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Porous Silicon Nanoparticles with Rare Earth as Potential Contrast Agents for MRI and Luminescent Probes for Bioimaging
Anastasiya D. Mironova1*, Yulia V. Kargina1,2, Olga S. Pavlova2,3, Alexander M. Perepukhov4, Igor O. Sobina2, and Victor Yu. Timoshenko1,2
1 Phys-Bio Institute, National Research Nuclear University "MEPhl", 31 Kashirskoe shosse, Moscow 115409, Russia
2 Faculty of Physics, Lomonosov Moscow State University, 1 Leninskie Gory, Moscow 119991, Russia
3 Faculty of Fundamental Medicine, Lomonosov Moscow State University, 1 Leninskie Gory, Moscow 119991, Russia
4 Institute of Physics and Technology, 9 Institutskiy per., Dolgoprudny 141700, Moscow Region, Russia
* e-mail: navonorim@mail.ru
Abstract. Nanoparticles of porous silicon with incorporated europium and gadolinium ions were prepared by using mechanical grinding of electrochemically grown mesoporous silicon films followed with impregnation with rare earth ions from aqueous solutions. The photoluminescence spectroscopy of europium doped porous silicon nanoparticles allowed us to reveal narrow lines associated with the 5D0^7F4 transitions in Eu3+ ions. Measurements of the proton relaxation in aqueous suspensions of nanoparticles with embedded Gd3+ ions showed an effect of the shortening of both the longitudinal and transverse relaxation times. Potential applications of rare earth doped porous silicon nanoparticles as contrast agents in MRI and fluorescent labels in bioimaging are discussed. © 2022 Journal of Biomedical Photonics & Engineering.
Keywords: nanoparticles; aqueous suspensions; proton relaxation; magnetic resonance imaging; contrast agent.
Paper #3477 received 19 Feb 2022; revised manuscript received 4 Apr 2022; accepted for publication 1 May 2022; published online 18 May 2022. doi: 10.18287/JBPE22.08.020304.
1 Introduction
Magnetic resonance imaging (MRI) is widely used in medical practice because this diagnostic method is safe and has a high spatial resolution [1]. Molecular substances called contrast agents (CAs) can significantly enhance the MRI contrast [2, 3]. Novel CAs based on nanoparticles (NPs) are being studied and developed [4]. Most clinically approved types of CAs consist of ions of gadolinium (Gd) [5-7] and iron (Fe) [8, 9]. Recently new types of CAs based on silicon (Si) NPs have been proposed [10, 11]. It was shown that CA properties of Si NPs could be significantly improved after incorporation of Fe3+ [12]. However, Gd-based substances are one of the most common CAs for clinical diagnostic purposes. They have a high relaxivity and increase the signal intensity due to a decrease in the longitudinal relaxation that is explained by the interaction of proton spins of water molecules with electron spins of rare earth ions [3, 13-15]. The disadvantage is the toxicity of free
Gd3+ ions, which can cause allergic reactions in some patients. To reduce the toxic effect of free Gd3+ ions, it is proposed to bind them with biocompatible Si NPs and reduce the concentration of ions in the CA. It is important that the CA remains effective. Besides Gd3+, trivalent ions Eu3+ are considered as potential labels in biomedical diagnostics [16]. The aim of our work is to study the effect of incorporated Gd3+ and Eu3+ ions in porous Si NPs on the proton relaxation in aqueous solutions as well the photoluminescence property rare earth doped MPSi NPs as possible CAs for MRI and probes for fluorescent bioimaging.
2 Materials and Methods
Aqueous suspensions of mesoporous Si (MPSi) NPs were prepared by electrochemical etching of crystalline Si (c-Si) wafers followed by mechanical grinding of the formed mesoporous films in water by using a planetary mill [10]. To obtain a uniform distribution of MPSi NPs
in water, all samples were exposed to ultrasound for 30 min. Then, suspensions of MPSi NPs with concentration about 10 mg/mL were mixed with aqueous solutions of lanthanide chlorides (EuCb and GdCb) at different concentrations of the latter varied from 10 to 50 mg/mL. The prepared mixtures were stored for 30 min followed with centrifugation and cleaning by deionized water two times to remove unbound lanthanide ions. Then the samples were diluted to obtain aqueous suspensions of MPSi:Gd and MPSi:Eu NPs with concentration of the order of 1 mg/mL.
The hydrodynamic diameter and surface charge (zeta-potential) of the MPSi NPs were estimated by using a methods of the dynamic light scattering (DLS) with a ZetasizerNano Malvern DLS spectrometer (Malvern Instruments, United Kingdom). A part of the prepared aqueous suspensions was dried at room temperature in air to analyze their chemical composition by means of the X-ray fluorescence (XRF) spectroscopy using a Renom apparatus (Expert Center Ltd., Russia). Photoluminescence (PL) spectra of MPSi NPs and those with Eu3+ ions under excitation with a semiconductor laser at 465 nm were measured by using a grating monochromator equipped with a CCD unit (Mightex, Canada). To measure the proton relaxation times in aqueous suspensions of MPSi NPs we used a Bruker Minispec NMR relaxometer (Bruker, Germany) with a constant magnetic field strength of about 0.5 T. The relaxation times were measured for 800 ^L of aqueous suspensions of NPs in glass ampoules placed in a thermostat at 40 °C.
3 Results and Discussion
DLS measurements reveal that prepared MPSi NPs are characterized by hydrodynamic diameter and zeta-potential of about 560 nm and -6 mV, respectively. The negative value of the latter indicates the predominant role of hydroxyl groups on the surface of MPSi NPs [10]. The
initial negative charge of NPs can obviously promote the electrostatic binding between the NP's surface and positively charged lanthanide ions, i.e. Gd3+ and Eu3+ in aqueous solutions.
Fig. 1 shows typical XRF spectra of MPSi NPs with incorporated rare earth and reference samples of dried lanthanide chlorides and initial MPSi NPs. The characteristic X-ray emission lines of the L-series in Europium at 5.82-5.85, 6.46, 6.84 and 7.48 keV [17] are detected for EuCl3 and MPSi:Eu NPs (see Fig. 1a). The characteristic L-shell lines of the Gadolinium at 6.03-6.06, 6.71, 7.10, and 7.79 keV [17] are also detected in MPSi:Gd NPs prepared at the initial rare earth concentration about 10 mg/mL (see Fig 2b). By comparing with the XRF line intensities the europium concentration in MPSi:Eu are estimated to be about 48.7 at.% that indicates a high efficiency of the incorporation of Eu3+ ions into MPSi NPs. This fact can be explained by high internal surface area and negative surface charge of the latter [18]. The Gd content is about 0.09 and 0.33 at.% in MPSi NPs samples prepared at the GdCl3 concentration of 10 mg/mL and 50 mg/mL, respectively. The smaller relative content of rare earth in MPSi:Gd NPs in comparison with that in MPSi:Eu ones can be related to different efficiency of the incorporation into oxidized internal surface of MPSi. Table 1 presents XRF data on the rare earth concentration in the samples under study.
Table 1 Rare earth content in MPSi NPs according to XRF data.
Sample Rare earth content, at.%
MPSi:Eu (46mg/mL) 48.70
MPSi:Gd (50 mg/mL) 0.33
MPSi:Gd (10 mg/mL) 0.09
Energy (keV) Energy (keV)
(a) (b)
Fig. 1 XRF spectra of the following samples: (a) EuCl3 (46 mg/mL) and MPSi NPs with Eu3+ ions incorporated from aqueous solution with the same rare earth concentration; (b) GdCl3 (10 mg/mL), MPSi NPs with Gd3+ ions incorporated from aqueous solutions at different concentrations and pure MPSi NPs.
MPSi NPs
MPSi NPs+Gd (0.09 at.%) MPSi NPs+Gd (0.33 at.%)
10000
15000
20000
Time (ms) (a)
ro
N
a> £= en ca
a) (/) i_
(D >
CO
10000
Time (ms) (b)
15000
20000
Fig. 2 Transients of the longitudinal (a) and transverse (b) components of the proton magnetization for aqueous suspensions of MPSi NPs and those with incorporated Gd3+ ions with different concentration.
Fig. 2 shows transients of the longitudinal and transverse components of the proton magnetization for suspensions of MPSi:Gd NPs. While pure MPSi NPs do not significantly affect the proton relaxation, Gd3+ ions introduced into the NPs lead to a shortening of both the longitudinal and transverse relaxation times. Table 2 summarizes the times of longitudinal and transverse proton relaxation for pure MPSi NPs and those with incorporated rare earth ions. It is seen that MPSi:Eu NPs are not effective CA for MRI because of the well-known low relaxivity efficiency of Eu3+ ions, in contrast to MPSi NPs with Gd3+ ions, which decrease the relaxation times by 3 orders of magnitude.
green-blue light with wavelength of 465 nm, which corresponds to relatively weak absorption due to the 7Fo ^5D2 transition in Eu3+[19]. The possibility to excite MPSi:Eu NPs by visible light seems to be useful for bioimaging application [16].
Table 2 Data on the longitudinal (Ti) and transverse (T2) relaxation times for aqueous suspensions of MPSi NPs without and with rare earth incorporation as well for the corresponding lanthanide salts and water for comparison.
Sample_Ti (ms)_T2 (ms)
Water 4360 ± 10 3500 ± 10
0,0
500
600
700
800
900
1000
Wavelength (nm)
Fig. 3 PL spectra of aqueous solution of EuCl3, suspensions of MPSi and MPSi:Eu NPs.
Fig. 3 shows PL spectra of MPSi:Eu NPs, EuCl3, and pure MPSi NPs. The spectra of EuCl3 and MPSi NPs with incorporated Eu3+ ions exhibit characteristic narrow lines associated with the 5Do^7F4 transition in Eu3+ [16, 19]. The broad PL band in the region from 550 nm to 900 nm can be related to the radiative emission processes in the hydroxide layer of MPSi NPs [18] as well as in optically excited Eu2+ions [19]. It is interesting to note that in our experiments the Eu3+PL is efficiently excited by the
MPSi, 1 mg/mL
3784 ± 10 3452 ±10
EuCl3, 46 mg/mL 2297 ±10 2012±10
GdCl3, 1 mg/mL 3.2 ± 0.6 2.77 ± 0.06
MPSi:Gd (0.09 at. %), 1 mg/mL 2769 ±10 2540 ±10
565 ± 5
514 ± 5
MPSi:Gd (0.33 at. %), 1 mg/mL
4 Conclusions
Aqueous suspensions of mesoporous Si NPs with incorporated Eu3+ and Gd3+ ions are prepared by using mechanical grinding of the electrochemically grown mesoporous silicon films followed with impregnation by the rare earth in concentrated aqueous solutions of the corresponding lanthanide chlorides. It is established that Gd3+ ions incorporated into MPSi NPs even at low concentration about 0.33 at.% can significantly decrease both the longitudinal and transverse times of the proton relaxation in water molecules in aqueous media. This observation opens new possibilities for Gd-doped MPSi NPs as potential CA in biomedical MRI diagnostics. The good degree of binding and characteristic PL emission in the visible spectral range of Eu3+ ions are found for MPSi
NPs impregnated with rare earth in EuCl3 solutions. This optical property can be of interest for applications of Eu-doped MPSi NPs, for example, as fluorescent probes in bioimaging.
Disclosures
The authors have no relevant financial interests in this article and no potential conflicts of interest to disclose.
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