Electrochemical sensors for the determination of hydrogen sulfide production in biological samples - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 221

18

20 m sulfide

16

37 C

pH 7.35

°

14

12

10

8

6

4

0 50 100 150 200 250 300

Polarizing voltage, mV

FIGURE 8.4 PHSS polarogram. A representative graph of PHSS current obtained for separate 20 µM
sulfi de samples as a function of polarizing voltage under anoxic conditions. Signal was stable between 100
and 200 mV as the ferricyanide to ferrocyanide ratio remained near unity (after [41]).

PNOS responded linearly to increasing concentrations of solution H 2 S and NO,
respectively, while the POS, unresponsive to either H 2 S or NO, reported a stable O 2
concentration near 2

M. In addition, the PHSS signal did not exhibit interference
from O 2 , NO (Fig. 8.5), millimolar concentrations of hydrogen peroxide, sulfi te, sul-
fate, ascorbate, other sulfhydryl-bearing compounds such as L-cysteine, glutathione,
homocysteine, or thiosulfate, or organosulfur compounds such as diallyl disulfi de, dial-
lyl trisulfi de, methyl sulfi de, or propyl disulfi de (not shown). The addition of 1 mM
KCN caused a small (relative to 10

µ

M H 2 S) positive offset of the baseline signal (not
shown), probably the result of HCN diffusion across the membrane increasing the elec-
trolyte conductance, which can be subtracted. The addition of Lucina pectinata metHb
I, which binds specifi cally to H 2 S [46], resulted in an abrupt stoichiometric drop of the
PHSS signal (Fig. 8.5). The rate of H 2 S consumption by the PHSS, calculated from
signal current at a 10

µ

M H 2 S concentration using Faraday's constant, was in the subpi-
comole s range and was thus negligible compared to the biological consumption rate.

µ

8.5.2 Sensitivity
Because the pK for the H 2 S/HS couple is near 6.8, the PHSS amperometric signal for
H 2 S is highly dependent on solution pH. Under physiological conditions near neutral
pH, small changes in pH can alter the PHSS signal. A pH titration of the PHSS signal
shown in Fig. 8.6 indicates that the sensor signal increased from 24 to 440 nA for an
80

M H 2 S injection over the pH range of 8 to 6, demonstrating that H 2 S becomes
the predominant sulfi de species at lowered solution pH. The pK for this protonation
is approximately 6.8 under the conditions tested. The PHSS sensitivity at pH 6, where
H 2 S is approximately 90% of the total sulfi de present, is 5.5 nA/

µ

M H 2 S. Although the
PHSS signal is less at pH above the pK, the sensor is responsive and can be calibrated

µ