New insights into the formation mechanism of gold nanoparticles using dopamine as a reducing agent

Journal of Colloid and Interface Science 523 (2018) 27–34

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New insights into the formation mechanism of gold nanoparticles using dopamine as a reducing agent Sinan Du 1, Yang Luo 1, Zhengfang Liao, Wei Zhang, Xinhua Li, Tianyu Liang, Fang Zuo ⇑, Keyi Ding ⇑ College of Chemistry & Environment Protection Engineering, Southwest Minzu University, Chengdu 610041, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 4 December 2017 Revised 21 March 2018 Accepted 23 March 2018 Available online 24 March 2018 Keywords: Dopamine Gold nanoparticle Quinone Semiquinone Mechanism

a b s t r a c t Dopamine (DA), a simplified mimic of mussel proteins, can be employed as a reductant in the preparation of Au nanoparticles (AuNPs) due to its inherent catechol building block. The widely accepted mechanism of AuNP formation using DA as the reductant assumes that the reduction of Au(III) ions involves the two-electron oxidation of DA, where the corresponding phenol and phenolates serve as the reductive species to yield quinone. We herein report a novel insight into the mechanism of formation of AuNPs using DA as the reductant. We demonstrate that the synthesis of AuNPs requires the prior oxidation of the DA to form quinone units, which then catalyze the formation of semiquinones. These semiquinone radicals (SMQs) reduce the Au(III) ions to form the initial AuNPs, and further growth is then catalyzed by the first AuNPs, with nucleation occurring where the SMQs, phenols, and phenolates can serve as reductive species. In addition, DA oxidizes and polymerizes to form a polydopamine capping layer on the AuNPs. We therefore expect that the novel mechanism proposed herein may promote us to furthermore explore the production of noble metal NPs using other polyphenols. Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction Recently, gold nanoparticles (AuNPs) have received significant attention in the areas of biomedical devices [1], biosensors [2], ⇑ Corresponding authors. E-mail addresses: [email protected] (F. Zuo), [email protected] (K. Ding). 1 These authors contributed equally to this work and should be considered co-first authors. https://doi.org/10.1016/j.jcis.2018.03.077 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

catalysis [3], and pharmaceuticals [4], due to their fascinating optical, electronic, and chemical properties [1–4]. To date, a variety of methods and techniques have been reported and reviewed for the preparation of AuNPs, including chemical [5–14], sonochemical [15], and photochemical [16] approaches. However, the chemical reduction method is the most common synthesis route for AuNPs due to its simplicity. This method is based on the chemical reduction of gold precursors by a reductant [5–14].

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In this context, the commonly used mussel protein mimic, dopamine (DA) [17,18], has been found to be an effective reductant in the synthesis of AuNPs [8–13]. Besides its reducing ability, DA and the generated polydopamine (PDA) can also be used as binding agents [17,18], which help stabilize the produced NPs [14]. In these reports, the authors considered that the initial reductive species are the phenols or phenolates of DA, and they also propose that the reduction of Au ions involves a two-electron oxidation of DA to yield the quinone [10–13]. Moreover, the oxidation mechanism of DA remains controversial [19]. Many researchers have reported that the oxidation of DA is a two-electron reaction, which produces the corresponding dopamine-quinone (DAQ) [20–22]. However, DA has also been reported to undergo one-electron oxidation, wherein semiquinone radicals (SMQs) are the main intermediates [23,24]. On the other hand, SMQs also form via disproportionation involving DA and DAQ [25,26]. This lack of consensus encourages us to further explore the formation mechanism of AuNPs using DA as the reductant. Herein we present new insights into the formation mechanism of AuNPs using DA as the reductant. We consider that the initial reductive species is not the phenol group itself, but rather the SMQs formed by electron transfer from o-quinone to the catechol

units [25,26]. In particular, we propose that the DA-based synthesis of AuNPs requires that DA is first oxidized to form quinone units, which then catalyze the formation of SMQs [25,26]. 2. Results and discussion We first examined the effect of the pH of the reaction system on the formation of the AuNPs using DA as a reductant. Initially, the pH of an aqueous DA solution was varied between 4.0 and 8.0 and allowed to stand under air for 30 min prior to the addition of a solution of HAuCl4 (i.e., Au(III)). Ultraviolet–visible (UV–Vis) spectroscopy was then employed to monitor the reaction between the Au(III) ions and the DA solution at a range of pH values. For example, at pH 8.0, addition of the gold salt led to a rapid color change with surface plasmon resonances (SPR) centered at 520 nm, which is consistent with the formation of AuNPs (Fig. 1A) [5]. As shown in the transmission electron microscopy (TEM) image of the obtained product (Fig. 1B), mainly spherical particles (in addition to a few anisotropic particles) exhibiting a uniform size distribution were observed. Upon decreasing the pH of the DA solution to 7.0, a broader SPR band was observed at 600 nm (Fig. 1C). In this case, a small amount of larger and aggregated spherical AuNPs were clearly observed in the TEM image

Fig. 1. Time dependent UV–Vis spectra of the AuNPs prepared at (A) pH 8.0, (C) pH 7.0, and (E) pH 4.0 via the reduction of HAuCl4 by DA. The insets show the corresponding AuNP solutions after reduction. TEM images of the AuNPs prepared at (B) pH 8.0, (D) pH 7.0, and (F) pH 4.0.

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Fig. 2. UV–Vis spectra of (A) DA solution over different time intervals at pH 8.0, and (B) the AuNPs synthesized by adding HAuCl4 to the DA solution at different time intervals (pH 8.0).

(Fig. 1D). In addition, upon further lowering the initial pH of the DA solution to 4.0, the UV–Vis absorption intensity between 500 and 600 nm was very low (Fig. 1E), and the solution appeared colorless to the eye. As indicated in the TEM image (Fig. 1F), only a few irregularly shaped NPs formed under these conditions. Furthermore, the obtained kinetics curve indicates that the formation rate of AuNPs increased with an increase in pH of the DA solution (Fig. S1). These results therefore confirm the important role of pH in controlling the formation rate, quantity, shape, and size of AuNPs when employing DA as the reductant. More specifically, the pH dependence of the reduction process indicates that the phenolic moiety of DA likely plays an important role in the formation of AuNPs. Indeed, various studies have proposed that the pH dependence of the reduction rate is closely related to the acid dissociation of the phenolic groups of DA, since the phenolates have a stronger reduction ability than the phenolic

groups [27,28]. In this work, we present a different viewpoint on the effect of pH on the DA-based synthesis of AuNPs. Surprisingly, we also found that the oxidation of DA significantly affects the formation of AuNPs. When the pH of the DA solution was maintained at 8.0, a new peak at 448 nm appeared in the UV–Vis spectra of the samples, which was ascribed to the overlapping peak at 390 nm (assigned to DAQ) and 480 nm (assigned to aminochrome), thus indicating the formation of both DAQ and aminochrome (arising from the cyclization of DAQ) [29,30] (Fig. 2A). Under these conditions, we also observed that the absorption intensity increased with longer standing times. We also investigated the effect of adding the HAuCl4 solution to the alkaline DA solution at different time intervals. In this case, the intensity of the absorption band at 520 nm (Fig. 2B) correlated with that of the 448 nm band shown in Fig. 2A, thereby indicating that as the levels of DAQ increase, AuNP formation is

Fig. 3. (A) UV–Vis spectra of the DA solutions at different DA:MEC molar ratios. (B) Cyclic voltammograms of DA at different DA:MEC molar ratios in Na2HPO4-citric acid buffer solution (pH 8.0) containing 0.2 M NaCl (scan rate = 50 mV/s). (C) UV–Vis spectra of the AuNPs synthesized by the reduction of HAuCl4 by DA with different DA:MEC molar ratios (pH 8.0).

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Fig. 4. TEM images of the AuNPs synthesized by the reduction of HAuCl4 by DA in the presence of MEC. (A) DA:MEC molar ratio of 1:0.5, and (B) DA:MEC molar ratio of 1:1.

Fig. 5. UV–Vis spectra of the AuNPs synthesized by the reduction of HAuCl4 by DA (a) in air, (b) with O2 purging, and (c) under N2 at pH 7.0. Fig. 6. ESR spectra of the DA solution at pH 8.0 in the absence (a) and presence (b) of MEC at pH 8.0.

promoted. These interesting findings motivated us to examine the underlying mechanism for this unexpected result. Thus, p-benzoquinone (p-BQ) was introduced into the mixed solution of DA and HAuCl4. In this case, a new band at 566 nm appears (Fig. S2, curve b). On the other hand, the TEM image (Fig. S3) demonstrates that the AuNPs prepared in the presence of p-BQ and DA exhibit a different morphology compared with that prepared used DA as the reductant. These results indicated that quinones do indeed play an important role in the synthesis of AuNPs. Because thiols are known to eliminate quinones via Michael addition reactions, [31–34] we introduced mercaptoethanol (MEC) into the alkaline DA solution. We found that the band at 448 nm, which corresponds to DAQ and its cyclization product, gradually disappears with the increasing MEC content (Fig. 3A). This reaction is rapid, and the absorption peak of DAQ disappears completely with a DA-to-MEC molar ratio of 1:5. The reaction between DAQ and MEC was also studied electrochemically using cyclic voltammetry (CV). As shown in Fig. 3B, the reduction peak corresponding to the electrogenerated DAQ [29] disappears gradually upon increasing the MEC content, which indicates that DAQ is removed upon reaction with MEC [29]. As expected from the UV–Vis spectra, the DAQ reduction peak completely disappeared at a DA-to-MEC molar ratio of 1:5. Subsequently, the HAuCl4 solution was added to the DA/MEC mixture at pH 8.0, resulting in a decrease in intensity of the UV–Vis absorbance at 520 nm

(i.e., corresponding to the AuNPs) at higher MEC contents (Fig. 3C), with the formation of AuNPs being completely suppressed at a DA-to-MEC molar ratio of 1:5. This result is consistent with the decrease in the DAQ content of the reaction solution. The TEM images in Fig. 4 show direct evidence that MEC hindered the formation of AuNPs, as an increase in the MEC content corresponded with decreased formation of the AuNPs. These results therefore indicate that the formation of AuNPs is closely related to the presence of DAQ, because the absence of DAQ, no AuNPs formed. As previously mentioned, amines react slowly with quinones [29]; thus, we examined the effect of monoethanolamine (MEA) addition on the reaction system. As shown in the UV–Vis spectra (Fig. S4A) and the CV curves (Fig. S4B), adding MEA slightly decreased the DAQ content, thereby resulting in little variation in AuNP formation (Fig. S4C). As a food additive, NaHSO3 is often employed to inhibit enzymatic browning, as SO23 reacts with the quinone structure [35]. We therefore examined the effect of NaHSO3 adding to the alkaline DA solution, and found that NaHSO3 completely inhibits the formation of DAQ (Fig. S5A), thereby suppressing the formation of the AuNPs (Fig. S5B). Indeed, the Ma group previously confirmed that the quinone moiety plays an important role in the photoreaction of Au ions with phenolic compounds; thus, the quinones are

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Fig. 7. UV–Vis spectra of the AuNPs synthesized by the reduction of HAuCl4 using DA as a reductant in the absence (a) and presence of different volumes of Au seeds (b–d) at pH 4.0 (A) and pH 7.0 (B). The insets show the samples corresponding to the UV–Vis spectra.

Scheme 1. Proposed synthetic mechanism for the formation of Au@PDA NPs.

reduced by NaBH4, no AuNPs can be formed [36]. In addition, the Kapoor group proposed that the hydroquinone must initially be oxidized by O2 or MnO2, after which point silver ions can be reduced to form AgNPs [37]. It is therefore apparent that the formation of DAQ is a prerequisite to the formation of AuNPs when DA is employed as the reductant. Researchers generally seem to agree that during the synthesis of AuNPs using DA and its derivatives as the reductants, the initial reductive species are the phenols and phenolates [10–13]. If this is indeed the case, O2 should not play a role in the synthesis of these NPs when DA is employed as the reductant. However, in the absence of O2, the generation of AuNPs decreases significantly (see Fig. 5), and later increases again upon the reintroduction of O2. These results therefore indicate that DA must first be oxidized before the oxidized species initiate the reduction of Au(III) to form AuNPs [37], thereby suggesting that the initial reductive species are neither phenols nor phenolates. In contrast, it has been reported that during the oxidation of DA, O2 and SMQs could be generated [38,39]. In addition, the synthesis of AgNPs and AuNPs via a reaction initiated by O2 or SMQ radical species has also been reported

[40–42]. We could therefore speculate that these radicals play a role in the synthesis of AuNPs when DA is employed as the reductant. To examine this in further detail, we examined the effect of the radical scavenger 2, 2-diphenyl-1-picrylhydrazyl (DPPH) on AuNPs formation. As the color of the product formed upon reaction of DPPH with a radical species is similar to that of the AuNPs, we employed Ag(I) instead of Au(III). Upon the addition of a DPPH solution to the reaction system containing AgNO3 and DA (at pH 8.0), the absorption maximum of the solution at kmax = 404 nm corresponding to the AgNPs [37] decreased in intensity and the color of the solution faded compared to the system without DPPH (Fig. S6). This result therefore confirms that the radicals take part in reduction of the metal ions. Furthermore, we investigated the effect of superoxide dismutase (SOD), an enzyme which catalyses the decomposition of O2 [41], on the formation of AuNPs. Interestingly, the addition of SOD led to a slight reduction in the rate of formation of the AuNPs (Fig. S7), which illustrates that O2 is not the key reductive species in Au(III) ion reduction. Furthermore, electron spin resonance (ESR) spectroscopy was employed to characterize the free radicals during the reaction process. In view of only dopamine, the g-factor is around 2.0038 (experimental valu

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e = 2.0063) (curve a, Fig. 6), which is reported to corresponds to a typical SMQs [43,44]. However, several researchers considered that the existence of g-factor near 2.004 was due to the simultaneous presence of carboncentered radicals and SMQs [45,46]. Further study is needed to carry out to make clear whether carboncentered radicals exist in the dopamine solution. As expected, this signal was not observed following the addition of MEC (curve b, Fig. 6). This result is in good agreement with the above observation that MEC suppresses the formation of AuNPs. It is therefore apparent that SMQ is responsible for the initial reduction activity, and is essential for the synthesis of AuNPs using our system. We could therefore conclude that DAQ is the prerequisite and the SMQs are the initial reductive species for the synthesis of AuNPs using DA as the reductant. As mentioned above, during the synthesis of AuNPs using DA as the reductant, DAQs are the prerequisites and SMQs are the initial reductive species. However, the relationship between DAQs and SMQs remains unclear. The catechol quinone species has previously been reported to serve as a catalyst to promote catechol oxidation by participating in comproportionation to yield the SMQs [25,26,47]. In addition, the Tatsumi group demonstrated that the fully reduced phenol is inert to O2, with only the presence of quinones initiating the oxidation of phenol, thereby generating SMQs [47]. Our observation that quinones can accelerate the formation of AuNPs also supports the idea that quinones can catalyze the formation of SMQs. These results therefore indicate that DA must first be oxidized to DAQ, and subsequent reaction of the formed DAQ with DA produces SMQs, which initiate reduction of the Au(III) ions. Upon the examination of DA solutions at different pH values by UV–Vis spectroscopy (Fig. S8), it was apparent that the formation of DAQs is promoted by an increase in pH. We next recorded CV curves to explore the oxidation of DA at a range of pH values (see Fig. S9). The anodic peak potential shifted negatively upon increasing the pH from 4.0 to 8.0, indicating that at higher pH values, DAQs form easily. Indeed, the Rich group previously reported that the autoxidation rate of DA depends on pH, and is stimulated by the presence of small amounts of quinone in the reaction mixture [48]. They found that when the reaction is performed at lower pH values, little or no autoxidation was observed [48]. In addition, the Kano group considered that this stabilization of SMQ at increased pH values may be responsible for the pH dependence of the autoxidation process [27]. We could therefore conclude that the influence of pH on AuNP formation originated from the pH dependence of DAQ and SMQ formation. As shown in Fig. S1, the sigmoidal nature of the absorption-time plot indicates that the formation of AuNPs is autocatalytic in nature [49,50]. A number of groups have demonstrated that during the synthesis of AgNPs or AuNPs using phenol derivatives as the reductants, nanoseeds must be present to initiate the reduction of Au(III) ions by the phenols [37,51,52]. Interestingly, although we observed that DA is not very effective in reducing Au(III) ions to metallic gold under acidic and neutral conditions, a small quantity of Au seeds added to the reaction mixture promoted the formation of AuNPs under both acidic and neutral conditions (Fig. 7). This observation indicates that the initially formed AuNPs can catalyze the reduction of Au(III) ions when phenols, phenolates, and SMQs serve as the reductive species. We therefore concluded that after the initial AuNPs form, catalytic growth and nucleation take place, with the SMQs, phenols, and phenolates of DA participating in the reduction reaction. We next examined the stability of the prepared AuNPs. Interestingly, the AuNPs prepared at pH 8.0 using DA as the reductant were stable in water for approximately 6 months. In addition, they gave a zeta-potential of 33 mV (Fig. S10), thereby indicating their high dispersibility in aqueous solution. It should be noted, however, that during the synthesis of the AuNPs, no additional stabilizing agent

was added. We then employed Fourier transform infrared (FT-IR) spectroscopy to examine the nature of the surface-bound ligands of AuNPs to determine their potential influence on NP stability. As shown in Fig. S11, the FT-IR spectrum of the AuNPs is similar to that of pure PDA, which suggests the formation of PDA on the AuNPs through self-polymerization [13]. An amorphous coating with no crystallinity was observed on the surface of the AuNPs, as shown in the HRTEM image (Fig. S12), which confirmed the formation of the PDA shell [53]. Indeed, it has been previously considered that AuNPs can be stabilized by the formed quinones when polyphenols are employed as the reductant [54,55]. In addition, it has been reported that quinones can be reduced by sodium borohydride (NaBH4) [56]. In our case, adding NaBH4 to an aqueous dispersion of AuNPs resulted in the precipitation of NPs after several minutes, thereby confirming that PDA molecules adhere to the AuNPs through gold–quinone interactions to yield Au@PDA NPs, and resulting in good stability in aqueous solution [57]. Furthermore the dissociation of quinone and imine groups under alkaline conditions led to the PDA surface exhibiting a negative charge [58]. Based on the above results, we proposed a new mechanism of formation for the AuNPs using DA as the reductant. As shown in Scheme 1, DA is initially oxidized to DAQ, and the resulting DAQ reacts with DA to produce SMQ. These radicals then reduce the Au(III) ions to form the initial Au nanoseeds and AuNPs. Further growth is subsequently catalyzed by the initial AuNPs and new nucleation takes place simultaneously, where the SMQs, phenols, and phenolates of DA can act as the reductive species. Finally, the DA oxidizes and polymerizes to form a PDA capping layer on the AuNPs, yielding PDA-coated AuNPs (Au@PDA NPs). A control experiment using catechol as the starting material was performed to support the above result (see Fig. S13).

3. Conclusions In summary, we herein presented a novel insight into the mechanism of formation of gold nanoparticles (AuNPs) using dopamine (DA) as the reductant. We demonstrated that the Au(III) ions are only reduced following the oxidation of DA to quinones (DAQs), which subsequently react with DA to form the semiquinones (SMQs). These SMQs then serve as the initial reductive species to reduce Au(III) ions and produce the first AuNPs. Further NP growth is then catalyzed by the initial AuNPs, with simultaneous nucleation taking place where the SMQs, phenols, and phenolates of DA serve as the reductive species. In addition, we confirmed that a film of PDA, the oxidation-polymerization product of DA, was formed on the surface of the AuNPs, which yielded AuNPs that were stable in aqueous solution for up to 6 months. Moreover, the proposed mechanism was confirmed using catechol as the reductant. We expect that our new understanding of the formation mechanism of AuNPs using DA as the reductant will promote us to furthermore explore the production of noble metal NPs using other polyphenols.

Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding sources This work was supported by the National Natural Science Foundation of China (51273220) and the Functional Polymer Innovation Team Project, Southwest University for Nationalities (14CXTD04).

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Notes The authors declare no competing financial interest. Acknowledgment The work was supported by College of Chemistry & Environment Protection Engineering, Southwest Minzu University, National Natural Science Foundation of China (51273220) and the Functional Polymer Innovation Team Project, Southwest Minzu University (14CXTD04). Appendix A. Supplementary material Detailed experimental procedure and characterization; Figures showing additional UV-Vis spectra, TEM image, cyclic voltammograms curves, and Zeta potential curve. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2018.03.077. References [1] N. Mustafaoglu, T. Kiziltepe, B. Bilgicer, Site-specific conjugation of an antibody on a gold nanoparticle surface for one-step diagnosis of prostate specific antigen with dynamic light scattering, Nanoscale 9 (2017) 8684–8694. [2] X. Wu, Y. Xia, Y. Huang, Y. Huang, J. Li, H. Ruan, T. Chen, L. Luo, Z. Shen, A. Wu, Improved SERS-active nanoparticles with various shapes for CTC detection without enrichment process with supersensitivity and high specificity, ACS Appl. Mater. Inter. 8 (2016) 19928–19938. [3] E. Seo, J. Kim, Y. Hong, Y.S. Kim, D. Lee, B. Kim, Double hydrophilic block copolymer templated Au nanoparticles with enhanced catalytic activity toward nitroarene reduction, J. Phys. Chem. C 117 (2013) 11686–11693. [4] L.C. Kennedy, L.R. Bickford, N.A. Lewinski, A.J. Coughlin, Y. Hu, E.S. Day, J.L. West, R.A. Drezek, A new era for cancer treatment: gold-nanoparticlemediated thermal therapies, Small 7 (2011) 169–183. [5] P. Dauthal, M. Mukhopadhyay, Noble metal nanoparticles: plant-mediated synthesis, mechanistic aspects of synthesis, and applications, Ind. Eng. Chem. Res. 55 (2016) 9557–9577. [6] S. Wang, K. Qian, X. Bi, W. Huang, Influence of speciation of aqueous HAuCl4 on the synthesis, structure, and property of Au colloids, J. Phys. Chem. C 113 (2009) 6505–6510. [7] V.M. Kariuki, J.C. Hoffmeier, I. Yazgan, O.A. Sadik, Seedless synthesis and SERS characterization of multi-branched gold nanoflowers using water soluble polymers, Nanoscale 9 (2017) 8330–8340. [8] J. Ho, H. Chang, W. Su, DOPA-mediated reduction allows the facile synthesis of fluorescent gold nanoclusters for use as sensing probes for ferric ions, Anal. Chem. 84 (2012) 3246–3253. [9] C. Lu, M. Zhang, A. Li, X. He, X. Yin, 3, 4-Dihydroxy-L-phenylalanine for preparation of gold nanoparticles and as electron transfer promoter in H2O2 biosensor, Electroanalysis 23 (2011) 2421–2428. [10] P. Huang, W. Ma, P. Yu, Dopamine-directed in-situ and one-step synthesis of Au@Ag core-shell nanoparticles immobilized to a metal-organic framework for synergistic catalysis, Chem. Asian J. 11 (2016) 2705–2709. [11] R. Baron, M. Zayats, I. Willner, Dopamine-, L-DOPA-, adrenaline-, and noradrenaline-induced growth of Au nanoparticles: assays for the detection of neurotransmitters and of tyrosinase activity, Anal. Chem. 77 (2005) 1566– 1571. [12] Y. Lee, T. Park, Facile fabrication of branched gold nanoparticles by reductive hydroxyphenol derivatives, Langmuir 27 (2011) 2965–2971. [13] Y. Ni, G. Tong, J. Wang, H. Li, F. Chen, C. Yu, Y. Zhou, One-pot preparation of pomegranate-like polydopamine stabilized small gold nanoparticles with superior stability for recyclable nanocatalysts, RSC Adv. 6 (2016) 40698– 40705. [14] G. Su, C. Yang, J. Zhu, Fabrication of gold nanorods with tunable longitudinal surface plasmon resonance peaks by reductive dopamine, Langmuir 31 (2015) 817–823. [15] T. Sakai, H. Enomoto, H. Sakai, M. Abe, Hydrogen-assisted fabrication of spherical gold nanoparticles through sonochemical reduction of tetrachloride gold(III) ions in water, Ultrason. Sonochem. 21 (2014) 946–950. [16] F. Masing, A. Mardyukov, C. Doerenkamp, H. Eckert, U. Malkus, H. Nusse, J. Klingauf, A. Studer, Controlled light-mediated preparation of gold nanoparticles by a norrish type I reaction of photoactive polymers, Angew. Chem. Int. Ed. 54 (2015) 12612–12617. [17] H. Lee, S. Dellatore, W. Miller, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426–430. [18] H. Li, Y. Aulin, L. Frazer, E. Borguet, R. Kakodkar, J. Feser, Y. Chen, K. An, D. Dikin, F. Ren, Structure evolution and thermoelectric properties of carbonized polydopamine thin films, ACS Appl. Mater. Inter. 9 (2017) 6655–6660.

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