The use of nanoparticles (NPs) in scientific applications has attracted the attention of many researchers in the last few years. The use of NPs can help researchers to tune the physical characteristics of the sensing coating (thickness, roughness, specific area, refractive index, etc.) leading to enhanced sensors with response time or sensitivity better than traditional sensing coatings. Additionally, NPs also offer other special properties that depend on their nanometric size, and this is also a source of new sensing applications. This review focuses on the current status of research in the use of NPs within coatings in optical fiber sensing. Most used sensing principles in fiber optics are briefly described and classified into several groups: absorbance-based sensors, interferometric sensors, fluorescence-based sensors, fiber grating sensors, and resonance-based sensors, among others. For each sensor group, specific examples of the utilization of NP-embedded coatings in their sensing structure are reported.
For the last few decades optical fiber sensors have experimented an important growth and relevance in sensing technologies field. Recently, many applications have been developed to monitor or detect a wide range of parameters in different fields such as biomedicine, aeronautics, environmental control, and other industries. This interest of the scientific community in optical fiber sensors is motivated by their already well-known advantages, as immunity to electromagnetic interferences, remote sensing, small dimensions, low weight, biocompatibility, real time monitoring, or multiplexing capabilities [
Currently, optical fiber sensors field has increased in its research lines and possibilities with the use of nanocoating deposition techniques. Nanostructured thin films and nanocoatings have been applied to the diverse optical fiber configurations for the fabrication of new sensors. Thanks to these combinations, many devices have been developed obtaining the detection and monitoring of multiple parameters such as a wide range of gases [
One of the latest steps in the search for improved novel sensors is the inclusion of nanoparticles (NPs) within coatings. In diverse new researches, it has been demonstrated that selected NP-embedded coatings enhance some parameters of previous devices, for example, sensitivity [
In the following sections, most used sensing principles and optical fiber configurations will be described, and then their combination with diverse NP-embedded coatings will also be presented. The optical fiber sensors described in this paper are classified into several groups depending on their detection method. Intensity-based sensors, interferometric sensors, fluorescence-based sensors, fiber grating sensors, and resonance-based sensors are the most typical ones.
Intensity-based optical fiber sensors have been reported since the 70s in literature, and their development has been widely used to these days. Generally, the underlying phenomenon of such sensors is the light transmission-absorption in materials. Absorbance is based on the attenuation of light due to the characteristics of the material that light is guided through. The sensitive materials change the absorbance in presence of a specific parameter or analyte, and therefore a change on the guided light can be observed. The absorption mechanisms are described by the Lambert-Beer Law, where the transmission of the light through an analyte, material, or sensitive region (
The incorporation of sensitive materials to optical fiber sensors can be performed by embedding them into coatings or thin films. The propagation of light through the optical fiber presents two contributions: the guided field in the core and the evanescent field in the medium surrounding this core. This evanescent field is not accessible in unmodified standard cladded fibers, and therefore it is not relevant to sensing. Thereby, the external medium cannot interact with the guided light through the core, nor the evanescent contribution in the cladding. Nevertheless, when the cladding is intentionally replaced by sensitive coatings, there could be a significant interaction between the external medium and the evanescent field to the guided light. The optical properties of the selected coating materials determine the changes in this evanescent-field interaction. In many cases, optical configurations are developed to enlarge this interaction with the evanescent field by removing the cladding, bending, or tapering the fiber, as it is shown in Figure
Schematic of the most used optical configurations used for the development of absorbance-based sensors with NP-embedded coatings: (a) cladding-removed fiber; (b) U-shape or bent fiber; (c) tapered fiber. Evanescent field is also depicted as tails that penetrate and interact within the coating.
Cladding-removed optical fiber (CROF) is one of the simplest structures used in optical fiber sensing (shown in Figure
During the last two decades many CROF based approaches have been developed. Nevertheless, as it was previously commented, the use of NPs in coatings has not been reported until the last few years.
CROF sensors with NP-based coatings have been reported in several works, detecting humidity [
Bent optical fiber sensors can be considered as a particular case of the CROF sensors, when the modified fragment of the fiber is submitted to a significant bending (see Figure
An alternative strategy for exposing the evanescent field to an external sensitive coating is fiber tapering. This technique modifies the optical fiber geometry and its structure (see Figure
Recently, tapered optical fiber sensors with Ag NPs-based coatings have been developed for ammonia sensing [
The combination of diverse special fibers with NPs-embedded coatings is also reported. Examples of this are such as hollow core fibers with Fe3O4 NPs for magnetic field sensing and optical filter purposes [
Optical fiber interferometers have been widely used in the development of optical fiber sensors. They can be mainly classified into four types: Fabry-Perot, Mach-Zehnder, Michelson, and Sagnac [
The fabrication of Fabry-Perot interferometers (FPI) in optical fibers has provided different sensing structures. There are numerous works that use the simplest FP configuration: an air gap between two perpendicularly cleaved optical fibers [
Schematic of the Fabry-Perot interferometer configuration in optical fiber sensing.
This reflective phenomenon has been used for sensing applications. Some of them have performed Fabry-Perot cavities including NPs within thin films. For instance, silica NPs were used in FPI based sensors for humidity [
The multiple configurations provided by the Mach-Zehnder (MZ) interferometers had led to a wide variety of sensing applications. At their beginning, these types of interferometers were composed of two separate light paths or arms: the sensing path and the reference path. The light entered into the device and was split into two beams by a fiber coupler. Then, light passed through both paths reaching to another fiber coupler, where lightbeams were reunited and both contributions create the interference. The traditional MZ structure was scaled down as it was applied to optical fiber devices. In Figure
Schematic of the mainly used MZ configurations in optical fiber sensing: (a) based on Long-Period Gratings (LPGs); (b) based on tapered fibers; and (c) based on a PCF.
Since the introduction of fiber gratings in sensing, many sensors have been performed including them in the MZ configuration [
Regarding the rest of MZ configurations, there are some relevant works for diverse applications. For instance, Li et al. [
Another interesting type of interferometers is that called Michelson interferometers (MI). Their optical structure is quite similar to the MZ devices, but in this case, the light is reflected at the end of each arm by a mirror addition. Also this approach can be developed in a compacted configuration, commonly known as in-line Michelson interferometers. As in the case of MZ interferometers, LPGs have been mainly used in MI configurations. There are recent advances in MZ with NP-embedded coatings for concretes applications. One of the most relevant works is reported by Carrasquilla et al., who design a LPG based MI interferometer [
Sagnac interferometers present an interesting alternative to other sensing structures, due to advantages as easy fabrication and simple set-up and robustness. These types of interferometers consist of an optical fiber loop, along which two beams are propagating in counter directions with different polarization states, providing the desired interference. A more detailed description of those interferometers can be found in the bibliography [
Sagnac interferometers designed with NP-embedded coatings have not been reported. However there are some advances where the sensing fiber has been coated with polymers. Hence, humidity sensors based in chitosan [
The use of fluorescence as a sensing mechanism for optical fiber sensors has been studied for decades because of two main reasons. On one hand fluorescence has been a daily life tool for scientific disciplines such as microbiology, and therefore researchers have an abundant repertoire of different fluorescent labels and dyes and a good knowledge of how to bond them to other target molecules. On the other hand the optical nature of the fluorescent signal is ideal to be collected and transmitted through a medium such as an optical fiber. The wide variety of fluorescent dyes together with the benefits of the optical fiber as transmission medium (low losses, wide broadband, multiplexing, small size, biocompatibility, etc.) has encouraged the research in this field for decades.
Although there are a lot of works in the bibliography that reports fluorescent based optical fiber sensors [
As it was commented in the previous introduction, one of the main advantages of fluorescence-based sensors is that after decades of research in fields such as microbiology there is an enormous available diversity of available fluorophores [
Fluorescent quantum dots (QDs) are nanoparticles of semiconductor material with a diameter of typically around 3–8 nm. Such nanosize of the semiconductor particle induces the phenomenon of the quantum confinement. The excited electron-hole pair behaves as a quasiparticle called exciton and this quasiparticle has some physical dimensions related to its Böhr radius that depend on the specific properties of the semiconductor material. When the exciton size is constrained by potential barriers, the density of energy state distribution (DOS) is significantly altered, changing from a continuous DOS distribution of the bulk materials to a discrete DOS typical of the QDs [
Depending on the size of the QD nanoparticles the fluorescence emission can be tuned from the near infrared to the blue region of the visible spectrum. Although there are different approaches for achieving quantum confinement and consequently the QDs, the wet-synthesis routes of semiconductor nanoparticles are the most used ones. Such wet chemistry approaches are reproducible and cost-effective, and currently there are several synthesis routes available using organic solvents or even water-based approaches. Typically QDs are chalcogenide semiconductors, most of them from group VI: CdTe, CdSe, ZnSe, ZnS, and so forth. One of the most important advantages of nanoparticle QDs is that they can be easily functionalized using well developed surface chemistry, and they can be embedded or bonded to a wide variety of surfaces and matrices [
There are a lot of sensing applications based on QDs luminescence. Their high quantum yield has made possible applications such as single particle tracking using fluorescence microscopy [
Temperature sensor using CdSe QDs embedded into LbL thin films fabricated inside the inner holes of a PCF. Reprinted with permission from [
There is another phenomenon that involves fluorescence that is a direct consequence of the nanostructure of certain particles. This phenomenon is known as metal-enhanced fluorescence (MEF), and it is caused by the alteration of the normal radiative and nonradiative decay rates caused by the close proximity of metal nanoparticles. The MEF phenomenon is caused by the singular concentration of the local electrical field in the surroundings of certain metallic nanoparticles as a consequence of a resonant phenomenon known as localized surface plasmon resonance (LSPR). LSPR is the collective oscillation of the free electrons of metallic nanoparticles due to their resonant coupling with incident light at a specific wavelength. More detailed information about the nature and applications of the LSPR phenomenon can be found in the bibliography [
The electrical field in the medium surrounding the metallic nanoparticles is altered, and as it is shown in Figure
Electric field intensity in the vicinity of two Ag NPs of 25 nm diameter separated 30 nm between centers. Simulated using Greensym. It is possible to see that the region between both particles (pointed to with an arrow) shows a significant increasing in the electrical field intensity.
It has been probed that the distance of the fluorophores to the nanoparticles surface is a critical parameter to achieve MEF. The fluorophore is needed to be close enough to the plasmonic nanostructure, since the field enhancement decays nearly exponentially with distance from the metallic surface. Nevertheless if the fluorophore is too close to the NP (less than 5 nm) its fluorescence would be quenched significantly due to the nonradiative decay through energy and/or charge transfer to the metal. Consequently the distance of the fluorophores should be controlled in a range of 5 to 30 nm in general.
Liu’s group had reported a DNA-detecting platform based on MEF, using Ag NPs, PDDA/PSS LbL films, and conjugated polyelectrolytes [
It has also been demonstrated that sharp shapes and edges of metallic nanoparticles induce more intense electromagnetic field concentrations and consequently higher MEF rates. Consequently nonspherical nanoparticles are frequently used in the development of optical sensors based on MEF. For example, gold nanorods have been successfully used to create glucose sensors [
(a) Schematic configuration of a gold nanorod dual MEF and SERS probe. (b) Fluorescence spectra showing a 2-fold enhancement of the Rose Bengal emission. Reprinted with permission from [
Sensors are devices designed for the quantitative identification of analytes but there are other applications in which the qualitative characterization of the analyte is crucial, such as in molecule identification. In such applications there are several analytic techniques available (High Pressure Liquid Chromatography (HPLC) and other chromatography techniques) that helps to determine the composition of the chemicals present in the sample. Nevertheless there are techniques that provide information about the structure, chemical bonds, or presence of certain functional groups and moieties as far as they are based on the excitation of the natural vibrational frequencies of the molecules. The most used ones are Fourier Transform Infrared (FTIR) and Raman spectroscopy. In fact Raman spectroscopy is especially useful because it makes it possible to distinguish between very similar structures but generally it requires powerful lasers and long acquisition times to get a weak Raman scattering signal.
As it is has been previously commented the electrical field concentrations in the vicinity of metallic nanoparticles by means of LSPR coupling allow the apparition of two different enhancement phenomena, MEF and SERS. Therefore when the LSPR induced electromagnetic field concentration occurs near metallic nanoparticles, the molecules nearby the surface experiment an enhancement in their Raman scattering cross section, making more efficient their excitation. Enhancements up to 8 orders of magnitude in the Raman scattering emission are typically observed from the molecules surrounding the metallic nanoparticles [
The very first approaches used highly rough metallic substrates obtained by several oxidation-reduction cycles of the surface of the metal, but the electrical field concentration spots were randomly distributed throughout the surface and this made difficult the utilization of SERS as a tool for quantitative determination of chemical species.
More sophisticated structures such as the so-called Nanosphere Lithography (NSL) technique or the Metal Film Over Nanosphere (MFON) have been successfully used to fabricate the metallic structures that allow the electromagnetic field concentrations that make the SERS phenomenon possible (Figure
(a) AFM image of a silver coated assembly of polystyrene nanospheres of 540 nm diameter. A continuous metallic film was created over the nanospheres. (b) 3D reconstruction of the AFM micrography where the sharp edges in the metallic film can be observed. It is in those regions where SERS takes place. Reprinted with permission from [
Although most of the applications are focused on planar substrates for Lab-On-a-Chip (LOC) applications [
(a): (A) low- and (B) high-magnification SEM images of a SERS probe made from a tapered optical fiber. It is possible to see the rough profile of the silver nanoparticles synthesized onto the surface of the taper. (b): SERS spectra of 4-ATP (10−7 M) detected by the tapered fiber probes with different cone angles; (A) 3.5, (B) 9.6, (C) 15.8, and (D) 22.6. Reproduced with permission from [
Fiber gratings are optical fibers that present a periodic perturbation of their optical properties, namely, the core refractive index. Since the 80s decade fiber gratings have contributed to the development of many devices for diverse applications in research fields such as communications, instrumentation, and sensing. There are several techniques for fabricating optical fiber gratings based on UV laser [
On one hand, for FBGs, the resonance wavelength obeys the Bragg condition described as [
More details about FBGs can be found in relevant works reported by Hill and Meltz [
On the other hand, for LPGs the resonance condition is given by [
Illustration of a LPG coated with a NP-embedded coating.
Both FBGs and LPGs have been widely used for the fabrication of optical fiber sensor devices and sensor networks. The following sections describe and enumerate briefly several research works based on FBGs and LPGs. Also, several recent applications with the use of NPs within coatings as sensitive regions onto these fibers are presented.
FBG sensors have been widely reported in literature during the last 25 years for the monitoring of numerous physical parameters, including vibration [
An important particular type of FBGs is the tilted FBGs (TFBGs), where their grating has a shift in angle with respect to the fiber axis [
All FBG sensors reported in the two previous paragraphs do not present NPs in their coatings, or even in some cases they do not have any coating as sensitive region. Works with coated FBGs as sensor have also been reported recently with gold nanofilms [
As in FBGs, the inherent LPG structure also permits the development of sensors for temperature, bending, strain, or external RI depending on their fabrication settings [
Cusano et al. studied theoretically and experimentally the effects of the cladding modes along the LPG structure when this was coated with nanoscale overlays [
During the last few years, the inclusion of NPs in coated LPGs has been also reported for different sensing applications. In Table
Summary of optical fiber sensors with NP-embedded coatings based on LPGs. Specific terms of sensitivity parameters, relative humidity (RH), limit of detection (LoD), parts per billion (ppb), refractive index units (RIU), and enhancement with respect to the same device without NPs (ENH).
Target | Nanoparticles | Deposition technique | Sensitivity parameters | Reference |
---|---|---|---|---|
Humidity | SiO2 NPs | LbL | 0.2 nm/RH% | [ |
Ion chloride | Au colloids | LbL | 0.46 nm/RH% | [ |
Ethanol | ZnO nanorods | Aqueous chemical growth | LoD ≈ 0.04% | [ |
Ammonia | SiO2 NPs | LbL | — | [ |
Low-molecular chemicals | TiO2 NPs | LbL | LoD = 140 ppb | [ |
RI | SiO2 NPs | LbL | 10−7 M | [ |
Proteins | SiO2 and Au NPs | LbL | 1927 nm/RIU | [ |
Aromatic carboxylic acids | SiO2 NPs | LbL | 19 nM | [ |
Low-molecular analytes | Au NPs | Sol-gel | 1 nM | [ |
Corrosion | Fe and SiO2 NPs | Dip-coating | ~2- and ~2.5-fold ENH for ATP and QDNA |
[ |
Sucrose, RI | Au NPs | LbL | — | [ |
Copper | Cibacron blue | LbL | 20 nm ENH/RIU | [ |
An interesting work about how to improve the sensitivity in humidity LPG sensors is reported by Viegas et al. [
Resonance wavelength shift dependence with the humidity for LPG with (black spots) and without (white spots) SiO2 NPs intermediate coatings [
Resonance-based sensors are another important group within optical fiber sensing field. Their development has been reported for more than 20 years. When an optical waveguide is coated by a nanostructured coating, the transmission of light along the overall structure can be affected. Depending on the properties of the different materials involved in the system (the waveguide, the coating, and the surrounding medium), three different kinds of electromagnetic resonances can be recognized. To distinguish these types of resonances, the relationship of the permittivity of the coating (
Schematic of a waveguide coated with a nanostructured film and the required conditions to generate SPR and/or LMR. Adapted from [
The first resonant phenomenon happens when the real part of
The second type of resonance occurs when the real part of
Finally, a third case happens when the real part of
According to the optical structure, resonance-based sensors could be englobed as a subgroup of absorbance sensors, grating sensors, or interferometric sensors if their coatings satisfy the concrete resonance conditions. Generally in literature, resonance-based sensors are considered as a group by themselves because of the importance of the resonance phenomena. However, they could also be classified as CROF sensors, U-shape sensors, tapered sensors, LPG sensors, FBG sensors, and so forth, depending on their optical configuration.
As the SPR and LMR based sensors with NP-embedded coatings have being widely reported in the last few years, they will be described in separate sections.
Since the introduction of optical fiber technology in the research of the technique of SPR, fiber-optic SPR sensors have presented a lot of advancements. Jorgenson and Yee published in 1993 [
The unique optical properties of metal NPs have also attracted the sensor community to develop LSPR based sensors [
LMR theory is very recent, and its development in sensing has been reported since 2009 by some authors [
In these few years, LMR based sensors with embedded NPs have been published to measure parameters such as the surrounding RI [
Rivero et al. have recently developed the first sensor where both LSPR and LMR phenomena appear [
UV-Vis absorption of the sensor in function of the number of LbL bilayers: (a) 1–30 bilayers; (b) 35−40 bilayers. Reprinted from [
Dynamic response of the sensor. The wavelength shifts of both LSPR and LMR 1 are monitored simultaneously to RH cycles from 20 to 70% RH at 25°C. Reprinted from [
The same research group also developed another refractometer based on both types of resonances, here using Au NP-embedded coatings [
Finally, a summary with different approaches of fiber-optic sensors based on LSPR and LMR is shown in Table
Summary of optical fiber sensors based on LSPR and LMR phenomena.
Resonance phenomenon | Target | Coating | Deposition technique | Sensitivity parameters | Reference |
---|---|---|---|---|---|
LSPR | HF | Au NPs and silica matrix | Sol-gel | 1% to 5% | [ |
|
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LSPR | Hydrogen peroxide | Ag NPs embedded in polyvinyl(alcohol) | Dipping and sintering | 10−8 M | [ |
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LSPR | Proteins | APTMS, glutaraldehyde/cysteamine + Au NPs (nanocages or nanospheres) | LbL | 11 pM (nanospheres) |
[ |
|
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LSPR | Anti-IgG | Amino silane + Au NPs | Silanization | 0.8 nM | [ |
|
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LSPR | Proteins | Poly(ethyleneimine)/Au NPs + poly(sodium 4-styrenesulfonate) | LbL | — | [ |
|
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LSPR | Blood glucose | Au NPs + glucose oxidase | LbL | Blood min. volume ~ 150 |
[ |
|
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LSPR | Explosive vapours | Au NPs functionalized with 4-mercaptobenzoic acid, l-cysteine, and cysteamine | Silanization with APTES | <100 nM (23 ppb) |
[ |
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LSPR | DNA sequences | Au NPs functionalized with oligonucleotides | LbL | <100 nM | [ |
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LSPR & LMR | Humidity | Poly(allylamine hydrochloride)/poly(acrylic acid) + Ag NPs | LbL | ~1 nm/RH% | [ |
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LSPR & LMR | RI | Poly(allylamine hydrochloride)/poly(acrylic acid) + Au NPs | LbL | 4037 nm/RIU LMR 2 |
[ |
|
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LMR | RI | TiO2 NPs/poly(sodium 4-styrenesulfonate) | LbL | 2872.73 nm/RIU |
[ |
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LMR | Humidity | TiO2 NPs/poly(sodium 4-styrenesulfonate) | LbL | 1.43 nm/RH% LMR 1 |
[ |
|
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LMR | Humidity | Poly(allylamine hydrochloride)/poly(acrylic acid) + Ag NPs | LbL | 0.455 nm/RH% | [ |
|
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LMR | VOCs | Poly(allylamine hydrochloride)/Au-Ag nanocompound + sodium dodecyl sulfate | LbL | 0.131 nm/ppm for methanol | [ |
In this review, a classification of optical fiber sensors based on nanoparticle-embedded coatings is proposed; this list of sensors has been ordered according to their sensing principles, which are briefly described in separated sections. Absorbance, interferometry, fluorescence, gratings, and resonances phenomena were briefly reported. The introduction of new specialty fibers combined to these coatings has plenty of potential applications. Moreover, LSPR and LMR technologies in fiber sensing are experiencing a great degree of development these days. All these advances are likely to drive future trends in the research and development of optical fiber sensors.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported in part by the Spanish Ministry of Economy and Competitiveness CICYT-FEDER TEC2013-43679-R Research Grant and a UPNA predoctoral research grant.