Silica-Gold Core-Shell Nanoparticles

Updated 4 December 2025

Silica-gold core-shell nanoparticles are hybrid structures with a silica core and a tunable gold shell, enabling controlled plasmonic resonances.
They leverage seeded growth and inverse synthesis methods alongside Mie theory to achieve precise optical and thermoplasmonic optimization.
Their engineered design enhances SERS, photothermal therapy, and biosensing performance in nanophotonic and biomedical applications.

Silica-gold core-shell nanoparticles are a class of hybrid nanoscale structures comprising a central silica (SiO₂) core encapsulated by a concentric gold (Au) shell. Their combination of tunable electromagnetic response, distinctive thermoplasmonic behavior, and compatibility with both active and passive optical functionalities renders them a versatile material platform for nanophotonics, photothermal therapy, biosensing, and catalysis. Modulating core dimensions and shell geometry enables precise control over their optical, thermal, and radiative properties, with emergent behaviors that exceed those of either component in isolation (Hajj et al., 5 Jun 2025, Passarelli et al., 2016, Alkurdi et al., 2019, Nikbakht, 2017).

1. Core-Shell Geometry, Synthesis, and Material Structure

Silica-gold core-shell nanoparticles typically consist of an amorphous or crystalline SiO₂ core of radius $a$ , surrounded by a gold shell of thickness $t$ (total outer radius $R = a + t$ ). The core acts as a low-loss, dielectric scaffold; the gold shell imparts strong plasmonic response and interfacial electronic coupling.

Two principal routes to synthesis predominate:

Seeded Growth on Silica Cores: Silica spheres are prepared by base-catalyzed hydrolysis and condensation of tetraethyl orthosilicate (Stöber process), followed by surface functionalization (e.g., APTMS silanization), adsorption of Au seeds, and reduction of gold precursors (HAuCl₄) to yield a contiguous Au shell. Control over shell thickness is achieved by stoichiometry and precursor addition protocol. This yields SiO₂@Au architectures (Saini et al., 2014).
Gold Core, Silica Shell (Inverse): Synthesis of citrate- or surfactant-stabilized Au nanospheres, then overgrowth of thin SiO₂ via TEOS hydrolysis, yields Au@SiO₂ particles. Porosity in the silica shell can be controlled by surfactants or ammonia etching (Hajj et al., 5 Jun 2025, Alkurdi et al., 2019).

Representative parameters:

Core	Shell	Typical Radii	Shell Thickness Range	Reference
Silica	Gold	$a$ = 50–150 nm	$t$ = 5–30 nm	(Passarelli et al., 2016, Saini et al., 2014)
Gold	Silica	$R_{core}$ = 20–50 nm	$t_{shell}$ = 2–20 nm	(Alkurdi et al., 2019, Hajj et al., 5 Jun 2025)

Complex hybrid structures—e.g., mesoporous shells for drug loading (Garcia et al., 2021), or fluorescent/multilayer composites (Blondot et al., 2022)—extend this paradigm.

2. Electromagnetic Response and Mie Theory

The optical properties of silica-gold core-shell nanoparticles originate from the interplay of dielectric confinement and plasmonic resonances. Classical electromagnetic analysis employs the solution to Maxwell’s equations for layered spheres (Aden–Kerker or Mie theory). For a core of permittivity $\varepsilon_c$ and shell $\varepsilon_s$ ,

The scattered field for $r > R$ is constructed from vector spherical harmonics, with $a_n, b_n$ (Mie coefficients) determined by matching field continuity at $r = a, R$ .
The scattering ( $\sigma_{\text{sca}}$ ) and extinction ( $\sigma_{\text{ext}}$ ) cross sections are given by

$\sigma_{\text{sca}}(\lambda) = \frac{2\pi}{k^2} \sum_{n=1}^\infty (2n+1) (|a_n|^2 + |b_n|^2)$

$\sigma_{\text{ext}}(\lambda) = \frac{2\pi}{k^2} \sum_{n=1}^\infty (2n+1) \text{Re}[a_n + b_n]$

where $k = 2\pi n_{\text{med}}/\lambda$ (Passarelli et al., 2016, Hajj et al., 5 Jun 2025).

Plasmon resonances manifest as poles in the dominant Mie denominators. In SiO₂@Au, the dipole mode often dominates and can be tuned over the visible or NIR by varying $a$ and $t$ . Thinner shells red-shift the resonance to longer wavelengths, supporting operation in the biological “water window” (700–900 nm) (Duong et al., 2017).

In active core systems (optically pumped SiO₂), the shell supports a “spaser” (surface plasmon amplification by stimulated emission of radiation) when gain offset equals the combined radiative and ohmic losses, i.e., when $\sigma_{\text{ext}}=0$ coincides with a scattering pole (Passarelli et al., 2016).

3. Thermoplasmonic Behavior and Nanoscale Heat Transfer

Silica-gold core-shells display unique thermoplasmonic dynamics under pulsed illumination. Absorption of femtosecond to nanosecond laser pulses in the gold shell leads to non-equilibrium heating of electrons ( $T_e$ ), followed by phonon ( $T_p$ ) and lattice thermalization, with energy transferred outward through the shell to the surrounding medium.

The principal modeling framework is the two-temperature model (TTM), extended to account for interfacial conductances:

$\begin{align*} V_ec_e(T_e)\,\frac{\partial T_e}{\partial t} &= -V_eG(T_e-T_p) - S_c\sigma_{es}(T_e - T_s(R_c, t)) + P(t) \ V_pc_p\,\frac{\partial T_p}{\partial t} &= +V_eG(T_e-T_p) - S_c\sigma_{ps}(T_p - T_s(R_c, t)) \ c_s \frac{\partial T_s}{\partial t} &= k_s\nabla^2 T_s \ c_w \frac{\partial T_w}{\partial t} &= k_w\nabla^2 T_w \end{align*}$

(Hajj et al., 5 Jun 2025, Alkurdi et al., 2019)

Key findings include:

Silica shells introduce an additional, direct electron–silica interfacial conductance channel ( $\sigma_{es}$ ), which accelerates heat transfer from hot electrons to the shell, bypassing the slower phonon–phonon pathway. This can reduce water heating times by up to a factor of two relative to bare gold (Alkurdi et al., 2019, Hajj et al., 5 Jun 2025).
Dense silica shells (thickness 5 nm; $k_s \sim 1$ W m $^{-1}$ K $^{-1}$ ) outperform porous shells (50% porosity, $k_s \sim 0.6$ W m $^{-1}$ K $^{-1}$ ) in terms of both speed and magnitude of water interfacial heating for or sub-ps to ns pulses. Thicker shells introduce additional thermal resistance and degrade photothermal response (Hajj et al., 5 Jun 2025).
Maximal photothermal efficiency is achieved for core $R_c = 20$ –40 nm and shell $d = 3$ –8 nm excited by ultrashort pulses ( $\tau_p \sim$ 100 fs–1 ps), with efficiencies up to 1–1.5% of incident energy stored in the first few nanometers of water (Alkurdi et al., 2019).

4. Radiative, Optical, and Near-Field Effects

Radiative and near-field enhancement in silica-gold core-shell nanostructures is critical for sensing and nonlinear optics:

Surface Enhanced Raman Scattering (SERS): Thin, continuous Au shells on SiO₂ maximize electromagnetic field “hot spots” while reducing total Au mass. SERS enhancement factors (EF) of $10^6$ – $10^7$ have been reported—one order of magnitude higher than for pure Au nanoparticles of equivalent diameter—with detection of analytes down to $10^{-11}$ M (Saini et al., 2014). Tuning LSPR via shell thickness optimizes SERS for specific Raman excitation wavelengths.
Purcell Effect & FRET Suppression: In complex architectures (e.g., semiconductor core/spacer/Au shell), external Au nanoshells produce strong Purcell enhancements (up to $F_P \sim 8$ ) and reduce rates of Förster resonance energy transfer (FRET) by $\sim$ 3×, as the enhanced radiative decay outcompetes non-radiative transfer (Blondot et al., 2022).
Spasing and Loss-Compensation: In optically active cores, lasing-type operation (spaser action) is realized when the gain parameter $\kappa$ achieves full loss compensation and coincides with a scattering resonance pole. For silica cores of $a=75$ nm, $t=5$ nm, the threshold $\kappa$ reaches $-0.54$ at $\lambda_p=1086$ nm (dipolar mode) (Passarelli et al., 2016).

5. Biomedical and Thermoplasmonic Applications

Silica-gold core-shell nanoparticles are engineered for a range of photothermal and photonic biomedical applications due to their tunable NIR absorption and favorable thermal properties.

Photoacoustic Imaging (PA): Silica shells provide biocompatibility and platform stability but reduce PA signal proportional to shell thickness; for $d=10$ –40 nm, PA amplitude is reduced by 10–40% relative to uncoated Au of the same core diameter. However, thin shells ( $\sim$ 10 nm) maintain substantial signal while affording surface chemistry advantages (Pang et al., 2019).
Photothermal Therapy (PTT) and Controlled Release: Core–shell nanorods (AuNR@MSN) loaded with drugs and NO donors achieve synergistic PTT and antimicrobial action against Staphylococcus aureus biofilms. NIR irradiation yields $\sim$ 90% biofilm reduction compared to $\sim$ 30% for drug only, underscoring the efficacy of photothermally modulated drug delivery (Garcia et al., 2021).
Subwavelength Light Sources & Nanoscale Heating: Nanoparticle parameters can be optimized to maximize NIR absorption and minimize thermal relaxation times in tissues, with protocols based on combined Mie-theory and Pennes bioheat modeling enabling deterministic design for target photothermal profiles (Duong et al., 2017).

6. Radiative Heat Transfer and Energy Coupling

Radiative thermal coupling between core-shell nanoparticles is highly sensitive to geometry and material choice:

SiO₂@Au Dimers: Thin Au shells ( $t \sim 1$ –3 nm) dramatically suppress radiative conductance (by up to $10^3$ ) compared to pure SiO₂; as the shell thickens, plasmonic modes in the shell restore and even surpass the conductance of dielectric-only spheres by mid-IR hybridization (Nikbakht, 2017).
Au@SiO₂ Dimers: Adding a silica shell on gold cores enhances radiative conductance by orders of magnitude (up to $10^3$ for $t=20$ nm), due to strong coupling of SiO₂ phonon-polaritons into the thermal energy window (Nikbakht, 2017).
This behavior is governed by competition between electric (phonon-polariton) and magnetic (eddy-current) dipolar channels, governed by the composite polarizability expressions for layered spheres.

7. Design Guidelines and Application-Specific Optimization

Optimization of silica-gold core-shell nanoparticles demands balancing electromagnetic, thermal, and chemical parameters to meet target applications:

For maximum photothermal transduction: Utilize dense, chemically bonded, thin silica shells ( $d=3$ –8 nm) on Au cores ( $R_c=20$ –40 nm), excited by fs–ps pulses at NIR wavelengths. Avoid excessive porosity and thick shells, which degrade heating rate. Confirm LSPR placement via Mie theory incorporating size- and temperature-dependent corrections (Hajj et al., 5 Jun 2025, Alkurdi et al., 2019, Duong et al., 2017).
For SERS and nonlinear optical substrates: Select shell thickness that matches the LSPR to the desired laser line; thinner, faceted shells increase hot-spot density and field enhancement (Saini et al., 2014).
For spasing and nanoscale lasing: Tailor gain medium dispersion, core size, and shell thickness to match loss-compensation and resonance conditions. Full loss compensation is necessary but not sufficient; resonance with a scattering pole is also required (Passarelli et al., 2016).
For bioimaging or controlled drug delivery: Balance shell thickness for stability and modifiable surface chemistry with maintenance of desired photothermal or PA signal amplitude (Pang et al., 2019, Garcia et al., 2021).

In summary, silica-gold core-shell nanoparticles constitute a highly tunable platform where electromagnetic confinement, thermoplasmonic response, radiative loss/gain management, and interfacial chemistry integrate to yield superior performance across photonic, sensing, biomedical, and energy applications (Hajj et al., 5 Jun 2025, Passarelli et al., 2016, Alkurdi et al., 2019, Saini et al., 2014, Duong et al., 2017, Garcia et al., 2021, Blondot et al., 2022, Nikbakht, 2017, Pang et al., 2019).