Quantification of Humic Substances in Natural Water Using Nitrogen

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Quantification of Humic Substances in Natural Water Using Nitrogen-Doped Carbon Dots Yan-Fang Guan, Bao-Cheng Huang, Chen Qian, and Han-Qing Yu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04430 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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Environmental Science & Technology

Quantification of Humic Substances in Natural Water Using Nitrogen-Doped Carbon Dots

Yan-Fang Guan†, Bao-Cheng Huang†, Chen Qian, Han-Qing Yu* CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, Hefei 230026, China

1

ACS Paragon Plus Environment


1

Dissolved organic matter (DOM) is ubiquitous in aqueous environments and plays a

2

significant role in pollutant mitigation, transformation and organic geochemical

3

circulation. DOM is also capable of forming carcinogenic by-products in the

4

disinfection treatment processes of drinking water. Thus, efficient methods for DOM

5

quantification are highly desired. In this work, a novel sensor for rapid and selective

6

detection of humic substances (HS), a key component of DOM, based on fluorescence

7

quenching of nitrogen-doped carbon quantum dots was developed. The experimental

8

results show that the HS detection range could be broadened to 100 mg/L with a

9

detection limit of 0.2 mg/L. Moreover, the detection was effective within a wide pH

10

range of 3.0 to 12.0, and the interferences of ions on the HS measurement were

11

negligible. A good detection result for real surface water samples further validated the

12

feasibility of the developed detection method. Furthermore, a non-radiation electron

13

transfer mechanism for quenching the nitrogen-doped carbon-dots fluorescence by HS

14

was elucidated. In addition, we prepared a test paper and proved its effectiveness. This

15

work provides a new efficient method for the HS quantification than the frequently

16

used modified Lowry method in terms of sensitivity and detection range.

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INTRODUCTION

18 19

Humic substances (HS) are prevalent in aqueous environment and the main

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component of dissolved organic matter (DOM) with a typical concentration range of

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0.2-20 mg/L in surface water.1 HS can affect the dissolution, mitigation,

22

transformation and bioavailability of heavy metals2-5 and are involved in the microbial

23

electron transfer processes.6 Moreover, they are able to form carcinogenic by-products

24

(e.g., trihalomethanes) in the disinfection treatment process of drinking water.7-9

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Therefore, it is of great importance to determine the HS concentration in natural water

26

and further understand their potential environmental effects. The development of a

27

sensitive HS quantification method with a high reliability and stability is prerequisite

28

for achieving this aim.

29

Several methods such as elemental analysis, electrochemical analysis,

30

chemiluminescence, UV-Vis absorption spectroscopy, and high performance size

31

exclusion chromatography have been developed for HS quantification.10-16 However,

32

each of these detection methods has its own drawbacks. Elemental analysis, e.g.,

33

nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy

34

(FTIR), is unsuitable for quantitative analysis and requires a large amount of sample

35

(usually more than 100 mg is needed for nuclear magnetic resonance).10,11 Although

36

cathodic stripping voltammetry is effective for HS measurements,12 it can be easily

37

interfered by metal ions such as Fe3+ and Ca2+, leading to inaccurate results.

38

Chemiluminescence can be accomplished with the help of glycine and oxidants

39

(1,3-dibromine-5,5-dimethylhydantion). Unfortunately, it leads to the formation of

40

bromides and hypobromites, which are harmful to the environment.13,14 UV-vis

41

spectroscopy method has been widely used to characterize the HS content in natural 3



42

waters. However, it shows no selectivity to HS and other aromatic compounds in

43

water might cause interferences.15 Other methods such as dissolved organic carbon

44

quantification are unable to distinguish HS from other organics either. In comparison,

45

high performance size exclusion chromatography is able to isolate and detect the HS

46

from other organics, but it is time consuming and costly columns are needed.16 In fact,

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HS contain many chromophoric (light absorbance) and fluorophoric (light emission)

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moieties because of their aromatic chromophores and π bonding, which could be

49

characterized by fluorescence.17 However, fluorescence measurement could be

50

interfered by instrumental state, sample themselves and data analysing.18 Furthermore,

51

fluorescence will be self-quenched once DOM is present at a high concentration.

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Recently, fluorescent quantum dots, especially carbon quantum dots or carbon

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nanoparticle dots (C-dots), have attracted considerable research interests due to their

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high biocompatibility, low cost, low toxicity and chemical inertness.19-21 C-dots have

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been applied for chemical sensors, biosensors, bio-imaging, nanomedicine and

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photoelectrocatalysis because of their superior properties as mentioned above.22,23

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Doping of a nitrogen atom into C-dots for forming N-doped C-dots could reduce the

58

contamination of the C-dot surface and increase their stability. However, so far

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N-doped C-dots have not been applied for the quantitative detection of HS.

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Therefore, this work aims at establishing a stable and sensitive HS detection

61

method by using the N-doped C-dots. The work was conducted to: 1) evaluate the

62

stability of the N-doped C-dots under extreme pH and ion strength conditions; 2)

63

determine the detection range and detection limit; 3) validate the practical application

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of this method for real water samples; and 4) explore the fluorescence quenching

65

mechanism. In this way, a simple, anti-interference and reliable method for HS

66

quantification was established. 4


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MATERIALS AND METHODS

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Chemicals. Ethylene glycol, sodium alginate (SA), ethylenediamine, hydroquinone

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(H2Q), KCl, MgCl2, FeCl3, CuCl2, Mn(NO3)2, AlCl3, CaCl2, HF, H2SO4, HNO3 and

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NaHCO3 were purchased from Sinopharm Chemical Reagent Co., China. Disodium

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anthraquinone-2,6-disulfonate (AQDS), bovine serum albumin (BSA) and quinine

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sulfate were purchased from Aladdin Co., China. Phosphate buffered saline tablet was

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purchased from Sigma-Aldrich Co., USA. Suwannee River fulvic acid (Standard I)

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was purchased from International Humic Substance Society. Commercial humic acid

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(HA) (Sigma-Aldrich Co., USA) was purified prior to use as reported previously.24

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Briefly, the commercial HA was dissolved in a NaOH solution (pH=13.0) and filtrated,

79

followed by the adjustment of pH to 7.0 with HCl and filtration; the final solution was

80

freeze-dried. Mili-Q ultrapure water was used in this work. All chemicals were

81

analytical grade.

82

Preparation of N-Doped C-Dots. The N-doped C-dots were prepared using a

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modified hydrothermal method reported by Hu et al.25 Briefly, 25 mL ethylene glycol

84

was placed into a 50 mL poly(tetra-fluoroethylene) (Teflon)-lined autoclave and

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heated at 200 °C for 5 h. After cooling to 20 °C, ethylenediamine of 1.35 mL was

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added into the obtained homogeneous solution. After stirring for 30 min, the mixtures

87

were heated at 180 °C for 5 h. After cooling to room temperature, the N-doped C-dots

88

were obtained.

89

Characterization of N-Doped C-Dots. The functional groups of the N-doped

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C-dots were characterized by FTIR (Bruker Co., Germany) with air as the background.

91

The sample spectra were scanned from 400 cm-1 to 4000 cm-1 with a resolution of 4 5



92

cm-1. The main surface chemical elements were characterized by ESCALAB 250

93

X-ray photoelectron spectroscopy (XPS, VG Instrument Co., USA). The

94

morphologies of the N-doped C-dots were imaged by transmission electron

95

microscopy (TEM, GATAN Instrument Co., USA). All ultraviolet-visible absorption

96

spectra were obtained using a UV-Vis spectrophotometer (Model 2450, Shimadzu Co.,

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Japan). The fluorescence spectra and excitation emission (Ex/Em) matrix were

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measured using a fluorescence spectrophotometer (Aqualog, Horiba Co., Japan),

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while the fluorescence lifetime was determined with a steady state/transient

100

fluorescence spectrometer (JY Fluorolog-3-Tou, Jobin Yvon Co., France).

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To explore the quenching mechanism of the N-doped C-dots by AQDS, HA and

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H2Q, electrochemical experiments were conducted on an electrochemical workstation

103

(CHI760D, CHI Instruments Co., China). As reported previouly,26,27 a 1-mm diameter

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planar platinum disk was used as the working electrode. Pt wire and Ag/Ag+ electrode

105

were used as the counter electrode and the reference electrode, respectively. The Pt

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disk electrode was sequentially polished, washed by water, sonicated three times and

107

activated with H2SO4 before each experiment. N2 was purged for 15 min before the

108

tests.

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Measurement of HS in Natural Water Samples. Before the measurement, the

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anti-interference capacity of this method was evaluated. BSA and SA, as the dominant

111

organics in DOM, were separately selected as the interfering substances and the

112

impacts of their concentration on the HA detection was examined. Moreover, the

113

impacts of several ions, which were selected according to the reference,28 on the

114

fluorescence intensity of the N-doped C-dots were also evaluated.

115

In a typical HS detection procedure, 50 µL of the as-prepared N-doped C-dots

116

was added to 5-mL HS solution to give a final C-dots concentration of 1% (v/v). The 6


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optimal concentration of the N-doped C-dots was selected according to its

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fluorescence performance (Fig. S1). Then, the mixture was sufficiently shaken by

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vortex and the fluorescence of the N-doped C-dots was recorded by a

120

spectrophotometer (JY Fluorolog-3-Tou, Jobin Yvon Co., France). In order to evaluate

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the feasibility of the proposed method, three sample sets from artificial water sources,

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wetland sources, municipal wastewater treatment plant (WWTP) sources and pool

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sources were collected and tested in May, 2017. Samples from Chaohu Lake

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(31°32.081' N, 117°38.845' E), Anhui Province, China, were used as the natural water.

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The wetland sample series was collected from the Artificial Wetland Park (31°71.803'

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N, 117°40.102' E), Hefei City, China. The influent and effluent samples of the WWTP

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were collected from the Jingkaiqu WWTP (Hefei City, China). The last water sample

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series were collected from three different pools in the campus of our university

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(31°84.302' N, 117°27.536' E), Hefei City, China. The standard addition recovery

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method was adopted to measure the HS in natural water samples and triplicate

131

measurements were conducted for each sample. The recovery of HS was calculated as

132

follows: P=(C2-C1)/C3×100%

(1)

133

where P (%) is the HS recovery, C1, C2, and C3 (mg/L) are the detected HS

134

concentrations in the pristine sample, the measured HS concentration after dose of a

135

certain amount of standard HS into sample solution, and the quantity of standard HS

136

added to the solution, respectively.

137 138

RESULTS AND DISCUSSION

139 140

Properties of the N-Doped C-Dots. The as-prepared N-doped C-dots were well 7



141

mono-dispersed and uniform (Figure S2). Their average diameters were 5.7 nm and

142

they were distributed predominantly in a narrow range of 3.5-7.5 nm (Fig. 1A). The

143

N-doped C-dots exhibited a quantum yield of approximately 15% (Fig. S3, for

144

detailed procedures, SI). Their full range XPS analysis (Fig. 1B) clearly showed C 1s

145

(284.8 eV), N 1s (399.9 eV) and O 1s (531.7 eV) signals.29 The C 1s energy peak was

146

divided into 3 peaks for the N-doped C-dots ascribed to the C=C (284.5 eV), C-N

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(286.0 eV) and C-O species (287.8 eV) (Fig. S4A).23 The N 1s energy peak reveals

148

that nitrogen was mostly in the form of graphite-like structure (N-C3) (398.3 eV),

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pyridinic N (399.9 eV) and N-H (401.2 eV) (Fig. S4B).30,31 The O 1s exhibits one

150

fitted peak, which was ascribed to C=O (531.7 eV) (Fig. S4C).30 The obtained XPS

151

results validate the successful doping of nitrogen atom into the C-dots. FTIR

152

spectroscopy shows that the three peaks present for the N-doped C-dots were ascribed

153

to the C-N (ca. 1544 cm-1), CO=NH (ca. 1650 cm-1), and C=NH- (or –C≡NH) (ca.

154

2098 cm-1) stretching vibrations (Fig. S5).32 These functional groups also demonstrate

155

that the nitrogen atom was successfully doped into the C-dots. The absorption band ca.

156

3276 cm-1 was attributed to the N-H stretching vibration,33,34 and the peaks ca. 2939

157

cm-1, 2879 cm-1, 1452 cm-1 and 1375 cm-1 were assigned to the C-H stretching

158

vibration.35 The analyses also suggest that the N-doped C-dots were highly

159

hydrophilic.

160

The optical properties of the N-doped C-dots were characterized by fluorescence

161

spectra under different excitation wavenumbers (Fig. 1C). The emission wavelength

162

was red-shifted from 390 nm to 480 nm when the excitation wavelength was

163

increased from 300 nm to 400 nm, suggesting the excitation-dependent fluorescence

164

behavior.36 This result was ascribed to the different sizes and surface states of the

165

as-prepared N-doped C-dots, similar to the behavior observed for the most 8


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luminescent graphite quantum dots and carbon dots.37-40

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To evaluate the stability of the N-doped C-dots, the effects of pH and ionic

168

strength on the fluorescence intensity were further examined. The fluorescence

169

intensity of the prepared C-dots was stable within the pH range of 3.0-12.0 (Fig. 2A).

170

Moreover, the fluorescence strength remained constant as the ionic strength was

171

increased from 10-7 to 1 M (Fig. 2B), implying that there was no ionization of surface

172

groups.41 These results show that the N-doped C-dots were optically stable.

173

Fluorescence Response of N-Doped C-Dots to HS. HA was selected as the

174

representative HS in this work and its fluorescence property was explored. Initially,

175

the fluorescence strength of HA increased as its concentration was increased from 0 to

176

50 mg/L and subsequently decreased with a further increase in its concentration (Fig.

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3A). This was caused by the inherent property of fluorescence quenching under high

178

concentration conditions. Although HA could be detected by using its fluorescence,

179

the linear range of the fluorescence method was 0-20 mg/L (R2=0.9692). When the

180

N-doped C-dots were added into the HA solution, the peak intensity of the C-dots

181

decreased (Fig. S6) and a wide detection range with 0-100 mg/L HA was achieved

182

(Fig. 3B). To validate the stability of this quantification method, the impact of pH on

183

its accuracy was further evaluated. Although the HA fluorescence changed

184

significantly in the tested pH range of 4.2~9.2, the linear decrease in the fluorescence

185

of the N-doped C-dots was stable (Fig. S7). These results suggest the potential of this

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method for practical applications.

187

The complex compositions of natural water poses a great challenge to the

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detection methods not only in sensitivity but also more significantly in selectivity.42

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Thus, the interferences of proteins and polysaccharides, which are the two widespread

190

organics in DOM,44 to the HA detection were explored. BSA and SA were 9



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respectively selected as the representative proteins and polysaccharides. As shown in

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Fig. 3C and D, the fluorescence strength of the N-doped C-dots did not substantially

193

decrease after adding SA and BSA. Moreover, the effect of their mixture was found to

194

be negligible (Fig. 3E). The selectivity of this detection method was further validated

195

in the presence of metal ions and anions. Several ions extensively present in natural

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water and wastewater were selected based on the previous reports,28,44 and their

197

dosing concentrations were adopted from the data of Zhou et al.28As shown in Fig. 3F,

198

the interferences of the ions were negligible. These results clearly indicate that the

199

N-doped C-dots were highly sensitive and selective for HA over other organic and

200

inorganic ions in natural water, further implying that the N-doped C-dots could meet

201

the detection requirements for practical use.

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Possible Fluorescence Quenching Mechanism of N-Doped C-Dots by HS. To

203

characterize the quenching mechanism of the N-doped C-dots by HS, the impact of

204

HA concentration on its own absorbance intensity was examined firstly. The

205

absorbance intensity showed an excellent linear response to the HA concentration

206

regardless the presence of the N-doped C-dots or not (Fig. 4A). This result indicates

207

that there was no static complexation between HA and the N-doped C-dots. In

208

comparison, the time-correlated single photon counting detection results show that the

209

average fluorescence lifetime of the N-doped C-dots was found to decrease with the

210

increasing HA concentration (Fig. 4B). Therefore, the quenching of C-dots

211

fluorescence by HA was a dynamic quenching process, which was possibly caused by

212

the collision between fluorescent and quencher.45

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HS are able to accept and donate electrons simultaneously,6,46 which is mainly

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attributed to the existence of the electron-accepting (quinone) and electron-donating

215

(phenol) moieties. To better understand the quenching mechanism, AQDS and H2Q 10


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were respectively selected as the quinone and phenol moieties47 and their impacts on

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the fluorescence of C-dots were explored. Both of them could linearly quench the

218

fluorescence of the N-doped C-dots (Fig. 4C-D). To find out the effect of the

219

quenching, the UV-Vis absorption spectra of AQDS, HA and H2Q were recorded and

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the fluorescence emission spectrum of the N-doped C-dots at the excitation

221

wavelength of 320 nm was also recorded (Fig.4E). There was no obvious overlapping

222

region between the absorption spectra (AQDS, HA and H2Q) and the fluorescence

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emission spectrum of the N-doped C-dots, suggesting that the fluorescence resonance

224

energy transfer was not the quenching mechanism of the N-doped C-dots by HS,

225

because there was no energy transfer from the excited C-dots to AQDS, HA and H2Q.

226

The light-excited fluorophore (donor) is able to transfer its excited electrons to

227

another substance (i.e., acceptor) when they are sufficiently close, e.g., via van der

228

Waals contact.48 Therefore, non-radioactive electron-transfer was proposed as a

229

possible quenching mechanism of the N-doped C-dots by HS (Fig. 5). For validation,

230

electrochemical experiments were conducted. A reduction peak at -0.52 V (vs.

231

Ag/AgCl) was observed for the N-doped C-dots in the cyclic voltammetry spectrum

232

while no oxidation peak was observed (Fig. 4F). This is in accordance with the

233

previous observations.41 In comparison, both AQDS (-0.4 V) and HA (-0.36 V)

234

showed a more positive reduction potential and were likely to accept electrons from

235

the C-dots. As a result, the light-induced electron was possibly absorbed by HS and

236

the fluorescence of the C-dots was quenched accordingly.

237

Application of Our Method to Water Samples. Before applying our method to

238

measure water samples, a stability test comparing with the Modified Lowry method

239

was conducted by mixing different proportions of HA and fulvic acid. The determined

240

HS concentrations by these two methods were of almost the same level (Table 1), 11



241

implying that the proposed method was appropriate for the quantification of HS in

242

natural water samples. Then, the nine water samples from various sources were also

243

analyzed by using our method. The recoveries for these actual samples ranged from

244

92.64% to 131.55% with a relative standard deviation (RSD) of 0.38%-25.00% range

245

(Table 2). Moreover, the linear range and detection limit of the proposed method are

246

comparable to those of the other methods (Table S1), further demonstrating the

247

suitability of this method for the HS detection of natural water samples.

248

Implications of This Work. A new method with a high detection range (0-100

249

mg/L) and a great selectivity for HS quantification was established by using the

250

N-doped C-dots as a probe in this work. The detection limit of this method was 0.2

251

mg/L, which was sufficient for HS quantification in surface water as its typically is in

252

a range of 0.5-20 mg/L.1 Currently, the HS content is quantified by the modified

253

Lowry method.49 However, the steps are complicated and the detection limit of the

254

modified Lowry method is approximately 1.3 mg/L. Compared to the modified Lowry

255

method, the C-dots fluorescence quenching-based method developed in this work is

256

easier and more appropriate for high-concentration HS determination. Additionally,

257

this method exhibits a wider detection range with a low background noise compared

258

to the other methods. More importantly, this developed detection method is universal

259

because the other N-doped C-dots synthesized by different methods are also able to

260

quantify HS in water (SI, Section S3), as evidenced by the HS detection results for the

261

N-doped C-dots synthesized with the microwave-assisted method (Fig. S8).

262

Furthermore, since C-dots could be extracted and deposited on the

263

non-fluorescence responsive surface, a portable test paper would be expected. We

264

fabricated a C-dots based test paper (for details, please see SI). The test paper was

265

successfully applied for the quantification of HS at a concentration of 0~20 mg/L (Fig. 12


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6). By coupling with the fluorescent excitation and emission, e.g., a Light Emitting

267

Diode lamp, a portable HS detection prototype will be developed in the future.

268 269

AUTHOR INFORMATION

270

†

271

*Corresponding Author: Prof. Han-Qing Yu, Fax: +86-551-63601592. E-mail:

272

[email protected]. ORCID: Han-Qing Yu: 0000-0001-5247-6244

273

Notes: The authors declare no competing financial interest.

These authors contributed equally to this work.

274 275

ACKNOWLEDGMENTS

276

We thank the National Natural Science Foundation of China (21261160489 and

277

51538011), and the Collaborative Innovation Center of Suzhou Nano Science and

278

Technology of the Ministry of Education of China for the support of this work.

279 280

ASSOCIATED CONTENT

281

Supporting Information Available. Experimental details, quantum yield calculation

282

of N doped C-dots (Section S1), preparation of the portable HA test paper (Section

283

S2), preparation of the N-doped C-dots by microwave-assisted method (Section S3),

284

comparison between this work and published methods for HS quantification (Table

285

S1), fluorescence intensity variations of the N-doped C-dots at diffrent concentrations

286

(Fig. S1), transmission electron microscopy (TEM) images of N doped C-dots (Fig.

287

S2), photoluminescence and absorbance of the quinine sulfate and N doped C-dots

288

(Fig. S3), (A) C 1s, (B) N 1s and (C) O 1s of N-doped C-dots (Fig. S4), Fourier

289

transform

290

three-dimensional excitation emission fluorescence spectroscopy (EEM) of N doped

infrared

spectroscopy

(FTIR)

of

N

doped

13


C-dots

(Fig.

S5),


291

C-dots with increasing HA concentration (Fig. S6), influence of pH on (A)

292

fluorescence of HA and (B) quantification of HA by N doped C-dots (Fig. S7), HA

293

quantification by the N-doped C-dots synthesized with the microwave-assisted

294

method (Fig. S8). This information is available free of charge via the Internet at

295

http://pubs.acs.org/.

296 297

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Table 1 HS Quantification Using Different Methods (5 mg/L theoretical concentration) sample

measured HS concentration (mg/L)

humic acid

fulvic acid

(mL)

(mL)

modified Lowry method

this method

0

5

4.51±0.11

4.87±0.15

1

4

4.92±0.10

5.26±0.24

2

3

4.92±0.21

4.37±0.33

3

2

5.40±0.20

5.04±0.10

4

1

5.35±0.18

4.60±0.22

measured

21



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Table 2 Detection of HS in Water Samples sample

concentration of HA (mg/L) amount measured

Chao Lake 1

1.14

Chao Lake 1 Chao Lake 2

1.64

Chao Lake 2 Chao Lake 3

0.92

Chao Lake 3 wetland

2.21

wetland WWTP influent

3.62

WWTP influent WWTP effluent

1.36

WWTP effluent campus pond 1

2.38

campus pond 1 campus pond 2

2.13

campus pond 2 campus pond 3 campus pond 3

2.08

recovery

RSD

amount found

(%) (%)

(n=3; %)

5

6.41±0.45

105.30

6.98

10

11.27±0.24

101.19

2.17

5

6.27±0.33

92.64

5.20

10

11.48±0.10

98.38

0.88

5

7.50±2.00

131.55

25.00

10

12.24±1.71

113.23

13.97

5

6.82±0.06

92.19

5.80

10

11.78±0.06

95.69

0.48

5

8.30±1.13

93.66

15.06

10

13.37±0.11

97.55

0.84

5

6.22±0.21

97.28

3.50

10

11.86±0.26

104.99

2.22

5

7.29±0.72

98.20

9.95

10

12.27±0.60

98.88

4.86

5

7.21±0.03

101.70

0.38

10

11.97±0.24

98.38

2.04

5

7.19±0.51

102.14

7.60

10

12.07±0.46

99.83

3.82

amount added

22


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Figure Captions

Figure 1.Characteristics of the as-prepared N-doped C-dots. (A) Particle size distribution, (B) XPS survey and (C) Fluorescence spectra of the N-doped C-dots at different excitation wavelengths.

Figure 2. Fluorescence responses of the N-doped C-dots to the variations of pH (A) and ionic strength (B) with Ex/Em of 320 nm/400 nm.

Figure 3. Selective and sensitive response of the N-doped C-dots to HA. Fluorescence intensity variation of (A) HA with rising concentrations; (B) N-doped C-dots with different HA dosages, inset: the optical image under 302 nm UV lamp; impacts of (C) BSA, (D) SA, (E) mixture of SA, BSA and HA, and (F) metal ions on the fluorescence of the N-doped C-dots.

Figure 4. Fluorescence quenching between HA and the N-doped C-dots. (A) Variation of absorbance at 255 nm with increasing HA concentrations. (B) Average fluorescence lifetime variation of C-dots with increasing HA concentrations. Fluorescence intensity variation of C-dots with the increasing concentration of (C) H2Q and (D) AQDS. (E) Fluorescence spectrum of the N-doped C-dots at the excitation wavelength of 320 nm and UV-Vis absorbance spectra of HA, AQDS and H2Q. (F) Cyclic voltammetry of the N-doped C-dots, AQDS and HA at pH 7.0.

Figure 5. Fluorescence quenching mechanism of the N-doped C-dots by HA.

Figure 6. Fluorescence intensity of HA by the test paper, inset: the optical image images at 302 nm (UV lamp).

23



Figure 1. Characteristics of the as-prepared N-doped C-dots. (A) Particle size distribution, (B) XPS survey and (C) Fluorescence spectra of the N-doped C-dots at different excitation wavelengths.

24


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Figure 2. Fluorescence responses of the N-doped C-dots to the variations of pH (A) and ionic strength (B) with Ex/Em of 320 nm/400 nm.

25



Figure 3. Selective and sensitive responses of the N-doped C-dots to HA. Fluorescence intensity variation of (A) HA with rising concentrations; (B) N-doped C-dots with different HA dosages, inset: the optical image under 302 nm UV lamp; impacts of (C) BSA, (D) SA, (E) mixture of SA, BSA and HA, and (F) metal ions on the fluorescence of the N-doped C-dots.

26


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Figure 4. Fluorescence quenching between HA and the N-doped C-dots. (A) Variation of absorbance at 255 nm with increasing HA concentrations. (B) Average fluorescence lifetime variation of C-dots with increasing HA concentrations. Fluorescence intensity variation of C-dots with the increasing concentration of (C) H2Q and (D) AQDS. (E) Fluorescence spectrum of the N-doped C-dots at the excitation wavelength of 320 nm and UV-Vis absorbance spectra of HA, AQDS and H2Q. (F) Cyclic voltammetry of the N-doped C-dots, AQDS and HA at pH 7.0.

27



Figure 5. Fluorescence quenching mechanism of the N-doped C-dots by HA.

28


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Figure 6. Fluorescence intensity of HA by the test paper, inset: the optical image at 302 nm (UV lamp).

29



Table of Contents (TOC) Art

30


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Quantification of Humic Substances in Natural Water Using Nitrogen

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