Multi-Stimuli Responsive FRET Processes of Bifluorophoric AIEgens in an Amphiphilic Copolymer and Its Application to Cyanide Detection in Aqueous Media

Abstract

A novel amphiphilic aggregation-induced emission (AIE) copolymer, that is, poly(NIPAM-co-TPE-SP), consisting of N-isopropylacrylamide (NIPAM) as a hydrophilic unit and a tetraphenylethylene-spiropyran monomer (TPE-SP) as a bifluorophoric unit is reported. Upon UV exposure, the close form of non-emissive spiropyran (SP) in poly(NIPAM-co-TPE-SP) can be photo-switched to the open form of emissive merocyanine (MC) in poly(NIPAM-co-TPE-MC) in an aqueous solution, leading to ratiometric fluorescence of AIEgens between green TPE and red MC emissions at 517 and 627 nm, respectively, via Förster resonance energy transfer (FRET). Distinct FRET processes of poly(NIPAM-co-TPE-MC) can be observed under various UV and visible light irradiations, acid-base conditions, thermal treatments, and cyanide ion interactions, which are also confirmed by theoretical studies. The subtle perturbations of environmental factors, such as UV exposure, pH value, temperature, and cyanide ion, can be detected in aqueous media by distinct ratiometric fluorescence changes of the FRET behavior in the amphiphilic poly(NIPAM-co-TPE-MC). Moreover, the first FRET sensor polymer poly(NIPAM-co-TPE-MC) based on dual AIEgens of TPE and MC units is developed to show a very high selectivity and sensitivity with a low detection limit (LOD = 0.26 μM) toward the cyanide ion in water, which only contain an approximately 1% molar ratio of the bifluorophoric content and can be utilized in cellular bioimaging applications for cyanide detections.

Ratiometric fluorescent chemosensor based on the block copolymer of poly(N‐isopropylacrylamide)‐b‐poly(N‐vinylcarbazole) containing rhodamine 6G and 1,8‐naphthalimide moieties

“…Poly(N‐isopropylacrylamide) (PNIPAM) is a frequently‐used polymer for thermometers and chemosensors due to its specific water‐solubility, which can be controlled by temperature 21–23 . The donor and acceptor fluorophores have been introduced into PNIPAM and designed as multi‐functional ratiometric fluorescent sensors based on FRET mechanism 24–26 . A series of block copolymers, poly(N‐isopropylacrylamide)‐ b ‐poly(N‐vinylcarbazole) (PNIPAM‐ b ‐PVK), have been synthesized via RAFT polymerization and investigated their temperature, stimuli, and pH responsive behaviors 27 .…”

Photo‐Responsive Fluorescent Materials with Aggregation‐Induced Emission Characteristics

“…[58] In addition to the above-mentioned organic AIE-active molecules with photochromic properties, many gels, polymers, supramolecular systems, and metal complexes are also photoresponsive fluorescent materials with AIE characteristics. [59][60][61][62][63] Li et al synthesized a class of photo-responsive liquid crystal gelators, with AIE properties and gel-enhanced fluorescence, by linking TPE and cholesterol to a central Azo moiety with different alkyl spacer lengths. Based on the trans-cis photoisomerization of Azo, these compounds showed gel-sol transitions and a color change from yellow to orange after UV light irradiation.…”

“…[60] Lin and coworkers synthesized an amphiphilic copolymer (poly (NIPAM-co-TPE-SP), consisting of N-isopropylacrylamide and a TPE-SP monomer), with AIE properties and photo-responsiveness, which showed green emission attributed to the TPE moiety (Figure 2A). [61] Upon UV irradiation, the copolymer switched to red fluorescence due to photoisomerization from the closed SP to the open MC form and the FRET process between TPE and MC. Poly (NIPAM-co-TPE-MC) could be used as a fluorescent sensor to detect CN − , owing to blocking of the FRET process by the reaction between CN − and MC.…”

Supramolecular oligourethane gel as a highly selective fluorescent “on–off–on” sensor for ions

“…16 The emergence of polymers/oligomers with aggregation-induced emission (AIE) properties has been a breakthrough in the field. 17,18 In addition to their excellent emission characteristics, AIE-active supramolecular gels show strong absorption activity and synergistic effects because of their large contact area with analytes. 19 Recently, our group has explored AIE-active poly/oligourethane-based unconventional luminophores, which are without typical polycyclic p-conjugated units.…”