Im Rahmen dieser Arbeit wurden schalenartig aufgebaute Nanoheterostrukturen aus CdTe, CdS und CdSe hergestellt und insbesondere im Hinblick auf ihre optischen Eigenschaften untersucht. Eine neue Variante der CdTe Nanokristallsynthese in ODE als hochsiedendem, nicht koordinierendem Lösungsmittel wurde entwickelt. Diese weist deutliche Vorteile im Bezug auf Reproduzierbarkeit und Wachstumskinetik gegenüber der älteren Synthese in TOP/TOPO auf, da durch ein deutlich verlangsamtes Wachstum die gezielte Synthese von Nanokristallen definierter Größe deutlich erleichtert wird. Diese CdTe Nanokristalle wurden sowohl mit CdS als auch mit CdSe beschichtet. Dabei wurden für beide Arten der Beschichtung verschiedene Methoden getestet und es konnte jeweils eine Synthesemethode gefunden werden, welche Kern-Schale Nanokristalle mit geringer Polydispersität und guten optischen Eigenschaften (schmale Emissionssignale und hohe Quantenausbeuten) liefert. Weiterhin wurden Kern-Schale-Schale Nanokristalle aus CdTe/CdS/CdSe synthetisiert. Dieses System besitzt besonders interessante Eigenschaften, die sich vor allem in Form einer über die eingebettete CdS Schicht einstellbare Ladungsträgertrennung im angeregten Zustand der Nanokristalle und der damit einhergehenden verlängerten Emissionslebensdauer zeigen. Die optischen Eigenschaften (erster elektronischer Übergang) der hergestellten Kern-Schale und Kern-Schale-Schale Teilchen wurden mit theoretischen Berechnungen im Rahmen der „Effektive-Masse-Näherung“ verglichen. Dieser Vergleich ergab für die mit CdSe beschichteten Teilchen eine gute, für die mit CdS beschichteten Teilchen allerdings nur eine mäßige Übereinstimmung. Auch die transmissionselektronenmikroskopischen Aufnahmen legen nahe, dass insbesondere die Beschichtung mit CdS nicht quantitativ im Bezug auf die eingesetzte Precursormenge abläuft. Die Emissionslebensdauern der verschiedenen Systeme zeigen nichts desto trotz, dass die theoretisch zu erwartende Ladungsträgertrennung im angeregten Zustand der CdTe/CdS/CdSe Kern-Schale-Schale Nanokristalle tatsächlich eintritt. Durch Einführen der CdS Schicht konnte eine Verlängerung der Emissionslebensdauer der Nanokristalle erreicht werden. Insbesondere wurde gezeigt, dass es möglich ist, verschiedene Nanoheterostrukturen herzustellen, deren Emissionslebensdauern unterschiedlich sind, obwohl sie bei derselben Wellenlänge Licht emittieren. Diese Eigenschaft kann mit Nanokristallen aus nur einem Material nicht erreicht werden. Durch die richtige Kombination von Halbleitermaterialien ist es also möglich, neben der bereits lange bekannten Möglichkeit der Einstellbarkeit der Emissionswellenlänge von Nanokristallen nun auch die Emissionslebensdauer der Nanokristalle zu beeinflussen.

Research activities towards the self-assembly of small organic molecules building blocks which lead to form supramolecular gel has increased extensively during the past two decades. The fundamental investigations of the morphological properties and the mechanical properties of these supramolecular gels are crucial for understanding gelation processes. Most supramolecular gelators were discovered by serendipity, but nowadays ratiional design of new gelators has become somewh at feasible. As a consequence, an increasing number of multi stimuli-responsive and functional molecular gels are reported, offering great prospects with myriads of applications includ ing drug delivery and smart materials as shown in scheme 1. Scheme 1 Part 2: Synthesis of semiconductor nanocrystals In the last two decades, the synthetic development of semiconductor col loidal nanocrystals has been extended from the adjustment of their size, shape, and composition of the particles at the molecular level. Such adjustments of nanocrystals at the molecula r level might open different fields of applications in materials and biological sciences. I n this chapter, the concept of the shape contr ol synthesis of colloidal nanocrystals with a narrow size distribution, and the synthesis of composition dependent alloy type mat erials are described (Scheme 2). Scheme 2 Chapter 2: Synthesis of luminescent semiconductor nanocrystals Part 1: Cadmium deoxycholate: a new and efficient precursor for high ly luminescent CdSe nanocrystals This part demonstrates the sy nthesis of Cadmium deoxycholate (CdDCh2), an efficient Cd-precursor for the synthesis of high quality, monodisperse, multi color emittting CdSe Scheme 3 nanocrystals, while maintaining their high photoluminescent quantum efficiency (Scheme 3). The high thermal stability of CdDCh2 (decomposition temperature: 332 °C) was utilized to achieve high injection and growth temperatures (∼300 °C) for the syntheesis of red emitting nanocrystals with a sharp f ull width at half maximum (FWHM) and multiple excitonic absorption features. We believe that CdDCh2 can be useful for the prreparation of other nanomaterials such as CdS, CdTe and CdSe@CdS core-shell QDs. Part 2: Ligand mediated exccited state carrier relaxation dynamics of Cd1-xZnxSe1-ySy NCs derived from bile salts Bile salts of Cadmium and Zinc provide a convenient and inexpensive single step synthetic route for highly photoluminescent and stable semiconductor nanocrystals (NCs). The high thermal stabilities of Cadmium and Zinc deoxycholates (CdDCh2 and ZnDCh2) allowed us to fine-tune the synthesis of the NCs at high temperatures while maintaining the monodispersity, crystallinity and reproducibility (Scheme 4). Organic capping agent induced lattice strain affects the excited Scheme 4 state relaxation processes of the NCs. The analysis of photoluminescence decay profiles revealed that the average lifettime decreased with the increasing lattice strain of the NCs. A kinetic stochastic model of photoexcited carrier relaxation dynamics of NCs was employed to estimate the values of the radiative recombination rates, the photoluminescence quenching rates and the non-radiative recombination rates of the NCs. These data showed that the non-radiative relaxation rates and the numbeer of surface trap states increased with the incrreasing lattice strain of the NCs. Such types of NCs can have great potential in nonlinear optics, photocatalysis and solar cells. Chapter 3: Synthesis of organic-inorganic hybrid materials Part 1: Hierarchical self-assembly of photoluminescent CdS nanoparticles into bile acid derived organogel: morphological and photophysical properties In this part a strategy towards integrating photoluminescent semiconductor nanoparticles into a bio-surfactant derived organoggel has been reported. A facially amphiphilic bile thiol was used for capping CdS nanoparticless (NPs) which were embedded in a gel derived from a new bile acid organogelator in order to furnish a soft hybrid material (Scheme 5). The presence of CdS NPs in a well-ordered 1D array on the organogel network was confirmed using microscopic Scheme 5 techniques. Photophysical stuudies of the gel–NP hybrid revealed resolved excitation and emission characteristics. Time resolved spectroscopic studies showed that the average lifetime value of the CdS NPs increased in the gel state compared to the sol phase. A kinetic model was utilized to obtain quantitative information about the different decay pathways of the photoexcited NPs in the sol and gel states. Part 2: A novel strategy towards designing a CdSe quantum dot–metallohydrogel composite material This section describes an efficiient method to disperse hydrophobic CdSe quaantum dots (QDs) in an aqueous phase using cetyltriimethylammonium bromide (CTAB) micelles without any surface ligand exchange. The water soluble QDs were then embedded in the 3D self-assembled fibrillar networks (SAFINs) of a hydrogel showing homogeneous dispersibility as eviidenced by Scheme 6 optical and electron microscopico techniques (Scheme 6). The photophyssical studies of the hydrogel–QD from composite are reported for the first time. These composite materials may have potential applications in biology, optoelectronics, sensors, non-linear optics and materials science. Part 3: Photophysical aspectts of self-assembled CdSe QD-organogel hyybrid and its thermoresponsive properties A luminescent hybrid gel was constructed by incorporating CdSe quantuum dots (QDs) in a facially amphiphilic bile acid derived dimeric urea organogel throough non-covalent interaction between ligands capped on QDs surface and hydrophobic pockets of the gel (Scheme 7). The optical transparency of the hybrid materials and the dirrectionalities of the QDs in the gel medium were confirmed by photophysical and microscopic studies. The detailed excited state dynamics of the QD–organogel hybrid has been reported for the first time with the help of lifetime analysis and a kinetic decay model, and thee data revealed that the average lifetime of the QDs decreased in the gel medium. The reversible thermoresponsive behavior of the QD doped organogel was investigated by steady-state fluorescence spectroscopy. W e believe that the results obtained herein provides a route to develop a thermoresponsive system for practical application, especially because of the spatial assembly between soft organic scaffolds and colloidal QDs. Scheme 7 Part 4: In-situ formation of luminescent CdSe QDs in a metallohydrogel: a strategy towards synthesis, isolation, storage and re-dispersion of the QDs A one step, in-situ, room temperature synthesis of yellow luminesce nt CdSe QD was achieved in a metallohydrog el derived from a facially amphiphilic bile salt, resulting in a QD-gel hybrid (Scheme 8). T he ordered self-assembly and homogeneous distribution of the CdSe QDs in the hydrogel network was observed from optical and electro n micrographs. The different excited state behav iors of the hybrid were revealed for the fir st time using time resolved spectroscopy. Ad ditionally, we described the successful isolation of the photoluminescent CdSe QDs from the gel followed by their re-dispersion in an organic solvent using suitable capping ligands. Scheme 8 Chapter 4: Facially a mphiphilic bile acid derived meta llohydrogel: an efficient template for th e enantioselective Diels-Alder reactio n An enantioselective Diels-Ald er reaction mediated by a facially amphiphilic bile acid derived metallogel scaffold has been a chieved (Scheme 9). Different hydrophobic domains present in Scheme 9 the gel appear to facilitate the enantioselective reaction. Various spectro scopic and electron microscopic techniques were employed to understand the possible reasons for the stereoselectivity in the gel. Subsequently, different counter anion s dependent rate accelerations and induced enantioselectivity in the ZnCh2 gel were studied in detail. These preliminary results of the non-covalent based supramolecular heterogeneous catalysis offer new possibilities for using metallogels as nanoreactors for different stereoselective reactions.

Im Rahmen dieser Arbeit wurden schalenartig aufgebaute Nanoheterostrukturen aus CdTe, CdS und CdSe hergestellt und insbesondere im Hinblick auf ihre optischen Eigenschaften untersucht. Eine neue Variante der CdTe Nanokristallsynthese in ODE als hochsiedendem, nicht koordinierendem Lösungsmittel wurde entwickelt. Diese weist deutliche Vorteile im Bezug auf Reproduzierbarkeit und Wachstumskinetik gegenüber der älteren Synthese in TOP/TOPO auf, da durch ein deutlich verlangsamtes Wachstum die gezielte Synthese von Nanokristallen definierter Größe deutlich erleichtert wird. Diese CdTe Nanokristalle wurden sowohl mit CdS als auch mit CdSe beschichtet. Dabei wurden für beide Arten der Beschichtung verschiedene Methoden getestet und es konnte jeweils eine Synthesemethode gefunden werden, welche Kern-Schale Nanokristalle mit geringer Polydispersität und guten optischen Eigenschaften (schmale Emissionssignale und hohe Quantenausbeuten) liefert. Weiterhin wurden Kern-Schale-Schale Nanokristalle aus CdTe/CdS/CdSe synthetisiert. Dieses System besitzt besonders interessante Eigenschaften, die sich vor allem in Form einer über die eingebettete CdS Schicht einstellbare Ladungsträgertrennung im angeregten Zustand der Nanokristalle und der damit einhergehenden verlängerten Emissionslebensdauer zeigen. Die optischen Eigenschaften (erster elektronischer Übergang) der hergestellten Kern-Schale und Kern-Schale-Schale Teilchen wurden mit theoretischen Berechnungen im Rahmen der „Effektive-Masse-Näherung“ verglichen. Dieser Vergleich ergab für die mit CdSe beschichteten Teilchen eine gute, für die mit CdS beschichteten Teilchen allerdings nur eine mäßige Übereinstimmung. Auch die transmissionselektronenmikroskopischen Aufnahmen legen nahe, dass insbesondere die Beschichtung mit CdS nicht quantitativ im Bezug auf die eingesetzte Precursormenge abläuft. Die Emissionslebensdauern der verschiedenen Systeme zeigen nichts desto trotz, dass die theoretisch zu erwartende Ladungsträgertrennung im angeregten Zustand der CdTe/CdS/CdSe Kern-Schale-Schale Nanokristalle tatsächlich eintritt. Durch Einführen der CdS Schicht konnte eine Verlängerung der Emissionslebensdauer der Nanokristalle erreicht werden. Insbesondere wurde gezeigt, dass es möglich ist, verschiedene Nanoheterostrukturen herzustellen, deren Emissionslebensdauern unterschiedlich sind, obwohl sie bei derselben Wellenlänge Licht emittieren. Diese Eigenschaft kann mit Nanokristallen aus nur einem Material nicht erreicht werden. Durch die richtige Kombination von Halbleitermaterialien ist es also möglich, neben der bereits lange bekannten Möglichkeit der Einstellbarkeit der Emissionswellenlänge von Nanokristallen nun auch die Emissionslebensdauer der Nanokristalle zu beeinflussen.

Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors question the origin of this ferromagnetism, i.e. if the observed ferromagnetism stems from ferromagnetic precipitates rather than from carriermediated magnetic coupling of ionic impurities, as required for a diluted magnetic semiconductor. In this thesis, this question will be answered for transition-metal implanted ZnO single crystals. Magnetic secondary phases, namely metallic Fe, Co and Ni nanocrystals, are formed inside ZnO. They are - although difficult to detect by common approaches of structural analysis - responsible for the observed ferromagnetism. Particularly Co and Ni nanocrystals are crystallographically oriented with respect to the ZnO matrix. Their structure phase transformation and corresponding evolution of magnetic properties upon annealing have been established. Finally, an approach, pre-annealing ZnO crystals at high temperature before implantation, has been demonstrated to sufficiently suppress the formation of metallic secondary phases.

M. Tech. (Department of Chemical Engineering, Faculty of Engineering and Technology), Vaal University of Technology. / Heavy metal ions are one of the most toxic materials in the environment. Adsorption is the most used process for the removal of heavy metals from wastewater. Much research has been conducted into processes to remove heavy metals using different adsorbents. Various adsorbents have been used to remove heavy metal ions from wastewater especially those that are harmful to mankind. Zeolite, clay, activated carbon and biopolymers are the most common adsorbents used. In this research, gelatin, and cellulose nanocrystals (CNCs) were used to synthesize a hydrogel membrane to remove Cu(II) and Co(II) metal ions from mining processes wastewater. The synthetic wastewater was prepared in the laboratory to conduct the experiments. Batch experiments were conducted to obtain the optimum conditions for the Cu(II) and Co(II) metal ions. The effect of parameters such as pH, ratio, contact time, and temperature were also determined. The optimum conditions obtained were 120 min contact time for both metal ions at the temperature of 30oC, pH 5 for copper and pH 7 for cobalt. The high removal of both metals ions was obtained using the ratio 3:1 (75% Gelatin and 25% CNCs) at the temperature of 303K. The maximum adsorption capacity of Cu(II) and Co(II) was 7.6923 mg/g and 10.988 mg/g, respectively. The high percentage removal of Cu(II) and Co(II) metal ions obtained was found to be 70.5% for Cu(II) at pH 5 and 74.5% for Co(II) at pH 7. The experimental data fit well to Pseudo-first-order kinetic and Freundlich isotherm models (KF= 1.89x103 mg/g for copper and 3.7x102 mg/g for cobalt) for both metal ions. The values of energy (E) from D-R model have shown that the adsorption of both metal ions was of physical nature (E<8kJ/mol) then confirmed by the thermodynamic results (ΔH°). The kinetic diffusion models have shown that the experimental data fit well with the film diffusion (R2= 0.977 and 0.989) for both metal ions at pH 5. Negative values of ΔG°obtained for both metal ions indicate that the adsorption process was spontaneous. The positive values of ΔH° obtained showed a physical adsorption process and also indicate that the adsorption process of both metal ions was endothermic. The positive values of ΔS° indicate an increase in randomness at the solid/solution interface during adsorption.

Non-ordered porous networks, so-called aerogels, can be achieved by the 3D assembly of quantum dots (QDs). These materials are well suited for photonic applications, however a certain quenching of the photoluminescence (PL) intensity is observed in these structures. This PL quenching is mainly attributed to the energy transfer mechanisms that result from the close contact of the nanoparticles in the network. Here, we demonstrate the formation of a novel aerogel material with non-quenching PL behaviour by non-classical, reversible gel formation from tetrazole capped silica encapsulated QDs. Monitoring of the gelation/degelation by optical spectroscopy showed that the optical properties of the nanocrystals could be preserved in the 3D network since no spectral shifts and lifetime shortening, which can be attributed to the coupling between QDs, are observed in the gels as compared to the original colloidal solutions. In comparison with other QD-silica monoliths, QDs in our gels are homogeneously distributed with a distinct and controllable distance. In addition we show that the silica shell is porous and allows metal ions to pass through the shell and interact with the QD core causing detectable changes of the emission properties. We further show the applicability of this gelation method to other QD materials which sets the stage for facile preparation of a variety of mixed gel structures.

Nanomaterials such as carbon nanotubes, gold nanorods, magnetic nanocrystals, and binary nanocrystal superlattices have exciting potential applications. However, before these ideas can be applied, it is imperative to fully understand the materials synthesis. Multiwall carbon nanotubes were synthesized in supercritical toluene using cobaltocene, nickelocene, ferrocene, or metal nanocrystals as catalysts. Toluene served as both the solvent and carbon source for nanotube growth. The reaction was optimized by introducing supplemental carbon sources; either hexane or ethanol increased the yield relative to pure toluene and catalytic amounts of water minimized carbon filament and amorphous carbon formation. Gold nanorods were synthesized by the colloidal seed-mediated, surfactantassisted approach using cetyltrimethylammonium bromide (CTAB) obtained from ten different suppliers. The gold nanorod yield depended strongly on the CTAB used: with the same recipe, three of the CTABs produced only spherical particles, whereas the other CTABs produced nanorods with nearly 100% yield. Inductively coupled plasma mass spectrometry revealed a trace iodide impurity in the CTABs that did not yield nanorods. Further experiments introducing potassium iodide to the nanorod synthesis verified the detrimental effect of iodide on nanorod formation. Multifunctional colloidal core-shell nanoparticles of magnetic nanocrystals or gold nanorods coated with a fluorescent dye (Tris(2,2 -bipyridyl)dichlororuthenium(II) hexahydrate) doped silica shells were also synthesized. The as-prepared magnetic nanocrystals were initially hydrophobic and silica coated using a microemulsion approach, while the gold nanorods were hydrophilic and silica coated using a Stöber process. These colloidal heterostructures have the potential to be used as dual-purpose tags, exhibiting a fluorescent signal that could be combined with either dark-field optical contrast or enhanced contrast in magnetic resonance imaging. Binary superlattices (BSLs) of large iron oxide and small gold nanocrystals were assembled by slow evaporation of colloidal dispersions on tilted substrates. SEM and grazing incidence small angle X-ray scattering (GISAXS) confirmed the BSLs were simple hexagonal AB2 superlattices with long range order. GISAXS also revealed that the superlattice was slightly contracted perpendicular to the substrate as a result of solvent drying during the deposition process. Additionally, in some BSLs nearly periodic superlattice dislocations consisting of inserted half-planes of gold nanocrystals were observed. / text

The majority of pharmacologically active chemotherapeutics are poorly water soluble. Solubilization enhancement by the utilization of organic solvents often leads to adverse side effects. Nanoparticle-based cancer therapy, which is passively targeted to the tumor tissue via the enhanced permeation and retention effect, has been vastly developed in recent years. Nanocrystals, which exist as crystalline and carry nearly 100% drug loading, has been explored for delivering antineoplastic agents. Additionally, the hybrid nanocrystal concept offers a novel and simple way to integrate imaging agents into the drug crystals, enabling the achievement of theranostics. The overall objective of this dissertation is to formulate both pure and hybrid nanocrystals, evaluate their performance in vitro and in vivo, and investigate the extent of tissue distribution and tumor accumulation in a murine model. Pure and hybrid nanocrystals of several model drugs, including paclitaxel (PTX), camptothecin, and ZSTK474, were precipitated by the antisolvent method in the absence of stabilizer, and their size was further minimized by homogenization. The nanocrystals of PTX, which is the focus of the study, had particle size of approximately 200 nm and close-to-neutral surface charge. Depending on the cell type, PTX nanocrystals exerted different level of cytotoxicity. In human colon and breast cancer xenograft models, nanocrystals yielded similar efficacy as the conventional formulation, Taxol, at a dose of 20 mg/kg, yet induced a reduced toxicity. Biodistribution study revealed that 3H-PTX nanocrystals were sequestered rapidly by the macrophages upon intravenous injection. Yet, apparent toxicity was not observed even after four weekly injections. The sequestered nanocrystals were postulated to be released slowly into the blood circulation and reached the tumor. Tritium-labeled-taxol, in contrast, was distributed extensively to all the major organs, inducing systemic toxicity as observed in significant body weight loss. The biodistribution results obtained from radioactive analysis and whole-body optical imaging was compared. To some degree, the correlation was present, but divergence in the quantitative result, due to nanocrystal integrity and limitations associated with the optical modality, existed. Despite their promising properties, nanocrystal suspensions must be securely stabilized by stealth polymers in order to minimize opsonization, extend blood-circulation time, and efficiently target the tumor.

Nanometer-sized semiconductor crystals, termed ‘quantum dots’, are of fundamental interest because of their size-tunable properties. Three-dimensional quantum confinement of charge carriers by the small crystal size results in discrete atomic-like electronic states. This dissertation describes the synthesis and in-depth characterization of lead chalcogenide colloidal quantum dots for forthcoming applications as near-infrared single photon emitters. An efficient single photon source that operates at telecommunication wavelengths (between 1.3 and 1.6 µm) is a basic requirement for many photonic quantum technologies, such as quantum computing and quantum cryptography. Chapters 1 and 2 of this work provide an introduction to colloidal quantum dots and their use as single photon emitters. It includes a description of photonic crystal microcavities and their ability to enhance the spontaneous emission rate of quantum dots. The synthesis and basic characterization of PbSe and PbS quantum dots is then discussed in chapter 3. In particular, a new synthetic method for the preparation of highly photoluminescent PbS quantum dots is presented. PbSe/CdSe core/shell quantum dots prepared by a cation exchange reaction are also described and a significant improvement in photo-stability is shown. Chapter 3 concludes with a description of three different surface modification techniques. PbSe core and PbSe/CdSe core/shell materials are investigated further in chapter 4 by advanced characterization techniques that include high-angle annular dark field (HAADF) imaging, energy-filtered transmission electron microscopy (EF-TEM) imaging, energy-dependent X-ray photo-electron spectroscopy (XPS), small angle X-ray scattering (SAXS), and small angle neutron scattering (SANS). The information obtained from these techniques is combined to form a structural model of the PbSe core and PbSe/CdSe core/shell quantum dots with greater complexity than previously reported. In chapter 5, the temperature-dependent photoluminescence from PbSe and PbSe/CdSe core/shell quantum dots is discussed and a thermal model is presented that accounts for the large (non-trivial) temperature dependence of the Stokes shift and photoluminescence lineshape over the entire temperature range (4.5 to 295 K). Chapter 6 examines two scalable methods to integrate the colloidal quantum dots into silicon two-dimensional photonic crystal slab microcavities (a requirement for efficient single photon emission). Finally, conclusions and possible future work are discussed in chapter 7. / Graduate

In this thesis, luminescent lanthanide-doped nanocrystals, and lead-based quantum dots nanocrystals are explored as alternative bioimaging agents to fluorescent proteins and organic fluorophores for deep-tissue imaging. The first chapter gives a brief introduction on the aforementioned nanocrystals and their special optical properties. In chapter 2 the simple changes in the drying and baking temperature of the Yb3+ and Ho3+ doped LaF3 nanocrystals-silica sol-gel mixture aid in the explanation of the formation of two types of silica. The difference in the phonon energies of the two types of silica is found to control effectively the ratio of red to green emissions obtained from the upconversion process. However, the nanocrystals do not disperse in water making them unsuitable for bioimaging. Chapter 3 describe the synthesis of NaYF4 nanocrystals doped with Yb3+/Er3+ or Yb3+/Tm3+ ions followed by two surface modification strategies (intercalation and crosslinking) to disperse them in physiological buffers and biological growth media. Of the two methods, the crosslinked polymer coating of the nanocrystals alone exhibits stability in aforementioned media. In chapter 4 the applicability of lanthanide-doped NaYF4 nanocrystals are studied as bioimaging agents in two-photon upconversion laser scanning microscopy for deep-tissue imaging. Their performance as bioimaging agents was not better than fluorescent proteins and organic molecules. On the other hand with two-photon upconversion wide field microscopy (TPUWFM), brain blood vessels over a depth of 100 µm could well be separated. Furthermore, with the 800 nm emission from Tm3+ ions one can image twice as deep as the green emission with TPUWFM. In chapter 5, probing the NaYF4 nanocrystals with energy-dependent XPS shows that, the Y3+ ions on the surface of the nanocrystals are different from the ones present inside the nanocrystals. This chapter is concluded with a preliminary investigation of Yb3+ and Tm3+ doped NaYF4 with resonant XPS. Chapter 6 examines four different types of surface modification strategies to transfer hydrophobic lead-based quantum dots to physiological buffers and biological growth media. Of the four methods, the crosslinked polymer coating of quantum dots alone exhibits colloidal stability and the QDs retain their luminescence in aforementioned media over several months. The conclusions and future outlook for the work are elucidated in chapter 7. / Graduate