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Nov 30, 2011 - Cui , X.; Hetke , J. F.; Wiler , J. A.; Anderson , D. J.; Martin , D. C. Electrochemical deposition and characterization of conducting ...
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Electrospun 1,4-DHAQ-Doped Cellulose Nanofiber Films for Reusable Fluorescence Detection of Trace Cu2+ and Further for Cr3+ Meiling Wang,† Guowen Meng,*,† Qing Huang,*,‡ and Yiwu Qian† †

Key Laboratory of Materials Physics, and Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, China ‡ Key Laboratory of Ion Beam Bioengineering, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, 230031, China S Supporting Information *

ABSTRACT: 1,4-Dihydroxyanthraquinone (1,4-DHAQ, a fluorophore) doped cellulose (CL) (denoted as 1,4-DHAQ@CL) microporous nanofiber film has been achieved via simple electrospinning and subsequent deacetylating, and used for highly sensitive and selective fluorescence detection of Cu2+ in aqueous solution. As the resultant byproduct of Cu2+contaminated 1,4-DHAQ@CL nanofiber film showed recovered fluorescence by extra addition of Cr3+ nitrate solution, 1,4DHAQ and Cu2+ codoped CL (denoted as (1,4-DHAQ)-Cu2+@ CL)) microporous nanofiber film has been further fabricated for the detection of Cr3+ in aqueous solution. It was found that the fluorescence intensity of the 1,4-DHAQ@CL microporous nanofiber film linearly decreases with Cu2+ concentration ranging from 2.5 × 10−9 to 3.75 × 10−8 M, while that of the codoped (1,4-DHAQ)-Cu2+@CL nanofiber film linearly increases with Cr3+ concentration from 2.5 × 10−9 to 2.5 × 10−8 M, both with high selectivity over many other common heavy metal ions. The sensing mechanism for Cu2+ is ascribed to the formation of phenolate between 1,4-DHAQ and Cu2+, while that for Cr3+ is attributed to the reversing reaction from Cu2+-based phenolate to Cu2+ and Cr3+-based excited complex with recovered fluorescence. The sensitive and selective detection of Cu2+ and Cr3+ by using the 1,4-DHAQ@CL and the (1,4-DHAQ)-Cu2+@CL nanofiber films was further demonstrated in polluted lake waters, thus indicating their potential applications in environmental monitoring of Cu2+ and Cr3+ in polluted water. Additionally, both the 1,4-DHAQ@CL and (1,4-DHAQ)-Cu2+@CL microporous nanofiber films are reusable for the detection of Cu2+ and Cr3+, respectively, after simple treatment. The design concept in this work might also open a door to the design of effective fluorescence probes for other heavy metal ions.

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INTRODUCTION Fluorescence sensors work via the interaction between the fluorophore-modified sensing building blocks and the target molecules on their surface,1 thus their sensitivity increases with the surface area per unit mass (S/M ratio) of the sensor materials.1,2 Much effort has been made to raise the S/M ratio of the fluorescence sensor materials, however the present synthetic approaches involve perplexing fabrication processes and can only be suitable to particular materials,1−4 thus it is necessary to develop facile and versatile approaches to sensor materials with high S/M ratio. One-dimensional (1D) nanomaterials have a huge S/M ratio, thus they have been used as fluorescence sensor building blocks.1 Electrospinning, an effective technique for producing long and continuous 1D nanofibers with different structures, morphologies, and functionalities have been used for a variety of polymers, biomolecules, and inorganic/polymer composites,5−7 to achieve films consisting of nanofibers with high S/M ratio.8 It has been also reported that quantum dots (or fluorescent dyes) could be © 2011 American Chemical Society

uniformly dispersed into the nanofibers via electrospinning and their fluorescence quantum efficiency could be improved by reducing aggregation and thus the corresponding Föster Resonance Energy Transfer among the dye molecules.9,10 Therefore, fluorophore-doped electrospun nanofiber films have promising potential as building blocks in ultrasensitive fluorescence sensors. Copper, a very important element for hemopoiesis, metabolism, growth, and immune system,4 plays important roles in various biological processes.11 However, exposure to high levels of copper can cause gastrointestinal disturbance, and even liver or kidney damage.12 Chromium, an essential trace element of human nutrition, has great impact on the metabolism of carbohydrates, fats, proteins, and nucleic Received: Revised: Accepted: Published: 367

June 23, 2011 September 27, 2011 November 30, 2011 November 30, 2011 dx.doi.org/10.1021/es202137c | Environ. Sci. Technol. 2012, 46, 367−373

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Figure 1. (A) Schematic showing the electrospinning setup (left) and sensing mechanism of the 1,4-DHAQ@CL microporous nanofiber film for Cu2+ (right), and (B) sensing mechanism of the (1,4-DHAQ)-Cu2+@CL microporous nanofiber film for Cr3+.

acids.13 But chromium is highly carcinogenic and mutagenic.14,15 High levels of Cr3+ can bind to DNA, destroy the cellular structure, and damage the cellular components,14,15 although toxicity of Cr3+ observed in vivo is less serious than that of Cr6+.13 Thus trace detections of copper and chromium in living organisms are important. It is well-known that analytical methods based on fluorescence spectroscopy show advantages such as simplicity, rapidity, high sensitivity, and selectivity. Up to now, several fluorescence-based probes for Cu2+ using aminoquinoline and coumarin,4,16 and for Cr3+ using rhodamine derivatives and glutathione modified quantum dots,17−19 have been developed. However, fabrications of those fluorescence sensor materials are complex, and all of them rely on solutions. In comparison, solid-state sensor materials have some preferred advantages such as portability, operational simplicity, and reusability, which make rapid online detection possible at low cost. Thus it is very attractive but also a challenge to develop solid-state fluorescence sensor materials for direct detection of trace Cu2+ and Cr3+ in aqueous solution in real time. In this work, first 1,4-dihydroxyanthraquinone (1,4-DHAQ) fluorophore doped cellulose acetate (CA) (denoted as

1,4-DHAQ@CA) nanofiber film was achieved via electrospinning; then the CA was deacetylated to cellulose (CL), and accordingly the as-electrospun 1,4-DHAQ@CA nanofiber film was turned into the final 1,4-DHAQ@CL microporous nanofiber film with much higher S/M ratio as there would be much more micropores in the 1,4-DHAQ@CL nanofibers after deacetylating treatments (Figure 1A).20 This deacetylating treatment makes more 1,4-DHAQ fluorophore molecules exposed on the fiber surface, thus increasing the amount of 1,4-DHAQ molecules that can interact with the target analytes and further improving the sensitivity of the nanofiber film. As expected, this 1,4-DHAQ@CL fluorescent microporous nanofiber film shows effective sensing to Cu2+ in aqueous solution, with remarkable fluorescence quenching and a lower detection limit of 3 nM. Moreover, the 1,4-DHAQ@CL nanofiber film exhibits good selectivity to Cu2+ over many other common heavy metal ions. The sensing mechanism can be attributed to the formation of Cu2+-based phenolate with 1,4-DHAQ. Furthermore, we unexpectedly found that, after the fluorescence detection of Cu2+ using the 1,4-DHAQ@CL nanofiber film, the final byproduct of Cu2+-contaminated 1,4-DHAQ@CL nanofiber film showed effective sensing to Cr3+ with recovered 368

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fluorescence of the 1,4-DHAQ fluorophores. Based on this, another kind of nanofiber film codoped with 1:2 (1,4-DHAQ)/ Cu2+ (denoted as (1,4-DHAQ)-Cu2+@CL) was fabricated and used for fluorescence detection of Cr3+ in aqueous solution via reversing the reaction between Cu2+ and 1,4-DHAQ (Figure 1B). It was found that the fluorescence intensity of the (1,4DHAQ)-Cu2+@CL microporous nanofiber film increases with [Cr3+] in aqueous solution in the concentration range of 2.5 × 10−9 to 2.5 × 10−8 M with high selectivity over other common metal ions. Both the 1,4-DHAQ@CL and the (1,4-DHAQ)Cu2+@CL have been explored to the fluorescence detection of Cu2+ and Cr3+ in polluted lake waters, showing their superior sensitivity to Cu2+ and Cr3+ in environmental waters at low concentration even down to the order of nM. As CL is biosafe, the 1,4-DHAQ@CL and (1,4-DHAQ)-Cu2+@CL microporous nanofiber fluorescent films (with detection lower limits much lower than that of the drinking water health standard, i.e., 3.1 × 10−5 M for Cu2+, and 1.15 × 10−8 M for Cr3+) might have potentials for the detection of trace Cu2+ and Cr3+ in biological, medical, and environmental fields.4 The designing concept for the two fluorescence sensor materials might also offer a new approach to the building of highly sensitive and selective fluorescence sensors for other heavy metal ions.

Figure 2. SEM observations of the 1,4-DHAQ@CA nanofiber film.

maintained.20 Furthermore, the bonding energy peak corresponding to C1s(CO) at 287.63 ev in the X-ray photoelectron spectra (XPS, performed on a Thermo ESCALAB 250 instrument and shown in Figure S2) of the 1,4-DHAQ@CL nanofiber film demonstrate that 1,4-DHAQ molecules have been adsorbed onto the surface of the 1,4-DHAQ@CL nanofibers. To characterize the fluorescence responses of the 1,4DHAQ@CL and (1,4-DHAQ)-Cu2+@CL nanofiber films to Cu2+ and Cr3+ respectively, the nanofiber films were immersed in the cuvette containing varied concentrations of Cu2+ and Cr3+ in deionized-water (DI-water) solutions and the fluorescence spectra were recorded after a short incubation time (about 3 min). The details of the fluorescence measurements are given in Part S3 in the Supporting Information.

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EXPERIMENTAL SECTION Fabrication of the 1,4-DHAQ@CL Microporous Nanofiber Film. For the synthesis of the electrospun nanofiber film, spinnable polymer must be selected as the raw material of the supporting substrate. Considering the requirements of hydrophobicity for the fluorophore (1,4-DHAQ) being well dispersed, insolubility for detecting Cu2+ in aqueous solution, nonspecific absorption, and negligible fluorescence emission in the interested wavelength range of the fluorophore 1,4-DHAQ, CA was thus selected as the raw polymer for electrospinning. Moreover, the phenyl groups on the CA side chains may interact with the rigid backbone of 1,4-DHAQ through π−π stacking, thus additionally reducing the aggregation of the fluorophore molecules. For electrospinning, a syringe containing the electrospinning solution was attached to a needle connected with a high voltage, as shown schematically in Figure 1A. Details of the electrospinning conditions can be found in Part S1 in the Supporting Information. To achieve nanofibers with much higher S/M ratio, the as-electrospun CA nanofiber film was deacetylated in 50 mM NaOH at 25 °C for 24 h, rinsed, and dried. This also makes cellulose acetate (CA) transform into cellulose (CL) (Figure 1A), which has been proved by the Fourier transform infrared spectroscopy (FTIR) spectroscopy (see Part S2 in the Supporting Information). Morphological and Spectral Characterization of the 1,4-DHAQ@CL Nanofiber Film. Typical scanning electron microscopy (FE-SEM, Sirion 200) observations (Figure 2) reveal that the as-electrospun product consists of a huge amount of smooth nanofibers with uniform diameters about 300−500 nm (inset in Figure 2). This nanofiber film shows porous structure as a result of the disordered arrangement of the CA nanofibers, providing a S/M ratio about 1−2 orders of magnitude higher than that of continuous film20 and being beneficial to the sensitivity of sensor materials. The deacetylation treatment of the CA to CL nanofiber further improves its S/M ratio by generating uniformly distributed micropores throughout the nanofiber backbone, while the mechanical and morphological properties of the nanofibers are

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RESULTS AND DISCUSSION Fluorescent Responses of the 1,4-DHAQ@CL and (1,4DHAQ)-Cu2+@CL Nanofiber Films to Cu2+ and Cr3+, Respectively. The fluorescence responses of the 1,4DHAQ@CL nanofiber film to different concentrations of Cu2+ in aqueous solution are shown in Figure 3A and B. It can be observed that the fluorescence intensity (I1) of the 1,4DHAQ@CL nanofiber film decreases steadily with [Cu2+]. As depicted in the inset of Figure 3B, there exists an approximate linear relationship between I1 and [Cu2+] in the given concentration range as follows:

I1 = 1.86 − 4 × 107[Cu 2 +] ([Cu 2 +] in the range of 2.5 × 10−9 to 3.75 × 10−8 M)

(1)

This detection range is much wider and lower than those for 10−5 M 1,4-DHAQ solution and the 1,4-DHAQ@CA nanofiber film (Part S4 and Figure S3), indicating its superior sensitivity as a fluorescence probe for Cu2+. Furthermore, to characterize the selectivity of the 1,4DHAQ@CL nanofiber film to Cu2+, fluorescence responses of the 1,4-DHAQ@CL nanofiber film to various nitrates of metal ions such as Ni2+, Cd2+, Pd2+, Hg2+, Cu2+, Cr3+, Zn2+, Na+, K+, Al3+, Mg2+, Ca2+, Co2+, and Fe2+ were recorded. The histogram of the relative fluorescence intensities of the 1,4-DHAQ@CL nanofiber film immerged in different metal ions is shown in Figure 3C. It can be seen that all other metal ions have little or 369

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[Cr3+], and there also exists a linear relationship between I2 and [Cr3+] in the low concentration range (inset in Figure 3E), i.e., from 2.5 × 10−9 to 2.5 × 10−8 M, as shown in eq 2.

I2 = 2.9 × 107[Cr 3 +] + 0.44 ([Cr 3 +] in the range of 2.5 × 10−9 − 2.5 × 10−8M)

(2) 2+

Moreover, from the selectivity of the (1,4-DHAQ)-Cu @CL nanofiber film to Cr3+ (Figure 3F), it can be seen that any of the other metal ions including Cu2+ have little impact on the fluorescence intensity of the (1,4-DHAQ)-Cu2+@CL nanofiber film. Thus it can be used to selectively detect Cr3+ in practical samples. Equations 1 and 2 can be served as the basis of quantitative analysis for the determination of Cu2+ and Cr3+ in the contaminated samples, respectively. Under the current experimental conditions, lower detection limits of 3 × 10−9 M (much lower than that for 10−5 M 1,4-DHAQ solution and the 1,4-DHAQ@CA nanofiber film to Cu2+, Part S4) and 3.75 × 10−9 M have been achieved for Cu2+ and Cr3+, respectively, given that the common criterion of 10% fluorescence quenching or enhancement is used to define the sensitivity of the solid state fluorescence sensor;4 which are much lower than those of the drinking water health standard (3.1 × 10−5 M for Cu2+, and 1.15 × 10−8 M for Cr3+). More importantly, the 1,4-DHAQ@CL and the (1,4DHAQ)-Cu2+@CL nanofiber films have also been explored for the possible fluorescence detection of Cu2+ and Cr3+ in lake water solutions. As the fluorescence spectra shown in Figure 4,

Figure 3. (A and B) Fluorescence spectra and titration curves of the 1,4-DHAQ@CL nanofiber film to Cu2+. (C) Relative fluorescence intensity histogram of the 1,4-DHAQ@CL nanofiber film immersed in 3 × 10−7 M different metal ions. (D and E) Fluorescence spectra and titration curves of the (1,4-DHAQ)-Cu2+@CL nanofiber film to Cr3+. (F) Relative fluorescence intensity histogram of the (1,4-DHAQ)Cu2+@CL nanofiber film immersed in 3 × 10−7 M different metal ions. (λex = 470 nm).

no impact on the fluorescence intensity of the 1,4-DHAQ fluorophore embedded in the CL nanofibrous film, thus the 1,4-DHAQ@CL nanofiber film shows very good selectivity to Cu2+. Moreover, even when the concentrations of the interference ions were 100-higher than that of Cu2+ in solution, there still exists good selectivity for the 1,4-DHAQ@CL nanofiber film to Cu2+ (fluorescence spectra shown in Figure S4), further proving the ultrahigh selectivity of the 1,4-DHAQ@CL nanofiber film to Cu2+. To evaluate the practicability of the nanofiber sensor film, coexistent systems of several metal ions with Cu2+ were also tested. Upon excessive addition of any other of the above-mentioned metal ions except Cr3+, there is little change in the fluorescence intensity of the nanofiber film with Cu2+ alone. Therefore, the 1,4-DHAQ@CL nanofiber film can realize practical, highly sensitive, and selective detection of Cu2+. However, in the presence of Cr3+, it was much different as follows: upon extra addition of Cr3+ to the Cu2+contaminated 1,4-DHAQ@CL nanofiber film with a low fluorescence emission, the fluorescence of 1,4-DHAQ@CL was recovered. Therefore, we propose that the 1,4-DHAQ and Cu2+ system as a whole may be used to detect Cr3+ by showing enhanced fluorescence intensity. Thus another electrospun and deacetylated CL nanofiber film codoped with 1:2 (1,4DHAQ)/Cu2+ (denoted as (1,4-DHAQ)-Cu2+@CL) was fabricated (detailed fabrication process is shown in Part S5 in the Supporting Information) and used to detect Cr3+. As shown in Figure 3D, the fluorescence intensity (I2) of the (1,4DHAQ)-Cu2+@CL nanofiber film steadily increases with

Figure 4. Fluorescence spectra of (A) 1,4-DHAQ@CL and (B) (1,4DHAQ)-Cu2+@CL nanofiber films immersed in Cu2+ and Cr3+ polluted lake waters, respectively.

even for nM Cu2+ and Cr3+, the fluorescence changes can still be distinguished, ensuring practicability for fluorescence detections of environmental heavy metals. Additionally, as the deacetylated CL is biosafe, the 1,4-DHAQ@CL and (1,4DHAQ)-Cu2+@CL nanofiber films might be even applicable in food and medical areas. 370

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Sensing Mechanism of the Two Kinds of Fluorescence Sensor Films for Cu2+ and Cr3+. The sensing mechanism of the 1,4-DHAQ@CL nanofiber film for Cu2+ is based on the coordination of Cu2+ with 1,4-DHAQ to form quinizarin complex (as shown by the chemical equation with red color in the lower-right corner of Figure 1A). To prove this further, we did the following experiments. Upon addition of the light blue Cu(NO3)2 in dimethylformamide (DMF) solution to the yellow 1,4-DHAQ/DMF solution, a purple solution appears (Figure 5A), indicating that the chemical reaction of Cu2+ with

fluorescence quantum efficiency and quenching its emission. However, with the successive addition of Cr3+, the abovementioned two peaks of phenolate characteristic absorption decrease again together with recovered fluorescence emission (as fluorescence photos show in Figure S5), and a new peak around 424 nm (corresponding to Cr3+ UV−vis absorption) appears and further increases with [Cr3+] (Figure 5C). All the above results reveal that the extra addition of Cr3+ makes the complexation between Cu2+ and 1,4-DHAQ reverse. To clarify it further, UV−vis absorption spectra of 1,4-DHAQ solution with addition of Cr3+ (Figure S6), and XPS spectra of 1,4DHAQ and its complex with Cr3+ (Cu2+) were recorded (Figure S7). Although no obvious changes were observed in the UV−vis spectra, changes of the XPS spectra clearly indicate the excited complex formation between Cr3+ and 1,4-DHAQ (Part S6 in the Supporting Information). The possible different bonding models of 1,4-DHAQ with Cu2+ and Cr3+ are suggested in Figure S8. Furthermore, the bonding constant between 1,4-DHAQ and Cr3+ was evaluated (Part S7), i.e., K = 3.67 × 105 M−1. And the high K value ensures the high sensitivity of the (1,4-DHAQ)-Cu2+@CL as a fluorescence sensor for Cr3+. This process is also shown schematically in the purple equation in the lower part of Figure 1B. Thus after the excess addition of Cr3+, the emission of 1,4-DHAQ recovered and its fluorescence was enhanced. Superiority of the 1,4-DHAQ@CL Microporous Nanofiber Film to the 1,4-DHAQ@CA Nanofiber Film and Even to Bare 1,4-DHAQ in Solution. To further evaluate the coordination ability of Cu2+ with 1,4-DHAQ, the Stern− Volmer constants Ksv for 10−5 M 1,4-DHAQ/ethanol solution, the as-electrospun 1,4-DHAQ@CA and the deacetylated 1,4DHAQ@CL nanofiber films were obtained from the slopes of their Stern−Volmer plots (Figure 6) to different concentrations of Cu2+ ((I0/I-1)∼[Cu2+]), i.e., Ksv = 2.2 × 105, 8.1 × 106, and 4.2 × 107 M−1 for the 10−5 M 1,4-DHAQ/ethanol solution, the 1,4-DHAQ@CA and 1,4-DHAQ@CL nanofiber films, respectively. Higher Ksv denotes higher sensitivity of a fluorescence sensor, thus the electrospun and deacetylated 1,4DHAQ@CL nanofiber film exhibits ultrahigh sensitivity and superior sensing performance. The significant enhancement of Ksv is attributed to the ultrahigh S/M ratio of the microporous 1,4-DHAQ@CL nanofiber film after 1,4-DHAQ@CA is deacetylated. Reversibility and Reusability of 1,4-DHAQ@CL and (1,4-DHAQ)-Cu2+@CL Microporous Nanofiber Films. The final products of 1,4-DHAQ@CL and the (1,4-DHAQ)-Cu2+@ CL nanofiber sensor films are reusable. From Figure 7A it can be seen that the 1,4-DHAQ@CL nanofiber film has a very low fluorescence emission after being used to detect Cu2+, however, when it was treated with high concentrations of chromium nitrate solution and repeatedly rinsed with DI-water, its fluorescence intensity could be recovered as high as 94%. Similarly, for the (1,4-DHAQ)-Cu2+@CL nanofiber film, after being used for the detection of Cr3+ and a subsequent treatment with high concentrations of copper nitrate solution and repeated washimg with DI-water, it was renewed and reusable, and the experimental errors were within 5% (Figure 7B). Thus once the fluorescence sensor materials were properly calibrated, they could be repeatedly used for further detection of Cu2+ and Cr3+ in real time. As compared to liquid phase fluorescence sensor,12,13,16−19 the 1,4-DHAQ@CL and (1,4-DHAQ)-Cu2+@CL nanofiber films exhibit the advantages of portability, versatility, simplified

Figure 5. (A) Color variations of the 10−2 M (1,4-DHAQ)/DMF solution, 10−2 M Cu(NO3)3/DMF solution, and their complex (shown in cuvettes from left to right and marked with “1, 2, 3” respectively). (B) Color variations of the 10−4 M 1,4-DHAQ solution, with titration of 1 equiv. Cu(NO3)2 and successive titration of 1 and 2 equiv. Cr(NO3)3, respectively (from left to right). (C) UV−vis absorption spectra of the 10−4 M (1,4-DHAQ))/DMF solution (black dotted line); with titration of 10−4 (red solid line), 2 × 10−4 (green solid line), 7 × 10−4 (blue solid line) M Cu2+; and successive titration of 1−3, 8, 13, 18, 28 × 10−4 M Cr3+, respectively (short dashed lines, [Cr3+] increases as the arrowhead shows). Inset: UV−vis absorption spectra of the 2.5 × 10−3 M Cr(NO3)3/DMF solution.

1,4-DHAQ has taken place. However, for successive addition of Cr(NO3)3, the color comes back slowly, as can be seen from Figure 5B, indicating that the addition of Cr3+ reversed the reaction between 1,4-DHAQ and Cu2+. The UV−vis absorption spectra of the 1,4-DHAQ/DMF solution with successive addition of Cu2+ and Cr3+ are shown in Figure 5C, and it can be seen that, upon addition of Cu2+, the characteristic absorption band of 1,4-DHAQ centered at 480 nm decreases, while two new characteristic absorption peaks of phenolate at 534 and 574 nm appear21 and further increase with [Cu2+], confirming the formation of phenolate. After the formation of complex between 1,4-DHAQ and Cu2+, photoinduced electrontransfer or energy transfer occurs from the excited hydroxyanthraquinon moiety to Cu2+, thus reducing 1,4-DHAQ 371

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(down to nM) and selectivity superior to previously reported fluorescence probes for Cu2+ and Cr3+.12,13,16−19 Lastly, both the 1,4-DHAQ@CL and the (1,4-DHAQ)-Cu2+@CL nanofiber films are biosafe and reusable, which ensures their wide application and further reduces expense. In summary, two prototypes of highly sensitive and selective solid state biosafe and biocompatible fluorescence sensor materials for Cu2+ and Cr3+, based on 1,4-DHAQ-doped nanofiber films, have been achieved via electrospinning and subsequent deacetylating, and used for the detection of Cu2+ and Cr3+ in the nM range with high selectivity over thirteen other common metal ions. The 1,4-DHAQ@CL nanofiber film exhibits a linear dynamic detection range of 2.5 × 10−9 to 3.75 × 10−8 M for Cu2+, while the (1,4-DHAQ)-Cu2+@CL nanofiber film shows a linear detection of Cr3+ in the low concentration ranging from 2.5 × 10−9 to 2.5 × 10−8 M in aqueous solution. Both the 1,4-DHAQ@CL and (1,4-DHAQ)Cu2+@CL nanofiber films have been tested to detect Cu2+ and Cr3+ in polluted lake waters, demonstrating their potential application as practical environmental probes. The sensing mechanism for Cu2+ is ascribed to the formation of phenolate between Cu2+ and 1,4-DHAQ, and that for Cr3+ is attributed to the reverse of the reaction between Cu2+ and 1,4-DHAQ by forming Cr3+-based excited complex. Moreover, both the 1,4DHAQ@CL and (1,4-DHAQ)-Cu2+@CL nanofiber films are reusable after additional simple treatment. To the best of our knowledge, these are the first developed solid-state fluorescence sensor materials consisting of nanofiber films that can realize practical and real-time detection of Cu2+ and Cr3+ in aqueous solutions. The design concept used in this work might open a door to the development of reusable, highly sensitive, and selective probes for other heavy metals.

Figure 6. Stern−Volmer plots of (A) 10−5 M 1,4-DHAQ solution, (B) 1,4-DHAQ@CA, and (C) 1,4-DHAQ@CL nanofiber films for different concentrations of Cu2+.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed fabrications of the 1,4-DHAQ@CA and (1,4-DHAQ)Cu2+@CA nanofiber films; IR absorption spectra of the CA nanofiber film before and after deacetylation; XPS spectra of the 1,4-DHAQ@CL nanofiber film; fluorescence responses of the 1,4-DHAQ in solution and the 1,4-DHAQ@CA nanofiber film to Cu2+ and Cr3+, respectively; the interaction mechanism between 1,4-DHAQ and Cr3+, and the evaluation of their bonding constant; fluorescence spectra measurement conditions; and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (G.M.), [email protected] (Q.H.). ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (Grant 2007CB936601), the National Natural Science Foundation of China (50972145, 50525207 and 10975152), and the Key Innovative Project of CAS (Grant KJCX2YWN341).

Figure 7. Reusability of (A) 1,4-DHAQ@CL and (B) (1,4-DHAQ)Cu2+@CL nanofiber films after simple treatments. The marked arabic numerals of 1, 2, 3, and 4 refer to the treatment status of the asprepared, the Cu2+/(Cr3+)-contaminated, the renewed, and the recontaminated 1,4-DHAQ-doped CL microporous nanofiber films, respectively.

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REFERENCES

(1) Yang, J. S.; Swager, T. M. Fluorescent porous polymer films as TNT chemosensors: Electronic and structural effects. J. Am. Chem. Soc. 1998, 120, 11864−11873. (2) Cui, X.; Hetke, J. F.; Wiler, J. A.; Anderson, D. J.; Martin, D. C. Electrochemical deposition and characterization of conducting

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optical setup, and larger dynamic range, with trivial expense for online detection.22 Furthermore, the 1,4-DHAQ@CL and the (1,4-DHAQ)-Cu2+@CL nanofiber films show sensitivity 372

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dx.doi.org/10.1021/es202137c | Environ. Sci. Technol. 2012, 46, 367−373