3D hierarchical hollow hydrangea-like Fe3+@ɛ-MnO2 microspheres with excellent electrochemical performance for dopamine and hydrogen peroxide

A novel electrochemical aptasensor based on AgPdNPs/PEI-GO and hollow nanobox-like Pt@Ni-CoHNBs for procymidone detection

Herein, an aptasensor based on a signal amplification strategy was developed for the sensitive detection of procymidone (PCM). AgPd nanoparticles/Polenimine Graphite oxide (AgPdNPs/PEI-GO) was weaned as electrode modification material to facilitate electron transport and increase the active sites on the electrode surface. Besides, Pt@Ni-Co nanoboxes (Pt@Ni-CoHNBs) were utilized to be carriers for signaling tags, after hollowing ZIF-67 and growing Pt, the resulting Pt@Ni-CoHNBs has a tremendous amounts of folds occurred on the surface, enables it to carry a larger quantity of thionine, thus amplify the detectable electrochemical signal. In the presence of PCM, the binding of PCM to the signal probe would trigger a change in electrical signal. The aptasensor was demonstrated with excellent sensitivity and a low detection limit of 0.98 pg·mL-1, along with a wide linear range of 1 μg·mL-1 to 1 pg·mL-1. Meanwhile, the specificity, stability and reproducibility of the constructed aptasensor were proved to be satisfactory.

One-pot synthesis and hydrogen peroxide electrochemical sensing of 3D TiO2/MnO2 nanorods assembled microspheres

Nanorods assembled 3D microspheres of TiO2/MnO2 were prepared via a simple one-pot hydrothermal approach. The resultant composite material exhibited remarkable electrocatalytic activity for hydrogen peroxide (H2O2) in comparison to each single component. The electrochemical sensor constructed with TiO2/MnO2 exhibited a linear relationship within the range 0.0001-5.6 mmol·L-1 for H2O2. The limit of detection (LOD) and sensitivity for H2O2 were 0.03 µmol·L-1 (S/N = 3) and 316.6 µA (mmol·L-1)-1 cm-2. Moreover, this sensor can be employed to detect trace amount of H2O2 in serum and urine samples successfully, supporting an insight and strategy for a more sensitive electrochemical sensor.

Alkaline liquid-derived NaxTi11.5MoVOx/C-40 material with controlled electron transfer rate for sensitive electrochemical detection of dopamine

The neurotransmitter dopamine (DA) is associated with many physiological and pathological processes, so the importance of low detection limits and high sensitivity analysis cannot be overstated, especially for early disease detection. Here, 2 M NaOH aqueous solution is used to precipitate metal ions in an ethanol solution containing carbon black (CB), and then nanocomposite catalysts (NaxTi11.5MoVOx/C-40 (40 denoted as 40 mg CB)) were obtained by calcining the precipitation. When used for DA detection, NaxVOx acts as the main active site for electrochemical oxidation of DA and NaxTi11.5MoOx plays a role in facilitating the binding of DA to the active site and stabilizing the active site. The NaxTi11.5MoVOx/C-40 electrochemical biosensor has a limit of detection (LOD) of 0.003 μM with a linear range of 0.005-51.665 μM for DA. This sensor can be used to sensitively identify the concentration of DA in human blood and urine. Catalysts containing varying amounts of CB exhibit diverse electron transfer rates, and surprisingly, we found that the appropriate electron transfer rate is optimal for the detection of low concentrations of DA. Because the performance of the electrochemical biosensors is affected by both the activity of the catalysts and the accuracy of the electrochemical testing instrumentation. To better explain this phenomenon, we propose the concept of resolution (Rn) and present the formula to derive it, offering a new approach to evaluating the performance of electrochemical biosensors.

A Review on Recent Trends and Future Developments in Electrochemical Sensing

Electrochemical methods and devices have ignited prodigious interest for sensing and monitoring. The greatest challenge for science is far from meeting the expectations of consumers. Electrodes made of two-dimensional (2D) materials such as graphene, metal-organic frameworks, MXene, and transition metal dichalcogenides as well as alternative electrochemical sensing methods offer potential to improve selectivity, sensitivity, detection limit, and response time. Moreover, these advancements have accelerated the development of wearable and point-of-care electrochemical sensors, opening new possibilities and pathways for their applications. This Review presents a critical discussion of the recent developments and trends in electrochemical sensing.

Ultrasensitive electrochemical sensing of catechol and hydroquinone via single-atom nanozyme anchored on MOF-derived porous carbon

Single-atom nanozymes (SANs) can significantly enhance the sensitivity and selectivity of electrochemical sensing platforms due to the homogeneity of their active sites, full atom utilization, and high catalytic activity. In this study, we demonstrate the synthesis and characterization of a high-density Co-based single-atom nanozyme anchored on activated MOF-derived porous carbon (Co-AcNC-3) via a cascade anchoring strategy for ultrasensitive, simultaneous electrochemical detection of catechol (CC) and hydroquinone (HQ). The Co-AcNC-3 displays a large specific surface area, high defectivity, and abundant oxygen-containing groups, with Co atoms being atomically dispersed throughout the carbon support via Co-N bonds. The Co-AcNC-3 biosensor exhibits superior electrochemical signals for CC and HQ, with linear ranges of 4.0 μM-300.0 μM. and detection limits of 0.072 μM and 0.034 μM, respectively. Moreover, the Co-AcNC-3 biosensor has shown excellent performance in accurately detecting CC and HQ in actual samples. Our findings highlight the potential of the proposed Co-AcNC-3 biosensor as a reliable and promising sensing platform for determining CC and HQ.

Hybridizing Ti3C2Tx Layers with Layered Double Hydroxide Nanosheets at the Molecular Level: A Smart Electrode Material for H2O2 Monitoring in Cancer Cells

Vertically stacked artificial 2D superlattice hybrids fabricated through molecular-level hybridization in a controlled fashion play a vital role in scientific and technological fields, but developing an alternate assembly of 2D atomic layers with strong electrostatic interactions could be much more challenging. In this study, we have constructed an alternately stacked self-assembled superlattice composite through integration of CuMgAl layered double hydroxide (LDH) nanosheets having positive charge with negatively charged Ti₃C₂T_x layers using well-controlled liquid-phase co-feeding protocol and electrostatic attraction and investigated its electrochemical performance in sensing early cancer biomarkers, i.e., hydrogen peroxide (H₂O₂). The molecular-level CuMgAl LDH/Ti₃C₂T_x superlattice self-assembly possesses superb conductivity and electrocatalytic properties, which are significant for obtaining a high electrochemical sensing aptitude. Electron penetration in Ti₃C₂T_x layers and rapid ion diffusion along 2D galleries have shortened the diffusion path and enhanced the charge transferring efficacy. The electrode modified with the CuMgAl LDH/Ti₃C₂T_x superlattice has demonstrated admirable electrocatalytic abilities in H₂O₂ detection with a wide linear concentration range and low real-time limit of detection (LOD) of 0.1 nM with signal/noise ratio (S/N) = 3. Practically, an electrochemical sensing podium based on the CuMgAl LDH/Ti₃C₂T_x superlattice has been effectively applied in real-time in vitro tracking of H₂O₂ effluxes excreted from different live cancer cells and normal cells after being encouraged by stimulation. The results exhibit that molecular-level heteroassembly holds great potential in electrochemical sensors to detect promising biomarkers.

Nonenzymatic Flexible Wearable Biosensors for Vitamin C Monitoring in Sweat

Nutritional status monitoring plays an important role in the maintenance of human health and disease prevention. Monitoring the intake of vitamins can support the improvement of diet behavior. In this work, a polyaniline (PANI) film-based nonenzymatic electrochemical sensor was prepared to track the vitamin C level in sweat. The PANI film was modified with organic acids (ethylformic acid, malic acid, tartaric acid, and phytic acid). The phytic acid-modified PANI film based on sensor has a wide detection range (0.5-500 μmol·L^-1), high sensitivity (665.5 and 326.2 μA·(mmol·L^-1)^-1·cm^-2), and low detection limit (0.17 μmol·L^-1) toward vitamin C in sweat. The phytate enhances the band transport between PANI chains, which increases the electrical conductivity of the film to improve the electrochemical properties of the sensor. In addition, we monitored changes of vitamin C levels in human body after taking vitamin C pills by detecting sweat or saliva. The ability to track the pharmacological profile demonstrates the potential of PANI film-based sensors for applications in personalized nutritional intake and tracking. And a simple and portable vitamin C detection system was developed to improve the practicability of the sensor. This work provides an idea for the application of wearable electrochemical sensing devices in nutrition guidance.

An Au/SnO-SnO2 nanosheet based composite used for rapid detection of hydrogen sulphide

In this work, a new type of H₂S sensor was fabricated by means of drop-coating of an Au/SnO-SnO₂ nanosheet material, which was prepared by a one-pot hydrothermal reaction, onto a gold electrode in an alumina ceramic tube with the formation of a thin nanocomposite film. The microstructure and morphology of the nanosheet composites were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A gas-sensitivity study presented good H₂S-sensing performance of such Au/SnO-SnO₂ nanosheet composites. At an optimal operating temperature of 240 °C and ambient temperature of 25 °C, the resulting sensor showed a good linear response to H₂S in a range of 1.0 to 100 ppm with a low detection limit of 0.7 ppm, and a very fast response-recovery time of 22 s for response and 63 s for recovery, respectively. The sensor was also unaffected by ambient humidity and had good reproducibility and selectivity. When being applied to the monitoring of H₂S in an atmospheric environment in a pig farm, the response signal to H₂S was only attenuated by 4.69% within 90 days, proving that the sensor had a long and stable service lifetime for continuous running and showing its important practical application prospects.

Facile preparation of MnO2-TiO2 nanotube arrays composite electrode for electrochemical detection of hydrogen peroxide

The MnO₂-TNTA composite electrodes were obtained through depositing MnO₂ into TiO₂ nanotube arrays (TNTA) by successive ionic layer adsorption reaction (SILAR) and subsequent hydrothermal method. The MnO₂-TNTA nanocomposites were used as electrochemical sensors for the detection of hydrogen peroxide (H₂O₂). The preparation conditions of MnO₂-TNTA electrodes and test conditions affect the electrochemical detection performance significantly. The optimal conditions are listed as follows: the number of SILAR cycles, 6 times; KMnO₄ solution temperature, 50 °C; supporting electrolyte, 0.5 M NaOH. Under these conditions, the MnO₂-TNTA electrode exhibits the best performance for detecting H₂O₂. The optimized MnO₂-TNTA electrode has a minimum detection limit of 0.6 μM (S/N = 3) and a linear range of 5 μM ∼ 13 mM, which is much superior to the previously-reported electrodes. Moreover, the optimized MnO₂-TNTA electrode possesses high selectivity, excellent stability and good reproducibility in the detection of H₂O₂. When used in the determination of H₂O₂ content in actual samples including disinfectant and milk, it also shows good accuracy, ideal recovery (96.00% ∼ 102.67%) and high precision (RSD < 4.0%).

Non-enzymatic nitrite amperometric sensor fabricated with near-spherical ZnO nanomaterial

A unique near-spherical ZnO nanostructure was synthesized by using mixed solvents composed of polyethylene glycol-400 (PEG-400) and water at the volume ratio of 12:1 via the solvo-thermal method, and it possessed an ideal morphology with higher uniformity, better dispersion and small particle size. Such ZnO was employed to modify glass carbon electrode (GCE) for the construction of electrochemical sensor, i.e. near-spherical ZnO/GCE, whose nitrite sensing performance was evaluated by Chronoamperometry (CA) and Linear Sweep Voltammetry (LSV). In order to emphasis the superior sensing property and extensive suitability for different electrochemical detection techniques, the excellent but not the same nitrite detection performance obtained from CA and LSV was individually given in detail. This sensor based on CA showed broad linearity range of 0.6 μM-0.22 mM and 0.46 mM-5.5 mM, improved sensitivity of 0.785 μA μM^-1 cm^-2 accompanied with low LOD of 0.39 μM. With regard to LSV, wide linearity response of 1.9 μM-0.8 mM and 1.08 mM-5.9 mM, high sensitivity of 0.646 μA μM^-1 cm^-2 with LOD of 0.89 μM were obtained. Meanwhile, this sensor displayed outstanding repeatability with RSD of 2.96% (n = 4), high reproducibility with low RSD (1.72%-2.35%, n = 4), strong selectivity towards nitrite with the concentration set at one-tenth of the interfering substances, ideal stability with the peak current intensity above 90% of its initial value after storage for one month and acceptable recovery of 1.72-2.35% to actual samples including ham sausage, pickle and tap water. The near-spherical ZnO nanomaterial may be a preferred candidate for the fabrication of nitrite electrochemical sensor, which may exhibit a fascinating application in terms of food analysis and environmental monitoring.

Monodispersed Ni active sites anchored on N-doped porous carbon nanosheets as high-efficiency electrocatalyst for hydrogen peroxide sensing

Metal active species combined with N-doped porous carbon nanosheets usually own excellent electrochemical activity and sensing performance owing to its unique microstructure and composition. In this work, monodispersed Ni active sites anchored on N-doped porous carbon nanosheets (Ni@N-PCN) were facilely prepared via rational metal-organic frameworks (MOFs) route. Firstly, zeolitic imidazolate frameworks-8 (ZIF-8) was in situ grown on physically-exfoliated graphene nanosheets (GN) with homogeneous sandwich-like structure (ZIF-8@GN). Secondly, nickel bonded ZIF-8@GN hybrids (Ni/ZIF-8@GN) were obtained by ionic exchange reaction, and then transformed into Ni@N-PCN by high-temperature pyrolysis. Benefiting from the monodispersed Ni active sites and highly reactive N-doped porous carbon nanosheets (N-PCN), the as-prepared Ni@N-PCN hybrids displayed superior catalytic performance toward hydrogen peroxide (H₂O₂) sensing. As a result, a highly sensitive electrochemical sensing platform for H₂O₂ was fabricated with low detection limit (0.032 μM), wide detection linearity (0.2-2332.8 μM), and high sensitivity (6085 μA cm^-2 mM^-1). Besides, the as-developed electrochemical sensing platform was successfully applied to detect H₂O₂ contents in biological medicine and food specimens with satisfied results. This study will provide effective guidance for the preparation of novel metal/N-doped carbon nanomaterials and establishment of high-performance electrochemical sensors.

Exonuclease III-Driven Dual-Amplified Electrochemical Aptasensor Based on PDDA-Gr/PtPd@Ni-Co Hollow Nanoboxes for Chloramphenicol Detection

Herein, a hierarchically porous Zr-MOF-labeled electrochemical aptasensor based on the composite of PtPd@Ni-Co hollow nanoboxes (PtPd@Ni-Co HNBs) and poly (diallyldimethylammonium chloride)-functionalized graphene (PDDA-Gr) was developed for ultrasensitive detection of chloramphenicol (CAP). PtPd@Ni-Co HNBs have excellent conductivity and provide binding sites for aptamers; the functionalized PDDA-Gr improves its dispersibility and conductivity as a substrate material, which can be successfully used to increase the electrode surface area and support more PtPd@Ni-CoHNBs. Besides, hierarchically porous Zr-MOFs (HP-UiO-66) were utilized as signal probes and showed a stronger load capacity for signal molecules than conventional UiO-66. In the presence of CAP, two ingeniously designed Exo III-assisted cyclic amplification strategies further improved the sensitivity of the aptasensor: CAP causes cycle I to release a large amount of trigger DNA (Tr DNA), and then, Tr DNA initiated cycle II, which causes the exposed capture DNA to further bind the signal probes. With these advantages, the constructed aptasensors performed with satisfactory sensitivity in a wide linear range (10 fM-10 nM) and a detection limit of 0.985 fM. Several signal amplification strategies adopted in this study have effectively improved the performance of the sensor, providing a new avenue for the development of ultrasensitive sensors in the food analysis field.

Molybdenum-containing polypyrrole self-supporting hollow flexible electrode for hydrogen peroxide detection in living cells

A flexible electrode based on polypyrrole-supported free-standing molybdenum oxide-molybdenum disulfide/polypyrrole nanostructure (MoO₃-MoS₂/PPy) was synthesized. The petal-like MoO₃-MoS₂ sheets grown on PPy were prepared step by step through simple electrodeposition and hydrothermal methods. The corresponding surface morphological and structural characterizations were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results showed that the prepared petal MoO₃-MoS₂ hybrid nanomaterials were uniformly distributed on the PPy skeleton and exhibited a three-dimensional porous network structure. The flexible electrode was used for non-enzymatic detection of hydrogen peroxide (H₂O₂), and the developed MoO₃-MoS₂/PPy nanomaterials exhibited high electrochemical sensing performance in the range of 0.3-150 μM, with the detection limit of 0.18 μM (S/N = 3). The excellent detection properties enabled the MoO₃-MoS₂/PPy flexible electrode to detect H₂O₂ released by living cells. The resulting MoO₃-MoS₂/PPy flexible electrode also has the advantages of customizable shape and adjustability, which provides a potential platform for constructing clinically diagnosed in vivo portable instruments and real-time environmental monitoring.

Polydopamine/graphene/MnO2 composite-based electrochemical sensor for in situ determination of free tryptophan in plants

The in vivo detection of small active molecules in plant tissues is essential for the development of precision agriculture. Tryptophan (Trp) is an important precursor material for auxin biosynthesis in plants, and the detection of Trp levels in plants is critical for regulating the plant growth process. In this study, an electrochemical plant sensor was fabricated by electrochemically depositing a polydopamine (PDA)/reduced graphene oxide (RGO)-MnO₂ nanocomposite onto a glassy carbon electrode (GCE). PDA/RGO-MnO₂/GCE exhibited high electrocatalytic activity for the oxidation of Trp owing to the combined selectivity of PDA and catalytic activity of RGO-MnO₂. To address the pH variability of plants, a reliable Trp detection program was proposed for selecting an appropriate quantitative detection model for the pH of the plant or plant tissue of interest. Therefore, a series of linear regression curves was constructed in the pH range of 4.0-7.0 using the PDA/RGO-MnO₂/GCE-based sensor. In this pH range, the linear detection range of Trp was 1-300 μM, the sensitivity was 0.39-1.66 μA μM^-1, and the detection limit was 0.22-0.39 μM. Moreover, the practical applicability of the PDA/RGO-MnO₂/GCE-based sensor was successfully demonstrated by determining Trp in tomato fruit and juice. This sensor stably and reliably detected Trp levels in tomatoes in vitro and in vivo, demonstrating the feasibility of this research strategy for the development of electrochemical sensors for measurements in various plant tissues.

N-doped Cu-MOFs for efficient electrochemical determination of dopamine and sulfanilamide

Fast and efficient tracking of micropollutants in aquatic environment by developing novel electrode materials is of great significance. Herein, a polyvinylpyrrolidone (PVP) assisted strategy is applied for synthesis of nitrogen doped Cu MOFs (N-Cu-MOF) for micropollutants electrochemical detection. The designed N-Cu-MOFs possess uniform octahedral shape with large surface area (1184 m² g^-1) and an average size of roughly 450 nm, exhibiting the excellent electroanalytical capability for the detection of multipollutants. In the case of dopamine (DA) and sulfonamides (SA) as typical microcontaminants, the designed N-Cu-MOFs exhibited wide linear ranges of 0.50 nM-1.78 mM and low detection limit (LOD, 0.15 nM, S/N = 3) for the determination of DA, as well as a linear range of 0.01-58.3 μM and LOD (0.003 μM, S/N = 3) for monitoring SA. The improved performance is attributed to the heteroatom introduction and good dispersion stability of N-Cu-MOF with PVP-decorated. The good electroanalytical ability of N-Cu-MOF for detection of DA and SA can provide a guide to efficient and rapid monitor other micropollutants and construct novel electrochemical sensors.

FeMn layered double hydroxides: an efficient bifunctional electrocatalyst for real-time tracking of cysteine in whole blood and dopamine in biological samples

A peculiar clock-regulated design of FeMn-LDHs (FMH) with specific physiochemical attributes has been developed and used for highly sensitive detection of cysteine (CySH) and dopamine (DA).