We have derived the general solution of a wave equation describing the dynamics of two-layer viscoelastic polymer materials of arbitrary thickness deposited on solid (quartz) surfaces in a fluid environment. Within the Voight model of viscoelastic element, we calculate the acoustic response of the system to an applied shear stress, i.e. we find the shift of the quartz generator resonance frequency and of the dissipation factor, and show that it strongly depends on the viscous loading of the adsorbed layers and on the shear storage and loss moduli of the overlayers. These results can readily be applied to quartz crystal acoustical measurements of the viscoelasticity of polymers which conserve their shape under the shear deformations and do not flow, and layered structures such as protein films adsorbed from solution onto the surface of self-assembled monolayers.

https://iopscience.iop.org/article/10.1238/Physica.Regular.059a00391/pdf

Viscoelastic Acoustic Response of Layered Polymer Films at Fluid-Solid Interfaces: Continuum Mechanics Approach

Nanomechanical resonators enable the measurement of mass with extraordinary sensitivity. Previously, samples as light as 7 zeptograms (1 zg = 10(-21) g) have been weighed in vacuum, and proton-level resolution seems to be within reach. Resolving small mass changes requires the resonator to be light and to ring at a very pure tone-that is, with a high quality factor. In solution, viscosity severely degrades both of these characteristics, thus preventing many applications in nanotechnology and the life sciences where fluid is required. Although the resonant structure can be designed to minimize viscous loss, resolution is still substantially degraded when compared to measurements made in air or vacuum. An entirely different approach eliminates viscous damping by placing the solution inside a hollow resonator that is surrounded by vacuum. Here we demonstrate that suspended microchannel resonators can weigh single nanoparticles, single bacterial cells and sub-monolayers of adsorbed proteins in water with sub-femtogram resolution (1 Hz bandwidth). Central to these results is our observation that viscous loss due to the fluid is negligible compared to the intrinsic damping of our silicon crystal resonator. The combination of the low resonator mass (100 ng) and high quality factor (15,000) enables an improvement in mass resolution of six orders of magnitude over a high-end commercial quartz crystal microbalance. This gives access to intriguing applications, such as mass-based flow cytometry, the direct detection of pathogens, or the non-optical sizing and mass density measurement of colloidal particles.

/pdf/weighing-of-biomolecules-single-cells-and-single-35bbp7ev6j.pdf

Weighing of biomolecules, single cells and single nanoparticles in fluid

Sensor technology is one of the most important key technologies of the future with a constantly increasing number of applications, both in the industrial and in the private sectors. More and more gas sensors are used for the control of technical processes, in environment monitoring, healthcare, and automobiles. Consequently, the development of fast and sensitive gas sensors with small cross sensitivity is the subject of intense research, propelled by strategies based on nanoscience and -technology. Established systems can be improved and novel sensor concepts based on bottom-up approaches show that the sensor properties can be controlled by molecular design. This Review highlights the recent developments and reflects the impact of nanoscience on sensor technology.

Metal and Metal Oxide Nanoparticles in Chemiresistors: Does the Nanoscale Matter?

Conventional approaches to chemical sensors have
traditionally made use of a “lock-and-key” design,
wherein a specific receptor is synthesized in order to
strongly and highly selectively bind the analyte of
interest.1-6 A related approach involves exploiting a
general physicochemical effect selectively toward a
single analyte, such as the use of the ionic effect in
the construction of a pH electrode. In the first
approach, selectivity is achieved through recognition
of the analyte at the receptor site, and in the second,
selectivity is achieved through the transduction
process in which the method of detection dictates
which species are sensed. Such approaches are appropriate
when a specific target compound is to be
identified in the presence of controlled backgrounds
and interferences. However, this type of approach
requires the synthesis of a separate, highly selective
sensor for each analyte to be detected. In addition,
this type of approach is not particularly useful for
analyzing, classifying, or assigning human value
judgments to the composition of complex vapor
mixtures such as perfumes, beers, foods, mixtures of
solvents, etc.

/pdf/cross-reactive-chemical-sensor-arrays-38dz1buu0b.pdf

Cross-reactive chemical sensor arrays.

We have measured the time-resolved adsorption kinetics of the mussel adhesive protein (Mefp-1) on a nonpolar, methyl-terminated (thiolated) gold surface, using three independent techniques:  quartz crystal microbalance with dissipation monitoring (QCM-D), surface plasmon resonance, and ellipsometry. The QCM-D and ellipsometry data shows that, after adsorption to saturation of Mefp-1, cross-linking of the protein layer using NaIO4 transforms it from an extended (∼20 nm), water-rich, and hydrogel-like state to a much thinner (∼5 nm), compact, and less water-rich state. Furthermore, we show how quantitative data about the thickness, shear elastic modulus, and shear viscosity of the protein film can be obtained with the QCM-D technique, even beyond the Sauerbrey regime, if frequency (f) and energy dissipation (D) measurements measured at multiple harmonics are combined with theoretical simulations using a Voight-based viscoelastic model. The modeling result was confirmed by substituting H2O for D2O. As expect...

Variations in Coupled Water, Viscoelastic Properties, and Film Thickness of a Mefp-1 Protein Film during Adsorption and Cross-Linking: A Quartz Crystal Microbalance with Dissipation Monitoring, Ellipsometry, and Surface Plasmon Resonance Study

The sensitivity of acoustic-wave microsensors coated with a viscoelastic film to mass changes and film modulus (changes) is examined. The study analyzes the acoustic load at the interface between the acoustic device and the coating. The acoustic load carries information about surface mass and film modulus; its determination has no restrictions in film thickness. Two regimes of film behavior can be distinguished:  the gravimetric regime, where the sensor response is mainly mass sensitive, and the nongravimetric regime, where viscoelasticity gains influence on the sensor response. We develop a method, which allows the assignment of the sensor signal to a gravimetric or a nongravimetric response. The critical value can be determined from oscillator measurements. The related limits for the coating thickness are not the same for the coating procedure and mass accumulation during chemical sensing. As an example, we present results from a 10 MHz quartz crystal resonator.

Role of mass accumulation and viscoelastic film properties for the response of acoustic-wave-based chemical sensors.

Continuous process monitoring in gaseous, liquid or molten media is a fundamental requirement for process control. Besides temperature and pressure other process parameters such as level, flow, concentration and conversion are of special interest. More qualified information obtained from new or better sensors can significantly enhance the process quality and thereby product properties. Ultrasonic sensors or sensor systems can contribute to this development. The state of the art of ultrasonic sensors and their advantages and disadvantages will be discussed. Commercial examples will be presented. Among others, applications in the food, chemical and pharmaceutical industries are described. Possibilities and limitations of ultrasonic process sensors are discussed.

https://iopscience.iop.org/article/10.1088/0957-0233/13/8/201/pdf

Application of ultrasonic sensors in the process industry

Quartz crystal resonators (QCR) respond to surface mass and material properties of a film coated on their surface. The acoustic load acting at the surface of the resonator is a more general parameter to describe this dependence. It can be represented by a mass factor and an acoustic factor. The quotient of resistance increase and frequency shift can be used for the determination of the acoustic factor, if the loss tangent of the coating is known. Viscoelastic properties of sensitive coatings can enhance the mass sensitivity of quartz crystal microbalance (QCM) sensors. Acoustic factor and acoustic amplification effective during chemical sensing are not the same. We further suggest a sensor concept, which is based on a bilayer arrangement. Acoustic amplification with a viscoelastic film and chemical sensitivity is separated. With a proper selection of materials, the first layer realizes acoustic amplification while the (chemical) sensitive layer acts as a pure mass detector. Major sensor design parameters are the shear modulus and the thickness of the first layer; major challenge is the preparation of a homogeneous and uniform first film.

The quartz crystal microbalance: mass sensitivity, viscoelasticity and acoustic amplification

Acoustic microsensors are commonly known as high-resolution mass-sensitive devices. This is a restricted view in many chemical and biosensor applications, especially in liquids. Sensitivity to non-gravimetric effects is a challenging feature of acoustic sensors. In this review we give an overview of recent developments in resonant sensors including micromachined devices and also list recent activity relating to the (bio)chemical interface of acoustic sensors. Major results from theoretical analysis of quartz crystal resonators, descriptive for all acoustic microsensors are summarized, and non-gravimetric contributions to the sensor signal from viscoelasticity and interfacial effects are discussed. We finally conclude with some future perspectives.

Acoustic microsensors—the challenge behind microgravimetry

A transmission line equivalent circuit for piezoelectric transducers has been modified to provide modeling of lossy piezoceramic transducers. A lossy transmission line is used to model the mechanical losses. The equivalent circuit parameters are derived from analogies between electrical transmission lines and acoustic wave propagation. Implementation of the equivalent circuit model in SPICE is shown. Simulations and measurements in the time and frequency domain of a low-Q material and a multilayered ultrasonic sensor using a low-Q piezoceramic transducer are presented.

Peter Hauptmann

Papers

Role of mass accumulation and viscoelastic film properties for the response of acoustic-wave-based chemical sensors.

Application of ultrasonic sensors in the process industry

The quartz crystal microbalance: mass sensitivity, viscoelasticity and acoustic amplification

Acoustic microsensors—the challenge behind microgravimetry

SPICE model for lossy piezoceramic transducers