Metasurface-Enhanced Raman Spectroscopy (mSERS) for Oriented Molecular Sensing

Surface-enhanced Raman spectroscopy (SERS) is a widely used sensing technique for ultrasensitivity chemical sensing, biomedical detection, and environmental analysis. Because SERS signal is proportional to the fourth power of the local electric field, several SERS applications have focused on the design of plasmonic nanogaps to take advantage of the extremely strong near-field enhancement that results from plasmonic coupling, but few designs have focused on how SERS detection is affected by molecular orientation within these nanogaps. Here, we demonstrate a nanoparticle-on-metal metasurface designed for near-perfect optical absorption as a platform for Raman detection of highly oriented molecular analytes, including two-dimensional materials and aromatic molecules. This metasurface platform overcomes challenges in nanoparticle aggregation, which commonly leads to low or fluctuating Raman signals in other colloidal nanoparticle platforms. Our metasurface-enhanced Raman spectroscopy (mSERS) platform is based on a colloidal Langmuir-Schaefer deposition, with up to 32% surface coverage density of nanogaps across an entire sensor chip. In this work, we perform both simulations of the local electric field and experimental characterization of the mSERS signal obtained for oriented molecular layers. We then demonstrate this mSERS platform for the quantitative detection of the drinking-water toxin polybrominated diphenyl ether (BDE-15), with a limit of detection of 0.25 μM under 530 μW excitation. This detection limit is comparable to other SERS-based sensors operating at laser powers over 3 orders of magnitude higher, indicating the promise of our mSERS platform for nondestructive and low-level analyte detection.

Colloidal Nanoantennas for Hyperspectral Chemical Mapping

Tip-enhanced Raman spectroscopy enables access to chemical information with nanoscale spatial resolution and single-molecule sensitivities by utilizing optical probes that are capable of confining light to subwavelength dimensions. Because the probes themselves possess nanoscale features, they are notoriously difficult to fabricate, and more critically, can result in poor reproducibility. Here, we demonstrate high-performance, predictable, and readily tunable nanospectroscopy probes that are fabricated by self-assembly. Shaped metal nanoparticles are organized into dense layers and deposited onto scanning probe tips. When coupled to a metal surface, these probes behave like nanoantenna by supporting a strong optical resonance, producing dramatic Raman field enhancements in the range of 10(8)-10(9) with sub-50 nm spatial resolution. In contrast to other nanospectroscopy probes, our colloidal probes can be fabricated in a scalable fashion with a batch-to-batch reproducibility of ∼80% and serve as an important demonstration of bottom-up engineering.

Investigating the effect of Ag nanocube polydispersity on gap-mode SERS enhancement factors

Colloidal polydispersity has a significant impact on the high Raman enhancement factors (EFs) for nanoparticle-based surface-enhanced Raman spectroscopy (SERS) substrates.

Plasmonic nanocomposites: polymer-guided strategies for assembling metal nanoparticles

Noble metal nanoparticles that support localized surface plasmon resonances (LSPRs) have the unique ability to manipulate and confine light at subwavelength dimensions. Utilizing these capabilities in devices and coatings requires the controlled organization of metal nanoparticles into ordered or hierarchical structures. Polymer grafts can be used as assembly-regulating molecules that bind to the nanoparticle surface and guide nanoparticle organization in solution, at interfaces, and within condensed phases. Here, we present an overview of polymer-directed assembly of plasmonic nanoparticles. We discuss how polymer grafts can be used to control short-range nanoparticle interactions that dictate interparticle gap distance and orientation. We also discuss how condensed polymer grafts can be used to control long-range order within condensed nanoparticle-polymer blends. The assembly of shaped plasmonic nanoparticles that have potential applications in enhanced spectroscopy and optical metamaterials is highlighted. We end with a summary of promising new directions toward the fabrication of plasmonic nanocomposites that are responsive and possess three-dimensional order.