Resistive-pulse sensing with biological or solid-state
nanopores
and nanopipettes has been widely employed in detecting single molecules
and nanoparticles. The analytical signal in such experiments is the
change in ionic current caused by the molecule/particle translocation
through the pipet orifice. This paper describes a new version of the
resistive-pulse technique based on the use of carbon nanopipettes
(CNP). The measured current is produced by electrochemical oxidation/reduction
of redox molecules at the carbon surface and responds to the particle
translocation. In addition to counting single entities, this technique
enables qualitative and quantitative analysis of the electroactive
material they contain. Using liposomes as a model system, we demonstrate
the capacity of CNPs for (1) conventional resistive-pulse sensing
of single liposomes, (2) electrochemical resistive-pulse sensing,
and (3) electrochemical identification and quantitation of redox species
(e.g., ferrocyanide, dopamine, and nitrite) contained in a single
liposome. The small physical size of a CNP suggests the possibility
of single-entity measurements in biological systems.
Resistive-pulse
sensing is a technique widely used to detect single
nanoscopic entities such as nanoparticles and large molecules that
can block the ion current flow through a nanopore or a nanopipette.
Although the species of interest, e.g., antibodies, DNA, and biological
vesicles, are typically produced by living cells, so far, they have
only been detected in the bulk solution since no localized resistive-pulse
sensing in biological systems has yet been reported. In this report,
we used a nanopipette as a scanning ion conductance microscopy (SICM)
tip to carry out resistive-pulse experiments both inside immobilized
living cells and near their surfaces. The characteristic changes in
the ion current that occur when the pipet punctures the cell membrane
are used to monitor its insertion into the cell cytoplasm. Following
the penetration, cellular vesicles (phagosomes, lysosomes, and/or
phagolysosomes) were detected inside a RAW 264.7 macrophage. Much
smaller pipettes were used to selectively detect 10 nm Au nanoparticles
in the macrophage cytoplasm. The in situ resistive-pulse
detection of extracellular vesicles released by metastatic human breast
cells (MDA-MB-231) is also demonstrated. Electrochemical resistive-pulse
experiments were carried out by inserting a conductive carbon nanopipette
into a macrophage cell to sample single vesicles and measure reactive
oxygen and nitrogen species (ROS/RNS) contained inside them.
The development of more intricate devices for the analysis of small molecules and protein activity in single cells would advance our knowledge of cellular heterogeneity and signaling cascades. Therefore, in this study, a nanokit was produced by filling a nanometersized capillary with a ring electrode at the tip with components from traditional kits, which could be egressed outside the capillary by electrochemical pumping. At the tip, femtoliter amounts of the kit components were reacted with the analyte to generate hydrogen peroxide for the electrochemical measurement by the ring electrode. Taking advantage of the nanotip and small volume injection, the nanokit was easily inserted into a single cell to determine the intracellular glucose levels and sphingomyelinase (SMase) activity, which had rarely been achieved. High cellular heterogeneities of these two molecules were observed, showing the significance of the nanokit. Compared with the current methods that use a complicated structural design or surface functionalization for the recognition of the analytes, the nanokit has adapted features of the well-established kits and integrated the kit components and detector in one nanometer-sized capillary, which provides a specific device to characterize the reactivity and concentrations of cellular compounds in single cells.
SignificanceThe quantification of protein activity in individual lysosomes in living cells is realized using a nanocapillary designed to electrochemically analyze internal solution, in which a single lysosome is sorted from the cell and the target protein is reacted with the corresponding kit components to generate hydrogen peroxide for measurement. The ability to sort and assay multiple lysosomes from the same cell allows direct study of protein function at subcellular resolution and provides unprecedented information about the homogeneity within the lysosomal population of a single cell.
For realizing scalable solar hydrogen synthesis, the development of visible-light-absorbing photocatalysts capable of overall water splitting is essential. Metal sulfides can capture visible light efficiently; however, their utilization in water splitting has long been plagued by the poor resilience against hole oxidation. Herein, we report that the ZnIn 2 S 4 monolayers with dual defects (Ag dopants and nanoholes) accessed via cation exchange display stoichiometric H 2 and O 2 evolution in pure water under visible light irradiation. In-depth characterization and modeling disclose that the dual-defect structure endows the ZnIn 2 S 4 monolayers with optimized light absorption and carrier dynamics. More significantly, the dual defects cooperatively function as active sites for water oxidation (Ag dopants) and reduction (nanoholes), thus leading to steady performance in photocatalytic overall water splitting without the assistance of cocatalysts. This work demonstrates a feasible way for fulfilling "all-in-one" photocatalyst design and manifests its great potential in addressing the stability issues associated with sulfide-based photocatalysts.
Here we show a novel strategy for tailoring the synergistic electrical properties of metal@semiconductor hybrid nanocrystals (HNCs) based on cation exchange-enabled electronic doping.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.