Biomedical Engineering Reference
In-Depth Information
Fig. 1.11
OH
yield
function from (A) a
NaH
2
PO
4
film and (B) a
self-assembled monolayer of
20 base-pair single-stranded
DNA [
68
]. The inset shows
the time dependence of the
OH
signal from the
phosphate film recorded at an
incident electron energy of
8 eV. In the inset, the solid
line is an exponential fit to the
data and the vertical scale is
the same as that in the figure.
(Reprinted with permission
from reference 68. Copyright
2006 Elsevier)
appears in the rectangles on the left of Fig.
1.3
and the corresponding units in a DNA
strand on the right. An example is provided in the case of sodium dihydrophosphate
in this subsection.
To understand LEE-induced damage to the phosphate group in DNA, Pan
and Sanche [
68
] measured the electron-stimulated desorption (ESD) of H
;
O
and OH
anions emitted from thin films of sodium dihydrogen phosphate under
bombardment with 0-19 eV electrons. The yield functions exhibit a single broad
peak with maxima at
eV, respectively, and
a continuous rise above 15 eV. In each curve, the structure was attributed to DEA
and the continuous rise to dipolar dissociation, both causing scission of the O-H,
P
8:8 ˙ 0:3
eV
;8:0˙ 0:3
eV, and
7:3 ˙ 0:3
O and P-O bonds, respectively. Rupture of these bonds was accompanied by
the corresponding desorption of the stable anions H
;
D
O
and OH
. The incident
electron energy dependence of the OH
yields is shown in A of Fig.
1.11
. The yield
function exhibits a single broad peak with a maximum at 7.3 eV. Such a resonant
peak is a typical signature of the DEA process [
10
,
69
], which can be expressed as
e
OH
. The inset shows the time
dependence of the anion signal at an incident electron energy of 8.0 eV. The solid
line in the inset is an exponential fit to the data. From measurements of this time
dependence and those of H
and O
, the effective cross section to damage the
molecule in the film was found to be very high, about
NaH
2
PO
4
C
NaH
2
PO
4
! Œ
! Œ
NaHPO
3
C
10
15
cm
2
[
68
].
Curve B of Fig.
1.11
exhibits the OH
yield function obtained from LEE-
bombardment of a 40-base oligonucleotide (i.e., a short single DNA strand)
chemisorbed on a gold substrate [
70
] and having a thickness similar to that of
the sodium dihydrogen-phosphate film
. The similarity and coincidence in
energy of the results obtained from the short DNA strands and that recorded from
.5
nm
/
