21
5. Conclusions and future perspectives.
Potentiometric ion-selective platforms in different configurations (i.e., on-board, on-site,
in situ
) are
likely a suitable alternative to the traditional sampling and ion measurements that rely on
extracting samples at different depths and locations and analysing them in centralized laboratories
using expensive instrumentation. The employment of non-submersible or submersible probes
(containing, for example, ISEs, optical sensors, CTD and spectroscopy measurement equipment)
may serve as a way to preserve the sample from any alteration (e.g., pollution, degradation,
destabilization, adsorption), modifying the concentration, the speciation and, ultimately, the
judgment of and the inter-correlation with ongoing processes.
Noticeably, alkali and alkali earth-metal ions, halides and oxanions 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
]. With ISEs functioning as
chemical
ion eyes,
it may be not only possible to contribute new foundations but also correlate this
information with processes, such as nitrification, calcification, accumulation, toxicity and
photosynthesis. [17] Some recent examples have been illustrated throughout this review.
In particular, ISEs have multiple strengths in terms of
in situ
ion monitoring, including an excellent
capacity for being implemented in submersible probes, low cost electronics,
selective response to
free-metal activity, fast response times and insensitivity to light, pressure and turbidity changes
among others. However, ISEs also feature certain disadvantages that are extremely pertinent to
particular applications and none strictly relevant for others. For example, lack
of sensitivity and
selectivity, inadequate long-term stability and lifetime and insufficient precision are selection of
them. Curiously, most of these issues are partially related to the lack of selective and stable
ionophores accompanied with polymeric membranes that present fouling and water adsorption
over prolonged exposure to environmental conditions.
22
Researchers that currently work on the development of ISEs (and derivatives) are aware of these
weaknesses and efforts to those ends are being carried out in collaboration with organic and
material chemists. Today, we are closer and closer to delivering great ISEs
sensors made out of
paper and rubbers on very small scale with implantable or submersible properties, from sensors
that do not require frequent calibration to some where calibration is no longer needed. Extremely
robust ISEs for environmental water research will be the next accomplishment.
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