Fig. 8 shows the in situ ATR-FTIR spectra of the oxidized iron surface with pre-adsorbed Mefp-1 film obtained after exposure to the 10 μM FeCl3 solution for different times. Comparing with the 15 min Mefp-1 spectrum (Fig. 6a), the intensity of negative water band is relatively larger after exposing the Mefp-1 layer to the FeCl3 solution (Fig. 8a), which indicates that water is released from the pre-adsorbed Mefp-1 layer. Therefore, it is reasonable to conclude that the addition of the Fe3+ makes the Mefp-1 film more compact, in agreement with the QCM-D and AFM results. It should be noted that the spectra obtained with the addition of FeCl3 are remarkably similar to those without the FeCl3 addition but after sufficiently long exposure in the Mefp-1 solution, which indicates that Fe3+ could also be generated from the oxidized iron surface. We note that the broad and unresolved 1540 cm−1 peak is more clearly split into two separate peaks at 1550 cm−1 and 1536 cm−1 (Fig. 8b). This is not surprising since the DFT analysis showed that the 1540 cm−1 peak had contributions from both the backbone and the catechol unit, and that these bands may be influenced differently by the change in aqueous solvation when the water content of the protein film becomes reduced. It should also be recognized that the effect of subtracting the water spectrum may influence the two spectra slightly differently in this region. However, more importantly the peaks, at 1485 cm−1 and 1260 cm−1 (Fig. 8b), which we have associated with the complexation between catechol and Fe3+, remain largely unaltered both in terms of frequencies and intensities. This supports our earlier hypothesis that Fe3+ could be generated from the oxidized iron surface during the exposure to Mefp-1 solution and forms [Fe(Cat)3]3− complexes.. The shoulder at 1423 cm−1 (marked with arrow in Fig. 8b) seems to be slightly enhanced upon addition of FeCl3, and this could indicate some differences in the geometry of the complexes.
FeCl3
10 μM
μM
in situ ATR-FTIR spectra
peak
1540 cm−1
cm−1
in situ ATR-FTIR spectra
peaks
1550 cm−1
in situ ATR-FTIR spectra
peaks
1536 cm−1
cm−1
in situ ATR-FTIR spectra
peak
1540 cm−1
cm−1
in situ ATR-FTIR spectra
peaks
1485 cm−1
cm−1
in situ ATR-FTIR spectra
peaks
1260 cm−1
cm−1
in situ ATR-FTIR spectra
shoulder
1423 cm−1
cm−1
Mefp-1 spectrum
15 min
min