Synthesis of Au@GdCP nanocomposites
The preparation method of Au@GdCP composite nanoparticles contains the following processes. Firstly, AuNPs were synthesized by reduction of chloroauric acid (HAuCl4, 1 mL, 0.025 mol/L) under sodium citrate solution (3 mL, 0.035 mol/L) and hexadecyl trimethyl ammonium bromide (CTAB, 20 mL, 0.125 mol/L) was used as stabilizer, the gold nanoparticles (AuNPs) with an average diameter of about 20 nm was diluted with DI water to 5 mL. Afterwards, 0.2 mmol rare earth ions solution which contains Gd(NO3)3, Yb(NO3)3 and Er(NO3)3 (molar ratio of Gd: Yb: Er = 0.16: 0.03: 0.01) were add into allantoin solution (0.3 mmol) stirring for 15 min at room temperature. Subsequently, the above solution was mixed with together and transferred into a Teflon lined stainless-steel autoclave with capacity of 50 mL, heated at 160 ℃ for 15 h. As the autoclave was cooled to room temperature naturally. Finally, the product was isolated by centrifugation and washed with deionized water and absolute ethanol for three times, and dried in vacuum at 60 ℃ for 12 h. The products marked as Au@GdCP. Meanwhile, the GdCP prepared with same method without Au NP introduced.
Characterization
The morphologies and sizes of the samples were obtained through scanning electron microscope (SEM, HITACHI S-3400N, 30 kV) and transmission electron microscope (TEM, Hitachi JEM2100, 200 kV). XRD pattern were performed on a Rigaku X-ray diffractometer with Cu Kα radiation (λ = 154178 Å) with the range of 5–90°, FT-IR data was obtained with Nicolet 6700 FT-IR spectrometer using KBr pellet between 4000 cm-1 and 500 cm-1 at room temperature. Photoluminescence (PL) emission spectra were recorded with an FLS 980 (Edinburgh Instruments, England). Elemental analysis was carried out on an EA3000 elemental analyzer. Magnetic properties were measured using a SQUID magnetometer equipment. The relaxation information was performed on a 0.5T NMI20-Analyst NMR system (Niumag Corporation, Shanghai, China).
Characterization of GdCP and Au@GdCP
The morphologies and sizes of the as-obtained GdCPs were initially characterized by scanning electron microscopic (SEM). As can be seen in Figure. 1(a), the products were discrete and spherical, with a mean diameter of ~ 179 nm and relative narrow size distribution, as determined by dynamic light scattering (DLS, insert of Figure. 1a). TEM images under different magnifications (Figure. 1b and c) further verify that GdCP is uniformly round and monodispersed. Interestingly, after combination with Au nanoparticles, A decrease in the particle size was obtained, with an average size of 150 nm (Figure. 1d) for Au@GdCP, which is helpful for the biomedical applications. The reduction in diameter probably due to the Au particles acting as the nuclei, leading to a decrease in the size of the final product, which is always observed in the growth of the crystal. The TEM image in Figure. 1e shows the internal structure of Au@GdCP. It can be clearly observed that each nanosphere encapsulating several Au nanoparticles. Magnification of the TEM image in Figure. 1f demonstrate that the Au@GdCP nanosphere has an outer layer with thickness of approximately 50 nm. In addition, the energy dispersive spectroscopy (EDS) mapping analysis of a single nanosphere are illustrated in Figure. 1g, the C, O, N, Gd, Yb, and Er elements coexist and are distributed homogeneously throughout the particle. However, the Au element is aggregated and located in the internal part of the Gd-based coordination polymers. All this evidence verifies that AuNPs have successful embedded within GdCP.
To assess the structure of the compounds, powder X-ray diffraction (XRD) patterns of GdCP spheres and Au@GdCP nanocomposites are presented in Figure. 2. No obvious diffraction peak is observed for GdCP, indicating that GdCP is amorphous and not crystalline. While the Au@GdCP exhibits four distinct peaks at \(2{\theta }\)= 38.37º, 44.56º, 64.75º, 77.72º, which can be assigned to the (111), (200), (220) and (311) planes of the standard data for hexagonal phase of Au (JCPDS 04-0784). The HRTEM image (Figure.1f) shows lattice fringe with the interplanar spacing of 0.252 nm, corresponding to the (200) planes of Au. The result was further proved the presence of AuNPs in the product and the formation of Au@GdCP.
Magnetic properties
The magnetic properties of GdCP and Au@GdCP nanocomposites were measured by SQUID magnetometer. Figure. 3 shows the magnetization of the magnetic field (M-H) curves of samples at 300 K. The magnetization of GdCP and Au@GdCP are 5.19 emu/g and 683.52 emu/g at 300 k, respectively. With the introduction of AuNPs, the magnetization was enhanced remarkably. No hysteresis loop was observed. The magnetization curve of the samples displayed typical paramagnetic properties. These results show that even after introduction of AuNPs, the particle keep well paramagnetic properties.
Luminescence properties of Au@GdCP
The upconversion emission spectrum of Au@GdCP under 980 nm laser excitation is shown in Figure. 4. Simultaneously, GdCP was used as the control under the same conditions. In this system, Yb3+ acts as a sensitizer and transfers the absorbed excitation energy to the activator ions of Er3+. First, Yb3+ is excited from the state level (7F7/2) to the exciting level (5F5/2) under the continuous 980 nm laser excitation. Subsequently, Er3+ ions are excited to the intermediate state (4I11/2) and then transfer energy to the excited state (4F7/2), which resulting the energy transfer of Er3+ ions from 4F7/2 levels to the levels of 2I11/2, 4S3/2, and 4H9/2 with nonradioactive process, respectively [37–39]. The emission spectra exhibited three bands, attributed to green emission (525 nm and 545 nm) and red emission (655 nm). Compared with GdCP, an obvious enhanced emission intensity of 38.4% was obtained after Au ions modification, coupled with a slight blue shift for 4 nm, mainly attributed to the LSPR effect of the AuNPs [38]. LSPR strengthens the local crystal field, resulting in the enhancement of excitation field intensity [41]. Meanwhile, coupling between the broad LSPR peaks of AuNPs and the upconversion luminescence enhances the energy transfer effect, improving the decay rate of luminescent centres and the efficiency of the emission intensity [42, 43]. Therefore, AuNPs play a role in enhancing the fluorescent intensity in Au@GdCP.
Relaxation information and T1-weighted MRI maps
Gd-based composites have good kinetic and thermodynamic stabilities. They can accelerate the T1 relaxation of water protons, and improve the brightness of MRI signals. Approximately 40% of MRI scans employ Gd3+ chelates for contrast agent in clinical [44]. However, free Gd3+ ions are known to have serious toxic effect due to the replacement of Ca2+ ions in the body, further improvement is required. Thanks to the biocompatibility and tuneable morphology, AuNPS have been frequently used in clinical applications. In recent years, they have been exploited to create very promising tools for multimodal imaging and MRI-guided therapies [45]. To estimate the performance of GdCP and Au@GdCP as MRI contrast agent, the relaxation rate and T1-weighted MRI were investigated in vitro at the low field of 0.5T. As shown in Figure. 5(a, c), with the increase of Gd concentration, the longitudinal relaxation time and transverse relaxation time of GdCP showed a good linear relationship. Based on the slop, the longitudinal relativity (r1) and transverse relativity (r2) were calculated to be 0.248 mM–1S–1 and 1.292 mM–1S–1, respectively. In comparsion with GdCP, Au@GdCP exhibited increased values with 2.853 mM–1S–1, 13.797 mM–1S–1 for r1, r2, respectively. Calculated to be 11.5 fold and 10.8 fold enhancements. The appealing improvement of the Au@GdCP is mainly due to the confined tumbling of Gd3+ in a biomacromolecule, which result a longer rotational correlation time. Besides, compared with previous reported modified UCNPs materials (such as the UCNPs@mSiO2-PEG nanocomposite with a r1 of 0.6361 mM–1S–1), our results exhibited stronger signals. [46]. The excellent longitudinal and transverse relaxation information with relativel high r2/r1 ratio (r2/r1 = 4.83 > 3) of Au@GdCP is expected to achieve T1-T2 dual mode MRI.
The T1-weighted MRI of Au@GdCP and GdCP nanocomposites were also obtained (Fig. 3b, d), in which T1-weighted MRI signals got brighter and brighter with the increasing concentration of Gd3+. Compared with the T1-weight MRI signal of both nanocomposites, the contrast effect of Au@GdCP is much better than GdCP, demonstrating that the existence of AuNPs greatly enhances the MRI signal intensity and the contrast of the MRI signal.
It is well known that reducing the inversion rate of the contrast agent in the aqueous proton solution can reduce the average spin-nuclear spin and the dipole-dipole interaction between the contrast agent, and it can prolong the inversion time, which can improve the relaxation efficiency of the contrast agent [25]. With the introduction of AuNPs, the LSPR of the AuNPs may strengthen the external magnetic field, which results in intermolecular dipole-dipole interactions, increasing the contrast relaxation rate and the T1-weighted MRI signal [47–48]. Moreover, the strong light scattering of AuNPs translates the absorbed light to the plasmon wavelength, resulting in enhancement of T1-weighted MRI signals [49].
Biocompatibility of the Au@GdCPs
As is well known, good biocompatibility is an important prerequisite and crucial factor for biological applications. In this work, the in vitro biocompatibility of Au@GdCP was evaluated through the 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide (MTT) assay with incubated HeLa cells. Meanwhile, GdCP was tested as a control under the same conditions. Figure. 6 shows that with the increasing concentration of Au@GdCP and GdCP, the relative cell viability of HeLa gradually decreased. When the concentration of Au@GdCP and GdCP reached 1.0 mg/mL, the relative cell viability is 80% and 82.2%, respectively. The MTT results indicate that Au@GdCP exhibit low toxicity and good biocompatibility.