biosensors lecuture 12

electrochemical sensors

electrochemical sensors

electrochemical transducers

• convert the concentration of ions or chemicals into an electrical signal

• electrochemical sensors + bioreceptors = electrochemical biosensors

types of electrochemical sensors

potentiometric ( voltage)

amperometric (current)

conductometric (resistance/conductance

impedimetric (impedance)

electrochemical cell

device that generates electrical voltage and current

• REDOX reaction ( reduction and oxidation )

oxidation and reduction

oxidation = loss of electrons

reduction = gain or electrons

daniell cell vs galvanic cell

daniell cell has 1 tube with 2 solutions seperated by bridge

galvanic cell has 2 tubes with different solutions connected by bridge

Ion Selective Electrodes (ISE’s)

• potentiometric

half cell design to take only a specific type of ions to pass through a ion-selective membrane and measure the concentration of that specific ion

• very low detection limit

• not available for all ions

Saturated Calomel Electrode (SCE)

Colomel: mercury chloride

• used to determine the standard half-cell electrode potential

• porous plug is immersed into the other half-cell to determine the electrode potential

• potential health hazards from mercury

Silver/Silver Chloride Electrode

• used to determine the standard half-cell electrode potential

• porous plug is immersed into the other half-cell to determine the electrode potential

Potentiometric biosensors

ISEs in conjunction with immobilzed enzymes are used to selectively detect specific enzyme substrates

what are enzymes and what do they do

• enzymes are globular proteins that work as catalysts of chemical reactions

• they speed up chemical reactions without being altered themselves

how to enzymes and substrates interact

enzymes act on the substrate (S) to yield the product (P)

enzymes active sites

active site is the region on the surface of the enzyme to which substrate(s) bind and which catalyzes the reaction

enzyme-substrate specificity

lock-and -key mechanism:

the substrate and the active site match each other in two ways

• structurally: 3D active site structure is specific to the substrate, if substrates dont fit they wont react

• chemically: if substrates are not chemically attracted to the active site, they wont react

Denaturation

a structural change in a protein that results in the loss (usually permanent) of its biological properties

• enzymes can be denatured by pH and temperature

what is Cyclic Voltammetry (CV)

linear scanning of potential of a stationary electrode using a triangular (or square) waveform

how does CV work

the current resulting from an applied potential is measured during a potential sweep

what type of data does CV generate

current - potential plot results and is known as cyclic voltammogram

what is CV most commonly used for

CV is the most widely used technique for quantitative analysis of redox reductions

CV triangular waveform

• Forward scan: positive potential sweep from negative to positive results in the oxidation

• Reverse scan: negative potential sweep (from high to low to negative) results in the reduction

know the characteristic parameter of the CV

Anodic peak current

Cathodic peak current

Anodic peak potential

Cathodic peak potential

Amperometric biosensors

• measures electrical current (with no voltage) or electric current change (with a voltage source) caused by REDOX reaction

most well known amperometric biosensor

electrochemical glucose sensor

• enzyme catalyzed redox reactions

glucose

simple sugar that exists in the blood stream

• comes from the food we eat

• fuel that energizes the cells in humans bodies

normal glucose levels

between 65 and 104 mg/dL

purpose of a glucose sensor

for diabetic patients to monitor thier blood glucose levels several times a day

GOx + co-factor FAD+

• GOx requires a co-factor to truly function as a catalyst

• FAD+ : fully oxidized

• FADH2: fully reduced

• H2O2 is oxidized back to H2O at an electrode, generating electrons

• whole cycle generates a change in electrical current (with constant voltage applied)

• required oxygen!!

GDH + co-factor NAD+

• reduced NADH is oxidized back to NAD+ at the electrode, while generating H+ and two electrons

• whole cycle generates a change in electrical current (with constant voltage applied)

• does NOT require oxygen!!

GDH + cofactor PQQ

• reduced PGG is oxidized back at the electrode, while generating H+ and 2 electrons

• whole cycle generates a change in electrical current (with constant voltage applied)

• oxygen is NOT required !

• faster electron transfer rate!

• most common method of commercial glucose meter!

continuous glucose monitoring (CGM)

• insert the electrode into the body

• measure the glucose level of the interstitial fluid (instead of blood)

• replace meter type electrode every 3-7 days

first FDA approved CGM

Eversense CGM system

• june 21st 2018

GlucoTrack

noninvasive glucose sensor

• personal ear clip clipped to earlobe and contains sensors to measure glucose levels in the tissue

• uses ultrasonic, electromagnetic, and thermal technologies

GlucoWise

noninvasive glucose sensor

• use low-power radio waves through a section of the body (fingers or earlobe)

• still under development

Conductometric biosensors

conductivity (inverse of resistivity) is measured to monitor the redox reactions of enzymatic oxidation or other chemical reactions

what does a conductivity cell do

can quantify the extent of the enzymatic redox reaction

• typically measured by a wheatstone bridge

advantages of conductometric biosesensors

• do not need the use of a reference electrode

• operate at a low-amplitude alternating voltage

• inexpensive

• insensitive to light

what are buffers

solutions that resist changes in pH

• mixture of acid and base form of a chemical

• can be adjusted to a particular pH value

why use buffers?

• enzyme reactions and cell functions have optimum pH’s for performance

• important anytime the structure/activity of a biological material must be maintained

PBS

• buffer solution commonly used in biological research

• osmolarity and ion concentration usually matches that of the human body

• water based salt solution

osmolarity

concentration of a solution

what is impedance spectroscopy effective for

effective method for probing the features of surface-modified electrodes

• directly measure the analyte immobilized on the electrodes

how do impedimetric biosensors calculate impedance

impedance is calculated as the ratio between the system voltage phasor and the current phasor, which are generated by a frequency response

• does NOT require a REDOX reaction !