A Dual Sensor for pH and Hydrogen Peroxide Using Polymer-Coated Optical Fibre Tips

Abstract

This paper demonstrates the first single optical fibre tip probe for concurrent detection of both hydrogen peroxide (H₂O₂) concentration and pH of a solution. The sensor is constructed by embedding two fluorophores: carboxyperoxyfluor-1 (CPF1) and seminaphtharhodafluor-2 (SNARF2) within a polymer matrix located on the tip of the optical fibre. The functionalised fibre probe reproducibly measures pH, and is able to accurately detect H₂O₂ over a biologically relevant concentration range. This sensor offers potential for non-invasive detection of pH and H₂O₂ in biological environments using a single optical fibre.

Keywords: dual sensor; fibre tip sensor; hydrogen peroxide probe; optical fibre; pH sensor.

Figure 1

Chemical structures of fluorescent probes used in this study. Carboxyperoxyfluor-1 (CPF1) reacts with H₂O₂ to form the fluorescent 6-carboxyfluorescein. Seminaphthorhodofluor-2 (SNARF2) is found in the protonated (open) form and lactone (closed) at low and high pH respectively.

Figure 2

Experimental configuration for optical measurements of the combined pH/peroxide sensor. The blue LED source was used to illuminate the peroxide sensitive carboxyperoxyfluor-1 (CPF1) fluorophore, while the green excites the pH sensitive seminaphtharhodafluor-2 (SNARF).

Figure 3

(A) Integrated fluorescence intensity from CPF1 using blue excitation with varied peroxide concentration in pH 7.5 buffer; 100 μM H₂O₂ shows an increased fluorescent response from the fibre without H₂O₂; (B) Slope of integrated fluorescence for increasing concentrations of H₂O₂ (0, 50, 75 and 100 μM). Error bars indicate the standard error of the calculated slope.

Figure 4

Response of CPF1 to 100 μM H₂O₂ in solutions that varied in pH. (A) Integrated fluorescent responses of probes to 100 μM H₂O₂ in pH 7.05, 7.55 and 8.05 over 20 min using blue excitation; (B) Rate of increase in fluorescence of each probe with H₂O₂ in each of the 3 pH solutions. Error bars indicate the standard error of the calculated slope.

Figure 5

pH response of SNARF-2 embedded in polyacrylamide on fibre tip to varied pH. (A) Emission spectra of SNARF in various pH buffers; (B) Ratio of emission peak intensities 600/660nm shown with over multiple trials. The effect of noise was reduced by taking the mean of eight values between 598–602 nm and 558–662 nm. Error bars represent the standard deviation of these values.

Figure 6

Sensing of pH before and after immersion in H₂O₂. Each graph plots the ratio of emission peaks of SNARF2 at 600/660 nm with the pH of the buffer solution tested, before and after solutions: (A) 100 μM solution of H₂O₂ in pH 7.55 buffer; (B) 50 μM H₂O₂ in pH 7.55 buffer; (C) 100 μM H₂O₂ in pH 7.05 buffer. Three different samples were trialled before and after H₂O₂ solutions to demonstrate the independence of the result to concentration of H₂O₂ or the pH of the H₂O₂ solution. To reduce any effect of noise, the mean of eight values between 598–602 nm and 558–662 nm is given. Error bars represent the standard deviation of these values.