Fabricating process-electrochemical property correlation of laser-scribed graphene and smartphone-based electrochemical platform for portable and sensitive biosensing

Laser-scribed graphene (LSG), a promising electrode material has attracted special research interest in recent years. Here, the fabricating process-electrochemical property correlation of laser-scribed graphene (LSG) devices was discussed emphatically and a pertinent optimization was performed to achieve better electroanalytical performance. Experiment results indicated that the laser scribing technique possessed great process latitude and reducing laser power and scribing speed facilitated fabricating high-quality graphene electrodes. Benefiting from its binder-free 3D porous network structure and high active/geometric area ratio, the optimized LSG electrode was superior to the screen-printed carbon electrode (SPCE) on electrochemical performance in the [Fe(CN)6]3-/4- redox system. Integrating the LSG electrode with a homemade hand-held detector, a portable electrochemical sensing platform with smartphone readout was developed. It realized a specific detection of H2O2 (linear range: 0.02-3.4 mM, sensitivity: 24.56 μA mM-1 cm-2), glucose (linear range: 0.04-4.0 mM, sensitivity: 16.35 μA mM-1 cm-2) by directly decorating biological enzymes without artificial redox mediator and featured a satisfactory comprehensive performance. The constructed immunosensor for tumor necrosis factor-α exhibited a wide linear range (2-500 pg mL-1) and a 4.3-fold enhancement in sensitivity compared with that of SPCE. With satisfactory selectivity, reproducibility, and sensitivity, the developed smartphone-based electrochemical sensing platform held great promise in accurate detection on the spot. This work also provided a significant reference for tailoring binder-free carbonaceous electrode materials toward the desired application.

A ratiometric electrochemical strategy based on Fe (III) and Pt (IV) for immobilization-free detection of Escherichia coli

A new ratiometric electrochemical strategy for immobilization-free detection of Escherichia coli (E. coli) was constructed by using a capture DNA-polyaniline/copper ferrite nanoparticles/graphene oxide (cDNA-PANI/CuFe₂O₄/GO) composite as capture probes, which has a high specific surface area and good magnetic properties. Then trigger DNA/Au nanoparticles (tDNA/Au NPs) were used as signal amplification labels, and Pt (IV) and Fe (III) were chosen as the signal probes. In the presence of targets, the sandwich format among cDNA-PANI/CuFe₂O₄/GO, E. coli and auxiliary DNA (aDNA) was realized by using the aptamer recognition system. Then, the tDNA/Au binding could be anchored on the sandwich format due to the principle of base complementation between unpaired aDNA and tDNA. And the unbounded tDNA of tDNA/Au NPs could bind an amount of Pt (IV). After separation using a magnet, a handful of unbound Pt (IV) which remained in the supernatant reacted with a large number of Fe (III) ions, leading to a markedly increased I_Fe(III)/I_Pt(IV) value. Oppositely, the sandwich format could not appear in the absence of targets, and even the tDNA/Au could not be immobilized on it. So, the redox reaction between a large amount of Pt (IV) residue in the supernatant and Fe (III) was significantly successful, causing a low I_Fe(III)/I_Pt(IV) value. Under optimal conditions, we found that I_Fe(III)/I_Pt(IV) was linearly related to the logarithmic E. coli concentration with a low limit of detection (1.862 × 10³ cfu mL^-1). This devised ratiometric electrochemical method may develop into a powerful and effective means for the detection of E. coli in real samples, which may also be developed as a universal tool for another microorganism.

A ratiometric electrochemical biosensor via alkaline phosphatase mediated dissolution of nano-MnO2 and Ru(III) redox recycling for the determination of dimethoate

A sensitive and ratiometric electrochemical biosensor was developed for the determination of dimethoate via alkaline phosphatase (ALP) mediated dissolution of nano-MnO₂ and [Ru(NH₃)₆]³⁺(Ru(III)) redox recycling. The electroactive probe Ru(III) was adsorbed on the nano-MnO₂ with the high specific surface area through electrostatic interaction to form the MnO₂-Ru(III) nanocomposite, which was then fixed on the surface of the glassy carbon electrode. When the dimethoate inhibited the catalytic activity of ALP in a homogeneous system, the hydrolysate L-ascorbic acid (AA) produced by ALP hydrolysis of L-ascorbic acid-trisodium 2-phosphate (AAP) decreased. The solution was then incubated with a glassy carbon electrode modified by MnO₂-Ru(III). At this time, only a small amount of MnO₂-Ru(III) was decomposed and Ru(III) was rapidly electroreduced to Ru(II) on the surface of the electrode. The in-situ produced Ru(II) was chemically oxidized back to Ru(III) by Fe(III). The redox recycling of Ru(III) was completed and the Ru(III) reduction current signal was amplified. The process consumed part of Fe(III) to reduce the reduction current signal of Fe(III), and the ratio of the two reduction currents (I_Ru(III)/I_Fe(III)) increased significantly. The I_Ru(III)/I_Fe(III) value increased with the increase of dimethoate concentration in the linear range of 0.01-300 ng mL^-1, and the detection limit was 6.3 pg mL^-1. It has been successfully applied to the determination of dimethoate in oilseed rape and lettuce with a satisfactory result.

An eNose-based method performing drift correction for online VOC detection under dry and humid conditions

Our recently demonstrated innovative concept of electronic nose (eNose) based on a combination of gas sensors is further tested and benchmarked in the present study. The system is a test bed for gas sensors of different principal technologies distributed within three compartments, which share a compact, very low volume, temperature-controlled sensing chamber. Here, the eNose-based analyser contains three sensing arrays of commercially available semiconducting metal oxide (MOX) gas sensors: one compartment contains 8 analog MOX sensors, while the other two compartments comprise 10 digital MOX sensors. The presented instrument is explicitly tested for the discrimination between mid-range (3-18 ppm) concentrations of different volatile organic compounds (VOCs) including acetaldehyde, acetone, ethanol, ethyl acetate, isoprene and n-pentane under dry and humid conditions, which are all considered relevant gases in future breath diagnostic applications. Since the experiments were performed in periods of time separated by around 20 days, they are affected by drift. For this reason, we explore the opportunity of drift mitigation using methods based on component removal computed by linear discriminant analysis, partial least squares discriminant analysis and direct orthogonalization, which lend themselves to future in-field applications of the developed device and sensing methodology.

Platinum Nanozyme-Triggered Pressure-Based Immunoassay Using a Three-Dimensional Polypyrrole Foam-Based Flexible Pressure Sensor

This work describes a novel and portable pressure-based point-of-care (POC) testing strategy for the sensitive and rapid detection of carcinoembryonic antigen (CEA) via a flexible pressure sensor constructed by three-dimensional (3D) polypyrrole (PPy) foam. Initially, platinum nanoparticles (PtNPs) were conjugated to the detection antibodies, which were used to form sandwich-type immunocomplexes with targets and capture antibodies in the reaction cell. Then, the carried PtNPs catalyzed the dissociation of hydrogen peroxide (H₂O₂) for the generation of oxygen (O₂) in a sealed device, translating the biomolecule recognition event into gas pressure. With the increase of pressure, a flexible pressure sensor with 3D polypyrrole foam as the sensing layer was used to sensitively monitor the pressure variations in this system. Thus, the concentration of the target could be quantitatively determined by the pressure response. Under optimal conditions, the pressure-based immunosensor showed good sensing performance for CEA in the dynamic working range from 0.2 to 60 ng/mL with a detection limit of 0.13 ng/mL. The reproducibility, specificity, and accuracy compared with commercial enzyme-linked immunosorbent assay (ELISA) kit were also acceptable. Therefore, this work provides a promising approach for developing portable and sensitive POC testing in the future.

Ultrasensitive split-type electrochemical sensing platform for sensitive determination of organophosphorus pesticides based on MnO2 nanoflower-electron mediator as a signal transduction system

Organophosphorus pesticides (OPs) are extensively used worldwide as agrochemicals; however, excess use may threaten the health of humans. Thus, it is an urgent need to develop a sensitive method for determination of OPs. Herein, a simple and sensitive split-type electrochemical method was developed by using MnO₂ nanoflower-electron mediator as a signal transduction element. The MnO₂ nanoflower-electron mediator was synthesized and shows an excellent electrochemical signal attributed to the high specific surface area of MnO₂ nanoflower. Meanwhile, the inhibition of OPs on butyrylcholinesterase (BChE) was carried out in the homogeneous system. In the absence of target molecule, a large number of thiocholines (TCh) were yielded from hydrolysis of acetylthiocholine (ATCh) by BChE. The MnO₂ nanoflower was cracked, and subsequently, multiple electron mediator molecules were released from the platform after treated with TCh, thus decreasing the electrochemical response. Furthermore, the inhibition of OPs on BChE resulted in the reduced generation of TCh, thus inducing the recovery of electrochemical signal. Under the optimal experimental, dichlorvos can be detected in a wide range of 10^-6-10^-10 M, with a detection limit of 3 × 10^-10 M. Moreover, the assay was successfully used to analyze dichlorvos in cucumber juice and pear juice, showing a great promising potential for detecting organophosphorus pesticides in complex samples. Graphical abstract In this assay, a split-type electrochemical biosensor was proposed for the ultrasensitive determination of organophosphorus pesticides based on the MnO₂ nanoflower-electron mediator as an electrochemical signal component.

Carbon black as an outstanding and affordable nanomaterial for electrochemical (bio)sensor design

Advances in cutting-edge technologies including nanotechnology, microfluidics, electronic engineering, and material science have boosted a new era in the design of robust and sensitive biosensors. In recent years, carbon black has been re-discovered in the design of electrochemical (bio)sensors thanks to its interesting electroanalytical properties, absence of treatment requirement, cost-effectiveness (c.a. 1 €/Kg), and easiness in the preparation of stable dispersions. Herein, we present an overview of the literature on carbon black-based electrochemical (bio)sensors, highlighting current trends and possible challenges to this rapidly developing area, with a special focus on the fabrication of carbon black-based electrodes in the realisation of sensors and biosensors (e.g. enzymatic, immunosensors, and DNA-based).

Recent developments of nanoenzyme-based colorimetric sensors for heavy metal detection and the interaction mechanism

This work highlights the application and interaction mechanism of metal nanoparticles, metal oxides, metal sulfides, graphene-based nanomaterials and G-quadruplex, etc. in nanoenzyme-based colorimetric sensors.

Ultrafine nanoparticles of W-doped SnO2 for durable H2S sensors with fast response and recovery

Ultrafine nanoparticles of W-doped SnO2 with an average diameter of 6 nm were fabricated via a facile hydrothermal method. The material shows a reduced particle size and enhanced response to H2S gas as compared to the pristine SnO2 nanoparticles. The detection limit can be down to 100 ppb while the response time and recovery time of the 5%-doped one are reduced to 17 s and 7 s respectively. In addition, the material shows impressive long-term stability of the response through 40 cycles of injection with 10 ppm H2S, which is attractive for designing a durable hydrogen sulfide sensor. The doping of W results in the reduction of size and modification of the electronic band structure of SnO2, which reduces the response time and recovery time and further improves the sensing durability of the materials.

Phytic acid-based copper(ii) ion-selective electrode on graphene oxide for potentiometric immunoassay of breast cancer antigen 15-3

A potentiometric immunoassay based on a copper(ii) ion-selective electrode (Cu-ISE) was designed for the quantitative determination of carbohydrate antigen 15-3 (CA 15-3; a tumor marker associated with breast cancer).