Electrochemical Assay for a Total Cellulase Activity with Improved Sensitivity

The organo-metal-like nature of long-range conduction in cable bacteria

Cable bacteria are filamentous, multicellular microorganisms that display an exceptional form of biological electron transport across centimeter-scale distances. Currents are guided through a network of nickel-containing protein fibers within the cell envelope. Still, the mechanism of long-range conduction remains unresolved. Here, we characterize the conductance of the fiber network under dry and wet, physiologically relevant, conditions. Our data reveal that the fiber conductivity is high (median value: 27 S cm-1; range: 2 to 564 S cm-1), does not show any redox signature, has a low thermal activation energy (Ea = 69 ± 23 meV), and is not affected by humidity or the presence of ions. These features set the nickel-based conduction mechanism in cable bacteria apart from other known forms of biological electron transport. As such, conduction resembles that of an organic semi-metal with a high charge carrier density. Our observation that biochemistry can synthesize an organo-metal-like structure opens the way for novel bio-based electronic technologies.

Electrochemical Cellulase-Linked ELASA for Rapid Liquid Biopsy Testing of Serum HER-2/neu

Non-invasive liquid biopsy assays for blood-circulating biomarkers of cancer allow both its early diagnosis and treatment monitoring. Here, we assessed serum levels of protein HER-2/neu, overexpressed in a number of aggressive cancers, by the cellulase-linked sandwich bioassay on magnetic beads. Instead of traditional antibodies we used inexpensive reporter and capture aptamer sequences, transforming the enzyme-linked immuno-sorbent assay (ELISA) into an enzyme-linked aptamer-sorbent assay (ELASA). The reporter aptamer was conjugated to cellulase, whose digestion of nitrocellulose film electrodes resulted in the electrochemical signal change. ELASA, optimized relative aptamer lengths (dimer vs monomer and trimer), and assay steps allowed 0.1 fM detection of HER-2/neu in the 10% human serum in 1.3 h. Urokinase plasminogen activator and thrombin as well as human serum albumin did not interfere, and liquid biopsy analysis of serum HER-2/neu was similarly robust but 4 times faster and 300 times cheaper than both electrochemical and optical ELISA. Simplicity and low cost of cellulase-linked ELASA makes it a perspective diagnostic tool for fast and accurate liquid biopsy detection of HER-2/neu and of other proteins for which aptamers are available.

A simple and sensitive electrochemical sensor for the detection of peptidase activity

In this work, a simple and sensitive electrochemical sensor was proposed for the detection of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) activity. Firstly, the BACE1 specific peptide was modified onto the Au electrode to graft a single-strand DNA with polycytosine DNA sequence (dC₁₂) via amide bonding between peptide and dC₁₂. Because the dC₁₂ is abundant in phosphate groups, thus it can react with molybdate to form redox molybdophosphate, which can generate electrochemical current. Using BACE1 as a model peptidase, the proposed sensor shows a linear response range from 1 to 15 U/mL and limit of detection down to 0.05 U/mL. The sensor displays good performance for the BACE1 activity detection in human serum samples, which may have potential applications in the clinical diagnostics of Alzheimer's disease.

Electrochemical ELASA: improving early cancer detection and monitoring

The discovery of new molecular biomarkers of cancer during the last decades and the development of new diagnostic devices exploiting those have significantly contributed to the clinical analysis of cancer and to improve the outcomes. Among those, liquid biopsy sensors exploiting aptamers for the detection of cancer biomarkers in body fluids are useful and accurate tools for a fast and inexpensive non-invasive screening of population. The incorporation of aptamers in electrochemical sandwich biosensors using enzyme labels, a so-called ELASA, has demonstrated its utility to improve the detection schemes. In this review, we overview the existing ELASA assays for numerous cancer biomarkers as alternatives to the traditional ELISA and discuss their possibilities to reach the market, currently dominated by optical immunoassays.

A novel chemiluminescence imaging immunosensor for prostate specific antigen detection based on a multiple signal amplification strategy

A novel chemiluminescence (CL) imaging platform was constructed for prostate specific antigen (PSA) detection in a multiple signal amplifying manner. To construct the platform, the primary antibody for PSA was firstly immobilized on a O-ring area of a glass slide for recognizing the PSA. The horseradish peroxidase (HRP) and the secondary antibody of PSA (Ab₂) functionalized Au NPs (HRP-Au NPs-Ab₂) were modified on the platform through immunoreaction between PSA and Ab₂. The excellent catalytic effect of Au NPs and HRP on the HRP-Au NPs-Ab₂ to the luminol-H₂O₂ CL system provided the dual-signal amplification for PSA detection. To further enhance the sensitivity, tyramine signal amplification (TSA) strategy was introduced: tyramine-HRP conjugates were added into the O-ring reservoir and thus tyramine-HRP repeats formed in the presence of H₂O₂, generating a multiple signal amplification because of the large amounts of HRP on the sensing interface. The excellent performance of HRP-Au NPs-Ab₂ and TSA strategy endows the CL platform with high sensitivity. The PSA was detected with a photomultiplier tube (PMT) and visually analyzed by a charge coupled device (CCD), respectively. The linear ranges of PMT and CCD for PSA are 0.1-100.0 ng mL^-1 with a detection limit of 0.05 pg mL^-1 and 0.5 - 100.0 ng mL^-1 with a detection limit of 0.1 pg mL^-1, respectively. The levels of PSA in several human serum samples were determined and the recoveries are ranged from 82.5% - 117.0%. This CL immunosensing platform holds great potential for bioactive molecules detection visually and sensitively.

Electrochemical Immuno- and Aptamer-Based Assays for Bacteria: Pros and Cons over Traditional Detection Schemes

Microbiological safety of the human environment and health needs advanced monitoring tools both for the specific detection of bacteria in complex biological matrices, often in the presence of excessive amounts of other bacterial species, and for bacteria quantification at a single cell level. Here, we discuss the existing electrochemical approaches for bacterial analysis that are based on the biospecific recognition of whole bacterial cells. Perspectives of such assays applications as emergency-use biosensors for quick analysis of trace levels of bacteria by minimally trained personnel are argued.

Cellulase-Linked Immunomagnetic Microbial Assay on Electrodes: Specific and Sensitive Detection of a Single Bacterial Cell

Pathogen-associated infections represent one of the major threats to human health and require reliable methods for immediate and robust identification of pathogenic microorganisms. Here, an inexpensive cellulase-linked immunomagnetic methodology was developed for the specific and ultrasensitive analysis of bacteria at their single-cell levels within a 3 h procedure. Detection of a model bacterium, Escherichia coli, was performed in a sandwich reaction with E. coli-specific either aptamer or antibody (Ab)-modified magnetic beads (MBs) and Ab/aptamer reporter molecules linked to cellulase. The cellulase-labeled immuno-aptamer sandwich applied onto nitrocellulose-film-modified electrodes digested the film and changed its electrical conductivity. Electrode's chronocoulometric responses at 0.3 V, in the absence of any redox indicators, allowed a single E. coli cell detection and from 1 to 4 × 10⁴ CFU mL^-1 E. coli quantification. No interference/cross-reactivity from Salmonella enteritidis, Enterobacter agglomerans, Pseudomonas putida, Staphylococcus aureus, and Bacillus subtilis was observed when the assay was performed on Ab-modified MBs, and E. coli could be quantified in tap water and milk. This electrochemically label-free methodology is sufficiently fast, highly specific, and sensitive to be used in direct in-field applications. The assay can be adapted for specific detection of other bacterial strains of either the same or different species and offers new analytical tools for fast, specific, and reliable analysis of bacteria in the clinic, food, and environment.

Design and fabrication of a label-free aptasensor for rapid and sensitive detection of endoglucanase

Endoglucanase (endocellulase, EC 3.2.1.4) is one of the most widely used enzymes in industry. Despite its importance, improved methods for the rapid, selective, quantitative assay of this enzyme have been slow to emerge. In this work, we have designed an aptasensor platform for ultrasensitive endoglucanase II detection based on DNA aptamer and reduced graphene oxide/Au nanoparticles (RGO/AuNPs). The surface morphology of RGO/AuNPs was characterized by various techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) methods. The aptasensor characterization was monitored with electrochemical techniques. Using RGO/AuNPs as a nanomaterial can effectively increase the conductivity of biosensor electrode. Moreover, using an RGO/AuNPs/aptamer platform in the presence of endoglucanase, the sensor system is able to generate a signal, which significantly improves the selectivity of the aptasensor. The fabricated aptasensor exhibits high sensitivity and selectivity to endoglucanase II with a low limit of detection (LOD) ˂0.1 nM.

Femtomolar electroanalysis of a breast cancer biomarker HER-2/neu protein in human serum by the cellulase-linked sandwich assay on magnetic beads

In cancer diagnostics, specific analysis of blood-circulating proteins biomarkers of cancer is often complicated both by their inherently low concentrations and by strong interference from serum/blood proteins. Here, we report a simple and robust electrochemical cellulase-linked sandwich assay on magnetic beads (MBs) for fM-sensitive analysis of the Human Epidermal growth factor Receptor-2 HER-2/neu protein that is over-expressed in most aggressive breast cancers. In the assay, a sandwich is assembled by capturing HER-2/neu on either antibody (Ab) or aptamer-modified MBs accompanied by reaction with the second Ab or aptamer labelled with cellulase. On application of the sandwiches assembled on MBs onto a cost-effective graphite electrode modified with an insulating nitrocellulose film, the cellulase label digests the film. This results in the pronounced changes in the electrical properties of the modified electrodes. The chronocoulometrically-measured extent of the produced changes was proportional to the 10^-15-10^-10 M HER-2/neu in the analyzed samples, and down to 1 fM of HER-2/neu could be detected in human serum samples in an overall less than 3 h assay. The developed simple and electrochemically label-free methodology is general and can be easily adapted for testing of any other protein.

Electrochemical Enzyme-Linked Sandwich Assay with a Cellulase Label for Ultrasensitive Analysis of Synthetic DNA and Cell-Isolated RNA

Electrochemical enzyme-linked sandwich assays on magnetic beads (MBs) refer to one of the most sensitive approaches for analysis of nonamplified nucleic acid samples, with redox enzymes being routinely used as labels. Here, we report a sensitive and inexpensive electrochemical nucleic acid sandwich assay on MBs that exploits a hydrolytic enzyme cellulase as a label, while MBs are used for preconcentration and bioseparation of analyzed samples. Binding of target DNA or RNA to capture DNA-modified MB triggers sandwich assembly and its labeling with cellulase. Application of the assembled sandwich to the electrodes covered with insulating nitrocellulose films induces film digestion by the cellulase label and pronounced changes in the electrical properties of the electrodes, the extent of the changes being proportional to the concentration of the analyzed nucleic acids. Down to 1 amol of Lactobacillus brevis specific synthetic DNA and rRNA isolated from L. brevis cells could be detected in 1 mL samples in the overall from 2 to 3 h assay. The assay is universal and can be adapted for point-of-care-testing and for in-field environmental and microbiomic analysis of unamplified samples of any DNA/RNA sequences.