold in the resuspended fraction shows the high yield and efficiency in the synthesis of AuNSs, presenting about 88.62% Au in colloidal solution. Additionally, the gold concentration of AuNS-supernatant fractions is determined as 2.13 mg L^-1, the low concentration possibly due to the unreacted ions form (Au⁺) remaining in the liquid fractions. The quantification result of Au concentration determined by ICP-MS exhibits a high correlation with the UV-Vis characterization of the colloidal sample fractions, thus could prove that microwave-assisted acid digestion is suitable for evaluating the yield of synthesis AuNS-collagen.

Table 7

Gold quantification in AuNS total, AuNS resuspended, and AuNS supernatant by ICP-MS.

	Au concentration (mg.L-1)	% Au in colloidal solution
AuNS total	19.67	99.84
AuNS resuspended	17.46	88.62
AuNS supernatant	2.13	10.81

3.7. Circular Dichroism Analysis

The secondary structure contents, including α-helix, β-sheet, and the random coil of protein, were determined through CD spectroscopy to understand the conformation of collagen molecules when interacting with the gold nanoparticles. The CD spectra of collagen and Au-collagen are presented in Figure 12. The absorption peak in the range from 200 to 250 nm was decreased in the spectra of Au-collagen samples. An increase in molar ellipticity at 220 nm is observed in the Au-collagen spectra. The calculation data based on adsorption peaks reveal the apparent changes in collagen secondary structure contents. Compared to the collagen, the secondary structure of the Au-collagen sample resulted in the decrease of α-helix, β-turn, and random coil by 16.8%, 10.5%, and 8.7%, respectively. It could be inferred that the interaction with nanoparticles might reduce intramolecular hydrogen bonding and lead to hydrophobic group exposure, resulting in a decrease in α-helix structure [23, 61]. The random coil content of AuNS-collagen was slightly decreased, indicating that gold particles’ binding might be attributed to the formation of a more compact secondary structure [62]. The CD data show that the deviations of collagen secondary structure conformation after interacting with gold nanoparticles are consistent with the FT-IR results.

[figure omitted; refer to PDF]

3.8. Stability of Collagen Stabilized Gold Nanoparticles

The colloidal stability of the AuNS-collagen samples against salt concentration was evaluated by UV-Vis spectroscopy for their further applications in the biomedical field. From the UV-Vis spectra presented in Figure 13(a), the AuNS-collagen is stable in NaCl solutions up to the concentration of 2.0% (w/v) (curves A and B). A blue shift of plasmon bands and slightly reducing intensity (0.767) were observed at 592 nm for the AuNS-collagen after testing with 2.0% (w/v) NaCl solution (curve B). As increased in the NaCl concentration to 3.0% (w/v), the plasmon peak for gold nanoparticles was unable to be detected (curve D). This change could be a sign of nanoparticle aggregation caused by the electrostatic repulsion force on the surface charge of nanoparticles in a high ion concentration environment. Furthermore, the solubility of collagen is strongly affected by the NaCl concentration, and it was reported that when in the high NaCl concentrations (>3.0% (w/v)), the collagen solubility decreased dramatically [49]. Increasing ionic salts could enhance the hydrophobic interaction of collagen chains, leading to protein precipitation [57, 63]. The stabilizing ability of collagen on the gold nanoparticles was limited due to the low protein coverage, resulting in the self-aggregation of particles.

[figures omitted; refer to PDF]

The AuNS-collagen colloidal solutions were also investigated for their stability under acidic and alkaline medium (1, 4, 7, and 10) in the aqueous solution by monitoring the changes in the surface resonance plasmon peaks of UV-Vis spectra exhibited in Figure 13(b). The plasmon peak with high intensity (0.785) was observed at 632 nm for AuNS-collagen sample prepared at pH of 4 (curve E). The displacement of the plasmon peak at the shorter wavelength of 562 nm was obtained in the spectrum of pH 1 sample with a slight reduction in intensity (0.562). It could be relevant to the change in size and morphology of gold nanoparticles, but the colloidal solution was considered stable in acidic conditions. After the increasing testing pH to 7 and 10, only a plasmon peak with the intensity of 0.642 was observed at the wavelength of 617 nm for neutral condition (curve F), and almost no sign of plasmon peak for gold nanoparticles under alkaline condition (curve H). The plasmon peak position and intensity changes are taken as proof that the colloidal solution remains stable under neutral pH and tends to aggregate in an alkaline medium. This colloidal instability could be explained based on the reduction of the collagen solubility in alkaline pH [47, 64], directly resulting in a decrease in the protective ability of collagen on the gold nanoparticles.

The influence of the temperature and storage time was further analyzed to get more insight into the colloidal stability of AuNS-collagen. The colloidal samples were investigated with various temperatures (25, 30, 35, 40, 45, and 50°C) and behaved differently regarding the change of plasmon peaks for gold nanoparticles (Figure 13(c)). In the UV-Vis spectra of the AuNS-collagen sample heating at 25 and 30°C, there is no significant change in the absorbance intensity of plasmon peak with the decrease from 0.893 to 0.867 (Table 8, curves I and J), indicating that the formation of gold nanoparticles in the presence of collagen is relatively stable at these temperatures. As increasing to higher temperatures (from 35 to 50°C), a significant drop in the intensity from 0.841 to 0.251 (Table 8, curves K–N) was detected, suggesting the reduction of colloidal stability. It could be related to collagen denaturation at high temperatures mentioned in the Komsa-Penkova et al. work [65]. The effect of storage times on the AuNS-collagen colloidal stability was investigated at 25°C for over 21 days postsynthesis and further analyzed based on the UV-Vis spectrum. As observed in Figure 13(c), there is only a small change in peak intensity of 0.811 (curve O) and 0.788 (curve P) for 14 and 21 days of storage time. Additionally, blue shifts to the shorter wavelength were also detected in the spectrum of sample storage over 14 and 21 days, which could be due to the change in size and shape of nanoparticles [1, 66]. These observation results showed that the AuNS-collagen colloidal solutions were stable over the storage time without occurring aggregation.

Table 8

UV-Vis data of AuNS-collagen sample spectra for determining the colloidal stability at different pH, NaCl concentrations, temperatures, and storage times.

Sample condition	Curve	Wavelength of plasmon peak (nm)	Absorbance intensity (a.u.)
NaCl 0% (w/v) (0.05 mM collagen)	NaCl 0%	634	0.809
NaCl 1.0% (w/v)	A	633	0.785
NaCl 2.0% (w/v)	B	592	0.767
NaCl 3.0% (w/v)	C	588	0.257
NaCl 4.0% (w/v)	D	–	–
pH 1	G	562	0.562
pH 4	E	632	0.785
pH 7	F	617	0.642
pH 10	H	–	–
25 °C	I	602	0.893
30 °C	J	618	0.867
35°C	K	610	0.841
40 °C	L	633	0.809
45 °C	M	588	0.501
50 °C	N	576	0.251
14 days postsynthesis	O	564	0.811
21 days postsynthesis	P	534	0.788

4. Conclusion

In summary, AuNSs have been successfully prepared with collagen through the new and direct one-pot reduction approach. Our study revealed that the AuNSs with controlled size and shape could be effectively obtained using appropriate collagen concentrations and acidic pH conditions. More importantly, the tips of star-liked crystal could be elongated with the increase of collagen concentration to 0.05 mM at a pH condition of 4.0. The UV-Vis spectrum showed that the light absorbance depended on the morphology of gold nanoparticles and shifted to a longer wavelength with the increase of collagen concentration from 567 nm for 0.02 mM to 603 nm for 0.05 mM. The EDS spectrum associated with HRTEM results confirms the formation of pure gold nanocrystals, and the SAED shows good agreement with the planes that appear in the XRD pattern. The FT-IR and CD analysis indicated the collagen interaction to gold nanoparticles mainly through the amine group and acid amine residues, followed by the collagen secondary structure change.

Furthermore, the use of collagen is vitally essential for the one-pot controlled synthesis AuNSs. The colloidal AuNS-collagen exhibited good stability in the low salt concentrations (below 2.0%, w/v), mildly acidic, and neutral pH (from 4 to 7), but not qualified in the alkaline environment. Besides, the prepared AuNS-collagen was also stable in the low-temperature range from 25 to 40°C. We found that collagen showed high feasibility in the preparation of AuNS, and this integrating nanomaterial could be favorable for application in the optical biosensor. Efforts in the biosensor application should be developed specifically for AuNS-collagen material, mediated through the optical properties of gold nanostars, and various protocols for testing AuNS-collagen in the biological environment need to be studied.

Authors’ Contributions

Investigation was contributed by H T Ho, T P P Nguyen, Q K Vo, and A T Nguyen. Writing (original draft) was contributed by Q K Vo.

Acknowledgments

This research is funded by the University of Science VNU-HCM under grant number HH 2021-02.

References

[1] K. L. Kelly, E. Coronado, L. L. Zhao, G. C. Schatz, "The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment," The Journal of Physical Chemistry. B, vol. 107 no. 3, pp. 668-677, 2003.

[2] M. Grzelczak, J. Pérez-Juste, P. Mulvaney, L. M. Liz-Marzán, "Shape control in gold nanoparticle synthesis," Chemical Society Reviews, vol. 37 no. 9, pp. 1783-1791, DOI: 10.1039/b711490g, 2008.

[3] C. L. Nehl, H. Liao, J. H. Hafner, "Optical properties of star-shaped gold nanoparticles," Nano Letters, vol. 6 no. 4, pp. 683-688, 2006.

[4] V. H. Segtnan, T. Isaksson, "Temperature, sample and time dependent structural characteristics of gelatine gels studied by near infrared spectroscopy," Food Hydrocolloids, vol. 18 no. 1, 2004.

[5] L. A. Dykman, N. G. Khlebtsov, "Multifunctional gold-based nanocomposites for theranostics," Biomaterials, vol. 108, pp. 13-34, 2016.

[6] J. Xiao, L. Qi, "Surfactant-assisted, shape-controlled synthesis of gold nanocrystals," Nanoscale, vol. 3 no. 4, pp. 1383-1396, 2011.

[7] L. Lu, K. Ai, Y. Ozaki, "Environmentally friendly synthesis of highly monodisperse biocompatible gold nanoparticles with urchin-like shape," Langmuir, vol. 24 no. 3, pp. 1058-1063, 2008.

[8] Y. Xia, N. J. Halas, "Shape-controlled Synthesis and surface plasmonic Properties of Metallic nanostructures," MRS Bulletin, vol. 30 no. 5, pp. 338-348, DOI: 10.1557/mrs2005.96, 2005.

[9] P. Priecel, H. Adekunle Salami, R. H. Padilla, Z. Zhong, J. A. Lopez-Sanchez, "Anisotropic gold nanoparticles: preparation and applications in catalysis," Chinese Journal of Catalysis, vol. 37 no. 10, pp. 1619-1650, DOI: 10.1016/s1872-2067(16)62475-0, 2016.

[10] P. Vijayaraghavan, C. H. Liu, K. C. Hwang, "Synthesis of multibranched gold nanoechinus using a gemini cationic surfactant and its application for surface enhanced Raman scattering," ACS Applied Materials & Interfaces, vol. 8 no. 36, pp. 23909-23919, 2016.

[11] I. Baginskiy, T. C. Lai, L. C. Cheng, Y. C. Chan, K. Y. Yang, R. S. Liu, M. Hsiao, C. H. Chen, S. F. Hu, L. J. Her, D. P. Tsai, "Chitosan-modified stable colloidal gold nanostars for the photothermolysis of cancer cells," Journal of Physical Chemistry C, vol. 117 no. 5, pp. 2396-2410, DOI: 10.1021/jp311271p, 2013.

[12] T. T. V. Phan, V. T. Nguyen, S. H. Ahn, J. Oh, "Chitosan-mediated facile green synthesis of size-controllable gold nanostars for effective photothermal therapy and photoacoustic imaging," European Polymer Journal, vol. 118, pp. 492-501, DOI: 10.1016/j.eurpolymj.2019.06.023, 2019.

[13] Y. Zhang, B. Wang, S. Yang, L. Li, L. Guo, "Facile synthesis of spinous-like Au nanostructures for unique localized surface plasmon resonance and surface-enhanced Raman scattering," New Journal of Chemistry, vol. 39 no. 4, pp. 2551-2556, 2015.

[14] M. D. Shoulders, R. T. Raines, "Collagen structure and stability," Annual Review of Biochemistry, vol. 78 no. 1, pp. 929-958, DOI: 10.1146/annurev.biochem.77.032207.120833, 2009.

[15] R. Parenteau-Bareil, R. Gauvin, F. Berthod, "Collagen-based biomaterials for tissue engineering applications," Materials, vol. 3 no. 3, pp. 1863-1887, 2010.

[16] O. S. Rabotyagova, P. Cebe, D. L. Kaplan, "Collagen structural hierarchy and susceptibility to degradation by ultraviolet radiation," Materials Science and Engineering: C, vol. 28 no. 8, pp. 1420-1429, 2008.

[17] P. Raveendran, J. Fu, S. L. Wallen, "Completely ‘green’ synthesis and stabilization of metal nanoparticles," Journal of the American Chemical Society, vol. 125 no. 46, pp. 13940-13941, 2003.

[18] R. M. Galazzi, E. D. B. Santos, T. Caurin, G. D. S. Pessôa, I. O. Mazali, M. A. Z. Arruda, "The importance of evaluating the real metal concentration in nanoparticles post-synthesis for their applications: a case-study using silver nanoparticles," Talanta, vol. 146, pp. 795-800, 2016.

[19] N. V. Godoy, R. M. Galazzi, K. Chacón-Madrid, M. A. Arruda, I. O. Mazali, "Evaluating the total gold concentration in metallic nanoparticles with a high content of organic matter through microwave-assisted decomposition platform and plasma-based spectrometric techniques (ICP-MS and ICP OES)," Talanta, vol. 224, article 121808,DOI: 10.1016/j.talanta.2020.121808, 2021.

[20] U. M. 3052, "Microwave assisted acid digestion of siliceous and organically based matrices," Tetrahedron, vol. 52 no. 44, pp. 13837-13866, 1996.

[21] B. L. Batista, D. Grotto, J. L. Rodrigues, V. C. de Oliveira Souza, F. Barbosa, "Determination of trace elements in biological samples by inductively coupled plasma mass spectrometry with tetramethylammonium hydroxide solubilization at room temperature," Analytica Chimica Acta, vol. 646 no. 1–2, pp. 23-29, DOI: 10.1016/j.aca.2009.05.022, 2009.

[22] W. Wu, L. He, Y. Liang, L. Yue, W. Peng, G. Jin, M. Ma, "Preparation process optimization of pig bone collagen peptide-calcium chelate using response surface methodology and its structural characterization and stability analysis," Food Chemistry, vol. 284, pp. 80-89, DOI: 10.1016/j.foodchem.2019.01.103, 2019.

[23] X. Yang, X. Yu, A. G. Yagoub, L. Chen, H. Wahia, R. Osae, C. Zhou, "Structure and stability of low molecular weight collagen peptide (prepared from white carp skin) -calcium complex," LWT, vol. 136, article 110335,DOI: 10.1016/j.lwt.2020.110335, 2021.

[24] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou, S. E. Hunyadi, T. Li, "Anisotropic metal nanoparticles: synthesis, assembly, and optical applications," The Journal of Physical Chemistry B, vol. 109 no. 3–4, pp. 13857-13870, DOI: 10.1021/jp0516846, 2005.

[25] L. C. Cheng, J. H. Huang, H. M. Chen, T. C. Lai, K. Y. Yang, R. S. Liu, M. Hsiao, C. H. Chen, L. J. Her, D. P. Tsai, "Seedless, silver-induced synthesis of star-shaped gold/silver bimetallic nanoparticles as high efficiency photothermal therapy reagent," Journal of Materials Chemistry, vol. 22 no. 5, pp. 2244-2253, DOI: 10.1039/C1JM13937A, 2012.

[26] F. Hao, C. L. Nehl, J. H. Hafner, P. Nordlander, "Plasmon resonances of a gold nanostar," Nano Letters, vol. 7 no. 3, pp. 729-732, 2007.

[27] X. Mo, Y. An, C. S. Yun, S. M. Yu, "Nanoparticle-assisted visualization of binding interactions between collagen mimetic peptide and collagen fibers," Angewandte Chemie, vol. 45 no. 14, pp. 2267-2270, DOI: 10.1002/ange.200504529, 2006.

[28] D. Wang, J. Ye, S. D. Hudson, K. C. K. Scott, S. Lin-Gibson, "Effects of nanoparticle size and charge on interactions with self-assembled collagen," Journal of Colloid and Interface Science, vol. 417, pp. 244-249, 2014.

[29] H. Imai, "Self-organized formation of hierarchical structures," Topics in Current Chemistry, vol. 270, pp. 43-72, 2006.

[30] M. Liu, P. Guyot-Sionnest, "Mechanism of silver(I)-assisted growth of gold nanorods and bipyramids," The Journal of Physical Chemistry. B, vol. 109 no. 47, pp. 22192-22200, 2005.

[31] A. Rostek, K. Loza, M. Heggen, M. Epple, "X-ray powder diffraction to analyse bimetallic core-shell nanoparticles (gold and palladium; 7-8 nm)," RSC Advances, vol. 9 no. 46, pp. 26628-26636, 2019.

[32] K. Xin Lee, K. Shameli, M. Miyake, N. Kuwano, N. B. Bt Ahmad Khairudin, S. E. Bt Mohamad, Y. P. Yew, "Green synthesis of gold nanoparticles using aqueous extract of Garcinia mangostana fruit peels," Journal of Nanomaterials, vol. 2016,DOI: 10.1155/2016/8489094, 2016.

[33] W. H. Bragg, W. L. Bragg, "The reflection of X-rays by crystals," Proceedings of the Royal Society of London, Series A, vol. 88 no. 605, pp. 146-152, DOI: 10.1098/rspa.1913.0040, 1913.

[34] E. R. Jette, F. Foote, "Precision determination of lattice constants," The Journal of Chemical Physics, vol. 3 no. 10, pp. 605-616, 1935.

[35] C. Wang, H. Wang, T. Huang, X. Xue, F. Qiu, Q. Jiang, "Generalized-stacking-fault energy and twin-boundary energy of hexagonal close-packed Au: a first-principles calculation," Scientific Reports, vol. 5 no. 5988, 2015.

[36] S. Pedireddy, H. K. Lee, W. W. Tjiu, I. Y. Phang, H. R. Tan, S. Q. Chua, C. Troadec, X. Y. Ling, "One-step synthesis of zero-dimensional hollow nanoporous gold nanoparticles with enhanced methanol electrooxidation performance," Nature Communications, vol. 5 no. 1,DOI: 10.1038/ncomms5947, 2014.

[37] L. Castaneda, J. Valle, N. Yang, S. Pluskat, K. Slowinska, "Collagen cross-linking with Au nanoparticles," Biomacromolecules, vol. 9 no. 12, pp. 3383-3388, 2008.

[38] J. O. Avila-Alejo, A. K. González-Palomo, G. Plascencia-Villa, M. José-Yacamán, H. R. Navarro-Contreras, I. N. Pérez-Maldonado, "Low cytotoxicity of anisotropic gold nanoparticles coated with lysine on peripheral blood mononuclear cells “ in vitro ”," Environmental Toxicology and Pharmacology, vol. 56 no. 550, pp. 210-218, DOI: 10.1016/j.etap.2017.09.014, 2017.

[39] H. I. Khan, M. U. Khalid, A. Abdullah, A. Ali, A. S. Bhatti, S. U. Khan, W. Ahmed, "Facile synthesis of gold nanostars over a wide size range and their excellent surface enhanced Raman scattering and fluorescence quenching properties," Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics, vol. 36 no. 3, article 03E101, 2018.

[40] S. Sasidharan, D. Bahadur, R. Srivastava, "Rapid, one-pot, protein-mediated green synthesis of gold nanostars for computed tomographic imaging and photothermal therapy of cancer," ACS Sustainable Chemistry & Engineering, vol. 5 no. 11, pp. 10163-10175, 2017.

[41] D. Vanhecke, L. Rodríguez-Lorenzo, C. Kinnear, E. Durantie, B. Rothen-Rutishauser, A. Petri-Fink, "Assumption-free morphological quantification of single anisotropic nanoparticles and aggregates," Nanoscale, vol. 9 no. 15, pp. 4918-4927, 2017.

[42] S. Suarasan, M. Focsan, D. Maniu, S. Astilean, "Gelatin-nanogold bioconjugates as effective plasmonic platforms for SERS detection and tagging," Colloids and Surfaces B: Biointerfaces, vol. 103, pp. 475-481, DOI: 10.1016/j.colsurfb.2012.10.046, 2013.

[43] S. Unser, S. Holcomb, R. Cary, L. Sagle, "Collagen-gold nanoparticle conjugates for versatile biosensing," Sensors, vol. 17 no. 2,DOI: 10.3390/s17020378, 2017.

[44] W. Moukarzel, J. Fitremann, J. D. Marty, "Seed-less amino-sugar mediated synthesis of gold nanostars," Nanoscale, vol. 3 no. 8, pp. 3285-3290, 2011.

[45] W. Wang, H. Cui, "Chitosan-luminol reduced gold nanoflowers: from one-pot synthesis to morphology-dependent SPR and chemiluminescence sensing," Journal of Physical Chemistry C, vol. 112 no. 29, pp. 10759-10766, 2008.

[46] S.-H. Park, T. Song, T. S. Bae, G. Khang, B. H. Choi, S. R. Park, B.-H. Min, "Comparative analysis of collagens extracted from different animal sources for application of cartilage tissue engineering," International Journal of Precision Engineering and Manufacturing, vol. 13 no. 11, pp. 2059-2066, DOI: 10.1007/s12541-012-0271-4, 2012.

[47] H. Cruz-López, S. Rodríguez-Morales, L. M. Enríquez-Paredes, L. J. Villarreal-Gómez, L. Olivera-Castillo, Y. Cortes-Santiago, L. M. López, "Comparison of collagen characteristic from the skin and swim bladder of Gulf corvina ( Cynoscion othonopterus )," Tissue & Cell, vol. 72,DOI: 10.1016/j.tice.2021.101593, 2021.

[48] S. Morozova, M. Muthukumar, "Electrostatic effects in collagen fibril formation," The Journal of Chemical Physics, vol. 149 no. 16, article 163333,DOI: 10.1063/1.5036526, 2018.

[49] W. H. Zhao, C. F. Chi, Y. Q. Zhao, B. Wang, "Preparation, physicochemical and antioxidant properties of acid- and pepsin-soluble collagens from the swim bladders of miiuy croaker (miichthys miiuy)," Marine Drugs, vol. 16 no. 5,DOI: 10.3390/md16050161, 2018.

[50] W. Wang, X. Ding, Q. Xu, J. Wang, L. Wang, X. Lou, "Zeta-potential data reliability of gold nanoparticle biomolecular conjugates and its application in sensitive quantification of surface absorbed protein," Colloids and Surfaces. B, Biointerfaces, vol. 148, pp. 541-548, DOI: 10.1016/j.colsurfb.2016.09.021, 2016.

[51] K. Kumari, P. Singh, G. K. Mehrotra, "A facile one pot synthesis of collagen protected gold nanoparticles using Na-malanodialdehyde," Materials Letters, vol. 79, pp. 199-201, 2012.

[52] A. Kandamchira, S. Selvam, N. Marimuthu, S. Kalarical Janardhanan, N. N. Fathima, "Influence of functionalized nanoparticles on conformational stability of type I collagen for possible biomedical applications," Materials Science and Engineering: C, vol. 33 no. 8, pp. 4985-4988, DOI: 10.1016/j.msec.2013.08.018, 2013.

[53] C. Stani, L. Vaccari, E. Mitri, G. Birarda, "FTIR investigation of the secondary structure of type I collagen: new insight into the amide III band," Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, vol. 229, article 118006,DOI: 10.1016/j.saa.2019.118006, 2020.

[54] M. Jackson, L. P.'. Choo, P. H. Watson, W. C. Halliday, H. H. Mantsch, "Beware of connective tissue proteins: assignment and implications of collagen absorptions in infrared spectra of human tissues," Biochimica et Biophysica Acta-Molecular Basis of Disease, vol. 1270 no. 1,DOI: 10.1016/0925-4439(94)00056-V, 1995.

[55] D. V. Svintradze, G. M. Mrevlishvili, N. Metreveli, K. Jariashvili, L. Namicheishvili, J. Skopinska, A. Sionkowska, "Collagen–DNA complex," Biomacromolecules, vol. 9 no. 1, pp. 21-28, DOI: 10.1021/bm7008813, 2008.

[56] K. Purna Sai, M. Babu, "Studies on Rana tigerina skin collagen," Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, vol. 128 no. 1, pp. 81-90, 2001.

[57] Z.-R. Li, B. Wang, C.-f. Chi, Q.-H. Zhang, Y.-d. Gong, J.-J. Tang, H.-y. Luo, G.-f. Ding, "Isolation and characterization of acid soluble collagens and pepsin soluble collagens from the skin and bone of Spanish mackerel (Scomberomorous niphonius)," Food Hydrocolloids, vol. 31 no. 1, pp. 103-113, DOI: 10.1016/j.foodhyd.2012.10.001, 2013.

[58] Y. Abe, S. Krimm, "Normal vibrations of crystalline polyglycine I," Biopolymers, vol. 11 no. 9, pp. 1817-1839, 1972.

[59] K. J. Payne, A. Veis, "Fourier transform ir spectroscopy of collagen and gelatin solutions: deconvolution of the amide I band for conformational studies," Biopolymers, vol. 27 no. 11, pp. 1749-1760, 1988.

[60] A. L. Fabricius, L. Duester, B. Meermann, T. A. Ternes, "ICP-MS-based characterization of inorganic nanoparticles-sample preparation and off-line fractionation strategies," Analytical and Bioanalytical Chemistry, vol. 406 no. 2, pp. 467-479, 2014.

[61] C. Wen, J. Zhang, J. Zhou, Y. Duan, H. Zhang, H. Ma, "Effects of slit divergent ultrasound and enzymatic treatment on the structure and antioxidant activity of arrowhead protein," Ultrasonics Sonochemistry, vol. 49, pp. 294-302, DOI: 10.1016/j.ultsonch.2018.08.018, 2018.

[62] K. Zhang, J. Li, H. Hou, H. Zhang, B. Li, "Purification and characterization of a novel calcium-biding decapeptide from Pacific cod ( Gadus Macrocephalus ) bone: Molecular properties and calcium chelating modes," Journal of Functional Foods, vol. 52, pp. 670-679, DOI: 10.1016/j.jff.2018.11.042, 2019.

[63] Y. Tan, S. K. C. Chang, "Isolation and characterization of collagen extracted from channel catfish ( Ictalurus punctatus ) skin," Food Chemistry, vol. 242, pp. 147-155, DOI: 10.1016/j.foodchem.2017.09.013, 2018.

[64] T. M. T. Truong, V. M. Nguyen, T. T. Tran, T. M. T. Le, "Characterization of acid-soluble collagen from food processing by-products of snakehead fish ( Channa striata )," Processes, vol. 9 no. 7,DOI: 10.3390/pr9071188, 2021.

[65] R. Komsa-penkova, R. Koynova, G. Kostov, B. G. Tenchov, "Thermal stability of calf skin collagen type I in salt solutions," vol. 1297 no. 2, pp. 171-181, DOI: 10.1016/S0167-4838(96)00092-1, 1996.

[66] P. N. Njoki, S. I-Im, D. M. Lim, H.-Y. Park, B. Khan, S. Mishra, R. Sujakumar, J. Luo, C.-J. Zhong, "Size correlation of optical and spectroscopic properties for gold nanoparticles," Journal of Physical Chemistry C, vol. 111 no. 40, pp. 14664-14669, DOI: 10.1021/jp074902z, 2007.