Label-Free Imaging of Dynamic and Transient Calcium Signaling in Single Cells

Combining plasmonic and electrochemical biosensing methods

The idea of combining electrochemical (EC) and plasmonic biosensor methods was introduced almost thirty years ago and the potential of electrochemical-plasmonic (EC-P) biosensors has been highlighted ever since. Despite that, the use of EC-P biosensors in analytics has been rather limited so far and the search for unique applications of the EC-P method continues. In this paper, we review the advances in the field of EC-P biosensors and discuss the features and benefits they can provide. In addition, we identify the main challenges for the development of EC-P biosensors and the limitations that prevent EC-P biosensors from more widespread use. Finally, we review applications of EC-P biosensors for the investigation and quantification of biomolecules, and for the study of biomolecular and cellular processes.

Polarization-Sensitive Asymmetric Scattering at the Single-Particle Scale via Surface Plasmon Resonance Microscopy

Surface plasmon resonance microscopy (SPRM), based on the scattering of single molecules at the interface, is a highly efficient analytical platform widely used in the fields of biology and chemistry. Due to the interference scattering, the imaging pattern exhibits typical parabolic tail and phase transition features, providing a quantitative means of observing the changes in the physical and chemical properties of single molecules. In this work, we reported another unique asymmetric parabolic distribution pattern resulting from polarization conversion in the experiment based on SPRM. This microscopic-level feature is derived from the switching between SPR resonant and nonresonant states. Starting from energy flux theory, we constructed an analysis model and conducted full-wave numerical simulations to verify the experimental results. Furthermore, we demonstrate that the optical rotation induced by chiral thin films can be directly measured through imaging with asymmetric features, providing valuable insights into the field of chiral materials. The quantitative interpretation of asymmetric scattering not only advances the fundamental understanding of the imaging mechanism of SPRM, but also opens up possibilities for utilizing this polarization-sensitive characteristic for single-particle detection and sensing in the future.

Plasmonic Scattering Microscopy for Label-Free Imaging of Molecular Binding Kinetics: From Single Molecules to Single Cells

Measuring molecular binding kinetics represents one of the most important tasks in molecular interaction analysis. Surface plasmon resonance (SPR) is a popular tool for analyzing molecular binding. Plasmonic scattering microscopy (PSM) is a newly developed SPR imaging technology, which detects the out-of-plane scattering of surface plasmons by analytes and has pushed the detection limit of label-free SPR imaging down to a single-protein level. In addition, PSM also allows SPR imaging with high spatiotemporal resolution, making it possible to analyze cellular response to the molecular bindings. In this Mini Review, we present PSM as a method of choice for chemical and biological imaging, introduce its theoretical mechanism, present its experimental schemes, summarize its exciting applications, and discuss its challenges as well as the promising future.

Label-Free Optical Imaging of the Electron Transfer in Single Live Microbial Cells

Measurement of electron transfer at the single-particle or -cell level is crucial to the in situ study of basic chemical and biological processes. However, it remains challenging to directly probe the microbial extracellular electron transfer process due to the weakness of signals and the lack of techniques. Here, we present a label-free and noninvasive imaging method that is able to measure the electron transfer in microbial cells. We measured the extracellular electron transfer processes by imaging the redox reaction of c-type outer membrane cytochromes in microbial cells using a plasmonic imaging technique, and obtained the electrochemical activity parameters (formal potential and number of electrons transferred) of multiple individual microbial cells, allowing for unveiling ample heterogeneities in electron transfer at the single-cell level. We anticipate that this method will contribute to the study of electron transfer in various biological and chemical processes.

Light-Addressable Electrochemical Sensors toward Spatially Resolved Biosensing and Imaging Applications

The light-addressable electrochemical sensor (LAES) is a recently emerged bioanalysis technique combining electrochemistry with the photoelectric effect in a semiconductor. In an LAES, a semiconductor substrate is illuminated locally to generate charge carriers in a well-defined area, thereby confining the electrochemical process to a target site. Benefiting from the unique light addressability, an LAES can not only detect multiple analytes in parallel within a single sensor plate but also act as a bio(chemical) imaging sensor to visualize the two-dimensional distribution of specific analytes. An LAES usually has three working modes: a potentiometric mode using light-addressable potentiometric sensors (LAPS) and an impedance mode using scanning photoinduced impedance microscopy (SPIM), while an amperometric mode refers to light-addressable electrochemistry (LAE) and photoelectrochemical (PEC) sensing. In this review, we describe the detection principles of each mode of LAESs and the concept of light addressability. In addition, we highlight the recent progress and advance of LAESs in spatial resolution, sensor system design, multiplexed detection, and bio(chemical) imaging applications. An outlook on current research challenges and future prospects is also presented.

Plasmonic-based impedance microspectroscopy of optically heterogeneous samples

A robust impedance microscopy technique is presented. This optical tool enables high resolution imaging of electrical properties with promising biophysical applications. The underlying principle is that surface plasmon resonance (SPR) sensors are able to measure perturbations of surface charge density and therefore can be used to compute the impedance of surface-adhered cells. However, the ability to perform reliable quantitative impedance imaging is affected by the optical heterogeneity of the cell-sensor interface. To address this issue, a novel method for quantitative time-resolved resonance angle tracking is developed and applied to correct for the effect of the optical properties. To demonstrate the capability of this technique, impedance microspectroscopy of bovine serum albumin (BSA) patterns was performed enabling measurements of capacitance with submicroscopic resolution. The work presented offers an impedance microspectroscopy method that will create new avenues in studying the electrical properties of single cells and biomolecules as well as bio-electrical currents.

Moving Electrons Purposefully through Single Molecules and Nanostructures: A Tribute to the Science of Professor Nongjian Tao (1963-2020)

Electrochemistry intersected nanoscience 25 years ago when it became possible to control the flow of electrons through single molecules and nanostructures. Many surprises and a wealth of understanding were generated by these experiments. Professor Nongjian Tao was among the pioneering scientists who created the methods and technologies for advancing this new frontier. Achieving a deeper understanding of charge transport in molecules and low-dimensional materials was the first priority of his experiments, but he also succeeded in discovering applications in chemical sensing and biosensing for these novel nanoscopic systems. In parallel with this work, the investigation of a range of phenomena using novel optical microscopic methods was a passion of his and his students. This article is a review and an appreciation of some of his many contributions with a view to the future.

Plasmonic scattering imaging of single proteins and binding kinetics

Measuring the binding kinetics of single proteins represents one of the most important and challenging tasks in protein analysis. Here we show that this is possible using a surface plasmon resonance (SPR) scattering technique. SPR is a popular label-free detection technology because of its extraordinary sensitivity, but it has never been used for imaging single proteins. We overcome this limitation by imaging scattering of surface plasmonic waves by proteins. This allows us to image single proteins, measure their sizes and identify them based on their specific binding to antibodies. We further show that it is possible to quantify protein binding kinetics by counting the binding of individual molecules, providing a digital method to measure binding kinetics and analyze heterogeneity of protein behavior. We anticipate that this imaging method will become an important tool for single protein analysis, especially for low volume samples, such as single cells.

Surface Plasmon Resonance Microscopy: From Single-Molecule Sensing to Single-Cell Imaging

Surface plasmon resonance microscopy (SPRM) is a versatile platform for chemical and biological sensing and imaging. Great progress in exploring its applications, ranging from single-molecule sensing to single-cell imaging, has been made. In this Minireview, we introduce the principles and instrumentation of SPRM. We also summarize the broad and exciting applications of SPRM to the analysis of single entities. Finally, we discuss the challenges and limitations associated with SPRM and potential solutions.

Label-free imaging of epidermal growth factor receptor-induced response in single living cells

Epidermal growth factor receptor (EGFR), which belongs to the second-largest protein family for cell signal transduction, plays crucial roles in homeostasis, cellular organized patterns and most human cancers. In EGFR-activated signaling networks, the detection of the spatial and temporal dynamics of cascades that encode the many cell fates is still a challenge. Here, we report real-time imaging of epidermal growth factor (EGF)-induced EGFR activation and its signaling cascade in single A431 cells using surface plasmon resonance (SPR) microscopy. A two-phase SPR response pattern was observed within 30 min after EGF treatment, including a positive SPR response that was related to the EGFR-activated mass redistribution in the first 600 s, and a subsequent negative SPR signal caused by the morphological change of the cells. Furthermore, the inhibitor analysis verified that AG1478 inhibited the response from the whole the cell, whereas cytochalasin B strongly inhibited the response from the cell edge region.

Nanometer-Resolved Mapping of Cell-Substrate Distances of Contracting Cardiomyocytes Using Surface Plasmon Resonance Microscopy

It has been shown that quantitative measurements of the cell-substrate distance of steady cells are possible with scanning surface plasmon resonance microscopy setups in combination with an angle resolved analysis. However, the accuracy of the determined cell-substrate distances as well as the capabilities for the investigation of cell dynamics remained limited due to the assumption of a homogeneous refractive index of the cytosol. Strong spatial or temporal deviations between the local refractive index and the average value can result in errors in the calculated cell-substrate distance of around 100 nm, while the average accuracy was determined to 37 nm. Here, we present a combination of acquisition and analysis techniques that enables the measurement of the cell-substrate distance of contractile cells as well as the study of intracellular processes through changes in the refractive index at the diffraction limit. By decoupling the measurement of the cell-substrate distance and the refractive index of the cytoplasm, we could increase the accuracy of the distance measurement on average by a factor of 25 reaching 1.5 nm under ideal conditions. We show a temporal and spatial mapping of changes in the refractive index and the cell-substrate distance which strongly correlate with the action potentials and reconstruct the three-dimensional profile of the basal cell membrane and its dynamics, while we reached an actual measurement accuracy of 2.3 nm.

Emerging tools for studying single entity electrochemistry

Electrochemistry studies charge transfer and related processes at various microscopic structures (atomic steps, islands, pits and kinks on electrodes), and mesoscopic materials (nanoparticles, nanowires, viruses, vesicles and cells) made by nature and humans, involving ions and molecules. The traditional approach measures averaged electrochemical quantities of a large ensemble of these individual entities, including the microstructures, mesoscopic materials, ions and molecules. There is a need to develop tools to study single entities because a real system is usually heterogeneous, e.g., containing nanoparticles with different sizes and shapes. Even in the case of "homogeneous" molecules, they bind to different microscopic structures of an electrode, assume different conformations and fluctuate over time, leading to heterogeneous reactions. Here we highlight some emerging tools for studying single entity electrochemistry, discuss their strengths and weaknesses, and provide personal views on the need for tools with new capabilities for further advancing single entity electrochemistry.

Electrochemical impedance spectroscopy of single Au nanorods

Monochromatic dark-field microscopy coupled with high-frequency potential modulation leads to non-faradaic electrochemical impedance spectroscopy of single Au nanorods.

We propose monochromatic dark-field imaging microscopy (DFM) to measure the non-faradaic electrochemical impedance spectroscopy (EIS) of single Au nanorods (AuNRs). DFM was utilized to monitor the plasmonic scattering of monochromatic incident light by surface-immobilized individual AuNRs. When modulating the surface potential at a certain frequency, non-faradaic charging and discharging of AuNRs altered their electron density, leading to periodical fluctuations in the scattering intensity. Analysis of the amplitude and phase of the optical intensity fluctuation as a function of modulation frequency resulted in the EIS of single AuNRs. High-frequency (>100 Hz) modulation allowed us to differentiate the intrinsic charging effect from other contributions such as the periodic migration and accumulation of counterions in the surrounding medium, because the latter occurred at a longer timescale. As a result, single nanoparticle EIS led to the surface capacitance of single AuNRs being closer to the theoretical value. Since interfacial capacitance has been proven sensitive to molecular interactions, the present work also offers a new platform for single nanoparticle sensing by measuring the single nanoparticle capacitance.

Plasmonic Imaging of Electrochemical Impedance

Electrochemical impedance spectroscopy (EIS) measures the frequency spectrum of an electrochemical interface to resist an alternating current. This method allows label-free and noninvasive studies on interfacial adsorption and molecular interactions and has applications in biosensing and drug screening. Although powerful, traditional EIS lacks spatial resolution or imaging capability, hindering the study of heterogeneous electrochemical processes on electrodes. We have recently developed a plasmonics-based electrochemical impedance technique to image local electrochemical impedance with a submicron spatial resolution and a submillisecond temporal resolution. In this review, we provide a systematic description of the theory, instrumentation, and data analysis of this technique. To illustrate its present and future applications, we further describe several selected samples analyzed with this method, including protein microarrays, two-dimensional materials, and single cells. We conclude by summarizing the technique's unique features and discussing the remaining challenges and new directions of its application.

Label-Free Imaging of Histamine Mediated G Protein-Coupled Receptors Activation in Live Cells

G protein-coupled receptors (GPCRs) are the largest protein family for cell signal transduction, and most of them are crucial drug targets. Conventional label-free assays lack the spatial information to address the heterogeneous response from single cells after GPCRs activation. Here, we reported a GPCRs study in live cells using plasmonic-based electrochemical impedance microscopy. This label-free optical imaging platform is able to resolve responses from individual cells with subcellular resolution. Using this platform, we studied the histamine mediated GPCRs activation and revealed spatiotemporal heterogeneity of cellular downstream responses. Triphasic responses were observed from individual HeLa cells upon histamine stimulation. A quick peak P1 in less than 10 s was attributed to the GPCRs triggered calcium release. An inverted P2 phase within 1 min was attributed to the alternations of cell-matrix adhesion after the activation of Protein Kinase C (PKC). The main peak (P3) around 3-6 min after the histamine treatment was due to dynamic mass redistribution and showed a dose-dependent response with a half-maximal effective concentration (EC₅₀) of 3.9 ± 1.2 μM. Heterogeneous P3 responses among individual cells were observed, particularly at high histamine concentration, indicating diverse histamine H1 receptor expression level in the cell population.

Label-free versus conventional cellular assays: Functional investigations on the human histamine H1 receptor

A set of histamine H₁ receptor (H₁R) agonists and antagonists was characterized in functional assays, using dynamic mass redistribution (DMR), electric cell-substrate impedance sensing (ECIS) and various signaling pathway specific readouts (Fura-2 and aequorin calcium assays, arrestin recruitment (luciferase fragment complementation) assay, luciferase gene reporter assay). Data were gained from genetically engineered HEK293T cells and compared with reference data from GTPase assays and radioligand binding. Histamine and the other H₁R agonists gave different assay-related pEC₅₀ values, however, the order of potency was maintained. In the luciferase fragment complementation assay, the H₁R preferred β-arrestin2 over β-arrestin1. The calcium and the impedimetric assay depended on G_q coupling of the H₁R, as demonstrated by complete inhibition of the histamine-induced signals in the presence of the G_q inhibitor FR900359 (UBO-QIC). Whereas partial inhibition by FR900359 was observed in DMR and the gene reporter assay, pertussis toxin substantially decreased the response in DMR, but increased the luciferase signal, reflecting the contribution of both, G_q and G_i, to signaling in these assays. For antagonists, the results from DMR were essentially compatible with those from conventional readouts, whereas the impedance-based data revealed a trend towards higher pK_b values. ECIS and calcium assays apparently only reflect G_q signaling, whereas DMR and gene reporter assays appear to integrate both, G_q and G_i mediated signaling. The results confirm the value of the label-free methods, DMR and ECIS, for the characterization of H₁R ligands. Both noninvasive techniques are complementary to each other, but cannot fully replace reductionist signaling pathway focused assays.

Multifunctional-layered materials for creating membrane-restricted nanodomains and nanoscale imaging

Experimental platform that allows precise spatial positioning of biomolecules with an exquisite control at nanometer length scales is a valuable tool to study the molecular mechanisms of membrane bound signaling. Using micromachined thin film gold (Au) in layered architecture, it is possible to add both optical and biochemical functionalities in in vitro. Towards this goal, here, I show that docking of complementary DNA tethered giant phospholiposomes on Au surface can create membrane-restricted nanodomains. These nanodomains are critical features to dissect molecular choreography of membrane signaling complexes. The excited surface plasmon resonance modes of Au allow label-free imaging at diffraction-limited resolution of stably docked DNA tethered phospholiposomes, and lipid-detergent bicelle structures. Such multifunctional building block enables realizing rigorously controlled in vitro set-up to model membrane anchored biological signaling, besides serving as an optical tool for nanoscale imaging.