Nitric oxide (NO) electrochemical sensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 12

a sensor's sensitivity is clearly important, its detection limit is often more important to
the investigator. High sensitivity of a sensor does not necessarily equate to a low detec-
tion limit. For example, a highly sensitive NO sensor may have a high background
noise level, which at a high NO concentration may not be a problem. However, at lower
NO concentrations, measurement can be hindered by excessive noise. Accordingly,
in evaluating the performance of an NO sensor, the ultimate detection limit is usually
more critical than the sensitivity. Fortunately, most commercial NO sensors can detect
NO at levels of 1 nM or less and are therefore well suited for the majority of research
applications.

1.5.2 Selectivity
An NO sensor is practically useless unless it is immune to interference from other spe-
cies likely to be present in the measurement environment. Selectivity is usually con-
trolled by both the voltage applied between the working and reference electrode (poise
voltage) and the selective membrane used to coat the sensor. Many species present in
a biological matrix are easily oxidized at the poise voltage employed to detect NO (i.e.

860 mV vs Ag/AgCl). For example, monoamines such as dopamine (DA), 5-hydrox-
ytryptamine (5-HT), and norepinephrine (NE), as well as their primary metabolites,
can be oxidized at 0.3 V (and higher) vs Ag/AgCl. Ascorbic acid can be oxidized at
0.4 V (and higher). A Clark type NO sensor (e.g. ISO-NOP) is covered with a gas
permeable membrane, hence the selectivity of such sensors in biological samples is
extremely good. With other types of NO sensors selectivity is usually achieved by
coating the sensor surface with Nafi on and other gas permeable membranes. Nafi on is
widely used to eliminate interference caused by anions, such as ascorbate and nitrite,
during measurement of catecholamine species. When used for NO detection the nega-
tively charged Nafi on layer can stabilize NO formed upon the oxidation of NO and
prevent a complicated pattern of reactions that could lead to the formation of nitrite
and nitrate. However, the main drawback with Nafi on is that it does not eliminate
interference from cationic molecules such as dopamine, serotonin, epinephrine and
other catecholamines. Consequently, selectivity of the conventional Nafi on coated NO
sensors is very poor. Nafi on coated NO sensors also exhibit other undesirable char-
acteristics including unstable background current, continuous drift in the base line,
and extended polarization requirements. These problems signifi cantly limit the use of
Nafi on coated carbon fi ber electrodes for measurement of NO. During the late 1990s
this lab developed a unique multi-layered proprietary membrane confi guration. NO
sensors coated with this membrane exhibited increased selectivity and sensitivity for
NO, and moreover were shown to be immune from interference caused from a wide
range of potentially interfering species [43].

1.5.3 Response time
Response and recovery times of NO sensors are extremely important for their use
in vivo . Theoretically, since the rate of mass transport at a microelectrode is very