Recent Developments in Porous Silicon Nanovectors with Various Imaging Modalities in the Framework of Theranostics.

The number of in vitro, ex vivo, and in vivo studies on porous silicon (PSi) nanoparticles for biomedical applications has increased extensively over the last decade. The focus of the reports has been on the carrier properties of PSi concerning the therapeutic aspect due to several beneficial nanovector characteristics including high payload capacity, biocompatibility, and versatile surface chemistry. Recently, increasing attention has been paid to the diagnostic aspects of PSi, which is typically attributed to the biotraceability of the nanovector. Also, PSi has been studied as a contrast agent. When both these aspects, therapy and diagnosis, are integrated into one nanovector, we can discuss a real nanotheranostics approach. Herein, we review the recent progress developing PSi for various imaging modalities, specifically focusing on optical imaging, magnetic resonance imaging, and nuclear medicine imaging. Furthermore, we summarized the knowledge gaps that must be covered before applying PSi in clinical imaging, highlighting future research trends.

Comparison between Fluorescence Imaging and Elemental Analysis to Determine Biodistribution of Inorganic Nanoparticles with Strong Light Absorption.

Black porous silicon nanoparticles (BPSi NPs) are known as highly efficient infrared light absorbers that are well-suitable for photothermal therapy (PTT) and photoacoustic imaging (PAI). PTT and PAI require a sufficient number of effectively light-absorbing NPs to be accumulated in tumor after intravenous administration. Herein, biodistribution of PEGylated BPSi NPs with different sizes (i.e., 140, 200, and 300 nm in diameter) is investigated after intravenous administration in mice. BPSi NPs were conjugated with fluorescent dyes Cy5.5 and Cy7.5 to track them in vitro and in vivo, respectively. Optical imaging with an in vivo imaging system (IVIS) was found to be an inadequate technique to assess the biodistribution of the dye-labeled BPSi NPs in vivo because the intrinsic strong absorbance of the BPSi NPs interfered fluorescence detection. This challenge was resolved via the use of inductively coupled plasma optical emission spectrometry to analyze ex vivo the silicon content in different tissues and tumors. The results indicated that most of the polyethylene glycol-coated BPSi NPs were found to accumulate in the liver and spleen after intravenous injection. The smallest 140 nm particles accumulated the most in tumors at an amount of 9.5 ± 3.4% of the injected dose (concentration of 0.18 ± 0.08 mg/mL), the amount known to produce sufficient heat for cancer PTT. Furthermore, the findings from the present study also suggest that techniques other than optical imaging should be considered to study the organ biodistribution of NPs with strong light absorbance properties.

Low-Load Metal-Assisted Catalytic Etching Produces Scalable Porosity in Si Powders.

The recently discovered low-load metal-assisted catalytic etching (LL-MACE) creates nanostructured Si with controllable and variable characteristics that distinguish this technique from the conventional high-load variant. LL-MACE employs 150 times less metal catalyst and produces porous Si instead of Si nanowires. In this work, we demonstrate that some of the features of LL-MACE cannot be explained by the present understanding of MACE. With mechanistic insight derived from extensive experimentation, it is demonstrated that (1) the method allows the use of not only Ag, Pd, Pt, and Au as metal catalysts but also Cu and (2) judicious combinations of process parameters such as the type of metal, Si doping levels, and etching temperatures facilitate control over yield (0.065-88%), pore size (3-100 nm), specific surface area (20-310 m2·g-1), and specific pore volume (0.05-1.05 cm3·g-1). The porous structure of the product depends on the space-charge layer, which is controlled by the Si doping and the chemical identity of the deposited metal. The porous structure was also dependent on the dynamic structure of the deposited metal. A distinctive comet-like structure of metal nanoparticles was observed after etching with Cu, Ag, Pd, and, in some cases, Pt; this structure consisted of 10-50 nm main particles surrounded by smaller (<5 nm) nanoparticles. With good scalability and precise control of structural properties, LL-MACE facilitates Si applications in photovoltaics, energy storage, biomedicine, and water purification.

Thermal dose as a universal tool to evaluate nanoparticle-induced photothermal therapy.

Thermal isoeffect dose (TID) is a widely applied concept to evaluate the safety of medical devices that can expose patients to heat. However, it has rarely been used in photothermal therapy (PTT), where nanoparticles are used as light absorbers. Utilizing TID in an appropriate way would make it feasible to compare the results obtained with different light absorbers as well as clarifying their cellular effects. Herein, we apply TID as a definitive parameter to evaluate the outcomes of a nanoparticle-induced PTT in vitro. We show that cell death measured with an ATP-based viability assay and flow cytometry can be correlated with TID if time-temperature data is available. As an experimental model, black porous silicon nanoparticles were studied as photothermal agents to kill HeLa cancer cells. The results indicate that as the critical TID of 70 min is reached, the cells start to undergo apoptosis independently of the way in which the TID was attained: by long heating at low temperatures or by short heating at high temperatures. Overall, TID is proposed as a valid parameter which could be determined in the PTT studies to allow a straightforward comparison of the published results and the elucidation of the cell death mechanisms.

Controlling the Nature of Etched Si Nanostructures: High- versus Low-Load Metal-Assisted Catalytic Etching (MACE) of Si Powders.

Metal-assisted catalytic etching (MACE) involving Ag deposited on Si particles has been reported as a facile method for the production of Si nanowires (Si NWs). We show that the structure of Si particles subjected to MACE changes dramatically in response to changing the loading of the Ag catalyst. The use of acetic acid as a surfactant and controlled injection of AgNO3(aq) enhanced Ag deposition. The use of acetic acid and controlled injection of H2O2 not only facilitated optimization of the etching step but also allowed us to identify a previously unobserved etching regime that we denote as low-load MACE (LL-MACE). Material produced by LL-MACE exhibits dramatically different yield and structural characteristics as compared to conventionally produced material. We demonstrate the production of Si NWs as well as mesoporous Si nanoparticles from an inexpensive metallurgical-grade Si powder. High loading of Ag (HL-MACE) generates parallel etch track pores created by the correlated motion of Ag nanoparticles. The uniform size distribution (predominantly 70-100 nm) of the Ag nanoparticles is generated dynamically during etching. The walls of these etch track pores are cleaved readily by ultrasonic agitation to form Si NWs. Low loading of Ag (LL-MACE) creates 10-50 nm Ag nanoparticles that etch in an uncorrelated (randomly directed) fashion to generate a bimodal distribution of mesoporosity peaking at ∼4 and 13-21 nm. The use of a syringe pump to deliver the oxidant (H2O2) and Ag+ is essential for increased product uniformity and yield. Different process temperatures and grades of Si produced significantly different pore size distributions. These results facilitate the production of Si NWs and mesoporous nanoparticles with high yield, low cost, and controlled properties that are suitable for applications in, e.g., lithium-ion batteries, drug delivery, as well as biomedical imaging and contrast enhancement.

Radiofrequency Hyperthermia of Cancer Cells Enhanced by Silicic Acid Ions Released During the Biodegradation of Porous Silicon Nanowires.

The radiofrequency (RF) mild hyperthermia effect sensitized by biodegradable nanoparticles is a promising approach for therapy and diagnostics of numerous human diseases including cancer. Herein, we report the significant enhancement of local destruction of cancer cells induced by RF hyperthermia in the presence of degraded low-toxic porous silicon (PSi) nanowires (NWs). Proper selection of RF irradiation time (10 min), intensity, concentration of PSi NWs, and incubation time (24 h) decreased cell viability to 10%, which can be potentially used for cancer treatment. The incubation for 24 h is critical for degradation of PSi NWs and the formation of silicic acid ions H+ and H3SiO4 - in abundance. The ions drastically change the solution conductivity in the vicinity of PSi NWs, which enhances the absorption of RF radiation and increases the hyperthermia effect. The high biodegradability and efficient photoluminescence of PSi NWs were governed by their mesoporous structure. The average size of pores was 10 nm, and the sizes of silicon nanocrystals (quantum dots) were 3-5 nm. Degradation of PSi NWs was observed as a significant decrease of optical absorbance, photoluminescence, and Raman signals of PSi NW suspensions after 24 h of incubation. Localization of PSi NWs at cell membranes revealed by confocal microscopy suggested that thermal poration of membranes could cause cell death. Thus, efficient photoluminescence in combination with RF-induced cell membrane breakdown indicates promising opportunities for theranostic applications of PSi NWs.

Cavitation Induced by Janus-Like Mesoporous Silicon Nanoparticles Enhances Ultrasound Hyperthermia.

The presence of nanoparticles lowers the levels of ultrasound (US) intensity needed to achieve the therapeutic effect and improves the contrast between healthy and pathological tissues. Here, we evaluate the role of two main mechanisms that contribute to the US-induced heating of aqueous suspensions of biodegradable nanoparticles (NPs) of mesoporous silicon prepared by electrochemical etching of heavily boron-doped crystalline silicon wafers in a hydrofluoric acid solution. The first mechanism is associated with an increase of the attenuation of US in the presence of NPs due to additional scattering and viscous dissipation, which was numerically simulated and compared to the experimental data. The second mechanism is caused by acoustic cavitation leading to intense bubble collapse and energy release in the vicinity of NPs. This effect is found to be pronounced for as-called Janus NPs produced via a nano-stopper technique, which allow us to prepare mesoporous NPs with hydrophobic inner pore walls and hydrophilic external surface. Such Janus-like NPs trap air inside the pores when dispersed in water. The precise measurement of the heating dynamics in situ enabled us to detect the excessive heat production by Janus-like NPs over their completely hydrophilic counterparts. The excessive heat is attributed to the high intensity cavitation in the suspension of Janus-like NPs. The present work elicits the potential of specifically designed Janus-like mesoporous silicon NPs in the field of nanotheranostics based on ultrasound radiation.

Biodegradable Porous Silicon Nanocontainers as an Effective Drug Carrier for Regulation of the Tumor Cell Death Pathways.

Nanocontainers based on solid materials have great potential for drug delivery applications. However, since nanocontainer-mediated delivery can alter the drug internalization pathways and metabolism, it is important to find out what are the mechanisms of cancer cell death induced by nanocontainers and, moreover, is it possible to regulate them. Here, we report on the detailed investigation of the internalization kinetics and intracellular spatial distribution of porous silicon nanoparticles (PSi NPs) loaded with doxorubicin (DOX) and response of cancer cells to treatment with DOX-PSi NPs as well as studies of nanocontainer biodegradation by applying various microscopy methods, Raman microspectroscopy and biological experiments with cancer cells of different etiology. The obtained results revealed the absence of toxicity of unloaded PSi NPs to cancer cells up to a concentration of 700 µg/mL during the prolonged incubation time. Thus, given the fact that the nanocontainers themselves are not toxic, it is easy to adjust the dose of the drug that they deliver to the cells. It is shown, that the treatment with DOX-loaded PSi NPs more efficiently eliminates cancer cells in comparison with the free DOX. At the same time, the obtained results demonstrate the possibility of regulating the initiation of apoptosis or necrosis in tumor cells after treatment with different concentrations of DOX-PSi NPs, as revealed by the analysis of the caspase-3 processing, the accumulation of sub-G1 cell fraction, and morphological changes determined by electron and light microscopy. The obtained results are important for future applications of porous silicon nanocontainers in drug delivery for apoptotic pathway-targeted cancer therapy.

Scalable Synthesis of Biodegradable Black Mesoporous Silicon Nanoparticles for Highly Efficient Photothermal Therapy.

Porous silicon (PSi) has attracted wide interest as a potential material for various fields of nanomedicine. However, until now, the application of PSi in photothermal therapy has not been successful due to its low photothermal conversion efficiency. In the present study, biodegradable black PSi (BPSi) nanoparticles were designed and prepared via a high-yield and simple reaction. The PSi nanoparticles possessed a low band gap of 1.34 eV, a high extinction coefficient of 13.2 L/g/cm at 808 nm, a high photothermal conversion efficiency of 33.6%, good photostability, and a large surface area. The nanoparticles had not only excellent photothermal properties surpassing most of the present inorganic photothermal conversion agents (PCAs) but they also displayed good biodegradability, a common problem encountered with the inorganic PCAs. The functionality of the BPSi nanoparticles in photothermal therapy was verified in tumor-bearing mice in vivo. These results showed clearly that the photothermal treatment was highly efficient to inhibit tumor growth. The designed PCA material of BPSi is robust, easy to prepare, biocompatible, and therapeutically extremely efficient and it can be integrated with several other functionalities on the basis of simple silicon chemistry.

Nano Air Seeds Trapped in Mesoporous Janus Nanoparticles Facilitate Cavitation and Enhance Ultrasound Imaging.

The current contrast agents utilized in ultrasound (US) imaging are based on microbubbles which suffer from a short lifetime in systemic circulation. The present study introduces a new type of contrast agent for US imaging based on bioresorbable Janus nanoparticles (NPs) that are able to generate microbubbles in situ under US radiation for extended time. The Janus NPs are based on porous silicon (PSi) that was modified via a nanostopper technique. The technique was exploited to prepare PSi NPs which had hydrophobic pore walls (inner face), while the external surfaces of the NPs (outer face) were hydrophilic. As a consequence, when dispersed in an aqueous solution, the Janus NPs contained a substantial amount of air trapped in their nanopores. The specific experimental setup was developed to prove that these nano air seeds were indeed acting as nuclei for microbubble growth during US radiation. Using the setup, the cavitation thresholds of the Janus NPs were compared to their completely hydrophilic counterparts by detecting the subharmonic signals from the microbubbles. These experiments and the numerical simulations of the bubble dynamics demonstrated that the Janus NPs generated microbubbles with a radii of 1.1 µm. Furthermore, the microbubbles generated by the NPs were detected with a conventional medical ultrasound imaging device. Long systemic circulation time was ensured by grafting the NPs with two different PEG polymers, which did not affect adversely the microbubble generation. The present findings represent an important landmark in the development of ultrasound contrast agents which possess the properties for both diagnostics and therapy.

Electrolytic conductivity-related radiofrequency heating of aqueous suspensions of nanoparticles for biomedicine.

The development of suitable contrast agents can significantly enhance the efficiency of modern imaging and treatment techniques, such as thermoacoustic (TA) tomography and radio-frequency (RF) hyperthermia of cancer. Here, we examine the heating of aqueous suspensions of silicon (Si) and gold (Au) nanoparticles (NPs) under RF irradiation in the MHz frequency range. The heating rate of aqueous suspensions of Si NPs exhibited non-monotonic dependency on the electrical conductivity of the suspension. The experimental results were explained by the mathematical model considering oscillating solvated ions as the main source of Joule heating. These ions could be the product of the dissolution of Si NPs or organic coating of Au NPs. Thus, the ions governed the conductivity of the suspensions, which in turn governs both the heating rate and the near-field RF TA response. The model predicted the contrast in different tissues taking into account both Joule heating and dielectric losses.

Temperature responsive porous silicon nanoparticles for cancer therapy - spatiotemporal triggering through infrared and radiofrequency electromagnetic heating.

One critical functionality of the carrier system utilized in targeted drug delivery is its ability to trigger the release of the therapeutic cargo once the carrier has reached its target. External triggering is an alluring approach as it can be applied in a precise spatiotemporal manner. In the present study, we achieved external triggering through the porous silicon (PSi) nanoparticles (NPs) by providing a pulse of infrared or radiofrequency radiation. The NPs were grafted with a temperature responsive polymer whose critical temperature was tailored to be slightly above 37°C. The polymer coating improved the biocompatibility of the NPs significantly in comparison with their uncoated counterparts. Radiation induced a rapid temperature rise, which resulted in the collapse of the polymer chains facilitating the cargo release. Both infrared and radiofrequency radiation were able to efficiently trigger the release of the encapsulated drug in vitro and induce significant cell death in comparison to the control groups. Radiofrequency radiation was found to be more efficient in vitro, and the treatment efficacy was verified in vivo in a lung carcinoma (3LL) mice model. After a single intratumoral administration of the carrier system combined with radiofrequency radiation, there was clear suppression of the growth of the carcinoma and a prolongation of the survival time of the animals. TOC IMAGE: The temperature responsive (TR) polymer grafted on the surface of porous silicon nanoparticles (PSi NPs) changes its conformation in response to the heating induced by infrared or radiofrequency radiation. The conformation change allows the loaded doxorubicin to escape from the pores, achieving controlled drug release from TR PSi NPs, which displayed efficacy against malignant cells both in vitro and in vivo.

Radio frequency radiation-induced hyperthermia using Si nanoparticle-based sensitizers for mild cancer therapy.

Offering mild, non-invasive and deep cancer therapy modality, radio frequency (RF) radiation-induced hyperthermia lacks for efficient biodegradable RF sensitizers to selectively target cancer cells and thus avoid side effects. Here, we assess crystalline silicon (Si) based nanomaterials as sensitizers for the RF-induced therapy. Using nanoparticles produced by mechanical grinding of porous silicon and ultraclean laser-ablative synthesis, we report efficient RF-induced heating of aqueous suspensions of the nanoparticles to temperatures above 45-50 °C under relatively low nanoparticle concentrations (<1 mg/mL) and RF radiation intensities (1-5 W/cm(2)). For both types of nanoparticles the heating rate was linearly dependent on nanoparticle concentration, while laser-ablated nanoparticles demonstrated a remarkably higher heating rate than porous silicon-based ones for the whole range of the used concentrations from 0.01 to 0.4 mg/mL. The observed effect is explained by the Joule heating due to the generation of electrical currents at the nanoparticle/water interface. Profiting from the nanoparticle-based hyperthermia, we demonstrate an efficient treatment of Lewis lung carcinoma in vivo. Combined with the possibility of involvement of parallel imaging and treatment channels based on unique optical properties of Si-based nanomaterials, the proposed method promises a new landmark in the development of new modalities for mild cancer therapy.

Photoluminescent biocompatible silicon nanoparticles for cancer theranostic applications.

Silicon nanoparticles (SiNPs) obtained by mechanical grinding of porous silicon have been used for visualization of living cells in vitro. It was found that SiNPs could penetrate into the cells without any cytotoxic effect up to the concentration of 100 µg/ml. The cell cytoplasm was observed to be filled by SiNPs, which exhibited bright photoluminescence at 1.6 eV. SiNPs could also act as photosensitizers of the singlet oxygen generation, which could be used in the photodynamic therapy of cancer. These properties of SiNPs are discussed in view of possible applications in theranostics (both in therapy and in diagnostics).