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29 Seiten, Note: 70
3. Literature Review
4. Experimental Section
4.5. Schematic diagram
5. Results and Discussion
5.1. Gold surface stability
5.2. Electropolymerisation of pyrrole around hydrogen bubble template on bare gold surface
5.3. Surface hydrophobicity
5.4. Electropolymerisation of pyrrole around n-decane droplet template on highly oriented pyrolitic graphite
5.5. Electropolymerisation of pyrrole around n-decane droplet template on thiol-coated gold
Simple electrochemical aided polymerization was done using pyrrole and beta- napthalenesulfonic acid as electrolyte cum anionic surfactant to attempt creating microstructures on sputtered gold surface, highly oriented pyrolitic graphite, and thiol- modified gold surface. Hydrogen bubble template was used on gold, but results showed doubted polymerization around this. Instead, one tenth structure size of hydrogen bubbles was commonly observed in two different electrolyte concentrations. No interesting microstructure was formed on highly oriented pyrolitic graphite. This was due to polarization of large size graphite. Irregular microcapsules were formed around n-decane template on thiol-coated gold, but other structures also appeared. Lack of control on many variables due to limitation of equipments and materials lead to inaccuracy and inability to do more detailed observation. However, some results showed research aim was achieved.
Since last decades, extensive research on fabrication of microstructures of conducting polymers has been aimed to exploit their semiconductive, optical, flexibility, biocompatibility, nano/microscale, low density, and large surface area properties. The term conducting polymers refers to polymers with conjugated chain structure with semiconductive property due to alternating single and double bond. Microstructures differ with their size, shape, and applications. Also, chemical and electrochemical pathways and various templating methods offer their own unique advantages and disadvantages in forming desired microstructures. Obviously, narrowing down the scope of this research is a sensible step to focus investigating fabrication of microstructures of conducting polymer.
One feasible research proposal was to investigate gaps in what have been found on microcontainers fabricated by simple electrochemical method using polypyrrole as conducting polymer. Microcontainers could be defined as polymerized shape that could be further encapsulated and reopened (e.g cups, bowls, bottles, capsule, spheres), mostly, reversibly. Many studies have been able to control morphology and properties of microcontainers by modifying electrochemical condition, applied potential, concentration, and techniques (for example, solidified droplet, etc). Conflicting and inconsistent evidence have been seen, for example, on applied potential to form soap bubble and start polymerization. Most importantly, gaps appeared in no formation of microcontainers on some electrodes: gold and highly orientated pyrolitic graphite (HOPG). Also, there has not been any electrochemical polymerization around oil-droplet template (n-decane is used in this project).
This research project aims to (1) electropolymerise pyrrole around hydrogen bubble template formed on gold surface (2) electropolymerise pyrrole around n-decane template on highly oriented pyrolitic graphite and thiol-coated gold surface, and (3) generally link some conditions of polymerization to microcontainer shapes formed. If sputtered gold was stable under polymerization potential, this surface would produce products similar to that of stainless steel surface. If working electrode size should be equal or less than counter electrode to avoid polarization, then highly oriented pyrolitic graphite would possibly be polarized because it was significantly bigger than platinum counter electrode. And if anionic surfactant beta-naphthalenesulfonic acid coated n-decane droplet, polypyrrole backbone would attach around n-decane template during electropolymerisation.
Narrowed down by the project scope, this review only included literatures promoting microstructure formation using simple equipments, materials, and methods. One example is electrochemical polymerisation of pyrrole around soft-templates in beta naphthalenesulfonic acid as electrolyte with various working electrodes and conditions. Whereas, more advanced equipments, materials, and methods were outside this project scope and hence, excluded. This review would consider all aspects of a simple electrochemical process such as available equipments and materials, electrochemical conditions, electrode material, hydrophobicity, and size. Conflicting evidence was noted in applied potential to both produce soap bubble templates and start polymerisation, and gold hydrophobicity as working electrode. Gaps appear in no formation of microcontainers on gold and highly oriented pyrolitic graphite, and no electrochemical polymerisation around oil-droplet template.
Materials and equipments were available to do simple controlled electrochemistry. Potentiostat and three electrode cell (working, counter, and reference electrode), pyrrole, beta-naphtalenesulfonic powder, OTS-silicon, gold and chromium sputter, highly oriented pyrolitic graphic, and 1mM thiol solution were main basic materials and equipments available to control variables to determine product properties. Stainless steel as common working and counter electrode was not available. As counter electrode, platinum wire-coil could be used as common substitute to stainless steel. Gold surface was used as substitute for working electrode because it is inert to electrolyte has good conductivity. Polymerisation could be done using chemical or electrochemical paths. Electrochemical path was chosen because it offers greater control over variables to produce desired microstructures and could be done in ambient environment. Using available potentiostat, electrochemical polymerisation reaction rate can be controlled by the applied potential or current density. Additionally, the amount of product can be controlled by the integrated charges used for electrosynthesis. Furthermore, product's morphology and property can be modified using electropolymerisation conditions such as electrolyte and pyrrole concentration^], and different templating methods [55,56,1].
Usage of pyrrole as monomer and beta-naphtalenesulfonic acid as electrolyte cum surfactant, hydrogen bubbles, and n-decane as soft-template was feasible for this project. Pyrrole was chosen due to its well known conductivity, chemical stability, mechanical properties, and most importantly, availability[4,5]. Template shape, to which pyrrole backbone will attach to during polymerisation, would also determine microstructure shape as product[1,5,4,49,54-56]. Beta-naphthalenesulfonic acid is anionic surfactant and electrolyte at the same time. This was known to coat hydrogen bubble with negative charges, and allow deposition of bubbles on working electrode when positive potential was applied at the working electrode. At this same instant, pyrrole underwent redox reaction to become conductive polypyrrole which acquired anion by attaching its backbone to surfactant-coated bubble. Thus, polymerization occurred around hydrogen bubble template. The available microscope limits clear observation of product's morphology only up to 10 micrometer scale. Various template methods with size^O micrometer that have been recognised were layer by layer, oil droplet, solidified droplet, microbubbles [1,2,3], membrane and solid, nanosheets, and micelles[45,46]. This list was further reduced by selecting the ones offering easiness to form before/during and remove after each experiment. Finally, simplest soft-templates such as hydrogen bubbles (or soap bubbles) and n-decane were chosen due to considerable practicality they offer. Easily, soap bubbles could be produced during experiment by water electrolysis and n- decane could be prepared by direct surface adsorption in 20%v/v ethanol emulsion. Hydrogen bubbles and n-decane templates could be removed after each experiment by, respectively, exposure and drying with atmospheric air. Microstructures formed by corresponding soft-templates were known as "microcontainers" with potential application of sensing, attributed to their large conductive surface area, and reversible encapsulating and releasing organic species such as dyes and drugs.
There has been abundant studies linking electrochemical condition to produce gas bubbles and start polymerisation of pyrrole with microcontainer morphology. Most studies were in agreement with others and some were contradictive and inconsistent. Firstly, increasing either applied potential or pyrrole concentration would increase "deformation force". This deformed microcontainer shape from spherical to elliptical and then to cylindrical. Additionally, extensive proofs on increased density, shape, and wall thickness of polypyrrole were observed by, respectively, increasing either the width of applied potentials for generation of hydrogen bubbles or concentration of electrolyte, increasing either scanning rate, width of applied potentials to start polymerisation, or concentration of electrolyte, and increasing either cyclic voltametric scan number, concentration of pyrrole, or scanning rate. Contradictions appeared on effective applied potential to produce soap bubbles as template. Cyclic voltametry was commonly used because One study used cyclic voltametry from 0V to 1.2V for various cycles with onset oxidation potential of water of -1.25V (versus saturated calomel electrode) to form soap bubble template and microcontainers simultaneously. This method was judged to have a detrimental overoxidation effect because it directly generated oxygen gas on the working electrode under a relatively high positive potential (>0.8V)[47,2]. The detrimental effect was predicted to be polypyrrole film[4,48] or droplet deposition on working electrode. Pyrrole droplet was another templating method that produce microcontainer of around 10 micrometer size, but considered as a different method. Applied negative potential of -1.0 to -1.4 V and - 1.0V to -1.3 or -1.4 or -1.65 or -1.7V for first cycle were proven effective to generate hydrogen bubbles as template. Furthermore, there has been inconsistency in applied potential for polymerisation of pyrrole. One study claimed that no microstructures were formed under 0.8V applied potential. This is inconsistent to a finding that successfully formed microcontainers by applying -1.0V to -1.7V for first cycle, continued with 0.5V-0.75V for the next ten cycles using 100mV/s cyclic voltametry. The reason of this inconsistency seemed to be former study's weakness by not producing any soap bubble template under 0.8V potential because of the absence of first scan of negative potential.
Stainless steel[2,3,47] and platinum were common working electrodes in production of soap bubble-templated microcontainers. However, there has not been any (or, only a few, if existed) evidence of microcontainers produced on other electrode material such as gold and highly oriented pyrolitic graphite. Instead of microcontainers, evidence has shown film polymerisation on highly oriented pyrolitic graphite and gold. One possible explanation seemed to be these material was not suitable in electropolymerisation systems, purpose of experiments, surface properties, shapes, and costs to produce microcontainers. Nevertheless, it would be interesting to investigate whether microcontainers could actually be formed on gold and highly oriented pyrolitic graphite electrodes. This research aimed to form gas bubble-templated microcontainers on gold surface, but not on highly oriented pyrolitic graphite. The decision not to use soap-bubble template on highly oriented pyrolitic graphite would be discussed in later paragraphs.
As another soft-template than gas bubbles, oil droplet has received little attention. One reason could be the new method of solidified droplet seemed to be more promising than liquid droplet [and also possibly soap bubble] because it offered stronger microstructure to avoid wall collapse and inhibited degradation of dye/drugs while sealing the mouth of microcontainers. Nevertheless, both oil and solidified droplet templating lied on the same principle of surface adsorption, which created water-insoluable template on hydrophobic surface[55,56]. Surface hydrophobicity played important role in adsorbing nonpolar liquid[4,5], for example, n-decane, mineral oil, and heated tetradecanol. Two studies successfully adsorped dye and heated tetradecanol /mineral oil on quartz glass, as well as sealing them in hemispherical polypyrrole capsule by chemical polymerisation path[55,56]. There were two interesting questions that arised from this finding. Firstly, what caused the hemispherical shape? One claimed this was due to hydrophobic glass interaction that affect non-polar oil template shape. If this was true, one could observe oil-droplet shape on hydrophobic surface and expect this to be template shape for polymerisation. This research measured oil-droplet contact angle in atmosphere and check if this could be related to template shape. Secondly, would electrochemical polymerisation path also work? This research aimed to electrochemically polymerise pyrrole around n-decane template. Only positive cyclic voltametric scan was required because gas bubble production template was replaced by oil. n-decane was used because it fulfills the requirement of non-polarity and immiscibility in water. One unknown factor to be observed was whether anionic surfactant beta-naphthalenesulfonic would coat n-decane bubbles during direct surface adsorption. This would affect pyrrole backbone attachment, and hence, structure created. Considered surfaces are hydrophobic surfaces so n-decane could adsorb on it[55,56].
To do this experiment, while stainless steel would have been ideal as working electrode because its surface was well known to be highly hydrophobic (contact angle with water = 0иго r« 79°), gold hydrophobicity was found to be highly debated. Since 1934, 26 (or more) attempts have been made to establish the hydrophilic or hydrophobic nature of clean gold surfaces. Eighteen (including theoretical studies) concluded clean gold is hydrophobic (0иго r« 65°), whereas eight concluded clean gold is hydrophilic. Other studies showed that bare gold was hydrophobic with relatively large reported contact angle within the range of 40 to 79.5 degrees and 65.5 degrees, which was unexpectedly high for metal. Conclusive study explained this phenomena: less than a monolayer of carbonaceous contamination renders the gold surface hydrophobic (0H2o ~ 50°), whereas clean gold is hydrophilic. Because gold carbonaceous contamination is an uncontrolled factor during experiment, it was safer to modify gold surface. Firstly, it is assumed that bare gold was hydrophilic in water. Afterwards, thiol-coating was applied to create covalent bond between thiol and gold. Self assembly monolayer (SAM) of thiol made gold surface hydrophobic. This method could guarantee successful n-decane direct surface adsorption on hydrophobic surface to create oil droplet.
Highly oriented pyrolitic graphite was reported hydrophobic (0H2o 90° ), therefore it seemed suitable to use this as working electrode for n-droplet template. Furthermore, due to its limited availability and high cost, it was not used to polymerise hydrogen template.
The size of highly oriented pyrolitic graphite was bigger than platinum coil as counter electrode. Additionally, its effective surface areas were both sides of the electrode. One study implied this would cause polarisation (or often referred as overpotential) of working electrode because it is larger than the counter electrode. Polarisation was defined as production of thermodynamically irreversible potential on electrode surface, which speeds up reaction and might cause overoxidation. In other words, an expected effect due to larger size of working electrode relative to counter electrode is overpolymerisation of pyrrole on highly oriented pyrolitic graphite electrode. This occurence would seem to cause difficulty in observation of microstructures. This implication has been discussed before conducting experiment but this study was not taken into consideration^].
In conclusion, many studies have contributed to control morphology and properties of microcontainers by modifying electrochemical condition, applied potential, concentration, and formation techniques. The knowledge has been applied in many fields such as medicine, communication, etc. In scope of simple electrochemical approach, gaps were observed in no microcontainer formation on some common electrodes such as gold and highly oriented pyrolitic graphite. Also, electrochemical pathway to polymerise around oil-template seemed absent. This review had attempted to fill knowledge gap in understanding simple electrochemical approach of fabricating microstructures.
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