Surface ligands

In the presence of H2, perhydrocarbyl surface complexes loose their ligands through the hydrogenolysis of their metal carbon bonds to generate putative hydride complexes, which further react with the neighbouring surface ligands, the adjacent siloxane bridges (Eqs. 8-9) [46,47]. 

Uyeda, H.T., Medintz, I.L., and Mattoussi, H. (2003) Design of water-soluble quantum dots with novel surface ligands for biological applications. Mater. Res. Soc. Proc. 789, Symposium N. 

For undersaturated ([M] < Km) systems with relatively fast internalisation kinetics (kmt > k, ), the uptake of trace metals may be limited by their adsorption. Because the transfer of metal across the biological membrane is often quite slow, adsorption limitation would be predicted to occur for strong surface ligands (small values of k ) with a corresponding value of Km (cf. equations (35) and (36)) that imposes an upper limit on the ambient concentration of the metal that can be present in order to avoid saturation of the surface ligands. More importantly, as pointed out by Hudson and Morel [7], this condition also imposes a lower limit on the carrier concentration. Since the complexation rate is proportional to the metal concentration and the total number of carriers, for very low ambient metal concentrations, a large number of carriers are required if cellular requirements are to be satisfied. 

Instead of electrostatic (or physical) adsorption, metal uptake onto oxides might be considered chemical in nature. In chemical mechanisms, the metal precursor is envisioned to react with the oxide surface, involving as surface-ligand exchange [13,14] in which OH groups from the surface replace ligands in the adsorbing metal complex. In this section it will be shown that a relatively simple electrostatic interpretation of the adsorption of a number of catalyst precursors is the most reasonable one for a number of noble metal/oxide systems. 

The retention of hydration sheaths upon adsorption is more consistent with an electrostatic view of adsorption than a chemical one, since by remaining a relatively large distance away from the surface, the metal complexes are less likely to participate in surface-ligand exchange. 

This mechanism has been quantified by employing surface-ligand exchange of OH or O from the alumina surface with ligands from the adsorbing CPA complex [13], as follows ... 

Figure 6.16 Proposed chemical interaction of Pt complexes with an alumina surface, which involves surface-ligand exchange of either surface OH for Cl ligands (model B1) or Cl ligands from the CPA complex for surface hydroxyls (model B2). (From Shelimov, B., Lambert, J.-F., Che, M., and Didillon, B., J. Mol. Catal. 158, 2000, 91.)...

Since the coordination sphere of a complex of a metal on the surface of a hydrous oxide is only partially occupied by the surface ligands, further ligands may be acquired to form a ternary complex (type A) (Schindler, 1990). 

Surface hydrolysis Surface complexation Surface ligand exchange Hydrogen bond formation... 

The mechanism given is in support of the existence of inner-sphere surface complexes it illustrates that one of the water molecules coordinated to the metal ion has to dissociate in order to form an inner-sphere complex if this H20-loss is slow, then the adsorption, i.e., the binding of the metal ion to the surface ligands, is slow. 

The postulate of steady state during dissolution reaction (Table 5.1) implies a continuous reconstitution of the surface with the maintenance of a constant distribution of the various surface sites and the steady state concentration of the surface complexes. Fig. 5.7 presents experimental evidence that the concentration of the surface ligand - in line with Fig. 5.5a - remains constant during the surface controlled dissolution reaction. 

It is difficult to predict the effect of surface functionalization on the optical properties of nanoparticles in general. Surface ligands have only minor influence on the spectroscopic properties of nanoparticles, the properties of which are primarily dominated by the crystal field of the host lattice (e.g., rare-earth doped nanocrystals) or by plasmon resonance (e.g., gold nanoparticles). In the case of QDs, the fluorescence quantum yield and decay behavior respond to surface functionalization and bioconjugation, whereas the spectral position and shape of the absorption and emission are barely affected. 

Toxicity of nanoparticles is a much more complicated issue as compared with organic fluorophores Nanoparticles may be nanotoxic, they may contain cytotoxic elements or compounds, or their surface ligands/coating may contain toxic species. Nanotoxicity refers to the ability of a substance to be intrinsically cytotoxic due to its size (and independent of its constituent materials). The most prominent example of nanotoxicity is asbestos. Even though there are no systematic studies on the nanotoxicity of different nanocrystals available the results from several cytotoxicity studies suggest that nanotoxicity is not dominating for nanoparticular reporters [85, 86]. 

Munro AM, Plante IJL, Ng MS, Ginger DS (2007) Quantitative study of the effects of surface ligand concentration on CdSe nanocrystal photoluminescence. J Phys Chem C 111 6220-6227... 

Fig. 7 PDA-OTS viral detection systems incorporating OTS monolayer with polydiacetylene linked backbone, lipid side chains, and surface ligand for receptor binding [14]...

---