A method for rapidly identifying trace ligand molecules using SERS substrates and quantum dots

Abstract

The present invention belongs to the technical field of surface-enhanced Raman detection, and discloses a method for rapidly identifying trace ligand molecules by using a SERS substrate and quantum dots. A combined SERS substrate modified with PbS quantum dots is prepared by combining a SERS substrate and PbS quantum dots; the combined SERS substrate modified with PbS quantum dots is used for qualitative detection of the Raman spectrum of the quantum dot capping ligand molecules; the band gap excitation of the PbS quantum dots forms resonance Raman, and on the basis of the enhancement of the SERS on the spectral signal, the Raman spectral signal is enhanced secondly to identify trace capping ligand molecules on the PbS quantum dots. The present invention uses a very small amount of PbS quantum dots to produce secondary enhancement of the SERS substrate, compensating for the problem that the spectral signal is too low due to the small number of capping ligand molecules; by combining the SERS substrate and PbS quantum dots, a method with high sensitivity, strong selectivity, high accuracy and fast detection speed is established.

Description

Technical Field

[0001] The present invention belongs to the technical field of surface-enhanced Raman detection, and particularly relates to a method for rapidly identifying trace ligand molecules by using an SERS substrate and quantum dots. Background Art

[0002] At present, semiconductor quantum dots (QDs) have considerable advantages in the optoelectronic field because their optical and electrical properties can be regulated by adjusting their size, shape, and composition. In particular, III-V and IV-VI semiconductor colloidal quantum dots and core / shell quantum dots have been extensively explored in recent years in directions including biexciton quantum yield, fluorescence blinking, Auger process, and multiple exciton generation. In addition to the intrinsic properties of quantum dots, their surface and environmental properties also play important roles in many applications. During the synthesis of quantum dots, in order to stabilize the quantum dots and keep them in the nanoscale without aggregating into bulk materials, one or several layers of organic molecules and other surface passivation layers, i.e., capping ligands, are often coated around the quantum dots. The types of these capping ligands can affect the electronic or optical properties of quantum dots, such as the energy band position of quantum dots, exciton dissociation, surface-mediated charge trapping, and electron mobility, etc.; at the same time, the excitation of quantum dots will in turn affect the vibration modes of the capping ligand molecules. However, the current research on quantum dot capping ligand molecules is very limited.

[0003] Surface-enhanced Raman scattering (SERS) spectroscopy has become an effective method for molecular characterization. Compared with ordinary Raman scattering, as a noble metal nanostructure serving as an SERS probe, the enhanced electromagnetic field generated by its local surface plasmon can enhance the scattering cross-section of the molecules adsorbed on it, thereby enhancing the scattering signal intensity. In addition to noble metals, semiconductor quantum dots have also been proven to have the role of SERS probes, and their enhancement coefficients are as high as 10 3 ~10 6In particular, PbS quantum dots play a significant role in SERS both physically and chemically: lead sulfide (PbS) quantum dots can strongly enhance the electromagnetic field generated by noble metal local surface plasmons and transfer carriers to surface-capping ligand molecules, thereby changing the spectroscopic characteristics of Raman spectra. Although there are already technologies using semiconductor nanostructures such as PbS as SERS probes to identify surface-capping ligand molecules such as thiopyridine (authorized announcement number CN100498300C, 200610016841.7), using only semiconductor nanostructures as SERS probes has an unclear spectroscopic enhancement effect on ligand molecules with relatively complex vibration modes due to low detection sensitivity. Although the bonding mode between the surface-capping ligand molecules of PbS quantum dots and the quantum dots has been explored to a small extent through surface-enhanced Raman scattering (SERS) spectroscopy technology in recent years, the size effect of quantum dots under photoexcitation on the spectroscopic characteristics of surface-capping ligand molecules, that is, the size-dependent exciton-molecule vibration coupling, remains unknown. Therefore, there is an urgent need to design a new method for quickly identifying trace ligand molecules.

[0004] Through the above analysis, the problems and defects existing in the prior art are as follows: the current research on quantum dot surface-capping ligand molecules is very limited, and the size effect of quantum dots under photoexcitation on the spectroscopic characteristics of surface-capping ligand molecules, that is, the size-dependent exciton-molecule vibration coupling, remains unknown. Summary of the Invention

[0005] In view of the problems existing in the prior art, the present invention provides a method for quickly identifying trace ligand molecules using a SERS substrate and quantum dots, particularly relating to a SERS probe, quantum dots, and a method for identifying trace surface-capping ligand molecules of quantum dots using SERS.

[0006] The present invention is implemented as follows. A method for quickly identifying trace ligand molecules using a SERS substrate and quantum dots includes: combining a SERS substrate and PbS quantum dots to prepare a combined SERS substrate modified with PbS quantum dots; using the combined SERS substrate modified with PbS quantum dots to qualitatively detect the Raman spectrum of quantum dot surface-capping ligand molecules; the bandgap excitation of PbS quantum dots forms resonance Raman, and on the basis of the enhancement of the spectral signal by SERS, the Raman spectral signal is enhanced secondarily to identify trace surface-capping ligand molecules on PbS quantum dots.

[0007] Further, the quantum dot surface-capping ligand molecule is an oleic acid molecule.

[0008] Further, the number of oleic acid molecules surface-capped and coated on each PbS quantum dot is 20 - 40.

[0009] Further, the method for rapidly identifying trace ligand molecules using a SERS substrate and quantum dots includes the following steps:

[0010] Step 1, prepare a SERS substrate of a gold nanopyramidal dimer array;

[0011] Step 2, synthesize a PbS quantum dot colloidal solution coated with a capping ligand molecule;

[0012] Step 3, clean the SERS substrate obtained in Step 1 using an ultraviolet ozone cleaning chamber, immerse it in ultrapure water, and gently blow it dry with a micro nitrogen stream;

[0013] Step 4, deoxygenate n-hexane with high-purity argon, dilute the PbS quantum dot solution obtained in Step 2, and drop it onto the SERS substrate obtained in Step 3; after the n-hexane solvent evaporates, a SERS substrate loaded with PbS quantum dots is obtained.

[0014] Further, in Step 1, by combining the SERS substrate and PbS quantum dots, a combined SERS substrate modified with PbS quantum dots is obtained by dropping a low-concentration PbS quantum dot solution onto the SERS substrate.

[0015] Further, the synthesized size of the PbS quantum dot colloidal solution in Step 2 is controllable, coated with oleic acid as a capping ligand molecule, and dispersed in n-hexane.

[0016] Further, the first exciton absorption peaks of the PbS quantum dots in Step 2 are 1101, 750, and 728 nm, respectively, labeled as OP-1101, OP-750, and OP-728.

[0017] Further, the concentrations of the PbS quantum dot colloidal solution in Step 2 are 99.2, 96.2, and 132.1 μmol / mL.

[0018] Further, the cleaning time of the SERS substrate in Step 3 is 30 min.

[0019] Further, the loading amount of PbS quantum dots in Step 4 is 1.46×10 11 、5.19×10 11 、5.19×10 11 per cm 2 , and the loading layers of PbS quantum dots are all within a single layer.

[0020] Combined with the above technical solutions and the solved technical problems, the advantages and positive effects of the technical solution to be protected by the present invention are as follows:

[0021] First, in view of the technical problems existing in the above-mentioned prior art and the difficulty of solving such problems, closely combining with the technical solution to be protected by the present invention, as well as the results and data during the R & D process, etc., analyze in detail and profoundly how the technical solution of the present invention solves the technical problems and the creative technical effects brought about after solving the problems. The specific description is as follows:

[0022] The present invention provides a method for rapidly identifying trace ligand molecules by using a SERS substrate and quantum dots. The band gap excitation of PbS quantum dots is utilized to form resonance Raman, and on the basis of the enhancement of the spectral signal by SERS, the Raman spectral signal is enhanced for the second time to identify trace capping ligand molecules on the PbS quantum dots. The method of the present invention has the advantages of high sensitivity, strong selectivity, high accuracy and fast detection speed.

[0023] Compared with the prior art, the present invention has the following advantages compared with the prior art:

[0024] (1) The present invention provides a method for rapidly detecting capping ligand molecules of quantum dots. A very small amount of PbS quantum dots are used to produce a second enhancement of the SERS substrate, compensating for the problem that the spectral signal is too low due to the small number of capping ligand molecules. By combining the SERS substrate and PbS quantum dots, a method for rapidly identifying capping ligand molecules of quantum dots with high sensitivity, strong selectivity, high accuracy and fast detection speed is established.

[0025] (2) The method for identifying trace ligand molecules of the present invention utilizes the band gap excitation of quantum dots to enhance the detection sensitivity of the SERS substrate, greatly enhancing the vibration modes of a small number of capping ligand molecules. Compared with ordinary Raman, cryogenic Raman, and even conventional SERS, a more delicate and accurate molecular vibration spectrum can be obtained. For rapidly characterizing and detecting trace molecules, the method is simple and easy to implement, and the actual detection operation is convenient.

[0026] (3) The present invention provides a method for rapidly identifying trace ligand molecules by using a SERS substrate and PbS quantum dots with high sensitivity, strong selectivity, high accuracy and fast detection speed. The present invention has high scalability: the SERS substrate in the present invention is not limited to gold nanostructures and can be extended to any metal nanostructure probe with surface-enhanced Raman activity; the quantum dots in the present invention are not limited to PbS quantum dots and are applicable to any semiconductor quantum dots; the ligand molecules in the present invention are not limited to the capping ligand oleic acid molecules coating the quantum dots and are applicable to detecting any other type of capping ligand molecules.

[0027] (4) The technical solution of the present invention fills the technical gaps at home and abroad in the industry: So far, there is no invention patent at home and abroad that uses semiconductor quantum dots to improve the detection sensitivity of SERS substrates. The technical solution of the present invention combines SERS substrates and quantum dots, and utilizes the band gap excitation of quantum dots and the local surface plasmon resonance effect of metal nanostructures to achieve high-sensitivity detection of trace molecules. The detection method is simple and accurate, and the technical method has strong expansibility and wide applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments of the present invention. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0029] Figure 1 is a flowchart of a method for rapidly identifying trace ligand molecules using an SERS substrate and quantum dots provided by an embodiment of the present invention;

[0030] Figure 2A is an AFM image of the SERS substrate provided by an embodiment of the present invention;

[0031] Figure 2B is an extinction spectrum of the SERS substrate provided by an embodiment of the present invention;

[0032] Figure 2C is an absorption spectrum of PbS quantum dots of different sizes provided by an embodiment of the present invention; wherein, the dashed line represents the excitation laser at 785 nm;

[0033] Figure 3 are surface-enhanced Raman spectra provided by Embodiments 1, 2, 3 and Comparative Examples 1, 2 of the present invention, and Raman spectra and infrared spectra of Reference Examples 1, 2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] In order to make the objectives, technical solutions and advantages of the present invention clearer, the following further elaborates on the present invention in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0035] Aiming at the problems existing in the prior art, the present invention provides a method for rapidly identifying trace ligand molecules using an SERS substrate and quantum dots. The following describes the present invention in detail with reference to the drawings.

[0036] This part is an explanatory embodiment that expands and explains the technical solutions of the claims in order to enable those skilled in the art to fully understand how the present invention is specifically implemented.

[0037] As Figure 1 shown, the method for rapidly identifying trace ligand molecules by using a SERS substrate and quantum dots provided by the embodiments of the present invention includes the following steps:

[0038] S101, preparing a PbS quantum dot-modified combined SERS substrate by combining a SERS substrate and PbS quantum dots;

[0039] S102, using the combined SERS substrate modified with PbS quantum dots to qualitatively detect the Raman spectrum of the quantum dot capping ligand molecules;

[0040] S103, forming resonance Raman by the band gap excitation of PbS quantum dots, and secondarily enhancing the Raman spectrum signal on the basis of the enhancement of the SERS spectrum signal to identify the trace capping ligand molecules on the PbS quantum dots.

[0041] The quantum dot capping ligand molecule provided by the embodiments of the present invention is oleic acid molecule, and the number of oleic acid molecules capped and coated on the surface of each PbS quantum dot is 20 - 40.

[0042] As a preferred embodiment, the method for rapidly identifying trace ligand molecules by using a SERS substrate and quantum dots provided by the embodiments of the present invention specifically includes the following steps:

[0043] Step 1, preparing a SERS substrate of a gold nanopyramidal dimer array.

[0044] In the embodiments of the present invention, by combining a SERS substrate and PbS quantum dots, a combined SERS substrate modified with PbS quantum dots is obtained by dropping a low-concentration PbS quantum dot solution onto the SERS substrate.

[0045] Step 2, synthesizing a colloidal solution of PbS quantum dots with controllable size, capped with oleic acid as a capping ligand molecule, and dispersed in n-hexane.

[0046] The first exciton absorption peaks of the PbS quantum dots obtained in the embodiments of the present invention are 1101, 750, and 728 nm respectively, labeled as OP-1101, OP-750, and OP-728. The concentration of the colloidal solution of PbS quantum dots provided by the embodiments of the present invention is 99.2, 96.2, and 132.1 μmol / mL.

[0047] Step 3, cleaning the SERS substrate obtained in Step 1 with an ultraviolet ozone cleaning box, immersing it in ultrapure water after 30 min, and gently drying it with a micro nitrogen stream.

[0048] Step 4: After deoxidizing n-hexane with high-purity argon, dilute the PbS quantum dot solution obtained in Step 2, and drop it onto the SERS substrate obtained in Step 3. After the n-hexane solvent evaporates, a SERS substrate loaded with PbS quantum dots is obtained.

[0049] The PbS quantum dot loading amount provided by the embodiment of the present invention is 1.46×10 11 、5.19×10 11 、5.19×10 11 pieces / cm 2 , and the number of loaded layers is within a single layer.

[0050] The specific implementation principle of the method for rapidly identifying trace ligand molecules using the SERS substrate and quantum dots provided by the embodiment of the present invention is as follows: Under laser irradiation, the local surface plasmon of the gold nanodimer array generates a local electromagnetic field within the gap of several nanometers of the triangular pyramid dimer. Due to the dielectric property of PbS semiconductor, the local electromagnetic field can be further enhanced, which is beneficial to enhancing the Raman scattering signal of the PbS quantum dot capping ligand molecules. In addition, when the bandgap of the PbS quantum dot is close to the incident laser energy, the PbS quantum dot undergoes bandgap excitation, which can form resonance Raman and further enhance the Raman signal of the capping ligand molecules, thereby realizing highly sensitive detection of a small number of molecules.

[0051] Some positive effects have been achieved during the research and development or use of the embodiment of the present invention, and it indeed has great advantages compared with the prior art. The following content will be described in combination with the data, charts, etc. of the experimental process.

[0052] Example 1

[0053] On the pretreated ITO conductive glass, a gold nanotriangular column dimer array is prepared by angle-resolved nanosphere lithography technology and evaporation technology to obtain a SERS substrate with polarization characteristics and a local surface plasmon effect under 785 nm laser, as shown in Figures 2A to 2C .

[0054] Figure 2A And Figure 2B are the AFM image and extinction spectrum of the SERS substrate respectively. It can be seen from Figure 2A that the SERS substrate formed by the gold nanodimer array is arranged regularly and orderly, and has obvious double triangular pyramid shapes within the size range of 100 nm. It can be seen from Figure 2BIt can be seen that when the SERS substrate is in the longitudinal polarization extinction spectrum (solid line), that is, when the electric field direction of the incident light is parallel to the long axis direction of the gold nanodimer, a maximum peak is generated near the laser wavelength of 785 nm emitted by the infrared laser; while in the transverse polarization extinction spectrum (dashed line), that is, when the electric field direction of the incident light is parallel to the short axis direction of the gold nanodimer, no extinction occurs near 785 nm. This indicates that the gold nanodimer array has a polarization effect. The local surface plasmon resonance frequency of its long axis is at 785 nm, which can amplify the optical field of the 785 nm incident laser to form surface enhanced Raman scattering, and as a SERS substrate, it can amplify the Raman spectral signals of surrounding molecules.

[0055] A single layer of PbS quantum dots is coated on the surface of the above SERS substrate. Specifically, a colloidal dilution solution of PbS quantum dots coated with oleic acid ligands and having a first exciton absorption peak at 1101 nm is dropped. After the quantum dot solution is evenly dispersed and the solvent evaporates, the resulting Example 1 is subjected to SERS measurement.

[0056] Example 2

[0057] A single layer of PbS quantum dots is coated on the surface of the SERS substrate in the same manner as in Example 1. The difference is that the colloidal dilution solution of quantum dots coated with oleic acid ligands dropped has a first exciton absorption peak at 750 nm.

[0058] Example 3

[0059] A single layer of PbS quantum dots is coated on the surface of the SERS substrate in the same manner as in Example 1. The difference is that the colloidal dilution solution of quantum dots coated with oleic acid ligands dropped has a first exciton absorption peak at 728 nm.

[0060] Figure 2C are the absorption spectra of quantum dots of different sizes. From Figure 2C It can be seen that as the size of the PbS quantum dots decreases, its first exciton absorption peak undergoes a blue shift and the band gap gradually expands. Among them, the first exciton absorption peak of Example 2 is around 750 nm, which is closest to the 785 nm incident laser and can produce a resonance enhancement effect on the basis of surface enhanced Raman scattering of the SERS substrate; while the first exciton absorption peak of Example 1 is at 1101 nm and the first exciton absorption peak of Example 3 is near 728 nm, which deviate greatly from the 785 nm incident laser, and both only have a signal amplification effect of surface enhanced Raman scattering on the SERS substrate.

[0061] Figure 3 are the surface enhanced Raman spectra of Example 1, 2, 3 and Comparative Example 1 and the ordinary spectra of Comparative Example 2, Reference Example 1, 2. From Figure 3It can be seen that the SERS signal of oleic acid molecules on the Au nanorod array SERS substrate (Comparative Example 1) and as the surface modification of PbS quantum dots (Examples 1, 2, and 3) has higher sensitivity to the vibrations detected by ordinary Raman (Comparative Example 2), including olefin groups (out-of-plane bending of C═C-H at about 700 cm -1 and in-plane bending of C═C-H at about 1260 cm -1 ), as well as the methylene long chain (rocking vibration of CH3 at 846 cm -1 , stretching vibration of C-C at 1060 - 1200 cm -1 , twisting vibration of CH2 at 1295 cm -1 , and scissoring vibration of CH2 at 1432 cm -1 ). Compared with the Raman spectrum of oleic acid crystals at -18 °C (Reference Example 1) and the infrared spectrum at room temperature (Reference Example 2), the SERS spectrum of PbS quantum dots on the Au nanorod array SERS substrate exhibits unique vibration modes, including the olefin group mode (bending vibration of C═C-C at 570 cm -1 ) in the low Raman shift region, the carboxyl group mode (stretching vibration of C-C at 900 cm -1 , bending vibration of α-CH2 at 1410 cm -1 ), and the methylene long chain mode (rocking vibration of CH2 at 1170 cm -1 ). These vibration modes are significantly enhanced, especially in Example 2, where a more refined vibration spectrum of oleic acid molecules is obtained, indicating that resonance Raman formed by bandgap excitation of PbS quantum dots further enhances the SERS signal of the capping ligand molecules, thus achieving highly sensitive detection of a small number of molecules (about 20 - 40 oleic acid ligand molecules on the surface of each quantum dot). These phenomena demonstrate the effectiveness of local surface plasmons in improving spectral detection, and such highly sensitive spectra could only be obtained at low temperatures in the past.

[0062] Table 1 Raman Shifts and Vibration Mode Assignments of Oleic Acid Molecules

[0063]

[0064]

[0065] Comparative Example 1

[0066] To compare with the SERS spectrum of oleic acid molecules coated around PbS quantum dots, pure oleic acid liquid was dropped on the SERS substrate in the present invention, and its SERS spectrum was measured at 25 °C. The results are shown in Figure 3 .

[0067] Comparative Example 2

[0068] To compare with the SERS spectrum of oleic acid molecules coated around PbS quantum dots, the present invention dropped pure oleic acid liquid on a cleaned glass slide and tested its SERS spectrum at 25 °C. The results are shown in Figure 3 .

[0069] Reference Example 1

[0070] To compare with the SERS spectrum of oleic acid molecules coated around PbS quantum dots, the present invention found the Raman spectrum of oleic acid molecules at -18 °C and the infrared absorption spectrum of oleic acid molecules at room temperature in the literature. The comparison results are shown in Figure 3 .

[0071] Reference Example 2

[0072] To compare with the SERS spectrum of oleic acid molecules coated around PbS quantum dots, the present invention found the infrared absorption spectrum of oleic acid molecules at room temperature in the literature. The comparison results are shown in Figure 3 .

[0073] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention shall be covered by the protection scope of the present invention.

Claims

1. A method for rapidly identifying trace ligand molecules using an SERS substrate and quantum dots, characterized in that, The method for rapidly identifying trace ligand molecules by using a SERS substrate and quantum dots comprises the following steps: Step 1, preparing a SERS substrate of a gold nanopyramidal dimer array; Step 2, synthesizing a PbS quantum dot colloidal solution coated with a capping ligand molecule; Step 3, cleaning the SERS substrate obtained in Step 1 with an ultraviolet ozone cleaning box, then immersing it in ultrapure water, and gently drying it with a micro nitrogen stream; Step 4, deoxidizing n-hexane with high-purity argon, diluting the PbS quantum dot solution obtained in Step 2, and dropping it onto the SERS substrate obtained in Step 3; after the n-hexane solvent volatilizes, a SERS substrate loaded with PbS quantum dots is obtained; In Step 1, a combined SERS substrate and PbS quantum dots are used, and a combined SERS substrate modified with PbS quantum dots is obtained by dropping a low-concentration PbS quantum dot solution onto the SERS substrate; The synthesized size of the PbS quantum dot colloidal solution in Step 2 is controllable, it is coated with oleic acid as a capping ligand molecule, and is dispersed in n-hexane; The first exciton absorption peaks of the PbS quantum dots in Step 2 are 1101, 750, and 728 nm respectively, and are labeled as OP-1101, OP-750, and OP-728; The loading amount of PbS quantum dots in Step 4 is 1.46×10 11 , 5.19×10 11 , 5.19×10 11 pieces / cm 2 , and the number of loaded layers of PbS quantum dots is within a single layer.

2. The method for rapidly identifying trace ligand molecules by using an SERS substrate and quantum dots according to claim 1, wherein The number of oleic acid molecules capped and coated on the surface of each PbS quantum dot is 20-40.

3. The method for rapidly identifying trace ligand molecules by using an SERS substrate and quantum dots as claimed in claim 1, wherein The concentrations of the PbS quantum dot colloidal solution in Step 2 are 99.2, 96.2, and 132.1 μmol / mL.

4. The method for rapidly identifying trace ligand molecules by using an SERS substrate and quantum dots according to claim 1, characterized in that, The time for cleaning the SERS substrate in Step 3 is 30 min.

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