The Q-Sense E4 is a real-time analytical instrument for studies of molecular events occurring on surfaces. The E4 measures mass and viscoelastic properties of molecular layers as they build up or change on the sensor surface. The Q-Sense E4 instrument is based on the eQuartz Crystal Microbalance with Dissipation (QCM-D) technology. The heart of the instrument is a sensor that oscillates at a specific frequency when voltage is applied. The frequency of the oscillation changes as the mass on the sensor changes (1). Turning off the voltage causes the oscillation to decay. The decay rate or Dissipation factor is related to the elasticity and viscosity of the molecular layer on the sensor (2). By measuring the frequency and dissipation, it becomes possible to analyze the state of molecular layers bound to the sensor surface, their mass, thickness and structural (viscoelastic) properties.
A recorded webinar on QCM-D technology, products and applications can be found here.
(Very) brief introduction to QCM-D technology
With QCM-D, two parameters - frequency and dissipation - are monitored simultaneously, in real-time, as molecular layers form on the sensor surface. Here is a demonstration of the principles behind the QCM-D technology.
Sauerbrey relation for rigid films
The mass of the adhering layer is calculated by using the Sauerbrey relation:
|C = 17.7 ng Hz-1 cm-2 for a 5 MHz quartz crystal.
n = 1,3,5,7 is the overtone number.
It is also possible to get an estimation of the thickness (d) of the adhering layer:
|where peff is the effective density of the adhering layer.|
Soft films and the importance of "D"
In most situations the adsorbed film is not rigid and the Sauerbrey relation becomes invalid. A film that is "soft" (viscoelastic) will not fully couple to the oscillation of the crystal, hence the Sauerbrey relation will underestimate the mass at the surface.
A soft film dampens the sensor's oscillation. The damping or energy dissipation (D) of the sensor's oscillation reveals the film's softness (viscoelasticity).
D is defined as:
|where Elost is the energy lost (dissipated) during one oscillation cycle and Estored is the total energy stored in the oscillator.|
The energy dissipation of the sensor is measured by recording the response of a freely oscillating sensor that has been vibrated at its resonance frequency. This also gives the opportunity to jump between the fundamental frequency and overtones (e.g. 15, 25 and 35 MHz). By measuring at multiple frequencies and applying a viscoelastic model (e.g. the so called Voigt model) incorporated in Q-Sense software QTools, the adhering film can be characterized in detail; viscosity, elasticity and correct thickness may be extracted even for soft films when certain assumptions are made.
Location of the instrument
The instrument is located in Dr. Tarabara's laboratory (3570 Engineering Building).
The instrument is available for use by MSU researchers on Mondays, Tuesdays, and Wednesdays. Time of scheduled training sessions and time reserved by users are reflected in the online calendar:
QCM-D training and usage fee schedule
|User group||Cost of training||Cost of usage|
|Members of the MSU materials science community||$26.80 / hr||$ 4.25 / hr|
|Other MSU users||$26.80 / hr||$10.80 / hr|
To schedule training on the equipment or to reserve instrument time please contact:
Dr. Volodymyr Tarabara
Department of Civil and Environmental Engineering
Michigan State University
East Lansing, MI 48824 USA