2.1.

Optical Absorbance-Based Sensor

Absorption spectroscopy optical fiber sensing is a technique used to gather information about a target substance by measuring the degree to which the substance absorbs light at a specific wavelength. This sensing principle is commonly used in chemical, biological, and environmental fields to detect and analyze the presence of chemicals or other specific targets in a sample.^5,43,44 As light of a specific wavelength travels through a sample, molecules or atoms in the sample may absorb the light energy. The absorbed photons excite the electrons of the molecules or atoms, causing them to jump to a higher energy level. The number of photons absorbed is proportional to the concentration of the target substance. The extent to which a sample absorbs light of a specific wavelength can be calculated by measuring the intensity of light coming back from the sample and the initial intensity of light emitted by the light source. This degree of absorption is usually represented by a plot of spectral intensity versus wavelength, known as an absorption spectrum. Based on the absorption spectrum, the properties of the target substance, such as concentration, composition, and other physical properties, can be analyzed. By comparing it with a pre-established standard spectrum, the target substance can be analyzed qualitatively. The fundamental principle of optical fiber absorption spectroscopy is illustrated in Fig. 2(a).⁴⁵ Pd nanoparticles (Pd NPs) were coated on tapered optical fibers, and when exposed to ${\mathrm{H}}_2$ gas, ${\mathrm{H}}_2$ molecules migrate toward the Pd-coated surface, where they interact with Pd atoms. The potential energy of these gas molecules reaches a minimum at approximately one molecular radius, causing the adsorption of ${\mathrm{H}}_2$ gas in the conical region of the fiber. Subsequently, the adsorbed gas molecules undergo dissociation into hydrogen atoms, which then diffused within the metal structure, and culminating in the observed hydrogen absorption by the Pd structure.

Fig. 2

Optical fiber-sensing principle: (a) optical absorbance-based sensor,⁴⁵ (b) SPR sensor,³¹ (c) Raman sensor,⁴⁶ (d) fluorescence sensor,⁴⁷ and (e) optical fiber interferometric sensor.⁴⁸

2.2.

Surface Plasmon Resonance Sensor

SPR optical fiber sensing is a highly sensitive and real-time biochemical sensing technique. The fundamental principle of SPR optical fiber sensing is based on the coupling phenomenon between light at the metal–medium interface and the freely moving electrons (plasma) on the metal surface.^49,50 Adjusting the angle of light incidence enables observation of changes in the reflected light’s intensity. When the angle of incidence aligns with the plasma resonance angle of the metal surface, a pronounced alteration in the reflected light occurs, and this angle is known as the resonance angle. Upon the specific binding of a target molecule (e.g., protein, DNA, or chemical molecule) within the sample to a biomolecule (e.g., an antibody, oligonucleotide, or affinity ligand) immobilized on the metal film, the refractive index (RI) of the sample surface alters.^51–53 This, in turn, affects the conditions governing light propagation across the metal–medium interface, leading to a shift in the resonance angle. As shown in Fig. 2(b),³¹ the decomposition of the cylindrical surface in the fiber offers an explanation for SPR phenomenon at each point, outlining a three-layer structure comprising the substrate layer, the intrinsic layer, and the dielectric layer. The substrate may consist of commercial fiber types [such as multimode fiber (MMF), photonic crystal fiber (PCF), and hollow fiber], custom fiber (such as suspended core fiber), fiber Bragg grating (FBG), etc. The intrinsic layer is composed of single or bimetallic films (such as Au, Ag, Cu, and Al) with high reflectivity and low absorptivity.^54,55 The medium layer refers to the analyte layer. It leverages the interaction of light with surface plasma at the metal–medium interface to monitor various processes on the sample surface, including molecular interactions, adsorption, and desorption. This technology finds extensive applications in biosensing, medical diagnostics, and food testing.

2.3.

Raman Sensor

Raman spectroscopy, derived from molecular vibrations, offers the benefits of providing rich molecular information and being impervious to water environment, earning it the moniker “fingerprint spectrum.”^56,57 A laser light source is guided to the sample through the fiber, capturing the scattered light signal and yielding the Raman scattering spectrum, replete with valuable chemical insights encompassing the sample’s chemical composition, molecular structure and functional group. Comparing these spectra to reference standards stored in the database enables the effective identification and quantification of chemical substances within the target sample.

Furthermore, the intensity of the Raman scattering spectrum correlates directly with the concentration of the target molecule in the sample, facilitating precise quantitative analysis of the target compound, thereby enabling the accurate analysis and detection of samples. Optical fiber Raman scattering sensors have extensive applications in chemical analysis, biomedicine, environmental monitoring, and other domains.

Combined with the powerful enhancement effect of nanostructured metals, it becomes feasible to create a surface-enhanced Raman scattering (SERS) sensor that is both highly specific and sensitive, allowing for the non-contact, real-time collection of chemical data in samples.^58–60 As shown in Fig. 2(c),⁴⁶ the dual-mode fiber sensor was crafted through a two steps: involving tapering and half-coating. In this design, only half of the fiber tip’s surface is coated with Ag film, enabling the enhancement and detection of Raman signal on the coated half surface. SERS predominantly operates as a near-field phenomenon, finely attuned to factors, such as intermolecular distances, hot spot geometry, and the efficacy of nanostructure coupling.⁶¹ Consequently, enhancements in signal intensity can vary significantly across multiple orders of magnitude.

2.4.

Fluorescence Sensor

Optical fiber fluorescence sensing is an advanced technology that harnesses the fluorescence phenomenon for sensing purposes. It involves introducing a fluorescent probe or substance into an optical fiber to detect the presence of a target molecule and gather valuable information such as its concentration and pH value.^62,63 This versatile sensing technology finds extensive applications in biomedicine, environmental monitoring, food safety, and other fields. Typically, an OFS comprises an optical fiber, where one end is connected to a light source to excite the fluorescent substance, and the other end is linked to a fluorescence detector to measure the intensity of the fluorescence signal. Tariq et al.⁶² innovated an optode for pH measurement in alkaline environments, utilizing a pH-sensitive molecular fluorescent probe. This fluorescent molecular probe, Naphth-AlkyneOMe, is encapsulated within a cross-linked polyvinyl alcohol-glutaraldehyde (GA) matrix, yielding thin films characterized by well-defined physicochemical and photophysical properties. These sensing films, boasting an average thickness of $150\mu \mathrm{m}$, exhibit responsive and reversible behavior in the face of pH fluctuations. The optode’s applicability to authentic cementitious materials has been empirically demonstrated through pH measurements conducted during the initial hydration stage of low-pH cement paste. These measurements yield precise results with an accuracy of $\pm 0.1\mathrm{pH}$ units and a rapid response time of $\sim 100\mathrm{s}$. Liu et al.⁴⁷ designed a smartphone-based optical fiber fluorescence sensor (SOFFS), as shown in Fig. 2(d). This system comprises a semiconductor laser for excitation of fluorescence signals, a fiber probe modified with quantum dots for ${\mathrm{Hg}}^{2+}$ sensing, a smartphone equipped with a filter for collecting fluorescence signals, and a fiber coupler to connect the fiber probe, laser, and smartphone. Due to the evanescent wave on the optimized combination of a tapered fiber probe’s surface, SOFFS achieves an ultralow detection limit of 1 nM and a broad detection range spanning from 1 to 1000 nM. SOFFS requires only 3 min for a single ${\mathrm{Hg}}^{2+}$ detection procedure, encompassing fiber probe replacement, recording fluorescence images, and conducting data analysis.

2.5.

Interferometric Sensor

Optical fiber interferometric sensors are a cutting-edge sensing technology based on the principle of light interference. They offer numerous advantages, including high sensitivity, real-time, and remote transmission capabilities and find extensive applications in various fields, such as industry, aerospace, medical, and environmental monitoring.^64,65 The sensing fiber interacts with the physical quantity to be measured, and the reference fiber remains unaffected by the said physical quantity. When light passes through the sensing region of the optical fiber, the light waves in the sensing fiber combine with those in the reference fiber. If the sensing fiber experiences a change in its optical range due to a physical quantity, such as altering the fiber’s length or RI, it leads to a modification in the phase difference between the light waves in the sensing fiber and the reference fiber. Subsequently, the combined beam of the sensing fiber and the reference fiber is directed into an interferometer, which precisely measures the phase difference between them. This alteration in phase difference is directly related to fluctuations in the optical path length within the sensing fiber, and this, in turn, corresponds to the measured physical quantity, such as pressure, temperature, or displacement. By analyzing the interference signal’s variation, the value of the physical quantity can be accurately calculated. Optical demodulation is commonly employed to extract information from the interference signal and convert it into the corresponding physical quantity value.

As shown in Fig. 2(e), Yan et al.⁴⁸ applied a multilayer film comprising epoxy resin (ER), multilayer chitosan (CS), and polyacrylic acid (PAA) onto a ceramic tube. This process was undertaken to fabricate an optical fiber Fabry–Perot (F–P) sensor designed for the detection of diverse metal ions. The conventional optical fiber interferometric sensor is constructed by inserting a single-mode fiber (SMF) into a ceramic tube, which is equipped with ER on the end face to reflect light, forming the F–P interferometric cavity. Additionally, a multilayer of CS and PAA is applied to the ER’s surface, serving as the sensitive film. This configuration yielded sensitivities of $9.95\times {10}^{-4}\mathrm{nm}{\mathrm{ppb}}^{-1}$ for ${\mathrm{Ni}}^{2+}$, $2.31\times {10}^{-4}\mathrm{nm}{\mathrm{ppb}}^{-1}$ for ${\mathrm{Zn}}^{2+}$, and $4\times {10}^{-4}\mathrm{nm}{\mathrm{ppb}}^{-1}$ for ${\mathrm{Na}}^{+}$. Interferometric sensors are further categorized based on their principle of operation into Mach–Zehnder interferometers (MZI),^66,67 Fabry–Perot interferometers (FPI),^68,69 Michelson interferometers (MI),^70,71 and Sagnac interferometers,^72,73 each with specific applications and advantages.

3.1.

Heavy Metal Ion Detection

3.1.1.

Lead

Lead (Pb) contamination poses one of the most prevalent and widespread water quality issues. Various sources contribute to lead contamination, including industrial wastewater, lead pipes, pesticides, and contaminated soil. Accumulation of lead in water bodies adversely affects aquatic organisms, disrupting the delicate balance of the entire ecosystem.⁷⁵ The consequences of lead contamination can be severe, leading to the mortality of aquatic organisms, reproductive problems, and significant ecological impacts in water bodies. Moreover, exposure to elevated lead levels raises significant concerns, particularly in the case of children, as it may potentially lead to neurological and intellectual development challenges.⁷⁶ In adults, heightened lead exposure can give rise to health problems, including hypertension, renal impairment, and anemia.

To address this problem, international organizations, such as the World Health Organization (WHO) and the U.S. Environmental Protection Agency have established standards for lead levels in water bodies. WHO’s recommendation for drinking water sets a limit of $0.01\mathrm{mg}/\mathrm{L}$ of lead, aiming to safeguard the health of children and pregnant women.^76,77 Ghosh et al. developed a hybrid fiber grating sensor combining high-order long-period grating (LPG) and FBG for the simultaneous detection of ${\mathrm{Pb}}^{2+}$ ions and water temperature.⁷⁸ Figure 3(a) illustrates the schematic diagram of a hybrid fiber grating sensor system. The sensor’s working principle relies on the strong light–matter interactions between the abrupt field of the high-order cladding modes and the change in the RI of the surroundings caused by the selective adsorption of heavy metal ions. The researchers created a label-free, highly selective ${\mathrm{Pb}}^{2+}$ ion sensor based on LPG by modifying its surface with cross-linked CS and nitrogen-doped graphene oxide-based nanocomposites (CCS-NGO) and PAA polymers using a layer-by-layer electrostatic adsorption method [see Fig. 3(b)]. This modification resulted in a lower detection limit of 0.5 nM in the ${\mathrm{Pb}}^{2+}$ concentration range of 0 to 1000 nM [see Fig. 3(c)]. CS is a linear cationic aminopolysaccharide hydrophilic biosorbent with a high content of active amino ($-{\mathrm{NH}}_2$) and hydroxyl ($-\mathrm{OH}$) groups. The crosslinked CS formed a three-dimensional network using an optimized amount of GA, enhancing its stability without compromising its inherent metal adsorption capacity. The interaction between the nitrogen atoms of the amino group and the valence ${\mathrm{Pb}}^{2+}$ ions led to the formation of ${\mathrm{NH}}_2{\mathrm{Pb}}^{2+}$ complexes. Additionally, water uptake in the CCS-NGO/PAA sensing layer may cause protonation of the amino group and the formation of $-{\mathrm{NH}}_3$, facilitating the ion exchange and adsorption of ${\mathrm{Pb}}^{2+}$ ions on the nitrogen atoms of $-{\mathrm{NH}}_3$. Yap et al.⁷⁹ developed a cost-effective and portable ${\mathrm{Pb}}^{2+}$ sensor by utilizing reflected optical power measurements from two FBGs. The surface of the tapered optical fiber-sensing region was coated with a layer of L-glutathione (GSH), which induced a small change in the effective RI when ${\mathrm{Pb}}^{2+}$ bound to the GSH-modified surface.

Fig. 3

(a) Schematic diagram of a high-order LPG and an FBG hybrid fiber grating sensor system for simultaneous detection of (i) ${\mathrm{Pb}}^{2+}$ ions and water temperature and (ii) operating schematic diagram of LPG. (b) LPG surface modification materials and their self-assembly characteristics. (c) Spectrogram used for the online monitoring of ${\mathrm{Pb}}^{2+}$ ion concentration (0 to 1000 nM).⁷⁸ (d) Schematic diagram of (i) portable optical fiber ${\mathrm{Pb}}^{2+}$ sensing mechanism and (ii) microfiber-sensing platform based on FBG demodulation system. (e) Selectivity.⁷⁹ (f) Schematic diagram of fiber structure and principle for the detection of ${\mathrm{Pb}}^{2+}$ and ${\mathrm{Cu}}^{2+}$. (g) Optical fiber SPR absorption spectrum for the detection of ${\mathrm{Pb}}^{2+}$ and ${\mathrm{Cu}}^{2+}$.⁸⁰

Figure 3(d) illustrates (i) the portable optical fiber ${\mathrm{Pb}}^{2+}$ sensing mechanism and (ii) microfiber-sensing platform based on the FBG demodulation system. GSH acts as a potent metal chelator, containing glutamate groups of amine and carboxylic acid donors, peptide bonds, and multiple active binding sites in the thiol group, enabling specific recognition of ${\mathrm{Pb}}^{2+}$. The thiol group in the cysteine residue ($\mathrm{pK}4=9.65$) exhibits a high affinity at pH 6.5 to 8.3, resulting in the sensor’s highest sensitivity at pH 6 to 7. The cysteine residues of GSH can form metal complexes, leading to stronger metal bonds, particularly through thiolate ligands with high affinity for soft metal ions, thereby providing relatively high binding efficiency for ${\mathrm{Pb}}^{2+}$ and excellent selectivity [see Fig. 3(e)]. During actual water sample testing, the GSH-modified microfiber sensor exhibited a slight response to blank samples due to interference from background ions, but this false positive result remained below the ${\mathrm{Pb}}^{2+}$ safety limit. A two-channel optical fiber SPR platform based on the modification with heavy metal ion-imprinted nanoparticles was presented by Shrivastav and Gupta.⁸⁰ Figure 3(f) shows the optical fiber structure and principle for the detection of ${\mathrm{Pb}}^{2+}$ and ${\mathrm{Cu}}^{2+}$. Channel I and channel II are equipped with thin films of copper and silver on the fiber core surfaces, respectively. Subsequently, ${\mathrm{Pb}}^{2+}$ ion-traced nanoparticles and ${\mathrm{Cu}}^{2+}$ ion-imprinted nanoparticles are applied to the surfaces of the respective channels. Channel I is dedicated to detecting ${\mathrm{Pb}}^{2+}$ ions, whereas channel II is employed for detecting ${\mathrm{Cu}}^{2+}$ ions. Figure 3(g) shows the optical fiber SPR absorption spectrum for the detection of ${\mathrm{Pb}}^{2+}$ and ${\mathrm{Cu}}^{2+}$. The sensitivities of channel I and channel II are $8.19\times {10}^4\mathrm{nm}/(\mu \mathrm{g}/\mathrm{L})$ and $4.07\times {10}^5\mathrm{nm}/(\mathrm{mg}/\mathrm{L})$, respectively. The sensor exhibits remarkable detection capabilities, with the ability to detect ${\mathrm{Pb}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ ion concentrations as low as $4.06\times {10}^{-12}\mathrm{g}/\mathrm{L}$ and $8.18\times {10}^{-10}\mathrm{g}/\mathrm{L}$, respectively. This probe offers simplicity, cost-effectiveness, and selectivity, making it suitable for on-line monitoring of heavy metal ions in water quality. Suhailin et al. introduced a D-type optical fiber SPR sensor for detecting the low concentrations of ${\mathrm{Pb}}^{2+}$ in aqueous solutions.⁸¹ The sensor’s configuration involved sputtering an Au film on the D-type fiber surface as a plasma excitation layer, followed by modification with a layer of $\gamma {\mathrm{Fe}}_2{\mathrm{O}}_3/\text{reduced}$ graphene oxide (rGO) nanocomposites to serve as the sensing layer. The sensor exhibited a ${\mathrm{Pb}}^{2+}$ sensitivity of $1.2\mathrm{nm}/\mu \mathrm{g}/\mathrm{L}$, and the lower limit of detection was 0.001 ppm. Nazari et al.⁸² developed an efficient sulfonated UiO-66(Zr) optical fiber for the rapid ${\mathrm{Pb}}^{2+}$ detection. UiO-66(Zr) metal-organic frameworks (MOFs) functionalized with ${\mathrm{SO}}_3\mathrm{H}$ groups $[{\mathrm{SO}}_3\mathrm{H}\text{-}\mathrm{UiO}\text{-}66(\mathrm{Zr})]$ was modified on the end face of the optical fiber to detect lead cations in water at 25.2, 43.5, and 64.0 ppm levels. The functionalized MOF was coated on the end face of an SMF, serving as an in-fiber FPI to detect changes in the MOF’s optical properties caused by ${\mathrm{Pb}}^{2+}$ adsorption, manifested as wavelength shifts in the interference spectrum. The optical fiber and ${\mathrm{SO}}_3\mathrm{H}\text{-}\mathrm{UiO}\text{-}66(\mathrm{Zr})$ material exhibit excellent stability in the pH range of 2 to 9, rendering them suitable for long-term ${\mathrm{Pb}}^{2+}$ monitoring in aqueous environments.

3.1.2.

Mercury

Mercury (Hg), in the forms of methylmercury and inorganic mercury, is a toxic heavy metal. It can contaminate water through various channels, such as industrial wastewater and coal combustion, eventually entering the human body through the food chain. Prolonged ingestion of high concentration of mercury can lead to detrimental effects on the nervous, immune, and circulatory systems, and in severe cases, it may impair brain development and intelligence.⁸³ Accurate detection of ${\mathrm{Hg}}^{2+}$ concentration in water is crucial for monitoring and controlling mercury pollution in the environment, thus safeguarding natural ecosystems and biodiversity.⁸⁴ The WHO recommends that the ${\mathrm{Hg}}^{2+}$ concentration in drinking water should not exceed $0.001\mathrm{mg}/\mathrm{L}$.⁸⁵

Commonly employed optical fiber-sensing methods for ${\mathrm{Hg}}^{2+}$ in water include SPR sensing and fluorescence optical fiber sensing.^86,87 Lu et al. presented a dual-mode detection technique combining plasmon resonance spectroscopy and electrochemistry (EC-SPR) for real-time quantitative detection of ${\mathrm{Hg}}^{2+}$ concentrations with remarkable selectivity and sensitivity.⁸⁸ Figure 4(a) shows the schematic diagram of the optical fiber-based EC-SPR sensor structure and system. To achieve this, they first deposited a 3-nm chromium film as a bonding layer onto the polished optical fiber’s surface. Subsequently, a 45-nm thick gold film was sputtered on top, followed by the modification of 1,6-hexanedithiol (HDT) to form a monolayer of dithiols on the gold surface, providing additional free thiol groups for sensing. The gold-coated fiber served both as a working electrode for EC sensors and as an extremely sensitive SPR sensor. To enhance ${\mathrm{Hg}}^{2+}$ detection sensitivity, they employed cyclic voltammetry (CV) to activate the HDT-modified gold film electrode. The interaction of ${\mathrm{Hg}}^{2+}$ ions with the free thiol groups on the HDT monolayer led to a significant shift in the SPR wavelength, which was further accelerated and amplified by the CV treatment. The researchers found that multiple CV scans on the HDT monolayer resulted in an increased acceleration of the SPR wavelength shift, ultimately improving the lower limit of ${\mathrm{Hg}}^{2+}$ concentration monitoring from 0.47 to $0.22\mu \mathrm{M}$ [see Figs. 4(b) and 4(c)]. Additionally, the CV scans promoted the redox reaction of ${\mathrm{Hg}}^{2+}$ ions, facilitating their detachment from the HDT monolayers. This redox process restored the active sites on the sensor, allowing for effective regeneration. This solution offers great potential for monitoring heavy metal ion concentrations in various applications, including water quality assessment, agricultural irrigation, and drinking water safety.

Fig. 4

(a) The schematic diagram of optical fiber-based EC-SPR sensor for ${\mathrm{Hg}}^{2+}$ detection. (b)–(c) The comparison of SPR responses with and without CV treatments;⁸⁸ (d) SOFFS hardware design; and (e) applications of SOFFS in the fresh and sea water samples.⁴⁷

Yuan et al.⁸⁹ developed an OFS for ${\mathrm{Hg}}^{2+}$ detection based on the SPR effect. 4-mercaptopyridine (4-MPY)-functionalized gold nanoparticles (Au NPs/4-MPY) were used as a signal-amplifying tag. 4-MPY was modified on the surface of gold-plated fibers through the formation of an Au-S bond. In this configuration, the pyridine nitrogen becomes exposed to the solution, facilitating its coordination with ${\mathrm{Hg}}^{2+}$. Initially, ${\mathrm{Hg}}^{2+}$ interacted with 4-MPY assembled on the Au membrane, followed by the capture of Au NPs/4-MPY by ${\mathrm{Hg}}^{2+}$, resulting in the formation of a sandwich structure comprised of (Au film/4-MPY)-${\mathrm{Hg}}^{2+}$-(Au NPs/4-MPY).

The coupling effect between the Au film and the Au NPs produces a strong localized SPR. The sensor was able to detect the ${\mathrm{Hg}}^{2+}$ concentration in the range of 8 to 100 nM with a detection limit of 8 nM. The practicality of the sensor for ${\mathrm{Hg}}^{2+}$ detection in tap water samples was also verified. Liu et al. designed an SOFFS for long-range, real-time online quantitative determination of ${\mathrm{Hg}}^{2+}$ concentration using fluorescence burst method.⁴⁷ The handheld sensor incorporates a built-in power supply, enabling rapid ${\mathrm{Hg}}^{2+}$ concentration detection in just 3 min, which includes the replacement of the optical fiber probe, fluorescence image recording, and data analysis. The sensor’s operation is straightforward and efficient. It consists of a semiconductor laser for exciting the fluorescence signal, a quantum dot-modified optical fiber probe for ${\mathrm{Hg}}^{2+}$ sensing, and a smartphone equipped with a filter for acquiring the fluorescence signal, as shown in Fig. 4(d). Upon verifying SOFFS’s performance, it was employed for ${\mathrm{Hg}}^{2+}$ detection in both freshwater and seawater environments, using water samples with a pH of $\sim 7$, Fig. 4(e) shows the applications of SOFFS in the fresh and sea water samples. Notably, SOFFS boasts an ultralow detection limit of 1 nM and an extensive detection range spanning from 1 to 1000 nM for ${\mathrm{Hg}}^{2+}$ concentration.

3.1.3.

Cadmium

Cadmium (Cd), primarily originating from battery manufacturing, mining activities, and industrial wastewater, is a profoundly toxic metal. Once it enters water, cadmium can contaminate water sources through bioaccumulation and sedimentation, posing a threat to ecological equilibrium. Long-term exposure to cadmium can lead to health problems, such as kidney damage, osteoporosis, and cancer.⁹⁰ In accordance with WHO standards, the cadmium concentration in drinking water should not exceed $0.01\mathrm{mg}/\mathrm{L}$. Cai et al. developed an exceptionally sensitive plasma fiber probe by utilizing a gold-plated tilted fiber Bragg grating (TFBG).⁹¹ Acinetobacter spp. bacteria were modified on the surface of the gold film, allowing for the precise determination of cadmium ion (${\mathrm{Cd}}^{2+}$) concentration in a solution. Figures 5(a)–5(c) show the ${\mathrm{Cd}}^{2+}$ detection principle and performance based on a gold-plated TFBGs. The detection limit for ${\mathrm{Cd}}^{2+}$ was impressively close to 1 ppb, and the detection process was achieved within a rapid 10-min timeframe. Li et al.⁹² developed a high-precision ${\mathrm{Cd}}^{2+}$ sensor based on a twisted helical fiber (THF). The sensor incorporates a propylene thiourea membrane modification on the surface of the twisted fiber, resulting in the formation of a cross-linked “-S-Cd-S-” structure with ${\mathrm{Cd}}^{2+}$. The twisted fiber structure and sensing mechanism are depicted in Figs. 5(d) and 5(e), wherein the twisting effect allows light in the core to leak into the cladding. Consequently, part of the light propagates forward along the core, while another part leaks into the cladding, reflects back to the core, and the two optical paths exist simultaneously. This coupling and interference occur within an SMF, leading to multiple resonance depressions, as illustrated in Fig. 5(f). The prepared band film THF demonstrates high sensitivity, achieving $0.07252\mathrm{nm}/({10}^{-11}\mathrm{mol}/\mathrm{L})$ in wavelength modulation and $0.08975\mathrm{dB}/({10}^{-11}\mathrm{mol}/\mathrm{L})$ in amplitude modulation. Furthermore, dual-wavelength modulation can be achieved through dual-mode calibration, which significantly enhances the accuracy of ${\mathrm{Cd}}^{2+}$ concentration monitoring.

Fig. 5

(a) SPR excitation with TFBGs; (b) time response plot of heavy metal ion detection; (c) selectivity at the same concentration of 10 ppb;⁹¹ (d) the sensing structure and (e) mechanism of THF-based sensor; and (f) transmission spectra in ${\mathrm{Cd}}^{2+}$ concentration detection.⁹²

3.2.

Microbial Surveillance

Microorganisms in water comprise bacteria, algae, fungi, and viruses. Among these, bacteria are prevalent in various water bodies. Certain bacteria play crucial roles, such as decomposing organic matter and participating in nitrogen cycling, while pathogenic bacteria, such as E. coli and Salmonella, can cause water contamination and waterborne diseases.^93,94 Escherichia coli (E. coli), a group of intestinal bacteria, is frequently employed as an indicator microorganism for assessing water body contamination levels.⁹⁴ The detection of E. coli can reveal the existence of fecal contamination, consequently determining whether a water is contaminated with pathogenic microorganisms.⁹⁵ Kaushik et al.⁹⁶ designed an E. coli sensor based on the optical fiber SPR effect. Molybdenum disulfide (${\mathrm{MoS}}_2$) nanosheets were linked to the gold-coated optical fiber via Au-S bonds, and E. coli monoclonal antibodies were attached to the ${\mathrm{MoS}}_2$-functionalized sensing surface through hydrophobic interactions, enabling precise, specific, and rapid ($\sim 15\min$) label-free detection of E. coli. Figure 6(a) depicts the diagram of the optical fiber SPR immunoassay system designed for detecting E. coli, whereas Fig. 6(b) illustrates the E. coli sensing performance both with and without ${\mathrm{MoS}}_2$ sensitizing layer. Nevertheless, the practical utility of this system is hampered by the decline in sensor performance stemming from the instability of E. coli antibodies. Kaushik et al.⁹⁸ developed an immunosensor for label-free detection of Escherichia coli based on cascaded chirped LPGs. It works in a manner similar to a MZI, where the resonance wavelength is shifted by changing the RI of the intergrating space region without affecting the actual grating region. The limiting bandwidth of the interference fringes improves the resolution of the resonance wavelength analysis of transmission spectra. The proposed immunosensor is capable of selectively detecting E. coli in real samples including lake water. Halkare et al.⁹⁹ developed an E. coli immunosensor based on wild-type phage T4 as a biorecognition element. Bacteria (analytes) were first immobilized on the sensing surface, followed by phage exposure for specific bacteria detection. The key advantage of this approach is that phages can freely diffuse and orient themselves, efficiently binding to the bacterial surface immobilized on the sensing platform. This sensing strategy finds extensive applications in the specific detection of bacterial species in environmental samples.

Fig. 6

(a) Diagram of optical fiber SPR E. coli immunodetection system; (b) E. coli sensing performance with/without ${\mathrm{MoS}}_2$ sensitizing layer;⁹⁶ (c) fabrication of the fluorescence microarray biosensing platform (FMB) system; (d) the forward and back photo of the FMB system; and (e) the fluorescence intensities of four channels.⁹⁷

The detection of microcystins in water is of paramount importance. Microcystins are highly toxic compounds produced by certain cyanobacteria, such as cyanobacteria or microcystis. Elevated levels of microcystins in water can trigger a cyanobacterial bloom, commonly known as an algal bloom outbreak, resulting in a significant decline in water quality. Prolonged exposure to high microcystins concentrations in water may lead to poisoning or even the death of fish and other aquatic organisms. Moreover, human exposure to microcystin-contaminated water can cause gastrointestinal problems, skin irritation, and neurotoxic effects. Long et al.⁹⁷ developed a portable automated fluorescence microarray biosensing (FMB) platform for microcystin-LR (MC-LR) sensing. Figure 6(c) depicts the fabrication of the FMB platform system. Through the control of optical fiber switches, only one excitation light and a photodiode detector can achieve four-channel parallel fluorescence analysis, making it suitable for real-time parallel monitoring of various analytes. The front and back photo of the FMB system is shown in Fig. 6(d). The optical structure of the FMB significantly enhances light transmission and fluorescence collection efficiency, resulting in improved detection sensitivity. Figure 6(e) depicts the fluorescence intensities of four channels in the FMB. Moreover, fewer optical separators and the elimination of strict optical alignment contribute to the system’s compactness and portability. As a multiplexed, portable, rapid, and quantitative detection platform, the FMB holds promise for early warning of contaminants in pollution incidents and water quality management. Yang et al.¹⁰⁰ developed a portable optical fiber chemiluminescence biosensor (FOCB) system. They utilized an optical fiber bioprobe, modified with hapten-carrier protein conjugates as the biorecognition element. The detection of MC-LR was achieved through an indirect competitive sandwich chemiluminescence immunoassay, demonstrating high sensitivity for MC-LR. The detection limit of MC-LR was $0.03\mathrm{g}/\mathrm{L}$ under the optimal conditions.

Optical fiber-sensing techniques in water quality environments mainly include SPR, fluorescence, and interferometric techniques. These sensing technologies offer several advantages, such as high sensitivity, real-time monitoring, non-invasiveness, and remote control. These advantages play a crucial role in effectively monitoring the concentration and contamination of heavy metal ions and microorganisms in water. Table 1 illustrates the comparative performance of OFSs for heavy metals in water quality. Table 2 illustrates the comparative performance of OFSs for microorganisms in water quality.

Table 1

Overview of optical fiber sensors for heavy metals in water quality.

Table 2

Overview of optical fiber sensors for microorganisms in water quality.

4.1.

Nitrogen Oxides

Nitrogen oxides (${\mathrm{NO}}_x$) are widely recognized as one of the primary air pollutants, and frequent exposure to these highly reactive gases can lead to various health problems.^108,109 Nitric oxide, a colorless and odorless gas, is insoluble in water. Its production can be categorized into two types: enzymatic nitric oxide and non-enzymatic nitric oxide. Due to its association with free radicals, nitric oxide exhibits high reactivity and paramagnetic properties. When reacting with oxygen, it transforms into the corrosive gas nitrogen dioxide (${\mathrm{NO}}_2$).

${\mathrm{NO}}_2$ appears as a red-brown gas at room temperature and can readily dissolve in water, resulting in the formation of nitrous acid and nitric acid. ${\mathrm{NO}}_2$ has a strong irritant effect on human eyes and respiratory organs, leading to symptoms such as cough, yellow phlegm, breathing difficulties, and, in severe cases, even pulmonary edema, ultimately leading to death. When the air’s nitrogen dioxide concentration reaches 0.006%, individuals may experience sore throat, cough, and lung pain in a short period. At 0.01% concentration, strong coughing, vomiting, and nerve paralysis may occur rapidly. A concentration of 0.025% of nitrogen dioxide can prove fatal quickly. In addition to natural sources, ${\mathrm{NO}}_2$ mainly originates from fuel combustion, urban vehicle exhaust, industrial production processes, and lightning discharges. It is a significant contributor to acid rain and plays a major role in the formation of photochemical smog. Detecting, eliminating, and preventing air pollution are crucial for maintaining air quality standards. Given the adverse health effects of nitrogen dioxides, continuous monitoring, and quantitative detection of these gases at trace levels are essential to control exposure to these pollutants effectively.^22,110,111 Reliable sensors for measuring nitrogen oxides concentrations are of utmost importance in this regard.

Zou and Wang¹¹² proposed an optimized long path differential optical absorption spectrometry (LPDOAS) system with enhanced UV light intensity, enabling the simultaneous quantification of ${\mathrm{SO}}_2$ and ${\mathrm{NO}}_2$. In contrast to conventional configurations employing telescope mirrors, this system adopted a Y-type optical fiber structure. The maximum calibration errors for ${\mathrm{SO}}_2$ and ${\mathrm{NO}}_2$ were 4.19% and 5.22%, respectively, with the combined error for both gases was falling well below 10%. The real-time measurement of ${\mathrm{SO}}_2$ and ${\mathrm{NO}}_2$ demonstrated strong alignment with the data recorded at monitoring stations, both in order of magnitude and variation trends. Laboratory calibration and field measurements confirmed the effectiveness of online air pollution monitoring based on optical fiber LPDOAS. Peng et al.¹¹³ used spectrum superposition theory to decompose the NO differential optical density from the gas mixture, enhancing the system’s selectivity by overcoming cross-sensitivity to ${\mathrm{SO}}_2$ and NO in band-I. The achieved detection limits were 60 ppb for ${\mathrm{SO}}_2$ and 7 ppb for NO, with non-linear deviations of 0.79% FS and 0.47% FS for ${\mathrm{SO}}_2$ and NO, respectively. The system employed optical fiber as the signal transmission medium for gas detection. Sangeetha and Madhan²² designed and fabricated a ${\mathrm{MoS}}_2/\mathrm{graphene}$-based hybrid sensor, evaluating its gas sensing performance for various gases (ethanol, methanol, acetone, CO, ${\mathrm{NO}}_2$, and formaldehyde) using a clad-removed optical fiber method. Figure 7(a) depicts the experimental setup for the optical fiber clad-modified gas sensor. Mohanraj et al.¹¹⁴ demonstrated an all fiber-optic multigas (FOMS) (${\mathrm{NH}}_3$, ${\mathrm{NO}}_2$, and CO) sensor by coating synthesized molybdenum tungsten disulfide (${\mathrm{MoWS}}_2$) nanosheets on a side-polished fiber (SPF). The ${\mathrm{MoWS}}_2$-coated SPF sensor was used for sensing three hazardous gases: ${\mathrm{NH}}_3$, ${\mathrm{NO}}_2$, and CO. Figure 7(b) illustrates the diagram of the all FOMS sensor with the ${\mathrm{MoWS}}_2$ sensitizing layer.

Fig. 7

(a) The diagram of the experimental setup for the optical fiber clad-modified gas sensor.²² (b) Diagram of the all FOMS (${\mathrm{NH}}_3$, ${\mathrm{NO}}_2$, and CO) sensor with the ${\mathrm{MoWS}}_2$ sensitizing layer.¹¹⁴ (c) Gas sensing mechanism of the CdS-coated clad-modified optical fiber sensor.²⁴ (d) Gas sensing response of the 2D $\mathrm{WO}x$-based optical fiber sensor.¹¹⁵ (e) Schematic of the experimental setup used for two different optical fiber sensor designs for the detection of trace amounts of ${\mathrm{NO}}_2$ in a mixture of gases.¹¹⁶

Theoderaj et al.²⁴ fabricated a clad-modified OFS, coating CdS over the clad-modified region using chemical bath deposition as the ${\mathrm{NO}}_2$ gas-sensing medium. Figure 7(c) illustrates the gas sensing mechanism of the CdS-coated clad-modified OFS. Yao et al.¹¹⁵ implemented a 2D near-infrared plasmonic tungsten oxide (${\mathrm{WO}}_x$)-enabled ultrasensitive optical fiber gas sensor on a side-polished D-shape single-mode optical fiber. Figure 7(d) displays the gas sensing response of the 2D $\mathrm{WO}x$-based OFS. A fluorescent probe composed of carbon quantum dots with o-phenylenediamine groups was prepared by the microwave method to selectively detect NO.¹¹⁷ Mechery and Singh¹¹⁶ designed two types of reflective optical fiber-sensing probes based on the signal transduction mechanism of spectral changes at the nanopore of sol–gel elements. The extrinsic design utilized the optical fiber transflection probe geometry with its sensor element placed at the distal end of the fiber bundle. The nitrogen dioxide-mediated coloration reaction ensured high specificity of detection when tested in a mixture of gas samples. Figure 7(e) shows the schematic of the experimental setup used for the two different OFS designs for the detection of trace amounts of ${\mathrm{NO}}_2$ in a mixture of gases. Niu et al.¹¹⁸ prepared a fibrous $\mathrm{rGO}/{\mathrm{MoS}}_2$ composite through a wet-spinning and a hydrothermal method.

4.2.

Carbon Monoxide

Under normal conditions, carbon monoxide (CO) is a colorless, odorless, and tasteless toxic gas, which is also insoluble in water, rendering it a neutral gas. When inhaled with air, CO enters the bloodstream through the alveoli and combines with hemoglobin to form carbon oxygen hemoglobin (HbCO). This compound is difficult to decompose and hinders the blood’s ability to carry oxygen, leading to hypoxia. Moreover, HbCO not only fails to transport oxygen itself but also interferes with the dissociation of ${\mathrm{HbO}}_2$, causing double hypoxia in the tissues. Consequently, tissue hypoxia and carbon dioxide retention ensue, resulting in the manifestation of oxygen toxicity symptoms. The carbon atom within the CO molecule exhibits a $+2$ oxidation state, susceptible to further oxidation to $+4$. Thus the imperative quest for real-time, quantitative, and accurate CO gas detection remains paramount.^119,120

Peng et al. developed an optical fiber dual F–P interferometric CO gas sensor by coating a $\mathrm{PANI}/{\mathrm{Co}}_3{\mathrm{O}}_4/\mathrm{GO}$ (PCG) sensing membrane on the end face of the optical fiber. The FPI sensor was created by splicing one end of endlessly photonic crystal fiber (EPCF) with an SMF and applying the PCG sensing membrane to the other end. This sensor offers compact size, cost-effectiveness, high sensitivity, strong selectivity, and a simple structure, making it suitable for detecting low concentrations of CO gas.¹²¹ Figure 8(a) illustrates the structural diagram of the FPI sensor, where a section of EPCF with a core RI of $n_{\mathrm{EPCF}}$ is spliced to a conventional SMF with a core RI of $n_{\mathrm{SMF}}$. Zhou et al.¹²² proposed and fabricated a Michelson interferometric CO OFS based on Co/Ni-MOF-74, a bimetallic-ion-coordination MOFs material, using the solvothermal method. Figure 8(b) presents the diagram of the Michelson interferometric optical fiber CO sensor with Co/Ni-MOF-74 as the sensitizing layer. He et al.¹²³ developed a CO sensor based on a Michelson interferometer combined with $\alpha \text{-}{\mathrm{Fe}}_2{\mathrm{O}}_3/\text{reduced}$ graphene oxide quantum dots (rGOQDs) composite film. Figure 8(c) depicts the diagram of a Michelson interferometric optical fiber CO sensor employing $\alpha \text{-}{\mathrm{Fe}}_2{\mathrm{O}}_3/\mathrm{rGOQDs}$ as the sensitizing layer. Chen et al. successfully developed an innovative optical fiber-based intensity-modulated CO gas sensor, utilizing a cerium dioxide (${\mathrm{CeO}}_2$) sensing film applied to a thin-core fiber (TCF). The sensor structure includes symmetrically spliced SMF, no-core fiber (NCF), and TCF, forming an SMF-NCF-TCF-NCF-SMF Mach–Zehnder interference configuration.¹²⁴

Fig. 8

(a) Diagram of the dual FPI sensor and the corresponding experimental setup.¹²¹ (b) Michelson interferometric optical fiber CO sensor with the Co/Ni-MOF-74 sensitizing layer.¹²² (c) Michelson interferometric optical fiber CO sensor with the $\alpha \text{-}{\mathrm{Fe}}_2{\mathrm{O}}_3/\mathrm{rGOQDs}$ sensitizing layer.¹²³ (d) Intensity-modulated CO gas sensor with the ${\mathrm{CeO}}_2$ film.¹²⁴ (e) Experimental setup of the PTI CO sensor using a HC-NCF.¹²⁵

Figure 8(d) illustrates the diagram of the intensity-modulated CO gas sensor with the ${\mathrm{CeO}}_2$ film. Yao et al.¹²⁵ developed the first CO photothermal sensor using a hollow core negative curvature fiber (HC-NCF), effectively enhancing the photothermal signal by incorporating a water vapor additive. Figure 8(e) displays the experimental setup of the PTI CO sensor utilizing a HC-NCF. Liu et al. introduced a rapid gas sensing scheme based on non-dispersive frequency comb spectroscopy (ND-FCS) capable of measuring multiple gas components. The researchers experimentally investigated its capacity for multicomponent gas measurement using the time-division-multiplexing (TDM) method to achieve specific wavelength selection of the fiber laser optical frequency comb. With TDM, the quasi-synchronous dynamic measurement of the three gases was achieved. The proposed ND-FCS exhibits exceptional sensing ability without the need for low-efficiency spectrum scanning and complex laser mode locking configuration, making it a promising candidate for long-term, high sensitivity, fast response gas sensing applications and multicomponent measurement requirements.¹²⁶

4.3.

Volatile Organic Compound

VOCs constitute a group of organic substances known for their high vapor pressure at room temperature. They readily evaporate into the air, giving rise to flammable and explosive mixtures. Commonly encountered VOCs include benzene, formaldehyde, acetone, xylene, and ethyl acetate, finding applications in various industrial processes, building materials, household products, cosmetics, and cleaning agents. Long-term exposure to elevated levels of VOC pollution may result in various health issues, including headaches, breathlessness, and eye irritation.^127,128 Furthermore, specific VOCs are associated with chronic respiratory diseases and neurological problems. Thus it becomes imperative to closely monitor VOC levels. To address this, OFSs emerge as the most promising gas detection devices. Their compact size, high sensitivity, resistance to electromagnetic interference, and ability to withstand harsh environments make them ideal for VOC monitoring applications.^16,17,129

Benzene, a colorless and transparent liquid with a distinct aromatic odor, finds extensive applications as a solvent and chemical raw material, despite its profound toxicity and hazardous nature. It poses serious threats to the human nervous and immune systems, the WHO recommends maintaining airborne benzene concentrations below 5 ppm for an 8-h average. Wang et al.¹³⁰ developed a photonic crystal OFS utilizing the MZI, as shown in Fig. 9(a). The sensor incorporated a silicon fiber array as the fiber cladding, air holes as the core, and carbon nanotubes (CNTs) as the coating material. Figure 9(b) displays the spectral response of the sensor at various concentrations, wavelength variation for different materials, and specificity. The study revealed that CNTs exhibited a more pronounced response to benzene ($0.8\mathrm{nm}/10\mathrm{ppm}$) compared to other gases due to $\pi -\pi$ conjugation between the CNTs and the benzene ring, which enhances their selectivity for benzene. The structural features of CNTs (sp2 hybridization) create a conjugated system with the benzene ring’s off-domain $\pi$-electronic structure, leading to increased benzene adsorption. Dong et al.¹³¹ designed and fabricated a long-period fiber grating sensor with graphene/IDTC1 as the sensitive fiber cladding for quantitative detection of benzene vapor. Figure 9(c) illustrates the schematic diagram of the sensing device and the optical fiber interface. Figure 9(d) exhibits the transmission spectra in the presence or absence of IDTC1 dye and in different gas atmospheres. The sensor demonstrated quantitative benzene detection over a broad dynamic range (20 to 23,000 ppm) and exhibited high sensitivity ($0.36\mathrm{pm}{\mathrm{ppm}}^{-1}$). Benzene-induced discoloration caused a change in the IDTC1 film’s RI on the optical fiber, leading to a shift in the resonance center’s wavelength of the long-period fiber grating.

Fig. 9

(a) MZI photonic crystal optical fiber benzene sensor based on CNT modification. (b) Concentration and specificity test plots of benzene sensors.¹³⁰ (c) Graphene/IDTC1-modified long-period optical fiber grating benzene sensors. (d) Transmission spectra in the presence or absence of IDTC1 dye and in different gas atmospheres.¹³¹

Acetone is a colorless, flammable, and volatile organic solvent. In high concentrations, its vapors can form flammable mixtures with air, presenting a risk of fire and explosion. Pawar et al.¹³² developed a dual-cavity F–P interferometric sensor that utilizes graphene/PMMA composite-modified polarization maintaining fibers (PMFs) to detect various volatile gases. The remarkable properties of graphene, such as its RI change, combined with the swelling of PMMA polymer and the effective RI change and swelling of graphene/PMMA composites, make these composites highly appealing for the detection of various VOCs, including acetone, chloroform, toluene, methanol, ethanol, and isopropanol. Bhise et al.¹³³ introduced an optical acetone detector based on a palladium-modified ${\mathrm{WO}}_3$ nanostructure ($\mathrm{Pd}@{\mathrm{WO}}_3$) with high sensitivity and reproducibility across a wide concentration range of 20 to 1000 ppm. Table 3 illustrates the comparative performance of OFSs for gases.

Table 3

Overview of optical fiber sensors for gases.