Relevant properties of gold nanoparticles, such as stability and biocompatibility, together with their peculiar optical and electronic behavior, make them excellent candidates for medical and biological applications. This review describes the different approaches to the synthesis, surface modification, and characterization of gold nanoparticles (AuNPs) related to increasing their stability and available features useful for employment as drug delivery systems or in hyperthermia and photothermal therapy. The synthetic methods reported span from the well-known Turkevich synthesis, reduction with NaBH4 with or without citrate, seeding growth, ascorbic acid-based, green synthesis, and Brust–Schiffrin methods. Furthermore, the nanosized functionalization of the AuNP surface brought about the formation of self-assembled monolayers through the employment of polymer coatings as capping agents covalently bonded to the nanoparticles. The most common chemical–physical characterization techniques to determine the size, shape and surface coverage of AuNPs are described underlining the structure–activity correlation in the frame of their applications in the biomedical and biotechnology sectors.

Synthesis, Chemical-Physical Characterization, and Biomedical Applications of Functional Gold Nanoparticles: A Review.

In this work, authors have developed a portable, sensitive, and quick-response fiber optic sensor that is capable of detection of Aflatoxins B1 (AFB1) quantitatively and qualitatively. Using multi-mode fiber (MMF) and multi-core fiber (MCF), the MMF-MCF-MCF-MMF fiber structure based on symmetric transverse offset splicing and waist-expanded taper is fabricated. The evanescent waves are enhanced to form a strong evanescent field by etching the fiber surface with hydrofluoric acid. To successfully excite the localized surface plasmon resonance phenomenon, gold nanoparticles are deposited on the optical fiber probe's surface. Further, to modify the fiber optic probes, Niobium carbide (Nb2CTx) MXene and AFB1 antibodies are functionalized. Nb2CTx MXene is employed to strengthen the biocompatibility of the sensor and increase the specific surface area of the fiber probe, while AFB1 antibody is used to identify AFB1 micro-biomolecules in a specific manner. The reproducibility, reusability, stability, and selectivity of the proposed fiber probe are tested and validated using various concentration of AFB1 solutions. Finally, the linear range, sensitivity, and limit of detection of the sensing probe are determined as 0 - 1000 nM, 11.7 nm/µM, and 26.41 nM, respectively. The sensor offers an indispensable technique, low-cost solution and portability for AFB1-specific detection in agricultural products and their byproducts with its novel optical fiber structure and superior detecting capability. It is also useful for marine species like fish and consequently affecting health of human body.

Plasmonic sensor based on offset-splicing and waist-expanded taper using multicore fiber for detection of Aflatoxins B1 in critical sectors.

In this paper, a review of recent studies on the optical fiber-based surface plasmon resonance (SPR) sensor and the sensitivity improvement based on specialty optical fibers and two-dimensional (2D) materials is given. The configurations and working principles of various SPR excitation methods are introduced. Several types of specialty optical fibers, including polymer fiber, multi-core fiber, hollow fiber, photonic crystal fiber, are discussed in detail with enhanced sensing configurations. Furthermore, some hybrid configurations using 2D materials like graphene, transition metal dichalcogenides, and black phosphorus are reviewed in terms of properties, sensing structures, and application fields. The review aims to provide an overview of the latest achievements in optical fiber-based SPR sensing technology, with emphasis on research towards sensitivity improvement. 

Specialty optical fibers and 2D materials for sensitivity enhancement of fiber optic SPR sensors: A review

Overview and emerging trends in optical fiber aptasensing.

/pdf/novel-optical-fiber-based-structures-for-plasmonics-sensors-ri9n5vuk.pdf

Novel Optical Fiber-Based Structures for Plasmonics Sensors

In the present study, an optical fiber surface plasmon resonance (SPR) biosensor was developed for measuring time- and concentration-dependent DNA hybridization kinetics. Its design complies with a 3D Au/Al2O3 multilayer composite hyperbolic metamaterial (HMM), a graphene film, and a D-shaped plastic optical fiber. According to the numerical simulation and the experimental demonstration, the SPR peak of the designed biosensor can be effectively altered in the range of visible to near-infrared by varying the HMM structure. The sensitivity of the appliance was shown to achieve a value of up to 4461 nm/RIU, allowing its applicability for bulk refractive index sensing. Furthermore, a biosensor designed in this work displayed high-resolution capability (ranging from 10 pM to 100 nM), good linearity, and high repeatability along with a detection limit down to 10 pM, thus suggesting a vast potential for medical diagnostics and clinical applications.

/pdf/optical-fiber-spr-biosensor-complying-with-a-3d-composite-2xvv14ue5o.pdf

Optical fiber SPR biosensor complying with a 3D composite hyperbolic metamaterial and a graphene film

Highly efficient SERS substrates with different Ag interparticle nanogaps based on hyperbolic metamaterials

/pdf/bulk-plasmon-polariton-in-hyperbolic-metamaterials-excited-5cb70axejl.pdf

Bulk plasmon polariton in hyperbolic metamaterials excited by multilayer nanoparticles for surface-enhanced Raman scattering (SERS) sensing

Since localized surface plasmon (LSP) is capable of generating strong electromagnetic fields, it has achieved extensive applications in surface-enhanced Raman scattering (SERS). As opposed to this, surface plasmon polariton (SPP) has been rarely employed for its weak electric field enhancement. The present study proposed an Ag nanoparticles (AgNPs) and multilayer Au/Al2O3 film (MLF) hybrid system, acting as an efficient SERS substrate by coupling LSPs and SPPs resonances. The dispersion relationship indicates that the light scattered by the AgNPs excites the SPP in the MLF, while the electric field is bound to the Au/Al2O3 interface and is significantly enhanced. As revealed from the simulated results, SPPs were generated in the MLF and then coupled with each other to generate a bulk plasmon polariton (BPP). As impacted by BPP, the electric fields stimulated by LSP displayed a dramatic increase. Besides, the electric field exhibited increased intensity with the layer of film. As rhodamine 6G (R6G) and malachite green (MG) were employed as the probe molecules, the AgNPs/MLF hybrid structure demonstrated highly sensitive SERS performance, complying with the theoretical simulations. Specific to the mentioned SERS substrate, R6G and MG had the limit of detection of 1.2 × 10−10M and 7.9 × 10−9M, respectively, demonstrating the prominent prospects of the NPs/MLF hybrid structure in SERS.

Coupling of multiple plasma polarization modes in particles–multilayer film system for surface-enhanced Raman scattering

In the present study, a nanoparticle-multilayer metal film substrate was presented with silver nanoparticles (Ag NPs) assembled on a multilayer gold (Au) film by employing alumina (Al2O3) as a spacer. The SERS performance of the proposed structures was determined. It was suggested that the SERS effect was improved with the increase in the number of layers, which was saturated at four layers. The SERS performance of the structures resulted from the mutual coupling of multiple plasmon modes [localized surface plasmons (LSPs), surface plasmon polaritons (SPPs), as well as bulk plasmon polaritons (BPPs)] generated by the Ag NP-multilayer Au film structure. Furthermore, the electric field distribution of the hybrid system was studied with COMSOL Multiphysics software, which changed in almost consistency with the experimentally achieved results. For this substrate, the limit of detection (LOD) was down to 10−13 M for the rhodamine 6G (R6G), and the proposed SERS substrate was exhibited prominently quantitatively detected capability and high reproducibility. Moreover, a highly sensitive detection was conducted on toluidine blue (TB) molecules. As revealed from the present study, the Ag NP-multilayer Au film structure can act as a dependable SERS substrate for its sensitive molecular sensing applications in the medical field.

Surface-enhanced Raman scattering by the composite structure of Ag NP-multilayer Au films separated by Al2O3.

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