Highly biocompatible, monodispersed and mesoporous La(OH)3:[email protected]2 core-shell nanospheres: Synthesis and luminescent properties

Accepted Manuscript Title: Highly biocompatible, monodispersed and mesoporous La(OH)3 :Eu@mSiO2 core-shell nanospheres: Synthesis and luminescent properties Authors: Anees A. Ansari, Ali Aldalbahi, Joselito P. Labis, Ahmed Mohamed El-Toni, Maqusood Ahamed, M.Aslam Manthrammel PII: DOI: Reference:

S0927-7765(17)30868-8 https://doi.org/10.1016/j.colsurfb.2017.12.026 COLSUB 9049

To appear in:

Colloids and Surfaces B: Biointerfaces

Received date: Revised date: Accepted date:

8-7-2017 6-12-2017 14-12-2017

Please cite this article as: Anees A.Ansari, Ali Aldalbahi, Joselito P.Labis, Ahmed Mohamed El-Toni, Maqusood Ahamed, M.Aslam Manthrammel, Highly biocompatible, monodispersed and mesoporous La(OH)3:Eu@mSiO2 core-shell nanospheres: Synthesis and luminescent properties, Colloids and Surfaces B: Biointerfaces https://doi.org/10.1016/j.colsurfb.2017.12.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highly

biocompatible,

monodispersed

and

mesoporous

La(OH)3:Eu@mSiO2 core-shell nanospheres: Synthesis and luminescent

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properties

Anees A. Ansari*1, Ali Aldalbahi1, Joselito P. Labis1, Ahmed Mohamed El-Toni1, Maqusood Ahamed1, M. Aslam Manthrammel2 1

King Abdullah Institute for Nanotechnology, King Saud University,

Department of Physics and Astronomy, King Saud University, Riyadh 11451,

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Riyadh 11451, Saudi Arabia

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Saudi Arabia

___________________________________________________________________ Fax. +966-1-4670662

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Phone +966-1-4676838;

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*Email: [email protected]

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H -O

Si-

OH

O

O

O

-O H

O O

O O

Si--O HO H

La(OH)3:Eu@mSiO2 nanospheres 1

H -O SiOH

La(OH)3:Eu

Si-OH

-OH

HOSi -O H

Si-O H

O

O

Si

La(OH)3:Eu

H

-O H

-OH Silica shell

O Si-

Si

H -O

H

TEOS /70oC

Core

-O

CTAB + H2O

-OH

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Graphical abstract

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Highlights 

Monodispersed luminescent La(OH)3:Eu@mSiO2 core-shell nanospheres.



Highly

biocompatible,

mesoporous,

nanospheres

aqueous

soluble

core-shell

Influence of surface functionalization on physiochemical properties.

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In-vitro cytotoxicity was evaluated by MTT assay

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

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Abstract

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Monodispersed La(OH)3:Eu nanospheres(core-NSs) were synthesized by urea-based

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homogeneous co-precipitation process, where mesoporous silica layer was coated over the surface of luminescent La(OH)3:Eu core-NSs. The XRD data exhibit the high single

hexagonal-shaped

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crystalline,

La(OH)3:Eu

core

and

silica

modified

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La(OH)3:Eu@mSiO2 (core-shell) NSs. Monodispersibility, spherical shaped, high surface area and mesoporosity were identified by TEM analysis and were further confirmed by

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BET analysis. The as-synthesized samples are highly soluble in aqueous media at ambient conditions. Spectroscopic analyses were also carried out to examine the impact of surface modification on structural, surface chemistry, optical and luminescence behavior of the as-designed silica coated core-shell NSs. The emission spectral study 2

revealed that the luminescence intensity of magnetic-dipole transition (590 nm, 5D0→7F1) is dominant with respect to electric-dipole (614 nm, 5D0→7F2) transition. The high crystallinity of the hydroxide products supports the existence of good photoluminescence

through

hypersensitive

emission

(614

nm,

5

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intensity, a good indication for their future use in detection of biomacromolecules D0→7F2)

transition.

Excellent

biocompatibility, cell viability and good luminescence properties suggested that the as-

prepared core-shell NSs are an ideal candidate for luminescence biolabeling/ bioimaging

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and as an optical bio-probe.

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Keyword: La(OH)3:Eu nanospheres, silica shell, optical properties, photoluminescence,

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biocompatibility

1. Introduction Recently, nano-phosphor is a material of growing interest among researchers in nanotechnology, due to its potential applications in biomedical sciences, such as in 3

biolabeling, optical bio-probe and in applied material sciences, such as light emitting diodes, laser, and solar energy materials[1-11]. Among the nano phosphor materials, luminescent lanthanide ion-doped rare-earth nano phosphors have an advantageous

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optical properties, such as sharp absorption & emission lines, long decay time, high quantum yield and superior photo-chemical and thermal stability at ambient

conditions[12, 13]. Lanthanum oxide is the most suitable host ceramic matrix for luminescent applications because of their high thermal stability, diamagnetic in nature, high k (~27) value, and low-cost compared to other trivalent lanthanide host matrices

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[14-19]. Additionally, lanthanum oxide has a substitution ability to accommodate similar

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valence and optically active lanthanide ion into the crystal lattice[16]. Due to the unique

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characteristics of the lanthanum oxide, there is a need to explore the La3+ base host

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ceramic materials. In the recent past, high resolution and excellent luminescence efficiency of the nano phosphors are in demand in the market for their use in biological

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windows, as well as in material sciences, where they can perform at low voltage.

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Therefore, for highly efficient phosphors, the specific morphology of the luminescent

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nanomaterials plays a key role in a particular application. Among the morphologies, spherically shaped phosphors are of interest because they offer the possibility of higher

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brightness and high resolution at ambient conditions. Furthermore, good packing densities and low scattering of the light can also be obtained by using spherically shaped

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nanophosphors[1]. Because of high brightness and high resolution of spherically shaped phosphors, therefore, there is an urgent need to develope alternate synthesis method for preparation of large-scale spherical shaped rare-earth nano-phosphors. Lin and his coworkers developed solvo-thermal synthesis method followed by subsequent heat 4

treatment for preparation of yttrium oxide micro-spheres[20]. Jia et al., synthesized uniform yttrium oxide hollow spheres by sacrificial template route[21]. Atabaev et al., prepared Eu and Tb-doped submicron yttrium oxide particles using urea based co-

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precipitation process[22]. To the best of our knowledge, most of the commonly employed spherical shaped morphologies are for yttrium oxide matrices [20, 21, 23-29]. None of the previous reports is available in the literature on spherically shaped lanthanum hydroxide. Here, we applied urea-based homogeneous co-precipitation process for the synthesis of lanthanum hydroxide NSs. It is expected that the large surface area,

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mesoporosity and spherical shape of the luminescent nanostructured materials are

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sensitive and more favorable for biological applications. Additionally, spherically shaped

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luminescent biolabeling ,etc[25, 30].

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materials are widely used as controlled-released capsules for drugs delivery and

In the present study, we prepared monodispersed trivalent europium ion doped

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lanthanum hydroxide NSs. We have modified the surface of luminescent seed core-NSs

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through mesoporous silica shell to enhance their solubility, biocompatibility, and toxicity

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at physiochemical pH. Because of it hydrophobic surface, mesoporous silica layer easily grafted over the surface of colloidal spherical shaped core-NSs to form core-shell

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nanostructure. X-ray diffraction pattern, transmission electron microscopy, dynamic light scattering (DLS), BET, optical absorption and photoluminescence spectroscopic

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techniques were used to examine the influence of mesoporous silica surface coating on crystal structure, crystallinity, morphology, hydrodynamic size, zeta potential and optical behavior. The mono dispersibility, mesoporosity, high surface area, good solubility along

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with biocompatibility and luminescent properties make them an ideal candidate for their future use in luminescent biolabeling, optical bio-detection, and drug delivery, etc.

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2. Experimental 2.1. Materials

Lanthanum oxide (99%, BDH Chemicals Ltd, England), Europium oxide (99.99%, Alfa Aesar, Germany), ethanol (E-Merck, Germany), Tetraethyl orthosilicate

(TEOS, 99 wt% analytical reagent A.R.), cetyltrimethylammonium bromide(CTAB),

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HNO3 and NaOH were used as the starting materials without any further purification.

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La(NO3)36H2O and Eu(NO3)36H2O were prepared by dissolving the corresponding

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oxides in diluted nitric acid. Milli-Q (Millipore, Bedford, MA, USA) water was used for

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synthesis and characterization of the samples.

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2.2. Preparation of La(OH)3:Eu@mSiO2 core-shell NSs In preparing the samples, 9.5 ml from a 2M stock solution of lanthanum nitrate and

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0.5 ml europium nitrate from 2M stock solution along with 2 g urea were mixed with 50 ml distilled water on a hot plate under constant mechanical stirring at 80 oC. Urea was

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used as the weak base for slow decomposition of lanthanum nitrate into lanthanum

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hydroxide. After homogeneous mixing at ambient condition, the solution was transferred to a bottle flask under refluxed condition. The resultant reaction mixture was reacted at 150 oC under refluxed condition for 3-4 h. The resulting precipitate was separated by centrifugation, washed with distilled water and further dried in an oven at 60 oC.[21, 23, 24, 26, 27] 6

For the silica surface modification of the mesoporous, a versatile sol-gel chemical route was followed. The 500 mg as-prepared La(OH)3:Eu NSs were first dispersed in 20 ml distilled water with the help of ultrasonication for 30 min. The ultra-sonicated NSs

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were centrifuged and re-dispersed in a solution containing 200 ml distilled water along with 0.500 g CTAB and 1N solution of NaOH on a hot plate at 70 oC under constant mechanical stirring., 1.0 ml TEOS was then slowly introduced into the vigorously stirred solution on a hot plate. The obtained precipitate was centrifuged, washed with distilled water, and dried in an oven at 60 oC.

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2.3. MTT assay

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Human breast cancer (MCF-7) were cultured in Dulbecco`s modified eagle`s

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medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 100 U/ml

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penicillin-streptomycin at 5% CO2 and 37 0C. Cell viability of core and core-shell NSs was examined by MTT assay as described in our previous work [31]. This assay

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determines the function of mitochondrial by measuring the ability of live cells to reduce

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MTT into formazon. In brief, 1×104 cells/well were seeded in 96-well plates and exposed

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to different concentrations (5-100 µg/ml) of NSs for 24 h. After the completion of exposure time, culture medium was taken out from each well to avoid interference of NSs

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and replaced with new medium containing MTT solution in an amount equal to 10% of culture volume and incubated for 3 h at 37°C until a purple-colored formazan product

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was developed. The resulting formazan product was dissolved in acidified isopropanol. Then, 96-well plate was centrifuged at 2300g for 5 min to settle down the remaining NPs. Further, 100 µl supernatant was transferred to new 96-well plate, and the absorbance was taken at 570 nm utilizing a microplate reader (Synergy-HT, BioTek, USA). Cells not 7

exposed to NSs served as control group. Statistical analysis for MTT assay was performed by one-way analysis of variance followed by Dunnett’s multiple comparison tests. Significance was ascribed at p<0.05.

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2.4. Characterization Powder X-ray diffraction was carried out using the PANalytical X’Pert X-ray diffractometer equipped with Ni filter Cu K (=1.5404Å) radiation. The morphology of the materials was examined by Field Emission-Transmission Electron Microscope (FE-

TEM- JEM-2100, JEOL, Japan) operating at an accelerating voltage 200kV. Specific

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surface area, porosity, and pore volume were measured from Brunauer-Emmett-

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Teller(BET) method by using the Barrett-Joyner-Halenda(BJH) model. Zeta potential

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and particle size distribution of the core and core-shell NSs were recorded by Dynamic

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Light Scattering (DLS) on Malvern Nanosizer ZS instrument. FTIR spectra were

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recorded on Vortex80 (Bruker, USA) spectrometer, using KBr pellet technique in the

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range 400-4000 cm-1. Absorption spectra were recorded from Cary 60 (Agilent Technologies, USA) UV/Visible spectrophotometer within the 200-600 nm wavelength.

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Photoluminescence spectra were recorded by Fluorolog-3(Model FL3-11, Horiba

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JobinYvon, USA) spectrophotometer at room temperature.

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3. Results and Discussion Crystal structure, phase purity, and crystallinity of the samples were determined

from the X-ray diffraction patterns of the samples. Figure 1 illustrates the powder X-ray diffraction pattern of the as-prepared luminescent La(OH)3:Eu and silica-coated mesoporous La(OH)3:Eu@SiO2 NSs at room temperature (Fig.1). Both diffractograms 8

contain all reflection peaks assigned to 110, 101, 200, 111, 201, 210, 002, 211, 112, 220, 311 and 401, which can be indexed to the pure hexagonal La(OH)3 crystal, as identified using the standard data JCPDS card No. 36-1481[15, 16, 19]. As seen in Fig. 1, the

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reflection peak positions and intensities are in good agreement with the crystalline hexagonal La(OH)3 phase. The absence of any additional peaks or any other phase

indicates successful doping of Eu ion into the La(OH)3 crystal lattice and formation of high purity homogeneous La-O-Eu solid solution. It is worth noticing that the diffraction planes intensity was decreased along with an increase in bandwidth in core-shell NSs, it

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could be due to the effect of amorphous silica surface coating [32]. An average

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crystalline size of the as-prepared NSs was estimated by the Debye-Scherrer formula with

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core and core-shell NSs, respectively.

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the most dominant reflection planes (101), (201) and (211) are to be 40 and 90 nm for

Figure 2 displays the high-resolution TEM micrographs of the as-prepared

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samples to get the shape, size, and meso-porosity of the NSs. The low magnification

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TEM micrographs clearly revealed the monodispersed, mesoporous, irregular and

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spherically shaped particles with mean diameter of 40±5 nm, non-aggregated, and highly distributed core-shell NSs. The uniform mesoporous silica shell possesses a wormhole-

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like channel structure with a thickness of about 20 nm. Additionally, the core-shell structure can be easily distinguished from the different electron penetrability between the

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luminescent core and silica shells. As shown in Fig.2(a) and (b), the luminescent seed core is dark black with an average size of about 40-46 nm and silica shell shows light gray with an average thickness of the shell is about 40-43 nm. As seen in low magnification and high magnification image each nanosphere has a luminescent seed 9

core and silica shells confirming the successful coating over the core NSs. The easy growth of homogeneous amorphous silica shell on the surface of La(OH)3:Eu core NSs could be attributed to the hydrated surface of the core particles. As a matter of fact,

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because of the hydrated surface of the core-NSs, TEOS can easily react with the surface of luminescent core NSs forming a mesoporous silica monolayer on top of the core, where silica can easily grow.

Brunauer-Emmett-Teller analysis was performed to ascertain the mesoporosity,

high surface area of the as-designed core-shell NSs. The nitrogen adsorption-desorption

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isotherm and pore-size distribution of core and core-shell NSs are shown in Figure 3. The

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measured typical tetragonal hysteresis loop for core-shell NSs revealed the shape that is

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the main characteristic of the uniform mesoporous structured nanomaterials (Fig.3b). The

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observed BET surface area 10.10 m2/g and pore volume 8.34x10-2 cc/g are for core and 627.85m2/g and pore volume 2.72x10-1 cc/g for core-shell NSs, respectively. As shown in

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Fig.3, after deposition of mesoporous silica layer surrounding the luminescent seed core-

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NSs, the surface area, pore size and pore volume remarkably increase, it indicated that

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mesoporous silica layer altered the surface characteristics of the as-designed luminescent nanomaterials. Furthermore, BET analysis further provides the confirmation for the

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formation of mesoporous structure within the single particle. Physiochemical pH was applied to measure the zeta potential of the both samples. The zeta potential was

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observed at -18.5 mV and -11.7 mV for core and core-shell NSs, respectively (Fig. 4). As seen in the graphs the zeta potential value is greatly reduced from core to core-shell NSs, it is a strong evidence of silica surface modification of the NSs. Additionally, the recorded zeta potential suggested that after silica surface modification deprotonating 10

value was increased (-11.7 mV), so that mesoporous silica surface is more easily available for conjugation with biomacromolecules such as proteins and oligonucleotides. FTIR spectra were performed to determine the surface chemistry of the as-

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prepared NSs and after that silica coating over the luminescent seed La(OH)3:Eu core NSs. The infrared spectra of core and core-shell NSs shows the characteristic broad infrared absorption band in between 3100-3650 cm-1 along with weak intensity peaks at

about 1450, and 645 cm-1 correspond to the stretching, bending and wagging vibrational

modes of hydroxyl groups(OH) of La(OH)3 and silanol (Si-OH)molecules(Fig. 5)[33-35].

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A strong doublet intensity band located at 1090 cm-1 along with weak intensity bands

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located at 802 and 542 cm-1 are attributed to the Si-O-Si, Si-O and Si-OH vibrational

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modes[36-38]. The existence of these infrared absorption bands clearly indicates the

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successful silica surface coating around the luminescent seed core-NSs. We believe that high intensity of infrared O-H bands could be due to the abundant surface anchored

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hydroxyl groups (La(OH)3:Eu) and silanol (Si-OH) molecules on the top of silica

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modified core-shell NSs. These surface attached hydroxyl groups are easily available for

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bonding with biomacromolecules for their use in multifunctional bio-applications. It indicated that surface silanol (Si-OH) groups play a vital role in solubility, colloidal

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stability, biocompatibility and non-toxic nature of the nanomaterials. Absorption spectra of the core and core-shell NSs were recorded in aqueous

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media for examining the solubility and colloidal stability character of the samples. As shown in Fig. 6, the absorption peak intensity and band edge was highly improved in core-shell samples, it suggested that amorphous silica layer has been effectively grafted around the core-shell NSs. The appearance of broad absorption peak maxima at 244 nm 11

in core-shell NSs sample originates from the silica shell, which is in good agreement with previous literature reports [36, 39]. The high absorbance of core-shell NSs in aqueous media reflected that the surface functionalized silanol(Si-OH) groups interact with the

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luminescent seed core hydroxyl groups through covalent bonding. This covalent bonding between the core and silica shell enhanced their solubility and colloidal stability character in aqueous media.

Photoluminescence spectra were recorded to investigate the europium ion doping

in La(OH)3 and silica surface coating over the luminescent seed core-NSs. The excitation

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spectra of core and core-shell NSs displays the all characteristic 4f-4f intra-

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configurational excitation transitions of Eu3+ ion observed at 316, 359,373, 394,434 and

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464 nm assigned to 7F0→5H3,6, 7F0→5D4, 7F0→5G3, 7F0→5L6, 7F0→5D3, and 7F0→5D2

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transitions, respectively, by monitoring the emission at 590 nm(Fig.7)[38, 40]. As shown in Fig 7(b), the intensity of excitation transitions was greatly suppressed in core-shell

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NSs, because of the influence of amorphous silica surface coating.

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Figure 8 illustrate the emission spectra of both samples under direct monitoring

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394 nm (7F0→5L6) excitation wavelength at ambient conditions, which is most commonly used excitation for trivalent europium ion. The emission spectra exhibited several at

431(5D3→7F2),

442(5D3→7F3),

456-465(5D2→7F0),

478(5D2→7F2),

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transitions

508(5D2→7F3), 519-535(5D1→7F1), 550-565(5D1→7F2), 589(5D0→7F1), 614(5D0→7F2),

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646(5D0→7F3) and 687-694(5D0→7F4) transitions of trivalent Eu ion, respectively [41]. Among all assigned transitions located in the middle of the spectrum at 589 (5D0→7F1) and 614(5D0→7F2) are well pronounced, whereas, the observed peaks at 646(5D0→7F3) and 687-697(5D0→7F4) are weaker. However, on comparing the luminescence peak 12

intensities in the core NSs spectrum, the emission intensity of magnetically-dipole allowed (5D0→7F1) transition is slightly stronger than their respective electric-dipoleallowed (5D0→7F2) transition causing the alteration of symmetry in local environment

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surrounding the Eu ion[42]. The spectra of core and core-shell NSs have similar peak positions expect their luminescent intensities. As seen in Fig. 8, the emission intensities of the core-shell NSs are greatly suppressed with respect to core-NSs, indicating that the mesoporous silica layer has been effectively grafted over the core-NSs, due to the surface

attached abundant silanol (Si-OH) groups, which effectively suppressed the luminescent

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intensity via non-radiative transition relaxation process. A similar impact of amorphous

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silica surface coating has been observed in previous literature reports [16, 37].

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Figure 9 represented the dose-dependent cell viability of core and core-shell NSs in MCF-7 cells. As illustrated in Fig 9, the core-shell NSs revealed higher number of cell

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viability compared to non-modified core NSs. These results suggested that silica surface modified NSs are highly biocompatible in respect to the bare core-NSs, which are in

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4. Conclusions

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good agreement with the zeta potential observed results.

In summary, we demonstrated a simple synthesis process for the preparation of

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spherically shaped, monodispersed, mesoporous silica modified La(OH)3:Eu core-shell NSs. Mesoporous silica layer has been successfully encapsulated over the surface of core

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NSs were observed from TEM images and further confirmed by FTIR spectral analysis. The core NSs with an average size of 40-45 nm and 43-nm thick mesoporous silica shell has been effectively grafted over the surface of core-NSs, revealing the improve solubility and biocompatibility of the as-designed NSs. Because of high crystalline nature 13

of the nano-products as observed from XRD pattern, they revealed good luminescence efficiencies even at ambient conditions. Analyzing the microscopic and spectroscopic results, the core-shell NSs possess good solubility, well-ordered hexagonal mesopores,

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high surface area, bright luminescence, good biocompatibility (zeta potential) and cell viability (MTT assay). These multifunctional properties can be potentially applied in

detection of bio-analytes and drug delivery systems, in which analyte detection limit can

be monitored by the change of the hypersensitive transition emission intensity. Due to the promising result of this study, further research can be extended to investigate the impact

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of surface coating on cell proliferation and luminescence biolabeling.

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Acknowledgment: Authors are thankful to the King Abdullah Institute for

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Nanotechnology, Deanship of Scientific Research, King Saud University, Riyadh, Saudi

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Arabia

References

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Figure Captions

Fig.1. X-ray diffraction pattern of La(OH)3:Eu (core) and La(OH)3:Eu@mSiO2 (coreshell) NSs.

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Fig.2. TEM micrographs of (a) low resolution La(OH)3:Eu@mSiO2 (core-shell) NSs (b) high resolution La(OH)3:Eu@mSiO2 (core-shell) NSs and (c) single particle of of La(OH)3:Eu@mSiO2 (core-shell) NSs.

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Fig.3. Nitrogen adsorption/desorption isotherm of (a) La(OH)3:Eu (core) and (b) La(OH)3:Eu@mSiO2 (core-shell) NSs.

Fig.4. Zeta potential of La(OH)3:Eu (core) and La(OH)3:Eu@mSiO2 (core-shell) NSs in aqueous media.

Fig.5. FTIR spectra of La(OH)3:Eu (core) and La(OH)3:Eu@mSiO2 (core-shell) NSs.

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Fig.6. UV-vis absorption spectra of La(OH)3:Eu in H2O and C2H5OH and

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La(OH)3:Eu@mSiO2 (core-shell) NSs suspended in C2H5OH.

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Fig.7. Excitation spectra of La(OH)3:Eu (core) and La(OH)3:Eu@mSiO2 (core-shell) NSs.

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Fig.8. Emission spectra of La(OH)3:Eu (core) and La(OH)3:Eu@mSiO2 (core-shell) NSs. Fig.9. Cell viability of core and core-shell NSs in MCF-7 cells. Data represented are

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mean±SD of three identical experiments (n = 3) made in three replicate.

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*Statistically significant difference in comparion of control cells (p<0.05)

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