Colorful “Stars” in the Dark
Plasmonic nanoparticles such as Au and Ag with localized surface plasmon resonance (LSPR) property exhibit unique scattering and absorption features. The plasmonic scattering and absorption bands are mainly located at visible light region which can be easily applied in visual detections. By modulating the size, shape, and composition of gold and silver colloid solutions, plenty of colorimetric methods have been constructed for the detection of metal ions, biomolecules, and environmental contaminants, etc. For many years, the LSPR-based measurements are implemented in reagent tubes. Since 2000, the plasmon resonance scattering (PRS) light of metal nanoparticles captured by dark-field microscopy enables the investigation at the nanoscale dimension. Mono-dispersed nanoparticles under a dark-field microscope showed distinct scattering light spots, like colorful stars in the dark sky. The PRS light of a single nanoparticle opens a new way for ultra-sensitive sensing which eliminates the average effects in bulk and provides more accurate reaction information. Thus, individual nanoparticles with specific scattering colors are excellent nanoprobes in the applications of biology, physics, and chemistry. In this review, the plasmonics based colorimetric nanosensors are presented. Particularly, the application of in-situ PRS in the dynamically monitoring of electrocatalytic reactions is highlighted. We firstly introduce a short history of the discovery and development of plasmonic nanoparticles from the ancient artwork to the modern characterization techniques. Some factors including morphology and dielectric constants that are correlated to the LSPR bands and scattering light colors are listed. Secondly, we demonstrate the use of single plasmonic nanoparticles as visualized color-coded nanoprobes. As the morphology of particles has strong effect on the PRS light, elegant sensors have been conceived by the etching and growth of nanoparticles with different size and shape. On the other hand, the real-time monitoring of particle structure evolution could also reveal the mechanism of the material fabrication at the nanoscale. In addition, core-satellite nanostructures with various linker are proposed as ultra-sensitive sensors according to the inter-particle coupling effect. Subsequently, we summarize several advanced techniques for nanoscale signal extraction and amplifications. For instance, to expand the application of the colorimetric nanosensors, converting the colors into RGB values could clearly distinguish the subtle color changes. Combining with high-throughput signal processing method, thousands of nanoparticles can be rapidly analyzed which can greatly enhance the measurement efficiency. Except the PRS color, the PRS intensity could also provide abundant information and is easier to be captured. A facile method by converting the PRS intensity of single nanoparticles into visible colors are presented which is mighty suitable for the in-situ monitoring of fast electrochemical process with high time resolution.