The extracellular spaces were probed with fluorescent nanoprobes with a radius from 1 to over 100 nm, encompassing the size of most therapeutic agents used in cancer treatment. I used TRITC-dextrans and PEG-coated fluorescent silica nanoparticles filled with rhodamine B, custom-synthesised by Siliquan (Siliquan, Poland). Listed nanoparticles along with their radii are presented in Tab. 5.1.
Table 5.1: Nanoprobes used in the experiments and their values of radius.
Sample name rp [nm]
TRITC-dextran 4.4 kDa 1.3 ± 0.2
TRITC-dextran 20 kDa 3.8 ± 0.3
TRITC-dextran 40 kDa 4.9 ± 0.5
TRITC-dextran 155 kDa 8.6 ± 0.7
PEG coated S34(1) silica nanoparticles 20.6 ± 1.3 PEG coated S43(2) silica nanoparticles 66.2 ± 3.1 PEG coated S44(3) silica nanoparticles 110.7 ± 3.3
The physical properties of the nanoparticles, such as their charge and density, may have an impact on mobility measurements.
Dextrans are neutral molecules. The degree of substitution of TRITC in dextran ranges from 0.001 to 0.008. The charge contribution from the tertiary amino groups on the rhodamine moiety is negligible at these low degrees of substitution. In the case of silica nanoparticles, they are coated with neutral surface charge polyethylene glycol (PEG). The neutral charge of all used nanoprobes eliminates the possible interaction with extra- and
80 5.6. Fluorescent tracers
intracellular components.
The below considerations prove that due to the low density of silica, the measured motion of the nanoparticles in the ECM of spheroids results from their Brownian motion and is not altered by the sedimentation process.
The main competing forces acting on the motion of an individual uncharged particle in a viscous fluid are gravity, buoyancy and hydrody-namic drag. For a spherical particle, sedimentation force (gravity and buoyancy) are given by:
F~g = 4
3πrp3(ρp−ρf)~g (5.3) where ρp is the density of the particle, ρf is the density of the fluid, rp is the particle radius, and g is the acceleration of gravity.
The hydrodynamic drag (for small Reynolds numbers) on a spherical particle is given by:
F~d = 6πηrp~v (5.4)
where η is the viscosity of the fluid, and v is particle velocity.
A force-balance of gravity, buoyancy and hydrodynamic drag yields the sedimentation velocity of a single particle sedimenting in the fluid of viscosity η as:
v = 2rp2(ρp−ρf)
9η (5.5)
The sedimentation speed depends on the physical properties of a particle – its size and density. In addition to motion related to settling of particles,
stochastic motion occurs and is characterised by a self-diffusion coefficient, which does not depend on the density and is the same for any (i.e. carbon, silica or gold) same-sized nanoparticles:
D = kT 6πηrp
(5.6)
Where k is the Boltzmann constant, T (K) is temperature. The diffusion coefficient is defined by the mean square displacement:
Dx2E= 6Dt (5.7)
To estimate how the density of particles affects the calculation of effective viscosity, I consider which effect dominates - the molecular displacement due to the sedimentation of particles or due to Brownian motion?
The missing parameter to estimate the exact value of sedimentation velocity is the density of the fluid ρf – in our case, the density of the ex-tracellular matrix. Here, we assume the density of water, ρf = 997kg/m3, the actual (for sure higher) density value will increase the sedimentation velocity, since v ∼ ∆ρ.
If we consider silica nanoparticles (ρp = 2650kg/m3) with radius rp=111 nm (the biggest nanoprobe used in experiments) moving in the ECM of HeLa cells whose viscosity I found to be ηmacro = 3.32mP as (more details in the next section, 5.7), the sedimentation speed is approximately 13nm/s. Therefore, during the 120 seconds (the time of data acquisition) the particle will cross a distance of L = 1.6µm (L = vt). At the same time, the root mean displacement x of the same nanoprobe resulting from Brownian motion (the determined from experiment diffusion coefficient equals to D = 0.66µm2/s) is around 22 µm. We can clearly see that x > L and the sedimentation does not influence our probes’ Brownian motion.
Since carbon nanoparticles have a similar density (ρp=2260 kg/m3) as silica nanoparticles, the sedimentation is also negligible. In the case of gold nanoparticles (ρp = 19320kg/m3) with the same radius, moving in the same environment, the sedimentation velocity is 148nm/s. After a time t=120 s, the molecular displacement resulting from settling equals 18 µm – it is a comparable distance with the root mean displacement resulting from diffusion, x ∼ L.
To conclude, the physical properties of nanoparticles, such as density, may contribute to their transport properties. However, in the case of
82 5.6. Fluorescent tracers
silica nanoparticles used in our experiments, diffusion was a prevailing process, and, as a consequence, the calculation of effective viscosity was done correctly [10].
Another problem that can arise in relation to nanoparticles is their cytotoxicity. All the probes used in the study can be uptaken by cells through endocytosis or pinocytosis. Nevertheless, the cellular uptake of the tracers in no way interferes with the measurements of their self-diffusion in the extracellular space. The only consequence of the penetration of nanoparticles into cells was their lower concentration in the ECM. To avoid a too low fluorescence signal, the increased concentration of nanoprobes was added to the medium with spheroids (to a final concentration 100 nM). Since FCS enables an estimate of the concentration of fluorescent particles in the sample, I additionally quantified the cellular uptake from the relation 2.5, which was around 90 nM of nanoprobes.
In the assessment of nanoprobe cytotoxicity, apart from their concentra-tion, time of exposure is an important parameter. In the experiments, nanoprobes were added a maximum of 24 h before FCS experiments, which took another 8 h.
In order to examine the cytotoxicity of nanoprobes, I performed an MTT assay in accordance with the protocol described in A.11. The MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) assay is based on the conversion of MTT into chromogenic formazan crystals by living cells, which determines mitochondrial activity. Since for most cell populations, the total mitochondrial activity is related to the number of viable cells; this assay is broadly used to measure the in vitro cytotoxic effects of substances on cell lines. The mitochondrial activity of the cells is reflected by the conversion of the tetrazolium salt MTT into formazan crystals. Thus, any increase or decrease in viable cell number can be detected by measuring formazan concentration reflected in optical density (OD) using a plate reader at 540 nm [132].
Since the readout of the formazan concentration is based on absorbance, I was unable to use the nanoprobes with dye absorbing in the readout wavelength as this would result in erroneous results. For this reason, I used non-labelled dextran 10 kDa and two different-sized PEG-coated silica nanoparticles filled with fluorescein isothiocyanate (FITC) (Siliquan, Poland). The fluorescence properties allowed me to measure their concen-tration (by means of FCS), and in contrast to rhodamine B, FITC does not absorb light at 540 nm.
The cytotoxicity profiles of these three types of nanoprobes against the cell lines used in my study are shown in 5.7.
Figure 5.7: Cytotoxicity of dextran 10 kDa and two different-sized (27 and 89 nm in diameter) PEG-coated silica nanoparticles filled with FITC at different concentrations with HeLa, MCF-7 and fibroblasts cells after 24 and 72 h exposition. Results are expressed as mean ± SD. Each substance was tested in 9-fold replicates at each concentration level.