Magnetic nanoparticles (MNPs) have many attractive applications for a wide range of disciplines, including magnetic fluids, catalysis, biotechnology/biomedicine, magnetic resonance imaging, data storage, and environmental remediation [1
]. While a number of suitable methods have been developed for the synthesis of magnetic nanoparticles, successful application is highly dependent on the stability of the particles under a range of different conditions.
However, among the different kinds of (MNPs), only maghemite (γ-Fe2
) and magnetite (Fe3
) NPs are mainly taken into account due to their lower toxicity for biomedical or environmental applications, with respect to other magnetic materials (Co- and Ni- based nanoparticles (NPs) [13
]. Both iron oxides are ferrimagnetic with a magnetic moment of 2.5 μB
/f.u. (formulae units) for γ-Fe2
and 4.0 μB
/f.u. for Fe3
, respectively [13
]. Below a characteristic dimension (critical diameter ds
), in single-domain magnetic nanoparticles, the magnetic behavior called superparamagnetism, is reached when the thermal energy is greater with respect to the anisotropy energy, so the magnetization vector can fluctuate easily between the up and down directions, with a time-averaged net moment equal to zero. The time required to have a change from the up to the down state is called relaxation time
) and the temperature at which a change from the spin -flip free state to the blocked state is observed is called blocking temperature (TB
). Since the time required to make a measurement is dependent on the equipment used, the blocking temperature for the same system of NPs could be different depending on the technique used. The superparamagnetic state is characterized by a zero value of the coercivity and this is the magnetic state commonly used for the above-cited applications.
Fe3O4 NPs are currently preferred to γ-Fe2O3 ones, due to larger magnetic moment and relatively simple synthetic methods. These NPs are also widely used as the starting point for complex functionalization in order to reach different properties. A typical “nano-architecture” is formed by a magnetic core plus one or more intermediate coatings before the final functionalization.
Usually, polymer or inorganic coatings are widely used when the core of functionalized “nano-architectures” needs to be protected from oxidation/corrosion processes, or when toxic or pollution problems, arising from the core, must be prevented. Furthermore, the use of inorganic structures (silica, alumina, or titania coatings) around the magnetic NPs increases their stability in solution, preventing or minimizing the formation of agglomerates due to magnetic dipole–magnetic dipole interactions. However, this step is always time consuming and is often quite expensive. Functionalized magnetic nanoparticles could be used to capture, for instance, toxic heavy metals or molecules, arising from relatively large quantities of waste water; this is a typical example where a fast and economic production of functionalized MNPs (without the presence of an intermediate coating) seems to be highly advisable.
The possibility of obtaining the same functionalization of the nanostructures with or without the presence of the intermediate coating is a desirable goal, but a comparison between the two complex nanosystems is fundamental to verify the achieved functionalization and its efficiency.
The purpose of the present paper is, thus, the functionalization of magnetite NPs with two different organo-silane molecules (OSM), namely (3-aminopropyl)triethoxysilane (APTES) and (3-mercaptopropyl)trimethoxysilane (MPTMS). Two different paths were used to achieve the desired functionalization: the first one is a direct reaction between the OSM and the surface of the magnetite nanoparticle (Fe3
@OSM) and the second one is the use of an intermediate silica coating (using tetraethoxysilane (TEOS) as a reagent) with the silane derivative (Fe3
@OSM). The two paths are shown in the following:
Fe3O4 + TEOS → Fe3O4@SiO2 + OSM → Fe3O4@SiO2@OSM
In order to verify the achieved functionalization and its efficiency, the different products have been analyzed by field emission scanning electron microscopy (FE-SEM) equipped with an energy dispersion X-ray spectrometer (EDX), IR spectroscopy, TGA (thermogravimetric analysis) and AC susceptibility measurements, adopting the physical model already proposed for a system of magnetic nanoparticles in suspension [14
]. The presence of the primary amine of APTES on the surface of the functionalized NPs was confirmed by UV-VIS spectroscopy analyses using ninhydrin as the revealing agent [15
]. The success of the thiol functionalization (MPTMS) was confirmed by the well-known reactivity of the -SH group with respect to Au. In fact Au NPs were successfully conjugated on the surface of Fe3
@MPTMS and Fe3
@MPTMS, thus leading to a nanocomposite formed by a magnetic core of Fe3
surrounded by Au NPs.
We demonstrated here that Fe3O4@OSM particles could be used with an efficiency comparable to Fe3O4@SiO2@OSM ones, for all the applicative uses not requiring a high dispersion in a fluid, thus avoiding a further synthetic step.
3. Materials and Methods
3.1. Materials and Synthetic Method
All chemicals (99.9 wt % purity minimum) were purchased from Sigma Aldrich (St. Louis, MO, USA) and used without further purification.
Magnetite nanoparticles were synthesized using a modified Massart method [24
] based on the co-precipitation of stoichiometric iron (II) and iron (III) chloride hexa-hydrate salts in aqueous ammonia solution. The obtained nanoparticles were washed several times in hot water and magnetically collected, following the protocol previously described [17
]. The NPs were finally dispersed in water to be used for further functionalization.
Silica-coated magnetite NPs were prepared following the Stöber process [25
]. Briefly, to an alcoholic suspension of magnetite (60 mg/L), water, ammonia, and TEOS are added. The reaction was then maintained for two hours at 40 °C and, finally, the product is magnetically collected. Details about the synthesis were already presented [26
The two different paths used to functionalize the magnetite NPs are reported in Scheme 1
, where OSM is the generic organo-silane molecule and X is the functional group (–NH2
3.1.1. APTES Functionalization
The protocol used for the direct APTES [(3-Aminopropyl)triethoxysilane] functionalization of magnetite nanoparticles is described here: 40 mL of a dispersion of Fe3O4 NPs in water (2 g/L) were added to 40 mL of ethanol and to 1.6 mL of 2% v/v solution of APTES. The temperature was kept constant at 50 °C and the reaction was carried out for 24 h.
The very low concentration of silica coated magnetite nanoparticles suggested the use of smaller volumes for reactants: 1 mL of Fe3
(60 mg/L) in water was added to 1 mL ethanol and to 43 μL of APTES (2% v/v); temperature and time were the same as in the previous functionalization [27
Each sample has been magnetically washed several times using first ethanol and then Milli-Q water (MilliQ Academic from Merck Millipore, Darmstadt, Germany).
3.1.2. MPTMS Functionalization
The method for the direct functionalization of Fe3O4 NPs with MPTMS [(3-mercaptopropyl) trimethoxysilane] is reported here: 40 mL of a dispersion of magnetite NPs in water (0.5 g/L) were added to 40 mL of ethanol and 1.6 mL of MPTMS (2% v/v). The reaction was carried out in the same conditions reported above.
The standard reaction used to functionalize silica-coated magnetite NPs is: to 1 mL of ethanol in Eppendorf, 1 mL of Fe3
NPs (60 mg/L) in water, and 20 μL of a solution 1% v/v of MPTMS and 11.5 μL of acetic acid were added. The reaction was carried out in the same conditions reported above [28
]. As for APTES-functionalized samples, each sample has been magnetically washed several times using ethanol and then Milli-Q water.
3.1.3. Synthesis of Au NPs
Gold NPs were prepared by the reduction of an aqueous solution of HAuCl4
(0.01 M) with sodium citrate (0.01 M) in a ratio of 1:3 (v/v) at room temperature under vigorous sonication for 15 min. After a few minutes, a color change from yellow to purple is easily detectable: the solution so obtained is stable for a very long time and does not require further treatments. A characteristic image of the synthesized Au NPs is shown in Figure 10
3.1.4. MPTMS-Au Conjugation
The gold NP conjugation to [email protected]
@MPTMS and Fe3
@MPTMS) (1:1 wt/wt ratio) is performed at RT in Eppendorf under vigorous mechanical stirring for 24 h. The NPs are then washed three times in Milli-Q water to completely eliminate the chemical environment, and then magnetically separated.
3.2. Characterization Techniques
Field emission scanning electron microscopy (FE-SEM) analyses were carried out using a FE-SEM JEOL ZEISS SUPRA 40 VP (Carl Zeiss AG, Oberkochen, Germany). Transmission electron microscopy (TEM) analyses were performed with the model JEOL JEM 2010 200 kV microscope from JEOL Ltd. (Peabody, MA, USA). The specimen powders for FE-SEM and TEM analyses were suspended in ethanol, and then exposed to ultrasonic vibrations to decrease the aggregation. A drop of the resultant mixture was deposited on a copper grid covered with a thin carbon lace and then carefully dried before the analysis.
Information on the organic molecules linked to the surface of the particles have been obtained by means of FT-IR spectrometry. For our purpose, we investigated dried samples with a Perkin Elmer Spectrum 65 FT-IR spectrometer (Perkin Elmer, Waltham, MA, USA) equipped with universal ATR (Attenuated Total Reflectance) sampling accessory.
Gold particle dimensions have been evaluated by means of FE-SEM and TEM observation, and indirectly determined by UV measurements of the absorbance in 0.01 M aqueous solution, using a Varian CARY50 UV-VIS spectrometer (Varian Medical Systems, Palo Alto, CA, USA). The organic compound ninhydrin (2,2-dihydroxyindane-1,3-dione) was used as the revealing agent. This chemical, when reacting with primary or secondary amines, produces a deep blue or purple color known as a Ruhemann complex [16
AC susceptibility measurements were obtained using an Oxford Maglab2000 (Oxford Instruments, Abingdon on Thames, UK), operating in the 1–104 Hz frequency range with 10 Oe applied a.c. magnetic field. The resolution of the AC signal was better than 10−7 emu.
Thermogravimetric analyses (TGA) were performed using a Labsys EVO Setaram instrument (Setaram Instrumentation, Caluire, France). Approximately 5 mg of each sample were weighed in an open alumina crucible (Setaram Instrumentation, Caluire, France) and heated from 50 °C to 1000 °C in a He atmosphere (20 mL/min) with a heating rate equal to 10 °C/min.
In this paper the results of the functionalization of magnetite nanoparticles with organo-silane molecules (APTES and MPTMS) using two different paths have been presented.
The functionalized magnetic NPs have been analyzed by different experimental techniques and the results are completely in agreement with each other. Furthermore, the magnetic susceptibility confirms the ability to achieve dimensional information about systems of magnetic nanoparticles in solution.
We demonstrated here the possibility of obtaining the same functionalization on the nanoparticles’ surfaces with and without the presence of the intermediate coating. Thus, the Fe3O4@OSM particles could be used with an efficiency comparable to the Fe3O4@SiO2@OSM one, for all of the applicative uses, for instance in the analytical and/or environmental fields, not requiring a high dispersion in a fluid, thus avoiding a further synthetic step.