Continuous monitoring of oxygen concentration is of great
importance in many different areas of research which range from medical
applications to food packaging. In the last three decades, significant progress
has been made in the field of optical sensing technology and this review will
highlight the one inherent to the development of oxygen indicators. The first
section outlines the bioanalytical fields in which optical oxygen sensors have
been applied. The second section gives the reader a comprehensive summary of
the existing oxygen indicators with a critical highlight on their photophysical
and sensing properties. Altogether, this review is meant to give the potential
user a guide to select the most suitable oxygen indicator for the particular
application of interest.
Oxygen
is by far one of the most important chemical species on earth since it is
essential for life. Measurements of its concentration are of extreme importance
in many different research fields such as: medicine, chemistry, environmental
and marine analysis, molecular biotechnology, bioprocess control, food
packaging, and industrial production monitoring. In the majority of the cases,
it would be ideal to monitor oxygen concentration continuously which implies
the use of oxygen sensors: a class of chemical sensors and by definition “a miniaturized device that can deliver real-time and
on-line information on the presence of specific compounds or ions in even
complex samples” .
Several methods
for oxygen detection exist and can be classified on the basis of the principle
used in electrochemical (amperometric, potentiometric, or conductometric),
optical (absorption changes or photoluminescence), and chemical (Winkler
titration).Since its development, the
Optical chemical sensors can be divided in several subgroups depending on the working principle applied; practically all spectroscopic methods have been used (absorption spectroscopy, reflectometry, luminescence, infrared and Raman spectroscopies, interferometry, and surface plasmon resonance). The majority of the optical sensors developed for oxygen detection rely on quenching of the luminescence of an indicator dye by molecular oxygen.
Typical layouts consist of a luminescent dye, whose optical properties are reversibly influenced by the presence of molecular oxygen, which is usually incorporated into a polymeric matrix and deposited on a solid support (planar waveguide, microtitre plate, or optical fiber). Nano- and microparticle-based oxygen probes have also proved to be important analytical tools.
The field of application plays an important role in the choice of the indicator dye and, as a consequence, of the matrix material and detection method. For example, when measuring oxygen in live cells or in tissues, it is necessary to take into account the autofluorescence generated by the presence of biological substances such as proteins, DNA, and melanin. In such cases, in order to minimize absorption and scattering of the excitation and emission light in the tissue, it is preferable to employ indicators that show longwave-shifted absorption (590–650 nm) and emission (730–900 nm) bands. On the other hand, when measuring ultrafast oxygen dynamics, for example in breath monitoring application , it is crucial to use optodes with a very fast response time which can be achieved by employing very thin sensing layers and indicator dyes possessing exceptional brightness.
The
scope of this review is not only to provide the reader with a selection of
recently developed oxygen indicators but also to give a feeling about the area
of applicability with a special focus on bioanalysis.
没有评论:
发表评论