The localized surface plasmon resonance of metal nanoparticles is the collective oscillation of electrons on particle surface, induced by incident light, and is a particle composition-, morphology-, and coupling-dependent property. Plasmonic engineering deals with highly precise formation of the targeted nanostructures with targeted plasmonic properties (e.g., electromagnetic field distribution and enhancement) via controlled synthetic, assembling, and atomic/molecular tuning strategies. These plasmonically engineered nanoprobes (PENs) have a variety of unique and beneficial physical, chemical, and biological properties, including optical signal enhancement, catalytic, and local temperature-tuning photothermal properties. In particular, for biomedical applications, there are many useful properties from PENs including LSPR-based sensing, surface-enhanced Raman scattering, metal-enhanced fluorescence, dark-field light-scattering, metal-mediated fluorescence resonance energy transfer, photothermal effect, photodynamic effect, photoacoustic effect, and plasmon-induced circular dichroism. These properties can be utilized for the development of new biotechnologies and biosensing, bioimaging, therapeutic, and theranostic applications in medicine. This Perspective introduces the concept of plasmonic engineering in designing and synthesizing PENs for biomedical applications, gives recent examples of biomedically functional PENs, and discusses the issues and future prospects of PENs for practical applications in bioscience, biotechnology, and medicine.

The application scope of plasmonic nanostructures is rapidly expanding to keep pace with the ongoing development of various scientific findings and emerging technologies. However, most plasmonic nanostructures heavily depend on rare, expensive, and extensively studied noble metals such as Au and Ag, with the limited choice of elements hindering their broad and practical applications in a wide spectral range. Therefore, abundant and inexpensive nonnoble metals have attracted attention as new plasmonic nanomaterial components, allowing these nonnoble-metal-based materials to be used in areas such as photocatalysis, sensing, nanoantennas, metamaterials, and magnetoplasmonics with new compositions, structures, and properties. Furthermore, the use of nonnoble metal hybrids results in newly emerging or synergistic properties not observed from single-metal component systems. Here, the synthetic strategies and recent advances in nonnoble-metal-based plasmonic nanostructures comprising Cu, Al, Mg, In, Ga, Pb, Ni, Co, Fe, and related hybrids are highlighted, and a discussion and perspectives in their synthesis, properties, applications, and challenges are presented.

Observation of individual single-nanoparticle reactions provides direct information and insight for many complex chemical, physical, and biological processes, but this is utterly challenging with conventional high-resolution imaging techniques on conventional platforms. Here, we developed a photostable plasmonic nanoparticle-modified supported lipid bilayer (PNP-SLB) platform that allows for massively parallel in situ analysis of the interactions between nanoparticles with single-particle resolution on a two-dimensional (2D) fluidic surface. Each particle-by-particle PNP clustering process was monitored in real time and quantified via analysis of individual particle diffusion trajectories and single-particle-level plasmonic coupling. Importantly, the PNP-SLB-based nanoparticle cluster growth kinetics result was fitted well. As an application example, we performed a DNA detection assay, and the result suggests that our approach has very promising sensitivity and dynamic range (high attomolar to high femtomolar) without optimization, as well as remarkable single-base mismatch discrimination capability. The method shown herein can be readily applied for many different types of intermolecular and interparticle interactions and provide convenient tools and new insights for studying dynamic interactions on a highly controllable and analytical platform.

Multiplexed real-time analysis on multiple interacting molecules and particles is needed to obtain information on binding patterns between multiple ligands and receptors, specificity of bond formations, and interacting pairs in a complex medium, often found in chemical and biological systems, and difference in binding affinity and kinetics for different binding pairs in one solution. In particular, multiplexed profiling of microRNA (miRNA) in a reliable, quantitative manner is of great demand for the use of miRNA in cell biology, biosensing, and clinical diagnostic applications, and accurate diagnosis of cancers with miRNA is not possible without detecting multiple miRNA sequences in a highly specific manner. Here, we report a multiplexed molecular detection strategy with optokinetically (OK) coded nanoprobes (NPs) that show high photostability, distinct optical signals, and dynamic behaviors on a supported lipid bilayer (SLB) (OK-NLB assay). Metal NPs with three distinct dark-field light scattering signals [red (R), green (G), and blue (B)] and three different target miRNA half-complements were tethered to a two dimensionally fluidic SLB with mobile (M) or immobile (I) state. In situ single-particle monitoring and normalized RGB analysis of the optokinetically combinatorial assemblies among three M-NPs and three I-NPs with dark-field microscopy (DFM) allow for differentiating and quantifying 9 different miRNA targets in one sample. The OK-NP-based assay enables simultaneous detection of multiple miRNA targets in a highly quantitative, specific manner within 1 h and can be potentially used for diagnosis of different cancer types. We validated the OK-NLB assay with single-base mismatched experiments and HeLa cell-extracted total RNA samples by comparing the assay results to the quantitative reverse transcription polymerase chain reaction (qRT-PCR) results.

LAT assembly into a two-dimensional protein condensate is a prominent feature of antigen discrimination by T cells. Here, we use single-molecule imaging techniques to resolve the spatial position and temporal duration of each pMHC:TCR molecular binding event while simultaneously monitoring LAT condensation at the membrane. An individual binding event is sufficient to trigger a LAT condensate, which is self-limiting, and neither its size nor lifetime is correlated with the duration of the originating pMHC:TCR binding event. Only the probability of the LAT condensate forming is related to the pMHC:TCR binding dwell time. LAT condenses abruptly, but after an extended delay from the originating binding event. A LAT mutation that facilitates phosphorylation at the PLC-γ1 recruitment site shortens the delay time to LAT condensation and alters T cell antigen specificity. These results identify a function for the LAT protein condensation phase transition in setting antigen discrimination thresholds in T cells.