Biology Reference
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In addition, hybridization of the liposome-labeled detection probe
DNAtotheelectrodeledtoasignificantnegativelychargedelectrode
interface, which repelled the negatively charged [Fe(CN)
6
]
3
-
redox
probe. This increase in resistance to charge transfer to the redox
probe was monitored using FIS resulting in a detection limit of
1.2 pM. The authors also used a biotinylated detection probe DNA
that after binding to target DNA on the electrode surface (see
Fig. 14.15B), was treated with avidin, allowing subsequent capture
of biotinlyated liposomes. This was again followed by treatment
with avidin and biotinylated liposomes forming large aggregates of
liposomes yielding a very high negatively charged surface density.
The detection limitfor this systemwas 50 fM.
In an extension of this work biotin-labeled liposomes were also
modifiedwithhorseradishperoxidase(HRP)viaperiodateoxidation
chemistry [74]. The HRP loaded biotin-labeled liposomes catalyzed
oxidationof4-chloro-1-naphtholinthepresenceofH
2
O
2
yieldingan
insoluble product which precipitated onto and fouled the electrode.
FIS was used to monitor resistance of electron transfer to the
[Fe(CN)
6
]
3
−
redoxproberesultingasimilardetectionlimitof650fM
foraDNAsequencerelevanttoTay-Sachsdisorder.Theauthorsalso
extended these various approaches to probe and amplify the signal
from single-basemismatches in analyte DNA [75].
Loading the aqueous interior of liposomes with electroactive
molecules has also been reported. Liposomes prepared with
cholesterol-labeled detection probe DNA and loaded with
[Fe(CN)
6
]
3
−
were used in a magnetic bead based sandwich assay
in a glass-chip PDMS microfluidic device [76]. Collected by the
magnet upstream of an interdigitated ultramicroelectrode array
(IDUA), the liposomes were lysed by addition of detergent. The
released[Fe(CN)
6
]
3
−
wassubsequentlydetectedatthedownstream
IDUA. The assay took less than 30 minutes to perform, including
hybridization time, and could detect 1 fmol DNA. This electroactive
liposome magnetic bead-based sandwich assay was also recently
used by the authors to detect the mRNA amplified from a single
oocyst (an immature ovum) within a PMMA biosensor [77].
Liposomes functionalized with reporter DNA and loaded with
[Ru(NH
3
)
6
]
3
+
werealsorecentlyreportedforeffectiveelectrochem-
ical DNA detection [78]. Used in a competitive assay format on Au
