Transducers

Transducers in Biosensors I(Week 4)

Centre for NanoBioengineering & Spintronics, Chungnam National University,Daejeon,Korea

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Transducers in Biosensors I

Various types of transducers: Principles and applications

• Potentiometric/amperometric conductrometric/resistormetric

• Optical CalorimetricMiscellaneous

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1. Brief over view of Transducers

Transducer: a device that converts one form of energy to another

• Energy forms as input and output– Thermal: temperature, heat, heat flow,...– Mechanical: position, velocity, acceleration,

force, pressure, ...– Chemical: concentration, composition, reaction

rate…– Optical: intensity, wavelength, phase, polarization– Magnetic: field intensity, flux, magnetization, etc.– Electrical: voltage, current, charge, …

Many measuring and sensing devices, as well as loudspeakers, thermocouples, microphones, and phonograph pickups, may be termed transducers.

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TransducerIUPAC Definition :

• Transduction of the biosensor signal is a process that is concurrent,and within the special environment of the biosensing element.

• The transduction efficiency determines many of the analytical characteristics of the biosensors, such as the signal stability, reproducibility, detection limit, and in many cases the operational stability and selectivity.

Transducers in biosensors

Rasoolyet al, Biosensors , 37, 1 (2005)

The transducer converts the biochemical interactions intomeasurable electronic signals. Electrochemical, electro-optical, acoustical, and mechanical transducers are amongthe many types used in biosensors. The transducer workseither directly or indirectly.

Advances in Biosensors, B.D. Malhotra & A. P. F. Turner10/5/2009 5

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Responsivity = Output signal / Input signal( Transducing effciency : if signals have same units )

Detectivity = Signal/Noise of output signal/size of output signal

= Responsivity/output noisesignal

Responsivity and Detectivity


Sensor and ActuatorSensor and actuator are kinds of transducersSensors: Magnify/Demagnify an environmental perturbation (signal/noise) and transform into an observable energy formActuators: Use small controlled energy to cause an observable (or controllable) perturbation (movement/energy radiation) to the environment


Types of Transducers commonly used in biosensors

Transducers Examples

Electrochemical1.Amperometric2. Potentiometric3.Conductometric

Clark oxygen electrode, mediated electrode systemsRedox electrodes, ion selective electrodes, field effect transistors, light addressable potentiometric sensorsPlatinum or gold electrodes for the measurement of change in conductivity of the solution due to the generation of ions

Enzymes, immunological systemsGases, enzyme, organelle, cell or tissue; electrodesIons in biological media, enzyme electrodes

Optical Photodiode, waveguide systems, intergrate optical sensorspH; enzymes; immunological analytes

Acoustic (mass) Piezoelectric crystals, surface acoustic devices, Volatile gases and vapors, antibodies

Calorimetric Thermistor or thermopileEnzyme, organelle, gases, pollutants, antibiotics, vitamins10/5/2009 9

1. Electrochemical sensor

Advances in Biosensors, B.D. Malhotra & A. P. F. Turner

There are three types: 1) Potentiometric 2) Amperometric and 3) Conductometric.

Principle: Electrochemical reactions take place at electrode-electrolyte interfaces and provide a switch for electricity to flow between two phases of different conductivity, i.e. the electrode (electrons or holes are the charge carriers) and solid or liquid electrolyte (ions are the main charge carriers)

e.g Oxidation of Fe(CN)6

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1. AMPEROMETRIC BIOSENSORS

• With amperometric sensors, the electrode potential is maintained at a constant level sufficient for oxidation or reduction of the species of interest (or a substance electrochemically coupled to it).

• The current that flows is proportional to the analyte concentration.

Id = nFADsC/d


AMPEROMETRIC BIOSENSORS

• Amperometric enzyme electrodes basedon oxidases in combination with hydrogenperoxide indicating electrodes havebecome most common among biosensors.

• With these reactions, the consumption of oxygen or the production of hydrogenperoxide may be monitored.

• The first biosensor developed was basedon the use of an oxygen electrode.


e flow

Working Electrode

Auxiliary Electrode

Reference Electrode(e.g. Ag/AgCl, SCE)

(e.g. Pt wire)

(e.g. Pt, Au, C)

Stirbar

Buffer solution (e.g. Tris, DPBS, Citrate)incorporating electrolyte(e.g. KCl, NaCl)

Typical Design


GlucoseOxidase Gluconic Acid + H2O2

The product, H2O2, is oxidised at +650mV vs a Ag/AgCl reference electrode.

Thus, a potential of + 650mV is applied and the oxidation of H2O2 measured. This current is directly proportional to the concentration of glucose.

Example


Glucose + O2

0

50

100

150

5 10 15 20

I (nA)

[Glucose], mM

Typical curveTypical Spectra

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-+

Platinum cathode

Polyethylene membrane

Silver anode

Electrode body

KCl soln.

Clark Oxygen Electrode



• The drawback of oxygen sensors is that they arevery prone to interferences from exogenousoxygen.

• H2O2 is more commonly monitored. It is oxidisedat +650mV vs. a Ag/AgCl reference electrode.

• At the applied potential of anodic H2O2 oxidation,however, various organic compounds (e.g.ascorbic acid, uric acid, glutathione,acetaminophen ...) are co-oxidised.

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• Various approaches have been taken toincrease the selectivity of the detectingelectrode by chemically modifying it by theuse of:

– Membranes– Mediators– Metallised electrodes– Polymers



1. Membranes.Various permselective membranes have beendeveloped which controlled species reaching theelectrode on the basis of charge and size.

Examples include cellulose acetate (charge and size), Nafion (charge) and polycarbonate (size).

The disadvantage of using membranes is, however, their effect on diffusion.



2. MediatorsMany oxidase enzymes can utilise artificial electron acceptor molecules, called mediators.

A mediator is a low molecular weight redox couplewhich can transfer electrons from the active site of the enzyme to the surface of the electrode, thereby establishing electrical contact between the two.

These mediators have a wide range of structures andhence properties, including a range of redox potentials.


AMPEROMETRIC BIOSENSORS, CV of FeCN


• Examples of commonly used mediators are:

– Ferrocene (insoluble)– Ferrocene dicarboxylic acid (soluble)– Dichloro-indophenol (DCIP)– Tetramethylphenylenediamine (TMPD)– Ferricyanide – Ruthenium chloride– Methylene Blue (MB)



3. Metallised electrodes

The purpose of using metallised electrodes is to createconditions in which the oxidation of enzymaticallygenerated H2O2 can be achieved at a lower appliedpotential, by creating a highly catalytic surface.

In addition to reducing the effect of interferents, dueto the lower applied potential, the signal-to-noiseratio incresaes due to an increased electrochemicallyactive area.



Metallization is achieved by electrodepositing therelevant noble metal onto a glassy carbon electrodeusing cyclic voltammetry.

Successful results have been obtained from a fewnoble metals - platinum, palladium, rhodium andruthenium being the most promising.



Potential Potential

Glassy carbon electrode Metallised GCE

Glassy carbon electrodes do not catalyse the oxidation of hydrogen peroxide.

GCEs metallised with ruthenium, rhodium, palladium orplatinum do.


4. Polymers

As with membranes, polymers are used to prevent interfering species from reaching the electrode surface. Polymers differentiate on the basis of size and charge.

An example is that of polypyrrole. A polypyrrolefilm has to be in the reduced state to becomepermeable for anions. If the film is oxidised, no anion can permeate.



• Examples of commonly usedpolymers are:

– Polypyrrole– Polythiophene– Polyaniline– Diaminobenzene– Polyphenol



1.1 Amperometric transductionAmperometry encompasses a group of electroanalyticaltechniques. It monitors the current generated at a fixed bias potential.

Linear relation between the concentration of the analyteand the current generated is obtained.

Several techniques like cyclic voltammetery, flow injection analysis studies, etc. are commonly employed.

Most biosensors are based on amperometric-type detection.

Amperometric biosensors have been at the focus of electroanalyticalresearch since the first report of the enzyme electrode by Updikeand Hicks in 1967 for the detection of glucose.


Clark-type oxygen electrode

Clark and Lyons 1962

Example of Amperometricsensor


1.2 Potentiometric transductionA potentiometric biosensor monitors

the potential under zero current conditions. The potential generated is directly proportional to the logarithm of the analyte concentration.

The basis of this type of electrochemical monitoring is the

Potentiometric biosensors - these use ion-selective electrodes to determine changes in concentration of chosen ions, e.g., hydrogen ions.


POTENTIOMETRIC BIOSENSORS• In potentiometric sensors, the zero-

current potential (relative to a reference)developed at a selective membrane orelectrode surface in contact with a samplesolution is related to analyteconcentration.

• The main use of potentiometrictransducers in biosensors is as a pH electrode.


POTENTIOMETRIC BIOSENSORS• E = Eo + RT/nF ln[analyte]

– Eo is a constant for the system– R is the universal gas constant– T is the absolute temperature– z is the charge number– F is the Faraday number– ln[analyte] is the natural logarithm of the

analyte activity.


POTENTIOMETRIC BIOSENSORS• The best known potentiometric sensor is

the Ion Selective Electrode (ISE).

• Solvent polymeric membrane electrodesare commercially available and routinelyused for the selective detection of severalions such as K+, Na+, Ca2+, NH4

+, H+, CO32-)

in complex biological matrices.

• The antibiotics nonactin and valinomycinserve as neutral carriers for thedetermination of NH4

+ and K+, respectively.


Ag/AgCl reference electrode

Internal aqueousfilling solution

Membrane/salt bridge

Porous membrane containing ionophore

Liquid ion exchanger

Typical Cell

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POTENTIOMETRIC BIOSENSORS• ISEs used in conjunction with immobilized

enzymes can serve as the basis of electrodes that are selective for specific enzyme substrates.

• The two main ones are for urea and creatinine.

• These potentiometric enzyme electrodes are produced by entrapment of the enzymes urease and creatinase, on the surface of a cation sensitive (NH4

+) ISE.


POTENTIOMETRIC BIOSENSORS

Urea + H2O + H+ urease 2NH4+ + HCO3

-

Creatinine + H2O creatininase N-methylhydantoin + NH4+

Penicillin penicillinase Penicillonic Acid

In contact with pH electrode.


1.3 Conductometric transduction

This is a technique where the changesin ionic concentrations are measured. If the biocatalyst producesionic products, or consumes ions, and the support solution haslow electrical conductivity, this is often a convenient and simpletechnique.

Conductivity is a measurement of the ability of a solution to conduct anelectric current. Instruments measure conductivity by placing two plates of conductive material with known area and distance apart in a sample. Then a voltage potential is applied and the resulting current is measured. Using Ohms law


Present status of electrochemical biosensors


Some electrochemical biosensors for detection and estimation of analytes


2. Optical Transducers

Principle: Optical sensors rely on the optical transduction of the signal and comprise ultraviolet, visible and infrared spectrophotometry in transmission or reflectance modes. The relationship between the incident light intensity and the transmitted radiation is given by the Lambert-Beer law


Optical Transducers

Optical methods have been used classically to monitoranalyte concentrations. Properties like absorption, refractiveindices, fluorescence, phosphorescence,chemiluminescence, etc., can be used in order to monitorthe biological recognition in biosensors. The devices can beminiaturized by using optical fibres, which act as lightguides. The detectors are often semiconductor photodiodes.These devices are often used for remote analysis as the lightsignal is resistant to electrical noise.


Examples of Optical Transducers• Optical fibers• Surface plasmon resonance sensors (SPR)• Waveguide based SPR• Integrated interferometers (Mach-Zehnder and Young

interferometers)• Differential mode interferometry• Resonant mirror• Grating coupler• Bidifractive couplers• Optical waveguide lightmode spectroscopy system

(OWLS)• Reflactometric interference spectroscopy (RIfS)


OPTICAL BIOSENSORS• The area of biosensors using optical detection

has developed over the last number of years.

• The basis of these systems is that enzymatic reactions alter the optical properties of some substances allow them to emit light upon illumination.

• Means of optical detection include fluorescence, phosphorescence, chemi/bioluminescence...


OPTICAL BIOSENSORS• Advantages of optical biosensors include

due to fibre optics, miniaturisation is possiblein situ measurements are possiblein vivo measurements are possiblediode arrays allow for multi-analyte detectionsignal is not prone to electromagnetic interference

• Disadvantages include:ambient light is a strong interferentfibres are very expensiveindicator phases may be washed out with time


• Fibre optics are a sub-class of optical waveguides which operate using the principle of total internal reflection.

• Light incident on the interface between two dielectric media is either reflected or refracted according to Snell’s Law.

OPTICAL BIOSENSORS


Working of SPRThe SPR is an optical phenomenon due to a charge density oscillation at the interface of a metal and a dielectric, which have dielectric constants of opposite signs


Surface Plasmon Resonance(i) Design: prisms coated with a thin film of metal, usually

gold or silver (~55-nm thick), typically known as the Kretschmann

configuration;by prism coupling: Working: a light wave passes through a high-refractive-index prism and is totally reflected at the prismϪmetal layer;

(ii) Design waveguide coupled with a thin film of metal;

Working by optical waveguide: the light wave is guided by an optical waveguide and, when entering the region with a thin metal layer, it evanescently penetrates through the metal layer exciting an SPR at its outer boundary;

(iii) Design metallized diffraction gratings where the metal thickness can be much greater (up to 150- nm thick)

Working: by light diffraction on a diffraction grating: thecomponent of the wave vector of the diffracted waves parallel to theinterface is diffraction-increased by an amount that is inverselyproportional to the period of the grating and can be matched to that ofan SPR.


Typical signal verses time curve

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Typical Response Curve


Overview of optical biosensors


Comparison of sensitivities for different optical biosensors

SPR has been one of the leading transducer techniques due to its extremely high sensitivity, offering detection limits up to few ppt (pg mL−1 )


Bacteriorhodopsin:Biophotonic Material

Natural Transducer: Converts light into chemical energy

Artificial Retina10/5/2009 52

4. CalorimetricPrinciple

Calorimetric sensors are based on measurement of the heat produced by

the molecular recognition reaction and the amount of heat produced is cor-

related to the reactant concentration. Calorimetry can be used for direct measurment of heat changes associated with thermochemical proceses ( Grime, 1985).

The metabolic activity of the biocomponent causes an increase in temperature, which is transformed into a detectable electrical signal.

• Enzyme-catalysed reactions exhibit thesame enthalpy changes as spontaneouschemical reactions.

• Considerable heat evolution is noted (5-100kJ/mol).

• Thus, calorimetric transducers areuniversally applicable in enzyme sensors.

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Calorimetric Transducers

• The thermal biosensors constructed have been based on:

direct attachment of the immobilised enzyme or cell to a thermistor

Immobilisation of the enzyme in a column in which the thermistor has been embedded.

ΔT = nΔH/cp


Enzyme Substrate -ΔΗ (kJ/mol)Catalase Hydrogen peroxide 100.4

Cholesterol oxidase Cholesterol 52.9

Glucose oxidase Glucose 80.0

Hexokinase Glucose 27.6

Lactic dehydrogenase Pyruvate 62.1

β - Lactamase Penicillin G 67.0

Urease Urea 6.6

Uricase Uric acid 49.1

Typical Values of enthalpy

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RecorderBridge/Amplifier

Enzyme reactor

Thermistor

Aluminium block

Heat exchanger

Bufferstream

Sample

polyurethane insulation10/5/2009 56


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Thermistor

They are devices used to monitor the enthalpy change of an enzyme-based reaction. If the enthalpy change in the biocatalytic process is significant, then the temperature of the transducer (thermistor) is changed and this change can be monitored. A thermistor is nothing but a miniature resistance thermometer with high sensitivity.

Bimetallic strip Two metals with different coefficient of expansion are adjoined. On heating the entire strip bents toward one end.Liquid and gas expansionMetal-resistance

PyroelectricThermal transducersRadiant heat energy sensing


Semiconductor-based Electrodes

These are transistor-like devices (usually npn type), and themost common configuration is the field effect transistor(FET). The biological element can be immobilized at thesurface of the gate of the FET to obtain EnFET (EnzymeField Effect Transistor). They operate in the potentiometricMode.


Liquid and gas expansion

Bimetallic strip


Thermocouples

The EMF (in mV) at 100 C different for platinum/metal thermocouples.


Metal-resistance sensors


Sound, infrasound and ultrasound

Audio to electrical sensors and transducersTHE CARBON MICROPHONETHE MOVING IRON (VARIABLE RELUCTANCE) MICROPHONEMOVING COIL MICROPHONERIBBON MICROPHONEPIEZOELECTRIC MICROPHONESCAPACITOR MICROPHONES


Sound, infrasound and ultrasound

Electrical to audio transducersTHE MOVING-IRON TRANSDUCERTHE MOVING-COIL TRANSDUCERRIBBON LOUDSPEAKERSPIEZOELECTRIC LOUDSPEAKERSCAPACITOR TRANSDUCERSULTRASONIC TRANSDUCERSINFRASOUND


Summary• Transducer• Types of Transducers : Electrochemical, Optical,

Thermal..• Examples• Next Week.....

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Transducers in Biosensors II(Week 5 )Transducers Examples

Piezoelectric Acoustic-Wave Microsensors, quartz crystal microbalance (QCM), Film bulk acoustic wave resonators (FBARs)

SemiconductorLED, p-n diodes or bipolar junctiontransducers, metal oxide semiconductor (MOS) capacitor, field effect transistor (FET)

Impedimetric

Mechanical Cantilevers

Molecular electronics based transducers Nanotransducers


Key ReferencesAdvances in Biosensors: B. D. Malhotra and Anthany P. F. Turner, Volume 5, 2003, Perspectives in biosensorsEnzyme and microbial biosensors: techniques and protocols , Ashok Mulchandani, Kim R. Rogers - 1998 - Science - 264 pages, Principles of Enzyme Biosensors, Ashok Mulchandani & Kim Rogers, Humana PressSensor Technology Handbook, Jon S. Wilson - 2005 -Technology & Engineering - 691 pages,6.5.Transduction Mechanisms in Biosensors Conventional Transducers.Mediated Biosensors,Asha Chaubey and B.D.Malhotra,Biosensors & Bioelectronics, 2002, 17,Nos.6-7,pp441-456


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Transducers

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