Design of Stable Plasmonic Dimers in Solution: Importance of

Article pubs.acs.org/JPCC

Design of Stable Plasmonic Dimers in Solution: Importance of Nanorods Aging and Acidic Medium Israa Haidar,† Jean Aubard,† Georges Lévi,† Stéphanie Lau-Truong,† Ludovic Mouton,† Daniel R. Neuville,‡ Nordin Félidj,*,† and Leïla Boubekeur-Lecaque*,† †

Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, University of Paris Diderot, 15 rue J-A de Baïf, 75205 Paris Cedex 13, France IPGP-CNRS, Sorbonne Paris Cité, 1 rue Jussieu, 75238 Paris Cedex 05, France

‡

S Supporting Information *

ABSTRACT: We describe a simple and effective strategy to couple gold nanorods (GNRs) into end to end dimers and freeze the assembly in water. The assembly is initiated using cysteine and driven by hydrogen bonding between two cysteine. We show that the aging of GNRs samples impacts both the assembly kinetics and the final yield of GNRs dimers. The addition of an appropriate amount of silver nitrate induces the immediate termination of GNRs dimerization and stabilizes the small aggregates in solution for at least 24 h. for many chemical and biological sensing applications.4,5,11It is still the subject of extensive investigations aiming to gain a better understanding of this phenomenon and allow a finer control over the resulting coupling.12,13 Thus far, many coupled plasmonic structures and strategies have been investigated such as aggregated colloidal particles, molecularly mediated assembly of colloids, and electron beam lithography (EBL). The simplest approach relying on randomly aggregated colloidal nanoparticles has been widely exploited for the first SERS experiments, providing huge enhancement but suffering from a poor reproducibility and from the complexity to model even small aggregates. EBL is one of the most powerful nanofabrication techniques that produces by direct writing regular nanopattern arrays on solid substrates with high resolution (∼10 nm) and reproducibility.14 The coupled plasmonic nanostructures generated by EBL represent excellent model structures. However, for the design of coupled particles, the interparticle gaps are generally limited to 10 nm. Alternatively, the chemically driven self-assembly techniques in solution are nowadays actively explored to obtain stable dispersion of small colloidal clusters with well-defined hot spot

1. INTRODUCTION Excitation of localized surface plasmon (LSP) modes in noble metal nanostructures results from the coherent collective oscillation of conduction electrons at the surface of metal particles with the incident light. The major consequence of this peculiar effect is a strong absorption in the visible and near infrared regions at the resonance (mainly for gold, silver, and copper). In addition, LSP excitation induces extremely intense and strongly localized electromagnetic (EM) fields near the nanostructure surface that decays exponentially with the distance away from the nanoparticle surface.1 Consequently, molecules placed in the vicinity of the surface experience this confined and intense EM field which enhances considerably the molecular spectral characteristics such as absorption and emission, nonlinear optical properties, and Raman scattering (surface-enhanced Raman scattering or SERS).2−5 When metallic nanoparticles (NPs) are placed adjacent to each other, their LSP properties are significantly modified in the far and near field, due to a strong electromagnetic coupling.6 In the near field, this coupling may give rise to a giant local electric field in the gap (called hot spot) up to 103−104 times the electric field associated with the isolated nanoparticles.7 The resulting local enhancement factor at the interparticle junction is highly sensitive to the distance between the coupled particles.8−10 Such electromagnetic coupling has been exploited © XXXX American Chemical Society

Received: July 23, 2015 Revised: September 7, 2015

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DOI: 10.1021/acs.jpcc.5b07135 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. GNRs Assembly Induced by Cysteine in Acidic Medium and Frozen in the Presence of Silver Ions

conducted seeking a better control of the number of particles per aggregate (mainly dimers) and higher dimerization yields.

architecture generating high SERS activity and reproducibility. In such strategy the use of molecular bridge between NPs allows fine tuning the interparticle junction down to 1 nm.15 However, the colloidal suspension being a metastable system, the synthesis of homogeneous small colloidal clusters (dimers/ trimers) implies controlling the aggregation kinetics and freezing the assembly to prevent further aggregation or undesired dissociation. The most efficient approach to adjust the interparticle gap for assembly of NPs in solution relies on molecular cross-linker such as symmetric bifunctional molecule (dithiol, diamine, ...)16−18 or molecule with recognition ability (DNA, macrocyclic cavity, amino acids, ...).10,19−24 In these cases, the assembly process is driven by hydrogen bonding, coordination chemistry,25−27 hydrophobic forces,28 or electrostatic interactions.29 The considerable effort that has been devoted by the scientific community to devise robust assembly and quenching strategies in solution highlights the challenge of mastering NPs aggregation to form stable dimers in solution. Despite recent advances in the field, the selective termination of nanoparticles assembly, for example, through encapsulation of the aggregates in polymer,30 lipidic,31 or silica shells,32,33 remains challenging for assemblies in suspension. The practical use of SERS platforms in solution consisting of small NPs clusters with controlled interparticle distance and reproducible enhancement properties requires primarily one to develop simple assembling strategies. In order to preserve the SERS enhancement characteristics, the coupled NPs in suspension must also be stable over time. In the present paper, we report on an efficient strategy to freeze the end to end assembly of gold nanorods (GNRs) initiated by cysteine in solution by taking advantage of hydrogen-bond formation between cysteine adsorbed onto distinct nanorods. If the generation of nanorods chains aligned end to end has been reported by other groups, the assembly of GNRs relying on cysteine reported so far leads to the irreversible aggregation of oligomers in solution.34−36 Given the high affinity of cysteine for silver, the addition of silver ions was explored in this work to freeze the assembly of GNRs in solution (Scheme 1). The stability over time of the quenched assemblies was investigated by extinction spectroscopy and SERS. A detailed kinetic study of GNRs assembly was

2. EXPERIMENTAL SECTION Chloroauric acid (HAuCl4) was purchased from Alfa Aesar. Hexadecyltrimethylammonium bromide CTAB (98%) and sodium oleate (NaOL) were purchased from TCI. Silver nitrate, L-cysteine, L-ascorbic acid (AA), and sodium borohydride (99%) were purchased from Sigma-Aldrich. All solutions used in the colloidal synthesis were prepared with 18.2 MΩ· cm−1 resistivity nanopure water. 2.1. Gold Nanorods Synthesis. AuNRs were synthesized by using seed-mediated growth methods reported previously.37 The seed solution for gold NR growth was prepared as follows: 5 mL of 0.5 mM HAuCl4 was mixed with 5 mL of 0.2 M CTAB solution in a 20 mL scintillation vial. NaBH4 (0.6 mL of fresh 0.01 M) was diluted to 1 mL with water and then injected to the Au(III)−CTAB solution under vigorous stirring (1200 rpm). The solution color turned from yellow to brownish, and the stirring was stopped after 2 min. The seed solution was aged at room temperature for 30 min before use. To prepare the growth solution, CTAB (3.5 g, 0.037 M in the final growth solution) and NaOL (0.617g) were dissolved in 125 mL of warm water (∼50 °C). The solution was allowed to cool down to 30 °C, and 9 mL of 4 mM AgNO3 solution was added. The mixture was kept undisturbed at 30 °C for 15 min, after which 125 mL of 1 mM HAuCl4 solution was added. The solution became colorless after 90 min of stirring (at 700 rpm), and 1.05 mL of HCl (37 wt % in water, 12.1 M) was then introduced to adjust the pH. After another 15 min of slow stirring at 400 rpm, ascorbic acid (0.625 mL of 0.064 M) was added and the solution was vigorously stirred for 30 s. Finally, a small amount of seed solution was injected into the growth solution. The resultant mixture was stirred for 30 s and left undisturbed at 30 °C for 12 h for NR growth. The GNRs were purified by repeated centrifugations (2×) at 6000 rpm for 20 min. 2.2. Gold Nanorods Assembly Initiation/Termination. For the assembly of Au nanorod, the pH of the GNR solution (2 mL of gold nanorod 0.97 nM) was adjusted through adding 0.6 mL of HAc aqueous solution (VHAc:Vwater = 1:10). A 15 μL volume of cysteine (5 mM) aqueous solution was then added to the gold nanorods solution. Before losing the isosbestic B


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The Journal of Physical Chemistry C point, a small amount of 4 mM AgNO3 (6.5, 25, 32 μL) was added into the above solution to freeze the assembly process. 2.3. Characterization. Visible−NIR Spectroscopy. Extinction spectra were recorded from GNRs samples in a cuvette (path length = 2 mm) under a bright field inverted optical microscope (Nikon, TI-DH) equipped with a spectrometer (Andor, SR-303i) and a CCD camera (ANDOR, DU401A-BRPD) through a 100× objective (Nikon, NA:0.7) using a halogen lamp as a light source. Raman Spectroscopy. Raman spectra were recorded using a confocal Raman microspectrometer Jobin-Yvon LABRAM HR 800 equipped with a thermoelectric-cooled CCD and a motorized XY displacement stage. In the present study, a continuous wave (CW) 785 nm laser was used for SERS excitation. The laser line was focused onto samples through a microscope 50× objective (Olympus, NA: 0.75), and SERS were collected through the same objective in backscattering mode. The Rayleigh scattering was filtered with a long-pass edge filter. Incident laser power was adjusted with neutral density filters and measured at the sample position with a power meter (Coherent, laser checker). All spectra were collected from colloidal nanorods samples in a cuvette and recorded with an integration time of 5s (GNRs-Cyst) or 15s (GNRs-DTTCI) within the 150−3500 cm−1 spectral range. TEM. Transmission electron microscopy (TEM) characterizations were performed using a Jeol 100-CX II microscope, operating at 100 kV. The concentrated GNRs were first dispersed in water, and 1 drop of this dispersion was deposited onto carbon-coated Cu grid.

Figure 1. Extinction spectra for GNRs in water at pH ≈ 3.0 (adjusted with acetic acid) following the addition of cysteine: (A) recorded between 0 and 4 min and (B) after 3 min.

3. RESULTS AND DISCUSSION 3.1. Gold Nanorods Assembly. Gold NRs were synthesized and purified by following the well-established seed-mediated growth approach, based on the reduction of HAuCl4 with a weak reducing agent (ascorbic acid) on premade CTAB-stabilized gold seeds in the presence of AgNO3. The gold nanorods used in our experiments have aspect ratios (AR) ranging from 2.5 to 3.9 (see Figure S1, Supporting Information). Due to their anisotropy, the GNRs possess two distinct LSP modes clearly identified by UV−vis spectroscopy. For instance, GNRs with AR ≈ 3.6 exhibit a short wavelength band at 508 ± 3 nm corresponding to the transverse LSP mode (short axis) and a stronger band at 740 ± 5 nm corresponding to the longitudinal LSP mode (main axis). The assembly of GNRs was initiated by adding cysteine (Cys) in an acidic medium. In fact, gold nanoparticles functionalized with cysteine are known to self-assemble into aggregated networks due to hydrogen bonding between carboxylate and protonated amine groups of two distinct cysteines.24,35,36 The progression of cysteine-induced assembly was monitored by UV−vis absorption spectroscopy. The GNRs concentration was fixed at 0.7 nM considering an extinction coefficient of 4.1 × 109 M−1·cm−1 at the λmax of the longitudinal plasmon resonance mode.38 Figure 1 shows the time-dependent extinction spectra of GNRs recorded in the presence of 27 μM cysteine at a pH of 3. Over time, the peak corresponding to the longitudinal LSP mode at 740 nm gradually decreases while a new peak at 890 nm rises concomitantly. A clean isosbestic point located at 822 nm is indicative of the coexistence of only two species in solution (isolated GNRs and GNRs dimers, Figure 1A). The new feature at 890 nm is assigned to the contribution in the far field of the electromagnetic coupling

between two rods.16,34,39 The transverse LSP mode located at 508 nm is barely affected by cysteine-induced self-assembly. At early stages of the assembly, changes in extinction spectra of GNRs in the presence of cysteine are consistent with the formation of dimers. As revealed by previous studies, the redshifted plasmon band of the coupled particles as compared to the isolated GNRs points to end to end assembled gold nanorods, and the amplitude of this red shift (ca. 150 nm) suggests that the particles are strongly coupled with a small gap distance (1 nm or less).9,34,40,41 Upon longer reaction time between GNRs and cysteine, the peak assigned to dimers red shifts and broadens, indicating the formation of larger GNRs assemblies (see Figure 1B). After a sufficient time, the peaks for both isolated and assembled GNRs progressively decrease, with a loss of isosbestic point, suggesting the precipitation of nanorods aggregates from the solution. To further assess the assembly process, the kinetics was monitored by following the LSP bands at 740 and 890 nm corresponding, respectively, to the isolated GNRs and coupled particles. As can be noticed in Figure 2, the assembly starts after a short induction period, allowing the chemisorption of cysteine on the gold surface. The extinction changes at 890 nm (corresponding to the plasmon band of coupled particles) were monitored over time after addition of cysteine. As can be seen in Figure 2A, the extinction intensity increases to reach a maximum at ti and gradually decreases, reflecting two distinct assembly regimes. Close examination of the kinetics from 0 min to ti showed that the extinction traces over time intersect in a clean isosbestic point and the assembly rate follows the second-order rate law, in accordance with a dimerization mechanism (see Figure 2B). C


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for Ag+ (log K ≈ 11.9) prevents further cysteine-induced assembly by hydrogen bonding.42,43 In the absence of silver nitrate, the extinction corresponding to the coupled GNRs increases to reach a plateau at 15 min and decreases after 30 min (see Figures 3 and S4). Once the

Figure 3. Extinction changes of cysteine-assisted assembly at 865 nm as a function of time. The assembly process was terminated by addition of silver nitrate at t1 and t2 after cysteine initiated dimerization. t1 = 3 min and t2 = 15 min located by vertical lines with the corresponding extinction spectra recorded immediately (black) and 24 h (red) after addition of silver nitrate.

Figure 2. (A) Extinction changes of cysteine-assisted assembly at 890 nm as a function of time. Before ti, all extinction spectra intersect in a clean isosbestic point. (B) Kinetic analysis of GNRs cysteine-induced assembly based on a second-order rate law. The red line is the best linear fit (correlation factor > 0.99) of the experimental data points to the second-order rate law.

kinetics is characterized, it is possible on the same batch of colloids to modulate the dimers/monomers ratio. The latter is in fact easily modified by adjusting the time at which silver is added during the assembly process. As illustrated in Figure 3, the addition of silver nitrate 3 and 15 min after cysteineinitiated assembly yields stable assemblies in solution for at least 24 h with two different monomers/dimers ratios. The efficiency of the freezing assessed by the stability over time of the extinction spectra after addition of Ag+ is strongly dependent on the concentration of silver ions as compared to the initial cysteine concentration. As illustrated in Figure 4, for a substoichiometric amount of Ag+ (versus cysteine), the GNR oligomerization leads to nonstable dimer assemblies and finally to the complete aggregation of the sample. For stoichiometric and an excess amount of silver, the dimerization is efficiently frozen without further modifications of GNR concentration. Transmission electron microscopy (TEM) images were recorded by drop casting the solution of dimers onto the carbon-coated Cu grid, confirming the formation of GNRs assembly (Figure 5). The two nanorods in each dimer are connected at the tips and form different angles. The gap size between two adjacent GNRs was estimated to be ∼1 nm, which is consistent with both the expected value for a molecular linker featuring two cysteines interacting by hydrogen bonds34,44and

Indeed, assuming that the band at 740 nm is mainly attributed to individual particles, the evolution of absorbance at this wavelength is directly related to GNRs concentration via the Beer−Lambert law (see Supporting Information). From a quantitative point of view, kinetic analysis gives a good fit to a second-order rate law as expected for dimerization process. As depicted in Figure 2B, plotting the integrated second-order rate law (εl(A0 − At)/A0At) versus time yields a linear function (correlation factor > 0.99) whose slope is the kinetic rate constant for the dimerization process. Beyond ti, the assembly rate deviates strongly from the second-order rate law, pointing to the formation of large GNRs aggregates which finally precipitate from the medium as revealed by the extinction decrease at 890 nm. 3.2. Nanorods Assembly Termination. The comparative kinetic studies contributed to optimize the assembly conditions in order to obtain the maximum amount of dimers in solution and locate the characteristic time ti before the irreversible aggregation of GNRs. The immediate termination of the assembly process at the dimer state requires freezing the system just before reaching ti. The approach retained here to stop the GNRs chain growth and stabilize the formed dimers relies on the addition of silver ions. Indeed, the high affinity of cysteine D


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Figure 4. Normalized extinction maximum of GNRs LSP band vs time after addition of various silver nitrate concentration [Ag+]/[cyst]0 = 0, 1/3, 1, and 2. [cyst]0 corresponds to the initial cysteine concentration used to initiate the dimerization process.

Figure 6. (A) Solution-based SERS spectra recorded with 785 nm laser excitation for GNRs before addition of cysteine (black line) and after cysteine-induced GNRs dimerization (red line). (Inset) Magnified view of the SERS spectrum displaying two bands at 663 (b) and 743 cm−1 (c) assigned to C−S stretching mode. (B) Schematic view of cysteine adsorption on metal surface in PH conformation as tridentate ligand and in PN (and PC) conformations through NH3+/thiolate (and COO−/thiolate).45−47

exhibits a strong band at 178 cm−1, commonly assigned to the stretching vibration Au−Br originating from the surface adsorption of bromide (from the CTAB layer) on the gold surface.50,51 Following cysteine-initiated dimerization, a shoulder at 220 cm−1 and two bands at 665 and 743 cm−1 are clearly evidenced on the SERS spectrum. On the basis of previous studies of thiol adsorption on metal surfaces, the shoulder at 220 cm−1 is attributed to the Au−S stretching mode resulting from the chemisorption of cysteine on gold. The absence of the strong and characteristic bands expected for disulfide S−S stretching vibration (close to 510 cm−1) and for thiol S−H (about 2500 cm−1) are clear evidence of the chemisorption of the thiolate form of cysteine on gold. It is worth mentioning that the frequency and intensity of the spectral features in the range 630−760 cm−1 corresponding to ν(C−S) are strongly influenced by the orientation and conformation of cysteine on the surface (see Figure 6B).45−47 The two very characteristic bands at 665 and 743 cm−1 originate from C−S stretching vibrations for the gauche rotamer PH, whereas the less intense band ν(C−S) at 743 cm−1 corresponds to the conformers PC and PN. The cysteine is thus mainly coordinated in a tridentate fashion (PH) on the gold surface, and this particular conformer is not able to form hydrogen bonds with other cysteine. Therefore, the dimerization of GNRs involving hydrogen bonding between two cysteine groups is only possible for the PC and PN rotamers. More quantitative SERS studies are required in order to assess the enhancement for coupled GNRs in comparison to monomers. The weak Raman cross section of cysteine

Figure 5. Transmission electron micrographs of cysteine-initiated assembly of gold nanorods in water at pH ≈ 3.0 after addition of silver nitrate.

the large red shift observed for the additional LSP band of the coupled GNRs (∼150 nm).9,34,40,41 3.3. SERS Analysis on GNRs Dimers. The adsorption of cysteine monolayers on gold and silver surfaces was thoroughly studied by surface-enhanced Raman scattering (SERS) on roughened substrates and randomly aggregated colloids.45−48 Although cysteine-induced GNRs assembly was actively studied, the Raman signature of adsorbed cysteine on gold nanorods has been reported only very recently on PSS-wrapped GNRs polymers.49 To the best of our knowledge, solutionbased SERS studies of cysteine on GNRs dimers have not been described so far. Taking advantage of the stability of GNRs dimers after addition of silver, solution-based surface-enhanced Raman scattering studies were performed using laser excitation at 785 nm to gain more insights into the adsorption of cysteine on gold surface. Figure 6A shows the SERS spectra recorded for GNRs frozen dimers assemblies in solution. Before assembly initiation (in the absence of cysteine), the SERS spectrum E


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The Journal of Physical Chemistry C precludes such quantitative SERS activities comparisons between GNRs monomers and dimers. Therefore, SERS investigations in the presence of 3,3′-diethylthiatricarbocyanine iodide (DTTCI), a better Raman probe than cysteine, were envisaged. DTTCI dye features a strong electronic absorption band at λmax = 765 nm close to the laser excitation used in our SERS studies (λexc = 785 nm), thus operating in resonance Raman conditions (SERRS). Besides, DTTCI was found to partition into the CTAB bilayer with a very high efficiency while maintaining colloidal stability.30,52 Prior to dimer initiation, GNRs stock solution was incubated overnight with a low DTTCI concentration (0.1 μM) and then purified by centrifugation to remove unbound DTTCI remaining in solution. Interestingly, the dye concentration proved to be critical as the high affinity of DTTCI for CTAB tends to saturate the GNRs surface especially on nanorods ends. For dye concentrations above 1 μM, the cysteine-induced assembly of DTTCI-GNRs conjugates is completely inhibited due to complete saturation of GNRs ends. It is noteworthy that given the very low DTTCI dye concentration used for incubation (∼10−7 M before centrifugation of GNRs suspension), the GNRs selected for these SERS studies have their longitudinal LSP band centered at 780 nm. The latter being very close to the laser excitation wavelength used for SERS experiments (785 nm) together with the resonant Raman conditions (DTTCI) maximize the enhancement and thus the signal-to-noise ratio for the measured intensity. In the presence of cysteine, the extinction was monitored to check whether the DTTCI influences the assembly of GNRs. In particular, the extinction at 780 nm (λmax longitudinal plasmon band of GNRs) following the expected second-order rate law and the red-shifted position of the LSP band corresponding to the assembled GNRs are clear evidence of the end to end dimer formation. It is worth mentioning that the kinetics is strongly slowed down compared to the assembly without DTTCI (see Figure S6, Supporting Information). Besides, a longer induction period is required for the chemisorption of cysteine on the gold surface, which is likely due to the DTTCI already occupying a fraction of the gold surface. The solution-based SERRS spectra recorded for the GNRs-DTTCI monomers and dimers with laser excitation at 785 nm are depicted in Figure 7. In the presence of DTTCI, a clear extra enhancement is observed upon dimerization. The SERRS signal is enhanced by a factor of 3, which could seem very low compared to the expectations for hot spots in dimeric plasmonic structures, but this has to be carefully discussed. Indeed, the GNRs samples before and after dimerization have distinct plasmon characteristics in the far field (LSP mode at λmax= 780 and 960 nm for monomer and dimer, respectively). The LSP resonance of the monomer more closely matches the 785 nm laser excitation used for the near field (SERS) than the LSP resonance of the dimer. Besides, as is clearly seen on extinction spectra, the GNRs sample after dimerization is a mixture composed of monomers mainly and dimers. Therefore, the relative enhancement evaluated previously from experimental SERRS data underestimates the benefit of GNRs dimer structures in comparison to isolated GNRs. Using the extinction results it is possible to roughly estimate the proportion of GNRs that are monomers (67%) and those that are involved in clusters such as dimers. The contributions of assembled and isolated GNRs particles could be separated in the recorded SERRS intensity after dimerization (see Supporting Information for more details). We reasoned

Figure 7. Solution-based SERS spectra (excitation at 785 nm) recorded for DTTCI-labeled GNRs before (black line) and after (red line) addition of cysteine. Raman data has been baseline corrected. (Inset) Extinction spectra for the corresponding DTTCI-labeled GNRs solution before (black line) and after addition of cysteine (red line).

that the comparison with the recorded intensity before dimerization requires one to normalize intensities per particle in solution. In this framework, the normalized SERRS intensity per particle is more than 25-fold higher for GNRs dimer compared to GNRs monomer. 3.4. Role of Sample Aging on the Assembly. The aging of the GNRs samples before adding cysteine proved to be critical for the assembly process. In fact, the freshly prepared GNRs samples do not display assembly in the presence of cysteine over time scales of hours. Conversely, our GNRs samples that have been aged for 5−30 days exhibit controlled assembly into dimers in the presence of cysteine within less than 10 min. With colloids being inherently quasi-stable systems, aging is an important parameter. For instance, the influence of aging on nanorods growth has been clearly demonstrated and extensively studied.53−58 Conversely, the influence of nanoparticles aging on their assembly is scarcely studied despite being mentioned in the literature. Indeed, the groups of Pelton for nanorods and Guyot-Sionnest for gold nanobipyramids39,59 mentioned that an aging period of colloidal stock solution was required to observe the end to end assembly induced by alkanedithiol. The well-controlled assembly developed here encouraged us to further explore the influence of aging times of colloids on their assembly in solution. To that purpose, a large quantity of gold nanorods was synthesized to conduct all of the studies on the same batch of colloids. The assembly was initiated using cysteine, and the kinetics were characterized by extinction spectroscopy as described above. Figure 8 depicts the results obtained after 5 and 30 days of aging and using two different acids (HCl and acetic acid, HAc) to adjust the solution pH. The dimerization starts after a short induction period, allowing the chemisorption of cysteine on the gold surface. The rate of dimer formation, reflected by the slope of the linear fit, is faster for a longer aging period. As suggested by Pelton, the aging effect is likely due to the reduction of CTAB coverage at the ends of the rods over time, leading to a higher reactivity toward the anchoring thiol group of cysteine. Interestingly, kinetic analysis on samples from one specific batch and with exactly the same aging process showed that the F


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Figure 8. Kinetic analysis of GNRs cysteine-induced assembly in pH ≈ 3 solution adjusted with HCl or acetic acid and for different aging periods.

nature of the acid (acetic or hydrochloric acid) used to adjust the solution pH is also crucial. The acid influences both the kinetics and the final dimerization yields. In particular, dimerization kinetics is significantly slower in the presence of HAc than HCl for short aging period. In an attempt to evaluate the dimerization efficiency, the extinction changes at 890 nm (corresponding to the longitudinal resonance band of the dimer) were monitored over time after addition of cysteine and exploited as a simple relative estimation of dimerization yield. Given that the initial gold nanorods concentration (fixed optical density) before the assembly was the same for all experiments, we reasoned that plotting the extinction intensity at 890 nm as a function of time would allow the comparison of the dimerization process depending on the nature of acid and the aging period. As can be seen on Figure 9 for all conditions, the extinction intensity increases to reach a maximum at ti and gradually decreases reflecting two distinct assembly regimes (dimerization followed by the oligomeriation−aggregation). The time ti does not depend on the nature of the acid but is strongly influenced by the aging of the nanorods. As discussed in the previous section, close examination of the kinetics from 0 min to ti showed that the extinction traces over time intersect in a clean isosbestic point and the assembly rate follows the second-order rate law, in accordance with a dimerization mechanism. Beyond ti, the assembly rate deviates strongly from the second-order rate law, pointing to the formation of large GNRs aggregates which finally precipitate from the medium as revealed by the extinction decrease at 890 nm. Considering GNRs samples aged for 5 days, the maximum amount of dimers formed in HCl-acidified solution is twice as much as in HAcacidified solution. Conversely, the final amounts of dimers obtained with GNRs samples aged for 30 days are almost identical for HAc and HCl. It was noticed that the dimerization yields in HCl-acidified GNRs solution are barely sensitive to the aging time. The latter is a more critical parameter for HAcacidified solution. Besides its influence on the kinetics, the nature of the acid used to adjust the pH (HCl or HAc) proved to be critical for the termination induced by silver ions. Indeed, the dimer assembly after addition of silver was only stable in acetic acid medium. In the presence of chloride ions provided by hydrochloric acid, the addition of silver even in large excess failed to freeze the assembly process for more than 2 h. This is very likely due to the preferential reaction of silver with

Figure 9. Extinction changes of cysteine-assisted assembly at a wavelength of 890 nm as a function of time for acetic acid (HAc, red) and hydrochloric acid (HCl, black). Kinetics were conducted on gold nanorods of the same synthesis batch but with different aging periods: (A) 5 and (B) 30 days after gold nanorods synthesis.

chloride instead of cysteine with the precipitation of silver chloride as the driving force. Finally, to illustrate the importance of the sample aging in the assembly kinetics, two GNRs samples with distinct aging time and aspect ratios (2.5 and 3.9) were mixed in a 1:1 ratio (see Figure 10A and 10B), and the assembly was monitored by UV−vis spectroscopy. The GNRs with AR = 3.9 and 2.5 are characterized by their longitudinal plasmon resonance bands at 785 and 700 nm, respectively. The kinetic trace obtained by monitoring the extinction maximum at 785 nm (GNRs monomer AR ≈ 3.9) after addition of cysteine in the GNRs mixture is depicted in Figure 10C. The kinetic traces fitted in the second-order rate law model show clearly two consecutive linear regimes. The slopes extracted from the linear fit model (kinetic rate constant) as well as graphical observation unambiguously point to homodimerization. The assembly proceeds gradually starting with the more reactive GNRs (the older sample with AR = 3.9) followed after 20 min by the less reactive GNRs. The selected GNRs have two very distinct aspect ratios, allowing, on one hand, the monitoring of monomer concentration by extinction spectroscopy without interference of the dimers. On the other hand, based on their respective size, the GNRs are easily discriminated by scanning electron microscopy. An aliquot of the colloidal assembly after 30 min was drop casted on a silicon wafer. A typical SEM micrograph (Figure 10D) does not exhibit statistical oligomers but is rather consistent with the homooligomerization of GNRs based on their aging time as evidenced by extinction spectroscopy. G


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Figure 10. (A) Extinction spectra of selected GNRs with two aspect ratios (AR = 3.9 and 2.5). (B) Evolution of extinction spectra over time after addition of cysteine to GNRs mix 1:1. (C) Kinetic analysis of GNRs cysteine-induced assembly for different aging periods and aspect ratio: AR = 3.9 (blue, monitoring at 785 nm) and 2.5 (red, monitoring at 700 nm). Kinetic trace at 785 nm for the 1:1 mixture of GNRs (AR = 3.9 and 2.5) cysteine-induced assembly. (D) SEM micrograph 30 min after addition of cysteine in the 1:1 GNRs mixture.

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4. CONCLUSIONS In summary, we explored metal-induced termination of GNRs end to end assembly in aqueous solution using cysteine as assembling initiator. The fine control of the colloidal assembly kinetics together with the termination using silver ions provided stable GNRs solutions containing dimers and monomers for more than 24 h and up to 6 days with less than 20% loss of GNRs. The influence of gold nanorods aging together with the nature of the acid used to adjust the pH in the medium proved to be critical for the assembly initiation and termination. In this work, we demonstrated how the control of kinetic and thermodynamic processes is a fundamental prerequisite to master the assembly and form stable gold nanorods dimers in solution. The strongly coupled dimer structures as demonstrated by the induced plasmon shift (more than 150 nm) were characterized by SERS, evidencing the chemisoption of cysteine on gold surface and revealing significant enhancement increase for GNRs dimer as compared to GNRs monomer.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the CNRS, the University ParisDiderot and Sorbonne Paris Cité through the funded project ANR-11-IDEX-05-02 (joint project Univ Paris Diderot and IPGP).

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ABBREVIATIONS GNRs, gold nanorods; SERS, surface-enhanced raman scattering; AR, aspect ratio; HAc, acetic acid; LSP, localized surface plasmon; DTTCI, 3,3′-diethylthiatricarbocyanine iodide

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ASSOCIATED CONTENT

REFERENCES

(1) Willets, K. A.; Van Duyne, R. P. Localized Surface Plasmon Resonance Spectroscopy and Sensing. Annu. Rev. Phys. Chem. 2007, 58, 267−297. (2) Kauranen, M.; Zayats, A. V. Nonlinear Plasmonics. Nat. Photonics 2012, 6, 737−748. (3) Alvarez-Puebla, R. A.; Liz-Marzan, L. M. Sers Detection of Small Inorganic Molecules and Ions. Angew. Chem., Int. Ed. 2012, 51, 11214−11223.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07135. Additional MEB images, extinction spectra, and kinetic traces (PDF) H


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The Journal of Physical Chemistry C (4) Bauch, M.; Toma, K.; Toma, M.; Zhang, Q.; Dostalek, J. Plasmon-Enhanced Fluorescence Biosensors: A Review. Plasmonics 2014, 9, 781−799. (5) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111, 3828−3857. (6) Halas, N. J.; Lal, S.; Chang, W.-S.; Link, S.; Nordlander, P. Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev. 2011, 111, 3913−3961. (7) Wei, H.; Xu, H. Hot Spots in Different Metal Nanostructures for Plasmon-Enhanced Raman Spectroscopy. Nanoscale 2013, 5, 10794− 10805. (8) Huang, W.; Qian, W.; Jain, P. K.; El-Sayed, M. A. The Effect of Plasmon Field on the Coherent Lattice Phonon Oscillation in Electron-Beam Fabricated Gold Nanoparticle Pairs. Nano Lett. 2007, 7, 3227−3234. (9) Jain, P. K.; Huang, W.; El-Sayed, M. A. On the Universal Scaling Behavior of the Distance Decay of Plasmon Coupling in Metal Nanoparticle Pairs: A Plasmon Ruler Equation. Nano Lett. 2007, 7, 2080−2088. (10) Ma, W.; Kuang, H.; Xu, L.; Ding, L.; Xu, C.; Wang, L.; Kotov, N. A. Attomolar DNA Detection with Chiral Nanorod Assemblies. Nat. Commun. 2013, 4, 2689. (11) Vigderman, L.; Khanal, B. P.; Zubarev, E. R. Functional Gold Nanorods: Synthesis, Self-Assembly, and Sensing Applications. Adv. Mater. 2012, 24, 4811−4841. (12) Klinkova, A.; Choueiri, R. M.; Kumacheva, E. Self-Assembled Plasmonic Nanostructures. Chem. Soc. Rev. 2014, 43, 3976−3991. (13) Yu, X.; Lei, D. Y.; Amin, F.; Hartmann, R.; Acuna, G. P.; Guerrero-Martinez, A.; Maier, S. A.; Tinnefeld, P.; Carregal-Romero, S.; Parak, W. J. Distance Control in-between Plasmonic Nanoparticles Via Biological and Polymeric Spacers. Nano Today 2013, 8, 480−493. (14) Merk, V.; Kneipp, J.; Leosson, K. Gap Size Reduction and Increased Sers Enhancement in Lithographically Patterned Nanoparticle Arrays by Templated Growth. Adv. Opt. Mater. 2013, 1, 313− 318. (15) Guerrini, L.; Graham, D. Molecularly-Mediated Assemblies of Plasmonic Nanoparticles for Surface-Enhanced Raman Spectroscopy Applications. Chem. Soc. Rev. 2012, 41, 7085−7107. (16) Shao, L.; Woo, K. C.; Chen, H.; Jin, Z.; Wang, J.; Lin, H.-Q. Angle- and Energy-Resolved Plasmon Coupling in Gold Nanorod Dimers. ACS Nano 2010, 4, 3053−3062. (17) Pramod, P.; Thomas, K. G. Plasmon Coupling in Dimers of Au Nanorods. Adv. Mater. 2008, 20, 4300−4305. (18) Shibu Joseph, S. T.; Ipe, B. I.; Pramod, P.; Thomas, K. G. Gold Nanorods to Nanochains: Mechanistic Investigations on Their Longitudinal Assembly Using Alpha,Omega-Alkanedithiols and Interplasmon Coupling. J. Phys. Chem. B 2006, 110, 150−157. (19) Lan, X.; Wang, Q. DNA-Programmed Self-Assembly of Photonic Nanoarchitectures. NPG Asia Mater. 2014, 6, e97. (20) Clark, A. W.; Thompson, D. G.; Graham, D.; Cooper, J. M. Engineering DNA Binding Sites to Assemble and Tune Plasmonic Nanostructures. Adv. Mater. 2014, 26, 4286−4292. (21) Barrow, S. J.; Funston, A. M.; Wei, X.; Mulvaney, P. DNADirected Self-Assembly and Optical Properties of Discrete 1d, 2d and 3d Plasmonic Structures. Nano Today 2013, 8, 138−167. (22) Guerrini, L.; McKenzie, F.; Wark, A. W.; Faulds, K.; Graham, D. Tuning the Interparticle Distance in Nanoparticle Assemblies in Suspension Via DNA-Triplex Formation: Correlation between Plasmonic and Surface-Enhanced Raman Scattering Responses. Chem. Sci. 2012, 3, 2262−2269. (23) Jones, S. T.; Taylor, R. W.; Esteban, R.; Abo-Hamed, E. K.; Bomans, P. H. H.; Sommerdijk, N. A. J. M.; Aizpurua, J.; Baumberg, J. J.; Scherman, O. A. Gold Nanorods with Sub-Nanometer Separation Using Cucurbit N Uril for Sers Applications. Small 2014, 10, 4298− 4303. (24) Liu, X.-L.; Liang, S.; Nan, F.; Yang, Z.-J.; Yu, X.-F.; Zhou, L.; Hao, Z.-H.; Wang, Q.-Q. Solution-Dispersible Au Nanocube Dimers with Greatly Enhanced Two-Photon Luminescence and Sers. Nanoscale 2013, 5, 5368−5374.

(25) Selvakannan, P. R.; Dumas, E.; Dumur, F.; Pechoux, C.; Beaunier, P.; Etcheberry, A.; Secheresse, F.; Remita, H.; Mayer, C. R. Coordination Chemistry Approach for the End-to-End Assembly of Gold Nanorods. J. Colloid Interface Sci. 2010, 349, 93−97. (26) Chan, Y.-T.; Li, S.; Moorefield, C. N.; Wang, P.; Shreiner, C. D.; Newkome, G. R. Self-Assembly, Disassembly, and Reassembly of Gold Nanorods Mediated by Bis(Terpyridine)-Metal Connectivity. Chem. Eur. J. 2010, 16, 4164−4168. (27) Velu, R.; Jung, S.; Won, N.; Im, K.; Kim, S.; Park, N. Fluorescence Enhancement and End-to-End Assembly of Bisacridinedione-Gold Nanorods by Calcium Ions. ChemPhysChem 2012, 13, 3445−3448. (28) Grzelczak, M.; Sanchez-Iglesias, A.; Mezerji, H. H.; Bals, S.; Perez-Juste, J.; Liz-Marzan, L. M. Steric Hindrance Induces Crosslike Self-Assembly of Gold Nanodumbbells. Nano Lett. 2012, 12, 4380− 4384. (29) Li, W.; Kanyo, I.; Kuo, C.-H.; Thanneeru, S.; He, J. PhProgrammable Self-Assembly of Plasmonic Nanoparticles: Hydrophobic Interaction Versus Electrostatic Repulsion. Nanoscale 2015, 7, 956−964. (30) McLintock, A.; Hunt, N.; Wark, A. W. Controlled Side-by-Side Assembly of Gold Nanorods and Dye Molecules into PolymerWrapped Serrs-Active Clusters. Chem. Commun. 2011, 47, 3757−3759. (31) Stewart, A. F.; Lee, A.; Ahmed, A.; Ip, S.; Kumacheva, E.; Walker, G. C. Rational Design for the Controlled Aggregation of Gold Nanorods Via Phospholipid Encapsulation for Enhanced Raman Scattering. ACS Nano 2014, 8, 5462−5467. (32) Nepal, D.; Park, K.; Vaia, R. A. High-Yield Assembly of Soluble and Stable Gold Nanorod Pairs for High-Temperature Plasmonics. Small 2012, 8, 1013−1020. (33) Shanthil, M.; Thomas, R.; Swathi, R. S.; Thomas, K. G. Ag@ Sio2 Core-Shell Nanostructures: Distance-Dependent Plasmon Coupling and Sers Investigation. J. Phys. Chem. Lett. 2012, 3, 1459− 1464. (34) Sun, Z.; Ni, W.; Yang, Z.; Kou, X.; Li, L.; Wang, J. PhControlled Reversible Assembly and Disassembly of Gold Nanorods. Small 2008, 4, 1287−1292. (35) Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. Selective Detection of Cysteine and Glutathione Using Gold Nanorods. J. Am. Chem. Soc. 2005, 127, 6516−6517. (36) Ni, W.; Mosquera, R. A.; Perez-Juste, J.; Liz-Marzan, L. M. Evidence for Hydrogen-Bonding-Directed Assembly of Gold Nanorods in Aqueous Solution. J. Phys. Chem. Lett. 2010, 1, 1181−1185. (37) Ye, X.; Zheng, C.; Chen, J.; Gao, Y.; Murray, C. B. Using Binary Surfactant Mixtures to Simultaneously Improve the Dimensional Tunability and Monodispersity in the Seeded Growth of Gold Nanorods. Nano Lett. 2013, 13, 765−771. (38) Orendorff, C. J.; Murphy, C. J. Quantitation of Metal Content in the Silver-Assisted Growth of Gold Nanorods. J. Phys. Chem. B 2006, 110, 3990−3994. (39) Wang, Y.; DePrince, A. E., III; Gray, S. K.; Lin, X.-M.; Pelton, M. Solvent-Mediated End-to-End Assembly of Gold Nanorods. J. Phys. Chem. Lett. 2010, 1, 2692−2698. (40) Sonnichsen, C.; Reinhard, B. M.; Liphardt, J.; Alivisatos, A. P. A Molecular Ruler Based on Plasmon Coupling of Single Gold and Silver Nanoparticles. Nat. Biotechnol. 2005, 23, 741−745. (41) Reinhard, B. M.; Siu, M.; Agarwal, H.; Alivisatos, A. P.; Liphardt, J. Calibration of Dynamic Molecular Rule Based on Plasmon Coupling between Gold Nanoparticles. Nano Lett. 2005, 5, 2246−2252. (42) Adams, N. W. H.; Kramer, J. R. Potentiometric Determination of Silver Thiolate Formation Constants Using a Ag2s Electrode. Aquat. Geochem. 1999, 5, 1−11. (43) Liu, J.; Sonshine, D. A.; Shervani, S.; Hurt, R. H. Controlled Release of Biologically Active Silver from Nanosilver Surfaces. ACS Nano 2010, 4, 6903−6913. (44) Wu, J.; Lu, X.; Zhu, Q.; Zhao, J.; Shen, Q.; Zhan, L.; Ni, W. Angle-Resolved Plasmonic Properties of Single Gold Nanorod Dimers. Nano-Micro Lett. 2014, 6, 372−380. I


Article

The Journal of Physical Chemistry C (45) Lee, H.; Kim, M. S.; Suh, S. W. Raman-Spectroscopy of SulfurContaining Amino-Acids and Their Derivatives Adsorbed on Silver. J. Raman Spectrosc. 1991, 22, 91−96. (46) Podstawka, E.; Ozaki, Y.; Proniewicz, L. M. Part Iii: SurfaceEnhanced Raman Scattering of Amino Acids and Their Homodipeptide Monolayers Deposited onto Colloidal Gold Surface. Appl. Spectrosc. 2005, 59, 1516−1526. (47) Lopez-Tobar, E.; Hernandez, B.; Ghomi, M.; Sanchez-Cortes, S. Stability of the Disulfide Bond in Cystine Adsorbed on Silver and Gold Nanoparticles as Evidenced by Sers Data. J. Phys. Chem. C 2013, 117, 1531−1537. (48) Zhu, Z.; Liu, W.; Li, Z.; Han, B.; Zhou, Y.; Gao, Y.; Tang, Z. Manipulation of Collective Optical Activity in One-Dimensional Plasmonic Assembly. ACS Nano 2012, 6, 2326−2332. (49) Zhai, D.; Wang, P.; Wang, R.-Y.; Tian, X.; Ji, Y.; Zhao, W.; Wang, L.; Wei, H.; Wu, X.; Zhang, X. Plasmonic Polymers with Strong Chiroptical Response for Sensing Molecular Chirality. Nanoscale 2015, 7, 10690−10698. (50) Ahmad, R.; Boubekeur-Lecaque, L.; Mai, N.; Lau-Truong, S.; Lamouri, A.; Decorse, P.; Galtayries, A.; Pinson, J.; Felidj, N.; Mangeney, C. Tailoring the Surface Chemistry of Gold Nanorods through Au-C/Ag-C Covalent Bonds Using Aryl Diazonium Salts. J. Phys. Chem. C 2014, 118, 19098−19105. (51) Boca, S. C.; Astilean, S. Detoxification of Gold Nanorods by Conjugation with Thiolated Poly(Ethylene Glycol) and Their Assessment as Sers-Active Carriers of Raman Tags. Nanotechnology 2010, 21, 235601. (52) McLintock, A.; Lee, H. J.; Wark, A. W. Stabilized Gold Nanorod-Dye Conjugates with Controlled Resonance Coupling Create Bright Surface-Enhanced Resonance Raman Nanotags. Phys. Chem. Chem. Phys. 2013, 15, 18835−18843. (53) Gole, A.; Murphy, C. J. Seed-Mediated Synthesis of Gold Nanorods: Role of the Size and Nature of the Seed. Chem. Mater. 2004, 16, 3633−3640. (54) Jiang, X. C.; Brioude, A.; Pileni, M. P. Gold Nanorods: Limitations on Their Synthesis and Optical Properties. Colloids Surf., A 2006, 277, 201−206. (55) Kaur, P.; Chudasama, B. Study of Seed Aging in Seed-Mediated Synthesis of Ir Responsive Gold Nanorods. Solid State Physics. AIP Conf. Proc. 2013, 1591, 543−545. (56) Liu, J.; Duggan, J. N.; Morgan, J.; Roberts, C. B. Seed-Mediated Growth and Manipulation of Au Nanorods Via Size-Controlled Synthesis of Au Seeds. J. Nanopart. Res. 2012, 14.10.1007/s11051-0121289-3 (57) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (Nrs) Using Seed-Mediated Growth Method. Chem. Mater. 2003, 15, 1957−1962. (58) Perez-Juste, J.; Liz-Marzan, L. M.; Carnie, S.; Chan, D. Y. C.; Mulvaney, P. Electric-Field-Directed Growth of Gold Nanorods in Aqueous Surfactant Solutions. Adv. Funct. Mater. 2004, 14, 571−579. (59) Malachosky, E. W.; Guyot-Sionnest, P. Gold Bipyramid Nanoparticle Dimers. J. Phys. Chem. C 2014, 118, 6405−6412.

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