TEM image of Mn2
core-shell nanostructures shows cores with size ranging from 25 to 100 nm with a shell thickness of 5 nm (Figure ). The presence of amorphous silica shell was clearly observed in the TEM image. The synthetic methodology utilizes already synthesized Mn2
nanoparticles which has been prepared from the route known in the literature [32
]. HRTEM image (Figure ) shows lattice fringes corresponding to (111) plane of Mn2
. The amorphous silica shell was clearly observed surrounding the crystalline core in the high resolution TEM image (Figure ). Thus HRTEM of Mn2
core-shell nanostructures confirms the chemical composition of core as Mn2
and shell as amorphous silica.
TEM and HRTEM image. (a) TEM and (b) HRTEM images of Mn2O3@SiO2 core-shell nanostructures.
core-shell nanostructures are present in an aggregated form as observed from TEM images in Figure . The presence of aggregates could be attributed to the formation of H-bond between the silica shells due to the presence of Si-OH bond over the shell surface. These Si-OH bonds were formed by the hydrolysis of TEOS in the presence of ammonia and water at room temperature. We have also discussed the aggregation effect in silica-coated core-shell nanostructures in our earlier report [33
]. It is also to be noted that the starting material (Mn2
nanoparticles) used for the synthesis of silica shell is a magnetic material, present in powder form. Thus, there is an inherent tendency of these oxide nanoparticles to agglomerate. However, a challenge still remains to form silica shell over individual nanoparticles (for the oxides present in powder form with high degree of agglomeration). The main emphasis in this article is on the enhancement of functional groups on the surface of core-shell nanostructures by using an organosilane precursor to form the shell and compared with our studies of shell formation by the post-grafting method which has been the common procedure in earlier studies [25
]. This point has been discussed in later sections.
Figure shows TEM image for Mn2O3@amino-functionalized silica particles with core diameter of 25-30 nm and shell thickness of 5 nm. Nanoparticles of Mn2O3@vinyl-functionalized silica (Figure ) show core-shell nanostructures with a core diameter of 25-30 nm and shell thickness of 5-10 nm. Cores with diameter of 25-30 nm with a shell thickness of 10-15 nm were observed (Figure ) for Mn2O3@allyl-functionalized silica. It is to be noted that the shell in the above three core-shell nanostructures is formed by the hydrolysis of organosilane precursors, which ensures that these core-shell nanostructures bear the respective functional groups (amine, vinyl, and allyl) on their surface. Core-shell nanostructures (amine groups over the shell) were obtained (Figure ) when the synthesis was carried out with TEOS and APTMS. The core size varied from 20 to 25 nm and a shell thickness was found to be 10 nm.
TEM images of functionalized core-shell. TEM images of (a) Mn2O3@amino-functionalized silica (without TEOS), (b) Mn2O3@vinyl-functionalized silica, (c) Mn2O3@allyl-functionalized silica, and (d) Mn2O3@amino-functionalized silica (with TEOS).
Bands at 3429, 1632, 572, and 520 cm-1
corresponding to O-H stretching, O-H bending, and Mn-O stretching were observed in IR spectrum of Mn2
nanoparticles. Additional bands at 1123 and 1079 cm-1
corresponding to Si-O-Si stretching were observed for the silica-coated nanostructures. This gives further evidence for the coating of silica over Mn2
nanoparticles corroborating with the TEM studies. Table S1 in Additional file 1
summarizes the IR bands for the functionalized core-shell nanostructures. Note that in all the three core-shell nanostructures, Si-O-Si stretching band was observed even though TEOS was not added
. This confirms that the stretching band was observed due to the functionalized silica shell formed as a result of hydrolysis of the organosilane precursors. Thus, IR spectrum gives us an additional proof for the formation of core-shell nanostructures with functionalized shells. In addition to the above we also observed C=C stretching vibrations in the IR spectrum of vinyl- and allyl-functionalized core-shell nanostructures which also suggest the proper functionalization of the shell.
Zeta potential studies for uncoated and coated Mn2O3 nanoparticles were carried out with varying pH (Figure ). Increase in the negative zeta potential values were observed for the coated particles compared to the uncoated particles, which suggests a uniform coating of silica over Mn2O3 nanoparticles. The negative surface charge of silica is expected due to the presence of hydroxyl groups on the surface of silica.
Figure 3 Zeta potential vs. pH plot. Zeta potential versus pH plot for bare Mn2O3, Mn2O3@SiO2, Mn2O3@amino-functionalized silica (with TEOS), Mn2O3@amino-functionalized silica (without TEOS), Mn2O3@vinyl-functionalized silica, and Mn2O3@allyl-functionalized silica (more ...)
Figure shows zeta potential versus pH curves for bare Mn2
@amino-functionalized silica (with TEOS), Mn2
@amino-functionalized silica (without TEOS), Mn2
@vinyl-functionalized silica, and Mn2
@allyl-functionalized silica core-shell nanostructures. The silica-coated Mn2
bears a negative surface charge at pH > 3. It has been reported in an earlier study [34
] that the presence of amine shifts the iso-electric point (IEP) toward higher pH values as the pKa
of aminopropyl group is 9.8. The amine group is protonated at pH < 9. In Mn2
@amino-functionalized silica (without TEOS), the IEP was found to be 9.6 which suggests that the amino groups are present on the surface of the core-shell particles. At pH > IEP, deprotonation of the positively charged R-NH3+
groups results in a negative surface charge while the presence of R-NH3+
groups at pH < IEP results in a positive surface charge. The zeta potential depends on two main factors viz. pH and concentration of the sample [35
]. In our study we have fixed the concentration of the sample from 1 to 2 mg in 10 ml of 10 mM NaCl and have studied the zeta potential as a function of pH.
Zeta potential values are sensitive to the surface charge of the outer particle surface and hence our result suggests that the amine groups are located on the outer surface of the core-shell nanostructures. It is also to be noted that the values of the obtained zeta potential do not refer to a single particle but represent an ensemble of particles present in the system. In order to ensure that more functional groups are present over the shell, zeta potential studies were carried out on Mn2O3@amino-functionalized silica (with TEOS) wherein amino functionalization was carried out by post-grafting method using APTMS. It was observed that the zeta values were less positive than Mn2O3@amino-functionalized silica (without TEOS). Zeta values as earlier mentioned are dependent on the surface charge of the outer particle, which suggests that the number of amine groups over the functionalized core-shell nanostructures synthesized using post-grafting method is less than the one synthesized using APTMS as the shell forming agent. The IEP for Mn2O3@amino-functionalized silica (with TEOS) also shifts to low pH (=6.3), which also suggests the presence of less number of amine groups and more number of hydroxyl groups over the surface of these core-shell nanostructures. The above inference was further confirmed by using fluorescamine dye. The concentration of amine groups was found to be 0.302 μmol/g in the case of Mn2O3@amino-functionalized silica (without TEOS) and 0.274 μmol/g for Mn2O3@amino-functionalized silica (with TEOS).
Surface charge density was calculated using Guoy-Chapman equation [36
]. The surface charge density was calculated at two pH value viz. 5.4 and 6.5 and was found to be 3.96 mC/m2
(at pH 5.4) and 3.14 mC/m2
(at pH 6.5) for Mn2
@amino-functionalized silica (without TEOS). The surface charge density for Mn2
@amino-functionalized silica (with TEOS) was found to be 3.31 mC/m2
(at pH 5.4) and -0.37 mC/m2
. Thus, both calculations (using fluorescamine and zeta potential) suggest that the core-shell nanostructures (amino-functionalized) synthesized using the hydrolysis of 3-APTMS only bear high density of amino groups on the shell as compared to the core-shell nanostructures synthesized using post-grafting method.
The zeta potential of allyl- and vinyl-functionalized silica was higher than that of silica-coated and bare nanoparticles, which also suggests the presence of allyl and vinyl groups on the surface of the core-shell nanostructures.
Zeta potential studies for the amino-functionalized core-shell nanostructures immobilized with glucose and L-methionine were carried out by dispersing the particles in 10 mM NaCl solution (Table ). The zeta potential values changed from positive to negative suggesting that glucose and L-methionine have been immobilized onto the surface of the core-shell nanostructures. Thus, the change in zeta potential values can be used to detect the immobilization of bio-molecules over nanoparticles. The immobilization of biomolecules (glucose and L-methionine) is just to show the use of functionalized silica core-shell structures for possible applications.
Zeta potential values for amino-functionalized and bio-molecule immobilized core-shell nanostructures