Synthesis, characterization, and incorporation of benzoperylene monoimides into stimuli-responsive polyacrylamide nanogels.

摘要

Research into "smart" polymeric materials has shown promise in improving the quality of non-invasive temperature sensors. Smart polymers are materials that engineered to analytically respond to small changes in their local environment. In this work, we present research towards novel polymeric molecular thermometers as non-invasive temperature sensors with a ratiometric fluorescent read-out. The new smart polymer systems combine the thermo-responsive phase change behavior of polyacrylamide nanogel hosts with solvatochromic benzoperylene monoimide (BPI) fluorophore dopants.;The individual components of the acrylamide-BPI molecular thermometers were studied with steady-state and time-correlated spectroscopies and dynamic light scattering. BPIs were synthesized via condensation of benzoperylene-1,2-dicarboxylic anhydride (BPA) in the presence of bulky primary amines. Structurally, an orthogonal orientation of sterically bulky imide substituents inhibits aggregation in solution and increases their versatility as fluorophores for analytical sensing. Absorbance spectra and geometry optimization of the BPIs show that at least 4 Å of distance is required of the imide substituent in order to prohibit planar stacking and aggregation. The BPIs exhibit strong UV absorbance (ϵ334 ∼80000 M-1cm-1) and fluorescence quantum yield (0.27–0.44) in many solvents. Large Stokes shifts are observed due to the asymmetrical substitution into the bay-region of the aromatic core. The BPI Stokes shifts were examined according to general and specific solvent models. It is determined that BPIs undergo a 3.9 D increase in dipole moment upon excitation and that protic solvents induce a more exaggerated fluorescence red-shift. The local polarity weakly influences the BPI ground state, but strongly influences the excited state.;Fluorescent molecular thermometers were prepared via encapsulation of the BPIs within polyacrylamide nanogels. The water-insoluble BPIs were emulsified within the nonpolar gel network and the temperature-responsive fluorescent behavior was investigated. The BPI-doped acrylamides exhibit temperature-responsive fluorescence at the lower critical solution temperature (LCST) chain collapse of the polymers. Ratiometric emission intensity measurements of the nonpolar (500 nm) and polar (600 nm) BPI emission increased 4.7- to 14.4-fold at the LCST. Changing the acrylamide host compositions yielded molecular thermometers with response ranges spanning 27 °C to 60 °C with high sensitivity (32% C-1 to 51% C-1) at the LCSTs.