Biosensors Flashcards
What is biosensor
A device that converts a clinically or biologically relevant signal to a measurable signal and provides quantitative or semi-quantitative analytical information
Functions of a biosensor
May detect
- Chemicals or ions related to bio-functions
- Small biomolecules (e.g. glucose, hormones)
- Macromolecules (e.g. proteins or DNAs)
- Mammalian cells or microorganisms
- a biological function or activity (e.g. bioelectricity, enzymatic reaction)
At:
- Single-molecule, single cell, tissue, or body level
In vitro or in vivo
Components of a typical biosensor
- Recognition element: immobilized on (or integrated in) sensor and specifically interact with target analyte
- Sensing element: translate the interaction between recognition element and analyte into a measurable and quantifiable signal
- Output component: output signal in user-friendly; often with associated electronics
What are the different signals
- Optical: fluorescence, color, absorbance, reflection, raman
- Physical: temperature, weight, displace, vibration
- Chemical: pH, chemicals
- Electrical: current, voltage
What is the operation of electrochemical sensors based on
Charge transfer or charge accumulation due to certain electrochemical reactions occurring at the electrode surface
What is amperometry
Measures the current between the working electrode and the reference electrode while a constant voltage is applied. The applied voltage can be a defined time-varying waveform
What is potentiometry
Measures the potential difference between the working electrode and the reference electrode while a constant current is applied. The applied current can be a defined time-varying waveform
What is a clark-type O2 electrode
It is an amperometric sensor. The measured current is proportional to O2 concentration.
Glucose sensor
Use of enzyme (glucose oxidase - GOx) and its co-factor (FAD) and mediator (e.g. ferrocene)
What is a pulse oximeter
A medical device that indirectly monitors the oxygen saturation of blood
The working principle of pulse oximetry
Based on the red (R) and infrared (IR) light absorption characteristics of oxygenated and deoxygenated hemoglobin.
Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through.
Deoxygenated (or reduced) hemoglobin absorbs more red light and allows more infrared light to pass through.
How do surface plasmonic resonance (SPR) based sensors work
Detect refractive index changes close to the surface
Working principle of SPR sensor
At metal-dielectric interface, surface plasmon forms (periodic oscillation of electrons on metal surface).
When the incident light beam has the right incidence angle, surface plasmon resonance occur.
At this so-called ‘resonance angle’, optical energy is coupled into the metal surface. As a result, reflection is decreased at this resonance angle. Molecular binding a the interface changes the angle.
Advantages of SPR sensor
- Sensitive, fast
- Without need to label the targeted molecule.
- Real-time monitoring binding/dissociation kinetics
- Only require a small amount of sample
Working principle of Ion-Sensitive Field-Effect Transistor (ISFET)
Target ions, which pass through the ion-selective membrane and accumulate at the gate, change the conductance of the FET due to field-effect hence leading to measurable electrical signal
Working principle of piezoelectric biosensors
Utilize crystals, such as quartz, which vibrate under the influence of an electric field.
The frequency of this oscillation changes upon binding of target biomolecules due to mass increase
Characterizing a biosensor
- Selectivity (or specificity): Interference from other chemicals or molecules must be minimized
- Lower detection limit (LDT): Minimal detectable target concentration (signal to noise ratio >3)
- Sensitivity: ratio of change between response and concentration
- Dynamic range of detection: LDT to the threshold concentration that causes saturation of response (or Higher detect limit - HDT)
- Response time: Time necessary for achieving 95% of the stable response
- Linearity: response is linearly scaled with concentration
- Reproducibility: consistency of response to a given sample
- Cost: for both raw materials and fabrication
- Convenience of measurement
Advantages of nanobiosensors
- Offering new sensing possibilities: because new phenomena arise at nanoscale
- Better sensitive and selectivity: because of large surface-volume ratio, ability to intimately interact with molecular targets, taking advantage of new phenomena
- Faster response, less power and sample consumption; because they are miniaturized
Disadvantages of Field-Effect Transistor (FET) sensors based on silicon nanowires (SiNW) or carbon nanotubes (CNTs)
As the current carries flow completely (as for CNT) or largely (as for SiNW) on the surface, their conductance is extremely sensitive to the minute electrical or electrochemical perturbation at the vicinity
How does immunosensor base on SiNW- or CNT-FET work? (nanoelectric FET sensors)
- Binding of electrically charged antigens onto immobilized antibodies alters the FET conductance due to electrostatic gating (field-effect), whereby leading to measurable electrical current change from the voltage biased (between D and S) FET sensor
How does pH sensor based on SiNW-FET work
De-protonation of APTES groups functionalized on p-type SINW increase nanowire conductance
How does protein sensor based on SiNW-FET work
Negatively-charged streptavidin increases the conductance of biotin-functionalized p-type SiNW
How to detect biopotentials using SiNW-FET
- Extracellular biopotential caused by an action potential in a neuron is translated into SiNW current signal
- The ionic current flowing through the resistive nanogap between SiNW and cell membrane changes the extracellular potential at the gap which, in turn, gates the SiNW current (field-effect)
How does nano-cantilever based biosensors work?
- Cantilever is coated with a chemically selective layer (e.g. antibodies)
- Deflection of cantilever can be measured precisely deflecting a light beam from the surface
- Cantilever bends upon molecular binding due to surface stress
Examples of nanopore technology
Translocation of a molecule (E.g. DNA) produces a characteristic ionic blockage current
How does colorimetric biosensors based on gold nanoparticles work
- Gold nanoparticles (AuNPs) are colored due to localized surface plasma resonance
- The color depends on the particle size and the distance between particles.
- Aggregated blue AuNPs (crosslinked by DNAs) turn to red as they separate apart after DNase cleaves the linking DNA strands
