Analytical Chemistry Chemical Cytometry Quantitates Superoxide

Figure 2. SEM images of Au substrates patterned by printing with
(a) NBD/1 M H2SO4, (b)1MH2SO4, (c) ABD/0.5 M HCl, (d) 0.5 M
HCl, and (e) CBD/1 M H2SO4 inks.
the corresponding controls. The aryldiazonium salt inks give
clearly defined patterns with feature sizes down to 20 µm, whereas
the blank inks give faint patterns, which are attributed to PDMS
residues.
SEM is not a good technique for imaging organic films on
carbon substrates. Consequently, the image contrast for PPF
patterned with NP (Figure 3a) is just marginally better than that
for the control sample (Figure 3b). To overcome this inherent
limitation, alternative methods were used to image surfaces
patterned with AP and CP groups. For the former, after printing
with ABD/0.5 M HCl (or blank 0.5 M HCl), the surfaces were
immersed for 40 min, at pH ∼ 5, in a solution of citrate-capped
Au nanoparticles. Preferential assembly of nanoparticles (via
electrostatic interactions) on the AP-modified areas clearly revealed
the patterns (compare Figure 3c,d). For surfaces patterned
with CP groups (by printing with CBD/1 M H2SO4 ink) and the
corresponding blanks, condensation figures were imaged by
optical microscopy (Figures 3e,f). Although the pattern is welldefined
in both the CP-printed sample and the blank, the relative
sizes of water droplets in the stamped and “bare” areas are the
opposite in the sample and blank. Water droplets are larger in
the CP areas than on bare PPF, consistent with addition of
hydrophilic CP groups to the surface; in contrast, areas contacted
by the stamp inked with 1MH2SO4 only are smaller than on
bare PPF, suggesting hydrophobic contaminants have been
transferred to the surface by the stamp. (Note that the sizes
of water droplets on bare PPF in Figures 3e,f are not the same
because the size depends on the extent of evaporation prior to
image capture.)
Figure 3. SEM images (a-d) and optical micrographs (e, f) of PPF
surfaces printed with (a) NBD/1 M H2SO4, (b, f) 1 M H2SO4, (c) 20
mM ABD/0.5 M HCl, (d) 0.5 M HCl, and (e) CBD/1 M H2SO4 inks.
The surfaces shown in (c) and (d) were immersed in Au nanoparticle
solution for 40 min before imaging; surfaces shown in (e) and (f) were
treated with water vapor before imaging.
Patterning of NP and AP groups on Si was also successful
as revealed by the SEM images of Figure 4a-d. There is strong
contrast between the grafted and bare areas in Figure 4a,c, in
comparison with the faint patterns of the controls (Figure
4b,d).
As a final example to demonstrate the wider applicability of
MCP with aqueous aryldiazonium salt inks, a Cu surface was
patterned with NP groups. Copper is known to react spontaneously
with NBD at OCP in both aqueous and nonaqueous conditions. 38,45
The SEM image in Figure 4e shows strong contrast between NPprinted
areas and bare Cu, whereas only a very faint pattern is
seen on the control (Figure 4f). The roughness of the Cu surface,
evident in both images, leads to incomplete contact with the stamp
and accounts for the “patchy” appearance of the pattern in Figure
4e.
These examples confirm that MCP is a very simple route to
patterning conducting surfaces. The scope of the method and the
characteristics of the patterned layers will be determined by the
substrate-diazonium salt combination as described in the previous
section.
Printed Tether Layers for Further Immobilization Chemistry.
The utility of MCP can be enhanced by printing layers that
act as tethers for further immobilization reactions. Two examples
are demonstrated here, on the basis of coupling of secondary
reagents (4-nitroaniline (NA) and SWCNTs) to primary layers of
CP or AP printed on PPF. The selection of these reagents was
based on their ease of detection: NA by its redox chemistry and
SWCNTs by AFM imaging.
(45) Hurley, B. L.; McCreery, R. L. J. Electrochem. Soc. 2004, 151, B252–B259.
Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
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