Determination of thiourea based on the reversion of fluorescence quenching of nitrogen doped carbon dots by Hg2+

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117666

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Determination of thiourea based on the reversion of fluorescence quenching of nitrogen doped carbon dots by Hg2þ Cengceng Zhang, Shu Wu, Yuanyuan Yu, Fang Chen* Hubei Key Laboratory of Bioinorganic Chemistry & Materia Medica, Key laboratory of Material Chemistry for Energy Conversion and Storage (HUST), Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 430074, Wuhan, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 July 2019 Received in revised form 15 October 2019 Accepted 15 October 2019 Available online 19 October 2019

Herein, a facile and quick strategy to detect thiourea was conducted based on the reversion of fluorescence quenching of nitrogen doped carbon dots (NCDs) by Hg2þ. The NCDs with good water solubility and 17% of quantum yield was synthesized by one-step hydrothermal method, using ammonium citrate and dextrin as carbon source and nitrogen source, respectively. The fluorescence of NCDs was obviously quenched by Hg2þ and can be recovered, due to stronger interaction between thiourea and Hg2þ. There was a good linear relationship between the recovered fluorescence and the concentration of thiourea within range of 0.90e10.0 mM and the detection limit for thiourea detection was 0.15 mM. The asprepared NCDs can be used for determination of thiourea in tap water, lake water and rice flour products, and the spike recoveries were between 91.6 and 108%. © 2019 Elsevier B.V. All rights reserved.

Keywords: Nitrogen doped carbon dots Thiourea Hg2þ Determination Fluorescence quenching Fluorescence recovering

1. Introduction Thiourea (TU), as one kind of organic sulfide, is widely applied for food which need bleaching and preservation, such as rice flour and its products. Thiourea can also be used as ripening agent, fungicide and fertilizer for fruits [1,2]. Despite its important role, the hazard of thiourea is inevitable. When thiourea is ingested in vivo, it will inhibit the function of thyroid and hematopoietic organs, cause central nervous paralysis, respiratory and cardiac function decline, damage the skin, and even cause death [3]. For these purpose, thiourea is considered to be carcinogenic to humans by the US Environmental Protection Agency and the International Agency for Research on Cancer [4]. Thus, developing a simple, economical and highly selective method to detect thiourea is of great importance. Traditional methods for determination of thiourea generally include UVevis spectrophotometry [5,6], Infrared spectroscopy [7], electrochemical method [8e11], flow injection chemiluminescence method [12], high performance liquid chromatography [13] and fluorescence method [1,14]. For example, a colorimetry method based on the color change of AuNPs for detection of thiourea was designed by Cao et.al [5]. The principle was mainly the leaching of

* Corresponding author. E-mail address: [email protected] (F. Chen). https://doi.org/10.1016/j.saa.2019.117666 1386-1425/© 2019 Elsevier B.V. All rights reserved.

thiourea to AuNPs under the catalysis of Fe3þ. Using electrochemical reduction phenomena, Rahman et al. [9] reported a highly sensitive electrochemical sensor based on cobalt oxide co-doped manganese oxide nanoparticles fabricated GCE for the determination of thiourea. However, Fluorometric determination of thiourea is relatively rare. According to the reaction between O-phenanthroline-5,6-dione and thiourea, Wang et al. [1] mixed [Ir (ppy)2(PD)]þ and thiourea, enhancing the fluorescence, thereby enabling rapid detection of thiourea and bioimaging. Meanwhile, other sulfur-containing compounds can also be determined based on the spectral properties of iridium (III) complexes [15e17]. Chen et.al [14] found that the fluorescence of fluorescein could be quenched by the absorption of AuNPs because of the fluorescence resonance energy transfer of fluorescein-AuNPs. The fluorescence of fluorescein was recovered by adding thiourea because the interaction of fluorescein-AuNPs can be inhibited by thiourea. CDs are a kind of carbon material with low toxicity and unique optical property. It has been widely used as a fluorescence probe for quantitative detection of analyte [18e22], bio-imaging [23e25] and drug delivery [26,27], especially metal ions [20,28e30], biomolecules [22,31e33] and some drugs [21,34]. Also, CDs can be applied as emitting species for Chemiluminescence to detect thiourea. Liu et.al [12] found that weak KMnO4-NCDs system was strongly increased by thiourea and had highly selectivity for thiourea. However, few reports have been published for detecting TU by fluorophotometry based on carbon quantum dots.

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In this work, a quick, highly selective approach for determination of thiourea based on the reversion of fluorescence quenching of NCDs by Hg2þ was established. NCDs, ammonium citrate and dextrin as carbon source and nitrogen source, was prepared by hydrothermal method, QY of which was 17%. Under pH ¼ 6.09, the fluorescence of NCDs can be selectively quenched by Hg2þ, and the fluorescence of NCDs was recovered by adding thiourea. The NCDs can selectively recognized Hg2þ and was used for optional determination of thiourea. The diagram for thiourea detection is shown in Scheme 1. 2. Experimental 2.1. Chemicals and materials Ammonium citrate, dextrin, acetic acid, phosphoric acid, boric acid, sodium hydroxide, thiourea were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Rice flour was supplied from local market. Tap water was from our laboratory. Deionized water was used throughout. 2.2. Apparatus LS-55 spectrofluorometer (PerkinElmer, USA), Cary 60 UVevis spectrophotometer (Agilent Technologies, Palo Alto, CA, USA), High resolution transmission electron microscope (FEI, USA), Fourier transform infrared spectroscopy (FTIR) spectra (Bruker, Germany), X-ray Photoelectron Spectroscopy (ThermoFischer, USA) 2.3. Synthesis of NCDs According to relevant literature [35] and corresponding modification, NCDs was synthesized using one-step hydrothermal method. Briefly, 0.7789 g ammonium citrate and 0.4024 g dextrin were dissolved into 25 mL H2O, mixed homogeneously. Then the above solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 165  C for 5.5 h. The obtained solution was flited with a 0.22 mm filter to remove large aggregates. Finally, the NCDs solution was diluted 100 times for following experiments. Effect of the synthesis conditions were shown in Fig.S1. 2.4. Fluorescence sensing of thiourea The NCDs was mixed with certain volume of Hg2þ solution in BR buffer solution, causing the fluorescence of NCDs quenched. Next, with the addition of different concentration of thiourea, the fluorescence was recovered. The detected range of concentration of

thiourea was from 0.90 to 10.0 mM. The fluorescence measurement conditions were 340 nm of excitation wavelength and 440 nm of emission wavelength. After the above experiment, we can choose Na2S which binds strongly with Hg2þ to remove Hg2þ [36,37]. 2.5. Thiourea detection in real samples 1.5088 g rice flour was put in 50 mL deionized water, heated and boiled for 20 min. After cooling to room temperature naturally, the suspension was flited with a 0.22 mm of membrane. The tap water and lake water were directly flited to remove large particles. In 7.5 mL BR buffer solution, 500 mL NCDs solution and 1 mL Hg2þ were added, mixed to stable fluorescence intensity. 1 mL sample solution was added in the above solution for FL detection. Each sample was measured in parallel three times. 3. Results and discussion 3.1. Morphology and spectral characterization of NCDs The morphology and spectral properties of NCDs were observed by HRTEM, UVevis spectroscopy and fluorescence spectrum and were shown in Fig. 1. The diameter of as-prepared NCDs is about 1 nm, spherical or spheroidal morphology (Fig. 1A). It can be seen that UVevis spectroscopy show two strong UV absorption peaks at 240 nm and 340 nm (Fig. 1B), which might be caused by p-p* and n-p* transition. FT-IR spectrum presented eOH, eNH2, eCONH of the surface functional groups in Fig. 1C. Meanwhile, the optical property of NCDs was studied and the result was shown in Fig. 1D. The FL intensity of NCDs was increased gradually and then decreased with the increasing of excitation wavelength, and the strongest emission occurred while excitation wavelength was 340 nm. Additionally, the as-prepared NCDs had excitation-dependent emission because the emission position red shifted with excitation wavelength increasing. The constituted elements of NCDs were studied by XPS, as shown in Fig.S2. The results indicated that there were three typical peaks, C1s, N1 s and O1s. The XPS spectra of C1s, N1s, and O1s proved the existence of CeC/C¼C, CeN, C¼O, CeNeC, NeC3, NeH, and CeOH bonds. 3.2. The influence of metal ions on fluorescence of NCDs The selectivity and interference ability for fluorescence quenching of NCDs by different metal ions was investigated. As shown in Fig. S3A and S3B, the FL intensity of NCDs was obviously quenched by Hg2þ, and besides slightly quenched by Fe3þ, other metal ions had rarely influence. Thus, the selectivity and anti-interference ability of NCDs was good relatively, which can further study. 3.3. Spectral properties of NCDs, Hg2þ, thiourea

Scheme 1. The schematic illustration for thiourea determination by NCDs.

Fig. 2A and B represented the UVeVis spectra and fluorescence spectra of NCDs-Hg2þ with and without thiourea. As shown in Fig. 2A, There was a characteristic absorption peak at 350 nm of NCDs solution (curve a). But with the addition of Hg2þ, the absorption peak at 350 nm red shifted and characteristic absorption peak of Hg2þ appeared (curve d). Meanwhile, there were no characteristic absorption peaks of thiourea between 300 nm and 500 nm (curve c). Continuing to add thiourea, characteristic absorption peak of Hg2þ at 300 nm disappeared while the absorption peak of NCDs at 350 nm had hardly changed (curve e). It can be seen that the interaction between Hg2þ and thiourea was stronger than that of NCDs and Hg2þ. Simultaneously, the annihilated fluorescence of NCDs by Hg2þ can be almost restored completely with the addition of thiourea, as

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Fig. 1. (A)HRTEM image of NCDs; (B) UVevis absorption spectra of NCDs; (C) IR spectra of NCDs; (D) Fluorescence excitation and emission spectra of NCDs, the maximum excitation/ emission is at 340/448 nm.

Fig. 2. (A) Absorption spectra of the (a) NCDs solution, (b) Hg2þ, (c) thiourea, (d) NCDs þ Hg2þ, (e) NCDs þ Hg2þ þthiourea; (B) fluorescence spectra of (a) NCDs solution, (b) NCDs þ Hg2þ, (c) NCDs þ Hg2þ þthiourea (the concentration of NCDs was 1.56 ppm, Hg2þ: 4 mM, the concentration of was thiourea was 10 mM).

depicted in Fig. 2B, which was consistent with the above phenomenon of UVeVis spectra. 3.4. Optimization of experimental conditions for thiourea detection 3.4.1. Influence of pH As indicated in Fig. 3, the FL intensity of NCDs with and without Hg2þ were influenced by various pH values. In acid medium, NCDs

had weak fluorescence. As pH was increased, the FL intensity of NCDs was gradually increased and tended to be stable between 6 and 10 (curve a). With pH varied from 2 to 6, the quenched degree of NCDs by Hg2þ was constantly enhanced. The quenched degree was decreased while pH proceeded to increase. This might be due to the fact that under alkaline medium, Hg2þ was bound to OH more strongly than interaction with NCDs. As a result, BR buffer solution of pH 6.09 was chosen to the optimist reaction medium.

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3.4.3. Effect of concentration of Hg2þ In order to more sensitive detection of thiourea, different concentration of Hg2þ was optimized (Fig. 4A and B). From Fig. 4A, the fluorescence of NCDs decreased by increasing the Hg2þ concentration and was stable basically up to 4.00 mM. So, Hg2þ concentration at 4.00 mM was selected for further research. The quenched mechanism of Hg2þ towards the FL signal of carbon dots was discussed. As shown in Table S1, the quenched effect of fluorescence of carbon dots was decreased with temperature increased. According to UVevis spectral properties of NCDs and Hg2þ, new compound between NCDs and Hg2þ was formed. The new compound was separated partly after the temperature increased to 323K, causing that the quenched effect was decreased. The results demonstrated that the quenched mechanism of Hg2þ towards the FL signal of carbon dots was a static quenching process. Fig. 3. Effect of pH on the FL intensity of NCDs (a) and NCDs-Hg2þ (b).

3.4.2. Effect of concentration of NCDs Quenching effect of different concentration of NCDs by Hg2þ was observed because the NCDs concentration can affect the FL intensity, as seen in Fig. S4. It found that the quenching effect of NCDs was better when the concentration of NCDs was quite low. The FL intensity of NCDs at a concentration of 1.56 mg mL1 was quenched nearly 90% by Hg2þ. So, 1.56 mg mL1 was chosen to the best NCDs concentration.

3.5. Analytical performance for detecting thiourea Under the optimized conditions, the fluorescence of NCDs quenched Hg2þ can be restored gradually with the increasing concentration of thiourea (Fig. 5). The recovered fluorescence of NCDs (F/F0) was proportional to the thiourea concentration in range of 0.90e10.0 mM. The linear equation was F/ F0 ¼ 0.915cþ0.175 (c: 106 mol L1, R2 ¼ 0.9997). The calculated limit of detection was 0.15 mM. Moreover, the relative standard deviation (RSD) for detecting thiourea at concentration of 3.00 mM was 2.65%, which was obtained by repeatedly measuring 10 times. The result demonstrated that this analysis method had a good linearity, high precision and simple operation.

Fig. 4. Fluorescence spectra of NCDs in the presence of different concentration of Hg2þ (A) and relation between quenched fluorescence and Hg2þ concentration(B).

Fig. 5. (A) Fluorescence emission spectra of NCDs-Hg2þ with the addition of different concentration of thiourea in BR buffer solution (pH ¼ 6.09); (B) The concentration of thiourea was from 0.9 to 14 mM (image of inset: The linear relationship between F/F0 and the concentration of thiourea (n ¼ 3)).

C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 227 (2020) 117666

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Fig. 6. (A) Fluorescence response of NCDs-Hg2þ system on different biomolecules; (B) Effect of coexisting substances on FL intensity of NCDs in the presence of Hg2þ (the concentration of NCDs was 1.56  106 g mL1, BR buffer solution: pH ¼ 6.09, the concentration of Hg2þ was 4 mM).

Table 1 Determination of thiourea in tap water, lake water and rice flour. Real sample

Detected (mM)

Added (mM)

Founded (mM)

recovery (%)

RSD (%)

Tap water

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

1.00 3.00 6.00 1.00 3.00 6.00 1.00 3.00 6.00

0.956 3.26 5.68 1.03 2.82 5.50 1.07 2.97 6.25

95.6 108 94.7 103 94.0 91.6 107 99.0 104

0.33 0.70 0.47 2.80 1.60 1.10 2.80 2.00 1.30

Lake water Rice flour

citrate and dextrin. An “on-off-on” fluorescence probe for determination of thiourea was established by simply mixing NCDs solution quenched by Hg2þ and thiourea. The fluorescence of NCDs solution quenched by Hg2þ can be recovered after adding thiourea. Also, this probe possessed good selectivity and strong antiinterference. The proposed method had potential application in food detection. Declaration of competing interest The authors declare that they have no conflict of interests. Acknowledgements

3.6. Effect of coexisting substance To research selectivity and anti-interference ability for determination of 10 mM thiourea, some possible coexisting metal ions and bio-molecules were studied in real sample (tap water, lake water and rice flour). As shown in Fig. 6, the results demonstrated that the presence of 100-fold for glutamate, fructose, glucose, sucrose, urea, DNA and cholesterol had no obvious interference for detection of thiourea. In terms of 10-fold for folic acid, vitamin B1, glycine, serine, histidine, bovine serum albumin, Ca2þ, Kþ, Naþ, Cu2þ, Mn2þ, Zn2þ, Mg2þ, Fe3þ, these substances also did not interfere. It can be seen that the analytical method had good selectivity and anti-interference so that it can be conducive to detect thiourea in real samples. 3.7. Thiourea detection of real samples To verified feasibility and applicability of this method, the proposed method was used for detecting thiourea in tap water, lake water and rice flour. As shown in Table 1, the spiked recoveries were 93.0e108% in tap water, 91.6e103% in lake water and 99.0e107.7% in rice flour for three replicates, of which the relative standard deviation were less than 1%. It could be seen that this method had good applicability for tap water, lake water and rice flour. 4. Conclusions In a word, a new fluorescence approach based on the recovery of Hg2þ quenching fluorescence of NCDs is developed for rapid detection of thiourea in actual samples. By one-step hydrothermal method, the NCDs with QY of 17% was prepared using ammonium

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