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Mesoporous silica thin films
One of interesting type of morphology is film. The big interest to it is given due to their multiple applications in optical, electronic and sensing devices, in separation and hosting [41,42], to which powder samples are not accessible [43].
The first notions about sol-gel deposited silica films on support appeared in 1996 [44–46]. An approach of growing the film from acidic solution containing silica precursor and alkyltrimethylammonium chloride as surfactant template was used by Aksay et al. on graphite, mica and silica support [44]. Authors showed that synthesised films have the same alignment as mica and graphite substrates, but film growth at silica/water interphase triggered a random orientation. The same way of depositing films of 0.2 – 1.0 µm thick was also used by Yang et al. on mica surface [46]. For all described cases, the pore size was typically defined by chosen surfactant and the time of synthesis was defined from hours to weeks. Ogawa [45] presented an utilisation of a spin-coating technique to deposit film on glass substrate from solution of the same composition. For this purpose, liquid was placed on the rotating support which distributes solution homogeneously around the surface (via centrifugal force) with subsequent evaporation of the solvent. As reported, obtained coating is 2 µm thick and possesses hexagonal mesostructure. These described films exhibited pore orientation parallel to the electrode substrate. Dip coating was also used to obtain porous silica films with wide pore distribution and pore diameter from 1.5 to 3.1 nm depending on time of aging [47]. An idea behind this method of deposition is in dipping a substrate for inside the solution with precursor and surfactant for the certain time with following slow vertical lifting of it back on air. The faster substrate is pulled out, the thicker the deposit is obtained. Finally, the liquid is evaporated and silica network undergoes a cross-linking.
Later, Brinker [30] introduced an Evaporation-Induced Self-Assembly (EISA) approach for generation of highly ordered mesostructured thin films. Most of the drawbacks of the firstly proposed methods, such as time-consuming deposition procedure, scale limit, and poorly-defined thin film morphology, which was closer to powders, were overcome. The key requirement for EISA is presence of volatile solvent (normally ethanol), which can be easily evaporated. EISA film deposition includes four main steps presented on example of dip coating film deposition technique illustrated on Figure 6 [48]:
1. Preparation of homogeneous sol (hydro-alcoholic media) with silica precursor for solid network creation around soft template (introduced in low concentration) and its hydrolysis. Condensation of silica is optimally slowed down by keeping low pH (Figure 1).
2. Evaporation of solvent, which causes an increase of surfactant concentration. Evaporation continues reaching the concentration of surfactant at which the transition from individual molecules to molecular assemblies starts (CMC). In case of dip coating, this process occurs much faster pace while pulling out the film from solution due to volatile nature of solvent.
3. Evaporation is complete and the film mesostructure is tuned by adjusting relative humidity (RH). At higher rates of RH more water molecules are present in silica matrix. In this way an increase of water content causes a transition of mesostructure from disordered (RH 20%) to 2D-hexagonal (RH 40%) and cubic (RH > 40%) [49].
4. Condensation of inorganic network and final stabilisation of hybrid mesostructure to increase porosity by template extraction. Condensation of silica network successfully occurs from 130oC for 48 h in air for functionalised films (with no degradation of organic groups [50]) up to 800oC (for unmodified mesoporous silica film with cubic orientation [51]).
Electrode modification by mesoporous silica films
Surface modification of electrodes by mesoporous silica thin films gives wide spectrum of advantages in terms of application. The use of mesoporous silica systems was studied for anti-corrosion coatings for Zn [76]. Thicker films were protecting metal surface from degradation much better and it shows that this approach can help to increase the durability of metal electrodes. For the analytical application comparison between bulk and porous silica coatings demonstrates that porous analogues provide much easier access of analysed species and, hence, improved the mass transport [77].
Owing to these properties and to feasibility of homogeneous modification by one-pot co-condensation mesoporous silica expanded the areas of film usage for detection of various analytes like gases (including humidity sensors and sensors for alcohol detection), heavy metals and organic molecules. Mesoporous films modified by β-cyclodextrin were used for benzene detection using quartz crystal microbalances [78], unmodified films showed sensitivity to relative humidity from 30 to 70% based on conductivity changes as a result of water adsorption [79] and to alcohols by current variation after potential application [80]. Modification of mesoporous silica surface by co-condensation with triethoxysilyl-propyl-5-mercapto-1-methyl-tetrazole allowed accumulation of Hg(II) at low values and its following analysis by anodic stripping square wave voltammetry [81]. The limits of Hg(II) detection by using this electrode is 2 nM. Targeted preconcentration of mercury ions was also provided by 3-mercaptopropyltrimethoxysilane-functionalised silica film [82]. Furthermore, it was shown that templating by organic molecules instead of surfactant (i.e. dopamine) can improve the recognition of this particular analyte when using sol-gel silica film [83]. In all cases, films were deposited by spin-coating [78,81–83] or dip-coating technique [79,80]. Moreover, immobilisation of proteins such as cytochrome c [84] and haemoglobin [85] triggered a development of biosensors for reduction of oxygen [86] or hydrogen peroxide [84–86] same as oxidation of ascorbic acid [84]. In the case of cytochrome c, the use of block copolymer was enough to provide amine-functionalised silica film with suitable diameter of pores to accommodate protein by impregnation without losing its electrochemical activity [84]. Immobilisation of haemoglobin required larger pores, hence more complicated system based on silica particles with bimodal pore distribution and chitosan was used [85]. Films were obtained by placing and drying a drop of suspension containing bimodal silica particles, chitosan and haemoglobin on the glassy carbon electrode. Composite materials based on mesoporous silica films were also studied extensively. Incorporation of nanoparticles inside silica matrix makes material with high surface area a perfect choice for catalytic systems. It was demonstrated on example of Au-modified mesosystem for glucose oxidation, where due to gold nanoparticles oxidation of analyte became possible [87]. Under the optimised conditions, limit of detection of glucose was estimated to be 0.1 mM.
Mesoporous silica films with vertically oriented pores for electrochemical application
The fact that conductive electrode surface was covered by non-conductive film may have a negative impact on electrochemical response in solution [59]. A presence of mesoporous film hindered the mass transport of analysed species to the electrode surface because the coating was acting as an obstacle due to the charge of the material surface, the pore shape and orientation. In spite of these limitations, mesoporous films stay interesting because of the provided access for guest species of organic and inorganic nature and preconcentration behaviour which increases sensitivity of electrochemical detection [59]. Since the pore size was an easily tuneable parameter [90,91], the main complications which were noticed were related to morphology and orientation of pores [92]. This is why it was suggested that perpendicular arrangement of channels would be more favourable for mesoporous silica films use as electrode coatings.
Silica films with perpendicular pore orientation were obtained by application of a high magnetic field [93,94]. The vertical film organisation from acidic aqueous solution containing TEOS silica precursor and template (be it CTAB, Brij 56 or P123 triblock copolymer) takes place owing to the influence of magnetic field on rod-shaped self-assembled surfactants which possess magnetic anisotropy. Pore diameter was determined by the chain length of used template. The disadvantage of this method is a necessity to have a specific apparatus for the film generation.
Another presented approach used block copolymer to form porous template by spin-coating with subsequent deposition of TEOS precursor in humidified supercritical CO2 [95]. This method did not find a wide application: in addition to two-step procedure, which implies pre-deposition of copolymer, pores of formed silica material are not perfectly homogeneous.
An interesting method of deposition of mesoporous platinum films using potential-controlled assembly of surfactant coupled with an electrodeposition process was reported back in 2003 [96]. It was successfully shown that at certain potential two parallel processes can occur: organisation of surfactant-inorganic matrix and reduction of metal ions, leading to generation of porous film. In this light, classical potentiostats can be used for the film deposition in one step.
Practical application of vertically oriented mesoporous silica films
Owing to specific pore morphology and orientation, and feasibility to functionalise films with vertical pore alignment, multiple ways of their utilisation were discovered [8]. Oriented mesoporous silica films can be used in four main conditions: with surfactant inside pores, as semi-permeable membranes after support removal, after its extraction, and after functionalisation.
The use of surfactant-containing films on the electrode support is presented on Figure 12A. Due to their anti-biofouling capacity, small pore size, lipophilicity, charge selectivity and preconcentration abilities they can be used for detection of small molecules in complex systems (i.e. chloramphenicol in human blood [115]). Another study [116] showed their utility for trace nitroaromatic explosives as well as for pesticide detection. Incorporated surfactant in described cases assists in preconcentration of analysed species increasing the sensitivity of voltammetric detection.
After the support removal film was used as a membrane (Figure 12B) for selective transport of species (specifically shown for metoprotol [117]) between two liquid phases promoted by electrochemically-induced transfer. Films obtained by oil-induced Stöber deposition exhibited an excellent performance in separation of nanosized gold nanoparticles and proteins [99].
X-Ray photoelectron Spectroscopy (XPS)
X-Ray photoelectron Spectroscopy (XPS) is a standard surface characterisation technique. It requires an X-Ray source of light with constant excitation energy during the experiment and high vacuum. After contact of photons with sample, an emission of excited electrons from the core levels (such as s, p and d) occurs. Each electron has its own binding energy, which defines a kinetic energy with which they are passing through electron analyser to be further detected (Figure 21). Obtained plot is represented as the number of electrons at a specific binding energy. Since each type of element has a unique distribution of core levels, each registered peak corresponds to a particular element, providing semi-qualitative data about elemental composition of upper layer (in the range of nanometres) of solid samples. Additionally, XPS delivers the information about quantity of elements which are present.
Grazing-incidence small-angle X-Ray scattering (GISAXS)
GISAXS is a non-destructive method which allows to identify the morphological properties of the thin layers in particular. The idea of method lays in reflection of the incident (vector ki) X-ray beam, which grazes the sample under a very small angle ai, with following recording of scattering (vector kr) coming from sample. It gives the specific patterns (detector plane q) with respect to material structure (Figure 23).
Experiments were carried out on MSF samples to verify and confirm the organisation of a pore structure, especially during the etching process. The apparatus “SAXSess mc2” (Anton Paar) was used with GISAXS equipped accessory “VarioStageXY” (X-ray tube from PANalytical, λ Cu, Kα=0.1542 nm) set to 0.25o incident angle).
Table of contents :
Chapter 1 Literature review
1.1 Introduction
1.1.1 Sol-gel process
1.1.2 Mesoporous silica
1.1.3 Mesoporous silica thin films
1.2 Main approaches for electrode modification
1.2.1 Electrode modification by mesoporous silica films
1.2.2 Mesoporous silica films with vertically oriented pores for electrochemical application
1.2.3 Functionalisation of vertically oriented mesoporous silica films
1.2.4 Practical application of vertically oriented mesoporous silica films
1.3 Our research project
Chapter 2 Experimental part
2.1 Instrumental techniques
2.1.1 Electrochemistry
2.1.2 Spectroscopy
2.1.3 Microscopy
2.1.4 Profilometry
2.2 Protocol for synthesis of (3-azopropyl)triethoxysilane (AzPTES)
2.3 Mesoporous silica film preparation by electrochemically-assisted self-assembly
2.3.1 Sol preparation and film deposition
2.3.2 Surfactant extraction
2.3.3 Click reaction to prepare modified films
2.4 Electrochemical characterisation of mesoporous silica films
2.4.1 Characterisation with conventional probes
2.4.2 Characterisation with haemoglobin
2.5 Protocol for etching experiments
Chapter 3 Tuning the thinness of well-organised mesoporous silica film
3.1 Introduction
3.2 Investigating the deposition time parameter for obtaining the thinnest flawless films
3.3 Wet etching approach to control the thickness of mesoporous silica film
3.3.1 Control experiments
3.3.2 Effect of mesoporous silica film nature
3.3.3 Effect of the NH4F concentration
3.3.4 The effect of the etching agent
3.4 Conclusions
Chapter 4 Development of system based on Ru(bpy)32+ modified mesoporous silica thin film for glyphosate detection by electrochemiluminescence
4.1. Introduction
4.1.1 General information about Ru(bpy)32+ as electrochemiluminescence reactant
4.1.2 Immobilisation of Ru(bpy)32+ onto solid matrix
4.1.3 Glyphosate and its detection
4.2 Glyphosate detection by ECL – Ru(bpy)32+ complex system: solution approach
4.2.1 ITO working electrode
4.2.2 ITO working electrode modified by mesoporous silica film
4.3 Impregnation of Ru(bpy)32+ complex onto silica film surface
4.3.1 Electrode preparation
4.3.2 Characterisation of mesoporous silica films with physically adsorbed Ru(bpy)32+ complex
4.3.3 Electrochemical response in presence of glyphosate
4.4 Covalent immobilisation of Ru(bpy)2(bpy’)2+ onto mesoporous silica film surface
4.4.1 Ru(bpy)2(bpy’)2+ complex preparation
4.4.2 Huisgen “click” reaction
4.4.3 Characterisation of Ru(bpy)2(bpy)’-modified mesoporous silica films
4.4.4 Electrochemical response in presence of glyphosate
4.5 Analytical aspect of electrochemical detection of glyphosate
4.6 ECL measurements: challenges and complications in course of the project
4.7 Conclusions
General conclusion and perspectives
Appendix 1: Chemicals
Appendix 2: Determination of ammonium fluoride concentration
Appendix 3: Surfactant extraction from mesopores during etching
Appendix 4: Effect of pH on glyphosate detection using Ru(bpy)32+ glyphosate co-reactant system on ITO electrode
Appendix 5: NMR characterisation
Bibliography
