This article presents the research methodology, results, and discussion of a project to demonstrate the optical response of plasmonic rulers (PRs) by dipolar coupling between the closed ring spiropyran (SP) and nanostructures by monitoring their localized surface plasmon resonance properties; the authors investigate the coupled-dipole mechanism, determining that a very large dipole moment of merocyanine structure induces strong dipole polarizability; and suggest that this next-generation PR has positive implications for advanced, plasmonic-based sensors and optoelectronic device fabrications.
Plasmonic rulers (PRs) linking nanoscale distance dependence spectral shifts are important for studying cellular microenvironments and biomarker detection. The traditional PR design employs tethering metal nanoparticle pairs using synthetic and biopolymers that severely suffer from reproducibility issues, as well as lack reversibility. Here, the fabrication of novel PRs is reported through the formation of self-assembled monolayers (SAMs) of photoswitchable molecular machines chemically tethered onto sharp-tip gold nanostructures (Au NSs). This unique and highly sensitive PR utilizes localized surface plasmon resonance (LSPR) properties of Au NSs to spectroscopically evaluate dipole–dipole coupling between NSs and photoisomerizable spiropyran (SP)-merocyanine (MC) conjugates in the solid-state. It is observed that the SAM-modified NSs are extremely sensitive to the photoisomerization of SP-to-MC, resulting in LSPR shifts as large as 5.6 nm for every 1.0 Å change in distance. The highly dipolar MC changes the NS-SAM interfacial polarizability and alters the dipole–dipole coupling leading to the ultrasensitive PR is hypothesized. The hypothesis is supported theoretically by calculating dipole polarizability of an inorganic-organic heterodimer model and experimentally by determining work function and interfacial dipole values. Taken together, this work represents the fabrication of next-generation PRs, which hold great promise for advanced, plasmonic-based sensors and optoelectronic device fabrication. (Published Abstract Provided)