Synthesis and photophysics of functionalized silicon nanoparticles

Abstract

Nanotechnology is an emerging multidisciplinary science that involves the formation, investigation and manipulation of nanoobjects (1 - 100 nm). It has a huge potential to revolutionize diverse fields as engineering and medicine since the basis of many different physical processes can now be controlled up to the nanometer-scale and since the nanoworld yields access to novel processes. For almost two decades, research into semiconductor nanoclusters has been focused on the properties of quantum dots (QDs) - semiconductor fragments consisting of hundreds to thousands of atoms.1 Due to their unique size–range, characteristically on the boundary between quantum mechanics and Newtonian physics, the properties of nanoparticles differ from those of the bulk and of single atoms. Quantum dots have exceptional optical and electronic properties such as: size dependent - tunable light emission wavelengths,1-3 intense fluorescence,4-7 resistance against photobleaching8 and simultaneous excitation of multiple fluorescent colors.9-12 All these qualities make QDs in many respects superior over organic dyes and fluorescent proteins that are used for bioimaging purposes so far. QDs can be synthesized from a variety of materials with different sizes, shapes and morphologies. The most studied are complex core/shell/coat (CdS/ZnS/silane) QDs which can be produced in a variety of colors (sizes). However, their high intrinsic toxicity and minimal sizes (10 – 20 nm due to their complex structure) are an issue when considering them as candidates for in vivo bioimaging. Therefore, a huge interest arose in the possible development of smaller, non-toxic, stable and more versatile nanoparticles (NPs). On the other side, application of Si was limited only to (micro-)electronics and its photoemission potential has not been realized for years. Due to the indirect band gap in bulk Si, light absorption and emission occur only when the absorption or the emission of a photon and a specific change in a lattice vibration mode occurs simultaneously. Consequently, the photoluminescence of bulk silicon is very weak. Nonetheless, by creating Si with nanoscale dimensions (silicon nanoparticles), it can be coaxed to emit visible light with relatively high efficiencies. 1-10 nm Si NPs luminesce intensely over a wide range of wavelengths, from UV (for NPs 8 nm). Silicon surfaces can be well passivated, by creating stable Si-C bonds. The methods to tailor silicon surfaces were developed on porous and planar Si and can also be applied for the functionalization of Si NPs surfaces. Such a coating of Si NPs could prevent surface oxidation. A high luminescence, well-developed surface passivation principles, and a low inherent toxicity of Si initiated the enthusiasm for the research in Si NPs. The goals of the work described in this thesis are: - the development and optimization of methods for the preparation of stable and monodisperse Si NPs, - photophysical characterization of such NPs, also in dependence on their functionalization , - the exploration of their possible applications, specifically in the realm of bioimaging. Chapter 1 describes general properties of semiconductor quantum dots and Si NPs, in particular. The origin of Si NPs luminescence is described in detail, and an overview of published methods for the synthesis of Si NPs is given, with a discussion of the advantages and drawbacks of each method. Experimental work is described in Chapter 2, 3, 4 and 5. Chapter 2 describes the first gramscale preparation of highly monodisperse alkyl-terminated Si NPs (diameter of 1.57 ± 0.21 nm) in reverse micelles.13 Both steady-state and time-resolved absorption and emission techniques, as well as FTIR (Fourier Transform InfraRed Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy) measurements are used to study their photophysical properties and chemical composition in detail. For the first time, due to the relatively efficient synthesis, the molar extinction coefficient of alkyl-terminated Si NPs is experimentally determined to be 261= 1.7 x 10-4 M-1cm-1, only a factor 4 lower than that of CdS and CdSe NPs of that size. The measured quantum yields of emission ranged from 0.12 (C10H21 capping) to 0.23 (C16H33 capping). Their UV-Vis absorption and emission spectra display clear vibrational progressions in each sample of alkyl-terminated Si NPs. The vibrational peaks were almost equidistant and appeared after 974 ± 14 cm-1. These vibrational bands evidently resemble bulk SiC phonons. These defined peaks confirm the monodispersity of Si NPs as also observed by TEM. Time-resolved fluorescence anisotropy measurements, displayed a strictly monoexponential decay that can only be indicative of monodisperse, ball-shaped nanoparticles. The thus developed method was further applied to obtain Si NPs with different functionalizations that could be used to develop various applications. The preparation of very stable and bright water-soluble amine-terminated Si NPs is described in Chapter 3. The Si NPs were synthesized with different alkyl-chain lengths between the amine group and the surface of the NP: Si-C3H6NH2, Si-C6H12NH2 and Si-C11H22NH2 NPs.14 A highly improved synthesis method is presented, and the photophysical characterization of alkyl-amine coated Si NPs, including steady-state and time-resolved fluorescence, as well as STS (Scanning Tunneling Spectroscopy) measurements is discussed. The topography of amine-terminated Si NPs is examined using transmission electron microcopy (TEM) and scanning tunneling microscopy (STM) techniques. TEM measurements indicate a homogeneous size distribution (1.57 ± 0.24 nm). This size was independent of the alkyl spacer length, which excludes effects of the Si NP core size on their optical properties, and also shows that the method used to make such NPs is very reproducible. Size distribution histograms obtained by TEM and STM display highly similar height distributions. All synthesized amine-terminated Si NPs show a broad continuous absorption between 200 and 380 nm. The absorption spectrum of the 3-amino-propyl-terminated Si NPs has a distinctive broad peak at 300 nm, while for Si NPs with longer alkyl chains between the Si core and the amine group, this peak was not pronounced. There is no appearance of a vibrational structure in the absorption spectra, as was observed for the alkyl-terminated Si NPs. The electronic band gap of propyl-amine-terminated Si NPs is determined by STS measurements of individual NPs; ~ 60% of the observed particles have a band gap of ~4 eV, which exactly corresponds to an absorption peak at 300 nm. All NPs under study displayed an intense, well-defined emission in the 350 – 600 nm range. The emission spectra, and specifically the wavelength at which the maximum emission intensity is observed, depend on the length of the alkyl spacer. With a short (-C3H6) spacer the maximum emission is observed in the blue with a maximum centered at 475 nm, when exciting Si NPs with 390 nm. For longer chains the maximum emission gradually shifts to the UV: for the C11H22 spacer excmax = 320 nm, with the maximum emission is centered at 385 nm. Two factors may contribute to this shift: the amine-to-Si core distance, and a higher degree of oxidation with shorter alkyl chains, as longer alkyl chains may provide a better passivation of the Si NPs surface. A Stokes shift of ~1 eV also points out to the formation of trapped states. The excited-state lifetimes are in the ns range and indicate direct band-gap processes. The stability of NH2-terminated Si NPs is exceptionally good over a wide pH range (1 - 13!) and high temperatures (120 C), which is of great importance for a variety of possible applications including cell contents of various pH values and sterilization conditions. The prepared amine-terminated Si NPs are shown to be highly suitable for bio-imaging studies. They display a very high mobility in water (D = 3.13•10-10 m2s-1), and are readily taken up by BV2 nerve cells. After the uptake Si NPs are located in the cell cytosol. For the first time, proliferation of BV2 cells stained with Si NPs is observed by confocal microscopy. The newly formed daughter cells are also stained with Si NPs, indicating their minimal toxicity. In this way, amine-terminated Si NPs can be successfully used for staining multiple cell generations by only staining the mother cells. The possible use of Si NPs as an active energy donor in energy-transfer processes is described in Chapter 4. To that aim, a ruthenium-containing dye was attached to Si NPs, while the chain length of the alkyl spacer between the possible donor and acceptor was varied. In order to examine if an energy transfer occurs, Si NPs emission intensities and lifetimes were measured with and without a Ru dye attached to Si NPs. In this way, it was shown that Si NPs can act as a very efficient energy donor, with energy transfer efficiencies up to 55%. Also, energy transfer efficiency is shown to be distance dependent. This opens up a venue for numerous applications of Si NPs as efficient energy donors in various systems. As the fast and efficient synthesis procedure of amine- and alkyl-terminated Si NPs was established, it became highly convenient to functionalize Si NPs via an easy and versatile route. Therefore, Chapter 5 deals with the development of a method for the production of stable azide-terminated Si NPs and their further functionalization using “click” chemistry. The produced azide-terminated Si NPs were studied by FTIR, steady-state and time-resolved absorption and emission spectroscopy. They absorb light in the UV spectral region with a pronounced vibrational structure, and emit light at ~ 350 nm. Stable azide-terminated Si NPs were further functionalized with a number of functional terminal alkynes: undec-10-yn-1-ol, propargyl amine, undec-10-ynoic acid, undec-10-ynyl lactoside, undec-10-ynyl hepta-O-acetyl lactoside and rhodamine-labeled lactoside. In this way, once synthesized, the azide-terminated Si NPs can be rapidly functionalized with many moieties of interest. The cellular uptake of rhodamine-labeled Si NPs was successfully demonstrated by using yeast cells (Rhodotorula glutinis). Yeast cells were allowed to actually grow in the presence of Si NPs. It was shown by confocal microscopy that rhodamine-labeled Si NPs are located in mitochondria, and no effect on the cells proliferation rate was observed. Our findings clearly demonstrate that Si NPs are biocompatible and can be internalized by growing cells. The cellular uptake and internal localization under varying physiological conditions can now be studied in more detail, and with more complex functionalities attached onto the Si NPs. The results of this study open the way for further progress in research on Si NPs as imaging agents and effective delivery vehicles for the transport of biologically active compounds into specific cells of interest. <br/