Competitive Immunoassays for the Detection of Small Molecules Using Single Molecule Arrays

Nanoscale Metal-Organic Frameworks as a Photoluminescent Platform for Bioimaging and Biosensing Applications

The tracking of nanomedicines in their concentration and location inside living systems has a pivotal effect on the understanding of the biological processes, early-stage diagnosis, and therapeutic monitoring of diseases. Nanoscale metal-organic frameworks (nano MOFs) possess high surface areas, definite structure, regulated optical properties, rich functionalized sites, and good biocompatibility that allow them to excel in a wide range of biomedical applications. Controllable syntheses and functionalization endow nano MOFs with better properties as imaging agents and sensing units for the diagnosis and treatment of diseases. This minireview summarizes the tunable synthesis strategies of nano MOFs with controllable size, shape, and regulated luminescent performance, and pinpoints their recent advanced applications as optical elements in bioimaging and biosensing. The current limitations and future development directions of nano MOF-contained materials in bioimaging and biosensing applications are also discussed, aiming to expand the biological applications of nano MOF-based nanomedicine and facilitate their production or clinical translation.

Recent advances and challenges in the analysis of natural toxins

Natural toxins (NTs) are poisonous secondary metabolites produced by living organisms developed to ward off predators. Especially low molecular weight NTs (MW<∼1 kDa), such as mycotoxins, phycotoxins, and plant toxins, are considered an important and growing food safety concern. Therefore, accurate risk assessment of food and feed for the presence of NTs is crucial. Currently, the analysis of NTs is predominantly performed with targeted high pressure liquid chromatography tandem mass spectrometry (HPLC-MS/MS) methods. Although these methods are highly sensitive and accurate, they are relatively expensive and time-consuming, while unknown or unexpected NTs will be missed. To overcome this, novel on-site screening methods and non-targeted HPLC high resolution mass spectrometry (HRMS) methods have been developed. On-site screening methods can give non-specialists the possibility for broad "scanning" of potential geographical regions of interest, while also providing sensitive and specific analysis at the point-of-need. Non-targeted chromatography-HRMS methods can detect unexpected as well as unknown NTs and their metabolites in a lab-based approach. The aim of this chapter is to provide an insight in the recent advances, challenges, and perspectives in the field of NTs analysis both from the on-site and the laboratory perspective.

Detection of estradiol with a digital immunoassay using an anti-immunocomplex antibody and single-molecule observation of gold nanoparticles

Gold nanoparticles (AuNPs) have been widely applied to molecular sensors due to their optical properties. We previously reported a molecular detection by observing the scattered light of AuNPs at a single nanoparticle level using dark field microscopy (DFM). Recently, a molecular detection method using digital immunoassay has been reported, taking advantage of the characteristics of DFM. However, the digital immunoassays reported so far have been performed by a conventional sandwich immunoassay, which is difficult to apply to the detection of small molecules. In this study, with the aim of small molecule detection, we developed a digital immunoassay method using an anti-immunocomplex antibody that specifically recognizes immunocomplexes of small molecules with antibodies. The number of AuNPs modified with anti-immunocomplex antibody bound to immunocomplex of estradiol and anti-estradiol antibody was counted at a single nanoparticle level using DFM. We demonstrated for the first time that estradiol molecule can be detected by digital immunoassay using DFM and an anti-immunocomplex antibody with a detection sensitivity of 1 pg/mL.

Production of antibody and development of enzyme-linked immunosorbent assay for therapeutic drug monitoring of eravacycline

Eravacycline (ERC) was approved for clinical use in 2018. It is more potent than other tetracyclines and can overcome resistance, making it an attractive option for combating multidrug-resistant bacterial infections. Intensive pharmacokinetic (PK) studies are currently being conducted to ensure the effectiveness and safety of ERC in various groups of patients, including those undergoing extracorporeal therapies. This study is the first attempt to develop a simple, efficient, and high-throughput immunoassay for quantifying ERC in human or animal serum. BSA-ERC conjugate as immunogen elicited antibody production in rabbits. Monitoring of the immune response and comparison of homologous and heterologous coating antigens allowed selection of immunoreagents and development of an assay that was selective for ERC possessing sensitivity (IC50), dynamic range (IC20-IC80) and detection limit equal to 3.3 ng/mL, 0.27-54 ng/mL and 0.09 ng/mL, respectively. The developed ELISA showed acceptable recovery of ERC (85-105 %) from rabbit and human serum in the clinically relevant concentration range of 0.1-3.0 mg/L. The method was used to quantify serum ERC concentration in the pilot PK study in Soviet chinchilla rabbits. The results were confirmed by HPLC-MS/MS.

Antibody-Bridged DNAzyme Walker for Sensitive Detection of Small Molecules

Sensitive detection of small molecules with biological and environmental interests is important for many applications, such as food safety, disease diagnosis, and environmental monitoring. Herein, we propose a highly selective antibody-bridged DNAzyme walker to sensitively detect small molecules. The antibody-bridged DNAzyme walker consists of a track, small-molecule-labeled DNAzyme walking strand, and antibody against small molecules. The track is built by co-modifying fluorophore-labeled substrates and small-molecule-labeled DNA linkers onto a gold nanoparticle (AuNP). In the absence of the target molecule, the antibody binds small molecule labels at the DNAzyme walking strand and the DNA linker, driving the DNAzyme walking strand on the surface of the AuNP. The attached DNAzyme walking strand moves along the track and cleaves substrates to generate high fluorescence signals to achieve signal amplification. As target molecules exist, they competitively bind with antibody to displace the small-molecule-labeled linker and DNAzyme walking strand, rendering the DNAzyme walker inactive in substrate cleavage and causing weak fluorescence. By using this antibody-bridged DNAzyme walker, we achieved sensitive detection of two biologically important small molecules, digoxin and folic acid. This work provides a new paradigm by combining the signal amplification strategy of a DNA walker and immunorecognition for sensitive and selective detection of small molecules.

Single-drop electrochemical immunosensor with 3D-printed magnetic attachment for onsite smartphone detection of amoxicillin in raw milk

Amoxicillin is a beta-lactam group of antibiotics used against bacterial infections. The growth of the antibiotic market and abuse of amoxicillin led to its accumulation in nature, especially in foods. Thus, rapid amoxicillin tests are needed to protect human health and increase food safety. In this study, a single drop, mobile electrochemical amoxicillin immunosensor was developed for the first time. The immunosensor platform is based on amoxicillin antibodies conjugated magnetic nanoparticles. A 3-Dimensional (3D) cell capable of accumulating amoxicillin molecules, which interact with antibody-conjugated magnetic nanoparticles on the electrode surface was designed and 3D-printed. A small potentiostat connected to an Android mobile phone was used to detect amoxicillin in a single-drop raw milk sample in a 3D-printed cell. Results demonstrated linearity between 0.5 and 100 µM with an LOD of 0.44 µM. Due to its rapid and portable application, the designed immunosensor shows good potential in on-site amoxicillin detection from milk samples.

Changes of neurofilament light chain in patients with alcohol dependence following withdrawal and the genetic effect from ALDH2 Polymorphism

Neurofilament light chain (NFL), as a measure of neuroaxonal injury, has recently gained attention in alcohol dependence (AD). Aldehyde dehydrogenase 2 (ALDH2) is the major enzyme which metabolizes the alcohol breakdown product acetaldehyde. An ALDH2 single nucleotide polymorphism (rs671) is associated with less ALDH2 enzyme activity and increased neurotoxicity. We examined the blood NFL levels in 147 patients with AD and 114 healthy controls using enzyme-linked immunosorbent assay and genotyped rs671. We also followed NFL level, alcohol craving and psychological symptoms in patients with AD after 1 and 2 weeks of detoxification. We found the baseline NFL level was significantly higher in patients with AD than in controls (mean  ±  SD: 264.2  ±  261.8 vs. 72.1 ± 35.6 pg/mL, p < 0.001). The receiver operating characteristic curve revealed that NFL concentration could discriminate patients with AD from controls (area under the curve: 0.85; p < 0.001). The NFL levels were significantly reduced following 1 and 2 weeks of detoxification, with the extent of reduction correlated with the improvement of craving, depression, and anxiety (p < 0.001). Carriers with the rs671 GA genotype, which is associated with less ALDH2 activity, had higher NLF levels either at baseline or after detoxification compared with GG carriers. In conclusion, plasma NFL level was increased in patients with AD and reduced after early abstinence. Reduction in NFL level corroborated well with the improvement of clinical symptoms. The ALDH2 rs671 polymorphism may play a role in modulating the extent of neuroaxonal injury and its recovery.

A Facile Strategy for Multiplex Protein Detection by a Fluorescent Microsphere-Based Digital Immunoassay

The digital immunoassay is a highly sensitive detection technique based on single-molecule counting and is widely used in the ultrasensitive detection of biomarkers. Herein, we developed a fluorescent microsphere-based digital immunoassay (FMDIA) by employing fluorescent microspheres as both the carriers for immunoreaction and fluorescent reports for imaging. In this approach, the target protein in the sample was captured by fluorescent microspheres to form a biotin-labeled sandwich immunocomplex, and then, the fluorescent microspheres containing the target protein molecules were captured by adding streptavidin-coated magnetic beads (SA-MBs). By counting the proportion of fluorescence-positive magnetic beads, the concentration of the target protein can be precisely quantified. As a proof of concept, α fetoprotein (AFP) and human interleukin-6 (IL-6) were used to assess the analytical performance of the proposed FMDIA, and limit of detection (LOD) values of 21 pg/mL (0.30 pM) and 0.19 pg/mL (7.3 fM) were achieved, respectively. The results of AFP detection in serum samples of patients and healthy people were consistent with the reference values given by the hospital. Furthermore, by adding fluorescent microspheres of various colors for encoding, the proposed FMDIA can easily realize the simultaneous detection of multiple proteins without the need to introduce multiple modified magnetic beads. This multiplex protein detection strategy, in which the reactions are first carried out on the fluorescent microspheres and then magnetic beads are used to capture the fluorescent reporters containing the target molecules, provides a new idea for digital assays.

A smartphone aptasensor for fipronil detection in honey samples

In this study, an electrochemical smartphone-based aptasensor for the determination of fipronil was developed by modifying a screen-printed carbon electrode (SPCE). Fipronil is a broad-spectrum insecticide that has been widely used in various applications such as agriculture, veterinary, and household pest control. Recently, its use has raised concerns over the potential impact on the environment and human health. The absence of effective methods for this purpose poses a significant obstacle. To tackle this problem, we have developed a cutting-edge aptamer-based portable sensor capable of rapidly and conveniently detecting fipronil in situ. Considering that the detection of small molecules, such as fipronil, can be a challenging task, a competitive replacement assay was set up based on the aptamer's preference for the free form of fipronil over the immobilized one on the electrode. The analytical performance provided by the sensor on standard solutions of a known fipronil content made it possible to estimate a limit of detection (LOD) equal to 1.07 μg kg-1 and a limit of quantification (LOQ) of 3.21 μg kg-1. Selectivity tests were conducted using atrazine as a possible interferent. The use and performance of the developed portable aptasensor was assessed on honey samples, which were simultaneously analyzed using an HPLC-MS method. This aptasensor could be an affordable and effective tool for accurately quantifying fipronil not only in honey samples but also in other food products.

Harnessing Small-Molecule Analyte Detection in Complex Media: Combining Molecularly Imprinted Polymers, Electrolytic Transistors, and Machine Learning

Small-molecule analyte detection is key for improving quality of life, particularly in health monitoring through the early detection of diseases. However, detecting specific markers in complex multicomponent media using devices compatible with point-of-care (PoC) technologies is still a major challenge. Here, we introduce a novel approach that combines molecularly imprinted polymers (MIPs), electrolyte-gated transistors (EGTs) based on 2D materials, and machine learning (ML) to detect hippuric acid (HA) in artificial urine, being a critical marker for toluene intoxication, parasitic infections, and kidney and bowel inflammation. Reduced graphene oxide (rGO) was used as the sensory material and molecularly imprinted polymer (MIP) as supramolecular receptors. Employing supervised ML techniques based on symbolic regression and compressive sensing enabled us to comprehensively analyze the EGT transfer curves, eliminating the need for arbitrary signal selection and allowing a multivariate analysis during HA detection. The resulting device displayed simultaneously low operating voltages (<0.5 V), rapid response times (≤10 s), operation across a wide range of HA concentrations (from 0.05 to 200 nmol L-1), and a low limit of detection (LoD) of 39 pmol L-1. Thanks to the ML multivariate analysis, we achieved a 2.5-fold increase in the device sensitivity (1.007 μA/nmol L-1) with respect to the human data analysis (0.388 μA/nmol L-1). Our method represents a major advance in PoC technologies, by enabling the accurate determination of small-molecule markers in complex media via the combination of ML analysis, supramolecular analyte recognition, and electrolytic transistors.

A molecularly imprinted sensor for single-molecule detection of pesticide metabolite at the amol/L level sensitized by water-soluble luminol derivative encapsulated liposome via click reaction

A novel luminol derivative, 4-[(1,4-dioxo-1,2,3,4-tetrahydrophthalazin-5-yl)amino]-4-oxobut-2-enoic acid (ALD) with electrochemiluminescence intensity and stability characteristics similar to luminol, but higher solubility in near neutral solution, was designed and synthesized in this study. Using this derivative, a molecular imprinted electrochemiluminescence sensor (MIECLS) was prepared for the sensitive and selective determination of 2-amino-5-mercapto-1,3,4-thiadiazole (AMT), a metabolite of bismerthiazol, thiediazole copper, thiazole zinc, and other pesticides. The ALD probes encapsulated in liposomes are immobilized on the molecularly imprinted film by light-triggered click reaction, and the concurrent release of multiple probes allows for highly sensitive detection. In the AMT concentration range of 1.00 × 10-18 - 5.00 × 10-13 mol/L, the relation between ECL response and log AMT concentration is linear. With a detection limit of 5.25 × 10-19 mol/L (about 4 - 6 molecules in 10 μL of the sample), the sensor allows for high sensitivity analysis of ultra-trace amounts of small organic compounds. In general, the ECL-based single-molecule detection technique proposed herein might be a promising alternative to fluorescence single-molecule detection.

Two-dimensional nanomaterials as enhanced surface plasmon resonance sensing platforms: Design perspectives and illustrative applications

Both increasing demand for ultrasensitive detection in the scientific community and significant new breakthroughs in materials science field have inspired and promoted the development of new-generation multifunctional plasmonic sensing platforms by adopting promising plasmonic nanomaterials. Recently, high-quality surface plasmon resonance (SPR) sensors, assisted by two dimensional (2D) nanomaterials including 2D van der Waals (vdWs) materials (such as graphene/graphene oxide, transition metal dichalcogenides (TMDs), phosphorene, antimonene, tellurene, MXenes, and metal oxides), 2D metal-organic frameworks (MOFs), 2D hyperbolic metamaterials (HMMs), and 2D optical metasurfaces, have emerged as a class of novel plasmonic sensing platforms that show unprecedented detection sensitivity and impressive performance. This review of recent progress in 2D nanomaterials-enhanced SPR platforms will highlight their compelling plasmonic enhancement features, working mechanisms, and design methodologies, as well as discuss illustrative practical applications. Hence, it is of great importance to describe the latest research progress in 2D nanomaterials-enhanced SPR sensing cases. In this review, we present some concepts of SPR enhanced by 2D nanomaterials, including the basic principles of SPR, signal modulation approaches, and working enhancement mechanisms for various 2D materials-enhanced SPR systems. In addition, we also demonstrate a detailed categorization of 2D nanomaterials-enhanced SPR sensing platforms and comment on their ability to realize ultrasensitive SPR detection. Finally, we conclude with future perspectives for exploring a new generation of 2D nanomaterials-based sensors.

Quantitative detection of RAS and KKS peptides in COVID-19 patient serum by stable isotope dimethyl labeling LC-MS

Angiotensin and kinin metabolic pathways are reported to be altered by many diseases, including COVID-19. Monitoring levels of these peptide metabolites is important for understanding mechanisms of disease processes. In this paper, we report dimethyl labeling of amines in peptides by addition of formaldehyde to samples and deutero-formaldehyde to internal standards to generate nearly identical isotopic standards with 4 m/z units larger per amine group than the corresponding analyte. We apply this approach to rapid, multiplexed, absolute LC-MS/MS quantitation of renin angiotensin system (RAS) and kallikrein-kinin system (KKS) peptides in human blood serum. Limits of detection (LODs) were obtained in the low pg mL-1 range with 3 orders of magnitude dynamic ranges, appropriate for determinations of normal and elevated levels of the target peptides in blood serum and plasma. Accuracy is within ±15% at concentrations above the limit of quantitation, as validated by spike-recovery in serum samples. Applicability was demonstrated by measuring RAS and KKS peptides in serum from COVID-19 patients, but is extendable to any class of peptides or other small molecules bearing reactive -NH2 groups.

M13 Bacteriophage-Assisted Recognition and Signal Spatiotemporal Separation Enabling Ultrasensitive Light Scattering Immunoassay

The demand for the ultrasensitive and rapid quantitative analysis of trace target analytes has become increasingly urgent. However, the sensitivity of traditional immunoassay-based detection methods is limited due to the contradiction between molecular recognition and signal amplification caused by the size effect of nanoprobes. To address this dilemma, we describe versatile M13 phage-assisted immunorecognition and signal transduction spatiotemporal separation that enable ultrasensitive light-scattering immunoassay systems for the quantitative detection of low-abundance target analytes. The newly developed immunoassay strategy combines the M13 phage-assisted light scattering signal fluctuations of gold nanoparticles (AuNPs) with gold in situ growth (GISG) technology. Given the synergy of M13 phage-mediated leverage effect and GISG-amplified light scattering signal modulation, the practical detection capability of this strategy can achieve the ultrasensitive and rapid quantification of ochratoxin A and alpha-fetoprotein in real samples at the subfemtomolar level within 50 min, displaying about 4 orders of magnitude enhancement in sensitivity compared with traditional phage-based ELISA. To further improve the sensitivity of our immunoassay, the biotin-streptavidin amplification scheme is implemented to detect severe acute respiratory syndrome coronavirus 2 spike protein down to the attomolar range. Overall, this study offers a direction for ultrasensitive quantitative detection of target analytes by the synergistic combination of M13 phage-mediated leverage effect and GISG-amplified light scattering signal modulation.

Digital-like Enzyme Inhibition Mechanism-Based Strategy for the Digital Sensing of Heparin-Specific Biomarkers

A new microbead (MB)-based digital flow cytometric sensing system is proposed for the sensitive detection of heparin-specific biomarkers, including heparin-binding protein (HBP) and heparinase. This strategy takes advantage of the inherent space-confined enzymatic behavior of T4 polynucleotide kinase phosphatase (T4 PNKP) around a single MB and the heparin's digital-like inhibitory effect on T4 PNKP. By integrating with an on-bead terminal deoxynucleotidyl transferase (TdT)-catalyzed fluorescence signal amplification technology, the concentration of HBP and heparinase can be digitally determined by the number of fluorescence-positive/-negative MBs which can be easily counted by flow cytometry. This is not only the first test to expand the application scenario of T4 PNKP to the digital detection of different biomarkers but also pioneers a new direction for fabricating digital biosensing platforms based on the enzyme inhibition mechanism.

Amyloid-Beta Co-Pathology Is a Major Determinant of the Elevated Plasma GFAP Values in Amyotrophic Lateral Sclerosis

Recent studies reported increased plasma glial acidic fibrillary protein (GFAP) levels in amyotrophic lateral sclerosis (ALS) patients compared to controls. We expanded these findings in a larger cohort, including 156 ALS patients and 48 controls, and investigated the associations of plasma GFAP with clinical variables and other biofluid biomarkers. Plasma GFAP and Alzheimer's disease (AD) cerebrospinal fluid (CSF) biomarkers were assessed by the single molecule array and the Lumipulse platforms, respectively. In ALS patients, plasma GFAP was higher than in controls (p < 0.001) and associated with measures of cognitive decline. Twenty ALS patients (12.8%) showed a positive amyloid status (A+), of which nine also exhibited tau pathology (A+T+, namely ALS-AD). ALS-AD patients showed higher plasma GFAP than A- ALS participants (p < 0.001) and controls (p < 0.001), whereas the comparison between A- ALS and controls missed statistical significance (p = 0.07). Plasma GFAP distinguished ALS-AD subjects more accurately (area under the curve (AUC) 0.932 ± 0.027) than plasma p-tau181 (AUC 0.692 ± 0.058, p < 0.0001) and plasma neurofilament light chain protein (AUC, 0.548 ± 0.088, p < 0.0001). Cognitive measures differed between ALS-AD and other ALS patients. AD co-pathology deeply affects plasma GFAP values in ALS patients. Plasma GFAP is an accurate biomarker for identifying AD co-pathology in ALS, which can influence the cognitive phenotype.

Small-Molecule Analysis Based on DNA Strand Displacement Using a Bacteriorhodopsin Photoelectric Transducer: Taking ATP as an Example

A uniformly oriented purple membrane (PM) monolayer containing photoactive bacteriorhodopsin has recently been applied as a sensitive photoelectric transducer to assay color proteins and microbes quantitatively. This study extends its application to detecting small molecules, using adenosine triphosphate (ATP) as an example. A reverse detection method is used, which employs AuNPs labeling and specific DNA strand displacement. A PM monolayer-coated electrode is first covalently conjugated with an ATP-specific nucleic acid aptamer and then hybridized with another gold nanoparticle-labeled nucleic acid strand with a sequence that is partially complementary to the ATP aptamer, in order to significantly minimize the photocurrent that is generated by the PM. The resulting ATP-sensing chip restores its photocurrent production in the presence of ATP, and the photocurrent recovers more effectively as the ATP concentration increases. Direct and single-step ATP detection is achieved in 15 min, with detection limits of 5 nM and a dynamic range of 5 nM-0.1 mM. The sensing chip exhibits high selectivity against other ATP analogs and is satisfactorily stable in storage. The ATP-sensing chip is used to assay bacterial populations and achieves a detection limit for Bacillus subtilis and Escherichia coli of 102 and 103 CFU/mL, respectively. The demonstration shows that a variety of small molecules can be simultaneously quantified using PM-based biosensors.

Micro- and nanochamber array system for single enzyme assays

Arrays of small reaction containers, ranging from 624 femtoliters (10-15 L) to 270 attoliters (10-18 L), for capturing a single enzyme molecule and measuring the activity were developed along with a new reversible sealing system based on a pneumatic valve actuator made of polydimethylsiloxane (PDMS). The valve was actuated by PBS solution, effectively preventing evaporation of the solution from the micro- and nanochambers and allowing the assay to be performed over a long period of time. The hydrolysis rates of β-D-galactosidase (β-gal), kcat, were decreased according to the decrease of the chamber size, and the overall tendency seems to be symmetrically related to the specific surface area of the chambers even under the prevented condition of non-specific adsorption. The spatial localization of the protons in the chambers, which might could affect the dissociation state of the proteins, was also investigated to explain the decrease in the hydrolysis rate. The developed chamber system developed here may be useful for artificially reproducing the confined intracellular environment and molecular crowding conditions.

Photo-Enhanced Chemo-Transistor Platform for Ultrasensitive Assay of Small Molecules

Compared with traditional assay techniques, field-effect transistors (FETs) have advantages such as fast response, high sensitivity, being label-free, and point-of-care detection, while lacking generality to detect a wide range of small molecules since most of them are electrically neutral with a weak doping effect. Here, we demonstrate a photo-enhanced chemo-transistor platform based on a synergistic photo-chemical gating effect in order to overcome the aforementioned limitation. Under light irradiation, accumulated photoelectrons generated from covalent organic frameworks offer a photo-gating modulation, amplifying the response to small molecule adsorption including methylglyoxal, p-nitroaniline, nitrobenzene, aniline, and glyoxal when measuring the photocurrent. We perform testing in buffer, artificial urine, sweat, saliva, and diabetic mouse serum. The limit of detection is down to 10^-19 M methylglyoxal, about 5 orders of magnitude lower than existing assay technologies. This work develops a photo-enhanced FET platform to detect small molecules or other neutral species with enhanced sensitivity for applications in fields such as biochemical research, health monitoring, and disease diagnosis.

The Applications of Electrochemical Immunosensors in the Detection of Disease Biomarkers: A Review

Disease-related biomarkers may serve as indicators of human disease. The clinical diagnosis of diseases may largely benefit from timely and accurate detection of biomarkers, which has been the subject of extensive investigations. Due to the specificity of antibody and antigen recognition, electrochemical immunosensors can accurately detect multiple disease biomarkers, including proteins, antigens, and enzymes. This review deals with the fundamentals and types of electrochemical immunosensors. The electrochemical immunosensors are developed using three different catalysts: redox couples, typical biological enzymes, and nanomimetic enzymes. This review also focuses on the applications of those immunosensors in the detection of cancer, Alzheimer's disease, novel coronavirus pneumonia and other diseases. Finally, the future trends in electrochemical immunosensors are addressed in terms of achieving lower detection limits, improving electrode modification capabilities and developing composite functional materials.

FRET Based Biosensor: Principle Applications Recent Advances and Challenges

Förster resonance energy transfer (FRET)-based biosensors are being fabricated for specific detection of biomolecules or changes in the microenvironment. FRET is a non-radiative transfer of energy from an excited donor fluorophore molecule to a nearby acceptor fluorophore molecule. In a FRET-based biosensor, the donor and acceptor molecules are typically fluorescent proteins or fluorescent nanomaterials such as quantum dots (QDs) or small molecules that are engineered to be in close proximity to each other. When the biomolecule of interest is present, it can cause a change in the distance between the donor and acceptor, leading to a change in the efficiency of FRET and a corresponding change in the fluorescence intensity of the acceptor. This change in fluorescence can be used to detect and quantify the biomolecule of interest. FRET-based biosensors have a wide range of applications, including in the fields of biochemistry, cell biology, and drug discovery. This review article provides a substantial approach on the FRET-based biosensor, principle, applications such as point-of-need diagnosis, wearable, single molecular FRET (smFRET), hard water, ions, pH, tissue-based sensors, immunosensors, and aptasensor. Recent advances such as artificial intelligence (AI) and Internet of Things (IoT) are used for this type of sensor and challenges.

Digital detection of proteins

This paper reviews methods for detecting proteins based on molecular digitization, i.e., the isolation and detection of single protein molecules or singulated ensembles of protein molecules. The single molecule resolution of these methods has resulted in significant improvements in the sensitivity of immunoassays beyond what was possible using traditional "analog" methods: the sensitivity of some digital immunoassays approach those of methods for measuring nucleic acids, such as the polymerase chain reaction (PCR). The greater sensitivity of digital protein detection has resulted in immuno-diagnostics with high potential societal impact, e.g., the early diagnosis and therapeutic intervention of Alzheimer's Disease. In this review, we will first provide the motivation for developing digital protein detection methods given the limitations in the sensitivity of analog methods. We will describe the paradigm shift catalyzed by single molecule detection, and will describe in detail one digital approach - which we call digital bead assays (DBA) - based on the capture and labeling of proteins on beads, identifying "on" and "off" beads, and quantification using Poisson statistics. DBA based on the single molecule array (Simoa) technology have sensitivities down to attomolar concentrations, equating to ∼10 proteins in a 200 μL sample. We will describe the concept behind DBA, the different single molecule labels used, the ways of analyzing beads (imaging of arrays and flow), the binding reagents and substrates used, and integration of these technologies into fully automated and miniaturized systems. We provide an overview of emerging approaches to digital protein detection, including those based on digital detection of nucleic acids labels, single nanoparticle detection, measurements using nanopores, and methods that exploit the kinetics of single molecule binding. We outline the initial impact of digital protein detection on clinical measurements, highlighting the importance of customized assay development and translational clinical research. We highlight the use of DBA in the measurement of neurological protein biomarkers in blood, and how these higher sensitivity methods are changing the diagnosis and treatment of neurological diseases. We conclude by summarizing the status of digital protein detection and suggest how the lab-on-a-chip community might drive future innovations in this field.

Microwell array chip-based single-cell analysis

Single-cell profiling is key to uncover the cellular heterogeneity and drives deep understanding of cell fate. In recent years, microfluidics has become an ideal tool for single-cell profiling owing to its benefits of high throughput and automation. Among various microfluidic platforms, microwell has the advantages of simple operation and easy integration with in situ analysis ability, making it an ideal technique for single-cell studies. Herein, recent advances of single-cell analysis based on microwell array chips are summarized. We first introduce the design and preparation of different microwell chips. Then microwell-based cell capture and lysis strategies are discussed. We finally focus on advanced microwell-based analysis of single-cell proteins, nucleic acids, and metabolites. The challenges and opportunities for the development of microwell-based single-cell analysis are also presented.

Surface modification for improving immunoassay sensitivity

Immunoassays are widely performed in many fields such as biomarker discovery, proteomics, drug development, and clinical diagnosis. There is a growing need for high sensitivity of immunoassays to detect low abundance analytes. As a result, great effort has been made to improve the quality of surfaces, on which the immunoassay is performed. In this review article, we summarize the recent progress in surface modification strategies for improving the sensitivity of immunoassays. The surface modification strategies can be categorized into two groups: antifouling coatings to reduce background noise and nanostructured surfaces to amplify the signals. The first part of the review summarizes the common antifouling coating techniques to prevent nonspecific binding and reduce background noise. The techniques include hydrophilic polymer based self-assembled monomers, polymer brushes, and surface attached hydrogels, and omniphobicity based perfluorinated surfaces. In the second part, some common nanostructured surfaces to amplify the specific detection signals are introduced, including nanoparticle functionalized surfaces, two dimensional (2D) nanoarrays, and 2D nanomaterial coatings. The third part discusses the surface modification techniques for digital immunoassays. In the end, the challenges and the future perspectives of the surface modification techniques for immunoassays are presented.

Supersensitive detection of single-histamine molecule on nanoplates by turn-on small molecule fluorescence sandwich immunoassay

We develop a supersensitive "turn-on format" fluorescence sandwich immunoassay for detecting small single molecules. Gold nanoplate-based biotin antibodies and streptavidin-fluorophores were used instead of streptavidin-horseradish peroxidase reacting with a biotin tracer in a microplate-based competitive enzyme-linked immunosorbent assay (ELISA). Our platform showed a low detection limit of 5 zeptomolar (5 × 10^-21 M), 5.4 × 10¹⁰ times higher detection sensitivity than the conventional tune-off format ELISA.

Investigation of Charged Small Molecule-Aptamer Interactions with Surface Plasmon Resonance

Investigating the interactions between small, charged molecules and aptamers using surface plasmon resonance (SPR) is limited by the inherent low response of small molecules and difficulties with nonspecific electrostatic interactions between the aptamer, analyte, and sensor surface. However, aptamers are increasingly being used in sensors for small molecule detection in critical areas like healthcare and environmental safety. The ability to probe these interactions through simple, direct SPR assays would be greatly beneficial and allow for the development of improved sensors without the need for complicated signal enhancement. However, these assays are nearly nonexistent in the current literature and are instead surpassed by sandwich or competitive binding techniques, which require additional sample preparation and reagents. In this work, we develop a method to characterize the interaction between the charged small molecule serotonin (176 Da) and an aptamer with SPR using streptavidin-biotin capture and a high-ionic-strength buffer. Additionally, other methods, such as serotonin immobilization and thiol-coupling of the aptamer, were investigated for comparison. These techniques give insight into working with small molecules and allow for quickly adapting a binding affinity assay into a direct SPR sensor.

Advances in optical counting and imaging of micro/nano single-entity reactors for biomolecular analysis

Ultrasensitive detection of biomarkers is of paramount importance in various fields. Superior to the conventional ensemble measurement-based assays, single-entity assays, especially single-entity detection-based digital assays, not only can reach ultrahigh sensitivity, but also possess the potential to examine the heterogeneities among the individual target molecules within a population. In this review, we summarized the current biomolecular analysis methods that based on optical counting and imaging of the micro/nano-sized single entities that act as the individual reactors (e.g., micro-/nanoparticles, microemulsions, and microwells). We categorize the corresponding techniques as analog and digital single-entity assays and provide detailed information such as the design principles, the analytical performance, and their implementation in biomarker analysis in this work. We have also set critical comments on each technique from these aspects. At last, we reflect on the advantages and limitations of the optical single-entity counting and imaging methods for biomolecular assay and highlight future opportunities in this field.

Ultrasensitive Automatic Detection of Small Molecules by Membrane Imaging of Single Molecule Assays

Determination of trace amounts of targets or even a single molecule target has always been a challenge in the detection field. Digital measurement methods established for single molecule counting of proteins, such as single molecule arrays (Simoa) or dropcast single molecule assays (dSimoa), are not suitable for detecting small molecule, because of the limited category of small molecule antibodies and the weak signal that can be captured. To address this issue, we have developed a strategy for single molecule detection of small molecules, called small molecule detection with single molecule assays (smSimoa). In this strategy, an aptamer is used as a recognition element, and an addressable DNA Nanoflower (DNF) attached on the magnetic beads surface, which exhibit fluorescence imaging, is employed as the output signal. Accompanied by digital imaging and automated counting analysis, E2 at the attomolar level can be measured. The smSimoa breaks the barrier of small molecule detection concentration and provides a basis for high throughput detection of multiple substances with fluorescence encoded magnetic beads.

Gold Nanoparticles-MWCNT Based Aptasensor for Early Diagnosis of Prostate Cancer

Prostate cancer is one of the most frequently diagnosed male malignancies and can be detected by prostate-specific antigen (PSA) as a biomarker. To detect PSA, several studies have proposed using antibodies, which are not economical and require a long reaction time. In this study, we propose to use self-assembled thiolated single-strand DNA on electrodes functionalized by multi-walled carbon nanotubes (MWCNT) modified with gold nanoparticles (AuNPs) to realize a low-cost label-free electrochemical biosensor. In this regard, the PSA aptamer was immobilized via electrostatic adsorption on the surface of a screen-printed MWCNT/AuNPs electrode. The immobilization process was enhanced due to the presence of Au nanoparticles on the surface of the electrode. Surface characterization of the electrode at different stages of modification was performed by electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) and contact angle for surface tension properties. The results showed an increase in surface roughness due to the absorbance of the aptamer on the electrode surfaces. The developed sensor has an extended linear range of 1-100 ng/mL, and a very low limit of detection down to 1 pg/mL. In addition, the reaction has a binding time of only five minutes on the developed electrodes. Investigations of the biosensor selectivity against several substances revealed an efficient selectivity for PSA detection. With this approach, low-cost biosensors with high sensitivity can be realized which have a wide linearity range and a low limit of detection, which are necessary for the early detection of prostate cancer.

Nano-Engineered Surface Comprising Metallic Dendrites for Biomolecular Analysis in Clinical Perspective

Metallic dendrites, a class of three-dimensional nanostructured materials, have drawn a lot of interests in the recent years because of their interesting hierarchical structures and distinctive features. They are a hierarchical self-assembled array of primary, secondary, and terminal branches with a plethora of pointed ends, ridges, and edges. These features provide them with larger active surface areas. Due to their enormous active areas, the catalytic activity and conductivity of these nanostructures are higher as compared to other nanomaterials; therefore, they are increasingly used in the fabrication of sensors. This review begins with the properties and various synthetic approaches of nanodendrites. The primary goal of this review is to summarize various nanodendrites-engineered biosensors for monitoring of small molecules, macromolecules, metal ions, and cells in a wide variety of real matrices. Finally, to enlighten future research, the limitations and future potential of these newly discovered materials are discussed.

Single-molecule displacement assay reveals strong binding of polyvalent dendrimer ligands to telomeric G-quadruplex

Binding between a ligand and a receptor is a fundamental step in many natural or synthetic processes. In biosensing, a tight binding with a small dissociation constant (Kd) between the probe and analyte can lead to superior specificity and sensitivity. Owing to their capability of evaluating competitors, displacement assays have been used to estimate Kd at the ensemble average level. At the more sensitive single-molecule level, displacement assays are yet to be established. Here, we developed a single-molecule displacement assay (smDA) in an optical tweezers instrument and used this innovation to evaluate the binding of the L2H2-6OTD ligands to human telomeric DNA G-quadruplexes. After measuring Kd of linear and dendrimer L2H2-6OTD ligands, we found that dendrimer ligands have enhanced binding affinity to the G-quadruplexes due to their polyvalent geometry. This increased binding affinity enhanced inhibition of telomerase elongation on a telomere template in a Telomerase Repeated Amplification Protocol (TRAP). Our experiments demonstrate that the smDA approach can efficiently evaluate binding processes in chemical and biological processes.

Normalizing the Optical Signal Enables Robust Assays with Lateral Flow Biosensors

Lateral flow assays (LFAs) are widely adopted for fast, on-site molecular diagnostics. Obtaining high-precision assay results, however, remains challenging and often requires a dedicated optical setup to control the imaging environment. Here, we describe quick light normalization exam (qLiNE) that transforms ubiquitous smartphones into a robust LFA reader. qLiNE used a reference card, printed with geometric patterns and color standards, for real-time optical calibration: a photo of an LFA test strip was taken along with the card, and the image was processed using a smartphone app to correct shape distortion, illumination brightness, and color imbalances. This approach yielded consistent optical signal, enabling quantitative molecular analyses under different illumination conditions. We adapted qLiNE to detect cortisol, a known stress hormone, in saliva samples at point-of-use settings. The assay was fast (15 min) and sensitive (detection limit, 0.16 ng/mL). The serial qLiNE assay detected diurnal cycles of cortisol levels as well as stress-induced cortisol increase.

Improved ELISA for linoleate-derived diols in human plasma utilizing a polyHRP-based secondary tracer

Dihydroxyoctadecenoic acids (DiHOMEs) are cytochrome P450 pathway-derived metabolites of linoleic acid, a highly abundant dietary fatty acid. They serve thermogenic functions at low concentrations but, at high concentrations, are involved in proinflammatory and deleterious outcomes in a wide range of pathologies. Hence, the development of a reliable analytical method is critical to elucidate their potential as biomarkers of health, and enzyme-linked immunoassay (ELISA)-based approaches offer unique benefits as alternatives to traditional liquid chromatography-tandem mass spectrometry (LC-MS/MS) systems. Accordingly, an earlier ELISA for DiHOMEs was dramatically improved employing new secondary tracers and geared towards use in human plasma, a universal matrix in biomedical applications, as well as urine. Three ELISA formats, two utilizing polyHRP-based secondary labels for signal amplification, were compared. The best format involved a biotinylated detection antibody and a polyHRP-conjugated streptavidin tracer. Assay detectability was enhanced 20-fold, relative to the original immunoassay, and performance assessments validated precision, selectivity, and robustness. Fast and easy extraction-clean up steps yielded high analytical recovery and permitted the assay to operate in moderate concentrations (up to 20%) of plasma, expanding its practical relevance. Finally, the ELISA was applied towards detection of DiHOMEs in clinical samples and authenticated with complementary LC-MS/MS analysis. Hence, the method provides a valuable analytical tool to investigate the diverse and extensive roles of DiHOMEs in regulatory biology.

A Dual-Modal Single-Antibody Plasmonic Spectro-Immunoassay for Detection of Small Molecules

Small molecules play a pivotal role in regulating physiological processes and serve as biomarkers to uncover pathological conditions and the effects of therapeutic treatments. However, it remains a significant challenge to detect small molecules given the size as compared to macromolecules. Recently, the newly emerging plasmonic immunoassays based on surface-enhanced Raman scattering (SERS) offer great promise to deliver extraordinary sensitivity. Nevertheless, they are limited by the intrinsic SERS intensity fluctuations associated with the SERS uncertainty principle. The single transducer that relies on the intensity change is also prone to false signals. Additionally, the prevailing sandwich immunoassay format proves less effective towards detecting small molecules. To circumvent these critical issues, a dual-modal single-antibody approach that synergizes both the intensity and shift of the peak-based immunoassay with Raman enhancement, coined as the INSPIRE assay, is developed for small molecules detection. With two independent transduction mechanisms, it allows better prediction of analyte concentration and attenuation of signal artifacts, providing a new and robust strategy for molecular analysis. With a proof-of-concept demonstration for detection of free T4 and testosterone in serum matrix, the authors envision that the INSPIRE assay could be expanded for a wide spectrum of applications in biomedical diagnosis, discovery of new biopharmaceuticals, food safety, and environmental monitoring.

Single molecule detection; from microscopy to sensors

Single molecule detection is necessary to find out physical, chemical properties and their mechanism involved in the normal functioning of body cells. In this way, they can provide a new direction to the healthcare system. Various techniques have been developed and employed for their successful detection. Herein, we have emphasized various traditional methods as well as biosensing technology which offer single molecule sensitivity. The various methods including plasmonic resonance, nanopores, whispering gallery mode, Simoa assay and recognition tunneling are discussed in the initial part which has been followed by a discussion about biosensor-based detection. Plasmonic, SERS, CRISPR/Cas, and other types of biosensors are focused in this review and found to be highly sensitive for single molecule detection. This review provides an overview of progression in different techniques employed for single molecule detection.

Recombinant Histidine-Tagged Nano-protein-based Highly Sensitive Electro-Sensing Device for Salivary Cortisol

We have developed a powerful biosensing strategy for immobilizing histidine-tagged (His-Tag)-oriented recombinant nano-protein immobilization on a chemically modified glassy carbon electrode (GCE) surfaces via (S)-N-(5-amino-1-carboxypentyl)iminodiacetic acid (ANTA) acting as a chelating Ni2+ centered interaction. Here, we introduce a label-free electro-sensor to quantify cortisol levels in saliva samples for point-of-care testing (POCT). The high specificity of the chemically modified GCE was established by genetically bio-engineered metal-binding sites on the selected recombinant apoferritin (R-AFTN) nano-protein to impart functionality to its surface and by coating the carbon surface with the self-assembled monolayers of 4-aminobenzoic acid (4-ABA) attached to ANTA groups complexed with Ni2+ transition metal ions. Despite the variety of conventional assays available to monitor cortisol levels, they require bulky exterior outfits, which hinders use in the healthcare systems. Therefore, we performed a rapid, easy-to-implement, and low-cost quantitative electro-sensor to enable the real-time detection of cortisol levels in saliva samples. As a result, the cortisol electro-sensor fabricated with high specificity utilizing a GCE could measure cortisol levels with a detection limit of 0.95 ng/ml and sensitivity of 7.91 μA/(ng/mL), which is a practical approach in human saliva. Thus, protein nanoprobe-based cortisol biosensing showed high sensitivity and selectivity for the direct electro-sensing of cortisol for POCT.

"Ready-to-use" immunosensor for the detection of small molecules with fast readout

Immunoassays are commonly used methods for detection of small molecules that typically require numerous steps of the labeling between immune-recognition reagents and tracers, immobilization and recurrent washing, making them time consuming and difficult to adapt into point of care formats. Here we describe a "ready-to-use" homogeneous competitive immunosensor with an assay time of 10 min that is based exclusively on recombinant reagents. The signal is produced when the split fragments of the nano luciferase (Nluc) are brought together by the interaction of a heavy chain only variable domain (VHH) with a peptidomimetic of the target small molecule. A VHH to 2,4-dichlorophenoxyacetic acid (2,4-D) was used to isolated the peptidomimetic (NGFFEPWQVVYV) from phage display libraries using six panning conditions. Then the peptidomimetic and VHH were fused with the larger (LgN) and smaller piece (SmN) of split fragments of Nluc, respectively. In order to optimize the signal and sensitivity of the immunosensor, we explored the effects of the spacer between the peptidomimetic and LgN, the copy number of peptidomimetics, and the spacer between SmN and VHH, generating 24 combinations that allowed to conclude on their respective roles. Eventually, the developed "ready-to-use" immunosensor performed excellent signal-to-noise ratio and sensitivity, and could be applied to the detection of 2,4-D in real samples. Meanwhile, the immunosensor totally realizes labeling-free, immobilization-free and washing-free, also can be produced in a highly cost effective way.

Aggregation-Induced Emission Luminogen-Based Dual-Mode Enzyme-Linked Immunosorbent Assay for Ultrasensitive Detection of Cancer Biomarkers in a Broad Concentration Range

The enzyme-linked immunosorbent assay (ELISA) is one of the most commonly used methods for measuring antibodies and antigens in biological samples. However, developing new ELISAs with high detection sensitivity and broad detection dynamic ranges without resorting to complicated signal processing and equipment setups remains a challenge. In this work, we report a strategy to simultaneously improve the detection sensitivity and broaden the dynamic range by replacing the chromogenic reagents used in traditional ELISAs with an aggregation-induced emission luminogen (AIEgen). The developed AIE-ELISA could generate complementary absorbance and fluorescence signals with a linear detection range of 1.6-25,000 pg/mL. The application of this dual-mode AIE-ELISA in the detection of the prostate-specific antigen (PSA) realized a limit of detection of 1.3 pg/mL (3.78 × 10^-14 M) and dynamic range improvement of approximately 2 orders of magnitude compared to a single-mode ELISA, which enabled it to discriminate a minor PSA difference in a patient's serum. The simpler experimental operation, faster enzyme response speed, and better photostability of AIEgen than the traditional chromogenic reagents used in ELISAs showed that our developed AIE-ELISA holds great potential in the fields of immunoassay, immunohistochemistry, and immunocytochemistry.

Plasmonically-enhanced competitive assay for ultrasensitive and multiplexed detection of small molecules

Novel methods that enable facile, ultrasensitive and multiplexed detection of low molecular weight organic compounds such as metabolites, drugs, additives, and organic pollutants are valuable in biomedical research, clinical diagnosis, food safety and environmental monitoring. Here, we demonstrate a simple, rapid, and ultrasensitive method for detection and quantification of small molecules by implementing a competitive immunoassay with an ultrabright fluorescent nanolabel, plasmonic fluor. Plasmonic-fluor is comprised of a polymer-coated gold nanorod and bovine serum albumin conjugated with molecular fluorophores and biotin. The synthesis steps and fluorescence emission of plasmonic-fluor was characterized by UV-vis spectroscopy, transmission electron microscopy, and fluorescence microscopy. Plasmon-enhanced competitive assay can be completed within 20 min and exhibited more than 30-fold lower limit-of-detection for cortisol compared to conventional competitive ELISA. The plasmon-enhanced competitive immunoassay when implemented as partition-free digital assay enabled further improvement in sensitivity. Further, spatially multiplexed plasmon-enhanced competitive assay enabled the simultaneous detection of two analytes (cortisol and fluorescein). This simple, rapid, and ultrasensitive method can be broadly employed for multiplexed detection of various small molecules in research, in-field and clinical settings.

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.

Monovalent Antigen-Induced Aggregation (MAA) Biosensors Using Immunomagnetic Beads in Both Sample Separation and Signal Generation for Label-Free Detection of Enrofloxacin

Exploring new functions of nanomaterials can help facilitate the development of biosensors for the detection of antibiotics. Herein, a new detection modality based on monovalent antigen-induced aggregation (MAA) of immunomagnetic beads (IMBs) was proposed for rapid and label-free detection of enrofloxacin (ENR), which endowed IMBs with the abilities of both sample separation and signal generation. In the presence of ENR, the initially well-dispersed IMBs were aggregated and the degree of aggregation was in a concentration-dependent manner. After exploring the mechanism underlying IMB aggregation and investigating the key parameters affecting it, a label-free biosensing platform was developed for rapid and sensitive detection of ENR. Based on the significant differences in the magnetic separation speed and size between the aggregated and well-dispersed IMBs, two methods were proposed for quantitatively determining ENR, i.e., measuring the turbidity of the IMB supernatant after magnetic separation for a given time and visualizing and calculating the grayscale value of the aggregated IMBs trapped on the surface of a nitrocellulose membrane. A three-dimensional (3D)-printed syringe was designed and fabricated for automatic filtration of IMBs. This immunosensor allowed for sensitive detection of ENR in less than 15 min without any labels. It exhibited a satisfactory limit of detection of 0.79 ng mL^-1 and showed the feasibility for ENR detection of spiked chicken meat with recovery rates ranging from 74.8 to 98.3%. The MAA immunosensor can act as a promising tool to detect trace levels of ENR and has the potential to be applied to complex food samples.

Utility of the Specialized Pro-Resolving Mediators as Diagnostic and Prognostic Biomarkers in Disease

A precision medicine approach is widely acknowledged to yield more effective therapeutic strategies in the treatment of patients with chronic inflammatory conditions than the prescriptive paradigm currently utilized in the management and treatment of these patients. This is because such an approach will take into consideration relevant factors including the likelihood that a patient will respond to given therapeutics based on their disease phenotype. Unfortunately, the application of this precision medicine paradigm in the daily treatment of patients has been greatly hampered by the lack of robust biomarkers, in particular biomarkers for determining early treatment responsiveness. Lipid mediators are central in the regulation of host immune responses during both the initiation and resolution of inflammation. Amongst lipid mediators, the specialized pro-resolving mediators (SPM) govern immune cells to promote the resolution of inflammation. These autacoids are produced via the stereoselective conversion of essential fatty acids to yield molecules that are dynamically regulated during inflammation and exert potent immunoregulatory activities. Furthermore, there is an increasing appreciation for the role that these mediators play in conveying the biological actions of several anti-inflammatory therapeutics, including statins and aspirin. Identification and quantitation of these mediators has traditionally been achieved using hyphenated mass spectrometric techniques, primarily liquid-chromatography tandem mass spectrometry. Recent advances in the field of chromatography and mass spectrometry have increased both the robustness and the sensitivity of this approach and its potential deployment for routine clinical diagnostics. In the present review, we explore the evidence supporting a role for specific SPM as potential biomarkers for patient stratification in distinct disease settings together with methodologies employed in the identification and quantitation of these autacoids.

Nano-Bio Interface-Guided Nanoparticle Protein Corona Antigen for Immunoassays and Immunoimaging in a Complex Matrix

Engineered nanoparticles are widely used in biological imaging and drug delivery because of their excellent physical and chemical properties, but almost all the original functions of engineered nanoparticles suffer from a complex matrix. Herein, we proposed a strategy of preparing nanoparticle protein corona antigens (NPCAgs) through exposing a magnetic core silicon shell (Fe₃O₄@SiO₂) fluorescent probe to an antigen protein solution, which could reduce the adsorption of nanoparticles (NPs) with other proteins in serum. In the presence of target anti-BSA IgG, a competitive-type displacement reaction was implemented between NPs@BSA and other proteins by target anti-BSA IgG through the specific antigen-antibody reaction. In addition, secondary structure analysis showed that almost all of the NPCAgs retained their natural conformation, which ensured the function of the NPCAgs, specifically capturing an antibody. Therefore, the NPCAgs showed good performance in immunoassays and immunoimaging, which should shed light on the application in imaging and identification of other nanomaterials.

Large-Area Interfaces for Single-Molecule Label-free Bioelectronic Detection

Bioelectronic transducing surfaces that are nanometric in size have been the main route to detect single molecules. Though enabling the study of rarer events, such methodologies are not suited to assay at concentrations below the nanomolar level. Bioelectronic field-effect-transistors with a wide (μm²-mm²) transducing interface are also assumed to be not suited, because the molecule to be detected is orders of magnitude smaller than the transducing surface. Indeed, it is like seeing changes on the surface of a one-kilometer-wide pond when a droplet of water falls on it. However, it is a fact that a number of large-area transistors have been shown to detect at a limit of detection lower than femtomolar; they are also fast and hence innately suitable for point-of-care applications. This review critically discusses key elements, such as sensing materials, FET-structures, and target molecules that can be selectively assayed. The amplification effects enabling extremely sensitive large-area bioelectronic sensing are also addressed.

Aptasensors versus immunosensors—Which will prevail?

Since the invention of the first biosensors 70 years ago, they have turned into valuable and versatile tools for various applications, ranging from disease diagnosis to environmental monitoring. Traditionally, antibodies have been employed as the capture probes in most biosensors, owing to their innate ability to bind their target with high affinity and specificity, and are still considered as the gold standard. Yet, the resulting immunosensors often suffer from considerable limitations, which are mainly ascribed to the antibody size, conjugation chemistry, stability, and costs. Over the past decade, aptamers have emerged as promising alternative capture probes presenting some advantages over existing constraints of immunosensors, as well as new biosensing concepts. Herein, we review the employment of antibodies and aptamers as capture probes in biosensing platforms, addressing the main aspects of biosensor design and mechanism. We also aim to compare both capture probe classes from theoretical and experimental perspectives. Yet, we highlight that such comparisons are not straightforward, and these two families of capture probes should not be necessarily perceived as competing but rather as complementary. We, thus, elaborate on their combined use in hybrid biosensing schemes benefiting from the advantages of each biorecognition element.

Organic Cation Receptor for Colorimetric Lateral Flow Device: Detection of Zearalenone in Food Samples

As per the WHO reports, it has been estimated that almost 25% of food crops contain mycotoxins as the major contaminant. In this work, we developed a paper-based colorimetric lateral flow device (CLFD) impregnated with an organic cation receptor (OCR) for sensitive and selective detection of zearalenone (ZEN). Various techniques such as ultraviolet (UV)-visible absorption, cyclic voltammetry, and fluorescence spectroscopy were used for the detection of mycotoxins, and it was observed that OCR shows sensitivity and selectivity toward zearalenone (ZEN) only, irrespective of any other analytes. Furthermore, the colorimetric test revealed that the developed OCR shows a change in color with the addition of ZEN from greenish-gray to blue that is visible to the naked eye. The quantification of ZEN was also achieved using RGB analysis and compared with UV-visible spectroscopy data. Further, for the on-site detection of ZEN, a paper-based CLFD was also developed and used to evaluate the spiked corn sample containing ZEN, and it provided significant results with a limit of detection (LOD) of 0.31 nM (3σ method), good linearity (R² = 0.9702), good reproducibility (SD = ±6%, triplicate), and good recovery of ZEN of 95-102% with a variation coefficient (VC) varying from 1.56 to 4.62%. Therefore, the device has the potential to check the mycotoxin toxicity in food products and is helpful in remote and developing areas.