Self-assembly of graphene oxide with a silyl-appended spiropyran dye for rapid and sensitive colorimetric detection of fluoride ions

Article abstract of DOI:10.1021/ac402592c

Fluoride ion (F^-), the smallest anion, exhibits considerable significance in a wide range of environmental and biochemical processes. To address the two fundamental and unsolved issues of current F^- sensors based on the specific chemical reaction (i.e., the long response time and low sensitivity) and as a part of our ongoing interest in the spiropyran sensor design, we reported here a new F^- sensing approach that, via assembly of a F^--specific silyl-appended spiropyran dye with graphene oxide (GO), allows rapid and sensitive detection of F^- in aqueous solution. 6-(tert-Butyldimethylsilyloxy)-1′,3′,3′-trimethylspiro [chromene- 2,2′-indoline] (SPS), a spiropyran-based silylated dye with a unique reaction activity for F^-, was designed and synthesized. The nucleophilic substitution reaction between SPS and F^- triggers cleavage of the Si-O bond to promote the closed spiropyran to convert to its opened merocyanine form, leading to the color changing from colorless to orange-yellow with good selectivity over other anions. With the aid of GO, the response time of SPS for F^- was shortened from 180 to 30 min, and the detection limit was lowered more than 1 order of magnitude compared to the free SPS. Furthermore, due to the protective effect of nanomaterials, the SPS/GO nanocomposite can function in a complex biological environment. The SPS/GO nanocomposite was characterized by XPS and AFM, etc., and the mechanism for sensing F^- was studied by ¹H NMR and ESI-MS. Finally, this SPS/GO nanocomposite was successfully applied to monitoring F^- in the serum.

Full text of DOI:10.1021/ac402592c

Article
pubs.acs.org/ac
Self-Assembly of Graphene Oxide with a Silyl-Appended Spiropyran
Dye for Rapid and Sensitive Colorimetric Detection of Fluoride Ions
Yinhui Li, Yu Duan, Jing Zheng, Jishan Li, Wenjie Zhao, Sheng Yang, and Ronghua Yang*
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Collaborative
Innovation Center for Chemistry and Molecular Medicine, Hunan University, Changsha 410082, China
S
* Supporting Information
ABSTRACT: Fluoride ion (F−), the smallest anion, exhibits
considerable significance in a wide range of environmental and
biochemical processes. To address the two fundamental and
unsolved issues of current F− sensors based on the specific
chemical reaction (i.e., the long response time and low
sensitivity) and as a part of our ongoing interest in the
spiropyran sensor design, we reported here a new F− sensing
approach that, via assembly of a F−-specific silyl-appended
spiropyran dye with graphene oxide (GO), allows rapid and
sensitive detection of F− in aqueous solution. 6-(tert-
Butyldimethylsilyloxy)-1′,3′,3′-trimethylspiro [chromene- 2,2′-
indoline] (SPS), a spiropyran-based silylated dye with a unique
reaction activity for F−, was designed and synthesized. The
nucleophilic substitution reaction between SPS and F− triggers
cleavage of the Si−O bond to promote the closed spiropyran to convert to its opened merocyanine form, leading to the color
changing from colorless to orange-yellow with good selectivity over other anions. With the aid of GO, the response time of SPS
for F− was shortened from 180 to 30 min, and the detection limit was lowered more than 1 order of magnitude compared to the
free SPS. Furthermore, due to the protective effect of nanomaterials, the SPS/GO nanocomposite can function in a complex
biological environment. The SPS/GO nanocomposite was characterized by XPS and AFM, etc., and the mechanism for sensing
F− was studied by 1H NMR and ESI-MS. Finally, this SPS/GO nanocomposite was successfully applied to monitoring F− in the
serum.
he development of new methodologies for determination
of anions is emerging as a research area of great
development of a new method for multipurpose determination
of the anion. In the last few decades, simple and effective
fluorescent or colorimetric sensors have been devised and
utilized for F− recognition and detection based on noncovalent
or covalent interactions between the sensor molecules and F−,
employing hydrogen bonding,12 lewis acid coordination,13
chemical reaction,14−16 and quantum dots,17 as the attractive
forces. Compared to noncovalent interaction, molecule
recognition based on specific chemical reaction displays higher
selectivity and stability15,18 and thus has recently attracted more
attention by researchers.19
Pioneered by the seminal work of Yamaguchi and Tamao in
2000,14,15 it has been demonstrated that the Si−O bond can be
broken by F− through the specific nucleophilic substitution
reaction between F− and silica, making the Si−O containing
derivatives attractive materials for F− recognition and sensing
with excellent selectivity. Hence, a number of sensors based on
silylated dye have been designed.19−21 tert-Butyldimethylsilyl
(TBDMS) or tert-butyldiphenylsilyl (TBDPS) was usually
T
importance because of the roles of anions in chemical and
biological processes. Fluoride ion (F−), the smallest anion,
exhibits considerable significance in a wide range of environ-
mental and biochemical processes.1−3 The soluble fluoride is
considered as an essential micronutrient and is essential for
body growth, but it is greatly toxic to the biological tissue if the
concentration exceeds cellular requirements, which will cause
many serious neurodegenerative diseases. For example, sodium
fluoride can affect a number of essential cell-signaling
components and disturb normal cellular metabolism at a
higher concentration.4 Excess F− could also produce ecological
damage5 and cause dental or skeletal fluorosis,6 even
nephrotoxic changes7 and urolithiasis8 in humans. Therefore,
the detection of F− has been the main focus of several research
groups.
For the F− detection, ion-selective electrode,9 a standard
fluoride-indicating methodology, has been developed and
recommended by the World Health Organization for critical
information.10 However, this sensor responds to the fluoride
activity in the internal electrode solution, not to the
concentration. In addition, significant interference could appear
with the presence of hydroxide ions.11 There is still need for the
Received: August 14, 2013
Accepted: October 29, 2013
Published: October 29, 2013
© 2013 American Chemical Society
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chosen as the F− acceptor that was covalently attached to the
chromophore or fluorophore molecule to render the dye
unreactive to potentially interfering compounds and thus
sensitive only to the presence of F−. Even though the silylated
dye has high selectivity and affinity toward F−, it is difficult to
overcome the strong hydration effect of fluoride in aqueous
environments, making the time necessary to reach equilibrium
tens of minutes or even hours.22 For advanced, sophisticated
approaches, employing surfactant micelles23 and hydrogels24
have recently been developed by Yang et al. to achieve the
desired reaction speed; however, the systems are known as
organic media and suffer from their intrinsic limits, such as poor
long-term stability due to physical entrapment. Therefore, to
further improve the performance of F− detection in the
complex biological environment, the development of a new
sensing method that can alleviate these problems is highly
desirable.
mica surface, allowing them to dry in air. Tapping mode was
used to acquire the images under ambient conditions. Energy-
dispersive X-ray (EDX) analysis was carried out using a
HITACHI S-4500 instrument. UV−vis absorption spectra were
recorded in 1.0 cm path length quartz cuvettes on a Hitachi U-
4100 UV−vis spectrometer (Kyoto, Japan). pH was measured
by model 868 pH meter (Orion).
Synthesis of SPS. The synthesis route of SPS is shown in
Scheme 1 (for details see the Supporting Information). 6-
Scheme 1. Synthetic Pathways for SPS
The current trend of nanoscale science has been the
integration of functionalized nanomaterial and technology
with biology and chemistry to develop new analytical tools.25
Graphene oxide (GO), a chemically exfoliated graphene
derivative, exhibits good biocompatibility and low cytotox-
icity,26,27 making GO attractive nanomaterials in analytical
chemistry for biological applications, such as recognizing
various analytes28 and improving the catalytic activity of
enzymes,29 delivering drugs,30 and exploiting intracellular
imaging studies.31 Inspired by the success of the aforemen-
tioned work, and as a continuation of our studies on sensors
based on carbon nanomaterials,32 herein, we present a new F−
sensing approach, coupling the specific molecule recognition
ability of silylated dyes and the unique features of GO to
develop a nanocomposite for rapid and sensitive colorimetric
detection of F− in aqueous solution. In this paper, a TBDMS-
functionalized chromophore, 6-(tert-butyldimethylsilyloxy)-
1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] (SPS), was de-
signed, and the SPS and GO hybrid nanocomposite (SPS/GO)
was prepared and applied to F− sensing. Significantly, the
response time of SPS/GO for F− was shortened from 180 to 30
min, and the detection limit was lowered more than 1 order of
magnitude compared to the free SPS. Furthermore, due to the
protective effect of nanomaterials, the SPS/GO nanocompo-
sites can function in a complex biological environment, making
it a promising material for bioanalytical applications.
Hydroxyl spiropyran (SP−OH) was synthesized via the
Knoevenagel condensation reaction of 2,4-dihydroxybenzalde-
hyde and 1, 2, 3, 3-tetramethyl-3H-indolium in an 80% yield.
To introduce the F− reaction site to the spiropyran compound,
tert-butyldimethylsilyl chloride was linked to SP−OH by the
Williamson reaction, giving rise to the product of SPS in a 60%
yield. The new synthesized compound was characterized by 1H
NMR and ESI-MS.
Preparation of SPS/GO Nanocomposite. GO nano-
sheets were synthesized from natural graphite by a modified
Hummers’ method,33 and the characterization data as shown in
Figure S1 (see the Supporting Information). The stock solution
of GO (4.0 mg/mL) was obtained by sonicating the final
product of GO for 2 h in Tris-HCl aqueous solution. The
mixture solution of SPS and GO was obtained by the addition
of 10 μL of GO to 0.5 mL of SPS (10.0 μM) solution. Then,
the mixture was stirred for about 2 h. After centrifugation for
three times with water, the SPS/GO nanocomposite and free
SPS were further separated by the dialysis method, and then the
product was dried under vacuum at 60 °C overnight. The
structure feature and chemical composition of GO and SPS/
GO were characterized by AFM and EDX spectroscopy.
F− Detection. GO was first sonicated in doubly deionized
water for 2 h to give a homogeneous black solution. After the
pretreatment, an aliquot of the freshly made GO suspension
(less than 1%, v/v) was added to 1.0 mL of Tris-HCl buffer
containing 10 μM of SPS and the absorption spectra were then
measured. Different concentrations of F− were added to the
complex system of GO and SPS for incubating at 25 °C for 30
min, and the absorption spectra were then measured. The
addition was limited to 20 μL so that the volume change was
insignificant. To study the kinetics and time-dependence of the
reaction between SPS and F− in the absence and presence of
GO, the absorption peak of mero form at 490 nm was recorded
in 10% (v/v) CH3CN/Tris-HCl solution. For determination of
F− in the real samples, bovine serum was diluted 10-fold, and
the measurements were conducted 30 min after the addition of
F−.
EXPERIMENTAL SECTION
■
Materials and Apparatus. All chemicals were of analytical
reagent grade and were supplied by Alfa Aesar or Sigma
Aldrich. GO used in this work was purchased from Carbon
Nanotechnologies, Inc. The solutions of anions were prepared
from NaF, NaCl, NaBr, NaI, NaNO3, NaHSO4, AcONa,
NaH2PO4, NaClO4, respectively. They were dissolved in
sterilized Milli-Q ultrapure water (18.2 MΩ) as stock solutions.
Working solutions were prepared by successive dilution of the
stock solution with Tris-HCl buffer (20 mM, pH 8.0).
Nuclear magnetic resonance spectra were recorded at 400
MHz and carbon spectra were recorded at 100 MHz on an
Invoa-400 (Invoa 400) spectrometer. Mass spectra were
obtained on an LCQ/Advantage high-performance liquid
chromatography (HPLC)−mass spectrotometer. Atomic force
microscopy (AFM) measurements were performed by using a
Nanoscope Vmultimode atomic force microscope (Veeco
Instruments). Samples for AFM images were prepared by
depositing a dispersed SPS/GO solution onto a freshly cleaved
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Due to the specific nucleophilic substitution reaction between
SPS and F− that occurs on the GO surface, the reaction rate
between F− and silicon ether was greatly accelerated.
RESULTS AND DISCUSSION
■
Design of SPS/GO Sensor Platform. Spiropyrans, an
important class of photoswitchable molecules, are an attractive
starting point in construction of optical molecular sensors,34
due to their reversible structure conversion between the closed
spiropyran (spiro-) form and the opened merocyanine (mero-)
form upon external optical, thermal, or chemical stimulation. In
recent years, our group has developed a number of spiropyran
fluorescent or colorimetric sensors for metal ions,35 anions,36
amino acids,37 and GSH38 by empolying the noncovalent host−
guest interaction. However, this strategy has generally displayed
moderate sensitivity and specificity over other competitive
species, even interference. In the continuation of our studies of
spiropyran sensors, we designed a new spiropyran derivative
containing a silylate moiety as a unique reaction activity for F−.
Scheme 2A showed the molecular structure of the new
F− Induced SPS Ring Opening. The photophysical
property of a spiropyran is related to the chemical structure
of the molecule and the surrounding media, such as the solvent
polarity and the nature of the substituents, displaying reversible
structural transformation from the closed spiro form to the
opened mero form, which has been shown to exhibit extremely
sensitive absorption and color changes in the visible range.
Figure S2 of the Supporting Information showed the UV−vis
absorption spectra of free SPS in different organic solvents, and
one could see that the free SPS was colorless with a strong
absorption band at around 300 nm in either the polar or the
nonpolar organic solvent, but no obvious absorption could be
observed in the longer wavelength, reflecting that SPS mainly
existed as the closed spiro form. Figure 1a showed the UV−vis
Scheme 2. Possible Scheme of the Structure Conversion of
SPS in (A) Polar Solvent and on (B) GO Surface Triggered
by F−
synthesized spiropyran compound, 6-(tert-butyldimethylsily-
loxy)-1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] (SPS),
and the mechanism of structure conversion of SPS triggered
by F−. The design strategy exploits the specific nucleophilic
substitution reaction between F− and SPS that triggers the
cleavage of the Si−O bond and promotes the closed spiropyran
to convert to the opened merocyanine form in the polar
solvent, leading to the color changing from colorless to orange-
yellow. However, in our primary experiment, we found that this
F− recognition process, as the common challenge, usually
needed a few hours in aqueous solution to complete the
detection process due to the low concentrations of the probe,
which severely reduced the reaction rate between F− and the
silyl moieties.
In recent years, GO has been used to construct a sensing
platform for detection of biomolecules by adsorbing lipophilic
organic molecules on the surface. The abundance of carboxy,
hydroxy, and epoxy groups on the surface of GO sheets makes
the system more water-soluble, and these groups are the
essential chemical skeletons for an ideal adsorbent,39,40 making
the sensor molecule and the target enriched in GO surface with
high concentration to bring some positive effects on the
reaction rate of the detection process. Moreover, the high
specific surface area of GO makes GO a promising material for
a catalyzing reaction.41 Therefore, GO was employed to
construct the rapid F− sensing platform of SPS. The basic
concept of the proposed approach was shown in Scheme 2B.
Figure 1. (A) UV−vis absorption spectra of (a) 10 μM SPS in 10%
(v/v) CH3CN/Tris-HCl solution; (b) a + 5.0 × 10−4 M F−; (c)
CH3CN/Tris-HCl solution containing 40 μg/mL GO; (d) c + SPS;
and (e) d + 5.0 × 10−4 M F−. (B) Corresponding solution colors of a,
b, c, d, and e.
absorption spectrum of SPS (10 μM) in the Tris-HCl buffer
solution (20 mM, pH 8.0) containing 10% CH3CN (v/v); the
absorbance was obviously weaker than that in the organic
solvent due to the poor solubility in the aqueous solution, and
no absorption in the visible region was observed. When NaF
(500 μM) was added to the SPS solution, as shown in Figure
1b, a new peak centered at around 490 nm (ε = 2.6 × 104 M−1
cm−1) appeared, concomitant with the solution color changed
from colorless into an orange-yellow color (Figure 1B), which
indicated that closed SPS was activated by specific nucleophilic
substitution reaction between the F− and Si−O bond to give a
free phenoxy anion. Then, the p−π conjugation effect between
the electron pair at the oxygen atom and the aromatic rings
caused the structural transition from the closed form to the
open mero form in the polar solvent (Scheme 2A).
The reaction mechanism of the as-proposed in Scheme 2A
was confirmed by 1H NMR and ESI-MS spectra. The 1H NMR
(Figure S3 of the Supporting Information) of SPS were
exhibited upon reaction with F−; the chemical shift of the vinyl
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protons at δ 5.60 ppm and δ 6.81 ppm were upfield shifted to δ
7.02 ppm and δ 7.52 ppm, respectively. And the corresponding
protons of the t-butyldimethylsilyl group at the upfield (δ 0.7−
1.1) disappeared after its removal by F−. Besides, the ESI-MS
spectra for both the sensor and the product as shown in Figure
S4 (see the Supporting Information). After treatment of SPS
with F−, a new peak at m/z 293.20 appeared, corresponding to
the merocyanine structure of SPS.
addition of F− (500 μM) in the absence and presence of GO
was examined. As shown in Figure 2A, in the absence of GO,
To assess the feasibility of SPS/GO nanocomposite for F−
detection, the effects of GO on the absorption spectra of SPS in
the absence and presence of F− were investigated. The GO
dispersion displayed a maximum absorption at 231 nm and a
shoulder at 290−300 nm (Figure 1c and Figure S5 of the
Supporting Information), corresponding to the π−π* transition
of aromatic CC bonds and the n−π* transition of the CO
bond,42 respectively. Upon addition of GO to the SPS solution,
no new absorption band could be observed, but the intensities
of the 285−330 nm bands obviously increased (Figure 1d).
After further increasing the GO concentrations, the absorbance
increased gradually concomitant with the absorption band red
shift (Figure S5 of the Supporting Information). We also noted
that the intensities at the two bands of Figure 1d were greater
than the sum of the absorbance of the above two individual
systems of SPS (Figure 1a) and GO (Figure 1c), indicating that
the solubility of SPS in aqueous solution has a considerable
improvement due to the decoration of SPS on the GO surface
to form the SPS/GO nanocomposite. Next, the effect of GO on
the Si−O cleavage of SPS was studied. As shown in Figure 1e,
reaction of SPS with 500 μM F− in the presence of GO, a very
strong absorption band maximum centered at 490 nm appeared
(ε = 5.8 × 104 M−1 cm−1), and the absorbance was larger than
that of SPS at this wavelength after reaction with F−. On the
other hand, there was no difference in the position of the
maximum absorption between the SPS system and SPS/GO
system, suggesting that no structural perturbation of
merocyanine molecules of SPS occurs in the presence of GO.
This absorbance enhancement clearly indicated that it could
achieve higher response sensitivity with the aid of GO than free
SPS due to the improvement of the solubility and signal
intensity of SPS in aqueous solution.
Effect of GO on the Si−O Cleavage Rate. Not only the
response sensitivity but also the Si−O cleavage rate of of SPS
by F− was promoted by GO. To demonstrate, we evaluated the
absorption enhancements of SPS by F− as a function of time in
the absence and the presence of GO. From Figure S6 (panels A
and B) of the Supporting Information, the SPS absorbance
around 490 nm increased gradually with time upon addition of
F− in the both cases, but there was a rather large difference in
the response time and the intensity change for the same
concentration of F−. Figure S6C of the Supporting Information
compared the change of absorbance at 490 nm with time in the
two cases. In the absence of GO, the SPS absorption
enhancement reached maximum (ΔA = 0.27) within 3.0 h
after interaction with 500 μM F−, while in the presence of GO
in the SPS solution, a maximal absorbance enhancement of 0.49
was observed within and around 30 min. Moreover, rapid color
change from colorless to orange-yellow in a very short time
(about 10 min) could be observed in the SPS/GO system after
addition of F− (Figure S6D of the Supporting Information),
while producing an observable yellow color in the SPS system
need about 2.0 h (Figure S6E of the Supporting Information).
To further study the effect of GO on the sensing system, the
reaction kinetics of the structure conversion of SPS after
Figure 2. Effect of GO on the reaction kinetics of F− with SPS. (A)
The time courses of the ring-opening reaction between 10 μM SPS
and 500 μM F− in the presence of different amounts of GO (0, 10, 25,
and 40 μg/mL) in 10% CH3CN/Tris-HCl solution at 25 °C.
Absorbance was recorded at 490 nm. (B) Kinetic plot of reaction of 10
μM SPS with 500 μM F− in the presence of different amounts of GO
( , 0; ,10; , 25; and , 40 μg/mL). (C) Kinetic curves of SPS/
GO (10 μM) with different concentrations of F− (0, 50, 100, 250, and
500 μM) in the presence of 40 μg/mL GO. (D) Plot of the apparent
rate constant k′ vs the concentrations of F−.
■
●
▲
▼
the absorbance at 490 nm reached its saturation value in about
3.0 h upon reaction with F− at 25 °C. Then the effect of
temperature on the response time was investigated, and the
reaction rate constants43,44 (pseudo-first-order rate constants
k′SPS and pseudo-second-order rate constants kSPS) at different
temperature were calculated as shown in Table S1 of the
Supporting Information, which indicated that the reaction rate
could be improved as the temperature went up. However, high
temperature is not conducive to the analysis of practical
samples. When GO was presented to SPS solution at the fixed
F− concentration, the time to reach the equilibrium value sped
up with the increasing GO amount, indicating that the reaction
kinetics of SPS/GO with F− was a GO-concentration-
dependent manner. When the amount of GO reached 40.0
μg/mL, the recognition process could be complete within 30
min with k′SPS/GO values as 2.45 × 10−3 s−1, which was 24-fold
faster than that of free SPS (1.02 × 10−4 s−1) at 25 °C (Figure
2B). Then, the reaction kinetics of SPS with F− were measured
by varying the concentrations of F− at the fixed GO amount of
40.0 μg/mL (Figure 2C) to determine the pseudo-second-
order rate constants kSPS/GO for the reaction of SPS/GO with
different concentrations of F−. As shown in Figure 2D, the plot
of k′SPS/GO versus [F−] was a straight line passing through the
origin, indicating that the reaction is second-order overall, first-
order with respect to nucleophilic substitution reaction, and
second-order with respect to the ring-opening in the polar
solvent, with kSPS/GO = 0.95 M−1 s−1. These results can most
reasonably be attributed to rate-limiting attack of F− at the Si−
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Figure 3. AFM images and line profile of (A) GO, (B) A + SPS, and (C) B + F− and the corresponding EDX data of (D) GO, (E) D + SPS, and (F)
E + F−.
O bond followed by cleavage of the C−O bond in the polar
solvent to afford merocyanine form. Therefore, the results
further proved that GO could accelerate the reaction rate for
sensing F−.
absorbance of SPS/GO before and after centrifugation and
dialysis.
Then, the morphology and thickness of the GO and SPS/
GO nanocomposite were studied by AFM observation. As
shown in Figure 3, the average thickness of the GO sheets is
about 0.9 nm (Figure 3A), which is consistent with that
observed by other researchers.45 In comparison, the average
thickness of SPS/GO in the absence and presence of F− were
determined to be about 1.8 nm (Figure 3B) and 1.4 nm (Figure
3C), respectively. There was a 0.9 nm increment compared
with that of the unmodified GO, owing to the presence of SPS
with a three-dimensional structure on the GO sheet surfaces.
While, after reaction with F−, SPS was converted to the
merocyanine form (MC) with a planar structure, and the
average thickness of MC/GO decreased about 0.4 nm
compared with SPS/GO. Considering that the thicknesses of
the closed and opened forms of one SPS molecule were
calculated to be 1.0 and 0.4 nm respectively, we could assume
that GO sheets were covered by a monolayer of SPS and MC.
Energy-dispersive X-ray (EDX) spectroscopy was also
employed to further explore the interactions between GO
and SPS in the absence and presence of F−. The survey of GO
showed the absence of any detectable amounts of Si at about
1.74 KeV and N at about 0.392 KeV (Figure 3D). Compared
with GO, the survey of SPS/GO showed the presence of Si and
N originate from SPS (Figure 3E), indicating that the
noncovalent functionalization of GO by SPS successfully
occurred. After reaction with F−, F−-mediated cleavage of the
Si−O bond from SPS/GO led to nearly no detectable peak of
Si (Figure 3F), due to the disappearance of the π−π stacking
interaction between the tert-butyldimethylsilyl group and GO.
Performance of F− Sensing with SPS/GO Nano-
composite. As spiropyran is sensitive to pH, the effect of
The F− sensing kinetics of SPS/GO was analyzed to gain
insight into the sped-up process. It could be explained by three
possible mechanisms. First, GO shows good adsorption ability
to aromatic compounds though π−π stacking interactions, so
GO can improve the solubility of SPS in aqueous solution
which leads to the reaction concentration of SPS increase
effectively. Second, GO provides a relatively hydrophobic
reaction interface for the Si−O cleavage reaction, as that
observed by Yang in organic medium.23,24 Finally, the high
active surface area, multiple surface oxygen-containing func-
tional groups, and unique electronic structure of GO presented
the good catalysis toward organic reactions.29a
Preparation and Characteristics of SPS/GO Nano-
composite. To achieve a real F− sensor, the SPS/GO
nanocomposite was prepared. The homogeneous GO dis-
persion was mixed with SPS solution, after being vigorously
stirred for two hours; the SPS/GO nanocomposite and free
SPS were separated by centrifugation and dialysis method. The
prepared SPS/GO nanocomposite was characterized by UV−
vis absorbance measurements, atomic force microscopy (AFM),
and energy-dispersive X-ray (EDX) spectroscopy. Firstly, the
SPS/GO nanocomposite was established through UV−vis
absorbance measurements. The dried nanocomposites dis-
persed in aqueous solution again, as shown in Figure S7 of the
Supporting Information; the distinct characteristic absorption
band of spiro form was observed, indicating that SPS was
assembled onto the surface of GO successfully. The coverage of
SPS on GO was calculated to be about 62.5% by comparing the
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the pH (from 4.0 to 10.0) on the signal response was first
investigated. Figure S8 of the Supporting Information showed
the absorbance at 490 nm of SPS/GO before and after the
addition of F− as a function of different pH values. At acidic
conditions, spiropyran converted to its protonated merocyanine
form (HME+), which was not in favor of sensing F− because of
the strong background signal at 456 nm. On the contrary, at
alkaline conditions, spiropyran existed in the closed spiro form,
resulting in the lack of appearance of an absorption band in the
visible range, which would get a high signal-to-background after
addition of F−. Therefore, the results indicated that the
performance of SPS toward F− work under a pH range from 7.0
to 10.0.
To demonstrate the applicability of the proposed approach
for quantitative detection of F−, we measured the absorption
spectra of SPS/GO containing F− at varied concentrations. As
shown in Figure 4A, the SPS/GO nanocomposite exhibited
very weak absorption in the absence of F−; however, with the
addition of F−, the absorbance around 490 nm gradually
increased. Moreover, the obvious change could be observed at a
sufficiently low concentration of 1.0 × 10−6 M in the absorption
spectra, and the absorbance leveled off when the concentration
of F− reached 4.0 × 10−4 M. This dynamic response range of
SPS/GO was obviously better than the free SPS system which
needed as high as 1.0 × 10−4 M F− to produce an obvious
change in the absorption spectra (Figure S9 of the Supporting
Information). The calibration curves were also established by
plotting the increment of absorbance at 490 nm versus F−
concentration (Figure 4B). One can see that the absorbance
increased considerably as F− with a good linearity concen-
tration was raised from 5.0 × 10−6 to 1.0 × 10−4 M. The
detection limit that was taken to be 3 times the standard
derivation of a blank solution was estimated to be 8.6 × 10−7
M, which was much lower than the free SPS system for the
detection of F− (1.0 × 10−5 M), indicating that the SPS/GO
nanocomposite is potentially appropriate for highly sensitive
quantification of F− in aqueous solution.
The selectivity of the nanocomposite described herein has
been determined by examining the absorption response toward
the common anions in the form of sodium salts, such as Cl−,
Br−, I−, HSO4 , NO3 , ClO4 , AcO−, HCO3 , and H2PO4 . As
shown in Figure 4C, the characteristic absorption peak at 490
nm accompanied with remarkable color response from colorless
to orange-yellow was found only in SPS/GO holding F−
solution, and no other anions caused observable color changes,
which indicated that nanocomposite can selectively target F−
due to the highly special affinity between fluorine and silicon.
To further characterize the binding specificity of SPS/GO for
F−, competition experiments were conducted in which SPS/
GO was first mixed with the competitive anion, and F− was
then added to the mixture. Figure 4D showed the absorption
responses of SPS/GO toward F− in the presence of other
competitive anions. Evidently, the coexistence of these anions
did not interfere with the sensing system of SPS/GO for F− as
well as the subsequent absorption enhancement. These results
suggested that the SPS/GO nanocomposite can function as a
high selective and anti-inference sensor for F−.
−
−
−
−
−
Preliminary Application. To extend the practical
application of the SPS/GO nanocomposite, we performed the
F− sensing in 10% bovine serum. As shown in Figure S10 of the
Supporting Information, SPS/GO exhibited excellent stability
and anti-interference ability in the serum compared with free
SPS, due to the protective effect of nanomaterials.46 Upon
addition of F− to the serum-containing sample, the absorption
spectra of SPS/GO displayed an obvious increase around 490
nm (Figure S10A of the Supporting Information), while only a
slight change was observed in serum-containing SPS alone
(Figure S10B of the Supporting Information). So the SPS/GO
nanocomposite can serve as a good candidate for F− detection
in a complex system. Then, the nanocomposite was applied to
quantitative determination of F− in 1:10 dilution of bovine
serum samples. The bovine serum samples were spiked with F−
at concentrations of 20.0, 50.0, and 100.0 μM and then
analyzed by the proposed method and compared with the
traditional F− ion-selective electrode (ISE), which served as a
referential standard. The results were shown in Table 1, which
revealed high consistency in determination of the F− in
biological samples by the present approach and conventional
instrument, demonstrating the excellent performace of this
sensor in practical application.
Figure 4. Sensitivity and selectivity of F− detection with GO/SPS
nanocomposite. (A) UV−vis absorption spectra of SPS/GO (10 μM
SPS, 40 μg GO) in the presence of increasing amounts of F− in 10%
CH3CN/Tris-HCl (v/v) aqueous solution (20 mM, pH 8.0). The
arrow indicates that the signal changes with increases in F−
concentrations (0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140,
160, 180, 200, 250, 300, and 400 μM). (B) Absorbance enhancement
(A − A0) of SPS and SPS/GO at 490 nm as a function of the
concentrations of F−, respectively. Here, A0 and A are the absorbance
of SPS in the absence and the presence of different concentrations of
F−, respectively. The magnitudes of the error bars were calculated
from the uncertainty given by three independent measurements. (C)
Absorption spectra of SPS/GO after addition of selected competing
anions (1.0 mM) in the 10% CH3CN/Tris-HCl solution. Inset: the
color change of SPS/GO solution after addition of selected anions
−
−
−
(from left to right: F−, Cl−, Br−, I−, HSO4 , NO3 , ClO4 , AcO−,
CONCLUSION
−
−
■
HCO3 , and H2PO4 ), respectively. (D) Absorbance enhancement, A
− A0, of SPS/GO at 490 nm by selected competing anions (1.0 mM, x
axis markers) and the mixture of each competing anion (1.0 mM) and
F− (500 μM).
In conclusion, a SPS/GO nanocomposite has been successfully
prepared and applied to the F− sensing in aqueous media. The
nanomaterial as a novel nanosensor exhibited good solubility
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Article
Table 1. Detection of F− in Bovine Serum Samples
(10) (a) World Health Organization. Fluoride In drinking-water;
WHO: Geneva, 2006. (b) Carton, R. J. Fluoride 2006, 39, 163−172.
(11) Frant, M. S.; Ross, J. W., Jr. Science 1966, 154, 1553−1555.
(12) (a) Black, C. B.; Andrioletti, B.; Try, A. C.; Ruiperez, C.; Sessler,
J. L. J. Am. Chem. Soc. 1999, 121, 10438−10439. (b) Miyaji, H.; Sato,
W.; Sessler, J. L. Angew. Chem., Int. Ed. 2000, 39, 1777−1780.
(c) Furuta, H.; Maeda, H.; Osuka, A. J. Am. Chem. Soc. 2001, 123,
6435−6436. (d) Kumar, S.; Luxami, V.; Kumar, A. Org. Lett. 2008, 10,
5549−5552.
sample F− added (μM)
ISE (μM)
this method (μM) recovery (%)
1
2
3
4
0
−
−
−
a
20.0
50.0
100.0
19.6 0.5
51.0 1.2
101.5 1.6
18.5 1.0
47.8 0.9
96.2 1.2
92.5
95.6
96.2
a
Mean standard deviation of three determinations.
(13) (a) Xu, S.; Chen, K.; Tian, H. J. Mater. Chem. 2005, 15, 2676−
2680. (b) Cooper, C. R.; Spencer, N.; James, T. D. Chem. Commun.
̀
1998, 1365−1366. (c) Sole, S.; Gabbaï, F. P. Chem. Commun. 2004,
and stability in the aqueous solution. Additionally, the SPS/GO
could accelerate the reaction rate of F− and silicon ether and
showed higher sensitivity and a lower detection limit for
detecting F− in comparison with most of reported fluoride
sensors. Furthermore, it showed an excellent anti-interference
ability in complicated real samples. This nanosensor showed a
great potential application in sensing analytes in a complex
system rapidly and sensitively. More importantly, this approach
could provide a new design strategy for developing a general
platform to make the water-insoluble sensors detect the
components in aqueous solution.
1284−1285. (d) Rao, M. R.; Ravikanth, M. J. Org. Chem. 2011, 76,
3582−3587.
(14) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000,
122, 6793−6794.
(15) Kim, T. H.; Swager, T. M. Angew. Chem., Int. Ed. 2003, 42,
4803−4806.
(16) (a) Liu, X. Y.; Bai, D. R.; Wang, S. Angew. Chem., Int. Ed. 2006,
45, 5475−5478. (b) Hudnall, T. W.; Gabbaï, F. P. Chem. Commun.
2008, 4596−4597.
(17) Liu, J. B.; Yang, X. H.; Wang, K. M.; Yang, R. H.; Ji, H. N.; Yang,
L. Y.; Wu, C. L. Chem. Commun. 2011, 47, 935−937.
(18) Descalzo, A. B.; Jimen
Amoros, P.; Marcos, M. D.; Martínez-Man
Commun. 2002, 562−563.
́
ez, D.; Haskouri, J. E.; Beltran
́
, D.;
́
́
̃
ez, R.; Sotoa, J. Chem.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and additional spectroscopic data as noted
in text. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
(19) (a) Yang, X. F. Spectrochim. Acta, Part A 2007, 67, 321−326.
(b) Yang, X. F.; Qi, H. P.; Wang, L. P.; Su, Z.; Wang, G. Talanta 2009,
80, 92−97. (c) Kim, S. Y.; Hong, J. I. Org. Lett. 2007, 9, 3109−3112.
(20) (a) Bao, Y. Y.; Liu, B.; Wang, H.; Tian, J.; Bai, R. Chem.
Commun. 2011, 47, 3957−3959. (b) Bozdemir, O. A.; Sozmen, F.;
Buyukcakir, O.; Guliyev, R.; Cakmak, Y.; Akkaya, E. U. Org. Lett. 2010,
12, 1400−1403.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Yangrh@pku.edu.cn. Tel: 86-731-88822523. Fax: 86-
731-88822523.
■
(21) (a) Sokkalingam, P.; Lee, C. H. J. Org. Chem. 2011, 76, 3820−
3828. (b) Zhang, J. F.; Lim, C. S.; Bhuniya, S.; Cho, B. R.; Kim, J. S.
Org. Lett. 2011, 13, 1190−1193. (c) Ke, B. W.; Chen, W. X.; Ni, N. T.;
Cheng, Y. F.; Dai, C. F.; Dinh, H.; Wang, B. H. Chem. Commun. 2013,
49, 2494−2496.
Notes
The authors declare no competing financial interest.
(22) Kim, S. Y.; Park, J.; Koh, M.; Park, S. B.; Hong, J. I. Chem.
Commun. 2009, 4735−4737.
ACKNOWLEDGMENTS
■
(23) Hu, R.; Feng, J.; Hu, D. H.; Wang, S. Q.; Li, S. Y.; Li, Y.; Yang,
G. Q. Angew. Chem., Int. Ed. 2010, 49, 4915−4918.
We are grateful for the financial support from the National
Natural Science Foundation of China (Grants 21305036,
21075032, 21005026, 21135001, and J1103312), the Founda-
tion for Innovative Research Groups of NSFC (Grant
21221003), and the ‘‘973’’ National Key Basic Research
Program (Grant 2011CB91100-0).
(24) Xiong, L.; Feng, J.; Hu, R.; Wang, S. Q.; Li, S. Y.; Li, Y.; Yang, G.
Q. Anal. Chem. 2013, 85, 4113−4119.
(25) (a) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng,
S.; Cho, K.; Dai, H. Science 2000, 287, 622−625. (b) Cui, Y.; Wei, Q.;
Park, H.; Lieber, C. M. Science 2001, 293, 1289−1292.
(26) (a) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K.
S.; Booth, T. J.; Roth, S. Nature 2007, 446, 60−63. (b) Novoselov, K.
S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.;
Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666−669. (c) Geim,
A. K. Science 2009, 324, 1530−1534.
REFERENCES
■
(1) Gale, P. A. Coord. Chem. Rev. 2001, 213, 79−128.
(2) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486−516.
́ ́
(3) Martinez-Manez, R.; Sancenon, F. Chem. Rev. 2003, 103, 4419−
̃
(27) Zhu, Y. W.; Murali, S.; Cai, W. W.; Li, X. S.; Suk, J. W.; Potts, J.
R.; Ruoff, R. S. Adv. Mater. 2010, 22, 3906−3924.
4476.
(4) Barbier, O.; Arreola-Mendoza, L.; Del Razo, L. M. Chem.-Biol.
Interact. 2010, 188, 319−333.
(5) (a) Bowman-James, K. Acc. Chem. Res. 2005, 38, 671−678.
(b) Gale, P. A. Acc. Chem. Res. 2006, 39, 465−475.
(6) Weatherall, J. A. Pharmacology of Fluorides. In Handbook of
Experimental Pharmacology XX/1; Springer-Verlag: Berlin, Part 1,
1969; pp 141−172.
(7) Cittanova, M. L.; Lelongt, B.; Verpont, M. C. Anesthesiology 1996,
84, 428−435.
(8) Singh, P. P.; Barjatiya, M. K.; Dhing, S.; Bhatnagar, R.; Kothari,
S.; Dhar, V. Urol. Res. 2001, 29, 238−244.
(9) (a) Hutchinson, J. P.; Evenhuis, C. J.; Johns, C.; Kazarian, A. A.;
Breadmore, M. C.; Macka, M.; Hilder, E. F.; Guijt, R. M.; Dicinoski, G.
W.; Haddad, P. R. Anal. Chem. 2007, 79, 7005−7013. (b) Breadmore,
M. C.; Palmer, A. S.; Curran, M.; Macka, M.; Avdalovic, N.; Haddad,
P. R. Anal. Chem. 2002, 74, 2112−2118.
(28) (a) Lu, C. H.; Yang, H. H.; Zhu, C. L.; Chen, X.; Chen, G. N.
Angew. Chem., Int. Ed. 2009, 48, 4785−4787. (b) Balapanuru, J.; Yang,
J. X.; Xiao, S.; Bao, Q.; Jahan, M.; Polavarapu, L.; Wei, J.; Xu, Q. H.;
Loh, K. P. Angew. Chem., Int. Ed. 2010, 49, 6549−6553. (c) Jang, H. J.;
Kim, Y. K.; Kwon, H. M.; Yeo, W. S.; Kim, D. E.; Min, D. H. Angew.
Chem., Int. Ed. 2010, 49, 5703−5707. (d) Xu, Y. X.; Zhao, L.; Bai, H.;
Hong, W. J.; Li, C.; Shi, G. Q. J. Am. Chem. Soc. 2009, 131, 13490−
13497. (e) Chou, S. S.; De, M.; Luo, J.; Rotello, V. M.; Huang, J.;
Dravid, V. P. J. Am. Chem. Soc. 2012, 134, 16725−16733.
(29) (a) Wang, D. M.; Zhang, Y.; Zheng, L. L.; Yang, X. X.; Wang, Y.;
Huang, C. Z. J. Phys. Chem. C 2012, 116, 21622−21628. (b) Song, Y.
J.; Qu, K. G.; Zhao, C.; Ren, J. S.; Qu, X. G. Adv. Mater. 2010, 22,
2206−2210.
(30) (a) Sun, X. M.; Liu, Z.; Welsher, K.; Robinson, J. T.; Goodwin,
A.; Zarc, S.; Dai, H. J. Nano Res. 2008, 1, 203−212. (b) Liu, Z.;
11462
dx.doi.org/10.1021/ac402592c | Anal. Chem. 2013, 85, 11456−11463

Analytical Chemistry
Article
Robinson, J. T.; Sun, X. M.; Dai, H. J. J. Am. Chem. Soc. 2008, 130,
10876−10877.
(31) (a) Li, J. L.; Bao, H. C.; Hou, X. L.; Sun, L.; Wang, X. G.; Gu,
M. Angew. Chem., Int. Ed. 2012, 51, 1830−1834. (b) Wang, H. B.;
Zhang, Q.; Chu, X.; Chen, T. T.; Ge, J.; Yu, R. Q. Angew. Chem., Int.
Ed. 2011, 50, 7065−7069.
(32) (a) Yang, R. H.; Jin, J. Y.; Chen, Y.; Shao, N.; Kang, H. Z.; Xiao,
Z. Y.; Tang, Z. W.; Wu, Y. R.; Zhu, Z.; Tan, W. H. J. Am. Chem. Soc.
2008, 130, 8351−8358. (b) Yang, R. H.; Tang, Z. W.; Yan, J. L.; Kang,
H. Z.; Kim, Y. M.; Zhu, Z.; Tan, W. H. Anal. Chem. 2008, 80, 7408−
7413. (c) Ouyang, X.; Yu, R. Q.; Jin, J. Y.; Li, J. S.; Yang, R. H.; Tan,
W. H.; Yuan, J. L. Anal. Chem. 2011, 83, 782−789. (d) Liu, J. H.; Li, J.
S.; Jiang, Y.; Yang, S.; Tan, W. H.; Yang, R. H. Chem. Commun. 2011,
47, 11321−11323. (e) Liu, J. H.; Wang, C. Y.; Jiang, Y.; Hu, Y. P.; Li, J.
S.; Yang, S.; Li, Y. H.; Yang, R. H.; Tan, W. H.; Huang, C. Z. Anal.
Chem. 2013, 85, 1424−1430.
(33) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80,
1339−1339.
(34) (a) Fischer, E.; Hirshberg, Y. J. Chem. Soc. 1952, 4522−4524.
(b) Day, J. H. Chem. Rev. 1963, 63, 65−80. (c) Taylor, L. D.;
Nicholson, J.; Davis, R. B. Tetrahedron Lett. 1967, 8, 1585−1588.
(d) Byrne, R.; Kevin, J.; Fraser, K. J.; Izgorodin, E.; MacFarlane, D. R.;
Forsyth, M.; Diamond, D. Phys. Chem. Chem. Phys. 2008, 10, 5919−
5924.
(35) (a) Shao, N.; Zhang, Y.; Cheung, S. M.; Yang, R. H.; Chan, W.
H.; Mo, T.; Li, K. A.; Liu, F. Anal. Chem. 2005, 77, 7294−7303.
(b) Shao, N.; Jin, J. Y.; Wang, H.; Zhang, Y.; Yang, R. H.; Chan, W. H.
Anal. Chem. 2008, 80, 3466−3475.
(36) Shao, N.; Wang, H.; Gao, X. D.; Yang, R. H.; Chan, W. H. Anal.
Chem. 2010, 82, 4628−4636.
(37) (a) Shao, N.; Jin, J. Y.; Cheung, S. M.; Yang, R. H.; Chan, W. H.;
Mo, T. Angew. Chem., Int. Ed. 2006, 45, 4944−4948. (b) Li, Y. H.;
Duan, Y.; Li, J. S.; Zheng, J.; Yu, H.; Yang, R. H. Anal. Chem. 2012, 84,
4732−4738.
(38) Shao, N.; Jin, J. Y.; Wang, H.; Zheng, J.; Yang, R. H; Chan, W.
H.; Abliz, Z. J. Am. Chem. Soc. 2010, 132, 725−736.
(39) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc.
Rev. 2010, 39, 228−240.
(40) (a) Slabaugh, W. H.; Seiler, B. C. J. Phys. Chem. 1962, 66, 396−
́ ́
401. (b) Dekany, I.; Kruger-Grasser, R.; Weiss, A. Colloid Polym. Sci.
̈
1998, 276, 570−576.
(41) Scheuermann, G. M.; Rumi, L.; Steurer, P.; Bannwarth, W.;
Mulhaupt, R. J. Am. Chem. Soc. 2009, 131, 8262−8270.
̈
(42) Ang, P. K.; Wang, S.; Bao, Q.; Thong, J. T. L.; Loh, K. P. ACS
Nano 2009, 3, 3587−3594.
(43) Dale, T. J.; Rebek, J. J. Am. Chem. Soc. 2006, 128, 4500−4501.
(44) Yuan, L.; Lin, W. Y.; Yang, Y. T. Chem. Commun. 2011, 47,
6275−6277.
(45) (a) Chou, S. S.; De, M.; Luo, J.; Rotello, V. M.; Huang, J.;
Dravid, V. P. J. Am. Chem. Soc. 2012, 134, 16725−16733. (b) Lu, C.
H.; Yang, H. H.; Zhu, C. L.; Chen, X.; Chen, G. N. Angew. Chem., Int.
Ed. 2009, 48, 4785−4787.
(46) He, X. X.; Wang, K. M.; Tan, W. H.; Liu, B.; Lin, X.; He, C. M.;
Li, D.; Huang, S. S.; Li, J. J. Am. Chem. Soc. 2003, 125, 7168−7169.
11463
dx.doi.org/10.1021/ac402592c | Anal. Chem. 2013, 85, 11456−11463