Enzymatic Hydrogelation-Induced Fluorescence Turn-Off for Sensing Alkaline Phosphatase in Vitro and in Living Cells

Ratiometric detection of alkaline phosphatase based on aggregation-induced emission enhancement

Alkaline phosphatase (ALP) is an important enzyme that is associated with many human diseases, so the quantitative detection of ALP is vital from a clinical perspective. Nevertheless, most fluorescent assays for monitoring ALP depend on aggregation-induced quenching (ACQ), single-signal modulation, or a "signal off" mode, which suffer from poor sensitivity, a "false positive" problem, and low signal output. In this work, we utilized the electrostatically driven self-assembly of glutathione-capped gold nanoclusters (GSH-AuNCs, which show aggregation-induced emission, AIE) and amino-modified silicon nanoparticles (SiNPs) to create a hybrid probe (SiNPs@GSH-AuNCs). This nanohybrid probe showed emission from the SiNPs at around 470 nm as well as aggregation-induced emission enhancement (AIEE) of the GSH-AuNCs at 580 nm. The AIEE of the GSH-AuNCs was quenched in the presence of KMnO₄, but the AIEE was recovered by adding ascorbic acid as an oxidation-reduction reaction occurred between KMnO₄ and the ascorbic acid. The fluorescence of the SiNPs remained constant whether the AIEE was quenched or not, meaning that the fluorescence of the SiNPs could be used as an internal reference. In a typical enzymatic reaction, ascorbic acid 2-phosphate is hydrolyzed by ALP to produce ascorbic acid. Therefore, the hybrid probe was shown to allow the ratiometric detection of ALP, with a linear range of 0.5-10 U L^-1 and a limit of detection (LOD) of 0.23 U L^-1. Finally, the proposed analytical strategy was successfully applied to detect ALP in human serum samples and to determine the concentration of an ALP inhibitor. Graphical Abstract.

Carboxylesterase-Cleavable Biotinylated Nanoparticle for Tumor-Dual Targeted Imaging

Near-infrared (NIR) nanoprobes with fluorescence "Turn-On" property are advantageous in cancer diagnosis but, to the best of our knowledge, "smart" nanoprobe that simultaneously targets both biotin receptor and carboxylesterase (CES) for HepG2 tumor-dual targeted imaging has not been reported. Methods: Using CBT-Cys click condensation reaction, we rationally designed a "smart" NIR fluorescence probe H2N-Cys(StBu)-Lys(Biotin)-Ser(Cy5.5)-CBT (NIR-CBT) and used it to facilely prepare the fluorescence-quenched nanoparticle NIR-CBT-NP. Results: In vitro results indicated that, after NIR-CBT-NP was incubated with CES for 6 h, its fluorescence was turned "On" by 69 folds. Cell experiments verified that NIR-CBT-NP was uptaken by HepG2 cells via biotin receptor-assisted endocytosis and its fluorescence was turned "On" by intracellular CES hydrolysis. Moreover, NIR-CBT-NP was successfully applied to image both biotin receptor- and CES-overexpressing HepG2 tumors. Conclusion: Fluorescence-quenched nanoparticle NIR-CBT-NP was facilely prepared to actively target biotin receptor-overexpressing HepG2 cancer cells and turn the fluorescence "On" by intracellular CES hydrolysis for tumor-dual targeted imaging. We anticipate that our fluorescence "Turn-On" nanoparticle could be applied for liver cancer diagnosis in clinic in the near future.

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.

Self-assembling peptide-based nanodrug delivery systems

Self-assembling peptide-based nanodrug delivery systems (NDDs), consisting of naturally occurring amino acids, not only share the advantages of traditional nanomedicine but also possess the unique properties of excellent biocompatibility, biodegradability, flexible responsiveness, specific biological function, and synthetic feasibility. Physical methods, enzymatic reaction, chemical reaction, and biosurface induction can yield versatile peptide-based NDDs; flexible responsiveness is their main advantage. Different functional peptides and abundant covalent modifications endow such systems with precise controllability and multifunctionality. Inspired by the above merits, researchers have taken advantage of the self-assembling peptide-based NDDs and achieved the accurate delivery of drugs to the lesion site. The present review outlines the methods for designing self-assembling peptide-based NDDs for small-molecule drugs, with an emphasis on the different drug delivery strategies and their applications in using peptides and peptide conjugates.

Smart probe for simultaneous detection of copper ion, pyrophosphate, and alkaline phosphatase in vitro and in clinical samples

Wilson's disease (WD), which might lead to acute liver failure, is an inherited disorder characterized by accumulation of copper (Cu2+) in the brain, the liver, and other vital organs. In the clinic, decreased serum alkaline phosphatase (ALP) concentration is used for WD diagnosis. But to the best of our knowledge, using a fluorescent probe to simultaneously detect multiple factors in WD (e.g., Cu2+, pyrophosphate (PPi), and ALP) has not been reported. Herein, we rationally designed a fluorescent switch (E)-8-((4-methylbenzylidene)amino)napthalen-1-amine (L) and successfully applied it for sequential and selective detections of Cu2+, PPi, and ALP in vitro, in living cells and synovial fluid samples with "Off," "On," and "Off" fluorescence signals, respectively. Considering the obvious correlations among Cu2+, PPi, and ALP in WD, we envision that our fluorescent probe L could be applied to in vitro diagnosing WD in the near future.

In situ chemical redox and functionalization of graphene oxide: toward new cathodic photoelectrochemical bioanalysis

This report outlines the first exploration of graphene oxide (GO) itself as a light harvesting material with an innovative in situ chemical redox and functionalization (CRF) strategy for versatile and high-throughput cathodic photoelectrochemical (PEC) bioanalysis.

A ratiometric multicolor fluorescence biosensor for visual detection of alkaline phosphatase activity via a smartphone

Herein we designed a selective and smartphone-based strategy for visual detection of alkaline phosphatase (ALP) by utilizing the property of amino-functionalized copper (II)-based metal-organic frameworks (NH₂-Cu-MOFs) with oxidase mimic property and fluorescence property. Surprisingly, the oxidase mimic property of NH₂-Cu-MOFs can work well at a high pH value 8.0. Thus, a cascade reaction between ALP and NH₂-Cu-MOFs was realized for the construction of a ratiometric multicolor sensing platform through the controllable catalytic activity of NH₂-Cu-MOFs by pyrophosphate (PPi) and ALP. The catalytic activity of NH₂-Cu-MOFs was greatly inhibited because of the binding ability of Cu²⁺ with PPi. When the ALP was added, the catalytic activity of NH₂-Cu-MOFs was restored and then further catalyzed the o-phenylenediamine to form the 2, 3-diaminophenazine due to the hydrolysis function of ALP towards PPi into orthophosphates. RGB analysis of the fluorescent sample images was adopted for ALP quantitative analysis. Besides, a hydrogel test kit and mobile app for ALP detection were designed as conceptual products for point-of-care. The LODs of the fluorescence sensing platform was 0.078 mU mL^-1 and 0.35 mU mL^-1 by solution analysis and hydrogel test kit analysis, respectively. This fluorescent visual method was applied to ALP detection in serum samples with satisfying results, which opened a promising horizon for the diagnosis of other biomarkers in clinical serum samples based on ALP-mediated enzyme-linked immunosorbent assay for the development of biomedicine and clinical diagnosis.

A review on emerging principles and strategies for colorimetric and fluorescent detection of alkaline phosphatase activity

Alkaline phosphatase (ALP) is a natural enzyme that is able to catalyze the dephosphorylation of phosphate esters. It participates in a great number of biological processes ranging from various metabolisms to signal transduction and cellular regulation. Since the abnormality of ALP activity in body is closely associated with many diseases, it has become an important biomarker for clinical diagnosis and treatment. Besides, it is often utilized in enzyme-linked immunosorbent assays. Given these demands, in the last few years considerable interest has been focused on exploring new materials and methods for ALP activity detection. In this review, we first made a clear classification on the principles that could be used for ALP activity determination. After that, emerging colorimetric and fluorescent strategies designed on the basis of these principles were systematically summarized. Finally, some perspectives on ALP activity analysis were discussed, hoping to inspire future efforts in the field.

Single-Particle Enzyme Activity Assay with Spectral-Resolved Dark-Field Optical Microscopy

In a clinical assay, enzymes are essential biomarkers for human disease diagnosis. In this work, a spectral-resolved single-particle detection (SPD) method is introduced to quantify alkaline phosphatase (ALP) activity in human serum with a supraparticle (SP) based on MnO₂-modified gold nanoparticle (denoted as GNP@MnO₂ SP) as the probe. In the presence of ALP, 2-phospho-l-ascorbic acid trisodium salt can be hydrolyzed into l-ascorbic acid, which serves as a good reduction agent to trigger the decomposition of the MnO₂ shell on the GNP surface. Given that a trace amount of ALP exists, noticeable scattering color change can be detected at the single-particle level due to the sensitive localized surface plasmon resonance (LSPR) effect from GNPs. With spectral-resolved dark-field optical microscopy, a linear dynamic range of 0.06 to 2.48 mU/mL ( R² = 0.99) and a very low limit of detection of 5.8 μU/mL for the ALP assay are readily achieved, which is more sensitive over the methods based on ensemble sample measurement. As a consequence, this strategy opens a new avenue for the design of an ultrasensitive detection method for disease-correlated biomarker diagnosis in the future.

A sequentially responsive and structure-transformable nanoparticle with a comprehensively improved 'CAPIR cascade' for enhanced antitumor effect

An intravenously administered drug delivery system should undergo a five-step 'CAPIR' cascade (circulation, accumulation, penetration, internalization and release), and the maximal efficiency of each step is of great importance to obtain the improved final therapeutic benefits and overall survival rate. Here, a pH/matrix metalloproteinase-9 (MMP9) sequentially responsive and continuously structure-transformable nanoparticle assembled from a doxorubicin (DOX)-conjugated peptide was exploited for comprehensively improving the 'CAPIR cascade' and eventually enhancing the therapeutic efficacy. The chimeric peptide can self-assemble into spherical nanoparticles (RGD-sNPs) at pH 7.4 with a particle size of 45.7 ± 5.4 nm. By a combination of passive and active targeting mechanisms, RGD-sNPs achieved efficient accumulation at the tumor site (∼15.1% ID g-1 within 24 h). Both in vitro and in vivo experiments revealed that RGD-sNPs can be transformed into rod-like nanoparticles (S-NFs) triggered by MMP9 that overexpressed in the tumor microenvironment, demonstrating remarkable advantages of deep tumor penetration, prolonged drug retention with ∼3.7% ID g-1 at 96 h, and 2-fold enhanced internalization. Subsequently, S-NFs would respond to the intracellular weakly acidic stimuli to rapidly release DOX for induction of cytotoxicity and apoptosis. Meanwhile, the remaining peptide was further converted into long fibers (length >5 μm) with significant cytotoxicity, thereby exerting a synergistic antitumor effect. Thus RGD-sNPs displayed superior antitumor efficacy and extended the median survival period to 55 days. This provides a new horizon for the exploration of high-performance antitumor nanomedicines.

An Allosteric-Probe for Detection of Alkaline Phosphatase Activity and Its Application in Immunoassay

A fluorescence strategy for alkaline phosphatase (ALP) assay in complicated samples with high sensitivity and strong stability is developed based on an allosteric probe (AP). This probe consists of two DNA strands, a streptavidin (SA) aptamer labeled by fluorophore and its totally complementary DNA (cDNA) with a phosphate group on the 5′ end. Upon ALP introduction, the phosphate group on the cDNA is hydrolyzed, leaving the unhydrolyzed cDNA sequence for lambda exonuclease (λ exo) digestion and releasing SA aptamer for binding to SA beads, which results in fluorescence enhancement of SA beads that can be detected by flow cytometry or microscopy. We have achieved a detection limit of 0.012 U/mL with a detection range of 0.02~0.15 U/mL in buffer and human serum. These figures of merit are better than or comparable to those of other methods. Because the fluorescence signal is localized on the beads, they can be separated to remove fluorescence background from complicated biological systems. Notably, the new strategy not only applies to ALP detection with simple design, easy operation, high sensitivity, and good compatibility in complex solution, but also can be utilized in ALP-linked immunosorbent assays for the detection of a wide range of targets.

Immune Responsive Release of Tacrolimus to Overcome Organ Transplant Rejection

Transplant rejection is the key problem in organ transplantation and, in clinic, immunosuppressive agents such as tacrolimus are directly administered to the recipients after surgery for T-cell inhibition. However, direct administration of tacrolimus may bring severe side effects to the recipients. Herein, by rational design of two hydrogelators NapPhePheGluTyrOH (1) and Nap d-Phe dPheGluTyrOH (2), a facile method of immune responsive release of tacrolimus is developed from their hydrogels to overcome organ transplantation rejection. Upon incubation with protein tyrosine kinase, which is activated in T cells after organ transplantation, the tacrolimus-encapsulating Gel 1 or Gel 2 is disassembled to release tacrolimus. Cell experiments show that both Gel 1 and Gel 2 have better inhibition effect on the activated T cells than free drug tacrolimus. Liver transplantation experiments indicate that, after 7 days of treatment of same dose tacrolimus, the recipient rats in the Gel 2 group show significantly extended median survival time of 22 days while the recipients treated with conventional tacrolimus medication have a median survival time of 13 days. It is expected herein that this "smart" facile method of immune responsive release of tacrolimus can be applied to overcome organ transplantation rejection in clinic in the near future.

Primer dephosphorylation-initiated circular exponential amplification for ultrasensitive detection of alkaline phosphatase

Alkaline phosphatase (ALP) is an important diagnostic indicator for various human diseases including bone diseases, liver dysfunction, diabetes, breast and prostatic cancers. However, the conventional methods for ALP assay are usually cumbersome and time-consuming with low sensitivity. Here, we develop a new fluorescent method for ultrasensitive detection of ALP activity on the basis of primer dephosphorylation-initiated isothermal circular exponential amplification. We design two dual-functional hairpin probes (HP1 and HP2), which function as both the templates for exponential amplification reaction (EXPAR) and the generators for signal output. In the presence of ALP, the 3'-phosphorylated primer is dephosphorylated and subsequently hybridizes with the 3' protruding end of HP1 to initiate the first strand displacement amplification (SDA), producing trigger 1 and fluorescence signal. The released trigger 1 is complementary to the 3' protruding end of HP2 for the initiation of the second SDA, producing trigger 2 and fluorescence signal. Notably, trigger 2 is complementary to the 3' protruding end of HP1 and may subsequently initiate two consecutive SDAs, enabling circular EXPAR to generate an amplified fluorescence signal. This method exhibits high sensitivity with a detection limit of 2.0 × 10^-10 U μL^-1 and a large dynamic range of 5 orders of magnitude from 1.0 × 10^-9 to 1.0 × 10^-4 U μL^-1, and it can measure ALP at the single-cell level. Importantly, this method can be applied for the measurement of kinetic parameters and the screening of potential inhibitors, providing a powerful tool for ALP-related biomedical research and clinical diagnosis.

Supramolecular Analytical Chemistry in Cancer Research

Supramolecular interactions, such as those observed between antibodies and antigens, have been employed in developing analytical methods for several decades. One major area of interest concerns cancer research, where intricate supramolecular designs have emerged to tackle difficult analytes in complex tumor systems. Our increasing knowledge toward supramolecular systems have elicited profound interest in creating more efficient analytical approaches, evidenced by the ever-growing body of literature in the field. Some of the novel tools have indeed facilitated our understanding of cancer biology, through providing previously inaccessible information. In this review, we describe common strategies of developing supramolecular analytical methods and their implementations in cancer research. We provide an overview for each of the approaches and discuss representative examples in recent literature.

Eu/Tb luminescence for alkaline phosphatase and β-galactosidase assay in hydrogels and on paper devices

Simple technologies for efficient detection of important (bio)molecules are always in great demand. We now report the detection and assay of two biologically important enzymes, alkaline phosphatase and β-galactosidase, in Eu- or Tb-based cholate hydrogels, respectively, and on filter paper discs coated with such hydrogels. Pro-sensitizers derived from 1-hydroxypyrene and 2,3-dihydroxynaphthalene were incorporated into Eu or Tb cholate hydrogels, respectively. Upon enzyme action, these artificial substrates liberate free sensitizers both in the gel and on gel-coated discs, resulting in turn-on luminescence, red/magenta for Eu, and green for Tb. The detection of enzymes was also demonstrated in natural/biological samples using low-cost systems.

Polydopamine nanodots are viable probes for fluorometric determination of the activity of alkaline phosphatase via the in situ regulation of a redox reaction triggered by the enzyme

The authors describe an environmentally friendly and fast (~14 min) method for the synthesis of homogeneously distributed fluorescent polydopamine nanodots (PDA-NDs) using KMnO₄ as the oxidant. Alkaline phosphatase (ALP) catalyzes the hydrolysis of ascorbic acid 2-phosphate to release free ascorbic acid which undergoes an in-situ redox reaction with KMnO₄. Depending on the activity of ALP, more or less KMnO₄ is consumed, and this affects the formation of the PDA-NDs. Based on this finding, a sensitive method was worked out to quantify the activity of ALP via real-time formation of fluorescent PDA-NDs. The fluorometric signal (best measured at excitation/emission peaks of 390/500 nm) is linear in the 1 to 50 mU·mL^-1 ALP activity range, and the limit of the detection is as low as 0.94 mU·mL^-1 (based on 3 σ/m). The method was successfully applied to the determination of ALP activity in spiked human serum and in MCF-7 cell lysates. It was also applied in a method to screen for inhibitors of ALP. Graphical abstract Schematic of a fluorometric method for the determination of alkaline phosphatase (ALP) activity. The method is based on the in-situ regulation of the formation of fluorescent polydopamine nanodots (PDA-NDs) through the competition between the KMnO₄-induced polymerization of dopamine and ALP-directed ascorbic acid 2-phosphate (Asc-2P) hydrolysis. AA: Ascorbic acid.

Intracellular Peptide Self-Assembly: A Biomimetic Approach for in Situ Nanodrug Preparation

Most nanodrugs are preprepared by encapsulating or loading the drugs with nanocarriers (e.g., dendrimers, liposomes, micelles, and polymeric nanoparticles). However, besides the low bioavailability and fast excretion of the nanodrugs in vivo, nanocarriers often exhibit in vitro and in vivo cytotoxicity, oxidative stress, and inflammation. Self-assembly is a ubiquitous process in biology where it plays important roles and underlies the formation of a wide variety of complex biological structures. Inspired by some cellular nanostructures (e.g., actin filaments, microtubules, vesicles, and micelles) in biological systems which are formed via molecular self-assembly, in recent decades, scientists have utilized self-assembly of oligomeric peptide under specific physiological or pathological environments to in situ construct nanodrugs for lesion-targeted therapies. On one hand, peptide-based nanodrugs always have some excellent intrinsic chemical (specificity, intrinsic bioactivity, biodegradability) and physical (small size, conformation) properties. On the other hand, stimuli-regulated intracellular self-assembly of nanodrugs is quite an efficient way to accumulate the drugs in lesion location and can realize an in situ slow release of the drugs. In this review article, we provided an overview on recent design principles for intracellular peptide self-assembly and illustrate how these principles have been applied for the in situ preparation of nanodrugs at the lesion location. In the last part, we list some challenges underlying this strategy and their possible solutions. Moreover, we envision the future possible theranostic applications of this strategy.

An impedimetric determination of alkaline phosphatase activity based on the oxidation reaction mediated by Cu2+ bound to poly-thymine DNA

A novel impedimetric assay for the accurate determination of alkaline phosphatase (ALP) activity is developed based on the Cu²⁺-mediated oxidation of ascorbic acid on a poly-thymine DNA-modified electrode.

Alkaline phosphatase-triggered assembly of etoposide enhances its anticancer effect

With the assistance of a hydrogelator precursor 1P, etoposide phosphate (EP) was subjected to alkaline phosphatase-triggered assembly which obviously enhanced its anticancer efficacy.

VS2 quantum dot label-free fluorescent probe for sensitive and selective detection of ALP

As an essential enzyme highly associated with various human diseases, alkaline phosphatase (ALP) plays an important role in human tissues. Developing new materials and strategies for monitoring ALP is thus important. We have developed a novel label-free fluorescent sensing system for ALP activity that is based on the "turn-on" fluorescence of VS₂ quantum dots. The fluorescence of VS₂ quantum dots quenched by Fe³⁺ can be restored by ascorbic acid, which is generated by hydrolysis of L-ascorbic acid 2-phosphate catalyzed by ALP. Rapid, convenient, and sensitive detection of ALP is achieved in the range from 3 to 1000 U/L (R ² =0.9985), with a detection limit of 0.27 U/L. The proposed sensor exhibits excellent selectivity for ALP compared with other enzymes and proteins, such as glucose oxidase, lysozyme, trypsin, human serum albumin, and bovine serum albumin. The reliability for ALP determination in human serum plasma has been demonstrated with satisfactory recovery, revealing promising application in clinical diagnosis and biomedical research. Graphical abstract Hydrothermally synthesized VS₂ quantum dots serving as a novel turn-on fluorescent probe for detection of alkaline phosphatase (ALP) activity. AA L-ascorbic acid, AAP L-ascorbic acid 2-phosphate, NAC N-acetyl-L-cysteine.

Intracellular Proteolytic Disassembly of Self-Quenched Near-Infrared Nanoparticles Turning Fluorescence on for Tumor-Targeted Imaging

The design of tumor-targeting, intracellular protease-activatable near-infrared fluorescence (NIRF) nanoprobes is broadly interesting but remains challenging. In this work, we report the rational design of a NIR probe Cys(StBu)-Lys(Biotin)-Lys-Lys(Cy5.5)-CBT (1) to facilely prepare the self-quenched nanoparticles 1-NPs for tumor-targeted imaging in vitro and in vivo. The biotinylated 1-NPs could be actively uptaken by biotin receptor-overexpressing tumor cells via receptor-mediated endocytosis. Upon intracellular proteolytic cleavage, 1-NPs were disassembled to yield the small molecular probe Lys(Cy5.5)-Luciferin-Lys(Biotin)-Lys-OH (1-D-cleaved), accompanied by fluorescence "Turn-On". With this NIRF "Turn-On" property, 1-NPs were successfully applied for tumor-targeted imaging. We envision that our nanoparticles could be applied for fluorescence-guided tumor surgery in the near future.

Fluorine substitution enhances the self-assembling ability of hydrogelators

When supramolecular hydrogels are applied as tissue culture scaffolds, their mechanical strength and biocompatibility are the two most important factors that must be considered. However, systematic studies on the structure-mechanical property (or structure-cytotoxicity) relationship of hydrogels are rare. Herein, we rationally designed three hydrogelators and their corresponding phosphate precursors, and systematically studied their self-assembling ability and cytotoxicity. The results indicated that fluorine substitution, but not trifluoromethyl substitution with more fluorine atoms, to the phenylalanine motif enhanced the self-assembling ability and cytotoxicity of the hydrogelators (or precursors). We envision that our preliminary study of hydrogelator fluorination would provide a strategy for the development of supramolecular hydrogels for wider biomedical applications.

Nitrogen-doped Carbon Dots Mediated Fluorescent on-off Assay for Rapid and Highly Sensitive Pyrophosphate and Alkaline Phosphatase Detection

In this report, a novel fluorescent sensing platform using nitrogen-doped carbon dots (N-CDs) as probes for fluorescence signal transmission has been designed for the detection of significant biomolecules pyrophosphate (PPi) and alkaline phosphatase (ALP). The high fluorescent N-CDs could be selectively quenched by Cu2+, and recovered by the addition of PPi because PPi preferentially binds to Cu2+. Once ALP was introduced into the system, ALP can specifically hydrolyze PPi into Pi, the intense fluorescence of N-CDs could be quenched again due to the recombination of the as-released Cu2+ with N-CDs. So, fluorescence of N-CDs is regulated by an ALP-triggered reaction. Based on this strategy, we demonstrated that N-CDs could serve as a very effective fluorescent sensing platform for label-free, sensitive and selective detection of PPi and ALP with low detection limit of 0.16 μM and 0.4 U/L for PPi and ALP, respectively. Moreover, the assay time is just around 0.5 min for PPi and 30 min for ALP. This developed strategy shows remarkable advantages including sensitive, rapid, simple, convenient, and low-cost and so forth. Furthermore, this method was also successfully applied to monitor ALP in human serum, which indicates its great potential for practical applications in biological and clinical diagnosis.

Alkaline Phosphatase-Instructed Self-Assembly of Gadolinium Nanofibers for Enhanced T2-Weighted Magnetic Resonance Imaging of Tumor

Alkaline phosphatase (ALP) is an important enzyme but using ALP-instructed self-assembly of gadolinium nanofibers for enhanced T₂-weighted magnetic resonance imaging (MRI) of tumor has not been reported. In this work, we rationally designed a hydrogelator Nap-FFFYp-EDA-DOTA(Gd) (1P) which, under the catalysis of ALP, was able to self-assemble into gadolinium nanofibers to form hydrogel Gel I for enhanced T₂-weighted MR imaging of ALP activity in vitro and in tumor. T₂ phantom MR imaging indicated that the transverse relaxivity (r₂) value of Gel I was 33.9% higher than that of 1P and both of them were 1 order of magnitude higher than that of Gd-DTPA. In vivo T₂-weighted MR imaging showed that, at 9.4 T, ALP-overexpressing HeLa tumors of 1P-injected mice showed obviously enhanced T₂ contrast. We anticipate that, by replacing ALP with other enzymes, our approach could be applied for MR diagnosis of other diseases in the future.

Reaction-Based Off-On Near-infrared Fluorescent Probe for Imaging Alkaline Phosphatase Activity in Living Cells and Mice

Alkaline phosphatases are a group of enzymes that play important roles in regulating diverse cellular functions and disease pathogenesis. Hence, developing fluorescent probes for in vivo detection of alkaline phosphatase activity is highly desirable for studying the dynamic phosphorylation in living organisms. Here, we developed the very first reaction-based near-infrared (NIR) probe (DHXP) for sensitive detection of alkaline phosphatase activity both in vitro and in vivo. Our studies demonstrated that the probe displayed an up to 66-fold fluorescence increment upon incubation with alkaline phosphatases, and the detection limit of our probe was determined to be 0.07 U/L, which is lower than that of most of alkaline phosphatase probes reported in literature. Furthermore, we demonstrated that the probe can be applied to detecting alkaline phosphatase activity in cells and mice. In addition, our probe possesses excellent biocompatibility and rapid cell-internalization ability. In light of these prominent properties, we envision that DHXP will add useful tools for investigating alkaline phosphatase activity in biomedical research.

Self-assembling peptide-based building blocks in medical applications

Peptides and peptide-conjugates, comprising natural and synthetic building blocks, are an increasingly popular class of biomaterials. Self-assembled nanostructures based on peptides and peptide-conjugates offer advantages such as precise selectivity and multifunctionality that can address challenges and limitations in the clinic. In this review article, we discuss recent developments in the design and self-assembly of various nanomaterials based on peptides and peptide-conjugates for medical applications, and categorize them into two themes based on the driving forces of molecular self-assembly. First, we present the self-assembled nanostructures driven by the supramolecular interactions between the peptides, with or without the presence of conjugates. The studies where nanoassembly is driven by the interactions between the conjugates of peptide-conjugates are then presented. Particular emphasis is given to in vivo studies focusing on therapeutics, diagnostics, immune modulation and regenerative medicine. Finally, challenges and future perspectives are presented.

A New Two-Photon Ratiometric Fluorescent Probe for Detecting Alkaline Phosphatase in Living Cells

Alkaline phosphatase (ALP) is an important diagnostic indicator of many human diseases. To quantitatively track ALP in biosystems, herein, for the first time, we report an efficient two-photon ratiometric fluorescent probe, termed probe 1 and based on classic naphthalene derivatives with a donor-π-acceptor (D-π-A) structure and deprotection of the phosphoric acid moiety by ALP. The presence of ALP causes the cleave of the phosphate group from naphthalene derivatives and the phosphate group changes the ability of the intramolecular charge transfer (ICT) and remarkably alters the probe's photophysical properties, thus an obvious ratiometric signal with an isoemissive point is observed. The fluorescence intensity ratio displayed a linear relationship against the concentration of ALP in the concentration range from 20 to 180 U/L with the limit of detection of 2.3 U/L. Additionally, the probe 1 is further used for fluorescence imaging of ALP in living cells under one-photon excitation (405 nm) or two-photon excitation (720 nm), which showed a high resolution imaging, thus demonstrating its practical application in biological systems.

Laccase-mediated formation of mesoporous silica nanoparticle based redox stimuli-responsive hybrid nanogels as a multifunctional nanotheranostic agent

In this work, we designed a new hybrid nanogel with redox responsive polymer gel shells and mesoporous silica nanoparticles (MSNs) cores via laccase-mediated radical polymerization. The successful coating of the responsive gel shells on the MSNs was confirmed by the morphology and increased diameters of the particles as determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). As observed by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS), the presence of the element S around the MSNs further confirmed the formation of the gel shell. When loaded with doxorubicin (DOX), these hybrid nanogels had a significantly higher cumulative DOX release in a reductive environment than that found under physiological conditions. The MSNs with mesoporous channels were loaded with perfluorohexane (PFH) for ultrasound imaging, which was enhanced by the presence of the elastic gel shells.

Using L-STM to directly visualize enzymatic self-assembly/disassembly of nanofibers

Self-assembly/disassembly is ubiquitous in nature and plays an important role in many biological events. But noninvasive characterization of this process in real time at molecular resolution remains challenging. Herein, using homebuilt liquid-phase scanning tunneling microscopy (L-STM) with ultrahigh stability, we directly visualized enzymatic self-assembly/disassembly of oligopeptide nanofibers in real time for the first time. Static high-resolution L-STM images clearly showed the molecular packing details in the supramolecular nanofiber and the diameter of the nanofiber was consistent with that of cryo transmission electron microscopy (cryo-TEM) observations. Moreover, the self-repairing behavior of the supramolecular nanofibers was also directly observed at high resolution for the first time. This work unprecedentedly revealed new insights into Nature-mimic self-assembly and disassembly at the molecular level. It also illustrates the potential of our homebuilt L-STM in studying delicate biological processes in physiological solution with high resolution.