One of the most representative physical parameters is the refractive index, whose detection is of great interest in the industry. According to this, refractometers are a very powerful tool in the field of sensing devices because they can be used for a direct measurement of the surrounding medium refractive index (SMRI) or even they can be also used combined with a sensitive coating whose refractive index depends on a specific parameter. The use of resonance-based optical fiber refractometers can take advantage of the intrinsic properties associated with optical fiber as well as the advantage of wavelength detection technique associated with the resonance phenomenon of the LSPR of the metallic nanoparticles. A clear example can be found in [6] where the development of an LSPR-based U-bent plastic optical fiber based on the immobilization of gold nanoflowers (denoted as AuNFs) is presented for measuring refractive index changes. The experimental results have corroborated that the U-bent LSPR sensor has shown an 8-fold improvement in refractive index sensitivity in comparison with the bare sensor. Other interesting approach for the detection of refractive index changes is also presented in [7]. In this work, the immobilization of five-branched gold nanostars (denoted as GNS) onto the part of the core is performed (see Figure 3), which exhibits three localized surface resonances (LSPR1, LSPR2 and LSPR3). Two of these LSPR bands have presented a wavelength dependence as a function of variations of the refractive index of the surrounding medium, being the sensitivity for LSPR3 (580 nm/RIU) greater than LSPR2 (175 nm/RIU), as shown in the linear fitting in Figure 4. Finally, a relevant aspect to remark is that GNS can be also used as a powerful tool for nanomedicine, exploiting the 700–1000 nm transparent window of biological matter for treatments against tumors or even multidrug resistant bacterial infections, among others.

A similar wavelength-based LSPR optical fiber sensor can be found in [8], although in this work two different types of gold morphologies such as gold nanospheres (GNSs) and gold nanorods (GNRs) have been immobilized, being the sensitivity to refractive index variation of 914 nm/RIU for GNSs and 601 nm/RIU for GNRs, respectively. A very promoting result is presented in [9] because the sensitivity of the LSPR optical fiber sensor has been increased up to 1933 nm/RIU by using hollow gold nanostructures (denoted as gold nanocages, AuNCs).

Other works are devoted to the use of silver nanoparticles for sensing applications. In this sense, presented in [10] is a comparative study based on the immobilization of silver nanoparticles with different shapes such as triangular and spherical onto U-shaped fiber sensors. The results have indicated that triangle silver nanoparticles have shown the highest sensitivity (1116.8 nm/RIU) in comparison with the spherical silver nanoparticles (342.7 nm/RIU). In other works, it is simulated by using theoretical studies the influence in the resultant refractive index of two input parameters such as the thickness layer and particle size of different types of metal nanoparticles (gold, silver, copper, platinum) [11,12].

The combination of different fiber structures or different coupling effects can also be employed for the detection of refractive index changes. According to this, an optical fiber hetero-core sensor consisting of a piece of single mode fiber longitudinally placed between two multimode fibers is used for refractive index sensing [13]. Finally, other interesting approach based on the electric field coupling effect between both Au film surface plasmon resonance (SPR) and gold nanoparticle localized surface plasmon resonance (LSPR) is presented in [14]. In this work, the SPR-LSPR coupling effect has considerably improved the resultant refractive index sensitivity with a value of 3074 nm/RIU, showing a considerable improvement in the sensitivity in comparison with only Au film SPR optical fiber.

Other interesting research field based on the design of LSPR optical fibers is for the detection and quantization of heavy metals (i.e., lead, cadmium, mercury) because they are considered as harmful to the human body. A clear example is presented in [15] where a monoclonal antibody-functionalized fiber-based biosensor using the LSPR effect has been developed to evaluate the concentration of lead ions (Pb²⁺). As observed in Figure 5, a change of 12.2% in absorbability has been observed for detecting 10 to 100 ppb Pb(II)-EDTA complex, showing a limit of detection of 0.27 ppb. In addition, an important aspect to remark is that the biosensor retains 92.7% of its original activity, giving reproducible results after storage in Trehalose dehydrate solution at 4 °C for 35 days. This same solution is used in [16] in order to retain the corresponding activity for a long period of time, although in this work is presented a fiber-optic biosensor for the detection of cadmium. In this work, the resultant LSPR biosensor has been fabricated by using phytochelatines (PCs) onto AuNPs, and the experimental results have shown a good limit of detection (0.16 ppb) and sensitivity (1.24 ppb⁻¹), respectively.

Other interesting work based on LSPR optical fiber biosensor for the detection of heavy metals is presented in [17]. In this work, AuNPs have been succesfully functionalized with mercapto-undecanoic acid (MUA), showing a sensitivity of 0.28 nm/mM and a limit of detection of 65 ppm for lead ions. In addition, a similar response has been also observed for the detection of cadmium ions. Finally, one aspect to remark is that among all the known heavy metals, mercury ions can easily enter the human body through skin, respiratory or even gastrointestinal tissues, and due to this, their detection is of great interest because an excess can damage brain or nervous system. According to this, an interesting approach is presented in [18] where a U-shaped optical fiber sensor is fabricated for mercury ions detection by using glucose capped silver nanoparticles. In this work, a good limit of detection (2 ppb) has been obtained, although this limit has been improved in [19] by using chitosan capped AuNPs, obtaining 0.1 ppb in tap water and 0.2 ppb in sea fish and vegetable samples with a negligible cross sensitivity towards other metal ions.

More recently, some authors have focused their research on the development of optical fiber sensors involving novel materials that allow plasmonic interactions (beyond Au and Ag nanoparticles [20]). In this sense, the 2-D plasmonic materials are very promising due to their intense light-matter interaction due to the plasmon confinement, leading to very intense plasmonic bands. Nevertheless, currently most of the 2D materials locate intrinsically at terahertz or mid-infrared range, which is a limitation for their practical application nowadays, especially for optical fiber sensing applications. Nevertheless, there are transition metal oxide 2-D crystals whose properties can be adjustable in the VIS-NIR range, and their plasmonic interactions are far more intense than in traditional materials. For example, in [21] the authors report the use of 2-D MoS₂ crystals to modify and enhance the plasmonic behavior of a Ti thin film over a partially uncladded SMF fiber. The authors have proven that the plasmonic bands of the sensor are sensitive to the external refractive index, opening the door to highly sensitive chemical sensors, if such structures are properly combined with further sensitive layers. Another interesting work was published in [22] where it is reported how 2-D WOx flat nanocrystals can show plasmonic bands matched with conventional telecommunications wavelength in the 1550 nm range. In this particular case, the authors report that the integration of such 2-D nanocrystals over a D-shape optical fiber can lead to highly sensitive gas sensors, in this case showing a LOD as low as 8 ppb of NO₂.

Finally, Surface-enhanced Raman Scattering (SERS)-based molecular sensing can be also used as an emerging tool for remote test in trace chemicals or even in the detection of food additives. According to this, a very representative work can be found in [23], where the development of a fiber-optrode is presented by using silver-coated gold nanostars (Au@Ag NS) as highly enhancing SERS active substrate. In this work, a chemical colorant such as Rhodamine B has been chosen as an analytical target due to its illegal use as food additive and rinsing concerns on the food safety [24]. In addition, the optimization of the optrode configuration has been performed by variating the density of the gold nanostars and the resultant roughness of the fiber end-face with the aim to maximize the SERS signal. Finally, the experimental results have demonstrated that the optrode can perfectly detect different types of analytes with a LOD for Rhodamine B between 10⁻⁷ and 10⁻⁸ M, respectively.

Other aspect to remark is that the use of plasmonic fiber-optic biosensors can be considered as a very promising alternative in comparison with traditional methods for biomolecule detection, showing important advances in clinical diagnostics [25,26,27]. This is possible because the metallic nanoparticles can be further functionalized in order to immobilize specific enzymes or antibodies for the detection of specific targets. Figure 6 shows the immobilization of AgNPs with a further functionalization in order to obtain a biosensing approach for monitoring antigen-antibody interactions [26]. The optical setup as well as the morphology and optical properties of the AgNPs are observed in Figure 7, whereas the biosensing results for anti-human IgG are presented in Figure 8.

A very interesting work based on a functionalized Long Period Grating plasmonic fiber sensor for the detection of glyphosate in water is presented in [27]. The sensor operation is based on the reaction between glyphosate molecules and cysteamine-functionalized gold nanoparticles that modifies the effective refractive index of the long-period grating cladding modes, showing a limit of detection about 0.02 µM. Other representative study can be found in [28] where an optical tapered fiber-based biosensor is developed in order to detect uric acid concentration in human serum. In this work, a comparative study in sensitivity by immobilizing gold nanoparticles of two different sizes (10 and 30 nm) is presented. In [29], by using the same tapered optical fiber configuration, an LSPR biosensor is presented for variable monoclonal mouse IgE-antiDNP antibody detection, showing an LSPR transmission intensity change as a function of the analyte concentration with a limit of detection of 4.8 pM. A cholesterol biosensor is presented in [30] by using a single-mode (SMF) and a hollow core fiber (HCF) with the aim to detect cholesterol concentration in the human body. In this work, AuNPs have been functionalized with a specific enzyme such as cholesterol oxidase (ChOx), showing a detection limit of 25.5 nM. The use of aptamers can be another novel alternative route for functionalizing the surface of the nanoparticles. A clear example is observed in [31] where gold nanorods (GNRs) were modified by an aptamer for the aim to detect ochratoxin A (OTA) which is a very harmful mycotoxin produced by several fungi. A spectral red-shift in the concentration range from 10 pM to 100 nM has been obtained with a limit of detection of 12 pM.

An approach based on co-immobilization of both enzyme glutamate dehydrogenase and coenzyme nicotamide adenine nucleotide onto AuNPs is presented in [32] for the detection of a specific amino acid such as the glutamate which plays important roles in the formation of synapses, learning and memory. The experimental results indicate that the resultant biosensor has presented a limit of detection of 0.36 mM with a sensitivity of 0.0048 AU/mM, showing a good selectivity for the analyte of interest. A similar approach can be found in [33] where a highly sensitive biosensor for the detection of taurine is presented by immobilizing taurine dioxygenase enzyme over the AuNPs. In this case, the LSPR biosensor has presented a higher limit of detection of 53 µM with a sensitivity of 0.0190 AU/mM, respectively. In addition, the LSPR biosensor is highly selective to taurine because a minimum change in the absorbance related to the LSPR with other interfering agents (creatinine, lactate or glutamate) has been observed. Presented in [34] is an LSPR-based fiber-optic sensor for the detection of triacylglycerides by immobilizing lipase enzyme on silver nanoparticles (AgNPs), showing a sensitivity of 28.5 nm/mM with a good selectivity, stability and reproducibility in the entire physiological range. Presented in [35] is a U-shaped fiber optic biosensor for the detection of blood glucose, showing the maximum sensitivity for a bending radius around 0.982 mm, respectively. Other interesting work can be found in [36] where an LSPR biosensor has been fabricated for the detection antibody-antigen reaction of interferon-gamma (IFN-γ) by the immobilization of AuNPs at the end-face of an optical fiber, showing a limit of detection of 2 pg/mL. This same biosensor has also been used for the detection of a prostate-specific antigen (PSA), showing a limit of detection for PSA 1 pg/mL below, respectively.

Alternatively there are some works that report optical fiber biosensors based on 2D plasmonic materials such as MoO₃ [37]. Such Molybdenum trioxide nanoflakes can be chemically modified to heavily dope them with free electrons MoO_3−x so their plasmonic properties can be tuned, in this case, placing their resonance band at 735 nm. The authors functionalized a D-shaped optical fiber with these nanoflakes, showing a good affinity and optical sensitivity to negatively charged biomolecules. As a proof of concept, the authors report Bovine Serum Albumin (BSA) detection with a LOD of 1 pg/mL.

Finally, evanescent wave-based excitation of noble metal structures on fiber core surface are also highly suitable for the design and development of SERS-based biosensor applications. A versatile approach toward a shape-controlled noble metal nanostructure-sensitized tapered fiber probe for SERS-based detection is presented in [38]. According to this, charged plasmonic structures with various morphologies (Au nanospheres, Ag nanocubes, Au nanorods and Au@Ag core-shell nanorod) can be perfectly assembled onto tapered silica fiber probes and the in situ Raman measurements indicate a detection concentration of methyl parathion down to 10⁻⁸ M can be performed. Presented in [39] is a comparative study based on plamonically active U-bent plastic optical fibers (POF) obtained by different techniques such as electroless deposition, sputtering and chemisorption of AuNPs. The experimental results indicate that the AuNPs immobilized by chemisorption process has shown a higher enhancement factor (1.24 × 10⁹) in comparison with gold sputtered films (3.29 × 10⁸) or electroless (5.42 × 10⁷), respectively. In a later work [40], it is demonstrated a biosensor application of the POF-based SERS sensor based on the chemisorption of AuNPs by realizing a sandwich immunoassay with 4-mercapto benzoic acid (4-MBA) as Raman label.

A summary of the different optical fiber sensors based on Chemisorpted nanoparticles monolayers is presented in Table 1.

The use of the optical fiber technology for monitoring the Relative Humidity (RH %) has been continuously increasing in industrial and environmental control processes. Rivero et al. have reported the first works based on the immobilization of AgNPs by using the Layer-by-Layer assembly for this purpose [51,52,53]. As the metal nanoparticles are immersed in a polyelectrolyte multilayer (PEM) structure, a change in the optical response is associated with a swelling/deswelling phenomenon derived from this PEM structure, showing a change in the their aggregation state and the refractive index of the surrounding medium. In addition, using AgNPs has shown an additional advantage because the growth of microorganisms or even bacteria can be prevented due to the intrinsic properties of AgNPs when the sensitive regions is exposed in high humidity ambient [54]. Furthermore, the integration of both metal nanoparticles and fiber technology makes it possible to combine the plasmonic interactions associated with the metal structures with other optical resonant phenomena denoted as Lossy-Mode Resonances (LMR). LMRs consist of the optical coupling of certain guided modes to an external optical fiber overlay, when certain optical conditions are fulfilled [55]. This approach can be found in [52] where it is presented for the first time the fabrication and characterization of an optical fiber humidity sensor based on the simultaneous observation of LSPR and LMR absorption bands. As can be appreciated in Figure 11, a considerable difference in sensitivity to RH changes is presented. In this sense, LSPR related to the immobilization of AgNPs has shown a very slight wavelength variation in comparison with the LMRs, which present a strong wavelength response to RH changes. In a later work [53], also evaluated is this high wavelength response associated with the LMR band for high relative humidity performance such as human breathing.

An interesting aspect associated with the LbL assembly is its versatility, making possible the incorporation of different types of metal nanoparticles into the polyelectrolyte multilayer structure. According to this, presented in [56] is a self-assembled monolayer of gold nanospheres coated LSPR fiber sensor for the measurement of the refractive index. An increase in the size and density of the nanoparticles enables a better sensitivity, being around 2016.224 nm/RIU. Another aspect to remark is the possibility of encapsulating nanoparticles of different shapes into the LbL films. Rivero et al. have also incorporated AuNPs with a spherical shape by using the Layer-by-Layer Embedding (LbL-E), and as a result, by controlling the resultant thickness coating, an optical fiber device based on LSPR and LMR has been successfully designed for the detection of refractive index changes [57]. Furthermore, in a later work, other type shapes of metal structures such as gold nanorods (GNRs) have been also deposited onto the optical fiber core by using the same nanodeposition technology (LbL-E). In this work, an optical fiber sensor is presented for the simultaneous detection of refractive index and relative humidity changes [58]. These GNRs present two representative LSPR peaks (corroborated by UV-Vis spectra) which are denoted as LSPR-T (transversal plasmon resonance) and LSPR-L (longitudinal plasmon resonance), showing different sensitivities to refractive index changes (see Figure 12). When the thickness coating of the LbL-E films is increased, it makes possible the appearance of a new LMR band. The experimental results showed an excellent sensitivity of 11.2 nm/%RH for the LMR, confirming the potential of this type of optical fiber sensor based on the combination of LSPRs and LMR bands.

Finally, in other works, the Layer-by-Layer assembly has been used as an effective supporting layer for a further immobilization of the metallic nanoparticles onto the polyelectrolyte structure. In [59], gold nanorod colloids with the aspect ratio of 3 were synthesized and fixed onto a polyelectrolyte structure on the sidewall of an optical fiber. The resultant LSPR sensor shows a sensitivity of 468 nm/RIU, when the sensitive coating was immersed in sucrose solutions with variable refractive indices from 1.33 up to 1.3749, respectively. In addition, a red shift of the LSPR band is observed when the concentration of sucrose solutions is increased, whereas a blue shift is observed when the concentration of sucrose solution is decreased.

As previously commented, the presence of heavy ions such mercury ions (Hg²⁺) in the environment is a real concern because it is considered as a highly toxic element that can cause DNA damage or central nervous system disorders due to its toxicity and carcinogenicity. The immobilization of the metal nanoparticles into LbL thin films deposited onto optical fiber is also used for its detection thanks to the sensing signal derived from LSPR. According to this, presented in [60] is a highly sensitive optical fiber sensor using the LbL nano-assembly technique with AuNPs for mercury (Hg²⁺) detection. The resultant sensor has presented a limit of detection of 0.7 ppb and low cross-sensitivity towards other heavy metal ions (see Figure 13). Other novel work to detect Hg²⁺ ions can be found in [61]. In this work, the optical fiber has been functionalized with gold nanoparticles (AuNPs) by using a flame-brushing technology and a polyelectrolyte structure composed of chitosan (CS) and polyacrylic acid (PAA) bilayers, which facilitates Hg²⁺ ions adsorption on the sensor for a further chemical detection. A summary of the synthetic route for the functionalization of the optical fiber is presented in Figure 14. The experimental results have demonstrated a linear shift with concentrations from 1 to 30 µM (see Figure 15) and a sensitivity of around 0.51 nm/µM, showing a good specificity and longtime stability, respectively.

This LSPR sensing signal can be employed for the detection of volatile organic compounds (VOCs) or chemical substances, which can be dangerous for the human health. In this sense, an interesting approach based on a fiber optic LSPR sensor anchored with metal organic framework film is presented for acetone sensing [62]. In this work, it has been observed a redshift of the resonance wavelength with a total reversibility to acetone which is directly associated with an increase in the local refractive index induced by the acetone adsorption into the sensitive coating.

The analytical detection of hydrogen peroxide (H₂O₂) is a key factor in the biosensing field because it is considered as an important indicator of several diseases such as Parkinson, Alzheimer, asthma or breast cancer, among others. According to this, the use of sensitive coatings onto optical fiber can be used as an efficient technology for its detection. A representative work is presented in [50] where a simple and robust hydrogen peroxide (H₂O₂) optical fiber sensor is proposed based on LSPR sensitivities of AgNPs and AuNPs, respectively. In Figure 16, it can be appreciated the difference in sensitivity of both LSPR bands, where it is practically maintained at the same maximum absorbance for AuNPs, whereas the maximum absorbance for AgNPs is decreased by a high magnitude when the analyte concentration is continuously increased (from 1 to 1000 ppm). The novelty of this work is that the LSPR signal associated with the AuNPs can be used as a stable reference to get a differential measurement estimator that is noticeably more robust than the simple typical intensity-based measurements. The experimental results (see Figure 17) have shown a good selectivity towards this specific analyte in comparison with other interfering agents (glucose, ascorbic acid) or even chemical solutions (ultrapure water, Hank’s balanced salt solution).

Finally, LbL-E of metal nanoparticles technique deposited onto optical fiber can be used for the chemical detection of pH variations. According to this, a representative work based on the immobilization of AuNPs into LbL films can be found in [63] for the detection of pH changes by using both LSPR and LMR phenomena. In concordance with previous works, there is a similarity in the optical behavior of these absorption bands because the dynamic range for LMR band (67.35 nm/pH unit) is greater than LSPR band (0.75 nm/pH unit) in the pH range from 4.0 to 6.0, as shown in Figure 18.

The use of the sensing signal of the LSPR into LbL films can be also extrapolated as a powerful tool in protein sensing applications by using different optical fiber configurations. An interesting approach is found in [64] where a highly sensitive protein sensor based on tapered optical fiber modified with AuNPs is presented. The basis of the sensing methodology is related to changes in the refractive index of the polyelectrolyte structure by the streptavidin (SV) binding to the biotin, showing a limit of detection (LOD) of 271 pM, respectively. However, in [65] it is coated the end-face of optical fiber and the reflection mode has been performed to the detection of the biotin-streptavidin bioconjugate pair, showing a sensitivity of around 800 pg/mm², respectively. Evaluated in [66] is the effective plasmon penetration depth by using AuNPs of variable size with the aim to optimize and improve the sensitivity of the LSPR biosensor. Another representative application of the LSPR is the detection of different types of antigens such as Immunoglobulins (Ig). Presented in [67] are gold nanoparticle multilayers which are highly sensitive and selective to surface modifications, detecting stable binding of antigen (Immunoglobulin G, IgG), whereas other novel works are focused on the immobilization of silica core gold shell nanoparticles (denoted as SiO₂@AuNPs) onto optical fiber long period gratings (LPGs) for the detection of human IgM [68]. In this work, the dynamic binding of IgM on the LPG can be observed at a concentration of 0.3 µM, showing a limit of detection of 0.0218 ng/mm². In addition, this same sensitive coating has been employed in [69] for streptavidin detection, showing a high sensitivity with a limit of detection of 0.86 pg/mm², respectively. The effective plasmon penetration depth of hollow gold nanostructures (HGNS) immobilized onto U-bent optical fiber has been studied in [70] for the detection of E. Coli B40 strain using bacteriophage T4. The results corroborate that the response of the sensor has been better for the HGNS in comparison with spherical gold nanoparticles.

As it was previously commented, the Layer-by-Layer polyelectrolyte multilayer structure can be also employed as an efficient matrix for a further assembly of the metallic nanoparticles [71]. The experimental results indicate that the LSPR sensor shows good refractive index sensitivity and is also used to conduct real-time and label free monitoring of Ribonuclease and Concavidine A (Con A) biomolecular interaction. Other interesting work for conducting real-time and label free monitoring of both IgG/anti-IgG and Con A/RNase B biomolecular interaction is presented in [72]. Finally, in [73] gold nanoparticles with spherical shape and variable size (48 ± 6 nm and 23 ± 2 nm, respectively) are successfully self-assembled onto a previously fabricated trilayer polyelectrolyte structure, as observed in the SEM images in Figure 19. These assembled AuNPs are uniformly dispersed onto the LbL film without showing any aggregates. In addition, the response of LSPR optical fiber sensors to different concentrations of sucrose solutions is presented in Figure 20. In both cases, a red shift of the LSPR band has been observed and the transmission peak intensities have been reduced when the sucrose concentration has been increased, showing a good and linear correlation. This optical fiber LSPR sensor has been also used for the immunoassay of goat anti-rabbit IgG and the lowest detection concentration for this biosensing analyte has been 11.1 ng/mL.

A summary of the different optical fiber sensors based on LbL nanoparticles assembled thin films is presented in Table 2.