Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
Browse
13 results
Search Results
Now showing 1 - 10 of 13
Master Thesis Investigation of Ion Transport Properties of Organic Electrochemical Transistors(01. Izmir Institute of Technology, 2022) Küçüktartar, Tuğçe; Yıldız, Ümit HakanOrganic electrochemical transistors (OECTs) comprise large amplification in current response while operating at low voltages and have high transconductance due to its volumetric capacitance created by ion injection from electrolyte through the whole organic semiconductor channel. OECTs are switchable by doping and de-doping of active channel via application of positive or negative gate bias. One of the most common organic material for OECTs is the conductive polymer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). PEDOT:PSS offers prominent advances because of their coupled electronic and ionic conductance, morphology and optical properties. Although the complex working principle of OECT has been tried to be elaborated with several models in literature, the conduction of ions and electrons in the channel has not been fully elucidated. In this thesis, the transformations between un-doped, doped and de-doped state investigated systematically based on the electrical (OECT), structural and morphological characterization of PEDOT:PSS thin film. Measurements were conducted with different dopant molecules and the repeatability of the device was investigated. As a result, the most stable drain and gate voltage range in which the device works has been determined. In addition, the X-ray photoelectron spectroscopy (XPS) investigation performed which is revealed that the density of the bipolaron formation of PEDOT:PSS in the doped state increase as compared to its natural form in the de-doped state. XPS mapping on OECT devices suggested that ions migration is homogeneously generated by applied bias.Master Thesis Investigation of Microbial Biofilm Formation Using Electrochemical Impedance Spectroscopy and Equivalent Circuit Modelling(01. Izmir Institute of Technology, 2021) Kuş, Anılcan; Yıldız, Ümit HakanBacterial biofilm is like a cooperative form of planktonic bacteria that colonize to acquire more nutritious and become resistant to surroundings. The communal organization results from the connection of bacteria by polysaccharides, lipids, or the extracellular matrix, which can provide a protective environment for living cells and communicate between them or allow specific types of chemicals inside through the matrix. 60%-80% of the infections are known to be biofilm-related. Bacterial biofilms are more resistant to antibiotics, and treating them with the wrong antibiotics might result in a thicker biofilm. In order to overcome these difficulties and researching new treatments for biofilm inflammation understanding the formation process is essential. For this manner, Electrochemical Impedance Spectroscopy (EIS) has potential uses in various fields such as biosensors, corrosion studies, healthcare owing to its facile operation and affordable devices to conduct electroanalysis. EIS calculates the excitation voltage and current generated with the oscillating frequency. Developing impedimetric methods are gaining attention due to the operation being label-free. Considering its label-free nature, EIS is a possible candidate to explain the electrodynamics of living systems such as cell-matrix interaction, biofilm formation in vitro. Detection of those is essential to prevent infections and to develop medical needs to cure them. The thesis focuses on understanding the electrodynamics of bacterial biofilm formation via electrochemical methods such as square wave voltammetry (SWV), Open Circuit Potential (OCP), and EIS. After carrying out the experiments, time-dependent circuit models for EIS were built, and the data were extracted to demonstrate changes in the bacterial system.Master Thesis Design and Development of Paper-Based Microfluidics for Point-Of Applications(01. Izmir Institute of Technology, 2020) Özefe, Fatih; Arslan Yıldız, Ahu; Yıldız, Ümit HakanPaper-based microfluidics is a subarea of microfluidics which is recently used in various applications from diagnostics to environmental monitoring, and to food safety. In such microfluidic systems, a test platform is formed from a paper substrate instead of silicon and polymers, such as poly-dimethylsiloxane, poly-methyl methacrylate, and etc. The main goal of this thesis is the development and fabrication of a paper-based microfluidic device (μPAD), which could be used in point-of-care (POC) applications. The characterizations of μPADs, which were fabricated via laser ablation methodology, were performed in terms of their surface and barrier characteristics, and liquid sample flows within μPADs. Depending on the characterization, nine different fabrication parameters, 10P40S (10%Power & 40%Speed), 10P60S, 20P90S, 30P50S, 30P100S, 40P80S, 40P100S, 70P80S, and 70P100S, were identified as optimized fabrication parameters. Also, two designed models of μPADs, 1S4T-Type2 and 1S4T-Type3, were selected to be used in the detection of BSA and recombinant Hepatitis C Virus (HCV) protein. The BSA and HCV (1 mg/ml) in PBS solution were successfully detected via naked eye depending on the colorimetric sensing through micro-paper enzyme linked immunosorbent assay (μP-ELISA) protocol. Moreover, the limit of detection (LoD) values for HCV were determined in 1S4T-Type2 μPAD as 1.000, 0.883, and 0.796 ng/ml when the detection was performed via naked eye, smart-phone, and bright-field microscope, respectively. Also, the easily-disposable 1S4T-Type2 μPAD provided 14 times faster and 45 times cheaper detection of HCV compared to conventional ELISA techniques. Consequently, the developed 1S4T-Type2 μPAD presented low-cost, easy-to-use, and rapid detection of HCV as POC devices.Master Thesis Synthesis of Polythiophene-Polyurethane Soft Nanoparticles for Bioimaging Applications(Izmir Institute of Technology, 2020) Karabacak, Soner; Yıldız, Ümit HakanIn this study, synthesis of fluorescent polythiophene and polyurethane soft nanoparticles carrying them was carried out for bioimaging applications. Conjugated polythiophenes and polyurethane derivatives were obtained by changing their structural properties such as size, charge, and group, and were characterized by nuclear magnetic resonance, infrared spectroscopy, dynamic functional light scattering, fluorescent microscopy, methods. In the study, changing characteristics of polyurethane nanoparticle materials were investigated depending on the synthesis conditions. Synthesis of polyurethane nanoparticles was carried out by miniemulsion polymerization technique and synthesized as nanosphere to be more controllable on nanoparticle size, morphology and stability. The sizes of the polyurethane nanospheres varied in the range from 10 to 500 nm. Polyelectrolyte-polyelectrolyte complexation was investigated using cationic polythiophene-anionic polythiophene for use in bioimaging applications. Synthesized conjugated polythiophenes are divided into two groups as anionic and cationic polythiophenes. In this thesis, three kinds of cationic polythiophene synthesis are included, but poly (1,4-dimethyl-1- (3- ((4-methylthiophene-3-yl) oxy) propyl) piperazine-1-ium bromide) (PT4) was used as cationic conjugated polythiophene in the studies. Anionic polythiophene acetic acid (PTAA) was used as a counter-charged polythiophene. In addition to these studies, dual-mode imaging agents were prepared, consisting of PTAA as fluorescent agent and gadolinium metals as magnetic agent. The bioimaging studies continued under conditions imitating biological systems, and the potential of the proposed bioimaging agents was investigated.Master Thesis Modification of Gold Surface by Layer-By Reactive Coating of Polyester-Polyethyleneimine Based Gel(Izmir Institute of Technology, 2020) Yıldırımkaraman, Öykü; Yıldız, Ümit HakanPolymeric gels defined as soft and solid-like systems that enable to retain a large volume of solvent and their high molecular weight provides long-term stability without crystallization. Therefore, the use of polymeric gels in the fields of energy and sensor technologies has become advantageous. In this thesis, the polymeric gel is successfully synthesized on the gold surface by Aza-Michael addition reaction of the polyester scaffold with a triple covalent bond and branched polyethyleneimine which is a secondary amine source. The polyester-polyethyleneimine based gel was generated on the isocyanate functionalized gold surface by using the grafting-to methodology. The morphology of the surface and thickness of the coating can be adjusted by layer-by-layer reactive coating on the gold surface of polymer structures. Electroactive properties are acquired for different application areas of the synthesized gels. To provide modular electron transfer, polyethyleneimine was modified with ferrocene carboxaldehyde prior to obtaining gel on the surface. The gel interface on the gold surface will increase the surface area and activity due to its three-dimensional structure and adjustable morphology. The number of the immobilized structures, the electroactive species in a unit area and electron transfer increases. The modified electrode surfaces coating yields and electroactivity examined with electrochemical methods, Cyclic Voltammetry and Electrochemical Impedance Spectroscopy. The morphological properties investigated by Atomic Force Microscopy. Additionally, polyester-based gels lithium-ion conductivity was investigated. Dissociation of lithium perchlorate in the gel and enhancing the conductivity was investigated. Indium tin oxide coated glasses were used as an electrode to characterize lithium ion conductivity.Master Thesis Fabrication of Polymer Nanofiber / Poly (3,4 Ethylene Dioxythiophene) / Metal Particle Hybrid Composite for Volatile Organic Compound Sensing Applications(Izmir Institute of Technology, 2020) Acar, İrem; Yıldız, Ümit HakanThis study aims to produce polymer nanofiber / poly (3, 4 ethylene dioxythiophene) / metal particle hybrid composite as a bioelectronic interface for the detection of volatile organic compounds in human breath. The sensor platform consists of two layers: polymeric nanofiber structure and conductive layer. Polyurethane (PU), polycaprolactone (PCL) and poly L-lactide-co-?-caprolactone (PLLCL) were selected to form polymeric nanofibers with electrospinning. For electrospinning process, solutions of polyurethane (PU) (25wt%) in DMF, polycaprolactone (PCL) (20wt%) in DCM (4) -DMF (1) and poly L-lactide-co-?-caprolactone (PLLCL) (10wt%) in DCM (9) -DMF (1) are prepared. PU, PCL and PLLCL polymer solutions are subjected to 25 kV, 29kV and 25 kV electrical potential, respectively, to produce electrospinning fibers. Poly(3,4-ethylenedioxythiophene) (PEDOT) and multi-walled carbon nanotubes (MWCNTs) are used to produce conductive layers on PU, PCL and PLLCL polymer nanofibers. The produced sensor platforms are tested by the electrochemical station, which records the electrical current change over time. The sensing mechanism is assumed to be the adsorption of VOCs to the conductive PEDOT and CNT layer, thus blocking the electron current on the PEDOT and CNT network and causing resistance change. More clearly; swelling of the polymer structure in the sensor causes destruction in the upper layer and micro-dimensional cracks in the PEDOT and CNT network, increasing resistance to electron flow and decreasing current. Organic volatile compounds (acetone, toluene, ethanol, isopene etc.) are detected from ppm to ppb range and reproducible and reliable responses are recorded.Master Thesis A Novel Approach for Fabrication of Free-Standing Conductive Network: Pedot: Pss Based Bendable Chemo and Photoresistor(Izmir Institute of Technology, 2019) Mutlu, Mustafa Umut; Yıldız, Ümit Hakan; Demir, Mustafa MuammerElectrospinning is a simple and versatile technique for the fabrication of polymeric nanofibrous substrate with high surface to volume ratio. Besides high surface to volume ratio, their dimensional stability and flexibility make it a perfect candidate for conductive network for various sensor applications. Free-Standing conductive network can be fabricated by deposition of PEDOT:PSS or MWCNT through bendable nanofibrous substrate. As a simple example for sensor applications, the moving object has been sensed through the electrostatic interactions between fibers and object. The sensing range has been found to be 1-5 cm above the surface of fabric. By the controlled combination of conductive polymers and electrospun polymer nanofibers effective device miniaturization has been provided without loss of performance. The noncontact motion sensor platform has unique flexibility and light weight holding a potential for wearable sensor technology. For another application as a wearable electronics, the controlled combination of conductive network and light-matter interaction provides opportunities to fabricate photo-resistor exhibits broad band response 400 to 1600 nm that holding promises for ultra-thin sensors used in telecommunication. As a final example, we report the effect of gold and iron oxide nanoparticles on the selectivity and sensitivity of MWCNT or PEDOT:PSS based chemiresistor responsive to VOCs. The interplay between conductive layer by gold and iron oxide nanoparticles resulted a significant conductivity improvement that affecting selectivity which is governed by the interaction between electron-donating VOCs and NP doped conductive layer due to variation in charge carrier densities in conductive layer lattice.Master Thesis Ultra-Porous Interconnected Hydrogel Structures for Tissue Engineering Applications(Izmir Institute of Technology, 2018) Yıldız, Büşra; Yıldız, Ümit Hakan; Arslan Yıldız, AhuTissue engineering aims to repair and regenerate tissue and organs with functional defects. The most significant developments in tissue engineering emerging as modification of the scaffold used to mimic native extracellular matrix (ECM) and support cell proliferation and differentiation. Hydrogel-based biomaterials are one of the most utilized materials as scaffold providing excellent chemical, physical/biophysical properties, high biocompatibility and functionality necessary for the applications in tissue engineering. In this study, Gelatin methacryloyl hydrogel (GelMA) and Gelatin-urethane hydrogels (GelatinK) are successfully synthesized as scaffold material for tissue engineering applications. Gelatin is modified with methacrylic anhydride for GelMA polymer and with 2-isocyanatoethly methacrylate for GelatinK polymer. The hydrogels of these two novel polymer are produced with photopolymerization reactions in aqueous media using Irgacure 2959 as redox initiator. Hydrogels are freeze-dried to remove solvent in the gel matrix and then they immersed in distilled water to reach equilibrium swelling ratio. The swelling capacity of GelMA hydrogels ranges between 1200 and 300% whereas GelatinK hydrogels has swelling capacity in between 1900-380%. Also, morphology of the hydrogels were investigated with Scanning Electron Microscopy (SEM). GelMA hydrogels has pore sizes between 142-14 µm while GelatinK hydrogels has between 160-56 µm pore sizes. The cell viability assay were also conducted using GelMA and GelatinK hydrogels. The results showed that both hydrogels provide high viability as compared to 2D control assay.Master Thesis Detection of Dna Methylation of Multiple Tumor Supressor P16ink4a Gene by Polythiophene Based Optical Sensor(Izmir Institute of Technology, 2018) Kaya, Hakan; Yıldız, Ümit Hakan; Elmacı Irmak, NuranDNA methylation is epigenetic events commonly occurs in mammalian genome starting from formation of embryo to the end of life. Especially, hypermethylation in tumor suppressor genes, corresponds the cancer growth and detection of DNA methylation in these genes crucial for the diagnosis of cancer. Water soluble polythiophenes are frequently used for the detection of biomolecules through the optoelectronic properties. In this study, detection of DNA methylation of multiple tumor suppressor p16INK4A gene via polythiophene based optical sensor was achieved. Newly designed, synthesized and characterized poly(1-(3-((4-methylthiophen-3-yl) oxy) propyl)-1,4diazabicyclo [2.2.2] octan-1-ium bromide) was used during characterization of DNA sequences and detection of DNA methylation. The target sequence position is +137 to +156 in p16INK4A gene which have three potential CG dinucleotide to be methylated. Detection of DNA methylation based on sodium bisulfite treatment, complementary sequence of unmethylated ssDNA and the conformational change of water soluble polythiophene. In our fluorometric analysis, unmethylated sequence/complementary successfully hybridized and dsDNA Io/I ratio is under the 1.40 while the methylated sequence/complementary hybridization failed due to different base content and remain as ssDNA and, Io/I ratio is higher than 1,60. The novelty of work is detection mechanism is PCR and FRET free with a range of 300 ng to 700 ng sample requirement. Characterization of homopurines, homopyrimidines, methylated and unmethylated sequence with cationic polythiophenes also accomplished. PolyG (10), polyG (20) and polyA (10) yielded a no signal in UV-VIS region while the polyA (20) yielded a 100 nm red shift. Furthermore, PolyC (10), PolyC (20), PolyT (10), PolyT (20) yielded three vibrionic peaks at 505 nm, 545 nm and 595 nm with different intensities and unique isosbestic points. All 10 bases long homopyrimidine and homopurine have a unique quencher character with cationic polythiophene. Lastly, conformational change of polythiophene investigated with computational methods and heptamer used as a model.Master Thesis Green Synthesis of Metal Nanoparticles and Their Applications as Plasmonic Substrates(Izmir Institute of Technology, 2018) Elveren, Beste; Arslan Yıldız, Ahu; Yıldız, Ümit HakanGold nanoparticles (GNPs) have been widely used in diagnostic, tissue engineering, and drug delivery fields, in the last decades. Generally, reducing gold salts to zero valent gold has been accomplished by harsh chemicals and strong reducing agents, which cause toxicity and eventually limiting the bioapplications. Green synthesis is a newly developing methodology to synthesize GNPs. Especially natural products and plants extracts are commonly preferred for green synthesis based on their natural content. Biological molecule-capped GNPs, are more biofriendly and biocompatible nano-materials that can be used for varied applications.1-3 Sensor applications; varying from biosensing to environmental analysis, are an important field that GNPs were intensively utilized.4-5 Cyanide ion (CN-) has been considered as one of the main pollutants of water, because of its rapid discharge. CN- is currently being used in industry such as; polymer synthesis6, noble metal mining7, pest control8, plastics production etc., at large scale. However, there is an unmet need for CN- detection and monitoring. Colorimetric detection of CN- that utilizes GNPs has been done by several researchers.9-10 However, in all these studies reduction of GNPs were done by strong reducing agents. Green synthesis of GNPs eliminates the toxic side-products that can be harmful to both environment and human health. To overcome this problem green synthesized GNPs were used to establish the sensor platform, which can be further employed for CN- detection. Oxidation of GNPs in the presence of cyanide molecules is a direct-forward, colorimetric and optical method that requires no toxic chemicals; therefore it is a greener approach towards CN- detection in water resources.