High-Performance Fiber-Optic pH Microsensors for Practical Physiological Measurements Using a Dual-Emission Sensitive Dye

Abstract

A fast and durable ratiometric pH microoptode that is highly accurate, precise, sensitive, reversible, and reproducible over the physiological ranges of pH, ionic strength, and temperature has been developed. The sensing site consists of 5 (and 6)-carboxynaphthofluorescein (CNF) entrapped in a polyacrylamide gel matrix via photopolymerization at the silanized end of an optical fiber with a diameter of 2 (pulled) or 125 microns (unpulled). The optode's precision for the pH 6.3-8.4 range in rat embryos, sera, or physiological (Earle's and Tyrode's) buffers was found to be better than +/- 0.03 pH unit. The pulled and unpulled optodes have respective upper limit response times of 1 and 400 ms for 1-pH-unit change. Over a 7-week period, they retain sensitivity for 600 and 10,000 measurements, respectively. Ratiometric measurements are made using a pH-sensitive emission peak on each side of an isosbestic point. The CNF microoptode is most suitable for biological applications because of its essentially linear response over the pH 7-8 range, its high sensitivity (slope about 2), and its almost perfect correlation with a pH macroelectrode. Furthermore, errors introduced by photobleaching, leaching, quenching, optode movement, and excitation source fluctuations are minimal.

Transmitted light pH optode for small sample volumes

“…In most cases, the spectra are sharpened with the aid of monochromatic filters before reaching the sample. Within the sample, specific effects like absorbance (Jin et al, 2000), fluorescence (Song et al, 1997) or phosphorescent emission may occur. These effects are used in many research approaches as sensing elements.…”

Cancer Detection in Human Tissue Samples Using a Fiber-Tip pH Probe

“…Optical fibre based sensors have found extensive use in the areas of chemical (20)(21)(22)(23)(24), biological sensing (25)(26)(27), physical sensing for temperature (28)(29)(30), or structural health monitoring (31,32). Typically they are deployed in situations where measurements along the length of the fibre (distributed measurements) are required (33), however they also have strong potential for use in applications in difficult to reach locations.…”

“…The optical fibre probe measures pH rapidly, in less than one minute, is simple to use, and does not leave a residue or stain that would affect later pathology testing. The probe consists of the pH sensitive fluorophore 5,6-carboxynapthofluorescein (CNF) (23), which changes colour with a change in the environmental pH. This fluorophore is embedded in an acrylamide polymer on the optical fibre tip, which is readout remotely via a laser at the other end of the fibre.…”

Plastic Optical Fiber pH Sensor Using a Sol-Gel Sensing Matrix

“…Photopolymerization (Barnard, 1991;Bronk, 1994;Ferguson, 1997;Song, 1997) and sol-gel chemistry (Lin, 1997 andMcDonagh, 2008) have been the mainly exploited reactions, and adsorption, covalent binding and entrapment are the most widely used approaches to immobilize indicator dyes.…”

“…The covalent method can overcome leaching problems as the pH indicator is strongly bound to the solid substrate (Baldini, 2008;Barnard, 1991;Bronk, 1994;Ferguson, 1997;Song, 1997). However, the immobilization by covalent attachment requires a previous surface modification of the optical fibers and the control of the surface reaction of the modified substrate with a suitable indicator.…”