The Unsung Hero of Molecular Spycraft

Rhodium Revolutionizes Raman Spectroscopy

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Introduction: The SERS Sensitivity Crisis

Surface-enhanced Raman spectroscopy (SERS) stands as one of analytical science's most powerful tools—a technique capable of detecting chemical fingerprints at the single-molecule level. For decades, gold and silver dominated SERS substrates, leveraging their plasmonic properties in visible light. Yet these metals falter in harsh conditions: silver tarnishes in air, gold absorbs UV light poorly, and both fail in corrosive environments. Enter rhodium, a corrosion-resistant platinum-group metal now emerging as a revolutionary SERS substrate. Recent breakthroughs reveal its unparalleled performance in ultraviolet regimes and real-world applications—from deep-sea vents to cancer diagnostics—heralding a new era of molecular detection 1 6 9 .

Why Rhodium? The Physics of UV Plasmonics

Most SERS substrates rely on localized surface plasmon resonance (LSPR), where light excites collective electron oscillations on metal surfaces, amplifying Raman signals by factors up to 1011. Traditional substrates operate optimally in visible-to-near-infrared light, but many critical biomolecules (e.g., DNA bases, proteins) absorb ultraviolet light. Their Raman cross-sections in visible light are notoriously weak, necessitating UV-compatible materials 1 9 .

Rhodium's edge lies in its UV plasmonic properties:

  • Optical resilience: Unlike silver/gold, rhodium maintains plasmonic activity below 300 nm, enabling excitation of UV-absorbing biomolecules.
  • Chemical inertness: Resists oxidation and sulfurization, critical for biological/environmental sampling.
  • Hot electron generation: Efficiently catalyzes photochemical reactions, allowing simultaneous detection and reaction monitoring 1 6 .
Table 1: Plasmonic Metals Compared
Metal Optimal Wavelength Corrosion Resistance SERS Enhancement Factor
Silver (Ag) 400–600 nm Low (tarnishes) 10⁷–10⁸
Gold (Au) 500–700 nm Moderate 10⁶–10⁷
Rhodium (Rh) <300 nm Extreme 10⁵–10⁶ (UV)

The Pivotal Experiment: Decoding Rhodium's Photodegradation Shield

A landmark 2025 study by Zou et al. (Nanoscale Advances) revealed rhodium's dual role as a SERS enhancer and biomolecular protector. The team engineered nanostructured aluminum platforms decorated with rhodium nanoparticles (RhNPs) to probe adenine—a DNA building block—under UV laser excitation 1 2 .

Methodology: Precision Engineering

  1. Substrate Fabrication:
    • Created nanoporous aluminum (Al) films via electrochemical etching.
    • Decorated surfaces with RhNPs via galvanic displacement, immersing Al in Na₃RhCl₆/NaCl solutions (0.5 mM Rh³⁺). Varied reaction times (0–4 min) to control NP density 1 .
    • Validated nanostructures using scanning electron microscopy (SEM): Rh coverage ranged from 0% (pristine Al) to 11.49% (4-min reaction).
Rhodium metal
  1. Biomolecule Deposition:
    • Applied adenine (0.5 mM) and bovine serum albumin (BSA) via drop-casting (not vapor deposition), ensuring molecules infiltrated Rh-Al "hot spots" 1 .
  2. UV-SERS Testing:
    • Acquired Raman spectra under 325 nm UV laser.
    • Performed six consecutive scans (2×2 μm² areas) to track photodegradation kinetics 1 2 .
Breakthrough Results
  • Unexpected intensity drop: Higher RhNP density (11.49% coverage) reduced SERS signal intensity by 60% vs. bare Al. Counterintuitively, this confirmed molecules were positioned in electromagnetic "hot spots" between RhNPs and Al, where photodegradation initiates 1 .
  • Photoprotection effect: Adenine on RhNP-coated substrates degraded 3× slower than on pure Al. RhNPs absorbed UV energy, generating "hot holes" that slowed oxidative damage 1 2 .
Table 2: Key Experimental Outcomes from Zou et al. (2025)
RhNP Coverage NP Density (particles/μm²) Adenine Signal Loss (6 scans) Photodegradation Rate
0% (bare Al) 0 98% Fast
6.46% 3.73 ± 0.37 85% Moderate
11.49% 8.59 ± 0.89 40% Slow

Rhodium's catalytic activity initially seemed detrimental—reducing signal intensity by accelerating photodegradation. Instead, it acted as a protective relay:

"RhNPs generated hot holes that suppressed reactive oxygen species, slowing adenine oxidation to azupurine. This enables longer spectral acquisition without sample destruction" 1 .

The Scientist's Toolkit: Building a Rhodium SERS Platform

Critical reagents and materials for Rh-based SERS substrates, as used in Zou et al.'s experiment:

Table 3: Essential Reagents for Rh-SERS Fabrication
Reagent/Material Function Key Property
Nanoporous Aluminum Base substrate High UV reflectivity, forms "hot spots" with RhNPs
Rhodium(III) hexachloride (Na₃RhCl₆) RhNP precursor Spontaneously reduces on Al, forming nanoparticles
Sodium chloride (NaCl) Electrolyte Controls Rh³⁺ reduction kinetics during galvanic displacement
Adenine / BSA Probe molecules Test biomolecules with low visible-range Raman cross-sections
Phosphate buffer Solvent Preserves biomolecule integrity during drop-casting
Cerium;copper12157-57-2CeCu6
ApsSodiumsaltC10H14N5Na2O10PS
6,10,11-HexolC30H47NO9
Glabrescone CC19H22O7
Tedizolid HClC17H16ClFN6O3

Beyond the Lab: Rhodium's Real-World Edge

1. Conquering Extreme Environments

Deep-sea chemosynthetic ecosystems host unique extremophiles, but in-situ SERS detection faces interference from sulfides, carbonates, and pressure shifts. In 2025, researchers deployed Rh-coated SERS probes at Formosa Ridge (South China Sea). Rhodium's corrosion resistance enabled detection of thiol-bearing metabolites in cold seeps—unlike silver, which degrades in sulfides 6 .

2. Biomedical Diagnostics

Rhodium's UV compatibility unlocks early cancer detection:

  • Serum albumin from multiple myeloma patients shows distinct SERS profiles at 635 cm⁻¹, 723 cm⁻¹, and 1052 cm⁻¹ under UV excitation.
  • Rh-SERS achieved 96% accuracy in discriminating cancer vs. healthy serum—surpassing visible-range gold/silver substrates 7 .
3. Dynamic Reaction Monitoring

Unlike static SERS, dynamic SERS tracks molecular transformations in real time. Rhodium's stability supports millisecond-scale acquisition, revealing:

  • Protein folding intermediates
  • Catalytic reaction pathways
  • Host-pathogen interactions 9 .

Future Horizons: Making Rhodium Accessible

Rhodium's rarity (price: ~$15,000/oz) remains a hurdle. Innovations to watch:

1. Nanostructuring

Sub-10-nm RhNPs reduce material needs while boosting enhancement via high-curvature hotspots 1 .

2. Hybrid substrates

Rhodium-gallium or Rh-MoO₃ composites combine UV enhancement with cost efficiency 3 9 .

3. Electrochemical recycling

Applying -0.4 V potential desorbs analytes, enabling substrate reuse >50 times 8 .

"Rhodium forces us to rethink SERS design principles. It trades raw enhancement for something more valuable: robustness in real-world scenarios."

Denis Garoli, co-author of Advances in Dynamic SERS 9

Conclusion: The Rhodium Renaissance

Once overshadowed by gold and silver, rhodium has carved a niche where stability matters more than sheer enhancement. From shielding DNA bases from UV damage to resisting deep-sea sulfides, it expands SERS into domains once deemed inaccessible. As nanostructuring techniques mature, this underdog metal may soon democratize single-molecule detection—proving that in science, durability can be as transformative as sensitivity.

Further Reading
  • Zou et al., Nanoscale Adv. 2025, 7, 00486 (Rh-Al photodegradation study)
  • Garoli et al., Nanoscale 2025, 17, 3656 (Dynamic SERS review)

References