Sculpting the analytical volume in and around nanoparticle sensors using a multilayer geometry

The use of structured nanoparticles as optical contrast agents has led to new sensing opportunities in localizing the analytical volume within or outside the particle. Here we examine the use of structured nanoparticles for controlling the sensed analytical volume and figures of merit for their use. Nanolayered alternating metal-dielectric particles (nanoLAMPs), consisting of metal-dielectric nanospheres, are a flexible and highly tunable structure and used here to illustrate the concept of sculpting the analytical volume associated with a nanoparticle. The alternating metal and dielectric shells in LAMPs are designed such that, when illuminated, the plasmonic coupling of metal shells results in amplified electric fields in specific volumes. The strength and extent of regions with amplified fields (hot spots) in and around a LAMP are at the expense of other regions with depleted fields. A rigorous Mie theory formulation is used to model electric field redistributions. A genetic algorithm-based strategy is then employed to design LAMPs that selectively enhance the response of analyte molecules located either outside or in various dielectric layers through electric field redistribution. We demonstrate that it is possible to localize the analytical volume to within or outside the particle quite efficiently. Further, the analytical figures of merit (localization and amplification of signal as well as contrast between sensed species and background) are optimized and limits to the same are described. The strategy proposed here is a general route to engineer a palette of probes with highly specific detection capabilities using spectroscopy techniques based on surface-enhanced scattering, absorption, or emission processes.