ES2992723B2 - METHOD FOR SYNTHESIS OF POROUS SILICON NANOPARTICLES WITH LONG-LIFE PHOTOLUMINESCENCE - Google Patents

Description

METHOD OF SYNTHESIS OF POROUS SILICON NANOPARTICLES WITH

LONG-LASTING PHOTOLUMINESCENCE

FIELD OF INVENTION

The present invention belongs to the technical field of photoluminescent materials and, more specifically, to a method of synthesizing porous silicon nanoparticles with long-lasting photoluminescence.

BACKGROUND OF THE INVENTION

Nanoparticles are gaining increasing importance in biomedicine and biotechnology due to their small size, multifunctionality, surface customization, and biocompatibility. Because of these characteristics, they are primarily used in these sectors as fluorescent biological markers or as optimal media for controlled drug release, tissue engineering, and cancer treatments, among other applications [1].

Among the wide variety of nanoparticles that are already applied or are currently being investigated, luminescent porous silicon nanoparticles (LPSiNPs) are those with the greatest development potential in the short, medium and long term, mainly due to their physicochemical and structural characteristics [2]:

- Due to their porosity, they have a high surface-to-volume ratio, making them an ideal material for storing medications in their structure and using them in controlled drug-release systems.

- The synthesis method, based on the anodization of monocrystalline silicon wafers, results in the formation of silicon quantum dots (QDs) that exhibit quantum confinement. These nanoparticles exhibit intense photoluminescence in the visible spectrum at room temperature when illuminated with ultraviolet light. This visible light emission makes them ideal for use as biological markers in both in vivo and in vitro applications.

- Due to the high reactivity of the porous silicon surface, nanoparticles exposed to an oxidizing environment form a matrix of silicon suboxides (SiO<x>, x = 2) that house the silicon quantum dots inside. This material turns out to be perfectly biocompatible [3].

- Porous silicon is a biodegradable material, as it degrades in the body to form orthosilicic acid, which is eliminated through urine.

- The process of synthesizing LPSiNPs by electrochemical anodization of monocrystalline silicon wafers is simple, economical, and very rapid.

There are some examples in the state of the art of the use of these LPSiNPs.

US20110300222A1 describes luminescent porous silicon nanoparticles (LPSiNPs) that can carry a drug payload and whose intrinsic near-infrared (NIR) photoluminescence allows monitoring of both accumulation and degradation in vivo. However, such photoluminescence of the nanoparticles lasts only 8 hours and the nanoparticles self-destruct into components that can be easily excreted or eliminated in a relatively short period of time.

All of this is because porous silicon is a very unstable material and degrades very easily. During the porous silicon synthesis process, the entire surface of the sheet is terminated in extremely reactive silicon-hydrogen (Si-H) bonds, causing the material to oxidize from the moment it is exposed to an oxidizing atmosphere.

On the one hand, if the oxidation is not optimal, dangling bonds will appear, which cause the recombination of electrons from excitons within the material, drastically reducing light emission (quenching).

On the other hand, if the film is oxidized excessively, the quantum dots can disappear, losing the quantum confinement effect (which means losing photoluminescence at room temperature). This problem is even greater when forming nanoparticles, since porous silicon films must be broken down into small particles, resulting in a much larger surface area exposed to the environment, which causes them to degrade even more rapidly.

Preserving the photoluminescence of LPSiNPs has been the subject of study in recent decades, as it opens the door to the effective implementation of this material in different applications in biomedicine and biotechnology where photoluminescence is of interest, especially in the case of in vivo and in vitro biological markers.

State-of-the-art documents can be found describing different methods to stabilize LPSiNPs to preserve their photoluminescence.

In the article by Park et al. [4], LPSiNPs were used for the controlled release of drugs in vivo. In this case, the surface of the nanoparticles was stabilized in deionized water for two weeks. There is no report on the evolution of the luminescence, but it is described that the monitoring of the nanoparticles lasted 4 weeks, during which time they were degraded in orthosilicic acid.

The article by La Ferrara et al. [5] describes the stabilization of porous silicon nanoparticles for transporting ascorbic acid. The stabilization is carried out with deionized water following the methodology of the article by Park et al. [4]. This article shows a study of the evolution of photoluminescence as a function of time, concluding that it stabilizes after the fifteenth day.

WO2011086210A1 describes the formation of porous silicon particles with magnetic material inside. Although the ability to respond magnetically is attractive when combined with the photoluminescence of porous silicon, the photoluminescence only lasts for two days.

In the study reported by Gallach et al. [6], porous silicon nanoparticles were functionalized using polyethylene glycol 600 (PEG-600) and 3-aminopropyltriethoxysilane (APTS) molecules, both in toluene solutions. In both cases, a decrease in luminescence over time was observed. Specifically, a 20% drop in luminescence was reported after 11 days in the case of PEG and 85% in the case of APTS.

US7371666B2 describes a method for the synthesis of porous silicon nanoparticles. The method involves reacting a silicon precursor in the presence of a gas (hydrogen, helium, argon, or nitrogen) with the heat provided by a CO2 laser to produce silicon nanoparticles, followed by electrochemical etching with a solution of hydrofluoric acid and nitric acid to produce photoluminescent silicon nanoparticles.

This patent also details methods for stabilizing the photoluminescence of photoluminescent silicon nanoparticles through surface oxidation. However, the graphs (see Figures 10 and 11) show that the photoluminescence decays over time (at 5 hours, the intensity is at low levels) relative to its original signal.

Therefore, there is a need for a method of obtaining these porous silicon nanoparticles that stabilizes the luminescence of the material and achieves long-lasting luminescence.

DESCRIPTION OF THE INVENTION

The present invention solves the problems existing in the state of the art by means of a method of synthesis of porous silicon nanoparticles, which exhibit long-lasting photoluminescence.

As discussed above, to achieve long-lived photoluminescence it is necessary to protect the quantum dots from the porous silicon matrix by surface passivation, such that two conditions are met: the formation of a protective oxide layer of SiO<x>(x = 2) and the disappearance of dangling bonds on the surface that can act as electron traps.

In the method of the present invention, these problems are solved by a method comprising two steps that ensure the formation of a protective oxide layer, as well as the elimination of dangling bonds.

In a first aspect, the present invention provides a method for synthesizing porous silicon nanoparticles with photoluminescence characterized in that it comprises the following steps:

i) provide porous silicon sheets and perform a heat treatment at 250-350°C for 4-6 minutes, and

ii) scrape the slides obtained from step i) and sonicate the slides dissolved in deionized water.

In step i), the formation of a protective oxide begins. However, the formation of this oxide layer is incomplete, so when the Si-H bonds are replaced by Si-O bonds, some incomplete bonds (dangling bonds) remain, which cause the initial decrease in photoluminescence (also called "quenching").

In a preferred embodiment, the heat treatment of step i) is carried out at 300°C for 5 minutes.

Next, in stage ii), a second oxidation process takes place that eliminates the dangling bonds, gradually restoring the photoluminescence and making it last over time.

In a preferred embodiment, the sonication of step ii) is performed for 15-30 minutes.

The synthesis method of the present invention is simple, inexpensive, rapid, and accessible, making it easy to implement at the industrial level. Furthermore, the method for preserving the particles is straightforward, as the nanoparticles are dissolved in deionized water and stored at room temperature. This method makes the resulting nanoparticles biocompatible and suitable for use in in vivo or in vitro applications as biological markers.

In a second aspect of the invention, the synthesis method of the present invention further comprises a step iii) in which the porous silicon nanoparticles with photoluminescence obtained in step ii) are subjected to a silanization process by bringing the nanoparticles into contact with a silane.

In a preferred embodiment of the second aspect of the invention, the silane is 3-aminopropyltriethoxysilane (APTS).

In a third aspect of the invention, the synthesis method of the present invention further comprises a step iii) in which the porous silicon nanoparticles with photoluminescence obtained in step ii) are subjected to a pegylation process by bringing the nanoparticles into contact with polyethylene glycol (PEG).

In the second and third aspect methods, porous silicon nanoparticles functionalized with organic molecules, such as PEG and APTS, are obtained, which do not lose luminescence.

In a final aspect of the invention, the porous silicon nanoparticles with photoluminescence obtained by the synthesis methods described above can be used as selective biological markers and nanodevices for controlled release of drugs systemically or in target cells.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1. Emission spectra of a sample of luminescent porous silicon nanoparticles synthesized by the method of the present invention, measured at time = 1 day, 4 days, 16 days, 37 days, 42 days and 310 days.

Figure 2. Photographs of a sample of luminescent porous silicon nanoparticles synthesized by the method of the present invention, taken at time = 1 day, 4 days, 16 days, 37 days and 42 days where the increase and conservation of luminescence over time is observed.

Figure 3. Emission spectra of a sample of luminescent porous silicon nanoparticles synthesized by the method of the present invention (without functionalization) and two samples synthesized by the method of the present invention, but with functionalization using polyethylene glycol (PEG) and 3-aminopropyltriethoxysilane (APTS).

DESCRIPTION OF IMPLEMENTATION MODES

Having described the present invention, it is further illustrated by the following examples. The purpose of the examples set forth below is to illustrate the invention without limiting its scope.

Example 1. Method for synthesizing porous silicon nanoparticles with photoluminescence

Porous silicon wafers were initially synthesized by electrochemical anodization of low-resistivity (0.01 < p < 0.02 Q-cm) p<+>-type Si wafers (B-doped) in a 48% hydrofluoric acid:Ethanol (1:2) solution. The electrochemical process was carried out at room temperature, without using additional illumination on the Si wafer. In this electrochemical process, a constant current density of 120 ± 9 mA-cm<-2> was applied for 30 ± 2 minutes, resulting in porous silicon wafers 22.4 ± 0.3 pm thick.

After electrochemical anodization, the steps of the method of the present invention took place.

The first step was a preliminary oxidation, in which the porous silicon wafers were annealed in air in a muffle furnace at 300 ± 50 °C for 5 ± 0.5 minutes. The objective of this passivation was to promote the growth of silicon suboxides (SiO<x>, with x = 2), producing an initial protective oxide layer. However, this layer was not sufficient to stabilize the entire porous silicon wafer due to inhomogeneous oxidation. Therefore, a second step was carried out.

After the annealing process, in a second step, the porous silicon wafers from the first step were stored in deionized water and sonicated in an ultrasonic bath for 20 minutes, producing a colloidal suspension of porous silicon nanoparticles.

The objective of this second stage is to provide greater oxidation of the particles and achieve a protective oxide layer that preserves photoluminescence over time by reducing the dangling bonds on the surface that remain after oxidation in the muffle furnace.

The resulting nanoparticles are dissolved in deionized water and stored at room temperature. The literature reports that porous silicon particles can be preserved in ethanol, but this method of preservation poses a challenge when using them in biomedicine.

Therefore, preservation in deionized water is advantageous, since, unlike ethanol, it is non-toxic to the biological environment. Therefore, the preserved product is biocompatible.

The preservation of this photoluminescence over time was evaluated using emission spectra measured at different times using a spectrofluorometer. Specifically, and as shown in Figure 1, measurements were taken after 1 day, 4 days, 16 days, 37 days, 42 days, and 310 days.

Figure 1 shows the emission spectra, showing how the luminescence reaches maximum intensity at 37 days and remains stable for at least 42 days. The measurement taken at 310 days shows a decrease in intensity, but it is still sufficient and is longer than reported in the literature. Furthermore, it is important to note that the nanoparticle emission does not shift, maintaining an emission range between 500 nm and 675 nm, which together results in a yellow-orange emission color.

Additionally, Figure 2 shows photographs of the samples in which the increase and conservation of luminescence over time is clearly observed.

Example 2. Method for the synthesis of porous silicon nanoparticles with photoluminescence functionalized with 3-aminopropyltriethoxysilane (APTS).

Porous silicon nanoparticles synthesized by the method of the present invention described in Example 1 were used as starting material.

These nanoparticles were subjected to a silanization process by contacting them with a silane. In this example, aminopropyltriethoxysilane (APTS) was used as the silane.

The nanoparticles were deposited on 2 mL of a 1:1 APTS:deionized water solution, then heat-treated and sonicated for approximately 30 minutes. Approximately 30 hours later, the samples were centrifuged three to five times, replacing the medium with ethanol to clean them. During the final medium change, deionized water was added instead of ethanol, and the samples were sonicated for approximately 45 minutes.

Example 3. Method for the synthesis of porous silicon nanoparticles with photoluminescence functionalized with polyethylene glycol (PEG)

Porous silicon nanoparticles synthesized by the method of the present invention described in Example 1 were used as starting material.

These nanoparticles were subjected to a pegylation process by placing the nanoparticles in contact with a polyethylene glycol (PEG).

The nanoparticles were deposited onto 2 mL of a 1:1 PEG:deionized water solution, then heat-treated and sonicated for approximately 1 hour. The sample was then washed twice with deionized water, and the supernatant was removed.

Figure 3 shows the photoluminescent emission spectra of samples of stabilized porous silicon nanoparticles (PS), stabilized porous silicon nanoparticles functionalized with polyethylene glycol (PEG), and stabilized porous silicon nanoparticles functionalized with 3-aminopropyltriethoxysilane (APTS). The luminescence of the functionalized nanoparticles is observed to be in the same range as that of porous silicon and persists 310 days after synthesis. However, it is interesting to note that, unlike those functionalized with polyethylene glycol, the APTS-functionalized nanoparticles have lower emission at wavelengths close to red, which makes their appearance more yellow. In the case of PEG-functionalized nanoparticles, the contribution of the different emitted wavelengths is very similar to that of the unfunctionalized porous silicon nanoparticles (orange-red).

BIBLIOGRAPHIC REFERENCES

[1] Salata, O. V. (2004). Applications of nanoparticles in biology and medicine. Journal of Nanobiotechnology, 2(1), 3.

[2] Granitzer, P., & Rumpf, K. (2010). Porous Silicon-A Versatile Host Material. Materials, 3(2), 943-998.

[3] Low, S. P., Voelcker, N. H., Canham, L. T., & Williams, K. A. (2009). The biocompatibility of porous silicone in tissues of the eye. Biomaterials, 30(15), 2873-2880.

[4] Park, J.-H., Gu, L., von Maltzahn, G., Ruoslahti, E., Bhatia, S. N., & Sailor, M. J. (2009).

Biodegradable luminescent porous silicon nanoparticles for in vivo applications. Nature Materials, 8(4), 331-336.

[5] La Ferrara, V., Fiorentino, G., Rametta, G., & Di Francia, G. (2012). Luminescence quenching of porous silicon nanoparticles in presence of ascorbic acid. physica status solidi (a), 209(4), 736-740.

[6] Gallach, D., Recio Sanchez, G., Muñoz Noval, A., Manso Silvan, M., Ceccone, G., Martin Palma, R. J., Martinez Duart, J. M. (2010). Functionality of porous silicon particles: Surface modification for biomedical applications. Materials Science and Engineering B-Advanced Functional Solid-State Materials, 169(1-3), 123-127.

Claims (7)

1. Method for synthesizing porous silicon nanoparticles with photoluminescence characterized in that it comprises the following steps: i) provide porous silicon wafers and heat treat them at 250-350 °C for 4-6 minutes, and ii) scrape the slides obtained from step i) and sonicate the slides dissolved in deionized water. 2. Synthesis method according to claim 1, wherein the heat treatment of step i) is carried out at 300°C for 5 minutes. 3. Synthesis method according to claim 1 or 2, wherein the sonication of step ii) is carried out for 15-30 minutes. 4. Synthesis method according to any of claims 1 to 3, wherein the method further comprises a step iii) in which the porous silicon nanoparticles with photoluminescence obtained in step ii) are subjected to a silanization process by bringing the nanoparticles into contact with a silane. 5. Synthesis method according to claim 4, wherein the silane is 3-aminopropyltriethoxysilane. 6. Synthesis method according to any of claims 1 to 3, wherein the method further comprises a step iii) in which the porous silicon nanoparticles with photoluminescence obtained in step ii) are subjected to a pegylation process by bringing the nanoparticles into contact with polyethylene glycol (PEG). 7. Use of porous silicon nanoparticles with photoluminescence obtained by the synthesis method according to any of claims 1 to 6, as selective biological markers and nanodevices for controlled release of drugs systemically or in target cells.