Elsevier

Particuology

Volume 52, October 2020, Pages 97-104
Particuology

Differently synthesized gold nanoparticles respond differently to functionalization with L-amino acids

https://doi.org/10.1016/j.partic.2019.12.010Get rights and content

Highlights

  • Three differently prepared AuNPs were functionalized with amino acids.

  • Chemical reduction did not produce nanoparticles suitable for functionalization.

  • Biogenic AuNPs were the most suitable for functionalization.

  • Bacteria-mediated AuNP biogenesis enabled functionalization with six amino acids.

  • pH of the functionalization reaction influences AuNP size/electrophoretic mobility.

Abstract

The potential utility of gold nanoparticles (AuNPs) synthesized via different methods for biomedical applications vary greatly due to inherent differences in their surface properties. In the present study, we investigated the functionalization of AuNPs synthesized by chemical reduction, plant extract, and bacteria-mediated methods with 22 l-amino acids. Nanoparticles produced by bacteria-mediated (B_AuNPs) and plant-mediated (P_AuNPs) methods showed good potential, as they were able to be functionalized with six (histidine, lysine, methionine, phenylalanine, tryptophan, and tyrosine) and four (cystine, tryptophan, tyrosine, and valine) amino acids, respectively. In contrast, AuNPs produced by chemical reduction (C_AuNPs) were not found to be suitable for functionalization. Optimal functionalization conditions were found to be amino acid concentration of 20–25 mM and neutral pH (7) for P_AuNPs, whereas B_AuNPs tolerated more variable conditions. The electrophoretic mobility of P_AuNPs after functionalization indicated that these nanoparticles were less sensitive than B_AuNPs to the deviations from optimal conditions. A significant change in mobility was observed when B_AuNPs were functionalized with either methionine or tryptophan. Overall, the results of this study suggest that the suitability of the three differently synthesized AuNPs with amino acids is in the following order: B_AuNPs > P_AuNPs > C_AuNPs.

Introduction

A phenomenal revolution has taken place in the field of nanotechnology over the past two decades (Zhu & Xu, 2016). At present, nanoparticles (NPs) are being used in a range of scientific and industrial applications and are also found in consumer products. The unique properties of NPs are attributable to their size and surface-area-to-volume ratio, and make them highly valuable for to science and technology (Soto-Alvaredo et al., 2017). Metal NPs (MNPs), typically particles of noble metals, are the subject of considerable research due to their unique optical, electrical, and magnetic properties (Csapó et al., 2014) and the large number of accessible active sites per unit area (Zhu & Xu, 2016).

Nanoparticles of gold, which are a widely studied group of NPs, have attracted much attention because of their potential utility in catalysis (Nita et al., 2016), electronics and sensor technologies (Han, Park, Chun, & Yoon, 2015), and solar cells (Chen, Wang, Han, Cheng, & Qian, 2015). Gold NPs (AuNPs) are of particular interest as they have wide range of properties including a high surface-area-to-volume ratio, biocompatibility (Yeh, Creran, & Rotello, 2012), and low toxicity (Khlebtsov and Dykman, 2011, Murphy et al., 2008). These unique properties make them an effective tool for various biomedical applications such as drug and gene delivery (Hussain & Hussain, 2015), tissue engineering (Vial, Reis, & Oliveira, 2017), and microbe detection and identification (Syed & Bukhari, 2011).

Scientists have been working to further improve the surface properties of NPs to create multifunctional NPs functionalized with specific recognition moieties like enzymes, antigens, antibodies, proteins (Subbiah, Veerapandian, & Yun, 2010). Surface functionalization of AuNPs alters the physico-chemical properties (Baptista et al., 2008, Radwan and Azzazy, 2009, Uehara, 2010), which may play a crucial role in a number of applications, particularly in biomedical treatments such as cancer therapy (Muddineti, Ghosh, & Biswas, 2015; Yamada, Foote, & Prow, 2015), radiotherapy (Ngwa et al., 2014), or enable their use as drug carriers (Ghosh, Han, De, Kim, & Rotello, 2008). The size of nanoparticles limits their use for targeted drug delivery to particular cells, which they can enter simply by translocating across the membrane. To resolve this issue, nanoparticles have been functionalized with a number of biomolecules including ligands, amino acids, and proteins (Tiwari, Vig, Dennis, & Singh, 2011). The conjugation of proteins or amino acids with MNPs stabilizes the system and increases biocompatibility (Bohara, Thorat, & Pawar, 2016).

It is noteworthy that the degree to which MNPs, such as AuNPs, can be surface functionalized vary between differently synthesized MNPs. For example, AuNPs synthesized via chemical and physical methods may be difficult to surface functionalize owing to the absence of surface-bound functional groups. In comparison, biologically synthesized (biogenic) AuNPs may be more easily functionalized, enabling their decoration with molecules for imaging or therapeutic applications (Mukherjee & Patra, 2017). Biogenic AuNPs are highly stable and monodisperse compared with NPs synthesized via chemical or physical methods, which increases their potential utility in various biomedical applications (Mukherjee & Patra, 2017). The ease of surface functionalization of biogenic AuNPs may stem from the inclusion of biomolecules such as proteins, lipids, and carbohydrates, which act as reducing and capping agents during biogenic synthesis. The presence of these biomolecules or their functional groups creates an external biomatrix around the surface of the MNPs, which enables easy and effective binding of drug molecules, thus avoiding the need for chemical capping agents (Mukherjee & Patra, 2017).

To date, there have been no systematic studies on the surface functionalization of MNPs such as AuNPs synthesized by different methods. To address this paucity of information, the present study investigated the interactions and responses of differently synthesized AuNPs that were functionalized with 22 natural l-amino acids (AA). To this end, AuNPs were synthesized by three different routes; namely, chemical (C_AuNPs), plant-mediated (P_AuNPs), and bacteria-mediated (B_AuNPs) synthesis, and then the success of functionalization with AAs was tested. Functionalization was characterized and monitored with ultraviolet visible (UV–vis) spectroscopy, Fourier transform infrared spectroscopy (FTIR), agarose gel electrophoresis (AGE), and transmission electron microscopy (TEM) imaging.

Section snippets

Materials

Gold (III) chloride trihydrate (HAuCl4·3H2O) was purchased from Hi Media (Mumbai, India). Trisodium citrate (Na3C6H5O7·2H2O), Luria-Bertani (LB) medium, sodium hydroxide (NaOH), hydrochloric acid (HCl), and the 22 AAs were procured from Merck Bioscience (Mumbai, India). All chemicals were of pure analytical grade, solutions and reagents were prepared in sterile double distilled water (SDDW).

Chemical synthesis of gold nanoparticles

Chemical synthesis of colloidal AuNPs was carried out using Turkevich’s method (Turkevich, Stevenson, &

Synthesis and characterization of gold nanoparticles

In the present study, we successfully synthesized C_AuNPs, P_AuNPs, and B_AuNPs using the Turkevich, ASC-extract, and P. stutzeri-mediated methods, respectively. The appearance of a ruby-red color demonstrating the synthesis of colloidal AuNPs was observed during all syntheses (Fig. 1, inset images). Synthesis by the three methods was also confirmed by the characteristic absorption spectra in the range of 520–540 nm that were recorded. Precisely, SPR peaks were recorded with maximum intensities

Conclusions

We synthesized AuNPs via three different routes to study their functionalization with 22 AAs. Our results demonstrate that P_AuNPs can be successfully functionalized with four particular AAs: Cys, Trp, Tyr, and Val, while B_AuNPs are most easily functionalized with six AAs: His, Lys, Met, Phe, Trp, and Tyr. Notably, C_AuNPs did not show any potential for conjugation with AA. Functionalized B_AuNPs had increased electrophoretic mobility compared with functionalized P_AuNPs. Functionalization of

Conflicts of interest

None.

Acknowledgements

This research did not receive any specific grants from funding agencies in the public, commercial, or-not-for profit sectors.

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