Plasmonic hollow fibers with distributed inner-wall hotspots for direct SERS detection of flowing liquids

Multifold Enhanced Raman Detection of Organic Molecules as Environmental Water Pollutants

Organic molecules, including the benzene series, have been identified as pollutants in environmental water. Due to their very low solubility, they have very small concentrations in water, and they are difficult to be detected by conventional techniques. In particular, there is a lack of real-time, accurate, and rapid detection methods for such molecules in water. However, they are detrimental to human health in many aspects. Toluene has been an important indicator of such environmental pollution detections. In this work, we propose a 3D SERS scheme consisting of a hollow fiber that is coated on the inner wall with densely arranged silver nanoparticles, which supplies multifold Raman enhancement by the plasmonic microcavity. Strong confinement of excitation laser energy and strongly enhanced Raman signals with the bidirectional collection are utilized to achieve high-sensitivity detection of toluene molecules in water. Raman signal with a reasonable signal-to-noise ratio has been measured for a concentration of 0.53 mg/L, indicating a detection limit even lower than this value for such a Raman spectroscopic technique. The corresponding enhancement factor is higher than 6 × 10³ with respect to the available systems. Thus, this device not only enables direct trace detection and real-time monitoring of the water-polluting status by organic molecules but also supplies a practical approach for biological sensing.

Direct Laser Writing of SERS Hollow Fibers

We report the direct laser writing (DLW) of surface-enhanced Raman scattering (SERS) structures on the inner wall of a hollow fiber. Colloidal gold-silver alloy nanoparticles (Au-Ag ANPs) are firstly coated onto the inner wall of a hollow fiber. A green laser beam is focused through the outer surface of the hollow fiber to interact with colloidal Au-Ag ANPs so that they become melted and aggregated on the surface of the inner wall with strong adhesion. Such randomly distributed plasmonic nanostructures with high density and small gaps favor the SERS detection of low-concentration molecules in liquids flowing through the hollow fiber. Such a SERS device also supplies a three-dimensional microcavity for the interaction between excitation laser and the target molecules. The DLW system consists mainly of the flexible connection between the motor shaft and the hollow fiber, the program-controlled translation of the hollow fiber along its symmetric axis and rotation about the axis, as well as the mechanical design and the computer control system. This DLW technique enables high production, high stability, high reproducibility, high precision, and a high-flexibility fabrication of the hollow fiber SERS device. The resultant microcavity SERS scheme enables the high-sensitivity detection of R6G molecules in ethanol with a concentration of 10-7 mol/L.

Synergistic double laser beam-boosted liquid-NIR-SERS for ultralow detection of non-adsorptive polycyclic aromatic hydrocarbons in lake water

Ultrasensitive trace-detection of toxic and carcinogenic polycyclic aromatic hydrocarbons (PAHs) can ceaselessly propel the environmental surveillance in aqueous ecosystems. Due to the intrinsic nonadsorptive feature of PAHs, the promising technique of surface-enhanced Raman scattering (SERS) spectroscopy has been restricted to diverse functional ligands-based surface modifications of nano-substrates. However, it is not suitable for practical ultralow liquid analysis. Herein, we propose an extraordinary strategy to boost liquid-near infrared (NIR)-SERS activity of plasmonic Au/Ag nano-urchins (NUs) by introducing extra 808 nm laser-triggered an additional strong electromagnetic enhancement into routine 785 nm laser-Raman system. The synergistic double laser-excited NIR-SERS of colloidal Au/Ag NUs enables the Raman signals of crystal violet to be significantly enhanced, approaching a maximum of ∼34-fold increase than that of traditional bare 785 nm laser-excitation. More importantly, the improved liquid-NIR-SERS enables the in-situ detection limit of pyrene molecules in lake water to be achieved at ∼10-9 M, which is already better than many previous SERS results based on the complicated functionalized nano-substrates. The established double laser-boosted NIR-SERS can also be easily extended to the simultaneous trace-detection of three PAHs-contaminated mixtures, supporting well distinguishable capability. Undoubtedly, the present work opens a new versatile and innovative avenue for ultrasensitive NIR-SERS monitoring of nonadsorptive toxic pollutants in wastewater.

Polarization Maintaining Fiber Temperature and Stress Gradient Sensitization Sensor Based on Semiconductor-Metal-Polymer Three-Layer Film Coating

Increasing sensitivity, measuring points, and stability have always been the pursuit of sensors. ZnSe₉:CO₁ and Ag composite nano films were coated on polarization maintaining fiber (PMF). Then, the coated PMF was nested in capillary and hose which was encapsulated with polydimethylsiloxane (PDMS) and epoxy resin. The integrated capillary sensor and thermoplastic hose sensor were prepared. The gradient sensitization of various measurement parameters such as temperature, stress, and micro bending is realized. The temperature sensitivity is 1.49 nm/°C, the micro bending sensitivity is 1.72 nm/10² g, and the stress sensitivity is 6.27 nm/mε. The sensors maintain good linearity and instantaneous response while having high sensitivity. By adjusting the length of PMF, the number of troughs is increased in the same band range, and different troughs have different sensitivities, which solves the inherent problem of cross sensitivity and realizes multiparameter measurement. Capillary sensors are used for remote safe real-time monitoring of mechanical overheating, and hose sensors are used for real-time monitoring of bridge load and human joint bending. This work is of great significance to the extension of the application range of optical fiber sensor.

SERS Platform Based on Hollow-Core Microstructured Optical Fiber: Technology of UV-Mediated Gold Nanoparticle Growth

Surface-enhanced Raman spectroscopy (SERS) is a powerful technique for biosensing. However, SERS analysis has several concerns: the signal is limited by a number of molecules and the area of the plasmonic substrate in the laser hotspot, and quantitative analysis in a low-volume droplet is confusing due to the change of concentration during quick drying. The usage of hollow-core microstructured optical fibers (HC-MOFs) is thought to be an effective way to improve SERS sensitivity and limit of detection through the effective irradiation of a small sample volume filling the fiber capillaries. In this paper, we used layer-by-layer assembly as a simple method for the functionalization of fiber capillaries by gold nanoparticles (seeds) with a mean diameter of 8 nm followed by UV-induced chloroauric acid reduction. We also demonstrated a simple and quick technique used for the analysis of the SERS platform formation at every stage through the detection of spectral shifts in the optical transmission of HC-MOFs. The enhancement of the Raman signal of a model analyte Rhodamine 6G was obtained using such type of SERS platform. Thus, a combination of nanostructured gold coating as a SERS-active surface and a hollow-core fiber as a microfluidic channel and a waveguide is perspective for point-of-care medical diagnosis based on liquid biopsy and exhaled air analysis.