Facile synthesis of polyaniline/carbon dot nanocomposites and their application as a fluorescent probe to detect mercury

Abstract

In this work, for the first time, novel polyaniline/carbon dot (PANI/Cdot) nanocomposites have been synthesized using a simple method. Meanwhile, the bright fluorescence of Cdots is effectively quenched by PANI through fluorescence resonance energy transfer. With the addition of Hg²⁺, the strong binding affinity between Hg²⁺ and the amino groups of PANI makes the Cdots break away and release from the PANI, resulting in the restoration of Cdot fluorescence, which can be used as a fluorescent probe for Hg²⁺ detection. This fluorescence “off–on” signal is sensitive to the concentration of Hg²⁺, and there is a good linear relationship between the fluorescence intensity of Cdots and Hg²⁺ concentrations in the range of 0.05 to 1.0 μM. The detection limit for Hg²⁺ at a given concentration is 0.8 nM. Moreover, it turns out that the nanoprobes represent a rather high selectivity for Hg²⁺ detection. We believe that PANI/Cdot composites will emerge as a new class of fluorescence materials that could be very likely to be suitable for practical applications.

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1. Introduction

The mercury(II) ion (Hg²⁺) is a hazardous environmental contaminant due to its accumulative and highly toxic properties and causes harm to the human body mainly via the central nervous system, digestive system and internal organs.¹ Even in low concentrations, it is a threat to the environment and human health because mercury is non-biodegradable and can enter the food chain. The maximum allowable level of Hg²⁺ in drinking water, defined by United States Environmental Protection Agency, is 10 nM .² So, various sensors have been developed for the detection of Hg²⁺, including colorimetry, photoelectrochemistry, atomic absorption/emission spectroscopy, electrochemical techniques, SERS spectroscopy and so forth.^3–7 But it is still a great challenge to develop new methods for aqueous Hg²⁺ analysis with high sensitivity, selectivity and simplicity.

Because of their ultrasensitivity, and rapid and easy operation, in recent years, much effort has been devoted to develop fluorescent sensors derived from various nanomaterials including gold clusters, silica nanoparticles, polymer nanoparticles, carbon dots, graphene oxide, and graphene quantum dots, etc.^8–14 Among these fluorescent materials, Cdots have recently received much attention due to their many unique and novel properties. Compared with conventional semiconductor nanodots and organic dyes, Cdots have good water solubility, low cytotoxicity, robust chemical inertness, low photobleaching, cost effectiveness, and excellent biocompatibility, etc.¹⁵ They have broad application prospects in many technologies, such as bioimaging, sensing, photocatalysis, and as fluorescent inks.^16–19 So far, a variety of simple, fast, and cheap synthetic routes to prepare Cdots, such as pyrolysis, combustion, hydrothermal processes, templating, electrochemistry, ultrasonic and microwave synthesis.^20–26 Cdots display photoluminescence properties reminiscent of those of quantum dots which are widely used as energy donors in fluorescence resonance energy transfer (FRET) applications.²⁷ Cdots as a new class of fluorescent nanomaterials are expected to behave as better FRET donors and acceptors than other organic fluorescent molecules. However, research on FRET using Cdots is still in the initial stages and there have been few reports concerning the potential applications of such materials. As a consequence of the health concerns and the known environmental and biological hazards of Cdots, Cdots are at the center of significant research efforts for the development of novel FRET sensors.

PANI is a very important polymer, and it is also the most extensively investigated conducting polymer in recent years, owing to its facile and low cost synthesis, good environmental stability, non-toxicity, reversible electrochemistry, corrosion protection, and high instinct redox properties, etc.²⁸ It has a large variety of applications such as in sensors, electrochromic devices, secondary batteries, catalysis and electrostatic discharge protection.^7,29–32 On the other hand, PANI is also a class of aromatic conjugated polymers with a rigid, planar π–π electronic conjugated system, showing absorption in the region of ultraviolet and visible light. So it should be a quencher of Cdots, as a result of nonradiative transfer of electronic excitation energy from Cdot excited states to its π system. Inspired by this property, PANI may be used as a FRET acceptor with Cdots as energy donors, in which PANI exhibits high efficiency in quenching the donor emission and consequently provides good sensitivity. Hence, this has made it possible and important to establish FRET systems using PANI/Cdot nanocomposites.

Here, we have synthesized PANI/Cdot nanocomposites using a simple method and established a novel FRET system. With the addition of Hg²⁺ into the system, the quenched fluorescence of Cdots is restored. This fluorescence “off–on” signal is sensitive to the concentration of Hg²⁺, and there is a good linear relationship between the fluorescence intensity of Cdots and the Hg²⁺ concentration. The detection limit for Hg²⁺ at a given concentration is 0.8 nM. The PANI/Cdot probe is also successfully applied for the determination of Hg²⁺ ions in real-water samples. To the best of our knowledge, this work represents the first example based on PANI/Cdot nanocomposites for Hg²⁺ detection and it would have great potential for applications in monitoring Hg²⁺ in the environment and in food.

2. Results and discussion

2.1. Synthesis and characterization of PANI/Cdot composites

The UV-vis absorption spectra of as-prepared samples are shown in Fig. 1a. The Cdot solution has an absorption peak at 340 nm and a feature at 235 nm in the UV-vis absorption spectrum of the Cdots (curve 1). The UV-vis absorption spectrum of PANI is shown as curve 2 (Fig. 1a). Two bands at about 420 nm and 760 nm observed in the UV-vis absorption spectrum of the sample are attributed to the π–π* transition and the exciton-like transition from the benzenoid rings, to the quinoid rings, respectively. This is consistent with the formation of a conventional emeraldine base, indicating that the prepared PANI is in a proton doped state with positive charge on the surface of the molecules.³³ In the UV-vis absorption spectrum of the PANI/Cdot composite (curve 3), the absorption peaks have some changes, except for the absorption peak of the Cdots at 340 nm. The peak of PANI at 420 nm is not observed, which may be overlapped by that of the Cdots. The band at 760 nm moves to 590 nm. These all show that there are interactions between PANI and the Cdots.

Fig. 1 UV-vis (a) and FTIR (b) spectra of Cdots (1), PANI (2), and PANI/Cdots (3).

Fig. 1b shows the FTIR spectra of the prepared samples. For the Cdots (curve 1), the peaks at 1580 and 1396 cm⁻¹ correspond to the asymmetric and symmetric stretching vibrations of the carboxylate anions, respectively. The bands at 3200–3600 cm⁻¹ are attributed to the stretching vibration of hydroxyl groups. The results reveal that negatively charged carboxylates and hydroxyl groups are mainly on the surface of Cdots. In the spectrum of pure PANI (curve 2), the characteristic peaks at 1579 and 1491 cm⁻¹ are due to the C=C stretching vibrations of the quinoid and benzenoid rings, respectively, and the peaks at 1297 and 1235 cm⁻¹ are related to the C–N and C=N stretching modes. The bands at 1143 and 819 cm⁻¹ are the distinctive features of C–H in-plane and C–H out-of-plane bendings, respectively. The FTIR spectrum of the PANI/Cdot composite shows all the bands of PANI and the Cdots (curve 3). However, it can be found that there are also some changes compared with pure PANI and Cdots. The intensities of some peaks in the composite are lower than those of pure PANI, while the intensity of the Cdots peak at 1580 cm⁻¹ decreases, which further confirms that there are interactions between PANI and Cdots in the composite.

The morphology of the Cdots was investigated using TEM. From Fig. 2a, it can be clearly seen that the as-synthesized Cdots are uniform in size and possess a nearly spherical shape. No obvious lattice fringes are found in the high resolution HRTEM of the Cdots (Fig. 2b), which is indicative of their amorphous nature. The indiscernible diffuse rings in the selected-area electron-diffraction pattern (inset in Fig. 2b) further confirm the poor crystallization of the Cdots. Cdots have a narrow size distribution. Fig. 2c reveals that the average size of the Cdots is about 1.7 nm. The Cdot solution exhibits strong bright blue luminescence under excitation at 365 nm, which can be easily seen with the naked eye and recorded with a digital camera, as shown in the inset (Fig. 2d). The emission peaks of the Cdots at various excitation wavelengths from 300 to 440 nm are shown in Fig. 2d. It is noted that with an increasing of the excitation wavelength, the intensity of the luminescence increases to the maximum (340 nm excitation), then decreases. The full width at half-maximum is about 68 nm. The peaks do not shift, indicating that the luminescence origin of the Cdots might be different from most other Cdots that show excitation wavelength dependent fluorescence. The maximum emission wavelength at different excitation wavelengths remains at 448 nm. Therefore, 340 nm was selected as the excitation wavelength for the following experiments.

Fig. 2 (a) TEM and (b) HRTEM images of the synthesized Cdots, inset: SAED pattern of the Cdots, (c) size distribution, and (d) FL spectra of the Cdots excited at different wavelengths, inset: photograph of the Cdot solution under a UV lamp, λ_ex = 365 nm.

As shown in Fig. 3a, the synthesized PANI is in the form of uniform nanofibers, and the surface of the nanofibers is very smooth. The average diameter of PANI is about 40 nm and the length is several micrometers. When PANI with a positive charge and carboxyl group functionalized Cdots are mixed in aqueous solution, the Cdots successfully attach onto the surface of the PANI nanofibers through electrostatic interactions and do not aggregate (Fig. 3b). So, the TEM analyses confirm the formation of a PANI/Cdot nanocomposite.

Fig. 3 TEM images of PANI (a), and the PANI/Cdot composite (0.28 : 0.20, v/v) (b), the fluorescence quenching of Cdots with different volume ratios of PANI/Cdots (from top to bottom: 0 : 0.2, 0.04 : 0.2, 0.08 : 0.2, 0.12 : 0.2, 0.16 : 0.2, 0.2 : 0.2, 0.24 : 0.2, and 0.28 : 0.2) (c) and the photoluminescence (red) and UV-vis spectra (black) of Cdots and PANI (d).

When the Cdots and PANI are mixed, the fluorescence of the Cdots is quenched in direct proportion to the volume ratio of PANI/Cdots (denoted by X), (Fig. 3c). When X is 0.04 : 0.2, the fluorescence intensity of the Cdots is quenched about 72%. With a further increase of X to 0.28 : 0.2, over 98% of the fluorescence is quenched, it reaches an equilibrium value, and the bright blue fluorescence under the UV lamp disappears. For assays based on such a fluorescence “off–on” model, higher quenching rates are generally preferable in terms of detection sensitivity. Here, PANI with a positive charge helps to attach carboxyl-stabilized Cdots. The highly efficient quenching by PANI should originate from the FRET from PANI to the Cdots owing to spectral overlapping. The absorption spectrum of PANI and the emission spectrum of the aqueous Cdot solution under excitation at 340 nm are shown in Fig. 3d, and the emission peak of the Cdots is at about 448 nm. While, the PANI exhibits an absorption centered at 420 nm, which has a partial spectral overlap with the Cdot emission. The spectral relationship indicates that the adsorption of the Cdots at the surface of PANI may lead to the highly efficient quenching of the Cdot fluorescence by FRET.

2.2. PANI/Cdot fluorescent probe for the detection of Hg²⁺

Based on the above fluorescence properties of the PANI/Cdot nanocomposites, we explored the feasibility of using the as-prepared PANI/Cdots for Hg²⁺ detection. As shown in Fig. 4a, it can be seen that the fluorescence of Cdots continuously recovers with the addition of Hg²⁺ to the PANI/Cdot solution. The fluorescence enhancement is closely related to the amount of Hg²⁺ added to the PANI/Cdot probe solution. About 8.7-fold fluorescence enhancement is measured when the concentration of Hg²⁺ reaches 1.0 μM. Even if the Hg²⁺ concentration is as low as 0.05 μM, the fluorescence intensity is still enhanced ca. 1.5-fold. Fig. 4b shows a plot of the fluorescence enhancement percentage vs. the concentration of Hg²⁺. As shown in the inset, the best linear response concentration range of Hg²⁺ is from 0.05 to 1.0 μM, while the quantification of Hg²⁺ can be achieved with a standard deviation R = 0.9929, and the lowest detection limit (LOD) is about 0.8 nM based on the standard deviation of the response (σ) and the slope of the calibration curve (S) at levels approximating the LOD according to the formula: LOD = 3.3(σ/S), which is lower than previously reported results.³⁴ Notably, the ultrasensitive detection of Hg²⁺ using this novel probe can be achieved in aqueous solution.

Fig. 4 (a) Evolution of the fluorescence spectra of PANI/Cdots (0.28 : 0.20, v/v) with the addition of Hg²⁺ (0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.6, 2.0 and 2.4 μM), (b) plot of the fluorescence intensity vs. Hg²⁺ concentration (the inset shows the linear correlation), and (c) photographs of the PANI/Cdot solution in the fluorescence “off–on” process under a 365 nm UV lamp.

The effect of pH on the fluorescence recovery of the sensor system was studied with a Hg²⁺ concentration of 0.8 nM. The results show that there is a slight variation in the value of F/F₀ in the pH range of 2.0–9.0 (Fig. S1†). So, the sensor system has a wide pH range for Hg²⁺ detection.

In order to evaluate the selectivity of the PANI/Cdot probe for Hg²⁺, the fluorescence response in the presence of 10 other common metal ions was also investigated including Na⁺, Cu²⁺, Mg²⁺, Cd²⁺, Mn²⁺, Zn²⁺, Ni²⁺, Fe³⁺, Co²⁺, and Pb²⁺. As shown in Fig. 5, only Hg²⁺ ions can significantly enhance the ratiometric luminescence output of the PANI/Cdots. The fluorescence intensity of the PANI/Cdot composite remains almost unchanged upon adding metal ions such as Na⁺, Mg²⁺, Cu²⁺, Fe³⁺, Co²⁺, Ni²⁺, Mn²⁺, and Pb²⁺. In addition, a slight fluorescence enhancement can be observed upon the addition of Cd²⁺ and Zn²⁺ to the PANI/Cdot composite. This is because Cd²⁺ and Zn²⁺ have the same electronic configuration and similar chemical properties as Hg²⁺, and they can react with the N atoms in PANI polymeric chains to form the corresponding complex. However, the slight fluorescence enhancement of Cd²⁺ and Zn²⁺ in this study does not affect the application of the present PANI/Cdot probe for Hg²⁺ detection. According to the literature,⁷ the main adsorption sites for the metal ions are the nitrogen atoms in the PANI chains because the nitrogen atom has a lone pair of electrons that can bind a metal ion to form a metal complex. Compared with other metal ions, Hg²⁺ has a larger ionic radius, and polarization and deformation happen more easily when it interacts with the nitrogen atoms of PANI. So, the high selectivity of the PANI/Cdot probe for Hg²⁺ may be due to the fact that Hg²⁺ ion has a higher thermodynamic affinity and faster chelating process with the “N” of PANI than other metal ions. These results suggest that the PANI/Cdot probe exhibits a high selectivity for Hg²⁺. A reasonable explanation for this high selectivity for Hg²⁺ needs to be further discussed.

Fig. 5 Selectivity investigation of the PANI/Cdot probe for Hg²⁺ detection (ion concentration: 20 μM).

The practical application of the designed probe for detecting Hg²⁺ in river water samples was also tested. To evaluate the PANI/Cdot probe in an artificial system, the performance of the present probe for real-water sample analysis was conducted using lake water samples obtained from the Caohu of Hefei, Anhui province, China. First, the water sample was filtered to get rid of any insolubles. Subsequently, standard Hg²⁺ solutions with different concentrations were added to the pretreated water sample, and it was then analyzed using the proposed method. Fig. 6 shows the fluorescence response of the PANI/Cdots in the presence of lake water containing different concentrations of Hg²⁺ ions (a), and the corresponding relationship between the fluorescence intensity and the concentrations of Hg²⁺ (b). It can be seen that the fluorescence intensity gradually recovers with an increase in the concentration of the Hg²⁺ in the lake water from 0 to 1.2 μM. The calibration curve for determining the Hg²⁺ in the lake water was obtained by plotting the values of F/F₀ versus the concentrations of Hg²⁺. As shown in Fig. 6b, a good linear correlation has been obtained in the range of 0–1.0 μM for the lake water. The results indicate that the PANI/Cdot probe may be a promising sensing platform for the detection of Hg²⁺ in real environmental samples.

Fig. 6 (a) Fluorescence response of PANI/Cdots in the presence of lake water containing different concentrations of Hg²⁺ (0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.6, and 2.0 μM), and (b) a plot of the fluorescence intensity vs. Hg²⁺ concentrations.

Fluorescence “off–on” mechanism for Hg²⁺ detection. Scheme 1 illustrates the fluorescence “off–on” mechanism of the PANI/Cdot probe for the detection of Hg²⁺. Here, Cdots have been used as energy donors, while PANI is a FRET acceptor. PANI possesses π–π stacking and amino groups with a positive charge, while the surface of the Cdots has a negative charge because of carboxyl groups, which all bring the donor (Cdots) and the acceptor (PANI) into close proximity (Fig. 3b). So, the emission of the Cdots is effectively quenched by PANI through FRET, and it consequently provides good sensitivity. In the presence of Hg²⁺, the binding between the amino groups of PANI and Hg²⁺ disturb the interaction between the Cdots and PANI. Such interactions release the Cdots from the PANI (Fig. S2†), resulting in the restoration of Cdot fluorescence (Fig. 4c). This design results in a fluorescence-enhanced detection that is sensitive and selective for Hg²⁺.

Scheme 1 Fluorescence “off–on” mechanism for the detection of Hg²⁺.

3. Experimental section

3.1. Materials

Aniline monomer, ammonium persulfate, sodium citrate, and ammonium bicarbonate were all analytical grade reagents (Shanghai Chemical Reagent Co. Ltd., China). The other reagents were used as received without further purification. Double distilled water was used throughout the experiment to prepare the solutions.

3.2. Synthesis of Cdots

Cdots were prepared according to a previously described method.³⁵ Briefly, sodium citrate (0.2 g), NH₄HCO₃ (1.5 g) and water (10 mL) were sealed in a Teflon equipped stainless steel autoclave, which was then placed in a drying oven followed by hydrothermal treatment at 180 °C for 4 h. After the reaction, the autoclave was cooled to room temperature. The purification of the Cdots was conducted through a dialysis tube (1000 Da, molecular weight cut-off) for about 24 h in the dark.

3.3. Preparation of PANI

In a typical synthesis, APS (0.22 g, 0.965 mmol) was dissolved in 8 mL of deionized water to prepare an oxidant solution at room temperature. The oxidant solution was then added dropwise to the aniline monomer (0.03 mL, 0.322 mmol) solution. Then, the reaction mixture was continuously stirred for 6 h in a water bath at room temperature. Finally, the polymerization system was immobilized for 48 h at 0–5 °C. The remaining precipitate was washed several times with deionized water, and ethanol, and then dried under vacuum for 24 h at 50 °C. The PANI powder was redispersed to make a solution with a concentration of about 6 mg mL⁻¹.

3.4. Formation of the PANI/Cdot composite

0.2 mL of a Cdot solution and certain volumes of PANI dispersions (0, 0.04, 0.08, 0.12, 0.16, 0.2 and 0.24 mL) were mixed with deionized water in a centrifuge tube with a final volume of 2 mL, and incubated for 30 min at room temperature before the fluorescence measurements. Then, the fluorescence spectra were measured in a 2 mL quartz cuvette at room temperature.

3.5. Detection of Hg²⁺

A typical procedure for the detection of Hg²⁺ is described as follows: the fluorescence spectrum of 2 mL of the PANI/Cdot suspension (PANI/Cdots: 0.28 : 0.20, v/v) was recorded subsequently and the fluorescence intensity was set as F₀. Then, different concentrations of Hg²⁺ (0, 1.0, 2.0, 4.0, 6.0, 10.0, 15.0, 20.0, 30.0, 50.0, and 100.0 mM) were mixed with the above solution with gentle shaking. The corresponding fluorescence spectra were recorded and a series of F/F₀ values were obtained. The other metal ions, such as Na⁺, Mg²⁺, Cu²⁺, Cd²⁺, Fe³⁺, Co²⁺, Ni²⁺, Mn²⁺, Pb²⁺, Zn²⁺, etc., were also measured. All of the detection experiments were carried out under the same conditions at room temperature. To evaluate the PANI/Cdot-based probe for Hg²⁺ detection in a practical application, the performance of the present method for real-water sample analysis was examined using lake water samples from Caohu (Anhui, China). The lake water samples were filtered through a 0.20 μm filter membrane and centrifuged at 10 000 rpm for 10 min to get rid of any insolubles. Different concentration levels of Hg²⁺ were added to the lake water samples and analyzed using the proposed method.

3.6. Characterization

UV-vis absorption spectra were recorded at room temperature using a TU-1800PC spectrometer. FTIR spectra were obtained using a NEXUS-870 spectrophotometer (frequency range from 3600 to 400 cm⁻¹) as KBr pellets. TEM was performed using a JEM-2100F instrument with a field emission gun operating at 200 kV. The fluorescence spectra were recorded at room temperature using an F-7000 fluorescence spectrophotometer (Hitachi) with a quartz cell (1 mm). Both the excitation and emission slit widths were fixed at 5 nm. Photographs were taken with a Canon 350D digital camera.

4. Conclusions

In conclusion, we have developed a PANI/Cdot “off–on” fluorescent probe for the simple, ultrasensitive and selective detection of Hg²⁺ in aqueous media. PANI/Cdot composites have been prepared via an electrostatic interaction. Meanwhile, the fluorescence of the Cdots is effectively quenched by FRET. The fluorescence intensity of the Cdots is switched on with the addition of Hg²⁺. A good linear fit between the fluorescence response and Hg²⁺ concentration is obtained in the range 0.05 to 1.0 μM. The LOD is found to be 0.8 nM for Hg²⁺. In addition, the probe shows a high selectivity for Hg²⁺ over other metal ions. Most importantly, the PANI/Cdots probe exhibits the promising application of Hg²⁺ detection for real-water samples. Thus, it is believed that the present strategy may offer a new approach for developing low-cost, highly sensitive and selective Hg²⁺ sensors for environmental applications.

Acknowledgements

This work was supported by the Natural Science Foundation of Anhui Province (1308085MB29, 1308085QB33, 1408085MB27), the Natural Science Foundation of China (21301004, 21201005), the Natural Science Foundation of Anhui Educational Committee (KJ2014ZD080), and the China Postdoctoral Science Foundation funded project (2013M5418100, 2013M530301).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03832d

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