Alkaline phosphatase assay using a near-infrared fluorescent substrate merocyanine 700 phosphate

Folic acid functionalized molybdenum oxide quantum dots for the detection of Cu2+ ion and alkaline phosphatase via fluorescence turn off-on mechanism

The assay of alkaline phosphatase (ALP) plays a key role in the diagnosis of various diseases. Herein, folic acid functionalized molybdenum oxide quantum dots (FA-MoO_x QDs) are explored as fluorescence "turn- off and on" probes for assaying of Cu²⁺ ion and ALP, respectively. This fluorescence sensing strategy was based on the quenching of emission peak of FA-MoO_x QDs at 445 nm by Cu²⁺ ion, followed by restoring of emission peak selectively with ALP. Based on the quenching and restoring of FA-MoO_x QDs emission intensity, quantitative assay was developed for the detection of Cu²⁺ ion (0.20 - 500 µM) and ALP (0.06 - 150 U/L) with detection limits of 29 nM and 0.026 U/L, respectively. The developed FA-MoO_x QDs-based fluorescence "turn- off and on" strategy exhibited satisfactory results for assaying of ALP in biofluids.

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.

FRET Effect between Fluorescent Polydopamine Nanoparticles and MnO2 Nanosheets and Its Application for Sensitive Sensing of Alkaline Phosphatase

As an essential and universal hydrolase, alkaline phosphatase (ALP) has been identified as a crucial indicator of various diseases. Herein, we, for the first time, expanded the application of fluorescent polydopamine (F-PDA) nanoparticles to nanoquencher-based biosensing system, as well as discovered the reversible quenching effect of manganese dioxide (MnO₂) nanosheets on the fluorescence of F-PDA nanoparticles and intensively confirmed the quenching mechanism of Förster resonance energy transfer by using transmission electron microscopy, UV-vis, Fourier transform infrared spectroscopy, and fluorescence lifetime experiments. By means of the ALP-triggered generation of ascorbic acid (AA) from the substrate ascorbic acid 2-phosphate, the AA-triggered reduction of MnO₂ nanosheets to Mn²⁺, as well as the clear quenching mechanism of F-PDA nanoparticles by MnO₂ nanosheets, we have developed a label-free, low-cost, visual, and facile synthetic fluorescent biosensor for convenient assay of ALP activity. The fluorescent bioassay shows a good linear relationship from 1 to 80 mU/mL (R² = 0.999), with a low detection limit of 0.34 mU/mL, and the excellent applicability in human serum samples demonstrates potential applications in clinical diagnosis and biomedical research.

Molybdenum oxide quantum dots prepared via a one-step stirring strategy and their application as fluorescent probes for pyrophosphate sensing and efficient antibacterial materials

MoO_x QDs were prepared using a one-step stirring treatment of MoO₃ powder in DMSO. They can be used as efficient fluorescent probes and antibacterial materials.

A novel single-fluorophore-based ratiometric fluorescent probe for direct detection of isocyanates in air

Airborne isocyanates were conveniently detected by a test paper loaded with a novel single fluorophore ratiometric fluorescent probe.

Carbon quantum dots-based recyclable real-time fluorescence assay for alkaline phosphatase with adenosine triphosphate as substrate

A convenient, reliable, and highly sensitive real-time assay for alkaline phosphatase (ALP) activity in the continuous and recyclable way is established on the basis of aggregation and disaggregation of carbon quantum dots (CQDs) through the competitive assay approach. CQDs and adenosine triphosphate (ATP) were used as the fluorescent indicator and substrate for ALP activity assessment, respectively. Richness of carboxyl groups on the surface of CQDs enables their severe aggregation triggered by cerium ions, which results in effective fluorescence quenching. Under the catalytic hydrolysis of ALP, ATP can be rapidly transformed to phosphate ions. Stronger affinity of phosphate ions to cerium ions than carboxyl groups is taken advantage of to achieve fluorescence recovery induced by redispersion of CQDs in the presence of ALP and ATP. Quantitative evaluation of ALP activity in a broad range from 4.6 to 383.3 U/L with the detection limit of 1.4 U/L can be realized in this way, which endows the assay with high enough sensitivity for practical detection in human serum. The assay can be used in a recyclable way for more than three times since the generated product CePO4 as a precipitate can be easily removed from the standard assay system. This strategy broadens the sensing application of fluorescent CQDs with excellent biocompatibility and provides an example based on disaggregation in optical probe development.

A ratiometric fluorescent probe for in vivo tracking of alkaline phosphatase level variation resulting from drug-induced organ damage

Clinical drug-induced organ toxicity and damage have been recognized as an important public health issue, and an effective approach capable of in vivo detection of biomarkers resulting from drug-induced organ damage is being actively pursued. Herein, we demonstrate a ratiometric fluorescent probe that can trace the variation in alkaline phosphatase (ALP, an organ damage biomarker) levels spatially in vivo. The probe was synthesized by incorporating a phosphate group and an amine-N-oxide group on a 1,8-naphthalimide derivative. The presence of ALP cleaves the phosphate group from naphthalimide and remarkably alters the probe's photophysical properties, thus achieving ratiometric detection of ALP. The incorporation of amine-N-oxide ensures excellent water solubility and biocompatibility, which guarantees the ratiometric detection of ALP in aqueous media and in the cells overexpressed with ALP. With a detection limit of 0.38 U L^-1, the probe was successfully used in detecting ALP in human serum samples. Moreover, the probe can be employed to monitor and spatially map the endogenous variation in ALP levels in zebrafishes. This is the first observation, to our knowledge, of organ-scale ALP pattern in vivo as a result of clinical drug (APAP) induced damage.

Development of a near-infrared fluorescence ELISA method using tyramide signal amplification

In this study, we applied tyramide signal amplification (TSA) to fluorescence enzyme-linked immunosorbent assay (ELISA) employing horseradish peroxidase (HRP) as the detection enzyme. When used with a human epidermal growth factor ELISA kit, the TSA method led to a >100-fold increase in fluorescence signal intensity in comparison to an unamplified method. It also showed wider dynamic range and better sensitivity compared to a conventional method using tetramethylbenzidine as the HRP substrate.

Near-infrared fluorescence imaging of mammalian cells and xenograft tumors with SNAP-tag

Fluorescence in the near-infrared (NIR) spectral region is suitable for in vivo imaging due to its reduced background and high penetration capability compared to visible fluorescence. SNAP(f) is a fast-labeling variant of SNAP-tag that reacts with a fluorescent dye-conjugated benzylguanine (BG) substrate, leading to covalent attachment of the fluorescent dye to the SNAP(f). This property makes SNAP(f) a valuable tool for fluorescence imaging. The NIR fluorescent substrate BG-800, a conjugate between BG and IRDye 800CW, was synthesized and characterized in this study. HEK293, MDA-MB-231 and SK-OV-3 cells stably expressing SNAP(f)-Beta-2 adrenergic receptor (SNAP(f)-ADRβ2) fusion protein were created. The ADRβ2 portion of the protein directs the localization of the protein to the cell membrane. The expression of SNAP(f)-ADRβ2 in the stable cell lines was confirmed by the reaction between BG-800 substrate and cell lysates. Microscopic examination confirmed that SNAP(f)-ADRβ2 was localized on the cell membrane. The signal intensity of the labeled cells was dependent on the BG-800 concentration. In vivo imaging study showed that BG-800 could be used to visualize xenograph tumors expressing SNAP(f)-ADRβ2. However, the background signal was relatively high, which may be a reflection of non-specific accumulation of BG-800 in the skin. To address the background issue, quenched substrates that only fluoresce upon reaction with SNAP-tag were synthesized and characterized. Although the fluorescence was successfully quenched, in vivo imaging with the quenched substrate CBG-800-PEG-QC1 failed to visualize the SNAP(f)-ADRβ2 expressing tumor, possibly due to the reduced reaction rate. Further improvement is needed to apply this system for in vivo imaging.