1138
doi: 10.1017/S1431927609095178
Microsc Microanal 15(Suppl 2), 2009
Copyright 2009 Microscopy Society of America
Single-Molecule Electron Transfer Reaction in Nanomaterials
Dehong Hu,* Chenghong Lei,* and Eric J. Ackerman*
*Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352
Interfacial electron transfer processes play an important role in many chemical and biological
processes.[1] However, interfacial electron transfer processes are usually very complex due to high
dependence on its local environments.[2][3] Here we report the study of single molecule electron
transfer dynamics by coupling fluorescence microscopy at a conventional electrochemical cell. The
single-molecule fluorescence spectroelectrochemistry of cresyl violet in aqueous solution and on
nanoparticle surface were studied. We observed that the single-molecule fluorescence intensity of
cresyl violet was modulated synchronously with the cyclic voltammetric potential scanning. We
attribute the fluorescence intensity change of single cresyl violet molecules to the electron transfer
reaction driven by the electrochemical potential.
In this work, an electrochemical cell was designed to allow cyclic voltammetry measurement and
fluorescence microscopy measurement simultaneously. We first studied the cyclic voltammetry of
36 M cresyl violet solution and fluorescence intensity on this sample cell. Cresyl violet underwent
quasi-reversible single step reduction/oxidation reaction. The fluorescence intensities of the reduced
and oxidized states of cresyl violet were measured by a fluorescence microscope. The fluorescence
intensity and electrochemical potential were recorded synchronously. Fig. 1 shows that the
fluorescence intensity level increases and decreases with respect to ramping up and down the
voltage. This result suggests that the oxidized state of cresyl violet emits strong fluorescence and its
reduced state yields very weak or no fluorescence.
To examine the electrochemical dynamics of single molecules, we conducted cyclic voltammetrysingle molecule fluorescence experiments of cresyl violet adsorbed on nanoclay surface. The
topography of nanoclay imaged by AFM was shown in Fig. 2. Then 120pM cresyl violet solution
was used to incubate the surface. The buffer solution washed away un-adsorbed cresyl violet
molecules. The fluorescence of adsorbed cresyl violet molecules were imaged by a confocal
microscope. (Fig. 3) The image demonstrates that there were many cresyl violet molecules
immobilized on the clay-modified surface for a long period (longer than seconds). Then the laser
was focused on one molecule at a time. The fluorescence intensity of single molecules during
electrochemical potential modulation was recorded. (Fig. 4) The long trajectories permit detailed
statistical analyses of the on-off emission intensity of single molecules. By statistical analysis of
multiple molecules, we conclude that the on-off emission intensity blinking is related to the
electrochemical potential. Each emission on-off event is mainly due to the redox chemical reactions.
The reduction and oxidation reaction rates were derived from the on-off emission periods and were
in agreement with the ensemble averaged electrochemical measurement.
References
[1] P. F. Barbara et al., J. Phys. Chem. 100 (1996) 13148
[2] M. W. Holman et al., J. Am. Chem. Soc. 125 (2003) 12649
[3] V. Biju et al., J. Am. Chem. Soc. 126 (2004) 9374
[4] This work was supported by a grant from DOE BES. DH and CL contributed equally.
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https://doi.org/10.1017/S1431927609095178
1139
Microsc Microanal 15(Suppl 2), 2009
E (V)
0.5
30 nm
0
Flu. Inten (arb unit)
I (A)
-0.5
10
0
-10
-20
-30
5
4
3
2
1
0
0
5
10
15
20
25
200nm
30
Time (s)
0 nm
Fig. 2 AFM topography image of nanoclay on
ITO glass
E (V)
Fig. 1 Ensemble averaged fluorescence
spectroelectrochemistry of 36 M cresyl violet.
(Top) The voltage vs. time plot; (Middle) the
current vs. time plot; (Bottom) The fluorescence
intensity vs. time plot.
0
-0.2
-0.4
-0.6
Int. (Cts)
1000
500
0
0
1m
Fig. 3 The single molecule fluorescence image
of cresyl violet adsorbed on clay.
10
20
30
Time (s)
40
50
Fig. 4 Voltage E (V) vs. time plot of three
cyclic voltammetry scans (top) and the plot of
fluorescence intensity of a single cresyl violet
molecules adsorbed on SM/ITO vs. time
during the scans (bottom).
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https://doi.org/10.1017/S1431927609095178