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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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

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

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significant role in pollutant mitigation, transformation and organic geochemical

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

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

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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

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

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

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

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

13

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

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

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

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

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INTRODUCTION

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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,

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

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

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(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

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

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

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for achieving this aim.

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Several methods such as elemental analysis, electrochemical analysis,

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

31

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

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

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nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy

34

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

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

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

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

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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

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

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

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

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

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

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

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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

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characterized by fluorescence.17 However, fluorescence measurement could be

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

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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

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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

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

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

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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

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

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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,

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

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

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analytical grade.

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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

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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

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

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were obtained.

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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.

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

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

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

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

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

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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

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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

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(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

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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

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

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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

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

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

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

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

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

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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

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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

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

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follows: P=(C2-C1)/C3×100%

(1)

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

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

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

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added to the solution, respectively.

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RESULTS AND DISCUSSION

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

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

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

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

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

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

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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

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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

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

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

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

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

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

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

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

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

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

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hydrophilic.

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

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

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

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

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

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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

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strength on the fluorescence intensity were further examined. The fluorescence

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

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

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

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

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

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

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

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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

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

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

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

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

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

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its accuracy was further evaluated. Although the HA fluorescence changed

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

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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.

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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

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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

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

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

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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

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

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

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

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

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the detection requirements for practical use.

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

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

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

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absorbance intensity showed an excellent linear response to the HA concentration

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

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

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

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

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

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

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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

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(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

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

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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

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

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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

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

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

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

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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

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

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

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

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

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

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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

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the fluorescence of the C-dots was quenched accordingly.

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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

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

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

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

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

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

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

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

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

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

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

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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

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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

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

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

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

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

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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

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

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

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

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

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

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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

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

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AUTHOR INFORMATION

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

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

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Notes: The authors declare no competing financial interest.

These authors contributed equally to this work.

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ACKNOWLEDGMENTS

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

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

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

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ASSOCIATED CONTENT

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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

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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

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C-dots

(Fig.

S5),

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

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

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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

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http://pubs.acs.org/.

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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

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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

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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).

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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.

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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.

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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.

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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.

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

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

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Table of Contents (TOC) Art

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