Nano and Microsensors for Mammalian Cell Studies

Micro- and nano-devices for electrochemical sensing

Electrode miniaturization has profoundly revolutionized the field of electrochemical sensing, opening up unprecedented opportunities for probing biological events with a high spatial and temporal resolution, integrating electrochemical systems with microfluidics, and designing arrays for multiplexed sensing. Several technological issues posed by the desire for downsizing have been addressed so far, leading to micrometric and nanometric sensing systems with different degrees of maturity. However, there is still an endless margin for researchers to improve current strategies and cope with demanding sensing fields, such as lab-on-a-chip devices and multi-array sensors, brain chemistry, and cell monitoring. In this review, we present current trends in the design of micro-/nano-electrochemical sensors and cutting-edge applications reported in the last 10 years. Micro- and nanosensors are divided into four categories depending on the transduction mechanism, e.g., amperometric, impedimetric, potentiometric, and transistor-based, to best guide the reader through the different detection strategies and highlight major advancements as well as still unaddressed demands in electrochemical sensing.

Graphical Abstract

In Vitro Methods for Measuring the Permeability of Cell Monolayers

Cell monolayers, including endothelial and epithelial cells, play crucial roles in regulating the transport of biomolecules to underlying tissues and structures via intercellular junctions. Moreover, the monolayers form a semipermeable barrier across which leukocyte transmigration is tightly regulated. The inflammatory cytokines can disrupt the epithelial and endothelial permeability, thus the reduced barrier integrity is a hallmark of epithelial and endothelial dysfunction related with numerous pathological conditions, including cancer-related inflammation. Therefore, the assessment of barrier function is critical in in vitro models of barrier-forming tissues. This review summarizes the commercially available in vitro systems used to measure the permeability of cellular monolayers. The presented techniques are separated in two large groups: macromolecular tracer flux assays, and electrical impedance measurement-based permeability assays. The presented techniques are briefly described and compared.

Label-free cell based impedance measurements of ZnO nanoparticles-human lung cell interaction: a comparison with MTT, NR, Trypan blue and cloning efficiency assays

Background

There is a huge body of literature data on ZnOnanoparticles (ZnO NPs) toxicity. However, the reported results are seen to be increasingly discrepant, and deep comprehension of the ZnO NPs behaviour in relation to the different experimental conditions is still lacking. A recent literature overview emphasizes the screening of the ZnO NPs toxicity with more than one assay, checking the experimental reproducibility also versus time, which is a key factor for the robustness of the results. In this paper we compared high-throughput real-time measurements through Electric Cell-substrate Impedance-Sensing (ECIS®) with endpoint measurements of multiple independent assays.

Results

ECIS-measurements were compared with traditional cytotoxicity tests such as MTT, Neutral red, Trypan blue, and cloning efficiency assays. ECIS could follow the cell behavior continuously and noninvasively for days, so that certain long-term characteristics of cell proliferation under treatment with ZnO NPs were accessible. This was particularly important in the case of pro-mitogenic activity exerted by low-dose ZnO NPs, an effect not revealed by endpoint independent assays. This result opens new worrisome questions about the potential mitogenic activity exerted by ZnO NPs, or more generally by NPs, on transformed cells. Of importance, impedance curve trends (morphology) allowed to discriminate between different cell death mechanisms (apoptosis vs autophagy) in the absence of specific reagents, as confirmed by cell structural and functional studies by high-resolution microscopy. This could be advantageous in terms of costs and time spent. ZnO NPs-exposed A549 cells showed an unusual pattern of actin and tubulin distribution which might trigger mitotic aberrations leading to genomic instability.

Conclusions

ZnO NPs toxicity can be determined not only by the intrinsic NPs characteristics, but also by the external conditions like the experimental setting, and this could account for discrepant data from different assays. ECIS has the potential to recapitulate the needs required in the evaluation of nanomaterials by contributing to the reliability of cytotoxicity tests. Moreover, it can overcome some false results and discrepancies in the results obtained by endpoint measurements. Finally, we strongly recommend the comparison of cytotoxicity tests (ECIS, MTT, Trypan Blue, Cloning efficiency) with the ultrastructural cell pathology studies.

Graphic Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-01033-w.

MEMS biosensor for monitoring water toxicity based on quartz crystal microbalance

This paper presents the use of a commercial quartz crystal microbalance (QCM) to investigate live-cell activity in water-based toxic solutions. The QCM used in this research has a resonant frequency of 10 MHz and consists of an AT-cut quartz crystal with gold electrodes on both sides. This QCM was transformed into a functional biosensor by integrating with polydimethylsiloxane culturing chambers. Rainbow trout gill epithelial cells were cultured on the resonators as a sensorial layer. The fluctuation of the resonant frequency, due to the change of cell morphology and adhesion, is an indicator of water toxicity. The shift in the resonant frequency provides information about the viability of the cells after exposure to toxicants. The toxicity result shows distinct responses after exposing cells to 0.526 μM of pentachlorophenol (PCP) solution, which is the Military Exposure Guidelines concentration. This research demonstrated that the QCM is sensitive to a low concentration of PCP and no further modification of the QCM surface was required.

An evaluation of the impact of clinical bacterial isolates on epithelial cell monolayer integrity by the electric Cell-Substrate Impedance Sensing (ECIS) method

Virulence is the relative capacity of a pathogenic microorganism to cause damage in susceptible host cells such as those found in airway passages and the gut. In this study, the effect of clinical bacterial isolates on the monolayer integrity of cultured human alveolar basal epithelial cells (A549) was evaluated using the Electric Cell-Substrate Impedance Sensing (ECIS) system. ECIS is a morphological biosensor which records electrical properties of cell-covered microelectrodes in an AC circuit including impedance (ohm), resistance (ohm), and capacitance (μFarad). In the current study, fluctuations in the electrical properties of cell-covered microelectrodes reflect dynamic changes in cell morphology resulting from disrupted cell monolayers following exposure to bacteria. Using the ECIS system, real-time changes of cell morphology and disruption of monolayer integrity of cell-cultures in vitro were revealed for A549 cells infected with either Pseudomonas aeruginosa, ESBL Escherichia coli, Staphylococcus aureus (MRSA), or Enterococcus (VRE). We determined empirically that the optimal signal response was obtained for resistance (ohm) measurements at 4000 hertz. Following infection of A549 cells, the data revealed that Pseudomonas aeruginosa resulted in little change in microelectrode resistance (ohm @4 kHz) as compared to pathogen-free controls within the first 12 h. In contrast, E. coli, MRSA, and VRE caused significant changes in electrode resistance (ohm @4 kHz) values in the infected cells compared to controls over the first 5 h. Resistance (ohm @4 kHz) changes were also observed in cell monolayers infected with different bacterial concentrations for all isolates over 24 h. The highest concentration of bacteria caused the measured resistance (ohm @4 kHz) to drop faster than its' immediate lower concentration, suggesting a dose-dependent effect. Compared to live bacteria, cells exposed to heat-killed bacteria did not show significant changes in resistance (ohm @4 kHz) over 48 h post-exposure. Functionally, cytokine responses were different between cells treated with live and heat-killed bacteria. Of note, live bacteria induced IFNγ, IL-13, and IL-1β production in A549 cells, whereas heat-killed bacteria induced IL-8 production suggesting a differential interaction with cells that could reveal the underlying causes of resistance (ohm @4 kHz) changes. Our findings indicate that ECIS provides a means to quantify, automate, and measure bacterial virulence, which may have broader implications governing the course of treatment compared to traditional methods alone.

Nanocalorimeters for biomolecular analysis and cell metabolism monitoring

Nanocalorimeters, or microfabricated calorimeters, provide a promising way to characterize the thermal process of biological processes, such as biomolecule interactions and cellular metabolic activities. They enabled miniaturized heat measurement onto a chip device with potential benefits including low sample consumption, low cost, portability, and high throughput. Over the past few decades, researchers have tried to improve nanocalorimeters' performance, in terms of sensitivity, accuracy, and detection resolution, by exploring different sensing methods, thermal insulation techniques, and liquid handling methods. The enhanced devices resulted in new applications in recent years, and here we have summarized the performance parameters and applications based on categories. Finally, we have listed the current technical difficulties in nanocalorimeter research and hope for future solutions to overcome them.