Luminescent Porous Silicon Research Articles

Plasmonic-enhanced photoluminescence in porous silicon with pore-embedded gold nanoparticles fabricated by direct reduction of chloroauric acid

The low efficiency of porous silicon (p-Si) luminescence hinders the development of silicon-based optoelectronic devices. The increase in p-Si emission using near-field enhancement, owing to the incorporation of gold nanoparticles (AuNPs) into the photonic structure, is probably the most viable alternative. However, the coupling of plasmon resonance to p-Si emission is challenging because of the difficulty in controlling the size and location of the AuNPs with respect to the emissive p-Si layer. In this study, AuNPs were synthesized by clean direct reduction of chloroauric acid inside a p-Si photonic structure. As a result, AuNPs could be synthesized all along the pores of the p-Si structure, allowing for a six-fold enhancement of the p-Si photoluminescence, specifically for the emission band at 567 nm owing to the plasmon effect. Possible applications of this hybrid material include light-emitting devices and photoluminescence-based sensors.

Metal Filled Nanostructured Silicon a Platform to Interlink Magnetism and Optics

This presentation deals with the utilization of nanostructured silicon (porous silicon and silicon nanotubes) for deposition of various metals, especially magnetic ones, within the pores/tubes. The novel magnetic properties of the semiconducting/magnetic composites which arise due to the nanoscopic size of the used materials are investigated with respect to optical and magnetic on-chip applications.Porous silicon is fabricated by anodization of a silicon wafer in an aqueous hydrofluoric solution. The morphology of the porous structures depends on the doping density of the used wafer and on the applied current density as well as the electrolyte concentration. The ferromagnetic metals are deposited within the porous structures electrochemically in using the corresponding metal salt solution as electrolyte. A modification of the electrochemical parameters results in adjustable size and shape of the deposits. Ni and Co, both metals are electrodeposited within the nanostructured silicon in using aqueous NiSO4 and CoSO4 solutions by applying a current density between 10 and 20 mA/cm2 and a frequency between 0.05 and 0.2 Hz.One of our key topics is luminescent porous silicon loaded with magnetic metals to enhance the photoluminescence, with the final aim to influence/control the optical properties by a magnetic field. The metal deposits affect the optical properties but also give rise to specific magnetic behavior (1). Due to the metal filling of the porous silicon the photoluminescence is blue-shifted and furthermore an increase of the intensity is observed. The influence of the magnetic metal filling on the optical properties (photoluminescence, decay time) is discussed, and the magnetic characterization of the nanocomposites is presented.A further issue, the deposition of hard and soft magnetic materials within the nanostructures, aiming in the fabrication of arrays of permanent nanomagnets is presented (2). Here the investigation of the magnetic behavior of bi-metal nanostructures within nanostructured silicon with the aim to exploit the magnetic properties of both metals and gain control of the exchange coupling between the two metals especially with respect to their volume ratio is discussed. Furthermore, a variation of the structure size and the proximity of the metal deposits modify the exchange coupling and thus the energy product. Nanocomposite systems with an energy product as high as possible should be achieved to give rise to on-chip applications using permanent nanomagnets, especially arranged in arrays.(1) P. Granitzer, et al, Frontiers in Physics, 8 (2020) 121(2) K. Rumpf, et.al, Phys. Stat. Sol. A, 217 (2020) 1901040

Detection of chemical warfare agent simulants based on luminescent distributed Bragg reflector porous silicon

Detection of chemical warfare agent simulants based on luminescent distributed Bragg reflector porous silicon

Study of the Effect of Nitric Acid in Electrochemically Synthesized Silicon Nanocrystals: Tunability of Bright and Uniform Photoluminescence

In this work, we show a correlation between the composition and the microstructural and optical properties of bright and uniform luminescent porous silicon (PSi) films. PSi films were synthesized by electrochemical etching using nitric acid in an electrolyte solution. PSi samples synthesized with nitric acid emit stronger (up to six-fold greater) photoluminescence (PL) as compared to those obtained without it. The PL peak is shifted from 630 to 570 nm by changing the concentration ratio of the HF:HNO3:(EtOH-H2O) electrolyte solution, but also shifts with the excitation energy, indicating quantum confinement effects in the silicon nanocrystals (Si-NCs). X-ray photoelectron spectroscopy analysis shows a uniform silicon content in the PSi samples that emit the strongest PL. High-resolution transmission electron microscopy reveals that the Si-NCs in these PSi samples are about ~2.9 ± 0.76 nm in size and are embedded in a dense and stoichiometric SiO2 matrix, as indicated by the Fourier transform infrared analysis. On the other hand, the PSi films that show PL of low intensity present an abrupt change in the silicon content depth and the formation of non-bridging oxygen hole center defects.

Fabrication and performance of a superhydrophobic fluorine-modified porous silicon based on photocatalytic hydrosilylation

Porous silicon was widely used in displays, optical sensors, and biological imaging due to its high surface areas, adjustable microstructure, large adsorption capacity and excellent optical performance. Unfortunately, there are numerous reactive silane groups on the freshly etched porous silicon surface, which are susceptible to oxidization in the air, thereby affecting the optical performance and stability of porous silicon. This study provides an efficient strategy of using photocatalytic reaction to graft dodecafluoroheptyl methacrylate to freshly prepared n-type porous silicon surface, thereby generating a fluorine-modified porous silicon with high stability and superhydrophobic surface. The structure and composition of porous silicon before and after modification were analyzed by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). The effects of UV light, reactant mass fraction and reaction time on the surface properties of porous silicon were studied by water contact angle (WCA) and photoluminescence (PL). The results indicated that the dodecafluoroheptyl methacrylate was successfully grafted onto n-type porous silicon surface by photocatalytic reaction. The superhydrophobic and luminescent porous silicon was prepared by reacting 40 min in a dichloride solution containing 10 wt% dodecafluoroheptyl methacrylate under UV light. The WCA of the fluorine-modified porous silicon surface could reach 153° and had good photoluminescence intensity. This modified porous silicon surface had great stability in alkaline and air environment. After treated with alkaline droplets for 30 min, the WCA of modified porous silicon could maintain above 150° and had good photoluminescence. And under room temperature within a week, the porous silicon surface still showed great hydrophobic properties and photoluminescence intensity. In addition, the modified porous silicon has different responsiveness to ethanol solutions of different concentrations, which could be applied to alcohol sensors and has great development prospects. • A fluorine-modified porous silicon was synthesized by a simple and efficient photocatalytic reaction. • The WCA of the fluorine-modified porous silicon surface could reach 153°. • The photoluminescence intensity showed a great stability in alkaline and air environment.

Enhanced explosive sensing based on silole-modified luminescent porous silicon

Enhanced explosive sensing based on silole-modified luminescent porous silicon

Metal Filled Light Emitting Porous Silicon as Platform for Tunable Optical and Magnetic Properties

The electrochemical filling of luminescent microporous silicon (PSi) with magnetic metals (Ni, Co) is presented which results in a composite system with specific magnetic and optical properties. The achieved physical properties are compared with mesoporous silicon loaded with Ni nanoparticles (NPs) of comparable size.The light emitting PSi samples are fabricated by electrochemical etching of a moderately doped p-type silicon wafer in a 10 wt% hydrofluoric acid solution. The achieved porous silicon offers a branched morphology with structure sizes in the range of a few nanometers. The observable visible luminescence is due to quantum confinement effects [1]. The challenging metal filling of these pores is carried out under cathodic conditions by pulsed electrodeposition. As electrolytes aqueous NiSO4 and CoSO4 solutions are employed. Correlated to the branched morphology of the PSi and thus also the deposited nanosized metal structures are interconnected. The structure size of the porous samples is estimated by photoluminescence measurements to 5 nm which also determines the diameter of the deposits.Mesoporous silicon is produced by anodization of a highly n-doped silicon wafer resulting in oriented pores with an average diameter of 50 nm [2]. The loading of these samples with Ni NPs is also carried out by electrodeposition. In contrast to microporous silicon the morphology of mesoporous silicon (mPSi) shows straight pores, grown perpendicular to the sample surface with a diameter up to 50 nm.Magnetic filling of meso- and microporous silicon, respectively means a change in the morphology as well as in the structure size of the magnetic precipitations and both influence the magnetic behavior of the composite system drastically.In the case of light emitting PSi also the luminescence is influenced in its intensity and wavelength by the magnetic filling. With enhanced metal filling a blueshift of the photoluminescence peak is observed.The magnetic properties are investigated with a Vibrating Sample Magnetometer (VSM) and a SQUID, respectively. A luminescence spectrometer is used to measure the intensity and the peak position with respect to the wavelength of the photoluminescence spectrum. The structure is investigated by SEM and TEM.Field dependent magnetization measurements show a distinct difference between metal filled luminescent PSi and mPSi loaded with Ni NPs. Considering luminescent PSi filled with Ni and Co, respectively a strong magnetic anisotropy between the two magnetization directions applying a magnetic field perpendicular and parallel to the surface, is observed, whereas in the case of Co this behavior is even more pronounced. The coercivities do not vary in a broad range between Ni and Co samples, mostly around 10% but the saturation field differs drastically between the two materials. Figure 1 depicts field dependent magnetization curves of luminescent porous silicon filled with Ni and Co, respectively. In contrast mesoporous silicon loaded with Ni NPs offers only a small magnetic anisotropy and also the role of easy axis and hard axis is changed. This different behavior of the two systems can be explained by the differing magnetic interactions between the metal deposits in the two systems. The presented systems which merge optical and magnetic properties within on system are of interest for optoelectronics and magneto optical on-chip devices.[1] Handbook of Porous Silicon, Ed. L. Canham, Springer Int. Publishing (2018).[2] P. Granitzer, K. Rumpf, Front. Phys. 8, 121 (2020). Figure 1

Graphene quantum dot-decorated luminescent porous silicon dressing for theranostics of diabetic wounds

Diabetic wound healing is highly desirable but remains a great challenge owing to the continuous damage of excess reactive oxygen species (ROS) and degradation of therapeutic peptide drugs by over-expressed matrix metalloproteinase (MMP). Herein, we developed a stimuli-responsive smart dressing for theranostics of diabetic wounds using graphene quantum dots-decorated luminescent porous silicon (GQDs@PSi), which was further loaded with peptide and embedded in chitosan (CS) film. The confinement of GQDs in nanochannels of PSi endowed GQDs@PSi with efficient fluorescence resonance energy transfer (FRET) effect, leading to initial red fluorescence of PSi with complete quench of GQD's blue fluorescence. Furthermore, the decoration of GQDs on PSi surface significantly enhanced the loading capacity for peptide drugs including epidermal growth factor (EGF) and insulin (Ins) which can promote diabetic wounds healing. The peptides coloaded in GQDs@PSi exhibited sustained release behavior and could be protected in presence of MMP owing to size exclusion of PSi's nanochannels. As H2O2-triggered oxidation of PSi lead to weakened FRET effect and degradation of PSi, GQDs@PSi demonstrated H2O2-responsive ratiometric fluorescence change (from red PSi to blue GQDs) and drug release behavior. In combination with CS's degradation in the acidic and oxidation microenvironment, the smart dressing also showed stimuli-responsive drug release toward slightly acid and highly oxidative conditions in diabetic wounds. In vitro and in vivo results demonstrated the smart dressing enhanced the proliferation and migration of cells as well as significantly healed diabetic wounds. Real-time indicating of the exacerbation or healing of diabetic wounds was also realized using the rate of fluorescent discoloration of the dressing. Statement of significanceIn this work, a dual luminescent nanomaterial was created by hosting graphene quantum dots (GQDs) in the nanochannel of porous silicon (PSi), which was further applied for theranostics of diabetic wound. The synergistic effect of the host-guest nanohybrid is significant. The GQDs can significantly improve the capacity for peptide drug loading and form a stimuli-response visual ratiometric sensor with luminescent PSi, which can also protect and sustain release of peptide drugs for effective diabetic wounds treatment. After embedded in a chitosan film, the smart dressing displayed H2O2-responsive visual ratiometric fluorescence change and drug release behavior. In vitro and in vivo results demonstrated the smart dressing enhanced the proliferation and migration of cells as well as significantly healed diabetic wounds.

Absorption and emission of porous silicon based on quantum dots models by TD-DFT: Experimental and theoretical approach

In the present work, the relationship between experimental and theoretical analysis of optical properties of Si-nanostructures is presented. Coral-like luminescent porous silicon (p-Si) thin films were synthesized, and the main photoluminescence peak was found at around 690 nm. Infrared spectroscopy revealed mainly the presence of Si–H and Si–O bonds. The structural characterization determined an average nanocrystal size of 5.4 nm. A model based on Si quantum dots (QDs) with sizes ranging from 1.0 to 2.2 nm by TD-DFT (PBE/6-31G) in the Gaussian-16 package was used to simulate p-Si nanostructures. Equations were proposed for QD-like morphology of p-Si to calculate absorption/emission energies that fit with experimental data with high accuracy. The present research will help to predict the optical properties of complex and heterogeneous Si nanostructures.

A co-delivery platform for synergistic promotion of angiogenesis based on biodegradable, therapeutic and self-reporting luminescent porous silicon microparticles

Insufficient angiogenesis happened in body defects such as ulceration, coronary heart disease, and chronic wounds constitutes a major challenge in tissue regeneration engineering. Owing to the poor bioactivity and maintenance of pro-angiogenic cells and factors during transplantation, new bioactive materials to tackle the barrier are highly desirable. Herein, we demonstrate a co-delivery platform for synergistic promotion of angiogenesis based on biodegradable, therapeutic, and self-reporting luminescent porous silicon (PSi) microparticles. The biodegradable and biocompatible PSi microparticles could quickly release therapeutic Si ions, which is bioactive to promote cell migration, tube formation, and angiogenic gene expression in vitro. To construct a highly efficient angiogenesis treatment platform, vascular endothelial growth factor (VEGF) was electrostatically adsorbed by PSi microparticles for effective drug loading and delivery. The dual therapeutic components (Si ions and VEGF) could release with the dissolution of Si skeleton, accompanying by the decay of photoluminescence (PL) intensity and blue shift of the maximum PL wavelength. Therefore, real-time drug release could be self-reported and assessed with the two-dimensional PL signal. The co-delivery of Si ions and VEGF displayed synergistic effect and highly efficient angiogenesis, which was evidenced by the enhancement of endothelial cell migration and tube formation in vitro with approximately 1.5–5 times higher than control. The blood vessel formation in vivo was also significantly improved as shown by the chick chorioallantoic membrane (CAM) model, in which the total length, size and junctions exhibited 2.1 ± 0.4, 4 ± 0.4, and 3.9 ± 0.3 times in comparison to control, respectively. The PSi and VEGF co-delivery system display great potential in tissue engineering as a biodegradable and self-reporting theranostic platform to promote angiogenesis.

The EPR and Luminescence of Porous Silicon

The data on the EPR, photoluminescence (PL), and current transfer in porous silicon (PS) on KDB-0.3 and KES-0.01 Si, which was oxidized by 10-min isochronous thermal annealing in air at temperatures Tann from 20 to 900°C, as well as in HNO3, are presented in order to further clarify the nature of Pb centers of nonradiative recombination. The maximum PL quantum yield was observed during the chemical oxidation of PS on KDB-0.3 silicon. An anticorrelation of the PL and EPR intensities of Pb centers is observed in the range Tann = (20–300)°C. A nonmonotonic dependence of the EPR intensity of Pb centers on Tann with a minimum at approximately 700°C is revealed. The weak PL of PS with Tann of ~700°C accompanied by a minimum EPR signal from Pb centers means that other nonradiative-recombination centers arise after annealing. A decrease in the PS conductivity with an increase in Tann is associated with the decomposition of Si fibers in PS into small granules, through which discrete tunneling of current carriers occurs.

Optical Properties and Oxide Growth during Chemical and Electrochemical Oxidation of Luminescent Porous Silicon in Liquid Electrolytes Monitored by In-Situ Photoconduction

The progress of chemical and electrochemical oxidation of porous silicon (PSi), formed from lightly-doped p-type silicon, in aqueous electrolytes, was monitored using in situ photocurrent from a monochromatic illumination, which can be used as a signature of the optical transmission through PSi. Models are proposed to explain the results quantitatively. For anodic oxidation, the valence of reaction was obtained as ∼1.5, which is consistent with non-stoichiometric oxide growth. Electroluminescence and photoluminescence during anodic oxidation were consistent with previous reports. For chemical oxidation, monitoring can be used up to a certain stage after which the oxide grown at the PSi/substrate interface prevents further use of electrical conduction. This initial stage was exploited to get model parameters, allowing derivation of time evolution of oxidation rate, oxide thickness, porosity, and relative quantum efficiency. Concurrent dissolution of oxide was also taken into account when the oxidation rate was low. The luminescence efficiency was improved in the first stages of oxidation, and then decreases for heavy oxidation, when the silicon content had drastically decreased. The rate of oxidation seems to play a role in the luminescence efficiency, a high oxidation rate leading to lower efficiency for a given fraction of silicon content in the layer.

Luminescent Porous Silicon Filled with Nanoscopic Magnetic Structures – Assessment of the Optical and Magnetic Properties

Luminescent porous silicon which is denoted as microporous silicon offers a structure size of a few nanometers. It is filled with magnetic metals resulting in a composite system with specific optical and magnetic properties. The morphology of microporous silicon offers a branched network of pores. The luminescence which is observed in the red-orange region due to the small structure size is influenced by the metal filling. The magnetic response of the samples is related to the interconnected pores and differs significantly to mesoporous silicon filled with magnetic nanostructures of similar size.The luminescent porous silicon is etched in aqueous hydrofluoric acid solution by applying a current density of 10 mA/cm2 for 20 min resulting in a porous layer of about 0.8 µm thickness. The metal filling of microporous silicon is a challenging procedure which is carried out under cathodic conditions by pulsed electrodeposition. As electrolytes adequate metal salt solutions are used. The deposited nanosized metal structures are of the same interconnected sponge-like morphology as the template material.Mesoporous silicon offers oriented pores whereas the diameter is in the range of 50 nm [1]. Also in this case the metal nanoparticles are electrodeposited within the matrix material.The magnetic properties of the different systems rely on the morphology of the template material and the size and geometry of the incorporated metal deposits.The magnetic response offers a significant difference between filled luminescent and mesoporous silicon samples. Luminescent metal filled samples show strong magnetic anisotropy between the magnetization parallel and perpendicular to the surface. For these specimens also significant differences of the anisotropy can be observed depending on the kind of deposited metal, e.g. Ni or Co. These differences are not so distinctive in the case of mesoporous metal filled silicon because of weak magnetic interactions between the metal nanoparticles.Due to the merged optical and magnetic properties in the case of microporous silicon the nanocomposite could be promising for integrated magneto-optic devices.[1] P. Granitzer, K. Rumpf, Semiconductor Science and Technology 31, 4004, 2016.

(Invited) In-Situ Monitoring of Luminescence and Oxidation of Porous Silicon in Liquid Electrolytes with Photoconduction

Silicon can be made porous by electrochemical (anodization) etching in hydrofluoric acid (HF). This is the most controlled and used technique for the preparation of porous silicon (PSi). The porous structure depends on several factors, such as silicon resistivity and type, as well as HF concentration and anodization current. In the case of lightly-doped p-type silicon, uniform sponge-like nano-structures can be formed. This type of PSi layer is characterized in particular by its porosity and thickness. PSi is interesting for a variety of properties, some related to quantum confinement in its nanostructure, such as luminescence and porosity-dependent optical absorption, refractive index and surface area. It has potential applications in optoelectronics, photovoltaics, medicine and sensing.Recently, we have made use of the particular behavior of PSi photoconduction in an electrolyte: when PSi is illuminated, only the charge carriers photogenerated in the Si substrate contribute to the measured photocurrent, and thus, the photocurrent is a measure of the optical transmission of PSi. This behavior allowed us to measure the PSi optical constants for various optical wavelengths and PSi porosities in situ in HF and thus for the most pristine form of PSi and avoiding all the problems related to the drying and handling of high-porosity PSi free-standing layers1. We have also used the photoconduction to study the chemical etching of PSi in HF. We could monitor the etching as it proceeded and developed a model that allowed us to derive the HF-dependent etching rates and the absorption coefficient of PSi for a large range of porosities not explored so far, from as-prepared to close to 100% porosity (PSi fully dissolved)2. Finally, we have also made use of the photoconduction technique to understand the photoetching of PSi in HF by monochromatic light.3 In this paper, we use the photoconduction technique to monitor the evolution of different types of oxidation processes of PSi in acid solutions. The photoluminescence is also acquired in-situ. The different oxidation processes studied here are: chemical oxidation in different solutions, photo-assisted oxidation by monochromatic light, and electrochemical oxidation. The photoconduction technique allowed for a good understanding of the oxidation progress and the evolution of the oxidation rates over reaction time. Moreover, the monitoring of the photoluminescence, coupled to the in-situ derivation of the optical absorption provides a way to optimize the reaction time for achieving maximum luminescence efficiency.An example is shown in Fig. 1. It shows the photocurrent (at 405 nm illumination)-time plot for a 5-μm thick PSi layer of initial porosity 68% oxidized in H2SO4 0.5M. The photocurrent, signature of the optical absorption, increases as the oxidation progresses since the formed oxide is transparent at the 405 nm light. Here, the oxidation does not proceed up to full PSi oxidation, therefore the photocurrent does not reach full saturation. The photoluminescence was also recorded and reached its maximum at ~ 65 h (blue star on the plot). 1 B. Gelloz, H. Fuwa, and L. H. Jin, Ecs Journal of Solid State Science and Technology 5 (3), P190 (2016). 2 B. Gelloz, K. Ichimura, H. Fuwa, E. Kondoh, and L. Jin, Ecs Journal of Solid State Science and Technology 6 (1), R1 (2017). 3 B. Gelloz, H. Fuwa, E. Kondoh, and L. Jin, Ecs Journal of Solid State Science and Technology 7 (12), P730 (2018). Figure 1

Catalytic parameters and thermal stability of chondroitinase ABCI on red porous silicon nanoparticles

The bacterial enzyme chondroitinase ABC, which digests extracellular chondroitin sulfate proteoglycans, has been shown to enhance axonal regeneration. However, the utilization of this enzyme as therapeutics is notably restricted due to its thermal instability. Therefore, red luminescent porous silicon that hold promise for potential applications in biological/medical imaging was used as a carrying matrix for chondroitinase with the aim of enhancing its stability. Porous Si nanoparticles were prepared by electrochemical etching of silicon wafers in ethanolic HF solution. The size of nanoparticles (210 nm) and the mean pore diameter (8 −20 nm) were determined using dynamic light scattering and scanning electron microscopy. Purified chondroitinase was then incorporated into the silicon pores. Results revealed similar Km and lower Vmax value for the immobilized enzyme when compared with the free enzyme. The immobilized chondroitinase exhibited about a 4 fold increase in stability at 37 °C after 50 min. It is likely possible that, the enzyme was protected inside the pores resulted in higher stability. Moreover, porous silicon was seen to be capable of holding the chondroitinase for repeated cyclic tests for three times. The cell viability assay exhibited no significant cytotoxicity for Psi-chondroitinase up to 24 h.

Luminescent Porous Silicon Filled with Nanoscopic Magnetic Structures – Assessment of the Optical and Magnetic Properties

Luminescent porous silicon which is denoted as microporous silicon (LPSi) offers a structure size of a few nanometers. The pores are arranged in a branched network-like morphology. These structures are filled with magnetic metals resulting in a composite system with specific optical and magnetic properties. In this work Ni and Co, respectively are used. The luminescence is observed in the red-orange region due to the small structure size. The photoluminescence spectra of initial LPSi offer a peak around 620 nm, which is blue-shifted after Ni-filling with increasing deposition time and the intensity of the luminescence is increased. In the case of Co deposition the spectrum is adequate but with increasing metal filling the luminescence intensity decreases. Magnetic field dependent measurements performed for easy and hard axis magnetization show a high magnetic anisotropy between these two magnetization directions, even more pronounced in the case of Co. The magnetic response of the samples is related to the interconnected pores and differs significantly to mesoporous silicon filled with magnetic nanostructures.

(Invited) In-Situ Monitoring of Luminescence and Oxidation of Porous Silicon in Liquid Electrolytes with Photoconduction

The progress of the chemical and electrochemical oxidation of porous silicon (PSi), formed from lightly-doped p-type silicon, in aqueous electrolytes, was monitored by recording in situ the photocurrent from a monochromatic illumination, which can be used as a signature of the optical transmission through PSi. Models are proposed to explain the data and derive valuable quantities. For galvanostatic anodic oxidation, the model gave the valence of reaction, ~1.5, which is consistent with non-stoichiometric oxide growth. The electroluminescence and photoluminescence during anodic oxidation were mostly consistent with other reports. For chemical oxidation, our model gives the oxidation rate, which was found to decrease as the oxidation progresses, and the optimum PSi porosity for efficient luminescence, about 83%. The simultaneous probing of the photoluminescence, photocurrent (and therefore electrical state) and optical absorption (also related to effective porosity) allows for a good control of a range of PSi properties during chemical oxidation experiments.

Ratiometric Fluorescent Nanohybrid for Noninvasive and Visual Monitoring of Sweat Glucose.

Noninvasive and visual monitoring of glucose is highly desirable for diabetes diagnostics and long-term home-based health management. Owing to the correlation of the glucose level between blood and sweat, on-body sweat glucose detection provides potential for noninvasive healthcare but is highly challenging. Herein, we for the first time demonstrate a wearable skin pad based on the ratiometric fluorescent nanohybrid, which can realize noninvasive and visual monitoring of sweat glucose. Luminescent porous silicon (PSi) particles, which have a porous structure and oxidation-responsive photoluminescence decay, are chosen to load (adsorb or entrap) carbon quantum dots (CQDs) for the construction of the dual fluorescence nanohybrid. Bimetallic (Au and Ag) nanoparticles (BiM) are also co-decorated on the PSi particle to improve detection sensitivity by enhancing PSi's initial fluorescence and oxidation kinetics. Owing to the efficient fluorescence resonance energy transfer effect, BiM-CQDs@PSi initially exhibits PSi's red fluorescence with complete quenching of CQDs's blue fluorescence. The oxidation of PSi triggered by hydrogen peroxide (H2O2) weakens the FRET effect and decays PSi's fluorescence, causing ratiometric fluorescence to change from red (PSi) to blue (CQDs). A wearable skin pad is easily fabricated by co-immobilization of BiM-CQDs@PSi and glucose oxidase (GOX) in a transparent and biocompatible chitosan film supported by an adhesive polyurethane membrane. When the skin pad is attached on the body, the same ratiometric fluorescence transition (red → blue) is observed upon the stimulation of H2O2 generated in GOX-catalyzed oxidation of sweat glucose. Based on the strong correlation between the ratio of the fluorescence change and sweat glucose level, clinical tests toward diabetics and healthy volunteers can clearly indicate hyperglycemia.

Size, porosity, and surface-termination dependence of the radiative and nonradiative relaxation processes of porous silicon

The role of porosity and surface-termination on the radiative and the nonradiative relaxation processes of luminescent porous silicon is investigated using temperature-dependent, time-resolved photoluminescence spectroscopy. We show that, for porous silicon having low- to mid-porosity, radiative relaxation times should be associated with the quantum confinement of excitons (the confined photo-excited electron–hole pairs), while nonradiative relaxation processes are related to the state of the surface (e.g., the surface chemistry), in agreement with previous reports. However, for high-porosity films of porous silicon, we have found much faster low-temperature relaxation times, which are associated with radiative transitions from the triplet excitonic state. This state becomes partially allowed due to a strong coupling and mixing with the singlet state in high-porosity films of porous silicon containing fairly small silicon nanocrystallites.

The effects of drying technique and surface pre-treatment on the cytotoxicity and dissolution rate of luminescent porous silicon quantum dots in model fluids and living cells.

Tailoring of the biodegradation of photoluminescent silicon quantum dots (Si QDs) is important for their future applications in diagnostics and therapy. Here, the effect of drying and surface pretreatment on the dissolution rate of Si QDs in model liquids and living cells was studied in vitro using a combination of photoluminescence and Raman micro-spectroscopy. Porous silicon particles were obtained by mechanical milling of electrochemically etched mesoporous silicon films, and consist of interlinked silicon nanocrystals (QDs) and pores. The samples were subjected to super-critical drying with CO2 solvent (SCD) or air drying (AD) and then annealed at 600 °C for 16 hours in 1% oxygen to obtain nano-sized Si QDs. The obtained samples were characterized by a core-shell structure with a crystalline silicon core and a SiO2 layer on the surface. The sizes of the crystalline silicon cores, calculated from Raman scattering spectra, were about 4.5 nm for the initial AD-SiQDs, and about 2 nm for the initial SCD-SiQDs. Both the AD-Si QDs and the SCD-Si QDs exhibited visible photoluminescence (PL) properties due to quantum confinement effects. The dissolution of the nanocrystals was evaluated through their PL quenching, as well as by the presence of a low-frequency shift, broadening, and a decrease in the intensity of the Raman signal. The stability of the AD-Si QDs and the complete dissolution of the SCD-Si QDs during 24 hours of incubation with cells have been demonstrated. This might explain the apparent lower cytotoxicity observed for SCD-Si QDs.