Dynamics of Gold Nanoparticle Assembly and Disassembly Induced

Article pubs.acs.org/JPCC

Dynamics of Gold Nanoparticle Assembly and Disassembly Induced by pH Oscillations Hideki Nabika,*,† Tetsuro Oikawa,† Keisuke Iwasaki,‡ Kei Murakoshi,‡ and Kei Unoura† †

Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12, Kojirakawa, Yamagata 990-8560, Japan ‡ Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo, 060-0810, Japan ABSTRACT: Gold nanoparticles protected by carboxylated alkane thiols were coupled with a BrO3−-SO32−/HSO3− pH oscillator which had large-amplitude and long-period oscillations. Reversible, repetitive, and sustainable pH-responsive switching between the assembled and disassembled states of the gold nanoparticles was achieved and examined via simultaneous in situ pH and optical absorption measurements. Spectral changes caused by the assembly/disassembly process revealed that the switch included at least three processes: (1) a rapid assembling process with gradual red-shifts in the localized surface plasmon resonance (LSPR), (2) a rapid disassembling process with gradual blue-shifts in the LSPR, and (3) a slow disassembling process with an isosbestic point at the middle wavelength, whose origins were closely related to the pH change dynamics of each process. The disassembly processes were found to have a delayed response to the pH change to propagate the disassembling process from the periphery to the inside of the assembly, whereas assembly proceeded in a wellsynchronized manner with the pH change. These new findings indicate the importance of the pH oscillation rate as a controlling factor for the assembly/disassembly dynamics of pH-responsive materials.

1. INTRODUCTION Gold nanoparticles are of fundamental and practical interest because their physicochemical properties can be tuned via the modification of their size and shape, the dielectric constant of the medium, and the interparticle distance.1 Among these techniques, controllable switching between the assembled and disassembled state of nanoparticle constructs induced by dynamic changes in the interparticle distance can be used in diverse practical applications such as nanosensors,2−5 bright coloring switches,6,7 switchable catalysts,8 and bioanalytical nanosystems.9−15 Among several approaches allowing the control of assembly/ disassembly switchingincluding temperature jump techniques and DNA addition methodsthe pH jump method offers a simple yet versatile approach. In this method, the addition of a pH modifier (acid or base) causes rapid switching between the assembled and disassembled nanoparticle states, dominantly driven by the protonation/deprotonation switching of surface-bound molecules on the nanoparticles. Such reversible, pH-dependent switching can be achieved simply by the repeated addition of acid and base solutions. However, as the ionic strength is increased during these repeated additions, nanoparticle aggregates are irreversibly formed, making the nanoparticles pH-insensitive.9−11 For example, carboxylated peptide-functionalized gold nanoparticles exhibited large shifts in their optical absorption spectrum from 520 to 600 nm upon the first pH jump.11 However, the nanoparticle assembly was © 2012 American Chemical Society

not completely reversed with the second pH jump, due to the presence of pre-existing ions. Thus, the irreversible formation of aggregates strongly disturbs the utility of repetitive pH jump methods for nanoparticle assembly/disassembly switching. To avoid ion accumulation, the solutions must be dialyzed before each pH jump cycle.9 This drawback is unavoidable and partly offsets the simplicity and versatility of the pH jump method. However, since the problem in the pH jump method lies in the repeated addition of acid and base solutions to induce cyclic pH changes, the realization of cyclic pH change without the need for repetitive acid/base addition would make it possible to maintain the sensitivity of the switching process. Growing interest has been paid to the use of pH oscillators for driving the repetitive switching of pH-responsive materials, due to the feasible and conceptually novel nature of this approach. pH oscillators in continuous stirred-tank reactors (CSTR) enable the continuous exchange of the reaction solution with fresh solution, during which the pH of the reaction solution shows continuous oscillations with amplitudes as large as 4 pH units. Unlike the pH jump method, this technique does not require repeated acid/base additions nor subsequent repetitive dialysis. The use of pH oscillators in diverse functional systems has demonstrated repetitive changes Received: January 19, 2012 Revised: February 15, 2012 Published: February 21, 2012 6153

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in pH-responsive gels, 16−19 polymer brushes, 20,21 drug release,22 salt formation,23 vesicles,24 and DNA.25 Recent studies have successfully demonstrated the ability to induce the repetitive assembly/disassembly switching of pHresponsive gold nanoparticles, using a methylene glycol/sulfite/ gluconolactone (MGSG) pH oscillator.26 The MGSG oscillator is the first example of an organic-based oscillator that is tunable in the pH 7 to 9 range.27,28 In those studies, the gold nanoparticles were protected with 2-fluoro-p-mercaptophenol, which has a pKa value of 8; this falls within the pH range of the MGSG oscillator. When the pH was oscillated between 7 and 9, the nanoparticles exhibited clear and synchronized switching between the assembled and disassembled states. However, the details of the assembly/disassembly process remained unclear for this system, partially because the oscillation period (∼2 min) seemed to be too fast to track the details of the switching dynamics. Furthermore, the use of 2-fluoro-p-mercaptophenol resulted in a comparatively small spectral shift in LSPR between the assembled and disassembled states, indicating relatively larger interparticle distance even in the assembled state when compared with the case when carboxylated alkane thiol was used as the protecting agent.29 This would be a disadvantage when using assembly/disassembly constructs as nanodevices and nanomachines capable of pH-responsive physicochemical switching since changes in the physicochemical properties such as optical, electrical, magnetic, and catalytic properties during the assembly/disassembly process would become more effective when the interparticle distance in the assembled state can be smaller. Although the uses of carboxylated alkane thiol require a pH oscillator with much lower pH range since its pKa is below the MGSG oscillator, it is desirable from a technological viewpoint to use such protecting agents that can influence nanoparticles to assemble in close proximity with each other, which can be characterized by large spectral shifts in the optical absorption of gold nanoparticles. To satisfy these criteria, we show that the use of gold nanoparticles protected with 12-mercaptododecanoic acid (MDA) (pKa = 5) in the BrO3−-SO32−/HSO3− oscillator system (oscillation range of pH 4 to 7 and cycle period of 40 min) results in a time resolution sufficient to be used to evaluate the dynamics of assembly/disassembly switching. The examination of time-dependent spectral changes revealed that there were at least three stages in the assembly/disassembly process but that only the disassembly process displayed a timedelayed response to the pH change. Furthermore, the present system showed spectral shifts up to twice as large as those reported previously because of hydrogen bond formation in the assembled state. These findings offer a deep insight into the assembly/disassembly switching process and dynamics of nanoparticles induced with pH oscillators.

Figure 1. Experimental setup for pH oscillation and in situ measurements.

washes, an aqueous dispersion of MDA-protected gold nanoparticles (MDA-Au) was obtained. 2.2. pH Oscillation Experiments. The experimental setup for the pH oscillation and in situ measurements is shown in Figure 1. In the CSTR reaction system, we used three independent peristaltic pumps that fed KBrO3 (75 mM), a mixture of Na2SO3 (75 mM) and H2SO4 (1.875 mM), and MDA-Au dispersion solutions, each at a constant rate of 200 μL/min. The pH of the reaction solution was monitored in situ with a pH meter (HORIBA, D-52S). The reaction solution was drained with another peristaltic pump, to keep the total volume of the reaction solution constant. The drained solution was directed into a cuvette equipped with temperature and stirring control in a spectrometer (Jasco, FP-6300) to obtain spectral information before being pumped into the waste cell. The optical absorption spectrum between 400 and 800 nm was acquired at intervals of 1 min.30

3. RESULTS AND DISCUSSION Under CSTR conditions, our pH oscillator system displayed large amplitude and large period oscillations, as shown in Figure 2. Typically, the amplitudes were as high as 2.5 pH units, ranging from pH 4.5 to 7.0, and the periods were as long as 40 min. Rapid pH changes were apparent at around 40, 80, 120, 160, and 200 min. Although the limitations of our CSTR system required us to stop the experiments at 240 min, the oscillatory pH behavior should persist infinitely with a continuous flow. The first cycle started at around 40 min, as evidenced by an abrupt pH decrease from 7 to 4.5. The system then recovered to the initial pH and reached near-saturation at 60 min. The processes underlying the pH oscillations were described by the protonation equilibria of SO32− (eqs 1 and 2) and the oxidation of SO32− by BrO3− (eqs 3 and 4)31

2. EXPERIMENTAL SECTION 2.1. Preparation of Gold Nanoparticles. Gold nanoparticles were prepared via the reduction of HAuCl4 with sodium citrate, followed by ligand exchange to MDA.29 Sodium citrate solution (38.8 mM) was added quickly to a boiling aqueous solution of HAuCl4 (1 mM). After initially stirring and heating for 10 min, stirring was continued without heating for 60 min. The obtained solution exhibited a bright red color, indicating the formation of gold nanoparticles. The solution pH was adjusted to 10.5 with NaOH, and a solution of MDA in dichloromethane was added and stirred for 24 h. After the removal of excess MDA with at least two fresh dichloromethane

SO32 − + H+ ↔ HSO3−

(1)

HSO3− + H+ ↔ H2SO3

(2)

3HSO3− + BrO3− → 3SO4 2 − + Br− + 3H+

(3)

3H2SO3 + BrO3− → 3SO4 2 − + Br− + 6H+

(4) +

The oxidation reactions 3 and 4 are autocatalytic in H , which causes rapid H+ production and thus a rapid pH decrease, as observed at 40 min. After the rapid pH decrease, the high proton concentration initiated the oxidation of SO32− to S2O62− by consuming H+ (eq 5) 6SO32 − + BrO3− + 6H+ → 3S2O62 − + Br− + 3H2O (5) 6154

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Figure 2. Time evolution of (top) pH and (bottom) absorption spectra during pH oscillations. The absorption spectra are shown as a contour plot. The color indicates the absorbance; from black (0) to yellow (0.7), via red (0.35).

reversible assembly/disassembly process was clearly synchronized with the pH oscillations and continued through at least 5 cycles, maintaining it repetitive response. Further evidence confirming that the assembled/disassembled state switching was induced by the pH oscillations is given in the time evolution of the absorbance intensities at 530 and 700 nm, corresponding to the disassembled and assembled states, respectively (Figure 3a). The absorbance at 700 nm was used to quantitatively characterize the assembled state because the absorption of the disassembled state was centered at 530 nm and could therefore be neglected at 700 nm. Periodic, synchronized, and negatively correlated oscillations were clearly observed for these two absorbances, indicating conversion between the assembled and disassembled states. Furthermore, characteristic changes in the absorbance intensities appeared at the same times as rapid pH changes at 40, 80, 120, 160, and 200 min, strongly suggesting that the changes in the optical properties, i.e., assembly/disassembly switching, were induced by a strong coupling to the pH oscillations. The gradual, longterm increases in absorbance observed for both wavelengths can be explained by the increased adsorption of MDA-Au with assembled and disassembled states on the cuvette walls during the continuous measurements. A further example of the synchronized switching dynamics is shown in Figure 3b, which depicts the relationship between the first-order derivatives of the pH and the optical absorbance as a function of time. As was expected from the periodicity of the pH (Figure 2) and absorbance (Figure 3a) oscillations, the three derivatized curves also showed periodic features. For clarity, the first oscillation cycle is shown in Figure 3c. The rapid pH decrease appeared as a sharp negative peak centered at 43 min. At the same time, the absorbances at 530 and 700 nm displayed sharp negative and positive peaks, respectively, indicating that the transition from the disassembled to the assembled state proceeded in a synchronous manner with the changes in pH. On the other hand, a large delay was observed between increases in the pH and the resultant absorbance changes. That is, an increase in the pH started at 44 min and reached a maximum at 53 min via a small maximum at 46 min, whereas the corresponding changes in the absorbances began at 48 min

In low-pH conditions, reaction 5 increased the solution pH to the initial state. These reactions proceeded periodically until the reaction components were exhausted, which implies that the pH oscillation proceeded infinitely under the CSTR configuration. Along with in situ pH measurements, we performed in situ optical absorption measurements. The time evolution of the absorption spectrum is shown as a contour plot (Figure 1 bottom), which can be directly compared with the pH oscillations described above. In the initial state, at 25 min, the absorption spectrum showed a maximum absorbance at 530 nm. Since individual gold nanoparticles without aggregation exhibit strong absorption at around these wavelengths because of their LSPR properties, the spectrum for the gold nanoparticles in the initial state indicated that they were in a disassembled state. After maintaining an absorption maximum at 530 nm until 35 min, the maximum shifted toward the red, to wavelengths longer than 600 nm. The transition starting at 35 min evidently corresponded to the pH decrease at ∼35−45 min. Since the pKa of MDA is about 5,29 the MDA on the Au nanoparticles in the initial state at pH 7 existed in a deprotonated and charged state. The surface charges on each Au nanoparticle caused interparticle electrostatic repulsive forces, which kept the nanoparticles in a disassembled state. When the pH dropped below its pKa, MDA transitioned into its protonated form, and the surface charge on the Au nanoparticles was diminished. Furthermore, protonated MDA can potentially participate in intermolecular H-bonding between two carboxylic acid groups bound to two adjacent Au nanoparticles.11 The combination of the decreasing surface charge and the H-bond formation between nanoparticles induced the assembly process, which manifested as the characteristic red-shift seen in the LSPR of the assembled gold nanoparticles. After this red-shift, the absorption maximum returned to its original position at 530 nm, stimulated by the subsequent increases in pH beginning at 45 min. This blue-shift was indicative of the system returning from the assembled state back to its initial disassembled state since the MDA was again deprotonated as the pH increased, and strong interparticle electrostatic repulsion forces recurred. This 6155

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decreased. In contrast, disassembly required a longer time to complete because nanoparticles embedded inside the assembly were initially insensitive to pH changes in the bulk solution. The disassembly started at the periphery of the assembly, where the outermost nanoparticles are deprotonated first. These deprotonated nanoparticles were released from assembly under the influence of increasingly repulsive electrostatic forces. The successive deprotonation and release of subsequent nanoparticles proceeded from the periphery toward the assembly core step by step and gave rise to the time delay in the absorbance changes only for the disassembly process. Spectral features provided additional information on the process of the assembly/disassembly switching, in which each switch was found to be divided into three processes: (1) an assembly process with an optical absorbance red-shift (Figure 4a), (2) a disassembly process with absorbance blue-shift (Figure 4b), and (3) a disassembly process with an isosbestic point (Figure 4c). During process (1), the absorption maximum shifted ∼90 nm from 530 to 620 nm; this shift was almost twice as large as the peak absorbance shifts seen in a previously reported assembly/disassembly switching system coupled with a MGSG pH oscillator.26 An important reason that our system yields such a large shift is partially attributable to the H-bonding ability of MDA that can bridge adjacent Au nanoparticles under low pH conditions,11 which helped to enhance the close contact distance between the nanoparticles and subsequently resulted in large spectral shifts. Contrary to the assembly process which progressed with a monotonic redshift, the disassembly process proceeded via two steps: one was characterized by a blue-shift in the absorption maximum and the other by the presence of an isosbestic point. Process (2) was stimulated by a rapid pH increase at 50 min. Changes in the absorption spectra were opposite to those observed with the rapid pH decrease after 40 min; i.e., the maximum wavelength shifted toward shorter wavelengths (Figure 4b). Process (3) was then observed by following a gradual increase in the absorbance ratio between 530 and 700 nm, with an isosbestic point at 600 nm (Figure 4c). During process (3), the pH was observed to be nearly invariant, as seen in Figure 3c, where dpH/dt ≈ 0 between ∼60 and 75 min, and the slow and delayed disassembly process proceeded as discussed above. The appearance of an isosbestic point in the assembly/disassembly switching of gold nanoparticle systems has been reported in several cases and was explained by a gradual change in the assembly size.13,32 Decreasing the assembly size intensifies the disassembled state absorbance and reduces the assembled state absorbance with an isosbestic point. On the basis of these previous studies, and taking into account differences in the rate

Figure 3. (a) Time evolution of the absorbance intensity at (blue) 530 nm and (red) 700 nm. (b) Derivatized curves of the time dependent (black) pH, (blue) absorbance at 530 nm, and (red) absorbance at 700 nm. (c) Derivatized curves of the first cycle of (b).

and reached their maxima at around 58 min. Even though the assembly process (resulting from the decreasing pH) proceeded as an immediate response to the pH change, additional time was required after the pH increase to complete the disassembly process. Upon the decrease in the pH, the deprotonated MDA species on the Au nanoparticle surface were protonated by protons diffusing in the reaction solution. This process proceeded without delay because there is no energetic barrier for the proton to access the nanoparticle surface. As the protonation process occurred, the interparticle electrostatic repulsive forces instantaneously and drastically decreased, and (with the additional influence of H-bonding capability) the overall interparticle interaction became more heavily attractive as soon as the pH decreased, resulting in a reduction of the average distance between particles. The assembly process therefore proceeded in a single, rapid step when the pH was

Figure 4. Time dependency of the optical absorption spectra divided into three processes: (a) a fast assembly observed at 42−48 min, (b) a fast disassembly observed at 48−58 min, and (c) a slow disassembly observed at 58−76 min. 6156

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potential energy as a function of interparticle distance occasionally reveals bistability where the potential energy curve has two minima at both large and small interparticle distances. The former stabilize the disassembled state, and the latter stabilize the assembled state.33 The ratio of deprotonated MDA species on the Au nanoparticle surface (γ) can be given by γ = 1/(1 + 10pKa‑pH). In the present case with pKa = 5, and oscillations between pH 4.5 and 7, γ oscillates between 0.07 and 0.99. Thus, the MDA on Au nanoparticles transitioned between almost fully protonated and fully deprotonated forms during the pH oscillations. In such a fully alternating system, the total interaction energy oscillates between bistable states at the intermediate pH region. In the bistable pH region, the nanoparticles can adopt either assembled or disassembled states depending on their pH history; i.e., the disassembled state is favored as the pH decreases, whereas the assembled state is favored as the pH increases.33 The pH-dependent bistability in the nanoparticle assembly processes partially explains the observed pH−absorbance hysteresis. Figure 5. Relationship between pH and absorbance at (blue) 530 nm and (red) 700 nm. Schematic illustrations of the nanoparticle assemblies are inset, using gray dots to represent the nanoparticles. Assembled and disassembled states are indicated at low, middle, and high pH regions. The assembly state is different in the middle pH region depending on its pH history.

4. CONCLUSION We observed and provided an explanation for the dynamics of Au nanoparticle assembly/disassembly switching induced by large amplitude, long period pH oscillations. Simultaneous in situ pH and optical measurements enabled us to reveal the presence of three processes in the assembly/disassembly switch, including one assembly and two disassembly processes, not a simple assembly/disassembly two-process switch. Furthermore, whereas the assembly process proceeded in a synchronized manner with the pH oscillations, the disassembly process showed a delayed response to the pH changes. This delay was explained by the fact that it takes a finite time for the disassembly process to propagate from the periphery to the center of the assembly. We suggest the importance of considering the rate of pH change at each step when pH oscillators are used as the driving force for the assembly/ disassembly switching processes of pH-responsive materials for functional nanodevices.

of pH change and the rapid pH change involved in processes (1) and (2), it is likely that the nanoparticles participating in rapid assembly/disassembly switching were those that formed assemblies of inhomogeneous size and shape, resulting in gradual changes in the spectral shape and peak position without an isosbestic point. On the other hand, during process (3) the nearly invariant pH meant that the delayed and slow disassembly process proceeded via a near-equilibrium state, causing a homogeneous change into a disassembled structure throughout the entire solution, resulting in spectral changes with an isosbestic point. It remains important to consider and account for the effect of not just protons but other ions such as Br− or SO32− because the interparticle interactions can be modulated by the presence of such ions. We will investigate this issue by comparing experimental and theoretical results in future reports. However, the present results alone suggest the presence of three distinct processes during the assembly/ disassembly switch including one assembly and two disassembly processesas opposed to a simple assembly/disassembly twoprocess switchfor the pH-responsive gold nanoparticles. We also observed a large hysteresis in the pH−absorbance diagram (Figure 5). The hysteresis was found when monitoring the absorbance at both 530 and 700 nm and during any cycle of oscillations. When assembly and disassembly were executed in reverse sequence with the same process, the pH−absorbance diagram appeared as a simple line with no hysteresis. The large hysteresis observed can be explained by the fact that only disassembly processes showed a time-delayed pH response, as discussed above, which offsets the pH−absorbance curve to a higher pH only for the disassembly process. However, there is one other possibility for the appearance of hysteresis during the oscillation. The classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO) indicates that the assembly and disassembly processes are governed by a subtle interplay between electrostatic and van der Waals forces. Taking into account these two interactions, examination of the interparticle

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AUTHOR INFORMATION

Corresponding Author

*Tel. & Fax: +81-23-628-4589. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Hatakeyama Foundation and Grants-in-Aid for scientific research 22655001 from the Ministry of Education, Science and Culture, Japan.

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REFERENCES

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