A Portable 'Plug-and-Play' Fibre Optic Sensor for In-Situ Measurements of pH Values for Microfluidic Applications

Abstract

Microfluidics is used in many applications ranging from chemistry, medicine, biology and biomedical research, and the ability to measure pH values in-situ is an important parameter for creating and monitoring environments within a microfluidic chip for many such applications. We present a portable, optical fibre-based sensor for monitoring the pH based on the fluorescent intensity change of an acrylamidofluorescein dye, immobilized on the tip of a multimode optical fibre, and its performance is evaluated in-situ in a microfluidic channel. The sensor showed a sigmoid response over the pH range of 6.0-8.5, with a maximum sensitivity of 0.2/pH in the mid-range at pH 7.5. Following its evaluation, the sensor developed was used in a single microfluidic PDMS channel and its response was monitored for various flow rates within the channel. The results thus obtained showed that the sensor is sufficiently robust and well-suited to be used for measuring the pH value of the flowing liquid in the microchannel, allowing it to be used for a number of practical applications in 'lab-on-a-chip' applications where microfluidics are used. A key feature of the sensor is its simplicity and the ease of integrating the sensor with the microfluidic channel being probed.

Keywords: fluorescent sensor; microfluidics; optical fibre sensor; pH sensor.

Figure 1

Schematic showing the equilibrium of protonated and deprotonated forms of the AAF dye in solution.

Figure 2

(a) Coated multimode fibre with a core diameter of ~1 mm. Inset shows a magnified version of the coated area. The coated area is fully inserted into the sensing chamber. (b) Fibres sealed in the microfluidic channel. (c) Top and side schematic view.

Figure 3

(a) Schematic of the experimental setup. Toggling of LED was controlled by sending pulse signal to electromechanical relay using a spectrometer. (b) Photograph showing several of the important components used in the setup.

Figure 4

(a) Typical fluorescence curve showing two peaks corresponding to AAF dye and perylene red at the excitation wavelength of 375 nm for pH 6.5 and pH 8.5. The spectrum is normalized with respect to the reference peak. (b) The change in the ratio of signal to reference peak versus pH.

Figure 5

(a) The cyclical response of the sensor at two pH values 3 and 11. The vertical dotted-dashed lines show the pH change time. (b) The first 100 min of the sensor response shows the rise and fall times, t₁₀ and t₉₀, which, respectively, represent the time taken to reach 10% of the lower and 90% of the higher value of the measured pH.

Figure 6

(a) The experimental and sigmoid curve fitting showing the change in the ratio (of signal to reference peak as a function of change in pH. (b) Cyclical response of sensor on three consecutive days with different flow rates. The pH varied from 3 to 11 and back. (Note: the red and yellow curves have been shifted in the y-direction by +1 and +2, respectively, for clarity.).