Nitric Oxide Sensors obtained through the entrapment of iron complexes in sol-gel matrix
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Nitric Oxide Sensors obtained through the entrapment of iron complexes in sol-gel matrix Juliana C. Biazzotto, João F. Borin, Roberto Mendonça Faria1 and Carlos F.O Graeff Departamento de Física e Matemática-FFCLRP-USP, Av. Bandeirantes 3900, 14040-901 Ribeirão Preto, Brazil 1-Instituto de Física de São Carlos-USP, C.P. 369, 13560-970 São Carlos, Brazil ABSTRACT Iron(III)-diethyldithiocarbamate (Fe3DETC) or iron(III)-tetra-pentafluorophenyl porphyrin (FeTFPP) was entrapped within a silica matrix by the sol-gel process. The obtained sol-gel materials SGFeDETC and SGFeTFPP were investigated as sensors for nitric oxide (NO). UV/Vis spectra of the SGFeTFPP present a Soret band at 410 nm similar to that found in the solution. The binding of gaseous NO resulted in a red shift in the Soret absorption band (410 to 419 nm) of the FeTFPP in the matrix unlike FeTFPP:NO in solution. In the case of SGFeDETC, after addition of sodium dithionite solution and bubbling NO we have good evidence that the complex is formed. The EPR spectrum of the SGFeDETC:NO in solid form exhibited a signal similar to that found in a solution of FeDETC:NO at 77K. The UV/Vis spectrum of SGFeDETC:NO shows a band at 367 nm also found in FeDETC:NO solutions. It is observed that the FeDETC:NO is more stable entrapped in the sol-gel than in aqueous solution. In the former the EPR signal decreases by a factor of 4 after one week, in the latter in 2 days the EPR signal cannot be observed anymore. INTRODUCTION Nitric oxide (NO) is an important signaling molecule that acts in many tissues to regulate a diverse range of physiological processes including neurotransmission and immune defense [1,2]. Several methods for NO detection have been developed and one of the most powerful methods for directly measuring NO production in biological systems is electron paramagnetic resonance (EPR) spin trapping techniques [3]. Iron complexes with dithiocarbamates and porphyrins are used as spin traps due to the high affinity between NO and the iron complexes, however these complexes are unstable for long-term measurements. The physical entrapment of molecules using the sol-gel process [4,5] has been used for the development of NO electrochemical and optical sensors. Sensors entrapped in gels offer numerous advantages when compared with liquid based systems: they are easier to manipulate, allow species detection and concentrations measurements with less contamination of the sample, can be used for continuous sensing, and are normally more stable. In the case of silica gels a further advantage is that it is transparent in the UV/visible/near infrared range, which makes this material especially interesting for optical sensors. For EPR measurement of special interest is the fact that solutions are in general hard to measure since liquids have high dielectric losses, or in other words they absorb microwaves thus killing the Q of the resonance cavity. EPR quartz liquid cells are necessary when liquids are to be measured by EPR, which are expensive and impose restrictions in
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