A microfluidic card-based electrochemical assay for the detection of sulfonamide resistance genes

Most electroanalytical detection schemes for DNA markers require considerable time and effort from expert personnel to thoroughly follow the analysis and obtain reliable outcomes. This work aims to present an electrochemical assay performed inside a small card-based platform powered by microfluidic manipulation, requiring minimal human intervention and consumables. The assay couples a sample/signal dual amplification and DNA-modified magnetic particles for the detection of DNA amplification products. Particularly, the sul1 and sul4 genes involved in the resistance against sulfonamide antibiotics were analyzed. As recognized by the World Health Organization, antimicrobial resistance threatens global public health by hampering medication efficacy against infections. Consequently, analytical methods for the determination of such genes in environmental and clinical matrices are imperative. Herein, the resistance genes were extracted from E. coli cells and amplified using an enzyme-assisted isothermal amplification at 37 °C. The amplification products were analyzed in an easily-produced, low-cost, card-based set-up implementing a microfluidic system, demanding limited manual work and small sample volumes. The target amplicon was thus captured and isolated using versatile DNA-modified magnetic beads injected into the microchannel and exposed to the various reagents in a continuously controlled microfluidic flow. After the optimization of the efficiency of each phase of the assay, the platform achieved limits of detections of 44.2 pmol L-1 for sul1 and 48.5 pmol L-1 for sul4, and was able to detect down to ≥500-fold diluted amplification products of sul1 extracted from E. coli living cells in around 1 h, thus enabling numerous end-point analyses with a single amplification reaction.

Progesterone and β-hCG Determination Using an Electrochemical Combo-Strip for Pregnancy Monitoring

The development of analytical devices that can allow an easy, rapid and cost-effective measurement of multiple markers, such as progesterone and β-hCG, could have a role in decreasing the burden associated with pregnancy-related complications, such as ectopic pregnancies. Indeed, ectopic pregnancies are a significant contributor to maternal morbidity and mortality in both high-income and low-income countries. In this work, an effective and highly performing electrochemical strip for a combo determination of progesterone and β-hCG was developed. Two immunosensing approaches were optimized for the determination of these two hormones on the same strip. The immunosensors were realized using cost-effective disposable electrode arrays and reagent-saving procedures. Each working electrode of the array was modified with both the IgG anti-β-hCG and anti-progesterone, respectively. By adding the specific reagents, progesterone or β-hCG can then be determined. Fast quantitative detection was achieved, with the analysis duration being around 1 h. Sensitivity and selectivity were assessed with a limit of detection of 1.5 × 10-2 ng/mL and 2.45 IU/L for progesterone and β-hCG, respectively. The proposed electrochemical combo-strip offers great promise for rapid, simple, cost-effective, and on-site analysis of these hormones and, thus, for the development of a point-of-care diagnostic tool for early detection of pregnancy-related complications.

Characterization of a Ruthenium(II) Complex in Singlet Oxygen-Mediated Photoelectrochemical Sensing

A water-soluble ruthenium(II) complex (L), capable of producing singlet oxygen (¹O₂) when irradiated with visible light, was used to modify the surface of an indium-tin oxide (ITO) electrode decorated with a nanostructured layer of TiO₂ (TiO₂/ITO). Singlet oxygen triggers the appearance of a cathodic photocurrent when the electrode is illuminated and biased at a proper reduction potential value. The L/TiO₂/ITO electrode was first characterized with cyclic voltammetry, impedance spectroscopy, NMR, and Raman spectroscopy. The rate constant of singlet oxygen production was evaluated by spectrophotometric measurements. Taking advantage of the oxidative process initiated by ¹O₂, the analysis of phenolic compounds was accomplished. Particularly, the ¹O₂-driven oxidation of hydroquinone (HQ) produced quinone moieties, which could be reduced back at the electrode surface, biased at -0.3 V vs Ag/AgCl. Such a light-actuated redox cycle produced a photocurrent dependent on the concentration of HQ in solution, exhibiting a limit of detection (LOD) of 0.3 μmol dm^-3. The L/TiO₂/ITO platform was also evaluated for the analysis of p-aminophenol, a commonly used reagent in affinity sensing based on alkaline phosphatase.

Paper-based genetic assays with bioconjugated gold nanorods and an automated readout pipeline

Paper-based biosensors featuring immunoconjugated gold nanoparticles have gained extraordinary momentum in recent times as the platform of choice in key cases of field applications, including the so-called rapid antigen tests for SARS-CoV-2. Here, we propose a revision of this format, one that may leverage on the most recent advances in materials science and data processing. In particular, we target an amplifiable DNA rather than a protein analyte, and we replace gold nanospheres with anisotropic nanorods, which are intrinsically brighter by a factor of ~ 10, and multiplexable. By comparison with a gold-standard method for dot-blot readout with digoxigenin, we show that gold nanorods entail much faster and easier processing, at the cost of a higher limit of detection (from below 1 to 10 ppm in the case of plasmid DNA containing a target transgene, in our current setup). In addition, we test a complete workflow to acquire and process photographs of dot-blot membranes with custom-made hardware and regression tools, as a strategy to gain more analytical sensitivity and potential for quantification. A leave-one-out approach for training and validation with as few as 36 sample instances already improves the limit of detection reached by the naked eye by a factor around 2. Taken together, we conjecture that the synergistic combination of new materials and innovative tools for data processing may bring the analytical sensitivity of paper-based biosensors to approach the level of lab-grade molecular tests.

A simple and selective electrochemical magneto-assay for sea lice eDNA detection developed with a Quality by Design approach

Environmental DNA (eDNA) is a novel, non-invasive sampling procedure that allows the obtaining of genetic material directly from environmental samples without any evidence of biological sources. The eDNA methodology can greatly benefit from coupling it to reliable, portable and cost-effective tools able to perform decentralized measurements directly at the site of need and in resource-limited settings. Herein, we report a simple method for the selective analysis of eDNA using a magneto-assay with electrochemical detection. The proposed method involves the polymerase chain-reaction (PCR) amplification of mitochondrial eDNA of parasitic Salmon lice (Lepeophtheirus salmonis), extracted from seawater samples. The eDNA sequence was targeted via sandwich hybridization onto magnetic beads and enzymatic labeling was performed to obtain an electroactive product measured by differential pulse voltammetry. Quality by Design (QbD), a recent concept of science- and risk-oriented quality paradigm, was used for the optimization of the different parameters of the assay. Response surface methodology and Monte Carlo simulations were performed to define the method operable design region. The optimized electrochemical magneto-assay attained a limit of detection of 2.9 amol μL^-1 of the short synthetic sea louse DNA analogue (43 bp). In addition, robustness testing using a further experimental design approach was performed for monitoring eDNA amplicons. Seawater samples spiked with individuals of free-swimming L. salmonis copepodite stages and seawater collected from tanks with sea lice-infested fish were analyzed.

Sustainable Printed Electrochemical Platforms for Greener Analytics

The development of miniaturized electrochemical platforms holds considerable importance for the in situ analytical monitoring of clinical, environmental, food, and forensic samples. However, it is crucial to pay attention to the sustainability of materials chosen to fabricate these devices, in order to decrease the amount and the impact of waste coming from their production and use. In the framework of a circular economy and an environmental footprint reduction, the electrochemical sensor production technology must discover the potentiality of innovative approaches based on techniques and materials that can satisfy the needs of environmental-friendly and greener analytics. The aim of this review is to describe some of the printing technologies most used for sensor production, including screen-printing, inkjet-printing, and 3D-printing, and the low-impact materials that are recently proposed for these techniques, such as polylactic acid, cellulose, silk proteins, biochar.

Innovative Biocatalysts as Tools to Detect and Inactivate Nerve Agents

Pesticides and warfare nerve agents are frequently organophosphates (OPs) or related compounds. Their acute toxicity highlighted more than ever the need to explore applicable strategies for the sensing, decontamination and/or detoxification of these compounds. Herein, we report the use of two different thermostable enzyme families capable to detect and inactivate OPs. In particular, mutants of carboxylesterase-2 from Alicyclobacillus acidocaldarius and of phosphotriesterase-like lactonases from Sulfolobus solfataricus and Sulfolobus acidocaldarius, have been selected and assembled in an optimized format for the development of an electrochemical biosensor and a decontamination formulation, respectively. The features of the developed tools have been tested in an ad-hoc fabricated chamber, to mimic an alarming situation of exposure to a nerve agent. Choosing ethyl-paraoxon as nerve agent simulant, a limit of detection (LOD) of 0.4 nM, after 5 s of exposure time was obtained. Furthermore, an optimized enzymatic formulation was used for a fast and efficient environmental detoxification (>99%) of the nebulized nerve agent simulants in the air and on surfaces. Crucial, large-scale experiments have been possible thanks to production of grams amounts of pure (>90%) enzymes.

Electrochemical detection of miRNA-222 by use of a magnetic bead-based bioassay

MicroRNAs (miRNAs, miRs) are naturally occurring small RNAs (approximately 22 nucleotides in length) that have critical functions in a variety of biological processes, including tumorigenesis. They are an important target for detection technology for future medical diagnostics. In this paper we report an electrochemical method for miRNA detection based on paramagnetic beads and enzyme amplification. In particular, miR 222 was chosen as model sequence, because of its involvement in brain, lung, and liver cancers. The proposed bioassay is based on biotinylated DNA capture probes immobilized on streptavidin-coated paramagnetic beads. Total RNA was extracted from the cell sample, enriched for small RNA, biotinylated, and then hybridized with the capture probe on the beads. The beads were then incubated with streptavidin-alkaline phosphatase and exposed to the appropriate enzymatic substrate. The product of the enzymatic reaction was electrochemically monitored. The assay was finally tested with a compact microfluidic device which enables multiplexed analysis of eight different samples with a detection limit of 7 pmol L(-1) and RSD = 15 %. RNA samples from non-small-cell lung cancer and glioblastoma cell lines were also analyzed.

PBDEs in Italian sewage sludge and environmental risk of using sewage sludge for land application

Polybrominated diphenyl ethers (PBDEs) were determined in sewage sludge samples collected from eight Italian wastewater treatment plants (WWTPs) between June 2009 and March 2010. Total PBDE concentrations ranged from 158.3 to 9427 ng g(-1) dw, while deca-BDE (BDE-209) (concentrations ranging from 130.6 to 9411 ng g(-1) dw) dominated the congener profile in all the samples, contributing between 77% and 99.8% of total PBDE. The suitability of using a magnetic particle enzyme-linked immunoassay (ELISA) to analyse PBDEs in sewage sludge was also tested. The ELISA results, expressed as BDE-47 equivalents, were well correlated with those obtained by GC-NCI-MS, with correlation coefficients (r(2)) of 0.899 and 0.959, depending on the extraction procedure adopted. The risk assessment of PBDEs in sewage sludge addressed to land application was calculated. PEC(soil) values compared to the relative PNEC(soil) for penta and deca-BDE suggests that there is a low risk to the soil environment.

Enzyme-amplified electrochemical hybridization assay based on PNA, LNA and DNA probe-modified micro-magnetic beads

In the present study, we investigated the properties of PNA and LNA capture probes in the development of an electrochemical hybridization assay. Streptavidin-coated paramagnetic micro-beads were used as a solid phase to immobilize biotinylated DNA, PNA and LNA capture probes, respectively. The target sequence was then recognized via hybridization with the capture probe. After labeling the biotinylated hybrid with a streptavidin-enzyme conjugate, the electrochemical detection of the enzymatic product was performed onto the surface of a disposable electrode. The assay was applied to the analytical detection of biotinylated DNA as well as RNA sequences. Detection limits, calculated considering the slope of the linear portion of the calibration curve in the range 0-2 nM were found to be 152, 118 and 91 pM, coupled with a reproducibility of the analysis equal to 5, 9 and 6%, calculated as RSD%, for DNA, PNA and LNA probes respectively, using the DNA target. In the case of the RNA target, the detection limits were found to be 51, 60 and 78 pM for DNA, PNA and LNA probes respectively.

Electrochemical biosensor technology: application to pesticide detection

In recent years, electrochemical sensors and biosensors are becoming an accepted part of analytical chemistry since they satisfy the expanding need for rapid and reliable measurements. An area in which electrochemical biosensors perhaps show the greatest diversity and potential for development involves the measurement of environmentally significant parameters. The increasing number of pollutants in the environment calls for fast and cost-effective analytical requirements. In this context, biosensors appear as suitable alternative or complementary analytical tools. The aim of this chapter is to review some basic concept concerning the electrochemical biosensors and to illustrate a protocol for the detection of environmental organic pollutants on the basis of electrochemical biosensors. In particular, a method based on the inhibition of the enzyme acetylcholinesterase (AChE) for the detection of organophosphorus and carbamate pesticides will be described in detail.

Electrochemical Imaging of Localized Sandwich DNA Hybridization Using Scanning Electrochemical Microscopy

Imaging of localized hybridization of nucleic acids immobilized on gold-DNA chip was performed by means of the feedback mode of scanning electrochemical microscopy (SECM). Thiol-tethered oligodeoxynucleotide (HS-ODN) probes, spotted on a gold surface, were hybridized with unmodified target sequence via sandwich hybridization with a biotinylated signaling probe. Spots where sequence-specific hybridization had occurred were developed by adding a streptavidin-alkaline phosphatase conjugate and biocatalyzed precipitation of an insoluble and insulating product. As a consequence, the surface conductivity of the spotted region of the chip where hybridization had taken place changed. These changes in conductivity were sensitively detected by the SECM tip. The proposed method allows imaging of a DNA array in a straightforward way. Analysis of real samples was also performed coupling this method with polymerase chain reaction. The imaging of 60 nM PCR amplicon (255 bp) was demonstrated.

Polychlorinated biphenyls (PCBs) detection in milk samples by an electrochemical magneto-immunosensor (EMI) coupled to solid-phase extraction (SPE) and disposable low-density arrays

An electrochemical immunosensor for polychlorinated biphenyl (PCB) detection based on graphite screen-printed low-density arrays and on magnetic beads is reported. The immunological reaction for the detection of PCBs is based on a direct competitive assay using alkaline phosphatase (AP) as enzymatic label. After the immunochemical recognition, the modified magnetic beads are captured by a magnet on the surface of the graphite working electrode. The electrochemical detection is thus achieved through the addition of the AP substrate (alpha-naphthyl-phosphate). Two different antibodies (sIgG anti-PCB28 and rIgG anti-PCB77) were tested and compared in terms of sensitivity and ability to recognise different congeners. The developed electrochemical magneto-immunosensor (EMI) was successfully combined with solid-phase extraction (SPE) for the analysis of PCBs in milk samples. In spiked samples a recovery of 80% was obtained. The proposed strategy offers great promise for rapid, simple, cost-effective, and on-site analysis of clinical, food and environmental samples, considering also that low-density arrays allow the simultaneous analysis of different processed samples.

One-shot screen-printed thylakoid membrane-based biosensor for the detection of photosynthetic inhibitors in discrete samples

Screen-printing technology offers the possibility to produce a large number of sensors at low cost. Thus, due to their intrinsic characteristics and reproducibility, screen-printed electrodes can be used in the development of disposable electrochemical devices. In the present work, carbon-based screen-printed electrodes (SPCEs) have been used to develop a one-shot-measure biosensor for the detection of photosynthetic inhibitors in discrete samples. The measurement was based on the electrochemical evaluation of the activity of photosystem II (PSII), a protein complex present in photosynthetic organisms and involved in the photosynthesis. The biosensor was prepared by the modification of the working electrode of a SPCE, using thylakoid membranes extracted from spinach leaves. The modified sensors were then used as one-shot system to measure the presence of PSII activity inhibitors in discrete standard solutions. The coupling of the developed biosensor with a custom-made cell made it possible to perform tests using only 50 microL of total sample volume with a measurement time of 10 min. Inhibition curves were recorded for some photosynthetic inhibitors in a concentration range of 10(-6) to 10(-8) molL(-1). A reproducibility (relative standard deviation, R.S.D.%) of 10% was found and the calculated limit of detections (LODs) were in the nanomolar range. The effect of storage on sensitivity and reproducibility of a biosensor prepared by direct lyophilisation of thylakoid membranes on the electrode surface was also evaluated, confirming the possible use of the modified sensor up to one week after the preparation. Measurements on real samples were also reported, comparing the results with those obtained using a fluorescence-based commercial instrument for the analysis of photosynthetic inhibitors.

Planar electrochemical sensors for biomedical applications

Electrochemistry has superior properties respect to the other measurement systems because of the rapid, simple and sensitive characteristics. For all these reasons, electrochemical sensors are playing a key role in many scientific sectors. Planar electrochemical sensors present many advantages respect to the three-dimensional devices, but this technology requires the use of specialty chemicals to meet the new functional requirements of planarization and miniaturization. In this paper, some production techniques and procedures to obtain planar electrochemical sensors are reported, together with their applications.

Polychlorinated biphenyls (PCBs) detection in food samples using an electrochemical immunosensor

In this work, a disposable electrochemical immunosensor, based on a competitive assay scheme, was applied to detect polychlorinated biphenyls (PCBs) in food. For this purpose, antibodies against PCBs were directly immobilized onto the carbon surface of a disposable screen-printed electrode. A competition between the PCBs present in the sample and a fixed concentration of an enzyme-labeled PCB was realized and evaluated by electrochemical detection. Alkaline phosphatase was used as the enzyme label, coupled with differential pulse voltammetry (DPV) as the electrochemical technique. The immunosensor was tested on aroclor mixture detection (1242 and 1248) and then on some typologies of food samples to evaluate the possible application for real sample analysis. Samples analyzed were from different matrixes, such as sheep milk, bovine adipose tissue, and bovine muscle. Results obtained were compared with the accredited results according to ISO 17025 methods for PCB detection (HRGC-LRMS) as a confirmatory analysis. Preliminary results show the possibility to use this device as a screening method in food sample analysis. The negligible matrix effect observed may lead to a simplified extraction procedure, and considerable time and consumable savings are the immediate benefits given by the proposed method.

Disposable electrochemical immunosensor for environmental applications

An Immunosensor for the detection of polychlorinated biphenyls (PCB) has been developed, using carbon-based screen-printed electrodes as solid-phase and signal transducers. The immunosensor realised is based on a direct competitive immunoassay scheme, in which the antibody against PCB was directly immobilised onto the carbon surface of the screen-printed electrode. Competition between the PCBs present in the sample and a fixed concentration of an enzyme-labelled congener was realised and evaluated by electrochemical detection. The immunosensor developed was tested on Arochlor mixtures (1242 and 1248), and was applied in environmental and food analysis by testing some real samples (from animal tissues and marine sediments). Results obtained demonstrate the ability of this device to detect PCBs in complex matrices.