Missing Piece of the Mechanism of the Turkevich Method: The Critical

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The missing piece of the mechanism of the Turkevich method: The critical role of citrate protonation Frieder Kettemann, Alexander Birnbaum, Steffen Witte, Maria Wuithschick, Nicola Pinna, Ralph Kraehnert, Klaus Rademann, and Jörg Polte Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01796 • Publication Date (Web): 17 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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The missing piece of the mechanism of the Turkevich method: The critical role of citrate protonation Frieder Kettemann†Δ, Alexander Birnbaum†Δ, Steffen Witte†, Maria Wuithschick†, Nicola Pinna†, Ralph Kraehnert‡, Klaus Rademann† and Jörg Polte*,†

†

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany, ‡Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany Δ Both authors contributed equally

Abstract This contribution investigates the growth mechanism of the Turkevich method. The experimental results provide the important missing piece of the mechanistic puzzle which enables the actual control of particle growth in the common Turkevich method. Applying the gained knowledge, the boundary conditions for a successful Turkevich synthesis are deduced. Moreover, the conditions under which the Turkevich method is highly reproducible are derived. Following these conditions, the Turkevich synthesis is modified to reveal small monodisperse particles with an unprecedented reproducibility of ±0.1 nm.

Introduction

Gold nanoparticles (AuNPs) are one of the most studied nanomaterials due to their unique optical, catalytic and biological properties.1,2 The most common AuNP synthesis in aqueous solution is the reduction of HAuCl4 with trisodium citrate (Na3Cit) at elevated temperatures. The synthesis is commonly denoted as the Turkevich method, named after John Turkevich who described the synthesis in the 1951.3 However, the synthesis was already described by Ernst A. Hauser and J. Edward Lynn in 1940.4 The synthesis yields monodisperse spherical particles which are electrostatically stabilized by nontoxic citrate ions. The AuNPs obtained by the Turkevich synthesis can be easily functionalized making them ideal candidates for biomedical applications.2,5 The synthesis is often claimed to be reproducible.6–8 However, several publications have shown that the Turkevich synthesis is in terms of size not well reproducible.9,10 For example, Schulz et al. and Wuithschick et al. showed in extensive studies that the final particle size (in radius) can vary between 5 to 9 (Schulz et al. ) and 6.5 to 10.5 nm (Wuithschick et al. ) for the apparent same reaction conditions.9,10 Thus, reproducibility still remains a major issue for the Turkevich synthesis. In our recent publications, the growth mechanism of the Turkevich method and corresponding physico-chemical processes were investigated in detail.9,11,12 It was found that the growth mechanism can be described as a seed-mediated growth. The growth mechanism is schematically displayed in Figure 1. Within the first seconds of the synthesis approx. 1-2% of the precursor is reduced (step 1) 1 ACS Paragon Plus Environment

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and particles grow due to coalescence which leads to the formation of stable seed particles with a radius of approx. 1.5 nm (step 2). 9,11,12 The remaining precursor is subsequently attached and reduced in the electric double layer (EDL) of these seed particles (steps 3 and 4). Thus, the seed formation is the size determining step in the Turkevich method: If more seed particles are formed in steps 1 and 2 (e.g. due to a change of reaction conditions), the remaining precursor is distributed on more seed particles in steps 3 and 4 leading to smaller final particles. In contrast, if fewer seed particles are formed in step 1 and 2 more Au precursor is distributed on each seed particle in steps 3 and 4 which results in fewer, yet larger particles. Polte deduced that the number of the initially formed seed particles is determined by two factors: The amount of Au3+ reduced in step 1 and the colloidal stability of the seed particles which determines their minimal stable size.12 Wuithschick et al. found that the colloidal stability is relatively insensitive towards typical parameter changes.9 Wuithschick et al. deduced that this seed-mediated growth mechanism is a fortunate coincidence and the consequence of a favorable interaction of many physico-chemical processes.9 First, only a fraction of the Au3+ precursor is reduced in the initial stage of the synthesis enabling the formation of “few” seed particles. Second, after steps 1 and 2 the reactivity of the reactants (Au3+ complexes and Cit) changes and the reduction of Au3+ does not occur in solution during steps 3 and 4. The remaining precursor is exclusively reduced in the EDL of the seed particles. Overall, this results in a separation of seed particle formation and growth – a seed-mediated growth mechanism. The final AuNPs are monodisperse due to the size focusing effect.13 The origin of the fortunate coincidence is an optimal interplay of chemical equilibrium reactions and redox potentials of the different citrate and gold species.9 It was shown that the addition of Na3Cit to HAuCl4 results in a shift of pH value of the reaction mixture from acidic to neutral conditions. This change in pH value affects the composition of the Au3+ species: While [AuCl4]- is favored in the initial gold precursor solution, [AuCl4-x(OH)x]- (with x ≥ 1) is the dominant species after the pH shift initiated by the addition of Na3Cit. It was shown that the shift of [AuCl4]- to [AuCl4-xOHx]- (with x > 1) initiated by the increasing pH value occurs on the same time scale as the seed particle formation. Thus, Wuithschick et al. concluded that mainly [AuCl4]- is reduced during the formation of seed particles. However, it still remains unclear which Cit species reacts with [AuCl4]- during the seed formation period. Furthermore, it remains uncertain to which extent the speciation of Au3+ determines the reduction in step 1 of the growth mechanism. In this contribution, we present the important missing piece of the mechanistic puzzle which enables the actual control of particle growth. With the profound mechanistic picture, the key to improve the reproducibility of the Turkevich method is deduced. Exemplarily, it is shown how the Turkevich synthesis needs to be modified to produce small monodisperse particles with an unprecedented reproducibility of ±0.1 nm. We structured the article into nine sections. The first two sections investigate the speciation of the precursors. In section 3 to 6 the influences of precursor species on the particle growth in the Turkevich synthesis are studied. In section 7 and 8, the Turkevich synthesis is modified to yield small monodisperse particles with a high reproducibility. In the final section the growth mechanism of the Turkevich synthesis is refined and the boundary conditions for a successful seed-mediated growth are summarized. Moreover, the conditions under which the Turkevich method is highly reproducible are derived.

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Experimental All chemicals were used as purchased without any purification. Ultrapure water (18.2 MΩ cm) was used as a solvent for all reactions. The labware was cleaned after each synthesis using aqua regia and Milli-Q water. The UV-vis spectra of HAuCl4 at certain pH values and Cl- concentrations were obtained by diluting a 10 mM HAuCl4 solution with Milli-Q water and a defined amount of HClO4 or NaOH, and KCl solution (p.a., Carl Roth) to a concentration of 0.125 mM. The solutions were allowed to equilibrate 90 min at room temperature (RT) before measurements. Standard Turkevich synthesis: A volume of 199 ml of a 0.251 mM HAuCl4 (99.9%, Sigma Aldich) was refluxed in a 250 ml round bottom flask for 15 min. One milliliter of a freshly prepared 0.5 M Na3Cit (≥99%, Sigma Aldirch) solution was added quickly (final concentration of Na3Cit 2.5 mM). The reaction mixture was refluxed under continuous stirring for approx. 15 min until a solution with a characteristic ruby red color was obtained. Syntheses at different pH values were performed as described above, with the addition of a certain amount of HCl (p.a., Carl Roth), HClO4 (99.999%, Sigma Aldich) or NaOH (p.a., Carl Roth) to either HAuCl4 or Na3Cit prior to mixing of the reactants. pH adjusted HAuCl4 solutions were allowed to equilibrate for 2 hours. The pH values of the solutions were determined using a pH electrode (Mettler Toledo). Turkevich synthesis with improved reproducibility: 47.2 ml of MilliQ water was refluxed in a 250 ml round bottom flask equipped with a reflux condenser. 2.5 ml of a 10 mM HAuCl4 solution were added and the resulting 0.5 mM HAuCl4 was refluxed for 1 h. 0.3 ml of the 0.5 M reducing agent (which can consist of Na3Cit or a mixture of Na3Cit and H3Cit) were quickly added (final concentration of citrate 3 mM). The reaction mixture was refluxed for approx. 5-10 min until a solution with a characteristic ruby red color was obtained. For the variation of reaction parameters (e.g. final gold concentration or reducing agent concentration) see SI. Instrumentation: UV-vis spectra were measured with an Evolution 220 spectrometer (Thermo Fischer Scientific) using either quartz cuvettes or disposable PMMA cuvettes. The SAXS experiments of the final colloidal solutions were conducted with a lab scale SAXS instrument (SAXSess, Anton Paar GmbH) using a quartz flow cell. Details on the evaluation of SAXS data can be found in the in S1 in the Supporting Information (SI).14 TEM Transition Electron Microscopy (TEM) analysis was carried out using a CM200 LaB6 Philips microscope operated at 200 kV. A few drops of a AuNP solution (containing Polyvinylpyrrolidone MW58000 with a concentration of 40mg/l) were deposited on a copper grid coated with an amorphous carbon film and dried in air.

Results and discussion The possible combinations of [AuCl4]- and Cit species during the seed formation are shown in Figure 1. Path 1 refers to the reduction of [AuCl4]- by fully protonated H3Cit, path 2 to H2Cit-, path 3 to HCit2and path 4 to fully deprotonated Cit3-. Path 1 can a priori be excluded since the fully protonated species does not have free coordination sites which are necessary for the coordination and subsequent reduction of Au3+.15 The understanding of which path (2, 3 or 4) is dominant in the seed formation period enables the deliberate adjustment of the species for a reproducible synthesis.


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Figure 1: Schematic illustration of the growth mechanism of the Turkevich synthesis; during the seed particle formation 0 [AuCl4] reacts with a citrate species (path 1-4) to give Au in solution; after the seed formation [AuCl4] is not present in the reaction mixture and [AuCl4-x(OH)x] complexes are reduced at the surface of the seed particles.

The fortunate coincidence depends on the chemical equilibria of Au3+ speciation and citrate protonation. These two equilibria and their kinetics are strongly dependent on pH value, Clconcentration and temperature.9 The speciation of Au3+ can be described as a ligand exchange and the speciation of Cit as a protonation/deprotonation. Both, pH value and Cl- concentration, change throughout the synthesis. In Eq. 1 to 3 the speciation equilibria of Au3+ and Cit are shown.

AuCl xH O ⇋ AuCl OH H Cl [AuCl4] ⇋ [AuCl3(OH)]- ⇋ [AuCl2(OH)2]- ⇋ [AuCl(OH)3]- ⇋ [Au(OH)4]Cit H ⇋ H Cit -

(1) (2) (3)

At first glance, the speciation of Cit is only dependent on the pH value. However, in case of the Turkevich synthesis it is also dependent on the Cl- concentration because it is coupled with Eq.1. A change of Cl- concentration affects the Au3+ speciation and therefore changes also the pH value. Thus, the speciation of Au3+ and citrate cannot be considered independently of each other. 1. Speciation of Au3+ complexes. The [AuCl4]- complex has a specific absorption maximum at 313 nm which can be assigned to a pσ →5dx²-y² ligand-metal transition.6,16–18 This band can be used to determine the amount of [AuCl4]- in a HAuCl4 solution. For the separate investigation of the influence of pH value and Cl- concentration on the different Au3+ species, the pH value of HAuCl4 solutions was adjusted with HClO4 and the Cl- concentration was adjusted with KCl. In general, the ligand exchange of Au3+ complexes is a rather slow process that proceeds within minutes to hours at RT. Even at 95°C the equilibration of Au3+ complexes takes about 20 seconds.9



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For the investigation of the Cl- influence, UV-vis spectra of a 0.125 mM HAuCl4 solution with different amounts of KCl at several pH values were measured. Figure 2 shows the spectra of the HAuCl4 solution at pH 3 (a) and pH 6 (b). The UV-vis spectra of HAuCl4 solutions adjusted at further pH values can be found in Figure S 1 in the SI. The data suggest that for low pH values, the speciation of Au3+ strongly depends on the presence of Cl-. High Cl- concentrations favor the formation of [AuCl4]-. The addition of KCl to a HAuCl4 solution at pH 6 has only a negligible effect on the [AuCl4]- concentration. Even for high Cl- concentrations of 62 mM, [AuCl4]- is not observed at pH 6. The effect of Cl- is often neglected although HCl is mostly used for the pH adjustment of HAuCl4 solutions which alters both pH and Cl- concentration. In Figure 2c) the spectra of HAuCl4 solutions at pH 2 are shown, once adjusted with HCl and once with HClO4. The resulting spectra show a different absorption at 313 nm. The sample adjusted with HCl contains a higher amount of [AuCl4]-. In contrast to Cl-, ClO4- is a weakly coordinating anion which does not affect the speciation of Au3+.19 With the addition of the corresponding amount of KCl to the HAuCl4 solution with HClO4, the same spectrum as for the sample with HCl can be obtained (see Figure 2c). In summary, the amount of [AuCl4]- in HAuCl4 solutions depends strongly on the Cl- concentration for acidic conditions. For neutral and basic conditions, the Cl- concentration only has a negligible effect on the presence of [AuCl4]- in HAuCl4 solutions. Accordingly, in acidic conditions the pH adjustment of HAuCl4 with HCl results in higher amounts of [AuCl]4- than the addition of the corresponding amount of HClO4.

-

Figure 2: UV-Vis spectra of a 0.125 mM HAuCl4 solution at different pH values (adjusted with HClO4) and with different Cl concentrations; a) HAuCl4 at a pH value of 3 with the addition of KCl; b) HAuCl4 at a pH value of 6 with the addition of KCl, c) HAuCl4 at a pH value of 2 adjusted with (i) HCl, (ii) HClO4 and (iii) HClO4 with KCl to yield similar Cl concentrations.

2. Speciation of Citrate. Trisodium citrate can undergo two processes in the Turkevich synthesis. It can be irreversibly oxidized to i.a. dicarboxy acetone (DCA) and acetone, either thermally or via the reaction with Au3+.3,7,20–25 Additionally, its protonation state can be changed reversibly (see Eq. 3).9 It has been shown that the ligand exchange and the subsequent hydrolysis of the Au3+ complex is a rather slow process compared to the protonation of Cit.9 Thus, after mixing HAuCl4 and Na3Cit in the Turkevich synthesis the protonation of the Cit is equilibrated fast (within mixing reactants), while the speciation of Au3+ is prone to change slower (about 20 s at 100°C).9 In the following sections experiments are described in order to determine which Cit and Au3+ species react during the seed formation (step 1 and 2 in Figure 1). The influence of Au3+ and Cit speciation on the seed formation period is investigated by using pH and Cl- adjusted HAuCl4 and Na3Cit solutions for the Turkevich synthesis. The herein used Turkevich synthesis comprises the reaction of 0.25 mM HAuCl4 with 2.5 mM Na3Cit at 100°C (for details see experimental part) which in the following is denoted as standard Turkevich synthesis.


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3. pH adjustment of HAuCl4 in the Turkevich synthesis. In order to determine the influence of the Au3+ speciation on the seed particle formation step of the standard Turkevich synthesis the pH value and Cl- concentration were adjusted in a HAuCl4 solution prior to mixing with Na3Cit. Several publications report that a mild acidification of HAuCl4 results in smaller final AuNPs.6,9,10,15 It has to be noted that for these previous studies, the pH value of HAuCl4 was adjusted with HCl (which, as shown before, alters the pH value and the Cl- concentration). In solutions adjusted with HCl the initial amount of [AuCl4]- is higher than in solutions which are pH adjusted with HClO4 (see Figure 2c). In order to determine the influence of the [AuCl4]- amount on the seed formation period, the pH value of the HAuCl4 solution was adjusted to 2.9 once with HCl and once with HClO4. The final size of the obtained nanoparticles was determined with SAXS. The corresponding scattering curves and their mathematical fits can be found S 4 in the SI. The use of a HAuCl4 solution with a pH value of 2.9 (either adjusted with HCl or HClO4) for the Turkevich synthesis results in small AuNPs with a radius of about 5.6 nm. This is smaller than the particles that are obtained under the standard conditions (approx. 7.5 nm) in which the HAuCl4 solution has a pH value of about 3.3. Thus, the acidification of the precursor leads to an increased number of initially formed seeds due to a higher reduction rate in the initial phase of the synthesis. However, a significant difference of the final size between the two samples was not observed even though the initial [AuCl4]- concentration is higher in the precursor solution adjusted with HCl. 4. Cl- adjustment of HAuCl4 in the Turkevich synthesis. The addition of acid to a HAuCl4 solution strongly affects the speciation of Au3+ and the protonation of Cit in the reaction mixture. At first glance, the addition of Cl- to the reaction mixture does primarily affect the speciation of Au3+ and should only have a minor effect on the citrate protonation. The increase of the Cl- concentration in a 0.25 mM HAuCl4 solution results in an increased concentration of [AuCl4]- (cf. Eq. 1). Assuming that the seed formation depends strongly on the [AuCl4]- concentration, the addition of NaCl to the precursor solution should result in an increased reduction rate in step 1 and therefore in a decreased final particle size. However, the opposite is observed. The mean particle radius is 10.0 nm ±0.5 nm (at a polydisperity of 13%) if the Turkevich synthesis is performed with the addition of NaCl (final NaCl concentration: 8 mM). The scattering curves can be found in S5 in the SI. This NP size is even larger than expected for the standard synthesis. Previous studies by Zhao et al. confirm that the addition of Cl- to a Turkevich synthesis results in an increasing particle size. They attributed this to a decreased colloidal stability of primary particles which leads to their aggregation to nanowires with subsequent inner particle ripening.26 However, their interpretations are in contrast to the findings of our recent publications.9,11,12,27 The often proposed formation of intermediate nanowires with subsequent inner particle ripening were shown to be nonexistent in solution. Furthermore, we could show that the addition of KCl to AuNPs with a radius of approx. 1.9 nm obtained by the reduction of HAuCl4 with NaBH4 (their final size is comparable to the seed particles size in the Turkevich method) does not lead to a significant decrease of colloidal stability (and thus an aggregation of the seed particles) for KCl concentrations of up to 10 mM.27 Consequently, an aggregation of the seed particles is very unlikely and cannot explain the larger final particles size obtained by a Turkevich synthesis with the addition of NaCl. As shown in Eq. 1, the addition of Cl- to a HAuCl4 solution shifts the Au3+ equilibrium towards [AuCl4]-. In turn, less H+ is released by hydrolysis of the Au3+ complex resulting in a less acidic HAuCl4 solution. This means that the protonation equilibrium of Cit is shifted towards less protonated states by addition of NaCl. Thus, the increase of the final particle size due to NaCl addition indicates that less protonated states of Cit appear to be less favorable for the reduction of [AuCl4]- (because less [AuCl4]6 ACS Paragon Plus Environment


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reduction in step 1 leads to fewer seed particles and therefore larger final particles). As a consequence, the reduction of [AuCl4]- during the seed formation period proceeds via path 2 or 3 in Figure 1. The increased final particle size for syntheses with increased Cl- concentrations shows that the protonation of Cit is a crucial factor for the seed particle formation. An increase in the [AuCl4]concentration alone (favorable for the seed formation) cannot promote the seed formation when the speciation of Cit is simultaneously shifted towards less protonated states. In summary, slight acidifications of the reaction mixture favor the formation of seed particles leading to smaller final particle sizes. In addition, increasing the [AuCl4]- concentration on the expense of higher pH values (as for the addition of Cl- to HAuCl4) hinders the seed particle formation. Therefore, protonated species of Cit (path 2 or 3) seem to play a crucial role for the seed particle formation. 5. pH adjustment of citrate in the Turkevich synthesis. In order to determine if H2Cit-, HCit2- (path 2 or 3) or both are responsible for the reduction of [AuCl4]-, the pH value of Cit was adjusted with HClO4 and NaOH prior to mixing the reactants. The pH adjustment of Cit prior to mixing with HAuCl4 results in a different pH value throughout the synthesis and therefore a different protonation state of Cit compared to the standard synthesis. However, the initial speciation of Au3+ is the same as in the standard synthesis. Thus, the pH adjustment of Na3Cit allows the variation of the Cit protonation without altering the initial speciation of Au3+. In Figure 3a) the molar fraction of citrate species in a 2.5 mM Na3Cit solution (corresponding to the concentration in the standard Turkevich synthesis) are plotted against the pH value (note that in the Turkevich synthesis the pH value is almost constant after reactant mixing). The molar fractions are calculated using the pKa values at RT .15,23 The distribution of Cit species during the Turkevich synthesis will be slightly different than that given in Figure 3 since it is performed at 100°C and not at RT (for which the pKa values are valid). Yet, the distribution at RT can serve as a good estimation of the speciation of Cit in the Turkevich synthesis. HCit2- is the dominant species at mild acidic conditions (approx. 4.8-6.5) while H2Cit- is the dominant species at acidic conditions (approx. 3-4.8). The pH value in a standard Turkevich synthesis is approx. 6.5.


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15,23

Figure 3: a) Speciation of citrate as a function of the pH value, derived from pKa values taken from ; b) final size and polydispersity (determined by SAXS) of AuNPs obtained by the standard Turkevich synthesis with a pH adjusted Na3Cit solution (with HClO4 or NaOH) plotted against the final pH value of the final AuNP solution (measured at RT).

In Figure 3b) the size and polydispersity of final AuNPs (representative scattering curves an TEM images can be found in S6 in SI) obtained from the Turkevich synthesis with pH adjusted Cit solutions are plotted against the final pH value of the AuNP solution (measured at RT). It has to be noted that the pH value during the synthesis at 100°C might differ slightly from the values measured at RT of the final colloidal solution since [AuCl4-x(OH)x]- species are present in solution and the pKa values of Cit and [AuCl4-x(OH)x]- species are temperature dependent. However, the pH of a 0.5 mM HAuCl4 solution remains at around pH 3.08 when heated from 24 °C to 60°C (see S10 in SI). The smallest AuNPs with a radius of about 5.5 nm are obtained for a solution with a pH value of approx. 5.4 (corresponds to a mild acidification of the pure Na3Cit solution). Both, a further decrease and increase of the pH value lead to larger particles. The particle size trend is approx. inversely proportional to the amount of mono-protonated citrate (HCit2-) whereby the polydispersity is low if a substantial amount of HCit2- is in solution or higher if not. The highest amount of HCit2- in the reaction mixture corresponds to smallest final particles (at low polydispersity). Thus, it appears that conditions under which the amount of HCit2- is maximized favor the reduction in the first two growth steps and therefore the formation of seed particles. As a consequence, the seed formation is most likely driven via the reduction of [AuCl4]- by HCit2- (path 3 in Figure 1). For a discussion of the pH influence on the growth mechanism and therefore the final size, the diagram in Figure 3b) is divided into four sections. Section III (final pH between 5.4 and 6.8) corresponds to the classical Turkevich synthesis and the presented growth mechanism. During the seed particle formation period [AuCl4]- and HCit2- are present. The seed particle formation is terminated by the shift of [AuCl4]- to [AuCl4-x(OH)x]- after approx. 20 s. The remaining precursor is then attached and reduced in the EDL of the seed particles resulting in monodisperse AuNPs due to the size focusing effect. 8 ACS Paragon Plus Environment


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In section IV (pH values above approx. 7) the final particles are large and rather polydisperse (polydispersity around 20%). The concentration of HCit2- in the reaction mixture is relatively low at pH values above approx. 7 which results in the formation of few seed particles and large final particles. The increased polydispersity is counterintuitive at first glance, since high pH values should facilitate the shift from [AuCl4]- to [AuCl4-x(OH)x]- and the seed-mediated growth mechanism should be maintained. Yet, pH values above approx. 7 appear to change the growth mechanism in step 3 and 4 of the growth mechanism. Thus, high pH values seem to be unfavorable for the reduction of Au3+ in the EDL of the seed particles. At pH values above approx. 7, Cit3- and [Au(OH)4]- are the dominant species (cf. Figure 4a) and Figure 5).23,28–30 The speciation of either Au3+ and Cit might obstruct the seed-mediated growth mechanism at high pH values. The presence of at least one Cl-ligand at the gold complex might be necessary for the reduction of Au3+ by Cit. This seems to be in accordance with DFT calculations that revealed that the replacement of a Cl- ligand by Cit is the first step in the molecular reduction mechanism of [AuCl4-x(OH)x]- by Na3Cit.15 If [Au(OH)4]- is the predominant Au species in the seed-mediated growth period, the reduction mechanism might change. In section I (final pH ≤3) the obtained AuNPs are large and rather polydisperse (polydispersity around 20%). At these low pH values the concentration of HCit2- is extremely low and at pH values ≤ 3 [AuCl4]- is not shifted to [AuCl4-x(OH)x]-. Both factors lead to an unspecified growth of particles without a well separated step of seed particle formation. The actual fortunate coincidence of the classical Turkevich synthesis does not exist. Section II (pH 3-5.4) represents the transition between the seed-mediated growth mechanism (section III) and a growth mechanism without the separation of seed formation and growth (section I). The particles obtained in section II are still in the size range of the standard Turkevich synthesis (69 nm), but show an increasing polydispersity with decreasing pH. The conditions for a seed particle formation (presence of [AuCl4]- and HCit-) are in principal given for these conditions. However, the seed particle formation is not terminated and small amounts of [AuCl4]- are still present in the reaction mixture after the equilibration of Au3+. Thus, the separation between the seed particle formation and the seed-mediated growth becomes more and more blurry with decreasing pH values resulting in final particles with increased polydispersity. In summary, it could be shown that in steps 1 and 2 of the growth mechanism [AuCl4]- and HCit2react to give the seed particles (path 3), whereby the amount of HCit2- appears to be the more relevant factor for the final size of AuNPs. Furthermore, the seed-mediated growth (step 3 and 4) is only maintained when the pH value of the final solution is between 5.4 and 6.7. Thus, as long as the seed-mediated growth mechanism is maintained (i.e. [AuCl4]- is not present in the reaction mixture after the equilibration of Au3+), the final size of the obtained particles is inversely proportional to the concentration of HCit2- in the reaction mixture. 6. Influence of HCit2- concentration on the Turkevich synthesis. In the previous sections, it could be deduced that the speciation of Cit is a crucial factor for the first two steps of the growth mechanism which determine the final size of the NPs. Figure 4a) shows the estimated concentration of HCit2- in a 2.5 mM Na3Cit solution versus pH value at RT. Over a wide pH range (pH 4-5.4 and pH 6-7.5, red segments) the concentration of HCit2- changes significantly with the pH value. Thus, small changes in the pH value can have a strong influence on the HCit2- concentration in a Na3Cit solution. The conditions for most synthetic protocols of the Turkevich synthesis conditions which have been found empirically over many decades are within the pH range (of about 6.5) in which the concentration of HCit2- is strongly dependent on the pH. 9 ACS Paragon Plus Environment

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2-

Figure 4: a) Molar fraction of monoprotonated citrate (HCit ) plotted against the pH value in a reaction mixture in the Turkevich synthesis after reactant mixing; pH values at which the molar fraction varies strongly (red segments) the synthesis is not reproducible and can yield polydisperse AuNPs; in areas where the distribution is rather constant the synthesis is reproducible (green area); the molar fraction of HCit2- suggests the reproducibility zone is around a pH value of 5.6; b) Size of AuNPs (polydispersity is 10% for all samples) obtained by the Turkevich method with pH adjusted Na3Cit solutions plotted against the final pH value of the solution, the synthesis is found to be reproducible at pH values between approx. 5.4 and 6.

This is most likely a main reason for the lack of reproducibility in these syntheses. Small changes of reactant concentrations (caused by mixing conditions, the evaporation of the solvent, weighing errors etc.) result in a different pH value of the reaction mixture (in almost neutral conditions) under supposedly same reaction conditions. This leads to distinct different HCit2- concentrations which in turn result in different final particle sizes. In order to improve the reproducibility of the Turkevich synthesis the conditions have to be chosen in a way that the pH value during the seed formation period is in a range where the concentration of HCit2- does not change strongly for slight variations of the pH value. As shown in Figure 4a) the areas in which the HCit2- fraction is only slightly dependent on the pH are acidic condition (pH below 4), mild acidic conditions (pH 5.4-6), and basic conditions (pH above 7). Yet, at acidic and basic conditions, the seed-mediated growth mechanism is not maintained (cf. previous section) and a synthesis would result in polydisperse final particles. The theoretical calculations of the Cit speciation suggest the reproducibility zone to be between 5.4 and 6 (see Figure 4a). It has to be noted that these pH values are calculated from pKa values valid at RT whereas the Turkevich synthesis is conducted at elevated temperatures. This suggestion is in accordance with the pH dependent synthesis (see Figure 4b). For a pH of about 5.9 the final particle size is very reproducible. To achieve a pH value in the reproducibility zone for the Turkevich synthesis several approaches are possible. For example the pH value of the Na3Cit or HAuCl4 solution can be adjusted prior mixing as described in the previous sections by the addition of HClO4 or with HCl as described by Schulz et al.10 In summary, a highly reproducible synthesis of AuNPs appears to be achieved when the final pH value of the reaction mixture is between 5.4 and 6. This can be achieved by (i) the addition of acid to Na3Cit, (ii) an appropriate mixture H3Cit and Na3Cit or (iii) a certain ratio of Na3Cit and HAuCl4 (with a lower Na3Cit concentration than generally used).



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7. Influence of gold precursor aging on reproducibility. As derived in the previous sections, a highly reproducible Turkevich synthesis demands the presence of [AuCl4]- in the HAuCl4 precursor solution (i.e. pH value ≤ 5) and a pH value of the final solution which maximizes the HCit2- concentration. From the pH dependent speciation of citrate (see Figure 3a) this region is approx. between 5.4 and 6. It is deduced that this can be achieved by acidification of the HAuCl4 or Na3Cit solution. As a more practical approach, the pH value of the reaction mixture might also be controlled by adjusting the ratio of the acidic HAuCl4 precursor solution and the basic Na3Cit. The synthesis using a HAuCl4 concentration of 0.5 mM with a Na3Cit concentration of 2.5 mM has a final pH value of around 6. Consequently, this modified synthesis would meet the beforehand deduced criteria for a highly reproducible synthesis. Indeed, this synthesis reveals monodisperse particles (polydispersity of 10%) having a mean radius of 5.5 nm with an unprecedented reproducibility of ±0.1 nm. The reproducibility of this modified Turkevich synthesis was tested by performing the synthesis several times with different glassware and different stock solutions (see S 6 in the SI). However, for these experiments a relatively old (4-8 month old) 10 mM stock solution (purchased from Sigma Aldrich) was used. Repeating this synthesis with a freshly prepared stock solution from a different HAuCl4 batch (also purchased from Sigma Aldrich) revealed a different final size. Therefore, this procedure was repeated for three different gold batches (also from different suppliers) as well as for a different aging of the stock solution (the detailed description can be found in S8 in SI). Although the size reproducibility for syntheses with a certain stock solution was still relatively good (in maximum ± 0.3 nm), the final particle size differed for every different gold stock solution between 5.5 and 7 nm. For these syntheses the gold precursor was refluxed for 15 min before citrate addition. Since an influence of the citrate solution is very unlikely (a longer description can be found in S9 in SI) the gold precursor solutions seem to be a further key to reproducibility. The gold precursor solution can comprise of different gold complexes depending on the pH or temperature. These include the gold species with a ligand exchange of Cl- by OH- (i.e [AuCl4-x(OH)x]-) or by H20 (i.e. [AuClx(OH)y(H20)z with x+y+z=4). In reproducible syntheses it can be assumed that the composition of the gold complexes is approximately the same. Although, it is very difficult to determine experimentally the exact speciation of gold precursor solutions, the same composition of gold species should show the same UV-vis spectra. Therefore, we prepared a large set of precursor solutions with different age, temperature, temperature treatments and measured their absorption spectra (see S10 in SI). It is found that the UV-vis spectra of 0.5 mM HAuCl4 solutions: (i) are different at RT if heated previously to different temperatures (from 25 °C to 100°C); (ii) are different at RT if refluxed for different durations; (iii) are different at RT if heated to 100°C and refluxed for a certain duration or if the stock solution is added to boiling water and the obtained gold solution is then refluxed for a certain duration; and (iv) changes with time at 80°C. Furthermore, HAuCl4 solutions with different concentrations (0.16mM, 0.5mM, 1mM), from different HAuCl4 batches and different chemical suppliers (Alfa Aeasar and Sigma Aldrich) have only shown a slight difference at RT if the precursor was previously not heated. As a consequence, the actual composition does not only depend on the pH and the temperature but also on precursor age as well as the kind and duration of temperature treatments. It appears as if there are several entropic barriers for the ligand exchanges. However, the actual differences between the UV-vis spectra of the heated precursor solutions differentiate less if boiling durations are longer than 30 min and if the stock solution is added to the boiling water. Thus, the Turkevich synthesis (0.5 mM HAuCl4, 3 mM Na3Cit) was conducted with a refined synthetic procedure and with different precursors (i.e. different suppliers, purity and age). For the refined 11 ACS Paragon Plus Environment

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synthesis 2.5 ml of 10 mM HAuCl4 stock solution was added to 47.2 ml of boiling Milli-Q water. The boiling gold precursor solution was then refluxed for 1h or 15h prior addition of 0.3 ml of 0.5 M Na3Cit solution. The mean final size derived from 9 syntheses made with different gold precursors is 6.4 nm with a standard deviation of ±0.25 nm (for details see S11 in SI). A significant difference of the final size between syntheses with gold precursor refluxed for 1h or 15h was not observed. Hence, the differences of the gold precursor speciation at boiling temperatures can be decreased by a relatively long boiling duration of about 1h which leads to a relatively high reproducibility of ±0.25 nm. 8. Synthesis protocol for the Turkevich method with improved reproducibility. As derived in the previous sections, maximizing the HCit2- concentration and boiling the gold precursor solution for around 1h improve the reproducibility of the Turkevich method. Concerning the citrate protonation (see Figure 3a) it can be deduced that HCit2- will have its maximum in a pH range of 5.4-6. Thus, for optimal conditions, a pH dependent reproducibility study was conducted. However, at an pH ≤ 5.4 the concentration of [AuCl4]- is not effectively 0. The presence of [AuCl4]- throughout the synthesis interferes with an optimal separation of the seed particle formation since it can lead to the formation of new particles. As a consequence, the polydispersity of the final particles increases as can be seen in Figure 3b for pH values below 5.4. Indeed, the improved synthetic protocols should be robust against slight changes of reaction conditions (e.g. due to weighing errors). Therefore, the study was restricted to the pH range of 5.6 -6. For the pH dependent reproducibility study, syntheses were conducted with a gold precursor / citrate ratio of 1:6 and 1:12 and the gold precursor was refluxed for 1h. The pH was adjusted by an appropriate ratio of citric acid and sodium citrate. For a certain parameter set, the synthesis was repeated 9 times with at least three different gold stock solutions. For all results and details of the synthetic procedure see S12 in SI. It is found that at pH 5.6 the synthesis is highly reproducible with the lowest standard deviation of 0.1 nm for the 1:6 ratio and 0.15 nm for the 1:12 ratio. Hence, the herein found optimal conditions comprise (i) 1h boiling of gold precursor solution and (ii) a pH of 5.6 for the reaction mixture adjusted with an appropriate H3Cit/Na3Cit ratio. These conditions were subsequently applied for different gold and citrate concentrations. For this study the HAuCl4/citrate ratio is restricted to 1:6 and 1:12. A further increase of citrate concentration did not show a significant improvement (see S13 in SI). In Table 1 and Table 2 the H3Cit:Na3Cit ratio to obtain a pH of 5.6 for different HAuCl4 to Cit ratios is displayed. Furthermore, the final particle size and polydispersity with the corresponding standard deviation are displayed. Each data point is based on at least 6 syntheses with three different stock solutions. The results of the individual experiments can be found in S14 in SI. Table 1: Mean radius and polydispersity of Au NPs obtained by the improved synthesis protocol using a HAuCl4:Cit ratio of 1:6; the final pH value of the colloidal solution was adjusted to approx. 5.6 using a mixture of H3Cit and Na3Cit in the given ratio as the reducing agent.

final c(Au0) [mM]

ratio H3Cit:Na3Cit

0.125 0.25 0.5 0.75 1

0.59:5.41 0.55:5.45 0.55:5.45 0.49:5.51 0.41:5.59

mean radius [nm] 7.7 7.3 6.2 5.7 5.8

polydispersity 10% 10% 10% 10% 14%

standard deviation [nm] 0.5 0.2 0.1 0.15 0.25 12

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Table 2: Mean radius and polydispersity of Au NPs obtained by the improved synthesis protocol using a HAuCl4:Cit ratio of 1:12; the final pH value of the colloidal solution was adjusted to approx. 5.6 using a mixture of H3Cit and Na3Cit in the given ratio as the reducing agent.

final c(Au0) [mM]

ratio H3Cit:Na3Cit

0.125 0.25 0.5 0.75

1.1:4.9 1.08:4.92 1:5 1:5

mean radius [nm] 7.9 6.3 5.6 5.5

polydispersity 10% 10% 10% 14%

standard deviation [nm] 0.6 0.3 0.15 0.2

9. Refining the fortunate coincidence of the Turkevich synthesis. The results of this contribution allow a refinement of the growth mechanism of AuNPs in the Turkevich synthesis. Summarizing, the fortunate coincidence of the Turkevich synthesis can be described as follows. The initial pH value of the used HAuCl4 solutions are approx. between 3 and 3.6. At these pH values a substantial amount of [AuCl4]- is present in solution. By the addition of Na3Cit the pH value of the reaction mixture changes to more or less neutral conditions due to Cit protonation. The Cit protonation equilibrates almost instantly to give a certain amount of HCit2- while the shift of [AuCl4]- to [AuCl4-x(OH)x]- (with x = 1-3) takes about 20 s. During this time, both [AuCl4]- and HCit2- are present in the reaction mixture and react to give Au0. This reduction results in the formation of the seed particles mainly due to coalescence. The initial reduction is ultimately terminated when the remaining gold precursor is converted completely to [AuCl4-x(OH)x]- (with x = 1-3). Thereby the pH value of the reaction mixture is between 5.4 and 6.8. Thus, the growth mechanism corresponds to a seed-mediated growth if (i) [AuCl4]- and HCit2- are present after mixing HAuCl4 and Na3Cit solutions; (ii) all remaining Au precursor is converted to [AuCl4-x(OH)x]- (with x = 1-3) within few seconds; and (iii) the pH value of the final solution is between 5.4 and 6.8 (measured at RT). If the reaction conditions do not meet these requirements, the seedmediated growth mechanism is not maintained and polydisperse particles result from the synthesis.

Figure 5: Speciation of a 0.25 mM HAuCl4 solution in dependence of the pH value; the yellow area represents the demanded pH range of the initial HAuCl4 solution (approx. 2.6-4). This pH range ensures the presence of [AuCl4] during


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the seed particle formation; the blue area (approx. 5.5-7) represents the demanded pH range of the reaction mixture 2(after addition of Na3Cit to HAuCl4 or vice versa). This pH range ensures the presence of HCit during the seed particle formation and that all remaining [AuCl4]- is shifted to [AuCl4-x(OH)x]- (with x=1-3). The green area represents the pH range 2of the final reactant mixture (5.8-6.1) in which the synthesis is highly reproducible. In this pH range, the HCit concentration is maximized during the seed-formation.

Figure 5 shows the pH dependent speciation of a HAuCl4 solution in typical concentrations used for the Turkevich synthesis.28 The colored areas indicate the pH ranges that are required for a seedmediated growth mechanism (to obtain monodisperse AuNPs). The yellow area represents the demanded pH range of the initial HAuCl4 solution (approx. 2.6-5). This pH range ensures the presence of [AuCl4]- during the seed particle formation (see molar fraction of [AuCl4]- in Figure 1Figure 5 and S15 in SI). The blue area (approx. 5.4-6.8) represents the demanded pH range of reaction mixture (after addition of Na3Cit to HAuCl4 or vice versa). This pH range ensures the presence of HCit2- during the seed particle formation and that all remaining [AuCl4]- is shifted to [AuCl4-x(OH)x]- (with x=1-3). However, a highly reproducible synthesis is only possible when the HCit2- concentration is maximized (c.f. Figure 4). A reproducible synthesis (±0.3 nm) is achieved with a final pH value between 5.5 and 6 (light green area in Figure 5) and if the gold precursor is refluxed for at least 1 h prior to synthesis. However, a high reproducibility (± 0.1 nm) is only achieved for a final pH value of 5.6 (adjusted with a mixture of H3Cit and Na3Cit). This is indicated by the dark green line in Figure 5. In summary, the requirements for a Turkevich synthesis that follows the seed-mediated growth mechanism and yields monodisperse NPs are the presence of [AuCl4]- and HCit2- in the seed formation period and a pH value between 5.4 and 6.8 during the seeded growth period in order to shift the Au3+ equilibrium towards [AuCl4-x(OH)x]- (with x=1-3). The obtained particles with a radius of 5.5 nm represent approximately the lower size limit of AuNPs accessible using only HAuCl4 and citrate. A further decrease of the AuNP size in the Turkevich synthesis is difficult to achieve since the Au3+ speciation and Cit protonation cannot be altered independently of each other. Additionally, a further substantial decrease of the final particle size (i.e. radii below 4 nm) will most likely be only possible with an increase of polydispersity. Polte et al. showed that the seed particles (formed in step 1 and 2) are rather polydisperse (polydispersity around 50%).11,31 Only through the seed-mediated growth in step 3 and 4 (due to the size focusing effect) the polydispersity decreases and the particles become monodisperse at sizes of about 4-5 nm in radius (cf. Figure 2a) in31 or Figure 2 d) and f) in 11).

Conclusion In this contribution the growth mechanism of the common Turkevich synthesis is decisively refined and the requirements for the proper interaction of several physico-chemical processes necessary to obtain monodisperse AuNPs are enlightened. It is shown that the seed particles in the Turkevich synthesis result exclusively from the reduction of [AuCl4]- by HCit2- whereby the reduction rate is mainly determined by the amount of HCit2- in solution. The varying amount of HCit2- under supposedly same reaction conditions in the standard Turkevich synthesis is the main reason for its low reproducibility. Therefore, the synthesis is reproducible if the HCit2- concentration is relatively insensitive to slight changes of the pH value. This is found to be the case for the pH range of 5.5 to 6 of the final reaction mixture whereby the highest reproducibility is at pH 5.6. The according pH of the reaction mixture can easily be achieved by a proper ratio of HAuCl4 and Na3Cit solutions or by a proper ratio of Na3Cit and H3Cit. However, slight changes of the gold precursor speciation are also 14 ACS Paragon Plus Environment


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shown to affect the reproducibility which can be minimized by refluxing the gold precursor for 1h prior citrate addition. With this profound mechanistic understanding, the Turkevich synthesis was modified to produce monodisperse AuNPs (polydispersity around 10%) with mean radii between 5.5 nm and 7.3 at unprecedented reproducibilities between ±0.1 nm and ±0.25 nm.

Acknowledgement J.P. acknowledges generous funding by the Deutsche Forschungsgemeinschaft under the project PO 1744/1-1. F.K. acknowledges support by the IMPRS “Functional Interfaces in Physics and Chemistry”. RK thanks in particular Einstein Foundation Berlin for generous support provided by an EinsteinJunior-Fellowship (EJF-2011-95). RK also acknowledges funding from BMBF (FKZ 03EK3009). Supporting Information Available: Details on the evaluation of SAXS data; additional SAXS scattering curves and TEM images; additional data of the reproducibility studies; UV-vis studies on the speciation and aging of HAuCl4. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (2) (3) (4) (5) (6) (7)

(8)

(9)

(10)

Zhao, P.; Li, N.; Astruc, D. State of the Art in Gold Nanoparticle Synthesis. Coord. Chem. Rev. 2013, 257, 638–665. Zeng, S.; Yong, K.-T.; Roy, I.; Dinh, X.-Q.; Yu, X.; Luan, F. A Review on Functionalized Gold Nanoparticles for Biosensing Applications. Plasmonics 2011, 6, 491–506. Turkevich, J.; Stevenson, P. C.; Hillier, J. A Study of the Nucleation and Growth Processes in the Synthesis of Colloidal Gold. Discuss. Faraday Soc. 1951, 11, 55–75. Hauser, E. A.; Lynn, J. E. Experiments in Colloid Chemistry; Mcgraw-Hill, 1940. De, M.; Ghosh, P. S.; Rotello, V. M. Applications of Nanoparticles in Biology. Adv. Mater. 2008, 20, 4225–4241. Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X. Size Control of Gold Nanocrystals in Citrate Reduction: The Third Role of Citrate. J. Am. Chem. Soc. 2007, 129, 13939–13948. Ojea-Jiménez, I.; Bastús, N. G.; Puntes, V. Influence of the Sequence of the Reagents Addition in the Citrate-Mediated Synthesis of Gold Nanoparticles. J. Phys. Chem. C 2011, 115, 15752– 15757. Zabetakis, K.; Ghann, W. E.; Kumar, S.; Daniel, M.-C. Effect of High Gold Salt Concentrations on the Size and Polydispersity of Gold Nanoparticles Prepared by an Extended Turkevich–Frens Method. Gold Bull. 2012, 45, 203–211. Wuithschick, M.; Birnbaum, A.; Witte, S.; Sztucki, M.; Vainio, U.; Pinna, N.; Rademann, K.; Emmerling, F.; Kraehnert, R.; Polte, J. Turkevich in New Robes: Key Questions Answered for the Most Common Gold Nanoparticle Synthesis. ACS Nano 2015, 9, 7052–7071. Schulz, F.; Homolka, T.; Bastús, N. G.; Puntes, V.; Weller, H.; Vossmeyer, T. Little Adjustments Significantly Improve the Turkevich Synthesis of Gold Nanoparticles. Langmuir 2014, 30, 10779–10784.


Page 17 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11)

(12) (13)

(14) (15) (16)

(17) (18) (19) (20) (21) (22) (23) (24)

(25)

(26) (27)

(28)

(29) (30)

(31)

Polte, J.; Ahner, T. T.; Delissen, F.; Sokolov, S.; Emmerling, F.; Thuenemann, A. F.; Kraehnert, R. Mechanism of Gold Nanoparticle Formation in the Classical Citrate Synthesis Method Derived from Coupled in Situ XANES and SAXS Evaluation. J. Am. Chem. Soc. 2010, 132, 1296–1301. Polte, J. Fundamental Growth Principles of Colloidal Metal Nanoparticles – a New Perspective. CrystEngComm 2015, 17, 6809–6830. Peng, X.; Wickham, J.; Alivisatos, A. P. Kinetics of II-VI and III-V Colloidal Semiconductor Nanocrystal Growth: “Focusing” of Size Distributions. J. Am. Chem. Soc. 1998, 120, 5343– 5344. Kotlarchyk, M.; Stephens, R. B.; Huang, J. S. Study of Schultz Distribution to Model Polydispersity of Microemulsion Droplets. J. Phys. Chem. 1988, 92, 1533–1538. Ojea-Jiménez, I.; Campanera, J. M. Molecular Modeling of the Reduction Mechanism in the Citrate-Mediated Synthesis of Gold Nanoparticles. J. Phys. Chem. C 2012, 116, 23682–23691. Peck, J. A.; Tait, C. D.; Swanson, B. I.; Brown Jr., G. E. Speciation of Aqueous gold(III) Chlorides from Ultraviolet/visible Absorption and Raman/resonance Raman Spectroscopies. Geochim. Cosmochim. Acta 1991, 55, 671–676. Goia, D.; Matijević, E. Tailoring the Particle Size of Monodispersed Colloidal Gold. Colloids Surf. Physicochem. Eng. Asp. 1999, 146, 139–152. Wang, S.; Qian, K.; Bi, X.; Huang, W. Influence of Speciation of Aqueous HAuCl4 on the Synthesis, Structure, and Property of Au Colloids. J. Phys. Chem. C 2009, 113, 6505–6510. Johansson, L. The Role of the Perchlorate Ion as Ligand in Solution. Coord. Chem. Rev. 1974, 12, 241–261. Kuyper, A. C. The Oxidation of Citric Acid. J. Am. Chem. Soc. 1933, 55 (4), 1722–1727. Barbooti, M. M.; Al-Sammerrai, D. A. Thermal Decomposition of Citric Acid. Thermochim. Acta 1986, 98, 119–126. Kumar, S.; Gandhi, K. S.; Kumar, R. Modeling of Formation of Gold Nanoparticles by Citrate Method. Ind. Eng. Chem. Res. 2007, 46, 3128–3136. Patungwasa, W.; Hodak, J. H. pH Tunable Morphology of the Gold Nanoparticles Produced by Citrate Reduction. Mater. Chem. Phys. 2008, 108, 45–54. Sivaraman, S. K.; Kumar, S.; Santhanam, V. Monodisperse Sub-10 Nm Gold Nanoparticles by Reversing the Order of Addition in Turkevich Method – The Role of Chloroauric Acid. J. Colloid Interface Sci. 2011, 361, 543–547. Doyen, M.; Bartik, K.; Bruylants, G. UV–Vis and NMR Study of the Formation of Gold Nanoparticles by Citrate Reduction: Observation of Gold–citrate Aggregates. J. Colloid Interface Sci. 2013, 399, 1–5. Zhao, L.; Jiang, D.; Cai, Y.; Ji, X.; Xie, R.; Yang, W. Tuning the Size of Gold Nanoparticles in the Citrate Reduction by Chloride Ions. Nanoscale 2012, 4, 5071–5076. Wuithschick, M.; Witte, S.; Kettemann, F.; Rademann, K.; Polte, J. Illustrating the Formation of Metal Nanoparticles with a Growth Concept Based on Colloidal Stability. Phys. Chem. Chem. Phys. 2015, 17, 19895–19900. Moreau, F.; Bond, G. C.; Taylor, A. O. Gold on Titania Catalysts for the Oxidation of Carbon Monoxide: Control of pH during Preparation with Various Gold Contents. J. Catal. 2005, 231, 105–114. Machesky, M. L.; Andrade, W. O.; Rose, A. W. Adsorption of gold(III)-Chloride and gold(I)Thiosulfate Anions by Goethite. Geochim. Cosmochim. Acta 1991, 55, 769–776. Kunze, J.; Burgess, I.; Nichols, R.; Buess-Herman, C.; Lipkowski, J. Electrochemical Evaluation of Citrate Adsorption on Au(1 1 1) and the Stability of Citrate-Reduced Gold Colloids. J. Electroanal. Chem. 2007, 599, 147–159. Polte, J.; Erler, R.; Thünemann, A. F.; Emmerling, F.; Kraehnert, R. SAXS in Combination with a Free Liquid Jet for Improved Time-Resolved in Situ Studies of the Nucleation and Growth of Nanoparticles. Chem Commun 2010, 46, 9209–9211.



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Missing Piece of the Mechanism of the Turkevich Method: The Critical

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