Ion selective sol-gel film synthesis and usage as optical sensor

Determination by fluorescence spectroscopy is an interesting way to create an effective chemosensor because of its ease of use and sense of concentrations below the micromolar level. Sensing approaches based on the fluorescence property offer high sensitivity, while optical sensing methods are not the only simple way and do not require an expensive assay but also allows for optical determination with the sample to be analyzed on a relatively small scale. However, in order to be able to form the selectivity properties, the technique of template derivation according to the target metal ion is required. Therefore, it is remarkably investigated that fluorescence-guiding molecules, which give chromophore information, are using for sensor designs. Sol-gel is the most common support material for chemical sensors because the sol-gel can easily be processed into small particles and stored as thin films on optical fibers. Sol-gel technology is a chemical synthesis technique used for the production of both glassy and ceramic materials at room temperature. This technology is based on the transition from solution form to solid form. Moreover, heat-resistant inorganic materials have been used for many years as natural raw materials in production processes requiring high temperature. This synthesis technique allows a transition from a colloidal fluid system to a solid gel phase. Sol-gel technology is used to produce a wide variety of ceramic and glass materials such as ultra-fine or spherical powders, thin film coatings, ceramic fibers, microporous inorganic membranes, single-piece or highly porous aerogels. The sol-gel process consists of three main steps: The first step is the hydrolysis and polycondensation of the alkoxide precursor followed by the supercritical drying. The second is the gelation reactions of colloidal particles. In the third step, similar to the second, the colloidal particles are gelled. The only difference is, however, at ambient temperatures in the drying process. The development of selective chemosensors for toxic heavy metal ions due to the importance of their usage in industrial applications and environmental impact is becoming important in recent years. Conventionally, atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), ion-selective potentiometer and X-ray fluorescence method is used for the detection of metal ions. However, these methods also have some disadvantages, such as complicated sample preparation steps, high equipment cost, and determining the appropriate method for each sample to be analyzed. For this reason, it is very important to determine low concentrations of metal ions by simple fluorescence technique with high sensitivity, good selectivity, short response time and real-time monitoring. Fluorescent chemosensors capable of selectively detecting metal ions in the environment or biological systems were of current interest in supermolecular chemistry because of their high selectivity, sensitivity, and simplicity. In this regard, many research reports have appeared that explored fluorescence chemosensor in the detection and discrimination of transition metal ions owing to their extremely toxic impact on our environment or biological systems. The fluorescent sensor absorbs and emits in the spectral range of visible light, exhibits a reversible response, is widely variable concerning receptor groups, and its optical properties also allow a miniaturization including LED's and photodiodes. The detection of the analyte, metal ions is based on a static quenching of fluorescence upon binding to metal ions. Optical chemosensors have been developed to be a useful tool to sense in vitro and in vivo biologically important species such as metal ions and anions because of the simplicity and high sensitivity of fluorescence assays. A typical fluorescent chemosensor contains a receptor linked to a fluorophore which translates the recognition event into the fluorescence signal. Phthalocyanines (Pcs) was a very important class of compounds, which was discovered by chance in 1907, and whose structure was illuminated in the 1930s. Phthalocyanine is an aromatic macrocyclic compound with the C32H18N8 formula. The phthalocyanine has the ability to bind to more than 70 cations in the periodic table instead of two hydrogen atoms at the center of the macrocyclic compound. For example, metal cations such as Cu (II), Co (II) and Fe (II) are strongly bound to the macrocycle and they have very stable form. The electronic, optical, structural and coordination properties of phthalocyanines and their ability to be modified according to the purpose by modifying the central metal atom or by attaching various substituents have created different application areas. Examples of these applications are semiconductors, gas sensors, pigments, molecular recognition systems, photocatalytic substances and photodynamic therapy (PDT). Otherwise, phthalocyanines are highly versatile and stable chromophores with unique physicochemical properties that make them, alone or in combination with many other functional structures for the construction of molecular materials or electro and photoactive moieties. Therefore, the synthesis of precursors bearing one or more functionality is vital in the preparation of new photoactive phthalocyanines and the design of Pcs linked to strong fluoroprobe moieties appears particularly promising. The immobilization of fluorescence probe into sol-gel silica supports for determination of metal ions has been recently attracted great attention. The porosity of sol-gel matrices allows fluoroprobe to diffuse into the solid support. There are three methods of fluoroprobe immobilization: sol-gel impregnation, covalent binding, and chemical doping. A covalent binding of the phthalocyanine (Pc) sensor on carrier matrices improves the practical use because it enables long-term measurements, avoiding leaching and a possible contamination of the sample under investigation. The sol-gel polymeric material is prepared by a conventional one-pot method with the co-condensation of such Pc-based precursor and the cross-linker, tetramethoxysilane. The advantages of this glassy substrate, compared to hydrogels or polymers, are a wide variability of sol compositions, the formation of very thin films and a relatively simple synthesis of the sols. The sol-gel layers are applied on conventional glass slides by a spin-coating process. These resulting glassy substrates show a good mechanical stability and a transparency for visible light. In this thesis, the physical doping method was used for the immobilization of thiol-substituted phthalocyanine molecule (ZnPc) into sol-gel network. Synthesis involves hydrolysis of tetraethylorthosilicate by a catalyst in the presence of ZnPc. Surfactant, N-LP was added into the sol-gel solution for homogeneous dispersion of ZnPc. In the first part of this study, absorption and fluorescence spectra of ZnPc were analyzed in mixtures of an ethanol-water solution. An optimum solution mixture was determined for ZnPc in which spectral behavior was analyzed. Absorption and fluorescence spectra of ZnPc were obtained at different pH values in the presence and absence of metal ions in predetermined solution mixture. The optimal pH value for analysis was determined by examining the difference in absorbance and emission spectra displayed in the presence and absence of metal ions. Fluorescence emission spectra were obtained according to different metal ion concentrations in 100% ethanol solution while ZnPc was used as a fluorescence probe. Fluorescence quenching was observed depending on the metal ion concentration and similar values in titrations of heavy metal ions such as Hg2+, Ag+, Pd2+ were obtained. Therefore, titration of metal ions was performed at different pH in a mixture of water: ethanol to determine the optimum conditions in order to ensure the selective sensing of metal ions. Furthermore, enhancement in fluorescence of ZnPc was observed upon addition of complex-disrupting substance for reusability of ZnPc in solution. In the second part of a study, the optimum conditions and gelation time in the presence of ZnPc were determined depending on metal ion, different catalyst, and surfactant. It was revealed that gelation times could be determined upon recording the fluorescence emission intensity of pre-gel solution containing ZnPc. The glass plate was spin-coated using pre-gel solution after determination of recipe for the synthesis of sol-gel. The spin-coated glass was dried under vacuum at 40 0C for the condensation of all of the obtained films, and the thickness of the film was determined by using the ellipsometry and found to be 0,9 µm. The addition of different metal ions to the film-coated glass, which was placed diagonally into the quartz tube, resulted in the signal change in fluorescence of ZnPc. The selective and precise detection ability of the film towards metal ion were obtained by means of immobilization of ZnPc in the sol-gel polymeric matrix. In conclusion, we described the complete synthesis and characterization of a new family of peripherally functionalized zinc phthalocyanine bearing (1-hydroxyhexan-3-ylthio)-substituents that could be used as a fluorescent chemosensor. Fluoroprobe, ZnPc exhibits distinct changes in both spectra of absorbance and fluorescence upon titration with metal ions. Q-band absorption peak of ZnPc gradually disappeared in response to treatment with metal ions in solution. Fluorescence emission intensity of ZnPc at longer wavelength region was quenched which indicates the complex formation between metal ions and ZnPc and also showing that metal ions-induced ZnPc aggregation plays a major role in the amplified fluorescence quenching.