Photoreduced silver nanoparticles grown on femtosecond laser ablated, D-shaped fiber probe for surface-enhanced Raman scattering

Microfluidic integrated D-shaped optical fiber SERS probe with high sensitivity and ability of multi-molecule detection

In order to enhance the sensitivity, integration, and practical application capability of Raman detection systems, we propose a multi-channel microfluidic integrated D-shaped optical fiber SERS (Surface-enhanced Raman scattering) probe structure. Firstly, a microfluidic polydimethylsiloxane (PDMS) channel was fabricated using a self-designed multi-channel microfluidic template. Secondly, a uniform layer of silver nanoparticles was deposited on the D-shaped optical fiber using the liquid-liquid interface method. Finally, the D-shaped optical fiber was plasma-bonded to the multi-channel microfluidic channel and a cover glass, resulting in a microfluidic integrated D-shaped optical fiber SERS probe. The prepared sample exhibited excellent detection performance for R6G (rhodamine 6 G) with a detection limit as low as 10-11 mol/L and an enhancement factor of 1.14 × 109. Moreover, the multi-channel configuration enables simultaneous detection of multiple molecules, demonstrating excellent multi-channel multiplexing capability.

Plasmonic volcano-like fiber-optic probe for Raman enhancement

Light-matter interaction is a fascinating topic extensively studied from classical theory, based on Maxwell's equations, to quantum optics. In this study, we introduce a novel, to the best of our knowledge, silver volcano-like fiber-optic probe (sensor 1) for surface-enhanced Raman scattering (SERS). We employ the emerging quasi-normal mode (QNM) method to rigorously calculate the Purcell factor for lossy open system responses, characterized by complex frequencies. This calculation quantifies the modification of the radiation rate from the excited state e to ground state g. Furthermore, we use and extend a quantum mechanical description of the Raman process, based on the Lindblad master equation, to calculate the SERS spectrum for the plasmonic structure. A common and well-established SERS probe, modified by a monolayer silver nanoparticle array, serves as a reference sensor (sensor 2) for quantitatively predicting the SERS performance of sensor 1 using quantum formalism. The predictions show excellent consistency with experimental results. In addition, we employ the FDTD (finite-difference time-domain) solver for a rough estimate of the all-fiber Raman response of both sensors, revealing a reasonable range of SERS performance differences compared to experimental results. This research suggests potential applications in real-time, remote detection of biological species and in vivo diagnostics. Simultaneously, the developed FDTD and quantum optics models pave the way for analyzing the response of emitters near arbitrarily shaped plasmonic structures.

Single-Step Process for Titanium Surface Micro- and Nano-Structuring and In Situ Silver Nanoparticles Formation by Ultra-Short Laser Patterning

Ultra-short laser (USL)-induced surface structuring combined with nanoparticles synthesis by multiphoton photoreduction represents a novel single-step approach for commercially pure titanium (cp-Ti) surface enhancement. Such a combination leads to the formation of distinct topographical features covered by nanoparticles. The USL processing of cp-Ti in an aqueous solution of silver nitrate (AgNO3) induces the formation of micron-sized spikes surmounted by silver nanoparticles (AgNPs). The proposed approach combines the structuring and oxidation of the Ti surface and the synthesis of AgNPs in a one-step process, without the use of additional chemicals or a complex apparatus. Such a process is easy to implement, versatile and sustainable compared to alternative methodologies capable of obtaining comparable results. Antimicrobial surfaces on medical devices (e.g., surgical tools or implants), for which titanium is widely used, can be realized due to the simultaneous presence of AgNPs and micro/nano-structured surface topography. The processed surfaces were examined by means of a scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM) and Raman spectroscopy. The surface morphology and the oxidation, quality and quantity of AgNPs were analyzed in relation to process parameters (laser scanning speed and AgNO3 concentration), as well as the effect of AgNPs on the Raman signal of Titanium oxide.

Fabrication of a three-dimensional (3D) SERS fiber probe and application of in situ detection

Surface-enhanced Raman scattering (SERS) fiber probes are useful for remote and online detection of harmful molecules using the SERS effect. In this study, a 3-dimensional (3D) SERS optical fiber probe is proposed. The formation of the 3D optical fiber probe mainly included three steps: construction of monolayer polystyrene (PS) spheres as a mask on the end face of the fiber, reactive ion etching (RIE) for PS spheres and fibers, and metal sputtering deposition. Compared with flat surface fiber probes, these 3D SERS fiber probes are composed of ordered nanocolumn arrays, which have the advantages of a simple manufacturing process, low cost, high sensitivity, and good stability. The structures of the 3D SERS fiber probe can be well controlled by changing the size of the PS sphere and etching time. The formation of the nanocolumn was studied using time evolution experiments. The obtained fiber SERS probe has good stability and high sensitivity for the in situ detection of 4-aminothiophenol (4-ATP) in solution. Therefore, these 3D SERS fiber probes have potential applications in harmful molecules for real-time detection.

Recent Advances in Biomedical Photonic Sensors: A Focus on Optical-Fibre-Based Sensing

In this invited review, we provide an overview of the recent advances in biomedical photonic sensors within the last five years. This review is focused on works using optical-fibre technology, employing diverse optical fibres, sensing techniques, and configurations applied in several medical fields. We identified technical innovations and advancements with increased implementations of optical-fibre sensors, multiparameter sensors, and control systems in real applications. Examples of outstanding optical-fibre sensor performances for physical and biochemical parameters are covered, including diverse sensing strategies and fibre-optical probes for integration into medical instruments such as catheters, needles, or endoscopes.

Optimized Tapered Fiber Decorated by Ag Nanoparticles for Raman Measurement with High Sensitivity

A tapered fiber decorated by Ag nanoparticles is prepared as a surface-enhanced Raman scattering (SERS) substrate. There are two key parameters during the preparation process, the fiber cone angle and the density of decorated AgNPs on the fiber tip surface. Their theoretical analysis on the forming mechanism and the optimization process is studied in detail. The tapered fibers with angles from 0.5 to 30° are successfully prepared, with a chemical method in a small tube using a bending interface. AgNPs with different densities are decorated on the surface of the tapered fibers with an electrostatic adsorption method. The optimized tapered fiber SERS probe with an angle of 12° and AgNPs density of 26.67% provides the detection of Rhodamine 6G (R6G) with 10⁻¹⁰ mol/L.

Development of monolayer AuNPs decorated on an optical fiber facet for SERS analysis

In this paper, we present a novel strategy for fabricating surface-enhanced Raman scattering (SERS) optical probe modified monolayer gold nanoparticles (AuNPs) by a seed-mediated growth method. The morphology and optical properties of the samples were characterized by transmission electron microscopy, scanning electron microscopy, and UV-visible absorption spectroscopy. The results show that the resulting probes exhibit high sensitivity with a detection limit down to 10^-9mol/L for Methylene Blue solution and 10^-8mol/L for both Crystal Violet and Rhodamine 6G solutions. Furthermore, the probes show an excellent reproducibility (relative standard deviation of 9.2% at 1621cm^-1) and good stability, and the SERS spectra can be reproduced after storing the probes for one month in air. Finally, by finite-element simulations, we investigate the electromagnetic field distribution of the fiber facet modified with AuNPs. This work provides a promising potential of prepared SERS fiber probes and has broad application prospects in food safety, pesticide residue analysis, and environmental surveillance.

Lab on D-shaped fiber excited via azimuthally polarized vector beam for surface-enhanced Raman spectroscopy

We present a method for Raman examination using a silver-nanoparticles (Ag-NPs) coated D-shaped fiber (DSF) internally excited via an in-fiber azimuthally polarized beam (APB) generated by an acoustically induced fiber grating. Simulation results show that an electric-field intensity enhancement factor can be effectively improved under APB excitation compared with the linear polarization beam (LPB) excitation, because the strong gap-mode is uniformly generated between two adjacent Ag NPs on the surface of the DSF planar side. Experimental results show that the Raman signal intensity of the methylene blue (MB) detected by DSF in the case of APB excitation is ∼4.5 times as strong as that of LPB excitation, and the Raman detection sensitivity is ∼10^-9 M. The time stability of this method is also tested to be guaranteed.

Au-nanorod-clusters patterned optical fiber SERS probes fabricated by laser-induced evaporation self-assembly method

Optical fiber surface-enhanced Raman scattering (SERS) probes provide a novel platform for liquid-phase in situ and remote SERS detections. However, it is still a challenge to fabricate noble metal nanostructures with large SERS enhancement factor (EF) onto optical fiber surfaces. In this article, we successfully prepare Au-nanorod cluster structures on optical fiber facets by a laboratory-developed laser-induced evaporation self-assembly method. It is demonstrated that the optimized optical fiber SERS probes show high detection sensitivity (10^-10 M for rhodamine 6G solution, and 10^-8 M for malachite green or crystal violet solution) and excellent reproducibility (relative standard deviation less than 6%). As the laser-induced evaporation self-assembly method is a simple and low-cost method capable of achieving automatic and reproducible preparations of cluster patterned optical fiber SERS probes, this work may find important application prospects in various liquid-phase SERS detection areas.