Electrochemical cells measurement challenges

Ellipsometry can measure films from subnanometer to a few micrometers, depending on material properties and wavelength of the light source. It has been widely used for thin film measurement in various applications, from biology to semiconductor, and from solid/solid to solid/liquid interfaces [24,25]. Ellipsometer with electrochemical cell for in situ thin film analysis is available from J.A. Woollam Co., Inc. and has been used in the research on electrochemical deposition [26]. However, in situ measurement of anodic films is more challenging because the films are usually metal complexes with unknown optical properties and difficult to verify with other ex situ techniques. 

Molten salts or ionic liquids (also referred to as fused salts by some authors) were among the very first media to be employed for electrochemistry. In fact, Sir Humphrey Davy describes electrochemical experiments with molten caustic potash (KOH) and caustic soda (NaOH) [1] as early as 1802 A wide variety of single molten salts and molten salt mixtures have been used as solvents for electroanalytical chemistry. These melts run the gamut from those that are liquid well below room temperature to those melting at more than 2000°C. The former present relatively few experimental challenges, whereas the latter can present enormous difficulties. For example, commercially available Teflon- and Kel-F-shrouded disk electrodes and Pyrex glass cells may be perfectly adequate for electrochemical measurements in ambient temperature melts such as the room-temperature chloroaluminates, but completely inadequate for use with molten sodium fluoroaluminate or cryolite (mp = 1010°C), which is the primary solvent used in the Hall-Heroult process for aluminum electrowinning. 

Microscale fluidic systems use small volumes so sensitivity of detection can be a challenge. Any detector for chip-based LC needs to be small and ideally have low power consumption. It is generally a problem of interfacing. Flow cell geometry is also a big factor, e.g. a U cell instead of linear flow cell can give a ten-fold increase in sensitivity for absorbance measurements. Electrochemical detection is very common, mainly ampero-metric and potentiometric, and very amenable to detection on chip. Fluorescence is more sensitive than UV-Vis absorbance and chemiluminescence is sensitive down to a single molecule, similar to LIF. 

Cell responses to physical or chemical cues are measured in microfluidic devices primarily via optical or electrochemical means. Huorescence is the most widely used optical detection technique, because absorbance detection (commonly used for macroscale assays) is of limited value in microchannels because of the short path lengths. Fluorescence detection, characterized by its unparalleled sensitivity, is easy to implement in microfluidic systems. Chemiluminescence and bioluminescence also offer low detection limits and have less background noise than fluorescence [8]. Electrochemical detectors are even more easily integrated with microfluidic devices and often are much less expensive than optical systems. However, fabrication of electrodes in microchannel devices is a technical challenge, and the electrical fields used in detection can interfere with on-chip processes such as electrophoresis. Electrochemical techniques include potentiometry, amperometry, and... 

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