Functional Plasmonic Microscope: Characterizing the Metabolic Activity of Single Cells via Sub-nm Membrane Fluctuations.

Metabolic abnormalities are at the center of many diseases, and the capability to film and quantify the metabolic activities of a single cell is important for understanding the heterogeneities in these abnormalities. In this paper, a functional plasmonic microscope (FPM) is used to image and measure metabolic activities without fluorescent labels at a single-cell level. The FPM can accurately image and quantify the subnanometer membrane fluctuations with a spatial resolution of 0.5 µm in real time. These active cell membrane fluctuations are caused by metabolic activities across the cell membrane. A three-dimensional (3D) morphology of the bottom cell membrane was imaged and reconstructed with FPM to illustrate the capability of the microscope for cell membrane characterization. Then, the subnanometer cell membrane fluctuations of single cells were imaged and quantified with the FPM using HeLa cells. Cell metabolic heterogeneity is analyzed based on membrane fluctuations of each individual cell that is exposed to similar environmental conditions. In addition, we demonstrated that the FPM could be used to evaluate the therapeutic responses of metabolic inhibitors (glycolysis pathway inhibitor STF 31) on a single-cell level. The result showed that the metabolic activities significantly decrease over time, but the nature of this response varies, depicting cell heterogeneity. A low-concentration dose showed a reduced fluctuation frequency with consistent fluctuation amplitudes, while the high-concentration dose showcased a decreasing trend in both cases. These results have demonstrated the capabilities of the functional plasmonic microscope to measure and quantify metabolic activities for drug discovery.

Progressive and instantaneous nature of lithium nucleation discovered by dynamic and operando imaging.

The understanding of lithium (Li) nucleation and growth is important to design better electrodes for high-performance batteries. However, the study of Li nucleation process is still limited because of the lack of imaging tools that can provide information of the entire dynamic process. We developed and used an operando reflection interference microscope (RIM) that enables real-time imaging and tracking the Li nucleation dynamics at a single nanoparticle level. This dynamic and operando imaging platform provides us with critical capabilities to continuously monitor and study the Li nucleation process. We find that the formation of initial Li nuclei is not at the exact same time point, and Li nucleation process shows the properties of both progressive and instantaneous nucleation. In addition, the RIM allows us to track the individual Li nucleus's growth and extract spatially resolved overpotential map. The nonuniform overpotential map indicates that the localized electrochemical environments substantially influence the Li nucleation.

Tunable ultra-narrow linewidth diode laser for multiple metastable rare gas pumping.

We present what we believe to be a novel external cavity feedback structure based on a double-layer laser diode array with volume Bragg grating (VBG). Diode laser collimation and external cavity feedback result in a high-power and ultra-narrow linewidth diode laser pumping source with a central wavelength of 811.292 nm, spectral linewidth of 0.052 nm, and output power exceeding 100 W, with external cavity feedback and electro-optical conversion efficiencies exceeding 90% and 46%, respectively. The temperature of VBG is controlled to tune the central wavelength from 811.292 nm to 811.613 nm, covering the Kr* and Ar* absorption spectra. We believe this is the first report of an ultra-narrow linewidth diode laser that can pump two metastable rare gases.

Imaging solid-electrolyte interphase dynamics using operando reflection interference microscopy.

The quality of the solid-electrolyte interphase is crucial for the performance of most battery chemistries, but its formation dynamics during operation are not well understood due to a lack of reliable operando characterization techniques. Herein, we report a dynamic, non-invasive, operando reflection interference microscope to enable the real-time imaging of the solid-electrolyte interphase during its formation and evolution processes with high sensitivity. The stratified structure of the solid-electrolyte interphase formed during four distinct steps includes the emergence of a permanent inner inorganic layer enriched in LiF, a transient assembly of an interfacial electrified double layer and a consequent emergence of a temporary outer organic-rich layer whose presence is reversible with electrochemical cycling. Reflection interference microscope imaging reveals an inverse correlation between the thicknesses of two interphasial subcomponents, implying that the permanent inorganic-rich inner layer dictates the organic-rich outer layer formation and lithium nucleation. The real-time visualization of solid-electrolyte interphase dynamics provides a powerful tool for the rational design of battery interphases.

Three-dimensional Zn-based alloys for dendrite-free aqueous Zn battery in dual-cation electrolytes.

Aqueous zinc-ion batteries, in terms of integration with high safety, environmental benignity, and low cost, have attracted much attention for powering electronic devices and storage systems. However, the interface instability issues at the Zn anode caused by detrimental side reactions such as dendrite growth, hydrogen evolution, and metal corrosion at the solid (anode)/liquid (electrolyte) interface impede their practical applications in the fields requiring long-term performance persistence. Despite the rapid progress in suppressing the side reactions at the materials interface, the mechanism of ion storage and dendrite formation in practical aqueous zinc-ion batteries with dual-cation aqueous electrolytes is still unclear. Herein, we design an interface material consisting of forest-like three-dimensional zinc-copper alloy with engineered surfaces to explore the Zn plating/stripping mode in dual-cation electrolytes. The three-dimensional nanostructured surface of zinc-copper alloy is demonstrated to be in favor of effectively regulating the reaction kinetics of Zn plating/stripping processes. The developed interface materials suppress the dendrite growth on the anode surface towards high-performance persistent aqueous zinc-ion batteries in the aqueous electrolytes containing single and dual cations. This work remarkably enhances the fundamental understanding of dual-cation intercalation chemistry in aqueous electrochemical systems and provides a guide for exploring high-performance aqueous zinc-ion batteries and beyond.

Electrochemical Impedance Imaging on Conductive Surfaces.

Electrochemical impedance spectroscopy (EIS) is a powerful tool to measure and quantify the system impedance. However, EIS only provides an average result from the entire electrode surface. Here, we demonstrated a reflection impedance microscope (RIM) that allows us to image and quantify the localized impedance on conductive surfaces. The RIM is based on the sensitive dependence between the materials' optical properties, such as permittivity, and their local surface charge densities. The localized charge density variations introduced by the impedance measurements will lead to optical reflectivity changes on electrode surfaces. Our experiments demonstrated that reflectivity modulations are linearly proportional to the surface charge density on the electrode and the measurements show good agreement with the simple free electron gas model. The localized impedance distribution was successfully extracted from the reflectivity measurements together with the Randles equivalent circuit model. In addition, RIM is used to quantify the impedance on different conductive surfaces, such as indium tin oxide, gold film, and stainless steel electrodes. A polydimethylsiloxane-patterned electrode surface was used to demonstrate the impedance imaging capability of RIM. In the end, a single-cell impedance imaging was obtained by RIM.

Stable, high-performance, dendrite-free, seawater-based aqueous batteries.

Metal anode instability, including dendrite growth, metal corrosion, and hetero-ions interference, occurring at the electrolyte/electrode interface of aqueous batteries, are among the most critical issues hindering their widespread use in energy storage. Herein, a universal strategy is proposed to overcome the anode instability issues by rationally designing alloyed materials, using Zn-M alloys as model systems (M = Mn and other transition metals). An in-situ optical visualization coupled with finite element analysis is utilized to mimic actual electrochemical environments analogous to the actual aqueous batteries and analyze the complex electrochemical behaviors. The Zn-Mn alloy anodes achieved stability over thousands of cycles even under harsh electrochemical conditions, including testing in seawater-based aqueous electrolytes and using a high current density of 80 mA cm-2. The proposed design strategy and the in-situ visualization protocol for the observation of dendrite growth set up a new milestone in developing durable electrodes for aqueous batteries and beyond.

Plasmonic Imaging of Oxidation and Reduction of Single Gold Nanoparticles and Their Surface Structural Dynamics.

Gold nanoparticles (AuNPs) have been widely used in catalytic electrochemistry. Heterogeneity in size, shape, and surface sites leads to variable, particle-specific catalytic activities. Conventional electrochemical methods can only obtain the collective responses from all the catalytic nanoparticles on the electrode surface; the heterogeneity of particle performance will be averaged. Alternatively, plasmonic electrochemical imaging (PECi) is capable of imaging the electrochemical activity at individual nanoparticles. In this work, PECi was used to image the oxidation and reduction of the gold surface at individual AuNPs, and their associated structural alterations were successfully measured. We have studied the electrochemical responses from gold nanocubes, gold nanorods, and gold nanowires with PECi and observed different surface redox activities. We have also demonstrated the capability of monitoring the surface dynamics at individual AuNPs utilizing characteristic PECi derived cyclic voltammograms (CVs).

Imaging the Electrochemical Impedance of Single Cells via Conductive Polymer Thin Film.

Many fundamentally important biological phenomena involve the cells to establish and break down the adhesive interactions with the substrate. Here, we report a novel optical method that could directly image the electrochemical impedance of cell-substrate interactions at the single cell level with conventional microscopes and cameras. A thin conductive polymer layer on top of the ITO substrate (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), PEDOT:PSS) is used as the impedance imaging and sensing layer. A sinusoidal electrochemical potential is applied to the conductive polymer film, and the ion intercalation and transportation in the PEDOT:PSS layer will change the absorption spectrum of the polymer film. The attachment of the cells to the substrate will block and affect the ion doping and dedoping process, and therefore change the color of the polymer film. This process can be captured by any upright or inverted microscope with a simple camera. Utilizing this method, we have successfully imaged the impedance of single-cell attachment, observed the variations of cell-substrate interactions, and measured the impedance changes at different stages of the attachment process. This paper has proposed and successfully demonstrated a new strategy that translates the electrochemical impedance information to an optical signal that could be imaged and used to quantify the local responses. In addition, this method does not need any specially designed optical setup, which may lead to its broad applications in the clinics and biological research laboratories.

Phase imaging of transition from classical to quantum plasmonic couplings between a metal nanoparticle and a metal surface.

When a metal nanoparticle is brought near to a metal surface within electron tunneling distance (∼1 nm), classical electromagnetic coupling between the nanoparticle and the metal is expected to transition to quantum coupling. We show that this transition can be observed as a drastic phase change in the surface plasmon resonance (SPR) images of the gold nanoparticles. We study the transition by controlling the distance between the nanoparticles and electrode surface, modeling the impact of the transition on the SPR image in terms of a phase shift and demonstrating detection of microRNA based on the transition from classical to quantum coupling. The work shows that the quantum coupling can be directly visualized in SPR, and the extremely sensitive dependence of the transition on distance leads to a biosensing principle with SPR.

A Paper Based Milli-Cantilever Sensor for Detecting Hydrocarbon Gases via Smartphone Camera.

Hydrocarbon gases, especially toxic ones like benzene and xylene, pose threats to human health and the environment. But existing detection techniques, like bulky GC-MS or portable PID, cannot fulfill people's requirement of affordable and reliable hydrocarbons monitoring for the purpose of personal exposure assessment. Here, a simple, low cost, and light hydrocarbon gases sensor using a smartphone camera as a readout was developed based on the paper based milli-cantilever bending induced by polymer swelling. Its sensing cantilever was composed of three layers: functional layer of polyethylene film, adhesive layer of double-side tape, and a substrate of weighing paper. And the dimensions of the milli-fabricated sensing cantilever are 8 mm long, 0.5 mm wide, and 50 µm thick. The sensor response was the displacement of milli-cantilever free end. As proof of concept, its performance to typical hydrocarbons of xylene, hexane, and BTEX was carefully examined. For all of them, the sensor showed good performance of linear response to hydrocarbon concentrations, wide detection range, low detection, and fast response. Taking xylene for example, the sensor showed wide detection range of 15-140 ppm, low detection limit of 15 ppm, and fast response of 30 s. The sensor cross-sensitivity to other hydrocarbons was consistent with polymer swelling theory that the more carbons the hydrocarbon has, the higher the sensor sensitivity. Taking advantage of the rough materials chosen and simple fabrication procedure, the developed sensors also had high stability with time, low cost, and good uniformity. The developed sensor is affordable both physically and financially, has good performance, could meet hydrocarbons monitoring requirements for occupational safety or air pollution in petroleum industry, and would benefit people's health.

Integrating Electrochemical and Colorimetric Sensors with a Webcam Readout for Multiple Gas Detection.

Multisensor detectors have merits of low cost, compact size, and capability of supplying accurate and reliable information otherwise hard to obtain by any single sensors. They are therefore highly desired in various applications. Despite the advantages and needs, they face great challenges in technique especially when integrating sensors with different sensing principles. To bridge the gap between the demand and technique, we here demonstrated an integration of electrochemical and colorimetric sensors with a webcam readout for multiple gas detection. Designed with two parallel gas channels but independent sensor cells, the dual-sensor detector could simultaneously detect each gas from their gas mixture by analysis of the group photo of the two sensors. Using Ag electro-dissolution as reporter, the bipolar electrochemical sensor achieved quantitative analysis for the first time thanks to application of pulse voltage. The sacrificed Ag layer used in the bipolar electrochemical (EC) sensor was recycled from CD, which further decreased the sensor cost and supplied a new way of CD recycling. The EC O2 sensor response, edge displacement of Ag layer due to electrochemical dissolution, has a linear relationship with O2 concentration ranging from 0 to 30% and has good selectivity to common oxidative gases. The colorimetric NO2 sensor linearly responded to NO2 concentrations ranging from 0 to 230 ppb with low detection limit of 10 ppb, good selectivity, and humidity tolerance. This integration method could be extended to integrating other gas sensors.

Quantifying Ligand-Protein Binding Kinetics with Self-Assembled Nano-oscillators.

Measuring ligand-protein interactions is critical for unveiling molecular-scale biological processes in living systems and for screening drugs. Various detection technologies have been developed, but quantifying the binding kinetics of small molecules to the proteins remains challenging because the sensitivities of the mainstream technologies decrease with the size of the ligand. Here, we report a method to measure and quantify the binding kinetics of both large and small molecules with self-assembled nano-oscillators, each consisting of a nanoparticle tethered to a surface via long polymer molecules. By applying an oscillating electric field normal to the surface, the nanoparticle oscillates, and the oscillation amplitude is proportional to the number of charges on the nano-oscillator. Upon the binding of ligands onto the nano-oscillator, the oscillation amplitude will change. Using a plasmonic imaging approach, the oscillation amplitude is measured with subnanometer precision, allowing us to accurately quantify the binding kinetics of ligands, including small molecules, to their protein receptors. This work demonstrates the capability of nano-oscillators as an useful tool for measuring the binding kinetics of both large and small molecules.

Plasmonic Measurement of Electron Transfer between a Single Metal Nanoparticle and an Electrode through a Molecular Layer.

We study electron transfer associated with electrocatalytic reduction of hydrogen on single platinum nanoparticles separated from an electrode surface with an alkanethiol monolayer using a plasmonic imaging technique. By varying the monolayer thickness, we show that the reaction rate depends on electron tunneling from the electrode to the nanoparticle. The tunneling decay constant is ∼4.3 nm-1, which is small compared to those in literature for alkanethiols. We attribute it to a reduced tunneling barrier resulting from biasing the electrode potential negatively to the hydrogen reduction regime. In addition to allowing study of electron transfer of single nanoparticles, the work demonstrates an optical method to measure charge transport in molecules electrically wired to two electrodes.

Gold-implanted plasmonic quartz plate as a launch pad for laser-driven photoacoustic microfluidic pumps.

Enabled initially by the development of microelectromechanical systems, current microfluidic pumps still require advanced microfabrication techniques to create a variety of fluid-driving mechanisms. Here we report a generation of micropumps that involve no moving parts and microstructures. This micropump is based on a principle of photoacoustic laser streaming and is simply made of an Au-implanted plasmonic quartz plate. Under a pulsed laser excitation, any point on the plate can generate a directional long-lasting ultrasound wave which drives the fluid via acoustic streaming. Manipulating and programming laser beams can easily create a single pump, a moving pump, and multiple pumps. The underlying pumping mechanism of photoacoustic streaming is verified by high-speed imaging of the fluid motion after a single laser pulse. As many light-absorbing materials have been identified for efficient photoacoustic generation, photoacoustic micropumps can have diversity in their implementation. These laser-driven fabrication-free micropumps open up a generation of pumping technology and opportunities for easy integration and versatile microfluidic applications.

Smartphone Nanocolorimetry for On-Demand Lead Detection and Quantitation in Drinking Water.

Lead ions (Pb2+) contamination in drinking water, a major source of lead poisoning to the general population, is typically detected by bulky and costly laboratory analytical instrument. A mobile analytical device for rapid Pb2+ sensing is a growing demand. Herein, we report smartphone nanocolorimetry (SNC) as a new technique to detect and quantify dissolved Pb2+ in drinking water. Specifically, we have employed a single-step sedimentation approach by mixing a controlled quantity of chromate ion (CrO42-) to react with Pb2+ containing solutions to form highly insoluble lead chromate (PbCrO4) nanoparticles as vivid yellow precipitates. This is followed by microscopic color detection and intensity quantitation at nanoscale level using dark-field smartphone microscopy. The sum of the intensity of yellow pixels bears a highly reproducible relationship with Pb2+ concentration between 1.37 and 175 ppb in deionized water and 5-175 ppb in city tap water. In contrast to traditional colorimetric techniques analyzing bulk color changes, SNC achieves unparalleled sensitivity by combining nanocolorimetry with dark-field microscopy and mobilized the metal ions detection by integrating the detection into the smartphone microscope platform. SNC is rapid and low-cost and has the potential to enable individual citizens to examine Pb2+ content in drinking water on-demand in virtually any environmental setting.

Measuring Ligand Binding Kinetics to Membrane Proteins Using Virion Nano-oscillators.

Membrane proteins play vital roles in cellular signaling processes and serve as the most popular drug targets. A key task in studying cellular functions and developing drugs is to measure the binding kinetics of ligands with the membrane proteins. However, this has been a long-standing challenge because one must perform the measurement in a membrane environment to maintain the conformations and functions of the membrane proteins. Here, we report a new method to measure ligand binding kinetics to membrane proteins using self-assembled virion oscillators. Virions of human herpesvirus were used to display human G-protein-coupled receptors (GPCRs) on their viral envelopes. Each virion was then attached to a gold-coated glass surface via a flexible polymer to form an oscillator and driven into oscillation with an alternating electric field. By tracking changes in the oscillation amplitude in real-time with subnanometer precision, the binding kinetics between ligands and GPCRs was measured. We anticipate that this new label-free detection technology can be readily applied to measure small or large ligand binding to any type of membrane proteins and thus contribute to the understanding of cellular functions and screening of drugs.

Modeling the surface of fast-cured polymer droplet lenses for precision fabrication.

Optical lenses with diameter in the millimeter range have found important commercial use in smartphone cameras. Although these lenses are typically made by molding, recent demonstration of fast-cured polymer droplets by inkjet printing has gained interest for cost-effective smartphone microscopy. In this technique, the surface of a fast-cured polydimethylsiloxane droplet obtains dynamic equilibrium via the interplay of surface tension, gravity, thermalization, and a steep viscosity hike. The nature of surface formation involves multiple physical and chemical domains, which represent significant challenges in modeling with the Young-Laplace theory, assuming constant surface tension and viscosity. To overcome these challenges, we introduce the concept of effective surface tension, which allows fast-cured polymer droplets to be modeled as normal liquid droplets with constant viscosity.

Fast Electrochemical and Plasmonic Detection Reveals Multitime Scale Conformational Gating of Electron Transfer in Cytochrome c.

Conformational fluctuations play a central role in the electron transfer reactions of molecules. Because the fluctuations can be extremely fast in kinetics and small in amplitude, a technique with fast temporal resolution and high conformational sensitivity is needed to follow the transient electron transfer processes. Here we report on an electrochemically controlled plasmonic detection technique capable of monitoring conformational changes in redox molecules with ns response time. Using the technique, we study the electron transfer reaction and the associated conformational gating of a redox protein (cytochrome c). The study reveals that the conformational gating takes place over a broad range of time scales, from microsecond to millisecond.

Achieving High Spatial Resolution Surface Plasmon Resonance Microscopy with Image Reconstruction.

Surface plasmon resonance microscopy (SPRM) is a powerful platform for biomedical imaging and molecular binding kinetics analysis. However, the spatial resolution of SPRM along the plasmon propagation direction (longitudinal) is determined by the decaying length of the plasmonic wave, which can be as large as tens of microns. Different methods have been proposed to improve the spatial resolution, but each at the expense of decreased sensitivity or temporal resolution. Here we present a method to achieve high spatial resolution SPRM based on deconvolution of complex field. The method does not require additional optical setup and improves the spatial resolution in the longitudinal direction. We applied the method to image nanoparticles and achieved close-to-diffraction limit resolution in both longitudinal and transverse directions.