A naked-eye chemosensor for fluoride ions: A selective easy-to-prepare test paper

Article abstract of DOI:10.1039/c3ob27131e

A new naked-eye chromogenic chemosensor based on 2-thiohydantoin shows high selectivity for fluoride ions and is used to develop a test paper for detection of fluoride ions in the solid state.

Full text of DOI:10.1039/c3ob27131e

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A naked-eye chemosensor for fluoride ions: a selective
easy-to-prepare test paper†
Cite this: DOI: 10.1039/c3ob27131e
Received 1st November 2012,
Accepted 14th February 2013
Xue Yong,a Minjian Su,a Wen Wang,a Yichen Yan,b Jinqing Qu*a and Ruiyuan Liu*b
DOI: 10.1039/c3ob27131e
www.rsc.org/obc
A new naked-eye chromogenic chemosensor based on 2-thio-
hydantoin shows high selectivity for fluoride ions and is used to
develop a test paper for detection of fluoride ions in the solid
state.
Scheme 1 Synthesis of probe 1.
There has been considerable interest in anion recognition and
sensing because of the fundamental roles of anions in many
chemical and biological processes.1 For example, fluoride ions
show beneficial effects on dental health and the treatment of
osteoporosis,2 while it is accused for several human patho-
logies.3 A number of compounds that are able to bind fluoride
ions with high affinity and selectivity have been reported.4 Of
particular interest in this regard are colorimetric sensors that
would allow the naked-eye detection of anions without resort-
ing to any spectroscopic instrumentation.5 The common recog-
nition moieties for anion sensors include amide,6 urea and
thiourea,7 amidourea,8 phenol,9 pyrrole,10 imidazolium,11 and
indole.12 In these subunits, N–H⋯X− and O–H⋯X− hydrogen
bonding played very important roles in selective sensing of
anions.
2-Thiohydantoin has received some attention in the past,
mainly due to its use for medical purposes as an anticonvul-
sant, antimutagenic, anticarcinogenic, antiandrogen and anti-
viral agent,13 but also recently as a polymerization catalyst,14
as a corrosion inhibitor and as an analytical reagent.15 Due to
the presence of a hydrogen bonding donor and acceptor in the
2-thiohydantoin ring, N–H⋯O intermolecular hydrogen
bonding can be formed,16 which means that 2-thiohydantoin
can act as an anion receptor to detect anions.
such as OH and NH. The colorimetric response of this probe
towards fluoride ions in solution or in solid state was also
investigated.
Probe 1 was prepared by reaction of 2-amino-2-(4-hydroxy-
phenyl) acetic acid methyl ester hydrochloride with butyl iso-
thiocyanate (Scheme 1). The structure of probe
confirmed from its spectroscopic and analytical data (see ESI,
S1–S4†).
We first evaluated the ability of probe 1 to detect fluoride
ions in solution. The titration of TBAF with the probe 1 was
performed in CH3CN. The UV-vis spectrum of probe 1 exhib-
ited a strong absorption band at around 275 nm. With the
addition of F− up to 3.0 molar equiv., the color of CH3CN solu-
tion of probe 1 dramatically changes from colorless to yellow,
which was accompanied by a new intense absorption band
centered at about 447 nm in the UV-vis spectrum (Fig. 1a).
With further addition of 3 more molar equiv. of F−, the color
of CH3CN solution of probe 1 changes to purple. At the same
time, the absorption band at 447 nm decreases gradually
accompanying the formation of a new band at 552 nm with a
shoulder at 600 nm in the UV-vis spectrum (Fig. 1b). It seems
that the interaction between probe 1 and F− was a two step
process with the help of hydrogen bonding.
1 was
Herein, we designed probe 1 based on 2-thiohydantoin,
which contains different types of hydrogen bonding donors
To confirm this assumption, 1H NMR F− titration experi-
ment was carried out. It was found that the phenol OH proton
signal (9.07 ppm) was broadened and completely disappeared
upon addition of 1.0 equiv. of fluoride ions, whereas the NH
proton signal was broadened with increasing F− concentration
from 0 to 1.0 equiv., then underwent a continuous downfield
shift from 10.05 to 10.48 to 10.86 ppm with increasing F− con-
centration from 1.0 to 2.0 to 4.0 equiv. (Fig. 2). Upon addition
of 16 equiv. of TBAF, the NH proton signal disappeared. The
addition of a large excess of F− may induce the consecutive
aSchool of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, P.R. China. E-mail: cejqqu@scut.edu.cn;
Fax: +8602087111344; Tel: +8602087111344
bSchool of Pharmaceutical Science, Southern Medical University, Guangzhou
510515, P.R. China. E-mail: ruiyliu@smu.edu.cn; Fax: +8602061648196;
Tel: +8602061648196
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3ob27131e
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of phenol OH groups, the second step is the hydrogen
bonding interaction between F− and a thiohydantoin NH motif
and deprotonation of NH groups. It seems that the different
acidity and hydrogen bond formation ability of phenol OH
groups and thiohydantoin NH subunits results in the two
step process. The acidity and high hydrogen bonding donor
ability of a phenol OH group have made it preferable to inter-
act with F−.
To examine the selectivity, the probe 1 (1.0 mM) was treated
with various relevant species (Cl−, Br−, HSO4−, NO3−) in
CH3CN. The photograph in Fig. 3 showed the color changes
after addition of these anions to the CH3CN solution of probe
1. The color changes occurred only when fluoride ions were
added, other anions failed to cause any significant color
change. Considering the high selectivity for F− over other
anions and the convenience of not requiring a spectrometer,
the probe 1 provided a great advantage for detecting fluoride
ions. The sensing of probe 1 for anions such as Cl−, Br−,
−
−
HSO4 and NO3 was also studied through UV-vis spectra in
CH3CN solutions (Fig. S5†). A red shift was observed in the
absorption spectrum of probe 1 upon addition of F−, while the
absorption spectrum changes slightly upon addition of Cl−,
−
Br−, NO3 and HSO4−. Thus, the system caused a distinct
host–guest interaction for only F−. It is suggested that the high
selectivity might be attributed to the high charge density and
small size of fluoride anions, which enables F− to be a strong
hydrogen bonding acceptor that interacts with 2-thiohydantoin
derivatives.
Generally, probes based on hydrogen bonding interactions
cannot act as efficient anion sensors in aqueous media, due to
the strong solvent competition.18 A number of colorimetric
and/or fluorescent sensors were embedded in polymer
matrices or hydrogel to avoid the solvation of anions in
water.19 Immobilizing sensors in paper to prepare test papers
which detect anions in solid state can also avoid the compet-
ing solvation effect of water.18 In addition to the “wet”
measurements, we wonder whether the probe 1 could also
work in the solid state. The probe 1 was adsorbed onto filter
papers.18 First, we prepared a paper film containing probe 1 by
immersing a filter paper in the CH3CN solution of probe 1
(1.0 mM) and then drying it in the air. For detecting the fluor-
ide ions, the paper film was submersed into the aqueous fluor-
ide ion containing solution for several seconds then dried in
Fig. 1 (a) UV absorption changes of probe 1 in CH3CN upon addition of F− ([1]
= 100 μM, [F−]/[1] = 0–3.0). (b) UV absorption changes of probe 1 in CH3CN
upon addition of F− ([1] = 100 μM, [F−]/[1] = 3.0–6.0).
Fig. 2 Partly 1H NMR spectra of probe 1 in the presence of (a) 0, (b) 0.5,
(c) 1.0, (d) 2.0, (e) 4.0, and (f ) 16.0 equiv. of TBAF in CH3CN-d3. **means
NH proton. *means OH proton.
deprotonation of N–H fragments, resulting in a bright color
change.17 These observations clearly indicated that the inter-
action between F− and probe 1 involves two steps. The first
Fig. 3 Caption visible color changes of probe 1 in CH3CN after the addition of
step includes the hydrogen bond formation and deprotonation TBA salts of a series of anions ([1] = 1.0 mM, [anion]/[1] = 10).
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aqueous solution with different F− concentrations.
the air. As displayed in Fig. 4, the color of the paper films
changed from pale yellow to violet. This demonstrates a proto-
type device using the probe 1 for detecting F− in real world
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limit of about 1.9 ppm (1.9 μg mL−1). We also try to confirm
the selectivity of paper films to the anions. Other anions such
as Cl−, Br−, HSO4 and NO3 did not cause any detectable
changes. To confirm the influence of other anions on the flu-
oride ion detection, test papers were immersed in aqueous solu-
tions having different fluoride concentrations in the presence
of 1.9 mg mL−1 chloride anions. Similar color changes to
those of solutions containing only fluoride were observed. This
demonstrates a prototype device using the probe 1 for detect-
ing fluoride ions in the wilderness.18
−
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In conclusion, we have reported a “naked-eye” colorimetric
probe 1 for selective detection of F−. Furthermore, probe 1 can
also be used as a chemosensor for detection of F− in solid
state. The easy-to-prepare fluoride ion test paper can detect F−
at a low limit of about 1.9 ppm (1.9 μg mL−1).
We gratefully thank the “National Natural Science Foun-
dation of China” (21074073, 51173050, and 21244007) for
financial support of this work.
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