JP7650456B2 - Salt sensor device and manufacturing method thereof - Google Patents

Description

The present invention relates to a salt sensor device and a manufacturing method thereof.

Conventionally, as a sensor device using optical fiber, for example, an optical fiber device in which gold nanoparticles are fixed to the outer surface of the cladding of the heterocore part of the optical fiber is known (for example, see Patent Document 1).

The optical fiber device described in Patent Document 1 is configured such that, when light is transmitted from the light transmission section through the heterocore section to the optical fiber, the degree of absorption of light of a predetermined wavelength in the near-infrared region in the heterocore section changes according to the refractive index of the material surrounding the heterocore section. As a result, the optical fiber device makes it possible to measure with high sensitivity the characteristics of the material surrounding the heterocore section, such as the refractive index and concentration, by measuring the degree of attenuation of the propagation intensity of light of a wavelength in the near-infrared region in the optical fiber.

Patent No. 6391305

Incidentally, it is known that sodium chloride contained in seawater and antifreeze agents can cause deterioration of concrete and steel frames in public infrastructure such as bridges and tunnels, and in architectural facilities such as buildings, and it is necessary to capture changes in the amount of sodium chloride attached over time in order to manage these conditions.

Conventional methods for detecting sodium chloride include measuring the electrical conductivity of a sodium chloride solution, optical spectroscopy, flame color reaction, and atomic absorption spectrometry.

However, all of the above detection methods require sampling of a sample containing sodium chloride from the target object, and have the problem that it is not possible to observe the change in the amount of sodium chloride attached to existing infrastructure or buildings over time in situ over a long period of time.

To solve the above problem, the inventors have proposed a salt sensor device that can monitor the amount of salt, such as sodium chloride, in situ over a long period of time (see the specification of Japanese Patent Application No. 2019-149024).

The salt sensor device includes an optical transmission section having a core and a clad, a heterocore section having a core and a clad respectively connected to the core and clad of the optical transmission section, an optical fiber having a core of the heterocore section having a smaller diameter than the core of the optical transmission section, a metal nanoparticle layer dispersed and fixed to the outer peripheral surface of the clad of the heterocore section, and an alternating laminated film layer covering the metal nanoparticle layer. In the salt sensor device, the metal nanoparticle layer is only one layer, and the alternating laminated film layer is formed by alternatingly laminating at least one layer each of a polycation film and a polyanion film.

In the salt sensor device, when light is transmitted through the core of the optical transmission section, evanescent light seeps out to the outside world when the light leaking into the cladding in the heterocore section is totally reflected at the interface between the cladding and the outside world. Here, when the evanescent light resonates with the collective vibration of electrons in the metal nanoparticles of the metal nanoparticle layer, the evanescent light is absorbed and the reflected light totally reflected at the interface is reduced, but the degree of reduction depends on the refractive index of the polyion layer.

Therefore, the salt sensor device can detect the degree of reduction in the light totally reflected at the interface between the cladding of the heterocore portion and the outside world, thereby making it possible to know the concentration of salt present in the outside world of the heterocore portion.

However, it is desirable to develop a salt sensor device that can further increase sensitivity to salt concentration.

In view of the above circumstances, the present invention aims to provide a salt sensor device and a manufacturing method thereof that can monitor the amount of salt such as sodium chloride in situ over a long period of time and further increase the sensitivity to salt concentration.

In order to achieve this object, the salt sensor device of the present invention is a salt sensor device comprising an optical fiber having a core and a clad, and a heterocore portion having a core and a clad respectively connected to the core and clad of the optical transmission portion, the core of the heterocore portion having a smaller diameter than the core of the optical transmission portion, and an inorganic-organic composite laminated film formed by alternately laminating a plurality of metal nanoparticle layers and a plurality of polyion layers covering the metal nanoparticle layers on the outer peripheral surface of the clad of the heterocore portion, the polyion layer being characterized in that it is made of a polycation selected from the group consisting of vinylpyrrolidone-N,N-dimethylaminoethyl methacrylic acid copolymer diethyl sulfate, chitosan, diethylaminoethyl dextran, diallylamine hydrochloride polymer, methyldiallylamine hydrochloride polymer, diethylaminoethyl cellulose, and dextran chloride hydroxypropyltrimethylammonium ether, or a polyanion selected from the group consisting of polyglutamic acid and salts thereof, carboxymethyl dextran, carboxymethyl cellulose, hyaluronic acid and salts thereof, and alginic acid and salts thereof .

The "salt" detected by the salt sensor of the present invention means a compound formed by ionic bonding between a cation and an anion. Examples of the cation include hydrogen ions, sodium ions, potassium ions, calcium ions, etc., and examples of the anion include fluoride ions, chloride ions, bromide ions, iodide ions, sulfate ions, nitrate ions, etc. Examples of "salt" as a compound formed by ionic bonding between the cation and anion include sodium chloride and calcium chloride.

In the salt sensor device of the present invention, similar to the salt sensor device described in Japanese Patent Application No. 2019-149024, when light is transmitted through the core of the light transmission section, when the light leaking into the clad in the heterocore section is totally reflected at the interface between the clad and the outside world, the evanescent light that has leaked out to the outside world resonates with the collective vibration of the electrons in the metal nanoparticles of the metal nanoparticle layer, and the evanescent light is absorbed, and the reflected light that is totally reflected at the interface is reduced. Here, the degree of reduction in the reflected light depends on the refractive index of the polyion layer, and the refractive index of the polyion layer changes depending on the concentration of salt present in the outside world of the heterocore section. Therefore, both the salt sensor device of the present invention and the salt sensor device described in Japanese Patent Application No. 2019-149024 can detect the degree of reduction in the light totally reflected at the interface between the clad of the heterocore section and the outside world, thereby knowing the concentration of salt present in the outside world of the heterocore section.

In this case, the salt sensor device described in Japanese Patent Application No. 2019-149024 has only one metal nanoparticle layer on the outer peripheral surface of the clad of the heterocore portion, whereas the salt sensor device of the present invention has multiple metal nanoparticle layers stacked vertically as the inorganic-organic composite laminated film, and the amount of metal nanoparticles is significantly increased compared to the salt sensor device described in Japanese Patent Application No. 2019-149024. Therefore, according to the salt sensor device of the present invention, it is possible to further increase the sensitivity to salt concentration compared to the salt sensor device described in Japanese Patent Application No. 2019-149024.

In the salt sensor device of the present invention, the inorganic-organic composite laminated film is preferably formed by alternately laminating 2 to 2,000 layers of the metal nanoparticle layers and the polyion layers in order to further increase the sensitivity to salt concentration compared to the salt sensor device described in Japanese Patent Application No. 2019-149024.

In addition, in the salt sensor device of the present invention, the metal nanoparticles constituting the metal nanoparticle layer preferably have a particle diameter in the range of, for example, 5 to 100 nm, and the metal nanoparticles are preferably gold nanoparticles.

In the salt sensor device of the present invention, when the polyion layer is a polycation layer, the polycation layer is positively charged and has anions as counterions. When salt is present in the external environment of the heterocore portion, the anions constituting the salt undergo ion exchange with the anions contained in the polycation layer, and depending on the salt concentration, the anions constituting the salt penetrate into the polycation layer. As a result, the polycation layer swells or shrinks, changing the refractive index.

On the other hand, in the salt sensor device of the present invention, when the polyion layer is a polyanion layer, the polyanion layer is negatively charged and has cations as counter ions. Therefore, when salt is present outside the heterocore portion, the cations constituting the salt undergo ion exchange with the cations contained in the polycation layer, and depending on the salt concentration, the cations constituting the salt penetrate into the polyanion layer. As a result, the polyanion layer swells or shrinks, changing the refractive index.

The method for producing a salt sensor device of the present invention is a method for producing a salt sensor device comprising an optical fiber comprising a light transmission section having a core and a clad, and a heterocore section having a core and a clad respectively connected to the core and clad of the light transmission section, the core of the heterocore section having a smaller diameter than the core of the light transmission section, the method comprising the steps of forming a metal nanoparticle layer on the outer peripheral surface of the heterocore section, and then repeating an operation of forming a polyion layer on the metal nanoparticle layer at least twice to form an inorganic-organic composite laminated film in which a plurality of metal nanoparticle layers and a polyion layer covering the metal nanoparticle layer are alternately laminated, and is characterized in that it comprises a polycation selected from the group consisting of vinylpyrrolidone-N,N-dimethylaminoethyl methacrylic acid copolymer diethyl sulfate, chitosan, diethylaminoethyl dextran, diallylamine hydrochloride polymer, methyldiallylamine hydrochloride polymer, diethylaminoethyl cellulose, and dextran chloride hydroxypropyltrimethylammonium ether, or a polyanion selected from the group consisting of polyglutamic acid and salts thereof, carboxymethyl dextran, carboxymethyl cellulose, hyaluronic acid and salts thereof, and alginic acid and salts thereof .

FIG. 2 is an explanatory cross-sectional view showing the configuration of a sensor unit of the salt sensor device of the present invention. FIG. 2 is an explanatory diagram showing the device configuration of the salt sensor device of the present invention. 4 is an explanatory cross-sectional view showing a method for measuring the concentration of a sodium chloride solution using the salt sensor device of the present invention. FIG. 1 is a graph showing the relationship between the intensity (loss) of transmitted light (reflected light) and the concentration of an aqueous sodium chloride solution 5 minutes after a predetermined concentration of the aqueous sodium chloride solution is added to distilled water.

Next, an embodiment of the present invention will be described in more detail with reference to the attached drawings.

As shown in FIG. 1, the salt sensor device 1 of this embodiment includes an optical fiber 4 having a heterocore portion 3 of a predetermined length sandwiched between light transmission portions 2, 2. The light transmission portion 2 is a multimode optical fiber consisting of a core 5a and a cladding 6a that covers the outer peripheral surface of the core 5a, and the heterocore portion 3 is a single-mode optical fiber consisting of a core 5b with a smaller diameter than the core 5a and a cladding 6b that covers the outer peripheral surface of the core 5b.

The salt sensor device 1 includes an inorganic-organic composite laminated film 7 formed on the outer peripheral surface of the clad 6b of the heterocore portion 3. In this embodiment, the inorganic-organic composite laminated film 7 is formed by alternately laminating a plurality of metal nanoparticle layers, for example, 2 to 2000 layers of metal nanoparticle layers, and a plurality of polyion layers, for example, 2 to 2000 layers of polyion layers, and the metal nanoparticle layers are made of metal nanoparticles having a particle diameter in the range of 5 to 100 nm. The polyion layers may be positively charged polycation layers or negatively charged polyanion layers.

In the salt sensor device 1 of this embodiment, when light is transmitted through the core 5a of the light transmission section 2, evanescent light seeps into the outside world when the light leaking into the cladding 6b in the heterocore section 3 is totally reflected at the interface between the cladding 6b and the outside world. When the evanescent light resonates with the collective vibration of electrons in the metal nanoparticles of the metal nanoparticle layer that constitutes the inorganic-organic composite laminated film 7, the evanescent light is absorbed and the reflected light that is totally reflected at the interface is reduced. Here, the degree of reduction in the reflected light depends on the refractive index of the polyion layer that constitutes the inorganic-organic composite laminated film 7, and the refractive index changes in response to the concentration of salt present in the outside world of the heterocore section.

Therefore, with the salt sensor device 1, the concentration of salt present in the outside world of the heterocore portion 3 can be known by detecting the degree of reduction in the light totally reflected at the interface between the cladding 6b of the heterocore portion 3 and the outside world.

As shown in FIG. 2, in the salt sensor device 1 of this embodiment, when the heterocore portion 3 of the optical fiber 4 is buried inside the concrete 21 of a bridge, tunnel, building, etc., a light source 22 such as an LED is connected to one end of the optical fiber 4, and a detection device 23 such as a spectrum analyzer is connected to the other end of the optical fiber 4. In addition, an analysis device 24 such as a personal computer that analyzes the degree of reduction in reflected light detected by the detection device 23 is connected to the detection device 23. The light source 22, the detection device 23, and the analysis device 24 can be placed at a location away from where the heterocore portion 3 is buried.

According to the salt sensor device 1 shown in FIG. 2, light having a wavelength in the range of 300 to 1000 nm, for example 425 nm, is irradiated from the light source 22 and enters the core 5a of the light transmission section 2, and the reflected light that is totally reflected at the interface between the cladding 6b of the hetero-core section 3 and the outside world exits the core 5a of the light transmission section 2 and is received by the detection device 23. In the salt sensor device 1 shown in FIG. 2, the reflected light received by the detection device 23 is analyzed by the analysis device 24, and the concentration of salt in the concrete 21 can be detected from the degree of reduction.

It is known that concrete and the steel frame inside it deteriorate due to sodium chloride contained in seawater and calcium chloride contained in antifreeze, but with the salt sensor device 1 shown in Figure 2, the concentration of salts such as sodium chloride and calcium chloride in the concrete 21 can be detected by InSuti, so the deterioration of the concrete and the steel frame inside it can be properly managed.

The salt sensor device 1 can be manufactured, for example, as follows:

First, the cladding 6b of the heterocore portion 3 of the optical fiber 4 is treated with a silane coupling agent such as 3-aminopropyltrimethoxysilane to introduce amino groups onto the surface of the cladding 6b. Next, the heterocore portion 3 with the amino groups introduced onto the surface of the cladding 6b is immersed in pure water to protonate the amino groups.

Next, the heterocore portion 3 in which the amino group introduced on the surface of the clad 6b has been protonated is immersed in a metal nanoparticle suspension. In this way, a metal nanoparticle layer made of the metal nanoparticles can be formed on the outer surface of the clad 6b due to the interaction between the positive charge of the protonated amino group and the negative charge on the surface of the metal nanoparticle. For example, when the metal nanoparticles are gold nanoparticles, the metal nanoparticle suspension can be prepared by adding a predetermined concentration of citric acid, sodium citrate, or potassium citrate and a predetermined concentration of gold tetrachloride (III) in boiling water in predetermined amounts and stirring.

Next, the heterocore portion 3 with the metal nanoparticle layer formed on the outer peripheral surface of the clad 6b is immersed in a polyion aqueous solution of a predetermined concentration for a predetermined time, and then pulled out and washed, thereby forming the polyion layer on the metal nanoparticle layer. Washing of the heterocore portion 3 after immersing it in the polyion aqueous solution can be performed by immersing the heterocore portion 3 in pure water and gently moving it. Note that when the polyion layer is a polycation layer, the polyion aqueous solution is a polycation aqueous solution, and when the polyion layer is a polyanion layer, the polyion aqueous solution is a polyanion aqueous solution.

Examples of the polycation used in the polycation aqueous solution include polycations selected from the group consisting of vinylpyrrolidone-N,N-dimethylaminoethyl methacrylic acid copolymer diethyl sulfate, chitosan, diethylaminoethyl dextran, diallylamine hydrochloride polymer, methyldiallylamine hydrochloride polymer, diethylaminoethyl cellulose, and dextran chloride hydroxypropyltrimethylammonium ether .

The polycation layer formed by the polycation aqueous solution has anions as counter ions, and when salt is present in the external environment of the heterocore portion 3, the anions constituting the salt undergo ion exchange with the anions contained in the polycation layer, and depending on the salt concentration, the anions constituting the salt penetrate the polycation layer, causing the polycation layer to swell or shrink, changing its refractive index. Therefore, when the salt sensor device 1 of this embodiment is used to detect sodium chloride, it is preferable that the counter anion is other than chloride ions, and it is preferable that the polycation layer does not contain chloride ions as counter ions.

On the other hand, examples of the polyanion used in the polyanion aqueous solution include polyanions selected from the group consisting of polyglutamic acid and its salts, carboxymethyldextran, carboxymethylcellulose, hyaluronic acid and its salts, and alginic acid and its salts .

The polyanion layer formed by the polyanion aqueous solution has cations as counter ions, and when salt is present in the external environment of the heterocore portion 3, the cations constituting the salt exchange with the cations contained in the polyanion layer, and depending on the salt concentration, the cations constituting the salt penetrate the polyanion layer, causing the polyanion layer to swell or shrink, changing its refractive index. Therefore, when the salt sensor device 1 of this embodiment is used to detect sodium chloride, it is preferable that the counter cation is other than sodium ion, and it is preferable that the polyanion layer does not contain sodium ion as the counter ion.
In the manufacturing method of the salt sensor device 1 of this embodiment, the operation of forming the metal nanoparticle layer and the operation of forming the polyion layer are repeated 2 to 2000 times, whereby the inorganic-organic composite laminated film 7 can be formed in which 2 to 2000 layers of metal nanoparticle layers and 2 to 2000 layers of polyion layers are alternately laminated one by one.

The following are examples and comparative examples.

[Example]
In this example, 100 ml of distilled water was first boiled, and 1 ml of a 1% by mass aqueous solution of citric acid and 1 ml of a 1% by mass aqueous solution of tetrachloroauric acid (III) were added thereto, followed by stirring for 15 minutes while still boiling to prepare a gold nanoparticle suspension. The gold nanoparticle suspension contains gold nanoparticles with particle diameters ranging from 5 to 100 nm and negatively charged surfaces.

Next, 50 milliliters of a 1% by mass aqueous solution of acetic acid was heated to a temperature of 40°C, 0.5 milliliters of 3-aminopropyltrimethoxysilane as a silane coupling agent was added, and the heterocore portion of the optical fiber 4 was immersed in the solution for 2 minutes while gently stirring. Next, the heterocore portion 3 was heated at a temperature of 110°C for 2 hours, and amino groups were introduced onto the surface of the cladding 6b of the heterocore portion 3.

Next, the heterocore portion 3 with amino groups introduced on the surface of the cladding 6b was immersed in pure water to protonate the amino groups.

Next, the heterocore portion 3, in which the amino groups introduced on the surface of the clad 6b were protonated, was immersed in the gold nanoparticle suspension prepared in this example, to form a gold nanoparticle layer on the outer surface of the clad 6b.

Next, the heterocore portion 3 with the gold nanoparticle layer formed on the outer peripheral surface of the clad 6b was immersed in a polycation aqueous solution of vinylpyrrolidone-N,N-dimethylaminoethyl methacrylic acid copolymer diethyl sulfate with a concentration in the range of 20 mmol/L, then pulled out and washed with pure water to form a polycation layer on the gold nanoparticle layer.

In this example, the operation of forming the gold nanoparticle layer and the operation of forming the polycation layer were repeated five times to form an inorganic-organic composite laminated film 7 in which five gold nanoparticle layers and five polycation layers were alternately laminated one by one, thereby obtaining the salt sensor device 1 shown in FIG. 1.

Next, in the salt sensor device 1 shown in FIG. 1, the heterocore portion 3 having the inorganic-organic composite laminated film 7 on the outer peripheral surface of the cladding 6b was immersed in distilled water 26 contained in a container 25 as shown in FIG. 3, instead of being embedded in the concrete 29 shown in FIG. 2. Next, aqueous sodium chloride solutions of predetermined concentrations were added to the distilled water 26 in sequence to turn the distilled water 26 into an aqueous sodium chloride solution, while varying the concentration.

Next, 5 minutes after adding the sodium chloride aqueous solution of a predetermined concentration, light with a wavelength of 425 nm was irradiated from the light source 22 onto the core 5a of the optical transmission section 2 of the heterocore section 3 immersed in the sodium chloride aqueous solution of each concentration. The reflected light totally reflected at the interface between the cladding 6b of the heterocore section 3 and the outside world was emitted from the core 5a of the optical transmission section 2, and the reflected light received by the detection device 23 was analyzed by the analysis device 24.

Next, the loss in the intensity of the propagated light (reflected light) when the heterocore portion 3 is immersed in the sodium chloride aqueous solution (hereinafter abbreviated as "loss in the propagated light intensity") was calculated using the following formula (1) for each concentration of the sodium chloride aqueous solution, relative to the propagated light (reflected light) intensity when the heterocore portion 3 is in air. The results are shown in Table 1 and Figure 4.

In Table 1, the values for the examples and comparative examples are the loss of propagation light intensity (dB). In Figure 4, the loss of propagation light intensity is shown as "Propagation light intensity (dB) 5 minutes after addition of NaCl."

Loss of propagation light intensity (dB)=10×log ₁₀ (propagation light intensity when the heterocore portion 3 is immersed in the sodium chloride aqueous solution 26/propagation light intensity when the heterocore portion 3 is in air) (1)
Comparative Example
In this comparative example, 100 ml of distilled water was first boiled, and 1 ml of a 1% by mass aqueous solution of citric acid and 1 ml of a 1% by mass aqueous solution of tetrachloroauric acid (III) were added thereto, followed by stirring for 15 minutes while still boiling to prepare a gold nanoparticle suspension. The gold nanoparticle suspension contains gold nanoparticles with particle diameters ranging from 5 to 100 nm and negatively charged surfaces.

Next, 50 milliliters of a 1% by mass aqueous solution of acetic acid was heated to a temperature of 40°C, 0.5 milliliters of 3-aminopropyltrimethoxysilane as a silane coupling agent was added, and the heterocore portion of the optical fiber 4 was immersed in the solution for 2 minutes while gently stirring. Next, the heterocore portion 3 was heated at a temperature of 110°C for 2 hours, and amino groups were introduced onto the surface of the cladding 6b of the heterocore portion 3.

Next, the heterocore portion 3 with amino groups introduced on the surface of the cladding 6b was immersed in pure water to protonate the amino groups.

Next, the heterocore portion 3, in which the amino groups introduced on the surface of the clad 6b were protonated, was immersed in the gold nanoparticle suspension prepared in this comparative example, and a gold nanoparticle layer was formed on the outer surface of the clad 6b.

Next, the heterocore portion 3 with the gold nanoparticle layer formed on the outer peripheral surface of the clad 6b was immersed in a vinylpyrrolidone-N,N-dimethylaminoethyl methacrylic acid copolymer diethyl sulfate aqueous solution with a concentration in the range of 20 millimoles/liter, pulled out and washed with pure water, and then immersed in a polyvinyl potassium sulfate aqueous solution with a concentration in the range of 20 millimoles/liter as a polyanion aqueous solution, pulled out and washed with pure water 20 times, forming an alternating laminated film consisting of 20 layers of polycation layers and 20 layers of polyanion layers stacked alternately. As a result, a salt sensor device was obtained that had only one gold nanoparticle layer on the outer peripheral surface of the clad 6b and the alternating laminated film covering the gold nanoparticle layer.

Next, the loss in the transmitted light intensity for each concentration of the sodium chloride aqueous solution was calculated in exactly the same manner as in the above example, except that the salt sensor device obtained in this comparative example was used. The results are shown in Table 1 and Figure 4.

表１に示す数値から、伝搬光強度の損失の最大値と最小値との差で示される変化幅を算出した。前記変化幅は、実施例では、
（－２．２１４）－（－２．６２８６）＝０．４１４６（ｄＢ）
であり、比較例では、
（－２．０３９５）－（－２．３１１３）＝０．２７１８（ｄＢ）
である。

From the values shown in Table 1, the variation width indicated by the difference between the maximum and minimum values of the loss in the propagation light intensity was calculated.
(-2.214)-(-2.6286)=0.4146(dB)
In the comparative example,
(-2.0395)-(-2.3113)=0.2718(dB)
It is.

Therefore, it is clear that the sensitivity of the salt sensor device 1 of the embodiment is increased by 0.4146/0.2718 = 1.53 (times) compared to the salt sensor device of the comparative example.

1...salt sensor device, 2...light transmission section, 3...heterocore section, 4...optical fiber, 5a, 5b...core, 6a, 6b...clad, 7...inorganic-organic composite laminated film.

Claims (5)

A salt sensor device comprising an optical fiber including a light transmitting section having a core and a clad, and a hetero core section having a core and a clad respectively connected to the core and the clad of the light transmitting section, the core of the hetero core section having a smaller diameter than the core of the light transmitting section,
a plurality of metal nanoparticle layers and a plurality of polyion layers covering the metal nanoparticle layers are alternately laminated on an outer peripheral surface of the clad of the heterocore portion ,
The polyion layer is made of a polycation selected from the group consisting of vinylpyrrolidone-N,N-dimethylaminoethyl methacrylic acid copolymer diethyl sulfate, chitosan, diethylaminoethyl dextran, diallylamine hydrochloride polymer, methyldiallylamine hydrochloride polymer, diethylaminoethyl cellulose, and dextran chloride hydroxypropyltrimethylammonium ether, or a polyanion selected from the group consisting of polyglutamic acid and its salts, carboxymethyl dextran, carboxymethyl cellulose, hyaluronic acid and its salts, and alginic acid and its salts. The salt sensor device according to claim 1, characterized in that the inorganic-organic composite laminated film is formed by alternately laminating 2 to 2000 layers of the metal nanoparticle layers and the polyion layers. The salt sensor device according to claim 1 or 2, wherein the metal nanoparticles constituting the metal nanoparticle layer have a particle diameter in the range of 5 to 100 nm. The salt sensor device according to claim 3, wherein the metal nanoparticles are gold nanoparticles. A method for manufacturing a salt sensor device comprising an optical fiber including a light transmitting section having a core and a clad, and a hetero core section having a core and a clad respectively connected to the core and the clad of the light transmitting section, the core of the hetero core section having a smaller diameter than the core of the light transmitting section,
a step of forming a metal nanoparticle layer on the outer peripheral surface of the heterocore portion, and then repeating an operation of forming a polyion layer on the metal nanoparticle layer at least twice to form an inorganic-organic composite laminated film in which a plurality of metal nanoparticle layers and polyion layers covering the metal nanoparticle layers are alternately laminated ,
The method for manufacturing a salt sensor device is characterized in that the polyion layer is made of a polycation selected from the group consisting of vinylpyrrolidone-N,N-dimethylaminoethyl methacrylic acid copolymer diethyl sulfate, chitosan, diethylaminoethyl dextran, diallylamine hydrochloride polymer, methyldiallylamine hydrochloride polymer, diethylaminoethyl cellulose, and dextran chloride hydroxypropyltrimethylammonium ether, or a polyanion selected from the group consisting of polyglutamic acid and its salts, carboxymethyl dextran, carboxymethyl cellulose, hyaluronic acid and its salts, and alginic acid and its salts .

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