Biomedical Engineering Reference
In-Depth Information
micromolar levels, but it again remains unclear if the use of reagents with strong affi n-
ity for H
2
S cause a signifi cant equilibrium shift to release H
2
S that was previously
bound as persulfi des.
A recent comprehensive review of the methods used to determine tissue H
2
S lev-
els details the multiple sources of measured H
2
S as acid labile, bound or free [37].
Mammalian tissue H
2
S concentrations tabulated in the review cover a large range from
not detectable to
M, indicating that method, tissue, and metabolic state all have
a strong infl uence on the actual measurement. Although the presence of tissue H
2
S is
generally accepted, how much is freely dissolved is an important unanswered ques-
tion. Furthermore, kinetic interactions of H
2
S with any other cellular signals in real
time are not captured with single time point determinations. It is clear that available
H
2
S measurement techniques are not applicable to dynamic biological experiments. To
address the problems of H
2
S stability and kinetics, we have developed a novel polaro-
graphic H
2
S sensor (PHSS) that allows real-time determination of H
2
S levels in bio-
logical samples under simulated physiological conditions of pH, temperature, and O
2
tension [36, 41].
100
ยต
8.2 ADVANTAGES OF ELECTROCHEMICAL SENSORS
FOR H
2
S DETERMINATION
8.2.1 Electrochemistry
The development of sensitive and selective polarographic NO sensors has greatly
advanced investigations by allowing the continuous real-time measurement of the
highly labile NO under physiological conditions. Ideally, an H
2
S sensor appropriate for
biological samples would report H
2
S continuously, not be contaminated by numerous
biological salts and compounds, and be suffi ciently sensitive to detect H
2
S at physi-
ological levels. Our polarographic H
2
S sensor (PHSS) has these characteristics. The
PHSS anode, cathode, and electrolyte are protected from solution constituents by an
H
2
S-permeable polymer membrane so that only free H
2
S is able to diffuse across the
membrane and interact electrochemically with the appropriately polarized anode. The
PHSS electrolyte consists of 0.05 M K
3
[Fe(CN)
6
] in alkaline 0.5 M carbonate buffer
pH 10 (Fig. 8.2). The electrochemical reaction is initiated as H
2
S diffuses from the
sample solution through the membrane and dissociates to HS
. HS
then reduces fer-
ricyanide to ferrocyanide, which subsequently donates electrons to the anode, polar-
ized at 100 to 200 mV, creating a current proportional to sample H
2
S concentration.
The PHSS background current resulting from electrolytic conduction of current from
cathode to anode occurs as ferricyanide is reduced at the cathode and oxidized at the
anode. Because the cathode potential, 0 mV, is not as negative as the equilibrium poten-
tial of HS
,
270 mV, the ferricyanide reduction rate and hence background current is
less than signal current resulting from sample H
2
S. The background current, which is
subtracted from signal current, dictates the PHSS lower limit of detection.
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