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A biotransducer is the recognition-transduction component of a
biosensor
A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector.
The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell recep ...
system. It consists of two intimately coupled parts; a bio-recognition layer and a
physicochemical
Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical mecha ...
transducer
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, measurement, and contr ...
, which acting together converts a
biochemical signal to an
electronic
Electronic may refer to:
*Electronics, the science of how to control electric energy in semiconductor
* ''Electronics'' (magazine), a defunct American trade journal
*Electronic storage, the storage of data using an electronic device
*Electronic co ...
or
optical
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
signal. The bio-recognition layer typically contains an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
or another binding protein such as
antibody
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
. However,
oligonucleotide
Oligonucleotides are short DNA or RNA molecules, oligomers, that have a wide range of applications in genetic testing, research, and forensics. Commonly made in the laboratory by solid-phase chemical synthesis, these small bits of nucleic acids c ...
sequences, sub-cellular fragments such as organelles (e.g.
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
) and receptor carrying fragments (e.g.
cell wall
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
), single whole cells, small numbers of cells on synthetic scaffolds, or thin slices of animal or plant tissues, may also comprise the bio-recognition layer. It gives the
biosensor
A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector.
The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell recep ...
selectivity and specificity. The physicochemical transducer is typically in intimate and controlled contact with the recognition layer. As a result of the presence and
biochemical
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology an ...
action of the analyte (target of interest), a physico-chemical change is produced within the biorecognition layer that is measured by the physicochemical transducer producing a signal that is proportionate to the concentration of the analyte.
The physicochemical transducer may be electrochemical, optical, electronic, gravimetric, pyroelectric or piezoelectric. Based on the type of biotransducer, biosensors can be classified as shown to the right.
Electrochemical biotransducers
Electrochemical biosensors contain a biorecognition element that selectively reacts with the target analyte and produces an electrical signal that is proportional to the analyte concentration. In general, there are several approaches that can be used to detect electrochemical changes during a biorecognition event and these can be classified as follows: amperometric, potentiometric, impedance, and conductometric.
Amperometric
Amperometric transducers detect change in current as a result of electrochemical oxidation or reduction. Typically, the bioreceptor molecule is immobilized on the
working electrode
The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode sys ...
(commonly gold, carbon, or platinum). The potential between the working electrode and the
reference electrode
A reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each ...
(usually Ag/AgCl) is fixed at a value and then current is measured with respect to time. The applied potential is the driving force for the electron transfer reaction. The current produced is a direct measure of the rate of electron transfer. The current reflects the reaction occurring between the bioreceptor molecule and analyte and is limited by the mass transport rate of the analyte to the electrode.
Potentiometric
Potentiometric sensors measure a potential or charge accumulation of an
electrochemical cell
An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
. The transducer typically comprises an ion selective electrode (ISE) and a reference electrode. The ISE features a membrane that selectively interacts with the charged ion of interest, causing the accumulation of a charge potential compared to the reference electrode. The reference electrode provides a constant half-cell potential that is unaffected by analyte concentration. A high impedance voltmeter is used to measure the electromotive force or potential between the two electrodes when zero or no significant current flows between them. The potentiometric response is governed by the
Nernst equation
In electrochemistry, the Nernst equation is a Thermodynamics#Chemical thermodynamics, chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction (half-cell or electrochemical cell, full cell reacti ...
in that the potential is proportional to the logarithm of the concentration of the analyte.
Impedance
Electrochemical impedance spectroscopy (EIS) involves measuring resistive and capacitive changes caused by a biorecognition event. Typically, a small amplitude sinusoidal electrical stimulus is applied, causing current to flow through the biosensor. The frequency is varied over a range to obtain the impedance spectrum. The resistive and capacitive components of impedance are determined from in phase and out of phase current responses. Typically, a conventional three-electrode system is made specific to the analyte by immobilizing a biorecognition element to the surface. A voltage is applied and the current is measured. The interfacial impedance between the electrode and solution changes as a result of the analyte binding. An impedance analyzer can be used to control and apply the stimulus as well as measure the impedance changes.
Conductometry
Conductometric sensing involves measuring the change in conductive properties of the sample solution or a medium. The reaction between the biomolecule and analyte changes the ionic species concentration, leading to a change in the solution electrical conductivity or current flow. Two metal electrodes are separated at a certain distance and an AC potential is applied across the electrodes, causing a current flow between the electrodes. During a biorecognition event the ionic composition changes, using an ohmmeter the change in conductance can be measured.
Optical biotransducers
Optical biotransducers, used in optical biosensors for signal transduction, use photons in order to collect information about analyte. These are highly sensitive, highly specific, small in size and cost effective.
The detection mechanism of optical biotransducer depends upon the enzyme system that converts analyte into products which are either oxidized or reduced at the working electrode.
Evanescent field detection principle is most commonly used in an optical biosensor system as the transduction principle . This principle is one of the most sensitive detection methods. It enables the detection of fluorophores exclusively in the close proximity of the optical fiber.
FET-based electronic biotransducers
Electronic
Electronic may refer to:
*Electronics, the science of how to control electric energy in semiconductor
* ''Electronics'' (magazine), a defunct American trade journal
*Electronic storage, the storage of data using an electronic device
*Electronic co ...
biosensing offers significant advantages over
optical
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
,
biochemical
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology an ...
and
biophysical
Biophysics is an interdisciplinary science that applies approaches and methods traditionally used in physics to study biological phenomena. Biophysics covers all scales of biological organization, from molecular to organismic and populations. Bi ...
methods, in terms of high sensitivity and new sensing mechanisms, high spatial resolution for localized detection, facile integration with standard wafer-scale semiconductor processing and label-free, real-time detection in a nondestructive manner
Devices based on
field-effect transistors
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of Electric current, current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''dra ...
(FETs) have attracted great attention because they can directly translate the interactions between target biological molecules and the FET surface into readable electrical signals. In a FET, current flows along the channel which is connected to the source and the drain. The channel conductance between the source and the drain is switched on and off by gate electrode that is capacitively coupled through a thin dielectric layer
In FET-based biosensors, the channel is in direct contact with the environment, and this gives better control over the surface charge. This improves the sensitivity of surface FET-based biosensors as biological events occurring at the channel surface could result in the surface potential variation of the semiconductor channel and then modulate the channel conductance. In addition to ease of on-chip integration of device arrays and the cost-effective device fabrication, the surface ultrasensitivity of FET-based biosensors makes it an attractive alternative to existing biosensor technologies
Gravimetric/Piezoelectric biotransducers
Gravimetric biosensors use the basic principle of a response to a change in mass. Most gravimetric biosensors use thin piezoelectric quartz crystals, either as resonating crystals (
QCM), or as bulk/surface acoustic wave (
SAW
A saw is a tool consisting of a tough blade, wire, or chain with a hard toothed edge. It is used to cut through material, very often wood, though sometimes metal or stone. The cut is made by placing the toothed edge against the material and mo ...
) devices. In the majority of these the mass response is inversely proportional to the crystal thickness. Thin polymer films are also used in which biomolecules can be added to the surface with known surface mass.
Acoustic wave
Acoustic waves are a type of energy propagation through a medium by means of adiabatic loading and unloading. Important quantities for describing acoustic waves are acoustic pressure, particle velocity, particle displacement and acoustic intensit ...
s can be projected to the thin film to produce an oscillatory device, which then follows an equation that is nearly identical to the Sauerbrey equation used in the QCM method. Biomolecules, such as proteins or antibodies can bind and its change in mass gives a measureable signal proportional to the presence of the target analyte in the sample.
Pyroelectric biotransducers
Pyroelectric biosensors generate an electric current as a result of a temperature change. This differential induces a
polarization in the substance, producing a
dipole moment in the direction of the temperature gradient. The result is a net voltage across the material. This net voltage can be calculated by the following equation.
[Heimlich et al. Biosensor technology: Fundamentals and applications, Marcel Dekker, INC.: New York, 1990. PP. 338]
where V = Voltage,
ω = angular frequency of the modulated incident,
P = pyroelectric coefficient,
L = film thickness,
ε = film dielectric constant,
A = area of film,
r = resistance of the film,
C = capacitance of the film,
τE = electrical time constant of the detector output.
See also
*
Biosensor
A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector.
The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell recep ...
*
DNA field-effect transistor A DNA field-effect transistor (DNAFET) is a field-effect transistor which uses the field-effect due to the partial charges of DNA molecules to function as a biosensor. The structure of DNAFETs is similar to that of MOSFETs, with the exception of th ...
*
Biointerface
A biointerface is the region of contact between a biomolecule, cell, biological tissue or living organism or organic material considered living with another biomaterial or inorganic/organic material. The motivation for biointerface science stems f ...
*
Electrochemiluminescence
Electrochemiluminescence or electrogenerated chemiluminescence (ECL) is a kind of luminescence produced during electrochemical reactions in solutions. In electrogenerated chemiluminescence, electrochemically generated intermediates undergo a highl ...
*
Bioelectronics
Bioelectronics is a field of research in the convergence of biology and electronics.
Definitions
At the first C.E.C. Workshop, in Brussels in November 1991, bioelectronics was defined as 'the use of biological materials and biological architectu ...
*
Nanobiotechnology
Nanobiotechnology, bionanotechnology, and nanobiology are terms that refer to the intersection of nanotechnology and biology. Given that the subject is one that has only emerged very recently, bionanotechnology and nanobiotechnology serve as blan ...
References
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Biosensors
Biotechnology
Molecular biology