3. Results and discussion
3.1 Synthesis and characterization of Au@MgF 2 core-shell nanoparticles
3.1.9 Attempts to synthesize magnesium citrate stabilized gold nanoparticles for Au@MgF 2
synthesis
Conventional methods known for the synthesis of metal@metal oxides core-shell nanoparticles are not suitable for metal@metal fluorides core-shell nanoparticles fabrication due to significant differences between metal oxides and metal fluorides. Therefore, an unconventional approach based on using magnesium containing stabilizing agent for gold nanoparticles synthesis will be examined.
Looking for already known ones from the literature working as stabilizing agent for gold nanoparticles synthesis was not successful. Therefore, sodium citrate in the procedure 4.1 was replaced by a equivalent amount of magnesium citrate. The synthesis was performed according to the procedure 4.1. Magnesium citrate was dissolved in MQ water and heated until the boiling point under magnetic stirring. Afterwards tetrachloroauric acid was added to the solution under intensive magnetic stirring and kept in the same conditions for 15 min. Within this time the solution became black. During cooling down the solution to 90 °C a black cloggy sediment was formed. The solution became transparent and no nanoparticles were found in it (UV-Vis measurements, not shown in this work). Due to the presence of a different cation, magnesium fluoride does not exhibit stabilizing properties unlikely to the magnesium citrate.
3.1.10 Synthesis of Au@SrF2 and Au@ZrF2 core-shell nanoparticles and their characterization As mentioned above, problems with the core-shell formation can be connected to the citrate stabilized gold nanoparticles, as well as to magnesium fluoride properties. Ritter et al. successfully synthesized core-shell nanoparticles containing several metal fluorides164 besides magnesium fluoride; therefore, attempts to synthesize Au@SrF2 and Au@ZrF2 were taken. Applying SrF2 and ZrF2 should be beneficial for TEM investigations, because of their, higher than MgF2, molar masses (higher contrast on the TEM images is expected).
Both strontium and zirconium fluoride were synthesized in the presence of citrate stabilized gold nanoparticles. Strontium fluoride was synthesized from two different precursors: strontium acetate and strontium chloride (procedure 4.13). Both strontium precursors have limited solubility in ethanol and required special treatment. Strontium acetate was dissolved in mixture of denatured ethanol and TFA in order to clear up the sol and magnetically stirred for 30 min. Strontium chloride was dissolved in mixture of ethanol and acetic acid in the volume ratio 3:1.172 Obtained solutions were transparent
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and clear. Zirconium acetate is soluble in methanol; therefore, fabrication of Au@ZrF2 nanoparticles was performed in methanol (procedure 4.13). 173
Figure 14: TEM images samples 12.1-21.2 prepared by addition of strontium acetate solution to gold nanoparticles ethanolic solution and fluorinated afterwards (a, b) and strontium chloride solution to gold nanoparticles ethanolic solution and fluorinated afterwards (c, d). The gold to strontium fluoride ratio was fixed as 1:5 for all samples. Scale bar 50 nm.
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Figure 15: TEM images of samples 12.1 and 12.3 prepared by addition of zirconium acetate solution to the gold nanoparticles solution and fluorinated afterwards. The gold to zirconium fluoride ratio 1:2 for sample 12.1 (a-c) and 1:10 for sample 12.3 (d-e). Scale bar 50 nm.
As expected, figures 14 and 15 show that both strontium fluoride and zirconium fluoride exhibit a higher contrast, to the carbon coated TEM grids, than magnesium fluoride. Thus TEM investigations of the formed structures are significantly easier. Figures 14 a and b indicates that core-shell nanoparticles were not formed; gold nanoparticles are randomly distributed in strontium fluoride matrix. Figure 14c also show random distribution of gold nanoparticles in strontium fluoride matrix;
however, at this sample a few core-shell looking particles were found (figure 14 d). Regarding to the previous TEM investigations described in this work, figure 14 d indicates rather location of the gold nanoparticles on the surface strontium fluoride, than core-shell nanoparticles formation. In contrast to the strontium fluoride, zirconium fluoride forms network structures. Some gold nanoparticles are embedded into this network (figures 15 a, b, d, e) and some are not (figures 15 c, f). A correlation between location of gold nanoparticles (inside or outside the zirconium fluoride matrix) and the zirconium fluoride concentration was not observed. Structures indicating a core-shell structure formation were not found. It indicates that sodium citrate stabilized gold nanoparticles are not suitable for core-shell nanoparticles synthesis containing metal fluorides shell. Experiments performed for magnesium fluoride suggest, that even exchanging the stabilizing agent for stabilizing agents known from a successful metal@metal oxides core-shell nanoparticles formation, does not allow core-shell nanoparticles formation in the case of metal fluorides.
3.1.11. Summary
Combination of the plasmonic properties of gold nanoparticles and the extraordinary optical properties of magnesium fluoride can be very beneficial for a desired plasmonic substrate. The approach of Au@MgF2 core-shell nanoparticles synthesis, described in this chapter, turned out to be
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extremely difficult. To the best of our knowledge, such structures has not been described is the literature. Therefore, undertaken attempts of Au@MgF2 core-shell nanoparticles synthesis were based on the knowledge of metal@metal oxides and metal fluorides@metal fluorides doped with rare earth metals core-shell nanoparticles synthesis. Despite the use of different magnesium precursors, several gold nanoparticles stabilizing agents, different solvents and different reaction conditions, a suitable approach of Au@gF2 was not developed. Moreover, nanoscopic characterization of obtained structures was very difficult due to the presence an excess of magnesium fluoride. The undertaken attempts to remove redundant magnesium fluoride were not successful. For such systems, obtaining a solid proof of core-shell nanoparticles formation using conventional analytical is very challenging.
Excess of magnesium fluoride in the samples hinders TEM and EDX investigations of a separated gold nanoparticles (e. g. chapter 3.1.8). The excess of magnesium fluoride and in some cases aggregation of gold nanoparticles, exclude the possibility of DLS or XPS investigations. The presence of gold nanoparticles in the samples excludes NMR measurements. However, based on the obtained TEM and in some cases EDX data presented in this work, can be concluded that Au@MgF2 nanoparticles were not formed. This is in line with conclusions made by Ritter et al. that magnesium fluoride does not form a shell even around metal fluoride cores.164 Studies performed by Karg indicate that formation of new seeds of magnesium fluoride is preferable over increasing size of the existing MgF2 nanoparticles or eventual shell formation.163 The reason of a different than other metal fluorides behavior of magnesium fluoride can be only speculated. One potential reason can be its, different than other metal fluorides, crystalline structure of rutile. Another possible scenario is a lack of interactions between gold nanoparticles and metal fluorides (core-shell nanoparticles formation was also not observed for strontium fluoride and zirconium fluoride). Investigated in this work linkers (agents stabilizing the gold nanoparticles), suitable for metal oxides, turned out to be not suitable for metal fluorides.
Due to many difficulties during the synthesis and characterization of the core-shell nanoparticles, this approach is not considered as a suitable approach for coating of the plasmonic substrates with magnesium fluoride. Therefore, another approach, based on magnesium fluoride dip-coating of plasmonic nanoparticles immobilized on the glass substrate was proposed.
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