determining at high-resolution the positions of the active nanoelectronic sensory
elements with respect to detail structures within the host. In the future, we also note
that the resolution could be even further improved by incorporating point-like
transistor photoconductivity detectors [ 10 , 17 ],p-n photodiodes [ 18 ] andp-i-
navalanche photodiodes [ 19 ] nanowire building blocks within the 3D macroporous
nanoelectronic network.
3.3.2 Mapping Chemical Diffusion in Gel.................
We have used 3D macroporous nanoelectronic networks hybridized with agarose
gel to map pH changes in 3D through the gel. The p-type nanowire FET device was
used as chemical sensor. The hybrid nanoelectronic/gel material was prepared as
description in experimental section. A reconstructed 3Dfluorescence image of the
hybrid material (Fig.3.4a) shows clearly the 3D macroporous chemical sensors
network fully embedded within an agarose gel block without phase separation. To
carry out sensing experiments the 3D nanoelectronic/gel hybrid material was
contained within a microfluidic chamber (Fig.3.4b). Positions of nanowire tran-
sistor devices, which can function as very sensitive chemical/biological sensors [ 10 ,
11 , 20 ], were determined by the photocurrent mapping method described above. As
a comparison, 3D macroporous nanoelectronic chemical sensors network without
gel was placed the chamberfilled by aqueous solution. For both samples, we
recorded signals simultaneously from 4 sensors chosen to span positions from upper
to lower boundary of network or gel, where representative z-coordinates of the
devices positions within the hybrid sample are highlighted in Fig.3.4c; a similar
z-range of devices for the free nanoelectronic network was also used.
Representative data recorded from chemical sensors in 3D macroporous nano-
electronic network without gel (Fig.3.4d, I) and in the hybrid 3D nanoelectronic/
agarose gel hybrid (Fig.3.4d, II) highlight several important points. First, the
device within the 3D macroporous nanoelectronic network without gel showed fast
stepwise conductance changes (<1 s) with solution pH changes. The typical sen-
sitivity of these devices was ca. 40 mV/pH, and is consistent with values reported
for similar nanowire devices [ 21 ]. Second, the device within the 3D nanoelectronic/
gel hybrid exhibited substantially slower transition times with corresponding
changes of the solution pH; that is, signal change required on order of 2000 s to
reach steady-state, and thus was 1000-fold slower than in free solution. Third, the
time to achieve one-half pH unit change for the four different devices in 3D
macroporous network without gel (Fig.3.4e, I) is ca. 0.5 s and the difference
between devices is only ca. 0.01 s. We note that the time delay in the data recorded
from device d4 is consistent with the down-stream position of this device within the
fluidic channel. In contrast, the time to achieve one-half for the four devices in the
3D nanoelectronic/gel hybrid (Fig.3.4e, II) range from ca. 280 to 890 s for devices
d1 to d4, respectively. The results show that the device response time within the
agarose is ca. 500–1700 times slower than in solution and is proportional to the
32 3 Integration of Three-Dimensional Macroporous Nanoelectronics...