Cyclodextrin supramolecular complex as a water-soluble ratiometric sensor for ferric ion sensing

Article abstract of DOI:10.1021/la9033244

Heightened concern for human health and environmental protection has stimulated active research on the potential impact of transition-metal ions and their toxic effects, thus it is very demanding to design transition-metal ion detection methods that are cost-effective, rapid, facile, and applicable to the environmental and biological milieus. In this study, we demonstrated an alternative strategy for constructing a water-soluble FRET-based ratiometric sensor for ferric ion detection by forming a supramolecular β-cyclodextrin/ dye complex. This water-soluble FRET system consists of a dansyl-linked β-cyclodextrin (β-CD-DNS) and a spirolactam rhodamine-linked adamantane (AD-SRhB). The dansyl moiety serves as the donor, and the spirolactam-rhodamine B derivative (SRhB) was chosen as a sensitive, selective chemosensor for Fe(III) ions and a very efficient ring-opening reaction induced by Fe(III) generates the long-wavelength rhodamine B fluorophore that can act as the energy acceptor. Moreover, the adamantyl (AD) group, which is known for its capability to form stable host-guest inclusion complexes with β-CD derivatives, was covalently linked to the spirolactam rhodamine, thus the adamantyl moiety of the ion-recognition element can be anchored inside theCDcavity. In this way, the donor-acceptor separation can be kept within the critical Foerster distance; accordingly, energy transfer can take place from the donor (dansyl) to the acceptor (rhodamine derivative/Fe(III) complex), and thus ratiometric detection for Fe(III) in an aqueous medium can be fulfilled. This FRET-based supramolecular sensor can be readily formed via an inclusion process using the donor part and the acceptor part, hence this strategy could afford a robust approach for constructing a wide range of FRET-based water-soluble sensing systems simply by assembling a specifically predesigned donor-linked CD and acceptor-linked adamantane.

Full text of DOI:10.1021/la9033244

pubs.acs.org/Langmuir
© 2009 American Chemical Society
Cyclodextrin Supramolecular Complex as a Water-Soluble Ratiometric
Sensor for Ferric Ion Sensing
Meiyun Xu,† Shuizhu Wu,*,†,‡ Fang Zeng,† and Changmin Yu†
†College of Materials Science & Engineering and ‡State Key Laboratory of Pulp & Paper Engineering ,
South China University of Technology, Guangzhou 510640, China
Received September 3, 2009. Revised Manuscript Received October 9, 2009
Heightened concern for human health and environmental protection has stimulated active research on the potential
impact of transition-metal ions and their toxic effects, thus it is very demanding to design transition-metal ion detection
methods that are cost-effective, rapid, facile, and applicable to the environmental and biological milieus. In this study,
we demonstrated an alternative strategy for constructing a water-soluble FRET-based ratiometric sensor for ferric ion
detection by forming a supramolecular β-cyclodextrin/dye complex. This water-soluble FRET system consists of a
dansyl-linked β-cyclodextrin (β-CD-DNS) and a spirolactam rhodamine-linked adamantane (AD-SRhB). The dansyl
moiety serves as the donor, and the spirolactam-rhodamine B derivative (SRhB) was chosen as a sensitive, selective
chemosensor for Fe(III) ions and a very efficient ring-opening reaction induced by Fe(III) generates the long-wavelength
rhodamine B fluorophore that can act as the energy acceptor. Moreover, the adamantyl (AD) group, which is known for
its capability to form stable host-guest inclusion complexes with β-CD derivatives, was covalently linked to the
spirolactam rhodamine, thus the adamantyl moiety of the ion-recognition element can be anchored inside the CD cavity.
€
In this way, the donor-acceptor separation can be kept within the critical Forster distance; accordingly, energy transfer
can take place from the donor (dansyl) to the acceptor (rhodamine derivative/Fe(III) complex), and thus ratiometric
detection for Fe(III) in an aqueous medium can be fulfilled. This FRET-based supramolecular sensor can be readily
formed via an inclusion process using the donor part and the acceptor part, hence this strategy could afford a robust
approach for constructing a wide range of FRET-based water-soluble sensing systems simply by assembling a
specifically predesigned donor-linked CD and acceptor-linked adamantane.
1. Introduction
external interference and/or ambiguities by the self-calibration of
two emission bands.12,13 The heightened concern for human
health and environmental protection has stimulated active re-
search on the potential impact of transition-metal ions and their
toxic effects, thus it is very demanding to design transition-metal-
ion detection methods that are cost-effective, rapid, facile, and
applicable to the environmental and biological milieus.14-17 In
recent years, the spirolactam rhodamine derivatives have been
found to be ideal for constructing off-on fluorescent chemosen-
sors for transition-metal ions because of their particular structural
property.18,19 As is well known, rhodamine derivatives with spiro-
lactam structure are nonfluorescent whereas ring-opening of the
spirolactam gives rise to strong fluorescence emission.18,19 To
date, several rhodamine-modified chemosensors (including some
rhodamine-based FRET sensors) for Hg2þ and Cu2þ ions have
been developed.18,19 In addition, some rhodamine-based sensors
for Fe3þ via fluorescence enhancement have been reported.20,21
Traditionally, many of the FRET-based sensors are created in
the form of a dyad or triad in which the donor and the acceptor
Fluorescence detection has become a powerful tool for quanti-
tative measurements of various analytes such as Hþ,1 metal
ions,2-4 and glucose5 in environmental, industrial, medical, and
biological applications because of its sensitivity, specificity, and
real-time monitoring with fast response time.1-9 In particular, a
fluorescence resonance energy transfer (FRET)-based sensing
technique, which does not directly produce redox-active ions that
could lead to photodamage or other undesirable processes, has
recently attracted considerable attention.6-11 In addition, FRET
can be utilized to realize ratiometric detection that can avoid
*To whom correspondence should be addressed. Phone: (þ86)-20-
22236262. Fax: (þ86)-20-87114649. E-mail: shzhwu@scut.edu.cn.
(1) Almutairi, A.; Guillaudeu, S. J.; Berezin, M. Y.; Achilefu, S.; Frechet,
J. M. J. J. Am. Chem. Soc. 2008, 130, 444–445.
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Langmuir 2010, 26(6), 4529–4534
Published on Web 10/23/2009
DOI: 10.1021/la9033244 4529

Article
Xu et al.
Scheme 1. Formation of the AD-SRhB/β-CD-DNS Supramolecular
Complex and Its FRET-Based Ratiometric Sensing of the Ferric
Ion in Aqueous Media
intensity, water solubility (and probably biocompatibility), and
photostability of the guest fluorophores could be enhanced.36-41
Herein, we demonstrated a novel alternative strategy for
constructing a FRET-based ratiometric sensor for ferric ion
detection by forming a supramolecular β-cyclodextrin/dye com-
plex, as shown in Scheme 1. This water-soluble FRET system
consists of a dansyl-linked β-cyclodextrin (β-CD-DNS) and an
adamantyl-linked spirolactam rhodamine (AD-SRhB). The
dansyl moiety, with its fluorescence spectrum, well matches the
absorption spectrum of rhodamine B and serves as the donor
whereas the spirolactam-rhodamine B derivative (SRhB) was
chosen as a sensitive and selective recognition element for Fe3þ
and a highly efficient ring-opening reaction induced by Fe3þ
generates the long-wavelength rhodamine B fluorophore, which
can act as the energy acceptor. Moreover, the adamantyl (AD)
group, which is known for its capability to form stable host-guest
inclusion complexes with β-CD derivatives,42-48 was covalently
linked to the spirolactam rhodamine, thus the adamantyl moiety
of the ion-recognition element (AD-SRhB) can be well anchored
inside the cyclodextrin cavity. In this way, the donor-acceptor
€
separation can be kept within the critical Foster distance; accord-
ingly, energy transfer can take place from the donor (dansyl
moiety covalently linked to β-CD) to the acceptor (rhodamine
derivative/Fe3þ complex in the CD cavity). Consequently, the
ratiometric detection of Fe3þ in an aqueous medium can be
fulfilled.
are covalently linked through a spacer with a certain length.22
However, FRET-based sensors can also be created within colloi-
dal nanoparticles, such as capped quantum dots,22-25 dye-doped
silica26,27 and polymeric nanoparticles,28-34 and these particle-
based FRET systems were found by some researchers23-29 and
our group30-34 to exhibit high brightness and improved photo-
stability. For the FRET systems to be used in biomedical and
environmental applications, it is essential that they are nontoxic
and water-soluble to prevent aggregation and precipitation and
should have a substantial photoluminescence quantum yield in
water.35 However, it is well known that cyclodextrin (CD) has a
unique configuration that makes its outer surface hydrophilic and
its inner surface hydrophobic, thus it can form supramolecular
inclusion complexes with hydrophobic small-molecule fluoro-
phores that fit into its 5-8 A cavity;36-40 the fluorescence
2. Experimental Section
2.1. Materials. Rhodamine B (RhB), diethylenetriamine, β-
cyclodextrin (β-CD), chloride salts of a metal ion (Kþ, Mg2þ
,
Ca2þ, Cr3þ, Mn2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Cd2þ, and
Hg2þ), the nitrate salt of a metal ion (Agþ), tris(hydroxymethyl)-
aminomethane (Tris), sodium acetate buffer concentrates, potas-
sium hydrogen phthalate buffer concentrate, HEPES sodium salt
buffer concentrate, hydrochloric acid/borax buffer concentrate,
and sodium tetraborate/sodium hydroxide solution buffer con-
centrates were purchased from Sigma-Aldrich. Sodium azide
(N3Na), p-toluenesulfonyl chloride (TSO-Cl), triphenylpho-
sphine (PPh3), dansyl chloride, N,N0-dicyclohexylcarbodiimide
(DCC), N-hydroxysuccinimide (NHS), and triethylamine were
obtained from Acros. 1-Adamantanecarboxylic acid (AD-CO-
OH) was obtained from Alfa. N,N-dimethylformamide (DMF)
was dried with CaH2 and vacuum distilled. Triethylamine was
dried over molecular sieves and vacuum distilled. Methanol,
ethanol, and dichloroethane were analytically pure solvents and
distilled before use.
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2.2. Synthesis of the Dansyl-Containing β-Cyclodextrin
(β-CD-DNS). First, mono-6-O-(p-tolylsulfonyl)-β-cyclodextrin
(6-TSO-β-CD) was prepared according to the literature.49 β-
Cyclodextrin (5 g, 4.4 mmol) was dissolved in a solution of 2.5 g
of sodium hydroxide in 150 mL of water. The solution was stirred
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4530 DOI: 10.1021/la9033244
Langmuir 2010, 26(6), 4529–4534

Xu et al.
Article
1
at 0-5 °C in an ice-water bath, and p-toluenesulfonyl chloride
(1.5 g, 7.90 mmol) was added in one portion. The reaction mixture
was stirred vigorously for 1.5 h at 0-5 °C, and then another
portion of p-toluenesulfonyl chloride (1.5 g, 7.90 mmol) was
added and the reaction mixture was stirred at this temperature
for another 1.5 h. Finally, the last portion of p-toluenesulfonyl
chloride (2 g, 10.53 mmol) was added, and the reaction mixture
was stirred at this temperature for another 2 h. The reaction
mixture was filtered through Celite in a fritted glass funnel
to separate unreacted tosyl chloride. The filtrate was cooled at
0-5 °C while 10% aqueous hydrochloric acid (HCl, 35 mL) was
added to acidify to pH 1. The resulting solution was stored
overnight in a refrigerator at 0 °C and then filtered. The product
was dried in a vacuum to yield a white solid. This material was
recrystallized (three times) by dissolvingit in 20 mL of water at the
boiling point and then cooling to room temperature. Storage in a
refrigerator overnight provided 1.76 g (yield 31%) of 6-O-p-
Yield: 0.87 g, 79%. H NMR (CDCl3, 300 MHz) δ: 7.88-7.85
(m, 1H), 7.45-7.42 (m, 2H), 7.09-7.07 (m, 1H), 6.44-6.37 (m,
4H), 6.30-6.26 (m, 2H), 3.37-3.26 (m, 13H), 2.83-2.79 (t, 2H),
2.61-2.57 (t, 2H), 2.33-2.38 (t, 2H), 1.18-1.14 (t, 12 H). ESI m/z
[MþH]þ 528.5 .
Second, AD-NHS was synthesized as follows. Under nitrogen,
adamantane-1- carboxylic acid (1.0961 g, 6.085 mmol) and
N-hydroxysuccinimide (NHS) (0.7 g, 6.085 mmol) were dissolved
in 10 mL of anhydrous DMF and then dicyclohexylcarbodiimide
(DCC) (2.36 g, 12.17 mmol) was added and the solution soon
became turbid. The suspension was stirred for 12 h. The reaction
mixture was processed by DCU byproduct removal via filtration,
and then the solvent was evaporated under vacuum and the
residue was recrystallized from CH2Cl2/hexane to give purified
ester AD-NHS. Afterwards, SRhB (0.248 g, 0.471 mmol) was
dissolved in 5 mL of anhydrous DMF and then 1 mL of triethyl-
amine was added. The solution was stirred for 10 min under
nitrogen, and AD-NHS (0.16 g, 0.614 mmol) in 10 mL of
anhydrous DMF was added. The reaction mixture was stirred
for 12h at50°C. The solvent was then evaporated, and the residue
was redissolved in CH2Cl2 and extracted with water (3 ꢀ 50 mL)
and dried with anhydrous Mg2SO4. After filtration and evapora-
tion, the residue was purified by silica gel column chromato-
graphy using 24:1 CH2Cl2/CH3OH as the eluent to give AD-
SRhB. Yield 83%. 1H NMR (300 MHz, CDCl3) δ: 7.89-7.86 (m,
1H), 7.46-7.44 (m, 2H), 7.11-7.08 (m, 1H), 6.47 (s, 1H, amide),
6.44-6.37 (m, 4H), 6.29-6.26 (m, 2H), 3.37-3.27 (m, 10 H),
3.20-3.19 (q, 2H), 2.58-2.54 (t, 2H), 2.42-2.38 (t, 2H),
2.27-1.70 (16H, adamantyl, imine), 1.19-1.14 (t, 12H). ESI
m/z [M þ H]þ 690.2.
1
toluenesulfonyl-β-cyclodextrin as a white solid. H NMR (300
MHz, DMSO-d6) δ: 7.75 (d, 2H), 7.43 (d, 2H), 5.63-5.82 (m,
14H), 4.77-4.84 (m, 6H), 4.31-4.49 (m, 6H), 4.19 (m, 1H),
3.19-3.66 (m, 42H), 2.42 (s, 3H). ESI MS m/z [M þ H]þ 1287.5.
Second, mono-6-azido-6-deoxy-β-cyclodextrin was prepared.
Mono-6-(p-tolylsulfonyl)-β-CD (1.73 g, 1.34mmol) wasdissolved
in DMF (325 mL), and sodium azide (174 mg, 2.68 mmol) was
added. The reaction mixture was heated to 105-110 °C and
stirred for 4 h at this temperature. The almost-clear solution
was treated with acetone at room temperature, and a white crys-
talline precipitate was formed. The crude product was recrystalli-
zed from 1:10 water-acetone. Yield: 1.56 g, 97%. 1H NMR (300
MHz, DMSO-d6) δ: 5.68-5.80 (m, 14H, OH2, OH3), 4.87 (m, 1H,
H1a), 4.82 (m, 6H), 4.46-4.70 (m, 6H, OH6), 3.33-3.83 (m, 42H).
Third, mono-6-deoxy-6-amino-β-CD (6-NH2-β-CD) was
synthesized as follows. A solution of mono-6-azido-6-deoxy-
β-cyclodextrin (400 mg, 0.345 mmol) and Ph3P (362 mg, 1.38
mmol) inDMF(5 mL) wasstirred for 1 h under anN2 atmosphere
at room temperature. The reaction mixture was cooled to 0 °C,
and concentrated aqueous ammonia (2 mL, approximately 28%)
was added to the solution and the solution was stirred for 48 h.
The resulting suspension was concentrated under reduced pres-
sure, and the triphenylphosphin oxide was then precipitated by
the addition of distilled water (100 mL) and filtered. After
evaporation of the water, the crude product was washed with
acetone and dried under vacuum to yield a white solid. Yield: 0.40
g, 95%. 1H NMR (300 MHz, D2O) δ: 5.00 (d, 7H, H1), 3.33-4.13
(m, 42H), 3.03-3.07 (m, 2H).
2.4. Synthesis of Benzene-SRhB. SRhB (0.884 g, 1.52
mmol) was dissolved in 10 mL of CH2Cl2, and then Et3N(1 mL)
was added. The solution was stirred vigorously at 0 °C, and
benzeneacetyl chloride (0.2 mL, 0.234 g) in 10 mL of CH2Cl2 was
added dropwise over 1 h. The solution was allowed to rise to room
temperature and stirred for 24 h at this temperature. The solvent
was then evaporated, and the residue was purified by silica gel
column chromatography using 30:1:0.5 (v/v/v) CH2Cl2/CH3OH/
Et3N as the eluent to give benzene-SRhB. Yield: 0.86 g, 88%. 1H
NMR (300 MHz, CDCl3) δ: 7.89 (m, 1H), 7.48-7.45 (m, 2H),
7.30-7.20 (m, 5H), 7.11-7.10 (m, 1H), 6.72 (s, 1H, amide),
6.41-6.37 (m, 4H), 6.27-6.23 (m, 2H), 3.57 (s, 2H), 3.37-3.19
(m, 12H), 2.60-2.58 (t, 2H), 2.37-2.27 (t, 3H), 1.18-1.14
(t, 12H). ESI MS m/z [M þ H]þ 646.6.
2.5. Preparation of the Supramolecular-Complex Aqueous
Solution. The solution of β-CD-DNS (4 ꢀ 10-5 mol) in 40 mL of
deionized water was prepared by being stirred at 45 °C until the
solution was clear. Then, a solution of AD-SRhB (4 ꢀ 10-5 mol)
in 10mL of ethanol was added slowly. The solutionwas stirred for
5 days at room temperature. Then ethanol was evaporated under
reduced pressure. Afterwards, the solution was fixed to a volume
of 200 mL in a volumetric flask with deionized water to give the
supramolecular-complex aqueous stock solution (2 ꢀ 10-4 M).
When preparing the solutions for spectral measurement, we first
added the supramolecular-complex aqueous stock solution to the
flask containing buffer concentrates, and then ferric ion aqueous
solution was added and the system was stirred for 10 min. Finally,
fluorescence and absorption measurements were performed. The
pH of the solution was adjusted as follows: sodium acetate buffer
for pH 2.6, 3.8, 4.6, and 5.2; Tris/HCl for pH 7.0; potassium
hydrogen phthalate buffer for pH 4.0; HEPES sodium salt buffer
for pH 7.2; hydrochloric acid/borax buffer for pH 8.0; and
sodium tetraborate/sodium hydroxide solution buffer for pH 9.0
and 10.0.
Dansyl chloride (71.2 mg, 0.265 mmol) was added to a stirred
solution of 6-NH2-β-CD (300 mg, 0.265 mmol) and triethylamine
(1.5 mL) in 5 mL of dry DMF. The reaction was stirred at
room temperature for 24 h in the dark. The solvent was evapo-
rated mostly under reduced pressure. Acetone was poured into
the residue, and the precipitate was collected and dried in vacuo.
Then the residue was recrystallized from water/ethanol (1:1) and
dried in vacuo to give β-CD-DNS (206.4 mg). Yield: 206.4 mg,
1
57%. H NMR (300 MHz, DMSO-d6) δ: 8.41-8.43 (d, 1H),
8.27-8.30 (d, 1H), 8.01 (t, 1H),7.56 (q, 2H), 7.23 (d, 1H),
5.64-5.85 (m, 14H), 4.79-4.83 (m, 6H), 4.45-4.50 (m,
6H), 4.27 (m, 1H), 3.13-3.63 (m, 42 H), 2.83 (s, 6 H). ESI m/z
[M þ H]þ 1365.2.
2.3. Synthesis of AD-SRhB. First, the spirolactam-rhoda-
mine derivative (SRhB) was prepared. Under nitrogen, rhoda-
mine B (1 g, 1.717 mmol) was dissolved in 20 mL of anhydrous
methanol, and then diethylenetriamine (4.5 mL) was added as
soon as possible at 40 °C and the temperature was slowly raised to
70 °C. After 10 h, the solvent was evaporated under reduced
pressure and then CH2Cl2 (50 mL) and water (100 mL) were
added and the organic layer was separated, washed five times with
water, and dried over anhydrous Mg2SO4. After the filtration of
sodium sulfate, the solvent was removed under reduced pressure
to give an orange powder and purified by silica gel column
chromatography (CH2Cl2/MeOH/Et3N 40:2:1) to give SRhB.
2.6. Measurements. 1H NMR spectra were recorded on a
Bruker Avance 300 MHz NMR spectrometer. UV-vis spectra
were recorded on a Hitachi U-3010 UV-vis spectrophotometer.
Fluorescence spectra were recorded on a Hitachi F-4600 fluore-
scence spectrophotometer. Mass spectra were obtained through a
Bruker Esquire HCT Plus mass spectrometer. The fluorescence
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DOI: 10.1021/la9033244 4531

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Xu et al.
Figure 2. Fluorescence spectra (λexc 410 nm) of the AD-SRhB/
β-CD-DNS supramolecular-complex aqueous solution (1ꢀ 10-4
M) upon addition of ferric ion. (pH 7.0). (Inset) Photographs of
the supramolecular-complex aqueous solution before and after
the addition of ferric ion (1.4 ꢀ 10-4 M) under UV light irradia-
tion.
Figure 1. Absorption spectra of the AD-SRhB/β-CD-DNS
supramolecular-complex aqueous solution upon addition of ferric
ion (pH 7.0).
emission lifetime was measured on an Edinburgh FL920 fluore-
scence lifetime spectrometer.
induces the ring opening of spirolactam, which is responsible for
the above dual color and fluorescence changes. It was clear that
the FRET process was switched on by Fe3þ as the excitation of
DNS at 410 nm resulted in the emission of rhodamine with a
3. Results and Discussion
3.1. Synthesis and Characterization. The dansyl-contain-
ing cyclodextrin β-CD-DNS was synthesized by first preparing
mono-6-O-(p-tolylsulfonyl)-β-cyclodextrin, then mono-6-deoxy-
6-amino-β-CD was prepared, and finally dansyl chloride was
reacted with mono-6-deoxy-6-amino-β-CD to render β-CD-
DNS. The ion-recognition element AD-SRhB was synthesized
from the reaction of 1-adamantane carboxylic acid with rhoda-
mine B-modified amine. And then the chromophore formed
supramolecular complex with the cyclodextrin β-CD-DNS, as
shown in Scheme 1. The molecular structure of the AD-SRhB
and β-CD-DNS were confirmed by NMR and MS spectra
(Figure S1-S6).
3.2. Absorption Spectra. Ion-recognition element AD-
SRhB is insoluble in water; however, when it formed a supra-
molecular complex with β-CD-DNS in water, the whole system
was soluble in water. The supramolecular-complex aqueous
solution is clear and transparent, and the UV/vis spectrum of
the supramolecular-complex solution showed only the absorption
profile of the donor (dansyl), which has absorptions at about 330,
380, and 410 nm (Figure S7). The addition of Fe3þ ions im-
mediately induced an increase in the absorption intensity at about
560 nm, which corresponds to the absorption of rhodamine B, as
shown in Figure 1. A significant color change from clear colorless
to pink could be easily visually observed (Figure S8). This
confirms that the addition of Fe3þ can promote the formation
of the open-ring state of the SRhB moieties.
€
maximum of 587 nm. The calculated Forster radius R0 (the
distance at which the energy-transfer efficiency is 50%) was
29.9 A (Table S1), and this distance is longer than the donor-
acceptor separation (<20 A) in this supramolecular complex,
indicating that the excited energy of dansyl can transfer to
rhodamine B. In addition, the emission lifetime of the supra-
molecular-complex aqueous solution (dansyl moiety) was mea-
sured, and the results showed that upon addition of ferric ion to
the supramolecular-complex aqueous solution the emission life-
time of the donor decreased from 10.01 to 5.15 ns (Figure S10),
thus providing additional evidence that in this system the FRET
process was turned on by the ferric ion because the FRET process
usually leads to a fluorescence lifetime decrease in the donor
directly induced by energy transfer from the donor to the
acceptor.50,51
This FRET system with the donor and acceptor located inside
and outside the cyclodextrin cavity, respectively, can keep the
€
donor and acceptor within the effective distance of the Forster
energy transfer; therefore, it can be used for ratiometric sensing of
the ferric ion. The FRET system can also avoid any undesirable
interaction between the donor and acceptor, which might exist if
they were in the unbound form, for example, the π-π interactions
between the donor and acceptor (both the donor and acceptor are
aromatic), which could lead to dye aggregation through the
stacking of aromatic units and accordingly changes in spectral
properties.52
3.3. Emission Spectra and Detection. Figure 2 shows the
fluorescence spectral changes in the supramolecular-complex
aqueous solution upon addition of Fe3þ. In the absence of
Fe3þ, excitation of the complex at 410 nm resulted in the emission
profile of DNS at around 530 nm (Φ = 0.11, the spectroscopic
data are shown in Table S1). Upon addition of Fe3þ, the DNS
emission at 530 nm decreased and a new emission band with a
maximum at 587 nm (rhodamine) gradually emerged (Figure 2).
Adamantyl is a well-known guest that forms a stable supra-
molecular inclusion complex with β-CD, and the association
constant (binding constant or equilibrium constant) between
the adamantyl group and β-CD can reach as high as 105 M-1
.
42-48 In this study, the binding constant for AD-SRhB and β-CD-
DNS has been determined to be 3.88 ꢀ 104 M-1 (Figure S11),
indicating that AD-SRhB bound strongly with β-CD-DNS, and
the concentration for unbound AD-SRhB (dissolved in water)
was very low, hence the contribution of singlet energy transfer
The ratio of the emission intensities at 587 and 530 nm (I587/I530
)
increased steadily as the concentration of ferric ion was increased
(Figure S9), rendering the supramolecular complex a sensitive
ratiometric fluorescent sensor for Fe3þ with a detection limit that
can reach 1 μM Fe3þ (Figure 2 and Figure S9). Moreover, upon
addition of Fe3þ, the fluorescence of the complex solution clearly
changed from green to orange (Figures 2 and S8). It is explicit that
the binding interaction between AD-SRhB/β-CD-DNS and Fe3þ
(50) Domingo, B.; Sabariegos, R.; Picazo, F.; Llopis, J. Microsc. Res. Tech.
2007, 70, 1010–1021.
(51) Biskup, C.; Zimmer, T.; Kelbauskas, L.; Hoffmann, B.; Klocker, N.;
Becker, W.; Bergmann, A.; Benndorf, K. Microsc. Res. Tech. 2007, 70, 442–451.
(52) Gabriel, G. J.; Iverson, B. L. J. Am. Chem. Soc. 2002, 124, 15174–15175.
4532 DOI: 10.1021/la9033244
Langmuir 2010, 26(6), 4529–4534

Xu et al.
Article
Figure 4. Fluorescence intensity changes (I587/I530) of the AD-
SRhB/β-CD-DNS supramolecular-complex aqueous solution
(1 ꢀ 10-4 M) upon addition of ferric ion (8 ꢀ 10-5 M) only, and
the aqueous solution containing both ferric ion (8 ꢀ 10-5 M) and
Hg2þ, and Agþ (8 ꢀ 10-5 M) (λexc 410 nm, pH 7.0).
Kþ, Mg2þ, Ca2þ, Cr3þ, Mn2þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Cd2þ
,
Figure 3. Fluorescence intensity changes (I587/I530) of the AD-
SRhB/β-CD-DNS supramolecular-complex aqueous solution
(1 ꢀ 10-4 M) upon addition of different metal ions (8 ꢀ 10-5 M)
(λexc 410 nm, pH 7.0).
from the donor to the unbound AD-SRhB can be neglected in the
spectral measurements. To verify further the role of the adaman-
tyl moieties in forming a stable supramolecular inclusion complex
and accordingly the efficient FRET process, we covalently linked
the spirolactam rhodamine B to the phenyl group (instead of
the adamantyl group) to form another ion-recognition element
(benzene-SRhB, with characterization in Figures S12 and S13).
The phenyl group is smaller than the adamantyl group, and the
supramolecular complex formed by benzene-SRhB and β-CD-
DNS is less stable than the supramolecular complex formed by
AD-SRhB and β-CD-DNS because the phenyl group can get in
and out of the cyclodextrin cavity more easily. The fluorescence
spectra of the supramolecular-complex benzene-SRhB/β-CD-
DNS aqueous solution in the presence and absence of ferric ion
were measured (Figure S14). It can be seen that, compared with
that in the AD-SRhB/β-CD-DNS system, benzene-rhodamine/
Fe3þ in this complex system could not efficiently quench the
fluorescence emission of dansyl, suggesting that some of the
acceptors (benzene-SRhBs) were not within the effective energy-
transfer distance but probably moved out of the CD cavity. In
addition, the sensitivity of the supramolecular-complex benzene-
SRhB/β-CD-DNS aqueous solution is much lower than that of
the AD-SRhB/β-CD-DNS aqueous solution, as indicated in
Figure S14.
Figure 5. Variation of fluorescence intensity at 587 nm for the
AD-SRhB/β-CD-DNS supramolecular-complexaqueoussolution
(1 ꢀ 10-4 M) with and without Fe3þ (1 ꢀ 10-4 M) as a function of
pH (λexc 410 nm; black curve, with Fe3þ; red curve, without Fe3þ).
fluorescence emission intensity of the supramolecular-complex
solution increased significantly because under highly acidic con-
ditions protons may also induce the ring-opening reaction of
spirolactam rhodamine. In the presence of ferric ion, the supra-
molecular-complex aqueous solution exhibits a higher fluore-
scence intensity, which remained fairly stable from pH 3.8 to 7, as
shown in Figure 5. These data establishthat the AD-SRhB/β-CD-
DNS supramolecular-complex system could act as a fluorescent
probe for Fe3þ over a pH span of 3.8-7.
3.4. Selectivity. The novel AD-SRhB/β-CD-DNS supramo-
lecular-complex system showed high selectivity toward Fe3þ
.
From Figure 3, it is clear that Fe3þ induced a prominent fluore-
scence increment, whereas some metal ions such as Kþ, Mg2þ
4. Summary and Conclusions
,
Ca2þ, Mn2þ, Co2þ, Ni2þ, Zn2þ, Cd2þ, Hg2þ, and Agþ gave a
negligible response; only Cr3þ had a slight effect (Figure S15).
3.5. Antidisturbance. In addition, further experiments for
Fe3þ-selective sensing were performed with the supramolecular-
complex aqueous solution in the presence of multifarious cations
We have developed a FRET-based cyclodextrin supramole-
cular-complex system as a ratiometric fluorescent probe that can
selectively detect Fe3þ in an aqueous medium. The key to the
successful formation of this FRET system is that the donor and
acceptor can reside inside and outside the cyclodextrin cavity,
respectively, because of its hydrophilic outer surface and hydro-
phobic cavity surface, thus not only keeping the donor and
including Kþ, Mg2þ, Ca2þ, Cr3þ, Mn2þ, Co2þ, Ni2þ, Cu2þ, Zn2þ
,
Cd2þ, Hg2þ, and Agþ (1 equiv), as shown in Figure 4. The
coexistence of most selected metal ions does not interfere with
Fe3þ binding to the chromophore or the subsequent fluorescence
turn on. The AD-SRhB/β-CD-DNS supramolecular-complex
system showed very good antidisturbance.
€
acceptor within the effective distance of the Forster energy
transfer but also preventing any undesirable interaction between
the donor and acceptor (e.g., π-π interactions between donor
and acceptor). This strategy provides a water-soluble ratiometric
fluorescent sensor for Fe3þ, allowing for a large shift (>100 nm)
between donor excitation and acceptor emission, which rules out
any influence of excitation backscattering effects on fluorescence
detection. Moreover, this FRET-based supramolecular sensor
3.6. Suitable pH Range. For practical applicability, we also
investigated the suitable pH range for ferric ion sensing. In the
absence of ferric ion, no remarkable fluorescence emission of the
solution was observed between pH 3.8 and 10; at pH 2.6, the
Langmuir 2010, 26(6), 4529–4534
DOI: 10.1021/la9033244 4533

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Xu et al.
can be readily formed via an inclusion process using the donor
part and the acceptor part, hence this strategy could afford a
robust approach for constructing a wide range of FRET-based
sensing systems for other analytes by preparing a variety of donor
parts and acceptor parts in advance and then selecting a suitable
donor-acceptor pair for a specific sensing purpose and assem-
bling them according to the component assembly approach in
industrial processes.
Acknowledgment. This work was supported by the NSFC
(project no. 50973032, 20974035, and 50773022) and the GDN-
SFC (project no. 07006497).
Supporting Information Available: Characterizations,
schemes, absorption and fluorescence spectra, photographs,
€
and calculation of Forster critical radius. This material is
available free of charge via the Internet at http://pubs.acs.org.
4534 DOI: 10.1021/la9033244
Langmuir 2010, 26(6), 4529–4534