Core-shell particles with enhanced optomechanical properties, method for enhancing optomechanical force, and applications

Abstract

The present invention belongs to the field of optical precision measurement technology, and specifically relates to a core-shell type particle with optomechanical enhancement characteristics, a method and application for enhancing optomechanical force. The core material is a nanocrystal, and the shell material is a metal material. The nanocrystal is NaYF4:Yb 3+ , Er 3+ , Nd 3+ or YLF:Yb 3+ , and the metal material is gold or silver; the present invention has the advantages of tunable absorption band, high upconversion efficiency, good chemical stability, easy synthesis, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical precision measurement, and particularly relates to a core-shell type particle with optical force enhancement characteristics, a method for enhancing optical force, and an application thereof. Background Art

[0002] The interaction between light and matter, including light absorption, scattering, and the generation of harmonic photons, is characterized by the dielectric constant of the material in the electromagnetic field. Low refractive index nanoparticles, such as silica, polystyrene, and the lipid membranes of organelles, have a low dielectric constant, which fundamentally limits the application of technologies that rely on the refractive index mismatch between nanoparticles and the surrounding medium, such as quantitative phase imaging, dark-field microscopy, interferometric scattering microscopy, and optical tweezers. In particular, the optical manipulation of low refractive index nanoparticles for nanomaterial assembly and biomolecular force sensing remains a challenge.

[0003] Research on the Enhancement and Stable Trapping of Optical Force of Metal Nanoparticles, Liu Hongshuang et al., Semiconductor Optoelectronics, Vol. 40, No. 4, August 2019, describes the influencing factors of optical force enhancement. Section 2.2 points out that for micro-nano structures composed of noble metals such as gold, silver, and copper, when the structural parameters and illumination parameters meet specific conditions, the structural light field can undergo a local surface plasmon resonance (LSPR) effect, realizing the localization and enhancement of the electromagnetic field on the metal surface. For the gold nanosphere structure on a gold substrate, when the incident light wavelength is fixed, gold nanospheres with a specific radius can generate the LSPR effect. In addition, gap surface plasmons (GSPP) will be excited in the nano-gap between the gold substrate and the gold nanosphere, which can localize the electromagnetic field in the nano-gap, further enhancing the electromagnetic field in the gap, increasing the optical force on the gold nanosphere, and realizing stable trapping. Therefore, for a gold nanosphere with a radius r = 142 nm on a gold substrate, under the irradiation of radially polarized light, due to the LSPR effect and the excitation of GSPP, a relatively large optical trap stiffness and the absolute value of the axial optical force can be obtained. However, the system complexity is high with the scheme relying on the structural light field and the substrate, which limits the application range of the optical tweezer system. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a core-shell type particle with optical force enhancement characteristics, a method for enhancing optical force, and an application thereof, which have the advantages of tunable absorption band, high up-conversion efficiency, good chemical stability, easy synthesis, etc.

[0005] The content of the present invention includes a core-shell type particle (also called a core-shell type microsphere) with optical force enhancement characteristics. The core material is a nanocrystal, and the outer shell is made of a metal material. The nanocrystal is NaYF4:Yb 3+ , Er 3+ , Nd 3+ (Lanthanide ions Yb can be doped in the NaYF4 nanocrystal 3+, Er 3+ , Nd 3+ (one of the following), or YLF:Yb 3+ , the metal material is gold or silver (any metal material capable of exciting surface plasmons is acceptable, preferably gold or silver).

[0006] Preferably, the nanocrystal is NaYF4: 20% Yb 3+ , the fluoride has a low phonon energy (~350 cm -1 ), and relatively stable chemical properties. Moreover, NaYF4 is the most widely studied upconversion matrix material at present, and 20% Yb 3+ doped NaYF4 is the material with the highest upconversion efficiency at present.

[0007] Preferably, the metal material is gold.

[0008] Preferably, the condition for determining the thickness of the metal shell is to tune the thickness of the metal shell. When the surface plasmon resonance wavelength of the metal shell matches the absorption resonance of the nanocrystal, the thickness of the metal shell at this time is the optimal thickness.

[0009] Preferably, the thickness of the metal shell is 2 - 20 nm.

[0010] Preferably, when the ratio of the core (nanocrystal) radius to the shell thickness is about 9:1, SPR of 900 nm can be obtained.

[0011] The present invention provides a method for enhancing optical force. The steps are as follows: a strongly focused Gaussian beam is irradiated onto the core-shell type particle with the property of enhancing optical force, and the thickness of the shell material in the core-shell type particle is controlled so that when the SPR wavelength of the metal shell matches the absorption resonance wavelength of the nanocrystal, the metal shell and the nanocrystal resonate, realizing the enhancement of optical force.

[0012] Preferably, the Gaussian beam is a strongly focused Gaussian beam, which is obtained by outputting after a Gaussian beam passes through an oil immersion objective lens with an NA value of 1.4.

[0013] The present invention provides an application of the core-shell type particle with the property of enhancing optical force in enhancing optical force.

[0014] Using core-shell particles (microspheres) of metal-coated lanthanide upconversion nanocrystals (UCNs@Metals), the diameter of which is much smaller than the incident light wavelength; the optical forces acting on the UCNs@Metals core-shell microspheres are divided into gradient force and scattering force. Among them, the gradient force is mainly used to stably capture the UCNs@Metals core-shell microspheres, while the scattering force mainly pushes the UCNs@Metals core-shell microspheres away from the stable capture point. Therefore, in order to achieve the stable capture of the microspheres, it is necessary to enhance the optical gradient force. According to the Rayleigh model, the magnitude of the optical gradient force is proportional to the real part of the polarizability of the microsphere:

[0015]

[0016] where F grad is the optical gradient force, Re(α cs ) is the real part of the equivalent polarizability of the UCNs@Metals core-shell microsphere, c is the speed of light in vacuum, ε0 is the permittivity of vacuum, is the square of the electric field gradient. The enhancement of the optical force can be achieved by increasing Re(α cs ). The physical quantity that measures the stable capture of the microsphere at the center of the optical trap is the optical trap stiffness, which represents the slope of the relationship curve between the displacement of the microsphere in the optical trap and the optical force in the linear part.

[0017] The beneficial effect of the present invention is that the present invention proposes a method for enhancing the optical force by using UCNs@Metals core-shell particles (microspheres). By coating a metal shell on the periphery of low-refractive-index lanthanide upconversion nanocrystals (UCNs) and adjusting the thickness of the metal shell so that its SPR wavelength is close to the absorption resonance wavelength of the UCNs, then at this time the energy of the SPR is exactly matched with the energy required for the electron excitation of the UCNs, and the energy is transferred from the surface plasmon to the electrons in the UCNs. Under this resonance condition, the electric field vector of the plasma wave is effectively coupled with the transition dipole in the UCNs, resulting in an increase in the absorption cross-section of the UCNs. A large absorption cross-section will generate more free charges, which will increase the polarizability of the UCNs@Metals, thereby achieving the enhancement of the optical force.

[0018] The whole process of realizing the enhancement of the optical force in this application does not change the refractive index of the particle itself. It is achieved by the coupling of the SPR of the metal shell and the absorption resonance of the UCNs, resulting in the enhancement of the real part of the polarizability of the UCNs@Metals core-shell microsphere, and has the advantages of tunable absorption band, high upconversion efficiency, good chemical stability, easy synthesis, etc.

[0019] The UCNs@Metals core-shell microspheres can bypass the limitations caused by the refractive index mismatch between the object and the surrounding medium, providing a new method for designing far-field optical tweezers at the nanoscale. This method enables high-spatial-resolution and high-sensitivity force sensing and offers great potential for the application of nanoscale optofluidics. So far, there has been no report on achieving enhanced optical force using UCNs@Metals core-shell microspheres.

[0020] Compared with pure UCNs crystals, using the core-shell particles of this application, the optical force can be increased by an order of magnitude and has near-infrared tunable characteristics. By adjusting the thickness of the outer shell, the conversion from reverse, stable trapping, optical thrust to optical traction can be achieved.

[0021] The nanocrystals in the core-shell particles are upconversion nanocrystals doped with lanthanide ions, which are usually used to enhance the upconversion luminescence intensity. Generally speaking, in order to improve the luminescence intensity, the nanocrystals are coated. Common coating materials are silica, NaYF4, etc. Common methods to improve the upconversion crystal luminescence efficiency are: 1 changing the type and concentration of doped ions, 2 coating an inorganic shell, 3 plasmon resonance, 4 increasing the crystal size, 5 nanoscale heterojunctions. In this application, the nanocrystals are coated with a simple metal material as the shell material and applied to enhance the optical force, effectively improving the optical force.

[0022] The present invention effectively improves the optical force by defining the thickness of the shell and the ratio between the shell and the nanocrystal core radius. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic structural diagram of the core-shell particles of the present invention, wherein 1 is a light source, 2 is a nanocrystal core, and 3 is a metal shell.

[0024] Figure 2 It is a schematic diagram of the trapping principle of the UCNs@Metals core-shell microspheres of the present invention.

[0025] Figure 3 It is a relationship curve between the optical force and the axial displacement of the microsphere under the irradiation of a strongly focused Gaussian beam in an embodiment of the present invention.

[0026] Figure 4 It is a two-dimensional diagram of the relationship between the optical trap stiffness and the volume fraction of the microsphere and the illumination wavelength under the irradiation of a strongly focused Gaussian beam in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] The following describes a specific embodiment of the present invention in detail with reference to the drawings, but the protection scope of the present invention should not be limited thereby.

[0028] Example 1

[0029] A specific and detailed preparation method of UCNs@Metals core-shell particles, including:

[0030] (1) Preparation of NaYF4:20% Yb 3+ microspheres

[0031] Add 1 mmol of RECl3·6H2O (RE = Yb) to a flask containing 6 mL of oleylamine (OA) and 15 mL of octadecane (ODE) according to the required molar ratio. Heat the mixture under an argon stream for 30 min to 160 °C to obtain a clear solution, then cool it to about 50 °C, and add 5 mL of NH4F (4 mmol) and NaOH (2.5 mmol) methanol solution. After stirring for 30 min, heat it under an argon stream for 20 min to 80 °C to expel methanol, then heat it for 90 min to 310 °C, and finally cool it to room temperature. Precipitate with ethanol, centrifuge at 9000 rpm for 5 min, and wash it 3 times with cyclohexane, ethanol, and methanol to obtain nanoparticles.

[0032] (2) Polyacrylic acid PAA-modified NaYF4:20% Yb 3+ microspheres

[0033] After the synthesis of NaYF4:20% Yb 3+ is completed, use polyacrylic acid PAA as a multidentate ligand to replace the hydrophobic ligand on the surface of NaYF4:20% Yb 3+ to obtain water-soluble NaYF4:20% Yb 3+ through a ligand exchange process. Heat PAA (0.200 g) and diethylene glycol DEG (16.0 mL) in a four-necked flask to 110 °C. Disperse the prepared NaYF4:20% Yb 3+ microspheres (38.7 mg) in toluene, and add the suspension to the four-necked flask. Continuously heat the system for 1.5 hours to 150 °C. Inject HCl (0.100 M) until the solution cools naturally to room temperature. This experiment is completed with vigorous stirring in a nitrogen atmosphere. The product is obtained by centrifugation and washed multiple times with ultrapure water.

[0034] (3) Preparation of Au colloid seeds

[0035] Add 0.06 g of K2CO3 to 240 mL of ultrapure water, stir continuously, and quickly add 4.80 mL of 1.00 wt% HAuCl4 to the mixture to obtain an aged Au solution. After a few minutes, the solution becomes colorless. Then, transfer the obtained Au nanoparticle seed solution to an opaque container and store it in the refrigerator for one day.

[0036] (4) Attachment of Au nanoparticles

[0037] To facilitate the subsequent attachment of negatively charged Au nanoparticles, a layer of poly(allylamine hydrochloride) PAH was added onto the surface of PAA-coated NaYF4:20% Yb 3+ to make it positively charged. Then, 2.00 mL of NaYF4:20% Yb 3+ (1.29 mg / mL) solution was added to 20.0 mL of the aged Au solution and stirred, and then 0.0400 mL of formaldehyde HCHO and 0.100 mL of hydroxylamine NH2OH were injected and stirred. After a few minutes, the solution changed from colorless to dark brown. It was centrifuged and washed multiple times with ultrapure water. NaYF4:20% Yb 3+ @Au core-shell microspheres were obtained.

[0038] Example 2

[0039] For ease of description, a coordinate system O-XYZ is defined: taking the geometric central axis of the UCNs@Metals core-shell microsphere as the Z-axis, the direction of light output as the positive direction of the Z-axis, the horizontal direction of the surface where the center of the UCNs@Metals core-shell microsphere is located as the Y-axis, the vertical direction of the surface where the center of the UCNs@Metals core-shell microsphere is located as the X-axis, and the center of the UCNs@Metals core-shell microsphere as the origin O, as Figure 1 described.

[0040] A method for realizing enhanced optical force using UCNs@Metals core-shell microspheres. In an optical tweezer system, the UCNs@Metals core-shell microsphere is captured near the waist position of the light source beam, and the beam output by the light source irradiates the UCNs@Metals core-shell microsphere; the light source emits a Gaussian beam, and the wavelength of the Gaussian beam is equal to the resonance wavelength of the UCNs@Metals core-shell microsphere; after the Gaussian beam passes through an oil immersion objective lens with a numerical aperture (NA) of 1.4, a strongly focused Gaussian beam is output (i.e., the spot size of the strongly focused Gaussian beam is: the spot radius in the x direction is 0.41 μm, and the spot radius in the y direction is 0.28 μm), the UCNs@Metals core-shell microsphere is placed at the focal position of the strongly focused Gaussian beam, and the strongly focused Gaussian beam irradiates the UCNs@Metals core-shell microsphere; the UCNs@Metals core-shell microsphere includes a metal shell and a UCNs core, and has the function of enhancing the polarizability by absorbing the energy of the strongly focused Gaussian beam emitted by the light source to produce a resonance effect.

[0041] As Figure 1As shown, in this embodiment, the light source uses a wavelength-tunable laser light source. After passing through an oil-immersion objective lens with a numerical aperture (NA) of 1.4, a strongly focused Gaussian beam is output. The UCNs@Metals core-shell microspheres are composed of a UCNs core and a metal shell. Among them, the radius a of the UCNs core is 23.3 nm, and the material is NaYF4: 20% Yb 3+ , the thickness (b - a) of the metal shell is tunable, and the variation range is 2 - 20 nm, and the material is Au.

[0042] As Figure 2 shown, since the absorption resonance wavelength of the NaYF4: 20% Yb 3+ crystal is 980 nm, by tuning the thickness of the Au shell to make the surface plasmon resonance (SPR) wavelength of the shell 980 nm, then at this time the energy of the SPR is exactly matched with the energy required for the electron excitation of the NaYF4: 20% Yb 3+ crystal. The energy is transferred from the surface plasmon of Au to the electrons in the NaYF4: 20% Yb 3+ crystal (the incident light wavelength is 980 nm). Under this resonance condition, the electric field vector of the plasma wave is effectively coupled with the transition dipole in the NaYF4: 20% Yb 3+ crystal, resulting in an increase in the absorption cross-section of the NaYF4: 20% Yb 3+ crystal. A large absorption cross-section will generate more free charges, which will increase the polarizability of NaYF4: 20% Yb 3+ @Au, thereby realizing the enhancement of the optical force.

[0043] The light source is placed at the rear focal plane of the oil-immersion objective lens. After the Gaussian beam with a wavelength of 980 nm is output and passes through the oil-immersion objective lens with a NA of 1.4, a strongly focused Gaussian beam is generated, which irradiates the NaYF4: 20% Yb 3+ @Au core-shell microspheres located at the focal position. For the NaYF4: 20% Yb 3+ crystal, when the illumination wavelength matches its absorption wavelength, it will cause the electrons inside it to produce a resonance effect, and its polarizability under the resonance effect is:

[0044]

[0045] where ω is the frequency of the illumination light, ω p-NY is the plasma angular frequency of the NaYF4: 20% Yb 3+ crystal, σ is the damping coefficient of the NaYF4: 20% Yb 3+ crystal, j is the imaginary unit, and ω0 is the resonance angular frequency of the NaYF4: 20% Yb 3+ crystal.

[0046] As Figure 3 shown, Yb 3+The ion has three resonant transitions with resonant angular frequencies of ω 01 , ω 02 , ω 03 , which will create three polarizabilities χ1, χ2, χ3. Therefore, considering the polarizabilities introduced by all resonant transitions, the dielectric constant ε 3+ of the NaYF4:20% Yb LnNP crystal is:

[0047]

[0048] where 1 + χ0 ≈ n 2 is the relative dielectric constant of the pure NaYF4 crystal. Under 976.5 nm illumination, n = 1.46, and χ1, χ2, χ3 are the polarizabilities corresponding to the NaYF4:20% Yb 3+ crystal at three different resonant angular frequencies respectively, and ε0 is the vacuum dielectric constant.

[0049] Subsequently, the Drude model considering the influence of the shell thickness on the microsphere is used to analyze the polarizability of Au. For noble metals at optical frequencies, the dielectric function can be expressed in the form of the following sum:

[0050] ε Au (ω) = ε0(1 + χ ∞ + χ D (ω)) (4)

[0051] where χ ∞ is the background magnetic susceptibility of Au with a value of 8.5, and χ D (ω) is the Drude response of the conduction electrons:

[0052]

[0053] where ω p_Au is the plasma resonance frequency of the Au shell, ω is the frequency of the illumination light, and Г is the derivative of the relaxation time of Au, which is related to the shell thickness:

[0054] Γ = Γ Au + Av F / (b - a) (6)

[0055] where Г Au has a value of 1.05×10 14 rad s -1 , v F is the Fermi velocity, which has a value of 1.43×10 8 cm / s for Au, A is a parameter determined by the geometry and theory used to derive this expression, A = 1 for a simple Drude model and isotropic scattering, b is the radius of the particle, and a is the radius of the nanocrystal.

[0056] For the core-shell microspheres trapped in the optical trap, their equivalent polarizability is as follows:

[0057]

[0058] where k m is the wave vector of the incident light in the ambient medium, and α CM is the polarizability of the core-shell microsphere in the static limit (i.e., the static response of electrons when the atomic nucleus remains unchanged):

[0059]

[0060] where ε m is the dielectric constant of the ambient medium, p = a 3 / b 3 represents the volume fraction, ε LnNP and ε Au are terms related to the incident light frequency ω, and, ε Au is also related to the shell thickness.

[0061] The optical forces on the NaYF4:20% Yb 3+ crystal and the NaYF4:20% Yb 3+ @Au core-shell microspheres are calculated as shown in Figure 3 . Among them, the maximum optical force values of the NaYF4:20% Yb 3+ @Au core-shell microsphere and the NaYF4:20% Yb 3+ crystal are 12.8 pN and 0.21 pN respectively, and the axial optical trap stiffnesses are 4.3 pN / μm / mW and 22.56 pN / μm / mW respectively. The axial optical trap has increased by an order of magnitude.

[0062] If the illumination light wavelength and the Au shell thickness are tuned, the conversion of the NaYF4:20% Yb 3+ @Au core-shell microsphere from reverse, stable trapping, optical thrust to optical traction can be achieved, as shown in Figure 4 . The black area indicates that the axial optical force curve is reverse, the white area indicates that the axial optical force is optical thrust, the gray points indicate that optical traction appears at this position, and the area changing from light gray to dark gray indicates stable trapping. The change in the color scale indicates the change in the optical trap stiffness.

[0063] As can be seen from the above, in addition to the advantages of simple structure, easy synthesis, and light system mass, the present invention also has the following advantages:

[0064] (1) The NaYF4:20% Yb 3+ @Au core-shell microspheres designed by the present invention have the characteristic of enhanced optical force, compared with the NaYF4:20% Yb3+ Compared with the crystal, its optical force can be increased by an order of magnitude.

[0065] (2) The designed NaYF4: 20% Yb 3+ @Au core-shell microspheres have near-infrared tunable characteristics. By adjusting the thickness of the Au shell, the conversion from reverse, stable trapping, optical thrust to optical traction can be achieved.

[0066] Due to the low cytotoxicity and high chemical stability of lanthanide upconversion nanocrystals and metal nanoparticles such as Au and Ag, UCNs@Metals core-shell microspheres can become an efficient and multifunctional tool for biological imaging and therapy. In addition, this resonance effect can also enhance scattering by increasing the absolute value of the CM polarizability by adjusting the illumination wavelength, which may increase the signal-to-noise ratio of scattering-based microscopy techniques such as dark-field microscopy and interference scattering microscopy.

[0067] Those of ordinary skill in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of protection of this application is limited to these examples; under the concept of this application, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of one or more embodiments in this application as described above. For the sake of brevity, they are not provided in detail.

[0068] One or more embodiments of this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of one or more embodiments of this application shall be included within the scope of protection of this application.

Claims

1. A core-shell type particle with the characteristic of enhanced optical force, characterized in that, The nuclear material is a nanocrystal with a metal shell, and the nanocrystal is NaYF4: 20% Yb 3+ , and the metal material is gold; The thickness of the metal shell is 2 - 20 nm, and the ratio of the radius of the nanocrystal to the thickness of the shell is 9:

1.

2. A method for enhancing optical force, characterized in that, strong A focused Gaussian beam is irradiated onto the core-shell particles with the optical force enhancement property as described in Claim 1, and the thickness of the shell material in the core-shell particles is controlled so that the metal shell and the nanocrystal resonate to achieve optical force enhancement.

3. The method for enhancing optical force according to claim 2, characterized in that, The Gaussian beam is a strongly focused Gaussian beam, which is obtained by outputting after a Gaussian beam passes through an oil immersion objective lens with an NA value of 1.

4.

4. Application of a core-shell particle with the optical force enhancement property as described in Claim 1 in enhancing optical force.

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