Scanning Electrochemical Microscopy Imaging of DNA Arrays for High Throughput Analysis Applications - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 126

Here, I T ins and I T cond are given in Eqs. (2) and (3), respectively.
The parameter N = k eff r T / D , is the dimensionless rate constant de-
rived from the first-order rate constant k eff , the UME radius r T and
the diffusion constant D of the mediator. For the particular
DNA-electrode system, this type of estimation provided more fa-
vorable results leading to a rough estimation of N = 1.8 as shown
in Fig. 8 .

2. Examples of Negative Feedback Mode Imaging

As previously pointed out, 6 the first SECM imaging of DNA in-
volved a scanning tunneling microscopy (STM) experiment that
took advantage of the lateral conductivity in surface-adsorbed wa-
ter layers. 35 Later, STM experiments towards DNA self-assembly
were extended to an electrochemical-mode of imaging. 36 Con-
versely, in an early stage of DNA array development, SECM was
used to examine such a device, including the detection of hybridi-
zation. Takenaka et al. reported SECM imaging of spotted-type
DNA microarrays by a combined use of an electrochemical-
ly-active, DNA binding molecule (intercalator) and a mediator
ion. 37 Wang and Zhou reported SECM imaging of poly[G] and
calf-thymus DNA by using a ruthenium bipyridine complex,
Ru(bpy) 3 3+/2+ , as both a mediator for SECM imaging and simulta-
neously, an oxidizing agent of the guanine bases found in the
DNA. 38 However, these achievements suffered from either lower
detection sensitivity compared with fluorometry, or one-way use of
the sample due to the irreversible nature of the guanine oxidation.
As described below, progress has been made in this area.
Schuhmann's group developed an electrostatic approach to
visualizing the surface bound DNA dots including their hybridiza-
tion response ( Fig. 9 ). 39,40 This group used Fe(CN) 6 3 Ѹ as a media-
tor, which can report changes in the surface-charge status. The
sample DNA was a 20-mer synthetic oligonucleotide and the DNA
molecules were attached as 150-μm-diameter dots through stand-
ard 5'-modified chemisorption on gold-covered glass slides. Over
the blank areas pretreated with propanethiol, the particular SECM
confirmation gave responses in the positive feedback mode. In
contrast, the tip current decreased considerably over the DNA