Nanogap tip-enhanced single-molecule Raman spectroscopy system

Abstract

The present invention provides a single-molecule Raman spectroscopy system based on nanogap tip enhancement. The system includes a laser generator and collimator, a polarizer, a half-wave plate, a dichroic mirror, a lens, a reflector, a tube lens, a microscope objective, a sample cell, a probe with a nanogap tip, an aperture, and a spectrometer. The present invention uses a nanogap tip, rather than a conventional plasma tip, as a substrate. The nanogap tip alone can achieve a highly localized enhanced electromagnetic field, enabling Raman detection with single-molecule sensitivity. Compared to conventional single-molecule tip-enhanced Raman spectroscopy systems, the present invention eliminates the need for complex tip control systems and metal substrates, providing a novel solution for single-molecule Raman spectroscopy and broadening its application scenarios.

Description

Technical Field

[0001] The present invention relates to the field of Raman spectroscopy, in particular to a single-molecule Raman spectroscopy system based on nano-gap needle tip enhancement. Background Art

[0002] Spontaneous Raman scattering from materials under non-resonant conditions is extremely weak, and obtaining high signal-to-noise ratio Raman spectra remains a challenge. Metallic nanostructures, under optical excitation, generate localized plasmon resonances that enhance Raman scattering from molecules. This phenomenon, known as surface-enhanced Raman scattering, overcomes the sensitivity limitations of traditional Raman spectrometers. Tip-enhanced Raman spectroscopy (TERS) employs a technique called tip-enhanced Raman spectroscopy, which brings the plasmon tip close to the sample to generate an enhanced Raman signal.

[0003] The electromagnetic field enhancement provided by a single metal nanoparticle is often insufficient for single-molecule Raman spectroscopy. To achieve single-molecule sensitivity in Raman spectroscopy, the sample must be fixed in the nanometer gap between two metal nanostructures to obtain a stronger electromagnetic field enhancement. Single-molecule-sensitive tip-enhanced Raman spectrometers utilize a scanning probe microscope control system to maintain a gap of less than 5 nm between the plasmonic tip and the metal substrate. This near-field coupling between the tip and the metal substrate enables single-molecule Raman spectroscopy. Summary of the Invention

[0004] The purpose of the present invention is to address the shortcomings of the existing technology and provide a single-molecule Raman spectroscopy system based on nanogap tip enhancement. Without the need for a metal substrate and a complex control system, the tip itself can form a highly localized enhanced electromagnetic field, providing a new solution for single-molecule Raman spectroscopy detection.

[0005] The object of the present invention is achieved through the following technical solutions: a single-molecule Raman spectroscopy system based on nanogap tip enhancement, comprising a laser generating and collimating device, a polarizer, a half-wave plate, a dichroic mirror, a first lens, a reflector, a tube lens, a microscope objective lens, a sample cell, a probe with a nanogap tip, a second lens, a pinhole and a spectrometer;

[0006] The light beam emitted by the laser generating and collimating device is converted into linearly polarized light by the polarizer, and the polarization direction of the linearly polarized light is rotated by the half-wave plate; the light beam is reflected by the dichroic mirror, and sequentially passes through the first lens, the reflector, the tube lens, and the microscope objective lens to be focused into the sample cell;

[0007] The nanogap tip of the probe consists of two plasmonic nanostructures with a spacing of less than 5 nm. The probe is suspended above the sample pool, the tip is immersed in the sample solution, and the tip is aligned with the laser focal spot.

[0008] Under light excitation, a highly localized enhanced electromagnetic field is generated in the nanogap. The molecules in the sample solution diffuse randomly through Brownian motion. When the molecules diffuse into the nanogap of the probe, their Raman spectral signals are enhanced.

[0009] The Raman scattered light of the molecule to be tested is collected again by the optical system, focused by the second lens into a small hole conjugated with the nanogap needle tip, and connected to the spectrometer via an optical fiber to obtain the Raman spectrum data of the molecule to be tested.

[0010] Furthermore, the half-wave plate is rotated to match the polarization direction of the light beam with the plasma resonance direction of the nanogap, thereby obtaining the strongest local electromagnetic field.

[0011] Furthermore, a water immersion objective lens is used as a microscope objective lens, and the area between the microscope objective lens and the bottom of the sample cell is immersed in water to eliminate the aberration introduced by refraction from the air surface to the water surface, thereby improving the focusing quality of the laser.

[0012] Furthermore, electrodes are inserted on both sides of the sample pool solution, and the sample pool becomes an electrochemical cell. When an AC voltage is applied across the electrodes, the molecules to be tested in the solution are enriched near the nanogap needle tip under the action of dielectrophoresis, thereby improving detection efficiency.

[0013] Furthermore, the Raman scattered light is focused on a small hole, which is used to eliminate out-of-focus stray light and is achieved by using a pinhole or multimode optical fiber.

[0014] Furthermore, the needle tip of the probe is composed of two plasma nanospheres, and the gap between the two spheres is less than 5 nm.

[0015] Compared to existing technologies, this invention offers the following beneficial technical advantages: By utilizing a nanogap tip, rather than a conventional plasmonic tip as a substrate, the nanogap tip alone can achieve a highly localized enhanced electromagnetic field, enabling Raman detection with single-molecule sensitivity. Compared to conventional single-molecule tip-enhanced Raman spectroscopy systems, this invention eliminates the need for complex tip control systems and metal substrates, providing a novel solution for single-molecule Raman spectroscopy and broadening its application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A schematic diagram of a single-molecule Raman spectroscopy system based on nanogap tip enhancement provided by an exemplary embodiment of the present invention;

[0017] Figure 2 A schematic diagram of a nanogap tip according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0018] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0019] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0020] The following is a specific implementation example of the system of the present invention, but is not limited to this. The single-molecule Raman spectroscopy system based on nanogap tip enhancement in this example is as follows: Figure 1 As shown, the system includes a laser generating and collimating device 1, a polarizer 2, a half-wave plate 3, a dichroic mirror 4, a first lens 5, a reflector 6, a tube lens 7, a microscope objective 8, a sample cell 9, a probe with a nano-gap tip 10, a filter 11, a second lens 12, a pinhole 13, a single-mode optical fiber 14, and a spectrometer 15. In this example, a grating spectrometer is used.

[0021] When the system is working, the light beam emitted by the laser generation and collimation device 1 is converted into linear polarized light by the polarizer 2, and the polarization direction of the linear polarized light is rotated by the half-wave plate 3; the light beam is reflected by the dichroic mirror 4, and passes through the first lens 5, the reflector 6, the tube lens 7, and the microscope objective lens 8 in sequence and is focused in the sample cell 9.

[0022] A probe 10 with a nano-gap needle tip is suspended above a sample pool 9 . The sample pool 9 is filled with a sample solution. The needle tip is immersed in the sample solution and is aligned with the laser focal spot.

[0023] The nano-gap tip structure of the probe is as follows Figure 2 As shown in Figure 1, the tip consists of two plasmonic nanostructures with a spacing of less than 5 nm. When excited by light, the nanogap generates a highly localized enhanced electromagnetic field, which greatly enhances the Raman spectroscopy signal of the molecules being measured in the nanogap.

[0024] The molecules to be measured in the sample solution diffuse randomly through Brownian motion. When the molecules to be measured diffuse into the nanogap of the probe, their Raman spectral signals are enhanced.

[0025] The Raman scattered light of the molecule to be measured is collected again by the optical system. Specifically, the Raman scattered light passes through the microscope objective 8, the tube lens 7, the reflector 6, the first lens 5 in sequence, is transmitted by the dichroic mirror 4, and is focused into the small hole 13 conjugated with the nanogap needle tip through the second lens 12.

[0026] The Raman scattered light is transmitted through the single-mode optical fiber 14 and coupled into the spectrometer 15. The Raman scattered light is split in the spectrometer 15 and recorded by a camera to obtain Raman spectrum data of the molecule to be measured.

[0027] Furthermore, the half-wave plate is rotated to match the polarization direction of the light beam with the plasma resonance direction of the nanogap, thereby obtaining the strongest local electromagnetic field.

[0028] Furthermore, a water immersion objective lens is used as a microscope objective lens, and the area between the microscope objective lens and the bottom of the sample cell is immersed in water to eliminate the aberration introduced by refraction from the air surface to the water surface, thereby improving the focusing quality of the laser.

[0029] Furthermore, electrodes are inserted on both sides of the sample cell solution, turning it into an electrochemical cell. Applying an AC voltage across the electrodes causes the molecules in the solution to accumulate near the nanogap tip through dielectrophoresis, improving detection efficiency.

[0030] Furthermore, the Raman scattered light is focused on a small hole, which is used to eliminate out-of-focus stray light and is achieved by using a pinhole or multimode optical fiber.

[0031] In one embodiment, the probe tip consists of two plasmonic nanospheres with a radius of 400 nm and a gap of less than 5 nm between them. The probe body has a radius of 50 μm and a total length of 6 cm. Under optical excitation, a highly localized electromagnetic field is generated within the nanogap, significantly enhancing the Raman spectroscopy signal of the molecules detected within the gap.

[0032] The above description is only a preferred embodiment of the present invention. Although the present invention has been disclosed as a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art can use the above disclosed methods and technical contents to make many possible changes and modifications to the technical solution of the present invention without departing from the scope of the technical solution of the present invention, or modify it into an equivalent embodiment with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of protection of the technical solution of the present invention.

Claims

1. A nanogap tip-enhanced single-molecule Raman spectroscopy system, characterized in that: The device comprises a laser generating and collimating device, a polarizing plate, a half-wave plate, a dichroic mirror, a first lens, a reflecting mirror, a tube lens, a microscope objective lens, a sample cell, a probe with a nano-gap needle tip, a second lens, a pinhole and a spectrometer; The light beam emitted by the laser generating and collimating device is converted into linearly polarized light by the polarizer, and the polarization direction of the linearly polarized light is rotated by the half-wave plate; the light beam is reflected by the dichroic mirror, and sequentially passes through the first lens, the reflector, the tube lens, and the microscope objective lens to be focused into the sample cell; The nanogap tip of the probe consists of two plasmonic nanostructures with a spacing of less than 5 nm. The probe is suspended above the sample pool, the tip is immersed in the sample solution, and the tip is aligned with the laser focal spot. A highly localized enhanced electromagnetic field is generated in the nanogap under optical excitation; The molecules in the sample solution diffuse randomly through Brownian motion. When the molecules diffuse into the nanogap of the probe, their Raman spectral signals are enhanced. The Raman scattered light of the molecule to be tested is collected again by the optical system, focused by the second lens into a small hole conjugated with the nanogap needle tip, and connected to the spectrometer via an optical fiber to obtain the Raman spectrum data of the molecule to be tested.

2. The nanogap tip-enhanced single-molecule Raman spectroscopy system according to claim 1, characterized in that: The half-wave plate is rotated to match the polarization direction of the light beam with the plasma resonance direction of the nanogap, thereby obtaining the strongest local electromagnetic field.

3. The nanogap tip-enhanced single-molecule Raman spectroscopy system according to claim 1, characterized in that: A water immersion objective is used as the microscope objective. The area between the microscope objective and the bottom of the sample cell is immersed in water to eliminate the aberration introduced by refraction from the air surface to the water surface and improve the focusing quality of the laser.

4. The nanogap tip-enhanced single-molecule Raman spectroscopy system according to claim 1, characterized in that: Electrodes are inserted on both sides of the sample pool solution, and the sample pool becomes an electrochemical cell. When an AC voltage is applied across the electrodes, the molecules to be tested in the solution are enriched near the nanogap needle tip under the action of dielectrophoresis, thereby improving detection efficiency.

5. The nanogap tip-enhanced single-molecule Raman spectroscopy system according to claim 1, characterized in that: The Raman scattered light is focused on a small hole, which is used to eliminate out-of-focus stray light and is achieved using a pinhole or multimode fiber.

6. The nanogap tip-enhanced single-molecule Raman spectroscopy system according to claim 1, characterized in that: The needle tip of the probe consists of two plasma nanospheres, and the gap between the two spheres is less than 5nm.

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