Organic-inorganic molecular nano-sensors: A bis-dansylated tweezer-like fluoroionophore integrating a polyoxometalate core

Article abstract of DOI:10.1002/ejoc.201101122

Functionalization of the bis-lacunary Keggin polyoxotungstate [γ-SiW₁₀O₃₆]^8- has been achieved with a two-step synthesis, by the covalent attachment of a 3-aminopropylsilane spacer and further linkage of the dansyl (5-dimethylamino-1-naphthalenesulfonyl-) residue. The resulting bis-decorated molecular hybrid [{{(CH₃) ₂N}C₁₀H₆SO₂NH(CH₂) ₃Si}₂O(γ-SiW₁₀O₃₆)] ^4- has been isolated and characterized in solution and in the solid state by FTIR, multinuclear NMR, ESI-MS, UV/Vis, luminescence spectroscopy, dynamic light scattering (DLS), and Scanning Electron Microscopy (SEM). The inorganic polyoxometalate provides a molecular nano-surface where the dansyl fluorophores are anchored with a tweezer-type arrangement. The merging of the organic and inorganic domains of the bis-dansylated complex dictates its fluorescence features, which are observed in the range 375-600 nm, its amphiphilic properties, and the multi-site recognition/signaling of cationic analytes due to the complementary effect of the tungsten oxide polyanionic surface. Indeed, the interplay of the appended sulfonamide moieties and of the molecular metal oxide fosters an enhanced selectivity for Cu²⁺ and Pb²⁺ ion sensing, even in the presence of potentially interfering cations such as Co²⁺. These latter are preferentially captured by the inorganic platform. By virtue of its hybrid nature, the title fluoroionophore evolves to supramolecular architectures and extended systems in mixed organic solvent/aqueous environment, yielding spherical vesicles and macroporous thin films. Flat polymeric membranes incorporating the hybrid fluorophore can also be obtained, suggesting the generation of heterogeneous sensing devices that integrate both filtration and separation functionalities. Copyright

Full text of DOI:10.1002/ejoc.201101122

FULL PAPER
DOI: 10.1002/ejoc.201101122
Organic-Inorganic Molecular Nano-Sensors: A Bis-Dansylated Tweezer-Like
Fluoroionophore Integrating a Polyoxometalate Core
Mauro Carraro,*[a] Gloria Modugno,[a] Giulia Fiorani,[a] Chiara Maccato,[a]
Andrea Sartorel,[a] and Marcella Bonchio*[a]
Dedicated to Professor Gianfranco Scorrano on the occasion of his retirement
Keywords: Hybrid materials / Nanostructures / Sensors / Trace analysis / Copper / Polyoxometalates
Functionalization of the bis-lacunary Keggin polyoxotung-
its amphiphilic properties, and the multi-site recognition/sig-
naling of cationic analytes due to the complementary effect
of the tungsten oxide polyanionic surface.
8–
state [γ-SiW10O36
]
has been achieved with a two-step syn-
thesis, by the covalent attachment of a 3-aminopropylsilane
spacer and further linkage of the dansyl (5-dimethylamino-
1-naphthalenesulfonyl-) residue. The resulting bis-decorated
molecular hybrid [{{(CH3)2N}C10H6SO2NH(CH2)3Si}2O(γ-
SiW10O36)]4– has been isolated and characterized in solution
and in the solid state by FTIR, multinuclear NMR, ESI-MS,
UV/Vis, luminescence spectroscopy, dynamic light scattering
(DLS), and Scanning Electron Microscopy (SEM). The inor-
ganic polyoxometalate provides a molecular nano-surface
where the dansyl fluorophores are anchored with a tweezer-
type arrangement. The merging of the organic and inorganic
domains of the bis-dansylated complex dictates its fluores-
cence features, which are observed in the range 375–600 nm,
Indeed, the interplay of the appended sulfonamide moieties
and of the molecular metal oxide fosters an enhanced selec-
tivity for Cu2+ and Pb2+ ion sensing, even in the presence of
potentially interfering cations such as Co2+. These latter are
preferentially captured by the inorganic platform. By virtue
of its hybrid nature, the title fluoroionophore evolves to sup-
ramolecular architectures and extended systems in mixed or-
ganic solvent/aqueous environment, yielding spherical vesi-
cles and macroporous thin films. Flat polymeric membranes
incorporating the hybrid fluorophore can also be obtained,
suggesting the generation of heterogeneous sensing devices
that integrate both filtration and separation functionalities.
mental for advanced catalytic applications[9] and can direct
the supramolecular organization of the hybrid molecules
evolving to extended nanostructures.[10] The interplay of or-
ganic chromophores and POMs by covalent linkage and
ionic assembly has been recently proposed to promote
charge separation states, being instrumental for solar energy
conversion and storage, as well as for the development of
molecular capacitors and photosensitized catalytic pro-
cesses.[11,12] Considering the interdisciplinary frontier of in-
organic nano-clusters in medicinal chemistry, the design
and synthesis of POM hybrids is appealing because it
should be possible to enhance the bio-availability, modify
delivery strategies, and to introduce tailored recognition/sig-
naling sites.[13] In this latter field, luminescent POMs have
been proposed for bio-imaging/sensors and could eventu-
ally couple the diagnostic potential with innovative therapy
protocols.[14] To this end, the use of totally inorganic lan-
thanide-substituted POMs suffers from extensive lumines-
cence quenching in aqueous environment coupled to a nar-
row-range pH stability.[15] A promising alternative stems
from decoration of the POM scaffold with resident lumino-
phores, whereby covalent bonds guarantee long-term per-
formance under physiological conditions.
Introduction
Polyoxometalates (POMs) are nano-dimensional molecu-
lar, multi-metal oxides, which have found applications in
catalysis, materials science, and nano-medicine.[1–4] Tailored
control of their properties is readily achieved by tuning the
elemental composition, structure, and charge density of the
inorganic scaffold.[1] Moreover, the introduction of surface-
bound organic arms is a valuable tool for shaping the
stereoelectronic features of the resulting hybrid complexes,
as well as their solubility, redox behavior, spectroscopic re-
sponse, and hydrolytic stability in various media.[5,6] Indeed,
the merging of organic and inorganic domains on POM
nano-scaffolds is a developing field of investigation focusing
on the design of new functional molecules and materials.[7,8]
POM-appended organic/organometallic moieties are instru-
[a] Istituto per la Tecnologia delle Membrane del CNR, Sezione di
Padova and Department of Chemical Sciences, University of
Padova,
1, via Marzolo, 35131 Padova, Italy
E-mail: mauro.carraro@unipd.it
marcella.bonchio@unipd.it
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101122.
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We report herein the synthesis of a luminescent POM
hybrid, obtained by selective functionalization of the la-
cunary Keggin-type polyoxotungstate [γ-SiW10O36]8– with
5-dimethylamino-1-naphthalenesulfonyl (generally indi-
cated as dansyl) chromophore. The dansyl label carries
some key optical properties, such as: (i) intense fluorescence
in the visible region; (ii) relatively long emission wavelength;
(iii) a high Stokes shift behavior preventing self quenching;
(iii) solvatochromism, by effect of a twisted intramolecular
charge-transfer (TICT) excited state, and (iv) selective exci-
tation/absorption response of the protonated state, leading
to antenna-effects in polydansylated arrays.[16,17] These
unique features have been successfully exploited for the mo-
lecular design of fluorescent chemosensors in which the
dansyl chromophore is used in combination with receptor
domains, including multi-dentate ligands,[18] supramolec-
ular hosts such as calix[4]arenes,[19] cyclodextrins,[20] crown
ethers,[21] and also nano-silica surfaces.[22] Depending on
the dansyl environment, selective fluoroionophores have
been obtained with interesting sensitivity towards ions such
as Pb2+ [23] Hg2+ [18,24] and Cu2+ [19,25]
, . In particular, a
,
strong signal amplification results from multi-cooperative
fluorescence quenching involving a dansylated surface-ar-
ray attached to tailored nanosensors.[17,26] Our innovative
approach targets the molecular scaffold of inorganic poly-
oxometalates with the final aim to (i) shape a tweezer-like
arrangement of the luminophore residues at the optical sig-
naling site; (ii) provide a polyanionic environment for com-
plementary charge recognition events; (iii) evaluate the am-
phiphilic properties of the luminescent hybrid and its supra-
molecular/thin film self-organization for sensitivity en-
hancement and membrane processes. A comparison with
the monomeric 3-(dansylamido)propyl(triethoxy)silane (1)
has also been included to evaluate the impact of the POM-
assisted signaling phenomena.
Scheme 1. Synthetic route to 3.
The FTIR spectrum of 3 (see the Supporting Infor-
mation, Figure S8), features diagnostic bands between 982
and 737 cm–1 (W–O bonds) and weak absorption at 1101
Results and Discussion
Covalent derivatization of POM surfaces occurs and 1045 cm–1 (Si–O), furthermore, comparison with the
smoothly according to well-established synthetic strategies, spectra of [γ-SiW10O36]8– and 2 confirms the integrity of
in analogy to those applied for the functionalization of ex- the hybrid POM network upon dansylation. The new ab-
tended metal oxides. Herein, selective dansylation of the di- sorption bands observed at 1318 and 1143 cm–1 result from
vacant polyoxotungstate [γ-SiW10O36]8– has been achieved the sulfonamidic substituents. The ESI-MS spectrum of 3,
after introduction of an aminopropyl trialkoxysilane spacer. recorded in the negative mode, shows a dominant cluster
The reaction entails a double functionalization of the POM centered at m/z = 773.9 that can be ascribed to [{{(CH3)2-
precursor at the tetra-oxygenated nucleophilic site, which N}C10H6SO2NH(CH2)3Si}2O(γ-SiW10O36)]4–, confirming
occurs readily in acetonitrile under phase-transfer condi- the expected functionalization pattern, and the introduction
tions, by addition of nBu4NBr.[27,6a]
of two dansyl units through a bis-sulfonamide linkage in-
The resulting intermediate [{NH2(CH2)3Si}2O(γ- volving both amino pendants on the POM surface (Figure
SiW10O36)]4– (2),[6a,11a] which was characterized by hetero- S9).
nuclear NMR and FTIR spectra (see the Supporting Infor-
The novel hybrid POM 3 exhibits 29Si and 183W NMR
mation), reacts in a post-functionalization step with dansyl spectral patterns confirming, respectively, the bis-organosil-
chloride (3.5 equiv.) in the presence of triethylamine (TEA, ane anchoring (29Si NMR: δ = –62.5 ppm, 2Si), in addition
3.5 equiv.), in acetonitrile at 50 °C for 2.5 h (Scheme 1). The to the central tetrahedral SiO4 group (29Si NMR: δ = –88.4
final product (nBu4N)4[{{(CH3)2N}C10H6SO2NH(CH2)3- ppm, 1Si; Figure S10), and the overall C2v symmetry of the
Si}2O(γ-SiW10O36)] (3) was isolated in 69% yield, after pre- POM structure with a 2:1:2 intensity ratio of the three ex-
cipitation with water and purification by extensive washing/ pected 183W NMR signals (183W NMR: δ = –107.9, –136.2,
extraction cycles with diethyl ether and acetone.
–142.1; Figure S11).[21,22]
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Organic–Inorganic Molecular Nano-Sensors
1
The H and 13C NMR spectra were also consistent with
As verified for the dansyl-free POM 2, the polyoxo-
the expected covalent conjugate and, in particular, the sig- tungstate component shows only negligible fluorescence so
nals of the aromatic carbons of 3 are sensibly deshielded that the excitation spectrum of 3 (λem = 450 nm) is domi-
(115–153 ppm), with respect to the dansyl chloride precur- nated by the dansyl contribution, exhibiting well-resolved
sor (115–135 ppm), the α methylene protons resonate at δ bands at 223, 245, and 329 nm.
= 2.84 ppm (Δδ = –0.7 ppm with respect to dansyl-free 2),
Protonation Equilibria of Hybrid 3
and the amidic (SO2NH) proton resonates at δ = 5.95 ppm,
which is a chemical shift close to that obtained for the refer-
ence derivative 1 in the same solvent (δ =5.82 ppm).[28]
The UV/Vis spectrum of 3 (Ͼ200 nm) displays a shoul-
der at 218 and a maximum at 250 nm, together with a broad
absorbance tail extending up to 450 nm. This spectral be-
havior is the result of the POM oxygen-to-tungsten charge-
transfer bands overlapping with the typical absorption fea-
tures of the fluorophore moiety, as can be seen from the
superimposed spectra of dansyl-free 2 and of the reference
dansyl monomer 1 (Figure 1, a). In particular, the three ab-
sorption bands at 218, 252, and 338 nm can be ascribed to
the appended dansyl tweezer.
Hybrid 3 (10 μm) undergoes protonation equilibria in
acetonitrile/water (97.5:2.5 v/v) upon addition of acid (HCl,
1.6ϫ10–3 m). The spectrophotometric titration shows
major modifications in the spectral range 210–410 nm,
whereby a gradual decrease of the bands at 342 and 250 nm
is accompanied by a progressive increase in a new absorp-
tion at 293 nm, giving rise to four isosbestic points at 222,
236, 274, and 308 nm (Figure 2, a).
Figure 2. Titration of 3 (10 μm in CH3CN) with aqueous HCl
(1.6ϫ10–3 m). (a) UV/Vis spectra; (b) Excitation (λem = 450 nm)
and emission (λexc = 324 nm) spectra. Inset: relative emission inten-
sity at 450 nm upon excitation at 236 nm (one of the isosobestic
points).
Figure 1. (a) UV/Vis spectra of 1 (20 μm), 2 (10 μm), and 3 (10 μm)
in CH3CN; (b) fluorescence spectra of 1 [2 μm, dashed lines: exci-
tation (λem = 551 nm) and emission (λexc = 336 nm)] and 3 [10 μm,
solid lines: excitation (λem = 450 nm) and emission (λexc = 324 nm)],
in CH3CN.
A parallel modification is registered in the emission and
excitation spectra (Figure 2, b), where a progressive de-
The fluorescence spectra of 3 (Figure 1, b) retain the typ- crease of the dansyl-centered bands is observed respectively
ical pattern expected for the dansyl fluorophore with one at 450 (λexc = 324 nm) and at 247 and 330 nm (λem
=
emission band centered at 449 nm (λexc = 324 nm), albeit 450 nm). This behavior is consistent with the protonation of
somewhat blue-shifted and with diminished intensity (20- both dansyl amino groups, and is responsible for a gradual
fold) when compared to the reference emission observed for disappearance of the charge-transfer bands involving the ni-
the singlet n,π* excited state of 1, upon excitation at trogen lone-pair, as experimentally observed in both the ab-
336 nm.
sorption and fluorescence spectra.[17]
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The protonated H-3 complex can be selectively excited b). These findings support the involvement of the POM
upon irradiation at 293 nm (Figure 2, a), thus promoting scaffold in protonation equilibria.
emission at 344 and 461 nm (Figure S14–15). During acid
titration, the band at 344 nm, ascribed to the naphthalenic
emission of H-3, increases, while the longer wavelength
band, due to the emission of non-protonated 3, decreases.
The spectrophotometric titrations determined either in ab-
sorption, emission, or excitation modes, display similar pro-
files, as a function of the added acid equivalents, and high-
light the independent behavior of the dansyl units (Figure 3
and S16).[17]
Figure 3. Spectrophotometric titration of 3 (10 μm in CH3CN) by
stepwise addition of aqueous HCl (1.6 μm). Normalized values of
Figure 4. (a) Map of the electrostatic potential and (b) representa-
absorbance (A) and fluorescence intensity (I) variations monitored
tion of the HOMO and HOMO–1 orbitals of 3, delocalized in the
at different wavelengths.
dansyl moieties.
In all cases, the observed spectral variations can mainly
Fluoroionophore Properties of 3
be ascribed to the dansyl probe, and they follow a sigmoidal
behavior, with an initial “silent” phase, which was clearly
determined in the fluorescence titration, followed by a lin-
ear behavior to a plateau value reached at two equivalents.
This is likely explained by an initial buffering effect of the
inorganic polyanion, which acts as a competing proton
scavenger in the system.[29]
To evaluate the optical response of hybrid 3 towards
other cationic analytes and its potential as a fluorescent
chemosensor for metal ions, emission quenching experi-
ments were performed in the presence of Cu2+ Fe2+, Ni2+
,
Hg2+, Co2+, Cd2+, Zn2+, and Pb 2+ by addition of an aque-
ous analyte solution (1.6 mm) to 3 (10 μm in CH3CN). The
bar-graph in Figure 5 highlights the dependence of the fluo-
This hypothesis has been addressed by DFT calculations
by using the ZORA-BP86 functional with the TZ2P basis
set, including solvent and relativistic effects. Due to the
large size of the molecules and to the high number of heavy
metals, the internal or core electrons were kept frozen (see
Exp. Sect.). These methods provide the optimized geometry
for 3, together with the molecular electrostatic potential
(MEP), and frontier molecular orbital (FMO) analysis
(Figure 4). MEP analysis allowed the relative basicity of the
different sites in the molecular hybrid to be estimated and
indicated that a significant and extended electron density
was localized at the oxygen centers of the polyoxometalate
framework (Figure 4, a). Moreover, inspection of the calcu-
lated FMO distribution showed that the filled orbitals were
delocalized on the oxygen POM centers (oxo band) with
energy levels close to the dansyl HOMO and HOMO–1,
mostly identified by the π orbitals of the naphthalenic units
with approximately 30% contribution of the p orbital of the
Figure 5. Optical sensing screening determined as the ratio between
the fluorescence intensity observed for 3 (10 μm in CH3CN with
2.5% v/v H2O) in the absence of any metal ion (I0) and in the
presence of 40 μm metal ions (IM) (λexc = 324 nm; λem = 457 nm,
nitrogen atoms of the dimethylamino functions (Figure 4, for Pb2+ λem = 525).
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Organic–Inorganic Molecular Nano-Sensors
rescence intensity ratio IM/I0 determined at λem = 457 (λex
=
For comparison, the fluorescence response of monomeric
324 nm) as a function of the metal ion added in the system. reference 1 was evaluated with respect to Cu2+, which
In most cases, no appreciable change in the emission in- showed negligible activity. This observation highlights the
tensity occurred (IM/I0 ≈ 1), with the remarkable exceptions interplay between the two domains in the sensing mecha-
of Cu2+ and Pb2+, which are two ions that are typically nisms. Indeed, at variance with the majority of previously
recognized as hazardous contaminants for drinking reported Cu-chemosensors, 3 does not contain any ad-
water.[30] The optical sensing by 3 clearly entails diverse re- ditional binding site tailored for a specific transition
cognition mechanisms, because opposite effects are induced metal.[31] Selective sensing by 3 and the tuning of its optical
in the fluorescence emission upon addition of either Cu2+ response stems from its multi-functional molecular nature
or Pb2+ analyte.
integrating a dansyl-based optical transducer with a nano-
In particular, sensing of aqueous PbSO4 (40 μm), was de- dimensional tungsten-oxide polyanionic surface. This latter
termined by a redshift (68 nm) of the maximum emission feature is instrumental for the geometrical constrains/spac-
wavelength (λ = 525 nm), together with a progressive ten- ing of the bis-fluorophore tweezer, the shaping of its sol-
fold increase of luminescence at that wavelength (Figure vation environment, and the occurrence of competing cat-
S17). Fluorescence enhancement has been previously re- ion scavenging equilibria that can be exploited in the analy-
ported for dansyl-based Pb2+ ionophores, which was as- sis of complex mixtures. Indeed, the selective recognition of
cribed to metal ligation upon deprotonation of the NH Cu2+ by 3 has also been validated in the presence of poten-
amide function.[25] In this specific case, however, Pb2+ ions tially interfering bivalent metal ions such as Fe2+, Ni2+
,
may trigger a change either in the electronic structure or in Co2+, and Zn2+. The presence of these latter ions did not
the interactions between the chromophoric units, such as result in any detrimental effects in the system, even when
a decreased stacking of paired naphthalenic moieties. The added in large excess (40 μm). On the other hand, the com-
similar emission of the Pb2+ adduct and of derivative 1 may bined presence of “dansyl-silent” metal cations, for instance
also suggest a reduced interaction between the dansyl units Co2+, turns out to improve the optical signaling of Cu2+
,
and the polyanionic domain.
resulting in a linearization of the sigmoidal fluorescence ti-
On the other hand, stepwise addition of CuSO4 tration curve (Figure 6). This can be explained by a prefer-
(1.6ϫ10–3 m, ca. 4 μL aliquots) induced a progressive ential electrostatic interaction between Co2+ and the POM
quenching of the emission band, monitored at 457 and at surface, thus switching off its masking effect for the compet-
525 nm (λexc = 324 nm), down to a plateau value of about ing Cu2+, and overriding the initial lag-response of the dan-
5–14% with respect to the initial conditions (Figure 6).
syl fluorophore.[32] Under such optimized conditions, a lin-
ear correlation is obtained, featuring a micromolar detec-
tion limit with maximum quenching at [Cu2+] above 12 μm,
pointing to a 1:1 interaction with 3.
Supramolecular Properties of Hybrid 3 and Thin Film
Heterogenization
Because of its composite but well-defined molecular
structure, hybrid 3 provides a functional model vis-à-vis the
emerging area of nano-supported chemosensors.[17,33] The
key motif of surface-implemented systems is the organiza-
tion of fluorophore arrays where spatial proximity is such
that multiple electron/energy transfer events are leveraged,
thus enhancing optical sensing.[17]
Aggregation/surface phenomena can be envisaged for hy-
brid 3 to take place as a result of its organic–inorganic and
amphiphilic nature, which is likely to yield extended supra-
molecular architectures as a function of the solvent compo-
Figure 6. Normalized values of fluorescence intensity (I) obtained
for the titration of hybrid 3 (10 μm in CH3CN) with an aqueous
CuSO4 solution (0–20 μm), in the presence and in the absence of
40 μm CoSO4.
The Cu2+ titration profile resembles the protonation sition. To shed light into the solution structure of 3 (10 μm)
curve, determined for 3 with a similar procedure, again under the sensing conditions, Dynamic Light Scattering
showing a sigmoidal shape, with a turning point at about (DLS) experiments were performed in CH3CN with added
1:1 equiv. ratio. Moreover, the UV/Vis spectrum registered water (2.5% v/v). In this medium, DLS analysis allowed the
for 3 in the presence of the cuprous salt retains isosbestic characterization of spherical aggregates with average hydro-
points (albeit with a lower absolute ΔAbs) similar to those dynamic radius Rh = 800 nm and a broad size distribution
ascribed to the protonation equilibria (Figure S18). Accord- (Figure S19); control analysis in neat CH3CN give no evi-
ingly, a plausible trigger for the Cu2+-induced emission dence of any supramolecular phenomena.
quenching could originate from its binding at the electron-
rich amine sites of the luminophore moiety.
Scanning electron microscopy (SEM) images confirmed
the formation of vesicular structures upon addition of more
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than 0.5% water to a CH3CN solution (Figure 7c). Their materials, which hold great promise for applications in or-
size range is consistent with the DLS evidence, displaying ganic/inorganic-based molecular devices.[40]
an average diameter of approximately 1 μm.[34,35]
To evaluate the potential of 3 as a supramolecular syn-
thon for three dimensional networks and sensing devices,
thin film formation was preliminarily addressed by SEM
studies after inclusion within polymeric membranes.[41]
To this end, vesicle aggregates of 3 were embedded within
a hydrophobic fluorinated polymer by blending polyvinyl-
idene difluoride (PDVF) and POM 3 in dimethylacetamide
solution (DMA) (16:1:83% w/w). After solvent casting,
membrane separation was obtained by phase inversion in
an aqueous bath.[42] The resulting film (300 μm thickness),
featured a homogeneous distribution of nanoclusters of 3
(SEM image in Figure 8, a) with average diameter 90 nm
that were roughly distributed in pillared arrays (Figure 8, b)
thus fostering a high surface area and an irregular cross-
section. It is noteworthy that the membrane luminescence
was maintained upon irradiation with a 366-nm lamp (Fig-
ure S22), emitting blue–green fluorescence. Moreover, fine
tuning of the polymer blend vis-à-vis surface properties and
wettability is expected to allow the preparation of ON/OFF
luminescent devices.[41]
Figure 7. (a) and (b) SEM micrographs of 3, drop casted from a
10 μm CH3CN solution [backscattered signal (left image) highlights
dark regions, without POMs], and (c) from a CH3CN solution con-
taining 0.5% H2O.
Amphiphilic POMs are known to yield well-defined sup-
ramolecular architectures, including vesicles, bilayer struc-
tures, and nano-objects with different morphology.[36] These
are mainly hybrid salts that are obtained by a tailored
choice of the organic counterion, as in the well-known sur-
factant encapsulated POMs.[37]
In the present system, the molecular unit is engineered
to incorporate directional covalent bonds connecting the in-
organic scaffold to an organic area with hydrophobic aro-
matic substituents.[38] These latter moieties can drive re-
cognition/association phenomena while also providing the
functional site of the system.
A solvent-casting film of 3 deposited on a silicon wafer
from a wet acetonitrile solution (10 μm) featured an ex-
tended area of POM-containing material (as highlighted by
the SEM, acquired with both secondary and backscattered
electron signals, Figure 7 a–b) with a surface distribution of
nanostructured cavities displaying a 200 nm diameter and
POM exposed on the inner walls (inset in Figure 7, b).
A porous architecture is instrumental for combining
transport/separation properties with sensing/recognition
functions, and this is likely ascribed to the templating effect
Figure 8. SEM micrographs of 3@PVDF at different magnifica-
tions (top side of the membrane). Bright areas identify the POM-
containing material.
Conclusions
The two-step synthetic procedure reported herein targets
of POM amphiphiles organized as supramolecular vesicles the organic–inorganic covalent assembly of innovative poly-
and acting as porogenic sites on solvent evaporation. Such oxometalate-based molecular chemosensors displaying re-
behavior has been previously observed for hybrid POM cognition and luminescent signaling functions. The selective
salts,[39] and can be exploited for the tailored fabrication functionalization of the lacunary Keggin-type polyoxotung-
of flat membranes implementing mechanical resistance and state [γ-SiW10O36]8–, which provides a highly robust, rigid,
selective separation/optical sensing functions.
and polyanionic nano-scaffold, has been successfully ex-
The covalent strategy used in POM hybrids is therefore ploited to shape a tweezer-like arrangement of a bis-dansyl-
expected to ultimately shape innovative multi-component sulfonamide moiety. The resulting hybrid 3 has been char-
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Organic–Inorganic Molecular Nano-Sensors
C94H182N8O41S2Si3W10 (4067.38): calcd. C 27.75, H 4.51, N 2.75,
acterized in the solid state and in solution by a combination
of spectroscopic and electron microscopy techniques to as-
certain its covalent bond connectivity, protonation state,
optical properties, and supramolecular aggregation state.
The formation of vesicles was detected in acetonitrile/aque-
ous media and was likely driven by the organic/inorganic
amphiphilic nature of the POM hybrid. The POM-based
bis-dansyl tweezer acts as a selective fluorescence sensor for
Cu2+ and Pb2+ ions, in quenching and enhancing mode,
S 1.58; found C 25.87, H 4.45, N 2.82, S 1.06. FTIR (KBr): ν =
˜
2961 (m), 2938 (m), 2873 (m), 634 (w,b), 1476 (m), 1394 (w), 1316
(w), 1146 (m), 1102 (m), 1043 (m,b), 967 (m), 906 (s), 885 (s), 821
(s), 792 (s), 741 (s, m), 684 (m), 625 (m), 569 (m), 544 (m), 508
1
(m) cm–1. H NMR (300 MHz, CD3CN, 301 K): δ = 0.24 (m, 4 H,
NCH2CH2CH2Si), 0.97 [t, 3J = 8.7 Hz, 48 H, N(CH2CH2-
CH2CH3)4], 1.31 [m, 36 H, N(CH2CH2CH2CH3)4 and
SiCH2CH2CH2N], 1.62 [m, 32 H, N(CH2CH2CH2CH3)4], 2.84
[m, 16 H, N(CH3)2 and SiCH2CH2CH2N], 3.14 [m, 32 H,
respectively, with tunable intensity that depends on the N(CH2CH2CH2CH3)4], 5.95 (t, 3J
=
6.0 Hz,
2
H,
NHCH2CH2CH2Si), 7.22 (m, 2 H, Ar-H), 7.58 (m, 4 H, Ar-H),
emission wavelength, POM-aided scavenging of potentially
interfering cations, and a detection limit in the micromolar
range. Whereas the reference monomer 1 displays negligible
sensing activity, 3 shows some remarkable benefits with re-
3
3
8.16 (m, 2 H, Ar-H), 8.30 (d, J = 8.7 Hz, 2 H, Ar-H), 8.48 (d, J
= 8.4 Hz, 2 H, Ar-H) ppm. 13C{1H} NMR (75.47 MHz, CD3CN,
301 K):
δ = 13.01 (2 C, SiCH2CH2CH2N), 14.07 [16 C,
N(CH2CH2CH2CH3)4], 20.47 [16 C, N(CH2CH2CH2CH3)4], 24.51
[16 C, N(CH2CH2CH2CH3)4], 24.73 (2 C, SiCH2CH2CH2N), 45.83
[4 C, N(CH3)2], 46.83 (2 C, SiCH2CH2CH2N), 59.40 [16 C,
N(CH2CH2CH2CH3)4], 124.53, 129.27, 129.99, 130.50, 130.70,
136.82, 152.87 (20 C, Ar) ppm. 183W NMR (16.67 MHz, CD3CN/
spect to previously known nano-colloid analogues: (i) Cu2+
/
Pb2+ sensing is achieved by implementing the luminescent
components in the tweezer arrangement, without the need
to integrate a tailored multi-dentate fragment; (ii) tuning of
the solubility as a function of the POM counterions can be
CH3CN, 298 K):
δ = –107.9 (4 W), –136.2 (2 W), –142.1
used to exploit typical dansyl-induced solvatochromism; (4 W) ppm. 29Si NMR (79.49 MHz, CD3CN/CH3CN, 298 K): δ =
–62.5 (2 Si), –88.4 (1 Si) ppm. ESI-MS (–ve, CH3CN): calcd. for
[C30H36N4O41S2Si3W10]4– 773.3; found 773.9. UV: λ (log ε) = 251
(4.7), 211 (5.1) nm.
(iii) POM-templated three-dimensional porous architec-
tures in thin films and flexible membranes have potential in
optical devices.
Our effort is now directed towards the surface decoration
of POM molecules with asymmetric fluorescent dyads exhi-
biting tailored photophysical properties with the aim of ex-
panding the field of applications.
Fluorimetric Titrations: CuSO4·5H2O, Zn(NO3)2·6H2O, HgSO4,
PbSO4, NiSO4·6H2O, FeSO4, CdSO4·H2O, and CoSO4·6H2O were
analytical-grade products and were used to prepare 1.6 mm aque-
ous solutions. 2 mL of a 10 μm CH3CN solution of 3 were placed
in a fluorescence quartz cell. A variable volume of concentrated
metal ion solution (up to 40 μL) was added, and water was added
to provide a fixed 2.5% water content. The resulting solutions were
allowed to equilibrate until stable emission spectra were obtained
(less than 5 min were usually required, except for Pb2+).
Experimental Section
General: All reagents were purchased from commercial sources and
used as received, without further purification. K8[γ-SiW10O36] was
prepared as described in the literature.[43] 1H NMR spectra were
recorded with Bruker AV300 instruments operating at 300.13 MHz.
13C NMR spectra were recorded with a Bruker AV300 operating
at 75.4 MHz; Si(CH3)4 was used as reference. 183W and 29Si NMR
spectra were recorded with a Bruker Avance DRX 400 instrument
operating at 16.67 and 79.50 MHz, respectively, using 2 m Na2WO4
in D2O and Si(CH3)4 in CDCl3 as external references. FTIR (KBr)
spectra were collected with a Thermo Quest Nicolet 5700 instru-
ment. ESI-MS spectra were obtained with a Agilent LC/MSD Trap
SL spectrometer, by using a capillary potential of 1500 V. Fluori-
metric analyses were performed with a Perkin–Elmer LS50B instru-
ment with 1 cm quartz cell. Dynamic light scattering was per-
formed with a Malvern Zetasizer Nano-S instrument. Field emis-
sion scanning electron microscopy (FE-SEM) measurements were
carried out at acceleration voltages between 5.0 and 20.0 kV with
a Zeiss SUPRA 40VP instrument.
Membrane Preparation: In a vial, 3 (20 mg), PVDF (300 mg), and
dimethylacetamide (1.6 g) were stirred overnight. The viscous mix-
ture was cast on a glass surface using a casting knife set at 300 μm.
The mixture was then immersed in water until the membrane,
formed by phase inversion, separated from the glass surface. The
film was thoroughly washed with water, dried under vacuum at
65 °C for 2 h and then placed in a oven at 65 °C for 12 h.
DFT Calculations: Computational resources and assistance were
provided by the Laboratorio Interdipartimentale di Chimica Com-
putazionale (LICC) at the Department of Chemical Sciences of the
University of Padova. DFT calculations were carried out using the
Amsterdam density functional (ADF) code;[44] scalar relativistic ef-
fects were taken into account by means of the two-component zero-
order regular approximation (ZORA) method,[45] adopting the
Becke 88 exchange plus the Perdew 86 correlation (BP) func-
tional.[46] The basis functions for describing the valence electrons
are triple-zeta quality, doubly polarized (TZ2P), specially opti-
mized for ZORA calculations. Due to the large size of the mole-
cules under investigation, the internal or core electrons (C, N and
O: 1s; Si: 1s to 2sp; W: 1s to 4spdf) were kept frozen. Geometries
were optimized without symmetry constraints. The solvent effect
was modeled by means of the ADF implementation[47] of the CO-
SMO method.[48]
(nBu4N)4[{{(CH3)2N}C10H6SO2NH(CH2)3Si}2O(γ-SiW10O36)]: In
a round-bottomed flask, 2 (400 mg, 0.12 mmol) was suspended in
acetonitrile (3 mL). Triethylamine (58 μL, 0.42 mmol) was slowly
added under vigorous stirring. After for 5 min, dansyl chloride
(112 mg, 0.415 mmol) was dissolved in acetonitrile (4 mL) and
added to the solution whilst stirring. The mixture was heated at
40 °C under reflux for 2.5 h and then centrifuged to remove insolu-
ble reagents and byproducts. The volume of the solution was re-
duced to 1 mL by evaporation under vacuum, then diethyl ether
was added to precipitate the product. The solid was washed with
water and diethyl ether on a fritted funnel and dried for several
hours under vacuum. Yield: 366 mg (0.09 mmol, 69%).
Supporting Information (see footnote on the first page of this arti-
cle): Synthesis and characterization of compounds 1 and 2, spectra
not reported in the paper, other SEM images, selectivity test by
fluorimetric analysis, optimized geometry coordinates.
Eur. J. Org. Chem. 2012, 281–289
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
287

M. Carraro, M. Bonchio et al.
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Acknowledgments
Financial support from the University of Padova (PRAT
CPDA084893/08),
Progetto
Strategico
2008
HELIOS
(STPD08RCX), Ministero dell’Università e della Ricerca (MIUR)
(PRIN contract number 20085M27SS and FIRB code RBAP11-
ETKA_006) and Fondazione Cariparo (Nano-Mode, progetti di
eccellenza 2010) are gratefully acknowledged. We thank Antonio
Sorarù for DLS analysis.
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Received: July 30, 2011
Published Online: November 24, 2011
Eur. J. Org. Chem. 2012, 281–289
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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