Magnetic particle-labeled biomaterial detection has attracted much attention in recent years for a number of reasons; easy manipulation by external magnetic fields, easy functionalization of the surface, and large surface-to-volume ratio, to name but a few. In this review, we report on our recent investigations into the detection of nano-sized magnetic particles. First, the detection by Hall magnetic sensor with lock-in amplifier and alternative magnetic field is summarized. Then, our approach to detect sub-200 nm diameter target magnetic particles via relatively large micoro-sized “columnar particles” by optical microscopy is described. Subsequently, we summarize magnetic particle detection based on optical techniques; one method is based on the scattering of the magnetically-assembled nano-sized magnetic bead chain in rotating magnetic fields and the other one is based on the reflection of magnetic target particles and porous silicon. Finally, we report recent works with reference to more familiar industrial products (such as smartphone-based medical diagnosis systems and magnetic removal of unspecific-binded nano-sized particles, or “magnetic washing”).

We describe wavelength tuning in a one dimensional (1D) silicon nitride nano-grating guided mode resonance (GMR) structure under conical mounting configuration of the device. When the GMR structure is rotated about the axis perpendicular to the surface of the device (azimuthal rotation) for light incident at oblique angles, the conditions for resonance are different than for conventional GMR structures under classical mounting. These resonance conditions enable tuning of the GMR peak position over a wide range of wavelengths. We experimental demonstrate tuning over a range of 375 nm between 500 nm˜875 nm. We present a theoretical model to explain the resonance conditions observed in our experiments and predict the peak positions with show excellent agreement with experiments. Our method for tuning wavelengths is simpler and more efficient than conventional procedures that employ variations in the design parameters of structures or conical mounting of two-dimensional (2D) GMR structures and enables a single 1D GMR device to function as a high efficiency wavelength filter over a wide range of wavelengths. We expect tunable filters based on this technique to be applicable in a wide range of fields including astronomy and biomedical imaging.

Fluorescent magnetic nanoparticles (f-MNPs) show potential as magnetic labels for detection of biomarkers, with their magnetic properties enhancing biomolecular interaction with external magnetic manipulation and fluorescence enabling optical detection of individual nanometer particles. These features translate into improvements in speed of detection, sensitivity, and low cost when both fluorescent and magnetic properties are utilized for biomarker detection. Specifically, f-MNPs show tremendous potential for developing smartphone based real-time diagnostic systems for monitoring potentially infectious diseases in remote areas, which is important for public healthcare to prevent epidemics, where rapid and sensitive medical diagnostic protocols designed to not only detect biomarkers but also to enable real-time information sharing are critical. We describe a new approach to biosensing by combining the advantages of magnetic and optical detection f-MNPs with the fluorescent imaging capabilities of CCD cameras of smartphones and the information sharing with cloud computing technology. Our protocol is designed to not only provide rapid sensing but also enable real-time sharing of medical diagnostics information between healthcare professionals and remote patients. As a feasibility study, we demonstrate the use of f-MNPs functionalized with prostate specific antigen (PSA) which is detected via the interactions of f-MNPs with the antibody coated silicon oxide surface. Images of 'dry samples' were obtained with a smartphone and analysed remotely with dedicated image processing programs on a cloud computer. Our approach enabled the detection of 100 pg ml −1 PSA in less than 60 s, which is an order of magnitude lower than the sensitivity required for clinical detection of PSA in public healthcare.

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