ch3 Flashcards
two main groups of affinity-based electrochemical biosensors
- three-electrode electrochemical cell
- field-effect transistor
what are some electroanalytical methods
voltammetry, electrochemical impedance spectroscopy (EIS)
in voltammetry, what are the two determining experimental factors that can be manipulated
1) the way the potential of WE is changed over time
2) when the current is chosen to be measured
Voltammetry examples
square wave voltammetry (SWV), differential pulse voltammetry (DPV), cyclic voltammetry (CV)
Explain EIS
AC voltage is applied to the WE in a range of frequencies, while the frequency dependent current is being recorded
leads to changed in the impedance across the surface
surface characterization: what is it used for?
nanomaterials used to modify electrodes can either accelerate or obstruct electron transfer between the redox probe and WE, these can be characterized by the electroanalytical techniques
CV and EIS are commonly used
Sensing: what is it used for?
DPV, SWV and EIS can provide high sensitivity, enabled researchers to measure extremely low concentrations of the target of interest
Sensing strategies used in potentiostatic affinity-based biosensors
based on BRE conformational switch:
- rely on change in electron transfer distance between electroactive intercalators of the BREs and WE surface
- caused by BRE conformational switch after capturing the target
based on target molecule displacement:
- the target binding with the BRE can disassemble the helix double SSDNA and release the electroactie group-modified complementary from the WE surface.
- displacement of this complementary sequence may result in a change in electrochemical signals
based on steric hindrance:
- when BRE is captures, surface of WE is partially blocked due to aptamer target complex, thus reducing exposure of WE surface to redox proobe.
- as s result, higher steric hindrance, leading to increased resistance, resulting in reduced current peak.
FET (field-effect transducer) biosensor
- composed of 3 terminals: source, drain, and gate electrodes, and semiconducting channel between drain and source
- current flowing between drain and source is modulated by voltage between gate and source
- sensing film consisting of bioreceptor to target is formed on the semiconductor channel
- overall, FET relies on the conductivity changes of the semiconductor channel connecting the source and drain electrodes
how do semiconductors work
electrons in conduction band are free to move and conduct current
valence band is right below - in a metal, they overlap, in an insulator they are separate, not allowing the electrons to transfer. in a semiconductor, the band gap is small enough for electrons to jump to the valance band
what is the electrostatic gating effect vs direct charge transfer
EGE - the charges induced in graphere are opposite to and hence in balance with the charges carried by the target molecules
DCT - from either charged target molecule of the BRE to the semiconducting channel material
List optical transducers
surface enhanced raman scattering (SERS)
colorimetry
fluorescence
surface plasmon resonance (SPR)
chemiluminescence (CL)
explain SPR
relies on refractive index variations connected with the binding of the tarted to the BRE on the transducer
when polarized light illuminates a metal surface at the interface between two media of different refractive indices, at a certain angle, the resonance angle - it produces electron charge density waves called plasmons.
SPR results in the declined intensity of the reflected light relative to the incident light
the decrease in intensity is proportional to the mass on the surface
explain the colorimetric aptasensor for detectino of gentamicin
LSPR property of gold nps w/ specificity of aptamers has been used in colorimetric assay
aptamer-coated gold NPs demonstrated a strong absorbance peak due to excitation of plasmons. when gentamicin concentrations are introduced, gentamicin affinity for aptamer» AuNP, leading AuNPs bare and susceptible to salt induced colour change
fluorescence spectroscopy: what is?
analyzes fluorescence (property of an object absorbing light at one wavelength and reemitting it at another) from a molecule based on its fluorescent properties
