Direct detection of small molecules using a nano-molecular imprinted polymer receptor and a quartz crystal resonator driven at a fixed frequency and amplitude.

Small molecule detection is of wide interest in clinical and industrial applications. However, its accessibility is still limited as miniaturisation and system integration is challenged in reliability, costs and complexity. Here we combined a 14.3 MHz quartz crystal resonator (QCR), actuated and analysed using a fixed frequency drive (FFD) method, with a nanomolecular imprinted polymer for label-free, realtime detection of N-hexanoyl-L-homoserine lactone (199 Da), a gram-negative bacterial infection biomarker. The lowest concentration detected (1 µM) without any optimisation was comparable with that of a BIAcore SPR system, an expensive laboratory gold standard, with significant enhancement in sensitivity and specificity beyond the state-of-the-art QCR. The analytical formula-based FFD method can potentially allow a multiplexed "QCR-on-chip" technology, bringing a paradigm shift in speed, accessibility and affordability of small molecule detection.

Probing biomolecular interaction forces using an anharmonic acoustic technique for selective detection of bacterial spores.

Receptor-based detection of pathogens often suffers from non-specific interactions, and as most detection techniques cannot distinguish between affinities of interactions, false positive responses remain a plaguing reality. Here, we report an anharmonic acoustic based method of detection that addresses the inherent weakness of current ligand dependant assays. Spores of Bacillus subtilis (Bacillus anthracis simulant) were immobilized on a thickness-shear mode AT-cut quartz crystal functionalized with anti-spore antibody and the sensor was driven by a pure sinusoidal oscillation at increasing amplitude. Biomolecular interaction forces between the coupled spores and the accelerating surface caused a nonlinear modulation of the acoustic response of the crystal. In particular, the deviation in the third harmonic of the transduced electrical response versus oscillation amplitude of the sensor (signal) was found to be significant. Signals from the specifically-bound spores were clearly distinguishable in shape from those of the physisorbed streptavidin-coated polystyrene microbeads. The analytical model presented here enables estimation of the biomolecular interaction forces from the measured response. Thus, probing biomolecular interaction forces using the described technique can quantitatively detect pathogens and distinguish specific from non-specific interactions, with potential applicability to rapid point-of-care detection. This also serves as a potential tool for rapid force-spectroscopy, affinity-based biomolecular screening and mapping of molecular interaction networks.

Anharmonic surface interactions for biomolecular screening and characterization.

The acoustic response of conventional mechanical oscillators, such as a piezoelectric crystal, is predominantly harmonic at modest amplitudes. However, here, we observe from the electrical response that significant motional anharmonicity is introduced in the presence of attached analyte. Experiments were conducted with streptavidin-coated polystyrene microbeads of various sizes attached to a quartz crystal resonator via specific and nonspecific molecular tethers in liquid. Quantitative analysis reveals that the deviation of odd Fourier harmonics of the response caused by introduction of microbeads as a function of oscillation amplitude presents a unique signature of the molecular tether. Hence, the described anharmonic detection technique (ADT) based on this function allows screening of biomolecules and provides an additional level of selectivity in receptor-based detection that is often associated with nonspecific interactions. We also propose methods to extract mechanical force-extension characteristics of the molecular tether and activation energy using this technique.

Anharmonic interaction signals for acoustic detection of analyte.

The challenges with frequency-based acoustic detection systems in sensitive, selective, and reliable quantitative estimation of surface-bound analyte are well-known. These systems are traditionally used in their linear incarnations; i.e., the measurement frequency is the same as the driving frequency. However, it was found in this work that interactions of adsorbents with sensor surface show significant anharmonicity even at low drive amplitudes. In particular, using streptavidin-coated polystyrene microbeads on an oscillating quartz surface in air, it has been demonstrated through modeling and experiments that the anharmonic signal from microparticle to surface interaction is significantly higher relative to that from bare quartz and orders of magnitude higher than relative shifts in resonant frequency. The signal is proportional to the number of microparticles and holds a well-defined functional relationship with the amplitude of oscillation, distinct to the nature of interaction with the surface for a given analyte. This approach, thus, can be used for ultrasensitive and quantitative detection of surface adsorbents and characterization of different kinds of surface interactions, distinguishing specific from nonspecific adsorbents. The modeling also reveals a direct functional relationship between the measured anharmonic signal and the interaction potential of the adsorbent with the surface.