In 2014, Prof. Daniel Mandler and I wrote a book chapter titled “Remote Sensing”. This term is often used to describe a research where the information was obtained using a satellite or a high-flying aircraft. A good example for traditional remote sensing is the use of satellite images to investigate global warming trends. This practice is also referred to as Geographical Information Science (GI Science) not to be confused with Geographical Information System (GIS). However, we choose to focus on remote electrochemical sensing. The description we choose was the ability to acquire information about an object or phenomenon without physically contacting the object or place from which this information is obtained. This might sound a broader and more general description, but there is a huge difference between the two. In the common use of remote sensing, like in the example I gave, you take an aerial map of a specific location and investigate or analyze the data. In electrochemical remote sensing, you need to have a physical device that monitors a specific element (or elements) or measure electron transfer reactions and transmit/record that data for you to analyze. An example for remote electrochemical sensing is placing a sensor in a remote stream and measure the concentration of specific contaminants during a chosen period. The following scheme illustrate that difference where (a) is the traditional remote sensing method and (b) is the electrochemical one.

There are a few reasons why electrochemical flow systems raise high interest in the scientific community as well as the relevant industries. These systems can be adjust to provide results that are extremely accurate and precise. Furthermore, the use of a flow system allows simplifying a process that requires many steps and enables automation, which is very appealing to the industry. Coupling a flow system with an electrochemical method allows a measurement of a fast phenomena or an analytical measurement of very low concentrations of electroactive species like heavy metals. Currently the biggest driving force for the development of these systems is legislation. New laws and regulations, require a more accurate measurement of lower concentrations that could be preform in the lab or on-site (online monitoring). This is an area where electrochemical flow system excel.

In our review, we talked about the different sensor types that are available and different sensing methods. Two major aspects deduct the efficiency or type of measurement those were: The nature of the flow system and the electrochemical cell. The liquid delivery systems or pumps that are use (if needed) did not change significantly since our publication therefore I will focus on the electrochemical cell were there were interesting developments.

The majority of electrochemical cells still relays on flow-by (thin layer) or wall-jet techniques (for more information about these types I refer you to our book chapter). While there are several developments in these systems, they are mostly due to electrode modifications or electrochemical techniques and not to the design of the electrochemical flow cell. One example is this excellent paper from R. G. Compton transforming the electrode to a random array of microelectrodes (RAM) for the detection of silver nanoparticles. The most interesting development (in terms of cell design) are in system that involve a flow-through methodology. Flow-through allows an improve mass-transfer of species to the electrode and therefore should be the most accurate and sensitive flow method; however it is also the most complex one.

In this update, I decided to illustrate three systems that show different approach and creativity:

A Modular System: While system that are based on a net shapped electrode were previously investigated, the system that is described in the following article is unique for two main reasons. The first is the modular nature of the system; by changing one part of the system, you change the flow path and the nature of measurement thus making it not a system but a platform. The second reason this is a dual measurement ability, which enables electrochemical and spectroscopic measurements to be performed simultaneously. If you look at the picture below, the system is comprise from two honeycomb electrodes (WE and CE) and it is designed in a “plug and play” configuration. Unit C1 design is for pure electrochemistry measurements, while unit C2 design is for Electrochemiluminescence (ECL) as can be seen in this short video. Unit C3 designed especially for dual electrochemistry and UV-Vis measurements. The authors show how species that form during the electrochemistry process can be measured using UV-Vis spectroscopy. This has great application for reaction mechanism investigations as well as HPLC detectors and pollution monitoring. This system is a great example of the abilities and flexibility electrochemical flow systems provide.

Filter Based: The second system is based on a filter paper saturated with carbon nanotubes (CNT) also known as a buckypaper. This is not a new concept and in 2015 there was a review discussing the different application of this technology for biosensing. Systems that are based on packed carbon beads were also tested for various applications. The idea is to use the increased surface area, absorbing abilities and electrochemical properties of the carbon particles (beads or nanotubes) to remove or detect pollutants (biological, organic or inorganic). The majority of the system that use CNT relay on the ability of the buckypaper to absorb the species and release them later in a different environment for the detection. However, in this article they constructed a flow system for direct detection of copper. This system is very attractive since it allows to use the buckypaper both as a filter to remove contamination but more important as a replaceable and renewable detector for heavy metals.

Electrode Array: The use of more than one working electrode (usually with a bi-potentiostat) had been exploited in the past by several researchers for example the works of P. R Unwin and R. M. Crooks, which utilized a channel flow setup. Yet, not many imbedded an electrode array in a flow system (I had given the example of Compton previously), and it is extremely rare to find an example that also exploit a flow through methodology do to the complexity of the required set-up. The advantage of an electrode array is an increase in the sensitivity and recorded response yielding a better S/N. This work describe a system that relays on replaceable ultramicroelectrodes (UMEs) which allow not only to control the amount of electrodes in the array (1-4) but also to measure simultaneously with two types of electrodes (as WE). The system was evaluated for analytical measurements as well all stochastic nanoparticle measurements.

There is no doubt that we will see much more developments in this field, especially in microfluidics, nano-fluidics and lab-on-chip devices. None of these were intentionally covered in this article. It is clear that in this age, humanity focus more and more on live and measurable qualities (such as sugar levels, hydration level etc.) , therefore online monitoring devices the can transmit results will be a big part of the IoT and lab-on-chip devices will have a major part in the consumer wearable devices as well as medical ones. I choose to focus on slightly bigger systems that may have a bigger effect on the industry and home usage, as opposed to individual use. Municipalities will use these systems to monitor the quality of city waters, water bodies and air pollution. Industrial facilities will employ them for online process monitoring and medical personals will be able to monitor patients during operations. As technology advance, fabrication methods improve and legislation drives research to reach new goals, Flow-through electrochemical flow systems will become more common and take a more vital part in our lives.