Highly selective and sensitive sensing for Al³⁺ and F⁻ based on green photoluminescent carbon dots

Abstract

Ascorbic acid and glycol were employed as the carbon resource and co-solvent, respectively, to prepare extraordinary green photoluminescent carbon dots (CDs) by means of a hydrothermal method. Furthermore, the resulting CDs could be successfully employed to establish highly sensitive and selective sensing systems for Al³⁺ ions with a photoluminescence (PL) enhancement response and for F⁻ anions with a PL “ON–OFF” model.

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Introduction

Aluminum (Al) is widely employed in various areas, such as food packaging, water supply, antiperspirants, as well as in the manufacture of cars and computers.¹ However, with an excessive intake of Al³⁺ ions the human nervous system will be damaged, which can result in some serious diseases, such as Alzheimer’s disease, Parkinson’s dementia, breast cancer and Wilson’s disease.^2–5 Therefore, developing sensitive and feasible detection methods for Al³⁺ ions are important for human health.

Numerous analytical methods, such as atom absorption spectrometry and atom fluorescence spectrometry, have been established for probing Al³⁺ ions.^6–11 In particular, constructing photoluminescence (PL)-based sensors towards Al³⁺ ions has attracted considerable attention, due to their high sensitivity.¹² On the other hand, a deficiency of fluoride (F⁻) anions (which possess the smallest ionic radius, highest charge density and a hard Lewis basic nature) can induce dental disease and osteoporosis.¹³ Therefore, F⁻ anions have also become one of the important detecting targets. It should be noted that most of these reported PL-based sensing systems for Al³⁺ ions or F⁻ anions focused on designing sophisticated organic dye receptors based on various sensing mechanisms. For example, some specific interaction mechanisms, including hydrogen bonding between probes and F⁻ anions, reactivity between F⁻ anions and boron atoms, and a promoted cleavage reaction by F⁻ anions, have been developed for sensing F⁻ anions.^14,15 However, these artificial sensors showed some drawbacks, such as complicated synthesis procedures, employing an environmentally unfriendly organic medium, and photobleaching. Therefore, it is of importance to develop novel, simple, stable, and environmentally friendly sensing systems for Al³⁺ ions or F⁻ anions.

It should be pointed out that carbon dots (CDs) have widely attracted scientist’s interests due to their extraordinary optical/electric properties,^16,17 including a high PL quantum yield (QY), biocompatibility and environmental friendliness. Therefore, CDs have been successfully employed in a series of applications, such as PL-based sensing and detecting,^18–22 in vivo cell imaging,^23,24 enhancing the performance of optoelectronic devices^25–27 and biological theranostic probes.^28–30 However, in general two shortcomings existed in these reported cases, (i) only blue emissive CDs with a short emission wavelength could be successfully prepared and employed to develop sensing systems, (ii) all reported emissive CD-based sensing systems were established on the basis of a PL quenching response, such as in Cu²⁺ detection.³¹ Therefore, developing CDs with a long emission wavelength for the sensing of Al³⁺ ions or F⁻ anions, especially with a PL enhancement response, is important for extending the application scope of CDs and obtaining a high sensing sensitivity. Herein, a green photoluminescent CD-based sensing system was successfully established towards Al³⁺ ions, with a PL enhancement response and high selectivity. Furthermore, the as-prepared CDs have also been employed to construct a PL-based sensing system towards F⁻ anions with a high selectivity and sensitivity, due to the strong complexing interaction of F⁻ anions towards Al³⁺ ions.

Experimental

Ascorbic acid (AA, 99%), ethanol (99.7%, AR), glycol (99%, AR), oxalate acid (99.8%, AR), tartaric acid (99.5%, AR), and Al(NO₃)·9H₂O (99%, AR), were obtained from Guo Yao (Shanghai, China). NaF (99%, AR) and glycerol (99%, AR) were purchased from Sigma-Aldrich. All chemicals were used in the experiments without further purification.

Instruments

Deionized water, purified by a Millipore system (18.0 MΩ cm at 25 °C), was employed for all experiments. pH values were measured by a Model1828 digital pH meter. PL spectra were acquired by a Hitachi F-7000. UV-vis absorption spectra were recorded by a Unico UV-2012PC spectrophotometer. An FTIR-4800S spectrometer was employed to obtain IR spectra in KBr discs in the 4000–400 cm⁻¹ region. X-ray diffraction (XRD) results were recorded on a Rigaku Smart lab with a speed of 6° per minute. Transmission electron microscopy (TEM) experiments were done on a TECNAI-F30 system. The zeta potential and DLS size distribution were obtained by a Zeta sizer Nano.

Preparation of CDs

AA (0.6 g, 3.4 mmol), glycol (10 mL, 0.16 mol) and distilled water (10 mL) were mixed in a beaker. The solution was stirred for 20 min, then sealed in a 50 mL PPL equipped stainless steel autoclave, followed by a hydrothermal treatment at 160 °C for 70 min. The color of the solution gradually turned to pale yellow from colorless in appearance. When the resulting solution had cooled to room temperature, the solution was placed in a refrigerator for further application. The product can be directly used without any further passivation or purification.

Al³⁺ ion and F⁻ anion sensing

For the sensing of the Al³⁺ ions or F⁻ anions a 100 μL CD solution and 8 mL 0.2 M NaOAc–HOAc buffer solution of pH 5.8 were mixed with a solution containing Al³⁺ ions of various concentration or 10 μM Al³⁺ or F⁻ ions of various concentration, to afford a fixed volume of 10 mL. After stirring, the mixed solution was maintained at room temperature for 10 min and PL spectra were measured. Moreover, experiments with coexisting interfering anions or heavy metal ions were conducted to further investigate the selectivity of this sensing system under similar experimental conditions. The excitation wavelength was set as 400 nm.

Results and discussion

The green photoluminescent CDs were prepared by a reported hydrothermal method with minor modification.³²

In order to probe the effect of co-solvents and carbon sources on the formation of photoluminescent CDs, several co-solvents and carbon sources were chosen to synthesize CDs (Fig. 1). Experimental results indicated that the green photoluminescent CDs could be successfully prepared when AA and glycol were employed as the carbon resource and co-solvent, respectively. The as-prepared CDs from ascorbic acid and glycol exhibited a green emission with a maximum peak centered at 530 nm under 365 nm wavelength excitation, which could be easily observed with the naked eye and seen with a digital camera (Fig. S1†). These observations were obviously different from those of CDs prepared by other carbon sources and co-solvents (Fig. 1). Therefore, the green photoluminescent CDs were expected to be employed to establish sensing systems, due to their intriguing long emission wavelength. The detailed characterization (Fig. 2, S2 and S3†) and sensing application of the CDs prepared from AA and glycol were conducted.

Fig. 1 PL spectra of CDs prepared from various co-solvents (glycerol, ethanol, water and glycol) in the presence of AA as a carbon resource (a), and from various carbon sources (citric acid, oxalic acid, tartaric acid and ascorbic acid) in the presence of glycol as a co-solvent (b).

Fig. 2 TEM image (a) (the scale bar is 20 nm), size distribution (b), zeta potential (c) and XRD (d) of the as-prepared CDs from AA and glycol.

TEM images exhibited that the CDs prepared from AA and glycol have a spherical morphology and nearly monodisperse size with an average diameter of approximately 2.85 ± 0.15 nm, calculated from its Gaussian fitting curve (Fig. 2a and b). The observable TEM results further supported the formation of the CDs. The zeta potential results also supported this conclusion, and showed a value of −0.04 mV (Fig. 2c), probably resulting from the slight ionization of the hydroxyl groups of the surface of the resulting CDs.

The XRD pattern of the resulting CDs (Fig. 2d) showed one broad peak at 22° (2θ), indicative of the turbostratic carbon phase.³³ This XRD result again confirmed the formation of CDs. These results and FT-IR spectra (Fig. S4†) indicated that the surface of the as-prepared CDs should be capped by glycol, thereby resulting in a good dispersal of the as-prepared CDs in water.

It was surprisingly found that the as-prepared CDs could show a high sensitivity and selectivity towards Al³⁺ ions, with a PL enhancement response, considerably different from those of the reported photoluminescent II–VI group quantum dots (QDs) such as CdS and CdTe QDs, since the PL of these QDs were in general quenched by most metal ions.^34–38 Furthermore, when the PL response of other CDs (prepared from other co-solvents such as glycerol, water, and ethanol and from other carbon sources including citric acid, oxalate acid, and tartaric acid) towards Al³⁺ ions were further investigated, the appearance of an observable PL enhancement wasn’t present. Therefore, to construct a sensing system towards Al³⁺ ions the green photoluminescent CDs prepared from AA and glycol were chosen.

Further investigation of the PL spectral response of the CDs towards Al³⁺ ions revealed that, with an increasing Al³⁺ ion concentration, the PL intensity of the CDs was linearly enhanced, accompanied by a blue shift of the maximum emission peaks (Fig. 3). The maximum PL intensity enhancement could reach nearly 4 times that of the original PL intensity of the CDs. A linear correlation curve could be fitted between the Al³⁺ ion concentration and PL intensity ratio, I/I₀ (Fig. 3), and the linear correlation coefficient could be obtained as 0.9951. The limit of detection (LOD) was also calculated to be 0.39 μM, according to the equation of LOD = 3σ/k, in which σ is the standard deviation from 11 blank solutions and k is the linear slope fitted from Fig. 3. The LOD is much lower than the tolerable limit for Al³⁺ ions in drinking water (this value is 7.4 μM), as defined by the World Health Organization,³⁹ and is also lower than most reported cases (Table S1†).^40–42

Fig. 3 (a) The PL spectral response of the CDs upon addition of Al³⁺ ions of various concentration (0.0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0 μM, respectively) in a 0.2 M NaOAc–HOAc buffer solution of pH 5.8 with an excitation wavelength of 400 nm. (b) The corresponding linear relationship between the PL intensity ratio at maximum emission of the CDs and the Al³⁺ ion concentration. Note that the error bars were obtained from three parallel measurements; I₀ and I represent the maximum PL intensity of the CDs in the absence and presence of 10 μM Al³⁺ ions, respectively.

The selectivity of the CDs towards Al³⁺ ions was evaluated by introducing a series of metal ions, such as Fe²⁺, Fe³⁺, Cu²⁺, Mg²⁺, Ca²⁺, Cd²⁺, Zn²⁺, Mn²⁺, Co²⁺, Ni²⁺ and Hg²⁺ ions (the concentrations of Cu²⁺, Al³⁺ and Fe²⁺ ions were 10 μM, while the concentrations of other tested metal ions were 100 μM) into the CD solution. Experimental results are shown in Fig. 4 and reveal that the effects of the other tested metal ions on the PL of the CDs were almost negligible. The PL of the CDs showed significant tolerability towards these tested metal ions in the presence of Al³⁺ ions. Therefore, the green emissive as-prepared CDs could be considered to have a high selectivity towards Al³⁺ ions.

Fig. 4 The PL intensity ratio at 530 nm of the CDs in the absence (black) and presence (10 μM, red) of Al³⁺ ions in a 0.2 M NaOAc–HOAc buffer solution of pH 5.8 containing various metal ions. The excitation wavelength was set as 400 nm. Cu²⁺, Al³⁺ and Fe²⁺ ion concentrations were 10 μM, while other tested metal ion concentrations were 100 μM. Note that the black column and red column of “Blank” refer to the maximum PL intensity of the CDs in the absence and presence of 10 μM Al³⁺ ions, respectively.

The pH effect experiments revealed that both the maximum PL intensity of the original CDs and the correspondingly enhanced PL intensity of the CD–Al³⁺ ion system were pH-dependent (Fig. 5). These observations further supported the idea that introducing Al³⁺ ions could facilitate the combination with hydroxyl groups on the surface of the CDs under nearly neutral pH conditions, resulting in the formation of much more surface states of the CDs. However, the exact mechanism should be further explored.

Fig. 5 The PL intensity @535 nm of the CDs in the absence (black) and presence (red) of 10 μM Al³⁺ ions in 0.2 M NaOAc–HOAc or 0.1 M Tris–HCl buffer solutions of various pH values. The excitation wavelength was set as 400 nm.

To probe the interaction mechanism between the Al³⁺ ions and the CDs, PL lifetimes of the CDs were measured before and after adding Al³⁺ ions (Fig. S5†). The results revealed that the original PL lifetime of the CDs was 4.7 ns with single exponential decay, however, the value changed to 6.8 ns after 10 μM Al³⁺ ions were introduced. The enhanced PL lifetime was indicative of a much more stable excited-state.⁴³ The UV-vis absorption spectra of the CDs showed a red-shift when the Al³⁺ ion solution was added. Correspondingly, the excitation spectra of the CDs were found to be gradually enhanced in their intensity and their maximum peaks were red-shifted after increasing the Al³⁺ ion concentration (Fig. S6†). Taken together, this suggested that the surface passivation of the CDs by Al³⁺ ions occurred, which should be responsible for the observation of the enhancement of PL intensity of the CDs, since a unique five-numbered cyclic structure between glycol and Al³⁺ ions was suggested, according to our calculations (Fig. 6).⁴⁴

Fig. 6 The calculated five-numbered cyclic structure between glycol and Al³⁺ ions by Gaussian 03. “Pink”, “Red”, “Gray” and “White” represent Al, O, C, and H, respectively.⁴⁵

It is considered that quenching of the PL intensity will occur if a certain amount of F⁻ anions are added into the CD–Al³⁺ ion system, because the binding constant between the Al³⁺ ions and F⁻ anions is extremely large,^46,47 up to 10^19.84. Experimental results revealed that when the F⁻ anion concentration was enhanced from 0 to 10.0 μM, the PL intensity of the CD–Al³⁺ ion system gradually decreased, accompanying the red-shift of the maximum PL wavelength, and a linear plot of lg I/I₀ = 0.006 − 0.04 [F⁻] with a regression coefficient R² of 0.996 could be fitted (Fig. 7). Further investigations on the F⁻ anion concentration-dependent PL spectral response were conducted. The LOD was also calculated to be 0.14 μM, according to the equation of LOD = 3σ/k. The LOD is less than 1.5 mg L⁻¹ of F⁻ anions, which is the WHO’s recommended limit for drinking water (1.5 mg L⁻¹ = 7.9 μM).⁴⁸ The LOD is also found to be much lower than most reported cases (Table S1†).^49–51

Fig. 7 PL spectral response of the CD–Al³⁺ ion (10 μM) solution towards F⁻ anions of various concentration (0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0 μM, respectively) in a 0.2 M NaOAc–HOAc buffer solution of pH 5.8, with an excitation wavelength of 400 nm (a), and the corresponding linear plot of lg(I/I₀) and F⁻ anion concentration (b). Note that the error bars were obtained from three parallel measurements; I₀ and I represent the maximum PL intensity of the CD–Al³⁺ ion solution in the absence and presence of 10 μM F⁻ anions, respectively.

The selectivity of the CD–Al³⁺ ion system towards F⁻ anions was evaluated by introducing a series of anions, such as SO₄²⁻, S₂O₃²⁻, CO₃²⁻, SCN⁻, S²⁻, Cl⁻, IO₃⁻, IO₄⁻ and Cr₂O₇²⁻ (the concentrations of F⁻, S²⁻ and CO₃²⁻ anions were 10 μM, while the concentrations of other tested anions were 100 μM) into the CD–Al³⁺ ion solution (Fig. 8). The results indicated that these tested anions did not significantly affect the PL intensity of the CD–Al³⁺ ion system. Moreover, experiments with coexisting interfering anions for the CD–Al³⁺ ion sensing system were further conducted. Under similar experimental conditions to the selectivity of this sensing system, both the 10 μM F⁻ anions and one of the tested anions of a certain concentration (the concentrations of S²⁻ and CO₃²⁻ anions were 10 μM, while the concentrations of SO₄²⁻, S₂O₃²⁻, SCN⁻, Cl⁻, IO₃⁻, IO₄⁻, and Cr₂O₇²⁻ anions were 100 μM) were mixed and added to the CD–Al³⁺ ion solution to obtain their PL spectra (Fig. 8). Experimental results further indicated the high selectivity of the established sensing system.

Fig. 8 The PL intensity ratio at 530 nm of the CD–Al³⁺ ion system containing one of the tested anions (the concentrations of S²⁻ and CO₃²⁻ anions were 10 μM, while the concentrations of other tested anions were 100 μM) in the absence (black) and presence (10 μM, red) of F⁻ anions. Note that the black column and red column of “Blank” refer to the maximum PL intensity of the CD–Al³⁺ system in the absence and presence of 10 μM F⁻ anions, respectively.

Conclusions

In conclusion, a green photoluminescent CD-based sensing system for Al³⁺ ions has been successfully developed, with a PL intensity enhancement response. This sensing system shows a high sensitivity and selectivity with a LOD of ca. 0.39 μM, which is much lower than most of the previously reported cases. This is an example of sensing metal ions by CDs with a PL enhancement response. Furthermore, the CD–Al³⁺ ion system could be employed to probe F⁻ anions based on a PL “on–off” model, with a LOD of 0.14 μM.

Acknowledgements

This work was supported by the Natural Science Foundation of China (No. 21275059, 21377124, and 21575044), the Natural Science Foundation of Fujian Province (No. 2015J01054, 2016J01062, 2013J01047 and 2014J01048).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19370f

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