Preparation of biomass fluorescent carbon dots nanomaterial and application thereof in latent fingerprint development

Abstract

The application discloses a preparation method of biomass fluorescent carbon dot nanomaterials, which is prepared from natural plant ophiopogon japonicus leaves by a solvent thermal method to obtain fluorescent carbon quantum dots dispersed in a solution, and dried to obtain solid fluorescent powder. A fingerprint brush is used to take a small amount of fluorescent carbon dot nanomaterial powder to brush and develop latent fingerprints on different objects, and clear fingerprint detail features can be observed under ultraviolet light, and the method is suitable for fingerprint identification. The biomass fluorescent carbon dot nanomaterials are mainly prepared from natural plants, have simple preparation process, low manufacturing cost, convenient operation, environmental protection, non-toxicity, small harm to operators, good latent fingerprint development effect on different object surfaces, and are suitable for industrialized production and conventional fingerprint development work, and not only have a great driving effect on the development of fingerprint development technology, but also have great practical application value.

Description

Technical Field

[0001] This invention belongs to the field of trace evidence inspection technology, and relates to the preparation of a biomass fluorescent carbon dot nanomaterial and its application in latent fingerprint development. Background Technology

[0002] Fingerprints are present in everyone from birth, remaining unchanged throughout life, unique, and leaving a trace upon contact with objects. Often used as a personal "identity card" and "information database," fingerprints provide invaluable evidence in criminal cases and have been hailed as the "king of evidence." Fingerprints are divided into three categories: two types, visible fingerprints and shaped fingerprints, which can be directly observed with the naked eye; and the third type, latent fingerprints. Because human skin contains substances such as sweat, amino acids, and oils, when a finger touches an object, it leaves invisible latent fingerprints at the point of contact, especially on smooth surfaces (such as metal, glass, ceramics, and painted surfaces). Latent fingerprint examination can accurately identify criminals, playing a role that no other evidence can replace, and is crucial for investigation and solving cases. Only by effectively revealing latent fingerprints using special methods can further individual identification be carried out. Therefore, criminal investigation departments both domestically and internationally attach great importance to the research and development of latent fingerprint revealing technology. With the advancement of technology, the methods for revealing latent fingerprints have become increasingly diverse. To date, latent fingerprint revealing methods have covered fields such as physical revealing, chemical revealing, biological revealing, and optical revealing.

[0003] While traditional fingerprint development methods are widely used, most existing methods suffer from drawbacks such as health hazards, long development times, sample destruction, complex operation, and susceptibility to background fluorescence interference. In the 21st century, with the advent of the nanotechnology era, nanomaterials have also been applied to fingerprint development. Carbon quantum dots (CDs), with their nanoscale dimensions (less than 10 nm) across all three dimensions, possess unique physicochemical properties (such as size quantum effects and quantum confinement effects), offering significant advantages for developing latent fingerprints. Specifically, firstly, carbon quantum dots have good biocompatibility; amino acids, lipids, or proteins in fingerprints can react with modified functional groups (amino or carboxyl groups, etc.) on the surface of quantum dots through covalent coupling or electrostatic attraction. Secondly, carbon quantum dots exhibit excellent fluorescence properties, enabling fingerprint development using photoluminescence technology. The basic principle involves first combining quantum dots with residual substances in the fingerprint, then exciting the quantum dots with ultraviolet light to produce fluorescence, thereby obtaining a latent fingerprint image. Finally, as a novel fluorescent quantum dot nanomaterial, carbon quantum dots, with carbon as their main element, have minimal or no toxicity to organisms or environmental impact. Carbon quantum dot fluorescent nanomaterials are considered ideal fingerprint detection reagents due to their unique optical properties, good chemical stability, and biocompatibility.

[0004] In 2004, Xu et al. discovered CDs (Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments) while purifying single-walled carbon nanotubes. In 2012, Kelarakis et al. synthesized a series of dual-wavelength luminescent carbon nanoparticles using citric acid and ethanolamine as raw materials via high-temperature pyrolysis; in 2015, they mixed these with silica powder for latent fingerprint detection (Carbon dot-based nanopowders and their application for fingerprint recovery). In 2018, Li et al. synthesized carbon dots emitting blue light under ultraviolet irradiation using citric acid and glycine as raw materials via a hydrothermal method (Carbon dots for promoting the growth of ZIF-8 crystals to obtain fluorescent powders and their application for latent fingerprint imaging). In 2023, Wu et al. synthesized blue fluorescent carbon dots using *Ivy* as a biomass raw material via a hydrothermal method for detecting iron ions in water (Preparation of blue fluorescent carbon quantum dots from *Ivy* and their application for Fe...). 3+ (Detection).

[0005] According to current literature, researchers have found that the inherent advantages of carbon dots (CDs) make them very suitable for latent fingerprint detection. However, the current state of research in this field also has the following shortcomings: First, most CDs exhibit blue fluorescence under ultraviolet light excitation, which is similar to the background emission of many objects, resulting in low contrast in fingerprints; second, due to resonant energy transfer and π-π interactions, as the concentration of CDs in the liquid phase increases, there will be a redshift in emission wavelength and a decrease in emission intensity. Removing the solvent and drying them into carbon dot powder will cause aggregation, leading to fluorescence quenching.

[0006] As a perennial herb, Liriope muscari can generally grow for three to five years after a single planting. It has strong shade tolerance and cold resistance, and can grow normally in environments with little sunlight and cold. In addition, Liriope muscari is relatively easy to manage, does not require frequent watering, and rainwater can meet its needs. It is very common in life and often appears on school lawns.

[0007] This invention is the first to use dried Ophiopogon japonicus leaf powder as raw material to prepare solid biomass green fluorescent carbon quantum dots (G-CDs) via a solvothermal method. Surface modification with NaOH effectively mitigates the fluorescence aggregation quenching effect, achieving solid-state luminescence. This synthesizes a low-cost, low-toxicity fingerprint developing powder with superior fluorescence performance, overcoming the shortcomings of currently synthesized materials in developing latent fingerprints on both permeable and non-permeable surfaces. We anticipate that this green-synthesized biomass fluorescent carbon dot nanomaterial will provide new opportunities for establishing non-toxic, simple, effective, and widely applicable latent fingerprint developing methods, and will also bring greater convenience to criminal investigation work. Summary of the Invention

[0008] In order to solve the problems existing in the above-mentioned latent fingerprint development technology, the purpose of this invention is to provide a method for preparing non-toxic, harmless, simple to manufacture and low-cost biomass fluorescent carbon dot nanomaterials.

[0009] Another object of the present invention is to provide the application of the fluorescent carbon dot nanomaterial in latent fingerprint development.

[0010] The detailed technical solution adopted in this invention is as follows:

[0011] I. Preparation of Biomass Fluorescent Carbon Dot Nanomaterials

[0012] The method for preparing fluorescent carbon dot nanomaterials of the present invention includes the following steps:

[0013] (1) After crushing the natural Ophiopogon japonicus leaves into powder, ultrasonically wash, filter to remove impurities mixed in during the collection process, and vacuum dry, use the dried purified Ophiopogon japonicus leaf powder as carbon source, water and ethanol as dispersants, and add sodium hydroxide as a modifier. Use ultrasonic / vibration / magnetic stirring to make the plant powder and sodium hydroxide uniformly dispersed in the water and ethanol system. Heat the uniformly dispersed liquid at 100~200℃ for 6~18h. After heating, the solution after natural cooling is centrifuged and filtered to remove larger particles of impurities that have not been fully reacted. After centrifugation, the supernatant is taken and ultrasonicated. Finally, dialyze with a dialysis bag for 24~32h to obtain fluorescent carbon dot solution.

[0014] The mass ratio of the natural Ophiopogon japonicus leaf powder, sodium hydroxide and ethanol solution is (1~10):1:(60%~100%).

[0015] Ophiopogon japonicus leaves mainly contain steroidal saponins, isoflavones, and various types of polysaccharides, protochromones, and other compounds, which can be directly used as a carbon source in carbon dot synthesis. Using Ophiopogon japonicus leaves as a carbon source eliminates the need for complex pretreatment procedures; they can be directly used after washing and pulverizing, significantly reducing production costs and avoiding the generation of byproducts. Ophiopogon japonicus has high landscaping value, possessing excellent traits such as evergreen foliage, shade tolerance, cold resistance, drought resistance, and disease and pest resistance. It reproduces quickly and is inexpensive. The Ophiopogon japonicus leaves in this invention remain green year-round, unlike some other plants whose leaves change color with the seasons, indicating a more stable composition. Compared to carbon dot solutions prepared using other biomass as carbon sources, the synergistic effect of Ophiopogon japonicus leaf powder and sodium hydroxide in this invention avoids fluorescence aggregation quenching, directly generating solid fluorescent carbon dot powder. This has significant potential for application in latent fingerprint development, as the fluorescence color of the carbon dot solution is consistent with that of the solid carbon dot powder, broadening its application range.

[0016] (2) The fluorescent carbon dot solution was freeze-dried to obtain solid fluorescent carbon dot nanomaterials.

[0017] This invention prepares biomass fluorescent carbon quantum dots dispersed in solution from natural plants using a solvothermal method, and then freeze-dries them to obtain solid biomass fluorescent carbon dot nanomaterials. Ophiopogon japonicus leaf powder is used as the carbon source for synthesizing the biomass fluorescent carbon dots, and sodium hydroxide is used as a modifier to effectively avoid the fluorescence aggregation quenching effect. This ensures that the prepared carbon dot solution, after drying into a solid state, emits bright green fluorescence under ultraviolet light excitation, and the fluorescence color is consistent with the color of the carbon dots dispersed in solution under ultraviolet light.

[0018] The biomass fluorescent carbon dot nanomaterials prepared in this invention are used for the detection of latent fingerprints. The detection of latent fingerprints using biomass fluorescent carbon dot nanomaterials includes the following steps:

[0019] (1) Grind the biomass fluorescent carbon nanoparticles into a fine powder without any particle feel using a mortar and pestle. Use an ordinary fingerprint brush to pick up the powder and flick it onto the surface of the object where the latent fingerprint is located. Gently sweep the powder. After the fingerprint lines appear, continue to gently sweep the powder along the fingerprint lines until the fingerprint is fully revealed. Then, gently blow away the excess powder that has not been bonded with the ear syringe to obtain a fingerprint that is visible to the naked eye. The object includes black glass, stainless steel, aluminum sheet, transparent glass, bottle cap, leather or cardboard.

[0020] (2) Excite the surface of the object containing the latent fingerprint with an ultraviolet light source with a wavelength of 250~400nm, and a high-resolution latent fingerprint image can be obtained by taking a picture. Biomass fluorescent carbon dot nanomaterial powder can be used to develop sweat latent fingerprints on different objects. Clear fingerprint details can be observed under ultraviolet light, including third-level fingerprint features such as sweat pores, which is suitable for fingerprint identification.

[0021] Compared with the prior art, the advantages and effects of the present invention are as follows:

[0022] This invention relates to a biomass fluorescent carbon dot nanomaterial latent fingerprint development powder, which not only possesses advantages such as readily available raw materials, simple preparation process, low production cost, and convenient operation, but also exhibits low biotoxicity, good stability, rapid development, and wide applicability. Employing a "top-down" method for carbon dot preparation, the raw materials are simple, and the prepared carbon dots have controllable surface functional groups, containing abundant functional groups (hydroxyl and carboxyl groups), which is beneficial for the sensing applications of carbon dots and their dispersion in aqueous media. Typically, when carbon dots transition from solution to solid, luminescence quenching can occur due to resonant energy transfer or direct collisions. To achieve solid-state luminescence of carbon dots, they need to be dispersed in a solid matrix to suppress luminescence quenching caused by aggregation. Figure 4 As shown in the inset, the carbon dot solid powder is photographed under sunlight and 365 nm ultraviolet light, respectively. The prepared carbon dot solid powder exhibits bright green luminescence under ultraviolet light radiation, further demonstrating its resistance to self-quenching. The PL emission spectrum of the carbon dots exhibits typical excitation-independent behavior; that is, the PL emission band does not change with the excitation wavelength. The largest emission peak appears at an excitation wavelength of 330 nm, with an emission wavelength of 498 nm. This phenomenon indicates that the composition and structure of our G-CDs are homogeneous, and the corresponding luminescent centers are also homogeneous. They exhibit long-term stable fluorescence and a considerable PLQY. When used for latent fingerprint recognition via powder brushing, the resulting images show high contrast, high sensitivity, and low background interference detail. Therefore, our current work provides a new strategy for the large-scale production of biomass fluorescent carbon dot powders, which has broad application prospects in latent fingerprint recognition. The carbon dot powder prepared in this invention can produce solid-state luminescence resistant to self-aggregation quenching without the addition of any solid dispersion matrix; moreover, using natural materials as a carbon source has environmental value and economic benefits. Compared to existing fingerprint development tools, its most prominent advantages are: high precision in developing latent fingerprints, achieving the effect of revealing third-level fingerprint features; and low cost and non-toxicity in powder production. This invention's biomass fluorescent carbon dot nanomaterials not only greatly promote the development of fingerprint development technology but also have significant practical application value. Attached Figure Description

[0023] Figure 1 This is a transmission electron microscope image of the biomass fluorescent carbon dots obtained in Example 1 of the present invention;

[0024] Figure 2 This is a particle size distribution diagram of biomass fluorescent carbon dots obtained in Example 1 of the present invention;

[0025] Figure 3The fluorescence emission spectra of biomass fluorescent carbon dots prepared in Example 1 of this invention at different wavelengths are shown.

[0026] Figure 4 The ultraviolet spectrum, excitation and emission diagrams of the biomass fluorescent carbon dots obtained in Example 1 of this invention are shown below.

[0027] Figure 5 The infrared spectrum of the biomass fluorescent carbon dots obtained in Example 1 of this invention;

[0028] Figure 6 This is a latent fingerprint image on black glass obtained from the biomass fluorescent carbon dot nanomaterial powder prepared in Example 1 of the present invention.

[0029] Figure 7 This is a latent fingerprint image of the biomass fluorescent carbon dot nanomaterial powder prepared in Example 1 of the present invention on stainless steel.

[0030] Figure 8 This is a latent fingerprint image of the biomass fluorescent carbon dot nanomaterial powder prepared in Example 1 of the present invention on an aluminum sheet.

[0031] Figure 9 This is a latent fingerprint image of the biomass fluorescent carbon dot nanomaterial powder prepared in Example 1 of the present invention on transparent glass.

[0032] Figure 10 This is a latent fingerprint image of the biomass fluorescent carbon dot nanomaterial powder prepared in Example 1 of the present invention used in a bottle cap.

[0033] Figure 11 This is a latent fingerprint image of leather made from biomass fluorescent carbon dot nanomaterial powder prepared in Example 1 of the present invention. Figure 12 The biomass fluorescent carbon dot nanomaterial powder prepared in Example 1 of this invention is used for latent fingerprint imaging on cardboard.

[0034] (Note: Figure 6-12 From left to right in the middle are: a normal image taken under sunlight, a magnified image taken under sunlight, a magnified image taken under ultraviolet light, and a magnified detail image taken under ultraviolet light. Detailed Implementation

[0035] The preparation method and application of the biomass fluorescent carbon dot nanomaterials of the present invention will be described in detail below through specific embodiments.

[0036] Example 1: Preparation of biomass fluorescent carbon dot nanomaterials

[0037] (1) After washing and drying the natural Ophiopogon japonicus leaves, pulverize them into powder, ultrasonically wash and filter to remove impurities mixed in during the collection process, and vacuum dry them. Add 3g of dried Ophiopogon japonicus powder to 60ml of ethanol solution (70%), then add 3g of sodium hydroxide, and disperse evenly by magnetic stirring. Transfer the evenly dispersed mixture to a high-temperature and high-pressure reaction vessel and heat at 160℃ for 8 hours. After heating is completed and the temperature is naturally cooled down, first centrifuge to remove the lower precipitate, filter the supernatant with a filter membrane, centrifuge again, take the supernatant and ultrasonicate for 10 minutes, dialyze with a dialysis bag for 32 hours to obtain biomass fluorescent carbon dot solution.

[0038] Transmission electron microscopy image of carbon dots as shown Figure 1 The prepared carbon dots are spherical, uniform in size, well-dispersed, and have a narrow size distribution range of 0 to 2 nm. The average particle size is approximately 1 nm. Figure 2 The fluorescence emission pattern of carbon dots is shown below. Figure 3 Under different excitation wavelengths, the carbon dots exhibited a strong single emission peak, and the peak did not shift significantly with increasing excitation wavelength, indicating that the photoluminescence of these carbon dots is not wavelength-dependent. Furthermore, a maximum emission peak appeared at an excitation wavelength of 330 nm, with an emission wavelength of 498 nm, exhibiting strong green fluorescence. The ultraviolet spectrum of the carbon dots is shown below. Figure 4 The peak at 280 nm corresponds to the π-π* transition of the sp2 hybridized domain in the carbon dot, and the peak at 390 nm can be attributed to the n-π* transition of the surface states of the carbon dot with C=O bonds. The excitation peak is at 330 nm, and the emission peak is at 495 nm. The infrared spectrum of the carbon dot is shown below. Figure 5 This allows for the characterization of the chemical composition and functional groups of CD, 3432.2 cm. -1 The strong absorption peak at 1644 cm⁻¹ originates from the stretching vibration of OH / NH in the carbon dots. -1 The characteristic vibrational absorption at this point corresponds to the stretching vibration of C=O. Furthermore, the stretching vibration caused by C=C / C=N occurs at 1594.4 cm⁻¹. -1 A strong absorption band is observed at this point. Furthermore, the CN stretching vibration appears at 1124.75 cm⁻¹. -1 The CO stretching vibration is located at 883-855.7 cm. -1 FT-IR results showed that -COOH, -NH2 and -OH groups were successfully introduced into the surface of G-CDs.

[0039] (2) The biomass fluorescent carbon dot solution was freeze-dried at -40℃ for 12h. The resulting dry solid was ground into a fine powder without any grainy texture using a mortar and pestle and stored in a brown glass bottle for later use.

[0040] Example 2: Development of latent fingerprints on glass using biomass fluorescent carbon dot nanomaterials

[0041] Using a regular fingerprint brush, the fluorescent carbon dot nanomaterial powder prepared in Example 1 was picked up and flicked onto the glass surface where the latent fingerprint was located. The powder was then gently swept onto the fingerprint ridges with a brush. After the fingerprint was fully developed, excess unbound powder was swept away. The area containing the latent fingerprint was then irradiated with sunlight and a 365nm ultraviolet light source, respectively, and photographs were taken to obtain clear images of the latent fingerprint on the black glass. Figure 6 As shown, clear fingerprint images can be obtained under both sunlight and ultraviolet light. Under ultraviolet light, the fingerprint appears bright green, and the details of the fingerprint lines are clear.

[0042] Example 3: Development of latent fingerprints on stainless steel using biomass fluorescent carbon dot nanomaterials

[0043] Following the latent fingerprint development effect test method of Example 2, the fluorescent carbon dot nanocomposite powder prepared in Example 1 was used to brush the latent fingerprint on the aluminum sheet. The development results are as follows: Figure 7 As shown, clear fingerprint images can be obtained under both sunlight and ultraviolet light. Under ultraviolet light, the fingerprint appears bright green, and the sweat pores are clearly visible.

[0044] Example 4: Development of latent fingerprints on aluminum sheets using biomass fluorescent carbon dot nanomaterials

[0045] Following the latent fingerprint development effect test method of Example 2, the fluorescent carbon dot nanomaterial development powder prepared in Example 1 was used to develop latent fingerprints on stainless steel. The development results are as follows: Figure 8 As shown, clear fingerprint images can be obtained under both sunlight and ultraviolet light. Under ultraviolet light, the fingerprint appears bright green with clear fingerprint patterns.

[0046] Example 5: Development of latent fingerprints on transparent glass using fluorescent carbon dot nanomaterials

[0047] Following the latent fingerprint development effect test method of Example 2, the fluorescent carbon dot nanocomposite development powder prepared in Example 1 was used to develop latent fingerprints on plastic. The development results are as follows: Figure 9 As shown, clear fingerprint images can be obtained under both sunlight and ultraviolet light. Under ultraviolet light, the fingerprint appears bright green with clear fingerprint patterns.

[0048] Example 6: Development of latent fingerprints on bottle caps using biomass fluorescent carbon dot nanomaterials

[0049] Following the latent fingerprint development effect test method in Example 2, the fluorescent carbon dot nanocomposite development powder prepared in Example 1 was used to brush latent fingerprints on graphics card paper. The development results are as follows: Figure 10 As shown, clear fingerprint images can be obtained under both sunlight and ultraviolet light, with a bright green hue under ultraviolet light.

[0050] Example 7: Development of latent fingerprints on leather using biomass fluorescent carbon dot nanomaterials

[0051] The latent fingerprint development effect of the fluorescent carbon dot nanocomposite material prepared in Example 2 on plastic was tested. Following the latent fingerprint development effect test method of Example 4, the fluorescent carbon dot nanocomposite material powder prepared in Example 2 was used to develop latent fingerprints on plastic. The development results are as follows: Figure 11 As shown, clear fingerprint images can be obtained under both sunlight and ultraviolet light. Under ultraviolet light, the fingerprints appear bright green, and scar features are clearly visible.

[0052] Example 8: Development of latent fingerprints on cardstock using biomass fluorescent carbon dot nanomaterials

[0053] Following the latent fingerprint development effect test method of Example 2, the fluorescent carbon dot nanocomposite development powder prepared in Example 1 was used to develop latent fingerprints on plastic. The development results are as follows: Figure 12 As shown, clear fingerprint images can be obtained under both sunlight and ultraviolet light. Under ultraviolet light, the fingerprint appears bright green with clear fingerprint patterns.

[0054] like Figure 6-12 As shown, although some objects themselves have fluorescent background interference, the fingerprint image is still clearly visible, and fingerprint details can be observed. It is evident that the fluorescent carbon dot nanocomposite material of this invention exhibits excellent latent fingerprint imaging effects on various objects.

Claims

1. A method for preparing biomass fluorescent carbon dot nanomaterials, comprising the following steps: (1) The natural Ophiopogon japonicus leaves were pulverized into powder, ultrasonically washed, filtered, and vacuum dried. The dried purified Ophiopogon japonicus leaf powder was used as a carbon source, ethanol and water were used as dispersants, and sodium hydroxide was added as a modifier and dispersed evenly. The evenly dispersed mixture was heated at 150~200℃ for 6~18h. After heating, the naturally cooled liquid was centrifuged and the supernatant was ultrasonicated. It was filtered using a 0.22um microporous membrane filter and finally dialyzed for 24~48h to obtain a fluorescent carbon dot solution. The mass ratio of the purified Ophiopogon japonicus leaf powder to sodium hydroxide was 1:1; the volume ratio of water to alcohol was 3:

7. (2) The dialysis-treated fluorescent carbon dot solution was freeze-dried to obtain solid fluorescent carbon dot nanomaterials: The fluorescence emission of the solid fluorescent carbon dot nanomaterial exhibits a strong single emission peak, and the emission peak does not shift significantly with increasing excitation wavelength. The photoluminescence of this carbon dot does not have an excitation wavelength dependent property. Solid fluorescent carbon dot powder exhibits bright green luminescence under ultraviolet light radiation and has anti-self-quenching properties. Moreover, the carbon dot powder can produce solid-state luminescence that resists self-aggregation quenching without the addition of any solid dispersion matrix.

2. The application of fluorescent carbon dot nanomaterials prepared by the method described in claim 1 in latent fingerprint development.

3. The application of the fluorescent carbon dot nanomaterials as described in claim 2 in latent fingerprint development, characterized in that: The detection of latent fingerprints using fluorescent carbon dot nanomaterials includes the following steps: (1) Grind fluorescent carbon nanomaterials into fine powder without particle feel using a mortar and pestle, dip the powder into a fingerprint brush, flick the powder onto the surface of the object where the latent fingerprint is located, and lightly sweep the surface of the object where the latent fingerprint is located with a brush to obtain a fingerprint visible to the naked eye. (2) Irradiate the surface of the object containing the latent fingerprint with an ultraviolet light source and take a picture to obtain a high-resolution latent fingerprint image.

4. The application of the fluorescent carbon dot nanomaterials as described in claim 3 in latent fingerprint development, characterized in that: The wavelength range of the ultraviolet light source is 250~400nm.

5. The application of the biomass fluorescent carbon dot nanomaterials as described in claim 3 in latent fingerprint development, characterized in that: The objects include black glass, stainless steel, aluminum sheet, transparent glass, bottle caps, leather, and cardboard.

Images