A SERS-active capillary for direct molecular trace detection in liquids

Construction of dense film inside capillary wall and SERS application research

The high-density particle distribution in capillary was a crucial factor for enhancing SERS properties and a difficult point in the preparation process. The direct high-temperature method was used to fuse the particles and form a uniform and dense particle distribution on the capillary's inner wall, providing a foundation for enhancing Raman signals. The prepared capillary SERS substrate strongly enhances the rhodamine 6G (R6G) signal, and the RSD values of several characteristic peaks of R6G are about 10 %, demonstrating high sensitivity, uniformity, and stability. Using capillary SERS substrate for detecting goat serum. Embedding precious metal particles into capillary SERS substrate can effectively encapsulate the tested liquid and avoid contamination, which improves the disadvantage of traditional substrates exposing the liquid to air. The prepared capillary SERS substrate could be used for field and biomedical sensitivity detection, providing a theoretical and experimental basis for developing the capillary SERS substrate.

Capillary-force-assisted self-assembly of gold nanoparticles into highly ordered plasmonic thin films for ultrasensitive SERS

In this study, a capillary device based on the surface plasmon-enhanced Raman scattering effect was prepared by a simple and easy method. First, the capillary was treated with APTES solution. Due to the electrostatic effect, gold nanoparticles could be easily and tightly assembled in the capillary inner wall. On this basis, the effects of changing the concentration of APTES, the concentration of colloids and the soaking time of the capillary in the colloids on the assembly of gold nanoparticles on the inner wall of the capillary were studied, and the SERS enhancement effect under different conditions was analyzed, and the optimal solution was successfully found. At the same time, the reason why the capillary substrate shows better SERS performance than the traditional planar substrate is deeply discussed. Since the nanoparticles can be attached to the upper and lower surfaces of the inner wall of the capillary, the utilization rate of nanoparticles and laser is improved, thereby achieving higher enhancement. For the detection of the probe molecule rhodamine 6G, it was proved that the substrate has good uniformity and the lowest detection limit can reach 10^-10 M. Finally, the real-life pesticide thiram and the food additive aspartame were tested, and the detection limits could reach 10^-6 M and 0.25 g L^-1. It is confirmed that the prepared capillary shows excellent SERS performance and can be used for rapid detection in various fields.

Controlled fiber core mode and surface mode interaction for enhanced SERS performance

Three-dimensional surface-enhanced Raman scattering (SERS) platform based on microstructure fibers has many advantages for rapid liquid detection due to its microfluidic channels and light guidance. The fiber mode field distribution determines the light-analyte interaction strength but has rarely been studied in SERS applications. In this paper, we numerically and experimentally investigate the mode field distribution in suspended-core fibers decorated with gold nanoparticles. The interaction between the core mode and surface mode is controlled by changing the density of gold nanoparticles on the inner surface. The avoided crossing wavelength shifts linearly to red with the decrease of the nanoparticle spacing. With an optimized nanoparticle spacing of 20 nm, the avoided crossing occurs near the laser wavelength of 633 nm, which greatly increases the power ratio in the liquid channels and hence improves the SERS performance. The detection limit for crystal violet was 10^-9 M, and the enhancement factor was 10⁸. The avoided crossing mechanism can be applied to all fiber SERS probes for sensitivity improvement.

Molecular trace detection in liquids using refocusing optical feedback by a silver-coated capillary

Surface-enhanced Raman scattering (SERS) has been widely used owing to its high sensitivity and rapid response. In particular, 3D SERS-active platforms greatly extend the interaction area and ensure the ability to directly detect trace amounts of molecules in liquids. A silver-coated capillary, with the ability of liquid sampling and light guiding, provides a new platform for high-performance SERS substrates. In this paper, the silver mirror reaction was used for coating silver on the outer wall of the capillary. PDMS was used as a coating material to protect the silver film. Because of the silver coating, Mie scattering and Raman scattering in the liquid channel can be refocused and reflected back which greatly reduces the propagation loss and extends the interaction length. An enhancement factor as high as 10⁸ and a detection limit of 10^-10 M of rhodamine 6G in aqueous solution have been achieved. Moreover, the SERS intensity is homogeneous across the end face of the liquid channel, with the relative standard deviation (RSD) value changing within 7%. The large area and high homogeneity greatly reduce the requirement of light coupling precision and liquid injection pressure. Using a common flange optical fiber connector, the capillary can be simply connected and aligned with a multimode fiber with a detection limit of 10^-8 M. The experiment results show great potential for the development of an optofluidic integrated system in the future.