Photostable and Low-Toxic Yellow-Green Carbon Dots for Highly

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Functional Nanostructured Materials (including low-D carbon)

Photo-stable and Low-toxic Yellow-green Carbon Dots for Highly Selective Detection of Explosive 2,4,6Trinitrophenol Based on Dual Electron Transfer Mechanism Bo Ju, Yi Wang, Yu-Mo Zhang, Ting Zhang, Zhihe Liu, Minjie Li, and Sean Xiao-An Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02330 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Applied Materials & Interfaces

Photo-stable and Low-toxic Yellow-green Carbon Dots for Highly Selective Detection of Explosive 2,4,6-Trinitrophenol

Based

on

Dual

Electron

Transfer Mechanism Bo Ju,1 Yi Wang,2 Yu-Mo Zhang,1 Ting Zhang,1 Zhihe Liu,3 Minjie Li,1,* and Sean Xiao-An Zhang1 1

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun 130012, P.R. China. 2

Research Center of Energetic Material Genome Science, Institute of Chemical Materials, China

Academy of Engineering Physics (CAEP), Mianyang, 621900, P.R. China. 3

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and

Engineering, Jilin University, Changchun, Jilin 130012, P.R. China.

KEYWORDS fluorescent carbon dots, highly selective detection, fluorescence quenching mechanism, electron transfer, environmental sensor applications

ACS Paragon Plus Environment

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ABSTRACT

Advances in the development of fluorescent carbon dots (CDs) for detecting nitro-explosives have attracted great interest. However, developing long-wavelength luminescence CDs for highly selective determination of 2,4,6-Trinitrophenol (TNP) and getting insight into the detection mechanism remain further to be investigated. Here, excitation independent yellowgreen emission CDs with good photostability and low biotoxicity were introduced for detecting TNP selectively. Then, two types of electron transfer (ET) processes including hydrogen-bond interaction assisted ET and proton transfer assisted ET are suggested to be responsible for their photophysical behavior. Finally, the visual detection of TNP has been successfully developed by a CDs-based indicator paper. The facile, highly sensitive and selective detection for TNP in both of a solution and a solid phase makes CDs potentially useful in environmental sensor applications.

INTRODUCTION

Although peace and development are the themes of our times, the incessant regional bombing terrorist attacks in public places (such as airports, metro stations and business centers) heavily threatens national and global security.1-3 The reliable and rapid detection of chemical explosives has become a burning issue in nowadays.2 The present widely-used industrial explosives are mostly polynitro compounds, such as 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol (TNP), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX),

2,4,6-triamino-1,3,5-trinitrobenzene (TATB),

1,3,5,7-tetranitro-1,3,5,7-tetrazocan (HMX) and so on. Among them, TNP is one of the most powerful explosives and have been extensively used in landmines for decades.4 However, the wide use and relatively high water solubility of TNP have made it a significant environmental


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pollutant now.5-6 Therefore, it is of great significance to develop simple and effective ways for detecting TNP sensitively and selectively.

Among various detecting methods, fluorescence-based sensors on explosives have become more and more popular owing to their high sensitivity, portability, rapid and easy manipulation. From traditional fluorescent organic molecules/conjugated polymers, to new metal/covalentorganic frameworks and chemically-modified CdSe/ZnS quantum dots/Ag nanocluster, several fluorescent detecting platforms for explosive TNP with have been established.7-16 But these fluorescent materials generally suffer from some drawbacks such as sophisticated synthetic procedures, high toxicity, and poor photostability. In contrast, CDs as a versatile member of fluorescent nanomaterial, have the fascinating advantages of facile preparation, good photostability and low biotoxicity and been widespread applications in bioimaging,17-20 photovoltaic cells,21-24 information security systems,25-28 medical diagnosis29-32 and light-emitting diodes33-35. In recent years, some new tries of carbon dots for detecting explosive TNP have been performed. However, these related studies are still in infancy stage, only limited successful examples of carbon dots with short-wavelength blue emission are used to detect explosive TNP and the detected mechanism are mostly focused on Forster resonance energy transfer (FRET) and unclear ET.36-38 Thus, there is an urgent demand to develop new CDs materials for high detection of explosive TNP, accompanied with further investigating the photophysical mechanism between CDs and explosive TNP. Herein, we have synthesized a new excitation independent yellow-green emission CDs with high photo-stability and low biotoxicity by a simple solvolthermal method from chloroform and o-phenylenediamine. These as-prepared CDs exhibit highly selective detection towards TNP by fluorescence quenching mode with a remarkably high fluorescence quenching efficiency (~


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97.6%), the detection limit can reach 2 µM. This high selectively detecting TNP in the presence of other nitro explosives can be ascribed to the special dual ET processes including hydrogenbond interaction assisted ET and proton transfer assisted ET between TNP and CDs. A good compatibility of high selective detection on TNP in both solution and solid media endows these yellow-green CDs with great potentials for TNP detection in environmental monitoring.

RESULTS AND DISCUSSION As we know, the CDs have a great diversity in structure (chemical compositions, crystallinity, sizes) depending on the synthetic methods, therefore it can be understood that various CDs with different surface functional groups may have many highly potential applications. Here, CDs were synthesized by a facile solvolthermal route using chloroform and o-phenylenediamine as precursors (see the Experimental Section). The optical properties of the as-prepared CDs are shown in Figure 1. The UV-vis spectrum of CDs exhibited absorption peak at 427 nm (Figure 1a). In the fluorescence spectra, the CDs show excitation-independent photoluminescence (PL) behavior with emission wavelength at 536 nm (Figure 1b). The CDs in ethanol solution exhibited yellow-green color under a 365 nm UV lamp (Figure 1a). And a much stronger yellow-green PL could be observed from the CDs prepared by doping 3.7 mmol ophenylenediamine (Figure S1a). The PL quantum yield of CDs was determined to be 13.2 % under the optimal excitation wavelength (λ=450 nm, based on both of PL emission and excitation spectra; see Figure S1b). Then the PL decay curve of CDs, as shown in Figure S2, can be fitted by a mono-exponential function with lifetime of 4.54 ns.39 The as-synthesized CDs have good solubility in different solvents, which can be observed that solvent-dependent tunable blue to orange-yellow emission dependent on the variation of solvent polarity with maximum 71 nm red-shifts (Figure S3a), and this phenomenon has been reported in other work, which is due to


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the nonspecific (e.g., dipole-dipole) and specific (e.g., hydrogen-bond) interactions between CDs and solvent environment.40 In addition, the emission intensity of CDs became decay with the variation of solvent polarity, which is attributed to the increase of nonradiative energy loss of excited state CDs through the interactions between CDs and solvent molecules (Figure S3b). Meanwhile, the change of the emission intensity of CDs is irregular, which is due to other interactions (such as π-π stacking interaction, etc) among the complicated structural CDs. And the CDs also have excellent photostability (Figure 1c), only slight attenuations are found after 8h continuous 365 nm UV light illumination (12 W). Compared with other traditional sensors, the CDs exhibited much lower biotoxicity, which can be good candidate as fluorescent probe in practice (Figure 1d).

Figure 1. (a) UV-vis absorption spectra of CDs in ethanol solution. Inset: photographs of CDs in ethanol under visible (left) and UV (right) light. (b) Fluorescence spectra of CDs with excitation-


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independent PL behavior. (c) Photostability measurements of CDs solution. (d) Effects of CDs on the viability of MCF-7 cells.

Transmission Electron Microscopy (TEM) image presented in Figure 2a reveals the CDs were well dispersed. By statistics of more than 110 particles, the CDs have a broad size distribution with the size of about ~5-30 nm (Figure 2b). High-resolution TEM image (HRTEM) of CDs shown the amorphous structure without well-resolved lattice fringes (Figure 2a).41 Due to their disordered carbon structure, no typical G bands were observed in the Raman spectra of CDs (Figure S4).41 Compared with the raw material o-phenylenediamine, more complicated structure was observed in 1H NMR spectra of CDs (Figure S5a, 5b), indicating derivative(s) of ophenylenediamine has been generated with complicated reaction routes as we have shown in our previous work.18 There are only two molecular weight of m/z= 282.2787 and its double m/z= 563.5511 are observed in mass spectra of CDs, which indicates the composition of CDs is relatively simple (Figure S5c). Although we haven’t so far figured out the molecular formula with the information of NMR and mass spectra, we suggest that the CDs are formed by molecular aggregates of some o-phenylenediamine derivatives via intermolecular interactions, which is similar to the reported polymer-like CDs through polymerizing small molecule/polymer precursors with multiple active species.42 Then the PL is due to the molecular fragments in these aggregates, that’s why our CDs do not show excitation dependent emissions.41


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Figure 2. (a) TEM image of CDs. Scale bar = 100 nm. Inset: HRTEM image of CDs. Scale bar = 10 nm. (b) Diameter distribution of CDs by statistics of more than 110 particles. The chemical bonds and functional groups on CDs are further investigated. In Fourier transform infrared (FT-IR) spectra, the following were observed: stretching vibrations of N-H and O-H at 3192 cm-1, 3305 cm-1 and 3396 cm-1, stretching vibrations of C-H at 2851 cm-1, 2921 cm-1 and 2961 cm-1, the vibrational absorption band of C=O and C=N at 1710 cm-1 and 1647 cm1

, stretching vibrations of C=C and C-N at 1473 cm-1 and 1261 cm-1, asymmetric and symmetric

stretching vibrations of C-O at 1098 cm-1 and 1026 cm-1, deformation vibration of N-H at 803 cm-1 (Figure S6).18,31 X-ray photoelectron spectroscopy (XPS) survey further confirm the FT-IR results. Carbon (C 1s, 284 eV), nitrogen (N 1s, 399 eV), oxygen (O 1s, 532 eV) are elemental compositions in CDs (Figure 3a).17 The high-resolution C 1s spectra can be deconvoluted into three peaks (Figure 3b), which are attributed to C-C groups (284.7 eV), C-N/C-O groups (285.8 eV) and C=O/C=N groups (287.7 eV).26 High-resolution N 1s spectra reveal the existence of CN/C=N groups (399.5 eV) (Figure 3c). The O 1s spectrum exhibits two peaks at 531.9 eV and 532.9 eV for C=O groups and C-O groups, respectively (Figure 3d).43 Hence, these CDs with various oxygen and nitrogen-related surface states have great potentials in detecting hazardous substances by supramolecular interactions.


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Figure 3. (a) XPS survey of CDs. (b-d) High-resolution XPS C 1s, N 1s, and O 1s spectra, respectively. Although the specific luminescence mechanism of CDs has always been debated, the fascinating optical properties of CDs continuously motivate researchers to explore their meaningful applications. In this work, the selective detection of TNP in the presence of other nitro-explosives by yellow-green emission CDs could be found. In Figure 4a, the emission intensity of CDs can be quenched by adding different concentrations of TNP. Inset photographs also exhibited that the yellow-green fluorescence of CDs became gradually weak with the increase of TNP concentration (Figure 4a). However, the fluorescence emission of CDs in the presence of other nitro-explosives (TATB, HMX, TNT, RDX) have negligible change (Figure


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S7). Therefore, compared with other nitro-explosives, the fluorescence quenching percentage of CDs upon addition TNP is extraordinarily high (~ 97.6%, Figure 4b). Further, the fluorescence quenching efficiency for TNP was analyzed using Stern-Volmer plots (Figure S8). Nonlinear curve were fitted by an exponential quenching equation: I0/I =Aexp(k[α]) + B, where A, B and k are constants.44-45 The result exhibits that the plots can be fitted to I0/I = 0.2775exp(0.0833[α])0.1461. The quenching constant value (Ksv) was calculated to be 2.31 x 104 M-1 and the detection limit was estimated to be 2 µM. Besides, the obvious color change of CDs solution with the addition of TNP, indicating the fast and naked-eye sensitive detection for TNP could be also observed in visible light (Figure S9).

Figure 4. (a) Fluorescence spectra of CDs in the presence of different concentrations of TNP. Inset: photographs of CDs by adding different contents of TNP. (b) Fluorescence quenching percentage of CDs upon addition different nitro-explosives, indicating high selectivity for TNP.

Because the reported CDs for TNP detection are all short-wavelength blue emission, which shows good spectral overlap between UV absorption spectra of TNP and the fluorescence emission spectra of CDs, the present popular detection mechanism of CDs on TNP are FRET.36-


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But for our as-prepared CDs, their emission is long-wavelength yellow-green color, obviously

non-overlapping with the UV-vis absorption spectra of TNP (Figure S10). A new detection mechanism of our as-prepared CDs on TNP should be addressed. To investigate the photophysical mechanism between as-prepared CDs and TNP, we have utilized cyclic voltammetry (CV) and theoretical simulations to obtain the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of CDs and nitro-explosives (Figure S11).13,41,46 The energy gap of CDs comes from the molecular o-phenylenediamine derivatives. As shown in Figure 5, the lowest unoccupied molecular orbital (ELUMO = -3.82 eV) levels of CDs is obviously higher than that (ELUMO = -3.92 eV) of TNP, as a result, excited electrons of CDs in its LUMO could easily be transferred to the LUMO of TNP to finally cause the fluorescence quenching. In contrast, LUMO energy levels of other nitro-explosives are all higher than that (ELUMO = -3.82 eV) of CDs, the ET process from CDs to other nitro-explosives are hardly taking place and CDs show no obvious fluorescence changes upon the addition of other nitro-explosives. Thus, the high sensitivity and selectivity of CDs towards TNP can be assigned to their efficient ET.


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Figure 5. HOMO-LUMO energy levels of CDs, TNP, TATB, HMX, TNT and RDX, respectively. Further ET behavior between CDs and TNP are investigated by UV-vis absorption and FTIR spectra. Upon adding small amount of TNP (less than 2×10-5 mol/L), the absorption peak at 427 nm of CDs is gradually blue-shift with appearance of characteristic absorption peak of TNP at 357 nm, indicating TNP mostly takes neutral forms in such low concentration (Figure 6a,6b). The blue-shift of the absorption peak at 427 nm of CDs might come from the hydrogen bond interaction between the -OH groups of TNP and the -NH2 groups of CDs (Figure S12a), which could be confirmed by the fact that the stretching vibrations of N-H/O-H (3396 cm-1) in FT-IR spectrum of CDs-TNP mixture is obviously blue-shift compared with single CDs (Figure S13a).47 In low concentration of TNP, the fluorescence quenching mechanism can be described as hydrogen-bond interaction assisted ET.48-49 Upon adding high concentration of TNP (more than 2×10-5 mol/L), two new absorption shoulders at 453 nm and 480 nm appears, which are well identical to the absorption spectra of protonated CDs (Figure 6a, 6b). This indicates that partial TNP have transferred the proton to CDs in high concentration of TNP (Figure S12b, 14). The protonated structure of CDs can be confirmed in FT-IR spectrum by the disappearance of the stretching vibrations of N-H at 3305 cm-1 and the occurrence of the corresponding protonated structure at 1658 cm-1 in CDs-TNP mixture (Figure S13b).50


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Figure 6. (a) UV-vis absorption spectra of CDs with the addition of different contents of TNP. (b) Comparison the UV-vis absorption spectra between CDs + TNP, TNP and CDs + acid. Although the emission intensity of CDs can also be quenched by adding different concentration trifluoroacetic acid (Figure S15, 16), the fluorescence quenching efficiency for TNP is much higher than trifluoroacetic acid (Figure S17) with adding the same concentration of trifluoroacetic acid (pKa = -0.23) and TNP (pKa = 0.38), which indicates that the effect of protonation of the CDs and ET process (denote as proton transfer assisted ET) could be also responsible for fluorescence quenching of TNP on CDs besides hydrogen-bond interaction assisted ET. After TNP give proton to CDs, the corresponding orbital levels of deprotonated TNP (TNP-) and protonated CDs (CDs+H) have altered to 0.75 eV (LUMO level) and (-2.85 eV (HOMO level) for TNP-, -4.33 eV and -6.58 eV for the LUMO and HOMO energy levels of CDs+H (Figure S18, S19). The HOMO energy level of TNP- (-2.85 eV) is higher than the LUMO energy level of CDs (-3.82 eV), indicating the possibility of ET from TNP- to CDs and lead to the fluorescence quenching. And other phenolic nitro-aromatics with acidity (mnitrophenol, p-nitrophenol and dinitrophenol) could hardly quench the fluorescence of CDs (Figure S20). Even though the variation of acidic environments, the CDs also exhibited


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absolutely fluorescence quenching towards TNP (quenching efficiency~ 98%), compared with other phenolic explosive (Figure 21). Similarly, LUMO energy levels of m-nitrophenol, pnitrophenol and dinitrophenol are also higher than that of CDs, the ET process from CDs to them are hardly taking place (Figure S22). Then, a series of interference experiments in the presence of analogous phenolic nitroaromatics were further performed to illustrate the selective detection of CDs towards TNP. Figure S23 showed the negligible effect on the emission intensity of CDs by the addition of m-nitrophenol, p-nitrophenol and dinitrophenol, however, an obvious fluorescence quenching happened after the addition of TNP into the mixture of CDs and phenolic nitroaromatics. And the CDs also exhibited highly sensitive detection on TNP with high fluorescence quenching efficiency (>97 %). Based on above discussion, we can clearly disclose that the highly efficient fluorescence quenching of TNP on CDs comes from the dual ET modes by hydrogen-bond interaction assisted ET and proton transfer assisted ET (Scheme 1).

Scheme 1. Schematic of the detection mechanism for TNP. To demonstrate the wide practicability of CDs on TNP detection, an indicator paper for the visual detection of TNP has been constructed by immobilization of the CDs on filter paper. And


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the highly selective detection for TNP was carried out in solid phase (Figure 7a). Because of the good solubility of CDs, the indicator paper can be used for single test (Figure S24). However, the CDs-based detection papers prepared by uniform manipulation have good reproducibility in the fluorescence properties and better repeatability on detecting TNP (Figure S25). In order to further investigate the applicability of CDs-based indicator paper, competition experiments of TNP mixed with other nitro-explosives were performed. Figure 7b shows the sensitive detection for TNP among different explosives can also be successfully achieved. As fluorescent sensors, the CDs exhibited lower cyctoxicity. Therefore, the as-prepared CDs have great potentials for TNP detection in environmental monitoring.

Figure 7. Indicator papers immoblized with CDs for the visual detection of nitro-explosives. (a) Photographs of CDs-based indicator papers with analysis of different nitro-explosives in solid phase. (b) with analysis of mixed nitro-explosives. CONCLUSIONS


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In conclusion, we have reported a facile solvolthermal method for preparing excitationindependent yellow-green emission CDs from chloroform and o-phenylenediamine. The assynthesized CDs exhibit good photostability, low biotoxicity and better solubility, which can be dispersed in different polar solvents and show tunable blue to orange-yellow emission. Meanwhile, as a fluorescent probe, the CDs don’t need further chemical modification and have high sensitivity for TNP over other nitro-explosives. The prominent selective detection of TNP can be attributed to the presence of two specific ET processes (hydrogen-bond interaction assisted ET and proton transfer assisted ET) through spectroscopic studies. Then, this high selectivity for TNP was further proved by theoretical calculations. Moreover, an indicator paper for the visual detection of TNP has been constructed successfully by immobilization of the CDs on filter paper. Our work will pave the way towards the development of sensing environmental hazards by fluorescent carbon-based materials.

METHODS AND EXPERIMENTAL SECTION Materials.

o-phenylenediamine was purchased from Energy Chemical Company. Chloroform was purchased from Beijing Chemical Company. Polynitro explosives (TNT, TNP, TATB, RDX, HMX) were obtained from Research Center of Energetic Material Genome Science, Institute of Chemical

Materials,

China

Academy

of

Engineering

Physics.

Tetrabutylammonium

hexafluorophosphate (TBAPF6) was purchased from Aladdin Reagent Company. 3-(4,5Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) were obtained from Beijing Dingguo Company. TBAPF6 was recrystallized from ethanol and dried under vacuum at 50 °C. Other chemicals were used as received without further purification.


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Instruments and Characterization.

Fluorescence spectroscopy was measured using a Shimadzu RF-5301 PC spectrometer. UVvis absorption spectra was characterized in a 1 cm cell using Shimadzu UV-2550 spectrometer. The size and morphology of CDs were recored by a JEM-2100F electron microscope at an acceleration voltage of 200 kV. FT-IR spectrum were measured on a BRUKER 80/80v spectrometer using KBr as reference. XPS investigation was carried out using ESCALAB 250 spectrometer with a mono X-ray source Al Kα excitation. The fluorescence quantum yields (Фf) and lifetime were measured on FLS 920 lifetime and steady state spectrometer. Raman spectrum were performed on a Renishaw Raman system model 1000 spectrometer with radiation at 633 nm. 1H NMR spectra was recorded on a 500MHz BrukerAvance. CV were obtained from Biologic electrochemical work station. Preparation of CDs.

0.4g o-phenylenediamine (3.7 mmol) was dissolved in chloroform (20 mL). The mixture was then transferred into a Teflon autoclave and heated at 160 °C for 12 h. After the autoclave was cooled to room temperature naturally, the supernatant was concentrated and then separated by thin-layer chromatography from unreacted raw material (developer: methylene chloride: methanol =5:1, v/v, Rf= 0.648). The collecting samples were redispersed in ethanol with ultrasonic treatment and filtered by 0.22 µm filter. Then, the obtained solutions were further dried under vacuum condition and the purified CDs were obtained. Procedures for Detecting TNP.

Various concentrations of TNP (0, 2, 5, 10, 15, 20, 25, 30, 35, 40, 50, and 60 µM) were added to CDs (0.025 mg/mL) in ethanol solution. Then, the fluorescence emission spectra of mixtures were measured under 450nm excitation at room temperrature. The detection limit and Ksv can be calculated from the Stern–Volmer plots.


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Theoretical Calculations.

All of the calculations for nitro-explosives (TNT, TNP, HMX, RDX, TATB) were investigated by the Gaussian 09 program.13 The structural model for corresponding explosives were optimized using the B3LYP/6-31G(d,p) method and the related electronic information such as the energy levels of HOMO and LUMO orbitals were also obtained by the same method. Biotoxicity Measurements of CDs.

The cytotoxicity of CDs was evaluated by the standard MTT assays.51 MCF-7 cells were seeded in 96-well U-bottom culture plates (Costar, IL, USA) at a density of about 5 x 104 cells/well, and incubated with CDs at varied concentrations (0-500 µg/mL) at 37 °C under 5% CO2 for 24 h. Then, the culture media were discarded, and MTT solution (0.02 mL, 5 mg/mL) was added to each well, followed by incubation at 37 °C for additional 4 h. After the addition of 150 µL dimethyl sulfoxide (DMSO) to each well, the absorbance values of the each well were measured with a microplate reader (BioTek Cytation 3) at 570 nm. The cell viability of CDstreated cells is defined as the mean absorbance value of CDs treatment group divided by the mean absorbance value of untreated group.

ASSOCIATED CONTENT Supporting Information Photoluminescence excitation spectra, lifetime curve, Raman spectra, 1H NMR spectra, mass analysis, cyclic voltammetry of CDs, Stern-Volmer plots for detecting TNP, Uv-vis absorption spectra, fluorescence spectra, FT-IR spectrum, photographs of CDs, TNP, CDs + TNP and CDs + acid, fluorescence quenching efficiency curve of CDs + TNP and CDs + acid AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] ORCID

Minjie Li: 0000-0002-1458-5690 Author Contributions Bo Ju and Yi Wang contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the financial support from the National Science Foundation of China (Grant No. 21574058 for M. Li, No. 21572079 and 51373068 for Pro. Zhang), the program of Changjiang Scholars and Innovative Research Team in University (IRT101713018) and the program for JLU Science and Technology Innovative Research Team.

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Photostable and Low-Toxic Yellow-Green Carbon Dots for Highly

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