Operation mechanism of the nanocarriers under magnetic stimulus

Because this magnetic-induced release profile is believed to be a result of the removal of the Fe3O4 NPs from the MSN surface, and from Figure 4.7b, a time-dependent (magnetic-induced) drug-release profile is suggestive of a time-dependent removal of the Fe3O4 NPs. Thus, it is interesting to explore the correlation between the release profile and the amount of Fe3O4 NPs magnetically removed. Upon magnetic stimulus, the amount and

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distribution of Fe3O4 NPs on the surface of MSN was carefully monitored, as shown in Figure 4.8. As the time of stimulus increased, the quantity of the capped Fe3O4 NPs decreased, and, accordingly, more pores of MSN were exposed to the diluting medium, resulting in a greater amount of drug dissolved and diffused outward.

Figure  4.7  (a)  Cumulative  drug‐release  of  MSN  and  MSN@Fe3O4  nanocarriers.  The  MSN@Fe3O4  nanocarriers  showed  no  drug  leakage  compared  to  MSN  nanoparticles.  (b)  Cumulative  drug  release  profiles  of  CPT  from  MSN@Fe3O4 nanocarriers  triggered  by  magnetic stimulus for 1‐5 min. 

Figure  4.8  TEM  images  of  nanostructures  of  MSN@Fe3O4  after  magnetic  stimulus  for  (a)  1‐min (b) 3‐min and (c) 5‐min duration. 

Because it is hard to obtain an accurate estimate of the exposed surface area of the MSN@Fe3O4 nanocarriers upon magnetic stimulus, from, for instance, microscopy examination, we employed the weight loss of the MSN@Fe3O4 as a parameter for the estimation by assuming an average of monolayer coverage of Fe3O4 was measured upon a

Magnetic

magnetic stimulus indicating that the magnetic nanoparticles were separated apart from the MSN surface. The weight loss (%) of the MSN@Fe3O4 nanocarriers is given in Figure 4.9 using the Eq. (1) below.

After a 1-min exposure under the field, the Fe3O4 NPs on the surface of MSN lost 10.86% in weight and became more pronounced as the time of stimulus elapsed. To investigate the relationship between the weight loss of nano-Fe3O4 caps and amount of CPT released, two timespans were chosen, i.e., 10 min and 12 hours of CPT release, as shown in Figure 4.10a.

The former timespan assumes that the releasing profile is

Figure 4.9 The weight loss (%), numbers and unoccupied surface area of Fe3O4 NPs on one  MSN@Fe3O4 nanocarriers surface under exposure to a magnetic stimulus for 0 to 5 min. 

approaching steady-state kinetics after a short time period when the nano-caps were removed, while the 12-h timespan suggests a steady-state profile reached after the nano-caps were magnetically lifted off. A linear correlation between the amount of drug released and the weight change of the MSN@Fe3O4 nanocarriers was measured for the two timespans. The slope of the straight lines is nearly the same, which suggests (1) the mechanism for the drug

surface

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release remained the same across these two timespans, which is considered diffusion kinetics, and (2) the release amount of the drug can be linearly correlated with the weight loss of the MSN@Fe3O4 nanocarriers.

Such a magnetic-induced removal of the nano-caps implies a cleavage of the chemical bond between the DMSA-Fe3O4 nano-caps and silanized MSN, forming a chemical link as schematically drawn in Figure 4.10b. The bond energy of such a chemical link can be a sum or a portion of individual bonds including C – C (83 kcal/mole), C – N (74 kcal/mole), C – Si (83 kcal/mole), O – CO (110 kcal/mole), and C = O (179 kcal/mole). To effectively break the bond, a higher magnetic-induced power is indispensible, and the magnetic power employed is 2.0 kW (=0.48 kcal/sec) in this study. For a period of stimulus from 1, 2, 3, 4 to 5 minutes, an energy of 28.8, 57.6, 86.4, 115.2 and 144 kcal, was produced, respectively. The molar ratio of MSN: Fe3O4 NPs = 1 : 2.07, which corresponds to a total of 3.45 x 10^-4 mole of Fe3O4

nano-caps. The surface area of one Fe3O4 NP projected onto the MSN is 49.24 nm², and the average radius of one DMSA molecule is 0.36 nm, corresponding to a surface area of 0.41nm² covering the Fe3O4 NP. Therefore, there are 121 molecules of DMSA on one side of the Fe3O4

NP attached to the MSN surface, and it is also suggestive of 121 units of chemical bonds linked between one Fe3O4 NP and MSN. As such, a total of 4.17 x 10^-2 mole chemical bonds can be estimated between MSN and Fe3O4 NPs. The bonding energy of these chemical bonds is multiplied by 4.17 x 10^-2, which gives the total bond energy anchored between the DMSA-Fe3O4 nano-caps and functionalized MSN nanoparticles: C – C: 83 x 4.17 x 10^-2 = 3.46 kcal, C – N: 3.09 kcal, C – Si: 3.46 kcal, O – CO: 4.59 kcal and C = O: 7.46 kcal.

Accordingly, the energy induced from the magnetic stimulus over various timespans is sufficiently large to cleave the chemical bond, resulting in an effective removal of the nano-caps (Figure 4.8). The rate of nano-cap removal can then be a function of the magnetic energy.

Figure  4.10  (a)  The  relationship  between  the  weight  loss  of  Fe3O4  NPs  and  the  released  amount of CPT for 10‐min and 12‐hours releasing duration. (b) Schematic illustration of the  chemical  bonding  between  Fe3O4  NPs  and  MSN.  (c)  and  (d)  The  FT‐IR  spectrum  of  DMSA‐Fe3O4 NPs,MSN@Fe3O4 and MSN@Fe3O4 after 3min of magnetic stimulus. 

The DMSA-Fe3O4 NPs, MSN@Fe3O4 and MSN@Fe3O4 with 3-min magnetic stimulus were verified using infrared spectroscopic analysis, as shown in Figure 4.10c. For the DMSA-Fe3O4 NPs, the typical Fe-O stretch is at 601 cm^-1, and the peak at 2374 cm^-1 is assigned to S-H stretching. In addition, there are two C = O stretches; the one at 1624 cm^-1 is referred to the stretching mode of carboxylate due to the interaction for the carboxylate anion with the Fe3O4 surface. The other, the C = O stretching of carboxylic acid, appears at 1732 cm^-1. For the MSN@Fe3O4, Si-OH (1085 cm^-1) in the IR spectrum was identified due to MSN.

The C = O stretching mode from carboxylic acid was also detected, but its intensity was reduced because of amide bond formation. The finding indicates that both the N - H (1538

50 

cm^-1) and C = O (1636 cm^-1) bonds were present because of the chemical reaction between the carboxylic acid group of DMSA-Fe3O4 and the amino group of MSN to form the amide linkage. After the 3-min magnetic stimulus, the N-H stretch mode was reduced in intensity, and the C = O stretch band shifted from 1636 to 1733 cm^-1, indicating that the amide bond was cleaved to form carboxylic acid. An enlarged version in the IR spectra between 3100 cm^-1 and 3500 cm^-1, shown below (Figure 4.10d), clearly indicates that N - H (3350 cm^-1) bond was present because the secondary amines of amide bond were formed. However, after the 3-min magnetic stimulus, the N-H stretch mode changed from singlet peak to doublet peak (3300 cm^-1 and 3400 cm^-1), indicating that the secondary amines of amide bond were cleaved to form the primary amine. This observation does provide evidence for bond cleavage of the amide linkage. From the bond strength information, the amide bond is the weakest among others. Combining the bond strength and IR analysis, it is reasonable to believe that the amide bond was broken under magnetic stimulus and takes major responsibility for the removal of the Fe3O4 NPs from the MSN surface.

Further estimation on the exposed surface area of the MSN upon magnetic stimulus was approached using the Eq. (2) described below.

where w is the total weight of Fe3O4 NPs for an MSN@Fe3O4 nanocarrier, P is a random probability of 64% (for a randomly packing configuration of the nano-caps on a given MSN surface area), R is the radius of MSN, ρ is the density of Fe3O4 NPs5.17g/cm³, h is the thickness of Fe3O4, r is the radius of Fe3O4 NPs, and N is the numbers of Fe3O4 NPs on the surface of a MSN nanoparticles. From TEM analysis it was clear that R is 45.8 nm, h is 7.1 nm, and r is 2.8 nm, which suggests an average of 1.2 nano-caps deposited on the surface, which is reasonably close to an earlier assumption of monolayer coverage. On this basis, the exposed surface area should be correlated in a quantitative manner with the weight change of

N

the nanocarriers after being subjected to the magnetic stimulus of various timespans. Figure 4.10 gives resulting calculations and, as expected, a linear relationship; Figure 4.11 can be well-estimated between the weight change (and drug release) of the MSN@Fe3O4

nanocarriers and corresponding surface area being exposed as a result of nano-cap removal.

This finding strongly implies the technical potential for advanced use of such capped functionalized nanocarriers for various industrial sectors where a programmable chemical or biochemical reaction, patient-specific therapy, or controlled catalysis can be magnetically tunable. However, as the main research theme of this work, a drug delivery nanosystem is tested, and the cellular uptake and cytocompatibility are prime concerns to be clarified.