Highly cooperative fluorescence switching of self-assembled squaraine dye at tunable threshold temperatures using thermosensitive nanovesicles for optical sensing and imaging

Abstract

Thermosensitive fluorescent dyes can convert thermal signals into optical signals as a molecular nanoprobe. These nanoprobes are playing an increasingly important part in optical temperature sensing and imaging at the nano- and microscale. However, the ability of a fluorescent dye itself has sensitivity and accuracy limitations. Here we present a molecular strategy based on self-assembly to overcome such limitations. We found that thermosensitive nanovesicles composed of lipids and a unique fluorescent dye exhibit fluorescence switching characteristics at a threshold temperature. The switch is rapid and reversible and has a high signal to background ratio (>60), and is also highly sensitive to temperature (10–22%/°C) around the threshold value. Furthermore, the threshold temperature at which fluorescence switching is induced, can be tuned according to the phase transition temperature of the lipid bilayer membrane forming the nanovesicles. Spectroscopic analysis indicated that the fluorescence switching is induced by the aggregation-caused quenching and disaggregation-induced emission of the fluorescent dye in a cooperative response to the thermotropic phase transition of the membrane. This mechanism presents a useful approach for chemical and material design to develop fluorescent nanomaterials with superior fluorescence sensitivity to thermal signals for optical temperature sensing and imaging at the nano- and microscales.

A rapid and highly sensitive biomarker detection platform based on a temperature-responsive liposome-linked immunosorbent assay

“…However, when the temperature was increased to > 42 °C, the liposomes changed to be in a liquid crystalline state and the fluorescence intensity dramatically increased. This fluorescence switching at the phase transition temperature of the liposomes was in good agreement with previous reports of liposomes containing SQR22, which correspond to the PEG-TLip in this study 18 . The results showed that the attachment of biotin on the surface of the liposomes had little effect on the temperature-responsive fluorescence properties, indicating that the PEG-TLip can be used as a negative control for the biotin-TLip.…”

“…DPPC and DHSG were purchased from Nippon Fine Chemical (Tokyo, Japan), while PEG 5000 -DSPE was purchased from NOF Corporation (Tokyo, Japan). The fluorescent dye SQR22 was synthesized according to a previous report 18 . Tert-butyl alcohol (t-BuOH) was used to lyophilize the lipids and was purchased from Kanto Chemical (Tokyo, Japan).…”

“…The liposomes change from an ordered gel state to a disordered liquid crystalline state, where the lipid tails become less packed, and more lipids are present in gauche conformations rather than all trans, as the liposomes reach the phase transition temperature 16,17 . The optical properties of the amphiphilic fluorescent dye, SQR22, have a specific response to the liposomal phase transition when the dye is mixed with lipids to form liposomes 18 . In the lipid bilayer membrane in the gel phase, the SQR22 molecules are concentrated to form self-aggregates and the luminescence is quenched because of the aggregation-caused quenching (ACQ) effect 19 .…”

“…When the liposomes are in a liquid crystalline state, the SQR22 molecules diffuse into the membrane bilayer and emit far-red fluorescence that can be detected. As a result, the liposomes containing SQR22 (SQR liposomes) have temperature-responsive fluorescence characteristics, and the fluorescence intensity of the liposomes markedly changes when the liposomes experience phase transition 18 . A single liposome can incorporate thousands of fluorescent molecules in the hydrophobic region of the bilayer membrane.…”

Temperature-Responsive Liposome-Linked Immunosorbent Assay for the Rapid Detection of SARS-CoV-2 Using Immunoliposomes

“…The synthesis and characterization of the fluorescent dye (2-[4-(dibutylamino)phenyl]-4-[(3,3-dimethyl-3 H -indol-2-yl)methylene]-3-hydroxycyclobut-2-en-1-one, SQR22) were reported previously. 28 SQR22 was obtained by custom synthesis from the NARD Institute (Kobe, Japan). Octyl glucoside (OG), which was used to destroy the liposomal structure for further lipid concentration analysis, was obtained from Carbosynth (Berkshire, UK).…”