Luminescence functionalization of MCM-48 by YVO₄:Eu³⁺ for controlled drug delivery

Abstract

A bifunctional (mesoporous, luminescent) composite was realized by depositing a YVO₄:Eu³⁺ phosphor layer onto the surface of MCM-48 spheres via a simple sol–gel process. This composite was employed as a drug delivery/release carrier using captopril (CapH₂) as a model drug. X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), N₂ adsorption/desorption and photoluminescence spectra were employed to characterize the as-prepared materials. The results indicated that the bifunctional composite possesses an ordered mesoporous structure and strong red luminescence of Eu³⁺. A drug release test revealed that the composite has favorable drug release properties. Additionally, the CapH₂-loaded composite still shows the red luminescence under UV irradiation. Also, the emission intensity of Eu³⁺ increases with an increase in the cumulative released amount of CapH₂, making the extent of drug release easy to identify, track and monitor by the change of luminescence during the release process and disease therapy.

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1. Introduction

Recently, luminescence-functionalized mesoporous composites have drawn much attention due to their multifunctional properties, such as a high pore volume, large surface area and luminescence properties. Thus, they are believed to have great potential applications in the fields of bioimaging, drug delivery, diagnosis, therapy, sensing, optical memories and light collectors in solar cells.¹ Especially in controlled drug delivery systems, these composite materials not only maintain a constant drug release rate and minimize drug toxic effects throughout the dosage period,² but also provide photoluminescence properties, which can be monitored to evaluate the efficiency of drug release.

Among the various mesoporous structures, their non-toxic nature, good biocompatibility and suitable pore size make mesoporous silica materials excellent candidates for controlled drug delivery. Research has been extensively focused on MCM-41 with unconnected 2D hexagonal pores³ and SBA-15 with lateral connected pores.⁴ There are few papers about the cubic Ia3d mesostructural MCM-48 with 3D interconnected pores,⁵ which is probably due to the relative difficulty in producing this mesostructure, which consists of two interpenetrating continuous networks of chiral channels.⁶ This interesting 3D channel network is considered to offer a highly open mesoporous host, resulting in easy and direct access for guest species, thus facilitating incorporation or diffusion throughout the pore channels.^6c

In addition, for the safe and efficient luminescent materials, rare earth (RE) doped oxide phosphor which can be used in biological applications is an attractive choice. Compared with organic dyes (which exhibit rapid photobleaching and a low fluorescence quantum yield (QY)⁷) and quantum dots (which are less chemically stable, potentially toxic with fluorescence intermittence⁸), RE-doped oxide nanophosphors possess many wonderful characteristics, including sharp emission lines, a long lifetime and high luminescence quantum yield (QY).⁹ In particular, Eu³⁺-activated yttrium vanadate (YVO₄), which displays outstanding luminescent properties arising from the 4f electron configuration, is an important commercial red phosphor (⁵D₀→⁷F₂ of Eu³⁺ at 617 nm) used in optical devices, laser host materials, cathode ray tubes and high-pressure mercury lamp.¹⁰

Although self-activated luminescent hydroxyapatite-coated MCM-48 has been reported for drug release,^6d their fluorescent efficiency and decay time should be much lower than those of the samples with RE-derived luminescence based on their luminescence mechanism.^9e,10e,11 In particular, the PL intensity and color of RE-based composites can easily be tuned by doping with different kinds of RE ions. Therefore, the design of a bifunctional composite combining the mesoporous properties, spherical shape and RE-based luminescence should have high potential in drug delivery and biomedical fields.

Herein, spherical MCM-48-type mesoporous silica was synthesized through a simple and modified Stöber method, and luminescence functionalization was realized by depositing YVO₄:Eu³⁺ onto the surface of MCM-48 via a Pechini sol–gel process,¹² resulting in the formation of a luminescent and mesoporous material. The obtained composite was well characterized by means of XRD, TEM, FT-IR, N₂ adsorption/desorption and luminescence spectra. In addition, water-soluble drug captopril (CapH₂) with a molecule size of 9.0 × 5.7 × 3.3 Å was selected as a model drug to study the drug release properties of the composite in the release media of simulated body liquid. It is shown that the emission intensity of Eu³⁺ increases with increasing cumulative drug released in the system, making the extent of drug release easy to identify, track and monitor by the change of luminescence.

2. Experimental section

2.1. Synthesis of MCM-48@YVO₄:Eu³⁺

In a modified process,¹³ 2.6 g of cetyltrimethylammonium bromide (CTAB) was dissolved in a mixture of 50 mL of ethanol, 120 mL of deionized water and 12 mL of aqueous ammonia (32 wt%) by stirring. Then, 3.4 g of tetraethyl orthosilicate (TEOS) was added into the mixture and stirred for 2 h at room temperature. The resulting solid was collected by filtration, washed several times with distilled water and dried in air at ambient temperature. The as-synthesized material was calcined from room temperature to 550 °C with a heating rate of 1 °C min⁻¹, and kept at 550 °C for 6 h to remove the templates, then the MCM-48-type mesoporous silica was obtained.

Deposition of a YVO₄:Eu³⁺ phosphor layer onto the surface of MCM-48 was prepared by a Pechini sol–gel process.¹² The doping concentration of Eu³⁺ was 5 mol% of Y³⁺ in YVO₄.^10e Typically, stoichiometric weights of Y₂O₃ (99.99%, Sinopharm Chemical Reagent Co., Ltd), Eu₂O₃ (99.99%, Sinopharm Chemical Reagent Co., Ltd) and NH₄VO₃ (A. R., Tianjin Damao Chemical Instrument Company) were dissolved in dilute HNO₃ and then added to a water–ethanol solution. Then, citric acid, with a molar ratio of 2 : 1 to metal ions, and polyethylene glycol (PEG, M_w = 10 000), at a concentration of 0.04 g mL⁻¹, were added. Subsequently, the mixture was stirred for 1 h to form a stable sol. Then, the desired amount of MCM-48 powder was added into the sol. This was further stirred for another 3 h and the resulting material was separated by centrifugation. After being dried at 100 °C, the sample was calcined from room temperature to 500 °C with a heating rate of 1 °C min⁻¹ and maintained at 500 °C for 2 h for the crystallization of YVO₄:Eu³⁺. The coating process was repeated three times. In this way, the luminescence functionalized MCM-48 materials were obtained (denoted as MCM-48@YVO₄:Eu³⁺).

2.2. Preparation of the drug storage/release system

The CapH₂ storage system was prepared according to the reported route with some modifications.¹⁴ Typically, 0.215 g of MCM-48@YVO₄:Eu³⁺ (or MCM-48) was added to 10 mL of 0.1 M CapH₂ solution by stirring for 2 h at room temperature. The CapH₂-loaded sample was separated by centrifugation, dried at 60 °C for 12 h and denoted as CapH₂–MCM-48@YVO₄:Eu³⁺ (or CapH₂–MCM-48).

The in vitro delivery test was performed by immersing 0.15 g of the CapH₂-loaded sample in 50 mL of the release media of simulated body fluid (SBF) under gentle stirring and the solution temperature was maintained at 37 °C. The ionic composition (Na⁺/K⁺/Ca²⁺/Mg²⁺/Cl⁻/HCO₃⁻/HPO₄²⁻/SO₄²⁻ = 142.0/5.0/2.5/1.5/147.8/4.2/1.0/0.5) of the SBF was similar to that of human body plasma (pH = 7.2–7.4).¹⁵ At selected time intervals, 0.5 mL of the solution was withdrawn and then centrifuged. The small amount of solid was immediately washed with an equal volume of fresh SBF and then added into the release media. The obtained clear solution was properly diluted and the amount of CapH₂ was monitored at 200 nm using a UV-vis spectrophotometer.

The experimental process for the luminescence functionalization of MCM-48 by YVO₄:Eu³⁺, and the subsequent loading and release of CapH₂ are schematically depicted in Scheme 1.

Scheme 1 The experimental process for the luminescence functionalization of spherical MCM-48 by YVO₄:Eu³⁺, and the subsequent loading and release of captopril.

2.3. Characterization

X-ray diffraction (XRD) was carried out on a Rigaku-Dmax 2500 diffractometer using Cu-Kα radiation (λ = 0.15405 nm). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on a FEI Tecnai G² S-Twin transmission electron microscope with a field emission gun operating at 200 kV. Fourier transform IR (FT-IR) spectra were measured on a Perkin-Elmer 580B IR spectrophotometer using the KBr pellet technique. The exact loading level of YVO₄:Eu³⁺ on MCM-48 was determined by inductively coupled plasma (ICP, Thermo Electron XSeries II). The N₂ adsorption/desorption isotherm was obtained at 77 K using a Micromeritics ASAP 2010 instrument. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The pore size distribution was measured using the Barret–Joner–Halenda (BJH) method. The excitation and emission spectra were obtained on a Hitachi F-4500 spectrofluorometer equipped with a 150 W xenon lamp as the excitation source. Luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a 250 nm laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). The UV-vis absorption was performed on a Shimadzu UV-2450 spectrophotometer. All of the measurements were performed at room temperature.

3. Results and discussion

3.1. Phase, structure and morphology

Fig. 1 shows the XRD patterns of MCM-48 and MCM-48@YVO₄:Eu³⁺, respectively. In the low-angle XRD patterns (Fig. 1A), all of the samples exhibit three well-resolved diffraction peaks that can be indexed to (211), (220) and (420) reflections associated with the Ia3d cubic symmetry, confirming a well-ordered mesoporous structure in these materials. In addition, the results also indicate that the ordered cubic mesoporous structure of MCM-48 can be maintained after the deposition of the YVO₄:Eu³⁺ phosphor layer. However, the intensity of these characteristic diffractions decreases apparently after the deposition of YVO₄:Eu³⁺, indicating that the integrity of the mesoporous structure (short range order) is decreased due to the deposition of YVO₄:Eu³⁺ onto the mesoporous framework of MCM-48.

Fig. 1 (A) Low-angle and (B) wide-angle XRD patterns of (a) MCM-48 and (b) MCM-48@YVO₄:Eu³⁺.

In the wide-angle XRD patterns (Fig. 1B), the broad band centered at 2θ = 22° for the MCM-48 sample can be assigned to amorphous SiO₂ (JCPDS No. 29-0085). While for the YVO₄:Eu³⁺@ MCM-48 sample, besides the broad peak assigned to amorphous silica, the main diffractions at 2θ = 18.82° (101), 24.99° (200), 33.56° (112), 35.63° (220), 38.15° (202), 40.65° (301) and 49.78° (312) can be indexed to a pure tetragonal phase for YVO₄ (JCPDS No. 17-0341), indicating the successful crystallization of YVO₄:Eu³⁺ on the surface of mesoporous MCM-48. Additionally, no other phase related to the doped Eu³⁺ can be detected, revealing the successful substitution of Eu³⁺ for Y³⁺ in the YVO₄ host. The exact loading level of YVO₄:Eu³⁺ on MCM-48 silica was determined to be 5.9 wt% by ICP measurement.

The morphology, size details, pore geometry and composition of the samples were investigated by TEM, which is exhibited in Fig. 2. The low-magnification TEM image (Fig. 2A) shows that MCM-48 silica consists of spherical particles with a diameter of 200–400 nm. The corresponding high-magnification TEM image (Fig. 2C) clearly shows the typical ordered mesostructure of MCM-48. For the MCM-48@YVO₄:Eu³⁺ sample (Fig. 2B), the spherical morphology of MCM-48 was basically maintained. The high-magnification TEM image of MCM-48@YVO₄:Eu³⁺ (Fig. 2D) also exhibits similar cubic ordered channels of MCM-48, indicating that the mesostructure of MCM-48 is well preserved. In the HRTEM image for the MCM-48@YVO₄:Eu³⁺ sample (Fig. 2E), the distance (0.35 nm) between the adjacent lattice fringes corresponds well to the d(200) spacing (0.355 nm) of YVO₄ (JCPDS No. 17-0341). These results further confirm the presence of crystalline YVO₄:Eu³⁺ on the surface of MCM-48, agreeing well with the wide-angle XRD results (Fig. 1B). The EDS result (Fig. 2F) confirms the presence of silicon (Si), oxygen (O), yttrium (Y) and vanadium (V) in the MCM-48@YVO₄:Eu³⁺ sample. The Eu element can't be detected clearly due to its low concentration, but it can be confirmed by the emission spectra in following section.

Fig. 2 Low-magnification TEM images of (A) MCM-48 and (B) MCM-48@YVO₄:Eu³⁺, high-magnification TEM images of (C) MCM-48 and (D) YVO₄:Eu³⁺@MCM-48 along the [110] direction (insets are the corresponding TEM images along the [111] direction), (E) HRTEM image and (F) EDS of MCM-48@YVO₄:Eu³⁺.

The FT-IR spectra of MCM-48, MCM-48@YVO₄:Eu³⁺, CapH₂-MCM-48@YVO₄:Eu³⁺ and pure CapH₂ are shown in Fig. 3. For the MCM-48 sample (Fig. 3A), the characteristic bands of Si-O (δ, 462 cm⁻¹), Si-OH (υ_s, 967 cm⁻¹), Si-O-Si (υ_s, 1089 cm⁻¹, υ_as, 798 cm⁻¹), OH (3444 cm⁻¹) and H₂O (1639 cm⁻¹) (where υ_s represents symmetric stretching, υ_as asymmetric stretching and δ bending) are present.¹⁶ For the MCM-48@YVO₄:Eu³⁺ sample (Fig. 3B), all of the characteristic absorption bands of SiO₂ can be obviously found. However, the absorption band of the V–O bond in the VO₄³⁻ group at 833 cm⁻¹ can't be detected due to its relatively low intensity with respect to that of Si-O-Si at 798 cm⁻¹ (but it can be detected by XRD and emission spectra). The strong bands of OH and H₂O reveal that a large amount of OH groups and H₂O exist on the surface of MCM-48@YVO₄:Eu³⁺, which play a key role in bonding CapH₂ molecules from CapH₂–water solution, as shown in the FT-IR spectrum of CapH₂–MCM-48@YVO₄:Eu³⁺ sample (Fig. 3C). Three new and strong bonds located at 1721 cm⁻¹ (COO⁻), 1476 cm⁻¹ and 1443 cm⁻¹ (C–C) assigned to the introduced CapH₂ (Fig. 3D) confirm the successful incorporation of CapH₂ into the channels of MCM-48@YVO₄:Eu³⁺ sample.¹⁷

Fig. 3 FT-IR spectra of (A) MCM-48, (B) MCM-48@YVO₄:Eu³⁺, (C) CapH₂–MCM-48@YVO₄:Eu³⁺ and (D) pure CapH₂.

N₂ adsorption/desorption isotherms of MCM-48, MCM-48@YVO₄:Eu³⁺ and the corresponding CapH₂-loaded samples are depicted in Fig. 4. As shown, all of the samples exhibit IV-type isotherms, indicating the typical mesoporous structure for all of the samples. It can also be deduced that the coating of YVO₄:Eu³⁺ and further loading of CapH₂ molecules doesn't change the basic pore structure of MCM-48, which coincides with the low-angle XRD result. The textural characteristics of above materials are summarized in Table 1. It can be seen from the table that MCM-48 possesses a very high BET surface area, large pore volume and suitable pore size for application as a drug carrier, even after coating of a YVO₄:Eu³⁺ layer. As expected, the specific surface area, pore size and pore volume are markedly reduced after CapH₂ loading. This result further proves that CapH₂ molecules were successfully incorporated into the channels of mesoporous MCM-48.

Fig. 4 N₂ adsorption/desorption isotherms of MCM-48, MCM-48@YVO₄:Eu³⁺ and CapH₂–MCM-48@YVO₄:Eu³⁺.

Table 1 Textural parameters of MCM-48, MCM-48@YVO₄:Eu³⁺ and CapH₂-loaded MCM-48@YVO₄:Eu³⁺

Samples	D (nm)	S BET (m^2 g^−1)	V P (cm^3 g^−1)
MCM-48	2.50	1634	1.005
MCM-48@YVO4:Eu^3+	2.37	1321	0.775
CapH2–MCM-48@YVO4:Eu^3+	1.30	309	0.30

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3.2. Luminescent properties

The photoluminescent (PL) properties of the samples were characterized by the excitation and emission spectra in Fig. 5. In the excitation spectra of MCM-48@YVO₄:Eu³⁺ and CapH₂–MCM-48@YVO₄:Eu³⁺ (Fig. 5A), the strong band at 279 nm can be attributed to the VO₄³⁻ group.^16a,18 The general f–f transition lines of Eu³⁺ in the longer wavelength region can't be observed due to their weak intensity compared to that of the VO₄³⁻. Therefore, it can be deduced that the excitation of Eu³⁺ is mainly caused by the energy transfer from VO₄³⁻ to Eu³⁺, which is consistent with the previous reports.^16a,18 Upon excitation into VO₄³⁻ at 279 nm, the characteristic transition lines from the excited ⁵D₀ level of Eu³⁺ can be observed in the emission spectra (Fig. 5B). As shown, four main lines at 539, 593, 617, 649, and 698 nm are obvious, which are assigned to the ⁵D₁→⁷F₁ and ⁵D₀→⁷F_J (J = 1, 2, 3, 4) transitions of Eu³⁺, respectively. Obviously, the emission spectra are dominated by the red ⁵D₀→⁷F₁ (593 nm) and ⁵D₀→⁷F₂ (617 nm) transitions of the Eu³⁺.¹⁹ It should be noted that no emission can be found from pure MCM-48 (Fig. 5B(c)). The luminescent intensity of CapH₂–MCM-48@YVO₄:Eu³⁺ sample is decreased to a half of MCM-48@YVO₄:Eu³⁺, which may be related to the loading of a CapH₂ molecule. However, this luminescent intensity is still strong enough to be easily tailored, tracked and monitored during the drug release process, suggesting its potential application in the biomedical field.

Fig. 5 (A) Excitation spectra of (a) MCM-48@YVO₄:Eu³⁺ and (b) CapH₂–MCM-48@YVO₄:Eu³⁺; (B) emission spectra of (a) MCM-48@YVO₄:Eu³⁺, (b) CapH₂–MCM-48@YVO₄:Eu³⁺ and (c) pure MCM-48.

The decay curves for the ⁵D₀→⁷F₂ (617 nm) of Eu³⁺ in MCM-48@YVO₄:Eu³⁺ and CapH₂–MCM-48@YVO₄:Eu³⁺ samples are presented in Fig. 6. It can be seen that both decay curves can be fitted into a single-exponential function as I = I₀ exp(−t/τ) (I₀ is the initial emission intensity at t = 0 and τ is the 1/e lifetime of the emission center). The calculated average lifetimes are 0.78 ms and 0.49 ms for MCM-48@YVO₄:Eu³⁺ and CapH₂–MCM-48@YVO₄:Eu³⁺, respectively, which basically agree with the reported lifetime values for Eu³⁺.^10e,20

Fig. 6 Decay times of (A) MCM-48@YVO₄:Eu³⁺ and (B) CapH₂–MCM-48@ YVO₄:Eu³⁺.

3.3. Drug storage/release properties

The CapH₂ loading amounts for CapH₂–MCM-48 and CapH₂–MCM-48@YVO₄:Eu³⁺ are 35.5 wt% and 28.6 wt%, as determined by TG analysis. Although the coating of the YVO₄:Eu³⁺ layer occupies or decreases the pore size to a certain extent, the drug loading amount is only reduced by 6.9%. Therefore, it can be concluded that the CapH₂–MCM-48@YVO₄:Eu³⁺ system still shows a relatively high CapH₂ storage capacity.

During the in vitro CapH₂ release study, the corrected concentration of released CapH₂ was calculated based on the following equation:²¹

Where C_tcorr is the corrected concentration at time t, C_t is the apparent concentration at time t, v is the volume of sample taken and V is the total volume of dissolution medium.

Fig. 7A shows the cumulative drug release profiles of CapH₂–MCM-48@YVO₄:Eu³⁺ and CapH₂–MCM-48 systems. In the figure, both of the systems show a slow release of CapH₂, which may be attributed to the interaction between CapH₂ molecules and the mesopore surface, suggesting their potential application in the area of drug delivery. In comparison with the CapH₂–MCM-48 system, the CapH₂–MCM-48@YVO₄:Eu³⁺ drug release system shows a relatively faster release profile over 72 h. This can be explained as the decrease of surface area and then reduction of the silanol and VO₄ groups presented on the mesoporous composite surface, which should be the sites to form hydrogen bonds with the carboxyl groups in CapH₂. However, for the CapH₂–MCM-48@YVO₄:Eu³⁺ system, 50% of the adsorbed CapH₂ is released from the system within 10 h and almost 90% is released within 24 h, exhibiting an obvious sustained property.

Fig. 7 (A) Cumulative release of CapH₂ from CapH₂–MCM-48@YVO₄:Eu³⁺ and CapH₂–MCM-48 as a function of the release time; (B) PL intensity of CapH₂–MCM-48@YVO₄:Eu³⁺ as a function of cumulative release of CapH₂.

The PL emission intensity of the CapH₂–MCM-48@YVO₄:Eu³⁺ sample was investigated as a function of cumulative released amount of CapH₂, as shown in Fig. 7B. It can be seen that the PL intensity increases with the cumulative released amount of CapH₂ and reaches a maximum when CapH₂ release is complete. It is well accepted that the emission of Eu³⁺ will be quenched to some extent in the environment, which contains a high phonon frequency.²² As for the CapH-loaded sample, the organic groups in CapH₂ with tremendous vibration frequencies from 500–4000 cm⁻¹ (shown in Fig. 3D) will greatly quench the emission of Eu³⁺, which was proved by the PL spectra (Fig. 5). However, with the release of CapH₂, its quenching effect will be weakened, resulting in the recovery of the emission intensity. This correlation between the emission intensity and cumulative released amount of CapH₂ can be potentially used as a probe for monitoring the drug release process and efficiency in the course of the disease therapy.

4. Conclusion

In summary, we synthesized a novel luminescent and mesoporous MCM-48@YVO₄:Eu³⁺ composite as a carrier for controlled drug release by depositing a YVO₄:Eu³⁺ phosphor layer into the channel surface of mesoporous MCM-48. This system possesses sustained release properties of a drug in an in vitro assay. In addition, the MCM-48@YVO₄:Eu³⁺ system exhibits strong red luminescence, even after the loading of the drug molecules. It is important that the PL intensity increases with the cumulative released amount of drug molecules, which makes the functional drug carrier easy to identify, track and monitor during the drug release and disease therapy process.

Acknowledgements

Financial support from National Natural Science Foundation of China (NSFC 20871035), Research Fund for the Doctoral Program of Higher Education of China (20112304110021) and the Fundamental Research Funds for the Central Universities of China (HEUCF201210009) are greatly acknowledged.

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