The Sol – Gel process of silica
The Sol – Gel processing of silica and silicates is a two-stage process which involves (i) hydrolysis with the final aim to reach sol (small particle dispersion in the liquid medium) and (ii) condensation, which results is the gel phase (relatively rigid polymerized and non-crystalline network possessing different in size pores). Apart from hydrolysis and condensation steps, one has to take into account other reactions that might occur, as for instance silica dissolution at higher pH values. Moreover, the gel phase usually needs further post-treatment to cure not fully cross-linked silica matrix in order to obtain the solid material. All these steps have been well studied and described,88 hence only a brief description will be given in here.
The rate of hydrolysis of alkoxysilane species as a function of the pH is shown on Figure 1.9 (blue curve). The reaction rate is slowest at a near neutral pH. Increasing concentration of protons or hydroxides species in the aqueous media leads to increase in hydrolysis rate. The reaction mechanism can be referred to as the nucleophilic substitution of SN2 type, namely nucleophilic attack on the positively charged silicon atom, which takes place synchronously with the cleavage of the Si–OR bond. The hydrolysis reaction is shown on Figure 1.8 (1). The rate of the hydrolysis reaction can be affected by the reaction medium – polar and protic solvents through formation of hydrogen bond may increase the efficiency of Si–OR bond cleavage – whereas the increasing hydrophobic character of R substituent shows the opposite effect.
Templates – towards surface engineering
The idea behind template technology is very straightforward and assumes the use of the geometrically well-defined ‘mold’, which is then used to grow the material of interest, usually on the basis of the bottom-up approach. In electrochemistry, an electrode surface modification has many advantages: increase in the electroactive surface area (once conductive material is grown), enhancement in the mass transport via diffusion or improvement in the catalytic efficiency for the increasing number of nucleation sites (especially for near-atomic range of roughness). Deposition of electrically insulating materials, as for instance porous silica, is of highest interest from the analytical point of view, especially when the deposit exhibits some degree of selectivity towards the analyte in the presence of a contaminant. Depending on pores dimension three main class of porosity can be distinguish: (i) microporous materials – are when the pore widths is less than 2 nm; (ii) mesoporous materials – with the pore widths between 2 and 50 nm and (iii) macroporous materials – with the pore widths greater than 50 nm.
Walcarius has divided the templates into three main groups: hard templates, colloidal crystal assemblies and soft templates.90 Hard template is the term reserved for a solid porous substrate that is modified within the pores and the deposit is then released by removal of surrounding pore walls (see Figure 1.10 A). Template route employing colloidal particles was classified as an intermediate option bearing the rigidity of the hard templates and self-assembly properties of soft templates (see Figure 1.10 B). The last group – shown on Figure 1.10 C – belongs to ‘soft’ mater, in this case liquid crystals formed by amphiphilic species (able to form variety of spatial arrangements: micelles, vesicles, cubic, hexagonal or lamellar liquid crystal structures), which is the most versatile method as the template extraction can be performed under mild condition without affecting deposit properties.
Silica or silicate materials can exhibit dual role since they were used as templates (in form of colloidal assemblies for instance)91 and were easily templated as shown in the following subsection.
A soft template for a Sol-Gel process of mesoporous silica thin films
Evaporation Induced Self-Assembly (EISA) is the method, which allows the formation of a silica film with controlled mesostructure and pore size by varying: chemical parameters and processing conditions. In general, in such a method, the sol solution (containing template and silica precursor species) is contacted with the solid support interface and the volatile components (e.g. H2O, EtOH and HCl) of the reaction media are left to evaporate. Reducing the volume of the sol solution results in condensation of the silica precursor around the template matrix (which tend to form liquid crystal phases once its critical micelle concentration is reached). Once solvent is evaporated the silica film is formed.92 Variety of processing methods were developed for EISA process, examples include: dip-coating – with the substrate being immersed to the sol solution and subsequently pulled out at a known rate;93 spin-coating – centrifugal force is used to spread the sol solution over the support; 94 casting – when the sol is simply poured on the support and left for evaporation95 or spraying – transferring the sol solution on the material in the form of aerosol.96 Under proper conditions, all the above methods allow the formation of ordered films with pores oriented horizontally to the support plane.
Deposition of mesoporous silica at solid electrodes in the manner which does not exclude the electrical contact between the medium phase and a conductive substrate is challenging. Deposition of mesoporous silica films with high symmetry and controlled pore orientation – preferentially perpendicular to the substrate plane – is achievable by Electrochemically Assisted Self-Assembly (EASA).97,98 In such an approach, the condensation of silica precursor is catalyzed by OH- electrogeneration at sufficiently low cathodic potential. Under these conditions, the cationic surfactants (cetyltrimethylammonium cations) present in the reaction media form hexagonally packed liquid crystal phase growing perpendicular to the electrode surface. The condensation of silica and self-assembly of soft templates occur simultaneously. The resultant thin film, after thermal curing, shows highly ordered silica network with the pores oriented normal to the underlying support.
Functionalized mesoporous silica films prepared by Sol-Gel processing
The introduction of chemical functionalities possessing different physico-chemical properties allows of material chemical and physical properties to be altered. It is especially important in analytical chemistry since it improves the selectivity once the system is designed to favors the detection of analyte and in parallel, inhibits the detection of contaminant (e.g. based on charge, hydrophilic/hydrophobic or host – ligand interactions). The sensor becomes even more versatile when surface nanoarchitecture is additionally adjusted. Such attributes are easily feasible for highly ordered mesoporous silica films with pores oriented normal to the surface plane prepared by EASA method.99 The functionalized mesoporous silica prepared by the Sol – Gel process has two possible synthetic routs: (i) direct co-condensation with organosilanes and (ii) co-condensation and deposition followed by chemical reaction. The first method involves hydrolysis of alkoxy silanes with organosilanes bearing the functional group of interest. An electrochemical deposition leads to functionalized silica film formation. Such an approach allowed the introduction of simple functionalities as for instance methyl – up to 40% mmol,100 amine – up to 10% mmol99 or thiol – up to 10% mmol101 groups without lost in mesostructure order. The second method is composed from two steps. Initially the organosilanes with the reactive organic group are co-condensate with the alkoxy silanes and electrodeposited at the solid electrode surface. The second step involves the reaction between the organic functionalities from the silica framework and properly selected reagent. A pioneering example was developed by Vilá et al. who electrogenerated azide functionalized oriented and ordered mesoporous silica films (for up to 40% mmol of azide group bearing silanes in the initial sol solution) that were further modified by alkyne-azide ‘click’ reaction with ethynylferrocene or ethynylpyridine.
Pd and Pt deposition at the electrified ITIES
Deposition of metallic Pd and Pt NPs at the ITIES was usually performed under potentiostatic interfacial reduction of metal-chloro-complex present in the aqueous phase by heterogenus electron transfer reaction from organic phase containing reductant – for instance 1,1’-dimethylferrocene122 or butylferrocene.123 An electrochemically induced liquid – liquid interface modification with both metals attracted a lot of attention concerning mechanistic studies of nucleation process. Johans et al. have proven on the basis of theoretical model that Pd NPs nucleation is free from preferential nucleation sites.124 The size of the Pd particles was also found to have an effect on their surface activity and based on quantitative thermodynamic considerations it was shown that only particles exceeding a critical radius can stay at the interface and further grow. When the interfacial tension was lowered by the adsorption of phospholipid molecules following changes were noticed: (i) the nucleation kinetics was significantly decreased (kinetics of growth was found to be unaffected); (ii) the critical radius and consequently, the particles size of the same surface activity had to increase and (iii) more energy was required to trigger the electron transfer reaction.
In order to eliminate NPs agglomeration found in all previous works, the ITIES was miniaturized using an alumina membrane with the mean pore diameter of 50 nm126 and 100 nm.127 Interestingly the growth of NPs was observed only in some of the pores, which was explained by autocatalytic effected followed by an interfacial nucleation. Other explanation of such a behavior was given by Trojánek et al. who studied initial nucleation rates of the Pt NPs and with the wide range of values obtained (from nucleation rate approaching zero up to 207 ∙ 10−5 −2 −1), they concluded that the presence or lack of nucleus formation is dictated by probability.
Pd and Pt are known for a long time as extremely versatile catalysts. This feature was also feasible at the liquid – liquid interface. For instance, pre-formed Pd NPs activated electrochemically by heterogeneous electron transfer reaction from the organic phase containing decamethylferrocene were used to catalyze the dehalogenation reaction of organic substrate being dissolved in the aqueous phase.128 Hydrogen evolution reaction was also catalyzed by Pd and Pt NPs (with the first having slightly better efficiency) electrogenerated in situ at the ITIES by metal precursor reduction with decamethylferrocene – electron donor which also served as a reductant for the aqueous phase protons.129 Trojánek et al. show the catalytic effect of Pt NPs modified ITIES on the oxygen reduction reaction obtaining the rate constant one order of magnitude greater as compared with the unmodified liquid – liquid interface.
Phospholipids at the electrified liquid – liquid interface
Phospholipids are a class of lipids being a part of all biological cell membranes. Phospholipids with the phosphate polar head group and the fatty acid tails are amphiphilic and biological membranes owe a unique double layer structure to this particular property. The nature of the functional groups attached to the phosphate groups results in five main types of compounds: phosphatidylcholines (PC), phosphatidylserines (PS), phosphatidylethanolamines (PE), phosphatidic acids (PA) and phosphatidylinositides (PI). Among each family, the number and saturation of carbon in alkyl chain may differ, with the exception of phosphatidylinositides where additionally inositol group can be substituted with one, two or three phosphate groups.131 Modification with phospholipids were successfully applied to the solid/liquid,132,133 air/liquid134 and at the liquid – liquid interfaces.135 Model phospholipid monolayers are well-defined and controllable systems, which represents half part of the biological membrane. The phospholipid modified liquid – liquid interface can also provide the information about pH equilibrium, adsorption – desorption reaction of the lipids at and from the liquid – liquid interface, association – dissociation interaction of phsopholipids with charged species from both sides of the interface as discussed in series of papers from Mareček et al..136,137,138 The interface modified with a phospholipid monolayer can be studied by the simple ion transfer process across the artificial half part of biological membrane. For instance, adsorption of phosphatidylcholines on the water-1.2-dichloroethane interface had almost no effect on the transfer of tetraethylammonium cations.139 As discussed in mentioned work, the phospholipids at the ITIES form ‘island-like clusters’ which partially cover the interface and the electrochemical signal is due to the transfer of electroactive species through cluster free domains. In order to control a compactness of the adsorbed monolayer, the surface pressure control – with the Langmuir trough technique – as an additional degree of freedom was introduced.140,141 Even though the monolayer quality could be controlled by lateral compression, the large planar area gave rise to a potential distribution and become unstable due to the dissolution of the adsorbed phospholipids in the organic phase. To overcome such difficulties, the Langmuir trough used to control surface pressure of the adsorbed phospholipid monolayer was used as the aqueous half-part of electrochemical cell. The second, organic phase was specially designed PTFE cell containing gelled o-nitrophenyloctylether (o-NPOE) – poly(vinyl chloride) (PVC), which was immersed into the monolayer, resulting in gel-liquid interface (see. Figure 1.15).
The effect of 1,2-dioctadecanoyl-sn-glycero-3-phosphocholine (DSPC) monolayer onto the adsorption and the kinetics of charge transfer for TEA+, porpanolol, metoprolol and tacrine has been studied by cyclic voltammetry and AC voltammetry.142 Comparison of the calculated values of admittance and apparent capacitance in the presence and absence of ion transfer through DSPC monolayer allowed concluding what follows: (i) all studied ions tend to interact with the phospholipid membrane; (ii) rate constant of TEA+, propranolol and metoprolol decrease with increasing phospholipid deposition surface pressure. No change was observed for tacrine and (iii) calculated apparent capacitance values in the presence and the absence of ion transfer indicated that charge transfer reaction of tacrine and partially the metoprolol are coupled with the adsorption process. In subsequent work, electrochemical impedance spectroscopy was used to evaluate interaction between four similar in structure therapeutics (aminacrine, tacrine, velnacrine and proflavine) and – different in composition – phospholipid monolayers adsorbed at the ITIES. The results indicated that the preferable adsorption site in the organic phase for velnacrine and proflavine is the polar head group region whereas tacrine and aminacrine prefer hydrocarbon tail domains.143
Table of contents :
1. Chapter I. Bibliographical introduction
1.1. Electrified interface between two immiscible electrolyte solutions
1.1.1. Liquid – liquid interface structure
1.1.2. Charge transfer reactions at the ITIES
22.214.171.124. Simple ion transfer reaction
126.96.36.199. Assisted/facilitated ion transfer
188.8.131.52. Electron transfer across ITIES
184.108.40.206. Electrochemically induced interfacial adsorption
1.1.3. Potential window and limiting current
1.1.4. Electrochemical instability at the electrified liquid – liquid interface in the presence of ionic surfactants
1.1.5. Miniaturization of the ITIES
1.2. Sol – Gel Process of Silica employing Template Technology
1.2.1. Nomenclature and physicochemical properties of silicon and silicon containing compounds
1.2.2. The Sol – Gel process of silica
1.2.3. Templates – towards surface engineering
1.2.4. A soft template for a Sol-Gel process of mesoporous silica thin films
1.2.5. Functionalized mesoporous silica films prepared by Sol-Gel processing
1.3. Liquid – liquid interface modification
1.3.1. Metals at the electrified liquid – liquid interface.
220.127.116.11. Au deposition at the ITIES
18.104.22.168. Ag deposition at the electrified ITIES
22.214.171.124. Pd and Pt deposition at the electrified ITIES
1.3.2. Phospholipids at the electrified liquid – liquid interface
1.3.3. Organic polymers at the polarized liquid – liquid interface
1.3.4. Carbon based materials at/near polarized liquid – liquid interface
1.3.5. Silica modified liquid – liquid interface
126.96.36.199. Three phase junction systems
188.8.131.52. Neat, non-polarized liquid – liquid interface in situ modification with silica material..
184.108.40.206. Electrified Interface between Two Immiscible Electrolyte Solutions modification with silica materials
220.127.116.11.1. Ex situ modification
18.104.22.168.2. In situ modification
2. Chapter II. Experimental part
2.2. Electrochemical set-ups
2.3. Composition of electrochemical cells. The aqueous and the organic phase preparation
2.5.1. Preparation procedure of BTPPA+TPBCl-
2.5.2. Preparation procedure of CTA+TPBCl-
2.5.3. Preparation procedure of TBA+TPBCl-
2.5.4. Preparation procedure of PH+TPBCl-
2.5.5. Protocol of organic counter electrode preparation
2.5.6. Single pore microITIES protocol of preparation
2.5.7. Protocol of preparation of trimethylbenzhydrylammonium iodide
3. Chapter III. Templated Sol – Gel process of silica at the electrified liquid – liquid interface
3.1. Results and discussion
3.1.1. Electrochemical study
3.1.2. Characterization of silica deposits electrogenerated at the ITIES
3.1.3. Spectroscopic analysis
3.1.4. BET analysis
3.1.5. Morphological characterization
4. Chapter IV. Silica electrodeposition using cationic surfactant as a template at miniaturized ITIES
4.1. Electrochemical and morphological study of silica deposits at the array of microITIES
4.1.1. Surfactant-template assisted Sol-Gel process of silica at the microITIES
22.214.171.124. Factors affecting silica deposition at the array of microITIES
126.96.36.199.1. Influence of [CTA+]org and [TEOS]aq
188.8.131.52.2. Influence of the pore center-to-center distance
184.108.40.206.3. Influence of the scan rate
4.1.2. Morphological study
4.1.3. Spectroscopic and electrochemical characterization of deposits
4.2. In situ confocal Raman spectroscopy study of interfacial silica deposition at microITIES
4.2.1. Raman spectroscopy analysis of the liquid – liquid interface at open circuit potential
4.2.2. Ion transfer followed by Raman spectroscopy
4.2.3. Interfacial silica deposition followed by Raman spectroscopy
4.3. Electrochemical evaluation of microITIES modified with silica deposits
4.3.1. Blank experiment before and after modification
4.3.2. Single charge ion transfer before and after modification
4.3.3. Multicharged ion transfer before and after modification
4.3.4. Electroanalytical properities of microITIES modified with silica deposits
5. Chapter V. Local pH change at the ITIES induced by ion transfer and UV photolysis
5.1. Synthesis and characterization of trimethylbenzhydrylammonium iodide
5.2. Electrochemical characterization of PH+ transfer at macroITIES
5.3. Study of photodecomposition of PH+ species
5.4. Local pH change induced by electrochemical transfer and photodecomposition of PH+ species
5.5. Silica deposition induced by local pH decrease
6. General conclusions
7. Further directions
7.1. Silica deposits – SECM characterization
7.2. Silica deposits functionalization
7.3. Interfacially active base
Appendix I. Nanopipette preparation and silanization
Appendix II. Protocol of preparation of 3-azidopropyltrimethoxysilane