This work demonstrates the preliminary results of rapid and direct detection of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) using the quartz crystal microbalance (QCM) method. Coronavirus Disease 2019 (COVID-19)-specific RNA-dependent RNA polymerase (RdRP) gene-dependent probe DNA was used as a selective agent toward target DNA, the inactivated SARS-CoV-2 virus, and RNAs extracted from clinical samples. This study developed and utilised a unique dry-QCM approach with a mitigated experimental procedure. Contact angle measurements, Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) measurements were employed to investigate the surface during probe immobilisation and target hybridisation. This study also investigates the effect of temperature on probe immobilisation and target hybridisation. The estimated probe density was 0.51 × 10¹² probes/cm², which is below the critical limit. The estimated hybridisation efficiency was about 58.9%. The linear detection range with a Limit of Detection (LoD) was about ~1.22 nM with high selectivity toward SARS-CoV-2 target DNA. The sensor shelf-life was found to be extended to 25 days. The novelty of using a new dry-QCM approach for SARS-CoV-2 detection was proven with the results.

Distal-less homeobox 1 (DLX1) has emerged as a promising urinary biomarker for prostate cancer. This study developed a label-free piezoelectric acoustic biosensor for selective DLX1 detection using a quartz crystal microbalance (QCM). The QCM gold electrode was sequentially functionalized with L-cysteine, graphene oxide (GO), and an amine-terminated DLX1 capture probe covalently immobilized through EDC–NHS-mediated amide bond formation. Stepwise surface functionalization was characterized by X-ray photoelectron spectroscopy (XPS), supported by contact angle measurements, X-ray diffraction, and field-emission scanning electron microscopy. XPS provided multi-element evidence for Au–S thiolate formation, GO deposition, amide coupling, probe immobilization, and Watson–Crick hybridization with the synthetic DLX1 target. Under optimized conditions, the biosensor exhibited a linear response to DLX1 concentrations and achieved a limit of detection of 81.19 nM. Non-complementary sequences, including PCA3 and SARS-CoV-2, produced frequency shifts below 9 Hz, confirming high selectivity. Comparative experiments showed that GO-mediated covalent immobilization was essential for reliable detection, whereas direct DNA attachment to bare Au generated anomalous positive frequency shifts, which were attributed to weak physisorption. The proposed platform offers a sensitive and selective strategy for quantitative DLX1 detection and may support future point-of-care nucleic acid diagnostics.

This study investigates the acoustic response of ZnO-based SAW devices fabricated on ZnO/Si and ZnO/SiO₂/Si substrates through a combined finite element analysis and experimental approach. Two-dimensional FEA was used to analyze scattering parameters, three-dimensional FEA was used to determine eigenfrequencies and mode shapes, and ZnO thin films were deposited by RF magnetron sputtering with interdigital transducers defined by UV lithography. The ZnO/SiO₂/Si device exhibited Rayleigh and Sezawa-type mode resonances at 145 MHz (4350 m/s) and 234 MHz (7020 m/s), respectively, in close agreement with simulation, while the ZnO/Si device resonated at 166 MHz with a phase velocity of 4980 m/s. Incorporation of the Au sensing layer improved signal transmission by approximately 2 dB, consistent with modeling predictions. For biosensing evaluation, the device was functionalized with a thiolated ssDNA probe targeting the EGFR L858R point mutation, a clinically relevant lung cancer biomarker. Probe immobilization and target hybridization were confirmed by contact angle measurements and resonance frequency shifts, with the sensor demonstrating a linear detection range of 0.1 to 0.6 µM and LOD of 0.09 µM. These findings establish an integrated framework of acoustic modeling, microfabrication, and biofunctionalization for ZnO-based SAW biosensors toward label-free nucleic acid detection.