Nanostructured porous Si optical biosensors: effect of thermal oxidation on their performance and properties

Bare Eye Detection of Bacterial Enzymes of Pseudomonas aeruginosa with Polymer Modified Nanoporous Silicon Rugate Filters

The fabrication, characterization and application of a nanoporous Silicon Rugate Filter (pSiRF) loaded with an enzymatically degradable polymer is reported as a bare eye detection optical sensor for enzymes of pathogenic bacteria, which is devoid of any dyes. The nanopores of pSiRF were filled with poly(lactic acid) (PLA), which, upon enzymatic degradation, resulted in a change in the effective refractive index of the pSiRF film, leading to a readily discernible color change of the sensor. The shifts in the characteristic fringe patterns before and after the enzymatic reaction were analyzed quantitatively by Reflectometric Interference Spectroscopy (RIfS) to estimate the apparent kinetics and its dependence on enzyme concentration. A clear color change from green to blue was observed by the bare eye after PLA degradation by proteinase K. Moreover, the color change was further confirmed in measurements in bacterial suspensions of the pathogen Pseudomonas aeruginosa (PAO1) as well as in situ in the corresponding bacterial supernatants. This study highlights the potential of the approach in point of care bacteria detection.

Botulinum Neurotoxin C Dual Detection through Immunological Recognition and Endopeptidase Activity Using Porous Silicon Interferometers

Botulinum neurotoxins (BoNTs) are the most potent toxins known in nature produced by Clostridium botulinum strains, which can cause life-threatening diseases in both humans and animals. The latter is of serious environmental and economic concern, resulting in high mortality, production losses, and rejection of contaminated animal feed. The available in vivo mouse assay is inadequate for real-time and on-site assessment of outbreaks. Herein, we present a reflective-based approach for the detection of BoNT/C while estimating its activity. Two adjacent porous Si Fabry-Pérot interferometers are simultaneously utilized to quantify minute BoNT/C concentrations by a competitive immunoassay and to assess their endopeptidase activity. The reflectivity signals of each interferometer are amplified by biochemical reaction products infiltration into the scaffold or by peptide fragments detachment from the nanostructure. The optical assay is highly sensitive in compliance with the in vivo approach by presenting a detection limit of 4.24 pg mL^-1. The specificity and selectivity of the designed platform are cross-validated against BoNT/B and BoNT/D, also relevant to animal health. Finally, the analytical performances of both interferometers for real-life scenarios are confirmed using actual toxins while depicting excellent compliance to complex media analysis. Overall, the presented sensing scheme offers an efficient, rapid, and label-free approach for potential biodiagnostic elucidation of botulism outbreaks.

Porous Silicon-Based Aptasensors: Toward Cancer Protein Biomarker Detection

The anterior gradient homologue-2 (AGR2) protein is an attractive biomarker for various types of cancer. In pancreatic cancer, it is secreted to the pancreatic juice by premalignant lesions, which would be an ideal stage for diagnosis. Thus, designing assays for the sensitive detection of AGR2 would be highly valuable for the potential early diagnosis of pancreatic and other types of cancer. Herein, we present a biosensor for label-free AGR2 detection and investigate approaches for enhancing the aptasensor sensitivity by accelerating the target mass transfer rate and reducing the system noise. The biosensor is based on a nanostructured porous silicon thin film that is decorated with anti-AGR2 aptamers, where real-time monitoring of the reflectance changes enables the detection and quantification of AGR2, as well as the study of the diffusion and target-aptamer binding kinetics. The aptasensor is highly selective for AGR2 and can detect the protein in simulated pancreatic juice, where its concentration is outnumbered by orders of magnitude by numerous proteins. The aptasensor's analytical performance is characterized with a linear detection range of 0.05-2 mg mL^-1, an apparent dissociation constant of 21 ± 1 μM, and a limit of detection of 9.2 μg mL^-1 (0.2 μM), which is attributed to mass transfer limitations. To improve the latter, we applied different strategies to increase the diffusion flux to and within the nanostructure, such as the application of isotachophoresis for the preconcentration of AGR2 on the aptasensor, mixing, or integration with microchannels. By combining these approaches with a new signal processing technique that employs Morlet wavelet filtering and phase analysis, we achieve a limit of detection of 15 nM without compromising the biosensor's selectivity and specificity.

Botulinum Neurotoxin-C Detection Using Nanostructured Porous Silicon Interferometer

Botulinum neurotoxins (BoNT) are the most potent toxins, which are produced by Clostridium bacteria and cause the life-threatening disease of botulism in all vertebrates. Specifically, animal botulism represents a serious environmental and economic concern in animal production due to the high mortality rates observed during outbreaks. Despite the availability of vaccines against BoNT, there are still many outbreaks of botulism worldwide. Alternative assays capable of replacing the conventional in vivo assay in terms of rapid and sensitive quantification, and the applicability for on-site analysis, have long been perused. Herein, we present a simple, highly sensitive and label-free optical biosensor for real-time detection of BoNT serotype C using a porous silicon Fabry–Pérot interferometer. A competitive immunoassay coupled to a biochemical cascade reaction was adapted for optical signal amplification. The resulting insoluble precipitates accumulated within the nanostructure changed the reflectivity spectra by alternating the averaged refractive index. The augmented optical performance allowed for a linear response within the range of 10 to 10,000 pg mL−1 while presenting a detection limit of 4.8 pg mL−1. The practical aspect of the developed assay was verified using field BoNT holotoxins to exemplify the potential use of the developed optical approach for rapid bio-diagnosis of BoNT. The specificity and selectivity of the assay were successfully validated using an adjacent holotoxin relevant for farm animals (BoNT serotype D). Overall, this work sets the foundation for implementing a miniaturized interferometer for routine on-site botulism diagnosis, thus significantly reducing the need for animal experimentation and shortening analysis turnaround for early evidence-based therapy.

Real-time detection of copper contaminants in environmental water using porous silicon Fabry-Pérot interferometers

Water sources are vulnerable to intentional and inadvertent human pollution with thousands of synthetic and geogenic trace contaminants, posing long-term effects on the aquatic ecosystem and human health. Thus, early and rapid detection of water pollutants followed by corrective and preventive actions can lead to the reduction of the overall polluting impact to safeguard public health. This study presents a generic sensing assay for the label-free detection of copper contaminants in environmental water samples using multilayered polyethylenimine (PEI) functionalized porous silicon Fabry-Pérot interferometers. The selective chelating activity of PEI thin-films was monitored in real-time by reflective interferometric Fourier transform spectroscopy (RIFTS) while assessing the improved optical responses. The optimized scaffold of two sequential PEI layers depicted a linear working range between 0.2 and 2 ppm while presenting a detection limit of 0.053 ppm (53 ppb). The specificity of the developed platform was cross-validated against various metallic pollutants and cations commonly found in water bodies (i.e., Cd2+, Pb2+, Cr3+, Fe3+, Mg2+, Ca2+, Zn2+, K+ and Al3+). Finally, as a proof of concept, the analytical performance of the porous interferometers for real-life scenarios was demonstrated in three water samples (tap, ground and irrigation), presenting sufficient adaptability to complex matrix analysis with recovery values of 85-106%. Overall, the developed sensing concept offers an efficient, rapid and label-free methodology that can be potentially adopted for routine on-site detection using a simple and portable device.

Porous Silicon Optical Devices: Recent Advances in Biosensing Applications

This review summarizes the leading advancements in porous silicon (PSi) optical-biosensors, achieved over the past five years. The cost-effective fabrication process, the high internal surface area, the tunable pore size, and the photonic properties made the PSi an appealing transducing substrate for biosensing purposes, with applications in different research fields. Different optical PSi biosensors are reviewed and classified into four classes, based on the different biorecognition elements immobilized on the surface of the transducing material. The PL signal modulation and the effective refractive index changes of the porous matrix are the main optical transduction mechanisms discussed herein. The approaches that are commonly employed to chemically stabilize and functionalize the PSi surface are described.

A Regenerable Biosensing Platform for Bacterial Toxins

Waterborne diarrheal diseases such as travelers' diarrhea and cholera remain a threat to public health in many countries. Rapid diagnosis of an infectious disease is critical in preventing the escalation of a disease outbreak into an epidemic. Many of the diagnostic tools for infectious diseases employed today are time-consuming and require specialized laboratory settings and trained personnel. There is hence a pressing need for fit-for-purpose point-of-care diagnostic tools with emphasis in sensitivity, specificity, portability, and low cost. We report work toward thermally reversible biosensors for detection of the carbohydrate-binding domain of the Escherichia coli heat-labile enterotoxin (LTB), a toxin produced by enterotoxigenic E. coli strains, which causes travelers' diarrhea. The biosensing platform is a hybrid of two materials, combining the optical properties of porous silicon (pSi) interferometric transducers and a thermoresponsive multivalent glycopolymer, to enable recognition of LTB. Analytical performance of our biosensors allows us to detect, using a label-free format, sub-micromolar concentrations of LTB in solution as low as 0.135 μM. Furthermore, our platform shows a temperature-mediated "catch-and-release" behavior, an exciting feature with potential for selective protein capture, multiple readouts, and regeneration of the sensor over consecutive cycles of use.

Porous Silicon Fabry-Pérot Interferometer for N-Acetyl-β-d-Glucosaminidase Biomarker Monitoring

Bovine mastitis (BM) is a prominent inflammatory disease affecting the dairy industry worldwide, originated by pathogenic agent invasion onto the mammary gland. Early detection of new BM cases is of high importance for infection control within the herd. Conventional analytical techniques lack the ability to detect BM-predicting biomarkers, used as analytical indicators for health status evaluation, in real time or outside the laboratory boundaries. Herein, we describe a biosensing platform for label-free detection and identification of BM onset through targeting N-acetyl-β-d-glucosaminidase (NAGase) for potential evidence-based therapy. The lysosomal activity in dissimilar milk qualities was monitored by a gelatin-functionalized porous Si Fabry-Pérot interferometer, while estimating the biochemical reaction precipitating products within the nanostructure. The optical response was proportional to the inherent NAGase concentration found in real milk samples, influenced by two dominant BM causative pathogens (i.e., Escherichia coli and Streptococcus dysgalactiae) at various somatic cell counts. Quantitative analysis of NAGase levels within the entire inflammatory spectrum (healthy, subclinical, and clinical BM) was obtained within the range of 1.0-4.2 μM/min (enzymatic activity per volume unit), while presenting a detection limit of 0.51 μM/min. The optical performances correspond with standardized biochemical activity assay in dissimilar milk qualities. Overall, the presented sensing concept exhibits the potential of BM-predicting biomarker detection using a simple and portable experimental setup for convenient early biodiagnostics and health status evaluation.

Amplified Fluorescence by ZnO Nanoparticles vs. Quantum Dots for Bovine Mastitis Acute Phase Response Evaluation in Milk

Bovine mastitis (BM) is a prominent inflammatory disease affecting the dairy industry worldwide, originated by pathogenic agent invasion onto the mammary gland. The early detection of new BM cases is of high importance for infection control within the herd. During inflammation, various biomarkers are released into the blood circulation, which are consequently found in milk. Herein, the lysosomal activity of N-acetyl-β-D-glucosaminidase (NAGase), a predominant BM indicator, was utilized for highly sensitive clinical state differentiation. The latter is achieved by the precise addition of tetraethyl orthosilicate-coated zinc oxide nanostructures (quantum dots or nanoparticles, individually) onto a conventional assay. Enhanced fluorescence due to the nanomaterial accumulative near-field effect is achieved within real milk samples, contaminated with Streptococcus dysgalactiae, favoring quantum dots over nanoparticles (> 7-fold and 3-fold, respectively), thus revealing significant differentiation between various somatic cell counts. The main advantage of the presented sensing concept, besides its clinically relevant concentrations, is the early bio-diagnostic detection of mastitis (subclinical BM) by using a simple and cost-effective experimental setup. Moreover, the assay can be adapted for BM recovery prognosis evaluation, and thus impact on udder health status, producing an alternative means for conventional diagnosis practices.

Bone Morphogenic Protein 2-Loaded Porous Silicon Carriers for Osteoinductive Implants

Bone morphogenetic proteins (BMPs) are probably the most important growth factors in bone formation and healing. However, the utilization of BMPs in clinical applications is mainly limited due to the protein poor solubility at physiological pH, rapid clearance and relatively short biological half-life. Herein, we develop degradable porous silicon (PSi)-based carriers for sustained delivery of BMP-2. Two different loading approaches are examined, physical adsorption and covalent conjugation, and their effect on the protein loading and release rate is thoroughly studied. The entrapment of the protein within the PSi nanostructures preserved its bioactivity for inducing osteogenic differentiation of rabbit bone marrow mesenchymal stems cells (BM-MSCs). BM-MSCs cultured with the BMP-2 loaded PSi carriers exhibit a relatively high alkaline phosphatase (ALP) activity. We also demonstrate that exposure of MSCs to empty PSi (no protein) carriers generates some extent of differentiation due to the ability of the carrier's degradation products to induce osteoblast differentiation. Finally, we demonstrate the integration of these promising BMP-2 carriers within a 3D-printed patient-specific implant, constructed of poly(caprolactone) (PCL), as a potential bone graft for critical size bone defects.

Porous Silicon Bragg Reflector/Carbon Dot Hybrids: Synthesis, Nanostructure, and Optical Properties

Carbon dots (C-dots) exhibit unique fluorescence properties, mostly depending upon their physical environments. Here we investigate the optical properties and nanostructure of Carbon dots (C-dots) which are synthesized in situ within different porous Silicon (PSi) Bragg reflectors. The resulting hybrids were characterized by photoluminescence, X-ray photoelectron, and Fourier Transform Infrared spectroscopies, as well as by confocal and transmission electron microscopy. We show that by tailoring the location of the PSi Bragg reflector photonic bandgap and its oxidation level, the C-dots emission spectral features can be tuned. Notably, their fluorescence emission can be significantly enhanced when the high reflection band of the PSi host overlaps with the confined C-dots' peak wavelength, and the PSi matrix is thermally oxidized at mild conditions. These phenomena are observed for multiple compositions of PSi Bragg reflectors/C-dots hybrids.

Alginate modified with maleimide-terminated PEG as drug carriers with enhanced mucoadhesion

The goal of this study was to generate a new mucoadhesive carbohydrate-based delivery system composed of alginate (Alg) backbone covalently attached to polyethylene glycol (PEG) modified with a unique functional end-group (maleimide). The immobilization of PEG-maleimide chains significantly improved the mucoadhesion properties attributed to thioether bonds creation via Michael-type addition and hydrogen bonding with the mucus glycoproteins. Mucoadhesion studies using tensile and rotating cylinder assays revealed a 3.6-fold enhanced detachment force and a 2.8-fold enhanced retention time compared to the unmodified polymer, respectively. Additional indirect studies confirmed the presence of polymer-mucus glycoproteins interactions. Drug release experiments were used to evaluate the release profiles from Alg-PEG-maleimide tablets in comparison to Alg and Alg-SH tablets. Viability studies of normal human dermal fibroblasts cells depicted the non-toxic nature of Alg-PEG-maleimide. Overall, our studies disclose that PEG-maleimide substitutions on other biocompatible polymers can lead to the development of useful biomaterials for diverse biomedical applications.

In situ investigation of mesoporous silicon oxidation kinetics using infrared emittance spectroscopy

The kinetics of mesoporous silicon oxidation is investigated by means of infrared emittance measurements. The chemical mechanisms are put in perspective with temperature.

Fabrication of Optical Multilayer Devices from Porous Silicon Coatings with Closed Porosity by Magnetron Sputtering

The fabrication of single-material photonic-multilayer devices is explored using a new methodology to produce porous silicon layers by magnetron sputtering. Our bottom-up methodology produces highly stable amorphous porous silicon films with a controlled refractive index using magnetron sputtering and incorporating a large amount of deposition gas inside the closed pores. The influence of the substrate bias on the formation of the closed porosity was explored here for the first time when He was used as the deposition gas. We successfully simulated, designed, and characterized Bragg reflectors and an optical microcavity that integrates these porous layers. The sharp interfaces between the dense and porous layers combined with the adequate control of the refractive index and thickness allowed for excellent agreement between the simulation and the experiments. The versatility of the magnetron sputtering technique allowed for the preparation of these structures for a wide range of substrates such as polymers while also taking advantage of the oblique angle deposition to prepare Bragg reflectors with a controlled lateral gradient in the stop band wavelengths.

Tethered Lipid Bilayers within Porous Si Nanostructures: A Platform for (Optical) Real-Time Monitoring of Membrane-Associated Processes

The importance of cell membranes in biological systems has prompted the development of artificial lipid bilayers, which can mimic the cellular membrane structure. Supported lipid bilayers (SLBs) have emerged as a promising avenue for studying basic membrane processes and for possible biotechnological applications. Conventional methods for SLB formation involve the spreading of lipid vesicles on hydrophilic solid supports. Herein, a facile approach for the construction of tethered SLB within an oxidized porous Si (pSiO2) nanostructure, avoiding liposome preparation, is presented. We employ a two-step lipid self-assembly process, in which a first lipid layer is tethered to the pore walls resulting in a highly stable monolayer. A subsequent solvent exchange step induces the self-assembly of the unbound lipids into a robust SLB. Formation of pSiO2-SLB is confirmed by fluorescence resonance energy transfer (FRET), and the properties of the confined SLB are characterized by environment-sensitive fluorophores. The unique optical properties of the pSiO2 support are employed to monitor in real time the partitioning of a model amphiphilic molecule within the SLB via reflective interferometric Fourier transform spectroscopy (RIFTS) method. These self-reporting SLB platforms provide a highly generic approach for bottom-up construction of complex lipid architectures for performing biological assays at the micro- and nanoscale.

Label-free optical biosensors based on aptamer-functionalized porous silicon scaffolds

A proof-of-concept for a label-free and reagentless optical biosensing platform based on nanostructured porous silicon (PSi) and aptamers is presented in this work. Aptamers are oligonucleotides (single-stranded DNA or RNA) that can bind their targets with high affinity and specificity, making them excellent recognition elements for biosensor design. Here we describe the fabrication and characterization of aptamer-conjugated PSi biosensors, where a previously characterized his-tag binding aptamer (6H7) is used as model system. Exposure of the aptamer-functionalized PSi to the target proteins as well as to complex fluids (i.e., bacteria lysates containing target proteins) results in robust and well-defined changes in the PSi optical interference spectrum, ascribed to specific aptamer-protein binding events occurring within the nanoscale pores, monitored in real time. The biosensors show exceptional stability and can be easily regenerated by a short rinsing step for multiple biosensing analyses. This proof-of-concept study demonstrates the possibility of designing highly stable and specific label-free optical PSi biosensors, employing aptamers as capture probes, holding immense potential for application in detection of a broad range of targets, in a simple yet reliable manner.

Detection of trace heavy metal ions in water by nanostructured porous Si biosensors

Specific and sensitive detection and quantification of heavy metals in real water using label-free optical biosensors.