Analytical Chemistry Chemical Cytometry Quantitates Superoxide

Table 1. Surface Concentration and Film Thickness
Data for NP Films Printed on PPF
1 (5 ± 1) × 10
film
thickness/nm
-10 1.5 ± 0.4
2 (6 ± 1) × 10-10 2.4 ± 0.6
3 (16 ± 3) × 10-10 1.7 ± 0.4
sample ΓNP /mol cm -2
first-scan, along with the corresponding film thicknesses
obtained by AFM depth profiling.
Surface---C6H4-NO2 + 6H + + 6e - f
Surface---C6H4-NH2 + 2H2O (1)
Surface---C6H4-NO2 + 4H + + 4e - f
Surface---C6H4-NHOH + H2O (2)
Surface---C 6 H 4 -NHOH h Surface---C 6 H 4 -NO + 2H + + 2e -
Previously, we have found that single layers of NP groups
electrografted to PPF have ΓNP ) (2.5 ± 0.5) × 10 -10 mol cm -2 , 36
and similar values were obtained for methylphenyl and CP
groups electrografted to flat Au substrates. 41 A monolayer of
vertically oriented NP groups has a calculated thickness of 0.8
nm; 36 hence, both the concentration and thickness data in Table
1 indicate the formation of multilayer domains with an average
thickness of 2 to 3 layers. 18 Table 1 also shows significant
variations between films prepared under the same conditions and
no systematic relationship between ΓNP and average AFMdetermined
film thickness. The initial OCPs differ significantly
between PPF samples, particularly from different preparation
batches, and we attribute the sample-to-sample variations in
Table 1 to this variability of “activity”. We, 37 and others, 37,42,43
have observed that the substrate potential increases as the
spontaneous reduction of aryldiazonium cations proceeds and that
film growth stops when the potential becomes too positive to
sustain reduction. The initial OCP, therefore, influences the
amount of charge that can be transferred before the “cutoff”
potential is reached, and consequently, it helps to determine the
amount of material that can be attached to the surface in the
absence of an externally applied potential.
The lack of a clear relationship between Γ NP and the “average”
film thickness is attributed to differences in the measurement
scales of the associated methods. The surface concentration
data are averages over a large area (0.18 cm 2 ), whereas the
AFM film thicknesses are derived from three depth profiles
across two 10 × 1.25 µm 2 scratches per sample. For films of
highly variable thickness, the average results of a few measurements
performed over a small area may not yield results that
are representative of the whole-sample average. The possibility
of multilayer grafting, coupled with an inhomogeneous distri-
(41) Paulik, M. G.; Brooksby, P. A.; Abell, A. D.; Downard, A. J. J. Phys. Chem.
C 2007, 111, 7808–7815.
(42) Le Floch, F.; Simonato, J.-P.; Bidan, G. Electrochim. Acta 2009, 54, 3078–
3085.
(43) Smith, R. D. L.; Pickup, P. G. Electrochim. Acta 2009, 54, 2305–2311.
7030 Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
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bution of aryldiazonium cations on the inked stamp, is likely
to lead to films of variable thickness, and the large uncertainties
for the AFM thickness values are consistent with this. Clearly,
such variability will be a limitation when highly reproducible
and homogeneous films are required.
Successful modification of PPF after printing with CBD/1 M
H2SO4 ink was confirmed by a decrease of the water contact
angles from 68 ± 2° to 31 ± 2°, consistent with the attachment
of hydrophilic CP groups. In contrast, control surfaces gave
an unchanged postprinting contact angle of 68 ± 11°. Cyclic
voltammograms (not shown) revealed no signals between 0.8
and -0.5 V, confirming the absence of physisorbed CBD
(expected reduction peak at -0.3 V). AFM depth-profiling
measurements on two films gave an average film thickness of
1.0 ± 0.2 nm, close to that expected for a monolayer. Printed
CP films are, thus, on average, thinner and significantly more
uniform than printed NP films. The different morphology can
be attributed to the lower reduction potential for CBD. Because
CBD is more difficult to reduce than NBD, its reduction ceases
at a lower substrate OCP and a correspondingly smaller amount
of CP is grafted to the surface. The more limited degree of
spontaneous reduction diminishes the prevalence of significant
multilayer “outgrowths”, and a more uniform film thickness
results. Vautrin-Ul and co-workers have reported results
consistent with this interpretation, 44 finding that spontaneous
reduction of NBD rapidly gave thick, nonuniform films on zinc
but formed thin homogeneous layers more slowly on a lessreducing
nickel surface. Hence, for substrates that act as the
reducing agent for aryldiazonium cation-based grafting, when
the potential driving force for reduction is large (the aryldiazonium
derivatives are easily reduced in comparison with the
reducing power of the substrate), thicker and more irregular
films will be formed than when the driving force is lower.
The feasibility of printing using other aryldiazonium salt/
substrate combinations was also examined in experiments described
in the Supporting Information. Electrochemical (Figures
S-1, S-2) or AFM depth-profiling measurements confirmed that
printing of ABD on PPF, NBD on Au, and NBD on Si gave the
expected modified surfaces. For Si samples, the oxide layer was
removed or significantly thinned by HF treatment prior to printing.
Samples with an intact native oxide layer could not be modified.
To summarize, we expect printing to be successful for all
aryldiazonium salt-substrate combinations for which the grafting
reaction proceeds spontaneously at OCP in solution. As is found
for layers grafted from solution, the stability of attachment will
depend mainly on the substrate; for example, very stable layers
are formed by grafting onto graphitic carbon, 2,15 but the stability
of the layers on Au is significantly less. 16
Patterned Microcontact Printing of Gold, PPF, Silicon,
and Cu. Having successfully demonstrated that printing with
aqueous aryldiazonium salt inks leads to surface modification, we
next investigated patterning of surfaces. Figures 2-4 show images
of Au, PPF, Si, and Cu substrates printed with aryldiazonium salt
inks and with blank inks.
In Figure 2, SEM images of Au substrates patterned with NP,
aminophenyl (AP), and CP groups are compared with images from
(44) Adenier, A.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson,
J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491–501.