Brust method

Three general preparative schemes are of particular interest due to their success in preparing nanoparticles on the order of 1 to 3 nm. The first, commonly known as the Brust method for preparing thiol stabilized Au nanoparticles, is discussed in detail in the chapter by Zhong et al. in Section IV of this book. The second method, which originates from Prof El-Sayed s group, is noteworthy for preparing particles with extremely well-defined shapes (tetrahedra, cubes, etc). ° The third method. 

One year later, Brust and Schiffrin [28,29] published a method for AuNPs synthesis which has a considerable impact on the overall field in less than a decade because it allowed the facile synthesis of thermally stable and air-stable AuNPs of reduced dispersity and controlled size for the first time. In this method, the gold colloids are sterically stabilized by organic molecules having thiol, amide or acid groups in contrast to the citrate reduction method where the gold colloids are kinetically stabilized in aqueous solutions by an electrical double layer [28,29], The main advantage of the Brust method is that the gold particles behave in a way as chemical compounds. These AuNPs can be repeatedly isolated and... 

Fig. 2.2. TEM image of An nanocrystals synthesized by the Brust method (reproduced with permission from [151])...

Dodecanethiol-capped Ag nanocrystals, prepared by the Brust method self-assemble into two-dimensional arrays (see Fig. 3.7) [156]. From the separation distance of 1.5 nm seen between the nanocrystal surfaces, one can deduce that the alkane chains projecting out of neighboring nanocrystals are inter-digitated. The ordered arrangement of nanocrystals typically covers an area of several square microns. Thiol-capped Au nanocrystals prepared by the Brust method have been size-fractionated in order to organize into well-ordered two-dimensional arrays [581]. Two-dimensional arrays of noble metal nanocrystals... 

There have been a number of synthetic protocols for the preparation of transition-metal nanoparticles, for example, vapor condensation, sonochemical reduction, chemical liquid deposition, reflux alcohol reduction, decomposition of organometallic precursors, hydrogen reduction, etc. Of these, the colloidal reduction route provides a powerful platform for the ready manipulation of particle structure and functionalization. One excellent example is the biphasic Brust method, in which nanoparticles are formed by chemical reduction of a metal salt precursor in the presence of stabilizing ligands. In a typical reaction, a calculated amount of a metal salt precursor is dissolved in water, and the metal ions are then transferred into the toluene phase by ion-pairing with a... 

The synthesis of Pd MPCs usually involves a Brust method very similar to that for An MPCs wherein a Pd salt and ligand are reduced. In a study comparing the ET rate constants for MPCs (of various core metals) attached to the electrode surface via dodecanethiol SAMs, the Pd cluster exhibited slower ET rates than the An and Pt clusters. Differences in MPC electronic conductivity among core metals represent yet another opportunity for the design of clusters to serve a desired purpose. Also, Pd clusters can be coated using either thiol ligands or covalent Pd-C bonds. The latter is unique to Pd and presents a new opportunity for surface functionalization that is distinct from An clusters. 

Like Au and Pd MPCs, the syntheses of Pt MPCs usually involve coreduction of a metal salt with the desired ligand, as in the Brust method. An early study by van Kempen et al. measured, via STM, the charging steps and Coulomb blockade of a single particle of phenanthroline-protected Ptgog MPCs. A claim to fame of Pt MPCs is their good electrocatalytic activity for ORRs, showing an improvement over traditional Pt black catalysts. ... 

The use of tetraoctylammonium salt as phase transfer reagent has been introduced by Brust [199] for the preparation of gold colloids in the size domain of 1-3 nm. This one-step method consists of a two-phase reduction coupled with ion extraction and self-assembly using mono-layers of alkane thiols. The two-phase redox reaction controls the growth of the metallic nuclei via the simultaneous attachment of self-assembled thiol monolayers on the growing clusters. The overall reaction is summarized in Equation (5). 

In 2001, we developed a simple and quite useful method to manipulate the size of Au nanoparticles by using the heat-treatment of small Au nanoparticles [9,10], which is far from the conventional techniques. The 1-dodecanethiol-protected Au nanoparticles (C12S-AU) of 1.5 0.2 nm in size synthesized by the Brust s two-phase (toluene/water) reaction procedure [3] were heat treated at 150-250 °C at the heating rate of 2°Cmin and held for 30min. This heat treatment of as-synthesized C12S-AU nanoparticles... 

Two phase reduction (Brust synthesis) This method is usually employed to make organic soluble quantum clusters [8]. This method consists of two steps. The first is the transfer of AuCLi from aqueous to the organic layer by a phase transfer reagent such as tetraoctyl ammonium bromide (TOABr). The next step is the subsequent reduction of AuCU in the presence of suitably selected ligands such as thiols, phosphines, etc. 

Considerable research effort was focused on systems of colloidal gold of which a broad variety of synthetic procedures were reported [140 b, fj. While native colloidal gold solutions are only stable for a restricted time, Brust et al. [141] were able to overcome this problem by developing a simple method for the in situ preparation of alkyl thiol-stabihzed gold nanoparticles. This synthetic route yields air-stable and easy to handle passivated nanoparticles of moderate polydispersity, and is now commonly employed for the preparation of inorganic-organic core-shell composites. Such composites are used as catalytic systems with principally two different functions of the protective 3D-SAM layer. Either the metal nanoparticle core can be used as the catalytically active center and the thiol layer is only used to stabihze the system [142], or the 3D-SAM is used as a Hnker system to chemically attach further catalytic functions [143]. 

Figure3.9 Brust-Schiffrin method for the synthesis of monolayer protected gold clusters. Reproduced from reference [62] by permission of The Royal Society of Chemistry.

Other modifications to the reaction conditions of the Brust-Schiffrin method, such as a reduction temperature of — 78 °C and the use of a hyperexcess of hexanethiol, results in an Au38(thiolate)24, based on observations, LDI-TOF mass spectrometry, TGA analysis and elemental analysis [69]. The influence of preparation temperature on the size and monodispersity of dodecylthiol monolayer protected gold clusters has also been reported. Both and SAXS measurements show that higher temperatures increase polydispersity. This modification of poly-dispersity may be related to the existence of a dynamic exchange of thiols at the particle surface with thiols in the solvent [70]. 

The modification to the Brust-Schiffrin method through the addition of lauryla-mine (LAM) or octadecylamine (ODA) instead of thiols to colloidal gold particles... 

As mentioned above, the grafting to technique enables in a one-pot reaction the synthesis of Au NPs stabilized by sulfur-containing polymers, which bear functional groups such as dithioester, trithioester, thiol, thioether and disulfide at the end of a polymer chain or in the middle. This method leads to nanoparticles similar to those obtained by the Brust-Schiffrin method in which alkanethiol-protected Au NPs of small size are obtained. This grafting to technique leads to very stable nanomaterials that also present a high surface graft density of polymer brush on the Au NP surface. 

For example, Wuelfing et al. reported on the synthesis of Au NPs using the thiolated polymer, a-methoxy-co-mercapto-poly(ethylene glycol) (PEG-SH), as stabilizer in a modification of the Brust-Schiffrin method using a 1/12 polymer thiol/ AuC14 ratio. Transmission electron microscopy showed that the product had modestly polydisperse Au cores of average diameter 2.8 1 nm. This nanomaterial led to characteristics uniquely different from alkanethiolate MPCs, notably aqueous solubility, thermal and chemical stability, ligand footprint size, and ionic conductivity [66]. 

The fourth method for the preparation of polymer stabilized Au NPs is the postmodification of pre-formed Au NPs . This method is used to avoid broad distribution of sizes of Au NPs stabilized with polymers through any of the methods described previously. As we have mentioned before, in a first step very monodisperse Au NPs are obtained by common methods, such as the citrate reduction or the Brust-Schiffrin method. In a second step, the exchange of weakly bound citrate ions with polymer or modification ofend-functionalized thiols with polymers is performed. 

The third class of AuNPs-dendrimer nanomaterials is Nanoparticle Cored Dendrimers (NCDs). The first report on this topic describes a new strategy in which Frechet-type dendrons with a single thiol group at the focal point were used as a surface stabilizer of Au NPs. These dendronized Au NPs were synthesized using a modification of the two-phase Brust-Schiffrin method, giving rise to highly stable and very monodisperse Au NPs of small size (about 2.4—3.1 nm) [129] (Figure 3.13). 

Brust et al. have developed methods to fabricate MPC and MMPC systems that rely on the reduction of metal salts (Pd, Au, Ag, Pt) in the presence of capping ligands.5 The approach uses mild conditions and moderate reducing agents that are compatible... 

Scheme 1 The one- and two-phase Brust-Schiffrin method for the synthesis of thiol-capped gold nanoparticles, and the first step of the reaction prior to the reduction step with NaBIL [63]...

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