No abstract

Surface-enhanced Raman spectroscopy (SERS) is an effective approach for ultrasensitive molecular detection due to the significantly enhanced electromagnetic field near the nanogaps of metallic nanostructures. However, the fabrication of large-scale SERS substrates simultaneously with high enhancement and uniform response in practical applications remains a challenge. Here, we propose the use of hierarchical plasmonic hotspot arrays consisting of long-range ordered Au nanoclusters. Each Au cluster contains a high density of plasmonic hotspots, which ensure ultrasensitive detection. We adopt modified colloidal lithography by introducing a rotational coordinate during the thermal evaporating process to fabricate the hierarchical plasmonic arrays. Under the effect of centrifugal potential, plenty of nanogaps directly form in the Au nanoclusters. The plasmonic coupling in nanogaps results in the ultrahigh Raman enhancement factor (EF) up to 2.3 × 10 9 and the limit of detection (LOD) of R6G can reach down to 10 −12 M, which is adequate for single molecular detection. As a highly uniform and sensitive SERS substrate, the ingredient in transformer oil including gas and furfural has been clearly identified, which proves to be an accurate detection tool for assessing mechanical degradation in oil paper insulation. With high sensitivity and mapping ability, this hierarchical plasmonic hotspot array offers a promising platform for ultrasensitive molecular detection.

The controllable nanogap structures offer an effective way to obtain strong and tunable localized surface plasmon resonance (LSPR). A novel hierarchical plasmonic nanostructure (HPN) is created by incorporating a rotating coordinate system into colloidal lithography. In this nanostructure, the hot spot density is increased drastically by the long‐range ordered morphology with discrete metal islands filled in the structural units. Based on the Volmer–Weber growth theory, the precise HPN growth model is established, which guides the hot spot engineering for improved LSPR tunability and strong field enhancement. The hot spot engineering strategy is examined by the application of HPNs as the surface‐enhanced Raman spectroscopy (SERS) substrate. It is universally suitable for various SERS characterization excited at different wavelengths. Based on the HPN and hot spot engineering strategy, single‐molecule level detection and long‐range mapping can be realized simultaneously. In that sense, it offers a great platform and guides the future design for various LSPR applications like surface‐enhanced spectra, biosensing, and photocatalysis.