Abstract
We report on the synthesis and characterization of folic acid-conjugated silica-modified TbPO4·H2O nanorods for biomedical applications. The uniform shape TbPO4·H2O nanorods with a hexagonal phase were successfully synthesized by wet chemical methods. A novel TbPO4·H2O@silica-NH2 nanocomplex was then formed by functionalizing these nanorods with silica and conjugating with biological agents. The field emission scanning electron microscopy, energy-dispersive X-ray, and X-ray diffraction reveal the morphology and structure of the nanorods, with their controllable sizes (500-800 nm in length and 50-80 nm in diameter). The Fourier transform infrared spectroscopy was employed to identify chemical substances or functional groups of the TbPO4·H2O@silica-NH2 nanocomplex. The photoluminescence spectra show the four emission lines of TbPO4·H2O@silica-NH2 in folic acid at 488, 540, 585, and 621 nm under 355 nm laser excitation, which could be attributed to the 5D4-7 () transitions of Tb3+. The TbPO4·H2O@silica-NH2 nanorods were conjugated with folic acid for the detection of MCF7 breast cancer cells. The obtained results show a promising possibility for the recognition of living cells that is of crucial importance in biolabeling.
1. Introduction
Rare earth containing luminescent nanophosphors with many advantages such as high stability, strong luminescence, noncomplex fabrication, easy surface functionalization, and friendly to environment and human body have been very promising materials for healthcare application, especially for biomedical fluorescence labeling [1–10]. Recently, the development of rare earth containing luminescent nanorods for application in biomedicine is attracted topic for scientists in materials science field [11–15]. According to some previous studies, compared with nanospheres, nanorods could have a short coherent time in the interband transitions. Therefore, the decay time of nanorods is relatively long that allows for more stable luminescence intensity.
Additionally, the nanorods indicate very small radiation damping because of small volumes [16, 17]. Besides, nanorods are also very interest for scientific community because the morphology is related with an intrinsic multifunction arising in different contact areas [18]. These characteristics are also very useful potential objects for application in highly functional devices. Some rare earth containing luminescent nanorods could be synthesized by wet chemical routes [19–21] including our successful work with TbPO4 nanorods [22]. These results and some other previous studies have shown that the morphology of materials can have important impacts on luminescent intensity. In biomedical applications, high luminescent yield with nanosize and biomedical compatibility is still required as indispensable property for biolabeling materials. Therefore, it is necessary for the development of a stable and high-quality synthesized procedure to fabricate a higher luminescent intensity of TbPO4·H2O nanorods with possible ability in biomedical application.
Folates are salts of folic acid. Folate is also the name of the anionic form of folic acid, and it has a high-affinity binding reagent that could recognize folate receptor. Moreover, folic acid is founded to many promises due to low cost, high stability, its faster internalization kinetics through cellular membrane, and its nonimmunogenicity. Coupling of folates to nanomaterials advanced their cellular uptake by folate receptor remediated endocytosis which can avoid their nonspecific attacks to normal tissues, also which can increase their cellular uptake within target cells. Folic acid was attached to the TbPO4·H2O@silica-NH2 nanorods via the formation of amide linkage between the amine group of TbPO4·H2O@silica-NH2 nanorods and the carboxyl group of folic acid.
In this work, we focus on the development of TbPO4·H2O nanorods prepared by wet chemical methods with strong luminescent that is potential for biolabeling application. The TbPO4·H2O nanorods were covered by a silica layer, attached with amine groups, and then linked with folic acid by a coupling reaction to form the TbPO4·H2O@silica-NH2 nanocomplex. The nanocomplex optical properties were discussed in detail. The biocomplex emitted strongly in green at 540 nm. The primary test result showed that TbPO4·H2O@silica-NH-folic acid nanocomplex could be used for both detection and recognition of MCF7 breast cancer cells.
2. Experimental Procedure
2.1. Materials
Terbium (III) nitrate pentahydrate (Tb(NO3)3·5H2O) (Aldrich, 99.9%), ammonium dihydrogen phosphate (NH4H2PO4) (Merck, 99%), sodium hydroxide (NaOH) (Merck, 99%), tetraethyl orthosilicate (Merck, 99%), 3-aminopropyltriethoxysilane (Aldrich, 98%), dimethyl sulfoxide (Aldrich, 99.9%) with N-hydroxysuccinimide (Aldrich, 98%), N,N-dicyclohexylcarbodiimide (Aldrich, 99%), and folic acid (C19H19N7O6).
2.2. Synthesis of TbPO4·H2O Nanorods
Wet chemical methods which include hydrothermal, sol-gel, and coprecipitation synthesis were developed to obtain ultrafine, very homogeneous, and high purity powders. Among these various wet chemical synthesis methods, hydrothermal method is a very simple method with cost effectiveness, easy control over material size, and compositions. Figure 1 illustrates the scheme of synthesis procedure by hydrothermal method for TbPO4·H2O nanorods.
The TbPO4·H2O nanorods were synthesized as the following procedure. 15 ml (0.05 M) terbium (III) nitrate pentahydrate and 225 ml (0.05 M) ammonium dihydrogen phosphate were mixed in stirred vigorously solution. Afterward, sodium hydroxide was added slowly to the solution and stirred using a magnetic pellet for 6 hours with adjusted pH in the range of . Meanwhile, the reactant Tb3+/PO43- molar ratio was changed from 1/15 to 1/1 by increasing the amount of terbium nitrate. Next, the mixture solution was put in a Teflon-lined stainless-steel autoclave and heated up at 200°C for 24 hours. The TbPO4·H2O was separated by centrifuge at 5800 rpm. The achieved product had been washed several times by deionized water and dried at 60°C for 6-24 hours.
2.3. Synthesis of TbPO4·H2O@Silica and TbPO4·H2O@Silica-NH2 Nanorods
Silica coating is a strategy for inorganic surface treatment to make nanorods water-dispersible and biocompatible as well as to introduce new therapeutic functionalities when using silica to entrap a therapeutic agent and deliver it to the treatment destination. Silica is highly stable, biocompatible, and optically transparent. When it is utilized as a coating material, surface silanization can flexibly offer abundant functional groups (e.g., –COOH, –NH2, and –SH), allowing for conjugation with biomolecules and exploiting the optical properties of functionalized materials.
TbPO4·H2O@silica nanorods were prepared by sol-gel Stöber process as the following: (i) 5 ml of tetraethyl orthosilicate, 5 ml of ethanol, 0.5 ml of acetic acid, and 1 ml of deionized water were stirred and mixed at room temperature for 6 hours (solution 1); (ii) then, 20 ml of as-synthesized TbPO4·H2O solution were dispersed in a mixed solution of 20 ml ethanol and 1 ml deionized water using a vortex mixer for 30 minutes (solution 2). Solution 2 was slowly dripped into solution 1; then, the mixture had been continuously stirred for 24 hours. The TbPO4·H2O@silica was achieved as a result of the separation using centrifugation. The product was washed and dried at 60°C for 20 hours.
TbPO4·H2O@silica-NH2 nanorods were prepared as the following: TbPO4·H2O@silica mixed with 20 ml of ethanol, 3 ml of 3-aminopropyltriethoxysilane, and 1 ml of tetraethyl orthosilicate was poured in a three-necked flask (size 50 ml) by stirring with a magnetic pellet for 60 minutes. This solution was heated up to 60°C in the reflux during continuous stirring for 20 hours. Finally, TbPO4·H2O@silica-NH2 products were separated by centrifuge, then washed with deionized water and dried again.
2.4. Conjugation of TbPO4·H2O@Silica-NH2 Nanorods with Folic Acid
The conjugation procedure of TbPO4·H2O@silica-NH-folic acid was prepared as the following. First, 0.5 mg folic acid was dissolved in 15 ml of dimethyl sulfoxide with N-hydroxysuccinimide (NHS) in the presence of N,N-dicyclohexylcarbodiimide (DCC) at room temperature by using a vortex mixer for 30 minutes (the molar ratio of folic acid/NHS/DCC is 1/5/3). After that, this activated folic acid was continuously stirred in the darkness for 20 hours. Next, 10 ml dispersed TbPO4·H2O@silica-NH2 nanorods in ethanol have been mixed in the above-activated folic acid solution. The mixed solution was continuously stirred again in the darkness for 20 hours. The final conjugated TbPO4·H2O@silica-NH-folic acid product was filtrated and separated by the centrifuge with rotation speed of 5800 rpm.
2.5. Cell Culture Experiments
The MCF7 breast cancer cell line was kindly provided by Dr. J M Pezzuto-US. They were cultured in MEME medium (Sigma Chemical Co.) and supplemented with 10% bovine embryonic serum (FBS) (Sigma Chemical Co.) and 50 μg/ml gentamicin. Cells were inoculated after 3-5 days and incubated in an incubator at 37°C and 5% CO2.
The biocomplex TbPO4·H2O@silica-NH-folic acid and the MCF7 breast cancer cells were seeded in 24-well plates with a density of 104 cells·ml-1; then, these biological samples have been incubated for 24 hours. Polyethylene glycol 1500 (Sigma) and the biocomplex TbPO4·H2O@silica-NH-folic acid with the concentration of 20 μg/ml were supplemented to the cell-seeded wells for 6 hours. Finally, remove the cultured medium; the fixed time cells and MCF7 breast cancer cells were poured in falcon tubes (size of 15 ml). Then, the achieved product was cleaned by phosphate buffer saline three times. Fluorescent images have been observed by an Olympus Scan^R 100X fluorescent microscope.
2.6. Characterization Techniques
The morphology and structure of the synthesized nanorods have been investigated by a Field Emission Scanning Electron Microscope (FE-SEM Hitachi S-4800), X-ray diffractometer (XRD) using a Siemens D5000 with CuKα radiation ( Å in the 2-theta range of ), and energy-dispersive X-ray (EDX). The chemical substances or functional groups of the TbPO4·H2O@silica-NH2 have been identified by Fourier transform infrared spectroscopy (FTIR NEXUS 670). The photoluminescence spectroscopy (Horiba iHR550 setup) has been used for photoluminescence property characterization of the synthesized biological nanosamples. The cells were observed under an Olympus Scan^R fluorescence microscope (Olympus Europa SE & Co.KG, Hamburg, DE).
3. Results and Discussion
3.1. The Morphology of Synthesized Samples and Energy-Dispersive X-Ray Analysis
Figures 2(a)–2(e) show FESEM images of TbPO4·H2O nanophosphors with the different reactant Tb3+/PO43- molar ratios and TbPO4·H2O@silica-NH2 nanophosphors. These images provide information for the morphological analysis of the synthesized samples. It was observed from the FE-SEM images that the samples are formed in the nanorod shapes with molar ratios of Tb3+/PO43- which are 1/15, 1/10, 1/5, and 1/1. The shape of these nanorods is very uniform proving the stable synthesized procedure. The size of synthesized nanorod can be estimated in the range of 20-30 nm in diameter and 200-300 nm in length from Figures 2(a) and 2(b). The reactant Tb3+/PO43- molar ratio for the products of Figures 2(a) and 2(b) is 1/15 and 1/10, respectively. When these molar ratios are increased to 1/5 and 1/1, the as-synthesized TbPO4 exhibits more uniform nanorod morphology (Figures 2(c) and 2(d), respectively). The length of nanorods increases in the range of 300-500 nm, and their observed diameters are bigger in the range of 30 to 50 nm with the same reaction time. We produce TbPO4·H2O nanorod biomedical nanocomposite complexes by several steps. First, it is necessary to treat the surface of the nanorods. Then, we functionalized the samples with amine groups. Finally, conjugation with biological agents of nanocomposite complexes has been done.
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Currently, silica coating is the main inorganic surface treatment. The silica shell may offer sites for the loading of functional molecules and allows the −NH2, −COOH, or –SH functionalization necessary for further chemical reaction with biological molecules and other functionalized compounds. Therefore, because of its versatility, silica constitutes an excellent intermediate layer, which allows the combination of TbPO4·H2O nanorods and additional molecules that need to be kept separate from one another. In these results, the nanorods with a 1/1 mole ratio of Tb3+/PO43- have been chosen for the next study including protection with silica and functionalization with amine. After TbPO4·H2O nanorods are protected with silica and attached with amine groups (forming TbPO4·H2O@silica-NH2 samples), the sizes of the products are slightly increasing with diameters in the range of 50-80 nm and lengths in the range of 500-800 nm (see Figure 2(e)).
The nanorods of TbPO4·H2O and TbPO4·H2O@silica-NH2 analyzed the elemental composition on material surfaces by energy-dispersive X-ray technique (EDX). The results of these analyses are shown in Figure 3. We can see the presence of Tb, P, and O in the EDX spectrum as the basic elements in original nanorods (Figure 3(a)). Obviously, the additional Si and N peaks have been observed in the EDX spectrum of silica-coated samples with the amine functionalized group (Figure 3(b)). The EDX analysis confirms that there are no other impurities detected in the analyzed nanorods (both are original and functionalized samples).
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3.2. The Fourier Transform Infrared Spectra and Crystalline Phase Identification
The Fourier transform infrared spectrum of the nanorods of TbPO4·H2O (1) and TbPO4·H2O@silica-NH-folic acid (2) is shown in Figure 4. The O–H vibrations were observed here as a shoulder at ~3417 cm-1. It can be observed that H2O, Tb-O, PO43- vibrations were defined at 1643 cm-1, 951.8 cm-1, and 670 cm-1, respectively (Figure 4, line (1)).
After TbPO4·H2O nanorods were protected with silica and then functionalized with amine and folic acid groups, there are three absorption peaks at 2091 cm-1, 1643 cm-1, and 1438 cm-1 corresponding to the vibrations of the N-H, C=O group [20, 23].
The asymmetric vibration of Si-O-Si is represented by strong absorption peaks at 1318 cm-1 and 1014 cm-1 (Figure 4, line (2)). Therefore, the information from infrared absorption spectroscopy proved clearly the existence of amine groups, Si-O-Si bond, and folic acid groups in the TbPO4·H2O@silica-NH-folic acid nanorods. This also confirms the formation of linkage between the luminescence nanorods of TbPO4·H2O@silica-NH2 and the folic acid.
Figure 5 presents the X-ray diffraction pattern of the nanorods of TbPO4·H2O (line 1), TbPO4·H2O@silica-NH2 (line 2), and standard data of hexagonal TbPO4·H2O (space group: P3121, PDS card No. 20-1244, Å; Å) as a reference. We can see that the all diffraction peaks of the nanorods TbPO4·H2O (line 1) and TbPO4·H2O@silica-NH2 (line 2) are high intensity, narrow width at 2θ angles of 14.85°, 20.83°, 25.95°, 30.01°, 32.10°, 38.75°, 42.78°, 48.13°, 49.70°, 53.21°, and 54.51°, and equivalent to the hexagonal phase of TbPO4·H2O (shown by PDF card No. 20-1244). This means both obtained TbPO4·H2O and TbPO4·H2O@silica-NH2 nanorods are almost in a hexagonal phase form. The XRD results also revealed the amorphous nature of the shell layer (silica-NH2) with no extra peaks was detected in these samples.
3.3. Photoluminescence Properties
Figure 6 shows the photoluminescence spectra of the TbPO4·H2O nanophosphors with various reactant Tb3+/PO43- molar ratios under an excitation wavelength of 355 nm at the room temperature.
TbPO4·H2O nanophosphors exhibit the significant luminescence peaks at 488 nm, 540 nm, 585 nm, and 621 nm, which relate to 5D4-7F6, 5D4-7F5, 5D4-7F4, and 5D4-7F3 optical transitions of Tb3+, respectively. The results were achieved emission characteristics of Tb3+ from the TbPO4·H2O nanophosphor samples with different Tb3+/PO43- molar ratios of 1/15 (line 1), 1/10 (line 2), 1/5 (line 3), and 1/1(line 4), in which the strongest emission peaks are yielded by the 5D4-7F5 transition at 540 nm (green light).
Moreover, we observed the increasing of TbPO4·H2O nanophosphor luminescence intensity when we raise up the amount of terbium nitrate in the synthesis. It seems that a highest luminescent intensity could be achieved in case of the synthesized TbPO4·H2O nanophosphors with the Tb3+/PO43- molar ratio of 1/1. This result is evidence for there is no luminescent quenching effect occurring at the high concentration of Tb3+ in the synthesized nanophosphors.
Figure 7 shows the comparison of achieved photoluminescence spectra from TbPO4·H2O nanorods and TbPO4·H2O@silica-NH-folic acid conjugate at 355 nm excitation wavelength. The luminescence peaks of TbPO4·H2O@silica-NH-folic acid conjugate appear at the same positions and remain a similar shape as TbPO4·H2O nanorod peaks. This result confirms that the luminescence properties of TbPO4·H2O nanorods have not changed after the functionalization and biological conjugation. Thus, these nanorods are very promising for biolabeling application.
3.4. In Vitro Cellular Imaging
A fluorescence microscope had been used for the observation tool to evaluate the linking ability between TbPO4·H2O@silica-NH-folic acid conjugates and MCF7 breast cancer cells after the incubation process described in the above experiment part. Figure 8 shows the fluorescent images for three cases: (a) MCF7 breast cancer cells (negative control), (b) incubated MCF7 breast cancer cells with TbPO4·H2O@silica-NH2, and (c) incubated MCF7 breast cancer cells with TbPO4·H2O@silica-NH-folic acid; the concentration of 20 μg/ml for 3 hours was observed by the Olympus Scan^R 100X fluorescent microscope that used the excitation wavelength in the violet region (with filter ranged from 340 to 385 nm). For the first case, we did not observe luminescence emission in the reference sample (Figure 8(a)). In the second case, the incubated MCF7 breast cancer cells with TbPO4·H2O@silica-NH2 sample shows low luminescence intensity (not bright) in Figure 8(b) that allow to propose about the weak bonds between TbPO4∙H2O@silica-NH2 and the cancer cells. The expected result has been achieved in the third case. We observed bright green pixels in Figure 8(c). This evidence demonstrates a strong coupling between MCF7 breast cancer cells and TbPO4·H2O@silica-NH-folic acid after we incubate these cells with the luminescence biological nanorod conjugates.
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According to Soheila Kashanian, folic acid receptors are highly overexpressed on the surface of numerous tumour types including also breast cancer cells. Otherwise, folate receptors are selectively overexpressed in a number of tumour cell types but present in low or nondetectable levels in the most normal cells [24]. Our cellular images in Figure 8 shows clearly that the green emission of MCF7 breast cancer cells labeled with luminescent nanocomplex TbPO4·H2O @silica-NH-folic acid is much brighter (stronger luminescence) in comparison with the other reference cases, i.e., MCF7 breast cancer cells (negative control) or MCF7 breast cancer cells incubated with TbPO4·H2O@silica-NH2. In vitro cellular imaging tests have proved the localization of the functionalized TbPO4·H2O@silica-NH-folic acid within the cytoplasm of breast cancer cells. As seen in the fluorescence microscope images, the TbPO4·H2O@silica-NH-folic acid has localized within the cell cytoplasm. A short explanation for this result could be as follows. The high folic acid-folate receptor affinity allows the ligand binding between cell and luminescent labeling rod. After that, the TbPO4·H2O@silica-NH-folic acid has internalized into the cell via invagination process. Therefore, the TbPO4·H2O@silica-NH-folic acid could potentially use as biolabels for MCF-7 breast cancer cells. This test method based on fluorescent materials and a microscope could provide a quick tool to detect the cancer cells. In comparison with the other techniques, it is a nonsophisticated method without requirements of complex apparatus, e.g., spectral equipment and data analysis. Additionally, this provides a visual tool needed for some specific study in the biology.
4. Conclusions
We have successfully synthesized the uniform shape TbPO4·H2O nanorods with hexagonal phase using wet chemical methods. TbPO4·H2O@silica-NH-folic acid nanocomplex has been achieved by protecting these nanorods with silica layer, attaching with amine groups and then linking with folic acid by a coupling reaction. The sizes of this product were estimated at about 50-80 nm in the diameter and 500-800 nm in the length. The main characteristic emission bands of TbPO4·H2O @silica-NH-folic acid nanocomplex are located at 488 nm, 540 nm, 585 nm, and 621 nm regions in photoluminescence spectra, which attributed to the 5D4- () transitions of Tb3+ ions, respectively, in which a very intense green luminescence peak at 540 nm has been observed. The preliminary in vitro test indicated that the TbPO4·H2O@silica-NH-folic acid nanocomplex could use for detecting and recognizing MCF7 breast cancer cells under a fluorescence microscope. This result allows us to propose TbPO4·H2O@silica-NH-folic acid nanocomplex as a very promising candidate with noncomplex apparatus for developing a quick fluorescent label and image tool in biomedicine.
Data Availability
The concerning data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
Acknowledgments
This work is funded by the Vietnamese Ministry of Education and Training under grant number B2019-MDA-04 and partly supported by the Institute of Materials Science, Vietnam Academy of Science and Technology under grant number CSPN.01/20-21.