Chemical Bio Sensors

8/4/2019 Chemical Bio Sensors

1/23

A chemical Biosensor is sensor that producesan electrical signal proportional to theconcentration of biochemical analytes (Asubstance or chemical constituent that isundergoing analysis)

These biosensors use chemical as well asphysical sensors principles in their operation


2/23

The important critical care chemical analytesin our human body are the blood levels ofpH,PO2,PCO2,hematocrit,totalhaemoglobin,o2 saturation etc

The fast and the accurate diagnosis of theseblood levels are vital in diagnosis andtreatment of many pathological conditions

These measurements are usually made withspecimens of arterial blood


3/23

Oxygen is carried in the blood in two states,98%of oxygen is combined with hemoglobin in thered blood cells, and the remaining is in theplasma

The saturation of oxygen bound to Hb arterialblood is defined as So2%=(hbo2)/(total Hb)* 100

The total content of o2 in the blood is directlyrelated to the so2 for any given Hb concentration

Arterial Po2 determines the efficiency of alveolarventilation, while so2 indicates mount of o2 perunit of blood


4/23

For young adult the normal range of po2 isfrom 90 to 100mm of Hg

Decrease in po2 level can cause decrease inoverall ventilation, obstruction of airways,lung disease

The pco2 level in young adults varies from 35to 40 mm Hg,it is also an indicator of

ventilation


5/23

pH is a quantitative unit of measure whichdescribes the degree of acidity or alkalinity ofa substance. It is measured on a scale of 0 to14.

The term pH is derived from p themathematical symbol of the negativelogarithm, and H the chemicalsymbol ofHydrogen.

The formal definition of pH is the negativelogarithm of the hydrogen ion concentrationi.e. pH = -log10[H]+


6/23

A galvanic or voltaic cell produces an electric currentbecause of spontaneous reactions at the twoelectrodes which may be in contact with the same ordifferent electrolytes.

In the latter case, the two electrode compartments

are connected by a salt bridge for electrical contactbetween the electrolytes. A salt bridge is a double bent glass tube containing a

concentrated solution of an electrolyte withcomparable ionic mobilities of the cation and theanion.

The solution is, in general, made up in a gel tominimize the mixing of the solutions of the two halfcells.


7/23

An electrode, i.e., a metallic conductor incontact with an electrolyte, develops anelectrical potential called the electrodepotential.

In a galvanic or voltaic cell, the e.m.f. of thecell is the algebraic sum of the electrodepotentials of the two electrodes withuniversally agreed sign conventions.

Each of the electrodes in contact with theelectrolyte solution is also called a half cell.


8/23

As a galvanic cell functions, i.e., delivers current,at the two electrodes spontaneous chemicalreactions take place, which when combined givesthe overall chemical reaction called the cellreaction.

It is the energy of this chemical reaction thatgenerates the e.m.f. of the cell.

In such cases, the cell is called a chemical cell. Insome cells there is no overall chemical reactionbut the e.m.f. is generated because of thedifference in the concentrations of either theelectrolytes or the electrode


9/23

The pH value of a solution is measured by usingpH electrode.

It essentially consists of a pair of electrodes:measuring and reference electrode, both dippedin the solution of unknown pH.

These two electrodes essentially form two half-cells; the total potential developed is thedifference between the individual electricpotential developed in each half cell.

While the potential developed in the reference

cell is constant, the measuring cell potential isdependent on the hydrogen ion concentration ofthe solution and is governed by Nernstsequation


10/23

E=Eo+RT/nf *ln( a*c)whereE= e.m.f of the half cell

Eo= e.m.f of the half cell under saturated condition

R= Gas constant (8.314 J/0C)

T= Absolute temperature (K)

N= valance of the ion

F= Faraday Constant = 96493 Ca= Activity co-efficient

C= molar concentration of ions.


11/23

A half cell is a structure that contains aconductive electrode and a surroundingconductive electrolyte separated by anaturally-occurring double layer

A double layer (DL, also called an electricaldouble layer, EDL) is a structure that appearson the surface of an object when it is placedinto a liquid.


12/23

HCL

Ag/Agcl


13/23

Most modern pH electrodes consist of a singlecombination reference and sensing electrodeinstead of separate electrodes.

Any pH electrode requires both a sensingelectrode and a reference electrode.

The sensing electrode consists of a thinhydrogen permeable membrane containing aknown solution and an electrode. The knownsolution used is hydrochloric acid and generally aion sensitive(specific) electrode made up of glass

is used. This is a electrode which will react to anyextend only with a specific ion. A solution whose pH is to be measured is kept

outside the electrode or membrane


14/23

The membrane of the sensing electrode allowshydrogen ions to slowly pass, creating apositive voltage across the membrane.

The voltage created in this electrode This voltage is then compared to the voltage in

the reference electrode. A reference electrodeis usually an Ag/Agcl electrode or a calomelelectrode The voltage difference between thetwo electrodes is then used to determine thepH of the unknown solution using the Nernstequation

The potential developed is generally measuredby using a PH meter


15/23

A salt bridge is placed in the reference electrode to preventchemical constituents of specimen from affecting voltage ofreference electrode

The voltage produces is affected by the temperature change

,thus generally a water bath is used to maintain a constanttemperature or a temperature correction has to be provided


16/23


17/23


18/23

Advantages of po2 measurement:


19/23


20/23


21/23

A MOSFET is a voltage controlled device, and requires only a smallinput current

The switching speed is very high and the switching time are of theorder nanoseconds

It has three terminals: Gate, source and drain

The gate to source voltage is either positive or negative ,If VGS isnegative the value of RDS is high and IDS=0


22/23

flows


23/23

Chemical Bio Sensors

Documents

Transcript of Chemical Bio Sensors