An “off-on” fluorescent sensor for copper ion using graphene quantum dots based on oxidation of l -cysteine

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 214 (2019) 320–325

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An “off-on” fluorescent sensor for copper ion using graphene quantum dots based on oxidation of L-cysteine Longhua Ding a,⁎, Zhongyao Zhao b, Dongjun Li b, Xue Wang b, Jialin Chen b a b

Institute for Advanced Interdisciplinary Research, University of Jinan, Jinan 250022, PR China Department of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, PR China

a r t i c l e

i n f o

Article history: Received 9 September 2018 Received in revised form 25 November 2018 Accepted 16 February 2019 Available online 18 February 2019 Keywords: Copper ion Grapheme quantum dots Catalytic oxidation Spectral overlap Fluorescent analysis

a b s t r a c t A simple and highly efficient “off-on” fluorescent sensor based on grapheme quantum dots (GQDs) for Cu2+ was developed. In this sensing platform, the fluorescence of GQDs was quenched in the presence of 2,4dinitrophenylcysteine (DNPC), which is the reaction product of 1-chloro-2,4-dinitrobenzene (CDNB) and L-cysteine, owing to the spectral overlap between the absorption of DNPC and the excitation of GQDs. In the presence of Cu2+, L-cysteine was catalytically oxidized to L-cystine by O2, resulting in the reduction of DNPC. Thus, the fluorescence of GQDs was recovery. Based on this, the fluorescent detection of Cu2+ could be achieved. The proposed sensing strategy offered a selective identification of Cu2+ with a detection limit of 4.5 nM. Additionally, the practical application of this assay for Cu2+ determination in real water samples was also demonstrated. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Copper is an essential trace element to living organisms and plays an important role in most pathological and physiological processes [1]. It severs as a vital cofactor for a number of enzymes [2], including zinc‑copper superoxide dismutase, lysyl oxidase, cytochrome c oxidase, tyrosinase and so on. Excessive intake of Cu2+ disturbs the cellular homeostasis and is highly toxic to human health, which can cause some serious diseases, such as prion, Wilson's, Parkinson's and Alzheimer [3–6]. However, along with the wide use of Cu2+ in agriculture and industry, the contamination of Cu2+ to environment is becoming serious, which may induce the excessive Cu2+ in the body due to the enrichment of Cu2+ in organism. Therefore, it is highly important to develop a simple and efficient method to detect Cu2+. In recent years, many methods have been developed for Cu2+ detection, including atomic absorption/emission spectroscopy (AAS/AES), inductively coupled plasma mass spectroscopy (ICP-MS), surfaceenhanced Raman scattering, electrochemistry, colorimetry, fluorimetric detection and so on [7–14]. Among them, fluorescent technique attracts much attention due to its simplicity and high sensitivity. In the last few decades, a number of fluorescent probes have been designed for Cu2+ detection [15], such as organic fluorescent dye [16,17], metal nanoclusters [18,19], semiconductor quantum dots [20,21], metal-organic frameworks (MOFs) [22,23] etc. However, the ⁎ Corresponding author. E-mail address: [email protected] (L. Ding).

https://doi.org/10.1016/j.saa.2019.02.048 1386-1425/© 2019 Elsevier B.V. All rights reserved.

fluorescent probes mentioned above suffer from some drawbacks, such as complex synthetic steps, poor water solubility, toxicity of metal based solid probes, photobleaching of organic dye, which limit their practical applications [24]. Thus, exploring novel fluorescent nanomaterials with high fluorescent, good photo-stability and low toxicity are urgently needed. Grapheme quantum dots (GQDs) as a new member of the grapheme family, exhibit unique optical and electronic properties, which make them show promising prospects in a wide range of fields, such as sensors, bioimaging, optoelectronic and so on [25]. Due to their considerable photostability, hypotoxicity, good biocompatibility and solubility, GQDs have been considered as an excellent candidate of fluorescent sensor materials. Up to now, GQDs have been used as fluorescent probe for various analytes, such as free chlorine [26], metal ions [27–32], organic molecules [33–35], various biological molecules [36–40] and nuclei acids [41,42]. However, most of the reported Cu2+ sensors based on GQDs involve a complex process, post-modification or “turn-off” mode [43–45]. Therefore, it is challenging to develop a simple and efficient method for Cu2+ detection with “turn-on” pattern. Herein, we developed a GQDs-based analysis platform for effective and selective detection of Cu2+. Scheme 1 illustrates the proposed mechanism for Cu2+ sensing. The fluorescence intensity of GQDs can be quenched by 2,4-dinitrophenylcysteine (DNPC) via inner filter effect (IFE) resulting from the spectral overlap between the absorption bands of DNPC and excitation bands of GQDs. Upon addition of Cu2+, Lcysteine is oxidized to L-cystine, which reduces the production of DNPC, resulting in the fluorescence recovery of GQDs. Therefore, GODs

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Scheme 1. Illustration the sensing principle for Cu2+ detection based on the fluorescent GQDs. (a) Generation of DNPC and catalytic oxidation of L-cysteine; (b) Fluorescence quenching of GQDs.

could behave as an “off-on” fluorescent sensor for Cu2+ determination. The proposed assay was also applied successfully for Cu2+ detection in real water samples. 2. Experimental sections

beaker and heated to 200 °C until CA changed to the pale yellow liquid. Then 100 mL NaOH (10 mg mL−1) solution was added drop by drop under stirring and the pH of the solution was adjusted to neutral (pH 7.0). The as prepared GQDs were stored at 4 °C for further use.

2.1. Reagents and chemicals 2.4. Reaction between CDNB and L-cysteine All chemicals and reagents were analytical grade or higher and used as received without further purification. NaCl, KCl, CaCl 2 , ZnCl 2 , CuSO 4 ·5H 2 O, FeCl 3 , Cd(NO 3 ) 2 , Ni(NO 3 ) 2 , Citric acid (CA) and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). L -Cysteine and 1chloro-2,4-dinitrobenzene (CDNB) were bought from Aladdin. 4(2-hydroxyethyl)piperazine- L -ethanesulfonic acid (HEPES) was purchased from Sigma. The pH of 0.1 M HEPES buffer was adjusted to 7.0 by NaOH. 2.2. Instruments Ultraviolet-visible (UV–vis) absorption and fluorescence spectra were conducted on a UV-2550 spectrophotometer (Shimadzu, Japan) and LS-55 spectrophotofluorometer (P. E. USA), respectively. The time-resolved photoluminescence decay spectra were conducted on a Fluomax up-conversion fluorimeter (IB Photonics), as described in previously report [46]. Fourier transform infrared (FT-IR) spectrum was collected on a Fourier transform infrared spectrum RX (PekinElmer Spectrometer). Transmission electron microscopy (TEM) was carried out on JEOL 4000 EX microscopy. 2.3. Synthesis of GQDs GQDs were synthesized by directly pyrolyzing CA according to the previously report [47]. Typically, 2.0 g CA was added into a

To measure the absorbance of the reaction product between CDNB and L-cysteine, the same concentration of CDNB and L-cysteine (0.2 mM) were added into 3.0 mL HEPES buffer solution and incubated at 90 °C for different period.

2.5. Florescence assay of Cu2+ All the fluorescent measurements were all performed in HEPES buffer solution. Firstly, different concentrations of Cu2+ were added into 0.2 mM L-cysteine at 90 °C for different time; then, 0.2 mM CDNB was added into the above solution and incubated at 90 °C for another 50 min; Finally, 0.5 mL GQDs were added into the mixture to measure the fluorescence of GQDs. The final volume of the solution was 3.0 mL. Control experiment was conducted as the same procedure without the addition of Cu2+.

2.6. Selectivity measurement of Cu2+ To investigate the selectivity of Cu2+ of the developed method, several metal ions were chosen as interfering substances. All the concentration of the metal ions (Na+, K+, Ca2+, Zn2+, Fe3+, Cd2+ and Ni2+) were 1.0 μM. The detection procedure and conditions were as same as mentioned above.

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Fig. 1. (a) TEM image (Inset: size distribution of GQDs) and (b) FT-IR spectrum of GQDs.

3. Results and discussion 3.1. Characterization of GQDs The morphology and composition of GQDs prepared by CA pyrolyzation were characterized. Fig. 1a shows the TME image and size distribution of GQDs. The results shown that the obtained GQDs are spherical with good monodispersity and their diameter is in the range of 3–5 nm. The surface functional groups of GQDs were investigated by FT-IR spectroscopy, as shown in Fig. 1b. The peak at 3420 cm−1 is assigned to the stretching vibrations of O\\H. The absorption at around 2928 and 2856 cm−1 are corresponded to the vibration of C\\H. And the absorption signal at 1600 and 1390 cm−1 are assigned to the stretching vibrations of –COO [42]. All these results indicate that there are hydroxyl and carboxyl groups on the surface of GQDs.

To reveal the spectroscopic properties of GQDs, UV–vis absorption and fluorescence emission spectra of GQDs were studied. As displayed in Fig. 2a, GQDs present only a broad absorption peak centered at 357 nm. Under excitation wavelengths from 300 to 400 nm, there was one emission band at a wavelength of 462 nm (Fig. 2b). And the most intense peak of 462 nm was obtained under an excitation wavelength at 370 nm. Hence, 370 nm was chosen as the optimum excitation wavelength in the following experiments. 3.2. Sensing principle for the detection of Cu2+ It had been reported that CDNB can react with L-cysteine through nucleophilic aromatic substitution reaction to produce 1-chloro-2,4-dinitrobenzene (DNPC) at nearly neutral pH [48,49]. CDNB itself is colorless (Fig. S1a) and only exhibits absorption at wavelength shorter than 300 nm (Fig. 3A-a). While, DNPC which is the reaction product of

Fig. 2. (a) UV–vis absorption spectrum of GQDs. (b) The emission spectra of GQDs under different excitation wavelengths ranging from 300 nm to 400 nm.

Fig. 3. (A) UV–vis absorption spectra of (a) CDNB; (b) mixture of CDNB, L-cysteine and DNPC; (c) excitation spectrum of GQDs. (B) Normalized fluorescence intensity of GQDs after adding some other relative substances.

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oxidation of L-cysteine. That is to say, Cu2+ can inhibit the fluorescence quenching of GQDs, producing a “turn-on” signal, which has been confirmed by the fluorescent measurement (Fig. 3B). Based on this, the concentration of Cu2+ can be quantitatively detected. 3.3. Fluorescent detection toward Cu2+

Fig. 4. Time-resolved decay of GQDs in the absence (a) and presence of DNPC (b).

CDNB and L-cysteine is yellow (Fig. S1b) and has an absorption at 355 nm (Fig. 3A-b). And the absorption of DNPC is highly overlapped with the excitation of GQDs (Fig. 3A-b and A-c). Hence, DNPC can absorb the exciting light of GQDs, leading to the fluorescence quenching of GQDs. And this conclusion was verified by Fig. 3B. It is found that both CDNB and L-cysteine have no influence on the fluorescence of GQDs (Fig. 3B). In contrast, the reaction product of CDNB and L-cysteine can obviously quench the fluorescence of GQDs (Fig. 3B). It is well know that several mechanisms can result in fluorescence quenching of fluorophores, such as fluorescence resonance energy transfer (FRET), electronic energy transfer (EFT), intramolecular charge transfer (ICT), inner filter effect (IFE) and so on [50–56]. Both in the FRET an IFE process, there need a spectral overlap between the absorption spectrum of a quencher and the excitation/emission spectrum of a fluorophore [57]. In order to ensure the main reason for fluorescence quenching of GQDs, the lifetime of GQDs in the absence and presence of DNPC was measured. As displayed in Fig. 4, the lifetime of GQDs with or without DNPC were fitted to be 1.69 ns and 1.68 ns, respectively. The fluorescence lifetime of GQDs almost remains unchanged after addition of DNPC, which indicates that the fluorescence reduction of GQDs is result from IFE more than FRET process. As we known, L-cysteine can be catalytically oxidized to L-cystine by O2 in the presence of Cu2+ [48,58,59]. Therefore, the reaction between L-cysteine and CDNB can be dramatically inhibited owing to the catalytical

To obtain the ideal analytical performance, the experimental conditions were optimized. All the experiments were conducted within neutral buffer solution. Firstly, the effects of temperature and time on the oxidation of L-cysteine were investigated. As displayed in Fig. S2a, the catalytic efficiency of Cu2+ increased with increasing temperature and kept level off at 90 °C. As a result, all the following experiments were carried out at 90 °C. Fig. S2b shows that L-cysteine can be oxidized by O2 within 30 min at the greatest extent. Following the oxidation of Lcysteine, CDNB was added and continually incubated for different time points. It can be seen form Fig. S2c, the reaction of L-cysteine and CDNB can be completed within 50 min. Thus, 30 min and 50 min were chosen as the incubation time of L-cysteine oxidation and reaction with CDNB, respectively. To evaluate the sensitivity of the present sensing platform, different concentrations of Cu2+ were added into L-cysteine solution and the fluorescence changes of GQDs were measured. Fig. 5 shows the dependence of normalized fluorescent intensity of GQDs on the concentration of Cu2+. It can be seen that the fluorescent intensity at 462 nm increased with the addition of Cu2+ (Fig. 5a). And the fluorescence increased more rapidly with the concentration of Cu2+ below 10 μM, while the florescence increased slowly with continuously increasing Cu2+ concentration (Fig. 5b). In addition, there is a good linear relationship between the quenching efficiency (F/F0) and the concentration of Cu2+ in the range from 0.01 to 10 μM (inset in Fig. 5b) via a linear equation of F/F0 = 0.064 × c[Cu2+] + 0.24 (R = 0.997). The limit of detection of this assay for Cu2+ is estimated to be 4.5 nM based on the 3σ calculation. The obtained experimental results were compared with some previously reported fluorescent methods. From Table S1, it can be found that our proposed method exhibits a wider concentration linear range with low detection limit. Although the limit of detection of our proposed method is not the lowest, it is much lower than the highest concentration of Cu2+ (~20 μM) permitted by the US Environmental Protection Agency (EPA) [60] and can satisfy the practical application. 3.4. Selectivity for Cu2+ detection In order to evaluate the selectivity of this sensor for Cu2+ analysis, fluorescence changes of GQDs in the presence of other metal ions (Na+, K+, Ca2+, Zn2+, Fe3+, Cd2+ and Ni2+) were investigated. All the

Fig. 5. (a) Normalized fluorescence intensity of GQDs with addition of various concentration of Cu2+ (from bottom to top: 0.01, 0.1, 0.5, 1.0, 3.0, 5.0, 7.0, 10.0, 50.0 and 100.0 μM). (b) The dependence of F/F0 on the concentration of Cu2+ within the range of 0.01–100 μM. Inset shows the linear relationship between F/F0 and concentration of Cu2+. F0 and F are the fluorescence intensities of GQDs in the absence and presence of Cu2+, respectively.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.02.048. References

Fig. 6. Normalized fluorescence responses of GQDs in the absence (blank) and presence various metal ions.

experiments were conducted under the optimal conditions and the concentration of all metal ions is 1.0 μM. As shown in Fig. 6, no obvious fluorescence increases were observed in the presence of other metal ions other than Cu2+, indicating a high specificity for Cu2+. 3.5. Real sample analysis To evaluate the feasibility of this method in real sample, it was applied for the detection Cu2+ in tap water. All the samples were diluted with 0.1 M HEPES buffer and detected under the optimal condition. As shown in Table 1, high recoveries were obtained for all real samples, indicating that this assay is potential for practical analysis of Cu2+. 4. Conclusions In conclusion, the one-pot synthesized GQDs were found to be excellent fluorescent probes for Cu2+ determination. The fluorescence of the as prepared GQDs can be intensively quenched by DNPC due to the IFE. The absorption of DNPC can be reduced due to Cu2+ catalytic oxidation of L-cysteine, resulting in the fluorescent recovery of GQDs. Under the optimal experimental conditions, this proposed sensing method can detect Cu2+ in the range of 0.01–10 μM with a detection of 4.5 nM. Furthermore, it has been demonstrated to have promising practical application in the detection of Cu2+ in real water samples. Conflict of interest There is no conflict of interest. Acknowledgments This work is financially supported the National Natural Science Foundation of China (51802118) and the Natural Science Foundation of Shandong Province (2018GSF118166, ZR2016BB29).

Table 1 Recovery of Cu2+ in real samples (n = 3). Sample

Added (μM)

Found (μM)

Recovery (%)

1 2 3

0.02 0.7 5

0.019 0.68 4.9

95.0 97.1 98.0

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