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Zinc is an important micronutrient but the biological function of its labile form is poorly understood. Zinc selective fluorescence sensors, recognized as the major tool to gain information about the role of zinc in living systems, have been attracting more and more interest. The most promising solution currently being studied comes in the form of ratiometric sensors. Unlike sensors based on the switch-on mechanism, ratiometric sensors determine the free metal concentration directly from the ratio of the emission intensities at two wavelengths. The major restriction on the design of this type of sensor is from the necessity for a spectral-shift upon binding metal ions. To develop novel ratiometric sensors, we have developed designs based on excited-state intramolecular proton transfer (ESIPT).
In this dissertation, we established a new approach assisted by computational chemistry to design fluorescent sensors. The approach is applicable to predict the behavior of a fluorophore-bridge-receptor sensor based on photoinduced electron transfer (PET). Our first designed rhodamine based pH sensor exhibits strong fluorescence under acidic conditions and very weak fluorescence under basic conditions, just as the computations predicted.
Chemistry has become an important tool in solving problems in biology and medicine. Chemists are engaged in developing new devices at the nanoscale level to aid in clinical diagnosis, real time monitoring of analytes, and analyzing neural activity, etc. Our research is aimed at designing and synthesizing fluorescent sensors to detect various divalent metal ions under physiological conditions. We designed and synthesized a chemosensor based on the principle of photoinduced electron transfer as a signal transduction mechanism in order to detect the presence of various divalent metal cations, particularly zinc. We report the synthesis, characterization and its spectrophotometric titrations with various metal ions. This sensor signals the presence of metal ions by a fluorescence signal and offers a significant advantage as it exhibits "off-on" behavior in the presence and absence of metal ions. The primary sensor was found to be very sensitive and selective toward zinc ions. We also aimed at optimizing the performance of this chemosensor by taking advantage of electron donating/withdrawing abilities of different functional groups by using computational methods. We devised a new scheme to synthesize it, unfortunately the modified sensor was not realized for unknown reason. We are currently involved in devising an alternate strategy to synthesize and develop new sensors for neutral molecules.
The main theme of this thesis is to develop a fluorescent probe for imaging the subcellular distribution of kinetically labile copper pools that might play a critical role in copper homeostasis. Various copper-selective sensors were designed by combining 1,3,5-triaryl-2-pyrazoline fluorophores with polythioethers as receptor moieties. A series of donor-substituted 1,3,5-triaryl-2-pyrazoline fluorophores were synthesized and characterized in terms of their photophysical and electrochemical properties. Interestingly, the aryl substituents attached to the 1- and 3-position of the pyrazoline ring influence the photophysical properties of the fluorophore in distinctly different ways. The excited-state equilibrium energy is primarily influenced by changes of the substituent in the 1-position, whereas the reduction potential of the fluorophore is determined by the 3-aryl group. Results from computational analyses agree well with the experimental data. A pyrazoline fluorophore library was synthesized, and their photophysical and electrochemical properties were studied. The compounds cover a broad range of excited state energies and reduction potentials, and allow for selective and differential tuning of these two parameters. A series of thiazacrownethers and tripodal aniline copper(I) receptors were synthesized and their copper binding stoichiometries, stability constants, and copper-self-exchange kinetics were investigated. The measured self-exchange activation parameters revealed for all studied ligands a negative activation entropy, suggesting a predominant associative exchange mechanism. With detailed knowledge of the fluorophore platform and copper receptors, sensor CTAP-1 was designed, synthesized and characterized. The probe shows a 4.6-fold emission enhancement and reaches a quantum yield of 14% upon saturation with Cu(I). The sensor exhibits excellent selectivity towards Cu(I) and is insensitive towards millimolar concentrations of Mg(II) or Ca(II). Mouse fibroblast cells (3T3) incubated with the sensor produced a copper-dependent perinuclear staining pattern, which colocalizes with the subcellular location of the mitochondria and the Golgi apparatus. The subcellular topography of copper was further determined by synchrotron-based x-ray fluorescence (SXRF) microscopy. Furthermore, microprobe x-ray absorption measurements at various subcellular locations showed a near-edge feature that is characteristic for low-coordinate monovalent copper. The data provide a coherent picture with evidence for a kinetically labile copper pool, which is predominantly localized in the mitochondria and the Golgi apparatus.
Over the past few decades, there has been significant interest in developing fluorescent probes, because they are useful tools for biological studies. As effective analytical techniques, fluorescent probes utilize distinct advantages offered by fluorescence detection in terms of sensitivity, selectivity, and fast response time. When fluorescent probes interact selectively with target molecules, ions or biological specimens, they can generate large optical responses. Since most ions or molecules, such as Zn2+, Ca2+, or pyrophosphate ion (PPi), are non-fluorescent, chemosensors having analyte binding-triggered fluorescence are appealing in many fields, like analytical chemistry, clinical biochemistry, medicine, and environmental science.This dissertation is devoted to the design, synthesis, and characterization of novel fluorescent sensors for Zn2+ and its associated applications. Chapter II of this dissertation presents several novel terpyridine-based fluorescent sensors with different substituents affecting the electronic and steric nature of the terpyridine (tpy) fluorophore. Sensors are designed to establish the correlation between sensor structure and its photophysical properties. Low temperature fluorescence is used to evaluate the essential role of intramolecular charge transfer (ICT) in zinc binding-induced fluorescence changes. The tpy molecular fragment has a relatively large [pi]-conjugated system which enables the potential [pi-pi] interaction between two tpy platforms and affects the fluorescence of tpy ligands. Chapter III introduces a dimeric tpy ligand containing two tpy fragments connected via a meta-phenylene unit. The detailed spectroscopic study shows that this ligand displays an attractive fluorescence turn-on, in sharp contrast to mono(tpy) ligand that shows fluorescence quenching upon binding Zn2+. The result suggests the existence of delicate structural influences on fluorescence of tpy derivatives.Chapter IV is devoted to 2-(2'-hydroxyphenyl)-1,3-benzoxazole (HBO) and 2-(2'-hydroxyphenyl)-1,3-benzothiazole (HBT) derivatives featured with a structural potential of excited-state intramolecular proton transfer (ESIPT). The study reveals additional information on the binding of HBO or HBT to metal cations, which aids the sensor design for Zn2+ and PPi detection. The molecular design aims to realize ESIPT process control upon complexation with an analyte. Chapter V is devoted to the synthesis of bis(HBO) derivatives which bind Zn2+ selectively and emit near-infrared (NIR) fluorescence as a consequence of metal ion binding-induced ESIPT turn-on. Preliminary cell stain experiment was conducted and indicated the potential biological applications.
Advanced Sensor Technology: Biomedical, Environmental, and Construction Applications introduces readers to the past, present and future of sensor technology and its emerging applications in a wide variety of different fields. Organized in five parts, the book covers historical context and future outlook of sensor technology development and emerging applications, the use of sensors throughout many applications in healthcare, health and life science research, public health and safety, discusses chemical sensors used in environmental monitoring and remediation of contaminants, highlights the use of sensors in food, agriculture, fire prevention, automotive and robotics, and more. Final sections look forward at the challenges that must be overcome in the development and use of sensing technology as well as their commercial use, making this book appropriate for the interdisciplinary community of researchers and practitioners interested in the development of sensor technologies. Covers a range of environmental applications such as protection and improvement of water, air, soil, plants, and agriculture and food production; biomedical applications including detection of viruses, genes, hormones, proteins, bacteria, and cancer, and applications in construction such as fire protection, automotive, robotics, food packing and micro-machining Provides an outlook on opportunities and challenges for the fabrication and manufacturing of sensors in industry and their applicability for industrial uses Demonstrates how cutting-edge developments in sensing technology translate into real-world innovations in a range of industry sectors
Chapter 1. Introduction A variety of inorganic molecules and ions participate in complex biological signaling networks. Three of these species are nitric oxide (NO), nitroxyl (HNO), and mobile zinc. Maintaining the homeostasis of these signaling molecules is vital and a deeper comprehension of their roles could help in understanding the pathology of specific diseases associated with their dysregulation. One method used to monitor levels of these analytes in biological samples is fluorescence microscopy. Shifting the fluorescence emission to longer wavelengths would improve these already existing probes. Having access to red and near-infrared (NIR) sensors is particularly useful for investigating the interplay of multiple analytes using fluorescence microscopy in conjunction with other probes that emit at shorter wavelengths. Chapter 2. Synthesis and Characterization of a Fluorescent Sensor with a Dihydrothioxanthene Fluorophore and a Quinoline Based Cu(II) Binding Site A NIR probe designed to detect NO was synthesized and its photophysical properties were fully characterized. Analysis of the photophysics of this sensor revealed that the quinoline-binding site might be quenching the fluorescence of the fluorophore and preventing a turn-on response upon addition of NO. Chapter 3. Synthesis, Characterization, and Implementation of a Near-Infrared Fluorescent Sensor for Detection of Nitroxyl (HNO) A NIR sensor for the detection of HNO was synthesized, fully characterized, and used in live HeLa cells to detect exogenously applied HNO. This probe is selective for HNO over thiols and many other biologically relevant analytes. This sensor was used in combination with the green, zinc-specific probe ZP1 to investigate the relationship between exogenously applied HNO and the release of mobile zinc in HeLa cells. Chapter 4. Characterization and Targeting of a Red Zinc Sensor To investigate the levels of mobile zinc in specific cellular organelles, attempts were made to target a red zinc-specific probe to acidic vesicles, the mitochondria, and the nucleus. A combination of peptide-based and small molecule-based targeting approaches was explored, including the vesicle-targeting R9 peptide, the mitochondria-targeting triphenylphosphonium ion, and the DNA-binding Hoechst dye.
This is the third volume in the Reviews in Fluorescence series. To date, two volumes have been both published and well received by the scientific community. Several book reviews have also favorably described the series as an "excellent compilation of material which is well balanced from authors in both the US and Europe". Of particular mention we note the recent book review in JACS by Gary Baker, Los Alamos. In this 3rd volume we continue the tradition of publishing leading edge and timely articles from authors around the world. We hope you find this volume as useful as past volumes, which promises to be just as diverse with regard to content. Finally, in closing, we would like to thank Dr Kadir Asian for the typesetting of the entire volume and our counterparts at Springer, New York, for its timely publication. Professor Chris D. Geddes Professor Joseph R. Lakowicz August 20*^ 2005.