A fully synthetic ion-sensitive mesoporous interface opens a new branch in the applications of this kind of system and exhibits characteristics so far believed to be a distinctive feature of biological systems
One of the most intriguing features of living systems is the selectiveness of biological membranes. They can let the necessary substances pass while blocking others, and when their needs change the “rules” are inverted. Is this a specific behavior of living organisms or one can emulate such behavior in synthetic systems? If that is possible, it would be very useful for a number of applications, as well as an interesting source of basic investigations about physico-chemistry and biophysics.
Biological membrane selective ability was already partially reproduced in human-made artifacts. As an example, it can be done with mesoporous films, a thin layer deposited onto a substrate that has pores with sizes between 2 and 50 nm. Scientists have already built artificial systems that can select if an anion or a cation (or both of them) will pass, depending on the situation – as an example, on the pH in the vicinties. This is similar to the acid-sensing ion channels (ASICs) in neurons of the peripherical sensory and central nervous systems. However, these neurons can also block both of them, cations and anions – what the available synthetic systems couldn't do. Full selectiveness like this remained a distintive feature of biological organisms.
Until now. It was done for the first time by a team of researchers led by Galo Soler-Illia, from the Centro Atómico Constituyentes, in Argentina, with scientists from the Max-Planck-Institute für Polymerforschung, in Germany, Tenaris (steel company) and the Institute of Theoretical and Applied Physicochemical Researches (INIFTA), the last two also in Argentina. The results were published in the Journal of the American Chemical Society (JACS) 131, 10866 (2009).
What we already knew
The paper describes two experiments. The first presents a partial selectiveness as described above – no novelty yet –, but the second behaves exactly like the neuronal ASICs, due to a modification in the mesoporous film. To understand how it works, let's see how the emulations made of mesoporous films are commonly done. In a normal situation – if nobody is worried about selectiveness –, molecules of substances of interest can pass easily through the pores. In order to block the passage of ions, the idea is to build the film in such a way that it has electric charges fixed at the inner walls of its pores. If these charges are negative, negative ions will not be able to pass and positive ions will be attracted through the pores. And vice-versa.
The first experiment described in the paper was of this kind. They deposited a silica (SiO2) mesoporous film onto a silicon and indium tin oxide (ITO) substrate. The resulting surface inside the pores was covered by Si–OH chemical groups. However, these groups release several hydrogens ions (positive) to the environment, depending on the pH nearby. So the Si–OH groups at the surfaces became silanolate groups (Si–O–), that kept negative charges, as in the Figure 1.
Figure 1 – Simplified scheme of two pores in the silica film (at left) and the inner surface of one of them (center and right). At right, hydrogens have been liberated from silica molecules to contribute to the pH of the environment; silica has then become negatively charged (as silanolates).
To understand what was happening, the authors had first to characterize the system they were studying, specially to know its pore architecture. Measurements performed at the Brazilian Synchrotron Laboratory (LNLS), such as grazing-incidence small-angle X-ray scattering (GI SAXS) and analysis of X-ray reflectivity, were essential for this knowledge.
One has to control the electric charges in the pores, so that the film permeability may be varied at will. This can be achieved by varying the pH. A very low pH means a large amount of hydrogen ions available, that tends to neutralize the negative charge of the silanolate groups and consequently to decrease the number of blocked ions.
The selective ability of this arrangement was tested with Ru(NH3)63+ positive ions and Fe(CN)63– negative ions. The behavior of the system in these situations was probed by means of cyclic voltammetry (CV). CV is a widely used method to measure the concentration and the diffusion dynamics of ions in solution, by measuring the current generated by the oxidation or reduction processes of such ions on a conductive electrode. Mesoporous films deposited onto the conductive Si or ITO substrates behave as a very thin membrane. A cyclic voltage perturbation leads to oxidation and reduction peaks when the ions travel across the pore system and exchange electrons at the electrode surface, giving rise to a cyclic voltammogram as in Fig. 2.
Figure 2 – Cyclic voltammograms corresponding to a mesoporous silica film deposited on an ITO electrode in the presence of 1 mM Ru(NH3)63+ (red trace) and 1 mM Fe(CN)63– (blue trace). (a) pH = 8, (d) pH = 1.
Charged ions can be detected because they leave “marks” in the voltammogram – the up and down peaks in the closed curves. The voltage at which these peaks appear is a signature of the ion. It happens because, as the voltage approaches to the reduction potential of the ion, electrolysis at the electrode becomes more intense and so the measured electric current increases. The same happened when the voltage is driven back to the original value, but with inverted polarization – so a closed curve results.
One can see that, with low pH, there is a small decrease in the concentration of Ru(NH3)63+ inside the pores and a small increase in Fe(CN)63–. This was expected, because low pH means few hydrogens out from the Si–OH groups and few negative charges in the pores (and so less negative ions blocked and less positive ions atracted through the pores).
Just like neurons
Now it comes the innovation. An interesting thing happened when the authors modifyed the silica mesoporous films by growing poly(methacryloyl-L-lysine) (PML) brushes in the substrate. A polymer brush is a layer of polymer deposited onto a substrate in such a way that long molecular chains are fixed at the surface at one of their ends, resulting in a structure similar to a brush. The authors used PML because it is a zwitterionic molecule – what means that it has positive and negative net charges in different parts of it. The characterization of the new films demanded some additional measurements, like the fraction of the pore volumes filled with the polymer, that was determined by X-ray reflectometry (XRR) at the LNLS.
Again, the charges may be controlled by varying the pH. Because of the zwitterionic character of PML, one may even change the charges from positive to negative, as schemed in Fig. 3.
Figure 3 – One extremity of the PML molecule. Normally it is positively charged in the nitrogen group and negatively in the carboxyl; but one of these charges may be preferred depending on the pH.
Figure 3 – One extremity of the PML molecule. Normally it is positively charged in the nitrogen group and negatively in the carboxyl; but one of these charges may be preferred depending on the pH.
So one might expect that at high pH positive ions would pass and negative ions would be blocked – and at low pH the opposite phenomenon would happen. However, the voltammogram obtained by the authors was surprising:
Figure 4 – The same as in Figure 2, but with mesoporous film modified with a PML polizwitterionic brush.
For low pH, the positive ions didn't pass, but the negative ones didn't either! Why did this unexpected behavior happen?
Charges versus charges
The researchers formulated an explanation. The film had two charges inside its pores: the one due to silanolate and the one due to PML. When the pH was grater than 5, PML had the same charge signal as silanolate (negative) and so the system blocked the negative ions and let the positive ones pass. But, when the pH was less than 5, PML and silanolate charges were opposite. As a result, they blocked both positive and negative ions. See fig. 5.
Figure 5 – Schematic description of the microscopic processes responsible for the results in Figure 4. Compare the right sides with Figure 1 – now there are the PML molecules linked to the silanolates. In (a), PML has the same charge signal as silanolate; negative ions are blocked and positive ions are attracted. In (b), PML is positive and silanolate negative; both ions are blocked.
The importance of this achievement does not lie only in the reproduction of a behavior until now believed to be specific of biological systems. It may also open a whole new branch of applications of mesoporous matrices, that have been so far considered as just scaffolds to create nanoscopic channels. The scientists concluded the paper with very optimistic remarks: “We consider that these results can lead to a new way of looking at interdisciplinary research in molecular materials science and trigger a cascade of new, refreshing ideas in nanochemistry aimed at the rational design of hyperfunctional assemblies with unprecedented properties.”
This work was published in the Journal of the American Chemical Society (JACS) 131, 10866 (2009) by Alejandra Calvo, Basit Yameen, Federico J. Williams,Galo J.A.A. Soler-Illia and
Omar Azzaroni.
Omar Azzaroni.
All figures were taken from the original article above.
