Comparisons of Figure 4. 13 to Figure 4. 17 shows that there is one significant difference between the two images; the uneven electron density across the sphere in Figure 4. 13 compared to the even density of the sphere in Figure 4. 17. Although it appears as though the dark spots in the image produced from the H2[PtCl6]-PDMAEMA solution are area of reduced Pt nanoparticles,
the electronic spectra of the solution taken immediately prior to deposition onto the TEM grid confirms that the Pt is still in its Pt(IV) state. These dark areas are also seen in images of K2[PtCl6]-PDMAEMA solution, Figure 4. 14, where the Pt is also observed to be Pt(IV) by UV-vis
spectroscopy. One explanation for this phenomenon is that the molecular Pt is acting as an electron stain for the PDMAEMA structures.
Pt complexes have previously been utilized as staining agents in biological systems to allow for the use of TEM to obtain images at the subcellular level.18-20 In these systems, the Pt complexes, are substituted for the traditional Gram stain, iodine-potassium iodide, as the complexing anion for crystal violet(CV)18, 20, 21 or the organism is washed with Pt, or another metal complex such as uranium or gallium, supplemented buffers19. For the Gram stain systems, CV and the Pt complex migrate into the cell and the CV+ carbocation forms a precipitate through the complexation with the Pt anion18. The samples are then washed to remove any excess CV and Pt from outside of the cells18, 20, 21. What remains is electron opaque Pt-CV precipitates within
124 Figure 4. 17 - TEM Image of PDMAEMA and Succinic Acid
125 the cellular structure which allow for TEM; providing images of the subcellular structures21. In the traditional stain experiments, cells were washed with metal supplemented PIPES buffer and incubated for several hours and finally washed with PIPES buffer to remove excess metal ions19. TEM images of these organisms reveal detailed structural features due to metal associated with the membranes of the cells compared with a barely discernable structure when an unstained cell is imaged19.
Both staining techniques result in systems strikingly similar to the Pt-PDMAEMA system being studied. While the biological applications are used to image cellular structure, these are analogous to the polymer structures in the inorganic system. Cell walls and other cellular structures are composed of biological polymers such as polysaccharides19 and peptidoglycan20, therefore, the present a similar system to our organic polymers. As with the biological stains, it is believed that in the Pt-polymer process the Pt molecules are aggregating within the polymer structure and acting as an electron opaque stain. While the polymer is electron dense enough to be imaged via TEM, the addition of the Pt aggregates creates darker areas within the large structures seen in the images much like the metals in the biological stains provide more electron dense areas within the cells structures with which they have associated18-21. In the case of K2[PtCl6]-PDMAEMA, the Pt is seen in higher contrast since less polymer will be present in each
oblong structur. Because the Pt-polymer structure is much weaker so fewer strands will be interlinked with each other which creates less electron dense polymer areas.
4.3.4 Conclusions
The observations presented within this section provide insight into the early portion of the mechanism by which PDMAEMA is able to direct the formation of Pt nanoparticles. Scheme 4. 1 A outlines the formation of large Pt-PDMAEMA particles formed through ionic interactions between the cationic protonated PDMAEMA polymer chain and the di-anion Pt unit. DLS data confirms the presence of large particles when H2[PtCl6] is used and lack thereof for K2[PtCl6],
Figure 4. 15, providing evidence of the importance of an available proton and therefore of the creation of cationic polymer chains. The use of organic acids provide a means to investigate the
126 importance of the di-anion by removing any Pt intermolecular forces that may have been influencing the formation of the structures seen in Figure 4. 13 and Figure 4. 14. From the DLS data, it is shown that the diacids, succinic and sulfuric, result in the formation of large particles, >200nm, while the monoacids, hydrochloric and acetic, show nothing significantly larger in size than the neat polymer. This study confirms the role of the dianion as a cross-linker of the protonated PDMAEMA since the diacids were able to create particles with the polymer and the monoacids did not show this ability.
TEM analysis of the succinic acid-PDMAEMA resulted in images very closely resembling those seen in images of H2[PtCl6]-PDMAEMA providing evidence that the shape of the large
macrostructure is driven by the Pt-polymer ionic bonds rather than ion-dipole intermolecular forces responsible for the shapes seen in images of K2[PtCl6]-PDMAEMA. The only substantial
difference seen between the succinic acid-PDMAEMA and H2[PtCl6]-PDMAEMA images is the
variation in contrast across the large particles with the succinic acid images having almost no variation and the presence of many small dark areas in the images of the particles containing platinum. This can be rationalized by examining techniques found in biological imaging where metals are used as electron opaque stains to allow TEM to be utilized to image at the subcellular level. These methods utilize metals as Gram or traditional staining agents adding them to organisms where the Pt-CV complex or metal complex associate with cellular structures, which can are biological polymer, allowing for TEM imaging. From these experiments a direct correlation can be seen with Pt acting as a electron opaque stain for PDMAEMA even as Pt(IV). All of these studies present ample verification of the formation of the structure seen in Scheme 4. 1 A which will be termed the nanogel from this point forward.