LECTURE OF SUBJECT LECTURE 3 Dr sharafaldin Almusawi

LECTURE OF SUBJECT : LECTURE: 3 Dr. sharafaldin Al-musawi SUBJECT: Biosensors & Biochips : College of Biotecholgy LEVEL: 4 Immobilization Of Biomolecules On Biosensors

Quartz Crystal Microbalance (QCM) Ø The quartz crystal microbalance (QCM) is an extremely sensitive mass sensor, capable of measuring mass changes in the nanogram range.

Quartz Crystal Microbalance (QCM) Ø QCM’s are piezoelectric devices fabricated of a thin plate of quartz, with gold, platinum (Pt) or silver (Ag) electrodes affixed to each side of the plate.

Oscillator: is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave. Oscillators convert direct current (DC) to an alternating current (AC) signal.

The main applications of QCMs are the Determination the adsorption properties of biomaterials and functional surfaces, for proteins, lipids, polymers, (MOFs), cells and bacteria.

The main applications of QCMs

Affinity Interactions used in QCM

Measuring principle Ø A spontaneous excitation of the sensor with an AC voltage is used to oscillate the quartz disc with a frequency that is dependent on the total oscillating mass.

Measuring principle Ø The deposition of a thin film increases the oscillation and the resonant frequency decreases. This dependence is described by the Sauerbrey equation:

Immobilization Molecules may be immobilized either passively through: Ø Hydrophobic Ø Ionic interactions Ø Covalently by attachment to activated surface groups.

Immobilization Ø Hydrophobic Immobilization

Immobilization Ø Ionic interactions

Immobilization Ø Noncovalent surfaces are effective for many applications; however, passive adsorption of receptors fails in many cases.

Immobilization ü Covalent immobilization is often necessary for binding of molecules that: • Do not adsorb, • Adsorb very weakly • Adsorb with improper orientation

Immobilization ü Covalent immobilization may result in reduced nonspecific adsorption, and greater stability.

Immobilization ü The immobilization process should occur selectively in the presence of common functional groups, including amines, thiols, carboxylic acids, and alcohols.

Immobilization v Surface density of the ligand should be optimized. § Low density surface coverage will yield a correspondingly low frequency. § High surface densities may result steric interference between the covalently immobilized receptor molecules, impending access to the target molecules.

Immobilization 1) unhindered binding. 2) inaccessible binding site. 3) hindered binding site when adjacent site is occupied. 4) restricted access binding site.