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In-Depth Information
UV excitation. As in the GFP case, this requirement of an excitation source is
a disadvantage for the types of systems considered here.
Although the intensity of the fluorescent signal produced by the UMT system
is comparable to that of GFP [118], the red fluorescent properties may provide
a greater signal-to-noise ratio because autofluorescence and light scattering of
endogenous materials are lower in this spectral region [118]. Furthermore, the
quantum efficiency of chip-based photodetectors typically reaches a maximum
in this spectral region while having almost no sensitivity to the UV excitation
[93]. For these reasons, UMT may prove to be a useful reporter protein for
cell-to-chip communication.
Ionic/Electronic Communication
Techniques for performing noninvasive recordings from cultured cells (e.g.,
cardiomyocytes, neuronal networks) using microelectrode arrays have been
developed during the last two decades [80]. Action potentials generated by the
cells are capacitively coupled to electrodes arranged in a 2D array on a substrate
surface [48]. Recordings from neuronal networks cultured on these microelec-
trode arrays exhibit highly complex spatially and temporally distributed signal
patterns that are highly sensitive to their environment. Recent work has focused
on using such systems for chemical and environmental sensing and for phar-
maceutical screening [32, 97]. It should be noted that for networks of cells,
in addition to cell-to-chip coupling, cell-to-cell communication is involved in
generating the complex signals found at the electrodes.
Electrochemical Communication
Microelectrophysiological electrochemical measurements (i.e., electroanalyt-
ical detection techniques) performed at ultramicroelectrodes (<50
m) are
emerging as a promising avenue for examining and monitoring chemical
dynamics at the single-cell level [53]. These electrodes exhibit fast (milli-
second) response times, high mass sensitivity (zeptomole), small size, large
linear dynamic range (up to 4 orders of magnitude), and selectivity. Further-
more, molecules of interest can be followed without the need for derivitization
as is necessary in fluorescence microscopy. The microelectrodes reported to
date for single-cell analysis typically consist of a linear carbon nanotube bun-
dle surrounded by an insulating layer of glass. Macroscopic carbon bundles
are placed into glass capillary tubes, which are then pulled down to microscale
dimensions (0.5-10
µ
µ
m). Microscopic inspection allows the cleaving of the
pulled capillary in the vicinity of the entrained carbon bundle, providing a
conductive carbon tip, surrounded by an insulating sheath of glass. This micro-
electrode can be placed in close proximity to an extracellular or intracellular
region of interest and used to analyze the local microenvironment for a large
variety of electroactive species. For easily oxidizable substances, such as cat-
echolamines [117], indoleamines [31], oxygen [61], and doxorubicin [64], the
