Smartphone enabled medical diagnostics by optically tracking electromagnetically induced harmonic oscillations of magnetic particles suspended in analytes

Abstract

Smartphone based point of care (POCT) medical diagnostic technology could potentially facilitate healthcare to remote locations with limited medical infrastrucure. However, such protocols exhibit background noise due to non-specific interactions as well as requiring long measurement times that limit their applications. We propose a procedure based on magnetic particle (MPs) that combines three dimensional electromagnetically induced oscillation of MPs with high precision optical tracking of the MPs. The measurement consisted of dropping MPs functionalized with streptavidin onto gold thin film actuators fabricated on silicon nitride substrates. Sinusoidal currents were passed through the actuators to move the MPs by horizontal dielectrophoretic forces. Simultaneously, a vertical magnetic field was applied to promote interaction between the MPs with biotin functionalized onto the substrates. The surfaces of MPs were functionalized with competing biotin whose concentration was varied. A high-resolution (4 K) video of the sensing surface was recorded with a smartphone and simultaneous tracking of 8000–10,000 MPs allowed us to identify MPs that interacted specifically with probes on the surface and ceased to show harmonic oscillations. Importantly, the dielectrophoretic forces reduced non-specific interactions and enhanced the probability of specific interactions while vertical magnetic forces accelerated the interaction of MPs with the functionalized surface. Our approach enabled a detection limit of 1 nM for biotin with a dynamic range of four orders of magnitude with a measurement time of approximately 2 min and video analysis time of 7 min. These results show that combining MPs and dynamic particle tracking is a promising method for practical POCT.

Keywords

• Point of care diagnostics
• Smartphone
• Particle tracking
• Magnetic particles