Non-Fluorescence label protein sensing with track-etched nanopore decorated by avidin/biotin system

Electrochemical sensor for ultrasensitive sensing of biotin based on heme conjugated with gold nanoparticles and its electrooxidation mechanism

We report the fabrication of a facile sensor using heme conjugated with gold nanoparticles (AuNPs) in situ on a glass carbon electrode (GCE) for the ultrasensitive determination of biotin without antibody or streptavidin. The use of heme and AuNPs as dual amplifiers allows a very broad detection range from 0.0050 to 50.0000 μmol·L-1 and a very low detection limit of 0.0016 μmol·L-1. The mechanistic aspects were elucidated using electrochemical analyses and frontier orbital calculations showing that the electrooxidation of biotin involves a one-electron and a one-proton transfer, generating biotin sulfoxide. The heme/AuNPs/GCE sensor exhibited excellent selectivity, reproducibility and stability, indicating high robustness. The recovery was between 97.20 and 105.70% with RSD less than 8.71%, suggesting good practicability. Our studies demonstrate that this approach can be used to detect and quantify biotin in a range of foods, including milk, infant formula, flour, orange juice, mango juice, egg white and egg yolk. Furthermore, all measurements do not require any intricate preparation or pre-treatment of the foods, thus representing a great potential for point-of-care testing.

Switchable Ion Current Saturation Regimes Enabled via Heterostructured Nanofluidic Devices Based on Metal-Organic Frameworks

The use of track-etched membranes allows further fine-tuning of transport regimes and thus enables their use in (bio)sensing and energy-harvesting applications, among others. Recently, metal-organic frameworks (MOFs) have been combined with such membranes to further increase their potential. Herein, the creation of a single track-etched nanochannel modified with the UiO-66 MOF is proposed. By the interfacial growth method, UiO-66-confined synthesis fills the nanochannel completely and smoothly, yet its constructional porosity renders a heterostructure along the axial coordinate of the channel. The MOF heterostructure confers notorious changes in the transport regime of the nanofluidic device. In particular, the tortuosity provided by the micro- and mesostructure of UiO-66 added to its charged state leads to iontronic outputs characterized by an asymmetric ion current saturation for transmembrane voltages exceeding 0.3 V. Remarkably, this behavior can be easily and reversibly modulated by changing the pH of the media and it can also be maintained for a wide range of KCl concentrations. In addition, it is found that the modified-nanochannel functionality cannot be explained by considering just the intrinsic microporosity of UiO-66, but rather the constructional porosity that arises during the MOF growth process plays a central and dominant role.

Ionic Signal Enhancement by the Space Charge Effect through the DNA Rolling Circle Amplification on the Outer Surface of Nanochannels

DNA species are recognized as a powerful probe for nanochannel analyses to address the issues of specific target recognition and highly efficient signal conversion due to their programmable and predictable Watson-Crick bases. However, in the conventional view, abundant sophisticated DNA structures synthesized by DNA amplification strategies are unsuitable for use in nanochannel analyses owing to their low probability to enter a nanochannel restricted by the smaller opening of the nanochannel, as well as the faint ion signal produced by the steric effect. Here, we present an integrated strategy of nanochannel analyses that combines the target recognitions by encoded rolling circle amplification (RCA) in solution and the ionic signal enhancement by the space charge effect through the immobilization of highly negative-charged RCA amplicons on the outer surface of the nanochannels. Owing to the highly negative-charged RCA amplicons with 100 nm sizes, a sharp increase of ionic current up to 7454% has been achieved. The RCA amplicon triggered by mRNA-21 on the outer surface of the poly(ethylene terephthalate) membrane with a single nanochannel realized the single-base mismatch detection of mRNA-21 with a sensitivity of 6 fM. The DNA amplicon endows the nanochannel with high sensitivity and selectivity that could extend to other applications, such as DNA sequencing, desalination, sieving, and water-energy nexus.

Solid-state and polymer nanopores for protein sensing: A review

In two decades, the solid state and polymer nanopores became attractive method for the protein sensing with high specificity and sensitivity. They also allow the characterization of conformational changes, unfolding, assembly and aggregation as well the following of enzymatic reaction. This review aims to provide an overview of the protein sensing regarding the technique of detection: the resistive pulse and ionic diodes. For each strategy, we report the most significant achievement regarding the detection of peptides and protein as well as the conformational change, protein-protein assembly and aggregation process. We discuss the limitations and the recent strategies to improve the nanopore resolution and accuracy. A focus is done about concomitant problematic such as protein adsorption and nanopore lifetime.

Ultrasensitive and Selective Protein Recognition with Nanobody-Functionalized Synthetic Nanopores

The development of flexible and reconfigurable sensors that can be readily tailored toward different molecular analytes constitutes a key goal and formidable challenge in biosensing. In this regard, synthetic nanopores have emerged as potent physical transducers to convert molecular interactions into electrical signals. Yet, systematic strategies to functionalize their surfaces with receptor proteins for the selective detection of molecular analytes remain scarce. Addressing these limitations, a general strategy is presented to immobilize nanobodies in a directional fashion onto the surface of track-etched nanopores exploiting copper-free click reactions and site-specific protein conjugation systems. The functional immobilization of three different nanobodies is demonstrated in ligand binding experiments with green fluorescent protein, mCherry, and α-amylase (α-Amy) serving as molecular analytes. Ligand binding is resolved using a combination of optical and electrical recordings displaying quantitative dose-response curves. Furthermore, a change in surface charge density is identified as the predominant molecular factor that underlies quantitative dose-responses for the three different protein analytes in nanoconfined geometries. The devised strategy should pave the way for the systematic functionalization of nanopore surfaces with biological receptors and their ability to detect a variety of analytes for diagnostic purposes.

Selective target protein detection using a decorated nanopore into a microfluidic device

Solid-state nanopores provide a powerful tool to electrically analyze nanoparticles and biomolecules at single-molecule resolution. These biosensors need to have a controlled surface to provide information about the analyte. Specific detection remains limited due to nonspecific interactions between the molecules and the nanopore. Here, a polymer surface modification to passivate the membrane is performed. This functionalization improves nanopore stability and ionic conduction. Moreover, one can control the nanopore diameter and the specific interactions between protein and pore surface. The effect of ionic strength and pH are probed. Which enables control of the electroosmotic driving force and dynamics. Furthermore, a study of polymer chain structure and permeability in the pore are carried out. The nanopore chip is integrated into a microfluidic device to ease its handling. Finally, a discussion of an ionic conductance model through a permeable crown along the nanopore surface is elucidated. The proof of concept is demonstrated by the capture of free streptavidin by the biotins grafted into the nanopore. In the future, this approach could be used for virus diagnostic, nanoparticle or biomarker sensing.

Conformation of Polyethylene Glycol inside Confined Space: Simulation and Experimental Approaches

The modification of the inner nanopore wall by polymers is currently used to change the specific properties of the nanosystem. Among them, the polyethylene glycol (PEG) is the most used to prevent the fouling and ensure the wettability. However, its properties depend mainly on the chain structure that is very difficult to estimate inside this confined space. Combining experimental and simulation approaches, we provide an insight to the consequence of the PEG presence inside the nanopore on the nanopore properties. We show, in particular, that the cation type in the electrolyte, together with the type of electrolyte (water or urea), is at the origin of the ion transport modification in the nanopore.

Surface coatings for solid-state nanopores

Since their introduction in 2001, solid-state nanopores have been increasingly exploited for the detection and characterization of biomolecules ranging from single DNA strands to protein complexes. A major factor that enables the application of nanopores to the analysis and characterization of a broad range of macromolecules is the preparation of coatings on the pore wall to either prevent non-specific adhesion of molecules or to facilitate specific interactions of molecules of interest within the pore. Surface coatings can therefore be useful to minimize clogging of nanopores or to increase the residence time of target analytes in the pore. This review article describes various coatings and their utility for changing pore diameters, increasing the stability of nanopores, reducing non-specific interactions, manipulating surface charges, enabling interactions with specific target molecules, and reducing the noise of current recordings through nanopores. We compare the coating methods with respect to the ease of preparing the coating, the stability of the coating and the requirement for specialized equipment to prepare the coating.

Single conical track-etched nanopore for a free-label detection of OSCS contaminants in heparin

The heparin contamination by oversulfated chondroitin (OSCS) was at the origin of one major sanitary problem of last decade. Here we propose a novel strategy to detect OSCS from heparin solution based on conical nanopore functionalized with poly-L-lysine deposition to ensure its re-usability. This sensor is an excellent to detect low heparin concentration (from 25 ng/ml to 3 μg/ml) using the modification of ionic current rectification. It also allows following the kinetic of heparin degradation by heparinase with a good correlation with results obtained by classical methods. The sensor is sensitive to the inhibition of heparinase by OSCS until a concentration of 200 pg/ml representing 0.01% in weight in a heparin. This resolution is one order of magnitude lower than the one obtained by chromatography. For the first time, it was reached without fluorescence labeling.

A handheld platform for target protein detection and quantification using disposable nanopore strips

Accessible point-of-care technologies that can provide immunoassay and molecular modalities could dramatically enhance diagnostics, particularly for infectious disease control in low-resource settings. Solid-state nanopores are simple and durable sensors with low-energy instrumentation requirements. While nanopore sensors have demonstrated efficacy for nucleic acid targets, selective detection and quantification of target proteins from sample background has not been demonstrated. We present a simple approach for electronic detection and quantification of target proteins that combines novel biomolecular engineering methods, a portable reader device and disposable nanopore test strips. The target of interest can be varied by swapping the binding domain on our engineered detection reagent, which eficiently binds in the bulk-phase to the target and subsequently generates a unique signature when passing through the pore. We show modularity of the detection reagent for two HIV antibodies, TNFα and tetanus toxin as targets. A saliva swab-to-result is demonstrated for clinically relevant HIV antibody levels (0.4–20 mg/liter) in under 60 seconds. While other strip-like assays are qualitative, the presented method is quantitative and sets the stage for simultaneous immunoassay and molecular diagnostic functionality within a single portable platform.

Unexpected ionic transport behavior in hydrophobic and uncharged conical nanopores

We investigated ionic transport behavior in the case of uncharged conical nanopores. We observed unexpected ionic transport behaviour, which is attributed to a predominant effect of slippage due to water organization at the solid/liquid interface.

Functionalization of single solid state nanopores to mimic biological ion channels: A review

In nature, ion channels are highly selective pores and act as gate to ensure selective ion transport, allowing ions to cross the membrane. By mimicking them, single solid state nanopore devices emerge as a new, powerful class of molecule sensors that allow for the label-free detection of biomolecules (DNA, RNA, and proteins), non-biological polymers, as well as small molecules. In this review, we exhaustively describe the fabrication and functionalization techniques to design highly robust and selective solid state nanopores. First we outline the different materials and methods to design nanopores, we explain the ionic conduction in nanopores, and finally we summarize some techniques to modify and functionalize the surface in order to obtain biomimetic nanopores, responding to different external stimuli.

Mimicking pH-Gated Ionic Channels by Polyelectrolyte Complex Confinement Inside a Single Nanopore

Biological channels have served as inspiration to design stimuli-response artificial nanopores. Here we propose an original approach to design a pH-gate nanopore based on polyethylenimine and chondroitin-4-sulfate (ChS) layer-by-layer self-assembly. This approach is interesting because it is rapid and permits monitoring in real time of functionalization. The study of ionic transport through these single nanopores reveals a selectivity on anions and pH-gate properties at low salt concentration. It is open at pH below 4 or 5 depending on salt concentration. These properties are explained by the modification of both charge and conformation of ChS as well as swelling of the polyelectrolyte complex.