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3 Chapter 3 Particle size control using solvent shift technique
57 Figure 15 Jiang et al. results showing the effect of particle size of gold NPs on the endocytosis and uptake of the NPs in breast cancer cell lines.
Figure 16 Cabral et al. results showing the effect of particle size of polymeric micelles on the tumour volume and dose accumulation in prostate cancer cell lines
58 Surface charge
Regarding the surface charge on particle, positively charged particles could decrease blood circulation time, due to opsonisation and clearance by reticuloendothelial system (RES), compared to the negatively charged and uncharged particles. Interestingly, it has also been reported that, the positively charged particles possess better accumulation in tumour tissues due to the favourable interaction between the positively charged NPs and the tumour blood vessels, hence preventing their redistribution in the systemic circulation (172).
Nanoparticle shape
Meanwhile, the shape of the nanoparticle could have an influence on the behaviour of the nanoformulation. Chauhan et al.(173) compared the tumour distribution kinetics of nanorods with a length of 44 nm and nanospheres of 35 nm in mammary tumours in mice. Their hydrodynamic diameters were found to be same. As compared to the nanospheres, they found that the nanorods were transported across vessel walls 4.1-times faster. The penetration inside the tumour was also 1.7-times higher for nanorods as compared to the volume to which the nanospheres were distributed.
Accordingly, developing LDL-like nanoparticles with a controlled size in the range of <50 nm is still a hurdle. This size range is required not only to mimic the real LDL in their size and properties (20-25 nm), but also to ensure the best accumulation and penetration to the tumours and consequently reaching the best therapeutic outcome.(174) Therefore, rapid solvent shift technique was studied as a simple and straightforward method for NPs preparation with particle size control.
3.1.2 Parameters that Govern Nanoparticle Size Prepared through Rapid Solvent Shifting
Rapid solvent shifting technique is a simple technique that depends on the fact that when a solute dissolved in a specific solvent and then swiftly exposed to an excess of anti-solvent(175),
59 it precipitates out of solution due to change in the solubility. However, this entire process of precipitation driven by solvent shifting, can be subdivided into discrete events in time, characterized by the solute physico-chemical properties. Assessing these parameters from the basic principles of solution, fluid-flow, mixing, nucleation, growth, and colloid stabilization can thereby provide the necessary understanding of the nanoparticle fabrication by rapid solvent shifting process. These parameters include:
1. Organic Solvent: the used organic solvent influences the nature of the organic solution of test materials, drugs and/or lipids, governing the amount of the materials used and hence the degree of supersaturation.
2. Fluid Flow: The nature of the fluid flow and the dependence of turbulence on the Reynaud’s number. The nature of the fluid flow is determined by parameters such as the geometry of the flow channel, pipe’s cross-sectional area, volumetric flow rate, the viscosity of the flowing liquid. These parameters can be normalized through one dimensionless number called Reynolds number (Re). Re is the ratio of inertial forces and the viscous forces for organic and aqueous streams of flow(176).
3. Mixing: The extent of mixing of organic and aqueous phase under either diffusion limited or turbulent flow greatly affect the growth and condensation of the formed nuclei.
4. Solubility and Supersaturation in Anti-Solvent: The solubility of the test material in the anti-solvent and its degree of supersaturation governs the stability of the formed nuclei and controls the rate of particles growth.
5. Nucleation: Nanoparticle nucleation and its initial critical radius of the employed material will determine the final particle size
6. Post nucleation events: Collection of the nanoparticles and their colloid stability, and any further aggregation, coalescence, or ripening, especially against time and ionic strength, will affect the properties of the formed NPs.
60 7. Surface Properties: The presence of a surface monolayer and/or coating of the NPs will have a direct influence on the initial precipitation and subsequent colloid stability
8. Phase separation: Any phase separation for solute mixtures.
With the help of first six parameters, the mechanism of nanoparticle formation through rapid solvent shifting was understood. Last two parameters may help in controlling the nanoparticle size and designing the nanoparticle Formulation.
3.1.3 Effect of concentration on the size of nanoparticles
For the sake of understanding the effect of the concentration of the hydrophobic materials added in organic solvent on the size of nanoparticles, it is reported that an increase in the size of nanoparticles due to increased supply of solute monomers is expected.
Moreover, the interparticle distance could be studied by utilizing Wigner-Seitz radius (spherical). This ‘Wigner-Seitz radius’ is often used in atomic physics and is defined as the radius occupied by one atom, and is also reported as a measure of interparticle distance. For instance, in a 10 ml volume, the density of molecules could be calculated for a range of concentrations using following equation (177).
Equation 13 𝑹 = ( 𝟑
𝟒𝝅𝒏)𝟏/𝟑
Where R is Wigner Seitz radius and n is the particle density.
Though this is considered an approximation, it is expected that with increasing the molar concentration of the solute in the solution, the time averaged intermolecular distance decreases, thus leading to more encounters of the solute molecules resulting into increase in size.
Therefore, for a given set of parameters (such as the geometry used for rapid solvent shifting, at lab temperature 20 ○C), an increase in the measured size of the nanoparticle with increase in concentration of solute is expected, in absence of any coating or stabilisers.
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