Researcher: Karahan, Hüseyin Enis
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Karahan, Hüseyin Enis
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Publication Metadata only pH-stability and pH-annealing of H-bonded multilayer films prepared by layer-by-layer spin-assembly(Elsevier, 2014) N/A; Department of Chemical and Biological Engineering; Department of Chemical and Biological Engineering; Department of Chemistry; Karahan, Hüseyin Enis; Eyüboğlu, Lütfiye; Kıyılar, Deniz; Demirel, Adem Levent; PhD Student; Undergraduate Student; Undergraduate Student; Faculty Member; Department of Chemical and Biological Engineering; Department of Chemistry; Graduate School of Sciences and Engineering; College of Engineering; College of Engineering; College of Science; N/A; N/A; N/A; 6568Spin-assisted assembly and pH-induced disintegration of H-bonded multilayers of poly(N-vinylpyrrolidone) and tannic acid (PVPON/TA) have been investigated. Multilayers showed significantly different pH-induced disintegration profiles depending on how long the multilayers were kept in the dry state between exposures to buffer solutions indicating relaxations of the molecules. To understand the effect of kinetic trapping and the consequent relaxations towards equilibrium on the pH-stability of H-bonded PVPON/TA spin-LbL multilayers, spin-coated multilayers were prepared at different nonequilibrium states by varying the spinning rate and the solution-film static contact time. The pH-stability of multilayers increased with increasing spinning rate and decreasing static contact time. The gradual decrease of bilayer thickness with increasing spinning rate indicated the flattening of the molecules. The enhancement of the pH-stability of multilayers was attributed to both the flattening and the relaxations of molecules having nonequilibrium conformations. A transition from linear to exponential growth profiles was observed with increasing static contact times indicating in and out diffusion in PVPON/TA system. pH-treatment of multilayers just below the critical disintegration pH, a process we call pH-annealing, enhanced their pH-stability significantly. Multilayers annealed longer at pH 7.0 showed better pH-stability (up to 1 h) at pH 9.0 whereas nonannealed films disintegrated within 10 min. The introduced pH-annealing process is promising for practical applications of H-bonded systems such as controlled drug delivery. (C) 2014 Elsevier Ltd. All rights reserved.Publication Metadata only Simultaneous DLS-SLS study of titanium and titanium/silicon oxide sol growth(Springer, 2017) N/A; N/A; N/A; N/A; Karahan, Hüseyin Enis; Karakuş, Kerem; Birer, Özgür; PhD Student; PhD Student; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; N/A; N/A; N/AN/APublication Metadata only Simultaneous DLS-SLS study of titanium and titanium/silicon oxide sol growth(Springer, 2015) N/A; N/A; N/A; N/A; Karahan, Hüseyin Enis; Karakuş, Kerem; Birer, Özgür; PhD Student; PhD Student; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; N/A; N/A; N/AA commercial DLS setup was used for simultaneous DLS/SLS analysis of sol growth of titanium and titanium/silicon oxides. The scattering data were analyzed in dynamic and static modes which allowed evaluating particle size and concentration simultaneously. A binary solvent (acetone/ethanol mixture) was introduced which effectively controls monodisperse growth behavior by simply adjusting its ratio. Fixing the solvent composition to the ratio which delayed gelation the most, the effect of the amount of catalyst (acetic acid), hydrolyzing agent (water) and titanium oxide precursor (titanium tetraisopropoxide) on growth kinetics were studied. Taking the advantage of extra functionalities of the catalyst used, acetic acid, i.e., decreasing the reactivity of titanium tetraisopropoxide and increasing the reactivity of tetraethyl orthosilicate, hybrid titanium/silicon oxide growth was also studied. Here, we step-by-step showed that particle size, particle concentration and sol-to-gel transition time of titanium and titanium/silicon oxide systems can be well controlled by adjusting the composition of formulations in ambient conditions. We also showed how practical the laser light scattering is to evaluate even the early onsets of growth profiles long before visual identification of clouding. The findings reported here are particularly important for practical applications of sol-gel technology where the control of particle size/concentration and gelation time is advantageous.Publication Metadata only Shadow-casted ultrathin surface coatings of titanium and titanium/silicon oxide sol particles via ultrasound-assisted deposition(Elsevier, 2016) N/A; N/A; N/A; N/A; N/A; Karahan, Hüseyin Enis; Birer, Özgür; Karakuş, Kerem; Yıldırım, Cansu; PhD Student; Researcher; PhD Student; Master Student; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; N/A; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; N/A; N/A; N/AUltrasound-assisted deposition (USAD) of sol nanoparticles enables the formation of uniform and inherently stable thin films. However, the technique still suffers in coating hard substrates and the use of fast-reacting sol-gel precursors still remains challenging. Here, we report on the deposition of ultra thin titanium and titanium/silicon hybrid oxide coatings using hydroxylated silicon wafers as a model hard substrate. We use acetic acid as the catalyst which also suppresses the reactivity of titanium tetraisopropoxide while increasing the reactivity of tetraethyl orthosilicate through chemical modifications. Taking the advantage of this peculiar behavior, we successfully prepared titanium and titanium/silicon hybrid oxide coatings by USAD. Varying the amount of acetic acid in the reaction media, we managed to modulate thickness and surface roughness of the coatings in nanoscale. Field-emission scanning electron microscopy and atomic force microscopy studies showed the formation of conformal coatings having nanoroughness. Quantitative chemical state maps obtained by x-ray photoelectron spectroscopy (XPS) suggested the formation of ultrathin (<10 nm) coatings and thickness measurements by rotating analyzer ellipsometry supported this observation. For the first time, XPS chemical maps revealed the transport effect of ultrasonic waves since coatings were directly cast on rectangular substrates as circular shadows of the horn with clear thickness gradient from the center to the edges. In addition to the progress made in coating hard substrates, employing fast-reacting precursors and achieving hybrid coatings; this report provides the first visual evidence on previously suggested "acceleration and smashing" mechanism as the main driving force of USAD.Publication Open Access Hydrogen-bonded multilayers of micelles of a dually responsive dicationic block copolymer(Royal Society of Chemistry (RSC), 2012) Tuncer, Cansel; Bütün, Vural; Department of Chemistry; Erel-Göktepe, İrem; Karahan, Hüseyin Enis; Demirel, Adem Levent; PhD Student; Faculty Member; Department of Chemistry; College of Sciences; N/A; N/A; 6568We report the fabrication of hydrogen-bonded multilayers of micelles of a dually responsive, dicationic block copolymer, poly[2-(N-morpholino)ethyl methacrylate-block-2-(diisopropylamino)ethyl methacrylate] (PMEMA-b-PDPA). By taking advantage of the difference in the hydrophilicity of PMEMA and PDPA blocks, micelles with a PMEMA-corona and a PDPA-core were obtained above pH 6.5 and were assembled layer-by-layer at the surface with tannic acid (TA) at pH 7.4 through hydrogen bonding interactions between morpholino units of PMEMA and hydroxyl groups of TA. Destruction of PMEMA-b-PDPA micelles/TA films could be controlled at both acidic and basic conditions. At basic pH (pH = 8.75), multilayers disintegrated due to ionization of TA and disruption of hydrogen bonding interactions between layers of micelles and TA. At moderately acidic pH values, partially dissolved PMEMA-b-PDPA micelles and monomers underwent a restructuring with TA molecules and remained adsorbed at the surface. Complete dissolution of the multilayers occurred at around pH 3.6 due to further protonation of the tertiary amino groups on both blocks of PMEMA-b-PDPA, resulting in a charge imbalance between PMEMA-b-PDPA and TA layers followed by disintegration of the films. We have also encapsulated pyrene in the micellar cores and found that pyrene released from PMEMA-b-PDPA micelles/TA films increased 1.5- and 2.5-fold when the pH was decreased from 7.5 to 6 and 5, respectively. Such an increase in the amount of pyrene released was due to pH-controlled dissolution of the micellar cores. We have also found that at pH 7.5, increasing the temperature to 40 degrees C enhanced the release of pyrene by approximately 2-fold. Such an increase is due to lower critical solution temperature (LCST) behaviour of coronal PMEMA chains leading to temperature-induced conformational changes on the coronal chains, facilitating the release of pyrene through the coronal chains into the solution. Hydrogen bonded multilayers of micelles of a dicationic block copolymer are interesting due to the response of both multilayers and micellar cores at different pH paving the way for multiple pH-controlled delivery of functional molecules from surfaces.