Pyridoxal Derived AIEgen for Fluorescence Turn-off Sensing of Cu2+ and Fe2+ Ions and Fluorescence Imaging of Latent Fingerprints

A substituent-modified new salicylaldehyde-diphenyl-azine based AIEgen: A promising skeleton for copper ion sensor

In this study, we have reported a novel 4-bromo-salicylaldehyde-diphenyl-azine (B-1), a new member of salicylaldehyde-diphenyl-azine (SDPA) family known for its excellent sensing properties. In contrast to the previously reported AIEgens, we found that the bromo-substitution at the 4th position of the salicylaldehyde moiety blue-shifted the emission by 10 and 15 nm as compared to the unsubstituted (Tong et.al 2017) and Bromo at the 5th position (Jain et.al 2023) respectively. Moreover, B-1 crystallizes instantly as the cooling process starts, which was not observed in the previously reported scaffolds. The sensing investigation again demonstrated the precise and ultrasensitive behavior of B-1 for copper ions. B-1 has a very low LOD value i.e. 29.2 x 10-8 M with a high association constant and binds with copper ion in 2:1 mode. This time we also analyzed the practical applicability in the solid phase using cotton swabs and performed the real-time estimation of copper ions in water and biological samples like urine and blood serum. The excellent percentage recovery and the RSD value suggest the precision of the experiments. Further, we also perform the sensing in living cancer HeLa cells. Altogether, we found that the SDPA skeleton is precise and ultrasensitive for copper ions and versatile which can be used variously to detect copper ions in the real world. This research will surely help in developing new specific skeleton-based AIEgens with desirable emission properties and precise applications in the future.

A Novel Rhodamine B Derivative as a "Turn-on" Fluorescent Sensor for Cu2+ with High Selectivity and Sensitivity

The number of "turn-on" fluorescent probes for Cu2+ is relatively limited, and interference from other metal cations presents a significant challenge for these sensors. In this study, we synthesized and characterized a rhodamine B-based sensor, designated as RBHP, using 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) and rhodamine B hydrazide. Selectivity, sensitivity, solvent effects, water content, and pH of RBHP in relation to Cu²⁺ were conducted. RBHP exhibited an exceptionally low fluorescence background signal in acetonitrile and demonstrated a "turn-on" fluorescent response to Cu²⁺. The PMBP-based acylhydrazone moiety and acetonitrile as the detection solvent are crucial for the selective detection. RBHP shows potential as a highly selective and sensitive fluorescent sensor for Cu2+.

Aggregation-induced emission luminogens for latent fingerprint detection

For over a century, fingerprints have served as a pivotal tool for identification of individuals owing to their enduring characteristics and easily apparent features, particularly in the realm of criminal investigations. Latent fingerprints (LFPs) are "invisible fingerprints" that are most commonly available at crime scenes and require a rapid, selective, sensitive, and convenient method for detection. However, existing fingerprint development techniques harbour limitations, prompting the exploration of novel approaches that prioritize investigator safety and environmental sustainability. Leveraging the unique photophysical properties of aggregation-induced emission luminogens (AIEgens) has emerged as a promising strategy for on-site analysis of LFP visualization. In this highlight, we have presented a comparative analysis of various AIEgens (organic compounds, metal complexes, nanoparticles, and polymers) for the development and detection of LFPs. Through this examination, insights into the efficiency and potential applications of AIE-based fingerprint development techniques are provided. In addition, several strategies have been proposed for circumventing the limitations of existing AIEgens. We hope that this highlight article will encourage more researchers to investigate AIEgens in LFP detection, contributing to forensic science.

Improving Copper(II) Sensitivity by Combined use of AIEE Active and Inactive Schiff Bases

An aggregation-induced emission enhancement (AIEE) active Schiff base PNN was synthesized by condensing benzidine with 2-hydroxynaphthaldehyde. The green-fluorescent PNN (λem = 510 nm) in DMF turned to yellow-fluorescent PNN (λem = 557 nm) upon increasing the fractions of HEPES buffer (10 mM, pH 7.4) above 40%. The DLS study supports the self-aggregation of PNN that restricts the intramolecular rotation and activates the excited-state intramolecular proton transfer (ESIPT) process. The fluorescence emission of AIEE active PNN was quenched by Cu2+ with an estimated detection limit of 2.1 µM. Interestingly, the detection limit of PNN towards Cu2+ was improved in the presence of an AIEE inactive Schiff base PBPM obtained by reacting 1,4-diaminobenzene with pyridine-4-carbaldehyde. The mixed PNN-PBPM showed a detection limit of 0.49 µM. The practical utility of PNN-PBPM was validated by quantifying Cu2+ ions in real environmental water samples and green tea.

Aggregation-induced Emission Active Naphthalimide Derived Schiff Base for Detecting Cu2+ and Its Applications

Herein, an aggregation-induced emission (AIE) active Schiff base (NHS) was synthesized by condensing naphthalimide hydrazide with salicylaldehyde. The non-fluorescent solution of NHS in DMSO turned to emissive NHS upon increasing the HEPES fraction in DMSO from 70 to 95%. The UV-Vis absorption and DLS studies supported the self-aggregation of NHS that restricted the intramolecular rotation and activated the ESIPT process. The blue fluorescence of AIE luminogen NHS in DMSO:HEPES (5:95, v/v, pH = 7.4) was examined by adding different metal ions (Al3+, Ca2+, Cd2+, Co2+, Cu2+, Cr2+, Fe2+, Fe3+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+ and Zn2+). NHS showed a selective fluorescence switch-off response for Cu2+ due to the chelation enhancement quenching effect (CHEQ). The quenching of NHS by Cu2+ was explored by using density functional theory (DFT) and Stern-Volmer plot. The practical utility of NHS was examined by quantitative and qualitative analysis of Cu2+ in real water samples.

Fluorescence Sensing of pH and p-Nitrophenol Using an AIEE Active Pyridoxal Derived Schiff Base

An easy-to-prepare aggregation-induced emission enhancement (AIEE) active Schiff base NPY was synthesized by condensing vitamin B6 cofactor pyridoxal with 3-hydroxy-2-naphthoic hydrazide, and employed for the fluorescent sensing of pH and p-nitrophenol (p-NP). The AIEE phenomenon of NPY was investigated in mixed DMSO/H2O medium. The weakly yellow-fluorescent NPY (λem = 535 nm) in pure DMSO turned to a bright cyan-fluorescent NPY (λem = 490 nm) upon addition of poor solvent water. The DLS and SEM analyses supported the self-aggregation of NPY that restricted the intramolecular rotation and activated the excited state intramolecular proton transfer (ESIPT) process. The AIEE luminogen (AIEEgen) NPY containing 90% of water fraction (fwater) was employed for the fluorescent sensing of pH. AIEEgen NPY displays three distinct fluorescent pH windows: non-fluorescent below pH 3.0 and above pH 10.0, cyan fluorescent between pH 3.0 to 8.0, and yellow fluorescent between pH 8.0 to 10.0. AIEEgen NPY was also applied for the detection of nitroaromatics in HEPES buffer (10% DMSO, 10 mM, pH 7.0). The addition of p-NP selectively quenched the fluorescent intensity of AIEEgen NPY with an estimated detection limit of 1.73 µM. The analytical utility of AIEEgen NPY was examined by quantifying p-NP in different real water samples.