Colorimetric and fluorescent nanofibrous film as a chemosensor for Hg 2+ in aqueous solution prepared by electrospinning and host-guest interaction

Article abstract of DOI:10.1039/c2cc17664e

A fluorescent sensing film for Hg²⁺ ions was fabricated by host-guest interaction and electrospinning. When the nanofibrous film was put into a solution of Hg²⁺ ions, it gave rise to orange fluorescence, causing a clear color change from white to pink-red.

Full text of DOI:10.1039/c2cc17664e

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COMMUNICATION
Colorimetric and fluorescent nanofibrous film as a chemosensor for Hg2+ in
aqueous solution prepared by electrospinning and host–guest interactionw
Wei Wang, Yapeng Li, Mingda Sun, Chen Zhou, Yue Zhang, Yaoxian Li and
Qingbiao Yang*
Received 7th December 2011, Accepted 26th January 2012
DOI: 10.1039/c2cc17664e
A fluorescent sensing film for Hg2+ ions was fabricated by
host–guest interaction and electrospinning. When the nanofibrous
film was put into a solution of Hg2+ ions, it gave rise to orange
fluorescence, causing a clear color change from white to pink-red.
Mercury is a highly toxic element in ecosystems. Mercury-
induced toxicity can cause a number of severe adverse effects
on human health.1 Highly selective and sensitive chemosensors
for Hg2+ are hence demanded.2
Fluorescence techniques have become powerful tools for
sensing and imaging of trace amounts of metal ions because of
their simplicity, sensitivity, and real-time monitoring with fast
response time.3 Rhodamines are dyes extensively employed in
fluorogenic-labeling and fluorescent chemosensors owing to
their high absorption coefficient, high fluorescence quantum
yield and long-wavelength absorption and emission. By virtue
of these fascinating properties, excellent examples of rhodamine-
based turn-on fluorescent Hg2+ sensors have been reported.4,5
However, their application in related analytical techniques in
the homogeneous phase is not suitable for the separation,
removal and enrichment of target species.6
Scheme 1 Chemical and schematic illustration for the preparation of
Nowadays, the electrospinning technique has been found to be
nanofibrous film fluorescent sensors for Hg2+ ions.
a unique and cost-effective approach to fabricate membranes
with large surface areas for fluorescent sensor application. It is
fluorophore moiety 3 is loaded on the cross-linked adamantane-
expected that this large available surface area has the potential
functionalized nanofiber 2 surface via host–guest interaction.
to provide unusually high sensitivity and fast response time in
sensing application.7–10 However, these films prepared by
One major advantage of this method is that guest molecules
grafted on the surfaces result in a fluorescent receptor with
covering and doping or other physical methods suffer from
minimal changes in surface properties such as hydrophilicity
issues such as fluorophore leaking, thickness control, and
inner-layer analyte diffusion.11–14 The use of supramolecular
and charge. In addition, since the immobilization occurs by
inclusion complex formation, the functionalized surfaces can be
interactions (i.e., host–guest interactions) is an attractive
readily synthesized and characterized. Fluorescent receptor
alternative to physical adsorption methods as these interactions
immobilization then takes place by self-assembly on the
can be easily tuned by the appropriate selection of geometrically
surfaces without additional need for chemical conjugation
complementary host and guest molecules, such as b-cyclodextrin
(b-CD) (host) and adamantane (guest) (Scheme 1).
In this work, we have successfully developed a novel fluorescent
and reagents added. The response of these fluorescent sensing
films fabricated in a self-assembling way is significantly faster
due to the direct exposure of fluorophore moieties, avoiding,
sensing system (Scheme 1), in which the rhodamine–cyclodextrin
at least in principle, the inner-layer diffusion problem encountered
by cyclodextrin-based physically fabricated sensing films.
Department of Chemistry, Jilin University, Changchun, 130021,
People’s Republic of China. E-mail: yangqb@jlu.edu.cn;
nanofibrous film 1 can be found in S3–S7 (ESIw). To confirm that
Fax: +86 431 88499576; Tel: +86 431 88499576
The detailed procedures for the synthesis of inclusion complex
the fluorophore moiety 3 was successfully loaded on the surface of
w Electronic supplementary information (ESI) available: Synthesis
scheme and H NMR spectrum. See DOI: 10.1039/c2cc17664e
1
nanofibrous film 2, FTIR was investigated. Fig. 1 exhibits the
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6040 Chem. Commun., 2012, 48, 6040–6042
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nanofiber 2 doesn’t show any serious cracks or degradation.
Fiber surface after self-assembly of fluorophore moiety 3 shows
slight fiber adhesion, and fibers become thick and curved
(Fig. 2(b)). The average diameter of nanofibers 2 is 495 nm.
After loading fluorophore moiety 3 on the surface of nano-
fiber 2, the nanofibrous film becomes conglutinate, coarsened,
tortuous slightly, and the average diameter of nanofiber 1 is
520 nm. Moreover, the BET surface areas of the nanofibers
produced with and without fluorophore moiety 3 were measured
to be 3.1893 and 2.9505 m2 gÀ1 respectively. This network
structure of electrospun film provides a surface area-to-volume
ratio roughly 1 to 2 orders of magnitude higher than that of
known continuous thin films.16 This unique porous structure
could greatly accelerate the targets to diffuse close to the
sensing elements and increase the complexation efficiency.
Fluorophores are usually disturbed by the protons in the
detection of metal ions, so their low sensitivity to the opera-
tional pH value was expected and investigated. Fig. S10 (ESIw)
shows the fluorescence response of nanofibrous film 1 without
and with Hg2+ ions as a function of pH. Experimental results
show that for Hg2+-free nanofibrous film 1, under acidic condi-
tions (pH o 6), an obvious off–on fluorescence appeared due to
the formation of the open-ring state of the thiocarbamido-
SRhB-b-CD based on strong protonation. In a pH range
from 6.0 to 13.5, an almost negligible fluorescence signal
(excited at 584 nm) was observed, suggesting that the mole-
cules prefer the spirocyclic form. On the contrary, upon
addition of Hg2+ ions, there was an obvious fluorescence
off–on change of nanofibrous film 1 at different pH values.
And the pH-control emission measurements reveal that nano-
fibrous film 1 is able to respond to Hg2+ ions in a wide pH range
from 6 to 13.5 with only minor changes in fluorescence intensity,
suggesting that the nanofibrous film 1 facilitates the quantifica-
tion of the concentration of Hg2+ ions in aqueous solution over
a broad pH range. Since most samples for the nanofibrous film 1
analysis of Hg2+ ions are neutral, pH 7.2 buffer solution is
chosen as the medium for Hg2+ ions quantification.
Fig. 1 FT-IR spectra of pure MMA (A), nanofibrous film 2 (B),
nanofibrous film 1 (C) and fluorophore moieties 3 (D).
FTIR spectra of pure MMA (A), nanofibrous film 2 (B), nano-
fibrous film 1 (C) and fluorophore moiety 3 (D). The spectrum
of nanofiber 2 (B) differs considerably from that of pure MMA
(A) in a range of 2800–2900 cmÀ1. Fig. 1 (B) exhibits bands at
2900 cmÀ1 and 2849 cmÀ1, which are attributed to methylene
asymmetric C–H stretching and methylene symmetric C–H
stretching from the adamantine molecule. In particular, two
distinct bands measured at 805 cmÀ1 and 1675 cmÀ1 for
monomer MMA have disappeared in the sample of nano-
fibrous film 2. These bands are ascribed to the bending
vibration of QC–H out-of-plane and CQC vibrations from the
allyl group in MMA. Therefore, we confirmed that adamantane
was successfully introduced into nanofibrous film 2. In addition,
FTIR measurement shows that the characteristic bands of free
moiety 3 at 3379–3389 cmÀ1 (NH stretching), 1730 cmÀ1
(aromatic CQO stretching), 1548 cmÀ1 (secondary amide
N–N bending), 1360–1250 cmÀ1 (aromatic C–N stretching),
1200–1050 cmÀ1 (CQS stretching), 1148 cmÀ1 (aliphatic C–N
stretching) and 700–665 cmÀ1(secondary amide N–H wagging)
are exhibited in spectra of nanofibrous film 1 (C). These bands,
however, are not present in nanofibrous 2 (B). The present
FTIR data indicate that the attachment of fluorophore moiety
3 to the surface of nanofibrous film has indeed taken place via
host–guest interaction. Notably, the inclusion of adamantine
in a b-CD cavity shifts methylene stretching vibration to higher
frequency i.e. from 2849 to 2870 cmÀ1 during the formation
of the thiocarbamido-SRhB-b-CD/poly (MMA-co-ADMA)
complex (Fig. 1(C)). The frequency shift was explained by
the breakdown of the intermolecular hydrogen bonding asso-
ciated with these molecules and the establishment of stronger
binding in the complex system.15
In order to gain insight into the signaling properties of the
film toward Hg2+, fluorescence titrations were conducted.
Detailed information of fluorescence titration is provided
in S3(ESIw). Fluorescence titration of the film was carried
out in DMF–H2O (1 : 10, v/v) solution ([Hg2+] = 0–2.0 Â
10À4 mol LÀ1, 0.1 M KH2PO4–NaOH buffer at pH 7.20, lex
525 nm, lem = 584 nm).
=
Solid UV-vis spectra of the inclusion complex nanofibrous
film 1 upon addition of Hg2+ are shown in Fig. S8 (ESIw).
When Hg2+ was added in the solution of the film in
DMF–H2O, the emission intensity at 584 nm was significantly
enhanced (Fig. 3), and a clear change in solution color from
white to pink-red was observed as shown in Fig. S5 (ESIw).
According to the molecular structure and spectral results of
fluorophore moiety 3, it is concluded that the addition of the
Hg2+ ion induced the N atom of spirolactam to attack the
C atom of thiourea, and thus a ring opening of the spirolactam
of rhodamine took place, followed by the removal of HgS and
the formation of intramolecular guanylation.3c Finally, a stable
cyclic product was formed through an irreversible desulfuriza-
tion reaction, as depicted in Fig. S6 (ESIw). It demonstrates that
inclusion complex nanofibrous film 1 can sensitively detect
A set of typical SEM images of nanofibrous film 2 are
shown in Fig. 2(a). It can be seen that the film is composed of
numerous, randomly oriented nanofibers. The surface of
Fig. 2 SEM images of nanofibrous film 2 (a) and 1 (b), insets show
further magnified images of several electrospun nanofibers.
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This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6040–6042 6041

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added to a solution of Hg2+ in the presence of other metal
ions (white bars in Fig. 4). Experimental results indicate that
these ions have no obvious interference in the Hg2+ detection.
Thus, the excellent selectivity toward Hg2+ makes the practical
application of our nanofibrous film 1 feasible. Further stability
experiments showed that there is no leakage of the dye from the
thiocarbamido-SRhB-b-CD/poly (MMA-co-ADMA) inclusion
complex nanofibrous film after soaking in DMF–H2O for 24 h.
After the soaking test, the nanofibrous film can be repeatedly
used for fluorescent titration without losing its sensitivity (ESIw,
Fig. S9).
Fig. 3 Fluorescent spectra of the thiocarbamido-SRhB-b-CD fluoro-
phore moiety of nanofibrous film in the absence and presence of Hg2+
(1.0 Â 10À5–2.0 Â 10À4 mol LÀ1). The inset shows fluorescence
intensity change as a function of Hg2+ concentration.
In conclusion, a simple approach for the production of
nanocomposite fibers was developed based on self-assembly
and the electrospinning technique. The selectivity and sensitivity
of the nanofibrous film for Hg2+ were satisfactory and the
detection limit was found to be 6.0 Â 10À5 mol LÀ1 (based on
S/N = 3). We believe that this technique would provide a very
promising alternative for developing high-performance sensing
materials for metal-ion detection in aqueous solution.
This work was supported by the Doctoral Program of
Higher Education of China (No. 20100061110008).
Hg2+ with high selectivity over other interfering metal ions,
especially the thiophilic metal ions including Cu2+, Ag+ and
Fe3+. The reaction responsible for these changes reaches comple-
tion well within the time frame (o1 min) of the measurement. The
fluorescence intensity was well proportional to the amount of
Hg2+ (1.0 Â 10À5–2.0 Â 10À4 mol LÀ1) with a linear correla-
tion (R2 = 0.9950) (ESIw, Fig. S7). The maximum fluorescence
intensity induced by Hg2+ was retained when the concentration
of Hg2+ solution was further increased to be higher than
2.0 Â 10À4 mol LÀ1. The local concentration of thiocarbamido-
SRhB-b-CD in nanofibrous film was also determined to be
0.0143 mol mÀ2 based on the spectral results.
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The fluorescence responses of the film to various cations and
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of Hg2+ at 2.0 Â 10À4 mol LÀ1. As can be seen from the black
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,
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Fig. 4 Black bars: fluorescent emission response of the film in the presence
of different metal ions in DMF–H2O solution. White bars: fluorescent
response of the film upon addition of 2.0 Â 10À4 mol LÀ1 Hg2+ in the
presence of 1.0 Â 10À3 mol LÀ1 each of background metal ions.
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6042 Chem. Commun., 2012, 48, 6040–6042
This journal is The Royal Society of Chemistry 2012