Synthesis Using Thiol Stabilizers

Reproduced with permission from Ref. [22] 2002 American Chemical Society. 

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Although thiol ligands are the most widely used as stabilizers in the synthesis of Au NPs, other organic ligands bearing amine, phosphine, phosphine oxide, carboxylate or other functional groups have been employed alone or in mixtures. 

Trityl resins are particularly suitable for immobilization of nucleophilic substrates such as acids, alcohols, thiols, and amines. They are quite acid-sensitive and are cleavable even with acetic acid this is useful when acid-labile protecting groups are used. The stability of trityl resin can be tailored by use of substituted arene rings, as shown by chlorotrityl resin, which furnishes a more stable linker than the trityl resin itself. Steric hindrance also prohibits formation of diketopiperazines during the synthesis of peptides. Orthogonality toward allyl-based protective groups was demonstrated in the reverse solid-phase peptide synthesis of oligopeptides [30] (Scheme 6.1.4). 

In addition to chemical methods variety of physical methods has been employed for the synthesis of AuNPs. UV irradiation is used to improve the quality of the AuNPs when it is used in synergy with micelles or seeds [32,33], Near-IR laser irradiation provokes an enormous size growth of thiol-stabilized AuNPs [34], The presence of an ultrasonic field (200 kHz) allowed the control of the rate of AuC14" reduction in an aqueous solution containing only a small amount of 2-propanol and the sizes of the formed AuNPs are controlled by varying the parameters such as the temperature of the solution, the intensity of the ultrasound, and the positioning of the reactor [35,36], Sonochemistry was also used for the synthesis of AuNPs within the pores of silica and for the synthesis of Au/Pd bimetallic particles [37,38], Radiolysis has been used to control the size of AuNPs [39], Laser photolysis has been used to form AuNPs in block copolymer micelles. Laser ablation is another technique of AuNP synthesis that has been used under various conditions whereby size control can be induced by the laser [40,41],... 

Thiols have also been used in an elaborate fashion to prepare spherical, hoUow Pd nanoparticles. Hyeon s group reported the fabrication of uniform hollow Pd spheres using thiols as stabilizers and sihca particles as templates [22]. The template-assisted method used (see Scheme 9.1) first involves the synthesis of uniform silica spheres which were then refluxed with mercaptopropyltrimethoxysilane in toluene in order to functionahze the sihca surface with mercaptopropylsilyl (MPS)... 

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]. 

Daniel et al. have also reported on the use of thiol-functionalized dendrons as stabilizers for the synthesis of NCDs of about 2.1 nm. The synthetic procedure... 

The three-component method is applicable to the synthesis of various C(6)- or C(7)-functionalized PGs. Scheme 11 illustrates the tandem conjugate addition-aldol reaction that affords 7-hydroxy-PGE derivatives (18). Both saturated and unsaturated C7 aldehydes can be used as a side-chain units. The aldol adducts can be transformed to naturally occurring PGs (5a, 19) and, more importantly, to a variety of analogues such as tumor-suppressing A7-PGA, (20) or 7-fluoro-PGI2, a stabilized prostacyclin (21). The unique cellular behavior displayed by A7-PGA methyl ester is well correlated to its chemical reaction with thiols (20). 

Depending on the kind of synthesis, these quantum dots can be prepared or size separated into batches covering almost the entire visible spectral range from 400 to 750 nm with, in part, high photoluminescence quantum efficiencies (some stable in air [106], others not [107]). Weller et al. reported on a very efficient synthesis for hydrophilic, thiol-capped CdTe quantum dots [108,109], which can be transformed to lipophilic, alkanethiol-stabilized CdTe quantum dots using a place exchange reaction similar to that for metal nanoparticles described above [110]. A related strategy has also been successfully employed to produce hydrophobic or otherwise functionalized CdS [111] or CdSe quantum dots [112] (Fig. 1). 

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