3. pH sensors

New fluorescent pH indicators were used as basis for preparation of optical pH sensors for different applications. These dyes were (non)covalently immobilized into hydrogel matrices. In most cases these sensors show “on”-“off” behaviour (Fig. 3.1) due efficient quenching of dye fluorescence via photoinduced electron transfer. Such behaviour is observed for sensors based on rhodamines, [20] perylenes [21, 22] or aza-BODIPY dyes. [23, 24] Therefore, in order to ensure reliable measurement, reference phosphor particles (ruby, Egyptian blue, Cr(III)-doped gadolinium aluminium borate) have been added into the sensing film. These materials possess long-lived luminescence (µs-ms time domain) which in combination with short-lived fluorescence (nanoseconds) enables referenced measurement of the overall phase shift in so called dual lifetime referencing (DLR) read-out. The sensors prepared cover visible (rhodamines, perylenes) [20, 21] or NIR part of the spectrum (perylenes, aza-BODIPYs). [22-24]
Figure 3.1. A: Calibration curves for different pH sensing materials based on a rhodamine dye; B: photographic images of the poly(HEMA) sensor foils in acidic (pH 4) and basic (pH 9) buffer. Reproduced from ref. [20].
Since most optical pH sensors only cover a limited dynamic range (~3 pH units) it is evident that tailor maid indicators are necessary to cover different applications. We prepared pH sensing materials on basis of aza-BODIPY dyes [24] which covered virtually all pH range (Fig. 3.2). Since all the dyes feature virtually identical spectral properties and photostability, a wide-dynamic range sensor was possible to manufacture by simply mixing several indicators in the hydrogel matrix. Indeed, mixing of four indicators extended the dynamic range to 7 pH units (pH 2 to 9, Fig. 3.2). 
Figure 3.2. (a) Calibration curves for different aza-BODIPY indicators immobilized into hydrogel D4. (b) Response of the wide dynamic range sensor based on combination of four different dyes. Reprodcued from [24].
Often, covalent immobilization is essential to achieve stable sensing materials. Thus, a NIR perylene dye was modified with acrylamido groups via chlorosulfonation of the aromatic moieties in the imide position. [22] A new hydrogel was prepared via cross-linking of acryloylmorpholine using a UV photoinitiator and immobilizing the pH indicator into the network (Fig. 3.3). Covalent immobilization was very helpful for avoiding the hysteresis effects and the sensor showed fast and reproducible response (Fig. 3.3). For comparison, the material based on a reference indicator which was non-covalently immobilized into the hydrogel proved to be unsuitable for practical applications. 
Figure 3.3. Dynamic response of the pH sensor based on a perylene dye covalently grafted into cross-linked poly(acryloylmorpholine) compared to a D4 hydrogel sensor where it was physically entrapped. Reprodcued from [22].

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References:

[20] Aigner, D.; Borisov, S. M.; Orriach Fernández, F. J.; Fernández Sánchez, J. F.; Saf, R.; Klimant, I. New Fluorescent PH Sensors Based on Covalently Linkable PET Rhodamines. Talanta 2012, 99, 194–201. [21] Aigner, D.; Borisov, S. M.; Klimant, I. New Fluorescent Perylene Bisimide Indicators—a Platform for Broadband PH Optodes. Anal Bioanal Chem 2011, 400 (8), 2475–2485. [22] Aigner, D.; Borisov, S. M.; Petritsch, P.; Klimant, I. Novel near Infra-Red Fluorescent PH Sensors Based on 1-Aminoperylene Bisimides Covalently Grafted onto Poly(Acryloylmorpholine). Chem. Commun. 2013, 49 (21), 2139–2141. [23] Jokic, T.; Borisov, S. M.; Saf, R.; Nielsen, D. A.; Kühl, M.; Klimant, I. Highly Photostable Near-Infrared Fluorescent PH Indicators and Sensors Based on BF2-Chelated Tetraarylazadipyrromethene Dyes. Anal. Chem. 2012, 84 (15), 6723–6730. [24] Strobl, M.; Rappitsch, T.; Borisov, S. M.; Mayr, T.; Klimant, I. NIR-Emitting Aza-BODIPY Dyes – New Building Blocks for Broad-Range Optical PH Sensors. Analyst 2015, 140 (21), 7150–7153.

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