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One-step synthesis of novel photoluminescent nitrogen-rich carbon nanodots from allylamine for highly sensitive and selective fluorescence detection of trinitrophenol and fluorescent Ink Xin Ran, Qing Qu, Lei Li, Limei Zuo, Shihong Zhang, Jingwei Gui, Yaxin Kang, and Long Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01977 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Sustainable Chemistry & Engineering

One-Step Synthesis of Novel Photoluminescent Nitrogen-Rich Carbon Nanodots from Allylamine for Highly Sensitive and Selective Fluorescence Detection of Trinitrophenol and Fluorescent Ink

Xin Rana, Qing Qua,*, Lei Lib,*, Limei Zuoa, Shihong Zhanga, Jingwei Guia, Yaxin Kanga, Long Yanga,c,*

a

School of Chemical Science and Technology, Yunnan University, Kunming 650091, China.

b

Laboratory for Conservation and Utilization of Bio-Resources, Yunnan University, Kunming, 650091, China.

c

Key Laboratory for Forest Resources Conservation and Utilization in the Southwest Mountains of China, Ministry of Education, Yunnan Province Key Lab of Wood Adhesives and Glued Products, School of Materials Science and Engineering, Southwest Forestry University, Kunming, 650224, China.

*Corresponding authors. Tel.: +86 871 65035798; fax: +86 871 65036538. E-mail: [email protected] (Q. Qu); [email protected] (L. Li); [email protected] (L. Yang)

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ABSTRACT: Monitoring of nitroaromatic explosives plays a vital role in homeland security and public safety. In this work, by employing allylamine as both the sources of carbon and nitrogen, photoluminescent nitrogen-rich carbon nanodots (NC-dots) was prepared via a peaceable and inexpensive hydrothermal route for the first time. The average size of the present synthesized NC-dots is 2.88 ± 0.4 nm. The NC-dots show outstanding blue fluorescence and exhibit a dramatically high fluorescence quantum yield of 15%. The NC-dots were completely characterized using various techniques. The NC-dots could be employed as a fluorescent material for the sensing of TNP with distinguished selectivity. The addition of TNP into the NC-dots resulted in splendid quenching, which was ascribed to the integrative action of fluorescence resonance energy transfer, electron transfer, and the hydrogen-bond interactions between NC-dots and TNP. In parallel, the proposed NC-dots could be applied as a new type of fluorescent ink. Besides, a detailed mechanism for the production of NC-dots via self-polymerization and carbonization was proposed.

KEYWORDS: carbon nanodots, photoluminescent, nitrogen-rich, trinitrophenol, fluorescent ink

INTRODUCTION Nitroaromatic explosives, for instance trinitrotoluene (TNT) and trinitrophenol (TNP), have been of widespread concern over the years. Monitoring of explosives plays a vital role in homeland security and public safety.1,2 Especially, because of its very

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high explosive power, TNP has attracted the urgent attention, which has been extensively utilized for the production of fireworks and matches.3 In parallel, TNP is identified as a highly poisonous environmental contaminant. TNP pollutants in source of water not only threaten human health but also lead to severe pollution of environment.4 Thus, there is an urgent need for determination of TNP. Because the structure and property of TNP and other nitro explosives are very similar, the development of highly selective sensing methods of TNP still faces some challenges. Existing detection methods of explosives not only need sophisticated instruments, but also are high-cost and time-consuming.5,6 Recently, fluorescence-based explosives detection methods have been paid much focus due to their distinct benefits, for instance simpleness, high sensitivity, easily detectable signal, and short response time.7,8 Diverse fluorescence materials such as conjugated polymers,9 metal–organic frameworks,6,10 and carbon nanodots (C-dots)

2,8

have been used in assembling

high-performance fluorescence sensing platforms for detection of nitro explosives. Among the various fluorescence materials, C-dots have been considered as one of the most outstanding carbon-based nanomaterials that have attracted a lot of interest due to their significant advantages, such as simple and convenient functionalization, excellent aqueous solubility, chemical inertness, very low toxicity, and favourable biocompatibility, compared with traditional semiconductor quantum dots that heavy metals were mainly used.11-13 C-dots have received great interest in a lot of fields due to their splendid fundamental performances. They have shown potential exciting applications in a large number of fields, such as catalysis,14,15 photocatalysis,16,17

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medical

diagnosis,18

bioimaging,11,12,19,20

sensing,21-24

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lasers,25

and

energy

conversion/storage devices.26 The quantum yield (QY) is one of the most crucial parameters of fluorescence C-dots. It could be enhanced by using appropriate functional groups via surface passivation although such a passivation process is complicated and time-consuming.22 Conversely, nitrogen-doping can improve the QY of C-dots because nitrogen atoms have five valence electrons, which is very useful for bonding with carbon atoms. Also, the electronic and optical properties of C-dots can also be significantly tuned by nitrogen doping.13,27 Generally, C-dots or nitrogen-doped carbon nanodots (NC-dots) can be prepared by top-down or bottom-up route. Compared with the top-down route, the bottom-up techniques including hydrothermal, microwave, and wet-chemical method have advantages of simpleness, lower cost, and less influence on the environment. In all these bottom-up methods, the hydrothermal approach is one of the most useful and easy to prepare in large quantities method. And the C-dots obtained via hydrothermal treatment also have a very high graphitization degree.28 In the current work, we report a nimble effective, economical, and hydrothermal synthesis of fluorescent nitrogen-rich C-dots using a single precursor allylamine as both the nitrogen and carbon sources. The prepared NC-dots revealed a splendid fluorescence with a 10.7% nitrogen content and a 15% QY. The fluorescence intensity of the obtained C-dots was optimized by controlling the reaction time and temperature. The detailed mechanism for the production of the NC-dots was proposed. Furthermore, it was demonstrated that the NC-dots had exhibited potential and

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promising applications as fluorescent ink and fluorescence probe for sensing of nitroaromatic explosive TNP. The synthesis route for the NC-dots and their applications for TNP sensing and fluorescent fingerprint on commercial filter paper were illustrated in Fig. 1.

EXPERIMENTAL Chemicals and materials. Allylamine hydrochloride was bought from Adamas Reagent Co., Ltd. (Shanghai, China). TNP, TNT, 2-nitrotoluene

(2-NT),

4-nitrotoluene (4-NT), 2,4-dinitrotoluene (2,4-NT), 2,6-dinitrotoluene (2,6-NT), and nitrobenzene (NB) were also obtained from Adamas Reagent Co., Ltd. (Shanghai, China). All the chemicals were used as obtained. Deionized water was used in all of these experiments. The synthetic process of the NC-dots. Typically, NC-dots was prepared by a hydrothermal method as follow: firstly, 0.5 M allylamine hydrochloride transparent aqueous solution was prepared. Then twenty milliliter of the allylamine aqueous solution was moved into a 25 milliliter Teflon-lined stainless steel autoclave, which was heated at 220 °C for 1.5 h in an oven. After the hydrothermal reaction, the autoclave was allowed to cool. The obtained yellow and transparent solution was subjected to dialysis to obtain the NC-dots. The reaction temperature (200, 220, 240 °C) and time (1.5, 3.0, 4.5, 6.0 h) were optimized. The same process was adopted for the time and temperature optimization experiments. Detection of TNP. The fluorescence sensing of TNP was performed as follows:

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upon the addition of target TNP explosives, the fluorescence quenching of NC-dots was detected via a fluorescence spectrometer. Briefly, 2.0 ml of the present obtained NC-dots solution was transfered in a cuvette, then the TNP explosive solution (pH 7.0 PBS) was added continuously. Every titration experiment was repeated three times. The fluorescence emission spectra data were recorded in the range of 360–650 nm at an excitation wavelength of 350 nm.

RESULTS AND DISCUSSION Characterization and composition of the synthesized NC-dots. By using a hydrothermal reaction of allylamine at 220 °C for 1.5 h, the NC-dots were obtained. The microstructure of the obtained NC-dots was investigated using transmission electron microscopy (TEM). Fig. 2A illustrates the TEM image of the NC-dots. It could be found that the C-dots had a uniform size of 2.88 ± 0.4 nm and excellent dispersion without any aggregation. The size distribution for the present NC-dots is provided in inset of Fig. 2A. As revealed in Fig. 2B, the high-resolution TEM (HRTEM) indicated that a small number of dots had well-definite lattice fringes, which may be due to the reason that the instrument can not reach the requirements of high-resolution for the such small carbon dots. The X-ray diffractometry (XRD) pattern of the NC-dots (Fig. 2C) reveals a broad peak centered at 25o (0.34 nm), which is caused by the disordered carbon atoms.12 In the Fourier transform infrared (FTIR) spectra of NC-dots, the following bands including O−H stretching vibrations at 3428 cm−1, N−H stretching vibrations at 3116 cm−1, asymmetric bending vibrations

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of NH3+ at 1620 cm−1, and bending vibrations of CH2 at 1403 cm−1, as illustrated in Fig. 2D.12,29 The present NC-dots were further investigated using X-ray photoelectron spectroscopy (XPS) analysis. Fig. 3A displayed the XPS survey spectrum of the NC-dots, which implied that the NC-dots are primarily made up of C, N, and O elements. As shown in Fig. 3B, four types of peaks at 288.80 eV, 286.11 eV, 285.11 eV, and 284.64 eV are found in the C 1s high resolution spectrum, which were ascribed to the C–C, C–N, C–O, and O–C=O, respectively.8,22,30 The N 1s high resolution spectrum (Fig. 3C) implies the existing of C3–N (graphite like, 402.10 eV), NH3+ (401.51 eV), N–H (400.82 eV), pyrrolic N (399.43 eV), and pyridinic N (398.32 eV) groups in the NC-dots sample.8,13,31,32 Thus, the synthesized NC-dots are indeed N-riched owing to the fact that the allylamine was used both as the C and N source. The high resolution XPS spectrum of O 1s (Fig. 3D) can be divided into peaks at 530.20 eV, 531.29 eV, 532.97 eV, and 533.14 eV, certifying the appearance of O–H, *O=C–O, O–C, and O=C–O*.13 A higher zeta potential corresponds to a higher surface charge density, which generally makes the suspensions maintain a higher stability due to electrical repulsion. The zeta potentials of the N-C-dots at various pH values were studied. As shown in Fig. S1, the zeta potential measurements show that the N-C-dots have high surface charge in the pH range of 2–8. The pKa was approximately 11. Photoluminescence performances of the NC-dots. In the ultraviolet visible (UV-vis) spectrum (Fig. S2), the absorbance peak of the NC-dots solution (220 oC-1.5 h) was centered at 350 nm. The spectra both excitation and emission of the proposed

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NC-dots are provided in Fig. 4A. The optimal emission intensity was found at 425 nm, which was excitated at 350 nm. Insets of Fig. 4A show the NC-dots under naked eye and UV light under an exciting wavelength of 365 nm, which exhibited a strong blue fluorescence. The QY (15%) of the present obtained NC-dots was tested at an exciting wavelength of 350 nm. The fluorescence spectra of NC-dots (220 °C-1.5 h) was excitated at different wavelength as revealed in Fig. 4B. The emission peak exhibited a clear bathochromic-shift with the increasing of the excitation wavelength.33 The photoluminescent (PL) emission intensity shows a largest value at an excitation wavelength of 350 nm, then decreases gradually with the increase of the excitation wavelength.34 Owing to the different sizes and the significant distribution of emissive trap sites of NC-dots, the PL emission spectra exhibited the dependence on excitation wavelength.35 As exhibited in Figs. S3–S5, the NC-dots obtained at 220 °C for 3.0 h, 4.5h, and 6.0 h were also investigated at different exciting wavelengths varying from 300 to 480 nm, which also implied an optimal emission intensity at 425 nm. Fig. S6 shows the thermogravimetric analysis (TGA) curve of allylamine hydrochloride, which suggested that the pyrolysis of allylamine started from 200 oC. Thus, three temperatures (200, 220, 240 oC) were selected for studying the hydrothermal reaction of allylamine aqueous solution. As displayed in Fig. 4C, the emission intensity of NC-dots obtained at 220 oC was higher than that obtained at 200 oC. However, emission intensity of NC-dots obtained at 220 oC is approximately equal to that obtained at 240 oC. Thus, 220 oC was selected for the synthesis of NC-dots. This may be due to the fact that the reaction of allylamine with water according to

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Markovnikov’s rule needs a relatively high temperature to produce the intermediate, then self-polymerization to obtain the N-C-dots. Conversely, at a low temperature, the Markov addition reaction can not happen. The dimer of allylamine produced by deamination reaction can not be further polymerized, which is very unfavorable for the prodution of N-C-dots. Insets show the NC-dots solutions obtained at 200 oC, 220 o

C, and 240 oC for 1.5 h under UV light under an exciting wavelength of 365 nm. The

time dependent emission spectra for the N-riched carbon dots solution obtained at 220 o

C for 0.5, 1.0, 1.5, 3.0, 4.5, and 6.0 h are illustrated in Fig. 4D. The PL emission

intensity of NC-dots increases with increasing the hydrothermal reaction time up to 1.5 h and tends to be stable at the range of 1.5–4.5 h, then decreases at 6.0 h. Insets show the NC-dots solutions under UV light (365 nm). Synthesis mechanism of the N-C-dots. In many previous studies,12,13 due to the complicated and harsh preparation conditions as well as carbonization reaction, the mechanisms for formation of C-dots were stated based on speculation. In this work, the mechanism for the production of the present NC-dots from allylamine was also proposed based on hypothesis, as shown in Fig. 5. Firstly, allylamine reacted with water according to Markovnikov’s rule, whereupon they formed polymers by dehydration and deamination through self-polymerization, which were then carbonized to form the N-riched carbon dots. The weight percentage of N based on Scheme 2 is calculated to be 13.08%, which is basically in accordance with that of the results of XPS (10.68%). TNP fluorescence sensing and fluorescent ink applications. The influence of

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pH on the fluorescent intensity towards the NC-dots was studied in the pH range of 2-12. As depicted in Fig. S7, the fluorescent intensity of the NC-dots was almost unaffected by the pH value in the 3-11 range. Thus, the as-proposed NC-dots are highly stable in a wide pH range and thus desirable for applying in the sensitive detection of TNP. The addition of TNP leads to the significantly quenching of N-C-dots’ FL over the pH range of 2-12; nevertheless, the quenching efficiencies are different at pH values of 2-12. The quenching efficiencies of TNP towards the N-C-dots are much higher in the pH 6−7. This indicates that the pH range of 6−7 is suitable for sensing. Thus, pH 7.0 was selected. The fluorescent C-dots have been found extensive applications in chemical/bio sensing because of their distinct fluorescence properties and excellent water solubility. 7,36-38

The PL emission intensity of the NC-dots was studied when different amount of

TNP was added. As shown in Fig. 6A, the PL emission intensity of the NC-dots decreased in succession via the increasing of the TNP concentration from 0.0 to 50.0 µM, which suggested that the fluorescence of the NC-dots was prominently quenched by the TNP explosives. It can be seen that there is an obvious red shift in the emission spectra with the increased TNP. This may be ascribed to polarity enhancement of the microenvironment in the system. On the other hand, the FRET results in the self-absorption, which may also lead to the red shifts. This obvious red shift in the emission spectra via the addition of TNP also indicates the molecular interactions between the NC-dots and TNP. As illustrated in the Fig. 6B, the analytical signal of PL emission intensities of the NC-dots before and after addition of TNP was measured.

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The (F0–F)/F0 value vs. the concentration of TNP was plotted (Fig. 6B). It can be seen that the the (F0–F)/F0 value levels off with the increasing of TNP concentration. Generally, the fluorescence quenching was attributed to the static quenching effect, the dynamic quenching effect, or simultaneously of static and dynamic effect. The fluorescence quenching could be studied by the Stern–Volmer (S-V) equation (F0/F = 1+Ksv[Q]) (equation 1), which was exhibited in Fig. S8. The nonlinear S–V plot indicates that energy transfer between TNP and N-C-dots or an integration of dynamic and static quenching exists in this system.6,8,24 Thus, the (F0–F)/F0 value levels off with the increasing of TNP concentration, which may be due to the above both dynamic and static quenching effect in this system. As exhibited in inset of Fig. 6B, the (F0–F)/F0 increases with the TNP amount and shows an excellent linear relationship in the concentration ranging from 1.0 to 10.0 µM. The LOD for the detection of TNP was 0.2 µM (S/N = 3). The selectivity of the NC-dots toward TNP was also investigated by evaluating the influence of other explosives including TNT, 2-NT, 4-NT, 2,4-NT, 2,6-NT, and NB at the same concentration of 40 µM. The fluorescence quenching experiments were carried out. The corresponding PL emission spectra were illustrated in Fig. 6C. The results demonstrate that the fluorescence of the NC-dots are highly quenched by the TNP compared to other nitro explosives. The (F0–F)/F0 value of the NC-dots with 40 µM of different nitro explosives is exhibited in Fig. 6D. This result indicates obviously indicates the very high affinity force of the TNP with the N-riched carbon dots, which is favorable for the highly selective detection of TNP. The NC-dots could

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also be utilized as fluorescent ink, which is one of the fascinating inks for paper printing.11-13 The NC-dots solution could be employed as a new type of fluorescent ink, which was directly utilized to write these letters of “NC-dots”, “TNP”, and “YNU” on a filter paper by using a Chinese writing brush and the obtained handwritings could be distinctly observed under UV-light (365 nm) as illustrated in Fig. 6E. Besides, by pressing fingerprints with the NC-dots aqueous solution on a filter paper, at 365 nm UV light, fingerprints were left on the filter paper, as illustrated in Fig. 1. The NC-dots aqueous inks is colorless at naked eye whereas shows wonderful fluorescence at UV-light. Therefore, NC-dots ink has potential application as fluorescent pens. Sensing mechanism for TNP determination. To illuminate the reason of the excellent selectivity and sensitivity of the fluorescent NC-dots towards TNP, the quenching mechanism was investigated. It has been reported that photoinduced electron transfer contributes to the explosive quenching because of the electron-deficient nature of nitroaromatic explosives.39,40 The excited-state of a fluorophore tends to give out electrons to the ground-state of the nitroaromatic explosives when the electron transfer occurs, resulting in the quenching of fluorescence.8 The electron transfer may be caused by the energy gap between LUMO of a donor and the LUMO of a receptor. So far, the quenching reason of electron transfer from the conduction band of C-dots to the LUMO of electron-deficient nitroaromatic explosive, has been widely investigated.2,8,22,41 The LUMOs energies of electron-rich NC-dots were higher than that of electron-deficient nitroaromatic

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explosives. Thus, the excited-state electron transfered from conduction band to the nitro explosives LUMOs, leading to the fluorescence quenching. The TNP has the lowest LUMO energy, which resulted in the maximum quenching of the NC-dots (Fig. 7). Nevertheless, the observed quenching sequence (TNP > TNT > 2,6-NT > 4-NT > 2-NT > NB > 2,4-NT) is not in accordance with the electron deficiency degree (TNP > TNT > 2,4-NT > 2,6-NT > NB > 4-NT > 2-NT). This result strongly suggests that electron transfer was not the only reason that results in the NC-dots quenching. Other factor for the quenching of the fluorescence of NC-dots should be fluorescence resonance energy transfer.6 It has been reported that the greater spectral overlap of adsorption of nitroaromatic explosives and emission spectrum of N-riched carbon dots, the higher degree of energy transfer.41,42 Fig. 6F reveals the typical UV−vis absorption spectra of TNP, TNT, NB, 2-NT, 4-NT, 2,4-NT, 2,6-NT (2.0 µM) and the PL emission spectrum of the NC-dots, which indicates that TNP and the NC-dots have the maximal spectral overlap, resulting in the obvious red shift in the emission spectrum with the increased TNP. The absorption spectra of o-nitrophenol (o-NP), m-nitrophenol (m-NP), p-nitrophenol (p-NP), and 2,4-dinitrophenol (DNP) were also investigated. As revealed in Fig. S9, the absorption spectra of o-NP, m-NP, p-NP, DNP have relatively small spectral overlap with emission spectrum of the NC-dots. Thus, the TNP’s absorption spectrum still has the obviously greater spectral overlap with the emission spectrum of NC-dots. In addition, the hydrogen-bond interactions between the –NH2 of NC-dots and the -OH of TNP may also resulted in the PL emission quenching as demonstrated by the very different quenching efficiencies at

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pH values of 2−12.6,8 However, other nitroaromatic explosives have no acidic -OH group to bind with -NH2 in the NC-dots. Thus, they exhibite very weak quenching effects. Therefore, the NC-dots show highly selective and sensitive determination of TNP beyond other nitroaromatic explosives, which is caused by the occurrence of electron-transfer, energy-transfer, and hydrogen-bond interactions.

CONCLUSIONS In summary, a kind of novel and photoluminescent NC-dots was successfully synthesized by using allylamine as a single reactant source by one-step hydrothermal method for the first time, demonstrating a convenient and low-cost route to prepare the N-riched carbon dots. The results from various characterizations suggested the successful production of fluorescent NC-dots. The NC-dots have an average diameter of approximately 2.88 ± 0.4 nm. The synthesized NC-dots reveal very high fluorescence QY (15%) and can also be employed as a fluorescent probe for the sensing of TNP with high selectivity. The splendid quenching ability of TNP towards the NC-dots was caused by the synergistic effect of electron transfer, fluorescence resonance energy transfer, and the hydrogen-bond interactions between the NC-dots and TNP. This finding demonstrates a convenient way to synthesize strong fluorescence N-riched C-dots, which also shows outstanding and potential application for the construction of selective and sensitive sensing platform for TNP detection. The proposed outstanding NC-dots could be applied as a new type of fluorescent ink. These results proved that the present synthesized NC-dots were indeed N-doped

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carbon dots and exhibited optimal and distinct optical performances as comparing with traditional C-dots. The NC-dots have revealed promising applications as fluorescent inks and TNP sensing platform. In addition, the mechanism for the production of the N-riched carbon dots via self-polymerization and carbonization was illuminated.

ASSOCIATED CONTENT Supporting Information Characterization methods for the N-C-dots; quantum yields (QY) measurements method; UV-vis absorption spectra of the N-C-dots; emission spectra of N-C-dots (220 oC for 3.0 h, 4.5 h, and 6.0 h) at various excitation wavelengths; TGA curve of allylamine hydrochloride; effect of pH on the PL intensity of the N-C-dots.

AUTHOR INFORMATION Corresponding Author *Tel.: +86 871 65035798. Fax: +86 871 65036538. E-mail: [email protected] (Q. Qu); [email protected] (L. Li); [email protected] (L. Yang)

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

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This work was financially supported by the National Natural Science Foundation of China under the Grant No. 51361028, 51161025, 51661033, and 31660538.

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Figure captions: Fig. 1 Schematic diagram for synthesis of N-riched carbon dots and their applications for selective fluorescence quenching sensing of TNP and fluorescent fingerprint on commercial filter paper.

Fig. 2 (A) The TEM image of NC-dots, inset shows the size distribution. (B) HR-TEM of the N-riched carbon dots. (C) XRD pattern of the NC-dots; (D) FTIR spectrum of the NC-dots.

Fig. 3 The XPS spectra of the NC-dots. The XPS survey spectrum (A); high resolution spectra of C1s (B), N1s (C), and O1s (D).

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Fig. 4 (A) Normalized excitation and PL emission spectra of the NC-dots solution. Insets show the N-riched carbon dots solution under visible and UV light at an excitation wavelength of 365 nm. (B) PL emission spectra of the NC-dots solution at different exciting wavelengths ranging from 300 to 480 nm. (C) The influence of temperature on the emission intensities of the N-riched carbon dots solution. Insets show the NC-dots solutions obtained at 200 oC, 220 oC, and 240 oC for 1.5 h under UV light excitated at 365 nm. (D) Time dependent emission spectra of the N-riched carbon dots solution. Insets show the NC-dots solutions obtained at 220 oC for 0.5, 1.0, 1.5, 3.0, 4.5, and 6.0 h under UV light excitated at 365 nm.

Fig. 5 Proposed mechanism for the synthesis of the N-riched carbon dots from allylamine.

Fig. 6 (A) The PL emission spectra of the NC-dots with the addition of different concentration of TNP (0.0, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0, 14.0, 18.0, 22.0, 26.0, 30.0, 34.0, 38.0, 42.0, 46.0, and 50.0 µM). Insets show the photos of the NC-dots solution before and after addition of 40 µM TNP excitated at a 365 nm UV light. (B) Stern-Volmer plot of the quenching of the fluorescence of N-riched carbon dots with the addition of TNP (0–50 µM). Inset is the kinetic plot of the quenching of the fluorescence of N-riched carbon dots with the addition of TNP (1–10 µM). (C) Fluorescence emission spectra of NC-dots with the absence and presence of 40 µM various nitroaromatic explosives. (D) The fluorescence intensity ratio (F0-F)/F0 of the

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N-riched carbon dots solution in the presence of various nitroaromatic explosives. (E) NC-dots fluorescent ink used to write the letters of “NC-dots”, “TNP”, and “YNU” with a writing brush on a filter paper under the exciting of UV lamp of 365 nm. (F) Absorption spectra of different explosives and the emission spectrum of the NC-dots solution.

Fig. 7 HOMO and LUMO energies of the investigated nitroaromatic explosives. The HOMO and LUMO energies of the the nitroaromatic explosives were calculated based on the DFT theory by using Gaussian 09 program at the B3LYP/6-311G** level.

TOC Graphic: A novel nitrogen-rich C-dots used for selective fluorescence quenching sensing of trinitrophenol and fluorescent fingerprint on a commercial filter paper.

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Figures:

Fig. 1 Ran et al.

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Fig. 2 Ran et al.

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Fig. 3 Ran et al.

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Fig. 4 Ran et al.

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Fig. 5 Ran et al.

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Fig. 6 Ran et al.

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Fig. 7 Ran et al.

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One-step synthesis of novel photoluminescent ... - ACS Publications

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