Gold-Functionalized Diatom Frustules: A Hierarchical Nanodevice for Sensitive SERS Detection of Biomolecules and Environmental Pollutants

Abstract

Silica diatom frustules are naturally occurring, silicon‑dioxide shells featuring highly ordered pore lattices that can be leveraged as low‑cost, high‑performance platforms for molecular capture and detection. We present a scalable photo‑deposition method that uniformly coats these frustules with 20‑nm gold nanoparticles, creating a hierarchical nanodevice (D24) that combines macroscopic manipulability, micrometer‑scale analyte harvesting, and nanometer‑scale surface‑enhanced Raman spectroscopy (SERS). The D24 devices capture bovine serum albumin (BSA) and mineral oil at concentrations as low as 10⁻¹⁶ M and 50 ppm, respectively, demonstrating their potential for biomedical diagnostics, safety monitoring, and environmental surveillance.

Background

Diatoms, ubiquitous unicellular algae, contribute to 40–50 % of oceanic organic material and 20 % of global CO₂ fixation. Their frustules—micrometer‑scale silica cylinders with diameters ≈ 8 µm and heights > 10 µm—exhibit exceptional specific strength (~ 1700 kN m⁻¹) and natural photonic properties arising from their regular pore lattices (200 ± 40 nm diameter). These attributes make diatoms attractive for nanotechnology, yet practical applications remain limited, largely because additional functionalization is required to impart sensing capabilities.

Here we demonstrate a straightforward, scalable photo‑deposition process that uniformly deposits gold nanoparticles onto diatom shells, producing a multi‑scale architecture that enables efficient analyte capture and SERS‑based detection. The method uses diatomaceous earth—a food‑grade, cost‑effective feedstock—to generate large quantities of D24 devices in under an hour.

Results

Functionalization with Au Nanoparticles

After cleaning diatomaceous earth with piranha solution and treating with 2 % HF to roughen the surface, shells were suspended in a 0.1 % HAuCl₄ solution in isopropanol and illuminated with UVA/UVB light. By controlling the shell concentration, gold precursor injection, and irradiation time (1 h, 20 mg shells in 50 mL, 30 µL HAuCl₄ every 5 min), we achieved a dense, uniform coating of 20 nm Au nanoparticles distributed both on the external surface and within the pores.

Scanning electron microscopy (SEM) confirmed that the nanoparticles occupy the pores and the external shell surface (Figure 1b,c). X‑ray photoelectron spectroscopy (XPS) showed the presence of metallic gold (Au 4f_5/2 at 84 eV) and characteristic valence band features, confirming successful functionalization (Figure 2).

Electromagnetic Field Enhancement

Finite‑element simulations modeled the D24 system as a dielectric (n = 1.3) with a hexagonal pore lattice and 125 Au nanoparticles per pore. Illuminated with a 633 nm TM plane wave (1 W), the local electric field near the nanoparticles reached |E| ≈ 3 × 10⁸ V m⁻¹, yielding a field enhancement factor Q ≈ 10² and a SERS enhancement factor Q_SERS ≈ 10⁸. The enhancement varied with incidence angle θ, oscillating between |E| ≈ 1.5 × 10⁸ V m⁻¹ (θ = 20°) and |E| ≈ 4 × 10⁸ V m⁻¹ (θ = 50°), but remained sufficient for robust detection in all orientations (Figure 3).

SERS Analysis of BSA

Incubating D24 devices in 10⁻¹⁶ M BSA solution (1 mg mL⁻¹) for 10 min resulted in BSA capture within the 200‑nm pores. Raman spectra collected with a 633 nm laser (0.18 mW) revealed distinct SERS peaks at 1392, 1556–1576, and 1670 cm⁻¹ corresponding to aromatic ring vibrations, amide bands, and COO⁻ stretching (Figure 4a). Raman mapping confirmed homogeneous distribution of BSA across the device (Figure 4c,d).

SERS Analysis of Mineral Oil

D24 devices captured mineral oil (MO) from water emulsions down to 50 ppm. Raman spectra displayed characteristic CH₂ scissor (1450 cm⁻¹) and CH stretching (2850–2923 cm⁻¹) bands that scaled linearly with MO concentration (Figure 5a). Mapping at 1450 and 2900 cm⁻¹ resolved MO distribution with sub‑micrometer resolution (Figure 5c,d).

Discussion

The D24 platform integrates capture, retention, and detection within a single, portable device. Unlike conventional SERS substrates that require external analyte deposition, D24 simultaneously serves as a target concentrator and sensor, enabling real‑time monitoring in complex matrices. The hierarchical design allows the devices to be introduced into biological circulatory systems, where they can sequester biomarkers and report on disease states with high spatial and temporal resolution.

Previous work has demonstrated diatom‑based SERS substrates using silver or electroless gold deposition, but these approaches either immobilize the frustules or involve more complex chemistry. Our photo‑deposition method is rapid, scalable, and requires minimal post‑processing, making it suitable for large‑scale production.

Conclusions

We have established a rapid, cost‑effective protocol to produce gold‑coated diatom frustules (D24) that capture molecules and provide SERS amplification. The devices detect BSA at 10⁻¹⁶ M and mineral oil at 50 ppm, highlighting their potential for analytical chemistry, biomedical diagnostics, food safety, and environmental monitoring.

Methods

Scanning Electron Microscopy

D24 samples were mounted on carbon tape and imaged with a Zeiss Auriga Compact FE‑SEM (secondary electron and back‑scattering detectors) to visualize morphology and Au distribution.

Fluorescence Analysis

Devices were incubated with 50 nm Fluoresbrite® Yellow Green microspheres (1:10 ratio), then imaged on a Leica TCS‑SP2 confocal microscope (ArUV laser, 441 nm excitation, 485 nm emission). Images spanned 975 × 750 µm² and were averaged to reduce noise.

X‑ray Photoelectron Spectroscopy

XPS was performed on a PHI Versa Probe II using a 100 W Al Kα source (11.7 keV). Survey spectra (≥ 20 min) and high‑resolution spectra (0.1 eV resolution) were referenced to C 1s (284.8 eV).

Electromagnetic Simulations

COMSOL Multiphysics 5.3 modeled a unit cell containing 125 20‑nm Au spheres on a 1.3‑index dielectric. A 633 nm TM plane wave (1 W) illuminated the cell, and field distributions were computed using perfectly matched layers and magnetic conductor boundaries.

Raman Spectroscopy

Raman mapping used a Renishaw inVia micro‑Raman microscope with a 633 nm HeNe laser (0.18 mW). Spectra were acquired with a 20× objective and integrated for 20 s per spot.

Abbreviations

BSA

Bovine serum albumin

D24 systems

Silicon dioxide diatom shells functionalized with gold nanoparticles

DE

Diatomaceous earth

MO

Mineral oil

SERS

Surface‑enhanced Raman spectroscopy