Broadband <i>LC</i> Oscillator as a High-Precision Radio Frequency Refractometer for Nanoparticle Quantification

This work uses a custom oscillator circuit to explore an inductive–capacitive approach to nanoparticle (NP) quantification. The development of quantitative lateral flow tests has prompted the design of devices that can assess the number of detection markers at the test line. These markers include latex, gold, silver, and magnetic NPs (MNPs). The aim is to provide quantification without compromising the typical advantages of lateral flow assays: ease of use, portability, and low cost. In the previous work, we described a sensor based on a coil’s self-resonant frequency (SRF) for high-precision detection of both magnetic and nonmagnetic NPs. The realization of such a device requires a reliable measurement of the SRF. Here, we face this challenge by introducing a new broadband LC oscillator that transforms the sensing coil’s impedance curve into a frequency power spectrum. This allows for straightforward, fast, and robust SRF detection. The oscillator is based on an ad hoc astable circuit supported by theoretical modeling, simulations, and experimental validation. We call this oscillator an astable tuned oscillator (ATO). Coupled with a frequency counter, the system enables quantification of microgram levels of various NPs. A detection limit of 650 ng was achieved for magnetite NPs using a compact and cost-effective setup. Beyond biosensing, the oscillator can support inductive sensing applications, where slight variations in coil response are used to detect changes in materials or environments, such as in structural monitoring, material inspection, or soft matter characterization.