Characterization and Application of Controllable Local Chemical Changes Produced by Reagent Delivery from a Nanopipet

The Modification and Applications of Nanopipettes in Electrochemical Analysis

Nanopipette, which is fabricated by glasses and possesses a nanoscale pore in the tip, has been proven to be immensely useful in electrochemical analysis. Numerous nanopipette-based sensors have emerged with improved sensitivity, selectivity, ease of use, and miniaturization. In this minireview, we provide an overview of the recent developments of nanopipette-based electrochemical sensors based on different types of nanopipettes, including single-nanopipettes, self-referenced nanopipettes, dual-nanopipettes, and double-barrel nanopipettes. Several important modification materials for nanopipette functionalization are highlighted, such as conductive materials, macromolecular materials, and functional molecules. These materials can improve the sensing performance and targeting specificities of nanopipettes. We also discuss examples of related applications and the future development of nanopipette-based strategies.

pH-sensing hybrid hydrogels for non-invasive metabolism monitoring in tumor spheroids

The constant increase in cancer incidence and mortality pushes biomedical research towards the development of in vitro 3D systems able to faithfully reproduce and effectively probe the tumor microenvironment. Cancer cells interact with this complex and dynamic architecture, leading to peculiar tumor-associated phenomena, such as acidic pH conditions, rigid extracellular matrix, altered vasculature, hypoxic condition. Acidification of extracellular pH, in particular, is a well-known feature of solid tumors, correlated to cancer initiation, progression, and resistance to therapies. Monitoring local pH variations, non-invasively, during cancer growth and in response to drug treatment becomes extremely important for understanding cancer mechanisms. Here, we describe a simple and reliable pH-sensing hybrid system, based on a thermoresponsive hydrogel embedding optical pH sensors, that we specifically apply for non-invasive and accurate metabolism monitoring in colorectal cancer (CRC) spheroids. First, the physico-chemical properties of the hybrid sensing platform, in terms of stability, rheological and mechanical properties, morphology and pH sensitivity, were fully characterized. Then, the proton gradient distribution in the spheroids proximity, in the presence or absence of drug treatment, was quantified over time by time lapse confocal light scanning microscopy and automated segmentation pipeline, highlighting the effects of the drug treatment in the extracellular pH. In particular, in the treated CRC spheroids the acidification of the microenvironment resulted faster and more pronounced over time. Moreover, a pH gradient distribution was detected in the untreated spheroids, with more acidic values in proximity of the spheroids, resembling the cell metabolic features observed in vivo in the tumor microenvironment. These findings promise to shed light on mechanisms of regulation of proton exchanges by cellular metabolism being essential for the study of solid tumors in 3D in vitro models and the development of personalized medicine approaches.

A pH-sensor scaffold for mapping spatiotemporal gradients in three-dimensional in vitro tumour models

The detection of extracellular pH at single cell resolution is challenging and requires advanced sensibility. Sensing pH at high spatial and temporal resolution might provide crucial information in understanding the role of pH and its fluctuations in a wide range of physio-pathological cellular processes, including cancer. Here, a method to embed silica-based fluorescent pH sensors into alginate-based three-dimensional (3D) microgels tumour models, coupled with a computational method for fine data analysis, is presented. By means of confocal laser scanning microscopy, live-cell time-lapse imaging of 3D alginate microgels was performed and the extracellular pH metabolic variations were monitored in both in vitro 3D mono- and 3D co-cultures of tumour and stromal pancreatic cells. The results show that the extracellular pH is cell line-specific and time-dependent. Moreover, differences in pH were also detected between 3D monocultures versus 3D co-cultures, thus suggesting the existence of a metabolic crosstalk between tumour and stromal cells. In conclusion, the system has the potential to image multiple live cell types in a 3D environment and to decipher in real-time their pH metabolic interplay under controlled experimental conditions, thus being also a suitable platform for drug screening and personalized medicine.

Scanning Ion Conductance Microscopy

Scanning ion conductance microscopy (SICM) has emerged as a versatile tool for studies of interfaces in biology and materials science with notable utility in biophysical and electrochemical measurements. The heart of the SICM is a nanometer-scale electrolyte filled glass pipette that serves as a scanning probe. In the initial conception, manipulations of ion currents through the tip of the pipette and appropriate positioning hardware provided a route to recording micro- and nanoscopic mapping of the topography of surfaces. Subsequent advances in instrumentation, probe design, and methods significantly increased opportunities for SICM beyond recording topography. Hybridization of SICM with coincident characterization techniques such as optical microscopy and faradaic electrodes have brought SICM to the forefront as a tool for nanoscale chemical measurement for a wide range of applications. Modern approaches to SICM realize an important tool in analytical, bioanalytical, biophysical, and materials measurements, where significant opportunities remain for further exploration. In this review, we chronicle the development of SICM from the perspective of both the development of instrumentation and methods and the breadth of measurements performed.

Scanning Ion Conductance Microscopy: Surface Charge Effects on Electroosmotic Flow Delivery from a Nanopipette

Scanning ion conductance microscopy (SICM) is a powerful and versatile technique that allows an increasingly wide range of interfacial properties and processes to be studied. SICM employs a nanopipette tip that contains electrolyte solution and a quasi-reference counter electrode (QRCE), to which a potential is applied with respect to a QRCE in a bathing solution, in which the tip is placed. The work herein considers the potential-controlled delivery of uncharged electroactive molecules (solute) from an SICM tip to a working electrode substrate to determine the effect of the substrate on electroosmotic flow (EOF). Specifically, the local delivery of hydroquinone from the tip to a carbon fiber ultramicroelectrode (CF UME) provides a means of quantifying the rate of mass transport from the nanopipette and mapping electroactivity via the CF UME current response for hydroquinone oxidation to benzoquinone. EOF, and therefore species delivery, has a particularly strong dependence on the charge of the substrate surface at close nanopipette-substrate surface separations, with implications for retaining neutral solute within the tip predelivery and for the delivery process itself, both controlled via the applied tip potential. Finite element method (FEM) simulations of mass transport and reactivity are used to explain the experimental observations and identify the nature of EOF, including unusual flow patterns under certain conditions. The combination of experimental results with FEM simulations provides new insights on mass transport in SICM that will enhance quantitative applications and enable new possibilities for the use of nanopipettes for local delivery.

High-resolution label-free 3D mapping of extracellular pH of single living cells

Dynamic mapping of extracellular pH (pHe) at the single-cell level is critical for understanding the role of H+ in cellular and subcellular processes, with particular importance in cancer. While several pHe sensing techniques have been developed, accessing this information at the single-cell level requires improvement in sensitivity, spatial and temporal resolution. We report on a zwitterionic label-free pH nanoprobe that addresses these long-standing challenges. The probe has a sensitivity > 0.01 units, 2 ms response time, and 50 nm spatial resolution. The platform was integrated into a double-barrel nanoprobe combining pH sensing with feedback-controlled distance dependance via Scanning Ion Conductance Microscopy. This allows for the simultaneous 3D topographical imaging and pHe monitoring of living cancer cells. These classes of nanoprobes were used for real-time high spatiotemporal resolution pHe mapping at the subcellular level and revealed tumour heterogeneity of the peri-cellular environments of melanoma and breast cancer cells.

Nanopipette delivery: influence of surface charge

In this report, transport through a nanopipette is studied and the interplay between current rectification and ion delivery for small pipettes is examined. First, surface charge dependence of concentration polarization effects in a quartz nanopipette was investigated. Electrical characterization was performed through current-potential (I-V) measurements. In addition, fluorescein (an anionic fluorescent probe) was utilized to optically map ion enrichment and ion depletion in the nanopipette tip. Bare nanopipettes and polyethylenimine (PEI)-modified nanopipettes were examined. Results confirm that concentration polarization is a surface charge dependent phenomenon and delivery can be controlled through modification of surface charge. The relationship between concentration polarization effects and voltage-driven delivery of charged electroactive species was investigated with a carbon ring/nanopore electrode fabricated from pyrolyzed parylene C (PPC). Factors such as surface charge polarity of the nanopipette, electrolyte pH, and electrolyte concentration were investigated. Results indicate that with modification of surface charge, additional control over delivery of charged species can be achieved.

Nanopipette exploring nanoworld

Nanopipettes, with tip orifices on the order of tens to hundreds of nanometers, have been utilized in the fields of analytical chemistry and nanophysiology. Nanopipettes make nanofabrication possible at liquid/solid interfaces. Moreover, they are utilized in time-resolved measurements and for imaging biological materials, e.g., living cells, by using techniques such as scanning ion-conductance microscopy and scanning electrochemical microscopy. We have successfully fabricated ion-selective nanopipettes that can be used to identify targeted ions such as sodium and potassium in- and outside of living cells. In this review, we discuss the extent of utilization of nanopipettes in investigating the nanoworld. In addition, we discuss the potential applications of future nanopipettes.

Subcellular glucose exposure biases the spatial distribution of insulin granules in single pancreatic beta cells

In living tissues, a cell is exposed to chemical substances delivered partially to its surface. Such a heterogeneous chemical environment potentially induces cell polarity. To evaluate this effect, we developed a microfluidic device that realizes spatially confined delivery of chemical substances at subcellular resolution. Our microfluidic device allows simple setup and stable operation for over 4 h to deliver chemicals partially to a single cell. Using the device, we showed that subcellular glucose exposure triggers an intracellular [Ca(2+)] change in the β-cells. In addition, the imaging of a cell expressing GFP-tagged insulin showed that continuous subcellular exposure to glucose biased the spatial distribution of insulin granules toward the site where the glucose was delivered. Our approach illustrates an experimental technique that will be applicable to many biological experiments for imaging the response to subcellular chemical exposure and will also provide new insights about the development of polarity of β-cells.

Local delivery of molecules from a nanopipette for quantitative receptor mapping on live cells

Using nanopipettes to locally deliver molecules to the surface of living cells could potentially open up studies of biological processes down to the level of single molecules. However, in order to achieve precise and quantitative local delivery it is essential to be able to determine the amount and distribution of the molecules being delivered. In this work, we investigate how the size of the nanopipette, the magnitude of the applied pressure or voltage, which drives the delivery, and the distance to the underlying surface influences the number and spatial distribution of the delivered molecules. Analytical expressions describing the delivery are derived and compared with the results from finite element simulations and experiments on delivery from a 100 nm nanopipette in bulk solution and to the surface of sensory neurons. We then developed a setup for rapid and quantitative delivery to multiple subcellular areas, delivering the molecule capsaicin to stimulate opening of Transient Receptor Potential Vanilloid subfamily member 1 (TRPV1) channels, membrane receptors involved in pain sensation. Overall, precise and quantitative delivery of molecules from nanopipettes has been demonstrated, opening up many applications in biology such as locally stimulating and mapping receptors on the surface of live cells.

Scanning ion conductance microscopy for studying biological samples

Scanning ion conductance microscopy (SICM) is a scanning probe technique that utilizes the increase in access resistance that occurs if an electrolyte filled glass micro-pipette is approached towards a poorly conducting surface. Since an increase in resistance can be monitored before the physical contact between scanning probe tip and sample, this technique is particularly useful to investigate the topography of delicate samples such as living cells. SICM has shown its potential in various applications such as high resolution and long-time imaging of living cells or the determination of local changes in cellular volume. Furthermore, SICM has been combined with various techniques such as fluorescence microscopy or patch clamping to reveal localized information about proteins or protein functions. This review details the various advantages and pitfalls of SICM and provides an overview of the recent developments and applications of SICM in biological imaging. Furthermore, we show that in principle, a combination of SICM and ion selective micro-electrodes enables one to monitor the local ion activity surrounding a living cell.

Scanning ion conductance microscopy

Scanning ion conductance microscopy (SICM) is a versatile type of scanning probe microscopy for studies in molecular biology and materials science. Recent advances in feedback and probe fabrication have greatly increased the resolution, stability, and speed of imaging. Noncontact imaging and the ability to deliver materials to localized areas have made SICM especially fruitful for studies of molecular biology, and many examples of such use have been reported. In this review, we highlight new developments in the operation of SICM and describe some of the most exciting recent studies from this growing field.

A vertical microfluidic probe

Performing localized chemical events on surfaces is critical for numerous applications. We earlier invented the microfluidic probe (MFP), which circumvented the need to process samples in closed microchannels by hydrodynamically confining liquids that performed chemistries on surfaces (Juncker et al. Nat. Mater. 2005, 4, 622-628). Here we present a new and versatile probe, the vertical MFP (vMFP), which operates in the scanning mode while overcoming earlier challenges that limited the practical implementation of the MFP technology. The key component of the vMFP is the head, a microfluidic device (∼1 cm(2) in area) consisting of glass and Si and having microfluidic features fabricated in-plane in the Si layer. The base configuration of the head has two micrometer-size channels that inject/aspirate liquids and terminate at the apex which is ∼1 mm(2). In scanning mode, the head is oriented vertically with the apex parallel to the surface with typical spacing of 1-30 μm. Such length scales and using flow rates from nanoliters/second to microliters/second allow chemical events to be performed on surfaces with tens of picoliter quantities of reagents. Before scanning, the head is clipped on a holder for leak-free, low dead volume interface assembly, providing a simple world-to-chip interface. Surfaces are scanned by mounting the holder on a computer-controlled stage having ∼0.1 μm resolution in positioning. We present detailed steps to fabricate vMFP heads having channels with dimensions from 1 μm × 1 μm to 50 μm × 50 μm for liquid localization over areas of 10-10,000 μm(2). Additionally, advanced design strategies are described to achieve high yield in fabrication and to support a broad range of applications. These include particulate filters, redundant aperture architectures, inclined flow-paths that service apertures, and multiple channels to enable symmetric flow confinement. We also present a method to characterize flow confinement and estimate the distance between the head and the surface by monitoring the evolution of a solution of fluorescently labeled antibody on an activated glass surface. This flow characterization reveals regimes of operation suitable for different surface topographies. We further integrate the dispensing of immersion liquid to the vMFP head for processing surfaces for extended periods of time (∼60 min). The versatility of the vMFP is exemplified by patterning fluorescently labeled proteins, inactivation of cells using sodium hypochlorite, and staining living NIH fibroblasts with Cellomics. These applications are enabled by the compact design of the head, which provides easy access to the surface, simplifies alignment, and enables processing surfaces having dimensions from the micrometer to the centimeter scale and with large topographical variations. We therefore believe that ease-of-operation, reconfigurability, and conservative use of chemicals by the vMFP will lead to its widespread use by microtechnologists and the chemical and biomedical communities.

Functionalized nanopipettes: toward label-free, single cell biosensors

Nanopipette technology has been proven to be a label-free biosensor capable of identifying DNA and proteins. The nanopipette can include specific recognition elements for analyte discrimination based on size, shape, and charge density. The fully electrical read-out and the ease and low-cost fabrication are unique features that give this technology an enormous potential. Unlike other biosensing platforms, nanopipettes can be precisely manipulated with submicron accuracy and used to study single cell dynamics. This review is focused on creative applications of nanopipette technology for biosensing. We highlight the potential of this technology with a particular attention to integration of this biosensor with single cell manipulation platforms.

Topographic imaging of convoluted surface of live cells by scanning ion conductance microscopy in a standing approach mode

Scanning ion conductance microscopy (SICM) using a nanopipette as a probe and ionic current as a feedback signal was introduced as a novel technique to study live cells in a physiological environment. To avoid contact between the pipette tip and cells during the conventional lateral scanning mode, we adopted a standing approach (STA) mode in which the probe was moved vertically to first approach and then retracted from the cell surface at each measurement point on an XY plane. The STA mode ensured non-contact imaging of the topography of live cells and for a wide range of uneven substrates (500 x 300 microm to 5 x 5 microm). We also used a field-programmable gate array (FPGA) board to enhance feedback distance regulation. FPGA dramatically increased the feedback speed and decreased the imaging time (450 s per image) with enhanced accuracy and quality of live cell images. To evaluate the potential of the STA mode for SICM, we carried out imaging of a convoluted surface of live cell in various scan ranges and estimated the spatial resolutions of these images.

Analyte concentration at the tip of a nanopipette

Concentration of molecules within the tips of nanopipettes when applying a DC voltage is herein investigated using finite-element simulations. The ion concentrations and fluxes due to diffusion, electro-migration, and electro-osmotic flow, and the electric potential are determined by the simultaneous solution of the Nernst-Planck, Poisson, and Navier-Stokes equations within the water solution containing sodium and chloride ions and negatively charged molecules. The electric potential within the pipette glass wall is at the same time determined by the Poisson equation together with appropriate boundary conditions and accounts for a field effect through the wall. Fixed negative surface charge on both the internal and external glass surfaces of the nanopipette is included together with the field effect through the glass wall to account for the electric double layer and the electro-osmosis. The inclusion of the field effect through the pipette wall is new compared to previous modeling of similar structures and is shown to be crucial for the behavior at the tip. It is demonstrated that the concentration of molecules is a consequence of ionic charge accumulation at the tip screening the electric field, thereby slowing down the electrophoretic motion of the molecules, which is further slowed down or stopped by the oppositely directed electro-osmosis. It is also shown that the trapping is very sensitive to the properties of the molecule, that is, its electrophoretic mobility and diffusion coefficient, the properties of the pipette, the ionic strength of the solution, and the applied electric field.

Applications of nanopipettes in bionanotechnology

At present, technical hurdles remain in probing biochemical processes in living cells and organisms at nanometre spatial resolution, millisecond time resolution and with high specificity and single-molecule sensitivity. Owing to its unique shape, size and electrical properties, the nanopipette has been used to obtain high-resolution topographic images of live cells under physiological conditions, and to create nanoscale features by controlled delivery of biomolecules. In the present paper, I discuss recent progress in the development of a family of new methods for nanosensing and nanomanipulation using nanopipettes.