I also thank the other colleagues of the department for their concern and motivation for my efforts. Sincere thanks to all colleagues of the Department of Chemistry for all kinds of help.
Metal Nanoparticles
These particles, individually as well as in their core-shell architecture, give rise to attractive chemical, optical, magnetic, electronic and biochemical properties in the presence of light, magnetic field, electric potential and biological environments. Many applications (such as sensors, medical diagnostics, homogeneous catalysis, etc.) make use of the properties of the individual or core-shell NPs.
Organic Nanoparticles
The choice of shell material of the core-shell NP is generally dependent on the end application and use. Furthermore, the occurrence of the well-known Fano resonance was demonstrated using metal NP core-shell structures with a silica spacer layer in between [120].
INORGANIC–ORGANIC HYBRIDS
SUPERSTRUCTURES OF NANOPARTICLES
- Strategies for Self-Assembly
In solution, the assembly of NPs is governed by the balance of attractive and repulsive forces. Self-assembly of NPs on templates leads to geometries that are complementary to those of the templates.
In the Langmuir-Blodgett technique, NP monolayers are formed at the water–air
Properties of Nanoparticle Self-Assembled Structures
The coordinating ligands play the role of mediating the electronic coupling between NPs and thus the properties of the solids composed of NPs. An exciton generated in a smaller NP quickly migrates to the larger NP toward the smaller energy gap.
Applications of Self-Assembled Nanoparticles
“Hot spots” generated by localized electromagnetic fields from assemblies of metal NPs have been used for the detection of molecules via surface-enhanced Raman scattering (SERS) [327–329] . Multifunctional hybrid assemblies of magnetic and metallic NPs have also been used for protein separation [336] and catalyst recovery [337] .
MOTIVATION BEHIND THE THESIS
The separation of self-assembled NPs into polymersomes, liposomes or polymer capsules paves the way for the production of multifunctional carriers for the delivery of biologically active species or sensory applications in biological, medical and pharmacological fields [338–340]. Separation of NPs into capsules allowed remote manipulation of carriers by applying an external magnetic field, identification of carriers by measuring their fluorescence, and controlled release of cargo molecules by locally heating the system to deform or dissolve the capsule walls. [340].
OUTLINE OF THE THESIS
The formation of the crystalline shell of pH was determined using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. Furthermore, upon treatment of the core-shell particles with a solution consisting of NaCl and HCl (pH < . 3), the Au shell can be dissolved, which subsequently releases pH molecules.
TEM observations also indicated melting of the organic layer in the presence of a high-energy beam and at longer exposure times. The appearance of a longitudinal SPR band at a longer wavelength was an indicator of assembly formation.
On the other hand, TEM images of the dispersion on day 3 indicated shell formation around the NPs (Figure 2.5). However, the width of the peaks at these angles indicated the presence of Ag NPs.
CONCLUSIONS
The formation of the crystalline shell of the organic part was established using XRD and TEM studies. Delineation of the different steps involved in the formation of pHA@Au core-shell NPs and the release of pHA molecules after Au shell etching. The appearance of color was indicative of the formation of Au NPs in the reaction medium.
NaCl/HCl aqueous solution was prepared by having the total Cl ion concentration equal to 0.375 M, while the ratio of concentrations of NaCl to HCl, i.e. [NaCl]/[HCl] was kept as 1, and such that the pH of the solution was less than 3.
RESULTS AND DISCUSSION
The behavior of the plasmon band, ie. change in the presence of CTAB, was similar for Au NPs prepared from different concentrations of HAuCl4. For example, they observed that the typical size of a core-shell core particle is about . Overall, XRD measurements confirmed the presence of pHA and Au in the core-shell structures formed in the medium.
To further confirm the release of pHA moieties in the medium, FTIR examination was carried out on the final solution (in its dried form) and the result is shown in Figure 3.10, i.e.
CONCLUSIONS
The extent of assembly formation depended on the concentration of pAA in the medium. UV/Vis spectra showed the appearance of a second peak at a longer wavelength - the position of which shifted towards the red with increasing pAA concentration. The concentration of HAuCl4, amount of citrate solution and the pH of the final solutions are given in Table 4.1.
It may be mentioned that the amount of different Au NP solutions was kept constant.
RESULTS AND DISCUSSION
57 The addition of pAA to citrate-stabilized Au NPs of different sizes led to a gradual change in the color of the Au NPs. The UV/Vis spectra of the solutions were recorded immediately after the addition of pAA to the Au NP solutions. The dipolar nature of the chains of citrate-stabilized Au NPs makes them stable in the environment.
This result suggests that pAA molecules were attached to the surface of Au NPs.
CONCLUSIONS
The formation of composite hybrid (3D) crystals consisting of inorganic Au NPs and organic p-hydroxyacetanilide (pHA) is discussed. Crystals can be grown from an aqueous mixture of Au NPs and pHA at 35 °C. The dimensions of the crystals were in the order of several millimeters and the optical properties of the Au NPs were preserved in the crystals.
Thus, by changing the ratio of Au NPs to pH in the original medium, two types of crystals (referred to as Process I and Process II) were obtained, as shown in Scheme 5.1.
EXPERIMENTAL
- RESULTS AND DISCUSSION
The UV/Vis spectrum of as-made citrate-stabilized Au NPs showed an extinction maximum at the 520 nm region (Figure 5.1a). The UV/Vis spectrum of the Au NP-pHA solution containing a lower ratio of the concentration of Au NP to pHA used to grow crystals by process I (Figure 5.2a) appeared to be identical in terms of line position and shape, to the spectrum of as-made Au NP solution (Figure 5.1a). However, the spectrum of the Au NP-pHA solution that had a lower concentration of pHA (or higher ratio of the concentration of Au NPs to pHA) tended to grow.
UV/Vis spectra of Au NP–pHA solutions used for crystal growth by (a) process I and (b) process II.
It may also be possible that upon redispersion in water (only) due to very low concentration of pH, additional agglomeration may occur compared to those in the crystals, leading to a large shift in the longitudinal plasmon resonance (observed to occur at 682 nm as shown in Figure 5.3b). In addition, the presence of some NPs in close proximity could be observed (both in Figure 5.4a and Figure 5.4b), possibly responsible for the second peak in the UV/Vis spectrum (Figure 5.3a). SAED patterns obtained from both crystals indicated the presence of NPs in close proximity within the crystals.
The AFM image of the composite crystal showed the presence of particles that are aggregated on the surface (Figure 5.5a).
The extent of aggregation will depend on the ratio of Au NPs to pHA present in the original medium. Ground form TGA of the composite crystals (generated by Process I and Process II) showed no significant difference in pHA decomposition temperature in the presence of Au NPs. Further, diffraction patterns due to the presence of crystalline pHA can be observed at the same positions (except for peaks occurring at other angles).
This was due to the presence of a lower content of Au NPs than pH in the composite crystals (as evidenced by TGA results presented above).
In other words, in the presence of excess pH, the NPs retained their original color and there may have been no further significant fusion of NP crystals during the process of crystallization of pH with them. In contrast, in the presence of a lower amount of pH in the medium, significant coalescence of NPs could have occurred during crystallization and thus the appearance of color features of larger NP crystals. Furthermore, it was observed that the solid dry crystals were stable in the atmosphere for weeks without any visible change in color or texture, indicating no further change in the agglomeration behavior after crystallization.
Thus, by controlling the concentration of pH in the medium before crystallization, one could possibly control the plasmonic properties of the composite crystals due to varying proximity of the NPs present in the solid.
- CONCLUSIONS
- OVERVIEW OF THE THESIS
- FUTURE OUTLOOK
More TEM images of the supercrystals grown in the presence of 0.19 T field can be found in Figure 6.7. The optical microscopy images of the crystals generated under 0.19 T field and in the absence of field are given in Figure 6.13. The color was indistinguishable for the crystals grown in the presence and absence of magnetic fields.
Optical micrographs of Fe3O4@Au NP supercrystals grown in (a) the presence (0.19 T) and (b) the absence of a magnetic field.
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