Fiber optic fluorescent sensors for the detection of heavy metal ions in aqueous media
The risks associated to water contamination are very important for human health and environmental protection. Therefore, water quality monitoring is a major concern of current society. In this sense, the efforts of the last years have been dedicated to the development of fast, sensitive, and selective sensors for the detection of heavy metal ions. Among all of them, optical fiber sensors offer the possibility of continuously monitoring the environment, even under hazardous conditions, and the feasibility of being integrated into a microfluidic system. Apart from that, some of the most popular optical sensors are those based on fluorescence quenching (see our previous post for more information).
Fluorescent sensors have gained in popularity due to their interesting features, such as high specificity, sensitivity, and reversibility. Devices based on fluorescent aptasensors, quantum dots, and organic dyes for heavy metal ion detection in aqueous media are thoroughly reviewed in . The utilization of these fluorescent markers requires the selection of the adequate wavelength excitation source for a correct fluorescence excitation. We can see in Figure 1 a typical fiber optic reflection setup used for the sensor monitoring. Here, the sensor was connected to the common branch of a 600 μm-core bifurcated fiber, whilst the light source and the spectrometer were connected to each one of its independent branches. The excitation source (Pyroistech COB-365) is selected according to the spectral features of the fluorescent dye. Fluorescence is monitored using a handheld portable spectrometer (Pyroistech).
Figure 1: Typical reflection setup used for the fluorescence measurement (CC license)
A particular case consists of the utilization of an optical fiber luminescent aptasensor for the detection of Hg2+ ions in aqueous media as it is presented in the work performed by Nerea de Acha . Here, the sensor was fabricated on the tip of a tapered 1000µm core plastic cladding silica fiber by means of a salinization process with APTMS followed by the linkage of the aptamer terminated in a fluorophore (Atto 390) at both 5’ and 3’.
The sensing working principle basically consist of the measurement of the quenching of the fluorescence induced in presence of the Hg2+ ion as it is schematically represented in Figure 2.
Figure 2. Structure of the aptamer sequence in the absence and in the presence of Hg2+ ions.
Fluorescence was characterized in this case using a reflection architecture, as it is shown in Figure 1 showing a clear decrease in fluorescence for increasing concentrations of Hg2+ (from 4·10-8 M to 8·10-8 M) as well as represent the dynamic response of the device (normalized luminescence intensity at 460nm) that probe the feasibility of the utilization of this kind of devices.
REFERENCES Nerea De Acha 1, César Elosúa, Jesús M. Corres and Francisco J. Arregui, “Fluorescent Sensors for the Detection of Heavy Metal,” Sensors, vol. 19(599), pp. 1-34, 2019 doi:10.3390/s19030599
 Nerea De Acha, César Elosúa and Francisco J. Arregui, “Optical fiber luminescent aptasensor for the detection of Hg2+ ions in aqueous media,” Procceedings of Sensors, 7th International Symposium on Sensor Science, 9-11 May 2019, Napoli, Italy.