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Sensing Systems and Signal Processing
Dr Sidahmed Abayzeed
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Dr Kevin chemical effects
Chemical Sensors
Chemical sensors are self contained devices that measure the concentration of one or more chemical species.
Chemical sensors provide two operations – recognition of the target and transduction of the signal into a measurable physical quantity.
Transduction
Recognition
Chemical Sensors – Recognition
Many different approached are used, in general we have the following scheme
Where A is the sample and R is the sensing element and P is the product of the interaction of the sample with the sensor. The ⇌symbol shows that this process reaches an equilibrium as it can go both ways. Interactions are usually via forces that involve non covalent bonding (e.g. van der waals, ionic bonds, hydrogen bonds)
This equilibrium is described by the affinity constant and depends on the concentration of the species in the reaction. High affinity means that sample and sensor interact strongly and the product is stable.
Selectivity is important, if we have:
Ideally, there will be a big difference This means that this sensor will select only sample A. If they are similar then the sensor is not very selective. Which is a problem if B is present in the environment. You can think of this a being similar to other sensors being sensitive to more than one thing (e.g. pressure and temperature).
Chemical Sensors – Recognition
Affinity interactions – this involves the reversible binding of two chemical species through non-covalent bonding. (e.g. ionic bond, hydrogen bonds, va der Waals interactions). The product of this reaction is a molecular association complex, for these to form they need to be compatible in terms of shape and chemical composition (reactivity). This is very common form of integration in biological systems, for example, antibody – antigen reactions – in the body this is how your immune system recognises pathogens and neutralises them. In sensing we often use antibodies to detect proteins as these are very selective as they are ‘matched’ to react with each other as the shape and form of the components are compatible with each other but not with other molecules.
Ion recognition – uses electric charge of ions and the interaction with the sensor to determine concentration. Selectivity is obtained by additional properties of the sensing material (for example the size or shape of sensing sites). These were some of the earliest forms of chemical sensors produced on a large scale. (the pH glass electrode is one such example). Transduction is typically performed by potentiometric or optical methods.
https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/11%3A_Immunology/11.05%3A_The_Adaptive_Immune_Response/11.5A%3A_Humoral_Immune_Response
Chemical Sensors – Recognition
Nucleic Acid recognition – using strands of RNA or DNA to form a sensor system. One part is attached to form the receptor site and the specific sequence used will have high affinity for a specific pattern sequence. This is taking advantage of the base pair matching in DNA/RNA to build a precise pattern selector. Recall we have 4 bases in DNA, Adenine (A) cytosine (C) Guanine(G) and Thymine(T) hydrogen bonds will only form between two sets of these giving 2 base pairs G-C and A-T.
Chemical Sensors – Recognition
Enzyme recognition
Enzymes are protein compounds that function as catalysts in chemical reactions in living systems.
They are selective in that they only catalyse reactions of specific compounds (substrates) or with specific functional groups. Recognition is different using enzymes as it is a dynamic process (unlike the usual equilibrium cases for the other methods discussed).
It involves 3 steps – first the target is bound to the active site of the enzyme, then the target undergoes a further chemical conversion (usually involving a co-reagent in the vicinity), the converted compound is now not matched to the enzyme so it is released and the enzyme returns to its initial state. Transduction is often performed by monitoring the steady state concentration of the product compounds.
Chemical Sensors – Recognition
Monitoring of gases and their composition is an important area for chemical sensing.
For example monitoring of air quality, detection of dangerous components e.g carbon monoxide. Sensing methods are based on sorption either at a surface (adsorption) or within a solid material (absorption).
A whole host of materials, from metals, polymers, inorganic materials are used for vapour recognition. It can involve chemical reactions or physical effects so sensor operation is quite broad.
http://soft-matter.seas.harvard.edu/index.php/Gas_adsorption_applications
Chemical Sensors – Transduction
General Approach
There are two main transduction options for chemical sensing – physical transduction or chemical transduction
Physical transduction monitors a physical property of the sensing element, for example mass, refractive index, electrical resistivity. These are typically label free
Chemical transduction relies on monitoring the chemical composition of the sensing element in response to the recognition process. Essentially monitoring the concentration or presence of a specific component. Ideally this is the thing you want to measure, but it could be a chemical label that bonds to the thing you want to measure.
Chemical Sensors – Transduction
Thermometric transduction – the reactions that take place between the target and sensor changes the temperature, typical this requires a catalyst to produce a measurable effect.
Mechanical Effects – the reaction leads to an overall change in mass at the sensor surface. This can be detected in a number of ways (quartz micro balance). The change of the properties of the sensing surface can also change the way vibrations propagate (speed of sound).
Chemical Sensors – Transduction
Resistive and Capacitive effects – the sensors electrical properties can change leading to a change in resistance or capacitance which can be measured in a number of different ways.
Electrochemical Transduction – Electrochemical reactions can be driven by passing a current through a solution containing ions. The reaction rate depends on the ions and concentration and can be sensed either though monitoring the potential for given current, or monitoring the current for a fixed driving potential.
Optical Transduction – the optical properties of the sensor can change due to the reaction,
This could change the way different colours of light interact with the sensor – spectrographic changes, or how light is reflected or transmitted through the sensor (via changes in the refractive index)
Chemical Sensors – Examples
Carbon monoxide detector
Carbon monoxide (CO) gas is a colourless and odourless gas produced by incomplete combustion.
It is often called the silent killer as it is hard to detect. In the UK, carbon monoxide poisoning in the home accounts for an average of 50 recorded deaths a year and up to 4,000 medical visits, according to the Department of Health and Social Care, 2018
Carbon monoxide detectors monitor the accumulation of carbon monoxide over time. Often this is based on either the resistance change or the colour change which blocks light transmission of a chemical reactive strip. If the change reaches some threshold then the alarm is triggered.
https://www.firesafetyuk.co.uk/size/compact-design-carbon-monoxide-alarm/2916?gclid=Cj0KCQjwjpjkBRDRARIsAKv-0O2MpjOnhTRuuUiOX2yNuZySkkGfrKW3eM-22Y7SP7ZUjq8K5DTfW7caAjFtEALw_wcB
Chemical Sensors – Examples
Pregnancy detector
Pregnancy tests look for the hormone human chorionic gonadotropin, or hCG. hCG is produced by the placenta and so can be found as soon as implantation of a fertilized egg has occurred.
A drop of urine on the test strip is wicked (flows) along the strip where any hCG it can react with anti –hCG antibodies,
these flow to the first test site where they are trapped by another hCG antibody immobilised at the test site,
at this site and enzyme on the first antibody causes the location to change colour showing positive result.
At the end of the test strip the antibody system is checked by binding any of the free antibodies in the test to the second line to confirm the test was working – there is no hCG here but shows the test was working as expected
http://webphysics.iupui.edu/webscience/bio_archive/goodfor3.html
Chemical Sensors – Examples
Glucose detector (diabetes)
Basic blood glucose monitors have three parts, a lancer to extract a drop of blood, a test strip where the blood reacts and the meter itself to measure the change.
Blood glucose detectors measure the amount of sugar in blood by using a multistage chemical process.
The glucose reacts with glucose oxidase in the strip to make gluconic acid.
Gluconic acid reacts with ferricyanide to produce ferrocyanide.
A current is passed through the blood sample and the ferrocyanide reacts at the electrode changing the current
So the current is related to the ferrocyanide concentration which is determined by the glucose level.
Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=29738536
Chemical Sensors – Examples
Surface plasmon resonance is an optical sensing technique. Electromagnetic surface waves can be generated at the interface between a dielectric and a noble metal. The conditions for coupling into these waves are very sensitive to the conditions around the boundary, the incident angle, the wavelength of the light, refractive index and the properties of the metal layer – anything that changes effects the amount of coupling and therefore the amount of light reflected from the surface.
Gas/ solution
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3045209/
Surface plasmon resonance
Chemical Sensors – Examples
Efficient coupling into the plasmon for a saline buffer solution using 632nm light source and a gold film (50nm) is ~55 degrees
We attach an antibody to the gold film and then flow a solution containing a protein that will bind to the antibody with different concentrations. This will change the refractive index in the vicinity of the plasmon changing the boundary conditions and therefore the amount of light reflected.
Chemical Sensors – Examples
Analyte in
Analyte out
Binding trace for samples with different concentrations of protein, using the same sensor surface. After each concentration we regenerate the antibody surface by using an acid wash to remove the protein.
Lower concentrations bind more slowly
If you measure the output after a specific time you can work out the concentration of an unknown solution.
Can be used to detect incredibly low concentrations of proteins.
Chemical Sensors – Voltage Sensing
To detect voltage optically, a three-electrode system is combined with SPR sensing system.
Resonance position shift (blue) is tracked using a differential detector.
Sidahmed A. Abayzeed, et al. Opt. Express 25, 31552-31567 (2017);
Chemical Sensors – Electrochemical Imaging
J (pA/pixel)
Current source
Optical map of current density
Chemical Sensors – Impedance Imaging
Impedance spectroscopy
Electrical properties
Scaling factor
Charge density
Current density
Resonance position modulation
Abayzeed, Sidahmed A. Biomedical Optics Express 11.11 (2020): 6168-6180.
Chemical Sensors – Fiber Optic Sensors
Fiber optics can be used for many different types of sensing applications
They typically exploit different effects that cause changes to the optical properties of the fibre.
Different types of fibre : single mode and multimode
Chemical Sensors – Fiber Optic Sensors
Interaction of light and matter:
Light can be absorbed, reflected, transmitted or scattered.
We can deliver a wide range of wavelengths with fibers to a sample and either look at the transmitted light or the reflected light.
Different chemical components will absorb different colours of light. Leaving finger prints of their presence in the spectrogram. Allowing us to measure their presence and their concentration.
http://www.astro.ucla.edu/~wright/fluxplot.html
Chemical Sensors – Examples
Another way for the light to interact with the environment without having a break in the fiber is to remove the cladding. We can then build sensors where the cladding was so we have high specificity to particular chemicals.
(ii) Polycation deposition
and drying
repeat (ii)
(iii) TSPP deposition
(i) KOH treatment
and drying
and drying
Optical fiber: silica core/plastic clad
200 mm clad 500 mm
PDDA: 0.5 wt% in water
TSPP: 1 mM in water
Chemical Sensors – Examples
Light source
Spectrophotometer
Sensor cell
Chemical Sensors – Examples
Light intensity at a specific wavelength is linearly related to the humidity, as when water molecules are absorbed by the sensing layers the refractive index changes which changes the amount of light transmitted by the fiber.
Other wavelengths change but not in an easy to interpret way.
Change of intensity, mV
Wavelength, nm
region of linear response
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