Trends Sci. 2025; 22(10): 10383

Optical fiber sensors apply optical methods to measure various physical and chemical properties in modern technological developments. Recent studies have examined the potential of optical fibers as sensors, as documented by many publications reporting their applications in chemical sensing [1-4], gas detection [5,6], humidity monitoring [7,8], biosensing [9,10], salinity measurement [11,12], and many other fields. The use of nanoplasmonic materials play critical role in the advancements of optical fiber sensing technology. Recent advances have also showed how Au/Ag NCs improved optical fiber sensing sensitivity compared to conventional sensors [11]. The integration of plasmonic Au/Ag nanostructures enabled detection limits down to picomolar concentrations for biosensing applications [13]. Moreover, optical fiber sensors have a relatively long service life, typically at least ten years, as demonstrated in previous study [14]. Based on the Verified Market Research report, the global optical fiber sensor market was valued at $ 4.506 billion in 2024 and is forecasted to grow $8.56 billion by 2031 having a compound annual growth rate (CAGR) of 10.80%. Nanocomposite-enhanced optical fiber sensors represent the fastest-growing segment, accounting for approximately 25% of this market [8]. Cost-benefit analyses show that these advanced sensors reduce operational costs by 30% - 40% in industrial monitoring applications due to their durability and precision [9]. Furthermore, the healthcare biosensing market utilizing Au/Ag NCs sensors is expected to reach $1.2 billion by 2028, driven by point-of-care diagnostics demand [10].

The optical fiber sensing technology development is in line with the goals of the United Nations, including building resilient infrastructure (SDG 9), where optical fiber sensors are applicable in the industrial fields [15]. In addition, the potential use of optical fiber sensors in the fields of healthcare, such as in biomedical diagnostics and patient monitoring [16], as well as in in environmental monitoring, for example water quality analysis and air pollution detection [17]. These applications are well aligned with SDG 3 (good health and well-being), SDG 6 (clean water and sanitation), SDG 13 (climate action), and SDG 14 (life below water). Thus, optical fiber sensing technology contributes significantly to multiple SDGs. In the context of green and sustainable development, optical fiber sensors provide to real-time [18], energy-efficient [19], and low-carbon monitoring systems [20]. Their compact size, resistance to electromagnetic interference, and long-term stability make them ideal for integration into green technologies, such as renewable energy systems [21], smart cities [22], and ecological conservation efforts [23]. For example, they can be deployed in smart irrigation systems to optimize water usage in agriculture [24] or in offshore platforms to detect marine pollutants [25], thereby supporting environmental sustainability. Through these diverse applications, optical fiber sensing technology not only enhances technical performance across sectors but also fosters a greener, more sustainable future in alignment with global environmental and development goals.

The growing demand for high transmission speed and data accuracy has driven researchers to develop optical fiber sensors with improved capabilities. One such development techniques involves modifying the optical fiber structure. For instance, Ding et al. [7] utilized single mode fiber (SMF) forming a balloon-like interferometer for humidity sensor. Tang et al. [2] employed no-core fiber (NCF) connected to an SMF to detect structural steel corrosion. Besides structural modification, coating optical fiber with plasmonic nanomaterials is also effective to enhance the sensing operation. For this purpose, Kong et al. [26] reported that the plasmonic nanomaterials significantly increased the tumor cell sensing sensitivity up to 5 times higher than that of uncoated fiber. Various nanostructures have since been applied in optical fiber-based sensors, including gold nanoparticles (AuNPs) for refractive index sensing [27], Au@Ag nanohybrid structures for biosensors [9], Ag@Fe₃O₄ NCs for H₂S detection [28], Ag/Cu NCs for effective petroleum sensing [29], and Au-assisted SiO₂-TiO₂ NCs for pH sensor [30]. These examples highlight that Au and Ag nanostructures are among the most applied materials in optical fiber sensors. This results from the fact that AuNPs and AgNPs exhibit unique LSPR peaks [31], and their combination can expand the wavelength range especially when applied in optical fiber sensors [32]. Therefore, the development of Au/Ag NCs offers a promising strategy for enhanced optical fiber sensing performance. Numerous studies have exclusively investigated Au/Ag NCs into various optical fiber sensing applications, such as for quick sensing of C-reactive protein [9], vitamin A [33], cyanide in water [34], refractive index [35], and many more.

Plasmonic materials, notably Au and Ag, are capable of generating optical phenomena - including localized and surface plasmon resonances (LSPR and SPR) - which contribute to enhancing the sensitivity of optical fiber sensors. LSPR occurs in plasmonic nanoparticles having size less than 100 nm, where incident light excites collective electron oscillations that couple with confined light waves around the nanostructures [36]. This interaction significantly enhances the local electromagnetic field near the nanoparticle surface, which in turn increases the sensitivity to the surrounding refractive index alteration [37]. While LSPR is typically associated with plasmonic nanoparticles, SPR occurs in continuous plasmonic films. SPR takes place when the momentum of the incident electromagnetic light wave matches that of the surface plasmon, leading to resonance [38].

Many other reviews have been published especially in optical fiber sensors. For instance, Mahmud et al. [39] reviewed SPR-activated optical fiber for biosensing applications. Zhang et al. [40] discussed various approaches for heavy metal ion concentration measurements using optical fiber sensors, including plasmonic-based approach. However, their review provided only the general overview of the role plasmonic materials. In other words, there remains a lack of exclusive reviews specifically focusing on the role of Au/Ag NCs in enhancing the performance of optical fiber sensors. To address this gap, this review offers a comprehensive overview of the impact of Au/Ag NCs for improved optical fiber sensors. It also discusses various optical fiber structures and nanocomposite coating techniques. Furthermore, the review explores potential applications of Au/Ag NCs-coated optical fiber sensors, outlines current limitations and challenges, and suggests directions for future research.

Gold/silver nanocomposites

Au/Ag NCs are composite materials composed of AuNPs and AgNPs with sizes less than 100 nm. Au/Ag NCs are classified as noble metal nanocomposites that have superior biocompatibility [41], good photoluminescence [42,43], excellent photostability, and versatile for functionalization [44]. Therefore, Au/Ag NCs can be applied in various fields, including in optical fiber sensing technology. Au/Ag NCs have been widely studied for various sensor applications, including biosensors for quick observation of C-reactive protein [9], vitamin A [33] and B1 [45], cyanide in water [34], and refractive index [35,46,47].

Figure 1 (a) XRD profiles, (b) HR-TEM photograph, and (c) UV-Vis spectra of Ag/Au NCs produced by Nd:YAG laser ablation method [48].

There are 2 main approaches to synthesize Au/Ag NCs: The top-down and bottom-up routes. In the top-down approach, Au/Ag NCs are produced by breaking down bulk materials into nanoscale structures. An example of a top-down approach is pulsed laser ablation [48]. In contrast, the bottom-up approach involves the formation of Au/Ag NCs through chemical reactions from molecular precursors. Ultrasonic-assisted growth and chemical reduction, as demonstrated by Phuong et al. [9]; Bi et al. [49], are representative bottom-up routes. Bi et al. [48] described that Au/Ag NCs synthesized in polyvinyl alcohol (PVA) by Nd:YAG laser ablation method had X-ray diffraction (XRD) peaks with decreasing intensity along with increasing AgNPs content and varying ablation times (Figure 1(a)). Morphological analysis via high resolution-transmission electron microscopy (HR-TEM) revealed that the shape of Au/Ag NCs were spherical with some agglomeration and exhibited core-shell structures with size ranges between 7 and 29 nm (Figure 1(b)) [48]. Furthermore, an increase in the absorption peak was observed by ultraviolet-visible (UV-Vis) spectroscopy with longer ablation times (Figure 1(c)), indicating enhanced complexation between the metal nanoparticles and the polymer matrix [48].

Figure 2 Plasmonic characteristics of (a) AuNPs [50] and (b) AgNPs [51].

The synthesis duration of Au/Ag NCs significantly influences their LSPR characteristics. One method of making Au/Ag NCs is by mixing Au and Ag nanocolloidal solutions. In this sense, temporal changes in the LSPR peak could be observed during the individual synthesis processes of AuNPs and AgNPs. A previous study [50] reported that AuNPs synthesized by the laser ablation method, with ablation times ranging from 10 to 30 min, exhibited a red shift of about 7 nm in the LSPR peak (Figure 2(a)). Similarly, another study investigating the effect of synthesis time on AgNPs reported time-dependent shifts in the LSPR peak position (Figure 2(b)) [51]. Furthermore, the HR-TEM image of Au@Ag nanostructures is shown in Figure 3(a), showing core-shell structure. The LSPR profiles of AuNPs and Au@Ag NCs are given in Figure 3(b), indicating the coexistence of the plasmonic peak of AuNPs and AgNPs.

Figure 3 (a) Morphological and (b) optical properties of Au@Ag NPs synthesized using bottom-up approach [9].

Table 1 presents a comparative summary of Au/Ag NCs synthesized using different methods, focusing on their particle sizes and corresponding LSPR peak positions (λ). From Table 1, it is obvious that different synthesis techniques yield in different particle sizes and LSPR characteristics. Nd:YAG laser ablation produced PVA-stabilized Au/Ag NCs with a size range of 7 - 29 nm. This relatively wide size distribution suggested some degree of agglomeration, which might arise from the laser ablation process in a PVA matrix. The LSPR peaks for AgNPs and AuNPs were occurred at 412 and 531 nm, respectively [48]. Furthermore, the ultrasonic-assisted growth and chemical reduction method yielded Au@Ag core - shell nanostructures with averaging 20 nm in diameter, as depicted also in Figure 3(a) [9]. As shown in Table 1 and Figure 3(b), this method produced multiple LSPR peaks corresponding to AgNPs (407 nm), AuNPs (529 nm), and Au@Ag core-shell structures (491 nm). In another study, simultaneous reduction of gold and silver ions using produced Au/Ag NCs with a larger particle size of approximately 55 nm [49]. The LSPR peaks were located at 417 nm for AgNPs and 525 nm for AuNPs. In addition, the wet chemical synthesis Au@Ag NCs with a particle size of 24 nm and LSPR peaks at 498 nm (AuNPs) and 404 nm (AgNPs) [52].

Table 1 Characteristics of Au/Ag NCs produced by different synthesis methods.

Optical fiber structures for sensing application

Optical fiber sensors represent a significant innovation in measurement technology based on optical methods. Optical fibers offer multiple benefits when used as sensors, such as resistance to corrosion, immunity to electromagnetic interference, combined sensing and signal transmission capabilities, and suitability for operation in harsh environments [54]. Various types of optical fibers have been studied and modified for sensor applications across diverse fields, as summarized in Table 2. These modifications aim to optimize sensor performance for specific detection requirements.

Table 2 Optical fiber structures and their sensing applications.

Optical fiber sensors can be designed with various structural configurations to suit specific application requirements. Each structure typically consists of a combination of several types of optical fibers. For example, in humidity sensors, the optical fiber structure generally uses the Single Mode Fiber (SMF) type, while the sensing region - the part that directly interacts with the target - may utilize different type of optical fiber tailored for enhanced sensitivity. Various structures were investigated to produce the best sensor performance. SMFs were widely used by previous researchers because they can measure small changes in the phase of light propagating through the sensing region [62]. In addition, SMFs can transmit signals at higher speeds compared to that of Multimode Fibers (MMFs), as it avoids modal dispersion and modal noise [63]. Dispersion occurs in MMFs because different propagation modes travel at varying speeds, leading to modal dispersion [64]. Modal noise is also associated with MMF and does not occur in SMF, where the optical power is distributed unevenly among a number of modes resulting in speckle patterns on the detector. SMF has a very small core diameter (5 - 10 μm) which serves to limit the transmission to a single mode, but incorporating light into SMF also requires tighter tolerances compared to optical fibers that have a larger core diameter [63].

Based on Table 2, MMFs are also used in several optical fiber sensor applications, such as humidity sensors [55] and ethanol gas detection sensors [61]. MMFs have a larger core compared to the wavelength of light, so that the optical phenomena that occur can be described with a geometric optics approach [65]. In this approach, light is treated as a collection of rays traveling in straight lines within the fiber. When these rays encounter interfaces separating media of differing refractive indices, they undergo reflection and/or refraction, as illustrated in Figure 4.

.

Figure 4 Illustration of light refraction and reflection.

The reflection and refraction of light that occurs in different media can be explained by the law of geometric optics which states that

where denotes the reflection angle, represents the incidence angle, indicates the refraction angle, is the refractive index of medium 1, and that of medium 2.

Based on Eq. (2), if , then radians, and this occurs when . As increases beyond a certain critical value, refraction no longer occurs, causing the incoming light to be entirely reflected within the medium - a process termed total internal reflection [65]. In optical fibers, this effect takes place at the interface between the cladding and the core.

As depicted in Figure 5, total internal reflection can be described using Snell’s Law. When the incident angle surpasses the critical threshold, the light does not transmit into the cladding but remains fully reflected inside the core. It has a consequence that

where is the refractive index of air and is the incidence angle. Due to total internal reflection, light is retained within the core and can effectively transmit along the entire optical fiber [65]. A larger core diameter affects both the attenuation and dispersion of light propagating through the fiber. In MMFs, multiple modes propagate simultaneously, each experiencing different attenuation and dispersion characteristics due to their distinct reflection patterns within the core [66]. The difference in attenuation and dispersion of the beam light passing through the MMF will impact the signal captured by the detector.

Figure 5 Illustration of total internal reflection in optical fiber.

Some previous studies have used MMFs as sensor structures that interact with the target, for example in the applications of humidity [55] and ethanol gas [61] sensors. When light beams from different types of optical fibers enter the MMF core, some rays may also enter the cladding. The light that couples into the MMF core can excite higher-order core modes. This change is used as a basis to determine the sensitivity of optical fiber sensors in detecting an analyte. The interference intensity in optical fibers I, can be represented as [55]:

where i and j respectively represents the core and cladding. , λ, and L indicate the core-cladding refractive index difference, the light wavelength, and the length of the optical fiber, respectively.

No-Core Fiber (NCF) is another type of optical fiber that is also widely applied as sensors. It is an optical fiber without a defined core that enables the guided light to interact directly with the external medium or analyte. When a light beam from another type of optical fiber enters the NCF, multiple high-order modes are excited. these modes interfere along the propagation path, forming self-image effect [60,67]. A portion of the propagated light in the NCF interacts with the surrounding analyte, while the remainder continues to transmit through the fiber. This results in different optical paths for the 2 light beams. The difference in optical path length introduces a phase difference between them, which can be quantified using Eq. (5) [33].

Accordingly, the interference intensity in the NCF is formulated in Eq. (4), while the self-imaging effect, driven by the superposition of excited high-order modes, is represented by Razali et al. [67]:

where is the electric field profile originating from the previous optical fiber (e.g. SMF). While is the spatial field and is the excitation coefficient. The subscript m indicates the of the order of field eigenmode. is defined by Eq. (8) [67].

where corresponds to the longitudinal propagation constant associated with the m^th-order mode.

In addition, there are also several other types that are used and modified to obtain maximum sensing performance, such as Fiber Bragg Grating (FBG) and Tilted Fiber Bragg Grating (TFBG). TFBG is characterized by a periodic grating structure inscribed at a specific tilt angle relative to the optical fiber axis [68]. This angular tilt provides distinctive functions such as coupling, dispersion, and light deflection [69]. Furthermore, the strong core-cladding mode coupling one of the prominent characteristics of TFBG [70]. The high sensitivity and good sensing ability [71], for example in chemical and biological sensing, make TFBGs widely used as sensors in the chemical and biological fields. TFBG is a modified form of FBG that is commonly used for strain sensing. FBGs possess excellent multiplexing capabilities and immunity to electromagnetic interference, which has led to their widespread use in measuring temperature and strain across various engineering applications [72]. FBG is characterized by modulation of periodic refractive index modulation in the core [73]. When broadband light propagates in the fiber grating, the Bragg wavelength is reflected and defined by Eq. (10) [74].

where , and represent respectively the Bragg wavelength, the core’s refractive index, and the periodic spacing of the grating.

When an FBG is exposed to external influences such as temperature, pressure, strain, or vibration, shifts in the Bragg wavelength occur, accompanied by variations in the core’s refractive index and periodic spacing [72]. The Bragg wavelength shift can be represented by Eq. (11) [75].

where ζ, and ΔT indicate the electrooptic constant, strain, thermal expansion coefficient, thermo-optic coefficient, and temperature change, respectively.

Au/Ag NCs coating on optical fibers

Plasmonic materials have been extensively employed in optical fiber sensors to improve sensing sensitivity. Numerous studies have documented the application of Au/Ag NCs coatings on different optical fiber sensor types. For example, they were coated on optical fiber for C-reactive protein rapid detection in biosensing field [9]. Huang et al. [13] and Phuong et al. [34] also coated Au@Ag-Pt NCs and Au@Ag on optical fibers as copper (II) ion and cyanide detections. Various methods have been used to coat Au/Ag NCs on optical fiber sensors. These methods include self-assembly monolayer (SAM) [36], electroless plating [76], and photochemical deposition [77].

SAM involves the use of coupling agents with specific terminal groups to modify the optical fiber surface, enabling the attachment of nanoparticles, as illustrated in Figure 6 [36]. Some materials coated on optical fibers using this approach include Au nanospheres [36], AuNPs capped with palladium [78], and chitosan-coated AuNPs [79]. Before the coating takes place, activation of the optical fiber surface is carried out. Hidayat et al. [36] in their research cleaned the optical fiber to be activated using methanol by soaking. After cleaning, the optical fiber was soaked using NaOH for OH groups activation on the fiber surface. In addition to using NaOH, other studies have also used a piranha solution consisting of hydrogen peroxide and sulfuric acid for the activation of hydroxyl groups [78]. Prior to the subsequent step, the optical fiber was initially dried and subsequently immersed in a 1% MPTMS solution prepared in ethanol [36] with the aim of activating thiol (-SH) functionalization or nanoparticle coupling agents. In activating coupling agents, some researchers also employed ethanol containing 1% APTES [79] or APMES (20 μL) contained in isopropyl alcohol (4 mL) [78]. After the coupling agent is activated, the optical fiber is immersed into the colloidal nanoparticles to be immobilized for several days. The duration of immersion starting from the surface activation process of the optical fiber until the activation of the coupling agent can be varied to produce maximum final results.

Figure 6 Illustration of nanoparticles coating using SAM method.

Electroless plating is another coating method commonly used optical fiber surface functionalization purposes. This method is widely adopted due to its ease of implementation, low-cost equipment requirements, and flexibility in application [80]. This nanoparticle coating technique operates without electrical power, relying instead on the chemical reduction of metal ions using reducing agents, for example, hydrazine, formaldehyde, hydroxylamine, and glucose [76]. This method can be applied to conductive or non-conductive objects (such as optical fibers). It is able to produce discontinuous layers that enables evanescent waves to interact with the outer layer and the analyte [76]. This method relies on electron kinetics during the coating process, in which the slow transfer of electron transfer from the reducing agent to metal ions limits the reduction of other ions. The use of a catalyst on the surface of substrates like optical fibers during the coating process serves to increase the reduction rate of metal ions, thereby enabling the formation and growth of the coating [76]. For instance, in the study reported by Loyez et al. [80], reducing agents to convert Au³⁺ to Au⁰ through electron transfer process producing Au thin film deposition. Illustration of the electroless coating method applied in a sandwich assay for thrombin detection is shown in Figure 7.

Figure 7 Illustration of the electroless plating method on optical fiber and its application in a sandwich assay for thrombin detection [81].

Furthermore, photochemical deposition can be applied as a method of nanoparticles coating or thin films formation on optical fibers. This technique controls the photosensitive properties and SPR response ability of certain elements such as Au and Ag to synthesize nanoparticles with customized size and shape [82]. Photochemical deposition enables the nanoparticles coating on the optical fiber tip and core [77]. It has several advantages, such as simple, low cost, controllable during coating, selective deposition site, and fast deposition process [83]. However, it is limited by the range of materials that can be effectively deposited. Materials commonly used with this method include AgNPs [82] and AuNPs [77].

As illustrated in Figure 8, a semiconductor laser is used during the deposition process, and the form of nanoparticles to be deposited should be in colloidal form. However, samples in colloidal form have some drawbacks, especially when applied to Surface-Enhanced Raman Spectroscopy (SERS) substrates, namely poor stability in suspension [82]. An investigation was conducted using a semiconductor laser emitting at 638 nm with a fiber tip output power of 35 mW to achieve the AgNPs deposition on the ends of the fibers [83]. Another study utilized a semiconductor laser with a working wavelength of 635 nm and power of 15 mW [77]. In this technique, the optical fiber intended for deposition must first be cleaved to produce a flat end face [83]. Subsequently, the fiber tip is immersed in the reaction solution containing Na₃Ct and gNO₃ [83]. Following this, laser light is transmitted into the reaction solution via the fiber core. Thus the laser irradiation will trigger the gradual formation and deposition of nanoparticles [83]. The deposition of nanoparticles can be controlled based on the laser irradiation time

Figure 8 Illustration of nanoparticles coating by photochemical deposition method.

Functionalization of optical fiber sensors is not limited to surface coating; other methods involve incorporating additional materials during fiber fabrication, which can influence sensor performance. This method is 3D Printing [84,85]. Based on previous research [65], optical fibers could be printed using a 3D printer and begin by feeding the prepared resin into the resin barrel. The printing process commences with UV light emitted from the printer’s lower section, which cures the first layer of resin onto the printing platform. After each layer has dried, the printing plate moves up slightly, according to the thickness of the layer. The process of multimaterial printing on optical fibers entails first depositing one material, then temporarily halting the operation to change the barrel to a different material and subsequently completing the printing of the remaining sections [65]. The illustration of this method is shown in Figure 9.

Figure 9 Illustration of 3D printing in optical fiber sensors [65].

Various approaches of functionalization of plasmonic materials on optical fibers have been discussed. These affect the thickness of the functionalized plasmonic material layer. The performance of optical fiber sensors is influenced by the thickness of the plasmonic material layer. The SAM technique offers potential for precisely controlling this layer’s thickness on optical fibers. However, if the coating is carried out when manufacturing optical fibers, then the 3D printing method is a suitable for the coating purposes.

Potential applications

Owing to their distinct advantages, optical fiber sensors have become the focus of extensive research and are now applied in various domains, such as healthcare, industrial regimes, and environmental monitoring. Efforts to improve the performance of these sensors often involve the functionalization of optical fibers with plasmonic materials. Au and Ag, in particular, are widely utilized plasmonic materials, and many investigations have explored the integration of Au/Ag NCs into optical fiber sensors. The potential applications for such sensors are summarized in Table 3.

Table 3 Applications of optical fiber sensor functionalized with Au/Ag-based structures.

The combination of Au and Ag can elevate the optical fiber sensing performances. The functionalization of these materials trigger plasmonic effects. For example, previous research [46] applied a gold coating followed by silver, which effectively activated the SPR phenomenon on the optical fiber. Confinement loss occurs and plays a crucial role in the capability of the optical fiber sensor. This loss increases due to the SPR effect, enhancing the sensor’s ability to detect analytes, including those at very low concentrations with high accuracy [46]. The study also suggested that the dimension of Au and Ag layers also influences the confinement loss, where the thicker the Au and Ag layers, the greater the confinement loss [46]. Furthermore, the sensing sensitivity is also influenced by the gap distance between the nanoparticles coated on the optical fiber. As in a previous report [13], when Au@AgPt NSs were immersed for too long on the optical fiber, the coated nanostructures became thicker and formed agglomerations which result in changes in the transmission spectrum and thus affect the sensitivity.

Applications of optical fiber sensors in the health sector such as in detecting vitamins A and B1 [33,45]. Au@Ag nanostructures incorporated into SiO₂-TiO₂-ZrO₂ ternary matrix could selectively detected vitamin A at concentrations between 10 and 1,000 μM [33]. Another study showed that an optical fiber sensor sequentially coated with AuNPs followed by an AgNPs layer was capable of sensing vitamin B1 within the detection range of 2 - 10 μM [45]. In addition, several previous researchers also stated that optical fiber sensors with Au-Ag coating were able to detect levels of cortisol, glucose (0 - 0.1 mM), and cholesterol (0 - 15 nM) [90,91]. SPR-based optical fiber using Au-Ag@Au NPs film was reported as effective device for the observing antibiotic residues in milk, meat, and aquatic products [92]. During the COVID-19 pandemic, which was announced by the World Health Organization (WHO) in March 2020 as the global pandemic and led to over 7 million deaths globally, researchers further explored the potential of this technology. Optical fiber sensors coated with Au/Ag nanoparticles film to detect SARS-CoV-2 virus [93]. Thus, the advance of optical fiber sensors has great potential in improving the quality of technology in the health sector.

The utilization of optical fiber sensors in industry such as in detecting copper ion (II) and cyanide has also been successfully carried out [13,34]. Cyanide is harmful to the body and is usually used in various industries such as Au and Ag mining. Au@Ag nanostructures coated optical fiber sensor could detect cyanide at the lowest concentration of 8×10¹¹ M [13]. PEI-Au@AgPt nanostructures functionalized on optical fibers could measure Cu²⁺ ions at the lowest concentration 10⁻¹⁶ mol/L [34]. Optical fiber-based temperature sensors can be applied in the industrial world, where temperature is one of the important parameters in an industry [86]. In food industry sector, Ag-coated Au nanostars were coated on MMF for the SERS detection purpose [97]. In addition, temperature sensors can also be applied in the aerospace field, where aircraft engine temperatures need to be monitored to prevent aircraft accidents. Based on previous research, optical fiber-based temperature sensors were able to detect temperatures with a range of 0 - 80 °C [32]. Furthermore, plastic optical fiber decorated with Au-Ag nanostructures for LSPR activation could be used for optofluidic nanosensor [94]. Therefore, optical fiber sensors have the potential to improve monitoring or sensing technology in the industrial world.

Measuring refractive indices is the fundamental application of Au/Ag NCs optical fibers, and therefore it has been widely investigated by many scientists [32,35,46,86-88,95,96]. The optical fiber sensor demonstrated a high sensitivity up to 3,309 nm/RIU for refractive index measurements [46].It relies on changes in the local refractive index near the optical fiber surface, which alter the optical properties of the nanocomposites and shift the resonance wavelength or transmitted light intensity. Such sensitivity makes these optical fibers valuable for further applications, for example in chemical detection [88], plasmoni sensing [96], and environmental monitoring [32,87]. Some environmental applications of optical fiber sensors included detection of pH [87], salinity [11], and temperature [32,89]. Recent advancements have led to the development of dual Au/Ag NCs-coated optical fiber sensors for simultaneously measuring multiple parameters. For instance, these sensors could detect both refractive index and pH changes [87], or concurrently monitor refractive index and temperature variations [32,89]. It implies that Au/Ag NCs-coated optical fiber sensors exhibit excellent versatility and practical utility in complex sensing environments.

Limitations and challenges

The advantages and potential applications of optical fiber sensors have attracted the attention of many experts. Nonetheless, they continue to face several limitations. Applications of optical fiber sensors involve various fiber structures tailored to specific uses. This indicates that the optical phenomena vary between different fiber structures. For example, the optical phenomenon that occurs in optical fibers with FBG structures is core mode coupling [98], while in NCF it is multi-modal interference (MMI) [55]. Thus, a more in-depth study is needed regarding the optical phenomena in optical fibers before being applied to certain optical fiber sensors. In addition, the small size and fragile nature of optical fibers [99], require modifications to enhance their mechanical strength for practical applications.

Plasmonic materials also play a critical role in elevating the performance of optical fiber sensors [97]. Materials with good optical properties are able to activate optical phenomena, such as SPR [32] and LSPR [34] in optical fiber sensors. Functionalization of plasmonic materials, especially Au/Ag NCs, on optical fibers has been widely used to trigger the SPR effect. However, in this field, LSPR using Au/Ag NCs is not as widely explored as SPR. Furthermore, the functionalization method used still mainly relied on SAM. Despite the promising optical responses of Au/Ag NCs, challenges remain in achieving uniform coatings with maintained stability [36].

Future research

Previous studies suggested that Au/Ag NCs coating could enhance the optical fiber sensing performance. Different synthesis methods have been carried out to obtain Au/Ag NCs with excellent characteristics. In addition to the nanomaterials, the structure of the optical fiber sensor itself significantly contributes to better performance. As a result, different fiber sensor configurations have been investigated to identify the most effective designs. Functionalization of Au/Ag NCs onto optical fiber sensors can be achieved through several techniques, with SAM being one of the most commonly employed methods due to their simplicity and effectiveness.

While optical fiber sensors have been increasingly applied in various fields, the number of applications involving sensors functionalized specifically with Au/Ag NCs remains relatively limited. Therefore, further studies are needed to develop optical fiber sensors functionalized with Au/Ag NCs across a broader range of applications, especially for simultaneous sensing purposes. Continued development of optical fiber structures is also essential to further maximize their potential. Additionally, alternative functionalization techniques beyond SAM are needed to improve adhesion, reproducibility, and long-term sensor performance. These actions will improve the potential of plasmonic nanocomposites in next-generation optical fiber sensors.

Conclusions

Au/Ag NCs have demonstrated the potential to improve the performance of optical fiber sensors. The characteristics of the synthesized Au/Ag NCs, which are influenced by the chosen synthesis method, play a crucial role in determining their effectiveness for coating in optical fiber sensors. The synthesis of Au/Ag NCs can be done by top-down or bottom-up approaches. Numerous optical fiber sensor designs have been engineered and optimized for specific applications, where both the structural configuration and nanomaterial coatings significantly influence sensing performances. To functionalize Au/Ag NCs onto optical fiber surfaces, several techniques have been employed, including self-assembly monolayer, electroless plating, and photochemical deposition. Different coating techniques provide specific advantages, including simplicity of the process, affordability, and the ability to precisely control coating parameters. Although optical fiber sensors functionalized with Au/Ag NCs have been developed in various fields, including healthcare, industry, and the environment, their implementation remains limited. Therefore, further research is crucial to broaden the scope of applications and to continue advancing the design and functionality of these promising sensing platforms.

Acknowledgements

This work is supported by the Matching Fund Research Grant, jointly funded by Universitas Negeri Malang and Universiti Teknologi Malaysia, awarded to N.H. and H.B. for the fiscal year 2025 under contract number 24.2.77/UN32.14.1/LT/2025.

Declaration of Generative AI in Scientific Writing

Generative artificial intelligence was not used for content creation or data analysis. The authors take full responsibility for the content and conclusions presented in this work.

CRediT Author Statement

Siti Azimatul Luthfiyyah: Methodology, Investigation, Writing – Original Draft, Writing – Review & Editing.

Sunaryono: Supervision, Validation, Writing – Review & Editing.

Hazri Bakhtiar: Resources, Funding Acquisition, Project Administration.

Arif Hidayat: Data Curation, Formal Analysis, Visualization.

Nurul Hidayat: Conceptualization, Supervision, Writing – Review & Editing, Funding Acquisition.

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