Coordination Polymers as Template for Mesoporous Silica Films: A Novel Composite Material Fe(Htrz)₃@SiO₂with Remarkable Electrochemical Properties

Abstract

While attempting to confine Fe(Htrz)3 (Htrz = 1,2,4,-1H-triazole) into a mesoporous silica matrix during its formation by electrochemically assisted self-assembly (EASA), we have discovered that such spin crossover complex is likely to act as the template (in place of the surfactant species) to form in one step a composite mesoporous material (Fe(Htrz)3@SiO2). The EASA method usually leads to the vertical growth of mesoporous silica thin films owing to the electro-induced condensation of silica precursors (i.e., tetraethoxysilane, TEOS) around tubular micelles (i.e., made of cetyltrimethylammonium bromide, CTAB) oriented orthogonally to the underlying support. In the presence of Fe(Htrz)3 in the starting sol (in addition to TEOS and CTAB), two distinct situations can be reached. At low Fe(Htrz)3 concentration (≤ 3 mM), the vertically aligned mesostructure is formed and Fe(Htrz)3 complexes are incorporated along with the surfactant phase, but most of them are released upon surfactant removal. At high Fe(Htrz)3 concentration (typically 5 mM), a wormlike mesoporous film is obtained in which Fe(Htrz)3 species act as a real template for the formation of a mesoporous Fe(Htrz)3@SiO2 film. Interestingly, the iron-triazole 2 complex is strongly entrapped in the silica matrix as it cannot be removed by solvent extraction (contrary to CTAB), as evidenced from X-ray photoelectron spectroscopy (XPS). Even more attractive are the electrochemical properties of the composite Fe(Htrz)3@SiO2 material, exhibiting highly stable operational stability (i.e., identical voltammetric signals upon multiple successive measurements), contrary to Fe(Htrz)3 species incorporated by impregnation of the surfactant phase which are found to leach out in solution upon use. Such in situ elaborated mesoporous composite made of Fe(Htrz)3 template strongly immobilized in a silica host is thus promising for bridging the gap between soft and hard functional organic-inorganic materials, which is briefly illustrated here for the mediated detection of hydrogen peroxide thanks to the electrocatalytic properties of the Fe(Htrz)3 complex, which are maintained even when immobilized in the composite Fe(Htrz)3@SiO2 film.

Non‐covalent Immobilization of Iron‐triazole (Fe(Htrz)3) Molecular Mediator in Mesoporous Silica Films for the Electrochemical Detection of Hydrogen Peroxide

“…Figure A gathers top SEM views of the film electrode, indicating a uniform morphology, yet with some particles or aggregates on top of the materials (as common for such deposits generated by EASA ), which are mostly eliminated after 15 min treatment in stirred solution (HCl/ethanol). In agreement to previous observations , TEM micrographs indicate the existence of a quite regular wormlike mesostructured (Figure B) with mesochannels having the tendency to orient vertically (see inset in Figure B) although not so overwhelming as for non‐functionalized mesoporous silica films . The film thickness is 110 nm, a constant value over the whole film area.…”

“…Again, no noticeable difference can be observed before and after HCl/ethanol treatment, confirming that the iron‐triazole coordination polymer acting as template is strongly embedded in the mesostructure and can be thus used as a durably‐immobilized redox catalyst, as shown hereafter for the amperometric detection of H 2 O 2 . Further characterization (XPS, diffraction patterns) can be found in our previous work .…”

“…Fe(Htrz) 3 ‐containing mesoporous silica films were generated by adapting the previously reported EASA method . A sol solution was first prepared by adding TEOS (100 mM), CTAB (32 mM) and the Fe(Htrz) 3 complex (5 mM, optimized value ) to a 50 : 50 water:ethanol medium containing also 0.1 M NaNO 3 as electrolyte. Its pH was adjusted to 3 by adding 0.1 M HCl and the sol was aged for 2.5 h to ensure proper hydrolysis of the silica precursors.…”

“…This confirms that a crack-free film recovers well the whole ITO surface and that Fe(Htrz) 3 species are not removed by solvent extraction (no possibility for the FcMeOH probe to cross the film and reach the electrode surface, contrary to what happened for the same kind of films pepared by EASA using a classical surfactant template [24]). Actually, for such Fe(Htrz) 3 @SiO 2 film, almost no CTAB is incorporated during synthesis and the coordination polymer made of Fe(Htrz) 3 species is playing itself the role of template which is strongly entrapped in the mesopore channels [38], explaining why the nanocomposite film is totally impermeable to external redox probes ( Figure 3B) and in that potential window the Fe(Htrz) 3 complex is not electrochemically accessible. Figure 4A gathers top SEM views of the film electrode, indicating a uniform morphology, yet with some particles or aggregates on top of the materials (as common for such deposits generated by EASA [24]), which are mostly eliminated after 15 min treatment in stirred solution (HCl/ethanol).…”

“…Long‐range charge transfer by electron hopping has been demonstrated for a vertically‐aligned ferrocene‐functionalized film , but the rate of charge transfer remains limited , except in special configuration . Promising ways to promote charge transport in mesoporous silica films are via conducting polymers electropolymerized in the mesopore channels or via the use of redox‐active coordination polymers as template in the synthesis of the films because these strategies ensure the electroactive moieties being very close to each other in the hybrid film. Here, we plan to use the iron‐triazole complex, Fe(Htrz) 3 (Htrz=1,2,4,–1 H ‐triazole), as coordination polymer template to generate Fe(Htrz) 3 ‐doped mesoporous silica films by electrochemically‐assisted self‐assembly (EASA) on indium‐tin oxide (ITO) electrode and its subsequent use for the electrocatalytic detection of hydrogen peroxide.…”