**3.1 Surface conditioning with PAH**

The choice of pH = 11 for PAH adsorption was inspired by the following considerations. The pKa of ethyl ammonium is 10.6. The apparent pKa of the PAH used here is 8.7 in water and 9.3 in 0.5 M NaCl (Petrov et al., 2003; Choi and Rubner, 2005); however it is expected to be appreciably higher after adsorption by a negatively charged surface (Tagliazucchi et al., 2008). Conditions of low degree of protonation were chosen in order to allow adsorption of a thick layer. The adsorbed amount was indeed reported to be higher for polymers with low and intermediate cationicities, such as PAH or PLL compared to PEI, and to increase with the molecular mass (Roberts, 1996; Lafuma, 1996), owing to a more coiled conformation. Highly charged polycations, such as PEI, form flat adsorbed layers at low ionic strength (Claesson et al., 2005) and the adsorbed amount increases with pH (Meszaros, 2004).

The surface chemical composition of glass and glass conditioned with PAH (3 independent sets of results) is presented in Table 1. Non-conditioned glass showed the expected organic contamination and a low concentration of nitrogen. In addition, low concentrations of potassium (1-2 %), boron (2-3 %), sodium (1-1.5 %), and traces (< 1 %) of zinc, titanium and aluminium were found. As the analyzed depth decreased (increase of ), the C/Si concentration ratio increased as expected, but N/Si did not vary significantly and N/C decreased. This indicates that nitrogen is associated with the glass matrix or the glass surface and not with the organic contaminants.

After PAH conditioning, polycation adsorption was evidenced by the increase of carbon and nitrogen concentrations and the decrease of silicon and oxygen concentrations. The N1s peak showed components at 401.5 eV and 399.3 eV due to protonated and non protonated nitrogen, respectively. As the analyzed depth decreased (higher ), the apparent concentration of carbon and nitrogen increased as expected, the N/C ratio remained constant, but the proportion of protonated nitrogen decreased appreciably.

The N/C ratio of about 0.14 must be compared with the value of 0.33 expected for pure PAH. This difference may not be due to an orientation of the polymer segments at the surface, given the structure of the repeat unit [-CH2-CH(-NH-CH3)-] and the analyzed depth. It is attributed to the simultaneous presence at the surface of PAH and organic contaminants (Caillou et al., 2008). As the N/C ratio does not vary with , the adsorbed organic layer appears to be a mere mixture, with no preferential accumulation of a component at the outermost surface. In an alternative way, the elemental composition of the adsorbed layer may be estimated by the difference of nitrogen concentration before and after PAH conditioning ratioed to the total carbon concentration, which provides a N/C ratio of 0.11. The adventitious contaminants observed on silica, which showed the same C1s peak shape as that observed here on glass, were modeled by the generic formula C15H28O4

Fabrication of Surfaces with Bimodal Roughness Through Polyelectrolyte/Colloid Assembly 59

(1.000). The superscripts ad and su designate the adsorbed overlayer and the substrate, respectively. The concentrations C are: ad CC = 49.6 mmol/cm3 for contamination layer deposited on glass; ad CC = 46.9 mmol/cm3 for the layer made by a mixture of 67 % contaminents and 33 % PAH (N/C = 0.11); su C = 39 mmol/cm Si 3 for Si in glass (considering that Si concentration in glass is about 90 % of that in pure SiO2; density of 2.6 g/cm3). The electron inelastic mean free paths were calculated according to Tanuma et al. (1997) with an

Si = 3.6 nm, ad

C/Si molar ratios computed using different values of t, as a function of . The curves are only slightly different if a N/C ratio of 0.14 is considered. Comparison with the experimental data indicates that the organic layer adsorbed after conditioning is about 3 nm, which is about 3 times thicker than the contamination layer. This does not fit closely the apparent proportions of contaminants and PAH in a homogeneous adlayer. Such approach cannot be exploited further owing to the simplicity of the model (homogeneous, flat and compact adlayer) and limited data accuracy. However it indicates that the organic adlayer obtained after conditioning has a thickness of about 3 nm and that PAH coexists with contaminants. The latter may adsorb from the surrounding atmosphere between

The degree of protonation of PAH in a solution at pH 11 is less than 10 %. The XPS spectra recorded after PAH conditioning (Table 1) gave values of 31 and 49 % for the degree of nitrogen protonation at a photoelectron collection angle = 0°. This is in agreement with expectations regarding the effect of local potential on protonation and with data indicating that the apparent pKa of PAH may be appreciably increased on a negatively charged surface

Fig. 2. Plot of computed C/Si molar concentration ratios vs the angle of photoelectron collection, for non-conditioned glass with a 0.8, 1.0, 1.2 nm thick (tcont) contamination layer, and for PAH-conditioned glass with a 2.5, 3.0 and 3.5 nm thick (tPAH+cont) adlayer made of PAH and contamination. Experimental data for glass (open symbols) and for PAHconditioned glass (dark symbols). Circles: samples 1 and 4, triangles: samples 2 and 5,

C = 4.4 nm. Figure 2 presents plots of

energy gap of 7 eV: ad

(Tagliazucchi et al., 2008).

squares: samples 3 and 6 (see Table 1).

C = 3.9 nm, ad

sulfochromic cleaning and PAH conditioning or after conditioning.


(Gerin et al., 1995). According to a N/C ratio of 0.11, the adsorbed layer obtained after PAH conditioning would be 33 wt% PAH (density 1.04 g/m3) and 67 wt% C15H28O4 (density of 0.9 g/cm3).

Table 1. Surface chemical composition determined by XPS (mole fraction with respect to all elements excluding hydrogen) at different photoelectron collection angles .

In order to evaluate the thickness of the organic adlayer, the experimental C/Si ratios were compared with ratios computed by considering a layer of constant thickness (t) on top of glass and using equation 1:

$$\frac{\mathbf{C}}{\mathbf{S}} = \frac{\mathbf{i}\_{\rm Si}}{\mathbf{i}\_{\rm N}} \frac{\sigma\_{\rm C}}{\sigma\_{\rm Si}} \frac{\mathcal{J}\_{\rm C}^{\rm ad} \ C\_{\rm C}^{\rm ad} \left[1 - \exp\left(-\mathbf{t} / \mathcal{J}\_{\rm C}^{\rm ad} \cos \phi\right)\right]}{\mathcal{J}\_{\rm Si}^{\rm su} \ C\_{\rm Si}^{\rm su} \exp\left(-\mathbf{t} / \mathcal{J}\_{\rm Si}^{\rm ad} \cos \phi\right)}\tag{1}$$

where iSi and iN are the relative sensitivity factors provided by the manufacturer for Si (0.328) and C (0.278); Si and N are the photoionization cross sections for Si (0.817) and C

(Gerin et al., 1995). According to a N/C ratio of 0.11, the adsorbed layer obtained after PAH conditioning would be 33 wt% PAH (density 1.04 g/m3) and 67 wt% C15H28O4 (density of

 Concentration (Mole fraction, in %) Concentration ratios Substrate C O Si N S C/Si N/C N+/Ntot

 sample 1 0° 12.6 57.2 22.8 0.6 -a 0.55 0.05 0.47 45° 15.6 58.4 19.7 0.4 - a 0.79 0.03 0.37 60° 20.5 56.5 18.0 0.4 - a 1.14 0.02 0.33 sample 2 0° 10.1 59.4 24.9 0.9 - a 0.41 0.09 - b 60° 16.5 59.6 20.1 0.8 - a 0.82 0.05 - b sample 3 0° 11.9 57.7 23.6 0.9 - a 0.50 0.07 0.46

 sample 4 0° 24.1 46.6 18.6 3.5 - a 1.30 0.15 0.31 45° 31.9 43.3 14.3 4.8 - a 2.23 0.15 0.18 60° 42.5 36.9 11.3 5.6 - a 3.76 0.13 0.12 sample 5 0° 22.8 48.5 19.5 3.4 - a 1.17 0.15 - b 60° 37.6 40.9 12.5 5.2 - a 3.01 0.14 - b sample 6 0° 30.4 42.5 18.6 4.5 - a 1.63 0.15 0.49

0° 64.1 21.8 8.1 1.6 0.3 7.91 0.02 0.38

0° 41.1 38.0 13.9 2.7 0.2 2.96 0.06 - b

0° 52.2 30.4 11.3 2.7 0.4 4.62 0.05 - b

 

  (1)

Table 1. Surface chemical composition determined by XPS (mole fraction with respect to all

In order to evaluate the thickness of the organic adlayer, the experimental C/Si ratios were compared with ratios computed by considering a layer of constant thickness (t) on top of

> ad ad ad C C <sup>C</sup> Si C su su ad N Si Si Si Si C 1 exp t/ cos C i Si i C exp t/ cos

where iSi and iN are the relative sensitivity factors provided by the manufacturer for Si (0.328) and C (0.278); Si and N are the photoionization cross sections for Si (0.817) and C

elements excluding hydrogen) at different photoelectron collection angles .

PAH pre-coated glass with adherent colloidal particles

0.9 g/cm3).

 Glass coverslips

 sample 7 (470 nm)

 sample 8 (470 nm)

 sample 9 (65 nm)

glass and using equation 1:

PAH pre-coated glass

(1.000). The superscripts ad and su designate the adsorbed overlayer and the substrate, respectively. The concentrations C are: ad CC = 49.6 mmol/cm3 for contamination layer deposited on glass; ad CC = 46.9 mmol/cm3 for the layer made by a mixture of 67 % contaminents and 33 % PAH (N/C = 0.11); su C = 39 mmol/cm Si 3 for Si in glass (considering that Si concentration in glass is about 90 % of that in pure SiO2; density of 2.6 g/cm3). The electron inelastic mean free paths were calculated according to Tanuma et al. (1997) with an energy gap of 7 eV: ad C = 3.9 nm, ad Si = 3.6 nm, ad C = 4.4 nm. Figure 2 presents plots of C/Si molar ratios computed using different values of t, as a function of . The curves are only slightly different if a N/C ratio of 0.14 is considered. Comparison with the experimental data indicates that the organic layer adsorbed after conditioning is about 3 nm, which is about 3 times thicker than the contamination layer. This does not fit closely the apparent proportions of contaminants and PAH in a homogeneous adlayer. Such approach cannot be exploited further owing to the simplicity of the model (homogeneous, flat and compact adlayer) and limited data accuracy. However it indicates that the organic adlayer obtained after conditioning has a thickness of about 3 nm and that PAH coexists with contaminants. The latter may adsorb from the surrounding atmosphere between sulfochromic cleaning and PAH conditioning or after conditioning.

The degree of protonation of PAH in a solution at pH 11 is less than 10 %. The XPS spectra recorded after PAH conditioning (Table 1) gave values of 31 and 49 % for the degree of nitrogen protonation at a photoelectron collection angle = 0°. This is in agreement with expectations regarding the effect of local potential on protonation and with data indicating that the apparent pKa of PAH may be appreciably increased on a negatively charged surface (Tagliazucchi et al., 2008).

Fig. 2. Plot of computed C/Si molar concentration ratios vs the angle of photoelectron collection, for non-conditioned glass with a 0.8, 1.0, 1.2 nm thick (tcont) contamination layer, and for PAH-conditioned glass with a 2.5, 3.0 and 3.5 nm thick (tPAH+cont) adlayer made of PAH and contamination. Experimental data for glass (open symbols) and for PAHconditioned glass (dark symbols). Circles: samples 1 and 4, triangles: samples 2 and 5, squares: samples 3 and 6 (see Table 1).

Fabrication of Surfaces with Bimodal Roughness Through Polyelectrolyte/Colloid Assembly 61

coverage by colloidal particles; PAH was as efficient as, or better than other polymers tested; a polycation concentration of 10-5 M was more efficient than 10-7 M. However in all cases,

Fig. 4. SEM images obtained on PAH-conditioned (10-5M, pH=11, I=10-2M) glass samples, submitted to latex particle adhesion (470 nm in diameter, 0.1%, pH=7, I=10-3M), rinsed with

A more homogeneous particle distribution was obtained when the sample was rinsed first with water then with isopropanol and left to dry in air overnight, as shown in Figure 4b. Both images of Figure 4 present the same degree of particle coverage (~ 40 %), but in image b, no aggregation is present and the particle distribution is more random. The influence of rinsing and drying procedures on the layer of latex particles may be due to their gathering together by the movement of the liquid film or by the nitrogen flow. In order to avoid particle aggregation at high coverage, Hanarp et al. (2001) attempted to retain particles in place during drying by adsorbing smaller silica particles between the latex particles or by heating the samples with adhering particles in boiling water in order to deform the particles and increase the contact area with the substrate. In the present work, rinsing with isopropanol was found to be efficient (Figure 4b) ; this is attributed to the reduction of capillary forces owing to the 3 times lower surface tension of isopropanol compared to

Particle adhesion was further studied using substrates conditioned with PAH as described above, rinsing 3 times with water and 3 times with isopropanol, followed by air drying. Figure 5 presents representative results obtained by incubating the conditioned substrate during 2 hours with the colloids at 0.1 % concentration, using 3 pH values and 2 ionic strengths. At pH 7 with a low ionic strength and at pH 11, substrates were covered by a homogeneous layer of particles. In contrast, a high degree of aggregation was observed at

Figure 6 (a, b) presents micrographs obtained after adhesion of the two kinds of latex particles separately (470 nm or 65 nm; 0.1%, pH 7, I 10-3 M), with isopropanol rinsing and air drying. Two counting methods were used in order to assess the degree of substrate coverage. Binarization and automatic processing of SEM images allow counting to be made more rapidly and on a large number of images in comparison with manual counting.

water and dried under a gentle nitrogen flow (a) or rinsed with water, then with

isopropanol and air-dried (b). Scale bars: 2 µm.

water ((21.7 mN/m compared to 73.0 mN/m (Weast, 1972).

pH 3 whatever the ionic strength, and at pH 7 with a high ionic strength.

the adhering particles were under the form of aggregates as described above.

Figure 3 presents the variation of the zeta potential of glass and of PAH-conditioned glass as a function of pH (in 0.01 M KNO3). As expected, the glass substrate was found to be negatively charged above pH 2. After polycation adsorption, the zeta potential became less negative, but did not reach positive values. It appears (Figure 3) that adsorbed PAH is shielding the negative charge of glass surface but is not reversing the surface charge. This is in contrast with data reported for substrates such as mica and titanium oxide (Adamczyk et al., 2006, 2007) and is attributed to the fact that here the samples were dried between adsorption and streaming potential measurement, which provoked irreversible shrinkage of the adsorbed layer. It may be related to the decrease of the degree of nitrogen protonation as increased (Table 1), revealing that it is much higher for segments in close contact with the glass surface. Note that drying after adsorption was a choice made in the context of a fabrication process.

Fig. 3. Zeta potential (mV) of glass (open symbols) and PAH-conditioned glass (dark symbols) as a function of pH in 0.01 M KNO3. Three experiments were performed on samples prepared independently.

### **3.2 Nanostructured surfaces with monomodal roughness**

Figure 4a shows representative micrographs of samples prepared with substrates conditioned as described above (PAH 10-5 M, pH 11, I 10-2 M), incubated with 470 nm particles (0.1%, pH 7, I 10-3 M), rinsed 6 times with water and dried with a nitrogen flow. The adhering particles were under the form of aggregates, usually bidimensional along the surface plane, sometimes tridimensional (i.e. forming multilayers). Attempts were made to improve the homogeneity of the colloidal particle distribution, keeping the colloidal particle treatment unchanged but using polycations which differ according to functional groups, hydrophobicity and size, and changing polycation treatment conditions : (i) PAH, PLL, PEI, lPDDA and hPDDA; 10-7 M at pH 3 and 10-5 M at pH 11; I 10-3and 10-1 M; (ii) PAH and lPDDA; 10-7 and 10-5 M; pH 3, 7 and 11; I 10-3, 10-2 and 10-1 M. In summary, polycation adsorption at pH 7 or 11 and low ionic strength (10-3, 10-2 M) provided a higher degree of

Figure 3 presents the variation of the zeta potential of glass and of PAH-conditioned glass as a function of pH (in 0.01 M KNO3). As expected, the glass substrate was found to be negatively charged above pH 2. After polycation adsorption, the zeta potential became less negative, but did not reach positive values. It appears (Figure 3) that adsorbed PAH is shielding the negative charge of glass surface but is not reversing the surface charge. This is in contrast with data reported for substrates such as mica and titanium oxide (Adamczyk et al., 2006, 2007) and is attributed to the fact that here the samples were dried between adsorption and streaming potential measurement, which provoked irreversible shrinkage of the adsorbed layer. It may be related to the decrease of the degree of nitrogen protonation as increased (Table 1), revealing that it is much higher for segments in close contact with the glass surface. Note that drying after adsorption was a choice made in the context of a

Fig. 3. Zeta potential (mV) of glass (open symbols) and PAH-conditioned glass (dark symbols) as a function of pH in 0.01 M KNO3. Three experiments were performed on

Figure 4a shows representative micrographs of samples prepared with substrates conditioned as described above (PAH 10-5 M, pH 11, I 10-2 M), incubated with 470 nm particles (0.1%, pH 7, I 10-3 M), rinsed 6 times with water and dried with a nitrogen flow. The adhering particles were under the form of aggregates, usually bidimensional along the surface plane, sometimes tridimensional (i.e. forming multilayers). Attempts were made to improve the homogeneity of the colloidal particle distribution, keeping the colloidal particle treatment unchanged but using polycations which differ according to functional groups, hydrophobicity and size, and changing polycation treatment conditions : (i) PAH, PLL, PEI, lPDDA and hPDDA; 10-7 M at pH 3 and 10-5 M at pH 11; I 10-3and 10-1 M; (ii) PAH and lPDDA; 10-7 and 10-5 M; pH 3, 7 and 11; I 10-3, 10-2 and 10-1 M. In summary, polycation adsorption at pH 7 or 11 and low ionic strength (10-3, 10-2 M) provided a higher degree of

**3.2 Nanostructured surfaces with monomodal roughness** 

fabrication process.

samples prepared independently.

coverage by colloidal particles; PAH was as efficient as, or better than other polymers tested; a polycation concentration of 10-5 M was more efficient than 10-7 M. However in all cases, the adhering particles were under the form of aggregates as described above.

Fig. 4. SEM images obtained on PAH-conditioned (10-5M, pH=11, I=10-2M) glass samples, submitted to latex particle adhesion (470 nm in diameter, 0.1%, pH=7, I=10-3M), rinsed with water and dried under a gentle nitrogen flow (a) or rinsed with water, then with isopropanol and air-dried (b). Scale bars: 2 µm.

A more homogeneous particle distribution was obtained when the sample was rinsed first with water then with isopropanol and left to dry in air overnight, as shown in Figure 4b. Both images of Figure 4 present the same degree of particle coverage (~ 40 %), but in image b, no aggregation is present and the particle distribution is more random. The influence of rinsing and drying procedures on the layer of latex particles may be due to their gathering together by the movement of the liquid film or by the nitrogen flow. In order to avoid particle aggregation at high coverage, Hanarp et al. (2001) attempted to retain particles in place during drying by adsorbing smaller silica particles between the latex particles or by heating the samples with adhering particles in boiling water in order to deform the particles and increase the contact area with the substrate. In the present work, rinsing with isopropanol was found to be efficient (Figure 4b) ; this is attributed to the reduction of capillary forces owing to the 3 times lower surface tension of isopropanol compared to water ((21.7 mN/m compared to 73.0 mN/m (Weast, 1972).

Particle adhesion was further studied using substrates conditioned with PAH as described above, rinsing 3 times with water and 3 times with isopropanol, followed by air drying. Figure 5 presents representative results obtained by incubating the conditioned substrate during 2 hours with the colloids at 0.1 % concentration, using 3 pH values and 2 ionic strengths. At pH 7 with a low ionic strength and at pH 11, substrates were covered by a homogeneous layer of particles. In contrast, a high degree of aggregation was observed at pH 3 whatever the ionic strength, and at pH 7 with a high ionic strength.

Figure 6 (a, b) presents micrographs obtained after adhesion of the two kinds of latex particles separately (470 nm or 65 nm; 0.1%, pH 7, I 10-3 M), with isopropanol rinsing and air drying. Two counting methods were used in order to assess the degree of substrate coverage. Binarization and automatic processing of SEM images allow counting to be made more rapidly and on a large number of images in comparison with manual counting.

Fabrication of Surfaces with Bimodal Roughness Through Polyelectrolyte/Colloid Assembly 63

Fig. 6. SEM images obtained on PAH-conditioned (10-5M, pH=11, I=10-2M) glass samples, submitted to adhesion of latex particles (0.1%, pH=7, I=10-3M, rinsing with water then with isopropanol) with 470 nm (a) and 65 nm diameter (b), and submitted to sequential adhesion of 470 nm and 65 nm particles without (c, bimodal I roughness) and with (d, e, f, bimodal II roughness) PAH adsorption after adhesion of the 470 nm particles. Image f was obtained on a sample prepared in the same experimental set as d, then submitted to immersion in PBS at 37°C for 24h. Images d and e were obtained on samples fabricated independently. The inset shows an enlargement of structures present on surfaces with bimodal II roughness. Scale

bars: 1 µm. Scale bar inset: 500 nm.

However, it involves operations which could possibly create systematic variations. Binarization of SEM images gave degrees of coverage of 50.2 ± 5 % and 14.6 ± 4 % on surfaces layered with 470 and 65 nm particles, respectively. A consistent degree of substrate coverage was obtained using manual counting (470 nm : 45.2 ± 2 %; 65 nm : 10.7 ± 1 %). The differences between the two modes of counting were thus in the range of the standard deviation. Samples with adherent particles were also analyzed by XPS (Table 1). For samples 7 and 8 , 470 nm particles were used and the degree of coverage was 53 and 27 %, respectively; for sample 9, 65 nm particles were used and the degree of coverage was 15 %, as assessed by SEM image analysis. As compared with PAH conditioned substrates with no adhering latex, these three samples gave a higher concentration of carbon, a lower concentration of silicon, oxygen and nitrogen and indicated the presence of sulfur, characterized by a S 2p peak (showing the 2p3/2 and 2p1/2 components with S 2p3/2 observed at 169 eV), characteristic of sulfate and due to the latex surface.

Fig. 5. SEM images obtained on PAH-conditioned (10-5M, pH=11, I=10-2M) glass samples, submitted to latex particle adhesion in different conditions of pH and ionic strength (470 nm in diameter particles, 0.1%), rinsed with water, then with isopropanol and air dried. Scale bars: 2 µm.

A variation of the duration of incubation of the PAH-conditioned substrate with the 470 nm latex (1 to 6 hours, 1 to 6 days) showed that an incubation time of at least 2 hours was required in order to obtain these degrees of coverage. However no increase of coverage was obtained when incubating for longer periods of times. Using a 1 % latex concentration instead of 0.1 % did not provide any rise of the degree of coverage (data not shown).

Obtaining a layer of adhering particles deserves discussion, considering not only interfacial interactions but also the amount of colloidal particles involved and mass transfer. Table 2 presents data computed for the experimental conditions of the treatments with 0.1 % colloidal suspensions. Similar data for PAH 10-5 M solution are given for comparison, considering the polycation as a non hydrated sphere of 3.0 nm radius; note that the hydrodynamic radius at pH 7.4 is about 5 times larger (Adamczyk et al., 2006).

However, it involves operations which could possibly create systematic variations. Binarization of SEM images gave degrees of coverage of 50.2 ± 5 % and 14.6 ± 4 % on surfaces layered with 470 and 65 nm particles, respectively. A consistent degree of substrate coverage was obtained using manual counting (470 nm : 45.2 ± 2 %; 65 nm : 10.7 ± 1 %). The differences between the two modes of counting were thus in the range of the standard deviation. Samples with adherent particles were also analyzed by XPS (Table 1). For samples 7 and 8 , 470 nm particles were used and the degree of coverage was 53 and 27 %, respectively; for sample 9, 65 nm particles were used and the degree of coverage was 15 %, as assessed by SEM image analysis. As compared with PAH conditioned substrates with no adhering latex, these three samples gave a higher concentration of carbon, a lower concentration of silicon, oxygen and nitrogen and indicated the presence of sulfur, characterized by a S 2p peak (showing the 2p3/2 and 2p1/2 components with S 2p3/2 observed

Fig. 5. SEM images obtained on PAH-conditioned (10-5M, pH=11, I=10-2M) glass samples, submitted to latex particle adhesion in different conditions of pH and ionic strength (470 nm in diameter particles, 0.1%), rinsed with water, then with isopropanol and air dried. Scale

A variation of the duration of incubation of the PAH-conditioned substrate with the 470 nm latex (1 to 6 hours, 1 to 6 days) showed that an incubation time of at least 2 hours was required in order to obtain these degrees of coverage. However no increase of coverage was obtained when incubating for longer periods of times. Using a 1 % latex concentration

Obtaining a layer of adhering particles deserves discussion, considering not only interfacial interactions but also the amount of colloidal particles involved and mass transfer. Table 2 presents data computed for the experimental conditions of the treatments with 0.1 % colloidal suspensions. Similar data for PAH 10-5 M solution are given for comparison, considering the polycation as a non hydrated sphere of 3.0 nm radius; note that the

instead of 0.1 % did not provide any rise of the degree of coverage (data not shown).

hydrodynamic radius at pH 7.4 is about 5 times larger (Adamczyk et al., 2006).

at 169 eV), characteristic of sulfate and due to the latex surface.

bars: 2 µm.

Fig. 6. SEM images obtained on PAH-conditioned (10-5M, pH=11, I=10-2M) glass samples, submitted to adhesion of latex particles (0.1%, pH=7, I=10-3M, rinsing with water then with isopropanol) with 470 nm (a) and 65 nm diameter (b), and submitted to sequential adhesion of 470 nm and 65 nm particles without (c, bimodal I roughness) and with (d, e, f, bimodal II roughness) PAH adsorption after adhesion of the 470 nm particles. Image f was obtained on a sample prepared in the same experimental set as d, then submitted to immersion in PBS at 37°C for 24h. Images d and e were obtained on samples fabricated independently. The inset shows an enlargement of structures present on surfaces with bimodal II roughness. Scale bars: 1 µm. Scale bar inset: 500 nm.

Fabrication of Surfaces with Bimodal Roughness Through Polyelectrolyte/Colloid Assembly 65

repulsion between particles. For this latex, a surface coverage of 10 to 15 % corresponds to an apparent area of 37 103 to 24 103 nm2/ particle, corresponding to an occupational diameter of 207 to 166 nm. The difference (142 to 101 nm) with the real particle size is larger that the latter, which demonstrates that the degree of coverage is limited by particle-particle

If the particles were in hexagonal close packing, the area occupied per particle would be as presented in Table 2. A surface coverage of 45 % for the 470 nm latex thus corresponds to an apparent area per particle of 4.2 105 nm2 and an occupational diameter of 700 nm. The difference of 230 nm between the occupational diameter and particle size is lower than the real particle diameter. Accordingly the degree of coverage is limited by the space available, considering a random distribution of particles. Consequently, the degree of coverage would be limited by the space available for 470 nm particles and by electrostatic repulsion for the 65 nm latex. Hanarp (2003) used the ionic strength (10-5 to 10-2 M), to control the density of 110 nm polystyrene particles adhering on titanium oxide conditioned with a solution of aluminium chloride hydroxide. Our data indicate that this method would not work for

Figure 5 shows aggregation of 470 nm particles when using a 10-1 M ionic strength at pH 3 or 7, or when using a 10-3 M ionic strength at pH 3. The surface charge of colloids used here is not affected by pH as confirmed by Schulz et al. (1994a, 1994b), who used latex particles of 131 nm from the same provider and with the same surface specifications as the particles used here. If the aggregates were formed in the suspension, the adhering amount would be expected to increase with time in the range of a few hours or when increasing the concentration from 0.1 to 1 %, in contrast with observations. This suggests that aggregation may take place at the sol - substrate interface owing to low particle - particle repulsion, in addition to the tendency to gather together induced by subsequent solvent evaporation. The aggregation may also be favored by partial desorption of the polycation, as it is enhanced at a pH 3 and 7, at which the polycation is more highly charged (Choi and Rubner, 2005).

Surfaces with bimodal I roughness (Figure 6c) were created by adding 65 nm particles subsequently to the 470 nm particles (cf Figure 1c,d). The degree of substrate coverage with 470 nm particles was of 46.9 ± 1 % on this sample. This approach is similar to that of Takeshita et al. (2004) using poly(ethylene terephthalate) conditioned with PAH and adhesion of 350 nm carboxylated polystyrene latex particles followed by 100 nm particles. However, the layer obtained in that case was not regular, despite sonication of the solution to prevent latex aggregation. We attribute this to the fact that the samples were rinsed with

Surfaces with bimodal II roughness (Figure 6d) were prepared by adsorbing PAH after the formation of the first adhering layer (cf Figure 1b,c,e,f). In this way, 65 nm particles were adhering not only to the glass substrate but also to the 470 nm particles, providing raspberry-like structures. This procedure was repeated five times independently and 3 to 15 samples were prepared in each experiment, demonstrating the repeatability as well as the reproducibility of the method. Figure 6 (d, e) presents the extreme results obtained, corresponding to degrees of coverage by 470 nm particles of 50 ± 3 % and 22 ± 1 %,

repulsion.

particles above 200 nm diameter.

water before drying.

respectively.

**3.3 Nanostructured surfaces with bimodal roughness** 


It appears that the particles are in large excess of what is needed to make a monolayer. Mass transport by diffusion is fast for polycations and 65 nm latex particles. In contrast, a period of one day is expected to be needed for insuring the formation of a monolayer of 470 nm particles if diffusion is the rate controlling process. Thus sedimentation or convection should contribute to bringing the 470 nm particles to the surface. Considering a density of 1.05 g/cm2, the rate of sedimentation vs of 470 nm particles is about 6.0 nm/s according to Stokes law. Accordingly the time ts required to produce a monolayer if the rate is entirely controlled by sedimentation is ts = e / vs = 13 hours. Actually the amount of adhering 470 nm particles does not increase significantly if the incubation time is extended from 1 hour to 6 days, suggesting that convection due to manipulation insures mass transport. The data of Table 2 also help to figure out in what limits it would be possible to control the adsorption of polycations and small latex particles by playing with incubation time and concentration.


Table 2. Particle characteristics relevant to experimental conditions. See text for details.

While mass transport is not a limiting factor for adhesion of 65 nm particles, the density of the monolayer remains fairly low (10 to 15 % degree of coverage). For this latex at the ionic strength of 10-2 M, the product κa, where κ is the inverse of the Debye length, is about 10. For this κa value, maximum coverages of about 35 % were reported for positively charged latex particles on mica (Johnson and Lenhoff, 1996) and for negatively charged latex particles on titanium oxide modified by successive adsorption of PDDA, polystyrene sulfonate and aluminium chloride hydroxide (Hanarp et al., 2001), in agreement with predictions based on repulsion between the particles considered as hard spheres. It is thus reasonable to consider that the coverage by the 65 nm latex is controlled by double layer

Area A occupied per particle in hexagonal close packing (3.46 a2, a being the particle

Amount Q of particles required to cover the surface with a monolayer in hexagonal

Concentration C of particles in the liquid phase, which, if expressed in ml-1, is also the

Thickness e of the liquid layer containing the amount of particles required to make a

Diffusion coefficient D of the particles (D = kT / 6a), where k, T and are the

 Time td required to produce a monolayer if the rate of adsorption is entirely controlled by diffusion through a concentration gradient created by adsorption itself (Crank, 1957):

It appears that the particles are in large excess of what is needed to make a monolayer. Mass transport by diffusion is fast for polycations and 65 nm latex particles. In contrast, a period of one day is expected to be needed for insuring the formation of a monolayer of 470 nm particles if diffusion is the rate controlling process. Thus sedimentation or convection should contribute to bringing the 470 nm particles to the surface. Considering a density of 1.05 g/cm2, the rate of sedimentation vs of 470 nm particles is about 6.0 nm/s according to Stokes law. Accordingly the time ts required to produce a monolayer if the rate is entirely controlled by sedimentation is ts = e / vs = 13 hours. Actually the amount of adhering 470 nm particles does not increase significantly if the incubation time is extended from 1 hour to 6 days, suggesting that convection due to manipulation insures mass transport. The data of Table 2 also help to figure out in what limits it would be possible to control the adsorption of polycations and small latex particles by playing with

Boltzmann constant, the temperature and the viscosity, respectively.

**PAH 10-5 M** 

A (nm2 part-1 ) 32 3.7 103 1.91 105 Q (part. cm-2) 3.1 1012 2.7 1010 5.2 108 C (part. cm-3) 6.0 1015 7.0 1012 1.8 1010 e (µm) 5.2 39 284 D (cm2s-1) 7.2 10-7 6.7 10-8 9.3 10-9 td 0.29 s 2.9 min 19 hours Table 2. Particle characteristics relevant to experimental conditions. See text for details.

While mass transport is not a limiting factor for adhesion of 65 nm particles, the density of the monolayer remains fairly low (10 to 15 % degree of coverage). For this latex at the ionic strength of 10-2 M, the product κa, where κ is the inverse of the Debye length, is about 10. For this κa value, maximum coverages of about 35 % were reported for positively charged latex particles on mica (Johnson and Lenhoff, 1996) and for negatively charged latex particles on titanium oxide modified by successive adsorption of PDDA, polystyrene sulfonate and aluminium chloride hydroxide (Hanarp et al., 2001), in agreement with predictions based on repulsion between the particles considered as hard spheres. It is thus reasonable to consider that the coverage by the 65 nm latex is controlled by double layer

**Latex 65 0.1 %** 

**Latex 470 0.1 %** 

radius).

close packing.

monolayer.

td = Q2 / 4 C2 D.

incubation time and concentration.

amount of particles involved in the preparation.

repulsion between particles. For this latex, a surface coverage of 10 to 15 % corresponds to an apparent area of 37 103 to 24 103 nm2/ particle, corresponding to an occupational diameter of 207 to 166 nm. The difference (142 to 101 nm) with the real particle size is larger that the latter, which demonstrates that the degree of coverage is limited by particle-particle repulsion.

If the particles were in hexagonal close packing, the area occupied per particle would be as presented in Table 2. A surface coverage of 45 % for the 470 nm latex thus corresponds to an apparent area per particle of 4.2 105 nm2 and an occupational diameter of 700 nm. The difference of 230 nm between the occupational diameter and particle size is lower than the real particle diameter. Accordingly the degree of coverage is limited by the space available, considering a random distribution of particles. Consequently, the degree of coverage would be limited by the space available for 470 nm particles and by electrostatic repulsion for the 65 nm latex. Hanarp (2003) used the ionic strength (10-5 to 10-2 M), to control the density of 110 nm polystyrene particles adhering on titanium oxide conditioned with a solution of aluminium chloride hydroxide. Our data indicate that this method would not work for particles above 200 nm diameter.

Figure 5 shows aggregation of 470 nm particles when using a 10-1 M ionic strength at pH 3 or 7, or when using a 10-3 M ionic strength at pH 3. The surface charge of colloids used here is not affected by pH as confirmed by Schulz et al. (1994a, 1994b), who used latex particles of 131 nm from the same provider and with the same surface specifications as the particles used here. If the aggregates were formed in the suspension, the adhering amount would be expected to increase with time in the range of a few hours or when increasing the concentration from 0.1 to 1 %, in contrast with observations. This suggests that aggregation may take place at the sol - substrate interface owing to low particle - particle repulsion, in addition to the tendency to gather together induced by subsequent solvent evaporation. The aggregation may also be favored by partial desorption of the polycation, as it is enhanced at a pH 3 and 7, at which the polycation is more highly charged (Choi and Rubner, 2005).
