Ultrasound assisted Eu3+–doped strontium titanate nanophosphors: Labeling agent useful for visualization of latent fingerprints
Graphical abstract
Introduction
Finger or thumb–impressions are the most authoritative tool for personal identification since the ridge patterns of every print are unique and indisputable. Researchers think that fingerprints can be classified into three categories/classes according to the surface type on which they are found and whether they are visible or not: Latent (invisible), visible (are also called patent prints), and plastic prints (ridge–impressions left on soft–material such as soap, wax, wet paint, etc) [1]. Locating a plastic or visible print is a comparatively simpler task since they are generally visible to searching naked eyes. However, finding and observing latent fingerprints (LFPs) is more difficult and requires special techniques for visualization. Forensic investigations often require fingerprinting powders (fine powders) to develop LFPs from a variety of surfaces, which then provide a useful source of evidence for suspect/subject identification.
Trivalent rare earth (RE) doped luminescent nanocrystals have attracted vast interest and come to the forefront in nanophotonics owing to their distinct luminescence properties, as well as their potential application in a diversity of fields, from display devices to biomedical and forensic applications [2]. In 1989, Mitchell and colleagues developed the first application of lanthanide complexes for fingerprint imaging [3], which inspired many research teams to study the lanthanide–doped luminescent materials for latent fingermarks recognition. Trivalent europium doped materials have drawn special attention as good emitting phosphor candidates for display and forensic applications. In this kind of materials, Eu3+ ions typically possess a rather strong absorption due to admixing of charge transfer and Eu3+ excited states. The choice of a suitable host matrix plays a crucial role in realizing rich photoluminescence properties since the activated luminescence of the RE–doped phosphors depends on the composition and energy levels of the host matrix as well as the surrounding environment. This is due to the crystal field strength, covalence, and the active energy transfer from the host material to dopant ions. Thus the research on Eu3+–doped luminescent materials still receives great attention from scientists.
In recent years, RE–doped perovskites (with ABO3 general–formula) have gained abundant interest for light–emitting diode (LED) applications because of their structural versatility in accommodating differently sized ions. Especially red–luminescence of Eu3+–doped perovskites under UV radiation has been widely investigated, for example, BaTiO3: Eu3+, CaTiO3: Eu3+ and AZrO3: Eu3+ (A = Ca, Sr, Ba) [4]. A variety of lanthanide ion–doped perovskites such as YAlO3: Sm3+ [5], CaZrO3: Eu3+ [6], and YBO3: Ln3+ (Ln= Eu, Tb) [7] have been also used for LFPs visualization. Dhanalakshmi et al. recently demonstrated the utilization of BaTiO3: Eu3+ nanoprobes for fingerprint detection on several complex surfaces [8]. Even though several Eu3+ doped oxides have been exploited by using a different material or other preparation techniques, some issues such as the optimization of the synthesis and luminescence, quenching of Eu3+ luminescence and impacts of the Eu3+ doping on the host lattice need to be addressed. Also, a comprehensive study of the dependence of the luminescence intensity on Eu3+ concentration, the quantum yield, and the brightness on the concentration of the active Eu3+ ions is still missing.
Over the last few decades, much attention has been given to the use of the host SrTiO3 (STO) – a mixed Sr2+/Ti4+ oxide – because of its chemical and thermal stability and nontoxic nature. The value of the optical band–gap energy (Eg) for STO, estimated by UV–visible absorption, varies between 3.44 eV to 3.73 eV [9]. Thus, STO dielectric material plays an important role in magnetic, photocatalytic, ferroelectric, and thermoelectric applications [[10], [11], [12]]. The wide Eg of STO makes it ideal for RE incorporation as RE luminescence thermal quenching is inversely proportional to Eg of a host. STO: Eu3+ is a known good absorber in the long UV region along with high chemical stability as compared to the undoped STO, which is not suitable for visible light absorption due to its wide band–gap properties [13]. It was also demonstrated that Eu doping to a certain extent (< 10%) does not change the structure of the STO host lattice and its stability remains unaltered. There is thus tremendous value attributed to Eu–doped STO as novel red luminescent material with low cost, ultra–high luminescence efficiency, and good color efficiency, which can be used for the development of LFPs. It is important to note that many existing labeling agents are only able to visualize level I and level II ridge details. However, the analysis of level II (bifurcation, ridge end, crossover, etc.) and level III (sweat pores and scar) details require more efficient fine particles with uniform size and morphology [14]. In theory, the use of nanopowders improves the light emission efficiency and could result in better ridge pattern resolution than using the micron–sized particles in traditional powders that normally contain particles in the submicron to micrometer range [15].
To date, various methods have been used for LFPs detection, namely, powder dusting, metal deposition, cyanoacrylate fuming, and staining with ninhydrin and fluorescent compounds, for example. Powder dusting remains the simplest and most effective method for LFP detection on challenging surfaces, employing regular, metallic, and luminescent powders. Despite this, however, fingerprinting powders are limited as having low sensitivity, low contrast, high background noise, and high autofluorescent interference [16], which need to be addressed. These shortcomings may be mitigated by using luminescent nanopowders such as those doped with the Eu3+ ions for the beneficial properties of lower phonon energy, low synthetic power consumption, and an excellent intrinsic luminescent performance [17].
In this work, excellent properties of photochemical stability and high emission efficiency are achieved in the red phosphor STO: Eu3+ synthesized by ultrasonication of their bulk powder materials, which has not been reported to date. The crystalline phases of the synthesized STO: Eu3+ (1–5 mol%) nanoparticles (NPs) were confirmed by XRD. Eu3+ ions concentration was optimized for achieving maximum luminescence by the photoluminescence (PL) spectroscopy measurements. Morphology parameters of the optimized product were investigated by scanning electron microscopy (SEM). Then, we successfully employed the optimized NPs as rapid identification of LFPs using the powder dusting method. Revealing the LFPs on the stainless steel surface was possible using STO: Eu3+ (3 mol%) despite the high–reflectivity of the metal surface, which is considered a hard surface for fingerprint detection.
Section snippets
Preparation of SrTiO3: Eu3+ NPs phosphors
Low–dimensional Eu3+–doped STO nanostructures were prepared via a two–step method. For the first step, we adopted the Pechini sol–gel technique to prepare the parent STO compound as described in our previous experiment [18]. The synthesis procedure of STO is detailed in Fig. 1 (a): High purity SrCO3 and TiO2 (>99.99%, Sigma–Aldrich chemicals) were considered as starting materials. Initially, these oxide and carbonate precursors were converted into their corresponding nitrates via the addition
Structural properties (XRD studies)
Powder XRD analysis was conducted to determine the crystalline–structure of the synthesized specimens. Fig. 2 (a) displays the XRD–patterns of the STO: δEu3+ NPs with different low concentrations (δ= 1, 2, 3, 4, and 5 mol%). The results stipulated that the XRD–peaks of all the synthesized compounds may be indexed to the pure phase of undoped STO [19]. No additional peaks of unreacted Eu2O3, SrCO3, and TiO2 were observed in the XRD–data, indicating that the reaction between the raw materials was
Conclusion
A simple and inexpensive route for the preparation of Eu3+ doped STO nanostructures was developed by ultrasonication of their bulk perovskite powder. Initially, ceramic pellets of the nominal compositions STO: Eu3+ (1–5 mol%) were prepared by the Pechini sol–gel technique. The resultant pellets were then ground into micrometer–sized particles and mixed with isopropanol in an ultrasonic bath to prepare nanoparticles. Compounds obtained from ultrasonication showed cubic symmetry and good
Declaration of Competing Interest
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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