Vertically aligned MXene bioelectrode prepared by freeze-drying assisted electrophoretic deposition for sensitive electrochemical protein detection

Two-dimensional (2D) carbides, MXenes, have attracted attention as electrode materials of electrochemical biosensors because of their metallic conductivity, hydrophilicity, and mechanical stability. However, when fabricating electrodes, the nanosheets tend to re-stack and generally align horizontally with respect to the current collector due to the highly anisotropic nature of MXene, resulting in low porosity and poor utilization of the MXene surface. Here we report the electrochemical biosensing of antibody-antigen reactions with a vertically aligned Ti3C2Tx MXene (VA-MXene) electrode prepared by freeze-drying assisted electrophoretic deposition. The macroporous VA-MXene electrode exhibited a better electrochemical response towards the immunoreaction between the allergenic buckwheat protein (BWp16) and the antibody compared to a non-porous, horizontally (in-plane) stacked MXene (HS-MXene) and the sensors reported in the literature. The sensor responsiveness, defined as the ratio of the obtained current density of the electrode to the antigen concentration, was much higher for the VA-MXene electrode (238 μA cm-2 (ng mL-1) -1) than for the HS-MXene electrode. The proposed technique is applicable to other exfoliated nanosheets, and will open a new avenue for porous nanosheet electrodes to improve the sensing characteristics of electrochemical biosensors.

Zero-Overpotential Redox Reactions of Quinone-Based Molecules Confined in Carbon Micropores

Quinone-based aromatic compounds have been studied as electrode materials for various energy-storage devices. However, the relatively large activation barrier of the charge-transfer process of these redox-active molecules causes sluggish reactions and a decrease in energy efficiency. To lower the activation barrier, aromatic compounds must be strongly adsorbed on the electrode surface, preferably via π-π stacking interactions. Molecules in slit-shaped micropores strongly adsorb on the graphitic walls, thus experiencing unique micropore-confinement properties. In this study, the micropore-confinement effect is extended to the adsorption of quinone-based redox-active molecules in 0.8 nm slit-shaped micropores of activated carbon, which produces a drastic reduction in the activation barrier of the charge-transfer process and creates a zero-overpotential redox reaction. The property originates from the short distance (approximately 0.3 nm) between the quinone molecules and the graphitic wall due to the strong adsorption of the aromatic compound. Our results provide the first demonstration that the micropore-confinement effect can reduce and nearly eliminate the activation barrier of an electrochemical reaction. We also demonstrate the applicability of this approach via the charge/discharge performance of a two-electrode cell. Cells comprising the aromatic compound/activated carbon material as positive and negative electrodes exhibit a greater retention capacity than those without activated carbon. The technique described herein can guide the development of high-performance, rapid charging/discharging electrodes for energy-storage devices such as batteries, supercapacitors, and hybrid devices using organic materials.

Enhancement in the Charge-Transfer Kinetics of Pseudocapacitive Iridium-Doped Layered Manganese Oxide

Birnessite manganese oxide is a promising candidate as an electrode material for aqueous supercapacitors owing to its pseudocapacitance associated with fast redox processes. While manganese oxides are semiconductive, the conductivity is much lower than that of typical materials used for capacitive electrodes such as activated carbon or ruthenium oxide. In an attempt to increase the electronic conductivity of birnessite, a new solid solution phase, K_y(Mn_1-xIr_x)O₂, was synthesized, and the electrochemical charge storage capability of Ir-doped birnessite was studied in aqueous Li₂SO₄. Structural characterization revealed that the single-phase K_y(Mn_1-xIr_x)O₂ could be synthesized up to x = 0.1. An increase in the pseudocapacitive charge was observed with the increase in Ir content. In addition to the increase in the pseudocapacitive charge, an unusual change in the peak potential was observed. The peak-to-peak difference for the Mn⁴⁺/Mn³⁺ redox decreased with increasing Ir content, indicating an increase in the reversibility of the pseudocapacitive process. The decrease in peak-to-peak difference was observed only by Ir substitution and was not observed for physical mixtures of K_0.28MnO₂ and IrO₂, suggesting a strong electronic interaction between the host Mn ion and the substituting Ir ion.

Correction: Stimuli-responsive magnetic nanoparticles for tumor-targeted bimodal imaging and photodynamic/hyperthermia combination therapy

Correction for 'Stimuli-responsive magnetic nanoparticles for tumor-targeted bimodal imaging and photodynamic/hyperthermia combination therapy' by Kyoung Sub Kim, et al., Nanoscale, 2016, DOI: 10.1039/c6nr02273a.

Stimuli-responsive magnetic nanoparticles for tumor-targeted bimodal imaging and photodynamic/hyperthermia combination therapy

Despite magnetic nanoparticles having shown great potential in cancer treatment, tremendous challenges related to diagnostic sensitivity and treatment efficacy for clinical application remain. Herein, we designed optimized multifunctional magnetite nanoparticles (AHP@MNPs), composed of Fe3O4 nanoparticles and photosensitizer conjugated hyaluronic acid (AHP), to achieve enhanced tumor diagnosis and therapy. Fe3O4 nanoparticles (MNPs) were synthesized by a facile hydrolysis method. MNPs have higher biocompatibility, controllable particle sizes, and desirable magnetic properties. The fabricated AHP@MNPs have enhanced water solubility (average size: 108.13 ± 1.08 nm), heat generation properties, and singlet oxygen generation properties upon magnetic and laser irradiation. The AHP@MNPs can target tumors via CD44 receptor-mediated endocytosis, which have enhanced tumor therapeutic effects through photodynamic/hyperthermia-combined treatment without any drugs. We successfully detected tumors implanted in mice via magnetic resonance imaging and optical imaging. Furthermore, we demonstrated the photodynamic/hyperthermia-combined therapeutic efficacy of AHP@MNPs with synergistically enhanced efficacy against cancer.

Label-free detection of Cu(ii) in a human serum sample by using a prion protein-immobilized FET sensor

We report a simple and label-free detection of Cu²⁺utilizing prion proteins as the recognition molecules, which can provide highly sensitive and selective performance at the nanomolar level in a human serum sample.

Field Effect Transistor Biosensor Using Antigen Binding Fragment for Detecting Tumor Marker in Human Serum

Detection of tumor markers is important for cancer diagnosis. Field-effect transistors (FETs) are a promising method for the label-free detection of trace amounts of biomolecules. However, detection of electrically charged proteins using antibody-immobilized FETs is limited by ionic screening by the large probe molecules adsorbed to the transistor gate surface, reducing sensor responsiveness. Here, we investigated the effect of probe molecule size on the detection of a tumor marker, α-fetoprotein (AFP) using a FET biosensor. We demonstrated that the small receptor antigen binding fragment (Fab), immobilized on a sensing surface as small as 2–3 nm, offers a higher degree of sensitivity and a wider concentration range (100 pg/mL–1 μg/mL) for the FET detection of AFP in buffer solution, compared to the whole antibody. Therefore, the use of a small Fab probe molecule instead of a whole antibody is shown to be effective for improving the sensitivity of AFP detection in FET biosensors. Furthermore, we also demonstrated that a Fab-immobilized FET subjected to a blocking treatment, to avoid non-specific interactions, could sensitively and selectively detect AFP in human serum.

A label-free electrical assay of fibrous amyloid β based on semiconductor biosensing

Simple electrical assay discriminates between fibrous and non-fibrous amyloid β (Aβ) proteins, and determines the fibrous Aβ concentration with high sensitivity.

Detection of biomolecular interaction between biotin and streptavidin on a self-assembled monolayer using magnetic nanoparticles

For developing a magnetic bioassay system, an investigation to determine the presence of a specific biomolecular interaction between biotin and streptavidin was done using magnetic nanoparticles and a silicon substrate with a self-assembled monolayer. Streptavidin was immobilized on the magnetic particles, and biotin was attached to the monolayer-modified substrate. The reaction of streptavidin-modified magnetic particles on the biotin-modified substrate was clearly observed under an optical microscope. The magnetic signals from the particles were detected using a magnetic force microscope. The results of this study demonstrate that the combination of a monolayer-modified substrate with biomolecule-modified magnetic particles is useful for detecting biomolecular interactions in medical and diagnostic analyses.