Hydroxyethyl Starch-Based Functionalization of Gold Nanorods: A Possible Alternative to Polyethylene Glycol as a Surface Modifier

Pandey, Asmita; Khadka, Sujan; Wan, Ying

doi:https://doi.org/10.1155/2021/5555448

Journal of Nanomaterials

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Data Availability Conflicts of Interest Authors’ Contributions Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2021 | Article ID 5555448 | https://doi.org/10.1155/2021/5555448

Hydroxyethyl Starch-Based Functionalization of Gold Nanorods: A Possible Alternative to Polyethylene Glycol as a Surface Modifier

Asmita Pandey,^1,2,3,4Sujan Khadka,^3,4,5and Ying Wan¹

Academic Editor: Marinella Striccoli

Received26 Jan 2021

Revised26 Feb 2021

Accepted09 Mar 2021

Published23 Mar 2021

Abstract

PEGylation refers to the process of functionalizing nanoparticles with polyethylene glycol (PEG) to avoid unspecific uptake by the mononuclear phagocyte system and prolong the circulation half-life of nanomedicines. Immunogenicity and nonbiodegradability are the major limitations in PEGylation that can be resolved by substituting PEG with biofriendly polymers, such as hydroxyethyl starch (HES). In the current study, thiolated hydroxyethyl starch (HES-SH, 130/0.4) was harnessed to stabilize gold nanorods (AuNRs) and compared with PEG-SH-coated AuNRs at different aspects of characterization and photothermal analysis. Our results confirm that AuNRs were functionalized successfully with both HES-SH and PEG-SH, where the initial spectra and colloidal stability of gold nanorods were restored after functionalization. In addition, the photothermal conversion stability of gold nanorods was maintained during both HESylation and PEGylation without affecting the heat generation. In summary, we presume that HES-SH can be used as a surface modifier to stabilize gold nanorods and might be used as a promising alternative to PEG.

1. Introduction

Surface modifiers are extensively added to functionalize desired nanoparticles in order to achieve diagnostic and targeted therapeutic goals in the nanomedicine field [1]. Most of the common surface modifiers are hydrophilic polymers that inhibit the aggregation between nanoparticles in an aqueous solvent [2]. The functionalization of nanoparticles using surfactant polymers began in the 1980s, where surfactant micelles were used and exhibited higher targeting efficiency for neuroleptic action [3]. The appropriate selection of surface modifiers is essential to functionalize the surface of nanoparticles to lower the nontargeted distribution into healthy cells [4]. Polyethylene glycol (PEG), one of the common and traditional polymeric compounds [5] and polysaccharides [6], has gained remarkable attention recently.

PEG is widely used to modify nanoparticles by the process called PEGylation, which involves the functioning of nanoparticles with PEG to prevent nanoparticles from being engulfed by phagocytes and to improve their stability as well as plasma life [5, 7]. High stability has made PEG more inclined to targeted delivery as a surface modifier of nanoparticles, which is a “gold standard” approach to alleviate the cytotoxicity in anticancer therapy [8]. PEGylation has been reported to offer the nanoparticles an extended circulation time in vivo and an improved solubility rate to hydrophobic drugs with low nonspecific cytotoxicity [9, 10].

Despite PEG’s various advantages, its practical application is hindered by several limitations. First of all, PEG causes hypersensitivity reactions in the body [11]. A plethora of clinical as well as animal studies reported that a high amount of anti-PEG antibodies were produced after repetitive parenteral dose, which resulted in a comparatively shorter plasma life in the microvascular system compromising the functions of PEG [12–15]. Moreover, the nondegradability of PEG in vivo is another issue that leads to severe toxicity due to assemblage inside the body, if given in maximum dose [16]. Likewise, PEG-based chemotherapeutics lose stability upon rehydration due to PEG crystallization upon lyophilization [17]. Furthermore, the structure of PEG has been shown to contain relatively less reactive sites, which do not allow any ligands or conjugates to get attached to the PEG surface in a convenient manner [18]. These limitations of PEG have motivated the search for efficient and reliable methods to functionalize nanoparticles, such as by using varieties of polysaccharides [6, 19, 20]. Polysaccharides and PEG bear similar characteristics of enhancing plasma life in microvascular circulation as well as resisting the adsorption of proteins [18]. However, polysaccharides offer better biocompatibility and biodegradability compared to PEG along with their significant virtues, such as low cost and low cytotoxicity [6, 20].

Hydroxyethyl starch (HES), a polysaccharide, has been proven to possess superior performances as a nanoparticle modifier in a targeted drug delivery system compared to other nondegradable biopolymers such as PEG [18, 21]. The application of HES was initiated in the early 2000s and was reported to be highly biodegradable and tunable [22]. HES is naturally found in the form of “waxy starch” and is synthesized by hydroxyethylation of amylopectin [23]. HES has been used as an excellent plasma-volume expander for decades and recently has been proved to enhance the plasma half-life of nanomedicines in vivo [24–27]. In addition, there are mounting evidences that HES offers biofriendly advantages over PEG, such as low immunogenicity in vivo [26, 28]. HES is a highly soluble polymer that contains negligible immunological reactions to cells [29]. Its structure analogy to human glycogen is a key reason for minimal immunogenicity [28]. Moreover, HES is reported to be more stable compared to PEG in the gene delivery system. For example, Noga et al. [21] used HES-decorated polyplex and reported higher stability with minimal aggregation effect compared to PEG-polyplex. In a further study, HES exhibited potential shielding property for the targeted delivery of nucleic acid and increased transfection efficiency compared to PEG [10, 17, 21].

Several modification strategies have been attempted on HES for the selective delivery of drugs as well as for stabilizing the nanoparticles, such as chemical modification [4, 30, 31] and biological modification [4, 32]. Herein, HES was chemically modified into HES-SH and was used to functionalize AuNRs by Tris-based loading method. Further, it was compared with PEG-SH-coated AuNRs at different aspects. “Seed mediated growth method” was applied to synthesize AuNRs using cetyltrimethylammonium-bromide (CTAB) as a precursor surfactant. The optical property was determined by UV-visible spectrophotometry, and the hydrodynamic size and morphology were characterized by dynamic light scattering (DLS) and transmission electron microscope (TEM), respectively. The structural characterization of the HES-SH structure was determined by FT-IR (Fourier transform infrared) spectroscopy and ¹H NMR (nuclear magnetic resonance) spectroscopy. Then, the functionalization of AuNRs by HES-SH at different ratios was carried out. Finally, photothermal analysis of AuNRs, HES-S-AuNRs, and PEG-S-AuNRs was performed to confirm the effect of functionalization on the photothermal conversion property of gold nanorods. Our results confirmed that gold nanorods were functionalized successfully with both PEG-SH and HES-SH. Furthermore, we showed that AuNRs retained their colloidal stability and preliminary spectra after functionalization. In addition, functionalization of AuNRs with HES-SH maintained the stability of photothermal conversion by gold nanorods as PEG-SH. In summary, HES-SH can be used as a promising alternative to PEG for the surface modification of gold nanorods.

2. Materials and Methods

2.1. Materials

Sodium hydroxide (NaOH), monochloroacetic acid (MCA), methanol, 2-mercaptoethylamine hydrochloride, 2,2-dipyridyl disulfide, ether, silver nitrate (AgNO₃), ascorbic acid, 2-amino-2-hydroxymethyl-propane-1,3-diol (Tris), N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDCI), N-hydroxysuccinimide (NHS), DL-dithiothreitol (DTT), dimethyl sulphoxide (DMSO), cetyltrimethylammonium-bromide (CTAB), and sodium borohydride (NaBH₄) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Hydrogen tetrachloroaurate (HAuCl₄) and 5-bromosalicyalic acid were purchased from Aladdin Industrial Corporation (Shanghai, China). Thiolated polyethylene glycol (PEG-SH) with an average molecular weight of 2000 Da and hydroxyethyl starch (HES) with an average molecular weight of 130 kDa and molar substitution 0.4 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). The rest of the chemicals used here were of analytical grade (%) purchased from Sinopharm, Beijing, China, and used without any purifications.

2.2. Synthesis of 2-(Pyridyldithio)-Ethylamine Hydrochloride (PDA-HCl)

The protocol published by Zugates et al. was applied to synthesize PDA-HCl [33]. Briefly, 2,2-dipyridyl disulfide (4.4075 g, 40 mM) dissolved in methanol-glacial acetic acid (20.8 mL) and 2-mercaptoethylamine hydrochloride (1.144 g, 20 mM) dissolved in methanol (8.75 mL) were mixed dropwise under stirring. Then, the reaction mixture was kept under an argon atmosphere for 48 h to lower thiol oxidation and was concentrated under reduced pressure on the following day. The product thus obtained was approximately 5-10 mL of yellow-colored oil-like liquid, which was further precipitated by the addition of cold ether (100 mL) following the purification by redissolving in methanol (20 mL). The same purification process was repeated five times and further dried for approximately 2 h to get the final product and stored at 4°C for further use. The synthetic scheme of PDA-HCl is shown in Figure S1.

2.3. Synthesis of HES-COOH

HES-COOH was synthesized based on the protocols suggested by El-Sheikh [34] and Wu et al. [18] with slight modifications. Briefly, HES (2 g, 130/0.4) was dissolved in water (40 mL) in a round-bottom flask, and sodium hydroxide (1.6 g) dissolved in a suitable amount of water was (approximately 5-10 mL) added to it. Then, the reaction was allowed for 3 h at 70°C using a condenser pipe after the addition of monochloroacetic acid (MCA). The resultant mixture was cooled, added into methanol (approximately 300 mL) under stirring, and waited for a while allowing them to precipitate. Supernatant was discarded, and the product collected was further dissolved in water (15 mL) under stirring. Later, the product was dialyzed against water for two days at least four times. Finally, lyophilization was carried out for 3 days to get the final product.

2.4. Synthesis of HES-PDA

The methods to synthesize HES-PDA were adopted from Wu et al. [18]. In brief, all of the three compounds, namely, EDCI (416 mg, 1.09 mM), NHS (125 mg, 0.54 mM), and PDA-HCl (242 mg, 0.54 mM) were alternately added to aqueous HES-COOH (1000 mg, 40 mL) under mild stirring. Then, the reaction was left for 24 h at room temperature, and dialysis was carried for three days. At last, the final dried product (HES-s-s-R) was obtained through lyophilization at -50°C [18].

2.5. Synthesis of HES-SH

HES-SH was synthesized using DTT following the methods by Wu et al. with slight modifications [18]. Briefly, HES-PDA (200 mg) dissolved in DMSO (5 mL) and DTT dissolved in DMSO (1 mL) were mixed. Later, the solution mixture was kept in the argon environment to eliminate the oxygen and left under stirring for 24 h at room temperature following the dialysis for four times and freeze-drying at -50°C. Collected products were stored below -20°C for the structural characterization by ¹H NMR spectroscopy (model-AscendTM, 600 MHz, Bruker) and FT-IR spectroscopy (model-Vertex 70, Bruker).

2.6. Synthesis of AuNRs by Seed-Mediated Growth Method

AuNRs were synthesized by a seed-mediated growth method following the protocol prepared by Ye et al. [35]. Firstly, all the glasswares were well cleaned by aqua regia. For the synthesis of seed solution, HAuCl₄/Au III (5 mL, 0.5 mM) and CTAB (5 mL, 0.2 M) were mixed under mild stirring. Then, freshly prepared cold NaBH₄ (0.6 mL, 0.01 M) was added into the previous solution under stirring for 2 min at a speed of 1200 rpm. The yellow color was changed to brownish yellow, and the solution was kept for 30 min at room temperature. For the synthesis of growth solution, CTAB (1.8 g, 0.05 M) and 5-bromosalicyalic acid (0.22 g) were dissolved in warm water (50 mL, 50-70°C). The solution was cooled and kept at 30°C under stirring. In the meantime, AgNO₃ (3.8 mL, 4 mM) was added and left undisturbed for 15 min. After then, HAuCl₄ (50 mL, 1 mM) was added under stirring and kept for another 15 min under slow stirring (400 rpm). Later, ascorbic acid (600 μL, 0.064 M) was added to the solution under vigorous stirring for 30 sec. Finally, seed solution (126 μL) was added and vigorously stirred for 30 sec. The final solution was kept for 12 h at 30°C to avoid the crystallization of CTAB.

2.7. Characterization of AuNRs by UV-Visible Spectrophotometry and TEM

Previously prepared AuNRs solution was transferred into one-third volume of disposable-cuvette. Then, the characterization was processed using a UV-vis spectrophotometer (model-TU-1901, Beijing Purkinje General Instrument Co., Ltd., China). AuNRs were then purified by double centrifugation with the speed of 9500 rpm for 25 min and redispersed in PBS. AuNRs solution (1 mL) was purified in ultrapure water after centrifugation for 5 min before performing TEM characterization. Finally, very little drop of solution (10 μL) was put on copper grids. The sample was left for air-drying at room temperature for TEM characterization by Hitachi microscope at voltage 100 kV (model-HT7700, Hitachi, Japan).

2.8. Functionalization of AuNRs by mHES-SH (HESylation) and mPEG-SH (PEGylation)

The functionalization of AuNRs with HES-SH and PEG-SH was carried out in presence of Tris-HCl following the protocol published by Zhang and Lin with slight modifications [36]. At first, the AuNRs were mixed with HES-SH (2 mM)/PEG-SH (2 mM) in different ratios by weight at constant volume 200 μL, namely, 1AuNRs : 2HES-SH, 1AuNRs : 2PEG-SH, 1AuNRs : 4HES-SH, 1AuNRs : 4PEG-SH, 1AuNRs : 8HES-SH, and 1AuNRs : 8PEG-SH. Secondly, the mixture of HESylated/PEGylated AuNRs were mixed in Tris-buffer solution (600 μL). Then, the solution mixture was stirred with a speed of 180 rpm for 30-45 min at room temperature. Later, the AuNRs were centrifuged at 9500 rpm for 25 min. Lastly, the supernatants were decanted to remove left HES-SH/PEG-SH, and pellets were redispersed in PBS. The resultant solution was characterized by UV-visible spectrophotometry to examine the changes in optical properties. TEM characterization was performed to determine the change in size after functionalization. Finally, a dynamic light scattering technique was carried out to determine the size distribution and zeta potential of AuNRs in a solution.

2.9. Photothermal Effect Analysis

To perform photothermal effect analysis, the solution with 1AuNRs : 8PEG-SH, 1AuNRs : 8HES-SH, and AuNRs with the same concentration (91.27 mg L⁻¹) was chosen. Laser-808 nm (model: ADR-1860, Changchun Laser Optoelectronics Technology Co., Ltd., China) was used to check the photothermal effect generated by AuNRs in all solutions. Each sample (3 mL) was prepared, and 1 mL sample was exposed to near-infrared (NIR) laser each time. The temperature was recorded by the thermal imaging camera (model-E50, FLIR Systems, United States) each 30 sec. Ultrapure water (3 mL) was used as a control, and the same protocols were followed for the previous three samples. The initial temperature for all solutions was set at 27°C.

2.10. Data Presentation and Analysis

Data were presented as . Data presentation and statistical analyses were performed in Origin 8 software, OriginLab Corporation, Northampton, Massachusetts, USA.

3. Results and Discussion

3.1. Synthesis of HES-SH and Characterization

HES with molecular weight 130 kDa and molar substitution (MS) 0.4 (HES 130/0.4) was taken for the study. On the other hand, thiolated PEG (PEG-SH) with molecular weight 2 kDa was selected to compare the surface modification effect on gold nanorods (AuNRs) with HES-SH. The synthetic route of HES-SH was carried out according to the synthetic scheme displayed in Figure 1. The synthetic protocol prepared by El-Sheikh [34] was applied to our study to synthesize carboxymethyl hydroxyethyl starch (HES-COOH). Briefly, HES (130/0.4) was mixed with monochloroacetic acid (MCA) in the presence of an alkaline solution under the application of heat. ¹H NMR characterization was performed and analyzed as suggested by Wu et al. [18] using NMR spectrometer, and the similar patterns of NMR spectra were observed. ¹H NMR spectra of HES and HES-COOH are shown in Figures 2(a) and 2(b), respectively. The newly emerged peak at the range 3.9-4.2 ppm in Figure 2(b) confirms the successful production of HES-COOH (2). This range represents to the methylene group’s protons present in MCA [37]. In the second step, PDA was added to HES-COOH. As a result, there was a formation of an amide bond. Figure 2(c) exhibits the successful synthesis of HES-PDA. The lately emerged three peaks at the range from 7 to 8.5 ppm in Figure 2(c) determine the protons present in the pyridyl group (4, 5, 6, 7). Similarly, the appearance of two new peaks to the right side (range 2.8 to 3.2 ppm) in Figure 2(c) represents to the methylene group’s protons present in ethylamino (2, 3) [33]. In the third step, DTT was added to HES-PDA. As DTT is a highly reducing agent, it is responsible for breaking disulfide (s-s) bonds to form 2-mercaptopyridine [18]. In Figure 2(d), almost all the pyridyl proton signals are vanished, which confirms the cleavage of disulfide bonds to produce HES-SH. The FT-IR characterization was performed and analyzed as suggested by Wu et al. [18] using FT-IR spectrometer, and the patterns of FT-IR spectra we observed were similar as they reported. The FT-IR spectra of HES (130/0.4), HES-COOH, and HES-SH are shown in Figure 2(e). While comparing the bands a and b, the remarkable change appeared around the peak 1606 cm^-1. This change determines the -COO¯ (carboxylate ion) stretch in the reaction, thus confirming the synthesis of HES-COOH. Similarly, while comparing the bands b and c, the significant difference was shown around the peak 1637 cm^-1, which displays that there is a stretch at -CONH- bond in the amide group of HES-PDA indicating the presence of PDA in the reaction. None of any notable peak within the range from 1513 cm^-1 to 1637 cm^-1 confirms the absence of a pyridyl group in HES-PDA due to the disulfide bond cleavage by DTT to produce HES-SH [18]. Based on both ¹H NMR and FT-IR spectra analyses, it is confirmed that HES-SH was synthesized successfully.

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3.2. Synthesis of AuNRs

AuNRs were synthesized by a simple and effective seed-mediated growth method reported by Ye et al. [35]. The synthetic scheme of AuNRs is displayed in Figure 3(a). Two solutions, namely, “seed solution” and “growth solution” were prepared and used cautiously. The seed solution was prepared reducing Au³⁺ (gold III) ion with NaBH₄ (sodium borohydride) and surfactant cetyltrimethylammonium-bromide (CTAB, 0.2 M). The growth solution was prepared with CTAB (0.05 M) and gold III ion solution (HAuCl₄) in the presence of additive 5-bromosalicyalic acid, silver nitrate (AgNO₃), and ascorbic acid. The hydrophobic benzene ring of 5-bromosalicyalic acid favors the penetration into the hydrophobic alkyl tail of CTAB molecules thereby allowing the rod-like transition of CTAB micelles due to the reduced electrostatic repulsion between micellar surface charge and COO^– ions [35]. Ag⁺ ions allow the anisotropic growth of gold nanorods by breaking the symmetry of gold seeds [38]. The addition of ascorbic acid in the CTAB-HAuCl₄ solution led to the disappearance of yellow color due to the reduction of Au³⁺ ions [35]. It is crucial to address the multiple parameters, such as temperature, pH of growth solution, concentration of reagents, and amount of Ag⁺ as well as the amount of VC (ascorbic acid) in order to maintain the aspect ratio of gold nanorods [35, 36, 39]. The injection of a little amount of seed solution in growth solution at the final step acted as a precursor to yield the growth of AuNRs. The resulted red color after 12 h of incubation displayed that there is a production of AuNRs with longitudinal surface plasmon resonance (LSPR) around 800 nm. For the further confirmation of LSPR wavelength, UV-vis spectrophotometry was conducted.

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Figure 3

(a) Synthetic route of AuNRs in the presence of CTAB, (b) HESylation and PEGylation of AuNRs in the presence of Tris-HCl at pH 3 followed by 808 nm laser irradiation, (c) UV-vis spectra of AuNRs-CTAB and AuNRs before and after centrifugation, respectively, (d) TEM image of AuNRs after centrifugation, (e) UV-vis spectra of HESylated AuNRs at different ratios, (f) TEM image of HES-S-AuNRs, (g) UV-vis spectra of PEGylated AuNRs at different ratios, (h) TEM image of PEG-S-AuNRs, (i) zeta potential of AuNRs, HES, HES-SH, HESylated AuNRs, and PEGylated AuNRs, and (j) hydrodynamic diameter of AuNRs, HESylated AuNRs, and PEGylated AuNRs.

3.3. Characterization of Optical Properties of AuNRs by UV-Visible Spectrophotometry

The “anisotropic” behavior of AuNRs caused the formation of two surface plasmon resonance bands [38]. The “absorption maxima” in longitudinal plasmon resonance of AuNRs was measured 804 nm in UV-visible spectrophotometry, whereas 527 nm of maximum absorption was found in transverse plasmon resonance mode. The gold nanorods prepared at the NIR region have potential applications in photothermal ablation of cancer cells, where they strongly absorb NIR light and generate heat upon excitation thereby resulting in a selective heating of the tumor cells [40–42]. The optical image and UV spectra of synthesized AuNRs with 804 nm are shown in Figure S2 and Figure 3(c), respectively. Synthesized AuNRs were centrifuged in falcon tubes to remove excess CTAB (Figure S3). Then, samples were centrifuged two times at the speed of 9500 rpm for 25 min. The supernatants were discarded in each step and finally redispersed in PBS. These steps removed almost all the excess CTAB present in the solution. The UV-visible spectrophotometry was again carried out in centrifuged samples, and the longitudinal plasmon resonance band was observed at 777 nm (Figure 3(c)). It is because double centrifugation led to the sedimentation of AuNRs with larger volume and smaller aspect ratio, thus resulting in the blue shift of longitudinal surface plasmon resonance band [43]. Our results demonstrated that there were no aggregations between AuNRs (Figure 3(c)).

3.4. Characterization of Morphology of AuNRs by TEM

TEM image of centrifuged AuNRs is displayed in Figure 3(d). The scale bar is 100 nm. The morphology of AuNRs was rod-shaped having the length nm and width nm. TEM image also showed that little spherical particles were present along with AuNRs (Figure 3(d)).

3.5. HESylation and PEGylation of AuNRs

HESylation and PEGylation were conducted according to the protocol published by Zhang and Lin [36]. HESylation and PEGylation were conducted simultaneously using the same method with the same concentration of AuNRs in the presence of Tris buffer. The experiment was performed for different ratios of AuNRs and HES-SH and compared. The ratios were as follows: 1AuNRs : 2HES-SH, 1AuNRs : 2PEG-SH, 1AuNRs : 4HES-SH, 1AuNRs : 4PEG-SH, 1AuNRs : 8HES-SH, and 1AuNRs : 8PEG-SH. Tris buffer contains an amine (-NH₂) as its functional group, which has a strong affinity toward the gold (Au) surface [36]. Thus, -NH₂ group might have displaced the CTAB-coated cationic bilayers [36]. As a result, the thiol (-SH) group of HES-SH and PEG-SH bounds to the gold surface by strong covalent bond [44]. Another reason for successful functionalization might be due to the neutralization of anions present in HES-SH and PEG-SH molecules, which facilitates HES-SH and PEG-SH molecules to be bonded quickly with the gold surface [36]. The application of 808 nm laser in functionalized gold nanorods resulted in a dissipation of heat without affecting the photothermal conversion stability of gold nanorods. The process of HESylation and PEGylation is shown in Figure 3(b).

3.6. Characterization of HESylated AuNRs and PEGylated AuNRs by UV-Visible Spectrophotometry and TEM

UV-visible spectrophotometer was used to characterize optical behaviors of HESylated gold nanorods (HES-S-AuNRs) and PEGylated gold nanorods (PEG-S-AuNRs). The UV-visible spectra of HES-S-AuNRs and PEG-S-AuNRs at different concentrations are shown in Figures 3(e) and 3(g), respectively. The maximum absorption occurred at 794 nm longitudinal mode when 1AuNRs : 8HES-SH ratio was used, which is the closest longitudinal resonance absorption point to 808 nm, the wavelength of NIR laser used in the current study. The “absorption maxima” of longitudinal resonance for 1AuNRs : 2HES-SH and 1AuNRs : 4HES-SH ratios were 792.5 nm and 781 nm, respectively. On the other hand, while comparing different ratios of AuNRs in PEG-S-AuNRs in Figure 3(g), 1AuNRs : 8PEG-SH and 1AuNRs : 4PEG-SH obtained the maximum absorption at 781.5 nm and 783 nm, respectively. The ratio 1AuNRs : 2PEG-SH exhibited the highest absorption at 778 nm. The red shift in the plasmon band position after functionalization might be due to a change in the refractive index of surrounding after the adsorption of both polymers on the surface of gold nanorods [45]. From the obtained AuNRs spectra, it is demonstrated that no aggregations between the AuNRs were observed after CTAB displacement with both HES-SH and PEG-SH. It means that the functionalization with both polymers restored the colloidal stability and initial spectra of AuNRs, presumably due to the steric hindrance caused by polymer coating [46]. It is reported that HES-SH or PEG-SH has strong-affinity towards water molecules. Surface functionalization of gold nanorods with HES-SH or PEG-SH forms “hydration layer” with water that prevents the aggregation of gold nanorods [2].

TEM characterizations of HES-S-AuNRs and PEG-S-AuNRs are shown in Figures 3(f) and 3(h). TEM images were taken by Hitachi microscope at a voltage 100 kV, and the scale bar is 200 nm. The morphology of AuNRs in both specimens was rod-shaped. The length and width of HES-S-AuNRs were measured nm and nm, respectively. The length and width of PEG-S-AuNRs were measured nm and nm, respectively. Our result showed that the length and width of both HES-S-AuNRs and PEG-S-AuNRs were increased by ~10 nm and ~3 nm, respectively, compared to centrifuged AuNRs alone (Figures 3(d), 3(f), and 3(h)). This is ascribed to the successful coating of HES-SH and PEG-SH after functionalization.

3.7. DLS Characterization (Zeta Potential and Hydrodynamic Size Measurement)

DLS characterization of AuNRs, HESylated AuNRs, and PEGylated AuNRs was carried out by Dynamic light scattering (DLS) (model-zetasizer Nano, Malvern Instrument-UK). The zeta potential () values of AuNRs, HES (130/0.4), HES-SH, HESylated AuNRs, and PEGylated AuNRs are shown in Figure 3(i). The result displays that the zeta potential of CTAB-stabilized AuNRs was +38.3 mV. The reason for the positive zeta potential is due to the presence of highly positive cationic CTAB in AuNRs solution [47]. HES (130/0.4) and HES-SH produced -7.29 mV and -13 mV zeta potential, respectively, whereas HES-S-AuNRs exhibited -25.1 mV surface charge. The result revealed that there was a huge reduction in the zeta potential charge (from +38.3 mV to -25.1 mV). This suggests that the CTAB bilayer in AuNRs was replaced and properly coated by HES-SH. On the other hand, the zeta potential of PEG-S-AuNRs was also decreased to -9.08 mV suggesting that the PEGylation approach was also completed while displacing the micelle bilayer formed by CTAB on AuNRs surface.

The hydrodynamic diameters (D H) of AuNRs, HESylated AuNRs, and PEGylated AuNRs are displayed in Figure 3(j). The original hydrodynamic diameter of AuNRs was found 81 nm. After HESylation and PEGylation, the diameter was increased to 188.2 nm and 183.1 nm, respectively. The hydrodynamic diameter is obtained as a result of the spherical approximation of gold nanorods. Thus, it cannot be compared directly to the “effective” size obtained by TEM [48]. It is observed that HESylation provides a narrower size distribution in comparison with PEGylated samples (Figure 3(j)).

3.8. Photothermal Effect Analysis

The photothermal effect was analyzed by NIR laser diode 808 nm and thermal imaging camera at AuNRs concentration 91.27 mg L⁻¹. Photothermal analysis images of AuNRs, HES-S-AuNRs, and PEG-S-AuNRs are displayed in Figures 4(a)–4(c), respectively. The water sample was taken as a control (Figure 4(d)). The images were captured at the highest temperature generated by each sample. The initial temperature was set to 27°C, and the delta of temperature ( T) was calculated by subtracting the initial temperature from the maximum temperature generated by each sample. The ideal temperature needed for photothermal therapy in clinical application is around 50°C, and the basal temperature in the human body is 37°C. Hence, the jump in the temperature needed is >13°C [49]. The maximum heat dissipation achieved by AuNRs, HES-S-AuNRs, and PEG-S-AuNRs was 50.8°C, 50.6°C, and 50.3°C, respectively (Figures 4(a)–4(c)), while the temperature in water remained constant (Figure 4(d)). The T achieved was ~23°C in 3.5 min for all the three samples. While comparing the temperature-time curve by HES-S-AuNRs and PEG-S-AuNRs, the increment rate of temperature is almost similar, and both functionalization techniques did not affect the heat dissipation process by AuNRs, thereby maintaining the photothermal conversion stability of AuNRs (Figure 4(e)).

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Figure 4

Photothermal effect analysis images of (a) AuNRs, (b) HES-S-AuNRs, (c) PEG-S-AuNRs, and (d) water. The maximum heat dissipation achieved by AuNRs, HES-S-AuNRs, and PEG-S-AuNRs was 50.8°C, 50.6°C, and 50.3°C, respectively, while the temperature in water remained constant. The T achieved was ~23°C in 3.5 min for all the three samples with AuNRs concentration 91.27 mg L⁻¹. (e) The temperature-time curve of AuNRs, HES-S-AuNRs, and PEG-S-AuNRs at different time intervals at AuNRs concentration 91.27 mg L⁻¹ (the number of experimental replicates ).

4. Conclusion

In conclusion, surface functionalization strategy by polymer surfactants is essential to improve the stability of gold nanorods. HESylation is one of the potential functionalization approaches that can overcome the limitations over PEGylation, such as hypersensitivity and nonbiodegradability. In the current study, we reported that the functionalization with both PEG-SH and HES-SH restored the colloidal stability and initial spectra of gold nanorods. In addition, both HESylation and PEGylation did not affect the photothermal effect produced by gold nanorods to exceed the optimal jump of the temperature, thereby maintaining the photothermal conversion stability of AuNRs. Owing to the significant advantages of HES over PEG, especially biocompatibility and biodegradability, HESylation can be a promising alternative for PEGylation. However, further in vitro and in vivo experiments must be performed to confirm the biocompatible and biodegradable behavior of HESylated gold nanorods for the clinical application.

Data Availability

All data are integrated in manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Authors’ Contributions

Y.W. conceived the concept; A.P. performed the experiments and wrote the manuscript; S.K. formatted, edited, and revised the manuscript in collaboration with A.P. All authors read and approved the final manuscript.

Acknowledgments

The authors would like to appreciate Mr. Sanjib Adhikari for checking the paper’s consistency and correcting the language.

Supplementary Materials

The supplementary material contains three figures: synthetic route of PDA-HCl (Figure S1) and an optical image of AuNRs-CTAB solution in Erlenmeyer flask and in centrifugal tubes (Figure S2 and Figure S3, respectively). (Supplementary Materials)

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Copyright © 2021 Asmita Pandey et al. 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.

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