Crooks et al.
2. Intradendrimer Metal Displacement Reactions In the previous section it was shown that dendrimer-encapsulated metal nanoclusters can be prepared by direct reduction if the corresponding metal ions can be extracted into the dendrimer interior. However, this approach is not suitable for encapsulation of Ag particles, because the equilibrium between PAMAM dendrimers and Ag ions does not strongly favor the intradendrimer complex. Although it is not possible to prepare Ag particles inside Gn-OH by direct reduction of interior ions, stable, dendrimer-encapsulated Ag particles can be prepared by a metal exchange reaction. In this approach, dendrimer-encapsulated Cu nanoclusters are prepared as described in a previous section (57), and then upon exposure to Ag the Cu particles oxidize to Cu2 ions, which stay entrapped within the dendrimer at pH values larger than 5.5, and Ag is reduced to yield a dendrimer-encapsulated Ag nanoparticle (Fig. 8). Figure 9 shows UV-vis spectra of an aqueous G6-OH(Cu2)55 solution before and after reduction. At pH 7.5 a strong LMCT band is evident at 300 nm (4000 M1 cm1), which (as discussed earlier) is not present in the absence of the dendrimer or Cu2 (90). As signaled by the loss of the band at 300 nm, chemical reduction of G6-OH(Cu2)55 with excess of BH4 results in formation of intradendrimer Cu nanoclusters (Fig. 9a). The postreduction spectrum is dominated by a monotonic and nearly exponential increase toward shorter wavelengths, which results from the interband transition of intradendrimer Cu clusters (57). Ag is a stronger oxidizing agent than Cu2, and therefore Eq. (1) leads to conversion of a dendrimer-encapsulated Cu nanoparticle to Ag.* Cu 2Ag Cu2 2Ag
Figure 9b shows the spectroscopic evolution of such a reaction. Spectrum 1 is the UV-vis spectrum of G6-OH(Cu55), which is the same as spectrum 2 in Fig. 9a. When Ag is added to a G6-OH(Cu55) solution at pH 3.0, a new absorbance band centered at 400 nm appears (spectrum 2), which corresponds to the plasmon resonance of Ag particles (82). When the pH of the solution is adjusted to 7.5 (spectrum 3), the Ag plasmon peak does not change much, but a new peak at 300 nm ( 4000 M1 cm1) appears. This is the G6-OH(Cu2)55 LMCT band discussed earlier, and it indicates that when the interior tertiary amines are deprotonated Cu2 resides within the dendrimer after intradendrimer oxidation of Cu. Thus, both the metallic Ag nanoparticles and the Cu2 ions generated by the displacement reaction are present within the dendrimers simultaneously. Such versatile and well-defined microenvironments are unique and should find a variety of
* Because the dendrimer-encapsulated nanoparticles are so small, they do not have the properties of bulk metals. Therefore, it is not possible to calculate the reduction potential for the exchange reactions from tabulated literature data.
Chemistry of metal nanoparticles