Encapsulation of Strongly Fluorescent Carbon Quantum Dots in

Heena Sammi , Deepak Kukkar , Jaskaran Singh , Preeti Kukkar , Rajwinder Kaur , Harmanpreet Kaur , Mohit Rawat , Gurjinder Singh , Ki-Hyun Kim. Sensor...
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Encapsulation of Strongly Fluorescent Carbon Quantum Dots in Metal−Organic Frameworks for Enhancing Chemical Sensing Xiaomei Lin,† Gongmin Gao,† Liyan Zheng,‡ Yuwu Chi,*,† and Guonan Chen† †

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Ministry of Education Key Laboratory of Analysis and Detection for Food Safety, Fujian Provincial Key Laboratory of Analysis and Detection for Food Safety, and Department of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China ‡ Key Laboratory of Medicinal Chemistry for Natural Resource (Yunnan University), Ministry of Education, School of Chemical Science and Technology, Yunnan University, Kunming, Yunnan 650091, China S Supporting Information *

ABSTRACT: Novel highly fluorescent (FL) metal−organic frameworks (MOFs) have been synthesized by encapsulating branched poly-(ethylenimine)-capped carbon quantum dots (BPEI-CQDs) with a high FL quantum yield into the zeolitic imidazolate framework materials (ZIF-8). The as-synthesized FL-functionalized MOFs not only maintain an excellent FL activity and sensing selectivity derived from BPEI-CQDs but also can strongly and selectively accumulate target analytes due to the adsorption property of MOFs. The selective accumulation effect of MOFs can greatly amplify the sensing signal and specificity of the nanosized FL probe. The obtained BPEI-CQDs/ZIF-8 composites have been used to develop an ultrasensitive and highly selective sensor for Cu2+ ion, with a wide response range (2−1000 nM) and a very low detection limit (80 pM), and have been successfully applied in the detection of Cu2+ ions in environmental water samples. It is envisioned that various MOFs incorporated with FL nanostructures with high FL quantum yields and excellent selectivity would be designed and synthesized in similar ways and could be applied in sensing target analytes.

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inorganic components or the generated luminescence from the weak metal−ligand charge transfer in MOFs. Therefore, introducing guest luminescent materials with high luminescence quantum yield into MOFs while basically maintaining the original structures and properties of host MOFs is an important alterative way to synthesize ideal luminescence-functionalized MOFs for chemical sensing. The functional MOFs will take advantage of both guest luminescent materials in high luminescence quantum yield and excellent sensing selectivity and host MOFs in highly selective adsorption and efficient accumulation of target analytes, which may lead to the construction of a designable and tunable luminescent sensing platform for various analytes with high sensing sensitivity and selectivity. Most recently, an encapsulation strategy that allows several types of nanoparticles to be incorporated within crystals of zeolitic imidazolate framework materials (ZIF-8) MOFs in a well-dispersed fashion has been developed.21 ZIF-8 MOFs are zeolitic imidazole-based MOFs with large cavities and small apertures.22 They combine the advantages of MOFs (i.e., high porosity and surface area, transition metal centers, and tailored linkers) with high stability, chemical robustness, and framework

etal−organic frameworks (MOFs) are permanently microporous materials synthesized by assembling metal ions with organic ligands in appropriate solvents.1 In the past years, studies on MOFs have focused on the construction of MOFs with permanent porosity for catalysis,2 separations,3 gas storage,4 ion-exchange,5 and sensors.6 Recently, MOFs have attracted more and more attention since they have unique host matrices for various functional species7−10 and thus promise to develop new types of composite materials that display enhanced or new functions in comparison to the parent MOF counterparts. Among the functional MOFs, luminescence-functionalized MOFs may exhibit various and tunable optical properties, since both organic and inorganic components can provide the platform to generate luminescence, while metal−ligand charge transfer-based luminescence within MOFs can add other dimensional luminescent functionalities; moreover, some guest molecules within MOFs can emit or induce luminescence.11 Several studies on luminescent MOFs have shown that these emerging luminescent composite materials are attractive in constructing chemical sensors.12−20 However, up to date, relatively low luminescence activities of MOFs limit their further applications in chemical sensing. The low luminescence activity limitation may be due to the difficulty in obtaining MOFs with high luminescence quantum yield merely through the utilization of the intrinsic luminescence of organic or © 2013 American Chemical Society

Received: November 1, 2013 Accepted: December 16, 2013 Published: December 16, 2013 1223

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Scheme 1. Concept and Process for Sensing Cu2+ Based on Fluorescent MOFs

China) were used to prepare BPEI-CQDs. All other reagents were of analytical grade and used as received. Doubly distilled water was used throughout the experiments. Phosphatebuffered saline (PBS) solutions of different pH values were prepared by titrating 0.01 mol L−1 phosphoric acid solutions with a concentrated sodium hydroxide solution (1 mol L−1) to the required pH values. Apparatus. Thermogravimetric analysis (TGA) was performed using the TG209F1 thermal analysis system. All TGA experiments were performed under a N2 atmosphere from 30 to 650 °C at a rate of 10 °C/min. Data were analyzed using a TA Universal Analysis software package. X-ray powder diffraction patterns were taken using a Rigaku Miniflex X-ray diffractometer at 30 kV, 15 mA for Cu Kα (λ = 1.5406 Å) with scan speed of 0.1 s/step and a step size of 0.01°. The specific surface area of the products was measured by the Brunauer− Emmett−Teller (BET) method using nitrogen gas adsorption/ desorption at 77 K (BET, Micromeritics ASAP 2020, USA). The Fourier transform infrared (FT-IR) spectra were obtained on a FT-IR spectrophotometer (Thermo Nicolet 360). UV−vis absorption was characterized by a UV/vis/NIR spectrophotometer (Lambda 750). High resolution transmission electron microscopy (HRTEM) observations were made on a Tecnai G2 F20S-TWIN 200KV electron microscope. Fluorescence spectra were recorded on a Hitachi High-Technologies Corporation Tokyo Japan 5J2-0004 model F-4600 FL spectrofluorometer. An Agilent 7500 ICPMS system (Agilent Technologies, Santa Clara, CA) was used to detect the Cu2+ concentration of the Min River water sample. Synthesis of BPEI-CQDs/ZIF-8 Composites. BPEICQDs/ZIF-8 composites were synthesized by the following synthetic route. First, the BPEI-capped CQDs with sizes mainly distributed in the range of 4−8 nm (see TEM images in Figure S-1 of the Supporting Information) and FL quantum yield of >40% were prepared by a previously reported method.27 Second, 0.5 mL of BPEI-CQDs (4.14 mg mL−1) solution, 5 mL of 2-methylimidazole (25 mM) solution, and 5 mL of Zn(NO3)2·6H2O (25 mM) solution were mixed and then allowed to react at room temperature for 24 h without stirring. BPEI-CQDs/ZIF-8 composites were collected by centrifugation, washed several times with methanol, and vacuum-dried overnight (50 °C).

diversity. Recently emerging carbon-based quantum dots (CQDs) are carbon nanocrystals of less than 10 nm in size. They not only exhibit many fascinating optical properties, such as photoinduced electron transfer, photoluminescence, and electrochemiluminescence, but also present some additional advantages over the heavy metal-containing semiconductorbased QDs, such as chemical inertness, good stability, biocompatibility, low toxicity, easy preparation, and environmental friendliness.23−26 Inspired by this reported work on MOFs and our recent research on highly fluorescent (FL) CQDs,27,28 we reported for the first time encapsulating branched poly(ethylenimine)-capped CQDs (BPEI-CQDs) with strong FL activity (quantum yield >40%), high sensitivity, and excellent selectivity for sensing Cu2+, into ZIF-8 MOFs (Scheme 1). The as-prepared FL-functionalized MOFs (i.e., BPEI-CQDs/ZIF-8) not only exhibit an excellent FL activity and sensing selectivity derived from CQDs but also can strongly and selectively accumulate target analytes such as metal ions and some molecules due to the well-known adsorption property of MOFs, leading to the development of ultrasensitive and highly selective sensors for Cu2+ ion. The developed FL senor has been applied for the detection of Cu2+ ions in the local surface water, since copper is essential to all living organisms, including human beings, as a trace dietary mineral, and appropriate intake of copper in the daily diet is very important for preserving our health.29−31 Compared with FL sensors without the amplifying effect of MOFs and MOF-based optical sensors without introduction of guest luminophores, the presently developed FL-functionalized MOF sensors have a much lower limit of detection (LOD), approximately a 2 orders of magnitude decrease for target analytes. It is envisioned that various ultrasensitive and highly selective FL sensors for target analytes can be developed by encapsulating corresponding FL nanomaterials into MOFs in appropriate ways, thus spreading applications of MOFs in analytical chemistry.

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EXPERIMENTAL SECTION Chemicals and Materials. Zn(NO3)2·6H2O (Tianjin, China) and 2-methylimidazole (Alfa Aesar) were used to synthesize ZIF-8. Citric acid (Alfa Aesar) and branched poly(ethylenimine) (BPEI, M = 1800) (Aladdin, Shanghai, 1224

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dependent on their surface states.32,33 This suggests that the surface states of BPEI-CQDs are well maintained when they are encapsulated in ZIF-8 MOFs. FI-IR spectra (Figure 1E) show that the BPEI-CQDs/ZIF-8 composites (curves (a)) have many characteristic absorption bands associated with BPEICQDs (curves (b)) and ZIF-8 parent counterparts (curve (c)). The above experimental results indicate that BPEI-CQDs are well encapsulated in ZIF-8 and retain their original excellent properties. Transmission electron microscopy (TEM) images reveal that the as-prepared BPEI-CQDs/ZIF-8 particles are defined as rhombic dodecahedron with an average diameter of 400 nm (Figure 1F). The comparison of the X-ray powder diffraction data of BPEI-CQDs/ZIF-8 (see Figure S-3 and Table S-1, Supporting Information) with those of undoped ZIF-8,19 indicates that the CQD-doping does not impact the crystalline integrity of ZIF-8. It should be noted here that the peak associated with BPEI-CQD nanoparticles can not be observed in BPEI-CQDs/ZIF-8, which might be explained by reason that the peaks of small-size (ca. 6 nm) BPEI-CQDs are too weak to be observed clearly under the strong peaks originating from big-size (400 nm) ZIF-8 in diffraction patterns. The Brunauer−Emmett−Teller (BET) isotherm of BPEI-CQDs/ZIF-8 shown in Figure S-4, Supporting Information, exhibits a typical type-I isotherm characteristics of microporous materials. The BET surface area of BPEICQDs/ZIF-8 is ∼1247 m2/g. Interactions of BPEI-CQDs/ZIF-8 with Cu2+. First, the adsorption interaction between Cu2+ ions and the FLfunctionalized MOFs was investigated. Cu2+ ions were chosen for investigation due to their sensitive and selective quenching effect on the fluorescence of BPEI-CQDs. After addition of BPEI-CQDs/ZIF-8 particles into blue Cu2+ solution (right test tube in Figure 1G) and centrifugation of the mixed solution, it was found that blue Cu2+ ions were almost all adsorbed into the deposited BPEI-CQDs/ZIF-8 particles, resulting in colorless supernatant (left test tube in Figure 1G). Apparently, the fluorescent CQD-functionalized MOFs exhibit strong adsorption ability toward copper ions, which provides an efficient way to accumulate the analytes (i.e., copper ions) around the fluorescent probes and thus increases both sensitivity and selectivity of detection. The strong adsorption ability of BPEICQDs/ZIF-8 toward copper cations might be attributed to the cation-adsorption properties of the zeolite-like parent ZIF-8 counterparts34 and well-known complexation between Cu2+ and imidazole in MOFs.35 Subsequently, the fluorescence interaction between Cu2+ ions and BPEI-CQDs/ZIF-8 MOFs was studied. The strong blue FL emission of BPEI-CQDs/ZIF-8 composites can be quenched obviously after adding Cu2+ ions into the solution (see the inset of Figure 2). The corresponding FL spectra indicate that the addition of Cu2+ ions decreases the FL intensity of BPEICQDs/ZIF-8 composites at 440 nm but has no effect on the maximum FL emission and excitation wavelengths (curves (a), (b), (c), and (d) in Figure 2). UV−vis absorption spectra (curves (e) and (f) in Figure 2) indicate that the presence of Cu2+ ion leads to the decrease in light absorption at 360 nm, which is exactly the maximum excitation wavelength of CQD FL (curves (c) and (d)). Simultaneously, a new shoulder UV adsorption associated with FL-inactive amine-copper group occurs in the range between 270 and 400 nm, which partially overlays the excitation spectra of CQDs in MOFs and thus decreases their quantum yield. Obviously, the quenched FL mechanism of the BPEI-CQDs/ZIF-8 composites is similar

Analysis of a Real Sample. A water sample was collected from Min River (Fujian, China). The sample was filtered through a 0.22 μm membrane (Millipore) prior to the detection. Ten microliters of the above water sample and an equal volume of different concentrations of standard Cu2+ solutions (final concentrations of 0.005, 0.075, 0.15, 0.3, 0.4, and 0.5 μM) were, respectively, added to 1000 μL of PBS (10 mM, pH 8.0) containing BPEI-CQDs/ZIF-8 (15 μg mL−1) and then analyzed using the developed sensing technique.

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RESULTS AND DISCUSSION Characterization. The as-prepared BPEI-CQDs/ZIF-8 materials not only display good dispersity in aqueous solution (Figure 1A) but also emit strong blue fluorescence (440 nm)

Figure 1. Photographs of the BPEI-CQDs/ZIF-8 composite suspended in water illuminated with ambient light (A) and 365 nm ultraviolet light (B). (C) Fluorescence image of the BPEI-CQDs/ZIF8. (D) Normalized fluorescence emission spectra of BPEI-CQDs/ZIF8 (dash line) and BPEI-CQDs (solid line) under the excitation of 360 nm ultravioltet light. (E) FT-IR spectra obtained for BPEI-CQDs/ ZIF-8 (a), BPEI-CQDs (b), and ZIF-8 (c). (F) TEM images of BPEICQDs/ZIF-8. (G) Cu2+ solution (0.05 M) before (right) and after (left) adding BPEI-CQDs/ZIF-8 (0.017 g).

under the excitation of 365 nm (Figure 1B,C). The profile of the normalized fluorescent emission spectrum of BPEI-CQDs/ ZIF-8 (the red dash line in Figure 1D) is almost the same as that of BPEI-CQDs (the black solid line in Figure 1D), and the FL activity of BPEI-CQDs/ZIF-8 (Figure S-2C, Supporting Information) is comparable with that of BPEI-CQDs (Figure S2A, Supporting Information), indicating that the optical properties of BPEI-CQDs in ZIF-8 have not been changed. It should be noted here that the FL mechanisms of carbon-based dots including CQDs are very complicated and still argumentative. However, more and more evidence indicates that the FL properties of carbon-based dots are mainly 1225

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(ΔI) of BPEI-CQDs/ZIF-8 composites induced by Cu2+ ions is much larger than that of BPEI-CQDs in the presence of the same amounts Cu2+ ions (Figure 3A). This suggests that BPEICQDs incorporated in MOFs have a much stronger sensing ability than BPEI-CQDs simply dispersed in solution. Establishment of BPEI-CQDs/ZIF-8 Composite-Based Sensor for Cu2+ Ions. Since the strong blue emission of the BPEI-CQDs/ZIF-8 composites can be sensitively quenched by Cu2+, the as-prepared BPEI-CQDs/ZIF-8 was applied for detection of copper ions in water. The sensing system is based on quenching the fluorescence of BPEI-CQDs/ZIF-8, so the concentration of BPEI-CQDs/ZIF-8 should fall within the linear response range. As shown in Figure S-5, Supporting Information, there is a good linear relationship between the FL intensity and the concentration of BPEI-CQDs/ZIF-8 composites in the range of 0−0.5 mg mL−1, but the FL response deviates from linearity in the higher concentration range. In general, the lower the concentration of fluorophore, the higher the quenching sensitivity at a given concentration of quencher. 36 However, too low of a concentration of fluorophore would lead to the increase of noise fraction. Finally, 15 μg mL−1 BPEI-CQDs/ZIF-8 was eventually chosen in the detection of Cu2+ to obtain the maximum signal-to-noise ratio (S/N) and thus the lowest limit of detection. Because of the strong collection capacity of copper ions by BPEI-CQDs/ZIF-8, the response rate of the FL signal of the BPEI-CQDs/ZIF-8 to Cu2+ was then investigated. As shown in Figure 3B, the FL intensity of the BPEI-CQDs/ZIF-8 is quenched by 90% in 30 min when 10 μM Cu2+ is added into the BPEI-CQDs/ZIF-8 solution (15 μg mL−1) and then remains stable over the next 30 min, suggesting that the reaction can be completed within 30 min. We then investigated the fluorescence response of BPEICQDs/ZIF-8 at different pH. As shown in Figure 3C, the pH value of the solution influenced both the initial FL intensity (in

Figure 2. FL spectra (emission spectra, a, b; excitation spectra, c, d) and UV absorption spectra (e, f) of 0.15 mg mL−1 BPEI-CQDs/ZIF-8 solution in the absence (a, c, e) and presence (b, d, f) of 100 μM Cu2+ ions. The inset shows the photos of 0.15 mg mL−1 BPEI-CQDs/ZIF-8 solutions in the absence (left) and presence (right) of 100 μM Cu2+.

with that of BPEI-CQDs proposed in our previous report.23 Briefly, Cu2+ ions can react with the amino groups at the surface of the BPEI-CQDs to form a FL-inactive but UV-active complex, resulting in the sensitive quenching of CQDs’ FL by an inner filter effect. The above-mentioned experimental results indicate that BPEI-CQDs/ZIF-8 composites can act as novel bifunctional materials; i.e., they not only can strongly adsorb and accumulate Cu2+ ions but also have selective FL response to Cu2+ ions. The synergetic effect of accumulation and quenching may lead to a significant improvement in the probe sensing sensitivity, which has been proved by the following designed experiment. The solution of BPEI-CQDs/ZIF-8 composites and the solution of BPEI-CQDs with the same FL intensity (i.e., with the same concentration of FL species) were added, respectively, with the same amounts of Cu2+ (the final concentration of Cu2+ was 1 μM). The comparison shows that the quenched FL intensity

Figure 3. (A) FL spectra of BPEI-CQDs/ZIF-8 solution (solid line) and BPEI-CQDs (dash line) in the absence and presence of 1 μM Cu2+ ions in pH 8 PBS solution. (B) Time-dependent fluorescence response of the 15 μg mL−1 BPEI-CQDs/ZIF-8 to 10 μM Cu2+ ions in pH 8 PBS. (C) Fluorescence responses of 15 μg mL−1 BPEI-CQDs/ZIF-8 in the absence (black) and presence (red) of 10 μM Cu2+ ions at different pH values. (D) Selectivity of the BPEI-CQDs/ZIF-8-based sensor for Cu2+ ions over other ions in pH 8 PBS solution. The concentrations of BPEI-CQDs/ZIF-8 and metal ions were15 μg mL−1 and 10 μM, respectively. 1226

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the absence of Cu2+) and the quenched FL intensity (in the absence of Cu2+), suggesting that pH is another key factor that affects the sensing system. First, the BPEI-CQDs/ZIF-8 (black columns in Figure 3C) has strong FL activities in the range of pH 7−10, with the maximum value at pH 8.0, but has weak FL activities at other pH values. This indicates that the weakly alkaline media (pH 7−10) are suitable for sensing. Second, the addition of Cu2+ can inhibit the BPEI-CQDs/ZIF-8′ FL in the wide pH range from 3 to 12 (see red columns in Figure 3C). However, the quenching efficiencies (F0/F) at these pH values are quite different. In strongly acidic media (pH = 3.0), the addition of Cu2+ has nearly no effect on the FL of BPEI-CQDs/ ZIF-8, which may be attributed to the fact that the amino groups at the surfaces of the BPEI-CQDs/ZIF-8 are well protonated, and thus are unable to complex with Cu2+ to form FL-quenching cupric amine moieties. It is generally appreciated that partial Cu2+ ions may hydrolyze in strongly alkaline media, which inhibits the complex reaction between Cu2+ and the amines of BPEI-CQDs/ZIF-8 and causes unsatisfied quenching efficiencies. In contrast, in the weakly alkaline media (pH 7− 10), the quenching efficiencies are obviously higher than other pH values due to easy formation of FL-quenching cupric amine moieties under the weakly alkaline media. The maximum S/N ratio can be obtained in pH 8.0 solution; thus, the pH was chosen for the sensitive detection of Cu2+. Along with the sensitivity requirement, high selectivity is crucial in most scenarios, especially in real sample detections. Therefore, the selectivity of the BPEI-CQDs/ZIF-8 FL sensing system was estimated and shown in Figure 3D. Besides Cu2+, the effects of 10 other kinds of cations, including Hg2+, Cd2+, Mn2+, Mg2+, Pb2+, Al3+, Ag+, Ba2+, and Ca2+ at the same concentration of Cu2+, on the FL response of BPEI-CQDs/ ZIF-8 containing 10 μM Cu2+ at the same time were investigated. From the figure, we can find that the FL intensities are significantly quenched by 10 μM Cu2+, whereas almost no additional inhibition of the FL intensities happens in the presence of Cd2+, Pb2+, Al3+, Ag+, and Ca2+ ions, and only little enhancement or weakness of the FL intensities happens in the presence of Hg2+, Mn2+, Mg2+, and Ba2+ ions. Considering the complex components of the water sample, we also found the selectivity of the BPEI-CQDs/ZIF-8-based sensor for Cu2+ ions over some commonly existing anions and organic components. Experimental results (Figure S-6, Supporting Information) indicate that the investigated anions and organic components have no effect on the sensor performance. The high selectivity of the BPEI-CQDs/ZIF-8-based sensor for Cu2+ ions over the anions might be attributed to the electrostatic repulsion between anions and the anionic ZIF-8 MOF. Apparently, these results clearly indicate that the BPEICQDs/ZIF-8 FL sensing system exhibits high selectivity for Cu2+. The high selectivity may be attributed to the synergetic effect of selective adsorption of heavy metal ions by MOFs and selective sensing of Cu2+ ion by BPEI-CQDs. The fluorescence intensity of BPEI-CQDs/ZIF-8 is sensitive to the presence of nanomolar concentrations of Cu2+. To evaluate the applicability of the sensitive sensing system for Cu2+ ions, the linear response range and LOD were investigated under the above-mentioned optimum experimental conditions. As shown in Figure 4, the FL intensity of the BPEI-CQDs/ZIF8 is quenched obviously with increasing the concentration of Cu2+. There is a good linear correlation between the quenching level, (F0 − F)/F0, and the concentration of Cu2+ in the range

Figure 4. Fluorescence emission spectra of the BPEI-CQDs/ZIF-8based sensor exposed to various concentrations of Cu2+: 1, 0.8, 0.6, 0.4, 0.2, 0.1, 0.08, 0.02, 0.002, and 0 μM from bottom to top. The inset is the linear plot of the fluorescence intensity versus the concentration of Cu2+. These spectra were measured in pH 8.0 PBS solution. The concentration of BPEI-CQDs/ZIF-8 was 15 μg mL−1. The excitation wavelength was 360 nm.

from 2 to 1000 nM (see the inset of Figure 4) with the following equation: (F0 − F )/F0 = 0.3688C + 0.03165

(R2 = 0.999)

(1)

where F0 and F represent, respectively, the FL intensities of BPEI-CQDs/ZIF-8 in the absence and presence of Cu2+ and C is the concentration of Cu2+. The LOD of Cu2+ (at the ratio of signal-to-noise = 3) was calculated to be 80 pM, which is much (4.0 × 105) lower than the allowed concentration of copper ion (32 μM or 2 mg L−1) in drinking water permitted by the World Health Organization (WHO).37 The LOD of Cu2+ by the presently developed BPEI-CQDs/ZIF-8-based FL sensor has been decreased by approximately 2 orders of magnitude, compared with the previously reported BPEI-CQDs-based FL sensor,28 showing the important role of the ZIF-8 MOFs in accumulating targets. In comparison with other reported FL methods,26,38−41 this sensing approach also shows higher sensitivity (1 or 2 orders of magnitude decrease in LOD) for the detection of Cu2+. The utra-high sensing sensitivity can be attributed to the use of highly sensitive FL carbon quantum dots and the strong signal-amplifying effect from the excellent metal ion-adsorbing ability of MOFs. The precision and reproducibility of the FL sensor were evaluated by calculating the variation coefficients (CVs) for intra-assay and interassay, respectively. Experimental results indicate that the CVs (n = 10) of the assay using the sensor from the same batch and the CVs (n = 7) of the assay using the sensor from the various batches were, respectively, 1.4% and 5.2% at the 0.1 μM Cu2+ level. The long-term stability of the present FL sensor for Cu2+ was also investigated. It has been found that the sensor can be stable for at least 20 days (Figure S-7, Supporting Information), demonstrating the robustness of the Cu2+ FL sensor. Sensing Application. The practicability of the developed FL-functionalized MOF-based sensors for Cu2+ ion was assessed by applying it to the real analysis for Cu2+ in water from the Min River (the largest river in Fujian Province, China). The concentration of Cu2+ ions in the Min River sample (n = 10) detected using this new approach is 2.201 ± 0.084 μM. The value is consistent with the result obtained by the ICPMS method, namely, 2.164 ± 0.052 μM, which 1227

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indicates the accuracy and reliability of our probe for Cu2+ determination in environmental samples.

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CONCLUSIONS In summary, we reported for the first time encapsulating a kind of highly fluorescent amine-capped CQD into the zeolitic imidazolate framework materials. The as-synthesized FLfunctionalized MOFs not only exhibit an excellent fluorescent activity and sensing selectivity derived from CQDs but also can strongly and selectively accumulate target analytes due to the adsorption property of MOFs. The accumulation effect of MOFs can greatly amplify the sensing signal from the nanosized FL probe. The obtained BPEI-CQDs/ZIF-8 composites have been used to develop an ultrasensitive and highly selective sensor for Cu2+ ion, with a wide response range (2 to 1000 nM) and a very low detection limit of 80 pM, and have been successfully applied in the detection of Cu2+ ions in environmental water samples. It is envisioned that various MOFs incorporated with fluorescent nanostructures with high FL quantum yields and excellent selectivity would be synthesized in similar ways and could be applied in sensing target analytes.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S-1−S-7 and Table S-1. 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] (Yuwu Chi). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by National Natural Science Foundation of China (21375020, 21075018), Program for New Century Excellent Talents in Chinese University (NCET-10-0019), National Basic Research Program of China (2010CB732400), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1116).

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dx.doi.org/10.1021/ac403536a | Anal. Chem. 2014, 86, 1223−1228