Li Chen

Surface-enhanced Raman scattering (SERS) holds great promise for the real-time detection of microbial metabolites. However, pH fluctuations in fermentation environments can induce probe corrosion, aggregation, or precipitation, severely compromising the stability and reproducibility of SERS signals and thus diminishing system reliability for real-time analysis. To address this challenge, this study proposes a “material-system co-design” strategy to develop a wide pH-tolerant microdroplet SERS detection platform based on Ag@SiO₂@PVP nanoprobes. First, SERS nanoprobes with inherent pH tolerance were constructed through the synergistic effects of physical isolation provided by the SiO₂ shell and steric hindrance from the outer polyvinylpyrrolidone (PVP) coating. Subsequently, to achieve integrated operations including rapid mixing of nanoprobes and metabolites, homogeneous droplet generation, in-situ trapping, and SERS detection, a microfluidic chip incorporating a herringbone-shaped mixer, a droplet generation zone, an in-situ trapping zone, and a reflective substrate was designed. Using L-DOPA as a model molecule for performance validation, the results demonstrated a limit of detection as low as 9.5 × 10⁻⁹ g/mL (based on the 3σ method). The SERS signals remained highly stable and reproducible across the pH 3–11 range, with a linear correlation coefficient R² > 0.99 and a relative standard deviation (RSD) of characteristic peak intensities below 5% (n = 20). In tests using real samples from E. coli fermentation broth, the method maintained clear spectral recognition capability across the concentration range of 10⁻⁸ ~ 10⁻⁴ g/mL, exhibiting significantly superior detection performance and stability compared to bare silver nanoparticles. Preliminary tests with L-tyrosine and arbutin further validated the platform’s universality. Through its synergistic “dual-protection probe–multifunctional chip” strategy, this study provides a powerful tool for reliable, high-throughput analysis of metabolites in complex biological environments. The alternative text for this image may have been generated using AI.

Melanin acts as a versatile biological pigment crucial for disease prevention and health preservation. In the process of melanin biomanufacturing, a specific ternary metabolic network is formed by L-tyrosine (Tyr), L-DOPA, and Arbutin, functioning as a “substrate – intermediate – inhibitor” system. To clarify metabolic pathways and enhance synthesis efficiency, tracking the concentrations of these compounds is vital. In this work, we present a droplet-based microfluidic surface-enhanced Raman scattering (DMF-SERS) system designed for the non-invasive, real-time, and high-throughput analysis of these three metabolites for the first time. The system features an integrated microfluidic chip capable of mixing samples, generating droplets, trapping them, and performing in-situ detection. We utilized polyvinylpyrrolidone (PVP) as a dispersant to optimize Ag@SiO ₂ nanoparticles (Ag@SiO ₂ NPs), which prevented the aggregation or precipitation often triggered by pH fluctuations, thereby creating high-density Raman ”hot spots” within the microdroplets. Real-time monitoring was facilitated by a portable detection setup. The findings revealed a Raman enhancement factor reaching 1.2 × 10 ⁷ . The DMF-SERS platform successfully quantified concentration gradients for all three analytes with a limit of detection (LOD) at 10 ⁻⁶ g/mL. It exhibited a linear range from 10 ⁻³ to 10 ⁻⁶ g/mL (R ² > 0.99) and maintained a relative standard deviation (RSD) below 5%. With a throughput capacity of 4000 droplets per minute, the system demonstrated superior reproducibility and stability. Furthermore, the platform was effectively utilized to analyze metabolic samples from engineered strains, offering essential data for the optimization of melanin metabolic flux. This tool shows immense promise for establishing closed-loop monitoring systems and serves as a robust instrument for strain screening and product regulation in biomanufacturing and synthetic biology.

Nanoparticle multilayer substrates usually exhibit excellent SERS activity due to multi-dimensional plasmon coupling. However, simply increasing the layers will lead to several problems, such as complex manufacturing procedures, reduced uniformity and poor reproducibility. In this paper, the local electric field (LEF) characteristics of a Ag nanoparticle (AgNP) multilayer were systematically studied through finite element simulations. We found that, on the glass support, the LEF intensity improved with the increase in the layers of AgNPs. However, the maximum LEF could be obtained with only two layers of AgNPs on the Au film support, and it was much stronger than the optimal value of the former. To verify the simulation results, we have successfully prepared one to four layers of AgNPs on both supports with a liquid–liquid interface self-assembly method, and carried out a series of SERS measurements. The experimental results were in good agreement with the simulations. Finally, the optimized SERS substrate, the 2-AgNP@Au film, showed an ultra-high SERS sensitivity, along with an excellent signal uniformity, which had a detection ability of 1 × 10⁻¹⁵ M for the Rhodamine 6G (R6G) and a relative standard deviation (RSD) of 11% for the signal intensity. Our study provides important theoretical guidance and a technical basis for the optimized design and application of high-performance SERS substrates.

In this study, the alkaline phosphatase (ALP)-like activity of zeolitic-imidazolate framework-8 (ZIF-8) is reported for the first time. Then, colorimetric sensors for the ascorbic acid oxidase (AAO) and copper ion (Cu²⁺) detection were developed based on the acetylcholinesterase (AChE)- and ALP-like activities of ZIF-8. The ZIF-8 has good mimetic enzyme activity and exhibits high affinity to the substrates. Its AChE- and ALP-like activities also have good reusability and storage stability. Good linear dependences are obtained in the range of 1.3−250.0 μM (AChE-like activity-based) and 4.5−454.5 μM (ALP-like activity based) for Cu²⁺ detection. Furthermore, good linear dependence is also obtained based on the ALP-like activity of ZIF-8 for AAO detection in the range of 2.3−454.5 U/L. Their limits of detection (LODs) are calculated to be 0.7 µM, 2.8 µM, and 1.8 U/L, respectively. Finally, the sample spiked recoveries of Cu²⁺ in tap water, Cu²⁺, and AAO in human serum and rabbit plasma were measured, and the results are in the range of 80.0−119.3%. In short, the preparation of ZIF-8 is simple, environmentally friendly, and harmless, and can realize highly selective detection of AAO and Cu²⁺ in an efficient and fast process.

A sandwich SERS detection system was constructed. Due to the optical convergence, the synergistic enhancement effects, and the “Au film–molecules–AgNPs” sandwich structure, the Raman intensity is greatly improved.

To improve the sensitivity of disease biomarker detection, we proposed a high-performance surface-enhanced Raman spectroscopy (SERS) chip integrated with a micro-optical system (MOS). The MOS, which is based on the micro-reflecting cavity and the micro-lens, optimizes the optical matching characteristics of the SERS substrate and the Raman detection system, and greatly improves the SERS detection sensitivity by improving the collection efficiency of the Raman scattering signal. A uniform single layer of silver nanoparticles on a gold film was prepared as the SERS substrate using a liquid-liquid interface self-assembly method. The micro-reflecting cavity and micro-lens were prepared using micro-processing technology. The SERS chip was constructed based on the MOS and the Au film-based SERS substrate, and experimental results showed an EF of 1.46×10⁸, which is about 22.4 times higher than that of the Si-based SERS substrate. The chip was used for the detection of creatinine and the detection limit of creatinine in aqueous solution was 1 µM while the detection limit in serum was 5 µM. In addition, SERS testing was conducted on serum samples from normal people and patients with chronic renal impairment. Principal component analysis and linear discriminant analysis were used for modeling and identification, and the results showed a 90% accuracy of blind sample detection. These results demonstrate the value of this SERS chip for both research and practical applications in the fields of disease diagnosis and screening.

To improve the sensitivity of surface-enhanced Raman spectroscopy (SERS) detection, we propose a three-dimensional (3D) SERS chip based on an inverted pyramid micro-reflector (IPMR) that converges Raman scattering light signals to improve the signal collection efficiency. The influence of the geometric parameters of the inverted pyramid structure on the Raman signal collection efficiency was analyzed by simulation for the determination of the optimal design parameters. The inverted pyramid through-hole structure was prepared on the silicon wafer through an anisotropic wet etching process, followed by the sputtering of a gold film to form the IPMR. The 3D SERS chip was constructed by bonding the IPMR and the active substrate that assembled with silver nanoparticles. Using Rhodamine 6G molecules, the Raman intensity measured with the 3D SERS chip was threefold greater than that of the silicon-based SERS substrate under the same test conditions. These experimental results show that the 3D SERS chip can significantly improve the SERS signal intensity. Its 3D structure is convenient for integration with microfluidic devices and has great potential in biochemical detection applications.