Selective Fluorescence Turn-On and Ratiometric Detection of

Article pubs.acs.org/ac

Selective Fluorescence Turn-On and Ratiometric Detection of Organophosphate Using Dual-Emitting Mn-Doped ZnS Nanocrystal Probe Kui Zhang,†,∥ Tao Yu,‡,∥ Fei Liu,§ Mingtai Sun,† Huan Yu,† Bianhua Liu,† Zhongping Zhang,† Hui Jiang,*,‡ and Suhua Wang*,† †

Institute of Intelligent Machines, Chinese Academy of Sciences, 350, Shushanhu Road, Hefei, Anhui 230031, China Beijing Institute of Pharmaceutical Chemistry, State Key Laboratory of NBC Protection for Civilian, Beijing 102205, China § Fujian Inspection and Research Institute for Product Quality, Fuzhou, Fujian 350002, China ‡

S Supporting Information *

ABSTRACT: Semiconductor nanocrystals (NCs) possess unique photoluminescent properties which can be used to design fluorescence probes for chemo/biosensing applications. Several have recently emerged that offer excellent turn-on or ratiometric fluorescence chemosensory protocols by sophisticated procedures, but it has been challenging to realize all of these advantages in a single construct. Herein, we develop an intrinsic dual-emitting Mn-doped ZnS nanocrystal-based probe that achieves this goal with turn-on and ratiometric fluorescence response for the determination of organophosphate (diethylphosphorothioate, DEP). The probe relies on the modification of dopamine dithiocarbamate on the surface of NCs and the modulation of dual emission through a photoinduced electron transfer process, which makes use of red fluorescence of Mn2+ ions doped in the NCs as specific recognition for the target analyte and blue defect emission of the NCs as stable internal reference. In presence of DEP, the red emission of the probe is thus enhanced by switching off the electron transfer pathway, while the blue emission is almost unchanged. With the addition of different amounts DEP, the two emission intensity ratios gradually vary and display color changes from dark-blue to purple to red. Thus, this method generates turn-on and ratiometric fluorescence signals for quantitative and visual detection of the analyte. Significantly, the dual-emitting probe has been used to fabricate paper-based test strips for visual detection of DEP residues, which validate the method for its rapid, on-site, and visual identification.

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to a remarkable enhancement of luminescence intensity. This principle has been successfully demonstrated in our previous work.18 The emission of NCs is first quenched by the association with transition metal complexes at the surface of NC through FRET process, and subsequently the fluorescence is turned on through substituting the ligands by stimulation of NO. However, developing such NC-based turn-on sensors is also a challenging work because the generation of FRET or PET usually requires strict conditions and complex modification procedures on NC surface. On the other hand, most NCbased sensors employ either turn-off or turn-on fluorescence intensity as a sole responsive signal, but such a signal is readily perturbed by various experimental factors, such as fluctuation in probe concentration and instrumental efficiency. Ratiometric measurements can eliminate these environmental effects and give more precise measurement because of their self-referencing

emiconductor nanocrystals (NCs) are promising optical labels for chemo/biosensing and imaging since NCs offer distinct advantages over organic dyes such as high photoluminescence efficiency, narrow and symmetric emission with tunable colors, broad absorption, and good photostability.1−7 In the past two decades, NCs-based chemosensors have become one of the most significant fields in analytical chemistry.4−8 Most of these chemosensory protocols are focused on fluorescence turn-off, in which the fluorescence signal is diminished in the presence of the target analyte.9−13 Turn-on fluorescence protocol is less common than turn-off, but it is highly attractive because sensitivity is significantly greater by the enhancement of a fluorescence signal on a dark background.14−18 For NCs-based turn-on sensors, the work principles generally rely on the connection of a receptor unit to the surface of NCs. The receptor is engineered to quench the emission of NCs on the basis of fluorescence resonance energy transfer (FRET) or photoinduced electron transfer (PET) mechanism. In presence of the analyte associated with the receptor, the quenching mechanism is suppressed, leading © XXXX American Chemical Society

Received: August 21, 2014 Accepted: October 30, 2014

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Scheme 1. Illustration for Synthesis of the Dual-Emitting Probe and the Mechanism for Fluorescence Turn-On and Ratiometric Detection of DEPa

a The bottom panel shows the fluorescent spectral changes of the dual-emitting probe upon exposure to DEP and the corresponding fluorescence photographs of the probe solution taken under UV illumination.

capability by calculation of two emission intensity ratio.19−23 In order to build ratiometric sensors, however, two independent fluorophores are usually preconjugated or preassembled together by sophisticated procedures. Herein, a manganesedoped ZnS nanocrystal probe showing intrinsic dual-emission has been developed for fluorescence turn-on and ratiometric detection of the hydrolysis product of organophosphate, which makes use of red fluorescence of Mn2+ ions doped in the NCs as specific recognition for the target analyte and blue surface defect related emission of the NCs as reference signal. This probe first generates turn-on fluorescence and subsequently displays ratiometric fluorescence when exposed to the analyte DEP. Organophosphates (OPs) are highly toxic, and irreversible inhibition of acetylcholine esterases raises serious human health and environmental concerns.24,25 Among them, a few have been used as chemical warfare agents banned by the Chemical Weapons Conventions, most widely used as chemical pesticides.25,48 As estimated, millions of tons of organophosphate pesticides are used for the control of pests in agriculture across the world, and more than 200 000 people in the rural areas of developing countries die from organophosphate pesticides poisoning every year.26,27,49 Accordingly, owing to their widespread use, the development of reliable assay for the determination of OPs is highly desirable, because the necessity of monitoring of their residues is evident. A wide variety of analytical methods have been developed for detecting, measuring, and/or monitoring OPs and their metabolites. Conventional methods such as liquid chromatography, gas chromatography, and tandem mass spectrometer have been commonly used.28−30 However, these methods are usually time-consuming with employment of cumbersome and expensive equipment, and require labor-intensive preliminary treatment of samples and skilled personnel. In addition, several enzymatic and immunoassays methods have been investigated for the determination OPs by the readout of various optical or electrochemical approaches.31−33 Although a high selectivity and sensitivity can be achieved, the poor chemical/physical

stability of the enzymes or antibodies limits their widespread use. Therefore, it still has a huge demand to develop a simple, rapid, inexpensive but on-site detection method for OPs or their metabolites in environments. In this study, we have constructed a new type of dualemitting NC probe consisting of manganese-doped ZnS nanocrystals showing intrinsic dual-emission for the determination of the hydrolysis product of organophosphate (diethylphosphorothioate, DEP) via turn-on and ratiometric fluorescence. The strategy for designing the assembled probe is based on the formation of PET process between NCs and dopamine dithiocarbamate (DDTC: carbon disulfide formed a dithiocarbamate with amine group of dopamine) without the aids of costly antibody/enzyme. Scheme 1 provides a schematic depiction of the probe synthesis process and the working principle for fluorescence turn-on and ratiometric detection of DEP. The intrinsic dual-emitting fluorescence manganesedoped ZnS nanocrystals (ZnS:Mn NCs) were first prepared by chemical precipitation method according to our previous work.10 Dopamine was then conjugated to the surface of NCs via a simple covalent bonding. The detailed procedures are given in the Experimental Section. ZnS:Mn NCs show unique and well-resolved dual emission bands at 600 nm of red emission and 435 nm of blue emission under a single wavelength excitation, respectively. After functionalization with dopamine, the fluorescence of red emission is selectively quenched due to PET process from the excited NC to oxidized dopamine-quinone of DDTC. Upon the addition of DEP, DEP replaces DDTC at the surface of NCs due to its strongly coordinative interaction. The red emission of NCs is thus enhanced by switching off the PET pathway, while the blue emission is almost unchanged as stable internal reference. With the addition of different amounts of DEP, the dual emission intensity ratios (I600/I435) of the probe gradually increase, resulting in continuous color changes of the probe solution from dark-blue to red, which can be clearly observed by the naked eye. These fluorescence intensity ratios and the B

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dispersed into ultrapure water (10 mL). The mixture was shaken for several minutes to get a homogeneous solution. A strip of filter paper (26 mm × 22 mm) was submerged into the above mixture under the condition of ultrasonic agitation for 2 min. The strip was then removed from solution and kept still for several minutes in the dark. Thus, a fluorescence test strip was prepared as the filter paper dried, and it emitted strong dark-blue fluorescence under a portable UV lamp (8 W, λmax = 312 nm). To apply the test strip for visual detection of DEP residues in local lake water, 2 μL of lake water with different amounts of DEP spiked was first carefully dropped onto the test strip. It was then moved to under a 312 nm UV lamp, and distinct fluorescence color changes were observed. Instrumentations. All fluorescence measurements were carried out on a PerkinElmer LS-55 spectrofluorimeter equipped with a xenon lamp source and a 1.0 cm quartz cell. Infrared spectra were recorded with a Nicolet Nexus-670 Fourier transform infrared (FT-IR) spectrometer. The obtained product was examined by X-ray powder diffraction (XRD) using an X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The morphology of the dual-emission probe was investigated with use of a JEOL 2010 transmission electron microscope (TEM). Fluorescence photographs were taken with a Canon-350D digital camera.

subsequent color changes can be employed for quantitative analysis and visual recognition, respectively.

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EXPERIMENTAL SECTION Chemicals and Materials. Diethyl chlorothiophosphate (DTP), chlorpyrifos (CPF), ethoprophos (EPP), profenofos (PFF), paraoxon-ethyl (POE), trichlorphon (TCP), and dopamine hydrochloride (DA) were purchased from Sigma. Diethylphosphorothioate (DEP) was prepared by hydrolyzing of DTP as previously described.27 Analytical-grade sodium sulfide (Na2S·9H2O), zinc sulfate heptahydrate (ZnSO4·7H2O), manganese acetate tetrahydrate [Mn(CH3COO)2·4H2O], carbon disulfide (CS2), triethylamine (NEt3), and ethanol were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of these reagents were used without further purification. Ultrapure water (18.2 MΩ cm) was obtained from a Millipore water purification system and used in all experimental steps unless specifically noted in the text. Synthesis of Dual-Emitting Zinc Sulfide Nanocrystals with Manganese Dopant. The dual-emitting Mn-doped zinc sulfide nanocrystals (ZnS:Mn NCs) were synthesized in aqueous phase using a chemical precipitation method according to the published protocol with minor modifications.10 Equivalent moles of sodium sulfide and zinc sulfate were used for the synthesis. In a typical synthesis procedure, 50 mM of ZnSO4·7H2O and 4 mM of Mn(CH3COO)2·4H2O were first dissolved in 80 mL of ultrapure water in a flask fitted with a reflux condenser. After the mixture solution was stirred and refluxed for several minutes, 20 mL of sodium sulfide aqueous solution (50 mM) was rapidly poured into the flask, followed by vigorously stirring and continually refluxing for further 3 h. The resultant product were separated from the mixture with centrifugation and washed with ultrapure water several times. A stock solution of such purified ZnS:Mn NCs was prepared by dissolving in ultrapure water for further use. Preparation of Dual-Emitting Probe. The dual-emitting fluorescence of the purified ZnS:Mn NCs was first modulated by surface covalent coupling of a special bidentate ligands, DDTC, according to the following procedure. Briefly, a mixture of DA (52.1 mg), ethanol (1 mL), CS2 (30 μL), and NEt3 (30 μL) was sonicated and reacted for 10 min. Then, 80 mg of ZnS:Mn NCs in 2 mL of ultrapure water was added to the above mixture, and subsequently stirred for 72 h at room temperature. The target product was purified by washing with ethanol and ultrapure water to remove the excess of CS2, DA, and unbound DDTC. The resultant probe was redispersed in 5 mL of ultrapure water. Measurements of Fluorescence Response to Organophosphate Compounds. The stock solutions of different organophosphate compounds were prepared in ethanol and stored in a refrigerator at 4 °C to avoid long exposure to light. A 5 μL portion of the dual-emitting probe stock solution was diluted to 3 mL with ultrapure water in a quartz cuvette. Thus, a final concentration of 27 μg/mL of the probe was obtained for the fluorescence measurements. A 6 μL portion of the organophosphate ethanol solution with known concentration was carefully added into the probe solution in the cuvette. The solution was then gently stirred, followed by recording the fluorescence spectra with excitation at 320 nm. The scan speed was 600 nm/min, and the band-slits of excitation and emission were set as 10 and 15 nm, respectively. Preparation of Test Strips for Visual Detection of DEP. To fabricate the test strips, 200 μL of the probe solution was

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RESULTS AND DISCUSSION Figure 1A shows the TEM image of the prepared ZnS:Mn NCs probe, and the size is estimated to be 3−6 nm. The NCs probe

Figure 1. (A) TEM image of the prepared ZnS:Mn NCs probe. (B) XRD pattern of the prepared ZnS:Mn NCs probe. (C) FTIR spectra of (a) pure dopamine and (b) the prepared ZnS:Mn NCs probe.

exhibit a zinc-blende crystal structure as revealed by the distinguishable (111), (220), and (311) planes in X-ray diffraction (XRD) pattern (Figure 1B). The NC size was calculated to be 4.8 nm using the Debye−Scherrer equation based on the XRD data, which is consistent with TEM results. Figure 1C shows the FT-IR spectra of pure dopamine and the prepared ZnS:Mn NCs probe. Figure 1Ca shows that pure dopamine exhibits two characteristic peaks at 935 and 1115 cm−1 which can be assigned to OH and CH2NH2 functional C

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groups. The accompanying peaks at 1261 and 1320 cm−1 can be assigned to the COH symmetric and asymmetric bending. The peaks at 1499 and 3371 cm−1 are attributed to the aromatic CC symmetric stretching and OH asymmetric stretching, respectively. Figure 1C(b) represents the FTIR spectrum of the prepared ZnS:Mn NCs probe. The characteristic frequencies of dopamine at 935 and 1115 cm−1 can be clearly seen. Moreover, the peaks at 3446 cm−1, 1630 and 1385 cm−1 confirm the presence of oxidized aromatic conjugated ketones (CCCO) in the probe. These results evidenced the successful attachment of dopamine molecules on the surfaces of ZnS:Mn NCs. The as-prepared ZnS:Mn NCs have two intrinsic peaks of the photoluminescence (PL) spectra in the visible range under UV illumination. The two emssion intensity of ZnS:Mn NCs can be modulated by controlling refluxing time, as shown in Supporting Information Figure S1. The symmetric peak at 600 nm of red emission is ascribed to the characteristic transition between 4T1 and 6A1 energy levels of Mn2+ 3d states in the ZnS matrix, and the other broad peak around 435 nm (blue emission) can be attributed to surface defect related emission (schematic representation of the emission levels of ZnS:Mn NCs is briefly illustrated in Figure 2B).10,34,47 Figure 2A indicates the PL spectra of bare ZnS:Mn NCs and the dualemitting probe when excited at 320 nm. It can be noticed that functionalized ZnS:Mn NCs with DDTC have drastic quenching on the fluorescence of red emission at 600 nm, while the blue emission at 435 nm stays almost unchanged and even has a little enhancement. Meanwhile, the relative intensity

of two emissions (I600/I435) was changed from 11.8 to 0.17, and the corresponding bright red color of NCs solution changed to dark blue (inset photographs of Figure 2A). The fluorescence quenching of red emission is indicative of PET process from NCs to oxidized dopamine-quinone of DDTC functioning as an electron acceptor. This photophysics of dopamine-modified nanocrystals has been reported and documented in published articles, and is a property that has been exploited for sensing applications.35−38 Figure 2B schematically illustrates the fluorescence quenching mechanism due to electron transfer from nanocrystal to the surface DDTC ligands. The selective quenching of Mn2+ emission at 600 nm might be due to the fact that the lowest unoccupied molecular orbital (LUMO) energy level of oxidized dopamine-quinone of DDTC is more matched with 4T1 energy level of Mn2+ 3d states in the ZnS matrix than that of the ZnS conduction band. Therefore, the fluorescence quenching can be attributed to the transfer of the NC conduction band electron to the LUMO energy level of DDTC acceptor after photoexcitation, and subsequently shuttling back to the valence band of the NC or other nonradiative relaxation of the DDTC acceptor. According to Scheme 1, the dopamine molecules attach to the surface of the NCs via the surface complexing of the DDTC moiety, and subsequently modulate the NCs’ dual emission through a PET process, as demonstrated in the above text. After the construction and establishment of probe, it is very important to assess whether or not DEP can replace DDTC at the surface of NCs and close the PET pathway. Fortunately, DEP has been documented exhibiting a very strong coordinative ability with many transition metal ions, and thus has been widely used as flotation reagents for mining noble metals.27,39−41 Accordingly, the working principle for fluorescence turn-on and ratiometric detection can be confirmed through monitoring the fluorescence responses of the two emission bands of the dual-emitting probe to DEP. Figure 3

Figure 3. Fluorescence emission spectra of the dual-emitting probe solution upon the exposure to different concentrations of DEP. The inset photographs show the corresponding fluorescence color changes under UV illumination. The concentrations of DEP from bottom to top are 0, 20, 40, 60, 80, 100 μM, respectively.

clearly shows the PL spectra of the probe exposed to aqueous solution containing different amounts of DEP performed at excitation wavelength 320 nm and emission wavelength range 350−750 nm. Obviously, the emission intensity at 600 nm was turned on gradually due to switching off the PET process from NC to DDTC. The results show that the increase of DEP to the probe solution indeed induces a remarkable fluorescence

Figure 2. (A) Fluorescence emission spectra of bare ZnS:Mn NCs (dash dot line) and the dual-emitting probe (solid line). The inset shows the optical photographs of bare ZnS:Mn NCs and the dualemitting probe solution under UV illumination, respectively. (B) Schematic representation of the emission levels of NC and the electron transfer from NC to DDTC ligands. D

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enhancement at 600 nm whereas the emission intensity at 435 nm almost remains unchanged. The emission intensity ratio, I600/I435, was gradually increased with increasing the amounts of DEP. When DEP was added up to 100 μM, about 4.8-fold ratiometric fluorescence enhancement was observed. This turn on and ratiometric fluorescence enhancement can be easily visualized with the probe solution color changes from dark blue to purple to bright red under a 312 nm UV lamp (Figure 3 inset image), which provides a nice chance for the naked eye detection without the need for elaborate equipment except for a small UV lamp used as an excitation light source. It is well-known that a ratiometric fluorescence probe possesses excellent ability of quantitative analysis because of its self-calculation of dual emission intensity ratios independent of the probe concentration.23,42−44 So the plot of the emission intensity ratio (I600/I435) to DEP concentration yielded a calibration curve that gave a linear relationship with a correlation coefficient of 0.998 (Figure 4). The ratio of the Figure 5. (A) Fluorescence emission intensity ratio (I600/I435) of the probe solution upon the addition of 100 μM of various organophosphates, respectively. (B) The corresponding luminescence color changes under UV illumination.

that the dual-emitting probe has high selectivity for visual detection of DEP over other similar organophosphate category compounds, while the selectivity among these tiny different organophosphate compounds has not been reported by using biological recognition systems (see Table S1 in the Supporting Information). Although a number of publications based on chromatography/mass spectrometry for analysis and detecting DEP have been reported, there are just a few examples using visual fluorescence methods for the detection of DEP. Fenske and et al. developed a semiquantitative method to visualize dermal exposure of organophosphate metabolite with ultraviolet light.50,51 The method has a LOD (200 mg/L) and has been occasionally used in developing countries. We previously reported a fluorescence turn-on method which achieves a lower LOD down to nanomolarity.27 The sensitivity of the method is dependent on the various factors such as probe concentration, instrumental efficiency, and other environmental conditions. Therefore, this method represents the first generation of selective turn-on and ratiometric fluorescence sensing approach, which could be used for rapid and visual detection of DEP (see Table S1 in the Supporting Information for a comparison with existing DEP detection approaches). To assess the utility of the dual-emitting probe in practical applications, the detection of DEP in real water samples such as tap water and lake water (local Shushan Lake) has been carefully demonstrated by recovery tests. The real water samples collected from a local Shushan Lake were first filtered through 0.45 μm Supor filters to remove any particulate suspension, whereas the tap water was used directly for the recovery test. The water samples were then prepared by spiking with known concentrations of DEP. Before the test, the effect of water on the ratiometric fluorescence of the probe was first examined, and no such effect was found. With the addition of DEP in the lake water, the fluorescence emission ratios of the dual-emitting probe were enhanced, and the working curve showed a good linearity in a concentration range from 0 to 100 μM (Supporting Information Figure S4). The recovery tests on

Figure 4. Fluorescence emission intensity ratio (I600/I435) of the dualemitting probe as a function of DEP concentration.

emissions at the two wavelengths increased from 0.17 to 0.82. The responsive concentration of DEP ranges up to 100 μM, and a limit of detection (LOD) of 1.8 μM (3/slope) can be obtained. The LOD is much lower than the median reference level of 7.1 μM in the U.S. population reported by the Centers for Disease Control and Prevention, implying its potential for practical usage. The sensitivity of the method does not meet a few extreme requirements, but it shows potential as a low-cost and rapid sensor for DEP monitoring and screening in scenarios like pesticide contamination in surface water, farming environment, etc. Many other existing methods cannot meet the requirement for rapid and visual screening. The selectivity of the dual-emitting probe was carefully validated by recording the fluorescence emission ratios (I600/I435) of the probe in the presence of DEP and other organophosphate category pesticides, including chlorpyrifos (CPF), ethoprophos (EPP), profenofos (PFF), paraoxon-ethyl (POE), and trichlorphon (TCP), as shown in Figure 5A. These organophosphates almost did not turn on the fluorescence emission at 600 nm of the probe except a little enhancement by EPP. On the contrary, about 4.8-fold ratiometric fluorescence enhancement was obtained at the same concentration of DEP. Clearly, no fluorescence color changes happened after adding other pesticides in the probe solution, while an obviously bright red fluorescence color was observed in the presence of DEP on the original background (Figure 5B). Therefore, the results indicate E

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natural light, and almost nothing could be observed. The red fluorescence intensity gradually enhances on the test strip along with the increase of DEP amounts under UV illumination (Figure 6B). Even at the DEP concentration of 150 μM, an obvious fluorescence color change could be clearly observed. Such change of fluorescence color on the paper-based test strip can indicate roughly the range of DEP amounts, although the accurate quantitative detection using a digital camera has not yet been made in the present work. In addition, because organophosphate compounds are recognized as environmental contaminants, it is preferable for the test strips to be destroyed to avoid environmental contamination after detecting. Therefore, such paper-based test strips are more eco-friendly because they can be completely burned (Figure 6D).

the lake water with 0, 20, and 40 mM of DEP spiked were carried out. The averaged recovery was obtained with standard deviation based on three triplicate measurements for each concentration. Table 1 shows the analysis results for the Table 1. Detection of DEP in Spiked Tap Water and Real Lake Water by the Dual-Emitting Probe tap water add DEP conc (μM)

found (μM)

0 20 40

0.179 19.9 38.7

lake water

recovery (%)

found (μM)

recovery (%)

99.5 ± 4.9 96.8 ± 4.5

1.76 19.9 41.8

99.7 ± 2.4 104.4 ± 7.1

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CONCLUSIONS In summary, we have demonstrated a novel type of dualemitting probe by using intrinsic dual-emission manganesedoped ZnS NCs for the detection of DEP via turn-on and ratiometric fluorescence. The probe relies on the modification of dopamine dithiocarbamate quenchers on the surface of NCs, and the red fluorescence of the dual-emitting probe is selectively quenched due to the PET process. In the presence of DEP, the red emission of the probe is thus enhanced by switching off the PET pathway because DEP can replace quenchers at the surface of NCs, while the blue emission is almost unchanged as internal reference signal. Moreover, the paper-based test strips based on dual-emitting probe have been successfully achieved for the detection of DEP residues, which validates the application potential of rapid, on-site, visual identification. In fact, the present contribution will bring new thought and method for the construction of chemosensors, and also could improve the probe design for visual sensing and promote the practical application.

samples with and without DEP spiked. Because the content of DEP in the blank samples is below the limit of detection of the method, these results are not used in the recovery calculation. Clearly, the recoveries of DEP are statistically close to the spiked values, implying that there are no serious positive or negative interferences in real water samples for this method. Therefore, all the data presented in Table 1 are obtained for unpretreated complex sample matrices, and the results indicate that the method is statistically satisfied for DEP detection and possess the potential in practical applications. More interestingly, we prepared portable test strips by transferring the probe on common filter paper for rapid and visual detection of DEP. As a matter of fact, paper-based test strips have been of significant interest as an analytical platform because of their low cost, availability, ease of storage and transport, and ease of disposal.23,45,46 Here, a strip of filter paper was first immersed in the well-homogenized probe solution to efficiently adsorb the probes utilizing the hydrophilic interactions. The paper strip was then removed from the probe solution and kept in the dark for future use. Such asprepared test strip shows dark blue fluorescence under UV illumination. As can be seen in Figure 6A, a bright red spot appeared immediately against the clear dark-blue background after 2 μL of DEP-spiked lake water (200 μM) was dropped on the test strip. Figure 6C shows the identical test strip under

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ASSOCIATED CONTENT

S Supporting Information *

Fluorescence spectra of dual-emitting nanocrystals. Details regarding storage stability, probe’s time dependent responses, the working curve of lake water samples, detection reproducibility, and comparison of different detection approaches. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ∥

These two authors (K.Z. and T.Y.) contributed equally to this work. Notes

The authors declare no competing financial interest.

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Figure 6. Paper-based visual detection of DEP-spiked lake water at the concentrations of (A) 200 μM, and (B) 50, 100, 150, and 300 μM. The fluorescence images were taken under a 312 nm UV lamp. (C) The corresponding strip images are taken under natural light. (D) Being burned after detecting to avoid the spreading of organophosphate contaminants.

ACKNOWLEDGMENTS

This work was supported by the National Basic Research Program of China (2011CB933700), the National Natural Science Foundation of China (Nos. 21205120, 21228702, and 21302187), and the Technology Planning Project of Fujian Province (2013Y0031). F

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dx.doi.org/10.1021/ac503134r | Anal. Chem. XXXX, XXX, XXX−XXX