Pyrenyl peripheral-decorated polyamidoamine dendrimer for fluorescent temperature detection in aqueous phase

Abstract

A protonated polyamidoamine (PAMAM) dendrimer of generation 2 with pyrenyl attached to its periphery (G2 PAMAM-PyH) was designed and synthesized. G2 PAMAM-Py was synthesized by a condensation of the terminal amino group of the PAMAM dendrimer and the aldehyde group of 1-pyrenecarboxaldehyde followed by a reduction of Schiff base through "one pot" reaction. G2 PAMAM-Py was further protonated by adding HCl, giving the target product G2 PAMAMPyH. The structure of G2 PAMAM-PyH was characterized by NMR, FTIR, and MS. The functionalization extent of the peripheral amino groups of PAMAM by pyrenyl is 100% according to the 1H NMR and UV-visible spectra. The amphiphilic G2 PAMAM-PyH is soluble in water with a critical aggregation concentration of 3.3×10-7mol•dm-3. Absorption, dynamic light scattering (DLS), and transmission electronic microscopy (TEM) studies demonstrate that G2 PAMAM-PyH exists as vesicle with a bilayer membrane and an average hydrodynamic diameter of ca. 184 nm in aqueous phase. G2 PAMAM-PyH in aqueous phase exhibits dual fluorescence, pyrenyl monomer and excimer emission. The pyrenyl monomer fluorescence increases slightly and the pyrenyl excimer emission decreases monotonically upon temperature raising from 1 to 70℃. Meanwhile, the fluorescence color changes from green (low temperature) to blue (high temperature). The monomer emission enhancement is mainly attributed to less formation of excimer when rising temperature. The fluorescence intensity ratio of pyrenyl excimer to pyrenyl monomer (I495 nm/I398 nm) changes with varying temperature recoverably, and the relationship between I495 nm/I398 nm and temperature can be expressed as I495 nm/I398 nm=28.23-0.68t+3.21×10-3t2+1.83×10-5t3. The accuracy for the measurement of the temperature is better than 0.9℃ in the temperature range of 1~70℃, facilitating in situ gradient temperature measurement. The temperature gradient of aqueous phase in a glass tube is investigated by using G2 PAMAM-PyH, which is consistent with the detection result by using a thermocouple meter. This study provides a potential strategy for developing fluorescent temperature sensing system.