Differential Magnetic Biosensor using HTS SQUID Gradiometer

Doctoral thesis, 2019

A fundamental tool for containing an epidemic outbreak and mitigating its effects is early diagnostics. Currently, most of the diagnostic tests are performed by trained staff in centralized labs, which are expensive and time-consuming to establish and operate. Lack of access to such facilities could have devastating effects. The principal motivation behind point-of-care diagnostic systems is to provide a low cost, fast, sensitive, and specific test in the field which does not require highly skilled staff to operate.

This thesis describes a magnetic biosensor which takes advantage of a high-Tc superconducting quantum interference device (SQUID) gradiometer sensor and magnetic nanoparticles (MNPs) to develop a diagnostic unit for point-of-care. Rolling circle amplification (RCA) is used as the primary molecular amplification method. RCA is an isothermal process with very high specificity. It is, therefore, easy to implement in a mix and measure concept of a homogeneous assay. The specific binding of the MNPs to the products of the RCA (i.e., DNA coils) changes their relaxation dynamics which is detected by sensitive ac magnetic susceptibility measurement.

One of the issues with homogeneous magnetic assays, which limits their sensitivity, is the presence of excess MNP labels in the test sample solution. To mitigate this problem, a novel technique is introduced, which takes advantage of the geometry of our gradiometer sensor for a differential ac magnetic susceptibility measurement. In this technique, a negative control sample and a positive test sample are measured in a single measurement. The differential measurement virtually removes all of the unbound MNPs in the test sample and is analogous to the physical washing step typically used in conventional assays. This technique also provides better signal to noise ratio (SNR) and can detect target concentrations down to tens of femtomolar levels (45 fM).

To eliminate the use of liquid nitrogen (LN2) for cooling of the SQUID sensor (as it is not abundantly available in the field) we have shown the successful operation of a SQUID gradiometer sensor on a commercially available micro-cooler platform. The operation of the SQUID on the micro-cooler and the high sensitivity of the novel differential ac susceptibility technique, realized in this work, are critical steps towards a homogeneous magnetic nucleic acid biosensor for rapid detection of diseases. The methods and instruments that are adopted and presented here are generic and could, in principle, be used for other targets such as Influenza, Ebola, and Zika. With full implementation of the molecular amplification on a disposable lab-on-a-chip, the unit would be promising for rapid and highly sensitive diagnostics at the point-of-care.