Optical sensor for interstitial pH measurements

Abstract

An optical fiber sensor for measuring the pH in interstitial fluid is described. Microdialysis is the approach followed for extracting the sample from the subcutaneous adipose tissue. The interstitial fluid drawn flows through a microfluidic circuit formed by a microdialysis catheter in series with a pH glass capillary. The pH indicator (phenol red) is covalently immobilized on the internal wall of the glass capillary. An optoelectronic unit that makes use of LEDs and photodetectors is connected to the sensing capillary by means of optical fibers. Optical fibers are used to connect the interrogating unit to the sensing capillary. A resolution of 0.03 pH units and an accuracy of 0.07 pH units are obtained. Preliminary in vivo tests are carried out in pigs with altered respiratory function.

Colorimetric Sensor Design

“…The axial position of the boron is substituted with a good leaving group such as phenol, which is displaced in the presence of fluoride anions. The initial complex is red and turns colorless upon the addition of fluoride (19), which makes this an on-off sensor. The authors state that the irreversibility of the system results from the high affinity of the boron toward fluoride anions.…”

“…Molecules that exhibit a change in color due to a shift in pH have been utilized in everything from simple titrations [5], for the determination of acid/base concentrations, to pH-responsive films [6][7][8][9][10][11][12][13][14][15] and fiber-optic flow cells [16][17][18][19][20][21] for various applications. Furthermore, pHresponsive molecules have found utility in indicator displacement assays (IDAs), allowing for the detection of a wide variety of analytes [22,23].…”

“…Another useful triphenylmethane pH indicator is phenol red (5), which changes from yellow to red over a range 6.4-8.2, making it useful in various physiological applications. It has been incorporated into sol-gel matrices [11] and more commonly into fiber-optic flow cells [16][17][18][19][20][21] to continuously monitor pH. Xanthene dyes (6)(7)(8) are the smallest class listed here and are more commonly used as fluorescent dyes than colorimetric indicators.…”

In-vivo characterization of a microdialysis-based pH sensor

“…The pH sensing capillary is interrogated by optical fibres connected with an optoelectronic unit which makes use of a light emitting diode and a photodetector as source and detector, respectively. The immobilisation procedure, the optoelectronic unit and their laboratory characterisation have been already described 7,8 . Figure 1 shows the sketch of the system used for the tests with animals.…”

In-vivo continuous measurement of interstitial pH for intensive care applications

“…In the proposed system, the drawn interstitial fluids flow through the microfluidic circuit formed by a microdialysis catheter (CMA60) in series with a glass capillary on the internal wall of which the appropriate chemistry is immobilised. The laboratory characterisation of the sensor has been already described 25,26 and in the present paper in-vivo tests on animals and on volunteers are described.…”