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Rationally Designed an Efficient Dual-Mode Colorimetric/Fluorescence Sensor Based on Carbon Dots for Detection of pH and Cu2+ Ions Liang Wang, Ming Li, Weitao Li, Yu Han, Yijian Liu, Zhen Li, Baohua Zhang, and Dengyu Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01625 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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ACS Sustainable Chemistry & Engineering

Rationally

Designed

an

Efficient

Dual-Mode

Colorimetric/Fluorescence Sensor Based on Carbon Dots for Detection of pH and Cu2+ Ions Liang Wang,*,† Ming Li,† Weitao Li,† Yu Han,† Yijian Liu,‡ Zhen Li,§ Baohua Zhang, ‡,ǁ †

and Dengyu Pan‡

Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical

Engineering, Shanghai University, No.99 Shangda Road, BaoShan District, Shanghai 200444, P.R. China ‡

Department of Chemical Engineering, School of Environmental and Chemical

Engineering, Shanghai University, No.333 Nanchen Road, BaoShan District, Shanghai 200444, P.R. China §

Shanghai Institute of Applied Radiation, Shanghai University, No.333 Nanchen Road,

BaoShan District, Shanghai 200444, P.R. China ǁ

Key Laboratory of Science & Technology of Eco-Textile, Ministry of Education,

Donghua University, No. 2999 People's North Road, Songjiang District, Shanghai 201620, P. R. China To whom correspondence should be addressed: Tel: +86-21-66135276. *E-mail: [email protected] (L. Wang). ABSTRACT: A dual-mode colorimetric/fluorescence pH sensor was fabricated from carbon dots (CDs). It exhibited a pH response via both fluorescence and visible colorimetric observation. The CDs solution changed from red to yellow when the pH changed from acid to alkaline. Meanwhile, the color of pH test paper that incorporated the CDs changed from purple-red to orange and then to yellow. The CD fluorescence maximum was at 630 nm in acid solution and 590 nm under neutral and alkaline conditions. The fluorescence of the pH test paper changed from red to orange and then to yellow, which was better than that of the pure CDs solution. The CDs exhibited highly selective and reversible fluorescence quenching with respect to Cu2+ ions because of strong binding and fast chelating kinetics. There was a linear

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relationship between fluorescence quenching and Cu2+ concentration, suggesting the other promising practical usage of this sensing system. The versatile CDs-based pH sensor thus provides a basis for developing sustainable and fast-response dual-mode pH meters as well as Cu2+ sensing. KEYWORDS: Dual-mode, Colorimetric/fluorescence sensor, Carbon dots, pH sensor, pH test paper, Cu2+ ions INTRODUCTION Despite the robust performance of electrochemical pH-sensors, optical pH sensors based on fluorescent materials have advantages such as high sensitivity and direct visual (naked-eye) observation1-23. Optical pH sensors have been based on organic dyes1-3,

semiconductor

quantum

dots4-11,

nanoparticles12-19,

and

fluorescent

proteins20-23. However, semiconductor quantum dots and proteins generally require multi-step, complex preparations4-11,20-23. Sensors based on polymers and small molecules often have inadequate optical or thermal stabilities, and are often toxic1-3, which limits applicability. It is therefore important to develop stabile and environmentally-friendly materials that determination pH values using both colorimetry and fluorescence. Carbon dots (CDs) and graphene quantum dots (GQDs) are optical pH sensors with large extinction coefficients and high fluorescence quantum yields. They are also environmentally friendly and have quick responses, high sensitivities, and reversible behaviors24-30. CDs absorption and fluorescence strongly depend on pH because of cationic, neutral, and anionic forms in aqueous solution24-30. Shangguan et al.24 reported on the use of fluorescent CDs for intracellular ratiometric pH sensing. The CDs exhibited emission at 475 and 545 nm and the intensity ratio (I475/I545) was linear with pH values ranging over 5.2-8.8. Wang et al.25 investigated multi-doped CDs as ratiometric probes for monitoring pH variations during enzyme-catalyzed reactions. Park et al.27 investigated block-copolymer-integrated GQDs that simultaneously sensed temperature and pH, and had dose-dependent responses to various metal ions with excellent reversibility and stability. Despite the advantages of optical pH sensors based on CDs or GQDs, dual-mode ACS Paragon Plus Environment

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colorimetric and fluorescence sensing has not been deeply explored. In particular, the dual-mode colorimetric/fluorescence pH response of CDs or GQDs-based sensors has not been reported. Here, a versatile hydrothermal route using 1,2,4-triaminobenzene as a precursor in NaOH solution was used to fabricate CDs-based pH optical sensor that exhibited dual-mode colorimetric/fluorescence responses. The pH-dependent orange and red fluorescence was demonstrated for two different pH values. Most importantly, the synthesized CDs exhibited high binding selectivity for Cu2+ ions that induced quenching. A linear relationship between the quenching efficiency and the Cu2+ concentration was observed. RESULTS AND DISCUSSION

Scheme 1. Schematic illustration of the preparation procedure for the CDs. The CDs were synthesized by a one-pot hydrothermal handy way using 1,2,4-triaminobenzene and NaOH (Scheme 1). The 1,2,4-triaminobenzene provided carbon and nitrogen. The NaOH was a catalyst for the molecular fusion of 1,2,4-triaminobenzene and enabled oxygen doping. The product CDs had abundant -OH, -NH2, N-C, N-O, and C=O groups on the surface, which was corroborated with fourier transform infrared (FT-IR) and x-ray photoelectron (XPS) spectra in Figure 2a and Figure 3.



Figure 1. (a) TEM images and lateral size distributions of CDs. (b) High-resolution TEM images of CDs (Inset: fast fourier transform patterns). (c) AFM images of CDs (Inset: height distributions and height profile along the white line). (d) 3D lines of height of CDs. The CDs morphologies were characterized with transmission electron microscopy (TEM) and atomic force microscopy (AFM). As shown in Figure 1a, the CDs were mainly spherical and were well-dispersed in the aqueous solution. The size distribution was narrow, with a range of 0.4-2.6 nm and an average size of 1.5 nm (inset in Figure 1a). In high-resolution TEM images (inset of Figure 1b), the 0.22 nm lattice spacing corresponded to the (100) lattice planes of graphite. AFM images in Figure 1c revealed the CDs dispersion on a mica sheet. Figure 1d is a three-dimensional (3D) plot of Figure 1c, where the height ranged over 0.4-3.0 nm. This indicated that the CDs were equivalent to several layers of graphite.

C-N C-OH C=O C=C O-H

D G

Intensity (a. u.)

b

c Intensity (a. u.)

a Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

N-H

4000 3500 3000 2500 2000 1500 1000 500 800 Wavelength (cm-1)

1200 1800 2200 2400 Ramam shift (cm-1)

0


10 20 30 40 50 60 70 80 90 2θ (degree)

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Figure 2. FT-IR spectra (a), Raman spectra (b) and XRD patterns (c) of CDs. As shown in Figure 2a, the presence of amino and hydroxyl functional groups was observed with FTIR. The two broad absorption bands at 3430 cm-1 and 3220 cm-1 could be attributed to the stretching and bending vibrations of O-H and N-H, respectively31,32. The peaks at 1629 cm-1, 1139 cm-1, and 1029 cm-1 were C=O, C-OH, and C-N vibrations, respectively33-35. In addition, the C=C stretching vibrations at 1444 cm-1 were also observed36. In the Raman spectrum in Figure 2b, two exceptional peaks were appeared at 1387 cm-1 and 1555 cm-1, which agreed with the D and G bands of CDs, respectively37. Figure 2c shows the CDs phase structures via X-ray diffraction (XRD) analysis. The (002) CDs interlayer spacing was 3.36 Å36. a

b

C1s

N1s

400 600 200 Binding Energy (eV)

c

390

0 N1s

C-C C-O C=O

275

N-C N-O

400 405 410 415 Binding Energy (eV)

280 285 290 295 Binding Energy (eV)

d

NH2

395

C-N

420

300 O1s

Intensity (a. u.)

800

O1s

Intensity (a. u.)

Intensity (a. u.)

C1s

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O=C

O-N

O-C

525

530 540 535 Binding Energy (eV)

545

Figure 3. Survey XPS spectra (a), high-resolution C1s (b), N1s (c) and O1s (d) spectra of CDs. XPS spectra in Figure 3a revealed 66% C, 19.2% N, and 14.8% O in the CDs. Deconvolution of the C1s peak in Figure 3b exhibited four characteristic peaks at 284.8 eV, 285.9 eV, 287.0 eV, and 288.7 eV for C-C, C-N, C-O, and C=O, respectively. As highlighted in Figure 3c, three classes of N-related bonding were observed, including NH2 (399.5 eV), N-C (401.1 eV), and N-O (402.8 eV). The O1s spectrum (Figure 3d) had binding energies at 531.6 eV, 533.0 eV, and 533.8 eV that ACS Paragon Plus Environment


corresponded to O-C, O=C and O-N, respectively. The XPS data thus verified that the CDs had NH2, -OH, -COOH, and -N-OH functional groups on the surface.

Figure 4. The 3D UV-vis absorption spectra (a) and fluorescence spectra (b) of the CDs at different pH values. The ABS intensity ratio changes (c) and emission intensity ratio changes (d) varied with pH. (e) Photographs of the CDs solutions at different pH values under visible light (up) and UV lamp irradiation (down). (f) Photographs of pH test paper loaded with CDs at different pH values under visible light (up) and UV lamp irradiation (down). Figure 4 reveals the CDs colorimetric and fluorescence changes with pH. At pH < 7.0, large changes in the 500-nm absorption peak were observed. Additionally, the 440-nm absorption maximum decreased with increasing pH in the range 7-14 (Figure 4a). The visible color of the CDs solution changed from red to yellow with increasing pH (Figure 4e). A quantitative analysis (Figure 4c) was performed based on the ratio I500/I440 between the 500-nm and 440-nm absorption intensities at various pH values. ACS Paragon Plus Environment

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The ratio decreased with increasing pH. The pH-dependent fluorescence response is shown in Figure 4b. Under acidic conditions (pH 1-6), 630-nm emission was obtained with 510-nm excitation. For pH ranging over 7-14, the fluorescence emission slightly decreased and was blue-shifted to 590 nm. The PL intensity ratio (I630/I590) was plotted as a function of pH in Figure 4d. It required nonlinear curve fitting over the pH range 1-8. However, the I630/I590 ratio displayed no significant changes for pH 9-14. The CDs PL emission exhibited no significant dependence on excitation wavelength (Figure S1). It exhibited a slight 20-nm red-shift with red-shifted excitation. Furthermore, the PL decay (Figure S2) revealed a single-exponential with a 4.6-ns lifetime. Fluorescence of the CDs solution under 365-nm excitation changed from red to orange and then to yellow with increased pH (3-13) in the aqueous solution (Figure 4e). Under extremely acidic or alkaline conditions, the fluorescence gradually faded. As shown in Figure 4f, the color of pH test paper loaded with CDs under visible light varied from purple-red to orange and then to yellow with increasing pH over 1-14. Whereas, under UV lamp irradiation, the fluorescence changed from red to orange and then to yellow with increasing pH. The fluorescence faded under extremely acidic conditions, which was better than the pure CDs solution. In going from an acidic to a basic environment, the OH and COOH groups change to O- and COO-, which results in the observed fluorescence changes29 (Figure S3). The negative charge on the surface of the CDs in basic solution was revealed with zeta-potential measurements (Figure S4). The pH sensitivity may allow CDs to be used as dual-mode colorimetric/fluorescence probes.



Figure 5. (a) The fluorescent responses to various metal ions. (b) Fluorescence emission spectra of CDs (0.1 mg·mL-1) with the presence of different Cu2+ concentration. The UV-vis absorption spectra (c) and photographs (d) of the CDs with absence and presence of Cu2+ ions. Response surface (e) and 2D contour plots (f) for interaction between initial Cu2+ concentration and CDs concentration. Experimental conditions: pH = 7, at room temperature. Apart from pH sensing, the CDs exhibited metal-ion selectivity as revealed by fluorescence quenching. Various metal cations including Mg2+, Ni2+, Cu2+, Na+, Ca2+, Ba2+, Pb2+, Mn2+, Cd2+, Zn2+, K+, Ag+, and Li+ were tested. As shown in Figure 5a, only Cu2+ significantly quenches the PL intensity. The probable fluorescence quenching mechanism was most likely associated with strong binding and rapid chelaton kinetics of Cu2+ ions with the N and O functional groups on the CDs32,38. In the chelating interaction, the separation of the photoexcited electron-hole pair in the CDs resulted in rapid electron transfer to the Cu2+39-41. In Figure 5b, quantitative determination of the Cu2+ concentration was examined. When the initial CDs concentration was 0.1 mg·mL-1, the 590-nm PL intensity decreased with increasing Cu2+ concentrations. Figure 5d shows a photograph of a CDs solution (0.1 mg·mL-1) under visible and 365-nm light excitation for various Cu2+ concentrations. The orange CDs fluorescence gradually decreased with increasing Cu2+ concentrations. Additionally, the color of the CDs solution did not


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visibly change because of its pH stability in the presence of Cu2+. For a 200-µM Cu2+ concentration, the orange fluorescence of the CDs solution was almost completely quenched. A good linear relationship could be achieved between the Cu2+ concentration and the quenching efficiency I0-I/I0, where I0 and I were the PL intensities at 590-nm excitation in the absence and presence of Cu2+, respectively. The Cu2+ concentration was in the range 0-200 µM, and the linear regression equation had a correlation coefficient of 0.998. This was also observed in a solution with a pH ranging over 4-10 (Figure S5). Additionally, the detection limit under various pH values was examined. As shown in Table S1, the detection limit increased from 40 µM to 250 µM, indicating that the CDs fluorescence was easier to quench in acidic conditions than in basic conditions. The relative fluorescence intensity ratio in 40-µM Cu2+ was investigated under different pH conditions. The ratio increased when the pH changed from basic to acidic conditions (Figure S6). Furthermore, the ratio exhibited changes under pure acidic and pure basic conditions. Absorption and PL spectra of the CDs in the absence and presence of Cu2+ are shown Figures 5b and 5c, respectively. There were no clear shifts, and the intensity of the absorption spectra changed slightly. In contrast, the PL intensity changed with increasing Cu2+ concentration. Thus, the existence of Cu2+ in CDs solutions just affects the fluorescence intensity, without changing the absorbance. Parameters such as the initial CDs and Cu2+ concentrations affected detection. As shown in Figure 5b and Figure S7, the quenching of CDs PL was favored at low initial CDs concentrations because there was less to be quenched by Cu2+ at a known concentration. The addition of a suitable amount of Cu2+ was equally effective. Thus, when the initial CDs concentration was 0.03 mg·mL-1, the best quenching efficiency was accomplished with 200-µM·mL-1 Cu2+. A 3D response surface graph and a two-dimensional (2D) contour plot of the CDs quenching effect were plotted in Figures 5d, e, respectively. From the quality of the self-reliant variable coefficients in the model42, the main factors affecting the response were (in order): initial CDs concentration > Cu2+ concentration. As shown in Figure 5e, the interactions between the two meaningful factors were defined by the shape of ACS Paragon Plus Environment


the response surface curves. The results visually clarified a relationship between the initial CDs concentration and the quenching efficiency.

1.2 1.0

PL I/I0 (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.8 0.6 0.4 0.2 0.0 -0.2 1

2

3

4

5

6

7

Repeat Cycles

Figure 6. Reversible cycle of CDs with Cu2+ and EDTA by PL intensity changes. Reversible binding of the CDs to Cu2+ ions was examined via fluorescence titrations with various anions. Only EDTA bleached the “signal-on” emission band for Cu2+ ions; i.e., the fluorescence intensity was completely recovered by adding EDTA. The reversibility was observed for at least four cycles (Figure 6), confirming that the CDs were reusable for Cu2+ sensing. CONCLUSIONS In conclusion, a dual-mode colorimetric/fluorescence CDs-based pH sensor was fabricated. The pH values were determined instantly via both fluorescence and visible colorimetric changes. Under visible light, pH test paper incorporating CDs changed from purple-red to orange and then to yellow with increasing pH over the range 1-14. Similarly, under UV excitation, their fluorescence changed from red to orange and then to yellow. The CDs exhibited high selectivity in the binding of Cu2+ ions, which resulted in fluorescence quenching. This was due to a strong binding affinity and quick chelating kinetics that was reversible. The CDs quenching was linear with the Cu2+ concentration, indicating another potential application of this sensing system. Parameters such as the initial CDs and Cu2+ concentrations greatly affected the performance. The main factor was the CDs concentration, followed by the Cu2+ concentration. This dual-mode colorimetric/fluorescence pH sensor expanded the potential of sustainable and rapid-response pH meters and metal-ion detection.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCID Liang Wang: 0000-0002-3771-4627 ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (No. 21671129, 21571124, 21671131), the Shanghai Sailing Program (No. 16YF1404400), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT17R71). We thank the Laboratory for Microstructures of Shanghai University. REFERENCES (1) Zhang, X.; Rehm, S.; Safont-Sempere, M. M.; Wuerthner, F. Vesicular perylene dye nanocapsules as supramolecular fluorescent pH sensor systems. Nat. Chem. 2009, 1, DOI 10.1038/nchem.368. (2) Gotor, R.; Ashokkumar, P.; Hech, M.; Keil, K.; Rurack, K. Optical pH Sensor Covering the Range from pH 0-14 Compatible with Mobile-Device Readout and Based on a Set of Rationally Designed Indicator Dyes. Anal. Chem. 2017, 89, DOI 10.1021/acs.analchem.7b01903. (3) Jokic, T.; Borisov, S. M.; Saf, R.; Nielsen, D. A.; Kuehl, M.; Klimant, I. Highly Photostable Near-Infrared Fluorescent pH Indicators and Sensors Based on BF2-Chelated Tetraarylazadipyrromethene Dyes. Anal. Chem. 2012, 84, DOI



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For Table of Contents Use Only

Carbon dots as an efficient dual-mode colorimetric/fluorescence sensor was fabricated by a facile hydrothermal strategy for detection of pH and Cu2+ ions.


Fluorescence

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