1. Introduction

With the vigorous development of global industrialization, the problem of heavy metal pollution becomes increasingly prominent in the atmosphere, in soils, and especially in water systems, placing a serious burden on the environment. Due to their toxicity and carcinogenicity, heavy metal ions, which are non-biodegradable and accumulate easily in ecosystems, cause significant harm to human and animal health [1,2]. When heavy metal ions dissolved in water enter the biosphere and are ingested into an organism, they can be highly detrimental to health, causing kidney and liver disorders, skin and bone diseases, disorders of the central nervous system, and cancers. Therefore, the rapid, real-time and high-sensitivity monitoring of heavy metal ions in solution has become a vital issue for both environmental protection and disease prevention. A wide variety of monitoring methods for heavy metal ions in water have been developed, including spectroscopy [3], chromatography [4], electrochemical technique [5], inductively coupled plasma mass spectrometry (ICP-MS) [6], atomic absorption spectrometry (AAS) [7], and chemiluminescence [8]. Among them, spectroscopy and chromatography are bulky, time-consuming, and high cost. The electrochemical method requires prior chemical treatment of the sample and suffers from electrochemical noise and unavoidable environmental interference, greatly restricting its detection limit. The ICP-MS and AAS methods require complex sample pretreatment and expensive detection instruments, and they suffer from complicated operation and long detection times [7,9]. The chemiluminescence method has poor selectivity and is not suitable for detecting low concentration pollutants.

Recently, with the rapid development of fiber optic sensing technologies and functional materials, fiber optic sensors have exhibited excellent performance in the measurement of heavy metal ion concentration [10–15] and are being intensively developed as potential candidates for tracing the presence of metal ions in aquatic environments. Compared with traditional detection technologies, fiber optic sensors uniquely combine the advantages of remote sensing, miniaturization, sensor multiplexing, high flexibility, high sensitivity, low cost, and immunity to electromagnetic interference. Various fiber optic sensing technologies have been demonstrated, with working principles based on fluorescence [15], fiber grating [16], optical absorbance [17], plasmonic [18] and fiber modal interference [19,20]. Among these different types of fiber sensors, intermodal interference based sensors have attracted great interest owing to their high inherent stability, compact structure and large dynamic range [21]. Various fiber interferometer configurations have been reported including Mach Zehnder (MZI), Michelson (MI), Fabry-Perot (FPI), and Sagnac interferometer (SI) [19–25]. A disadvantage of MI, FPI and SI designs is that they usually require complicated fabrication processes involving additional components such as optical circulators or couplers to guide the reflected light. In contrast, intermodal MZI designs offer significant benefits over other interferometer configurations having simple setup, high sensitivity, and a relatively simple fabrication process without any requirement for circulators or couplers. These advantages have encouraged a variety of MZI sensors to be proposed and extensively applied to monitor the presence of metal ions in solution.

In this paper, we mainly propose a new type of MZI fiber optic sensor and experimentally demonstrate real-time, high-sensitivity monitoring of heavy metal ions. This alternative all-fiber MZI is simply constructed using a hollow optic fiber (HOF) spliced between lead-in and lead-out standard single mode fibers (SMF), in which two ultra-abrupt tapers near the spliced joints are formed. The two tapering sections excite the cladding modes of the HOF at the first SMF-HOF interface, and combine the interfering signal of the core mode and cladding modes into the lead-out SMF. Due to the ultra-abrupt taper configuration, most of the evanescent field interacts with the external environment. This causes the microfiber MZI become highly sensitive to small changes in the external environment and allows a sensing device to be constructed that can be optimized to suit a range of applications.

For validating the sensing performance of the proposed MZI fiber and real time monitoring the presence of the metal ion Cu²⁺concentration in liquids, the fiber surface is functionalized with a nanoscale chitosan/poly acrylic acid (CS/PAA) multilayer film deposited by layer-by-layer (LbL) self-assembly [26,27]. This pair of polyelectrolytes serve as adsorption sites for copper ions by offering chelating sites for direct Cu²⁺ adsorption on the fiber surface [13,27]. Due to the amino group in CS and the carboxyl group in PAA, this functionalized coating chelates with Cu²⁺ ions and causes an increase in the refractive index (RI) of the CS/PAA coating film when the Cu²⁺ concentration adsorbed on the fiber surface is increased [13,27]. As a result, the changes in the transmission spectra of the MZI sensor induced by the RI variation of the coating film are used to sense changes of the Cu²⁺ concentration in the external environment.

To evaluate the sensing characteristics of the MZI fiber, spectral shifts due to adsorption of Cu²⁺ at the fiber surface are measured in real-time for solutions of various concentrations. Results show that the MZI sensor has sensitivity of 42 nm/mM with a highly linear response in the range of 0­0.04 mM. Additionally, excellent reusability is demonstrated by monitoring the transmission spectrum of the MZI sensor over repeated tests. Our results show that HOF-based MZI fibers optic sensors can be applied to sensitively detect and measure heavy metal contamination in aqueous solutions, with good potential for measuring environmental water quality robustly and cost effectively. Thus, the proposed interferometer has potential application as a sensor with high sensitivity based on tracking the wavelength shift of a particular interference valley.

2. Principle and fabrication

2.1 Principle of MZI fiber for sensing heavy metal ions

In order to reduce the cost of an all-fiber MZI and simplify the manufacturing process, we propose a novel configuration of MZI fiber realized by arc-splicing a section of HOF between two standard SMFs, in which a micro-tapered structure is formed at the jointed end as shown in Fig. 1. As the light propagates through the first taper, a part of the fundamental core mode in the lead-in SMF is excited to higher order modes into the cladding layer of the HOF. After propagating a certain distance L, these cladding modes are recoupled back into the fiber core of the lead-out SMF by the second taper and interfere with core mode. The output interference light intensity of the MZI fiber can be expressed as [20]:

 

Fig. 1. (a) Schematic diagram of the proposed all-fiber MZI, and (b) the abrupt tapered structure constructed at the spliced end.

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Due to the amine group in the CS and the carboxyl group in the PAA, a bilayer of CS/PAA polyelectrolytes, providing a spongy surface, is commonly applied to chelate with heavy metal ions resulting in a change in RI of the bilayer coating [13]. Thus, to facilitate Cu²⁺ ion detection by the fiber sensor, functionalized bilayers of CS and PAA are deposited on the MZI fiber surface using the electrostatic self-assembly technique [26,27]. As the Cu²⁺ ions on the fiber sensor surface are adsorbed, the Cu²⁺ ions form coordination bonds with the amine groups of CS and the carboxyl groups of PAA on the fiber surface, thereby increasing the crosslinking of the CS/PAA network, decreasing the effective refractive index of the cladding modes and altering the effective index difference δn_eff of the MZI fiber [13,26–27]. As a result, the interference wavelength λ_m is sensitive to changes in RI induced by the absorption of Cu²⁺ ions on the CS/PAA bilayers. On this basis, the concentration of Cu²⁺ ions in aqueous solution can be quantitatively determined by monitoring the peak wavelength of the output spectrum and the sensitivity $\partial $λ_m/$\partial n\; $of MZI responding to metal ion solution is proportional to the length of the HOF according to Eq(2). Although the anti-resonant effect, which is helpful for improving the sensitivity of the MZI sensor, is not considered here. In the future, to enhance the sensitivity of the MZI fiber sensor, double phase matching condition of MZI, which satisfies both the destructive condition of double beam interference and the resonant condition of anti-resonant effect, will be performed.

2.2. Fabrication and functionalization of the MZI fiber sensor

To implement the HOF-based intermodal MZI fiber design, a segment of HOF (TSP010150; Polymicro Molex) is prepared and spliced in between SMF (8.2/125 µm SMF-28) lead-in and lead-out fibers using a commercial optical fiber splicer (FITEL S175, Japan). During the splicing process, the air hole at both splicing ends of the HOF collapses to generate abrupt tapering structures near the two spliced joints. The microscopic image of the tapered joint is shown in Fig. 1(b). The tapering sections function as a beam splitter and combiner; the former excites several cladding modes at the HOF input and the latter recombines the core-mode light and the cladding-mode light into the second SMF to generate the interference signals.

To make the proposed MZI for real time detection of Cu²⁺ ions, a bilayer film of CS/PAA is directly coated on the surface of the MZI fiber. In this work, to functionalize the proposed fiber sensor, 2% CS solution is first prepared by dissolving 2 g of CS in 4% acetic acid solution and 4% PAA solution is prepared by diluting 35% PAA solution in Milli-Q water (Millipore). Both the solutions are continuously stirred for 24 h at room temperature. Prior to depositing the LbL film, the MZI fiber is pretreated with piranha solution (concentrated sulfuric acid, H₂SO₄ (98%) and hydrogen peroxide, H₂O₂ (35%) in a 7:3 ratio) for 60 min, sonicated in distilled water and dried with nitrogen gas. This process removes organic residues and hydroxylates the fiber surface. Subsequently, the piranha pretreated MZI fiber is functionalized by LbL self-assembly as illustrated in Fig. 2. To conduct LbL self-assembly, the pretreated MZI fiber is first dipped into CS solution for 5 min and withdrawn at a rate of 70 mm/min by using a dip coater (DX-5; Sadhudesign Com.). The fiber is allowed to dry for 2 min, followed by washing with deionized water to remove incompletely bonded CS molecules. The same process is repeated for functionalizing a layer of PAA onto the CS layer and this completes a full deposition cycle of the CS/PAA bilayer on the fiber sensor surface as shown in Fig. 2. Here, to confirm that the polymer chains have sufficient charge to facilitate self-assembly, the pH of the CS and PAA solutions is adjusted to ∼2 and ∼5, by hydrochloric acid and sodium hydroxide solution, respectively. For increasing the adsorption of Cu²⁺ and thereby discriminate the presence of Cu²⁺ ions more effectively, multiple coatings of CS/PAA bilayers are required to generate a multilayer film, hereafter denoted as [CS/PAA]_n, where n is the number of bilayers. After the desired number of bilayers are deposited, the functionalized fiber is then dried overnight at room temperature. As the adsorption of metal ion Cu²⁺ on the functionalized sensor surface occurs, the refractive index of the [CS/PAA]_n film is increased resulting a phase shift of the interference spectrum of the sensor. Thus the concentration of heavy metal ion can be inferred by tracking the change of the interference wavelength.

 

Fig. 2. Schematic illustration of the self-assembly deposition process of the [CS/PAA]_n multi-bilayer.

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2.3. Sensing experimental setup

Figure 3 shows the schematic diagram of the experimental setup of the fiber sensing system, which consists of a broadband light source (BLSS101B-028002A; GIP Tech. Corp.) with emission wavelength range of 1250-1650 nm, an optical spectrum analyzer (OSA) (Anritsu MS9740A), and a functionalized MZI fiber sensor. The light emitted from the broadband source is coupled into the lead-in SMF and the lead-out SMF is coupled to the OSA. To achieve a controlled external environment for the MZI fiber probe during the process of monitoring the metal ions in solution, a polydimethylsiloxane (PDMS) polymer substrate with microfluidic channel structure is applied to hold the MZI fiber. For detecting the presence of metal ions, the fiber sensor is repeatedly immersed in 400µL of Cu²⁺ aqueous solutions at various concentrations, which are introduced through the microchannel, and the wavelength of the interference peak in response to the Cu²⁺ presence is monitored and recorded in real-time.

 

Fig. 3. Schematic diagram of the MZI fiber sensing system.

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3. Experimental results and discussion

To verify our notion of using HOF to realize an all-fiber MZI, the transmission spectra of microfiber MZIs with three HOF lengths (0.4 cm, 0.8 cm, and 1.2 cm) are measured as shown in Fig. 4. The measured spectra indicate that strong interference fringes are obtained, with dynamic range over 25 dB, which is adequate for most sensing applications. The background loss of the device is ∼8 dB, which is mainly caused by the two abrupt tapers. Average fringe spacing is influenced by the MZI length. Due to the accumulated phase difference between the core and the cladding increasing with the MZI length, the average fringe spacing declines as the length of the HOF section increases. As a result, the proposed all-fiber MZI can be successfully realized by the HOF. Since the interference properties of MZI fiber not only depends one HOF length, but also on the tapering microstructure (i.e., tapering angle and length), which considerably influences the optical power distributed between the core and cladding layers from the input SM fiber. The analysis of the impact of HOF length on the spectrum of the MZI is complicated. Therefore, the detailed analysis of the relationship between the average fringe spacing (FS) and HOF length will be performed in the future. Although the sensitivity of MZI fiber responding to metal ion solution increases with the length of the HOF. However, the transmission spectra of the proposed MZI fiber with 0.8 cm HOF was observed to have a highest dynamic range and sharpest wavelength peaks here. To explore the characteristics of the proposed MZI in sensing applications, the MZI fiber with HOF length of 0.8 cm is applied to monitor the presence of Cu²⁺ ions in aqueous solution.

 

Fig. 4. Measured transmission spectra of the proposed MZI fiber with different lengths of HOF segment.

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3.1 Optical characteristics of MZI fiber with multilayer [CS/PAA]_n coating

The resonant wavelength of the MZI fiber is very sensitive to changes in refractive index induced by Cu²⁺ ion adsorption on the CS/PAA bilayer. In this work, optimization of the sensing layer is investigated by depositing varying numbers of CS/PAA bilayers and recording the transmittance spectrum accordingly. Figure 5 demonstrates the spectral response of the MZI fiber sensor before and after multiple [CS/PAA]_n deposition processes. After the functionalization process, it is observed that there is a shift in the spectral dip towards longer wavelength, indicating a variation in the effective refractive index of the cladding modes of MZI. The red shift of the peak wavelength increases with increasing numbers of CS/PAA bilayers. For example, the red shift of peak wavelength for 5, 8, 10, and 12 CS/PAA bilayers is 4.0 nm, 7.6 nm, 10.2 nm, and 12.2 nm, respectively. The red shift of peak wavelength is caused by the reduction in the effective refractive index of the cladding mode, which is attributed to the functionalization of CS/PAA bilayers on the sensor surface. In addition, the amount of wavelength red shift can be used to determine whether the CS/PAA bilayer film is successfully electrostatically self-assembled on the fiber probe.

 

Fig. 5. The transmission spectra of the MZI fiber sensor before and after (a) 5, (b) 8, (c) 10, and (d) 12 layers of CS/PAA bilayers.

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To study the influence of the number of CS/PAA bilayers on the wavelength shift when exposed to Cu²⁺ ions, functionalized fiber probes with different numbers of LbL-coating cycles are immersed in 1mM Cu²⁺ solution and transient transmittance spectra are recorded at various time delays as shown in Fig. 6. It can be seen that the spectra shift towards longer wavelength with increasing exposure time as the adsorption of Cu²⁺ gradually increases the cross-linking level of the [CS/PAA]_n sensing film, increasing the RI of the [CS/PAA]_n film, which in turn decreases the effective refractive index of the cladding mode of MZI fiber probe and thereby red shifting the peak wavelength. The amount of wavelength shift induced by the adsorbed Cu²⁺ is also influenced by the bilayer number.

 

Fig. 6. The transmittance spectra of the functionalized MZI fiber sensor with (a) 5, (b) 8, (c) 10, (d) 12 (CS/PAA) bilayers immersed in 1 mM Cu²⁺solution.

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To more clearly and quantitatively evaluate the impact of multiple CS/PAA bilayers on the sensing characteristics of MZI probe, the corresponding transient resonant wavelength shift of MZI fibers functionalized by different numbers of CS/PAA bilayers is analyzed as shown in Fig. 7, where the peak wavelength shift is plotted as a function of exposure time for various numbers of bilayers. The peak wavelength red-shift of the MZI fiber initially increases with time and then saturates to a value determined by the number of bilayers, with more layers resulting in a larger wavelength shift. For example, the saturation wavelength shifts of the MZI fibers with 5, 8, and 10 bilayers are 0.8 nm, 1.6 nm, and 2.6 nm, respectively, indicating that [CS/PAA]₁₀ film obtains greatest sensitivity. However, the wavelength shift of the MZI fiber with functionalized [CS/PAA]₁₂ film is only 1.4 nm, suggesting that the thickness of the [CS/PAA]₁₂ sensing film may be too thick, causing the ability of the fiber sensor to detect the external environment to decline. Hereafter, to obtain the optimum sensing sensitivity, the MZI fiber functionalized with [CS/PAA]₁₀ multilayers is applied to monitor the copper ions in aqueous solution.

 

Fig. 7. The corresponding transient resonant wavelength shift of the functionalized MZI fiber sensors.

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3.2 Detection of Cu²⁺ ions

In order to evaluate the characteristics of the MZI sensor, the functionalized [CS/PAA]₁₀ fiber is tested with different concentrations of Cu²⁺ ions in the concentration range from 0.00 mM to 1.00 mM. The Cu²⁺ solutions are prepared by dissolving copper chloride powder in Milli-Q water. Due to the pH-dependent characteristics of the amino group in CS and the carboxyl group in PAA, the CS/PAA deposited on the fiber is inherently pH sensitive, exhibiting large expansion/contraction in response to pH of the external solution and has been utilized in various pH sensing applications [28]. The volume and RI of the PAA/CS multilayer are easily regulated by the pH of the external solution and in turn lead to a shift of interference spectrum. Thus for obtaining the stability of the sensor responding to the metal ion solution, The pH value of Cu²⁺ solution is controlled at ∼ 6 by using 2-(N-morpholino)ethanesulfonic acid (MES) buffer. Additionally, the ambient temperature disturbance could affect the pH value of the solution and the properties of the PAA/CS-sensitive film [29]. When the temperature of the solution increases, the pH value of the solution gradually increases, leading to an increase in the volume and a decrease in RI of the sensitive film. and thus resulting in a shift in the interference spectrum. To validate the detection ability of the proposed fiber sensor with high stability, the measurement experiment was performed at 25^◦C (the laboratory temperature is basically kept at 25 ^◦C). The spectral response of the MZI fiber is continuously measured and the transient peak wavelength shift induced by different concentrations of Cu²⁺ aqueous solutions is obtained as shown in Fig. 8.

 

Fig. 8. The transient wavelength shift of MZI fiber during Cu²⁺ detection in concentration changes from 0 mM to 1.0 mM.

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The wavelength shift of the MZI sensor increases rapidly for the first few minutes and then gradually reaches saturation after around five to ten minutes, at which time the interaction of Cu²⁺ with the CS/PAA bilayers reaches equilibrium. From Fig. 8, it can be observed that the wavelength shift with detection time could reveal the kinetic behavior of Cu²⁺ adsorption onto the MZI sensor, which is highly impacted by the Cu²⁺ concentration. The response time for the proposed sensor to reach 90% of the saturation wavelength shift becomes faster with increasing Cu²⁺ concentration. From the results in Figs. 7 and 8, the response time for Cu²⁺ ion detection is relatively slow, which is similar to many studies [30]. The kinetics of metal adsorption on chitosan is attributed to the significant influence of the diffusion rate of Cu²⁺ ions to the multilayer-aqueous solution interface and the chelation rate of Cu²⁺ ions by CS and PAA [12,13,29]. The diffusion properties may be improved by physical modification of chitosan and the affinity of the adsorbent for metals is enhanced by controlling the reactivity of the polymer. For example, the nano-sized CS possesses the advantages of high specific surface area, small size, low internal diffusion resistance, and quantum size effect that could enable them to exhibit higher capacities for the chelation of metal ions [30]

The dependence of the saturation wavelength shift on the Cu²⁺ concentrations is illustrated in Fig. 9, and linear fits are applied to the experimental data to obtain the sensitivity. It can be seen that the peak wavelength shift increases with the Cu²⁺ concentration, and the sensitivity of MZI fiber sensor in the range of 0.00 mM to 0.04 mM and 0.1 mM to 1.00 mM is 42 nm/mM and 0.67 nm/mM, respectively. The sensor sensitivity is observed to be most sensitive at low concentrations, demonstrating that highly dilute Cu²⁺ ions can be detected using the proposed fiber MZI sensor. Additionally, from the above experimental results, it can be observed the structural characteristics of the CS/PAA multilayer film significantly affect the amount of Cu²⁺ bonding to the sensing layer and then influences the sensitivity of the MZI fiber sensor. Therefore, the sensitivity of the proposed MZI fiber can be further improved by using a suitable polymer matrix and thickness, which would be the focus of future work.

 

Fig. 9. The linear fit of wavelength shifts and Cu²⁺ ion concentrations from 0 to 0.07 mM and 0.07 to 1.0 mM.

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For studying the reusability of the proposed MZI fiber probe, the [CS/PAA]₁₀ functionalized sensor is first immersed in 0.1 mM Cu²⁺ solution for 20 mins, and then immersed in pH 2.5 HCl solution for desorption for 30 s. Figure 10 shows five successive regeneration cycles of the sensor. Immersion in the 0.1 mM Cu²⁺ solution causes a wavelength shift of about 2 nm and subsequent immersion in the HCl buffer restores the resonant wavelength shift to the original value. This can be attributed to the reversible binding of Cu²⁺ on CS/PAA bilayers. At low pH, amine groups in CS are protonated resulting in the unbinding and release of Cu²⁺ ions [27], and the fiber sensor is regenerated. We find the sensor is regenerated without any significant loss in performance during these 5 cycles.

 

Fig. 10. Wavelength shifts of the sensor in 0.1 mM Cu²⁺ and HCl solutions for five successive regeneration cycles.

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Furthermore, for studying the specificity performance of the proposed MZI fiber sensor, the response of the functionalized fiber sensor was also examined for Na ⁺, Ca²⁺, Mg²⁺, and Ni²⁺ ions with concentration of 1 mM. The wavelength shifts of the sensor for different metal ions were compared as shown in Fig. 11 (line chart). It can be observed that the resonance wavelength shifts for Na ⁺, Ca²⁺, Mg²⁺, and Ni²⁺ ions are not as significant as for Cu²⁺. Figure 11 reveals that the sensor has good selectivity for Cu²⁺ over other metal ions. The cross-sensitivity for Na⁺, Ca²⁺ and, Mg²⁺ is attributed to the affinity of PAA to alkali and alkaline earth metals, although chitosan doesn’t chelate these alkali and alkaline earth metals ions [12]. In addition, the cross-sensitivity problem for transition metal Ni²⁺ is also created owing to chitosan can chelate the transition metals [12].

 

Fig. 11. Wavelength shifts of the sensor for Na⁺, Ca²⁺, Mg²⁺, Ni²⁺ and Cu²⁺ ions

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The purpose of this research is mainly concerned on proposing a simple and novel MZI fiber, and applying this MZI fiber to monitor the presence of the metal ion Cu²⁺ concentration in liquids. Due to the amine group in the CS and the carboxyl group in the PAA, a bilayer of CS/PAA polyelectrolytes is utilized as a sensing membrane to chelate with heavy metal ions resulting in a change in volume and RI of the bilayer coating. As a result, the interference wavelength λ_m is sensitive to changes in RI induced by the uptake of Cu²⁺ ions on the CS/PAA bilayers. Owing to amine groups are strongly reactive with metal ions, the wavelength shift of the transmission spectra is totally induced by the chelation of metal cations by CS. Thus the sensing characteristics (such as sensitivity, response time, specificity, etc.) of the sensor are highly dependent on the properties and microstructure of the CS/PAA membrane, in which the microstructure controls the swelling of sensing film and diffusion properties of metal cations. Chelating agents with enhanced capability to bind specific metals is an intense area of research in recent years. To control the reactivity of the polymer for enhancing the adsorption capacities of the metal ions and increasing adsorption selectivity for the target metal, or improve diffusion properties for shortening response time, the modification of chitosan has recently attracted intense interesting by chemical or physical processes to prepare chitosan derivatives obtained by grafting new functional groups or to condition the polymer prepared by of membranes, gel beads, fibers, nanoparticles) [30–32]. For example, diffusion properties can be improved by physically modifying the structure of the polymer by making gel beads or reducing crystallinity, selectivity may be enhanced by chemical modification (grafting, eg, sulfur compounds). Accordingly, optimizing cross-linked CS/PAA multilayer by choosing high adoption capacities of chitosan-based material and controlling the LBL process conditions would be the focus of future work for improving the sensitivity, response time and specificity of the proposed MZI sensor.

Based on the experimental results, this novel HOF-based MZI fiber sensor has high sensitivity, good reversibility and repeatability for sensing Cu²⁺ ions in aqueous solution, and simple fabrication process. Therefore, our proposed MZI fiber is an excellent candidate for biosensor, which is potentially useful in monitoring practical biomolecule and biomolecule interaction, due to its characteristics of fast response, label free, easy to manufacture, and a microliter volume requirement for the bio-sample solution.

4. Conclusion

A robust and simple to manufacture HOF-based MZI fiber sensor, functionalized with CS/PAA self-assembled multilayers, has been proposed and experimentally demonstrated for real-time and high-sensitivity detection of Cu²⁺ ions in the aquatic environment. The wavelength shifts are linear in the Cu²⁺ ions concentrations range from 0 to 0.04 mM and 0.1 to 1 mM, and the corresponding Cu²⁺ ion sensitivities are 42 nm/mM and 0.67 nm/mM, respectively. Experimental results show that the sensor has good measurement repeatability and reusability. With low cost, compact structure, and immunity to electromagnetic interference, the HOF-based MZI fiber sensor has many potential applications such as monitoring of heavy metal ions in drinking water and industrial wastewater, and could be easily extended to the detection of biomolecules by choosing appropriate sensing films.