Contents
2 Fingerprint development by solution⁃dispersed CDs
2.1 Solution⁃dispersed CDs as small particle reagents
2.2 Solution⁃dispersed CDs based on special effects
3 Fingerprint development by solid⁃state CDs powders
3.2 CDs⁃based composite powders
1 Introduction
Handprint is the trace left by the skin of the palm after touching an object, which can be used to identify the same person in forensic science and is recognized as "the first evidence". Most of the fingerprints left at the crime scene are latent fingerprints (LFPs) that cannot be observed with the naked eye and need to be processed to make them appear from the object. The basic principle of fingerprint development is to make the difference between the fingerprint ridge area and the background area that can be observed by the naked eye or identified by the instrument. Powder method, small particle suspension method, fumigation method, vacuum coating method and other fingerprint development technologies emerge as the times require. In recent years, the cross-research of new materials and new technologies in chemistry, physics, materials and other disciplines for fingerprint visualization has become a hot spot, in which the application of photoluminescent materials such as organic fluorescent dyes, rare earth luminescent materials and quantum dots has injected new vitality into traditional fingerprint visualization technology
[1,2][3,4]. In 2004, Xu et al. Discovered carbon dots (CDs) when purifying single-walled carbon nanotubes
[5]. Carbon dots (CDs) are nearly spherical carbon nanoparticles with particle size less than 10 nm, which have good photoluminescence properties, high chemical stability, good biocompatibility, easy chemical modification, and strong photobleaching resistance, and have been widely studied in the fields of bioimaging, optoelectronic devices, sensing, catalysis, and so on
[6⇓~8]. Initially, carbon dots entered the field of fingerprint as fluorescent ink for fingerprint collection, and then the research and application of carbon dots in potential fingerprint visualization began to attract people's attention
[9,10][11⇓⇓~14].
The microstructure (graphitization degree, element composition, etc.) And luminescent properties of carbon dots are closely related to the type of precursor and the synthesis method. The synthesis methods of carbon dots can be divided into two categories: "top down" and "bottom up"
[15]. The "top-down" method produces carbon dots by decomposing graphite, carbon nanotubes, carbon black, activated carbon and other large-size carbon skeletons, including laser ablation, arc discharge, chemical ablation and electrochemical oxidation, which usually require high energy and have low yield of carbon dots. The "bottom-up" method uses hydrolysis, polymerization, carbonization and other reactions of small molecules or polymers in the process of heat treatment to prepare carbon dots, including pyrolysis, combustion, microwave pyrolysis and hydrothermal/solvothermal methods, and the prepared carbon dots generally have abundant surface functional groups and surface defects
[16]. Comparatively speaking, the "bottom-up" synthesis method has more advantages in preparing carbon dot materials for fingerprint visualization, because the method has a wide range of precursors, mild reaction conditions, high yield of carbon dots, and easy control of product properties.
However, there are two disadvantages in using carbon dots for fingerprint visualization: one is that although the photoluminescence properties of carbon dots are affected by many factors such as particle size, excitation wavelength and surface functionalization, most carbon dots emit blue to green fluorescence under ultraviolet excitation, which is similar to the background luminescence of many objects, and the contrast of fingerprints is not high; Secondly, due to resonance energy transfer and π-π-interaction, with the increase of the dispersion concentration of carbon dots in the liquid phase, the emission wavelength will shift to the red and the emission intensity will decrease, and the aggregation of carbon dots will lead to fluorescence quenching after removing the solvent and drying into carbon dot powder, which greatly limits the application of carbon dots in the solid state
[30]. In order to make the structural properties of carbon dots suitable for the specific application of fingerprint visualization, the research work in this field has explored the effects of different synthesis methods and post-treatment processes on fingerprint visualization. In this paper, the research progress of carbon dots for fingerprint visualization at home and abroad is systematically reviewed from two aspects of liquid dispersed carbon dots and solid luminescent carbon dots, the current problems to be solved in this field are analyzed and summarized, and the research development trend is reviewed and prospected.
2 Liquid Dispersed Carbon Dots for Fingerprint Development
Table 1 summarizes the research overview of liquid dispersed carbon dots for fingerprint development. According to different fingerprint development principles, it can be divided into two categories: liquid dispersed carbon dots based on the principle of small particle suspension development and liquid disseminated carbon dots using special effects to develop fingerprints. Small particle suspension method is a common method to show the fat-rich fingerprints on the surface of impermeable objects (including wet and water-immersed objects). Its principle is that the grease, sweat and other components in the fingerprint material are insoluble in the suspension, while the suspended particles can be adsorbed on the surface of the fingerprint material, so that the fingerprint line area and the background area have a contrast. The traditional small particle suspension is prepared by suspending insoluble particles such as MoS
2, Fe
2O
3, ZnCO
3, TiO
2 and the like in a surfactant dilute solution; In recent years, some studies have combined metal nanoclusters, rare earth luminescent materials, quantum dots, etc. With the traditional principle of small particle suspension visualization, which has significantly improved the effect of fingerprint visualization with the help of the luminescent properties of materials, and carbon dots, as a new class of materials with excellent photoluminescence properties, have also made some progress in this direction
[31][32][33⇓~35]. On the other hand, some special effects (including coffee ring effect, segregation effect, etc.) In the process of fingerprint visualization by liquid dispersed carbon dots have also been studied, and good fingerprint visualization effect has also been achieved.
2.1 Liquid Dispersion of Carbon Dots Based on the Appearance Principle of Small Particle Suspension
Thongsai et al. Prepared nitrogen-doped carbon dots by pyrolysis using maleic anhydride and tetraethylenepentamine as precursors, and dispersed them in water to obtain 0.05 G/L liquid dispersed carbon dots.It can show the potential fingerprints on the glass surface, which emit blue fluorescence under ultraviolet light, but because the carbon dots are also easily adsorbed in the background area, the details of the fingerprints are not clear enough
[17]. Zhao et al. Synthesized nitrogen-doped carbon dots by hydrothermal method using DL malic acid and ethylenediamine as precursors, and dispersed them in acetonitrile aqueous solution to obtain 3.2 G/L liquid dispersed carbon dots, which can be used to display sebum fingerprints on impermeable guest surfaces. Good display effect can be obtained by soaking the liquid dispersed carbon dots with pH = 9 for 30 min, and the displayed fingerprints emit green fluorescence under 365 nm ultraviolet light
[18]. The soaking time of this method is much longer than that of the traditional suspension, which may be related to the surface functional groups of carbon dots and the hydrophilicity and hydrophobicity. Subsequently, Li et al. Synthesized green fluorescent carbon dots by a similar method, and carried out a study on the development of sebum fingerprints on the surface of aluminum foil by liquid dispersed carbon dots, but the immersion time and fingerprint development effect were not significantly improved
[19]. Tang et al. Synthesized orange fluorescent carbon dots (O-CDs) by hydrothermal method using rhodamine B as precursor, and the results of MTT colorimetric method showed that O-CDs had good biocompatibility
[20]. O: CDs are dispersed in water to form liquid dispersed carbon dots, which can be used to visualize sebum fingerprints on impermeable guest surfaces, showing that the fingerprints emit orange fluorescence under ultraviolet light. The immersion time of using the liquid to disperse carbon dots to display fingerprints is only 10 s (see Figure 1), and it also has a certain effect on the display of sebum fingerprints left for up to 4 months.
When the liquid dispersed carbon dots based on the principle of small particle suspension are used to visualize fingerprints, the adsorption between the carbon dots and the fingerprint material and the object background will directly affect the fingerprint visualization effect. On the other hand, although the carbon dots are not used in solid form in this method, when the fingerprint is photographed, the carbon dots will aggregate to a certain extent with the evaporation of the solvent, and the fluorescence quenching caused by aggregation is easy to weaken the fluorescence intensity of the fingerprint. In view of these two problems, it is very important to regulate the surface properties of carbon dots, such as surface charge, hydrophilicity and hydrophobicity. Jiang et al. Oxidized Tween 80 with concentrated sulfuric acid and concentrated phosphoric acid (see Figure 2) to prepare white fluorescent carbon dots (W CDs) with long alkyl chains on the surface
[21]. The long alkyl chain can promote the combination of carbon dots with grease and other components in the fingerprint material, and can also effectively inhibit the fluorescence quenching effect caused by aggregation. WCDs were dispersed in a mixture of ethanol and water (4 ∶ 1) to obtain liquid dispersed carbon dots, which could be used to show sebum fingerprints on the surface of impermeable objects such as glass, and the fingerprints emitted white fluorescence under ultraviolet irradiation. Dilag et al. Used activated carbon as a precursor to prepare carbon dots with hydroxyl groups on the surface by hydrothermal method. Through Steglich esterification, the hydroxyl groups on the surface of carbon dots were bonded with the carboxyl groups of chain transfer agent 2 (dodecyl trithiocarbonate) 2 methylpropionic acid, and then N, N dimethylacrylamide (DMA) was modified on the surface of carbon dots by reversible addition fragmentation chain transfer polymerization, and finally CDs/pDMA was obtained
[22]. The 1.5 G/L liquid dispersed carbon dots were prepared by dispersing them in water, and the aluminum foil with sebum fingerprints was immersed in the liquid dispersed carbon dots for 5 s and then rinsed with water, and the yellow-green fluorescent fingerprints could be observed under ultraviolet light. Because the full width at half maximum of the fluorescence emission peak of CDs/pDMA is 94.3 nm, green and red fluorescent fingerprints can be observed through 555 ± 24 and 640 ± 40 nm bandpass filters, respectively. However, the synthesis process of CDs/pDMA is relatively complicated, and the esterification reaction and reversible addition-fragmentation chain transfer polymerization take 48 H respectively, which greatly limits its application prospect in fingerprint visualization.
2.2 Liquid Dispersed Carbon Dots for Fingerprint Visualization Using Special Effect
For a sessile droplet containing a non-volatile solute and a volatile solvent, as the solvent evaporates, the "pinning effect" at the contact line induces capillary flow, which transports the solute to the contact line and eventually leaves a ring-shaped solid stain, a phenomenon known as the "coffee ring effect"
[36]. When the liquid dispersed carbon dots are sprayed on the impermeable object surface with potential fingerprints, the coffee ring effect will also be produced, which can significantly delay the attenuation of the fluorescent properties of fingerprints. Chen et al. Dispersed the red fluorescent carbon dots (RCDs) prepared by hydrothermal method in 0.1 mol/L hydrochloric acid to prepare 1 G/L liquid dispersion of RCDs. The amino groups on the surface of RCDs were easy to combine with the H
+ in the solution to form positively charged particles on the surface
[23]. When the liquid dispersed carbon dots were sprayed on impermeable objects such as glass, aluminum foil, leather and plastic sheets with sebum fingerprints, red fluorescent fingerprints could be seen under ultraviolet light after the water evaporated, and the fluorescent properties of the fingerprints could be stable for about 1 day, and finally the fluorescence disappeared due to the complete evaporation of water and the aggregation of carbon dots. As shown in fig. 3, through continuous observation by a laser scanning confocal fluorescence microscope, it is found that the rate of fluorescence quenching caused by the aggregation of carbon dots at the ridge edge is significantly lower than that in the background area.It is speculated that this phenomenon is due to two reasons: one is that the lipid in the fingerprint substance can delay the evaporation of water from the liquid dispersed carbon dots, and the other is that there is a coffee ring effect caused by water evaporation and electrostatic interaction between the carbon dots and the fingerprint substance. Thirumalaivasan et al. Prepared green fluorescent carbon dots rich in amino groups on the surface by solvothermal method using 4-bromoaniline and ethylenediamine as precursors.The sebum fingerprints on the aluminum foil surface were successfully displayed by the coffee ring effect of the liquid dispersed carbon dots, and the fingerprints emitted green fluorescence under ultraviolet light, and the fluorescence images of the local fingerprints under different wavelength excitation light were studied by confocal fluorescence microscope
[24]. Li et al. Synthesized hollow spherical carbon dots by hydrothermal method using 5-aminosalicylic acid as precursor, and used the liquid to disperse the carbon dots to display the potential fingerprints on the glass surface, which emitted red fluorescence under green light due to the coffee ring effect
[25].
Carbon dots were dispersed in polyvinyl alcohol (PVA) aqueous solution to prepare liquid dispersion CDs/PVA, which was dropped on the impermeable guest with sebum fingerprints. After the water evaporated, a layer of transparent CDs/PVA film was formed, which could be completely peeled off from the guest surface. Under natural light, the fingerprint lines on the film could be observed, indicating that the fingerprints had been transferred to the CDs/PVA film. As shown in Fig. 4, Wang et al. used porcine small intestine as a precursor to prepare amphiphilic carbon dots by pyrolysis and prepared them into liquid dispersion CDs/PVA, and transferred the fingerprints to CDs/PVA film.Bright blue fluorescent fingerprints can be seen under ultraviolet light, which is caused by the segregation of carbon dots at the interface between PVA and fingerprints due to the coffee ring effect induced by water evaporation and the entropy-driven ordering of nanoparticles/polymer system
[26]. At the same time, the PVA material has stable physical and chemical properties, which can effectively fix the distribution of carbon dots, so that the fingerprints transferred to the film are very stable, and the fluorescent fingerprints can still be observed under ultraviolet light after two and a half years of storage. The sensitivity of liquid dispersion CDs/PVA to extract fingerprints is very high, and it can transfer the same fingerprint many times.
Ffectively extract fingerprints left for up to 60 days. The carbon dots in the CDs/PVA film have excitation-dependent fluorescence characteristics, that is, they show different fluorescence emissions under different wavelengths of excitation light, specifically, fingerprints emit blue, green and red fluorescence under ultraviolet, blue and green light, respectively
[37]. Li et al. Prepared blue fluorescent carbon dots by hydrothermal method using citric acid and dimethylpyridylamine as precursors, and developed potential fingerprints on glass surface by similar methods, which verified the feasibility of using liquid dispersion CDs/PVA system to develop potential fingerprints on impermeable guest surface
[27]. Eskalen et al. Prepared blue fluorescent carbon dots by hydrothermal method using rosemary leaves as precursor, and prepared liquid dispersion CDs/PVA to show potential fingerprints on the glass surface, but only the outline of the fingerprints could be seen under ultraviolet light, which may be related to the insufficient affinity between carbon dots and fingerprints
[28]. Liu et al. Added cellulose to citric acid and ethylenediamine solution, and used the electrostatic interaction between negatively charged cellulose and positively charged carbon dots formed in the microwave process to obtain CDs/cellulose film that can transfer fingerprints
[29]. The transferred fingerprint was fumigated with 502 glue and dyed with black magnetic powder, and the black fingerprint lines and blue fluorescent background could be seen under ultraviolet light. Compared with the fumigated and dyed fingerprints extracted from fingerprint tape, especially the fingerprints on rubber objects, this method has certain advantages in sensitivity.
3 Solid State Luminescent Carbon Dots for Fingerprint Development
3.1 Carbon dot powder for fingerprint development
Powder visualization is a common method to visualize fresh fingerprints on the surface of impermeable or semi-permeable objects. Its principle is to use the adsorption of fingerprint substances on powder particles to contrast the fingerprint ridge area with the background area. An appropriate precursor and a preparation method are adopt to directly synthesize that carbon dots with good solid-state luminescent property, and then the carbon dot powder is used for showing potential fingerprint, so that the method has the advantage of convenient operation. At present, the preparation methods of carbon dot powder for fingerprint development mainly include pyrolysis/microwave pyrolysis and hydrothermal/solvothermal methods (see Table 2). Among them, the reaction time of pyrolysis/microwave pyrolysis method is shorter, while the reaction conditions of hydrothermal/solvothermal method are easier to control. In addition, compared with other methods, these two preparation methods require simple equipment and are relatively green and environmentally friendly. The surface of the prepared carbon dots usually contains abundant oxygen-containing groups, and the fluorescence quantum yield is high
[37].
3.1.1 Synthesis of carbon dot powder by pyrolysis/microwave pyrolysis
Bandi et al. Synthesized oleophilic carbon dot powder by pyrolysis using o-phenylenediamine and 2,6-pyridine dicarboxylic acid as precursors, which can be used to display sebum fingerprints on impermeable and semi-permeable objects. The displayed fingerprints emit green fluorescence under 365 nm ultraviolet light, which is conducive to overcoming blue background fluorescence interference
[38]. Wang et al. Prepared carbon dot powder by pyrolysis using sodium citrate and urea as precursors. The surface of the impermeable object showed fingerprints and emitted green fluorescence under ultraviolet light, but the contrast of the fingerprints on the object with background fluorescence was not high
[39]. Niu et al. Synthesized boron-doped CDs powder by pyrolysis using phloroglucinol and boric acid as precursors, which showed red emission in a wide range of excitation wavelengths. It was speculated that this property was due to the formation of luminescent centers after boron doping, and the impermeable guest surface showed fingerprints and emitted red fluorescence under green light
[40].
Wang et al. Synthesized nitrogen and sulfur co-doped carbon dot powders (N, S SFCDs) by microwave pyrolysis using citric acid and L-glutathione as precursors. In order to study the effect of heteroatoms on the luminescent properties of carbon dots, they also synthesized carbon dot powders using urea, ethylenediamine, polyethyleneimine and polyethylene glycol as precursors, but the solid-state luminescent properties were inferior to those of N, S SFCDs
[41]. As shown in Fig. 5, due to the excitation-dependent fluorescence characteristics of N, S SFCDs, the fingerprints on the aluminum foil emit blue to red fluorescence under different wavelengths of excitation light, but the effectiveness of this mode for objects with background fluorescence has not been verified. N, S SFCDs can also be used as liquid dispersed carbon dots, but the effect is not as good as the powder method. Wang et al. Synthesized carbon dot powder by microwave pyrolysis. When the mass ratio of citric acid to piperazine in the precursor was 1 ∶ 2, the prepared carbon dot had two emission peaks with the maximum emission wavelength of 436 nm and 530 nm, respectively, under 365 nm ultraviolet light. The carbon dot powder was used to display sebum fingerprints on impermeable and semi-permeable objects, and the displayed fingerprints emitted blue-green fluorescence under ultraviolet light
[42]. Then they used phthalic acid instead of citric acid as the carbon source, and the carbon dot powder prepared by a similar method had a maximum emission wavelength of 524 nm under 365 nm ultraviolet light, which could show sebum fingerprints on the surface of various objects
[43]. They also used (1s, 2S) (+) (1,2) diaminocyclohexane and citric acid as precursors to synthesize carbon dot powder emitting blue fluorescence under ultraviolet light by microwave pyrolysis, and the effect of fingerprint visualization was similar to that of the previous work
[44]. Bahadur et al., citric acid and polyethylene
[45]Amine was used as a precursor to synthesize positively charged carbon dot powder by microwave pyrolysis, which can visualize the latent fingerprint on the impermeable guest surface. The excitation-dependent fluorescence properties were studied by laser scanning confocal microscopy, and the three-dimensional information of the fingerprint was also analyzed.
3.1.2 Hydrothermal/solvothermal synthesis of carbon dot powder
Wang et al. Used wheat, corn, sorghum, rice and other grains as precursors to synthesize a series of carbon dot powders by hydrothermal method. The abundant hydroxyl groups on the surface of carbon dots and the template effect of ice crystals during freeze-drying inhibited the aggregation of carbon dots and ensured the good solid-state luminescence properties of carbon dots
[46]. The carbon dot powder can be used for visualizing latent fingerprints on the surface of an impermeable object, and the visible fingerprints emit bright blue fluorescence under ultraviolet light. Milenkovic et al. Used polyvinylpyrrolidone as a precursor to synthesize nitrogen-doped carbon dots with negative surface charges by hydrothermal method.The carbon dot powder obtained after drying can show potential fingerprints on the surfaces of impermeable objects such as metals, plastics and the like, and the fingerprints emit light orange fluorescence under ultraviolet light, but the contrast of the fingerprints is not high possibly due to the poor solid-state luminescent performance of the carbon dot
[47]. Pacquiao et al. Used Flammulina velutipes as the precursor to synthesize carbon dot powder emitting blue fluorescence under ultraviolet light by hydrothermal method. Because the operation method of fingerprint visualization is not conventional, the effectiveness of fingerprint visualization is doubtful
[48]. Ren et al. Synthesized carbon dot powder by solvothermal method using sodium citrate and urea as precursors. The surface of silicon wafer showed fingerprints and emitted blue fluorescence under ultraviolet light. The excitation-dependent fluorescence characteristics were studied by fluorescence microscopy
[49]. However, this study only used silicon wafers as the object, and the results did not support the conclusion that the tertiary characteristics were clearly displayed.
3.2 Carbon dot composite powder for fingerprint development
When the carbon dots are uniformly dispersed in the matrix material, the distance between the carbon dot particles can be increased, and the non-radiative energy transfer caused by the direct contact of the carbon dots can be effectively inhibited, so that the luminescence quenching caused by aggregation can be avoided. This kind of carbon dot composite powder with good luminescent properties is also suitable for powder method to develop fingerprints, as shown in Table 3. There are many kinds of matrix materials used in this direction, including starch, SiO2, lithium alginite, montmorillonite, B2O3, metal oxides (ZnO, Al2O3, TiO2) and molecular sieves. According to the characteristics of different preparation systems and the properties of matrix materials, the pre-synthesized carbon dots can be compounded with the matrix materials through post-treatment, and can also be compounded with the matrix materials during the formation of carbon dots, which can be divided into physical methods (dispersion, adsorption, intercalation) and chemical methods (coating, intercalation).
3.2.1 Preparation of carbon dots composite powder by physical dispersion/adsorption/intercalation
The preparation of carbon dots composite powder by physical method has the advantage of simple process. Generally speaking, the dispersion effect of physical dispersion, physical adsorption and physical intercalation on carbon dots increases in turn, and the specific method depends mainly on the physical and chemical properties of the matrix material.
Starch is a macromolecular compound with abundant hydroxyl groups. When carbon dots and starch are mixed in a certain proportion in liquid phase, the carbon dots adhere to the surface of starch granules after the solvent is evaporated, and the carbon dots/starch composite powder is formed by physical dispersion, which usually shows good luminescent properties. Li et al. Mixed the carbon dots prepared by pyrolysis with starch at a mass ratio of 1 ∶ 100 to obtain CDs/starch composite powder that emitted blue fluorescence under ultraviolet light, and studied the fingerprint development effects of different object types and remaining time, and compared the development effects of CDs/starch composite powder with those of iodine fumigation, 502 glue fumigation and traditional powder
[50]. Feng et al. Mixed carbon dots synthesized by hydrothermal method with starch at a mass ratio of 1:25 to obtain a composite powder that emits blue fluorescence under ultraviolet light, which can be used to show sebum fingerprints on the surfaces of permeable and impermeable objects
[51]. Li et al. Mixed the red fluorescent carbon dots prepared by solvothermal method with starch at a mass ratio of 1 ∶ 100 to obtain CDs/starch composite powder that emits red fluorescence under ultraviolet light, which can be used to display sebum fingerprints on the surface of impermeable objects. However, due to the low luminous intensity of the composite powder, the ability to resist background fluorescence interference is insufficient, and the contrast of fingerprints is not high
[52]. Dong et al. Mixed carbon dots prepared by solvothermal method with starch at a mass ratio of 1:40 to obtain R CDs/starch composite powder that emits red fluorescence under green light, as shown in Figure 6, which can be used to show sebum fingerprints on impermeable objects such as glass, paper and plastics
[53]. The displayed fingerprint image is binarized by MatLab, and the two-dimensional coordinates of the detail features are extracted, and the similarity between the displayed fingerprint and the sample fingerprint is quantified by Euclidean distance formula.
Starch particles are mostly in the micron size, and usually have a wide particle size distribution. When used as a matrix material for physical dispersion of carbon dots, the development performance of the powder will be limited by the above factors. Therefore, in addition to starch, chemically synthesized Fe
3O
4@SiO
2, ZnO, and Al
2O
3 have been studied as matrix materials for physical dispersion of carbon dots, which has the advantage of facilitating the design and control of the physicochemical properties of matrix materials. Ding et al. Used the layer-by-layer assembly technique to combine carbon dots synthesized by hydrothermal method on the surface of Fe
3O
4(Fe
3O
4@SiO
2) nanoparticles coated with SiO
2, and successfully prepared Fe
3O
4@SiO
2/CDs composite powder with magnetic and fluorescent properties, which could show sebum fingerprints on the surface of impermeable guest, and showed blue fluorescence under 365 nm ultraviolet light
[54]. However, the preparation process of the powder is relatively complicated, and it needs to assemble 7 layers of carbon dots to effectively overcome the quenching effect of Fe
3O
4, and each layer needs 3 H to assemble.
Prabakaran et al. Mixed carbon dots synthesized by hydrothermal method with ZnO nanoparticles in water at a mass ratio of 1 ∶ 5, and obtained CDs/ZnO composite powder after stirring, centrifugation, rinsing and drying at room temperature.The CDs/ZnO composite powder can be used for displaying sweat fingerprints on the surface of an impermeable object, the displayed fingerprints emit blue fluorescence under ultraviolet light, and the fluorescence intensity of the displayed fingerprints of the CDs/ZnO composite powder is higher than that of the displayed fingerprints of the pure carbon dot powder
[55]. Fouda Mbanga et al. Synthesized carbon dots by hydrothermal method using banana peel as precursor, and mixed them with Al
2O
3 prepared by sol-gel method at a mass ratio of 1.5 ∶ 10 in water. After stirring at room temperature, centrifugation, drying and grinding, CDs/Al
2O
3 composite powder was obtained
[56]. CDs/Al
2O
3 is an excellent adsorbent for Pb
2+, and the CDs/Al
2O
3 after adsorption of Pb
2+ was tried to be used as a fingerprint development powder for developing sebum fingerprints on permeable and impermeable objects, the developed fingerprints were white under natural light, and whether they had fluorescent properties was not studied, and the powder was not valuable for fingerprint development because it contained highly toxic Pb
2+.
Montmorillonite is a cheap and non-toxic natural silicate mineral, which has strong adsorption capacity and cation exchange properties. When carbon dots and montmorillonite are mixed in liquid phase, the carbon dots are fixed between the layers of montmorillonite silicate under the action of physical adsorption, and the carbon dots/montmorilonite composite powder with good luminescence properties can be obtained. Yu et al. Synthesized nitrogen-sulfur co-doped carbon dots with a Zeta potential of 0.80 mV by microwave pyrolysis using citric acid and L-cysteine as precursors. With the help of the electrostatic interaction between the negatively charged silicate layers of Na-montmorillonite (PGV) and the carbon dots,As well as the cation exchange interaction between sodium ions in the PGV layer and carbon dots, CDs/PGV composite powder was successfully prepared, which was used to develop sebum fingerprints on impermeable guest surfaces, and the developed fingerprints emitted blue fluorescence under ultraviolet light
[57]. Zhai et al. Used citric acid and urea as precursors to synthesize green fluorescent carbon dots by microwave pyrolysis, as shown in Figure 7. They were mixed with montmorillonite in water, and green fluorescent carbon dots/montmorillonite (GCDs/MMT) composite powder was prepared after filtration and freeze-drying by electrostatic attraction, hydrogen bonding and complexation between them.The powder can display sweat fingerprints on the surfaces of impermeable objects such as tin foil, glass, plastic and the like, and the displayed fingerprints emit green fluorescence under ultraviolet light, but the G | CDs/MMT composite powder is also easy to adhere to the background of the object, so that the displayed fingerprints are blurred
[58].
Silica gel or laponite can form colorless and transparent colloidal dispersion in water, pre-synthesized carbon dots are added into the dispersion, and carbon dot /SiO
2 or carbon dot/laponite composite powder can be prepared by using the physical intercalation effect in the freeze-drying process. Fernandes et al. Prepared by pyrolysis
[59]Mixing the carbon dots with silica gel or laponite dispersion liquid according to a mass ratio of 1:150, and then freeze-drying to obtain CDs/SiO
2 and CDs/laponite composite powder; and after fingerprints on impermeable objects such as glass, metal and the like appear,Using different wavelengths of excitation light under the fluorescence microscope, it can be observed that the local lines emit different colors of fluorescence, which is conducive to avoiding the interference of object background fluorescence, color and texture, but the influence of the fluorescence color adjustability of the complete fingerprint on the development effect has not been studied in depth. Sekar et al. Prepared ZnO-doped CDs by microwave pyrolysis using ghatti gum and zinc acetate as precursors, mixed it with silica gel at a mass ratio of 1 ∶ 100 in water, dried it by microwave, and then ground it to obtain CDs/SiO
2 composite powder with excitation-dependent fluorescence characteristics. The research mode of this powder for fingerprint development is similar to the previous work
[60]. Using citric acid and thiourea as precursors, Kathiravan et al. Synthesized nitrogen-sulfur co-doped carbon dots by pyrolysis, hydrothermal and solvothermal methods, respectively, among which the pyrolysis method had the best yield and fluorescence quantum efficiency of carbon dots
[61]. The CDs/SiO
2 composite powder with excitation-dependent fluorescence was obtained by mixing carbon dots and silica gel in water at a mass ratio of 1 ∶ 100, vacuum drying, and grinding. Fingerprints on impermeable objects emit blue fluorescence under ultraviolet light, but this work also lacks systematic research on the tunability of fingerprint fluorescence. In addition, because the composite powder is an efficient fluorescent probe for detecting picric acid, the fingerprint fluorescence intensity can qualitatively indicate whether the fingerprint substance contains picric acid; Using the RGB analysis software running on the Android system, the concentration of picric acid in the fingerprint material can also be quantitatively analyzed.
3.2.2 Preparation of carbon dots composite powder by chemical coating/intercalation
Different from the physical method, which can only compound the pre-synthesized carbon dots with the matrix material, the chemical method is more flexible in the strategy of preparing carbon dot composite powder, and the composite process is more controllable, but the reaction process of chemical coating or chemical intercalation is relatively complex.
Carbon dots are formed on the surface of the matrix material through chemical reaction, and the chemical coating can be realized to prepare the carbon dot composite powder. Fernades et al. Used dimethyloctadecyl [3 (trimethoxysilyl) propyl] ammonium chloride to treat silica gel through pyrolysis, surface oxidation, amine functionalization, dialysis and other steps.The carbon dot-coated SiO
2(SiO
2@CDs) composite powder with excitation-dependent fluorescence characteristics was prepared, as shown in fig. 8. The composite powder still showed good solid-state fluorescence properties at a higher carbon dot loading, and was suitable for use as a fingerprint development powder
[62]. Peng et al. Successfully prepared monodisperse SiO
2@CDs composite powder by forming carbon dots on the surface of SiO
2 microspheres synthesized by St Stöber method through hydrothermal reaction using citric acid and aminosilane as precursors
[63]. The aminosilane in the precursor can not only be used as a nitrogen source to passivate the surface of the carbon dots, but also can fix the carbon dots generated in the reaction through amide bonds, thereby inhibiting the fluorescence quenching caused by the agglomeration of the carbon dots in the product. The SiO
2@CDs can be used for showing sebum fingerprints on impermeable and semi-permeable objects such as glass, artificial leather, metal, paper money and the like, and the fingerprints show blue fluorescence under ultraviolet light.
A matrix material is formed outside the pre-synthesized carbon dots through a chemical reaction, and the chemical coating can also be realized to prepare the carbon dot composite powder. Chen et al. Used a method similar to that of Yu et al. To synthesize carbon dots, and the carbon dots were dispersed and fixed in the SiO
2 generated by the hydrolysis of tetraethyl orthosilicate (TEOS) by the sol-gel method. The product was evaporated to dryness at 50 ℃, ground, and sieved with a 500-mesh sieve to obtain CDs/SiO
2 composite powder
[64][57]. The magnetic fluorescent fingerprint powder can be prepared by mixing the magnetic fluorescent fingerprint powder with reduced iron powder in a ratio of 1:10, and can be used for displaying potential fingerprints on impermeable objects (glass, aluminum foil, etc.) And semi-permeable objects (tickets, paper money).The visible fingerprint emits blue fluorescence under ultraviolet light, but the visual contrast is not as good as that of CDs/PGV and traditional magnetic green phosphor, especially on the background fluorescent object. Zhao et al. Synthesized silane-functionalized carbon dots by pyrolysis using citric acid and N- (2-aminoethyl) -3-aminopropyltrimethoxysilane as precursors, and then added the carbon dots and TEOS into an alkaline solution of cetyltrimethylammonium bromide (CATB), and removed the CATB in the product after reaction to obtain spherical mesoporous CDs/SiO
2 composite powder with excitation-dependent fluorescence characteristics
[65]. By selecting different wavelength excitation light and appropriate filters, different colors of fluorescence can be observed on the aluminum foil. Among them, the use of 415 nm purple light as the excitation light has many advantages, such as overcoming the background fluorescence interference of some objects, and the DNA in the fingerprint material is not easily damaged. Fluorescent images of fingerprints on the surface of different impermeable objects were taken in this mode, and bright green fluorescence could be seen through the yellow filter. Compared with the traditional green fluorescent fingerprint powder, the mesoporous CDs/SiO
2 composite powder has higher sensitivity and can effectively show the fingerprint left on the surface of aluminum foil for 60 days, which may be due to the strong adhesion of fingerprint substances to mesoporous powder particles.
In addition to SiO
2, carbon dots can also be coated by matrix materials such as TiO
2, 2-methylimidazolium zinc salt (ZIF-8) and B
2O
3. Amith Yadav et al. Used pomegranate peel as precursor to synthesize "carbon dots" by hydrothermal method, but SEM photos showed that the "carbon dots" were micron-sized and did not belong to the category of carbon dots
[66]. Then a layer of TiO
2 was coated on the surface of the "carbon dots" by hydrothermal method to obtain micron-sized CDs/TiO
2 composite powder. Although fingerprints on different impermeable objects were visualized and the visualization effect of water-soaked and air-dried fingerprints was also studied, all images were taken under white light and did not give any fluorescent images, which may be due to the fact that the original synthetic "carbon dots" did not have fluorescent properties. Li et al. Synthesized glycine-modified carbon dots by hydrothermal method using citric acid and glycine as precursors, which could be used as a crystallizing agent for the synthesis of ZIF-8 in aqueous solution. Zinc acetate solution was added to the solution containing 2-methylimidazole and glycine-modified carbon dots, and the CDs/ZIF-8 composite powder was obtained by aging at room temperature for 48 H
[67]. The powder was used to visualize the latent fingerprints on the surfaces of impermeable objects such as glass and plastics, and the excitation-dependent fluorescence characteristics of the fingerprints were studied by fluorescence microscopy. He et al. Added boric acid solution to the carbon dots prepared by hydrothermal method, and fixed the carbon dots in the glassy B
2O
3 matrix by heat treatment, which formed carbon-boron bonds, which could promote intersystem crossing and prevent the quenching of triplet excitons, and obtained long afterglow CDs/B
2O
3 composite powder with both thermal activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) properties (see Fig. 9)
[68]. Long afterglow materials are a kind of materials that can continue to emit light after the excitation stops. Using these materials to display fingerprints can effectively avoid the interference of excitation light and object background fluorescence, so it is easy to obtain clear fingerprint images.The sebum fingerprint on the surface of an impermeable object is displayed by using the CDs/B
2O
3 composite powder, and the long afterglow luminescence in the fingerprint ridge area can be observed when the displayed fingerprint is switched to a dark room environment after being irradiated by visible light.
Different from the preparation of the carbon dot composite powder by chemical coating, the carbon dot composite powder suitable for fingerprint visualization can also be prepared by embedding the carbon dots into the matrix material formed synchronously during the synthesis of the carbon dots. Li et al. Synthesized CDs/SiO
2 composite powder by a one-step hydrothermal method using sodium citrate, 3-aminopropyltriethoxysilane (APTES) and TEOS as raw materials. While sodium citrate and APTES were carbonized into carbon dots, the cross-linked gel structure formed by co-hydrolysis of APTES and TEOS avoided the agglomeration of carbon dots
[69]. The composite powder can show sebum fingerprints on the surface of an impermeable object, and the fingerprints show blue fluorescence under ultraviolet light. Qin et al. Synthesized nitrogen and sulfur co-doped CDs/SiO
2 composite powder by one-step solvothermal method using citrate, thiourea and APTES as raw materials, and the fingerprints on the impermeable guest surface emitted green fluorescence under ultraviolet irradiation
[70]. Using rice husk and ethylenediamine as precursors, Sun et al. Prepared a long afterglow CDs/SiO
2 composite powder with both TADF and RTP properties by sol-gel method. The rigid three-dimensional structure of the SiO
2 makes the powder resistant to acids, alkalis, salts, oxidants and polar solvents. When irradiated by 365 nm ultraviolet light, the impermeable guest surface shows fingerprints, and when moved to a dark room, green long afterglow luminescence can be seen
[71].
4 Conclusion and prospect
From the early organic fluorescent dyes to the later rare earth luminescent materials, metal nanoclusters, quantum dots and so on, the use of photoluminescent materials for fingerprint display has a long history. Researchers discovered the photoluminescence properties of carbon dots in 2004, and compared with other photoluminescent materials, carbon dots have many advantages. Combining the two traditional fingerprint development methods of suspension method and powder method, there are two forms of carbon dots in fingerprint development: liquid dispersion carbon dots and solid luminescent carbon dots. Liquid dispersed carbon dots use various solutions as dispersants, which are of high value in the application of showing fingerprints on the surface of wet and water-soaked objects. The application range of solid luminescent carbon dots is similar to that of traditional powder method, and the effect of showing fresh fingerprints on the surface of impermeable or semi-permeable objects is better. According to the action principle, it can be divided into liquid dispersed carbon dots based on the principle of small particle suspension and liquid dispersed carbon dots based on special effect to develop fingerprints. According to the chemical composition, it can be divided into carbon dot powder for fingerprint development and carbon dot composite powder for fingerprint development, and the dispersion medium of the latter is various solid matrix materials. At present, there are three main problems in the use of carbon dots for fingerprint display.
(1) The physical morphology (morphology, particle size and distribution) and surface properties (functional group, hydrophilicity and hydrophobicity, charged state) of carbon dots are important factors affecting the fingerprint development effect, but there is no in-depth research on the optimization of physical morphology and surface properties of carbon dots for fingerprint development. For example, in the aspect of the morphology of solid-state luminescent carbon dots, many works only characterize the basic units of the particles without considering the influence of agglomeration. Latent fingerprints can be divided into sweat latent fingerprints, oil latent fingerprints, oil-sweat mixed fingerprints, protein fingerprints, blood latent fingerprints and so on according to the material composition. Most studies only select oil latent fingerprints as the research object, and need to further expand the scope of fingerprint samples. In the study of fingerprint visualization of luminescent materials, in addition to the physical visualization principles based on electrostatic adsorption and hydrophobic interaction, there are more selective chemical visualization principles based on chemical bonding, immune binding, aptamer recognition, etc. At present, there is a lack of research on the combination of carbon dots and chemical visualization.
(2) The use of carbon dot photoluminescence to display fingerprints is mostly based on the single fluorescence property of carbon dots, that is, to obtain fluorescent fingerprints by illuminating fingerprints with a certain wavelength of excitation light. Some carbon dot materials have excitation-dependent fluorescence characteristics, which are usually related to the surface state, size distribution and carbon core structure of carbon dots, and this characteristic has unique advantages in fingerprint visualization.Different excitation light can be used to obtain different fluorescent fingerprints, which is conducive to overcoming the background interference of different objects and improving the versatility of developing materials. However, the research in this area is mostly limited to local fingerprints, and does not involve objects with complex backgrounds. There are also some carbon dot materials with long afterglow properties, which can continue to emit light after stopping excitation, which is conducive to avoiding the interference of excitation light and object background at the same time, but the research work in this area is relatively scarce.
(3) The three levels of features contained in the spatial form of fingerprints can be used for individual identification. In addition, the chemical and biological information contained in the fingerprint material is also very important trace evidence. At present, there are many studies on the cytotoxicity of carbon dots, but the effect of the process of carbon dots showing fingerprints on the detection of fingerprints, especially DNA analysis, is not clear. On the other hand, the excitation light source (mostly ultraviolet excitation) used in the process of photographing carbon dot materials to show fingerprints may also affect the detection of DNA analysis. Therefore, it is an urgent research direction to clarify the compatibility between carbon dot materials and DNA analysis.
At present, the application of luminescent materials based on carbon dots in potential fingerprint visualization is still in the exploratory stage in general, and most of the related studies are based on the synthesis of carbon dots.It focuses on regulating the photoluminescence color of carbon dots and alleviating the fluorescence quenching caused by aggregation, and fingerprint visualization is only used as an effect display or verification link, which is also the current situation of many other new materials and technologies introduced into the field of fingerprint visualization. Only from the perspective of improving the extraction rate and utilization rate of fingerprints, can we make full use of the performance advantages of carbon dot materials and effectively integrate carbon dots into the basic and applied research of fingerprint visualization. In order to achieve the goal of efficient fingerprint visualization, three key points are proposed here around the physicochemical properties of future synthetic carbon dots. (1) Under the selected excitation conditions, the fluorescence signal of the carbon dot showing the fingerprint should be able to overcome the interference of noise (such as the background luminescence of the object); (2) The applicability of carbon dots for fingerprint development cannot be measured by the quantum yield, but should be evaluated in combination with the actual fingerprint development effect, because the state of carbon dots in the quantum yield test is often quite different from that in the fingerprint development; (3) In order to ensure the high definition of the fingerprint, the relative binding force between the carbon dot and the fingerprint material and the object background should be considered in the regulation of the surface properties of the carbon dot.
National Natural Science Foundation of China(21802169)
National Natural Science Foundation of China(21205139)
Scientific Research Foundation of the Educational Department of Liaoning Province(LJKZ0076)
Scientific Research Foundation of the Educational Department of Liaoning Province(LJKZ0068)
Fundamental Research Funds for the Central Universities(D2021021)
Liaoning BaiQianWan Talents Program in 2020, and the National Students Innovation Training Program(202110175005)
PDF
