Biomedical Engineering Reference
In-Depth Information
The second method used for the synthesis of silicon nanoparticles consists of a
laser ablation of a silicon wafer in water by using a titane-sapphire laser (Rioux
et al.
2009
). The exposure of a silicon wafer to a femtosecond radiation for 30 min
led to the formation of nanoparticles of mean size around 2.4 nm. The suspension
was stable for months in water. Irradiation of the suspension at 266 or 523 nm led
to the observation of
1
O
2
luminescence. The
1
O
2
production rate was estimated as
10% of the rate of photofrin. Si nanoparticles were also subjected to a 532 nm
pulsed laser for 1 h at 200 mW/cm
2
. No signs of photobleaching were observed
indicating a higher stability than conventional organic photosensitizers.
5
Carbon Nanotubes
Single (SWNTs) or multiwalled carbon nanotubes (MWNTs) can be used as deliv-
ery agents for PDT photosensitizers. Carbon nanotubes present several advantages:
it has been showed that they are internalized by mammalian cells (Pantarotto et al.
2004
) through endocytosis (Kam et al.
2004
) even if other mechanisms can occur
(Kostarelos et al.
2007
), they are excreted from the body rapidly and present no
significant cytotoxicity (Liu et al.
2008c
), they can be modified chemically (Hirsch
2002
; Qu et al.
2002
) but non covalent associations are also possible (Chen et al.
2001
). The combination of carbon nanotubes to organic molecules can lead to the
formation of unique composites with enhanced chemical and/or physical properties
(Niyogi et al.
2002
). SWNT have been also used as efficient fluorescence quench-
ing probes (Chen et al.
2003
; Kam et al.
2004
).
Zhu et al. (
2008
) have designed a new kind of
1
O
2
production probe by linking
non-covalently a single strand DNA aptamer - chlorin e6 PS moiety (Fig.
31
) with
a single-walled carbon nanotubes (SWNT) as it has already been described in a
recent review (Verhille et al.
In Press
). Zhu et al. supposed that they could be good
1
O
2
quenchers. Meanwhile, interactions of SWNTs with biomolecules, such as
proteins and DNA, have been intensively studied and applied to biosensing and as
intracellular transporters. This indicates SWNTs may also protect DNA probes
from digestion by nuclease. Aptamers are synthetic DNA/RNA probes that can
recognize and bind their targets with high affinity and specificity (Ellington and
Szostak
1990
; Tuerk and Gold
1990
). Thrombin aptamers have been widely inves-
tigated for the past decade. The aptamer coupled to a chlorin e6 (Fig.
31
) was able
to perform
1
O
2
production upon PS irradiation. Due to the p-stacking between the
aptamer and SWNT, the energy transfer occurs between the PS and the SWNT,
leading to 98%
1
O
2
quenching. DNA aptamers are good linkers to biological tar-
gets; they can bind biomolecules so strongly that the p-stacking, occurring between
the SWNT and the photodynamic probe breaks off, releasing the chlorin from the
1
O
2
quencher. This was illustrated by the chlorin e6 fluorescence which increased
up to 20-fold after the addition of 2.0 mM thrombin.
1
O
2
production was restored
when the construct probe was put in presence of thrombin and then irradiated at
chlorin e6 maximum absorption wavelength (404 nm). The high specificity of
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