Photonic crystals: emerging biosensors and their promise for point-of-care applications

All-printed point-of-care immunosensing biochip for one drop blood diagnostics

Designing and preparing a fast and easy-to-use immunosensing biochip are of great significance for clinical diagnosis and biomedical research. In particular, sensitive, specific, and early detection of biomarkers in trace samples promotes the application of point-of-care testing (POCT). Here, we demonstrate an all-printed immunosensing biochip with the characteristics of hydrodynamic enrichment and photonic crystal-enhanced fluorescence. Direct quantitative detection of cardiac biomarkers via one drop of blood is achieved in 10 min. After simulating the hydrodynamic behavior of one droplet serum on the printed assay, creatine kinase-MB (CK-MB) has been recognized and located on the photonic crystal arrays. Benefiting from the fluorescence enhancement effect, quantitative detection of CK-MB has been demonstrated from 0.01 ng ml^-1 to 100 ng ml^-1, which is superior to the conventional enzyme-linked immunosorbent assay (ELISA). This strategy provides a general and easy-to-use approach for fast quantitative detection of biomarkers, which would be improved further for portable clinical diagnostics and home medical monitoring.

Frequency-controlled electrophoretic mobility of a particle within a porous, hollow shell

The unique properties of yolk-shell or rattle-type particles make them promising candidates for applications ranging from switchable photonic crystals, to catalysts, to sensors. To realize many of these applications it is important to gain control over the dynamics of the core particle independently of the shell.

HYPOTHESIS

The core particle may be manipulated by an AC electric field with rich frequency-dependent behavior.

EXPERIMENTS

Here, we explore the frequency-dependent dynamic electrophoretic mobility of a charged core particle within a charged, porous shell in AC electric fields both experimentally using liquid-phase electron microscopy and numerically via the finite-element method. These calculations solve the Poisson-Nernst-Planck-Stokes equations, where the core particle moves according to the hydrodynamic and electric forces acting on it.

FINDINGS

In experiments the core exhibited three frequency-dependent regimes of field-driven motion: (i) parallel to the field, (ii) diffusive in a plane orthogonal to the field, and (iii) unbiased random motion. The transitions between the three observed regimes can be explained by the level of matching between the time required to establish ionic gradients in the shell and the period of the AC field. We further investigated the effect of shell porosity, ionic strength, and inner-shell radius. The former strongly impacted the core's behavior by attenuating the field inside the shell. Our results provide physical understanding on how the behavior of yolk-shell particles may be tuned, thereby enhancing their potential for use as building blocks for switchable photonic crystals.

Emerging Bioanalytical Devices and Platforms for Rapid Detection of Pathogens in Environmental Samples

The development of robust bioanalytical devices and biosensors for infectious pathogens is progressing well with the advent of new materials, concepts, and technology. The progress is also stepping towards developing high throughput screening technologies that can quickly identify, differentiate, and determine the concentration of harmful pathogens, facilitating the decision-making process for their elimination and therapeutic interventions in large-scale operations. Recently, much effort has been focused on upgrading these analytical devices to an intelligent technological platform by integrating them with modern communication systems, such as the internet of things (IoT) and machine learning (ML), to expand their application horizon. This review outlines the recent development and applications of bioanalytical devices and biosensors to detect pathogenic microbes in environmental samples. First, the nature of the recent outbreaks of pathogenic microbes such as foodborne, waterborne, and airborne pathogens and microbial toxins are discussed to understand the severity of the problems. Next, the discussion focuses on the detection systems chronologically, starting with the conventional methods, advanced techniques, and emerging technologies, such as biosensors and other portable devices and detection platforms for pathogens. Finally, the progress on multiplex assays, wearable devices, and integration of smartphone technologies to facilitate pathogen detection systems for wider applications are highlighted.

Fast Self-Assembly of Photonic Crystal Hydrogel for Wearable Strain and Temperature Sensor

Structural colors from photonic crystals (PCs) have attracted emerging attention in the research area of wearable sensors. Conventional self-assembly of PC takes days to weeks. Here, a fast self-assembly method of PC with horizontal precipitation of silica nanoparticles (NPs) in a polydimethylsiloxane fence, which can be completed within 1-4 h depending on the fence parameters, is introduced. The resultant PC exhibits tunable structural colors in the entire visible spectrum. With infiltration of composite hydrogels containing acrylic acid, acrylamide, chitosan, and carbon nanotubes (CNTs) into the gaps of NPs to form an inverse opal PC, a structural color hydrogel that can quickly respond to different stimuli, including strain and temperature, is obtained. Moreover, with the addition of CNTs, the composite PC hydrogel can also output an electronic signal together with optical color changes. Based on these extraordinary responsive behaviors, the PC hydrogel sensor for quantitative feedback to external stimuli of stretching, bending, pressing, and thermal stimuli, with brilliant color change and electronic signal outputs simultaneously, is demonstrated. This fast-assembled PC hydrogel with excellent responsive properties has great potential for applications in wearable devices, mechanical sensors, temperature sensors, and colorimetric displays.

Hydrogel Nanoarchitectonics: An Evolving Paradigm for Ultrasensitive Biosensing

The integration of nanoarchitectonics and hydrogel into conventional biosensing platforms offers the opportunities to design physically and chemically controlled and optimized soft structures with superior biocompatibility, better immobilization of biomolecules, and specific and sensitive biosensor design. The physical and chemical properties of 3D hydrogel structures can be modified by integrating with nanostructures. Such modifications can enhance their responsiveness to mechanical, optical, thermal, magnetic, and electric stimuli, which in turn can enhance the practicality of biosensors in clinical settings. This review describes the synthesis and kinetics of gel networks and exploitation of nanostructure-integrated hydrogels in biosensing. With an emphasis on different integration strategies of hydrogel with nanostructures, this review highlights the importance of hydrogel nanostructures as one of the most favorable candidates for developing ultrasensitive biosensors. Moreover, hydrogel nanoarchitectonics are also portrayed as a promising candidate for fabricating next-generation robust biosensors.

Advancement in Silicon Integrated Photonics Technologies for Sensing Applications in Near-Infrared and Mid-Infrared Region: A Review

Exploration and implementation of silicon (Si) photonics has surged in recent years since both photonic component performance and photonic integration complexity have considerably improved. It supports a wide range of datacom and telecom applications, as well as sensors, including light detection and ranging, gyroscopes, biosensors, and spectrometers. The advantages of low-loss Si WGs with compact size and excellent uniformity, resulting from the high quality and maturity of the Si complementary metal oxide semiconductor (CMOS) environment, are major drivers for using Si in photonics. Moreover, it has a high refractive index and a reasonably large mid-infrared (MIR) transparency window, up to roughly 7 μm wavelength, making it beneficial as a passive mid-IR optical material. Several gases and compounds with high absorption properties in the MIR spectral region are of prodigious curiosity for industrial, medicinal, and environmental applications. In comparison to current bulky systems, the implementation of Si photonics devices in this wavelength range might allow inexpensive and small optical sensing devices with greater sensitivity (S), power usage, and mobility. In this review, recent advances in Si integrated photonic sensors working in both near-infrared (NIR) and MIR wavelength ranges are discussed. We believe that this paper will be valuable for the scientific community working on Si photonic sensing devices.

Three-Dimensional Ordered Macroporous Magnetic Inverse Photonic Crystal Microsphere-Based Molecularly Imprinted Polymer for Selective Capture of Aflatoxin B1

Development of an efficient detection method to monitor residual mycotoxins in food is very important to ensure food safety, but the complex food matrix seriously affects the detection sensitivity and accuracy. Here, using a three-dimensional ordered macroporous magnetic inverse photonic crystal microsphere (MPCM) as the supporting material, a molecularly imprinted polymer (MIP) that can selectively recognize aflatoxin B₁ (AFB₁) was synthesized through the dummy template imprinting strategy. The MPCM@MIP prepared by employing 5,7-dimethoxycoumarin as the template and methacrylic acid as the functional monomer displayed selectivity toward AFB₁ (imprinting factor of 1.5) and could be used as a solid-phase extraction material. By coupling with high-performance liquid chromatography, an analytical method targeting AFB₁ was established and displayed a wide linear range of 5-1000 ng/mL with a low detection limit of 0.4 ng/mL. The method showed a good recovery rate of 73-92% in AFB₁-spiked soy sauce and vinegar samples. Moreover, the MPCM@MIP could be separated from the sample solution easily because of its magnetic performance, displaying a promising future not only in the enrichment of AFB₁ to improve the detection sensitivity and accuracy but also in the removal of AFB₁ from food and environmental samples.

Imprinted Photonic Crystal-Film-Based Smartphone-Compatible Label-Free Optical Sensor for SARS-CoV-2 Testing

The coronavirus disease (COVID-19) caused by SARS-CoV-2 has caused a global pandemic. To manage and control the spread of the infection, it is crucial to develop and implement technologies for the early identification of infected individuals and rapid informatization in communities. For the realization of such a technology, a widely available and highly usable sensor for sensitive and specific assay of the virus plays a fundamental role. In this study, we developed an optical sensor based on an imprinted photonic crystal film (IPCF) for quick, simple, and cost-effective detection of SARS-CoV-2 spike protein in artificial saliva. Our IPCF sensor enabled label-free and highly sensitive detection with a smartphone-equipped optical setup. The IPCF surface was functionalized with an anti-SARS-CoV-2 spike protein antibody for immunoassay. We evaluated the specificity and sensitivity of the IPCF sensor for quantitative detection of the spike protein in artificial saliva using simple reflectometry with a spectrometer-equipped optical setup. Specific and quantitative detection of the spike protein was successfully achieved, with a low detection limit of 429 fg/mL. In the demonstration of reflectometric detection with a smartphone-equipped setup, the sensitivity was comparable with that with a spectrometer-equipped setup. The test result is returned immediately and can be saved to cloud storage. In addition, it costs less than USD 1 for one IPCF to be used for diagnosis. Thus, the developed IPCF has the potential to realize a widely available and highly usable sensor.

Recent Advancement in Biofluid-Based Glucose Sensors Using Invasive, Minimally Invasive, and Non-Invasive Technologies: A Review

Biosensors have potentially revolutionized the biomedical field. Their portability, cost-effectiveness, and ease of operation have made the market for these biosensors to grow rapidly. Diabetes mellitus is the condition of having high glucose content in the body, and it has become one of the very common conditions that is leading to deaths worldwide. Although it still has no cure or prevention, if monitored and treated with appropriate medication, the complications can be hindered and mitigated. Glucose content in the body can be detected using various biological fluids, namely blood, sweat, urine, interstitial fluids, tears, breath, and saliva. In the past decade, there has been an influx of potential biosensor technologies for continuous glucose level estimation. This literature review provides a comprehensive update on the recent advances in the field of biofluid-based sensors for glucose level detection in terms of methods, methodology and materials used.

Multifunctional Photonic Janus Particles

Photonic Janus particles with a sphere fused to a cone are created from the phase separation of dendronized brush block copolymers (den-BBCP) and poly(4-vinylpyridine)-r-polystyrene (P4VP-r-PS) during the solvent evaporation of oil-in-water emulsions. Rapid self-assembly of den-BBCP generates well-ordered lamellar structures stacking along the long axis of the particles, producing structural colors that are dependent on the incident light angle. The colors are tunable over the visible spectrum by varying the molecular weight of den-BBCP. The P4VP-r-PS phase can undergo further surface modifications to produce multifunctional photonic Janus particles. Specifically, real-time magnetic control of the reflected color is achieved by coating the P4VP-r-PS phase with citric acid-capped Fe₃O₄ nanoparticles. Charged biomolecules (i.e., antibodies) are electrostatically immobilized to the Fe₃O₄ coating for potential applications in biosensing. As a demonstration, a new photonic sensor for the foodborne pathogen Salmonella is developed with antibody-modified photonic Janus particles, where the angle-dependent structural color plays a key role in the sensing mechanism.

Titanium nitride as an alternative and reusable plasmonic substrate for fluorescence coupling

The development of alternative plasmonic materials that can replace gold and silver is of long-standing interest in materials research. In this study, we have prepared and characterized thin films of TiN, an emerging plasmonic material, and examined its effectiveness for fluorescence coupling in metal-dielectric structures having TiN as the plasmonically active component. We have used a combination of experiment and reflectivity calculations to determine the nature and dispersion of the optical modes sustained by the metal-dielectric structures, which furthermore are adjustable by varying the thickness of the dielectric layer. Our results reveal that fluorophores placed on the TiN substrates can couple with the surface-plasmon mode and/or the waveguide modes supported by these structures, to provide polarized and directional emission over narrow angular ranges. The performance of TiN substrates for surface plasmon-coupled emission (SPCE) and waveguide-coupled emission (WGCE) is found to be comparable with conventional Au substrates. Importantly, the TiN thin films are reusable, which is certainly advantageous for their use in SPCE or WGCE-based fluorescence sensing applications.

Emerging biosensing and transducing techniques for potential applications in point-of-care diagnostics

With the deepening of our understanding in life science, molecular biology, nanotechnology, optics, electrochemistry and other areas, an increasing number of biosensor design strategies have emerged in recent years, capable of providing potential practical applications for point-of-care (POC) diagnosis in various human diseases. Compared to conventional biosensors, the latest POC biosensor research aims at improving sensor precision, cost-effectiveness and time-consumption, as well as the development of versatile detection strategies to achieve multiplexed analyte detection in a single device and enable rapid diagnosis and high-throughput screening. In this review, various intriguing strategies in the recognition and transduction of POC (from 2018 to 2021) are described in light of recent advances in CRISPR technology, electrochemical biosensing, and optical- or spectra-based biosensing. From the perspective of promoting emerging bioanalytical tools into practical POC detecting and diagnostic applications, we have summarized key advances made in this field in recent years and presented our own perspectives on future POC development and challenges.

Photonic crystals with split ring unit cells for subwavelength light confinement

Here we report a photonic crystal with a split ring unit cell shape that demonstrates an order of magnitude larger peak electric field energy density compared with that of a traditional photonic crystal. Split ring photonic crystals possess several subwavelength tuning parameters, including split ring rotation angle and split width, which can be leveraged to modify light confinement for specific applications. Modifying the split ring's parameters allows for tuning of the peak electric field energy density in the split by over one order of magnitude and tuning of the air band edge wavelength by nearly 10 nm in the near infrared region. Designed to have highly focused optical energy in an accessible subwavelength gap, the split ring photonic crystal is well suited for applications including optical biosensing, optical trapping, and enhanced emission from a quantum dot or other nanoscale emitter that could be incorporated in the split.

Highly Ordered, Plasmonic Enhanced Inverse Opal Photonic Crystal for Ultrasensitive Detection of Staphylococcal Enterotoxin B

Although there is considerable interest in self-assembly of ordered, porous "inverse opal" structures for optical, electronic, and chemical applications, uncontrolled defect formation limits the usefulness of such materials. Herein, we develop a highly ordered and plasmonic enhanced sensing inverse opal photonic crystal (IOPC) material. The co-assembly of the colloidal template with the matrix material avoids the need for liquid penetration into the preassembled colloidal crystals and minimizes the associated rupture and inhomogeneity of the resulting IOPC. Au nanoparticles (Au NPs) not only act as a "bridge" between recognition elements (aptamers) and IOPCs, but also can amplify optical signals. Furthermore, the enhancement mechanism of Au NPs is simulated by COMSOL. During the detection process, the optical signal of the sensing Au-Apt IOPC responds to the Staphylococcal enterotoxin B with a concentration ranging from 10^-2 to 10³ pg mL^-1, and the limit of detection is 2.820 fg mL^-1. Spiked real sample detection indicates that the as-proposed method possessed good accuracy. The sensing Au-Apt IOPC provides an extensive biosensor platform to detect a variety of toxic and harmful substances through replacing the aptamer by other recognition elements, such as antibodies or receptors.

Large-Scale Nanogrooved Photonic Crystals for Label-Free Biosensing by Guided-Mode Resonance

We have developed large-scale one-dimensional photonic crystals from standard recordable Blu-ray disks, tailored to sense unlabeled biorecognition events on their surface. These materials rely on coating, with layers of 80 nm of titanium oxide, nanogrooved polycarbonate plates obtained from regular disks. As a result, they present guided-mode resonances that we have demonstrated that can be exploited to quantify biorecognition events by means of the bandgap positions in the transmission spectra. These photonic crystals have displayed well-correlated dose-response curves in immunoassays to quantify IgGs, C-reactive protein, and lactate dehydrogenase. The detection limit reached is 16 ng/mL, 2μg/mL, and 18 ng/mL, respectively. Herein we describe the experimental procedures and methods to fabricate and functionalize these photonic crystals, perform immunoassays on them, set up an optical system to measure their response, and process the resulting data to perform bioanalytical determinations in label-free format.

Highly sensitive optical MEMS based photonic biosensor for colon tissue detection

<abstract> <p>Biological component of cells, protein has been effectively studied and investigated using biological sensors. Photonic crystal-based sensor is highly sensitive optical nanostructure it can be manipulated to affect the motion of photon for users' application. In the proposed work microcavity based photonic crystal biosensor has been designed and investigated for its different optical sensing evaluation parameters such as transmission efficiency, sensitivity, Q factor and peak resonant wavelengths. Sensor is designed and analyzed for early detection of colon cancer tissues in blood. Radius of defect micropillar has been increased from 0.16 µm to 0.19 µm. High Quality factor 10232 has been achieved with the micro pillar radius of 0.17 µm and sensitivity 700nm/RIU. Similarly, radius of 0.16 µm, 0.18 µm and 0.19 µm has attained quality factor and sensitivity such as 5324, 7232, 8343 and 111 nm/RIU, 320 nm/RIU and 340 nm/RIU respectively. Compared other work in literature, proposed work has shown better sensing capability. Designed sensor has shown remarkable output and feasibility for future fabrication.</p> </abstract>

Mechanomaterials: A Rational Deployment of Forces and Geometries in Programming Functional Materials

The knowledge of mechanics of materials has been extensively implemented in developing functional materials, giving rise to recent advances in soft actuators, flexible electronics, mechanical metamaterials, tunable mechanochromics, regenerative mechanomedicine, etc. While conventional mechanics of materials offers passive access to mechanical properties of materials in existing forms, a paradigm shift is emerging toward proactive programming of materials' functionality by leveraging the force-geometry-property relationships. Here, such a rising field is coined as "mechanomaterials". To profile the concept, the design principles in this field at four scales is first outlined, namely the atomic scale, the molecular scale, the manipulation of nanoscale materials, and the microscale design of structural materials. A variety of techniques have been recruited to deliver the multiscale programming of functional mechanomaterials, such as strain engineering, capillary assembly, topological interlocking, kirigami, origami, to name a few. Engineering optical and biological functionalities have also been achieved by implementing the fundamentals of mechanochemistry and mechanobiology. Nonetheless, the field of mechanomaterials is still in its infancy, with many open challenges and opportunities that need to be addressed. The authors hope this review can serve as a modest spur to attract more researchers to further advance this field.

A Portable Micro-Gas Chromatography with Integrated Photonic Crystal Slab Sensors on Chip

The miniaturization of gas chromatography (GC) systems has made it possible to utilize the analytical technique in various on-site applications to rapidly analyze complex gas samples. Various types of miniaturized sensors have been developed for micro-gas chromatography (µGC). However, the integration of an appropriate detector in µGC systems still faces a significant challenge. We present a solution to the problem through integration of µGC with photonic crystal slab (PCS) sensors using transfer printing technology. This integration offers an opportunity to utilize the advantages of optical sensors, such as high sensitivity and rapid response time, and at the same time, compensate for the lack of detection specificity from which label-free optical sensors suffer. We transfer printed a 2D defect free PCS on a borofloat glass, bonded it to a silicon microfluidic gas cell or directly to a microfabricated GC column, and then coated it with a gas responsive polymer. Realtime spectral shift in Fano resonance of the PCS sensor was used to quantitatively detect analytes over a mass range of three orders. The integrated µGC–PCS system was used to demonstrate separation and detection of a complex mixture of 10 chemicals. Fast separation and detection (4 min) and a low detection limit (ng) was demonstrated.

Design of highly sensitive refractive index sensors in the visible region utilizing metal layer assisted guided modes

We present a systematic comparison of the metal layer assisted guided mode resonance-based sensing structures with the traditional guided mode resonance-based sensing structures sharing identical design parameters for various two-dimensional square hole and pillar grating type lattice configurations. The surface and volume integrals of the electromagnetic field intensity profiles at resonance have been computed for all the considered structures to show that the waveguide-pillar-based structures offer the strongest interaction between the resonant modes and the sensing region, resulting in a superior sensitivity. Further insights into the nature of metal assisted guided mode resonance-based sensors and the ways to generate a strong resonant response are reported for the visible range of operation.

High-Q 1D rod-based nanocavities

We report an analysis of one-dimensional rod-based photonic crystal nanocavities. These cavities offer opportunities for dielectric materials which lack a matching low-refractive index substrate or are limited in under-etching possibilities to create slab-based PhC cavities. They offer high theoretical Q-values exceeding 10⁶ for transverse magnetic polarized modes with modal volumes below 2.5(λ/n)³. For practical implementations, we propose embedding these structures in a low-refractive index polymer. An analysis of intentionally introduced variations in a rod diameter reveals which design directions should be followed in order to create cavities that are most robust for fabrication-induced variations.

Emerging Optical Materials in Sensing and Discovery of Bioactive Compounds

Optical biosensors are used in numerous applications and analytical fields. Advances in these sensor platforms offer high sensitivity, selectivity, miniaturization, and real-time analysis, among many other advantages. Research into bioactive natural products serves both to protect against potentially dangerous toxic compounds and to promote pharmacological innovation in drug discovery, as these compounds have unique chemical compositions that may be characterized by greater safety and efficacy. However, conventional methods for detecting these biomolecules have drawbacks, as they are time-consuming and expensive. As an alternative, optical biosensors offer a faster, simpler, and less expensive means of detecting various biomolecules of clinical interest. In this review, an overview of recent developments in optical biosensors for the detection and monitoring of aquatic biotoxins to prevent public health risks is first provided. In addition, the advantages and applicability of these biosensors in the field of drug discovery, including high-throughput screening, are discussed. The contribution of the investigated technological advances in the timely and sensitive detection of biotoxins while deciphering the pathways to discover bioactive compounds with great health-promoting prospects is envisaged to meet the increasing demands of healthcare systems.

Towards Promising Platform by Using Annular Photonic Crystals to Simulate and Design Useful Mask

Human masks are considered the mainstay in air filtration and purification technologies and against the spreading of bacterial and viral infections. This paper introduces a novel design of a human mask to increase the ultraviolet germicidal irradiation effect on pathogens. The proposed design consists of a tube with an annular photonic crystal (APC) attached to the mask’s orifice, and a UV source is located in the tube’s center. The main role of this study is the enhancement of UV doses based on the reflectivity of the proposed APC. Therefore, increasing pathogens’ inactivation level in the incoming air to the mask’s orifice could be investigated. The numerical investigations demonstrated that the proposed APC could provide a complete photonic bandgap with a high reflectivity in the wavelength regime from 207 to 230 nm. In addition, we have considered the roles of the thickness of layers, inner core radius, and the azimuthal number. Meanwhile, the results showed the ability to use a wide range of core radius values without almost any variations in the optical properties of the proposed design. Such results could grant the advantage of using this design by the manufacturing of human masks with different sizes besides the inclusions in other ultraviolet germicidal irradiation applications.

Polymerization-Induced Hierarchical Self-Assembly: From Monomer to Complex Colloidal Molecules and Beyond

The nanoscale hierarchical design that draws inspiration from nature's biomaterials allows the enhancement of material performance and enables multifarious applications. Self-assembly of block copolymers represents one of these artificial techniques that provide an elegant bottom-up strategy for the synthesis of soft colloidal hierarchies. Fast-growing polymerization-induced self-assembly (PISA) renders a one-step process for the polymer synthesis and in situ self-assembly at high concentrations. Nevertheless, it is exceedingly challenging for the fabrication of hierarchical colloids via aqueous PISA, simply because most monomers produce kinetically trapped spheres except for a few PISA-suitable monomers. We demonstrate here a sequential one-pot synthesis of hierarchically self-assembled polymer colloids with diverse morphologies via aqueous PISA that overcomes the limitation. Complex formation of water-immiscible monomers with cyclodextrin via "host-guest" inclusion, followed by sequential aqueous polymerization, provides a linear triblock terpolymer that can in situ self-assemble into hierarchical nanostructures. To access polymer colloids with different morphologies, three types of linear triblock terpolymers were synthesized through this methodology, which allows the preparation of AX_n-type colloidal molecules (CMs), core-shell-corona micelles, and raspberry-like nanoparticles. Furthermore, the phase separations between polymer blocks in nanostructures were revealed by transmission electron microscopy and atomic force microscopy-infrared spectroscopy. The proposed mechanism explained how the interfacial tensions and glass transition temperatures of the core-forming blocks affect the morphologies. Overall, this study provides a scalable method of the production of CMs and other hierarchical structures. It can be applied to different block copolymer formulations to enrich the complexity of morphology and enable diverse functions of nano-objects.

Roadmap on Universal Photonic Biosensors for Real-Time Detection of Emerging Pathogens

The COVID-19 pandemic has made it abundantly clear that the state-of-the-art biosensors may not be adequate for providing a tool for rapid mass testing and population screening in response to newly emerging pathogens. The main limitations of the conventional techniques are their dependency on virus-specific receptors and reagents that need to be custom-developed for each recently-emerged pathogen, the time required for this development as well as for sample preparation and detection, the need for biological amplification, which can increase false positive outcomes, and the cost and size of the necessary equipment. Thus, new platform technologies that can be readily modified as soon as new pathogens are detected, sequenced, and characterized are needed to enable rapid deployment and mass distribution of biosensors. This need can be addressed by the development of adaptive, multiplexed, and affordable sensing technologies that can avoid the conventional biological amplification step, make use of the optical and/or electrical signal amplification, and shorten both the preliminary development and the point-of-care testing time frames. We provide a comparative review of the existing and emergent photonic biosensing techniques by matching them to the above criteria and capabilities of preventing the spread of the next global pandemic.

Softness-Enhanced Self-Assembly of Pyrochlore- and Perovskite-like Colloidal Photonic Crystals from Triblock Janus Particles

It remains extremely challenging to build three-dimensional photonic crystals with complete photonic bandgaps by simple and experimentally realizable colloidal building blocks. Here, we demonstrate that particle softness can enhance both the self-assembly of pyrochlore- and perovskite-like lattice structures from simple deformable triblock Janus colloids and their photonic bandgap performances. Dynamics simulation results show that the region of stability of pyrochlore lattices can be greatly expanded by appropriately increasing softness, and the perovskite lattices are unexpectedly obtained at enough high softness. Photonic calculations show that the direct pyrochlore lattices formed from overlapping soft triblock Janus particles exhibit even larger photonic bandgaps than the ideal nonoverlapping pyrochlore lattice, and proper overlap arising from softness can also dramatically improve the photonic properties of the inverse pyrochlore and perovskite lattices. Our study offers a new and feasible self-assembly path toward three-dimensional photonic crystals with large and robust photonic bandgaps.

Comprehensive Review Tapered Optical Fiber Configurations for Sensing Application: Trend and Challenges

Understanding environmental information is necessary for functions correlated with human activities to improve healthcare quality and reduce ecological risk. Tapered optical fibers reduce some limitations of such devices and can be considerably more responsive to fluorescence and absorption properties changes. Data have been collected from reliable sources such as Science Direct, IEEE Xplore, Scopus, Web of Science, PubMed, and Google Scholar. In this narrative review, we have summarized and analyzed eight classes of tapered-fiber forms: fiber Bragg grating (FBG), long-period fiber grating (LPFG), Mach-Zehnder interferometer (MZI), photonic crystals fiber (PCF), surface plasmonic resonance (SPR), multi-taper devices, fiber loop ring-down technology, and optical tweezers. We evaluated many issues to make an informed judgement about the viability of employing the best of these methods in optical sensors. The analysis of performance for tapered optical fibers depends on four mean parameters: taper length, sensitivity, wavelength scale, and waist diameter. Finally, we assess the most potent strategy that has the potential for medical and environmental applications.

Self-assembly of shape-tunable oblate colloidal particles into orientationally ordered crystals, glassy crystals and plastic crystals

The shapes of colloidal particles are crucial to self-assembled superstructures. Understanding the relationship between the shapes of building blocks and the resulting crystal structures is an important fundamental question. Here, we demonstrate that, by using particles whose shape interpolates between a flat disc and a sphere, not only are self-assembled superstructures but also their orientations sensitively dependent on the particle shape. By changing the shape gradually from a flat disc to a spherical shape, a crystal sequence from orientationally ordered crystals to orientationally disordered crystals with frozen and more free rotations are found. The latter two phases are identified as a glassy crystal and a plastic crystal, respectively. By combining theoretical model calculations, the formed crystal structures and the occurring transitions are found to be dictated by the interplay between particle shape and particle-particle interaction as well as particle-wall interaction. In particular, for quasi-spherical shapes, when the strong attraction dominates, a glassy crystal forms, or otherwise a plastic crystal forms. These results demonstrate that the interplay between the particle shape and the interaction can be used to tune crystallization and further fabricate colloid-based new structured and dynamic materials.

Lithographic Processes for the Scalable Fabrication of Micro- and Nanostructures for Biochips and Biosensors

Since the early 2000s, extensive research has been performed to address numerous challenges in biochip and biosensor fabrication in order to use them for various biomedical applications. These biochips and biosensor devices either integrate biological elements (e.g., DNA, proteins or cells) in the fabrication processes or experience post fabrication of biofunctionalization for different downstream applications, including sensing, diagnostics, drug screening, and therapy. Scalable lithographic techniques that are well established in the semiconductor industry are now being harnessed for large-scale production of such devices, with additional development to meet the demand of precise deposition of various biological elements on device substrates with retained biological activities and precisely specified topography. In this review, the lithographic methods that are capable of large-scale and mass fabrication of biochips and biosensors will be discussed. In particular, those allowing patterning of large areas from 10 cm² to m², maintaining cost effectiveness, high throughput (>100 cm² h^-1), high resolution (from micrometer down to nanometer scale), accuracy, and reproducibility. This review will compare various fabrication technologies and comment on their resolution limit and throughput, and how they can be related to the device performance, including sensitivity, detection limit, reproducibility, and robustness.

Bottom-Up Assembled Photonic Crystals for Structure-Enabled Label-Free Sensing

Photonic crystals (PhCs) display photonic stop bands (PSBs) and at the edges of these PSBs transport light with reduced velocity, enabling the PhCs to confine and manipulate incident light with enhanced light-matter interaction. Intense research has been devoted to leveraging the optical properties of PhCs for the development of optical sensors for bioassays, diagnosis, and environmental monitoring. These applications have furthermore benefited from the inherently large surface area of PhCs, giving rise to high analyte adsorption and the wide range of options for structural variations of the PhCs leading to enhanced light-matter interaction. Here, we focus on bottom-up assembled PhCs and review the significant advances that have been made in their use as label-free sensors. We describe their potential for point-of-care devices and in the review include their structural design, constituent materials, fabrication strategy, and sensing working principles. We thereby classify them according to five sensing principles: sensing of refractive index variations, sensing by lattice spacing variations, enhanced fluorescence spectroscopy, surface-enhanced Raman spectroscopy, and configuration transitions.

AIEgens enabled ultrasensitive point-of-care test for multiple targets of food safety: Aflatoxin B1 and cyclopiazonic acid as an example

Food safety is currently a significant issue for human life and health. Various fluorescent nanomaterials have been applied in the point-of-care test (POCT) for food safety as labeling materials. However, previous fluorescent nanomaterials can cause aggregation-caused quenching (ACQ), thus reducing the detection sensitivity. Conversely, aggregation-induced emission luminogens (AIEgens) are promising candidates for POCT in the food safety field because they can enhance detection sensitivity and throughput. Mycotoxins, such as aflatoxin B1 (AFB1) and cyclopiazonic acid (CPA), are a primary threat to human life and health and a significant food safety issue, and their on-site detection from farm to table is needed. Herein, an ultrasensitive point-of-care test was developed based on TPE-Br, a blue-emissive tetraphenylethylene derivative AIEgen. Under optimal conditions, this AIEgen-based lateral-flow biosensor (ALFB) allowed for a rapid response of 8 min toward AFB1 and CPA detection, with considerable sensitivities of 0.003 and 0.01 ng/mL in peanut matrices, respectively. In peanut matrices, the recoveries were 90.3%-110.0% for both mycotoxins, with relative standard deviations (RSDs) below 6%. The ALFB was further validated via UPLC-MS/MS using spiked peanut samples. AIEgens open an avenue for on-site, ultrasensitive, high-throughput detection methods and can be extensively used in point-of-care tests in food safety.

Supersoft Elastic Bottlebrush Microspheres with Stimuli-Responsive Color-Changing Properties in Brine

Solvent-free supersoft elastomer is highly desirable for building photonic structures with significant stimuli-responsive color changes. We report supersoft elastic porous microspheres with vivid structural colors obtained via self-assembly of amphiphilic bottlebrush block copolymers at the water/oil interface templated by ordered water-in-oil-in-water double emulsions. The porous structure is composed of cross-linked bottlebrush polydimethylsiloxane (PDMS) as the supersoft elastic skeleton and bottlebrush poly(ethylene oxide) (PEO) as the internal responsive layer. The obtained microspheres show large reversible volume changes through well-controlled dehydration or hydration of PEO in response to salt ions in an aqueous environment. As a result, full-spectrum colors are obtained dependent on different salt concentrations. In-situ observation of color reflection of a microsphere indicates a gradual structural transition from the outside to the inside corresponding to migration of water molecules and salt ions. Moreover, rod-like bottlebrush PEO exhibits an anion-induced salting-out behavior different from that of random coil polymers. The significantly responsive behaviors of bottlebrush block copolymer (BBCP) assemblies in the presence of salt ions primarily rely on the supersoft elastic skeleton of the porous structure, providing a facile route to the creation of stimuli-responsive photonic materials by low-cost self-assembly methods.

Rapid Responsive Mechanochromic Photonic Pigments with Alternating Glassy-Rubbery Concentric Lamellar Nanostructures

Photonic pigment particles prepared via self-assembly have been suffering from their poor mechanical performances; i.e., they can easily be damaged and lose structural color under a compression force. This greatly limits their uses as mechanochromic pigments. Here, a nanoscale concentric lamellar structure of alternating glassy-rubbery microdomains is successfully created within photonic microparticles through a confined self-assembly and photo-cross-linking strategy. The glassy domain is composed of polystyrene, and cross-linked bottlebrush polydimethylsiloxane served as the supersoft elastic domain. The obtained photonic structure not only shows large deformation and visible color changes under a loaded compression force but also rapidly recovers to its original state in less than 1 s (∼0.16 s) upon unloading. Continuously loading-unloading micro compression test indicates that no obvious damage can be identified after 250 cycles, indicating the high durability of the pigments against deformation. These pigments with different reflected colors are simply obtained using bottlebrush block copolymer formulations with tunable weight percentages of polymer additives. The mechanical robust photonic pigments may be useful in many important applications.

The anesthetic sevoflurane induces tau trafficking from neurons to microglia

Accumulation and spread of tau in Alzheimer's disease and other tauopathies occur in a prion-like manner. However, the mechanisms and downstream consequences of tau trafficking remain largely unknown. We hypothesized that tau traffics from neurons to microglia via extracellular vesicles (EVs), leading to IL-6 generation and cognitive impairment. We assessed mice and neurons treated with anesthetics sevoflurane and desflurane, and applied nanobeam-sensor technology, an ultrasensitive method, to measure tau/p-tau amounts. Sevoflurane, but not desflurane, increased tau or p-tau amounts in blood, neuron culture medium, or EVs. Sevoflurane increased p-tau amounts in brain interstitial fluid. Microglia from tau knockout mice took up tau and p-tau when treated with sevoflurane-conditioned neuron culture medium, leading to IL-6 generation. Tau phosphorylation inhibitor lithium and EVs generation inhibitor GW4869 attenuated tau trafficking. GW4869 mitigated sevoflurane-induced cognitive impairment in mice. Thus, tau trafficking could occur from neurons to microglia to generate IL-6, leading to cognitive impairment.

Spatially Shaping Waves to Penetrate Deep inside a Forbidden Gap

It is well known that waves with frequencies within the forbidden gap inside a crystal are transported only over a limited distance-the Bragg length-before being reflected by Bragg interference. Here, we demonstrate how to send waves much deeper into crystals in an exemplary study of light in two-dimensional silicon photonic crystals. By spatially shaping the wave fronts, the internal energy density-probed via the laterally scattered intensity-is enhanced at a tunable distance away from the front surface. The intensity is up to 100× enhanced compared to random wave fronts, and extends as far as 8× the Bragg length, which agrees with an extended mesoscopic model. We thus report a novel control knob for mesoscopic wave transport that pertains to any kind of waves.

Intelligent Packaging for Real-Time Monitoring of Food-Quality: Current and Future Developments

Food packaging encompasses the topical role of preserving food, hence, extending the shelf-life, while ensuring the highest quality and safety along the production chain as well as during storage. Intelligent food packaging further develops the functions of traditional packages by introducing the capability of continuously monitoring food quality during the whole chain to assess and reduce the insurgence of food-borne disease and food waste. To this purpose, several sensing systems based on different food quality indicators have been proposed in recent years, but commercial applications remain a challenge. This review provides a critical summary of responsive systems employed in the real-time monitoring of food quality and preservation state. First, food quality indicators are briefly presented, and subsequently, their exploitation to fabricate intelligent packaging based on responsive materials is discussed. Finally, current challenges and future trends are reviewed to highlight the importance of concentrating efforts on developing new functional solutions.

Optomechanical crystals for spatial sensing of submicron sized particles

Optomechanical crystal cavities (OMC) have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, bacteria and viruses. In this work we demonstrate the working principle of OMCs operating under ambient conditions as a sensor of submicrometer particles by optically monitoring the frequency shift of thermally activated mechanical modes. The resonator has been specifically designed so that the cavity region supports a particular family of low modal-volume mechanical modes, commonly known as -pinch modes-. These involve the oscillation of only a couple of adjacent cavity cells that are relatively insensitive to perturbations in other parts of the resonator. The eigenfrequency of these modes decreases as the deformation is localized closer to the centre of the resonator. Thus, by identifying specific modes that undergo a frequency shift that amply exceeds the mechanical linewidth, it is possible to infer if there are particles deposited on the resonator, how many are there and their approximate position within the cavity region. OMCs have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, viruses and bacteria.

Smartphone-based optical spectroscopic platforms for biomedical applications: a review [Invited]

Rapid advancements in smartphone technology have enabled the integration of many optical detection techniques that leverage the embedded functional components and software platform of these sophisticated devices. Over the past few years, several research groups have developed high-resolution smartphone-based optical spectroscopic platforms and demonstrated their usability in different biomedical applications. Such platforms provide unprecedented opportunity to develop point-of-care diagnostics systems, especially for resource-constrained environments. In this review, we discuss the development of smartphone systems for optical spectroscopy and highlight current challenges and potential solutions to improve the scope for their future adaptability.

Single-step, wash-free digital immunoassay for rapid quantitative analysis of serological antibody against SARS-CoV-2 by photonic resonator absorption microscopy

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the cause of Coronavirus Disease 2019 (COVID-19), poses extraordinary threats and complex challenges to global public health. Quantitative measurement of SARS-CoV-2 antibody titer plays an important role in understanding the patient-to-patient variability of immune response, assessing the efficacy of vaccines, and identifying donors for blood transfusion therapy. There is an urgent and ever-increasing demand for serological COVID-19 antibody tests that are highly sensitive, quantitative, rapid, simple, minimally invasive, and inexpensive. In this work, we developed a single-step, wash-free immunoassay for rapid and highly sensitive quantitative analysis of serological human IgG against SARS-CoV-2 which requires only a single droplet of serum. By simply incubating 4 μL human serum samples with antibody-functionalized gold nanoparticles, a photonic crystal optical biosensor coated with the recombinant spike protein serves as a sensing platform for the formation of sandwich immunocomplex through specific antigen-antibody interactions, upon which the detected IgG molecules can be counted with digital precision. We demonstrated a single-step 15-min assay capable of detecting as low as 100 pg mL-1 human COVID-19 IgG in serum samples. The calculated limit of detecting (LOD) and limit of quantification (LOQ) is 26.7 ± 7.7 and 32.0 ± 8.9 pg mL-1, respectively. This work represents the first utilization of the Activate Capture + Digital Counting (AC + DC)-based immunoassay for rapid and quantitative analysis of serological COVID-19 antibody, demonstrating a route toward point-of-care testing, using a portable detection instrument. On the basis of the sandwich immunoassay principle, the biosensing platform can be extended for the multiplexed detection of antigens, additional IgGs, cytokines, and other protein biomarkers.

Dynamic Colloidal Photonic Crystal Hydrogels with Self-Recovery and Injectability

Simulation of self-recovery and diversity of natural photonic crystal (PC) structures remain great challenges for artificial PC materials. Motivated by the dynamic characteristics of PC nanostructures, here, we present a new strategy for the design of hydrogel-based artificial PC materials with reversible interactions in the periodic nanostructures. The dynamic PC hydrogels, derived from self-assembled microgel colloidal crystals, were tactfully constructed by reversible crosslinking of adjacent microgels in the ordered structure via phenylboronate covalent chemistry. As proof of concept, three types of dynamic colloidal PC hydrogels with different structural colors were prepared. All the hydrogels showed perfect self-healing ability against physical damage. Moreover, dynamic crosslinking within the microgel crystals enabled shear-thinning injection of the PC hydrogels through a syringe (indicating injectability or printability), followed by rapid recovery of the structural colors. In short, in addition to the great significance in biomimicry of self-healing function of natural PC materials, our work provides a facile strategy for the construction of diversified artificial PC materials for different applications such as chem-/biosensing, counterfeit prevention, optical display, and energy conversion.

Directional and Reconfigurable Assembly of Metallodielectric Patchy Particles

Colloidal particles with surface patches can self-assemble with high directionality, but the resulting assemblies cannot reconfigure unless the patch arrangement (number, symmetry, etc.) is altered. While external fields with tunable inputs can guide the assembly of dynamic structures, they encourage particle alignment relative to its shape rather than the surface patterns. Here, we report on the synthesis of metallodielectric patchy particles and their assembly under the AC electric field, which gives rise to a series of structures including two-layer alternating chains, open-brick walls, staggering stacks, and vertical chains that are directed by the patches yet reconfigurable by the field. The configurations of the assemblies (e.g., the chains) can be further switched between a rigid and a flexible state emulating the conformations of polymers. Our work suggests that, for directed colloidal assembly, the particle complexities (patches and shapes) can be coupled with the external manipulations in a cooperative manner for creating materials with precise yet reconfigurable structures.

Biosensing platforms based on silicon nanostructures: A critical review

Biosensors are revolutionizing the health-care systems worldwide, permitting to survey several diseases, even at their early stage, by using different biomolecules such as proteins, DNA, and other biomarkers. However, these sensing approaches are still scarcely diffused outside the specialized medical and research facilities. Silicon is the undiscussed leader of the whole microelectronics industry, and novel sensors based on this material may completely change the health-care scenario. In this review, we will show how novel sensing platforms based on Si nanostructures may have a disruptive impact on applications with a real commercial transfer. A critical study for the main Si-based biosensors is herein presented with a comparison of their advantages and drawbacks. The most appealing sensing devices are discussed, starting from electronic transducers, with Si nanowires field-effect transistor (FET) and porous Si, to their optical alternatives, such as effective optical thickness porous silicon, photonic crystals, luminescent Si quantum dots, and finally luminescent Si NWs. All these sensors are investigated in terms of working principle, sensitivity, and selectivity with a specific focus on the possibility of their industrial transfer, and which ones may be preferred for a medical device.

Biosensing strategies for the electrochemical detection of viruses and viral diseases - A review

Viruses are the causing agents for many relevant diseases, including influenza, Ebola, HIV/AIDS, and COVID-19. Its rapid replication and high transmissibility can lead to serious consequences not only to the individual but also to collective health, causing deep economic impacts. In this scenario, diagnosis tools are of significant importance, allowing the rapid, precise, and low-cost testing of a substantial number of individuals. Currently, PCR-based techniques are the gold standard for the diagnosis of viral diseases. Although these allow the diagnosis of different illnesses with high precision, they still present significant drawbacks. Their main disadvantages include long periods for obtaining results and the need for specialized professionals and equipment, requiring the tests to be performed in research centers. In this scenario, biosensors have been presented as promising alternatives for the rapid, precise, low-cost, and on-site diagnosis of viral diseases. This critical review article describes the advancements achieved in the last five years regarding electrochemical biosensors for the diagnosis of viral infections. First, genosensors and aptasensors for the detection of virus and the diagnosis of viral diseases are presented in detail regarding probe immobilization approaches, detection methods (label-free and sandwich), and amplification strategies. Following, immunosensors are highlighted, including many different construction strategies such as label-free, sandwich, competitive, and lateral-flow assays. Then, biosensors for the detection of viral-diseases-related biomarkers are presented and discussed, as well as point of care systems and their advantages when compared to traditional techniques. Last, the difficulties of commercializing electrochemical devices are critically discussed in conjunction with future trends such as lab-on-a-chip and flexible sensors.

Fluorescence Coupling to Internal Modes of 1D Photonic Crystals Characterized by Back Focal Plane Imaging

The coupling of fluorescence with surface electromagnetic modes, such as surface plasmons on thin metal films or Bloch surface waves (BSW) on truncated one-dimensional photonic crystals (1DPC), are presently utilized for many fluorescence-based applications. In addition to the surface wave, 1DPCs also support other electromagnetic modes that are confined within the 1DPC structure. These internal modes (IMs) have not received much attention for fluorescence coupling due to lack of spatial overlap of their electric fields with the surface bound fluorophores. However, our recent studies have indicated that the fluorescence coupling with IMs occurs quite efficiently. This observed internal mode-coupled emission (IMCE) is (similar to BSW-coupled emission) indeed wavelength dependent, directional and S-polarized. In this paper, we have carried out back-focal plane (BFP) imaging to reveal that the IMs of 1DPCs can couple with surface bound excited dye molecules, with or without a BSW mode presence. Depending on the emission wavelength, the coupling is observed with BSW and IMs or only IMs of the 1DPC structure. The experimental results are well matching with numerical simulations. The occurrence of IMCE regardless of the availability of BSWs removes the dependence on just the surface mode for obtaining coupled emission from 1DPCs. The observation of IMCE is expected to widen the scope of 1DPCs for surface-based fluorescence sensing and assays.