Biomedical Engineering Reference
In-Depth Information
Fiber Optic pH Sensors
A fiber optic pH sensor can be designed by placing a reversible color-changing dye as an
indicator at the end of a pair of optical fibers. For example, the popular indicator phenol red
can be used because this dye changes its absorption properties from the green to the blue
part of the spectrum as the acidity is increased. The dye can be covalently bound to a
hydrophilic polymer in the form of water-permeable microbeads to stabilize the indicator
concentration. The indicator beads are contained in a sealed hydrogen ion-permeable enve-
lope made out of hollow cellulose tubing, forming a miniature spectrophotometric cell at
the end of the optical fibers.
The phenol red dye indicator is a weak organic acid, and its unionized acid and base
forms are present in a concentration ratio that is determined by the ionization constant
of the acid and the pH of the medium according to the familiar Henderson-Hasselbach
equation.
3
The two forms of the dye have different optical absorption spectra. Hence, the
relative concentration of one form, which varies as a function of pH, can be measured
optically and related to variations in pH. In the pH sensor, green and red lights that emerge
from the distal end of one fiber pass through the dye, where it is backscattered into the
other fiber by the light-scattering beads. The base form of the indicator absorbs the green
light. The red light is not absorbed by the indicator and is used as an optical reference.
The ratio of green to red light is related to the pH of the medium.
A similar principle can also be used with a reversible fluorescent indicator where the
concentration of one indicator form is measured by its fluorescence rather than by the
absorbance intensity. Light, typically in the blue or UV wavelength region, excites the fluo-
rescent dye to emit longer-wavelength light. The concept is based on the fluorescence of
weak acid dyes that have different excitation wavelengths for the basic and acidic forms
but the same emitted fluorescent wavelength. The dye is encapsulated in a sample chamber
that is permeable to hydrogen ions. When the dye is illuminated with the two different exci-
tation wavelengths, the ratio of the emitted fluorescent intensities can be used to calculate
the pH of the solution that is in contact with the encapsulated dye.
Fiber Optic pCO
2
Sensors
The
CO
2
of a sample is typically determined by measuring changes in the pH of a bicar-
bonate solution that is isolated from the sample by a CO
2
-permeable membrane but
remains in equilibrium with the CO
2
gas. The bicarbonate and CO
2
, as carbonic acid, form
a pH buffer system. By the Henderson-Hasselbach equation, the hydrogen ion concentra-
tion is proportional to the
p
CO
2
of the sample. This measurement can be done with either
a pH electrode or a dye indicator.
p
Mixed Venous Oxygen Saturation Sensors
Fiber optic catheters can be used in vivo to measure mixed venous oxygen saturation
(SvO
2
) inside the pulmonary artery, which represents the blood outflow from all tissue
log
HCO
3
CO
2
3
pH
¼
6
:
1
þ
