A nanocoaxial-based electrochemical sensor for the detection of cholera toxin

Caco-2 cell-derived biomimetic electrochemical biosensor for cholera toxin detection

Cholera is a highly contagious and lethal waterborne disease induced by an infection with Vibrio cholerae (V. cholerae) secreting cholera toxin (CTx). Cholera toxin subunit B (CTxB) from the CTx specifically binds with monosialo-tetra-hexosyl-ganglioside (GM1) found on the exterior cell membrane of an enterocyte. Bioinspired by the pathological process of CTx, we developed an electrochemical biosensor with GM1-expressing Caco-2 cell membrane (CCM) on the electrode surface. Briefly, the electrode surface was functionalized with CCM using the vesicle fusion method. We determined the CTxB detection performances of Caco-2 cell membrane-coated biosensor (CCB) using electrochemical impedance spectroscopy (EIS). the CCB had an excellent limit of detection of ∼11.46 nM and a detection range spanning 100 ng/mL - 1 mg/mL. In addition, the CCB showed high selectivity against various interfering molecules, including abundant constituents of intestinal fluid and various bacterial toxins. The long-term stability of the CCBs was also verified for 3 weeks using EIS. Overall, the CCB has excellent potential for practical use such as point-of-care and cost-effective testing for CTxB detection in developing countries.

Detection of cholera toxin with surface plasmon field-enhanced fluorescent spectroscopy

In this work, a biosensor based on surface plasmon field-enhanced florescence spectroscopy (SPFS) method was successfully constructed to detect the truncated form of cholera toxin, that is, its beta subunit (CTX-B). CTX-B is a relatively small molecule (12 kDa) and it was chosen as model analyte for the detection of protein toxins originated from waterborne pathogens. Recognition layer was prepared on gold-coated LaSFN9 glasses modified with 11-mercaptoundecanoic acid (11-MUA). Biotin-conjugated anti-CTX-B polyclonal antibody (B-Ab) was immobilized on streptavidin (SA) layer constructed on the 11-MUA-modified surface. CTX-B amount was determined with direct assay using B-Ab in surface plasmon resonance (SPR) mode and with sandwich assay in SPFS mode using Cy5-conjugated anti-CTX-B polyclonal antibody. Minimum detected CTX-B concentrations were 10 and 0.01 μg/ml with SPR and SPFS, respectively, showing the sensitivity of the SPFS system over the conventional one. The detection was done in 2-6 h, which was faster than both culture and polymerase chain reaction (PCR)-based methods. Stability tests were performed with SA-coated sensors (excluding B-Ab). In this form, the layer was stable after 30 days of storage in phosphate-buffered saline (PBS; 0.01 M, pH = 7.4) at +4°C. B-Ab layer was formed immediately on them before each measurement.

Recent advances and challenges in electrochemical biosensors for emerging and re-emerging infectious diseases

The rise of emerging infectious diseases (EIDs) as well as the increase in spread of existing infections is threatening global economies and human lives, with several countries still fighting repeated onslaught of a few of these epidemics. The catastrophic impact a pandemic has on humans and economy should serve as a reminder to be better prepared to the advent of known and unknown pathogens in the future. The goal of having a set of initiatives and procedures to tackle them is the need of the hour.

Rapid detection and point-of-care (POC) analysis of pathogens causing these diseases is not only a problem entailing the scientific community but also raises challenges in tailoring appropriate treatment strategies to the healthcare sector. Among the various methods used to detect pathogens, Electrochemical Biosensor Technology is at the forefront in the development of POC devices. Electrochemical Biosensors stand in good stead due to their rapid response, high sensitivity and selectivity and ease of miniaturization to name a few advantages.

This review explores the innovations in electrochemical biosensing based on the various electroanalytical techniques including voltammetry, impedance, amperometry and potentiometry and discusses their potential in diagnosis of emerging and re-emerging infectious diseases (Re-EIDs), which are potential pandemic threats.

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This review offers a detailed description of the latest developments in electrochemical biosensors for emerging and re-emerging infectious diseases.

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Advantages and limitations of various types of electrochemical biosensor techniques are demonstrated.

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Discusses the latest electrochemical biosensors for COVID-19.

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Challenges and future prospects of electrochemical biosensors have been discussed in this review.

Diagnostic techniques for rapid detection of Vibrio cholerae O1/O139

Cholera caused by the toxigenic Vibrio cholerae is still a major public health problem in many countries. This disease is mainly due to poor sanitation, hygiene and consumption of unsafe water. Several recent epidemics of cholera showed its increasing intensity, duration and severity of the illness. This indicates an urgent need for effective management and preventive measures in controlling the outbreaks and epidemics. In preventing and spread of epidemic cholera, rapid diagnostic tests (RDTs) are useful in screening suspected stool specimens, water/food samples. Several RDTs developed recently are considered as investigative tools in confirming cholera cases, as the culture techniques are difficult to establish and/or maintain. The usefulness of RDTs will be more at the point-of-care facilities as it helps to make appropriate decisions in the management of outbreaks or epidemiological surveillance by the public health authorities. Apart from RDTs, several other tests are available for the direct detection of either V. cholerae or its cholera toxin. Viable but non-culturable (VBNC) state of V. cholerae poses a great challenge in developing RDTs. The aim of this article is to provide an overview of current knowledge about RDT and other techniques with reference to their status and future potentials in detecting cholera/V. cholerae.

Aptamer lateral flow assays for rapid and sensitive detection of cholera toxin

Aptamers are envisioned to serve as powerful synthetic substitutes to antibodies in a variety of bioanalytical assay formats. However, lateral flow assays (LFAs) remain dominated by antibody-based strategies. In this study, a LFA for the detection of cholera toxin as a model analyte is developed and optimized using a synthetic aptamer and a naturally occurring receptor as biorecognition elements and directly compared with solely aptamer and aptamer and antibody-based alternative approaches. The aptamer (CT916) recently selected by our group, GM1 receptors and an anti-cholera toxin antibody were evaluated. Relying solely on molecules that can easily be synthesized while aiming for high sensitivity, we applied a novel combination of capture aptamer and GM1 cell receptor-labeled liposomes for cholera toxin detection, achieving a limit of detection (LOD) of 2 ng ml-1 (3σ)/10 ng ml-1 (visual) in ∼15 min. To put our novel aptasensor into perspective, we developed a competitive lateral flow assay, exploiting the competition of cholera toxin in solution with immobilized cholera toxin for binding of aptamer-coated gold nanoparticles (AuNPs) (LOD = 51 ng ml-1 (3σ)/100 ng ml-1 (visual), assay time ∼10 min). As dual simultaneously binding aptamers were not available, we designed aptamer antibody pair-based lateral flow assays using aptamer-coated AuNPs which yielded a LOD of 5 ng ml-1 (by the 3σ rule)/10 ng ml-1 (visual) in a 10 min assay and an even better LOD of 0.6 ng ml-1 (3σ)/1 ng ml-1 (visual), with an ∼20 min total assay time. All set-ups are highly specific and provide an excellent alternative for cholera toxin detection in places where professional knowledge and sophisticated equipment are not readily available and cost efficient, simple, and rapid tests are needed, while the combination of GM1 cell receptor-labeled liposomes and aptamers is clearly the most promising.

On-Chip Electrochemical Detection of Cholera Using a Polypyrrole-Functionalized Dendritic Gold Sensor

Rapid diagnosis of an infectious disease outbreak in the field is critical for limiting the escalation of an outbreak into an epidemic. Devices suited to point-of-care (POC) diagnosis of cholera must not only demonstrate clinical laboratory levels of sensitivity and specificity but do so in a portable and low-cost manner, with a simplistic readout. We report work toward an on-chip electrochemical immunosensor for the detection of cholera toxin subunit B (CTX), based on a dendritic gold architecture biofunctionalized via poly(2-cyanoethyl)pyrrole (PCEPy). The dendritic electrode has an ∼18× greater surface area than a planar gold counterpart, per electrochemical measurements, allowing for a higher level of detection sensitivity. A layer of PCEPy polymer generated on the dendritic surface facilitated the performance of an electrochemical enzyme-linked immunosorbant assay (ELISA) for CTX on-chip, which demonstrated a detection limit of 1 ng mL^-1, per a signal-to-noise ratio of 2.6. This was more sensitive than detection using a simple planar gold electrode (100 ng mL^-1) and also matched the diagnostic standard optical ELISA, but on a miniaturized platform with electrical readout. The ability to meet POC demands makes biofunctionalized gold dendrites a promising architecture for on-chip detection of cholera.

Highly affine and selective aptamers against cholera toxin as capture elements in magnetic bead-based sandwich ELAA

Aptamers are single-stranded DNA or RNA oligonucleotides, which have been emerging as recognition elements in disease diagnostics and food control, including the detection of bacterial toxins. In this study, we employed the semi-automated just in time-selection to identify aptamers that bind to cholera toxin (CT) with high affinity and specificity. CT is the main virulence factor of Vibrio cholerae and the causative agent of the eponymous disease. For the selected aptamers, dissociation constants in the low nanomolar range (23-56 nM) were determined by fluorescence-based affinity chromatography and cross-reactivity against related proteins was evaluated by direct enzyme-linked aptamer assay (ELAA). Aptamer CT916 has a dissociation constant of 48.5 ± 0.5 nM and shows negligible binding to Shiga-like toxin 1B, protein A and BSA. This aptamer was chosen to develop a sandwich ELAA for the detection of CT from binding buffer and local tap water. Amine-C6- or biotin-modified CT916 was coupled to magnetic beads to serve as the capture element. Using an anti-CT polyclonal antibody as the reporter, detection limits of 2.1 ng/ml in buffer and 2.4 ng/ml in tap water, with a wide log-linear dynamic range from 1 ng/ml to 1000 ng/ml and 500 ng/ml, respectively, were achieved.

Nanomaterials-based biosensors for detection of microorganisms and microbial toxins

Detection of microorganisms and microbial toxins is important for health and safety. Due to their unique physical and chemical properties, nanomaterials have been extensively used to develop biosensors for rapid detection of microorganisms with microbial cells and toxins as target analytes. In this paper, the design principles of nanomaterials-based biosensors for four selected analyte categories (bacteria cells, toxins, mycotoxins, and protozoa cells), closely associated with the target analytes' properties is reviewed. Five signal transducing methods that are less equipment intensive (colorimetric, fluorimetric, surface enhanced Raman scattering, electrochemical, and magnetic relaxometry methods) is described and compared for their sensory performance (in term oflimit of detection, dynamic range, and response time) for all analyte categories. In the end, the suitability of these five sensing principles for on-site or field applications is discussed. With a comprehensive coverage of nanomaterials, design principles, sensing principles, and assessment on the sensory performance and suitability for on-site application, this review offers valuable insight and perspective for designing suitable nanomaterials-based microorganism biosensors for a given application.

Shielded Coaxial Optrode Arrays for Neurophysiology

Recent progress in the study of the brain has been greatly facilitated by the development of new tools capable of minimally-invasive, robust coupling to neuronal assemblies. Two prominent examples are the microelectrode array (MEA), which enables electrical signals from large numbers of neurons to be detected and spatiotemporally correlated, and optogenetics, which enables the electrical activity of cells to be controlled with light. In the former case, high spatial density is desirable but, as electrode arrays evolve toward higher density and thus smaller pitch, electrical crosstalk increases. In the latter, finer control over light input is desirable, to enable improved studies of neuroelectronic pathways emanating from specific cell stimulation. Here, we introduce a coaxial electrode architecture that is uniquely suited to address these issues, as it can simultaneously be utilized as an optical waveguide and a shielded electrode in dense arrays. Using optogenetically-transfected cells on a coaxial MEA, we demonstrate the utility of the architecture by recording cellular currents evoked from optical stimulation. We also show the capability for network recording by radiating an area of seven individually-addressed coaxial electrode regions with cultured cells covering a section of the extent.