1378 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006 Quantum Dots-Based Optical Fiber Temperature Sensors Fabricated by Layer-by-Layer Gonzaga de Bastida, Francisco J. Arregui, Member,IEEE, Javier Goicoechea, and Ignacio R. Matias, Senior Member, IEEE Abstract—Two types of CdTe quantum dots of different sizes (4 and 5 nm) were successfully deposited on optical fibers using the layer-by-layer electrostatic self-assembly method. The sensors showed a linear and reversible variation of the emission wave- length for a temperature range from 30 ◦ C to 100 ◦ C, with a sensitivity of 0.2 nm/ ◦ C. Index Terms—Layer-by-layer electrostatic self-assembly (LbL ESA), optical fiber sensor, quantum dots (QDs). I. I NTRODUCTION Q UANTUM DOTS (QDs) have attracted an intense research activity in the last years due to their outstanding luminescent properties. QDs have already been used in several applications where more representatives can be biological labels, optical sensors, optoelectrochemistry (i.e., LEDs, laser or solar cells) and many others [1]–[3]. One of the most exciting properties of the QD is that they can be excited in a broad range of wavelengths and, at the same time, have a narrow emission spectrum. Moreover, the center wavelength of the emission peak depends on the geometrical size of the QD; therefore, the emission wavelength can be tuned by changing the size of the nanocrystals; that gives a large choice of emission wavelengths. The bigger the particle is, the higher the emission-peak wave- length is. In addition, QDs offer an exceptional photostability and have a high quantum yield compared to regular fluorescent dyes used for sensing applications: higher than 50%. Recently, it has been demonstrated the temperature depen- dence of the QD emission peak [4], [5] which opens the field for thermometry applications. On one hand, Bawendi and co-workers have published an extensive work about the utiliza- tion of QDs as temperature probes [5], on the other hand, Crisp and Kotov have also demonstrated the possibility of deposit- ing QDs by means of the Layer-by-Layer Electrostatic Self- Assembly (LbL ESA) technique on optical fibers [6]. The main advantages of this technique with respect to other deposition techniques such as sol-gel [4] are the control on the nanometer scale of the thickness of the coating and the possibility of using different combinations of anionic and cationic colloids for the fabrication of the coatings. Therefore, the QD can be incorporated in one nanostructured polymeric matrix which could affect the optical properties of the nanocrystals. In this Manuscript received April 6, 2006; revised July 22, 2006 and August 19, 2006. This work was supported by the Spanish CICYT Research Grants TIC2003-00909 and TIC2006-12170/MIC. The associate editor coordinat- ing the review of this paper and approving it for publication was Prof. Eugenii Katz. The authors are with the Departamento de Ingeniería Eléctrica y Electrónica, Universidad Pública de Navarra, 31006 Pamplona, Navarra, Spain (e-mail: parregui@unavarra.es). Color versions of Figs. 2–4 are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2006.884436 paper, we present for the first time a study about the response to temperature of a QD deposited by means of the LbL ESA. II. EXPERIMENTAL In order to fabricate the sensing coatings, an initial bi- layer of poly(diallyldimethylammonium chloride), PDDA, and poly(acrylic acid), PAA is deposited with the purpose of improving the stability and homogeneity of the rest of the deposition. The concentration of the PDDA and PAA solu- tions was 1% in weight with a pH of 8 and 6, respectively. After this first bilayer, successive bilayers of PDDA and CdTe QD are deposited, forming a sensitive coating denoted by [PDDA/CdTe] 20 , where 20 is the number of bilayers chosen for this paper. The solution of the polycation PDDA was the same as the one used for the fist bilayer. For the adsorption of the nanocrystals, CdTe QD functionalized with -S - CH 2 - CH 2 - COONa groups were purchased from American Dye Source, Inc. The concentration of the QD aqueous solutions was 0.025% in weight with a pH of 8. Two different types of CdTe QD with diameters of 4 and 5 nm, which correspond to the green and red emission bands, were used for this paper. It is important to remark that the pH of the pure water for rinsing had to be previously adjusted to pH = 10. More details about the deposition process can be found in earlier works [6], [7]. These films were deposited on hard clad silica (HCS) optical fibers, 200/230 micrometers of core and cladding diameter, respectively. Prior to the deposition of the coatings, the plastic cladding of the fiber end was removed and then the fiber was tapered by means of a splicing machine. The large core fibers and the tapered end geometry were chosen to enhance the emission light monitored using the experimental setup of Fig. 1. This setup is based on a laser at 470 nm as the light source (160 μW of optical power) and an Ocean Optics USB2000 spectrometer connected to a PC to measure both the reflected optical power (at 470 nm) as well as the emission fluorescence from the sensor head (at 570 or 620 nm depending of the QD used). Once the coatings are fabricated, if the sensors are kept under illumination and the presence of oxygen for several days; then, a premature photooxidation of the QD will happen. In order to avoid this degradation, the sensors were kept in a dark environment. In addition, prior to any measurement, the sensor heads were cured at 115 ◦ C in nitrogen atmosphere. After this, a permanent red shift was experienced by the emission peak with respect to the original emission-peak wavelength of the QD. Due to this thermal treatment, the sensors show a more repetitive and stable response when submitted to temperature changes. Then, in order to study their response to temperature, the sensors were placed on a Peltier cell together with a cali- brated thermocouple as reference. 1530-437X/$20.00 © 2006 IEEE