Developing an early rapid and highly sensitive diagnostic technology is of great significance to the prevention and treatment of diseases. However, current detection methods require tedious steps, long analytical time, high cost, advanced instruments, and skillful personnel, even with high sensitivity and specificity. Immunochromatographic assay (ICA) is currently recognized as the most promising method for point of care testing (POCT) due to its portability, simple operation, low cost, and short detection time. However, traditional ICA is judged by the visual detection results produced by colloidal gold nanoparticles, and can merely achieve qualitative and semi-quantitative detection. To overcome the disadvantages of low sensitivity and non-quantitative detection, researchers apply PCR, fluorescent probes, and surface-enhanced Raman scattering (SERS) to immunochromatographic systems, greatly improving the sensitivity and quantitative detection properties of ICA. SERS is an excellent analytical method with high sensitivity, against photobleaching, narrow bandwidth, and multi-channel detection.

SERS-ICA is the research on cutting-edge technologies and has become a research hotspot in related fields recently, which combines the advantages of SERS including high throughput and sensitivity, and ICA featuring simpleness and rapid speed. SERS immuno-tags prepared by nanomaterials are employed to replace the traditional colloidal gold nanoparticles and can provide SERS signals for quantitative detection. SERS immuno-tags are mainly composed of three parts containing noble metal nanomaterials (e.g., gold, silver), Raman report molecules, and specific recognition elements (e.g., antibodies, aptamer, nucleic acids). Notably, Raman report molecules are adopted to provide the characteristic Raman signals, and their SERS signal intensities will be greatly enhanced while approaching the rough surface of the noble metal nanomaterials. Additionally, the specific recognition elements are applied to specifically recognize and capture the targets from the sample solutions. Quantitative detection of SERS-ICA is achieved by collecting and analyzing the SERS signals on the test line of the strips produced by the intercepted SERS immuno-tags.

High-performance SERS tags play a key role in SERS-ICA detection. Many studies concentrate on developing the nanomaterials with high density "hot spots", and improving the SERS signals by constructing multi-dimensional and high-density "hot spots" on the SERS substrates. To improve the detection sensitivity, in recent years, researchers have synthesized the SERS substrates with strong SERS enhanced performances by designing and optimizing the particle sizes, morphology and structures of the nanomaterials (Figs. 2-5). Raman report molecules and antibodies are successively conjugated on the SERS substrates to prepare functionalized SERS immuno-tags. The as-prepared SERS immuno-tags can specifically capture the target antigen to form the immunocomplex of tags-antigen and migrate on the ICA strips by capillary action towards the absorbent pad. In addition, the detection antibodies precoated on the test line of the ICA strips can specifically identify the target antigen and capture the immunocomplex of tags-antigen. Therefore, visual band and SERS signals can be found on the test line due to the formation of antibody-antigen-antibody sandwich composite structure. Then, the SERS signals on the test line are collected by the Raman detector, and the calibration curves of the SERS signal intensities and the corresponding concentrations of target antigen are plotted for quantitative analysis of the unknown target concentration in the sample. For improving the detection efficiency of SERS-ICA, researchers have set up multiple testing lines or testing dots on one ICA strip (Figs. 6-10). Meanwhile, based on the characteristic Raman fingerprint spectra, different Raman report molecules with uncrossed characteristic Raman shifts are modified on the SERS substrates to distinguish different targets on the same sites and this feature is applied to SERS-ICA (Figs. 8-9). Moreover, the as-reported integrated multi-channel immunochromatography reaction column greatly improves the multiplexing and automation detection properties of SERS-ICA (Fig. 10).

We briefly introduce the basic principles of SERS and ICA and summarize several different SERS substrates for SERS-ICA and the application of SERS-ICA in different detection fields. It is of significance for increasing the detection sensitivity of SERS-ICA to improve the antigen-capture ability of the SERS tags and SERS enhanced properties of the SERS substrates. Finally, the future development trend of SERS-ICA detection technology is prospected.