Page 107 - Organic Electronics in Sensors and Biotechnology

84 Cha pte r T w o

replicate injections were carried out at any investigated concentration
level. At both the working temperatures (150 and 175°C) Au-NP-FET
sensors were able to detect NO in a concentration range ranging
2
from 50 to 200 ppm, with no significant difference when NO was
employed. Furthermore, the signal-to-noise (S/N) ratio was almost
doubled upon increasing the operating temperature from 150 to 175°C.
Note, however, that after the NO pulse, the sensor did not recover
x
back to the initial baseline, thus indicating that some irreversible
interactions take place between NO and the Au-NP film. Such evi-
x
dence has already been reported in Refs. 169 and 170, relevant to
similar Au-NPs, capped by other stabilizers. Data of Fig. 2.13 confirm
that slow sensor features are intrinsically related to the Au-NO system,
x
regardless of the capping agent and particle size.
The selectivity was evaluated by exposing the sensor to different
gases: CO, H , NH , and C H . Typical sensor response to interferent
2 3 3 6
and target species (at 175°C) is reported in Fig. 2.14.
Exposing the sensor to NO and NH caused quite similar peak
x 3
intensities, although changes of the output voltage occurred in oppo-
site directions, due to the different oxidizing/reducing character of
the analytes. The sensor showed also a small response to H , while
170
2
it did not respond at all to CO and C H .
3 6
As the operation temperature was changed to 150°C (data not
shown), the selectivity toward NO increased, but a reduction in the
x
response and recovery time was observed as well, with the sensing
response strongly influenced by the operation temperature.
–0.40
250 ppm NH 3

–0.30 500 ppm NH 3 250 ppm CO 500 ppm CO 100 ppm C 3 H 6 200 ppm C 3 H 6 250 ppm H 2 500 ppm H 2
Voltage (V)

–0.20 500 ppm NO 2
250 ppm NO 2

–0.10
20 40 60 80 100 120
Time (min)

FIGURE 2.14 Responses of the Au-NP sensor to NO and interfering species,
2
measured at 175°C. (Reprinted with permission from Ref. 175: E. Ieva, K.
Buchholt, L. Colaianni, N. Cioffi , L. Sabbatini, G. C. Capitani, A. Lloyd Spetz, P. O.
Käll, and L. Torsi, Sensor Letters, 6:577–584, 2008.)