Biomedical Engineering Reference
In-Depth Information
changing the concentration of nanoparticles in solution. For the avidin biosensor,
the sensitivity of the T
2
response increased and dynamic range, or the target con-
centration range over which the biosensor was responsive, decreased at lower
nanoparticle concentrations. Conversely, at higher nanoparticle concentrations the
sensitivity decreased and the dynamic range increased. This observation indicated
that the dynamic range and sensitivity of an assay could be tuned by means of the
nanoparticle concentration [61].
The second method introduced by Taktak
et al.
provided a means for controlling
nanoparticle precipitation. As discussed in the present and subsequent reports,
under certain conditions nanoparticle clusters can become unstable in solution
and precipitate [49, 61, 69]. Precipitation, which leads to an increase in T
2
, is most
often caused by the over-titration of analyte, which leads to extremely large clusters
[61, 69]. This may be detrimental to MRSw measurements because, in this case,
it leads to a different change in T
2
than would be expected for target-induced
clustering. Fortunately, T
1
can be used as an independent marker for particle
precipitation, because T
1
depends only on the total amount of soluble iron in solu-
tion, and not on the clustering state of the nanoparticles [61]. Taktak
et al.
dem-
onstrated that T
1
remained constant when T
2
changed from analyte-induced
nanoparticle clustering, a similar observation to that made for the fi rst MRSw
biosensor [1]. Upon over-titration, both the T
2
and T
1
increased, indicating a loss
of iron from solution and a shift of the assay conditions to outside the linear
response curve. Taktak and coworkers subsequently recommend a workfl ow to
validate that an assay is within the linear response curve by taking both T
2
and T
1
measurements [61]. Although this workfl ow was initially intended for manual
sample preparation, it could be integrated into an automated fl uidic handling
system for applications that require minimal user interaction.
A third method introduced by Taktak
et al.
was to monitor the coeffi cient of
variation between multiple T
2
measurements, and thus to determine if the biosen-
sor solution was within the linear response range, or if the reagents had precipi-
tated or degraded in some way. It was shown that, when three T
2
measurements
were obtained within several minutes, the coeffi cient of variation between mea-
surements was increased dramatically when particle precipitation or instability
had occurred. The utility of this approach was analogous to that of the T
1
measure-
ment system, in that it allowed for an independent verifi cation that the measured
T
2
value could be used to obtain the concentration of target via a calibration curve
[61]. These methods are important when independently validating the integrity of
biosensor measurements.
The methods introduced by Taktak
et al.
were later extended at T2 Biosystems
to demonstrate how splitting a sample between multiple nanoparticle reagent
chambers could provide validated results, and also expand the biosensor dynamic
range [49]. These methods were demonstrated with the same avidin-sensitive
biotinylated MRSw architecture. The addition of avidin led to nanoparticle cluster-
ing and a change in T
2
from 350 ms to 175 ms. The methods introduced by Taktak
et al.
were extended to include a control for variations in the background T
2
,
which can be observed in complex samples such as blood due to variations in the
