Mise en forme de structures à base de carbone pour des applications électrochimique

(1)

HAL Id: tel-01485076

https://tel.archives-ouvertes.fr/tel-01485076

Submitted on 8 Mar 2017

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

applications électrochimique

Oranit Phuakkong

To cite this version:

Oranit Phuakkong. Mise en forme de structures à base de carbone pour des applications électrochim-ique. Matériaux. Université de Bordeaux; Mahāwitthayālai Kasētsāt (Thaïlande), 2016. Français. �NNT : 2016BORD0305�. �tel-01485076�

(2)

THÈSE EN COTUTELLE PRÉSENTÉE POUR OBTENIR LE GRADE DE

DOCTEUR DE

L’UNIVERSITÉ DE BORDEAUX

ET DE L’UNIVERSITÉ DE KASETSART

ÉCOLE DOCTORALE SCIENCES CHIMIQUES

SPÉCIALITÉ : CHIMIE-PHYSIQUE

Par Oranit PHUAKKONG

DESIGN OF CARBON BASED STRUCTURES FOR ELECTROCHEMICAL APPLICATIONS

Sous la direction du Prof. Alexander KUHN et du Dr. Chompunuch WARAKULWIT

Soutenue le: 07.12.2016

Membres du jury:

Mme. DELVILLE, Marie-Hélène Directeur de Recherche, CNRS Bordeaux Présidente M. PEDERSEN, Steen Associate Professor, Aarhus University Rapporteur M. PHOMPHRAI Khamphee, Associate Professor, Vidyasirimedhi Institute of Science and Technology Rapporteur M. KUHN, Alexander, Professor, Bordeaux INP Directeur de thèse Mme. WARAKULWIT, Chompunuch, Lecturer, Kasetsart University Directrice de thèse M. ERVITHAYASUPORN, Vuthichai, Associate Professor, Mahidol University Examinateur M. LIMTRAKUL, Jumras, Professor, Vidyasirimedhi Institute of Science and Technology Examinateur M. ZIGAH, Dodzi, Maître de Conférences, Université de Bordeaux Examinateur

(3)

Design of carbon based structures for electrochemical applications

Abstract:

In this thesis, the design of advanced carbon materials via electrochemical techniques and for electrochemical applications have been studied. In the first part, the concept of bipolar electrochemistry, which allows carrying out electrochemical reactions on a free-standing conductive object in an electric field, was employed to generate Janus-type objects. These objects are modified with a thermoresponsive hydrogel of poly(N-isopropylacrylamide) (pNIPAM) on one side and an electrophoretic deposition paint (EDP) on the other side. The results show that the length and the thickness of the hydrogel can be controlled by varying the electric field and the time of the experiment. The concept can be further generalized to other micro- and nanometer-sized objects, thus opening up perspectives for various applications.

In the second part, the design of porous carbon structures for electrochemical applications was studied. The direct carbonization of non-porous zinc containing polymers was used to synthesize micro/mesoporous carbons with high surface area, pore volume. Non-porous zinc containing polymers with various types of dicarboxylic acid ligands prepared by solvothermal method were used as templates and starting materials. After carbonization porous carbons with various characteristics and properties were obtained. The synthesized porous carbon samples showed good electrochemical performance with high capacitance values. In addition, the derived materials exhibit excellent electrocatalytic activity with respect to the oxygen reduction reaction (ORR).

Keyword: bipolar electrochemistry, Janus particles, thermoresponsive hydrogel, porous

(4)

Mise en forme de structures à base de carbone pour

des applications électrochimique

Résumé:

Dans cette thèse nous avons étudié la mise en forme de matériaux carbonés par des méthodes électrochimiques pour des applications dans les domaines des capteurs et de l’énergie. Dans la première partie, l’électrochimie bipolaire, qui permet de réaliser des réactions électrochimiques sur un objet conducteur présent dans une solution et soumise à un champ électrique, a été utilisée pour générer des objets de type Janus. Ces objets asymétriques ont été modifiés à une extrémité par du poly(N-isopropylacrylamide (pNIPAM), un hydrogel sensible à la température, et par une peinture électrophorétique à l’autre extrémité. En contrôlant l’intensité du champ électrique ainsi que son temps d’application il a été possible de varier la longueur ainsi que l’épaisseur de l’hydrogel. Ces objets sensibles à la température, émettant de la lumière, ont des applications potentielles dans le domaine des capteurs ou dans le milieu médical.

Dans la seconde partie, la mise en forme de carbone poreux pour des applications électrochimiques a été étudiée. La carbonisation de polymères contenant du zinc a été utilisé pour synthétiser du carbone micro/mésoporeux possédant ainsi une grande surface spécifique. Les polymères contenant du zinc ont été préparés à partir de différents types de ligands d’acide dicarboxylique par une méthode solvothermique. Ils ont ensuite été carbonisés pour obtenir des matériaux poreux avec des caractéristiques et des propriétés particulières. Ils ont été utilisés comme matériaux d’électrode pour des supercondensateurs, montrant des capacités élevées. De plus ils possèdent également une activité électrocatalytique à la réaction de réduction de l’oxygène.

Mots clés: électrochimie bipolaire, particules Janus, hydrogels thermosensibles, carbone

(5)

UNITÉ DE RECHERCHE

Institut des Sciences Moléculaires (ISM) UMR 5255 CNRS Université de Bordeaux 351 cours de la libération 33405 Talence Cedex – France

Chemistry Department and Center of Nanotechnology

(6)

ACKNOWLEDGEMENTS

First of all I would like to take this opportunity to thank all members of the jury for having accepted to evaluate my thesis and for their useful comments and suggestions, particularly, Prof. Steen Pedersen and Assoc Prof. Khamphee Phomphrai for accepting the invitation as examiners (rapporteurs) in the jury.

I would like to express my deepest gratitude to Prof. Alexander Kuhn, my supervisor in France (Université de Bordeaux), for his valuable advice, guidance, support and fruitful discussion. I appreciate all what he has done for me, both academically and personally with his positive attitude. As a member of NSysA, he has shown me how to be a better chemist and teacher. I am very pleased by the warm welcome from him and his family during my stay in France.

I would like to express my sincere gratitude to Dr. Chompunuch Warakulwit, my supervisor in Thailand (Kasetsart University), for her excellent supervision, the continuous support, suggestion, inspiring talks and all she has done for me with her enthusiastic during my Ph.D. thesis. I really appreciate her tremendous help including non-academic assistance.

My sincere thanks also go to my co-advisor, Assistant Prof. Dodzi Zigah and Prof. Jumras Limtrakul. This thesis would not be possible without their suggestion and support. Special thanks go to Assistant Prof. Dodzi Zigah for his encouragement, guidance and patience. I am grateful for the confidence, all suggestion and big help about bipolar electrochemistry with his enthusiastic during my research in France. I am also very grateful to Prof. Jumras Limtrakul for his motivation, enthusiasm, and wide scientific knowledge.

My deep thanks for Prof. Valérie Ravaine for sharing her knowledge on thermoresponsive hydrogel and encouragement to accomplish my work. I would like to thank Prof. Neso Sojic for their kind ideas on Electrochemiluminescence.

My special, grateful thanks go to Dr. Milica Sentic for introducing me to the fundamentals of Electrochemiluminescence technique and supporting my experiments. My thanks are extended to Dr. Chawanwit Kumsapaya and Chaiyan Boonyuen for their kind ideas, helps and supporting experiment.

(7)

I gratefully thank all my friends and colleagues at “Nanosystèmes Analytiques (NSysA)” of the “Institut des Sciences Moléculaires (ISM)”, Université de Bordeaux, France for their kind help and support, Aline Simon-Lalande, Dr. Laurent Bouffier, Dr. Aleksandar Karajić, Dr. Anne Poulpiquet, Dr. Vasilica Lates, Dr. Aileen Justies, Dr. Suresh Vajrala, Gibran Hernández Moreno, Hélène Labie, Sabina Scarabino, Eugenio Gianessi, Pauline Lefrançois, Alessandra Zanut, Beatriz Díez Buitrago, Magdalena Murawska, Yuliia Malytska and Haidong Li. In addition, Dr. Bertrand Goudeau, Patrick Garrigue, and Véronique Lapeyre are also acknowledged for their technical support.

The member of laboratory for Computational and Applied Chemistry (LCAC) from Kasetsart University, both past and present, have been an important part of my studies. Thank you for their helps, supports and encouragement in both work and personal life, especially Dr. Ratsupa Thammaporn, Dr. Chularat Wattanakit, Dr. Saowapak Choomwattana, Dr. Sudarat Yadnum, Dr. Thittaya Yutthalekha, Sombat Ketrat, Dr. Somkiat Nokbin, Dr. Anawat Thivasasith, Dr. Krongkaew Navakul and Malinee Niamlaem.

I gratefully acknowledge financial support from the Royal Thai Government Scholarship supported by Ministry of Science and Technology (THAILAND), the National Nanotechnology Center (NANOTEC Center of Excellence) and Kasetsart University Research and Development Institute (KURDI). My workplace, Suratthani Rajabhat University, is also acknowledged.

Finally, I would like to thank my family for their endless love and supporting whatever I have done. I would not get the best education and have a wonderful life without them. Thank you very much.

Oranit Phuakkong December 7, 2016

(8)

CHAPTER 1: General Introduction

Carbon materials play a key role in human history. Due to their different allotropes with different chemical bonds, carbon materials have a variety of structural dimensions, such as fullerene (0D), carbon nanotube (1D), graphene (2D), graphite (3D), where carbon is sp2 hybridized and diamond (3D, where carbon is sp3 _{hybridized). The physical and chemical} properties of these carbon materials are thus very different. For example, graphite is a fragile, soft and black material that exhibits a good electrical conductivity. Diamond is colorless and the hardest material with a poor electrical conductivity. Fullerenes and nanotubes are structures which take the form of a hollow sphere, ellipsoid or tube, while glassy carbon is a non-graphitizing carbon with a highly disordered structure and good electrical and thermal conductivity. Furthermore, a wide range of applications has been reported for carbon, such as its use as catalyst support, for gas separation and water purification. Electrochemical techniques are not only very useful to investigate the properties of carbon materials but also to functionalize them for various applications. In this work, the design of advanced carbon materials for electrochemical applications was studied via two approaches, the surface modification of milli- or micrometer-sized carbon rods and the synthesis of porous carbon via a template carbonization.

Nowadays surface modifications of carbon materials have attracted considerable attention, both from a theoretical and an experimental point of view. The surface modifications can not only improve the properties of the carbon-based materials, but also bring novel and desirable properties, opening up a large variety of applications in the field of energy storage/transformation, sensors, medical devices etc. Among the various techniques used to modify carbon, bipolar electrochemistry is very attractive. This technique is based on the electrogeneration of two different reactive species at each extremities of the carbon material by applying a potential difference or a current to a system composed of electrolyte and two feeder electrodes. There is no physical contact between the carbon material and the feeder electrodes. Carbon materials with various sizes and shapes can be modified with different types of inorganic and organic materials via this technique. The development of new surface modification concepts using bipolar electrochemistry is very challenging. In this work, we explore the electrochemically induced free radical polymerization of a thermoresponsive hydrogel which was performed by the bipolar electrochemical approach. The physico-chemical properties of the resulting hydrogel deposited on one side of the carbon material were

(12)

investigated. Then a luminophore, that can produce light by an electrochemical reaction, was immobilized with the hydrogel. Bipolar electrochemistry was used as a dual wireless tool to generate and to activate a thermoresponsive electrochemiluminescence on the modified carbon material. The obtained material is expected to provide new opportunities for sensing and medical applications.

Apart from the surface modification of carbon materials via bipolar electrochemistry, the design of the carbon structure itself plays also a key role for obtaining desirable applications. In this thesis, a second focus is the design of porous carbon materials with hierarchical micro/mesopores, opening up interesting perspectives for applications in the field of energy storage and conversion. The controlled synthesis of different porous carbon structures was studied in detail. Although porous carbon can be obtained with various techniques, it is very important to develop a simple and efficient technique that allows tailoring porous carbons with desirable porosity and morphology. One of the most powerful synthesis techniques that meets these criteria is the carbonization of a template material. Among the templates used to fabricate porous carbons, metal organic frameworks (MOFs) and metal coordination polymers are of great interests as they can provide porous carbons with high porosity and high surface area. The preparation of micro/mesoporous carbon from various Zn-polymers with different dicarboxylic acid ligands was studied. Porous carbons with a high specific surface area and high pore volume could be obtained. The supercapacitive properties and the electrocatalytic behavior with respect to the oxygen reduction reaction (ORR) of the resulting materials were then investigated.

(13)

CHAPTER 2: Asymmetric Deposition on Carbon Materials

via Bipolar Electrochemistry

In this chapter, asymmetric deposition of thermoresponsive hydrogel and electrophoretic deposition paint on carbon materials via bipolar electrochemistry was studied. The concept of bipolar electrodeposition of NIPAM and anodic electrophoretic deposition paint is used as the first proof-of-principle experiments to fabricate Janus fibers. The materials, mechanisms and techniques including scientific background to obtain Janus particle will be briefly described for understanding the subsequent section. These materials are promising to be used as sensors.

1. INTRODUCTION

1.1 Asymmetric particles

Janus is an ancient Roman god usually depicted with two faces looking in opposite directions (Figure 2.1). Janus grains were firstly described by de Gennes in 1992, they had two sides, one side was nonpolar and the other one was polar, like an amphiphilic particle 1_{. The} particles exhibit different chemical properties at their two sides due to hydrophobic and hydrophilic features. Subsequently, the expression Janus particles was also used to describe micro- or nanoobjects that have at least two physical properties and exhibit different chemical properties or polarity on two opposite sides. Although, the term of Janus particle was widely used to refer to particles that are composed of at least two components of different nature, asymmetric particles can be classified into two types 2.The first one is Janus particles which is used for objects with equal phase separated domains. The second one is Patchy particles which is used for objects with non-equally separated domains (Figure 2.2). However, the term of asymmetric particles can be used for describing both Janus particles and Patchy particles.

(14)

Figure 2.1 Janus god on Raman coin (a) and Janus statue

Figure 2.2 Schematic representation of different type of asymmetric particles a) spherical Janus particle, b) Cylindrical Janus particles and c) Patchy particles

Up to now, a wide range of asymmetric objects with different chemical properties and shapes has been reported. For example, different Janus particle geometries were reported, such as spherical 3-5_{, cylindrical}6-8_{, sheets}9_{and discs}10_.

Due to a very wide range of asymmetric particles with different chemistries and shapes, these materials are very promising for various applications. Figure 2.3 shows the number of publications which reveals that the scientific interest for Janus particles has been growing in the last twenty years. These asymmetric particles have a lot of different applications. They can be used for:

(15)

Figure 2.3 Trends in the number of publications about Janus particles listed by Scopus on June 20, 2016 (www.scopus.cm)

- Display technology

Electronic paper is a display technology designed for electronic books or mobile phones. Yin and coworker reported the synthesis of Janus supraballs for display technology. The bifunctional magnetic-fluorescent Janus supraballs presented bicompartmental particles with a combination of two distinct hemispherical regions of Quantum dots (QDs) and magnetic nanoparticles which opened a new way to create flexible bead displays under electric field as shown in Figure 2.4 11.

- Water repellent

The amphiphilic Janus particles can be used for the water-repellent applications. Droplets of the photocurable resin ethoxylated trimethylolpropane triacrylate (ETPTA) were decorate with silica particles. After polymerization and etching, superhydrophobic particles were obtained and used for preparing flexible hydrophobic surfaces and liquid marble that can be manipulated with magnets or tweezers 12.

(16)

Figure 2.4 a) Schematic representation of a fluorescent switch of Janus particles controlled by varying the direction of an external magnetic field. b–d) Optical images of the magnetoresponsive bead display prepared from Janus particles. Adapted with permission from John Wiley and Sons: Advanced Materials11_{, copyright} 2011.

- Stabilizers in polymer system

Amphiphilic Janus particles have been applied for emulsion polymerization as stabilizers without additives. These amphiphilic particles were prepared by selectively cross-linking spherical polybutadiene microdomains within the lamella–sphere morphology of a microphase-separated template of a polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock terpolymer and subsequent hydrolysis of poly(methyl methacrylate) to poly(methacrylic acid). The size of polystyrene latex particles can be controlled by changing the content of Janus particles13.

- Micro- or nanoswimmer

There are many reports which aim to develop autonomous swimmers for medical applications such as drug delivery14-16 or DNA detection17. For example, the generation of motion of a metallic object, where one end is the site of metal deposition and the other is the site of metal dissolution. The propulsion of zinc macro- and microswimmers moving with speeds of up to 80 µm/s can be controlled under the influence of an external electric field 18.

(17)

- Sensing application

Asymmetric particles can also be used as probes or sensors19-21. For example, asymmetric fluorescent nanoparticles were prepared to investigate intermolecular interactions. The fluorescent particles fabricated by depositing thin gold film on one side of the particle, show intensity fluctuation under an optical microscope because of their rotational Brownian motion. Interactions between gold surfaces of the particle and the solid substrate restrict the rotation of the particle, thus modulating the intensity fluctuations. The intensity signals were used to investigate the weak interactions between unlabeled molecules by analyzing the angular distribution of the particle. Very small rotational distance on the order of nanometers through the change in fluorescence intensity was detected 21.

Nowadays, they are many strategies to produce asymmetric particles, such as templating, colloidal assembly, lithography techniques, spinning disk process, electrodynamic co-jetting process, and microfluidic methods. However, the main drawback of these techniques is the limitation in yield, thus preventing production at industrial scales. Recently, Loget et al. demonstrated that the production of Janus particles by bipolar electrochemistry can be performed at a large scale 22. Indeed, it is possible to synthesize them in the bulk of a solution. This allows a 3D production instead of using an interface or a surface to break the symmetry, which implies a 2D production, and therefore a lower time-space yield. This new approach based on bipolar electrochemistry to design a variety of asymmetric objects is used in this thesis. The details about bipolar electrochemistry will be described in section 1.4.

1.2 Hydrogel materials

Hydrogels are defined as crosslinked polymeric networks of hydrophilic polymer chains containing a large volume of water without dissolving. Depending on the properties of the polymer and the density of the polymeric network, the structure of hydrogels can contain various amounts of water in the swollen state 23,24_{. Therefore, the mass fraction of water can be} up to thousand times greater than the one of the hydrogel. In terms of classification of materials, hydrogels are solids that contain fluid. According to the starting materials, hydrogel can be classified into three types: natural hydrogels, synthetic hydrogels and natural/synthetic hybrid hydrogel. In terms of physical properties, hydrogels can be classified as non-responsive and responsive. Stimulus-responsive hydrogels undergo a physical change under the influence of different external stimuli such as magnetic field25,26, electrical current27,28, pH29, ionic

(18)

strength30, temperature31,32 and light33. With an external stimulus, responsive hydrogels are switched between swollen and collapsed state by changing the chain conformation.

In this thesis, the study is focused on thermoresponsive hydrogels. A change of chain conformation in the hydrogel structure happens when the temperature changes. The miscible polymer solution exhibits a phase transition at a certain temperature which is also called the critical temperature. Thermoresponsive hydrogels are classified into two main types. The lower critical solution temperature (LCST) is the critical temperature below which the components of a mixture are miscible for all components. And the upper critical solution temperature (UCST) is the critical temperature above which the polymer and solvent are miscible as shown in Figure 2.5. A polymer below the LCST is a homogeneous solution while a polymer solution above the LCST shows cloudy or phase separation. Conversely, a polymer above the UCST is a homogeneous solution and becomes solid at temperature below the UCST.

Figure 2.5 Phase diagram of polymer solutions a) the lower critical solution temperature (LCST) and b) the upper critical solution temperature (UCST)

Three interactions are involved in the solubility of polymer in water, which are polymer-polymer interactions, water-water interactions and polymer-polymer-water interactions. For polymer-polymers exhibiting an LCST, the polymer becomes solid or separates into two phases when the temperature is increased. The polymer appears cloudy which can be explained in terms of Gibbs free energy by the following equation:

ΔG = ΔH – TΔS (2.1)

(19)

where G is the Gibbs free energy, H is the enthalpy and S is the entropy. The phase separation is more favorable with increasing the temperature due to the entropy of system. In this case, the water-water association is the governing interaction in the system. Thus the main driving force is the entropy of the water, that when the polymer is not in solution the water is less ordered and has a higher entropy (ΔS). This is also called hydrophobic effect34,35_{. For this} reason, LCST is an entropically driven effect while UCST is an enthalpically driven effect36. The poly (N-isopropylacrylamide; pNIPAM) is one of the thermoresponsive hydrogels which is the most intensely studied for biomedical applications. It undergoes a phase transition from the swollen to the collapsed state upon heating above 32°C which is very close to body temperature. The details about poly(N-isopropylacrylamide) are described in the next section.

1.3 Poly N-isopropylacrylamide (pNIPAM)

Poly(N-isopropylacrylamide) is variously abbreviated PNIPA, PNIPAA, PNIPAAm or PNIPAM. In this thesis, pNIPAM is used. It was first synthesized in 1956 via radical polymerization from N-isopropylacrylamide37. Figure 2.6 shows the molecular structure of pNIPAM. It responds to an increasing of temperature above its lower critical solution temperature (LCST), 32C, by expelling water. It goes from a swollen hydrated state to a shrunken dehydrated state, losing around 90% of its volume. Therefore, pNIPAM is identified as thermoresponsive hydrogel.

This thermoresponsive hydrogel exhibits a hydrophobic and hydrophilic transition in the solution phase involving the hydrogen bonding interaction of the amide group. 37-40 During the transition, the intramolecular hydrogen bonds between the water and the amide groups in pNIPAM are broken. The water molecules are expelled from the pNIPAM structure, resulting in shrunken dehydrated state, this transition is reversible. At the temperature below LCST the water is absorbed due to new hydrogen bond formation between water molecules and the polar functional groups in the pNIPAM structure.

Poly(N-isopropylacrylamide) has been synthesized by a variety of techniques; redox initiation41, radiation polymerization42, anionic polymerization43, ultrasound irradiated polymerization44, plasma polymerization45. However, free radical polymerization is the most widely used for synthesis route for pNIPAM 40,46-50.

(20)

Figure 2.6 Chemical structure of poly(N-isopropylacrylamide)

Free radical polymerization is a chain polymerization from which a polymer grows by addition of monomer to a terminal free radical reactive site (active center). Generally, free radical polymerization consists of three steps which are initiation, propagation and termination as shown in Figure 2.7. The first step of the process of initiation is the formation of free radicals from initiator and the second step is addition of a free radical to a monomer molecule. During the propagation, the radical initiator attacks a monomer turning the molecule into another radical allowing the growth of the polymer chain. For termination, the polymer chain can be terminated by combination of two growing polymer chains or disproportionation in the polymer chain. The free radical polymerization of pNIPAM typically uses ammonium persulfate (APS) or potassium persulfate (KPS) as initiator.

(21)

pNIPAM exhibits thermosensitive and biocompatible properties because its LCST is between room temperature and body temperature. Hence, pNIPAM is attractive in biomedical applications such as drug release carrier 51-54, protein separation55 and biosensors56,57. Consequently, there are many reports involving the grafting of pNIPAM on solid surfaces for creating smart biocompatible materials 48,58,59_{as hybrid materials.}

For asymmetric particles, the synthesis of Mg/Pt-pNIPAM Janus micromotors was prepared by Mou and coworkers. The magnesium-water reaction in aqueous media with the assistance of concentrated inorganic salts produced H2 bubbles that could drive the Janus micromotor while drugs could be loaded in the gel layer, transported and further released upon the application of a stimulus14. These Janus micromotors can effectively uptake, transport, and temperature-controlled release drug molecules by taking advantage of the partial surface-attached pNIPAM layers that demonstrated the potential application in drug delivery system (Figure 2.8). Therefore, it is still a challenge to find an efficient approach to fabricate thermoresponsive hydrogels on various surfaces and investigate the sensitivity of these materials.

Figure 2.8 Scheme of drug release behavior of Mg/Pt-pNIPAM Janus particles. a) fluorescent images representing the drug release from the Mg/Pt-pNIPAM particles and b) the normalized average cumulative drug release profiles at 20 and 37 °C versus time. Adapted with permission from 14_{. Copyright 2014} American Chemical Society

(22)

In this thesis, we take advantage of the film responsiveness to amplify the electrogenerated chemiluminescence (ECL) signal of a ruthenium complex luminophore, Ru(bpy)32+ incorporated in the hydrogel film. ECL is a phenomenon of light emission by the excited state of a luminophore produced upon an initial electrochemical activation. The swell−collapse transition of hydrogel allows the reversible enhancement of ECL intensity32_. Therefore, the ECL of pNIPAM was a proof of principle experiment for designing new ultrasensitive assays taking advantage of thermoresponsive gels. Herein, thermoresponsive hydrogel will be fabricated on conductive material to produce hybrid materials where the light emission can be tune depending of the state of the polymer for sensing application.

1.4 Bipolar electrochemistry

1.4.1 Principles of bipolar electrochemistry

Bipolar electrochemistry on microobjects was first described by Fleischmann and co-workers in 1986 60. It is widely used to trigger electrochemical reactions at the extremities of a conductive object, without physical contact with the feeder electrodes. Briefly, the conductive object is immersed in solution under a strong electric field between two feeder electrodes. The electric field polarizes the solution and when the potential difference between the solution and the conductive object is high enough, electrochemical reactions occur at opposite sides of the object 61,62. These electrochemical reactions are the oxidation reaction at the anodic pole, simultaneously with the reduction reaction at the cathodic pole, and as a consequence the conductive object is called a bipolar electrode.

Let’s consider the following simple thought experiment based on the bipolar electrochemical cell shown in Figure 2.9. A conductive object is placed in an electrolyte solution between two feeder electrodes without direct contact with the feeder electrodes and the power supply. When a potential difference is applied between the feeder electrodes, the conductive object behaves as a cathode and an anode at the same time. It differs from conventional electrochemistrywhere cathode and anode are physically separated.

(23)

Figure 2.9 Scheme of bipolar electrochemistry principles. The conductive object (bipolar electrode) is localized between the two feeder electrodes in an electrolyte solution.

Considering the electrochemistry in bipolar configurations, the traditional electrochemical cell and the bipolar electrochemical cell are compared. Generally, a traditional three-electrode electrochemical cell consists of a working electrode, an auxiliary electrode, and a reference electrode. The potential of the working electrode is controlled (versus a reference electrode) using a potentiostat, thus the energy of the electrons in the electrode is controlled 63. The potential of the solution is at a floating value that depends on the composition of the solution without externally applied electric field.When the potential of the working electrode is driven to be more negative than the standard potential of the electroactive molecule in the solution, electrons may transfer from the electrode to the oxidized species in solution, inducing a reduction reaction (Figure 2.10a). Similarly, oxidation reactions occur at the counter electrode as shown in Figure 2.10b. Therefore, the faradaic current measured in the circuit connecting the working and auxiliary electrodes is a direct measurement of the rate of this electrochemical reaction. It means that the interfacial potential difference between the electrode and the solution is the driving force for electron transfer.

(24)

Figure 2.10 Scheme of electron transfer for (a) reduction and (b) oxidation of redox species

in solution.

The bipolar electrochemical configuration is opposite to the conventional system. The conducting object does not have any contact with the power supply, then its potential is not controlled. However, the potential of solution is controlled by the power supply 63_{. Therefore,} the potential difference between the bipolar electrode and the solution is determined by the electric field in solution.

Theoretically, a potential difference (Eapp) = Ea-Ec is applied between the two feeder electrodes that are spaced by a distance d. The potential of anode and cathode are represented by Ea and Ec, respectively. The electric field E is given by the following equation:

E = 𝐸𝑎−𝐸𝑐

𝑑 (2.2)

In a first order approximation, the potential drops linearly across the electrolyte solution. The interfacial potential difference between the substrate and the solution is the driving force for the bipolar electrochemical reactions. Figures 2.11 shows the solution potential distribution in the cell for a tubular and spherical bipolar electrode, respectively. When the electric field is applied, a polarization potential Vis induced at the solution/substrate interface and it is given by the difference between the solution potential and the potential of the conducting object64.

(25)

Figure 2.11 Scheme of the polarization of a linear (a), and spherical (b) conductive substrate positioned in the electric field in an open bipolar configuration.

For linear objects (Figure 2.11a), the value of V varies along the object/solution interface and can be calculated at a position x as:

V(x) = E (x-x0) (2.3)

For spherical object (Figure 2.11b), the value of V varies along the object/solution interface and can be calculated at a position x as:

V(x) = E ∙ l /2cos  (2.4)

where x is the position at bipolar electrode. where x0 is in the ideal case a position in the middle of the substrate and the polarization potential zero at this location, indicating that there is no electrochemical reaction. Therefore, the polarization potential (V) increases from x0 position

towards both extremities of the conductive objects. Therefore, the maximum polarization potential difference (∆V) is obtained at extremities of conductive objects, expressed by the equation below:

∆V = E ∙ l (2.5)

where l is the length of the bipolar electrode. In equation 2.5, the value of the polarization potential difference indicates the overall polarization at the ends of the conductive objects.

(26)

Generally, the electroactive species will undergo the following reactions at the cathode and anode, respectively:

Ox1 + n1e-_{ Red1} _(2.6)

Red2  Ox2 + n2e- (2.7)

where n1 and n2 are the number of electrons involved for the half reactions and the two redox couples Red1/Ox1 and Red2/Ox2 having a standard potential of E10 and E20, respectively.

In order to trigger the electrochemical reactions on the bipolar electrode, a minimum potential difference (ΔVmin) between the extremities of the bipolar electrode is

required. The minimum potential difference needs to be at least equal to the difference between the standard potentials of two redox couples.

ΔVmin = E20 - E10 (2.8)

If the applied electric field is high enough to provoke a sufficient polarization on the conductive object (ΔV >ΔVmin), then the electrochemical reactions can occur at both ends of the conductive

object. The reduction reaction (eq.2.6) occurs at the cathodic pole simultaneously with the oxidation reaction (eq. 2.7) at the anodic pole. In general, the ΔVmin is a constant

thermodynamic value for two half reactions, thus the length of substrate (l) and the applied electric field (E) are the most important parameters in the experiment.

In addition, for bipolar experiments we can distinguish between two types of configuration depending on the arrangement of the distinct electrolytic compartments 64_{. One} is the open configuration and the other one is the closed configuration.

Figure 2.12 shows the design for an open bipolar cell. The current can flow through both, the electrolyte and the bipolar electrode. The conductive object is immersed in an electrolyte solution between two feeder electrodes without direct physical connection between the substrate and the power supply.

(27)

Figure 2.12 Scheme of a conductive substrate positioned between two feeder electrodes in

an electrolyte solution in the open bipolar configuration.

In a closed bipolar electrochemical system (Figure 2.13) the bipolar electrode is connecting two separate compartments. As a consequence, anode and cathode are physically separated. In this case the total electrochemical current passes through the bipolar electrode and can be directly measured due to the lack of an ionic current path, therefore providing information about the rates of the faradaic processes. In this case the potential drop across the substrate is almost equal to the applied potential difference between the feeder electrodes, if the electrolyte has a low resistance 65,66. This electrochemical cell is very well suited for carrying out bipolar reactions with relatively low electric fields, thus enabling the synthesis of a variety of functionalized surfaces for materials with different sizes 3,5,22,67.

(28)

1.4.2 The applications of bipolar electrochemistry

Up to now, the bipolar electrochemical technique has been widely used in numerous applications (Figure 2.14-2.15) such as water electrolysis 68, metal electroplating 69, bipolar batteries70,71, establishing electrical contacts 72, controlling the motion of micro swimmer 15,16,18,73,74_{, studying the corrosion behavior of materials} 75_{and driving} electrochemiluminescent reactions 76-79. Moreover, bipolar electrochemistry has been used to design smart materials by surface modification.

Figure 2.14 Bipolar electrochemical technique used for establishing electrical contacts. a)

Scheme of Cu wire formation between Cu particles by simultaneous electrodeposition and electrodissolution and b) Cu deposits formed after 10 s and 29 s by applying 30.3 V/cm. Adapted by permission from Macmillan Publishers Ltd: Nature 72_{, copyright 1997.}

(29)

Figure 2.15 The optical micrographs of a zinc microswimmer in a glass capillary filled with a zinc sulfate solution at pH ≈ 5 under the influence of an external electric field. Reprinted with permission from 18_{. Copyright 2010 American Chemical} Society.

In 1999, bipolar electrochemical experiment was first used to trigger electrodeposition of a palladium catalyst by Bradley and Ma 80.The electrodeposition of Pd onto the cathodic pole was coupled with the oxidation of solvent (toluene/acetonitrile) at the anodic pole of micrometer-sized graphite particles as shown in Figure 2.16. From this pioneering work, palladium deposited on various carbon materials including graphite, carbon nanofibers, carbon nanotubes and carbon nanopipes was performed 81,82

(30)

Figure 2.16 Scheme of the preparation of bipolar electrodeposited catalyst. The electric field is applied between two graphite electrodes to deposit asymmetrically Pd on graphite particles. Adapted by permission from John Wiley and Sons: Angewandte Chemie International Edition80, copyright 1999.

During the last ten years, bipolar electrochemistry has shown attractive features for application in the field of synthesis of Janus objects. This technique has also been used to deposit conductive materials, either inorganic (metal, semiconductor) 5,7-9,22,83_or organic (conducting polymer) 3,4,84. Figure 2.17 shows the difference size of glassy carbon beads modified with metal followed by the electrodeposition of metal on semiconductor in Figure 2.18. Furthermore, the electrografting of an organic layer on conductive material is shown in Figure 2.19.

(31)

Figure 2.17 SEM images of the carbon/metal Janus particles prepared by bipolar

electrodeposition. a) Glassy carbon beads with diameters ranging from 200 to 400 m modified with gold, b) Glassy carbon beads with diameters ranging from 20 to 50 m modified with gold and c) Glassy carbon beads modified with silver. Adapted by permission from John Wiley and Sons:Advanced Materials 22_{, copyright 2012.}

Figure 2.18 SEM images of the metal deposited on semiconductor by bipolar

(32)

Figure 2.19 SEM images of an aryl diazonium layer deposited on a glassy carbon bead by

Although the bipolar electrochemical technique has been successfully used with macroscopic substrates 85, with micrometer or nanometer-size substrates new difficulties appear. For a small-sized substrate, a higher electric field or a shorter distance of feeder electrode is required. In this case, a very high electric field of the order of MV/m is required to produce the redox reactions at both ends of small substrates (nanometer range) which is the limitation of commercial power supplies. To overcome this limitation capillary assisted bipolar electrodeposition (CABED) has been developed 7. By using this technique, first millimeter-sized carbon fibers were modified with gold as proof of principle (Figure 2.20a). CABED is based on the use of capillary electrophoresis and can be performed with various ingredients as shown in Figure 2.20b-f. The experiment was even scaled down to nanometer-sized multi-wall carbon nanotubes (Figure 2.20b)7,86. Multi-wall carbon nanotubes were also modified with Copper, Gold and Nickel at one extremity, and even dumbbell-like CNT/Cu objects could be obtained as shown in Figure 2.20f.

(33)

Figure 2.20 Capillary assisted bipolar electrodeposition (CABED). a) Optical micrograph of a carbon fiber inside a glass capillary during asymmetric bipolar gold deposition and b) TEM image of a multi-wall carbon nanotubes modified with gold. Adapted with permission from 7. Copyright 2008 American Chemical Society. c-f) SEM image of CNT modified with copper, gold, nickel and a dumbbell-like object with copper deposits on both sides, respectively. Adapted with permission from 86_{. Copyright 2011 American Chemical Society.}

Apart from surface modification by direct electrooxidation or electroreduction at objects, indirect bipolar electrodeposition (IBED) was also reported by Loget and coworker 6. The approach does not involve a direct electron transfer between the substrate and the deposit precursor, but rather takes advantage of a local pH change around the conducting object, which leads to controlled polymerization and/or precipitation of the insulating deposit (Figure 2.21). The concept has been validated for particles with different characteristic sizes to generate Janus objects.

Recently, indirect bipolar electrodeposition (IBED) was applied to site-selective in situ synthesis of MOFs on metallic wires and particles. By using the IBED technique, the reactive metal ions are generated at the surface of a metallic substrate and further react with ligand species in the solution to form coordination-based network structures 5. Figure 2.22shows the modification of a zinc wire with ZIF-8 and a copper bead with HKUST-1. Therefore, this technique can be also applied to generate such micro- and nanoobjects with a variety of other materials.

(34)

Figure 2.21 Modification of various substrates with different materials by IBED. SEM images of a submillimeter-sized glassy carbon bead modified with silica using a TEOS-based sol−gel (a), silicone using a MTMOS-based sol-gel (b), titanate (c) and electrophoretic deposition paint (d). Carbon particles (600-1000 m) modified with EDP (e) and an amorphous Pt particle modified with EDP (f). Adapted with permission from6. Copyright 2012 American Chemical Society.

Figure 2.22 Site-selective modification of a zinc wire with ZIF-8 and a copper bead with

(35)

As mentioned above, the development of surface modification concepts using bipolar electrochemistry is still challenging. In this work, we aim to generate Janus particles with a thermoresponsive hydrogel selectively deposited at one side. The hydrogel is generated via indirect bipolar electrochemistry induced by free radical polymerization. Furthermore, the one-step simultaneous modification with thermoresponsive hydrogel and electrophoretic paint at both sides of a conductive material will be studied. In the first proof-of principle experiments, millimeter-sized graphite rods were used. Then this technique has been miniaturized for micrometer-sized carbon tubes. After that, potential applications of the resulting materials will be demonstrated.

2. ASYMMETRICALLY DEPOSITION OF THERMORESPONSIVE HYDROGEL ON CARBON MATERIALS VIA BIPOLAR ELECTROCHEMISTRY

Thermoresponsive hydrogels allow the production of original Janus particles that combine different properties. Most of the literature reports deal with the regioselective inclusion of particles inside hydrogel beads 87,88,89,90. In particular, magnetic beads are highly sought-after, giving an additional trigger to that of stimuli-responsive materials. Furthermore, such anisotropic materials can further self-assemble into 2D stable microstructures under the action of a magnetic field 89. Their fabrication takes advantage of the spatial separation of particles in a liquid phase (phase separation, external fields such as gravity, magnetic field) which is further stabilized by the polymerization of the hydrogel surrounding the preorganized particles. The liquid phase can be either confined in drops87,88 or shaped by stop-flow lithography89.

In this thesis, Janus particles based on responsive hydrogels were prepared by an unconventional method. The thermoresponsive hydrogel layer is coated on a conducting object using bipolar electrochemistry. The modification of the electrodes with a layer of responsive polymer allows the generation of materials with switchable electrochemical properties, no matter whether the film is a hydrogel layer91,92 or a polymer brush grafted on the surface58,93,94. The phase transition of the hydrogel layer can for example be used to improve the loading of immobilized enzymes for the construction of biosensors15. This kind of objects might also serve as swimmers with integrated drug delivery capacity.

(36)

This work demonstrates the possibility to prepare responsive Janus materials, by growing selectively a pNIPAM hydrogel on one side of carbon fibers using bipolar electrochemistry. The thickness and the length of the pNIPAM can be controlled by the experimental conditions used during the bipolar electrochemistry reactions. The mechanism of electropolymerization is shown is Figure 2.23.

Figure 2.23 Bipolar electrochemical production of free radicals and formation of pNIPAM

on the cathodic side of the bipolar electrode

The applicability of such materials to detect a change of temperature is illustrated with a further functionalization of the hydrogel layer with redox luminescent moieties, [Ru(bpy)]32+. Those species are well-known as electrochemiluminescent (ECL) emitters, which means that the light emission from an excited state is initiated through an electrochemical process at the surface of an electrode. These materials are thus used as bipolar electrodes, allowing a wireless readout of ECL emission, whose emission depends on the swelling state of the hydrogel.

2.1 Experimental methods 2.1.1 Materials

Major chemicals

- Graphite rods (99.9995%, diameter of 3mm, Alfa Aesar)

- Carbon fibers(Grade P100, diameter of10 µm, Goodfellow Cambridge Limited)

(37)

- N-Isopropylacrylamide (NIPAM, 97%, Sigma-Aldrich) - Potassium persulfate (KPS, K2S2O8, >98%, Sigma-Aldrich) - N,N’-Methylenebisacrylamide (BIS, Sigma-Aldrich) - Rhodamine 101inner salt for fluorescence (Sigma-Aldrich)

- Sodium phosphate monobasic monohydrate (99%, Sigma-Aldrich) - Sodium phosphate dibasic heptahydrate (98-102%, Sigma-Aldrich) - Tripropylamine (TPrA, ≥98%, Sigma-Aldrich)

- Ruthenium(II) (4-vinyl-4′-methyl-2,2′-bipyridine)bis(2,2′-bipyridine)bis (hexafluorophosphate)95

- Ethanol (C2H5OH, 96%, VWR International) - Hexane (C6H14, VWR International)

- Milli-Q water (resistivity of 18 MΩ cm, Millipore)

Major Equipment

- Power supply: DC power supply, ELC

- Scanning Electron Microscope (SEM) (performed with an accelerating voltage of 15 kv, TM-100, Hitachi)

- Optical microscopy (DM6000B, Leica) with filter cube N21 and I3 - Heat controller (TC 324C, Warner Instrument)

- Imaging chamber (RC-21BRW, Warner Instrument)

- Horizontal epifluorescence microscope (BXFM-ILHSPU, Olympus) equipped with a EM-CCD camera (Hamamatsu)

- Photomultiplier tube (R5070A, Hamamatsu)

2.1.2 Method

2.1.2.1 Purification of N-isopropylacrylamide

Commercial N-isopropylacrylamide was used as monomer. It was purified by recrystallization in Hexane (ICS) and dried in vacuum at room temperature for 24 h.

(38)

Figure 2.24 Schematic illustration showing the recrystallization of NIPAM

2.1.2.2 Electrochemical study of a solution of persulfate anions with NIPAM in 0.1 M KNO3 on glassy carbon electrodes

Cyclic voltammetry was used to estimate the applied potential required for the bipolar electrodeposition of pNIPAM. The working electrode was a glassy carbon electrode (diameter 3 mm), the counter electrode was a platinum wire and Ag/AgCl (3 M NaCl) the reference electrode. At the beginning, the working electrode was polished and cleaned before use to ensure good electron transfer. All electrodes were rinsed by Milli-Q water and put into the cell as shown in Figure 2.25. Cyclic voltammetry of a solution of persulfate anions (0.015 M KPS) and 0.5 M NIPAM in water was performed with 0.1 M KNO3 as supporting electrolyte, in a three electrode system. The solution was purged with N2 gas for 10 min prior to polymerization. The cyclic voltammetry was carried out at scan rate of 100 mV/s.

(39)

Figure 2.25 Schematic of the experimental setup for cyclic voltammetry

2.1.2.3 Bipolar Electrodeposition of pNIPAM on graphite rod

The bipolar electrochemical cell was made from silicone casting, containing the feeder electrodes (graphite rods), as shown in Figure 2.26. A bipolar electrode (BPE) was placed in the middle of the cell. The distance between two feeder electrodes was 2 cm while the BPE (graphite rod) was 3 mm in diameter. The length of the BPE was 10 mm.

Figure 2.26 Schematic of the experimental setup for electrodeposition of pNIPAM on

(40)

In order to deposit pNIPAM on a graphite rod, the details of the experiment are shown below: - Dissolve 0.340 g of NIPAM in 10 mL of Milli-Q water followed

by adding 0.041 g of KPS.

- The bipolar electrochemical cell filled with 2 mL of the above solution.

- Connect the feeder electrodes to a power supply and apply the desired voltage for a given deposition time.

- After the deposition, the sample was carefully removed from the cell and washed with Milli-Q water.

In addition, the influence of the applied electric field and the deposition time on electropolymerization of pNIPAM on graphite rod are investigated.

2.1.2.4 Bipolar Electrodeposition of pNIPAM on carbon fibers

In this case the bipolar electrochemical cell was made of two compartments, containing the feeder electrodes (graphite rods), connected with a glass capillary (outer diameter: 2 mm, inner diameter: 1.12; length: 25 mm) as shown in Figure 2.27. The BPE was placed in the middle of the glass capillary. The distance between the two feeder electrodes was 4.5 cm while the BPE (carbon fiber) was 10 µm in diameter. The length of the BPE was varied between 2 mm and 10 mm.

Figure 2.27 Schematic of the experimental setup for electrodeposition of pNIPAM on

carbon materials.

In order to deposit pNIPAM on the BPE, the details of the experiment are shown below: - Dissolve 0.566 g of NIPAM as a monomer and 0.057g of BIS as a

(41)

- The mixture was purged with N2 gas to remove O2 for 10 min followed by addition of 0.041 g of KPS as an initiator.

- The bipolar electrochemical cell was sealed with a septum and filled with 2.5 mL of the above solution.

- After the deposition, the sample was carefully removed from the cell and washed with Milli-Q water.

Furthermore, the experiment is performed with different applied electric fields and for different deposition times.

2.1.2.5 The pNIPAM/rhodamine modification of carbon materials

To visualize the electrodeposition of pNIPAM on carbon materials under a fluorescence microscope, a fluorophore (rhodamine 101) was entrapped inside pNIPAM. A solution of 0.5 mL ethanol containing 5 mg of rhodamine 101 was prepared and 30 µL of this ethanolic rhodamine solution were dropped on the sample for 1 min and then carefully washed with Milli-Q water to remove the excess of rhodamine.

2.1.3 Characterizations

2.1.3.1 Scanning Electron Microscopy (SEM)

SEM observation was carried out with a Hitachi TM-1000 (Figure 2.28) to study the surface morphology of pNIPAM on carbon materials. In the case of observation of pNIPAM on carbon fibers, all samples were lyophilized to prevent a collapsed state of pNIPAM. The sample was placed directly on a conductive double-sided sticky carbon tape, coated with gold by sputtering and observed at 15 kV accelerating voltage. For lyophilization of pNIPAM on carbon fiber, the sample was kept in Milli-Q water in a microtube and dipped into liquid nitrogen until it was frozen. After that the microtube was transferred to the freeze-drying flask connected with the freeze-drying machine as shown in Figure 2.30.

(42)

Figure 2.28 Photograph of typical Vacuum Sputter Coating and Scanning Electron

Microscopy.

Figure 2.29 Schematic illustration of lyophilization process

2.1.3.2 Imaging setup and fluorescence recording

In the case of the graphite rod, a UV lamp was used as detector in the first proof-of-principle bipolar experiments to observe the deposition of pNIPAM on graphite rod. Apart from the UV lamp, an epifluorescence microscopy (Leica DM6000B) was used for visual observation to study the thickness and the length of the polymer deposit on the carbon fiber. For the fluorescence mode with filter cube N21, an excitation filter in the range of 515 –

(43)

560 nm and an emission filter of 590 nm were used. In the case of the ruthenium complex copolymerized with pNIPAM on carbon fiber, a fluorescent microscope was used with a filter cube I3 containing an excitation filter in the range of 450 – 490 nm and an emission filter of 515 nm.

2.1.4 Study of thermoresponsiveness of the Janus fiber

To measure the effect of temperature on the thermoresponsive gel immobilized on the carbon fiber, swelling experiments were performed. The heat controller TC 324C (Warner Instrument) connected with an imaging chamber (RC-21BRW, Warner Instrument) was employed. Figure 2.30 shows the imaging chamber (diameter 25 mm) which was filled with Mill-Q water and the sample. A constant solution level can be controlled by the inlet-outlet tube. An imaging chambers require a PH-2 platform to complete the assembly for mounting onto a microscope. The sample was immersed in water and placed in an imaging chamber, then placed on the stage of the microscope (Figure 2.31). The thermoresponsive behavior of the gel on carbon fiber was studied in the temperature range from 25C to 45C.

(44)

Figure 2.31 Photograph of the imaging chamber equipped with the heat controller on the microscope used for the direct visualization of the swelling properties of pNIPAM on carbon fiber.

2.1.5 Electrogenerated chemiluminescence (ECL) measurement

The applicability of these materials to sensing is illustrated with a further functionalization of the hydrogel layer with a redox luminescent agent, [Ru(bpy)]32+_. Incorporating a ruthenium complex in the synthesized bipolar Janus electrodes allows a wireless readout of ECL emission, which depends on the swelling state of the hydrogel. To study the changing of ECL emission at the temperature above and below LCST, thermoresponsive ECL microgels were reported32_{. Thermoresponsive pNIPAM microgels} covalently incorporating [Ru(bpy)3]2+_{deposited on carbon fibers need to be prepared by} bipolar electrochemistry. In this case, the ruthenium complex acts as comonomer which is copolymerized with NIPAM monomer and ECL luminophore.

2.1.5.1 The incorporation of Ruthenium(II) complex with thermoresponsive pNIPAM

In order to prepare the sample, the same bipolar cell as the one used for the experiment described in the previous part (part 2.1.2.4) is used. In this experiment, ruthenium complex (Figure 2.32) acts as comonomer in the polymerization.

(45)

Figure 2.32 Chemical structure of ruthenium(II) (4-vinyl-4′-methyl-2,2′-bipyridine)bis(2,2′-bipyridine)bis(hexafluorophosphate).

Figure 2.33 shows the schematic of preparation of the ruthenium complex incorporated in thermoresponsive pNIPAM and the details of the experiment are described below:

- Dissolve 0.566 g of NIPAM and 0.057g of BIS in 10 mL of Milli-Q water.

- The mixture was purged with N2 gas to remove O2 for 10 min followed by adding 0.041 g of KPS.

- The bipolar electrochemical cell was sealed with a septum and filled with 2.5 mL of above solution.

- The electrodeposition was performed in the dark to prevent the photoexcitation of the Ru(bpy)3 monomer.

- To prepare the Ru(bpy)32+ solution, 0.0089 g of Ru(II) complex was dissolved in 100µL of acetone and kept away from light. Then 25 µL of Ru(bpy) 32+ solution was added to the cell.

- After the electropolymerization, the solution should be removed from the sample very quickly to prevent jellification and washed with Milli-Q water.

(46)

Figure 2.33 Schematic of preparation of the ruthenium complex incorporated in

thermoresponsive pNIPAM on a carbon fiber

2.1.5.2 The electrogenerated chemiluminescence of the ruthenium complex incorporated in thermoresponsive pNIPAM

The two black compartments containing the feeder electrodes (graphite rods) were connected with a glass capillary (outer diameter: 2 mm, inner diameter: 1.12 mm; length: 80 mm) as shown in Figure 2.34.

(47)

Figure 2.34 Schematic of the electrochemical cell for electrogenerated chemiluminescence

measurement.

First of all, the ruthenium complex incorporated in thermoresponsive pNIPAM on the carbon fiber was placed into the bipolar cell in the middle of the glass capillary between the two feeder electrodes.

Next, the cell was filled with 100 mM TPrA in phosphate-buffered solution (PBS) at 3 mM (pH 7.4). Then, the appropriate electric field was applied with a DC power supply. Finally, the ECL imaging was performed using a horizontal epifluorescence microscope Olympus BXFM-ILHSPU equipped with a Hamamatsu EM-CCD camera. The electrochemical measurement was combined with a simultaneous monitoring of the ECL intensity by using a Hamamatsu photomultiplier tube R5070A (Figure 2.35).

Figure 2.35 Photograph of the experimental setup for monitoring the ECL intensity by using

a Hamamatsu photomultiplier tube R5070A.

(48)

2.2 Results and discussion

2.2.1 The cyclic voltammetry of a solution of persulfate anions with NIPAM in 0.1 M KNO3 on glassy carbon electrodes.

First of all, the external applied potential required in the bipolar experiment needs to be determined because in addition to thermodynamic aspects the potential drop in solution also affects to the potential along the conductive object. In this step, cyclic voltammetry allowed to estimate the applied potential required to trigger the bipolar electrochemistry reaction. The electrochemical measurement was performed with a Autolab potentiostat/galvanostat with a three electrode system. The cyclic voltammetry set-up is shown in Figure 2.25. This experiment was performed in a solution of persulfate anions (0.015 M KPS) and 0.5 M NIPAM in water with 0.1 M KNO3 as supporting electrolyte. The solution was purged with N2 gas for 10 min prior to polymerization. Figure 2.36 shows the persulfate reduction at a scan rate 100 mV/s in the reaction mixture. During the cathodic scan, the current attributed to the reduction of persulfate was observed at a potential of about E1 = −1.0 V vs Ag/AgCl to generate a radical anion (Eq. 2.9), and the oxidation of water was observed at E2 = 1.3 V vs Ag/AgCl.

S2O82- + e-  SO42- + SO4- (2.9)

Therefore, in order to trigger the persulfate reduction at the cathodic side, and the water oxidation at the anodic side of the bipolar electrode, the minimum potential value (Vmin)

needed to induce the two redox reactions at both sides of the object is E2- E1 = 2.3 V. Therefore, in order to trigger the reduction of persulfate and water oxidation at both extremities of a BPE (carbon rod) with a length of 1 cm, an electric field of at least 2.3 V/cm had to be applied between the feeder electrodes. In this experiment, a 1 cm of graphite rod was placed between the two feeder electrodes spaced by 2 cm meaning that the voltage drop along the object should be higher than 4.6 V.

(49)

Figure 2.36 Cyclic voltammogram of the persulfate reduction (black line: first scan; blue

line: second scan; red line: tenth scan) at a scan rate of 100 mV/s in the reaction mixture (0.1 M KNO3 + 0. 5 M NIPAM + 0.015 M S2O82-).

The decreasing of reduction current during continuous cycling indicates the kinetics of the persulfate reaction is decreasing due to the formation of pNIPAM on the glassy carbon electrode leading to a blocking of the electrochemically active surface area. However it was not possible to observe the pNIPAM hydrogel on glassy carbon because it is transparent.

2.2.2 Bipolar Electrodeposition of pNIPAM on graphite rod

In order to demonstrate the electropolymerization of pNIPAM by bipolar electrochemistry, graphite rods were used in the first proof-of-principle bipolar experiments. The mixture of substance was not purged with N2 and the composition for electropolymerization was optimized as shown in table 1.