Nanoparticles Enhance Light-Based Detection

A new technique amplifies the light from small concentrations of molecules on a surface by maximizing the spectral overlap between the emission and the plasmon resonance of adjacent nanoparticles. The glowing molecules can be clearly seen with the naked eye when excited. (Source: J. Fitlow, Rice U.)

Rice University scientists have found revealing information where light from a molecule meets light from a nanoparticle. The labs of Rice chemists Christy Landes and Stephan Link demons­trated how to optimize a method that can sense small concen­trations of molecules by amplifying the light they emit when their spectral frequencies overlap with those of nearby plasmonic nano­particles. The surface plasmons, coherent electron waves that ripple across the surface of a metallic nano­particle, act as antennas and enhance the molecules’ emitted light up to 10 times when they’re in the “sweet spot” near a particle.

The work could help researchers analyze the active surfaces of catalysts and other materials at the nanoscale, an important step in improving their effi­ciency. The discovery relies on the pheno­menon of electrochemiluminescence (ECL), by which electricity drives chemical reactions that prompt molecules to emit light, said Thomas Heider­scheit, a Rice graduate student. It is often used to detect trace materials like heavy metals in water or the Zika virus in biological fluids.

Previous studies inferred that spectral overlap of the nano­particle and molecules would enhance the signal, but those studies could not account for the innate dif­ferences in nano­particle sizes and shapes that could mask the effects. The Rice researchers had set out to minimize these other impacts to focus only on the role of spectral frequency overlap on signal enhancement. “This study looks at what type of antenna is the best to use, because the properties of the nano­particle dictate the spectrum and its overlap with the molecule,” said Miranda Gallagher, a Rice post­doctoral research associate. “Should it be round or should it have sharp edges? Should it be smaller or larger?”

In experiments, the researchers combined either gold nano­spheres or sharp-tipped gold nano­triangles with a ruthenium-based dye molecule in a polymer shell that kept the molecules from migrating too far from the particles. “That’s essentially our electrode,” Heider­scheit said. “If we didn’t have the polymer, the dye molecules would be free to move and we’d see light diffused across the sample.” With the molecules constrained by the polymer, they could clearly see molecules emitting near particles. They determined signal enhancement is controlled by a combi­nation of size and frequency matching between the dye molecule and the nano­spheres, and just frequency matching for nano­triangles.

Single-molecule imaging is still a stretch for the nascent technique, Heider­scheit said. “Essen­tially, we’re imaging how active a surface is,” he said. “The Depart­ment of Energy – the main sponsor of the project – cares about this research because it could achieve super-reso­lution mapping of reac­tivity on a surface.” Super-resolution enables the capture of images below the diffraction limit of light. “For instance, if you have nano­particles in a battery system, you can use ECL to map where the reactions are most chemically active,” Heider­scheit said. “You’re essen­tially deter­mining what nano­particles make a good catalyst, and we can use this tool to design better ones.” (Source: Rice U.)

Reference: T. S. Heiderscheit et al.: Nanoelectrode-emitter spectral overlap amplifies surface enhanced electrogenerated chemiluminescence,J. Chem. Phys. 151, 144712 (2019); DOI: 10.1063/1.5118669

Link: Research group S. Link, Dept. of Chemistry, Rice University, Houston, USA

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