An ultra-wideband surface-enhanced Raman scattering chip and its application

Abstract

The present invention discloses an ultra-wideband surface enhanced Raman scattering chip and its application, which includes a substrate layer and a surface enhancement layer from the bottom layer to the top layer in sequence; wherein the surface enhancement layer is provided with a metal periodic micro-nano structure array, the metal periodic micro-nano structure array is composed of periodically distributed independent metal square units, the square units are diamond or square structures with thickness, and the vertex angles between adjacent structures are relative to form a chain structure. The present invention has ultra-wideband SERS enhancement characteristics, can keep the enhanced hot spot position unchanged within an ultra-wide bandwidth range, and continuously maintain a SERS enhancement coefficient higher than 10^5, and can even reach the level of 10^8; its enhanced bandwidth can reach the micron level, or even the level of 10 microns, and has high robustness to changes in the refractive index environment; it has extremely high practical value for Raman spectroscopy analysis and detection of substances, especially for the detection of unlabeled unknown substances, and has a simple structure and is easy to prepare, and has broad application prospects.

Description

Technical Field

[0001] The invention relates to the field of surface enhanced Raman scattering (SERS), in particular to a SERS chip with ultra-wideband enhancement characteristics. Background Art

[0002] Surface enhanced Raman spectroscopy inherits the rich chemical fingerprint information of Raman spectroscopy and achieves high sensitivity by enhancing the excitation and scattering capabilities through plasmons. In particular, the width of the Raman peaks of most substances is very narrow, which is suitable for multiple analysis, and can be easily measured under air and water conditions. These advantages make SERS a very promising technology for studying complex biochemical material systems.

[0003] From the mechanism, SERS enhancement includes electromagnetic enhancement and chemical enhancement. The electromagnetic enhancement mechanism is considered to be the dominant mechanism in SERS enhancement, which originates from the electromagnetic field "hot spot" generated by the micro-nanostructure / particle. The high electric field enhancement of the hot spot will greatly enhance the Raman scattering signal of the Raman molecule, including the enhancement of the incident light ( G 0 ) and the enhancement of Raman scattered light G’ Two parts

[0004] (1)

[0005] λ 0 is the wavelength of incident light, λ R is the wavelength of Raman scattered light; E 0 is the incident light field intensity, E’ is the field strength after the electromagnetic wave is acted upon by the SERS chip. Theoretical calculations show that under certain circumstances, the enhancement factor can be as high as 10^12, which can achieve single-molecule SERS detection; usually, if the molecule is directly adsorbed on the surface of the enhanced structure, charge transfer may occur between the molecule and the structure, similar to the process of resonant Raman scattering, which will lead to an enhancement in the effective polarizability of the molecule and thus also to an enhancement in Raman scattering, which is generally known as the chemical enhancement mechanism, and its enhancement factor varies from about 10 to 10^2.

[0006] However, the design of the surface enhanced Raman scattering (SERS) chip usually has a narrow enhancement wavelength, which is conducive to shielding sidelobe signals and background noise when detecting a single Raman peak, but is not conducive to detecting multiple Raman peaks, especially the identification of characteristic peaks with large Raman shifts. Therefore, chips with wide wavelength enhancement are particularly important. Broadband enhancement can obtain a wide range of Raman information, which is especially helpful for testing unknown substances and realizing label-free material analysis and identification; at the same time, broadband enhancement also means that even if the enhancement wavelength moves due to the environmental refractive index, it will not lead to a decrease in the enhancement effect, that is, the chip has good robustness to the environment.

[0007] A common approach to designing broadband-enhanced SERS chips is to use a random metal particle structure with particles of different average diameters. Different diameters and gaps mean different response wavelengths, which statistically achieves a broadband enhancement effect. The location of the enhancement hotspot in this approach will change due to different response wavelengths, so it has certain limitations in more precise detection (such as single molecules, single bacteria, single cells, etc.). Another design scheme is to adopt an ordered array structure, in which each structural unit is composed of small structures of different sizes to achieve multi-wavelength resonance, thereby splicing into a broadband enhancement effect; in publicly published literature, there are combinations of nanorods of different lengths (Hughes TW, Fan S H. NanoLett. 2016, 16(9): 5764-5769), and there are also multi-layer structure designs of different widths (Nie MT, Zhao YM,et al. Adv Funct Mater. 2022, 32(32): 2202231). These designs have the disadvantages of limited enhancement bandwidth, discrete response wavelength, and the position of the enhancement hotspot will change with different working wavelengths; in practical applications, this will affect the accuracy and repeatability of the measurement results, and limit the scope of use of the enhancement chip. Summary of the invention

[0008] In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide an ultra-wideband surface-enhanced Raman scattering chip and its application. The present invention utilizes the gradual change of the size of the array antenna structure to meet the response of different wavelengths, thereby achieving the effect of broadband enhancement, while ensuring that the enhanced hotspot position is fixed and does not change due to changes in the incident / scattering wavelength.

[0009] To achieve the above technical objectives, the technical solution of the present invention is as follows:

[0010] An ultra-wideband surface enhanced Raman scattering chip comprises a substrate layer and a surface enhancement layer in order from the bottom layer to the top layer; wherein the surface enhancement layer is provided with a metal periodic micro-nano structure array, the metal periodic micro-nano structure array is composed of periodically distributed independent metal square units, the square units are diamond or square structures with thickness, and the vertex angles between adjacent structures are opposite to each other to form a chain structure; when the period is fixed, by changing the spacing between the vertex angles and / or the thickness of the square unit, the enhancement hotspots are concentrated between the vertex angles of adjacent blocks, and the SERS enhancement coefficient is changed; the array period is smaller than the equivalent excitation wavelength, and the vertex angle spacing and the square unit thickness are both smaller than half of the period; the substrate layer is made of a low refractive index material.

[0011] For the broadband surface enhanced Raman scattering chip, when the period is fixed, the smaller the vertex angle spacing is, the greater the enhancement multiple is; the smaller the block unit thickness is, the fewer the number of longitudinal hot spots is.

[0012] When the top angle of the metal diamond blocks constituting the surface enhancement layer is a right angle, the enhancement multiple is the largest, and when it is an obtuse angle or an acute angle, the enhancement multiple is smaller.

[0013] The metal periodic micro-nano structure of the surface enhancement layer may have the same or different longitudinal and lateral periods, and the size of the period determines the minimum operating wavelength applicable to the chip.

[0014] The metal block material includes gold, silver, copper, platinum, aluminum, alkali metal, doped semiconductor, or a combination of the above materials having surface plasmon optical properties.

[0015] The substrate layer material is a low refractive index material, including aluminum oxide, silicon dioxide, or a combination of the above materials.

[0016] The enhanced chip has ultra-wideband SERS characteristics, with a bandwidth reaching the micron level, maintaining the enhanced hotspot position unchanged within the bandwidth range, and maintaining a SERS enhancement coefficient higher than 10^5.

[0017] The broadband surface enhanced Raman scattering chip is applied to chips with the same structural parameters and is applied to any Raman excitation wavelength within a bandwidth range.

[0018] The application of the broadband surface enhanced Raman scattering chip is highly robust to changes in the refractive index environment and can maintain performance in both air and liquid.

[0019] The broadband surface enhanced Raman scattering chip can effectively enhance Raman characteristic peaks with Raman shifts greater than 10,000, and is suitable for finding characteristic peaks with large Raman shifts of objects to be tested.

[0020] Beneficial effects of the present invention:

[0021] First, ultra-large bandwidth. The theoretical value can reach the micron level, or even 10 micron level, covering almost all Raman signal peaks of the object to be tested, which is particularly helpful for mining the hidden Raman features of unknown substances and identifying characteristic peaks with large Raman shifts, and can realize label-free material analysis and identification; at the same time, broadband enhancement also means that even if the enhancement wavelength moves due to the refractive index of the environment, it will not lead to a decrease in the enhancement effect, that is, the SERS chip of the present invention has good robustness to the test environment (such as air, liquid, etc.).

[0022] Second, the signal is repeatable. The enhanced hotspots are located in a constant and evenly distributed position, which ensures the repeatability of the measurement results and allows the concentration of the analyte to be accurately estimated using the signal intensity.

[0023] Third, the enhancement coefficient is high. When the excitation wavelength and Raman scattering wavelength signals are enhanced at the same hotspot, the enhancement coefficient is the product of the two, which can be higher than 10^5 within a very large bandwidth.

[0024] Fourth, it is easy to design and prepare. The main structure of the present invention is composed of a periodic array of metal blocks of equal thickness, and the structural unit has a simple shape, which is convenient for design optimization and large-scale processing and manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of the three-dimensional structure of the ultra-wideband surface-enhanced Raman scattering chip of the present invention.

[0026] Figure 2 It is a top view and a corresponding cross-sectional view of the first embodiment of the present invention.

[0027] Figure 3 This is a curve showing how the field enhancement factor at the hotspot position changes with wavelength in Example 1.

[0028] Figure 4 1 and 1 are field enhancement distribution diagrams corresponding to different wavelengths in Example 1 (the first row is a top view, and the second row is a corresponding cross-sectional view).

[0029] Figure 5 The graphs of the hot spot field enhancement factors corresponding to different materials in Example 1 as a function of wavelength are shown.

[0030] Figure 6 The graphs are the variation curves of the hot spot field enhancement factor with wavelength corresponding to different materials in Example 2.

[0031] Figure 7 The present invention is designed for application in three typical excitation wavelengths in actual Raman spectroscopy measurement.

[0032] Figure 8 It is an assembly diagram of an application example of the present invention in Raman spectroscopy measurement;

[0033] Among them, 1-surface enhancement layer (composed of array structure), 2-substrate layer.

[0034] Fig. 9 The theoretical Raman enhancement spectrum is an application example of the present invention in Raman spectrum measurement. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

[0036] For the convenience of description, field enhancement, that is, electric field enhancement, is defined as , the field intensity detection position is set as the center point between adjacent structures; the electromagnetic simulation analysis adopts the finite difference time domain method (FDTD), the incident direction of the light source is perpendicular to the paper surface and downward, and the electric field direction is x Polarization, i.e. horizontal polarization; unless otherwise specified, the SERS structure is placed in an air environment.

[0037] like Figure 1 , 2 As shown, a broadband surface enhanced Raman scattering chip comprises, from the bottom to the top, a substrate layer 2 and a surface enhancement layer 1. The substrate layer, wherein the surface enhancement layer is provided with a metal periodic micro-nano structure array, wherein the structure array is composed of periodically distributed independent metal square units, wherein the square unit is a diamond or square structure of a certain thickness; and the substrate layer is composed of a low refractive index material.

[0038] The functional area of ​​the chip is mainly concentrated in the surface enhancement layer, which is composed of periodically arranged metal square units. Adjacent metal squares are spaced a certain distance apart and arranged with their top angles relative to each other to form a chain structure. The enhanced hotspots are concentrated between the tips of adjacent squares.

[0039] Arranging adjacent metal blocks with their top angles relative to each other utilizes the tip effect, causing the enhanced hotspots to be concentrated between the tips of the blocks, ensuring that the position of the enhanced hotspot on the chip surface remains unchanged; at the same time, the relative top angles also mean that the structure size near the hotspot is gradual (starting from the tip and gradually increasing), thereby ensuring that the hotspot position can resonate or couple with light of different wavelengths, that is, the chip surface can capture light signals of different wavelengths, thereby achieving broadband enhancement and amplification.

[0040] By adjusting the size parameters of each unit structure (such as length, width, thickness), as well as the array period, a suitable electromagnetic enhancement factor and a starting resonance wavelength equivalent to the period can be obtained. The array period should be smaller than the minimum action wavelength.

[0041] Furthermore, to ensure that the enhanced hotspot on the chip surface does not change due to the wavelength of action, there should be only one enhanced hotspot between the tips of the blocks, both longitudinally and transversely. Therefore, the spacing between the tips (g) and the thickness of the blocks (H ) are of comparable size and less than half the period of the structure.

[0042] By changing the spacing between the tips (g), the electromagnetic enhancement factor (SERS enhancement factor) can be effectively changed. Generally, the smaller the g, the greater the enhancement factor; and the thickness of the block ( H ) will change the number and location of vertical enhancement hotspots.

[0043] Furthermore, the SERS chip mainly enhances the excitation and scattering capabilities of the structure through plasmon resonance. Therefore, the metal block material should be gold, silver, copper, platinum, aluminum, etc., which have surface plasmon optical properties, as well as alkali metals, doped semiconductors and other materials; the substrate layer material should be low-refractive index materials such as alumina and silica, in order to reduce interference with the enhanced hotspots. Example

[0044] The chip substrate material is alumina, and the surface enhancement layer is a square array structure of gold, such as Figure 1 , 2 As shown, the thickness H =35nm, period P =440nm, gap g =10nm (side length ). After electromagnetic simulation analysis, the field enhancement curve at the center point between adjacent structures is obtained as the incident wavelength changes, as shown in Figure 3 As shown in the figure (including both air and water environments), it can be seen that the field enhancement factor from the peak position (about 0.78μm) to the 10um or even longer band is greater than 10^3; according to the SERS enhancement mechanism, when the Raman excitation wavelength is selected in this band (such as the commonly used 785nm), its Raman enhancement coefficient (Formula (1)) must be greater than 10^5.

[0045] In addition, in a water environment, the broadband performance of the chip remains unchanged and the enhancement factor is even greater (this is because the refractive index difference between the water environment and the substrate material is small, and the electric field distribution is more concentrated in the center), fully demonstrating the high environmental robustness of the present invention.

[0046] at the same time, Figure 4 The electric field distribution at three characteristic wavelengths in Example 1 is demonstrated, and it can be clearly seen that the enhanced hot spots are located between adjacent structures and are not shifted due to the change of the action wavelength. That is, the SERS chip of the present invention can enhance both the excitation wavelength and the Raman scattering signal of the object to be measured at the same position, giving full play to the enhancement function of the SERS chip and ensuring the reliability and uniformity of the signal.

[0047] Furthermore, without changing the structural parameters and substrate material, the surface enhancement layer is replaced with other metal materials, and its field enhancement is calculated; Figure 5 , showing the field enhancement curves of gold, silver, aluminum and copper as a function of the incident wavelength. In terms of broadband optical response, they all have outstanding performance, reflecting the flexibility of the present invention in material selection and the stability of performance. Example

[0048] Furthermore, by changing the substrate material to silicon dioxide and setting the surface enhancement layer materials to gold, silver, aluminum, and copper, respectively, and adjusting the structural dimensions (thickness H = 30nm, period P = 500nm, gap g = 10nm), the present invention also exhibits excellent broadband enhancement performance. Figure 6 As shown, except for the decrease in enhancement factor near 10μm due to the absorption loss of the substrate material silicon oxide itself, the field enhancement factor is greater than 10^3 in a very wide range, especially for the three designs of gold, silver and copper, whose enhancement factors are all greater than 2×10^3; this further reflects the flexibility of the present invention in the selection of materials and structural dimensions and the stability of performance, and also shows that the present invention has room for further optimization and improvement (by adjusting the structural dimensions and materials) according to actual conditions, and has broad application development potential.

[0049] Furthermore, an example of the application of the present invention in Raman spectroscopy measurement is also shown. By changing the structural period P (Other structural parameters are consistent with those of Example 2, and the material is a combination of silver and silicon oxide), and a design suitable for three typical Raman excitation wavelengths is achieved, such as Figure 7 The field enhancement curve shown in the figure changes with the incident wavelength (the middle band is omitted for the convenience of display) has designs with minimum action wavelengths of 633nm, 785nm, and 1064nm respectively; the application scope of the present invention is compatible with the long-wave direction, that is, the design with a shorter excitation wavelength can also be used in the case of a longer excitation wavelength, such as the 633nm design, which is also suitable for use with an excitation wavelength of 785nm or 1064nm.

[0050] Furthermore, an application assembly diagram of the present invention in Raman spectroscopy measurement is also shown, such as Figure 8 As shown, the laser irradiates the SERS chip containing the object to be tested, stimulating the Raman scattering signal, which is then collected into the spectrometer and finally the measured spectrum is displayed on the screen.

[0051] In this example, assuming that the excitation wavelength is 785 nm, then according to the SERS enhancement mechanism, the enhancement coefficient corresponding to each wavelength is the enhancement coefficient at the excitation wavelength multiplied by the enhancement coefficient of the corresponding Raman wavelength. Therefore, the SERS enhancement coefficients of the Raman chips shown in Example 1 (gold and aluminum oxide combination) and Example 2 (silver and silicon oxide combination) are as follows: Fig. 9 It can be seen that in a very wide Raman shift range (greater than 10000cm -1 ), the enhancement coefficients are much higher than 10^5, and combined with the chemical enhancement of the object to be measured, the final measured Raman enhancement coefficient can reach the order of 10^7 to 10^8. Among them, the Raman shift is defined as , unit: cm -1 .

[0052] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. An ultra-wideband surface-enhanced Raman scattering chip, characterized in that: From the bottom layer to the top layer, it includes a substrate layer and a surface enhancement layer in sequence; wherein the surface enhancement layer is provided with a metal periodic micro-nano structure array, the metal periodic micro-nano structure array is composed of periodically distributed metal square units, the square units are independent islands, and the non-structured area is a blank substrate; the square units are diamond or square structures with thickness, and the top angles of adjacent structures are opposite to each other to form a chain structure; when the period is fixed, by changing the spacing between the top angles and / or the thickness of the square units, the enhancement hotspots are concentrated between the adjacent square top angles, and the SERS enhancement coefficient is changed; The array period is smaller than the equivalent excitation wavelength, and the top angle spacing and the thickness of the square unit are both smaller than half of the period; the substrate layer is flat and made of low-refractive index material, and the material of the square unit includes gold, silver, copper, platinum, and aluminum having surface plasmon optical properties.

2. The broadband surface-enhanced Raman scattering chip according to claim 1, characterized in that: When the period is fixed, the smaller the top angle spacing, the greater the enhancement multiple; the smaller the block unit thickness, the fewer the number of longitudinal hot spots.

3. The broadband surface enhanced Raman scattering chip according to claim 1, characterized in that: When the top angle of the metal diamond blocks constituting the surface enhancement layer is a right angle, the enhancement multiple is the largest, and when it is an obtuse angle or an acute angle, the enhancement multiple is smaller.

4. The broadband surface-enhanced Raman scattering chip according to claim 1, characterized in that: The metal periodic micro-nano structure of the surface enhancement layer may have the same or different longitudinal and lateral periods, and the size of the period determines the minimum operating wavelength applicable to the chip.

5. The broadband surface enhanced Raman scattering chip according to claim 1, characterized in that: The substrate layer material is a low refractive index material, including aluminum oxide, silicon dioxide, or a combination of the above materials.

6. The broadband surface-enhanced Raman scattering chip according to claim 1, characterized in that: The enhanced chip has ultra-wideband SERS characteristics, with a bandwidth reaching the micron level, maintaining the enhanced hotspot position unchanged within the bandwidth range, and maintaining a SERS enhancement coefficient higher than 10^5.

7. An application of the broadband surface enhanced Raman scattering chip according to claim 1, characterized in that: Chips with the same structural parameters can be used for any Raman excitation wavelength within the bandwidth range.

Images