3.1 Characterizations of the nanocomposites
The FT-IR spectra of the MSN, Al-MSN, MSN-PEI, and Al-MSN/PEI samples are shown in Fig. 1. The broad bands at about 3452 cm−1 are attributed to the O–H stretching vibrations of either silanol groups or adsorbed water [25-43]. The O-H and Si-O-Si bending vibrations have also appeared at 1640 cm-1. The symmetric and asymmetric Si–O–Si stretching vibrations were observed around the region of 1082 and 802 cm−1, respectively [25]. Two weak peaks of the C-H stretching vibrations are visible at about 2970 and 2890 cm-1 which are attributed to the presence of the PEI molecules in the MSN-PEI and Al-MSN/PEI samples [41-43]. Metal-oxide vibrations of either Si4+ or Al3+ species in tetrahedral holes also appeared at 467 cm-1 [43]. Although the C-N stretching vibrations of the PEI are anticipated to appear in the range of 1000-1250 cm-1, these were overlapped with the strong peaks of the Si-O-Si vibrations [43]. It can be concluded that PEI modification was successfully achieved and no considerable change in the metal-oxide bands were occurred.
The low and high angle XRD patterns for the MSN, Al-MSN, MSN-PEI, and Al-MSN/PEI are presented in Fig. 2. In all cases, a broad peak was observed at about 22o, which originates from amorphous silica [2]. In the low angle XRD patterns, three diffraction peaks of 100, 110, and 200 were observed at 2.65o, 4.3o, and 4.8o, respectively, for the MSN (Fig. 2A). These peaks are indicative of the mesoporous silica structure [36,37]. The intensity of these peaks were decreased after incorporating the metal and organic compounds which showed the less order mesoporous structure for these nanomaterials [2, 43].
The morphology of the MSN, MSN-PEI, Al-MSN, and Al-MSN/PEI samples was studied by SEM imaging (Fig. 3). All of the samples were made of uniform spherical particles in the range of 70-180 nm [2]. It is also noteworthy that PEI treatment did not change the morphology of the products as it is evident from Fig. 3b, and 3d.
The presence and homogeneity distribution of Al ions in the Al-MSN/PEI sample were investigated by EDX and mapping, respectively. Fig. 4a shows a homogeneous distribution of Al3+ in its mesoporous silica and Fig. 4b is evidence of the presence of Si, O, and Al species in Al-MSN/PEI.
The nitrogen adsorption-desorption isotherms and pore size distribution of the samples are shown in Fig. 5. All of the samples showed a type IV isotherm with distinct H4 hysteresis loops in the p/p0 range of 0.3-1.0 (Fig. 5A). The narrow pore size distribution of the samples are observed around 1.2 nm (Fig. 5B).
Table 1 Physicochemical properties of the MSN, MSN-PEI, Al-MSN, and Al-MSN/PEI samples
Catalyst
|
Sa (m2g-1)
|
Vpb (cm3g-1)
|
Wc(nm)
|
MSN
|
1062
|
0.53
|
2.00
|
Al-MSN
|
691
|
0.36
|
2.10
|
MSN-PEI
|
522
|
0.33
|
2.50
|
Al-MSN/PEI
|
393
|
0.34
|
3.44
|
aS, Surface area obtained from N2 adsorption-desorption isotherms; bVp Total pore volume; cW Pore diameter
|
The surface area of the MSN was reduced upon modification with either Al3+ or PEI (1062 m2g-1 for MSN vs. 691, 552, and 393 m2g-1 for the Al-MSN, MSN-PEI, and Al-MSN/PEI samples, respectively). Conversely, the total pore volume of the MSN was reduced after modification from 0.53 cm3g-1 for MSN to 0.34 cm3g-1 for Al-MSN/PEI which may be due to partial pore blockage by PEI, Al3+, or both of them [6]. The pore size was also increased upon modification with Al3+ and PEI from 2.00 nm for MSN to 3.44 nm for Al-MSN/PEI.
3.2. Zeta potential measurements
Zeta potential of the synthesized samples is presented in Fig. 6. The Zeta potential of the pure MSN was altered after introduction with aluminium ions and PEI species so the PEI-containing samples demonstrated positive zeta potentials [44]. The change in the zeta potential values from negative to positive indicated to increase in pure MSN surface isolation, due to the interactions between the MSN surface and PEI and aluminium ion modifiers [45]. The zeta potential of MSN was changed from −16.4 mV to 6.0 mV after amination with PEI, making it possible to load siRNA molecules through the electrostatic attractions. Metal ions are also known to increase siRNA adsorption onto silica surfaces due to the mediation of the electrostatic repulsion between the negatively charged
silica surface and the siRNA molecule [46].
Higher zeta potential of Al-MSN (+26 mV) can be related to the electronic nature of the Al3+ ions which carries a high positive charge. PEI, on the other hand, is a cationic polymer which induces a very positive charge on the modified MSN. Thus, we decided to use a combination of PEI and Al3+ to obtain the highest possible zeta potential. As anticipated, Al-MSN/PEI showed a high zeta potential of 54.1 mV, which confirmed that trivalent metals bind more strongly to siRNA [47].
3.3. Loading and release of the siRNA
The trend of the siRNA incorporation on the MSN and its modified forms was studied by UV-vis spectroscopy. First, a calibration curve was plotted according to the method described under the heading 2.2.4 (Fig. 7).
Fig. 8 illustrates the trend of siRNA loaded after each 120 min for the MSN, Al-MSN, MSN-PEI, and Al-MSN/PEI samples. The adsorption capacity of the Al-MSN and MSN-PEI samples was higher than the pure MSN.
The adsorption capacity was improved when a combination of Al3+ and PEI was used. These observations are in agreement with the zeta potential values of the synthesized samples and the physicochemical properties (Table 1). The results showed although the MSN has the highest surface area and the highest monolayer adsorbed volume among the synthesized samples, it has the lowest siRNA adsorption capacity. Therefore, the adsorption capacity of the Al-MSN/PEI sample can be related to its physicochemical properties, including surface area, pore volume and pore diameter. Under the optimized conditions, 47.19 µg of the siRNA was loaded on 1.0 mg of the Al-MSN/PEI, after 90 min, which is a promising result.
The siRNA release was investigated in a PBS (10 ml) at 260 nm, and at room temperature. The results were calculated as 1−(A1/A0), where A0 is the absorbance of the siRNA standard solution and A1 is the absorbance of siRNA in the filtrate. Fig. 9 depicts the trend of the siRNA release for 120 min from the Al-MSN, MSN-PEI, and Al-MSN/PEI samples. The sharp release of the siRNA was observed for all of the samples in the first 20 min. The pure MSN showed a release efficiency of about 13% of its initial siRNA loading after 120 min.
The MSN modification with aluminium or PEI alone or together had a significant positive effect on the release efficiency of the siRNA. The results showed 22, 30, and 36% siRNA release for the Al-MSN, MSN-PEI, and Al-MSN/PEI samples, respectively. According to the obtained results, the nanocomposite of Al-MSN/PEI exhibited the highest release due to the possessing the highest zeta potential on the surface of this composite.
Since the MSN surface and the siRNA chain possess negative charge at neutral media [48], the pure MSN cannot strongly adsorb the siRNA chain due to the repulsion force between these two materials [49]. The formation of bonds occurred among the molecules with donor electron atoms and also phosphate groups in the siRNA chain with the Al3+ atoms in the nanomaterial structure [34]. The phosphate buffer solution reduced the interaction between the metal ions and the siRNA species which results in the release of the siRNA molecules.
According to the Hard and Soft Acids and Bases (HSAB) theory, hard acids tend to hard bases and soft acids prefer binding with the soft bases. Al3+ is known as a hard acid due to its high positive charge and small radius, and it can easily form a stable complex with a phosphate group of the siRNA molecule. It is worth noting that the siRNA delivery systems are actively pursued by researchers around the world. A comparison of our results with the reported methods on siRNA delivery is summarized in Table 2.
In comparison with the other reported nanoparticles as siRNA carriers, Al-MSN/PEI nanoparticles show many advantages including ease of work-up, high performance in siRNA loading and release, and the use of affordable and non-toxic raw materials. The synthesized nanocomposites were able to undertake gene delivery in a high yield for the first time, and have superiority in comparison to the other reported nanomaterials. Another important advantage of our protocol is working at neutral pH. For example, calcium phosphate nanoparticles53 as siRNA carriers need the pH of solution to be adjusted at 4.0.
Table 2 A comparison of modified MSNs with the other vehicles for the siRNA delivery
Sample
|
Temp.
(oC)
|
Loading
(%)
|
Release
(%)
|
Ref.
|
Al-MSN/PEI
|
25
|
31.4
|
36
|
This study
|
Mannitol
|
25
|
2
|
2
|
50
|
Human Serum Albumin
|
37
|
8
|
40
|
51
|
PLGA
|
25
|
2
|
57
|
52
|
Chitosan nanoparticles (Ionic cross-linking)
|
25
|
8.3
|
9.5
|
53
|
Chitosan nanoparticles (Ionic gelation with TPP)
|
25
|
10
|
12
|
53
|
Mesoporous glass nanoparticles
|
25
|
2.5
|
40
|
54
|
Titania Nanotubes
|
25
|
2.2
|
90
|
55
|
Calcium phosphate nanoparticles
|
25
|
-
|
30
|
56
|