Potentiometric sensors for trace-level analysis

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Abstract

This review summarizes recent progress in the development and application of potentiometric sensors with limits of detection (LODs) in the range 10−8–10−11 M. These LODs relate to total sample concentrations and are defined according to a definition unique to potentiometric sensors. LODs calculated according to traditional protocols (three times the standard deviation of the noise) yield values that are two orders of magnitude lower. We are targeting this article at analytical chemists who are non-specialists in the development of such sensors so that this technology may be adopted by a growing number of research groups to solve real-world analytical problems.

We discuss the unique response features of potentiometric sensors and compare them to other analytical techniques, emphasizing that the choice of the method must depend on the problem of interest. We discuss recent directions in sensor design and development and present a list of 23 sensors with low LODs, with references. We give recent examples where potentiometric sensors have been used to solve trace-level analytical problems, including the speciation of lead and copper ions in drinking water, the measurement of free copper in sea water, and the uptake of cadmium ions by plant roots as a function of their speciation.

Section snippets

Introduction – What are potentiometric sensors good for?

In recent years, the well-established field of potentiometric sensors has undergone a quiet revolution that did not go unnoticed in the general analytical chemistry community. While it has been traditional wisdom that such sensors may reach only mediocre limits of detection (LODs) around the micromolar range, they have now been improved to make possible true trace-level analysis at sub-nanomolar (low parts per trillion) concentrations. Clearly, this improvement asks for new applications for

What does LOD mean?

In general, the lower LOD is defined as the concentration of the analyte at which the signal is increased relative to the background level by three times the standard deviation of the noise [7]. According to IUPAC recommendations [8], the definition of the lower LOD in potentiometry is unique. This is somewhat unfortunate because a direct comparison with corresponding figures of other methods is not appropriate. This is especially confusing because of the recent improvement of potentiometric

Potentiometric sensors at trace levels: the state of the art

Traditionally, ISEs are distinguished by the underlying membrane material. Polymeric membrane-based sensors are a group of very high chemical versatility and tunability because the selectivity is given by the extraction of ions into a polymer and complexation with a selective receptor that may be chemically designed [3]. Glass electrodes, including chalcogenide glasses, are an attractive material for a variety of ions, including H+, but the fine tuning of their electrochemical response is

Predictability of interference effects with potentiometric sensors

An important characteristic of potentiometry is that the response function may be predicted on the basis of fundamental relationships and measurable parameters. In the case of polymer membrane electrodes, the response function is related to thermodynamic constants and the composition of the membrane. The contribution of the individual ions to the EMF can be calculated with potentiometric selectivity coefficient KIJpot, which is determined from measurements on simple solutions (pure solutions or

Applications of ISEs with low LODs

The focused development of polymeric membrane electrodes for trace analysis started less than 10 years ago [39], [40] although some early examples of trace-level measurements with a Cu2+-selective electrode are known [56], [57]. More recently, different solid-state electrodes have been applied for trace-level measurements in seawater. The Cu2+-electrode based on an optimized jalpaite membrane in the rotating disk configuration was used to analyze San Diego Bay seawater samples [18]. The LODs

Future directions

The last few years have witnessed significant activity in understanding the principles that may dictate the low LODs of potentiometric sensors and in finding protocols and examples of successful improvements. Because of this, perhaps, a novice in the field may seem somewhat overwhelmed by the various choices. It will therefore be crucial to see a unified, simplified approach to producing potentiometric sensors with low LODs, rapid response time, sufficient chemical ruggedness and long lifetime,

Acknowledgments

The authors thank the US National Institutes of Health (Grant R01-EB02189 and R01-GM071623) for supporting their research on potentiometric sensors for trace-level analysis (joint grant) and instrumentally controlled ion sensors (E.B.). E.P. also acknowledges financial support from the Swiss National Science Foundation and an internal research grant from ETH Zurich.

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