Inorganic-organic hybrid vesicles with counterion- and pH-controlled fluorescent properties

Article abstract of DOI:10.1021/ja204034g

Two inorganic-organic hybrid clusters with one or two covalently linked pyrene fluorescent probes, [(n-C₄H₉)₄N] ₂[V₆O₁₃{(OCH₂)₃C(NH(CO) CH₂CH₂CH₂C₁₆H₉)} {(OCH₂)₃C-(NH₂)}] ((TBA⁺) ₂1) and [(n-C₄H₉)₄N] ₂[V₆O₁₃{(OCH₂)₃C(NH(CO) CH₂CH₂CH₂C₁₆H₉)} ₂] ((TBA⁺)₂2), respectively, are synthesized from Lindqvist type polyoxometalates (POMs). The incorporation of pyrene into POMs results in amphiphilic hybrid molecules and simultaneously offers a great opportunity to study the interaction between hybrid clusters and their counterions. 2D-NOESY NMR and fluorescence techniques have been used to study the role of counterions such as tetrabutyl ammonium (TBA) in the vesicle formation of the hybrid clusters. The TBA⁺ ions not only screen the electrostatic repulsions between the POM head groups but also are involved in the hydrophobic region of the vesicular structure where they interrupt the formation of pyrene excimers that greatly perturbs the luminescence signal from the vesicle solution. By replacing the TBA⁺ counterions with protons, the new vesicles demonstrate interesting pH-dependent fluorescence properties.

Full text of DOI:10.1021/ja204034g

ARTICLE
pubs.acs.org/JACS
InorganicꢀOrganic Hybrid Vesicles with Counterion- and
pH-Controlled Fluorescent Properties
Dong Li,†,§ Jie Song,‡,§ Panchao Yin,† Silas Simotwo,† Andrew J. Bassler,† YuYu Aung,† James E. Roberts,†
Kenneth I. Hardcastle,‡ Craig L. Hill,*,‡ and Tianbo Liu*,†
†Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States
‡Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States
S Supporting Information
b
ABSTRACT: Two inorganicꢀorganic hybrid clusters with one or
two covalently linked pyrene fluorescent probes, [(n-C4H9)4N]2-
[V6O13{(OCH2)3C(NH(CO)CH2CH2CH2C16H9)}{(OCH2)3-
Cꢀ(NH2)}] ((TBA+)21) and [(n-C4H9)4N]2[V6O13{(OCH2)3-
C(NH(CO)CH2CH2CH2C16H9)}2] ((TBA+)22), respectively,
are synthesized from Lindqvist type polyoxometalates (POMs).
The incorporation of pyrene into POMs results in amphiphilic
hybrid molecules and simultaneously offers a great opportunity to study the interaction between hybrid clusters and their
counterions. 2D-NOESY NMR and fluorescence techniques have been used to study the role of counterions such as tetrabutyl
ammonium (TBA) in the vesicle formation of the hybrid clusters. The TBA+ ions not only screen the electrostatic repulsions
between the POM head groups but also are involved in the hydrophobic region of the vesicular structure where they interrupt the
formation of pyrene excimers that greatly perturbs the luminescence signal from the vesicle solution. By replacing the TBA+
counterions with protons, the new vesicles demonstrate interesting pH-dependent fluorescence properties.
’ INTRODUCTION
environments and therefore can enhance the applications of the
functional clusters. Interesting supramolecular structures such as
micelles, vesicles, liquid crystals, and 2D-films have been reported
in various amphiphilic hybrid solutions.34ꢀ36 The self-assembly is
dependent upon solvent polarity,37 the hydrocarbon chain length,38
the type and length of organic linkers,39 and the architecture of the
hybrids, as well as temperature.
Meanwhile, the development of fluorescent tags or probes
opens new opportunities for smart molecules. Applying fluor-
escent probes to investigate biochemical microenvironments is a
powerful technique that can often provide unique and critical
information. Pyrene, a highly symmetrical polyaromatic hydro-
carbon fluorophore possessing restricted modes of motion, can
exhibit fine structure in its absorbance and fluorescence spectra at
room temperature.40 One important application of the pyrene
fluorescence stems from its ability to probe the polarity of the
local microenvironment, either in a hydrophobic or hydrophilic
media, from the change of specific emission peaks in the
spectrum.41,42 Therefore, amphiphilic hybrid POM clusters with
pendent pyrene fluorescent probes are of potential interest for
the construction of smart supramolecular assemblies via macro-
ion-counterion interaction.
Smart supramolecular assemblies that respond to single or
multiple external stimuli, such as temperature,1 ionic strength,2
pH,3 redox,4 light,5 ultrasound,6 and magnetic field,7 are of great
interest for their potential applications in drug delivery, oil
recovery, and design of new sensors and catalysts.8ꢀ13 While most
smart supramolecular assemblies are constructed from organic
molecules or block copolymers, incorporatingfunctional inorganic
components (such as quantum dots, silica, transition metal ions,
etc.) into organic building blocks to make functional hybrid
assemblies represents an interesting new direction.5,6,14 Polyox-
ometalates (POMs), a large group of early transition metal oxide
clusters, represents structurally well-defined inorganic molecules
with diversified structures and properties.15ꢀ17 The synergistic
combination of POMs and organic components gives these
hybrids noteworthy properties of potential utility.18,19 For in-
stance, polymers with grafted POMs can be used in photovoltaic
cells or in trapping magnetic nanoparticles,20,21 and surfaces that
are covered with hybrid POMs (SAMꢀPOMꢀpyrene) show
specific cell adhesion ability.22 Organo-POM hybrids23ꢀ27 have
been shown to exhibit unusual spectroscopic properties28 or
catalyze oxidation reactions,29ꢀ31 organometallic POM clusters
have been tested for norbornene and cyclohexene oxygenation as
precatalysts,32 and a recent study on a photoresponsive surfactant-
encapsulated POM complex has shown an interesting dynamic
structure transition in solution.33 The new hybrids with surfactant-
like amphiphilic solution properties are particularly interesting be-
cause they can be compatible with both hydrophilic and hydrophobic
It is an interesting question how the hybrid surfactants arrange
themselves to form closely packed regions in the supramolecular
structures. These hybrids differ from conventional surfactants in
Received: May 12, 2011
Published: July 27, 2011
r
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i
i
that they have large polar head groups making close packing of
hydrophobic domains very difficult due to the spatial obstruc-
tion. We proposed, but without solid evidence, that the coun-
terions might be important by perturbing the solvophobic layer
formation.39 The newly synthesized hybrid clusters with pyrene
as fluorescent probes offer a unique opportunity to study this
phenomenon.
by plotting ΓG(Γ) versus Rh, with ΓiG(Γ) being proportional to the
angular-dependent scattered intensity of particle i having an apparent
hydrodynamic radius Rh,i. The temperature in the sample chamber was
controlled to within (0.1 °C. More details about SLS and DLS can be
found in our previous publications.46
Transmission Electron Microscopy (TEM). Samples for elec-
tron microscopy characterization were prepared by pipeting 5 μL of
diluted solution onto a carbon-coated TEM grid. The TEM samples
were left under ambient conditions for several hours until the solvent
completely evaporated. Bright-field (BF) TEM imaging was performed
on a JEOL 2000FX transmission electron microscope having an accel-
erating voltage of 200 kV.
Zeta (ζ) Potential Analysis. All the ζ potential analysis measure-
ments were performed on a Brookhaven Instrument Inc. Zeta PALS
Analyzer. The instrument is equipped with a red laser operating at
660 nm wavelength and has an accuracy of (2% for filtrated samples.
The sample chamber was kept at 25 ( 0.1 °C, and all sample solutions
were loaded 30 min prior to measurements in order to achieve thermal
equilibrium with the chamber.
Nuclear Magnetic Resonance Spectroscopy (NMR). All 1D
and 2D 1H NMR measurements in the liquid state were performed on a
Bruker Avance 500 spectrometer equipped with a 5 mm triple-axis
gradient (TXI) probe. 2D nuclear Overhauser enhancement spectros-
copy (2D NOESY) spectra were recorded with 256 t1 increments and 64
scans under the pulse program of noesygpph19 provided with Topspin
1.3. The relaxation delay D1 varied from 1 to 2 s, and the mixing time D8
changed from 0.1 to 0.5 s. Baseline correction and noise reduction were
performed when appropriate. All spectra were taken at room tempera-
ture, and FIDs were processed and analyzed with the NMR software
provided by Bruker.
Fluorescence Measurements. Fluorescence spectra were recorded
at room temperature on a Cary Eclipse fluorescence spectrophotometer.
The excitation wavelength, λexc, used was 335 nm, and the spectrum width
was from 350 to 700 nm. An emission filter of 360ꢀ1100 nm was used.
Each spectrum was obtained by averaging three scans and corrected for
scatter of the equivalent blank sample. In calculations of the excimer-to-
monomer intensity ratio, the monomer (IM) and the excimer (IE) were
determined by taking the integrals under the fluorescence peaks from
350 to 430 nm for the pyrene monomer and from 430 to 700 nm for the
pyrene excimer.40
’ EXPERIMENTAL SECTION
X-ray Crystallography. X-ray quality crystals of [V6O13{(OCH2)
CNH3}] 2.4DMSO(NH3V6), [(n-C4H9)4N]2[V6O13{(OCH2)3CNH2}2]
(TBAꢀNH2V6), and (TBA+)22 were coated with Paratone N oil and
mounted on a small fiber loop for index and intensity data collection. The
X-ray diffraction data were collected under a nitrogen stream at 173 K on a
Bruker D8 SMART APEX CCD single-crystal diffractometer with graphite
monochromated Mo KR (λ = 0.71073 Å) radiation. Data collection,
indexing, and initial cell refinements were processed using the SMART43
software while frame integration and final cell refinements were carried out
using the SAINT44 software. The final cell parameters were determined from
the least-squares refinement of total reflections. The structures were deter-
mined through direct methods (SHELXS97) and difference Fourier maps
(SHELXL97). The final results of crystal data and structure refinement are
summarized in Table S1 of the Supporting Information. The Cambridge
Crystallographic Data Centre deposition number for NH3V6, TBAꢀNH2V6,
and (TBA+)22 are CCDC 821763, 821764, and 821765, respectively.
Cation Exchange. Counterion replacement of TBA+ with H+ was
achieved by using a cation-exchange resin column. For a typical experi-
ment, 5 mg of hybrid cluster 1 or 2 was dissolved in 2 mL of acetonitrile.
This solution was then applied to a prepacked, cation-exchange resin
column (Amberjet 1200 hydrogen form purchased from Sigma-Aldrich)
rinsed with D.I. water and acetonitrile. An additional 20ꢀ50 mL of
acetonitrile was used to elute the column, and the yellow fraction was
collected. The post elution was further washed with 5 mL of diethyl ether
to remove organic impurities. The final solution was transferred into a
glass culture plate and kept in the dark for several days to fully evaporate
the solvent. Yellow colored fine powders were collected and could be
easily dissolved in water or DMSO for further study.
Laser Light Scattering. Both dynamic light scattering (DLS) and
static light scatting (SLS) techniques were used to characterize the self-
assembly of hybrids in solution. A Brookhaven Instruments Inc. light
scattering spectrometer, equipped with a diode-pumped solid-state (DPSS)
laser operating at 532 nm and a BI-9000AT multichannel digital correlator
was used for all experiments. The SLS was performed over a broad range of
scattering angles from 30° to 130°, with a 2° interval. The radius of gyration
(Rg) and the weight-average molecular mass (Mw) of the large assemblies
were calculated using the RayleighꢀGansꢀDebye equation:45
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Hybrid (TBA+)21 and
(TBA+)22. The detailed synthesis and purification procedure for
precursors and hybrid clusters can be found in the Supporting
Information. The synthetic protocol was developed here to
efficiently construct a new type of POM-based inorganicꢀ
organic hybrid. Based on the idea that succinate (NHS) esters
show high reaction selectivity to N-termini in negatively charged
protein modification, the primary amine groups symmetrically
attached on the two opposite faces of a negatively charged
hexavanadate were further functionalized by reacting with a
pyrene-containing succinate ester (PASE) at room temperature,
to give the desired hybrids via efficient amide bond formation. By
tuning the PASE/TBAꢀNH2V6 molar ratio in the reaction,
asymmetric hybrid compounds, 1 (onlyone amine groupreacted),
and symmetric hybrid compounds, 2 (both of two amine groups
reacted), were obtained, respectively, in high yield and selectivity.
Their molecular structures were determined from single crystal
ꢁ
c
Hc=R90 ¼ 1=Mw þ 2A2
ð1Þ
where His an optical parameter, Mw is the weight-average molecular mass of
the solutes, A2 is the second virial coefficient, and c is the solute concentra-
tion. The 2A2c term is neglected during calculations because the sample
solutions examined in this study have very low concentrations.
For DLS measurements, the intensityꢀintensity time correlation
functions were analyzed by the constrained regularized (CONTIN)
method in order to ascertain the average hydrodynamic radius (Rh) of
the large assemblies. The average apparent translational diffusion coef-
ficient, Dapp, was determined from the normalized distribution function
of the characteristic line width, Γ(G). The hydrodynamic radius Rh is
converted from D through the StokesꢀEinstein equation:
R ¼ kBT=6πηD
ð2Þ
X-ray diffraction, H, 13C NMR, IR, and elemental analysis
1
(see Scheme S1 and Part 1 in the Supporting Information).
where kB is the Boltzmann constant and η is the viscosity of the solvent at
temperature T. The particle size distribution in solution can be obtained
Analysis of the packing in the unit cells of (TBA+)22 shows several
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Scheme 1. Molecular Structure of Two Novel Polyoxometa-
lates Hybrid Cluster Anions 1 (A) and 2 (B) Shown in
Combined Ball-and-Stick Representationa
the supramolecular structures have a hollow spherical vesicular
structure, which is also clearly confirmed by the TEM studies
(Figure 1C). The TEM image shown in Figure 1D reveals several
important aspects. First, the different contrast shown inside
(lighter color) and around (darker color) the vesicular structures
indicate they are hollow, which is quite similar to the vesicles
formed by phospholipids.51 Second, these vesicles collapsed on
the carbon film due to the evaporation of internal solvent under
high vacuum condition, indicating a soft and flexible nature of the
vesicles’ membrane. In order to form such vesicular structures in
the mixed solvents as shown here, it is reasonable to assume that
the hybrids use their polar POM clusters to face the hydrophilic
solvent while their organic tails form a hydrophobic domain. The
addition of water induces dissociation of the TBA+ counterions
from the POM cluster and increases the overall negative charge
on the polar head groups, which in turn changes the amphiphi-
licity of the hybrid molecules and leads to the vesicle formation.
The above discussions focus on one set of condition (0.1 mg/mL
hybrid 1 in 80:20 v/v water/DMSO); however, similar vesicular
structures are also observed in different solvents, that is, in water/
acetonitrile, or in water/methanol.
a Atoms are represented as follows: V (green), N (blue), O (red), C
(black), H (white).
supramolecular interactions between the polyanions in solid state,
which include limited pyreneꢀpyrene π stacking (the closest
distance between two adjacent carbons on two pyrene rings is ca.
0.335 nm) and H bonding between the amide ꢀNHꢀ and a ter-
minal POMoxygen (the N...O distanceisca. 0.286 nm) (FigureS2
of the Supporting Information). Such interactions could well be
operable in the vesicles and are expected to play a role in solution
aggregation. The length of the hydrophobic region is critical to the
overall amphiphilic propertyof thehybrid clusteraswell astheself-
assembled supramolecular structures.38,47ꢀ49 At the same time,
POMs are well-known electron acceptors so fluorescence quench-
ing by the clusters was a concern at the outset.50 Therefore, we
targeted complexes in which the pyrene fluorophores were
separated from the POM head groups by a four-carbon hydro-
carbon chain. Later, we determined that this hydrocarbon chain
maintains flexibility in the hydrophobic component and also
preserves the pyrene fluorescence.
When the original TBA+ counterions of 1 are replaced
by tetraethylammonium (TEA+) or tetramethylammonium
(TMA+), similar vesicular structures were observed. However,
compared with the case of TBA+, the total scattered light
intensity becomes lower, and more water is needed to trigger
the vesicle formation. The Rh of the large vesicular structures also
changes from 50 to 30 nm and 23 nm in the same solvent system
(80:20 v/v H2O/DMSO) (Figure 1A and B), respectively. The
decrease of the vesicle size should be attributed to the shorter
alkyl chains of the counterions, which decreases the size of the
hydrophobic domain and consequently increases the curvature
of the vesicles. These results indicate that the counterions play an
important role in regulating the amphiphilic nature of the hybrid
clusters and the electrostatic interactions between them.
Hybrid (TBA+)22 in mixed solvents (H2O/DMSO, H2O/
acetonitrile, etc.) shows similar self-assembly behavior as hybrid
(TBA+)21. However, the vesicular structures formed by hybrid 2
are smaller than those formed by hybrid 1 under the same
conditions. This is because an additional bending energy is
needed to properly fold the two hydrophobic tails of hybrid 2
into the vesicular structure and therefore leads to a higher
curvature or smaller vesicles (see the Supporting Information).52
Counterions Affect the Packing of Hybrid Clusters in
Vesicles. Fluorescent probes are highly useful in monitoring
polarity changes of microenvironments in macromolecules and
membranes. The general dependence of probe fluorescence on
polarity has been attributed to the dipoleꢀdipole interaction
between the singlet excited state of the fluorophores and the
solvent molecules. Specifically, the fine structural pattern in the
fluorescence of the pyrene monomers is found to be independent
of excitation conditions or collisional quenching but highly
dependent on solvent polarity.42 Therefore, any changes in the
pyrene fluorescence will reveal interactions between counterions
and the hybrid clusters. As shown in Figure 2, when counterions
are TBA+, TEA+, or TMA+, the fluorescence intensity primarily
comes from the pyrene monomer, and no excimer peak is
observed. Under theseconditions, vesicular structures havealready
formed in the solution. Therefore, the fluorescence spectra
indicate that the pyrene groups on the vesicle surface are not
spatially close enough to form excimers. However, the fluorescence
spectra are significantly different when H+ counterions are present
Characterization of the Amphiphilic Properties of Hybrids
1 and 2 in Polar Solvents. As shown in Scheme 1, two novel
POM based inorganicꢀorganic hybrids are constructed by
incorporating one or two pyrene functional groups onto one
Lindqvist-type polyoxovanadate [V6O13{(OCH2)3CNH2}]2ꢀ
.
The linkage of flexible, hydrophobic organic tails to the inorganic
POM head groups renders these new species amphiphilic.
Hybrids (TBA+)21 and (TBA+)22 are insoluble in water but
can be readily dissolved in dimethyl sulfoxide (DMSO), dimethyl
formamide (DMF), and other polar organic solvents. For
0.1 mg/mL of hybrid (TBA+)21 in DMSO, a very low scattered
intensity (∼45 Kcps) was collected from SLS measurements (for
comparison, the scattered intensity of pure solvent is ∼40 Kcps),
indicating that the hybrid molecules prefer to remain as mono-
mers in solution rather than large assemblies. However, when
additional water was introduced to make a solvent containing up
to 70 vol% water, the total scattered intensity starts to increase
significantly, and supramolecular structures are observed under
DLS, as shown in Figure 1. The CONTIN analysis of the DLS
measurement on this solution reveals a peak corresponding to
assemblies with a very narrow size distribution and an average
hydrodynamic radius (Rh) of 50 nm. The Rh value does not show
angular dependence that suggests that the supramolecular struc-
tures are likely spherical.45 The radius of gyration (Rg) of the
assemblies measured by SLS is 48 nm (see the Supporting
Information). The relation of Rh,0 ≈ Rg strongly suggests that
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Figure 1. (A) Total scattered intensity recorded by SLS for hybrid cluster 1 with different counterions in H2O/DMSO mixed solvents. (B) CONTIN
plot of the size distribution of vesicular structures formed by hybrid cluster 1 with different counterions in 80:20 v/v H2O/DMSO mixed solvents.
(C) TEM image of the vesicular structure formed in 80:20 v/v H2O/DMSO mixed solvents (bar = 0.2 μm). (D) Enlarged region of C in order to show
the structural details of the hollow spherical vesicular structures.
in the solution. The emission peaks of the monomer become less
well-defined, and the peak of the excimer centered at 480 nm
becomes the dominant one for both H21 and H22. Second, it has
been shown in the literature that the polarity of the microenviron-
ment around the pyrene group is reflected by the ratio of the
emission peak at 375 nm to the peak at 395 nm: the lower the ratio,
the less polar the pyrene environment.40,41 Figure 2B clearly shows
that the pyrene groups are more solvated when large counterions
(for example TBA+) are present in the assemblies. In contrast, when
H+ ions are the counterions, the pyrene groups stay closer to each
other. This data confirms that counterions with long alkyl chains
can prevent the close packing of the hybrid clusters in the vesicular
structures.
More direct evidence comes from the 2D NOESY NMR
measurements. In general, when a saturated or inverted proton
undergoes dipolar cross-relaxation, another spatially close proton
may experience an intensity enhancement, a common phenom-
enon termed the nuclear Overhauser effect (NOE).53 The NOE
is unique among the NMR phenomena because it does not rely
on through-bond J couplings but depends only on the spatial
proximity between protons.54,55 In other words, the strength of
the NOE can be used to estimate how close two protons are.56,57
In the current system, there are two possible scenarios for TBA
interaction with the vesicular structure, either strongly bound to
the vesicles or as free counterions in solution. Since no covalent
chemical bonds are present between TBA cations and the hybrid
cluster, it is quite possible that a dynamic exchange and equilib-
rium exists between bound and free cations. Free TBA cations
show very weak to no NOE, while the bound TBAs are explicitly
revealed through strong, negative NOE cross peaks. Moreover,
the strong, negative NOE from bound TBA cations will outweigh
that from free cations and dominate the NOESY spectrum, even
when the free TBAs are in considerable excess.54 Figure 3 shows
that in a 0.25 mg/mL (TBA)21 solution in pure DMSO-d6, the
TBA protons exhibit positive NOE cross peaks between adjacent
protons indicating that the TBA counterions exist as free ions.
The NOE cross peak pattern is identical to that of the control
compound TBAI (tetrabutylammonium iodide). This is because
there is no vesicle formation in solution. The TBA counterions
interact with the {V6O19}2ꢀ POM portion through electrostatic
interactions. However, when vesicular structures form in 90:10
v/v D2O/DMSO-d6 solution, the NOE spectra for (TBA+)21
dramatically changes. First, the previously positive NOE cross
peaks become negative, indicating strong binding between the
TBA cations and the vesicles. More interestingly, a set of new
negative NOE cross peaks appears between cations and the
fluorescent pyrene groups on 1 (Figure 3D), which do not appear
in Figure 3B. This clearly indicates that the amphiphilic TBA
cations interact with the hydrophobic domains in the vesicles. It
is almost certain that this interaction interrupts the formation of
pyrene excimers that in turn greatly affects the fluorescence
pattern of the pyrene (Scheme 2). Previously, we noticed that the
dumbbell shaped POM hybrids could form vesicles in water/
acetone mixed solvents.39 The interesting question is how these
hybrids could form closely packed hydrophobic layers in their
vesicles. The giant POM head groups make the close packing of
the alkyl chains very difficult due to spatial hindrance. We
speculate that the alkyl chains of the TBA cations interact with
the hydrophobic domains, but there is no direct evidence for this.
From the 2D NOESY NMR study, we can confirm that the
amphiphilic TBA cations are distributed partly into the hydro-
phobic regions of the vesicular structures.
pH Sensitive Vesicles. When two pyrene groups are spatially
close to each other (less than 0.5 nm) without any interruption,
an excimer peak appears.40 We have shown earlier in the paper
that replacement of the original TBA counterions by H+ through
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Figure 2. (A) Fluorescence spectra of hybrid clusters 1 and 2 with different counterions. (For the TBA, TEA, and TMA salts, the sample concentrations
are all at 0.1 mg/mL, and the solvent is 80:20 v/v H2O/DMSO; for the H salt, the sample concentrations are at ∼0.4 mg/mL, and the solvent is H2O.).
(B) Plot of the pyrene monomer fluorescence peak I(375 nm)/I(395 nm) versus the counterion size for hybrid clusters 1 and 2 with different
counterions.
Figure 3. (A) 2D NOESY spectrum of TBAI in DMSO-d6. (B) 2D NOESY spectrum of 0.25 mg/mL (TBA+)21 in DMSO-d6. (C) Enlarged region of B
showing the TBA cross peaks. (D) Enlarged region of E showing the TBAꢀpyrene cross peaks. (E) 2D NOESY spectrum of 0.2 mg/mL (TBA+)21 in
90:10 v/v D2O/DMSO-d6 mixed solvent. (F) Enlarged region of E show the TBA cross peaks. (Important positive NOE peaks with dark green color are
highlighted in solid red rectangular while important negative NOE peaks with light green color are highlighted in dashed red rectangular.).
cation exchange changes the fluorescence spectra of the
appended pyrene: the peaks due to pyrene excimers become
dominant relative to those from pyrene monomers, indicating
a much closer packing of the hybrid clusters in the vesicular
structure. This unique property gives us a great opportunity to
construct a smart, pH sensitive vesicular structure and to study
the effect of pH on the assembly and disassembly of vesicular
structures. Figure 4 shows that the initial pH for an aqueous
solution of hybrid cluster 2 (ca. 0.44 mg/mL based on UV
absorption calibration) after counterion exchange is 3.41 and the
dominant fluorescence comes from the pyrene excimers. The
molar ratio of hybrid 2 divided by free H+ in solution is 0.87
based on solution pH, which indicates only a portion of the H+
ions are released into solution and contribute to the pH, while a
large amount of H+ counterions are closely associated with the
hybrids. (If protons are free in solution, the molar ratio should
be 0.5). When dilute NaOH solution is slowly titrated into the
solution of POM hybrid, a gradual decrease of the excimer peak
along with a continuous increase in the pyrene monomer peaks
occurs, indicating that the distance between pyrene excimers
increases (larger than 0.5 nm). Because Na+ has a weaker affinity
for the hybrid macroanions in solution than H+ does,58 which
leads to less screening of the POM cluster from neighboring
groups, the drop of proton concentration in solution will increase
the repulsion between adjacent POM groups in the vesicles.
When the solution pH approaches 7, the decrease in the ratio of
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Scheme 2. Illustration of Possible Vesicular Structures Formed by Hybrid Clusters, 2, in Polar Solvents, and How the TBA
Counterions May Be Arranged in the Packing of Individual Clustersa
a The hexagons, parallelograms, and four-legged stars represent the POM, pyrene, and TBA cations, respectively.
indicating that the net charge on the vesicles increases with
increasing pH.
’ CONCLUSIONS
In summary, two novel POM-based hybrid clusters were
synthesized by connecting one or two organic pyrene tails with
a Lindqvist type polyoxovanadate head cluster through triester
capping groups. These two hybrid clusters demonstrate note-
worthy amphiphilic properties by forming spherical vesicular
structures in polar solvents. Four different counterions (TBA,
TEA, TMA, and H+) have been used to study the counterion
effect on the vesicular structures and their consequent role in the
fluorescent properties of the pyrene groups on vesicle surface.
TBA counterions not only interact with POM polar head groups
but also move into the hydrophobic regions and interrupt the
close packing of pyrene groups. More importantly, when TBA
counterions are replaced by protons, a dramatic change of the
Figure 4. (A) Fluorescence spectra of hybrid clusters (H+)22 in water at
pyrene fluorescence pattern occurs, and the vesicle size, the
different pH values (the fluorescence intensity has been normalized).
fluorescence pattern, and the effective charge on the vesicles
(B) Plot of pyrene excimer/monomer intensity ratio versus solution pH
change correspondingly and reversibly with solution pH. The
for hybrid clusters (H+)22. (C) Change in the vesicular structure size
construction of pH sensitive vesicular structures could well have
with solution pH for (H+)22. (D) Zeta potential of the vesicular
structure with solution pH for (H+)22. The sample concentration of
∼0.4 mg/mL was determined by UV absorption.
application to artificial cell studies, nanoreactors, as well as drug
and gene delivery systems.
’ ASSOCIATED CONTENT
[excimer]/[monomer] peak areas becomes much slower, indi-
cating that the distance between hybrid clusters becomes less
sensitive to pH due to the limited amount of available protons.
More interestingly, in the pH range of 1ꢀ7, the whole process is
reversible. In other words, the average pyrene-to-pyrene distance
between adjacent clusters is reversibly tunable by changing pH.
The average vesicle size recorded by DLS at 90° scattering
angle also shows pH dependence: the Rh of vesicles decreases
with increasing pH. Stronger electrostatic repulsion between
hybrid clusters will result in a high curvature, that is, a smaller
vesicle, for the assemblies; this trend shows how counterions
affect the close packing of hybrid clusters. Meanwhile, the zeta
potential of the vesicular structures becomes more negative
with increasing pH and becomes nearly neutral at a low pH,
S
Supporting Information. Crystallographic data for TBA,
b
(TBA+)21, and (TBA+)22. Details of the synthesis and charac-
terization of (TBA+)21 and (TBA+)22, vesicle characterization,
counterion exchange, and pH effects. This information is avail-
able free of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION
Corresponding Author
liu@lehigh.edu; chill@emory.edu
Author Contributions
§These two authors contributed equally to this paper.
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Journal of the American Chemical Society
ARTICLE
’ ACKNOWLEDGMENT
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