Basics Flashcards
HelmHoltz Double Layer
counter charges of the liquid bind directly to the surface and neutralize the charges there
- this model can’t describe the capacity of the electrical double layer
- strong electrical field
Gouy-Chapman Model
- includes thermal fluctuations of the counterions
- -> diffuse layer extends into the liquid
- can only be used for planar geometry
Debye-Hückel Model
- calculates the potential and ion distribution around spherical surfaces
Poisson-Boltzmann equation
electrical potential closed to a charged, planar surface depends on the distance to the surface
Simplifications used for Poisson-Boltzmann equation
- only electrical work is performed
- only monovalent salts
- ions are regarded as continuous charge distributions
- ion concentration of background salt is higher than the concentration of ions that dissociate from the surface or adsorb to the surface
Debye length
- surface potential decreases expontentially with the decay constant k
–> sweep fraction of this decay constant is the debye length (Kehrwert)
–> decreases with increasing ion concentration, because more ions in the solution can shield the surface charge more effectively
Hydration energy vs. dissociation energy of salt
1) dissociation energy > hydration energy:
- -> the salt is sparingly soluble or not soluble at all. The dissolution process requires ana additional energy
2) dissociation energy < hydration energy:
- -> salt is easily soluble, energy is released
What does an electrolytic cell consist of?
2 electrodes and an electrolye
–> electrical voltage can lead to the decomposition of the electrolyte
Fahradays Law
mass of a material converted on an electrode must be proportional to the charge Q (Q = I * t)
Q = z * n * F
z= chemical valency n = amount of substance [mol]
–> for double charged ions, twice the amount of electrons is needed
Chemical Potential
- a reactions, transformation or redistribution can only then happen, when the chemical potential in the starting state is higher than in the final state
chemical potential describes the work that has to be applied to transform a specific number of molecules
Metal/Electrolyte - Phase Boundary
–> equilibrium condition
- two phase system (half cell)
equilibrium when both components have equal chemical potential
- equilibrium may not be reached due to opposing electrical potential
Galvani Potential
the potential increases from the potential of the electrolyte to the potential of the metal linearly
- total potential difference is the Galvani Potential
Redox Electrodes and Redox reactions
- can be identified over their redox potentials (Nernst Equation)
- 2 electrodes needed to measure the redox potential
-
Reference Electrode
to compare the Galvani Potentials of different materials
- galvani potential of reference electrode to zero
e. g. Standard hydrogen electrode (SHE)
e. g. Ag/AgCl electrode
Silver/silver Chloride Electrode
used as reference electrode
- silver wire immersed in a highly concentrated potassium chloride solution
- diffusion of chloride ions in the electrode is suppressed
- potential of electrode is only dependent on the activity of the chloride ions
Point of zero charge
pH value, where the surface is neutral
Electrodes Type 3
Mixture of Type 1 and Type 2 Electrodes
- -> only these can be used as real reference electrodes
- -> electrochemical reference electrodes
e.g. Calomel electrode
Polarizability of electrodes
Type 1: polarizable –> frequency dependent, behave like capacitor, no charge transport
e.g. metal electrodes
Type 2: non-polarizable –> ion exchange in the reaction layer, current density is only diffusion-limited
Diffusion limit - electrode size
macroelectrodes –> planar diffusion
- reaction is faster than the diffusion
microelectrodes –> spherical diffusion
- diffusion is faster than the reaction –> good!
Capacity of the diffuse double layer
relationship with debye length
- dQ/dU
- -> between two areas of separated charges
- capacitance behaves in first approximation like a plate capacitor with a plate spacing in the size of the Debye length
Stern Layer
combination of theories of rigid Helmholtz layer and diffuse double layer
- -> 2 parts:
- inner layer (Stern layer) –> linear
- outer layer (diffuse double layer) –> exponential voltage drop
–> 2 capacities in series
- Stern Layer between inner and outer Helmholtz plane
Zeta Potential
Potential at the point where the Stern layer ends (linear voltage drop) and the diffuse layer begins (exponential voltage drop)
–> point = outer Helmoltz plane
Ion-sensitive field effect transistor (ISFET)
- 2 signal components
signal components:
1) potential change at gate caused by an AP of a cell
2) potential change at gate caused by changes of ionic concentration of the electrolyte near the gate (e.g. pH sensitivity)
- detection of DNA
DNA - what are SNPs?
SNPs: particular type of DNA microarrays
- responsible for genetic variations and the source of susceptibility to genetically caused diseases
Arguments for label-free DNA detection system
- labeling (fluorescence) can interfere with biomolecular structure and function
- aiming to minimize time and resource costs
- development of a minituarized and simplified readout system
Properties of ideal sensors
1) Linearity: large linear response range in the calibration curve
2) Sensitivity: size of sensor response
3) Selectivity: interference with other analytes must be minimized
4) Response Time
Requirements for a biosensor
- high specificity
- good stability
- no contamination of the sample
- high and reproducible signals
Detection limits
- limitation by quantity
- limitation through concentration
Parameters of sensors
- working range
- selectivity
- resolution
- response time
- hysteresis
- stability of sensor
- operational lifetime
What does the Grahame equation relate?
relates change in surface charge density to a change in surface potential
ISFET - detection principle
- gate potential is modulating the inversion channel between drain and source
- changes in drain-source current are proportional to changes of the gate potential
pH sensitivity - 2 possible ISFET operation modes
1) Saturation region
2) sub-threshold region
Conductive Polymers
1) PEDOT:PSS hole conducting polymer
2) organic electrochemical transistor (OECT):
- conductive polymer channel
- electrolyte between gate and channel
Polymer-nanomaterial composites
definition
combination of 2 materials with different physical and chemical properties
–> combined they create a material specialized for a certain job
(stronger, lighter, conductive…)
- one or more phases with nanoscale dimensions are embedded in a metal, ceramic or polymer matrix
Why use nanocomposites?
- nanoparticle properties
- multifunctional capabilities
- chemical functionalitation
- huge interphase zone
Second Phase of Polymer-matrix nanocomposites
- dispersed within the matrix
- has nanoscale dimensions
- small size of phase leads to unique properties
Electrical properties of polymer-matrix nanocomposits
- -> influence by amount of particles
- -> influence by size of particles
- can be improved by adding nanoparticles
- the smaller the nanoparticles the higher the electrical conductivity
Optical Properties
- to achieve transparency –> scattering must be minimized –> nanoparticles should be as small as possible
- index of refraction should be kept similar to that of the matrix
Metal-organic frameworks (MOF)
- subclass of coordination polymer
- special feature: often porous
- coordiation network with organic ligands containing potential voids
What are SNPs used for?
to identify genetic variations
forensic applications
What is DNA Detection used for?
criminal investigation
relationship identification
diagnosis of cancer
Characteristics of sensors - Sensitivity and Limit of Detection
Sensitivity: dS/dC –> Steigung des linearen Bereichs
Limit of detection: maximale Signalstärke
Carrier Ionophores
Channel-forming ionophores
Carrier-ionophores: form ions, diffuse through cell membrane and release the ions on the other side
channel forming ionophores: small proteins that form pores/transmission channels through which the corresponding ions can diffuse –> ion channels
OECT working principle
- ions from electrolyte can enter the channel
- cations compensate the negative PSS groups
- amount of holes in PEDOT goes down
- conductivity goes down
- -> fast OECTs needed for sensing APs
- downscaling channel length to obtain faster device with higher transconductance
What do the properties of composites depend on?
function of:
- properties of the constituent phases
- their relative amounts
- the geometry of the dispersed phase
