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The capacitive immunosensor, known for its label-free simplicity, has great potential for point-of-care diagnostics. However, the interaction between insulation and recognition layers on the sensing electrode greatly affects its performance. This study introduces a pioneering dual-layer strategy, implementing a novel combination of acrylic resin (AR) and nitrocellulose (NC) coatings on screen-printed carbon electrodes (SPCEs). This innovative approach not only enhances the dielectric properties of the capacitive sensor but also streamlines the immobilization of recognizing elements. Particularly noteworthy is the superior reliability and insulation offered by the AR coating, surpassing the limitations of traditional self-assembled monolayer (SAM) modifications. This dual-layer methodology establishes a robust foundation for constructing capacitive sensors optimized specifically for liquid medium-based biosensing applications. The NC coating in this study represents a breakthrough in effectively immobilizing BSA, unraveling the capacitive response intricately linked to the quantity of adsorbed recognizing elements. The results underscore the prowess of the proposed immunosensor, showcasing a meticulously defined linear calibration curve for anti-BSA (ranging from 0 to 25 μg/ml). Additionally, specific interactions with anti-HAS and anti-TNF-α further validate the versatility and efficacy of the developed immunosensor. This work presents a streamlined and highly efficient protocol for developing label-free immunosensors for antibody determination and introduces a paradigm shift by utilizing readily available electrodes and sensing systems. The findings are poised to catalyze a significant acceleration in the advancement of biosensor technology, opening new avenues for innovative applications in point-of-care diagnostics., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Celiac patients are required to strictly adhere to a gluten-free diet because even trace amounts of gluten can damage their small intestine and leading to serious complications. Despite increased awareness, gluten can still be present in products due to cross-contamination or hidden ingredients, making regular monitoring essential. With the goal of guaranteeing food safety for consuming labeled gluten-free products, a capacitive aptasensor was constructed to target gliadin, the main allergic gluten protein for celiac disease. The success of capacitive aptasensing was primarily realized by coating a Parylene double-layer (1000 nm Parylene C at the bottom with 400 nm Parylene AM on top) on the electrode surface to ensure both high insulation quality and abundant reactive amino functionalities. Under the optimal concentration of aptamer (5 μM) used for immobilization, a strong linear relationship exists between the amount of gliadin (0.01-1.0 mg/mL) and the corresponding Δ C response (total capacitance decrease during a 20 min monitoring period after sample introduction), with an R 2 of 0.9843. The detection limit is 0.007 mg/mL (S/ N > 5), equivalent to 0.014 mg/mL (14 ppm) of gluten content. Spike recovery tests identified this system is free from interferences in corn and cassava flour matrices. The analytical results of 24 commercial wheat flour samples correlated well with a gliadin ELISA assay ( R 2 = 0.9754). The proposed label-free and reagentless capacitive aptasensor offers advantages of simplicity, cost-effectiveness, ease of production, and speediness, making it a promising tool for verifying products labeled as gluten-free (gluten content <20 ppm).

A new automated, generic analytical approach for determining the clinical disinfectant o-phthalaldehyde (OPA) is reported in this study. The proposed sequential injection analysis (SIA) is based on the online reaction of the OPA with glycine/N-acetylcysteine (NAC) in a neutral medium (pH = 7.0) to form a highly fluorescent isoindole derivative. All critical flow and reaction variables were investigated, while validation was carried out in the linearity detection range (0.0075-0.02%). As a result, excellent linearity (R 2  > 0.99) and precision (1.5-2.4% for repeatability and 0.7-2.2% for reproducibility) were achieved for the reference OPA solutions. Furthermore, reasonable concentration verification of OPA disinfection (0.2-0.6%) in healthcare institutes can be achieved using the developed fluorescent SIA due to its good sensitivity (0.111 V/%) and precision (1.0-2.3% for intermediate precision) around the minimum effective concentration (MEC) of 0.3% for Cidex-OPA disinfectant., (© 2023. The Author(s), under exclusive licence to The Japan Society for Analytical Chemistry.)

The study proposes an o -phthalaldehyde (OPA) sensor for rapid and reliable detection of OPA in healthcare disinfection practices, based on a hydrogel-modified screen-printed carbon electrode strip. The hydrogel film, which contains glycine and N -acetylcysteine, reacts with OPA to produce a reductive isoindole derivative. The derivative is then oxidized for OPA determination using cyclic voltammetry. The proposed sensor achieves an optimal detection time of 20-30 s and requires only a small analyte volume of 5 µL. It exhibits good precision (10%) and sensitivity (3.3 μA/cm 2 mM) in a phosphate-buffered solution (pH 7.6), with excellent linearity (R 2 > 0.97) and precision (<3%) in the detection range (0.2-0.6%) required for clinical OPA solutions. Moreover, the sensor demonstrates good concentration verification of Cidex-OPA disinfection in healthcare institutes, with high sensitivity (18.28 μA/cm 2 mM) and precision around the minimum effective concentration (0.3%). Overall, the proposed sensor offers a promising and practical solution for accurate and reliable OPA detection in clinical disinfection practices.

Quality control of deep-frying oil is a global public health concern. A simple and economic electrochemical chamber composed of two bare screen-printed carbon electrodes (working area: 78.54 × 10 2 cm 2 ; distance: 0.0055 cm; cell constant: 0.70 × 10 -2 cm -1 ) was constructed for precisely acquiring the impedimetric responses of a high-resistance palm oil sample (RSD < 7%, n = 3). Good correlations between the measured impedance data (charge transfer resistance and logarithmic output impedance (Log Z) obtained in the frequency region <0.1 Hz) and the regulatory quality indicators (total polar compounds and acid value) were achieved ( R 2 > 0.97), suggesting that the proposed impedimetric sensing method is useful for accurately assessing the deteriorated condition of repeated frying oil. Applications for rapid screening can also be realized because the measurement times of Log Z at any given perturbation frequency from 0.01-1 Hz were all less than 3 min.

This investigation develops a label-free and reagentless aptasensor, based on a capacitive transducer with simple face-to-face electrode pairs. The electrode pairs of the transducer are composed of a gold electrode and an indium tin oxide film with micrometer separation with a double-side polyethylene terephthalate tape. Aptamers and 1-dodecanethiol are modified to form a self-assembled monolayer (SAM) on the gold electrode surfaces, and function as bio-recognition elements and preventers of non-specific protein binding, respectively. Electrochemical characterization results indicate that the SAM also forms an effective insulating layer, which is sufficient for capacitive sensing. The feasibility of the capacitive biosensor is validated using thrombin as a model analyte. The ultra-small value changes of capacitance originating from thrombin binding with the aptamers modified on the biosensor were measured with a home-made capacitance measuring circuit based on switched capacitor (SC) technology. The developed biosensor has detection limits of 1 pM and 10 pM of thrombin in phosphate buffered saline and mimic serum solution, respectively. The linear range for thrombin detection in human serum solution is from 10 pM to 1 μM, with a regression coefficient of 0.98. Additionally, the proposed aptasensor does not have significant levels of non-specific binding of bovine serum albumin and human serum albumin. Accordingly, the combination of SC and SAM bringing capacitive transduction at the forefront of ultrasensitive label-free and reagentless biosensing devices, particularly for point-of-care clinical analysis, which adopts small numbers of biological samples with low analyte concentrations., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Adulterating edible oil with copper chlorophyll derivatives (E141i) has made a substantial impact on the edible oil industry and food safety. This study demonstrates an efficient and reliable screening method to directly identify the color adulteration by the aid of a simple photobleaching pretreatment using a 365 nm ultraviolet-light-emitting diode working at a photon flux density of 480 mmol m -2 s -1 for 24 min. The content of copper chlorophyll [predominantly Cu-pyropheophytin a (Cu-py a)] can be calculated by A 600 , A 650 , and A 700 with satisfactory spike recovery [97.9-103.6%; six kinds of edible oils spiked with 1 ppm of Cu-py a; n = 3 for each kind of oil; relative standard deviation (RSD) < 5%], linearity ( R 2 = 0.9961 when spiking 0.1-10 ppm of Cu-py a into soybean oil standard; n = 3 for each concentration; RSD < 5%), and reproducibility (RSD < 5% for spiking 1 ppm of Cu-py a into soybean oil standard; n = 3 over 3 days). The detection limit (S/N > 5) was 0.05 ppm. The analytical results of 50 commercially available oil samples were verified by the official high-performance liquid chromatography method.

Besides being a crucial parameter for surgery and clinical diagnosis, hematocrit tends to affect the analytical results of point-of-care analytical devices. Therefore, an accurate and quick method for measuring hematocrit was developed on the basis of screen-printed carbon electrodes (SPCE). An impulse DC voltage of 3.0 V was imposed on ferricyanide-coated SPCE to induce hemolysis, and the released hemoglobin reduced the ferricyanide, generating a higher oxidation current for estimating hematocrit. Hematocrit ranging from 10 to 70% can be determined in 5 s by linear sweep voltammetry (r(2) = 0.9907) or 0.8 s by 3 V of potential step voltammetry (r(2) = 0.9833).

Tryptophan was detected with a flow-injection manifold after reacting with mM order of fluorogenic o-phthalaldehyde (OPA)/thiol reagent (pH 10.0) in the carrier stream (0.63 mL/min). Based on the intra-molecular fluorescence resonance energy transfer of OPA-tryptophan adduct, the difference in fluorescence intensity obtained at 280 and 300 nm excitation was used to detect tryptophan content with satisfactory precision (CV<6.5% for concentration higher than 0.5 μM), linearity (0.1-10 μM, R(2)=0.9893) and sensitivity (≈10 nM). Since tryptophan will decompose during manufacturing non-fermented soy sauce by acid-hydrolysis procedure, the method was used to discriminate pure fermented soy sauces, adulterated soy sauces and chemical soy sauces in less than 5 min. The ratio of tryptophan to total amino acid content served as the index for the judgment, and the results were validated by capillary electrophoresis., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The aim of the present study was to develop a simple and fast screening technique to directly evaluate the bactericidal effects of 5-aminolevulinic acid (ALA)-mediated photodynamic inactivation (PDI) and to determine the optimal antibacterial conditions of ALA concentrations and the total dosage of light in vitro. The effects of PDI on Staphylococcus aureus and Pseudomonas aeruginosa in the presence of various concentrations of ALA (1.0 mM, 2.5 mM, 5.0 mM, 10.0 mM) were examined. All bacterial strains were exponentially grown in the culture medium at room temperature in the dark for 60 minutes and subsequently irradiated with 630 ± 5 nm using a light-emitting diode (LED) red light device for accumulating the light doses up to 216 J/cm 2 . Both bacterial species were susceptible to the ALA-induced PDI. Photosensitization using 1.0 mM ALA with 162 J/cm 2 light dose was able to completely reduce the viable counts of S. aureus. A significant decrease in the bacterial viabilities was observed for P. aeruginosa, where 5.0 mM ALA was photosensitized by accumulating the light dose of 162 J/cm 2 . We demonstrated that the use of microplate-based assays-by measuring the apparent optical density of bacterial colonies at 595 nm-was able to provide a simple and reliable approach for quickly choosing the parameters of ALA-mediated PDI in the cell suspensions., (Copyright © 2013. Published by Elsevier B.V.)

An online dynamic pH junction preconcentration method was developed for quantification of 5-aminolevulinic acid (ALA) by CE with the separation time less than 6 min. The optimal dynamic pH junction of ALA was carried out between pH 9.3 borate buffer (BGE, 40 mM) and pH 2.5 phosphate buffer (sample matrix, 40 mM) when 4.1 cm of sample plug was hydrodynamically injected into an uncoated fused-silica capillary (48.5 cm in length, id of 50 μm). If a 24 kV separation voltage was applied, the calibration curve of ALA peak area (200 nm) showed good linearity (R(2) = 0.9991) ranging from 0.01 to 6.5 mg/mL. The reproducibility of this system was excellent with RSDs (n = 10) of 2.5% for peak area response and 0.6% for migration time at ALA concentration of 0.5 mg/mL. The LOD was evaluated as 1.0 μg/mL (S/N > 3). Compared to conventional CE procedure, the sensitivity was successfully improved over 50-fold. The analytical results of pharmaceutical formulations show a good agreement with those by HPLC (r = 0.94)., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A hand-held electronic tongue was developed for determining taste levels of astringency and umami in tea infusions. The sensing principles are based on quenching the fluorescence of 3-aminophthalate by tannin, and the fluorogenic reaction of o-phthalaldehyde (OPA) with amino acids to determine astringency and umami levels, respectively. Both reactions were measured by a single fluorescence sensing system with same excitation and emission wavelengths (340/425 nm). This work describes in detail the design, fabrication, and performance evaluation of a hand-held fluorometer with an ultra-violet light emitted diode (UVLED) and a photo-detector with a filter built-in. The dimension and the weight of proposed electronic tongue prototype are only 120×60×65 mm(3) and 150 g, respectively. The detection limits of this prototype for theanine and tannic acid were 0.2 μg/ml and 1 μg/ml, respectively. Correlation coefficients of this prototype compared with a commercial fluorescence instrument are both higher than 0.995 in determinations of tannin acid and theanine. Linear detection ranges of the hand-held fluorometer for tannic acid and theanine are 1-20 μg/ml and 0.2-10 μg/ml (CV<5%, n=3), respectively. A specified taste indicator for tea, defined as ratio of umami to astringency, was adopted here to effectively distinguish flavour quality of partially fermented Oolong teas., (Copyright © 2010 Elsevier B.V. All rights reserved.)

β(1→3)-d-Glucan sensors were fabricated using bi-enzyme and tri-enzyme immobilized systems with gold nanoparticles (GNPs) to increase sensitivity. The plant β(1→3)-D-glucanase (βG), glucose oxidase (GOD) or/and peroxidase (POD) in agarose-corn flour-gelatin (ACG) matrix were coated on platinum disc electrode to detect soluble β(1→3)-D-glucan. The atomic force microscopy (AFM) revealed that GNPs embedded in ACG formed tiny islands/clusters with enzymes. Both of bi-enzyme sensor (ACG-βG-GOD-GNPs/Pt) and tri-enzyme sensor (ACG-βG-GOD-POD-GNPs/Pt) had response time less than 20s for β(1→3)-D-glucan. A linear calibration plot for bi-enzyme sensor was obtained for β(1→3)-D-glucan concentration ranged from 100 to 1000 ngmL(-1) (R(2)=0.983). The lower detection limit was 30 ngmL(-1) using applied potential of 200 mV and scan rate of 50 mVs(-1); with signal to noise ratio (S/N) of 3. Fabricated tri-enzyme sensor was also operable under similar conditions with LOD of 50 ngmL(-1) (r(2)=0.989) at -175 mV applied potential and scan rate of 50 mVs(-1). Both sensors were durable and could be repeatedly used for at least 14 times. When the tri-enzyme sensor was employed to analyze β(1→3)-d-glucan content in alcoholic beverages, the results were comparable to those obtained by standard method., (Copyright 2010 Elsevier B.V. All rights reserved.)

A capillary electrophoresis method and a durable choline biosensor were developed for measuring serum cholinesterase (EC 3.1.1.8) activity, a useful clinical index for liver function. The former is based on separation of benzoate and benzoylcholine (the artificial substrate of cholinesterase) in an uncoated fused-silica capillary. The migration time of benzoylcholine and benzoate was 1.3 min and 5.5 min, respectively. By the peak areas of A(233) signals, the linear dynamic ranges for both analytes were 0.01-50.0 mM, and the relative standard deviations of 1.0 mM benzoylcholine and benzoate were less than 4% and 6%, respectively. The FIA-choline sensor was constructed with the working electrode of the flow cell covered with a natural chitinous membrane purified from Taiwanese soldier crab, Mictyris brevidactylus. The biomembrane served as the supporting material for enzyme immobilization (choline oxidase, EC 1.1.3.17), and also prevented protein adsorption on the electrode surface. The calibration curve was linear between 0.05 and 5.0 mM (r=0.999). The relative standard deviations for 1.0 mM choline (n=7) were less than 3%, and the activity of the bioactive membrane lasted for about 2 months. The analytical results of both methods correlated well (r=0.940).

An amperometric flow-injection choline biosensor was assembled utilizing natural chitinous membrane as the supporting material for biocatalyst immobilization, and the membrane was purified from Taiwanese soldier crab, Mictyris brevidactylus. The chitinous membrane (<50.0 microm in thickness) was covalently immobilized with choline oxidase (EC 3.1.1.17 from Alcaligenes sp.) and then attached onto the platinum electrode of an amperometric flow cell. The flow cell served as the choline sensing device of the proposed FIA system. The sensor signal (peak height of the FIAgram) was linearly related to choline concentration (r=0.999 for choline up to 5.0mM) with low detection limit (S/N>3 for 10.0 microM choline) and high reproducibility (CV<3% for 1.0mM choline, n=7). The system was proved to be useful in measuring cholinesterase inhibitory activities of synthetic chemicals or natural products.

This study investigated characteristics of a chitosan membrane from the carapace of the soldier crab Mictyris brevidactylus intended to construct an amperometric biosensor. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used in this study to characterize these chitosan membranes intended for constructing enzymatic biosensors. Chitosan membranes suffering various durations (>10 min) of deacetylation had small charge-transfer resistances (<7.88 kohms) but large double-layer capacitances (>0.55 microF). They were found in EIS where both the solution resistance and Warburg impedance upon electrode interface were almost independent of the durations and degree of deacetylation. The degree of deacetylation and the thickness of chitosan membranes were also determined. Membrane thickness was slightly dependent with the duration but degree of deacetylation was slightly dependent on the duration. Chitosan membranes with various thicknesses suffered various durations of deacetylation, but this did not influence their electrochemical characteristics. The chitinous membrane was covalently immobilized with glucose oxidase (EC 1.3.4.3) and then attached onto the platinum electrode of a homemade amperometric flow cell. Sensor signal was linearly related to glucose concentration (r=0.999 for glucose up to 1.0 mM). The system was sensitive (S/N>5 for 10 microM glucose) and reproducible (CV<1.3% for 50 microM glucose, n=5).

A simple spectrophotometric method is proposed for determining deacetylation degrees (DD) of chitinous materials using phosphoric acid as the UV-transparent solvent system. Calibrating by the extinction coefficients (A(210)) of D-glucosamine and N-acetyl-D-glucosamine, DD values (24-88%) were computed numerically. The results correlated well (R(2) = 0.9805, n = 50) with those obtained by solid-state (13)C NMR. Comparison of the results obtained by the proposed UV method and solid-state (13)C NMR.

Glucose oxidase (EC 1.3.4.3) was immobilized on chitosan membrane (<0.1 mm in thickness) prepared from the carapace of the soldier crab Mictyris brevidactylus. A glucose electrode was constructed by covering a platinum electrode (2.0 mm in diameter) with the enzyme membrane. The enzyme electrode sensed glucose amperometrically (1.0 micro A/mM, with linear range up to 0.5 mM, r = 0.999) when positively imposed with 0.6 V against an Ag/AgCl reference electrode. The glucose biosensor was sensitive (<0.1 micro M, S/N > 3), reproducible (CV for 55 micro M glucose <3%, n = 5), reagentless, and durable for months.