# Unveiling the Unprecedented Optical Properties of Citrate‐Stabilized Hollow AgAu Nanoshells Under Photothermal Irradiation

## TL;DR

Citrate-stabilized hollow gold nanoshells show unexpected optical changes and enhanced Raman signals when heated with light.

## Contribution

The discovery of a citrate-dependent, heat-induced blueshift in plasmon resonance and enhanced SERS in hollow gold nanoshells.

## Key findings

- Photothermal irradiation causes a blueshift in the localized surface plasmon resonance of citrate-stabilized hollow gold nanoshells.
- Surface-enhanced Raman scattering signal increases 8-fold during plasmonic heating despite the resonance shift.
- The effect is absent without citrate, silver, or heat, indicating their combined role in the phenomenon.

## Abstract

Metallic nanoshells heat efficiently on excitation of the localized surface plasmon resonance (LSPR). Whilst investigating the photothermal properties of citrate‐stabilized hollow gold nanoshells (HGNs) synthesized using a sacrificial silver nanoparticle (AgNPs), the LSPR undergoes a distinct blueshift (70 ± 20 nm (0.20 ± 0.06 eV)) when photothermally irradiated. Notably, when functionalized with a Raman reporter, the surface‐enhanced Raman scattering (SERS) signal unexpectedly and dramatically increases 8 ± 2‐fold upon plasmonic heating, despite the LSPR shifting away from the excitation wavelength. This unprecedented enhancement of the SERS signal is absent in samples lacking citrate or prepared using a cobalt nanoparticle template, underscoring the importance of citrate, heat, and AgNPs in eliciting these effects. It is hypothesized that aqueous silver ions near the surface of the HGNs react with the citrate and form a complex that is both light and temperature sensitive. The formation of silver deposits, observed by electron microscopy, alters the core‐to‐shell thickness ratio, resulting in a blueshift in the LSPR, and change the scattering to absorption properties, enabling an improved SERS performance. This new optical phenomenon has now been understood and will be of significant interest to future studies in harnessing the properties of HGNs.

When photothermally irradiated, hollow gold nanoshells prepared using silver nanoparticles as templates and stabilized by citrate, exhibit unexpected optical changes. The localized surface plasmon resonance is blueshifts, and more surprisingly, the surface‐enhanced Raman scattering intensity of an adsorbed analyte increases. In the absence of either silver, citrate, or heat, this phenomenon is not observed.© 2026 WILEY‐VCH GmbH