ESTUDIO DEMOGRÁFICO

D-glucose’s hydrophilicity creates an equilibrium where reducing agent is likely to be found both within the nanogel and in the bulk solution. This provides a route for reduction of the platinum complexes outside of the macroparticles which have been shown to provide growth control for the metal nanoparticles. In order to minimize the reduction of the platinum from occuring outside of the nanogel, valeraldehyde, Figure 4. 2 D, was chosen as the reducing agent for the system. Valeraldehyde’s five carbon tail lowers the solubility in water sufficiently to shift the equilibrium constant to a majority of the reducing agent residing within the nanogel. The electronic spectra of valeraldehyde is also transparent within the UV-vis region providing an unambiguous method for observing the reduction of Pt(IV).

Another advantage of valeraldehyde as a reducing agent is the clear oxidation reaction, Scheme 4. 3, it undergoes. One site is available for oxidation in this molecule opposed to the multiple sites in ascorbic acid, citric acid, and glucose. This allows for the number of electrons released via the oxidation of the reducing agent to be precisely known and controlled. The reduction mechanisms of ascorbic acid, citric acid, and glucose are well known in biological systems where they are oxidized via enzyme catalyzed chemical reactions. However, mechanisms for uncatalyzed reactions are unknown, making it difficult to predict the quantity of electrons released through oxidation of the reducing agents. Therefore, the use of valeraldehyde is advantageous since the number of electrons available for reduction of the platinum species can

145 O + H2O O OH 2H+ _{+ 2e}- + Pt+4 _Pt0 4e- ₊ 2 O ₊ 2(H2O) O OH 4H+ + + 2 + Pt+4 2 Pt0

146 be easily quantified providing a significant benefit for reactions where changes to the rate of reduction are required.

Initial runs with valeraldehyde were performed in a similar manner to the reactions in the previous sections with H2[PtCl6] as the cross-linker in a 7:1 donor to platinum ratio and a 10:1

reducing agent to platinum ratio. TEM grids were prepared in the same manner as described previously and a representative image is shown in Figure 4. 28. Sizing of 74 particles seen in the images obtained via TEM resulted in an average size of 1.18 ± 0.23nm. The histogram of the data shown in Figure 4. 28 shows a fairly normal distribution curve indicating that the average size of the particles is, in fact, trending towards the mean. These results are consistent with those seen for the reduction of H2[PtCl6] by the previous choices of reducing agent. This

indicates that reduction of Pt(IV) utilizing valeraldehyde remains controlled as in the systems employing D-glucose.

4.4.4 Conclusions

Ascorbic acid, citric acid, D-glucose, and valeraldehyde were assessed for their ability to produce well defined platinum particles when used as the reducing agent for the platinum- PDMAEMA system. In all cases, H2[PtCl6] was necessary as the complex provides both the

desired metal as well as the proton necessary to form the cross-linked nanogel. Initial results were obtained exploiting ascorbic acid as the reducing agent. While good results of the particles being synthesized were obtained, an intense peak in the UV-vis spectrum at approximately 260nm made ascorbic acid unsuitable for studying the reduction spectra of the Pt (IV) to Pt(0) process. However, this peak did provide evidence that the particles’ surface remains active since they are able to process the large excess of ascorbic acid. Citric acid was employed next since it the less absorption is seen in the region of interest for the platinum complexes. Once again, control of the particle synthesis is observed, however, a peak is observed in the UV-vis spectra in an important region of the platinum spectrum. For both of the previous reducing agents, destruction of the nanogel after their introduction is observed in the DLS analysis.

147 Figure 4. 28 - TEM Image and Analysis of Pt Particles Reduced from H2[PtCl6] by Valeraldehyde

148 a neutral reducing agent should provide a route to a mechanism that results in more control of the formation since the nanogel is never destroyed. D-glucose was chosen as the reducing agent for several reasons: it is highly soluble in water, will not release protons that will destroy the nanogel superstructure, and lack of absorption in the UV-vis region of importance for reduction of Pt(IV). Use of D-glucose resulted in the continued presence of the nanogel superstructure after the introduction of the reducing agent as shown in the DLS results. Particles of significantly smaller sizes were produced with this mechanism demonstrating the importance of maintaining the nanogel.

D-glucose as well as the platinum complex are highly soluble in water and while the desired reaction occurs in the nanogel an equilibrium will occur between the aqueous bulk and the less polar nanogel superstructure. Consequently, the reduction could be occurring in the bulk as well as in the controlled nanogel environment. In an effort to diminish the effect of the reaction occurring outside of the nanogel, valeraldehyde was chosen as a possible candidate for a lower solubility reducing agent. Small reasonably monodisperse particles were synthesized utilizing valeraldehyde as the reducing agent. This provides another reducing agent which is able to work with the PDMAEMA nanogel system to produce well defined nanoparticles. While all four compounds performed well for the platinum nanoparticle synthesis, ultimately the D-glucose and valeraldehyde proved most useful for these and further studies.