CsPbBr3 Perovskite Quantum Dots-Based Monolithic Electrospun

Letter pubs.acs.org/JPCL

CsPbBr3 Perovskite Quantum Dots-Based Monolithic Electrospun Fiber Membrane as an Ultrastable and Ultrasensitive Fluorescent Sensor in Aqueous Medium Yuanwei Wang, Yihua Zhu,* Jianfei Huang, Jin Cai, Jingrun Zhu, Xiaoling Yang, Jianhua Shen, Hao Jiang, and Chunzhong Li* Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China S Supporting Information *

ABSTRACT: Perovskite quantum dots with excellent optical properties and robust durability stand as an appealing and desirable candidate for fluorescence resonance energy transfer (FRET) based fluorescence detection, a powerful technique featuring excellent accuracy and convenience. In this work, a monolithic superhydrophobic polystyrene fiber membrane with CsPbBr3 perovskite quantum dots encapsulated within (CPBQDs/PS FM) was prepared via one-step electrospinning. Coupling CPBQDs with PS matrix, this CPBQDs/PS FM composite exhibits high quantum yields (∼91%), narrow half-peak width (∼16 nm), nearly 100% fluorescence retention after being exposed to water for 10 days and 79.80% fluorescence retention after 365 nm UV-light (1 mW/cm2) illumination for 60 h. Thanks to the outstanding optical property of CPBQDs, an ultralow detection limit of 0.01 ppm was obtained for Rhodamine 6G (R6G) detection, with the FRET efficiency calculated to be 18.80% in 1 ppm R6G aqueous solution. Electrospun as well-designed fiber membranes, CPBQDs/PS FM composite also possesses good tailorability and recyclability, showing exciting potential for future implementation into practical applications.

F

luminescence, due to peeling of loosely bound surfactants around perovskite crystals.11 Such drawback largely limits their applicability in fluorescence detection, which is commonly carried out in aqueous medium.12 The goal of stabilizing CPXQDs in water prompts for the idea of hybridizing CPBQDs with certain water-stable materials. Encapsulating CPXQDs within polymer is an attractive strategy for enhancing their stability in water. Polymer matrices may not only prevent agglomeration of CPXQDs, but may also provide mechanical and chemical stability of the QDs system.13−15 Additionally, tunable processability of polymers allows access toward the forms of thin films, microspheres, or fibers, with which high surface area and preferable mechanical properties can be further achieved.16−18 Herein, we prepared a monolithic polystyrene (PS) fiber membrane (FM) encapsulating CsPbBr3 quantum dots (CPBQDs) within through facile and industrially available electrospinning method, and demonstrated its extraordinary stability in aqueous and ethanol media for ultrasensitive detection of Rhodamine 6G (R6G). The CPBQDs were

luorescence resonance energy transfer (FRET)-based sensing platforms have been widely utilized in detection of organic dyes and bioanalysis fields.1−5 FRET, a nonradioactive process between an excited-state donor and a ground-state acceptor via a through-space dipole−dipole interaction (150°), which implies its superhydrophobic surface nature, which is beneficial for postcleaning and reuse.

pattern of CPBQDs/PS FM is displayed in Figure 1e, in which the broad peak appearing at ∼20° is ascribed to the semicrystalline PS, and all other diffraction peaks agree well with the characteristic pattern of CPBQDs (PDF#54−0752). This affirms the chemical stability of the CPBQD phase during the electrospinning process and successful combination of CPBQDs and PS fibers. As shown in Figure S1a,b, the electrospun CPBQDs/PS FM collected on aluminum foil is displayed as a uniform light green membrane, and it exhibits bright green PL under 365 nm UVlight irradiation, which agrees well with the laser confocal 4255

DOI: 10.1021/acs.jpclett.6b02045 J. Phys. Chem. Lett. 2016, 7, 4253−4258

Letter

The Journal of Physical Chemistry Letters

Figure 3. (a) Pb-4f and (b) Br-3d XPS analysis of CPBQDs/PS FM. (c) Time-resolved PL decay spectra and the corresponding fitting curves of CPBQDs (inset), CPBQDs/PS FM (red), and CPBQDs/PS FM in 1 ppm R6G aqueous solution (blue). (d) The fluorescence spectra of CPBQDs/ PS TF in R6G aqueous solution with concentrations of 0, 5, 10 ppm.

where k2 describes the relative orientation in space of the transition dipoles of the donor and acceptor (herein 0.476 for randomly oriented molecules fixed in space);20 n is the refractive index of the medium (1.6 for PS); ψ is the quantum yield of the donor; J(λ) is the integral overlapping of the peaks of donor emission and acceptor absorption, the Förster radius R0 is calculated to be 8.78 nm, which suggests the efficient FRET process occurring between CPBQDs/PS FM and R6G. On the basis of the above analysis, we carried out the fluorescence detection of R6G in aqueous medium with our CPBQDs/PS FM. First, the PL peak position for R6G aqueous solution under 365 nm was confirmed at ∼560 nm (Figure S3). As shown in Figure 2c, under 365 nm UV-light irradiation, with the increasing concentration of R6G, the PL intensity at ∼513 nm decreases gradually while the intensity at ∼560 nm increases accordingly, reflecting that the fluorescence group of R6G absorbs the energy transferred from CPBQDs via FRET process. It is notable that in the concentration range of 1−10 ppm, the FM sensor exhibits an excellent liner relationship between the concentration and PL intensities located at ∼513 and ∼560 nm for CPBQDs and R6G, respectively). Remarkably, the detection limit has been pushed down to 0.01 ppm (inset Figure 2d), which is superior to any previously reported QDs-based sensing systems.21,22 To gain information on the durability of CPBQDs/PS FM, its aqueous stability and photostability were investigated, respectively. The aqueous stability of CPBQDs/PS FM was

The PL characters of CPBQDs/PS FM were studied in comparison with the primitive CPBQDs toluene solution and CPBQDs/PS toluene solution. As shown in Figure 2a, after dissolving PS into primitive CPBQDs toluene solution, the PL peak position blue shifts by ∼2 nm, along with discernible inferior peak symmetry and a slight increase of fwhm by ∼2 nm, which may result from the moderate aggregation of CPBQDs in the high viscosity CPBQDs/PS toluene solution and possible unavoidable damage during dissolving because of their poor stability. After the mixture was electrospun into FM, the PL characters remain almost the same, which indicates that the fast resolidification of PS during electrostatic drawing process did not change the PL features of CPBQDs. Therefore, the optical properties of CPBQDs are well almost preserved in our CPBQDs/PS FM composite. In our case, R6G in aqueous solution serves as the analytical target for evaluation of the detection performance of CPBQDs/ PS FM, since the PL emission spectrum of CPBQDs/PS FM overlaps greatly with the absorption spectrum of R6G in water (Figure 2b), which meets the prerequisite for FRET process. Additionally, according to the following eqs 1 and 2,7 R 06 = 8.8 × 10−25k 2n−4ψJ(λ) J (λ ) =

∫0

∞

FD(λ)εA (λ)λ 4dλ

(1)

(2) 4256

DOI: 10.1021/acs.jpclett.6b02045 J. Phys. Chem. Lett. 2016, 7, 4253−4258

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presence of R6G verifies the occurrence of FRET convincingly.24 Detailed lifetime data are provided in Table S1. According to eq 5,25,26 where τDA and τD stand for the lifetime of donor in the presence or absence of acceptor, respectively, the FRET efficiency E is calculated to be 18.80% in 1 ppm R6G aqueous solution.

assessed by directly immersing the composite membrane into water, which would lead instant destruction of unprotected pure CPBQDs. As shown in Figure 2e, the PL intensity of CPBQDs/PS FM was measured every 12 h, and after 10 days, the PL intensity almost remained 100%. Additionally, the corresponding FWHMs and PL peak position were also recorded as shown in Figure S4, which were also quite steady during the test. These results favorably confirm the extraordinary long-term stability of CPBQDs/PS FM against aqueous medium, which is accredited to protection of the PS fiber host. The photostability of CPBQDs/PS FM was tested under 365 nm UV-light (1 mW/cm2) irradiation in comparison with CPBQD toluene solution contained in an airtight, fourclear-faces quartz cuvette. As shown in Figure 2f, the PL enhancement of CPBQD toluene solution in the first 1 h is known as the “photoactivation” phenomenon, which originates from the removal of some surface defects and dangling bonds.23 However, CPBQDs/PS FM does not show a similar phenomenon because of the immobilization of CPBQDs in the PS matrix. Upon further irradiation, the PL intensity of both CPBQDs/PS FM and CPBQDs toluene solution went through continuous decrease, but CPBQDs/PS FM shows much lower PL decay rate than CPBQDs toluene solution. After 60-h irradiation, the PL retention of CPBQDs toluene solution reduces to as low as 17.50% while that of CPBQDs/PS FM still maintains at as high as 79.80%. Also, the peak positions of CPBQDs/PS FM fluctuated only in a narrow range (less than 3 nm), while the peak positions of CPBQDs toluene solution blue-shifted to ∼522 nm. Compared to aqueous medium, UV-light shows higher impact on the optical property of CPBQDs/PS FM, mainly due to the aging effect of polymer PS under UV irradiation. For CPBQDs in toluene, yellow precipitate appeared after the 60-h irradiation, which was verified to be aggregates of nanoplates, whose formation mechanism will not be covered here (Figure S5). In summary, encapsulating CPBQDs into PS fiber matrix has significantly improved both the aqueous stability and photostability of CPBQDs. As energy transfer efficiency E is inversely proportional to the distance r between donor and acceptor, CPBQDs located in the near surface area of FM (less than 10 nm) play a key role in high-efficiency FRET process, whose existence can be confirmed from the TEM ultramicrotomed section image of CPBQDs/PS FM shown in Figure 1d. Considering the inspection depths of XPS (4−10 nm for polymer), the Pb 4f and Br 3d peaks can also attest the existence of CPBQDs in the near surface area of the PS fiber, as shown in Figure 3a,b. In order to further comprehend the FRET process between CPBQDs donor and R6G acceptor, time-resolved PL decay spectra at 513 nm of CPBQDs, CPBQDs/PS FM, and CPBQDs/PS FM in 1 ppm R6G aqueous solution were obtained. As shown in Figure 3c, the time-resolved PL decay curves are well second-order exponentially fitted according to eq 3. With eq 4, the average lifetime for CPBQDs is determined to be ∼7.16 ns, revealing the high ratio of radiative-tononradiative transition (inset Figure 3c). After encapsulating CPBQDs into PS fibers, the lifetime reduces to ∼5.16 ns. It is reasonable to consider two main causes behind this decrease in lifetime: one is fluorescence quenching resulted from unavoidable aggregation of CPBQDs in high viscosity CPBQDs/PS toluene solution, and the other is the energy transfer process between CPBQDs and PS. The further decreased lifetime of 4.19 ns for CPBQDs/PS FM in the

⎛ −t ⎞ ⎛ −t ⎞ A(t ) = A1 exp⎜ ⎟ + A 2 exp⎜ ⎟ ⎝ τ1 ⎠ ⎝ τ2 ⎠

(3)

tavg. = (A1t12 + A 2 t 22)/(A1t1 + A 2 t 2)

(4)

E = 1 − τDA /τD = 1 − IDA /ID

(5)

Figures S6 and S7 show the study on detection performance of CPBQDs/PS FM in ethanol solution of R6G, with the corresponding discussion in the Supporting Information. To further highlight superiority of the designed fiber membrane structure, we drop-coated a thin film (TF) of ∼1 mm in thickness with the identical precursor of the aforementioned electrospinning procedure and applied it to detecting R6G in aqueous medium. As shown in Figure 3d, the FRET process occurs in the case of TF-R6G system as well, but the detection limit of TF is much inferior to that of the FM sample, agreeing well with its FRET efficiency in 10 ppm aqueous solution of R6G calculated as 60.89%, much lower than that of the FM sample. The fiber membrane has larger surface area than the thin film, which contributes to higher degree of effective interaction between the sensor unit and the analyte, and thus obtaining better detection performance. In summary, we prepared a monolithic superhydrophobic PS FM with encapsulated CPBQDs through a simple electrospinning method and applied it to determination of ultralow concentration R6G in aqueous and ethanol media. The CPBQDs/PS FM elegantly combines the optical properties of CPBQDs and stabilizing ability of the PS matrix, exhibiting a remarkably sensitive detection limit of 0.01 ppm and ultrahigh optical stability in aqueous and ethanol media. Taking advantage of the well-designed fiber membrane structure, this flexible, tailorable and easy-cleaning composite membrane would be promising to be adapted for scalable production and wider application. More importantly, this proof-of-concept work reveals that electrospinning polymer fibers with perovskite QDs embedded is feasible to obtain highly stable perovskite QDs functional platforms in fluorescent detection, which can be further enriched by rational selection of QDs with various emission spectra for sensing different fluorescent molecules with corresponding absorptions. Thus, a wide range of fluorescent molecules in aqueous medium may be detected by such QDs/PS architecture. With technical readiness of surface functionalization of polymer matrix, the present work may open up a brand new approach to reliable detectors based on perovskite QDs for sensing other variables including biological protein, pH, temperature, etc.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02045. The preparation process of fiber membrane; fluorescence detection process; the result and discussion of the detection in ethanol medium; supporting figures and table (PDF) 4257

DOI: 10.1021/acs.jpclett.6b02045 J. Phys. Chem. Lett. 2016, 7, 4253−4258

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Water contact angle measurement of the fiber membrane wetted by ethanol (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (21236003, 21322607, 21406072, 21471056, 21676093 and 91534202), Shanghai Educational Development Foundation (14CG29), the Basic Research Program of Shanghai (15JC1401300), the Key Scientific and Technological Program of Shanghai (14521100800), the International Science and Technology Cooperation Program of China (2015DFA51220), Project funded by China Postdoctoral Science Foundation (2014M560307, 2015T80408), Program for New Century Excellent Talents in University (NCET-13-0796).

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DOI: 10.1021/acs.jpclett.6b02045 J. Phys. Chem. Lett. 2016, 7, 4253−4258