3.3 Conjugations
3.3.5 Fluorometer analisys
The most performanced conjugation SP33 was also analyzed with fluorometer and the result is reported in figure 3.57:
Figure 3.57: Photoluminescent spectra of SP33 which conjugates dialyzed C-Dots and hydrothermal Bismuth Oxide microparticles with PEG-4000 5%.
Differently to the absorption case, the emission spectrum of conjugation is quite similar to that of used C-Dots (figure??), as expected from high photolu-minescence of C-Dots.
CHAPTER 4
Conclusions
The bottom-up approach has been used for the C-Dots synthesis, which implies their formation starting from the precursors. The precursors in this work are DPA and CA. Initially they are polymerized and then nucleated. In the first step DPA and AC are placed in the aqueous solution and sonicated promoting in this way the aromatic heterocyclization. One of the possible reactions is ob-tained by the initial formation of the derivative of amide from AC and DPA reaction and its subsequent transformation in the random cross-linked polymer structure. Another possibility is that the amino groups can react with two car-boxyl groups of AC molecule forming an heterocyclic structure and so on. The second step, that is the carbonization, can be carried out by heat treatment and/or purification. On one side, the previously obtained precursor is heated to 250°C, 350°C and 450°C for 10 min. in this way the C-Dots condense and nucleation occurs. Subsequently they were dissolved in deionized (DI) water for 15 min in a ultrasonic bath in order to promote their dissolution and break any aggregations. To purify C-Dots, the solution thus obtained was centrifuged at 10000 rpm for 15 minutes. Only the solubilized part was taken and oven dried, instead the residue was removed away. In this way, only the soluble C-Dots were preserved. Comparing now C-Dots thermally treated at different temperature, it can be deduced that increasing the temperature the C-Dots structure becomes more and more aromatic, as could be seen from the from the band of νC=C at 1600cm1 and that of unsaturated νC−H at 3000cm1, characteristic of the aromatic structures. This result was expected because due to the thermal treatment the C-Dots condense and carbonization/nucleation takes place. On the other hand, the purification is performed by dialysis, explained though the concept of os-mosis. Thanks to the pressure gradient established between the inside and the outside of the membrane the precursor is purified, proportionally to the mem-brane cutoff (4000 g/mol molecular mass cutoff were used), from the too small organic molecules and oligomers which didn’t polymerize in the previous step.
The previously obtained C-Dots precursor was put inside the dialysis membrane and agitated. It can be hypothesized that in both cases the C-Dots are composed by sp2 and sp3 hybridized carbon structures with size of few nanometers and present a random cross-linked polymer structures which are organized in parallel
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planes with external functional groups are obtained and. Comparing the two purification methods it arise that the heat treatment results in a more heteroge-neous and disordered structure with many external functional groups, instead the dialyzed one has more compact structure, less flexible with an intense of C=C and C-H bonds, respectively at 1600cm1 and 3000cm1, typical of the aromatic rings. So, it is possible to choose the method to use basing on the properties one want to get Another set of C-Dots synthesis was repeated including PEG-4000 as a non-ionic reagent and adding it in 1:1, 0.5:1 and 0.1:1 proportions with respect to Citric Acid and subsequently treated at 250°C and purified by the centrifugation or directly by means of dialysis membrane. In this case PEG acts as a separator which avoids the C-Dots aggregation, promotes the aromatic compounds condensation and, reacting with their functional groups, maximizes the subsequent conjugation. Indeed, it is observed from obtained spectrums that increasing PEG concentration enhances the νC=C (1600cm1) of the aromatic ring.
Also the peak of the saturated νC−H (3000cm1) is increased due to its presence in PEG. From the Raman spectrum it can be observed that the C-Dots synthesized in these ways present a high number of well defined peaks, but are very fluores-cent due the presence of its functional groups. So even though the low intensity laser (green light 514 nm was used) is used the ration signal/noise remains low covering a part of the bands, therefore the presence of D and G bands common to all samples can be only supposed. Fluorometer was implemented to take cer-tainty of their photoluminescent features. Also absorption spectra was studied.
For the hydrothermal synthesis of the Bismuth Oxide particles, Bi(N O3)3×5H2O and HN O3 are used as precursors. The solution is made in an acidic environ-ment in order to promote the Bi(N O3)3 solubility, which is poorly soluble in water. Then a base, sodium hydroxide, is added at room temperature, because this reaction is exothermic, forming bismuth hydroxide which is extremely un-stable. The synthesis is stirred at heated at 80° for 3h forming the precipitate of Bismuth Oxide, which is recovered by filtering. This synthesis is repeated adding fourth different active surfactants (PEG-4000, SLS, LA and PVP) in order to test their ability to direct the planes growth of the synthesis, thanks to their hydrophilic and hydrophobic, and avoid the aggregation between microparticles.
From FESEM imagines, the particles of Bismuth Oxide without active surfac-tants show the irregular structures, there are breaks due to the planes that grow into a castle of paper and collapse. The planes grow is complicated and there is not preferential morphology and direction. Indeed active surfactants, thanks to their hydrophilic and hydrophobic, orient and aggregate the triangular planes either parallel one to each other or intertwined. The more regular growth were obtained adding PEG and SLS as active surfactants, the structures are organized in tetrahedral shaped. The situation is different for PVP and LA. Their mor-phology is irregular, disordered and without preferential planes of growth. LA contains different geometrical shapes inside its sample. This can be explained by the fact that maybe these active surfactants have not dissolved and consequently they have not performed their function. Analyzing Raman spectrums of these microparticles, it is evident that the characteristic peak of Bi(N O3)3 (1050cm1)
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disappears, suggesting that the reaction is occurred completely. Also the peaks of Bismuth Oxide (below 500cm1), denoting theνB−O in different proportions due to different active surfactants, are well evident. Furthermore, it is observed that for PVP the structure is grown bad and it is strongly defected. Also the solid state syntheses are performed starting from Bismuth Nitrate pentahydrate with-out solvent. Thermal decomposition of bismuth is carried with-out using different calcination temperatures in a gradual way (200°C for 1h, 300°C for 2h, 425°C for 2h obtaining the heterostructures containing Bi2O3 and Bi5O7(N O)3. Their Ra-man spectrum still contains the typical Bismuth Nitrate peak, so the conversion in Bismuth Oxide is only partial. Analyzing the same sample in different points, it is evident that the peaks are not all the same because this is an intermedi-ate level, where the structure is extremely heterogeneous. Conjugation the solid state synthesized particles are stirred up with different types of purified C-Dots for 48h. Subsequently the product is centrifuged in order to separate the soluble part, containing C-Dots which have not conjugate, and residue, which is logi-cal that contains the Bismuth Oxide particles conjugated with C-Dots through the weak interactions of H-bond with hydroxyl functions and between the π or-bitals of the C-Dots, due to their aromatic structure, and d oror-bitals present on the microparticles surfaces. The interaction with molecules of Bismuth Oxide stiffens the structure and changes the vibrational modes of the C-Dots, as is seen from their IR spectrums. The band due to unsaturated νC−H (3000cm1)) is inhibited due to the π − d interaction with the particles’ surface. Also νC=C (1600cm1) band decreases in intensity because the ring vibration is inhibited due to these interactions. The Raman spectrum reminds to that of C-Dots, but in this case it is difficult to appreciate differences in peaks due to strongly noised C-Dots Raman signal. Due to the fact that there no any characteristic peak of Bismuth Oxide or Bismuth Nitrate, it can be deduced that the C-Dots coverage layer is quite thick. Same syntheses were repeated for hydrothermally synthetized microparticles containing PEG-4000, since they demonstrate the best structural morphology. The microparticles synthesized with this active surfactant present a most regular thetraedrical morphology. Also in these cases IR and Raman spectroscopy confirm the conjugation between Bismuth Oxide microparticles and C-Dots. Differently to the solid state particles, in this case some Raman spectra present also peaks related to Bismuth Oxide, due to different vibration modes Bi-O aroused from interaction.
The direct synthesis of microparticles and their conjugation with C-Dots is made by repeating all steps of the hydrothermal synthesis but adding also C-Dots after the Bismuth Hydroxide formation. Also here the inhibition of νC=C (1600cm1) and unsaturated νC−H (3000cm1)) band is observed by IR inspection.
From Raman spectrum the Bismuth Oxide peaks can be observed, but there are also bands probably due to the C-Dots aromatic structures containing bands D and G.
FESEM images of conjugation containing dialyzed C-Dots and hydrother-mally synthesized microparticles revel the exfoliation of Bismuth Oxide planes as a consequence of conjugation. C-Dots absorption and emission bands were
produced and investigated, due to their excellent optical properties. All these aspect are confirmed with some experiments presented in literature . All the results of this project have been satisfactory and they stimulate improvement of the synthesis methods and investigation of the nanoparticles core structure and properties.
CHAPTER 5
Acknowledgements
First of all, I would like to express my gratitude to my supervisor professor Al-berto Tagliaferro, who assisted and guided me throughout this project. Thank you for giving me this opportunity to be a part of your research group, it was a great experience.
I am deeply grateful to dr. Mattia Bartoli for his assistance at every stage of my project, insightful comments and suggestions. Thank you for your patience and availability even on Sunday, I really appreciate it.
I would like to extend my sincere thanks to Andrea, Cristian, Filippo and Giovanni for having shared, for better or for worse, these months in laboratory with me.
I would also like to thank my beloved mother and Giovanni, whom without this would have not been possible. Thank you for your continuous support and understanding.
My heartfelt thanks to Vincenzo, my better half, for your understanding, your never-ending encouragement, unconditional love and for always being there for me. I am happy to have you by my side and share many experiences with you
Special thanks go to Martina infinite friendship, understanding, support and having shared so many experiences together.
Thanks should also go to Viktoriia, even if you are far away I know that I can always count on you.
Thanks to Giordano, to love me as a sister. I always remember with a smile the moments spent together.
I wish to extend my special thanks to Anna Chiara, Marta and Nicoletta, who can always make me smile.
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Thanks to Marta, a good friend rather than roommate, I have always appre-ciated your presence and support.
Heartfelt thanks go to my roommates Agata, Alessandra, Giada e Manuela for your support, good words when I needed them. Thank you above all for having endured my endless and annoying alarm clocks in this last period.
Thanks to all the people who have been close to me over the years: Alessio, Andrea, Linda, Rebecca e Simone. You are always in my heart.
CHAPTER 6
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