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A simple, sensitive SERS-based immunoassay realized in aqueous solution is demonstrated with a sandwich immune protocol. In such an immunoassay, antibodies-immobilized silica nanoparticles served as the immune substrate while 4MBA-labeled immuno-Au nanoparticles are used as the immune sensors. According to the TEM images, it is clear that the immune gold nanoparticles are embedded onto the surfaces of the silica nanoparticles specifically after the immunoreaction. As a result, the aggregations of gold nanoparticles have been formed with SERS-active "hot spots" on the dimers or multimers. The SERS results confirm that the method proposed in this paper is an effective way for SERS-based aqueous immunoassay and that the detection limit is as low as 0.1 ng/mL.

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The sandwich-type immunoassays have been developed by using electrochemical and surface-enhanced Raman scattering (SERS) techniques for the detection of carcinoembryonic antigen (CEA). Nile blue as a kind of Raman dye has been decorated on nanospheres with polydopamine resin (PDR) via $\pi$-stacking interaction. The Nile blue displays the strong SERS signals as well as a characteristic electrochemical reduction peak at $-$0.33V (versus Ag/AgCl). The implementation of the PDR nanospheres mixing with Au nanoparticles (AuNPs/PDR) exhibits a better electrical conductivity and large SERS enhancement. The immunoassays based on Nile blue-labeled AuNPs/PDR nanospheres have been fabricated by using electrochemical and SERS techniques for the detection of CEA. The proposed immunoassay shows higher sensitivity, high selectivity, lower detection limit and long-term stability. The performances of the electrochemical immunoassay are better than that of SERS immunoassay. For the electrochemical immunoassay, the linear range occurs from 1pg/mL to 100ng/mL ($R=0.995$) with a detection limit of 0.68pg/mL and signal-to-noise ratio of 3 in the detection of CEA. The data for the analysis of human serum samples by using the electrochemical method are acceptably consistent with those obtained from the enzyme-linked immunosorbent assay (ELISA). The proposed immunoassay exhibits a promising potential for application in clinical diagnosis.

Ultrasensitive detection of alpha-fetoprotein (AFP) is critical for the early diagnosis of liver cancer. In this work, a novel surface-enhanced Raman scattering (SERS)-based immunoassay complex has been successfully developed for the detection of AFP by using the Au-Ag alloy nanoparticals and the Ag/AgBr hybrid nanostructure. As the typical bimetal or metal/semiconductor plasmonic materials, besides the strong SERS enhancement characteristics, the Au-Ag alloy nanoparticals exhibit excellent monodispersity and the Ag/AgBr hybrid nanostructure demonstrates good stability. The experimental results show that the SERS-based immunoassay of AFP presents a low limit of detection of 1.86fg/mL and a broad dynamic range from 2fg/mL to 0.8$\mu$g/mL. Furthermore, the clinical applicability of the proposed SERS-based immunoassay has been assessed by the detection of AFP in the human serum samples of cancer patient and healthy person. The test data are consistent well with that of chemiluminescence immunoassay (CLIA) in the relative errors of $-$8.82–8.06% and show better detection sensitivity. It reveals that the proposed immunoassay protocol is significant for giving insight into the design of ultrasensitive biosensor and the point-of-care testing of cancers.

We propose an improved immunoassay method with EWOD-based microfluidic biochip. Electro-wetting on dielectric (EWOD) is based on contact angle-changed phenomenon of droplet on the channel surface. The reactant-containing droplet can be moved from reservoir to a modified electrode area, which a stir function can be provided by controlling AC electro-osmosis flow (ACEOF). The immunochemistry reaction takes place on our modified area via electro-wetting actuation, and the signal from fluorescent or electrochemical impedance spectroscopy (EIS) was used to confirm the performance of designed element.