WO2012086236A1

WO2012086236A1 - Photoenhancement element

Info

Publication number: WO2012086236A1
Application number: PCT/JP2011/064095
Authority: WO
Inventors: 三津夫川崎; 森本　幸裕; 昌博川▲崎▼; 祐山崎
Original assignee: 国立大学法人京都大学; ウシオ電機株式会社
Priority date: 2010-12-22
Filing date: 2011-06-21
Publication date: 2012-06-28
Also published as: JP2012132804A

Abstract

Provided is a photoenhancement element that obtains a sufficiently high photoenhancement effect. The photoenhancement element comprises: a substrate; a highly reflective layer formed on the substrate; a dielectric layer formed on the highly reflective layer; and an enhanced electromagnetic-field-forming layer formed on the dielectric layer and resulting from numerous metal microparticles. The thickness of the dielectric layer is at least 50 nm. The highly reflective layer is preferably configured from a metal selected from silver, gold, aluminum, and copper, and furthermore, the surface of the highly reflective layer on the dielectric layer side is preferably a rough surface. Also, the enhanced electromagnetic-field-forming layer preferably has a configuration wherein the metal microparticles are arrayed randomly.

Description

Light enhancement element

The present invention relates to a light enhancement element using localized surface plasmons of fine metal particles, and more specifically, to enhance the intensity of luminescence (fluorescence) emitted from a luminescent chemical species (fluorescent substance) by irradiation with excitation light, or The present invention relates to a light enhancement element capable of enhancing the intensity of Raman scattered light obtained from a Raman active chemical species by excitation light irradiation.

For example, a fluorescence analysis method for performing qualitative and quantitative analysis of an analysis object by exciting the analysis object with excitation light irradiation to generate fluorescence and analyzing the fluorescence is one of high-sensitivity analysis methods, It plays an important role in trace analysis.

For example, the fluorescence emitted from a fluorescent material not only reflects the characteristics of the fluorescent material itself, but is also easily affected by the environment around the fluorescent material. Therefore, the fluorescent material is used as a labeling agent (biosensor) for tissue and DNA in the body. It is also used for appraisal and is used in a wide range of fields including biomedical chemistry.

In addition, a fluorescent material excited by a field effect is applied to an organic EL element that has been remarkably developed as one of thin display devices.

Fluorescence is widely used in this way, but the fluorescence intensity varies depending on the substance. Usually, if the fluorescence of a substance having a weak fluorescence intensity can be observed, the application range is expected to be further expanded. For this reason, research aimed at enhancing the fluorescence of substances has been actively conducted.

As a fluorescence enhancement method, Patent Document 1 discloses a technique using an island-like sputtered film made of flat silver particles having an average particle size in the submicron region and a height of several tens of nanometers. According to this fluorescence enhancement method, significant emission enhancement can be obtained even in a chemical species located in a position in direct contact with the silver particle surface. Therefore, it is said that the quenching action by the metal is remarkably suppressed, and therefore, significant emission enhancement can be obtained even in the case of a chemical species that is relatively easy to shine.

On the other hand, Raman spectroscopy, which obtains a spectrum of Raman scattered light by dispersing Raman scattered light obtained by irradiating an analysis object with light of a single wavelength (laser light), is used for identification of substances. Usually, Raman scattered light obtained from an analysis object is difficult to detect with high sensitivity because the signal is weak.
In recent years, research on surface-enhanced Raman scattering (SERS) that increases the signal intensity of Raman-scattered light using the above-described light-enhancing element (using localized surface plasmons of metal fine particles) has been promoted. .

JP 2007-139540 A

However, the conventional light enhancement technology using a light-enhancing element using localized surface plasmons is still insufficient for practical use, and sufficient intensity is required unless high-power and large-scale laser light is irradiated as excitation light. The measurement light (fluorescence, Raman scattered light) cannot be obtained.
The present invention has been made based on the above-described circumstances, and an object thereof is to provide a light enhancement element capable of obtaining a sufficiently high light enhancement effect.

The light enhancement element of the present invention includes a substrate, a highly reflective layer formed on the substrate, a dielectric layer formed on the highly reflective layer, and a number of metals formed on the dielectric layer. It consists of an enhanced electromagnetic field forming layer made of fine particles, and the dielectric layer has a thickness of 50 nm or more.

In the light enhancement element of the present invention, the highly reflective layer is preferably made of a metal selected from silver, gold, aluminum, and copper.

In the light enhancement element of the present invention, it is preferable that the surface on the dielectric layer side of the high reflection layer is a rough surface.

Furthermore, in the light enhancement element of the present invention, it is preferable that the enhanced electromagnetic field forming layer has a configuration in which metal fine particles are randomly arranged.

According to the light enhancement element of the present invention, a highly reflective layer, a dielectric layer, and an enhanced electromagnetic field forming layer are formed in a multilayer structure formed on a substrate, and the thickness of the dielectric layer is set to a specific size. As a result, a strong enhanced electromagnetic field can be formed in the vicinity of each metal fine particle constituting the enhanced electromagnetic field forming layer, so that a sufficiently high light enhancement effect can be obtained.
Further, even when an extremely low output and small light source such as an LD (semiconductor laser) or an LED (light emitting diode) is used as the excitation light source, the desired light enhancement effect can be obtained.
Furthermore, the enhanced electromagnetic field forming layer itself does not have to have a structure (nanostructure) in which the light enhancement effect by the localized surface plasmon is dominant (dominant). The structure itself is simplified by eliminating the structural constraints of conventional light-enhancing elements using the mechanism based on the interaction between the localized surface plasmons of fine particles and the surface plasmon polaritons between the metal fine particles and the highly reflective layer. It is easy to handle and can be produced advantageously in terms of cost.

It is a schematic diagram which shows the outline of a structure in an example of the light enhancement element of this invention. It is a schematic diagram for demonstrating the mechanism of the optical enhancement in the optical enhancement element of this invention. In an experiment example, it is explanatory drawing which shows the outline of a structure of the measurement system constructed | assembled in order to perform a fluorescence measurement and a Raman scattered light measurement. 6 is a graph showing an example of a fluorescence spectrum measured using a light enhancement element produced in Experimental Example 2. It is a graph which shows an example of the Raman spectrum measured using the optical enhancement element produced in Experimental example 2.

Hereinafter, embodiments of the present invention will be described in detail.
FIG. 1 is a schematic diagram showing an outline of a configuration in an example of the light enhancement element of the present invention.
The light enhancement element 10 according to this embodiment includes, for example, a flat substrate 15, a high reflection layer 20 formed on the surface of the substrate 15, and a dielectric formed on the surface of the high reflection layer 20. The multilayer structure includes a layer 30 and an enhanced electromagnetic field forming layer 40 formed of a large number of metal fine particles 41 formed on the surface of the dielectric layer 30. For example, the light enhancement element 10 enhances light emission (for example, fluorescence) by irradiation of excitation light with respect to a luminescent chemical species supported directly or via a spacer on the surface of the enhanced electromagnetic field forming layer 40 (metal fine particles 41). Alternatively, the Raman scattered light by the excitation light irradiation to the Raman active chemical species carried directly or via a spacer on the surface of the metal fine particle 41 is enhanced.

The material of the board | substrate 15 is not specifically limited, For example, glass, ceramics, resin, a metal etc. can be illustrated. As will be described later, when heat treatment (for example, heating at 200 ° C. or higher) is performed in the manufacturing process of the light enhancement element 10, it is preferable to have heat resistance such as glass or polyimide resin.
Further, the surface of the substrate 15 on the high reflection layer 20 side does not have to be a flat surface, and may be a curved surface, a small spherical surface, or the like, for example.

The highly reflective layer 20 is highly reflective in, for example, the entire visible region or a wavelength region of at least 500 nm or more, specifically, a wavelength region of excitation light that excites an analyte such as a luminescent chemical species and a Raman active chemical species. It is preferable that the material has a ratio, and specific examples include silver, gold, aluminum, and copper.
The thickness of the highly reflective layer 20 is preferably large enough to obtain a reflectance of 90% or more in the entire visible region or in a wavelength region of 500 nm or more.
The surface of the highly reflective layer 20 on the dielectric layer 30 side may be an optically smooth surface, but is preferably a rough surface. Specifically, for example, a rough surface having a surface roughness Ra of about 10 to 30 nm is preferable. Thereby, as shown in the result of the experimental example to be described later, a higher light enhancement effect can be obtained.

The dielectric layer 30 is made of a material that is transparent to excitation light. As will be described later, heat treatment (for example, heating at 200 ° C. or higher) is performed in the manufacturing process of the light enhancement element 10. It is preferable that it is comprised with the material which has heat resistance. Specific examples include SOG (spin-on-glass) materials mainly composed of silicon oxide, siloxane-based materials such as tetraethoxysilane (TEOS) and dimethylsiloxane. Further, when the dielectric layer 30 is composed of a fired body (SOG film) obtained by firing an SOG material, the surface of the SOG film has a relatively strong hydrophobicity, so that an alkali treatment is performed as necessary. Or it is preferable that the hydrophilic treatment by plasma treatment was made.
The thickness of the dielectric layer 30 is 50 nm or more, more preferably 60 to 90 nm. When the thickness of the dielectric layer 30 is too small, a sufficient light enhancement effect cannot be obtained as shown in the result of an experimental example described later.

The enhanced electromagnetic field forming layer 40 is preferably composed of a single layer film composed of a large number of metal fine particles 41.
As the metal fine particles 41 constituting the enhanced electromagnetic field forming layer 40, for example, silver ultrafine particles can be preferably used, but any material that can be excited by irradiation with excitation light to excite surface plasmons, such as gold, Copper or the like may be used.
The metal fine particles 41 have a size equal to or smaller than the wavelength of the excitation light, for example, a cross-sectional particle size (a horizontal dimension in FIG. 1) d of 30 to 400 nm and a thickness t of 5 to 70 nm. Those having shape anisotropy such as a shape can be preferably used. Here, it is desirable that all of the metal fine particles 41 have a uniform size and shape, but there may be some variation in size and shape.

In the enhanced electromagnetic field forming layer 40, the metal fine particles 41 having the above-mentioned size (cross-sectional particle size, thickness) are, for example, two-dimensionally randomly, specifically, for example, a density of 10 ⁸ to 10 ¹⁰ particles / cm ² . And are arranged in an independent state without contacting each other.
Further, the enhanced electromagnetic field forming layer 40 may be configured such that the metal fine particles 41 are regularly arranged.

A method for forming such an enhanced electromagnetic field forming layer 40 is not particularly limited. For example, a method in which a dispersion in which metal fine particles are dispersed in an appropriate solvent is applied by a spin coating method and heated, dipping Then, a heating method, a vacuum deposition method, or the like can be suitably used.

When using this light enhancement element 10, as described above, the analysis object is carried on the enhanced electromagnetic field forming layer 40, specifically, on the surface of the metal fine particle 41 directly or via a spacer. Here, when the analyte is, for example, a fluorescent material, when the concentration of the fluorescent material increases, the excitation energy moves between the fluorescent materials, the probability of a non-radiative process increases, and the intensity of the fluorescence decreases. End up (self-quenching effect). Therefore, in order to reduce the self-quenching effect as much as possible, it is desirable that the fluorescent material be supported at the lowest possible concentration (thin). And spectroscopic analysis is performed by detecting the fluorescence emitted from an analysis object by excitation light irradiation with a suitable spectroscope.

The spacer is for adjusting the distance between the surface of the metal fine particle 41 and the analysis target, and can be composed of a dielectric such as an SOG film.
The spacer thickness is preferably, for example, 10 nm or less, and the light enhancement effect is maximized particularly when the thickness is about 1 nm or less.

Thus, according to the above-described light enhancement element 10, the highly reflective layer 20, the dielectric layer 30, and the enhanced electromagnetic field forming layer 40 have a multilayer structure formed on the substrate 15. By setting the thickness to 50 nm or more, a strong enhanced electromagnetic field can be formed in the vicinity of each metal fine particle 41 constituting the enhanced electromagnetic field forming layer 40, so that it is clear from the results of the experimental examples described later. In addition, a sufficiently high light enhancement effect can be obtained. The reason for this is that, as shown in FIG. 2, interference occurs between the excitation light and the reflected light of the excitation light by the high reflection layer 20 in the vicinity of each metal fine particle 41 constituting the enhanced electromagnetic field forming layer 40. A field is formed, and it is considered that the enhanced electromagnetic field (region surrounded by a broken line in FIG. 2) due to the localized surface plasmon of the metal fine particle 41 is further enhanced by the action of the interference field. The well-known interference effect alone cannot sufficiently explain the large light enhancement factor shown in the experimental example, and the secondary electromagnetic field itself radiated by the localized surface plasmon of the metal fine particles 41 is a highly reflective layer. It is presumed that the presence of 20 causes a high-order enhancement such as self-amplification, thereby generating a large enhanced electromagnetic field that cannot be imagined from conventional common sense.
Further, even when an extremely low output and small light source such as an LD (semiconductor laser) or an LED (light emitting diode) is used as an excitation light source, for example, fluorescence of a luminescent species, Raman active species, etc. For Raman scattered light, a sufficiently large light enhancement effect can be obtained. For example, in the measurement of Raman scattered light, the Raman spectrum of Raman active species can be measured even when low-power laser light having an energy density of 10 mW / cm ² or less is used as excitation light. Therefore, the desired Raman spectroscopic analysis can be performed with high reliability with a very simple structure such as the measurement system used in the experimental examples described later.
Furthermore, since it is not necessary for the enhanced electromagnetic field forming layer 40 itself to have a structure (nanostructure) in which the light enhancement effect by the localized surface plasmon is dominant (dominant), for example, as means for forming the enhanced electromagnetic field, The structure itself is simplified by eliminating the structural constraints of the conventional light enhancement element using the mechanism based on the interaction between the localized surface plasmon of the metal fine particle and the surface plasmon polariton between the metal fine particle and the highly reflective layer. Are easy to handle, and can be produced advantageously in terms of cost.

Hereinafter, experimental examples performed for confirming the effect of the light enhancement element according to the present invention will be described.

<Experimental example 1>
[Production of light enhancement element (10)]
In accordance with the configuration shown in FIG. 1, six light enhancement elements in which the thickness of the dielectric layer was adjusted according to Table 1 below were produced as follows.
A 1 cm square slide glass was used as the substrate (15), and a silver film as a highly reflective layer (20) was formed on the surface of the slide glass by a standard resistance heating vacuum deposition method. The thickness of the obtained silver film was 0.2 μm or more, and the transmittance was substantially zero in the visible region. It was also found that the measured reflectance shows a high reflectance that is equal to or more than 98% of the theoretical value calculated using the optical constant of bulk silver in almost the entire visible region.
Next, a solution obtained by appropriately diluting a commercially available dimethylsiloxane solution with ethanol is spin-coated on the surface of the silver film at 3000 revolutions, and then heat-treated on a hot plate at 200 to 250 ° C. for several minutes to thereby form a dielectric layer ( 30) was formed as a dielectric film (refractive index: 1.3 to 1.4). The thickness of the dielectric film was appropriately adjusted based on the relationship (calibration curve) between the concentration of the solution used for spin coating and the thickness of the dielectric film to be formed, which was actually measured by measuring the level difference using AFM. Here, when the stock solution of a commercially available dimethylsiloxane solution is spin coated as it is, the maximum film thickness obtained is 180 nm, and when forming a dielectric film with a large film thickness, the above treatment was repeated a plurality of times. . And the hydrophilization process by a plasma process was performed with respect to the surface (upper surface) of the obtained dielectric film.
Then, an acetone dispersion (concentration of about 0.4 wt%) of protective film-free silver nanoparticles (volume average particle size of about 15 nm) is spin-coated on the hydrophilic surface of the dielectric film at 3000 rpm, and then about 250 By heating on a hot plate at 0 ° C. for several minutes, a silver ultrafine particle monolayer film as an enhanced electromagnetic field forming layer (40) was formed. In the obtained silver ultrafine particle monolayer film, the cross-sectional particle diameter (d) of the ultrafine silver particles is about 150 nm on average and the thickness (t) is about 30 nm on average, with a density of 10 ⁸ to 10 ¹⁰ particles / cm ² . It is randomly arranged in a two-dimensional manner in a distributed state. Moreover, it was confirmed by XRD (X-ray diffraction) measurement that each silver ultrafine particle has high crystallinity similar to bulk silver.

[During sample (analysis object)]
A dilute ethanol solution of rhodamine 6G (Rh6G) dye is spin-coated at 3000 revolutions on the surface of the silver ultrafine particle monolayer film in the light enhancement element, thereby supporting the dye molecule on the surface of the silver ultrafine particle in the light enhancement element. I let you. Here, the relationship between the density of the dye molecules carried on the surface of the light enhancement element and the dye concentration of the solution used for spin coating is as follows. When the concentration of rhodamine 6G is 0.3 mM, the density of the dye molecules is 7 × 10 ¹³ pieces / cm ²

[Measurement of fluorescence and Raman scattered light]
For each light enhancement element fabricated as described above, the fluorescence emitted from the sample by irradiation with excitation light is measured by the measurement system having the configuration shown in FIG. 3, and is measured when the same dye is spin-supported directly on the glass. While calculating the increase intensity based on the fluorescence intensity, the Raman scattering intensity from the sample was measured. The results are shown in Table 1 below.
In FIG. 3, reference numeral 50 denotes a xenon lamp with an output of 150 W used as an excitation light source in fluorescence measurement. Light emitted from the xenon lamp 50 is transmitted by a spectroscope “SPG-120S” (manufactured by Shimadzu Corporation) 52. Spectroscopically, the light enhancement element 10 is irradiated with light having a wavelength of 530 nm as excitation light. 53A and 53B are condensing lenses, and 54A is a filter. Reference numeral 51 denotes a He—Ne laser (wavelength 632.8 nm) having an output of less than 1 mW used as an excitation light source in the measurement of Raman scattered light, and is not condensed (energy density is about 300 mW / cm) through the filter 54B. ² ) or the light enhancement element 10 is irradiated as anti-condensation (defocused, energy density of about 10 mW / cm ² or less) excitation light. In this measurement system, the light enhancement element 10 is rotatably provided, and the incident angle of the excitation light can be set. In the measurement of fluorescence, excitation light is incident on the light enhancement element 10 perpendicularly, and fluorescence emitted in a 45 ° angle (polar angle) direction from the dye carried on the light enhancement element 10 is converted into the light enhancement element 10. The light was condensed on the light receiving head 56 of the electronically cooled diode array detector (manufactured by Hamamatsu Photonics) 55 through the filter 54C by a condensing lens (caliber 35 mm) 53C disposed at a position approximately 13 cm away from the surface. In the measurement of Raman scattered light, the Raman scattering is performed by causing excitation light to enter the light enhancement element 10 at an incident angle of 45 ° and being scattered in the 90 ° angle direction by the dye carried on the light enhancement element 10. The light was condensed by the condenser lens 53C onto the light receiving head 56 of the electronically cooled diode array detector 55 via the filter 54C.

By forming a silver ultrafine particle monolayer film on a slide glass (substrate) by the same method as described above, a comparative light enhancement element was prepared, and the fluorescence intensity and the Raman scattering intensity were the same as in Experimental Example 1. As a result, the fluorescence enhancement was about 20 times at the maximum, and the Raman scattering intensity was not measurable.

As is clear from the above results, a silver film (highly reflective layer), a dielectric film (dielectric layer) and a silver ultrafine particle single layer film (enhanced electromagnetic field forming layer) are formed on a slide glass (substrate). It was confirmed that a high fluorescence enhancement effect and a Raman scattering light enhancement effect can be obtained when the multilayer structure is used and the thickness of the dielectric film is 50 nm or more.

<Experimental example 2>
In the manufacturing process of the light enhancement element in Experimental Example 1, the surface of the silver film (high reflection layer) on the dielectric film side was subjected to a roughening treatment by heat treatment, and the surface was optically roughened. Other than that, a light enhancement element having the same configuration as that of the light enhancement element according to Experimental Example 1 was manufactured by the same method as described above. Here, the surface roughness Ra of the surface of the silver film was about 10 to 30 nm by AFM measurement.
Then, the fluorescence intensity and the Raman scattering intensity were measured by the same method as in Experimental Example 1. The results are shown in FIG. 4 and FIG. FIG. 4 is a graph showing an example of the fluorescence spectrum of the light enhancement element produced in Experimental Example 2, and FIG. 5 is a graph showing an example of the Raman spectrum of the light enhancement element produced in Experimental Example 2. FIGS. (A) is due to a light enhancement element having a rough surface on the dielectric film side of the silver film, and (B) is a light enhancement element (experiment) where the surface is an optically smooth surface. Produced in Example 1).

As is clear from the above results, the surface of the silver film (highly reflective layer) on the dielectric film side is roughened, so that the surface of the silver film has an optically smooth surface. It was confirmed that both the fluorescence intensity and the Raman scattering intensity (Raman signal) were higher than those of the device, and a higher light enhancement effect was obtained.

DESCRIPTION OF SYMBOLS 10 Light enhancement element 15 Board | substrate 20 High reflection layer 30 Dielectric layer 40 Augmented electromagnetic field formation layer 41 Metal fine particle 50 Xenon lamp 51 He-Ne laser 52

Spectroscope

53A, 53B,

53C Condensing lens

54A, 54B, 54C Filter 55 Electronic cooling Type diode array detector 56 Light receiving head

Claims

A substrate, a highly reflective layer formed on the substrate, a dielectric layer formed on the highly reflective layer, and an enhanced electromagnetic field forming layer formed of a large number of metal fine particles formed on the dielectric layer. Become
The light enhancement element, wherein the dielectric layer has a thickness of 50 nm or more.
2. The light enhancement element according to claim 1, wherein the highly reflective layer is made of a metal selected from silver, gold, aluminum, and copper.
3. The light enhancement element according to claim 1, wherein a surface of the highly reflective layer on a dielectric layer side is a rough surface.
The light enhancement element according to any one of claims 1 to 3, wherein the enhanced electromagnetic field forming layer is formed by randomly arranging metal fine particles.