ENTRADA Y SALIDA ESTÁNDAR (TECLADO Y PANTALLA)

The synthetic procedure described above does not differ from other methods reported

elsewhere16,39_{in a sense that it involves the reduction of Au}III _{into Au}I _{by the thiol /}

thioether–containing stabilizer in the reaction vessel, and then, the complete reduction

of AuI _{to Au}0 _{by addition of a strong reducing agent (NaBH}

4).

The mechanism of gold particles formation and the causes of their dispersion in size

using different methods are not completely understood.17_{Therefore, the rationalization}

for the formation of highly monodisperse nanoparticles using the procedure described in this Chapter remains challenging.

However, from the various data described above, a number of basic design rules can be formulated for the effect of the polymeric ligands nature on the formation of gold nanoparticles.

Polymeric main chain. For the DDT–capped ligands, pMAA and pAA were found to be the most effective polymers with the pVSA–DDT ligand in particular showing incompatibility with this synthetic route (Figure 3.8). pAA–DDT ligands (Figures 3.6) gave somewhat narrower particle size distributions (7 – 9% standard deviation by TEM) than the pMAA–DDT ligand series (10 – 14% standard deviation) at all concentrations studied (Figure 3.3) possibly because of its higher water solubility. Both of these carboxylic-acid-containing polymers gave rise to much narrower particle size distributions than ligands pHEA–DDT, pEGMA–DDT and pVP–DDT, which led to standard deviations in the range 15 – 28% (Table 3.1 and Figures 3.4,3.5 and 3.7). pAA and pMAA are polyfunctional molecules bearing a carboxylic acid group on each monomer and, as such, have the potential to stabilize these particles by a combination

of both steric and electrostatic mechanisms (see Chapter 1 Section 1.4). Both pAA40,41

and pMAA42 _{(without thioether end groups) have been used previously as stabilizers}

for the preparation of stable nanodispersions of materials such as cerium oxide,40_nickel

ferrite,41 _{and zinc oxide.}42

Polymers concentrations. The resulting particle size is strongly dependent on the polymer concentration over the range 0.006 – 6.0 mM, with higher polymer concen- trations leading to smaller particles. It is quite common to produce small particles when using a high polymer concentration and can be explained by seeding and growth

mechanistic considerations (see Chapter 1 Section 1.6).2,8,9,43,44

Polymer molecular weight. A consistent but relatively modest molecular weight trend was observed for the pMAA–DDT ligands, and at a given molar concentration, lower molar mass pMAA–DDT chains give rise to smaller and more monodisperse particles (Figure 3.15). It should be stressed however that the four “standard” polymer concentrations (0.006, 0.06, 0.6 and 0.6 mM) were defined on the basis of the molar mass of the polymer repeat unit and not on the polymer molecular weight; as such, the number of polymer chains (and hence the number of available reactive thioether

end groups) depended on the total Mn for each sample. This effect was weak but

reproducible at all ligand concentrations. It has been predicted that, in the small

Mw) passivant molecules because they require more time to diffuse and passivate the

growing nanoparticle.45 _{This prediction was not supported by data for Me–PEG–SH}

ligands of various molecular weights,45_{but our observations are broadly consistent with}

this hypothesis. An alternative hypothesis, put forward by the same research,45 _{is that}

heteroatom–containing polymers such as Me–PEG–SH (and by analogy pMAA and pAA) act as a net which assists in particle nucleation by more rapidly supplying gold atoms to small, unstable gold clusters. In this case, larger polymers might enhance the

effect by being larger nets.45 _{Our data for pMAA–DDT ligands do not suggest that}

this is the main effect, although it is possible that both gold atom sequestration and

ligand diffusion play opposing roles in the system with ligand diffusivity (for lowerMn

species) dominating in this case.

End–group hydrophobicity. An increased particle monodispersity was observed

for the pMAA ligands with more hydrophobic ends groups (C12 and C18 containing

thioethers, Figures 3.21 and 3.3). It is known, that alkanethiols physisorb onto Au(111)

surfaces through van der Waals interactions and that this physisorption enthalpy de-

pends on the alkyl chain length.46 _{The physisorption enthalpy per CH}

2 group was

found to be of the order of 6.1 kJ/mol, which implies that for alkanethiols longer than about 14 carbon atoms the physisorption enthalpy may exceed the chemisorption en-

thalpy.46_{The pMAA ligands produced using monofunctional alkanethiol chain–transfer}

agents contain thioether,47 _{not thiol end–groups. Dialkyl sulfides bind less strongly to}

Au, having bond strengths near 60 kJ/mol as compared with the 130 kJ/mol value

typical of chemisorbed alkanethiol–Au bonds.46,48 _{Unlike alkanethiols, where both ph-}

ysisorbed (approximately 60 kJ/mol) and chemisorbed (approximately 120 kJ/mol)

forms have been observed, dialkyl sulfides do not readily chemisorb to Au(111) _{and only}

physisorption (60 kJ/mol) occurs.46,48 _{On the basis of these previous observations, it}

is conceivable that the pMAA ligands with longer hydrophobic alkyl end–groups ph- ysisorb more strongly to the growing Au particles and that this causes the enhanced control over particle size distribution.

End–group denticity. End–group!Multiple thiol containingThe multidentate ligand pMAA–PTMP gave rise to gold particles which were quite distinct from those obtained

with the other ligands studied here. In particular, the particles were much smaller at a given ligand concentration and more monodisperse. Remarkably narrow size distribu- tions were observed at a ligand concentration of 0.006 mM (Figure 3.26 and 3.27). At this stage, this effect could not be fully rationalized, but it is clear that the structure of the pMAA–PTMP ligand differs in a number of respects. First, one might expect the

tetradentate end group to have a larger footprint19_{and hence for each pMAA–PTMP}

ligand to passivate an increased area on the gold particle surface at a given molar cov- erage. Second, it is possible that the sticking probability for this ligand is enhanced and that this affects the kinetics of passivation. Third, the end–group, if multiply bound to the gold surface, may help to stabilize very small clusters, which is consistent with the marked decrease in particle size that was observed with this ligand at a given poly- mer concentration. These particles are also very stable toward the presence of salt, or

pH variations.31 _{The characteristics of those pMAA–PTMP gold particles are further}

investigated in Chapter 4.

In combination, these “rules” have allowed the production of ligands that are signifi-

cantly more effective than those reported previously.13_{The precise physical explanation}

of these various effects is more complex, although some trends may be rationalized quite

readily.31