Introduction

Colloidal gold nanoclusters (AuNCs) have recently been studied intensively due to their potential applications as quasi-homogeneous catalyst for many reactions,¹ such as C-C bond coupling,² selective-alcohol oxidation,³ oxidation of amines,⁴ and other bond forming reactions.⁵ One of the benefits is the size selective synthesis of the clusters by kinetic control, due to the homogeneity achieved during preparation. Therefore, colloidal clusters have often been used as precursors of the heterogeneous clusters with uniform cluster size. Control of the cluster size is indispensable in the investigation of AuNCs, especially in the field of catalysis, because of highly size-depending on catalytic activity NCs. A smaller cluster size generally results in better catalytic activity.^6,7 However, some results indicate an optimum size and smaller is not always better.⁸

Rademann and co-workers demonstrated the reduction of p-nitrophenol by AuNCs which were prepared by the seed-mediated growth method. The AuNCs with a diameter of 13 nm is the most efficient catalyst (Figure 2-1-1, left).^8b Furthermore, Tsukuda and co-workers reported AuNCs-supported on hydroxyapatite (Au:HAP) catalyzed the oxidation of cyclohexane. The Au:HAP was synthesized by mixing glutathione (GS)-stabilized AuNCs and hydroxyapatite (HAP) under basic conditions, then AuNCs were calcined at high temperature in order to remove of GS. The turnover frequency (TOF) increased with an increasing of the size as mentioned in the introduction part (Figure 2-1-1, right).^8c Although it cannot be explained in terms of geometries such as surface area or coordination on the surface, the observed optimum size may be associated with the electronic structure. Therefore, precise control of the size at the sub-nanometer scale is highly desirable.

The synthesis of AuNCs with size controlled at the atomic level has been achieved using strongly coordinating ligands such as thiol derivatives. The obtained AuNCs were found in discrete structure at the atomic level determined by MALDI-TOF analysis, for example Au10GS10, Au18GS14, Au25GS18, Au39GS24.^8c However, such strong coordination inhibits the catalytic activity.⁹ Thus, an additional process to remove the ligands, such as annealing, is required to activate the catalyst. By the composite of strong binding organic stabilizer-supported AuNCs as gold precursor and solid-stabilizer-supported was calcined in order to remove the organic stabilizer.^8b,9b,10 The obtained heterogeneous AuNCs were also found in the same discrete structure that the agglomeration process can be avoided during calcination. The preparation method is shown in Figure 2-1-2.

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Figure 2-1-1 Size-dependency properties of AuNCs catalyzed reaction; a) AuNCs catalyzed the reduction of p-nitrophenol to p-aminophenol. b) AuNCs catalyzed the oxidation of cyclohexane to corresponding cyclohexanol

and cyclohexanone. These figures were reprinted from reference 8b and 8c, respectively.

On the other hand, in AuNCs, which are protected by polymers to show high catalytic activity are stabilized through weak non-covalent interaction, which is poorly resistant to further agglomeration. However, such a trade-off between activity and structure could be customized to achieve a high level of kinetic control during the agglomeration process.

Figure 2-1-2 The activation of dodecanethiol-capped gold catalyst for CO oxidation by oxidation-calcination.

This figured was reprinted from reference 9b.

Indeed, various types of preparative method for the colloidal AuNCs have been reported, for example, the reduction by alcohol¹¹ or by ascorbic acid¹² in the presence of the polymers.

However, the obtained clusters have broad size distribution because of the reaction conditions under high temperature and slow reduction rate. To achieve the preparation with small size and narrow distribution, kinetic reduction is important. Thus, strong reducing agents as sodium

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borohydride (NaBH4) is used to reduce tetrachloroauric acid (HAuCl4) in the presence of PVP at 0 ^°C. By this kinetic process, the small NCs with unified size was obtained as a brownish dispersion,¹ since the shorter time of the nucleation process leads to the simultaneous formation of NCs.

Another problem in the synthesis of colloidal AuNCs is the viscosity of polymers, which also impedes the kinetic control of agglomeration due to inefficient physical mixing.

The small size of AuNCs cannot be obtained when using stabilizing polymer with high viscosity. As already described in the introduction part, a batch process, the reduction of tetrachloroauric acid (HAuCl4) by sodium borohydride (NaBH4) in the presence of PVP, afforded a mean cluster size of 1.3 nm when using poly-(N-vinylpyrrolidone) (PVP) K-15 (Mw = 10 kDa, Viscosity = 1 cps) or K-30 (Mw =40 kDa, Viscosity = 3 cps), while a mean size of only 1.6 nm was obtained using K-90 (Mw= 360 kDa, Viscosity = 150 cps).^2a As the 1.3 nm of Au:PVP was not available by batch method, the comparison of catalytic activity between polymer chain length with the same size was not able to study. Thus, alternative method to overcome the viscosity problem was required. Another example of the high viscosity polymer for the matrix of AuNCs is Aoshima and co-workers’ star-shaped polymers as also shown in Chapter 1. Their thermos-sensitive star-shaped polymers which were prepared by the introduction of appropriate moieties in the coronal arms afforded a micellar matrix with reversible gelation responsivity by lower critical solution temperature (LCST)-type phase-separation in water.¹² One of the most attractive applications of these polymers involves utilization as a stabilizing matrix for AuNCs, because the micellar structure is suitable for stabilization of AuNCs in water. Their thermosensitivity allows easy separation and reuse.

However, by the batch method, the AuNCs with core size more than 2 nm were obtained in the preparation of star-shaped polymers-stabilized AuNCs. The significant large size of AuNCs may possibly be caused by its large molecular weight of 900 kDa.

To solve two major problems as described above, the size distribution and preparation using polymer with high molecular weight as well as viscosity, recently, microflow reactors have been proposed for the synthesis of NCs due to their efficient and homogeneous mixing.

The microreactors are suggested for nanoparticle synthesis because the nucleation process continuously occurs during the reaction time in the lower volume of the reaction mixture, leads to sufficiently lower polydispersity of nanoparticles.^{11, 13} Microflow reactors have also been reported for the control of particle size, shape and aggregation rate in the preparation of AuNCs.¹⁴ Various types of microflow reactor have been designed and applied for the synthesis

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of gold starting from Au seed¹⁵ and Au solution.¹⁶ Wagner and co-workers demonstrated Au:PVP synthesized by ascorbic acid-reduced gold via continuous flow process using the IPHT microreactor. The obtained AuNCs were shown smaller size distribution than obtained from conventional batch method.^15,16 Tsukuda and co-workers also reported the efficient synthesis of monodisperse Au:PVP clusters with an average diameter of ∼1 nm by using sodium borohydride as reducing agent under continuous flow condition using a micromixer. These clusters exhibited higher catalytic activity for aerobic alcohol oxidation than prepared Au:PVP using a batch reactor (Scheme 2-1-1, left).^7b

Scheme 2-1-1 Schematic diagram for the synthesis of PVP-stabilized gold nanoclusters by using microflow reactor. These figures were reprinted from reference 7b and 15.

However, there is a severe requirement in the choice of the micro mixer for the preparation of AuNCs to avoid any metal contaminant. The common metal component in stainless^® steel, Ni and Mo, are found to leach out in the presence of acid such as HAuCl4. Contamination of impure metals must induce the drastic change of electronic structure of NCs as well as the activity toward catalytic reactions by doping effect.¹⁷ Therefore, among commercially available micromixers with reasonable price and free from stainless^® steel in the flow channel, two different types of microflow reactors were carefully selected for the investigation; Techno-Applications COMET X-1 and Sigma-Aldrich type S02 microflow reactors were shown in Figure 2-1-3.

Figure 2-1-3 Microflow reactors used in this study. a) Techno-Applications COMET X-1 and b) Sigma-Aldrich type S02.

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In order to carry out the polymer matrix effect, various and wide range of size of the clusters in the presence of the different types of polymers as well as the small size of them is necessary. Two important methods for the size-selective preparation of relatively large sized clusters have been reported, a seed-mediated growth method and a slow reduction method.

Therefore, applicability of these two methods to the preparation of the high viscosity polymer-protected AuNCs should also be tested.

In case of seed-mediated growth method, small metal particles are initially prepared and later used as seeds to prepare larger particles in the presence of a metal salt and a weak reducing agent. The concept of seed-mediated growth method was developed by Zsigmondy and Thiessen.¹⁸ Natan and co-workers investigated the use of citrate- and borohydride-reduced AuNCs as seeds for the preparation of larger AuNCs with diameters between 30-100 nm employing citrate or hydroxylamine as the growth stage reducing agent.¹⁹ Murphy and co-workers reported seed-mediated growth method by using 12 nm AuNCs as seeds to grow various sizes of larger AuNCs under the presence of ascorbic acid.²⁰ The size of AuNCs could be controlled by additional rate of reducing agent and seed concentration. A series of nearly monodisperse Au:PVP ranging from 1-10 nm has been prepared by seed-mediated growth method. By using 1.3 nm Au:PVP as seed, subsequence reduction of Au salt by Na2SO3 yielded a series of larger Au:PVP. Significantly larger size of Au:PVPs were obtained under basic condition (Figure 2-1-4).^3b

Figure 2-1-4 TEM images and core size distributions of Au:PVP (K-30) prepared by seed-mediated growth method; upper : in the absence of K2CO3; lower : in the presence of K2CO3. This figure was reprinted from

reference 3b.

The larger particle size can also be obtained by the slow reduction method.

Stellacci and co-workers reported shape-controlled growth of gold nanocrystal under reflux condition in the presence of oleylamine or dodecylamine. The morphology of gold nanocrystal was controlled by varying the concentration of amine.²¹ Tsukuda and co-workers recently

reported a slow reduction to prepare thiolate-protected AuNCs under basic conditions.

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The obtained AuNCs, which was larger than those prepared by sodium borohydride reduction, exhibited a strong near-infrared absorption band at 1340 nm.²²

In current studies, various methods were used to apply to size-control preparation of hydrophilic polymers-stabilized AuNCs. The two different types of micromixers, Techno-Applications COMET X-1 and Sigma-Aldrich type S02 microflow reactors (Figure 2-1-3), were selected for the preparation of AuNCs stabilized by hydrophilic polymers with high viscosity; a) PVP (K-90) and b) star 2-methoxyethyl vinyl ether (MOVE)200 (Scheme 2-1-2).

In addition, the various sizes of PVP (K-15, 30, 60 and 90) (Mw = 10, 40, 160, and 360 kDa, respectively)-stabilized AuNCs were prepared by both slow reduction under basic conditions and seed-mediated growth methods in order to obtain wide range of the cluster size from 1-9 nm.

Scheme 2-1-2 Structures of a) PVP and b) star(MOVE)200.

2.2 Experimental Section