Layered Bimetal Nanoporous Platforms for SERS Sensing

Technical Summary: Layered Bimetal Nanoporous Platforms for SERS Sensing

Problem Statement
Nanoporous metals (NPMs) are established platforms for plasmonic applications due to their high surface-to-volume ratios and tunable local electric fields. However, research has historically focused on single-element porous metals, with limited attention paid to the impact of alloy composition or bimetallic interactions. While chemical dealloying methods often result in residual "contamination" that alters electronic states, this is typically viewed as a limitation rather than a design feature. Furthermore, the specific interactions between two distinct plasmonic metals in a stacked configuration—such as plasmonic coupling, electron transfer, and electromagnetic field interactions—remain underexplored. There is a need for a controllable fabrication method to create pure, stacked bimetallic nanoporous layers to systematically investigate how metal choice and stacking order influence Surface Enhanced Raman Scattering (SERS) performance across the visible spectrum.

Methodology
The study employs a dry-synthesis fabrication method based on oblique angle deposition and template removal to create bilayer nanoporous structures.

• Fabrication: Poly(methyl methacrylate) (PMMA) is spin-coated onto silicon substrates. High-purity metals (Au, Ag, Cu) are deposited via electron-beam evaporation at an 80° oblique incidence angle. The PMMA template is subsequently removed via plasma treatment (O₂ for Au; N₂ for Ag and Cu to prevent oxidation), leaving behind a porous metal layer. This process is repeated to stack a second porous metal layer on top of the first, creating bilayers such as Au/Ag, Ag/Au, Au/Cu, Cu/Au, Ag/Cu, and Cu/Ag.
• Numerical Modeling: Electromagnetic (EM) field distributions were simulated using two approaches:
  1. Finite Element Method (FEM): Using COMSOL Multiphysics, 3D geometries were reconstructed directly from Scanning Electron Microscopy (SEM) images of the experimental samples to account for nanoscale discontinuities.
  2. Atomistic Modeling: A frequency-dependent fluctuating charges and dipoles (ωFQFμ) approach was used for cross-validation on idealized 7 × 7 × 4 nm³ structures containing thousands of atoms, providing a reliable description of noble metal nanoparticles at a lower computational cost than ab initio methods.
• Experimental Characterization: SERS performance was evaluated using Rhodamine 6G (R6G) as a probe molecule and the ADAMTS3 protein as a biological target. Measurements were conducted at excitation wavelengths of 532 nm, 633 nm, and 785 nm. Spectroscopic ellipsometry was used to determine the dielectric permittivity of the bilayer systems.

Key Results

• Morphology: SEM analysis confirmed the formation of distinct, stacked nanoporous layers with fully connected ligaments. In bimetallic samples, the layers remained largely unmixed, separated by a potential thin oxide gap, particularly for Ag and Cu.
• Simulated EM Fields:
  • Homometallic Systems: Ag exhibited the strongest EM field enhancement, particularly at 630 nm, with numerous "hot spots." Cu showed the weakest response, while Au displayed intermediate activity.
  • Bimetallic Systems: The stacking order significantly dictated the optical response.
    • In Au/Ag (Ag on top), the Ag layer showed reduced intensity compared to homometallic Ag but maintained strong activity. In Ag/Au (Au on top), the Au layer showed enhanced response at 550 nm due to interlayer coupling.
    • In Ag/Cu and Cu/Ag systems, the response was dominated by the Ag layer, with Cu acting primarily as a lossy substrate that could suppress resonances depending on its position.
    • Au/Cu systems showed that Au dominated the intensity, with coupling compensating for Cu's low plasmonic activity at longer wavelengths.
• SERS Performance (R6G):
  • At 532 nm, substrates with Ag as the bottom layer (Ag/Cu, Ag/Ag) yielded the highest signal enhancement.
  • At 633 nm, homometallic Ag outperformed Au by approximately 2.5 times.
  • At 785 nm, the Ag/Au structure (Ag beneath Au) exhibited the strongest enhancement. The authors attribute the initially weaker performance of homometallic Ag at this wavelength to oxidation of the exposed top layer over time, whereas the Au-capped Ag/Au structure remained protected.
  • Detection limits for R6G reached 10 nM (532 nm), 0.1 μM (633 nm), and 10 μM (785 nm).
• Biosensing (ADAMTS3): The optimized bilayer substrates successfully detected the ADAMTS3 protein (a hepatocellular carcinoma biomarker) at concentrations as low as 10⁻¹⁰ M using 633 nm excitation. The signal showed a linear relationship with the logarithm of concentration at 633 nm, though non-linearity and higher variability were observed at 532 nm and 785 nm, attributed to wavelength-specific electronic interactions between the protein and the alloy surface.
• Stability: The substrates demonstrated long-term stability and mechanical durability, maintaining SERS activity after 40 minutes of ultrasonication.

Significance and Claims
The paper claims to present the first detailed study combining morphological analysis, numerical modeling, and optical spectroscopy on perfectly stacked bimetallic nanoporous platforms prepared via a dry-synthesis method. The work demonstrates that:

1. Stacking Order Matters: The sequence of metal layers (e.g., Ag/Au vs. Au/Ag) critically influences resonance conditions and SERS enhancement, allowing for the tuning of plasmonic properties across the visible spectrum.
2. Interlayer Interactions: Bimetallic systems facilitate unique interlayer interactions that can enhance the performance of less active metals (like Au or Cu) when coupled with highly active ones (like Ag).
3. Practical Utility: These platforms offer a reproducible, simple, and low-cost fabrication route for SERS substrates with high uniformity and stability, suitable for both standard chemical sensing and the detection of low-concentration biological biomarkers.

The authors position this work as a foundational step toward understanding complex multi-material plasmonic platforms, highlighting the potential of controlling metal-metal interfaces to optimize sensing performance without relying on chemical dealloying.