Rhodamine-Triazole Functionalized Fe₃O₄@SiO₂ Nanoparticles as Fluorescent Sensors for Heavy Metal Ions

Introduction

Contamination of water sources by heavy metal ions (e.g. Co²⁺, Cu²⁺, Fe³⁺, Hg²⁺, Ni²⁺ and Pb²⁺)is a major environmental concern. In addition to their toxicity, accumulation of these in living organisms can have profound impacts on the overall ecology of environmental systems. Accordingly, great effort is being extended into devising ways of removing such pollutants, and in the development of new analytical tools for their detection. Although conventional analytical techniques such as atomic absorption spectrometry (AAS),¹ flame atomic absorption spectroscopy (FAAS),² inductively coupled plasma mass spectrometry (ICP-MS)³ and voltammetry⁴ are typically used in metal ion detection, these have drawbacks in requiring expensive and sophisticated hardware, long analysis time, and a high level of technical expertise. Fluorescence spectroscopy, on the other hand, is an emerging technique for metal ion detection in environmental science due to its high selectivity, rapid response allowing real-time detection for potential field use, and low cost.⁵

Magnetic nanoparticles, which exhibit large surface area to volume ratios, and are readily separated from liquid phases by application of an external magnetic field, have been extensively studied over the last decade. Conjugation of organic groups on the nanoparticle surface allows these systems to be utilized in analytical applications, such as metal ion detection. For example, amino-functionalized Fe₃O₄@SiO₂ magnetic nanomaterials were developed as sorbents for the toxic metal ions including Cu²⁺, Pb²⁺, and Cd²⁺ ions.^6–9 In addition, thiol-functionalized Fe₃O₄@SiO₂ nanoparticles were synthesized for Pb²⁺ and Hg²⁺ sequestration from water samples.^10–13 Appending fluorescent groups (dansyl, naphthalimide, Nile red, or Bodipy) to the nanoparticle surface allows the potential of fluorescence as a detection method to be built into the nanoparticle system. As an example, Ma’s group developed reusable dansyl-functionalized Fe₃O₄@SiO₂ nanoparticles for Hg detection, and subsequent removal from water samples.¹⁴ In this case, the fluorescence intensity of nanoparticles was quenched in the presence of Hg²⁺, with the limit of detection being 10 mM in HEPES buffer containing 50% (v/v) CH₃CN/H₂O. In 2013, “Turn-off” naphthalimide functionalized Fe₃O₄@SiO₂ nanoparticles were observed to aggregate in the presence of Hg²⁺, forming imide-Hg-imide complexes in aqueous solution.¹⁵ In another development, Zhu’s group designed thioether-crown conjugated naphthalimide modified Fe₃O₄@SiO₂ nanoparticles via click chemistry for Hg²⁺ sensing.¹⁶ Recently, magnetic nanoparticles appended with 4-acetamidobenzaldehyde functionalities were able to detect Hg²⁺ at the nanomolar leve.l¹⁷ Meanwhile, dipicolylamine-naphthalimide conjugated magnetic nanoparticles exhibited fluorescence emission allowing feasible Zn²⁺ detection in environmental water samples.¹⁸ In the case of Co²⁺ ion, chelation by Nile red-functionalities on the surface of Fe₃O₄@SiO₂ nanoparticles leads to fluorescence emission, through the inhibition of photoinduced electron transfer (PET).¹⁴

Rhodamine derivatives have been widely reported as fluorescent entities due to their long absorption and emission wavelengths, large molar extinction coefficients, high fluorescence quantum yields and appreciable photostability.¹⁹ In these systems, rhodamine having the closed-form of the spirolactam ring is colorless and shows no fluorescence emission, while the solutions of the open-form of rhodamine exhibit a pink color and fluorescence emission. As a consequence, rhodamine has been appended to the surface of magnetic nanoparticles to form sensors for Fe³⁺ and Hg²⁺. In 2010, rhodamine 6G-ethylenediamine was conjugated to the surface of Fe₃O₄@SiO₂ nanoparticles via a polyethylene linker, and they exhibited fluorescence emissions in the presence of Fe³⁺ ion.²⁰ Substituting the polyethylene linker in the previous system by an isocyanate resulted in the selectivity of the sensor with the binding of Hg²⁺, Cr³⁺ and Fe³⁺ ions.^21,22 In the context of Hg²⁺ removal, rhodamine hydrazide was grafting onto the magnetic nanoparticle surface via 3-glycidyloxy moiety,²³ or chloroacetyl aminopropane linkers.²⁴ From the previous literatures, rhodamine-based fluorescent sensor immobilized magnetic nanoparticles have not been widely developed for heavy metal ion detection.

This work reports the synthesis of a rhodamine-triazole sensor appended Fe₃O₄@SiO₂ nanoparticles for the detection of heavy metal ions (Cu²⁺, Ni²⁺, Hg²⁺, Co²⁺, Fe³⁺ and Pb²⁺). These functionalized nanoparticles could be easily removed from the solution by applying an external magnetic field. Additionally, the 1,2,3-triazole moiety plays several important roles, being both the metal chelator and the linker between the fluorescence reporter and the metal ion binding site.²⁵ The selectivity and sensitivity of the sensor system, and a rationale for the sensing mechanism, are also reported herein.

Materials and Method

Materials and Instruments

All chemicals and reagents of standard analytical grade were purchased from commercial suppliers and used without further purification. All solvents for column chromatography were distilled before use. Acetate salts of the following metal ions: Co²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, Ni²⁺ and Pb²⁺ were prepared in deionized water as stock solutions (10 mM).

¹H and ¹³C NMR spectra were recorded using an Advance Bruker-600AV spectrometer in CDCl₃, with tetramethylsilane (TMS) as an internal reference. Fourier transform infrared (FT-IR) spectra were collected on a Perkin Elemer Spectrum two spectrometer using KBr pellets in the 4000-370 cm^–1 region. High resolution mass spectra (HRMS) were obtained using a Bruker Micr OTOF mass spectrometer. Scanning electron microscope (SEM) images were obtained using a Jeol JAM-7610F with normal mode and EDS mode. Transmission electron microscopy (TEM) images were obtained using a Jeol JEM-2100Plus Electron Microscope. X-ray diffraction studies (XRD) were performed on a Bruker d8 Venture powder diffractometer (2theta 20 to 80°) with Cu Kα radiation. Thermogravimetric analysis (TGA) utilized a Linseis STA PT1600 instrument. UV-Vis absorption spectra were measured on a Hitachi U-2900/2910 UV-Visible spectrophotometer. Fluorescence spectra were performed on an Edinburgh Fluorescence Lifetime Spectrometer (FLS).

Synthesis of Silica–Coated Magnetic Nanoparticles (Fe₃O₄@SiO₂)

The magnetic nanoparticles were synthesized according to a previously reported procedure.²⁶ Briefly, a solution of Fe(acac)₃ (5.648 g, 0.016 mol), benzyl ether (80 mL) and oleylamine (80 mL) was heated to 110°C and allowed to reflux for 1 hour with vigorous stirring. The temperature was raised to 210°C and then refluxed for 2 hours under an argon atmosphere. After cooling to room temperature, the reaction mixture was centrifuged at 10,000 rpm for 20 min, affording the magnetic nanoparticles (Fe₃O₄) as a black solid. The magnetic nanoparticles were then dispersed in a mixture of 1-propanol (180 mL), conc. NH₄OH (25 mL) and deionized water (18 mL) in a 500 mL round-bottom flask by ultra-sonication for 30 min under an argon atmosphere. Following this tetraethylorthosilicate (TEOS) (6 mL, 0.027 mol) was added dropwise for 30 min with vigorous stirring. After stirring at room temperature for 6 hours, the silica-coated magnetic nanoparticles (Fe₃O₄@SiO₂) were separated from the solution using an external magnet, and then washed twice successively with 1-propanol and deionized water, respectively. The Fe₃O₄@SiO₂nanoparticles (brown solid) were then dried under vacuum at 60°C for 6 hours, prior to use.

Synthesis of 3–azidopropyl triethoxysilane

3-Chloropropyl triethoxysilane (5 mL, 0.021 mol) and sodium azide (NaN₃) (2.6998 g, 0.042 mol, 2 equiv.) were dissolved in DMF (40 mL), and the mixture heated at 90°C for 6 hours. After that, the mixture was filtered, and the filtrate was evaporated under reduced pressure to provide the product as a colorless liquid in 90% yield. ¹H NMR (600 MHz, CDCl₃) δ(ppm): 3.80 (d, J = 6 Hz, 6H), 3.24 (d, J = 6 Hz, 2H), 1.70 (d, J = 6 Hz, 2H), 1.21 (t, J = 6 Hz, 9H), 0.66 (t, J = 6 Hz, 2H); ¹³C NMR (150 MHz, CDCl₃) d: 58.5, 53.9, 22.8, 18.3, 7.7.

Synthesis of Rhodamine–n–Propargyl Alkyne

Rhodamine-N-propargyl alkyne was prepared according to a previously reported procedure.²⁷ The crude product was purified by silica column chromatography with 1% CH₃OH/CH₂Cl₂ system as a mobile phase to obtain a light brown solid in 75% yield. ¹H NMR (600 MHz, CDCl₃) δ(ppm): 7.91 (d, J = 6 Hz, 1H), 7.45 (s, 2H), 7.1 (d, J = 6 Hz, 1H), 6.41-6.46 (m, 4H), 6.27 (d, J = 8.4 Hz, 4H), 4.57 (s, 1H), 3.32 (t, J = 6.9 Hz, 10H), 2.10 (s, 1H), 1.16 (t, J = 6.6 Hz, 12H); ¹³C NMR (150 MHz, CDCl₃) d: 166.64, 153.70, 151.78, 149.52, 148.84, 132.75, 129.83, 128.54, 128.51, 128.10, 123.98, 122.89, 122.86, 109.74, 107.86, 105.65, 100.07, 98.46, 80.10, 72.42, 65.43, 45.53, 44.44, 44.34, 40.42, 12.63.

Synthesis of Chemosensor–Functionalized Magnetic Nanoparticles (RBT-Fe₃O₄@SiO₂)

The Fe₃O₄@SiO₂nanoparticles (1.5 g) were suspended in toluene (80 mL) under an argon atmosphere, and then sonicated for 30 min. 3-Azidopropyl triethoxysilane (1.5 g, 6.064 mmol) was slowly added dropwise into the mixture for 15 min, and the mixture was then heated at reflux for 12 hours. The Fe₃O₄@SiO₂-N₃nanoparticles were then separated by applying an external magnet, and then washed three times successively with toluene, 1-propanol and deionized water (30 mL), respectively. The Fe₃O₄@SiO₂-N₃nanoparticles were then dispersed in acetonitrile (50 mL) with the sonication for 30 min, while maintaining an argon atmosphere. In a separate flask, a solution of rhodamine-N-propargyl alkyne derivative (500 mg, 1 mmol) and CuBr(PPh₃)₃ (50 mg, 0.05 mmol) in acetonitrile (50 mL) was prepared and after being stirred for 30 min, it was added into the Fe₃O₄@SiO₂-N₃nanoparticle dispersion with vigorous stirring under an argon atmosphere. After 24 hours, the RBT–Fe₃O₄@SiO₂ nanoparticles were separated using an external magnet and washed twice successively with acetonitrile, methanol and dichloromethane (15 mL), respectively.

Results and Discussion

RBT-functionalized Fe₃O₄@SiO₂ nanoparticles were synthesized by the click chemistry protocol (copper (I)-catalyzed azide alkyne cycloaddition CuAAC, which involved grafting rhodamine-N-propargyl alkyne moieties on the surface of Fe₃O₄@SiO₂ nanoparticles.²⁷ Rhodamine-N-propargyl alkyne was synthesized in good yield from the condensation of rhodamine B with hydrazine to obtain rhodamine hydrazide, followed by nucleophilic substitution with electrophilic propargyl bromide, as shown in Scheme 1. Magnetic nanoparticles (Fe₃O₄) were prepared from a thermal decomposition process, and the surface of the Fe₃O₄ nanoparticles were coated with silica via a sol-gel process. Subsequent appending of azide groups to the silica surface, followed by application of the click chemistry protocol (CuAAC) with rhodamine-N-propargyl alkyne afforded RBT-Fe₃O₄@SiO₂ nanoparticles.

Scheme 1: Synthesis of RBT–functionalized Fe3O4@SiO2 nanoparticles. Click here to view scheme

 

The presence of RBT on the surface of the Fe₃O₄@SiO₂ nanoparticles was confirmed by FT-IR spectroscopy, as highlighted in Fig.1. Both Fe₃O₄@SiO₂and RBT -Fe₃O₄@SiO₂ nanoparticles exhibit characteristic Fe-O stretching frequencies at 592 cm^-1 and 586 cm^-1, respectively. Absorption peaks at 797 cm^-1,963 cm^-1 and 1104 cm^-1 in RBT-Fe₃O₄@SiO₂ nanoparticles are assigned to Si–O stretching vibrations, which are also observed in Fe₃O₄@SiO₂ nanoparticles (797 cm^-1, 956 cm^-1 and 1101 cm^-1). Strong intensity bands at 468 cm^-1 and 464 cm^-1, ascribed to Si–O–Si bending, are presented in Fe₃O₄@SiO₂ and RBT-Fe₃O₄@SiO₂ nanoparticles, respectively. The absorption bands around 1634 cm^-1 in Fe₃O₄@SiO₂, and 1628 cm^-1 in RBT-Fe₃O₄@SiO₂ nanoparticles can be attributed to O-H bonds in silanol groups. Furthermore, the FT-IR spectrum of RBT-Fe₃O₄@SiO₂ nanoparticles shows absorption peaks at 635 cm^-1, 1397 cm^-1, 1490 cm^-1 and 1573 cm^-1 which can be assigned to C-H bending in aromatic groups, alkyl (C-H) bending, aromatic C=C stretching, and aromatic C=C bending, respectively, consistent with surface functionalization of nanoparticles with rhodamine-triazole entities. TGA analysis of RBT-Fe₃O₄@SiO₂ nanoparticles further confirmed the presence of surface organic functional groups. From the TGA curves in Fig. 2, initial weight loss occurred below 200°C, consistent with removal of adsorbed water. Thermal decomposition of RBT-Fe₃O₄@SiO₂ occurs at 450-560°C with a total weight loss of 10.2%, in contrast to < 1% for both Fe₃O₄@SiO₂ and azide-Fe₃O₄@SiO₂ nanoparticles.

Figure 1: FT-IR spectra of Fe3O4@SiO2 (blue line) and RBT-Fe3O4@SiO2 (red line). Click here to view figure

Figure 2: TGA curves of (a) Fe3O4@SiO2 (blue line), (b) Fe3O4@SiO2-N3 (red line) and (c) RBT-Fe3O4@SiO2 nanoparticles (black line). Click here to view figure

 

The magnetic property of Fe₃O₄@SiO₂ nanoparticles was proved by XRD analysis. XRD pattern in Fig. 3 shows the characteristic diffraction peaks of magnetite Fe₃O₄ nanoparticles at (220), (311), (400), (511), (440) and (533), corresponding to previous reports.^28,29 Silica shows a broad peak around 20-28°C suggesting that Fe₃O₄ nanoparticles were coated with silica.

Figure 3: XRD pattern of Fe3O4@SiO2 nanoparticles. Click here to view figure

 

The morphologies of the RBT-functionalized Fe₃O₄@SiO₂ nanoparticles were observed using SEM imaging (Fig.4). Magnetic Fe₃O₄ nanoparticles are spherical with the diameter approximately 67 nm (Fig. 4a). After the modification, silica-coated Fe₃O₄ nanoparticles (Fig. 4b) and RBT-functionalized Fe₃O₄@SiO₂ nanoparticles (Fig.4c) also remain spherical, but they have larger sizes as expected from the addition of surface coatings, respectively. The size of RBT-Fe₃O₄@SiO₂ nanoparticles was elucidated by TEM image. Fig. 4d showed that the average diameter of the RBT-Fe₃O₄@SiO₂ nanoparticles was about 155 nm. EDX analysis and elemental mapping of the Fe₃O₄@SiO₂ nanoparticles provides a further insight into their composition (Fig. 5), with the elemental distribution of iron (Fe) and silica (Si) highlighted in Fig. 5b. The atomic percentages of carbon (C, 62.73%), oxygen (O, 30.06%), silica (Si, 4.33%), and iron (Fe, 2.87%) for RBT-Fe₃O₄@SiO₂ nanoparticles are indicative of the existence of organic and silica surface layers, as shown in Fig. 5c.

Figure 4: SEM images of (a) Fe3O4, (b) Fe3O4@SiO2, and (c) RBT-Fe3O4@SiO2 nanoparticles, and (d) TEM image of RBT-Fe3O4@SiO2 nanoparticles.Click here to view figure

Figure 5: SEM images of Fe3O4@SiO2 in (a) normal mode, (b) EDS mode showing Fe (red), Si (green), and (c) the EDS mapping profile of Fe3O4@SiO2 depicting elemental ratios. Click here to view figure

 

The selectivity of RBT-Fe₃O₄@SiO₂ nanoparticles towards sensing of heavy metal ions (Cu²⁺, Ni²⁺, Hg²⁺, Co²⁺, Fe²⁺, Fe³⁺ and Pb²⁺) in MeOH/H₂O (9:1, v/v) was investigated by UV absorption and fluorescence measurements. While initially exhibiting colorless, the RBT-Fe₃O₄@SiO₂ solutions exhibit a visible color change on the addition of metal ions, becoming pink in the presence of Cu²⁺, Co²⁺, Ni²⁺ ions, and pink-orange when challenged with Hg²⁺, Pb²⁺ and Fe³⁺ ions (Fig. 7b). However, no changes result from the presence of Fe²⁺ ion. From Fig. 7a, the RBT-Fe₃O₄@SiO₂ solutions exhibit an absorption maximum at 554 nm when exposed to all of the above metal ions, with Fe³⁺ ion resulting in the highest absorption intensities. Fluorescence spectra in Fig. 8a indicate that the RBT-Fe₃O₄@SiO₂(control) solution are only weakly fluorescent on excitation at 530 nm. Nevertheless, the addition of metal ions, with the exception of Fe²⁺, results in significant fluorescence enhancements, with maximum emission wavelengths at 572 nm on addition of Co²⁺, 574 nm for Cu²⁺ and Hg²⁺, 576 nm for Ni²⁺, 578 nm for Fe³⁺, and 580 nm for Pb²⁺ ions. Furthermore, fluorescence enhancement of the RBT-Fe₃O₄@SiO₂solution displayed 1.5-fold upon the addition of Cu²⁺ in MeOH/H₂O (9:1, v/v) by excitation at 530 nm, as shown in Fig. 8b. In case of Fe³⁺ ion, fluorescence emission revealed 0.8-fold compared to the absence of metal ions because of fluorescence quenching effect of Fe³⁺ ion. From these results, the RBT-Fe₃O₄@SiO₂nanoparticles can act as a “naked-eye” colorimetric and fluorescent sensor for certain heavy metal ions (Cu²⁺, Hg²⁺, Co²⁺, Fe³⁺ and Pb²⁺).

Figure 6: Separation method of the RBT–Fe3O4@SiO2 nanoparticles in the aqueous phase using external magnet. Click here to view figure

Figure 7: (a) UV-absorption spectra of a dispersion of RBT-Fe3O4@SiO2 (20 mg) in MeOH/H2O (9:1, v/v) in the presence of various metal ions (1 mM), and (b) Click here to view figure

 

a bar graph showing the relative absorbance (554 nm) of RBT-Fe₃O₄@SiO₂ (20 mg) in the presence of these metal ions (1 mM), in MeOH/H₂O (9:1, v/v); Inset: Color changes of RBT-Fe₃O₄@SiO₂ (20 mg) dispersions on the addition of metal ions (1 mM), in MeOH/H₂O (9:1, v/v).

Figure 8: (a) Fluorescence spectra obtained by excitation of RBT-Fe3O4@SiO2 (20 mg) in MeOH/H2O (9:1, v/v) at 530 nm in the presence of metal ions (1 mM), and (b) Click here to view figure

 

a bar graph depicting the relative fluorescence enhancement (574 nm) (DF/F₀) in dispersions of RBT-Fe₃O₄@SiO₂ (20 mg) in MeOH/H₂O (9:1, v/v) on the addition of metal ions (1 mM) in MeOH/H₂O (9:1, v/v).

Due to the fluorescence enhancement being greatest in the case of Cu²⁺, the fluorescence response of the RBT-Fe₃O₄@SiO₂ nanoparticles to this metal ion was investigated in more detail using titration experiments (Fig. 9). From the fluorescence spectra in Fig. 9a, increasing the Cu²⁺ concentration results in fluorescence enhancements, with the maximum intensity being reached at 800 mM. Excitation at 530 nm results in a linear relationship (Fig. 8b) (y = 82.986X + 8677.2, R² = 0.9909) at 574 nm between fluorescence intensity and Cu²⁺ concentration (range 100-800 mM). The detection limit was determined to be 9.8 mM based on 3s/S, where s is the standard deviation of the fluorescence intensity of the RBT-Fe₃O₄@SiO₂ solutionand S is the slope of the calibration curve. Moreover, naked eye color changes in the RBT-Fe₃O₄@SiO₂dispersion were observable for Cu²⁺ ion concentrations > 50 mM, as shown in Fig. 9c.

Figure 9: (a) Fluorescence spectra obtained by excitation of a dispersion of RBT-Fe3O4@SiO2 Click here to view figure

 

(20 mg) at 530 nm in MeOH/H₂O (9:1, v/v) with the addition of Cu²⁺ (10-1,000 mM), (b) calibration curve and (c) color changes in MeOH/H₂O (9:1, v/v) RBT-Fe₃O₄@SiO₂ (20 mg) dispersions on the addition of Cu²⁺ (10-1,000 mM).

The binding mechanism of Cu²⁺ to RBT-Fe₃O₄@SiO₂ nanoparticles in MeOH was further investigated by ESI–MS analysis, with the results shown in Fig. 10. As indicated above, the addition of these ions results in color changes fluorescence emission, both of which are the result of metal ion promoted ring-opening of the spirolactam ring in rhodamine.¹⁹ The mass spectrum of the RBT-Fe₃O₄@SiO₂ solution in the presence of Cu²⁺ (in MeOH) displayed a new molecular ion peak at m/z 457.2615, which is suggestive of formation of a rhodamine methyl ester (calcd for C₂₉H₃₃N₂O₃457.2486). This outcome is consistent with rhodamine ring–opening followed by hydrolysis, as postulated previously.²⁷

Figure 10: Mass spectrum of the RBT–Fe3O4@SiO2 (20 mg) solution upon the addition of Cu2+ in MeOH. Click here to view figure

 

Conclusion

We have successfully synthesized rhodamine-triazole functionalized Fe₃O₄@SiO₂ nanoparticles, based on the click chemistry approach. Upon addition of heavy metal ions (Cu²⁺, Co²⁺, Ni²⁺, Hg²⁺, Fe³⁺, and Pb²⁺), the solutions of these nanoparticles exhibit distinct naked eye color changes (maximum absorption wavelength 552 nm), and intense fluorescence emissions at 574 nm. For Cu²⁺, the ion displaying optimum binding behavior, the limit of detection was found to be 9.8 mM with fluorescence responses showing a linear correlation with Cu²⁺ concentration (range 100-800 µM).

Conflict of Interest

There is no conflict of interest.

Acknowledgements

This work was financially supported by the Institute of Research and Development, Rajamangala University of Technology Thanyaburi (Project No. NRF62D0602). We wish to thank Assoc. Prof. Dr. Palangpon Kongsaeree (Department of Chemistry, Mahidol University) for experimental supports and Assoc. Prof. Dr. Christopher Smith (Kamnoetvidya Science Academy) for any suggestion. We would also like to thank the Kamnoetvidya Science Academy (KVIS) and Vidyasirimedhi Institute of Science and Technology (VISTEC) for use of equipment for characterization purposes.

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