Sol–gel encapsulation of metalloproteins for the development of optical biosensors for nitrogen monoxide and carbon monoxide

Abstract

Three heme proteins; horse heart cytochrome c, myoglobin (Mb) and hemoglobin (Hb), have been encapsulated in a sol–gel matrix and investigated for their potential use as biological recognition centres for the biosensing of NO and CO. It has been shown that the encapsulation of these metalloproteins in sol–gels has no effect on the formation of nitrosyl or carbonyl complexes as observed using UV/VIS spectrophotometry. The metalloproteins in sol–gels could be readily reduced to the Fe^II state, using excess sodium dithionite, and re-oxidized to the Fe^III state on the addition of excess potassium ferricyanide. The observed redox capability within sol–gels enabled formation of the nitrosyl complex in both the Fe^III and Fe^II states of Mb and Hb and in the Fe^III state of cytochrome c. The formation of these nitrosyl complexes was shown to be reversible. Similarly, the reversible formation of the carbonyl complex with Mb and Hb in the reduced form has been achieved. Titration of a saturated solution of CO with Hb contained in a sol–gel showed a linear increase in intensity of the α(568 nm) and β(540 nm) absorption bands of the metalloprotein until saturation of the iron centres of the Hb occurred. A similar increase in intensity of the α(562 nm) absorption band of cytochrome c was observed when a saturated solution of NO was titrated against this metalloprotein contained in a sol–gel. The results show that sol–gels can be readily used as a host for the encapsulation of cytochrome c, Mb and Hb and that these metalloproteins can be used as the biological recognition centre for the biosensing of NO and CO at the micromolar level.