Highly selective fluorescent chemosensor for Zn2+ derived from inorganic-organic hybrid magnetic core/shell Fe3O4@SiO2 nanoparticles
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Magnetic nanoparticles with attractive optical properties have been proposed for applications in such areas as separation and magnetic resonance imaging. In this paper, a simple and novel fluorescent sensor of Zn2+ was designed with 3,5-di-tert-butyl-2-hydroxybenzaldehyde [DTH] covalently grafted onto the surface of magnetic core/shell Fe3O4@SiO2 nanoparticles [NPs] (DTH-Fe3O4@SiO2 NPs) using the silanol hydrolysis approach. The DTH-Fe3O4@SiO2 inorganic-organic hybrid material was characterized by transmission electron microscopy, dynamic light scattering, X-ray power diffraction, diffuse reflectance infrared Fourier transform, UV-visible absorption and emission spectrometry. The compound DTH exhibited fluorescence response towards Zn2+ and Mg2+ ions, but the DTH-Fe3O4@SiO2 NPs only effectively recognized Zn2+ ion by significant fluorescent enhancement in the presence of various ions, which is due to the restriction of the N-C rotation of DTH-Fe3O4@SiO2 NPs and the formation of the rigid plane with conjugation when the DTH-Fe3O4@SiO2 is coordinated with Zn2+. Moreover, this DTH-Fe3O4@SiO2 fluorescent chemosensor also displayed superparamagnetic properties, and thus, it can be recycled by magnetic attraction.
KeywordsSilica Shell Superparamagnetic Property Fe3O4 Core Magnetic Silica Magnetite Core
dynamic light scattering
diffuse-reflectance infrared Fourier transform
energy-dispersive X-ray spectrometer
transmission electron microscopy
thermal gravimetric analysis
X-ray power diffraction.
Zinc is the second abundant transition metal ion in the human body, which plays a vital role in various biological processes, such as gene expression , apoptosis , enzyme regulation , and neurotransmission [4, 5]. It is also believed that the Zn2+ homeostasis may have some bearing on the pathology of Alzheimer's disease and other neurological problems [6, 7, 8]. Therefore, there is an urgency to develop approaches to detect Zn2+in vivo. Besides, techniques for the separation and removal of metal ions and additives in the detection process are very important to prevent poisoning in environmental and biological fields. Conventional analytical methods including atomic absorption spectrophotometry , inductively coupled plasma atomic emission spectrometry , and electrochemical method  can hardly be applied for Zn2+ ion detection in biological systems due to their complicated pretreatment steps and expensive equipment. Hence, for convenience in future in vivo applications, various fluorescent probes based on small molecules have been designed. They were fairly efficient as reported [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]; however, the small molecules would be toxic , and it is impossible to recover or remove them from organisms . The limitation of recoverability blocked the practical applications of small molecular fluorescent probes. To resolve this challenge, the inorganic supports incorporated with small molecular fluorescent probes were applied for the improvement on recoverability.
Various mesoscopic or nanoscopic materials can be acted as the inorganic supports in the design of fluorescent probes, including magnetic nanoparticles, nanotubes, mesoporous silica, metal nanoparticles, and TiO2[25, 26, 27, 28, 29, 30, 31, 32, 33, 34]. Among all these inorganic materials, magnetic silica core/shell nanoparticles have advantages over other competitors for biological and environmental applications [35, 36, 37, 38, 39, 40, 41]. Firstly, they could be simply separated or recovered via external magnetic field. Besides, with magnetic silica core/shell nanoparticles as delivery, their low toxicity and biocompatibility also had advantages for the design of biological fluorescent probes. Furthermore, the silica shell around magnetic core has large surface area, and it can be grafted by fluorescent probes. Therefore, to develop nontoxic, biocompatible, and recoverable fluorimetric Zn2+ sensors, introducing the magnetic silica nanoparticles with small molecular fluorescent probes incorporated is very necessary and highly desirable.
Materials and methods
All reagents are purchased commercially. Besides, ethanol was used after purification by standard methods. Other chemicals were used as received without further purification.
Thermal gravimetric analysis [TGA] (P.E. Diamond TG/DTA/SPAECTRUN ONE thermal analyzer, PerkinElmer Inc., Waltham, MA, USA), dynamic light scattering (BI-200SM, Brookhaven Instruments Corporation, Holtsville, NY, USA), transmission electron microscopy [TEM] (Tecnai G2 F30, 300 kV, FEI Company, OR, USA), and energy-dispersive X-ray spectrometer [EDX] were used to characterize the materials. X-ray diffraction [XRD] pattern of the synthesized products was recorded with a Rigaku D/MAX 2400 X-ray diffractometer (Tokyo, Japan) using Cu Kα radiation (λ = 0.154056 Å). The scan range (2θ) was from 10° to 80°. Solid-state infrared [IR] using diffuse-reflectance infrared Fourier transform [DRIFT] spectroscopy was performed in the 400- to 4,000-cm-1 region using a Bruker Vertex 70v (Bremen, Germany) and IR-grade KBr (Sigma-Aldrich Corporation, St. Louis, MO, USA) as the internal standard. 1H NMR and 13C NMR spectra were measured on a Bruker DRX 400 spectrometer in a CDCl3 solution with TMS as the internal standard. Chemical shift multiplicities are reported as s = singlet, t = triplet, q = quartet, and m = multiplet. Mass spectra were recorded on a Bruker Daltonics esquire6000 mass spectrometer. UV absorption spectra were recorded on a Varian Cary 100 spectrophotometer (Palo Alto, CA, USA) using quartz cells of 1.0-cm path length. Fluorescence measurements were made on a Hitachi F-4500 spectrophotometer (Tokyo, Japan) and a Shimadzu RF-540 spectrofluorophotometer (Chorley, UK) equipped with quartz cuvettes of 1.0-cm path length with a xenon lamp as the excitation source. An excitation and emission slit of 10.0 nm was used for the measurements in the solution state. All spectrophotometric titrations were performed with a suspension of the sample dispersed in ethanol.
Synthesis of Fe3O4@SiO2 NPs
Fe3O4@SiO2 NPs were synthesized according to the study of Nigam et al. . The process can be briefly described in the following two steps: (1) FeCl2 and FeCl3 (molar ratio, 1:2) were added to a concentrated solution of base (25% ammonium hydroxide) under N2. The solution was mechanically stirred for 1 h at 20°C and then heated at 70°C for 1 h. The mixture was then stirred for 30 min at 90°C upon addition of citric acid (0.5 g/ml). After cooling the reaction mixture to room temperature, the magnetite NPs were obtained by permanent magnet, and then it was rinsed with deionized water to remove excess citric acid and other nonmagnetic particles thoroughly. (2) Then, the magnetite NPs were further coated with a thin silica layer via a modified Stöber method  to obtain stable Fe3O4@SiO2. Tetraethyl orthosilicate was hydrolyzed with magnetic NPs as seeds in an ethanol/water mixture. The resulting silica-coated magnetite NPs with an average diameter of 60 to 70 nm were used.
Synthesis of DTH-Fe3O4@SiO2 NPs
As shown in Figure 1, the synthetic procedure for 2,4-di-tert-butyl-6-((3-(triethoxysilyl)propylimino)methyl)phenol [DTH-APTES] followed the method previously described in the literatures [44, 45]. DTH (234 mg, 1 mmol) and (3-aminopropyl) triethoxysilane [APTES] (221 mg, 1 mmol) were mixed in dry ethanol (15 mL) at room temperature. Then, the solution was refluxed for 3 h under N2. After that, the solvent was evaporated, and the crude product was further purified by flash column chromatography (silica gel, ethyl acetate/petroleum ether 1:2) to produce 371 mg (84.9%) of DTH-APTES as yellow oil. ESI-MS: m/z 438.5 (M + H+). 1H NMR: (400 MHz, CDCl3): δ (ppm) 0.69 (t, 2H, CH2Si); 1.22 (t, 9H, CH3); 1.30 (s, 9H, C(CH3)3); 1.43 (s, 9H, C(CH3)3); 1.82 (m, 2H, CH2); 3.58 (t, 2H, NCH2); 3.82 (q, 6H, SiOCH2); 7.07, 7.36 (d, 2H, Ar); 8.34 (s, 1H, HC = N). 13C NMR (100 MHz, CDCl3): 7.92 (CH2Si); 18.30 (CH3); 24.38, 29.40, 29.70, 31.50 (CH3); 34.11 (C), 35.01 (C); 58.41 (CH2); 62.08 (CH2); 117.83, 125.69, 126.66, 136.65, 139.75, 158.27 (Ar); 165.80 (C = N). FT-IR (KBr pellet) (cm-1): 1,637 (νC = N), 1,275-1,252 (νC-O), 1,596-1,342 (νC = C), 1,106-1,085 (νSi-O).
One hundred milligrams of dried Fe3O4@SiO2 NPs and 356 mg (0.81 mmol) of DTH-APTES were suspended in 10 mL of anhydrous ethanol. The mixture was refluxed for 8 h at 80°C under N2 to obtain DTH-Fe3O4@SiO2. The nanoparticles were collected by centrifugation and repeatedly washed with anhydrous ethanol thoroughly. Unreacted organic molecules were removed completely and monitored by the fluorescence of the upper liquid. Then, the DTH-Fe3O4@SiO2 NPs were finally dried under vacuum over night. About 2.81% DTH-APTES in the precursors was finally grafted on the NPs, and the rest could be recycled if no hydrolysis occurred.
Results and discussion
Characterization of DTH-Fe3O4@SiO2
In addition, dynamic light scattering [DLS] was performed to further reveal the colloidal stability of NPs. According to DLS results (Figure 2B), DTH-Fe3O4@SiO2 presents good stabilization and a narrow size distribution with peak centered at 147 nm, confirming its good stabilization in ethanol. In a common sense, the diameter achieved by DLS is mostly higher than the one observed in TEM since the size of NPs identified by DLS includes the grafted molecules' steric hindering and the hydrodynamic radius of first few solvent layers [49, 50, 51]. Besides, according to the calculated size of DTH-APTES which covalently grafted on the surface of Fe3O4@SiO2, the grafted molecules' steric hindering could increase the diameter by about 2.72 nm.
Fluorescence response of DTH-Fe3O4@SiO2
The remarkable increase of fluorescence intensity can be explained as follows: DTH-Fe3O4@SiO2 is poorly fluorescent due to the rotation of the N-C bond of DTH-APTES part. When stably chelated with Zn2+, the N-C rotation of DTH-APTES part is restricted and the rigid plane with conjugation is formed and the fluorescence enhanced, which consists of our previous work . The emission spectra of DTH-Fe3O4@SiO2, which is excited at 397 nm, exhibit the emission maximum at 452 nm with a low quantum yield (Φ = 0.0042) at room temperature in ethanol. Upon the addition of excess Zn2+, the fluorescence intensity of DTH-Fe3O4@SiO2 increased by more than 25-fold, the emission maximum shifts from 452 to 470 nm, and the quantum yield (Φ = 0.11) results in a 26-fold increase.
In summary, we have successfully designed and synthesized functionalized magnetic core/shell Fe3O4@SiO2 NPs (DTH-Fe3O4@SiO2 NPs) which could act as a new type of fluorescent chemosensor for efficient sensing and separation of Zn2+ in ethanol. The inorganic-organic hybrid fluorescent chemosensor DTH-Fe3O4@SiO2 was able to recognize and adsorb Zn2+ with a selective and sensitive fluorescence response in ethanol. The magnetic separation capability of Fe3O4@SiO2 NPs and the reversibility of the combination between DTH-Fe3O4@SiO2 and Zn2+ would also provide a simple route to separate Zn2+ from the environment (Figure 6, inset).
The authors acknowledge the financial support from the NSFC (grant nos. 20931003 and 91122007) and the Specialized Research Fund for the Doctoral Program of Higher Education (grant no. 20110211130002).
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