SENSOR FOR SURFACE AND THIN FILM INTERACTIONS

The q-sense surface sensor measures the mass of thin films and simultaneously provides information about their structure. The principle is that of a quartz crystal microbalance with the additional measurement of dissipation. It is an extremely sensitive technique for the study of molecular interactions with biosurfaces. Applications include proteins, lipids, polymers and whole cells. QCM-D data can be used to determine kinetics - affinity - specificity - structure - mass and thickness.

The ping technique

The surface or thin film to be analysed is attached to one side of a quartz disc which is coated with a substrate such as gold. An AC voltage is applied to electrodes in contact with the quartz disc and a resonance oscillation is induced. When the AC voltage is turned off the oscillation dissipates, decaying exponentially.

Dissipation - viscoelastic films dampen the oscillation much more than rigid films

By using a multiple 'ping' (on-off voltage) the disk oscillation frequency and rate of dissipation are recorded up to 20 times a second. This technique is extremely sensitive and enables kinetics to be observed in real time.

Frequency-mass

The disc oscillation frequency is related to the total mass ie film plus water. If the film is thin and rigid the frequency can be used in the Sauerbrey relation to calculate the total mass on the disc surface. If the density is known then the film thickness can be calculated.

Mounting the quartz disc in the temperature controlled chamber is quick and easy

Dissipation-viscoelasticity

Viscoelastic films dampen the disc oscillation more than rigid films and the Sauerbrey relation is no longer valid. A compact globular protein gives low dissipation (rigid), while an elongated protein with coupled water results in higher dissipation (viscoelastic). It is the dissipation that gives the structural information. By driving the disc at multiple harmonic frequencies and using Q-tools software models, it is possible to calculate the values of film thickness, viscosity, elasticity and density.

Applications include the study of conformational changes enabling distinction between two similar binding events and recording protein unfolding. It is possible to determine the water content of molecular layers. Interactions can be studied on many substrates such as metals, polymers and functionalised coatings. The QCM-D is an invaluable tool for studying biomolecular adsorption/interaction and an important complement to other technologies.

The Q-Sense E4

The measurement chamber has four removable flow modules, each holding a sensor. The flow modules can be used in any serial or parallel configuration with simultaneous measurement of all four sensors.

Features of the Q-Sense E4

Measure the mass of molecular layers forming on the surface with nanogram sensitivity. For example, 1% or less of a protein monolayer can be detected.

Structural changes measured simultaneously so as to distinguish between two similar binding events or observe a phase transition in bound layers.

Real time measurements allowing recording and evaluation of kinetics

4-Sensor chamber allows higher throughput and better reproducibility.

Flow measurements in a temperature-controlled environment

Electrochemistry cell for the study of electrochemical reactions simultaneously.

Multi-frequency measurements up to 6 overtones can be measured in addition to the fundamental frequency. Overtones provide more reliable measurements and enable modelling of the film characteristics.

Flexible choice of surface including metals, polymers and chemically/biologically modified surfaces. Any surface that can be applied as a thin film can be used. The most commonly used sensor surface is pure gold as it is chemically inert and easy to modify to thiolated gold which creates a hydrophobic surface.

Gold sensor crystal

The Q-Sense E1
This has the same analytical performance as the E4 except that it incorporates a single sensor instead of 4 parallel sensors.

Features
The same as E4 plus:
Optional window module for optical access. This enables simultaneous measurements with microscopy and studies of UV induced reactions.
Optional ellipsometry module This enables simultaneous measurements with ellipsometry
Optional electrochemistry cell for the study of electrochemical reactions simultaneously.

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