Recent Advances in Ion-Selective Membrane Electrodes for In Situ Environmental Water Analysis
General Features of ISEs for Water Analysis
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| 2. General Features of ISEs for Water Analysis
The use of ISEs for ion detection is a well-established methodology in many laboratories. Characteristics, such as versatility (applicable to many cations and anions), simplicity (electrodes and electronic circuitry), low cost, fast response time (< 90 s) as well as a broad dynamic response range (~ 5 orders of magnitude) are responsible for the wide use of ISEs in many applications where precise and fast ion detection is demanded.[34] When analytical methodologies are assessed among each other, there is always a certain degree of dissent and acceptance across scientific communities. Accordingly, the analytical performance of ISEs for a number of scenarios may be questioned (i.e., precision, accuracy, robustness, sensitivity) and this healthy criticism aids in recognizing weaknesses for further progress in the field. However, there is one irrefutable point that makes ISEs unique and this relies on the origin of the potentiometric signal. Briefly, the phase boundary potential (derived from the equality of the electrochemical potentials of the same ion in both phases) is a function of the activity of the free ion and is well-described by the Nernst equation.[35] As a result, ISEs have the ability to provide direct information on so-called “free or uncomplexed ion activities”, even in complex environmental and biomedical matrixes.[36, 37] From the environmental perspective, this is particularly relevant given that the free metal ion activity is thought of as a master variable directly related to bioavailability (for more details, refer to De Marco et al. [20]). As an example, the possibility of measuring free CO 3 2 – and Ca 2+ concentrations is of interest, e.g., in calcification or decalcification studies because the sensor responds directly to the activity of the free CO 3 2 – , which, together with the activity of free Ca 2+ , determining the saturation of CaCO 3 .[38] With the recent incorporation of novel transducing materials, the so-called “all-solid-state ISEs” revolution has begun.[39] Figure 2 illustrates the three key elements that comprise the core of all- 7 solid-state ISEs, which mainly depend on: i) the membrane phase that, on the one hand, isolates the recognition portion of the sensor from the sample and, on the other hand, is responsible for the establishment of phase boundary potential (interface membrane/sample); ii) the receptor (also termed the ionophore in potentiometric jargon), which is involved in the selective recognition of the target; and iii) the transducer, which efficiently converts an ionic current into one that is electronic without leakage. The use of conducting polymers [40] and nanomaterials [39, 41] has facilitated excellent transduction accompanied with a large potential stability and the elimination of an undesired water layer at the metal/membrane interface. Probably, nanostructured transducers that do not display sensitivity to light are rather more interesting for environmental applications in which the light intensity either increase or decrease along the depth.[42] Notably, the more recent incorporation of nanostructured materials as transducers constitutes a valuable tool for the fabrication of long- term durable sensors with huge possibilities regarding substrate-embedding and implementation within different devices.[43-45] Research focusing on the development of novel solid-state electrodes is currently highly stimulating and novel, interesting applications appear daily.[42, 46- 51] In particular, the inherent benefits of the all-solid-state configuration, including the possibility of miniaturization, easy portability, remote control and in-line implementation, are highly suitable for decentralized measurements. In addition, implementation of all-solid-state ISEs into submersible devices is present relevant to in situ environmental water analysis.[21, 24, 32, 52] As an example, backpressure problems presented by classical ISEs based on an inner-filling solution implemented into submersible probes are totally suppressed using the all-solid-state configuration.[53] Despite the aforesaid features that make ISEs suitable for decentralized measurements, the critical assessment of key analytical performance indicators (limit of detection, selectivity, stability and robustness, Figure 2) is crucial to the aim of decentralized natural water analysis. This latter 8 aspect is related to the chemical behaviour of the sensor and its interaction with the complex environment, and thus limitations must be acknowledged. The experimental tải về 1.01 Mb. Chia sẻ với bạn bè của bạn:
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