Synthesis of Multibranched Gold Nanoechinus Using a Gemini

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Synthesis of Multibranched Gold Nanoechinus Using a Gemini Cationic Surfactant and Its Application for Surface Enhanced Raman Scattering Priya Vijayaraghavan, Cheng-Hong Liu, and Kuo Chu Hwang* Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan, R.O.C. S Supporting Information *

ABSTRACT: High-yield multibranched Au nanoechinus possessing lengthy and dense branched nanorods on the surface were synthesized using a seedmediated surfactant-directed approach in the presence of gemini cationic surfactant N,N,N′N′-tetramethyl-N,N′-ditetradecylethane-1,2-diaminium bromide (C14C2C14Br2), HAuCl4, AgNO3, and ascorbic acid. C14C2C14Br2 surfactant provides a versatile template in designing the unique morphology of Au nanoechinus with the assistance of AgNO3. UV−vis spectroscopic analysis proves that Au nanoechinus possess a unique intense broad localized surface plasmon resonance (LSPR) peak, which extends from 400 to 1700 nm in the NIR region making a highly potential platform for biomedical applications. Systematic time-dependent TEM, UV−vis−NIR, and XRD analysis were performed to monitor the morphological evolution of multibranched Au nanoechinus. It was found that the surface of branched nanorods of Au NE preferentially grew along (111) crystal planes. Furthermore, as-synthesized Au nanoechinus shows excellent SERS enhancement ability for dopamine inside HeLa cells. KEYWORDS: multibranched nanoparticles, gold nanoechinus, gemini cationic surfactant, NIR, SERS, dopamine

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morphologies such as nanoparticles,11 nanorods,12 nanowires,13 cubes,14 stars,15 flowers,16 nanorattles,17 and nanocorollas.18 Among all the morphologies of Au nanoparticles developed, complex nanostructures possessing elongated branches protruding outward from the core resembling flower-, urchin-, or echinus-like constructions have procured significant attention due to the distinct morphology and optical properties. As anisotropic nanoparticles evolve from bump-shaped nanoparticle to well-defined branched shape structures, there is a dramatic transformation in the main attractive optical properties, such as extending absorbance in the near-infrared (NIR) region due to LSPR coupling and scattering enhancement leading to surface-enhanced Raman scattering (SERS) making them a potential tool in medical diagnostics. To date, several groups have reported various synthetic protocols to design well-developed morphologies of gold nanoparticles, including UV irradiation,19 polyol synthesis,20 and the use of different template-mediated syntheses using hydroquinone,21 CTAB and CTAC,22 and gum arabic;23 a versatile procedure of a two-step, seed-mediated surfactantdirected route using a cationic surfactant CTAB or CTAC is widely used as it leads to step-by-step growth of nanoparticles making it easy to control their size and shape. By adopting a

INTRODUCTION Mankind has been deeply fascinated by the magnificent properties of noble metal nanostructures and has devoted significant effort to developing different strategies to synthesize exceptionally unique nanoarchitectures for prospective applications in catalytic and biomedical assays in many fields. Controlling the properties by fine-tuning the synthetic parameters, such as concentration, chemical composition of reagents, or temperature, helps in evolving nanomaterials with superior optical properties and functional capabilities. Noble metal nanoparticles, such as palladium (Pd), platinum (Pt), silver (Ag), ruthenium (Ru), gold (Au), and copper (Cu), have procured significant attention in recent eras due to their enthralling optical electronic and catalytic properties1−3 that extend their applications in numerous fields such as biomedical,4 sensors,5 imaging,6 electronics,7 water purification,8 and surface enhanced Raman scattering (SERS).9 Among the various noble metal nanoparticles, nanosized gold particles have gained the utmost attention owing to their unique optical properties that are dependent on size and shape, known as localized surface plasmon resonance (LSPR), which enables its use in various applications, especially in the field of biomedical research.10 Since the properties of Au nanoparticles widely vary with their size, shape, and surface topography, various synthetic strategies adopted for preparation of Au nanoparticles is also of vital importance. Previously, several groups have developed wide varieties of gold nanomaterials, which possess varying © XXXX American Chemical Society

Received: June 15, 2016 Accepted: August 24, 2016

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DOI: 10.1021/acsami.6b07218 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Brief Summary of Literature Reported Multibranched Au Nanostructures

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Absorbic acid.

photodynamic therapy,27,28 NIR-light-mediated upconversion fluorescence imaging,29 and so on. In addition to the synthetic methods reported, a comprehensive study of the mechanism of formation is still lacking. Earlier a series of multibranched nanoparticles with roughened surface morphology and long branches, such as nanostars,30 nanodendrites,9 and mesocrystals31 have been synthesized by various wet chemical methodologies for SERS applications. For example, Jiang et al. have fabricated Au nanodendrites of 100 nm in size with a multiamine surfactant with SPR absorption to 1100 nm for SERS studies of Rhodamine B.9 Yu and co-workers have utilized a pentanol/water interface growth “bed” approach for the synthesis of Au mesocrystals.31 Two-dimensional highly branched Au nanoparticles with a broad SPR absorption range of 400−900 nm were synthesized by Wang et al. by using C12C6C12Br2 gemini surfactant for SERS applications of Rhodamine 6G with 105 enhancement factor.42 Wang et al. have also utilized a stepwise reduction method to synthesize hollow nanostructures by using cationic C12C6C12Br2 gemini surfactant as a template.43 However, morphology with welldeveloped nanorods from the surface of gold with supreme optical absorbance in the NIR region until 1700 nm is yet to be reported. A list of branch-shaped nanoparticles was briefly summarized in Table 1.9,30−42

noncomplex method of reducing HAuCl4 by glucosamine or glucamine, Moukarzel et al. have demonstrated a seedless method of preparing nontoxic Au nanostars.24 Jing Li et al. have reported synthesis of urchin-shaped Au nanoparticles by altering the feed ratio of hydroquinone through a seedmediated approach.21 Another group led by Zhang et al. has recently demonstrated that by changing the ratio of surfactants CTAB/CTAC, one can synthesize from irregular shaped, quasispherical particle to spinouslike nanostructure in a seedmediated procedure.22 Branch-shaped nanoparticles are usually formed by the preferential adsorption of structure-directing species and capping agents on specific crystal planes. Although many studies have successfully demonstrated synthesis of anisotropic nanoparticles, most of the methods miss the mark to produce controllable, high yield, unimorphological, dense multi-branch-shaped nanoparticle with well-defined nanorods which might exhibit high LSPR coupling among branched nanorods, resulting in broad UV−vis−NIR absorbance in the NIR region, especially in the biological windows I (650−950 nm) and II (1000−1350 nm) with high extinction coefficients. NIR light has large tissue penetration depths. Nanomaterials with large extinction coefficients in the NIR region are urgently needed for biomedical applications, such as nanomaterialsmediated photothermal therapy,25,26 nanomaterial-mediated B


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ACS Applied Materials & Interfaces

followed by adding 10 μL of various concentrations of dopamine hydrochloride and allowing the sample to air-dry. The spectrometer was calibrated using Raman signal from a clean Si wafer at 520 cm−1. The SERS spectra of dopamine were recorded using a 10× objective with an acquisition time of 30 s. Intracellular SERS Experiments. HeLa cells were grown in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% heat-inactivated bovine serum, penicillin (100 U/mL), and streptomycin (100 U/mL). The cells were cultured in a humidified incubator at 37 °C (95% humidity, 5% CO2), and 2.0 × 105 cells/mL were loaded in a 6-well plate and incubated for 24 h. After 24 h, cells were supplemented with fresh medium consisting of Au nanoechinus with different concentrations of dopamine hydrochloride and incubated for 16 h.48 The cells were then washed thoroughly with PBS and fixed with paraformaldehyde solution (4%), followed by PBS wash. The coverslip was removed and mounted on a clean Si wafer for SERS measurements. Intracellular SERS measurements were recorded using 10× objective with an acquisition time of 30 s.

Herein we report a highly efficient seed-mediated procedure to synthesize unishaped, dense multibranched nanoparticles using a new gemini cationic surfactant with twin ammonium headgroups, consisting of a C14 hydrocarbon tail and a spacer of two methyl groups, C14C2C14Br2. Several kinds of gemini amphiphile have been reported in literature.44,45 We have utilized a modified approach by replacing the conventional single-chain C14TAB by a double-chain surfactant C14C2C14Br2 and performed a systematic study by fine-tuning reaction parameters to synthesize Au nanostructures. By using C14C2C14Br2 as a template to grow nanoparticle in the presence of HAuCl4, AgNO3, and ascorbic acid, we were able to produce high-yield, dense multibranched Au nanoechinus (Au NEs) with sharp tips and extendable NIR absorbance up to 1700 nm at room temperature. Morphological changes aided by the effect of varying reagents and reaction parameters on the growth of Au NEs have also been evaluated in this study. The mechanism of formation of Au NEs has also been investigated by isolating the intermediate species and studying their growth direction using XRD analysis. As-synthesized Au NEs have excellent surface-enhanced Raman scattering response for dopamine, suggesting its potential application in SERS.

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RESULTS AND DISCUSSION Synthesis and Characterization of Au NEs. Surfactant C14C2C14Br2 was prepared using a one pot synthesis procedure. N,N,N′,N′-Tetramethylethylenediamine (1 equiv) and 1bromotetradecane (2 equiv) were reacted under a reflux condition. As-synthesized C14C2C14Br2 was purified, recrystallized, and subjected to spectroscopic methods for further characterization. Figure S1a−c shows the synthetic scheme and results from 1H NMR and 13C NMR of C14C2C14Br2. Au NEs were synthesized using a wet chemical route of seed-mediated surfactant-directed approach using a newly synthesized doublechain cationic gemini surfactant C14C2C14Br2. Morphology of Au NEs was characterized using SEM and TEM. Figure 1a

EXPERIMENTAL SECTION

Materials. Chloroauric acid (HAuCl4·3H2O, Alfa Aeser, 99.99%), sodium citrate tribasic dihydrate (Riedel-de Haën, 99.9%), AgNO3 (Sigma-Aldrich, 99.0%), and ascorbic acid (AA, J. T. Baker, 99.5%) were used as received. All glassware was thoroughly cleaned with freshly prepared aqua regia, followed by ultrapure deionized water. All solutions were prepared using deionized water. N,N,N′,N′-Tetrame th yl -N ,N′ -di tetradecylethane-1 ,2-diamin ium b romide (C14C2C14Br2) was synthesized via a single-step procedure using 1bromotetradecane (Alfa Aeser, 98%) and N,N,N′,N′-tetramethylethylenediamine (Alfa Aeser, 99%). Absolute ethanol (Sigma-Aldrich, 99.5%) was used as received. Synthesis of N,N,N′,N′-Tetramethyl-N,N′-ditetradecylethane-1,2-diaminium Bromide (C14C2C14Br 2). N,N,N′,N′Tetramethylethylenediamine (1 equiv) was refluxed with 2 equiv of 1-bromotetradecane in absolute ethanol at 85 °C overnight. The crystalline powder obtained was washed thoroughly and recrystallized from ethyl acetate. Synthesis of Citrate-Coated Gold Nano Seeds. HAuCl4 (50 mL, 0.25 mM) and 5 wt % trisodium citrate solution (0.5 mL) were taken in a reaction vessel and brought to boiling until the color of solution turned red.46 Synthesis of Au NEs. Au NEs were synthesized according to the modified seed-growth procedure.47 In a typical experiment, an aliquot containing 47.5 mL of 0.001 M C14C2C14Br2, 2 mL of 0.01 M HAuCl4· 3H2O, and 0.3 mL of 0.01 M AgNO3 were taken; to this mixture was added 0.32 mL of 0.1 M AA upon gentle stirring. Finally, 0.1 mL of sodium citrate stabilized Au seed (average size: 5−10 nm) solution was added to the above reaction mixture and gently mixed for 10 s and left undisturbed for 24 h. After 24 h, the upper solution was removed carefully using a pipette, and to the lower part of the solution, distilled water was added. The resulting particles were collected via centrifugation at 3000 rpm for 7 min and finally washed three times with DI water. Characterization of Au NEs. As-synthesized Au NEs were characterized using UV−vis spectroscopy (Jasco V570), scanning electron microscopy (SEM, JSM-7000F field-emission SEM), transmission electron microscopy (TEM, JEM 2010, 200 kV), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS, ULVAC-PHI model Quantera SXM). SERS Measurements. Raman measurements were done using LabRAM HR Raman microscope (632.8 nm) using dopamine hydrochloride as a probe molecule. SERS substrate was prepared by drop-casting 20 μL of Au NEs onto a Si wafer and drying properly,

Figure 1. (a) TEM image of gold nanoechinus; (b and c) SEM images of single Au NE and Au NE at low magnification, respectively. Scale bar: 100 nm. (d) Average size distribution of gold nanoechinus measured using DLS.

shows the representative TEM image of Au NEs. Figure 1b shows the representative SEM image of a single Au NE clearly depicting the presence of multiple branches protruding in multiple directions from a dense core. The average size of the branches is length 110 ± 50 nm and width 20 ± 5 nm. The average size of Au NEs was found to be 350 ± 100 nm (see Figure 1d) as measured by dynamic light scattering (DLS), which is also in good agreement with the size distribution as evident from SEM and TEM images. C


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Figure 2. (a) Representative TEM image of gold nanoechinus. (b) Corresponding SAED pattern of the region (marked inside rectangle). (c) UV− vis−NIR spectrum; (d) XRD pattern of Au nanoechinus.

Figure 3. SEM images of Au nanostructures obtained under different concentration of C14C2C14Br2: (a) 0.25 mM, (b) 0.5 mM, (c) 1.0 mM, (d) 2.0 mM, (e) 3 mM, and (f) 4 mM. Scale bar: 100 nm. (g) Normalized UV−vis−NIR spectrum.

Figure 2a shows the representative TEM in Au NEs. Inset in Figure 2b shows a characteristic selected-area diffraction pattern (SAED) of a long branch of Au NEs marked in the yellow rectangle region, which indicates that the growth direction of single elongated planes is . Figure 2c exhibits the corresponding UV−vis−NIR spectra of Au NE, which shows broad extendable absorption until 1700 nm. As compared to Au nanoparticle and nanorods which possess single or dual narrow absorption bands, Au NEs possess a broadly extendable absorption peak ranging from 400 to 1700 nm. The presence of multiple sharp rods on the surfaces would allow inter-rods

LSPR coupling, which leads to an enhancement in a broad and extendable NIR absorbance until 1700 nm. The molar extinction coefficient of Au NEs was on the order of 1012 M−1cm−1 (see Figure S2). From the XRD pattern of Au NEs shown in Figure 2d, wellresolved peaks of {111}, {200}, {220}, {311}, and {222} were obtained indicating face centered-cubic ( fcc) structure of Au with particles consisting of independent orientation. The peak corresponding to {111} was more intense in comparison to other peaks. The intensity ratio between {111}/{200} was found to be 2.52, which is much larger than the ratio for Au D


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Figure 4. SEM images of Au nanostructures obtained at different temperatures: (a) 20 °C, (b) 25 °C, (c) 40 °C, and (d) 60 °C. (e) Normalized UV−vis−NIR spectra. Scale bar: 1 μm.

A higher concentration of surfactant yields nanostructures with broad SPR band with maxima position at 600 nm. Previously, it was reported that the adsorption of surfactant moieties on Au nanoparticles will reduce the interfacial energy of Au NPs seeds on the interface, resulting a favorable formation of branch-shaped nanoparticle.9 As the surfactant concentration increases, liquid crystal undergoes poor diffusion affecting solution properties, such as density, viscosity, and ionic strength, eventually inhibiting ordered anisotropic growth of crystals and giving irregular uncontrollable morphologies. Therefore, in order to promote a controlled growth of Au NEs, a low and ideal concentration of surfactant is required. To ensure the uniqueness of gemini surfactant in synthesizing Au nanoparticles, its monomeric analog C14TAB was also used as a growth-directing surfactant under the same growth condition. However, only partially developed stellated particles were obtained (see SEM images shown in Figure S4). The presence of stable bilayers using a gemini surfactant C14C2C14Br2 is an important factor in facilitating the formation of Au NEs. Effect of Temperature. The effect of temperature was systematically analyzed. The ideal condition to generate Au NEs was at room temperature, 25 °C. Figure 4 shows the SEM and UV−vis−NIR spectra of Au nanostructures produced at different temperatures. As observed, the ideal condition to favor the formation of Au NEs is at 25 °C. From the UV−vis−NIR spectra, a slight decrease in the intensity of absorption in the NIR region was observed. However, no significant morphological change was observed upon reducing the temperature to 20 °C. An increase in the reaction temperature has drastically diminished the yield of Au NEs and facilitated the formation of spherical shaped nanoparticles. As the temperature was increased from 25 to 40 °C, the broad SPR absorption of Au nanoechinus was diminished into a band profile with two absorption bands at shorter wavelengths, 579 and 851 nm. Further increase in temperature to 60 °C has effected in blueshifting the spectra with SPR band at 530 nm, which implies the complete disappearance of branched shape morphology into spherical shaped nanoparticles. Elevated temperature might accelerate intraparticle diffusion and ripening, which may induce the formation of spherical nanoparticles eventually affecting the anisotropy.49,50 Effects of Ascorbic Acid Concentration. Gold nanoechinus was prepared by the reduction of HAuCl4 using a weak reducing agent, ascorbic acid (AA), in the presence of

nanoparticles {111}/{200} = 1.9 (see Figure S3). This data implies that the preferential growth direction of Au NEs is presumably {110}. Both HRTEM and XRD pattern indicate that Au NEs were made of fcc crystalline structure. It is clearly evident from SEM and TEM images of Au nanoechinus that the synthetic method employed using double-chain cationic surfactant provides an excellent yield above 99% with welldefined multiple branches. This kind of reproducible method for synthesis of echinus-like gold nanostructures with excellent uniformity and nicely developed branches has been never been observed previously. Influence of C14C2C14Br2 Concentration. To evaluate the formation of branched morphology of Au NEs, several control experiments were performed by varying the concentration of reagents, such as C14C2C14Br2 surfactant, reducing agents, and capping agents. Figure 3a−g demonstrates the effects of surfactant concentration on the formation of Au nanoechinus. C14C2C14Br2 surfactant plays a vital role on the formation of multibranched Au NEs. Growing Au NEs in varying concentrations of C14C2C14Br2 while keeping all other parameters constant, we were able to witness a drastic change in the morphology of NEs. An ideal concentration of 1 mM C14C2C14Br2 was required to produce a high yield of Au NEs. As the concentration of surfactant was decreased to 0.5 mM from the ideal concentration (1 mM), the presence of bumpyshaped particles was observed, accompanied by a dramatic decrease in the yield of Au NEs. Upon increasing concentration of the surfactant to 4 mM, Au nanoseed developed into welldeveloped branched-shape Au NEs. Beyond the ideal concentration of surfactant, the average size of Au NEs also increased from 350 nm to several undefined gigantic micrometer-sized particles with bulky branches. Figure 3g presents the corresponding UV−vis−NIR spectra. Since the SPR property of Au is critically dependent on its respective size, shape, and electronic interactions of particles, there is an obvious change in the SPR absorptions of Au nanostructures in the UV−NIR region upon changing the surfactant concentration. When the concentration of surfactant is gradually reduced from the ideal concentration of 1mM to 0.5 mM and then to 0.025 mM, the SPR maxima significantly changes from the broad absorption profile to a blue-shifted spectra with SPR maxima position of 900 and 650 nm, respectively, owing to the formation of bumpy-shaped particles. E


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Figure 5. SEM images of Au nanostructures obtained under different concentrations of ascorbic acid: (a) 0.3 mM, (b) 0.6 mM, (c) 1.2 mM, (d) 1.8 mM, and (e) 2.5 mM, respectively. (f) Normalized UV−vis−NIR spectrum of Au nanostructures. Scale bar: 100 nm.

Figure 6. SEM images of Au nanostructures obtained at different concentrations of AgNO3: (a) 0.015 mM, (b) 0.06 mM, (c) 0.12 mM, (d) 0.18 mM, (e) 0.23 mM, and (f) 0.29 mM. (g) Corresponding normalized UV−vis−NIR spectra of Au nanostructures formed at different AgNO3 concentrations.

SPR absorptions of Au nanostructures was clearly seen. Accordingly, the broadened UV−vis−NIR band of Au NEs was reduced to a blue-shifted broad peak with an absorption band centered at 1100 nm for 1.2 mM AA. As compared to strong reducing agents like NaBH4 and sodium citrate that cause a surplus supply of Au(0) and promote secondary nucleation facilitating particles with random shape and size, the mild reducing agent AA reduces gold ions from Au(III) to Au(0); these gold seeds act as nucleation centers for latterly formed Au atoms to deposit leading to the crystallization on the surface by encouraging homogeneous structures. As the concentration of AA increases, a higher amount of Au atoms was produced, which can override such process leading to the formation of different kind of Au nanostructures. At higher concentrations of AA, a vast supply of Au ions was produced, followed by the formation of nucleation centers, which further promotes growth of seed particles in all directions and leads to the formation of flowerlike particles of smaller size.47 Effects of AgNO3 Concentration. AgNO3 plays a crucial role in the branched morphological evolution of Au NEs. We performed a parallel experiment by replacing silver nitrate with sodium nitrate (NaNO3) to have a further understanding about the importance of Ag+ over NO3−. In the absence of Ag+ ions, no Au NEs were formed, and the particles produced have asymmetric shape and size, without any definite morphology

C14C2C14Br2 and AgNO3. AA is a mild reducing agent commonly used in various seed-mediated synthesis of metal nanoparticles.49 The role of ascorbic acid is very crucial in synthesizing anisotropic nanoparticle. To evaluate the role of ascorbic acid in the formation of Au NEs, different concentrations of AA were added to the growth solution with fixed concentrations of C14C2C14Br2 and AgNO3. Figure 5a−f show the SEM images and UV−vis−NIR spectra of Au NEs at different concentrations of AA. The ideal concentration of AA required to produce Au NEs is 0.6 mM. As a result of lowering the concentration of AA to 0.3 mM, Au nanoechinus formed short and blunt nanorods on the surface (see Figure 5a). This effect may arise due to the insufficient supply of Au atoms for the sequential growth. When the concentration of AA was increased from 0.6 to 2.4 mM, the average size of Au NEs decreases gradually ranging from 350 to 100 nm. Even though Au nanostructures have a regular morphology and are anisotropic in nature, there is a drastic decrease in the aspect ratio of the rods on the surface, producing Au nanoflowers-like structures with short petals. A balanced amount of reducing agent is required for production and to facilitate symmetrical sequential growth of Au atoms toward a well-developed nanoechinus-like gold nanostructure. From the UV−vis−NIR spectra (Figure 5f), the differences in shape due to the effect of increasing concentration of AA on the F


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Figure 7. SEM images of Au nanostructures obtained at different concentrations of sodium halide (0.39 mM): (a) no halide (Au NEs), (b) NaCl, (c) NaBr, and (d) NaI. (e) Normalized UV−vis−NIR spectra of Au nanostructures.

indicating the vital role of Ag+ (see SEM images shown in Figure S5). In order to further elucidate the crucial role of AgNO3, a series of experiments were performed by varying the concentrations of AgNO 3 while keeping all the other parameters such as surfactant, gold precursor, and reducing agent constant. Figure 6 shows the SEM images and UV−vis− NIR spectrum of Au nanoparticle obtained at varying concentrations of AgNO3. As shown in Figure 6a, when the amount of AgNO3 was reduced to 0.015 mM than the ideal reaction concentration of 0.06 mM, only bumpy-shaped particles were formed. As the concentration of AgNO3 was increased beyond the ideal concentration, an increase in the size of Au NEs from 350 to 450 nm were observed; the multibranched structure is maintained, followed by a decrease in the average size upon further increasing the concentration of the Ag+ ion. When the concentration of AgNO3 increases, various kinds of anisotropic branched structures or quasi-spherical particles with short branches were observed as shown in the Figure 6c−f. Thus, an appropriate composition of silver ions with C14C2C14Br2 is required for the preparation of multibranched Au NEs. The LSPR absorption spectra of nanostructures produced at various concentrations of AgNO3 demonstrate a drastic change in shape and size of Au NEs. As the concentration of AgNO3 was lowered to 0.015 mM, the LSPR absorption of Au NEs changes from a broad band to a narrow band at around 650 nm. As the concentration of AgNO3 was increased, the NIR absorption beyond 1050 nm got reduced with a broadened absorption maxima centered at around 700−900 nm. The decrease of absorption peaks in the NIR region was attributed to the presence of short branched-shape particles. There are several explanations for the role played by Ag+ ions. One possible explanation was given by Guyot-Sionnest et al., regarding the underpotential deposition (UPD) of Ag+ ions on the surface of growing rods at a potential of less than the standard potential to create layers of Ag(0).30 In another report, the combination of silver and bromide ions from (C14TA)22+ and their preferential adsorption on {111} facet of Au nanoparticles, which prevented further growth in the {111} direction and allowed growth in {110} direction, was also proposed.51 Due to the UPD, chemisorption of Ag(0) and/or AgBr on the surface of the active sites of the seed particles for sequential growth of gold atoms occurs and leads to the formation of multibranched- or star-shaped particles. To further evaluate the crucial role of silver ions on the subsequent growth of Au NEs,

high concentrations of iodide ions were introduced. As the binding strength of halide ions on Au surface increases in the order Cl− > Br− > I−, an increased concentration of iodide ions will have a detrimental effect by destabilizing the Ag UPD.52 Figure S6 represents the SEM images of the nanoparticles prepared at different concentrations of iodide ions added to the growth solution. It is clearly evident from the SEM images that increasing concentration of sodium iodide (NaI) results in the evolution of particles from well-defined multibranched particles to undefined stellated structures by destabilizing the Ag UPD and thus by reducing the yield of Au NEs. In order to identify the chemical species adsorbed on the surface of Au NEs, we have performed XPS measurements. Figure S7 shows the XPS spectra of Au NEs. The XPS scan of Au NEs reveals the presence of Au 4f, Br 3d, Ag 3d, C 1s, and N 1s. In particular, Ag 3d3/2 and Ag 3d5/2 peaks were observed at 373.45 and 367.45 eV, respectively. In literature, it was reported that CTA−AgBr complex has assisted the synthesis of nanoparticles; our XPS results support such an argument. The XPS results of Br exhibit two prominent biding energies: One is at 67.72 eV, belonging to 3d5/2, and the second is at 68.62 eV, belonging to 3d3/2. The XPS band at 67.72 eV peak corresponds to the (C14TA)2+−AgBr complex, matching with previous reports. The XPS band at 68.62 eV correlates with the AgBr binding energy value.53−55 The C 1s exhibits two peaks at 284.9 and 286 eV. The XPS band at 284.9 eV is related to aliphatic carbons; the 286 eV band originates from the C−N bond. Hence, the XPS results clearly reveal that Au NEs formation is tightly associated with the presence of (C14TA)2+− AgBr and AgBr on the {100} facet of Au NP surface. Effect of Halide Ion Concentration. The role of halide ions in influencing the morphology of Au nanostructures has been widely reported in literature.57,58 From Figure 7 it can be clearly observed that when the same concentration of different halide ions (Cl−, Br−, and I−) was introduced to the growth solution of Au NEs, the morphology of Au NEs were completely changed. Addition of chloride ions into the growth solution of Au NEs promoted cubic-shaped multibranched nanoechinus, and the presence of bromide ions drastically reduced the yield of multibranched Au NEs. As explained before, iodide ions induced a sharp contrast to the morphology of Au NEs by producing spherical-shaped nanoparticles. The effect of various halide ions may be attributed to the affinity to stabilize or destabilize the Ag UPD layer. It has already been reported in literature Cl− has the ability to stabilize the Ag UPD G


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ACS Applied Materials & Interfaces Scheme 1. Proposed Growth Mechanism of Au Nanoechinus

layer and that Br − and I− have the ability to destabilize the Ag UPD layer.56 Isolation of Intermediates and Growth Mechanism. To gain a further understanding about the formation mechanism of Au NEs, the intermediate products at different time points were isolated and subjected to TEM, UV−vis−NIR spectroscopy, and XRD measurements. Figures S8 and S9 show TEM images, corresponding UV−vis−NIR spectra, and XRD patterns of isolated particles in the dynamic process of Au NEs. Figure S8a shows the TEM image after 1 h of reaction of Au nanoechinus formation, which clearly shows the presence of irregularly shaped Au nanoparticles approximately sized 2−4 nm. As time progresses to 4 and 6 h, clusters of irregularly shaped gold nanoparticles were found to evolve (see Figure S8c,d). It was clearly evident from the TEM images after 6 h of reaction process that crystalline growth of formation of anisotropic nanostructures has started by forming densely assembled layers of Au nanoparticles, giving rise to massive structures of few hundreds of nanometers in size. The multibranched structure formation progression is observed in all the samples collected until 20 h as illustrated from Figure S8a−f. In a detailed analysis of intermediate stages of the synthesis, involvement of three steps in the growth formation process can be obviously observed. The first step is the formation of clusters, followed by the second step where the aggregation of several irregularly shaped 2−4 nm Au nanocrystals, and the third step mainly facilitates the crystalline growth formation of gold nanoparticles in different orientations progressing toward a multibranched structure. Although a rough morphology of urchin-shaped structures were formed within 10 h of reaction, a well-developed gold nanoechinus composed of long tips on the surface were formed only after 24 h of reaction time. XRD analysis was performed for the products isolated after various reaction time points. The UV− vis−NIR spectra of various intermediates of Au NEs shown in Figure S8g clearly support such a three-step growth process. The primary crystals of aggregated clusters isolated at 1, 2, and 4 h do not show any significant SPR absorbance in the visible region. The SPR absorption of product isolated at 6 h shows a small absorption band at 670 nm, which was attributed to the initiation of anisotropic growth or protrusion of branches from the agglomerates. At 10 h, the SPR band starts to broaden and redshift to 850 nm with increasing absorbance in the NIR region as a result of the development of branches giving a rough morphology of Au NEs. At 20 h, there is a clear broadening of

UV−vis−NIR spectra upon attaining the morphology of Au NEs. On the basis of TEM and UV−vis−NIR absorption analysis of intermediate products isolated at various time points, it is clear that the evolution of Au NEs follows a threestage growth process. In order to gain a further understanding about the crystalline growth direction, the intermediate products were subjected to XRD analysis. Figure S9 shows the XRD patterns of intermediate particles formed during the growth of Au NEs. XRD patterns of various intermediate products indicate that the lattice parameters were in good agreement with fcc structure of bulk gold as explained before. The increase in the intensity ratio of {111} to {200} shows that anisotropic growth of multibranches of Au NEs might progress by the rapid deposition of Au(0) on the high-energy facet {111} lattice plane. In relevance to all of the above experiments, we can summarize the growth of Au NEs using gemini C14C2C14Br2 surfactant. A brief representation of growth mechanism of Au nanoechinus is shown in Scheme 1. At the beginning of synthesis, AuCl4− ions bind to C14C2C14Br2 surfactant forming AuCl4−−C14C2C14Br2 complex. Upon reduction of AuCl4− by AA, Au clusters are formed. The initial stage of growth of Au NEs was characterized by the presence of Au nanoparticle cluster formation and rapid aggregation of clusters to form large anisotropic multibranched structures. Anisotropic growth of multibranched structures proceeds via a controlled deposition of Au atoms on specific facets with the assistance of C14C2C14Br2 and Ag+ ions. C14C2C14Br2 forms stable bilayers which could combine with Ag+ ions and stabilize {110} or {100}. Surface energy of crystal planes decreases in the order of {110} > {100} > {111}, and the stability follows the reverse order due to the increase of interatomic distance or decreasing surface atom density.54,55 Hence, the preferential deposition of gold atoms on the {111} facet leads to surface growth of Au NRs on the core Au NP along the {110} direction. SERS Measurements and Enhancement Factor. Multibranched Au NE shows a very high intense SERS spectrum upon probing dopamine molecules absorbed onto its surface. SERS is a highly sensitive technique to analyze even ultralow concentrations of various probe molecules. It was clearly evident from Figure S10 that dopamine on Au NEs exhibits intense surface-enhanced Raman scattering signals at an ultralow concentration of 10−12 M. The SERS enhancement factor of 109 was achieved for dopamine (see Supporting H


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Figure 8. Stability of SERS spectra of dopamine in the presence of Au NEs in the aqueous solution at different runs: (a) first, (b) second, (c) fourth, and (d) fifth. The concentrations of dopamine are labeled.

Information). The efficiency to provide enhancement to dopamine molecules is mainly attributed to the multiple end tips morphology of Au NEs and the exceptionally high absorbance of Au NE at the excitation wavelength light of 633 nm. Stability of SERS substrates is an important factor for the detection of molecules at trace levels. Although nanoparticles, such as Au nanorods, have high optical absorbance in the NIR region, they are easily susceptible to photothermal reshaping which will affect the stability and SERS reproducibility.59 Particles like Ag nanostructures are easily oxidized in the air, which will affect their SERS reproducibility.60 The SERS enhancement signals of dopamine capability of Au NEs up to 5 consecutive runs has also been demonstrated (see Figure 8). Au NEs provided high sensitivity and reproducibility for the detection of trace amount of dopamine (10−12 M) even after five runs, indicating that the Au NEs structure is very stable toward light irradiation and that photoreshaping does not occur. To extend the sensing application of dopamine in living cells, HeLa cells were chosen as a suitable cellular system. Different concentrations of dopamine co-loaded with Au NEs were added to an incubator containing HeLa cells to investigate the SERS effects. Figure 9 shows the intracellular SERS signals obtained from HeLa cells incubated with Au NEs and dopamine. Typical SERS bands of 1279 and 1330 cm−1, which correspond to the catechol C−O stretching and the aromatic C−H bending vibrations of dopamine molecules, respectively, could be clearly observed from HeLa cells.

Figure 9. Optical images of HeLa cells from Raman microscopy treated with (a) dopamine (0.1 M) and (b) Au NE−dopamine complex. (c) SERS signals detected from HeLa cells incubated with different concentrations of dopamine−Au NEs complex in a medium solution.

successful synthesis of Au nanoechinus can be attributed to twin-tailed gemini amphiphile C14C2C14Br2, AgNO3 and weak reducing agent AA. The growth process of Au nanoechinus was also investigated in a very detailed manner. The broad and extendable NIR absorption covering the biological windows and 109-fold SERS enhancement ability for dopamine indicates its potential applications in various biomedical fields, such as biosensing, bioimaging, NIR light mediated phototherapies, and others.

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CONCLUSIONS We have successfully developed a versatile and facile synthetic methodology to produce high yield and stable gold nanoechinus with densely packed spikes by using a double-chain cationic gemini C14C2C14Br2 surfactant as the structuredirecting reagent. By modifying traditional single-chain surfactant with twin-tailed cationic surfactant, we are able to produce branched nanostructures with high LSPR coupling and very broad NIR absorption extending up to 1700 nm. The

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07218. NMR spectra of C14C2C14Br2, SEM images of Au nanostructures, XRD of Au nanoparticles (PDF) I


Research Article


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the financial support from the Ministry of Science & Technology, Taiwan. ABBREVIATIONS Au NEs = Au nanoechinus C14C2C14Br2 = N,N,N′,N′-tetramethyl-N,N′-ditetradecylethane-1,2-diaminium bromide AA = Ascorbic acid

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Synthesis of Multibranched Gold Nanoechinus Using a Gemini

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