Enhanced fluorescence detection of metal ions using light-harvesting mesoporous organosilica

Article abstract of DOI:10.1002/chem.201102492

Enhanced fluorescence detection of metal ions was realized in a system consisting of a fluorescent 2,2′-bipyridine (BPy) receptor and light-harvesting periodic mesoporous organosilica (PMO). The fluorescent BPy receptor with two silyl groups was synthesiz

Full text of DOI:10.1002/chem.201102492

DOI: 10.1002/chem.201102492
Enhanced Fluorescence Detection of Metal Ions Using Light-Harvesting
Mesoporous Organosilica
Minoru Waki,[a, b] Norihiro Mizoshita,[a, b] Yoshifumi Maegawa,[a, b] Takeru Hasegawa,[b, c]
Takao Tani,[a, b] Toyoshi Shimada,[b, c] and Shinji Inagaki*[a, b]
Abstract: Enhanced fluorescence de-
tection of metal ions was realized in a
system consisting of a fluorescent 2,2’-
bipyridine (BPy) receptor and light-
harvesting periodic mesoporous orga-
nosilica (PMO). The fluorescent BPy
receptor with two silyl groups was syn-
thesized and covalently attached to the
pore walls of biphenyl (Bp)-bridged
PMO powder. The fluorescence inten-
sity from the BPy receptor was signifi-
cantly enhanced by the light-harvesting
property of Bp-PMO, that is, the
energy funneling into the BPy receptor
from a large number of Bp groups in
the PMO framework which absorbed
UV light effectively. The enhanced
emission of the BPy receptor was
quenched upon the addition of a low
concentration of Cu2+ (0.15–1.2ꢀ
10ꢀ6 m), resulting in the sensitive detec-
tion of Cu2+. Upon titration of Zn2+
(0.3–6.0ꢀ10ꢀ6 m), the fluorescence exci-
tation spectrum was systematically
changed with an isosbestic point at
375 nm through 1:1 complexation of
BPy and Zn2+ similar to that observed
in BPy-based solutions, indicating
almost complete preservation of the
binding property of the BPy receptor
despite covalent fixing on the solid sur-
face. These results demonstrate that
light-harvesting PMOs have great po-
tential as supporting materials for en-
hanced fluorescence chemosensors.
Keywords: fluorescence · ligand ef-
fects · light harvesting · mesoporous
materials · metal-ion detection
Introduction
Periodic mesoporous organosilicas (PMOs), synthesized
by surfactant-directed self-assembly of bridged organosilane
Mesoporous silica is an ideal support material for sensitive
receptors because of its high surface area (ca. 1000 m2 gꢀ1)
and uniform porosity.[1–3] The high surface area allows
doping with high concentrations of sensitive receptors, and
the uniform porosity allows the facile diffusion of analytes
(e.g., molecules and ions). The mechanical robustness and
high processability of mesoporous materials are also great
advantages in their practical applications. A variety of sensi-
tive receptors have been anchored on the pore surfaces of
mesoporous silicas through post-synthetic grafting or co-
condensation for use in fluorescent chemosensors.[1–3]
precursors (R-[Si
(OR’)3]n: nꢁ2, R=bridging organic group,
R’=CH3, C2H5, etc.), are unique hybrid materials in which
functional organic groups (R) are densely and covalently
embedded within the silica walls.[4] There have been a large
number of reports on the synthesis of PMOs containing vari-
ous bridging organic groups in their frameworks,[5] including
highly luminescent organic fluorophores[6] such as biphen-
yl,[7] oligo(phenylenevinylene),[8] and tetraphenylpyrene.[9]
Chandra et al. reported the application of a luminescent
PMO bearing a diimine moiety to metal-ion detection.[10]
The organization of two or more different fluorophores in
nanometer-sized channel materials is important for the fab-
rication of highly functional donor–acceptor structures such
as energy-transfer and energy-funneling systems.[11] PMOs
allow the fluorophores to be located in two spatially sepa-
rated regions: in the pore walls and in the mesochannels.[12]
We recently reported unique light-harvesting antenna prop-
erties for PMOs doped with dye molecules in the mesochan-
nels.[13,14] The organic fluorophores in the pore walls ab-
sorbed light effectively and transferred their excitation
energy to the dye molecules in the mesochannels through
Fçrster resonance energy transfer (FRET). The efficient
funneling of the excitation energy into the dye by FRET sig-
nificantly enhanced the fluorescence emission from the dye.
For example, biphenyl (Bp)-PMO doped with coumarin 1
dye showed a remarkable enhancement of the emission
from the dye, in which light energy absorbed by a large
number of Bp groups in the pore walls was transferred to a
[a] Dr. M. Waki, Dr. N. Mizoshita, Y. Maegawa, Dr. T. Tani,
Dr. S. Inagaki
Toyota Central R&D Laboratories, Inc.
Nagakute, Aich 480-1192 (Japan)
Fax : (+81)561-63-6507
E-mail: inagaki@mosk.tytlabs.co.jp
[b] Dr. M. Waki, Dr. N. Mizoshita, Y. Maegawa, T. Hasegawa,
Dr. T. Tani, Dr. T. Shimada, Dr. S. Inagaki
Core Research for Evolutional Science and Technology (CREST)
Science and Technology Agency (JST)
Kawaguchi, Saitama 332-0012 (Japan)
[c] T. Hasegawa, Dr. T. Shimada
Department of Chemical Engineering
Nara National College of Technology
Yamatokoriyama, Nara 639-1080 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102492.
1992
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FULL PAPER
single dye molecule in the mes-
ochannel with a large quantum
efficiency.[13]
Light-harvesting
PMOs have been successfully
applied to an enhanced photo-
Scheme 1. Preparation of fluorescent bipyridine receptor 1.
catalytic system of CO2 reduc-
tion[15] and to highly luminescent PMO films with tunable
rescent receptor containing the BPy ligand 1 was newly de-
signed. The receptor 1 was synthesized by a coupling reac-
tion of 5,5’-dibromo-2,2’-bipyridine with molecular building
emission colors.[9,16]
Here, we report the first application of a light-harvesting
PMO to an enhanced metal-ion detection system. As illus-
trated in Figure 1, the emission from a fluorescent receptor
blocks
of
1-diallylethoxysilyl-4-ethynylbenzene[26]
(Scheme 1). The attachment of phenylethynyl groups on
both sides of the BPy expanded the p-conjugation length,
which resulted in a red shift of the absorption band of the
receptor from labs =280 to 350 nm. This red shift was favora-
ble for efficient FRET from Bp-PMO to the receptor be-
cause the absorption band overlapped well with the emis-
sion band of Bp-PMO (lem =385 nm) (Figure 2). A diallyle-
Figure 1. Schematic illustrations of enhanced fluorescence detection of
analyte using a light-harvesting PMO.
attached in the mesochannels of PMO can be enhanced by
the light-harvesting effect. The enhanced emission can be
quenched or shifted in wavelength through the interaction
between the receptor and an analyte, which results in sensi-
tive analyte detection. Similar enhanced fluorescence detec-
tions based on FRET have been reported for zeolites,[11]
silica[17] and polymer[18] nanoparticles, micelles,[19] self-assem-
bled monolayers,[20] Langmuir–Blodgett films,[21] and den-
drimers[22] functionalized with fluorophores and receptors.
Compared to these materials, PMOs have an ideal confor-
mation of fluorophores and receptors, that is, numerous flu-
orophores surround a receptor placed in the rigid nanochan-
nels, which offer great advantages such as a large enhance-
ment effect and easy access of the analyte to the receptors.
Amplified fluorescence chemosensors have also been re-
ported for conjugated polymers, in which a single interaction
with an analyte can quench a large number of fluorophores
through exciton transport along the polymer chain.[23] In this
study, a novel BPy-bridged organosilane was synthesized
and covalently attached on the pore walls of Bp-PMO
powder. The BPy receptor in Bp-PMO was confirmed to
show an enhancement of the fluorescence intensity by the
light-harvesting effect, which successfully emphasized the
change in emission intensity upon the addition of metal
ions. In addition, the BPy receptor was found to show a sim-
ilar binding property to that in BPy-based solution systems
despite its covalent fixing on the solid surface.
Figure 2. a) Fluorescence excitation (gray) and emission (black) spectra
of Bp-PMO dispersed in CH2Cl2. b) Absorption (gray) and emission
(black) spectra of BPy receptor 1 in CH2Cl2 (1.0ꢀ10ꢀ5 m).
thoxysilyl group was employed instead of a trialkoxysilyl
group to bind the receptor to the surface silanol groups (Si-
OH) in Bp-PMO. This substitution was made because the
diallylethoxysilyl group is more stable than a trialkoxysilyl
group during palladium-catalyzed coupling reactions and pu-
rification by silica gel chromatography,[27] but it behaves as
the synthetic equivalent of a trialkoxysilyl group, as reported
by Shimada et al., who demonstrated the functionalization
of silica surfaces using an allylorganosilane at the reflux
temperature of toluene without any acid or base catalysts.[28]
In our preliminary experiments, the functionalization of Bp-
PMO using receptor 1 with no catalyst was unsuccessful,
possibly because of the lower acidity of the silanol groups in
Bp-PMO than in silica.[29] Therefore, we added a catalytic
amount of trifluoroacetic acid to the starting suspension to
promote the surface coupling reaction during reflux
(Scheme 2). The amounts of BPy receptor loaded onto Bp-
PMO, which were adjusted by the amount of 1 added to the
Results and Discussion
Preparations of PMO with bipyridine receptor: The BPy
moiety was selected as a receptor because it has typically
been used as a strong ligand for transition metal ions, and
has been incorporated in various molecular[24] and poly-
mer[25] chemosensors for the detection of metal ions. A fluo-
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S. Inagaki et al.
Table S1 in the Supporting Information). The average pore
diameter also decreased, without a large decrease in the spe-
cific surface area. These results strongly suggest that 1 was
successfully attached onto the pore surface of Bp-PMO. The
29Si magic-angle spinning (MAS) NMR spectra of BPy/Bp-
PMO (0.038) showed three signals at ꢀ82.6, ꢀ73.0, and
ꢀ10.6 ppm, corresponding to T3, T2, and D1 silicon species
Scheme 2. Attachment of bipyridine receptor 1 onto the pore wall of Bp-
PMO in refluxing toluene containing trifluoroacetic acid.
(Tn: RSi(OH)
(OSi)n, n=0–3, Dm: R2Si(OH)
3ꢀnACHTUTGNRENNUG 2ꢀmACHTUNGTRENNUNG(OSi)m,
m=0–2), respectively (Figure 4). The Tn silicon species were
starting suspension, were finally determined to be 12, 27, 75,
and 140 mmolgꢀ1 by a Total Nitrogen Analyzer, assuming
that the detected nitrogen originated from the BPy receptor.
The Bp-PMOs with attached BPy receptors are represented
as BPy/Bp-PMO (X), where X is the BPy/Bp molar ratio of
0.003, 0.007, 0.020, or 0.038.
Figure 3a shows the X-ray diffraction (XRD) patterns of
BPy/Bp-PMO with different BPy/Bp ratios. All the samples
Figure 4. 29Si MAS NMR of a) BPy/Bp-PMO and b) Bp-PMO.
attributed to the framework biphenyl-silica of Bp-PMO.[7a]
The D1 silicon species was attributed to the silyl groups of
the BPy receptor, which were condensed with silanol groups
on the pore surface through elimination of the allyl groups
(Scheme 2). There was no detection of uncondensed silicon
species (S0: R3Si(OH), D0: (R2Si(OH)2, and T0: (RSi(OH)3),
suggesting that both sides of the silyl groups of the receptor
1 were attached on the surface. For the low BPy/Bp ratios
(0.003, 0.007, and 0.020), the signals of the D1 species were
not clearly observed because the amounts of BPy receptor
were too small to detect (Figure S1 in the Supporting Infor-
mation). Figures 5a and 5b show the 13C cross-polarization
(CP)-MAS NMR spectra of BPy/Bp-PMO (0.038) and Bp-
PMO, respectively. The signals of the BPy receptor over-
lapped with the original signals of Bp-PMO, so the differen-
tial spectrum was calculated between BPy/Bp-PMO and Bp-
PMO (Figure 5c). This was very consistent with the
13C NMR spectrum of the BPy receptor 1 in CDCl3 (Fig-
ure 5d), indicating the successful anchoring of 1 in Bp-
PMO.
Figure 3. a) XRD patterns of BPy/Bp-PMO with different BPy/Bp ratios.
b) Nitrogen adsorption isotherms of BPy/Bp-PMO with different BPy/Bp
ratios.
Enhancement of fluorescence: Figures 6a and 6b show the
fluorescence spectra of BPy/Bp-PMOs (0.003–0.038) excited
at 350 and 280 nm, respectively. The 350 nm light was direct-
ly absorbed by the BPy receptor in Bp-PMO because the
BPy receptor had much higher absorbance at this wave-
length than Bp-PMO, as shown in Figure 2. However, the
absorption efficiency of the light is not so high because of
the low concentration of BPy receptors in Bp-PMO, which
result in a relatively low-intensity fluorescence emission
showed clear diffraction patterns for both an ordered meso-
porous structure (2q=1.82–1.868, d100 =4.75–4.85 nm) and a
periodic arrangement of the Bp groups within the pore walls
(2q=7.4, 14.9, 22.5, 30.1, and 37.98, d=1.19, 0.59, 0.40, 0.30,
and 0.24 nm). The nitrogen adsorption/desorption isotherms
were type IV, which is typical for an ordered mesoporous
material, although the adsorption capacity gradually de-
creased with increasing BPy/Bp ratio (Figure 3b and
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Enhanced Fluorescence Detection of Metal Ions
FULL PAPER
Bp groups to the BPy receptors
was confirmed by the strong
quenching of the fluorescence
emission from the Bp groups
(lem =385 nm)
in
BPy/Bp-
PMOs (Figure 6b). The energy-
transfer efficiencies were esti-
mated to be 68%, 83%, and
99% at BPy/Bp=0.003, 0.007,
and 0.020–0.038, respectively,
from the quenching rates of the
Bp groups emissions (Table S2
in the Supporting Information).
The fluorescence intensity
from BPy/Bp-PMO decreased
with increasing BPy/Bp ratio
(Figure 6b). The decrease in
fluorescence intensity was at-
tributed to self-quenching of
the BPy receptors in the meso-
channels because of the inter-
molecular interaction at high
concentrations, which was con-
firmed by a large decrease in
the fluorescence quantum yield
of BPy in Bp-PMOs (F350 =0.73
! 0.20, Table S2 in the Sup-
porting Information) and the
red shift of the fluorescence
band. Thus, the increase in the
loading amount of BPy recep-
tor has a limitation for the en-
hancement of the fluorescence
intensity in the BPy/Bp-PMO
system. The light-harvesting
property of PMO has a great
merit for the enhancement of
the fluorescence intensity of the
receptor without increasing its
loading amount.
Figure 5. 13C CP-MAS NMR spectra of a) BPy/Bp-PMO (0.038) and b) Bp-PMO. c) Differential spectrum be-
tween BPy/Bp-PMO (0.038) and Bp-PMO. d) 13C NMR spectrum of BPy receptor 1 in CDCl3.
Metal-ion detection: Figure 7
shows the changes in the fluo-
rescence spectra of BPy/Bp-
PMO powder (0.003) dispersed
in CH2Cl2 upon titration of a
CuACHTNURTGNE(UNG OAc)2 solution. The en-
hanced fluorescence was largely
quenched at very low concen-
Figure 6. Fluorescence spectra of BPy/Bp-PMO powder dispersed in CH2Cl2 excited at a) 350 and b) 280 nm,
and schematic illustrations of the emission mechanism from BPy/Bp-PMO when excited at c) 350 nm and
d) 280 nm.
(Figures 6a and 6c). On the other hand, the 280 nm light
was effectively absorbed by Bp-PMO because of the high
density of Bp groups and its high absorbance at this wave-
length (Figure 2). The excitation energy on a large number
of Bp groups was transferred to a few BPy receptors
through FRET, which promoted a strong fluorescence emis-
sion from the receptor due to the light-harvesting effect
(Figures 6b and 6d). The efficient energy transfer from the
trations of Cu2+ ions (0–1.2ꢀ10ꢀ6 m) when excited at 280 nm
(Figure 7a), whereas the changes in the fluorescence intensi-
ty were very small when excited at 350 nm (Figure 7b). This
clearly indicates the enhanced detection of Cu2+ ions
through the light-harvesting effect of PMO. The weak fluo-
rescence at 385 nm observed after quenching was the un-
quenched emission from the Bp groups in Bp-PMO arising
from the low receptor concentrations (Figure 7a).
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S. Inagaki et al.
cence band at around 470 nm. The excitation spectrum (Fig-
ure 8b) also showed a decrease in the absorption band at
around 350 nm, with the concomitant formation of a new
red-shifted absorption band at around 400 nm through an
isosbestic point at 375 nm. A similar systematic change in
the spectra was observed for homogeneous solution systems
with dissolved BPy-containing molecules[24] and polymers[25]
upon the addition of Zn2+. The spectral red shift is under-
stood as an enhanced planarity of the BPy unit and the re-
sulting change in the electron density upon 1:1 metal com-
plexation, as shown in Figure 8c. The Jobꢂs plot of BPy/Bp-
PMO with Zn2+ was obtained from the fluorescence intensi-
ties at 400 nm (new band) in excitation spectra measured
under a constant combined BPy and Zn2+ concentration
([BPy] + [Zn2+]) of 3.0ꢀ10ꢀ6 m (Figure S2 in the Supporting
Information). The plot shows a typical curve with a maxi-
mum at the molar ratio of 0.5, which indicates typical 1:1
complexation between the BPy ligand and Zn2+. The itera-
tive least-squares curve-fitting method (Figure S3 in the
Supporting Information) gave an association constant of
2.8ꢀ106 mꢀ1 for BPy/Bp-PMO and Zn2+, which is higher
than the reference value (2.2ꢀ105 mꢀ1) of an unmodified
BPy solution for Zn2+ [24b] To the best of our knowledge, this
.
is the first example of a systematic spectral change of BPy
ligands attached onto the surface of a solid material upon
the titration of metal ions. The systematic spectral change
through 1:1 complexation in BPy/Bp-PMO suggests a very
weak interaction between the BPy ligands and the surface
silanol groups of Bp-PMO, in contrast to BPy adsorbed on
silica, which was reported to interact with the surface silanol
groups to form ternary surface complexes involving BPy, si-
lanol groups, and metal ions.[30] This was probably a result of
the well-defined pore surface structure and the low concen-
tration of silanol groups in the PMO. The systematic spec-
tral change with minimal influence from the surface silanol
groups of BPy/Bp-PMO is advantageous for applications in
chemosensors.
Figure 7. Changes in fluorescence spectra of BPy/Bp-PMO (BPy/Bp=
0.003) powder dispersed in CH2Cl2 upon titration of Cu(OAc)2. The con-
AHCTUNGTRENNUNG
centrations of CuACHTUNGTRENNUNG(OAc)2 in the dispersions were 0, 0.15, 0.3, 0.45, 0.6, 0.9,
and 1.2ꢀ10ꢀ6 m. The excitation wavelengths were a) 280 nm and
b) 350 nm.
Figure 8a shows the change in the enhanced fluorescence
spectrum of BPy/Bp-PMO dispersed in CH2Cl2 upon titra-
tion of a ZnACHTUNGTRENNUNG
(OAc)2 solution. The addition of Zn2+ caused a
gradual decrease in the fluorescence band at around
420 nm, with the concomitant formation of a new fluores-
Figure 9 shows the fluorescence spectra of BPy/Bp-PMOs
upon the addition of various metal ions. Red shifts similar
to that observed for Zn2+ were observed for Ag+, Hg2+
,
Cd2+, and Pd2+, but strong fluorescence quenching occurred
only for Cu2+. Complexes of Zn2+, Hg2+, Cd2+, and Ag+
with a closed-shell d10 configuration are often emissive,
whereas that of Cu2+ with a partially filled d9 shell often is
not.[24,25] Eu3+ gave almost no change in the fluorescence
spectrum because of its weak binding to the 2,2’-bipyridyl
unit in the presence of methanol.[31] The dependence of the
spectral change of the fluorescence on the metal ions was
similar to that observed for the BPy-derivative molecule[24]
and BPy-modified polymer[25] reported by Smithꢂs group.
They reported a 44 nm red shift of the emission upon the
addition of divalent d10 metal ions of Zn2+, Hg2+, and Cd2+
,
Figure 8. Changes in fluorescence a) emission (excitation wavelength of
280 nm) and b) excitation (detected emission wavelength of 420 nm)
spectra of BPy/Bp-PMO (BPy/Bp=0.007) powder dispersed in CH2Cl2
accompanied by an increase in the emission intensity when
the new absorption band was excited at 420 nm. This sug-
gests that the BPy ligand in Bp-PMO has similar binding
properties with metal ions to those of molecules and poly-
mers.
upon titration with ZnACHTUNGTRENNUNG(OAc)2. The concentrations of ZnHCATUNGTRNEN(UGN OAc)2 in the
dispersions were 0, 0.3, 0.6, 0.9, 1.5, 2.1, 3.0, 3.9, 4.8, and 6.0ꢀ10ꢀ6 m. The
detected emission and excitation wavelengths were 431 and 280 nm, re-
spectively. c) Planarization of BPy ligand upon binding with Zn2+
.
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Enhanced Fluorescence Detection of Metal Ions
FULL PAPER
heated under reflux for 40 h. After being cooled to room temperature,
the reaction mixture was quenched with water. The organic layer was di-
luted with CH2Cl2, and the solution was washed with water, dried over
anhydrous MgSO4, and concentrated under reduced pressure. The resi-
due was chromatographed on silica gel (hexane/CH2Cl2 =1:2) to give
5,5’-bis[4-(diallylethoxysilyl)phenylethynyl]-2,2’-bipyridine (1) (802 mg,
84%) as a yellow solid. IR (neat): n˜max =3076, 2972, 2920, 2877, 2216,
1630, 1585, 1529, 1495, 1462, 1417, 1390, 1363, 1153, 1101, 1078, 1020,
991, 949, 930, 895, 843, 822, 764 cmꢀ1 1H NMR (400 MHz, CDCl3): d=
;
8.82 (d, J=1.8 Hz, 2H), 8.44 (d, J=8.2 Hz, 2H), 7.95 (dd, J=8.2 Hz,
1.8 Hz, 2H), 7.61–7.56 (m, 8H), 5.87–5.76 (m, 4H), 4.99–4.91 (m, 8H),
3.79 (q, J=7.1 Hz, 4H), 1.95 (d, J=8.2 Hz, 8H), 1.23 ppm (t, J=7.1,
6H); 13C NMR (100 MHz, CDCl3): d=18.4, 21.1, 59.4, 87.3, 93.7, 115.0,
120.4, 120.6, 123.8, 130.8, 132.8, 134.0, 136.4, 139.4, 151.8, 154.1 ppm;
HRMS (FAB): m/z calcd for C42H45O2N2Si2 [M+H]+: 665.3014; found:
665.3027.
Preparation of BPy/Bp-PMOs: Bp-PMO was synthesized according to
the previous report.[7a] 1 (3, 6, 30, or 60 mg, 0.0045, 0.009, 0.045, or
0.090 mmol) and trifluoroacetic acid (0.14, 0.28, 1.4, or 2.8 mg) were
added to a Bp-PMO (0.14 g) suspension in toluene (12 mL). The suspen-
sion was heated under reflux for 24 h and filtered. The resulting solid was
washed with AcOEt to remove the unreacted 1, and dried under vacuum
at room temperature for 24 h. The amounts of the binding BPy receptor
were measured as follows. The nitrogen contents of BPy/Bp-PMO and
Bp-PMO were measured by a Total Nitrogen Analyzer (Mitsubishi, ND-
100). The nitrogen content (98 ppm) of Bp-PMO, which was attributed to
the residual template surfactant (octadecytrimethylammonium), was sub-
tracted from the total nitrogen content of BPy/Bp-PMO. The resultant
nitrogen content was assumed to originate from BPy.
Figure 9. Fluorescence spectra of BPy/Bp-PMO powder (0.007) dispersed
in CH2Cl2 upon the addition of Eu
Hg(OAc)2, Pd(OAc)2, or Cu
(OAc)2 (3.0ꢀ10ꢀ6 m). The excitation wave-
length was 280 nm.
ACHTUGNTRENNU(GN OTf)3, ZnACHTUNRTGENN(GUN OAc)2, CdACHTUNGRTEN(NUGN OAc)2, AgOTf,
A
R
ACHTUNGTRENNUNG
Conclusion
We have reported the enhanced fluorescence detection of
metal ions using a light-harvesting PMO. A newly designed
fluorescent receptor containing a BPy ligand was attached
onto the pore walls of Bp-PMO. The emission of the fluores-
cent BPy receptor in Bp-PMO was successfully enhanced by
the light-harvesting effect. The enhanced emission of the
BPy receptor was quenched upon the addition of a low con-
centration of Cu2+. Systematic changes in the enhanced
fluorescence emission and excitation spectra were also ob-
served upon the addition of Zn2+ and other metal ions.
PMOs have a merit in terms of the flexibility of sensor
design by appropriate selection of the fluorescent receptor
in the mesochannels and the framework organic group of
PMOs. It will be very interesting to explore the special fea-
tures of light-harvesting PMOs for applications as support-
ing materials for dedicated fluorescence chemosensors.
Metal-ion detection: BPy/Bp-PMO (BPy/Bp=0.003 or 0.007) (4.5 mg)
was dispersed in CH2Cl2 (100 mL). A MeOH solution of metal salt (e.g.,
CuACHTUNRTGNENUG(OAc)2 or ZnACHUTNGTREN(NUNG OAc)2) was added to the suspension, and the changes in
its fluorescence spectra were monitored.
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Experimental Section
General: All reagents and solvents were commercially available and used
without further purification. 1H and 13C NMR spectra were measured
using a JEOL ECX-400 spectrometer, operated at 400 and 100 MHz, re-
spectively. Solid-state NMR analyses were performed by 29Si MAS and
13C CP MAS NMR spectroscopy (Bruker ADVANCE 400). Nitrogen ad-
sorption and desorption isotherms were measured on a Quantachrome
Nova 3000e sorptometer at ꢀ1968C. Prior to the measurements, all the
samples were outgassed at 608C for 6 h. XRD patterns were recorded on
a
Rigaku RINT-TTR diffractometer using CuKa radiation (50 kV,
300 mV). UV/Vis absorption and fluorescence spectra were measured
using JASCO V-670 and JASCO FP-6500 spectrometers, respectively.
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Received: August 11, 2011
Revised: October 4, 2011
Published online: January 13, 2012
1998
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 1992 – 1998