AIEgens: Next Generation Signaling Source for Immunoassays?

Lateral flow analysis test strips based on aggregation-induced emission technique: Principle, design, and application

This review examines the potential of aggregation-induced luminescence (AIE) materials in lateral flow assays (LFA) to enhance the sensitivity and specificity of a range of assay applications. LFA is a straightforward and effective paper-based platform for the rapid detection of target analytes in mixtures. Its simple design, low cost, and ease of operation are among the most attractive advantages of LFA. The signal reporting label, which constitutes the core component of LFA detection, is of paramount importance for enhancing the sensitivity of the detection process. The sensitivity of traditional LFA signal labels is insufficient for the detection of biomarkers at low concentrations. To address this issue, AIE materials have been developed in recent years. These materials can significantly enhance the luminescence intensity at high concentrations or in aggregated states, exhibiting excellent photostability and a high signal-to-noise ratio. They possess the advantages of high quantum yields, good photostability, and strong fluorescence, rendering them suitable for a variety of applications, including medical diagnostics, food safety, and environmental monitoring. This review therefore provides an overview of the operational principles of AIE and LFA, details the selection of AIE materials, the design of the platform and their applications, and reviews the latest research. Notable examples include the detection of viral pathogens, bacterial and mycotoxin contamination, antibiotic residues, and pesticide residues. The integration of AIE materials in LFA technology has demonstrated the potential for rapid, accurate, and cost-effective diagnostics. However, challenges remain in large-scale synthesis and multiplexed assays. The paper concludes with a discussion of current limitations and future directions for the technology.

Aggregation-induced emission-based competitive immunoassays for "signal-on" detection of proteins with multifunctional metal-organic frameworks as signal tags

An aggregation-induced emission (AIE)-based strategy was proposed for fluorescence immunoassays of protein biomarkers using Cu-based metal-organic frameworks (Cu-MOFs) to load recombinant targets and enzymes for dual signal amplification. The immunosensing platform was built based on the sequestration and consumption of the substrates of pyrophosphate (PPi) ions by Cu-MOFs and enzymatic catalysis. The negatively charged PPi could trigger the aggregation of positively charged tetraphenylethene (TPE)-substituted pyridinium salt nanoparticles (TPE-Py NPs) by electrostatic interactions, lighting up the fluorescence due to the AIE phenomenon. The consumption of PPi by the captured Cu-MOFs through the Cu2+-PPi chelation interaction and ALP-enzymatic hydrolysis depressed the aggregation of TPE-Py NPs. Capture of the tested targets in samples by the antibodies on the plate surface could prevent the attachment of target/ALP-loaded Cu-MOFs due to the competitive immunoreactions. The "signal-on" competitive immunoassay was applied for the detection of procalcitonin (PCT) as the model analyte with a linear range of 0.01-10 pg/mL and a detection limit down to 8 pg/mL. The conceptual integration of AIE with enzymatic and MOFs-based dual signal amplification endowed fluorescence immunoassays with high sensitivity and selectivity. The surface modification of Cu-MOFs with hexahistine (His6)-tagged recombinant proteins through metal coordination interactions should be evaluable for the design of novel biosensors.

Advances in enhancement-type signal tracers and analysis strategies driven Lateral flow immunoassay for guaranteeing the agri-food safety

As a classical and continuously developing on-site sensor, Lateral flow immunoassay (LFIA) exhibits promising potential for advanced point-of-care testing (POCT). Especially given the significance of agri-food in human dietary structure and the ever-increasing agri-food safety concerns, improved analysis performance of LFIA is urgently required. Recently, flourishing enhancement-type signal tracers (STs) and brilliant enhancement-type analysis strategies have been actively pursued in the development of LFIA because these patterns endow immense feasibility in manufacturing target-oriented sensing platforms. To facilitate further advancements in this field, this review comprehensively examines the recent developments in enhancement-type STs (e.g., load-, green-, recognizable-, Janus-, and dyestuffs-type STs) and enhancement-type analysis strategies (e.g. immuno-network, in-situ growth, nanozymes, multi-signal readout, and software-assisted quantitative analytical strategies) that significantly improve precise analysis efficiency. Moreover, by conducting a comprehensive evaluation of the major advancements and aiming to identify future trends in LFIA-based sensor, the objective of this review is to provide recommendations for future research based on the challenges and opportunities of LFIA.

Ultrabright aggregation-induced materials for the highly sensitive detection of Ag+ and T-2 toxin

Heavy metals and mycotoxins are important contaminants in food pollution. Sensitive, reliable, and rapid detection of heavy metals and mycotoxins is crucial for human health. In this work, imidazole-functionalized aggregation-induced emission (AIE) molecule tetra-(4-pyridylphenyl) ethylene (TPPE) was used as a precise and specific probe for Ag+ detection, with a limit of detection (LOD) of 0.0318 μM. Meanwhile, a large amount of hydrophobic TPPE molecules were loaded into the amphiphilic block copolymer F127 to form ultrabright fluorescent microspheres (TPPENPs). Lateral flow immunochromatography (LFIA) based on immunoprobe (TPPENPs-Ab) has been successfully developed. The LOD of TPPENPs-LFIA for T-2 toxin was 0.13 μg/L, which was 13.31-fold and 8.62-fold more sensitive than that of gold nanoparticles-based LFIA and ordinary fluorescent microspheres-based LFIA, respectively. TPPENPs-LFIA was employed to detect T-2 toxin in actual grain samples, with a spiked recovery rate of 71.69 - 111.13 %. This assay offers a promising strategy and new idea for multi-target detection.

Development of a β-lactamase-based aggregation-induced emission lateral flow strip for the detection of clavulanic acid in Milk

Lack of biorecognition elements significantly hinders the development of rapid detection methods for clavulanic acid (CA). To address this, we expressed Class A β-lactamases PC1 in vitro and demonstrated its high affinity for CA. Then we investigated the recognition mechanisms of PC1 for CA and identified key contact amino acids: Ser70, Lys73, Ser130, Glu166, and Lys234. Furthermore, PC1 was utilized as a novel biorecognition element to establish a "pseudo-immuno" lateral flow strip (LFS) for CA detection. Aggregation-induced emission fluorescence microspheres (AIE@FM) and biotin-streptavidin (Bio-SA) were integrated to improve the detection performance of PC1-based LFS. Results showed that the sensitivity (cut-off value) of PC1-based AIE(Bio-SA)-LFS was enhanced 2-fold and 4-fold compared to basic AIE@FM-LFS and traditional Au-based LFS, respectively. Eventually, the proposed PC1-based AIE(Bio-SA)-LFS was successfully verified in milk samples with a cut-off value of 20 ng mL-1. This study provides a powerful tool for on-site CA monitoring for the first time.

Efficient green synthesis of biocompatible MPN fluorescent microspheres via hydrophobic-force-driven strategy for enhanced immunochromatographic assays

The unique fluorescence properties of aggregation-induced emission (AIE) fluorescent microspheres (FMs) make them ideal signal markers. Traditional synthesis methods are complex, labor-intensive, and hazardous, leading to AIEFMs that lack biocompatibility and require further modification for immunoprobe preparation. This study introduces a novel hydrophobic force-driven method for rapid synthesis of highly biocompatible FMs (H-FMs), demonstrating their benefits in immunochromatographic assay (ICA) applications. The metal-polyphenol network (MPN) shell around the AIEgen core structure of H-FMs is quickly and safely formed by depositing MPN onto AIEgen nano-aggregates, achieving high dye utilization, affordability, and design flexibility, while producing H-FMs with fluorescence across 300-800 nm. The excellent biocompatibility of H-FMs eliminates the need for additional modifications, allowing antibodies to be coupled swiftly (within 10 min) with a high coupling efficiency of 93.4 %. The resulting immunoprobes exhibit strong target recognition and 90.6 % fluorescence retention over 30 days. These features support their application in double antibody sandwich and competitive ICA formats, with detection limits of 9.62 × 10² CFU/mL for E. coli O157:H7 and 0.0081 ng/mL for AFM1. This study provides new insights into designing fluorescent probes for safety monitoring of hazardous materials in the environment.

Aggregation-Induced Emission Luminogen: Role in Biopsy for Precision Medicine

Biopsy, including tissue and liquid biopsy, offers comprehensive and real-time physiological and pathological information for disease detection, diagnosis, and monitoring. Fluorescent probes are frequently selected to obtain adequate information on pathological processes in a rapid and minimally invasive manner based on their advantages for biopsy. However, conventional fluorescent probes have been found to show aggregation-caused quenching (ACQ) properties, impeding greater progresses in this area. Since the discovery of aggregation-induced emission luminogen (AIEgen) have promoted rapid advancements in molecular bionanomaterials owing to their unique properties, including high quantum yield (QY) and signal-to-noise ratio (SNR), etc. This review seeks to present the latest advances in AIEgen-based biofluorescent probes for biopsy in real or artificial samples, and also the key properties of these AIE probes. This review is divided into: (i) tissue biopsy based on smart AIEgens, (ii) blood sample biopsy based on smart AIEgens, (iii) urine sample biopsy based on smart AIEgens, (iv) saliva sample biopsy based on smart AIEgens, (v) biopsy of other liquid samples based on smart AIEgens, and (vi) perspectives and conclusion. This review could provide additional guidance to motivate interest and bolster more innovative ideas for further exploring the applications of various smart AIEgens in precision medicine.

Construction of a monoclonal molecular imprinted sensor with high affinity for specific recognition of influenza a virus subtype

Although molecular imprinting technology has been widely used in the construction of virus sensors, it is still a great challenge to identify subtypes viruses specifically because of their high similarity in morphology, size and structure. Here, a monoclonal molecular imprinted polymers (MIPs) sensor for recognition of H5N1 is constructed to permit the accurate distinguishing of H5N1 from other influenza A virus (IAV) subtypes. Firstly, H5N1 are immobilized on magnetic microspheres to produce H5N1-MagNPs, then the high affinity nanogel H5N1-MIPs is prepared by solid phase imprinting technique. When H5N1-MIPs is combined with MagNP-H5N1, different concentrations of H5N1 are added for competitive substitution. The quantitative detection of H5N1 is realized by the change of fluorescence intensity of supernatant. As expected, the constructed sensor shows satisfactory selectivity, and can identify the target virus from highly similar IAV subtypes, such as H1N1, H7N9 and H9N2. The sensor was highly sensitive, with a detection limit of 0.58 fM, and a selectivity factor that is comparable to that of other small MIPs sensors is achieved. In addition, the proposed sensor is cheap, with a cost of only RMB 0.08 yuan. The proposed monoclonal sensor provides a new method for the specific recognition of designated virus subtype, which is expected to be used for large-scale screening and accurate treatment of infected people.

A strategy to enhance the water solubility of luminescent β-diketonate-Europium(III) complexes for time-gated luminescence bioassays

Luminescent β-diketonate-europium(III) complexes have been found a wide range of applications in time-gated luminescence (TGL) bioassays, but their poor water solubility is a main problem that limits their effective uses. In this work we propose a simple and general strategy to enhance the water solubility of luminescent β-diketonate-europium(III) complexes that permits facile synthesis and purification. By introducing the fluorinated carboxylic acid group into the structures of β-diketone ligands, two highly water-soluble and luminescent Eu3+ complexes, PBBHD-Eu3+ and CPBBHD-Eu3+, were designed and synthesized. An excellent solubility exceeding 20 mg/mL for PBBHD-Eu3+ was found in a pure aqueous buffer, while it also displayed strong and long-lived luminescence (quantum yield φ = 26%, lifetime τ = 0.49 ms). After the carboxyl groups of PBBHD-Eu3+ were activated, the PBBHD-Eu3+-labeled streptavidin-bovine serum albumin (SA-BSA) conjugate was prepared, and successfully used for the immunoassay of human α-fetoprotein (AFP) and the imaging of an environmental pathogen Giardia lamblia under TGL mode, which demonstrated the practicability of PBBHD-Eu3+ for highly sensitive TGL bioassays. The carboxyl groups of PBBHD can also be easily derivatized with other reactive chemical groups, which enables PBBHD-Eu3+ to meet diverse requirements of biolabeling technique, to provide new opportunities for developing functional europium(III) complex biolabels serving for TGL bioassays.

Dynamic Imaging of Lipid Droplets in Cells and Tissues by Using Dioxaborine Barbiturate-Based Fluorogenic Probes

Lipids are essential for various cellular functions, including energy storage, membrane flexibility, and signaling molecule production. Maintaining proper lipid levels is important to prevent health problems such as cancer, neurodegenerative disorders, cardiovascular diseases, obesity, and diabetes. Monitoring cellular lipid droplets (LDs) in real-time with high resolution can provide insights into LD-related pathways and diseases owing to the dynamic nature of LDs. Fluorescence-based imaging is widely used for tracking LDs in live cells and animal models. However, the current fluorophores have limitations such as poor photostability and high background staining. Herein, we developed a novel fluorogenic probe based on a push-pull interaction combined with aggregation-induced emission enhancement (AIEE) for dynamic imaging of LDs. Probe 1 exhibits favorable membrane permeability and spectroscopic characteristics, allowing specific imaging of cellular LDs and time-lapse imaging of LD accumulation. This probe can also be used to examine LDs in fruit fly tissues in various metabolic states, serving as a highly versatile and specific tool for dynamic LD imaging in cellular and tissue environments.

Synthesis, structure, photophysical property, stability of tetraphenylethylene-based boranil, and applications in cell imaging

A new family of tetraphenylethylene-based N,O-chelated boranil complexes (TPE-BAs) with aggregation-induced emission (AIE) characteristics were developed. X-ray crystallographic analysis indicated that the terminal substituents on the aniline moiety significantly affected the intermolecular stacking mode, thereby influencing the photophysical properties. The stabilities of these compounds are closely related to the substituents on the aniline moiety. Electron-donor-substituted TPE-BA-OMe exhibited the best stability, whereas the electron-acceptor-substituted compounds exhibited poor stability. Benefitting from its AIE properties and suitable lipophilicity, TPE-BA-OMe served as an excellent fluorescent probe for the specific bioimaging of lipid droplets in living cells.

High-Performance Fluorescent Microspheres Based on Fluorescence Resonance Energy Transfer Mode for Lateral Flow Immunoassays

The label with a large Stokes shift and strong fluorescence properties could improve the sensitivity of the lateral flow immunoassay (LFIA). Herein, two aggregation-induced emission (AIE) luminogens with spectral overlap were encapsulated in polymers by using the microemulsion method as a label, and the construction of a fluorescence resonance energy transfer mode was further verified via theoretical calculation and spectral analysis. Satisfactorily, the doped AIE polymer microspheres (DAIEPMs) exhibited a large Stokes shift of 285 nm and a 10.8-fold fluorescence enhancement compared to those of the AIEPMs loaded with acceptors. Benefiting from the excellent optical performance, DAIEPMs were applied to the LFIA for sensitive detection of chlorothalonil, which is an organochlorine pesticide. The limit of detection of the proposed DAIEPMs-LFIA was 1.2 pg/mL, which was 4.8-fold and 11.6-fold lower than those of quantum dot bead LFIA and gold nanoparticle LFIA, respectively. This work provides a new strategy to improve the optical properties of fluorescent materials and construct a sensitive and reliable detection platform.

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

AIEgens-Doped Photonic Crystals for High Sensitivity Fluorescence Detection of Tumor Markers

Detection of tumor markers is of great significance to preliminarily judge whether patients have malignant tumors. Fluorescence detection (FD) is an effective means to achieve sensitive detection of tumor markers. Currently, the increased sensitivity of FD has attracted research interest worldwide. Here, we have proposed a method of doping luminogens with aggregation-induced emission (AIEgens) into photonic crystals (PCs), which can significantly enhance the fluorescence intensity to achieve high sensitivity in the detection of tumor markers. PCs are made by scraping and self-assembling, which has the special effect of fluorescence enhancement. The combination of AIEgens and PCs can enhance the fluorescence intensity 4-7 times. These characteristics make it extremely sensitive. The limit of detection (LOD) for the detection of alpha-fetoprotein (AFP) in the AIE10 (Tetraphenyl ethylene-Br) doped PCs with a reflection peak of 520 nm is 0.0377 ng/mL. LOD for the detection of carcinoembryonic antigen (CEA) in the AIE25 (Tetraphenyl ethylene-NH₂) doped PCs with a reflection peak of 590 nm is 0.0337 ng/mL. Our concept offers a good solution for highly sensitive detection of tumor markers.