Owing to their excellent multiplexing ability, high sensitivity, and large dynamic range, immunoassays using surface-enhanced Raman scattering (SERS) as the readout signal have found prosperous applications in fields such as disease diagnosis, environmental surveillance, and food safety supervision. Various ever-increasing demands have promoted SERS-based immunoassays from the classical sandwich-type ones to those integrated with fascinating automatic platforms (e.g., test strips and microfluidic chips). As recent years have witnessed impressive progress in SERS immunoassays, we try to comprehensively cover SERS-based immunoassays from their basic working principles to specific applications. Focusing on several basic elements in SERS immunoassays, typical structures of SERS nanoprobes, productive optical spectral encoding strategies, and popular immunoassay platforms are highlighted, followed by their representative biological applications in the last 5 years. Moreover, despite the vast advances achieved to date, SERS immunoassays still suffer from some annoying shortcomings. Thus, proposals on how to improve the SERS immunoassay performance are also discussed, as well as future challenges and perspectives, aiming to give brief and valid guidelines for choosing suitable platforms according to particular applications.

Unlike widely used postsynthetic halide exchange for CsPbX (X is halide) perovskite nanocrystals (NCs), cation exchange of Pb is of a great challenge due to the rigid nature of the Pb cationic sublattice. Actually, cation exchange has more potential for rendering NCs with peculiar properties. Herein, a novel halide exchange-driven cation exchange (HEDCE) strategy is developed to prepare dually emitting Mn-doped CsPb(Cl/Br) NCs via postsynthetic replacement of partial Pb in preformed perovskite NCs. The basic idea for HEDCE is that the partial cation exchange of Pb by Mn has a large probability to occur as a concomitant result for opening the rigid halide octahedron structure around Pb during halide exchange. Compared to traditional ionic exchange, HEDCE is featured by proceeding of halide exchange and cation exchange at the same time and lattice site. The time and space requirements make only MnCl molecules (rather than mixture of Mn and Cl ions) capable of doping into perovskite NCs. This special molecular doping nature results in a series of unusual phenomenon, including long reaction time, core-shell structured mid states with triple emission bands, and dopant molecules composition-dependent doping process. As-prepared dual-emitting Mn-doped CsPb(Cl/Br) NCs are available for ratiometric temperature sensing.

We report an intracellular pH sensor based on surface enhanced Raman scattering (SERS) using the hydrochloric acid (HCl) treated gold nanorods (GNRs) as the SERS substrates and p-aminothiophenol (pATP) as the Raman reporter. Using the HCl treated GNRs previously reported by us, the biocompatibility and the SERS performance of GNRs have been greatly improved. Meanwhile, the adsorbed reporters are allowed to be directly exposed to the surrounding environments, which is very important for biosensors. It is found that the SERS spectrum of pATP is strongly dependent on the pH value. The intensities of SERS bands at 1142 cm(-1), 1390 cm(-1), and 1432 cm(-1) increased obviously with the pH value varying from 3.0 to 8.0. This pH-dependent SERS performance of pATP-functionalized HCl treated GNRs was well retained after the incorporation of the GNRs into living HeLa cells. Our experimental results indicate that such pATP-functionalized HCl treated GNRs can be used as an effective intracellular pH sensor. Thus, we show a good example that the bioapplications of the normal CTAB-stabilized GNRs can be expanded after the simple HCl treatment.

As a kind of most important cancer biomarker, exosomes are getting more frequently investigated in cancer diagnosis. In this study, we proposed an SERS-based method for the screening and simultaneous multiple detection of exosomes using magnetic substrates and SERS probes. Specifically, the capturing substrates are achieved using gold shell magnetic nanobeads modified by aptamers, which can capture most kinds of exosomes by recognizing the generic surface protein CD63. Moreover, the SERS probes are made of gold nanoparticles decorated with a Raman reporter and a specific aptamer for targeting exosomes. Further, for the simultaneous detection of multiple kinds of exosomes, three kinds of SERS probes were designed using different SERS reporters. While detecting specific kinds of exosomes, the capturing substrates were mixed with these three kinds of SERS probes. When the target exosome is present, an apta-immunocomplex can be formed among the target exosomes, the substrate, and the corresponding kind of SERS probes, and the other non-specific SERS probes remain in the suspension. Hence, an SERS signal with a decreased intensity will be detected in the supernatant, indicating the presence of the target exosomes. Finally, this detection method has also been successfully employed for the detection of exosomes in real blood samples; this proves that the proposed SERS-based method is a promising tool for clinical cancer screening based on exosomes.

A new concept of optical encoding approach, surface enhanced Raman scattering (SERS)-fluorescence joint spectral encoding method (SFJSE), was demonstrated by using organic-metal-quantum dot (QD) hybrid nanoparticles (OMQ NPs) with a nanolayered structure. This method has two distinct characteristics, which make it more feasible to achieve enormous codes in practice, compared with a sole fluorescence- or SERS-based encoding protocol. One of the two characteristics is to use the joint SERS and fluorescence spectra as the encoding elements instead of an individual optical signal, resulting in a broadened optical spectrum range for efficient encoding. The other is to assemble SERS reporters and fluorescent agents onto different layers of OMQ NPs, leading to an easier fabrication protocol when a large number of agents need to be involved into encoding carriers. By conjugating different antibodies to OMQ NPs with varied codes, the potential application of such an encoding system in high-throughput detection has been investigated by multiplex sandwich immunoassays. The high specificity and sensitivity of the assays suggest that the SFJSE method could be developed as a powerful encoding tool for high-throughput bioanalysis with the use of OMQ NPs.