In the present work, optical temperature sensors were prepared using a fluorescent dye, rhodamine B and different adsorbents, viz. silica gel and sol–gel. Silica gel (28–200 mesh) and a sol–gel consisting of a mixture of 3-amino-propyl-trimethoxy-silane (APTMS) and 3-glycidoxypropyl-tri-methoxy-silane (GPTMS) were used as the support matrices for the fluorescent dye. The polymerization of alkoxide silanes to produce a clear single phase sol–gel polymer at low temperature allows for the encapsulation of the fluorescence dye, and the rigidity of the silica gel prevents the movement and interaction of intermolecules, while still allowing them to retain their activity. The linear detection ranges of the fiber optic temperature sensors (FOTS) were between 10–95 °C and 0–60 °C when using silica gel and sol–gel as the support materials, respectively. The planar optic temperature sensors (POTS) showed high sensitivity in the temperature range of 25–40 °C. The interferences and life-time of the temperature sensors were also investigated.
Temperature is an essential physical parameter in all fields of science and process control technology [1]. Among the many types of temperature sensors, various optical temperature sensors have been designed and manufactured using optical fibers or optical waveguides. These optical fiber sensors usually showed many advantages over their electrical counterparts, such as high sensitivity, small size, safety in hazardous or explosive environments and the potential for signal processing over large distances [2].
The optical temperature sensors developed so far used a number of different sensing methods such as absorption [3], fluorescence [4] and [5], Raman [6], Bragg grating [7], interferometry and other spectroscopic techniques [8]. A number of optical temperature sensors have been developed based on fluorescence techniques, e.g. the fluorescence life-time, amplified spontaneous emission or fluorescence intensity, etc. The fluorescence method is one of the most sensitive ones, because the excitation and emission of light can be separated resulting in low background noise. Optical temperature sensors based on the fluorescence technique can also cover a relatively wide sensing range with reasonable resolution [9] and [10].
In order to fabricate a fluorescence-based temperature sensor, fluorescence materials which have a sufficiently strong dependence on temperature need to be used as the sensing element. Some ions of rare earth elements such as erbium, activated or doped in optical fibers, have been extensively used to develop optical temperature sensors [11] and [12]. Even though rare earth doped optical temperature sensors are applicable over a wide temperature range, e.g. −50 to 600 °C [13], the fabrication of sensor probes, i.e. the doping of rare earth elements, is somewhat complex. Therefore, a simple and reliable method of immobilizing fluorescence dyes is required for the development of optical temperature sensors.
Recently, an oxygen sensitive dye like Ru(bpy)32+ was used for temperature sensing after its encapsulation into poly(vinyl alcohol) (PVA) films [14]. Some fluorescent dyes such as tetraphenylporphyrin (TPP) have been also immobilized onto the tip or side of an optical fiber for the measurement of O2, pH and CO2 by the sol–gel techniques [15] and [16]. The sol–gel technique has several advantages over other immobilization techniques used for dye molecules. Apart from its simplicity, the encapsulation of dyes into sol–gels makes the sensor more practical and resistant than other methods in aggressive environments or biological systems.
In this work, optical temperature sensors were fabricated based on the quenching of the fluorescence of a dye, rhodamine B. As in theories, rhodamine B was well known as a sensitive temperature indicator. It was added to liquid flow [17] or attached on the surface of glass beads [18] to measure temperature based on the change in fluorescence intensity. However, these sensing systems showed their ability only in the detection range of 20–80 °C and could not recover the fluorescence intensity at high temperature due to limited capability of supporting material [18]. Therefore, in our sensing models rhodamine B was incorporated into a silica gel or sol–gel matrix since those are proved as useful supporting materials in many fields [19] and [20]. The rhodamine B entrapped into a silica gel or sol–gel based solid matrix is coated onto the tip of an optical fiber probe (fiber optic temperature sensor (FOTS)) or onto the surface of a well in a 96-well microtiter plate (planar optic temperature sensor (POTS)) and then they enabled to measure the temperature of aqueous solution. The design of these sensing models originate from the requirements of temperature sensing in fermentation processes, especially for testing cultivation conditions of microorganisms on microtiter plates since it needs small sample volume and simultaneous screening of high numbers of samples. Other characteristics of these sensing systems, such as their interferences and stability, were also investigated.
Optical temperature sensors were successfully prepared by incorporating rhodamine B in a silica gel or sol–gel matrix (GPTMS and APTMS). The use of silica gel or silanes with epoxy and amino groups preserved and improved the temperature sensitivities of the fluorescent dye (rhodamine B) entrapped in their matrix. Furthermore, these materials prevented the pH from interfering with the temperature measurement, reduced the effect of the ionic strength and extended the life-time of the sensors to more than 3 months. The results of this study demonstrate that it is possible to use optical temperature sensors in biological processes and environmental monitoring, as well as in geology.
