Transducers and sensor systems

2014-01-31

Outline

What is the transducer?

Electrochemical

Optical

Gravimetric

Micromechanical

Magnetic

Thermometric

Sensor System

Of what we will talk?

Main elements of a biosensor.

(a) Biorecognition element(b) Transducer

(c) amplifier(d) Signal converter(e) recorder

What is the Transducer?

The transducer is the component of a biosensor that transforms the chemical/physical changes, resulting from or associable to the interaction of the analyte with the biorecognition element, into another signal (i.e., electrical) that can be more easily measured and quantified.

Label or label-free transduction

Label-free transduction: When a direct measurement of the biorecognition event can be performed:

• Changes in mass (Quartz Crystal microbalance)

• Changes in surface properties (electrical or optical)

Label transduction: When an external (not involved in the analyterecognition process) element is added to generate a readable signal

• Enzymatic label (generate color or electrical signal)

• Fluorescent molecules

Electrochemistry

Electrochemistry is the science studying the chemical changes involving electrons flow (current) between the interface of an electron conductor (the electrode: a metal or a semiconductor) and an ionic conductor (the electrolyte).

Electron transfer plays a fundamental role in governing the pathway of several chemical and biological reactions.

Commonly used Electrochemical techniques

Potentiometric: Take advantage of changes in the equilibrium potential (no current flowing in the system) of the measuring electrode.

Voltammetric: Measure variation of current as the function of an applied potential.

Amperometric: Measure the current associated with a redox process induced my the application of a constant potential.

Impedimetric: Detect variation in the impedance (resistance) of the system interface.

Some Definitions

VOLTAGGE: The voltage is the total energy required to move an electric charge between two points of a circuit or between two electroactive molecules.Unit = Volt (V) Symbol = E

CURRENT: Is the measure of electrons flow through an electrical conductor. Current associated to electrochemical reactions are named Faradic.Unit = Amp (A) Symbol = I (or i)

RESISTANCE: Is the opposition to the passage of an electric current through a conductor. Unit = Ohm (Ω) Symbol = R

Voltage, Current and Resistance related by Ohm’s Law:

V = iR

ELECTRODE: Is the electro-conductive material at which electrochemical reactions take place.

ELECTROLYTE: Is a chemical compound (salt, acid or a base) that, upon dissociation in the solvent (water) provide the ions and allows the current flow in the solvent.

ELECTRICAL POTENTIAL: Is the difference in potential between two point in the circuit, working and reference electrodes, and induce the flow of current.

ANODE: Electrode at which oxidation take place. Current recorded at the anode are considered positive according to international conventions.

CATHODE: Is the electrode at which reduction takes place. The current flowing at the cathode should be considered negative according to international convention.

Reduction or cathodic reactions: result in the consumption of electrons (electrons from external circuit to species in solution).

Oxidation or Anodic reactions: result in the generation of electrons (electrons from the solution to the external circuit).

H2O2 2H+ + O2 + 2e-

O2 + H2O + 4e- 4OH-

Standard reduction potentials

Standard electrode (working electrode)potential is the potential at which anelectrochemical reaction (in the specificcase of the table a reduction) takesplace. This correspond to thework/energy needed to have thereaction occurring.

∆G=-nFE

E (Voltage) is a potential difference this must be measured between 2 points.

Fixed one (Reference electrode) – the potential of the other can be extrapolated.

Reference electrode

Absolute Standard = Standard Hydrogen ElectrodePt, H2 (g, 1 atm) / H+ (1.0M, aq) //

Standard Potential Defined as 0 Volts

Ag/AgCl electrode: most common reference electrode

REFERENCE ELECTRODE: Is an electrode having a well defined and stable equilibrium potential. It is used to set the potential of the working electrode.

Working electrodes

WORKING ELECTRODE: Is the electrode at which the electrochemical reaction takes place.Junction between ionic conductor and electronic conductor.

An interphase region:One side - current carried by ionsOther side - current carried by electrons

Interphase not Interface (2 adjacent regions)

Behavior governed by the Nernst Equation:

E = Eo + 2.303 RT log10(ao/aR)nF

n: number of moles of electrons exchanged in the reaction molR: the universal gas constant; 8.314 J K−1 mol−1

T: Absolute temperature (Kelvin).F: Faraday constant or the number of coulombs per mole of electrons 96467 C mol−1

a: is the activity of the chemical/ion in the solution.

How standard potential and

concentrations are correlated

Electrochemical measurements in which the current is

prevented from flowing (equilibrium condition).

Equilibrium Electrochemistry..Potentiometry

A simple potentiometric biosensor. A semi-permeable membrane (a) surrounds the biocatalyst (b) entrapped next to the active glass membrane (c) of a pH electrode (d). The electrical potential (E) is generated between the internal Ag/AgCl electrode (f) bathed in dilute HCl (g) and an external reference electrode (h).

,

,

,

Electrode response as a function of Urea concentration

Trivedi U.B., Sensors and Actuators B: 140 (1), 2009, 260-266

In the potentiometric urea sensor the urease is used to convert the urea into ammonium that change the local pH at the interface of a glass electrode.

UREASE

(NH2)2CO + 2H2O + H+ HCO3- +

2NH4+ 2NH3 + 2H+

The intensity of the faradaic current is proportional to the concentration of the electroactive species, its diffusion coefficient and diffusion properties (how this molecule move in the solvent to reach the electrode interphase), to the area of working electrode and to the potential applied.

Faradaic processes

Three electrodes electrochemical cell

Potentiodynamic measurements

Electrochemical measurements are performed by changing, at a constant rate, the potential of the working electrode as a function of the time.

Differential pulse voltammetry

Staircase voltammetry

Typical response

Current peak (ip) is proportional to the concentration of the molecule undergoing electrochemical reaction. Ep is a characteristic of the electroactive molecule.

Potentiostatic measurements

Electrochemical measurements in which the current is recorded for a fixed time at a fixed potential. Identification of adequate measurement potential is crucial for optimal performances of the biosensor (High sensitivity and high specificity).

Glucosensor (Clark’s electrode)

Typical response curve of a glucose sensor. Sensor is placed in a buffer solution and stirred using a magnetic stirrer. ( A ) Standard glucose solution is dropped into the solution. ( B ) Sensor is washed using a buffer solution. Enough of the buffer solution is added so that the glucose is totally washed away

D-glucose + H2O + O2 gluconic acid + H2O2

Reduction in O2 concentration

Electrochemical impedance spectroscopy

EIS studies the system response to the application of a small amplitude ac voltage. The measurements are carried out at different ac frequencies.

EIS provides information about the interphase, its structure and reactions taking place at it.

Impedance is the opposition to the flow of alternating current (AC) in a complex system and can be correlated with the Resistance of the system itself.

How can it be used?

Electrochemical device Potentiostat

Research laboratory potentiostat

Point of care electrochemical device

Potentiostat functions by maintaining the potential of the working electrode at a constant level with respect to the reference electrode by adjusting the current at an auxiliary electrode (Counter electrode).

Optical transduction

Optical transduction is taking advantages of some properties of light to transduce a recognition event.

-Absorption and emission of light (fluorescence, luminescence)-Surface plasmonics

Optical detection often do not require physical interphase to take place.

Region of interest

Fluorescence

Fluorescence: is the emission of light by a substance that has absorbed light or other electromagnetic radiation. Emission occurs at a wavelength higher than those absorbed.

DNA Chip; fluorescence detection

Cell imaging using biofunctionalised nano-particles

Fluorescence Resonance Energy Transfer (FRET)

•Donor and acceptor molecules must be in close proximity (typically 10–100 Å).•The absorption spectrum of the acceptor must overlap the fluorescence emission spectrum of the donor (Figure 1).•Donor and acceptor transition dipole orientations must be approximately parallel

Quenching

Colorimetric

Lateral flow: Pregnancy testsAggregation assay

Au nano-particles when aggregate change the Wavelength of the light that adsorb.Solution change colour; from red to blue.

ELISA Colourimetric assay

In ELISA colorimetric assay the colour is associated to the conversion of a substrate by a label (enzyme).

Hardware?

http://www.files.chem.vt.edu/chem-ed/spec/uv-vis/singlebeam.html

Laser,

Lamp

Array of Diodes

CCD Camera

Photo counter

Spectrophotometer Fluorescence microscopeConfocal microscope

Spectrophotometer platereader

Luminescence

Luminescence is the emission of light resulting from a chemical reactions, electrical stimulation.

• No need for a light source

• Chemically or electrically

generated light is measured

by the use of photo counter.

H. Dai et al. / Electrochemistry Communications 11 (2009) 1599–1602

Surface Plasmon Resonance (SPR)

Plasmonics are oscillations, along a preferential axes, of the electrons in a thin layer of a conductive metal (Au, Ag).

The phenomena of surface plasmon resonance occurs when a polarised light strikes, at a well defined angle (resonant angle), a thin layer of a metal after crossing a media with higher refractive index.

The resonance angle is strongly dependent from the refractive index of the metal surface: changes in the refractive index of the metal surface (for example binding of biomolecules) will result in changes in the resonance angle.

BIACORE 3000

Sample to sensor:

• Automatic sampler• Pumps• Injection system• Microfluidics

Piezoelectricity was discovered in ~1880 by the Curie brothers (Pierre and Jacques).

The word “Piezoelectricity” originates from the Greek word “piezein”, which means “to press”. The Piezoelectric effect occurs in crystals without a centre of symmetry.

When pressure is applied, the crystal lattice is deformed in such a manner that a dipole moment arises.

Piezoelectric Devices

Piezoelectric Quartz Crystals (PQC)

A PQC system consist of a piezoelectric crystal sandwiched between two electrodes (usually Au).

When an alternating potential difference is applied between the electrodes a distortion of the physical orientation of the crystal lattice is generated, resulting in a mechanical oscillation and of a shear wave across the quartz disk at a characteristic vibrational frequency (i.e. the crystal’s natural resonant frequency).

QCM Sensor in Gas Phase

QCM sensing device consists of an PCQ system coated with a substrate (recognition element) capable of adsorbing the compounds to be measured. The resonant frequency of the crystal will change as the mass of the device increases, according to the Sauerbrey equation (valid for gas phase):

Where: DF: measured frequency shift, in Hzf0

2: the fundamental resonant frequency (squared), in HzDm: mass change, in gA: piezoelectric active area, in cm2

µq: shear modulus of quartz (2.947 × 10 11g)ρq: density of quartz (2.648 g cm -3)C: mass sensitivity constant in sg-1

mCA

mfF

qq

∆−=∆−=∆ρµ

202

QCM Sensor in Liquid Phase

For QCM working in liquid phase the Sauerbrey’s mass relation cannot be applied.

The frequency shift is affected by many other difference parameters of the solution in contact to the transducer:

Viscosity (Interfacial viscosity described in terms of hydrophilicity and hydrophobicity)

Density

Temperature

Polarity

Uniformity of crystal coating

But relation between resonant frequency and mass still present.

mCF ∆−=∆

QCM for DNA hybridisation monitoring

Hybridisation-Regeneration Cycle

Shear-Horizontal Surface Acoustic Wave (SH-SAW) Devices

SH-SAW substrates:

Lithium niobate (LiNbO3), 41°°°°-

rotated Y-cut, X-prop.

Lithium tantalate (LiTaO3), 36°°°°-

rotated Y-cut, X-prop.

Quartz, ST-cut, Z-prop.

30 – 500 MHz operating frequency

Suitable for liquid testing More sensitive than QCM

Advantages

Acoustic methods are:• rapid,• use very small amount of sample,• don’t need the use of hazardous material, very easy to

miniaturise.• Comparing to the other physical transducers capable of

measuring surface mass changes the piezoelectric devices aresignificantly less expensive.

• In many cases acoustic sensors can accomplish the sameresults as the SPR (Surface Plasmon Resonance).

• Detection can be done in real time and in native conditions.

Disadvantages

A reproducible immobilisation of the biorecognition element on the crystal surface as well as the identification of methods to minimise the non-specific binding of interferences are not easily achievable.

Measurements noise might be present and needs to be minimised.

Laboratory based Quartz Crystal Mycrobalance

Reader(measure

frequency delay)

AmplifierGeneratethe Voltage

QCM

Nano-mechanical Biosensors Cantilever

Microcantilevers (MC) are typically 0.2-3 µm thick, 20-100 µm wide and 100-800 µm long.

Cantilevers biosensors are already been used for DNA hybridisation and detection of proteins, antibody, single virus particles and bacteria

(a) array of silicon-based cantilevers with individual functionalized surfaces (b) principle of a cantilever-based biosensor for oligonucleotide detection.

• The ‘optical lever’ technique has been traditionally applied to monitor the bending of the cantilever.

Measurement of the deflection (using an array of photo-diodes) of the source beam (laser) allows to quantify the biorecognition.

• Cantilevers with integrated piezoresistors have been developed.

The measurement of changing in resistance allow to quantify cantilever bending below the nanometrical scale and subsequently detects biorecognition.

• MOSFET (metal–oxide–semiconductor field-effect transistor) cantilevers have been recently proposed for low noise detection.

Cantilever BiosensorsHow I measure biorecognition?

Magnetic Biosensors

•Based on Magnetic Nanoparticles

•Use Magnetoresistive detectors: Giant Magnetoresistive type (GMR) or tunnelling magnetoresistive type (TMS) already widely used in hard disk drivers and in automotives.

•Advantages of magnetic biosensors:

–Lack of background signal

–Easiness to miniaturise and manipulate on chips format

–Easy removal of unspecific binding

–Easy regeneration by application of strong magnetic gradient field

Magnetic nanoparticles of rust (illustrated here in red and orange) tend to bind to arsenic. These properties make them ideal for

removing arsenic from contaminated well water using little more than a magnet. (Credit: CBEN Rice University)

Magnetic Sensors for Medical Diagnostics

J. Richardson, A. Hill, R. Luxton and P. Hawkins (2001) A novel measuring system for the determination of paramagnetic

particle labels for use in magneto-immunoassays.

Biosensors and Bioelectronics 16, 1127-1132

Dual detection coil

See also 1st suggestion: D.R. Baselt, G.U. Lee and M. Natesan et al. (1998), Biosensors and Bioelectronics 13,

739.

Magnetic DNA biosensor: Binding of themagnetic markers and detection of their

stray field by the GMR (Giant magnetoresistance)-sensor.

Schotter et al. / Biosensors and Bioelectronics 19 (2004)

1149–1156).

Thermal biosensors

These biosensors use thermistors (resistor whose resistance varies significantly with temperature) to measure changes in temperature due to heat generated during biological reactions (enzymatic reaction).

Examples of Applications

Glucose detection

Cholesterol/Cholesterol ester detection

K. Ramanathan, Biosensors & Bioelectronics, 16 (2001) 417

Sensor System

A functional sensor system cannot be considered in isolation from otherfunctional elements as for example:

• the sample collection and pretreatment system (imicrodialysis tube).

• microfluidics and, when needed, associated pumps and injection

devices.

• Stimulation sources (light source, pulse generators, potentiostat).

and signal collectors (photocounters, potentiostat….).

• electronics (signal amplification, conversion and display technology.

The practical / commercial success of a device is dependent on effective designand engineering to meet performance criteria and cost of goods (COGs)targets.

Devices intended for patient use (but not only) must not only be extremelyrobust, but must be easy to use and display results in a user-friendly way.