Molecular plasmonics for nanoscale spectroscopy.

Surface- and tip-enhanced Raman and LSPR spectroscopies have developed over the past 15 years as unique tools for uncovering the properties of single particles and single molecules that are unobservable in ensemble measurements. Measurements of individual events provide insight into the distribution of molecular properties that are averaged over in ensemble experiments. Raman and LSPR spectroscopy can provide detailed information on the identity of molecular species and changes in the local environment, respectively. In this review a detailed discussion is presented on single-molecule and single-particle Raman and LSPR spectroscopy focusing on the major developments in the fields and applications of the techniques.

A localized surface plasmon resonance imaging instrument for multiplexed biosensing.

Localized surface plasmon resonance (LSPR) spectroscopy has been widely used for label-free, highly sensitive measurements of interactions at a surface. LSPR imaging (LSPRi) has the full advantages of LSPR but enables high-throughput, multiplexed measurements by simultaneously probing multiple individually addressable sensors on a single sample surface. Each spatially distinct sensor can be tailored to provide data regarding different surface functionalities or reaction environments. Previously, LSPRi has focused on single-particle sensing where the size scale is very small. Here, we create defined macroscale arrays of nanoparticles that are compatible with common patterning methods such as dip-pen nanolithography and multichannel microfluidic delivery devices. With this new LSPR sensing format, we report the first demonstration of multiplexed LSPR imaging and show that the increased throughput of our instrument enables the collection of a complete Langmuir binding curve on a single sensor surface. In addition, the multiplexed LSPR sensor is highly selective, as demonstrated by the hybridization of single-stranded DNA to complementary sequences immobilized on the sensor surface. The LSPR arrays described in this work exhibit uniform sensitivity and tailorable optical properties, making them an ideal platform for high-throughput, label-free analysis of a variety of molecular binding interactions.

Single plasmonic nanoparticle tracking studies of solid supported bilayers with ganglioside lipids.

Single-particle tracking experiments were carried out with gold nanoparticle-labeled solid supported lipid bilayers (SLBs) containing increasing concentrations of ganglioside (GM(1)). The negatively charged nanoparticles electrostatically associate with a small percentage of positively charged lipids (ethyl phosphatidylcholine) in the bilayers. The samples containing no GM(1) show random diffusion in 92% of the particles examined with a diffusion constant of 4.3(±4.5) × 10(-9) cm(2)/s. In contrast, samples containing 14% GM(1) showed a mixture of particles displaying both random and confined diffusion, with the majority of particles, 62%, showing confined diffusion. Control experiments support the notion that the nanoparticles are not associating with the GM(1) moieties but instead most likely confined to regions in between the GM(1) clusters. Analysis of the root-mean-squared displacement plots for all of the data reveals decreasing trends in the confined diffusion constant and diameter of the confining region versus increasing GM(1) concentration. In addition, a linearly decreasing trend is observed for the percentage of randomly diffusing particles versus GM(1) concentration, which offers a simple, direct way to measure the percolation threshold for this system, which has not previously been measured. The percolation threshold is found to be 22% GM(1) and the confining diameter at the percolation threshold only ∼50 nm.

Advances in localized surface plasmon resonance spectroscopy biosensing.

In recent years, localized surface plasmon resonance (LSPR) spectroscopy advancements have made it a sensitive, flexible tool for probing biological interactions. Here, we describe the basic principles of this nanoparticle-based sensing technique, the ways nanoparticles can be tailored to optimize sensing, and examples of novel LSPR spectroscopy applications. These include detecting small molecules via protein conformational changes and resonance LSPR spectroscopy, as well as coupling LSPR with mass spectrometry to identify bound analytes. The last few sections highlight the advantages of single nanoparticle LSPR, in that it lowers limits of detection, allows multiplexing on the nanometer scale, and enables free diffusion of sensors in solution. The cases discussed herein illustrate creative ways that LSPR spectroscopy has been improved to achieve new sensing capabilities.