Browsing by Author "Yildirim, A."

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Open Access
Analysis of design parameters in safety-critical computers
(IEEE Computer Society, 2018) Ahangari, H.; Atik, F.; Ozkok, Y. I.; Yildirim, A.; Ata, S. O.; Ozturk, O.
Nowadays, safety-critical computers are extensively used in many civil domains like transportation including railways, avionics, and automotive. In evaluating these safety critical systems, previous studies considered different metrics, but some of safety design parameters like failure diagnostic coverage (C) or common cause failure (CCF) ratio have not been seriously taken into account. Moreover, in some cases safety has not been compared with standard safety integrity levels (IEC-61508: SIL1-SIL4) or even have not met them. Most often, it is not very clear that which part of the system is the Achilles heel and how design can be improved to reach standard safety levels. Motivated by such design ambiguities, we aim to study the effect of various design parameters on safety in some prevalent safety configurations, namely, 1oo2 and 2oo3, where 1oo1 is also used as a reference. By employing Markov modeling, we analyzed the sensitivity of safety to important parameters including: failure rate of processor, failure diagnostic coverage, CCF ratio, test and repair rates. This study aims to provide a deeper understanding on the influence of variation in design parameters over safety. Consequently, to meet appropriate safety integrity level, instead of improving some parts of a system blindly, it will be possible to make an informed decision on more relevant parameters. IEEE
Open Access
Bio-inspired hierarchically structured polymer fibers for anisotropic non-wetting surfaces
(Royal Society of Chemistry, 2017) Yunusa, M.; Ozturk, F. E.; Yildirim, A.; Tuvshindorj, U.; Kanik, M.; Bayındır, Mehmet
We demonstrate a rice leaf-like hierarchically textured polymer fiber array for anisotropic non-wetting surfaces. To provide superhydrophobicity in addition to the anisotropic behavior, fiber surfaces are spray coated with organically modified silica nanoparticles. The resulting micro/nano hierarchically structured fiber surfaces demonstrate anisotropic non-wetting properties. We designed various fiber architectures for droplet transportation, mixing, and guiding exploiting the scalability of the fiber texture during thermal drawing; optional nanoparticle surface modification; and inherent flexibility of the fibers.
Open Access
Bioinspired optoelectronic nose with nanostructured wavelength-scalable hollow-core infrared fibers
(Wiley, 2011) Yildirim, A.; Vural, M.; Yaman, M.; Bayındır, Mehmet
A digital photonic nose concept based on infrared absorption inside a hollow core infrared transmitting fiber array is presented. Wavelength-scalable photonic band gap fibers filter specific energy photons from a blackbody source, where volatile compounds selectively absorb photons depending on their chemical absorption spectrum. The pattern resulting in the detector array is processed as a binary signature. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Cytotoxicity of multifunctional surfactant containing capped mesoporous silica nanoparticles
(Royal Society of Chemistry, 2016) Yildirim, A.; Turkaydin, M.; Garipcan, B.; Bayındır, Mehmet
This paper reports the synthesis of silica capped surfactant (cetyltrimethylammonium bromide; CTAB) and dye (Rose Bengal; RB) containing mesoporous silica nanoparticles (MSNs). Capping the pores of the surfactant containing MSNs with a thin silica layer decreased the immediate surfactant originated cytotoxicity of these particles without affecting their long term (3 days) cytotoxicity. Also, the silica capping process almost completely prevented the hemolytic activity of the surfactant containing MSNs. In addition, improved uptake of silica capped MSNs compared to the uncapped particles by cancer cells was demonstrated. The delayed cytotoxicity, low hemolytic activity, and better cellular uptake of the silica capped MSNs make them promising for the development of safe (i.e. with fewer side effects) yet efficient theranostic agents. These nanocarriers may release the loaded cytotoxic molecules (CTAB) mostly after being accumulated in the tumor site and cause so minimal damage to the normal tissues and blood components. In addition, the nanoscale confinement of RB molecules inside the pores of MSNs makes the particles brightly fluorescent. Furthermore, it was demonstrated that due to the singlet oxygen generation capability of the RB dye the silica capped MSNs can be also used for photodynamic therapy of cancer. © 2016 The Royal Society of Chemistry.
Open Access
Enhanced performance of dye-sensitized solar cells by omnidirectional antireflective coatings
(S P I E - International Society for Optical Engineering, 2015) Ulusoy, T. G.; Daglar, B.; Yildirim, A.; Ghobadi, A.; Bayındır, Mehmet; Okyay, Ali Kemal
Organically modified silica (ORMOSIL)-coated dye-sensitized solar cells (DSSCs) with improved energy conversion efficiency are demonstrated. ORMOSIL-coated DSSC surfaces exhibit omnidirectional low reflectivity over a broad range of wavelengths (400-800 nm). The short-circuit current density (JSC) is enhanced up to 23% at normal incidence (θ=0 deg) as a result of ORMOSIL coating. In addition, JSC enhancement is even higher at larger angles of incidence; 84% enhancement was observed at θ=30 deg. Moreover, ORMOSIL coating is superhydrophobic with a contact angle of 155 deg. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its.
Open Access
Flexible and mechanically stable antireflective coatings from nanoporous organically modified silica colloids
(Royal Society of Chemistry, 2012) Budunoglu, H.; Yildirim, A.; Bayındır, Mehmet
We report the preparation of flexible and mechanically stable antireflective organically modified silica (ormosil) coatings at ambient conditions. Thin films are obtained from colloidal suspensions of ormosil gels which are prepared using methyltrimethoxysilane (MTMS) and tetraethyl orthosilicate (TEOS) monomers. The ormosil suspensions are directly applicable and suitable for the large-area deposition of nanoporous ormosil thin films. The nanoporosity of the films can be tuned by changing the monomer ratio of the starting solution. Thin films on flexible substrates retain their antireflective properties even after 100 cycles of excessive bending without a significant change in transmission. Furthermore, the films remained intact after water dripping and adhesive tape tests. In addition, thin films on glass substrates are found to exhibit antifogging properties after annealing at 600 °C for 30 min. The ease of fabrication and multifunctionality of these films make them ideal coatings for flexible electronic and optoelectronic devices, sensors, and solar cells. © 2012 The Royal Society of Chemistry.
Open Access
Formation of pyrene excimers in mesoporous ormosil thin films for visual detection of nitro-explosives
(American Chemical Society, 2014-04-01) Beyazkilic, P.; Yildirim, A.; Bayındır, Mehmet
We report the preparation of mesoporous thin films with bright pyrene excimer emission and their application in visual and rapid detection of nitroaromatic explosive vapors. The fluorescent films were produced by physically encapsulating pyrene molecules in the organically modified silica (ormosil) networks which were prepared via a facile template-free sol-gel method. Formation and stability of pyrene excimer emission were investigated in both porous and nonporous ormosil thin films. Excimer emission was significantly brighter and excimer formation ability was more stable in porous films compared to nonporous films. Rapid and selective quenching was observed in the excimer emission against vapors of nitroaromatic molecules; trinitrotoluene (TNT), dinitrotoluene (DNT), and nitrobenzene (NB). Fluorescence quenching of the films can be easily observed under UV light, enabling the naked-eye detection of nitro-explosives. Furthermore, excimer emission signal can be recovered after quenching and the films can be reused at least five times.
Open Access
High selectivity boolean olfaction using hollow-core wavelength-scalable Bragg fibers
(American Chemical Society, 2012) Yaman, M.; Yildirim, A.; Kanik, M.; Cinkara, T. C.; Bayındır, Mehmet
A new odorant detection scheme, based on infrared absorption of volatile organics inside an optofluidic channel array, is discussed in terms of its selectivity. The sensor unit of the array is a hollow core Bragg fiber that selectively (spectrally) guides an incident continuum radiation. The presence of infrared absorbing molecules in the channel results in the quenching of the otherwise transmitted signal. Each fiber unit in the array is designed and fabricated so that it is sensitive to specific chemical bonds and the bond environment, but at the same time, each fiber is also broadly sensitive to a large number of chemicals due to their infrared absorbance spectra. The cumulative array response data, using an appropriate threshold, enable selective binary sampling of the infrared fingerprint of hundreds of molecules. The selectivity of the system is quantitatively investigated with computer simulations and found to be exponentially increasing with the number of fibers in the array. Relatively simple data analysis using binary logic combined with the high selectivity of the novel scheme paves the way for ubiquitous application of electronic noses in toxic gas detection, food quality control, environmental monitoring, and breath analysis for disease diagnostics. © 2011 American Chemical Society.
Open Access
Highly transparent, flexible, and thermally stable superhydrophobic ORMOSIL aerogel thin films
(American Chemical Society, 2011) Budunoglu, H.; Yildirim, A.; Güler, Mustafa O.; Bayındır, Mehmet
We report preparation of highly transparent, flexible, and thermally stable superhydrophobic organically modified silica (ORMOSIL) aerogel thin films from colloidal dispersions at ambient conditions. The prepared dispersions are suitable for large area processing with ease of coating and being directly applicable without requiring any pre- or posttreatment on a variety of surfaces including glass, wood, and plastics. ORMOSIL films exhibit and retain superhydrophobic behavior up to 500 °C and even on bent flexible substrates. The surface of the films can be converted from superhydrophobic (contact angle of 179.9°) to superhydrophilic (contact angle of <5°) by calcination at high temperatures. The wettability of the coatings can be changed by tuning the calcination temperature and duration. The prepared films also exhibit low refractive index and high porosity making them suitable as multifunctional coatings for many application fields including solar cells, flexible electronics, and lab on papers. © 2011 American Chemical Society.
Open Access
Impact of mesoporous silica nanoparticle surface functionality on hemolytic activity, thrombogenicity and non-specific protein adsorption
(Royal Society of Chemistry, 2013) Yildirim, A.; Ozgur E.; Bayındır, Mehmet
Although numerous mesoporous silica nanoparticle (MSN) drug carriers and theranostic agents with various surface functionalities have been designed in the last decade, their biocompatibility remains a matter of intensive debate. Here, we systematically evaluated interactions of a series of MSNs possessing different surface functional groups (ionic, polar, neutral, and hydrophobic) with blood constituents, in terms of their hemolytic activity, thrombogenicity, and adsorption of blood proteins on their surfaces. Using a hemolysis assay we showed that surface functionalization can reduce or even completely prevent the hemolytic activity of bare MSNs. We investigated thrombogenicity of MSNs by measuring prothrombin time (PT) and activated partial thromboplastin time (aPTT). We observed that none of the MSNs used in this study exhibit significant thrombogenic activity. Lastly, we examined non-specific protein adsorption on MSN surfaces using human serum albumin (HSA) and gamma globulins (γGs) and found that surface functionalization with ionic groups can greatly reduce protein adsorption. Demonstration of the surface functionalization having a crucial impact on blood compatibility might serve as a guideline for further investigation related to the design of mesoporous silica systems for biomedical applications, and shed light on research towards the ultimate goal of developing smart theranostic systems. © The Royal Society of Chemistry 2013.
Open Access
Nanoconfinement of pyrene in mesostructured silica nanoparticles for trace detection of TNT in the aqueous phase
(Royal Society of Chemistry, 2014) Beyazkilic, P.; Yildirim, A.; Bayındır, Mehmet
This article describes the preparation of pyrene confined mesostructured silica nanoparticles for the trace detection of trinitrotoluene (TNT) in the aqueous phase. Pyrene confined mesostructured silica nanoparticles were prepared using a facile one-pot method where pyrene molecules were first encapsulated in the hydrophobic parts of cetyltrimethylammonium micelles and then silica polymerized around these micelles. The resulting hybrid particles have sizes of around 75 nm with fairly good size distribution. Also, they are highly dispersible and colloidally stable in water. More importantly, they exhibit bright and highly stable pyrene excimer emission. We demonstrated that excimer emission of the particles exhibits a rapid, sensitive and visual quenching response against TNT. The detection limit for TNT was determined to be 12 nM. Furthermore, excimer emission of pyrene shows significantly high selectivity for TNT. This journal is © The Royal Society of Chemistry.
Open Access
Noncovalent functionalization of mesoporous silica nanoparticles with amphiphilic peptides
(Royal Society of Chemistry, 2014) Sardan, M.; Yildirim, A.; Mumcuoglu, D.; Tekinay, A. B.; Güler, Mustafa O.
The surface of mesoporous silica nanoparticles (MSNs) has been modified for enhancing their cellular uptake, cell targeting, bioimaging, and controlled drug release. For this purpose, covalent anchorage on the silica surface was predominantly exploited with a wide range of bioactive molecules. Here, we describe a facile self-assembly method to prepare a hybrid peptide silica system composed of octyl-modified mesoporous silica nanoparticles (MSNs) and peptide amphiphiles (PAs). The hydrophobic organosilane surface of mesoporous silica was coated with amphiphilic peptide molecules. The peptide functionalized particles exhibited good cyto-compatibility with vascular smooth muscle and vascular endothelial cells. The peptide coating also improved the cellular uptake of particles up to 6.3 fold, which is promising for the development of highly efficient MSN based theranostic agents. © 2014 the Partner Organisations.
Open Access
One-pot preparation of fluorinated mesoporous silica nanoparticles for liquid marble formation and superhydrophobic surfaces
(American Chemical Society, 2011) Yildirim, A.; Budunoglu, H.; Daglar, B.; Deniz, H.; Bayındır, Mehmet
One-pot synthesis of fluorinated mesoporous silica nanoparticles (FMSNs) is reported. Uniform mesoporous nanoparticles are prepared by condensation of tetraethyl orthosilicate (TEOS) and fluoroalkyl containing organotriethoxy silane monomers, respectively. The method enables selective deposition of fluorine atoms on the surface of the particles. FMSNs are used to prepare stable liquid marbles with water. An organo-modified silica sol is used with FMSNs to prepare mechanically stable superhydrophobic surfaces (water contact angle of 161 degrees). The mechanical stability of the surface is investigated with water dripping and adhesive tape tests. The prepared FMSNs are promising building blocks for robust, large-area, and multifunctional self-cleaning surfaces.
Open Access
Photonic bandgap narrowing in conical hollow core Bragg fibers
(AIP Publishing, 2014) Ozturk, F. E.; Yildirim, A.; Kanik, M.; Bayındır, Mehmet
We report the photonic bandgap engineering of Bragg fibers by controlling the thickness profile of the fiber during the thermal drawing. Conical hollow core Bragg fibers were produced by thermal drawing under a rapidly alternating load, which was applied by introducing steep changes to the fiber drawing speed. In conventional cylindrical Bragg fibers, light is guided by omnidirectional reflections from interior dielectric mirrors with a single quarter wave stack period. In conical fibers, the diameter reduction introduced a gradient of the quarter wave stack period along the length of the fiber. Therefore, the light guided within the fiber encountered slightly smaller dielectric layer thicknesses at each reflection, resulting in a progressive blueshift of the reflectance spectrum. As the reflectance spectrum shifts, longer wavelengths of the initial bandgap cease to be omnidirectionally reflected and exit through the cladding, which narrows the photonic bandgap. A narrow transmission bandwidth is particularly desirable in hollow waveguide mid-infrared sensing schemes, where broadband light is coupled to the fiber and the analyte vapor is introduced into the hollow core to measure infrared absorption. We carried out sensing simulations using the absorption spectrum of isopropyl alcohol vapor to demonstrate the importance of narrow bandgap fibers in chemical sensing applications.
Open Access
Pluronic polymer capped biocompatible mesoporous silica nanocarriers
(Royal Society of Chemistry, 2013) Yildirim, A.; Demirel, G. B.; Erdem, R.; Senturk, B.; Tekinay, T.; Bayındır, Mehmet
A facile self-assembly method is described to prepare PEGylated silica nanocarriers using hydrophobic mesoporous silica nanoparticles and a pluronic F127 polymer. Pluronic capped nanocarriers revealed excellent dispersibility in biological media with cyto- and blood compatibilities. © 2013 The Royal Society of Chemistry.
Open Access
A porosity difference based selective dissolution strategy to prepare shape-tailored hollow mesoporous silica nanoparticles
(Royal Society of Chemistry, 2015) Yildirim, A.; Bayındır, Mehmet
This article reports a general method to prepare hollow mesoporous silica nanoparticles with tailored morphology. The method is based on selective dissolution of porous cores of solid silica shell/porous silica core nanoparticles under mild conditions without the need for corrosive or toxic etchants. First, core-shell nanospheres or nanorods are prepared in a one-pot reaction. Then, mesoporous cores of the nanoparticles are selectively dissolved by incubating them in phosphate buffered saline (PBS) at 65 °C for one day. Surprisingly, shells of the resulting hollow particles contain both small and large mesopores which makes the particles very suitable for adsorption and desorption of a wide range of molecules. In addition, we proposed a mechanism for selective dissolution of porous cores of the core-shell nanoparticles.
Open Access
Region covariance descriptors calculated over the salient points for target tracking
(IEEE, 2012) Çakir, S.; Aytaç, T.; Yildirim, A.; Beheshti, S.; Gerek Ö.N.; Çetin, A. Enis
Features extracted at salient points in the image are used to construct region covariance descriptor (RCD) for target tracking purposes. In the classical approach, the RCD is computed by using the features at each pixel location and thus, increases the computational cost in the scenarios where large targets are tracked. The approach in which the features at each pixel location are used, is redundant in cases where image statistics do not change significantly between neighboring pixels. Furthermore, this may decrease the tracking accuracy while tracking large targets which have background dominating structures. In the proposed approach, the salient points are extracted via the Shi and Tomasi's minimum eigenvalue method and a descriptor based target tracking structure is constructed based on the features extracted only at these salient points. Experimental results indicate that the proposed method provides comparable and in some cases even better tracking results compared to the classical method while providing a computationally more efficient structure. © 2012 IEEE.
Open Access
Robust cassie state of wetting in transparent superhydrophobic coatings
(American Chemical Society, 2014) Tuvshindorj, U.; Yildirim, A.; Ozturk, F. E.; Bayındır, Mehmet
This paper investigates the stability of the Cassie state of wetting in transparent superhydrophobic coatings by comparing a single-layer microporous coating with a double-layer micro/nanoporous coating. Increasing pressure resistance of superhydrophobic coatings is of interest for practical use because high external pressures may be exerted on surfaces during operation. The Cassie state stability against the external pressure of coatings was investigated by squeezing droplets sitting on surfaces with a hydrophobic plate. Droplets on the single-layer coating transformed to the Wenzel state and pinned to the surface after squeezing, whereas droplets on the double-layer micro/nanoporous coating preserved the Cassie state and rolled off the surface easily. In addition, the contact angle and contact-line diameter of water droplets during evaporation from surfaces were in situ investigated to further understand the stability of coatings against Wenzel transition. A droplet on a microporous coating gradually transformed to the Wenzel state and lost its spherical shape as the droplet volume decreased (i.e., the internal pressure of the droplet increased). The contact line of the droplet during evaporation remained almost unchanged. In contrast, a water droplet on a double-layer surface preserved its spherical shape even at the last stages of the evaporation process, where pressure differences as high as a few thousand pascals were generated. For this case, the droplet contact line retracted during evaporation and the droplet recovered the initial water contact angle. The demonstrated method for the preparation of robust transparent superhydrophobic coatings is promising for outdoor applications such as self-cleaning cover glasses for solar cells and nonwetting windows.
Open Access
Robust superhydrophilic patterning of superhydrophobic ormosil surfaces for high-throughput on-chip screening applications
(Royal Society of Chemistry, 2016) Beyazkilic, P.; Tuvshindorj, U.; Yildirim, A.; Elbuken, C.; Bayındır, Mehmet
This article describes a facile method for the preparation of two-dimensionally patterned superhydrophobic hybrid coatings with controlled wettability. Superhydrophobic coatings were deposited from nanostructured organically modified silica (ormosil) colloids that were synthesized via a simple sol-gel method. On the defined areas of the superhydrophobic ormosil coatings, stable wetted micropatterns were produced using Ultraviolet/Ozone (UV/O) treatment which modifies the surface chemistry from hydrophobic to hydrophilic without changing the surface morphology. The degree of wettability can be precisely controlled depending on the UV/O exposure duration; extremely wetted spots with water contact angle (WCA) of nearly 0° can be obtained. Furthermore, we demonstrated high-throughput biomolecular adsorption and mixing using the superhydrophilic patterns. The proposed superhydrophilic-patterned nanostructured ormosil surfaces with their simple preparation, robust and controlled wettability as well as adaptability on flexible substrates, hold great potential for biomedical and chemical on-chip analysis.
Open Access
Salient point region covariance descriptor for target tracking
(SPIE, 2013-02-22) Cakir, S.; Aytac, T.; Yildirim, A.; Behesti, S.; Gerek, O. N.; Çetin, A. Enis
Features extracted at salient points are used to construct a region covariance descriptor (RCD) for target tracking. In the classical approach, the RCD is computed by using the features at each pixel location, which increases the computational cost in many cases. This approach is redundant because image statistics do not change significantly between neighboring image pixels. Furthermore, this redundancy may decrease tracking accuracy while tracking large targets because statistics of flat regions dominate region covariance matrix. In the proposed approach, salient points are extracted via the Shi and Tomasi’s minimum eigenvalue method over a Hessian matrix, and the RCD features extracted only at these salient points are used in target tracking. Experimental results indicate that the salient point RCD scheme provides comparable and even better tracking results compared to a classical RCD-based approach, scale-invariant feature transform, and speeded-up robust features-based trackers while providing a computationally more efficient structure.