Anal. Chem. 2001, 73, 3993-3998
Microscopic Imaging with Electrogenerated Chemiluminescence Russell G. Maus and R. Mark Wightman*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290
The use of electrogenerated chemiluminescence (ECL) at microelectrodes as a light source for scanning optical microscopy is demonstrated. Cone-shaped microelectrodes were constructed by flame etching carbon fibers to a fine point. ECL generated in solution at such electrodes was forced to the apex of the conducting surface by using high-frequency (20-kHz) potential pulses and high concentrations of ECL reagents in the solution. ECL arose from the reaction of 9,10-diphenylanthracene radical cation with the radical anion of benzonitrile, the solvent. The electrode/light source was raster-scanned a finite distance above the sample surface, and images were generated with standard scanning probe software by collecting the transmitted light with a microscope objective. These images compared favorably to optical images of the same sample. A resolution of ∼600 nm was achieved with this arrangement even though a feedback loop was not employed to control the tip distance from the sample. The source was sufficiently bright (1.82 pW) that well-defined transmittance spectra could be obtained at individual locations on the sample. Electrogenerated chemiluminescence (ECL) arises from excited electronic-state species generated near the surface of electrodes.1 Previously, we demonstrated a very bright ECL system using 9,10-diphenylanthracene (DPA) and benzonitrile (BN) suitable for use at microelectrodes.2 During each step of a potential square wave, heterogeneous electron transfer, either oxidation or reduction, occurs at an electrode (reactions 1 and 2), creating radical cations and anions. Once created, these
BN + e- f BN-•
(1)
DPA - e- f DPA+•
(2)
+•
DPA
+ BN
-•
1
f DPA* + BN
1
DPA* f DPA + hν
(3) (4)
radicals diffuse from the electrode toward the previously generated species and undergo electron transfer. The product is an emissive excited state (reactions 3 and 4). The reaction involves the regeneration of the starting species via the creation of an excited(1) Knight, A. W. Trends Anal.Chem. 1999, 18 (1), 47-62. (2) Wightman, R. M.; Curtis, C. L.; Flowers, P. A.; Maus, R. G.; McDonald, E. M. J. Phys. Chem. 1998, 102, 49, 9991-6. 10.1021/ac010128e CCC: $20.00 Published on Web 07/19/2001
© 2001 American Chemical Society
state species and ground-state species. These are appropriately positioned to be reelectrolyzed. Because the photons from the relaxation of the excited-state species originate near the electrode surface, ECL has often been used as a tool for imaging.3,4 It has been used, for example, to visualize the electroactive area of very small electrodes.2,5 The resolution in such experiments is limited by the distance the reagents diffuse before reaction. This can be controlled by employing reagents with short lifetimes or by the use of transient generation pulses. Imaging of samples can also be obtained by the generation of ECL with an ultramicroelectrode that is scanned over the sample surface.6 Recently, near-field images were generated using ECL from the Ru(bpy)32+/tripropylamine system.7 In addition to the factors important in electrode imaging, the resolution possible with a scanning approach to imaging is governed by two factors. First, the size of the light source limits the absolute resolution that can be achieved. A 1-µm light source, for example, would be capable of scanned imaging at a resolution of, at best, 1 µm. Second, the distance between the light source and the surface to be imaged must be minimized to reduce diffraction of the light as it traverses this distance.8 Creating a small (
