Infiltration of biomineral templates for nanostructured polypyrrole

Biomineral templates like sea urchin spine, nacre or eggshell were applied in the polymerisation of pyrrole. The insufficient infiltration of pyrrole into the CaCO3 structure of the biomineral templates was improved using three different and universally applicable approaches and the electrochemical properties of the received polypyrrole were examined.


Introduction
Biominerals have developed in a variety of organisms for at least 3500 million years and have different functions e.g. a shell for protection against predators or the skeleton of vertebrates as a supporting structure. 2,3 Biominerals have complex morphologies and most of them show a hierarchical structure consisting of organic and inorganic components that results in special properties, like magnetic and optical properties or an increased mechanical stability, like in nacre. [2][3][4][5] Amongst 60 different biominerals CaCO 3 is the most abundant. 6 The CaCO 3 structure of some biominerals is composed of CaCO 3 nanocrystals with biomolecule inclusions (Fig. 1). 1,7,8 The inspiration from the complex structure of some biominerals is an intriguing approach to develop functional and multifunctional materials with remarkable properties. 9,10 To transfer the hierarchical structure of biominerals to new materials, it is possible to use biominerals as templates which can be realised with two different strategies. The rst strategy uses the organic matrix of a biomineral as a template to mineralize a new material inside. Siglreitmeier et al. demineralised the CaCO 3 structure of a shell to receive the organic matrix. Within this organic matrix they inltrated gelatine and synthesised magnetite nanoparticles to form a new organic-inorganic hybrid material. 11 The second strategy is to dissolve the organic matrix of a biomineral and to utilize the obtained mineral structure as a template aer inltration. Imai et al. demonstrated that it is possible to inltrate molecules like dyes between the nanocrystals inside a biomineral template. 12,13 Later, Imai et al. used the CaCO 3 structure of biominerals as templates for polymerisations. 14-17 Generally templates are used oen in polymerisations, as it is not easy to control the morphology of an organic polymer since they are oen poorly soluble or difficult to process. 12,15,18 A biomineral as a template allows to transfer the hierarchical structure of biominerals, consisting of the nano-and macrostructure, to polymer materials. In general the morphology has a huge impact on the properties of a material, as in the example of biominerals. 15,19 If the hierarchical structure with its nanostructure is transferred to a conductive polymer, it could lead to interesting electrochemical properties. 20,21 Nanostructured conductive polymers show advantages such as a high electrical conductivity, 22,23 a high surface area 24,25 or a short distance for the transport of ions. 26 The hierarchical structure could lead to a better passage of the electrolyte in a bulk material, because of its micrometre pores, which lack in a merely nanostructured material. Thus, in this work we tried to replicate the synthesis of the conductive polymer polypyrrole (PPy) with a sea urchin spine as a template as it is described elsewhere. 14,16

Results and discussion
The process consists of four steps: preparation of the template by removing the organic components, inltration of the pure monomer, polymerisation of the monomer and dissolving of the template. The entire process is described in detail in the ESI † and has been displayed by EDX measurements (Table 1 and S2 †). The polymerisation resulted in the coating of the sea urchin spine surface with PPy on the microscale and we were not able to observe PPy inside the CaCO 3 structure (Fig. 2). So, we assume that no polymerisation has taken place inside the CaCO 3 structure, meaning the pores between the nanocrystals of the biomineral (Fig. 1). These results suggest that the pyrrole (Py) monomer could not be inltrated into the nanopores of the CaCO 3 sea urchin spine structure. We also achieved comparable results with other biominerals, such as nacre, corals and eggshells which also consist of nanostructured building blocks with incorporated organic polymer and therefore are suitable as templates, too. 7 Nevertheless it would be desirable to inltrate the nanopores between the nanocrystals of the biominerals, to achieve a nanostructured PPy with expected better electronic properties. [22][23][24]26 Therefore, the goal of this work was to improve the inltration of the monomer Py into different biominerals to achieve a nanostructured PPy. The inltration could be enhanced with three independent methods which can be applied to various inltration problems.
In the rst approach we used an increased pressure of 1 bar, as an external force, to improve the inltration of the Py into the nanopores of the CaCO 3 sea urchin spine scaffold. Porous systems can't be inltrated spontaneously by a non-wetting liquid under atmospheric pressure, based on the capillary effect. 27 However it is possible to inltrate these systems under an increased pressure, which is dependent on the pore size. 28 Using this technique, we were able to partially polymerise Py monomers inside the CaCO 3 sea urchin spine structure (Fig. 3a). This technique is straightforward, yet an experimental set-up for the required higher pressures is needed.
In the second approach we improved the inltration ability of the Py monomer liquid by adding a polar solvent. The inltration ability can be described by the capillary eqn (1).
where h is the height of a liquid in a column, s is the liquid-air surface tension, q is the contact angle, r is the density of the liquid, g is the local acceleration due to gravity and r is the radius of the column. The capillary equation shows, that if we reduce the contact angle, by improving the wettability of CaCO 3 with Py, we can increase the ability of Py to enter the nanopores.
We learned from contact angle measurements that the wettability can be enhanced if the Py is mixed with a polar solvent such as methanol (Fig. S1 †). Aer improving the wettability, we successfully inltrated the CaCO 3 nanostructure of the sea urchin spine with Py and polymerised inside the structure ( Fig. 4a and d). The sample in Fig. 4d shows the complete sea urchin spine structure which consists solely of PPy, indicating that the polymerisation took place inside the nanopores, in contrast to the original polymerisation on the surface of the template (Fig. 5). The synthesis of PPy has been veried by ATR-    IR measurements (Fig. S3 and S4 †) and TGA measurements show that we were able to inltrate 1.38 wt% of PPy into the sea urchin spine template (Fig. S5 †). Aerwards we used other biominerals such as nacre, eggshell and coral to compare the results with the sea urchin spine. The inltration of the coral unexpectedly did not work, although they are known to have a mesocrystalline structure. 29 But the inltration and polymerisation inside nacre and eggshell worked well with the improved wettability (Fig. 4). The handling of this approach is straight forward. Nevertheless, the dilution of the monomer might be a limiting factor in other systems. The wetting behaviour has to be sufficient for an inltration but with a high enough concentration of the monomer. There will be one point, there the concentration of the monomer is too low for a sufficient polymerisation. In this case a possible strategy could be to use a supercritical uid as an inltration medium. 30,31 The third approach is based on the capillary equation as well. We turned around the procedure and rst inltrated the oxidant CuCl 2 . Aerwards the polymerisation was started by adding the monomer Py from the gaseous phase. 32 The contact angle measurements (Fig. S1 †) showed that CaCO 3 has a better wettability with isopropyl alcohol because it is more polar than Py. Therefore, the oxidant dissolved in isopropyl alcohol inltrates the CaCO 3 structure better than the pure monomer Py. An EDX measurement shows the inltration of the CuCl 2 into the CaCO 3 sea urchin spine structure (Fig. S6 †). The inltration of the CuCl 2 is more demanding, compared to the synthesis of the rst and second approach. But this method showed the best outcomes regarding the reproduction of the complete sea urchin spine structure as a result of a better inltration into the nanopores (Fig. 3b).   5 (a) SEM image of PPy structure (cross section) after the dissolution of the CaCO 3 template (sea urchin spine). The PPy was infiltrated and polymerised as described elsewhere. 14,16 The sketch under the picture shows the schematic structure of a sea urchin spine template. (left) The structure on the micrometre scale in dark blue. The light blue domains clarify the pores on the micrometre scale. The location of the PPy is drawn in red, its polymerisation resulted in the coating of the template structure. (right) The CaCO 3 nanocrystals with amorphous CaCO 3 and organic inclusions. After the removing of the organic inclusions the light blue parts between the nanocrystals are partly available as pores on the nanometre scale. With the original synthesis the Py could not be infiltrated into these nanopores. (b) As comparison on the right side the PPy structure (cross section) after the dissolution of the CaCO 3 template (sea urchin spine). The PPy was infiltrated as a mixture with the polar solvent methanol as described in the ESI 1.2. † The sketch under the picture shows, the Py could be infiltrated into the nanopores and thus into the complete CaCO 3 structure. After the dissolution of the CaCO 3 template we obtain the same macrostructure made of PPy.
Aer we successfully used the sea urchin spine and other biominerals as templates for the synthesis of nanostructured PPy, the electrochemical properties of the obtained PPy were analysed. The cyclic voltammetry curves ( Fig. 6a and b) display a symmetric shape with a pair of redox peaks indicating the pseudocapacitive behaviour of the prepared electrodes. The obtained specic capacitance value is 45 F g À1 at a scan rate of 5 mV s À1 . Furthermore, the electrochemical performance of the electrodes was evaluated using galvanostatic charge/ discharge tests and EIS measurements ( Fig. 6c and d). All the galvanostatic charge/discharge curves exhibit a nearly symmetric shape with a voltage plateau in the charge/ discharge process which is consistent with the results obtained from the CV curves. The Nyquist curve presents a semicircular arc in the high-frequency region and a straight line in the low-frequency region. The equivalent series resistance amounts to 1.06 U. The large diameter of the semicircle at the high-frequency region indicates a high interfacial charge transfer resistance. The electrochemical properties are not superior compared with the results of PPy synthesised in solution under the same conditions (Fig. 7). The possible reason is, that the PPy synthesised in solution, is already nanostructured. In addition, we determined via BET analysis a surface area of 16.43 m 2 g À1 of the PPy from the sea urchin spine (Table S1 †). The surface of the PPy from solution is comparable with 13.3 m 2 g À1 . The BET analysis shows, that the nanostructuring of the PPy from solution is comparable to the PPy synthesised in the sea urchin spine and therefore, we could not achieve an improvement in the electrochemical properties.

Conclusions
In summary we were able to improve the insufficient inltration of Py into CaCO 3 nanostructures of biomineral templates. This could be realised by three different methods as using an increased pressure during the inltration or enhancing the wettability of the CaCO 3 substrate with the inltration medium. These improvements are universally applicable for other inltration problems. With the improved inltration of Py into CaCO 3 nanostructures it was possible to polymerise the Py monomers inside the macro-and especially the nanopores of the biomineral templates. The hierarchical structure of the biominerals were transferred to the PPy polymer material and as PPy is a conductive polymer we analysed its electrochemical properties. Based on a working inltration it is promising to use biominerals as templates to produce functional materials. Besides further conductive polymers, like polythiophene, a conductive replacement with gold or a replacement with wax seem to be promising.

Conflicts of interest
There are no conicts to declare.