Ascorbic and Citric Acid

Initial results, section 4.2, confirmed that utilization of ascorbic acid in the Pt-PDMAEMA system will result in the formation of small monodisperse nanoparticles if protons are the counter- ion of the platinum complex and the platinum ion is a dianion. However, due to the large peak in the electronic spectra, Figure 4. 8, which overlaps the region of the Pt(IV) Lπ→M transition, it can

be used only for certain studies. DLS and TEM analyses were performed on Pt-PDMAEMA reactions reduced with ascorbic acid. From these observations, the mechanism seen in Scheme 4. 1 B was determined. This process is characterized by the destruction of the nanogel due to the protonation of every amine site in the PDMAEMA chains, releasing the platinum anions from their tight association with the polymer.

The acidic character of ascorbic acid is not immediately apparent as there is no carboxylic unit within the molecule. Ascorbic acid’s acidic character results from the conjugated system formed from the double bond, carbonyl group, and hydroxyl group. Conjugation within the molecule stabilizes the conjugate base by providing a route to two resonance structures, Figure 4. 18, greatly increasing the acidity of the hydroxyl group. Therefore, the use of ascorbic acid as a reducing agent will result in the proton injection and considering the large quantity of reducing agent utilized for the reduction of platinum in the Pt-PDMAEMA system this will create a large excess of protons in the reaction. Protonation of every unit within the PDMAEMA chains greatly increases solubility in aqueous solution creating interactions with water and the conjugate base that compete with the ionic bond formed with the platinum dianions breaking apart the nanogel.

The effect of the addition of ascorbic acid was monitored by performing DLS immediately after adding ascorbic acid to the reaction solutions. Spectra shown in Figure 4. 19 show the analysis of the Pt-PDMAEMA system at three different stages: prior to, immediately following, and three days after the addition of ascorbic acid to the reaction solution. Prior to the addition of ascorbic acid the average hydrodynamic diameter is 93.1nm representing the nanogel formation

128 Figure 4. 18 - Ascorbic Acid and Ascorbate

129 Figure 4. 19 - DLS Spectra of Ascorbic Acid Reduction of Pt-PDMAEMA

130 which is expected with the addition of H2[PtCl6] to PDMAEMA as seen in the previous studies.

Immediately following the addition of ascorbic acid to the reaction mixture, the average hydrodynamic diameter dramatically decreases to 5.5nm. The large decrease in particle size approximately to that of neat PDMAEMA solution confirms the destruction of the nanoparticles as hypothesized in the mechanism proposed in Scheme 4. 1 B.

By analyzing the reaction after complete reduction of the platinum, DLS data corresponding to the point where aliquots are removed for TEM analysis is obtained. Particles with average hydrodynamic diameters of 103nm are seen via DLS measurements of fully reduced solutions. These observations coincide with part C of Scheme 4. 1. During this stage of the synthesis, the platinum particles are formed as the polymer chains adsorb to the surface fully wrapping around the particle. At this point, 1.12 ± 0.25nm are observed in TEM images of the same solution. The ten-fold increase in size disparity between data reported in TEM and DLS measurements is directly related to the PDMAEMA coating the platinum particles. This coating will not be electron dense enough to be observed by TEM but will effect DLS measurements. Swelling of the PDMAEMA chains coating the platinum resulting from particles being analyzed while in a dilute aqueous solution will coincide with the larger particle size seen in DLS analysis.

Citric Acid, shown in Figure 4. 2 B, was utilized for several experiments since it does not have a intense peak in the same region as ascorbic acid and the Pt(IV) Lπ→M transfer band.

However, there is a less intense peak in the electronic spectra, Figure 4. 20, at ~205nm which overlaps the Lσ→M transfer band of Pt(IV). Therefore, while it is useful for observation of the

decline of the Lπ→M transfer peak, Figure 4. 21, interference from the electronic spectrum of

citric acid will make it unsuitable for analysis concerning the change in the Lσ→M throughout

reduction of the Pt(IV) species. Since this is also an acidic reducing agent, similar DLS spectra are expected. As in the experiments performed with ascorbic acid, the citric acid is in a 10:1 ratio with platinum, so there will be a large quantity of protons injected into the solution with the addition of citric acid.

131 Figure 4. 20 - Citric Acid UV-vis Spectrum

132 DLS analysis was performed at three separate points throughout the experiment, shown in Figure 4. 22: prior to, immediately following, and three days after the addition of citric acid to the reaction solution. In these spectra similar results are seen to those of ascorbic acid; initially, prior to the addition of citric acid large particles are seen with an average hydrodynamic diameter of 90.1nm, followed by destruction of the nanogel seen in the spectra of the solution immediately following the addition of citric acid where the average hydrodynamic diameter of the particle is reduced to 15.5nm, and finally after several days of reduction a return of large particles with an average hydrodynamic diameter of 71.8nm. These results further confirm the mechanism proposed in Scheme 4. 1 and broadens the applicability to any acidic reducing agent.

Determination of the platinum nanoparticle size was achieved by obtaining TEM images of the reaction solution; a representative image and size analysis is shown in Figure 4. 23. Analysis of 150 platinum nanoparticles resulted in an average size of 0.92 ± 0.26nm. These particles are slightly smaller and have a higher relative standard deviation compared with those produced using ascorbic acid as the reducing agent, Figure 4. 11 and Table 4. 1. Inspection of the histogram, Figure 4. 23, for the platinum nanoparticles reduced with citric acid reveal a much less normal size distribution than in the ascorbic acid reaction with significant high-end tailing. This indicates that there is wider variability in the speed at which the nanoparticles become fully stabilized by the PDMAEMA with the larger particles being produced when reduction of the platinum complexes occur further from the polymer chains while the very small particles were reduced while still in a close proximity to the polymer.

Adherence to the mechanism proposed in Scheme 4. 1 provides an adequate rationale for these observations. Once the nanogel is destroyed, dispersion of the platinum anions into the bulk will occur irregularly and reduction of some platinum will occur in vicinity of the polymer chains while some will begin reducing farther from the PDMAEMA chains. This behavior results in some particles taking an extended time to be fully enveloped and stabilized by the PDMAEMA chains forming larger particles. On the other hand, other particles reduce in a close vicinity to the PDMAEMA chains and are quickly stabilized forming very small particles. When these two

133 Figure 4. 22 - DLS Spectra of Citric Acid Reduction of Pt-PDMAEMA

134 Figure 4. 23 – TEM Image and Analysis of H2[PtCl6] Reduced By Citric Acid

135 processes are combined a larger size distribution will be observed. Previous studies shown in section 4.2, illustrate the importance of the nanogel in the ability of PDMAEMA to control the formation of small monodisperse platinum particles. In the work presented within this section it is apparent that during the reduction by acidic reducing agents of the Pt(IV) species to Pt(0) the nanogel is disrupted through the injection of a large number of protons, which results in the protonation of every DMAEMA unit of the polymer. Considering the impact of the nanogel on the control as seen in the cation studies, it appears beneficial to attempt to maintain the Pt- PDMAEMA particles throughout the reduction. For this reason utilization of a neutral reducing agent was pursued.