Rapid identification of latent fingerprints, security ink and WLED applications of CaZrO₃:Eu³⁺ fluorescent labelling agent fabricated via bio-template assisted combustion route

Highlights

• CaZrO₃:Eu³⁺ NPs are prepared via Aloe vera gel assisted combustion route.

• Simple and nondestructive powder dusting method was used for visualization of LFPs.

• Visualized LFPs exhibits high sensitivity, reproducibility, selectivity and reliability.

• The Cheiloscopy and anticounterfeiting applications are studied in detail.

Abstract

Currently the traditional powder dusting method has low resolution, less sensitive, low contrast and high background noise. To provide the suitable solution for the existing problems of powder dusting method, Eu³⁺ doped CaZrO₃ nanoparticles were prepared by a facile solution combustion method using Aloe vera gel was used as a dusting powder. Powder X-ray diffraction results showed the orthorhombic phase. Scanning electron microscopy supports for the Ostwald reopening phenomena and observed flakes type morphology. Transmission electron microscopy analysis supports for the flakes type morphology and shows the polycrystalline nature of the prepared samples. Photoluminescence studies revealed that the transitions from the excited states ⁵D₀ to the ground state⁷F_j (j = 0, 1, 2, 3, 4) of Eu³⁺ ions. The product showed intense red light emission with the color coordinates of (0.640, 0.359). The estimated correlated color temperature value was ∼1798 K also the estimated color purity and quantum efficiency are found to be 75.79% and 74% respectively. The optimum concentration of Eu³⁺ ions in CaZrO₃ host lattice has been found to be at 9 mol%. Using this optimized CaZrO₃: Eu³⁺ phosphors, various ridge shapes (arch, whorl and loop) of the latent fingerprints were observed clearly. Further, under 254 nm UV and normal light illumination, on substrates (glass, aluminum foil, stainless steel, compact disk and credit card) also the visualization was excellent and sensitivity was high to see the clear ridges. The results indicate that CaZrO₃: Eu³⁺ (9 mol%) phosphors can be effectively used in fingerprint identification, anti-counterfeiting applications and red component of WLEDs.

Introduction

Fingerprints have been extensively studied because of their uniqueness in identifying the individuals based on whorl, arch and loop shape patterns. Fingerprints have several minutia's such as ridge flow (level 1), core, delta, bifurcation (level 2) and sweat pore (level 3) which were different for each individual and independent of the age. Therefore, fingerprints have been used as the effective technique for identifying individual's information at crime scenes [[1], [2], [3], [4]]. There were three different types of fingerprint powders; regular, metallic and luminescent. Regular fingerprint powder consists of a resinous polymer and a colorant. It was impotent to develop latent fingerprint (LFP) on challenging surfaces. Use of metallic compounds was harmful for user health because of metallic powder containing meshed metals with lead, gold and silver. They were effective in the development of LFPs except the problems of low resolution, less sensitive, low contrast and high background noise. Low resolution was due to their material property (resinous polymer) and colorant components. Low contrast was due to the non-fluorescence nature of the samples. Lastly, they were harmful for users' health due to their composition. To provide the better solution to these problems, the development of luminescent materials was in demanded for replacing the metal and magnetic powders [[5], [6], [7], [8]].

Recently, the use of nanopowders (NPs) helped for the accurate identification of LFPs. This was because NPs were extremely small, generally 1000–10,000 times smaller than a fingerprint ridge width, ensuring excellent resolution upon detection [9]. So, the luminescent NPs were highly required as a solution to overcome the limitation for regular and metallic types of fingerprints [10]. Security ink was an important issue because it can be used to several things such as protection of the counterfeit money and the forgery of a private documents. The counterfeiting behavior has gradually increased that was causing significant damage. Particularly, the counterfeiting of currency has a bad influence on social economy, which causes the decrease of monetary value. As a result, countries around the world have lost hundreds of million dollars due to the illegal counterfeiting behavior. To prevent the counterfeiting, a number of anti-counterfeiting technologies (simple markers, security inks) have been studied. However, they have some problems which were injurious to user health and not transparent solution because it used the toxic solvents and materials such as the hexane, toluene, cadmium selenide and cadmium sulfide. When the security ink was transparent, it can prevent the counterfeit money and private documents. To solve the problems, it was necessary to develop luminescent materials with high efficiency, high contrast, high selectivity and nontoxicity [11,12].

In case of incomplete evidence of proof with FPs in crime scene, there was a need of extra confirmation like lip prints (LPs) to solve the crime scene. Lip prints play a vital role in many of the crime scenes namely, prints on a cigarette butt, prints on a glass that a person drank from, prints on a glass/window if they were pressed up against it and when a person has been bound or gagged. All of these were possible places where LPs may be found and used in the examination of a crime [13].

Nevertheless, the use of LPs in crime scene was incomplete due to the reliability of LPs has not been decisively recognized in the court of law. Further, LPs were unique and do not alter during the life span of an individual. Several factors such as, pathosis, surgical treatment to lips may distract their shape and size, which in turn alter the arrangement and morphology of grooves. The dissimilarity in the lip grooves help in identifying the, male and female. Further, the lipstick smears gives a secondary proof in connection between a victim and a suspect or a suspect and a crime scene. Henceforth, in adding to the LFPs the LPs were also considerably attracted more interest for the forensic scientists.

Alkali earth zirconates were found to be wide range of applications namely superior photonic conductivity, remarkable electrical properties including ferroelectricity and piezoelectricity. Among the perovskites, CaZrO₃ ceramic exhibit high permittivity and insulation resistance. Further, they show high ionic conductivity, excellent gas sensitivity at high temperatures. However, there were very few reports related to rare earth doped CaZrO₃ by low temperature synthesis routes and also CaZrO₃ has not received significant consideration as luminescent host in the fabrication of display devices [[14], [15], [16], [17]].

Till date a variety of methods were utilized for the fabrication of perovskites namely, CaZrO₃, SrTiO₃, SrZrO₃ etc. In the present work eco-friendly and cost effective green synthesis route was used for the fabrication of CaZrO₃. Herein, we report a simple solution combustion method to synthesize the novel red emitting CaZrO₃:Eu³⁺ phosphors with different concentrations (1–11 mol %) of Eu³⁺ ions using Aloe vera (A.V.) gel as fuel. The obtained products were well characterized. To the best of our knowledge, there were no reports available for the visualization of latent fingerprints and anti-counterfeiting applications using CaZrO₃:Eu³⁺ phosphors.