Porous Silicon-Based Biosensors: Towards Real-Time Optical Detection of Target Bacteria in the Food Industry

Fully Integrated Microfluidic Digital Chip for Simple and Highly Quantitative Detection of Norovirus

The prevalence of foodborne illnesses is a significant global concern, resulting in numerous illnesses, deaths, and substantial economic losses annually. Traditional detection methods for foodborne pathogens are often slow, limited, and impractical for field use, underscoring the need for rapid, sensitive, and portable assays. Microfluidic technology has emerged as a promising solution for sample preparation, reaction, and detection on a small scale. Our study introduces a novel microfluidic digital loop-mediated isothermal amplification (LAMP) assay platform, which employs digital microfluidic chips for absolute quantitative analysis of nucleic acids. This portable chip utilizes LAMP technology to achieve ultrasensitive detection of target nucleic acids within 30 min and reduces the detection limit to 1 fM without the need for complex instrumentation. By digitizing amplification signals directly from the target sample, our platform offers simplicity, affordability, portability, and quantitative molecular readouts. This innovation represents a crucial step toward the on-site detection of foodborne pathogens, thereby enhancing food safety and mitigating disease outbreaks.

Enzyme-Free Identification of Monosaccharide Enantiomers on TiO2 Nanotube Array-Based Fabry-Pérot Interferometer

Chirality is a vital property across various domains, especially for biological activity. Herein, an enzyme-free sensing platform for monosaccharide enantiomer identification was developed by utilizing the Fabry-Pérot interferometer feature of TiO2 nanotube arrays modified with enantioselective metal-organic framework and glucose oxidase-mimicking Au NPs. In this design, optical property is monitored by reflective interferometric Fourier transform spectroscopy (RIFTS), a highly sensitive technique for detecting changes in the average refractive index within nanotubular structures. Using glucose (Glu) enantiomers as the model targets, after the recognition of L-/d-Glu on mesoporous homochiral MIL-101 (Fe), Au NPs anchored in MIL-101(Fe) catalyze the oxidation of Glu molecules to produce hydrogen peroxide (H2O2). Benefiting from the confinement effect of frameworks, MIL-101(Fe), as an artificial enzyme with excellent peroxidase-like activity, catalyzes the conversion of 4-chloro-1-naphthol (4-CN) into insoluble precipitates. These gathered precipitates effectively change the average refractive index of the interferometric substrate. Based on the variation of effective optical thickness (ΔEOT) values, the enantioselective determination of l-Glu and d-Glu can be achieved. Moreover, the proposed RIFTS sensor also presents broad applicability for the identification of other monosaccharide enantiomers. As the enzyme-free homochiral interferometer is directly constructed on a Ti-metal sheet, the RIFTS platform offers a robust, sensitive, and low-cost device for monosaccharide enantiomer recognition.

Soft surface-enhanced Raman scattering sensing platform based on an oil-in-water emulsion stabilized by silver nanoparticles

Pickering emulsions are excellent candidates for developing soft biosensors utilized for detecting native biomolecules such as peptides and proteins through the Surface-Enhanced Raman Spectroscopy (SERS) transduction mechanism. Here, we have developed a SERS sensor based on oil-in-water Pickering emulsions stabilized by Ag nanoparticles (Ag-NPs) with the Raman active molecule (4-Aminothiphenol, 4ATP) adsorbed to their surface. The structural properties and composition of the Pickering emulsion were tuned to meet the demands of the maximal optical response. Our results show that the obtained SERS signals of the main studied Pickering emulsion (water: oil ratio 7:3, 1 wt% Ag-NPs) outperformed colloidal dispersions with the same Ag-NPs concentration by 10-fold at any studied content of 4ATP. The superior optical response of the Pickering emulsion compared to the colloidal dispersion can thus pave the way for the detection of a large variety of analytes at high sensitivity by a soft sensing device. This study innovates by comparing the SERS signals of Raman-active Ag-NPs when they are assembled at the oil/water interface of an emulsion to the case where the NPs are individually dispersed in the medium. The findings shed light on the edit value of utilizing Raman-active Pickering stabilizers for biosensing applications.

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.

Multifunctional Composite Hydrogels for Bacterial Capture, Growth/Elimination, and Sensing Applications

Hydrogels are cross-linked networks of hydrophilic polymer chains with a three-dimensional structure. Owing to their unique features, the application of hydrogels for bacterial/antibacterial studies and bacterial infection management has grown in importance in recent years. This trend is likely to continue due to the rise in bacterial infections and antimicrobial resistance. By exploiting their physicochemical characteristics and inherent nature, hydrogels have been developed to achieve bacterial capture and detection, bacterial growth or elimination, antibiotic delivery, or bacterial sensing. Traditionally, the development of hydrogels for bacterial/antibacterial studies has focused on achieving a single function such as antibiotic delivery, antibacterial activity, bacterial growth, or bacterial detection. However, recent studies demonstrate the fabrication of multifunctional hydrogels, where a single hydrogel is capable of performing more than one bacterial/antibacterial function, or composite hydrogels consisting of a number of single functionalized hydrogels, which exhibit bacterial/antibacterial function synergistically. In this review, we first highlight the hydrogel features critical for bacterial studies and infection management. Then, we specifically address unique hydrogel properties, their surface/network functionalization, and their mode of action for bacterial capture, adhesion/growth, antibacterial activity, and bacterial sensing, respectively. Finally, we provide insights into different strategies for developing multifunctional hydrogels and how such systems can help tackle, manage, and understand bacterial infections and antimicrobial resistance. We also note that the strategies highlighted in this review can be adapted to other cell types and are therefore likely to find applications beyond the field of microbiology.

Spectral-Free Double Light Detection of DNA Based on a Porous Silicon Bragg Mirror

To improve the detection sensitivity of a porous silicon optical biosensor in the real-time detection of biomolecules, a non-spectral porous silicon optical biosensor technology, based on dual-signal light detection, is proposed. Double-light detection is a combination of refractive index change detection and fluorescence change detection. It uses quantum dots to label probe molecules to detect target molecules. In the double-signal-light detection method, the first detection-signal light is the detection light that is reflected from the surface of the porous silicon Bragg mirror. The wavelength of the detection light is the same as the wavelength of the photonic band gap edge of the porous silicon Bragg mirror. CdSe/ZnS quantum dots are used to label the probe DNA and hybridize it with the target DNA molecules in the pores of porous silicon to improve its effective refractive index and enhance the detection-reflection light. The second detection-signal light is fluorescence, which is generated by the quantum dots in the reactant that are excited by light of a certain wavelength. The Bragg mirror structure further enhances the fluorescence signal. A digital microscope is used to simultaneously receive the digital image of two kinds of signal light superimposed on the surface of porous silicon, and the corresponding algorithm is used to calculate the change in the average grey value before and after the hybridization reaction to calculate the concentration of the DNA molecules. The detection limit of the DNA molecules was 0.42 pM. This method can not only detect target DNA by hybridization, but also detect antigen by immune reaction or parallel biochip detection for a porous silicon biosensor.

The interplay of chemical structure, physical properties, and structural design as a tool to modulate the properties of melanins within mesopores

The design of modern devices that can fulfil the requirements for sustainability and renewable energy applications calls for both new materials and a better understanding of the mixing of existing materials. Among those, surely organic–inorganic hybrids are gaining increasing attention due to the wide possibility to tailor their properties by accurate structural design and materials choice. In this work, we’ll describe the tight interplay between porous Si and two melanic polymers permeating the pores. Melanins are a class of biopolymers, known to cause pigmentation in many living species, that shows very interesting potential applications in a wide variety of fields. Given the complexity of the polymerization process beyond the formation and structure, the full understanding of the melanins' properties remains a challenging task. In this study, the use of a melanin/porous Si hybrid as a tool to characterize the polymer’s properties within mesopores gives new insights into the conduction mechanisms of melanins. We demonstrate the dramatic effect induced on these mechanisms in a confined environment by the presence of a thick interface. In previous studies, we already showed that the interactions at the interface between porous Si and eumelanin play a key role in determining the final properties of composite materials. Here, thanks to a careful monitoring of the photoconductivity properties of porous Si filled with melanins obtained by ammonia-induced solid-state polymerization (AISSP) of 5,6-dihydroxyindole (DHI) or 1,8-dihydroxynaphthalene (DHN), we investigate the effect of wet, dry, and vacuum cycles of storage from the freshly prepared samples to months-old samples. A computational study on the mobility of water molecules within a melanin polymer is also presented to complete the understanding of the experimental data. Our results demonstrate that: (a) the hydration-dependent behavior of melanins is recovered in large pores (≈ 60 nm diameter) while is almost absent in thinner pores (≈ 20 nm diameter); (b) DHN-melanin materials can generate higher photocurrents and proved to be stable for several weeks and more sensitive to the wet/dry variations.

Deep Learning Approach for Classifying Bacteria types using Morphology of Bacterial Colony

The significant bottlenecks in determining bacterial species are much more time-consuming and the biology specialist's long-term experience requirements. Specifically, it takes more than half a day to cultivate a bacterium, and then a skilled microbiologist and a costly specialized machine are utilized to analyze the genes and classify the bacterium according to its nucleotide sequence. To overcome these issues as well as get higher recognition accuracy, we proposed applying convolutional neural networks (CNNs) architectures to automatically classify bacterial species based on some key characteristics of bacterial colonies. Our experiment confirmed that the classification of three bacterial colonies could be performed with the highest accuracy (97.19%) using a training set of 5000 augmented images derived from the 40 original photos taken in the Hanoi Medical University laboratory in Vietnam.

2D Material-Based Optical Biosensor: Status and Prospect

The combination of 2D materials and optical biosensors has become a hot research topic in recent years. Graphene, transition metal dichalcogenides, black phosphorus, MXenes, and other 2D materials (metal oxides and degenerate semiconductors) have unique optical properties and play a unique role in the detection of different biomolecules. Through the modification of 2D materials, optical biosensor has the advantages that traditional sensors (such as electrical sensing) do not have, and the sensitivity and detection limit are greatly improved. Here, optical biosensors based on different 2D materials are reviewed. First, various detection methods of biomolecules, including surface plasmon resonance (SPR), fluorescence resonance energy transfer (FRET), and evanescent wave and properties, preparation and integration strategies of 2D material, are introduced in detail. Second, various biosensors based on 2D materials are summarized. Furthermore, the applications of these optical biosensors in biological imaging, food safety, pollution prevention/control, and biological medicine are discussed. Finally, the future development of optical biosensors is prospected. It is believed that with their in-depth research in the laboratory, optical biosensors will gradually become commercialized and improve people's quality of life in many aspects.

Peptide-Based Capture of Chikungunya Virus E2 Protein Using Porous Silicon Biosensor

The detection of pathogens presents specific challenges in ensuring that biosensors remain operable despite exposure to elevated temperatures or other extreme conditions. The most vulnerable component of a biosensor is typically the bioreceptor. Accordingly, the robustness of peptides as bioreceptors offers improved stability and reliability toward harsh environments compared to monoclonal antibodies that may lose their ability to bind target molecules after such exposures. Here, we demonstrate peptide-based capture of the Chikungunya virus E2 protein in a porous silicon microcavity biosensor at room temperature and after exposure of the peptide-functionalized biosensor to high temperature. Contact angle measurements, attenuated total reflectance-Fourier transform infrared spectra, and optical reflectance measurements confirm peptide functionalization and selective E2 protein capture. This work opens the door for other pathogenic biomarker detection using peptide-based capture agents on porous silicon and other surface-based sensor platforms.

Design considerations of aptasensors for continuous monitoring of biomarkers in digestive tract fluids

We present a porous Si (PSi)-based label-free optical biosensor for sensitive and continuous detection of a model target protein biomarker in gastrointestinal (GI) tract fluids. The biosensing platform is designed to continuously monitor its target protein within the complex GI fluids without sample preparation and washing steps. An oxidized PSi Fabry-Pérot thin films are functionalized with aptamers, which are used as the capture probes. The optical response of the aptamer-conjugated PSi is studied upon exposure to unprocessed GI fluids, originated from domestic pigs, spiked with the target protein. We investigate biological and chemical surface passivation methods to stabilize the surface and reduce non-specific adsorption of interfering proteins and molecules within the GI fluids. For the passivated PSi aptasensor we simulate continuous in vivo biosensing conditions, demonstrating that the aptasensor could successfully detect the target in a continuous manner without any need for surface washing after the target protein binding events, at a clinically relevant range. Furthermore, we simulate biosensing conditions within a smart capsule, in which the aptasensor is occasionally exposed to GI fluids in flow or via repeated cycles of injection and static incubation events. Such biosensor can be implemented within ingestible capsule devices and used for in situ biomarker detection in the GI tract.

Modern Analytical Techniques for Detection of Bacteria in Surface and Wastewaters

Contamination of surface waters with pathogens as well as all diseases associated with such events are a significant concern worldwide. In recent decades, there has been a growing interest in developing analytical methods with good performance for the detection of this category of contaminants. The most important analytical methods applied for the determination of bacteria in waters are traditional ones (such as bacterial culturing methods, enzyme-linked immunoassay, polymerase chain reaction, and loop-mediated isothermal amplification) and advanced alternative methods (such as spectrometry, chromatography, capillary electrophoresis, surface-enhanced Raman scattering, and magnetic field-assisted and hyphenated techniques). In addition, optical and electrochemical sensors have gained much attention as essential alternatives for the conventional detection of bacteria. The large number of available methods have been materialized by many publications in this field aimed to ensure the control of water quality in water resources. This study represents a critical synthesis of the literature regarding the latest analytical methods covering comparative aspects of pathogen contamination of water resources. All these aspects are presented as representative examples, focusing on two important bacteria with essential implications on the health of the population, namely Pseudomonas aeruginosa and Escherichia coli.

Environmental Management of Legionella in Domestic Water Systems: Consolidated and Innovative Approaches for Disinfection Methods and Risk Assessment

Legionella is able to remain in water as free-living planktonic bacteria or to grow within biofilms that adhere to the pipes. It is also able to enter amoebas or to switch into a viable but not culturable (VBNC) state, which contributes to its resistance to harsh conditions and hinders its detection in water. Factors regulating Legionella growth, such as environmental conditions, type and concentration of available organic and inorganic nutrients, presence of protozoa, spatial location of microorganisms, metal plumbing components, and associated corrosion products are important for Legionella survival and growth. Finally, water treatment and distribution conditions may affect each of these factors. A deeper comprehension of Legionella interactions in water distribution systems with the environmental conditions is needed for better control of the colonization. To this purpose, the implementation of water management plans is the main prevention measure against Legionella. A water management program requires coordination among building managers, health care providers, and Public Health professionals. The review reports a comprehensive view of the state of the art and the promising perspectives of both monitoring and disinfection methods against Legionella in water, focusing on the main current challenges concerning the Public Health sector.

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.

Fabrication techniques of biomimetic scaffolds in three-dimensional cell culture: A review

In the last four decades, several researchers worldwide have routinely and meticulously exercised cell culture experiments in two-dimensional (2D) platforms. Using traditionally existing 2D models, the therapeutic efficacy of drugs has been inappropriately validated due to the failure in generating the precise therapeutic response. Fortunately, a 3D model addresses the foregoing limitations by recapitulating the in vivo environment. In this context, one has to contemplate the design of an appropriate scaffold for favoring the organization of cell microenvironment. Instituting pertinent model on the platter will pave way for a precise mimicking of in vivo conditions. It is because animal cells in scaffolds oblige spontaneous formation of 3D colonies that molecularly, phenotypically, and histologically resemble the native environment. The 3D culture provides insight into the biochemical aspects of cell-cell communication, plasticity, cell division, cytoskeletal reorganization, signaling mechanisms, differentiation, and cell death. Focusing on these criteria, this paper discusses in detail, the diversification of polymeric scaffolds based on their available resources. The paper also reviews the well-founded and latest techniques of scaffold fabrication, and their applications pertaining to tissue engineering, drug screening, and tumor model development.

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.

A hybrid electronic nose system for discrimination of pathogenic bacterial volatile compounds

A hybrid electronic nose comprising an array of three organic-inorganic nanocomposite gas sensors [zinc tetra tert-butyl phthalocyanine (ZnTTBPc), zinc tetra-phenyl porphyrin (ZnTPP), and cobalt tetraphenyl-porphyrin (CoTPP)] coupled with three commercial metal-oxide semiconductor gas sensors (TGS 2444, TGS 2603 and TGS 2620) was developed to discriminate bacterial volatile compounds. Each type of gas sensor had its own strengths and weaknesses in terms of its capability to detect complex odors from the five different bacterial species tested. Bacterial samples were controlled at a fixed initial bacterial concentration by measuring the optical density at 600 nm of the culture suspensions. A comparative evaluation of the volatile compound fingerprints from five bacterial species grown in Luria-Bertani medium was conducted to identify the optimal incubation time for detection of volatile biomarkers to discriminate among bacteria. The results suggest that the hybrid electronic nose was indeed able to discriminate among the bacterial species and culture media, with a variance based on contributions of 92.4% from PC1 and 7.2% from PC2, at an incubation time of 6 hours. Furthermore, the results of hierarchical cluster analysis showed that bacterial odor data formed two major bacterial groups, with the maximum cluster distance close to 25. Intra-group similarity was demonstrated as the two bacterial species (E. cloacae and P. aeruginosa) from among the Gram-negative bacteria had a greater similarity with a cluster distance close to 4. Finally, the minimum distance between E. cloacae and S. Typhi was approximately 1, at an equal distance from E. coli and S. aureus.

Mass Transfer Limitations of Porous Silicon-Based Biosensors for Protein Detection

Porous silicon (PSi) thin films have been widely studied for biosensing applications, enabling label-free optical detection of numerous targets. The large surface area of these biosensors has been commonly recognized as one of the main advantages of the PSi nanostructure. However, in practice, without application of signal amplification strategies, PSi-based biosensors suffer from limited sensitivity, compared to planar counterparts. Using a theoretical model, which describes the complex mass transport phenomena and reaction kinetics in these porous nanomaterials, we reveal that the interrelated effect of bulk and hindered diffusion is the main limiting factor of PSi-based biosensors. Thus, without significantly accelerating the mass transport to and within the nanostructure, the target capture performance of these biosensors would be comparable, regardless of the nature of the capture probe–target pair. We use our model to investigate the effect of various structural and biosensor characteristics on the capture performance of such biosensors and suggest rules of thumb for their optimization.

A lectin-coupled porous silicon-based biosensor: label-free optical detection of bacteria in a real-time mode

Accuracy and speed of detection, along with technical and instrumental simplicity, are indispensable for the bacterial detection methods. Porous silicon (PSi) has unique optical and chemical properties which makes it a good candidate for biosensing applications. On the other hand, lectins have specific carbohydrate-binding properties and are inexpensive compared to popular antibodies. We propose a lectin-conjugated PSi-based biosensor for label-free and real-time detection of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) by reflectometric interference Fourier transform spectroscopy (RIFTS). We modified meso-PSiO₂ (10–40 nm pore diameter) with three lectins of ConA (Concanavalin A), WGA (Wheat Germ Agglutinin), and UEA (Ulex europaeus agglutinin) with various carbohydrate specificities, as bioreceptor. The results showed that ConA and WGA have the highest binding affinity for E. coli and S. aureus respectively and hence can effectively detect them. This was confirmed by 6.8% and 7.8% decrease in peak amplitude of fast Fourier transform (FFT) spectra (at 10⁵ cells mL⁻¹ concentration). A limit of detection (LOD) of about 10³ cells mL⁻¹ and a linear response range of 10³ to 10⁵ cells mL⁻¹ were observed for both ConA-E. coli and WGA-S. aureus interaction platforms that are comparable to the other reports in the literature. Dissimilar response patterns among lectins can be attributed to the different bacterial cell wall structures. Further assessments were carried out by applying the biosensor for the detection of Klebsiella aerogenes and Bacillus subtilis bacteria. The overall obtained results reinforced the conjecture that the WGA and ConA have a stronger interaction with Gram-positive and Gram-negative bacteria, respectively. Therefore, it seems that specific lectins can be suggested for bacterial Gram-typing or even serotyping. These observations were confirmed by the principal component analysis (PCA) model.

FCC-Net: A Full-Coverage Collaborative Network for Weakly Supervised Remote Sensing Object Detection

With an ever-increasing resolution of optical remote-sensing images, how to extract information from these images efficiently and effectively has gradually become a challenging problem. As it is prohibitively expensive to label every object in these high-resolution images manually, there is only a small number of high-resolution images with detailed object labels available, highly insufficient for common machine learning-based object detection algorithms. Another challenge is the huge range of object sizes: it is difficult to locate large objects, such as buildings and small objects, such as vehicles, simultaneously. To tackle these problems, we propose a novel neural network based remote sensing object detector called full-coverage collaborative network (FCC-Net). The detector employs various tailored designs, such as hybrid dilated convolutions and multi-level pooling, to enhance multiscale feature extraction and improve its robustness in dealing with objects of different sizes. Moreover, by utilizing asynchronous iterative training alternating between strongly supervised and weakly supervised detectors, the proposed method only requires image-level ground truth labels for training. To evaluate the approach, we compare it against a few state-of-the-art techniques on two large-scale remote-sensing image benchmark sets. The experimental results show that FCC-Net significantly outperforms other weakly supervised methods in detection accuracy. Through a comprehensive ablation study, we also demonstrate the efficacy of the proposed dilated convolutions and multi-level pooling in increasing the scale invariance of an object detector.

Aptamers vs. antibodies as capture probes in optical porous silicon biosensors

Over the past decade aptamers have emerged as a promising class of bioreceptors for biosensing applications with significant advantages over conventional antibodies. However, experimental studies comparing aptasensors and immunosensors, under equivalent conditions, are limited and the results are inconclusive, in terms of benefits and limitations of each bioreceptor type. In the present work, the performance of aptamer and antibody bioreceptors for the detection of a his-tagged protein, used as a model target, is compared. The bioreceptors are immobilized onto a nanostructured porous silicon (PSi) thin film, used as the optical transducer, and the target protein is detected in a real-time and label-free format by reflective interferometric Fourier transform spectroscopy. For the antibodies, random-oriented immobilization onto the PSi nanostructure results in a poor biosensing performance. Contrary, Fc-oriented immobilization of the antibodies shows a similar biosensing performance to that exhibited by the aptamer-based biosensor, in terms of binding rate, dynamic detection range, limit of detection and selectivity. The aptasensor outperforms in terms of its reusability and storability, while the immunosensor could not be regenerated for subsequent experiments.

Structural, optical and terahertz properties of graphene-mesoporous silicon nanocomposites

We investigate the structural, optical and terahertz properties of graphene-mesoporous silicon nanocomposites using Raman, terahertz time-domain and photoluminescence spectroscopy. The nanocomposites consist of a free-standing mesoporous silicon membrane with its external and pore surfaces coated with few-layer graphene. Results show a stabilization of the porous silicon morphology by the graphene coating. The terahertz refractive index and absorption coefficient were found to increase with graphene deposition temperature. Four bands in the 1.79-2.2 eV range emerge from the PL spectra of the nanocomposites. The broad bands centered at 1.79 eV and 1.96 eV were demonstrated to originate from Si nanocrystallites of different sizes. The narrower bands at 2.11 eV and 2.14 eV could be related to a thin SiC film at the Si/C interface.

Non-lytic M13 phage-based highly sensitive impedimetric cytosensor for detection of coliforms

A highly sensitive and selective non-lytic M13 phage-based electrochemical impedance spectroscopy (EIS) cytosensor for early detection of coliforms is introduced for the first time. Gold nanoparticles were electrochemically deposited on the surface of glassy carbon electrode, and the M13 phage particles were immobilized on them using 3-mercaptopropionic acid linker and zero-length crosslinking chemistry (EDC/NHS). Next, the sensor surface was blocked to avoid non-specific binding. The M13-EIS cytosensor was tested for detection of F⁺ pili Escherichia coli species, using XL1-Blue and K12 strains, as examples of coliforms. The selectivity against non-host strains was demonstrated using Pseudomonas Chlororaphis. The binding of E. coli to the M13 phage on the cytosensor surface increased the charge transfer resistance, enabling detection of coliforms. The biosensor achieved a limit of detection (LOD) of 14 CFU/mL, the lowest reported to-date using EIS-phage sensors, and exhibited a high selectivity towards the tested coliforms. The SEM micrographs confirmed the successful capturing of E. coli on the M13-based EIS cytosensor. Moreover, the sensor showed almost the same sensitivity in the simulated river water samples as in phosphate buffer, reflecting its applicability to real samples. On the other hand, this sensor system exhibited high stability under harsh environmental conditions of pH (3.0-10.0) and temperature as high as 45 °C for up to two weeks. Overall, this sensor system has excellent potential for real field detection of fecal coliforms.

Nanosilicon-Based Composites for (Bio)sensing Applications: Current Status, Advantages, and Perspectives

This review highlights the application of different types of nanosilicon (nano-Si) materials and nano-Si-based composites for (bio)sensing applications. Different detection approaches and (bio)functionalization protocols were found for certain types of transducers suitable for the detection of biological compounds and gas molecules. The importance of the immobilization process that is responsible for biosensor performance (biomolecule adsorption, surface properties, surface functionalization, etc.) along with the interaction mechanism between biomolecules and nano-Si are disclosed. Current trends in the fabrication of nano-Si-based composites, basic gas detection mechanisms, and the advantages of nano-Si/metal nanoparticles for surface enhanced Raman spectroscopy (SERS)-based detection are proposed.

Comparative study of macroporous silicon-based photovoltaic characteristics using indium tin oxide-silicon and pn-silicon junction based devices

We report highly ordered macroporous silicon (Si)-based photovoltaic characteristics using indium tin oxide (ITO)/n-Si and pn-Si junction-based devices. The detailed fabrication processes including new controlled ITO etching are presented. Theoretical device simulations are performed to understand the presented device structures and propose an optimum device design based on processing limitations. The performance of ITO/n-Si junction devices directly depends on the conformal ITO coating along the pore surface. While pn-Si junction device requires additional doping step, the device can overcome the limitation of ITO conformal coating, especially for a device with high-aspect-ratio macropore structures. Experimental results also support the simulation analysis. The three-dimensional structural properties of well-defined macroporous Si coupled with the formation of photovoltaic devices are attractive for multi-functional applications.

TiO2 Nanolayer-Enhanced Fluorescence for Simultaneous Multiplex Mycotoxin Detection by Aptamer Microarrays on a Porous Silicon Surface

A new aptamer microarray method on the TiO₂-porous silicon (PSi) surface was developed to simultaneously screen multiplex mycotoxins. The TiO₂ nanolayer on the surface of PSi can enhance the fluorescence intensity 14 times than that of the thermally oxidized PSi. The aptamer fluorescence signal recovery principle was performed on the TiO₂-PSi surface by hybridization duplex strand DNA from the mycotoxin aptamer and antiaptamer, respectively, labeled with fluorescence dye and quencher. The aptamer microarray can simultaneously screen for multiplex mycotoxins with a dynamic linear detection range of 0.1-10 ng/mL for ochratoxin A (OTA), 0.01-10 ng/mL for aflatoxins B₁ (AFB₁), and 0.001-10 ng/mL for fumonisin B₁ (FB₁) and limits of detection of 15.4, 1.48, and 0.21 pg/mL for OTA, AFB₁, and FB₁, respectively. The newly developed method shows good specificity and recovery rates. This method can provide a simple, sensitive, and cost-efficient platform for simultaneous screening of multiplex mycotoxins and can be easily expanded to the other aptamer-based protocol.

Unraveling Antimicrobial Susceptibility of Bacterial Networks on Micropillar Architectures Using Intrinsic Phase-Shift Spectroscopy

With global antimicrobial resistance becoming increasingly detrimental to society, improving current clinical antimicrobial susceptibility testing (AST) is crucial to allow physicians to initiate appropriate antibiotic treatment as early as possible, reducing not only mortality rates but also the emergence of resistant pathogens. In this work, we tackle the main bottlenecks in clinical AST by designing biofunctionalized silicon micropillar arrays to provide both a preferable solid-liquid interface for bacteria networking and a simultaneous transducing element that monitors the response of bacteria when exposed to chosen antibiotics in real time. We harness the intrinsic ability of the micropillar architectures to relay optical phase-shift reflectometric interference spectroscopic measurements (referred to as PRISM) and employ it as a platform for culture-free, label-free phenotypic AST. The responses of E. coli to various concentrations of five clinically relevant antibiotics are optically tracked by PRISM, allowing for the minimum inhibitory concentration (MIC) values to be determined and compared to both standard broth microdilution testing and clinic-based automated AST system readouts. Capture of bacteria within these microtopologies, followed by incubation of the cells with the appropriate antibiotic solution, yields rapid determinations of antibiotic susceptibility. This platform not only provides accurate MIC determinations in a rapid manner (total assay time of 2-3 h versus 8 h with automated AST systems) but can also be employed as an advantageous method to differentiate bacteriostatic and bactericidal antibiotics.

Biosensors of bacterial cells

Biosensors are devices which utilize both an electrical component (transducer) and a biological component to study an environment. They are typically used to examine biological structures, organisms and processes. The field of biosensors has now become so large and varied that the technology can often seem impenetrable. Yet the principles which underlie the technology are uncomplicated, even if the details of the mechanisms are elusive. In this review we confine our analysis to relatively current advancements in biosensors for the detection of whole bacterial cells. This includes biosensors which rely on an added labeled component and biosensors which do not have a labeled component and instead detect the binding event or bound structure on the transducer. Methods to concentrate the bacteria prior to biosensor analysis are also described. The variety of biosensor types and their actual and potential uses are described.