Nucleic acid-regulated perylene probe-induced gold nanoparticle aggregation: a new strategy for colorimetric sensing of alkaline phosphatase activity and inhibitor screening

The development of biosensors for alkaline phosphatase activity detection based on a phosphorylated DNA probe

Alkaline phosphatase (ALP) is a zinc-containing metalloprotein that shows very great significance in clinical diagnosis, which can catalyze the hydrolysis of phosphorylated species. ALP has the potential to serve as a valuable biomarker for detecting liver dysfunction and bone diseases. On the other hand, ALP is an efficient biocatalyst to amplify detection signals in the enzyme-linked assay. It has always been a major research focus to develop novel biosensors that can detect ALP activity with high selectivity and sensitivity. There have been numerous reports on the development of biosensors to determine ALP activity using a phosphorylated DNA probe. Among them, various beneficial strategies, such as λ exonuclease-mediated cleavage reaction, terminal deoxynucleotidyl transferase-triggered DNA polymerization, and Klenow fragment polymerase-catalyzed elongation, are employed to generate amplified and more intuitive signal. This review discusses and summarizes the development and advances of biosensors for ALP activity detection that use a well-designed phosphorylated DNA probe, aiming to provide some guidelines for the design of more sophisticated sensing strategies that exhibit improved sensitivity, selectivity, and adaptability in detecting ALP activity.

Specializing Carbon Nanozyme Active Sites for Sensitive Alkaline Phosphatase Activity Metal-Free Detection

As biological enzymes regulate metabolic processes, alkaline phosphatase (ALP) is a critical diagnostic indicator associated with many diseases. To accurately measure the enzyme activity, nanozymactic materials can offer sensitive strategies for ALP detection. However, nanozymes often lack specific target binding sites, and the presence of common co-components, e. g., metal ions, may cause false-positive or false-negative results in enzyme activity determination. Herein, we developed a colorimetric assay for ALP detection using metal-free nanozymatic carbon dots (CDs). The ALP hydrolysis of pyrophosphate ions (PPi) to phosphate ions (Pi) induces a "turn-on" response based on the nanozyme activity. This PPi-induced inhibition mechanism is extensively studied via the Michaelis-Menten model, revealing that PPi acts as a noncompetitive inhibitor for CDs at a binding site distinct from the common nanozyme active site. With superior responses to ALP substrates, a highly sensitive and selective method is established for sensing ALP activity with a linear range of 0.010-0.200 U/L and a detection limit of 0.009 U/L. This finding explores the recognition and binding behavior of nanozymes, allowing for precise and reliable measurements even in complex samples, and represents a significant breakthrough for nanozyme-based assays in biological analysis.

Synergistic effect-triggered fluorescence quenching enables rapid and sensitive detection of alkaline phosphatase

The development of biosensors mediated by synergistic quenching effect is of great significance for rapid and accurate clinical diagnosis. Hence, we prepared a cyan-emitting fluorescent Si dots for alkaline phosphatase (ALP) detection through the synergistic quenching effect of inner filter effect (IFE) and photo-induced electron transfer (PET). Si dots were prepared by microwave-assisted method, which displayed high quantum yield (28.7%), as well as good physiochemical properties, such as photo-stability, pH stability, and chemical stability. As the hydrolysate of 4-nitrophenyl phosphate disodium salt hexahydrate catalyzed by ALP, both IFE and PET of 4-nitrophenyl to Si dots were used for the turn-off mode detection of ALP. The linear relationships were established between the change of fluorescence intensity and ALP concentration in the range of 0.05 U L^-1 to 5.0 U L-1, and 5.0 U L^-1 to 80.0 U L^-1, respectively. The detection limit was 0.01 U L^-1. The synergistic quenching effect caused the turn-off mode detection to be more sensitive, and it can also be used for the accurate detection of ALP in human serum, thereby showing great anti-interference ability in complex environments.

Label-free fluorescence detection of alkaline phosphatase activity using a G-triplex based dumbbell-shaped probe

Based on the fluorescence enhancement property of the G-triplex (G3)-Thioflavin T (ThT) complex, a fluorescent biosensor was successfully constructed for detection of ALP using a G3-based dumbbell-shaped probe (DP). In this work, calf intestinal ALP (CIP) can act on the 5'-terminal phosphate of DP, thereby regulating the subsequent DNA ligation reaction and enzyme cleavage of the DP nick. When the DP is digested by exonuclease, the released G3 can bind to ThT, resulting in enhanced fluorescence signal. The linear range of the sensor for CIP detection is 0.00002-0.002 U/μL, and the detection limit is 1.8 × 10^-5 U/μL. The proposed method has the advantages of simplicity, no fluorophore labeling, and low cost, which was successfully applied to the screening of enzyme inhibitors and ALP determination in human serum samples. To the best of our knowledge, this is the first report of a biosensor using G3-ThT as the signal indicator for ALP detection, which should promote the further exploitation of applying G3-ThT complex in the field of various targets recognition and analysis.

Magnetic iron oxide cores with attached gold nanostructures coated with a layer of silica: An easily, homogeneously deposited new nanomaterial for surface-enhanced Raman scattering measurements

Nanostructures made of magnetic cores (Fe₃O₄) with many smaller plasmonic (Au) nanostructures attached were covered with a very thin layer of silica. The first example of the application of this type of material for surface-enhanced Raman scattering (SERS) measurements is presented. (Fe₃O₄@Au)@SiO₂ nanoparticles turned out to be very efficient substrates for SERS measurements. Moreover, due to the nanomaterial's strong magnetic properties, it can be easily manipulated using a magnetic field, and it is therefore possible to form homogeneous layers (with no significant 'coffee-ring' effect) of (Fe₃O₄@Au)@SiO₂ nanoparticles using a very simple procedure: depositing a drop of a sol of such nanoparticles and evaporating the solvent after placing the sample in a strong magnetic field. Synthesised (Fe₃O₄@Au)@SiO₂ nanostructures have been used for the SERS detection of penicillin G in milk. Good quality SERS spectra of penicillin G were obtained even at a concentration of penicillin G in milk of 1 nmol/l - this means that the SERS detection of penicillin G in milk is possible at a concentration lower than the maximum residue limit of penicillin G in milk established by the European Commission. .

The First Silver-Based Plasmonic Nanomaterial for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy with Magnetic Properties

Nanostructures made of magnetic cores (from Fe₃O₄) with attached silver plasmonic nanostructures were covered with a very thin layer of silica. The (Fe₃O₄@Ag)@SiO₂ magnetic–plasmonic nanomaterial can be manipulated using a magnetic field. For example, one can easily form homogeneous layers from this nanomaterial using a very simple procedure: deposition of a layer of a sol of such a nanostructure and evaporation of the solvent after placing the sample in a strong magnetic field. Due to the rapid magnetic immobilization of the magnetic–plasmonic nanomaterial on the investigated surface, no coffee-ring effect occurs during the evaporation of the solvent. In this contribution, we report the first example of a magnetic, silver-based plasmonic nanomaterial for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Nanoresonators based on silver plasmonic nanostructures locally enhance the intensity of the exciting electromagnetic radiation in a significantly broader frequency range than the previously used magnetic SHINERS nanoresonators with gold plasmonic nanostructures. Example applications where the resulting nanomaterial was used for the SHINERS investigation of a monolayer of mercaptobenzoic acid chemisorbed on platinum, and for a standard SERS determination of dopamine, are also presented.

Colorimetric detection of alkaline phosphatase activity based on pyridoxal phosphate-induced chromatic switch of polydiacetylene nano-liposomes

A colorimetric assay based on polydiacetylenes (PDA) nano-liposomes is reported for facile and sensitive detection of alkaline phosphatase (ALP) activity. The critical basis of this method is that the interaction of pyridoxal phosphate (PLP) with nitrogenous group functionalized PDA nano-liposomes induces distinct blue-to-red color changes of PDA nano-liposomes. In the presence of ALP, as a nature substrate, PLP is enzymatically hydrolyzed to form pyridoxal, which cannot interact with PDA nano-liposomes. As a result, the concentration of PLP is reduced and the color change of PDA nano-liposomes is retarded, which is associated with ALP level. Under optimal conditions, the proposed method showed good linear relationship with ALP activity in the range 10-200 U/L with a limit of detection of 2.8 U/L. The detection process could be vividly observed with the naked eye. Additional attempts by using the method for the evaluation of inhibitor efficiency were also achieved with satisfying results. The method was further challenged with real human serum samples, showing consistent results when compared with a commercial standard assay kit. Such simple and easy-to-use approach may provide a new alternative for clinical and biological detection of ALP.

Ratiometric fluorescent sensor based on MoS2 QDs and AuNCs for determination and bioimaging of alkaline phosphatase

Herein, a ratiometric fluorescent sensor based on MoS₂ quantum dots (QDs) and glutathione-capped gold nanoclusters (AuNCs) was developed for determination and imaging of alkaline phosphatase (ALP). The sensor was developed by covalently linking QDs with AuNCs to form stable MoS₂@AuNCs nanohybrids that exhibited the blue fluorescence of MoS₂ QDs and the red fluorescence of AuNCs. In the presence of Ce³⁺, the fluorescence intensity of AuNCs was increased due to the aggregation-induced emission enhancement (AIEE), while that of MoS₂ QDs remained unchanged, thus could be used as a reference signal. After adenosine 5'-monophosphate (AMP) and ALP were introduced into the system, AMP was hydrolyzed to adenosine and phosphate ions (PO₄^3-). Owing to higher affinity between Ce³⁺ and PO₄^3-, the AIEE effect was inhibited, in turn resulting in the decrease of AuNCs fluorescence. The developed ratiometric fluorescent sensor had a linear response to ALP concentration ranging from 0.5 to 50 U L^-1 with a detection limit (LOD) of 0.08 U L^-1. Moreover, the sensor had low cytotoxicity and was successfully employed in lysosome localization and bioimaging of intracellular ALP in living cells.

A feasible self-assembled near-infrared fluorescence sensor for acid phosphatase detection and cell imaging

The single signal amplification strategy is significant for detecting various disease biomarkers but is restricted by its limited accuracy. The multi-signal and multi-mode methods have overcome this deficiency. Acid phosphatase (ACP) is an important intracellular enzyme but one-step cell imaging material-based probes are scarce for ACP. Herein, we designed a one-step self-assembled polymer probe using neutral red (NR), modified-(pyridoxal-5'-phosphate (PLP)) and Eu³⁺. The polymer exhibited non-emission and excellent stability. Upon the catalytic hydrolysis reaction of ACP, the polymer exhibited two strong fluorescence signals at 373 nm and 613 nm and an appreciable decline of absorbance at 395 nm. The probe has excellent selectivity and higher sensitivity with a limit of detection as low as 0.02 mU mL^-1. It possesses favorable biocompatibility and has been successfully used to detect and image intracellular ACP in several living cells.

Restoring the Oxidase-Like Activity of His@AuNCs for the Determination of Alkaline Phosphatase

In this paper, we propose a simple colorimetric method for the sensitive and selective detection of alkaline phosphatase (ALP) activity based on the turn off/turn on oxidase mimic activity of His@AuNCs. His@AuNCs/graphene oxide hybrids (His@AuNCs/GO) were easily obtained using the self-assembly method with poly (diallyldimethylammonium chloride) (PDDA)-coated GO and showed high oxidase-like activity compared with His@AuNCs. We found that the pyrophosphate ion (P₂O₇^4-, PPi) could effectively inhibit the oxidase mimic activity of His@AuNCs/GO, and the hydrolysis of PPi by ALP restored the inhibited activity of His@AuNCs/GO, enabling them to efficiently catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to generate the blue oxidized product oxTMB. The intensity of the color showed a linear dependency with the ALP activity. ALP was detected in the linear range of 0-40 mU/mL with a low detection limit (LOD) of 0.26 mU/mL (S/N = 3). The proposed method is fast, easy, and can be applied to monitor the ALP activity in serum samples accurately and effectively, which suggests its practicability and reliability in the detection of ALP activity in clinical practice.

A dual-signal fluorescent probe for detection of acid phosphatase

Acid phosphatase has become a significant indicator of prognostic and medical diagnosis, and its dysfunction may lead to a series of diseases. A novel dual-signal fluorescence method for acid phosphatase detection based on europium polymer (europium-pyridine dicarboxylicacid-adenine) and pyridoxal phosphate (PLP) was proposed. PLP coordinated with europium polymer via Eu³⁺ and P-O bonds, and the fluorescence of europium polymer was quenched due to the photoinduced electron transfer (PET) effect between aldehyde and europium polymer. Upon addition of acid phosphatase, the PLP was transformed to phosphate (Pi) and pyridoxal (PL). The PL was released from the surface of europium polymer, and the blue emission was enhanced due to the formation of internal hemiacetal, while the fluorescence of europium polymer recovered. The blue (PL) and red emission (Eu³⁺) were positively correlated with acid phosphatase activity; thus the sensitive assay of acid phosphatase was effectively achieved. The two signals were applied to determine the acid phosphatase with limits of detection (LOD) of 0.04 mU/mL and 0.38 mU/mL, and the linear ranges were 0.13-5.00 mU/mL and 1.25-20.00 mU/mL, respectively. The probe can be used to trace the acid phosphatase in biological systems and holds promise for use in clinical diagnosis and early prevention.

Target-induced transcription amplification to trigger the trans-cleavage activity of CRISPR/Cas13a (TITAC-Cas) for detection of alkaline phosphatase

Herein, an ultra-sensitive alkaline phosphatase (ALP) sensing strategy is developed by target-induced transcription amplification to trigger the trans-cleavage activity of Cas13a (TITAC-Cas). A double-stranded DNA duplex integrating a T7 promoter with 5'-phosphate and a transcription template (5'P-dsDNA) serves as the ALP substrate. In the absence of ALP, 5'P-dsDNA can be degraded by the λexo, leading to the subsequent transcription failure. In the presence of ALP, dephosphorylation reaction converts the 5'P-dsDNA to 5'OH-dsDNA and provides the protection for T7 promoter against the λexo-digestion. The intact T7 promoter of 5'OH-dsDNA can activate T7 transcription to produce a mass of single-stranded RNA (ssRNA). The ssRNA products possess a full complementarity to the spacer of crRNA and activate the ssRNase activity of CRISPR/Cas13a. As a result, Cas13a exhibits the indiscriminate cleavage of collateral FQ-reporter to release significant fluorescence signal, realizing the ultra-sensitive detection of ALP. Due to the triple signal amplification (ALP self-catalysis, T7 transcription amplification, and trans-cleavage of CRISPR/Cas13a), TITAC-Cas assay shows the ultra-sensitive detection of ALP activity with a wide linear range from 0.008 to 250 U∙L^-1). The LOD is calculated to be 6 ± 0.52 mU∙L^-1. TITAC-Cas assay is also successfully applied for analysis of ALP activity in HepG2 cell lysate with high fidelity. In addition, this method is employed to screen ALP inhibitor.

A turn-on near-infrared fluorescent probe for visualization of endogenous alkaline phosphatase activity in living cells and zebrafish

Alkaline phosphatase (ALP) is an essential hydrolase and widely distributed in living organisms. It plays important roles in various physiological and pathological processes. Herein, a turn-on near-infrared (NIR) fluorescent probe (DXMP) was developed for sensitive detection of ALP activity both in vitro and in vivo based on the intramolecular charge transfer (ICT) mechanism. Upon incubation with ALP, DXMP exhibited a strong fluorescence increment at 640 nm, which was attributed to the fact that ALP-catalyzed cleavage of the phosphate group in DXMP induced the transformation of DXMP into DXM-OH. The probe exhibited prominent features including outstanding selectivity, high sensitivity, and excellent biocompatibility. More importantly, it has been successfully used to detect and image endogenous ALP in living cells and zebrafish.

Identification of milk adulteration by a sensor array based on cationic polymer induced aggregation of a perylene probe

A novel fluorescence sensor array based on cationic polymer induced self-assembly of a perylene probe is developed. Cationic polymer induced aggregation of the carboxyl modified negatively charged perylene probe, and resulted in large quenching of monomer emission and generation of excimer emission. Upon the addition of negatively charged protein, monomer fluorescence restored with a decrease in excimer fluorescence. Based on these observations, we developed a six-channel sensor array to discriminate five main proteins in milk. In addition, we successfully identified pure milk out of different drinks using the developed sensor array since different drinks contained distinct species and contents of proteins. Furthermore, the sensor array exhibited excellent performance to discriminate milk adulterated by different concentrations of adulterants with 100% accuracy of cross validation. The analysis results also presented excellent linear correlation of adulterants contents and thus the developed sensor array shows great potential for quantitative detection of milk adulteration.

A path-choice-based biosensor to detect the activity of the alkaline phosphatase as the switch

Alkaline phosphatase (ALP), which converts the phosphate group (-PO₄) in the substrate to the hydroxyl group (-OH), is a useful tool in the biological analysis, a good indicator of dissolved inorganic phosphorus levels and an important biomarker for several diseases. In conventional designs for ALP detection, both the interferent with a -PO₄ and the target with a -OH will go into the sensing path and give out the undesired background and the desired signal respectively. This limited the sensitivity of the method and required the complicated design to achieve a satisfying limit of detection (LOD) of ALP. Here, we provided a new sensing strategy for ALP detection design. We designed a path-choice-based biosensor with two DNA tracks in which ALP works as the switch to guide the reaction path of lambda exonuclease (λ exo). The path-choice character enlarged the difference between signal and background by separating the interferent removing path and the target sensing path. The substrate preference of ALP and λ exo was studied to optimize the structure of DNA tracks. The path-choice-based biosensor achieved simple, fast (30 min), sensitive (LOD 0.014 U L^-1) and selective detection of the activity of ALP. The method has been applied to detect the activity of ALP in cell lysates, which shows the potential application in ALP-related biological research.

Assessing the range of enzymatic and oxidative tunability for biosensor design

Development of multi-functional materials and biosensors that can achieve an in situ response designed by the user is a current need in the biomaterials field, especially in complex biological environments, such as inflammation, where multiple enzymatic and oxidative signals are present. In the past decade, there has been extensive research and development of materials chemistries for detecting and monitoring enzymatic activity, as well as for releasing therapeutic and diagnostic agents in regions undergoing oxidative stress. However, there has been limited development of materials in the context of enzymatic and oxidative triggers together, despite their closely tied and overlapping mechanisms. With research focusing on enzymatically and oxidatively triggered materials separately, these systems may be inadequate in monitoring the complexity of inflammatory environments, thus limiting in vivo translatability and diagnostic accuracy. The intention of this review is to highlight a variety of enzymatically and oxidatively triggered materials chemistries to draw attention to the range of synthetic tunability available for the construction of novel biosensors with a spectrum of programmed responses. We focus our discussion on several types of macromolecular sensors, generally classified by the causative material response driving ultimate signal detection. This includes sensing based on degradative processes, conformational changes, supramolecular assembly/disassembly, and nanomaterial interactions, among others. We see each of these classes providing valuable tools toward coalescing current gaps in the biosensing field regarding specificity, selectivity, sensitivity, and flexibility in application. Additionally, by considering the materials chemistry of enzymatically and oxidatively triggered biomaterials in tandem, we hope to encourage synthesis of new biosensors that capitalize on their synergistic roles and overlapping mechanisms in inflammatory environments for applications in disease diagnosis and monitoring.

A label-free G-quadruplex-based fluorescence assay for sensitive detection of alkaline phosphatase with the assistance of Cu2

The level of alkaline phosphate (ALP) is a significant biomarker index in organism. In this work, a label-free and sensitive G-quadruplex fluorescence assay for monitoring ALP activity has been developed with the assistance of Cu²⁺ based on the competitive binding effect between pyrophosphate (PPi) and G-quadruplex-N-methylmesoporphyrin (G4/NMM) complex to Cu²⁺. In the sensing assay, the G4/NMM complex is employed as a signal indicator, while the Cu²⁺ as a quencher and the PPi as recovery agent as well as the hydrolytic substance for ALP. In details, the fluorescence of the G4/NMM complex was efficiently quenched by introducing Cu²⁺ due to the proximal carboxylate groups of NMM coordinating with the Cu²⁺ as well as the unfolding of G-quadruplex by Cu²⁺, while the higher affinity between PPi and Cu²⁺ could lead to the fluorescence recovery. However, in the presence of ALP, the PPi was hydrolyzed to phosphate ions (Pi) which cannot integrate with Cu²⁺, resulting in the fluorescence quenching once again. Thus, a simple and facile way to inspect ALP has been exploited. The proposed assay shows a good linear relationship in the range from 0.5 to 100 U/L with the detection limit of 0.3 U/L. Moreover, the fabricated method is succeeded in detecting ALP in human serum samples, indicating the potential as a profitable candidate in biological and biomedical application.

Ratiometric sensing of alkaline phosphatase based on the catalytical activity from Mn-Fe layered double hydroxide nanosheets

Two-dimensional (2D) Mn-Fe layered double hydroxide (LDH) nanosheets are firstly examined to mediate an O-phenylenediamine (OPD) based fluorescent switch in the presence of ascorbic acid (AA). On one hand, Mn-Fe LDH mimicked the functions of oxidase to catalyze the oxidation of OPD to OPDox, emitting fluorescence at 565 nm. On the other hand, Mn-Fe LDH acts as a superior catalyst for the reaction between AA and OPD to generate 3-(1,2-dihydroxyethyl)furo[3,4-b]quinoxalin-1(3H)-one (N-heterocyclic compound) with an emergence of the maximum emission at 425 nm (ca. 6 fold). The presence of AA not only induces the enhanced emission at 425 nm from the N-heterocyclic compound, but also leads to decreased fluorescence at 565 nm due to the decomposition of Mn-Fe LDH nanosheets. On the basis of the reversed fluorescence response at 425 and 565 nm, ratiometric fluorescence sensing methods (ΔF425 nm/ΔF565 nm) are developed for the determination of AA. With the assistance of alkaline phosphatase (ALP), the activity of ALP can be monitored using the ratiometric platform based on the hydrolyzing ascorbic acid 2-phosphate to yield AA with the detection limit of 0.16 mU mL-1. Different from the traditional ratiometric sensing platform, where two fluorescent probes are often introduced, the present ratiometric system derived from one signal precursor holds great potential in developing a facile platform and broadens the application of 2D nanomaterials in the field of biology.

o-Phenylenediamine/gold nanocluster-based nanoplatform for ratiometric fluorescence detection of alkaline phosphatase activity

This study demonstrates a novel and convenient ratiometric fluorescent method for the detection of alkaline phosphatase (ALP) activity. Amino-functionalized mesoporous silica nanoparticle-gold nanoclusters (MSN-AuNCs) nanocomposites were integrated with o-phenylenediamine (OPD) to form a ratiometric fluorescence nanoplatform. The presence of ALP induced the generation of quinoxaline (QX) derivative which called 3-(dihydroxyethyl)furo[3,4-b]quinoxaline-1-one (DFQ) with strong fluorescence emission at 450 nm, while the orange-red fluorescence of MSN-AuNCs at 580 nm was slightly quenched. Meanwhile, an obvious fluorescence color change from orange-red to purple and finally to blue can be observed by naked eyes with the increasing of ALP concentration. Therefore, employing the fluorescence emission of DFQ at 450 nm as the reporter signal and the fluorescence emission of MSN-AuNCs at 580 nm as a reference signal, a sensitive ratiometric detection method for ALP was developed. Quantitative detection of ALP activity in the linear range from 0.2 to 80 U/L with a detection limit of 0.1 U/L can be realized in this way, which endows the assay with high sensitivity enough for practical detection of ALP in human serum samples.

Rational Design of High-Performance Donor-Linker-Acceptor Hybrids Using a Schiff Base for Enabling Photoinduced Electron Transfer

Donor-linker-acceptor (D-L-A)-based photoinduced electron transfer (PET) has been frequently used for the construction of versatile fluorescent chemo/biosensors. However, sophisticated and tedious processes are generally required for the synthesis of these probes, which leads to poor design flexibility. In this work, by exploiting a Schiff base as a linker unit, a covalently bound D-L-A system was established and subsequently utilized for the development of a PET sensor. Cysteamine (Cys) and N-acetyl-l-cysteine (NAC) costabilized gold nanoclusters (Cys/NAC-AuNCs) were synthesized and adopted as an electron acceptor, and pyridoxal phosphate (PLP) was selected as an electron donor. PLP can form a Schiff base (an aldimine) with the primary amino group of Cys/NAC-AuNC through its aldehyde group and thereby suppresses the fluorescence of Cys/NAC-AuNC. The Rehm-Weller formula results and a HOMO-LUMO orbital study revealed that a reductive PET mechanism is responsible for the observed fluorescence quenching. Since the pyridoxal (PL) produced by the acid phosphatase (ACP)-catalyzed cleavage of PLP has a weak interaction with Cys/NAC-AuNC, a novel turn-on fluorescent method for selective detection of ACP was successfully realized. To the best of our knowledge, this is the first example of the development of a covalently bound D-L-A system for fluorescent PET sensing of enzyme activity based on AuNC nanoprobes using a Schiff base.

An Effective Osteogenesis Porous CaP/Collagen Interface Compatible with Various Substrates Fabricated by Controlled Mineralization in a Delicately Adjustable Organic Matrix

Increasing bone formation on the surfaces of implants such as screws, plates, or shims holds great significance for clinical medicine. However, osteogenesis implant coatings that mimic natural bone in terms of both their components and structural features are still lacking. Here we report the biomimetic interface of calcium phosphate (CaP) in a collagen matrix fabricated by controlled mineralization that presents biomimetic porous features. The porous CaP/collagen interface, with a thickness of about 1 μm, significantly enhances osteogenesis, as verified at both the gene and protein levels as well as by in vivo experiments. Taking advantage of the generality of the method, the biomimetic interface was prepared on a variety of substrates, including conductive substrates, 3D metal meshes, plastic or elastic substrates, and even on filter papers. The adjustability and generality of the method have enabled new characterization tests to be developed during experiments on cells and thus should greatly facilitate clinical medicine and tissue engineering.

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 fluorescent assay for the detection and bioimaging of alkaline phosphatase based on near infrared Ag2S quantum dots and calcein

Herein, a ratiometric fluorescent method was developed for alkaline phosphatase (ALP) detection based on near-infrared (NIR) Ag₂S quantum dots (QDs) and calcein through the competitive approach. The system based on Ag₂S QDs and calcein shows green (maximum emission at 512 nm from calcein) and near infrared (NIR) fluorescence (maximum 798 nm from Ag₂S QDs) under the same excitation wavelength (468 nm). In the presence of Ce³⁺, the fluorescence intensity of calcein is decreased due to static quenching, while the fluorescence intensity of Ag₂S QDs is enhanced through aggregation induced emission (AIE). The p-nitrophenyl phosphate is hydrolyzed by ALP, and the yield phosphate ions bind with Ce³⁺ with higher affinity than these of Ag₂S QDs and calcein. Therefore, the green fluorescence from calcein is recovered while NIR fluorescence from Ag₂S QDs is decreased. On the basis of these findings, a ratiometric fluorescence assay was developed for the measurement of ALP activity. The ratio of fluorescence intensity at 512 and 798 nm (F₅₁₂/F₇₉₈) was well associated with the ALP concentration ranging from 2 to 100 mU/mL with the detection limit of 1.28 mU/mL. The method was successfully applied for detecting ALP in human serum with an acceptable recovery and bioimaging intracellular ALP with good performance. In addition, the approach was also employed for the screening ALP inhibitor.

An exonuclease-assisted fluorescence sensor for assaying alkaline phosphatase based on SYBR Green I

In this report, we propose a fast, reliable and convenient approach to determine the alkaline phosphatase (ALP) activity based on a label-free fluorescence strategy. Upon catalysis of ALP, dephosphorylated dsDNA hampers the λ exonuclease (λexo) cleavage, shows high affinity to SYBR Green I (SG I), resulting in a strong fluorescence emission peak at 520 nm. In the absence of ALP, the dsDNA with 5'-phosphoryl-termini could be employed as a substrate of λexo. After cleavage, a weak fluorescence emission peaks at 520 nm could be observed. The assay was both selective and sensitive, and the detection limit was found to be as low as 3 U/L. This method was utilized to evaluate Na₃VO₄ as ALP inhibitor. The method was successfully applied to the determination of the activity of ALP in spiked human serum samples.

An ALP-activatable and mitochondria-targeted probe for prostate cancer-specific bimodal imaging and aggregation-enhanced photothermal therapy

Tumor-derived alkaline phosphatase (ALP) is over-expressed in metastatic prostate cancer. The development of selective probes for ALP detection is therefore critical for early diagnosis and therapy of metastatic prostate cancer. Herein, we develop a mitochondria-targeted near-infrared activatable fluorescent/photoacoustic (NIR FL/PA) probe for the selective detection of prostate cancer-derived ALP and aggregation-enhanced photothermal therapy. Upon dephosphorylation, the probes are activated and they provide a red-shifted strong absorption and emission in the NIR window and thus enable NIR FL and PA imaging of ALP activity in tumor tissues. Particularly, the activated probes self-assemble in situ into a supramolecular network structure which induces cell apoptosis and significantly enhances the photothermal therapy efficacy.

A high-sensitivity fluorescent probe with a self-immolative spacer for real-time ratiometric detection and imaging of alkaline phosphatase activity

The probe APW uses a self-immolative mechanism to achieve a ratio response to ALP, which has the following advantages: fast response (in less than 15 min), high quantum yield (Φ = 0.6), low detection limit (0.46 U L⁻¹) and excellent selectivity.

Ligase amplification reaction-catalyzed assembly of a single quantum dot-based nanosensor for sensitive detection of alkaline phosphatase

We demonstrate the ligase amplification reaction-catalyzed assembly of a single quantum dot-based nanosensor for sensitive detection of alkaline phosphatase.

Peroxidase-like activity of acetylcholine-based colorimetric detection of acetylcholinesterase activity and an organophosphorus inhibitor

Colorimetric detection of acetylcholinesterase (AChE) and its inhibitor organophosphates (OPs) is attractive for its convenience, but the addition of exogenous catalyst to produce a chromogenic agent may result in complexity and interference. Herein, we first found that acetylcholine (ATCh) itself mimicked peroxidase's activity, based on which a simple and reliable colorimetric system containing ATCh- 3,3',5,5'-tetramethylbenzidine (TMB)-H₂O₂ was developed for the sensitive and selective assay of AChE activity and its inhibitor OPs. Due to the AChE-catalyzed hydrolysis of acetylcholine, the peroxidase-like activity was affected, which was used for highly sensitive detection of AChE activity with a low limit of detection (LOD) of 0.5 mU mL^-1 and a linear detection range from 2.0 to 14 mU mL^-1. Furthermore, due to the inhibition of OPs on AChE, OPs were also detected with the present ATCh regulated colorimetric system with LOD of 4.0 ng mL^-1 and a linear dynamic range from 10 to 10 000 µg L^-1. This strategy was also demonstrated to be applicable for pesticide detection in real samples. Meanwhile, the sensing platform can also be implemented on test strips for rapid and visual monitoring of OPs. Thus, this extremely simple colorimetric strategy without the addition of other exogenous catalysts holds great promise for on-site pesticide detection and can be further exploited for sensing applications in the environmental and food safety fields.

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.

Nondestructive Real-Time Monitoring of Enhanced Stem Cell Differentiation Using a Graphene-Au Hybrid Nanoelectrode Array

Stem cells have attracted increasing research interest in the field of regenerative medicine because of their unique ability to differentiate into multiple cell lineages. However, controlling stem cell differentiation efficiently and improving the current destructive characterization methods for monitoring stem cell differentiation are the critical issues. To this end, multifunctional graphene-gold (Au) hybrid nanoelectrode arrays (NEAs) to: (i) investigate the effects of combinatorial physicochemical cues on stem cell differentiation, (ii) enhance stem cell differentiation efficiency through biophysical cues, and (iii) characterize stem cell differentiation in a nondestructive real-time manner are developed. Through the synergistic effects of physiochemical properties of graphene and biophysical cues from nanoarrays, the graphene-Au hybrid NEAs facilitate highly enhanced cell adhesion and spreading behaviors. In addition, by varying the dimensions of the graphene-Au hybrid NEAs, improved stem cell differentiation efficiency, resulting from the increased focal adhesion signal, is shown. Furthermore, graphene-Au hybrid NEAs are utilized to monitor osteogenic differentiation of stem cells electrochemically in a nondestructive real-time manner. Collectively, it is believed the unique multifunctional graphene-Au hybrid NEAs can significantly advance stem-cell-based biomedical applications.

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.