Metabolic oxygen consumption measurement with a single-cell biosensor after particle microbeam irradiation

Advances in Electrochemistry for Monitoring Cellular Chemical Flux

The transport of organic and inorganic molecules, along with inorganic ions across the plasma membrane results in chemical fluxes that reflect the cellular function in healthy and diseased states. Measurement of these chemical fluxes enables the characterization of protein function and transporter stoichiometry, characterization of a single cell and embryo viability prior to implantation, and screening of pharmaceutical agents. Electrochemical sensors emerge as sensitive and non-invasive tools for measuring chemical fluxes immediately outside the cells in the boundary layer, that are capable of monitoring a diverse range of transported analytes including inorganic ions, gases, neurotransmitters, hormones, and pharmaceutical agents. Used on their own or in combination with other methods, these sensors continue to expand our understanding of the function of rare cells and small tissues. Advances in sensor construction and detection strategies continue to improve sensitivity under physiological conditions, diversify analyte detection, and increase throughput. These advances will be discussed in the context of addressing technical challenges to measuring chemical flux in the boundary layer of cells and measuring the resultant changes to the chemical concentration in the bulk media.

Real-time local oxygen measurements for high resolution cellular imaging

Single-cell metabolic investigations are hampered by the absence of flexible tools to measure local partial pressure of O₂ (pO₂) at high spatial-temporal resolution. To this end, we developed an optical sensor capable of measuring local pericellular pO₂ for subcellular resolution measurements with confocal imaging while simultaneously carrying out electrophysiological and/or chemo-mechanical single cell experiments. Here we present the OxySplot optrode, a ratiometric fluorescent O₂-micro-sensor created by adsorbing O₂-sensitive and O₂-insensitive fluorophores onto micro-particles of silica. To protect the OxySplot optrode from the components and reactants of liquid environment without compromising access to O₂, the micro-particles are coated with an optically clear silicone polymer (PDMS, polydimethylsiloxane). The PDMS coated OxySplot micro-particles are used alone or in a thin (~50 μm) PDMS layer of arbitrary shape referred to as the OxyMat. Additional top coatings on the OxyMat (e.g., fibronectin, laminin, polylysine, special photoactivatable surfaces etc.) facilitate adherence of cells. The OxySplots report the cellular pO₂ and micro-gradients of pO₂ without disrupting the flow of extracellular solutions or interfering with patch-clamp pipettes, mechanical attachments, and micro-superfusion. Since OxySplots and a cell can be imaged and spatially resolved, calibrated changes of pO₂ and intracellular events can be imaged simultaneously. In addition, the response-time (t_0.5 = 0.7 s, 0-160 mmHg) of OxySplots is ~100 times faster than amperometric Clark-type polarization microelectrodes. Two usage example of OxySplots with cardiomyocytes show (1) OxySplots measuring pericellular pO₂ while tetramethylrhodamine methyl-ester (TMRM) was used to measure mitochondrial membrane potential (ΔΨ_m); and (2) OxySplots measuring pO₂ during ischemia and reperfusion while rhod-2 was used to measure cytosolic [Ca²⁺]_i levels simultaneously. The OxySplot/OxyMat optrode system provides an affordable and highly adaptable optical sensor system for monitoring pO₂ with a diverse array of imaging systems, including high-speed, high-resolution confocal microscopes while physiological features are measured simultaneously.

A Horizontal Multi-Purpose Microbeam System

A horizontal multi-purpose microbeam system with a single electrostatic quadruplet focusing lens has been developed at the Columbia University Radiological Research Accelerator Facility (RARAF). It is coupled with the RARAF 5.5 MV Singleton accelerator (High Voltage Engineering Europa, the Netherlands) and provides micrometer-size beam for single cell irradiation experiments. It is also used as the primary beam for a neutron microbeam and microPIXE (particle induced x-ray emission) experiment because of its high particle fluence. The optimization of this microbeam has been investigated with ray tracing simulations and the beam spot size has been verified by different measurements.