Nitric oxide (NO) electrochemical sensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 8

platinized anode [66, 67] and its application for measurement of NO release from NO
donors. Scheler and coworkers explored using a myoglobin-clay modifi ed electrode
for NO detection [68]. Kamei and coworkers fabricated a NO sensing device for drug
screening using a polyelectrolyte fi lm [69]. Indium hexacyanoferrate fi lm-modifi ed
electrodes were used for NO detection by Casero and coworkers [70]. Schuhmann's
group [71] developed a device for both in-situ formation and scanning electrochemical
microscopy assisted positioning of NO sensors above human umbilical vein endothe-
lial cells for the detection of nitric oxide release.
Unfortunately, despite the novelty of the above approaches none of the sensors has
stood the test of time, mostly due to various practical diffi culties and/or poor sensitiv-
ity/selectivity. Furthermore, the lack of any published data describing the use of these
sensors in any biological research applications limits any conclusion that can be made
on their individual performance.
1.4 CALIBRATION OF NO ELECTRODES
Routine calibration of an NO sensor is essential in order to ensure accurate experi-
mental results. One of three calibration techniques is generally used, depending
on the sensor type, and will be described in the following section. Each of these
methods has already been the subject of several reviews [23, 72-74] and will there-
fore only be summarized here. NO sensors are typically sensitive to temperature.
Therefore, calibration is usually best performed at the temperature at which the
measurements will be made.
1.4.1 Calibration using an NO standard solution
This technique involves the production of an NO stock solution using a supply of com-
pressed NO gas. One advantage of this method is that it allows an NO sensor to be
calibrated in a similar environment in which the experimental measurements will be
made. However, the major drawback is that it requires a source of compressed NO gas
and since NO gas is toxic, the whole procedure must be performed in a fume hood.
This method can be summarized as follows. A vacutainer is fi rst fi lled with 10 mL
deionized water and agitated ultrasonically for 10 min. Purifi ed argon is then passed
through an alkaline pyrogallol solution (5% w/v) to scavenge any traces of oxygen
before being purged through a deionized water solution for 30min. NO stock solu-
tion is prepared by bubbling compressed NO gas through the argon-treated water for
30 min. The NO gas is fi rst purifi ed by passing it through 5% pyrogallol solution in
saturated potassium hydroxide (to remove oxygen) and then 10% (w/v) potassium
hydroxide to remove all other nitrogen oxides. The resultant concentration of saturated
NO in the water is 2 mM at 22ºC [75]. This can be confi rmed further by a photometric
method based on the conversion of oxyhemoglobin to methemoglobin in the presence
of NO [76]. NO standard solutions can then be freshly prepared by serial dilution of
saturated NO solution with oxygen-free deionized water prior to each experiment.