Highly photoluminescent nitrogen-rich carbon dots from melamine and citric acid for the selective detection of iron(III) ion

Abstract

A facile one-step hydrothermal strategy to fabricate nitrogen-rich carbon dots (N-rich CDs) with melamine and citric acid as the precursors has been successfully developed. The as-prepared N-rich CDs exhibit a uniform quasi-spherical morphology, high N content of about 24 wt% and very strong photoluminescence (PL) with a high quantum yield of ∼42%. Additionally, the PL emission of these N-rich CDs can be stabilized in a wide pH range from 4.0 to 11.0. More importantly, the N-rich CDs can be utilized as the label-free PL sensors for the highly selective and sensitive detection of Fe³⁺ ions in aqueous solution with a very low detection limit of 142 nM.

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Introduction

In recent years, photoluminescent carbon dots (CDs) have attracted considerable attention due to their alluring optical properties, high chemical stability, good biocompatibility as well as low toxicity, which have been proposed for various potential applications, such as energy-efficient displays and lighting, photovoltaic devices, bioimaging, drug delivery, medical diagnosis, catalysis, and environmental detection, etc.^1–5 Basically, the photoluminescence (PL) behaviors of CDs are related to their sizes, morphologies, compositions and crystalline degrees, which are decided by both the starting materials and the fabrication methods.^6–8 Especially, it is widely accepted that the doping of nitrogen (N) atoms in CDs provides the opportunities to adjust the surface features, the PL behaviors, as well as the electrochemical performance of the resulting CDs.^3,9–11

Due to these reasons, various synthesis methods have been developed to fabricate N-doped CDs, which generally can be categorized as two types: one is treating the as-made CDs with N sources such as ammonia and hydrazine.^12,13 Naturally, multiple steps are involved in such strategies, which will inevitably reduce the controllability over the compositions and the morphologies of the resulting N-doped CDs. In contrast, the other route is to directly utilize N containing molecules as the precursors of CDs, which thus provide more options to adjust the structure and property of the N-doped CDs.^14–17 For example, Zhu and co-workers found that the PL quantum yields (QYs) of CDs can be over 10% if amino molecule was used in the fabrication process. In contrast, the PL QYs of the prepared CDs are less than 10% if the reactant only contained –OH and –COOH groups.¹⁵ Qian et al. also attributed the improvement of the QYs of their CDs to the introduction of nitrogen atoms.¹⁷ Therefore, it is important to prepare unprecedented N-rich CDs for the further modification on the PL behaviors of CDs and the understanding of the relationship between the composition of CDs and their optical properties.

As a typical organic base, melamine, the trimer of cyanamide with a 1,3,5-triazine skeleton, is an ideal precursors for N-doped carbon materials since it contains 67% N atoms by mass and the three amine groups on it can easily react with other functional groups such as carboxyl, hydroxyl and ketone. For instance, Wang et al. obtained N containing CDs with a QY up to 22% by the thermal treatment of melamine and glycerol with the presence of acidic catalysts.¹⁸ In another case, Li and co-workers produced N-doped CDs via a hydrothermal method using melamine and glutaraldehyde. With the N/C atomic ratio of ca. 20.7%, the N-doped CDs manifested a very high QY of ∼31%.¹⁹

Herein, we report a facile one-pot approach towards highly photoluminescent N-rich CDs by the hydrothermal treatment of melamine and citric acid. The obtained CDs possess a uniform quasi-spherical morphology with a diameter of ∼4 nm and a height of ∼1.5 nm. More importantly, these CDs have a high N content of ∼24 wt% and excellent PL properties with a very high QY of ∼42%, which are superior to the previously reported N-doped CDs from melamine.^18,19 Furthermore, the PL of the N-rich CDs can be retained in a wide pH range from 4.0 to 11.0. Impressively, as the label-free PL sensors for the detection of Fe³⁺ in aqueous solution, the N-rich CDs exhibit outstanding selectivity and sensitivity with a very low detection limit of 142 nM, suggesting their appealing potential in chemical sensing and bioimaging.

Experimental part

Chemicals

Melamine and citric acid were purchased from Shanghai Chemical Corp. The other chemicals used in this work were purchased from Aladdin Reagent Co. Ltd. Unless otherwise stated, all the reagents were of analytical grade and used as received without further purification. The water used throughout all experiments was purified through a Millipore system.

Preparation of N-rich CDs

The N-rich CDs were prepared by the hydrothermal treatment of melamine and citric acid. In a typical procedure, melamine (1.26 g) and citric acid monohydrate (2.10 g) was dissolved in water (40 °C, 50 ml). Then the solution of the mixture was transferred to a Teflon-lined autoclave (100 ml) and thermally treated for 12 h (at 220, 240 or 260 °C). After cooling to room temperature, the light yellow suspension of N-rich CDs was obtained. The large particles in the suspension were removed by centrifugation (10 000 rpm for 10 min) and then vacuum filtrated through a microporous membrane (0.22 μm). The resulting suspension was further dialyzed against Milli-Q water with a cellulose ester membrane bag (M_w = 3500). The N-rich CDs were then concentrated by freeze-drying under vacuum and obtained as brown powders with the yields of ca. 34%. According to the different hydrothermal treatment temperatures (220, 240, and 260 °C), the obtained N-rich CDs were named as NCDs-220, NCDs-240, and NCDs-260, respectively.

Characterization

High-resolution transmission electron microscopy (HRTEM) was performed on JEM-2010F (JEOL, Japan) at operating voltage of 200 kV. The sample was dispersed in water and then drop-casting on carbon-coated copper grid. For atomic force microscopy (AFM) characterization, the aqueous solution of the N-rich CDs (2 × 10⁻⁵ mg ml⁻¹) was spotted onto freshly cleaved mica surface and dried in air. The samples were measured in air on Multimode NanoScope IIIa (Bruker, Germany). X-ray diffraction (XRD) patterns were recorded with Bruker AXS D8 Advance X-ray diffraction instrument using Cu-Kα irradiation. Fourier transform infrared spectroscopy (FTIR) spectra was measured using Avatar 370 (Thermo Nicolet Corporation, USA). The elemental analysis was measured on EA3000 (Dalton International, Italy). UV/Vis spectra were recorded at room temperature on a U3010 spectrophotometer (Techcomp LTD, Japan). Fluorescence spectra were measured with a F-70000 spectrometer (Hitachi, Japan).

Quantum yield (QY) measurements

The QY of the N-rich CDs was determined by using quinine sulfate (QY = 54% in 0.1 M H₂SO₄) as the standard sample and calculated according to the following equation:where Q is the QY of the samples, I is the measured integrated emission intensity, n is the refractive index, and E is the optical density, which is measured on a UV/Vis spectrophotometer. The subscript “R” refers to the standard values from quinine sulfate with the known QY.

Detection of Fe(III) ions

The dose-dependent response of the N-rich CDs to Fe³⁺ was investigated at room temperature. In a typical procedure, the suspension of the N-rich CDs (1 ml, 50 mg ml⁻¹) was first diluted with water (199 ml). The aqueous solutions of Fe³⁺ with other metal ions were freshly prepared before use. Upon the addition of various amount of Fe³⁺, the emission intensity of the N-rich CDs at 390 nm was recorded under 320 nm excitation. To evaluate the sensitivity of the N-rich CDs towards Fe³⁺, the aqueous solutions of Fe³⁺ at different concentrations (0–1000 μM) were added into the aqueous solution containing the same amount of the N-rich CDs and the mixed solutions were equilibrated for 5 min before spectral measurements.

The performance of the N-rich CDs for Fe³⁺ detection was also examined with environmental water samples in this work. The water samples were obtained from Nanchen River (Shanghai, China), which were filtered through a 0.20 μm filtered membrane before usage.

pH sensing capability

Detection of pH is critically important because it plays a vital role in many systems, especially in the chemical sensing, bioimaging and nanomedicine. The pH sensing capability of the aqueous solution of the N-rich CDs was studied under 320 nm excitation at room temperature for a wide range from pH = 1 to 13, which were adjusted by adding the aqueous solution of HCl or NaOH.²⁰

Results and discussion

The fabrication strategy of the N-rich CDs in this work is briefly illustrated in Fig. 1. The hydrothermal treatment of melamine and citric acid monohydrate with equivalent molar amount at various temperatures (220, 240, and 260 °C) can produce the N-rich CDs as products. According to the hydrothermal temperatures, the obtained N-rich CDs were denoted as NCDs-220, NCDs-240, and NCDs-260, respectively. It should be stressed that citric acid is a very popular candidate to fabricate CDs due to its ability to form carbonaceous materials during thermal treatment.^19,21–23 Compared with the hydroxyl groups in glycerol and aldehyde groups in glutaraldehyde, the carboxyl groups of citric acid are more easily to react with the amines in melamine.^18,19 The condensation between melamine and citric acid enable the formation of a carbon–nitrogen framework with excellent stability, which finally generate the N-rich CDs in high yields.

Fig. 1 Schematic illustration for the hydrothermal synthesis of the highly photoluminescent N-rich CDs with melamine and citric acid as starting materials.

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) were first carried out to investigate the morphology of the N-rich CDs. As indicated by their TEM images (Fig. 2a–c), the sizes of the three N-rich CDs are ranging from 2.0 to 5.5 nm. Based on the statistical analysis of 100 particles, the average diameters of NCDs-220, NCDs-240, and NCDs-260 were 3.84, 3.70, and 3.58 nm, respectively (Fig. S1†), which show a gradually decreasing trend with the rising hydrothermal temperatures. Moreover, the high-resolution TEM (HRTEM) images indicate that all of the three N-rich CDs have highly crystallized frameworks with the lattice fringe of ∼0.21 nm (Fig. 2d–f), which corresponds to the (100) facets of graphite.^1,8 It is interesting that the heights of NCDs-220, NCDs-240, and NCDs-260 from the AFM analysis are 1.62, 1.49, and 1.47 nm, respectively (Fig. S2†), implying that these N-rich CDs have a quasi-spherical morphology.

Fig. 2 TEM images of (a) NCDs-220, (b) NCDs-240, and (c) NCDs-260; HRTEM images of (d) NCDs-220, (e) NCDs-240, and (f) NCDs-260.

The chemical composition of the N-rich CDs was then studied by elemental analysis (EA) and the results are listed in Table 1. It can be found that the contents of N atoms in NCDs-220, NCDs-240, and NCDs-260 are 24.25, 22.40 and 19.57 wt%, respectively, which are higher than those of the previously reported N-doped CDs with melamine as N source.^18,19 The obvious decrease in the weight contents of N atoms in the samples indicates the escaping of N atoms with the increasing of the hydrothermal temperatures.

Table 1 The summary of element analysis results and QYs of the N-rich CDs

Samples	C (%)	H (%)	N (%)	O^a (%)	N/C (%)	QY (%)
a Calculated values.
NCDs-220	33.49	3.49	24.25	38.77	72.4	37.1
NCDs-240	30.58	3.53	22.40	43.60	73.3	41.7
NCDs-260	34.53	3.50	19.57	42.39	56.7	35.1

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X-ray diffraction (XRD) was applied to illustrate the microstructures of the N-rich CDs. In their XRD patterns (Fig. S3†), the three N-rich CDs show similar broad diffraction peaks with the 2θ value of ∼25°, corresponding to the (002) diffraction plane of graphite, which is in accordance with the TEM results. The role of melamine in the fabrication of the N-rich CDs was further explored with Fourier transform infrared spectroscopy (FTIR) measurements. As indicated in the FTIR spectra of NCDs-240 (Fig. S4a†), the broad absorption peaks between 3000 and 3500 cm⁻¹ are assigned to the stretching vibrations of –NH₂ and –OH, which are inherited from melamine and citric acid, respectively.^24,25 The typical peaks at 1570 and 1680 cm⁻¹ are associated with the bending vibration of –CONH–. The peak at 1400 cm⁻¹ is related to the stretching band of C–N (amine III).²⁵ These results confirm that melamine joins the hydrothermal carbonization process as one key component to generate the CDs with high content of N atoms. On the other hand, the three N-rich CDs manifest similar absorption bands in their FTIR spectra, implying that the temperatures of the hydrothermal treatment have no obvious influences on the types of their functional groups (Fig. S4b†).

The uniform quasi-spherical morphology and high N content of the N-rich CDs inspire us to further investigate their optical properties. In the UV/Vis absorption spectra of the N-rich CDs (Fig. 3a), two absorption bands locating at 270 and 320 nm can be found, which are related to C=N and C=O bonds respectively.¹⁴ The N-rich CDs manifest a graduate increasing adsorption at 270 and 320 nm with the elevated temperature, indicating the raised contents of C=N and C=O bonds in N-rich CDs obtained in higher temperature.²⁵ Besides, the N-rich CDs can give off strong PL spectra under irradiation. Along with the changed excitation wavelength from 260 to 380 nm, the emission peaks of the samples red-shift from 380 to 450 nm (Fig. 3b, S5a and b†). All the emission spectra of the three N-rich CDs exhibit similar excitation-dependent PL behavior, which might be due to optical selection of the surface functional groups in the N-rich CDs, consistent to the previously reported photoluminescent carbon nanomaterials.^3,26–30 Furthermore, the PL spectra shows that the maximum emission at ∼390 nm is obtained by excitation at 320 nm, implying that these N-rich CDs obtained at different hydrothermal temperatures have similar fluorescent centers.²⁶ As shown in Fig. 3c, the normalized emission spectra of NCDs-220, NCDs-240, NCDs-260 demonstrate a gradually red-shift from 387 to 392 nm, which might be induced by the enhanced degree of π conjugation and the decreased band gaps in the N-rich CDs with the increased temperature.^31,32 Additionally, the QYs of the N-rich CDs at 320 nm excitation are calculated to be 37.1, 41.7, 35.1% for NCDs-220, NCDs-240, NCDs-260, respectively (Table 1), which is superior to most of the N-doped CDs.^{4,15,16,22,33} Additionally, the N-rich CDs stored for one year still manifest almost identical emission spectra as the freshly prepared samples, indicating the high photo-stability of the N-rich CDs (Fig. S6†).

Fig. 3 (a) UV/Vis absorption spectra of the N-rich CDs; (b) PL emission spectra of NCDs-240 with progressively increased excitation wavelengths from 260 to 380 nm with 20 nm increment. Inset of (b) is the optical image of NCDs-240 suspensions with strong blue luminescence under the excitation at 365 nm; (c) normalized emission spectra of N-rich CDs under the maximum excitation wavelength of 320 nm; (d) PL intensity of NCDs-240 in the solutions with different pH values.

Furthermore, the PL sensing capacity of NCDs-240 was investigated for a wide range of pH values under 320 nm excitation at 25 °C (Fig. 3d). Interestingly, NCDs-240 shows almost unchanged PL intensities over a wide pH range (pH 4.0–11.0) with a maximum activity at pH 5.0, implying the good PL stability of NCDs-240. Even at pH 1.0 and 13.0, the intensities of its PL spectra still maintain 74.7 and 46.7% of the maximum value. The pH-dependent PL property of NCDs-240 may be attributed to the protonation and deprotonation of the amine and carboxylic functional groups at their surfaces.^9,34,35 The above results suggest that NCDs-240 has potential as PL sensors for pH measurement, especially in the physiological pH range.

In view of the excellent optical properties, the applications of NCDs-240 as the label-free fluorescent sensor for Fe³⁺ ions was further explored in this work. To evaluate the selectivity of the sensing system, the PL intensities of NCDs-240 under the excitation at 320 nm in the presence of representative metal ions were subsequently recorded, including Ag⁺, K⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Mg²⁺, Pb²⁺, Fe³⁺, and Al³⁺ (Fig. 4a). It is found that no tremendous decrease is observed by addition of other metal ions into the dispersion of NCDs-240 except Fe³⁺ (Fig. 4a). In comparison with other ions, the high selectivity of NCDs-240 for Fe³⁺ may be owing to the faster chelating processing of Fe³⁺ ions with the N containing functional groups in NCDs-240.^4,11 For the sensitivity study to Fe³⁺, the PL spectra of NCDs-240 with the addition of Fe³⁺ at different concentrations (0–1000 μM) were measured and compared. A gradual decrease in the PL intensity at 390 nm with the increasing of Fe³⁺ concentration can be easily found (Fig. 4b), revealing that Fe³⁺ ions can effectively quench the PL of the N-rich CDs. The dependence of F/F₀ on the concentrations of Fe³⁺ ions is further concluded in Fig. 4c, where F₀ and F represent the PL intensities of NCDs-240 at 390 nm in the absence and presence of Fe³⁺ (excitation at 320 nm), respectively. It is found that the resultant plot fits a linear equation over the concentration range of 0–100 μM (Fig. 4d). However, the plot cannot fit a linear equation over the whole concentration range of 0–1000 μM, indicating the N-rich CDs based sensing system involves both dynamic and static quenching processes.³⁶ It is notable that the quenching efficiency can be fitted to the Stern–Volmer equation:^37,38

where K_SV is the Stern–Volmer quenching constant, c is the concentration of Fe³⁺. Accordingly, the fitted Stern–Volmer equation is assigned as follow: F/F₀ = 0.99821 − 0.00585c[Fe³⁺] (R² = 0.993) (the unit of c[Fe³⁺] is μM), and the detection limit is estimated to be 142 nM at a signal-to-noise ratio of 3. Based on the above results, the PL quenching behavior of NCDs-240 with the presence of Fe³⁺ should be owing to the changed surface states of the N-rich CDs.³⁰ Upon the addition of Fe³⁺ ions, the strong coordination/chelation interactions between the N and O containing groups of NCDs-240 and Fe³⁺ will cause the non-radiative electron-transfer or energy transfer, which thus leads to the substantial PL quenching.^29,39

Fig. 4 (a) The difference in the relative PL intensity of N-rich CDs dispersion between the blank and solutions containing different metal ions (excitation at 320 nm; [M⁺] = 500 μM). (b) The PL spectra of NCDs-240 in the presence of different concentrations of Fe³⁺ (0–1000 μM). (c) The relative PL intensity of NCDs-240 versus the concentration of Fe³⁺. (d) The linear relationship between the F/F₀ and Fe³⁺ concentration (0–100 μM).

The sensing behavior of NCDs-240 on Fe³⁺ in environmental water sample was also explored in this work. Similar to the observation in Milli-Q water, the PL intensities of NCDs-240 in environmental water show a trend of decreasing with the increase of the concentration of Fe³⁺ ranging from 0–1000 μM (Fig. S7a and b†). Additionally, the linear relationship between the PL intensities of NCDs-240 and the concentrations of Fe³⁺ ions from 0–100 μM can also be found in environmental water (F/F₀ = 0.98088 − 0.00333c[Fe³⁺]; R² = 0.986) (Fig. S7c†).

Conclusions

In this work, we have successfully developed a hydrothermal strategy to fabricate N-rich CDs by using melamine and citric acid as the precursors. The as-prepared N-rich CDs exhibit uniform quasi-spherical morphology, high N content and outstanding PL performance. As the label-free PL sensors for the detection of Fe³⁺ ions, the N-rich CDs show high sensitivity with a very low detection limit of 142 nM. More importantly, our synthesis strategy towards N-rich CDs is expected to be further applied in the fabrication of other N containing carbon nanomaterials with excellent optical and electronic properties.

Acknowledgements

This work was financially supported by 973 Program of China (2013CB328804 and 2014CB239701), Natural Science Foundation of China (61235007, 61575121, 21572132 and 21372155), Professor of Special Appointment at Shanghai Institutions of Higher Learning (Eastern Scholar), and Aeronautical Science Foundation of China (2015ZF57016). The authors also acknowledge instrument analysis center of Shanghai Jiao Tong University for material characterization.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26521e

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