Sandwich-structured displays encapsulating polystyrene microspheres coated with Fe3O4 nanoparticles for label-free biosensing for electrorheological operation

Core/shell structures consisting of polystyrene microspheres (PM, similar to 2.2 mu m) and Fe3O4 nanoparticles (FeNPs, similar to 12.7 nm) were synthesized using a layer-by-layer process to vary the dielectric property. PM@FeNP solutions were encapsulated into home-made sandwich-structured displays (SSDs) to observe their transmittance change under an alternating electric field (AEF). The dispersed PM@FeNP blocked the light traveling through the SSD in the absence of AEF, resulting in an opaque state. In the presence of an AEF, the SSD could display the highest transmittance at the so-called characteristic frequency of the encapsulated microspheres, because of particle chaining. Furthermore, protein G and an antibody were coated onto the PM@FeNP, denoted as PM@FeNP@GIg, for use as a biosensor medium. The characteristic frequency of PM@FeNP@GIg shifted from 300 kHz to 750 kHz after antigen coupling, which could be visualized without any labeling because of the significant change in transmittance. The limit of detection (LOD), similar to 40 ng/mu L, could be identified within 30 s by the highest transmittance of SSD under AEF at 750 kHz. The transmittance was not affected by BSA or streptavidin, indicating high sensitivity and selectivity. This technique significantly simplifies common immunoassays, which may possess great potential for practical applications in label-free biosensors and visualizing antigen.