Surface-enhanced Raman scattering (SERS) is a spectral analysis technique that can effectively enhance Raman signals. This enhancement is based on the attachment of sample molecules to nanoscale metal structures [1,2]. Since its discovery, SERS has garnered widespread attention due to its wide range of potential applications in the fields of biology [3], sensors [4], catalysis [5], and environmental monitoring. Unlike traditional Raman spectroscopy, SERS provides a substantial intensity enhancement, making it a vital analysis tool with high sensitivity and low detection limit. It has been applied to many biological applicants, such as amino acids [6], glucose [7,8], and antioxidants [9], as well as large molecules, such as proteins [10] and DNA [11]. In recent years, SERS has been utilized in diagnosing cervical cancer [12], bladder cancer [13], breast cancer tumor resection [14], and brain tumor resections [15]. Mohs et al. used a SERS biosensor to detect prostate specificity and SpectroPen equipment to detect the tumor's edge [16]. Li et al. detected circulating tumor cells in human whole blood [17]. Compared with other detection processes, such as blood component analysis, SERS can perform rapid, non-invasive, and accurate analysis, thereby overcoming problems arising from complex sample preparation, long processing times, high costs, and inaccuracies.

Since 1974, scientists Fleischmann, Hendra, and McQuillan have observed considerable inelastic light scattering in the Raman spectra of pyridine on a rough silver electrode [18]. Thus, SERS has garnered widespread attention from scientists. SERS's primary enhancement mechanisms are electromagnetic enhancement (EM) and chemical enhancement mechanism. Localized surface plasmon resonance refers to the dominant effect of the EM in SERS, which can increase elastic (Rayleigh) and inelastic (Raman) light scattering from the sample [19]. When the incident laser frequency is close to the substrate surface plasma frequency, the electromagnetic field resonance occurs, and the Raman signal intensity of the molecular structure increases exponentially [20]. Currently, common metals, such as gold, silver, copper, and a few alkaline metals (lithium and aluminum), exhibit strong SERS effects. Consequently, the study of SERS beyond these three metals has yet to make practical progress for an extended period [21].

Silver nanoparticles are popular SERS substrate owing to their high efficiency compared with gold and copper nanoparticles [22,23]. This heightened efficiency is attributed to the sharp corners or large curvatures in metal nanostructures, which generate an excellent local electromagnetic field enhancement due to increased charge accumulation [24]. The unique structural properties of silver nanocubes (AgNCs) make them highly appealing as a SERS substrate due to their well-defined edges and corner structures [25,26]. However, various synthesis processes, such as oxidation, chemical reduction, or physical deposition, can lead to poor biocompatibility in noble metal nanomaterials [27]. Studies have found that silver nanoparticles can generate DNA damage, cell cycle arrest, oxidative stress, apoptosis, and necrosis [28]. Additionally, the release of Ag+ from silver nanoparticles upon absorption into the cell membrane has been linked to cellular toxicity [29]. As a result, noble metal nanomaterials are not commonly used for various biomedical applications [30]. To address this issue, precious metal nanomaterials with low toxicity, good stability, and high sensitivity must be prepared for the study of biomolecule sensing. Researchers have attempted to mitigate toxicity by coating silver nanoparticles with substances, such as silicon dioxide, to reduce the direct contact between silver nanoparticles and organisms. However, the biocompatibility of these materials is not ideal [31]. To reduce the toxicity of silver nanoparticles while maintaining biological activity and improving stability, we have chosen to modify AgNCs using polyethylene glycol (PEG), a polymer with good safety profile in humans [32]. Furthermore, our choice to focus on esophageal cancer as the detection target stemmed from the complexity of past detection methods and the utmost importance of early-stage detection in improving patient survival rates. Consequently, Raman detection was employed due its ability to achieve early, rapid, and accurate detection.

In this study, we synthesized AgNCs with good SERS efficiency and low toxicity toward biological tissues for detecting esophageal cancer. The surface of the AgNCs was coated with PEG to create an excellent SERS-based biosensor substrate. Cell viability was evaluated using CCK8 and AM/PI assays, confirming that the use of PEG reduced the toxicity of AgNCs. SERS performance was investigated using 4-mercaptobenzoic acid (4-MBA) as a Raman probe molecule. The experimental results showed that an optimal concentration of 16 μM PEG-AgNCs was most effective for the Raman detection of cells. To detect differences between human keratinocytes (HaCaT) and human esophageal carcinoma cell (TE-1) at the molecular level, we conducted Raman detection using PEG-AgNCs and analyzed the Raman spectrum between 588 cm−1 and 768 cm−1. Our findings revealed remarkable differences in molecular structure between HaCaT and TE-1 cells.

The experimental procedures are illustrated in Fig. 1. This SERS tool utilizes PEG-AgNCs and provides a novel approach for direct and sensitive detection of early-stage esophageal cancer through Raman spectrum analysis.