1 Introduction

In recent years, luminescent rare-earth inorganic materials have aroused rapidly growing interest for fluorescent-based biomedical applications because of their unique optical properties such as sharp absorption and emission lines invisible region, low phonon energy, good quantum yield, narrow bandwidth, large stocks shift, high photo-chemical and thermal stability, excellent biocompatibility, and non-toxicity [1,2,3,4,5]. These outstanding optical and photochemical properties make them promising candidates for their future applications in widespread bio-medical sciences. Amongst lanthanide nanomaterials, gadolinium oxide (Gd2O3) is an ideal host matrix for doping of luminescent ion due to their excellent photo-chemical and thermal stability and low vibrational energy [1,2,3, 5,6,7]. Additionally, Gd2O3 is magnetically active and used as magnetic resonance imaging contrast agent [2,3,4, 8]. Therefore, trivalent Eu substituted Gd2O3 is one of the most important red-emitting inorganic material because of its significant emission and exciton in UV/Visible regions [3, 4, 7]. Many efforts were made for the synthesis of Gd2O3 materials through various chemical routes such as polyol [9, 10], sono-chemical [11], micro-emulsion [12], sol-gel chemical [13], microwave decomposition [14, 15], thermal decomposition [16, 17], hydrothermal/solvothermal [18,19,20], co-precipitation [21,22,23], and aqueous self-combustion process. Among them, low-temperature thermal decomposition process can be the most efficient and appropriate synthesis route as it can yield high phase purity powder with excellent chemical homogeneity. So that, micro to nano-scale Gd2O3:Ln synthesis methods with different morphologies via wet chemical process are considered better for their molecular level proper mixing. This is because molecular level homogeneous mixing offers the possibilities for controlling the chemical composition, obtaining better-quality homogeneity, single phase, and higher surface area powders. Instead of ammonia or sodium hydroxide based co-precipitation process, urea-based thermal decomposition method is also considered as a better process [24,25,26,27,28]. Urea-based thermal decomposition process has an additional advantage; homogeneous precipitation generates fine small particles, which have a relatively narrow size distribution with the large surface area and high porosity compared to others. Additionally, it takes advantage of the exothermic, fast, and self-sustaining chemical reactions between lanthanide nitrates/chlorides and the weak base (Urea) reducing agent. As a consequence, it is particularly highly suitable for the elaboration of uniform nano-crystalline particles of lanthanides with the high specific surface area and superfine dimensions.

In the present study, we illustrated the single step procedure for the synthesis of europium-doped gadolinium oxide nanoparticles (Core-NPs) via thermal decomposition process at low temperature with grain size about 10–30 nm, which were well monodispersed and showed high dispersibility in aqueous media. Consequently, the prepared core-NPs were covered through amorphous silica layer to improve their solubility, biocompatibility, and toxicity character at ambient conditions. These core and silica surface modified core–shell NPs were fully characterized systematically via different physio-chemical techniques such as X-ray diffraction pattern (XRD), transmission electron microscopy (TEM), energy dispersive x-ray analysis (EDX), thermogravimetric analysis (TGA), and zeta potential, Fourier transform infrared (FTIR), UV/Visible, excitation and emission spectroscopy to examine their crystallinity, phase purity, crystal structure, surface morphology, thermal durability, surface chemistry, solubility, biocompatibility optical absorption, and photoluminescence properties. For their use in biomedical sciences, it is necessary to functionalized or cover the surface with active functional groups to enhance their aqueous solubility character, which can be easily available for binding with bio-macromolecules as per requirement. For surface coating or functionalization, silica shell coating process is well accepted to grow on the surface of the luminescent core-NPs by co-hydrolysis and poly-condensation with tetraethyl-orthosilicate (TEOS) [10, 29, 30]. The surface grew SiO2 layer is easily available for binding with other –OH groups of bio-macromolecules by condensation to form a Si–O–Si network; it would be applicable for highly fluorescent, sensitive, and reproducible biomedical applications. Additionally, the surface functionalized NPs with the high surface area and mesoporosity allow for designing multifunctional systems for simultaneous drug delivery and cell imaging.

2 Experimental

2.1 Materials

Gd2O3 (99.99%, BDH Chemicals, UK), Eu2O3 (99.99%, AlfaAesar, Germany), NH4OH, NaOH, ethyl alcohol, urea, tetraethyl-orthosilicate (TEOS), and N-cetyltrimethylammonium bromide (CTAB) were used directly as-received without further purification. Gadolinium nitrate and europium nitrate were prepared by dissolving the corresponding metal oxides in the diluted nitric acid. Milli-Q (Millipore, Bradford, USA) water was used for the synthesis and characterization of the samples.

2.2 Synthesis of Gd2O3:Eu (Core), Gd2O3:Eu@nSiO2(Core/nSiO2) and Gd2O3:Eu@ nSiO2@mSiO2 (core/nSiO2/mSiO2)NPs

In a typical procedure for the synthesis of Gd2O3:Eu NPs(core), 4.5 g gadolinium nitrate hexahydrate (9.5 ml, 2 M) and 0.223 g europium nitrate hexahydrate (0.05 ml, 2 M) were mixed together in 100 ml dist. water and kept on a hot plate at 80 °C under constant mechanical stirring. Then an appropriate amount of urea, dissolved in aqueous medium was introduced into the vigorously stirred solution mixture. After that, the transparent solution mixture was transferred into round bottle flask and refluxed at 150 °C for 3–4 h. The obtained white precipitate was separated by centrifugation, washed several times with distilled water, and dried in an oven overnight. This dried sample was further calcined at 750 °C under air condition. Stober sol-gel chemical method was followed for nanoporous silica (nSiO2) surface coating as discussed in the previously published literature [10, 29, 30]. In this method, 100 mg luminescent nanoparticles were treated with C2H5OH by ultra-sonication for half-an-hour and separated by centrifugation. The mixture was again re-dispersed in a mechanically stirred aqueous solution containing ethanol (80 ml), H2O (25 ml), and NH4OH (1.0 ml). Later TEOS (1.0 ml) solution was slowly introduced into the vigorously magnetically stirred solution. The reaction was proceeding for 5–6 h at environmental conditions. For mesoporous silica (mSiO2) surface coating, the modified sol–gel method was applied. A solution of 100 mg Gd2O3:Eu@nSiO2, 100 mg CTAB and 50 mg NaOH was made by dissolving in 250 ml distilled water on a hot plate at 80 °C with constant mechanical stirring [31,32,33,34,35]. Afterward 1.0 ml TEOS was introduced slowly into the vigorously mechanically stirred solution. The mixed solution was allowed to co-hydrolyze and condensate for 2 h to give a white precipitate, which was separated by centrifugation, washed several times with water, and dried in an oven to yield the mesoporous core–shell/SiO2 NPs.

2.3 Characterization

Powder X-ray diffraction pattern (PANalytical X’Pert, X-ray diffractometer) equipped with Ni filter CuKα (λ = 1.5404 Å) radiation was used for examining the phase purity and crystal structure of the samples. Field emission-transmission electron microscope (FE-TEM, JEM-2100F, JEOL, Japan), equipped with EDX analysis, operating at an accelerating voltage 200 kV, was applied for morphology inspection. Zeta potential was measured from zeta sizer (Brookhaven Instruments Corporation Holtsville, NY, USA). Thermal analysis was carried out by a thermogravimetric analyzer (Mettler Toledo, Analytical CH-8603 Schwarzenbach, Switzerland). FTIR spectra were measured from Vertex 80 (Bruker, USA) infrared spectrometer with KBr pellet technique. Absorption spectra were recorded by Cary 60 (Agilent Technologies, USA) UV–Vis spectrophotometer within the wavelength range 200–600 nm. Photoluminescence spectra were obtained from Fluorolog 3 (Model: FL3-11, Horiba Jobin Yvon, USA) photoluminescence spectrophotometer.

3 Results and discussion

X-ray diffraction pattern was reported to determine the phase purity, crystal structure, and crystallinity of the as-prepared samples. Figure 1 illustrates the XRD pattern of Gd2O3:Eu core, core/nSiO2 and core/nSiO2/mSiO2 samples. All reflection peaks in all three diffractograms are well indexed to the JCPDS card No. 012-0797 that resulted cubic Gd2O3 phase [3, 4, 6, 36, 37]. No impurity is detected over the entire XRD range. This indicated that the Eu3+ ion is homogeneously distributed inside the crystal lattice and there is the formation of single phase pure cubic Gd2O3 NPs. The observed broadening of reflection peak width is related to the nanocrystalline size of the as-synthesized samples. An observed reduction in reflection peak intensity is implying the influence of amorphous silica surface coating on the luminescent core-NPs [38]. The most prominent diffraction peak, observed at 2θ = 28.79o is used for calculating the average grain size of the NPs. The experimentally estimated the crystalline size of the core, core/nSiO2, and core/nSiO2/mSiO2 NPs are to be 10, 19, and 35 nm, respectively.

Fig. 1
figure 1

X-ray diffraction pattern of Core, Core/nSiO2 and Core/nSiO2/mSiO2 NPs

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Morphology of the as-synthesized core, core/nSiO2 and core/nSiO2/mSiO2 NPs was determined from TEM micrographs. TEM image in Fig. 2a illustrates the highly aggregated, quasi-spherical shaped, well distributed, porous, crystalline NPs with grain size about 10–30 nm. As seen in typical high-resolution TEM micrographs, spherical-shaped NPs have a large number of pores, which are randomly arranged and distributed homogeneously throughout the entire particle. This high porosity of the luminescent NPs is facilitating insolubility and high colloidal stability in an aqueous environment. We expected that the existence of the little amount of urea and surfactants enhanced the aggregation or clustering of the spherical shaped NPs. The lattice fringes in individual NPs are obvious as observed in the high-resolution TEM image (Fig. 2c), suggests the highly crystalline nature of the materials. The lattice planes are well separated and possessed a uniform lattice structure with the well-indexed lattice fringes d222 = 0.311 Å, which is well consistent with the d-spacing value of (222) lattice planes of the cubic phase Gd2O3 structure. This value is corroborated with the reported value for Gd2O3 NPs (Fig. 2c) [39,40,41]. As observed in Fig. 2d, e, a uniform and thick mesoporous silica layer possess a wormhole-channel like structure with a thickness of 55 nm that has been effectively grown over the surface of luminescent core-NPs. As seen in Fig. 2d, after encapsulation of silica layer around the core NPs, the morphology of the core NPs is still maintained; this indicates that mesoporous silica layer has no side effect on the uniformity of the core NPs. The mesoporous silica shell is a contrast in color due to different electron penetrability between the core and an amorphous silica layer. The silica shell is light gray and the core is dark black in color (Fig. 2e). EDX analysis was performed to verify the doping constituents and silica surface coating surrounding the luminescent core-NPs. The EDX profile confirmed the existence of gadolinium (Gd), oxygen (O), and europium(Eu) in the core NPs, suggesting the Eu3+ ion is homogeneously distributed inside the Gd2O3 crystal lattice (Fig. 2f, g). An additional peak of silica is observed in core/nSiO2/mSiO2 NPs at around 1.8 keV, indicate the successful silica coating around the core-NPs. The appearance of strong C and Cu peak in the spectra is belonging to the carbon coated copper grid. No other assigned peaks are detected in both EDX spectrums; it suggests the phase purity of the NPs.

Fig. 2
figure 2

TEM images of a low magnification core b high magnification core c lattice fringes in core image d low resolution core/nSiO2/mSiO2 image e high magnification core/nSiO2/mSiO2 image f EDX spectrum of core and g EDX spectrum of core/nSiO2/mSiO2 NPs

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The solubility character, colloidal stability, and surface charge of the as-synthesized luminescent NPs were further verified from zeta potential. As illustrate in Fig. 3, the zeta potential values of core and core/nSiO2 NPs at physiological pH (pH = 7.0) are 22.8 and −14.2 mV, respectively (Fig. 3a, b). Additionally, on increasing the pH values from 8 to 10 in aqueous solution, the zeta potential values gradually decrease to −13.9 and −19.9 mV, respectively (data not shown); whereas, the zeta potential values for silica modified core/nSiO2/mSiO2 NPs at 8 and 10 pH are −39.5 and −34.2 mV, respectively (Fig. 3c, d). Notably, the zeta potential value in core/nSiO2/mSiO2 NPs is greatly decreased with respect to the core-NPs. It suggests the successful silica surface modification on the luminescent core-NPs [42,43,44]. It is a fact that mesoporous silica is having abundant surface hydroxyl groups and provides powerful claw for easily binding with hydroxyl groups or bio-macromolecules [38, 42, 45, 46]. On increasing the pH value, the concentration of hydroxyl ion is increased, and these free hydroxyl (OH-) ions bind with positively charged silica modified core/nSiO2/mSiO2 NPs to form neutralized complex, which resulted in the continuous decrease of zeta potential values [1, 42, 43]. These facts obviously confirm the solubility and excellent colloidal stability character across a broad range of pH values and further verified the surface charged over the core and surface functionalized core/nSiO2/mSiO2 NPs [38, 42, 46].

Fig. 3
figure 3

Zeta-potential graphs of a core b core/nSiO2, and c core/nSiO2/mSiO2 NPs

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Thermal analysis was performed to analyze the surface adsorbed organic moieties and thermal stability of the as-synthesized NPs. The thermogram of core and core/nSiO2 NPs demonstrate two-stage thermal decomposition. The first weight loss approximately 2% in core-NPs occurs between 50 and 287 °C, which seems to the removal of surface adsorbed residual water molecules and organic moieties (Fig. 4). In the second stage, approximately 4% weight loss is observed in between 288 and 800 °C, which corresponds to the combustion and elimination of surface capping oxygen species along with bulk oxygen species and formation of Gd2O3 nano-product; whereas, in the case of core/nSiO2 NPs, initial weight loss (4%) is recorded below 200 °C. This implies the removal of surface adsorbed residual water molecules and organic moieties during silica shell formation. After that, a continuous weight loss is observed and the highest weight loss (9.2%) is recorded at 800 °C, which denotes the burning and elimination of surface modified silica [47]. It concluded that most of the organic and surface modified silica materials are decomposed before temperature reached 800 oC and the remaining product is an as-synthesized luminescent metal oxide.

Fig. 4
figure 4

Thermo-gravimetric analysis of core and core/nSiO2 NPs

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The silica surface coating was further validated by FTIR spectra with the characteristic peaks of amorphous silica molecules. Compared with the infrared spectrum of Core sample, nSiO2 and mSiO2 coated samples have doublet high-intensity band located at 1097 cm-1 along with weak sharp intensity bands at 798 and 600 cm-1, which are originating from stretching and bending vibrational modes of Si–O–Si, Si–O, and Si–OH surface modified amorphous silica (Fig. 5) [48,49,50,51,52,53]. A diffused broad intensity band is observed in all three spectra located at 3425 cm-1 along with middle-intensity infrared band located at 1517 and 1405 cm-1 are ascribed to the stretching and bending vibrational modes of surface adsorbed residual water molecules [29, 33, 43, 54]. These observations are supported well with the results of XRD, TEM, EDX, and TGA analysis and previously published literature reports [9, 42, 43, 51, 52]. A sharp infrared absorption peak is observed at 549 cm-1 in all three spectra, which is attributed to the stretching vibration of the M-O network [55]. Optical absorption spectra were performed to investigate the optical properties, solubility, and colloidal stability character in an aqueous environment. Absorption spectra of core and core/nSiO2 NPs were recorded in dist. water (Fig. 6). The core/nSiO2 NPs sample exhibits strong absorbance in the visible region, which is assigned to the 8S7/2 → 6Ij transition of Gd(III) ion and existence of the amorphous silica surrounding the core-NPs [29, 32, 45, 56].

Fig. 5
figure 5

UV–vis absorption spectra of Core and Core/nSiO2 NPs suspended in de-ionized water

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Fig. 6
figure 6

FTIR spectra of the as-prepared core, core/nSiO2 and core/nSiO2/mSiO2 NPs

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Photoluminescence measurement was performed to confirm the Eu(III) ion doping and amorphous silica surface modification around the core-NPs. Figure 7 illustrates the excitation spectra of core, core/nSiO2 and core/nSiO2/mSiO2 NPs under monitoring 610 (5D0 → 7F2) nm emission wavelength at room temperature. The excitation spectra consist of several sharp excitation electronic transitions assigned to 7F0 → 5H3,6(321), 7F0 → 5D4(362), 7F0 → 5G3(381), 7F0 → 5L6(394), 7F0 → 5D3(415), and 7F0 → 5D2(466), which are originating from 4f-4f-intra-configurationally transitions of Eu3+ ions [30, 37, 47, 54, 57,58,59]. Figure 8 demonstrates the emission spectra of core, core/nSiO2 and core/nSiO2/ mSiO2 NPs upon excitation at 395 nm at room temperature. The obtained spectra display characteristic emission transitions of Eu3+ions at 464-474 (5D2 → 7F0), 531-538 (5D1 → 7F1), 578(5D1 → 7F3), 585-599 (5D0 → 7F1), 605-631 (5D0 → 7F2), 644-662 (5D0 → 7F3), and 684-712 (5D0 → 7F4) transitions, respectively [6, 29, 30, 47, 59, 60]. Some weak intensity emission transitions located at 470 and 535 nm assigned to 5D2 → 7F0 and 5D1 → 7F1 transition of the Eu3+ ion. This indicates low vibrational energy of the Gd–O band [6, 58]. Consequently, the multiphonon relaxation by gadolinium-oxygen vibration is not able to bridge the gaps between the higher energy levels of 5D2 → 7F0 and 5D1 → 7F1 transition and the 5D0 → 7FJ level of Eu(III) ion completely, resulting in the emission from these levels [59]. A prominent emission transition is observed between 605 and 630 nm because of 5D0 → 7F2 forced electric dipole transition and is hypersensitive to the environment [30, 47, 60]. The emission band observed around 585-598 nm is assigned to magnetic dipole transition, which is weak with respect to forced electric dipole transition [30, 60]. It is a fact that forced electric dipole transition (5D0 → 7F2) is sensitive to the surrounding chemical environment; whereas, magnetic dipole transition (5D0 → 7F1) is independent of the surrounding environment [30, 47, 60, 61]. It is obvious from the emission spectra of all three samples that the emission efficiency of hypersensitive transition or electric dipole transition is higher than their respective magnetic dipole transition; it suggests that the trivalent europium ions are in the C2 site symmetry. We speculate that, in the cubic crystal structure of Gd2O3, Gd3+ have C2 and S6 symmetric site, which is both octahedral coordination. In the C2 site, four equal equatorial Gd–O and two apical Gd–O bonds have different bond distances. However, the S6 site has equal bond distances (Gd–O) and inversion center [6, 37, 57,58,59, 62]. In that case, if Eu(III) ions occupy the S6 site symmetry of Gd3+ ions, the magnetic dipole transition will be obtained. In fact, the emission spectra simplify that most of the Eu3+ ions had C2 symmetry compared to the S6 symmetry, resulting in the predominant high emission intensity transition is achieved. It is interesting to note that the 7FJ energy levels of Eu3+ split into some components under crystal field effects caused by the surrounding ions. Under C2 site symmetry, 5D0 → 7F1 and 5D0 → 7F2 are split in multiple times. It could be due to the completely lifted “2 J + 1” degeneracy of the ground state electronic configuration of Eu3+. These results are in excellent agreement with the previous reports [3, 6, 30, 37, 57,58,59, 62]. It is worth noticing that, the emission intensity is suppressed in core/nSiO2 NPs, which is further greatly quenched after mesoporous silica surface coating on the core/nSiO2 NPs. The reduction in emission and excitation intensity is related to the formation of interfacial high phonon energy surface quenching sites by the amorphous silica and organic moieties as supported by EDX, TGA, and FTIR analysis. These surface formed high vibrational energy quenching sites enhanced the non-radiative transition pathways and suppressed the photoluminescence efficiency of the NPs. These results are in excellent agreement with the previously published reports [30, 50, 52, 63,64,65,66]. Additionally, the luminescent intensity is also associated with the structural characteristics such as crystal field effect that is closely related to the structure and its symmetry, oxygen vacancies, which also act as a quenching center for photoluminescence [67]. It is the fact that amorphous silica surface suppressed the luminescence efficiency; whereas, enhanced the solubility, colloidal stability, biocompatibility, and non-toxic nature of the luminescent materials regardless non-silica modified luminescent core-NPs [30, 50, 68,69,70].

Fig. 7
figure 7

Excitation spectra of the as-prepared core, core/nSiO2, and core/nSiO2/mSiO2 NPs

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Fig. 8
figure 8

Emission spectra of the as-prepared core, core/nSiO2, and core/nSiO2/mSiO2 NPs

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4 Conclusion

In conclusion, colloidal and mesoporous silica surface modified Gd2O3:Eu@nSiO2@mSiO2 NPs were successfully synthesized by urea-based thermal decomposition process, and subsequently, amorphous silica layer was developed by sol–gel chemical route. TEM images clearly revealed the successful mesoporous silica surface coating on the core NPs. Absorption spectra demonstrate the high solubility and colloidal stability, which verified from zeta potential results. FTIR spectral results verified the successful silica layer coating on the core NPs. The core NPs revealed strong emission transition in the red region (612 nm) even after covering of amorphous silica layer. However, the emission intensity of the silica layer cover NPs has quenched relatively their uncoated counterpart because of multiphoton-relaxation pathways from high phonon energy hydroxyl groups. These high vibrational energy groups scattered the incident and emission light, resulting to suppress the emission intensity of the nanomaterials. The available Si-OH groups on the surface enhanced the solubility, colloidal stability, biocompatibility, and non-toxic nature of the luminescent nanomaterials. Our finding illustrates the successful approach for the synthesis of very bright red phosphors for their use in broad photonic based biomedical applications like optical bio-sensor/detection/labeling, bio-imaging etc.