Affinity Sensor Research Articles

Separation of shifts in surface plasmon resonance into refractive index and thickness of deposited layers

Routine Surface Plasmon Resonance (SPR) measurements, which are widely applied in affinity (bio)sensors and surface science, do not allow one to separate the obtained SPR signals into the change of the refractive index and thickness of deposited layers. For thin layers the signal is proportional to the product of the layer thickness and the difference in the refractive indices of this layer and of the aqueous media. In this study, we suggest an approach to separate these parameters. It is performed by subsequent measurements in the presence and in the absence of polyethylene glycol (PEG), an inert additive which modifies the refractive index of the liquid phase but do not penetrate into the adsorbed layer. Both parameters were successively determined for human serum albumin (HSA) as well as for anti-HSA IgG in three different settings: i) HSA adsorbed directly onto a gold surface, ii) HSA chemically immobilized on a gold surface coated with a self-assembled monolayer of 1,16 mercaptohexadecanoic acid and iii) chemically immobilized HSA with additional monomolecular layer of anti-HSA IgG antibodies bond to this protein layer. The suggested approach can be applied in most SPR devices with a flow cell for analysis of various adsorbed layers.

Biomimetic affinity sensor for the ultrasensitive detection of neonicotinoids

Multiple pesticides are often used in combination to protect crops from pests. This makes rapid on-site detection of pesticide contamination challenging. Herein, we describe a method for simultaneous detection of diverse neonicotinoid pesticides using a sensor that combines neonicotinoid-specific odorant-binding protein 2 (OBP2), which was cloned from an insect chemical sensing protein and modified gold nanoparticles with local surface plasmon resonance (LSPR)-based digital nanoplasmonometry (DiNM). When neonicotinoid pesticides bind to OBP2 on gold nanoparticles, the induced LSPR shift peak wavelength is too small to be measured using conventional LSPR immunoassays. DiNM records and compares the scattered image intensity in two adjacent wavelength bands, A and B, centered on the LSPR peak. It considers both the peak shift and the relative intensity change in these two bands, resulting in a significant LSPR signal enhancement. Then the spectral-image contrast was computed as the signal response. Using this approach, we obtained excellent limits of detection (LODs) of 1.4, 1.5, and 4.5 ppb for the neonicotinoids imidacloprid, acetamiprid, and dinotefuran, respectively. Blind tests demonstrated high positive and negative rates for teas, approximately 85 and 100%, respectively. Recombinant OBP2 produced in E. coli offers several advantages over antibodies, including high yield, time savings, and cost effectiveness. Moreover, this method is highly selective and sensitive to neonicotinoids, making it practical for field use.

Optimization of Vancomycin Aptamer Sequence Length Increases the Sensitivity of Electrochemical, Aptamer-Based Sensors In Vivo.

The measurement of serum vancomycin levels at the clinic is critical to optimizing dosing given the narrow therapeutic window of this antibiotic. Current approaches to quantitate serum vancomycin levels are based on immunoassays, which are multistep methods requiring extensive processing of patient samples. As an alternative, vancomycin-binding electrochemical, aptamer-based sensors (E-ABs) were developed to simplify the workflow of vancomycin monitoring. E-ABs enable the instantaneous measurement of serum vancomycin concentrations without the need for sample dilution or other processing steps. However, the originally reported vancomycin-binding E-ABs had a dissociation constant of 45 μM, which is approximately 1 order of magnitude higher than the recommended trough concentrations of vancomycin measured in patients. This limited sensitivity hinders the ability of E-ABs to accurately support vancomycin monitoring. To overcome this problem, here we sought to optimize the length of the vancomycin-binding aptamer sequence to enable a broader dynamic range in the E-AB platform. Our results demonstrate, via isothermal calorimetry and E-AB calibrations in undiluted serum, that superior affinity and near-equal sensor gain in vitro can be achieved using a one-base-pair-longer aptamer than the truncated sequence originally reported. We validate the impact of the improved binding affinity in vivo by monitoring vancomycin levels in the brain cortex of live mice following intravenous administration. While the original sequence fails to resolve vancomycin concentrations from baseline noise (SNR = 1.03), our newly reported sequence provides an SNR of 1.62 at the same dose.

Dynamic Regulation of Mitochondrial [Ca2+] in Hippocampal Neurons.

While neuronal mitochondria have been studied extensively in their role in health and disease, the rules that govern calcium regulation in mitochondria remain somewhat vague. In the present study using cultured rat hippocampal neurons transfected with the mtRCaMP mitochondrial calcium sensor, we investigated the effects of cytosolic calcium surges on the dynamics of mitochondrial calcium ([Ca2+]m). Cytosolic calcium ([Ca2+]c) was measured using the high affinity sensor Fluo-2. We recorded two types of calcium events: local and global ones. Local events were limited to a small, 2–5 µm section of the dendrite, presumably caused by local synaptic activity, while global events were associated with network bursts and extended throughout the imaged dendrite. In both cases, cytosolic surges were followed by a delayed rise in [Ca2+]m. In global events, the rise lasted longer and was observed in all mitochondrial clusters. At the end of the descending part of the global event, [Ca2+]m was still high. Global events were accompanied by short and rather high [Ca2+]m surges which we called spikelets, and were present until the complete decay of the cytosolic event. In the case of local events, selective short-term responses were limited to the part of the mitochondrial cluster that was located directly in the center of [Ca2+]c activity, and faded quickly, while responses in the neighboring regions were rarely observed. Caffeine (which recruits ryanodine receptors to supply calcium to the mitochondria), and carbonyl cyanide m-chlorophenyl hydrazine (CCCP, a mitochondrial uncoupler) could affect [Ca2+]m in both global and local events. We constructed a computational model to simulate the fundamental role of mitochondria in restricting calcium signals within a narrow range under synapses, preventing diffusion into adjacent regions of the dendrite. Our results indicate that local cytoplasmic and mitochondrial calcium concentrations are highly correlated. This reflects a key role of signaling pathways that connect the postsynaptic membrane to local mitochondrial clusters.

Using Cell Membranes as Recognition Layers to Construct Ultrasensitive and Selective Bioelectronic Affinity Sensors.

Conventional sandwich immunosensors rely on antibody recognition layers to selectively capture and detect target antigen analytes. However, the fabrication of these traditional affinity sensors is typically associated with lengthy and multistep surface modifications of electrodes and faces the challenge of nonspecific adsorption from complex sample matrices. Here, we report on a unique design of bioelectronic affinity sensors by using natural cell membranes as recognition layers for protein detection and prevention of biofouling. Specifically, we employ the human macrophage (MΦ) membrane together with the human red blood cell (RBC) membrane to coat electrochemical transducers through a one-step process. The natural protein receptors on the MΦ membrane are used to capture target antigens, while the RBC membrane effectively prevents nonspecific surface binding. In an attempt to detect tumor necrosis factor alpha (TNF-α) cytokine using the bioelectronic affinity sensor, it demonstrates a remarkable limit of detection of 150 pM. This new sensor design integrates natural cell membranes and electronic transduction, which offers synergistic functionalities toward a broad range of biosensing applications.

Analysing and meta-analysing time-series data of microbial growth and gene expression from plate readers.

Responding to change is a fundamental property of life, making time-series data invaluable in biology. For microbes, plate readers are a popular, convenient means to measure growth and also gene expression using fluorescent reporters. Nevertheless, the difficulties of analysing the resulting data can be a bottleneck, particularly when combining measurements from different wells and plates. Here we present omniplate, a Python module that corrects and normalises plate-reader data, estimates growth rates and fluorescence per cell as functions of time, calculates errors, exports in different formats, and enables meta-analysis of multiple plates. The software corrects for autofluorescence, the optical density’s non-linear dependence on the number of cells, and the effects of the media. We use omniplate to measure the Monod relationship for the growth of budding yeast in raffinose, showing that raffinose is a convenient carbon source for controlling growth rates. Using fluorescent tagging, we study yeast’s glucose transport. Our results are consistent with the regulation of the hexose transporter (HXT) genes being approximately bipartite: the medium and high affinity transporters are predominately regulated by both the high affinity glucose sensor Snf3 and the kinase complex SNF1 via the repressors Mth1, Mig1, and Mig2; the low affinity transporters are predominately regulated by the low affinity sensor Rgt2 via the co-repressor Std1. We thus demonstrate that omniplate is a powerful tool for exploiting the advantages offered by time-series data in revealing biological regulation.

Highly Selective Benzo[a]Pyrene Detection Even under Competitive Conditions with Molecularly Imprinted Surface Plasmon Resonance Sensor

In this study, a new approach for the detection of benzo[a]pyrene (BaP) in aqueous media is presented by combining molecularly imprinted polymers (MIPs) with surface plasmon resonance (SPR) sensors. Herein, BaP imprinted sensor was fabricated on a SPR chip in the presence of a pre-polymerization complex of BaP and N-methacryloyl-L-phenylalanine, 2-hydroxyethyl methacrylate (HEMA), and ethylene glycol dimethacrylate (EGDMA). The developed sensor was characterized by atomic force microscopy, ellipsometry, and contact angle measurements. Association kinetics analysis via Scatchard, Langmuir, Freundlich, Langmuir–Freundlich isotherms were applied to the data and Langmuir adsorption model was found as the most accurate model for the affinity sensor system. MIPs-based SPR sensor linearly responded to BaP in a concentration range of 20-400 ng mL−1 with an excellent LOD of 14.97 ng L−1. The SPR sensor was found to be highly selective toward BaP according to the selectivity study results in the presence of interfering agents such as benzo[b]floranthene (BbF), benzo[k]floranthene (BkF), indeno[1,2,3-cd]pyrene (IcdP), and 1-naphthol. The studies demonstrated that MIPs-based SPR sensor provides rapid and reliable detection and identification of polyaromatic hydrocarbons (PAHs) within a quite short time.

Confined adsorption within nanopatterns as generic means to drive high adsorption efficiencies on affinity sensors

Miniaturization of the sensor active areas to length scales of the order of the analyte has been sought as means to reduce device footprints, and to capitalize on the novel optical and electronic enhancements that arise at this scale. However, little is known on what to expect of the sensor behaviour if the sensor footprints are shrunk to the dimensions of the order of analyte themselves. Our results demonstrate the densities and kinetics of adsorption of analyte to significantly increase with decrease in the sensor footprints to dimensions of the order of few multiples of analyte dimensions. Such increase is found to be generic, irrespective of the nature of interactions that drive adsorption, exhibiting qualitative similarity for electrostatic adsorption of nanoparticles, chemisorption of primary oligonucleotides, or complementary base pairing with target nucleotides. The carryover of these benefits onto a macroscopic sensor however requires high density nanopatterns exhibiting significant fill factors withtout compromising inter-feature isolation. The impact of feature dimensions, pattern fill factors, analyte concentrations, presence of convective flow, or the density of receptors are investigated using quantitative and real-time measurements of the nanostructure-analyte interactions using nanopatterned QCM sensors. The results indicate significant opportunities for rational design of nanopatterned macroscopic sensors, as well as nanoscopic sensors with sensor active areas of the order of analyte dimensions, e.g., electromagnetic hot-spots, or nanowire sensors.

Confined Adsorption within Nanopatterns as Generic Means to Drive High Adsorption Efficiencies on Affinity Sensors

Confined Adsorption within Nanopatterns as Generic Means to Drive High Adsorption Efficiencies on Affinity Sensors

Double-opening metal ring based terahertz metamaterial absorber sensor

Terahertz metamaterial biosensor is a label-free affinity sensor that enhances the strength of the local electromagnetic field. It is extremely sensitive to changes in the dielectric constant of the surrounding environment, thereby providing a new method of detecting micro or trace biological samples. In this work, a highly sensitive terahertz refractive index metamaterial absorber sensor for detecting the biological sample is proposed. The sensor consists of two concentric open metal rings and is a multimode resonator. With two independent adjustable operating bands in a frequency range of 0.7–2.5 THz, i.e. 1.079 THz and 2.271 THz, the sensor can observe different electromagnetic effects of the sample in the terahertz band. We evaluate the performance of terahertz sensors with indicators such as absorption characteristics and sensitivity. The sensor possesses the absorption higher than 99.9% in free space. In addition, the large Q value indicates that the sensor provides high frequency selectivity characteristics. Especially, the sensitivity of the sensor achieves 693.7 GHz/RIU, with a minimum refractive index change of 0.004 for the detection of biological samples, which provides good sensing performance. In the proposed sensor, a flexible material with low dielectric constant is used, which has the advantages of biocompatibility and portability and shows high stability at the 0°–60° oblique incidence angle and within 4% fabrication error. Moreover, the detection effectiveness of the sensor is verified by simulation experiments with ethanol-water mixtures. The sensor units designed in this paper have small interactions among them, work stably and are easily fabricated The sensor can significantly enhance the interaction between light and matter and has broad application prospects in terahertz high-sensitivity biosensing detection.

Electrochemical Affinity Sensors Using Field Effect Transducer Devices for Chemical Analysis

AbstractThe review presents advances and main challenges of the affinity sensors based on field‐ effect transistors published during the last five years. The different nanomaterial‐based field‐effect transistors are classified according to the nature of the nanomaterials, beginning by silicon, the “gold‐standard” semiconductor, the gallium nitride semiconductor, the organic semiconductors, the silicon nanowires, the inorganic nanomaterials, the carbon nanotubes and the graphene. Due to its exceptional electrical properties, the main works are devoted to graphene. The obtained analytical performances for the detection of biomarkers, of DNA sequences and of miRNA are listed. The relation between the operational conditions ‐ nature of the nanomaterials, procedure of preparation, choice of the receptor molecule, method of immobilization – and the analytical performance are discussed. The perspective of industrialization of these affinity sensors based on field‐effect transistors is discussed.

Ag-Ni bimetallic film on CaF2 prism for high sensitive surface plasmon resonance sensor

Ag-Ni bimetallic film on CaF2 prism for high sensitive surface plasmon resonance sensor

Graphdiyne nanosheet as a novel sensing platform for self-enhanced electrochemiluminescence of MOF enriched ruthenium (II) in the presence of dual co-reactants for detection of tumor marker

Graphdiyne (GDY) is a new two-dimensional carbon material with high charge carrier mobility, excellent conductivity, more suitable band gap, and natural pores was introduced as a new electrochemiluminescent sensing platform. Herein, the metal organic framework (MOFs) used for enrichment of luminophore with grafting Ru(bpy)2(phen-NH2)2+(Ru-complex) and Ru-complex amine-rich nitrogen-doped carbon nanodots(Ru-NCNDs) via both encapsulating and external decoration and decoration of SmS2 QDs as coreactant. Then, the MOF enriched Ru-complex (Ru@MOF@NCNDs-Ru@SmS2 QD) located on a GDY modified ITO electrode developed as a novel and efficient ECL platform. According to the Density Functional Theory (DFT) calculation, the band gap of graphdiyne/Ru(bpy)2(phen-NH2)2+ system decreased compared to graphdiyne, Ru-complex and also graphene oxide/Ru(bpy)2(phen-NH2)2+system, which enhanced (2 folds) the signal response of the presented ECL platform. The ECL response signal of the suggested emitter with high ECL efficiency (13.34%) increased 8 and 4 folds compared to GDY/Ru-NCNDs and GDY/Ru@MOF@NCNDs-Ru as platforms, respectively. The proposed ECL platform applied for CA19-9 antigens detection at concentration range 0.0005 UmL−1 to 200 UmL−1 and detection limit of 0.00013 UmL−1.The development of GDY based platform for decorating nano luminophores, not only provides the design of ECL luminophores with high performance but also promises the application of the presented strategy for fabrication of ultrasensitive bio affinity sensors as candidates in clinical monitoring and diseases diagnostics.

Electrochemical Sensing in Contact Lenses

AbstractElectrochemical sensors embedded in hydrogel‐based contact lenses provide valuable health‐related information, enabling non‐invasive and real‐time continuous monitoring. Recently, considerable progress has been made in tear based electrochemical sensors. The scope of reported analytes is continuously expanding. This review identifies key chemical biomarkers (including metabolites, ions, proteins) that can be electrochemically detected in tears. The working principles of i) amperometric enzymatic biosensors, ii) ion‐selective sensors for pH and ions, iii) voltammetric sensors and iv) affinity sensors are summarized. This review provides guidelines for the future development of contact lens based electrochemical sensors.

Electrochemical Affinity Assays/Sensors: Brief History and Current Status.

The advent of electrochemical affinity assays and sensors evolved from pioneering efforts in the 1970s to broaden the field of analytes accessible to the selective and sensitive performance of electrochemical detection. The foundation of electrochemical affinity assays/sensors is the specific capture of an analyte by an affinity element and the subsequent transduction of this event into a measurable signal. This review briefly covers the early development of affinity assays and then focuses on advances in the past decade. During this time, progress on electroactive labels, including the use of nanoparticles, quantum dots, organic and organometallic redox compounds, and enzymes with amplification schemes, has led to significant improvements in sensitivity. The emergence of nanomaterials along with microfabrication and microfluidics technology enabled research pathways that couple the ease of use of electrochemical detection for the development of devices that are more user friendly, disposable, and employable, such as lab-on-a-chip, paper, and wearable sensors.

Single-Entity Electrochemistry for Digital Biosensing at Ultralow Concentrations.

Quantifying ultralow analyte concentrations is a continuing challenge in the analytical sciences in general and in electrochemistry in particular. Typical hurdles for affinity sensors at low concentrations include achieving sufficiently efficient mass transport of the analyte, dealing with slow reaction kinetics, and detecting a small transducer signal against a background signal that itself fluctuates slowly in time. Recent decades have seen the advent of methods capable of detecting single analytes ranging from the nanoscale to individual molecules, representing the ultimate mass sensitivity to these analytes. However, single-entity detection does not automatically translate into a superior concentration sensitivity. This is largely because electrochemical transducers capable of such detection are themselves miniaturized, exacerbating mass transport and binding kinetic limitations. In this Perspective, we discuss how these challenges can be tackled through so-called digital sensing: large arrays of separately addressable single-entity detectors that provide real-time information on individual binding events. We discuss the advantages of this approach and the barriers to its implementation.

Rapid and Easy-to-Use Method for Accurate Characterization of Target Binding and Kinetics of Magnetic Particle Bioconjugates for Biosensing.

The ever-increasing use of magnetic particle bioconjugates (MPB) in biosensors calls for methods of comprehensive characterization of their interaction with targets. Label-free optical sensors commonly used for studying inter-molecular interactions have limited potential for MPB because of their large size and multi-component non-transparent structure. We present an easy-to-use method that requires only three 20-min express measurements to determine the key parameters for selection of optimal MPB for a biosensor: kinetic and equilibrium characteristics, and a fraction of biomolecules on the MPB surface that are capable of active targeting. The method also provides a prognostic dependence of MPB targeting efficiency upon interaction duration and sample volume. These features are possible due to joining a magnetic lateral flow assay, a highly sensitive sensor for MPB detection by the magnetic particle quantification technique, and a novel mathematical model that explicitly describes the MPB-target interactions and does not comprise parameters to be fitted additionally. The method was demonstrated by experiments on MPB targeting of cardiac troponin I and staphylococcal enterotoxin B. The validation by an independent label-free technique of spectral-correlation interferometry showed good correlation between the results obtained by both methods. The presented method can be applied to other targets for faster development and selection of MPB for affinity sensors, analytical technologies, and realization of novel concepts of MPB-based biosensing in vivo.

Affinity Sensors for the Diagnosis of COVID-19.

The coronavirus disease 2019 (COVID-19) outbreak caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was proclaimed a global pandemic in March 2020. Reducing the dissemination rate, in particular by tracking the infected people and their contacts, is the main instrument against infection spreading. Therefore, the creation and implementation of fast, reliable and responsive methods suitable for the diagnosis of COVID-19 are required. These needs can be fulfilled using affinity sensors, which differ in applied detection methods and markers that are generating analytical signals. Recently, nucleic acid hybridization, antigen-antibody interaction, and change of reactive oxygen species (ROS) level are mostly used for the generation of analytical signals, which can be accurately measured by electrochemical, optical, surface plasmon resonance, field-effect transistors, and some other methods and transducers. Electrochemical biosensors are the most consistent with the general trend towards, acceleration, and simplification of the bioanalytical process. These biosensors mostly are based on the determination of antigen-antibody interaction and are robust, sensitive, accurate, and sometimes enable label-free detection of an analyte. Along with the specification of biosensors, we also provide a brief overview of generally used testing techniques, and the description of the structure, life cycle and immune host response to SARS-CoV-2, and some deeper details of analytical signal detection principles.

Advances in Molecularly Imprinted Polymers Based Affinity Sensors (Review).

Recent challenges in biomedical diagnostics show that the development of rapid affinity sensors is very important issue. Therefore, in this review we are aiming to outline the most important directions of affinity sensors where polymer-based semiconducting materials are applied. Progress in formation and development of such materials is overviewed and discussed. Some applicability aspects of conducting polymers in the design of affinity sensors are presented. The main attention is focused on bioanalytical application of conducting polymers such as polypyrrole, polyaniline, polythiophene and poly(3,4-ethylenedioxythiophene) ortho-phenylenediamine. In addition, some other polymers and inorganic materials that are suitable for molecular imprinting technology are also overviewed. Polymerization techniques, which are the most suitable for the development of composite structures suitable for affinity sensors are presented. Analytical signal transduction methods applied in affinity sensors based on polymer-based semiconducting materials are discussed. In this review the most attention is focused on the development and application of molecularly imprinted polymer-based structures, which can replace antibodies, receptors, and many others expensive affinity reagents. The applicability of electrochromic polymers in affinity sensor design is envisaged. Sufficient biocompatibility of some conducting polymers enables to apply them as “stealth coatings” in the future implantable affinity-sensors. Some new perspectives and trends in analytical application of polymer-based semiconducting materials are highlighted.

Challenges for impedimetric affinity sensors targeting protein detection

The review provides articles discussing big challenges behind successful design of impedimetric biosensors and the way such challenges were/could be addressed for highly sensitive affinity detection of proteins. In particular, the effect of gold etching by chloride ions or by CN − ions (released from the redox probe [Fe(CN) 6 ] 3-/4- ), the effect of ionic strength of the assay buffer; the effect of initial interfacial properties (capacitance) and the effect of the high analyte:bioreceptor ratio on electrochemical impedance spectroscopy–based biosensing are discussed. The review provides also short discussion related to other factors, which can significantly affect operational performance of electrochemical impedance spectroscopy–based biosensing including novel conductive interfaces, small bioreceptors, strategies for improved selectivity (nonbiofouling interfaces) and sensitivity (use of nanoparticles). • The review paper provides a short introduction to electrochemical impedance spectroscopy (EIS). • Three EIS modes of operation are discussed in the review paper. • Novel trends in EIS sensing are provided, as well.