Porous carbon nanowire array for surface-enhanced Raman spectroscopy

The study addresses long-standing challenges in SERS: poor reproducibility, uniformity, biocompatibility, and durability of metal-based substrates that rely on localized surface plasmon resonance and hot spots, which can generate photothermal damage and undergo oxidation. The research question is whether a metal-free, LSPR-free substrate can deliver high SERS enhancement while achieving high reproducibility, uniformity, biocompatibility, and durability suitable for biomedical applications. The authors propose a porous carbon nanowire array (PCNA) leveraging chemical (charge-transfer) enhancement and fluorescence quenching to enable reliable, sensitive SERS for diverse biomolecules.

Prior efforts to improve SERS have explored nonmetallic substrates including silicon and germanium nanostructures, two-dimensional materials (graphene, MoS2, h-BN), and semiconducting metal oxides. These systems provide enhancement via structural resonance or charge-transfer resonance, achieving up to ~10^5 enhancement. However, reproducibility remains problematic and biocompatibility can be compromised by photocatalytic activity and inherent material toxicity. Metal substrates, while highly enhancing through LSPR hot spots, suffer from nonuniformity, poor reproducibility, photothermal heating, and oxidation, especially limiting for biomedical use. This context motivates development of a metal-free, biocompatible, reproducible SERS platform.

Synthesis of PCNA: (1) Prepare a polypyrrole (PPy) nanowire array (PNA) via template-assisted electropolymerization using an anodized aluminum oxide (AAO) template. (2) Interchange working and counter electrodes and perform electrical degradation of the PNA in high-temperature dimethyl sulfoxide (DMSO) containing sulfur clusters under opposite bias to introduce numerous nanopores in each PPy nanowire, forming a porous PPy nanowire array (PPNA) and increasing specific surface area and roughness. (3) Remove template in 6 M NaOH. (4) Carbonize the PPNA in argon (800 °C) to obtain the SERS-active porous carbon nanowire array (PCNA). Morphology and materials characterization: Scanning electron microscopy (SEM) shows porous nanowires with average diameter ~140 nm, length ~15 µm, and surface holes ~50 nm, yielding fractal nanostructures with high specific surface area. Raman spectra confirm disappearance of PPNA characteristic peaks after carbonization. Current–voltage (I–V) measurements show increased conductivity and semiconducting behavior after carbonization. Energy-dispersive X-ray spectroscopy (EDS) shows increased carbon content post-carbonization with residual heteroatoms (H, N, S). SERS measurements and performance evaluation: R6G dye was measured on silicon, PNA, carbon nanowire array (CNA), and PCNA under identical conditions (10 µM in water, 30 s integration, 1 mW at 785 nm) to compare adsorptivity and enhancement; detection limit on PCNA assessed via concentration series. Fluorescence quenching (including anti-Stokes emission at 785 nm) by carbon was noted. Concentration–intensity calibration was constructed at Raman shifts 1185, 1309, 1361, 1507, and 1650 cm−1. Substrate-to-substrate reproducibility assessed across 20 independent PCNA samples by relative peak intensity statistics. Biomolecular detection: β-lactoglobulin measured on PCNA and silicon (ground truth) and on a commercial Ag–Au metal SERS substrate (SERSitive) at 785 nm, 2 mW. Enhancement factor (EF) calculated using mass fraction (Mf) ratios and integration times: for β-lactoglobulin, EF ≈ (300 s/1 s) × (100%/0.4%) × (60,000/2,000) ≈ 10^6. Spot-to-spot uniformity evaluated by SERS mapping at 999 and 1447 cm−1 with step sizes 1 µm and 0.1 µm; coefficient of variation (CV) quantified. Glucose SERS at 785 nm measured on silicon and PCNA; EF computed as (300 s/1 s) × (100%/0.1%) × (1600/500) ≈ 10^6; time-to-time stability tested hourly up to 4 h to assess durability (CV reported). Mechanistic studies: Density functional theory (Gaussian16) used to model energy levels of R6G adsorbed on a carbon sheet approximating PCNA, predicting charge-transfer pathways enabling resonance at 785 nm and 532 nm. Finite element method (FEM) simulations compared electric field magnitude for single CNA vs single PCNA nanowires to estimate electromagnetic (EM) contributions (max field enhancement ~2, average ~1.8), indicating small EM role and absence of strong structural resonance between 532 and 785 nm. UV–vis absorption spectra of PCNA with and without β-lactoglobulin were measured; difference spectrum yielded a broad charge-transfer band encompassing 785 nm. Raman spectra were extended to high-shift region (up to 3200 cm−1) to check for overtones/combination bands (none observed). EF dependence on excitation wavelength was measured (785 vs 532 nm).

• PCNA, a metal-free porous carbon nanowire array, delivers strong SERS enhancement dominated by chemical (charge-transfer) mechanisms, with average enhancement factors ~10^6 for β-lactoglobulin and glucose at 785 nm and ~10^5 at 532 nm.
• R6G detection on PCNA exhibits a detection limit of ~10 nM under 30 s integration and 1 mW at 785 nm; fluorescence (including anti-Stokes) is effectively quenched by carbon, enabling clear Raman spectra.
• Across 20 independently fabricated PCNA substrates, the standard deviation of relative Raman peak intensities (1185, 1309, 1361, 1507, 1650 cm−1 for R6G) is 5.7%, indicating high substrate-to-substrate reproducibility.
• SERS mapping of β-lactoglobulin at 999 and 1447 cm−1 shows spot-to-spot uniformity with average CV <7.8% on both large (1 µm step) and small (0.1 µm step) scales, outperforming a commercial metal substrate.
• For β-lactoglobulin, the PCNA provides ~10× higher Raman signal than a commercial Ag–Au metal SERS substrate under identical conditions, with spectral features closely matching the spontaneous Raman ground truth on silicon.
• Glucose SERS on PCNA clearly resolves characteristic peaks at 0.1% Mf (≈5.6 mM), relevant to physiological blood levels; time-to-time measurements over at least 4 h show temporal stability with CV ≈15.1%, reflecting durability and resistance to oxidation.
• Material characterization confirms porous morphology (nanowire diameter ~140 nm, length ~15 µm; pore diameter ~50 nm), increased conductivity post-carbonization (semiconducting behavior), and increased carbon composition.
• Mechanistic evidence supports chemical enhancement: DFT energy alignment indicates charge-transfer resonances enabling excitation at 785 and 532 nm; FEM shows small EM contribution (max field enhancement ~2, average ~1.8); absorption spectra exhibit a broad charge-transfer band; absence of overtones/combination bands up to 3200 cm−1 indicates negligible EM-driven nonlinear effects.

The results demonstrate that an LSPR-free, metal-free PCNA substrate can provide high SERS enhancement while resolving the key limitations of metal-based SERS. The porous carbon architecture increases adsorptivity and surface area, while the carbon–analyte charge-transfer resonance broadly enhances Raman scattering at common excitation wavelengths and suppresses fluorescence. Uniform enhancement across the chemically active surface eliminates the need for localized electromagnetic hot spots, yielding high spot-to-spot and substrate-to-substrate reproducibility. The absence of metal mitigates photothermal damage and oxidation, improving biocompatibility and durability. Mechanistic studies (DFT, FEM, absorption) corroborate that chemical enhancement dominates, explaining consistent spectral fidelity relative to spontaneous Raman ground truths. These advances enable reliable trace detection of sensitive biomolecules (e.g., proteins, glucose) with performance comparable to or exceeding commercial metal substrates, addressing reliability concerns in analytical and biomedical SERS.

This work introduces a metal-free porous carbon nanowire array (PCNA) as a robust SERS substrate that achieves high sensitivity (~10^6 enhancement), excellent uniformity and reproducibility, fluorescence quenching, and durability without oxidation. The enhancement originates from broadband charge-transfer resonance rather than electromagnetic hot spots, enabling faithful, biocompatible detection of diverse analytes, including proteins and glucose, with superior reliability. Future work could further increase enhancement by optimizing substrate composition (dopant types/levels) and porous morphology, selecting excitation wavelengths tuned to molecule-specific charge-transfer resonances (e.g., via wavelength-tunable lasers), and refining the theoretical understanding of chemical enhancement. These improvements can broaden applications to quantitative protein analysis, accurate glucose monitoring, and trace contaminant detection in complex samples.

While delivering ~10^6 enhancement and strong reproducibility, the demonstrated performance depends on the analyte and excitation wavelength, and was characterized primarily for R6G, β-lactoglobulin, and glucose. Time-to-time durability was validated over at least 4 hours; longer-term stability and performance in complex biological matrices were not reported. Enhancement optimization regarding composition/doping and nanowire morphology remains to be explored.