.
Angewandte
Communications
DOI: 10.1002/anie.201307232
Intracellular Localization
Hot Paper
Lactose as a “Trojan Horse” for Quantum Dot Cell Transport**
David Benito-Alifonso, Shirley Tremel, Bo Hou, Harriet Lockyear, Judith Mantell,
David J. Fermin, Paul Verkade, Monica Berry,* and M. Carmen Galan*
Abstract: A series of glycan-coated quantum dots were
prepared to probe the effect of glycan presentation in intra-
cellular localization in HeLa and SV40 epithelial cells. We
show that glycan density mostly impacts on cell toxicity,
whereas glycan type affects the cell uptake and intracellular
localization. Moreover, we show that lactose can act as
a “Trojan horse” on bi-functionalized QDs to help intra-
cellular delivery of other non-internalizable glycan moieties
and largely avoid the endosomal/lysosomal degradative path-
way.
the cell membrane unaided. These bifunctionalized QDs
escape the endosomal pathway and experience a different
intracellular fate that is dependent on the glycan pattern and
cell type.
Luminescent semiconductors, quantum dots (QDs), have
emerged as a versatile class of non-isotopic detection labels
suitable for live cells, in vivo imaging and immunoassays.[3]
Among the many advantages of quantum dots are their
narrow emission spectra and common excitation, a photo-
stability superior to organic fluorophores, their electron
density, and their bright visible emission. QDs are inherently
electron dense and therefore ideal cellular markers for
correlative light electron microscopy (CLEM).[4]
Carbohydrate–lectin recognition processes are mediated
by multivalent interactions that help achieve higher affinity,
as well as higher specificity.[5] Glycan-coated QDs provide
a powerful tool to screen for protein–carbohydrate interac-
tions, and consequently for the identification of carbohydrate
receptors or ligands associated with intercellular recognition
processes,[6] and in our case, for a glimpse at these processes.
To use glyco-QDs effectively in biomedical applications, it
is of the utmost importance to evaluate the parameters that
control particle stability in physiological media and the effect
that specific capping groups, that is, glycan type and glycan
surface density, have on particle cellular uptake, localization,
and toxicity.
Active cellular internalization is largely dependent on the
inorganic core composition of the particles, size, organic shell
used for glycan conjugation, and the type of glycan and cell
environment (culture conditions).[7] Functionalization with
mono- and oligosaccharides has been used to facilitate
cellular uptake of nanoparticles of different core composition
and linker coating in a variety of cell lines.[2–6,7b,e,8] However,
little attention has been paid to the effects of glycan type and
glycan surface density on cellular uptake, localization, and
toxicity in the short term (hours to days).
T
he ability to track functional biomolecules within the cell is
essential to understanding complex cellular processes. The
last few decades have seen an explosion of research in the
area of nanotechnology applied to biology.[1] Nanomaterials
with novel optical, electronic, and surface properties, as well
as size, geometry, distribution, and surface functionality have
become useful platforms for studying biological processes.[2]
Herein, we report a simple and convenient synthesis of sugar-
coated PEGylated CdSe/ZnS QDs with varying carbohydrate
types and surface density that were used to study the effect of
glycan type and presentation on cellular uptake and intra-
cellular localization. We show that these biophysical param-
eters are highly dependent on the type of sugar coating,
whereas carbohydrate surface density has an impact on
toxicity to cells. Moreover, we show that lactose can be used
as a “Trojan horse” on bifunctionalized QDs to help internal-
ize sugars, such as mannose and maltotriose, that do not cross
[*] Dr. D. Benito-Alifonso, S. Tremel, B. Hou, H. Lockyear,
Prof. D. J. Fermin, Dr. M. C. Galan
School of Chemistry, University of Bristol
Cantock’s Close, Bristol BS8 1TS (UK)
E-mail: M.C.Galan@bristol.ac.uk
Dr. M. Berry
School of Physics, University of Bristol
NSQI, Tyndall Ave, Bristol BS8 1F (UK)
E-mail: mon.berry@bristol.ac.uk
To that end, monodispersed lipophilic CdSe/ZnS nano-
particles coated with trioctylphosphine oxide were prepared
following literature procedures[9] (see the Supporting Infor-
mation for general experimental procedures) and QDs with
two different core sizes (2.7 Æ 0.2 nm and 4.0 Æ 0.4 nm) were
obtained. PEG-terminated dihydrolipoic acid (DHLA-PEG)
linkers with a bidentate thiol motif to provide enhanced
affinity for CdSe/ZnS core–shell QDs,[10] and either a hydroxy
group (as a spacer; 2) or an acid group (for sugar attachment;
3) were prepared (Scheme 1). Ligand exchange under reduc-
tive conditions with different ratios of HO-DHLA-PEG and
HOOC-DHLA-PEG linkers 2 and 3 produced water-soluble
QDs 5a–d. Similarly, QDs fully coated with mercaptoacetic
acid (1) were prepared (MAA-QDs; 4). The QDs were
Dr. J. Mantell, Dr. P. Verkade
Wolfson Bioimaging Facility, School of Biochemistry and Physiology
& Pharmacology, Medical Sciences Building, University of Bristol
University Walk, Bristol BS8 1TD (UK)
[**] We gratefully acknowledge financial support from EPSRC grant
number EP/J002542/1. We also thank Dr. M. Crump for help with
high field NMR analysis of QDs.
Supporting information for this article is available on the WWW
ꢀ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
810
ꢀ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 810 –814