Page 201 - Organic Electronics in Sensors and Biotechnology

178 Chapter Five

integration of the OLED/O sensor. This multiple bioanalyte sensor is
2
obviously important for various clinical, health, industrial, and envi-
ronmental applications.
The glucose, lactate, and ethanol sensing methods were similar,
so we describe the method for glucose only. The glucose level was
determined from the DO level following the enzymatic oxidation of
glucose by glucose oxidase (GOx) (the GOx was embedded in a thin
sol-gel film or dissolved in solution):

glucose + O + GOx → H O + gluconic acid (5.2)
2 2 2
Hence, in the presence of glucose, the PL quenching of the O -
2
sensitive dye is reduced (i.e., I and τ increase) due to O consumption
2
during the enzymatic oxidation. 13, 16, 62, 63 A similar reaction in the pres-
ence of lactate oxidase (LOx) or alcohol oxidase (AOx) results in lac-
tate or ethanol oxidation, respectively.
The multianalyte measurements were performed in sealed cells,
since the DO in such cells could not be replenished from the air, and,
consequently, its final level yielded the initial analyte level directly.
Indeed, the responses from cells open to air were more complex,
16
clearly due to such replenishment. The O -sensitive dye, embedded
2
in a PS film, was deposited on the bottom of the reaction cells. Each
cell contained a buffered solution of GOx, LOx, or AOx. The total
volume of each cell was 100 to 200 μL.
Measurements in sealed cells also resulted in a low LOD of ~0.01
to 0.02 mM for glucose, lactate, and ethanol. In our previous studies
on the OLED-based glucose sensor, the GOx was embedded in a sol-
gel film, rather than dissolved in solution. The sol-gel film was depos-
ited on the PtOEP:PS film, and the monitored sample was simply
dropped on the sol-gel film. While the dynamic range was a much
higher 0 to 5 mg/mL, the LOD was also higher. In reports on other
PL-based glucose sensors, the excitation sources have included Ar +
lasers (operated at 20 to 40 mW), Hg lamps, or a fluorimeter with a
150 W Xe lamp light source. 4, 62, 63 The glucose concentration c was
Gl
usually monitored via changes in I.
The OLED sensor array was assembled in a back-detection geom-
etry and operated in the τ mode. The analytes were monitored both
sequentially, using a single PD, and simultaneously, using a compat-
ible array of Si photodiodes. The sequential and simultaneous modes
yielded similar results. Calibration curves were obtained by using a
modified SV equation suitable for these analytes in sealed cells, where
there is no supply of DO beyond the initial concentration. The modi-
fied SV equation was based on the following considerations: Let the
initial analyte, initial DO, and final DO levels be [analyte] ,
initial
[DO] , and [DO] , respectively. Then if
initial final
[analyte] ≤ [DO] (5.3)
initial initial