The papillary patterns on human fingers remain basically unchanged in their topological structure from birth, possessing different characteristics from person to person, and the papillary patterns on each finger of the same person are also different. The papilla pattern on the fingers is ridged and distributed with many sweat pores. The human body continuously secretes water-based substances such as sweat and oily substances such as oil. These substances will transfer and deposit on the object when they come into contact, forming impressions on the object. It is precisely because of the unique characteristics of hand prints, such as their individual specificity, lifelong stability, and reflective nature of touch marks that fingerprints have become a recognized symbol of criminal investigation and personal identity recognition since the first use of fingerprints for personal identification in the late 19th century.
At the crime scene, except for three-dimensional and flat colored fingerprints, the occurrence rate of potential fingerprints is the highest. Potential fingerprints typically require visual processing through physical or chemical reactions. The common potential fingerprint development methods mainly include optical development, powder development, and chemical development. Among them, powder development is favored by grassroots units due to its simple operation and low cost. However, the limitations of traditional powder based fingerprint display no longer meet the needs of criminal technicians, such as the complex and diverse colors and materials of the object at the crime scene, and the poor contrast between the fingerprint and the background color; The size, shape, viscosity, composition ratio, and performance of powder particles affect the sensitivity of powder appearance; The selectivity of traditional powders is poor, especially the enhanced adsorption of wet objects on the powder, which greatly reduces the development selectivity of traditional powders. In recent years, criminal science and technology personnel have been continuously researching new materials and synthesis methods, among which rare earth luminescent materials have attracted the attention of criminal science and technology personnel due to their unique luminescent properties, high contrast, high sensitivity, high selectivity, and low toxicity in the application of fingerprint display. The gradually filled 4f orbitals of rare earth elements endow them with very rich energy levels, and the 5s and 5P layer electron orbitals of rare earth elements are completely filled. The 4f layer electrons are shielded, giving the 4f layer electrons a unique mode of motion. Therefore, rare earth elements exhibit excellent photostability and chemical stability without photobleaching, overcoming the limitations of commonly used organic dyes. In addition, rare earth elements also have superior electrical and magnetic properties compared to other elements. The unique optical properties of rare earth ions, such as long fluorescence lifetime, many narrow absorption and emission bands, and large energy absorption and emission gaps, have attracted widespread attention in the related research of fingerprint display.
Among numerous rare earth elements, europium is the most commonly used luminescent material. Demarcay, the discoverer of europium in 1900, first described sharp lines in the absorption spectrum of Eu3+in solution. In 1909, Urban described the cathodoluminescence of Gd2O3: Eu3+. In 1920, Prandtl first published the absorption spectra of Eu3+, confirming De Mare’s observations. The absorption spectrum of Eu3+is shown in Figure 1. Eu3+is usually located on the C2 orbital to facilitate the transition of electrons from 5D0 to 7F2 levels, thereby releasing red fluorescence. Eu3+can achieve a transition from ground state electrons to the lowest excited state energy level within the visible light wavelength range. Under the excitation of ultraviolet light, Eu3+exhibits strong red photoluminescence. This type of photoluminescence is not only applicable to Eu3+ions doped in crystal substrates or glasses, but also to complexes synthesized with europium and organic ligands. These ligands can serve as antennas to absorb excitation luminescence and transfer excitation energy to higher energy levels of Eu3+ions. The most important application of europium is the red fluorescent powder Y2O3: Eu3+(YOX) is an important component of fluorescent lamps. The red light excitation of Eu3+can be achieved not only by ultraviolet light, but also by electron beam (cathodoluminescence), X-ray γ Radiation α or β Particle, electroluminescence, frictional or mechanical luminescence, and chemiluminescence methods. Due to its rich luminescent properties, it is a widely used biological probe in the fields of biomedical or biological sciences. In recent years, it has also aroused the research interest of criminal science and technology personnel in the field of forensic science, providing a good choice to break through the limitations of traditional powder method for displaying fingerprints, and has significant significance in improving the contrast, sensitivity, and selectivity of fingerprint display.
Figure 1 Eu3+Absorption Spectrogram
1,Luminescence principle of rare earth europium complexes
The ground state and excited state electronic configurations of europium ions are both 4fn type. Due to the excellent shielding effect of the s and d orbitals around the europium ions on the 4f orbitals, the f-f transitions of europium ions exhibit sharp linear bands and relatively long fluorescence lifetimes. However, due to the low photoluminescence efficiency of europium ions in the ultraviolet and visible light regions, organic ligands are used to form complexes with europium ions to improve the absorption coefficient of the ultraviolet and visible light regions. The fluorescence emitted by europium complexes not only has the unique advantages of high fluorescence intensity and high fluorescence purity, but also can be improved by utilizing the high absorption efficiency of organic compounds in the ultraviolet and visible light regions. The excitation energy required for europium ion photoluminescence is high The deficiency of low fluorescence efficiency. There are two main luminescence principles of rare earth europium complexes: one is photoluminescence, which requires the ligand of europium complexes; Another aspect is that the antenna effect can improve the sensitivity of europium ion luminescence.
After being excited by external ultraviolet or visible light, the organic ligand in the rare earth complex transitions from the ground state S0 to the excited singlet state S1. The excited state electrons are unstable and return to the ground state S0 through radiation, releasing energy for the ligand to emit fluorescence, or intermittently jump to its triple excited state T1 or T2 through non radiative means; Triple excited states release energy through radiation to produce ligand phosphorescence, or transfer energy to metal europium ions through non radiative intramolecular energy transfer; After being excited, europium ions transition from the ground state to the excited state, and europium ions in the excited state transition to the low energy level, ultimately returning to the ground state, releasing energy and generating fluorescence. Therefore, by introducing appropriate organic ligands to interact with rare earth ions and sensitize central metal ions through non radiative energy transfer within molecules, the fluorescence effect of rare earth ions can be greatly increased and the requirement for external excitation energy can be reduced. This phenomenon is known as the antenna effect of ligands. The energy level diagram of energy transfer in Eu3+complexes is shown in Figure 2.
In the process of energy transfer from the triplet excited state to Eu3+, the energy level of the ligand triplet excited state is required to be higher than or consistent with the energy level of the Eu3+excited state. But when the triplet energy level of the ligand is much greater than the lowest excited state energy of Eu3+, the energy transfer efficiency will also be greatly reduced. When the difference between the triplet state of the ligand and the lowest excited state of Eu3+is small, the fluorescence intensity will weaken due to the influence of the thermal deactivation rate of the triplet state of the ligand. β- Diketone complexes have the advantages of strong UV absorption coefficient, strong coordination ability, efficient energy transfer with rare earths, and can exist in both solid and liquid forms, making them one of the most widely used ligands in rare earth complexes.
Figure 2 Energy level diagram of energy transfer in Eu3+complex
2.Synthesis Method of Rare Earth Europium Complexes
2.1 High temperature solid-state synthesis method
The high-temperature solid-state method is a commonly used method for preparing rare earth luminescent materials, and it is also widely used in industrial production. The high-temperature solid-state synthesis method is the reaction of solid matter interfaces under high temperature conditions (800-1500 ℃) to generate new compounds by diffusing or transporting solid atoms or ions. The high-temperature solid-phase method is used to prepare rare earth complexes. Firstly, the reactants are mixed in a certain proportion, and an appropriate amount of flux is added to a mortar for thorough grinding to ensure uniform mixing. Afterwards, the ground reactants are placed in a high-temperature furnace for calcination. During the calcination process, oxidation, reduction, or inert gases can be filled according to the needs of the experimental process. After high-temperature calcination, a matrix with a specific crystal structure is formed, and the activator rare earth ions are added to it to form a luminescent center. The calcined complex needs to undergo cooling, rinsing, drying, re grinding, calcination, and screening at room temperature to obtain the product. Generally, multiple grinding and calcination processes are required. Multiple grinding can accelerate the reaction speed and make the reaction more complete. This is because the grinding process increases the contact area of the reactants, greatly improving the diffusion and transportation speed of ions and molecules in the reactants, thereby improving the reaction efficiency. However, different calcination times and temperatures will have an impact on the structure of the crystal matrix formed.
The high-temperature solid-state method has the advantages of simple process operation, low cost, and short time consumption, making it a mature preparation technology. However, the main drawbacks of the high-temperature solid-state method are: firstly, the required reaction temperature is too high, which requires high equipment and instruments, consumes high energy, and is difficult to control the crystal morphology. The product morphology is uneven, and even causes the crystal state to be damaged, affecting the luminescence performance. Secondly, insufficient grinding makes it difficult for the reactants to mix evenly, and the crystal particles are relatively large. Due to manual or mechanical grinding, impurities are inevitably mixed to affect the luminescence, resulting in low product purity. The third issue is uneven coating application and poor density during the application process. Lai et al. synthesized a series of Sr5 (PO4) 3Cl single-phase polychromatic fluorescent powders doped with Eu3+and Tb3+using the traditional high-temperature solid-state method. Under near-ultraviolet excitation, the fluorescent powder can tune the luminescence color of the phosphor from the blue region to the green region according to the doping concentration, improving the defects of low color rendering index and high related color temperature in white light-emitting diodes. High energy consumption is the main problem in the synthesis of borophosphate based fluorescent powders by high-temperature solid-state method. Currently, more and more scholars are committed to developing and searching for suitable matrices to solve the high energy consumption problem of high-temperature solid-state method. In 2015, Hasegawa et al. completed the low-temperature solid-state preparation of Li2NaBP2O8 (LNBP) phase using the P1 space group of the triclinic system for the first time. In 2020, Zhu et al. reported a low-temperature solid-state synthesis route for a novel Li2NaBP2O8: Eu3+(LNBP: Eu) phosphor, exploring a low energy consumption and low-cost synthesis route for inorganic phosphors.
2.2 Co precipitation method
The co precipitation method is also a commonly used “soft chemical” synthesis method for preparing inorganic rare earth luminescent materials. The co precipitation method involves adding a precipitant to the reactant, which reacts with the cations in each reactant to form a precipitate or hydrolyzes the reactant under certain conditions to form oxides, hydroxides, insoluble salts, etc. The target product is obtained through filtration, washing, drying, and other processes. The advantages of co precipitation method are simple operation, short time consumption, low energy consumption, and high product purity. Its most prominent advantage is that its small particle size can directly generate nanocrystals. The drawbacks of the co precipitation method are: firstly, the product aggregation phenomenon obtained is severe, which affects the luminescent performance of the fluorescent material; Secondly, the shape of the product is unclear and difficult to control; Thirdly, there are certain requirements for the selection of raw materials, and the precipitation conditions between each reactant should be as similar or identical as possible, which is not suitable for the application of multiple system components. K. Petcharoen et al. synthesized spherical magnetite nanoparticles using ammonium hydroxide as a precipitant and chemical co precipitation method. Acetic acid and oleic acid were introduced as coating agents during the initial crystallization stage, and the size of magnetite nanoparticles was controlled within the range of 1-40nm by changing the temperature. The well dispersed magnetite nanoparticles in aqueous solution were obtained through surface modification, improving the agglomeration phenomenon of particles in the co precipitation method. Kee et al. compared the effects of hydrothermal method and co precipitation method on the shape, structure, and particle size of Eu-CSH. They pointed out that hydrothermal method generates nanoparticles, while co precipitation method generates submicron prismatic particles. Compared with the co precipitation method, the hydrothermal method exhibits higher crystallinity and better photoluminescence intensity in the preparation of Eu-CSH powder. JK Han et al. developed a novel co precipitation method using a non aqueous solvent N, N-dimethylformamide (DMF) to prepare (Ba1-xSrx) 2SiO4: Eu2 phosphors with narrow size distribution and high quantum efficiency near spherical nano or submicron size particles. DMF can reduce polymerization reactions and slow down the reaction rate during the precipitation process, helping to prevent particle aggregation.
2.3 Hydrothermal/solvent thermal synthesis method
The hydrothermal method began in the mid-19th century when geologists simulated natural mineralization. In the early 20th century, the theory gradually matured and is currently one of the most promising solution chemistry methods. Hydrothermal method is a process in which water vapor or aqueous solution is used as the medium (to transport ions and molecular groups and transfer pressure) to reach a subcritical or supercritical state in a high-temperature and high-pressure closed environment (the former has a temperature of 100-240 ℃, while the latter has a temperature of up to 1000 ℃), accelerate the hydrolysis reaction rate of raw materials, and under strong convection, ions and molecular groups diffuse to low temperature for recrystallization. The temperature, pH value, reaction time, concentration, and type of precursor during the hydrolysis process affect the reaction rate, crystal appearance, shape, structure, and growth rate to varying degrees. An increase in temperature not only accelerates the dissolution of raw materials, but also increases the effective collision of molecules to promote crystal formation. The different growth rates of each crystal plane in pH crystals are the main factors affecting the crystal phase, size, and morphology. The length of reaction time also affects crystal growth, and the longer the time, the more favorable it is for crystal growth.
The advantages of hydrothermal method are mainly manifested in: firstly, high crystal purity, no impurity pollution, narrow particle size distribution, high yield, and diverse product morphology; The second is that the operation process is simple, the cost is low, and the energy consumption is low. Most of the reactions are carried out in medium to low temperature environments, and the reaction conditions are easy to control. The application range is wide and can meet the preparation requirements of various forms of materials; Thirdly, the pressure of environmental pollution is low and it is relatively friendly to the health of operators. Its main drawbacks are that the precursor of the reaction is easily affected by environmental pH, temperature, and time, and the product has a low oxygen content.
The solvothermal method uses organic solvents as the reaction medium, further expanding the applicability of hydrothermal methods. Due to the significant differences in physical and chemical properties between organic solvents and water, the reaction mechanism is more complex, and the appearance, structure, and size of the product are more diverse. Nallappan et al. synthesized MoOx crystals with different morphologies from sheet to nanorod by controlling the reaction time of hydrothermal method using sodium dialkyl sulfate as the crystal directing agent. Dianwen Hu et al. synthesized composite materials based on polyoxymolybdenum cobalt (CoPMA) and UiO-67 or containing bipyridyl groups (UiO-bpy) using solvothermal method by optimizing synthesis conditions.
2.4 Sol gel method
Sol gel method is a traditional chemical method to prepare inorganic functional materials, which is widely used in the preparation of metal nanomaterials. In 1846, Elbelmen first used this method to prepare SiO2, but its use was not yet mature. The preparation method is mainly to add rare earth ion activator in the initial reaction solution to make the solvent volatilize to make gel, and the prepared gel gets the target product after temperature treatment. The phosphor produced by the sol gel method has good morphology and structural characteristics, and the product has small uniform particle size, but its luminosity needs to be improved. The preparation process of sol-gel method is simple and easy to operate, the reaction temperature is low, and the safety performance is high, but the time is long, and the amount of each treatment is limited. Gaponenko et al. prepared amorphous BaTiO3/SiO2 multilayer structure by centrifugation and heat treatment sol-gel method with good transmissivity and refractive index, and pointed out that the refractive index of BaTiO3 film will increase with the increase of sol concentration. In 2007, Liu L’s research group successfully captured the highly fluorescent and light stable Eu3+metal ion/sensitizer complex in silica based nanocomposites and doped dry gel using the sol gel method. In several combinations of different derivatives of rare earth sensitizers and silica nanoporous templates, the use of 1,10-phenanthroline (OP) sensitizer in tetraethoxysilane (TEOS) template provides the best fluorescence doped dry gel to test the spectral properties of Eu3+.
2.5 Microwave synthesis method
Microwave synthesis method is a new green and pollution-free chemical synthesis method compared to high-temperature solid-state method, which is widely used in material synthesis, especially in the field of nanomaterial synthesis, showing good development momentum. Microwave is an electromagnetic wave with a wavelength between 1nn and 1m. Microwave method is the process in which microscopic particles inside the starting material undergo polarization under the influence of external electromagnetic field strength. As the direction of the microwave electric field changes, the motion and arrangement direction of the dipoles change continuously. The hysteresis response of the dipoles, as well as the conversion of their own thermal energy without the need for collision, friction, and dielectric loss between atoms and molecules, achieves the heating effect. Due to the fact that microwave heating can uniformly heat the entire reaction system and conduct energy quickly, thereby promoting the progress of organic reactions, compared to traditional preparation methods, microwave synthesis method has the advantages of fast reaction speed, green safety, small and uniform material particle size, and high phase purity. However, most reports currently use microwave absorbers such as carbon powder, Fe3O4, and MnO2 to indirectly provide heat for the reaction. Substances that are easily absorbed by microwaves and can activate the reactants themselves need further exploration. Liu et al. combined the co precipitation method with the microwave method to synthesize pure spinel LiMn2O4 with porous morphology and good properties.
2.6 Combustion method
The combustion method is based on traditional heating methods, which use organic matter combustion to generate the target product after the solution is evaporated to dryness. The gas generated by the combustion of organic matter can effectively slow down the occurrence of agglomeration. Compared with solid-state heating method, it reduces energy consumption and is suitable for products with low reaction temperature requirements. However, the reaction process requires the addition of organic compounds, which increases the cost. This method has a small processing capacity and is not suitable for industrial production. The product produced by combustion method has a small and uniform particle size, but due to the short reaction process, there may be incomplete crystals, which affects the luminescence performance of the crystals. Anning et al. used La2O3, B2O3, and Mg as starting materials and used salt assisted combustion synthesis to produce LaB6 powder in batches in a short period of time.
3. Application of rare earth europium complexes in fingerprint development
Powder display method is one of the most classic and traditional fingerprint display methods. At present, the powders that display fingerprints can be divided into three categories: traditional powders, such as magnetic powders composed of fine iron powder and carbon powder; Metal powders, such as gold powder, silver powder, and other metal powders with a network structure; Fluorescent powder. However, traditional powders often have great difficulties in displaying fingerprints or old fingerprints on complex background objects, and have a certain toxic effect on the health of users. In recent years, criminal science and technology personnel have increasingly favored the application of nano fluorescent materials for fingerprint display. Due to the unique luminescent properties of Eu3+and the widespread application of rare earth substances, rare earth europium complexes have not only become a research hotspot in the field of forensic science, but also provide broader research ideas for fingerprint display. However, Eu3+in liquids or solids has poor light absorption performance and needs to be combined with ligands to sensitize and emit light, enabling Eu3+to exhibit stronger and more persistent fluorescence properties. Currently, the commonly used ligands mainly include β- Diketones, carboxylic acids and carboxylate salts, organic polymers, supramolecular macrocycles, etc. With the in-depth research and application of rare earth europium complexes, it has been found that in humid environments, the vibration of coordination H2O molecules in europium complexes can cause luminescence quenching. Therefore, in order to achieve better selectivity and strong contrast in fingerprint display, efforts need to be made to study how to improve the thermal and mechanical stability of europium complexes.
In 2007, Liu L’s research group was the pioneer of introducing europium complexes into the field of fingerprint display for the first time at home and abroad. The highly fluorescent and light stable Eu3+metal ion/sensitizer complexes captured by the sol gel method can be used for potential fingerprint detection on various forensic related materials, including gold foil, glass, plastic, colored paper and green leaves. Exploratory research introduced the preparation process, UV/Vis spectra, fluorescence characteristics, and fingerprint labeling results of these new Eu3+/OP/TEOS nanocomposites.
In 2014, Seung Jin Ryu et al. first formed an Eu3+complex ([EuCl2 (Phen) 2 (H2O) 2] Cl · H2O) by hexahydrate europium chloride (EuCl3 · 6H2O) and 1-10 phenanthroline (Phen). Through the ion exchange reaction between interlayer sodium ions and europium complex ions, intercalated nano hybrid compounds (Eu (Phen) 2) 3+- synthesized lithium soap stone and Eu (Phen) 2) 3+- natural montmorillonite) were obtained. Under excitation of a UV lamp at a wavelength of 312nm, the two complexes not only maintain characteristic photoluminescence phenomena, but also have higher thermal, chemical, and mechanical stability compared to pure Eu3+complexes.However, due to the absence of quenched impurity ions such as iron in the main body of lithium soapstone, [Eu (Phen) 2] 3+- lithium soapstone has better luminescence intensity than [Eu (Phen) 2] 3+- montmorillonite, and the fingerprint shows clearer lines and stronger contrast with the background. In 2016, V Sharma et al. synthesized strontium aluminate (SrAl2O4: Eu2+, Dy3+) nano fluorescent powder using combustion method. The powder is suitable for the display of fresh and old fingerprints on permeable and non permeable objects such as ordinary colored paper, packaging paper, aluminum foil, and optical discs. It not only exhibits high sensitivity and selectivity, but also has strong and long-lasting afterglow characteristics. In 2018, Wang et al. prepared CaS nanoparticles (ESM-CaS-NP) doped with europium, samarium, and manganese with an average diameter of 30nm. The nanoparticles were encapsulated with amphiphilic ligands, allowing them to be uniformly dispersed in water without losing their fluorescence efficiency; Co modification of ESM-CaS-NP surface with 1-dodecylthiol and 11-mercaptoundecanoic acid (Arg-DT)/ MUA@ESM-CaS NPs successfully solved the problem of fluorescence quenching in water and particle aggregation caused by particle hydrolysis in the nano fluorescent powder. This fluorescent powder not only exhibits potential fingerprints on objects such as aluminum foil, plastic, glass, and ceramic tiles with high sensitivity, but also has a wide range of excitation light sources and does not require expensive image extraction equipment to display fingerprints。In the same year, Wang’s research group synthesized a series of ternary europium complexes [Eu (m-MA) 3 (o-Phen)] using ortho, meta, and p-methylbenzoic acid as the first ligand and ortho phenanthroline as the second ligand using precipitation method. Under 245nm ultraviolet light irradiation, potential fingerprints on objects such as plastics and trademarks could be clearly displayed. In 2019, Sung Jun Park et al. synthesized YBO3: Ln3+(Ln=Eu, Tb) phosphors through solvothermal method, effectively improving potential fingerprint detection and reducing background pattern interference. In 2020, Prabakaran et al. developed a fluorescent Na [Eu (5,50 DMBP) (phen) 3] · Cl3/D-Dextrose composite, using EuCl3 · 6H20 as the precursor. Na [Eu (5,5 '- DMBP) (phen) 3] Cl3 was synthesized using Phen and 5,5′ – DMBP through a hot solvent method, and then Na [Eu (5,5 '- DMBP) (phen) 3] Cl3 and D-Dextrose were used as the precursor to form Na [Eu (5,50 DMBP) (phen) 3] · Cl3 through adsorption method. 3/D-Dextrose complex. Through experiments, the composite can clearly display fingerprints on objects such as plastic bottle caps, glasses, and South African currency under the excitation of 365nm sunlight or ultraviolet light, with higher contrast and more stable fluorescence performance. In 2021, Dan Zhang et al. successfully designed and synthesized a novel hexanuclear Eu3+complex Eu6 (PPA) 18CTP-TPY with six binding sites, which has excellent fluorescence thermal stability (<50 ℃) and can be used for fingerprint display. However, further experiments are needed to determine its suitable guest species. In 2022, L Brini et al. successfully synthesized Eu: Y2Sn2O7 fluorescent powder through co precipitation method and further grinding treatment, which can reveal potential fingerprints on wooden and impermeable objects.In the same year, Wang’s research group synthesized NaYF4: Yb using solvent thermal synthesis method, Er@YVO4 Eu core-shell type nanofluorescence material, which can generate red fluorescence under 254nm ultraviolet excitation and bright green fluorescence under 980nm near-infrared excitation, achieving dual mode display of potential fingerprints on the guest. The potential fingerprint display on objects such as ceramic tiles, plastic sheets, aluminum alloys, RMB, and colored letterhead paper exhibits high sensitivity, selectivity, contrast, and strong resistance to background interference.
4 Outlook
In recent years, the research on rare earth europium complexes has attracted much attention, thanks to their excellent optical and magnetic properties such as high luminescence intensity, high color purity, long fluorescence lifetime, large energy absorption and emission gaps, and narrow absorption peaks. With the deepening of research on rare earth materials, their applications in various fields such as lighting and display, bioscience, agriculture, military, electronic information industry, optical information transmission, fluorescence anti-counterfeiting, fluorescence detection, etc. are becoming increasingly widespread. The optical properties of europium complexes are excellent, and their application fields are gradually expanding. However, their lack of thermal stability, mechanical properties, and processability will limit their practical applications. From the current research perspective, the application research of the optical properties of europium complexes in the field of forensic science should mainly focus on improving the optical properties of europium complexes and solving the problems of fluorescent particles being prone to aggregation in humid environments, maintaining the stability and luminescence efficiency of europium complexes in aqueous solutions. Nowadays, the progress of society and science and technology has put forward higher requirements for the preparation of new materials. While meeting application needs, it should also comply with the characteristics of diversified design and low cost. Therefore, further research on europium complexes is of great significance for the development of China’s rich rare earth resources and the development of criminal science and technology.
Post time: Nov-01-2023