JP2009150749A - Surface plasmon sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively achieve the attachment of an objective substance to be detected and enhance sensitivity. <P>SOLUTION: A surface plasmon sensor makes an objective substance attach on a surface plasmon element equipped with a metal thin film 20 to which a plurality of periodic apertures has been formed on a dielectric substrate 10 and detects a change of transmission light 32 transmitting through the apertures 21 by irradiating the surface plasmon element with light depending on the attaching objective substance, in which through-holes 11 connecting with the apertures 21 are respectively provided at each site corresponding to the apertures 21 of the dielectric substrate 10 and a plurality of flow passages flowing the fluid 41 containing the objective substance is formed by these apertures and the through-holes 11 connecting with them. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a sensor using the effect of surface plasmons.

In the metal, free electrons collectively vibrate to generate a dense wave called a plasma wave. And the thing which quantized this density wave generated on the metal surface is called surface plasmon. Conventionally, various surface plasmon sensors that quantitatively analyze a substance in a sample using a phenomenon in which the surface plasmon is excited by a light wave have been proposed. Among them, one that uses a system called Kretschmann configuration is well known (for example, see Patent Document 1).
A surface plasmon sensor using the above system is basically a prism, a metal film formed on one surface of the prism and brought into contact with a sample, a light source for generating a light beam, a light beam passing through the prism, and the prism and the metal film. And an optical system that makes the incident light so that various incident angles can be obtained with respect to the interface with the optical detector, and a light detection means that can detect the intensity of the light beam totally reflected at the interface between the prism and the metal film at various incident angles. Is provided.

  In order to obtain various incident angles as described above, a so-called goniometer that rotates a light beam irradiation system is used (for example, see Patent Document 2), or the light beam is incident at various angles. An optical system is used in which a relatively thick beam is incident so as to be converged at the interface between the prism and the metal film so as to include a component. In the former case, a light beam whose reflection angle changes with the deflection of the light beam is detected by a small photodetector that moves in synchronization with the deflection of the light beam, or by an area sensor that extends along the direction of change of the reflection angle. can do. On the other hand, in the latter case, it can be detected by an area sensor extending in a direction in which each light beam reflected at various reflection angles can be received.

  When the light beam is incident on the metal film at an incident angle θ equal to or greater than the total reflection angle, a “bleeding wave” called an evanescent wave is generated in the metal film on the reflecting surface. The evanescent wave has an electric field distribution in the sample in contact with the metal film, and surface plasmons are generated at the interface between the metal film and the sample. When the wave vector of the evanescent wave generated by the incidence of the p-polarized light beam on the metal film is equal to the wave vector of the surface plasmon described above, when the wave number matching is established, both are in a resonance state and the energy of the light is changed to the surface plasmon. The plasmon is excited by moving to. At this time, the intensity of the totally reflected light is significantly reduced due to the transfer of light energy.

Therefore, in the surface plasmon sensor, the light intensity incident on the metal film at various incident angles θ is measured by measuring the intensity of the light beam totally reflected by the metal film, so that the reflection intensity is remarkably reduced. An incident angle θ _sp (total reflection elimination angle) at the time of occurrence is obtained, and the resonance wave number K _sp is obtained from the total reflection elimination angle θ _sp and the wave number vector K _{1 of the} incident light.
K _sp = K ₁ sinθ _sp
Guided by the relationship. If the wave number K _sp of the surface plasmon can be known, the dielectric constant of the sample can be determined. That is, when the angular frequency of the surface plasmon is ω, the speed of light in vacuum is c, and the dielectric constants of the metal and the sample are ε _m and ε _s , respectively, the following relationship is established.

K _sp (ω) = (ω / c) √ (ε _m (ω) · ε _s / (ε _m (ω) + ε _s ))
If the dielectric constant ε _s of the sample is known, the concentration of the specific substance in the sample can be known based on a predetermined calibration curve or the like, so that by knowing the total reflection elimination angle θ _{sp at} which the reflected light intensity decreases, The specific substance can be quantitatively analyzed.
On the other hand, in recent years, it has been discovered that when an opening having a size smaller than a wavelength is periodically provided in a metal thin film, the transmittance of light transmitted through the opening becomes equal to or higher than the opening ratio (see, for example, Patent Document 3). There is a correlation between the aperture period and the wavelength at which the transmittance reaches a peak, and it is considered that the transmittance is enhanced as a result of the interaction between the surface plasmon and the incident light. Patent Document 3 proposes a structure in which periodic circular openings are arranged in a metal thin film or a slit array structure. There has also been proposed a method of amplifying transmitted light passing through an opening by providing a periodic ring-shaped groove around a single opening (see, for example, Patent Document 4). Furthermore, as a means for utilizing such a surface plasmon enhancement effect, a surface having a structure in which a periodic ring-shaped groove is formed in a dielectric film, a metal film is formed on the surface, and an opening is provided in the metal film A plasmon sensor has been proposed (see, for example, Patent Document 5).
JP-A-6-167443 JP-A-6-50882 Japanese Patent No. 3008931 JP 2001-291265 A JP 2005-308658 A

When using a system called Kretschmann arrangement, which is the mainstream in conventional surface plasmon sensors using metal thin films, the measurement accuracy depends on the rotation angle accuracy and resolution of the sample, so precise (high sensitivity) measurement is required. There was a problem that the more it tried to do, the larger the optical system, such as the goniometer and the parallelism of the light beam.
On the other hand, the surface plasmon sensor described in Patent Document 5 does not require a large optical system and can be downsized. However, the surface plasmon sensor described in Patent Document 5 performs quantitative analysis of a target substance by detecting an intensity change of light (near-field light) due to adhesion of the target substance (specimen). Efficient adhesion of the target substance to the periodic uneven pattern of the metal film that causes plasmon resonance is required.

However, in Patent Document 5, since the periodic uneven pattern is a concentric groove, it is not easy to effectively flow a fluid (sample) containing the target substance in such a concentric minute groove. In other words, it was difficult to attach the target substance efficiently, and in that respect, it was not an optimal structure.
In view of these problems, an object of the present invention is to provide a surface plasmon sensor that can efficiently capture and attach a target substance and can improve detection sensitivity as compared with the conventional one. It is in.

According to the first aspect of the present invention, a target substance is attached to a surface plasmon element comprising a metal thin film having a plurality of periodic openings formed on a dielectric substrate, and the surface plasmon element is irradiated with light. In a surface plasmon sensor that detects a change in transmitted light that passes through an opening in accordance with adhesion of a target substance, a through hole that is connected to the opening is provided at each position corresponding to the opening of the dielectric substrate. And a plurality of flow paths through which a fluid containing the target substance flows are formed by the through holes connected to it.
According to the invention of claim 2, a target substance is attached to a surface plasmon element comprising a metal thin film having a plurality of periodic openings formed on a dielectric, and the surface plasmon element is irradiated with light to open the opening. In a surface plasmon sensor that detects a change in transmitted light that passes through a surface according to adhesion of a target substance, a dielectric is constituted by a fluid-permeable film, and the target passes through the fluid-permeable film and the plurality of openings. The fluid containing the substance is configured to flow.

In the invention of claim 3, in the invention of claim 1 or 2, L ′ = L / P when the thickness of the metal thin film is L and the period of the opening is P, and the unit area A of the film surface of the metal thin film The ratio L ′ / S ′ with S ′ = S / A when the area of the opening included in S is S satisfies L ′ / S ′> 1.6.
According to a fourth aspect of the present invention, in the third aspect of the present invention, the opening is a slit having an opening width d, and L ′ / S ′ is equal to a ratio L / d between the film thickness L and the opening width d.
According to a fifth aspect of the present invention, in any one of the first to fourth aspects, a molecular recognition layer is provided on the upper surface of the metal thin film and the side wall surface of the opening.

  According to the present invention, the fluid containing the target substance has a structure that can pass through the periodic opening that exhibits the surface plasmon enhancement effect formed in the metal thin film. Can be efficiently attached, and thus a highly sensitive surface plasmon sensor can be realized.

First, as shown in FIG. 1, a metal thin film 20 is formed on the dielectric substrate 10, and a transmission spectrum in the case where the metal thin film 20 is provided with a periodic opening 21 having a size equal to or smaller than the wavelength of light is shown. A result obtained by the simulation will be described. The opening 21 is a slit in this example, that is, in this example, a slit array is formed in the metal thin film 20 as a periodic opening structure. Two-dimensional FDTD (Finite Difference Time Domain) method calculation was used for the simulation. In FIG. 1, 31 indicates incident light, and 32 indicates transmitted light.
As already described, it has been pointed out that when a periodic opening having a size equal to or smaller than the wavelength of light is provided in a metal thin film, there is a strong correlation between the period and the wavelength of incident light. Therefore, in order to clarify the interaction between the aperture periodic structure and the incident wavelength when a substance adheres to the surface, the optical constant of the metal is constant regardless of the wavelength (refractive index n = 0.05, extinction) in the simulation. The calculation was performed with a coefficient k = 5.0).

FIG. 2 shows a transmission spectrum when the period (arrangement period) of the openings (slits) 21 is 1.0 μm, the opening width of the openings 21 is 0.5 μm, and the film thickness of the metal thin film 20 is 0.3 μm. It is known that the transmittance of an aperture smaller than the wavelength becomes smaller in proportion to (d / λ) ⁴ where d is the aperture width and λ is the wavelength. According to this law, for example, the transmittance at λ = 1.0 μm is on the order of 10 ⁻² , but according to the calculation result, the transmittance is as large as about 0.5. This is considered to be the effect of the surface plasmon pointed out in Patent Document 3 described above.
Unlike conventional surface plasmon sensors that use a uniform metal thin film when the metal thin film has openings, the metal thin film upper surface (front surface), the side wall surface inside the opening, and the upper surface (surface) of the dielectric substrate located in the opening Exposed outside. In order to clarify which part is attached with high sensitivity, the transmission spectrum when the substance is attached only to the upper surface of the metal thin film and when the substance is attached to the upper surface of the metal thin film and the side wall of the opening. Comparison of changes. The period of the opening 21, the opening width, and the film thickness of the metal thin film 20 were the same as those in FIG. The results are shown in FIGS. 3A and 3B, respectively. 3A and 3B are enlarged views of the partial wavelength regions shown in FIGS. 4A and 4B, respectively. Note that FIGS. 3A and 3B and FIGS. 4A and 4B show cases in which the thickness of the substance is 0 nm (no adhesion), 20 nm, and 100 nm.

When the substance is attached to the side wall surface of the opening 21, the change in the transmission spectrum is larger especially on the long wavelength side of 0.9 μm or more, and the substance is made to adhere to the side wall surface of the opening 21. Was found to be necessary for higher sensitivity.
Although the above calculation was performed with a slit array structure, even with a periodic array structure such as a circular opening, the effect of increasing the transmittance due to surface plasmons is observed. The array may use a surface plasmon element formed on the metal thin film 20, and even in this case, it is considered that adhesion of substances to the side wall surface of the opening is indispensable for improving detection sensitivity.

  By the way, in a minute opening whose opening width is equal to or smaller than the wavelength, if the ratio of the opening depth (= thickness of the metal thin film) to the opening width (opening aspect ratio) becomes large, it is simply immersed in a solution or gas. Then, it becomes difficult for a solution or gas to enter the opening. In the present invention, as shown in FIG. 5, through holes 11 connected to the openings 21 are provided in the dielectric substrate 10 at positions corresponding to the openings 21 of the metal thin film 20, respectively, and the openings 21 and the through holes connected to the openings 21 are provided. A plurality of flow paths through which the fluid containing the target substance flows are formed by the holes 11. As a result, the fluid can easily flow through the opening 21, and the target substance can easily adhere to the side wall surface of the opening 21. That is, by using the opening 21 connected to the through hole 11 like a filter, it is possible to efficiently capture and attach the target substance in the fluid. In FIG. 5, reference numeral 12 denotes a diaphragm formed in the dielectric substrate 10, and in this example, a through hole 11 is formed in the diaphragm 12. In FIG. 5, reference numeral 41 indicates a fluid (liquid) containing the target substance, and an arrow 42 indicates the flow of the fluid 41.

On the other hand, FIG. 6 shows another configuration according to the present invention. In this example, a gas in which the metal thin film 20 is formed (holding the metal thin film 20) is a gas that is a form of a fluid permeable film. A permeable membrane 50 is formed, and a gas containing a target substance flows through the gas permeable membrane 50 and the opening 21 of the metal thin film 20.
In this example, when a gas containing the target substance is passed through the opening 21, the gas permeates, but the target substance stays inside the opening 21. Therefore, the probability of adhering to the side wall surface of the opening 21 increases, and the target substance in the gas Can be efficiently attached. In FIG. 6, reference numeral 51 denotes a diaphragm formed on the gas permeable film 50, and the metal thin film 20 is formed on the diaphragm 51. Moreover, in FIG. 6, 60 shows a board | substrate and the arrow 43 shows the flow of the gas containing the target substance (solid). An arrow 44 indicates the flow of gas that has passed through the gas permeable membrane 50.

An embodiment in which the gas permeable membrane 50 in the example shown here is replaced with a liquid permeable membrane to allow the liquid containing the target substance to pass therethrough is also possible.
Specific examples will be described below.
[Example 1]
A silicon substrate was used as the dielectric substrate, and a 1.8 mm square aperture diaphragm was formed on the silicon substrate. The diaphragm was formed by using an inductively coupled plasma reactive ion etching apparatus and etching the silicon leaving a thickness of 1 μm.

A thin chromium film was formed as an adhesive layer on the surface opposite to the etched surface of the silicon substrate, and then a gold thin film was formed by sputtering to form a metal thin film. The film thickness was 1 μm. At the position where the diaphragm of the silicon substrate is formed using the focused ion beam etching apparatus, a slit having an opening width of 0.5 μm is formed in the metal thin film with a period of 1.0 μm, and subsequently the diaphragm having a thickness of 1 μm is formed below the slit. A through hole connected to the slit was formed. Thereby, the structure (surface plasmon element) as shown in FIG. 5 is obtained.
This element was cut out to a size of 4 mm square, and placed like a filter in the flow path as shown in FIG. In FIG. 7, 70 indicates a case, 71 indicates an inlet, and 72 indicates an outlet. A liquid containing a small amount of dioxin was allowed to flow through the channel as shown by the arrows with a pressure difference.

After flowing for 10 minutes, the 4 mm square element was taken out and the transmission spectrum in the infrared region was measured with a self-recording spectrophotometer. As a result, the transmission spectrum shape was changed as compared with the transmission spectrum before flowing the liquid.
[Example 2]
A plurality of the same elements as in Example 1 were produced on a silicon substrate.
A polymer film capable of forming molecular bonds with dioxin molecules was formed on the surface using molecular imprinting. The molecular imprinting method is a technique for imprinting (engraving) the shape of a molecule (target molecule) to be recognized by a polymer, and performing molecular recognition using the resulting hole. First, target molecule A (in this case, dioxin) and a molecule B having a polymerizable group and a site that specifically binds to this molecule are polymerized together with a crosslinking agent, and then target molecule A is released from within the polymer. By removing, a binding site complementary to the target molecule A is built in the imprint polymer. That is, the functional group derived from the molecule B is arranged in the matrix of the polymer derived from the crosslinking agent so as to recognize the characteristic functional group of the target molecule A according to the shape of the target molecule A. The area around the localized functional group becomes a site that specifically binds to the target molecule A (specific binding site), recognizes a specific molecule, and binds to the molecule. As a material of this molecular recognition film (molecular recognition layer), pyrrole was used, and a polymer film was uniformly formed on the upper surface of the metal thin film and the side wall surface of the opening by a spray coating method.

Two of these elements were cut out and installed in the flow path as shown in FIG. 7 in the same manner as in Example 1, and a liquid containing a small amount of dioxin and a liquid containing a small amount of trihalomethane were allowed to flow, respectively. Changes were observed with a self-recording spectrophotometer. As a result, it was observed that the transmission spectrum changed with time in the liquid containing a small amount of dioxin, but no change was observed in the transmission spectrum in the liquid containing trihalomethane.
[Example 3]
A polyimide film having a thickness of 10 μm was formed on the silicon substrate as a gas permeable film by vapor deposition polymerization. After forming a thin chromium film as an adhesive layer on the polyimide film surface, a gold thin film was formed by sputtering to form a metal thin film. The film thickness was 1 μm. Using a focused ion beam etching apparatus, slits with an opening width of 0.5 μm were formed in a metal thin film with a period of 1.0 μm. Then, mask the silicon substrate surface opposite to the polyimide film formation surface, partially remove the silicon substrate using an ion milling device, and further etch the polyimide film to leave a thickness of 0.5 μm. A polyimide diaphragm having an opening size of 10 to 100 μm square was formed. Thereby, a structure (surface plasmon element) as shown in FIG. 6 is obtained.

This element was cut out and installed in the flow path as shown in FIG. Nitrogen gas was used as a carrier gas in the flow path, and a small amount of ethanol was mixed to flow with a pressure difference.
After flowing for 10 minutes, the device was taken out and the transmission spectrum in the infrared region was measured with a self-recording spectrophotometer. As a result, the transmission spectrum shape was changed as compared with the transmission spectrum before flowing the gas.
[Comparative example]
A 4 mm square element was fabricated using the same method as in Example 1 except that the diaphragm and the through hole were not formed on the silicon substrate.

This element was installed in the flow path as shown in FIG. 7, and a liquid containing a small amount of dioxin was allowed to flow with a pressure difference. In this case, a required gap through which liquid flows is provided between the case 70 and the element.
In order to obtain the same change in the transmission spectrum shape as in Example 1, 40 minutes were required as the time for flowing the liquid.
As described above, according to the present invention, it is possible to realize a surface plasmon sensor in a transmission type optical system, and a structure that allows a fluid containing a target substance to pass through an opening. In other words, the target substance can be attached to the inner side wall surface of the opening in a shorter time, and in this respect, a highly sensitive surface plasmon sensor can be obtained. In addition, the amount of a specific molecule can be detected by forming a molecule recognition layer as in Example 2.

Next, the relationship between the thickness of the metal thin film and the opening width will be described.
8A shows the structure shown in FIG. 1 described above, in which the period P of the opening (slit) 21 is 1.0 μm, the opening width d is 0.5 μm, and the film thickness L of the metal thin film 20 is 0.1 μm, 0. The transmission spectra when 3 μm, 0.6 μm, 0.9 μm, and 1.2 μm are shown. From FIG. 8A, it is clear that as the film thickness L increases, in other words, as the ratio L / d between the film thickness L and the opening width d increases, the transmitted light intensity decreases rapidly in the region where the wavelength is 1 μm or more. Became. In other words, when L / d (aspect ratio) is 0.2 (when the film thickness L is 0.1 μm), the transmitted light intensity reaches a peak at a wavelength of 1.04 μm, but in a longer wavelength region. Although several peaks are shown, when L / d is 2.4 (when the film thickness L is 1.2 μm), the intensity of transmitted light rapidly decreases as the wavelength increases after the peak of 1.04 μm. Yes.

In order to clarify the relationship between the sharpness of the peak at a wavelength of 1.04 μm and the ratio L / d between the film thickness L and the opening width d, the transmitted light intensity when the wavelength is 1.4 μm with respect to L / d The relationship of the ratio of transmitted light intensity in the case of 1.04 μm was plotted as shown in FIG. 8B. It can be seen that the transmitted light intensity ratio rapidly increases in the region where L / d is 1.6 or more.
As a mechanism in which the ratio of transmitted light intensity suddenly increases in a region where the ratio L / d between the film thickness L and the opening width d is 1.6 or more, the film is larger than the thickness of the outermost surface layer in which surface plasmons are involved. As the thickness L increases, the properties of the metal as a bulk come to be involved, and it is assumed that the transmittance increasing effect due to the effect of the surface plasmon is less likely to be exhibited in a long wavelength region.

FIG. 9 shows the result of examining the change in the transmission spectrum when a substance adheres to the upper surface of the metal thin film 20 and the side wall surface of the opening 21 when the film thickness L of the metal thin film 20 is 1.2 μm. It was found that the wavelength curve of the region (wavelength 1.04 to 1.2 μm) in which the transmittance changes sharply as the adhesion thickness increases. Therefore, if the transmitted light intensity is monitored at a specific wavelength in this region, for example, a wavelength of 1.1 μm, it is possible to monitor how the transmittance increases as the target substance adheres.
From the above, in the configuration of the present invention shown in FIGS. 5 and 6, the ratio L / d between the film thickness L of the metal thin film 20 and the opening width d of the opening (slit) 21 is
L / d> 1.6 (1)
It can be said that it is preferable to set so as to satisfy.

The expression (1) shows the condition when the opening 21 is a slit (elongated rectangular opening). However, the opening is not limited to the slit as described above, and may be a circular opening. It is preferable to set to satisfy the following condition instead of the expression (1).
That is, L ′ = L / P when the thickness of the metal thin film is L and the period of the opening is P, and S ′ when the area of the opening included in the unit area A of the film surface of the metal thin film is S. = S / A ratio L ′ / S ′ is
L ′ / S ′> 1.6 (2)
To satisfy.

L ′ / S ′ in the equation (2) is a general concept in which the aspect ratio of the opening is enlarged. If the opening is a periodic slit (opening width d),
L ′ / S ′ = (L / P) / (d / P) = L / d
L ′ / S ′ becomes equal to L / d in the equation (1).

The figure for demonstrating the model of the surface plasmon sensor used for simulation. The figure which shows the transmission spectrum of the model of FIG. FIG. 1 shows a transmission spectrum when a substance adheres to the model of FIG. 1 (thickness of the metal thin film = 0.3 μm). When A is attached only to the upper surface of the metal thin film, B denotes the upper surface of the metal thin film and the side wall of the opening. If attached. 3A is an enlarged view of a partial wavelength region of FIG. 3A, and B is an enlarged view of a partial wavelength region of FIG. 3B. The figure for demonstrating the structure of one Example of this invention. The figure for demonstrating the structure of the other Example of this invention. The figure which shows the state in which the structure shown in FIG. 5 was installed in the flow path. A is a figure which shows the transmission spectrum at the time of changing the film thickness of a metal thin film in the model of FIG. B is a graph showing the relationship between the ratio L / d between the film thickness L and the opening width d and the transmitted light intensity ratio at wavelengths of 1.4 μm and 1.04 μm. In the model of FIG. 1, the figure which shows the transmission spectrum when a substance adheres to the metal thin film upper surface and the side wall surface of an opening (film thickness of a metal thin film = 1.2 micrometers).

Claims (5)

A target substance is attached to a surface plasmon element comprising a metal thin film having a plurality of periodic openings formed on a dielectric substrate, and the object of transmitted light that is transmitted through the opening by irradiating the surface plasmon element with light. In the surface plasmon sensor that detects changes according to the adhesion of substances,
Through-holes connected to the openings are provided at positions corresponding to the openings of the dielectric substrate, and a plurality of flow paths through which the fluid containing the target substance flows are configured by the openings and the through-holes connected to the openings. Surface plasmon sensor characterized by being made. A target substance is attached to a surface plasmon element comprising a metal thin film having a plurality of periodic openings formed on a dielectric, and the surface plasmon element is irradiated with light and transmitted through the opening. In the surface plasmon sensor that detects changes according to the adhesion of
A surface plasmon sensor, wherein the dielectric is constituted by a fluid-permeable membrane, and a fluid containing the target substance flows through the fluid-permeable membrane and the plurality of openings. The surface plasmon sensor according to claim 1 or 2,
L ′ = L / P where L is the thickness of the metal thin film and P is the period of the opening, and S is the area of the opening included in the unit area A of the film surface of the metal thin film. The ratio L ′ / S ′ with S ′ = S / A is
L '/ S'> 1.6
A surface plasmon sensor characterized by satisfying The surface plasmon sensor according to claim 3,
The opening is a slit having an opening width d;
The surface plasmon sensor, wherein L ′ / S ′ is equal to a ratio L / d between the film thickness L and the opening width d. The surface plasmon sensor according to any one of claims 1 to 4,
A surface plasmon sensor, wherein a molecular recognition layer is provided on an upper surface of the metal thin film and a side wall surface of the opening.

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