Martin Nirschl

CMOS-Integrated Film Bulk Acoustic Resonators for Label-Free Biosensing

The throughput is an important parameter for label-free biosensors. Acoustic resonators like the quartz crystal microbalance have a low throughput because the number of sensors which can be used at the same time is limited. Here we present an array of 64 CMOS-integrated film bulk acoustic resonators. We compare the performance with surface plasmon resonance and the quartz crystal microbalance and demonstrate the performance of the sensor for multiplexed detection of DNA.

The application of polythiol molecules for protein immobilisation on sensor surfaces

The immobilisation of bio-receptors on transducer surfaces is a key step in the development of biosensors. The immobilisation needs to be fast, cheap and most importantly should not affect the biorecognition activity of the immobilised receptor. The development of a protocol for biomolecule immobilisation onto a surface plasmon resonance (SPR) sensor surface using inexpensive polythiol compounds is presented here. The method used here is based on the reaction between primary amines and thioacetal groups, formed upon reaction of o-phthaldialdehyde (OPA) and thiol compounds. The self-assembled thiol monolayers were characterised using contact angle and XPS. The possibility to immobilise proteins on monolayers was assessed by employing BSA as a model protein. For the polythiol layers exhibiting the best performance, a general protocol was optimised suitable for the immobilisation of enzymes and antibodies such as anti-prostate specific antigen (anti-PSA) and anti Salmonella typhimurium. The kinetic data was obtained for PSA binding to anti-PSA and for S. typhimurium cells with a detection limit of 5x10(6) cells mL(-1) with minimal non-specific binding of other biomolecules. These findings make this technique a very promising alternative for amine coupling compared to peptide bond formation. Additionally, it offers opportunity for immobilising proteins (even those with low isoelectric point) on neutral polythiol layers without any activation step.

Investigation of in-vitro bacterial surface layer formation by FBARs

We report investigations on the adsorption and in-vitro recrystallization kinetics of the bacterial surface layer protein of Bacillus sphaericus NCTC 9602 on gold surfaces by means of film bulk acoustic resonators. The acoustic resonators were operated in shear mode at about 800 MHz. From the measured changes of frequency and in dissipation, the mass and the viscoelasticity of biomolecular films formed at the top electrode of the device could be derived, respectively. The measured data revealed that protein adsorption is a fast process while the time constant for the recrystallization of the monomers into ordered two-dimensional protein crystals is typically on the order of 1 h.

Frequency Response of Thin-Film Bulk Acoustic Resonators to the Deposition of Tungsten, Platinum, Aluminium Oxide and Carbon Nanotube Thin-Films

Thin-film bulk acoustic resonators (FBAR) can be used as mass sensors when the adsorbed mass is linear to the frequency shift caused by the adsorption. This is, however, only the case if the adsorbed layer is thin compared to the thickness of the resonator. In this paper, we investigate the adsorption of films with thicknesses of some nanometres up to few hundreds of nanometres. With this range, we cover films thicknesses being small compared to the resonator and thicknesses in the range of the resonator thickness. The adsorption of materials was simulated for materials with different mass densities and acoustic velocities. Thin films of platinum, aluminium oxide, tungsten and carbon nanotubes were deposited on the FBAR and the results were fitted to the model used in the simulations. The acoustic velocity of the carbon nanotube films was much lower than the other materials investigated in this study. With this interesting property, carbon nanotube thin-films are a promising material for acoustic devices where materials with particularly low acoustic impedance are desired. The paper shows that the FBAR can be a useful tool to characterise mechanical properties of thin films in situ in the micro- and nanoscale within a certain range of parameters.