Mudarri, Timothy C.
16 January 2004
Ionic electroactive polymers have been developed as mechanical sensors or actuators, taking advantage of the electromechanical coupling of the materials. This research attempts to take advantage of the chemomechanical and chemoelectrical coupling by characterizing the transient response as the polymer undergoes an ion exchange, thus using the polymer for ionic sensing. Nafion™ is a biocompatible material, and an implantable polymeric ion sensor which has applications in the biomedical field for bone healing research. An ion sensor and a strain gauge could determine the effects of motion allowed at the fracture site, thus improving rehabilitation procedures for bone fractures. The charge sensitivity of the material and the capacitance of the material were analyzed to determine the transient response. Both measures indicate a change when immersed in ionic salt solutions. It is demonstrated that measuring the capacitance is the best indicator of an ion exchange. Relative to a flat response in deionized water (±2%), the capacitance of the polymer exhibits an exponential decay of ~25% of its peak when placed in a salt solution. A linear correlation between the time constant of the decay and the ionic size of the exchanging ion was developed that could reasonably predict a diffusing ion. Tests using an energy dispersive spectrometer (EDS) indicate that 90% of the exchange occurs in the first 20 minutes, shown by both capacitance decay and an atomic level scan. The diffusion rate time constant was found to within 0.3% of the capacitance time constant, confirming the ability of capacitance to measure ion exchange. / Master of Science
01 June 2008
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Supramolecular chemistry is an emerging field of chemistry which has attracted much attention in recent years as a result of its broad applicability in many areas. Thus, the design of functional supramolecular systems is strongly in demand in this field. For this purpose, we have developed near-IR emitting ratiometric fluorescent chemosensors for transition metal ions. Judicious placement of dithiodioxaazamacrocycles on the BODIPY chromophore generates this chemosensor which is selective for Hg(II) ions and both absorption and emission spectra display large changes that would allow ratiometric sensing.
01 September 2008
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Rational design of fluorescent chemosensors is an active area of supramolecular chemistry, photochemistry and photophysics. Ratiometric chemosensors are even more important, as they have an internal system for selfcalibration. In order to develop a new methodology for a ratiometric chemosensor design, we proposed coupling of energy transfer phenomenon to ion sensing. In this study, we targeted energy transfer cassette type chemosensors, where the efficiency of transfer is modulated on the donor side, by metal ion binding which changes the spectral overlap. This work involves the synthesis of a number of EET systems with varying degrees of EET efficiency. The results suggest that this strategy for ratiometric ion sensing is a promising one, enabling a modular approach in chemosensor design.
The present thesis entitled, "Molecular expression through fluorescence: Studies in probe design and aggregation" describes very simple bi-functional donor-acceptor poly-aromatic fluorophores that have been shown to possess distinctive properties depending on the context in which they are studied. In a sense, this work is an effort in exemplifying the inherent diversity and power of "molecular expression", with the central theme here being the phenomenon of fluorescence. The work has been divided into four chapters, each having a self-contained introduction. Chapter 1: First instance of metal ion (Zn2+) sensing exclusively at amphiphilic interfaces. (1 -pyrenyl)rnethyl-bis- [(2-pyridyl)methyl]amine (Pybpa), a simple, bi-functional fluorophore was synthesized. Pybpa has the modular design of a photoinduced electron transfer (PET) based analyte sensor. In Pybpa, a photoinduced electron transfer (PET) operates from the pyrenyl nitrogen (PyCH2-iV) to the excited pyrenyl (Py*) chromophore leading to fluorescence quenching. Zn2+ ion binding to the bis-picolyl (bpa) unit of Pybpa stops the PET process and leads to fluorescence enhancement. Thus Pybpa was able to sense Zn2+" in organic solvents. In water, however, Pybpa showed pronounced aggregation and the probe did not sense any metal ion. Surfactant micelles provide hydrophobic regions in water and the dynamic rnicellar assemblies could disrupt Pybpa aggregates. Pybpa monomers solubilized in micelles were responsive to Zn2+ in the low micro molar concentration range. The metal ion sensing on micelles was reflective of the charge of the interface. The sensing is negligible on cationic surface (CTAB), moderate on negatively charged surface (SDS micelles) and is the most efficient on neutral interface provided by TWEEN-20 micelles. With the Pybpa 'sensor*, no sensing is possible in water and hence the sensing is exclusive to the interface. Pybpa doped in membranous aggregates like phosphatidylcholine (PC) lipid bilayers, exists in monomeric form, and was able to sense Zn . The sensing on phosphatidylcholine (PC) bilayer vesicles was found to depend on the fluidity of the membrane. Zn2^ sensing with interfacially bound probe "was extended to a globular protein bovine serum albumin (BSA). BSA, a carrier protein, can bind hydrophobic molecules as well as metal ions like Zn2f. BSA was shown to disrupt Pybpa aggregation and bind Pybpa in a facile manner. BSA bound Pybpa was able to sense externally added Zn2+. Biological sensing of trace amounts of Zn2+ has been considered important since Zn2+ is crucial for eukaryotic systems. This is the first example of such 'exclusive' interfacial sensing of a metal ion. Chapter 2: Towards understanding and modulating self-assembly of pyrenyl bis-picolyl a mine: Organic nanoparticles that show tunable emission. Pybpa was found to aggregate in water in the size range of 80-250 nm. Evidence of aggregation was seen at concentrations as low as 1 |iM. The nanoscopic particles formed were characterized through transmission electron microscopy (TEM) and dynamic light scattering (DLS). Pybpa in water showed dual emission bands, with one band resernhling the emission from 'monomeric' Pybpa (as seen in solutions in organic solvents) and a broad red-shifted emission band (A,max ~ 480 ran) designated as "aggregate/nanoparticle" emission. Distinct excitation spectra for the two emission bands indicate that the bands (the '390 nm' band and the '480 nm' band) originate through distinct excitation/emission channels. The time resolved emission decay for the 'monomer' emission (397 nm) showed a substantial contribution from a long-lived pyrene-like excited state (x = 103.9 ns, 40% relative amplitude). On the other hand, the decay at 475 nm (for the nanoparticle/aggregate emission band) was considerably faster, with no evidence of any pyrene-like long-lived state. The short lifetimes indicated an exciplex nature of the red-shifted emission band, X-,nax~480 nm. The effect of temperature and urea on these aggregates was examined. The nanoparticles formed even in a concentrated urea solution (7.8 M). The aggregates formed in urea were found to be more emissive, indicating a 'looser' aggregate with reduced fluorescence quenching. Similar results were obtained on heating the aggregate. Increasing the concentration of Pybpa in water causes a change in the nature of the colloids formed as exemplified by increase in aggregate size and a decrease in the polydispersity index. Also seen was a substantial red shift in the 'aggregate emission'. At higher concentrations, the presence of three independent excitation/emission channels was observed. It is likely that a new type of aggregated Pybpa species formed at higher concentration, which emits at longer wavelength (A,rnax~540 nm), In such a scenario, it is possible to tune the emission wavelength by the choice of appropriate wavelength of excitation. Further, there is an opportunity to tailor the emission properties by controlling the aggregation behavior. The modulation of emission is one of the primary goals of research on fluorescent organic nanoparticles. Chapter 3: Photophysical properties of aryl-terpyridines in solution, solid and aggregated state: Unique CT emission from nanoparticles in water. Two aryl terpyridines, 4T-(l-pyrenyl)-2,2l:6'52fl-terpyridine (Pytpy) and 4'-(9-anthryl)-2,2':6',2n-terpyridine (Antpy), where the fluorophoric pyrene or anthracene unit is directly coupled to the terpyridine unit, were synthesized. The aryl terpyridines conjugates can be viewed as donor-acceptor molecules that are conformationally labile, with the possibility of rotation around three single bonds. It was of interest to see as to how conformational effects express themselves in different environments, especially in relation to the possibility of charge separation. Crystal structure data and Serni-empirical AMI calculations revealed a twisted molecular conformation for each of the molecules. Absorption and emission (steady state as well as tirne-resolved) behavior of Pytpy and Antpy in various organic solvents have been presented. The molecules showed only limited conjugation between the two units in the absorption behavior with the degree of conjugation being greater for Pytpy. In the emission behavior, only a single emission band (with a single lifetime) was observed in all organic solvent. Steady state and time resolved fluorescence data suggest the existence of a mixed or coupled, largely 7t—7i* state, with only marginal charge separation. The various photophysical parameters have been determined for the two systems. It appears that in the excited state, the inesomeric interactions show an increase for each of the two aryl-terpyridines, indicating at least a partially planar geometry in the excited state. Some specific solvent effects were observed for the molecules in alcoholic solvents and there was evidence of excited state H-bonding occurs for the aryl terpyridines in polar protic organic solvents, especially methanol. Pytpy and Antpy self-assembled in water over a large concentration range (1-100 |xM) to form spherical nanoparticles in the size range of 150-200 nm, as characterized by TEM and DLS. The absorption spectra for both conjugates showed red shift of the absorption bands in water (-10 nrn) along with significant tailing of the long-wavelength bands. The change in emission behavior in going from solution to the aggregates in water was very dramatic. Multiple, broadened, highly red-shifted emission bands for both Antpy and Pytpy were observed. Quite significantly, a long lifetime component in the emission decay was shown by the conjugates in water as compared to the lifetimes observed in solution. The data points towards a unique CT emission for Antpy and Pytpy aggregates in water. The excitation spectra for the multiple emission bands seen for Pytpy (or Antpy) were observed to be identical. Thus a single ground state population is responsible for emission over the entire range (approximately 420 nin - 600 nm). The existence of multiple emission bands and the large bathochromic shifts are exclusively due to excited state effects in the aggregated state in water. It appears that excited state H-bonding of the tpy N with water helps facilitate the excited state CT. The solid-state behavior of Pytpy and Antpy lias been examined and the emission from the two crystalline solids is very distinct. Antpy emission showed a X,,nax at -430 nm while Pytpy emission peaked at ~ 560 nm. The difference in the solid-state emission behavior exhibited by Pytpy and Antpy is explained through a consideration of the crystal packing for the two molecules. The degree of n-facial stacking was observed to be much greater for Pytpy. The observation of the distinct packing and emission shown by solid Pytpy and Antpy is highly significant if one considers the identical emission shown by the aqueous nanoparticles of the two molecules and brings to fore the 'nanoparticle effect' in water as compared to a simple concentration effect. It was also demonstrated that it was possible to modulate the aggregation of the terpyridines through additives, like metal ions Chapter 4: Pyrenyl terpyridine as a ratiometric fluorescence probe for sensing order and polarity of membranous aggregates. Pytpy was examined for its utility in probing surfactant aggregates, particularly membranous assemblies. la lipid bilayer vesicles made of phosphatidylcholine (PC) lipids (like dimyristoyl phosphatidylcholine, DMPC or egg-yolk PC) Pytpy showed an emission profile with marked similarity to that shown by the probe in water. Specifically, a broad red-shifted emission with A,maxin. the 500 nm region was observed. In addition, a peak in the -420 nm region was also seen. Fluorescence anisotropy was used to confirm the presence of vesicle-bound probe. Excitation spectra confirmed the presence of two distinct probe populations, om responsible for the '420 nm9 emission and another population responsible for the multiple, red-shifted emission bands. The emission behavior was indicative of aggregation of Pytpy on the vesicle surface and CT effects operating in conjunction with H-bonding. Fluorescence lifetime measurements, carried out at different Is suggest the CT nature of the red-shifted emission. The aggregation of the probe on the bilayer interface was confirmed by concentration and temperature dependence of the emission profile. The role of water in stabilizing this CT emission on bilayer surfaces was shown with use of a surface dehydrating agent polyethylene glycol (PEG). All these results helped build a model for the behavior of Pytpy in water. Pytpy aggregates on bilayer surface and shows a red-shifted CT emission with stabilization by interfacial water. Thus, the Pytpy 'aggregate' has a shallow, water accessible location in the bilayer. In addition to this, there is another Pytpy population responsible for the emission in the 420 nm region, and this second population might have a comparatively deeper location. The wavelength of the CT emission was sensitive to the polarity of the interface as evidenced "by the results obtained with bilayers made of a number of PC lipids. In general, the X™ax of the CTband showed a red shift with increasing polarity. The increase in polarity also caused an increase in the average lifetime of the probe. Pytpy could distinguish between vesicles made of lipids of different head groups. Aggregates made of phosphatidylethanolamine (PE) head group are in general less hydrated than PC lipid assemblies and Pytpy emission reflected this when examined in vesicles made of related lipids (dioleoyl lipids, DOPC and DOPE; dirnyristoyl lipids, DMPC and DMPE). Pytpy emission from PE vesicles was quenched and showed a pronounced blue shift in the emission Xmax vis-a-vis PC bilayers. Thus, dehydration of the interface consistently led to the destabilization of the CT state. Further, Pytpy emission was also responsive to hydration in more complex mixed PC-PE assemblies. Pytpy emission "behavior was also used to probe fluidity in complex "mixed" lipid assemblies- The effect of cholesterol on DMPC bilayers in terms of its known ability to dehydrate the bilayer was reported through a blue-shift Xmax of CT emission band. Further, cholesterol also causes drastic change in the bilayer at concentrations greater than ~ 30 mol%. This change in the bilayer was sensed through a sudden reduction in fluorescence intensity. Also from a careful analysis of Pytpy in various PC and PE vesicles, it emerged that the more fluid aggregates showed larger quantum yields. Thus, Pytpy could simultaneously report on both the polarity and fluidity of lipidic aggregates. Pytpy could also provide information about the order of an assembly. While the probe aggregated in bilayers and other membranous assemblies and showed water assisted CT emission, in more dynamic assemblies like micelles, Pytpy aggregates were not sustained, Pytpy in micelles showed emission spectra very similar to that seen in solutions in aprotic organic solvents. Thus, Pytpy proved to be a very useful ratiometric sensor for vesicle-to-rnicelle transition. Also, it has been possible to study some surfactant-lipid mixed assemblies that show phase separation. Pytpy reported the formation of a 'rigid', bilayer-like phases in mixed assemblies that are called bicelles.
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Bile acid-based cation sensors involving through space photo-induced electron transfer (PET) processes have been synthesized. In this approach, appropriate known fluorophores and aza crown ether receptor units were attached on a suitable bile acid scaffold. A through space photo induced electron transfer from N-atom of the aza 18-crown-6 to the excited pyrene was responsible for quenching of the pyrene fluorescence. A fluorescence enhancement was observed with the addition of K+ due to the inhibition of fluorescence quenching by PET mechanism. In order check the relationship between the sensitivity and the molecular structure of the sensors, four different molecules with different geometries were synthesized. The changes in the fluorescence spectra for different sensors were recorded in MeOH. The binding constants calculated by curve fitting showed that while the binding constants did not significantly vary, the sensitivities were different depending on the structure of the sensors. The modular nature of the sensor design was verified by changing the receptor module from aza-18-crown-6 to aza-15-crown-5, keeping other parts of the sensor same, to prepare a sodium selective sensor using the same principle. Fluorescence titration in MeOH confirmed the Na+ selective sensing in the presence of K+. The modular design concept was further extended by replacing the fluorophore pyrene to a coumarin derivative. Coumarin sensors showed a behavior similar to that of the pyrene sensors. In order to check the possibility of sensing metal ions in water, non ionic surfactant, Triton X-100 was chosen to dissolve the sensor in water. Fluorescence titration of the sensors showed a desired selective fluorescence enhancement with the particular metal ions. Merrifield resin and water swellable Tentagel® was used to immobilize the sensor to fabricate reusable sensor beads for detecting the metal ions in non polar solvent and water respectively. Fluorescence enhancements of the sensor beads with the metal ions confirmed the process in the immobilized solid state. K+ and Na+ selective sensor beads successfully demonstrated the fluorescence enhancement with the respective cations. This general strategy can be extended to fabricate other sensors for practical uses.
The Copper(I)-catalyzed Azide–Alkyne Cycloaddition: A Modular Approach to Synthesis and Single-Molecule Spectroscopy Investigation into Heterogeneous CatalysisDecan, Matthew January 2015 (has links)
Click chemistry is a molecular synthesis strategy based on reliable, highly selective reactions with thermodynamic driving forces typically in excess of 20 kcal mol-1. The 1,3-dipolar cycloaddition of azides and alkynes developed by Rolf Huisgen saw dramatic rate acceleration using Cu(I) as a catalyst in 2002 reports by Barry Sharpless and Morten Meldal enabling its click chemistry eligibility. Since these seminal reports, the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) has become the quintessential click reaction finding diverse utility. The popularity of the CuAAC has naturally led to interest in new catalyst systems with improved efficiency, robustness, and reusability with particular focus on nanomaterial catalysts, a common trend across the field of catalysis. The high surface area of nanomaterials lends to their efficacy as colloidal and heterogeneous nanocatalysts, but the latter boasts the added benefit of easy separation and recyclability. With any heterogeneous catalyst, a common question arises as to whether the active catalyst species is truly heterogeneous or rather homogeneous through metal ion leaching. Differentiating these processes is critical, as the latter would result in reduced efficiency, higher cost, and inevitable environmental and heath side effects. This thesis explores the CuAAC from an interdisciplary approach. First as a synthetic tool, applying CuAAC-formed triazoles as functional, modular building blocks in the synthesis of optical cation sensors by combining azide and alkyne modified components to create a series of sensors selective for different metal cations. Next, single-molecule spectroscopy techniques are employed to observe the CuNP-catalyzed CuAAC in real time. Combining bench-top techniques with single-molecule microscopy to monitor single-catalytically generated products proves to be an effective method to establish catalysis occurs directly at the surface of copper nanoparticles, ruling out catalysis by ions leached into solution. This methodology is extended to mapping the catalytic activity of a commercial heterogeneous catalyst by applying super-localization analysis of single-catalytic events. The approach detailed herein is a general one that can be applied to any catalytic system through the development of appropriate probes. This thesis demonstrates single-molecule microscopy as an accessible, effective, and unparalleled tool for exploring the catalytic activity of nanomaterials by monitoring single-catalytic events as they occur.
Design, Synthesis, And Characterization Of Novel Hydrophilic Fluorene-based Derivatives For Bioimaging ApplicationsNguyen, Dao 01 January 2009 (has links)
In this work, hydrophilic fluorene-based derivatives that contain ethylene oxide substituents, have been synthesized and characterized for potential use as new fluorophores for bioimaging applications and for fluorescence sensing of heavy metals. Symmetrical and unsymmetrical fluorene derivatives based on structural types of acceptor-pi-acceptor, acceptor-pi-donor, and donor-pi-donor were characterized by TGA, UV-vis absorption, fluorescence emission, lifetime, anisotropy, and two-photon absorption (2PA) cross section. They were found to possess high thermal stability, high photostability, high fluorescence quantum yields, and generally large two-photon absorption cross sections, making them quite suitable for new probes in single-photon absorption and two-photon absorption fluorescence microscopy imaging. Novel hydrophilic fluorene derivatives were synthesized from fluorene in multiple steps employing the metal-catalyzed Heck coupling reaction, the Stille reaction, the Sonogashira reaction, the Ullmann condensation reaction, and "click" chemistry. To increase the hydrophilicity of the new compounds, ethylene oxide substituents were utilized for to impart water solubility. An alternative alkylation methodology using ethyleneoxy tosylates was introduced for the synthesis of ethylene oxide-containing fluorene derivatives. Several of these hydrophilic derivatives were incubated into various cell lines as new probes for both conventional and two-photon absorption fluorescence bioimaging. These compounds were biocompatible, exhibiting low cytotoxicity as determined by cell viability studies, and displayed colocalization for selected cellular organelles. In addition, hydrophilic bis(1,2,3-triazolyl)fluorene derivatives were found to exhibit sensitive fluorescence responses in the presence of certain heavy metal, and were selective for sensing zinc and mercury over other a number of other metal ions relevant to living cells or other biological environments. The UV-vis absorption and fluorescence emission spectra of the complexes exhibited a blue-shifted absorption and emission for selective metal chelation upon binding to zinc and mercury(II) ions, resulting in an approximately two-fold enhanced fluorescence response. Fluorescence titration studies revealed that the complexes of 1:2 and 1:3 ligand to metal formed with binding constant values of 108 and 1014 for zinc and mercury ions, respectively. Finally, preliminary experiments were performed to explore the possibility of employing select hydrophilic fluorene-based derivatives in the synthesis of hydrophilic fluorescent gold nanoparticles. Although results are very preliminary, the aim is to use such materials for other biomedical applications, such as surface enhanced scattering resonance and noninvasive photothermal therapy to diagnose and to treat cancers. Thus, this research had led to the discovery of alternative methodologies for synthesis of hydrophilic fluorene derivatives by alkylation with alkyl tosylates and synthesis of hydrophilic fluorescent molecule capped gold nanoparticles. Furthermore, several novel hydrophilic fluorene-based derivatives were synthesized and characterized for their linear and nonlinear photophysical properties, and are now available for further examination of their bioimaging and sensing applications.
New Supramolecular Ion Sensing Probes And Their Application In The Detection Of Environmentally Relevant IonsNamita Kumari, * 07 1900 (has links) (PDF)
The thesis entitled “New Supramolecular Ion Sensing Probes and their Application in the Detection of Environmentally Relevant Ions” deals with the design and synthesis of several small molecular probes which can specifically sense environmentally relevant ions of (anion or cation) particularly in aqueous or biological medium. The probes have been designed using four different molecular entities which include anthraquinone, oxidized bis-indolyl system, pyrene and rhodamine. The probes afford naked eye detection of a particular ion in the aqueous medium. This work has been divided into six chapters. Chapter 1. Introduction The first chapter gives a brief idea of ion sensor. It provides the description of various approaches used for designing molecular sensors. The chapter further presents an overview of the four different dyes (anthraquinone, oxidized-bis-indole, pyrene and rhodamine) used for designing probes in this work. The properties of these probes, their advantages and disadvantages to use as a signaling subunit have been discussed. This chapter also describes the use of micellar medium for solubilizing different organic dyes in water. Chapter 2. Colorimetric Probes based on Anthraimidazolediones for Selective Sensing of Fluoride and Cyanide ion via Intramolecular Charge Transfer. The second chapter describes the design and synthesis of four different probes based on anthra [1, 2-d] imidazole-6, 11-dione. The anthraquinone part of each molecule has an acceptor moiety whereas substituted nitrogen linked aromatic unit forms the donor site. Each probe acted as strong colorimetric sensor for fluoride and cyanide ion detection and exhibited intramolecular charge transfer (ICT) band which showed significant red-shifts after addition of either the F¯ or CN¯ ion. One of the probes 2 showed selective colorimetric sensing for both cyanide and fluoride ions. In organic medium 2 showed selective color change with fluoride and cyanide, whereas in aqueous organic medium it showed a selective ratiometric response towards cyanide ion. The effect of anionic charge (on the donor moiety) on ICT has been discussed. Among the various donor moieties, the donor site having negative charges on them was found to disperse greater electron density on them. Figure 1. Molecular structures of the sensors Chapter 3 deals with chemodosimetric detection of cyanide ion in water using various oxidized bis-indole based compounds. Chapter 3A. A Chemodosimetric Probe based on a Conjugated and oxidized Bis¬ indolyl System for Selective Naked Eye Sensing of Cyanide ion in Water. The chapter 3A describes the design and synthesis of a new water-soluble bis-indolyl based probe, 5 which possesses two –COOH groups. This probe specifically reacted with the CN¯ ion in pure water at ambient temperature and produced a remarkable change in color from red to colorless. The mechanism of this process was investigated by NMR (1H, 13C and DEPT-135) spectroscopy, mass spectrometry and kinetic studies. The mechanism investigation showed that the cyanide ion reacts with the probe and removes the conjugation of the bis-indolyl moiety of the probe with that of the 4-substituted aromatic ring which renders the probe colorless. Taken together a plausible mechanism of the reaction was presented which showed to operate via a Michael type adduct formation under ambient conditions of pH and temperature in water. The probe gave a detection limit of 0.38 ppm for detection of cyanide ion in water. Figure 2. Molecular structure of the probe 5. Chapter 3B. Micelle Assisted ppb level Detection of Cyanide ion in Water by Chemodosimetry and Visual detection of the Endogenous Cyanide. The chapter 3B deals with the synthesis of a bis-indole based colorimetric probe 6. The probe showed selective detection of the cyanide ion in water at ppb level and a visible detection of endogenous cyanide from cassava (a major staple food in the developing world) by chemodosimetry. The cyanide ion binds with the probe 6 in a chemodosimetric fashion and follows pseudo first-order kinetics in water under appropriate conditions. It showed a highly sensitive detection of the cyanide ion in water with a detection limit of 0.33 ppm. The use of the micellar medium improved the detection limit drastically and a ppb level detection limit was achieved. The probe also showed the detection of the endogenously bound cyanide in cassava both visually and by spectrophotometer. Figure 3. Molecular structure of the probe 6. Chapter 3C. Ratiometric Cyanide ion probe in Water and for the detection of the Endogenously bound cyanide. Chapter 3C presents the synthesis of two new bis-indolyl (7 and 8) based probes for colorimetric detection of cyanide ion in pure water. Compound 8 showed a ratiometric response with cyanide in water and a visual detection of the endogenously bound cyanide ion in cassava. Using compound 8 the selective detection of the cyanide ion in water was achieved with a detection limit of ~ 17 ppb which is almost 13 times lower than the permitted limit as specified by EPA, United States. 7; R = H 8; R = -(OCH2CH2)3CH3 Figure 4. Molecular structures of the probes 1 and 2. Chapter 4 deals with the colorimetric and ratiometric detection of the Cu2+and Hg2+ions using different small synthetic molecular probes. Chapter 4A. Colorimetric Sensors for Ratiometric Detection of Copper and Mercury ions in Biological media and below ppm level in Water. The chapter 4A deals with the synthesis of two novel colorimetric probes (9, 10) using bispicolyl unit as the binding moiety and anthraimidazolediones and bis-indolyl system as a signaling sub-unit. Using the two sensors, Cu2+ion can be detected below the permitted limit (1.3 ppm) in both drinking water and at physiological pH 7.4. Sensor 9 can detect both Cu2+and Hg2+ in water with very low detection limit. It showed specific binding with Cu2+ at physiological pH 7.4 and in presence of serum albumins. Chemosensor 10 can be used for the specific detection of both Cu2+and Hg2in water as well as for the contamination in microorganisms. Figure 5. Molecular structure of the sensors 9 and 10. Chapter 4B. A New Molecular Probe for the Selective Sensing of Cu2+ and Hg2+ ions in Micellar Media and in Live ells.This chapter describes a synthesis of a novel bispicolyl based sensor 11 which can detect Cu2+ ion specifically in water medium and both Cu2+ and Hg2+ ions selectivelyin Brij-58 micellar medium. In micellar medium both the ions can be detected in the ppb level. Using fluorescence spectroscopy these two metal ions can be discriminated.The probe is also be useful for checking metal ion contamination in cellular samples. Figure 6. Molecular structure of the sensor 11. Chapter 4C. Rhodamine based Sensors for Cu2+ and Hg2+ ions in Water and in Biological media. The chapter 4C presents the synthesis and the sensing properties of the three positional isomers of the pyridine end of the rhodamine-pyridine compounds (12-14). The three isomers only differ in the position of nitrogen of the pyridine moiety. Sensor 12, which contains the pyridine nitrogen at the ortho-position showed selective sensing toward Cu2+ ion in both pure water and in buffered physiological media of pH 7.4. It gave a detection limit of ~13 ppb which is 100 times lesser than the EPA permitted limit. The other two sensors 13 and 14, which possessed the pyridine ends with the nitrogen atom at the meta- and the para- positions respectively showed the selective sensing of Hg2+ ion in water and did not show any interaction with the Cu2+ ion. Probes 2 and 3 showed ‘turn-on’ detection of Hg2+ ion both in the UV-vis and the fluorescence emission spectroscopy. Compound 2 and 3 showed a detection limit of ~ 9 and 4 ppb respectively. The NMR titration showed the change in color was due to the opening of the spirolactam ring of the rhodamine. The sensors can also be used for the detection of Cu2+ and Hg2+ ion in real life water samples and in the live cells. Figure 7. Molecular structure of the sensors 12, 13 and 14. Chapter 5. Ratiometric and ppb level Detection of Toxic Transition Metal ions using a Single Probe in Micellar media. This chapter describes the selective sensing of multiple ions using a single probe 15. The probe incorporates pyrene and pyridine as signaling and interacting moiety respectively. The sensor showed different responses towards different metal ions just by varying the medium of detection. In organic solvent (acetonitrile), the probe showed selective detection of Hg2+ ion. In water the fluorescence quenching was observed with three metal ions, Cu2+, Hg2+ and Ni2+. Further just by varying the surface charge of different micellar media, the probe showed selective interaction with Hg2+ ion in neutral micelles (Brij-58). However, in anionic micellar medium (SDS), the probe showed selective changes with both Cu2+ and Ni2+ in the UV-vis spectroscopy. The discrimination between these two ions was achieved by emission spectroscopy, where it showed selective quenching only with Cu2+. Thus using a single probe all the three metal ions Cu2+, Hg2+ and Ni2+ can be detected and discriminated just by varying the surface charge of the micellar medium. Figure 8. Molecular structure of the sensors 15. Chapter 6. Highly sensitive Rhodamine Based Dual Probes for the Visual detection of F¯ and Hg2+ ions in Water. This chapter deals with the design and synthesis of two new rhodamine based probes (16-17) which act as dual probes for the ppb level selective detection of Hg2+ and F¯ ions in water and at physiological pH 7.4. The two probes were synthesized by coupling tert-butyldiphenylsilyl (TBDPS) protected forms of 4-hydroxybenzaldehyde and 2, 4- dihydroxy benzaldehyde with rhodamine hydrazone. The F¯ ion detection is based on the desilylation of the probe, whereas the spirolactam ring opening leads to the detection of Hg2+ ion. The two probes gave turn-on detection of both Hg2+ and F¯ ion selectively in aqueous medium with the detection limit well below the EPA permitted limits. The probes showed detection of both the ions by dual mode with visibly different color and fluorescence under UV-lamp. The F¯ ion interacts with the silyl bond of probe and the cleavage results into yellow color whereas; the addition of Hg2+ ion to the probe solution opened the spirolactam ring and resulted into appearance of pink color. Figure 9. Molecular structure of the probes 16 and 17. (For structural formula pl see the abstract file)
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Metal ion contamination in surface and ground water is a major threat as it has a direct implication on the health of terrestrial and aquatic flora and fauna. Lead (Pb2+), mercury (Hg2+), cadmium (Cd2+), nickel (Ni2+), copper (Cu2+) and cobalt (Co2+) are few of these metal ions which are classified under the high risk category. Of these, lead and mercury are of greater concern, as even nanomolar concentrations can be lethal, as they can be bio-accumulated and result in physiological as well as neurological disorders. In Asian countries like India and China, heavy metal pollution is more prevalent, as a consequence of poor governmental policies or ineffective or inadequate measures to combat this problem. In recent times, the monitoring and assessment of water pollution is a critical area of study, as it has a direct implication for its prevention and control. The major techniques used for metal ion detection are atomic absorption spectroscopy (AAS), X-ray fluorescence, ion chromatography, neutron activation, etc. Alternatively, the electrochemical, optical and electrical methods provide a platform for the fabrication of portable devices, which can facilitate the on-site analysis of samples in a rapid and cost-effective manner. This has led to a new field of research called chemical sensors or chemo sensory devices. The main aim of this study is to develop various chemosensory materials and test their response towards metal ion sensing. In this study, electroactive polymers have been synthesized for various sensor applications. The focus has been to design synthesize and test various functionalized electroactive polymers (FEAP) for the development of electrochemical, optical and chemoresistive sensors. Electroactive polymers like polyaniline, polypyrrole, polypyrrole grafted to exfoliated graphite oxide and dipyrromethene conjugated with p-(phenylene vinylene) have been synthesized and evaluated after functionalizing with metal coordinating ligands. These metal coordinating ligands were selected, in order to enhance their metal uptake capacity. Various metal ligands like imidazole, tertiary amine group, iminodiacetic acid, and dipyrromethene incorporated either in the polymer backbone or as a part of the backbone have been chosen for the metal binding. These functionalized electroactive polymers (FEAP) served as active material for metal ion sensing. The present investigation is subdivided into three sections. The first part includes design and chemical synthesis of the functionalized polymers by a series of organic reactions. The synthesis has been followed up by characterization using spectroscopic methods including NMR, FTIR, GCMS and Mass spectrometry. In the second part of the investigation, the synthesized polymer has been characterized for the changes in electronic, electric and optical properties after interaction with the selected metal ions. For this, the FEAP is allowed to interact with various metal ions and the changes in the relevant properties have been measured. This includes the study of changes in the conductivity, electronic properties like absorption or emission of the polymer, changes in the redox properties, etc. The third phase of investigation deals with the fabrication of the devices using the active FEAP. The sensor devices comprised of either films, or electrode modified with FEAP or solution of the FEAP, in combination with an appropriate technique has been used for the sensing. The major objectives are enumerated below 1. Functionalzation of polyaniline with imidazole functional group to get imidazole functionalized polyaniline (IMPANI) and study of the electronic, electrical and optical properties of the same. 2. Preparation of films of IMPANI and study of the change in conductivity of the film upon interaction with various metal ions, namely Cu2+, Co2+ and Ni2+ in their chloride form. 3. Synthesis of amine functionalized aniline monomer and chemical graft polymerization onto exfoliated graphite oxide as a substrate to synthesise the amine funtionalised polyaniline grafted to exfoliated graphite oxide (EGAMPANI). Modification of the carbon paste electrode (CPE) with EGAMPANI and study of the electrode characteristic. 4. Study of the electrode properties of EGAMPANI modified carbon paste electrode. 5. Evaluation of the EGAMPANI modified carbon paste electrode as a multi-elemental voltammetric sensor for Pb2+, Hg2+ and Cd2+ in aqueous system. 6. Functionalization of polypyrrole with iminodiacetic acid and characterization of the polymer to synthesis iminodiacetic acid functionalized polypyrrole (IDA-PPy). 7. Modification of the CPE with IDA-PPy by drop casting method and evaluation of the Pb2+ sensing properties. 8. Study of the effect of other metal ions say Hg2+, Co2+, Ni2+, Zn2+, Cu2+ and Cd2+ on the anodic stripping current of Pb2+ using EGAMPANI modified CPE. 9. Synthesis of dipyrromethene-p-(phenylene vinylene) conjugated polymer for heavy metal ion sensing. 10. Study of the changes in the optical absorption and emission properties of the polymer in THF and evaluation of the change in these optical properties upon interaction with the metal ions as analyte. The salient findings of the research work are highlighted as follows, In the first synthesis, aniline has been functionalized with imidazole group and this monomer has been chemical oxidatively polymerized to obtain imidazole functionalized polyaniline (IMPANI). The synthesized polymer possesses a nano-spherical structure, as confirmed from the morphological characterisation using scanning electron microscopy. The IMPANI has been interacted with a representative metal ion, copper (II) chloride, and the copper complexed polymer (Cu-IMPANI) has been subjected to various studies. The coordination of copper with IMPANI results in an increase of molecular weight of the polymer as a result of aggregation, as observed from dynamic light scattering measurements. Apart from this, a significant finding is the decrease of the pH of the system after copper ion coordination attesting to the generation of a secondary hydrochloride ion during the coordination of the copper to the imidazole side chain. This is further confirmed by an increase in conductivity of the Cu-IMPANI compared to IMPANI, measured using the four-probe technique. The increase of conductivity due to copper coordination is one order of magnitude higher. The films which have been prepared from IMPANI and Cu-IMPANI exhibit different morphology. The Cu-IMPANI film prepared by prior co-ordination of Cu ion with IMPANI powder shows a flaky structure, which is not preferable for the conductivity measurements, as a consequence of discontinuity in the medium. To overcome this problem, IMPANI films were initially prepared and then interacted with copper ions for a desired duration, before measurement of the conductivity. This latter procedure enabled the preparation of smooth films for the development of chemoresistive sensors. In continuation of the initial study highlighted above, IMPANI films of thickness 0.02 ± 0.001 mm have been prepared using IMPANI and PANI in DMPU in the ratio of 7:3 by mass. After exposure of the films with respective metal chlorides, such as Ni2+, Co2+ and Cu2+, a change in conductivity is observed in the concentration range of 10-2 to 1 M of metal chlorides. The sensor response may be arranged in the sequence: Ni2+ > Cu2+ > Co2+ at 1M concentration. On the contrary, films prepared from PANI-EB under identical conditions do not exhibit any appreciable change in conductivity. The optimum exposure time is determined to be 10 min for a maximum change in conductivity, after exposure to the chosen metal ions. In the second system taken up for investigation, a tertiary amine containing polyaniline (AMPANI) has been grafted to exfoliated graphite oxide. The amine containing polyaniline grafted to exfoliated graphite oxide (EGAMPANI) has been characterised for structural, morphological and elemental composition. The grafting percentage has been determined to be 7 % by weight of AMPANI on the EGO surface. The synthesized EGAMPANI (5 weight %) has been used to modify carbon paste electrode (CPE) for electrochemical sensor studies. Based on the differential pulse anodic stripping voltammetric studies, the electrochemical response may be arranged in the following sequence: Pb 2+>Cd 2+>Hg 2+ The minimum detection levels obtained are 5×10-6, 5×10-7, and 1.0×10-7 M for Hg2+, Cd2+ and Pb2+ ions respectively. In the next study, an iminodiacetic acid functionalized polypyrrole (IDA-PPy) has been synthesized and characterised for its elemental and structural properties. This has been further used to modify the CPE by drop casting method and used for the specific detection of Pb2+ in acetate buffer. Various parameters governing the electrode performance such as concentration of depositing solution, pH of depositing solution, deposition potential, deposition time, and scan rate, have been optimized to achieve maximum performance and found to be 20 μl, 4.5, -1.3 V, 11 min, 8 mV s-1 respectively for the chosen parameters. Additionally, the influence of other heavy metal ions on the lead response has been studied and it is observed that Co, Cu and Cd ions are found to be interfering. Further, the response of Cd, Co, Cu, Hg, Ni and Zn on IDA-PPy functionalized electrode has been evaluated. The selectivity of IDA-PPy modified electrode for Pb2+ is observed in the concentration range of 1 × 10-7 M and below. The IDA-PPy modified CPE shows a linear correlation for Pb2+ concentration in the range from 1×10-6 to 5×10-9 M and with a lowest limit of detection (LLOD) of 9.6×10-9 M concentration. The efficacy of the electrode for lead sensing has also been evaluated with an industrial effluent sample obtained from a lead battery manufacturing unit. The fourth synthesis pertained to the development of an optical sensor for Fe2+, and Co2+ ions. For this, dipyrromethene as a metal coordinating ligand in conjugation with p-phenylenevinylene has been synthesized and tested for its structural as well as optical properties. It is observed that the polymer shows three absorptions, namely at 294 nm, 357 nm and a major absorption observed as a broad band ranging from 484 to 670 nm. The emission spectrum of the polymer excited at 357 nm shows a characteristic blue emission with a maximum intensity centered at 425 nm. The emission quenching in the presence of various metal ions have been tested and are found to be quenched in presence of Fe2+ and Co2+ ions. All the other metal ions tested namely, Cr3+, Cu2+, and Zn2+ are not found to exhibit any change in the emission spectra below the concentration of 1 × 10-4 M. The linear correlation of the emission intensity with the concentration of the Co2+ and Fe2+ ions has been determined using Stern-Volmer plot. For Co2+ the Stern-Volmer regime is observed from 1×10-4 to 9×10-4 M concentration and the quenching constant Ksv is determined to be 8.67 ×103 M-1. For Fe2+, the linearity is found to be in the regime of 1×10-5 to 9×10-5 M and the quenching constant Ksv is determined to be 7.90 × 103 M-1. In conclusion, different electroactive polymers functionalized with metal coordinating ligands have been synthesized, characterised and evaluated for metal sensing applications. Techniques like electrochemical, optical and conductivity have been used to characterise the response of these FEAP towards metal sensing. It is can be concluded that the electrochemical sensors are more reliable for sensing especially at very low concentrations of metal ions such as Pb, Cd and other techniques like optical and conductimetric are good for detecting metal ions namely Fe, Co, Ni, Cu. The selectivity towards the metal ions is a function of the metal chelating ligand and the extent of sensitivity is dependent upon the technique employed.
Pendyala, Naresh Babu
The thesis describes the synthesis of PbS nano-, micro-structures by colloidal and hydrothermal techniques. Size and morphology dependent luminescence studies were carried out in detail. Application oriented studies like ion sensing and modulation of luminescence are carried out on colloidal PbS QDs. Photoelectrical studies are carried out on various morphologies of PbS microstructures. We observe the persistent photoconductivity, growth and quenching of photocurrent, and a few novel phenomena in flower shaped PbS microstructures. This work is presented in eight chapters inclusive of summary and directions for future work. CHAPTER 1 provides a brief introduction to optical and photoelectrical properties of semiconductor quantum dots and hydrothermal technique in preparation of quantum structures. A review of PbS nanostructures and its technological applications are discussed. CHAPTER 2 provides the experimental techniques used in this work. First, the synthesis of PbS nano-, micro-structures by various methods, and characterization tools used in this work are briefly presented. CHAPTER 3 deals with the synthesis of PbS quantum dots in poly vinyl alcohol with various precursor concentrations to identify the surface states by temperature dependent photoluminescence (PL) measurements. Average bandgap value calculated from absorption measurements was 2.1 eV. We have observed that high-energy PL bands (>1.3 eV) are due to electron traps (Pb dangling bonds) and low-energy bands (<1 eV) are due to hole traps (S dangling bonds). By capping with thiol compounds (mercaptoethanol-C2 H5OSH), absence of the 1.67 eV band indicates the passivation of Pb dangling bonds. To explain above observed results, we propose a band diagram with distributed shallow to deep states and attributed them to the specific surface related defects (Pb or S). CHAPTER 4 discusses the ion sensing applications of PbS quantum dots. We found that the sulfur related dangling bonds are quite sensitive to different metallic ions (since mercaptoethanol passivates only Pb atoms). Sulfur related PL band (~ 1 eV) have shown an order of magnitude improvement in its intensity for Hg, Ag ions and relatively low enhancement for Zn, Cd ions at 1 µmol concentrations. However Cu quenches the luminescence. An important distinction may have to be made between PbS and Cd related quantum structures. The PbS QDs can distinguish between Cu & Hg, however Cd related QDs couldn’t distinguish between these two ions. Photo-brightening and photo-darkening is an interesting phenomena indicative of photo-induced ionic migration that either helps in enhancing the emission of sulfur related defect emission or degrades the emission properties depending on the ion concentration. This report is the first of its kind in ion sensing applications using PbS QDs. CHAPTER 5 discusses the results of duel beam excitation on trap luminescence of PbS QDs. By using different lasers simultaneously (514 nm and 670 nm), we have observed the reversible luminescence quenching of trap emission. The high-energy PL band (1.67 eV) has double the quenching effect compared to low-energy PL band (1.1 eV). The luminescence quenching mechanism is attributed to the re-emission of the charge carriers from the traps (photo-ionization) due to the simultaneous excitation with the second beam. The dependence of the temperature, the effect of two beam excitation intensities and modulation frequency dependent quenching mechanism are primarily focused in this chapter. The quenching mechanism is considered to be quite useful in the optical modulation devices. CHAPTER 6 discusses the PL results on various morphologies of PbS nano-, microstructures. Interestingly, after protecting the surface with organic ligands such as mercaptoethanol (C2 H5OSH), dendrite structures have shown high-energy bands (~ 1.0 eV) in the PL spectra, which indicate the existence of various quantum confinement regimes in different branches of dendrites. The anomalous temperature dependent behavior of PL intensity is attributed to the size distribution. CHAPTER 7 discusses the results of photoconductivity measurements on various morphologies of PbS nano-, micro-structures. Flower shaped structures have shown persistent photoconductivity (PPC). This observed PPC is attributed to the presence of potential barriers, which are created by the different confinement regimes or due to the lattice relaxation, which occurs due to the carrier trapping at surfaces. In PPC, the estimated time constants of both build up and decay transients using the stretched exponentials are of the order of few tens of seconds. In PPC measurements, we observe the PC quenching below 40 K and growth above this temperature. PC quenching is attributed to the transfer of photo-excited carriers to a metastable state. The presence of metastable state is supported by the dark conductivity measurements in flower shaped structures. CHAPTER 8 presents the summary and directions for the future work.
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