Making a cup of carbon dots for ratiometric and colorimetric fluorescent detection of Cu2+ ions

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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa

Making a cup of carbon dots for ratiometric and colorimetric fluorescent detection of Cu2+ ions Wenjing Zhanga,1, Ning Lia,1, Qing Changa, Zhenfei Chenb,*, Shengliang Hua,* a b

North University of China, School of Energy and Power Engineering & School of Material Science and Engineering, Taiyuan 030051, PR China Tangshan Environmental Monitoring Centre of Hebei Province, Tangshan 063000, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon dots Fluorescence probe Metal ion detection Tea

Fluorescence carbon dots (CDs) are firstly obtained from microbial fermented Pu-erh tea by a simple brewing method without high temperature treatment as reported in literature. The obtained CDs contain abundant surface functional groups and exhibit a good ability to detect Fe and Cu ions. Moreover, a new sensing universal platform that can simultaneously apply fluorescent and colorimetric dual-readout for Cu2+ detection is constructed using the obtained CDs and o-phenylenediamine (OPD). Not only does this sensing platform exhibit much higher selectivity and sensitivity than pure CDs, but it also enhances detection scope for Cu2+. Its detection mechanism is dominated by synergistic combination of fluorescence quenching resulted from the coordination reaction of Cu2+ ions and fluorescence resonance energy transfer from CDs to the oxidation product of OPD.

1. Introduction Due to multiple health benefits, tea is appreciated and consumed popularly as a beverage in the world. Generally, it can be divided into green tea (non-fermented), oolong tea (semi-fermented), black tea (fully fermented by oxidizing enzyme) and dark tea (post-fermented by

microbe) according to the processing procedures utilized [1]. Among these teas, Pu-erh tea belonging to microbial fermented tea produced from the sun-dried leaves of large-leaf tea species in the Yunnan province of China is the most representative dark tea [2]. The synthesis of fluorescent carbon dots (CDs) by high temperature treatment of tea is well-reported [3–5]. However, there are no reports available to directly

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Corresponding authors. E-mail addresses: [email protected] (Z. Chen), [email protected] (S. Hu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.colsurfa.2019.124233 Received 26 September 2019; Received in revised form 12 November 2019; Accepted 14 November 2019 Available online 14 November 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Wenjing Zhang, et al., Colloids and Surfaces A, https://doi.org/10.1016/j.colsurfa.2019.124233

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measurements were conducted for three times, of which the data were averaged. For ratiometric and colorimetric FL sensing platform, the sensing system was firstly obtained by mixing 500 μL of as-prepared TCDs, 900 μL of Tris-HCl buffer, 50 μL of fresh OPD solution (60 mM) and 50 μL of H2O2 solution (10 mM); subsequently the metal ion solution (1 mM) with calculated volume was added into this system and then added deionized water to 2 mL. After incubated for 1 h at room temperature, the FL spectra were collected for quantitative detection of metal ions similar with the method mentioned above. The detection limit of our used sensing system was obtained by the FL titration. The emission intensity of the sensing system without any Cu and Fe ions was tested by several times to determine the S/N ratio, and then the standard deviation (σ) was determined by the blank measurement. Finally, the detection limit was calculated by the following equation: detection limit = 3σ/m, in which m represents the slope between signal intensity versus metal ion concentration.

search for CDs from various teas. Naturally occurring CDs have been discovered in honey, which is a sweet nutrient made by bees foraging nectars from flowers and traditionally known for its medicinal values [6]. Considering manufacture process of Pu-erh tea [7], the generation of CDs during microbial fermented processes is feasible. Because of their unique optical properties, low cost, multiple functional groups, non-toxicity, chemical and photo-stability, CDs have attracted a large amount of concerns for applications [8–12]. An interesting application of CDs is in the field of chemical sensing. As reported in the literatures [13–17], metal ions like Hg2+, Fe3+, Pb2+, Zn2+ and Cu2+ can usually quench the fluorescence (FL) of CDs through interacting with the functional groups (phenolic hydroxyl, carboxylic acid and/or amine) on the surface. Therefore, CDs with specific surface functionalization have been endowed with molecular recognition ability, and used as the chemical sensors respond to metal ions by decreasing the FL intensity [18–21]. However, such single-intensity-based sensing is usually compromised by some other factors including of drift of light source or detector, the concentration change of sensors, or environmental affects in complex samples. Colorimetric FL sensors can avoid these issues and have been developed recently by combining CDs with other fluorophores [22–25]. For these colorimetric nanosensors, fluorescence resonance energy transfer (FRET) effect plays a leading role in detection mechanism. In this work, the CDs were directly prepared by mixing Pu-erh tea (purchased from the market) with hot water (namely T-CDs) and used for detecting Cu and Fe ions. To improve their sensitivity and selectivity, a ratiometric and colorimetric FL universal platform was developed by mixing T-CDs with o-phenylenediamine (OPD). In this sensing platform, a cooperation combination of FL quenching resulted from the coordination reaction of Cu2+ ions with surface functional groups of T-CDs and FRET from T-CDs to the oxidation product of OPD (oxOPD) was implemented to apply colorimetric and ratiometric FL dual-readout for Cu2+ detection.

3. Results and discussion The dispersive solution of T-CDs displays a yellow color under white light and emits blue light under UV lamp of 365 nm (Inset of Fig. 1a). TEM image reveals that the solution of T-CDs consists of small nanoparticles well separated from each other (Fig. 1a). Two kinds of structures in these nanoparticles are observed from high-resolution TEM (HRTEM) images. One is an amorphous structure with a spherical shape (Fig. 1b), and the other is a crystalline structure with lattice spacing of 0.21 nm which may be attributable to the (102) diffraction planes of graphitic (sp2) carbon (Fig. 1c) [26]. This suggests that the as-prepared T-CDs are not well crystallized. Fig. 1d and e show the optical properties of the aqueous dispersion of T-CDs. FL emission spectra of T-CDs (Fig. 1d) show a broad and gradually increasing FL emission peak at around 470 nm as their concentration decreases, and a typically excitation-wavelength-dependent property (Fig. S1). The concentration dependent FL behavior of T-CDs may result from FL inner filter effect induced by the interaction among T-CDs, which arises from photon absorbers and scatters in the sample solution and its degree is highly associated with the sample absorption intensities at the excitation and emission wavelengths [27,28]. This case is supported by the UV–vis spectra of T-CD solution (Fig. 1e), which reveal gradual blue-shift and the absorbance in visible region continues decreasing with the addition of water. A strong peak at 275 nm appears finally in diluted solution. This trend implies that the interaction among T-CDs could influence their optical properties. What’s more, the FL intensity of T-CDs still holds 88.6 % of original intensity after 365 nm UV light irradiation for a period of 100 min, indicating that T-CDs have high FL stability (Fig. S2). The surface composition and element analysis for the T-CDs were characterized by Fourier-transform (FT)-IR and X-ray photoelectron spectroscopy (XPS). FT-IR spectrum (Fig. 2a) exhibits the characteristic absorption bonds of OeH or NeH stretching vibration at 3226−3652 cm-1 [29]. Typically, the peaks at 2086, 1647, 1388 and 1114 cm-1 are attributed to C]N, C]O, CeN and CeOH bonds, respectively [30,31]. The XPS C1 s spectrum (Fig. 2b) shows four peaks at

2. Experimental details 2.1. Preparation of T-CDs The preparation of T-CDs was shown in Scheme 1. Typically, 100 mg of Pu-erh tea was put into a beaker of 50 mL. Subsequently, 30 mL of boiling deionized water (about 100 ℃) was added in it and 20 min later a cup of CDs was produced. Finally, the supernatant liquid of T-CDs was obtained by vacuum suction filter for removing residue of tea. 2.2. Procedures for sensing metal ions The detection of metal ions was performed at room temperature in Tris-HCl (20 mM, pH 7.4) buffer. For pure T-CDs as the probe, metal ion solution (500 μL) with a calculated concentration and was added into the mixture of 1000 μL of Tris-HCl buffer and 500 μL of as-prepared suspension of T-CDs. The FL emission spectra were recorded after reaction for 10 min at room temperature. The sensitivity and selectivity

Scheme 1. Preparation schematic of T-CDs. 2

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Fig. 1. (a) TEM image of T-CDs, the inset showing the photos of T-CDs excited by white light and 365 nm light, respectively; (b) and (c) HRTEM image of the typical nanoparticle with different structures; (d) FL emission spectra of T-CDs with different concentrations obtained by diluting 1, 2, 3 times, respectively at 365 nm excitation; (e) UV–vis absorption spectra of TCDs with different concentrations.

Fig. 2. FT-IR spectrum of T-CDs (a); XPS C 1s (b), N 1s (c), and O 1s (d) spectra of the T-CDs. 3

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Fig. 3. Ratiometric FL responses to various metal ions with 1 mM under 365 nm excitation (a); FL emission spectra of T-CDs upon addition of various concentrations of Cu2+ (b), Fe2+ (d), Fe3+ (f) ions, and the corresponding plot of F/F0 at 470 nm against concentrations of Cu2+ (c), Fe2+ (e), Fe3+ (g) ions, respectively. The Inset of (g) shows the linear response range for Fe3+.

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Fig. 4. Selectivity and sensitivity of the new sensing system of mixing T-CDs and OPD at 365 nm excitation: (a) FL response to various metal ions (1 mM); (b) FL colors with different concentrations of Cu2+ under 365 nm UV light; (c) FL spectra in the presence of different contents of Cu2+ ions; (d) The plot of log I/I0 as a function of the Cu2+ concentration, the inset showing the linear range of 0–22 μM for Cu2+.

288.2, 286.4, 285.7, and 284.6 eV, which are ascribed to C]O/C]N, CeO, CeN, and C]C/CeC, respectively. The XPS N 1s spectrum (Fig. 2c) displays three peaks at 398.6, 399.5, and 400.7 eV, which correspond to pyridinic N, pyrrolic N and amino N, respectively. The XPS O 1s spectrum (Fig. 2d) exhibits two peaks at 531.3 and 532.5 eV, which are attributed to C]O and CeOH/CeOeC groups, respectively [32,33]. All of results demonstrate that T-CDs contain abundant O and N-related groups on their surface, which could be more conducive to coordination reaction with metal ions. To examine the specificity of T-CDs, various metal ions such as Al3+, Ba2+, Ca2+, Cd2+, Fe2+, Fe3+, Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Sr2+, Zn2+, Cu2+ were tested under the same conditions. Fig. 3a shows the FL intensity ratio of T-CDs before and after adding metal ions (their corresponding intensities are represented with F0 and F, respectively). It can be clearly seen that except for Cu2+, Fe2+ and Fe3+ ions, no obvious changes of the FL signal are observed for the other metal ions. Fig. 3b shows that the FL emission of T-CDs at 470 nm gradually decreases upon increasing the concentration of Cu2+ ions when excited at 365 nm. The FL signal ratio shows good linearity with the Cu2+ concentration in the range of around 0–800 μM (Fig. 3c). Similarly, the sensitivity of T-CDs to Fe ions was also examined as shown in Fig. 3d–g. T-CDs for Fe2+ ion recognition give a linear relationship ranging from 0–450 μM while for Fe3+ display good linearity in the range of 0–100 μM. The detection limits for Fe2+, Fe3+ and Cu2+ were calculated to be 0.31, 0.17, 0.62 μM, respectively.

Table 1 Comparison of various methods for detection limit and linear range. Methods

Detection range (μM)

Detection limit (μM)

References

FL FL Ratiometric FL FL FL FL FL Ratiometric FL FL FL FL FL FL Electrochemistry Electrochemistry Colorimetry Colorimetry Atomic Absorption Spectrometry Raman Spectrometry Ratiometric & Colorimetry FL

0–8 0–16 0.25–10.0 0–70 1–8 0.01–10 5–100 10–150 0.5–47.6 0.5–40 0–100 10–60 0.01–200 0.3–1.4 0.5–50 1.0–10 0.25–14 0.4–4.69

0.29 0.45 0.076 0.11–0.14 0.15–0.25 0.0043 2.4 3.5 0.0156 0.28 0.01 6.8 0.56 0.3 0.35 0.17 0.25 0.052

[35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]

0–50 0–170

0.5 0.051

[53] This work

5

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Fig. 5. (a) Zeta potentials of the T-CDs, OPD and oxOPD in the aqueous solution; (b) Zata potential response of T-CDs to the concentrations of Cu2+ solution; (c) UV–vis absorption spectrum of oxOPD and FL emission spectrum of T-CDs; (d) FL decays of T-CDs and the mixture of T-CDs and oxOPD; (e) FL emission spectra of TCDs without and with the indicated quenchers; (f) FL decays of T-CDs without and with the indicated quenchers.

Cu2+ ions can catalyze the oxidation of OPD participating in hydrogen peroxide (H2O2) to produce 2, 3-diaminophenazine (oxOPD), which exhibits a bright yellow color and yellow FL emission at around 573 nm [34]. When Cu2+ ions are added to the mixture of T-CDs and OPD, the catalytic activity of Cu2+ ions will be activated in the presence of H2O2 and then lead to the formation of oxOPD with yellow FL emission. With these insights, we can construct a universal platform that applies ratiometric and colorimetric dual-readout for Cu2+ detection by simultaneously measuring the FL signals of T-CD and oxOPD. In this sensing platform, the FL emission of T-CDs can’t be influenced by the addition of OPD (Fig. S3). If FL intensities of oxOPD and T-CDs at their characteristic emission peaks are assigned with the F573 and F470, respectively, their ratio (I = F573/F470) changes with adding metal ions can be used as the signal for ratiometric detection. Meanwhile, I0 represents the value in absence of Cu2+ ions. As shown in Fig. 4a, a distinct difference from the signal of I/I0 is observed with adding varied metal ions. Cu2+ ions in the sensing platform yield remarkably higher value than other metal ions. Nevertheless, the same case with Cu2+ ions is not observed from Fe ions because they can’t catalyze the oxidation of

OPD under the same conditions (Fig. S4). As a result, the potential interference of other metal ions has negligible effects on the signal for Cu2+ sensing. Unlike pure T-CDs, this new sensing platform not only exhibits high selectivity, but also it is more sensitive and reliable for visual detection of Cu2+ ions (Fig. 4b) than a single fluorescence quenching probe. As shown in Fig. 4c, FL emission spectra of the sensing system clearly illustrate the gradual increase of F573 accompanied by the decrease of F470 as Cu2+ ions increase. The appearance of F573 ascribed to the formation of oxOPD occurs at 22 μM of Cu2+ ions while the F470 from T-CDs is completely quenched at 111 μM of Cu2+ ions. The relationship between the log(I/I0) and Cu2+ concentrations can be calculated yet based on the above result, and displays different linear relationships in the concentration range of < 22, 22–111 and > 111 μM of Cu2+ ions, respectively (Fig. 4d). The detection limit was calculated using the slope determined from the linear relationship in the concentration range of < 22 μM of Cu2+ ions (The inset of Fig. 4d). The obtained value is 51 nM, which is significantly lower than that of pure T-CDs. In addition, this sensing system is capable of detecting a wider range of Cu2+ concentrations compared to the sensing method by the 6

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Fig. 6. Mechanism schematic of new sensing system for Cu2+ detection.

reaction of Cu2+ and OPD (Fig. S5). As for detection limit and linear range of this sensing system, we also compared it with other reported approaches and provided the result in Table 1. Our presented sensing system is markedly superior to other approaches in detection limit of Cu2+ ions. Although a few methods possess high sensitivity, their linear range is narrow for sensing Cu2+ ions. In order to explore the mechanism of Cu2+ detection using the presented sensing platform, a series of characterizations are implemented. The zeta potentials of T-CDs, OPD and oxOPD were conducted to confirm the interaction between them. As shown in Fig. 5a, their potentials are −28.85, −7.615 and −11.8 mV, respectively. Since the potential of T-CDs is more negative than OPD, Cu2+ ions with positive charges preferentially react with T-CDs. Nevertheless, the potential of T-CDs gradually decreases and tends to zero as the concentration of Cu2+ ions increases (Fig. 5b). According to this result shown in Fig. 5b, the adsorption reaction between OPD and Cu2+ ions is able to occur only beyond a critical concentration of Cu2+ ions, and then gives rise to the formation of oxOPD. The occurrence of FRET commonly requires a nonzero integral of the spectral overlap between donor FL and acceptor absorption [54]. That is, energy transfer suggests itself through quenching or decrease of the donor FL and a reduction of excited state lifetime accompanied also by an enhancement in acceptor FL intensity [55]. Fig. 5c clearly manifests that the absorption band of oxOPD partially overlaps with the FL emission spectrum of T-CDs. FL lifetimes of T-CDs alone and the T-CDs interacting with oxOPD were further measured to verify the occurrence of FRET. As shown in Fig. 5d, there is an obvious change in the FL decay time of T-CDs before and after adding oxOPD and the shorter decay time is observed from the mixed system of T-CDs and oxOPD. These results demonstrate the occurrence of FRET between T-CDs and oxOPD. What’s more, the photoinduced electron transfer properties of T-CDs were also examined based on the method reported previously [56]. The FL emission of T-CDs were distinctly quenched by the known electron acceptors 2,4-dinitrotoluene (DNT) and the corresponding lifetime was also attenuated comparing with the T-CDs alone, whereas this case did not take place when the electron donor N,N-diethylaniline (DEA) was added in T-CDs (Fig. 5e and f). In this regard, T-CDs could only serve as electron donor, hereby allowing electron transfer from T-CDs to oxOPD and FL quenching of T-CDs faster. Based above results and characterizations, we proposed a sensing

mechanism of Cu2+ ions shown in Fig. 6. In the sensing platform established by mixing T-CDs, OPD and H2O2, Cu2+ ions with positive charges will preferentially react to T-CDs with large negative potentials, resulting in FL quenching of T-CDs. This case mechanism should be similar with those reported in literatures [57,58]. The zeta potential of T-CDs will decrease with the gradual increase of Cu2+ concentration on the basis of above potential characterizations. When the potential of TCDs is close to that of OPD, the interaction of OPD with Cu2+ ions will occur yet and give rise to the formation of oxOPD in the presence of H2O2. The typical yellow FL emission of oxOPD comes out in this sensing system. Simultaneously, FRET from T-CDs to oxOPD begins to play a leading role in colorimetric fluorescent detection of Cu2+. FRET effect is able to not only accelerate the FL quenching process of T-CDs, but also promote the FL emission of oxOPD. When FL emission of T-CDs is completely quenched, so only FL emission from oxOPD appears in the sensing system. As a result, the sensitivity and scope of Cu2+ detection are improved significantly by this sensing system. Because of these phenomena mentioned above, two critical concentrations of Cu2+ ions are observed in the linear relationships of colorimetric readouts. 4. Conclusions In conclusion, we reported a simple method of acquiring CDs by brewing Pu-erh tea. T-CDs contained abundant oxygen and nitrogenrelated functional groups on their surface and exhibited a good ability to detect Fe and Cu ions. Moreover, we constructed a new sensing universal platform that simultaneously applied fluorescent and colorimetric dual-readout for Cu2+ detection by measuring the FL signals of TeCD and oxOPD. Not only does this sensing platform exhibit much higher selectivity and sensitivity than pure T-CDs, but also have larger detection scope than the chemical oxidation reaction FL probe of OPD for Cu2+ detection. We also found a cooperation work mechanism of both FL quenching resulted from the coordination reaction of Cu2+ ions with surface functional groups of T-CDs and FRET from T-CDs to oxOPD in this new sensing platform, opening a novel approach of designing ratiometric and colorimetric FL probes. Authors’ contributions Wenjing Zhang and Ning Li conducted the experiments, analyzed 7

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experimental results and wrote the manuscript. Qing Chang assisted in sensing metal ions and reviewing the manuscript. Zhenfei Chen assisted in analyzing experimental results and discussing mechanism of Cu2+ detection. Shengliang Hu conceived the idea and supervised the project. All authors read and contributed to the manuscript.

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