Synthesis and characterization of undoped and ZnO nanoparticles doped NIPAM nanogels

Polymer based materials have drawn attention in 20th century and after that their entrance to all aspects of daily life happened very rapidly. Through diversity of polymers in nature, by observing the ones that they can be used were identified and polymers got indispensably involved in life in a wide spectrum from industrial to daily indoor use. Due to their specifications such as programmability and usefulness with various dopands helped then to find a place in progressing technology. That is why they became critical in areas such as pharmacy, bio medicine, molecular biology and biophysics and interdisciplinary researches are increasing on synthetic and natural polymer structures. Especially polymer forms such as gels which can display specifications that encountered often in living structures became very popular. An important type of gels, which are composed of cross linked polymers, are nanogels. Some important aspects of it, causing so much attention, are easiness of synthesis, controllability by external factors and compatibility and low toxicity in human body. Nanogels, which have the property of swelling. Among the external factors that can be used to stimulate behaviors of nanogels can be sorted such as pH, redox potential, enzymes and light. Those factors can be very useful in a differentiating chemical environment like human body. Owe to the these specifications nanogels can be used as molecular sensor and drug carriers. NIPAM nanogels are known for their thermosensitive structure which can perform swelling behavior under its lower critical solution temperature (LCTS). LCTS of NIPAM nanogels are 32C which is near to the human body temperature. These gels also sensitive to the temperature changes over 40C and shrivels up by expelling the solvent which contains. These properties are the result of dominant hydrophobic interactions of the gels over LCTS. NIPAM nanogels can be prepared by different methods such as hetero-phase polymerization, radiation polymerization, micro-emulsion polymerization and precipitation polymerization. In these methods by varying cross-linker and mixing some interactive molecules to the solution helps to set gels sizes which is useful in different applications. During the synthesis of nanogels, understanding the gelation process is important to improve efficiency and modelling the process. During polymerization, the model that polymer clusters follows was thought to be as special lattice model that called Cayley tree. But recent works shows that polymers follows new model that called percolation theory. According to that during polymerization forming clusters shows fractal behavior. This behavior can be observed by measuring non-integer fractal dimensions during the process. That dimension change gives the observer to determine critical point of the process that sol-gel phase transition occurs. Different specifications can be assigned to nanogels by embedding various elements. Nanoparticles that are embedded to nanogels can contribute by their specifications such as drug carrier chaperons. Proteins can hold their structural integrity by attaching nanoparticles. By using these two structure together, more efficient drug carrier systems can be produced. These hybrid structures not only helps to carry a drug to the desired environment but also helps to maintaining structural integrity of nanoparticles. In order to construct useful hybrid structures some metals and compounds can be used to achieve different purposes. One of the most studied and useful nanoparticle in this area is zinc oxide. Zinc oxide is a semiconductor compound and is used for its optical, chemical, semiconducting, piezoelectric and thermal stability properties. Especially wide spectral optical and high chemical stability properties is useful in mentioned hybrid structures. In the first part of this thesis synthesis of N-isopropylacrylamide nanogels produced by following the method used by Koc and Alveroglu [1]. N,N-methylenebisacrylamide (BIS) and ammonium persulfate (APS) are used as cross-linker and initiator molecules respectively. Also pyranine molecule added to the structure in order to set nanogel sizes. Changing amount of pyranine in solution, different sized nanogels obtained and reduction of nanogel size observed by increasing pyranine amount . Characterization of the gels during gelation process is conducted. In order to trace the structural changes while polymerization and gel formation is going on, pyranine, which is a fluorescent molecule, is served for this purpose in NIPAM solution. By addition of that molecule, changes through gelation process is observed by spectrophotometry without any interference to the system. At the beginning of the polymerization maximum peaks were observed at 510 nm, which is originated from pyranine existence. As polymerization continued the movement of the peak points towards 410 nm were observed due to the bounding of Py to polymer chains. Obtained results gives information about critical point during phase transitions and fractal structures which is an important aspect in order to characterize the model that polymerization process follows. After the polymerization and fluorescence measurement obtained samples washed by centrifuge and prepared for further investigation. Fluorescence spectrometry results and SEM imaging methods is used to characterize obtained nanogels at the end of this part. As obtained from SEM and AFM images, nanogels sizes were measured approximately between 140-550 nm depending on different pyranine molarity. Also by using FL measurements depending on slit width of excitation window used to calculate fractal dimensions during polymerization. Graph of fractal dimension vs. polymerization time shows that at the begining of the process fractal dimension is calculated approximately 3, then decreases with time and after a threshold point dimension is fixed at 3. Observed threshold value occurs around 2.2 and changes depending on gel size. In addition in latter part of the thesis, in-situ synthesis and characterization of ZnO nanoparticle embedded nanogels are investigated. ZnO nanoparticles can be used as drug carriers. So synthesis of structure which ZnO nanoparticles are coated with nanogels is produced. ZnO formation in the nanogels are observed by spectroscopic methods. To synthesize nanoparticles in nanogels, same process was followed during the nanogels synthesis. In addition to process zinc acetate dihydrate is added to the solution instead of pyranine, before polymerization begins. Same polymerization steps followed and fluorescence spectrum is observed. At the end of the polymerization excess unreacted polymers washed away by centrifuging. After that nanogels structures that include zinc is held in 2M NaOH solution for two days in order to form ZnO nanoparticles inside the nanogels. As observed from florescence spectrum, FL emission of ZnO is not powerful due to the bonding by nanogels. For that reason zinc acetate amount is doubled during the experiment. Obtained structures is investigated by fluorescence spectrometry, SEM, XRD and EDS. Fluorescence measurements showed certain changes in emission band of ZnO nanoparticles as found in literature. Also XRD spectrum of dried samples shows ZnO nanoparticle crystal pattern along with amorphous nanogel structure. XRD values indicate the presence of ZnO in the samples. Also average nanoparticle size is calculated according to broadening of XRD peaks by using Scherrer equation. Average particle size was found approximately 14 nm. In addition EDS measurements show Zn and O presence in samples prepared for SEM measurements. EDS images also shows homogeneous distribution of ZnO particles in the samples. Changing gel size by using pyranine molecule is found to be a very easy and quick method to tune the gel size. Also addition of nanoparticle synthesis to the gels also possible by the same synthesis method. These nanogels can be used for biomedical purposes such as drug delivery.