Preparation and photoluminescent properties of magnetic Ni@SiO₂–CDs fluorescent nanocomposites

Abstract

Fluorescent Ni@SiO₂–CDs magnetic nanocomposite powder was prepared and its photoluminescence properties characterized. The photoluminescence properties are quite different in solid state and in ethanol solution.

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1. Introduction

Ever since carbon dots (CDs) were discovered in the purification of crude nanotubes in 2004,¹ these interesting luminescent nanoparticles have become increasingly popular because of their valuable qualities. Owing to their small size and tunable surface functionalities, they combine photostability, water solubility, biocompatibility and excellent cell membrane permeability.² Compared to luminescent semiconductor quantum dots, which are known for their toxicity and potential environmental hazards due to the contained heavy metals,^3,4 carbon based photoluminescent nanomaterials are environmentally and biologically compatible.⁵

When it comes to the synthetic aspects, a lot of methodologies to synthesize CDs have been reported, mainly including two types,“top-down” and “bottom-up” approaches.⁶ In the top-down methods CDs are prepared from larger carbon materials, like graphite, carbon nanotubes, carbon soot, and graphite oxide. In contrast for the bottom-up approach CDs are made from molecular precursors such as citric acid, glucose and resin.⁷

Magnetic nanoparticles are of increasing interest because of their special magnetic properties and high catalytic activity. Therefore, they have found various biomedical applications, in magnetic resonance imaging (MRI), bioseparation, targeted delivery, and stem cell labeling.⁸ Coating fluorescent CDs over magnetic nanoparticles provide even more attractive composite systems with magnetic, optical and biomedical applications. Among all these magnetic materials, nano-sized nickel remains one of the most important metals due to its potential applications, especially as catalysts, magnetic sensors, conducting materials and multilayered ceramic capacitors.⁹ The nickel nanoparticles were prepared by a modified polyol process,^10,11 and then, they were coated with silica shells by the Stöber method using tetraethyl orthosilicate (TEOS) as a silica source in water to yield Ni@SiO₂ shell particles.^12–18 In recent years, some research papers have focused on the syntheses, preparation of novel hybrids composed of carbon dots and inorganic nanoparticle cores (e.g. iron oxide, zinc oxide, silica, titania). Combining fluorescence carbon dots with metal nanoparticles that have magnetic, optical or mechanical properties can yield hybrid materials which combine magnetic and optical responses on a single platform.^19,20

In the present work, we report a Ni@SiO₂ shell nanoreactor framework as an excellent magnetic model system. After the Ni@SiO₂ magnetic nanoparticles were prepared via a sol–gel process, CDs were linked to the surface of Ni@SiO₂ in only one simple step. The synthesized Ni@SiO₂–CDs nanocomposites offer not only magnetic properties, but also optical properties. The photoluminescence properties were studied both in the solid sate and in ethanolic solution.

2. Experimental

2.1. Chemicals

BPEI (Branched Polyetherimide) with a molecular weight of 20 000 Da, 3-(triethoxysilyl)propyl isocyanate (96%), nickel(II) acetylacetonate (Ni(acac)₂, 99%), poly(vinylpyrrolidone) (PVP, M = 58.000), 1,5-pentanediol (PD, 98%) and tetraethyl orthosilicate (>96.0%) were purchased from TCI. Ammonium hydroxide (NH₄OH, 25% in water) and ethanol (>99.7%) were bought from Guangfu Reagent Company (Tianjin, China). All solvents and reagents were of analytical grade and directly used without further purification.

2.2. Instrument

XRD measurements were performed on a X-ray diffractometer (D/max-2400pc, Rigaku, Japan) with Cu Kα radiation (λ = 1.54178 Å), with operating voltage and current at 40 kV and 60 mA, respectively. The 2θ range was from 20 to 80° in steps of 0.02°. X-ray photoelectron spectra (XPS) were measured on a PHI-550 spectrometer using the Mg Kα radiation (hv = 1253.6 eV) photoemission spectroscopy with a base vacuum operated at 300 W. The transmission electron microscopy (TEM) was obtained on a JEM-2100 transmission electron microscope at an acceleration voltage of 120 kV. Samples were prepared by placing a drop of a dilute alcohol dispersion of the products on the surface of a copper grid. Fourier transform infrared (FTIR) spectra were conducted within the 4000–400 cm⁻¹ wavenumber range using a Nicolet 360 FTIR spectrometer with the KBr pellet technique. The magnetic behavior was investigated using a vibrating sample magnetometer (VSM, Lake Shore 7304, Lake Shore, USA) with an applied field between −11 kOe and 11 kOe at room temperature.

2.3. Steady-state UV–vis absorption and fluorescence spectroscopy

UV–vis absorption spectra were recorded on a Varian UV-Cary100 spectrophotometer, and for the corrected steady-state excitation and emission spectra, a FLS920 spectrofluorometer was employed. Freshly prepared samples in 1 cm quartz cells were used to perform all UV–vis absorption and emission measurements.

2.4. Time-resolved fluorescence spectroscopy

Fluorescence lifetimes were measured on an Edinburgh Instruments FLS920 equipped with different light emitting diodes (excitation wavelength 330 nm and 360 nm), using the time-correlated single photon counting technique^21,22 in 2048 channels at room temperature. The powder samples were measured by using a solid sample holder. For the solutions, the samples were dissolved in ethanol and the concentrations were adjusted to have optical densities at the excitation wavelength (330 or 360 nm) < 0.1. The monitored wavelengths were 420 nm, 440 nm, and 460 nm.

Histograms of the instrument response functions (using a LUDOX scatter) and sample decays were recorded until they typically reached 5.0 × 10³ counts in the peak channel. Obtained histograms were fitted as sums of the exponentials, using Gaussian-weighted nonlinear least squares fitting based on Marquardt–Levenberg minimization implemented in the software package of the instrument. The fitting parameters (decay times and preexponential factors) were determined by minimizing the reduced chi-square χ². An additional graphical method was used to judge the quality of the fit that included plots of surfaces (“carpets”) of the weighted residuals vs. channel number. All curve fittings presented here had χ² values <1.1.

2.5. Fluorescence quantum yield determination

Fluorescence quantum yields were determined by comparison of the integral of the emission bands with the one of quinine sulfate/0.5 M H₂SO₄ (Φ = 0.55).²² Typically, three absorption traces were recorded (and averaged) and three fluorescence emission traces, excited at 360 nm wavelengths. Three quantum yields were calculated and the mean value reported.

2.6. Synthesis of CDs

The CDs were synthesized according to a previous report.⁵ Briefly, 0.5 g BPEI and 1.0 g CA were dissolved uniformly with 10 mL hot water in a 25 mL beaker, and then heated moderately (<200 °C). About 20 min later, most water evaporated, remaining a uniform pale-yellow gel. Water (1.0 mL) was added before the gel was scorched, and the heating was continued for 3 h. Finally, the solution of the obtained CDs was adjusted to 10 mL using doubly distilled water. The resultant CDs solution was purified by silica gel column chromatography with 0.01 mol⁻¹ HCl solution as eluents. The purified CDs solution was stored under 4 °C before further study.

2.7. Synthesis of Ni magnetic nanoparticles (MNPs)

Ni magnetic nanoparticles were prepared according to a modification of the previously reported method.¹³ 1.0 g of Ni(acac)₂ and 5.40 g PVP were mixed in 50 mL of PD. The reaction mixture was slowly heated from room temperature to 473 K and, and stirring at 473 K was continued for 4 h. The resulting solution was heated to 513 K for 10 min and stirred at that temperature for 1 h. Then, the reaction mixture was cooled in an ice-water bath. The product was separated by centrifugation at 12 000 rpm for 20 min and was washed with ethanol by a repetitive dispersion/precipitation cycle in ethanol (100 mL) to remove excess poly(vinyl pyrrolidone).

2.8. Silica coating of Ni nanoparticles

Ni@SiO₂ nanospheres were synthesized according to a previous report.¹⁶ Typically, 25 mg of the prepared Ni MNPs were dispersed in a mixture of 40 mL ethanol and 10 mL of deionized water by ultrasonication for 15 min. Then, under continuous strong magnetic stirring, 1.5 mL of ammonia solution (25%) was added to the mixture, then 150 μL of TEOS was added drop wise. The reaction was allowed to proceed at 40 °C for 3 h. After the reaction was completed, the resulting black mixture was cooled to room temperature; the product was separated by centrifugation at 12 000 rpm for 15 min and was washed with ethanol three times and dried in air.

2.9. Synthesis of the Ni@SiO₂–CDs

30 mg of the Ni@SiO₂ was dispersed in 20 mL ethanol by ultrasonication for 15 min. Then, under continuous magnetic stirring, a solution (6 mL) of the prepared CDs was added to the mixture, then 1 mL TEOS and 1 mL 3-(triethoxysilyl)propyl isocyanate was added dropwise. The reaction was allowed to proceed at 60 °C for 4 h. After the reaction was completed, the resulting ivory-white mixture was cooled to room temperature. The product was separated by centrifugation at 12 000 rpm for 15 min and was washed with ethanol three times and dried in air.

3. Results and discussion

The Ni@SiO₂–CDs were synthesized via three steps shown in Scheme 1. The products of each step were characterized by TEM, XRD, XPS and FT-IR. The prepared Ni@SiO₂–CDs powder shows bright blue luminescence under excitation of a 365 nm UV lamp.

Scheme 1

As revealed by transmission electron microscopy (TEM), Fig. 1a demonstrates the prepared CDs have an average diameter of about 6 nm. The morphology of prepared nickel nanoparticles is irregularly spherical with an average diameter of about 40 nm (Fig. 1b). A selected area electron diffraction (SAED) pattern (Fig. S1†) shows two rings that related to the (111) plane and (220) plane. Through silica coating of nickel nanoparticles in the ethanol–ammonia mixture, the TEM image of Ni@SiO₂ nanoparticles revealed a smooth surface spherical structure and a clear core–shell structure with the average size of about 60 nm. However, when CDs were linked to the surface of Ni@SiO₂, it can be seen clearly that the surface of Ni@SiO₂–CDs was not smooth (Fig. 1c, d, e and f).

Fig. 1 TEM of CDs, Ni, Ni@SiO₂, Ni@SiO₂–CDs. (a) CDs; (b) nickel nanoparticles; (c and d) Ni@SiO₂; (e and f) Ni@SiO₂–CDs.

The X-ray diffraction (XRD) pattern is presented in Fig. S2.† Diffraction peaks at 44.6°, 51.9°, 76.4° are ascribed to the (111), (200), (220) of Ni in Ni magnetic nanoparticles (Fig. S2a†). There are two additional diffraction peaks at 33.2° and 59.4° which exist in the XRD pattern of Ni, we consider that they are the peaks of by-products NiO.²³ As shown in Fig. S2b,† the diffraction peak at 23.2° of Ni@SiO₂ is attributed to Si. XRD of Ni@SiO₂–CDs exhibits a typical pattern of nickel at 44.6°, 51.9° shown in Fig. S2c,† indicating that CDs have been linked to the Ni@SiO₂. The XPS analysis of the Ni@SiO₂–CDs, shown in Fig. S3,† reveals five atom peaks at 104.5, 287.4, 403.7, 534.5, 851.9 eV, attributed to Si, C, N, O, and Ni atoms, respectively. At room temperature, magnetic properties of Ni, Ni@SiO₂ and Ni@SiO₂–CDs nanostructures have also been studied (shown in Fig. 2). The saturation magnetization (M_s) of the Ni@SiO₂ was at 7.9 emu g⁻¹ less than that of the nickel nanoparticles (10.6 emu g⁻¹), which might be due to the decrease in the density of Ni in the obtained nanocomposites after coating of the SiO₂. However after immobilization of the CDs, the saturation magnetization (Ms) of the Ni@SiO₂–CDs is decreased clearly. The saturation magnetization (Ms) of the Ni@SiO₂–CDs was 1.4 emu g⁻¹. The lower value of Ni@SiO₂–CDs modified nanoparticles might be explained either by a more defected magnetic structure compared to the naked magnetic particles or by a diminished relative weight of the magnetic material in the sample.²⁴ On placing a magnet beside the solution of the Ni and Ni@SiO₂, the materials were quickly attracted to the side of the vial within a few seconds and the solution became transparent, which illustrated their magnetic nature. However, this phenomenon of the Ni@SiO₂–CDs was not observed clearly. The coercive force at 300 K is different due to their reduced size and dimensionality compared to bulk materials.¹²

Fig. 2 Magnetization curves of the Ni, Ni@SiO₂, Ni@SiO₂–CDs.

IR spectra of CDs, nickel nanoparticles, Ni@SiO₂, Ni@SiO₂–CDs are shown in Fig. S4.† The vibration bands at 463.21 and 798.09 cm⁻¹ of Ni@SiO₂ are ascribed to the Ni–O–Si vibrations and the characteristic absorption band of SiO₂. In the IR spectra of Ni@SiO₂–CDs, the vibration band at 3332.98 cm⁻¹ is attributed to δOH vibrations. Furthermore, for Ni@SiO₂–CDs, a new sharp peak appeared at 1578.04 cm⁻¹ indicating that the –NH₂ based on CDs should be coated at the surface of Ni@SiO₂. The vibration band at 1578.04 cm⁻¹ is assigned to the scissoring bending vibration of N–H.

Fig. S5† shows the UV/vis absorption spectra of CDs in ethanol solution. The absorption spectrum in ethanol exhibits two bands at 255 nm and 360 nm, shown in Fig. S5a.† The absorption spectrum is similar to that of the CDs reported in the literature.^6,20,25

It has been reported that the fluorescence emission spectra and photoluminescent intensity of CDs depend on the excitation wavelength.^6,22,25 The photoluminescent properties of Ni@SiO₂–CDs were examined in the solid state and in ethanolic solution at several excitation wavelengths shown in Fig. 3. It can be seen that, the photoluminescent emission spectra and photoluminescent intensity of Ni@SiO₂–CDs in ethanol are strongly excitation wavelength dependent. The emission bands are bathochromically shifted with an increase of the excitation wavelength, which is shown clearly in Fig. 3a. The strongest PL emission band with a maximum at 437 nm was observed with an excitation wavelength of 360 nm. Further increase of the excitation wavelength at λ_ex > 360 nm results in the decrease of the photoluminescent intensity, in accordance with the typical photoluminescent features of CDs. The mechanism of the PL behavior of carbon dots is very complicated and has not yet been clearly reported. The plausible reasons for the PL behavior are the presence of different particle sizes and the distribution of the different surface energy traps of the carbon dots .²⁵ The difference in the position of emission peak is due to the variation in size of the carbon dots. The intensity of the PL depends on the number of particles excited at a particular wavelength.

Fig. 3 (a) Fluorescence emission spectra of Ni@SiO₂–CDs in ethanol. (b) Fluorescence emission spectra of Ni@SiO₂–CDs in the solid state.

Owing to their small size, the as-prepared nano CDs are normally gel-like. To the best of our knowledge, the photoluminescence properties in the solid state have not been reported yet. Unlike the photoluminescent intensity that is strongly excitation wavelength dependent in ethanol, the photoluminescent intensity in the solid state is almost the same with increasing excitation wavelength even at λ_ex > 360 nm. The red shift of the emission bands in the solid state is not clearly seen in Fig. 3b. This finding may be due to different particle sizes of CDs in ethanolic solution and in the solid state. The quantum yields (ϕ_f) of the as-synthesized CDs and Ni@SiO₂–CDs are about 0.09 ± 0.01 and 0.25 ± 0.05, respectively.

The fluorescence decays for the Ni@SiO₂–CDs and CDs in the solid state and in ethanol were studied in order to gain some insight into the excited state properties shown in Fig. 4 and Fig. S6–S10 and Table S1.†

Fig. 4 The decay curves of Ni@SiO₂–CDs in the solid state and in ethanol solution collected at 440 nm when excited at 360 nm.

The fluorescence decay of Ni@SiO₂–CDs in the solid state at λ_ex = 330 nm can be described as bi-exponential function with the contributions of the τ₁ (∼3.0 ns) and τ₂ (∼9.0 ns) components ∼25% and ∼75%, respectively. Unlike the steady-state photoluminescent spectra of Ni@SiO₂–CDs which depend on the excitation wavelength. It is interesting to note that the time constants τ₁ (∼3.0 ns, 25%) and τ₂ (∼9.0 ns, 75%) obtained at the excitation wavelength 360 nm are almost the same as those obtained at 330 nm.

In ethanol, at the excitation of λ_ex = 330 nm, the fluorescence decays for Ni@SiO₂–CDs clearly show tri-exponential behavior at . The fast component (τ₁ ≈ 1.0 ns) has an amplitude of about 17%, whereas the contributions of the τ₂ (∼5.0 ns) and τ₃ (∼11.0 ns) components are about 50% and 33%, respectively. The change of the excitation wavelength to 360 nm does not induce a change in the fluorescence decay. The decay times are similar to those obtained at 330 nm. The fluorescence decays of Ni@SiO₂–CDs in ethanol are complicated probably due to the involvement of different particle sizes of Ni@SiO₂–CDs in solution and in the solid state. In ethanol, the decay has at least three different particle sizes.

For CDs in ethanol, a tri-exponential function (∼1.0 ns, ∼5.0 ns and ∼13.0 ns) was used to fit the decay at all three emission wavelengths. The decay times are similar to those reported in the literature.²⁶ The shorter decay time (τ₁ ≈ 1.0 ns) has the lowest contribution (<5%) when excited at 330 nm.

In general, the decay profiles are complicated due to the involvement of at least two different emitting electronic states, and the likely presence of different particle sizes of CDs in Ni@SiO₂–CDs samples. The photophysical results are in accordance with the steady state measurements.

Conclusions

In summary, we have synthesized Ni@SiO₂–CDs nanocomposites, with the CDs successfully decorated on the surface of Ni@SiO₂ magnetic nanoparticles. Carbon dots have recently attracted much attention because of their wide applications in many fields. However, there are only a few studies on the preparation of carbon dots-based composites with magnetic and photoluminescence properties. From the experimental results, the significant contribution of the newly prepared nanoparticles is that they offer not only magnetic properties but also optical properties. Moreover, to the best of our knowledge, the luminescent properties of carbon dots were determined in solution (ethanol or water et al.). In this contribution, we reported the photoluminescent properties of Ni@SiO₂–CDs not only in solution but also in the solid state. Such multifunctional nanoparticles should have great potential for nanoparticle-based diagnostic and therapeutic applications.

Acknowledgements

This work was supported by the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (Grant no. J1103307) and the “International Cooperation Program of Gansu Province” (1104WCGA182). The authors would like to thank the Natural Science Foundation of China (no. 21271094), and this study was supported in part by the “Key Program of National Natural Science Foundation of China” (20931003).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45310c

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