Portable Microelectrochemical Sensors for Rapid and Sensitive Determination of Hesperidin in Citrus reticulate 'Chachi' Peel.

Portable and low-cost analytical devices are essential for rapid detection of bioactive substrates in agricultural products. This study presents the first highly integrated microelectrochemical sensor based on pencil graphite for rapid and sensitive detection of hesperidin in Citrus reticulate 'Chachi' peel. The surface morphology and characterization as well as the electrochemical property of pencil graphite was investigated and discussed. A high electrocatalytic efficiency of hesperidin has been found at used pencil graphite-based microelectrodes. Kinetic analysis was carried out to further understand the electrochemical process of hesperidin at a pencil graphite microelectrode. Consequently, a portable and highly-integrated microelectrochemical sensor exhibits a sensitivity of 0.7251 µA cm-2 µM-1 and a detection limit as low as 25 nM (S/N = 3), and high selectivity was fabricated. Proposed microelectrochemical sensors were applied to electrochemically determinate the hesperidin content in the extract of Citrus reticulata "chachi" peel. As a result, the concentration of hesperidin in the actual real sample detected electrochemically with the proposed portable and low-cost microelectrochemical sensors is highly consistent to that obtained with a common chromatographic method, thus indicating the good reliability and that it can be used in practical applications.

Metaoptronic Multiplexed Interface for Probing Bioentity Behaviors.

Biointerface sensors have brought about remarkable advances in modern biomedicine. To accurately monitor bioentity's behaviors, biointerface sensors need to capture three main types of information, which are the electric, spectroscopic, and morphologic signals. Simultaneously obtaining these three types of information is of critical importance in the development of future biosensor, which is still not possible in the existing biosensors. Herein, by synergizing metamaterials, optical, and electronic sensing designs, we proposed the metaoptronic multiplexed interface (MMI) and built a MMI biosensor which can collectively record electric, spectroscopic, and morphologic information on bioentities. The MMI biosensor enables the real-time triple-monitoring of cellular dynamics and opens up the possibility for powerlessly monitoring ocular dryness. Our findings not only demonstrate an advanced multiplexed biointerface sensor with integrated capacities but also help to identify a uniquely significant arena for the nanomaterials, meta-optics, and nanotechnologies to play their roles in a complementary manner.

Tackling the Challenges of Enzymatic (Bio)Fuel Cells.

The ever-increasing demands for clean and sustainable energy sources combined with rapid advances in biointegrated portable or implantable electronic devices have stimulated intensive research activities in enzymatic (bio)fuel cells (EFCs). The use of renewable biocatalysts, the utilization of abundant green, safe, and high energy density fuels, together with the capability of working at modest and biocompatible conditions make EFCs promising as next generation alternative power sources. However, the main challenges (low energy density, relatively low power density, poor operational stability, and limited voltage output) hinder future applications of EFCs. This review aims at exploring the underlying mechanism of EFCs and providing possible practical strategies, methodologies and insights to tackle these issues. First, this review summarizes approaches in achieving high energy densities in EFCs, particularly, employing enzyme cascades for the deep/complete oxidation of fuels. Second, strategies for increasing power densities in EFCs, including increasing enzyme activities, facilitating electron transfers, employing nanomaterials, and designing more efficient enzyme-electrode interfaces, are described. The potential of EFCs/(super)capacitor combination is discussed. Third, the review evaluates a range of strategies for improving the stability of EFCs, including the use of different enzyme immobilization approaches, tuning enzyme properties, designing protective matrixes, and using microbial surface displaying enzymes. Fourth, approaches for the improvement of the cell voltage of EFCs are highlighted. Finally, future developments and a prospective on EFCs are envisioned.

Single Plasmonic Particle with Exposed Sensing Hot Spot for Exploring Gas Molecule Adsorption in Nanolocalized Space.

Single-particle (SP) sensing technology provides a methodology to explore the biochemical process in a micro/nanosize area (super-high resolution) with high sensitivity. Plasmonic nanoparticle is promising as a substrate for single-particle sensing. To realize specific sensing, a modification layer on the surface of the plasmonic nanoparticle is usually in need. However, a challenge stands in the way: the traditional coating of modification layer can deplete the highly enhanced electric field (EF) around the plasmonic particle and also, perhaps, hinder the analytes moving into the sensing hot spot with the most enhanced EF; thereby, the plasmonic particle cannot perform with super-high sensitivity. To solve this problem, we demonstrated an innovative single plasmonic particle sensing system in this work. In a convenient and controllable way, a single gold nanorod (AuNR) was successfully modified by monolayer WS2. There is an energy interaction between the AuNR and WS2, and thus, an exposed sensing hot spot with a nondepleted enhanced EF exists at the interface, which equips the as-prepared AuNR-WS2 SP with the ability to detect small changes in the local dielectric environment. Meanwhile, the monolayer WS2 also acted as a specific modification layer for detecting different analytes. We applied the AuNR-WS2 SP to explore the adsorption kinetics of different gas molecules, including ammonia, ethanol, and acetone for the first time. Through monitoring the scattering spectra under a microscope in dark-field, AuNR-WS2 SP could successfully differentiate the three small molecules, and help to explore the adsorption kinetics of them. Our experimental results were consistent with theoretical simulation in SP's EF distribution and its scattering spectra under different dielectric environments. Additionally, this proposed interaction-based modification strategy was also applied to other plasmonic nanoparticles, such as Au@Ag nanocube and Au nanodisk, suggesting the universality of this innovative SP sensing system.

Chemical elucidation and rheological properties of a pectic polysaccharide extracted from Citrus medica L. fruit residues by gradient ethanol precipitation.

Citron (Citrus. medica L.) fruits are commonly utilized in the production of essential oil, therefore, the fruits residues turn out to be industrial byproducts. In the present study, a crude polysaccharide was extracted from citron fruit residues by hot water extraction and precipitation of ethanol (95%), after deproteinization, a major polysaccharide component (CMLP-2) was obtained by gradient ethanol precipitation (20%-80%). The physicochemical properties of CMLP-2 such as surface morphology, functional groups, and thermostability were examined by FT-IR spectroscopy, SEM, and thermogravimetric analysis. Moreover, the chemical structure of CMLP-2 was elucidated that CMLP-2 is an acidic pectic polysaccharide consisting of arabinose (Ara), galacturonic acid (GalA), and rhamnose (Rha) in a molar ratio of 4:2:1 with a molecular weight of 202.18 kDa. CMLP-2 is a novel pectic polysaccharide rich in rhamnogalacturonan I (RG-I). Moreover, rheological tests revealed that CMLP-2 solution is pseudoplastic and temperature resistant. The result could be a good basis for the utilization of Citrus medica L. fruits residues as plant-derived food additive.

Comparative investigation of bioflavonoid electrocatalysis in 1D, 2D, and 3D carbon nanomaterials for simultaneous detection of naringin and hesperidin in fruits.

Electrocatalysis of bioflavonoids in carbon nanomaterials plays an important role in electrochemical sensors for the detection of their content in fruits. In this study, three types of carbon nanomaterials with 1D, 2D, and 3D structures, namely carbon nanotubes (CNTs), graphene oxide (GO), and Ketjen black (KB), were modified onto glassy carbon electrodes for the electrocatalysis of hesperidin and naringin, which are two important bioflavonoids in fruits. As a result, the CNT-modified electrodes showed the highest electrocatalytic activity for both hesperidin and naringin compared to GO and KB. The morphology and surface chemistry of the carbon nanomaterials were characterized. The structural defects and carbon status of carbon nanomaterials are proposed to be the most important factors affecting the electrocatalysis of hesperidin and naringin. Finally, a CNT-based electrochemical sensor was fabricated to simultaneously detect hesperidin and naringin. Real sample tests on the fruit extract of Citrus grandis "Tomentosa" show that the proposed electrochemical sensors with high recovery thus could be employed in practical applications.

In Situ Forming Epidermal Bioelectronics for Daily Monitoring and Comprehensive Exercise.

Conventional epidermal bioelectronics usually do not conform well with natural skin surfaces and are susceptible to motion artifact interference, due to incompatible dimensions, insufficient adhesion, imperfect compliance, and usually require complex manufacturing and high costs. We propose in situ forming hydrogel electrodes or electronics (ISF-HEs) that can establish highly conformal interfaces on curved biological surfaces without auxiliary adhesions. The ISF-HEs also have favorable flexibility and soft compliance comparable to human skin (≈0.02 kPa-1), which can stably maintain synchronous movements with deformed skins. Thus, the as-prepared ISF-HEs can accurately monitor large and tiny human motions with short response time (≈180 ms), good biocompatibility, and excellent performance. The as-obtained nongapped hydrogel electrode-skin interfaces achieve ultralow interfacial impedance (≈50 KΩ), nearly an order of magnitude lower than commercial Ag|AgCl electrodes as well as other reported dry and wet electrodes, regardless of the intrinsic micro-obstacles (wrinkles, hair) and skin deformation interference. Therefore, the ISF-HEs can collect high-quality electrocardiography and surface electromyography (sEMG) signals, with high signal-to-noise ratio (SNR ≈ 32.04 dB), reduced signal crosstalk, and minimized motion artifact interference. Simultaneously monitoring human motions and sEMG signals have also been implemented for the general exercise status assessment, such as the shooting competition in the Olympics. The as-prepared ISF-HEs can be considered as supplements/substitutes of conventional electrodes in percutaneously noninvasive monitoring of multifunctional physiological signals for health and exercise status.

Electrochemical Biosensor for Polycyclic Organic Compounds Screening Based on a Methylene Blue-incorporated DNA Polyion Complex Modified Electrode.

A reagent-less electrochemical DNA biosensor for rapid non-electroactive polycyclic organic compounds (POCs) screening and detection was proposed. In this method, methylene blue (MB) was incorporated into DNA/chitosan polyion complex membrane and then modified onto a glassy carbon electrode (GCE). The electrochemical analysis for the prepared DNA-MB/chitosan/GCE showed that the modified electrode exhibited high electrochemical activity and stability. The addition of tetracycline hydrochloride (TC), a model analyte of non-electroactive POCs, resulted in an obvious peak current decrease in DNA-MB/chitosan/GCE, and this electrochemical response was affected by the DNA type and MB/DNA ratio in the modified electrodes. Ultraviolet-visible (UV-Vis) absorption spectroscopy was utilized to furthermore investigate the interaction between TC and DNA-MB/chitosan/GCE. As a result, a competitive interaction and displacement effect between TC and the intercalated MB was proposed. In our condition, the prepared DNA-MB/chitosan/GCE showed high sensitivity to POCs and had almost no response to common interferences. Besides, the good stability and reproducibility of the prepared electrode made it suitable for practical use.

Carbon-nanotube-caged microbial electrodes for bioelectrocatalysis.

A method to stably immobilize microbes on electrodes was developed. Resting cells of Methylobacterium extorquens AM1(MeAM1) were caged within multiwalled carbon nanotubes (MWNTs)by adding the cells to a water dispersion of MWNTs then allowing the resulting mixture to dry on electrodes. The MeAM1-MWCNTs electrode thus obtained displayed excellent activities in the bidirectional bioelectrocatalysis due to formate dehydrogenase(s) in the resting cells; formate oxidation and carbon dioxide reduction proceeded at steady-state catalytic current densities of 0.6 ±â€¯0.1 and -0.8 ±â€¯0.1 mA cm-2, respectively, using methyl viologen as mediator under very mild conditions (pH 7.0, atmospheric pressure, and 37 °C). In addition, the catalytic signal was stable for more than one week under continuous operation.

Selected landscape phage probe as selective recognition interface for sensitive total prostate-specific antigen immunosensor.

The level of total prostate-specific antigen (t-PSA) is generally known as the key index of prostate cancer. Here, phage probes against t-PSA were selected from f8/8 landscape phage library. After three rounds of biopanning, four t-PSA-binding phage clones were isolated and identified by the DNA sequencing. Based on the phage capture assay, the phage clone displaying the fusion peptide ATRSANGM showed highest affinity and specificity against t-PSA. Subsequently, the t-PSA-specific phage was used as t-PSA capture probe in a sandwich enzyme-linked immunosorbent assay (ELISA) and differential pulse voltammetry (DPV) assay systems. Both assay methods showed high specificity and acceptable reliability for real serum samples analysis. By comparison, DPV method showed wider linear range (0.01-100 ng mL-1) and lower limit of detection (3 pg mL-1) than those (3.3-330 ng mL-1 and 1.6 ng mL-1) of ELISA. Moreover, DPV system showed smaller distinction to the authoritative method in real samples assay. Excitingly, the phage probe based DPV immunosensor showed high sensitivity for the detection of t-PSA and LOD achieved the pg mL-1 level, which was far lower than those values (usually above 0.1 ng mL-1) for reported immunosensors based on antibodies. Due to the biocompatibility, multivalency, stability, and high structural homogeneity, the t-PSA-specific landscape phage demonstrates as a novel specific interface in biosensors.

Direct Electron Transfer-type Bioelectrocatalysis of Peroxidase at Mesoporous Carbon Electrodes and Its Application for Glucose Determination Based on Bienzyme System.

Non-catalytic direct electron transfer (DET) signal of Compound I of horseradish peroxidase (POD) was first detected at 0.7 V on POD/carbon nanotube mixture-modified electrodes. Excellent performance of DET-type bioelectrocatalysis was achieved with POD immobilized with glutaraldehyde on Ketjen Black (KB)-modified electrodes for H2O2 reduction with an onset potential of 0.65 V (vs. Ag | AgCl | sat. KCl) without any electrode surface modification. The POD-immobilized KB electrode was found to be suitable for detecting H2O2 with a low detection limit (0.1 µM at S/N = 3) at -0.1 V. By co-immobilizing glucose oxidase (GOD) and POD on the KB-modified electrode, a bienzyme electrode was constructed to couple the oxidase reaction of GOD with the DET-type bioelectrocatalytic reduction of H2O2 by POD. The amperometric detection of glucose was performed with a high sensitivity (0.33 ± 0.01 µA cm-2 µM-1) and a low detection limit (2 µM at S/N = 3).

Factors affecting the interaction between carbon nanotubes and redox enzymes in direct electron transfer-type bioelectrocatalysis.

The effects of three types of water-soluble carbon nanotubes (CNTs) of different lengths on the direct electron transfer (DET)-type bioelectrocatalysis of redox enzymes were investigated. Bilirubin oxidase (BOD), copper efflux oxidase (CueO), and a membrane-bound NiFe hydrogenase (H2ase) were used as model redox enzymes for four-electron dioxygen (O2) reduction (in the case of BOD and CueO) and two-electron dihydrogen (H2) oxidation (in the case of H2ase). As a result, diffusion-controlled O2 reduction in an O2-saturated neutral buffer was realized by BOD on CNTs of a length of 1µm, but the catalytic current densities decreased as the length of CNTs increased. However, almost opposite trends were obtained when CueO and H2ase were utilized as the biocatalysts. Factors of the CNTs and the enzymes affecting the characteristics of the DET-type bioelectrocatalysis of the three enzymes were discussed. Finally, the electrostatic interaction between an enzyme (especially the portion near the redox active center) and CNTs is proposed as one of the most important factors governing the performance of DET-type bioelectrocatalysis.

Direct Electrochemistry and Electrocatalysis of Horseradish Peroxidase Immobilized in a DNA/Chitosan-Fe3O4 Magnetic Nanoparticle Bio-Complex Film.

A DNA/chitosan-Fe3O4 magnetic nanoparticle bio-complex film was constructed for the immobilization of horseradish peroxidase (HRP) on a glassy carbon electrode. HRP was simply mixed with DNA, chitosan and Fe3O4 nanoparticles, and then applied to the electrode surface to form an enzyme-incorporated polyion complex film. Scanning electron microscopy (SEM) was used to study the surface features of DNA/chitosan/Fe3O4/HRP layer. The results of electrochemical impedance spectroscopy (EIS) show that Fe3O4 and enzyme were successfully immobilized on the electrode surface by the DNA/chitosan bio-polyion complex membrane. Direct electron transfer (DET) and bioelectrocatalysis of HRP in the DNA/chitosan/Fe3O4 film were investigated by cyclic voltammetry (CV) and constant potential amperometry. The HRP-immobilized electrode was found to undergo DET and exhibited a fast electron transfer rate constant of 3.7 s-1. The CV results showed that the modified electrode gave rise to well-defined peaks in phosphate buffer, corresponding to the electrochemical redox reaction between HRP(Fe(III)) and HRP(Fe(II)). The obtained electrode also displayed an electrocatalytic reduction behavior towards H2O2. The resulting DNA/chitosan/Fe3O4/HRP/glassy carbon electrode (GCE) shows a high sensitivity (20.8 A·cm-2·M-1) toward H2O2. A linear response to H2O2 measurement was obtained over the range from 2 µM to 100 µM (R² = 0.99) and an amperometric detection limit of 1 µM (S/N = 3). The apparent Michaelis-Menten constant of HRP immobilized on the electrode was 0.28 mM. Furthermore, the electrode exhibits both good operational stability and storage stability.