Chapter 17 Immunoassays: Future Prospects and Possibilities Sandeep K. Vashist1, John H.T. Luong 1IDS Immunodiagnostic Systems Deutschland GmbH, Frankfurt am Main, Germany;2University College Cork, Cork, Ireland 1. IMMUNOASSAYS: TRENDS AND PROSPECTS Significant advances in immunoassay (IA) formats and technologies during the last few decades [1-3] have drastically shortened the duration of an IA procedure manyfold in comparison with conventional radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA). ELISA is also difficult to develop for point-of-care (POC) use and requires significant technical expertise. Furthermore, the IA procedure is simplified as many process steps are integrated and automated using dedicated instruments. The evolution of microfluidics [4] and lab-on-a-chip (LOC) technologies [5] based on automated microfluidic protocols [6-8] has resulted in prospective semi- and fully automated immunoassays, enabling point-of-care testing (POCT) of analytes at remote settings. As the number of steps in the IA has also been critically reduced, a wide range of rapid IA chemistries have resulted in novel IA formats to detect analytes in less than 30 min [9-14]. The use of emerging micro- and nanomaterials has paved the way for IAs to achieve detectivity in the range of fg/mL. Such low or even lower detection limits are needed for early detection of cancer or the prompt detection of relapse after cancer treatment and therapy. To date, advances together with novel detection concepts such as chemiluminescent resonance energy transfer and fluores- cence resonance energy transfer have further pushed down the limits of sensitivity to attogram levels. The use of nanomaterials and novel IA concepts results in the emergence of visualized IAs [15,16], which can read and analyze pertinent information based on the visual detection. This interesting trend in immunodiagnostics has instigated an interest toward the development of instrument-free IAs. The surface plasmon resonance (SPR)-based BiaCore instruments for label-free IAs from GE Healthcare (previously Pharmacia Biosensor AB, Handbook of Immunoassay Technologies. https://doi.org/10.1016/B978-0-12-811762-0.00017-7 Copyright @ 2018 Elsevier Inc. All rights reserved. 455
456 Handbook of Immunoassay Technologies Biacore AB Corporation) are another striking advance, leading to the emer- gence of real-time label-free IAs. These systems have become the global standard to probe biomolecular interactions, the development of rapid IAs for analytes, and screening of appropriate immunological components for an IA [17,18]. Besides GE Healthcare as the leader in the field of SPR, other competitors have also developed prospective SPR-based instruments for IAs. The main characteristic features of SPR-based IAs are critically reduced sample-to-answer time of only a few minutes; minimal samples and reagents requirement of only tens of microliters; a label-free microfluidic IA procedure; and a fully automated analysis with a high throughput. Moreover, a large number of surface-functionalized SPR chips, immobilization chemistries, regeneration buffers, and assay protocols further facilitate the rapid develop- ment of SPR-based IAs. The low-cost IA formats including the disposable paper-based IAs [19-22] have been the long-cherished vision as they would be ideal for the developing nations and remote settings. The developed complementary technologies during the last decade have led to the emergence of several prospective paper- based IAs. However,