Potentiometric biosensing of proteins with ultrasensitive ion-selective microelectrodes and nanoparticle labels

Operation-friendly and accurate naked-eye observation assay for fast zoonotic echinococcosis and pulmonary tuberculosis monitoring in clinics

Echinococcosis and tuberculosis are two common zoonotic diseases that can cause severe pulmonary infections. Early screening and treatment monitoring are of great significance, especially in areas with limited medical resources. Herein, we designed an operation-friendly and rapid magnetic enrichment-silver acetylene chromogenic immunoassay (Me-Sacia) to monitor the antibody. The main components included secondary antibody-modified magnetic nanoparticles (MNP-Ab2) as capture nanoparticles, specific peptide (EG95 or CFP10)-modified silver nanoparticles (AgNP-PTs) as detection nanoparticles, and alkyne-modified gold nanoflowers as chromogenic nanoparticles. Based on the magnetic separation and plasma luminescence techniques, Me-Sacia could completely replace the colorimetric assay of biological enzymes. It reduced the detection time to approximately 1 h and simplified the labor-intensive and equipment-intensive processes associated with conventional ELISA. Meanwhile, the Me-Sacia showed universality for various blood samples and intuitive observation with the naked eye. Compared to conventional ELISA, Me-Sacia lowered the detection limit by approximately 96.8 %, increased the overall speed by approximately 15 times, and improved sensitivity by approximately 7.2 %, with a 100 % specificity and a coefficient of variation (CV) of less than 15 %.

Aptamer-conjugated gold nanoparticles and their diagnostic and therapeutic roles in cancer

The burden of incidence rate and mortality of cancer is increasing rapidly, and the development of precise intervention measures for cancer detection and treatment will help reduce the burden and pain of cancer. At present, the sensitivity and specificity of tumor markers such as CEA and CA-125 used clinically are low, while PET, SPECT, and other imaging diagnoses with high sensitivity possess shortcomings, including long durations to obtain formal reports and the inability to identify the molecular pathological type of cancer. Cancer surgery is limited by stage and easy to recur. Radiotherapy and chemotherapy often cause damage to normal tissues, leading to evident side effects. Aptamers can selectively and exclusively bind to biomarkers and have, therefore, gained attention as ligands to be targeted for cancer detection and treatment. Gold nanoparticles (AuNPs) are considered as promising nano carriers for cancer diagnosis and treatment due to their strong light scattering characteristics, effective biocompatibility, and easy surface modification with targeted agents. The aptamer-gold nanoparticles targeting delivery system developed herein can combine the advantages of aptamers and gold nanoparticles, and shows excellent targeting, high specificity, low immunogenicity, minor side effects, etc., which builds a bridge for cancer markers to be used in early and efficient diagnosis and precise treatment. In this review, we summarize the latest progress in the application of aptamer-modified gold nanoparticles in cancer targeted diagnosis and delivery of therapeutic agents to cancer cells and emphasize the prospects and challenges of transforming these studies into clinical applications.

Deep Learning-Enhanced Potentiometric Aptasensing with Magneto-Controlled Sensors

Bioelectronic sensors that report charge changes of a biomolecule upon target binding enable direct and sensitive analyte detection but remain a major challenge for potentiometric measurement, mainly due to Debye Length limitations and the need for molecular-level platforms. Here, we report on a magneto-controlled potentiometric method to directly and sensitively measure the target-binding induced charge change of DNA aptamers assembled on magnetic beads using a polymeric membrane potentiometric ion sensor. The potentiometric responses of the negatively charged aptamer, serving as a receptor and reporter, were dynamically controlled and modulated by applying a magnetic field. Based on a potentiometric array, this non-equilibrium measurement technique combined with deep learning algorithms allows for rapidly and reliably classifying and quantifying diverse small molecules using antibiotics as models. This potentiometric strategy opens new modalities for sensing applications.

Beyond Nonactin: Potentiometric Ammonium Ion Sensing Based on Ion-selective Membrane-free Prussian Blue Analogue Transducers

The determination of ammonium ions (NH₄⁺) is of significance to environmental, agriculture, and human health. Potentiometric NH₄⁺ sensors based on solid-contact ion selective electrodes (SC-ISEs) feature point-of-care testing and miniaturization. However, the state-of-the-art SC-ISEs of NH₄⁺ during the past 20 years strongly rely on the organic ammonium ionophore-based ion selective membrane (ISM), typically by nonactin for the NH₄⁺ recognition. Herein, we report a Prussian blue analogue of copper(II)-hexacyanoferrate (CuHCF) for an ISM-free potentiometric NH₄⁺ sensor without using the ionophores. CuHCF works as a bifunctional transducer that could realize the ion-to-electron transduction and NH₄⁺ recognition. CuHCF exhibits competitive analytical performances regarding traditional nonactin-based SC-ISEs of NH₄⁺, particularly for the selectivity toward K⁺. The cost and preparation process have been remarkably reduced. The theoretical calculation combined with electrochemical tests further demonstrate that relatively easier intercalation of NH₄⁺ into the lattices of CuHCF determines its selectivity. This work provides a concept of the ISM-free potentiometric NH₄⁺ sensor beyond the nonactin ionophore through a CuHCF bifunctional transducer.

Prototype Digital Lateral Flow Sensor Using Impact Electrochemistry in a Competitive Binding Assay

This work demonstrates a lateral flow assay concept on the basis of stochastic-impact electrochemistry. To this end, we first elucidate requirements to employ silver nanoparticles as redox-active labels. Then, we present a prototype that utilizes nanoimpacts from biotinylated silver nanoparticles as readouts to detect free biotin in solution based on competitive binding. The detection is performed in a membrane-based microfluidic system, where free biotin and biotinylated particles compete for streptavidin immobilized on embedded latex beads. Excess nanoparticles are then registered downstream at an array of detection electrodes. In this way, we establish a proof of concept that serves as a blueprint for future "digital" lateral flow sensors.

Electrochemically Imaging the Response of Ion-Selective Membranes with an Ultralow Detection Limit

The development of ion-selective membranes for the selective response of a particular ion has been studied for many years; however, imaging the response of the membrane with a low detection limit is challenging. Here, high spatial-resolution electrochemical imaging of this response down to picomolar is achieved using scanning ion conductive microscopy. The detection strategy relies on the exclusion of a small amount of counter ions from the membrane in the presence of a low concentration of target ions in the solution. These excluded counter ions are adsorbed at the membrane-solution interface, leading to more positive charges at the surface. The resultant elevation of the ionic current in the approach curve behaves as the response for the target ions down to 10^-11 M, which is much more sensitive than that using potentiometric measurement. The constant-current scanning of the membrane exhibits the fluctuation of the apparent surface height that is correlated with the ionic concentration, permitting the imaging of the response at the nanoscale. The achievement of highly sensitive and spatial-resolution imaging for the ionic response enable the collection of spatial response at the ion-selective membrane, which will greatly advance the study of ion-selective electrodes.

Magnetic-Field-Driven Extraction of Bioreceptors into Polymeric Membranes for Label-Free Potentiometric Biosensing

We report here the concept of a magnetically controlled extraction of hydrophilic bioreceptors into polymeric membranes for bioassays. The potentiometric assay relies on the intrinsic charges of an antimicrobial peptide and its unique recognition abilities, which can eliminate the probe labeling and indicator addition. The target binding event could effectively prevent the extraction of the peptide into the polymeric membrane doped with an ion exchanger, thus resulting in a potential change. The potentiometric response properties of the peptide assembled on magnetic beads can be dynamically controlled and modulated by applying a magnetic field. Staphylococcus aureus, as a model of food-borne pathogens, was measured at levels down to 10 CFU mL^-1 . Based on this sensing strategy, a potentiometric array was developed for the pattern recognition of bacteria. The proposed general platform can be used for potentiometric biosensing using other hydrophilic bioreceptors.

Real time monitoring of in situ generated hydrogen peroxide in electrochemical advanced oxidation reactors using an integrated Pt microelectrode

This work propose the fabrication and characterization of a Pt microelectrode integrated with a silver quasi-reference counter electrode (Pt/AgQRCE) for real time amperometric measurements of hydrogen peroxide electrochemically generated by water oxidation on Nb-supported boron doped diamond (Ni/BDD) anode. The developed electroanalytical method requires a very small sample volume and has higher sensitivity when compared to the conventional spectrophotometric analysis using ammonium metavanadate. The experiments were performed with Nb/BDD anode applying current densities of 30, 60, 90 and 120 mA cm^-2 in 0.10 mol L^-1 HClO₄ supporting electrolyte showed that H₂O₂ production increase in the first 90 min of electrolysis and then reaches a plateau in both off-line and real time measurements. For the first 90 min, the electrogeneration of H₂O₂ exhibited a pseudo zero-order kinetics. The results obtained by the electrochemical amperometric analysis were compared to a spectrophotometric methodology reported on the literature and, at 95% confidence level the two methods do not demonstrated significant difference.

Printed Electrodes in Microfluidic Arrays for Cancer Biomarker Protein Detection

Medical diagnostics is trending towards a more personalized future approach in which multiple tests can be digitized into patient records. In cancer diagnostics, patients can be tested for individual protein and genomic biomarkers that detect cancers at very early stages and also be used to monitor cancer progression or remission during therapy. These data can then be incorporated into patient records that could be easily accessed on a cell phone by a health care professional or the patients themselves on demand. Data on protein biomarkers have a large potential to be measured in point-of-care devices, particularly diagnostic panels that could provide a continually updated, personalized record of a disease like cancer. Electrochemical immunoassays have been popular among protein detection methods due to their inherent high sensitivity and ease of coupling with screen-printed and inkjet-printed electrodes. Integrated chips featuring these kinds of electrodes can be built at low cost and designed for ease of automation. Enzyme-linked immunosorbent assay (ELISA) features are adopted in most of these ultrasensitive detection systems, with microfluidics allowing easy manipulation and good fluid dynamics to deliver reagents and detect the desired proteins. Several of these ultrasensitive systems have detected biomarker panels ranging from four to eight proteins, which in many cases when a specific cancer is suspected may be sufficient. However, a grand challenge lies in engineering microfluidic-printed electrode devices for the simultaneous detection of larger protein panels (e.g., 50-100) that could be used to test for many types of cancers, as well as other diseases for truly personalized care.

Interface engineering of microelectrodes toward ultrasensitive monitoring of β-amyloid peptides in cerebrospinal fluid in Alzheimer's disease

Aβ monomers directed the assembly of Cu²⁺-PEI/AuNPs-hemin nanoprobes into network aggregates on a microelectrode interface for enhanced electrochemical analysis.

Ultrasensitive Detection of Parathyroid Hormone through Fast Silver Deposition Induced by Enzymatic Nitroso Reduction and Redox Cycling

Enzymatically induced silver deposition and subsequent electrochemical oxidation have been widely used in electrochemical biosensors. However, this method is ineffective for producing highly enhanced silver deposition for use in ultrasensitive detection. Herein, we report a fast silver deposition method that simultaneously uses three signal amplification processes: (i) enzymatic amplification, (ii) chemical-chemical (CC) redox cycling, and (iii) chemical-enzymatic (CN) redox cycling. DT-diaphorase (DT-D) is used for enzymatic amplification to convert a nitroso compound, a species incapable of directly reducing Ag⁺ to an amine compound, which can directly reduce Ag⁺. NADH acts as a reducing agent for the indirect reduction of Ag⁺ via the two redox cycling processes. 4-Nitroso-1-naphthol is converted to 4-amino-1-naphthol (NH₂-N) in the presence of DT-D. NH₂-N initiates two redox cycling processes: NH₂-N, along with Ag⁺ and NADH, are involved in the CC redox cycling, whereas NH₂-N, along with Ag⁺, DT-D, and NADH, are involved in the CN redox cycling. Finally, the deposited silver is electrochemically oxidized to produce a signal. When this triple signal amplification strategy for fast silver deposition is applied to an electrochemical immunosensor for detecting parathyroid hormone (PTH), a detection limit as low as ∼100 fg/mL is obtained. The concentrations of PTH in clinical serum determined using the developed immunosensor are found to agree with those measured using a commercial instrument. Thus, the use of this strategy for fast silver deposition is highly promising for ultrasensitive electrochemical detection and biosensing applications.

A novel electrochemical strategy based on porous 3D graphene-starch architecture and silver deposition for ultrasensitive detection of neuron-specific enolase

This work was aimed at designing a novel and ultrasensitive electrochemical immunoassay strategy to detect neuron-specific enolase (NSE) with a triple signal amplification strategy.

A potentiometric immunosensor for enterovirus 71 based on bis-MPA-COOH dendrimer-doped AgCl nanospheres with a silver ion-selective electrode

Herein a new potentiometric immunoassay for the point-of-care detection of enterovirus 71 (EV71) was developed by using a silver (Ag⁺) ion-selective electrode (ISE). Initially, the carboxylated dendrimer-doped AgCl nanospheres were synthesized by the reverse micelle method. Then the synthesized nanospheres were used to label a polyclonal mouse anti-EV71 antibody via a typical carbodiimide coupling method. The immunoreaction was executed on a monoclonal anti-EV71 antibody-coated microplate by using biofunctional AgCl nanospheres as the detection antibody. With a sandwich-type immunoassay format, the carried AgCl nanospheres could be dissolved in the presence of NH₃·H₂O, and the released silver ions were determined with an external silver ion-selective electrode. Under optimal conditions, the shift in the potential increased with the increase in the EV71 concentration, in a wide linear range of 0.3-300 ng mL^-1, with a detection limit of 0.058 ng mL^-1. Intra- and inter-assay relative standard deviations with identical batches were less than 4.15% and 6.15%, respectively. By validating the spiked serum samples, our system shows consistency with the enzyme-linked immunosorbent assay (ELISA) kit.

Nanomaterials: Electrochemical Properties and Application in Sensors

The unique properties of nanoparticles make them an extremely valuable modifying material, being used in electrochemical sensors. The features of nanoparticles affect the kinetics and thermodynamics of electrode processes of both nanoparticles and redox reactions occurring on their surface. The paper describes theoretical background and experimental studies of these processes. During the transition from macro- to micro- and nanostructures, the analytical characteristics of sensors modify. These features of metal nanoparticles are related to their size and energy effects, which affects the analytical characteristics of developed sensors. Modification of the macroelectrode with nanoparticles and other nanomaterials reduces the detection limit and improves the degree of sensitivity and selectivity of measurements. The use of nanoparticles as transducers, catalytic constituents, parts of electrochemical sensors for antioxidant detection, adsorbents, analyte transporters, and labels in electrochemical immunosensors and signal-generating elements is described.

Quantum dot-based electrochemical biosensor for stripping voltammetric detection of telomerase at the single-cell level

Human telomerase is responsible for the maintenance of chromosome end structures and is a valuable biomarker for malignant growth. However, the accurate measurement of telomerase activity at the single-cell level has remained a great challenge. Here we develop a simple quantum dot (QD)-based electrochemical biosensor for stripping voltammetric detection of telomerase activity at the single-cell level. We designed a thiol-modified capture DNA which may be immobilized on the gold electrode by the gold-sulfur bond. The presence of telomerase enables the addition of the telomere repeats of (TTAGGG)_n to the 3' end of the primer, accompanied by the incorporation of abundant biotins in the extension product with the assistance of the biotin-tagged dATP. The subsequent hybridization of extension product with the capture DNA and the addition of streptavidin-coated QDs induce the assembly of large amounts of QDs onto the electrode via specific biotin-streptavidin binding. After the acidic dissolution of QDs, the released Cd (II) can be simply quantified by anodic stripping voltammetry (ASV). Due to the introduction of large amounts of QDs by telomerase-induced primer extension reaction and the synergistic signal amplification induced by the release of Cd (II) from the QDs, this biosensor can detect telomerase activity at the single-cell level without the involvement of any thermal cycling and extra enzymes for signal amplification. Moreover, this assay exhibits a large dynamic range over four orders of magnitude and it is very simple without the involvement of specific hairpin probe design and complicated labelling, holding great potential in point-of-need testing.

Molecular dynamics simulation of potentiometric sensor response: the effect of biomolecules, surface morphology and surface charge

The silica-water interface is critical to many modern technologies in chemical engineering and biosensing. One technology used commonly in biosensors, the potentiometric sensor, operates by measuring the changes in electric potential due to changes in the interfacial electric field. Predictive modelling of this response caused by surface binding of biomolecules remains highly challenging. In this work, through the most extensive molecular dynamics simulation of the silica-water interfacial potential and electric field to date, we report a novel prediction and explanation of the effects of nano-morphology on sensor response. Amorphous silica demonstrated a larger potentiometric response than an equivalent crystalline silica model due to increased sodium adsorption, in agreement with experiments showing improved sensor response with nano-texturing. We provide proof-of-concept that molecular dynamics can be used as a complementary tool for potentiometric biosensor response prediction. Effects that are conventionally neglected, such as surface morphology, water polarisation, biomolecule dynamics and finite-size effects, are explicitly modelled.

An ultrasensitive electrochemical immunosensor based on C60-modified polyamidoamine dendrimers and Au NPs for co-catalytic silver deposition

C₆₀-Modified polyamidoamine dendrimers and Au NPs for the co-catalytic deposition of silver, used for ultrasensitive electrochemical immunosensing.

A method for estimating intracellular ion concentration using optical nanosensors and ratiometric imaging

Optical nanoparticle (NP)-based sensors have been widely implemented as tools for detection of targeted ions and biomolecules. The NP sensing platform offer a modular design that can incorporate different sensing components for greater target specificity and the ability to tune the dynamic range, as well as encapsulation of multiple dyes to generate a ratiometric signal with varying spectra. Despite these advantages, demonstrating quantitative ion imaging for intracellular measurement still possess a major challenge. Here, we describe fundamentals that enable intracellular validation of this approach using ion-selective nanosensors for investigating calcium (Ca²⁺) as a model ion. While conventional indicators can improve individual aspects of indicator performance such as Kd, wavelength, and ratiometric measurements, the use of NP sensors can achieve combined benefits of addressing these issues simultaneously. The nanosensor incorporates highly calcium-selective ionophores and two fluorescence indicators that act as signal transducers to facilitate quantitative ratiometric imaging. For intracellular Ca²⁺ application, the sensors are fine-tuned to physiological sensing range, and live-cell imaging and quantification are demonstrated in HeLa cells loaded with nanosensors and their responsiveness to carbachol-evoked store release (~400 nM). The current nanosensor design thus provides a promising sensing platform for real-time detection and optical determination of intracellular ions.

Ultrasensitive electrochemical biosensor based on the oligonucleotide self-assembled monolayer-mediated immunosensing interface

Highly sensitive and selective quantitation of a variety of proteins over a wide concentration range is highly desirable for increased accuracy of biomarker detection or for multidisease diagnostics. In the present contribution, using human immunoglobulin G (HIgG) as the model target protein, an electrochemical ultrasensitive immunosensing platform was developed based on the oligonucleotide self-assembled monolayer-mediated (OSAM) sensing interface. For this immunosensor, the "signal-on" signaling mechanism and enzymatic signal amplification effect were integrated into one sensing architecture. Moreover, the thiolated flexible single-stranded DNAs immobilized onto gold electrode surface not only performs the wobbling motion to facilitate the electron transfer between the electrode surface and biosensing layer but also fundamentally prohibiting the direct interaction of proteins with gold substrate. Thus, the electrochemical signal could be efficiently enhanced and the unspecific adsorption or cross-reaction might be eliminated. As a result, utilizing the newly-proposed immunosensor, the HIgG can be detected down to 0.5 ng/mL, and the high detection specificity is offered. The successful design of OSAM and the highly desirable detection capability of new immunosensor are expected to provide a perspective for fabricating new robust immunosensing platform and for promising potential of oligonucleotide probe in biological research and biomedical diagnosis.

Integrated multi-ISE arrays with improved sensitivity, accuracy and precision

Increasing use of ion-selective electrodes (ISEs) in the biological and environmental fields has generated demand for high-sensitivity ISEs. However, improving the sensitivities of ISEs remains a challenge because of the limit of the Nernstian slope (59.2/n mV). Here, we present a universal ion detection method using an electronic integrated multi-electrode system (EIMES) that bypasses the Nernstian slope limit of 59.2/n mV, thereby enabling substantial enhancement of the sensitivity of ISEs. The results reveal that the response slope is greatly increased from 57.2 to 1711.3 mV, 57.3 to 564.7 mV and 57.7 to 576.2 mV by electronic integrated 30 Cl⁻ electrodes, 10 F⁻ electrodes and 10 glass pH electrodes, respectively. Thus, a tiny change in the ion concentration can be monitored, and correspondingly, the accuracy and precision are substantially improved. The EIMES is suited for all types of potentiometric sensors and may pave the way for monitoring of various ions with high accuracy and precision because of its high sensitivity.

Metal-linked Immunosorbent Assay (MeLISA): the Enzyme-Free Alternative to ELISA for Biomarker Detection in Serum

Determination of disease biomarkers in clinical samples is of crucial significance for disease monitoring and public health. The dominating format is enzyme-linked immunosorbent assay (ELISA), which subtly exploits both the antigen-antibody reaction and biocatalytic property of enzymes. Although enzymes play an important role in this platform, they generally suffer from inferior stability and less tolerant of temperature, pH condition compared with general chemical product. Here, we demonstrate a metal-linked immunosorbent assay (MeLISA) based on a robust signal amplification mechanism that faithfully replaces the essential element of the enzyme. As an enzyme-free alternative to ELISA, this methodology works by the detection of α-fetoprotein (AFP), prostatic specific antigen (PSA) and C-reactive protein (CRP) at concentrations of 0.1 ng mL(-1), 0.1 ng mL(-1) and 1 ng mL(-1) respectively. It exhibits approximately two magnitudes higher sensitivity and is 4 times faster for chromogenic reaction than ELISA. The detection of AFP and PSA was further confirmed by over a hundred serum samples from hepatocellular carcinoma (HCC) and prostate cancer patients respectively.

Sensitive electrochemical detection of copper ions based on the copper(II) ion assisted etching of Au@Ag nanoparticles

A new sensitive electrochemical sensor for the detection of copper ions based on the copper ion assisted etching of Au@Ag nanoparticles was developed in this work. Since copper ions could greatly catalyze the etching process of the silver shell of Au@Ag nanoparticles in the presence of thiosulfate solutions, leading to an obvious decrease of the linear sweep voltammetry (LSV) signals of silver, the concentration of the copper ions, therefore, can be measured. Under the optimized conditions, the electrochemical sensor exhibited excellent sensitivity and selectivity for Cu(2+), with wide linear ranges of 1 nM to 100 μM, and the detection limit of 0.3 nM. In addition, this method was successfully applied for the analysis of Cu(2+) in river water and exhibited good analytical performance.

Electrochemical Peptide Biosensor Based on in Situ Silver Deposition for Detection of Prostate Specific Antigen

In this work, we have demonstrated a novel electrochemical method based on target-induced cleavage of a specific peptide for sensitive analysis of prostate specific antigen (PSA) by using silver enhancement. First, multiwalled carbon nanotubes/poly(amidoamine) dendrimers (MWCNTs-PAMAM) nanohybrids were assembled on the electrode to bind the peptide. Subsequently, dithiobis(succinimidylpropionate) (DSP)@Au@SiO2 was prepared as a tracing tag and covalent bond with the peptides via the inherent interaction between DSP and the amino of peptide. In the presence of PSA, the peptide was specifically recognized and cleaved, resulting in the loss of the tracing tag in electrode surface. Thereafter, silver enhancement was performed on the left DSP@Au@SiO2 nanohybrids. The electrochemical stripping signal of the deposited silver was used to monitor this process. Under optimal conditions, the proposed biosensor achieved a wide line from 0.001 to 30 ng mL(-1) with a detection limit of 0.7 pg mL(-1). This work demonstrated the combination of the direct transduction of peptide cleavage events with the highly sensitive silver enhancement method, providing a promising effective strategy for PSA detection.

Pulsed galvanostatic control of a polymeric membrane ion-selective electrode for potentiometric immunoassays

Pulsed galvanostatic control of ion fluxes across polymeric membrane ion-selective electrodes (ISEs) is an emerging field for potentiometric sensing. Herein we report a novel potentiometric enzyme immunoassay based on current-controlled release of an enzyme substrate, which eliminates the addition of marker ions in the sample solution. In this method, the carboxylated poly(vinyl chloride) matrix at the outer layer of the ISE membrane is employed to attach a primary antibody. A sandwich immunoassay with an alkaline phosphatase labeled antibody (ALP-Ab) as the reporter is used for the determination of human IgG (as a model protein). The large difference between the lipophilicity of the substrate ion and that of the product ion allows p-nitrophenyl phosphate to be used as the enzyme substrate for potentiometric immunosensors. After the immunoreactions, the captured ALP-Ab catalyzes the hydrolysis of the substrate ions released at the sample-membrane interface by using the pulsed galvanostatic technique. This process can be potentiometrically determined by measuring the open circuit potential of the ISE. Under optimal conditions, the potential response of the proposed immunosensor is proportional to the concentration of human IgG in the range of 50-1000 ng/mL with a detection limit of 30 ng/mL (3σ). Owing to simplicity and independence of sample volume and sample turbidity, the proposed potentiometric immunoassay offers a viable alternative to those based on optical absorbance.

Potentiometric aptasensing based on target-induced conformational switch of a DNA probe using a polymeric membrane silver ion-selective electrode

In this article, we introduce a general, sensitive, facile, and label-free potentiometric assay based on metal-mediated DNA base pairs. A nucleic acid with one adenosine-5'-triphosphate (ATP) binding sequence (aptamer) in the middle and two cytosine(C)-rich sequences at the lateral portions was employed as a model. A rigid hairpin structure can be formed in the presence of Ag(+) ions, in which the C residues of the spatially separated nucleotides are linked by the ions. The strong interaction between Ag(+) ions and cytosines forms a stable C-Ag(+)-C structure, which could reduce the concentration of silver ions released from the polymeric membrane silver ion-selective electrode (ISE) at the sample-membrane interface and decrease the potential response. In the presence of its target, the aptamer (the loop sequence of the probe) binds specifically to the target via reaction incubation. Such target-binding induced aptamer conformational change prevents the formation of C-Ag(+)-C structure, leaving more silver ions at the sample-membrane interface, which can be detected by the silver ISE. ATP can be quantified in the range of 0.5-3.0 μM with a detection limit of 0.37 μM. The relative standard deviation for 5 μM ATP is 5.5%. For the proposed method, the combination of using ion fluxes of silver ions as modulating reagents and as signal reporters greatly simplifies the detection procedures. In addition, by changing the binding sequence in the middle of the probe, the present detection method will be able to explore new applications of ISE for the detection of a large variety of targets.