Porous Silicon Membranes Research Articles

Process for integrating multiple porous silicon membranes with variable characteristics into planar microfluidics

We present a fabrication process based on selective ion implantation to monolithically integrate several porous silicon membranes with different morphologies in the same planar fluidic chip through a single anodization step. By manipulating the dopant concentration of specific zones of a silicon on insulator wafer, the porous silicon elements formed through electrochemical anodization present different characteristics in terms of pore size and porosity. Using this technique, we are able to fabricate both lateral porous silicon membranes and standard porous silicon membranes with vertical pores that can be used in flow-through and flow-over configurations. Achieved mean pore sizes and porosities for the various membranes integrated in the same chip range from 25 nm to 50 nm, and 65–90 %, respectively. Because porous silicon membranes have been used for biosensing and for sample preparation in microfluidics devices, this method is foreseen to enable the use of porous silicon membranes to achieve all the functions involved in the analytical process on a single chip.

Design of a Porous Silicon Biosensor: Characterization, Modeling, and Application to the Indirect Detection of Bacteria.

The design of a porous silicon (PSi) biosensor is not often documented, but is of the upmost importance to optimize its performance. In this work, the motivation behind the design choices of a PSi-based optical biosensor for the indirect detection of bacteria via their lysis is detailed. The transducer, based on a PSi membrane, was characterized and models were built to simulate the analyte diffusion, depending on the porous nanostructures, and to optimize the optical properties. Once all performances and properties were analyzed and optimized, a theoretical response was calculated. The theoretical limit of detection was computed as 104 CFU/mL, based on the noise levels of the optical setup. The experimental response was measured using 106 CFU/mL of Bacillus cereus as model strain, lysed by bacteriophage-coded endolysins PlyB221. The obtained signal matched the expected response, demonstrating the validity of our design and models.

Filling fraction measurement around a drying drop onto nanoporous silicon using digital holographic microscopy

In this work, we implemented an off-axis digital holographic microscopy (DHM) setup to study the behavior of liquid drops on nanostructured porous silicon (PS) membranes. When a liquid comes into contact with the PS structure, capillary action and surface tension give rise to imbibition of the liquid into the intricate volume of the pores. At the same time, the vapor field developed around the drop enhances liquid condensation and evaporation into and from the pores. The complex interplay between these phenomena (capillary driven internal flows, condensation, and evaporation) is not yet fully understood but the methods we propose can provide new insights. We demonstrate how DHM allows determining the filling fraction of the porous membrane around drying drops of water and isobutyl alcohol. The phase information obtained from registered holograms provides quantitative information that permits to reconstruct the filling fraction along the porous membrane and its evolution during the drying process. Additionally, relevant parameters such as speed and contact angle during retraction of the drops can be measured. These methods can be further extended to other porous materials, such as porous alumina, micro- and macro-PS, to develop better fluid-dynamic models for describing fluid behavior in highly confined structures.

Morphological and mechanical characterization of a novel porous silicon membrane used in a lung-on-a-chip system

In the last decade, organ-on-a-chip technology has been researched as an alternative to animal and cell culture models (Buhidma et al. in NPJ Parkinson’s Dis, 2020; Pearce et al. in Eur Cells Mater 13:1–10, 2007; Huh et al. in Nat Protoc 8:2135–2157, 2013). While extensive research has focused on the biological functions of these chips, there has been limited exploration of functional materials that can accurately replicate the biological environment. Our group concentrated on a lung-on-a-chip featuring a newly fabricated porous silicon bio-membrane. This bio-membrane mimics the interstitial space found between epithelial and endothelial cells in vivo, with a thickness of approximately 1 μm (Ingber in Cell 164:1105–1109, 2016). This study aims to establish a fabrication method for producing a thin, uniform porous silicon membrane with a predictable reduced modulus. We conducted mechanical and morphological characterization using scanning electron microscopy and nanoindentation. A small, parametric study was conducted to determine the reduced modulus of the porous silicon and how it may relate to the morphological features of the membrane. We compare our results to other works.Graphical

A comparative study of Polarimetric sensing with freestanding n-type porous silicon and porous alumina membrane

Sensors are everywhere in the modern world, and every aspect of our lives, including security, domestic, and environmental factors, is being monitored. Improving the precision and quality of such analysis are highly valued if more sophisticated, intelligent, and capable systems are available. This paper discusses the practical use of polarimetric sensing on n-type porous silicon and porous alumina freestanding membranes (Whatman-anodic membranes with pore widths of 200 nm and thicknesses of 60 μm). “In-plane and out-of-plane birefringence” is investigated for both types of free-standing membranes. With NaCl solution and a constant flow rate in flow-through mode, the sensing capability of these freestanding membranes, namely porous silicon freestanding membranes (15.54 μm thick) and porous alumina freestanding membranes (60 μm thick), was investigated. These two sensor systems have a sensitivity of 0.25°/ %NaCl and 0.30°/ %NaCl, respectively. Freestanding membranes have 8.32°/RIU-μm and 3.01°/RIU-μm sensitivity, respectively. Porous silicon and porous alumina freestanding membranes are used in conjunction with flow-through bovine serum albumin (BSA) sensor as the principal model system for LOC devices. The results support the increased sensitivity achieved in porous silicon membranes over porous alumina membranes.

Wafer-Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle-Assisted Chemical Etching and Pseudomorphic Thermal Oxidation.

Many biological materials exhibit a multiscale porosity with small, mostly nanoscale pores as well as large, macroscopic capillaries to simultaneously achieve optimized mass transport capabilities and lightweight structures with large inner surfaces. Realizing such a hierarchical porosity in artificial materials necessitates often sophisticated and expensive top-down processing that limits scalability. Here, an approach that combines self-organized porosity based on metal-assisted chemical etching (MACE) with photolithographically induced macroporosity for the synthesis of single-crystalline silicon with a bimodal pore-size distribution is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 60 nm across. The MACE process is mainly guided by a metal-catalyzed reduction-oxidation reaction, where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the AgNPs act as self-propelled particles that are constantly removing silicon along their trajectories. High-resolution X-ray imaging and electron tomography reveal a resulting large open porosity and inner surface for potential applications in high-performance energy storage, harvesting and conversion or for on-chip sensorics and actuorics. Finally, the hierarchically porous silicon membranes can be transformed structure-conserving by thermal oxidation into hierarchically porous amorphous silica, a material that could be of particular interest for opto-fluidic and (bio-)photonic applications due to its multiscale artificialvascularization.

Design of a novel self-breathing micro fuel cell stack with a silicon membrane based on the demand for portable charging power sources

This study proposes a portable mobile power supply with a self-breathing micro-proton exchange membrane fuel cells (µPEMFCs) stack as the core for the application of µPEMFCs in portable charging power supplies. A porous silicon membrane etched with hydrofluoric acid was used to conduct protons. A series array of single cells was formed on a single printed circuit board (PCB) board. Four PCB boards are connected by rotation to form a PEMFC stack, and the cavity enclosed by the PCB boards was used to store hydrogen. The effects of hydrogen pressure and ambient conditions on stack performance were analyzed, and the discharging time of the stack was investigated at standard charging powers of 5 V and 1 A. The results revealed that the heat dissipation method of the self-breathing stack limits the operating current density of the stack. To ensure that the stack temperature was within 60–80 °C, the current density should be controlled within 600–820 mA·cm−2. The mobile power supply designed in this study can satisfy the standard charging power requirements of 5 V and 1 A for electronic devices. When the hydrogen pressure was 3 atm, the discharging time of the mobile power supply was 420 s, and the discharging time could be extended by increasing the hydrogen pressure.

Self-Supported Bragg Stacked Microcavities for DNA Sensing

Porous silicon (PSi) layers are prepared on the single-crystal silicon surface by electrochemical etching, and PSi membranes (PSiMs) are fabricated by separating the PSi layers from the single-crystal silicon substrate using high-current pulses. PSiMs have better adsorption and response time than nonseparated PSi layers, making them more suitable for sensing applications. PSi microcavity (PSM) structures are widely used for the detection of biomolecules in biosensors because of their high transmission resonance peaks with narrow full-width at half-maximum (FWHM) in the reflection spectra. However, when compared to PSiMs, the permeability of the PSM on the silicon substrate is low, resulting in a significant reduction in its detection sensitivity. In this study, an efficient self-supported Bragg stacked microcavity (SBSM) structure was prepared for the detection of DNA biomolecules by combining the advantages of PSM and PSiMs. The redshift of the reflectance spectra obtained in the concentration range of 0.1– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.5 ~\mu \text{M}$ </tex-math></inline-formula> shows a good linear relationship, with a detection limit of 2.2 nM. The detection sensitivity is improved by more than a factor of ten when compared to the PSM structure on the silicon substrate. The novel device allows DNA biomolecules to enter the porous layers more easily, especially the defective layer, thus improving its detection sensitivity.

Study and Characterization of the Porous Silicon Membrane Anode for LITHIUM-Ion Batteries

Study and Characterization of the Porous Silicon Membrane Anode for LITHIUM-Ion Batteries

Multilayer sensor structure based on porous silicon

The work is devoted to the creation of a sensor structure based on a porous silicon membrane. The structure under study integrates a porous layer used as a gas transport layer and a gas sensitive layer of non-stoichiometric tin oxide. The paper investigates the morphology of the structure and shows the gas permeability of the membrane on porous silicon. The gas sensitivity of the test structure obtained by passing a gas-air mixture containing NO2 has been studied. Keywords: porous silicon, membrane, non-stoichiometric tin oxide, thin films, resistive gas sensor.

Многослойная сенсорная структура на основе пористого кремния

Abstract The work is devoted to the creation of a sensor structure based on a porous silicon membrane. The structure under study integrates a porous layer used as a gas transport layer and a gas sensitive layer of non-stoichiometric tin oxide. The paper investigates the morphology of the structure and shows the gas permeability of the membrane on porous silicon. The gas sensitivity of the test structure obtained by passing a gas- air mixture containing NO2 has been studied.

Passivated Porous Silicon Membranes and Their Application to Optical Biosensing.

A robust fabrication method for stable mesoporous silicon membranes using standard microfabrication techniques is presented. The porous silicon membranes were passivated through the atomic layer deposition of different metal oxides, namely aluminium oxide Al2O3, hafnium oxide HfO2 and titanium oxide TiO2. The fabricated membranes were characterized in terms of morphology, optical properties and chemical properties. Stability tests and optical probing noise level determination were also performed. Preliminary results using an Al2O3 passivated membranes for a biosensing application are also presented for selective optical detection of Bacillus cereus bacterial lysate. The biosensor was able to detect the bacterial lysate, with an initial bacteria concentration of 106 colony forming units per mL (CFU/mL), in less than 10 min.

Lateral porous silicon interferometric transducer for on-chip flow-through sensing applications

Most porous silicon-based interferometric sensors targeting biosensing applications consist of vertical porous silicon layers created into a silicon wafer by electrochemical anodization and operate in a flow-over configuration. In this work, we present an alternative porous silicon interferometer based on porous silicon with horizontally oriented pores. This architecture permits the integration of flow-through porous silicon membranes within planar microfluidics. Fourier-transform infrared spectroscopy was used to obtain interference spectra from fabricated lateral porous silicon membranes and red shifts were observed upon filling microfluidic chips integrating the porous membranes with solvents of higher optical indices. This work proves that lateral porous silicon membranes are typical Fabry-Pérot interferometers with a sensitivity of more than 150 nm/RIU and a limit of detection less than 10−3 RIU, that is comparable to vertical porous silicon layers. Moreover, we have conducted simulation studies showing that the addition of Bragg mirrors on the membranes results in spectra with narrower fringes and lateral porous silicon interferometers with improved performances. After appropriate biofunctionalization of the porous silicon surface, lateral porous silicon membrane interferometers should offer alternative solutions for the development of porous silicon flow-through biosensors monolithically integrated on-chip.

Surface-enhanced Raman scattering sensors for biomedical and molecular detection applications in space.

The detection of molecular traces in the environment is a technical problem that is critical in pollutant control procedures at all stages of spacecraft assembly, in space flight, as well as in other technological processes such as food production, medical diagnostics, environmental control, warfare. However, in the aerospace industry, it is necessary to detect molecular traces of contaminants with extreme sensitivity, as even concentrations as low as part-per-billion (ppb) can be critical during long missions. The high sensitivity of the Volatile Organic Compounds (VOCs) detection within the air can be a challenge because of the poor affinity of VOC’s to the metal surface of the sensor substrate. In this work, we present a surface‐enhanced Raman scattering (SERS) spectroscopy technique as a highly sensitive and selective molecular sensor for gas trace detection not sensitive to molecules adsorbtion on sensing element. The developed hybrid SERS platform for molecular trace detection is supported by the hybrid nanoplasmonic porous silicon membrane in conjunction with micropump to achieve the trace level detection of VOCs in the environment. The combination of silicon membrane, made by electrochemical etching of the microchannels in the silicon chip, with chemical deposition of the silver nanoparticles inside the channels, produce a porous Ag nanoparticles membrane with a high density of plasmonic nanostructures (“hot spots”). The micropump integrated with the SERS sensor, pump the air with VOC’s molecules through the plasmonic membrane “hot spots” to increase the probability of interaction of VOC’s molecules with SERS substrate and to increase the enhancement factor. The sensor chip structure was designed, gas flow in the sensor was simulated, and the sensor was fabricated using 3D printing. The limit of detection of hydrazine with concentration level 10–12 M from solution and the vapor phase 0.1 ppm was demonstrated. The anisole vapors with concentration 0.5 ppb spectra in the air were recorded. Our results demonstrate that plasmonic membrane can be used as a high enhancement factor SERS sensor for many pollutants molecules detection with the nanomolar sensitivity and can be applied in the design of sensors for space applications, environment control, biomedical diagnostic.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12567-021-00356-6.

Porous Silicon Biosensor for the Detection of Bacteria through Their Lysate.

Porous silicon (PSi) has been widely used as a biosensor in recent years due to its large surface area and its optical properties. Most PSi biosensors consist in close-ended porous layers, and, because of the diffusion-limited infiltration of the analyte, they lack sensitivity and speed of response. In order to overcome these shortcomings, PSi membranes (PSiMs) have been fabricated using electrochemical etching and standard microfabrication techniques. In this work, PSiMs have been used for the optical detection of Bacillus cereus lysate. Before detection, the bacteria are selectively lysed by PlyB221, an endolysin encoded by the bacteriophage Deep-Blue targeting B. cereus. The detection relies on the infiltration of bacterial lysate inside the membrane, which induces a shift of the effective optical thickness. The biosensor was able to detect a B. cereus bacterial lysate, with an initial bacteria concentration of 105 colony forming units per mL (CFU/mL), in only 1 h. This proof-of-concept also illustrates the specificity of the lysis before detection. Not only does this detection platform enable the fast detection of bacteria, but the same technique can be extended to other bacteria using selective lysis, as demonstrated by the detection of Staphylococcus epidermidis, selectively lysed by lysostaphin.

Mechanical Strength Enhancement of Porous Nanocrystalline-Silicon (pnc-Si) Membrane via Titanium-oxide (Ti-O) Coating

Porous nanocrystalline silicon (pnc-Si) membrane is mainly studied as a blood filtration membrane, mimicking the glomerulus filtration membrane of a human kidney. However, the pnc-Si material itself is not hemocompatible and enormous membrane area to thickness ratio makes the membrane to be easily fractured. Silicon surface modification via titanium-oxide (Ti-O) thin film layer deposition has been proven to be hemocompatible and the presence of Ti-O layer has been numerically studied to give higher membrane flexural strength. In this work, square pnc-Si membranes of 2 mm × 2 mm × 20 nm size have been fabricated with and without Ti-O layer. Point loading-unloading nanoindentation method has been performed and the membranes’ displacement behaviour subjected to point loads is studied. The pnc-Si membranes with Ti-O layer were found to attain higher fracture strength, membrane bending stiffness and average hardness with the increase of ~20, ~11 and ~24%, respectively, compared to bare pnc-Si membranes. The mechanical strength of a free-standing pnc-Si membrane is improved by depositing a Ti-O thin film layer on the membrane structure.

Direct Observation of Homogeneous Cavitation in Nanopores.

We report on the evaporation of hexane from porous alumina and silicon membranes. These membranes contain billions of independent nanopores tailored to an ink-bottle shape, where a cavity several tens of nanometers in diameter is separated from the bulk vapor by a constriction. For alumina membranes with narrow enough constrictions, we demonstrate that cavity evaporation proceeds by cavitation. Measurements of the pressure dependence of the cavitation rate follow the predictions of the bulk, homogeneous, classical nucleation theory, definitively establishing the relevance of homogeneous cavitation as an evaporation mechanism in mesoporous materials. Our results imply that porous alumina membranes are a promising new system to study liquids in a deeply metastable state.

A novel electrochemical anodization cell for the synthesis of mesoporous silicon.

A novel design of an electrochemical anodization cell dedicated to the synthesis of mesoporous, single-crystalline silicon is presented. First and foremost, the design principle follows user safety since electrochemical etching of silicon requires highly hazardous electrolytes based on hydrofluoric (HF) acid. The novel cell design allows for safe electrolyte handling prior, during, and post-etching. A peristaltic pump with HF-resistant fluoroelastomer tubing transfers electrolytes between dedicated reservoirs and the anodization cell. Due to the flexibility of the cell operation, different processing conditions can be realized providing a large parameter range for the attainable sample thickness, its porosity, and the mean pore size. Rapid etching on the order of several minutes to synthesize micrometer-thick porous silicon epilayers on bulk silicon is possible as well as long-time etching with continuous, controlled electrolyte flow for several days to prepare up to 1000 μm thick self-supporting porous silicon membranes. A highly adaptable, LabVIEW™-based control software allows for user-defined etching profiles.

A novel proton-exchange porous silicon membrane production method for μDMFCs.

This study introduces a new production method to use as a porous silicon-based proton exchange membrane for μDMFCs. In this respect, EIS, fuel crossover test, and fuel cell performance test at the μDMFC sample cell are performed at room temperature on a porous silicon-based membrane that was produced for passive mode μDMFC as a proton exchange membrane. The reason for performing the fuel crossover test is to ensure the silicon opened pores along the silicon wafer and to examine the fuel permeability of the membrane. The fuel crossover test shows that the fuel cell provides energy for about 60 min with a 50 mL fuel. EIS reveals proton permeability of proton exchange membrane. The calculated value of the conductivity of the membrane is 0.0016 S/cm. OCV of the system is 0.4V, whereas values (with highest power density is 0.1 mW/cm²and with the highest current density is 0.39 mA/cm²) are low. However, porous silicon is not a natural proton conductor. Hence, these values can be increased by different ways such as porous silicon functionalized, or serial connection of fuel cells. On the other hand, the value of OCV is consistent with the previous studies. In sum, this study presents a simple, cost-effective, and short time-consuming method for the production of porous silicon as proton-conducting membrane behavior.

Fabrication of a Physiologically Relevant Novel Lung‐on‐a‐Chip for the Study of Human Pulmonary Disease and Drug Toxicity

In the scope of biomedical research, many advances have been made in the field of pulmonology with the use of multiple animal models and traditional cell culture experimentation. While these methods are useful when studying single cellular pathways or overarching biological processes, they often falter when studying the meticulous nature of human disease. In this work, we propose the development of a novel Lung‐on‐a‐Chip device that would more accurately mimic the physiological boundaries of human alveoli than animal models or traditional in vitro studies. Utilizing multiple microfabrication techniques, we are developing a microfluidic device with an ultra‐thin (25μm) biodegradable porous silicon membrane. Current data suggests that human pulmonary epithelial and endothelial cells are viable and prolific on the proposed microfabricated silicon membrane in extended studies (14 days). Due to the biodegradability of the fabricated novel silicon membranes, it has been observed in long term studies that cells can remodel and degrade the porous silicon membrane. This degradation allows for physiological cellular contact between membranes mimicking a true blood gas exchange interface as observed in vivo. To further validate this model’s ability to recapitulate human physiology, we have begun to co‐culture with immunological cell types and are monitoring for response to bacterial and tumor antigens. Preliminary data suggests that immunological cells are able to remodel the porous silicon membrane substrate and migrate across cellular boundaries in response to environmental cues as seen in vivo. Broadly, we believe that this model may be used to further characterize and study human pulmonary disease and drug toxicity.Support or Funding InformationMaximizing Access to Research Careers Undergraduate Student Training in Academic Research Award# T34GM092711NSF PREM for Functional Nanomaterials, Award# 1827847STROBE NSF Science and Technology Center on Real‐Time Functional Imaging, Award# 1548924.