Sulfur and Nitrogen Co-Doped Graphene Quantum Dots as a

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Sulfur and Nitrogen Co-Doped Graphene Quantum Dots as a Fluorescent Quenching Probe for Highly Sensitive Detection toward Mercury Ions Siyong Gu, Chien-Te Hsieh, Yi-Yin Tsai, Yasser Ashraf Gandomi, Sinchul Yeom, Kenneth David Kihm, Chun-Chieh Fu, and Ruey-Shin Juang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02010 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Applied Nano Materials

Sulfur and Nitrogen Co-Doped Graphene Quantum Dots as a Fluorescent Quenching Probe for Highly Sensitive Detection toward Mercury Ions

Siyong Gu1, Chien-Te Hsieh2,3,*, Yi-Yin Tsai2, Yasser Ashraf Gandomi3, Sinchul Yeom3, Kenneth David Kihm3,*, Chun-Chieh Fu4, Ruey-Shin Juang4,5,* 1

Fujian Provincial Key Laboratory of Functional Materials and Applications, School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, PR China 2

Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan 32003, Taiwan

3

Department of Mechanical, Aerospace, and Biomedical Engineering, University of Tennessee, Knoxville, TN 37996, U. S. A. 4

Department of Chemical and Materials Engineering, Chang Gung University, Guishan, Taoyuan 33302, Taiwan

5

Division of Nephrology, Department of Internal Medicine, Chang Gung Memorial Hospital, Linkou, Taiwan

1 ACS Paragon Plus Environment

ACS Applied Nano Materials 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

Prepared for submission to ACS Applied Nano Materials November 7, 2018 Revised January 5, 2019

*

Corresponding authors. E-mail address: [email protected] (Prof. C.T. Hsieh), [email protected] (Prof. K.D. Kihm), and [email protected] (Prof. R.S. Juang)


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ABSTRACT Sulfur and nitrogen co-doped graphene quantum dots (SN-GQDs) were synthesized through an efficient infrared (IR)-assisted pyrolysis of glucose, urea, and ammonia sulfate at 260°C. These served as a highly selective probe for the sensing of Hg2+ ions in an aqueous solution. The IR technique can also prepare N-doped graphene quantum dots (N-GQDs), which have been compared with SN-GQDs for their fluorescence (FL) quenching sensitivities by Hg2+ ions. The FL intensities of both GQDs show decreasing functions of concentration of Hg2+ ions within the entire concentration ranges of 10 ppb‒10 ppm. The sensitivity of SN-GQD is 4.23 times higher than that of N-GQD, based on the calculation of the Stern-Volmer equation. One inter-band gap structure of SN-GQDs for the detection of mercury ions is proposed. The S doping can coordinate with phenolic groups on the edge of SN-GQDs (i.e., the formation of (CxO)2Hg2+) and induce the cutting off or alleviation of photon injection paths, thereby leading to significant FL quenching. This work proves that SN-GQD offers sufficient sensitivity for probing the quality of drinking water to ensure that it contains less than 10 ppb of Hg2+ ions, as per the World Health Organization standard.

KEYWORDS: Infrared-assisted heating; Graphene quantum dots; Fluorescence quenching; Mercury detection; Sulfur doping; Nitrogen doping



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1. Introduction Graphene quantum dots (GQDs), generally produced either from graphene-based precursors or through a carbonization of hydrocarbon precursors, are receiving much attention for their applications to electronic, biological and optical devices.1-4 Although GQDs are not monolayer in nature, the term usually represents a few to tens of layers of graphene with a particle size of less than 10 nm.5 Interestingly, GQDs with tunable fluorescent emission are considered as next generation green nanomaterials and thus present a potential alternative to fluorescent semiconductor nanocrystals, which consist of toxic heavy metals such as Cd.4,6 Several methods have been proposed to synthesize GQDs during the last decade, and so far, great efforts have been devoted to the large-scale production of GQDs since the success of the carbonization of hydrocarbon precursors.7-9 The synthesis methods include hydrothermal/solvothermal treatment,10-12 microwave irradiation,13 thermal pyrolysis,14,15 and laser ablation.16 These achievements deliver commercial feasibility due to low cost, simplicity, and high efficiency. The present work is to look for an alternative high-throughput heating system with high energy efficiency, which is essential for large-scale production of GQD materials. Herein we have innovated an induction heating technique to synthesize GQDs via the pyrolysis of carbon precursors under infrared (IR) irradiation. This induction heating method has exhibited superior capabilities

of

synthesizing

ternary

electrode

materials


(e.g.,

LiFePO4,17

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LiNi1/3Co1/3Mn1/3O2,18 LiNi0.80Co0.15Al0.05O219) for energy storage devices. The reaction period has been reduced significantly to only 3 h, which is much shorter than that of traditional resistive heating, which requires 8‒24 h. Doping is an effective method to modify the electronic density of bulk semiconductor materials and to tune their optical and electrical properties.20 For example, recent progress in doping graphene with hetero-atoms such as electron-rich nitrogen (N), has enabled the in-plane substitution of N atoms to graphene as n-type semiconductor.20,21 The other sulfur (S) doping in graphene sheets also attracts considerable attentions due to S-doped materials with unique properties and the potential for widespread applications.22,23 In S-doped graphene composites, a mismatch of the outermost orbitals of S and C induces a non-uniform spin density distribution.22 Since the electronegativity difference between S (2.58) and C (2.55) is almost insignificant, there exists a significant polarization in the S‒C composite.24,25 However, the chemical doping of S into the framework of GQDs appears to be quite difficult, based on atomic engineering viewpoints. This is because the S atom is larger than the C and N atoms and because the C–S bond length (1.78 Å) is 22‒25% longer than those of the C–C22 and C‒N bonds. Previous efforts have been made to synthesize S and N co-doped GQDs using a hydrothermal synthesis of citric acid + thiourea.5,26 The unique design of S and N co-doped GQDs has been confirmed to exhibit their broad photo-absorption in a wide spectral range, which yields high carrier transport mobility and



superior chemical stability. In the present work, a fast IR approach is developed to synthesize S and N co-doped GQDs for the detection of mercury ions (Hg2+) in an aqueous solution. The one-step IR heating technique is capable of producing S and N co-doped GQDs through a thermal pyrolysis of citric acid and ammonium sulfate under IR irradiation. The IR heating process was efficiently carried out at 260°C for 10 min. Mercury is one of the most toxic heavy metals that exist in the environment, generating a large amount of binary compounds, many of which are toxic.27 The oxidized form of Hg2+ is the source of contamination and it needs to be sensitively detected and monitored at a very low concentration in water by a simple method. The fluorescence (FL) quenching of GQDs offers an efficient way to detect metal ions in aqueous media, as compared to the conventional chromatography/mass spectrometry and enzyme-linked immunosorbent assays.28 To meet the urgent demand, this work employs S and N co-doped GQDs as an FL probe for the rapid detection of Hg2+ ions in water. The FL quenching of S and N co-doped GQDs is efficiently induced by a charge transfer between the GQDs and mercury ions, enabling the advanced development of a low cost, time saving, sensitive, and reliable method for eco-environmental protection. To our knowledge, this is the first report for the highly sensitive detection of mercury ions by S and N co-doped GQDs that have been readily and efficiently prepared by an IR-assisted synthesis method. One inter-band gap structure of S and N co-doped GQDs for the


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detection of mercury ions is proposed. This work proves that S and N co-doped GQDs offer sufficient sensitivity for probing the quality of drinking water to ensure that it contains less than 10 ppb of Hg2+ ions, as per the World Health Organization standard.

2. EXPERIMENTAL SECTION IR synthesis of S and N co-doped GQDs. The chemical precursors used in this work, including citric acid (C6H8O7), ammonia sulfate ((NH4)2SO4), and urea (CON2H4) were of analytical reagents. For comparison, two kinds of samples, S and N co-doped and N-doped GQDs, were synthesized by the IR-assisted synthesis method (see Figure 1a). First, for the S and N co-doped GQDs, the solid mixture of citric acid (10 g), urea (10 g), and ammonia sulfate (10 g) was blended by a three-dimensional mixer using Zr balls for 5 min. Then the mixture was placed into a home-made IR furnace, which was equipped with six medium-wave IR heater with a near-IR wavelength range of 1.4−3.2 μm. Each IR filament was able to exhibit a maximum power density of 80 kW/m2. The heaters were composed of metallic wire as the filament and quartz tube as the cover. The near-IR heaters were capable of transferring energy to a body through electromagnetic radiation, i.e., no contact or medium was required for energy transfer.19 The IR calcination process was performed by heating the mixture from room temperature to 260°C in air with a ramping rate of 30 °C/min and remaining at this temperature for 10 min. After cooling back down to ambient



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temperature, the powder was sieved through a stainless foil mesh (Type: 300 mesh), thus collecting the S and N co-doped GQD sample. Finally, the sample was washed using ultrapure water, and finally centrifuged at 15,000 rpm for 0.5 h. As for the N-doped GQD sample, the similar IR-assisted synthesis method was performed under the same operating conditions. The major difference from the S and N co-doped GQD synthesis was to prepare the chemical precursor of citric acid and urea with the weight ratio 1:1 without any ammonia sulfate. Characterization of S and N co-doped GQDs. Fourier-transform infrared (FT-IR) spectra of GQD powders were measured using a Nicolet 380 spectrometer. High-resolution transmission electron microscopy (HR-TEM, FEI Talos F200s) was adopted to observe the micro-structural morphology of GQD samples. The crystalline structure of GQD samples was also characterized by Raman spectroscopy (Renishaw Micro-Raman spectrometer). An inductively-coupled

plasma

optical

emission

spectroscopy/mass

spectrometry

(ICP-OES/MS) was adopted to analyze the chemical compositions of GQD samples. X-ray photoelectron spectroscopy (XPS, Fison VG ESCA210) equipped with Mg‒Kα radiation emitter, was used to characterize chemical composition of the samples. The C 1s, N 1s and S 2p spectra were deconvoluted by using a non-linear least squares fitting program with a symmetric Gaussian function. The surface composition of N-GQD and SN-GQD powders was analyzed with an appropriate sensitivity factor. The absorbance spectra of GQD


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suspensions (100 ppm) were obtained by an ultraviolet-visible (UV-vis) spectrometer (Agilent Technology Cary 60). Finally, the FL spectrometer (Hitachi F-7000 FLS920P) was used to characterize the FL emission spectra of GQD suspensions. These FL measurements were performed three times at a fixed wavelength of 360 nm and the deviation was less than 0.25 %. The quantum yield (Θ) was quantified with reference to Rhodamine B (Θ: 95% at 552-nm excitation) using the following equation: Θ = Θr × [(FL area/OD)s/(FL area/OD)r] × ηs2/ηr2, where the subscripts “s” and “r” indicate the sample and standard quantum dots, respectively. The η is the reflective index of the solvent, and FL area and OD are the fluorescence area and absorbance value, respectively. Detection of mercury ions by S and N co-doped GQDs. The detection of mercury ion by GQDs was performed at pH = 7, adjusted by a phosphate buffer solution. Different amounts of Hg2+-containing solutions were added stepwise and mixed with the GQD suspensions (100 ppm). Afterwards, the resulting GQD solutions were uniformly shaken and incubated for 1 min at ambient temperature. The FL emission spectra were then recorded at 360 nm by the FL spectrometer. For accuracy, the detection measurements were carried out in triplicate. The relative FL intensity, F / F0, versus Hg2+ concentration was found to have a linear function as the calibration curve. Herein, F0 and F were the FL intensities of GQD suspensions in the absence and presence of different concentrations of Hg2+, respectively.



3. RESULTS AND DISCUSSION The two types of GQD samples are designated as SN-GQD and N-GQD, according to the thermal pyrolysis of citric acid + urea and citric + urea + ammonia sulfate under IR irradiation with or without ammonia sulfate, respectively (see Figure 1a). Fluorescence quenching test of N-GQD and SN-GQD samples in the presence of Hg2+ ions is illustrated in Figure 1b. FTIR spectra of both GQDs are illustrated in Figure 2a, showing several functional groups on the GQDs. First, an emerging peak unique to the SN-GQD sample at 570–600 cm-1 (i.e., C‒S bonding) reveals the effect of sulfur doping in the carbon dots.29,30 In contrast, the C‒S peak disappears for the N-GQD sample in the similar wavenumber range. The strong peak in the range of 1150–1300 cm-1 can be assigned to the contributions from the C‒O stretch of –COOH (on the N-GQD sample) or S=O stretch of sulfones (on the SN-GQD sample). This reflects the presence of hydroxyl and carboxyl functional groups31-33 on both GQD samples. In addition, for both GQD samples, the transmittance peaks occurring at approximately 1400 and 1706 cm-1 correspond to the existence of C‒N and C=O/S=O functional groups, respectively.29,34 The weak band at ca. 3400 cm-1 mainly originated from the physically adsorbed water molecules on the surface of both GQDs. However, it is worth noting that the increased band intensities at ca. 2920 (C‒H) and 3400 cm-1 (‒OH) on SN-GQD sample reveal an effective doping of sulfur due to the presence of


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more oxygen-containing functional groups and sulfur-related defects on GQD crystals.34 To ensure the N and S co-doping in GQD samples, ICP-OES/MS analysis was employed to uncover the chemical compositions of as-received samples. The ICP-OES/MS measurements reveal that the N-GQD sample shows N/C and O/C atomic ratio of 29.0 and 50.9 at.%, respectively, whereas the SN-GQD sample contains not only high amidation and oxidation levels and high N/C (30.2 at.%) and O/C (54.2 at.%) atomic ratios, but also a moderate sulfuration extent, i.e., S/C atomic ratio: 3.5 at.%. This result shows that the IR-assisted technique is capable of synthesizing the desired GQDs through the adjustment of the precursor recipe. Raman spectroscopy provides crucial information to identify intrinsic characteristics of sp2 carbon materials to investigate disorder. Raman spectra of both GQDs, as shown in Figure 2b, reveal that two prominent bands at ca. 1460 cm-1 and 1525 cm-1 are assigned to D and G bands, respectively. An obvious shift is observed as compared to typical D and G bands at 1350 cm-1 and at 1570 cm-1 for carbon-based materials. It is generally recognized that the D band is attributed to the disordered nature due to hetero-atoms doping defects and reduction in size, whereas the G band is related to the Raman active E2g mode and measures the crystalline nature of a carbon material.35 Due to the S and N -doping, the shift in D and G bands for both GQDs samples confirms the more disordered structure involving carbon-sulfur (C‒S) and carbon-nitrogen (C‒N) bonds. The intensity ratio of G to D band



serves as an important factor to figure out the graphite degree of carbon-based materials, strongly relative to the sp2 cluster sizes (La) and the distance between defects (Lb).36-38 The G/D intensity ratios of SN-GQD and N-GQD samples are estimated to be 0.85 to 0.93, respectively, indicating that the SN-GQD sample generates a more disordered structure. Since both SN-GQD and N-GQD have identical amidation and oxidation extents, the lower crystallinity of SN-GQD sample is attributed to the fact that the S dopant as a hetero-atom inevitably induces the decreased La and Lb values by disrupting the conjugated sp2 cluster and accelerating point-like defect density through the generation of disordered structures.38 This deduction is also supported by the fact that the C–S bond length is 22‒25% longer than that of the C–C and C‒N bond. XPS was employed to determine the chemical composition of N-GQD and SN-GQD samples, as shown in Figure 3. The survey-scan XPS spectra of both samples present three peaks at ca. 532, 400, and 284 169 eV, which corresponds to O 1s, N 1s, and C 1s, respectively.5,23 Two additional peaks at approximately 227 and 164 eV are viewed in the survey spectrum of SN-GQD sample, assigned to S 1s and S 2p. The S/C doping ratio is ca. 4.0 at%, which was doubly confirmed by elemental ICP-OES/MS analysis. The high-resolution C 1s XPS spectrum, as shown in Figures 3b and 3d can be divided into five peaks, namely, C–C/C=C (ca. 284.6 eV), C‒N/C‒S (ca. 285.2 eV), C‒OH (ca. 287.5 eV), C=O (ca. 288.5 eV), and O‒C=O (ca. 289.5 eV), indicating that the carbon is present in


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different chemical environments. The N 1s high-resolution XPS spectra in Figures 3c and 3e can be deconvoluted into signals for pyrrolic/pyridinic N (ca. 399.6 eV), quaternary N (ca. 400.2 eV), and N oxides (ca. 401.5 eV).27,39 It is worth noting that SN-GQD sample contains amino group, originates from the amide‒carbonyl group (O=C‒NH2), e.g., amino and amide carbonyl functional groups located at edges of graphene sheets.40,41 As expected, there are no S peaks observed for N-GQD sample, while high resolution of the S 2p XPS spectrum of SNGQD sample (see Figure 3f) clearly shows three peaks at 163.9, 165.1 eV and 168.3 eV, which represent S 2p3/2 and S 2p1/2 of thiophene and C‒SOx bonding.5,42,43 Accordingly, the doped S exists in two configurations: one is thiophene sulphur and the other is oxide-S,23 which are in good agreement with FTIR results. Low- and high-magnification HR-TEM micrographs and particle size distributions of both GQD samples, as shown in Figures 4a-f, confirm the presence of uniformly dispersed GQDs prepared by the IR-assisted pyrolysis of chemical precursors. Each quantum dot is displayed as a round shape, with an average diameter of 3‒5 nm. The influence of sulfur doping on the particle size of GQD sample seems to be insignificant. Without any impurities, both GQDs possess the multi-layered crystallinity with a lattice distance of 0.21‒0.22 nm, assigned to the (1120) lattice fringes of graphene sheets.20 The production yield of GQD samples through the IR synthesis method can achieve as high as 41 and 43 wt.% for SN-GQD and N-GQD, respectively. This demonstrates that the IR synthesis



method successfully enables the production of functionalized and highly-crystalline GQD powders, offering potential feasibility in large-scale production. To show that functionalized GQDs are effective and benign as imaging probes, we have demonstrated their performance as fluorescent plant labels. We carried out the growth experiments using bean sprouts and celery in an SN-GQD solution with a concentration of 200 mg/L. The two kinds of plants were incubated and grown in the SN-GQD suspension for three days. Figures 5a and 5b present that both the cultured bean sprouts and celery emit a bright blue color from both their stems and leaves under 360-nm UV irradiation, indicating that SN-GQDs enable the permeation throughout the plant cell with good bio-compatibility. This cultured period of more than three days reveals that the potential bio-toxicity on the growth of plants caused by SN-GQDs turns out to be negligible, which is beneficial for FL labeling possibly in biology and disease detection in the future.44 Figures 5c and 5d show the photographs of both N-GQD and SN-GQD aqueous solutions after adding different amounts of mercury ions, respectively. Under 360-nm UV irradiation, the original N-GQD and SN-GQD suspensions (0 ppm) display bright-green and blue colors, respectively. With a progressively increasing Hg2+ concentration, however, their colors tend to gradually become darker because of the increasing FL quenching effects. Accordingly, we demonstrate the feasibility of using both N-GQD and SN-GQD samples as fluorescent probes for the detection of mercury ions. Note that the remarkable FL


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quenching takes place after mixing 75 ppm Hg2+ in N-GQD solution but as low as 30 ppm Hg2+ ions in SN-GQD solutions. This implies that the SN-GQD sample shows a higher sensitivity with faster FL quenching than N-GQD. To support this argument, FL emission spectra of the two GQD solutions as functions of different Hg2+ ion concentrations are illustrated in Figures 6a and 6b. With no Hg2+ ions (0 ppm), both N-GQD and SN-GQD samples display an asymmetric peak with a full width at half maximum (FWHM) of approximately 110 nm. The strongest FL emissions of N-GQD and SN-GQD solutions appear at ca. 530 nm (2.34 eV) and ca. 510 nm (2.43 eV), respectively, excited under UV illumination (360 nm). This result shows one blue shift due to the S-doping/-decorating on the sp2 domain of GQDs. The Θ values of N-GQD and SN-GQD samples are 23.4 and 25.5 %, respectively. As expected, the FL peak intensity is found to progressively decrease with the increase in Hg2+ concentration. As viewed on the quenching ratio of FL intensity, the SN-GQD sample is capable of exhibiting a significantly higher sensitivity towards the detection of mercury ions, as compared to the N-GQD sample. The measured FL intensity shows a decreasing function of concentration of Hg2+ ions within the entire concentration ranges of 0‒100 ppm. This FL quenching of GQDs by Hg2+ ions can be interpreted by the formation of a GQD‒(Hg2+)x complex on the carbon surface. The GQD surface is highly hydrophilic due to the presence of surface oxygen, nitrogen,



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and sulfur functionalities, as analyzed by FT-IR and ICP-OES/MS measurements. Both GQD aqueous solutions can be stored at ambient temperatures without generating any sediment for several months. The hydrophilic surface allows better accessibility of Hg2+ ions in the aqueous solution to the carbon surface, leading to ample surface coverage and a more efficient adsorption.45,46 This significantly alters the surface-ion interaction at the edge or basal plane of functionalized GQDs by the modification of surface charge properties.47 Thus, the formation of a great number of surface oxygen groups, such as carboxyl, phenol, and lactone, favor the interaction between the carbon surface and the mercury ions during the adsorption process, as follows:48,49 2 CxOH+ + Hg2+ → (CxO)2Hg2+ + 2 H+

(R1)

The other pathway for adsorbing Hg2+ ions originates mainly from the presence of amide-carbonyl groups (O=C‒NH2), e.g., amino- and amid-carbonyl functionalization at the edges of graphene sheets. A strong interaction between mercury ions and the amide-carbonyl group takes place at the N-modified edges, confirmed by XPS analysis on SN-GQD sample. The formation of surface intermediates, i.e., the GQD‒(Hg2+)x complex, effectively hinders the photon rejection from the highest occupied molecular orbital (HOMO)‒lowest unoccupied molecular orbital (LUMO) gap, leading to FL quenching behavior. As compared to the N-GQD sample, the SN-GQD sample displays higher sensitivity


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toward Hg2+ detection. This is mainly due to the synergistic effect of S, N and O functional groups on SN-GQDs. That is, the SN-GQD sample possesses not only surface O and N functional groups, but also C‒SOx‒C (x = 2, 3, and 4) sulphone bridges.24 These sulphone bridges can facilitate the additional adsorption of mercury ions onto SN-GQD surface and then improve the sensitivity of SN-GQD. The S dopant shares doping configurations such as C‒S‒C, C‒SOx‒C (x = 2, 3, and 4), and C‒SH. Accordingly, the S-doping effectively adjusts the Fermi level and localized electronic state,22,24 promoting SN-GQDs’ the affinity toward mercury ions and thus inducing the higher sensitivity of said GQDs. To quantitatively investigate the FL intensity, the plot of relative FL quenching ratio, (F0 ‒ F) / F0, versus Hg2+ concentration (0.01‒10 ppm) is illustrated in Figure 6c. It shows that the FL quenching ratio on both GQD samples consistently increases when the Hg2+ concentration increases from 0.01 to 10 ppm. The SN-GQD sample serves as a better probing material toward Hg2+ detection since it facilitates a higher sensitivity than N-GQD. The FL quenching of both sensing materials (i.e., N-GQD and SN-GQD) by Hg2+ ions can be modeled by Stern-Volmer analysis, as described by the following relationship:24,50,51 F0/F = 1 + KSV [C]

(2)

where KSV is the Stern-Volmer quenching constant and [C] is the concentration of the quencher. The Stern-Volmer of FL quenching on the GQDs by Hg2+ ions shows a linear dependence with Hg2+ concentration in a low quencher concentration regime, as illustrated



in Figure 6d. Importantly, the linear fitting curves possess excellent correlations (r2= 0.997) within the entire concentration range, providing a good description for the quantitative analysis. The KSV values can be derived from the slope of these linear Stern-Volmer plots. The KSV value of SN-GQD is found to be 4.23 times higher than that of N-GQD, i.e., KSV value: 0.22 L/mg for SN-GQD versus 0.052 L/mg for N-GQD. It is generally recognized that KSV is the kq·τ0 product, where kq and τ0 represent the reaction rate constant and the lifetime of emissive excited state without the presence of quencher, respectively. Accordingly, the SN-GQD sample’s higher KSV value denotes that the S doping in GQDs efficiently enhances the FL reaction rate or prolongs the lifetime of the emissive excited state, making SN-GQD highly sensitive to Hg2+ detection. Additionally, it is worth mentioning that the detection limit of SN-GQDs can be as low as 10 ppb (i.e., ca. 5 × 10-8 M), which is very close to the standard of mercury ion concentration in drinking water according to the World Health Organization (WHO) guidelines.52 This detection limit also closely approaches to that of graphitic carbon nitride prepared by microwave mediated method from formamide27 and fluorescent carbon nanoparticles prepared by a microwave-assisted hydrothermal treatment of melamine and trisodium citrate dehydrate.53 Accordingly, this finding attests that SN-GQD offers a potential feasibility for probing the quality of drinking water. It is inferred from the above observations that the S, N, and O doping alter the energy


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levels of GQDs, inducing an improved sensitivity toward Hg2+ detection in the liquid phase. Accordingly, a schematic diagram of different electronic transitions on SN-GQD is proposed in Figure 7a. The individual transitions are indicated by arrows in the energy band diagram, i.e., a multiple chromophoric band-gap structure.22,34 The doping of S atoms is prone to create additional “defect sites”, thereby introducing additional energy levels and effectively creating new electron transition pathways in the inter-band structure of SN-GQD.34 Under UV-light irradiation, the absorption of UV photons by the localized π electron in double bonds (mainly C=C) produces an electron‒hole pair (exciton) after electron transition, i.e., path (i). The exciton may emit UV light (path (i’)) through radiative recombination after vibration relaxation. The excited electron may also undergo inter-band transition from a higher conduction band to a lower conduction band (e.g., path (ii) and (iii)), subsequently emitting visible light (paths (ii’) and (iii’)) by radiative recombination.54 Due to the highly atomic heterogeneity of N-GQD and SN-GQD, the excited electrons may go through the three pathways simultaneously. This inter-band structure uncovers the reason why both samples emit an asymmetric FL emission spectrum under 360-nm UV irradiation. Thus, both samples exhibit broad FWHM and wide FL spectra within the entire wavelength range of 420‒680 nm. When the FL emission is quenched by the adsorption of Hg2+ ions, three paths induced by UV photons would be partially or completely blocked by the surface intermediates (i.e., the GQD‒(Hg2+)x complex), prohibiting the photon rejection



from the HOMO‒LUMO band, as depicted in Figure 7b. To figure out the proposed mechanism, the exhausted SN-GQD sample was analyzed by XPS. The XPS Hg 4f peak is seen at 101.5 eV, as shown in the Electronic Supporting Information (Figure S1). The main component at 101.5 eV can be assigned to that most of the mercury is present as Hg2+, implying the presence of (CxO)2Hg2+) complex on the spent SN-GQD sample after detecting mercury ions in aqueous solution. Therefore, manipulating the current distribution across GQDs is feasible via incorporating S atoms within the molecular structure. Such a molecular structure modification results in an enhanced coordination between the surface oxygen functionalities and Hg2+ ions; in particular, towards the edges compared to the internal structure. Comparing sulfur and nitrogen atomic structure, lower electronegativity of sulfur (2.58 for S versus 3.04 for N) promotes stronger coordination between the oxygen and sulfur atoms compared to oxygen and nitrogen atoms. Such a biased coordination has already been confirmed in some previous works.25 To better demonstrate the Hg2+ detection mechanism, the internal band gap structure of SN-GQD and N-GQD samples under various light illuminations have been illustrated in Figure 7 before (7(a)) and after (7(b)) detection of Hg2+ ions. As shown, the incorporation of Hg2+ ions within SN-GQD, is strongly enhanced towards the edges where coordination with phenolic groups results in the formation of (CxO)2Hg2. Upon excitation, the excited


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electrons in the exterior orbital of SN-GQDs have higher tendency towards jumping on the 4f orbits of Hg2+. Such an electron transfer directly influences the excitation/emission pathways shown in Figure 7b ((i)‒(i’), (ii)‒(ii’), and (iii)‒(iii’)). Therefore, the non-radiative electron/vacancy charge recombination along with exciton annihilation reactions are promoted.25 Significant FL quenching is the direct influence of this interaction confirming the effectiveness of S and N co-doped GQDs on promoting the Hg2+ ions detection at ultra-low concentrations (at the acceptable levels for standard drinking water). To inspect the FL behavior, we observe the UV-vis absorption spectra of both samples before and after adding 10 ppm Hg2+ ionic solution, as shown in Figures 7c and 7d. Without adding Hg2+ solution, the UV-vis absorption spectra of both samples show a typical π–π* transition absorption peak due to the aromatic sp2 domains (C=C) around 260 nm, an n–π* transition absorption peak due to C=O bonding around 340 nm, a band-gap transition absorption band due to surface molecular center or absorption edge induced by n–π* transitions of nonbonding electrons of adatoms around 410 nm, and a long tail extending into the visible range.55,56 Apparently, the SN-GQD sample displays a stronger tailing effect in the visible range, which overrides the n–π* transitions around 410 nm. The overwhelming tailing effect mainly originates from the sulfur doping that takes place at adsorption bands within the range of 550‒595 nm which are ascribed to the π‒π* and n‒π* of C=S and S=O bonds, respectively.5 After adding a 10 ppm Hg2+ solution, the UV-vis



spectrum of N-GQD sample shows an obvious decay in absorbance at 260, 340, and 410 nm, indicating that a number of active sites are occupied by mercury ions and thus form a GQD‒(Hg2+)x complex. This means that N-GQD sample still maintains a surface coverage that is not poisoned by the Hg2+ complex. In contrast, we also observe this decreased absorbance in the UV-vis spectrum of the SN-GQD sample. However, the adsorption peak at 260 nm (i.e., C=C) almost disappears in the UV-vis spectrum of the SN-GQD sample. This striking result reveals that the SN-GQD sample is far more efficiently contaminated by Hg2+-containing complexes, causing an ultralow surface coverage and a strong FL quenching after adding Hg2+ ionic solution; that is, SN-GQD is more highly sensitive toward Hg2+ ions, as compared to N-GQD.

4. CONCLUSIONS We have successfully developed an efficient and high-throughput IR heating technique to rapidly synthesize both SN-GQD and N-GQD for the highly sensitive detection of mercury ions in an aqueous solution. The IR technique delivered commercial feasibility to produce both high-quality and high-yield GQD products. Both GQD samples showed their FL quenching abilities in the presence of Hg2+ ions within the tested concentration ranges of 10 ppb‒10 ppm. However, the sensitivity of SN-GQD was found to be 4.23 times higher than that of N-GQD. The corresponding Stern-Volmer coefficient KSV


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was determined to be 0.22 L/mg and 0.052 L/mg for SN-GQD and N-GQD, respectively. One inter-band gap structure of both GQDs was proposed for the FL quenching due to the detection of mercury ions in liquid phase. The improved sensitivity of SN-GQDs could be ascribed to the fact that S doping coordinates with phenolic groups on the edges of SN-GQDs (i.e., the formation of (CxO)2Hg2+) and that the electrons in the excited state of SN-GQDs tend to transfer to the 4f orbits of Hg2+. Therefore, this would facilitate added non-radiative electron/hole recombination annihilation, thereby additionally imparting significant FL quenching. Accordingly, the design of S and N co-doped GQDs could be considered as a feasible candidate for the highly sensitive detection of Hg2+ ions at ultralow concentration, favoring the FL labeling in biology and disease detection in the future.

ACKNOWLEDGMENTS Financial support from the Ministry of Science and Technology of Taiwan (MOST 105-2628-E-155-002-MY3 and MOST 105-2221-E-155-014-MY3) is greatly appreciated. Partial financial support for this work under the grant number CMRPD2E0082 by the Chang Gung Memorial Hospital, Linkou, Taiwan (Chang Gung Medical Foundation, Taiwan) is deeply appreciated. In addition, partial support from the Nano-Material Technology Development Program (R2011-003-2009) through the National Research Foundation of Korea is also acknowledged.



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Figure Captions Figure 1. (a) Schematic diagram for growth of N-GQD and SN-GQD samples by an IR-assisted technique. (b) Fluorescence quenching test of N-GQD and SN-GQD samples in the presence of Hg2+ ions. Figure 2. (a) FT-IR and (b) Raman spectra of SN-GQD and N-GQD samples. Figure 3. Survey-scan XPS spectra of N-GQD and SN-GQD samples. High-resolution XPS spectra of N-GQD sample: (b) C 1s and (c) N 1s, and SN-GQD sample: (d) C 1s, (e) N 1s, and (f) S 2p. Figure 4. HR-TEM micrographs, lattice fringes, and particle size distributions of (a, c, e) N-GQD and (b, d, f) SN-GQD samples. Figure 5. Blue-light emission from (a) bean sprouts and (b) celery grown in SN-GQD solution under 360-nm UV irradiation, indicating the permeation throughout the plant cell with good bio-compatibility. Photographs of (c) N-GQD and (d) SN-GQD suspension after adding different amount of Hg2+ ionic solutions. Figure 6. FL emission spectra from (a) N-GQD and (b) SN-GQD suspensions excited by 360-nm UV irradiation after adding different amounts of Hg2+ ionic solution. (c) Relative FL quenching ratio as a function of Hg2+ concentration and (d) Stern-Volmer plot of FL intensity ratio versus Hg2+ concentration under UV irradiation at 360 nm.



Figure 7. Inter-band gap structure models of SN-GQD and N-GQD samples under different light illuminations: (a) before and (b) after detecting Hg2+ ions. UV-vis spectra of (c) N-GQD and (d) SN-GQD suspensions before and after detecting Hg2+ ions.


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Figure 1. (a) Schematic diagram for growth of N-GQD and SN-GQD samples by an IR-assisted technique. (b) Fluorescence quenching test of N-GQD and SN-GQD samples in the presence of Hg2+ ions.



Figure 2. (a) FT-IR and (b) Raman spectra of N-GQD and SN-GQD samples.


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Figure 3. Survey-scan XPS spectra of N-GQD and SN-GQD samples. High-resolution XPS spectra of N-GQD sample: (b) C 1s and (c) N 1s, and SN-GQD sample: (d) C 1s, (e) N 1s, and (f) S 2p.



Figure 4. HR-TEM micrographs, lattice fringes, and particle size distributions of (a, c, e) N-GQD and (b, d, f) SN-GQD samples.


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Figure 5. Blue-light emission from (a) bean sprouts and (b) celery grown in SN-GQD solution under 360-nm UV irradiation, indicating the permeation throughout the plant cell with good bio-compatibility. Photographs of (c) N-GQD and (d) SN-GQD suspension after adding different amount of Hg2+ ionic solutions.



Figure 6. FL emission spectra from (a) N-GQD and (b) SN-GQD suspensions excited by 360-nm UV irradiation after adding different amounts of Hg2+ ionic solution. (c) Relative FL quenching ratio as a function of Hg2+ concentration and (d) Stern-Volmer plot of FL intensity ratio versus Hg2+ concentration under UV irradiation at 360 nm.


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Figure 7.


Inter-band gap structure models of SN-GQD and N-GQD samples under different light illuminations: (a) before and (b) after detecting Hg2+ ions. UV-vis spectra of (c) N-GQD and (d) SN-GQD suspensions before and after detecting Hg2+ ions.


Sulfur and Nitrogen Co-Doped Graphene Quantum Dots as a

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