Development of sulfur doped carbon quantum dots for highly selective and sensitive fluorescent detection of Fe2+ and Fe3+ ions in oral ferrous gluconate samples

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 226 (2020) 117602

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Development of sulfur doped carbon quantum dots for highly selective and sensitive fluorescent detection of Fe2þ and Fe3þ ions in oral ferrous gluconate samples Fuyou Du a, b, *, Zhenfang Cheng b, Wei Tan c, Lingshun Sun b, Guihua Ruan b, ** a

College of Biological and Environmental Engineering, Changsha University, Changsha, 410003, China Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin, 541004, China c Department of Food and Chemical Engineering, Lushan College of Guangxi University of Science and Technology, Liuzhou, 545616, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 November 2018 Received in revised form 16 August 2019 Accepted 6 October 2019 Available online xxx

Sulfur-doped carbon quantum dots (S-CQDs) with stable blue fluorescence were synthesized through a facile one-step hydrothermal method by using ascorbic acid and thioglycolic acid as carbon and sulfur sources. The prepared S-CQDs exhibited a sensitive and selective response to Fe3þ ions in comparison with Fe2þ and other metal ions, In the presence of adequate H2O2, Fe2þ was completely transformed to Fe3þ that is the determinable form of iron ions, and the difference in the change of the fluorescence intensity of S-CQDs before and after adding H2O2 was used for detection of Fe2þ and Fe3þ ions, respectively. Under the optimum experimental conditions, the fluorescence intensity of S-CQDs gradually decreased with increasing of Fe3þ concentration ranging from 0 to 200 mM. Good linearity was achieved over the range of 0-200 mM. The detection limit of the developed method was 0.050 mM for Fe3þ. The recoveries of Fe2þ and Fe3þ spiked in real samples ranged from 98.2% to 112.4%. Finally, the proposed SCQDs integrated with Fenton system was applied to the detection of Fe2þ and Fe3þ ions in oral ferrous gluconate samples, which presents potential applications in the speciation and determination of Fe2þ and Fe3þ ions in complex samples. © 2019 Published by Elsevier B.V.

Keywords: Carbon quantum dot Sulfur co-doped Fluorescence quenching Iron Iron supplement

1. Introduction Iron is one of the most essential trace elements in living biosystems and plays indispensable and versatile roles in many physiological and pathological processes, including enzyme catalysis, oxygen transport, cellular metabolism, electron transfer, cell death, disease, and DNA and RNA synthesis [1e5]. However, too little or too much iron in human body can cause various diseases, for example, too little iron produces iron-deficiency anemia, and excess iron causes siderosis and organ damage [4e7]. Iron deficiency anemia affects roughly a sixth of the world’s population, especially for children aged 0-5 years, women of childbearing age, and pregnant women, therefore, how to treat iron deficiency

* Corresponding author. College of Biological and Environmental Engineering, Changsha University, Changsha, 410003, China. ** Corresponding author. E-mail addresses: [email protected] (F. Du), [email protected] (G. Ruan). https://doi.org/10.1016/j.saa.2019.117602 1386-1425/© 2019 Published by Elsevier B.V.

anemia still remains challenging [7e9]. At present, many treatments of iron deficiency anemia are available, but oral iron is usually recommended as first-line therapy [7,8]. Since free Fe3þ is easy to form insoluble Fe(OH)3 in the neutral physiological environment, which is difficult to be assimilated by the human body, and in addition, free Fe3þ may also induce to produce extremely reactive hydroxyl radical which will cause severe damage to membranes, DNA and chelating with various regulatory proteins, therefore, oral iron supplements use Fe2þ as a source of iron, such as common ferrous sulfate, ferrous sulfate exsiccated, ferrous gluconate, and ferrous fumarate [8]. However, Fe2þ is likely to be oxidized into Fe3þ during the production and storage of oral iron supplementation samples, therefore, to develop selective and sensitive analytical method for determination and discrimination of Fe3þ and Fe2þ in iron preparations is of fundamental importance for the quality and safety of pharmaceuticals. At present, several analytical methods for determination and discrimination of Fe3þ and Fe2þ have been developed and applied including ion-exchange chromatography-flame atomic absorption spectrometry [10], inductively coupled plasma mass spectrometry

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[11], capillary zone electrophoresis-UV detection [12], Solid phase extraction coupled with liquid waveguide capillary cell [13], and colorimetric [13e15]. However, these instrumentally intensive methods often require lengthy pretreatment sample procedures, and also to some extent the sophisticated instrumentation, which will not be suitable for on-line or in-field analysis of Fe3þ and Fe2þ. Owing to the non-destructive character, high sensitivity, selectivity and fast detection, fluorescence analytical method has attracted considerable attention in the determination of Fe3þ and Fe2þ in recent years [16e22]. Carbon quantum dots (CQDs) are a new class of carbon nanomaterials that were discovered in 2004 [23] and have recently garnered much attention as potential competitors to conventional semiconductor quantum dots owing to their excellent optical properties, good biocompatibility, great water-solubility, low cost and cytotoxicity, easy synthesis, and environmental friendliness [24e26]. Thus, CQDs have been widely applied in chemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis, and several excellent reviews of CQDs and their applications have been reported in recent years [24e32]. However, low fluorescent quantum yields (FLQYs, usually less than 50%) and few functional surface groups for bioconjugation and further applications as two considerable problems for original CQDs have limited their potential applications based on their photoluminescent properties, therefore, the surface functionalization and chemical heteroatoms doping have been selected to improve the photoluminescence performance and expand the application scope of CQDs [33]. Heteroatom-doped CQDs, due to their excellent photoluminescence properties and potential applications, have attracted tremendous attention recently in many fields [26,27,33e35]. Among various non-metal-based doping, nitrogen doping (N-doping) and sulfur doping (S-doping) were found to be highly beneficial to improve the optical properties of CQDs [27,34,35], however, it was difficult to prepare S-CQDs by chemical doping of S into the framework of CQDs because the S atom is much larger than carbon (C) atom, and the difference of electronegativity between S (2.58) and C (2.55) is too small to offer significant charge transfer in CeS composites [36]. Thus, different precursors, including waste frying oil, sulfuric acid, sodium thiosulfate, sodium citrate, thiomalic acid, thioglycolic acid, and vitamin B1, have been used to fabricate S-CQDs [22,37e39] and N,S co-doped CQDs (N,S-CQDs) [40e43]. Compared with undoped CQDs, S-CQDs and N,S-CQDs showed more stable fluorescence, drastically improved electronic properties, and increased surface chemical reactivity, and thus have been applied for selective recognition of Fe3þ ions and other target analytes [34,37e44]. However, few works have been reported about the applications of CQDs, S-CQDs or N,S-CQDs for the simultaneous determination and discrimination of Fe3þ and Fe2þ in real samples [19,20,22]. Therefore, there is still a great need for the development of novel CQDs with high sensitivity and high selectivity for detection of Fe2þ and Fe3þ ions. In this work, we present a fluorescent sensor system for selective and sensitive detection of Fe2þ and Fe3þ in iron supplement oral liquids by use of S-CQDs, which was prepared by one step hydrothermal method. The prepared S-CQDs shows a high stability, high selectivity, high sensitivity and fast response for iron detection. The fluorescence intensity of the S-CQDs decreased linearly with increasing Fe3þ and Fe2þ concentration due to fluorescence quenching without and with addition of hydrogen peroxide (H2O2), respectively. The S-CQDs as fluorescent probe exhibited a linear range within 0-200 mM and a detection limit of 0.05 mM. The high recovery in spiked standard sample shows the potential of the proposed S-CQDs integrated with Fenton system for detection of Fe2þ and Fe3þ in real complex samples.

2. Materials and methods 2.1. Reagents and solutions Ascorbic acid (AA) was purchased from Chengdu Kelong Chemical Reagent Factory, (CHN). Thioglycolic acid (TGA) was purchased from Chinese medicine group chemical reagent co., Ltd (CHN), Quinine sulfate dihydrate, ferrous gluconate, potassium gluconate were obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). Cr(NO3)3$9H2O, Ba(NO3)2, Pb(NO3)2, Al(NO3)3$9H2O, Fe(NO3)3$9H2O, Ni(NO3)2$6H2O, Co(NO3)2$6H2O, Sr(NO3)2, KNO3, Cd(NO3)2$4H2O, Bi(NO3)3$5H2O, Zn(NO3)2$6H2O, Cu(NO3)2$3H2O, La(NO3)3$6H2O, Fe(NO3)2$6H2O, HCl, NaOH, H2O2, NaCl, methanol, anhydrous ethanol and other reagents were purchased from Xilong Chemical Reagent Co., Ltd. (Guangzhou, China). Aqueous solutions of metal ions were prepared from their metallic salts. Different oral ferrous gluconate, calcium gluconate, and zinc gluconate samples were purchased from Guilin pharmaceutical city (Guilin, China). Deionized water was used throughout the experiments. 2.2. Characterization Fluorescence measurements were performed on a Hitachi F7000 florescence spectrophotometer (Tokyo, Japan). Ultravioletevisible (UVevis) absorption spectra were obtained at room temperature on a TU-1950 UVevis spectrophotometer (Beijing Pu Analytical General instrument Co., Ltd, China). The functional groups of synthesized S-CQDs were investigated by a Nicolet IS10 Fourier transform-infrared (FT-IR) spectrometer (ThermoElectron Corp., USA) with the KBr pellet technique in the range of 400e4000 cm1. The microstructure of the S-CQDs was characterised by an FEI Tecnai G2 f20s-twin field emission transmission electron microscope (FEI Co., USA). X-ray photoelectron spectroscope (XPS) spectra were measured at an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Scientific, USA). All pH measurements were done on a PHS-3C pH meter (Shanghai Precision Scientific instrument equipment Co., Ltd., China). 2.3. Preparation of S-CQDs We proposed a simple one-step method for synthesizing SCQDs through the thermal treatment of AA and TGA at different temperature (160, 170, 180, 190, and 200  C). Briefly, 450 mg of AA was dispersed in 15 mL of 0.1 mol/L TGA. After thorough mixing, the mixture was transferred into poly(tetrafluoroethylene) autoclave and heated at high temperature for 6 h. After cooling to room temperature, a dark solid material was obtained and then centrifuged at 10000 rpm for about 10 min to remove black precipitate. The obtained supernate was collected and filtered with 0.22 mm Nylon membrane, and then dialyzed in a dialysis bag with amolecular weight cut-off of 500 Da against distilled water at room temperature for about 48 h to remove the remaining salts and tiny fragments. Finally, a clean and brown solution containing S-CQDs was obtained. In addition, the solid S-CQDs were obtained by drying the S-CQDs solution in Freeze dryer. Before use, 1 mg/mL SCQDs solution was prepared by dissolving S-CQDs in deionized water, and preserved in 4  C, which could retain stability for at least six months according to our experiment data. 2.4. Quantum yield measurement The quantum yield (QY) of the S-CQDs in aqueous solution was calculated by quinine sulfate solution (QR ¼ 54%, 0.10 mol/L H2SO4) as reference together with the following formula [44,45]:

F. Du et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 226 (2020) 117602

 Y ¼ YST G=G ST



h=h

2 ST

Where, YST and Y are the QY of the reference (quinine sulfate) and sample, respectively. G and GST are the slopes of fitting equation of the sample and reference, h and hSt are the refractive indexes of the sample and reference solution, respectively. In order to reduce the influence of the self-priming effect on the determination, the measured absorbance of individual solutions was maintained below 0.1 at the excitation.

2.5. Detection of Fe3þ and Fe2þ ions with S-CQDs The as-prepared S-CQDs exhibited strong blue fluorescence, and their fluorescence emission intensities increased obviously with the increasing concentrations of S-CQDs (see Fig. S1 in Supporting Information). In this work, 30 mg/mL homogeneous S-CQDs solution was prepared by dissolving the as-prepared in acidified deionized water (pH 3.0). The acidified carbon dot solution was selected in the following experiments in order to avoid the possible Fe(OH)3 precipitation at high concentrations of Fe3þ in aqueous solution [46]. For the detection of Fe3þ ions, 2.0 mL of Fe3þ sample solution was added to 2.0 mL of the above S-CQD solution in a 10 mL volumetric flask. The Fe3þ species that quenched the fluorescence intensity of each S-CQD solution were recorded immediately at 450 nm when excited at 360 nm. Then, the spectral measurements were used to plot the quenching calibration curve for Fe3þ. For the detection of Fe2þ species, 1.0 ml H2O2 was added in 2.0 mL of Fe2þ sample solution (ferrous gluconate sample solution used in this work), and then mixed with 2.0 mL of the above S-CQD solution in a 10 mL volumetric flask. After reaction for 2.0 min, the fluorescence intensity of each mixture solution were recorded according to the same procedure as the same of Fe3þ detection.

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The selectivity of the S-CQDs was determined by adding other related analogues, including Al3þ, La3þ, Cr3þ, Cd2þ, Co2þ, Fe2þ, Zn2þ, Sr2þ, Ba2þ, Ni2þ, Ca2þ, Kþ and Naþ in a similar manner. All experiments were performed at room temperature. In order to evaluate the S-CQDs as a sensor for Fe3þ and Fe2þ in real samples, oral ferrous gluconate samples were chosen as real sample. Before the detection, oral ferrous gluconate samples were diluted 10-fold for Fe3þ and 200-fold for Fe2þ detection with deionized water, without any other sample preparation, because the fluorescence intensity of S-CQDs was not obviously affected by the other possible components such as saccharose, citric acid, gluconic acid and sodium benzoate, according to our original experimental results shown in Fig. S2. The following analytical process was according to the above-described process for detection of Fe3þ and Fe2þ ions.

3. Result and discussion 3.1. Synthesis and characterization of S-CODs We proposed a simple one-step method for synthesizing SCQDs via solvothermal treatment of AA and TGA at high temperature (Fig. 1(A). The results shown in Fig. 1(B) indicated that the carbonization degree was elevated at high temperature and the QY values were increased with the increase of carbonization temperature from 160 to 180  C. According to the calculation method from literature [44,45], the highest QY values reached 32.07% at 180  C, which is comparable to those prepared using other carbon precursors [47]. The elemental composition and chemical state of different elements on the surface of S-CQDs was analyzed by XPS, and the obtained EDS spectra in Fig. 2(A) showed that there are three typical peaks for S-CQDs at 0.25, 0.5 and 2.35 KeV, which are corresponding to C1s, O1s and S2p, and the content of C, O, and S was 88.24%, 9.80% and 1.96%, respectively. In addition, FT-IR spectrum of

Fig. 1. (A) Schematic presentation of the synthesis of S-CQDs with a one-step hydrothermal process and its application for detection of Fe2þ and Fe3þ ions. The effect of synthesis temperature (B) and time (C) on the quantum yields of S-CQDs.

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Fig. 2. Characterization study of the S-CODs. (A) EDS spectra by XPS analysis, (B) FT-IR results, (C) TEM images, (D) size distribution results, (E) UVeVisible (black color), excitation (red color) and emission (blue color) spectra of S-CQDs dispersed in deionized water, (F) the fluorescence emission spectrum of S-CQDs under different excitation, (G) 3D matrices of the fluorescence spectra of S-CQDs under different excitation wavelength varying from 220 to 650 nm and the resulting emission wavelength varying from 220 to 900 nm, and (H) the photostability of S-CQDs after UV irradiation for 60 min.

S-CQDs shown in Fig. 2(B) revealed a broad peak at 3438 cm-1 correlated with the stretching vibration of eOH. The absorption peak of the C¼O group from TGA at 1720 cm-1 [48] conjugated with condensed aromatic carbons at 1640 cm-1 appeared, demonstrating that the carbon dots were synthesized through the aromatization steps under hydrothermal conditions [49]. The broad absorption peak at 2664-2528 cm-1 indicated to the existence of -SH. In addition, the absorption peak at 1391 cm-1-1187 cm-1 could be

assigned to the stretching vibration of the -CO group. The above results verified the successful preparation of sulfhydryl functionalized carbon quantum dots was success. The morphologies of the prepared S-CQDs was characterize by TEM, and the results were shown in Fig. 2(C). TEM images revealed that the S-CQDs were nearly spherical in shape, and the average diameter was about 2.3 nm. The size distribution results of S-CQDs indicated that the S-CQDs were uniformly dispersed (see Fig. 2(D)).

F. Du et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 226 (2020) 117602

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Fig. 3. Results of the selectivity and specificity of S-CQDs. (A) Fluorescence intensity of S-CQDs with various metal ions at concentrations of 1.0 mM, (B) Relative fluorescence intensities ((F0eF)/F0  100%) of the S-CQDs towards the various interfering metal ions (1.0 mM) in the absence (black column) and presence (red column) of Fe3þ (20 mM). Solutions of Fe2þ, Ca2þ, Zn2þ ions were prepared by ferrous gluconate, calcium gluconate, and zinc gluconate, respectively. Here F0 and F were the fluorescence emission intensity of the SCQDs before and after addition of other metal ions, respectively.

In UVeVis absorption spectrum of S-CQDs (see Fig. 2(E)), two absorption bands were observed at 233 and 285 nm, which are assigned to p-p* transition of the C¼C bond and n-p* transition of the C¼O bond [36,50,51], respectively. It can be seen from the Fig. 1(F) that the fluorescence intensity of S-CQDs increased with the increase of excitation wavelength from 260 to 360 nm, and then decreased with increasing excitation wavelength, and the fluorescence emission intensity at 450 nm reached a maximum when excited at 360 nm, therefore, the optimal excitation and emission wavelengths were 360 and 450 nm, respectively. Moreover, the excitation-dependent photoluminescence behavior was observed

in Fig. 2(G), which is due to the surface state and band gap of SCQDs. In addition, the light emitting characteristics at 700-900 nm shown in Fig. 2(G) represents the features with up-conversion luminescence of synthesized carbon dots. Besides, the stability of fluorescence intensity of S-CQDs as a probe plays a key role in their sensing applications. The photostability of S-CQDs was investigated after UV irradiation for 60 min. Fig. 2(H) shows that the fluorescence intensity of S-CQDs was stable within the studied irradiation time, indicating that the S-CQDs has an excellent photostability. Fig. S3 shows that the fluorescence intensity of S-CQDs almost had no change after kept for 65 days at

Fig. 4. Fluorescence spectra of S-CQDs in the presence of different concentrations of Fe3þ ions (from top to bottom: 0, 2.5, 5, 25, 50, 100, 150, and 200 mM) (A), and the relationship between DF/F0 (DF ¼ FeF0) and the concentration of Fe3þ ions in the range of 0e200 mM. Here F0 and F are the fluorescence intensity in the absence and presence of Fe3þ ions, respectively.

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Table 1 Comparison of S-CQDs with other fluorescent probes for Fe3þ and Fe2þ detection. Fluorescent probea

Fe2þ

CQDs CQDs ChPA GSH-doped GQDs S-CQDs

Fe3þ

Reference

Detection range

Detection limit

Recovery

Detection range

Detection limit

Recovery

1-500 mM 0-32 mM 0.1-1 nM 1-150 mM 0-200 mM

170 nM 20 nM 0.138 nM 100 nM 50 nMb

105.1%e109.4% e e 100.1%e104.1% 106.9%e112.4%

1-500 mM 0-50 mM 0.1-1 nM 1-150 mM 0-200 mM

170 nM 35 nM 0.124 nM 100 nM 50 nM

97.4%e102.3% e e 98.08e102.7% 98.2%e118.6%

[19] [20] [21] [22] This work

Note. a CQDs, carbon quantum dots; ChPA, fluorescent hydrogel based on chitosan, polyvinyl alcohol and 9-anthraldehyde; GSH-doped GQDs, glutathione doped graphene quantum dots; S-CQDs, Sulfur doped carbon quantum dots. b The detection limit of Fe2þ ions was estimated by the quenched fluorescent intensity with the oxidation of Fe2þ in H2O2 (Fe3þ).

Table 2 Results of determination in oral ferrous gluconate liquids (n ¼ 5)a. Sample

Labelled Fe2þ concentration (mM)

Detected Fe3þ concentration (mM)b

Detected Fe2þ concentration (mM)

Brand 1 Brand 2 Brand 3 Brand 4 Sample 5c

51.848 62.218 62.218 62.218 51.848

ND ND ND ND 0.749 (±0.197)

52.435 63.524 62.413 64.790 44.674

(±0.590) (±1.469) (±1.334) (±2.017) (±1.273)

Note. a All real samples were diluted 200 fold for Fe2þ and 10-fold for Fe3þ with acidified water (pH 3.0) before the detection, respectively. b All ND means Not detected. c Sample 5 was the Brand 1 sample after exposure to air for a three-day.

room temperature. In addition, the fluorescence intensity of SCQDs was measured over a wide pH range to investigate their stability. From Fig. S4, S-CQDs showed strong fluorescence intensity in the pH range from 3.0 to 11.0 (adjusting by using hydrochloric acid and sodium hydroxide solution), which exhibited a slight pHdependence fluorescence behavior in the pH value range of 3e11. The above results suggested that the S-CQDs possessed excellent fluorescence and good stability in various environmental conditions, which make them suitable for further sensing applications. 3.2. Selectivity and specificity of S-CQDs To further evaluate the selectivity and specificity of the S-CQDs for Fe detection, the fluorescence responses to the other potential interfering substances were tested under the same conditions with various other metal ions, including Cr3þ, La3þ, Al3þ, Fe3þ, Co2þ, Ni2þ, Zn2þ, Cd2þ, Fe2þ, Pb2þ, Sr2þ, Ba2þ, Ca2þ, Kþ and Naþ. As can be seen in Fig. 3 (A), the fluorescence intensity of S-CQDs were decreased by about 80% in the presence of Fe3þ ions (concentration at 1.0 mM), while influence of the other investigated metal ions (each concentration at 1.0 mM) was almost negligible. These observed results indicated that the selectivity of the proposed fluorescence method for Fe3þ detection was high, attributing to the high affinity of S and O on the surface of the carbon dots for complex formation with Fe3þ, fast electron transfer process between Fe3þ and CQDs, and the resulting nonradiative electron/hole recombination [36e38].

Fe3þ ions are highly quenched the fluorescence of S-CQDs compared to other metal ions, might attributing to the incorporation of S atoms into the S-CQDs that tuned the electronic local density of the CQDs and promoted the coordination interaction between Fe3þ and the hydroxyl groups on the edge of CQDs [22]. Moreover, the anti-interference test of Fe3þ detection was also performed in the coexistence of the other metal ions, and the results were shown in Fig. 3(B). Compared to the blank sample, the relative fluorescence intensity ((F0eF)/F0  100%) in the presence of Fe3þ (20 mM) was unaffected by 50-fold excesses of the interfering metal ions (1000 mM), suggesting that the interference of the coexisted metal ions was negligible. The results revealed that the SCQDs can provide credible anti-interference ability, demonstrating the high specificity for Fe3þ over the other metal ions. 3.3. Detection of Fe3þ and Fe2þ with S-CQDs In this work, the fluorescence intensity of S-CQDs was significantly decreased with the addition of Fe3þ ions, while Fe2þ ions displayed negligible effects on their fluorescence intensities, which indicated that the S-CQDs gave fast, sensitive and selective response to Fe3þ ions and therefore could be used for the detection of Fe3þ ions. In addition, Fe2þ ions could be oxidized to Fe3þ ions in presence of H2O2, and thus could be indirectly detected by a similar CQDs-Fenton hybrid system [22]. Fortunately, the fluorescence intensity of the S-CQDs retained unchanged with increase of the added Fe2þ concentration from 0 to 200 mM (Fig. S5) or H2O2

Table 3 Recovery results of Fe2þ and Fe3þ ions in oral ferrous gluconate liquids (n ¼ 3)*. Sample

Fe2þ ions

Fe3þ ions

Spiked (mM)

Found (mM)

Recovery(%)

RSD (%)

Spiked (mM)

Found (mM)

Recovery(%)

RSD (%)

1 2 3 4

0 6.5 13.0 20.0

15.530 22.871 29.427 37.332

e 112.4 106.9 109.0

0.98 1.1 2.4 1.7

0 0.50 1.0 10.0

0 0.543 0.982 10.173

0 108.6 98.2 101.7

0 3.6 2.0 2.5

Note: The oral ferrous gluconate samples were diluted 200 fold for Fe2þ and 10-fold for Fe3þ with acidified water (pH 3.0) before the detection. The standard Fe3þ ions were from the oxidation of standard ferrous gluconate with H2O2 at room temperature.

F. Du et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 226 (2020) 117602

concentration from 0 to 1500 mM (Fig. S6). However, the fluorescence intensity of S-CQDs was decreased with the increase of Fe2þ in the presence of H2O2 (concentration at 500 mM), attributing to the generated Fe3þ ions from the Fenton reaction between Fe2þ and H2O2. In this work, a high concentration of H2O2 at 500 mM was chosen, taking the complete oxidation of Fe2þ ions and unnoticeable change of the fluorescence intensity of S-CQDs into account. The above results suggested that Fe2þand Fe3þ ions could be discriminated and determined based on their fluorescence quenching of S-CQDs with and without the addition of H2O2. The fluorescence quenching of the S-CQDs at 450 nm was closely related to the concentration of Fe3þ ions (and Fe2þ ions with H2O2), and a relationship between fluorescence quenching of SCQDs and Fe3þ ions was presented in Fig. 4. The fluorescence intensity of S-CQDs decreased gradually as the concentration of Fe3þ ions (see Fig. 4), and a good linear correlation (y ¼ 8.744xþ30.265) over the range of 0e200 mM with a correlation coefficient (R2) of 0.9953 was achieved (see the inset Table of Fig. 4). The limit of detection (LOD) was calculated to be 0.05 mM (3s/S, where s is represents standard deviation of the blank S-CQDs and S is the slope of the calibration curve, n ¼ 10). The relative standard deviation (RSD) for five replicate determinations was 2.1% for 50.0 mM Fe3þ ions. Compared with the other fluorescent probes such as CQDs [19,20], ChPA [21], and GSH-doped GQDs [22] shown in Table 1, the proposed S-CQDs probes exhibited comparable and/or better performance for detection of Fe3þ and Fe2þ ions in terms of linear range, LODs, and the corresponding recoveries, demonstrating it was a promising probes for the detection of Fe3þ and Fe2þ by combining with Fenton reaction.

3.4. Analytical applications The practical feasibility of our proposed S-CQDs-based fluorescence probes for the detection of Fe3þ and Fe2þ in real iron supplement oral liquids has been investigated. The sample selected was oral ferrous gluconate liquids that are commercially available to the public in China. The results were summarised in Tables 2 and 3. As shown in Table 2, no Fe3þ ions were detected in the selected oral ferrous gluconate liquids, however, the Fe3þ ions were detected in the same oral liquids after exposure to air for a three-day, attributing to the oxidation of Fe2þ to Fe3þ by air. Compared to the ferrous gluconate amount labelled on their corresponding bottles, the content of Fe2þ determined by our proposed method was not obvious difference with the standard deviations lower than 7%. Table 3 shows that the recoveries of Fe2þ and Fe3þ ions at three different concentration levels were lower than 113% with RSDs lower than 4%, which indicated that the proposed method is reliable and can be applied for determination of Fe2þ and Fe3þ ions in real samples.

4. Conclusion In this study, we have developed a simple one-step hydrothermal method for the preparation of S-CQDs from vitamin C and mercapto glycolic acid for the first. The as-prepared S-CQDs were used as a novel fluorescence probe for the highly sensitive and selective detection of Fe2þ and Fe3þ ions by combining with Fenton reaction. Under the optimal conditions, good linearity was achieved in the range of 0-200 mM with the correlation coefficient of 0.9953, low LOD of 0.050 mM was obtained. Furthermore, the feasibility of the proposed method has been proven by the determination of Fe2þ and Fe3þ ions in real oral iron supplement samples with satisfactory results.

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Acknowledgements This work was jointly supported by the National Natural Science Foundation of China (21964006, 21465008 and 21665006), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (2018GXNSFAA138134 and 2015GXNSFAA139024), and the Guangxi Colleges and Universities Key Laboratory of Food Safety and Detection. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.saa.2019.117602. References [1] M.U. Muckenthaler, S. Rivella, M.W. Hentze, B. Galy, A red carpet for iron metabolism, Cell 168 (2016) 344e361. [2] S.J. Dixon, B.R. Stockwell, The role of iron and reactive oxygen species in cell death, Nat. Chem. Biol. 10 (2014) 9e17. [3] D.H. Manz, N.L. Blanchette, B.T. Paul, F.M. Torti, S.V. Torti, Iron and cancer: recent insights, Ann. N. Y. Acad. Sci. 1368 (2016) 149e161. [4] A.R. Bogdan, M. Miyazawa, K. Hashimoto, Y. Tsuji, Regulators of iron homeostasis: new players in metabolism, cell death, and disease, Trends Biochem. Sci. 41 (2016) 274e286. [5] C. Legendre, E. Garcion, Iron metabolism: a double-edged sword in the resistance of glioblastoma to therapies, Trends Endocrin. Met. 26 (2015) 322e331. [6] M.E. Conrad, J.N. Umbreit, Iron absorption and transportdan update, Am. J. Hematol. 64 (2000) 287e298. [7] A. Lopez, P. Cacoub, I.C. Macdougall, L. Peyrin-Biroulet, Iron deficiency anaemia, Lancet 387 (2016) 907e916. [8] M. Auerbach, J.W. Adamson, How we diagnose and treat iron deficiency anemia, Am. J. Hematol. 91 (2016) 31e38. [9] C. Camaschella, New insights into iron deficiency and iron deficiency anemia, Blood Rev. 31 (2017) 225e233.  [10] R. Ajlec, J. Stupar, Determination of iron species in wine by ion-exchange chromatography-flame atomic absorption spectrometry, Analyst 114 (1989) 137e142. [11] L.S. Huang, K.C. Lin, Detection of iron species using inductively coupled plasma mass spectrometry under cold plasma temperature conditions, Spectrochim. Acta, Part B 56 (2001) 123e128. [12] Z. Chen, R. Naidu, On-column complexation capillary electrophoretic separation of Fe2þ and Fe3þ using 2,6-pyridinedicarboxylic acid coupled with large-volume sample stacking, J. Chromatogr. A 1023 (2004) 151e157. [13] Y. Chen, Y. Huang, S. Feng, D. Yuan, Solid phase extraction coupled with a liquid waveguide capillary cell for simultaneous redox speciation analysis of dissolved iron in estuarine and coastal waters, Anal. Methods 7 (2015) 4971e4978. [14] Z. Yan, Y. Zhu, J. Xu, C. Wang, Y. Zheng, P. Li, L. Hu, J. You, A novel polydentate Schiff-base derivative developed for multi-wavelength colorimetric differentiation of trace Fe2þ from Fe3þ, Anal. Methods 44 (2017) 6240e6245. [15] G.R. You, G.J. Park, S.A. Lee, K.Y. Ryu, C. Kim, Chelate-type Schiff base acting as a colorimetric sensor for iron in aqueous solution, Sens. Actuators, B 215 (2015) 188e195. [16] P. Wu, Y. Li, X.P. Yan, CdTe quantum dots (QDs) based kinetic discrimination of Fe2þ and Fe3þ, and CdTe QDs-Fenton hybrid system for sensitive photoluminescent detection of Fe2þ, Anal. Chem. 81 (2009) 6252e6257. [17] S. Lee, H. Jang, J. Lee, S.H. Jeon, Y. Sohn, C.S. Ra, Selective and sensitive morpholine-type rhodamine B-based colorimetric and fluorescent chemosensor for Fe(III) and Fe(II), Sens. Actuators, B 248 (2017) 646e656. [18] Y. Yu, X. Cheng, H. Liu, S. Gu, Z. Jiang, H. Huang, J. Lian, Highly sensitive fluorescent polyamide for detection of Hg2þ, Hgþ, Fe3þ, and Fe2þ ions, J. Polym. Sci. Pol. Chem. 53 (2015) 615e621. [19] M.R. Hormozi-Nezhad, M. Taghipour, Quick speciation of iron(II) and iron(III) in natural samples using a selective fluorescent carbon dot based probe, Anal. Methods 8 (2016) 4064e4068. [20] A. Iqbal, Y. Tian, X. Wang, D. Gong, Y. Guo, K. Iqbal, Z. Wang, W. Liu, W. Qin, Carbon dots prepared by solid state method via citric acid and 1,10phenanthroline for selective and sensing detection of Fe2þ and Fe3þ, Sens. Actuators, B 237 (2016) 408e415. [21] S. Maity, N. Parshi, C. Prodhan, K. Chaudhuri, J. Ganguly, Characterization of a fluorescent hydrogel synthesized using chitosan, polyvinyl alcohol and 9anthraldehyde for the selective detection and discrimination of trace Fe3þ and Fe2þ in water for live-cell imaging, Carbohyd. Polym. 193 (2018) 119e128. [22] K. Saenwong, P. Nuengmatcha, P. Sricharoen, N. Limchoowong, S. Chanthai, GSH-doped GQDs using citric acid rich-lime oil extract for highly selective and sensitive determination and discrimination of Fe3þ and Fe2þ in the presence of H2O2 by a fluorescence “turnoff” sensor, RSC Adv. 8 (2018) 10148e10157. [23] X.Y. Xu, R. Ray, Y.L. Gu, H.J. Ploehn, L. Gearheart, K. Raker, W.A. Scrivens,

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