Biomedical Engineering Reference
In-Depth Information
Signal vs. protein concentration
1.6
Negative, conj : 0.5
µ
g/ml
Positive, conj : 0.5
µ
g/ml
Negative, conj : 1.0
1.4
µ
g/ml
Positive, conj : 1.0
µ
g/ml
1.2
1
0.8
0.6
0.4
0.2
0
1.0
0.5
0.1
Protein concentration (
µ
g/sample)
FIGURE 22.7
Optimization of antigen immobilized on the carbon particles: Amperometric response of the device at different
concentrations of conjugate (AP) and recombinant protein of Sin Nombre virus (SNV) envelope at an analyte
concentration of 1:1000.
the response of the device for negative and positive blood samples at various dilutions. It
clearly shows the differentiation between the positive and negative samples, which is
more than three times at a sample dilution of 1:1000.
The difference diminishes with increased dilution, due to the decrease in the antibodies
in the analyte solution with a decrease in the analyte concentration. This modification pro-
vides an improved lower detection limit than the one previously described [11], which
was confirmed by the working range of the sample dilutions. The present system was
clearly able to distinguish the positive and the negative sample even at a dilution of
1:10,000. The substrate solution was prepared fresh everyday to minimize nonenzymatic
hydrolysis.
22.4.4
Nafion Layer
The nonspecific sorption of the components in the hemolyzed blood samples on to the sur-
face of the working, reference, and counter electrodes is further reduced by Nafion layer
coating. We have studied the flow-through measurements with Ag/AgCl electrode covered
with a Nafion layer (by dipping and drying a commercially available Nafion solution).
Covering the Ag/AgCl electrode with a Nafion layer improved the results even more.
The Nafion layer protected the electrodes from contact with the interferents in the blood by
acting as a barrier, reducing the binding of the interferents to the electrodes, and also
minimizing fouling affect. A minor (twofold) decrease in the signal magnitude was
observed but a significant improvement in reproducibility was obtained, as seen in Figure
22.6. Even though covering the reference electrode surface with a Nafion layer diminished
the efficiency of the anodic oxidation process by twofold in comparison with the bare elec-
trode, the difference in signal for the negative and positive samples is still satisfactory
(more than threefold at 1:1000 sample dilution (Figure 22.6).
The decrease in signal is due to the presence of the Nafion layer, which restricts the sub-
strate supply to the inner layer, where the enzymatic reaction is verified. The use of Nafion
layer on the reference resulted in added stability to the device, also protecting the electrode
