Review-Recent Advances in Nanosensors Built with Pre-Pulled Glass Nanopipettes and Their Applications in Chemical and Biological Sensing

An Insulin-Modified pH-Responsive Nanopipette Based on Ion Current Rectification

The properties of nanopipettes largely rely on the materials introduced onto their inner walls, which allow for a vast extension of their sensing capabilities. The challenge of simultaneously enhancing the sensitivity and selectivity of nanopipettes for pH sensing remains, hindering their practical applications. Herein, we report insulin-modified nanopipettes with excellent pH response performances, which were prepared by introducing insulin onto their inner walls via a two-step reaction involving silanization and amidation. The pH response intensity based on ion current rectification was significantly enhanced by approximately 4.29 times when utilizing insulin-modified nanopipettes compared with bare ones, demonstrating a linear response within the pH range of 2.50 to 7.80. In addition, insulin-modified nanopipettes featured good reversibility and selectivity. The modification processes were monitored using the I-V curves, and the relevant mechanisms were discussed. The effects of solution pH and insulin concentration on the modification results were investigated to achieve optimal insulin introduction. This study showed that the pH response behavior of nanopipettes can be greatly improved by introducing versatile molecules onto the inner walls, thereby contributing to the development and utilization of pH-responsive nanopipettes.

Recent Advances in the Development and Characterization of Electrochemical and Electrical Biosensors for Small Molecule Neurotransmitters

Neurotransmitters act as chemical messengers, determining human physiological and psychological function, and abnormal levels of neurotransmitters are related to conditions such as Parkinson's and Alzheimer's disease. Biologically and clinically relevant concentrations of neurotransmitters are usually very low (nM), so electrochemical and electronic sensors for neurotransmitter detection play an important role in achieving sensitive and selective detection. Additionally, these sensors have the distinct advantage to potentially be wireless, miniaturized, and multichannel, providing remarkable opportunities for implantable, long-term sensing capabilities unachievable by spectroscopic or chromatographic detection methods. In this article, we will focus on advances in the development and characterization of electrochemical and electronic sensors for neurotransmitters during the last five years, identifying how the field is progressing as well as critical knowledge gaps for sensor researchers.

Glass Capillary-Based Nanopores for Single Molecule/Single Cell Detection

A glass capillary-based nanopore (G-nanopore), due to its tapered tip, easy tunability in orifice size, and especially its flexible surface modifications that can be tailored to effectively capture and enhance the ionic current signal of single entities (single molecules, single cells, and single particles), offers a powerful and nanoconfined sensing platform for diverse biological measurements of single cells and single molecules. Compared with other artificial two-dimensional solid-state nanopores, its conical tip and high spatial and temporal resolution characteristics facilitate noninvasive single molecule and selected area (subcellular) single cell detections (e.g., DNA mutations, highly expressed proteins, and small molecule markers that reflect the change characteristics of the tumor), as a small G-nanopore (≤100 nm) does negligible damage to cell functions and cell membrane integrity when inserted through the cell membrane. In this brief review, we summarize the preparation of G-nanopores and discuss the advantages of them as solid-state sensing platforms for single molecule and single cell detection applications as well as for cancer diagnosis and treatment applications. We also describe the current bottlenecks that limit the widespread use of G-nanopores in clinical applications and provide an outlook on future developments. The brief review will provide the reader with a quick survey of this field and facilitate the rapid development of a G-nanopore sensing platform for future tumor diagnosis and personalized medicine based on single-molecule/single-cell bioassay.

A DNA-Schiff base functional nanopore sensing platform for the highly sensitive detection of Al3+ and Zn2+ ions

The combination of DNA nanotechnology and nanopore sensing technology has greatly promoted research on target molecule or ion detection. The large solid-state nanopores/nanochannels show better mechanical stability and reproducibility, but metal ion detection in the large nanopores with diameters of hundreds of nanometers or several micrometers is rarely reported. Hence, it is meaningful and urgent to develop a large nanopore-based sensing platform for the detection of metal ions. Herein, we employed a salicylic aldehyde-modified DNA network in conjunction with a glass nanopipette (GN) with a diameter of hundreds of nanometers as a sensing platform for the detection of target metal ions. Upon the addition of different receptors with the amino group, the salicylic aldehyde could in situ specifically recognize and bind with Zn²⁺ and Al³, forming Schiff base-metal ion complexes at the four vertices of one face per nanocube unit. The steric hindrance effect of multiple Schiff bases and metal ion complexes leads to the blockage of internal structure and decrease of ion current in the GN. Owing to this signal amplification strategy, the detection limit of the target metal ion reaches a level of fM in the GN with a diameter of about 300 nm. In the future, this functional nanopore sensing platform is expected to realize highly sensitive detection for more biological metal ions by choosing appropriate receptors.

Target-Triggered Assembly in a Nanopipette for Electrochemical Single-Cell Analysis

Engineered nanopipette tools have recently emerged as a powerful approach for electrochemical nanosensing, which has major implications in both fundamental biological research and biomedical applications. Herein, we describe a generic method of target-triggered assembly of aptamers in a nanopipette for nanosensing, which is exemplified by sensitive and rapid electrochemical single-cell analysis of adenosine triphosphate (ATP), a ubiquitous energy source in life and important signaling molecules in many physiological processes. Specifically, a layer of thiolated aptamers is immobilized onto a Au-coated interior wall of a nanopipette tip. With backfilled pairing aptamers, the engineered nanopipette is then used for probing intracellular ATP via the ATP-dependent linkage of the split aptamers. Due to the higher surface charge density from the aptamer assembly, the nanosensor would exhibit an enhanced rectification signal. Besides, this ATP-responsive nanopipette tool possesses excellent selectivity and stability as well as high recyclability. This work provides a practical single-cell nanosensor capable of intracellular ATP analysis. More generally, integrated with other split recognition elements, the proposed mechanism could serve as a viable basis for addressing many other important biological species.

The double life of conductive nanopipette: a nanopore and an electrochemical nanosensor

The continuing interest in nanoscale research has spurred the development of nanosensors for liquid phase measurements. These include nanopore-based sensors typically employed for detecting nanoscale objects, such as nanoparticles, vesicles and biomolecules, and electrochemical nanosensors suitable for identification and quantitative analysis of redox active molecules. In this Perspective, we discuss conductive nanopipettes (CNP) that can combine the advantages of single entity sensitivity of nanopore detection with high selectivity and capacity for quantitative analysis offered by electrochemical sensors. Additionally, the small physical size and needle-like shape of a CNP enables its use as a tip in the scanning electrochemical microscope (SECM), thus, facilitating precise positioning and localized measurements in biological systems.