Rough Surface Enhanced Interfacial Synthesis of Core‐Shell Magnetic Fluorescent Microspheres for Enhanced Latent Fingerprint Visualization

Developing latent fingerprints is extremely significant for personal identification and criminal investigation. Sub‐micrometer core‐shell structured magnetic fluorescent composite which combines the merits of noncontact magnetic responsiveness and strong fluorescence emission is highly desired for the visualization of latent fingerprint on various substrates with high resolutions. However, it remains a great challenge to synthesize uniform magnetic fluorescent composites with well‐defined structure and functionalities due to the uncontrollable heterogeneous growth. Herein, a urea‐assisted rough interface precipitation method is proposed to controllably synthesize uniform core‐shell structured magnetic fluorescent composite microspheres (Fe3O4@mesoSiO2@Y2O3:Eu3+). The composite microspheres with magnetic core and mesoporous silica shell (Fe3O4@mesoSiO2) possess huge guest‐host interface, numerous nucleation sites, and rough surface morphology, facilitating the efficient adsorption of Y3+/Eu3+ ions, and further controllable interface deposition of metal (Y, Eu) hydroxides induced by slow release of OH− and CO32− anions via the in‐solution decomposition of urea. After subsequent thermal annealing, the obtained Fe3O4@mesoSiO2@Y2O3:Eu3+ microspheres possess high magnetization for convenient magnetic manipulation, strong fluorescence intensity and negligible quenching effect, enabling a superior performance in latent fingerprint visualization with high contrast and resolution on various substrates.

possess large particle size of the micrometer level with nonuniform morphologies, giving rise to reduced sensitivity in practical applications. [18] Magnetic particles like magnetite (Fe 3 O 4 ) can develop latent fingerprints by magnetic-powder dusting method through noncontact-model physical brushing. This method can preserve the integrity of the fingerprint patterns on most substrates in crime scenes, but the black magnetic particles are unsuitable for dark substrates. [19,20] Therefore, in order to solve the abovementioned drawbacks of the single component developer, it has been widely recognized that the development of uniform sub-micrometer multi-component magnetic fluorescent particles by deposition of phosphor shell on the magnetic core is favorable for visualization of latent fingerprints with detailed features since the minimum gap between the fine lines, wrinkles or sweat pores is at a size of 50 micrometer. [21][22][23][24] On the one hand, novel core-shell structure can provide synergistic enhancement effect for various applications, and unique core layer or shell layer formed by various components or functions can realize functional integration to optimize the response of materials, such as magnetic response, photo response, and so on. On the other hand, fluorescent particles usually possess strong luminous properties and light capture capability, which has been widely applied in medical imaging and information identification. Thus, exploiting multifunctional core-shell fluorescent materials is vital for latent fingerprint visualization. The sub-micrometer core-shell structured magnetic fluorescent composite which combines the merits of non-contact magnetic responsiveness and strong fluorescence emission is suitable for visualization of latent fingerprint on various substrates with high resolution for fine lines. Although great efforts have been made in developing novel bifunctional materials with the magnetic core and fluorescent shell through sol-gel method, [25,26] combustion strategy, [5,27] and hydrothermal method, [28] these methods failed to control the growth process of core-shell structure, especially for the uniform sub-micrometer magnetic fluorescent particles. Owing to the quick nucleation-growth kinetics of rare earth oxide and their relative weak interaction with the magnetic particle, it remains a great challenge to control the uniform deposition of fluorescent species onto the surface of magnetic particle. Thus, it is of highly importance to develop effective, convenient, and controllable method for fabrication of dual-functional magnetic fluorescent particles with uniform sub-micrometer size, well-defined core-shell structures, strong fluorescence intensity, and high magnetization for high-resolution latent fingerprint visualization.
In this study, a facile but effective rough interface-enabled heterogeneous precipitation method is developed to construct well-defined core-shell structured magnetic fluorescent microspheres with the core of mesoporous silica-coated Fe 3 O 4 particles (Fe 3 O 4 @mesoSiO 2 ), and the outer fluorescent layer of Y 2 O 3 :Eu 3+ . By adjusting the deposition kinetics via the slow decomposition of urea in solutions and the utilization of a highly active rough interface of Fe 3 O 4 @mesoSiO 2 core, a controllable heterogeneous deposition of uniform Y 2 O 3 :Eu 3+ fluorescent layer on magnetic silica core was achieved, resulting in bifunctional microspheres (denoted as Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ ). The obtained Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres have uniform size of ≈610 nm, the high magnetization of 29.3 emu/g, and strong fluorescence intensity with the obvious emission peak centered at 610 nm under excitation at 250 nm. Notably, the mesoporous silica shell provides a hydrophilic surface that is affinity to the precursors of Y 2 O 3 :Eu 3+ outer shell, and more importantly, its rough interface consisting of spiculate structured silica enhances the adsorption and deposition of metal (Y, Eu) hydroxides. Moreover, due to the insulation effect of silica, the as-formed fluorescent Y 2 O 3 :Eu 3+ shell can be effectively separated from Fe 3 O 4 core, avoiding occurrence of quenching effect. By using magnetic powder dusting method to Scheme 1. Synthesis procedure of the uniform Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres. www.advmatinterfaces.de develop latent fingerprints, the red fluorescent fingerprints with high contrast, high resolution, and high integrity are clearly visible to the naked eye under 254 nm ultraviolet light on various substrates without any background fluorescence interference.

Results and Discussion
The synthesis procedure of Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres is described in Scheme 1. Highly water-dispersible Fe 3 O 4 particles with a mean diameter of 230 nm were synthesized according to the previous reports. [29] Although the as-synthesized Fe 3 O 4 particles show affinity to the precursor of Y 2 O 3 :Eu 3+ in solutions, it is challenging to uniformly form a controllable rare earth oxide shell on Fe 3 O 4 particles due to the quick homogeneous nucleation and growth of oxides in the solution by precipitating agents. Especially, this nucleation growth is strongly dependent on the liquid phase environment, such as solvent, acid-base property, and so on, which can usually induce to form uncontrolled nucleation under the surface of the growth substrate. In addition, spherical structure can also induce nucleation and growth of precursor on a curved surface, and isotropic nucleation growth is usually more easily than anisotropic coating. To achieve the controlled interface deposition of rare earth oxide layer, a protective mesoporous silica shell with a rough surface is fabricated through coating magnetite particles with a hybrid layer of resorcinol-formaldehyde (RF)/ silica shell via sol-gel process, [30] followed by calcination in air, leading to Fe 3 O 4 @mesoSiO 2 microspheres with spiculate structured silica shell. Using the obtained rough Fe 3 O 4 @mesoSiO 2 microspheres as seeds, the interface precipitation of Y 3+ and Eu 3+ ions were carried out in a liquid-phase synthesis at 90°C. The spiculate structured silica with high exposed surface area possess large surface energy and have a strong interaction/ adsorption with Y 3+ /Eu 3+ ions in the solution. On the other hand, the decomposition of urea slowly releases OH − and CO 3 2− anions, which further interact with rare earth ions (mainly Y 3+ and Eu 3+ ) by electrostatic attractions, resulting in the homogeneous precipitation of the uniform rare earth precursors coated on the surface of Fe 3 O 4 @mesoSiO 2 microspheres. The composite microspheres are calcined at 800°C in air to decompose the precursor shell completely and transform the precursor into crystallized Y 2 O 3 : Eu 3+ shell. Notably, considering the partial oxidation of Fe 3 O 4 to Fe 2 O 3 at 800°C (denoted as FeOx@mes-oSiO 2 @Y 2 O 3 :Eu 3+ ), the microspheres are further annealed in hydrogen to reduce FeOx core into magnetite core with a high magnetization.
The X-ray diffraction pattern (Figure 1a) of FeOx@mes-oSiO 2 @Y 2 O 3 :Eu 3+ microspheres show peaks at 24.0°, 33.0°, 35.5° and 54.0° attributed to the rhombohedral phase of www.advmatinterfaces.de on the Fe 3 O 4 @mesoSiO 2 microspheres, also with the partial oxidation of Fe 3 O 4 into Fe 2 O 3 during high-temperature calcination. [31][32][33] After hydrogen reduction, the diffraction peaks of Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres are in well agreement with face-centered cubic phase of TEM observation (Figure 2c) further indicated that confirms a layer of Y 2 O 3 :Eu 3+ shell is successfully deposited onto the magnetic microspheres. The obtained as-made Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microsphere exhibits the spherical morphology and the distinct core-shell-shell structure with a particle size of 610 nm in diameter. The ultrathin sections of Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres were prepared and www.advmatinterfaces.de the profile structures were characterized by TEM ( Figure S2, Supporting Information), and the thickness of Y 2 O 3 :Eu 3+ shell is ≈75 nm, thanks to the rough surface morphology of Fe 3 O 4 @mesoSiO 2 facilitating the deposition of rare earth precursors. Notably, the surface morphology of the obtained Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres is smooth and uniform due to the well-controlled deposition of precursor component via the urea-assisted interface precipitation. Highresolution TEM image of Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microsphere (Figure 2d) shows high crystallinity of the as-synthesized Y 2 O 3 :Eu 3+ shell, and the crystal lattices space is measured to be 0.306 nm, which corresponds to the (222) lattice plane of cubic Y 2 O 3 . Energy dispersive X-ray spectroscopy (EDX) technique is applied on a single microsphere to study the element distribution of the multi-layer particles (Figure 2e-i). Element mappings indicate a clear separation of Si, Fe, and rare earth region, confirming a distinct core-shell-shell structure. Moreover, Y and Eu species are uniformly distributed on the surface of Fe 3 O 4 @mesoSiO 2 microsphere to form a smooth and thick fluorescent layer, which is significant to provide adequate fluorescence intensity for practical latent fingerprint development.
To investigate the structure control for Fe 3 O 4 @mesoSiO 2 @ Y 2 O 3 :Eu 3+ microspheres, the feeding amount of precursors was adjusted for the interface deposition process. First, the concentration of Fe 3 O 4 @mesoSiO 2 was optimized for synthesizing Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres while keeping other-parameter unchanged. As the amount of Fe 3 O 4 @ mesoSiO 2 is much small (0.33 mg mL −1 ), the rough Fe 3 O 4 @ mesoSiO 2 was completely covered by Y 2 O 3 :Eu 3+ nanoparticles to form smooth Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres. Besides, many excess free Y 2 O 3 :Eu 3+ nanoparticles were observed, which was formed in solution due to the lack of sufficient nucleate sites for heterogeneous growth ( Figure S3a,d, Supporting Information). With the increase of Fe 3 O 4 @mesoSiO 2 amount, the surface morphology gradually became rough due to the decreasing thickness of Y 2 O 3 :Eu 3+ shell and no free Y 2 O 3 :Eu 3+ nanoparticles formed, indicating the Y 2 O 3 :Eu 3+ was completely deposited onto the Fe 3 O 4 @ mesoSiO 2 core ( Figure S3b,c,e,f, Supporting Information). Besides, the influence of urea content (6.67-20.00 mg mL −1 ) on the surface morphology was further investigated with the content of Fe 3 O 4 @mesoSiO 2 microspheres and other reaction conditions unchanged. The obtained magnetic fluorescent composite is denoted as Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ -x (here "x" indicates the urea concentration). As shown in Figure S4, Supporting Information, Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ -6.67, and Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ -10 exhibit rough surfaces with massive nanoparticles of about 60 nm in diameter dispersed in the reaction system. It may be due to the phase separation of Fe 3 O 4 @mesoSiO 2 microspheres and Y 2 O 3 :Eu 3+ nanoparticles with the relatively lower feeding amount of urea. With the increase of urea amount from 13.33 to 16.67 mg mL −1 , the content of free Y 2 O 3 :Eu 3+ nanoparticles gradually decreased. Moreover, the surface of the as-made microspheres becomes smooth because the sufficient precipitating agent urea can anchor more metal ions to be precipitated for the growth and aggregation of small grains in the interface of Fe 3 O 4 @mesoSiO 2 microspheres. As the urea content increased to 20 mg mL −1 , the smooth surface of Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres became more clear and the microspheres show a distinct core-shellshell structure with 75 nm thick fluorescent Y 2 O 3 :Eu 3+ layer. Therefore, it can be seen that urea can slowly release NH 3 in solution which helps continuous generation of hydroxyl anions to achieve the controllable precipitation of Y 3+ and Eu 3+ ions. Thus, the interface nucleate-growth-crystallization kinetics can be finely tuned by varying the amount of urea, allowing an easy control over the final surface morphology of Fe 3 O 4 @mes-oSiO 2 @Y 2 O 3 :Eu 3+ microspheres.
To investigate the effect of surface roughness on the interface deposition of rare earth precursors, smooth Fe 3 O 4 @nSiO 2 microspheres with an average size of 450 nm were employed as a control sample, followed by the same urea-assisted homogeneous precipitation and thermal treatment ( Figure S5, Supporting Information). SEM images show Fe 3 O 4 @nSiO 2 microspheres have good dispersibility with the compact silica layer of 110 nm in thickness ( Figure S6, Supporting Information). As shown in Figure S7a, Figure S8, Supporting Information); however, the thick of Y 2 O 3 :Eu 3+ shell is less than 40 nm, much thinner than that (75 nm) of Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres, which is unfavorable for emission of fluorescence. It confirms that, the rough surface plays an important role in enhancing the interfacial deposition of Y 2 O 3 :Eu 3+ coating layer around the Fe 3 O 4 @mSiO 2 seeds.
To gain more insight into the formation process of Y 2 O 3 :Eu 3+ shell on the mesoporous silica layer, intermediate samples were withdrawn from the solution after reaction for 0, 10, 20, 30, and 60 min (Figure 3). TEM observation indicates that, after a reaction for 10 min (Figure 3b), the rare earth cations began to adsorb and nucleate on the rough surface of Fe 3 O 4 @mesoSiO 2 microspheres due to the electrostatic attraction between the positive rare earth ions and negative Si-OH groups. The rare earth cations further interact with the precipitating OH − and CO 3 2− anions which were slowly generated by continuous decomposition of urea. And the surface gradually becomes smooth and the mesopore structure almost disappears due to the continual interface deposition and formation of rare earth compounds along the longitudinal depth of porous channel after reaction for 20 min (Figure 3c). Further prolonging the reaction time to 30 min (Figure 3d), fluorescent precursors gradually grow thicker and the morphology of the microsphere tend to be "cauliflower like" structure, due to the isolated domains became continuous thin layers covering the Fe 3 O 4 @mesoSiO 2 seeds. After reaction for 60 min (Figure 3e), the microspheres exhibit the spherical morphology with a smooth surface. In addition, the well-defined core-shell-shell structure with a certain thickness of fluorescent precursors shell can be clearly observed. In contrast, for Fe 3 O 4 @nSiO 2 microspheres with smooth surface, www.advmatinterfaces.de no precipitates were found on the surface of compact silica shell in the first 20 min. (Figure 3f-h). After 30 min of reaction (Figure 3i), rare earth precursors initiate to deposit on the smooth surface. Further prolonging the reaction time to 60 min (Figure 3j), fluorescent precursors partially deposit on the surface, and the luminescent layer is relatively thinner. Thus, the surface roughness is the key factor to control the thickness of Y 2 O 3 :Eu 3+ shell during urea-assisted homogeneous precipitation method. The rough surface has a certain longitudinal depth with a huge host-guest interaction interface, which provides more nucleation site, facilitating the deposition and nucleation of fluorescent precursors to further enlarge shell thickness. Moreover, the mesoporous silica interlayer has two main functions, one is to prevent quenching effect of Fe 3 O 4 on Y 2 O 3 :Eu 3+ fluorescent shell, and the other is to provide rough surface for enhanced interface deposition of fluorescent precursors to ensure higher luminescent intensity.
The luminescent properties of Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres and Fe 3 O 4 @nSiO 2 @Y 2 O 3 :Eu 3+ microspheres are characterized by fluorescence spectroscopy. The excitation spectra show the same peak wavelength of Fe 3 O 4 @mesoSiO 2 @ Y 2 O 3 :Eu 3+ microspheres and Fe 3 O 4 @nSiO 2 @Y 2 O 3 :Eu 3+ microspheres in Figure 4a. The peak at 250 nm is attributed to the occurrence of charge transfer transition between O 2− and Eu 3+ ions. [34] In addition, both the samples exhibit strong visible fluorescence under excitation at 250 nm (Figure 4b). The obvious characteristic peak centered at 610 nm is assigned to the hypersensitive electronic transition of 5 D 0 → 7 F 2 of Eu 3+ ions. [ (Figure 4c). More significantly, latent fingerprint developed by Fe 3 O 4 @mesoSiO 2 @ Y 2 O 3 :Eu 3+ microspheres exhibit more distinct fingerprint minutiae and clear three levels of fingerprint features than Fe 3 O 4 @nSiO 2 @Y 2 O 3 :Eu 3+ microspheres, especially the island (c 1 ), sweat poles (c 2 ), the crease (c 3 ) and the ridge ending (c 4 ), and this is particularly benefit to increase the reliability and veracity for practical application of latent fingerprint detection. To further investigate the universality of the magnetic fluorescent powders, fingerprints remained on various substrates including glass, aluminum foil, marble, and porcelain, are developed by Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres ( Figure S9, Supporting Information). The latent fingerprint developed by Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ microspheres shows higher contrast and resolution. It further demonstrates that rough mesopores surface, providing more nucleation sites and certain longitudinal depth than compact surface, is more conducive to deposit rare earth precursors during the urea-assisted homo geneous precipitation. In addition, unique mesoporous structure can provide confinement effect for the deposition of fluorescence nanoparticles. Thus, a thicker Y 2 O 3 :Eu 3+ fluorescence layer is gained after calcination to ensure the more excellent property in latent fingerprints visualization.

Conclusions
In summary, bifunctional magnetic-fluorescent composite microspheres with core-shell structure (Fe 3 O 4 @mesoSiO 2 @Y 2 O 3 :Eu 3+ ) were synthesized via a rough interface assisted deposition method using urea as precipitating agent. Benefiting from the rough surface of Fe 3 O 4 @mesoSiO 2
Synthesis of Fe 3 O 4 @mesoSiO 2 Microspheres: Highly water-dispersible Fe 3 O 4 particles with an average size of 240 nm were synthesized via a solvothermal reaction according to the literature. Fe 3 O 4 @mesoSiO 2 microspheres were prepared according to the previous report with some modifications. Typically, 10 mg of Fe 3 O 4 was first dispersed in 128 mL mixed solution of 112 mL anhydrous ethanol and 16 mL distilled water. After adequate ultrasound and transfer to a 250 ml three-necked flask, 1.6 mL of ammonia solution was added with stirring for 20 min at 30 °C under the mechanical stirring. Subsequently, 1.6 mL of TEOS solution was added to the above solution, followed by adding 0.64 g of resorcinol and 0.8 mL of formaldehyde (37 wt.%). The mixture was stirred for another 8 h to give rise to fuscous precipitates. The obtained solid product was separated by a magnet, washed with deionized water and ethanol, and then dried at 40 °C with calcining in the air at 500 °C for 6 h.
Synthesis of Fe 3 O 4 @nSiO 2 Microspheres: 10 mg of Fe 3 O 4 with an average diameter of about 240 nm was dispersed in the mixed solution of 105 mL ethanol and 35 mL deionized water. After sonication, the dispersion was transferred to a 250 mL three-necked flask. Then, 2 mL of ammonia solution was added with stirring for 20 min at 30 °C under the mechanical stirring. Subsequently, 3 mL of TEOS solution was added, followed by stirring for another 8 h. The as-made Fe 3 O 4 @nSiO 2 particles were separated by a magnet and subsequently washed with ethanol and distilled water. Then these particles were obtained after drying at 40 °C in an oven.