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Mesoporous silica modified electrodes
In recent years, the emergence of electrodes modified with mesoporous materials as electrochemical sensors has gained much importance.37 IUPAC classifies the porous materials into microporous < 2nm, mesoporous 2-50 nm and macroporous > 50nm.38 Mesoporous silica materials generation by using surfactant molecules was pioneered by scientists at Mobil Oil Corporation in 1992.39 These materials from M41S family have been classified into three classes namely (MCM = Mobil Composition of Matter) (i) MCM-41 (ii) MCM-48 and (iii) MCM-50 giving rise to hexagonal, cubic and lamellar shaped materials respectively (Figure I-3).40 MCM-41 among other is the most commonly used mesoporous material in electrochemical applications due to its pore size, distribution and uniform distribution of hexagonal channels.
Composite electrodes
Composite electrodes are prepared by the mixing of mesoporous materials (mostly silica based and non-conducting) with a conducting compounds such as graphite powder. Carbon paste electrodes modified with silica compounds are the first examples of composite electrodes whereas modified screen printed electrodes (SPE) are the more recent ones.51,52 There is a considerable interest in production of SPE with different organic and inorganic compounds incorporated in carbon inks which are used to make SPE. Composite electrodes are widely applied in the field of electroanalysis.53 Composite electrodes have certain advantages such as close association of mesoporous materials and conducting matrix results in efficient electrochemical response and possibility of electrode modifications with compounds that usually does not form homogenous thin films.
Silica coated electrodes
Deposition of powdered mesoporous materials present in a suspension can result in the formation of thin films of respective materials at electrode surfaces. These thin films can be prepared either by dipping the electrode inside suspension or putting it at the surface and letting solvent to evaporate. Major drawback of thin films prepared this way is their mechanical stability and poor homogeneity at electrode surface making them unfit for electrochemical applications.42 To resolve these issues different approaches have been proposed including formation of strengthening layers of different organic polymers such as Nafion, Styrene and Poly (vinyl alcohol) over deposited mesoporous films and also mixing mesoporous silica materials with these polymers to form polymer films containing encapsulated mesoporous silica materials.54–58 However, these organic polymers are not very inert in nature and can interact with mesoporous silica materials in solution and hence can change its properties. Formation of thin films using template molecules result in homogenous and mechanically stable mesostructures at electrode surfaces. In addition to composite and thin films as electrodes modifiers, mesoporous silica based materials with different morphologies i.e. nanoparticles, monoliths, spheres and rods have found their applications in the field of drug delivery, catalysis, as a support, encapsulation and selective adsorption of biomolecules.59–62 In this study we focused on deposition of surfactant-templated mesoporous silica thin films on glassy carbon electrodes. Various methods available for production of mesoporous silica thin films using template route are discussed.
Mesoporous silica thin films
The deposition of mesoporous silica thin at electrode surfaces is based on the local control of sol-gel chemistry. Mesoporous silica thin films are synthesized by surfactant templating. A general method for the synthesis of mesoporous silica thin films with parallel arrangement to electrode surface is evaporation induced self-assembly (EISA).63 This process is used for depositing silica film by dip coating, spin coating and drop coating methods with the help of a sol prepared with surfactant and silica precursors in water and ethanol medium. In the next step, solvent evaporation results in the deposition of a homogenous film due to polycondensation of silica precursors and surfactant critical micelle concentration (CMC).64 Mesoporous silica thin films formed by EISA process have parallel arrangement of pores to electrode; this can result in poor performance of silica films in electrochemical applications due to transport limitations. Schematic representation of mesoporous silica films deposition by EISA process is presented in Figure I-4.
Vertically oriented mesoporous silica thin films
Vertically oriented mesoporous silica thin films are advantageous in electroanalytical application as the can offer fast mass and electron transfer due to normal pore arrangement towards electrode surface. Various strategies have been proposed to generate mesoporous silica thin films with hexagonally packed mesochannels aligned vertically to electrode surface such as (i) dimensional confinement and self-assembly process in exotemplates (pre-assembled block copolymer films65 or porous membranes66 (ii) combination of photoaligning and micropatterning techniques or surface mediated alignment via - interactions;67,68 (iii) epitaxial growth;69 (iv) magnetically induced orientation;70 (v) oil-induced co-assembly approach;71 (vi) electrochemically assisted self-assembly (EASA);72 and (vii) Stöber-solution growth method.73 Among them, the most robust ones are certainly the last two methods, as they have been reproduced by independent groups (especially the EASA method).74–83 These two methods lead to small-pore materials (2-3 nm in diameter) as a result of using cetyltrimethylammonium bromide (CTAB) as the surfactant template. The Stöber-solution growth method73 involves a controlled kinetic interface nucleation and slow growth (1-3 days’ time range, at 60-100°C) originating from the gradual transformation of silicate-CTAB composites from spherical to cylindrical micelles with the assistance of ammonia hydrogen bonding and progressive silicate polymerization, such films can be manufactured in mono- or multi-layered structures.
Electrochemically assisted self-assembly (EASA) process
The EASA method72,84, like Stöber-solution growth method operates under kinetic control but is much faster (5-30s deposition time) and film formation is achieved at room temperature. It is based on the application of a reductive potential to an electrode immersed in a hydro-alcoholic sol containing a hydrolysed tetraalkoxysilane as silicate source and CTAB surfactant as template, inducing concomitant self-assembly of surfactant hemi-micelles at the electrode surface and polycondensation of the silica precursors driven by the electrochemically generated pH increase.72 (under cathodic potentials, protons and water molecules are being reduced inducing a local increase of the pH at the electrode/solution interface and hence favouring the self-assembly condensation of silica around the surfactant template). Film growth occurs from the electrode surface, explaining the vertical arrangement of the hexagonally packed mesochannels. The mesoporosity is then revealed by the removal of surfactant molecules from the film. In this study, silica thin films were deposited at electrode surface by EASA technique which were subsequently Characterized by transmission electron microscopy (TEM). Figure I-5 is the schematic representation of mesoporous silica thin films formation at electrode surface with the help of EASA method.
Electrochemical detection of herbicides
Conventional methods for the detection of herbicides and pesticides in general include high performance liquid chromatography127, spectroscopy128, capillary electrophoresis129, gas chromatography mass spectrometry (GC/MS)130, radioimmunoassay131, fluorescent probe titration132, spectrophotometry133, and more recently flow injection colorimetric assay134 or surface-enhanced Raman spectroscopy.135 These methods may provide quite low detection limits but suffer from the disadvantages of requiring cumbersome instrumentation, time consuming, highly trained personnel, inability to perform real time analysis and can be costly as well. There is need for systems which can provide rapid, cost effective, reliable and on-site detection of pollutants. Electrochemical sensing of pollutants in aqueous samples is rapidly growing technique these days with having the advantage of electrode surface modification, ease of handling, cost effectiveness and possibility of on-site applications for real time detection.43,136,137 Electrochemical sensing with technological advancements can provide these detection systems for harmful compounds. This can lead to proper management of the risk for lowest possible damage to human health and environment. In table I-2 few examples related to electrochemical sensing of paraquat139,140,141,142–148,149–158,159 and isoproturon159–164 are presented.
3-Aminopropyltriethoxysilane (APTES) electrografting
APTES was electrografted on GCE surface (A = 3.4cm2) by cyclic voltammetry (200 mV s-1) using an EMStat2 potentiostat (Figure II-3 A) according to a procedure published for oxidation of aliphatic amines170 by scanning potentials between +0.7 V and +2 V. Acetonitrile as a solvent while 0.1 M Bu4N+BF4- as electrolyte and closed electrochemical cell for organic solvents (Figure II-2 C) was used. A concentration of APTES to be electrografted was optimized between 1 to 5 mM. Electrodes were rinsed with ethanol after electrografting to remove any remaining traces of electrolyte and non-grafted APTES molecules.
Sol gel preparation and thin film electrodeposition (monolayer films)
A typical sol solution was prepared by mixing ethanol and 0.1 M NaNO3 in 1:1 v/v ratio, tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) was added to this solution (100 mM and 32 mM respectively) according to previously optimized concentration.84 The pH of sol solution was adjusted to 3 by adding 0.1 M HCl and hydrolysed by stirring for 2.5 hours at room temperature before using it for electrodeposition of silica film; Sol was prepared on daily basis. The area of the GCE to be modified with silica film was 0.5cm2, electrochemical cell used for film deposition is shown in (Figure II-2 B). Film deposition was achieved through galvanostatic conditionsby using PGSTAT 100 apparatus (Figure II-3 C) from Ecochemie (Metrohm, Switzerland), Electrodes were immersed in hydrolysed sol and a current density, j, of -0.74 mA cm-2 was applied for 30 seconds, while in case of ITO electrodes silica thin films were deposited using potentiostatic conditions (-1.3 V for 30 seconds). Silver wire was used as pseudo reference and stainless steel rod as counter electrode. Modified electrodes were treated overnight at 130 °C for proper cross linking of silica network. Template was removed from film matrix under moderate stirring conditions in 0.1 M HCl-ethanol solution for 1 hour as previously described.97 Modification of electrodes was characterized at each step by hexaamine ruthenium as redox probe.
Sol preparation and thin film deposition (multi-layered films)
Sol for multi-layered films, different deposition time and microelectrode arrays modification was prepared according to the previously described procedure for chapter III and IV. For multi-layered and various deposition time GCE plates were used, microelectrode arrays at GC substrates were prepared in clean room followed by curing at high temperature in N2 presence. GCE plates were cleaned as described previously and microelectrode array were thoroughly rinsed with ultrapure water before use. GCE and microelectrode array were electrografted with 1 mM APTES by cyclic voltammetry (1 cycle) prior to silica film deposition. Silica film was deposited by applying a current density, j, of -0.74 mA cm-2 for 5-30 seconds for various deposition time experiments while 8 and 16 seconds deposition was selected for multi-layered films. Microelectrode arrays were modified with silica films for deposition time ranging from 10-45 seconds and 20 seconds modification was found to be suitable. Electrodes were dried in oven at 130 °C overnight for better cross linking of silica films. Electrochemical characterization by 0.5 mM Ruthenium hexaamine III was done at each step of modification. Template extraction was carried out in 0.1 M HCl ethanol solution from 15 to 90 minutes depending on the silica film deposition time.
Square wave voltammetry (SWV)
Square wave voltammetry (SWV) is a powerful electrochemical technique, more suited for quantitative applications and mechanistic study of electrode processes. It is one of the most rapid and advanced electrochemical technique that solely has the advantages of pulse voltammetry (high sensitivity) cyclic voltammetry (insight into electrode mechanisms) and impedance spectroscopy (kinetic information of electrode processes). Typical modulation in SWV consists of a staircase potential ramp modified with square shaped potential pulses. At each step of staircase ramp, two equal in height and opposite in direction potential pulse are imposed to give rise to a single potential cycle in SWV. This potential cycle in SWV is repeated at each step of the staircase ramp during the voltammetric experiment (Figure II-5).
Electrochemical impedance spectroscopy (EIS)
Electrochemical impedance is the response of an electrochemical system to an applied AC potential or current, frequency dependence of the impedance can reveal underlying chemical processes. EIS or AC impedance was initially applied for the determination of double layer capacitance. It is carried out at different frequencies that’s why its termed as impedance spectroscopy. Advantages of EIS include sensitivity, large amount of data, fine-tuning mechanisms and determination of kinetics of processes, resistance, capacitance and real surface areas in situ. Applications of EIS include:
(i) Interfacial processes: Redox reactions at electrodes, adsorption, electrosorption and kinetics of homogenous reactions in solution combined with redox processes.
(ii) Geometric effects: linear, spherical, cylindrical mass transfer, limited volume electrodes, porous electrodes and determination of solution resistance.
(iii) Applications in power sources systems: (Batteries, fuel cells, membranes) corrosion studies, electrocatalytic reactions, self-assembly monolayers and sensors.
EIS generated data interpretation is carried out most frequently by complex plane (Nyquist) and Bode plot. Nyquist plots are most used in electrochemical literature as they provide easy prediction of circuit elements and allows easy relation to electrical model.
In present study potentiostatic impedance was performed by PGSTAT 100 apparatus using (Frequency response analysis) FRA2 module. Measurements were carried out on both bare and mesoporous silica modified GCE, complex plane (Nyquist) and bode plots were constructed. Condition for impedance measurements were; Potential applied 0V, wait time 5 s, Highest frequency 100 kHz and lowest was 0.1 Hz, Amplitude 0.01 VRMS, and current ranges highest 100 μA and lowest 10 μA. Fitting and simulation was carried out using analysis tool in the software of Nova 2.1.
Table of contents :
Chapter I: Literature review
1.1 Electrochemistry and sensing
1.2 Types of electrochemical sensors
1.2.1 Potentiometric sensors
1.2.2 Conductometric sensors
1.2.3 Amperometric sensors
1.3 Types of working electrodes
1.3.1 Mercury electrodes
1.3.2 Solid electrodes
1.4 Modified electrodes
1.4.1 The Sol-gel process
1.4.2 Mesoporous silica modified electrodes
1.4.3 Configurations of mesoporous silica modified electrodes
1.4.4 Composite electrodes
1.4.5 Silica coated electrodes
1.5 Mesoporous silica thin films
1.5.1 Vertically oriented mesoporous silica thin films
1.5.2 Electrochemically assisted self-assembly (EASA) process
1.5.3 Mesoporous silica thin film applications
1.6 Use of pesticides in agriculture
1.6.1 Paraquat and diquat
1.6.2 Isoproturon
1.6.3 Other herbicides
1.7 Electrochemical detection of herbicides
1.8 Brief introduction of research project
Chapter II: Experimental section
2.1 Chemicals
2.2 Electrodes and electrochemical apparatus
2.2.1 Working electrodes
2.2.2 Reference electrodes
2.2.3 Counter electrode
2.2.4 Electrochemical cells
2.2.5 Electrochemical apparatus
2.2.6 Electrodes preparation
2.3 Protocols for mesoporous silica film preparation
2.3.1 3-Aminopropyltriethoxysilane (APTES) electrografting
2.3.2 Sol gel preparation and thin film electrodeposition (monolayer films)
2.3.3 Sol preparation and thin film deposition (multi-layered films)
2.4 Electrochemical techniques for analysis
2.4.1 Cyclic voltammetry (CV)
2.4.2 Square wave voltammetry (SWV)
2.4.3 Stripping voltammetry
2.4.4 Electrochemical impedance spectroscopy (EIS)
2.5 Analytical techniques for chemical and structural analysis
2.5.1 Ion chromatography
2.5.2 Scanning electron microscopy (SEM)
2.5.3 Transmission electron microscopy (TEM)
2.5.5 Silica thin film Characterization by profilometry
2.5.6 X-ray photoelectron spectroscopy (XPS)
Chapter III: Improved adhesion of mesoporous silica thin films on glassy carbon electrodes by electrografting of 3-aminopropyltriethoxysilane
3.1 Introduction
3.2 Electrochemical modification and characterization of glassy carbon electrodes
3.2.1 Electrochemical Characterization of APTES electrografting
3.2.2 Electrogeneration and Characterization of the mesoporous silica films
3.2.3 Characterization of modified GCE by neutral and anionic redox species
3.2.4 Characterization of modified electrodes by scanning electron microscopy (SEM) .
3.2.5 Silica film characterization by transmission electron microscopy (TEM)
3.2.6 X-ray photoelectron spectroscopy analysis of GCE modification
3.3 Functionalization of the mesoporous silica with click chemistry
3.4 Conclusions
Chapter IV: Electrochemical detection of paraquat at mesoporous silica thin films modified glassy carbon electrodes
4.1 Introduction
4.2 Electrode modification
4.3 Electrochemistry of paraquat
4.3.1 Effect of charge on redox analytes at their electrochemical behaviour
4.3.2 Electrochemical behavior of paraquat at mesoporous silica modified GCE
4.3.3 Investigation of paraquat electrochemical behavior by square wave voltammetry .
4.3.4 Influence of ionic strength on SWV signal intensity
4.3.5 Effect of electrolytes nature on SWV response
4.3.6 Electrochemical impedance spectroscopic (EIS) analysis of electrolyte solutions .
4.3.7 Effect of solution pH on SWV response
4.3.8 Effect of Debye length and mesochannels Interaction on sensitivity
4.4 Square wave voltammetric analysis with low concentration of paraquat
4.4.1 Analysis of real samples spiked with paraquat
4.5 Conclusions
Chapter V: Studies of electrode modification and electrode geometry effect on paraquat sensing
5.1 Introduction
5.2 Electrochemical deposition and characterization of monolayer films
5.2.1 Microscopic characterization of electrode modification
5.2.2 Film thickness measurements by profilometry
5.2.3 Paraquat accumulation by CV at modified electrodes
5.2.4 SWV analysis of paraquat on monolayer thin films
5.2.5 Conclusions
5.3 Multi-layered silica thin films
5.3.1 Characterization of multi-layered silica films
5.3.2 Microscopic characterization of multi-layered silica film modification
5.3.3 Paraquat electrochemistry at multilayer silica thin films
5.3.4 Paraquat detection on multilayer silica thin films
5.3.5 Conclusion
5.4 Microelectrode arrays fabrication and modification with silica thin films
5.4.1 Microelectrode arrays fabrication
5.4.2 Electrochemical characterization of microelectrode arrays
5.4.3 Microelectrode arrays modification by silica thin films
5.4.4 Electrochemical characterization of microelectrode arrays modification
5.4.5 Scanning electron microscopic characterization of microelectrode arrays modification
5.4.6 Paraquat detection at silica thin films modified microelectrode arrays
5.5 Conclusions
Chapter VI: Electrochemical Characterization and detection of isoproturon at mesoporous silica modified and unmodified glassy carbon electrodes
6.1 Introduction
6.2 Results and discussion
6.2.1 Cyclic voltammetric Characterization of isoproturon
6.2.2 Scan rate studies of isoproturon
6.2.3 Effect of repetitive scans at peak intensity
6.2.4 Effect of supporting electrolyte on electrochemical behavior of isoproturon
6.3 Analytical detection of isoproturon
6.3.1 Effect of preconcentration time
6.3.2 Quantitative analysis of isoproturon by SWV
6.4 Conclusions
Conclusions and perspectives
Conclusions et perspectives
References:
