Citrate reduction

Silver Nanowires In order to form the silver nanowires in high yield, control over the geometry of the seed is essential. Here, it is necessary to form multiple-twined 

Reproduced with permission from Ref [103] 2008 American Chemical Society. 

In brief, polyol synthesis has been successfully applied to the generation of spherical and anisotropic nanomaterials of gold and silver, with high yields. Moreover, the method exhibits good control over the morphology of the product, with quantitative reproducibility. 

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In addition, the catalyst preparation method plays an important role sol immobilization appears superior to incipient wetness or impregnation methods. Gold sol immobilization occurs via citrate reduction of HAuCU, yielding a narrow par-... 

Coreduction of Mixed Ions. Coreduction of mixed ions is the simplest method to synthesize bimetallic nanoparticles. However, this method cannot be always successful. Au/Pt bimetallic nanoparticles were prepared by citrate reduction by Miner et al. from the corresponding two metal salts, such as tetrachloroauric(III) acid and hexachloroplatinic(IV) acid (24). Reduction of the metal ions is completed within 4 h after the addition of citrate. Miner et al. studied the formation of colloidal dispersion by ultraviolet-visible (UV-Vis) spectrum, which is not a simple sum of those of the two monometallic nanoparticles, indicating that the bimetallic nanoparticles have an alloy structure. The average diameter of the bimetallic nanoparticles depends on the metal composition. By a similar method, citrate-stabilized Pd/Pt bimetallic nanoparticles can also be prepared. 

Kim et al. used the exchange reaction to synthesize cross-linked AuNP-PNIPAM core-shell hybrid structures, as well as a brush-type AuNP/PNIPAM hybrid through surface-initiated ATRP in an aqueous medium. The disulfide initiators, [BrC (CH3)2COO(CH2)iiS]2, were bound to AuNPs synthesized by citrate reduction. They have studied the effect of cross-linking on the thermo-responsiveness of the AuN / PNIPAM hybrids for potential use as a stimuli responsive optical device, such as surface plasmon resonance-based sensing materials [91]. 

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. 

Citrate reduction. Chloroauric acid is boiled under reflux with sodium citrate (5) until the solution turns red. Particle sizes achieved are in the range of 15-150 nm, and a particular diameter is chosen for a batch by adjusting the amount of citrate added to the boiling flask. This method has been used, therefore, for producing probes in the larger particle size range. 

Combined tannic acid-citrate reduction in 1985, Slot and Geuze (5) described a method of using two reducing agents, tannic acid and... 

Pong, B. K., Elim, H. I., Chong, J. X., Ji, W., Trout, B. L., Lee, J. Y., New insights on the nanoparticle growth mechanism in the citrate reduction of Gold(III) salt Formation of the au nanowire intermediate and its nonlinear optical properties. Journal of Physical Chemistry C2007, 111, 6281-6287. 

The rate of many catalyzed redox reactions decreases as the gold nanoparticle increases in size [9, 23, 24]. The work of Panigrahi et al. [9] demonstrates this in Fig. 18.1a, with particles synthesized by the Frens method (sodium citrate reduction of a gold salt and subsequent protection of the formed nanoparticles by citrate ions) [29]. This trend occurs because as the size of the nanoparticle decreases, the surface atoms are less coordinatively saturated and thus more reactive. Several authors [9, 30] have concluded this data is evidence for heterogeneous catalysis however, it is premature to conclude solely from catalytic kinetics that correlate with nanoparticle size that the catalysis is heterogeneous in nature. This increase in the coordination unsaturation of surface atoms would lead to their decreased stabihty... 

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... 

Figure 3.104 TEM image of monodispersed 18 nm gold particles, prepared by citrate reduction of HAuCU.

To the best of our knowledge, the previous 1-b-l nanoparticle modifications of latex have all been directed at incorporation into the walls of hollow capsules, as described below. However, if metal nanoparticles are stabilized by a surface charge then they can be adsorbed to appropriately modified 1-b-l latex by electrostatic attraction, as shown in Fig. 8.1b. In this figure, the gold nanoparticles were produced by citrate reduction and so had a negative charge... 

Silver colloid is popular due to the relative ease of manufacture, low cost, and stability. One widely used method of silver colloid preparation is citrate reduction of silver nitrate (Figure 3). This results in an overall net negative charge on the colloid, which is high compared with many preparations (Figure 4). As a result, the colloid is stable for months and even years, and in some cases, with careful attention to detail, quantitative SERRS can be obtained from this colloid with relative standard deviations of less than 5%. There is, however, a change in enhancement with time, and so a standard needs to be used. 

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