Hirsch Teil1. What are chemical sensors?- Definition !!!2. Selectivity- Definition- Equilibrium based selectivity: free energy, dielectric constant and distance,- Kinetic based selectivity: steady-state regime3. Recognition Methods- Ion recognition: recognition-electric charge, selectivity-size,transduction-potentiometric, optical methodse.g. PH electrode ----> part 3- Recognition by affinity interactions: reversible, non-covalent bonds-ionic bonds, hydrogen bonds, van der Waals interaction => result in a molecular assiciation complex; also respect to shape and chemical reactivity; indicated by stability constant (very stable)- antibody - antigen interaction => immunochemical reactionantibody: glycoprotein produced by immune system to identify and neutralizepathogen microorganisms.antigen: the part of the pathogen that reactions with the antibody.use specific antibody receptor => identify pathogenuse antigen receptor => identify antibody (the detection of infection byparticular pathogen)- lectin proteins recognize caborhydrates (agglutinins, hemagglutinin)carbohydrate-binding modules link to the catalytic part of glycosidehydrolases => result in degradation of cell wall, storage of polysaccharide- A Molecularly Imprinted Polymer (MIP) is a polymer that has been processed usingthe molecular imprinting technique which leaves cavities in polymer matrix withaffinity to a chosen "template" molecule.In chemistry, molecular imprinting is a technique to create template-shaped cavities in polymer matrices with memory of the template molecules to be used in molecular recognition.-Nucleic acid aptamers are nucleic acid species that have been engineered throughrepeated rounds of in vitro selection to bind to various molecular targets such assmall molecules, proteins, nucleic acids, and even cells, tissues and organisms.Aptamers are useful in biotechnological and therapeutic a pplications as they offer molecular recognition properties that rival that of the commonly used bimolecular antibodies.- Recognition by nucleic acids: hydrogen bonds between two distinct pairs of nucleobases => two complementary nucleic acids form a double strand association complex => called hybridizationnucleic acid sensors: short single strand NA as receptor to recognize a particular NA sequence in the analyte NA => detection of genetic anomalies and pathogen mircoorganism- Recognition by enzyme: dynamic processEnzyme: protein compound that function as catalysts in chemical reaction occurring in living system.- Recognition by cells and tissues: advantages of enzyme incorporated in biological materials => in their natural environmentsee part 3, Wegener - Recognition by gases and vapors: based on sorption at solid material => surface-adsorption, inner-absorption; purely physical phenomenon or chemical reaction.4. Transduction MetohdosChemical transduction: monitoring the change of chemical composition of the sensing element in response to the recognition process. => change in concentration/amount is measured => detect primary product -> secondary product or coreagent -> labeling productLABEL can be a simple molecular species or nanoparticals that can be detected by available physiochemical methods => enzyme, fluorescent dyes, luminescent dyes, electroactive compoundsPhysical transduction: a specific physical property of the sensing element that is affected by its interaction with the analyte is monitored. => mass, reflective index, dielectric properties, electrical resistivity => LABEL-FREE- Thermometric transductionRecognition of the analyte leads to change in temperature => only catalytical processes generate sufficient heat to the measurement => application: combustible gases react with O2 at the surface of a catalyst.- Transduction based on mechanical effectsRecognition leads to change in mass of the sensing element => monitored by mass tranducer based on quartz crystal microbalance (QCM)----------------------------------------------------------------------------------------------------------------- QCM, correct name: Thickness shear modePiezoelectric effect:generation of electrical charges on the surface of a solid by strain, pressure or torsion (mechanical deformation of solid) =>electricity resulting from pressureI nverse piezoelectric effect:application of charges to surfaces of piezoelectricsolid generates mechanical deformation (elongation, contraction, torsion)QCM is based on Inverse piezoelectric effect!# AT cut => 35`15`=> minimum temperature coefficient at 50~70 CIt makes the AT-cut well suited to applications requiring high degree of frequency stability over wide temperature ranges.## Electrodes are applied on both sides, and AC voltage applied.DC cannot flow across the crystal because it consists of an insulator material;however the crystal somewhat behaves as capacitor and allow an AC current to f low along the left-hand loop.AC voltage applied => leads to shear oscillation of crystal => when the voltage frequency matches the intrinsic vibration frequency of the crystal => the vibration amplitude is at maximal => the resonant => resonant frequency (f0) => depend on crystal thickness (e.g. d q= 330 um, f0= 5MHZ), density and elasticity of piezoelectric material### AT-cut resonator: thickness: ~0.2 mm, diameter of the active area: 5~20 mm #### Deposition of a homogenous mass film (a rigid overlay)Sauerbrey equation:Cf indicate sensitivity of QCMcondition of this equation: rigid deposited mass; △m<2% of crystal mass;operated in vacuum or in gaseous atmopphereIn liquid: the liquid breaks the vibration by friction => lessen f0Thickness of the layer must be greater than the wave decay lengththat is of 250 nm of 5 MHz resonator at water. ----> part 2!!!##### QCM in practice => see p.41----------------------------------------------------------------------------------------------------------------- - Resistive and capacitive transductionRecognition leads to changes in the electrical property of this materialResistive transduction: gases interact with MOS => change in electrical resistivity Capactive transduction => dielectric constant- Electrochemical transductionsee part 2, Matysik - Optical transductionOptical transduction can be based on light emission or light absorption, also by physical quantity (reflective index) and light scattering.5. Sensor Configuration and Fabrication- Lateral flow assayA typical test strip consists of the following components:1. Sample pad – an absorbent pad onto which the test sample is applied2. Conjugate pad –this contains antibodies specific to the target analyte;conjugated to coloured particles (e.g. gold nanoparticles)3. Reaction membrane –typically a hydrophobic nitrocellulose or celluloseacetate membrane onto which anti-target analyte antibodies are immobilized in a line across the membrane as a capture zone or test line, and a control zonecontaining antibodies specific for the conjugate antibodies.4. Wicking pad –a further absorbent pad designed to draw the sample acrossthe reaction membrane by capillary action and collect it.Double antibody sandwich assays: the sample migrates from the sample pad through the conjugate pad where any target analyte present will bind to the c onjugate.=> The sample then continues to migrate across the membrane until it reaches the test line where the target or conjugate complex will bind to the immobilized antibodies producing a visible line on the membrane. => The sample then migrates further along the strip until it reaches the control line, where excess conjugate will bind and producea second visible line on the membrane.This control line indicates that the sample has migrated across the membrane as intended. Two clear lines on the membrane is a positive result. A single line in the control zone is a negative result. Double antibody sandwich assays are most suitable for larger analytes, such as bacterial pathogens and viruses, with multiple antigenic sites. 6. Methods and Material in Sensor Preparation- Immobilization at solid surface => integration of a transducer with the receptor Physical adsorption at a solid supportNon covalent immobilization at solid surface => hydrophobic interaction, hydrogen bonding, electrostatic attraction; monolayer; no restrict access; not stable; Langmuir isotherm -> equilibrium interactionSupport material: silica, cellulose acetate, PVCCovalent bonding to the solid supportCovalent conjugation => stable, covalent bond, time consuming, expensiveCommon reactive group: -OH, -NH2, -C=O, -SH- Carboxylic acid with DCC- Glutaraldehyde reacts with the a.a. of lysine in protein => widely used Support: porous material => high specific area, high density of immobilized compounds => hydrogel: immobilized by entrapment/covalent corsslink - Natural polymers: Cellulose, Dextran- Synthetic polymers: Polystyrene- Active polymers: Epoxide (without preliminary activation) -->DNA array !!!- Inactive Polymers: Vicinal hydroxyls actived by CNBr- Inorganic support: Silica, AL2O3, TiO2 => stable at extreme PH- Metal support: noble metals, thiols on golds --> self assembled monolayers!Affinity reaction: avidin-biotin !!!Thin molecular layers: one or several molecular layers in solid support - Self-assembly of amphiphilic compounds: preparation of liposome andmicelles; liposome can be used of entrapment of molecular- Bilipid layer membranes: Langmuir-Blodgett technique- Layer by Layer assembly- Sol-Gel chemistry methods: silica gel => -O-Si-O-- Hydrogels: Xerogel, aerogel- Conducting polymers: Polyacetylene, polyaniline --> gas senor based on CP (----> part 3 !!!); also as entrapment matrix for biological receptors- Mesoporous materials: porous materials with pore (diameter: 2-50 nm,close to protein) => enzyme immobilization by entrapment (crosslinking withglutaraldehyde)- Deposition of polymers onto solid surfaces: dip coating, drop coating, spin coating ----> part 2 !!!Perm-selective memberanes: Nafin ----> Clark oxygen electrode Support-free crosslinkingEntrapment in a polymer networkEncapsulation7. Microfabrication Methodes- Spot Arraying: Contact-based & Noncontact-based; DNA microarray !!!!!Pros & Cons- Thick-film Technology: screen-printing technique (5-50 um thick layer)- Thin-film Technology: Photolithography (2 um)- Softlithography ----> experiment !!!!- Microcontact printing ----> experiment !!!!8. Optical Sensors- Electromagnetic RadiationOptical sensor => interaction of electromagnetic radiation with sensor layer - frequency; wavelength; photon energy (definition)- Structure: integration with wavelength-selection (optical filters) device and light sources (lasers), light detectors (phototransistors)- Optical Waveguides- Optical FibersOptical fibers' structuretotal internal reflection => evanescent wave- Spectrochemical Transduction MethodsSpectrochemical method analysis => light absorption or emission by sample => optical label performs absorption or emission (organic dye or metal complexes) - Light absorption: absorbance => concentration; sensitivity => thickness, absorpyivity, absorptivity => wavelength- Diffuse reflectance spectrometry: refelctance => concentration; suitable forsolid in near IR- Luminescence: Fluorescence spectromerty => fluorophore (label, organic dye or metal complexes, luminescent nanparticle ); steady-statefluorescence measurement, Time-resolved fluormetry; fluorescencequenching; resonance energy transfer (FRET); chemical- andbioluminescence => luminol; electrochemicaluminescence; Ramanspetrometry- Surface Plasmon Resonance Spectroscopy (SPR)。
