Recent advances in AIEgen-based chemosensors for small molecule detection, with a focus on ion sensing

Since the aggregation-based emission (AIE) phenomenon emerged in 2001, numerous chemical designs have been built around the AIE concept, displaying its utility for diverse applications, including optics, electronics, energy, and biosciences. The present review critically evaluates the broad applicability of AIEgen-based chemical models towards sensing small analytes and the structural design strategies adjusting the mode of action reported since the last decade. Various AIEgen models have been discussed, providing qualitative and quantitative estimation of cationic metal ions and anionic species, as well as biomolecular, cellular, and organelle-specific probes. A systematic overview of the reported structural design and the underlying working mode will pave the way for designing and developing the next generation of AIEgens for specific applications.

Tuning Molecular Motion Enhances Intrinsic Fluorescence in Peptide Amphiphile Nanofibers

Peptide amphiphiles (PAs) are highly tunable molecules that were recently found to exhibit aggregation-induced emission (AIE) when they self-assemble into nanofibers. Here, we leverage decades of molecular design and self-assembly study of PAs to strategically tune their molecular motion within nanofibers to enhance AIE, making them a highly useful platform for applications such as sensing, bioimaging, or materials property characterization. Since AIE increases when aggregated molecules are rigidly and closely packed, we altered the four most closely packed amino acids nearest to the hydrophobic core by varying the order and composition of glycine, alanine, and valine pairs. Of the six PA designs studied, C16VVAAK2 had the highest quantum yield at 0.17, which is a more than 10-fold increase from other PA designs including the very similar C16AAVVK2, highlighting the importance of precise amino acid placement to anchor rigidity closest to the core. We also altered temperature to increase AIE. C16VVAAK2 exhibited an additional 4-fold increase in maximum fluorescence intensity when the temperature was raised from 5 to 65 °C. As the temperature increased, the secondary structure transitioned from β-sheet to random coil, indicating that further packing an already aligned molecular system makes it even more readily able to transfer energy between the electron-rich amides. This work both unveils a highly fluorescent AIE PA system design and sheds insights into the molecular orientation and packing design traits that can significantly enhance AIE in self-assembling systems.

Development of the Colorimetric and/or Fluorescent Probes for Detecting Fluoride ions in Aqueous Solution

Fluoride ion is a strong Lewis base and one of the essential trace elements in human body. It plays a very important role in human health and ecological balance. The deficiency or excessive intake of fluoride ions will cause serious health problems, so the development of a sensitive and accurate detection method for fluoride ions is very important. The colorimetric and/or fluorescence sensing method has been a long standing attractive technique with high sensitivity and fast response. To date, most reported probes for fluoride ion are applicable only in organic solvents or organic-containing aqueous solutions. However, the probes for fluoride ion used in aqueous solution are more practically needed in view of environment protection and human health. In this paper, the materials and designing ideas of the colorimetric and/or fluorescent probes for fluoride ion based on different detection mechanisms in recent years were reviewed. Two main categories including formation of hydrogen bonds and formation of coordination covalent bonds were discussed. The latter one is further subdivided into three types, formation of B-F bond, formation of Si-F bond and formation of Mn+-F bond.

Bubble wall confinement-driven molecular assembly toward sub-12 nm and beyond precision patterning

Patterning is attractive for nanofabrication, electron devices, and bioengineering. However, achieving the molecular-scale patterns to meet the demands of these fields is challenging. Here, we propose a bubble-template molecular printing concept by introducing the ultrathin liquid film of bubble walls to confine the self-assembly of molecules and achieve ultrahigh-precision assembly up to 12 nanometers corresponding to the critical point toward the Newton black film limit. The disjoining pressure describing the intermolecular interaction could predict the highest precision effectively. The symmetric molecules exhibit better reconfiguration capacity and smaller preaggregates than the asymmetric ones, which are helpful in stabilizing the drainage of foam films and construct high-precision patterns. Our results confirm the robustness of the bubble template to prepare molecular-scale patterns, verify the criticality of molecular symmetry to obtain the ultimate precision, and predict the application potential of high-precision organic patterns in hierarchical self-assembly and high-sensitivity sensors.

A rhodamine based dual chemosensor for Al3+ and Hg2+: Application in the construction of advanced logic gates

A rhodamine-based compound (RBO), which has been constructed from the reaction between N-(rhodamine-6G)lactam-ethylenediamine and 2,1,3-benzoxadiazole-4-carbaldehyde, is reported here as a selective chemosensor for both Al³⁺ and Hg²⁺ ions in 10 mM HEPES buffer in water:ethanol (1:9, pH = 7.4). Absorption intensity of RBO increases considerably at 528 nm with these cations. It shows fluorescence enhancement at 550 nm by 1140- and 524-fold in the presence of Al³⁺ and Hg²⁺, respectively. LOD has been determined as 6.54 and 16.0 nM for Al³⁺ and Hg²⁺, respectively. Quantum yield and lifetime of RBO enhances with these metal ions. Fluorescence intensity of Al-probe complex or Hg-probe complex is quenched in the presence of fluoride or sulfide ion, respectively, opening a path for the construction logic gates. DFT analysis has been used to understand the spectral transitions. We have constructed a systematic development from single to five inputs complex circuit, and for the first time a time dependent five input complex logic circuit is reported herein.

Aggregation induced emission molecules for detection of nucleic acids

Aggregation-induced emission (AIE) is an ingenious concept in the field of luminescent molecules. AIE is the energy released in an excited state that in turn is converted into light irrespective of being in either liquid phase or solid phase. Aggregation or crystallization of AIE molecules impedes the free movement of molecules and it resultantly becomes highly fluorescent. It is currently being used for several applications including sensing, diagnostic, protein, DNA or RNA detection, cells and cell organelles imaging. AIEs are highly sensitive and specific for binding with target molecules. In this chapter, we underline different AIE molecules for detection of nucleic acids.

Phthalimide conjugation turns the AIE-active tetraphenylethylene unit non-emissive: its use in turn-on sensing of hydrazine in solution and the solid- and vapour-phase

Hydrazine is a vital precursor used in several pharmaceuticals and pesticide industries and upon exposure can cause severe health hazards. Herein, a new AIEgen, tetraphenylethylene phthalimide (TPE-PMI), is synthesized in a one-step solvent-free mechanochemical approach exploiting the simple condensation between TPE-NH2 and phthalic anhydride and used for the selective and sensitive detection of hydrazine. TPE-PMI with an AIE-active TPE-moiety is non-emissive in the solid phase by design. Hydrazine performs the cleavage of TPE-PMI in a typical "Gabriel synthesis" pathway to release AIE-active TPE-NH2 in an aqueous solution to emit blue fluorescence. A gradual rise in fluorescence intensity at 462 nm was due to the increasing hydrazine concentration and TPE-PMI showed a linear relationship with hydrazine in the concentration range from 0.2 to 3 μM. The selectivity study confirmed that the probe is inert to amines, amino acids, metal anions, anions and even common oxidants and reductants. The detection limit is 6.4 ppb which is lower than the US Environmental Protection Agency standard (10 ppb). The practical utilities of TPE-PMI were successfully demonstrated through quantitative detection of hydrazine vapour on solid platforms like paper strips and TLC plates. Furthermore, on-site detection of hydrazine in the solid phase was demonstrated by spiking the soil samples with measured quantities of hydrazine and quantitation through image analysis. This cost-effective sensing tool was successfully utilized in in vitro detection of hydrazine in live HeLa cells.

Tetraphenylethylene AIEgen bearing thiophenylbipyridine receptor for selective detection of copper(ii) ion

A new tetraphenylethylene (TPE) AIEgen appended with a thiophenylbipyridine moiety exhibits sensitivity and selectivity towards copper ions via a PET “turn on–turn off” mechanism.

Hydrazone Based Dual - Responsive Colorimetric and Ratiometric Chemosensor for the Detection of Cu2+/F- Ions: DNA Tracking, Practical Performance in Environmental Samples and Tooth Paste

Colorimetric sensors have attracted wide scope of attentions due to its fascinating advantages, like handy, equipment-free and naked eye detections. In this investigation, a new and novel hydrazone based dual-responsive ratiometric/colorimetric chemosensor have been developed for highly selective and sensitive detection of Cu²⁺ and F^- ions in dimethyl sulfoxide (DMSO) solvent. The probe showed highly selective sensing towards Cu²⁺ and F^- ions by exhibiting a color change from pale yellow to yellowish green and pale yellow to yellowish brown respectively., in DMSO without any interference of other ions at same concentration. These experimental results have also substantiated by the NMR, HR-MS, UV-Vis spectroscopic, cyclic voltammetry, differential pulse voltammetry techniques and DFT calculations. The detection limits are found to be 5.8 μM for Cu²⁺ and 0.025 μM for F^- ions which is far below to the values recommended by WHO. The stoichiometric ratios between NAPCBH and Cu2+/ F- ions were confirmed from the Job's plots and ¹H NMR titration experiments which are found to be 2:1 and 1:1 respectively. The tracking ability of the DNA with NAPCBH-Cu²⁺ was studied by UV-Vis titration and Cyclic voltammetry measurements. It shows efficient affinity towards DNA with NAPCBH-Cu²⁺. The probe can also quantitatively determine the Copper and fluoride ions present in environmental samples & toothpaste. The NAPCBH was promptly recovered by utilizing very low concentration of HCl, showing that was found feasible and re-usable sensor for the convenient detection of Cu²⁺ and F^- ions. Graphical Abstract.

Al3+ triggered aggregation induced emission of an anthracence based azine derivative in SDS medium

Single crystal X-ray structurally characterized anthracene appended unsymmetrical azine derivative, viz. 4-(anthracen-9-ylmethylene-hydrazonomethyl)-2-methoxy-phenol (L5) shows Al³⁺ assisted aggregation induced emission in SDS medium.

A sensitive OFF–ON–OFF fluorescent probe for the cascade sensing of Al3+ and F− ions in aqueous media and living cells

A simple Schiff-base ligand 2-hydroxy-1-naphthaldehyde semicarbazone (HNS) was synthesized and characterized. Based on the combined effect of inhibition of CH Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N isomerization and chelation-enhanced fluorescence (CHEF), HNS functions as a fluorescence “turn on” sensor for Al³⁺ in buffered aqueous media. Based on the strong affinity of Al³⁺ to F⁻ ions, the in situ generated Al³⁺–HNS complex can also be utilized as an effective chemosensor for F⁻ sensing by metal displacement approach, ensuing quenching of fluorescence by the reversible return of HNS from Al³⁺–HNS complex. Thus a method using a single probe for the detection of both Al³⁺ and F⁻ ions is developed. The system exhibits high selectivity and sensitivity for Al³⁺ and F⁻ ions and the detection limits were found to be as low as 6.75 × 10⁻⁸ M and 7.89 × 10⁻⁷ M, respectively. Furthermore, the practical applicability of this probe has been examined in living cells.

A simple Schiff-base ligand 2-hydroxy-1-naphthaldehyde semicarbazone (HNS) was synthesized and applied to the sequential sensing of Al³⁺ and F⁻ ions in aqueous media and live cells.

Turn-On Fluorescent Sensors for the Selective Detection of Al3+ (and Ga3+) and PPi Ions

Rationally designed multiple hydroxyl-group-based chemosensors L1-L4 containing arene-based fluorophores are presented for the selective detection of Al³⁺ and Ga³⁺ ions. Changes in the absorption and emission spectra of L1-L4 in ethanol were easily observable upon the addition of Al³⁺ and Ga³⁺ ions. Competitive binding studies, detection limits, and binding constants illustrate significant sensing abilities of these chemosensors with L4, showing the best results. The interaction of Al³⁺/Ga³⁺ ions with chemosensor L4 was investigated by fluorescence lifetime measurements, whereas Job's plot, high-resolution mass spectrometry, and ¹H NMR spectral titrations substantiated the stoichiometry between L4 and Al³⁺/Ga³⁺ ions. The solution-generated [L-M³⁺] species further detected pyrophosphate ion (PPi) by exhibiting emission enhancement and a visible color change. The binding of Al³⁺/Ga³⁺ ions with chemosensor L4 was further supported by density functional theory studies. Reversibility for the detection of Al³⁺/Ga³⁺ ions was achieved by utilizing a suitable proton source. The multiionic response, reversibility, and optical visualization of the present chemosensors make them ideal for practical applications for real samples, which have been illustrated by paper-strip as well as polystyrene film-based detection.

Recent Advances in Aggregation-Induced Emission Chemosensors for Anion Sensing

The discovery of the aggregation-induced emission (AIE) phenomenon in the early 2000s not only has overcome persistent challenges caused by traditional aggregation-caused quenching (ACQ), but also has brought about new opportunities for the development of useful functional molecules. Through the years, AIE luminogens (AIEgens) have been widely studied for applications in the areas of biomedical and biological sensing, chemosensing, optoelectronics, and stimuli responsive materials. Particularly in the application of chemosensing, a myriad of novel AIE-based sensors has been developed to detect different neutral molecular, cationic and anionic species, with a rapid detection time, high sensitivity and high selectivity by monitoring fluorescence changes. This review thus summarises the recent development of AIE-based chemosensors for the detection of anionic species, including halides and halide-containing anions, cyanides, and sulphur-, phosphorus- and nitrogen- containing anions, as well as a few other anionic species, such as citrate, lactate and anionic surfactants.

Multifunctional supramolecular self-assembly system for colorimetric detection of Hg2+, Fe3+, Cu2+ and continuous sensing of volatile acids and organic amine gases

A novel multifunctional gelator (1) based on an azobenzene derivative was designed and characterized. This compound could gelate some solvents including hexane, petroleum ether, DMSO, acetonitrile and ethanol through a heating-cooling procedure. The self-assembly process in different solvents was studied by means of UV-vis absorption and Fourier transform infrared (FTIR) spectra, field emission scanning electron microscopy (FESEM), rheological measurements, X-ray powder diffraction and water contact angle experiments. Interestingly, compound 1 had a high-contrast colorimetric detection ability towards Hg2+, Cu2+, Fe3+ and volatile acids and further organic amine gases in solution through its color change. At the same time, organogel 1 in acetonitrile also exhibited detection performance through a color or gel state change. In the response process, the self-assembly structures were changed from a nanofiber into a microsphere under induction by analytes. More significantly, film 1 could continuously detect volatile acids and organic amine gases. The number of cycles of film 1 for the detection of volatile acids and organic amine gases was at least seven times. The limit of detection (LOD) of film 1 towards TFA was calculated to be 0.0848 ppb. The sensing mechanisms were studied using 1HNMR, FESEM, UV-vis absorption spectra and HRMS. The intramolecular cyclization occurred on molecule 1 and a H2S molecule was lost during the detection process of Hg2+. It was proposed that the -N[double bond, length as m-dash]N- bonding could be coordinated by Fe3+ and Cu2+ and this further induced the absorption spectra and color change. For a volatile acid, it was possible that the volatile acid was combined with the N,N-dimethyl amine group of molecule 1. This research opens up a novel pathway to the fabrication of supramolecular self-assembly gels to detect polymetallic ions and trace volatile acids in the environment.