Micellar and vesicular nanoassemblies of triazole-based amphiphilic probes triggered by mercury(II) ions in a 100% aqueous medium

Article abstract of DOI:10.1039/c4cc06742h

ABA-type amphiphiles bearing a triazole-based aromatic block were easily synthesized using click chemistry, which act as fluorescent turn-off Hg²⁺-chemoprobes in an aqueous solution. Interestingly, the metal-binding process of amphiphiles induc

Full text of DOI:10.1039/c4cc06742h

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Micellar and vesicular nanoassemblies of
triazole-based amphiphilic probes triggered by
mercury(II) ions in a 100% aqueous medium†
Cite this: Chem. Commun., 2014,
50, 14006
Received 27th August 2014,
Accepted 22nd September 2014
Inhye Kim,a Na-Eun Lee,a Yoo-Jeong Jeong,a Young-Ho Chung,b Byoung-Ki Cho*c
and Eunji Lee*ab
DOI: 10.1039/c4cc06742h
www.rsc.org/chemcomm
ABA-type amphiphiles bearing a triazole-based aromatic block were
‘‘Click chemistry’’ has received much interest in green-
easily synthesized using click chemistry, which act as fluorescent chemistry and biomedicine6 due to its biocompatibility, syn-
turn-off Hg2+–chemoprobes in an aqueous solution. Interestingly, thetic simplicity, and mild reaction conditions, which results in
the metal-binding process of amphiphiles induced nanoassemblies fluorescent 1,4-disubstituted 1,2,3-triazole derivatives.7 This
even below the CMCs, and the binding stoichiometry affected the click-derived triazole moiety has been known to play a role as
morphologies of the resultant nanostructures.
a ligand offering several donor sites for metal coordination8 as
well as a part of a conjugated fluorophore providing a significant
Stimuli-responsive nanomaterials alter their physical properties number of chemosensors.9 Therefore, one can envision that the
according to a small change in the environment. These systems design of emissive amphiphiles mediated by click chemistry
have gained increasing attention in a variety of applications would provide a facile route for the fabrication of aqueous
such as catalysis, sensors, drug delivery, tissue engineering, and nanostructured probes in response to metal ions.
diagnostics.1 Among the molecular scaffolds, the fluorophore-
Meanwhile, the development of fluorescent chemosensors for
conjugated molecules, in response to specific metal ions, have selective detection of Hg2+ in the human body and the environment
contributed to the development of chemosensors possessing is of paramount importance because Hg2+ is toxic and hazardous to
fluorescence ‘‘on-off’’ signals which are attributed to the charge several species, causing disorders and damage to brain, the central
transfer, the photo-induced energy transfer (PET) process and nervous system, the endocrine system and kidney10 as well as
excimer formation.2 However, most of them have been designed bioaccumulation in the human body.11 However, the design of
on the basis of small organic fluorophores, and the signaling aqueous probes is still challenging due to the strong hydration
events have occurred at a single molecular level. Furthermore, ability of Hg2+ in an aqueous medium.12
they could not form nanoaggregates in an aqueous solution due
To this end, we designed ABA-type amphiphilic molecules
to low solubility,3 so it is hard to achieve easy processability of based on triazole-based aromatic segments and hydrophilic
aqueous metal ion sensors for a variety of nanomaterial-based tri(ethylene oxide) peripheries (Scheme 1), and investigated their
applications. In this context, aromatic amphiphiles containing assembly behaviour and optical response to various metal ions
an active block for metal ion binding can be considered as in 100% aqueous solution. The subtle change in the aromatic
aqueous fluorescent nanoprobes that can overcome the limits of segment caused significantly different aggregation (Fig. 1). In
single molecular systems.4 This is because amphiphiles based particular, a higher binding affinity of amphiphiles to Hg2+ was
on the emissive aromatic unit and flexible coil can self-assemble observed, and it reinforced the formation of well-defined nano-
into various core–shell nanostructures, including spheres, cylinders aggregates (even below their critical micellar concentrations (CMC))
and vesicles.5
accompanied by a fluorescence ‘‘turn-off’’. In this communication,
a Graduate School of Analytical Science and Technology,
Chungnam National University, Daejeon 305-764, Republic of Korea.
E-mail: eunjilee@cnu.ac.kr; Fax: +82 42 821 8541; Tel: +82 42 821 8548
b Division of Life Science, Korea Basic Science Institute, Daejeon 305-806,
Republic of Korea
c Department of Chemisty, Dankook University, Gyeonggi-Do 448-701,
Republic of Korea. E-mail: chobk@dankook.ac.kr
† Electronic supplementary information (ESI) available: Detailed synthetic and
experimental procedures. See DOI: 10.1039/c4cc06742h
Scheme 1 Molecular structure of ABA-type amphiphilic molecules 1, 2,
and 2N; the aromatic moiety is denoted as Ar.
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Considering the extended molecular length of 2N (ca. 4.8 nm,
Fig. S3a, ESI†), the evaluated wall thickness of B5 nm indicates
that the vesicle membrane consists of a monolayer packing
arrangement of 2N. The distinct aggregation ability between the
amphiphiles can be explained by a hydrophilic/hydrophobic
balance between the oligoether dendron and hydrophobic aro-
matic blocks.18 The aromatic block of 1 may be too short to form
aggregates so the increase in the number of aromatic rings
(2 and 2N) induced the self-organization of amphiphiles through
enhanced p-stacking. Molecules 2 and 2N formed the aggregates
to avoid unfavorable contact between the hydrophobic block and
water. However, 2 still has tenuous assembly capability, resulting
in the dimeric aggregates as shown in the DLS data (Fig. S4,
ESI†). Only 2N with enhanced coplanarity of the aromatic blocks
could assemble into well-defined nanostructures.
Fig. 1 Schematic representation of the Hg2+-induced nanostructure of 2
and 2N; CMC is defined as the concentration of amphiphiles above which
aggregates form.
the mechanistic aspects of the morphologies of selectively
Hg2+-captured amphiphiles are highlighted.
The metal-binding ability of the amphiphiles was examined
using the various metal ions, including Hg(ClO4)2, CuSO4, ZnSO4,
CaCl2Á2H2O, Pb(NO3)2, Al(NO3)3Á9H2O, and MgSO4 (Fig. 3a and b
and Fig. S5, ESI†). Since molecule 1 did not form any aggregates,
the binding selectivity of all molecules toward metal ions was
investigated at a concentration below CMC (5 mM) for fair
comparison. Overall, the emission of amphiphiles progressively
decreased with an increase in the amount of metal ions, which
indicates the coordination of the metal ion with the triazole
units.19 However, 1 did not show any noticeable binding affinity
(Fig. S5, ESI†). In particular, 2 exhibited a good selectivity to
Hg2+ (Fig. 3a). The suppression in the emission was observed
upon the addition of Hg2+ to the aqueous solution of 2 (ca. 2/5 (a.u.)
quenching upon the addition of 100 equiv. of Hg2+). This is due
to a heavy metal ion effect or a reverse PET (Fig. S6a, ESI†).20
Interestingly, 2N showed a high affinity to Hg2+ as well as
Cu2+.21 The strong quenching of the emission appeared even
upon addition of 1 equiv. of Hg2+ and Cu2+, respectively (ca. 4/5
and 9/10 (a.u.) quenching, Fig. S6b and c, ESI†). By comparing
the degrees of fluorescence quenching of 2 and 2N upon
The aromatic amphiphiles 1, 2, and 2N were prepared by Cu(I)-
catalyzed cycloaddition (click reaction) of an azide-terminated
benzyl ether dendron and diethynyl precursors.13 In an aqueous
solution (20 mM), the emission spectra of all molecules were
shown to be different from each other. Molecule 1 with the
shortest aromatic block showed a weak emission (Fig. S2b,
ESI†). It displayed only a slight quenching of fluorescence at
327 nm (lex = 276 nm) by replacement of chloroform with
water. Amphiphile 2 based on the biphenyl moiety exhibited an
obvious quenching of emission in the aqueous solution, as
compared to the chloroform solution (Fig. S2b, ESI†). This is
due to the energy transfer between the aromatic segments,
which indicates the aggregation of amphiphiles.14 In contrast, the
emission of 2N containing a naphthalene moiety was enhanced,
even though it has a similar conjugated aromatic length to 2
(Fig. S2d, ESI†). The emission enhancement of 2N may be attri-
buted to the conformational restriction by the fused ring structure
of the naphthalene, which is known as the aggregation induced
emission enhancement (AIEE).15
The CMC was determined using a fluorescence spectrometer
using pyrene as a hydrophobic substance (Fig. 2a).16 Amphiphile 1
did not show the CMC. In contrast, the CMCs of 2 and 2N were
calculated to be approximately 9.2 mM and 6.8 mM, respectively.
Interestingly, above the CMC (20 mM), the transmission electron
microscopy (TEM) image of 2 did not show any noticeable
aggregates, while that of 2N displayed well-defined vesicular
nanostructures with a wall thickness of about 5 nm (Fig. 2b),
which could be identified by the contrast between the peri-
pheral and central areas of the aggregates (Fig. 2b, inset).17
Fig. 3 (a and b) The binding selectivity of 2 and 2N toward Hg2+ and other
metal ions. The fluorescence measurements were performed in aqueous
solutions and in the absence and presence of 4.0 equiv. of a metal ion
(inset of (b), fluorescence photographs of a solution of sensors (5 mM) excited
by a commercially available UV lamp (lex = 365 nm)). TEM images of (c) 2–Hg2+
and (d) 2N–Hg2+, respectively (c), non-stained image; (d) negatively-stained
Fig. 2 (a) Determination of the CMCs of 2 and 2N. (b) TEM image showing
the presence of vesicular structures of 2N (yellow arrow in the inset image,
wall thickness).
image; inset of (d), cryo-TEM image of vesicular structures of 2N–Hg2+
,
scale bar = 10 nm.
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addition of the equal equivalent of Hg2+ (Fig. S6, ESI†), it was
concluded that 2N has a higher binding affinity to Hg2+ metal
ions than 2. In detail, the association constants (Ka) of 2 and 2N for
Hg2+ were evaluated to be 1.24 Â 104 MÀ1 and 3.06 Â 105 MÀ2 by a
non-linear fitting curve method, respectively (Fig. S8, ESI†),22,24
indicating that the interaction of 2N with Hg2+ is stronger than 2.
Remarkably, after the formation of the amphiphile–Hg2+
complex, the TEM images of both 2 and 2N showed the formation
of nanoaggregates even though the concentrations of the aqueous
solution were below their CMCs. The 2–Hg2+ and 2N–Hg2+ com-
plexes formed spherical aggregates with diameters of 10–20 nm
and 60–90 nm, respectively (Fig. 3c and d). In the conventional
TEM image of 2–Hg2+, no significant contrast was revealed in the
spherical aggregate, which suggests that the aggregate is not a
vesicle. Instead, the large spheres of 2–Hg2+ seem to be composed
of elemental spherical micelles.23 On the other hand, the aggre-
gates of 2N–Hg2+ displayed the deflated nature resulting from the
evaporation of water (Fig. 3d). The cryogenic-TEM (cryo-TEM)
image of the solution of 2N–Hg2+ in the inset of Fig. 3d clearly
showed that the inner cavity of the spheres is full of water, thus
confirming the vesicular structure.
To elucidate the different molecular assemblies between
2–Hg2+ and 2N–Hg2+, we first investigated the binding stoichio-
metries by Job’s plot method. The stoichiometric ratios of
Fig. 4 Fluorescence Job’s plot for determination of stoichiometries of
2 : Hg2+ and 2N : Hg2+ were determined to be 1 : 1 and 1 : 2, (a) 2–Hg2+ and (b) 2N–Hg2+, respectively. 1H NMR (300 MHz) spectra of
(c) 2 and (d) 2N (5 mM) in the absence (top) and presence (bottom) of 1 equiv.
respectively (Fig. 4a and b). Next, to understand such Hg2+
-
of Hg2+ in D2O. Possible Hg2+-binding modes of (e) 2 and (f) 2N.
binding modes on the molecular level, 1H-NMR experiments
were carried out and the chemical shift changes were examined.
Upon addition of 1.0 equiv. of Hg2+ to the solution of 2, the
In addition, direct evidence for the triazole-based amphiphile–
triazolyl Hc and benzylic Hb were shifted downfield by 0.14 ppm Hg2+ complexes could be obtained from Fourier transform infra-
and 0.06 ppm, respectively (Fig. 4c). These changes imply that red spectroscopy (FTIR) experiments.25 After the addition of Hg2+
Hg2+ was bound to a triazolyl nitrogen.23 Similarly, the triazolyl to the aqueous solution of 2 and 2N, the Hg–N stretching vibration
Hc and benzylic Hb of 2N were shifted downfield by 0.18 ppm band appeared at 587 cmÀ1 (Fig. S9a, ESI†), indicating the coordi-
and 0.10 ppm, respectively (Fig. 4d). It is worth noting that the nation of triazolyl nitrogen to the metal atom. Furthermore, the
extent of the chemical shift changes is in agreement with the high angle annular dark-field scanning TEM (HAADF-STEM)
association constants calculated from the emission spectra measurements were carried out to confirm the existence of Hg2+
results suggesting the stronger affinity of 2N toward Hg2+ than within the hybrid vesicles (Fig. S9b, ESI†). The image showed a clear
2 (Fig. S8, ESI†). Interestingly, in contrast to the small change contrast, showing the presence of organic–inorganic hybrid vesi-
(by 0.03 ppm) of the H* in the NMR spectra of 2, the H* of 2N cular structures against the black background (Fig. S9b, top left,
showed a significant downfield shift of 0.09 ppm upon addition ESI†). Energy dispersive X-ray spectroscopy (EDS) elemental maps
of Hg2+. This result means that Hg2+ in the 2N–Hg2+ complex could prove that Hg2+ ions were concentrated in the vesicles
also had a strong interaction with the oxygen atoms next to the (Fig. S9b, top right, ESI†). Therefore, we concluded that the Hg2+
phenyl group.23 Thus, we supposed that this binding mode may can join the neighboring amphiphiles by interacting with the triazolyl
be attributed to a strong tendency of 2N with enhanced p–p moieties which induces the molecular assembly to form the nano-
interaction to maintain the tight packing of aromatic blocks. structures with enhanced stability, in addition to the inherent ability
On the basis of these results, the plausible binding models of of the amphiphiles to form the aggregates in aqueous solution.
2–Hg2+ and 2N–Hg2+ are proposed in Fig. 4e and f, respectively.
Considering the growing applications of click chemistry in
As a result, the morphological differences between the 2–Hg2+ and material science,13 the nanostructure formation of triazole-
2N–Hg2+ are possibly explained in terms of the Hg2+-binding mode based amphiphile–Hg2+ conjugates in this study would provide
in aqueous solution. The 2–Hg2+ complex showing the 1 : 1 a useful and simple strategy for the fabrication of a nano-
stoichiometry of 2 to Hg2+ may pack loosely which leads to a chemosensor for the detection of toxic heavy metals in water.
micellar structure with high interfacial curvature.24 On the In addition, the resultant vesicular hybrids induced by the
other hand, the 1 : 2 stoichiometry of the 2N–Hg2+ complex response to a particular metal ion would lead to the develop-
enforces tight packing, which favours a better-organized flat ment of stimuli-responsive functional nanovehicles.
interface between the hydrophobic core and the hydrophilic
This research was supported by Basic Science Research
Program (2013R1A1A2061197) through the National Research
corona. Consequently, 2N–Hg2+ formed the vesicles.
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