Polyethylene glycol-modified cystamine for fluorescent sensing

Tedious polymeric modifications, laborious syntheses, low water solubility, high cytotoxicity and low quantum yields are still the major concerns related to fluorescent materials used for biosensing applications. Here we report one-pot facile synthesis of polyethylene glycol (PEG)-modified cystamine-based multifunctional fluorescent dyes named as CPEG-168, CPEG-1000 and CPEG-2000, which can be used for biosensing and detection of both cations and anions in pure aqueous medium. Rapid response toward thiols could be achieved due to the presence of disulfide functionality and reversible redox property between thiol and disulfide. A cyclic structure is formed by (NH–H) intramolecular hydrogen bonding to produce electron dense region for strong fluorescence emission. The presence of –NH group made them sensitive toward Cu²⁺ ions, while excellent sensitivity toward pH and F⁻ ions was provided by intramolecular hydrogen bonding between –NH groups. The increase in chain length of PEGs enhanced the fluorescence emission intensities due to the formation of more rigid structure, and hence, the desired quantum yields can be tuned by changing the chain length of PEG. The aforementioned properties along with high brightness in aqueous solution, at biological pH and temperature range, make these fluorescent dyes potential candidates for biosensing applications and detection of cations and anions.