7. Multi-parameter sensors

Light has a unique property of transporting multiple information simultaneously. This is very useful for design of multi-parameter sensors i.e. sensors capable of simultaneous sensing of several parameters with a single material and a single read-out device. In contrast to sensor arrays, true multi-parameter sensors measure the analytes of interest in exactly the same place. Combination of temperature with any other important parameter is of high interest due to the fact that all optical sensors show temperature cross-talk. Examples of muti-parameter sensors reported by our group include the ones for simultaneous measurement of pH and temperature, [40] oxygen and temperature [38] as well as for simultaneous measurement of 4 parameters (oxygen, temperature, pH and carbon dioxide). [41]   The oxygen/temperature dual sensor made use of Eu(III) and Gd(III) b-diketonate complexes immobilized into acridone-modified polystyrene acting as matrix and antenna excitable with visible light. The Gd(III) complex is mostly sensitive to oxygen and less sensitive to temperature, whereas the Eu(III) complex shows the opposite behaviour. Addition of a polyfluorene conjugated polymer, which fluorescence is virtually not affected by oxygen or temperature, allows for ratiometric sensing of both parameters (Fig. 7.1.).
Figure 7.1. Emission spectra of the dually-sensing material (λexc 390 nm) based on polystyrene-acridone conjugate doped with 2 % w/w of gadolinium(III), 0.5 % w/w of europium(III) complex  and 1 % w/w of polyfluorene conjugated polymer. Oxygen partial pressures are identical for all the temperatures. Based on source [38]
A different principle was used in dual sensor for temperature and pH. It made use of so called modified dual lifetime referencing read-out [42] which combines the fluorescent pH indicator (SNARF-decylester) with a phosphorescent reference (also serving as an optical temperature probe). [40] Here, a stable thermographic phosphor Cr(III) activated yttrium aluminium borate (Cr-YAB) was used. Whereas the pH indicator was simply dissolved in a polyurethane hydrogel, Cr-YAB was used in the form of a microcrystalline powder dispersed in the same matrix. Both SNARF-DE and Cr-YAB were excited with the red light (λmax 605 nm) and their NIR emission was simultaneously detected with a photomultiplier equipped with a long-pass filter. Importantly, Cr-YAB not only acts as a temperature probe, but also as a reference material operating via a dual lifetime referencing (DLR) scheme. The information about pH and temperature is extracted from the measurements of a luminescence phase shift at two modulation frequencies (Fig. 7.2). Such measurement allows calculation of the decay time of the temperature probe (and therefore temperature) which is used to calculate the pH (Fig. 7.2a).
Figure 7.2. 3D calibration plots for pH (a) and temperature (b) in the dual sensor based on SNARF-decylester as the pH probe and Cr-YAB as the temperature probe embedded into polyurethane hydrogel. Dots are the experimental points (average from 3 independent measurements), surfaces – a fit. Reproduced from ref. [38, 42].
The potential of optical technology for multi-parameter sensing has been demonstrated with a sensors enabling simultaneous sensing of 4 parameters. [41] The sensor combined both approaches (separation of the information via excitation and emission spectra and via decay time). Briefly, two pairs of probes were combined in two sensing layers. An oxygen and a carbon dioxide probes (an iridium(III) coumarin complex and HPTS, respectively) have very similar excitation and emission spectra but are fluorescent and phosphorescent, respectively. They were contained in the first layer (silicone rubber) coated onto a polyester support. The NIR pH and temperature probes (lipophilic SNARF derivative and Cr-YAB, respectively) have been embedded into a second layer which is proton and gas-permeable. Again, the spectral properties of these probes are very similar, and the information is separated via a phase shift measurement. Importantly, the spectral properties of the probes contained in the first and in the second layer are so different that optical cross-talks can be almost completely eliminated. In order to enable separation of analytical signals measurement at two different modulation frequencies was required for each sensing layer and the set-up contained two sets of LEDs, excitation and emission filters for each sensing layer. It was found, that despite that simultaneous sensing of 4 parameters is feasible, such multiparameter sensor has a lot of limitations: (i) complicated sensor design requiring preparation of multiple layers which has to ensure permeation selectivity; the layers may have poor adhesion on each other like the adhesion of the hydrophilic pH layer on a hydrophobic silicone; (ii) limited choice of indicator dyes since the spectral properties should be selected in order to avoid optical cross-talk between different parameters measured; (iii) rather complex design of the read-out set-up making it more bulky and expensive. These considerations make a 4-parameter optical sensor significantly less attractive for practical applications. Such applications may be better realizable with dual optical sensors or even by an array of 1-parameter sensors.

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References:

[40] Borisov, S. M.; Vasylevska, A. S.; Krause, C.; Wolfbeis, O. S. Composite Luminescent Material for Dual Sensing of Oxygen and Temperature. Advanced Functional Materials 2006, 16 (12), 1536–1542. [41] Borisov, S. M.; Wolfbeis, O. S. Temperature-Sensitive Europium(III) Probes and Their Use for Simultaneous Luminescent Sensing of Temperature and Oxygen. Analytical Chemistry 2006, 78 (14), 5094–5101. [42] Borisov, S. M.; Gatterer, K.; Klimant, I. Red Light-Excitable Dual Lifetime Referenced Optical PH Sensors with Intrinsic Temperature Compensation. Analyst 2010, 135 (7), 1711–1717.

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