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Microfluidics and Nanofluidics (2018) 22:63
https://doi.org/10.1007/s10404-018-2085-x
RESEARCH PAPER
A droplet-based microfluidic platform for rapid immobilization
of quantum dots on individual magnetic microbeads
Thu H. Nguyen
1
· Xiaoming Chen
1
· Abootaleb Sedighi
2
· Ulrich J. Krull
2
· Carolyn L. Ren
1
Received: 13 February 2018 / Accepted: 30 May 2018
© Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract
Quantum dots (QDs) provide opportunities for the development of bioassays, biosensors, and drug delivery strategies.
Decoration of the surface of QDs offers unique functions such as resistance to non-specific adsorption, selective binding
to target molecules, and cellular uptake. The quality of decoration has substantial impact on the functionality of modified
QDs. Single-phase microfluidic devices have been demonstrated for decorating QDs with biological molecules. The device
substrate can serve as a solid-phase reaction platform, with a limitation being difficulty in the realization of reproducible
decoration at high density of coverage of QDs. Magnetic beads (MBs) have been explored as an alternative form of solid-
phase reaction platform for decorating QDs. As one example, controlled decoration to achieve unusually high density can
be realized by first coating MBs with QDs, followed by the addition of molecules such as DNA oligonucleotides. Uniform-
ity and high density of coatings on QDs have been obtained using MBs for solid-phase reactions in bulk solution, with the
further advantage that the MBs offer simplification of procedural steps such as purification. This study explores the use of a
droplet microfluidic platform to achieve solid-phase decoration of MBs with QDs, offering control of local reaction condi-
tions beyond that available in bulk solution reactions. A microchannel network with a two-junction in-series configuration
was designed and optimized to co-encapsulate one single 1 µm MB and many QDs into individual droplets. The microdroplet
became the reaction vessel, and enhanced conjugation through the confined environment and fast mixing. A high density of
QDs was coated onto the surface of single MB even when using a low concentration of QDs. This approach quickly produced
decorated MBs, and significantly reduced QD waste, ameliorating the need to remove excess QDs. The methodology offers a
degree of precision to control conjugation processes that cannot be attained in bulk synthesis methods. The proposed droplet
microfluidic design can be widely adopted for nanomaterial synthesis using solid-phase assays.
Keywords Droplet microfluidics · Quantum dots · Electrostatic association · Magnetic beads
1 Introduction
In the last two decades, a promising nanomaterial, known
as a semiconductor nanoparticle or quantum dot (QD), has
demonstrated tremendous potential for enhancing imaging.
Examples include applications such as fixed cell labeling,
imaging of live cell dynamics, sensing, and in vivo animal
imaging (Bruchez et al. 1998; Zhang et al. 2005; Xing and
Rao 2008; Algar et al. 2010; Sun and Gang 2013; Chou et al.
2014; He et al. 2014; Cao et al. 2016). QDs are chemically
synthesized semiconductor nanocrystals with diameters of
a few nanometers (2–10 nm) and quantum yields that rou-
tinely approach 50–80%. They are 100 times more resistant
to photobleaching than molecular fluorescent dyes (Resch-
Genger et al. 2008).
Electronic supplementary material The online version of this
article (https ://doi.org/10.1007/s1040 4-018-2085-x) contains
supplementary material, which is available to authorized users.
* Carolyn L. Ren
c3ren@uwaterloo.ca
1
Department of Mechanical and Mechatronics Engineering,
University of Waterloo, 200 University Avenue West,
Waterloo, ON, Canada
2
Department of Chemical and Physical Sciences, University
of Toronto Mississauga, 3359 Mississauga Road,
Mississauga, ON, Canada
@Phil_Robichaud
@deepthiw
@JoseServera