Modified fractal iron oxide magnetic nanostructure: A novel and high performance platform for redox protein immobilization, direct electrochemistry and bioelectrocatalysis application

Electrochemical aptamer sensor based on AgNPs@PDANSs and "sandwich" structure guidance for the detection of tobramycin in water samples

In this study, an ultrasensitive detection platform for tobramycin (TOB) was developed, featuring a "sandwich" structure guided by AgNCs@PDANSs and Thi-AuNCs@ZnONSs. To address the issue of large background current peak signals in tagless sensors, Thi-AuNCs@ZnONSs composites were synthesized as signal tags. Zinc oxide nanosheets (ZnONSs) served as the loading agent, and AuNCs with the electroactive molecule Thi acted as carriers. Furthermore, AgNPs@PDANSs nanocomposites, possessing excellent electrical conductivity and large specific surface areas, were prepared as substrate materials for the modified electrodes. A "sandwich" structure strategy was also introduced to enhance the accuracy of the electrochemical aptasensor. This strategy, utilizing a dual sequence for target labeling and capture, yielded higher sensitivity and simplified the sensor construction compared to methods employing a single sequence. Under optimal conditions, the detection limit for TOB was established at 1.41 pM, with a detection range of 0.05-5000 nM. The aptasensor was effectively applied in the detection of TOB in tap and lake water, demonstrating outstanding reproducibility, selectivity, and stability. These results may serve as a reference for environmental TOB detection.

Recent strategies for electrochemical sensing detection of miRNAs in lung cancer

MicroRNAs (miRNAs) associated with lung cancer are diversifying. MiR-21, Let-7, and miR-141 are common diagnostic targets. Some new lung cancer miRNAs, such as miR-25, miR-145, and miR-126, have received increasing attention. Although various techniques are available for the analysis of lung cancer miRNAs, electrochemistry has been recognized for its high sensitivity, low cost, and rapid response. However, how to realize the signal amplification is one of the most important contents in the design of electrochemical biosensors. Herein, we mainly introduce the amplification strategy based on enzyme-free amplification and signal conversion, including non-linear HCR, catalytic hairpin assembly (CHA), electrochemiluminescence (ECL), and Faraday cage. Furthermore, new progress has emerged in the fields of nanomaterials, low oxidation potential, and simultaneous detection of multiple targets. Finally, we summarize some new challenges that electrochemical techniques may encounter in the future, such as improving single-base discrimination ability, shortening electrochemical detection time, and providing real body fluid samples assay.

A Comparative Study of Voltammetric vs Impedimetric Immunosensor for Rapid SARS-CoV-2 Detection at the Point-of-care

Here, a novel biosensing platform for the detection of SARS-CoV-2 usable both at voltammetric and impedimetric mode is reported. The platform was constructed on a multi-walled carbon nanotubes (MWCNTs) screen-printed electrode (SPE) functionalized by methylene blue (MB), antibodies against SARS-CoV-2 spike protein (SP), a bioactive layer of chitosan (CS) and protein A (PrA). The voltammetric sensor showed superior performances both in phosphate buffer solution (PBS) and spiked-saliva samples, with LOD values of 5.0±0.1 and 30±2.1 ng/mL, compared to 20±1.8 and 50±2.5 ng/mL for the impedimetric sensor. Moreover, the voltammetric immunosensor was tested in real saliva, showing promising results.

Highly stable acetylcholinesterase electrochemical biosensor based on polymerized ionic liquids microgel for pesticides detection

A highly stable electrochemical biosensor for pesticide detection was developed. For the first time polymeric ionic liquids (PILs) were introduced to construct an acetylcholinesterase (AChE) biosensor . AChE was entrapped in PILs microspheres through an emulsion polymerization reaction, where negatively charged Au nanoparticles (Au NPs) can be immobilized by the positively charged PILs, leading to improved catalytic performance. The results suggest that the positively charged PILs not only provide a biocompatible microenvironment around the enzyme molecule, stabilizing its biological activity and preventing its leakage, but also act as a modifiable interface allowing other components with electron transport properties to be loaded onto the polymer substrate, thus providing an efficient electron transport channel for the entrapped enzyme. More notably, when AChE was immobilized in a positively charged environment, the active site is closer to the electrode, promoting faster electron transfer. The detection limits of the constructed electrochemical biosensor AChE@PILs@Au NPs/GCE toward carbaryl and dichlorvos (DDVP) were 5.0 × 10^-2 ng ml^-1 and 3.9 × 10^-2 ng ml^-1, in a wide linear range of 6.3 × 10^-2-8.8 × 10² ng ml^-1 and 1.3 × 10^-1-1.4 × 10³ ng ml^-1, respectively. More importantly, the biosensor has high thermal and storage stability, which facilitates rapid field analysis of fruits and vegetables in a variety of climates. In addition, the biosensor reported has good repeatability and selectivity and has high accuracy in the analysis of peaches, tap water, and other types of samples.

RuO2/rGO heterostructures as mimic peroxidases for colorimetric detection of glucose

The successful synthesis of ruthenium oxide/reduced graphene oxide (RuO₂/rGO) heterostructures by one-pot hydrothermal method using graphene oxides and RuCl₃ as precursors is reported. The heterostructures had high peroxidase-like (POD-like) activities, which catalyzes the oxidation of classical peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to create a blue colored reaction product. The catalytic activity was significantly enhanced by the synergistic effect between RuO₂ nanoparticles and rGO. RuO₂/rGO had a low K_m of 0.068 mM and a high v_max of 1.228 × 10^-7 M·s^-1 towards TMB in the TMB-H₂O₂ catalytic oxidation system. In addition, the POD-like activity originating from the electron transfer mechanism was confirmed by cytochrome C (Cyt C) oxidation experiment. A colorimetric method based on RuO₂/rGO heterostructures was developed with good sensitivity and selectivity for glucose detection with a limit of detection of 3.34 μM and a linear range of 0-1500 μM. The RuO₂/rGO heterostructures have potential applications in the biomedical areas, such as biosensor and diagnostics.

Human serum albumin templated MnO2 nanosheets as an efficient biomimetic oxidase for biomolecule sensing

Herein, we have proposed a colorimetric biosensor for detection of acid phosphatase based on human serum albumin (HSA) templated MnO2 nanosheets (HSA-MnO2 NSs). HSA-MnO2 NSs as an efficient biomimetic oxidase could catalyze the oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) to the coloured oxidation product (oxTMB). Acid phosphatase (ACP) could hydrolyze l-ascorbic acid-2-phosphate (AAP) to produce ascorbic acid, and ascorbic acid could lead to the decomposition of MnO2 NSs to Mn2+ ions, inhibiting the production of oxTMB. On the basis of this, we have demonstrated a novel colorimetric approach for the detection of acid phosphatase with the linear range from 50 μU mL-1 to 1500 μU mL-1 and a detection limit of 40 μU mL-1. The MnO2 NS-based colorimetric method has been successfully used to determine the content of acid phosphatase in real samples with satisfactory results.

Meso-Cellular Silicate Foam-Modified Reduced Graphene Oxide with a Sandwich Structure for Enzymatic Immobilization and Bioelectrocatalysis

An integrated composite of meso-cellular silicate foam (MCF)-modified reduced graphene oxide (MCF@rGO) was designed and synthesized based on polyethylene oxide-polypropylene oxide-polyethylene oxide (P123)-modified rGO (P123-rGO). As the polymeric template for the fabrication of mesoporous silicates, modified P123 greatly improved the affinity between the nanosheet and the in situ formed MCFs, resulting in the formation of thin layers of MCFs on both sides of rGO. Therefore, the MCFs@rGO formed exhibited a unique sandwich structure with an inner skeleton of rGO and two outer layers of MCFs. The outer modification by MCFs, with the presence of large mesopores, not only shifted the surface property of rGO from hydrophobic to hydrophilic but also offered immobilized enzymes a favorable microenvironment to maintain their bioactivity. Meanwhile, the inner skeleton of rGO compensated for the weak conductivity of MCFs, providing a pathway for the direct electron transfer (DET) of various redox enzymes or proteins, such as hemoglobin (Hb), horseradish peroxidase, glucose oxidase (GOD), and cholesterol oxidase. It was found that the DET signal obtained from Hb-MCFs@rGO/glassy carbon electrode (GCE) was much larger than the sum of the signals from two components-based modified electrodes of Hb-P123-rGO/GCE and Hb-MCFs/GCE. A similar improvement in DET signal was also observed using GOD-MCFs@rGO/GCE. The significant enhancement of DET signals for both protein electrodes can be ascribed to the synergistic effects generated from the integration of the two components, one of which enhances biocompatibility and the other enhances conductivity. The bioelectrocatalytic performance of Hb and GOD electrodes was further investigated. As for Hb-MCFs@rGO/GCE, the GOD electrode displayed excellent analytical performance for the detection of hydrogen peroxide (H₂O₂), including a good sensitivity of 0.25 μA μmol^-1 L cm^-2, a low detection limit of 63.6 nmol L^-1 based on S/N = 3, and a low apparent Michaelis-Menten constant (K_M^app) of 49.05 μmol L^-1. GOD-MCFs@rGO/GCE also exhibited good analytical performance for the detection of glucose, with a wide linear range of 0.25-8.0 mmol L^-1. In addition, blood glucose detection in samples of human serum was successfully achieved using GOD-MCFs@rGO/GCE with a low quantification limit.

Bimetallic ZrHf-based metal-organic framework embedded with carbon dots: Ultra-sensitive platform for early diagnosis of HER2 and HER2-overexpressed living cancer cells

We report here a new bimetallic ZrHf metal-organic framework (ZrHf-MOF) embedded with abundant carbon dots (CDs) (denoted as CDs@ZrHf-MOF), which exhibits strong fluorescence and rich-amino-functionalization. The CDs@ZrHf-MOF can be applied as the scaffold for anchoring aptamer strands to determine human epidermal growth factor receptor-2 (HER2) and living HER2-overexpressed MCF-7 cells. The basic characterizations reveal that the CDs are embedded within the interior cavities of ZrHf-MOF without varying the nanostructure, leading to good biocompatibility, strong fluorescence, and high electrochemical activity of CDs@ZrHf-MOF. As compared with the pristine ZrHf-MOF, the CDs@ZrHf-MOF-based electrochemical aptasensor displays better sensing performances toward both HER-2 and MCF-7 cells, giving an extremely low detection limit of 19 fg mL^-1 (HER2 concentration range: 0.001-10 ng mL^-1) and 23 cell mL^-1 (cell concentration range: 1 × 10²~1 × 10⁵ cell mL^-1), with good selectivity, stability, reproducibility, and acceptable applicability. The proposed strategy for developing CDs@ZrHf-MOF-based aptasensor is promising for the early and sensitive detection of cancer markers and living cancer cells.

Novel approaches for biomolecule immobilization in microscale systems

This manuscript reviews novel approaches applied for biomolecule immobilization in microscale systems.

N-Carbamoylmaleimide-treated carbon dots: stabilizing the electrochemical intermediate and extending it for the ultrasensitive detection of organophosphate pesticides

To date, numerous methods have been reported for the detection of organophosphorus pesticides (OP) due to their severe potential hazard to the environment, public health and national security. However, very few works have ever found that the signal loss of thiocholine (TCh) during electrochemical processing is a key factor leading to the low sensitivity of acetylcholinesterase (AChE)-based OP electrochemical sensing platforms. Herein, we propose an ultrasensitive detection method for multiple OPs including parathion-methyl, paraoxon, dimethoate and O,O-dimethyl-O-2,2-dichlorovinyl-phosphate using N-carbamoylmaleimide-functionalized carbon dots (N-MAL-CDs) as a nano-stabilizer. For the first time, Michael addition is introduced into an AChE-based OP electrochemical sensing platform to enrich the electrochemical intermediate TCh. The Michael addition between TCh and N-MAL-CDs is demonstrated via XRD, FTIR, SEM and EDS elemental mapping experiments. Due to the stabilization and enhancement of TCh with N-MAL-CDs, the as prepared OP sensing platform achieves ultrahigh sensitivity by detecting the initial electrochemical signals of TCh without signal loss, showing a wide linear range of 3.8 × 10^-15-3.8 × 10^-10 M for parathion-methyl and 1.8 × 10^-14-3.6 × 10^-10 M for paraoxon, with a limit of detection of 1.4 × 10^-15 M for parathion-methyl and 4.8 × 10^-15 M for paraoxon.

Double-enhanced lateral flow immunoassay for potato virus X based on a combination of magnetic and gold nanoparticles

This study presents the joint use of magnetic nanoparticles (MNPs) and gold nanoparticles (GNPs) for double enhancement in a lateral flow immunoassay (LFIA). The study realizes two types of enhancement: (1) increasing the concentration of analytes in the samples using conjugates of MNPs with specific antibodies and (2) increasing the visibility of the label through MNP aggregation caused by GNPs. The proposed strategy was implemented using a LFIA for potato virus X (PVX), a significant potato pathogen. MNPs conjugated with biotinylated antibodies specific to PVX and GNPs conjugated with streptavidin were synthesized and characterized. The LFIAs with and without the proposed enhancements were compared. The double-enhanced LFIA achieved the highest sensitivity, equal to 0.25 ng mL^-1 and 32 times more sensitivity than the non-enhanced LFIA (detection limit: 8 ng mL^-1). LFIAs using one of the types of amplification (magnetic concentration without GNPs-causing aggregation or MNP aggregation without the concentration stage) showed intermediate levels of sensitivity. The double-enhanced LFIA was successfully used for PVX detection in potato leaves. The results for PVX detection in the infected plants were similar for the double-enhanced LFIA developed and the conventional LFIA based on the GNP conjugates; however, the new system provided significant coloring enhancement. This study confirmed that a simple combination of MNPs and GNPs has great potential for high-sensitivity detection and could possibly be adopted for LFIAs of other compounds.

Synthesis of a superparamagnetic ultrathin FeCO3 nanorods–enzyme bionanohybrid as a novel heterogeneous catalyst

A novel superparamagnetic ultrathin FeCO₃ nanorods–enzyme bionanohybrid heterogeneous catalyst has been developed.

Visual electrochemiluminescence biosensing of aflatoxin M1 based on luminol-functionalized, silver nanoparticle-decorated graphene oxide

A sensitive electrochemiluminescence (ECL) aptasensor for aflatoxin M1 (AFM1) detection by a closed bipolar electrode (BPE) array has been introduced. The thiolated AFM1 aptamer was immobilized on gold nanoparticle-coated magnetic Fe₃O₄ nanoparticles (Apt-GMNPs). Luminol-functionalized silver nanoparticle-decorated graphene oxide (GO-L-AgNPs) participates in π-π interactions with the unpaired bases of the immobilized aptamer (Apt-GMNPs-GO-L-AgNPs). After the Apt-GMNPs-GO-L-AgNPs were introduced to a gold anodic BPE array, the individual electrodes were subjected to different concentrations of AFM1. Upon the interaction of AFM1 with the aptamers, the GO-L-AgNPs detach from the aptamer; the resulting ECL of luminol and H₂O₂ at the anodic poles is monitored using a photomultiplier tube (PMT) or smartphone, and the images are analyzed using ImageJ software. This process triggers thionine reduction at the cathodic poles. Under the optimal conditions obtained by a face-centered central composite design (FCCD), the PMT-based detection of the BPE-ECL aptasensor exhibit a linear response over a wide dynamic range from 5 to 150ngmL^-1, with a detection limit of 0.01ngmL^-1. Additionally, smartphone-based detection shows a linear relationship between the ECL image gray value and the logarithmic concentration of the AFM1 target over a range of 10-200ngmL^-1, with a detection limit of 0.05ngmL^-1. Furthermore, the BPE-ECL aptasensor was successfully used to detect AFM1 in milk complex media without any serious interferences with reliable reproducibility (average relative standard deviation (RSD = 2.3%)). This smartphone-based detection opens a new horizon for bioanalysis that does not require a trained technician to operate and is a promising technology for point-of-care testing.

Graphene nanoribbon/FePt bimetallic nanoparticles/uric acid as a novel magnetic sensing layer of screen printed electrode for sensitive determination of ampyra

A novel electrochemical sensor for sensitive determination of ampyra (Am) based on graphene nanoribbons modified by iron-platinum bimetallic nanoparticles and uric acid (SPCE/FePtGNR/UA) dropped on the screen-printed carbon electrode (SPCE) surface and magnetically captured onto an SPCE working electrode surface is reported in the present work. The modified nanocomposite and sensing layer was characterized by different techniques, including cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray powdered diffraction (XRD). Am determination by conventional electrochemical methods is not possible, because of its high redox overpotential. Therefore, the differential pulse voltammetry (DPV) signals of UA were used as a redox probe for indirect electrochemical determination of Am. The limit of detection (LOD) and linear concentration range were obtained as 0.028 and 0.08-9.0µmolL^-1 (3S_b/m = 3), respectively. The feasibility of the proposed method was examined by the detection of Am in biological and pharmaceutical samples with satisfactory results. The constructed electrochemical sensor was applied for fast, simple and sensitive detection of Am in real environments.

Fabrication of a Novel Highly Sensitive and Selective Immunosensor for Botulinum Neurotoxin Serotype A Based on an Effective Platform of Electrosynthesized Gold Nanodendrites/Chitosan Nanoparticles

In this work, a novel nanocomposite consisting of electrosynthesized gold nanodendrites and chitosan nanoparticles (AuNDs/CSNPs) has been prepared to fabricate an impedimetric immunosensor based on a screen printed carbon electrode (SPCE) for the rapid and sensitive immunoassay of botulinum neurotoxin A (BoNT/A). BoNT/A polyclonal antibody was immobilized on the nanocomposite-modified SPCE for the signal amplification. The structure of the prepared nanocomposite was investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The charge transfer resistance (R_CT) changes were used to detect BoNT/A as the specific immuno-interactions at the immunosensor surface that efficiently limited the electron transfer of Fe(CN)₆^3−/4− as a redox probe at pH = 7.4. A linear relationship was observed between the %∆R_CT and the concentration logarithm of BoNT/A within the range of 0.2 to 230 pg·mL⁻¹ with a detection limit (S/N = 3) of 0.15 pg·mL⁻¹. The practical applicability of the proposed sensor was examined by evaluating the detection of BoNT/A in milk and serum samples with satisfactory recoveries. Therefore, the prepared immunosensor holds great promise for the fast, simple and sensitive detection of BoNT/A in various real samples.

An electrochemical sensor for the simultaneous determination of rifampicin and isoniazid using a C-dots@CuFe2O4 nanocomposite modified carbon paste electrode

In this study, a sensitive carbon paste electrode based on a novel nanocomposite of carbon dots/CuFe₂O₄ (C-dots@CuFe₂O₄) was developed for the simultaneous determination of rifampicin (RIF) and isoniazid (INZ).

Sensitive electrochemical determination of rifampicin using gold nanoparticles/poly-melamine nanocomposite

Green and facile method for fabrication of a conductive polymer–Au nanocomposite platform as a novel electrochemical sensing layer for rifampicin.