Slab Optical Waveguide High-Acidity Sensor Based on an Absorbance Change of Protoporphyrin IX

• Published in: Analytical chemistry (Washington) 2006-11, Vol.78 (21), p.7511-7516
• Main Authors: Umemura, Tomonari, Hotta, Hiroki, Abe, Takahiko, Takahashi, Yoshihito, Takiguchi, Hiromi, Uehara, Masayuki, Odake, Tamao, Tsunoda, Kin-ichi
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Biosensors; Chemistry; Electrochemical methods; Exact sciences and technology; General, instrumentation; Glass substrates; Measurement techniques; Optics; Spectrometric and optical methods; Spectrum analysis
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac0606150

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 ∼1 μm 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 HNO3 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 μL 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.