A reusable nanofibrous film chemosensor for highly selective and sensitive optical signaling of Cu2+ in aqueous media

Article abstract of DOI:10.1039/c3cc41317a

A novel surface modification strategy for an electrospun nanofibrous film was reported, allowing detection and removal of copper ions in the aqueous solution. This reusable dual fluorescent-colorimetric nanofibrous film can be utilized conveniently to achieve real-time naked-eye sensing in aqueous medium just like using a test paper.

Full text of DOI:10.1039/c3cc41317a

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A reusable nanofibrous film chemosensor for highly
selective and sensitive optical signaling of Cu2+ in
aqueous media†
Wei Wang,a Xiuling Wang,a Qingbiao Yang,*a Xiaoliang Fei,a Mingda Suna and
Yan Song*b
Cite this: Chem. Commun., 2013,
49, 4833
Received 20th February 2013,
Accepted 8th April 2013
DOI: 10.1039/c3cc41317a
www.rsc.org/chemcomm
A novel surface modification strategy for an electrospun nanofibrous nitrites,12 volatile gas13 etc.,14–18 in the aqueous phase, have been
film was reported, allowing detection and removal of copper ions in reported. However, these films prepared by covering, dye-doping, or
the aqueous solution. This reusable dual fluorescent–colorimetric by other physical methods suffer from issues such as aggregation and
nanofibrous film can be utilized conveniently to achieve real-time self-quenching of molecular fluorophores, fluorophore leakage, and
naked-eye sensing in aqueous medium just like using a test paper.
inner-layer analyte diffusion.19–22 A more straightforward method is
to modify the surface of polymer nanofibers without affecting bulk
Copper ions are significant environmental pollutants and copper properties of the treated nanofibers. The methods used to impart
is an essential trace element in biological systems.1 The unregu- surface modification usually depend strongly on the nature of the
lated Cu2+ ions can lead to oxidative stress, and their concentration fiber-forming polymer and include, but are not limited to, plasma
in the neuronal cytoplasm may cause the etiology of Alzheimer’s or treatment,23 physisorption,24 self-assembly,8 and covalent grafting.25
Parkinson’s disease.2 Thus, the development of highly selective Covalent grafting of functional compounds onto polymer fiber
sensors for detection of copper ions is of great interest.3
surfaces is a simpler and more convenient approach to introduce
Fluorescence detection techniques have become powerful tools for functionalities permanently with a reasonably high efficiency. One
sensing and imaging of trace amounts of metal ions because of their major advantage of this method is that by covalent grafting of organic
simplicity, sensitivity, and real-time monitoring with fast response fluorescent molecules onto the surface of nanofibers, these organic
time.4 However, currently, most reported examples of fluorescent fluorescent molecules could be rationally brought into a detection
sensing application in the homogeneous phase are not suitable for system by the nanofibers with minimal changes in surface properties
separation, removal and enrichment of target species.5 The solid such as hydrophilicity and charge. In addition, this kind of method
substrate sensor may generally be employed in basic laboratory can avoid the aforementioned problems.
assays, in measurement fields through portable devices, and in
In this work, we have successfully developed a novel sensing sys-
household use as commercial indictors.6–8 The capacity of the solid tem, in which the surfaces of polymer nanofibers are modified with
substrate for adsorbing metal ions can determine the sensitivity of rhodamine derivatives to form a colorimetric and ‘turn-on’ fluores-
fluorescence detection signals.9 Enlarging the specific surface areas of cence sensor for Cu2+ (Scheme 1). The experimental details and
the solid substrate can improve the sensitivity of such fluorescence characterization data are given in the ESI.† We prove that the nano-
detection. To achieve this goal, we plan to utilize a simple yet effective fibrous film (PMAR) as the solid substrate can dramatically improve
technology to fabricate a nanofibrous film and apply the film as the the sensitivity and response time of fluorescence detection of copper
solid substrate for fluorescent detection of metal ions.
ions in aqueous solution. Furthermore, the prepared nanofibrous film
Electrospinning is an effective and simple method for preparing could be utilized as an adsorbent to remove Cu2+ in aqueous solution
various composite nanofibers.10 The electrospinning nanofibers with efficiently, and can be reused if it is washed with EDTA solution.
small diameters have a large surface area per unit mass, which has
To confirm that the fluorophore moiety was successfully grafted
the potential to provide unusually high sensitivity and fast response onto the surface of PMAR nanofibers, FTIR was used. Fig. 1 exhibits
time in sensing application.11 At present, several electrospun fibrous the FTIR spectra of monomer AHPA (A), pure MMA (B), the poly-
membrane chemosensors for some analytes, including metal ions,5,7 (MMA-co-AHPA) nanofibrous film (C), the PMAR nanofibrous film (D)
and RhB–hydrazine (E). The spectrum of the poly(MMA-co-AHPA)
a Department of Chemistry, Jilin University, Changchun 130021, P. R. China.
nanofibrous film (C) differs considerably from that of pure PMMA (B)
in the range of 1800–1600 cmÀ1. Fig. 1(A) and (C) exhibits a strong
E-mail: yangqb@jlu.edu.cn; Fax: +86 431-88499576; Tel: +86 431-88499576
b Department of Chemical and Materials Engineering, Jilin Institute of Chemical
band at 1750 cmÀ1, which is ascribed to CQO of ester and carbonyl
Technology, Jilin, 132022, P. R. China. E-mail: songyan199809@163.com
groups in the AHPA molecule. In particular, a distinct band measured
at 800 cmÀ1 for the AHPA monomer disappeared in the sample of the
† Electronic supplementary information (ESI) available: Supplementary figures
and methods. See DOI: 10.1039/c3cc41317a
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Scheme 1 Chemical and schematic illustration of the preparation of nanofibrous film fluorescent sensors for Cu2+ ions.
poly(MMA-co-AHPA) nanofibrous film. This band is ascribed to the
bending vibration of QC–H out-of-plane from the allyl group in
AHPA. Therefore, we confirmed that aldehyde groups of AHPA were
successfully introduced into the nanofibrous film. In addition, FTIR
measurement shows that the characteristic bands of the RhB–
hydrazine fluorescent unit (E) at 1510 cmÀ1 (secondary amide N–N
bending), 1360–1250 cmÀ1 (aromatic C–N stretching) and 1148 cmÀ1
Fig. 2 SEM images of poly(MMA-co-AHPA) (a) and poly(MMA-co-AHPA)–RhB nano-
(aliphatic C–N stretching) are exhibited in spectra of the PMAR nano-
fibrous film (D). These bands, however, are not present in the spectra
of the nanofibrous film (C). The most obvious change in the FT-IR
spectra of the nanofibrous film (C) is the near disappearance of the
peak at 1670 cmÀ1 (CQO carbonyl groups). From the FT-IR spectra of
the PMAR nanofibrous film, the introduction of a fluorophore moiety
grafted onto the surface of PMAR nanofibers was confirmed.
fibrous film (b), the insets show further magnified images of several electrospun nanofibers.
Fluoroionophores are usually disturbed by a proton in the detec-
tion of metal ions, so their low sensitivity to the operational pH value
was expected and investigated. The fluorescence emission intensities
of the PMAR nanofibrous film in ethanol–water (1 : 1, v/v) as a
function of pH are shown in Fig. S7 (ESI†). We found that the
dispersion exhibits high fluorescence intensity ratios which remained
fairly stable from pH 7.0 to pH 12.0, indicating that the sensor can be
used under some environmental and most physiological conditions.
Considering that most samples for Cu2+ ion analysis were neutral, the
media for Cu2+ ion quantification were then buffered at pH 7.2.
In order to gain an insight into the signaling properties of the film
toward Cu2+, fluorescence titrations were conducted. The fluorescence
titration behavior of the film was investigated in an ethanol–water
(1 : 1, v/v) solution ([Cu2+] = 1.0 Â 10À6–2.0 Â 10À4 M, 0.1 M HEPES–
NaOH buffer at pH 7.20). Fig. 3a shows detailed fluorescence changes
of the PMAR nanofibrous film upon using different concentrations of
Cu2+ ions under the same conditions. The PMAR nanofibrous film
was white. After adding the aqueous solution of copper ions, it
became pink. Even if the concentration of copper ions was as low
as 5 Â 10À6 mol LÀ1, the color of the PMAR film was still visible
(Fig. 3b). According to the molecular structure and spectral results of
the fluorophore moiety, it was concluded that Cu2+ ions could chelate
with the imine N, carbonyl O and phenol O atoms of the fluorophore
Fig. 2 shows the typical SEM images of the poly(MMA-co-AHPA)
nanofibrous film before and after surface modification. It can be seen
that the poly(MMA-co-AHPA) nanofibrous film was composed of
numerous, randomly oriented nanofibers. Under the optimized condi-
tions, the surface of poly(MMA-co-AHPA) nanofibers does not show
any serious cracks or degradation. The average diameter of poly(MMA-
co-NAAP) nanofibers is 495 nm (Fig. 2a). After surface modification,
the nanofibers become conglutinate, tortuous, submicron bumps and
the average diameter of PMAR nanofibers was 520 nm as analyzed
from SEM images (Fig. 2b). The result indicated that the fiber struc-
tures were maintained during the modification process, but the fiber
diameter increased slightly. Moreover, the Brunauer–Emmett–Teller
(BET) surface areas of the nanofibers produced with and without a
fluorophore moiety were measured to be 3.3852 and 3.0445 m2 gÀ1
respectively. This network structure of the electrospun nanofibrous
film provides a surface area-to-volume ratio roughly 1 to 2 orders of
magnitude higher than that of known continuous thin films.26 This
unique porous structure could greatly accelerate the targets to diffuse
close to the sensing elements and increase the complexation efficiency.
Fig. 3 (a) Fluorescent spectra of the PMAR nanofibrous film in the absence and pre-
sence of Cu2+ (1.0 Â 10À6–2.0 Â 10À4 mol LÀ1). The inset shows fluorescence intensity
change as a function of Cu2+ concentration. (b) From bottom to top are photographs of
the nanofibrous film after 1 mM, 5 mM, 10 mM, 500 mM, 200 mM Cu2+ involvements.
Fig. 1 FT-IR spectra of AHPA (A), pure MMA (B), poly(MMA-co-AHPA) nano-
fibrous film (C), PMAR (D) and fluorophore moieties RhB–hydrazine (D).
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moiety, as depicted in ESI.† In addition, upon the addition of increas-
In conclusion, a simple approach for the production of a
ing concentrations of Cu2+ ions, a significant enhancement of the nanofibrous film is developed based on a surface modification
characteristic fluorescence of a rhodamine B fluorophore moiety in a and electrospinning technique. The surface of electrospun nano-
Cu2+ ion concentration-dependent manner is observed at 557 nm, fibers decorated with molecular fluorophores as a fluorescence
accompanied by an obvious orange fluorescence enhancement. When probe avoids the aggregation and self-quenching of molecular
more than 1 Â 10À4 mol LÀ1 Cu2+ ions were added, the maximum fluorophores effectively, and exhibits high fluorescence sensitivity,
fluorescence intensity was retained. From the fluorescence titration very short response and recovery time, good reversibility and
experiment, a linear relationship (R2 = 0.9818) is observed between the reproducibility. We believe that this technique will provide a
fluorescence intensity of the nanofibrous film and the concentration promising alternative for developing high-performance sensing
of Cu2+ (Fig. S8, ESI†). The reaction responsible for these changes materials for metal-ion detection and removal in aqueous solution.
reaches completion well within the time frame of the measurement
We are grateful for the financial support from the National
(o10 s, [Cu2+] = 1.0 Â 10À6–2.0 Â 10À4 mol LÀ1) and the detection Natural Science Foundation of China (No. 21174052) and the Doc-
limit was found to be 1.5 Â 10À6 mol LÀ1 (based on S/N = 3).
The fluorescence responses of the film to various cations and its
selectivity for Cu2+ are illustrated in Fig. S9 (ESI†). The experiments
were carried out by fixing the concentration of Cu2+ at 5.0 Â
10À5 mol LÀ1. As can be seen from the black bars in Fig. S9 (ESI†),
fluorescence almost did not change in the solutions of 1.0 Â
10À3 mol LÀ1 representative metal ions, such as Na+, K+, Ca2+,
Mg2+, Fe2+, Fe3+, Mn2+, Ni2+, Cd2+, Co2+, Pb2+, Zn2+, Pb2+, Hg2+ and
Ag+. The miscellaneous competitive cations did not lead to any
significant color in the visible region (Fig. S10, ESI†). In order to
further test the interference of other competitive cations in the
determination of Cu2+ competition experiments were performed in
which the PMAR nanofibrous film was added to a solution of Cu2+
in the presence of other metal ions (white bars in Fig. S9, ESI†).
Experimental results indicate that the increases in fluorescence
intensity resulting from the addition of the Cu2+ were not influ-
enced by the subsequent addition of miscellaneous cations. Thus,
the excellent selectivity toward Cu2+ makes the practical application
of the PMAR nanofibrous film feasible.
toral Program of Higher Education of China (No. 20100061110008).
Notes and references
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