Immobilization techniques for molecular rotors—Towards a solid-state viscosity sensor platform

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Molecular rotors are a subgroup of twisted intramolecular charge transfer fluorphores with high viscosity sensitivity. These fluorphores enjoy a high and growing popularity as in-solution nonmechanical viscosity sensors. A solid-state sensor with molecular rotor molecules covalently bound to a sensor substrate, such as glass, would be desirable to avoid contaminating the solution with a fluorescent solute and to obtain a more universal sensor platform. Although the proof-of-principle was provided [M.A. Haidekker, W.J. Akers, D. Fischer, E.A. Theodorakis, Optical fiber-based fluorescent viscosity sensor, Opt. Lett. 31 (2006) 2529–2531], a marked loss of sensitivity was observed with rotor immobilization. The influence of immobilization protocol and choice of linker was analyzed more systematically in this study. Among three silane linkers, ureidopropyl-trimethoxysilane was found to provide the highest sensitivity, and a higher immobilization density generally caused higher sensitivity. A glass slide serving as a substrate provided better sensor performance than an optical fiber because the glass-based sensor has a larger surface and no autofluorescence; conversely, an optical fiber is capable of measuring smaller volumes of fluid. Overall, we found that overall sensor performance strongly depends on the details of the immobilization protocol, and without specialized equipment, large variability between sensors exists. Sensitivity was measured as the exponent x in the intensity–viscosity power-law relationship. While in-solution molecular rotors exhibit values of x = 0.6, covalently linked molecular rotors did not exceed x = 0.15. This value is still sufficiently high to produce a sensor with a precision comparable to mechanical rheometers. However, further research is needed to examine the causes of between-sensor variability and to create more reproducible sensors.