Cylindrical core-shell tween 80 micelle templated green synthesis of gold-silver hollow cubic nanostructures as efficient nanocatalysts

Materials and Design 160 (2018) 169–178

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Cylindrical core-shell tween 80 micelle templated green synthesis of gold-silver hollow cubic nanostructures as efficient nanocatalysts Dickson Joseph a,⁎, Hoomin Lee b, Yun Suk Huh b,⁎, Young-Kyu Han a,⁎ a b

Department of Energy and Materials Engineering, Dongguk University, Seoul, Republic of Korea WCSL of Integrated Human Airway-on-a-Chip, Department of Biological Engineering, Inha University, Incheon, Republic of Korea

H I G H L I G H T S

G R A P H I C A L

• A green, direct, one-pot, seedless route towards the synthesis of hollow cubic nanostructures has been successfully developed. • The hollow cubic nanostructures are made up of a bimetallic gold-silver alloy, with gold as the major component. • Tween 80, a non-ionic surfactant's coreshell cylindrical micellar structure guides the shape of the nanostructures. • The prepared nanostructures are biocompatible and act as efficient nanocatalysts.

Tween 80, a biocompatible non-ionic surfactant as a soft-template for the synthesis of novel gold-silver hollow cubic nanostructures as nanocatalysts, using a direct one-pot seedless method in aqueous medium.

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 18 June 2018 Received in revised form 30 August 2018 Accepted 3 September 2018 Available online 5 September 2018 Keywords: gold-silver nanostructures Hollow cubic Tween 80 Cylindrical micelle Nanocatalysts

A B S T R A C T

The non-ionic surfactant, Tween 80 (T80) a material with promising applications in the pharmaceutical, cosmetic, and food industries, has been explored here for the design of novel nanostructures (NS). We report here a direct one-pot seedless green method for the synthesis of gold-silver (AuAg) hollow cubic NS, with gold (90%) being the major component. The concentration of T80 plays an important role in determining the shape of the NS. A minimum T80 concentration of 50 mM is required for the formation of hollow cubic NS, whose sizes are controlled by increasing the T80 concentration. The unique molecular structure of T80 directs its molecules to assemble into core-shell cylindrical micelles that act as a soft-template for the growth of the NS. Study of the mechanism for the formation of the NS suggests nucleation followed by co-reduction of metal ions and its growth into hollow cubic NS over T80 micelles. The AuAg hollow cubic NS shows greater potential as efficient nanocatalysts than the non-cubic NS obtained at lower T80 concentrations. The hollowness and the cubic shape of the NS contribute towards their effective surface area that facilitates efficient catalytic activity. The AuAg NS may have biological applications due to their cell viabilities. © 2018 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

1. Introduction Abbreviations: T80, Tween 80; Au, Gold; Ag, Silver; AA, ascorbic acid; NS, Nanostructure; CTAB, Cetyltrimethylammonium bromide; 4-NP, 4-nitrophenol; FETEM, field emission transmission electron microscope. ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Joseph), [email protected] (Y.S. Huh), [email protected] (Y.-K. Han).

High hopes have been pinned on discoveries and innovation in nanotechnology to alleviate the present concerns regarding environment and pollution. Chemical processes involving green chemistry principles have gained an overwhelming importance in recent decades

https://doi.org/10.1016/j.matdes.2018.09.003 0264-1275/© 2018 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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[1–3]. Efforts are aimed at eliminating or at least minimizing the waste generated by embracing the 12 green chemistry principles for the execution of sustainable processes. To achieve these goals, emphasis has been placed on the design of green synthetic routes, using nontoxic and renewable materials [1]. For nanomaterials targeted at biomedical applications, emphasis is placed on the development of protocols involving green chemistry and excluding toxic chemicals from the synthesis procedures to avoid adverse effects during their applications [4,5]. The growing need to develop environmentally benign technologies in nanomaterial syntheses has led to the incorporation of biocompatible molecules with nanoparticles. Biocompatible nanomaterials have attracted considerable attention due to their potential applications in biomedical science, including diagnosis and therapy [6,7]. Among nanomaterials, noble metal nanoparticles especially gold is vital due to their low reactivity, biocompatibility, intense color, distinct surface plasmon resonance bands and broad range of applications [8,9]. In addition, morphologically controlled synthesis of gold nanoparticles (AuNPs) is also essential because the structure plays an important role in tuning their properties [10,11]. Currently, one of the challenges in biocompatible nanostructured material synthesis is developing approaches with biocompatible molecules as templates. Surfactants are often used as templates for the shape-controlled synthesis of nanomaterials [12–16]. Cetyltrimethylammonium bromide (CTAB), a cationic surfactant is most commonly used due to its ability to assist in the formation of a wide variety of NS [13,15]. However, CTAB has cytotoxic effects especially the CTAB molecules that are not involved in the capping of the NS but remain free in the dispersion medium [17–20]. Due to the toxicity induced by the ionic surfactants, there is a demand for the use of biocompatible non-ionic surfactants. In this regard, we were keen on exploring the biocompatible non-ionic tween (polysorbate) surfactants for the preparation of gold NS. Tween surfactants comprise fatty acid

esters of polyethylenglycolated sorbitan, with low critical micellar concentrations (CMC = 0.012 mM), and hydrophile−lipophile balance (HLB), values above 14, which are suitable for use in cosmetics, food industries, bioremediation and pharmaceutical formulations [21,22]. A series of Tween surfactants (20, 40, 60, 80 and etc.) are commercially available consisting of 20 ethylene oxide groups attached to the sorbitan headgroup, with different long-chain saturated carboxylic acids, from lauric to stearic, and unsaturated oleic acid. The molecular structure of Tween 80 (T80) is interesting due to the presence of an unsaturated bond in the 18‑carbon atoms alkyl chain. The names Tween and polysorbate are used interchangeably in the literature. T80 is a nonionic polymeric amphiphilic surfactant, with the hydrophilic polar head containing a sorbitan ring structure, in which the 4 hydroxyl (OH) groups are replaced by polyoxyethylene (CH2CH2O) polymer groups (Fig. 1A). The fourth polyoxyethylene polymer replacing the secondary hydroxyl group is further attached to an oleic acid moiety to form the hydrophobic non-polar tail of the structure; therefore, this structure is named polyoxyethylene (20) sorbitan monooleate. The total number of polyoxyethylene subunits present at the z, y, x, and w positions of each polysorbate is 20 (w + x + y + z = 20). The molecular formula is expressed as C24H44O6(CH2CH2O)n, n = 20, with a molecular weight of 1310 g/mol. [23] Although T80 is known for its dual functionality as a reductant and a stabilizer for the preparation of spherical AuNPs, [24,25] its potential as a template for the fabrication of NS is still unexplored except for a couple studies, [14,26] despite its biocompatibility and unique structure. Hence, herein we study the role of surfactant T80 for the synthesis of gold NS. Generally, surfactant templated synthesis of gold NS is achieved by the seeded growth method, involving a two-step reduction process where initially small spherical nanoparticles are prepared as seeds and added to a growth solution. [13] The growth solution usually contains ascorbic acid (AA) as a weak reductant, silver ions (Ag+ ) to

Fig. 1. (A) Molecular structure, (B) MM2 energy minimized model of T80 drawn using ChemBio3D (w, x, y and z chains of T80 are not shown in the molecular model for clarity), and (C) Postulated molecular structure of two T80 molecule heads arranged in the upside down fashion connected through pi-pi interaction.

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facilitate symmetry breaking [27], surfactants as soft templates and additional gold ions for the growth of the NS. [28] However, our goal is to develop a green seedless one-pot chemical process for the reduction of Au3+ to a novel Au0 NS using T80, Ag+ and AA. Herein, we demonstrate the preparation of bimetallic AuAg hollow cubic NS by optimizing the reactant concentrations and report our understanding of the mechanism involved in the formation of the NS. Similarly, reductions were performed in the presence of surfactants Tween 20 and 40 as a comparative study for a clear understanding of the mechanistic route. In contrast to the previously reported studies on the use of T80 for the synthesis of “solid gold nanostructures” [14,26], we report here for the first time the use of T80 for the preparation of “bimetallic AuAg hollow nanocubes”. The galvanic replacement reaction between Ag templates and aqueous HAuCl4 is the most commonly used route towards producing controllable AuAg alloy hollow nanostructures, such as nanocages, nanotubes, and nanoframes [29]. However, this approach is limited by its prerequisite of the preformed silver template, organic solvents and high temperatures. Hence using our newly developed method for the synthesis of AuAg hollow nanocubes we have overcome the issues related to the galvanic replacement reaction protocol. We also show the potential of the hollow cubic NS as a nanocatalyst by testing its catalytic activity through the reduction of 4-nitrophenol to 4-aminophenol as a model reduction reaction. In the last decade plasmon-enhanced catalysis has gained considerable attention as it holds promises in the field of nanocatalysis towards the manufacture of chemicals, pharmaceuticals, and clean energy generation. Exploiting the surface plasmon resonance effects of the plasmonic NS holds the key to their enhanced catalytic performances. Hence, scalable and cost-effective design of controlled plasmonic NS with uniform size, shape and distribution of active sites is in demand. In this regard, engineering of hollow metallic NS by the galvanic replacement reaction for catalytic applications has gained attention [29]. However, our goal is to replace the two-step galvanic replacement reaction protocol by a direct one-pot green method. The cell viabilities of the NS were also tested as a potential for use in biological applications. 2. Experimental section 2.1. Materials and methods All reagents were of analytical reagent grade. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4.3H2O), Tween 80 (T80), silver nitrate (AgNO3), ascorbic acid (AA), 4-nitrophenol(4-NP) and sodium borohydride (NaBH4) were purchased from Sigma-Aldrich. All chemicals were used as received. Doubly distilled deionized (DI) water (H2O) was used throughout this investigation. All glassware was precleaned with aqua regia (a mixture of HCl:HNO3 in a 3:1 volume ratio) and rinsed thoroughly with DI H2O. The morphology of the NS was observed by JEM2100F (JEOL, Japan) field emission transmission electron microscope (FETEM) operated at a voltage of 200 kV, equipped with an energydispersive X-ray spectrometer (EDS, Oxford Instruments, Oxford, UK) for elemental analysis. For FETEM imaging the synthesized colloids were concentrated by centrifuging at 8000 rpm for 10 min, collecting the residue, and dispersing it in 1 mL H2O. The concentrated solution was dropped onto a carbon coated copper grid and dried in vacuum for 24 h before imaging. UV–Visible spectra for the as prepared colloidal solutions were recorded as such on an UV–Visible spectrophotometer (V-770 Jasco, Japan). Particle size by dynamic light scattering method was also measured with the as prepared colloidal solutions using Zeta-potential and Particle size Analyzer ELSZ-2000 series, Otsuka Electronics Co., Ltd., Japan. 2.2. Synthesis of the gold-silver hollow NS In a typical synthesis procedure, appropriate volumes of T80 was dissolved in DI H2O (8.45 mL) to prepare 25, 50, 75 and 100 mM of

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T80 and allowed to stir in a 70 mL glass vial until complete dissolution. The stirring was then stopped and, HAuCl4.3H2O (0.5 mL, 0.01 M) was added to the T80 solution and mixed by manual agitation. AgNO3 (0.05 mL, 0.02 M) was also added and mixed again by manual agitation for about 1 min. Finally, AA (1 mL, 0.01 M) was added and mixed until blue coloration was observed, the mixture was then left to stand under ambient conditions for 12 h. For larger volumes T80, DI H2O, metal precursors and AA were increased proportionally. 2.3. Catalytic reduction of 4-Nitrophenol using the NS To an aqueous solution of 4-NP (30 μL, 0.01 M) in a 3 mL disposable UV cuvette containing 2.72 mL of H2O, NaBH4 (150 μL, 0.1 M) and AuAg NS (100 μL) were sequentially added and mixed. Time resolved UV–Vis absorbance spectra were recorded using UV–Vis spectrometer in 1 min time intervals with 200–800 nm wavelength scanning range under ambient conditions. Prior to every experiment, solutions of 4-NP and NaBH4 were freshly prepared. 2.4. Cell culture and viability assay The monkey kidney fibroblast-like cell line (COS-7, ATCC, USA) was maintained in Dulbecco's Modified Eagles' Media (DMEM; 11965-092, Gibco, USA) supplemented with 10% fetal bovine serum (FBS; 16000044, Gibco, USA) and 1% penicillin–streptomycin (15070063, Gibco, USA). The cells were incubated at 37 °C in 5% CO2. The cell viability after exposure to the different samples was determined using a Cell Counting Kit-8 (CCK-8) assay (CK04-11, Dojindo, Japan). COS-7 cells were seeded in 12-well cell plates at 1.0 × 10^4 cells per well and incubated for 24 h at 37 °C and 5% CO2. After 24 h of cell attachment, the cells were incubated with different concentrations of AuAg NS (10, 50, and 100 μL/mL), 50 mM CTAB and 50 mM T80 (0.01, 0.1, 1, 10, and 100 μL/ mL) for 24 h. The AuAg NS samples were centrifuged at 8000 rpm for 10 min to remove the excess T80 and redispersed in DI H2O before treatment with the cells, whereas the neat T80 and CTAB were used as such. The samples were removed and the cells were washed three times with PBS. A 10 mL sample of CCK-8 dye and 100 mL of DMEM cell culture media were added to each well and incubated for 2 h at 37 °C. The absorbance of the supernatant was collected at 420 nm using a SpectraMax Plus 384 microplate reader. The percentage relative cell viability was calculated by normalizing the treated cells to that of the untreated control sample. The medium and CCK-8 reagent without the cells served as a method blank for all samples. For cell morphology studies after incubation for 24 h, the medium was removed, and cells were washed with PBS buffer three times. The cells were fixed with 4% paraformaldehyde in PBS (w/v) for 10 min, then incubated with 4′,6‑diamidino‑2‑phenylindole (DAPI) (H-1200, Vector Labs, USA). The cells were then covered with coverslips and photographed using an inverted fluorescence microscope. All experiments were carried out in triplicates. 3. Results and discussion Metal ions such as Ag+ and Au3+ catalyze the oxidation process of T80 along the polyoxyethylene chains and are reduced to elemental metals. [24,30] Taking advantage of T80’s unique structure, biocompatibility, and solubility in water with high hydrophilic–lipophilic balance, [23] we describe here a direct seedless one-pot green synthesis of hollow cubic NS. Hollow AuAg cubic NS were obtained merely by adding and manually mixing HAuCl4, AgNO3 and ascorbic acid in an aqueous solution of T80. To obtain distinct Au NS using T80 as a soft template, silver ions were used to break the symmetry [27] of the initially formed spherical gold nanoparticles and aid in the growth of the NS. Based on the seed-mediated approach, ascorbic acid (AA) has been used to regulate the reduction rate and enable fine control over nanocrystal morphology. [31–33] Hence, our reaction system contained four species in

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water: the gold precursor (HAuCl4.3H2O), the silver precursor (AgNO3), the reducing agent (ascorbic acid) and the template cum stabilizer (T80). Preliminary experiments were performed by choosing three different concentrations of T80 (25, 50, and 100 mM), while fixing the concentrations of the metal precursors and the reductant. Fig. 2A shows the UV spectra of the corresponding NS obtained using 25, 50, 75, and 100 mM concentrations of T80 with 5 mM of AA. The NS formed in the presence of 25 mM T80 (Fig. 2D) were irregularly shaped, with a broad plasmon band above 500 nm and showed poor stability over a period. On the other hand, the 100 mM led to the formation of almost spherically shaped NS (size = 46 ± 5 nm) (Fig. 2F), with a narrow plasmon band and good stability. In the case of 50 mM T80, rugged/ branched cubic NS (size = 70 ± 11 nm) (Fig. 2E) were obtained with an absorption spectrum lying between the previous two and exhibiting excellent stability. As observed in the UV–Vis absorption spectra (Fig. 2A), increasing the concentration of T80 causes the plasmon band to become narrower with blue shifts indicating more uniformly sized and shaped NS [34]. TEM images (Fig. 2) confirm the change in the shapes of the NS with increasing T80 concentration, implying that the micellar structure of T80 at different concentrations in an aqueous solution, containing metal salts and AA, plays an important role in shaping the NS. The UV–Vis absorptions and the TEM images clearly shows an increase in the order, from randomly shaped NS to cubic and then to a more uniform and stable spherical NS as the concentration of T80 increases. Thus, based on this observation, it can be hypothesized that under the same experimental conditions, changes in the T80 concentration leads to a change in the micellar structure, thereby forming a different NS. The TEM images (Fig. 2D, E, and F) show NS with lighter center that appeared hollow and a dark thick solid edge. The Au:Ag ratio plays an important role in controlling the shape or the hollowness of the NS. Solid Au nanoparticles are formed in the absence of Ag ions, whereas at 5:1 and 1:1 ratios well-defined hollow NS and some random broken non-uniform structures were formed, respectively (Fig. S1). Hence, in

order to obtain well-defined hollow NS it is crucial to maintain the Au:Ag ratio at 5:1. Although we could obtain a distinct hollow cubic NS using a one-pot seedless method, the NS (Fig. 2E) was non-uniform in size and shape, which could be due to the high reduction rate. Hence, to study the effect of the reduction rate, i.e. the concentration of AA in shaping the NS, experiments were performed with low concentrations (1 and 0.5 mM) of AA and were characterized by UV spectroscopic studies (Fig. 2B and C). As the concentration of AA decreases, the absorbance weakens, whereby low absorbance at 0.5 mM indicates incomplete reduction reaction with some unreacted metal ions. At 1 mM concentration the plasmon bands lie in-between that of the 5 and 0.5 mM, but narrower than those for 5 mM AA, indicating better uniformity in size and shape [34]. TEM images in Fig. 3 and Fig. S2 confirm the uniformity in size and shape of the NS formed for T80 concentrations above 50 mM in the presence of 1 mM of AA. An increase in T80 concentration from 25 mM to 50 mM led to the formation of well-defined cubic NS, while further increase in the concentration resulted in decreases in the size; however no shape changes were observed. The sizes of the NS formed using 25, 50, 75, and 100 mM T80 are 51 ± 14 nm, 85 ± 7 nm, 64 ± 9 nm and 58 ± 6 nm respectively. Although there is a significant difference in the NS formed between 25 mM and 50 mM of T80, there are no changes above 50 mM T80. Hence, at T80 concentration of 50 mM, the micellar structure is not altered significantly except in terms of size for lower AA concentrations. However, for higher AA concentrations, the micellar structure changes significantly leading to different structures. Thus, we believe that differently shaped micellar structures are formed at different T80 concentrations that act as soft templates in shaping the NS. The rate of reduction also influences the shape of the NS, which is clearly observed in the NS formed using 100 mM of T80. Reactions with 5 mM of AA produces hollow NS that look almost spherical (Fig. 2F) whereas in 1 mM of AA, cubic NS (Fig. 3C) are observed. Thus, at higher reduction rates, the gold ions are reduced rapidly to

Fig. 2. UV–Vis absorption spectra of NS formed at different concentrations of T80 by varying AA concentrations (A) 5 mM, (B) 1 mM, (C) 0.5 mM and TEM images of nanostructures formed using 5 mM of AA in (D) 25 mM, (E) 50 mM and (F) 100 mM of T80. Scale bar = 100 nm.

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Fig. 3. TEM images of NS formed using 1 mM concentration of ascorbic acid in (A) 25 mM, (B) 50 mM, (C) 75 mM, and (D) 100 mM concentrations of T80. Scale 100 nm.

the most stable spherical structure, while at lower reduction rates, the reduced nanoparticles have sufficient time to assemble into the cubic NS. To understand the micellar structure of T80 at different concentrations, their hydrodynamic sizes were studied using DLS and compared with that of the NS formed using T80 (Table S1). According to the intensity size distribution, the sizes of neat T80 in water at different concentrations are around 10 nm. The hydrodynamic sizes of the NS formed using the corresponding concentrations vary, for 25 to 50 mM an increase in size is observed, while for 75 and 100 mM the sizes drop below that of 50 mM. The same trend is observed in sizes extracted from the TEM images using imageJ software; however, the hydrodynamic sizes were larger as expected due to the apparent size of the dynamic hydrated/solvated particle. We postulate that the formation mechanism of the cubic NS can be described as follows. T80 contains a double bond in the 18‑carbon atoms alkyl chain tail. When two molecules come in contact with one another, the π-π interactions arrange the molecules in an upside down fashion (Fig. 1). These molecules then form micelles depending on the concentrations, which act as soft templates for the growth of the NS over it. The formation of differently shaped micellar structures at different concentrations can be correlated with the shape of the NS formed at T80 concentrations of 25 mM and 50 mM. Fig. 3A shows that random non-uniform NS were formed at 25 mM of T80, while cubic NS were observed for T80 concentrations above 50 mM (Fig. 3B, C and D). Thus, the micellar structure of T80 determines the shape of different NS synthesized in our system. T80 is known to form cylindrical core-shell micelles at concentrations of 0.5 g/mL in sodium phosphate buffer solution [35]. We believe that similar cylindrical core-shell like micelles are formed in our system at T80 concentrations above 50 mM, which act as a soft template over which the NS grows. A schematic representation of our postulation on the

micelle formation and the NS growth is shown in Fig. 4. At T80 concentrations less than 50 mM, there are not enough T80 molecules to arrange themselves into a cylindrical micelle. Instead, different randomly shaped micellar structures are formed. At T80 concentrations equal to and above 50 mM, we believe that sufficient molecules of T80 are present in the system to form the reported core-shell type of micelles. These micelles are then believed to act as templates for NS growth. For 25 mM of T80, a structure similar to that of its micelle is formed with no defined shape, whereas at concentrations equal to and above 50 mM a cubic NS grows adopting the cylindrical shape. A schematic representation of systems involving 5 and 10 molecules of T80, for concentrations of 25 and 50 mM respectively, are shown in Fig. S3 to explain the step by step formation of the micelle and the growth of NS over it. The electron diffraction (ED) pattern and HRTEM images were recorded to study the crystal structure and surface morphology of the cubic NS, respectively. The diffraction spots in ED pattern are superimposed on the rings, indicative of the poly crystalline cubic structure (Fig. 5A). The HRTEM images show that the edges or borders of the cubic structure are thicker, while the inner portions are thinner (Fig. 5B, C, and D). STEM-EDX mappings were obtained to estimate the distribution of Ag and Au elements in the cube at the edges and the center (Fig. 5C and D). The EDX results (Fig. 5E and F) shows that the weight percentages of Au in both the edge and center are above 90% (92:08 and 90:10 respectively), suggesting that the major portion of the NS is made up of Au. There is a minor difference in the atomic ratio of Au: Ag at the edges and centers (86:14 and 84:16 respectively), which can be attributed to the difference in the electron beam transmittance in the TEM images. The above elemental mapping done using point patterns suggests the elemental ratio between Au and Ag is 6:1, which is

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Fig. 4. Schematic representation of the proposed micelles formation from T80 molecules at their corresponding concentrations and the NS that are expected to grow over the micelles as a soft-template.

close to the feed ratio, 5:1. The ratio obtained using inductively coupled plasma optical emission spectrometry (ICP-OES), is 8:1. Thus the oxidation state of Au and Ag on the cubic NS exists in the form of elemental Au and Ag rather than the ions confirming the complete reduction of metal ions to metal with a ratio close to the feed ratio. Since the NSs are prepared in the aqueous colloidal form and extraction of it as a solid resulted in a gel formation due to the characteristic behavior of the surfactant, T80 (which in its native state exists in the liquid form at ambient conditions). It was not possible for us to examine the metallic oxidation states through X-ray photoelectron spectroscopy studies. Hence, elemental analysis was done using the EDX and ICP-OES studies. Hence, in this study, by fixing a ratio of 5:1 for Au:Ag and maintaining the concentrations of AA at 1 mM, hollow cubic NS were obtained for T80 concentrations above 50 mM. Hollow metallic NS are generally prepared by replacement reactions involving sacrificial templates like

silver, [36] cobalt, [37] or silica [38]. Here we use a direct one-pot seedless synthetic protocol, merely by manually mixing the precursors and allowing it to assemble into a hollow cubic NS on T80 micelles acting as soft-templates. Recently there have been some reports on the onepot synthetic protocols. He et al. prepared Au-Ag alloy nanoboxes similar to our cubic NS, [39] in the absence of a surfactant or a template; however at a later stage, surfactant was added to stabilize the NS. The surfactant (CTAB) does not play any role in shaping the NS; rather, it is the Au:Ag molar ratio that determines the cubic shape. An optimal 1:1 ratio of Au:Ag, produced uniform nanoboxes, below and above which incomplete and broken nanoboxes were formed, respectively. Although we also observed a similar behavior when we varied the Au:Ag ratio (Fig. S1), the dominant factor in obtaining the cubic NS was the T80 concentration. The primary difference here is the mechanism of the reduction reaction and the formation of the NS. He et al. obtained

Fig. 5. (A) ED pattern, (B) TEM images of a single AuAg Hollow Nanocube, (C and D) HRTEM images and (E and F) TEM-EDS patterns of the edge and the center of a single AuAg Hollow Nanocube respectively.

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nanoboxes with 1.9:1 atomic ratios, using Au:Ag precursor molar ratios of 1:1. They postulated that the mechanism involves two growth processes, one involving reduction of Au ions by AA to form the initial nuclei and the second involving the co-reduction of Au and Ag. At higher Ag concentrations, the Ag grows faster than Au leading to formation of alloys with high Ag contents that drives the Au3+ reduction through displacement reaction to form nanoboxes. In contrast, in our study, we used Au:Ag molar ratio of 5:1 to obtain a NS with atomic ratio of 6:1. Hence, we believe that in our study, the mechanism for the formation of cubic NS does not involve the displacement reaction. Instead, since T80 can reduce metal ions, [24,30] we believe that initially Au and Ag ions are co-reduced by T80 along the polyoxyethylene chains to form Au and Ag nuclei. The Ag nuclei are then involved in breaking the symmetry of the Au nuclei allowing the interaction of remaining Au and Ag ions with the Au and Ag nuclei. It is followed by further co-reduction of Au and Ag ions at the nuclei that grows into the NS. The high Ag content in the center compared to the edges suggests equal distribution of metal ions and that the Ag ions are co-reduced along with Au. However, as the NS grows into well-formed cubes the amount of Au is dominated along the polyoxyethylene chains that are present at the edges of the cylindrical micelle (Fig. S4), resulting in edges with higher Au distribution. The growth and assembly of the AuAg NS takes place inwards towards the center of the cylindrical micellar structure (Fig. S4), thereby forming a cubic NS. A time-resolved UV–Vis spectroscopic study on cubic NS was done to understand the formation mechanism. The reactants were added as mentioned in the experimental section in a vial and after the appearance of the blue coloration, 3 mL of the sample was withdrawn and placed in a UV cuvette. The absorbance was measured without disturbing the sample in 1-hour cycles until maximum intensity was achieved. Maximum absorbance was observed within 12 h, suggesting the completion of the reduction process. Soon after the addition of ascorbic acid, the reaction was initiated in 30 s, leading to the broad absorbance spectrum, which then increases with time over a period of 12 h. Two distinct plasmon bands are observed (Fig. S5): a shoulder at 525 nm and a strong band at 888 nm, which are plasmon resonance bands characteristic of Au NS. During the reaction there is a significant change in the intensity of plasmon band at 888 nm which became narrower with time, whereas there was no significant change in the peak at 525 nm. The above results confirm that the initial nucleation is followed by the growth and assembly of a NS over a 12-hour period which contains Au as the major component. A time-dependent TEM analysis was also carried by the imaging the NS formed at different intervals of time, to visualizing understand the mechanism. A clear observation in the evolution of the hollow cubic NS is depicted in the TEM images (Fig. S6) that shows the formation of the NS from 10 min to 15 h. 10 min after the initiation of the reaction large number of randomly shaped NS are observed with a few cubic NS. Until 6 h into the reaction a similar behavior is observed as the concentration of randomly shaped NS starts to increase along with a few cubic NS. As time progresses the randomly shaped NS organize into cubic structure that finally results in the formation of a nanostructure that is cubic in shape with a hollow core. After the 9th h (Fig. S6 D) a complete formation of the cubic NS is observed with a few randomly shaped NS, the reaction then comes to a completion in 15 h where monodisperse hollow cubic NS are formed as the final product. The TEM images correlates the UV–Vis spectroscopic studies on cubic NS examined at different intervals for time for understanding the formation mechanism. In conclusion, the formation mechanism for the cubic NS can be explained as follows. During the addition of the metal precursors, the metal ions are deposited on polyoxyethylene moieties of the cylindrical T80 micelles. The Au and Ag ions are initially reduced by T80 to Au nuclei arranged over the edges of the cylindrical micelle (Fig. S4). The Ag nuclei present in the system then interacts with the Au nuclei breaking its symmetry and allowing the growth of non-spherical NS by further reduction of Au and Ag ions onto their corresponding nuclei. The cubic

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NS grows in the inwards direction to form a complete cube with a hollow core. Once the hydrophobic NS are formed, the tails of the T80 are attracted by hydrophobic interactions to the surface of the NS to stabilize it. To further confirm the T80’s role in the shape control, comparative experiments were performed using two other tween surfactants, T20 and T40. The major difference among these surfactants is the molecular structure of their tails. T20, T40, and T80 are composed of tails containing oleate, palmitate and laurate, respectively (Fig. S7). The hydrodynamic size distributions obtained for neat 50 mM of T20 and T40 by DLS are 12 nm and 28 nm, respectively (Table S1). Using the same experimental conditions that formed cubic NS, experiments were performed by replacing T80 with 50 mM of T20 and T40. As expected the two tweens do not form the same cubic NS that is formed with T80; instead, aggregates of smaller nanoparticles were observed (Fig. S8). Small individual nanoparticles and a few aggregates were observed with T20, whereas with T40, larger aggregates were observed. This confirms that the difference in the micellar structure formed in the same system with different tweens plays an important role in directing the growth of the NS. Due to the absence of the double bond in the tails of T20 and T40, cylindrical core-shell micelles are not formed unlike T80, which does not favor the growth of cubic NS. We have simplified the two-step galvanic replacement synthesis protocol to a one-pot co-reduction synthesis to achieve hollow AuAg cubic NS. We therefore believe that our hollow NS has benefits over the galvanically produced NS as efficient nanocatalysts. We choose the reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) in the presence of a catalyst as a model catalytic reaction to investigate the catalytic properties of the as-prepared cubic NS. The reduction process was monitored using UV–Vis spectroscopy and visual examination. A typical 4-NP absorption band appears at 317 nm, which red shifts to 400 nm upon addition of NaBH4 due to the formation of 4nitrophenoloate ions (Fig. 6). The catalytic activity of the NS was estimated using 4 different samples as shown in TEM images (Fig. 3) based on T80 concentrations (25, 50, 75, and 100 mM) and the size of the NS. The concentrations of the reactants for the experiment were optimized to 30 μL of 0.01 M 4-NP, 150 μL of 0.1 M NaBH4 and 100 μL of Au NS as catalyst. It is worth mentioning here that the concentration of the Au ions used for the synthesis was 0.5 mM, resulting in a final Au concentration of ~0.017 mM in the 3-mL catalytic reaction vessel. On addition of the NS a sequential attenuation in the peak intensity at 400 nm is observed with increasing reaction time, with a gradual appearance of a new peak at 300 nm and increase in intensity indicating the formation of 4-aminophenol (4-AP) [40]. Visually, the light-yellow color of the 4NP solution changes rapidly to bright yellow on addition of NaBH4 aqueous solution indicating the formation of 4-nitrophenolate ions. Subsequently, on addition of the catalyst due to the successive reduction of 4-nitrophenolate ions to 4-aminophenol, the color changes from bright yellow to light yellow to colorless. The rates of the catalytic reduction reactions varied for the four NS (Fig. 6). The non-cubic randomly shaped rugged NS obtained using 25 mM of T80 has the least catalytic activity, taking 90 min for the complete reduction of 4-NP to 4-AP. Among the cubic NS the catalytic activity increases with decrease in the size of the nanoparticles, which could be attributed to their high effective surface areas. [41] The reduction times for the hollow cubic NS with sizes 85 ± 7 nm, 64 ± 9 nm and 58 ± 6 nm were 12, 10, and 8 min, respectively (Fig. 6). Thus, the effective surface area affects the catalytic activity based on the shape and size of the NS. Hollow cubic NS prepared using 100 mM of T80 (58 ± 6 nm) reduces 4-NP to 4-AP in 8 mins, whereas for nanoparticles/NS prepared with 25 mM of T80 (51 ± 14 nm) the complete reduction takes place in 90 mins. Despite the smaller size of solid rugged nanoparticles, the reduction rate here is slower than that of hollow cubic NS signifying the importance of the shape and hollowness of NS. Hence, we have shown here that our asprepared hollow cubic NS can act as efficient nanocatalysts due to their effective surface area contributed by the cubic shape and

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Fig. 6. UV–Vis absorption spectra recorded during the reduction reaction of 4-nitrophenol with NaBH4 in the presence of 100 μL of NS prepared using (A)25 mM, (B) 50 mM, (C) 75 mM and (D) 100 mM of T80.

hollowness. We believe a nanocatalysis molecular reduction mechanism takes places in the same manner as that explained by Mahmoud, et al. [42] Initially NaBH4 reacts with water on the surface of the nanocatalyst to produce hydrogen and an oxidized form of borohydride. When 4-NP molecule approaches the surface of the nanocatalyst, it is

reduced by the adsorbed oxidized NaBH4 species. Due to the difference in the effective surface areas of the NS the rate of the reduction varies depending on the available effective surface area. The AuAg NS's potential for application in biology was estimated by studying their in-vitro cytotoxicity using monkey kidney fibroblast-like

Fig. 7. Cell viability studies by using CCK-8 assay on cos-7 cells treated with (A) 50 mM surfactants and (B) AuAg NS prepared with different concentrations of T80. Error bars indicate the deviation between the triplicate experiments.

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cell line. The cell viabilities of neat 50 mM of T80 at different feed concentrations were assessed by comparing with 50 mM of CTAB (Fig. 7A). In addition, the cell viabilities of the nanostructures prepared using different concentrations of T80 were also studied (Fig. 7B). Compared to the neat CTAB, T80 showed better cell viabilities indicating lower toxicities towards cos-7 cells. The NS showed a minimum of 75% viability towards cells at all concentrations with no major differences, irrespective of the shape and size as the initial Au atom concentration in all the NS were the same. The cell morphology studies on neat T80 and CTAB (Fig. S9), shows that T80 has comparatively higher cell viability than CTAB with less cell damage or change in morphology. The above results signify better biocompatibility of the surfactant T80 with the cells compared to CTAB. The AuAg NS showed significant cell viability thereby making it a potential biocompatible nanomaterial for biological applications.

4. Conclusions Tween 80 has been shown to effectively influence the preparation of hollow cubic gold-silver NS using a one-pot seedless method, synthesized merely by the addition and manual mixing of the precursors. The effect of the concentration on the shape suggests the role of micelles in guiding the shape of the NS. A core-shell cylindrical micellar structure here is believed to act as a soft-template and is responsible for the formation of the cubic NS. The tail of T80 which contains a double bond plays a unique role in forming micelles and thereby shaping the NS. The elemental content of gold and silver (Au:Ag, 5:1) in the NS are the same as the ratio of the corresponding metal precursors used for the synthesis, ruling out the possibility of a galvanic replacement mechanism. The mechanism of the NS formation involves initial metal nucleation by T80 followed by the co-reduction of the remaining metal ions by AA at the nuclei and growth along the micellar template. Therefore, in this study we report a direct one-pot seedless green synthetic route towards the preparation of a hollow cubic NS, which does not involve the complex procedures used in the galvanic replacement method reported earlier. The prepared hollow cubic NS are known to act as efficient nanocatalysts for the reduction of 4-nitrophenol due to their effective surface area involved in the reaction. The cell viability studies confirm the possibility of the as prepared AuAg NS's potential use in biomedical applications.

Author contributions D. Joseph, Y.S. Huh, and Y.-K. Han planned and conducted all experiments. D. Joseph, H. Lee, Y.S. Huh, and Y.-K. Han examined the experimental results, performed data analysis and wrote the article draft together. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2016R1A2B4013374 & 2014R1A5A1009799) and Inha University WCSL research grant. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matdes.2018.09.003.

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References [1] H.C. Erythropel, J.B. Zimmerman, T.M. de Winter, L. Petitjean, F. Melnikov, C.H. Lam, A.W. Lounsbury, K.E. Mellor, N.Z. Janković, Q. Tu, L.N. Pincus, M.M. Falinski, W. Shi, P. Coish, D.L. Plata, P.T. Anastas, The green ChemisTREE: 20 years after taking root with the 12 principles, Green Chem. 20 (2018) 1929. [2] P.T. Anastas, E.S. Beach, S. Kundu, Handbook of green chemistry, Tools for Green Chemistry, 10, , 2017. [3] F.G. Calvo-Flores, M.J. Monteagudo-Arrebola, J.A. Dobado, J. Isac-García, Green and bio-based solvents, Top. Curr. Chem. 376 (2018) 18. [4] P.L. Lam, W.-Y. Wong, Z. Bian, C.-H. Chui, R. Gambari, Recent advances in green nanoparticulate systems for drug delivery: efficient delivery and safety concern, Nanomedicine 12 (2017) 357. [5] J.A. Dahl, B.L.S. Maddux, J.E. Hutchison, Toward greener nanosynthesis, Chem. Rev. 107 (2007) 2228–2269. [6] M. Adabi, M. Naghibzadeh, M. Adabi, M.A. Zarrinfard, S.S. Esnaashari, A.M. Seifalian, R. Faridi-Majidi, H. Tanimowo Aiyelabegan, H. Ghanbari, Biocompatibility and nanostructured materials: applications in nanomedicine, Artif. Cells Blood Substit. Immobil. Biotechnol. 45 (2017) 833–842. [7] N. Cuu Khoa, T. Ngoc Quyen, N. Thi Phuong, N. Dai Hai, Biocompatible nanomaterials based on dendrimers, hydrogels and hydrogel nanocomposites for use in biomedicine, Adv. Nat. Sci. Nanosci. Nanotechnol. 8 (2017), 015001. . [8] P.K. Jain, K.S. Lee, I.H. El-Sayed, M.A. El-Sayed, Calculated absorption and scattering properties of gold nanoparticles of different size, shape, and composition: applications in biological imaging and biomedicine, J. Phys. Chem. B 110 (2006) 7238–7248. [9] 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, J. Phys. Chem. B 107 (2003) 668–677. [10] X. Hong, C. Tan, J. Chen, Z. Xu, H. Zhang, Synthesis, properties and applications of one- and two-dimensional gold nanostructures, Nano Res. 8 (2015) 40–55. [11] J. Gu, Y.-W. Zhang, F. Tao, Shape control of bimetallic nanocatalysts through well-designed colloidal chemistry approaches, Chem. Soc. Rev. 41 (2012) 8050–8065. [12] K. Holmberg, Surfactant-templated nanomaterials synthesis, J. Colloid Interface Sci. 274 (2004) 355–364. [13] 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, J. Phys. Chem. B 109 (2005) 13857–13870. [14] D. Joseph, K.E. Geckeler, Surfactant-directed multiple anisotropic gold nanostructures: synthesis and surface-enhanced Raman scattering, Langmuir 25 (2009) 13224–13231. [15] J. Xiao, L. Qi, Surfactant-assisted, shape-controlled synthesis of gold nanocrystals, Nanoscale 3 (2011) 1383–1396. [16] Z. Wu, S. Yang, W. Wu, Shape control of inorganic nanoparticles from solution, Nanoscale 8 (2016) 1237–1259. [17] A.M. Alkilany, P.K. Nagaria, C.R. Hexel, T.J. Shaw, C.J. Murphy, M.D. Wyatt, Cellular uptake and cytotoxicity of gold nanorods: molecular origin of cytotoxicity and surface effects, Small 5 (2009) 701–708. [18] T. Niidome, M. Yamagata, Y. Okamoto, Y. Akiyama, H. Takahashi, T. Kawano, Y. Katayama, Y. Niidome, PEG-modified gold nanorods with a stealth character for in vivo applications, J. Control. Release 114 (2006) 343–347. [19] H. Takahashi, Y. Niidome, T. Niidome, K. Kaneko, H. Kawasaki, S. Yamada, Modification of gold nanorods using phosphatidylcholine to reduce cytotoxicity, Langmuir 22 (2006) 2–5. [20] C.M. Goodman, C.D. McCusker, T. Yilmaz, V.M. Rotello, Toxicity of gold nanoparticles functionalized with cationic and anionic side chains, Bioconjug. Chem. 15 (2004) 897–900. [21] J. Penfold, R.K. Thomas, P.X. Li, I. Tucker, J. Petkov, R.E. Petkova, Nature of the intermicellar interactions in ethoxylated polysorbate surfactants with high degrees of ethoxylation, Langmuir 32 (2016) 1319–1326. [22] M. Cheng, G. Zeng, D. Huang, C. Yang, C. Lai, C. Zhang, Y. Liu, Tween 80 surfactantenhanced bioremediation: toward a solution to the soil contamination by hydrophobic organic compounds, Crit. Rev. Biotechnol. 38 (2018) 17–30. [23] M. Graca, J.H.H. Bongaerts, J.R. Stokes, S. Granick, Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces, J. Colloid Interface Sci. 315 (2007) 662–670. [24] T. Premkumar, D. Kim, K. Lee, K.E. Geckeler, Polysorbate 80 as a tool: synthesis of gold nanoparticles, Macromol. Rapid Commun. 28 (2007) 888–893. [25] Y. Tang, W. Zhang, J. Liu, L. Zhang, W. Huang, F. Huo, D. Tian, A plasmonic nanosensor for lipase activity based on enzyme-controlled gold nanoparticles growth in situ, Nanoscale 7 (2015) 6039–6044. [26] M.R. Hormozi-Nezhad, P. Karami, H. Robatjazi, A simple shape-controlled synthesis of gold nanoparticles using nonionic surfactants, RSC Adv. 3 (2013) 7726–7732. [27] M.J. Walsh, S.J. Barrow, W. Tong, A.M. Funston, J. Etheridge, Symmetry breaking and silver in gold nanorod growth, ACS Nano 9 (2015) 715–724. [28] M.S. Bakshi, How surfactants control crystal growth of nanomaterials, Cryst. Growth Des. 16 (2016) 1104–1133. [29] A.G.M. da Silva, T.S. Rodrigues, S.J. Haigh, P.H.C. Camargo, Galvanic replacement reaction: recent developments for engineering metal nanostructures towards catalytic applications, Chem. Commun. 53 (2017) 7135–7148. [30] L.M. Liz-Marzán, I. Lado-Touriño, Reduction and stabilization of silver nanoparticles in ethanol by nonionic surfactants, Langmuir 12 (1996) 3585–3589. [31] Y. Yu, Q. Zhang, X. Lu, J.Y. Lee, Seed-mediated synthesis of monodisperse concave trisoctahedral gold nanocrystals with controllable sizes, J. Phys. Chem. C 114 (2010) 11119–11126.

178

D. Joseph et al. / Materials and Design 160 (2018) 169–178

[32] H.-L. Wu, C.-H. Kuo, M.H. Huang, Seed-mediated synthesis of gold nanocrystals with systematic shape evolution from cubic to trisoctahedral and rhombic dodecahedral structures, Langmuir 26 (2010) 12307–12313. [33] M. Eguchi, D. Mitsui, H.-L. Wu, R. Sato, T. Teranishi, Simple reductant concentrationdependent shape control of polyhedral gold nanoparticles and their plasmonic properties, Langmuir 28 (2012) 9021–9026. [34] C.J. Orendorff, T.K. Sau, C.J. Murphy, Shape-dependent plasmon-resonant gold nanoparticles, Small 2 (2006) 636–639. [35] H. Aizawa, Morphology of polysorbate 80 (Tween 80) micelles in aqueous 1,4-dioxane solutions, J. Appl. Crystallogr. 42 (2009) 592–596. [36] Y. Sun, B. Mayers, Y. Xia, Metal nanostructures with hollow interiors, Adv. Mater. 15 (2003) 641–646. [37] H.-P. Liang, L.-J. Wan, C.-L. Bai, L. Jiang, Gold hollow nanospheres: tunable surface plasmon resonance controlled by interior-cavity sizes, J. Phys. Chem. B 109 (2005) 7795–7800.

[38] P.B. Landon, A.H. Mo, C. Zhang, C.D. Emerson, A.D. Printz, A.F. Gomez, C.J. DeLaTorre, D.A.M. Colburn, P. Anzenberg, M. Eliceiri, C. O'Connell, R. Lal, Designing hollow nano gold golf balls, ACS Appl. Mater. Interfaces 6 (2014) 9937–9941. [39] W. He, X. Wu, J. Liu, X. Hu, K. Zhang, S. Hou, W. Zhou, S. Xie, Design of AgM bimetallic alloy nanostructures (M = Au, Pd, Pt) with tunable morphology and peroxidaselike activity, Chem. Mater. 22 (2010) 2988–2994. [40] N. Pradhan, A. Pal, T. Pal, Silver nanoparticle catalyzed reduction of aromatic nitro compounds, Colloids Surf. A Physicochem. Eng. Asp. 196 (2002) 247–257. [41] M. Guo, J. He, Y. Li, S. Ma, X. Sun, One-step synthesis of hollow porous gold nanoparticles with tunable particle size for the reduction of 4-nitrophenol, J. Hazard. Mater. 310 (2016) 89–97. [42] M.A. Mahmoud, B. Garlyyev, M.A. El-Sayed, Determining the mechanism of solution metallic nanocatalysis with solid and hollow nanoparticles: homogeneous or heterogeneous, J. Phys. Chem. C 117 (2013) 21886–21893.