Preparation of functional surfaces using zeolite nanocrystals for biosensor and biomedical applications

Kirdeciler, Salih Kaan
Zeolites are crystalline aluminosilicates which have highly ordered pore structures and high surface area. Also the tailorable surface properties, high ion-exchange capability, high chemical, thermal, and mechanical strength make these particles an important candidate for various application such as sensors, catalysis, dielectric materials, separation, and membrane technologies. Although zeolites have these unique properties, applications where zeolites are integrated into devices according to their application areas, are limited due to the powder form of the material. The purpose of the current study was to investigate the effect of zeolite nanoparticles on conductometric biosensor performance and cell viability measurements. Firstly, zeolite attachment on silicon surfaces was investigated by attaching silicalite and zeolite A nanoparticles onto the silicon substrates by direct attachment methodology in a closely packed monolayer form with perfect orientation and full coverage without using any chemical linker. Furthermore, the ability to pattern these zeolite crystals on silicon substrates with electron beam lithography and photolithography techniques was investigated. With the combination of electron beam lithography and direct attachment methodology, zeolite patterns were produced with feature sizes as small as a single silicalite nanoparticle thick line, that is approximately 500 nm. This approach has the ability of patterning very small features on silicon substrate, but the drawback is the long patterning time and lack of electron beam stability during long pattern formation process. Accordingly, it is almost impossible to form large patterns with electron beam lithography systems. Afterwards, to have full control on surfaces with differentiated areas on solid substrates, patterns of one type of zeolite crystals was formed on the monolayer of another type of zeolite layer with electron beam lithography for the first time. The same closed packed and highly oriented silicalite patterns were successfully formed on zeolite A monolayers and vice versa. Then photolithography technique was combined with direct attachment methodology to overcome the problem of the lack of total patterned area. With this technique, it was possible to pattern the whole silicon wafer in a couple of seconds, however the feature size of the zeolite patterns was limited with the infrastructures of the mask fabricated for photolithography studies. In this particular study, zeolite lines patterns with a minimum of 5 µm thickness were prepared and the total patterned area was kept constant at 1 cm2. Similar to what was obtained by electron beam lithography study, zeolite A patterns were formed on silicalite monolayers with the minimum feature size of 5 µm and vice versa. In the second part of the study, zeolite films were prepared on the transducers of conductometric biosensors using dip coating technique and named as Zeolite Coated Transducers (ZCT). Electrodes prepared using a mixture of zeolite and enzyme solution and then subjected to casting using glutaraldehyde were called Zeolite Membrane Transducers (ZMT). The operational and storage stabilities were determined to be in an acceptable range using ZCTs for conductometric urea biosensors. It was observed that using electrodes fabricated by the ZCT technique enhanced the biosensor signals up to two times and showed a rapid response after the addition of urea to the medium when it was compared with Standard Membrane Transducers (SMT). This enhancement can be explained by the lack of GA layer on top of the film, which acts as a diffusion barrier and inhibits the activity of the enzyme. On the second part of this conductometric biosensor study, effect of zeolite modification with methyl viologen (MV) and silver nanoparticles (Ag+ and Ag0), as well as the effect of changing Si/Al ratio was investigated with three different zeolite Beta particles which have Si/Al ratios of 40, 50, and 60. There were no significant effect of MV modification on ZMTs and there was no response observed with Ag+ and Ag0 modified zeolites. However, it was observed that conductometric responses increased with increasing Si/Al ratio for ZMTs. This behavior can be due to an increased hydrophobicity and/or the increasing acidic strength with the increasing Si/Al ratio within the zeolite crystals. Also ZCTs showed higher responses with respect to both SMTs and ZMTs. When compared with SMTs and ZMTs, ZCTs had higher reproducibility due to the controlled thickness of zeolite thin film by dip coating, and the controlled amount of enzyme adsorbed on this film. In the third part of the study, effect of zeolites on cell proliferation with MG63 osteoblast cells and NIH3T3 fibroblast cells were investigated. For that purpose, zeolite A, silicalite, and calcined forms of these zeolites were patterned with photolithography technique onto silicon wafers. Three different patterns prepared for this particular study, which has 0.125cm2, 0.08825cm2, and 0.04167cm2 zeolite patterned areas on 1 cm2 samples. In that way, not only the zeolite type and effect of calcination of zeolites, but also the effect of zeolite amount on MG63 osteoblast cells and NIH3T3 fibroblast cells were investigated. Silicalite coated samples were observed to have higher amount of cells than zeolite A coated samples after 24, 48, and 72 hours of incubation. This may be referred to the hydrophilic/hydrophobic properties, surface charge, and/or particle size of zeolites. Also it is observed that higher zeolite amount on samples resulted in an increase in the number of cells attached to the samples. There was also a significant increase in the number of cells upon using calcined silicalite samples. Accordingly, it can be hypothesized that zeolite pores result in an enhancement of protein adsorption and proliferation, even if this only occurs at the pore openings. On the other hand, there was no positive effect of calcining zeolite A. This result was expected since there is no structure directing agent used in synthesis procedure of zeolite A, which again supports the fact that pores might have some role in cell attachment.


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Citation Formats
S. K. Kirdeciler, “Preparation of functional surfaces using zeolite nanocrystals for biosensor and biomedical applications,” M.S. - Master of Science, Middle East Technical University, 2012.