Preparation method and application of luminescent nanocluster

Abstract

The invention relates to the technical field of developing materials, and discloses a preparation method and application of a luminescent nanocluster, the preparation method comprises the following steps: step 1, dissolving a metal precursor in methanol to prepare a metal precursor solution, the metal precursor being selected from silver nitrate, copper nitrate or a combination thereof; step 2, adding 4, 6-dimethyl-2-mercaptopyrimidine into the metal precursor solution, and carrying out a stirring reaction to form a complex; 3, adding sodium hydroxide into the reaction mixture, and continuously stirring until a luminescence phenomenon occurs; step 4, carrying out centrifugal treatment on the reacted solution, and separating to obtain an upper solution and a lower precipitate; step 5, respectively collecting the upper solution and the lower precipitate redissolved by methanol, and standing; step 6, obtaining a high-luminescence nanocluster crystal in a natural volatilization or standing and airing mode; the absolute luminous quantum efficiency of the material prepared by the invention is 60% or above, and some absolute luminous quantum efficiencies are even close to 100%; the light-emitting wavelength can be adjusted from yellow-green to red, the latent fingerprint showing precision is extremely high, the third-level characteristics of fingerprints can be clearly shown, the advantages of being simple in synthesis process, low in cost, small in pollution and the like are achieved, and wide application prospects are achieved in the technical field of new-generation fingerprint detection.

Description

Technical Field

[0001] The present invention relates to the technical field of developing materials, and more particularly to a preparation method of luminescent nanoclusters and applications thereof. Background Art

[0002] Latent fingerprints refer to invisible biological traces left on the surface of objects that need to be developed by physical or chemical methods. The current mainstream luminescent powder development technology enhances the recognizability of fingerprint signals under the excitation of a specific wavelength light source through specific interactions between substances. Although traditional organic luminescent dyes (such as rhodamine 6G and sodium fluorescein) have high quantum yields, they face application bottlenecks such as strong toxicity and poor photostability. In recent years, significant progress has been made in nanomaterial research: rare earth doped nanoparticles (YVO4:Eu 3+ ) optimizes luminescence performance through lattice regulation; metal organic frameworks (MOFs) improve adsorption selectivity with the help of porous structures; carbon quantum dots (CQDs) have attracted much attention due to their excellent optical properties and biocompatibility.

[0003] However, CQDs still face three technical obstacles in practical applications: first, the blue light emission (450-500nm) under ultraviolet excitation overlaps with the common substrate background emission spectrum, resulting in insufficient imaging contrast; second, the π-π stacking effect in the liquid phase system causes concentration-dependent luminescence red shift and intensity attenuation, and the aggregation-induced quenching effect in the solid state affects the development effect; third, the material synthesis lacks atomic-level precise control, making it difficult to establish a structure-activity relationship model of "structure-performance-development mechanism", which restricts in-depth mechanism research. Summary of the Invention

[0004] In order to solve the problems existing in the above-mentioned existing latent fingerprint revealing technology, the present invention provides a preparation method of luminescent nanoclusters and applications thereof.

[0005] A method for preparing luminescent nanoclusters comprises the following steps:

[0006] Step 1: dissolving a metal precursor in methanol to prepare a metal precursor solution, wherein the metal precursor is selected from silver nitrate, copper nitrate or a combination thereof;

[0007] Step 2: Add 4,6-dimethyl-2-mercaptopyrimidine to the metal precursor solution, and stir to react to form a complex;

[0008] Step 3: Add sodium hydroxide to the reaction mixture and continue stirring until photoluminescence occurs;

[0009] Step 4: Centrifuge the reaction solution to separate the upper solution and the lower precipitate;

[0010] Step 5: Collect the upper solution and the lower precipitate redissolved with methanol and let them stand;

[0011] Step 6: Obtain luminescent nanocluster crystals by natural evaporation or standing to dry.

[0012] Preferably, the metal precursor is silver nitrate, and the prepared luminescent nanoclusters are Ag6(SN2C6H7)6, which has the strongest emission peak at 550nm and exhibits yellow-green luminescence.

[0013] Preferably, the metal precursors are silver nitrate and copper nitrate, and the prepared luminescent nanoclusters are Ag2Cu4(SN2C6H7)6, which has the strongest emission peak at 695nm and exhibits red-orange luminescence.

[0014] Preferably, the metal precursor is copper nitrate, and the prepared luminescent nanoclusters are Cu6(SN2C6H7)6, which has the strongest emission peak at 745nm and emits red light.

[0015] Preferably, the stirring reaction time in step 2 is 2 hours.

[0016] Preferably, the stirring reaction time in step 3 is 2 hours.

[0017] Preferably, the time for natural volatilization and crystal growth in step 6 is 1-2 weeks.

[0018] Preferably, the particle size of the luminescent nanoclusters is less than 2 nm, and the luminescent quantum yield is greater than 60%, wherein the luminescent quantum yield of the copper nanoclusters is close to 100%.

[0019] 9. An application of the luminescent nanoclusters prepared by the above preparation method, wherein the luminescent nanoclusters are applied to latent fingerprint visualization, the method for applying the luminescent nanoclusters to latent fingerprint visualization comprising the following steps:

[0020] Step a: grinding the luminescent silver-copper nanoclusters according to claim 8 into powder;

[0021] Step b: Dip the powder using a fingerprint brush;

[0022] Step c: evenly scattering the powder on the surface of the object where the latent fingerprint is located and lightly sweeping;

[0023] Step d: Use an ear cleaning bulb to gently blow air on the surface of the object to remove excess powder that has not combined with the latent fingerprint;

[0024] Step e: Use an ultraviolet light source with a wavelength of 300-400 nm to illuminate the surface of the object and observe the luminescent latent fingerprint image.

[0025] Preferably, the object is selected from aluminum foil, a kitchen knife, a coin, a plastic bottle, transparent glass, wood or leather.

[0026] The beneficial effects of the present invention are that the material prepared by the present invention has extremely high latent fingerprint display accuracy and can clearly display the third-level features of fingerprints, making the fingerprint identification results more accurate and reliable, and providing strong technical support for criminal investigation, identity recognition and other fields.

[0027] Furthermore, the powder is inexpensive to produce and non-toxic, reducing operational costs while ensuring operator safety and health. In summary, the silver / copper nanocluster latent fingerprint revealing powder of the present invention exhibits significant advantages in terms of raw material availability, preparation process, ease of use, biotoxicity, stability, optical properties, and display accuracy. It not only significantly promotes the development of fingerprint revealing technology but also possesses immense value in practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the luminescent nanoclusters of the present invention being applied to multi-color visualization of latent fingerprints;

[0029] Figure 2 are the unit cell parameters of different clusters of the present invention;

[0030] Figure 3 is a crystal structure diagram of different clusters of the present invention; Figure 4 is the infrared spectra of different clusters of the present invention;

[0031] Figure 5 The UV-visible diffuse reflectance spectra of different clusters of the present invention and their corresponding Tauc plots;

[0032] Figure 6 The crystal luminescence and photoluminescence spectra of different clusters of the present invention are shown (PLE is the photoluminescence excitation spectrum, PL is the photoluminescence emission spectrum);

[0033] Figure 7 are the photoluminescence decay curves of different clusters of the present invention;

[0034] Figure 8 is the emission spectra of methanol solutions of different nanoclusters under N2 purification, ambient conditions and N2 re-purification conditions of the present invention; λ ex They are 380, 400, and 400 nm respectively;

[0035] Figure 9 is a table of luminescence spectroscopic parameters of different clusters of the present invention;

[0036] Figure 10Representative fingerprint images of the present invention taken on aluminum foil using commercial powders and different clusters under white light and ultraviolet light;

[0037] Figure 11 Representative fingerprint images of the present invention taken on a kitchen knife using commercial powder and different clusters under white light and ultraviolet light;

[0038] Figure 12 Representative fingerprint images of the present invention taken on coins using commercial powders and different clusters under white light and ultraviolet light;

[0039] Figure 13 Representative fingerprint images of the present invention taken on a plastic bottle using commercial powder and different clusters under white light and ultraviolet light;

[0040] Figure 14 Representative fingerprint images of the present invention taken on clear glass using commercial powders and different clusters under white light and ultraviolet light;

[0041] Figure 15 Representative fingerprint images of the present invention taken on wood using commercial powder and different clusters under white light and ultraviolet light;

[0042] Figure 16 Representative fingerprint images of the present invention taken on leather using commercial powder and different clusters under white light and UV light. DETAILED DESCRIPTION

[0043] The subject matter described herein will now be discussed with reference to example embodiments. It should be understood that these embodiments are discussed solely to enable those skilled in the art to better understand and implement the subject matter described herein, and that the functions and arrangements of the elements discussed may be varied without departing from the scope of this specification. Various examples may omit, substitute, or add various processes or components as needed. Furthermore, features described in some examples may be combined in other examples.

[0044] At least one embodiment of the present invention discloses a method for preparing a luminescent nanocluster, comprising the following steps:

[0045] Step 1: dissolving a metal precursor in methanol to prepare a metal precursor solution, wherein the metal precursor is selected from silver nitrate, copper nitrate or a combination thereof;

[0046] Step 2: Add 4,6-dimethyl-2-mercaptopyrimidine to the metal precursor solution, and stir to react to form a complex;

[0047] Step 3: Add sodium hydroxide to the reaction mixture and continue stirring until luminescence occurs;

[0048] Step 4: Centrifuge the reaction solution to separate the upper solution and the lower precipitate;

[0049] Step 5: Collect the upper solution and the lower precipitate redissolved with methanol and let them stand;

[0050] Step 6: Obtain luminescent nanocluster crystals by natural evaporation or standing to dry.

[0051] in:

[0052] The stirring reaction time in step 2 is 2 hours.

[0053] The stirring reaction time in step 3 is 2 hours.

[0054] The drying time in step 6 is 1-2 weeks.

[0055] The particle size of the luminescent nanoclusters is less than 2 nm, and the luminescence quantum yield is greater than 60%.

[0056] At least one embodiment of the present invention discloses that the metal precursor is silver nitrate, and the prepared luminescent nanoclusters are Ag6(SN2C6H7)6, whose strongest emission peak is at 550nm, exhibiting yellow-green luminescence.

[0057] At least one embodiment of the present invention discloses that the metal precursors are silver nitrate and copper nitrate, and the prepared luminescent nanoclusters are Ag2Cu4(SN2C6H7)6, whose strongest emission peak is at 695nm, exhibiting orange-red luminescence.

[0058] At least one embodiment of the present invention discloses that the metal precursor is copper nitrate, and the prepared luminescent nanoclusters are Cu6(SN2C6H7)6, whose strongest emission peak is at 745nm, exhibiting red luminescence.

[0059] At least one embodiment of the present invention discloses an application of the luminescent nanoclusters prepared by the above-mentioned preparation method, wherein the luminescent nanoclusters are applied to latent fingerprint visualization. The method of applying the luminescent nanoclusters to latent fingerprint visualization comprises the following steps:

[0060] Step a: grinding the luminescent silver-copper nanoclusters according to claim 8 into powder;

[0061] Step b: Dip the powder using a fingerprint brush;

[0062] Step c: evenly scattering the powder on the surface of the object where the latent fingerprint is located and lightly sweeping;

[0063] Step d: Use an ear cleaning bulb to gently blow air on the surface of the object to remove excess powder that has not combined with the latent fingerprint;

[0064] Step e: Use an ultraviolet light source with a wavelength of 300-400 nm to illuminate the surface of the object and observe the luminescent latent fingerprint image.

[0065] The object is selected from aluminum foil, kitchen knife, coin, plastic bottle, transparent glass, wood or leather.

[0066] The detailed technical solutions adopted in the present invention are as follows:

[0067] 1. Preparation of Highly Luminescent Nanocluster Materials

[0068] Preparation of Ag6: Dissolve silver nitrate in methanol, ultrasonically dissolve, transfer to a flask, and begin stirring. Then, add 4,6-dimethyl-2-mercaptopyrimidine (DMPT) and stir for 2 hours. Then, add sodium hydroxide and continue stirring for 2 hours until a strong green luminescence appears under UV light. Centrifuge the reaction solution, transfer the upper layer to a test tube, label it, and allow it to evaporate and dry naturally. Redissolve the lower precipitate in methanol, transfer it to a test tube, label it, and let it stand. After approximately 1-2 weeks, crystals will form by adhering to the wall.

[0069] Preparation of Cu6: Dissolve copper nitrate trihydrate in methanol in a flask and begin stirring. Add DMPT and stir for 2 hours. Add sodium hydroxide and continue stirring for 2 hours until a red glow appears under UV light. Centrifuge the resulting solution, transfer the upper layer to a beaker, and dissolve the lower precipitate in methanol and transfer it to a beaker. Mark the container, allow to evaporate, and air dry. After about two weeks, crystals will form at the bottom of the beaker.

[0070] Preparation of Ag2Cu4: Dissolve copper nitrate trihydrate in methanol, then silver nitrate in methanol, place each in a flask and begin stirring. Then add DMPT and stir for 2 hours. Then add sodium hydroxide and continue stirring for 2 hours until an orange glow appears under UV light. Centrifuge the resulting solution, transfer the upper layer to a beaker, and dissolve the lower precipitate in methanol. Transfer the solution to a test tube, label it, and let it sit. After approximately two weeks, crystals will form at the bottom of the test tube.

[0071] 2. Characterization of Highly Luminescent Nanocluster Materials

[0072] The three prepared clusters have precise atomic composition and structure, namely Ag6(SN2C6H7)6, Ag2Cu4(SN2C6H7)6 and Cu6(SN2C6H7)6. Their specific unit cell parameters and structures are shown in Figure 2 and Figure 3 As shown in the figure, it can be seen that the arrangement of Ag6 and Ag2Cu4 cluster molecules has the same crystal form, both belonging to the trigonal R3c space group, with 6 cluster molecules in one unit cell; while the Cu6 crystal phase arrangement belongs to the monoclinic P21 / c space group, with 4 cluster molecules in one unit cell.

[0073] Infrared spectroscopy: Figure 4 As shown, the copper / silver nanocluster material prepared in the present invention has a wavelength of 3000-3700 cm -1 and 1630cm -1 The absorption peaks appearing near 1500-400 cm are attributed to the characteristic absorption of water molecules. -1 The absorption peaks appearing within this range are attributed to the vibrational absorption of the -C=C, -CN, -C=N, -CS, and -C=S functional groups in the DMPT ligand. The infrared spectra of the three nanoclusters are very similar and significantly different from the absorption peaks of the pure ligand, indicating that all three clusters were successfully synthesized and stabilized by the DMPT ligand.

[0074] UV-visible diffuse reflectance spectrum: Figure 5 As shown in the figure, the absorption of Ag6 nanocluster materials is in the range of 200-600nm, and the optical band gap is calculated to be 2.78eV by the Tauc method; after being doped with copper atoms, the absorption red shift of Ag2Cu4 and Cu6 nanocluster materials is in the range of 200-700nm, which also indicates the red shift of the emission wavelength, and the optical band gap is 2.44eV.

[0075] 3. Photoluminescence properties of highly luminescent nanocluster materials

[0076] like Figure 6 As shown, the strongest excitation peaks (λ ex ) are located at 380nm, 400nm and 400nm respectively, and Figure 5 The UV-visible absorption results are consistent; the strongest emission peak (λ em ) are located at 550nm, 695nm and 745nm respectively, and can present three bright and clear luminous colors of yellow-green, orange-red and red respectively (see Figure 6 This rich color representation will help fingerprint details be clearly visible under different lighting conditions, greatly improving the accuracy and efficiency of fingerprint recognition.

[0077] like Figure 7 As shown in the figure, the luminescence lifetimes of the three nanocluster materials Ag6, Ag2Cu4 and Cu6 are 17.0μs, 12.2μs and 10.9μs respectively after single exponential fitting. Figure 3From the molecular structure of a single cluster, it can be seen that the six ligands of the Ag6 and Ag2Cu4 cluster molecules form a π-π stacking structure in pairs, while the Cu6 cluster molecule has only four ligands forming a π-π stacking structure in pairs, indicating that the Ag6 and Ag2Cu4 clusters have a stronger rigid structure than Cu6, and the rotation of the ligands themselves is greatly suppressed, which can reduce non-radiative transition channels (such as vibrational relaxation), thereby having a longer luminescence lifetime. The lifetimes of these three clusters are all in the microsecond level, and are most likely phosphorescent. Therefore, the luminescence spectrum was further tested under N2 purge-O2 dissolution conditions ( Figure 8 In the experiment, the methanol solutions of the three nanoclusters were purged with ultra-high-purity N2 for 5 minutes, then placed in an air environment for 5 minutes, and then purged with N2 for another 5 minutes. Figure 8 The results show that the three cluster materials purged with N2 all exhibited higher luminescence intensity than the ambient solution (containing dissolved air), which proves that the luminescence properties of these three clusters are phosphorescence.

[0078] like Figure 9 The three nanoclusters, Ag6, Ag2Cu4, and Cu6, all exhibit significant Stokes shifts of 170nm, 295nm, and 345nm, respectively. This indicates that after absorbing light energy, the cluster materials dissipate more energy through intersystem crossing (ISC), resulting in phosphorescent emission with a red-shifted wavelength. The phosphorescence quantum yields of the three nanoclusters, Ag6, Ag2Cu4, and Cu6, are 78.3%, 60.11%, and 95.53%, respectively. This is likely due to the strong intermolecular interactions, such as π…π and CH…π interactions. These weak interactions can inhibit molecular motion and vibration, keeping the molecules relatively stationary, thereby increasing ISC efficiency and enabling more singlet excitons to transition to triplet states, ultimately dissipating them as phosphorescent radiative transitions.

[0079] 4. The luminescent nanocluster material prepared by the present invention is used for latent fingerprint detection. The method comprises the following steps:

[0080] First, the three cluster materials need to be placed in a mortar separately, and through careful grinding operations, they are converted into fine powder substances without any granularity. This process requires strict control of the strength and uniformity of grinding to ensure that the quality and performance of the powder meet the experimental requirements. Then, use an ordinary fingerprint brush to dip an appropriate amount of the above powder. During the operation, pay attention to control the amount of dipping to avoid too much or too little affecting the subsequent fingerprint display effect. Then, gently flick the fingerprint brush dipped in powder to scatter the powder evenly on the surface of the object where the latent fingerprint is located. The types of objects mentioned here are rich and varied, covering a variety of materials such as aluminum foil, kitchen knives, coins, plastic bottles, transparent glass, wood or leather ( Figures 10-16Once the powder has landed on the object's surface, gently sweep it across the surface to ensure full contact and bonding with the latent fingerprint. As the sweeping motion continues, the lines of the latent fingerprint gradually become more clearly visible. Continue sweeping the powder along the fingerprint lines to ensure every detail is fully covered, until the fingerprint is completely and clearly visible. After completing this process, use an ear bulb to gently blow air onto the object's surface to remove excess powder that has not bonded to the latent fingerprint, thereby obtaining a clear fingerprint image that can be directly observed with the naked eye. To obtain a higher-resolution latent fingerprint image, we use an ultraviolet light source with a wavelength range of 300-400nm to excite the object surface where the latent fingerprint is located. Under UV illumination, the detailed features of the latent fingerprint are further highlighted, including the fingerprint's center point, trident, and other details. Finally, using professional camera equipment to capture the latent fingerprint, a high-resolution latent fingerprint image is obtained. Experimental verification shows that the silver / copper nanocluster powder is effective in revealing sweat latent fingerprints on various objects, comparable to commercial powder. Under ultraviolet light, rich detailed information including third-level fingerprint features such as sweat pores can be clearly observed, which provides reliable and effective technical support for fingerprint identification.

[0081] In summary, the research on silver / copper nanoclusters in multicolor luminescent fingerprint identification has achieved remarkable results. From material property research, preparation process optimization to luminescence mechanism research, as well as the progress in each stage of experimental use, it has provided strong evidence for the feasibility and beneficialness of this technology and opened up a new path for the future development of fingerprint identification technology.

[0082] Unique silver-copper nanocluster material: A unique silver-copper nanocluster material has been developed, comprising a specific composition of Cu6(SN2C6H7)6, Ag6(SN2C6H7)6, and Ag2Cu4(SN2C6H7)6. These clusters possess a precise atomic composition and structure, and their ultra-small particle size (1-3nm in diameter) facilitates close bonding with the minute concave and convex structures of fingerprint surfaces, enabling better adhesion to fingerprint ridges, thus demonstrating unique advantages in fingerprint visualization.

[0083] Rich pyrimidine structure: The prepared silver-copper nanocluster material contains a large amount of pyrimidine structure. This characteristic makes the cluster have a certain weak interaction with components such as amino acids in fingerprints, which helps to evenly contact with the fingerprint sample during the fingerprint development operation, thereby improving the effect and quality of fingerprint development.

[0084] Aggregation-enhanced luminescence: Utilizing the aggregation-enhanced luminescence properties of silver-copper nanoclusters, when clusters aggregate on the fingerprint surface, their luminescence intensity is significantly enhanced. This property enables even faint fingerprint traces to produce a strong luminescence signal during latent fingerprint visualization, effectively resolving the problem of traditional fingerprint visualization methods being ineffective when revealing faint fingerprints.

[0085] In today's latent fingerprint revealing technology field, the silver / copper nanocluster material latent fingerprint revealing powder involved in the present invention has shown many outstanding advantages, making it stand out among many technologies.

[0086] The silver / copper nanocluster latent fingerprint revealing powder of the present invention offers significant advantages in terms of raw materials and preparation process. Its raw materials are readily available, eliminating the need for specialized or scarce resources, significantly reducing production costs. Furthermore, the preparation process is simple and efficient, saving time and labor compared to other complex preparation methods while also facilitating large-scale production. This simple preparation process also ensures product stability and consistency, providing reliable support for subsequent applications.

[0087] It is particularly noteworthy that, to date, no other papers have reported clusters of the same composition, nor have clusters been applied to fingerprint identification. This makes the present invention groundbreaking in this field, filling a technological gap. When the silver / copper nanocluster powder of the present invention is used for latent fingerprint identification via powder brushing, the resulting image displays detailed features with high contrast, high sensitivity, and low background interference. Even without the addition of any solid dispersion matrix, the powder can produce aggregated luminescence, further enhancing the fingerprint visualization effect.

[0088] This research has pioneered the development of an innovative metal nanocluster system: through ligand engineering, the electronic structures of Ag6, Ag2Cu4, Ag2Cu4, and Cu6 clusters are precisely controlled to achieve green, orange, and red luminescence (quantum yields >60%), respectively. Their ultra-small size (<2nm) demonstrates remarkable fingerprint ridge enhancement. Compared to traditional materials, this system offers five unique advantages: 1) atomically precise composition control ensures batch stability; 2) excitation wavelength tunability overcomes background interference limitations; 3) surface ligand functionalization enhances specific binding to fingerprint residues; 4) an inorganic core imparts excellent photo- and chemical stability; and 5) a near-100% ultra-high luminescence quantum yield. This technology provides an innovative solution for the development of environmentally friendly, simple-to-operate, and sensitive fingerprint detection methods, with significant application value in the field of criminal evidence.

[0089] The above describes an embodiment of the present invention, but this embodiment is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Ordinary technicians in this field can also make more forms of equivalent embodiments based on the inspiration of this embodiment, all of which are protected by this embodiment.

Claims

1. A method for preparing luminescent nanoclusters, characterized in that: The steps include: Step 1: dissolving a metal precursor in methanol to prepare a metal precursor solution, wherein the metal precursor is selected from silver nitrate, copper nitrate or a combination thereof; Step 2: Add 4,6-dimethyl-2-mercaptopyrimidine to the metal precursor solution, and stir to react to form a complex; Step 3: Add sodium hydroxide to the reaction mixture and continue stirring until luminescence occurs; Step 4: Centrifuge the reaction solution to separate the upper solution and the lower precipitate; Step 5: Collect the upper solution and the lower precipitate redissolved with methanol and let them stand; Step 6: Obtain luminescent nanocluster crystals by natural evaporation or standing to dry.

2. The method for preparing a luminescent nanocluster according to claim 1, characterized in that: The metal precursor is silver nitrate, and the prepared luminescent nanoclusters are Ag6(SN2C6H7)6, whose strongest emission peak is located near 550nm, showing yellow-green luminescence.

3. The method for preparing a luminescent nanocluster according to claim 1, characterized in that: The metal precursors are silver nitrate and copper nitrate, and the prepared luminescent nanoclusters are Ag2Cu4(SN2C6H7)6, whose strongest emission peak is near 695nm, showing red-orange luminescence.

4. The method for preparing a luminescent nanocluster according to claim 1, wherein: The metal precursor is copper nitrate, and the prepared luminescent nanoclusters are Cu6(SN2C6H7)6, whose strongest emission peak is near 745nm, showing red luminescence.

5. The method for preparing a luminescent nanocluster according to claim 1, characterized in that: The stirring reaction time in step 2 is 2 hours.

6. The method for preparing a luminescent nanocluster according to claim 1, characterized in that: The stirring reaction time in step 3 is 2 hours.

7. The method for preparing a luminescent nanocluster according to claim 1, characterized in that: The time for natural volatilization and crystal growth in step 6 is 1-2 weeks.

8. The method for preparing a luminescent nanocluster according to claim 1, characterized in that: The particle size of the luminescent nanoclusters is less than 2 nm, and the luminescence quantum yield is greater than 60%.

9. An application of the luminescent nanoclusters prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The luminescent nanoclusters are applied to latent fingerprint visualization. The method for applying the luminescent nanoclusters to latent fingerprint visualization comprises the following steps: Step a: grinding the luminescent silver / copper nanoclusters according to claim 8 into powder; Step b: Dip the powder using a fingerprint brush; Step c: evenly scattering the powder on the surface of the object where the latent fingerprint is located and lightly sweeping; Step d: Use an ear cleaning bulb to gently blow air on the surface of the object to remove excess powder that has not combined with the latent fingerprint; Step e: Use an ultraviolet light source with a wavelength of 300-400 nm to illuminate the surface of the object and observe the luminescent latent fingerprint image.

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