Complexity of Gold Nanoparticle Formation Disclosed by Dynamics

Article pubs.acs.org/JPCC

Complexity of Gold Nanoparticle Formation Disclosed by Dynamics Study Christian Engelbrekt, Palle S. Jensen, Karsten H. Sørensen, Jens Ulstrup, and Jingdong Zhang* Department of Chemistry, Building 207, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark S Supporting Information *

ABSTRACT: Although chemically synthesized gold nanoparticles (AuNPs) from gold salt (HAuCl4) are among the most studied nanomaterials, understanding the formation mechanisms is a challenge mainly due to limited dynamics information. A range of in situ methods with down to millisecond (ms) time resolution have been employed in the present report to monitor time-dependent physical and chemical properties in aqueous solution during the chemical synthesis. Chemical synthesis of AuNPs is a reduction process accompanied by release of ions and protons, and formation of solid particles. Dynamic information from redox potential, pH, conductivity, and turbidity of the solution enables distinct observation of reduction and nucleation/growth of AuNPs phases. The dynamics of the electrochemical potential shows that reduction of gold salt (HAuCl4 and its hydrolyzed forms) occurs via intermediate [AuCl2]− to form Au atoms during the early stage of the synthesis process. pH- and conductivitydynamics point further clearly to formation of coating layers on AuNPs and adsorbate exchange between MES and starch.

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reported as an intermediate stage in the AuNP formation.8−11 Recently, in situ X-ray absorption near edge spectroscopy (XANES) and small-angle X-ray scattering (SAXS) have been introduced to study the AuNP formation mechanism using the citrate synthesis.12,13 The time resolution of SAXS has even reached milliseconds.14,20 Using these advanced and highly sensitive physical methods, size and composition of the nanoparticles can be directly obtained and used to understand the process of formation of many nanostructures. Such measurements are, however, based on complicated and expensive instrumentation and facilities such as synchrotrons, thus limiting their general applications. In addition, it is hard to describe the progress of the reduction process in solution solely with X-ray techniques.20 Chemistry involved and processes of nucleation and growth of particles are different from one another, depending on recipes. 21 Matters are further complicated by the inability to transfer mechanisms from one synthesis to another as evidenced by the variety of significantly different mechanisms reported for different syntheses. Understanding the processes and use of mechanistic studies to predict product structures and design synthetic routes for specific properties are therefore needed. In this report, we address the formation mechanisms of AuNPs by monitoring the formation dynamics via physical and chemical parameters during the formation process using a

INTRODUCTION The shape and size, and thus the properties of AuNPs1−3 are crucially determined by the reaction routes of the chemical syntheses of the AuNPs. Understanding the formation mechanism lifts the synthesis from “alchemy” to chemistry. In contrast to the large number of reports on synthesis, characterization, and function of AuNPs,1,4−7 very few studies of AuNP formation mechanisms have, however, been reported.8−16 A main reason is the limited availability of in situ techniques able follow reactions with multiple steps, elevated temperatures, and organic phases. For this reason, simple reactions such as the citrate synthesis of AuNP have predominantly been studied as a model system by various techniques. The synthesis recipe using sodium citrate was developed by Turkevich and his co-workers in 1950s.17 Due to its high reproducibility and uniform AuNPs, it becomes a standard wet method for AuNPs and a subject for many investigations including mechanism study of nucleation, growth, and coagulation.18,19 In spite of a number of works on mechanism study that have been reported,17−19 a satisfactory theory concerning the precise mechanism has not been reached yet due to lack of dynamic information. Ultraviolet−visible light (UV−vis) spectrophotometry and transmission electron microscopy (TEM) are the most widely used techniques showing the characteristic localized surface plasmon resonance (LSPR) band and the structure of the metallic core of the AuNPs, respectively. With a time resolution of seconds to minutes, these two techniques have been used in mechanistic studies where chainlike structures have been © 2013 American Chemical Society

Received: February 22, 2013 Revised: April 30, 2013 Published: May 9, 2013 11818

dx.doi.org/10.1021/jp401883h | J. Phys. Chem. C 2013, 117, 11818−11828

The Journal of Physical Chemistry C

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potential of the gold salt, that is, EAu dominates the solution potential.

variety of in situ techniques. The green saccharide-based approach to metallic nanostructure synthesis (SAMENS) introduced in our previous work22 is chosen due to its mild synthesis conditions and short reaction time even at room temperature. Buffers such as phosphate and 2-(N-morpholino)ethanesulfonic acid (MES) play important roles in the synthesis. Formation of AuNPs in MES solution can even be accomplished at room temperature within 10 min. This synthesis offers a unique opportunity to monitor physical and chemical parameters using affordable and broadly available methodologies in aqueous environment. The equilibrium electrochemical potential of the solution changes during the reaction since redox reactions dominate the AuNP formation process. Protons and chloride ions are, further released during the reaction. pH and conductivity reflect these changes. In addition to observation of the LSPR with UV−vis spectroscopy, changes of the optical properties are reflected by the turbidity of the solution. Time-dependent electrochemical potential, turbidity, pH, and conductivity of the synthesis solution are recorded in our experiments, with time resolution of a second or even milliseconds. Several drastically different trends are consistently discovered in all of these in situ observations. TEM and nanoparticle tracking analysis (NTA) were employed to characterize the Au cores as well as the total size of the AuNPs in solution including both the coating layer and the gold core of the particles. We have focused on two reactions, namely, (1) HAuCl4 in the MES (the OBS recipe) and (2) HAuCl4 with glucose and starch in MES buffered solution (the MOS recipe).22 A plausible mechanistic model with two major phases, each comprising several steps is proposed based on the experimental data.

Figure 1. Redox potentials of reagents used and AuNPs, measured on a BPG electrode at room temperature (21 °C). 2 mM HAuCl4, 10 mM MES (pH 7.0), 10 mM PB (pH 7.2), 10 mM ammonium acetate (AAc) (pH 6.4), 10 mM glucose, 0.6 wt % starch, and as synthesized MOS and OBS AuNPs.

A typical experimental time dependent potential evolution is shown in Figure 2. The potential first rises steeply to point “b”

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RESULTS AND DISCUSSION Potential Dynamic Time Evolution. The formation of AuNPs is a reduction process. The gold salt HAuCl4 with a bright yellow color is reduced to gold atoms which undergo nucleation and growth to form nanoparticles with a deep red color. The overall process can be described by eq 1.4,17,18,23 [AuCl4]− + 3e− ⇌ Au 0 + 4Cl− 0 = 1.00 V vs NHE E Au 3+ /Au 0

(1) Figure 2. Time dependent potential change for OBS. 2 mL of 20 mM HAuCl4 was injected (point “a” = 0 s) to a solution with 2 mL of 0.1 M MES (pH 7.0) and 16 mL of Millipore water at room temperature (20 °C). Final concentrations: 10 mM MES, 2 mM HAuCl4. Magnetic stirring was maintained during the measurement.

The equilibrium potential, E, follows the Nernst equation, eq 2. E = E0 −

RT [red] ln nF [ox]

(2)

(5 s), then increases more slowly until point “c” (35 s), and further with a slope approaching zero, to point “d” (65 s). A very short steep increase is then followed by a fast decay to point “e” (90 s), a slower further decay to point “f” (200 s), and a following faster decay through a shoulder at point “g” (225 s) to a stable level at point “h” (400 s). The potential thus decays twice with a time interval of ca. 100 s, Figure 2. This pattern is much more complex than direct expectations from eqs 1 and 2, meaning that multiple phases with an intermediate rather than a single reduction step24 are involved in the reduction of HAuCl4. To identify the intermediate, we considered many possibilities in the system. Depending on pH, [AuCl3(OH)]−, [AuCl2(OH)2]−, [AuCl(OH)3]−, and [Au(OH)4]− can be formed spontaneously in aqueous solution of HAuCl 4 spontaneously. According to the predominance diagram25 of

where E0 is the standard reduction potential. We use here concentration instead of activity to simplify the description. R is the gas constant, and F the Faraday constant. T is the temperature in Kelvin, and n is the number of electrons involved in the redox reaction. The time-dependent potential reflects the development of the ratio of the oxidized and reduced forms and, therefore, the dynamics of the reduction process. The time dependence of the potential is dominated by the reduction of HAuCl4, which is both significantly different from the potential values of the buffer and other molecules in the solution, and equilibrates much faster. The electrochemical potentials for related chemicals and AuNPs from both the MOS and OBS recipes are measured under conditions similar to the synthesis solution and summarized in Figure 1. Obviously, the 11819



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Au(III)−OH−−Cl−, the dominant Au(III) species apart from [AuCl4]− are [AuCl3(OH)]−, [AuCl2(OH)2]−, [AuCl(OH)3]−, and Au(OH)3 at pH 5−7 with ca. 8 mM Cl− in aqueous solution. At pH 7, the conversion of [AuCl4]− into [AuCl(OH)3]− and [AuCl2(OH)2]− occurs spontaneously in the solution under the experimental conditions presently used. Conversion of [AuCl4]− into [AuCl(OH)3]− and [AuCl2(OH)2]− is pHdependent, and occurs similarly at the same pH controlled by different buffers. At pH 7, controlled by either phosphate buffer (PB) or ammonium acetate (NH4Ac), the intermediate phase does not appear in the potential dynamic curves, Figure 3A.

The corresponding Nernst equations are given in eqs 5 and 6. 0 E = E Au − 3+ /Au+

0 E = E Au − + /Au 0

This experiment suggests strongly that the intermediate which gives an increase of potential in MES buffer cannot be caused by conversion of [AuCl4]− into any of [AuCl3(OH)]−, [AuCl2(OH)2]−, [AuCl(OH)3]−, or [Au(OH)4]−. Based on this analysis, we believe that intermediate is most likely due to formation of Au(I) in the form of [AuCl2]−. Reduction of Au(III) in HAuCl4, which is mixed with [AuCl(OH)3]− and [AuCl2(OH)2]− in the present work, thus proceeds to Au0 as AuNPs via formation of the Au(I) intermediate [AuCl2]−. To simplify the description of equations, HAuCl4 rather than its hydrolyzed forms is used in redox eqs 3−6 and related discussion. Reduction schemes of HAuCl4 to metallic gold via formation of intermediate [AuCl2]− are given in eqs 3 and 4:23,26 [AuCl4 ]− + 2e− ⇌ [AuCl 2]− + 2Cl− (3)

[AuCl 2]− + e− ⇌ Au 0 + 2Cl− 0 E Au = 1.154 V vs NHE + /Au 0

RT [Cl−]2 ln F [[AuCl 2]− ]

(5)

(6)

The reduction route through the consecutive reactions, eqs 3 and 4, with different standard redox potentials implies that at least two steps should appear in the time dependent potential curve as observed, Figure 2. MES solution gives a stable potential at 166 mV, while injection of HAuCl4, defined as time zero; that is, point “a” in Figure 2, drives the solution potential up until point “b” (5 s). From point “a” to “b”, the potential increases mainly due to diffusion of HAuCl4 in the solution facilitated by mechanical stirring, so that the chemical reduction process essentially begins at “b”. This is supported by addition of HAuCl4 to starch and glucose solution without buffer, in PB (pH 7.1), and in ammonium acetate (pH 7.2) all at room temperature, Figure 3B. The potential here increases and reaches a stable value within 900 s. The increase is caused by HAuCl4 but in the absence of MES, HAuCl4 is reduced by glucose, which is a very slow process, and the formation of AuNPs takes several days at room temperature. The potential increases from “b” to “c” and further with a slope approaching 0 at “d” (65 s). The slope of the potential versus time from “b” to “c” is significantly higher than that from “c” to “d”, suggesting two different reduction reactions. A potential increase suggests strongly formation of a new intermediate chemical species with a higher redox potential. By comparison with the possible chemical species in the system and their redox potential values, we believe that by far the most likely intermediate is [AuCl2]−. We assign these two reactions to [AuCl4]− → [AuCl2]− and [AuCl2]− → Au, respectively. As noted in eqs 3 and 4, E0Au+/Au (1.154 V vs NHE) is, however, higher than E0Au3+/Au+ (0.929 V vs NHE). This means that the potential control is shifted from predominantly eq 3 to predominantly eq 4 and that gold atoms begin to form at point “c”. This suggests a potential increase which is consistent with the experimental observations from “c” to “d”. Following the maximum after point “d”, the potential drops, suggesting that the oxidized forms (primarily Au(I)) is depleted as solid Au is formed. The potential starts to decrease at 65 s and reaches the plateau at point “h” after 6 min. Notably, the rate of the potential drop is not uniform, implying that the formation and growth of the AuNPs is itself a dynamic multistep process. Three pronounced potential drop phases are generally observed, that is, a fast one from “d” to “e”, a slow one from “e” to “f”, followed by a second fast phase with a large decay from “f” to “h” through a shoulder at “g”. Figure 2 is based on addition of gold salt into MES solution. Potential dynamic curves for addition of MES into gold solution give a similar result with distinct phases. Freshly formed AuNPs are covered by coating layers that prevent aggregation. It is evident in Figure 4 that this process does not influence the main features of the dynamic potential curves. The OBS recipe produces AuNPs by mixing HAuCl4 with MES, while in the MOS recipe AuNPs are synthesized from mixing HAuCl4 with glucose and starch in MES solution. These two recipes are chosen for comparison because both can be carried out at room temperature and at the same initial pH. The OBS procedure serves as a simplified reference system to

Figure 3. Time-dependent potential changes for the MOS synthesis where MES is exchanged with (A) 2 mL 0.1 M PB (pH 7.1) (solid line), 2 mL 0.1 M NH4Ac (pH 7.2) (dashed line) or (B) water. Inset in (B) shows the extinction spectra after 0, 1, 2, and 7 days.

0 = 0.929 V vs NHE E Au 3+ /Au+

[Cl−]2 [[AuCl 2]− ] RT ln 2F [[AuCl4]− ]

(4) 11820



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Figure 4. Comparison of MOS (top row) and OBS (bottom row). (A and B) Potential dynamic curves, with UV−vis spectra of the synthesized AuNPs inserted. (C and D) TEM images of the metal cores of the AuNPs (scale bars correspond to 100 nm). (E and F) Hydrodynamic size distributions of the AuNPs in solution including coating layers, measured immediately after completion of the synthesis using NTA. Large amounts of AuNPs are below detection limit of NTA in (E). AuNPs are synthesized from 2 mM HAuCl4 + 10 mM MES (pH 7.0) (OBS) or 2 mM HAuCl4 + 10 mM glucose +0.6 wt % starch +10 mM MES (pH 7.0) (MOS).

Table 1. Time Markers and Characteristic Features Observed in the Dynamic Measurements for MOS and OBS at Room Temperaturea phase a−b

b−c

c−d

d−e

e−f

f−g

g−h

MOS

time (s) potential pH conductivity turbidity

0−5 ↑↑↑ ↓↓↓ n.c. n.c.

5−25 ↑↑ ↓↓ ↑↑ n.c.

25−65 ↑ ↓ ↑ ↓↓↓

65−105 ↓↓ ↓ ↑↑ ↓↑

105−175 ↓ (↓) (↑) ↑↑

175−190 ↓↓ ... ... (↓)

190−300 ↓↓ ... n.c. n.c.

OBS

time (s) potential pH conductivity turbidity

0−5 ↑↑↑ ↓↓↓ n.c. n.c.

5−35 ↑↑ ↓↓ ↑↑ n.c.

35−65 ↑ ↓ ↑ ↓↓↓

65−90 ↓↓ ↓↓↓ ↑↑ ↓

90−200 ↓ ↓ ↑ (↑)

200−225 ↓↓ ... ... ...

225−400 ↓↓ ... ↑ ...

a

Addition of HAuCl4 is defined as t = 0. Arrows indicate either increase or decrease (arrow up and down), and the magnitude is indicated by the number of arrows. Arrows in brackets indicate that only a slight change is observed. Triple dots indicate that this step cannot be distinguished from the previous step, and “n.c.” denotes that no change is observed. The time intervals of the final steps of the OBS are not well-defined, indicated by approximate time markers (italics).

be measured precisely by NTA due to the detection limit of the instrument resulting in an inaccurate measurement of the MOS AuNPs, Figure 4E. UV−vis spectra show a well-defined LSPR peak at 523 nm for MOS AuNPs, while a shoulder at 290 nm, a peak at 365 nm, and a LSPR peak at 528 nm with a broad tail at higher wavelengths are found for the OBS AuNPs. In spite of different characteristic features of the AuNPs observed with TEM, UV−vis spectroscopy, and NTA analysis, the potential dynamic curves for OBS and MOS are, however, overall very similar in terms of the shape and amplitude of the different phases in the potential/time variations, Figure 4. The main difference is that the dynamic curve for MOS is smoother than for OBS, particularly around point “d”, “e”, and “f”. In addition, the development from “e” to “f” takes only 70 s for MOS and is very reproducible while the steps of the decreasing phases can

the standard SAMENS recipe (MOS). In the MOS procedure, MES and glucose serve as reducing agents, while starch is the coating agent. In contrast, MES serves as both reducing and stabilizing agent in the OBS. The differences between the OBS and MOS recipes reflect strongly on the AuNPs produced. Monodisperse and uniform 8 ± 2 nm diameter AuNPs are observed with TEM from MOS due to the presence of sterically stabilizing starch, Figure 4C. Less uniform and poorly dispersed nanoparticles, rods and sheets in the size range 5−50 nm are found in the OBS AuNP sample, Figure 4D. Correspondingly, the larger size (40 ± 10 nm) of the OBS AuNPs is reflected in the hydrodynamic AuNP diameter, Figure 4F. The difference between sizes measured with NTA and TEM is mainly attributed to the thickness of coating layers. We notice that AuNPs with a hydrodynamic diameter less than 10 nm cannot 11821



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In summary, the potential dynamic study shows a phase of potential increase and a phase of potential decrease during the synthesis of AuNPs from HAuCl4. The potential increase from ‘a’ to ‘d’ with two different slopes is mainly due to chemical reactions, suggestive of the intermediate [AuCl2]− with the higher equilibrium redox potential compared to [AuCl4]−. The potential decrease phases are assigned to depletion of [AuCl2]− followed by the formation of AuNPs from Au atoms. Fast and slow potential decays followed by a second fast potential drop are discovered for the potential decrease phase. The dynamic time evolution can be probed in a parallel study of the time dependence of pH and conductivity of the solution via protons and chloride or other ions liberated. The time-dependent turbidity and UV−vis LSPR peak development are also monitored and sensitive to the formation of the actual AuNPs. In the following, we present the dynamics of these four parameters, which reflects systematically chemical reactions as well as nucleation and growth of the AuNPs. The Time Evolution of pH. In general, pH was found to decrease during the AuNP synthesis using either MOS or OBS recipe, Figure 6A. pH decays rapidly in the beginning and reaches a stable value after 300 s. Characteristic features for both MOS (solid line) and OBS (dotted line) are indicated by arrows, Figure 6A and summarized in Table 1. The pH dynamic curves for MOS and OBS overlap almost completely from “a” to “c”. The difference between MOS and OBS begins at “c”, increases significantly from “d” to “e” and then reaches an approximately stable value. We cannot identify “f” on the pH dynamic curve, which is observed as a large potential drop on the potential dynamic curve. This is due to the fact that “f” is related to the reduction of the last amount of the gold ions which does not contribute significantly to pH and conductivity. The three main gold species in aqueous solution of HAuCl4 are tetrachloroaurate(III) ([AuCl4]−), the monoaqua-substituted complex ([AuCl3OH2]), and the deprotonated monohydroxo-analogue ([AuCl3OH]−),27 Scheme 1 top row. Among these, [AuCl3OH]− has been found to be the most active species in the reduction reaction.28−30 It is therefore reasonable to suggest that [AuCl3OH]− is the dominating oxidizing Au(III) form which is reduced into Au(I) in the form of [AuCl2]− by two electrons, and further to metallic gold, Scheme 1, middle and bottom rows. Chemical reactions to form of AuNPs are known to be favored in basic solution,22 suggesting that the conversion of [AuCl4]− to [AuCl3OH]− plays a role in triggering the subsequent chemical reactions. We should note that [AuCl4]− rather than [AuCl3OH]− is written

vary a bit between measurements for OBS. This is associated with the absence of a good stabilizing agent and therefore less controlled nucleation and growth and less well-defined distribution of shapes and sizes. The potential increase phases are, however, very similar from “a” to “e” in both MOS and OBS. Time scales of all the phases in the potential dynamic curves are given in Table 1 for both OBS and MOS. The prolonged slow phase indicates that starch as a strong coating agent plays a significant role already from point “e”. This implies that nucleation (initial stage of the AuNPs) starts around point “d”, assuming that the freshly formed gold surface are immediately coated by a starch layer. Figure 5 shows a drastic increase in reaction rate at moderately elevated temperature. Accordingly faster potential

Figure 5. Time dependence of the potentials for the MOS recipe at 60 °C for 900 and 30 s (inset). A water bath is used to control the temperature for the chemical synthesis. The potential is measured via a standard calomel electrode kept at room temperature through a long salt bridge. Corresponding AuNPs are imaged by TEM and presented in Supporting Information SI 1.

changes are observed and the potential dynamic curve becomes narrower. With high time resolution (inset Figure 5), a shape and amplitude of the potential changes similar to that at room temperature (Figure 4A) are observed at 60 °C, but at a much shorter time scale. These experiments suggest that the origin of the potential drops is neither the formation of coating layers nor the reducing agents, but reduction of [AuCl4]− and formation of gold nanostructures.

Figure 6. (A) Time dependent pH evolution for MOS (solid line) and OBS (dotted line). Conditions as in Figure 4. Addition of HAuCl4 defines t = 0 s. (B) Change in pH for OBS AuNPs upon addition of starch (0 s). Start concentration: AuNPs equivalent to 2 mM HAuCl4, 10 mM MES and final concentration after addition of starch solution: AuNPs equivalent to 1.6 mM HAuCl4, 8 mM MES and 0.4 wt % starch. 11822



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Scheme 1. Reduction of Gold Salt in Aqueous Solutiona

Overall the pH dynamic curves accord well with the timedependent potential curves. Most of the features on the pH dynamic curves such as mixing of the solution, chemical reactions via the intermediate [AuCl2]−, and formation of AuNPs appear at almost identical time markers as for the potential dynamic curves, Table 1. However, two differences should be noted. First, no feature can be identified on the pH dynamic curve where the potential of the solution decreases drastically (“f”). Second, a very small pH decrease, that is, “dip”, is observed around at 275 and 400 s for MOS and OBS, respectively. This “dip” is reproducible for both OBS and MOS and appears in the very final growth stage of the AuNPs, regardless of the coating molecule. So far we do not have a clear explanation for this observation. Time Evolution of the Ionic Conductivity. The ionic conductivity of a solution holds the combined transport contribution of ions in the solution including both anions and cations. During the synthesis of AuNPs, anions such as [AuCl4]−, [AuCl2]−, Cl−, and OH− as well as cations, mainly H+, are involved in the chemical reactions, Schemes 1 and 2, and eq 7. The time-dependent conductivity reflects changes of the total ion concentration in the chemical reactions. Starch used in this work contains trace amount of inorganic ions32 and therefore gives an increased baseline conductivity for MOS, Figure 7. The trace ion concentration is, however, constant and

a

Top row: three main gold species, tetrachloro-aurate(III) ion (left), monoaqua-substituted complex (middle), and the deprotonated monohydroxo-analogue (right) are equilibrated in aqueous solution of HAuCl4. Middle row: reduction of Au(III) in the form of [AuCl3OH]− to Au(I) in the form of [AuCl2]−. Bottom row: reduction of [AuCl2]− to metallic gold.

in eqs 3 and 7 to emphasize this species as the dominant Au(III) form. As noted, in this work, the reducing agent is MES or MES and glucose for OBS and MOS, respectively. MES has been used widely in biological/biochemical research due to its inert properties, but MES can donate two electrons and be oxidized by H2O2 to the corresponding amine oxide.31 As a reducing agent MES reacts efficiently with gold salts (such as [AuCl4]−, or [AuCl3OH]−) to form oxidized MES (MESox), while the gold salt is reduced to [AuCl2]− and further to metallic gold. The MES reactions are summarized Scheme 2. Scheme 2. Chemical Reactions of MESa

Figure 7. Time dependent conductivity at room temperature. Solid line, MOS; dotted line, OBS. Conditions as in Figure 4. Addition of HAuCl4 defines t = 0 s.

a

Top: buffer function of MES. Bottom: oxidation of MES to the corresponding amine oxide.

affects neither the conductivity dynamics nor the conclusions. Small ions contribute more strongly to the conductivity than large bulky ones. H+ and Cl− are thus expected to have the largest effect on the measurements. The hydronium concentration remains insignificant in the given pH ranges in terms of conductivity so the conductivity dynamics will reflect largely changes of the Cl− concentration. The feature “e” is slightly delayed in the MOS conductivity dynamics relative to the potential and pH dynamics. Equation 7 suggests that Cl− and H+ are released together in the chemical reaction, with the same development pace of pH and conductivity. The delay in the conductivity must be related to a process different from the chemical reaction, that is, nucleation and desorption. In fact, halogen ions can absorb on metal surfaces and form dense monolayers by chemical bonding.33 Large amounts of Cl− can be immobilized by adsorption on the freshly formed gold surfaces and subsequently released into solution, corresponding to “d” to “e”, when the nuclei grow into AuNPs the surfaces of which are covered by either MES or starch layers. This exchange causes “e” to be delayed on the conductivity curve compared to “e” on the pH dynamic curve, Figure 7. The conductivity reaches a stable value after “g”,

The overall chemical reaction can be represented by eq 7: 2[AuCl4 ]− + 3MES + 6H 2O → 2Au 0 + 3MESox + 8Cl− + 6H+

(7)

+

It is noted from eq 7 that protons (H ) and chloride anions (Cl−) are released during these chemical reactions, resulting in a decay in pH and an increase in ionic conductivity. This expectation is consistent with the experimental observations shown in Figure 6A and supports strongly that the phases “a”−”d” mainly constitute the chemical reactions. MES adsorbs on the freshly formed gold nuclei causing a drop in buffer concentration in OBS. However, the buffer molecules in coating layers are rapidly exchanged with starch in MOS minimizing this effect. Addition of starch solution to the OBS AuNP solution causes a jump in pH, indicative of starch replacing MES as coating agent, Figure 6B. Ultimately less MES is used in the MOS reaction and a higher pH is maintained. From “e”, MES and starch begin to form coating layers on AuNPs. Formation of coating layers around AuNPs is hardly detected by either X-ray techniques or electron microscopic techniques. 11823



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which resembles the pH dynamic measurement. This suggests that all chemical reactions are completed at “d”. Nucleation and growth of AuNPs occurs mainly afterward, that is, from “d”, while formation of coating layers mainly starts from “e”. UV−Vis Spectroscopy and Turbidity. AuNPs have unique optical properties that have stimulated a wealth of interesting investigations since their discovery.27,34−36 Among various optical techniques, UV−vis spectroscopy is the most common and has been employed thoroughly in the synthesis of AuNPs. In this work, UV−vis spectroscopy and turbidity were used for the dynamic study of optical properties to follow the synthesis process and AuNPs formation. The precursor HAuCl4 in water gives a well-defined absorption peak at 310 nm and a strong absorption peak at 224 nm in the UV−vis spectrum. [AuCl2]− has no absorption in the UV−vis range.23 This could be a reason that the presence of intermediate [AuCl2]− has been ignored in literature.4 Figure 8A shows a series of UV−vis spectra obtained at 11 different

The LSPR peak is caused by collective resonant oscillations of the free electrons in the metal and can be described by the socalled dipole approximation of Mie theory.35 AuNPs with a diameter of approximately 3 nm and above give rise to a LSPR peak.37 In general, the lower the wavelength of λmax, the smaller the AuNPs. UV−vis spectroscopy cannot match the time resolution level of the other dynamic studies in this communication, but still provides useful information in a broad wavelength window. The time-dependent extinction for a given wavelength, for example, 890 nm, can be obtained from the same set of data, Figure 8B. The extinction starts to arise around 85 s and reaches a maximum around 150 s followed by a drastic drop around 170 s. The time resolution is, however, poor, and it is difficult to determine the exact time-markers. The reason to investigate the extinction at 890 nm is to relate the UV−vis data to the turbidity measurements employing an infrared diode at 890 nm having much higher time resolution. Turbidity sensors have been used for the detection of particles in a fluid medium such as air or water to monitor the extent of pollution by suspended solid particles. The principle of the turbidity sensor is given in Supporting Information SI 3. A nephelometer is used in which light at 890 nm is emitted, transmitted through an aqueous solution and measured by a detector placed to one side at 90°. Clean water is defined as zero nephelometric turbidity units (NTU) in which only a certain amount of stray light scattered off the glass walls reaches the detector. If a solution contains particles that scatter light, more light reaches the detector and gives a positive turbidity. On the other hand, some of the stray light is blocked and the measurement gives negative values if the solution contains species that absorb light at the diode frequency, cf. Supporting Information SI 4. Nephelometry thus reflects at the same time scattering and adsorption, both of which depend on particle size, shape, concentration and state of aggregation. Time-dependent turbidity curves for the MOS and OBS procedures are shown in Figure 9. Characteristic features corresponding to “a”, “b”, “d”, and “f” are found for MOS, while “a”, “b”, and “d” are found for OBS.

Figure 8. (A) UV−vis spectra at eleven different times during MOS. The spectra have been normalized by baseline shifting to obtain zero absorbance at 1000−1100 nm, and hence, relative extinction is given as ordinates. (B) Extinction at 890 nm recorded with UV−vis versus time, for MOS. The spectra were recorded around 40 s (1), 60 s (2), 85 s (3), 110 s (4), 150 s (5), 170 s (6), 200 s (7), 225 s (8), 250 s (9), 275 s (10), and 460 s (11).

times during the MOS synthesis. UV−vis spectra for OBS are similar and are presented in Supporting Information SI 2. The HAuCl4 peak at 310 nm decays followed by the appearance of the LSPR peak at 553 nm (after spectra 5). Initially the peak is weak and broad but rapidly becomes narrow, is blue−shifted to 523 nm, and grows in maximum intensity (spectra 5−11). The development of the extinction at 890 nm is shown in Figure 8B as this wavelength is used in the turbidity measurements. A further increase in intensity is observed before reaching a stable spectrum after 225 s (spectra 8−11). OBS and MOS show a similar tendency in time-resolved UV−vis spectra. The differences start after spectra 4 where the LSPR band both narrows and is blue-shifted but the OBS starts to show a broad shoulder around 650 nm. This observation agrees well with the tendency of the UV−vis spectra observed for citrate AuNPs.10

Figure 9. Time-dependent turbidity for MOS (solid line) and OBS (dotted line). Synthesis conditions as in Figure 4.

The turbidity is stable from the beginning at “a” and “b”. MOS gives a lower turbidity (solid line) than OBS (dotted line) mainly due to baseline shifting resulting from calibration. This entails that ordinates cannot be compared quantitatively but this has no effect on quantification of the abscissa. The plateau represents the chemical reaction reducing the concentration of gold salt [AuCl4]− and increasing that of [AuCl2]− during which the absorbance of the solution does not change. As the reduction proceeds, individual gold atoms and 11824



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Figure 10. Two-phase mechanism of AuNP formation. In phase 1 (a−d), gold salt is chemically reduced to gold atoms via intermediate [AuCl2]−. In phase 2 (e−h), gold atoms form nuclei and grow to form coated AuNPs. Yellow and red balls represent gold atoms and AuNPs respectively. Light blue and green spheres represent MES and starch coating layers, respectively.

subsequently nuclei are formed. The solution turns completely black, absorbing light strongly and giving a steep drop in turbidity. The turbidity starts to increase again at “d”, as the AuNPs grow, Figure 9. The turbidity dynamic curve correlates well with the UV−vis spectral observations. The initial build-up of a very broad LSPR peak, spectra 3−5 in Figure 8A, corresponds to increased absorbance at 890 nm and the turbidity drop from “c” to “d”, Figure 9. The turbidity increases from “d” to “f” for MOS, Figure 9, which reflects the focusing and blue-shift of the LSPR peak. A slight decrease in the turbidity precedes the final stable value which correlates with the growing intensity of the LSPR peak in the final growth stage, spectra 6−8, Figure 8A and B. Overall, OBS and MOS give a similar time-dependent turbidity pattern, with a stable plateau and steep drop, from “a” to “d”. The main difference between OBS and MOS is that the turbidity increases slowly after “d” for OBS, in contrast to the rapid turbidity increase for MOS. This means that the absorbance at 890 nm is much higher for OBS than MOS. This agrees with UV−vis spectra of the products shown in the insets in Figure 4A and B. The polymorphous character of the OBS product and the tendency to flocculate generate a very broad LSPR stretching toward the infrared spectral region. Observations from the time-dependent turbidity studies are reproducible with subsecond resolution, and consistent with the UV−vis spectral results. The optical dynamic studies confirm that large amounts of gold nuclei are formed between “c” and “e”, which supports strongly the electrochemical dynamic observations. Mechanism. The current dynamic investigations monitor in situ physical and chemical properties of the process with a focus on reactants and chemical intermediates (potential dynamic), side products (pH and conductivity dynamic) and the main product, AuNPs (UV−vis spectroscopy and turbidity dynamics), during the synthesis. Two major phases, that is, reduction of gold salt to gold atoms, phase 1, and nucleation/growth of AuNPs, phase 2, are consistently distinct in these in situ observations. A plausible mechanism is proposed in Figure 10.

The reduction of gold salt to gold atoms consists of two steps, corresponding to “a”−“d” in the dynamic studies. The precursor HAuCl4 forms an active monohydroxoanalogue [AuCl3OH]− by hydrolysis in aqueous solution, which is first reduced into a colorless intermediate, [AuCl2]− by a reducing agent, either MES or glucose. As evident in the current dynamic study, [AuCl2]− triggers the whole nanoparticle formation. Intermediate [AuCl2]− is formed at an early stage of the chemical synthesis, corresponding to “c” in the dynamic curve but is not stable in aqueous environments and is further reduced to gold atom. In fact, X-ray photoelectron spectroscopy (XPS) has detected the presence of Au(I) in photoreduction kinetics of NaAuCl4 followed by final reduction product Au0.38 This strongly supports that formation of gold atoms by reduction of [AuCl4]− is via intermediate [AuCl2]− irrespective of reducing agents, that is, chemicals or photons. Freshly produced gold atoms are active and collide to form nuclei in a very focused time interval, corresponding to “d”−“e” in the dynamic study. The growth of the gold nuclei into AuNPs is observed via the main features “e” to “g”/“h”. MES and starch form thin and thick coating layers for OBS AuNPs and MOS AuNPs, respectively. Due to different coating agents, MOS AuNPs are smaller, more uniform and more stable than OBS AuNPs. For the same concentration of HAuCl4, MOS produce smaller AuNPs at a higher concentration than OBS. Ostwald ripening, in which small crystals or particles dissolve and redeposit onto larger crystals or particles, has also been found in nanoparticle solutions.11,39,40 The growth of large particles can be attributed to atoms received from dissolving small particles. Ostwald ripening, however, occurs at a much longer time scale than the other formation processes in the present systems.

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CONCLUSION Chemical synthesis of AuNPs is a complex system that includes chemical reactions, nucleation of solid nuclei and growth at solid/liquid interfaces. The current dynamic study offers 11825



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comprehensive in situ chemical and physical methods with focus on redox species, solution properties, and products, respectively. The aim has been at obtaining an overview of the whole AuNP synthesis process. Gold salt (HAuCl4 and its hydrolyzed forms) is reduced in two separate steps from Au(III) to Au(I) and further to Au(0), respectively. Toward the end of the second reduction, the concentration of Au0 passes the critical nucleation limit and a burst of nuclei occur, immobilizing large amounts of coating agents on the freshly formed surfaces, that is, MES (OBS) and starch (MOS). The rapid increase in concentration and growth of the nuclei deplete the concentration of Au0 until nucleation stops. The remaining Au0 diffuses onto the now formed AuNPs resulting in slow growth of the AuNPs. The presence of Au(I) as an intermediate during reduction of tetrachloroaurate(III) or its hydrolyzed form to gold atoms is evident in this study. This is also consistent with two recent reports, which suggest that Au(I) is present in two distinct reaction pathways for reduction Au(III) to Au0 in thiolate stabilized AuNP synthesis,41,42 different from the current synthesis recipe. As a perspective, a series of combined in situ methods described in this work enable us to follow a wide range of dynamic processes related to the formation of metallic nanostructures, providing useful information for understanding the mechanisms of formation, optimizing experimental conditions as well as being highly useful in the design of functional materials. Moreover, a large number of reports on AuNPs have shown many unique electronic properties42−45 for size controlled AuNPs. Recently sustainability has been regarded as a design criterion in nanoparticle synthesis.46 In all these respects, the synthesis of AuNPs using green chemistry holds potential application in biotechnology, medicine, and food industry.

Scheme 3. Molecular Structures of MES, Glucose, Tetrachloroaurate(III) and Starch Used in This Work

1.45r. Size analysis was achieved by generating black/white images, automated measurement of particle cross-sectional areas and conversion to diameters assuming sphericity. The equilibrium potential of the solution was measured using a PGSTAT12 potentiostat from Metrohm Autolab B.V. (Netherlands). The potential between the working electrode (WE) and reference electrode (RE) was recorded over time using chronopotentiometry (zero current). A basal plane graphite electrode (BPG) (Pine Instrument Company, Grove City, PA) was used as WE due to inertness toward anions involved in the synthesis reaction. A leakage free Ag/AgCl electrode (Innovative Instruments, Inc., Tampa, FL) or standard calomel electrode (SCE) was used as RE. The potential of Ag/AgCl electrode was calibrated vs SCE after each measurement. All potentials are referred to the normal hydrogen electrode (NHE). A representative homemade setup for in situ monitoring is shown in Figure 11. The

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METHODS Chemicals. Aqueous stock solutions were prepared from HAuCl4·3H2O (99.99%, CAS 16961-25-4, Sigma-Aldrich) and D-(+)-glucose (laboratory use, CAS 50-99-7, Sigma). Soluble starch (pro analysi, CAS 9005-84-9, Merck), 2-(Nmorpholino)ethanesulfonic acid hydrate (99.5%, CAS 443231-9, Sigma-Aldrich), and ammonium acetate (Ultrapure, CAS 631-61-8, Fluka). Phosphate buffer solutions were prepared by mixing K2HPO4 (99.99%, CAS 7758-11-4, Sigma-Aldrich) and KH2PO4 (99.99%, CAS 7778-77-0, Aldrich). pH of the MES solution was adjusted with KOH (99.995%, Merck). Synthesis Process. AuNPs for the dynamics studies were prepared by mixing HAuCl4 with MES (the “only buffer synthesis“(OBS)), or HAuCl4 with glucose and starch in MES solution (the “MES optimized synthesis” (MOS)). The OBS procedure was accomplished by addition of 2 mL 0.1 M MES (pH 7.0) to 16 mL of Millipore water under stirring and subsequently adding 2 mL of 20 mM HAuCl4. The MOS synthesis entails mixing of 2 mL of 0.1 M MES (pH 7.0), 2 mL of 0.1 M glucose, 6 mL of 2.0 wt % starch solution, and 8 mL of Millipore water. After equilibration for a few minutes 2 mL of 20 mM HAuCl4 is added. Instrumentation. UV−vis adsorption spectra were recorded using a 8453 spectrophotometer from Agilent Technologies (Santa Clara, CA). NTA was undertaken using an LM10 instrument from NanoSight (Wiltshire, U.K.) fitted with a red laser. TEM was measured on a Tecnai G2 T20 instrument from FEI Company (Hillsboro, OR) using copper grids with plain carbon films. Treatment of TEM micrographs and statistical analysis was carried out with the freeware ImageJ

Figure 11. Schematic illustration of the electrochemical glass cell with an inlet for injection of solution for the in situ measurements. RE and WE are reference electrode and working electrode, respectively. The RE is accommodated in a separate chamber to avoid interference from AuNPs. The solution is mixed uniformly by magnetic stirring. The entire cell can be kept in a water bath for temperature control.

solution can be injected and mixed uniformly using magnetic stirring. RE and WE are kept in separated chambers. When the solution is heated, a 15 cm salt bridge filled with the same concentration of MES at the same pH as in the synthesis chamber was used to connect the RE chamber with the RE in order to keep the RE at room temperature (21 °C). The basal plane pyrolytic graphite (BPG) electrode was freshly cleaned by mechanical polishing on SiC-papers followed by polishing with 1.0, 0.3, and 0.1 μm Al2O3 slurry on a felt disk and ultrasonicating twice in Millipore water. pH, conductivity, and turbidity were measured using sensors controlled by Logger Lite software via a LabPro interface, all from Vernier Software and Technology (Beaverton, OR). The pH Sensor (PH-BTA) is a glass electrode (Ag/AgCl) which was calibrated with standard pH solutions at pH 4.01, 7.00, and 10.01 (DIN 19266, NIST). The conductivity probe (CONBTA) is a parallel graphite electrode with a cell constant of 1.0 cm−1 and was calibrated using in-house standard KCl and NaCl solutions covering a conductivity range of 718−1990 μS/cm. 11826



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The turbidity sensor (TRB-BTA) is a nephelometer with an LED wavelength of 890 nm and was calibrated with a 100 NTU formazin standard (StablCal, supplied with the sensor) and Millipore water. All measurements were carried out in glassware, which had been freshly cleaned by boiling in 15% nitric acid and rinsed thoroughly with Millipore water. The system was calibrated prior to each measurement. For pH and conductivity dynamics measurements, all components apart from HAuCl4 were mixed in a glass beaker with a stir bar, the sensor immersed into the solution and the system allowed to stabilize under sirring for a few minutes. The measurement was then initiated and recorded when the HAuCl4 solution was added. The dynamics was recorded for up to 30 min. Turbidity dynamics was measured by mixing all components apart from HAuCl4 in the cleaned measurement glass vial containing a stir bar, recording a stable baseline, adding HAuCl4 instantaneously and continue recording for up to 30 min. The turbidity sensor was placed on a magnetic stirrer ensuring homogeneity during measurement.

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ASSOCIATED CONTENT

S Supporting Information *

Further characterization of reference experiments and descriptions of experimental setups. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 Financial support from the Lundbeck (R45-A3878) and Carlsberg Foundation is acknowledged. REFERENCES

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Complexity of Gold Nanoparticle Formation Disclosed by Dynamics

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