The primary focus of this dissertation is the development of novel fluorescent near-infrared molecules for various applications. In Chapter 1, a compound dU-BZ synthesized via Sonogashira coupling reaction methodology is described. A deoxyuridine building block was introduced to enhance hydrophilic properties and reduce toxicity, while an alkynylated benzothiazolium dye was incorporated for near-IR emission and reduce photodamage and phototoxicity that is characteristic of common fluorphores that are excited by UV or visible light. A 30-fold enhancement of fluorescence intensity of dU-BZ was achieved in a viscous environment. Values of fluorescence quantum yields in 99% glycerol/1% methanol (v/v) of varying temperature from 293 K to 343 K, together with fluorescence quantum yields, radiative and nonradiative rate constants and fluorescence lifetimes in glycerol/methanol solutions of varying viscosities from 4.8 to 950 cP were determined. It was found that both fluorescence quantum yields and fluorescence lifetimes increased with increasing viscosity, which is consistent with results predicted by theory. This suggests that the newly designed compound dU-BZ is capable of functioning as a probe of local microviscosity, and was later confirmed by in vitro bioimaging experiments. In Chapter 2, a new BAPTA (O,O*-bis(2-aminophenyl)ethyleneglycol-N,N,N*,N*-tetra acetic acid) and BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)-based calcium indicator, BAPBO-3, is reported. A new synthetic route was employed to simplify both synthesis and purification, which tend to be low yielding and cumbersome for BAPTA derivatives. Upon excitation, a 1.5-fold increase in fluorescence intensity in buffer containing 39 ?? Ca2+ and a 3-fold increase in fluorescence intensity in buffer containing 1 M Ca2+ was observed; modest but promising fluorescence turn-on enhancements. In Chapter 3, a newly-designed unsymmetrical squaraine dye, SQ3, was synthesized. A one-pot synthesis was employed resulting in a 10% yield, a result that is generally quite favorable for the creation of unsymmetrical squaraines Photophysical and photochemical characterization was conducted in various solvents, and a 678 nm absorption maximum and a 692 nm emission maximum were recorded in DMSO solution with a fluorescence quantum yield of 0.32. In vitro cell studies demonstrated that SQ3 can be used as a near-IR probe for bioimaging.

This thesis provides several nanomaterial systems that can be used as contrast agents in magnetic resonance imaging (MRI) and for optical fluorescence imaging. Nanoparticle systems described in this thesis fall under three categories: (a) graphene oxide-nanoparticle composites for MRI contrast agent application, (b) core-shell nanoparticles for MRI contrast agent application and (c) nanoparticle systems for both MRI and optical fluorescence imaging. In the case of graphene oxide based nano-composites, the following observations were made: (i) in the case of graphene oxide-Fe3O4 nanoparticle composite, it was observed that high extent of oxidation of the graphene oxide and large spacing between the graphene oxide sheets containing Fe3O4 nanoparticles provides the optimum structure for yielding a very high transverse proton relaxivity value, (ii) in the case of graphene oxide-Gd2O3 nanoparticle composite, it was observed that this composite exhibits high value for both longitudinal and transverse relaxivity values making it a potential materials for multi-contrast study of pathologies with a single agent, (iii) in the case of graphene oxide-CoFe2O4 nanoparticle composites, it was observed that an increase in the reflux time of the reaction mixture containing this composite led to appreciable variations in the proton relaxivity values. Transverse relaxivity value of the water protons increased monotonically with increase in the reflux time. Whereas, the longitudinal relaxivity value initially increased and then decreased with increase in the reflux time. In the case of coreshell nanoparticles for MRI contrast agent application two different core-shell systems were investigated. They are MnFe2O3-Fe3O4 core-shell nanoparticles and CoFe2O4-MnFe2O4 coreshell nanoparticles. Investigations of both the core-shell nanoparticle systems revealed that the proton relaxivity value obtained in the dispersion of the core-shell nanoparticles was considerably greater than the proton relaxivity value obtained in the presence of single phase nanoparticles of the core and shell phases. Very high value of transverse relaxivity in the case core-shell nanoparticles was due to the large magnetic inhomogeneity created by the core-shell nanoparticles in the water medium surrounding it. In the case of nanoparticle systems for both MRI and optical fluorescence imaging, two different systems were investigated. They were CoFe2O4-ZnO core-shell nanoparticles and Gd doped ZnS nanoparticles [Zn1-xGdxS, x= 0.1, 0.2 and 0.3] formed on graphene oxide sheets or coated with chitosan. In the case of CoFe2O4-ZnO core-shell nanoparticles it was observed that fluorescent CoFe2O4-ZnO core-shell nanoparticles with the unique geometry in which CoFe2O4 ferrite nanoparticles agglomerates were present within larger sized hollow ZnO capsules yields very high value of transverse proton relaxivity when compared to the proton relaxivity value exhibited by the individual CoFe2O4-ZnO coreshell nanoparticles. In the case of Gd doped ZnS nanoparticles, two different systems were synthesized and the values of the longitudinal and transverse proton relaxivity obtained were compared. These systems were (i) graphene oxide- Zn1-xGdxS (x= 0.1, 0.2 and 0.3) nanoparticle composites and (ii) chitosan coated Zn1-xGdxS (x= 0.1, 0.2 and 0.3) nanoparticles. It was observed that Gd doped ZnS nanoparticles in both cases exhibit both longitudinal and transverse relaxivity values. The relaxivity values showed a clear dependence on the composition of the nanoparticles and the nanoparticle environment (presence and absence of graphene oxide). It was also observed that Gd doped ZnS nanoparticle can be used for florescence imaging.

Herein we report the synthesis of an RGD-ruthenium bipyridine [Ru(Bpy)2(BpyRGD)]2+ complex aimed at the detection of angiogenesis. Angiogenesis plays a critical role in many pathophysiological processes, such as tumor growth. The αv-integrins (αv[beta]3, αv[beta]5) are currently used as molecular targeting sites for anti-angiogenic therapies. The [Ru(Bpy)2(BpyRGD)]2+ complex is an organometallic luminescent probe, which enables noninvasive, in vitro imaging of αv[beta]3 expression. Peptides containing the arginine-glycine-aspartic acid (RGD) sequence have been shown to bind strongly to the αvb3 integrin. The RuBpy probes are soluble in water, display long lifetimes, and are photochemically stable. These properties enable the Ru(tris-bpy) complexes to be useful in numerous applications in biophysical and cell biology. The [Ru(Bpy)2(BpyRGD)]2+ complex was synthesized by combining the succinimidyl ester on the RuBpy complex with the lysine of the c(RGDfK) peptide. The results of the one-photon fluorescence bioimaging showed selective binding of the cyclic RGD to αv[beta]3 integrin, which supports previous literature. The high luminescence intensity, long lifetimes, and low cell toxicity levels of dye [Ru(Bpy)2(BpyRGD)]2+, illustrates the potential usage of this probe for future biological applications.

Nanomaterials, typically less than 100 nm size in any direction have gained noteworthy interest from scientific community owing to their significantly different and often improved physical properties compared to their bulk counterparts. Semiconductor nanoparticles (NPs) are of great interest to study their tunable optical properties, primarily as a function of size and shape. Accordingly, there has been a lot of attention paid to synthesize discrete semiconducting nanoparticles, of where Group III-V and II-VI materials have been studied extensively. In contrast, Group IV and Group IV-V based nanocrystals as earth abundant and less-non-toxic semiconductors have not been studied thoroughly. From the class of Group IV, Ge1-xSnx alloys are prime candidates for the fabrication of Si-compatible applications in the field of electronic and photonic devices, transistors, and charge storage devices. In addition, Ge1-xSnx alloys are potentials candidates for bio-sensing applications as alternative to toxic materials. Tin phosphides, a class of Group IV-V materials with their promising applications in thermoelectric, photocatalytic, and charge storage devices. However, both aforementioned semiconductors have not been studied thoroughly for their full potential in visible (Vis) to near infrared (NIR) optoelectronic applications. In this dissertation research, we have successfully developed unique synthetic strategies to produce Ge1-xSnx alloy quantum dots (QDs) and tin phosphide (Sn3P4, SnP, and Sn4P3) nanoparticles with tunable physical properties and crystal structures for potential applications in IR technologies. Low-cost, less-non-toxic, and abundantly-produced Ge1-xSnx alloys are an interesting class of narrow energy-gap semiconductors that received noteworthy interest in optical technologies. Admixing of α-Sn into Ge results in an indirect-to-direct bandgap crossover significantly improving light absorption and emission relative to indirect-gap Ge. However, the narrow energy-gaps reported for bulk Ge1-xSnx alloys have become a major impediment for their widespread application in optoelectronics. Herein, we report the first colloidal synthesis of Ge1-xSnx alloy quantum dots (QDs) with narrow size dispersity (3.3±0.5 – 5.9±0.8 nm), wide range of Sn compositions (0–20.6%), and composition-tunable energy-gaps and near infrared (IR) photoluminescence (PL). The structural analysis of alloy QDs indicates linear expansion of cubic Ge lattice with increasing Sn, suggesting the formation of strain-free nanoalloys. The successful incorporation of α-Sn into crystalline Ge has been confirmed by electron microscopy, which suggests the homogeneous solid solution behavior of QDs. The quantum confinement effects have resulted in energy gaps that are significantly blue-shifted from bulk Ge for Ge1-xSnx alloy QDs with composition-tunable absorption onsets (1.72–0.84 eV for x=1.5–20.6%) and PL peaks (1.62–1.31 eV for x=1.5–5.6%). Time-resolved PL (TRPL) spectroscopy revealed microsecond and nanosecond timescale decays at 15 K and 295 K, respectively owing to radiative recombination of dark and bright excitons as well as the interplay of surface traps and core electronic states. Realization of low-to-non-toxic and silicon-compatible Ge1-xSnx QDs with composition-tunable near IR PL allows the unprecedented expansion of direct-gap Group IV semiconductors to a wide range of biomedical and advanced technological studies. Tin phosphides are a class of materials that received noteworthy interest in photocatalysis, charge storage and thermoelectric devices. Dual stable oxidation states of tin (Sn2+ and Sn4+) enable tin phosphides to exhibit different stoichiometries and crystal phases. However, the synthesis of such nanostructures with control over morphology and crystal structure has proven a challenging task. Herein, we report the first colloidal synthesis of size, shape, and phase controlled, narrowly disperse rhombohedral Sn4P3, hexagonal SnP, and amorphous tin phosphide nanoparticles (NPs) displaying tunable morphologies and size dependent physical properties. The control over NP morphology and crystal phase was achieved by tuning the nucleation/growth temperature, molar ratio of Sn/P, and incorporation of additional coordinating solvents (alkylphosphines). The absorption spectra of smaller NPs exhibit size-dependent blue shifts in energy gaps (0.88–1.38 eV) compared to the theoretical value of bulk Sn3P4 (0.83 eV), consistent with quantum confinement effects. The amorphous NPs adopt rhombohedral Sn4P3 and hexagonal SnP crystal structures at 180 and 250 °C, respectively. Structural and surface analysis indicates consistent bond energies for phosphorus across different crystal phases, whereas the rhombohedral Sn4P3 NPs demonstrate Sn oxidation states distinctive from those of the hexagonal and amorphous NPs owing to complex chemical structure. All phases exhibit N(1s) and ʋ(N-H) energies suggestive of alkylamine surface functionalization and are devoid of tetragonal Sn impurities.