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Using complementary acoustic and optical techniques for quantitative monitoring of biomolecular adsorption at interfaces.

Konradi R, Textor M, Reimhult E - Biosensors (Basel) (2012)

Bottom Line: In this tutorial review, different optical and acoustic evanescent techniques are used to illustrate how an understanding of the transducer principle of each technique can be exploited for further interpretation of hydrated and extended polymer and biological films.The case studies deal with representative examples of adsorption of protein films, polymer brushes and lipid membranes, and describe e.g., how to deal with strongly vs. weakly hydrated films, large conformational changes and ordered layers of biomolecules.The presented systems and methods are compared to other representative examples from the increasing literature on the subject.

View Article: PubMed Central - PubMed

Affiliation: BASF SE, Advanced Materials and Systems Research, D-67056 Ludwigshafen, Germany. rupert.konradi@basf.com.

ABSTRACT
The great wealth of different surface sensitive techniques used in biosensing, most of which claim to measure adsorbed mass, can at first glance look unnecessary. However, with each technique relying on a different transducer principle there is something to be gained from a comparison. In this tutorial review, different optical and acoustic evanescent techniques are used to illustrate how an understanding of the transducer principle of each technique can be exploited for further interpretation of hydrated and extended polymer and biological films. Some of the most commonly used surface sensitive biosensor techniques (quartz crystal microbalance, optical waveguide spectroscopy and surface plasmon resonance) are briefly described and five case studies are presented to illustrate how different biosensing techniques can and often should be combined. The case studies deal with representative examples of adsorption of protein films, polymer brushes and lipid membranes, and describe e.g., how to deal with strongly vs. weakly hydrated films, large conformational changes and ordered layers of biomolecules. The presented systems and methods are compared to other representative examples from the increasing literature on the subject.

No MeSH data available.


Schematics of the quartz crystal microbalance. An alternating electric field applied across a piezoelectric quartz crystal yields a resonant shear acoustic wave in the sensor crystal. The shear oscillation couples to the adsorbed film on top and shifts the resonant frequency and energy dissipation in response to the added mass and viscoelastic load.
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biosensors-02-00341-f003: Schematics of the quartz crystal microbalance. An alternating electric field applied across a piezoelectric quartz crystal yields a resonant shear acoustic wave in the sensor crystal. The shear oscillation couples to the adsorbed film on top and shifts the resonant frequency and energy dissipation in response to the added mass and viscoelastic load.

Mentions: The QCM sensor is made of a thin piece of quartz, which is a piezo-electric material. An oscillating electric field is applied across the crystal typically making it deform in thickness-shear mode, i.e., perpendicular to the applied electric field (Figure 3). Each crystal has a fundamental resonant frequency determined by , where νq is the speed of shear waves in the crystal and tq is the thickness of the crystal. When the driving electric field oscillates at the fundamental frequency the mechanical amplitude of the crystal shear oscillation is increased more than hundred-fold. The width of the crystal resonance is very narrow, which gives an extremely well-defined resonant frequency and the ability to measure changes in the resonant frequency very precisely. Since the resonant frequency is determined by the total oscillating mass, which also includes all mass that is coupled to the surface, it can be used to measure the mass adsorbed on the surface in real time without need for any labels. A good approximation for the mass adsorbed to the sensor surfaces is the Sauerbrey relation [37]:(3)where k is the mass sensitivity of the crystal (~18 ng/(Hz × cm2 for a typically used 4.95 MHz crystal). Resonance is also observed at so-called overtones of the fundamental resonant frequency, which respond to changes in mass in a manner similar to the fundamental frequency. Only odd-numbered overtones can be produced, yielding overtone resonances at, e.g., 15, 25, 35, … MHz for a 5 MHz crystal. The conditions that have to be fulfilled for the Sauerbrey relation to be strictly valid are that the mass of the adsorbed film must be: (i) much smaller than the mass of the crystal (<5%); (ii) evenly distributed over the active area of the sensor or with an inhomogeneous but symmetric and periodic distribution around the geometrical center of oscillation; (iii) rigidly attached to the surface (no slip); (iv) complete bulk rigidity (no visco-elastic deformation induced by the sensor motion in the overlayer to be measured); Of these, criterion (v) can mostly not be relaxed for biological molecules or polymers adsorbed from aqueous solution, while the other assumptions are less critical.


Using complementary acoustic and optical techniques for quantitative monitoring of biomolecular adsorption at interfaces.

Konradi R, Textor M, Reimhult E - Biosensors (Basel) (2012)

Schematics of the quartz crystal microbalance. An alternating electric field applied across a piezoelectric quartz crystal yields a resonant shear acoustic wave in the sensor crystal. The shear oscillation couples to the adsorbed film on top and shifts the resonant frequency and energy dissipation in response to the added mass and viscoelastic load.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4263558&req=5

biosensors-02-00341-f003: Schematics of the quartz crystal microbalance. An alternating electric field applied across a piezoelectric quartz crystal yields a resonant shear acoustic wave in the sensor crystal. The shear oscillation couples to the adsorbed film on top and shifts the resonant frequency and energy dissipation in response to the added mass and viscoelastic load.
Mentions: The QCM sensor is made of a thin piece of quartz, which is a piezo-electric material. An oscillating electric field is applied across the crystal typically making it deform in thickness-shear mode, i.e., perpendicular to the applied electric field (Figure 3). Each crystal has a fundamental resonant frequency determined by , where νq is the speed of shear waves in the crystal and tq is the thickness of the crystal. When the driving electric field oscillates at the fundamental frequency the mechanical amplitude of the crystal shear oscillation is increased more than hundred-fold. The width of the crystal resonance is very narrow, which gives an extremely well-defined resonant frequency and the ability to measure changes in the resonant frequency very precisely. Since the resonant frequency is determined by the total oscillating mass, which also includes all mass that is coupled to the surface, it can be used to measure the mass adsorbed on the surface in real time without need for any labels. A good approximation for the mass adsorbed to the sensor surfaces is the Sauerbrey relation [37]:(3)where k is the mass sensitivity of the crystal (~18 ng/(Hz × cm2 for a typically used 4.95 MHz crystal). Resonance is also observed at so-called overtones of the fundamental resonant frequency, which respond to changes in mass in a manner similar to the fundamental frequency. Only odd-numbered overtones can be produced, yielding overtone resonances at, e.g., 15, 25, 35, … MHz for a 5 MHz crystal. The conditions that have to be fulfilled for the Sauerbrey relation to be strictly valid are that the mass of the adsorbed film must be: (i) much smaller than the mass of the crystal (<5%); (ii) evenly distributed over the active area of the sensor or with an inhomogeneous but symmetric and periodic distribution around the geometrical center of oscillation; (iii) rigidly attached to the surface (no slip); (iv) complete bulk rigidity (no visco-elastic deformation induced by the sensor motion in the overlayer to be measured); Of these, criterion (v) can mostly not be relaxed for biological molecules or polymers adsorbed from aqueous solution, while the other assumptions are less critical.

Bottom Line: In this tutorial review, different optical and acoustic evanescent techniques are used to illustrate how an understanding of the transducer principle of each technique can be exploited for further interpretation of hydrated and extended polymer and biological films.The case studies deal with representative examples of adsorption of protein films, polymer brushes and lipid membranes, and describe e.g., how to deal with strongly vs. weakly hydrated films, large conformational changes and ordered layers of biomolecules.The presented systems and methods are compared to other representative examples from the increasing literature on the subject.

View Article: PubMed Central - PubMed

Affiliation: BASF SE, Advanced Materials and Systems Research, D-67056 Ludwigshafen, Germany. rupert.konradi@basf.com.

ABSTRACT
The great wealth of different surface sensitive techniques used in biosensing, most of which claim to measure adsorbed mass, can at first glance look unnecessary. However, with each technique relying on a different transducer principle there is something to be gained from a comparison. In this tutorial review, different optical and acoustic evanescent techniques are used to illustrate how an understanding of the transducer principle of each technique can be exploited for further interpretation of hydrated and extended polymer and biological films. Some of the most commonly used surface sensitive biosensor techniques (quartz crystal microbalance, optical waveguide spectroscopy and surface plasmon resonance) are briefly described and five case studies are presented to illustrate how different biosensing techniques can and often should be combined. The case studies deal with representative examples of adsorption of protein films, polymer brushes and lipid membranes, and describe e.g., how to deal with strongly vs. weakly hydrated films, large conformational changes and ordered layers of biomolecules. The presented systems and methods are compared to other representative examples from the increasing literature on the subject.

No MeSH data available.