[page 24↓]

2.  Experimental Techniques and Procedures

2.1. Electrochemical techniques

Throughout the work, electrochemical methods were used in which the electrode potential was controlled via a potentiostat. Two electrochemical techniques involved in the analysis of electrochemiluminescence (ECL) are cyclic voltammetry (CV) and potential step experiments. In these experiments, the potential of the working electrode (Wk) is controlled by a potentiostat (Fig. 2.1a) either directly or following the program from a function generator, as in the case of potential step experiments. Reference and counter electrodes are represented as Ref and Ctr in the figure.

Fig. 2.1a: Scheme for controlled potential experiments.

2.1.1. Cyclic voltammetry

Cyclic voltammetry (CV) is a very popular technique for initial electrochemical studies of new systems and even to derive information about fairly complicated electrode reactions. The technique is based on applying a triangular potential sweep at a rate ν = dE/dt, in V/sec, to the electrochemical system.


[page 25↓]

Fig. 2.1.1a. Triangular waveform of the potential in potential sweep techniques.

The potential (E) at any time is given by

(2.1.1a)

(2.1.1b)

where t is the time of the potential sweep, λ is the time corresponding to the switching potential, Eλ , Ei is the initial potential and ν is the scan rate [51].

Let us consider the oxidation of DPA to its radical cation DPA+. at a platinum (Pt) electrode in an electrolytic solution, with an initial concentration of DPA, CDPA. When the potential is swept in the positive direction, initially there is charging of the double layer at the electrode/electrolytic solution interface giving rise to a current that is proportional to the double layer capacitance, Cdl,

(2.1.1c)

As the potential approaches the standard potential E0/+, the oxidation current starts increasing. It reaches its maximum at the standard potential, when the surface concentration of DPA becomes zero at the Pt electrode. At higher magnitudes of the positive potential, the current progressively decreases [page 26↓]because of the depletion of oxidizable DPA in the layer of solution adjacent to the electrode, by diffusion. This can be understood from the definition of the faradaic diffusion controlled current, IF.

(2.1.1d)

Clearly, as the diffusion front extends further and further into the electrolyte, the concentration gradient at the electrode (x = 0) decreases, and so does the current. At the switching potential, the sweep becomes reverted, and the reduction current corresponding to the conversion of DPA+. to the neutral DPA, increases slowly and follows the same trend near and beyond the standard potential for reduction, E+/0. This is because, in the initial oxidation sweep, the diffusion layer becomes depleted of DPA and populated with DPA+. at a concentration of CDPA*. The conditions in the diffusion zone are similar for reduction as compared to oxidation in the initial sweep.

In the considered case of a reversible reaction, the surface concentrations of oxidized and reduced species at the electrode (Pt) are defined by the Nernst equation:

(2.1.1e)

This was used to derive the expressions for the anodic and cathodic peak currents and potentials:

(2.1.1f)

where ip is the current in amperes at 25 °C, A is the electrode area in cm2, DDPA +. is the diffusion coefficient in cm2 s-1, ν is the scan rate in Vs-1 and CDPA +. is the concentration in mol cm-3. For a reversible electrochemical reaction, the plot of ip vs. ν1/2 is a straight line, from which the diffusion coefficient of the reactant can be calculated, knowing the other parameters. In addition, the ratio for reversible oxidation and reduction reactions. Any deviation in this ratio reflects kinetic complications, i.e., irreversible or quasi-reversible electrochemical reactions at the electrode. The difference between Epa and Epc (ΔEp) is another useful tool to test the reversibility of the electrochemical process, which is close to 2.3 RT/nF at 25 °C for a Nernstian reaction.


[page 27↓]

The cyclic voltammetry of a polymer-coated electrode in the electrolytic solution is similar to that of a bare electrode, except with some important differences: The first change is brought about by the thickness of the polymer. This poses limitations to the progress of the diffusion front in the polymer. If the polymer thickness is lower than the diffusion layer thickness, the diffusion becomes hindered abruptly. This results in shortening of the diffusion tail in the cyclic voltammogram, and the current peak is now depicted by

(2.1.1g)

where CO* is the bulk concentration of the oxidized polymer moiety and V is the volume of the polymer. Thus, the peak current becomes proportional to ν, when the thickness of the polymer is smaller than the diffusion layer thickness (l), defined by the diffusion coefficients of the reactant and the time (l = ), for a linear diffusion. Thus, the cyclic voltammogram gives clues about the kinetic and transport parameters of the concerned electrochemical reaction.

2.1.2. Potential step experiments

2.1.2.i. Electrode reaction kinetics

Let us consider a single potential step to understand the current characteristics, by taking the example of 1 mM DPA in 0.1 M TEAClO4 in acetonitrile (AN).

Fig. 2.1.2a: Scheme of a single positive potential step.


[page 28↓]

ln these experiments the working electrode is initially kept at a potential where there will be no electrochemical reaction. And then the potential is stepped at t = 0, to a positive potential, which is above the standard potential E0/+ for the oxidation of the DPA in the electrochemical cell. The reactant at the electrode surface will become converted to positive radical ion, DPA+.. This provokes subsequent diffusion of DPA towards the electrode generating a concentration gradient within the diffusion layer. Since DPA becomes instantaneously oxidized to DPA+., the concentration gradient at the electrode surface defined by the diffusion, controls the current. At t = 0, the concentration gradient is maximum, because the concentration of DPA remains the same throughout the bulk except at the electrode surface, where it goes to 'zero'; this leads to maximum current in the chronoamperometry. Subsequently, decreasing current is recorded, since the concentration of DPA in the diffusion layer depletes with time due to the electrochemical reaction. The current is obtained by solving the concentration terms using the relationship,

(2.1.2a)

where O stands for the oxidized species, DPA+.. Even a sluggish electrochemical reaction can be made diffusion controlled, by increasing the applied potential, and the current is derived to be

(2.1.2b)

This equation is known as Cottrell equation. The plot of i(t) vs. t-1/2 is linear for a diffusion controlled reaction, from which the value of the diffusion coefficient can be calculated. As with the case of cyclic voltammetry, the thin layer condition imposed by the polymer thickness can be inferred from the break in linearity of the i(t) vs. t-1/2 plot. This is because when the diffusion layer exceeds the thickness of the polymer film, there is no oxidizable species, and hence the current drops to zero all of a sudden.


[page 29↓]

2.1.2.ii.  Measurement of double layer parameters

Electrode potentials, at which electrochemical charge transfer does not take place, the electrochemical systems: electrode/solution or electrode/polymer/solution behaves like an RC circuit upon application of potential to it. The double layer at the interface electrode/solution (or the electrode/polymer/solution) behaves like a capacitor. The potential drop across this capacitor supplies the potential for an electrochemical reaction. In addition, the current has to flow through the electrolytic solution (or polymer/electrolytic solution), which offers resistance to the current flow. This can be schematically shown as below:

Fig. 2.1.2.b: Equivalent circuit description of an electrochemical system in the absence of electrode reaction.

ET is the total potential applied to the electrochemical system. The values of the uncompensated resistance Ru and the double layer capacitance Cdl are essential for the quantitative analysis of the kinetics and transport properties. Their product, Ru Cdl = τ determines the time constant of electrochemical kinetics. Their measurement was achieved by stepping the potential to a value far below the standard potential for oxidation or reduction of the electrochemical system and by measuring the current. The current in potential step experiment in this potential range is due to double layer charging ( idl ) which is expressed as,

(2.1.2c)


[page 30↓]

When log idl is plotted against the time (t), the uncompensated resistance (Ru) can be calculated from the intercept; knowing which, the double layer capacitance Cdl can be estimated from the slope.

2.1.2.iii. Double layer charging and charge transfer

Fig. 2.1.2.c: Equivalent circuit description of an electrochemical system in the presence of an electrochemical reaction.

CT is the resistance for the charge transfer reaction. Due to the IRu drop in the electrochemical system, the electrode potential that is effective for the interfacial charge transfer reactions ECT is diminished from the applied potential ET [52]. Considering potential steps as in Fig. 2.1.2a, and defining the start and end potentials of a step as Estart and Eend, we can write

(2.1.2d)

The total current (I) at any time t is a sum of faradaic current IF and the charging current Idl:

(2.1.2e)

Following these equations, Eq. (2.1.2c) can be re-written as

(2.1.2f)


[page 31↓]

2.2. Experimental procedures

The polymer coating was done by dissolving the polymer in CHCl3 to make a saturated solution. Then, the polymer solution was applied to the electrode in drops with subsequent spinning of the electrode at a rate of 3 rps to ensure uniform coating. The Pt substrate was polished with 3, 1 and 0.7 μm diamond paste and cleaned ultrasonically. The surface was checked for uniformity with a Nikon PFX 104 optical microscope before and after the deposition of the polymer. The amount of polymer was estimated separately by evaporating part of the solution on a Pt disc and weighing it in a microbalance. The approximate polymer thickness was estimated from the amount of the polymer applied to an electrode using the value of dry density of the polymer.

Electrochemical experiments were carried out in a three-electrodes configuration with the polymer coated Pt as the working electrode and Pt wires as the quasi-reference (QRE) and counter electrodes. Anhydrous CH3CN obtained from Aldrich was used as the solvent and 0.1 M tetraethylammonium tetrafluoroborate (TEABF4), tetraethylammonium hexafluorophosphate (TEAPF6), tetrabutylammonium tetrafluoroborate (TBABF4), or tetraethylammonium perchlorate (TEAClO4) were used as the supporting electrolytes. The electrolytes were stored in desiccators over silica. Prior to the experiment they were dried in the oven for 12 hours to remove the traces of water present. Further care was taken for the removal of water, by heating the mixing cell containing the supporting electrolyte for half an hour, prior to mixing it with the solvent. The solvent acetonitrile, was stored in a closed vessel and was pressed inside the mixing cell by applying Ar pressure. Immediately after mixing the solvent and the supporting electrolyte, the solution was bubbled with argon and was also heated at intervals with proper care of the pressure development inside it, for nearly an hour [53]. The solution was then pumped to the electrochemical cell containing the working, counter and reference electrodes and was filled with Ar. Electrochemical measurements were accomplished with a homemade potentiostat. Potential pulses were applied by Agilent 33250 A function generator via the potentiostat. ECL emission intensities were measured with an RCA photomultiplier tube (type 7326), which was polarized at -1000 V by a Keithley 246 high voltage supply. The polymer [page 32↓]coated Pt working electrode was cycled in the respective potential range at 100 mV s-1. The range of potential used in the sequential potential step experiment to generate the ECL was determined from the oxidation and reduction peaks in the cyclic voltammetry, which was recorded using Volta Lab 4.

Fig. 2.2a: Scheme of the setup for the ECL experiment.

2.3. Optical methods

2.3.1. Absorbance and fluorescence spectroscopy

Every elementary system, whether nucleus, atom, or molecule has a number of discrete, quantized energy states. When an electromagnetic radiation of energy equivalent to one of the quantized energy levels of a substance is applied, there is interaction between the two, leading to absorption of the incident radiation. It can be schematically denoted for a substance X as

(2.3.1a)


[page 33↓]

where X * is the excited state of the substance. The term hν is the energy of the incident radiation. It is related to the energy level of the substance X by the Bohr equation,

(2.3.1b)

Here h is the Planck's constant, ν is the frequency of the incident radiation, Ef and Ei are the energies of the final (excited state X*) and initial (ground state X) states of the substance. The measurement of the intensity of the absorbed light against energy, the absorption spectrum, yields clues about the nature of the excited bond.

The average lifetime of an undisturbed excited atom or molecule X * is estimated to be on the order of 10-8 s or shorter than that. The excited state therefore dissipates the excess energy in some form and returns to the ground state. If the mode of relaxation is by emission of radiation, then it is called fluorescence or phosphorescence depending on the spin of the electron in the excited state configuration of the system X.

(2.3.1c)

In fluorescence, the delay between absorption and reemission is only about 10-4 to 10-8 s, while for phosphorescence it ranges from 10-4 to 10 s or more. Hence, fluorescence appears as instantaneous. Further, the emission occurs at longer wavelengths than the absorbed radiation. This is because part of the incident radiation is lost through other modes of deactivation, e.g., dissipation to vibrational, rotational levels leading to heating up of the substance, intersystem crossing, etc.. The plot of energy versus emission intensity yields the fluorescence spectrum, which is again a tool to investigate the nature of the excited bond.

The processes taking place in a substance upon application of an electromagnetic radiation can be understood from the following schematic picture.


[page 34↓]

Fig. 2.3.1a: Scheme of the interaction of radiation with energy levels of a substance.

The absorbance and fluorescence measurements were made with a dilute CHCl3 solution by using a blue laser, with the solvent CHCl3 as the blank. The absorbance measurement was made with Unicam UV/Vis spectrometer UV4, and the fluorescence was done with Aminco. Bowman® series 2 Luminescence spectrometer.

2.3.2. Raman spectroscopy

In Raman spectroscopy, when molecules are irradiated with monochromatic light, a portion of the light is scattered; most of this scattered radiation (about 99 %) has the original frequency (Rayleigh scattering), but a small portion (<1 %) is found at other frequencies. The difference in frequency between these new frequencies (Raman lines or bands) and the original frequency is characteristic [page 35↓]of the molecule irradiated and numerically identical with some of the vibrational and rotational frequencies of that molecule.

The cause of this phenomenon is that, in Raman spectroscopy, the molecules are also vibrationally and rotationally excited to a higher level. The energy of the incident radiation is not just equivalent to the electronic excitation energy, as with the case of molecular emission and absorption spectroscopy. This causes polarization of the bonds of the molecule. If the polarized bond resonates with the incident light, it emits radiation of the same frequency, called Rayleigh scattering. If emission occurs at a lower frequency, it is called Stoke's line, and that occurring at higher frequency than the exciting radiation is called anti-Stokes line. Stoke's lines are generally accounted since they are more intense than the anti-Stokes lines while providing the same information about the molecule. The plot of the emission intensity versus energy in wave numbers, yields clues about the nature of excited bonds [54].

For a particular mode of vibration to appear in the Raman spectrum, i.e., to be Raman-active, the molecule's polarizability must change during the course of this vibration. The polarizability of a molecule is the ability of the molecule to be polarized under the action of an electric field such as the alternating field of a light wave, and it can be defined in terms of the dipole moment μ produced by the electric field, E:

(2.3.2 a)

where α is the polarizability. The polarizability is thus a measure of the efficiency with which a varying electric field will induce a dipole moment in a molecule.

In measuring the Raman effect, visible light is almost always used as the incident radiation. Usually the 435.8 nm line from a mercury arc or the 632.8 nm line from a helium-neon laser is used. Ultraviolet light could be used, but it is not as widely applicable, not only because it is absorbed by many substances, but also because it may cause molecular dissociation and fluorescence.

Under the influence of visible (or ultraviolet) incident light, only electrons (not nuclei) oscillate, since nuclei cannot follow the rapid oscillations. Thus, [page 36↓]polarizability measures the ease of displacement of electrons by the electric field.

This spectroscopy has several advantages: Samples can be handled freely in any forms, solids, liquids or gases. Simple glass cells are sufficient for holding the sample. In addition, one instrument and a single continuous scan can be used to cover the entire range of molecular vibration frequencies.

Raman spectroscopic measurements were done with the solid polymer film on the electrode being subjected to red laser. The polymer coated electrode surface was analyzed before and after the ECL experiment ex-situ. The analyses were also made before and after cross-linking the polymer.

2.3.3. Photomultiplier Tubes

The ECL transients were recorded with the help of the photomultiplier tubes. Photomultiplier tubes are extremely sensitive, fast-responding types of vacuum phototube, designed so that an amplification of several million folds is achieved within one tube by the emission of secondary electrons. Radiation striking a photocathode causes the ejection of primary electrons, as in an ordinary vacuum phototube. In photomultiplier tube, these electrons are accelerated by a positive potential to a second sensitive surface, where each electron striking it causes the release of four to five secondary electrons. These electrons in turn are accelerated to another sensitive surface, where the number of electrons is again increased by a factor of 4 or 5. This practice can be repeated as many times as desired, though most commercial photomultipliers have about 10 target electrodes, or dynodes. The following figure schematically illustrates the cross section of the widely used circular-cage photomultiplier design.


[page 37↓]

Fig. 2.3.3a: Cross section of a photomultiplier tube.

The amplification factors achieved depend critically on the voltage applier to each dynode, and a very stable high-voltage power supply is required. Typically, each dynode is made 75 to 100 V more positive than the preceding dynode, and overall amplification factors of about 106 are common. In addition, the output of the photomultiplier can be further amplified. The limit of detection is set by the inherent dark-current noise, which is due to thermionic emission and other random noise. The response time is extremely short, responding to light pulses as brief as 10-9 s. The output current is linearly dependent on illumination over a fairly wide range but becomes nonlinear at high levels of illumination.

2.4. Polymer cross-linking by synchrotron radiation

Synchrotron radiation is produced, when charged particles, such as electrons, travel close to the speed of light and are deflected by a magnet. As a result of the radial acceleration in the magnetic field (Lorentz force), the particles emit electromagnetic radiation. For a particle of mass m in circular motion with velocity β = υ/c, energy E, and a radius of curvature ρ, the power (P) radiated by a single nonrelativistic accelerating particle with charge e (e.g., an electron) is given, following the Larmor formula as


[page 38↓]

(2.4a)

The practical units are Pin kW, E in GeV, ρ in meters, B in kiloGauss and I in amperes [55]. Thus, an intermediate energy storage ring with E = 1 GeV, B = 10 kG and I = 0.5 A would radiate 13.3 kW.

The secondary radiations are spread over a wide range of energy. By using grating or crystal monochromators, researchers can select any wavelength from the intense synchrotron radiation continuum. Thus, synchrotron radiation makes it possible for a research worker to select the wavelength most appropriate for the experiment, and also to scan the wavelength over a large range, because it provides five orders of magnitude more continuum vacuum ultraviolet (VUV) and X-ray radiations than the conventional sources such as X-ray tubes. It has several other properties that further add to its abilities as a research tool. These include an extremely broad spectral range (from infrared to X-rays), natural collimation, high polarization, pulsed time structure, high-vacuum environment, small source size, and high stability. Therefore it is preferred over many other radiation sources and has found varied application such as lithography, production of micro emulsions, chemical vapor deposition, and so on.

To stabilize a polymer in a solution and to decrease its dissolution, cross-linking of the side groups of the polymer is a known route. This cross-linking can be a chemical or radiation induced process. When a radiation of energy, higher than the energy of a particular bond, is applied to a molecule (or polymer), bond breaking results. This results in the production of a free radical at the site of a broken bond. As an example, if the polymer has a side group having C-H bond, then upon subjecting to a radiation of suitable wavelength, it breaks as

(2.4b)

where C and H are the free radicals. Two C radicals, of different bonds, can react with each other forming a C-C bond. Thus, the reaction between the free radicals leads to cross linking between the chains. This decreases the solubility of an otherwise slightly soluble polymer. The cross linked polymer will pose hindrance to the movement of ions, which is one adverse effect of this procedure.


[page 39↓]

The polymer-coated surface of the electrodes were mounted on a special aluminum holder and subjected to synchrotron radiation from the white beam line for lithography, of BESSY-II, Berlin, Germany. The energy range of the beam line is 1.5 - 6 keV, and the strength of the magnetic field is 0.4 T. A beryllium window (200 μm) and a graphite window (160 μm) were used. The dosage was varied from 100-500 mJ/cm2, corresponding to the dosage levels 19.4645 mA min/cm to 97.3225 mA min/cm.


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