Nanoparticle characterisation
Polystyrene particles (Polybead® Microspheres 0.20 μm, Polysciences) were used as model hydrophobic NMs. The same types of polystyrene particles modified with carboxyl groups (Polybead® Carboxylate Microspheres 0.20 μm, Polysciences) were used as a hydrophilic model.
Particle size distribution was measured by dynamic light scattering (DLS) using a Zetasizer Nano-ZS instrument (Malvern Instruments Ltd., UK) with temperature control (24.9 °C). Measurements of each sample were performed in duplicate with an equilibration step of 120 s. Acquisition time was 80 s. The software was set to automatic acquisition mode. Hydrodynamic diameters were calculated using the internal software analysis. Zeta potential was measured using the same instrument and recorded in a DTS1060C disposable cell with an equilibration time of 120 s. Measurements were done just after pH measurement. A Smulochowski model with a F(Ka) of 1.5 was used.
To confirm the difference in hydrophobicity of the different types of NM, their contact angles with water were measured. Briefly, the original dispersion provided by the supplier was centrifuged and washed in a solution of ethanol/water (30:70) for three times in order to remove the surfactants which might interact with wettability measurements. The NMs were then dispersed in ultrapure water and left to dry on the substrate to create a homogenous layer made of colloids. The contact angle measurements were performed with Milli-Q water as probe liquid, at room temperature. Also, in order to measure the NM dispersive (LW) component of the surface free energy, the contact angles with α-bromonaphtalene were measured with the same method.
Octanol/water partition was also done in order to evaluate the nanomaterials’ hydrophobicity/hydrophilicity. Briefly, 2 ml of octanol and 2 ml of water were added to a glass bottle; the octanol volume stays on top of the water being lighter and totally immiscible. A solution of 100 μl PS particles on the one hand and PS-COOH particles on the other hand were then added to the bi-phase solution at a concentration of 0.4% solid content. The bottle was strongly shaken in order to favour the dispersion of the NMs in the two media. The presence of NMs in the two phases was then observed by naked eyes.
Surface characterisation
In order to have a complete characterisation of the surfaces, different techniques have been used.
Thickness and refractive index of each deposited layer were measured by Ellipsometry (Vase VUV™ J.A. Woollam Co.). All measurements were performed in air at room temperature for different angles of incidence (between 40° and 70°) with a step width of 0.5° and a low-capacity laser with the wavelength λ = 554.3 nm used as a light source. Conventional polarizer-compensator-sample-analyser (PCSA) null-ellipsometric procedure was used to obtain maps of the Δ and ψ angles. The thickness and the complex refractive index were calculated from these two angle maps by point-by-point modelling using the software provided with the ellipsometer, using a two-layer model with the silicon wafer as first layer and a Cauchy layer as the second.
XPS measurements were carried out with an ultra-axis spectrometer (Kratos Analytical Ldt., Manchester, UK) equipped with a monochromatic Al Kα source (hν = 1486.6 eV), operated at 150 W with a spot of 100 μm in diameter. The base pressure was better than 3 × 10−9 mbar, and the analysis pressure better than 10−8 mbar. Survey spectra (0 to 1150 eV binding energy (BE) range) were collected at a 90° take-off angle (with respect to the sample surface) and with a pass energy of 160 eV. High-resolution spectra were recorded at the same conditions but with a pass energy of 20 eV. Surface charge was compensated by a magnetic charge compensation system, and the energy scale was calibrated by setting the C1s hydrocarbon peak to 285 eV. For each sample, at least three measurements were carried out in a non-superimposing region to investigate the film uniformity.
Data were processed using the Vision 2 software (Kratos Analytical). Curve fitting of C1s peaks were performed using the same initial conditions and inter-peak constraints for each spectra. The Gaussian to Lorentzian mix was varied between 0.7 and 0.9, while the full-width half maximum (FWHM) was kept constant. The area of the β-shifted carbon was constrained to be equal to that of the COOH/R component. The position of the C–O and C=O components were fixed at 1.35–1.5 eV and 2.7–2.85 eV from the CH or C–C component, respectively.
ToF-SIMS analysis was conducted using a reflection-type TOF-SIMS IV spectrometer (ION-TOF GmbH, Münster, Germany) equipped with a 25-keV liquid metal ion gun (LMIG) operating with bismuth primary ions. Spectra were acquired in static mode (primary ion fluence <1012 ions cm−2) in order to preserve the molecular information. During analysis, charging of the surface was compensated using low-energy (~20 eV) electron flood gun. For each samples, four positive and four negative spectra were acquired in the non-superimposing regions. Mass calibration of ToF-SIMS spectra was done by using the hydrocarbon peaks CH+ (13 u), CH3
+ (15 u), C2H3
+ (27 u), C3H5
+ (41 u), C5H7
+ (67 u) and C7H7
+ (91 u) for positive ion spectra in order to ensure a good relative mass accuracy. Analyses were obtained from square areas of 200 × 200 μm2 (1128 × 128 pixels) in high mass resolution burst mode (resolution M/ΔM>6000). Spectral interpretation was carried out using Surface Lab software v6.4 (ION-TOF GmbH, Münster, Germany).
The wettability of the modified substrates was measured using the sessile drop method with a static contact angle Goniometer (GBX Digidrop, France) employing Milli-Q water and α-bromonaphtalene separately as probe liquid at room temperature. In brief, a 2-μl drop of the probe liquid was dropped from a calibrated micro-syringe over each substrate (taken in triplicate) at three different locations, then the nine measurements were then averaged. The contact angle was measured after each step of the surface modification procedure, on the unmodified silicon wafer first then on PTFE and PAA, as well as after each polyelectrolyte layer on the different substrates.
Surface morphology was measured by scanning probe analysis with a commercial atomic force microscope (SMENA head, Solver electronics, NT-MDT, Russia). The positioning system was equipped with a 3-D closed loop, in order to correct the non-linear behaviour of the piezoelectric crystal. The topography measurements were carried out using a standard tapping mode silicon cantilever with a nominal force constant of 5 N/m.
For the collection of force-distance curves, a standard silicon tip mounted on a soft cantilever (force constant 0.01 N/m), with a nominal radius of curvature of 10 nm was used. Briefly, the tip was brought to contact with the surface in tapping mode, by setting the z-piezoelectric at the middle of its maximum extension (the maximum extension was around 6 μm). Then the system was switched to contact mode and the cantilever was moved away from the surface of about 1 μm, then approached to the surface at a constant speed of 1 μm/s and pushed against the surface for about 0.2 μm. The cantilever was then brought back to the original position (1 um above the surface). The cantilever deflection was recorded as a function of the position of the z-piezoelectric for the approach and the retract curve. The cantilever deflection is a direct measurement of the interaction forces occurring between the tip and the surface. In particular, the adhesion force (when present) is measured when the tip is retracted from the surface right after the indentation, making the cantilever deflect downwards (i.e. with a negative deflection value).
Finally, zeta-potential measurements were performed for a range of pH values from 3 to 10 in order to determine the surface charge using an ElectroKinetic Analyser (Anton Paar, Austria) with a rectangular clamping cell suitable for small flat substrates, based on the streaming potential method. Inside the cell, the sample was pressed against a PMMA spacer with seven rectangular channels. Therefore, the measured zeta potential includes a contribution from the PMMA spacer, which can be eliminated by measuring a reference PMMA surface. For this purpose, a PMMA reference curve was also determined by measuring its zeta potential under the same measuring conditions as the one used with the PAA- and PTFE-modified samples. The pH was adjusted by adding 0.1 M HCl or 0.1 M NaOH. The raw zeta-potential values for both samples were measured in a solution of 1 mM KCl and in steps of approximately 0.5 pH units by automatic titration with 0.1 M HCl. To ensure good statistics, four single measurements with alternating flow direction were taken for each stabilised pH. The zeta potential was calculated based on the Helmholtz–Smoluchowsky equation: ζ = (dU/dp) × (η/εε0) × K, where ζ is the zeta potential, dU is the streaming potential, dp is the pressure differential across the sample, η is the viscosity of the electrolyte solution, ε is the relative dielectric constant of the fluid, ε0 is the vacuum permittivity and K is the specific electrical conductivity of the electrolyte solution. The corrected zeta-potential (ζc) values for the different samples were obtained by using the equation ζc = 2 × ζsample − ζPMMA for each concerned pH.
Nanoparticle-binding study
The two model particles were incubated with collector surfaces with tuned properties to determine their binding, resulting from interaction forces between particles and surfaces. In order to tune the electrostatic forces, the experiments were performed under 16 different conditions of salt concentration ([NaCl] = 0/1/10/100 mM) and pH (2/4/7/10) in aqueous solution. The incubation with NMs was done by full immersion of the substrate in the different NM dispersions for 30 min. The surface is then rinsed thoroughly with Milli-Q water and dried under nitrogen flow before being imaged by scanning electron microscopy (SEM).
SEM measurements were performed by a FEI NOVA 600, Dual Beam, using 5 keV acceleration voltage and acquiring secondary electrons. The average size of particles was calculated through ImageJ software, from at least 100 particles. The surface coverage was calculated from the SEM images using the same software.
Calculation of the acting potential between nanoparticles and collectors
According to the XDLVO theory (van Oss
1993), the total interaction energy
G
tot between a flat surface and nanoparticles can be expressed as:
$$ {G}^{\mathrm{tot}}={G}^{\mathrm{el}}+{G}^{\mathrm{AB}}+{G}^{\mathrm{LW}} $$
(1)
where G
el
G
AB and G
LW are related to the electrostatic, acid-base and Lifshitz-Van der Waals interactions, respectively. The three potential depends on the distance between the NM and the surface.
Electrostatic interaction energy:
$$ {G}^{\mathrm{el}}=\pi \varepsilon {R}_N\left({\zeta}_N^2+{\zeta}_S^2\right)\left(\frac{2{\zeta}_N{\zeta}_S}{\zeta_N^2+{\zeta}_S^2}\times \ln \frac{1+ \exp \left(-\kappa d\right)}{1- \exp \left(-\kappa d\right)}+ \ln \left\{1- \exp \left(-2\kappa d\right)\right\}\right) $$
(2)
where
d is the separation distance between the NM and the surface and
ζ
N
and
ζ
S
are the zeta potential of the nanoparticle and the collector surface, respectively.
1/κ is the double-layer thickness, which is expressed from the equation:
$$ \kappa ={\left(\frac{e^2}{\varepsilon kT}\sum { i z}_i{n}_i\right)}^{1/2} $$
(3)
where
ε is the permittivity of the medium,
e is the charge of electron,
k is the Boltzmann constant,
T is the temperature,
z
i
is the valency of the ions
i, and
n
i
is their number per unit volume.
The Lifshitz-Van der Waals
ΔG
LW
components to the free energy of interaction between a nanoparticle and surface are calculated following the XDLVO theory:
$$ {G}^{\mathrm{LW}}=-\frac{H}{6}\left(\frac{2 r\left( d+ r\right)}{d\left( d+2 r\right)}- \ln \frac{d+2 r}{d}\right) $$
(4)
where
d is the separation distance between NM and surface, and
r is the nanoparticle’s radius.
H is the effective Hamaker constant for the NM-collector-water system, which can be expressed as:
$$ H=24\pi {d}^2\left(\sqrt{\gamma_N^{\mathrm{LW}}}-\sqrt{\gamma_w^{\mathrm{LW}}}\right)\left(\sqrt{\gamma_s^{\mathrm{LW}}}-\sqrt{\gamma_w^{\mathrm{LW}}}\right) $$
(5)
While the analytical expressions for the electrostatic potential and the Lifshitz-Van der Waals potentials are well known and commonly accepted, the acid-base interaction potential has mainly an empirical formulation based on experimental observations (Boks et al.
2008; van Oss
1993; Wood and Rehmann
2014) and on direct measurements of the interaction potential between two surfaces (sphere-sphere, sphere-plane, plane-plane) in a polar medium or in an electrolytic solution. The
G
AB includes all those forces, which involve the structural reorganisation of the water molecules around two surfaces, depending on the degree of wettability of the surfaces involved. For a sphere-plane system:
$$ {G}^{\mathrm{AB}}=\pi r\lambda F\left( r,\lambda \right)\varDelta {G}^{\mathrm{AB}}{e}^{\left(\left({d}_0- d\right)/\lambda \right)} $$
(6)
where
d
0 is the minimum separation distance between the NM and the surface, taken generally as 0.158 nm for many different kinds of substrates and
d the separation distance in nanometers.
G
AB is defined as a short-range acting potential, having an exponential decrease with the distance. The field of interaction of the potential is mainly determined by the correlation length
λ, expressed in nanometers. Various values for
λ have been reported in literature, ranging from 0.2 to 13 nm (van Oss
1993; Wood and Rehmann
2014). The AB interaction can range from distances less than 1 nm up to very few tenths of nanometers and thus compete with the long-range electrostatic and LW potentials. The
F(
r ,
λ) term is a function taking into account the shape and the size of the interacting objects. An analytical expression for
F(
r ,
λ) between a sphere and a plane can be found in the article from Wood and Rehmann (
2014).
F(
r ,
λ) depends on the ratio between the radius of the sphere
r and the correlation length
λ and tend to the unity when
r >>
λ. In this work, we have used relatively large NMs, with
r > 100 nm, so
F(
r ,
λ) can be considered equal to unity.
The nature of the two interacting surfaces intervenes in the AB potential with the term Δ
G
AB that can be expressed as:
$$ \varDelta {G}^{\mathrm{AB}}=-2\ \left(\sqrt{\gamma_N^{\mathrm{AB}}}-\sqrt{\gamma_W^{\mathrm{AB}}}\right)\times \left(\sqrt{\gamma_S^{\mathrm{AB}}}-\sqrt{\gamma_W^{\mathrm{AB}}}\right). $$
(7)
where the term
\( \sqrt{\gamma_i^{AB}} \) refers to the polar component of the surface free energy for the nanoparticle (
N), water (
W) and surface (
S). The values for the
\( \sqrt{\gamma_i^{\mathrm{AB}}} \) have been calculated using the Owen-Wendt-Fowkes equation (Eq.
8) and determined experimentally with the contact angle between the surface of the collector and two liquids.
$$ \sqrt{\gamma_{sv}^{\mathrm{LW}}{\gamma}_{lv}^{\mathrm{LW}}}+\sqrt{\gamma_{sv}^{\mathrm{AB}}{\gamma}_{lv}^{\mathrm{AB}}}=0.5{\gamma}_{lv}\left(1+ \cos {\theta}_y\right) $$
(8)
where s is solid (surface), l is the liquid (water or bromonaphtalene), v is the vapour (air) and θ is the contact angle.
The potentials were calculated using the function wizard included in the software OriginPro 2015.