4
One step even further, in particular concerning the aforementioned high spatial and temporal
resolution, involves the availability of submersible
in situ
analysers. This type of configuration is
not always possible to implement into analytical methodologies and is the likely reason for few
examples in the literature, though there is indeed a clear upward trend. Basically, the
development of a submersible prototype comprises a series of requirements that need to be
satisfied by both the analytical methodology and the corresponding instrumentation (i.e. possibility
of implementation, fast response time, low cost, simplicity, reduced size and weight, low power
consumption and very few calibration steps)[6], and, to date, few methodologies fit well with the
majority of those specifications. Of them, optical sensor (for more
details about optical
commercially available probes, refer to Moore et al. [7], [8]) spectroscopy methods based on
LED/photodiodes [9], voltammetric sensors for sulfides [10] and trace metals [11], ion-selective
electrodes (ISEs) [12-14], pH, salinity and conductivity [15, 16] are the most relevant within a
scientific framework.
A recent contribution by Moore et al. on
“sensors for marine water” listed the most pertinent
chemical
species to measure, such as O
2
, nutrients (N and P), macro- and micronutrients, pCO
2
,
dissolved inorganic carbon, pH and sulfide.[17] In this chemical context, ISEs and, in particular,
ionophore-based ISEs would be useful technology for detecting those species
with a specific
emphasis herein on alkali and alkali earth-metal cations, ammonium ions, halide anions, nutrients
(NO
2
–
, NO
3
–
), carbon (HCO
3
–
/CO
3
2
–
) and phosphorus (HPO
4
2
–
/H
2
PO
4
–
).
In particular, those ions are involved in many biogeochemical processes in water reservoirs, like,
for
instance, the sulphur cycle [S
2
–
/SO
4
2
–
], nitrogen cycle [NO
2
–
, NO
3
–
and NH
4
+
and N
2
], carbon
cycle [CO
3
2
–
, HCO
3
–
, CO
2
and organic carbon], eutrophication by phosphate enrichment [H
2
PO
4
–
/HPO
4
2
–
], calcification and decalcification [Ca
2+
, SO
4
2
–
and CO
3
2
–
] and water acidification [H
+
and
CO
2
]. Therefore, this information may be correlated with processes, such as nitrification,
calcification, mineralization, accumulation, toxicity and photosynthesis. [6, 17-19]
5
Beyond the scope of this review, De Marco et al.[20] comprehensively reviewed the topic of
crystalline and glass ISEs for environmental pollutants with examples for Cu, Fe, Cd, Hg, Pb and
Cr detection. These ISE devices are not amenable to alkali and alkali-earth metals as well as
environmentally important anions such as nitrate, nitrite, carbonate, phosphate, sulphate, etc. By
contrast to, in another review, Bakker [21, 22] provided an overview of available receptors
(selectivity coefficients and limits of detection) for trace metal environmental detection (including
the aforementioned alkali, alkali-earth and anions) using potentiometric polymeric membranes.
Accordingly, this approach has excellent potential for field deployment in the analytical
chemistry
of these environmentally important species.
Potentiometric ISEs feature most of the presented characteristics recommending for implementing
into submersible probes and this uniqueness makes them a very attractive alternative for
in situ
water research.[20, 23, 24] It is interesting to take a glance at other related fields in which a
successful implementation of ISEs for decentralized measurements (e.g., biomedical, agriculture)
is being performed efficiently [25-31] In contrast, it seems that the implementation of ISEs for
in
situ
water research has not been as effective as in other aforementioned fields, though, there is a
recent large number of ISEs that successfully work on the laboratory bench with water
analysis as
a final application [20, 24, 32]. Certain ISEs (i.e., pH, CO
2
and NH
4
+
, K
+
, NO
3
–
, Cu
2+
) have been
truly implemented for
in situ
environmental analysis [3, 13, 14, 33]. It is expected that a clear
assessment of ISEs for natural water monitoring may stimulate novel insights into further
development of submersible devices.
This review examines the current state of ISEs for decentralized water analysis.
Ex situ
and
in situ
configurations are detailed with the most updated examples reported in the literature. These
examples confirm that applications in real aquatic scenarios are possible by achieving a
compromise situation between ISE advantages and implementation challenges. Open questions
and further steps are clearly identified. Finally, the case of saline water
analysis is also considered
based on the difficulty of performing potentiometric measurements of minority ions in such a
6
complex matrix containing excessively high amounts of interfering ions (such as Cl
–
and/or OH
–
).
Elegant approaches that enable the elimination or minimization of these interferences for
unbiased potentiometric detection are briefly described.
Chia sẻ với bạn bè của bạn: