Doped Graphene Quantum Dot Nanocomposites for Highly Selective

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One-Step Synthesis of Size-Tunable Gold@Sulfur- Doped Graphene Quantum Dot Nanocomposites for Highly Selective and Sensitive Detection of Nanomolar 4-Nitrophenol in Aqueous Solutions with Complex Matrix Nguyen Thi Ngoc Anh, and Ruey-an Doong ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00210 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Applied Nano Materials

One-Step Synthesis of Size-Tunable Gold@SulfurDoped Graphene Quantum Dot Nanocomposites for Highly Selective and Sensitive Detection of Nanomolar 4-Nitrophenol in Aqueous Solutions with Complex Matrix Nguyen Thi Ngoc Anh1 and Ruey-an Doong 1, 2,* 1. Institute of Environmental Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan. 2. Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 30013, Hsinchu, Taiwan *: Corresponding Author. E-mail address: [email protected] (R.A. Doong)

ABSTRACT In this study, a simple one-step synthesis procedure for the fabrication of gold nanoparticle (Au

NP)@sulfur-doped graphene quantum dot (Au@S-GQD) with tunable size was developed for the sensitive and selective detection of 4-nitrophenol in water and wastewater with complex matrix. The particle size of 2 – 6 layered S-GQD is in the range of 1 – 7 nm with mean diameter of 4.0 ± 0.5 nm. The S-GQD serves as reducing agents for both Au ions and 4-nitrophenol reduction to generate

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Au@S-GQD nanocomposites as well as to produce absorption signal of 4-aminophneol at 307 nm. The particle size of Au@S-GQD is tunable by simply adjusting the Au precursor concentration and the mean diameter increases from 5 to 25 nm when the Au precursor concentration increases from 50 to 200 µM. Moreover, the Au@S-GQD nanocomposite exhibits good UV-visible absorption property, and the change of wavelength ratio between 307 and the SPR peak of Au at 530 nm (A307/A530) is used to detect nanomolar level of 4-nitrophenol after the interaction of 4-nitrophenol with S-GQD by π – π stacking interaction. A wide dynamic range of 4 orders of magnitude with the limit of detection (LOD) of 3.5 nM in deionized water is achieved. The UV-visible response of Au@S-GQD also shows good selectivity to 4-nitrophenol detection over other aromatic and nitroarene compounds. In addition, the Au@S-GQD sensing platform is successfully applied to the detection of 0.05 – 50 µM 4-nitrophenol in highly contaminated food wastewater with LOD of 8.4 nM. The recovery of 0.1 – 20 µM 4-nitrophenol in 3 different aqueous solutions with complex matrix is in the range of (97 ± 2) – (110 ± 3)%. Results obtained clearly indicate the superiority of using Au@S-GQD as the optical sensing probe for the detection of nano- to micro-molar level of 4-nitrophneols in aqueous solutions, which can be developed as a high performance and robust sensing platform for rapid detection of nitroaromatics in a wide variety of water and wastewater. Keywords: Sulfur-doped graphene quantum dot (S-GQD); gold nanoparticles (Au NPs); 4-nitrophenol; optical sensing; wastewater.

INTRODUCTION Nitroaromatic compounds including 2-nitrophenol, 4-nitrophenol and 2,4-dinitrophenol are one

of the most often used chemicals in production of explosives, pharmaceuticals, pesticides, dyes and rubber chemicals.1,2 These recalcitrant organics are hardly to be biodegraded in water and wastewater treatment plants, and result in the discharge to the receiving water to cause serious contamination and toxic effect in aquatic systems.1 For example, nitrophenols possesses health problems to human ACS Paragon Plus Environment

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beings including brain, kidneys, central nervous system, and endocrine system.2,

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However, only

limited studies have focused on the detection of 4-nitrophenol in aqueous solutions, especially in wastewater. Therefore, the development of cost-effective and sensitive method to rapidly detect 4-nitrophenol in aquatic system is highly needed for environmental safety and analysis. An intrinsic difficulty in real sample detection of 4-nitrophenol is the wide concentration range of 4-nitrophenol in different types of water and wastewater with complex matrix effect and, therefore, sample pretreatment before identification and quantification of 4-nitrophenol is required. Several

analytical

methods

including

gas

chromatography,

high

performance

liquid

chromatography and spectrophotometry have been applied to detect 4-nitrophenol.4 However, these methods are usually tedious and require expensive instruments. More recently, various sensing techniques such as fluorescence,5-7 electrochemical,4, 8 and photoluminescence9 methods with various sensing elements have been developed for the detection of 4-nitrophenol in deionized or river water. Although satisfactory analytical performance can be obtained using these techniques, the fluorescence method usually suffers the narrow dynamic range and photo-bleaching problem. Moreover, the electrochemical method is sensitive to matrix effect and hard to be used for the detection of 4-nitrophenol in aqueous solutions with complex matrix. Therefore, the colorimetric UV-visible sensors with gold nanoparticles (Au NPs) as the sensing probe is one of most effective and cost-effective method for the detection of a wide variety of chemicals because of their unique physiochemical properties and optical property including surface plasmon resonance (SPR), stability and sensitivity.10-13 However, the Au NPs are easy to aggregate without surface modification, and subsequently result in the reduction of sensing ability.14 Therefore, the surface modification of Au NPs to maintain the active surface area as well as the high stability is urgently demanded. Carbon-based nanomaterials such as carbon nanotube and graphene family have been widely used as the support or modification agent to enhance the sensing performance of Au NPs.4, 10, 15-17 The immobilization of Au NPs onto carbon support has been reported to prevent the aggregation of Au NPs as well as to enhance the detection sensitivity of analytes.15 More recently, graphene quantum dot ACS Paragon Plus Environment

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(GQD), one of the newly developed 0-dimensional graphene family, has received considerable attention on sensing application because of their unique optical and electrochemical properties. GQD and doped GQD have been successful fabricated for the rapid detection of a wide variety of chemicals including metal ions, biothiols and cancer markers.18-21 The combination of GQD with Au NPs is an effective sensing element for the detection of quercetin20 and hydrogen peroxide.22 However, the use of Au-GQD based materials for the optical detection of nitroaromatic using UV-visible method has received less attention. It is noteworthy that several reducing agents such as citric acid, NaBH4 and tert-butylamine have been used to fabricate Au NPs first and then the Au NPs are anchored onto the carbon based nanomaterials in the presence of thiol compounds to form Au-GQD nanocomposites.10, 11, 23, 24

Since carbon is a good reducing agent which can reduce Au3+ to Au NPs, one-step synthesis of

Au@S-GQD can be achieved by doping GQDs with sulfur atoms because of the strong interaction between Au and sulfur atoms. However, no work is attempted to use S-GQD as the reducing agent to fabricate Au@ S-QGD for nitrophenol sensing.

Citric acid

HAuCl4

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4-Nitrophenol

Scheme 1. The schematic diagram of optical detection of 4-nitrophenol using Au@S-GQD nanocomposites as the sensing probe.


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Herein, we have developed a one-step process to fabricate Au@S-GQD nanocomposites for highly sensitive and selective detection of 4-nitrophenol in aqueous solution with complex matrix using UV-visible method. As illustrated in Scheme 1, S-GQD prepared from the pyrolysis of citric acid and mercaptopropionic acid was simply mixed with HAuCl4, the Au precursor, to form Au@S-GQD nanocomposites at room temperature without adding any reducing or capping agent. To the best our knowledge, this is the first report to use S-GQD to directly synthesize Au@S-GQD in aqueous solution by Au-thiol linkage and then used as the sensing probe for the detection of 4-nitrophenol in different types of aqueous solutions with complex matrix. The S-GQD in Au@S-GQD nanocomposites serves as reducing agents for both Au ions and 4-nitrophenol to generate Au@S-GQD nanocomposites as well as to produce absorption signal of 4-aminophneol at 307 nm. Moreover, the SPR of Au NPs at 530 nm acts as sensing probe to detect 4-nitrophenol after the interaction of 4-nitrophenol with S-GQD by π – π stacking interaction. The 4-nitrophenol can be detected by the change of wavelength ratio between 307 and 530 nm (A307/A530) in UV-visible spectra with the limit of detection (LOD) of 3.5 nM. In addition, a wide dynamic range with recovery of 97 – 110 % in different matrices of water and wastewater is obtained. Results obtained in this study clearly indicate the superiority of as-prepared Au@S-GQD for the detection of nanomolar level of 4-nitrophenol in water and wastewater with complex matrix.

EXPERIMENTAL Synthesis of sulfur-doped graphene quantum dot (S-GQD). The S-GQD was prepared by

pyrolysis of 2 g citric acid and 300 µL of 3 mmol 3-mercaptopropionic acid (Sigma-Aldrich) at 200 °C in an oven for 45 min. The S-GQD was harvested from the obtained bright yellow suspension using 1kDa dialysis bag by adding deionized water (18.2 MΩ cm) as the dialysate for 24 h to remove the residual reagents. The final product was stored at 4 °C for further use. Preparation of Au@S-GQD nanocomposites. Au@S-GQD was prepared through a one-step process using S-GQD as the reducing agent at room temperature. 50–150 µM HAuCl4⋅3H2O (Sigma-Aldrich) were added into 0.17 mg/mL S-GQD solution to fabricate Au@S-GQD ACS Paragon Plus Environment

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nanocomposites. The color of mixture changed from yellow to pink after 30 min of incubation, indicating the formation of Au NPs. The final product of Au@S-GQD nanocomposites was obtained from centrifugation and stored at room temperature for further use. Surface characterization. The optical property including UV-visible and fluorescence spectra of as-synthesized S-GQD and Au@S-GQD was monitored by Hitachi U-4100 and F-7000 fluorescence spectrophotometer (Tokyo, Japan), respectively. Atomic force microscopy (AFM) images were examined using Agilent 5500 scanning probe microscope in tapping mode to elucidate the topography of S-GQD. The transmission electron microscopy (TEM) was carried out with a JEOL JEM-ARM200F transmission electron microscope and a JEOL JEM-2010 high-resolution TEM (HR-TEM) at 200 kV to identify the morphology and particle size distribution of S-GQD and Au@S-GQD. X-ray diffraction (XRD) patterns were performed with a Bruker D8 X-ray diffractometer with Ni filtered Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) was recorded on an ESCA Ulvac-PHI 1600 photoelectron spectrometer from Physical Electronics using Al Kα radiation photon energy at 1486.6 ± 0.2 eV. The Fourier transform infrared (FTIR) spectra of S-GQD and Au@S-GQD were determined by using a Horiba FT720 spectrophotometer. In addition, Raman spectra of S-GQD and Au@S-GQD were recorded by using Burker Senterra micro-Raman spectrometer equipped with an Olympus BX 51 microscope and an Andor DU420-OE CCD camera. Detection of 4-nitrophenol by Au@S-GQD nanocomposites. The detection of 4-nitrophenol (Sigma-Aldrich) by Au@S-GQD was performed in cuvette at room temperature. Typically, various volumes of 4-nitrophenol stock solution were added to the 1 mL of 1.5 µg/mL Au@S-GQD solution at pH 5 – 9 to get the final concentrations of 5 nM – 50 µM. After well mixing for 60 s, all the solutions were characterized by UV-visible spectrophotometer in the wavelength range of 250 – 700 nm at a scan rate of 60 nm/min. The concentration of 4-nitrophenol was determined using the absorption intensity ratio between 307 and 530 nm (A307/A530). Furthermore, all the experiments were run triplicate to obtain the precision of the developed method. ACS Paragon Plus Environment

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Detection of 4-nitrophenol in real wastewater samples. The applicability of the developed method was evaluated by spiking various concentrations of 4-nitrophenol and Au@S-GQD to real wastewater samples. Wastewater collected from food and textile wastewater treatment plants (Taoyuan City, Taiwan) and lake water (National Chiao Tung University, Hsinchu, Taiwan) were filtrated by 0.45 µm Millipore filter paper followed by C-18 SPE column to remove suspended solids and impurities.25 After filtration, Au@S-GQD was added into the solution, and then the solution was spiked with 0.1 – 20 µM 4-nitrophenol. The change in absorption ratio of A307/A530 was measured to evaluate the matrix effect and recovery of 4-nitrophenol in different types of wastewater. In addition, all the experiments were repeated three times to control the precision of the evaluation procedure of Au@S-GQD in wastewater.

RESULTS AND DISCUSSION Characterization of S-GQD and Au@S-GQD. The size distribution as well as surface

morphology of the as-prepared S-GQD and Au@S-GQD was first characterized. Figure 1a shows the TEM image and histogram of as-prepared S-GQD. The distribution of as-prepared GQD is homogeneous and well dispersed. The histogram of as-prepared S-GQD shown in the inset of Figure 1a indicates the narrow sized distribution of S-GQD. The particle size is in the range of 1 – 7 nm with average diameter of 4.0 ± 0.5 nm, which is in good agreement with the reported data.16, 25-27 The HRTEM image clearly shows that the fringe of carbon lattice spacing distance of as-prepared S-GQD is 2.4 Å, which can be assigned as the (120) plane of graphene (Figure 1b). In addition, the height found in the AFM topography is around 0.5 – 2.0 nm with an average value of 1.0 nm, confirming that GQD is a 2–6 layered graphene-based material (Figure S1, Supporting Information). After the successful preparation of S-GQD, 50 – 150 µM HAuCl4, the precursor of Au NPs, were added to the S-GQD solution for the formation of Au@S-GQD. Figure 1c shows the TEM image and histogram of Au@S-GQD produced from the addition of 150 µM HAuCl4. The spherical Au NPs are successfully generated after 120 min of reaction using S-GQD as the reducing agent. The


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particle size of Au@S-GQD nanocomposites is the range of 11 – 22 nm with the mean diameter of 17 nm (inset of Figure 1c), which is similar to those of Au NPs prepared from citrate reduction method.10, 11 In addition, the HRTEM image of Au@S-GQD nanocomposites clearly shows two distinct fringes in the outer and inner parts (Figure 1d). The lattice spacing of Au@S-GQD is 2.35 Å for the inner material (Figure 1e) and 2.1 Å for the outer shell (Figure 1f), which corresponds to the (111) plane of Au and (100) plane of graphene, respectively.28-30 The formation of core-shell like Au@S-GQD is mainly attributed to the fact that Au3+ ions can attached onto the surface of S-GQD by Au-thiol linkage. Since carbon material is a good reducing agent for Au3+ ions, the reduced gold species will be then aggregated into the Au NPs to form core-shell like Au@S-GQD nanocomposites where the inner Au NPs are covered with the shell-like S-GQD.

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Figure 1. The (a) TEM and (b) HRTEM images of as-prepared S-GQD, (c) TEM and (d) HRTEM images, fringes of (e) inner material and (f) outer shell of Au@S-GQD at 150 µM HAuCl4 and TEM ACS Paragon Plus Environment

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images of Au@S-GQD at (g) 50 and (h) 100 µM HAuCl4, respectively. Insets are the histogram of Au@S-GQD prepared by 50 – 150 µM HAuCl4 (n = 50). Similar to the Au@S-GQD nanocomposites in the presence of 150 µM HAuCl4, the addition of 50 – 100 µM Au precursor still can produce Au@S-GQD nanocomposites. As shown in Figure 1h and 1g, the Au@S-GQD nanocomposites are homogeneously generated after the addition of 50 and 100 µM HAuCl4. The histograms illustrated in the insets of Figure 1g and 1h show that the particle size distributions of Au@S-GQD are in the range of 1 – 8 and 6 – 18 nm with mean particle sizes of 5 and 12 nm when 50 and 100 µM HAuCl4 are added to the S-GQD solution, respectively. These results clearly indicate that the particle size of Au@S-GQD nanocomposites can be easily controlled by adjusting the Au precursor concentration. Furthermore, the Au@S-GQD can be well dispersed in aqueous solution for at least 2 months with less aggregation because of the negatively charged S-GQD on the surface of Au NPs (Figure S2, Supporting Information). In addition, the EDS spectrum indicates that the elemental composition of Au@S-GQD contains 30.8 wt% C, 2.5 wt% O, 65.7 wt% Au and 1 wt% S (Figure S3, Supporting Information), clearly indicating that S atoms are successfully doped into GQD and then the S-GQD serves as the reducing and stabilizing agent to fabricate the size-tunable Au@S-GQD nanocomposites. The optical property including fluorescence and UV-visible spectra of S-GQD and Au@S-GQD is further examined. Figure 2a shows the emission fluorescence intensity of S-GQD after excitation with the wavelength of 310 – 450 nm. It is clear that the S-GQD emits strong blue fluorescence at 460 nm after the irradiation (inset of Figure 2a). The emission fluorescence intensity increases from 310 to 360 nm and then decreases in the range of 370 – 450 nm, clearly showing that 360 nm is the optimal excitation wavelength of S-GQD. Moreover, the as-prepared S-GQD exhibits a distinct absorption bands at 280 nm in the UV-visible spectrum (Figure 2b), which is attributed to the doping of sulfur element along with the strong absorption band of graphitic structure.31 After addition of 50 – 150 µM HAuCl4 to form Au@S-GQD nanocomposites, the peak intensity of 280


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nm becomes weaker, presumably attributed to the fact that S-GQD serves as the reducing agent for Au precursor as well as the interaction of Au-thiol group between Au NPs and S-GQD.

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Figure 2. (a) Emission fluorescence spectra of S-GQD after excitation of 310 – 450 nm, (b) UV-visible spectra, (c) Raman spectra and (d) XRD patterns of the as-prepared S-GQD and Au@S-GQD nanocomposites. The SPR peak of Au NPs is also clearly observed for Au@S-GQD nanocomposites,32-34 indicating the successful formation of Au NPs (Figure 2b). Moreover, the SPR peak shifts from 519 to 530 nm when the Au precursor concentration increases from 50 to 150 µM. When further increasing the Au precursor concentration to 200 µM, the large Au NPs at mean diameter of 25 nm with some separated S-GQD is formed and the SPR peak slightly shifts to 535 nm (Figure S4, Supporting Information). The red-shift in SPR peaks of Au@S-GQD nanocomposites indicates the increase in particle size of Au@S-GQD, which is in good agreement with the results of TEM images where the average particle size of Au@S-GQD increases from 5 to 25 nm. Furthermore, the solution color changes from yellow to deep purple when the Au precursor concentration increases from 0 ACS Paragon Plus Environment

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(S-GQD only) to 150 µM (inset of Figure 2b), which corroborates with the change in particle size. These results demonstrate the easy fabrication of size-tunable Au@S-GQD nanocomposites by simply mixing HAuCl4 and S-GQD and the addition of 150 – 200 µM Au precursor can produce distinct optical property for sensing purposes. Therefore, the Au@S-GQD prepared by 150 µM HAuCl4 is further used for surface characterization. Raman spectroscopy is an ideal tool to characterize the atomic structure of carbon-based materials.35 Figure 2c shows the Rama spectra of S-GQD and Au@S-GQD after addition of 150 µM HAuCl4. The Raman spectrum of S-GQD exhibits two representative peaks located at 1348 and 1595 cm-1, which can be assigned as D and G bands arising from the presence of defect sites in the graphite and tangential C-C bond vibrations, respectively.36 After formation of Au@S-GQD nanocomposites in the presence of 150 µM HAuCl4, the peak intensity of D band decreases obviously, depicting the decrease in defects in S-GQDs. The D and G bands also blue-shift to 1338 and 1579 cm-1, respectively, and the ID/IG ratio decreases from 0.95 to 0.88. Furthermore, the crystallinity of S-GQD and Au@S-GQD nanocomposite was also examined. The XRD patterns shown in Figure 2d exhibit a broad peak centered at 28.7° 2θ, which can be assigned as the (002) plane of graphite.18 After addition of 150 µM Au precursor, another two strong peaks at 38.2° and 44.3° 2θ appear, which belong to the (111) and (200) planes of Au NPs. The crystalline size of Au NPs, calculated from the Scherrer equation, is around 12 nm, which is in good agreement with the results of TEM image. XPS was used to characterize the chemical species of elements in S-GQD and Au@S-GQD. The survey scan of S-GQD clearly shows the major peaks at 531 and 284 eV, which are the characteristic peaks of O 1s and C 1s, respectively (Figure S5a, Supporting Information). Additionally, the occurrence of S 1s (228 eV) and S 2p (162 eV) peaks confirms the existence of sulfur atom in S-GQD. After formation of Au@S-GQD nanocomposites, several additional peaks of Au 4f (85.8 and 83.9 eV), Au 4d (335 and 354 eV) and Au 4p (546.1 and 642.4 eV) appear.37-39 Furthermore, the XPS C 1s, O 1s and S 2p peaks are deconvoluted to understand the change in chemical species before and after the formation of Au NPs. After peak deconvolution of C 1s peak ACS Paragon Plus Environment

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of S-GQD, C=C (284.1 eV), C-O (287.4 eV) and C=O (288.5 eV) are clearly observed (Figure 3a), indicating the existence of carbonyl and carboxyl functional groups on the surface of S-GQD.25, 30 In addition, the contribution of C-S (286.1 eV) means the successful incorporation of S atoms onto the graphene structure. The deconvoluted O 1s and S 2p peaks of S-GQD also show similar results. The deconvoluted O 1s peaks at 530.5, 532.0 and 534.9 eV can be assigned as C=O, O-H and adsorbed water bonding, respectively (Figure 3b), while S-O (168.0 eV), S-H (162.7 eV) and S-S (160.2 eV) are observed in the deconvoluted S 2p peak (Figure 3c). These results clearly indicate the existence of oxygen-containing functional group on the surface and S atom is closely linked onto the GQD surface.

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Figure 3. The deconvoluted XPS spectra of (a) C 1s, (b) O 1s and (c) S 2p peaks of S-GQDs and (d) C 1s, (e) O 1s and (f) S 2p peaks of Au@S-GQDs nanocomposites. After the formation of Au@S-GQD nanocomposites, the deconvoluted XPS C 1s peak of Au@S-GQD shows similar patterns with C 1s peak of S-GQD and four peaks including C=C, C-O, C-S and C=O are deconvoluted (Figure 3d). The decrease in intensity of C=C and C=O is probably attributed to the fact that S-GQD serve as the reducing agent to convert Au3+ ions into Au NPs. Furthermore, the 285.2 eV is the characteristic peak of C-O-C bond. In addition to the C=O, O-H ACS Paragon Plus Environment

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and adsorbed water bonds shown in the deconvoluted O 1s peak of Au@S-GQD, an additional peak of C-O-C at 532.9 eV is observed (Figure 3e). The XPS S 2p peak also shows the Au-S bond at 162.5 eV along with the S-H at 164.3 eV and S-O at 168.2 eV (Figure 3f), which is in good agreement with the previously reported results.41, 42 Moreover, the Au signal shows the doublet peaks at 84 and 87.7 eV, which are the typical Au 4f7/2 and 4f5/2 (Figure S5b , Supporting Information). After deconvolution, the Au 4f spectra show the major Au0

peaks at 84.0 and 87.6 eV,43 depicting the

successful reduction of Au3+ into Au NPs by S-GQD. In addition, the C, O and S atoms contribute 55, 35 and 5 wt% to the S-GQD, and then decrease obviously to only 21, 19 and 3 wt% in Au@S-GQD, respectively (Table 1). It is noted that the O content determined by XPS is much higher than that by EDS, which indicates that oxygen is mainly contributed from the functional groups on the surface of Au@S-GQDs. Table 1. Change in chemical composition of S-GQD after addition of 150 µM HAuCl4 to generate Au@S-GQD nanocomposites.

Material

C 1s

Element (wt%) O 1s Au 4f

S 2p

S-GQD

55

35

-

5

Au@S-GQD

21

19

57

3

Figure S6 (Supporting Information) shows the FTIR spectra of S-GQD and Au@S-GQD. The S-GQD exhibits a strong intensity peak at 3472 cm-1, which is the stretching vibration of O–H group. This result is in good agreement with the XPS O 1s peak, indicating the good hydrophilic property of S-GQD because of the existence of hydroxyl groups on the surface. The broad peak at 2937 cm-1 is attributed to the stretching vibration of C-H in graphitic backbone of S-GQD. Another band located at 2287 cm-1 in the S-GQD based nanomaterials corresponds to the stretching vibration mode of S–H bond. Additionally, the bands at 1574 and 1485 cm-1 are from the bending vibration of


13


C=C bond of graphene. Band at 988 cm-1 can be assigned as the S=S bond and the peak at 851 cm-1 is the weak C–S stretching, which indicate the successful doping of S atoms on GQD. The Au@S-GQD also shows those peaks similar to S-GQD. In particular, peaks located at 721 and 683 cm-1 in Au@S-GQD are the Au-S and C-S stretching, respectively,44, 45 which confirm the close linkage of S-GQD onto the surface of Au NPs.

0.8

pH 5 pH 6 pH 7 pH 8 pH 9

Without 4-nitrophenol

0.6 0.4 0.2 0.0 0.9

With 50 µM 4-nitrophenol

0.6 0.3 0.0

300

400

500

600

pH 5 pH 6 pH 7 pH 8 pH 9

700

Wavelength (nm)

(b)

A307/A530 ratio

(a)

Absorbance

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

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3.0 2.5 2.0 1.5 1.0 0.5

pH 5

pH 6

pH 7

pH 8

pH 9

Figure 4. (a) The UV-visible spectra and (b) A307/A530 ratio of Au@S-GQD nanocomposites in the presence of 50 µM 4-nitrophenol at pH 5 – 9. Detection of 4-nitrophenol using Au@S-GQD. The pH value plays a vital role in determining the molecular form of 4-nitrophenol as well as the optical absorption behavior of Au@S-GQD. Therefore, the effect of pH on the sensing efficiency of 4-nitrophenol by Au@S-GQD was first investigated. Figure 4 shows the UV-visible spectra and change in absorption intensity ratio of Au@S-GQD nanocomposites in the absence and presence of 50 µM 4-nitrophenol at pH 5 – 9. The UV-visible spectra of Au@S-GQD at pH 5 – 9 in the absence of 4-nitrophenol only show SPR peak of Au NPs at 530 nm. After addition of 4-nitrophenol, the UV-visible spectra exhibit two major absorption peaks centered at 307 and 530 nm (Figure 4a). The absorption peak at 530 nm is the SPR peak of Au NPs in Au@S-GQD, while 307 nm is mainly attributed to the absorption of 4-aminophenol produced from the reduction of 4-nitrophenol by Au@S-GQD. Therefore, the absorbance ratio between 307 and 530 nm (A307/A530) can be used as an indicator to normalize the


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variation of absorption. Figure 4b shows the A307/A530 ratio as a function of pH. The A307/A530 ratio of 4-nitrophenol increases from pH 5 to 6 first and then decreases in the pH range of 6 – 9. The low sensitivity of Au@S-GQDs toward 4-nitrophenol detection at pH > 7 is mainly attributed to the poor interaction between Au@S-GQD and 4-nitrophenol. The pKa value of 4-nitrophenol is pH 7.2 and 4-nitrophenol would convert to 4-nitrophenolate under alkaline conditions, which can be observed at 400 nm. Since both 4-nitrophenolate and Au@S-GQD are negatively charged, the adsorbed amount of 4-nitrophenolate onto Au@S-GQD nanocomposites is low because of the negatively repulsive force. Therefore, pH 6 is selected for the further experiment. The analytical performance on optical sensing of 4-nitrophenol by Au@S-GQD was first examined in deionized water at pH 6. Figure 5a shows the UV-visible spectra of Au@S-GQD in the presence of 0.005 – 50 µM 4-nitrophenol. The interaction of various concentrations of 4-nitrophenol with Au@S-GQD nanocomposites causes the change in absorption peak at 307 nm along with the appearance of 317 nm peak and the peak intensity increases upon increasing 4-nitrophenol concentration from 5 nM to 50 µM. Figure S7 (Supporting Information) shows the optical property of 4-nitrophenol in the presence of bare S-GQD and Au NPs. In the presence of bare S-GQD, the UV-visible spectra of 4-nitrophenol exhibit a broad peak at 307 nm (Figure S7a, Supporting Information), which is similar to that of Figure 5a. However, the UV-visible spectra of 4-nitrophenol in the presence of bare Au NPs only show SPR peak of Au at 530 nm and 4-nitrophenolate peak at 400 nm (Figure S7b, Supporting Information). A small peak at 317 nm, which belongs to the absorption characteristic of 4-nitrophenol (Figure S7c, Supporting Information), is also observed. These results clearly indicate that S-GQD is the main reducing power, which can reduce the adsorbed 4-nitrophenol onto the surface of Au@S-GQD. It is noteworthy that the SPR peak intensity of Au NPs at 530 nm decreases slightly when 4-nitrophenol concentration increases (inset of Figure 5a). The slight decrease in SPR peak is mainly attributed to the adsorption of 4-nitrophenol onto the Au@S-GQD surface by π–π interaction. It means that the use of A307/A530 ratio as the quantitative


15


indicator can normalize the change in variation of Au NPs, and subsequently enhances the analytical performance of 4-nitrophenol detection in aqueous solutions. Figure 5b shows the calibration curve of 4-nitrophenol using Au@S-GQD as the sensing probe. A good two-stage linear relationship between A307/A530 ratio and 4-nitrophenol concentration is obtained. The 4-nitrophenol can be optically detected by Au@S-GQD at concentration range of 1 – 50 µM and an excellent linearity with a correlation coefficient (r2) of 0.995 is observed. The optical Au@S-GQD sensor also exhibits a good linear relationship in the low 4-nitrophenol concentration range of 5 – 1000 nM with a correlation coefficient of 0.997 (inset of Figure 5b). The LOD of 4-nitrophenol, which can be determined by the 3δ/S where δ is the standard deviation of the lowest fluorescence signal and S is the slope of linear calibration plot,18, 21 is calculated to be 3.5 nM. In addition, the analytical performance of 4-nitrophenol by Au@S-GQD prepared by 200 µM HAuCl4 was performed and compared. As shown in Figure S8 (Supporting Information), a linear range of 1 – 50 µM 4-nitrophenol with r2 of 0.987 is obtained. However, no absorption peak of 307 nm is observed when the 4-nitrophenol concentration is in the range of 50 – 500 nM. This result clearly indicates that the Au@S-GQD prepared by 150 µM Au precursor can exhibit superior analytical performance toward 4-nitrophenol detection.

(b) 3

530

0.10

/A

0.25

2.5

0.12

475

500

525

550

575

Wavelength (nm)

0

2 1.4

A

0.08

0.20

1 - 50 µM log(y) = 0.0065x+0.167 R2 = 0.995

307

50 µM

0.30

Absorbance

4-nitrophenol

0.35

A307/A530

(a)

Absorbance (a.u.)

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

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1.5

0.15

1.3

0.005 - 1 µM log (Y) = 0.12 X + 0.03 R2 = 0.997

1.2 1.1

0.10

0.0

0.2

0.4

0.6

0.8

1.0

Concentration (µM)

250

300

350

400

450

500

550

600

1

0

10

Wavelength (nm)

20

30

40

50

Concentration (µM)

Figure 5. (a)The UV-visible spectra of Au@S-GQD in the presence of various concentrations of 4-nitrophenol ranging from 0 – 50 µM and (b) the absorption intensity ratio of A307/A530 as a function of 4-nitrophenol concentration. ACS Paragon Plus Environment

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Several studies have depicted the 2-stage linear relationship for the detection of different analytes using nanomaterials.4, 21, 46 Tang et al.4 have developed the electrochemical sensor for the detection of 4-nitrophenol based on the rGO/Au nanomaterials. A two-linear range, 0.05 – 2.0 µM and 4.0 – 100 µM, with LOD of 10 nM was obtained. Ganganboina et al.21 have developed a turn off-on fluorescence N-GQD@V2O5 sensor for cysteine detection and a two-stage linear relationship between the change in fluorescence intensity and cysteine concentration was established. It is noteworthy that the reaction of 4-nitrophenol with Au@S-GQD nanocomposites is a surface-mediated reaction and 4-nitrophenol molecules need to attach onto the surface of Au@S-GQD first. The adsorption between 4-nitrophenol and S-GQD in Au@S-GQD nanocomposites by π–π interaction is rapid in the low concentration range because of the abundantly active sites of Au@S-GQD. However, the 4-nitrophenol molecules would compete the active sites on the Au@S-GQD surface at high concentration, and subsequently results in the flat slope of linearity in the concentration range of 1 – 50 µM. Table 2 compares the analytical performance of 4-nitrophenol detected by different types of sensing probes. Several studies have developed electrochemical sensors for the detection of 4-nitrophenol and the dynamic range is usually 2 – 3 orders of magnitude with LOD values of 0.26 – 60 nM.4, 8, 46-50 Ezhil Vilian et al. have fabricated Pd-gum arabic/rGO (Pd-GA/rGO) for catalytic reduction as well as electrochemical detection of 4-nitrophenol. The Pd-GA/rGO based sensor exhibits a low detection limit (9 fM) with a linear range of 2 – 80 pM.50 In addition, the organic and inorganic quantum dots (QD) including carbon dots and CdTe have been fabricated for the fluorescence detection of 4-nitrophenol in aqueous solutions based on the quenching effect.5-7 The working range of QD-based sensors is 2 orders of magnitude and the LOD is in the range of 1 – 60 nM. In this study, the UV-visible optical sensor is, for the first time, developed and the dynamic range can be up to 4 orders of magnitude with LOD of 3.5 nM in deionized water, which is superior to the most of the reported results shown in Table 2.


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Table 2. Analytical performance of optical and electrochemical sensors for the detection of 4-nitrophenol in aqueous solutions using different types of sensing elements.

Fluorescence

Sample matrixb DI water

Linear range (µM) 0.001– 0.5

LOD (nM) 1

MIP-CD

Fluorescence

DI water

0.2 – 50

60

5

CdTe

Fluorescence

DI water

1 – 30

40

6

rGO/Au NPs

Electrochemical

Lake water

10

4

ZnO/GCE

Electrochemical

DI water

1 – 400

20

8

rGO/GCE

Electrochemical

ABS solution

50–800

42

46

rGO/Fe3O4

Electrochemical

DI water

0.26

47

Au-CSiC-SH/ NH2CD/GCE

Electrochemical

DI water

0.01 - 150

23

48

Pd-GA/rGO

Electrochemical

DI water

2 – 80 pM

9 fM

50

Probea

Method

BSA Au-NCs

0.05–2 4 – 100

0.2–10 20 – 100

Reference 7

0.005–1 DI water Au@S-GQD

1– 50

3.5

UV-visible

This study Food wastewater

0.01–1.8 1.8– 50

8.4

a: BSA Au-NC: bovine serum albumin Au nanocrystal; MIP-CD: molecularly imprinted polymer-carbon dots; Au-CSiC-SH/NH2CD: thiol β-cyclodextrin and ethylenediamine; Pd-GA/rGO: Pd-gum arabic; β-cyclodextrin functionalized Au@carboxyl SiC; GCE: glassy carbon electrode; rGO: reduced graphene oxide. b: DI water: deionized water; ABS water: acetate buffered solution.


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(b)

(a) 3.0

100

80

Recovery (%)

2.5

A307/A530 ratio

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


2.0

60

40

20

1.5

0 nk Bla

P 4-N

P 2-N

l e eno oluen Ph T

T 2-N

P TN

T 4-N

B CN

nk Bla

P 4 -N

P ol ne 2-N Phen olue T

T 2-N

P TN

T 4-N

B CN

Figure 6. The (a) absorption intensity (A307/A530) ratio and (b) recovery of 50 µM aromatic and nitroarene interferences detected by Au@S-GQD in aqueous solution at pH 6. To understand the interference of aromatics and nitroarenes on the detection of 4-nitrophenol by Au@S-GQD in aqueous solutions, several organic compounds including 50 µM toluene, phenol, 2-nitrotoluene (2-NT), 4-nitrotoluene (4-NT), 2-nitrophenol (2-NP) , 2, 4, 6-trinitrophenol (TNP) and 1-chloro-4-nitrobenzene (CNB) were detected by Au@S-GQD under the identical conditions. Figure 6 shows the A307/A530 ratio and recovery of 50 µM aromatic and nitroarene interferences detected by Au@S-GQD in aqueous solution at pH 6. It is noteworthy that some aromatic compounds like CNB and TNP exhibit the obvious absorption background at 307 nm (Figure S9, Supporting Information). Therefore, the A307/A530 ratio of interfering substances is calculated by the difference in absorbance in the presence and absence of Au@S-GQD. Although the aromatic interferences can undergo the π – π interaction with S-GQD, the A307/A530 ratio of these chemicals is similar to that of blank control, which depicts that the interference of aromatic compounds on the detection of 4-nitrophenol can be neglected (Figure 6a). The addition of 50 µM 2-NP exhibits the slight increase in A307/A530 ratio. However, the recovery, calculated from the calibration curve, is only 22%, which is lower than that of 4-nitrophenol (97%) (Figure 6b). It is noteworthy that the negative charge of oxygen atom on nitro group of 4-nitrophenol would be delocalized throughout the benzene ring, and results in the enhancement of resonance stability and sensing sensitivity.49 In spite of the similarity of chemical structures between 4-nitrophenol and other nitroarenes, the steric ACS Paragon Plus Environment

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hindrance of substituent lowers the adsorption and reduction efficiency of 2-NP onto the surface of Au@S-GQD surface. These results clear indicate that Au@S-GQD is a promising probe for selective and sensitive detection of 4-nitrophenol over other aromatic and nitroarenes using UV-visible sensing system. Detection of 4-nitrophenol in aqueous solutions with complex matrix. To further evaluate the feasibility of using Au@S-GQD sensing probe to optically detect 4-nitrophenol in real water and wastewater samples, the analytical performance of Au@S-GQD nanocomposite was further examined by the standard addition of 4-nitrophenol in untreated food wastewater. As shown in Table S1 (Supporting Information), the food wastewater contains 6010 mg/L COD and 2320 mg/L suspended solid (SS), which is a highly polluted wastewater. Since the food wastewater contains high concentration of SS, the collected wastewater sample was centrifuged initially to minimize the interference of colloidal suspension. Figure 7a shows the UV-visible spectra of 0.05–50 µM 4-nitrophenol in food wastewater in the presence of Au@S-GQD. The A307/A530 ratio of Au@S-GQD in the filtrated samples is unchanged in comparison with that of blank control, indicating that the food wastewater contains no 4-nitrophenol. After spiking 0.05 – 50 µM 4-nitrophenol into the food wastewater, the absorption peaks at 307 and 530 nm are clearly observed. Furthermore, an additional peak at 400 nm, the absorption peak of 4-nitrophenolate, is observed because the pH value of food wastewater is 7.5, which can deprotonate 4-nitrophenol into 4-nitrophenolate. Similar to the analytical performance in DI water, the 4-nitrophenol detection by Au@S-GQD in food wastewater exhibits an excellent 2-stage linear relationship ranging from 0.05 – 50 µM with correlation coefficients of 0.986 – 0.989 (Figure 7b). It is noteworthy that the dynamic range of 4-nitrophenol in untreated food wastewater is 3 orders of magnitude and the LOD value is 8.4 nM, which is mainly due to the matrix effect and high COD value of food wastewater. Moreover, the pH value of food wastewater at pH 7.5 also reduces the sensitivity of Au@S-GQDs, and results in the decrease in sensitivity at ultra-low concentration of 4-nitrophenol. However, the analytical performance of


20

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4-nitrophenol by Au@S-GQD in food wastewater is superior to or comparable with those reported results shown in Table 2.

4-nitrophenol

(b) 2.2

10 - 50 µM y = 0.03log(x)+0.18 2 R = 0.989

2

50 µM

0.4

1.8

0.3

0

1.6 A307/A530

0.5

A307/A530

(a)

Absorbance

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


1.4

0.2

1.6

0.05 - 10 µM

1.5

y = 0.01log(x)+0.099 2 R = 0.986

1.4

1.3

1.2

1.2

0

2

4

6

8

10

Concentration (µM)

0.1 300

400

500

600

0

10

Wavelength (nm)

20

30

40

50

Concentration (µM)

Figure 7. The change in (a) the absorbance spectra and (b) absorption intensity ratio (A530/A307) as a function of 4-nitrophenol concentration in food wastewater. The matrix effect of aqueous solution was evaluated by spiking 0.1 – 20 µM 4-nitrophenol into 3 different aqueous solutions including food wastewater, textile wastewater and lake water with complex matrix. The physicochemical property of these 3 wastewaters is listed in Table S1 (Supporting Information). It is clear that food wastewater is an untreated raw wastewater containing high organic and SS concentrations. Textile wastewater is the treated effluent with low organic content but contain metal ions including Cu2+, Zn2+ and Ni2+. The COD value of lake water is 16 mg/L with low SS concentration, which suggests that the lake water is contaminated with the discharge of sewage. Table 3 shows the recovery of various concentrations of 4-nitrophenol in aqueous solutions detected by Au@S-GQD nanocomposites. The recovery of 4-nitrophenol at low (100 nM), medium (5 µM) and high (20 µM) concentrations is similar, which is in the range of (97 ± 2) – (110 ± 3)%. Moreover, the matrix effect of water samples has little influence on the recovery of 4-nitrophenol by Au@S-GQD. It is noteworthy that the COD concentration of the examined solutions ranges between 0.26 and 6010 mg/L, and the recovery of 4-nitrophenol by Au@S-GQDs is similar to that of DI water. These results clearly indicates the superiority of Au@ S-GQD probe for


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the optical detection of 4-nitrophenol in a wide variety of water and wastewater samples with complex matrix. Table 3. Recovery of 0.1 – 20 µM 4-nitrophenol by Au@S-GQD nanocomposites in different types of water and wastewater samples. The experiment was run triplicate (n = 3).

Sample Name

Added concentration

DI Water

100 nM

Detected concentration 105 nM

100 nM

98 nM

98 ± 3

5.0 µM

5.2 µM

104 ± 4

20 µM

20 µM

100 ± 5

100 nM

101 nM

101 ± 4

5.0 µM

4.9 µM

98 ± 5

20 µM

22 µM

101 ± 6

100 nM

103 nM

97 ± 2

5.0 µM

5.5 µM

110 ± 3

20 µM

20 µM

100 ± 6

Recovery (%) 105 ± 2

Food manufacturing wastewater

Textile wastewater

Lake water

CONCLUSIONS In this study, we have, for the first time, developed the size tunable Au@S-GQD nanocomposites

using one-step procedure by simply adjusting the Au precursor concentration in the S-GQD solutions for the selective and sensitive detection of a wide range of 4-nitrophenol in different matrices of aqueous solution. The 2 – 6 layered S-GQD is uniformly dispersed in solution and the particle size is in the range of 1 – 7 nm with mean diameter of 4.5 nm. The as-prepared S-GQDs can serve as the reducing agent to convert Au3+ ions into Au@S-GQD as well as to reduce 4-nitrophenol into 4-aminophenol. In addition, the particle size of Au@S-GQDs can be easily tuned from 1 – 8 nm to 11 – 22 nm when the gold precursor concentration increases from 50 to 150 µM. The XPS and FTIR ACS Paragon Plus Environment

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results indicate that the Au@S-GQD surface contains hydrophilic functional groups and can perform the highly sensitive detection of 4-nitrophenol at pH 6. The Au@S-GQD nanocomposites show the good UV-visible property, which can optically detect 4-nitrophenol. The sensing probe exhibit a two-stage linear response to 4-nitrophenol in the concentration range of 0.005 – 1 µM and 1 – 50 µM with the LOD of 3.5 nM. The Au@S-GQD has also a good selectivity toward 4-nitrophenol over other aromatic and nitroarene compounds. Moreover, the Au@S-GQD can be applied to detect 4-nitrophenol in aqueous solutions with complex matrix. The recovery of 0.1 – 20 µM 4-nitrophenol in different types of water and wastewater is in the range of (97 ± 2) – (110 ± 3) %. Results obtained in this study clearly demonstrate the superiority of using Au@S-GQD as the optical sensor for the highly sensitive and selective detection of 4-nitrophenol, which can open an avenue to design a platform for rapid and robust detection of nitroaromatic chemicals in a wide variety of aqueous solutions with complex matrix.

ASSOCIATED CONTENT

Supporting Information Physicochemical property of water and wastewater samples; AFM image of S-GQD; Zeta potential and EDS spectra of Au@S-GQD; TEM image and SPR of Au@S-GQD by 200 µM HAuCl4; full survey, deconvoluted XPS Au 4f peak and FTIR of S-GQD and Au@S-GQD. UV-visible spectra of 4-nitrophenol by bare S-GQD and Au NPs and UV-visible spectra of interfering substances. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author Corresponding author: Ruey-an Doong (Email: [email protected], Tel: +886- 3-5726785, Fax: +886-3-5725958.) Notes ACS Paragon Plus Environment

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The authors declare no competing financial interests.

ACKNOWLEDGEMENTS

The authors thank the Ministry of Science and Technology (MOST), Taiwan for financial support under grant Nos. MOST 104-2221-E-009-020-MY3 and 105-2113-M-009-023-MY3.

REFERENCES

(1) Wasi, S.; Tabrez, S.; Ahmad, M. Toxicological Effects of Major Environmental Pollutants, An Overview. Environ. Monit. Assess. 2013, 185, 2585-2593. (2) Pan, Y.; Zhang, X. R.; Li, Y. Identification, Toxicity and Control of Iodinated Disinfection Byproducts in Cooking with Simulated Chlor(am)inated Tap Water and Iodized Table Salt. Water Res. 2016, 88, 60-68. (3) Inglezakis, V. J.; Malamis, S.; Omirkhan, A.; Nauruzbayeva, J.; Makhtayeva, Z.; Seidakhmetov, T.; Kudarova, A. Investigating the Inhibitory Effect of Cyanide, Phenol and 4-Nitrophenol on the Activated Sludge Process Employed for the Treatment of Petroleum Wastewater. J. Environ. Manag. 2017, 203, 825-830. (4) Tang, Y.; Huang, R.; Liu, C.; Yang, S.; Lu, Z.; Luo, S. Electrochemical Detection of 4-Nitrophenol Based on a Glassy Carbon Electrode Modified with a Reduced Graphene Oxide/Au Nanoparticle Composite. Anal. Methods 2013, 5, 5508-5514. (5) Hao, T.; Wei, X.; Nie, Y.; Xu, Y.; Yan, Y.; Zhou, Z. An Eco-friendly Molecularly Imprinted Fluorescence Composite Material Based on Carbon Dots for Fluorescent Detection of 4-Nitrophenol. Microchim. Acta. 2016, 183, 2197-2203. (6) Jiang, L.; Liu, H.; Li, M.; Xing, Y.; Ren, X. Surface Molecular Imprinting on CdTe Quantum Dots for Fluorescence Sensing of 4-Nitrophenol. Anal. Methods 2016, 8, 2226-2232. (7) Yang, X.; Wang, J.; Su, D.; Xia, Q.; Chai, F.; Wang, C.; Qu, F. Fluorescent Detection of TNT and 4-Nitrophenol by BSA Au Nanoclusters. Dalton Trans. 2014, 43, 10057-10063.


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Determination

of

Nitrophenol

Isomers

Using

b-Cyclodextrin

Derivative-Functionalized Silicon Carbide. RSC Adv. 2018, 8, 775-784. (49) Lin, F. H.; Doong, R. A. Catalytic Nanoreactors of Au@Fe3O4 Yolk-Shell Nanostructures with Various Au Sizes for Efficient Nitroarenes Reduction. J. Phys. Chem. C. 2017, 121, 7844-7853. (50) Vilian, A. E.; Choe, S. R.; Giribabu, K.; Jang, S. C.; Roh, C.; Huh, Y. S.; Han, Y. K. Pd nanospheres decorated reduced graphene oxide with multi-functions: Highly efficient catalytic reduction and ultrasensitive sensing of hazardous 4-nitrophenol pollutant. J. Hazard. Mater. 2017, 333, 54-62.


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Citric acid

HAuCl4

Pyrolysis

Self assembly

@ Room temperature

200 C, 45 min

S-GQDs 3-Mercaptopropionic acid Absorbance

Au@S-GQDs

4-nitrophenol with Au@S-GQD

0.35 0.30 0.25

HO

0.20 0.15 0.10 250

300

350

400

450

500

550

600

Wavelength (nm) 3

N

0.005 - 50 M

2.5

A307/A530

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 30 of 30

O+

2

4-Nitrophenol

1.5

1

O-

0

10

20

30

40

50

Concentration (M)


Doped Graphene Quantum Dot Nanocomposites for Highly Selective

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