bio electronic devices

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BioelectronicDevices:Biosensors



What are Biosensors?

s Biosensors are small analyticaldevices which identify the analyte

and modify it into a productdetectable by a transducer whichtransforms it into an electrical orelectronic signal.



Principles of Biosensors

The first link of a biosensor is the bioreceptor, whichhas a particularly selective site that identifies the

analyte(substrate). The bioreceptor (biologically activesubstance) ensures molecular recognition and thentransforms it or modifies it into a product detectable bya transducer.

This modification of substrate by the receptor is abiochemical modification producing a physicochemicaleffect. Transducer then transforms this modificationinto an electrical or electronic signal.



Biosensor

Enzymes – Electrodes

microorginisms – Transistors

Antibodies – Thermistors Amp

Receptors optical fibers

Cells and Tissues

Sample Bioreceptor Transducer Signal

Schematic representation of biosensor



Transduction mechanism of biosensors: the total effect of a biosensor is to transform a

biological event into an electrical or electronic signal.

Biological

membrane

Bio-chemical signalS

H+

S D

P-Si

n+ n+

Bindingsites



Bioreceptors

Bioreceptor is made of highly specialized

macromolecules with the appropriatespecificity and sensitivity. The following

biocomponents are generally used as

BIORECEPTOR



Different Bioreceptors

s Enzymes: These are proteins thatlower the energy threshold at

which a given reaction takesplace.A large number of enzymesare commercially available, forexample, glucose oxidase and

urease.

s Antibodies(Immunoreceptors): Theantibodies are glycoproteins

produced by the immune system



.Receptors: The regulation of biological

processes at the molecular level is

based on specialized protein structures,called receptors, able to recognize a numbeof physiological signals. The acetylcholine

receptor is the best known receptor in thefield of neurotransmission.



Biodetection TechniquesA natural event is characterized by certainproperties, which can be exploited for its

identification using biodetection methods.Thesenses of living organisms are the most apttools for the specific recognition of theseproperties. Out of our five senses, sight, touchand hearing exploit preferentially theprinciples of physics, whereas smell and tastemake more use of chemical reactions.Thus in

living organisms, the information istransmitted through an assembly of physicochemical systems.In reality, biosensoris also an assembly of physicochemical

systems. In a biosensor, the transducer plays



Electrochemical Detection

(a) Amperometric techniques: Thistechnique measures the intensityof current,generated by oxidation-reductions induced by thebiological component, toward afixed electrode. Oxidation orreduction of a species is performedby a working electrode, and asecond electrode acts as a

reference.Example:-Glucose-



Gluconic acid + H2O2

H2O2 2H+

+ O2 +2e-

Glucose +O2

+0.65V

Glucose oxidase

Fig(a): Reactions in a glucosesensor

Here either the H2O2 or the amount of oxygen

consumed during the oxidation of glucose can bedetected. In the case of H2O2, during

electrolysis ,the working electrode(platinum) actsas an anode

because it is polarized to a positive potential+0.65 V



(b) Potentiometric technique:

This technique measures the

difference in potential between twoelectrodes immersed in solution.Oneof the electrodes probes the solution,

while the other serves as thereference. The reference electrodepotential is independent of itsenvironment while the potential of theprobe electrode is the potential at theinterface between the solid and liquidphases.Glass pH electrode, MOSFET

sensors are based on this technique.



Thermometric Detection

This method detects the

variations inenzymatic reactionenthalpies.Thermistors

or thermopiles are used to detectsmallvariations in temperature.



Photometric Detection

This method uses the principle of

emission, absorption orscattering of light and makeslarge use of optical fibers



Classification of Biosensors

Biosensors can be classified according to thBioreceptor used for detection and sensing o

can be classified according to the type

of transducers used.Table 1 shows theclassification of biosensors with an idea

of the approximate state of research as a

function of the different possibilities of combination between various bioreceptors

and transducers.



Bioreceptor/biosensor

Transducer

Enzyme/ Enzymatic Antibodies

/

immuno

Cell/ Cell Tissue/ Tissue

Amperome

tric

XXX XX XX X

Electroch

emicalcell

Potentiome

tric

XXX XX XX X

ISFET XX X

Thermometric X X

Optical X X

Piezoelectric X X

Table 1: Classification of Biosensors

X: State of fundamental research

XX: Research and Development of prototypes

XXX: Research and commercial Devices



Bioelectronic sensor



ISFET: Transducer for Bio-electronic

device

The Ion-Sensitive Field Effect transistors (ISFETS) aresensors to measure the level of specified ions inaqueous solution. In the case of ISFET the metallic gateof MOSFET is replaced by a specific oxide-coatedgate,that is sensitive to hydrogen ion concentration.

When immersed in a liquid, the electrical circuit Vgs isclosed with the reference electrode and the hydrogenion concentration in solution can influence the drain-source current. Ions or other charged particles do nothave to pass through the ion sensitive membrane. Thecontrol of drain current is solely based on electrostaticeffects. The interaction between the ions and the thin

film changes the properties of the insulator -electrolyteinterface, which leads to a modification of the thresholdvoltage, and hence a change in the drain current.



MOSFET to ISFET



What is BIOFET?

The biosensor which uses ISFET as atransducer is called BIOFET

s ENFET: Glucose sensor, Ureasensor, Creatinine sensor etc.

s IMMUNOFET:s DNAFET



What is ENFET

An ENFET is a biosensor which uses enzyme as

bioreceptor. It is constructed by placing anenzymatic membrane over the ion-selectivemembrane of an ISFET. The product of theenzymaticreaction chosen for measurement is detected by

the ISFET

nnSiO2

S D

P-S i

Enzyme

VG Reference electrode

ID

VD



S Penzyme

Ssolution

Transducer(ISFET)

Figure: Schematic representation of the diffusion of the substrate S and thproduct P in the enzymatic layer on a transducer.

Principle of operation

The sensitive surface of the transducer is in contact with the enzymatic lay The external surface of the enzymatic layer is immersed in the solutioncontaining the substrate.



Steps required during the operation:

transport of the substrate from the bulk of the solution towards the

ymatic layer.

iffusion of the substrate within this layer, accompanied by theymatic transformation of the substrate into reaction product.

igration of the product towards the transducer,

onversion of the concentration of the product at the interface of enzymatic layer and transducer into an electrical signal by the transd



Theoretical aspects

The presence of the enzyme ensures the transformation of the subsinto the reaction product according to the following reaction:

k+1 k+2

E + S ↔ ES → E + Pk-1

where E represents the enzyme, S the substrate and P the reactionproduct; k+1, k-1 and k+2 are the rate constants of the reactions.



One example

Glucose oxidase catalyses the transformation of glucose into gluconolactone which in turnhydrolyses into gluconic acid. Due to acid

formation, the local pH changes which can bedetected by the adjacent ISFET.

Glucose + O2 + H2O → gluconic acidH2O2

Glucose oxidase



Some basic enzyme reactions which areused for ENFET creation

H2N

Urease

C = O + 2H2O + H+ 2NH4+ + HCO3-

H2N

CD

Creatinine + H2O N-methylhydantion + NH4+

CD is the creatinine deiminase



These reactions generate or consumeprotons, changing the pH inside the

enzyme membrane, which isregistered by the ISFET



Figure: Schematic representation of ENFET from R&D Instituteof Microdevices (Kiev,Ukraine).

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