# Supramolecular Stabilization of Single-Molecule SERS: Cucurbit[7]uril Encapsulation of Thionine

## TL;DR

This paper shows how encapsulating thionine molecules in cucurbit[7]uril improves the stability of single-molecule SERS signals by reducing molecular motion.

## Contribution

The novel contribution is using cucurbit[7]uril to suppress molecular motion in plasmonic nanocavities, thereby enhancing the reliability of single-molecule SERS.

## Key findings

- CB[7] encapsulation reduces amplitude fluctuations in SM-SERS signals by suppressing molecular motion.
- Electronic-resonant excitation increases detection probability due to optimal alignment of the transition dipole moment.
- Spectral diffusion remains unaffected, indicating that substrate/adatom dynamics are unchanged by encapsulation.

## Abstract

Surface-enhanced Raman spectroscopy (SERS) in plasmonic
nanocavities
enables single-molecule detection through dramatic enhancement of
the local electromagnetic field. However, single-molecule SERS (SM-SERS)
signals exhibit pronounced fluctuations in both absolute and relative
band intensities, as well as abrupt signal dropouts, which complicate
reliable analyte detection and identification. A key contributor to
this temporal variability is the translational and rotational mobility
of molecules within the plasmonic cavity. In this work, we investigated
how confining thionine (Th) molecules within the macrocycle cucurbit[7]­uril
(CB[7]) suppresses molecular motion and improves spectroscopic stability.
We employed two high-field-enhancement geometries  nanoparticle-on-mirror
and spherical gold oligomers. The spectral analyses were supported
with density functional theory (DFT) calculations and simulations.
Our results demonstrate that CB[7] encapsulation improves SM-SERS
detection reliability by reducing amplitude fluctuations. Although
the average SERS intensity decreases by several tens of percent, signal
decay during initial illumination accelerates. Under electronic-resonant
excitation of the analyte, detection probability increases owing to
the CB[7]-enforced optimal alignment of Th’s transition dipole
moment with the nanocavity’s electromagnetic field. Limiting
analyte mobility through encapsulation diminishes amplitude fluctuations,
while spectral diffusion remains unaffected. These complementary results
disentangle two fluctuation mechanisms: molecular motion suppressed
by CB[7] and substrate/adatom dynamics unchanged by encapsulation.
These findings provide fundamental insights into molecule–nanocavity
interactions and establish new strategies for enhancing the reliability
of single-molecule detection. The approach opens promising avenues
for probing the dynamics of biologically and catalytically relevant
species with improved temporal stability and reduced measurement uncertainty.