Burcu Akata

A novel urea conductometric biosensor based on zeolite immobilized urease

A new approach was developed for urea determination where a thin film of silicalite and zeolite Beta deposited onto gold electrodes of a conductometric biosensor was used to immobilize the enzyme. Biosensor responses, operational and storage stabilities were compared with results obtained from the standard membrane methods for the same measurements. For this purpose, different surface modification techniques, which are simply named as Zeolite Membrane Transducers (ZMTs) and Zeolite Coated Transducers (ZCTs) were compared with Standard Membrane Transducers (SMTs). Silicalite and zeolite Beta with Si/Al ratios 40, 50 and 60 were used to modify the conductometric electrodes and to study the biosensor responses as a function of changing zeolitic parameters. During the measurements using ZCT electrodes, there was no need for any cross-linker to immobilize urease, which allowed the direct evaluation of the effect of changing Si/Al ratio for the same type of zeolite on the biosensor responses for the first time. It was seen that silicalite and zeolite Beta added electrodes in all cases lead to increased responses with respect to SMTs. The responses obtained from ZCTs were always higher than ZMTs as well. The responses obtained from zeolite Beta modified ZMTs and ZCTs increased as a function of increasing Si/Al ratio, which might be due to the increased hydrophobicity and/or the acid strength of the medium.

Application of enzyme/zeolite sensor for urea analysis in serum.

Urea biosensor based on zeolite-adsorbed urease was applied for analysis of blood serum samples. It should be noted, that this biosensor has a number of advantages, such as simple and fast performance, the absence of toxic compounds during biosensor preparation, high reproducibility and repeatability (RSD=9% and 4%, respectively). The linear range of urea determination by using the biosensor was 0.003-0.75 mM, and the limit of urea detection was 3 μM. The method of standard addition was used for analysis of serum samples with 500-fold dilution. Total time of analysis was 10 min. Good reproducibility of urea determination in real samples was demonstrated (RSD=10%). Biosensor results were verified by using a common method of urea determination (diacetyl monoxime reaction). It was shown that by using this biosensor distinguishing healthy people from people with renal dysfunction becomes easier.

Study of zeolite influence on analytical characteristics of urea biosensor based on ion-selective field-effect transistors

A possibility of the creation of potentiometric biosensor by adsorption of enzyme urease on zeolite was investigated. Several variants of zeolites (nano beta, calcinated nano beta, silicalite, and nano L) were chosen for experiments. The surface of pH-sensitive field-effect transistors was modified with particles of zeolites, and then the enzyme was adsorbed. As a control, we used the method of enzyme immobilization in glutaraldehyde vapour (without zeolites). It was shown that all used zeolites can serve as adsorbents (with different effectiveness). The biosensors obtained by urease adsorption on zeolites were characterized by good analytical parameters (signal reproducibility, linear range, detection limit and the minimal drift factor of a baseline). In this work, it was shown that modification of the surface of pH-sensitive field-effect transistors with zeolites can improve some characteristics of biosensors.

Effect of different modifications of BEA-zeolites on operational characteristics of conductometric biosensor

Effect of different modifications of zeolite Na(+)-BEA on working characteristics of urease-based conductometric biosensor was studied. As the biosensor sensitive elements were used bioselective membranes based on urease and various zeolites immobilised with bovine serum albumin on the surface of conductometric transducers. Influence of zeolites on sensitivity of urea biosensor was investigated as well as reproducibility of biosensor signal and reproducibility of activity of the bioselective element after different variants of urease immobilisation on the surface of conductometric transducer. The biosensors based on zeolites (NH4(+)-BEA 30 and H(+)-BEA 30) were shown to be the most sensitive. Concentration of these zeolites in the bioselective membrane was optimized. Use of zeolites modified with methyl viologen and silver was ascertained to be of no prospect for urea conductometric biosensors. It was demonstrated that characteristics of urea biosensors can be regulated, varying zeolites modifications and their concentrations in bioselective membranes.

Zeolite crystal growth on the international space station

Abstract Zeolites L, Beta, and ZK-5 were grown in the low gravity environment of the International Space Station from precursor solutions held unmixed until activation on orbit. Resultant particle sizes of Beta and ZK-5 were larger and crystals of L smaller on average for the flight-grown zeolites than for their earth-grown controls. Different purity of the flight and terrestrial/control zeolite L samples, which depended on the degree of mixing, is hypothesized to be due to the potentially different behavior of the poorly mixed synthesis mixtures under low gravity conditions (less convective mixing). Differences in mixing characteristics of the viscous zeolite ZK-5 synthesis mixture in the low and the normal gravity environment were inferred from the fact that several flight autoclaves were only partially activated. The flight and terrestrial/control zeolite Beta products were pure zeolite Beta, had identical morphology, surface and framework Si/Al ratio, and unit cell dimensions, but the less intense terminal silanol IR band acquired for the flight particles confirms their larger average size, and may also indicate their lower roughness. The flight and terrestrial/control zeolite ZK-5 had the same intergrown/twinned cubes morphology, however the flight particles were larger on average and composed of individual crystallites that appeared larger than these forming terrestrial controls. The flight and terrestrial/control products had the same surface Si/Al ratio and unit cell dimension, however, a less intense isolated internal silanol IR band after heat treatment for the flight ZK-5 indicates a lower concentration of structural defects and suggests a higher thermal stability of the flight framework.

The catalytic activity of space versus terrestrial synthesized zeolite Beta catalysts in the Meerwein Ponndorf Verley Reactions: Support for PFAL as the Lewis active site for cis-alcohol selectivity

The Lewis activity of the Meerwein-Ponndorf-Verley (MPV) reactions is hypothesized to be due to partial framework aluminum (PFAl) that is either octahedrally or tri-coordinated. Crystals grown in the free-fall environment of low earth orbit (LEO) are more uniform; that is, have fewer lattice “defects” compared to those grown in a gravity field (i.e., on earth). Therefore, crystals grown in orbit should be less catalytically active relative to their earth grown counterparts. The catalytic activity towards the MPV reaction, and the associated IR and XPS spectrum for zeolite Beta that was synthesized on earth (1g) and aboard the International Space Station (10−3–10−5g) were compared in their as-synthesized forms, and after applying heat treatment protocols designed to stress the crystal structure to generate Lewis acid sites (i.e., tri and octahedrally coordinated PFAl). The activity of the MPV reaction and cis-alcohol selectivity over the heat-treated flight samples was observed to be lower than the identically heat-treated terrestrial zeolite Beta samples. Higher MPV activity as well as cis-alcohol selectivity is related to both a higher number of partial framework Al atoms (PFAl), and a constrained pore volume. As PFAl are created by the destruction of the framework upon heat treatment, flight samples were shown to be thermally more stable with fewer lattice defects and less associated stress in zeolite Beta crystals. The changes observed in the IR spectra, as well as the XPS Al Auger and 2p peaks, of the terrestrial samples support this conclusion. Additionally, the flight samples showed higher tr-alcohol selectivity, which implies more pore volume and less channel blockage. This is consistent with the fact that crystals grown in space have less stress, fewer lattice defects, and thus there are fewer channel obstructions.

Controlled Assemble and Microfabrication of Zeolite Nanoparticles on SiO2 Substrates for Potential Biosensor Applications

Zeolite nanoparticles were organized into functional entities on SiO 2 substrates and microfabrication technique was tested to form patterns of zeolite nanoparticles on SiO 2 using the electron beam lithography (EBL). The effect of different techniques for efficient zeolite assembly on the SiO 2 substrates was investigated. For this purpose, three different assembly techniques were tested. The first two methods are spin-coating (SC) and ultrasound aided strong agitation (US) methods, which were tested using bare and silanized zeolite nanoparticles. The third technique is the manual assembly method, which was also investigated using bare zeolites. All methods were facile in terms of experimental approach. Full coverage of the substrate was obtained after all three methods, however strong agitation (US) leads to better organization of zeolite nanoparticles. Among all techniques, manual assembly method lead to the most organized zeolite nanoparticles with full coverage. Although strong agitation (US) also results in organized zeolite entities, it was not found to be a suitable technique for EBL studies. Using the manual assembly method, it was possible to form monolayers of zeolite nanoparticles on SiO 2 and to make patterns of zeolite nanoparticles by EBL, which offers a simple technique to engineer the surfaces for immobilization of biomolecules.