biosensor - PowerPoint PPT Presentation

Title: biosensor


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BIOSENSORS
Prepared by Samaneh Rahamooz Haghighi PHD
student
may2015
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BiosensorsSection 1
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SENSOR

• A small device used for direct measurement of a
  physical quantity of an analyte in a sample
  matrix
• Response is continuous and reversible
• Sample is not perturbed
• Does not require sample collection and
  preparation
• Consists of a transduction element covered by a
  recognition layer
• Recognition layer may be chemical or biological
• Recognition layer interacts with target analyte
• Transduction element translates the chemical
  changes into electrical signals

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History of Biosensors

• First described in 1962 by Dr. Leland Clark
• 1969 a sensor was invented to detect urea
• 1972 the first glucose biosensor commercialized
  by Yellow Springs Instruments

Dr. Leland Clark Jr Father of the biosensor
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1980s ---- Biosensors Would Solve the World's
Analytical Needs ? Industry -- process
monitoring and control, particularly food and
drink ? Medicine -- diagnostics, metabolites,
hormones ? Military -- battlefield monitoring of
poison gases, nerve agents people ? Domestic
-- home monitoring of non acute conditions
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Introduction

• Biosensors 3B
• 90 ? Glucose testing
• 8 - 10 increase in industry per year

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Market Size of Biosensors

• 7.3 Billion in 2003
• 10.2 Billion in 2007 with a growth rate of about
  10.4

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Biosensor Development

• 1916 First report on the immobilization of
  proteins adsorption of invertase on activated
  charcoal.
• 1956 Invention of the first oxygen electrode
  Leland Clark
• 1962 First description of a biosensor an
  amperometric enzyme electrode for glucose.
  Leland Clark, New York Academy of Sciences
  Symposium
• 1969 First potentiometric biosensor urease
  immobilized on an ammonia electrode to detect
  urea. Guilbault and Montalvo
• 1970 Invention of the Ion-Selective Field-Effect
  Transistor (ISFET).

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History of Biosensors

• 1975 First commercial biosensor ( Yellow
  springs
• Instruments
  glucose biosensor)
• 1975 First microbe based biosensor, First
  immunosensor
• 1976 First bedside artificial pancreas (Miles)
• 1980 First fibre optic pH sensor for in vivo
  blood gases (Peterson)
• 1982 First fibre optic-based biosensor
• 1983 First surface plasmon resonance (SPR)
  immunosensor
• 1984 First mediated amperometric biosensor
  ferrocene used with glucose oxidase for
  glucose detection

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• 1987 Blood-glucose biosensor launched by
  MediSenseExacTech
• SPR based biosensor by Pharmacia BIACore
• 1992 Hand held blood biosensor by
  i-STAT
• 1996 Launching of Glucocard
• 1998 Blood glucose biosensor launch by
  LifeScan FastTake
• 1998 Launch of LifeScan FastTake blood
  glucose biosensor
• 1998 Merger of Roche and Boehringer Mannheim to
  form Roche Diagnostics 1
• LifeScan purchases Inverness Medical's
  glucose testing business for 1.3billion
• 2001 To 2015 Microorganism and nano
  technology to biosensors
• Quantomdots,
  nanoparicles, nanowire, nanotube, etc

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Your welcome To this subject
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What are biosensors?

• Devices that analyze biological samples to better
  understand structure and function and for
  diagnostics
• Uses for biosensors
• Molecule analysis (DNA and proteins)
• Food safety
• Diagnostics
• Medical monitoring
• Detection of biological weapons
• Rapid analysis and detection

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Biosensors

• Advantages
• Rapid detection
• Small volumes of samples needed
• Can be used by the patient (blood glucose
  monitor)
• Disadvantages
• Cost
• May require expertise to use
• Sample collection can be painful

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Types of biosensors

• Electrochemical
• Temperature sensitive
• Photosensitive
• Pressure sensitive
• Motion sensitive
• Chemical sensitive

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Category biosensors for biochemical and
biological function and structure

• Biocatalytic (eg, enzymes)
• Immunological (eg, antibodies)
• (DNA Nucleic acid (eg,

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Common biosensors

• Blood glucose monitors
• Heart and blood pressure monitors
• Pacemakers
• HIV and pregnancy tests

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Blood glucose monitors

• Used by diabetics to measure blood glucose
  concentration
• Helps patients determine their insulin dose
• Uses electrochemistry for detection

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biosensors
A biosensor consists of two components a
bioreceptor and a transducer. The bioreceptor
is a biomolecule that recognizes the target
analyte whereas the transducer converts the
recognition event into a measurable signal.
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Schematic illustration of a Biosensor
signal prossing
monitor
amplification
transducer
bioreceptor

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Introduction

• Bioreceptor
• Incorporation of a biomolecule in order to detect
  something

Recognition Layer
Species to be detected (analyte)
Transducer
Electronics
Signal
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bio sample Analyte

• Sugar
• urea
• cholesterol
• ethanol

glutamic acid lactic acid Penicillin toxin

• many amino acids
• Peptide
• vitamin
• aspirin
• phosphate

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bioreceptor

• Enzyme
• Antibody
• (DNA)
• (receptor)
• (microorganism)
• ( tissue)
• (cell)
• (organel)

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Saraju P.Mohanty and Elias Kougianos,2006
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Bioreceptors

• Enzyme

Enzyme is a large protein molecule that acts as a
catalyst in chemical reactions. Enzymes are often
chosen as bioreceptors based on their specific
binding capabilities as well as their catalytic
activity
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Enzyme
Enzymes are folded polypeptides (polymers
of amino acids) which catalyze chemical
reactions without being used up in the
conversion of substrates to products.
Enzymes are proteins with high catalytic
activity and selectivity towards substrates.
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Enzyme
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Advantage and disadvantage of Enzyme
Advantage connected to the object High
selection catalytic activity increase
sensitivity Fastly performance highest consumption
Disadvantage Expensive When immobilization
enzyme on transducer, loses part of its
activitiesBecause inactivity , shortly lose its
activities
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Bioreceptors

• Antibody

Antibodies are biological molecules that exhibit
very specific binding capabilities for specific
structure (antigens).

• Antigen

membrane
It can be recognized by antibody.
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biosensors-based antibody also called
Immunosensors
Antibodies usually immobilize on level of
transducer by the amino, carboxyl, aldehyde,
sulfide groups. Bonding antibodies to the
antigen is stronger and more specific than
bonding substrate of the enzyme,.
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• Advantage
• High selectiveVery sensitiveTheir bond is
  very strong.

Disadvantage Loss of catalytic effect
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Bioreceptors

• DNA structure

Another biorecognition mechanism involves
hybridization of deoxyribonucleic acid (DNA) or
ribonucleic acid (RNA), which are the building
blocks of genetics.

• Four chemical bases
• adenine(A), guanine (G),
  
• cytosine (C), thymine (T)

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• Nucleic acid hybridization

Principles of DNA biosensors
(Target Sequence)
Probe DNA is useful for recognation genetic
disease , cancer, viral infection
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Mark

• Antibody
• Probe DNA

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The complementarity of adenine-thymine and
cytosine-guanosine pairing in DNA forms the
basis for the specificity of biorecognition in
DNA biosensors (Fig. 2).
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receptors
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Introduction to Biosensors
Bioreceptor
Transducer
Absorption
Fluorescence
Antibody
Optical
Interference
potentiometric
Enzyme
Electrochemical
amperometric
conductimetric
Nucleic Acid (DNA)
Mass based
Cell
Temperature based
Dielectric properties
Electric Magnetic
Permeability properties
MIP
Voltage or Current
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• Discriminative Membrance and membrance proce
  are essential component of a biosensors
• Selective prevalence Prevent foulingEliminate
  interferenceControl Emission of
  analyte Preserving the environment enzymes 
  Protection against mechanical stresses

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transducer
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transducer
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• Electrochemical

The basic principle for this class of biosensors
is that chemical reactions between immobilized
biomolecule and target analyte produce or
consume ions or electrons, which affects
measurable electrical properties of the
solution, such an electric current or potential
(Thevenot et al. 1999).

• electrochemical DNA biosensors

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Amperometric Amperometric biosensors are the
most widespread class of biosensors .
Amperometric biosensors are very sensitive
and more suitable for mass production than
the potentiometric ones (Ghindilis et al.
1998).
Platinum, gold, silver, rustproof steel or
carbon based material electrodes
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Potentiometric
This transducer measures difference
in potential that is generated across an
ion-selective membrane separating two solutions
at virtually zero current flow. Nearly all
potentiometric sensors, including glass
electrodes, metal oxide based sensors as well as
ion-selective electrodes, are commercially
available. Moreover, they can be easily
mass-fabricated in the miniature formats
using advanced modern silicon or thick-film
technologies (Koncki 2007).
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Conductometric

• A technique that works on ion changes and
  changes in ion concentration is
  measured.Solutions are containing electron
  conductor ions.The magnitude of the conductivity
  changed by chemical reaction.  In this method,
  platinum or gold electrodes are used to measure
  changes in ion.

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• Piezoelectric (mass-sensitive)

These biosensors are based on the coupling of the
bioelement with a piezoelectric component,
usually a quartz-crystal coated with gold
electrodes.
Many types of materials (quartz, tourmaline,
lithium niobate or tantalate, oriented zinc oxide
or aluminium nitride) exhibit the piezoelectric
effect.
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Calorimetric (thermometric)
These biosensors are constructed by
immobilization of biomolecules onto
temperature sensors. Once the analyte comes
in contact with the biocomponent, the
reaction heat which is proportional to the
analyte concentration is measured. The
measurement of the temperature is via a
thermistor, and such devices are called as
enzyme thermistors. Calorimetric biosensors
were used for food, cosmetics, pharmaceutical
and other component analysis (An- tonelli et al.
2008, Bhand et al. 2010, Ramanathan et al. 2001,
Vermeir et al. 2007).
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Opticometric

• Absorption spectroscopy
• fluorescence spectroscopy
• internal reflection spectroscopy
• Light scattering

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Immobilizatiom
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Immobilizatiom

• The most commonly used immobilization
  techniques for construction of biosensors
  are physical adsorption (Nanduri et al.
  1997), covalent binding (Schuhmann et al.
  1990), matrix entrapment (Gupta and Chaudhury
  2007), inter molecular cross-linking (Nenkova
  et al. 2010) and membrane entrapment (Pancrazio
  et al. 1998, Scouten et al. 1995, Sharma et al.
  2003).

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1. Adsorption

• The physical adsorption utilizes a
  combination of Van der Waals and hydrophobic
  forces, hydrogen bonds, and ionic forces to
  attach the biomaterial to the surface of the
  sensor.
• Many substrates such as cellulose,
  collodion, silica gel, glass, hydroxyapatite
  and collagen are well known to adsorb
  biocomponents.
• This method is very simple, however, employed
  forces are not very strong and biomolecules
  attached by this method may be released or not
  persist.

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2. Covalent binding

• The sensor surface is modified to acquire a
  reactive group to which the biological materials
  can be attached.
• formation of a stable covalent bond between
  functional groups of the bioreceptor components
  and the transducer.
• In case of enzymatic biosensors it is through the
  functional group in the enzyme which is not
  essential for its catalytic activity.
• Usually, nucleophilic functional groups
  present in amino acid side chains of
  proteins such as amino, carboxylic,
  imidazole, thiol, hydroxyl etc.
  (SH,OH,COOH,NH) are used for coupling.

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Covalent binding

• This method improves uniformity, density
  and distribution of the bioelements,
• as well as reproducibility and
  homogeneity of the surfaces.
• Covalent immobilization may decrease or
  eliminate some common problems such as
  instability, diffusion and aggregation, or
  inactivation of biomolecules. This occurs when
  biomolecules are immobilized on sensor surfaces
  by polymer matrices.
• In this method, the enzyme can not be
  washed out
• For this purposes the reagents such as
  glutaraldehyde, carbodiimide, succinimide
  esters, maleinimides and periodate are often
  used for covalent immobilization (Collings and
  Caruso Frank 1997).
• PH and temperature should be low

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3. Matrix entrapment

• In this case biomolecules are trapped
  within the polymeric gel matrix.
• For this method the polyacrylamide, starch,
  alginate, pectate, polyvinyl alcohol,
  polyvinyl chloride, polycarbonate,
  polyacrylamide, cellulose acetate and silica gel
  are often be used.
• Matrix entrapment has disadvantage of possible
  leakage of the biological species during use,
  resulting in a loss of activity (Collings and
  Caruso Frank 1997).

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• The immobilization is done either by physical
  entrapment or chemical attachment.

Physical Entrapment
Bioreceptor (Antibody, Enzyme, Cell, )
polymer solution ? polymerization
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4. Cross-linking

• For intermolecular cross-linking of
  biomolecules biofunctional or multifunctional
  reagents such as glutaraldehyde, hexamethylene
  di-isocyanate, 1,5-difluoro
  2,4-dinitrobenzene and bisdiazobenzidine-2,2-d
  isulphonic acid, etc., are used.
• The most common cross-linking agent in
  biosensor applications is glutaraldehyde,
  which couples with the lysine amino groups
  of enzymes.
• bridging between functional groups on the outer
  membrane of the receptor by multifunctional
  reagents to transducer. The cells can be bounded
  directly onto the electrode surface or on a
  removable support membrane, which can be placed
  on the transducer surface

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disadvantages the enzyme layer
formed is not rigid there are higher
demands for amount of biological material
cross-linking can cause the formation of
multilayers of enzyme, which negatively
affects the activity of the immobilized layers.
Moreover larger diffusional barriers may
delay interactions (Collings and Caruso
Frank 1997).
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• 5. Encapsulation
• In this method a porous encapsulation
  matrix (e.g. lipid bilayers) is formed around the
  biological material and helps in binding
  it to the sensor.
• -Cellulose acetate- Polycarbonate-
  Collagen- Teflon
• Other approach for encapsulation uses solgel
  method for the immobilization of biological
  molecules in ceramics, glasses, and other
  inorganic materials using.
• These matrices allow optical monitoring of the
  chemical interactions since they are optically
  transparent. The solgel process can be
  performed at room temperature and which
  protects biomolecules against
  denaturation.

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microencapsulation
Biomolecules immobilized by this procedure are
very stable but achieving of sol-gels with
reproducible pore sizes seems to be still
an obstacle. Problems such as diffusional
limitations inside the porous network,
brittleness of the glassy matrix,
reproducibility or discrepancies in the
preparation procedures has to be solved before
this procedure can be used for routine
application (Collings and Caruso Frank 1997).
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Two factors play a role in the design of a
suitable Biosensors

• Appropriate method immobilized bioreceptors in
  solid surface that will extend the life ,
  sensitivity and stability
• Select the appropriate trancducer

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• Amplifier

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The main tasks in the development of a
Biosensor selection an appropriate
bioreceptor molecules(biochemistry and biology)
Select an appropriate immobilization (chemistry)
Select a suitable transducer (electrochemistry
and physics) Biosensors are designed according
to the measurement range (kinetics and mass
transfer )To minimize interference and packaging
Biosensors
Thus, interdisciplinary cooperation is essential
for the successful development of Biosensors
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The accuracy and reproducibility
Insensitive to temperature
Insensitive to environmental interference
The response rate
Requirements needed for successful
commercialization of biosensors
Costs and capital Biosensor
Prevention of pollution
Physical strength
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Application in agriculture and food products
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• Biosensor Using a banana (bananatrod)Biosensors
  detector soil pathogensBiosensors tracking E.
  coliBiosensors mercuryBiosensors to detect
  neurological defectsGold Biosensor to detect
  cancer cellsWarning sensors thirstBiosensor for
  measuring aflatoxin Biosensors ureaBiosensors
  penicillinBlood Glucose Biosensors

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NANOBIOSENSORS
Section2
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Abstract

• The revolution of nanotechnology in molecular
  biology gives an
• opportunity to detect and manipulate atoms and
  molecules at the
• molecular and cellular level.

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What is a Nanobiosensor?

• A biosensor is a measurement system for the
  detection of an analyte that combines a
  biological component with a physicochemical
  detector, and a nanobiosensor is a biosensor that
  on the nano-scale size.
• Nanobiosensor
• Transducer Detector Biological Recognition
  Element (Bioreceptor)
• 
  Living biological system
• (cell, tissue or whole organism)
• 
  Biological molecular species
• (antibody, enzyme, protein)

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• Principle of Detection
• Piezoelectric Mass
• Electrochemical Electric distribution
• Optical Light intensity
• Calorimetric Heat
• Types of Nanobiosensors
• Optical Biosensors Nanotube Based Biosensors
• Electrical Biosensors Viral Nanosensors
• Electrochemical Biosensors Nanoshell
  Biosensors
• Nanowire Biosensors

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• Optical Nanobiosensors
• A sensor that uses light to detect the effect of
  a chemical on a biological system. Kopelman et
  al.
• The small size of the optical fibers allow
  sensing intracelular physiological and biological
  parameter in micro-environment.
• Two kind of fabrication methods for optical
  fiber tips
• 1) Heat and Pull
  Method
• 2) Chemical
  Etching

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• Nanowire Field Effect Nanobiosensors(FET)
• Sensing Element
• Semiconductor channel (nanowire) of
  the transistor.
• The semiconductor channel is fabricated using
  nanomaterials such
• a carbon nanotubes,metal oxide nanowires or Si
  nanowires.
• Very high surface to volume radio and very large
  portion of the atoms are located on the surface.
  Extremely sensitive to environment

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Applications of Nanobiosensors

• Biological Applications
• DNA Sensors Genetic monitoring, disease
• Immunosensors HIV, Hepatitis,other viral diseas,
  drug testing, environmental monitoring
• Cell-based Sensors functional sensors, drug
  testing
• Point-of-care sensors blood, urine,
  electrolytes, gases, steroids,
• drugs, hormones, proteins, other
• Bacteria Sensors (E-coli, streptococcus, other)
  food industry,
• medicine, environmental, other.
• Enzyme sensors diabetics, drug testing, other.
• Environmental Applications
• Detection of environmental pollution and toxicity
• Agricultural monitoring
• Ground water screening
• Ocean monitoring

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Future Application

• Cancer Monitoring
• Nanobiosensors play a very important role for
  early cancer detection in
• body fluids.
• The sensor is coated with a cancer-specific
  antibody or other
• biorecognation ligands. The capture of a cancer
  cell or a target protein
• yields electrical, optical or mechanical signal
  for detection. Professor
• Calum McNeil detection of cancer proteins that
  cause MRSA
• Identification of Biomarkers
• ?
• Validation of Cancer Biomarkers
• ?
• Cancer Biomarkers
• ?
• Ligands / Probes Developments
• ?
• Cancer Diagnostics Biosensor ? Detector

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Section 3

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• Extensive activity spread throughout Engineering
  and Science.

• Goals

• Biosensor devices (really biointerface devices
  since both sensing and actuating are of
  interest).
• Inference and control algorithms for use with
  such devices.
• Basic science to clinical medicine.

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Recent Work

• Development of rapid lateral flow assays for
• CD4 cells from human blood
• Cryptosporidium parvum
• Pathogenic bacteria (i.e.-Bacillus anthracis,
  Escherichia coli)
• Dengue virus (serotype specific)
• Herbicides (Alachlor, imazethapyr)
• Development of microtiter plate assays for cell
  culture supernatants
• Cholera toxin
• Insulin
• Visualization and quantification of cholera toxin
  binding to epithelial cells
• Encapsulation of DNA oligonucleotides for
  detection of
• protective antigen from B. anthracis allowed
  for multi-analyte
• analysis proof of principle

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Assay Overview

• Biorecognition elements can be conjugated to
  liposomal bilayer
• Antibodies
• Streptavidin or Protein A/G, Enzymes, Other
  Proteins
• Small-molecule analytes
• Fluorophores
• Hydrophilic molecules can be encapsulated within
  interior cavity
• Enzymes
• Fluorophores
• Electrochemical markers
• Oligonucleotides
• Assay types
• Sandwich immunoassays
• Sandwich hybridizations
• Competitive assays
• Assay formats
• Lateral-flow assays
• Microfluidic devices
• Sequential-injection analysis
• Microtiter plates

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mRNA detection

• mRNA extracted from culture and amplified using
  NASBA
• Sandwich-hybridization of amplified RNA target
  between reporter probe-tagged liposomes and
  immobilized capture probes
• Synthetic DNA analogue used for development work
• Assay proven successful for the detection of mRNA
  from E. coli, B. anthracis, Dengue virus and C.
  parvum

• DNA-tagged liposomes in a sandwich hybridization
  assay for B. anthracis atxA mRNA. Limit of
  detection (bkgd3xStDev) 0.11 nM, Assay range
  0.5-50 nM, CV 4.4, Assay time 1.75 hours

Analytical Bioanalytical Chemistry, vol. 386 (6),
p. 1613 1623 (2006)
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• Section4

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Electrochemical DNA Sensors

• Harnesses specificity of DNA
• Simple assembly
• Customizable
• Vast uses for small cost

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DNA Specificity

• Hydrogen bonding between base pairs
• Stacking interaction between bases along axis of
• double-helix

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Principles of DNA biosensors

• Nucleic acid hybridization

(Target Sequence)
(Hybridization)
(Stable dsDNA)
ssDNA (Probe)
Source http//cswww.essex.ac.uk
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Whole Cell Sensors
Source http//www.whatsnextnetwork.com/technology
/media/cell_adhesion.jpg
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Whole Cell Sensors

• Harness normal genetic processes
• May detect dozens of pathogens
• Modifiable/customizable
• Reports bioavailability
• Temperature/pH sensitive
• Short shelf-life

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Whole Cell Sensors
Source Daunert et al., 2000
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Summary

• Use of biomolecules in sensors offers
• Extreme sensitivity
• Flexibility of use
• Wide array of detection
• Universal application
• But still maintains challenges of
• pH/Temperature sensitivity
• Degradation
• Repeatable use
• Regardless of challenges
• Biosensors will permeate future society

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ELECTROCHEMICAL BIOSENSORS

• - Produces an electrical signal that is related
  to the concentration of an analyte
• - Biological recognition processes are converted
  into quantitative amperometric or potentiometric
  response
• - Two categories depending on the nature of the
  biological recognition process
• A. Biocatalytic Devices
• - Utilizes enzymes, cells, tissues as immobilized
  biocomponents
• B. Affinity Sensors
• - Utilizes antibodies, membrane receptors,
  nucleic acids

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ELECTROCHEMICAL BIOSENSORS

• A. Enzyme-Based Electrodes
• - Enzymes are proteins that catalyze chemical
  reactions in living things
• - Based on coupling a layer of an enzyme with an
  electrode
• (enzyme is immobilized on an electrode)
• - Electrode serves as a transducer
• - Very efficient and extremely selective

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A. Enzyme-Based Electrodes Enzymes
(biocatalytic) layer immobilized on an electrode
Electrode
Biocatalytic Layer
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ELECTROCHEMICAL BIOSENSORS
A. Enzyme-Based Electrodes - Polymeric films are
used to entrap enzyme (Nafion, polypyrrole) Enzym
e may be trapped - between electrode and a
dialysis membrane - by mixing with carbon paste -
by surface adsorption - by covalent
binding Applications - Useful for monitoring
clinical, environmental, food samples
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ELECTROCHEMICAL BIOSENSORS
AI. Glucose Sensors - For determination of
glucose in blood - For diagnosis and therapy of
diabetes Glucose O2 ? Gluconic acid
H2O2
Glucose
oxidase
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ELECTROCHEMICAL BIOSENSORS
AII. Ethanol Sensors (Ethanol Electrodes) AIII.
Urea Electrodes AIV. Other Enzyme
Electrodes AV. Tissue and Bacteria Electrodes
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ELECTROCHEMICAL BIOSENSORS
B. Affinity Biosensors - Based on selective
binding of certain biomolecules towards specific
species that triggers electrical signals -
Measures electrochemical signals resulting from
the binding process - Highly sensitive and
selective
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ELECTROCHEMICAL BIOSENSORS
BI. Immunosensors - Based on immunological
reactions - Useful for identifying and
quantifying proteins
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ELECTROCHEMICAL BIOSENSORS
BII. DNA Hybridization Biosensors - Nucleic acid
recognition layers are combined with
electrochemical transducers - Used to obtain
DNA sequence information - Electrochemical
response of DNA is strongly dependent on DNA
structure
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ELECTROCHEMICAL BIOSENSORS
BII. DNA Hybridization Biosensors Other
Applications - For chemical diagnosis of
infectious diseases - For environmental
monitoring - For detecting drugs, carcinogens,
food containing organisms - For criminal
investigations
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ELECTROCHEMICAL BIOSENSORS
BIII. Receptor-Based Sensors BIV. Molecularly
Imprinted Polymer Sensors
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GAS SENSORS

• -
• For monitoring gases such as CO2, O2, NH3, H2S
• - Device is known as compound electrode
• - Highly sensitive and selective for measuring
  dissolved gases
• - For environmental monitoring
• For clinical and industrial applications
• - Gas permeable membrane (teflon, polyethylene)
  is immobilized
• on a pH electrode or ion-selective electrode
• - Thin film of electrolyte solution is placed
  between
• electrode and membrane (fixed amount, 0.1 M)
• - Inbuilt reference electrode
• - The target analyte diffuses through the
  membrane and comes
• to equilibrium with the internal electrolyte
  solution

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GAS SENSORS
- The target gas then undergoes chemical reaction
and the resulting ion is detected by the
ion-selective electrode - Electrode response is
directly related to the concentration of gas in
the sample - Two types of polymeric materials
are used Microporous and Homogeneous - Membrane
thickness is 0.01 0.10 mm - Membrane is
impermeable to water and ions
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GAS SENSORS
CO2 Sensors NH3 Sensors Other Gas Sensing
Devices NO2 and SO2 H2S HF
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GAS SENSORS
Oxygen Sensors - Based on amperometric
measurements (gas sensors discussed earlier are
potentiometric) - Consists of a pair of
electrodes (Ag anode and Pt cathode) in an
electrolyte solution - Electrodes are separated
by a gas-permeable hydrophorbic membrane -
Membrane may be teflon, silicon rubber,
polyethylene
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114

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