Nitrogen doped carbon dots for turn-off fluorescent detection of alkaline phosphatase activity based on inner filter effect

Detection of alkaline phosphatase activity with a CsPbBr3/Y6 heterojunction-based photoelectrochemical sensor

An ultra-sensitive photoelectrochemical (PEC) sensor based on perovskite composite was developed for the determination of alkaline phosphatase (ALP) in human serum. In contrast to CsPbBr3 or Y6 that generated anodic current, the heterojunction of CsPbBr3/Y6 promoted photocarriers to separate and generated cathodic photocurrent. Ascorbic acid (AA) was produced by ALP hydrolyzing L-ascorbic acid 2-phosphate trisodium salt (AAP), which can combine with the holes on the photoelectrode surface, accelerating the transmission of photogenerated carriers, leading to enhanced photocurrent intensity. Thus, the enhancement of PEC current was linked to ALP activity. The PEC sensor exhibits good sensitivity for detection of ALP owing to the unique photoelectric properties of the CsPbBr3/Y6 heterojunction. The detection limit of the sensor was 0.012 U·L-1 with a linear dynamic range of 0.02-2000 U·L-1. Therefore, this PEC sensing platform shows great potential for the development of different PEC sensors.

Dual-emission ciprofloxacin-gold nanoclusters enable ratiometric sensing of Cu2+, Al3+, and Hg2

An innovative triple optical sensor is presented that utilizes gold nanoclusters (GNCs) stabilized with ciprofloxacin (CIP) and bovine serum albumin (BSA). The sensor is designed to identify three critical metal ions, namely Cu2+, Al3+, and Hg2+. Under 360 nm excitation, the synthesized CIP-BSA-GNCs demonstrate dual fluorescence emission with peaks at 448 nm (blue) and 612 nm (red). The red emission is associated with the interior of the CIP-BSA-GNCs, whereas the blue emission results from the surface-bound CIP molecules. The sensitive and selective fluorescent nanosensor CIP-BSA-GNCs were employed to detect Cu2+, Al3+, and Hg2+ ions. Cu2+ effectively quenched the fluorescence intensity of the CIP-BSA-GNCs at both peaks via the internal charge transfer mechanism (ICT). Cu2+ could be detected within the concentration range 1.13 × 10-3 to 0.05 µM, with a detection limit of 0.34 nM. Al3+ increased the intensity of CIP fluorescence at 448 nm via the chelation-induced fluorescence enhancement mechanism. The fluorescence intensity of the core CIP-BSA-GNCs at 612 nm was utilized as a reference signal. Thus, the ratiometric detection of Al3+ succeeded with a limit of detection of 0.21 nM within the dynamic range 0.69 × 10-3 to 0.07 µM. Hg2+ effectively quenched the fluorescence intensity of the CIP-BSA-GNCs at 612 nm via the metallophilic interaction mechanism. The fluorescence intensity of CIP molecules at 448 nm was utilized as a reference signal. This allowed for the ratiometric detection of Hg2+ with a detection limit of 0.7 nM within the concentration range 2.3 × 10-3 to 0.1 µM.

A ratiometric fluorescent probe for simultaneous detection of L-ascorbic acid and alkaline phosphatase activity based on red carbon dots/polydopamine nanocomposite

Herein, efficient red carbon dots (R-CDs) were synthesized by one-step hydrothermal treatment of N-(4-amino phenyl) acetamide and (2,3-difluoro phenyl) boronic acid. The optimal emission peak of R-CDs was at 602 nm (under 520 nm excitation) and the absolute fluorescence quantum yield of R-CDs was 12.9%. Polydopamine, which was formed by the self-polymerization and cyclization of dopamine in alkaline condition, emitted characteristic fluorescence with peak position of 517 nm (under 420 nm excitation) and affected the fluorescence intensity of R-CDs through inner filter effect. L-Ascorbic acid (AA), which was the hydrolysis product of L-ascorbic acid-2-phosphate trisodium salt under the catalytic reaction of alkaline phosphatase (ALP), effectively prevented the polymerization of dopamine. Combined with the ALP-mediated AA production and the AA-mediated polydopamine generation, the ratiometric fluorescence signal of polydopamine with R-CDs was correlated closely with the concentration of both AA and ALP. Under optimal conditions, the detection limits of AA and ALP were 0.28 μM during linear range of 0.5-30 μM and 0.044 U/L with linear range of 0.05-8 U/L, respectively. This ratiometric fluorescence detection platform can efficiently shield the background interference of sophisticated samples by introducing a self-calibration as reference signal in a multi-excitation mode, which can detect AA and ALP in human serum samples with satisfactory results. Such R-CDs/polydopamine nanocomposite provides a steadfast quantitative information and makes R-CDs be excellent candidate for biosensors via combining target recognition strategy.

Novel carbon nanozymes with enhanced phosphatase-like catalytic activity for antimicrobial applications

In this work, Sulfur and Nitrogen co-doped carbon nanoparticles (SN-CNPs) were synthesized by hydrothermal method using dried beet powder as the carbon source. TEM and AFM images indicated that these SN-CNPs form a round-shape ball with an approximate diameter of 50 nm. The presence of Sulfur and Nitrogen in these carbon-based nanoparticles was confirmed by FTIR and XPS analyses. These SN-CNPs were found to have strong phosphatase-like enzymatic activity. The enzymatic behavior of SN-CNPs follows the Michaelis-Menten mechanism with greater v_max and much lower K_m values compared to alkaline phosphatase. Their antimicrobial properties were tested on E. coli and L. lactis, with MIC values of 63 μg mL^-1 and 250 μg mL^-1, respectively. SEM and AFM images of fixed and live E. coli cells revealed that SN-CNPs strongly interacted with the outer membranes of bacterial cells, significantly increasing the cell surface roughness. The chemical interaction between SN-CNPs and phospholipid modeled using quantum mechanical calculations further support our hypothesis that the phosphatase and antimicrobial properties of SN-CNPs are due to the thiol group on the SN-CNPs, which is a mimic of the cysteine-based protein phosphatase. The present work is the first to report carbon-based nanoparticles with strong phosphatase activity and propose a phosphatase natured antimicrobial mechanism. This novel class of carbon nanozymes has the potential to be used for effective catalytic and antibacterial applications.

Polychromatic fluorescent MoS2 quantum dots: fabrication and off-on sensing for fluorine ions in water

Recently, MoS₂ quantum dots (QDs) have receive widespread attention as a promising luminescent material. However, so far, little effort has been made on the multicolor emission of MoS₂ QDs. Herein, an in situ iodine doping strategy is presented and used to synthesize tunable-photoluminescent (PL) MoS₂ QDs. By fine iodine doping, the PL of the MoS₂ QDs (I-MoS₂ QDs) can be tuned in the range from 423 nm to 529 nm, which exceeds the as-reported emission wavelength range. Studies using controlled experiments and density functional theory (DFT) reveal that the change in electronic state of MoS₂ QDs is responsible for the changing PL due to iodine doping. As-synthesized I-MoS₂ QDs combined with Fe³⁺ is developed as a "turn-off-on" fluorescence sensor for F^- ions in water. The fluorescence probe has a fine linear response to F^- ions in the concentration range of 2.5-80 μM, and the limit of detection is 1.4 μM (S/N = 3).

Nitrogen-doped carbon dots: Recent developments in its fluorescent sensor applications

Doped carbon dots have attracted great attention from researchers across disciplines because of their unique characteristics, such as their low toxicity, physiochemical stability, photostability, and outstanding biocompatibility. Nitrogen is one of the most commonly used elements for doping because of its sizeable atomic radius, strong electronegativity, abundance, and availability of electrons. This distinguishes them from other atoms and allows them to perform distinctive roles in various applications. Here, we have reviewed the most current breakthroughs in nitrogen-doped CDs (N-CDs) for fluorescent sensor applications in the last five years. The first section of the article addresses several synthetic and sustainable ways of making N-CDs. Next, we briefly reviewed the fluorescent features of N-CDs and their sensing mechanism. Furthermore, we have thoroughly reviewed their fluorescent sensor applications as sensors for cations, anions, small molecules, enzymes, antibiotics, pathogens, explosives, and pesticides. Finally, we have discussed the N-CDs' potential future as primary research and how that may be used. We hope that this study will contribute to a better understanding of the principles of N-CDs and the sensory applications that they can serve.

Novel dual-emission sulfur quantum dot sensing platform for quantitative monitoring of pesticide 2,4-dichlorophenoxyacetic acid

In this work, a novel environment-friendly dual-emission Rhodamine B modified sulfur quantum dots (RhB-SQDs) sensing platform was established to economically monitor organochlorine pesticide 2,4-dichlorophenoxyacetic acid (2,4-D) through regulating the activity of alkaline phosphatase (ALP). This dual emission RhB-SQDs exhibited excellent fluorescence and high photostability with emission wavelengths of 455 nm and 580 nm. ALP catalyzed the hydrolysis of the substrate p-nitrophenyl phosphate to p-nitrophenol, which quenched RhB-SQDs fluorescence at 455 nm due to the internal filtration effect, but had no effect the fluorescence intensity of RhB-SQDs at 580 nm. When 2,4-D was present, the activity of ALP was specifically inhibited and enzymatic reaction was interrupted, leading to the reduction of p-nitrophenol production, so the fluorescence of RhB-SQDs at 455 nm was restored. It demonstrated a good linear relationship between the concentration of 2,4-D and F₄₅₅/F₅₈₀ in the range of 0.050-0.500 μg mL^-1, with a detection limit of 17.3 ng mL^-1. The dual-emission fluorescent probe was successfully realized in the identification of 2,4-D in natural water samples and vegetables with the advantages of exceptional accuracy, immunity to interference, and selectivity. The platform offers a fresh look at pesticide monitoring and has the potential to prevent pesticide-related health issues.

Water-soluble long afterglow carbon dots/silica composites for dual-channel detection of alkaline phosphatase and multi-level information anti-counterfeiting

Water-soluble carbon dots/silica (CDs/SiO₂) composites with an ultra-long lifetime of 846.9 ms and an ultra-high afterglow quantum yield of 12.1% were successfully obtained by incorporating CDs into a SiO₂ network. Within the aqueous solution, the SiO₂ layer isolates CDs from the surrounding oxygen. Meanwhile, hydrogen bonds and covalent bonds are formed between the CDs and SiO₂, and these bonds restrict the movement and vibration of the CDs. Accordingly, the non-radiative inactivation rate of the triplet excitons of the CDs is reduced, thereby enhancing the room-temperature phosphorescence of the CDs and triggering thermally activated delayed fluorescence. The multiple properties of the material effectively protect the CDs from the external environment, making CDs/SiO₂ emit a long afterglow in the solid and liquid phases. The prepared CDs/SiO₂ composites have exceptional stability against strong oxidants, acids, bases, and polar solvents. The composites were successfully used in the dual-optical mode detection of alkaline phosphatase, as an anti-counterfeiting ink, and in multi-level information anti-counterfeiting and encryption.

Preparation of Fluorescent Carbon Dots Composites and Their Potential Applications in Biomedicine and Drug Delivery-A Review

Carbon dots (CDs), a new member of carbon nanostructures, rely on surface modification and functionalization for their good fluorescence phosphorescence and excellent physical and chemical properties, including small size (<10 nm), high chemical stability, biocompatibility, non-toxicity, low cost, and easy synthesis. In the field of medical research on cancer (IARC), CDs, a new material with unique optical properties as a photosensitizer, are being applied to heating local apoptosis induction of cancer cells. In addition, imaging tools can also be combined with a drug to form the nanometer complex compound, the imaging guidance for multi-function dosage, so as to improve the efficiency of drug delivery, which also plays a big role in genetic diagnosis. This paper mainly includes three parts: The first part briefly introduces the synthesis and preparation of carbon dots, and summarizes the advantages and disadvantages of different preparation methods; The second part introduces the preparation methods of carbon dot composites. Finally, the application status of carbon dot composites in biomedicine, cancer theranostics, drug delivery, electrochemistry, and photocatalysis is summarized.

A ratiometric fluorescence method based on nitrogen-doped carbon quantum dots for the determination of the activity of alkaline phosphatase

Herein, a simple and sensitive ratiometric fluorescence sensing platform to detect alkaline phosphatase (ALP) activity is developed on the basis of yellow fluorescent nitrogen-doped carbon quantum dots (YNCDs). The hydrolysis of ascorbic acid 2-phosphate (AAP) into ascorbic acid (AA) is catalyzed by ALP. Then, AA will react with o-phenylenediamine (OPD) to form 3-(1,2-dihydroxyethyl)furo[3,4b]-quinoxaline (QXD) which is a blue fluorescent quinoxaline derivative with emission at 435 nm in the presence of Cu²⁺. YNCDs have yellow fluorescence emission at 555 nm, and can maintain stable in QXD reaction system. Therefore, by utilizing the fluorescence of YNCDs at 555 nm as reference signal and the fluorescence of QXD at 435 nm as report signal, we can detect the ALP activity by monitoring the fluorescence ratio (F₄₃₅/F₅₅₅). The linear range is 0.5-5 U/L, and the limit of detection is 0.14 U/L. An application of this method for the analysis of ALP in human serum has given satisfactory results. A ratiometric fluorescent nanoprobe for ascorbic acid and alkaline phosphatase detection with excellent biocompatible and high sensitivity was successfully constructed based on YNCDs and QXD.

Glutathione-stabilized copper nanoclusters mediated-inner filter effect for sensitive and selective determination of p-nitrophenol and alkaline phosphatase activity

A simple and highly selective fluorescence biosensor has been exploited for p-nitrophenol (p-NP) and alkaline phosphatase (ALP) activity detection based on the glutathione-stabilized copper nanoclusters (GSH-CuNCs) mediated-inner filter effect (IFE). The GSH-CuNCs were prepared by employing GSH as stabilizer and ascorbic acid (AA) as reductant. The obtained GSH-CuNCs exhibited a strong blue fluorescence emission at 420 nm with an excitation wavelength of 365 nm, which overlapped largely with the absorption spectra of p-nitrophenol (p-NP). Therefore, the luminescence of GSH-CuNCs could be quenched by p-NP through inner filter effect. In addition, ALP catalyzed the substrate p-nitrophenyl phosphate (p-NPP) to form p-nitrophenol (p-NP), which also leading to the fluorescence quenching of GSH-CuNCs. The fluorescent strategy was realized for the sensitive determination of p-NP and ALP activity with the promising limit of detection of 20 nM (for p-NP) and 0.003 mU⋅mL^-1 (for ALP). Furthermore, the method could be applied to detect the p-NP content in river water samples and ALP activity in human serum samples.

Nitrogen-Doped Carbon Dots as a Fluorescent Probe for the Highly Sensitive Detection of Bilirubin and Cell Imaging

Nitrogen-doped carbon dots (NCDs) with bright blue fluorescence were constructed by a hydrothermal method using sucrose and L-proline as raw materials. The NCDs were characterized by transmitted electron microscopy, X-ray diffractometry, Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy, and ultraviolet-visible absorption (UV-vis) and fluorescence spectroscopy to investigate the morphology, elemental composition, and optical properties. The NCDs had good water solubility, high dispersibility with an average diameter of only 1.7 nm, and satisfactory optical properties with a fluorescence quantum yield of 23.4%. The NCDs were employed for the detection of bilirubin. A good linear response of the NCDs in the range 0.35-9.78 μM was obtained for bilirubin with a detection limit of 33 nM. The NCDs were also applied to the analysis of real samples, serum and urine, with a recovery of 95.34%-104.66%. The low cytotoxicity and good biocompatibility of the NCDs were indicated by an MTT assay and cell imaging of HeLa cells. Compared with other detection systems, using NCDs for bilirubin detection was a facile and efficient method with good selectivity and sensitivity.

Enhanced Fluorescence and Environmental Stability of Red-Emissive Carbon Dots via Chemical Bonding with Cellulose Films

The development of red emission carbon dots with bright solid-state fluorescence would significantly broaden their application in optoelectronic devices and sensors. Herein, a red-emissive carbon dot-based nanocomposite has been synthesized through chemical bonding with cellulose films. The red emission originating from the surface states of carbon dots was maintained in the cellulose films. Due to the stable chemical bonding, the photoluminescence intensity and emission wavelength remained unchanged for 12 months, and the quantum yield of the composite was enhanced over 4 times. It also showed outstanding stability in water or weak acid-base environments under pHs ranging from 2 to 11. Therefore, the mechanism of chemical bonding that eliminated the defects and preserved the efficient radiative process through surface states was proposed.

Copper ions assisted fluorescent detection of some dithiocarbamates based on nickel nanocluster with aggregation-induced emission enhancement behavior

Pesticide residue contamination has become an urgent issue since it threatens both the natural environment and public health. In this study, a fluorescent method for detecting dithiocarbamate (DTC) compounds was constructed based on novel nickel nanoclusters (Ni NCs) and copper ions (Cu²⁺). The water-soluble fluorescent Ni NCs were synthesized for the first time through a one-pot method using glutathione as stabilizer and ascorbic acid as reducing agent. The as-prepared Ni NCs exhibited a maximum fluorescence emission at 445 nm when excited by 380 nm. And they displayed aggregation-induced emission enhancement when ethylene glycol was introduced into the nanocluster aqueous solution. Based on the Ni NCs, a label-free fluorescence quenching sensor was established for sensitive and selective detection of DTC compounds with the assistance of Cu²⁺. The complex formed by DTC and Cu²⁺ led to fluorescence quenching of Ni NCs through inner filter effect. The sensing method was successfully applied to two typical DTC compounds, thiram and disulfiram, with good linearity over a wide linear range and a low detection limit. Moreover, the proposed approach was capable of thiram detection in real samples, which confirms the potential of this sensing method as a platform for DTC compound detection.

Carbon dot composites for bioapplications: a review

Recent advancements in the synthesis of carbon dot composites and their applications in biomedical fields (bioimaging, drug delivery and biosensing) have been carefully summarized. The current challenges and future trends of CD composites in this field have also been discussed.

Highly Sensitive Alkaline Phosphatase Biosensor Based on Internal Filtration Effect between G-Quadruplex/N-methylmesoporphyrin IX and p-Nitrophenol

In this work, an alkaline phosphatase (ALP) biosensor was established based on G-quadruplex/N-methylmesoporphyrin IX (G4/NMM) and p-nitrophenol (PNP). Because the absorption of PNP was close to the excitation wavelength of G4/NMM, PNP could reduce the fluorescence of G4/NMM. Meanwhile, PNP was the hydrolysis product of p-nitrophenylphosphate (PNPP) by ALP. Therefore, ALP could be detected. This ALP biosensor had a linear analytical range from 2.5 to 25 U/L a the detection limit of 0.81 U/L. Moreover, it showed a satisfactory selectivity and recovery rates.

An Exonuclease I-Aided Turn-Off Fluorescent Strategy for Alkaline Phosphatase Assay Based on Terminal Protection and Copper Nanoparticles

As an important DNA 3'-phosphatase, alkaline phosphatase can repair damaged DNA caused by replication and recombination. It is essential to measure the level of alkaline phosphatase to indicate some potential diseases, such as cancer, related to alkaline phosphatase. Here, we designed a simple and fast method to detect alkaline phosphatase quantitively. When alkaline phosphatase is present, the resulting poly T-DNA with a 3'-hydroxyl end was cleaved by exonuclease I, prohibiting the formation of fluorescent copper nanoparticles. However, the fluorescent copper nanoparticles can be monitored with the absence of alkaline phosphatase. Hence, we can detect alkaline phosphatase with this turn-off strategy. The proposed method is able to quantify the concentration of alkaline phosphatase with the LOD of 0.0098 U/L. Furthermore, we utilized this method to measure the effects of inhibitor Na₃VO₄ on alkaline phosphatase. In addition, it was successfully applied to quantify the level of alkaline phosphatase in human serum. The proposed strategy is sensitive, selective, cost effective, and timesaving, having a great potential to detect alkaline phosphatase quantitatively in clinical diagnosis.

Controlled synthesis of fluorescent carbon materials with the assistance of capillary electrophoresis

Carbon nanodots (CNDs) have been widely applied in variety of fields, while some evidences indicate their components may be complicated. In this work, capillary electrophoresis (CE) was used to evaluate the effect of synthetic conditions of fluorescent CNDs prepared through the hydrothermal method using citric acid (CA) and Triaminoguanidinium chloride (TG_Cl) as the starting materials. The results indicated that the fluorescent components of the products were affected by the ratio of the starting materials, the reaction temperature and reaction time. Under selected conditions, a ratio of TG_Cl to CA of 1:6, the reaction at 180 °C for 3 h, the product contains more than 4 fluorescent components with similar optical properties. CNDs were used for the determination of Cr(VI) in environmental samples with recoveries ranging in 95.3-107%, and the mechanism was also confirmed.

Synthesis of polyurethane-derived nitrogen-doped carbon dots for a high-performance fluorescence bioimaging probe

A universal and robust strategy using polyurethane sponge as a precursor was developed to synthesize highly-soluble nitrogen-doped carbon quantum dots via a solvothermal method.

Ratiometric ultrasensitive optical chemisensor film based antibiotic drug for Al(III) and Cu(II) detection

Herein, we developed and designed a novel ratiometric optical chemisensor film for determining Al(III) and Cu(II) in low concentration ranges. The chemisensor film consists of (a) antibacterial drug Ciprofloxacin (CPFX) [1-cyclopropyl-6-fluoro1,4-dihydro-4-oxo-7-(piperaziny-l-yl) quinolone-3carboxylic acid] and (b) a reference dye 5,10,15,20- tetrakis (pentafluorophenyl) porphyrin (TFPP) in a polyvinyl chloride (PVC) matrix. PVC was applied as a homogeneous system for mixing CPFX and TFPP. The emission intensity of the CPFX in the PVC matrix varies depending on the concentrations of the Al(III) and Cu(II) ions. When the sensor film is immersed in different Al(III) concentrations, a significant fluorescence enhancement of the CPFX at (427 nm) is observed. Furthermore, the fluorescence intensity of the red emission of the TFPP dye at (644 nm) does not alter. However, in the presence of Cu(II) ions, a considerable emission quenching of the CPFX peak at (427 nm) is observed. PVC provides a great permeability and penetration facilities of dissolved ions that make the sensor film sensitive to Al(III) or Cu(II) changes outside the matrix. The film displays immense sensitivity depending on their distinctive optical characteristics of CPFX and detection capabilities within a low detection limit LOD for Al(III) and Cu(II). The LOD values were estimated to be 2.05 x 10^-7 M and 1.04 x 10^-7 M respectively with a relative standard deviation RSD_r (1%, n=3). Density functional theory (DFT) and the time-dependent DFT (TDDFT) theoretical calculations were performed to study Cu(II) and Al(III) complexation structures and their electronic properties in solution and in the sensor film. The interference of the chemisensor film was examined using different cations and the chemisensor provides significant selectivity. We develop a new ratiometric chemisensor based on PVC polymer film for Al(III) and Cu(II) detection.

Hydrothermal synthesis of highly fluorescent nitrogen-doped carbon quantum dots with good biocompatibility and the application for sensing ellagic acid

Blue emissive nitrogen-doped carbon quantum dots (N-CQDs) with a high quantum yield as high as 84.79% were successfully synthesized via the hydrothermal treatment of citric acid and diethylenetriamine in one pot. The as-prepared N-CQDs displayed excellent stability in high-salt conditions, good photostability, promising the N-CQDs as potential probes for selectively detecting ellagic acid with a linear range of 0.01-50 μM on the basis of inner filter effect. And the hydroponics experiment of gardenia with N-CQDs suggested the good biocompatibility of the N-CQDs, indicating the potential applications in biomedical fields.

In Situ Formation of 2,3-Diaminophenazine for Evaluation of Alkaline Phosphatase Activity via the Inner Filter Effect

We report a fluorescence assay for alkaline phosphatase (ALP) detection using in situ generation of 2,3-diaminophenazine (OPDox) through an inner filter effect (IFE). AgNO₃ can oxidize o-phenylenediamine in a short time to obtain fluorescent OPDox. p-nitrophenol is obtained from ALP-catalyzed hydrolysis of p-nitrophenylphosphate, which can result in the fluorescence quenching of OPDox via the IFE. Consequently, in situ generation of OPDox can be utilized for evaluating the activity of ALP. The fluorescence of OPDox decreases linearly with increasing concentration of ALP in the range from 0.1 to 8.0 mU mL^-1, and the detection limit is calculated to be 0.05 mU mL^-1. More importantly, this assay has been used to construct a logic gate. The IFE-based fluorescence assay exhibits several distinctive advantages, including high sensitivity, good selectivity, and simple operation. Consequently, it may pave the way for the detection of ALP by utilization of the in situ generation of fluorophores.

Development of a facile and sensitive method for detecting alkaline phosphatase activity in serum with fluorescent gold nanoclusters based on the inner filter effect

In this work, a simple and sensitive method based on the inner filter effect (IFE) of p-nitrophenol (PNP) on the fluorescence of gold nanoclusters (AuNCs) has been developed for detecting alkaline phosphatase (ALP) activity.

Aggregation-Induced Emission Luminogens for Activity-Based Sensing

Fluorescent sensing has emerged as a powerful tool for detecting various analytes and visualizing numerous biological processes by virtue of its superb sensitivity, rapidness, excellent temporal resolution, easy operation, and low cost. Of particular interest is activity-based sensing (ABS), a burgeoning sensing approach that is actualized on the basis of dynamic molecular reactivity rather than conventional lock-and-key molecular recognition. ABS has been recognized to possess some distinct advantages, such as high specificity, extraordinary sensitivity, and accurate signal outputs. A majority of ABS sensors are constructed by modifying conventional fluorogens, which are strongly emissive when molecularly dissolved in solvents but experience emission quenching upon aggregate formation or concentration increase. The aggregation-caused quenching (ACQ) phenomenon leads to a limited amount of labeling of the analyte with the sensor and low photobleaching resistance, which could impede practical applications of the ABS protocol. As an anti-ACQ phenomenon, aggregation-induced emission (AIE) provides a straightforward solution to the ACQ problem. Thanks to their intrinsic advantages, including high photobleaching threshold, high signal-to-noise ratio, fluorescence turn-on nature, and large Stokes shift, AIE-active luminogens (AIEgens) represent a class of extraordinary fluorogen alternatives for the ABS protocol. The use of AIEgen-involved ABS can integrate the advantages of AIEgens and ABS, and additionally, the AIE process offers some unique properties to the ABS approach. For instance, in some cases of water-soluble AIEgen-involved ABS, chemical reaction not only leads to a chang in the emission color of the AIEgens but also causes solubility variations, which could result in specific "light-up" signaling. In this Account, the basic concepts and mechanistic insights of the ABS approach involving the AIE principle are briefly summarized, and then we highlight the new breakthroughs, seminal studies, and trends in the area that have been most recently reported by our group. This emerging sensing protocol has been successfully utilized for detecting an array of targets including ions, small molecules, biomacromolecules, and microenvironments, all of which closely relate to human health, medical, and public concerns. These detections are smoothly achieved on the basis of various reactions (e.g., hydrolysis, boronate cleavage, dephosphorylation, addition, cyclization, and rearrangement reactions) through different sensing principles. In these studies, the AIEgen-involved ABS strategy generally shows good biocompatibility, high selectivity, excellent reliability and high signal contrast, strongly indicating its great potential for high-tech innovations in the sensing field, among which bioprobing is of particular interest. With this Account, we hope to spark new ideas and inspire new endeavors in this emerging research area, further promoting state-of-the-art developments in the field of sensing.