title: Slab optical waveguide high-acidity sensor based on an absorbance change of protoporphyrin IX.
abstract: A sensitive and fast-responsive evanescent wave absorption sensor has been constructed for pH measurements in highly acidic ranges. This sensor is based on a pH-dependent color change of protoporphyrin IX (PPIX). For the sensitive detection, a visible attenuated total reflection spectrometer with a slab optical waveguide (SOWG) was laboratory-made, and the guiding layer surface was modified with a PPIX-immobilized acrylamide-based thin membrane. The sensing membrane with a thickness of approximately 1 mum was directly fabricated on the SOWG glass surface by copolymerization of acrylamide, N,N'-methylene bisacrylamide, and PPIX in the narrow space confined by a cover plate. PPIX possesses two double bonds in its structure, and so it can be covalently incorporated into the membrane. The response characteristics of the PPIX-immobilized optode membrane were explored using aqueous solutions with different concentrations of HNO(3) or HCl. The optode membrane provided characteristic Soret band absorption spectra depending on the hydrogen ion concentration; the absorbance at 410 nm increased with increasing the concentrations in the range of 0.15-2 M, corresponding to the range of pH -0.3 to 0.8. The absorption signal reached 90% of its final value within 10 s, while the absorption signal was quite readily returned to background level simply by passing 2 mL of distilled water through a flow cell with a volume of 16.5 muL placed on the SOWG. Due to the rapid response and reversibility, this sensor could be operated in a flow-through mode as well as in a conventional static mode, where deionized water was conveniently used as a carrier and conditioning solution. In terms of the stability and precision, this sensor showed no significant change in response even after 100 assays and after being stored in a dry condition for over 6 months. Relative standard deviations for 10 replicate measurements were less than 1.8% in the linear range, and the detection limit calculated from 3 times of the standard deviation was 0.02 pH unit.
authors.affiliation: Department of Chemistry, Faculty of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan. umemura@apchem.nagoya-u.ac.jp
authors.forename: Tomonari
authors.initials: T
authors.lastname: Umemura
authors.forename: Hiroki
authors.initials: H
authors.lastname: Hotta
authors.forename: Takahiko
authors.initials: T
authors.lastname: Abe
authors.forename: Yoshihito
authors.initials: Y
authors.lastname: Takahashi
authors.forename: Hiromi
authors.initials: H
authors.lastname: Takiguchi
authors.forename: Masayuki
authors.initials: M
authors.lastname: Uehara
authors.forename: Tamao
authors.initials: T
authors.lastname: Odake
authors.forename: Kin-Ichi
authors.initials: K
authors.lastname: Tsunoda