Surface-enhanced Raman spectroscopy (SERS) significantly enhances the inherently weak Raman scattering signal, providing much higher sensitivity for vibrational spectroscopy. This enhancement is achieved by exciting localized surface plasmon resonance (LSPR) on metal substrates. However, conventional SERS using metal nanoparticles suffers from several drawbacks that limit its broader application, particularly in biomedicine. The strong dependence on "hot spots"—regions of intense electromagnetic fields—leads to poor reproducibility and uniformity in signal enhancement. Furthermore, the large photothermal heat generated on the metal surface can damage biomolecules, and the metal surfaces are susceptible to oxidation, reducing their long-term stability and reliability. While various approaches have attempted to mitigate these issues, developing a SERS substrate that combines high sensitivity, uniformity, and reproducibility remains a significant challenge. Recent research has explored non-metallic alternatives such as silicon, germanium nanostructures, and two-dimensional materials, aiming to address the biocompatibility and durability problems. Although these materials offer improvements in some areas, the issue of reproducibility remains a significant hurdle, often due to inherent photocatalytic activity or material toxicity. This study presents a novel approach, employing a porous carbon nanowire array (PCNA) as a metal-free SERS substrate. The PCNA is designed to circumvent the limitations of conventional metal-based SERS while achieving high sensitivity, uniformity, reproducibility, biocompatibility, and durability. This unique combination of properties makes the PCNA a promising candidate for reliable SERS applications in various fields, especially those where inconsistent or non-reproducible spectra have been a major problem such as analytical chemistry, pharmaceutical science, food science, forensic science, and pathology.

The authors extensively review existing SERS techniques and their limitations. They highlight the challenges associated with metal-based SERS substrates, including the lack of reproducibility due to the dependence on hot spots, the generation of excessive photothermal heat, and the susceptibility to oxidation. They discuss the exploration of non-metallic alternatives such as silicon and germanium nanostructures, and two-dimensional materials like graphene, MoS2, and h-BN, noting their advantages in biocompatibility but persistent reproducibility issues. The review sets the stage for the introduction of the PCNA as a superior alternative, addressing the shortcomings of previously reported substrates.

The PCNA substrate was synthesized through a multi-step process. Initially, a polypyrrole (PPy) nanowire array (PNA) was fabricated using a template-assisted electropolymerization method with an anodized aluminum oxide (AAO) template. Subsequently, an electrical degradation process in a high-temperature dimethyl sulfoxide (DMSO) solution containing sulfur clusters, under an oppositely applied voltage, introduced numerous nanopores into each PPy nanowire, forming a porous polypyrrole nanowire array (PPNA). This step significantly increased the specific surface area (SSA) and roughness. Finally, a carbonization process at high temperature transformed the PPNA into the SERS-active PCNA. The resulting PCNA consisted of porous nanowires with an average diameter of 140 nm and a length of 15 µm. Scanning electron microscopy (SEM) confirmed the porous nanowire morphology and the presence of numerous holes (average diameter of ~50 nm), contributing to the high SSA. Raman spectroscopy demonstrated the complete removal of PPy-related peaks after carbonization. Current-voltage (I-V) curve measurements revealed the semiconducting behavior of the PCNA, showing significantly increased conductivity after carbonization. Energy-dispersive X-ray spectroscopy (EDS) confirmed the high carbon content after carbonization. The SERS performance of the PCNA was evaluated using various molecules, including rhodamine 6G (R6G), β-lactoglobulin, and glucose. SERS measurements were performed under various conditions to assess the sensitivity, reproducibility, uniformity, and durability of the PCNA. The authors compare their results with those obtained from silicon, PNA, CNA, and commercial metal SERS substrates. Theoretical analysis using density functional theory (DFT) calculations was conducted to understand the mechanism of SERS enhancement, specifically exploring charge-transfer pathways between the PCNA and analyte molecules. Finite element method simulations were employed to assess the electromagnetic contribution to the SERS enhancement.

The PCNA substrate demonstrated exceptionally high SERS enhancement factors (~10⁶), primarily attributed to a strong broadband charge-transfer resonance. This chemical mechanism (CM), unlike the electromagnetic mechanism (EM) prevalent in conventional metal-based SERS, was supported by theoretical calculations showing efficient charge-transfer pathways. The PCNA exhibited extraordinarily high reproducibility across different substrates, spots, samples, and time points. The absence of "hot spots" in the PCNA resulted in uniform signal enhancement across the entire substrate surface, eliminating the variability observed in traditional SERS. The metal-free nature of the PCNA conferred high durability, preventing oxidation issues that plague metal substrates. The PCNA's fluorescence quenching capability enhanced its compatibility with fluorescent biomolecules. The high sensitivity of the PCNA was demonstrated by detecting R6G molecules at concentrations as low as 10 nM. SERS measurements of β-lactoglobulin and glucose showcased the substrate's high sensitivity and compatibility with biomolecules. The enhancement factor for β-lactoglobulin was determined to be ~10⁶ by comparing the signal intensity on the PCNA with that on a silicon substrate. SERS mapping of β-lactoglobulin revealed high uniformity across the PCNA surface, with a coefficient of variation (CV) of <7.8%. The time-to-time consistency of the PCNA was demonstrated with glucose, showing negligible degradation over 4 hours, highlighting the high stability and reliability of the PCNA. The absorption spectroscopy and the absence of overtones and combination bands in Raman spectra further substantiated the dominance of CM over EM in the SERS enhancement.

The findings address the long-standing challenges of conventional SERS by demonstrating a metal-free substrate with superior properties. The high enhancement factor (~10⁶), excellent reproducibility, high biocompatibility, and superior durability offered by the PCNA significantly improve the reliability and applicability of SERS, particularly in biomedicine. The absence of hot spots and the broadband charge-transfer resonance mechanism are key to its superior performance. The results indicate a significant advancement in SERS technology, opening possibilities for broader applications requiring highly reliable and reproducible measurements. The observed fluorescence quenching effect further enhances the suitability of the PCNA for analyzing fluorescent biomolecules, making it a promising tool for quantitative analysis of biological systems. The study's findings contribute substantially to the field of SERS by offering a solution to the limitations of traditional metallic substrates.

This study successfully designed, fabricated, and characterized a novel metal-free SERS substrate based on a porous carbon nanowire array (PCNA). The PCNA exhibited significantly improved performance compared to traditional metal-based SERS substrates, demonstrating high sensitivity, exceptional reproducibility, remarkable biocompatibility, and high durability. The findings pave the way for reliable and reproducible SERS applications in various fields. Future research could focus on optimizing the PCNA's structure and composition to further enhance the SERS signal, exploring other excitation wavelengths, and expanding the range of detectable molecules.

While the PCNA demonstrated excellent performance, potential limitations include the synthesis process which might require further optimization for scalability and cost-effectiveness. Further exploration of the interaction between the PCNA and various biomolecules could provide a more comprehensive understanding of its biocompatibility. The current study focused on specific molecules; further testing with a broader range of analytes is needed to confirm the generalizability of the findings.