Silica nanoparticles functionalized via click chemistry and ATRP for enrichment of Pb(II) ion
© Li et al.; licensee Springer. 2012
Received: 5 June 2012
Accepted: 18 August 2012
Published: 29 August 2012
Silica nanoparticles have been functionalized by click chemistry and atom transfer radical polymerization (ATRP) simultaneously. First, the silanized silica nanoparticles were modified with bromine end group, and then the azide group was grafted onto the surface via covalent coupling. 3-Bromopropyl propiolate was synthesized, and then the synthesized materials were used to react with azide-modified silica nanoparticles via copper-mediated click chemistry and bromine surface-initiated ATRP. Transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis were performed to characterize the functionalized silica nanoparticles. We investigated the enrichment efficiency of bare silica and poly(ethylene glycol) methacrylate (PEGMA)-functionalized silica nanoparticles in Pb(II) aqueous solution. The results demonstrated that PEGMA-functionalized silica nanoparticles can enrich Pb(II) more quickly than pristine silica nanoparticles within 1 h.
In recent years, silica nanoparticles (SNPs) have received significant attention due to their chemical inertness, nontoxicity, optical transparency, and excellent thermal stability [1–3], which can be widely used in catalysis , chemical process industry , removal of metal ions , and metal ion preconcentration [7–9] through polymer coatings or other functional groups. For many applications, there are several chemical methods for controlling the nanoparticles’ surface functionality, such as chemisorptions , sol–gel process, and immobilization of organic molecules by silane coupling reagents , which can result in the immobility, mechanical stability, and water insolubility of functionalized SNPs . Polymerization methods involving living free radical nitroxide-mediated polymerization, reversible addition-fragmentation chain transfer (RAFT), and atom transfer radical polymerization (ATRP) were commonly considered as effective techniques to functionalize materials [13–16]. Especially, the ATRP technique has been developed rapidly due to the advantages of being simple, inexpensive, and more general for controlled radical polymerization when it was firstly proposed by Wang and Matyjaszewsi . There are a greater number of researchers using ATRP to functionalize SNPs [18–22].
With the appearance of the click chemistry proposed by Sharpless , the copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition reaction also becomes a particularly powerful approach to synthesize designed molecules or modify inorganic materials because only mild reaction conditions are required and the extreme selectivity toward molecules bearing azides and alkynes prevents unwanted side product [24–26]. Recently, some literatures reported that gold nanoparticles, silicon oxides, carbon nanotubes, and other materials were functionalized by click chemistry [27–30].
Based on the advantages of ATRP and click chemistry, the combination of these two methods has attracted attention to functionalize nanomaterials. Ranjan and Brittain  successfully grafted polymer chains onto SNPs through a combination of RAFT polymerization and click chemistry. Wang and co-workers  utilized ATRP and click chemistry to modify particles. Polystyrene brushes were firstly introduced on the surface via ATRP, and then the click reaction was generated on the surface of polymerized SNPs. The combination of surface-initiated ATRP and Huisgen [3 + 2] cycloaddition [33, 34] was also developed as a versatile method for the functionalizations of SNPs. Most of the literatures reported the combination of ATRP and click chemistry to modify nanomaterials in tandem.
3-Bromopropanoic acid and propargyl acid were obtained from Alfa Aesar (Ward Hill, MA, USA). Poly(ethylene glycol) methacrylate (PEGMA), 1,3-dicyclohexylcarbodiimide (DCC), N-hydroxysuccinimide (NHS), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), 4-dimethylaminopyridime (DMAP), and sodium azide (NaN3) were obtained from Sigma-Aldrich Corporation (St. Louis, MO, USA). 3-Aminopropyltriethoxysilane (APTES) was purchased from Dow Corning Corporation (Midland, MI, USA). CuBr and 3-bromo-1-propanol were obtained from Shanghai Jingchun Reagent Co., Ltd. (Aladdin; Shanghai, China). Tetraethyl orthosilicate (TEOS) and 2,2′-bypiridine (Bpy) were purchased from Tianjin Chemical Reagent Co. Ltd. (Tianjin, China).
Synthesis of 3-bromopropyl propiolate
3-Bromo-1-propanol (3 ml, 33 mmol), DCC (5 g, 25.5 mmol), and DMAP (0.6 g, 5.04 mmol) were dissolved in 100 ml of dichloromethane (DCM), then propargyl acid (4.5 ml, 42 mmol) was added into the solution slowly. The mixture was stirred at room temperature in a dark room for 24 h, diluted with DCM, filtered off, and washed with DCM until white particles were observed. Then, the liquid was evaporated completely using a rotary evaporator. The products were dried in a clean vacuum at 50°C overnight.
Preparation and modification of silica particles
The spherical silica particles were prepared from TEOS using NH3·H2O as catalyst according to the Stöber method . Then, 65 ml of ethanol, 3 ml of deionized water, and 10 ml of NH3·H2O (25%) were added into a flask with violent stirring at 40°C for 2 h; 5 ml of TEOS was added into the above solution dropwise. The mixture was kept at 40°C with stirring overnight. After the reaction, the mixture was separated by centrifugation and the white particles were washed with water, ethanol, and toluene, respectively. Then, the silica particles were dispersed in 50 ml of toluene by ultrasonication to produce a homogeneous suspension.
The surface of SiO2 has functioned as an amino group by a silanization reaction according to a previous report. Namely, the prepared SiO2 homogeneous suspension was put into a 100-ml round flask, and APTES (5.3 ml, 23 mmol) was added using a syringe. The reaction mixture was refluxed at 95°C in oil bath with stirring for 10 h. The obtained amino group-immobilized silica particles, defined as SiO2-NH2, were collected by centrifugation and washed with ethanol (2 × 50 ml) and N,N-dimethylformamide (DMF; 2 × 50 ml) in turn, then dispersed in 50 ml of DMF to immobilize the azide group.
3-Bromopropionic acid (2.8 g, 18.4 mmol), NHS (2.4 g, 7 mmol), and EDC (4 g, 22.3 mmol) were added to the SiO2-NH2 and DMF solution with stirring and protected from light at 80°C under N2. After 24 h, 3 g of NaN3 was added into the above system for 24 h. Then, particles called SiO2-N3 were collected by centrifugation and washed with water and DMF.
Surface-initiated atom transfer radical polymerization by click chemistry
The reaction involved in click reaction and atom transfer radical polymerization was described as follows: The prepared azide-modified SiO2 and 3-bromopropyl propiolate were dispersed in 50 ml of DMF, then CuBr (43 mg, 0.3 mmol), Bpy ligand (90 mg, 0.6 mmol), and PEGMA (10 ml, 30 mmol) were added quickly; the reaction was stirred at 80°C under N2 for 24 h. After the reaction, the grafted PEGMA silica particles were collected by centrifugation, referred as SiO2-PEGMA; the product was washed with DCM at least three times to remove the excess reactant, then washed with ethanol and deionized water, and dried in a clean vacuum oven at 50°C overnight.
Sensing of Pb2+
Sensing of Pb2+ experiments were carried out by the following steps: a 10 ml portion of the aqueous sample solution, containing 100 mg/l Pb2+, was prepared, and the pH value was adjusted to 6 with aqueous HCl. Then, 50 mg of SiO2 nanoparticles and SiO2-PEGMA were dispersed in eight portions of the above solution with an ultrasonic oscillator, and the solutions were stewed for 0.5, 1, 2, and 3 h, respectively. The particles were separated by centrifugation. The supernatant was detected by atomic absorption spectroscopy (AAS).
Fourier transform infrared (FT-IR) spectra were obtained using a Spectrum One FT-IR spectrometer (IR Prestige-21, Shimadzu Corporation, Kyoto, Japan) with a resolution of 4 cm−1. To characterize the layers formed on the surface of the nanoparticle, the powder was milled with KBr, and the mixture was pressed into a disk for analysis. The morphology and size of SNPs were studied by transmission electron microscopy (TEM) with a JEOL 200CX (Akishima-shi, Japan). X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5500 electron spectrometer (Physical Electronics, Inc., Chanhassen, MN, USA) using 200-W Mg radiations. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. Thermogravimetric analysis (TGA) was carried out using a Netzsch STA449 F3 thermogravimetric instrument (Wolverhampton, UK) at a heating rate of 10°C/min under a flow of nitrogen. AAS was carried out using an AA-6800 instrument (Shimadzu Corporation).
Results and discussion
After introducing amino groups onto silica surfaces, the ATRP initiator was successfully immobilized via click chemistry while poly(ethylene glycol) methacrylate chain-modified SNPs were obtained. This method takes advantage of click chemistry to optimize ATRP, which we have offered as a versatile pathway to functionalize SNPs. It will develop a series of desired functional groups and also extend this current different functional polymer chains which have potential application in the fabrication of SNP-based nanocomposites. Aside from the applications that require the enrichment of lead, because of the diversity and controllable ability of functional molecules on the surface of SNPs, efforts in our group are being made to extend current work to applications in selective removal of heavy metal ions such as Hg(II), Cu(II), As(III), and Cd(II).
WL and YX are MD students. YZ is an associate professor and WM is a professor and the Dean of the Faculty of Metallurgical and Energy Engineering. SW is a postdoctoral fellow in Kunming University of Science and Technology. YD is an academician of the Chinese Academy of Engineering.
Financial support of this work from NSFC (50903041), Natural Science Foundation of Yunnan Province (2009CD026 and 2010CA019), and Inspection and Quarantine of the People’s Republic of China (2009QK406) were gratefully acknowledged.
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