Biomedical Engineering Reference
In-Depth Information
poly(propyleneimine) dendrimers containing both ferrocene and cobaltocenium units
(33). Such dendritic macromolecules containing a controlled number of redox-active
organometallic units at the core, within the branches or at the periphery of the dendritic
structure can also be utilized for many practical applications such as catalysts, electron
transfer, ion sensors, or in electronics devices. Ashton et al. (35) also describe a new fam-
ily of globular-shaped fourth-generation dendrimers constructed with 5-hydroxyisoph-
thalic acid and diethanolamine. Both the convergent and divergent methods were used
in the synthesis.
Recently, dendrimers have been adsorbed onto gold substrates (36). These layers
revealed a high mechanical stability and can be functionalized without any loss of den-
drimers from the surface. The large area of dendrimers allows an increase in the number
of immobilized biomolecules, which can lead to greater sensitivities. Multiarrayed
enzyme films composed of a GOD) and of fourth-generation dendrimers have been
reported (G4) (37). Further improvement in the sensitivity of this biosensor has been
obtained by modification of the dendrimer molecules with ferrocene carboxaldehyde (37).
The biosensors showed good stability, and a 32% modification of the surface amines on the
dendrimers to ferrocenyls was found to be optimum. Yang et al. (38) report on a novel
route to prepare bioreactive surfaces on gold by the self-assembly of third-generation
hydroxyl-terminated dendron thiols (G3-OH) and subsequent bridging reactions using
generation two amine-terminated dendrimers (G2-NH
2
). It has been shown that G3-OH
dendron thiols form a stable and uniform self-assembled monolayer on gold and have
potential applications for DNA microarrays and protein chips.
Chang et al. (39) describe the detection of live
Pseudomonas aeruginosa
using a sensing
film containing a fourth-generation hydroxy-terminated polyamidoamine (PAMAM) den-
drimer (i.e., G4-OH) and SYTOX Green fluorescent nucleic acid stain. The films were pre-
pared on disposable plastic and placed on optical fibers. In the presence of PAMAM-OH
(G4-OH) in water, the bacterial cell becomes permeable to the SYTOX dye and the fluo-
rescence is significantly enhanced. The fluorescence increased with the bacteria concen-
tration and the intensity was greater compared to controls. The dendrimers stabilized the
sensing film. After drying and desiccation, the SYTOX Green/PAMALM-OH films were
still able to quantitatively detect
P. aeruginosa
in water. It is evident that dendrimers offer
a great potential for the design of “smart” biosensing layers and this area continues to be
of great interest for biosensors.
7.1.7
Nanoparticles and Microspheres
It is interesting to note that silicon technology has revolutionized the concepts of
intelligent or smart sensing, since the sensing technology and detection circuits can be
incorporated on single chips. Sensing technologies based on radiation or light (40),
mechanical (41), thermal signals (42,43), magnetic signals (44), or chemical signal detection
(45) have been previously reported. Silicon-based MOSFET devices have been applied for
chemical detection and as humidity sensors (46). Recently, Nather et al. (47) have
described pH- and penicillin-sensitive electrolyte-insulator-semiconductor sensors
(At/Si/SiO
2
/Ta
2
O
5
and penicillinase), which were integrated into a commercial flow-
injection analysis system. The FIA set-up contained miniature flow-through cells on sili-
con wafers. High throughput, minimum dispersion, fluid consumption, and sensitivity
and reproducibility of response were reported by the authors. One disadvantage of using
silicon-based integrated systems in biosensors is the corrosive environment they may have
to operate in leading to poor lifetimes. Several challenges that still need to be overcome in
the design of smart integrated biosensors include improving the signal-to-noise ratio;
