Ellipsometric measurement of bacterial films at metal-electrolyte interfaces

Abstract

Ellipsometric measurements were used to monitor the formation of a bacterial cell film on polarized metal surfaces (Al-brass and Ti). Under cathodic polarization bacterial attachment was measured from changes in the ellipsometric angles. These were fitted to an effective medium model for a nonabsorbing bacterial film with an effective refractive index (nf) of 1.38 and a thickness (df) of 160 +/- 10 nm. From the optical measurements a surface coverage of 17% was estimated, in agreement with direct microscopic observations. The influence of bacteria on the formation of oxide films was monitored by ellipsometry following the film growth in situ. A strong inhibition of metal oxide film formation was observed, which was assigned to the decrease in oxygen concentration due to the presence of bacteria. It is shown that the irreversible adhesion of bacteria to the surface can be monitored ellipsometrically. Electrophoretic mobility is proposed as one of the factors determining bacterial attachment. The high sensitivity of ellipsometry and its usefulness for the determination of growth of interfacial bacterial films is demonstrated.

FIG. 1

Variations of Δ (a) and Ψ (b) during cathodic polarization of Al-brass in 3.5% NaCl in the absence (•) and presence (○) of bacteria. The potential was kept constant at E = −0.6 V. The vertical lines show the times at which bacteria were added.

FIG. 2

Variations of Δ (a) and Ψ (b) during cathodic polarization of titanium in 3.5% NaCl in the absence (•) and presence (○) of bacteria. E = −0.6 V. The vertical lines show the times at which bacteria were added.

FIG. 3

Influence of the substrate optical constants on the calculated bacterial film refractive index and thickness for k_s = 3.35 (a) and n_s = 0.645 (b).

FIG. 4

Variations of Δ (a) and Ψ (b) with the growth of an oxide film on Al-brass in 3.5% NaCl at the free corrosion potential in the absence (•) and presence (○) of bacteria. Arrows indicate the times when the potential was allowed to reach its free corrosion value. The vertical line in b shows the time at which bacteria were added.

FIG. 5

Variations of Δ and Ψ with the growth of an oxide film on Al-brass in 3.5% NaCl. •, experimental data (from Fig. 4) from the initial phase of oxide growth at the free potential; □, calculated data (16) for n_f values of 2.3 to 3.0 in 7 steps (the arrow indicates the direction of increase) and for d_f values of 0 to 5 nm in 10 steps. k_f = 0.

FIG. 6

Variation of the effective refractive index for a bacterial film with the filling factor corresponding to the interfacial model in equation 1.

FIG. 7

Δ-Ψ signature corresponding to a one-film model on Al-brass with n_f values of 1.37 to 1.382 in 0.001 steps and d_f values of 100 to 250 nm in 10-nm steps (○) and experimental δΔ and δΨ values from the first 150 s at the free corrosion potential (•). The arrows indicate the direction of increase.

FIG. 8

Changes in the Δ-Ψ signature for a two-film model composed of a bacterial film (refractive index of 1.38, as previously found) over an oxide film on Al-brass. The values of n_f for the oxide film are changed from 2.8 to 2.0 in eight steps and the calculations were carried out for bacterial film thicknesses of 160 and 190 nm. ○, 1-nm oxide film thickness; •, 1.5-nm oxide film thickness. The arrow indicates the direction of change of Ψ giving values comparable with the experimental results.

FIG. 9

Variations of Δ (a) and Ψ (b) during cathodic polarization of titanium in 3.5% NaCl in the absence (•) and presence (○) of Cu-treated bacteria. E = −0.6 V. The vertical lines show the times at which bacteria were added.