Biomedical Engineering Reference
In-Depth Information
1M OxyHb
20 sec
100 nM NO
Time/S
FIGURE 1.5
Amperometric response of an NO nanosensor upon addition of 100 nM NO and 1µM
oxyhemoglobin to a stirred 0.1 M phosphate buffer solution (pH 7.4).
high it should have a response time on the order of
sec; however, the addition of
NO-selective membranes to the electrode surface decreases this response time signifi -
cantly. With a membrane present the response time is now dependent on the diffusion
rate of NO across the membrane, which is highly dependent on the nature of the mem-
brane as well as its thickness. The response time not only depends on the electrode
being used but also on the electronics being used to read out the current. For example,
since the current being read out is typically on the order of pAs an electronic fi lter is
usually applied, which also slows the system response. Since the half-life of NO is
from a few seconds to minutes in a biological system, a sensor response on the order
of 3-4 sec will work fi ne. Figure 1.5 is a typical response of an NO microsensor to the
addition of NO in PBS solution. It shows the 90% response of the sensor to a step from
0 to 100 nM NO and subsequently injection of NO scavenger 1
µ
M of oxyhemoglobin.
As can be seen, the response times to both additions of NO and oxyhemoglobin are
less than 4 seconds. This indicates that even with a heavy fi lter, the response is within
a few seconds, which is within the time frame required to measure NO. To decrease
the response time of NO sensors, fewer fi lters can be applied, which will sacrifi ce the
detection limit of NO sensors.
µ
1.5.4 Effect of temperature and pH on NO electrodes
Background currents of all NO electrodes are sensitive to changes of temperature and
pH. Depending on type of electrodes, the effect may be more or less. Clark type NO
electrodes are very sensitive to temperature change. The temperature induced response
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