Surface area of the different synthesised materials were determined using nitrogen adsorption – desorption test. BET analysis of adsorption isotherm determines the specific surface area of the powder sample by physical adsorption of nitrogen on the sample surface. Measurements were performed according to methods described in Section 2.32.3.The data was considered acceptable if the correlation coefficient was greater than 0.9975 (Naderi, 2015). Surface area of the different materials are summarised in Table 3-2.
Table 3-2. Summary of the surface area of the material
Material ID Surface area (m²/g)
ME32 133.9683 ± 0.5248
ME33 1187.7596 ± 6.7348
ME56 38.2521 ± 0.6845
ME60 358.0238 ± 0.5267
ME93 752.8064 ± 7.6538
ME94 574.5482 ± 3.5390
Figure 3-17 presents the Nitrogen gas adsorption-desorption isotherm of mesoporous silica coated magnetic nanoparticles (ME32). The BET plot is classified as type IV isotherm which is associated with mesoporous structures. The isotherm displayed a H1 hysteresis loop within the relative pressure range of 0.6 to 1 indicating the presence of agglomerates in the sample (Haul, 1982). The BET specific surface area was measured to be 133.96 m2/g. The presence of porous structures and agglomerates were consistent with the TEM images (see Figure 3-6).
Figure 3-17. Nitrogen gas adsorption-desorption isotherm of ME32, BET Surface Area: 133.9683 ± 0.5248 m²/g
Figure 3-18 shows the Nitrogen gas adsorption-desorption isotherm of the mesoporous silica coated magnetite (ME33). This material exhibited a type IV isotherm indicating the mesoporous structure. The hysteresis loop observed between relative pressures of 0.4 to 0.8 could be categorize to be type H2 indicating non-uniform shapes and sizes of mesopores (Haul, 1982). The BET surface area of the material was measured to be 1187.8 m2/g which is exceptionally high.
This presents the potential of this material to be used in drug loading or enzyme immobilisation.
The average pore diameter was measured to be around 4 nm which is expected from the porous structures generated using CTAB as pore templates (Liberman et al., 2014).
The surface area measurement was repeated after the enzyme immobilisation on ME33 to study the effects of the enzyme on the surface area of the material. The BET results after enzyme immobilisation shows a surface area of 146.93 m2/g for chemically immobilised PFL and 51.27 m2/g after chemically immobilisation of CRL. The drastic decrease observed in the surface area of the material confirms the presence of the enzyme molecules inside the pores. The BET isotherm for PFL immobilised nanoparticles are shown in Figure 3-19.
Figure 3-18. Nitrogen gas adsorption-desorption isotherm of ME33, BET Surface Area: 1187.7596 ± 6.7348 m²/g
Figure 3-19. Nitrogen gas adsorption-desorption isotherm of ME33 after chemical PFL immobilization. BET Surface Area: 146.9339 ± 2.5072 m²/g
The nitrogen gas adsorption-desorption isotherm of mesoporous silica coated magnetic nanoparticles (ME60), is presented in Figure 3-20. The BET plot shows Type IV isotherm and H2 hysteresis loop which is similar to the results obtained for ME33. The similar results was expected since ME60 was synthesised following the same method as ME33 with slight modification in synthesis method to reduce the size of the nanoparticles in order to make them suitable for drug delivery applications. The BET specific surface area of ME60 was measured to be 358 m2/g which is much lower in value compared to ME33. This could be the result of relatively thin silica shell of ME60.
Figure 3-20. Nitrogen gas adsorption-desorption isotherm of the ME60-BET Surface Area: 358.0238 ± 0.5267 m²/g
Figure 3-21 presents the nitrogen gas adsorption-desorption isotherm of ME93. A type IV isotherm was obtained from nitrogen gas adsorption-desorption isotherm plot for ME93 indicating mesoporous structure. The nitrogen adsorption–desorption isotherm exhibits a linear increase in the volume of adsorbed nitrogen at low relative pressure (P/P0 < 0.2), which can be assigned as the mono-layer adsorption of nitrogen on the sample surface. The sharp inflection in the volume of adsorbed nitrogen between relative pressures 0.2 and 0.4 could be the result of nitrogen capillary condensation inside the mesopores. The hysteresis loop observed for ME93 between relative pressure 0.3 and 1 could be classified as type H4 indicating the presence of narrow slit-shapedpores (Haul, 1982, Ursachi et al., 2011a). The BET surface area of the material was measured to be 752.80 m2/g. The high surface area indicates that ME93 is suitable to be used in drug delivery applications. The average pore diameter was measured to be 3 nm which is lower than pore size generated by either CTAB or Pluronic F127 templates. The lower pore diameter could be the result of OTS surface functionalization which is consistent with the results reported by Yildirim et al. (Yildirim et al., 2013).
The nitrogen gas adsorption-desorption isotherm of ME94 is shown in Figure 3-22. ME94 established a type IV isotherm indicating mesoporous structure. The vertical hysteresis loop observed between relative pressures 0.4 and 1 could be categorised to be a type H1 which could be obtained from either agglomerates (in case of ME32) or spherical nanoparticles with fairly uniform size (Haul, 1982) which is consistent with the TEM images of ME94 (Figure 3-13). The surface area of the material was measured to be 574.5 m2/g. The high surface area of the material suggests the potential of this material to be used for drug delivery applications. The average pore diameter was measured to be 6.3 nm which is consistent with pores generated using CTAB and triblock polymers together as network structuring templates (Liberman et al., 2014).
Figure 3-21. Nitrogen gas adsorption-desorption isotherm of ME93, BET Surface Area: 752.8064 ± 7.6538 m²/g
Figure 3-22. BET isotherm of ME94 BET Surface Area: 574.5482 ± 3.5390 m²/g
The surface area measurement was performed for the starting material for development of ME94 (ME56) before etching process which demonstrated a low surface area of 38.25 m2/g.
X-Ray Diffraction (XRD)
X-ray diffraction (XRD) was used to confirm the crystalline structure of the magnetite nanoparticles. Samples for X-ray diffraction analysis were prepared according to methods described in Section 2.32.1.
0 100 200 300 400 500 600 700
0 0.2 0.4 0.6 0.8 1 1.2
Volume absorbed (cm3/g)
Relative pressure (P/P0)
Ads Des
Figure 3-23 shows the XRD pattern of ME18 (bare magnetite nanoparticles). The powder X-ray diffraction of magnetite nanoparticles exhibited multiple peaks with miller indices of 220, 311, 400, 422, 511, and 440 similar to the fingerprint of pure magnetite (Fe3O4) ( JCPDS No. 19-0629) in the 2θ range of 20 to 70 (Sen et al., 2006, Sun et al., 2004).
While the patterns of Fe3O4 ( JCPDS No. 19-0629) and 𝛾-Fe2O3 phases (JCPDS No. 39-1346) are rather similar (Todaka et al., 2003) some peaks corresponding to 𝛾-Fe2O3 phase, such as (210) and (211) peaks, were not present in the XRD pattern of ME18 indicating that the synthesized nanoparticles were in Fe3O4 phase. The black colour of the powder further verifies that it contains mainly magnetite nanoparticles.
Figure 3-23. XRD pattern of bare magnetite (ME18)
The low intensity of the peaks could be an indication of ultra-small size of magnetite nanoparticles. The average particle diameters were calculated from the XRD pattern according to the linewidth of the (3 1 1) plane refraction peak using Scherrer equation as presented in Equation 3-1 (Sun et al., 2004, Schwertmann and Cornell, 2007)
𝐷 =𝑏𝑐𝑜𝑠 θ𝑘λ Equation 3-1
The equation uses the reference peak width at angle θ, where λ is the X-ray wavelength (1.5418 Å), b is the width of the XRD peak at half height and K is a shape factor (about 0.9 for magnetite). The calculated diameter of ME18 was about 10.4 nm which is consistent with the TEM images (see Figure 3-2).
Figure 3-24. XRD pattern of silica coated magnetite (ME33)
Figure 3-24 shows the XRD pattern of mesoporous silica coated magnetite nanoparticles in the wide angle region. As seen in the image the XRD pattern of silica coated nanoparticles has similar diffraction peaks to uncoated magnetite nanoparticles, which suggests the magnetite nanoparticles were conserved during mesoporous silica coating. The broad peak observed in mesoporous silica coated nanoparticles at around 2θ = 22° corresponds to the presence of amorphous silica (JCPDS No. 29-0085) (Souza et al., 2008, Souza et al., 2009b, Ursachi et al., 2011a)