Three-Mediator Enhanced Collisions on an Ultramicroelectrode for Selective Identification of Single Saccharomyces cerevisiae.

Selective detection of colliding entities, especially cells and microbes, is of great challenge in single-entity electrochemistry. Herein, based on the different cellular electron transport pathways between microbes and mediators, we report a three-mediator system [K3Fe(CN)6, K4Fe(CN)6, and menadione] to achieve redox activity analysis and selective identification of single Saccharomyces cerevisiae without the usage of antibodies. K4Fe(CN)6 in the three-mediator system will oxidize near the electrode surface and increase the local concentration of K3Fe(CN)6, which will promote the redox reaction of S. cerevisiae. The hydrophobic mediator─menadione─can selectively penetrate through the S. cerevisiae membrane and get access to its intracellular redox center and can further react with K3Fe(CN)6 in the bulk solution. In contrast, the mediator can only get access to the bacterial membranes of Escherichia coli and Staphylococcus aureus, which results in different electrochemical collision signals between the above microbes. In the three-mediator system, upward step-like collision signals were observed in S. cerevisiae suspension, which are related to their microbial redox activity. In comparison, E. coli or S. aureus only generated downward current steps because the blockage effect of mediator diffusion suppresses their redox activities. When S. cerevisiae co-existed with E. coli or S. aureus, transients generated by both blockage and redox activity were observed. The approach enables us to trace the collision behaviors of different microbes and distinguish their simultaneous collisions, which is the foundation for further application of electrochemical collision technique in the specific identification of single biological entities.

Superhydrophobic Nanodiamond-Functionalized Melamine Sponge for Oil/Water Separation.

Fabrication of absorbent materials foroil/water separation is an important ecological pursuit for oil spill clean-up and organic pollutants' removal. In this study, nanodiamonds (NDs), a promising member of the carbon family, were functionally modified by the covalent linking of octadecylamine (ODA). Subsequently, the superhydrophobic sponge with hierarchical microstructures was fabricated by embedding ND-ODA into a melamine sponge (MS) skeleton via a simple immersion-drying process. The as-prepared sponge (ND-ODA@PDMS@MS) showed superhydrophobic properties with a water contact angle of 155 ± 2°. For various oils and organic solvents, ND-ODA@PDMS@MS possesses excellent absorption capacity (26.65-55.64 g/g) and oil/water separation efficiency (above 98.6%). Furthermore, the adsorption capacity to crude oil remained relatively stable in highly acidic, alkaline, and salty conditions, ensuring the application in the clean-up of industrial oily sewages and marine oil spills. Besides absorbing oil for a single time, ND-ODA@PDMS@MS also exhibited satisfactory performance in continuous oil/water separation. Therefore, this work provides a facile strategy to produce robust and efficient absorbent materials for oil/water separation in large-scale oil and organic solvent clean-ups.

Photosensitizer Functionalized Nanodiamonds for Raman Imaging and Photodynamic Therapy of Cancer Cells.

One novel nanoplatform with multiple functions including Raman imaging and photodynamic therapy (PDT) capacities was constructed through modifying nanodiamonds (NDs) with photosensitizer chlorin e6 (Ce6). The NDs-Ce6 nanoparticles show enhanced singlet oxygen generation efficiency relative to free Ce6. Cytotoxicity tests indicate that NDs-Ce6 have negligible influence toward HeLa cells vitality under dark condition but enhanced photodynamic ablation upon 660 nm laser irradiation in comparison with free Ce6. In addition, the NDs-Ce6 could be used as Raman imaging probes toward HeLa cells. These results demonstrate that the NDs-Ce6 multifunctional nanoplatform have attractive features using for Raman imaging and PDT. Additionally, a new idea could be provided for designing the multifunctional platform from the work.

Nanodiamond-Based Theranostic Platform for Drug Delivery and Bioimaging.

Nanodiamonds (NDs) are promising candidates for biomedical application due to their excellent biocompatibility and innate physicochemical properties. In this Concept article, nanodiamond-based theranostic platforms, which combine both drug delivery features and bioimaging functions, are discussed. The latest developments of therapeutic strategies are introduced and future perspectives for theranostic NDs are addressed.

Toward the Detection and Identification of Single Bacteria by Electrochemical Collision Technique.

Fast and cost-efficient detection and identification of bacteria in food and water samples and biological fluids is an important challenge in bioanalytical chemistry. It was shown recently that bacteria can be detected by measuring the decrease in the diffusion current to the ultramicroelectrode caused by cell collisions with its surface. To add selectivity to the bacteria detection, herein we show the possibility of collision experiments with the signal produced by electrochemical activity of bacterial cells reducing (or oxidizing) redox species. The mediator oxidation/reduction rate can be used to identify different types of bacteria based on their specific redox activities. Here we report the analysis of electrochemical collision transients produced by two kinds of bacteria, Escherichia coli and Stenotrophomonas maltophilia. The effects of the charge and redox activity of bacterial cells on collision events are discussed. The current transients due to live cell collisions were compared to those produced by bacteria killed either by heavy metal ions (cobalt) or by an antibiotic (colistin). This approach is potentially useful for evaluating the effectiveness of antimicrobial agents. Finite-element simulations were carried out to model collision transients.

Simple preparation and highly selective detection of silver ions using an electrochemical sensor based on sulfur-doped graphene and a 3,3',5,5'-tetramethylbenzidine composite modified electrode.

A novel electrochemical sensor based on sulfur (S)-doped graphene (S-Gr) and a 3,3',5,5'-tetramethylbenzidine (TMB) composite (S-Gr-TMB) modified glassy carbon (GCE) electrode for highly selective quantitative detection of silver ions (Ag+) were fabricated. The S-Gr-TMB composite was first prepared via electrostatic interaction between TMB and S-Gr and then, the composite was coated on the surface of GCE. The resultant S-Gr-TMB/GCE electrode showed a significant voltammetric response to Ag+ at 0.3 V vs. Ag/AgCl due to the synergistic effect of S-Gr and TMB. The sensor showed good linearity from 50 µM to 400 µM with a detection limit of 2.15 µM towards the determination of Ag+. In addition, after the addition of Fe3+ and other metal ions, including Al3+, Ca2+, Cd2+, Co2+, Cu2+, K+, Mg2+, Na+, Ni2+, Pb2+ and Zn2+, in the same concentration, the current signal remained almost unchanged, revealing that the proposed electrochemical sensor exhibited a high selectivity for Ag+, which solves the nonselective problem of TMB as a spectral probe. This enhanced detection performance is attributed to two factors: (1) S-Gr has excellent electrical conductivity; (2) the coupling interactions between Ag-S are speculated to result in strengthened enrichment for Ag and good selective performance.

Adaptive use of a personal glucose meter (PGM) for acute biotoxicity assessment based on the glucose consumption of microbes.

In this study, a new method for acute biotoxicity assessment was proposed by measuring the glucose consumption of microbes with a personal glucose meter (PGM). To obtain an ideal biotoxicity assessment performance, an appropriate microbe was selected first, and then the relevant parameters, such as temperature and microbial concentration were optimized. Under the optimized parameters, the acute biotoxicity of four environmental pollutants (As(3+), Ni(2+), 4-chlorophenol, and 2,4-dichlorophenol), three wastewater samples and three soil samples were evaluated. This technology breakthrough will help us develop a low cost, easy to use water-environmental early-warning kit.

A novel integrated biosensor based on co-immobilizing the mediator and microorganism for water biotoxicity assay.

A novel integrated biosensor for biotoxicity assay has been developed by co-immobilizing microorganisms and mediators within a novel redox hydrogel. The proposed redox hydrogel acts as an immobilizing matrix both for microorganism E. coli and redox mediator, which was prepared by grafting the benzoquinone (BQ) redox mediator with gelatin/silica hybrid hydrogel (GSH). This redox hydrogel was characterized by UV-Vis, CV and EIS. The feasibility of the novel integrated biosensor for biotoxicity assay was demonstrated by measuring the heavy metal ions Hg(2+), Cu(2+) and Cd(2+) polluted water as the model toxicants. The results showed that the integrated biosensor was able to evaluate the water biotoxicity and the corresponding 50% inhibiting concentrations (IC50) are determined to be 21.2 µg mL(-1), 44 µg mL(-1) and 79 µg mL(-1), respectively. This integrated biosensor could achieve real-time monitoring of water quality and evaluation of biotoxicity. Moreover it avoids the waste and contamination of mediators, and also simplifies the assay process.

Diamond nanowires: fabrication, structure, properties, and applications.

C(sp(3) )C-bonded diamond nanowires are wide band gap semiconductors that exhibit a combination of superior properties such as negative electron affinity, chemical inertness, high Young's modulus, the highest hardness, and room-temperature thermal conductivity. The creation of 1D diamond nanowires with their giant surface-to-volume ratio enhancements makes it possible to control and enhance the fundamental properties of diamond. Although theoretical comparisons with carbon nanotubes have shown that diamond nanowires are energetically and mechanically viable structures, reproducibly synthesizing the crystalline diamond nanowires has remained challenging. We present a comprehensive, up-to-date review of diamond nanowires, including a discussion of their synthesis along with their structures, properties, and applications.

Feasibility investigation and development of microbial electrochemical biosensors for marine pollution monitoring.

Electrochemical biosensor, as a real-time and rapid detection method, has rarely been explored in marine monitoring. In present work, microbial electrochemical biosensors based on two design strategies: disperse system and integrated microbial electrode, were systematically discussed and their feasibility in marine biotoxicity assessment were investigated. An isolation method was initially investigated to eliminate the potential interference and detect the biological response accurately. The influence of water salinity on the current response was eliminated by adopting the salt-tolerant bacteria Staphylococcus aureus as test microorganism and buffer solution with sufficient ionic strength. The biotoxicity of heavy metal ions and pesticides were sensitively determined. Furthermore, a novel integrated microbial biosensor was designed by immobilizing S. aureus with a redox-active gel that consists of chitosan and poly (diallyl dimethyl ammonium chloride) mixture and confined potassium ferricyanide via electrostatic interaction. The IC50 values for Cu2+, Zn2+, Cr2O72- and Ni2+ were 3.01 mg/L, 1.34 mg/L, 7.64 mg/L and 9.41 mg/L, respectively. This work not only verified the feasibility of electrochemical biosensor in marine pollution monitoring, but also compared the pros and cons of two biosensor design strategies, which provide a guidance for the future development and application of marine monitoring devices based on electrochemical method.

Effect of cell settlement on the electrochemical collision behaviors of single microbes.

Electrochemical collision technique has emerged as a powerful approach to detect the intrinsic properties of single entities. The diffusion model, together with migration and convection processes are generally used to describe the transport and collision processes of single entities. However, things become more complicated concerning microbes because of their relatively large size, inherent motility and biological activities. In this work, the electrochemical collision behaviors of four different microorganisms: Escherichia coli (Gram-negative bacteria), Staphylococcus aureus, Bacillus subtilis (Gram-positive bacteria) and Saccharomyces cerevisiae (fungus) were systematically detected and compared using a blocking strategy. By using K4Fe(CN)6 as redox probe, the downwards step-like signals were recorded in the collision process of all the three bacteria, whereas the collision of S. cerevisiae was rarely detected. To further investigate the underlying reason for the abnormal collision behavior of S. cerevisiae, the effect of cell settlement was discussed. The results indicated that ellipsoidal S. cerevisiae with a cell size larger than 2 µm exhibited a cell sedimentation rate of 261.759 nm s-1, which is dozens of times higher than the other three bacteria. By further enhanced convection near the microelectrode or positioned the microelectrode at the bottom of electrochemical cell, the collision signals of S. cerevisiae were successfully detected, indicating cell sedimentation is a nonnegligible force in large cell transport. This study fully addressed the effect of cell settlement on the transport of microbial cells and provided two strategies to counteract this effect, which benefit for the deeper understanding and further application of electrochemical collision technique in single-cell detection.

Redox activity of single bacteria revealed by electrochemical collision technique.

This paper reports on an innovative strategy based on the electrochemical collision technique to quantify the redox activity of two bacterial species: the Gram-negative Escherichia coli and the Gram-positive Bacillus subtilis. Thionine (TH), as a redox mediator, was electrostatically adsorbed on bacterial surface and formed the bacterium-TH complexes. TH can receive electrons from bacterial metabolic pathways and be reduced. When a single bacterium-TH complex collides on the ultramicroelectrode, the reduced TH will be re-oxidized at certain potential and generate current spike. The frequency of the spikes is linearly proportional to the living bacteria concentration, and the redox activity of individual bacterium can be quantified by the charges enclosed in the current spike. The redox ability of Gram-negative E.coli to the TH mediator was 6.79 ± 0.26 × 10-18 mol per bacterial cell in 30 min, which is relatively more reactive than B. subtilis (3.52 ± 0.31 × 10-18 mol per cell). The spike signals, fitted by 3D COMSOL Multiphysics simulation, revealed that there is inherent redox ability difference of two bacterial strains besides the difference in bacterial size and collision position. This work successfully quantified the bacterial redox activity to mediator in single cells level, which is of great significance to improve understanding of heterogeneous electron transfer process and build foundations to the microorganism selection in the design of microbial electrochemical devices.

Nanodiamonds Inhibit Cancer Cell Migration by Strengthening Cell Adhesion: Implications for Cancer Treatment.

Nanodiamonds (NDs) are a type of biocompatible nanomaterial with easily modified surfaces and are considered as promising candidates in biomedicine. In this work, the inhibition of tumor cell migration by carboxylated nanodiamonds (cNDs) was investigated. AFM-based single cell adhesion and F-actin staining experiments demonstrated that cNDs treatment could enhance cell adhesion and impair assembly of the cytoskeleton. The mechanism analysis of the regulatory protein expression level also proved that cNDs could inhibit the migration of Hela cells by preventing the epithelial-mesenchymal transition (EMT) process through the transforming growth factor ß (TGF-ß) signaling pathway. The in vivo pulmonary metastasis model also showed that cNDs effectively reduced the metastasis of murine B16 melanoma cells. In summary, cNDs have been demonstrated to inhibit cancer cell migration in vitro and decrease tumor metastasis in vivo. Therefore, cNDs might have potential utility for specific cancer treatment.

Nanodiamond as the pH-responsive vehicle for an anticancer drug.

Cis-dichlorodiammineplatinum(II) (CDDP, cisplatin), a widely used anticancer drug, is successfully loaded onto nanodiamond (ND) by adsorption and complexation. The CDDP-ND composite is characterized by IR spectroscopy, atomic absorption spectroscopy, thermogravimetric analysis, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. CDDP is released from the composite in phosphate-buffered saline (PBS) of pH 6.0 at a rate higher than in PBS of pH 7.4. Therefore, it is predicted that the ND vehicle would deliver low concentrations of CDDP in the blood, but release much more drug after integration into the acidic cytoplasm, thereby reducing toxic side effects. The complexation between CDDP and the carboxyl groups on the ND surface is responsible for the pH-responsive release property. The drug released from the composite retains the same cytotoxicity as free CDDP against human cervical cancer cells.

Development of an Escherichia coli-based electrochemical biosensor for mycotoxin toxicity detection.

Mycotoxin contamination in food and feed is a global concern because mycotoxin contamination can cause both acute and chronic health effects in humans and animals. In the present work, an Escherichia coli-based biosensor is described for the toxicity assessment of aflatoxin B1 (AFB1) and zearalenone (ZEN). In this electrochemical biosensor, E. coli is used as the signal recognition element, p-benzoquinone is used as the mediator, and a two-step reaction procedure has been developed to separate the mediator from the mycotoxins. The current value of the as-prepared microbial biosensor exhibits a linear decrease with concentrations of AFB1 and ZEN in the range of 0.01-0.3 and 0.05-0.5 µg/mL, with detection limits reaching 1 and 6 ng/mL, respectively. The IC25 values obtained by the present method are 0.25 and 0.40 µg/mL for AFB1 and ZEN, which shows that the cytotoxicity of AFB1 to E. coli is more severe than the cytotoxicity of ZEN to E. coli. The combined toxic effect of these two mycotoxins has also been explored, and synergistic biotoxicity has been observed. Moreover, the biosensor is successfully applied to the toxicity evaluation of mycotoxins in real samples, including peanut and corn oils. This work could provide new insight into mycotoxin and microorganism interactions and could establish a new approach for future mycotoxin detection.

A portable instrument for monitoring acute water toxicity based on mediated electrochemical biosensor: Design, testing and evaluation.

A water quality early-warning instrument for evaluating acute water toxicity based on the electrochemical biosensor, Model ETOX18-01, was developed and manufactured with the features of low current detection (0.1 nA), precise thermostatic control, self-cleaning as well as remote data transmission. A sensitive integrated microbial electrode, made up of a glass carbon electrode that was modified by an active biofilm consisting of Escherichia coli, thionine, carbon nanodots and chitosan, has been fabricated as the biosensor. To validate the performance, multiple real water samples and artificial water samples were tested by Model ETOX18-01, and compared with ISO standardized luminescent bacterial test simultaneously. The correlation between the Model ETOX18-01 and luminescent bacterial test for these water samples showed good determination coefficient (R2 = 0.9827). In addition, Model ETOX18-01 is more sensitive to colored metal ionic samples. With its characteristics of high sensitivity, excellent repeatability and easy operation, the instrument Model ETOX18-01 provides a promising tool for large-scale water environmental assessment, and has a potential application in evaluating the water quality and early risk warning.

Non-enzymatic glucose detection using as-prepared boron-doped diamond thin-film electrodes.

Electrochemical oxidation of glucose in alkaline solution at as-prepared boron-doped microcrystalline diamond (BDMD) and nanocrystalline diamond (BDND) thin-film electrodes is investigated by cyclic voltammetry. The results demonstrate that glucose can be directly oxidized at as-prepared boron-doped diamond (BDD) thin-film electrodes and the curve of the negative scan traces onto the positive scan. The effect of sodium hydroxide concentration on the response of glucose is also studied in the range of 0.02-0.6 M and the optimum concentration of sodium hydroxide is found to be 0.1 M. The voltammetric signal of glucose and the mixture of ascorbic acid (AA) and uric acid (UA) can be observed well-separated at as-prepared BDD thin-film electrodes in 0.1 M sodium hydroxide solution. The peak current is proportional to the glucose concentration in the range 0.25-10 mM with a correlation coefficient of 0.9993 in the presence of AA and UA. Furthermore, the experiment results also show that the non-enzymatic glucose sensor based on as-prepared BDD thin-film electrodes has high sensitivity, good reproducibility and stability.

Electrochemical synthesis of multicolor fluorescent N-doped graphene quantum dots as a ferric ion sensor and their application in bioimaging.

A novel electrochemical strategy for simple and facile synthesis of semicarbazide functionalized nitrogen-doped graphene quantum dots (N-GQDs) was reported, based on direct exfoliation and oxidation from graphite rods. The average diameter of the as-synthesized N-GQDs is about 20 nm, and their dispersion is bright yellow due to the rich nitrogen and oxygen functional groups on their surface. The N-GQD dispersion was further applied in the selective detection of ferric ions (Fe3+) based on the photoluminescence (PL) quenching of N-GQDs after adding Fe3+. The fluorescent sensor has a wide linear range of 0-200 µM and a detection limit of 0.87 µM, which is much lower than the maximum level (0.3 mg L-1, equivalent to 5.4 µM) of Fe3+ permitted in drinking water by the U.S. Environmental Protection Agency (EPA). Moreover, these novel N-GQDs exhibit much wider emission bands, which extend into the entire visible region, and emit three primary color fluorescence independently. This distinctive behavior of the as-prepared GQDs not only breaks the limitation that traditional reported GQDs only exhibit blue emission in the short-wavelength region, but may also provide a new research platform for further applications of GQDs in real environmental detection and biological imaging systems.

Nanodiamonds conjugated upconversion nanoparticles for bio-imaging and drug delivery.

A multifunctional nanoplatform with simultaneous imaging and therapeutic capacities has been fabricated by covalently conjugating nanodiamonds (NDs) with upconversion NaYF4:Yb,Er nanoparticles (UCNPs). Green emission can be observed for the UCNP-NDs composites under 980 nm excitation, and in vitro imaging was carried out towards HeLa cells. The UCNP-NDs also shows good drug storage capability toward anticancer drug doxorubicin (Dox) and exhibits significant pH-dependent drug-release behavior. Dox-loaded UCNP-NDs shows higher therapy efficiency towards Hela cells than free Dox when the equivalent concentration of the Dox is more than 10 µg/mL. Thus, a favorable strategy may be provided to decrease the side effects of Dox, minimize drug dose, and improve its efficacy. These findings highlight the fascinating features of UCNP-NDs nanocomposites as upconversion fluorescence imaging contrast agents and doxorubicin loading carrier. In addition, this work may also provide a novel strategy for the design of multifunctional nanoplatforms.

Direct electrochemistry and electrocatalytic activity of cytochrome c covalently immobilized on a boron-doped nanocrystalline diamond electrode.

Cytochrome c (Cyt c) was covalently immobilized on a boron-doped nanocrystalline diamond (BDND) electrode via surface functionalization with undecylenic acid methyl ester and subsequent removal of the protecting ester groups to produce a carboxyl-terminated surface. Cyt c-modified BDND electrode exhibited a pair of quasi-reversible and well-defined redox peaks with a formal potential (E(0)) of 0.061 V (vs Ag/AgCl) in 0.1 M phosphate buffer solution (pH 7.0) and a surface-controlled process with a high electron transfer constant (ks) of 5.2 +/- 0.6 s(-1). The electrochemical properties of as-deposited and Cyt c-modified boron-doped microcrystalline diamond (BDMD) electrodes were also studied for comparison. Investigation of the electrocatalytic activity of the Cyt c-modified BDND electrode toward hydrogen peroxide (H2O2) revealed a rapid amperometric response (5 s). The linear range of response to H2O2 concentration was from 1 to 450 microM, and the detection limit was 0.7 microM at a signal-to-noise ratio of 3. The stability of the Cyt c-modified BDND electrode, in comparison with that of the BDMD and glassy carbon counterpart electrodes, was also evaluated.