Synthesis, functionalization and properties of uniform europium-doped sodium lanthanum tungstate and molybdate (NaLa(XO₄)₂, X = Mo,W) probes for luminescent and X-ray computed tomography bioimaging

Abstract

A one-pot simple procedure for the synthesis of uniform, ellipsoidal Eu³⁺-doped sodium lanthanum tungstate and molybdate (NaLa(XO₄)₂, X  = W, Mo) nanophosphors, functionalized with carboxylate groups, is described. The method is based on a homogeneous precipitation process at 120 °C from appropriate Na⁺, Ln³⁺ and tungstate or molybdate precursors dissolved in ethylene glycol/water mixtures containing polyacrylic acid. A comparative study of the luminescent properties of both luminescent materials as a function of the Eu³⁺ doping level has been performed to find the optimum nanophosphor, whose efficiency as X-ray computed tomography contrast agent is also evaluated and compared with that of a commercial probe. Finally, the cell viability and colloidal stability in physiological pH medium of the optimum samples have also been studied to assess their suitability for biomedical applications.

Introduction

The interest in Ln-based phosphors (Ln = lanthanide), usually consisting of nanoparticles made up of a crystalline host (oxides, fluorides, phosphates and vanadates [1]) doped with active Ln³⁺cations, has greatly increased during the last years because of their applicability as luminescent probes for several biomedical applications among which, imaging is the one most commonly investigated [2]. Such interest mainly arises from the higher chemical and optical stability of these phosphors when compared with other available luminescent materials along with their lower toxicity [3]. Other advantages of Ln-based nanoparticles include high luminescence quantum yield and narrow emission bands, which result in a high selectivity for bioassays [3].

The luminescent characteristics of this kind of materials are determined by the nature of the doping cations. Thus, down-shifting (emissions at lower energy than that of excitation) or up-converting (emissions at higher energy than that of excitation) phosphors can be obtained by a proper selection of the emitting cations. In the case of down-shifting phosphors, the most appropriated Ln doping cation for bioapplications is red emitting Eu³⁺, since possible interferences caused by the autofluorescence of tissues are minimized in this wavelength range [4]. Although Eu³⁺-based phosphors are not useful for in vivo applications because of the need of UV excitation, they have been used for many ex vivo and in vitro biomedical applications [5].

It is important to mention that for this kind of uses, the bioprobes must fulfil several requirements related to: (i) uniformity in size and shape, since these factors affect the interactions with cell and tissues [6], (ii) particle size, which must be kept within the nanometer range, especially for in vivo applications, to control the circulation behaviour of the nanoparticles in the body, their biodistribution and their excretion pathway [6], (iii) toxicity, since the nanoprobes must be obviously non-toxic and (iv) colloidal stability, which must be high in physiological media [7]. To meet the latter criteria, the nanoparticles may require surface coating with organic molecules having different functional groups (carboxylate, amino, imino, etc.), which confer to them a high colloidal stability by increasing their surface charge or by steric hindrance [8]. In addition, a high emission intensity is required for luminescent imaging. However, the absorption coefficient of the Ln³⁺ cations is low, which results in low intensity of the emissions [9]. To overcome this problem, several strategies can be adopted. One of them takes advantage of the large absorption coefficient of some inorganic anions, which are able to transfer the absorbed energy to the active Ln ions thus increasing the intensity of luminescence [9]. Among these anions are tungstates and molybdates, which efficiently absorb UV light that can be transferred to some Ln³⁺ ions such as Eu³⁺ [10], [11], [12], [13]. In particular, the scheelite family having general formula MR(XO₄)₂ (M = Na⁺ or K⁺, R = rare earth cation and X  = W or Mo) has been shown to be a promising host for doping with Ln³⁺ cations resulting in high efficient phosphors [14]. It should be noted that the composition of these phosphors confer to them an additional and useful functionality as contrast agent for X-ray computed tomography (CT), a commonly used technique for biomedical imaging [1]. The reason for that is that they contain elements with high atomic number (lanthanides, molybdenum and mainly tungsten), which, consequently, provide them with high X-ray attenuation capacity [1].

In spite of such advantages, the biomedical applications of MR(XO₄)₂-based compounds has been rarely addressed. Only a few reports have been found in literature for some members of the family like NaLa(MoO₄)₂ [15], NaGd(MoO₄)₂ [16] and NaGd(WO₄)₂ [17], [18], whereas no reports have been found for NaLa(WO₄)₂. A possible reason for that might be the unavailability of synthesis methods to produce this material with the needed characteristics mentioned above. Most previous reported methods for the synthesis of uniform NaLa(WO₄)₂ particles resulted in micron sized entities [19], [20], [21], [22], [23], [24], [25], which do not meet the size criteria. Only Wang et al., recently reported a procedure, which produced Dy and Sm doped NaLa(WO₄)₂ nanocubes based on a solvothermal reaction using oleic acid as dispersing and capping agent [26]. However, the resulting particles were covered by oleic acid molecules making them hydrophobic and therefore, not useful for bioapplications, and they have to be further treated following several procedures (for example, a 5-min treatment with NOBF₄) to remove the oleic acid cover [27]. Therefore, the development of synthesis procedures yielding uniform NaLa(WO₄)₂ nanoparticles doped with luminescent Ln³⁺ cations having colloidal stability in physiological media is highly desirable.

In this work, we describe for the first time a one-pot simple procedure for the synthesis of uniform Eu³⁺-doped NaLa(WO₄)₂ nanophosphors with ellipsoidal shape and functionalized with carboxylate groups provided by polyacrylic acid (PAA). This polymer has been suggested as one of the most useful functional coatings for biomedical applications due to its high affinity to the cellular membranes [8]. The employed method is based on a homogeneous precipitation process at 120 °C from appropriate Na⁺, Ln³⁺ and tungstate precursors dissolved in ethylene glycol (EG)/water mixtures containing PAA molecules. We also show that this method can be easily adapted for the synthesis of isomorphous Eu³⁺-doped NaLa(MoO₄)₂ nanoparticles. A comparative study of the luminescent properties of both luminescent materials as a function of the Eu³⁺ doping level has also been conducted to find the optimum nanophosphor, whose efficiency as CT contrast agent is evaluated and compared with that of a commercial probe. Finally, the cell viability and colloidal stability in a physiological pH media of the optimum samples have also been studied to assess their suitability for biomedical applications.