A Microfluidic Sensor for Continuous, in Situ Surface Charge Measurement of Single Cells

Rapid Surface Charge Mapping Based on a Liquid Crystal Microchip

Rapid surface charge mapping of a solid surface remains a challenge. In this study, we present a novel microchip based on liquid crystals for assessing the surface charge distribution of a planar or soft surface. This chip enables rapid measurements of the local surface charge distribution of a charged surface. The chip consists of a micropillar array fabricated on a transparent indium tin oxide substrate, while the liquid crystal is used to fill in the gaps between the micropillar structures. When an object is placed on top of the chip, the local surface charge (or zeta potential) influences the orientation of the liquid crystal molecules, resulting in changes in the magnitude of transmitted light. By measuring the intensity of the transmitted light, the distribution of the surface charge can be accurately quantified. We calibrated the chip in a three-electrode configuration and demonstrated the validity of the chip for rapid surface charge mapping using a borosilicate glass slide. This chip offers noninvasive, rapid mapping of surface charges on charged surfaces, with no need for physical or chemical modifications, and has broad potential applications in biomedical research and advanced material design.

Nanotechnology and Cancer Bioelectricity: Bridging the Gap Between Biology and Translational Medicine

Bioelectricity is the electrical activity that occurs within living cells and tissues. This activity is critical for regulating homeostatic cellular function and communication, and disruptions of the same can lead to a variety of conditions, including cancer. Cancer cells are known to exhibit abnormal electrical properties compared to their healthy counterparts, and this has driven researchers to investigate the potential of harnessing bioelectricity as a tool in cancer diagnosis, prognosis, and treatment. In parallel, bioelectricity represents one of the means to gain fundamental insights on how electrical signals and charges play a role in cancer insurgence, growth, and progression. This review provides a comprehensive analysis of the literature in this field, addressing the fundamentals of bioelectricity in single cancer cells, cancer cell cohorts, and cancerous tissues. The emerging role of bioelectricity in cancer proliferation and metastasis is introduced. Based on the acknowledgement that this biological information is still hard to access due to the existing gap between biological findings and translational medicine, the latest advancements in the field of nanotechnologies for cellular electrophysiology are examined, as well as the most recent developments in micro- and nano-devices for cancer diagnostics and therapy targeting bioelectricity.

Resistive Pulse Sensing on a Capillary-Assisted Microfluidic Platform for On-Site Single-Particle Analyses

Capillary-assisted flow is valuable for utilizing microfluidics-based electrical sensing platforms at on-site locations by simplifying microfluidic operations and system construction; however, incorporating capillary-assisted flow in platforms requires easy microfluidic modification and stability over time for capillary-assisted flow generation and sensing performance. Herein, we report a capillary-assisted microfluidics-based electrical sensing platform using a one-step modification of polydimethylsiloxane (PDMS) with polyethylene glycol (PEG). As a model of electrical sensing platforms, this work focused on resistive pulse sensing (RPS) using a micropore in a microfluidic chip for label-free electrical detection of single analytes, and filling the micropore with an electrolyte is the first step to perform this RPS. The PEG-PDMS surfaces remained hydrophilic after ambient storage for 30 d and assisted in generating an electrolyte flow for filling the micropore with the electrolyte. We demonstrated the successful detection and size analysis of micrometer particles and bacterial cells based on RPS using the microfluidic chip stored in a dry state for 30 d. Combining this capillary-assisted microfluidic platform with a portable RPS system makes on-site detection and analysis of single pathogens possible.

Recent Advances in Biomolecular Detection Based on Aptamers and Nanoparticles

The fast, accurate detection of biomolecules, ranging from nucleic acids and small molecules to proteins and cellular secretions, plays an essential role in various biomedical applications. These include disease diagnostics and prognostics, environmental monitoring, public health, and food safety. Aptamer recognition (DNA or RNA) has gained extensive attention for biomolecular detection due to its high selectivity, affinity, reproducibility, and robustness. Concurrently, biosensing with nanoparticles has been widely used for its high carrier capacity, stability and feasibility of incorporating optical and catalytic activity, and enhanced diffusivity. Biosensors based on aptamers and nanoparticles utilize the combination of their advantages and have become a promising technology for detecting of a wide variety of biomolecules with high sensitivity, reliability, specificity, and detection speed. Via various sensing mechanisms, target biomolecules have been quantified in terms of optical (e.g., colorimetric and fluorometric), magnetic, and electrical signals. In this review, we summarize the recent advances in and compare different aptamer-nanoparticle-based biosensors by nanoparticle types and detection mechanisms. We also share our views on the highlights and challenges of the different nanoparticle-aptamer-based biosensors.

Cell Surface Charge Mapping Using a Microelectrode Array on ITO Substrate

Many cellular functions are regulated by cell surface charges, such as intercellular signaling and metabolism. Noninvasive measurement of surface charge distribution of a single cell plays a vital role in understanding cellular functions via cell membranes. We report a method for cell surface charge mapping via photoelectric interactions. A cell is placed on an array of microelectrodes fabricated on a transparent ITO (indium tin oxide) surface. An incident light irradiates the ITO surface from the backside. Because of the influence of the cell surface charge (or zeta potential), the photocurrent and the absorption of the incident light are changed, inducing a magnitude change of the reflected light. Hence, the cell surface charge distribution can be quantified by analyzing the reflected light intensity. This method does not need physical or chemical modification of the cell surface. We validated this method using charged microparticles (MPs) and two types of cells, i.e., human dermal fibroblast cells (HDFs) and human mesenchymal stem cells (hMSC). The measured average zeta potentials were in good agreement with the standard electrophoresis light scattering method.

Principle Superiority and Clinical Extensibility of 2D and 3D Charged Nanoprobe Detection Platform Based on Electrophysiological Characteristics of Circulating Tumor Cells

The electrical characteristic of cancer cells is neglected among tumor biomarkers. The development of nanoprobes with opposing charges for monitoring the unique electrophysiological characteristics of cancer cells. Micro-nano size adsorption binding necessitates consideration of the nanoprobe's specific surface area. On the basis of the electrophysiological characteristics of circulating tumor cells (CTCs), clinical application and performance assessment are determined. To demonstrate that cancer cells have a unique pattern of electrophysiological patterns compared to normal cells, fluorescent nanoprobes with opposing charges were developed and fabricated. Graphene oxide (GO) was used to transform three-dimensional (3D) nanoprobes into two-dimensional (2D) nanoprobes. Compare 2D and 3D electrophysiological magnetic nanoprobes (MNP) in clinical samples and evaluate the adaptability and development of CTCs detection based on cell electrophysiology. Positively charged nanoprobes rapidly bind to negatively charged cancer cells based on electrostatic interactions. Compared to MNPs(+) without GO, the GO/MNPs(+) nanoprobe is more efficient and uses less material to trap cancer cells. CTCs can be distinguished from normal cells that are fully unaffected by nanoprobes by microscopic cytomorphological inspection, enabling the tracking of the number and pathological abnormalities of CTCs in the same patient at various chemotherapy phases to determine the efficacy of treatment. The platform for recognizing CTCs on the basis of electrophysiological characteristics compensates for the absence of epithelial biomarker capture and size difference capture in clinical performance. Under the influence of electrostatic attraction, the binding surface area continues to influence the targeting of cancer cells by nanoprobes. The specific recognition and detection of nanoprobes based on cell electrophysiological patterns has enormous potential in the clinical diagnosis and therapeutic monitoring of cancer.

Specially Resolved Single Living Cell Perfusion and Targeted Fluorescence Labeling Based on Nanopipettes

Targeted delivery and labeling of single living cells in heterogeneous cell populations are of great importance to understand the molecular biology and physiological functions of individual cells. However, it remains challenging to perfuse fluorescence markers into single living cells with high spatial and temporal resolution without interfering neighboring cells. Here, we report a single cell perfusion and fluorescence labeling strategy based on nanoscale glass nanopipettes. With the nanoscale tip hole of 100 nm, the use of nanopipettes allows special perfusion and high-resolution fluorescence labeling of different subcellular regions in single cells of interest. The dynamic of various fluorescent probes has been studied to exemplify the feasibility of nanopipette-dependent targeted delivery. According to experimental results, the cytoplasm labeling of Sulfo-Cyanine5 and fluorescein isothiocyanate is mainly based on the Brownian movement due to the dyes themselves and does not have a targeting ability, while the nucleus labeling of 4',6-diamidino-2-phenylindole (DAPI) is originated from the adsorption between DAPI and DNA in the nucleus. From the finite element simulation, the precise manipulation of intracellular delivery is realized by controlling the electro-osmotic flow inside the nanopipettes, and the different delivery modes between nontargeting dyes and nucleus-targeting dyes were compared, showcasing the valuable ability of nanopipette-based method for the analysis of specially defined subcellular regions and the potential applications for single cell surgery, subcellular manipulation, and gene delivery.

Mapping Surface Charge Distribution of Single-Cell via Charged Nanoparticle

Many bio-functions of cells can be regulated by their surface charge characteristics. Mapping surface charge density in a single cell's surface is vital to advance the understanding of cell behaviors. This article demonstrates a method of cell surface charge mapping via electrostatic cell-nanoparticle (NP) interactions. Fluorescent nanoparticles (NPs) were used as the marker to investigate single cells' surface charge distribution. The nanoparticles with opposite charges were electrostatically bonded to the cell surface; a stack of fluorescence distribution on a cell's surface at a series of vertical distances was imaged and analyzed. By establishing a relationship between fluorescent light intensity and number of nanoparticles, cells' surface charge distribution was quantified from the fluorescence distribution. Two types of cells, human umbilical vein endothelial cells (HUVECs) and HeLa cells, were tested. From the measured surface charge density of a group of single cells, the average zeta potentials of the two types of cells were obtained, which are in good agreement with the standard electrophoretic light scattering measurement. This method can be used for rapid surface charge mapping of single particles or cells, and can advance cell-surface-charge characterization applications in many biomedical fields.

A microfluidic cathodic photoelectrochemical biosensor chip for the targeted detection of cytokeratin 19 fragments 21-1

A microfluidic chip integrated with a microelectrode and a cathodic photoelectrochemical (PEC) biosensor for the ultrasensitive detection of non-small cell lung cancer cytokeratin fragments based on a signal amplification strategy was designed. The mechanism for signal amplification is developed based on the p-n junction of AgI/Bi₂Ga₄O₉, with dissolved O₂ as an electron acceptor to produce the superoxide anion radical (˙O₂^-) as the working microelectrode. By combining this with a novel superoxide-dismutase-loaded honeycomb manganese oxide nanostructure (SOD@hMnO₂) as the co-catalyst signal amplification label, ˙O₂^- can be catalyzed by SOD via a disproportionation reaction to produce O₂ and H₂O₂; then, hMnO₂ is able to trigger the decomposition of H₂O₂ to generate O₂ and H₂O. Therefore, the increased O₂ promotes the separation of electron-hole pairs via consuming more electrons, leading to an effective enhancement of the cathodic PEC behavior. Under optimum conditions, with the cytokeratin 19 fragments 21-1 (CYFRA 21-1) as the targeted detection objects, the microfluidic cathodic PEC biosensor chip exhibited excellent linearity from 0.1 pg mL^-1 to 100 ng mL^-1, with a detection limit of 0.026 pg mL^-1 (S/N = 3). The exciting thing that this work offers is a new strategy for the detection of other important cancer biomarkers for disease diagnosis and prognosis.