IJUP08 - Universidade do Porto

Fiber optic pH sensors based on CdTe quantum dots
Helena Gonçalves 1 , Conceição Mendonça 1 , Joaquim C.G. Esteves da Silva 1 ,
César Maule 2 and Pedro Jorge 2
1 Department of Chemistry, CIQ-UP, Faculty of Sciences, University of Porto, Portugal.
2 Optoelectronics Unit of INESC Porto, Department of Physics, University of Porto, Portugal.
Quantum dots (QDs) are gaining a great scientific interest due to their tunable optical
(absorption and emission of radiation) properties. Also, by modifying the surface of QDs
with hydrophilic capping ligands provides stability in water. One end of these hydrophilic
capping ligands contains a thio group that binds with the QDs surface, and the other, polar
end contains a carboxylic group that makes them water soluble. Because the carboxylic
acid group has acid-base properties the optical properties of QDs may be used as pH
sensing molecular devices.
In this communication the synthesis of thiglycolate capped CdTe QDs is described and
their luminescence properties (excitation and emission wavelength and lifetimes) obtained
as function of the reaction time, concentration and pH are presented. Moreover, the
immobilization of QDs in fiber optics using sol-gel techniques is described and their use as
fiber optic pH sensors assessed.
Fig. 1 shows the emission spectra of the synthesized CdTe QDs as a function of the
reaction time (from 15 minutes and up to 27 hours reaction time) (a) and as function of the
pH (from 3 to 9). The effect of the reaction time is to shift the emission wavelength of the
CdTe QDs to higher wavelengths (from yellow/ green up to red) and by changing the
reaction time the emission wavelength can be tuned. Decreasing the pH provokes a shift
towards higher wavelengths. Also, the lifetimes are reaction time and pH dependent. These
results suggest that the CdTe QDs have great potential as fiber optics pH sensors.
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Wavelength (nm)
a. b.
Fig. 1 – Fluorescence emission spectra of QDs as function of the reaction time (a) and pH (b).
Acknowledgements: Financial support from Fundação para a Ciência e Tecnologia
(Lisboa) (FSE-FEDER) (Project POCTI/QUI/44614/2007) is acknowledged.
References:
[1] Alivisatos, A.P. (1996), Semicondutor cluster, nanocrystals and quantum dots, Science, 217,
933-937.
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λ Emission/ nm
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