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4-nitrophenol optical sensing with N doped oxidized carbon dots
Journal Pre-proof 4-nitrophenol optical sensing with N doped oxidized carbon dots N.K.R. Bogireddy, R. Cruz Silva, Miguel A. Valenzuela, Vivechana Agarwal
PII:
S0304-3894(19)31597-3
DOI:
https://doi.org/10.1016/j.jhazmat.2019.121643
Reference:
HAZMAT 121643
To appear in:
Journal of Hazardous Materials
Received Date:
26 June 2019
Revised Date:
27 September 2019
Accepted Date:
7 November 2019
Please cite this article as: Bogireddy NKR, Cruz Silva R, Valenzuela MA, Agarwal V, 4-nitrophenol optical sensing with N doped oxidized carbon dots, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121643
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4-nitrophenol optical sensing with N doped oxidized carbon dots N. K. R. Bogireddy1, R. Cruz Silva2, Miguel A. Valenzuela3 and Vivechana Agarwal1* 1
Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autonoma del Estado de Morelos, Av. Univ. 1001, Col. Chamilpa, Cuernavaca, Morelos 62209, México. 2
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Global Aqua Innovation Center, Shinshu University, Nagano City, 380‐8553 Japan, and Institute of Carbon Science and Technology, Faculty of Engineering, Shinshu University, Nagano City, 380‐8553 Japan. Lab. Catálisis y Materiales, ESIQIE-Instituto Politécnico Nacional, Zacatenco, 07738 CDMX, México.
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*Corresponding author e-mail: [email protected]
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Graphical abstract
Highlights
Hydrothermally synthesized N-doped oxidized carbon dots from citric acid and urea
Emission from nanoprobes found to be sensitive towards 4-nitrophenol (4-NP)
Sensing mechanism attributed to simultaneous electron transfer and reduction of the probe structure
The developed material was successfully used to sense to 4-NP spiked tap and industrial water samples
Abstract: In this work, we report a facile strategy for 4-nitrophenol (4-NP) sensing using highly luminescent nitrogen doped oxidized carbon dots. The quenching of fluorescence (turn OFF),
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with the addition of trace amounts of organic pollutant (4-NP) in NOCDs, has been attributed to the complete reduction of nitrogen doped oxidized carbon dots (NOCDs) to reduced
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nitrogen doped oxidized carbon dots (rNOCDs) and its formation was confirmed by infrared, X-ray diffraction and X-ray photoelectron spectroscopy measurements.
The chemical
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changes in oxygen-containing functional groups of NOCDs, in the presence of 4-NP, are elucidated and corresponding characterization through XPS reveals the changes in the peak
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intensities of C-C (284.5 eV) and O-C=O (288.6 eV), indicating a decrement in hydroxyl groups that hinder its complete reduction to NOCDs. The sensitivity of NOCDs towards 4-
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NP has been tested in spiked tap water in the concentration range 2 µM to 2mM with the minimum detection limit of 2 µM (linear detection range from 2 to 100 µM with regression coefficient R2 =0.99). The proposed simple sensing platform can be used to reduce NOCDs
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and simultaneously sense low concentrations of 4-NP. Finally, an effective treatment to improve the reduction of nitrogen doped graphene oxide is proposed.
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KEYWORDS: Fluorescent probe, graphene, nitrogen doped oxidized carbon dots, 4nitrophenol, sensing
Introduction
In recent days pollutant free drinking water [1] is limited due to the constant release of organic contaminants into the environment, such as i.e., 4-nitrophenol (4-NP), direct blue 24 (DB24), methyl orange (MO) and methylene blue (MB). Among them 4-NP has been listed as “priority pollutant” by U. S. Environmental Protection Agency (EPA) due to the extent and severity of the environmental pollution, maximum allowable level in drinking water is
60 µg/L (~0.43 µM) [2-4] caused by this substance. Apart from being utilized in the manufacturing of many chemicals such as fenitrothion, paracetamol, parathion and methyl parathion, 4-NP is released by various herbicide/insecticide industries [5]. Resulting ground water pollution has been found to adversely effect the cognitive as well as nervous and ocular system functioning in all the living organisms including humans (cyto-embryotoxic and mutagenic effects in mammals) [2,5]. Due to the above-mentioned toxic effects of 4-NP, there is an increasing demand of portable sensing devices for reliable on-site observation of the pollutant. Among the existing standard analytical methods such as fluorimetry [6, 7],
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spectrophotometry [8,9], gas and liquid chromatography [10,11], fluorimetry based detection processes are relatively simple, fast, cost effective, sensitive and portable with an option of
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real-time detection. Therefore, the fabrication of fluorescent nanomaterials such as Mn doped ZnS quantum dots,[12] silver and gold nanocrystals,[13] carbon-based nanomaterials, [14-
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19] has been the focus of attention towards the detection of 4-NP at industrial level. Apart from 4-NP sensing reported with relatively expensive materials (such as Au NCs [13]
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and mesoporous molecular imprinting polymers containing manganese (II)-doped ZnS quantum dots [12]), among the non-toxic carbon dots based 4-NP sensors, H. Yuan et al.
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(2016)[14] were the first to report fluorimetry based optical (PL quenching) sensing of 4-NP employing nitrogen doped CDs with pH-independent emission. In addition, boron, nitrogen co-doped CDs were used in the recent past to develop fluorescence quenching assay for 4-
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NP in the concentration range of 0.5 - 200 µM (Na Xiao et al. (2018) [15]. Similar work
from G. Ren et al. (2018) [16] showed 4-NP induced PL quenching of hydrothermally prepared CDs from natural spinach. Recently, Ji-Min Yang et al. (2019) [17] reported the fluorescence quenching of CDs-immobilized zirconium-based metalorganic framework composite within a wide concentration range of almost three orders of magnitude.
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Additionally, J. Wei et al. (2019) [18] reported fluorometric determination of pesticides (including 4-NP) and organophosphates using nanoceria as a phosphatase mimic with an inner filter effect on carbon nanodots. Y. Y. Qu et al. (2019) [19] reported CDs as colorimetric and fluorescent dual-readout probe for 4-NP detection with 0.001-1µM range. However, understanding the changes in the characteristics (morphology and crystallinity) of the sensing probe, during the sensing of 4NP, still remains challenging. Here we report, a facile colorimetric and fluorescence based 4-NP sensing (turn OFF) using blue light emitting
nitrogen doped oxidized carbon dots (NOCDs) as probes, accompanied with the simultaneous reduction of the sensing probe itself. 4-NP induced reduction of NOCDs was confirmed by infrared, X ray diffraction and X-ray photoelectron spectroscopy measurements.
Materials and methods Citric acid (ACS reagent, ≥99.5%), Urea (ACS reagent, 99 to 100%) and 4-nitrophenol
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(spectrophotometric grade) were acquired from Sigma Aldrich and used without further
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treatment. Deionized water (18.2 MΩ cm) was used throughout the experiments.
Synthesis of nitrogen-doped oxidized carbon dots (NOCDs)
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Similar to the previously reported synthesis method, [20-22] nanoparticles were prepared by hydrothermal method, using 0.25 g citric acid and 0.37 g urea suspension in 50 mL deionized
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water and stirred for 15 min to make a clear solution, which was kept in a Teflon-lined hydrothermal autoclave at 180°C for 60 min. The final product (bright yellow color solution)
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was centrifuged several times by continuous washing with water/ethanol [23] and stored under ambient conditions for further use.
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Surface characterization
The optical properties including UV-visible and fluorescence spectra of as-synthesized NOCDs were measured in a dual beam Perkin-Elmer Lambda 950 and a Cary Eclipse Fluorescence Spectrophotometer, respectively. Transmission electron microscopy (TEM) was carried out with a JEOL JEM-ARM200F transmission electron microscope to identify
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the morphology and particle size distribution of NOCDs. Sample was prepared on lacey carbon film over 100 mesh Cu grid by drop-casting well diluted NOCDs and evaporated under ambient conditions. X-ray diffraction (XRD) patterns of NOCDs and reduced NOCDs (rNOCDs) were performed with a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å). XRD samples were prepared as a film by drop casting using 500 µL of 4nitrophenol with 5 µL of as-prepared NOCDs and then evaporated by using heating mantle at 45 ± 50C. X-ray photoelectron spectroscopy (XPS) was recorded on an ESCA Ulvac-PHI
1600 photoelectron spectrometer from Physical Electronics using Al Kα radiation with a photon energy of 1486.6 ± 0.2 eV. The Fourier transform infrared (FTIR) spectra of NOCDs and reduced nitrogen doped oxidized carbon dots (rNOCDs) were measured using Varian 660-IR FT-IR spectrophotometer.
Procedure for detection of 4-nitrophenol (simultaneous reduction of NOCDs)
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For the detection of 4-nitrophenol, 405 µL of reaction mixture contains different concentrations, from 2µM to 2mM, of 4-nitrophenol (4-NP) and 5 µL of as prepared NOCDs.
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Samples with the same concentration of 4NP, with the addition of NOCDs particles, the volume was made to 1ml in deionized water. After constant stirring for 30s, absorption and emission spectra of the solution was measured by UV-visible and fluorescence
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spectrophotometer (λex = 344 nm) in the wavelength range of 200 – 600 nm at a scan rate of
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240 nm/min at room temperature under ambient conditions. Detection of 4-NP in real tap water samples
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Three different tap water (TW) and industrial water (IW) samples were obtained from different places in Cuernavaca city (TW1 (google map view: 18.94926, -99.2709454), TW2 (18.977705, -99.247733), TW3 (18.9808869, -99.2401062), IW1 and IW2. The real water
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samples were directly spiked with different concentrations of 4-NP (0, 10, 30, 50, 80 and 100 μM) and the corresponding PL spectra were recorded.
RESULTS AND DISCUSSION
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The size distribution of as-prepared NOCDs, analyzed through TEM and HR-TEM (Figure 1(a-c)) shows a homogeneous distribution of NOCDs. The histogram reveals a narrow-size distribution of NOCDs (Figure 1a inset) in the range of 2 – 5.5 nm, with average diameter of 3.8 ± 1.7 nm [14]. Crystalline structure, with lattice spacings of 0.221 nm (G (100)), 0.110 nm (G (020)), and diamond like 0.203 nm (111), 0.129 nm (110) respectively, were also observed (Fig. 2c). These spacings are consistent with both graphitic [(100), (020)] and diamond-like [(111), (110)] carbon preventing an obvious structure assignment [24-26].
However, the absence of graphitic (002) spacings of 0.33 nm indicates the NOCDs may
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contain predominantly sp3 enriched carbon.
Figure 1. (a, c and d) TEM and HRTEM images of as-prepared NOCDs at different magnifications
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(b) Size distribution taken over 100 particles; Inset of (c) represents the FFT analysis of corresponding image; Measured d-spacing values are indicated in (d).
Fluorescence spectrum of NOCDs (Figure 2 (a, b)), at different excitation wavelengths [27], reveals an excitation-independent strong blue fluorescence band at 441 nm with a gradual increase only in fluorescence intensity (no shift in maximum emission peak), when excited with wavelength from 290 to 347 nm followed by decreasing emission signal in the range of 350 – 390 nm. The results clearly show that at λex = 347 nm, largest number of NOCDs
particles have been excited giving rise to maximum emission intensity. Photoluminescence excitation (PLE) measured at 441 nm (emission) and absorbance (Figure 2b) additionally confirms the same. Moreover, as-prepared NOCDs exhibit two distinct absorption bands at around 235 nm (π to π* transition corresponding to C=C group of the possibly localized sp2 clusters in the carbon quantum dot) and 347 nm (n to π*) in the UV-visible spectrum (Figure 2c). The absorption at 347 nm (in agreement with the PLE results) is attributed to the dopant element (nitrogen) along with the strong absorption band (235nm) of graphitic structure [27]. It is believed that the introduction of nitrogen as dopant introduces many surface states
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associated to the enhancement of the luminescence properties of NOCDs. [27] The PL characteristics of the proposed nanoprobes (NOCDs), are controlled simultaneously by
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surface defects as well as the dimensions (quantum confinement).
Figure 2. (a) Fluorescence emission spectra of NOCDs when excitation wavelength varies from 290 – 390 nm, with step of 20nm (b) Excitation fluorescence spectra (PLE) of NOCDs at 441 nm and emission spectra of NOCDs at 344 nm excitation, (c) UV-visible absorbance spectra of NOCDs; Inset shows the optical images of NOCDs illuminated under white (left:
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visible light) and UV light (middle at 254 nm and right one at 365 nm)
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Figure 3. (a) The XRD patterns of as-prepared NOCDs (inset shows enlarged view of NOCDs peak from 20 to 60˚) (sample was prepared as a film by drop casting 200 µL of NOCDs followed by heating at 45 ± 50C till dry); Deconvoluted XPS spectra of (b) C 1s, (c) O 1s (change in C-O/C=O 0.518) intensity ratio is studied before and after the addition of 4-NP to analyze the change in the probe structure) and (d) N1s peaks of NOCDs.
In conformity with the already reported literature [27-30], XRD pattern of NOCDs (Figure 3a) exhibits a sharp peak centered at 11.31°, which is attributed to an interlayer spacing (001) of 0.78 nm. Apart from this peak, we can see the patterns of sp2 -diamond, graphite and carbon. sp2 -Diamond possesses the same space group as diamond around 2Ɵ = 39.0 (004) and 40.5 (111). Similarly, carbon and G (002) peaks were also observed. [31, 32]
XPS was used for elemental and chemical state identification in NOCDs. The survey scan of
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NOCDs clearly shows the major peaks at 532, 400 and 285 eV, which are the characteristic peaks of O1s, N1s and C1s, respectively. The occurrence of N 1s (400 eV) peak confirms
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the existence of nitrogen atom in NOCDs (Figure S1, Supporting Information). Furthermore, the XPS C 1s, O 1s and N1s peaks are deconvoluted to understand the chemical species
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present in NOCDs. After deconvolution of C1s peak of NOCDs, C-C (284.5 eV), C-O (286.0 eV), C=O (287.1 eV) and O-C=O (288.6 eV) are clearly observed (Figure 3a), indicating the
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existence of carbonyl and carboxyl functional groups on the surface of NOCDs [33]. In addition, the contribution of C-N (286.5 eV) means the successful incorporation of N atoms
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onto the NOCDs structure. The deconvoluted O1s and N1s peaks of NOCDs also show similar results. The deconvoluted O1s peaks at 531.6 and 533.0eV can be assigned as C=O and C-O, respectively (Figure 3b), while pyridinic N (399.6 eV) and graphitic N/amine N
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(401.3 eV) are observed in the deconvoluted N1s peak (Figure 3c, Table S1). These results clearly indicate the existence of oxygen-containing functional groups and N atom is closely linked onto the NOCDs surface.
Figure S2 (Supporting Information) showing the FTIR spectra of NOCDs exhibits a strong intensity peak at 3320 cm-1, corresponding to the stretching vibration of O–H group [34].
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This result is in good agreement with the XPS O1s peak, indicating the good hydrophilic property of NOCDs suggesting the location of hydroxyl groups on the surface. Another band located at 2343 cm-1 correspond to the stretching vibration mode of N–H bond [34]. Additionally, the bands at 1644 cm-1 are from the bending vibration of C=C bond of graphene [33].
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Reduction of NOCDs with simultaneous detection of 4-nitrophenol
Figure 4. NOCDs in organic pollutants (a) photographic images under daylight (upper part) and UV lamp at 365 nm (lower part), (b) histogram of the fluorescence intensity. Phe= phenol; AP=aminophenol; NP=nitrophenol; Cat=catechol.
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Figure 5. (a) The PL emission and (b) PLE spectra of NOCDs at different concentration of 4nitrophenol from 2 µM to 2 mM with 3 different concentration ranges; (Range 1) 2 µM to 20 µM, (Range 2) 20 µM to 100 µM and (Range 3) 200 µM to 2 mM, (c) 4-NP concentration (µM) vs emission intensity corresponding to peak wavelength and (d) 4-NP (µM) concentration vs emission wavelength. Samples were prepared by the addition of NOCDs (5 µL) into different concentrations
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of 4-nitrophenol (total volume 500 µL)).
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Figure 6. Optical photographs of NOCDs containing various concentrations of 4-NP (12.5-
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1000 µM) solution under visible light (top) and UV light (bottom) at λex=365nm. Absorbance
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corresponding to each concentration of 4-NP is shown in Figure S4.
The selectivity of a sensing probe is an important criteria of a successful sensor nanoprobe. In order to check the selectivity, the fluorescence response of NOCDs was tested with respect
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to different organic pollutant molecules viz. phenol, 2-aminophenol, 4-aminophenol, catechol, and 4-nitrophenol. Except 4-nitrophenol, the pollutants remained inert towards
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optical response of the NOCDs (shown in Figure S3) and their corresponding digital photographic images and histogram of PL studies are presented in Figure 4.
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Figure 5a shows the fluorescence spectra of NOCDs dispersed in different concentrations of 4-NP (2 –2000µM; three orders of magnitude). An increase in 4-nitrophenol concentration from 2 µM-2 mM (Figure 5 (c, d)) decreases the PL intensity along with a red shift in the emission peak wavelength position from 441 to 475 nm. Furthermore, after the addition of 1mM 4-NP to the NOCDs, although there is no significant shift in the emission peak
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wavelength, a decrement in the emission peak intensity is observed. Similar characteristics were revealed in the PLE spectra as well, i.e., a decrement in the PL intensity (corresponding to 441nm) with a little blue shift (peak position) in the excitation wavelength from 347 to 340 nm. Interestingly, after the addition of >1 mM 4-NP to the NOCDs solution, the PLE wavelength shift to its actual position (347 nm). As it occurs at higher concentrations, it is possibly due to intermolecular interactions between 4-NP molecules. Additionally, a gradual
decrement in the full width half maximum (FWHM) of emission peak from 2 µM-2 mM is
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discerned from the PL emission spectra.
Figure 7. (a) The XRD patterns of the as-prepared NOCDs to rNOCDs by increasing the 4-NP concentration (0.6, 1.2 and 2 mM) into NOCDs; and deconvoluted XPS spectra of (b) C 1s, (c) O
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1s and (d) N1s peaks of rNOCDs.
The XRD patterns of NOCDs at different concentrations of 4-NP (0.6, 1.2 and 2mM), shown in Figure 7a exhibit a sharp peak centered at 11.31°, which is matching with above results of NOCDs. After the addition of 4-NP (0.6 and 1.2 mM), there is drastic decrement in the peak at 11.31°. Interestingly, an addition of 2 mM 4-NP to the NOCDs, there is a new peak observed around 28.1° and the peak around 11.3° disappeared. The sharp peak at 28.1º, corresponding to d-spacing of ca. 0.32 nm is attributed to the enlarged interlayer distance, resulting from steric hindrance of functional groups on the graphene edge or the plane
distortion from sp3 C in the graphene plane. The peak at 28.1º is the characteristic (002) diffraction peak of graphite [explained in conformity with Ref 29, 30]. Its appearance indicates the interlayer stacking within NOCDs with 2mM 4-NP, and is very close to that in graphite (≈ 0.35 nm), confirming the formation of reduced NOCDs (rNOCDs) in the present case. Additionally, the close packing (0.35 nm) of highly conjugated sp2 domains possibly takes dominant place within the core part of rNOCDs). On the edge of rNOCDs, the functional groups or sp3 C will enlarge the interlayer distance.
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XPS was used to characterize the chemical species of elements in rNOCDs. The survey scan of rNOCDs clearly shows the major peaks are similar to NOCDs (Figure S5 (supplementary
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information)). But after deconvolution of C1s peak of rNOCDs, there is a decrement in the peak intensities corresponding to C-C (284.5 eV) and O-C=O (288.5 eV) (Figure 7b),
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indicating the decrement of hydroxyl groups on the surface of NOCDs. In addition, the deconvoluted O1s and N1s peaks of rNOCDs also show similar results. The deconvoluted
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O1s peaks at 531.6 (C=O) and 533.0 eV (C-O) are decreased (Figure 7c), while pyridinic N (399.6 eV) and graphitic N/amine N (401.3 eV) are increased due to the presence of 4-NP in
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rNOCDs (Figure 7d, Table S1). These results clearly indicate the % decrement of oxygencontaining functional groups on the surface of NOCDs, which clearly confirms the reduction (loss of oxygens) of NOCDs [33].
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Furthermore, Figure S6 (Supporting Information) shows the FTIR spectra of 4-nitrophenol (2mM), rNOCDs (presence of 2mM of 4-NP into NOCDs) and its corresponding differential signal. The rNOCDs exhibit a weak intensity peak compared with NOCDs at 3320 cm-1, which implies the reduction of stretching vibration of O–H group in NOCDs. This result is in good agreement with the XPS C1s and O1s peak, indicating the reduced hydrophilic
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property of NOCDs because of the less transmittance % of hydroxyl groups on the surface [33, 35 and 36]. Additionally, the band at 1644 cm-1 shows a drastic decrement in the % transmittance corresponding to the bending vibration of C=C bond of graphene.
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Interference studies for 4-nitrophenol detection:
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Figure 8. Optical sensing of 4-nitrophenol in water spiked with different other contaminants (a) Phenol (using PL intensity, peak emission wavelength shift), (b) 2-aminophenol (full width half maxima) (c) 4-aminophenol (using PL intensity, peak emission wavelength shift) and (d) catechol (using PL intensity, peak emission wavelength shift). Furthermore, we also performed the interference studies of the mentioned organic pollutants (phenol, 2-aminophenol, 4-aminophenol, and catechol) with 4-nitrophenol at different concentrations (Fig. 4 and 8). A red shift in the PL emission wavelength (λem) as well as
decrease in the PL signal intensity of NOCDs by the addition of 4-nitrophenol (at different concentrations) is revealed. Moreover, in the case of 2-aminophenol a shift in the full width half maxima (FWHM) (instead of red shift in the λem of NOCDs) is observed.
Effect of pH on reduction of NOCDs The pH induced changes in the spectral properties of NOCDs with 4-NP (1mM) are shown
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in Figure S7. Similar to Figure 5a, the broad PL spectra of NOCDs with 4-NP (1mM) are observed around 475 nm (without maintaining pH) along with an increased intensity
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accompanied with a red shift in PL peak (Figure S6) with pH variation in the range 3.0 ≤ pH ≤ 4.5. Furthermore, when the pH is maintained between 5.0 ≤ pH ≤ 6.0, the PL intensity
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gradually decreased with red shift in the emission peak. Similarly, when the pH is adjusted
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6.5 ≤ pH ≤ 8.5, the PL emission is gradually increased without any further shifting of PL peak emission wavelength. The observed spectral changes can be useful in contemplating the
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response of the NOCD sensor at different pH.
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Figure 9. The XRD patterns of as-prepared NOCDs to rNOCDs by adding different pH conditions of 4-NP (1mM; pH 3.0, 6.5 and 9.0) into NOCDs
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The XRD patterns of NOCDs, at different pH conditions (3.0, 6.5 and 9.0) of 4-NP (1mM) (shown in Figure 9), exhibited a sharp peak centered at 11.31° and 28.2° at pH (4-NP) 3.0. This sharp peak at 11.31° is matching with above-mentioned NOCDs result (Ref to Figure 3a) but with less intensity. Additionally, the occurrence of the new peak at around 28.2° is attributed to the partial formation of rNOCDs. At pH = 6.5 with no observable change in the peak position (at 11.31°) as compared to Figure 3a, implies that without maintaining the pH of 4-NP solution, it is equivalent to maintaining the pH at around 6.5. Furthermore, a change
to basic pH = 9.0 (4-NP with NOCDs) reveals some changes in the intensity and peak position of rNOCDs (28.0°) along with appearance of the inherent peak at 11.31° (similar to NOCDs with relatively less intensity as compared with pH = 6.5). This may imply towards the possibility to have partial formation of rNOCDs at acidic as well as basic pH. Effect of NOCDs´ concentration on the detection of 4-nitrophenol As-prepared NOCDs concentration effect on the 4-NP (1mM) detection was tested by adding 5-27 µL (with 2 µL of NOCDs in each step) into reaction mixer. The quantity of NOCDs
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was shown to have an effect on the PL spectra of NOCDs at λex =347 nm (Fig. 2b). The emission intensities of NOCDs are found to gradually increase with an increase in the amount
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of NOCDs (≈27 µL) in the solution. It is accompanied with peak wavelength red-shift from
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approx. 475 to 480 nm with an increase in the concentration (of NOCDs) from 5 to 13 µL. Any further increase in the concentration fails to manifest any observable shift in the PL emission peak position. Interestingly, on the other hand PLE spectra (Figure S8,
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Supplementary) shows a continuous blue shift (from 347 to 330 nm) in the excitation wavelength, along with an increase in the signal intensity (PL emission at 475 nm) of the
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peak when the concentration of NOCDs increased from 5 to 27 µL. These shifts in electronic spectra as a function of concentration usually reflect the molecules interactions with themselves. Under much diluted conditions they are isolated, but as the concentration
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increase, they start to interact to each other.
Sensing response of 4-NP spiked tap and industrial water samples for practical applicability
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Furthermore, we have tested NCODs’ based nanoprobes on 4-NP spiked tap water and industrial samples from five different sources at different concentrations (0 to 1000 μM) of the organic pollutant (Figure 10, 11 and S9 - S14). Insignificant changes in the optical spectra are observed upon the incorporation of NCODs in the real water samples in the absence of 4-NP (control sample).
The measured PL spectra results are reliable at various
concentrations of 4-NP spiked tap water, which signifies a good precision and exactness of the proposed method. The 4-NP detection capability of the nanoprobes in tap water is found
to be very similar with respect to the tests performed with distilled water. A linear decrease (Figure 10, 11) at lower concentrations opens the possibility of its applicability in evaluating
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the level of contamination in actual industrial water from pesticide/textile industry.
Figure 10. Fluorescence spectra of NCODs in various concentrations of 4-NP (0 -1000µM)
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spiked five different tap and industrial water samples at λex=344nm; (a) deionized water, (b) tap water 1, (c) tap water 2, (c) tap water 3, (d) industrial water 1 and industrial water 2
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(corresponding contour graphs presented in supplementary information Figure S 8– S13).
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Figure 11. (a, c) Variation of peak emission wavelength and intensity (at λex=344nm) with 4-NP concertation (0 to 1000μM), (b, d) linear dependence of peak emission wavelength at and PL intensity with 4NP in the concentration range from 0 to 100μM. Corresponding PL
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signal vs wavelength for each tap and industrial water sample is given in Figure 7).
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Figure 12. (a) Optical spectra of NOCDs and 4-NP, before and after the addition of NOCDs into 4-NP; (b) Schematic representation of possible sensing mechanism of 4-NP
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(deprotonation).
Proposed fluorescence quenching mechanism
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Possible quenching of NOCDs PL by 4-NP could be due to an electron transfer process [37]. We understand that NOCDs promote the deprotonation of 4-NP to 4-nitrophenolate ion (4NP ion) as confirmed from the changes in the maximum absorption band of 4-NP centered
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around 314 to 400 nm corresponding to 4-NP ion [38, 39] (Figure 12 and S14) and the color change from pale yellow to dark yellow (Figure 6 and Figure S6e). Similar changes in 4-NP have been reported in the presence of NaBH4 [39, 40] during its catalytic reduction [40]. Due to the overlap between PLE bands of NOCDs and the absorbance band of 4-NP (Figure 12a) only part of the UV radiation is absorbed by the 4-NP itself (the effect more prominent at
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higher concentrations of 4NP). The absorption of the UV radiation initiates the deprotonation of 4-NP to 4-NP ion in the presence of NOCDs, giving rise to electron transfer process (contact quenching; direct contact between 4NP and NOCDs) which in turn promotes the reduction of the nanoprobes themselves. The quantum chemical calculations corresponding to 4nitrophenol and 4-nitrophenolate ion (frontier molecular orbitals, conformation HOMO-LUMO, density of states, vibrational analysis) are presented in the Figure S14 (in the supplementary information).
Conclusion: The fluorescent NOCDs, prepared with simple, low-cost bottom-up approach, have been demonstrated as optical sensors for 4-NP. The detailed XRD characterization of the nanoprobes (pristine and after 4NP sensing) revealed the disappearance of the peak at 11.3o
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(corresponding to graphene oxide) and the emergence of clear peak at 28.1 o (reduced graphene oxide), confirming the partial reduction of the probes in the presence of 4NP. A
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linear dependence of emission and absorption signal was exhibited in the wide concentration range of 4-NP. The loss of PL signal from NOCDs at higher concentrations of 4-NP is found to be due to the absorption of UV radiation by 4NP itself. The applicability of the proposed
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nanoprobes for the determination of 4-NP spiked tap and industrial water samples in the linear range of 0- 100 µM, establishes the potential applications of the proposed NOCDs in
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the field of environmental and biological surveillance.
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PII:
S0304-3894(19)31597-3
DOI:
https://doi.org/10.1016/j.jhazmat.2019.121643
Reference:
HAZMAT 121643
To appear in:
Journal of Hazardous Materials
Received Date:
26 June 2019
Revised Date:
27 September 2019
Accepted Date:
7 November 2019
Please cite this article as: Bogireddy NKR, Cruz Silva R, Valenzuela MA, Agarwal V, 4-nitrophenol optical sensing with N doped oxidized carbon dots, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121643
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4-nitrophenol optical sensing with N doped oxidized carbon dots N. K. R. Bogireddy1, R. Cruz Silva2, Miguel A. Valenzuela3 and Vivechana Agarwal1* 1
Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autonoma del Estado de Morelos, Av. Univ. 1001, Col. Chamilpa, Cuernavaca, Morelos 62209, México. 2
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Global Aqua Innovation Center, Shinshu University, Nagano City, 380‐8553 Japan, and Institute of Carbon Science and Technology, Faculty of Engineering, Shinshu University, Nagano City, 380‐8553 Japan. Lab. Catálisis y Materiales, ESIQIE-Instituto Politécnico Nacional, Zacatenco, 07738 CDMX, México.
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*Corresponding author e-mail: [email protected]
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Graphical abstract
Highlights
Hydrothermally synthesized N-doped oxidized carbon dots from citric acid and urea
Emission from nanoprobes found to be sensitive towards 4-nitrophenol (4-NP)
Sensing mechanism attributed to simultaneous electron transfer and reduction of the probe structure
The developed material was successfully used to sense to 4-NP spiked tap and industrial water samples
Abstract: In this work, we report a facile strategy for 4-nitrophenol (4-NP) sensing using highly luminescent nitrogen doped oxidized carbon dots. The quenching of fluorescence (turn OFF),
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with the addition of trace amounts of organic pollutant (4-NP) in NOCDs, has been attributed to the complete reduction of nitrogen doped oxidized carbon dots (NOCDs) to reduced
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nitrogen doped oxidized carbon dots (rNOCDs) and its formation was confirmed by infrared, X-ray diffraction and X-ray photoelectron spectroscopy measurements.
The chemical
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changes in oxygen-containing functional groups of NOCDs, in the presence of 4-NP, are elucidated and corresponding characterization through XPS reveals the changes in the peak
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intensities of C-C (284.5 eV) and O-C=O (288.6 eV), indicating a decrement in hydroxyl groups that hinder its complete reduction to NOCDs. The sensitivity of NOCDs towards 4-
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NP has been tested in spiked tap water in the concentration range 2 µM to 2mM with the minimum detection limit of 2 µM (linear detection range from 2 to 100 µM with regression coefficient R2 =0.99). The proposed simple sensing platform can be used to reduce NOCDs
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and simultaneously sense low concentrations of 4-NP. Finally, an effective treatment to improve the reduction of nitrogen doped graphene oxide is proposed.
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KEYWORDS: Fluorescent probe, graphene, nitrogen doped oxidized carbon dots, 4nitrophenol, sensing
Introduction
In recent days pollutant free drinking water [1] is limited due to the constant release of organic contaminants into the environment, such as i.e., 4-nitrophenol (4-NP), direct blue 24 (DB24), methyl orange (MO) and methylene blue (MB). Among them 4-NP has been listed as “priority pollutant” by U. S. Environmental Protection Agency (EPA) due to the extent and severity of the environmental pollution, maximum allowable level in drinking water is
60 µg/L (~0.43 µM) [2-4] caused by this substance. Apart from being utilized in the manufacturing of many chemicals such as fenitrothion, paracetamol, parathion and methyl parathion, 4-NP is released by various herbicide/insecticide industries [5]. Resulting ground water pollution has been found to adversely effect the cognitive as well as nervous and ocular system functioning in all the living organisms including humans (cyto-embryotoxic and mutagenic effects in mammals) [2,5]. Due to the above-mentioned toxic effects of 4-NP, there is an increasing demand of portable sensing devices for reliable on-site observation of the pollutant. Among the existing standard analytical methods such as fluorimetry [6, 7],
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spectrophotometry [8,9], gas and liquid chromatography [10,11], fluorimetry based detection processes are relatively simple, fast, cost effective, sensitive and portable with an option of
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real-time detection. Therefore, the fabrication of fluorescent nanomaterials such as Mn doped ZnS quantum dots,[12] silver and gold nanocrystals,[13] carbon-based nanomaterials, [14-
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19] has been the focus of attention towards the detection of 4-NP at industrial level. Apart from 4-NP sensing reported with relatively expensive materials (such as Au NCs [13]
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and mesoporous molecular imprinting polymers containing manganese (II)-doped ZnS quantum dots [12]), among the non-toxic carbon dots based 4-NP sensors, H. Yuan et al.
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(2016)[14] were the first to report fluorimetry based optical (PL quenching) sensing of 4-NP employing nitrogen doped CDs with pH-independent emission. In addition, boron, nitrogen co-doped CDs were used in the recent past to develop fluorescence quenching assay for 4-
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NP in the concentration range of 0.5 - 200 µM (Na Xiao et al. (2018) [15]. Similar work
from G. Ren et al. (2018) [16] showed 4-NP induced PL quenching of hydrothermally prepared CDs from natural spinach. Recently, Ji-Min Yang et al. (2019) [17] reported the fluorescence quenching of CDs-immobilized zirconium-based metalorganic framework composite within a wide concentration range of almost three orders of magnitude.
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Additionally, J. Wei et al. (2019) [18] reported fluorometric determination of pesticides (including 4-NP) and organophosphates using nanoceria as a phosphatase mimic with an inner filter effect on carbon nanodots. Y. Y. Qu et al. (2019) [19] reported CDs as colorimetric and fluorescent dual-readout probe for 4-NP detection with 0.001-1µM range. However, understanding the changes in the characteristics (morphology and crystallinity) of the sensing probe, during the sensing of 4NP, still remains challenging. Here we report, a facile colorimetric and fluorescence based 4-NP sensing (turn OFF) using blue light emitting
nitrogen doped oxidized carbon dots (NOCDs) as probes, accompanied with the simultaneous reduction of the sensing probe itself. 4-NP induced reduction of NOCDs was confirmed by infrared, X ray diffraction and X-ray photoelectron spectroscopy measurements.
Materials and methods Citric acid (ACS reagent, ≥99.5%), Urea (ACS reagent, 99 to 100%) and 4-nitrophenol
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(spectrophotometric grade) were acquired from Sigma Aldrich and used without further
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treatment. Deionized water (18.2 MΩ cm) was used throughout the experiments.
Synthesis of nitrogen-doped oxidized carbon dots (NOCDs)
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Similar to the previously reported synthesis method, [20-22] nanoparticles were prepared by hydrothermal method, using 0.25 g citric acid and 0.37 g urea suspension in 50 mL deionized
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water and stirred for 15 min to make a clear solution, which was kept in a Teflon-lined hydrothermal autoclave at 180°C for 60 min. The final product (bright yellow color solution)
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was centrifuged several times by continuous washing with water/ethanol [23] and stored under ambient conditions for further use.
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Surface characterization
The optical properties including UV-visible and fluorescence spectra of as-synthesized NOCDs were measured in a dual beam Perkin-Elmer Lambda 950 and a Cary Eclipse Fluorescence Spectrophotometer, respectively. Transmission electron microscopy (TEM) was carried out with a JEOL JEM-ARM200F transmission electron microscope to identify
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the morphology and particle size distribution of NOCDs. Sample was prepared on lacey carbon film over 100 mesh Cu grid by drop-casting well diluted NOCDs and evaporated under ambient conditions. X-ray diffraction (XRD) patterns of NOCDs and reduced NOCDs (rNOCDs) were performed with a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å). XRD samples were prepared as a film by drop casting using 500 µL of 4nitrophenol with 5 µL of as-prepared NOCDs and then evaporated by using heating mantle at 45 ± 50C. X-ray photoelectron spectroscopy (XPS) was recorded on an ESCA Ulvac-PHI
1600 photoelectron spectrometer from Physical Electronics using Al Kα radiation with a photon energy of 1486.6 ± 0.2 eV. The Fourier transform infrared (FTIR) spectra of NOCDs and reduced nitrogen doped oxidized carbon dots (rNOCDs) were measured using Varian 660-IR FT-IR spectrophotometer.
Procedure for detection of 4-nitrophenol (simultaneous reduction of NOCDs)
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For the detection of 4-nitrophenol, 405 µL of reaction mixture contains different concentrations, from 2µM to 2mM, of 4-nitrophenol (4-NP) and 5 µL of as prepared NOCDs.
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Samples with the same concentration of 4NP, with the addition of NOCDs particles, the volume was made to 1ml in deionized water. After constant stirring for 30s, absorption and emission spectra of the solution was measured by UV-visible and fluorescence
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spectrophotometer (λex = 344 nm) in the wavelength range of 200 – 600 nm at a scan rate of
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240 nm/min at room temperature under ambient conditions. Detection of 4-NP in real tap water samples
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Three different tap water (TW) and industrial water (IW) samples were obtained from different places in Cuernavaca city (TW1 (google map view: 18.94926, -99.2709454), TW2 (18.977705, -99.247733), TW3 (18.9808869, -99.2401062), IW1 and IW2. The real water
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samples were directly spiked with different concentrations of 4-NP (0, 10, 30, 50, 80 and 100 μM) and the corresponding PL spectra were recorded.
RESULTS AND DISCUSSION
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The size distribution of as-prepared NOCDs, analyzed through TEM and HR-TEM (Figure 1(a-c)) shows a homogeneous distribution of NOCDs. The histogram reveals a narrow-size distribution of NOCDs (Figure 1a inset) in the range of 2 – 5.5 nm, with average diameter of 3.8 ± 1.7 nm [14]. Crystalline structure, with lattice spacings of 0.221 nm (G (100)), 0.110 nm (G (020)), and diamond like 0.203 nm (111), 0.129 nm (110) respectively, were also observed (Fig. 2c). These spacings are consistent with both graphitic [(100), (020)] and diamond-like [(111), (110)] carbon preventing an obvious structure assignment [24-26].
However, the absence of graphitic (002) spacings of 0.33 nm indicates the NOCDs may
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Figure 1. (a, c and d) TEM and HRTEM images of as-prepared NOCDs at different magnifications
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(b) Size distribution taken over 100 particles; Inset of (c) represents the FFT analysis of corresponding image; Measured d-spacing values are indicated in (d).
Fluorescence spectrum of NOCDs (Figure 2 (a, b)), at different excitation wavelengths [27], reveals an excitation-independent strong blue fluorescence band at 441 nm with a gradual increase only in fluorescence intensity (no shift in maximum emission peak), when excited with wavelength from 290 to 347 nm followed by decreasing emission signal in the range of 350 – 390 nm. The results clearly show that at λex = 347 nm, largest number of NOCDs
particles have been excited giving rise to maximum emission intensity. Photoluminescence excitation (PLE) measured at 441 nm (emission) and absorbance (Figure 2b) additionally confirms the same. Moreover, as-prepared NOCDs exhibit two distinct absorption bands at around 235 nm (π to π* transition corresponding to C=C group of the possibly localized sp2 clusters in the carbon quantum dot) and 347 nm (n to π*) in the UV-visible spectrum (Figure 2c). The absorption at 347 nm (in agreement with the PLE results) is attributed to the dopant element (nitrogen) along with the strong absorption band (235nm) of graphitic structure [27]. It is believed that the introduction of nitrogen as dopant introduces many surface states
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associated to the enhancement of the luminescence properties of NOCDs. [27] The PL characteristics of the proposed nanoprobes (NOCDs), are controlled simultaneously by
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surface defects as well as the dimensions (quantum confinement).
Figure 2. (a) Fluorescence emission spectra of NOCDs when excitation wavelength varies from 290 – 390 nm, with step of 20nm (b) Excitation fluorescence spectra (PLE) of NOCDs at 441 nm and emission spectra of NOCDs at 344 nm excitation, (c) UV-visible absorbance spectra of NOCDs; Inset shows the optical images of NOCDs illuminated under white (left:
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visible light) and UV light (middle at 254 nm and right one at 365 nm)
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Figure 3. (a) The XRD patterns of as-prepared NOCDs (inset shows enlarged view of NOCDs peak from 20 to 60˚) (sample was prepared as a film by drop casting 200 µL of NOCDs followed by heating at 45 ± 50C till dry); Deconvoluted XPS spectra of (b) C 1s, (c) O 1s (change in C-O/C=O 0.518) intensity ratio is studied before and after the addition of 4-NP to analyze the change in the probe structure) and (d) N1s peaks of NOCDs.
In conformity with the already reported literature [27-30], XRD pattern of NOCDs (Figure 3a) exhibits a sharp peak centered at 11.31°, which is attributed to an interlayer spacing (001) of 0.78 nm. Apart from this peak, we can see the patterns of sp2 -diamond, graphite and carbon. sp2 -Diamond possesses the same space group as diamond around 2Ɵ = 39.0 (004) and 40.5 (111). Similarly, carbon and G (002) peaks were also observed. [31, 32]
XPS was used for elemental and chemical state identification in NOCDs. The survey scan of
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NOCDs clearly shows the major peaks at 532, 400 and 285 eV, which are the characteristic peaks of O1s, N1s and C1s, respectively. The occurrence of N 1s (400 eV) peak confirms
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the existence of nitrogen atom in NOCDs (Figure S1, Supporting Information). Furthermore, the XPS C 1s, O 1s and N1s peaks are deconvoluted to understand the chemical species
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present in NOCDs. After deconvolution of C1s peak of NOCDs, C-C (284.5 eV), C-O (286.0 eV), C=O (287.1 eV) and O-C=O (288.6 eV) are clearly observed (Figure 3a), indicating the
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existence of carbonyl and carboxyl functional groups on the surface of NOCDs [33]. In addition, the contribution of C-N (286.5 eV) means the successful incorporation of N atoms
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onto the NOCDs structure. The deconvoluted O1s and N1s peaks of NOCDs also show similar results. The deconvoluted O1s peaks at 531.6 and 533.0eV can be assigned as C=O and C-O, respectively (Figure 3b), while pyridinic N (399.6 eV) and graphitic N/amine N
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(401.3 eV) are observed in the deconvoluted N1s peak (Figure 3c, Table S1). These results clearly indicate the existence of oxygen-containing functional groups and N atom is closely linked onto the NOCDs surface.
Figure S2 (Supporting Information) showing the FTIR spectra of NOCDs exhibits a strong intensity peak at 3320 cm-1, corresponding to the stretching vibration of O–H group [34].
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This result is in good agreement with the XPS O1s peak, indicating the good hydrophilic property of NOCDs suggesting the location of hydroxyl groups on the surface. Another band located at 2343 cm-1 correspond to the stretching vibration mode of N–H bond [34]. Additionally, the bands at 1644 cm-1 are from the bending vibration of C=C bond of graphene [33].
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Reduction of NOCDs with simultaneous detection of 4-nitrophenol
Figure 4. NOCDs in organic pollutants (a) photographic images under daylight (upper part) and UV lamp at 365 nm (lower part), (b) histogram of the fluorescence intensity. Phe= phenol; AP=aminophenol; NP=nitrophenol; Cat=catechol.
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Figure 5. (a) The PL emission and (b) PLE spectra of NOCDs at different concentration of 4nitrophenol from 2 µM to 2 mM with 3 different concentration ranges; (Range 1) 2 µM to 20 µM, (Range 2) 20 µM to 100 µM and (Range 3) 200 µM to 2 mM, (c) 4-NP concentration (µM) vs emission intensity corresponding to peak wavelength and (d) 4-NP (µM) concentration vs emission wavelength. Samples were prepared by the addition of NOCDs (5 µL) into different concentrations
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of 4-nitrophenol (total volume 500 µL)).
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Figure 6. Optical photographs of NOCDs containing various concentrations of 4-NP (12.5-
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1000 µM) solution under visible light (top) and UV light (bottom) at λex=365nm. Absorbance
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corresponding to each concentration of 4-NP is shown in Figure S4.
The selectivity of a sensing probe is an important criteria of a successful sensor nanoprobe. In order to check the selectivity, the fluorescence response of NOCDs was tested with respect
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to different organic pollutant molecules viz. phenol, 2-aminophenol, 4-aminophenol, catechol, and 4-nitrophenol. Except 4-nitrophenol, the pollutants remained inert towards
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optical response of the NOCDs (shown in Figure S3) and their corresponding digital photographic images and histogram of PL studies are presented in Figure 4.
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Figure 5a shows the fluorescence spectra of NOCDs dispersed in different concentrations of 4-NP (2 –2000µM; three orders of magnitude). An increase in 4-nitrophenol concentration from 2 µM-2 mM (Figure 5 (c, d)) decreases the PL intensity along with a red shift in the emission peak wavelength position from 441 to 475 nm. Furthermore, after the addition of 1mM 4-NP to the NOCDs, although there is no significant shift in the emission peak
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wavelength, a decrement in the emission peak intensity is observed. Similar characteristics were revealed in the PLE spectra as well, i.e., a decrement in the PL intensity (corresponding to 441nm) with a little blue shift (peak position) in the excitation wavelength from 347 to 340 nm. Interestingly, after the addition of >1 mM 4-NP to the NOCDs solution, the PLE wavelength shift to its actual position (347 nm). As it occurs at higher concentrations, it is possibly due to intermolecular interactions between 4-NP molecules. Additionally, a gradual
decrement in the full width half maximum (FWHM) of emission peak from 2 µM-2 mM is
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discerned from the PL emission spectra.
Figure 7. (a) The XRD patterns of the as-prepared NOCDs to rNOCDs by increasing the 4-NP concentration (0.6, 1.2 and 2 mM) into NOCDs; and deconvoluted XPS spectra of (b) C 1s, (c) O
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1s and (d) N1s peaks of rNOCDs.
The XRD patterns of NOCDs at different concentrations of 4-NP (0.6, 1.2 and 2mM), shown in Figure 7a exhibit a sharp peak centered at 11.31°, which is matching with above results of NOCDs. After the addition of 4-NP (0.6 and 1.2 mM), there is drastic decrement in the peak at 11.31°. Interestingly, an addition of 2 mM 4-NP to the NOCDs, there is a new peak observed around 28.1° and the peak around 11.3° disappeared. The sharp peak at 28.1º, corresponding to d-spacing of ca. 0.32 nm is attributed to the enlarged interlayer distance, resulting from steric hindrance of functional groups on the graphene edge or the plane
distortion from sp3 C in the graphene plane. The peak at 28.1º is the characteristic (002) diffraction peak of graphite [explained in conformity with Ref 29, 30]. Its appearance indicates the interlayer stacking within NOCDs with 2mM 4-NP, and is very close to that in graphite (≈ 0.35 nm), confirming the formation of reduced NOCDs (rNOCDs) in the present case. Additionally, the close packing (0.35 nm) of highly conjugated sp2 domains possibly takes dominant place within the core part of rNOCDs). On the edge of rNOCDs, the functional groups or sp3 C will enlarge the interlayer distance.
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XPS was used to characterize the chemical species of elements in rNOCDs. The survey scan of rNOCDs clearly shows the major peaks are similar to NOCDs (Figure S5 (supplementary
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information)). But after deconvolution of C1s peak of rNOCDs, there is a decrement in the peak intensities corresponding to C-C (284.5 eV) and O-C=O (288.5 eV) (Figure 7b),
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indicating the decrement of hydroxyl groups on the surface of NOCDs. In addition, the deconvoluted O1s and N1s peaks of rNOCDs also show similar results. The deconvoluted
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O1s peaks at 531.6 (C=O) and 533.0 eV (C-O) are decreased (Figure 7c), while pyridinic N (399.6 eV) and graphitic N/amine N (401.3 eV) are increased due to the presence of 4-NP in
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rNOCDs (Figure 7d, Table S1). These results clearly indicate the % decrement of oxygencontaining functional groups on the surface of NOCDs, which clearly confirms the reduction (loss of oxygens) of NOCDs [33].
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Furthermore, Figure S6 (Supporting Information) shows the FTIR spectra of 4-nitrophenol (2mM), rNOCDs (presence of 2mM of 4-NP into NOCDs) and its corresponding differential signal. The rNOCDs exhibit a weak intensity peak compared with NOCDs at 3320 cm-1, which implies the reduction of stretching vibration of O–H group in NOCDs. This result is in good agreement with the XPS C1s and O1s peak, indicating the reduced hydrophilic
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property of NOCDs because of the less transmittance % of hydroxyl groups on the surface [33, 35 and 36]. Additionally, the band at 1644 cm-1 shows a drastic decrement in the % transmittance corresponding to the bending vibration of C=C bond of graphene.
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Interference studies for 4-nitrophenol detection:
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Figure 8. Optical sensing of 4-nitrophenol in water spiked with different other contaminants (a) Phenol (using PL intensity, peak emission wavelength shift), (b) 2-aminophenol (full width half maxima) (c) 4-aminophenol (using PL intensity, peak emission wavelength shift) and (d) catechol (using PL intensity, peak emission wavelength shift). Furthermore, we also performed the interference studies of the mentioned organic pollutants (phenol, 2-aminophenol, 4-aminophenol, and catechol) with 4-nitrophenol at different concentrations (Fig. 4 and 8). A red shift in the PL emission wavelength (λem) as well as
decrease in the PL signal intensity of NOCDs by the addition of 4-nitrophenol (at different concentrations) is revealed. Moreover, in the case of 2-aminophenol a shift in the full width half maxima (FWHM) (instead of red shift in the λem of NOCDs) is observed.
Effect of pH on reduction of NOCDs The pH induced changes in the spectral properties of NOCDs with 4-NP (1mM) are shown
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in Figure S7. Similar to Figure 5a, the broad PL spectra of NOCDs with 4-NP (1mM) are observed around 475 nm (without maintaining pH) along with an increased intensity
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accompanied with a red shift in PL peak (Figure S6) with pH variation in the range 3.0 ≤ pH ≤ 4.5. Furthermore, when the pH is maintained between 5.0 ≤ pH ≤ 6.0, the PL intensity
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gradually decreased with red shift in the emission peak. Similarly, when the pH is adjusted
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6.5 ≤ pH ≤ 8.5, the PL emission is gradually increased without any further shifting of PL peak emission wavelength. The observed spectral changes can be useful in contemplating the
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response of the NOCD sensor at different pH.
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Figure 9. The XRD patterns of as-prepared NOCDs to rNOCDs by adding different pH conditions of 4-NP (1mM; pH 3.0, 6.5 and 9.0) into NOCDs
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The XRD patterns of NOCDs, at different pH conditions (3.0, 6.5 and 9.0) of 4-NP (1mM) (shown in Figure 9), exhibited a sharp peak centered at 11.31° and 28.2° at pH (4-NP) 3.0. This sharp peak at 11.31° is matching with above-mentioned NOCDs result (Ref to Figure 3a) but with less intensity. Additionally, the occurrence of the new peak at around 28.2° is attributed to the partial formation of rNOCDs. At pH = 6.5 with no observable change in the peak position (at 11.31°) as compared to Figure 3a, implies that without maintaining the pH of 4-NP solution, it is equivalent to maintaining the pH at around 6.5. Furthermore, a change
to basic pH = 9.0 (4-NP with NOCDs) reveals some changes in the intensity and peak position of rNOCDs (28.0°) along with appearance of the inherent peak at 11.31° (similar to NOCDs with relatively less intensity as compared with pH = 6.5). This may imply towards the possibility to have partial formation of rNOCDs at acidic as well as basic pH. Effect of NOCDs´ concentration on the detection of 4-nitrophenol As-prepared NOCDs concentration effect on the 4-NP (1mM) detection was tested by adding 5-27 µL (with 2 µL of NOCDs in each step) into reaction mixer. The quantity of NOCDs
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was shown to have an effect on the PL spectra of NOCDs at λex =347 nm (Fig. 2b). The emission intensities of NOCDs are found to gradually increase with an increase in the amount
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of NOCDs (≈27 µL) in the solution. It is accompanied with peak wavelength red-shift from
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approx. 475 to 480 nm with an increase in the concentration (of NOCDs) from 5 to 13 µL. Any further increase in the concentration fails to manifest any observable shift in the PL emission peak position. Interestingly, on the other hand PLE spectra (Figure S8,
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Supplementary) shows a continuous blue shift (from 347 to 330 nm) in the excitation wavelength, along with an increase in the signal intensity (PL emission at 475 nm) of the
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peak when the concentration of NOCDs increased from 5 to 27 µL. These shifts in electronic spectra as a function of concentration usually reflect the molecules interactions with themselves. Under much diluted conditions they are isolated, but as the concentration
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increase, they start to interact to each other.
Sensing response of 4-NP spiked tap and industrial water samples for practical applicability
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Furthermore, we have tested NCODs’ based nanoprobes on 4-NP spiked tap water and industrial samples from five different sources at different concentrations (0 to 1000 μM) of the organic pollutant (Figure 10, 11 and S9 - S14). Insignificant changes in the optical spectra are observed upon the incorporation of NCODs in the real water samples in the absence of 4-NP (control sample).
The measured PL spectra results are reliable at various
concentrations of 4-NP spiked tap water, which signifies a good precision and exactness of the proposed method. The 4-NP detection capability of the nanoprobes in tap water is found
to be very similar with respect to the tests performed with distilled water. A linear decrease (Figure 10, 11) at lower concentrations opens the possibility of its applicability in evaluating
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the level of contamination in actual industrial water from pesticide/textile industry.
Figure 10. Fluorescence spectra of NCODs in various concentrations of 4-NP (0 -1000µM)
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spiked five different tap and industrial water samples at λex=344nm; (a) deionized water, (b) tap water 1, (c) tap water 2, (c) tap water 3, (d) industrial water 1 and industrial water 2
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(corresponding contour graphs presented in supplementary information Figure S 8– S13).
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Figure 11. (a, c) Variation of peak emission wavelength and intensity (at λex=344nm) with 4-NP concertation (0 to 1000μM), (b, d) linear dependence of peak emission wavelength at and PL intensity with 4NP in the concentration range from 0 to 100μM. Corresponding PL
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signal vs wavelength for each tap and industrial water sample is given in Figure 7).
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Figure 12. (a) Optical spectra of NOCDs and 4-NP, before and after the addition of NOCDs into 4-NP; (b) Schematic representation of possible sensing mechanism of 4-NP
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(deprotonation).
Proposed fluorescence quenching mechanism
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Possible quenching of NOCDs PL by 4-NP could be due to an electron transfer process [37]. We understand that NOCDs promote the deprotonation of 4-NP to 4-nitrophenolate ion (4NP ion) as confirmed from the changes in the maximum absorption band of 4-NP centered
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around 314 to 400 nm corresponding to 4-NP ion [38, 39] (Figure 12 and S14) and the color change from pale yellow to dark yellow (Figure 6 and Figure S6e). Similar changes in 4-NP have been reported in the presence of NaBH4 [39, 40] during its catalytic reduction [40]. Due to the overlap between PLE bands of NOCDs and the absorbance band of 4-NP (Figure 12a) only part of the UV radiation is absorbed by the 4-NP itself (the effect more prominent at
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higher concentrations of 4NP). The absorption of the UV radiation initiates the deprotonation of 4-NP to 4-NP ion in the presence of NOCDs, giving rise to electron transfer process (contact quenching; direct contact between 4NP and NOCDs) which in turn promotes the reduction of the nanoprobes themselves. The quantum chemical calculations corresponding to 4nitrophenol and 4-nitrophenolate ion (frontier molecular orbitals, conformation HOMO-LUMO, density of states, vibrational analysis) are presented in the Figure S14 (in the supplementary information).
Conclusion: The fluorescent NOCDs, prepared with simple, low-cost bottom-up approach, have been demonstrated as optical sensors for 4-NP. The detailed XRD characterization of the nanoprobes (pristine and after 4NP sensing) revealed the disappearance of the peak at 11.3o
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(corresponding to graphene oxide) and the emergence of clear peak at 28.1 o (reduced graphene oxide), confirming the partial reduction of the probes in the presence of 4NP. A
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linear dependence of emission and absorption signal was exhibited in the wide concentration range of 4-NP. The loss of PL signal from NOCDs at higher concentrations of 4-NP is found to be due to the absorption of UV radiation by 4NP itself. The applicability of the proposed
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nanoprobes for the determination of 4-NP spiked tap and industrial water samples in the linear range of 0- 100 µM, establishes the potential applications of the proposed NOCDs in
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