Transducers and sensor systems

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1 Transducers and sensor systems

2 Outline What is the transducer? Electrochemical Optical Gravimetric Micromechanical Magnetic Thermometric Sensor System

3 Of what we will talk? Main elements of a biosensor. (a) Biorecognition element (b) Transducer (c) amplifier (d) Signal converter (e) recorder

4 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.

5 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 analyte recognition process) element is added to generate a readable signal Enzymatic label (generate color or electrical signal) Fluorescent molecules

6 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.

7 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.

8 Some Definitions VOLTAGGE: CURRENT: 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 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

9 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.

10 Reduction or cathodic reactions: result in the consumption of electrons (electrons from external circuit to species in solution). O 2 + H 2 O + 4e - 4OH - Oxidation or Anodic reactions: result in the generation of electrons (electrons from the solution to the external circuit). H 2 O 2 2H + + O 2 + 2e -

Standard reduction potentials Standard electrode (working electrode) potential is the potential at which an electrochemical reaction (in the specific case of the table a reduction) takes place.

11 Standard reduction potentials Standard electrode (working electrode) potential is the potential at which an electrochemical reaction (in the specific case of the table a reduction) takes place. This correspond to the work/energy needed to have the reaction 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.

Ag/AgCl electrode: most common reference electrode Absolute Standard = Standard

12 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. Ag/AgCl electrode: most common reference electrode Absolute Standard = Standard Hydrogen Electrode Pt, H 2 (g, 1 atm) / H + (1.0M, aq) // Standard Potential Defined as 0 Volts

Working electrodes WORKING ELECTRODE: Is the

13 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 ions Other side - current carried by electrons Interphase not Interface (2 adjacent regions)

14 How standard potential and concentrations are correlated Behavior governed by the Nernst Equation: E = E o RT log 10 (a o /a R ) nf n: number of moles of electrons exchanged in the reaction mol R: the universal gas constant; J K 1 mol 1 T: Absolute temperature (Kelvin). F: Faraday constant or the number of coulombs per mole of electrons C mol 1 a: is the activity of the chemical/ion in the solution.

15 Equilibrium Electrochemistry..Potentiometry Electrochemical measurements in which the current is prevented from flowing (equilibrium condition). 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).,,,

16 Electrode response as a function of Urea concentration 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 (NH 2 ) 2 CO + 2H 2 O + H + HCO NH 4 + 2NH 3 + 2H + Trivedi U.B., Sensors and Actuators B: 140 (1), 2009,

Faradaic processes The intensity of the faradaic current is proportional to the concentration of the electroactive species, its diffusion coefficient and diffusion properties

17 Faradaic processes 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. Three electrodes electrochemical cell

18 Potentiodynamic measurements Electrochemical measurements are performed by changing, at a constant rate, the potential of the working electrode as a function of the time. Staircase voltammetry Differential pulse voltammetry

19 Typical response Current peak (i p ) is proportional to the concentration of the molecule undergoing electrochemical reaction. E p is a characteristic of the electroactive molecule.

20 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).

21 Glucosensor (Clark s electrode) D-glucose + H 2 O + O 2 gluconic acid + H 2 O 2 Reduction in O 2 concentration 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

22 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.

23 How can it be used?

reference electrode by adjusting the current at an auxiliary electrode

24 Electrochemical device Potentiostat 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). Research laboratory potentiostat Point of care electrochemical device

25 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.

26 Region of interest

27 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.

28 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).

29 Fluorescence Resonance Energy Transfer (FRET) Donor and acceptor molecules must be in close proximity (typically 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

30 Colorimetric Aggregation assay Lateral flow: Pregnancy tests Au nano-particles when aggregate change the Wavelength of the light that adsorb. Solution change colour; from red to blue.

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

32 Hardware? Laser, Lamp Array of Diodes CCD Camera Photo counter Spectrophotometer Spectrophotometer plate reader Fluorescence microscope Confocal microscope

33 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)

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

34 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.

$The resonance angle is strongly dependent from the refractive index of the metal surface: changes in the$

35 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.

36 BIACORE 3000 Sample to sensor: Automatic sampler Pumps Injection system Microfluidics

37 Piezoelectric Devices 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.

38 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).

39 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): F = 2 f A 2 0 µ q m ρ q = C m Where: DF: measured frequency shift, in Hz f 02 : the fundamental resonant frequency (squared), in Hz Dm: mass change, in g A: piezoelectric active area, in cm 2 µ q : shear modulus of quartz ( g) ρ q : density of quartz (2.648 g cm -3 ) C: mass sensitivity constant in sg -1

40 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. F = C m

41 QCM for DNA hybridisation monitoring Hybridisation-Regeneration Cycle

niobate (LiNbO 3 ), 41 rotated Y-cut, X-prop.

42 Shear-Horizontal Surface Acoustic Wave (SH-SAW) Devices Suitable for liquid testing More sensitive than QCM SH-SAW substrates: Lithium niobate (LiNbO 3 ), 41 rotated Y-cut, X-prop. Lithium tantalate (LiTaO 3 ), 36 rotated Y-cut, X-prop. Quartz, ST-cut, Z-prop MHz operating frequency

43 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 are significantly less expensive. In many cases acoustic sensors can accomplish the same results as the SPR (Surface Plasmon Resonance). Detection can be done in real time and in native conditions.

44 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.

45 QCM Amplifier Generate the Voltage Reader (measure frequency delay) Laboratory based Quartz Crystal Mycrobalance

Nano-mechanical Biosensors Cantilever Microcantilevers (MC) are typically 0.

Cantilevers biosensors are already been used for DNA hybridisation and detection of proteins, antibody,

46 Nano-mechanical Biosensors Cantilever Microcantilevers (MC) are typically µm thick, µm wide and 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 cantileverbased biosensor for oligonucleotide detection.

Cantilever Biosensors How I measure biorecognition? The optical lever technique has been traditionally applied to monitor the bending of the cantilever.

47 Cantilever Biosensors How I measure biorecognition? 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.

Magnetic Biosensors Based on Magnetic Nanoparticles Use Magnetoresistive detectors: Giant Magnetoresistive type (GMR) or tunnelling magnetoresistive type (TMS) already widely used in hard disk

48 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)

Biosensors and Bioelectronics 16, 1127-1132 Dual detection coil Magnetic DNA biosensor: Binding of the magnetic markers and detection of their stray

49 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, Dual detection coil Magnetic DNA biosensor: Binding of the magnetic markers and detection of their stray field by the GMR (Giant magnetoresistance)-sensor. See also 1 st suggestion: D.R. Baselt, G.U. Lee and M. Natesan et al. (1998), Biosensors and Bioelectronics 13, 739. Schotter et al. / Biosensors and Bioelectronics 19 (2004) ).

50 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 other functional elements as for example: the sample collection and pretreatment system (imicrodialysis tube).

51 Sensor System A functional sensor system cannot be considered in isolation from other functional 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 design and engineering to meet performance criteria and cost of goods (COGs) targets. Devices intended for patient use (but not only) must not only be extremely robust, but must be easy to use and display results in a user-friendly way.

BIOSENOSRS BIO 580. Electrochemical Biosensors - theory part 1 WEEK 1 Fall Semester

BIOSENOSRS BIO 580. Electrochemical Biosensors - theory part 1 WEEK 1 Fall Semester BIOSENOSRS BIO 580 Electrochemical Biosensors - theory part 1 WEEK 1 Fall Semester Faculty: Dr. Javed H. Niazi KM Faculty of Engineering & Natural Sciences Sabanci University Topics that will be covered