Chemical Functionalization of Plasmonic Surface Biosensors: A Tutorial Review on Issues, Strategies, and Costs

Detection of MicroRNA-155 Based on Lambda Exonuclease Selective Digestion and CRISPR/Cas12a-assisted Amplification

In numerous malignancies, miRNA-155 is overexpressed and has oncogenic activity because it is one of the most efficient microRNAs for inhibiting apoptosis in human cancer cells. As a result, the highest sensitive detection of the miRNA-155 gene is a technological instrument that can enable early cancer screening. In this study, a miRNA-155 biosensor was created to create a hairpin probe that can bind to the miRNA-155 gene using lambda nucleic acid exonuclease, which can cut the 5' phosphorylated double strand, and by the DNA probe is recognized by the Cas12a enzyme, which then activates Cas12a to catalyze trans-cutting produces strong fluorescence. Research finding, the target concentration's logarithm and corresponding fluorescence intensity have a strong linear connection, and the limit of detection (LOD) of the sensing system was determined to be 8.3 pM. In addition, the biosensor displayed exceptional specificity, low false-positive signal, and high sensitivity in detecting the miRNA-155 gene in serum samples. This study's creation of a biosensor that has high sensitivity, good selectivity, and is simple to operate provides promising opportunities for research into biosensor design and early cancer detection.

Optical fiber biosensors toward in vivo detection

This review takes stock of the various optical fiber-based biosensors that could be used for in vivo applications. We discuss the characteristics that biosensors must have to be suitable for such applications and the corresponding transduction modes. In particular, we focus on optical fiber biosensors based on fluorescence, evanescent wave, plasmonics, interferometry, and Raman phenomenon. The operational principles, implemented solutions, and performances are described and debated. The different sensing configurations, such as the side- and tip-based fiber biosensors, are illustrated, and their adaptation for in vivo measurements is discussed. The required implementation of multiplexed biosensing on optical fibers is shown. In particular, the use of multi-fiber assemblies, one of the most optimal configurations for multiplexed detection, is discussed. Different possibilities for multiple localized functionalizations on optical fibers are presented. A final section is devoted to the practical in vivo use of fiber-based biosensors, covering regulatory, sterilization, and packaging aspects. Finally, the trends and required improvements in this promising and emerging field are analyzed and discussed.

Exosome Isolation and Detection: From Microfluidic Chips to Nanoplasmonic Biosensor

Exosomes are becoming more widely acknowledged as significant circulating indicators for the prognosis and diagnosis of cancer. Circulating exosomes are essential to the development and spread of cancer, according to a growing body of research. Using existing technology, characterizing exosomes is quite difficult. Therefore, a direct, sensitive, and targeted approach to exosome detection will aid in illness diagnosis and prognosis. The review discusses the new strategies for exosome isolation and detection technologies from microfluidic chips to nanoplasmonic biosensors, analyzing the advantages and limitations of these new technologies. This review serves researchers to better understand exosome isolation and detection methods and to help develop better exosome isolating and detecting devices for clinical applications.

Plasmonic Fluorescence Sensors in Diagnosis of Infectious Diseases

The increasing demand for rapid, cost-effective, and reliable diagnostic tools in personalized and point-of-care medicine is driving scientists to enhance existing technology platforms and develop new methods for detecting and measuring clinically significant biomarkers. Humanity is confronted with growing risks from emerging and recurring infectious diseases, including the influenza virus, dengue virus (DENV), human immunodeficiency virus (HIV), Ebola virus, tuberculosis, cholera, and, most notably, SARS coronavirus-2 (SARS-CoV-2; COVID-19), among others. Timely diagnosis of infections and effective disease control have always been of paramount importance. Plasmonic-based biosensing holds the potential to address the threat posed by infectious diseases by enabling prompt disease monitoring. In recent years, numerous plasmonic platforms have risen to the challenge of offering on-site strategies to complement traditional diagnostic methods like polymerase chain reaction (PCR) and enzyme-linked immunosorbent assays (ELISA). Disease detection can be accomplished through the utilization of diverse plasmonic phenomena, such as propagating surface plasmon resonance (SPR), localized SPR (LSPR), surface-enhanced Raman scattering (SERS), surface-enhanced fluorescence (SEF), surface-enhanced infrared absorption spectroscopy, and plasmonic fluorescence sensors. This review focuses on diagnostic methods employing plasmonic fluorescence sensors, highlighting their pivotal role in swift disease detection with remarkable sensitivity. It underscores the necessity for continued research to expand the scope and capabilities of plasmonic fluorescence sensors in the field of diagnostics.

Exploiting the design of surface plasmon resonance interfaces for better diagnostics: A perspective review

Surface Plasmon Resonance based-sensors are promising tools for precision diagnostics as they can provide tests useful for early and, whenever possible, non-invasive disease detection and monitoring. The design of novel, robust and effective interfaces enabling the sensing of a variety of molecular interactions in a highly selective and sensitive manner is a necessary step to obtain both accurate and reliable detection by SPR. This review covers the recent research efforts in this area, specifically emphasizing well-designed interfaces and applications in real-life samples. In particular, after a short introduction which identifies some of the critical challenges, the emerging strategies for the integration of the linker, the metal substrate and the recognition element on the sensing interface will be explored and discussed in three sections, as well as the opportunities for building SPR biosensors, easy to use, and with excellent sensitivities. Finally, a summary of some of the more promising and latest diagnostic applications will be provided, presenting a new window into the near-future perspectives.

Surface chemistry applications and development of immunosensors using electrochemical impedance spectroscopy: A comprehensive review

Immunosensors are promising alternatives as detection platforms for the current gold standards methods. Electrochemical immunosensors have already proven their capability for the sensitive, selective, detection of target biomarkers specific to COVID-19, varying cancers or Alzheimer's disease, etc. Among the electrochemical techniques, electrochemical impedance spectroscopy (EIS) is a highly sensitive technique which examines the impedance of an electrochemical cell over a range of frequencies. There are several important critical requirements for the construction of successful impedimetric immunosensor. The applied surface chemistry and immobilisation protocol have impact on the electroanalytical performance of the developed immunosensors. In this Review, we summarise the building blocks of immunosensors based on EIS, including self-assembly monolayers, nanomaterials, polymers, immobilisation protocols and antibody orientation.

Calligraphy of Nanoplasmonic Bioink-Based Multiplex Immunosensor for Precision Immune Monitoring and Modulation

Immunomodulation therapies have attracted immense interest recently for the treatment of immune-related diseases, such as cancer and viral infections. This new wave of enthusiasm for immunomodulators, predominantly revolving around cytokines, has spurred emerging needs and opportunities for novel immune monitoring and diagnostic tools. Considering the highly dynamic immune status and limited window for therapeutic intervention, precise real-time detection of cytokines is critical to effectively monitor and manage the immune system and optimize the therapeutic outcome. The clinical success of such a rapid, sensitive, multiplex immunoanalytical platform further requires the system to have ease of integration and fabrication for sample sparing and large-scale production toward massive parallel analysis. In this article, we developed a nanoplasmonic bioink-based, label-free, multiplex immunosensor that can be readily "written" onto a glass substrate via one-step calligraphy patterning. This facile nanolithography technique allows programmable patterning of a minimum of 3 μL of nanoplasmonic bioink in 1 min and thus enables fabrication of a nanoplasmonic microarray immunosensor with 2 h simple incubation. The developed immunosensor was successfully applied for real-time, parallel detection of multiple cytokines (e.g., interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and transforming growth factor-beta (TGF-β)) in immunomodulated macrophage samples. This integrated platform synergistically incorporates the concepts of nanosynthesis, nanofabrication, and nanobiosensing, showing great potential in the scalable production of label-free multiplex immunosensing devices with superior analytical performance for clinical applications in immunodiagnostics and immunotherapy.

Delocalized Electric Field Enhancement through Near-Infrared Quasi-BIC Modes in a Hollow Cuboid Metasurface

The two main problems of dielectric metasurfaces for sensing and spectroscopy based on electromagnetic field enhancement are that resonances are mainly localized inside the resonator volume and that experimental Q-factors are very limited. To address these issues, a novel dielectric metasurface supporting delocalized modes based on quasi-bound states in the continuum (quasi-BICs) is proposed and theoretically demonstrated. The metasurface comprises a periodic array of silicon hollow nanocuboids patterned on a glass substrate. The resonances stem from the excitation of symmetry-protected quasi-BIC modes, which are accessed by perturbing the arrangement of the nanocuboid holes. Thanks to the variation of the unit cell with a cluster of four hollow nanocuboids, polarization-insensitive, delocalized modes with ultra-high Q-factor are produced. In addition, the demonstrated electric field enhancements are very high (103-104). This work opens new research avenues in optical sensing and advanced spectroscopy, e.g., surface-enhanced Raman spectroscopy.

Rising to the surface: capturing and detecting bacteria by rationally-designed surfaces

Analytical microbiology has made substantial progress since its conception, starting from potato slices, through selective agar media, to engineered surfaces modified with capture probes. While the latter represents the dominant approach in designing sensors for bacteria detection, the importance of sensor surface properties is frequently ignored. Herein, we highlight their significant role in the complex process of bacterial transition from planktonic to sessile, representing the first and critical step in bacteria detection. We present the main surface features and discuss their effect on the bio-solid interface and the resulting sensing capabilities for both flat and particulate systems. The concepts of rationally-designed surfaces for enhanced bacterial detection are presented with recent examples of sensors (capture probe-free) relying solely on surface cues.

Imprinted Hydrogel Nanoparticles for Protein Biosensing: A Review

Over the past decade, molecular imprinting (MI) technology has made tremendous progress, and the advancements in nanotechnology have been the major driving force behind the improvement of MI technology. The preparation of nanoscale imprinted materials, i.e., molecularly imprinted polymer nanoparticles (MIP NPs, also commonly called nanoMIPs), opened new horizons in terms of practical applications, including in the field of sensors. Currently, hydrogels are very promising for applications in bioanalytical assays and sensors due to their high biocompatibility and possibility to tune chemical composition, size (microgels, nanogels, etc.), and format (nanostructures, MIP film, fibers, etc.) to prepare optimized analyte-responsive imprinted materials. This review aims to highlight the recent progress on the use of hydrogel MIP NPs for biosensing purposes over the past decade, mainly focusing on their incorporation on sensing devices for detection of a fundamental class of biomolecules, the peptides and proteins. The review begins by directing its focus on the ability of MIPs to replace biological antibodies in (bio)analytical assays and highlight their great potential to face the current demands of chemical sensing in several fields, such as disease diagnosis, food safety, environmental monitoring, among others. After that, we address the general advantages of nanosized MIPs over macro/micro-MIP materials, such as higher affinity toward target analytes and improved binding kinetics. Then, we provide a general overview on hydrogel properties and their great advantages for applications in the field of Sensors, followed by a brief description on current popular routes for synthesis of imprinted hydrogel nanospheres targeting large biomolecules, namely precipitation polymerization and solid-phase synthesis, along with fruitful combination with epitope imprinting as reliable approaches for developing optimized protein-imprinted materials. In the second part of the review, we have provided the state of the art on the application of MIP nanogels for screening macromolecules with sensors having different transduction modes (optical, electrochemical, thermal, etc.) and design formats for single use, reusable, continuous monitoring, and even multiple analyte detection in specialized laboratories or in situ using mobile technology. Finally, we explore aspects about the development of this technology and its applications and discuss areas of future growth.

From Whispering Gallery Mode Resonators to Biochemical Sensors

Optical biosensors are frontrunners for the rapid and real-time detection of analytes, particularly for low concentrations. Among them, whispering gallery mode (WGM) resonators have recently attracted a growing focus due to their robust optomechanical features and high sensitivity, measuring down to single binding events in small volumes. In this review, we provide a broad overview of WGM sensors along with critical advice and additional "tips and tricks" to make them more accessible to both biochemical and optical communities. Their structures, fabrication methods, materials, and surface functionalization chemistries are discussed. We propose this reflection under a pedagogical approach to describe and explain these biochemical sensors with a particular focus on the most recent achievements in the field. In addition to highlighting the advantages of WGM sensors, we also discuss and suggest strategies to overcome their current limitations, leaving room for further development as practical tools in various applications. We aim to provide new insights and combine different knowledge and perspectives to advance the development of the next generation of WGM biosensors. With their unique advantages and compatibility with different sensing modalities, these biosensors have the potential to become major game changers for biomedical and environmental monitoring, among many other relevant target applications.

Biomarkers as Biomedical Bioindicators: Approaches and Techniques for the Detection, Analysis, and Validation of Novel Biomarkers of Diseases

A biomarker is any measurable biological moiety that can be assessed and measured as a potential index of either normal or abnormal pathophysiology or pharmacological responses to some treatment regimen. Every tissue in the body has a distinct biomolecular make-up, which is known as its biomarkers, which possess particular features, viz., the levels or activities (the ability of a gene or protein to carry out a particular body function) of a gene, protein, or other biomolecules. A biomarker refers to some feature that can be objectively quantified by various biochemical samples and evaluates the exposure of an organism to normal or pathological procedures or their response to some drug interventions. An in-depth and comprehensive realization of the significance of these biomarkers becomes quite important for the efficient diagnosis of diseases and for providing the appropriate directions in case of multiple drug choices being presently available, which can benefit any patient. Presently, advancements in omics technologies have opened up new possibilities to obtain novel biomarkers of different types, employing genomic strategies, epigenetics, metabolomics, transcriptomics, lipid-based analysis, protein studies, etc. Particular biomarkers for specific diseases, their prognostic capabilities, and responses to therapeutic paradigms have been applied for screening of various normal healthy, as well as diseased, tissue or serum samples, and act as appreciable tools in pharmacology and therapeutics, etc. In this review, we have summarized various biomarker types, their classification, and monitoring and detection methods and strategies. Various analytical techniques and approaches of biomarkers have also been described along with various clinically applicable biomarker sensing techniques which have been developed in the recent past. A section has also been dedicated to the latest trends in the formulation and designing of nanotechnology-based biomarker sensing and detection developments in this field.

Modular and Substrate-Independent Grafting-To Procedure for Functional Polymer Coatings

The ability to tailor polymer brush coatings to the last nanometer has arguably placed them among the most powerful surface modification techniques currently available. Generally, the synthesis procedures for polymer brushes are designed for a specific surface type and monomer functionality and cannot be easily employed otherwise. Herein, we describe a modular and straightforward two-step grafting-to approach that allows introduction of polymer brushes of a desired functionality onto a large range of chemically different substrates. To illustrate the modularity of the procedure, gold, silicon oxide (SiO₂), and polyester-coated glass substrates were modified with five different block copolymers. In short, the substrates were first modified with a universally applicable poly(dopamine) primer layer. Subsequently, a grafting-to reaction was performed on the poly(dopamine) films using five distinct block copolymers, all of which contained a short poly(glycidyl methacrylate) segment and longer segment of varying chemical functionality. Ellipsometry, X-ray photoelectron spectroscopy, and static water contact angle measurements confirmed successful grafting of all five block copolymers to the poly(dopamine)-modified gold, SiO₂, and polyester-coated glass substrates. In addition, our method was used to provide direct access to binary brush coatings, by simultaneous grafting of two different polymer materials. The ability to synthesize binary brush coatings further adds to the versatility of our approach and paves the way toward production of novel multifunctional and responsive polymer coatings.

Continuous flow microfluidics for colloidal particle assembly on porous substrates

Self-assembly of colloidal particles for 'bottom-up' fabrication of various patterns and structures is critical for a range of applications including, but not limited to, energy generation and storage, nanomaterial structures, biomimetics, and biosensing. Multiple self-assembly techniques, such as substrate templating-via topological or chemical patterning-and solvent evaporation were discussed in our previous papers and have been developed for the deposition of patterned self-assembled structures, such as bands of colloidal particles, on various substrates. While the templating techniques are limited in applications due to the requirements for pattern-specific prior substrate engineering to fabricate the desired structure, solvent evaporation requires longer assembly times and precise control over environmental conditions. In this paper, a template-free, continuous flow process, which is facilitated by continuous solvent drainage through porous substrates, is demonstrated for the self-assembly of colloidal particles into high-aspect ratio (>10³, length to width) structures, such as linear arrays or grid structures. Colloidal particles were assembled both on polymeric and metallic porous membranes, with rapid assembly times.

Polymer Brushes on Silica Nanostructures Prepared by Aminopropylsilatrane Click Chemistry: Superior Antifouling and Biofunctionality

In nanobiotechnology, the importance of controlling interactions between biological molecules and surfaces is paramount. In recent years, many devices based on nanostructured silicon materials have been presented, such as nanopores and nanochannels. However, there is still a clear lack of simple, reliable, and efficient protocols for preventing and controlling biomolecule adsorption in such structures. In this work, we show a simple method for passivation or selective biofunctionalization of silica, without the need for polymerization reactions or vapor-phase deposition. The surface is simply exposed stepwise to three different chemicals over the course of ∼1 h. First, the use of aminopropylsilatrane is used to create a monolayer of amines, yielding more uniform layers than conventional silanization protocols. Second, a cross-linker layer and click chemistry are used to make the surface reactive toward thiols. In the third step, thick and dense poly(ethylene glycol) brushes are prepared by a grafting-to approach. The modified surfaces are shown to be superior to existing options for silica modification, exhibiting ultralow fouling (a few ng/cm2) after exposure to crude serum. In addition, by including a fraction of biotinylated polymer end groups, the surface can be functionalized further. We show that avidin can be detected label-free from a serum solution with a selectivity (compared to nonspecific binding) of more than 98% without the need for a reference channel. Furthermore, we show that our method can passivate the interior of 150 nm × 100 nm nanochannels in silica, showing complete elimination of adsorption of a sticky fluorescent protein. Additionally, our method is shown to be compatible with modifications of solid-state nanopores in 20 nm thin silicon nitride membranes and reduces the noise in the ion current. We consider these findings highly important for the broad field of nanobiotechnology, and we believe that our method will be very useful for a great variety of surface-based sensors and analytical devices.

Biofunctionalization of Multiplexed Silicon Photonic Biosensors

Silicon photonic (SiP) sensors offer a promising platform for robust and low-cost decentralized diagnostics due to their high scalability, low limit of detection, and ability to integrate multiple sensors for multiplexed analyte detection. Their CMOS-compatible fabrication enables chip-scale miniaturization, high scalability, and low-cost mass production. Sensitive, specific detection with silicon photonic sensors is afforded through biofunctionalization of the sensor surface; consequently, this functionalization chemistry is inextricably linked to sensor performance. In this review, we first highlight the biofunctionalization needs for SiP biosensors, including sensitivity, specificity, cost, shelf-stability, and replicability and establish a set of performance criteria. We then benchmark biofunctionalization strategies for SiP biosensors against these criteria, organizing the review around three key aspects: bioreceptor selection, immobilization strategies, and patterning techniques. First, we evaluate bioreceptors, including antibodies, aptamers, nucleic acid probes, molecularly imprinted polymers, peptides, glycans, and lectins. We then compare adsorption, bioaffinity, and covalent chemistries for immobilizing bioreceptors on SiP surfaces. Finally, we compare biopatterning techniques for spatially controlling and multiplexing the biofunctionalization of SiP sensors, including microcontact printing, pin- and pipette-based spotting, microfluidic patterning in channels, inkjet printing, and microfluidic probes.

Design and Preparation of Sensing Surfaces for Capacitive Biodetection

Despite their high sensitivity and their suitability for miniaturization, biosensors are still limited for clinical applications due to the lack of reproducibility and specificity of their detection performance. The design and preparation of sensing surfaces are suspected to be a cause of these limitations. Here, we first present an updated overview of the current state of use of capacitive biosensors in a medical context. Then, we summarize the encountered strategies for the fabrication of capacitive biosensing surfaces. Finally, we describe the characteristics which govern the performance of the sensing surfaces, along with recent developments that were suggested to overcome their main current limitations.

Advanced Plasmonic Nanoparticle-Based Techniques for the Prevention, Detection, and Treatment of Current COVID-19

Coronavirus is an ongoing global pandemic caused by severe acute respiratory syndrome coronavirus 2. Coronavirus disease 2019 known as COVID-19 is the worst pandemic since World War II. The outbreak of COVID-19 had a significant repercussion on the health, economy, politics, and environment, making coronavirus-related issues more complicated and becoming one of the most challenging pandemics of the last century with deadly outcomes and a high rate of the reproduction number. There are thousands of different types - or variants - of COVID circulating across the world. Viruses mutate all the time; it emphasizes the critical need for the designing of efficient vaccines to prevent virus infection, early and fast diagnosis, and effective antiviral and protective therapeutics. In this regard, the use of nanotechnology offers new opportunities for the development of novel strategies in terms of prevention, diagnosis, and treatment of COVID-19. This review presents an outline of the platforms developed using plasmonic nanoparticles in the detection, treatment, and prevention of SARS-CoV-2. We select the best strategies in each of these approaches. The properties of metallic plasmon NPs and their relevance in the development of novel point-of-care diagnosis approaches for COVID-19 are highlighted. Also, we discuss the current challenges and the future perspectives looking towards the clinical translation and the commercial aspects of nanotechnology and plasmonic NP-based diagnostic tools and therapy to fight COVID-19 pandemic. The article could be of significance for researchers dedicated to developing suitable plasmonic detection tools and therapy approaches for COVID-19 viruses and future pandemics.

Nanotechnology‐based electrochemical biosensors for monitoring breast cancer biomarkers

Breast cancer is categorized as the most widespread cancer type among women globally. On-time diagnosis can decrease the mortality rate by making the right decision in the therapy procedure. These features lead to a reduction in medication time and socioeconomic burden. The current review article provides a comprehensive assessment for breast cancer diagnosis using nanomaterials and related technologies. Growing use of the nano/biotechnology domain in terms of electrochemical nanobiosensor designing was discussed in detail. In this regard, recent advances in nanomaterial applied for amplified biosensing methodologies were assessed for breast cancer diagnosis by focusing on the advantages and disadvantages of these approaches. We also monitored designing methods, advantages, and the necessity of suitable (nano) materials from a statistical standpoint. The main objective of this review is to classify the applicable biosensors based on breast cancer biomarkers. With numerous nano-sized platforms published for breast cancer diagnosis, this review tried to collect the most suitable methodologies for detecting biomarkers and certain breast cancer cell types.

New Trends in Fluorescent Nanomaterials-Based Bio/Chemical Sensors for Neurohormones Detection-A Review

The study of neurotransmitters and stress hormones allows the determination of indicators of the current stress load in the body. These species also create a proper strategy of stress protection. Nowadays, stress is a general factor that affects the population, and it may cause a wide range of serious disorders. Abnormalities in the level of neurohormones, caused by chronic psychological stress, can occur in, for instance, corporate employees, health care workers, shift workers, policemen, or firefighters. Here we present a new nanomaterials-based sensors technology development for the determination of neurohormones. We focus on fluorescent sensors/biosensors that utilize nanomaterials, such as quantum dots or carbon nanomaterials. Nanomaterials, owing to their diversity in size and shape, have been attracting increasing attention in sensing or bioimaging. They possess unique properties, such as fluorescent, electronic, or photoluminescent features. In this Review, we summarize new trends in adopting nanomaterials for applications in fluorescent sensors for neurohormone monitoring.

Diagnostic plasmonic sensors: opportunities and challenges

The medical fraternity is currently burgeoned and stressed with a huge rush of patients who have inflammatory conditions, metabolite diseases, and cardiovascular diseases. In these circumstances, advanced sensing technologies could have a huge impact on the quality of life of patients. Given plasmonic resonance effects significantly improve the ability to rapidly and accurately detect biological markers, plasmonic technology is harnessed to develop a fast and accurate diagnosis that can provide timely intervention with the diseases and can also aid the recovery process by complementing the therapy stage. In this short review, we provide an overlook of how the field of plasmonic sensing has revolutionized the field of medical diagnostics. This article reviews the fundamentals and development of plasmonics. In addition, we highlight the sensitivity of various SPR and LSPR sensors. The chemistry for functionalizing plasmonic sensors is also discussed. This review also outlines some general suggestions for future directions that we feel might be useful to advance our understanding of the universe or speed up the development of plasmonic sensors in the future.

Use of Cysteamine and Glutaraldehyde Chemicals for Robust Functionalization of Substrates with Protein Biomarkers—An Overview on the Construction of Biosensors with Different Transductions

Currently, several biosensors are reported to confirm the absence/presence of an abnormal level of specific human biomarkers in research laboratories. Unfortunately, public marketing and/or pharmacy accessibility are not yet possible for many bodily fluid biomarkers. The questions are numerous, starting from the preparation of the substrates, the wet/dry form of recognizing the (bio)ligands, the exposure time, and the choice of the running buffers. In this context, for the first time, the present overview summarizes the pre-functionalization of standard and nanostructured solid/flexible supports with cysteamine (Cys) and glutaraldehyde (GA) chemicals for robust protein immobilization and detection of biomarkers in body fluids (serum, saliva, and urine) using three transductions: piezoelectrical, electrochemical, and optical, respectively. Thus, the reader can easily access and compare step-by-step conjugate protocols published over the past 10 years. In conclusion, Cys/GA chemistry seems widely used for electrochemical sensing applications with different types of recorded signals, either current, potential, or impedance. On the other hand, piezoelectric detection via quartz crystal microbalance (QCM) and optical detection by surface plasmon resonance (LSPR)/surface-enhanced Raman spectroscopy (SERS) are ultrasensitive platforms and very good candidates for the miniaturization of medical devices in the near future.

Gas Cluster Ion Beams as a Versatile Soft-Landing Tool for the Controlled Construction of Thin (Bio)Films

Surface biofunctionalization with proteins is the key to many biomedical applications. In this study, a solvent-free method for the controlled construction of protein thin films is reported. Using large argon gas cluster ion beams, proteins are sputtered from a target (a pool of pure proteins), and collected on a chosen substrate, being nearly any solid material. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed the presence of intact protein molecules on the collectors. Furthermore, lowering the energy per atom in the cluster projectiles down to 1 eV/atom allowed more than 60% of bradykinin molecules to be transferred intact. This protein deposition method offers a precise control of the film thickness as the transferred protein quantity is proportional to the argon clusters ion dose reached for the transfer. This major feature enables building protein films from (sub)mono- to multilayers, without upper limitation of the thickness. A procedure was developed to measure the film thickness in situ the ToF-SIMS instrument. The versatility and potential of this soft-landing alternative for further applications is demonstrated on the one hand by building a protein thin film at the surface of paper, a substrate hardly compatible with solution-based adsorption methods. On the other hand, the possibility to achieve alternated multilayer buildup is demonstrated with the construction of a bilayer composed of bradykinin and Irganox, with the two layers well separated. These results lay the first stone toward original and complex multilayers that could previously not be considered with solution-based adsorption methods, and this regardless of the substrate nature.

Adiponectin-targeted SERS immunoassay biosensing platform for early detection of gestational diabetes mellitus

The anisotropic gold nanotriangles (AuNTs) were synthesized by a fast seedless growth process. The high-yield monodispersed AuNT colloids were obtained through a purification process based on depletion-induced interactions. AuNTs were modulated with a localized surface plasmon resonance (LSPR) peak of 638 nm wavelength coherent with the Raman excitation light. However, from finite element computation results, the AuNT clusters showed better performance for the 785 nm laser source due to a red shift in their LSPR properties, hence it was selected for the surface-enhanced Raman scattering (SERS) immunoassay. A self-assembly strategy using a thiol group and ON-OFF strategy in the heat map was performed to ensure the stability of SERS immunoassay platform. The sandwich SERS immunoassay biosensor platform for adiponectin detection demonstrated a wide assay range (10^-15-10^-6 g/mL), good reliability (R² = 0.994, clinically relevant range), femto-scale limit of detection (3.0 × 10^-16 g/mL), and excellent selectivity without interference from other biomarkers. This showed the possibility of effectively detecting adiponectin levels in the biofluids of pregnant women. Therefore, our technology is the first to quantitatively detect adiponectin based on SERS technology for early detection of gestational diabetes mellitus and has the potential to be used as a clinical biosensor capable of diagnosing various obstetric diseases during early pregnancy.

New Trends in Nanoarchitectured SERS Substrates: Nanospaces, 2D Materials, and Organic Heterostructures

This article reviews recent fabrication methods for surface-enhanced Raman spectroscopy (SERS) substrates with a focus on advanced nanoarchitecture based on noble metals with special nanospaces (round tips, gaps, and porous spaces), nanolayered 2D materials, including hybridization with metallic nanostructures (NSs), and the contemporary repertoire of nanoarchitecturing with organic molecules. The use of SERS for multidisciplinary applications has been extensively investigated because the considerably enhanced signal intensity enables the detection of a very small number of molecules with molecular fingerprints. Nanoarchitecture strategies for the design of new NSs play a vital role in developing SERS substrates. In this review, recent achievements with respect to the special morphology of metallic NSs are discussed, and future directions are outlined for the development of available NSs with reproducible preparation and well-controlled nanoarchitecture. Nanolayered 2D materials are proposed for SERS applications as an alternative to the noble metals. The modern solutions to existing limitations for their applications are described together with the state-of-the-art in bio/environmental SERS sensing using 2D materials-based composites. To complement the existing toolbox of plasmonic inorganic NSs, hybridization with organic molecules is proposed to improve the stability of NSs and selectivity of SERS sensing by hybridizing with small or large organic molecules.

Responsivity of Fractal Nanoparticle Assemblies to Multiple Stimuli: Structural Insights on the Modulation of the Optical Properties

Multi-responsive nanomaterials based on the self-limited assembly of plasmonic nanoparticles are of great interest due to their widespread employment in sensing applications. We present a thorough investigation of a hybrid nanomaterial based on the protein-mediated aggregation of gold nanoparticles at varying protein concentration, pH and temperature. By combining Small Angle X-ray Scattering with extinction spectroscopy, we are able to frame the morphological features of the formed fractal aggregates in a theoretical model based on patchy interactions. Based on this, we established the main factors that determine the assembly process and their strong correlation with the optical properties of the assemblies. Moreover, the calibration curves that we obtained for each parameter investigated based on the extinction spectra point out to the notable flexibility of this nanomaterial, enabling the selection of different working ranges with high sensitivity. Our study opens for the rational tuning of the morphology and the optical properties of plasmonic assemblies to design colorimetric sensors with improved performances.

Plasmonic Nanomaterials for Colorimetric Biosensing: A Review

In the last few decades, plasmonic colorimetric biosensors raised increasing interest in bioanalytics thanks to their cost-effectiveness, responsiveness, and simplicity as compared to conventional laboratory techniques. Potential high-throughput screening and easy-to-use assay procedures make them also suitable for realizing point of care devices. Nevertheless, several challenges such as fabrication complexity, laborious biofunctionalization, and poor sensitivity compromise their technological transfer from research laboratories to industry and, hence, still hamper their adoption on large-scale. However, newly-developing plasmonic colorimetric biosensors boast impressive sensing performance in terms of sensitivity, dynamic range, limit of detection, reliability, and specificity thereby continuously encouraging further researches. In this review, recently reported plasmonic colorimetric biosensors are discussed with a focus on the following categories: (i) on-platform-based (localized surface plasmon resonance, coupled plasmon resonance and surface lattice resonance); (ii) colloid aggregation-based (label-based and label free); (iii) colloid non-aggregation-based (nanozyme, etching-based and growth-based).

PLGA-Gold Nanocomposite: Preparation and Biomedical Applications

A composite system consisting of both organic and inorganic nanoparticles is an approach to prepare a new material exhibiting “the best of both worlds”. In this review, we highlight the recent advances in the preparation and applications of poly(lactic-co-glycolic acid)-gold nanoparticles (PLGA-GNP). With its current clinically use, PLGA-based nanocarriers have promising pharmaceutical applications and can “extract and utilize” the fascinating optical and photothermal properties of encapsulated GNP. The resulting “golden polymeric nanocarrier” can be tracked, analyzed, and visualized using the encapsulated gold nanoprobes which facilitate a better understanding of the hosting nanocarrier’s pharmacokinetics and biological fate. In addition, the “golden polymeric nanocarrier” can reveal superior nanotherapeutics that combine both the photothermal effect of the encapsulated gold nanoparticles and co-loaded chemotherapeutics. To help stimulate more research on the development of nanomaterials with hybrid and exceptional properties, functionalities, and applications, this review provides recent examples with a focus on the available chemistries and the rationale behind encapsulating GNP into PLGA nanocarriers that has the potential to be translated into innovative, clinically applicable nanomedicine.

Label-Free Physical Techniques and Methodologies for Proteins Detection in Microfluidic Biosensor Structures

Proteins in biological fluids (blood, urine, cerebrospinal fluid) are important biomarkers of various pathological conditions. Protein biomarkers detection and quantification have been proven to be an indispensable diagnostic tool in clinical practice. There is a growing tendency towards using portable diagnostic biosensor devices for point-of-care (POC) analysis based on microfluidic technology as an alternative to conventional laboratory protein assays. In contrast to universally accepted analytical methods involving protein labeling, label-free approaches often allow the development of biosensors with minimal requirements for sample preparation by omitting expensive labelling reagents. The aim of the present work is to review the variety of physical label-free techniques of protein detection and characterization which are suitable for application in micro-fluidic structures and analyze the technological and material aspects of label-free biosensors that implement these methods. The most widely used optical and impedance spectroscopy techniques: absorption, fluorescence, surface plasmon resonance, Raman scattering, and interferometry, as well as new trends in photonics are reviewed. The challenges of materials selection, surfaces tailoring in microfluidic structures, and enhancement of the sensitivity and miniaturization of biosensor systems are discussed. The review provides an overview for current advances and future trends in microfluidics integrated technologies for label-free protein biomarkers detection and discusses existing challenges and a way towards novel solutions.

Advances and applications of nanophotonic biosensors

Nanophotonic devices, which control light in subwavelength volumes and enhance light-matter interactions, have opened up exciting prospects for biosensing. Numerous nanophotonic biosensors have emerged to address the limitations of the current bioanalytical methods in terms of sensitivity, throughput, ease-of-use and miniaturization. In this Review, we provide an overview of the recent developments of label-free nanophotonic biosensors using evanescent-field-based sensing with plasmon resonances in metals and Mie resonances in dielectrics. We highlight the prospects of achieving an improved sensor performance and added functionalities by leveraging nanostructures and on-chip and optoelectronic integration, as well as microfluidics, biochemistry and data science toolkits. We also discuss open challenges in nanophotonic biosensing, such as reducing the overall cost and handling of complex biological samples, and provide an outlook for future opportunities to improve these technologies and thereby increase their impact in terms of improving health and safety.

Surface-enhanced mid-infrared absorption spectroscopy using miniaturized-disc metasurface

Surface-enhanced infrared spectroscopy is an important technique for improving the signal-to-noise ratio of spectroscopic material identification measurements in the mid-infrared fingerprinting region. However, the lower bound of the fingerprinting region receives much less attention due to a scarcity of transparent materials, more expensive sources, and weaker plasmonic effects. In this paper, we present a miniaturized metasurface unit cell for surface-enhanced infrared spectroscopy of the 15-[Formula: see text]m vibrational band of CO[Formula: see text]. The unit cell consists of a gold disc, patterned along the edge with fine gaps/wires to create a resonant metamaterial liner. In simulation, our plasmonic metamaterial-lined disc achieves greater than [Formula: see text] the average field intensity enhancement of a comparable dipole array and a miniaturized size of [Formula: see text] using complex, 100-nm features that are patterned using 100-kV electron-beam lithography. In a simple experiment, the metamaterial-lined disc metasurface shows a high tolerance to fabrication imperfections and enhances the absorption of CO[Formula: see text] at 15 [Formula: see text]m. The resonant wavelength and reflection magnitude can be tuned over a wide range by adjusting the liner feature sizes and the metasurface array pitch to target other vibrational bands. This work is a step toward low-cost, more compact on-chip integrated gas sensors.

Attomolar analyte sensing techniques (AttoSens): a review on a decade of progress on chemical and biosensing nanoplatforms

Detecting the ultra-low abundance of analytes in real-life samples, such as biological fluids, water, soil, and food, requires the design and development of high-performance biosensing modalities. The breakthrough efforts from the scientific community have led to the realization of sensing technologies that measure the analyte's ultra-trace level, with relevant sensitivity, selectivity, response time, and sampling efficiency, referred to as Attomolar Analyte Sensing Techniques (AttoSens) in this review. In an AttoSens platform, 1 aM detection corresponds to the quantification of 60 target analyte molecules in 100 μL of sample volume. Herein, we review the approaches listed for various sensor probe design, and their sensing strategies that paved the way for the detection of attomolar (aM: 10^-18 M) concentration of analytes. A summary of the technological advances made by the diverse AttoSens trends from the past decade is presented.

Toward the Development of Rapid, Specific, and Sensitive Microfluidic Sensors: A Comprehensive Device Blueprint

Recent advances in nano/microfluidics have led to the miniaturization of surface-based chemical and biochemical sensors, with applications ranging from environmental monitoring to disease diagnostics. These systems rely on the detection of analytes flowing in a liquid sample, by exploiting their innate nature to react with specific receptors immobilized on the microchannel walls. The efficiency of these systems is defined by the cumulative effect of analyte detection speed, sensitivity, and specificity. In this perspective, we provide a fresh outlook on the use of important parameters obtained from well-characterized analytical models, by connecting the mass transport and reaction limits with the experimentally attainable limits of analyte detection efficiency. Specifically, we breakdown when and how the operational (e.g., flow rates, channel geometries, mode of detection, etc.) and molecular (e.g., receptor affinity and functionality) variables can be tailored to enhance the analyte detection time, analytical specificity, and sensitivity of the system (i.e., limit of detection). Finally, we present a simple yet cohesive blueprint for the development of high-efficiency surface-based microfluidic sensors for rapid, sensitive, and specific detection of chemical and biochemical analytes, pertinent to a variety of applications.

Biochemical Sensing with Nanoplasmonic Architectures: We Know How but Do We Know Why?

Here, the research field of nanoplasmonic sensors is placed under scrutiny, with focus on affinity-based detection using refractive index changes. This review describes how nanostructured plasmonic sensors can deliver unique advantages compared to the established surface plasmon resonance technique, where a planar metal surface is used. At the same time, it shows that these features are actually only useful in quite specific situations. Recent trends in the field are also discussed and some devices that claim extraordinary performance are questioned. It is argued that the most important challenges are related to limited receptor affinity and nonspecific binding rather than instrumental performance. Although some nanoplasmonic sensors may be useful in certain situations, it seems likely that conventional surface plasmon resonance will continue to dominate biomolecular interaction analysis. For detection of analytes in complex samples, plasmonics may be an important tool, but probably not in the form of direct refractometric detection.

COVID-19 Biomarkers and Advanced Sensing Technologies for Point-of-Care (POC) Diagnosis

COVID-19, also known as SARS-CoV-2 is a novel, respiratory virus currently plaguing humanity. Genetically, at its core, it is a single-strand positive-sense RNA virus. It is a beta-type Coronavirus and is distinct in its structure and binding mechanism compared to other types of coronaviruses. Testing for the virus remains a challenge due to the small market available for at-home detection. Currently, there are three main types of tests for biomarker detection: viral, antigen and antibody. Reverse Transcription-Polymerase Chain Reaction (RT-PCR) remains the gold standard for viral testing. However, the lack of quantitative detection and turnaround time for results are drawbacks. This manuscript focuses on recent advances in COVID-19 detection that have lower limits of detection and faster response times than RT-PCR testing. The advancements in sensing platforms have amplified the detection levels and provided real-time results for SARS-CoV-2 spike protein detection with limits as low as 1 fg/mL in the Graphene Field Effect Transistor (FET) sensor. Additionally, using multiple biomarkers, detection levels can achieve a specificity and sensitivity level comparable to that of PCR testing. Proper biomarker selection coupled with nano sensing detection platforms are key in the widespread use of Point of Care (POC) diagnosis in COVID-19 detection.

State of the art in nonthermal plasma processing for biomedical applications: Can it help fight viral pandemics like COVID-19?

Plasma processing finds widespread biomedical applications, such as the design of biosensors, antibiofouling surfaces, controlled drug delivery systems, and in plasma sterilizers. In the present coronavirus disease (COVID-19) situation, the prospect of applying plasma processes like surface activation, plasma grafting, plasma-enhanced chemical vapor deposition/plasma polymerization, surface etching, plasma immersion ion implantation, crosslinking, and plasma decontamination to provide timely solutions in the form of better antiviral alternatives, practical diagnostic tools, and reusable personal protective equipment is worth exploring. Herein, the role of nonthermal plasmas and their contributions toward healthcare are timely reviewed to engage different communities in assisting healthcare associates and clinicians, not only to combat the current COVID-19 pandemic but also to prevent similar kinds of future outbreaks.

"CHicable" and "Clickable" Copolymers for Network Formation and Surface Modification

In this study, we present the generation of novel, multifunctional polymer networks through a combination of C,H-insertion cross-linking (CHic) and click chemistry. To this, copolymers consisting of hydrophilic N,N-dimethylacrylamide as matrix component and repeat units containing azide moieties, as well as benzophenone or anthraquinone groups, are generated. The benzophenone or anthraquinone groups allow photo-cross-linking, surface attachment or covalent immobilization of adjacent (bio)molecules through CHic reactions. The azide moieties either can react with available alkynes through conventional click reactions or can be activated to form nitrenes, which can also undergo CHic reactions. By choosing appropriate reaction conditions, the same polymer can be used to follow very different reaction paths, opening up a plethora of choices for the generation of functional polymer networks. In the exemplary presented case ("CHic-Click"), irradiation of the copolymers with UV-A light (λ_irr = 365 nm) leads to cross-linking (network formation) and surface attachment simultaneously. The azide units remain intact during this cross-linking step, and alkyne-modified (bio)molecules can be bound through click reactions. Biofunctionalization of the polymer network with alkynylated streptavidin, followed by application of biotin-conjugated antibody and a model analyte, highlights the potential of these surface architectures as a toolbox which can be adapted for diverse bioanalytical applications.

Recent Advancements in Receptor Layer Engineering for Applications in SPR-Based Immunodiagnostics

The rapid progress in the development of surface plasmon resonance-based immunosensing platforms offers wide application possibilities in medical diagnostics as a label-free alternative to enzyme immunoassays. The early diagnosis of diseases or metabolic changes through the detection of biomarkers in body fluids requires methods characterized by a very good sensitivity and selectivity. In the case of the SPR technique, as well as other surface-sensitive detection strategies, the quality of the transducer-immunoreceptor interphase is crucial for maintaining the analytical reliability of an assay. In this work, an overview of general approaches to the design of functional SPR-immunoassays is presented. It covers both immunosensors, the design of which utilizes well-known and often commercially available substrates, as well as the latest solutions developed in-house. Various approaches employing chemical and passive binding, affinity-based antibody immobilization, and the introduction of nanomaterial-based surfaces are discussed. The essence of their influence on the improvement of the main analytical parameters of a given immunosensor is explained. Particular attention is paid to solutions compatible with the latest trends in the development of label-free immunosensors, such as platforms dedicated to real-time monitoring in a quasi-continuous mode, the use of in situ-generated receptor layers (elimination of the regeneration step), and biosensors using recombinant and labelled protein receptors.

Boosting biomolecular interactions through DNA origami nano-tailored biosensing interfaces

The interaction between a bioreceptor and its target is key in developing sensitive, specific and robust diagnostic devices. Suboptimal interbioreceptor distances and bioreceptor orientation on the sensor surface, resulting from uncontrolled deposition, impede biomolecular interactions and lead to a decreased biosensor performance. In this work, we studied and implemented a 3D DNA origami design, for the first time comprised of assay specifically tailored anchoring points for the nanostructuring of the bioreceptor layer on the surface of disc-shaped microparticles in the continuous microfluidic environment of the innovative EvalutionTM platform. This bioreceptor immobilization strategy resulted in the formation of a less densely packed surface with reduced steric hindrance and favoured upward orientation. This increased bioreceptor accessibility led to a 4-fold enhanced binding kinetics and a 6-fold increase in binding efficiency compared to a directly immobilized non-DNA origami reference system. Moreover, the DNA origami nanotailored biosensing concept outperformed traditional aptamer coupling with respect to limit of detection (11 × improved) and signal-to-noise ratio (2.5 × improved) in an aptamer-based sandwich bioassay. In conclusion, our results highlight the potential of these DNA origami nanotailored surfaces to improve biomolecular interactions at the sensing surface, thereby increasing the overall performance of biosensing devices. The combination of the intrinsic advantages of DNA origami together with a smart design enables bottom-up nanoscale engineering of the sensor surface, leading towards the next generation of improved diagnostic sensing devices.

Atomistic simulations of gold surface functionalization for nanoscale biosensors applications

A wide class of biosensors can be built via functionalization of gold surface with proper bio conjugation element capable of interacting with the analyte in solution, and the detection can be performed either optically, mechanically or electrically. Any change in physico-chemical environment or any slight variation in mass localization near the surface of the sensor can cause differences in nature of the transduction mechanism. The optimization of such sensors may require multiple experiments to determine suitable experimental conditions for the immobilization and detection of the analyte. Here, we employ molecular modeling techniques to assist the optimization of a gold-surface biosensor. The gold surface of a quartz-crystal-microbalance sensor is functionalized using polymeric chains of poly(ethylene glycol) (PEG) of 2 KDa molecular weight, which is an inert long chain amphiphilic molecule, supporting biotin molecules (bPEG) as the ligand molecules for streptavidin analyte. The PEG linkers are immobilized onto the gold surface through sulphur chemistry. Four gold surfaces with different PEG linker density and different biotinylation ratio between bPEG and PEG, are investigated by means of state-of-the art atomistic simulations and compared with available experimental data. Results suggest that the amount of biotin molecules accessible for the binding with the protein increases upon increasing the linkers density. At the high density a 1:1 ratio of bPEG/PEG can further improve the accessibility of the biotin ligand due to a strong repulsion between linker chains and different degree of hydrophobicity between bPEG and PEG linkers. The study provides a computaional protocol to model sensors at the level of single molecular interactions, and for optimizing the physical properties of surface conjugated ligand which is crucial to enhance output of the sensor.

Catecholamine-Copper Redox as a Basis for Site-Specific Single-Step Functionalization of Material Surfaces

Realization of robust and facile surface functionalization processes is critical to biomaterials and biotechnology yet remains a challenge. Here, we report a new chemical approach that enables operationally simple and site-specific surface functionalization. The mechanism involves a catechol-copper redox chemistry, where the oxidative polymerization of an alkynyl catecholamine reduces Cu(II) to Cu(I), which in situ catalyzes a click reaction with azide-containing molecules of interest (MOIs). This process enables drop-coating and grafting of two- and three-dimensional solid surfaces in a single operation using as small as sub-microliter volumes. Generalizability of the method is shown for immobilizing MOIs of diverse structure and chemical or biological activity. Biological applications in anti-biofouling, cellular adhesion, scaffold seeding, and tissue regeneration are demonstrated, in which the activities or fates of cells are site-specifically manipulated. This work advances surface chemistry by integrating simplicity and precision with multipurpose surface functionalization.

Electrochemically triggered sensing and recovery of mercury over sodium alginate grafted polyaniline

An efficient potential triggering process has been established over chemically functionalized SA-g-PANI under optimum conditions to enable the sensing and extraction of residual mercury ions present in wastewater and soil samples.

Ultrasensitive antibody-aptamer plasmonic biosensor for malaria biomarker detection in whole blood

Development of plasmonic biosensors combining reliability and ease of use is still a challenge. Gold nanoparticle arrays made by block copolymer micelle nanolithography (BCMN) stand out for their scalability, cost-effectiveness and tunable plasmonic properties, making them ideal substrates for fluorescence enhancement. Here, we describe a plasmon-enhanced fluorescence immunosensor for the specific and ultrasensitive detection of Plasmodium falciparum lactate dehydrogenase (PfLDH)—a malaria marker—in whole blood. Analyte recognition is realized by oriented antibodies immobilized in a close-packed configuration via the photochemical immobilization technique (PIT), with a top bioreceptor of nucleic acid aptamers recognizing a different surface of PfLDH in a sandwich conformation. The combination of BCMN and PIT enabled maximum control over the nanoparticle size and lattice constant as well as the distance of the fluorophore from the sensing surface. The device achieved a limit of detection smaller than 1 pg/mL (<30 fM) with very high specificity without any sample pretreatment. This limit of detection is several orders of magnitude lower than that found in malaria rapid diagnostic tests or even commercial ELISA kits. Thanks to its overall dimensions, ease of use and high-throughput analysis, the device can be used as a substrate in automated multi-well plate readers and improve the efficiency of conventional fluorescence immunoassays.

Reliable plasmonic biosensors with high throughput and ease of use are highly sought after. Here, the authors report a plasmon-enhanced fluorescence antibody-aptamer biosensor based on a gold nanoparticle array, and demonstrate its use for effective specific detection of a malaria marker, at femtomolar level, in whole blood.

How Nanophotonic Label-Free Biosensors Can Contribute to Rapid and Massive Diagnostics of Respiratory Virus Infections: COVID-19 Case

The global sanitary crisis caused by the emergence of the respiratory virus SARS-CoV-2 and the COVID-19 outbreak has revealed the urgent need for rapid, accurate, and affordable diagnostic tests to broadly and massively monitor the population in order to properly manage and control the spread of the pandemic. Current diagnostic techniques essentially rely on polymerase chain reaction (PCR) tests, which provide the required sensitivity and specificity. However, its relatively long time-to-result, including sample transport to a specialized laboratory, delays massive detection. Rapid lateral flow tests (both antigen and serological tests) are a remarkable alternative for rapid point-of-care diagnostics, but they exhibit critical limitations as they do not always achieve the required sensitivity for reliable diagnostics and surveillance. Next-generation diagnostic tools capable of overcoming all the above limitations are in demand, and optical biosensors are an excellent option to surpass such critical issues. Label-free nanophotonic biosensors offer high sensitivity and operational robustness with an enormous potential for integration in compact autonomous devices to be delivered out-of-the-lab at the point-of-care (POC). Taking the current COVID-19 pandemic as a critical case scenario, we provide an overview of the diagnostic techniques for respiratory viruses and analyze how nanophotonic biosensors can contribute to improving such diagnostics. We review the ongoing published work using this biosensor technology for intact virus detection, nucleic acid detection or serological tests, and the key factors for bringing nanophotonic POC biosensors to accurate and effective COVID-19 diagnosis on the short term.