Julia Körner

Signal enhancement in cantilever magnetometry based on a co-resonantly coupled sensor

Summary Cantilever magnetometry is a measurement technique used to study magnetic nanoparticles. With decreasing sample size, the signal strength is significantly reduced, requiring advances of the technique. Ultrathin and slender cantilevers can address this challenge but lead to increased complexity of detection. We present an approach based on the co-resonant coupling of a micro- and a nanometer-sized cantilever. Via matching of the resonance frequencies of the two subsystems we induce a strong interplay between the oscillations of the two cantilevers, allowing for a detection of interactions between the sensitive nanocantilever and external influences in the amplitude response curve of the microcantilever. In our magnetometry experiment we used an iron-filled carbon nanotube acting simultaneously as nanocantilever and magnetic sample. Measurements revealed an enhancement of the commonly used frequency shift signal by five orders of magnitude compared to conventional cantilever magnetometry experiments with similar nanomagnets. With this experiment we do not only demonstrate the functionality of our sensor design but also its potential for very sensitive magnetometry measurements while maintaining a facile oscillation detection with a conventional microcantilever setup.

Introduction of a co-resonant detection concept for mechanical oscillation-based sensors.

Micro- and nanoelectromechanical oscillators driven at or close to their resonance frequency are used as sensors in many fields of science and technology. A decrease in the oscillators effective spring constant and/or mass holds great potential for an increase in the sensors sensitivity. This is usually accompanied by a reduction in spatial dimensions, which in most cases requires more complex detection methods. By analyzing the complex behavior of a simple asymmetric coupled harmonic oscillator model we propose a novel sensor concept which combines the advantages of bigger and smaller oscillators, i.e. ease of detection and high sensitivity. The concept is based on matching the resonance frequencies of two otherwise very different oscillators. To support our theoretical considerations, we present an experimental implementation of such a sensor and respective experimental data, verifying a substantial signal enhancement by several orders of magnitude.

Magnetic properties of individual Co 2 FeGa Heusler nanoparticles studied at room temperature by a highly sensitive co-resonant cantilever sensor

The investigation of properties of nanoparticles is an important task to pave the way for progress and new applications in many fields of research like biotechnology, medicine and magnetic storage techniques. The study of nanoparticles with ever decreasing size is a challenge for commonly employed methods and techniques. It requires increasingly complex measurement setups, often low temperatures and a size reduction of the respective sensors to achieve the necessary sensitivity and resolution. Here, we present results on how magnetic properties of individual nanoparticles can be measured at room temperature and with a conventional scanning force microscopy setup combined with a co-resonant cantilever magnetometry approach. We investigate individual Co2FeGa Heusler nanoparticles with diameters of the order of 35 nm encapsulated in carbon nanotubes. We observed, for the first time, magnetic switching of these nanoparticles in an external magnetic field by simple laser deflection detection. Furthermore, we were able to deduce magnetic properties of these nanoparticles which are in good agreement with previous results obtained with large nanoparticle ensembles in other experiments. In order to do this, we expand the analytical description of the frequency shift signal in cantilever magnetometry to a more general formulation, taking unaligned sensor oscillation directions with respect to the magnetic field into account.

Application of a co-resonant sensor concept in cantilever magnetometry

Cantilever magnetometry is a technique to study magnetic properties of particles and films. Decreasing sample size and thus a loss in signal strength require advances in this technique, regarding cantilever sensitivity and detection. We present a co-resonant magnetometry sensor concept based on a coupled harmonic oscillator, which employs a standard silicon AFM-cantilever with simple laser-deflection measurement but also allows the use of a very sensitive nanocantilever with low mass and spring constant as sensing element. For our magnetometry sensor, the nanocantilever is made of an iron-filled carbon nanotube (FeCNT), simultaneously acting as one of the oscillators and as magnetic sample. We demonstrate the functionality and significant increase in signal strength of our sensor-setup and compare it to other magnetometry experiments with similar FeCNTs.

Theory and application of a novel co-resonant cantilever sensor

Abstract Dynamic cantilever sensors have many applications, for example in material’s research, biology, as gas and magnetic field sensors. The sensing principle is based on the effect that a force gradient or mass change applied to the cantilever alter its oscillatory state which can be related to the parameter of interest. In order to detect very small interactions, the cantilever needs to have a low stiffness which is commonly achieved by a reduction of the beam’s dimensions, especially its thickness. However, this is limited by the commonly employed laser-based detection of the cantilever’s oscillatory state. In this paper, we describe a novel co-resonant cantilever sensor concept which is based on the coupling and eigenfrequency matching of a micro- and a nanocantilever. This approach allows to access a large fraction of the nanocantilever’s high sensitivity while ensuring a reliable oscillation detection with standard laser-based methods at the microcantilever. Experiments in cantilever magnetometry and magnetic force microscopy demonstrate the immense potential of the sensor concept. Furthermore, applications are not limited to material’s research, instead this concept creates a cantilever sensor platform with many potential applications, for example as gas, mass or pressure sensors.

Bidirectional scanning force microscopy probes with co-resonant sensitivity enhancement

In dynamic scanning force microscopy (dSFM) an oscillating micromechanical cantilever equipped with an interaction tip is used to provide sub-nanometer spatial resolution data on force gradient related properties of the tip-sample interaction. Our bidirectional dSFM probes provide lateral as well as perpendicular force gradient sensitivity with in-situ-switching capability between the operation directions by employing a special geometry and the first two flexural modes of the cantilever. These probes rely only on basic vertical excitation and detection schemes and are therefore compatible with standard dSFM equipment. A way to increase the sensitivity of the lateral mode is given by a co-resonant detection concept. In the framework of this concept the ease of detection of a micromechanical oscillator, in our case a microstructured cantilever, is combined with the high force gradient sensitivity of a nanomechanical low-stiffness and low-mass carbon nanotube oscillator. The resonance frequency of the nanotube is adjusted via mass deposition to be close to the lateral sensitivity resonance frequency of the cantilever. This ensures that the oscillatory state of the coupled cantilever-nanotube system has an increased sensitivity to force gradients but still retains the easy detection of the oscillation of the cantilever. The amplified sensitivity is experimentally verified by comparing data on magnetic measurements acquired with such a co-resonant sensor with calculations for a standard bidirectional sensor.

Low-Cost Microfluidic Sensors with Smart Hydrogel Patterned Arrays Using Electronic Resistive Channel Sensing for Readout

There is a strong commercial need for inexpensive point-of-use sensors for monitoring disease biomarkers or environmental contaminants in drinking water. Point-of-use sensors that employ smart polymer hydrogels as recognition elements can be tailored to detect almost any target analyte, but often suffer from long response times. Hence, we describe here a fabrication process that can be used to manufacture low-cost point-of-use hydrogel-based microfluidics sensors with short response times. In this process, mask-templated UV photopolymerization is used to produce arrays of smart hydrogel pillars inside sub-millimeter channels located upon microfluidics devices. When these pillars contact aqueous solutions containing a target analyte, they swell or shrink, thereby changing the resistance of the microfluidic channel to ionic current flow when a small bias voltage is applied to the system. Hence resistance measurements can be used to transduce hydrogel swelling changes into electrical signals. The only instrumentation required is a simple portable potentiostat that can be operated using a smartphone or a laptop, thus making the system suitable for point of use. Rapid hydrogel response rate is achieved by fabricating arrays of smart hydrogels that have large surface area-to-volume ratios.

Effective sensor properties and sensitivity considerations of a dynamic co-resonantly coupled cantilever sensor

Background: Co-resonant coupling of a micro- and a nanocantilever can be introduced to significantly enhance the sensitivity of dynamic-mode cantilever sensors while maintaining the ease of detection. Experimentally, a low-stiffness nanocantilever is coupled to an easy to read out microcantilever and the eigenfrequencies of both beams are brought close to one another. This results in a strong interplay between both beams and, hence, any interaction applied at the nanocantilever alters the oscillatory state of the coupled system as a whole and can be detected at the microcantilever. The amplitude response curve of the microcantilever exhibits two resonance peaks and their response to an interaction applied to the sensor depends on the properties of the individual beams and the degree of frequency matching. Consequently, while an individual cantilever is characterized by its eigenfrequency, spring constant, effective mass and quality factor, the resonance peaks of the co-resonantly coupled system can be described by effective properties which are a mixture of both subsystem’s characteristics. These effective properties give insight into the amount of sensitivity of the nanocantilever that can be accessed and, consequently, into the sensitivity gain associated with the co-resonance. In order to design sensors based on the co-resonant principle and predict their behaviour it is crucial to derive a description for these effective sensor properties. Results: By modeling the co-resonantly coupled system as a coupled harmonic oscillator and using electromechanical analogies, analytical expressions for the effective sensor properties have been derived and discussed. To illustrate the findings, numerical values for an exemplary system based on experimental sensor realizations have been employed. The results give insight into the complex interplay between the individual subsystem’s properties and the frequency matching, leading to a rather large parameter space for the co-resonant system’s effective properties. While the effective spring constant and effective mass mainly define the sensitivity of the coupled cantilever sensor, the effective quality factor primarily influences the detectability. Hence, a balance has to be found in optimizing both parameters in sensor design which becomes possible with the derived analytic expressions. Besides the description of effective sensor properties, it was studied how the thermal noise and, consequently, minimal detectable frequency shift for the co-resonantly coupled sensor represented by a coupled harmonic oscillator could be derived. Due to the complex nature of the coupled system’s transfer function and the required analysis, it is beyond the scope of this publication to present a full solution. Instead, a simplified approach to estimate the minimal detectable frequency shift for the co-resonant system based on the effective sensor properties is given. Conclusion: By establishing a theoretical description for the effective sensor properties of a co-resonantly coupled system, the design of such systems is facilitated as sensor parameters can easily be predicted and adapted for a desired use case. It allows to study the potential sensitivity (gain) and detectability capabilities before sensor fabrication in a fast and easy way, even for large parameter spaces. So far, such an analysis of a co-resonantly coupled sensor was only possible with numerical methods and even then only with very limited capability to include and understand the complex interplay between all contributions. The outlined calculation steps regarding the noise considerations in a coupled harmonic oscillator system can provide the basis for a thorough study of that question. Furthermore, in a broader scope, the investigations presented within this work contribute towards extending and completing the already established theoretical basics of this novel co-resonant sensor concept and open up new ways of studying the coupled system’s behaviour.