Rational Design of High-Performance Donor–Linker–Acceptor Hybrids Using a Schiff Base for Enabling Photoinduced Electron Transfer

Abstract

Donor−linker−acceptor (D-L-A)-based photoinduced electron transfer (PET) has been frequently used for the construction of versatile fluorescent chemo/biosensors. However, sophisticated and tedious processes are generally required for the synthesis of these probes, which leads to poor design flexibility. In this work, by exploiting a Schiff base as a linker unit, a covalently bound D-L-A system was established and subsequently utilized for the development of a PET sensor. Cysteamine (Cys) and N-acetyl-L-cysteine (NAC) costabilized gold nanoclusters (Cys/ NAC-AuNCs) were synthesized and adopted as an electron acceptor, and pyridoxal phosphate (PLP) was selected as an electron donor. PLP can form a Schiff base (an aldimine) with the primary amino group of Cys/NAC-AuNC through its aldehyde group and thereby suppresses the fluorescence of Cys/NAC-AuNC. The Rehm−Weller formula results and a HOMO−LUMO orbital study revealed that a reductive PET mechanism is responsible for the observed fluorescence quenching. Since the pyridoxal (PL) produced by the acid phosphatase (ACP)-catalyzed cleavage of PLP has a weak interaction with Cys/NAC-AuNC, a novel turn-on fluorescent method for selective detection of ACP was successfully realized. To the best of our knowledge, this is the first example of the development of a covalently bound D-L-A system for fluorescent PET sensing of enzyme activity based on AuNC nanoprobes using a Schiff base.

A dual-signal fluorescent probe for detection of acid phosphatase

“…Subsequently, the PLP was added to the dispersion of europium polymer, and they were linked together through hydrogen bonding or coordination interaction, which reduced the red emission of Eu 3+ owing to the PET effect between the PLP and europium polymer. When ACP was introduced, the PLP was hydrolyzed to PL and phosphate, and the blue emission at 372 nm was enhanced due to the internal hemiacetal formation of PL in HEPES buffer [23]. Simultaneously, the peak at 614 nm exhibited positive relation with the ACP concentration, which is…”

“…When ACP was added into the mixture of PLP and europium polymer, the emission intensity at 614 nm was significantly enhanced (green line) due to the elimination of the PET effect, and the emission at 372 nm appeared and was increased, which demonstrates that the PLP was hydrolyzed to PL in weak acid buffer [23]. Meanwhile, the significant color change can be observed (Fig.…”

“…Besides, pyridoxal phosphate (PLP) is a substance rich in functional groups such as aldehyde, hydroxyl, methyl, nitrogen heteroatom and phosphate groups, and it is more likely to bond to nanoparticles through hydrogen bonding or coordination interaction. It is hydrolyzed by phosphatase to release pyridoxal (PL) and phosphate [23], and the PL generates blue emission at 372 nm with excitation at 280 nm. Eu-coordination polymer or metal-organic frameworks are usually applied to achieve dual-emission fluorescence detection [24,25] based on its antenna effect and the phenomenon of organic ligand-absorbing photons to produce a triplet state [26].…”

Luminescent metal nanoclusters: Biosensing strategies and bioimaging applications

“…successfully designed a PET sensor for evaluating acid phosphatase (ACP) activity, where Cys/N‐acetyl‐ l ‐cysteine (NAC)‐Au NCs and pyridoxal phosphate (PLP) were used as electron acceptor and donor, respectively (Figure 5D). [84] Since PLP can be converted to pyridoxal (PL) by ACP, the electron transfer process established between Au NCs and PLP will be disturbed in the presence of ACP. The luminescence change of Au NCs can then reflect the activity of ACP.…”

An IMPLICATION-logic-based fluorescent probe for sequential detection of Cu2+ and phosphates in living cells

“…Thus, the ''on-off-on'' fluorescence process may be due to the fact that the coordination ability of phosphates toward copper ions is stronger than that of the ligand, leading to the decomposition of the metal ions in the complex L-Cu 2+ by phosphates, resulting in the activation of the PET process and the recovery of the fluorescence of ligand HL. 50 As a consequence, it is entirely possible to implement IMPLICATION gates in which Cu 2+ and phosphates are the energy modulators/inputs, and the ''on-off-on'' fluorescence response is the luminescence signal reporter/output for the IMPLICATION logic behavior. Using the PPi anion as an example, the fluorescence responses of L-Cu 2+ to various concentrations of PPi 4À (0-16.7 mM) in methanol/HEPES (10 mM, pH = 7.00, 7/3, v/v) showed that L-Cu 2+ had a sensitive response to PPi 4À (Fig.…”