Novel turn-on fluorescent detection of alkaline phosphatase based on green synthesized carbon dots and MnO 2 nanosheets

A Ratiometric Fluorescence Biosensor for Detection of Alkaline Phosphatase Via an Advanced Chemometric Model

In this paper, a ratiometric fluorescence biosensor was introduced for alkaline phosphatase (ALP) detection based on 2-aminopurine (2-Amp) and thioflavin T (ThT)-G-quadruplex system. We designed a special DNA (5'-AGGGTTAGGGTTAGGGTTAGGGAAA/i2-Amp/AAAA-PO4-3', AP) modified with a phosphate moiety at the 3'-end, G-quadruplex at the 5'-end, and a fluorophore (2-Amp) in the middle. In the absence of ALP, the G-rich AP strand could be prone to fold into G-quadruplex structures in the presence of K+. Then, ThT combined with G-quandruplex, resulting in the enhancement of fluorescence emission peak at 485 nm. However, ALP-mediated hydrolysis of the 3'-phosphoryl end promoted the cleavage of AP by the exonuclease I (Exo I), releasing 2-Amp which displayed a strong fluorescence emission peak at 365 nm. Moreover, the quantitative fluorescence model (QFM) was derived for the analysis of the fluorescence measurements obtained by the proposed ratiometric fluorescent biosensor. With the aid of the advanced model, the proposed ratiometric fluorescent biosensor possessed satisfactory results for the detection of ALP in the human serum samples, with accuracy comparable to that of the reference method-the commercial ALP assay kit. Under the optimized experimental conditions, this method exhibited good selectivity and higher sensitivity, and the detection limit was found to be as low as 0.017 U/L. Therefore, it is reasonable to expect that the method had a great potential to detect ALP quantitatively in clinical diagnosis.

Lignin-Based N-Carbon Dots Anchoring NiCo2S4/Graphene Hydrogel Exhibits Excellent Performance as Anodes for Hybrid Supercapacitor

A NiCo2S4/N-CDs/RGO ternary composite hydrogel was prepared via a one-step hydrothermal method, utilizing lignin-based nitrogen-doped carbon dots as a bridge connecting NiCo2S4 and graphene. The specific capacitance of NiCo2S4/N-CDs/RGO significantly outperforms that of the GH and NiCo2S4/RGO electrodes, achieving 1050 F g-1. The 3D mesh porous hydrogel structure mitigates NiCo2S4 nanoparticle aggregation, providing a larger specific surface area for enhanced charge storage. The abundant functional groups of N-CDs interact with Ni (II) and Co (III) cations, favoring NiCo2S4 particle synthesis. Additionally, an assembled solid-state asymmetric supercapacitor employing NiCo2S4/N-CDs/RGO as the positive electrode exhibited excellent energy density (68.4 Wh kg-1) and cycle stability (82% capacitance retention after 10,000 constant current charge-discharge cycles).

Blue light emitting graphene quantum dots/ Rhodamine B doped gold nanostars for ratiometric detection of methotrexate

In this work, an innovative ratiometric sensing platform was developed for the determination of methotrexate (MTX), an antifolate drug, a chemotherapy agent, and an immune system suppressant based on blue emission graphene quantum dots/Rhodamine B doped gold nanostars (B-GQDs/Au NSt-RB). The developed sensor was a dual-emission fluorescent probe with two major emission peaks at 440 nm (B-GQDs) and 580 nm (Au NSt-RB) by exciting at 330 nm. Based on the inhibiting effect of MTX on the system's fluorescence density, the stable ratiometric fluorescent probe was used for the rapid determination of MTX in aquatic solutions and spiked human serum samples. The results indicated good linear correlations over the logarithmic concentration range of 0.3 nM-50.0 μM. In addition, B-GQDs/Au NSt-RB can further realize highly sensitive detection of MTX with a low LOD value of 2.28 × 10-10 M. The RSD% values obtained for the intra-day and inter-day precision were 0.63-3.86 %. With recoveries of 98.2-100.1 % and 98.7-100.5 %, respectively. The short-term temperature and freeze-thaw tests confirmed the higher stability of the developed sensor. In addition, the calculated recoveries for MTX recognition in real samples were in the range of 98-102 %. These findings suggested the excellent potential of the ratiometric fluorescence B-GQDs/Au NSt-RB sensor for detecting MTX in real plasma samples.

Construction of CDs@β-CD@CCM ratiometric fluorescence probe for FRET-based ClO--sensing

β-Cyclodextrin (β-CD)-functionalized carbon quantum dots (CDs) loaded with curcumin (CCM) were used for ClO-sensing with high sensitivity and selectivity. This fluorescence resonance energy transfer (FRET)-based sensor was created through attaching CCM to the CDs via β-CD linker. CCM could get into the interior of β-CD triggering the FRET from CDs to CCM, providing an 'off' state of the CDs. However, the effect of FRET was weakened by the ClO-, because the o-methoxyphenol structure from CCM was oxidized to be benzoquinone. The fluorescence intensity of CDs@β-CD@CCM at 440 nm can be heightened and 520 nm from CCM can decrease along with the increased ClO-. Therefore, a ratiometric fluorescence probe for ClO-sensing is successfully constructed. It conforms to a polynomial curve equation which is I440/I520= -0.0268 + 0.0315 CClO-+ 0.0055[CClO-]2(R2= 0.9958) between 0 and 18.4μM ClO-. Furthermore, we also obtain excellent results using this spectrophotometric method for ClO--sensing in pure water and commercial disinfectants, which afford potential in the environment monitoring area. We expect this sensing platform could be helpful in other analogous probes in relevant fields.

Research progress of organic fluorescent probes for lung cancer related biomarker detection and bioimaging application

As one of the major public health problems, cancers seriously threaten the human health. Among them, lung cancer is considered to be one of the most life-threatening malignancies. Therefore, developing early diagnosis technology and timely treatment for lung cancer is urgent. Recent research has witnessed that measuring changes of biomarkers expressed in lung cancer has practical significance. Meanwhile, we note that bioimaging with organic fluorescent probes plays an important role for its high sensitivity, real-time analysis and simplicity of operation. In the past years, kinds of organic fluorescent probes targeting lung cancer related biomarker have been developed. Herein, we summarize the research progress of organic fluorescent probes for the detection of lung cancer related biomarkers in this review, along with their design principle, luminescence mechanism and bioimaging application. Additionally, we put forward some challenges and future prospects from our perspective.

A carbon dots-MnO2 nanosheet-based turn-on pseudochemodosimeter as low-cost probe for selective detection of hazardous mercury ion contaminations in water

Mercury is a highly hazardous element due to its profound toxicity and wide abundance in the environment. Despite the availability of various fluorimetric detection tools for Hg2+, including organic fluorophores and aptasensors, they often suffer from shortcomings like the utilization of expensive chemicals and toxic organic solvents, multi-step synthesis, sometimes with poor selectivity and low sensitivity. Whereas, biomass-derived fluorophores, such as carbon dots (CDs), present themselves as cost-effective and environmentally benign alternatives that exhibit comparable efficacy. Herein, we report a reaction-driven sensing assembly based on CDs, MnO2 nanosheets, and hydroquinone monothiocarbonate (HQTC) for the detection of Hg2+ ions, which relies on the formation of a CDs-MnO2 FRET-conjugate, resulting in the quenching of the intrinsic fluorescence of CDs. In a pseudochemodosimetric approach, the thiophilic nature of mercury was utilized for in-situ generation of the reducing species, hydroquinone from HQTC, resulting in the reduction of MnO2 nanosheets, the release of fluorescent CDs back to the solution. The low limit of detection (LOD) was achieved as 2 ppb (0.01 μM). The probe worked efficiently in real water samples like sea, river with good recovery of spiked Hg2+ and in some Indian ayurvedic medicines as well. Furthermore, solid-phase detection with sodium alginate beads demonstrated the ability of this cost-effective sensing assembly for onsite detection of Hg2+ ions.

β-Glucuronidase-triggered reaction for fluorometric and colorimetric dual-mode assay based on the in situ formation of silicon nanoparticles

BACKGROUND

β-Glucuronidase (GUS) is considered as a promising biomarker for primary cancer. Thus, the reliable detection of GUS has great practical significance in the discovery and diagnosis of cancer. Compared with traditional organic probes, silicon nanoparticles (Si NPs) have emerged as robust optical nanomaterials due to their facile preparation, superior photobleaching resistance and excellent biocompatibility. However, most nanomaterials-based methods only output a single signal which is easily influenced by external factors in complex systems. Hence, developing nanomaterial-based multi-signal optical assays for highly sensitive GUS determination is still urgently desired.

RESULTS

In this study, we developed a simple and efficient one-step method for the in situ preparation of yellow color and yellow-green fluorescent Si NPs. This was achieved by combining 3-[2-(2-aminoethylamino) ethylamino] propyl-trimethoxysilane with p-aminophenol (AP) in an aqueous solution. The obtained Si NPs showed yellow-green fluorescence at 535 nm when excited at 380 nm, while also exhibiting an absorption peak at a wavelength of 490 nm. Taking inspiration from the easy synthesis step regulated by AP, which is generated through the hydrolysis of 4-aminophenyl β-D-glucuronide catalyzed by GUS, we constructed a direct fluorometric and colorimetric dual-mode method to measure GUS activity. The developed fluorometric and colorimetric sensing platform showed high sensitivity and accuracy with detection limits for GUS determination as low as 0.0093 and 0.081 U/L, respectively.

SIGNIFICANCE

This study provides a facile dual-mode fluorometric and colorimetric approach for determination of GUS activity based on novel Si NPs for the first time. This designed sensing approach was successfully employed for the quantification of GUS in human serum samples and screening of GUS inhibitors, indicating the feasibility and potential applications in clinical cancer diagnosis and anti-cancer drug discovery.

Manganese dioxide nanosheet-modulated ratiometric fluoroprobe based on carbon quantum dots from okra for selective and sensitive dichlorvos detection in foods

Herein, we developed a ratiometric fluoroprobe by integrating okra-derived carbon quantum dots (CQDs) with amplex red (AR) using manganese dioxide nanosheets (MnO2 NSs) as a medium. Fluorescence intensities (FIs) of CQDs were sharply quenched by MnO2 NSs via an inner-filter effect processes, whereas the FIs of AR were significantly enhanced due to oxidation of AR to AR-ox by the oxidase-mimetic activity of MnO2 NSs. Acetyrylcholinesterase hydrolyzed acetylthiocholine to produce thiocholine, and the decomposition of MnO2 NSs to Mn2+ by thiocholine led to the FI recovery of CQDs, but decreased FIs of AR-ox. Based on the above phenomenon and the inhibitory effect of dichlorvos (DDVP) on acetyrylcholinesterase activity, a novel ratiometric fluoroprobe for DDVP quantification was pioneered. Under optimized conditions, this fluoroprobe gave a wide linear range (4-120 μg/L), low detection limit (1.2 μg/L), and satisfactory fortification recoveries (90.0-110.0%), thereby providing good prospects for routine DDVP monitoring in foods.

Ratiometric fluorescence determination of alkaline phosphatase activity based on carbon dots and Ce3+-crosslinked copper nanoclusters

A new ratiometric fluorescent probe for efficient determination of ALP was developed. The probe was constructed by combining Ce3+-crosslinked copper nanoclusters (Ce3+-CuNCs) which exhibit the aggregation-induced emission (AIE) feature with carbon dots (CDs). The introduction of phosphate (Pi) induced the generation of CePO4 precipitation, resulting in significant decrease of fluorescence emission of CuNCs at 634 nm. At the same time, the fluorescence of CDs at 455 nm was obviously enhanced, thus generating ratiometric fluorescence response. Based on the fact that the hydrolysis of pyrophosphate (PPi) by ALP can produce Pi, the CD/Ce3+-CuNCs ratiometric probe was successfully used to determine ALP. A good linear relationship between the ratiometric value of F455/F634 and ALP concentrations ranging from 0.2 to 80 U·L- 1 was obtained, with a low detection limit of 0.1 U·L- 1. The ratiometric responses of the probe resulted in the visible fluorescence color change from orange red to blue with the increase of ALP concentration. The smartphone-based RGB recognition of the fluorescent sample images was used for ALP quantitative determination. A novel ratiometric fluorescent system based on Ce3+-CuNCs with AIE feature and CDs were constructed for efficient detection of ALP.

A label-free ThT-assisted fluorescence detection strategy of alkaline phosphatase activity based on MnO2 nanosheets

Alkaline phosphatase (ALP) is a metalloenzyme, the level of which is clinically significant as an abnormality of ALP activity results in several diseases. In the present study, we introduced a MnO₂ nanosheet-based assay for ALP detection employing the adsorption and reduction characteristics of G-rich DNA probes and ascorbic acid (AA), respectively. Ascorbic acid 2-phosphate (AAP) was utilized to act as a substrate for ALP which hydrolyzes AAP generating AA. In the absence of ALP, MnO₂ nanosheets adsorb the DNA probe destructing the G-quadruplex formation and showing no fluorescence emission. On the contrary, being present in the reaction mixture ALP hydrolyzes AAP yielding AA, then the AA reduce the MnO₂ nanosheets into Mn²⁺, hence, the probe is free to react with a dye, thioflavin T (ThT), and synthesizes ThT/G-quadruplex to spark high fluorescence intensity. Therefore, under optimized conditions (250 nM DNA probe, 8 μM ThT, 96 μg/mL MnO₂ nanosheets, and 1 mM AAP) the sensitive and selective measurement of ALP activity can be achieved through the change of fluorescence intensity, with a linear range and a limit of detection of 0.1-5 U/L and 0.045 U/L. Our assay exhibited its potential to assess the ALP inhibitor when in an inhibition assay Na₃VO₄ inhibited ALP with an IC50 value of 0.137 mM and also was validated in clinical samples.

Folic Acid Adjustive Polydopamine Organic Nanoparticles Based Fluorescent Probe for the Selective Detection of Mercury Ions

Polydopamine fluorescent organic nanomaterials present unique physicochemical and biological properties, which have great potential application in bio-imaging and chemical sensors. Here, folic acid (FA) adjustive polydopamine (PDA) fluorescent organic nanoparticles (FA-PDA FONs) were prepared by a facile one-pot self-polymerization strategy using dopamine (DA) and FA as precursors under mild conditions. The as-prepared FA-PDA FONs had an average size of 1.9 ± 0.3 nm in diameter with great aqueous dispersibility, and the FA-PDA FONs solution exhibit intense blue fluorescence under 365 nm UV lamp, and the quantum yield is ~8.27%. The FA-PDA FONs could be stable in a relatively wide pH range and high ionic strength salt solution, and the fluorescence intensities are constant. More importantly, here we developed a method for rapidly selective and sensitive detection of mercury ions (Hg²⁺) within 10 s using FA-PDA FONs based probe, the fluorescence intensities of FA-PDA FONs presented a great linear relationship to Hg²⁺ concentration, the linear range and limit of detection (LOD) were 0-18 µM and 0.18 µM, respectively. Furthermore, the feasibility of the developed Hg²⁺ sensor was verified by determination of Hg²⁺ in mineral water and tap water samples with satisfactory results.

Photothermal visual sensing of alkaline phosphatase based on the etching of Au@MnO2 core-shell nanoparticles

Alkaline phosphatase (ALP), as a crucial enzyme involved in many physiological activities, is always used as one of the significant biomarkers in clinical diagnosis. Herein, a novel, simple, and effective photothermal quantitative method based on the etching of MnO₂-coated gold nanoparticles (Au@MnO₂ NPs) was established for ALP activity assay with a household thermometer-based visual readout. The photothermal effect of Au@MnO₂ NPs is much higher than that of MnO₂ NPs or Au NPs. The MnO₂ shell of Au@MnO₂ NPs can be etched by ascorbic acid, a product of ALP-catalyzed hydrolysis of 2-phospho-l-ascorbic acid. With the etching of Au@MnO₂ NPs, the photothermal conversion efficiency decreased gradually, causing the decrease of the temperature increment of the solutions by degrees. A household thermometer, instead of large-scale and professional instruments, was used as a signal reader to realize the visual quantitative detection. The photothermal platform was used successfully for the determination of ALP with a wide linear range from 2.0 to 50 U/L and a detection limit as low as 0.75 U/L. Moreover, the inhibition efficiency of sodium vanadate for ALP activity was investigated, proving the photothermal quantitative method will be a potential platform for screening enzyme inhibitors. Such a sensitive, facile, and low-cost sensing assay provides a new prospect to develop platforms for point-of-care testing.

Fluorescence Turn-on Detection of Alkaline Phosphatase Activity and Al3+ Using Vitamin B6 Cofactor Conjugated GSH Capped Mn-doped ZnS Quantum Dots

The glutathione (GSH) functionalized Mn-doped ZnS quantum dots (GSH_Mn_ZnS QDs) was conjugated with pyridoxal 5'-phosphate (PLP). The -CHO group of vitamin B₆ cofactor PLP interacted with the -NH₂ group of GSH functionalized Mn_ZnS QDs. The conjugation of PLP quenched the fluorescence emission of GSH_Mn_ZnS QDs at 601 nm. Addition of alkaline phosphatase (ALP) catalytically dephosphorylated the PLP into pyridoxal that restored the fluorescence emission of GSH_Mn_ZnS QDs. With a sensitivity of 0.035 U/L, the PLP conjugated GSH_Mn_ZnS QDs was applied to quantify ALP activity in human serum and plasma. Further, the developed nanoprobe PLP conjugated GSH_Mn_ZnS QDs was also applied to detect Al³⁺. The complexation-induced fluorescence enhancement was observed at 492 nm upon the interaction of Al³⁺ with the PLP conjugated GSH_Mn_ZnS QDs. Without any interference from other tested metal ions, this nanoprobe can be employed to detect Al³⁺ down to 2.30 µM.

Ultrarapid Microwave-Assisted Synthesis of Fluorescent Silver Coordination Polymer Nanoparticles and Its Application in Detecting Alkaline Phosphatase Activity

Fluorescent silver coordination polymer nanoparticles (Ag-TPA CPNs) were synthesized using a combination of terephthalic acid (TPA) and silver nitrate via an ultrarapid microwave-assisted strategy within 15 min. The Ag-TPA CPNs displayed a high fluorescent quantum yield (QY = 20.19%) and large Stokes shift (~200 nm), with two emission peaks at 490 nm and 520 nm under an excitation wavelength of 320 nm. A fluorescent "turn-off" method using fluorescent Ag-TPA CPNs was applied to detect the alkaline phosphatase (ALP) activity on the basis of the ALP-catalyzed hydrolysis of ascorbic acid 2-phosphate (AA2P) to ascorbic acid (AA), and the AA product triggered the reduction of Ag⁺ ions into silver nanoparticles. The fluorescent lifetime of Ag-TPA CPNs decreased from 3.93 ms to 3.80 ms after the addition of ALP, which suggests that this fluorescent "turn-off" detection of ALP activity is a dynamic quenching process. The fluorescent intensity had a linear relationship with the concentration of ALP in the range of 0.2-12 mU/mL (r = 0.991) and with a limit of detection (LOD) of 0.07 mU/mL. It showed high selectivity in ALP detection towards metal ions and amino acids, as well as other enzymes such as horseradish peroxidase, glucose oxidase, tyrosinase, trypsin, lysozyme, and superoxides. When it was applied for the fluorescent "turn-off" detection of ALP activity in serum samples, mean recovery levels ranging from 99.5% to 101.2% were obtained, with relative standard deviations (RSDs) lower than 4% accuracy. Therefore, it is an efficient and accurate tool for analyzing ALP levels in biosamples.

Silver ion-regulated ratiometric fluorescence assay for alkaline phosphatase detection based on carbon dots and o-phenylenediamine

In this work, a novel silver ion (Ag⁺)-regulated ratiometric fluorescence method for the effective and sensitive determination of alkaline phosphatase (ALP) was established based on carbon dots (CDs) and o-phenylenediamine (OPD). OPD can be oxidized by Ag⁺ to generate fluorescent 2, 3-diaminophenazine (DAP). Thus, based on inner-filter effect (IFE) or/and fluorescence resonance energy transfer (FRET) between CDs and DAP, the CDs-Ag⁺-OPD system can generate dual-emission at 454 nm and 570 nm respectively when excited at 360 nm. The introduction of ascorbic acid (AA) can react with Ag⁺ to produce dehydroascorbic acid (DHAA), which inhibits the generation of DAP, resulting in the fluorescence decrease at 570 nm and fluorescence recovery of CDs at 454 nm. Meanwhile, DHAA can react with OPD to generate quoxaline (QX), which emits strong blue fluorescence at 440 nm, further inhibiting the IFE or/and FRET between CDs and DAP. An obvious ratiometric fluorescence response was observed with the increase of the concentration of AA introduced. Due to the fact that AA can be generated by the enzyme catalysis reaction between ALP and 2-phospho-l-ascorbic acid (AAP), the CDs-Ag⁺-OPD ratiometric system was applied to the determination of ALP successfully. The ratiometric fluorescence value of F₄₅₄/F₅₇₀ increases with increasing ALP concentration, with a linear range of 0.2 to 40 U/L and detection limit of 0.1 U/L. In addition, the CDs-Ag⁺-OPD ratiometric system was successfully applied to the detection of ALP in human serum samples.

Fluorescence recovery based on synergetic effect for ALP detection

Alkaline phosphatase (ALP) is an important biomarker associated with diabetes, liver dysfunction, bone diseases, and breast cancer. Here we developed a method based on synergetic fluorescence recovery for the sensitive detection of ALP. Cadmium-zinc-selenium (CdZnSe) quantum dots (QDs) were prepared by one-pot water bath method without any complicated and rigorous conditions. CdZnSe QDs displayed high luminous efficiency, good stability, and good biocompatibility. KMnO₄ and ascorbic acid phosphate (AAP) can dynamically quench the fluorescence of CdZnSe QDs. Ascorbic acid, produced by ALP-catalyzed hydrolysis of AAP, reacted with KMnO₄, causing the synergetic fluorescence recovery of CdZnSe QDs. The synergetic recovery efficiency correlates well with the logarithmic ALP concentration in the range of 2.5-250 U/L with a detection limit of 0.21 U/L. In addition, good recoveries were obtained in the detection of ALP in human serum. This method provided a new research idea to improve the detection sensitivity and selectivity of ALP detection.

Novel nitrogen-doped carbon dots for "turn-on" sensing of ATP based on aggregation induced emission enhancement effect

In this work, a nitrogen-doped carbon dots (CDs) was successfully synthesized by hydrothermal synthesis of polyethylenimine (PEI) and citric acid. The as-prepared CDs suffered from aggregation-caused quenching (ACQ) with a high concentration, but after adding adenosine triphosphate (ATP), the CDs aggregated. The generation of aggregates caused the rotation of the surface groups on CDs and reduced the non-radiation decay. The QY of CDs in water was 9.25 %, and increased to 16.60 % and 63.38% in the addition of 100 and 1000 μM ATP. And then, the enhancement of the radiation rate led to the aggregation induced enhancement effect (AIEE). Moreover, we also found that the proportion of precursors for CDs synthesis was a key factor in the occurrence of AIEE. Therefore, such CDs would be excellent candidates as fluorescent probes for the label-free detection of ATP. Our proposed method exhibited simple and easy preparation of nanoprobe, quick response (3 min), wide range of linear rage (1-2000 μM) and eco-friendly. In addition, the method performed successfully as a "turn-on" sensor for detection of ATP in the tablet with a recovery of 100.1~106.9% and RSD below 3.5%.

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.

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.

A ratiometric fluorescence platform composed of MnO2 nanosheets and nitrogen, chlorine co-doped carbon dots and its logic gate performance for glutathione determination

Illustration of the principle of a dual-emission ratiometric fluorescence strategy for the selective detection of GSH based on an N, Cl-CD-assisted MnO2 nanosheet–OPD system.

A turn off-on fluorometric and paper based colorimetric dual-mode sensor for isoniazid detection

In the present study, Cobalt oxyhydroxide (CoOOH) nanosheets were applied for establishing a dual fluorometric and smartphone-paper-based colorimetric method to detect isoniazid. CoOOH nanosheets quenched the fluorescence emission of sulfur and nitrogen co-doped carbon dots (S,N-CDs) due to inner filter effect (IFE). The quenched fluorescence intensity of S,N-CDs restored in the presence of isoniazid due to destroying CoOOH nanosheets by this drug. Moreover, with adding isoniazid the solution color of CoOOH nanosheets altered from brownish yellow to pale yellow. We exploited these facts to design a turn off-on fluorometric and paper based colorimetric sensor for isoniazid measurement at the range of 0.5-10 and 5-100 μM with detection limits of 0.28 μM and 4.0 μM, respectively. The introduced dual sensor was used for pharmaceutical, environmental and biological analysis of isoniazid with satisfactory results. The paper based colorimetric sensor can be applied for isoniazid portable monitoring by smartphone as a detector and even nocked eyes.

A BCNO QDs-MnO2 nanosheets based fluorescence "off-on-off" and colorimetric sensor with smartphone detector for the detection of organophosphorus pesticides

In this work, boron carbon oxynitride quantum dots (BCNO QDs) were prepared by a one-step hydrothermal process of ethanolamine and boric acid. BCNO QDs exhibited blue fluorescence with the optimal excitation/emission fluorescence peak at 335 and 420 nm, respectively. As an efficient fluorescence quencher, manganese dioxide (MnO₂) nanosheets can effectively quench the fluorescence of BCNO QDs via the inner filter effect (IFE). Acetylcholinesterase (AChE) catalyzes the hydrolysis of acetylcholine (ATCh) to produce thiocholine (TCh). TCh can reductively degrade MnO₂ nanosheets to generate Mn²⁺, thereby recovering the fluorescence of BCNO QDs. Organophosphorus pesticides (OPs) can inhibit the activity of AChE enzymes, thereby preventing the production of TCh and the decomposition of MnO₂ nanosheets, resulting in the fluorescence "turn-off". Therefore, the concentration of OPs can be detected by measuring the fluorescence intensity change of AChE-ATCh-MnO₂-BCNO-QDs system. Under optimal experimental conditions, the dynamic detection range of paraoxon is 0.1-250 ng mL^-1, and the detection limit is 0.03 ng mL^-1. Meanwhile, the reaction system also showed concentration-dependent visual color changes from colorless to brownish. Furthermore, we prepared a portable BCNO QDs test paper. By using a smartphone to identify the RGB values of the reaction solution and the corresponding test paper, we carried out the digital image chromaticity analysis, which can shorten the detection time and reduce the detection cost, and provide an effective solution for the rapid detection of OPs on site.

Manganese-based advanced nanoparticles for biomedical applications: future opportunity and challenges

Nanotechnology is the most promising technology to evolve in the last decade. Recent research has shown that transition metal nanoparticles especially manganese (Mn)-based nanoparticles have great potential for various biomedical applications due to their unique fundamental properties. Therefore, globally, scientists are concentrating on the development of various new manganese-based nanoparticles (size and shape dependent) due to their indispensable utilities. Although numerous reports are available regarding the use of manganese nanoparticles, there is no comprehensive review highlighting the recent development of manganese-based nanomaterials and their potential applications in the area of biomedical sciences. The present review article provides an overall survey on the recent advancement of manganese nanomaterials in biomedical nanotechnology and other fields. Further, the future perspectives and challenges are also discussed to explore the wider application of manganese nanoparticles in the near future. Overall, this review presents a fundamental understanding and the role of manganese in various fields, which will attract a wider spectrum of the scientific community.

A Review of Off-On Fluorescent Nanoprobes: Mechanisms, Properties, and Applications

With the development of nanomaterials, fluorescent nanoprobes have attracted enormous attention in the fields of chemical sensing, optical materials, and biological detection. In this paper, the advantages of "off-on" fluorescent nanoprobes in disease detection, such as high sensitivity and short response time, are attentively highlighted. The characteristics, sensing mechanisms, and classifications of disease-related target substances, along with applications of these nanoprobes in cancer diagnosis and therapy are summarized systematically. In addition, the prospects of "off-on" fluorescent nanoprobe in disease detection are predicted. In this review, we presented information from all the papers published in the last 5 years discussing "off-on" fluorescent nanoprobes. This review was written in the hopes of being useful to researchers who are interested in further developing fluorescent nanoprobes. The characteristics of these nanoprobes are explained systematically, and data references and supports for biological analysis, clinical drug improvement, and disease detection have been provided appropriately.

Nitrogen-doped carbon quantum dots fabricated from cellulolytic enzyme lignin and its application to the determination of cytochrome c and trypsin

A sensitive and effective strategy for the detection of cytochrome c (Cyt c) and trypsin was developed using biomass nitrogen-doped carbon quantum dots (N-CQDs) as the fluorescence probe. N-CQDs were synthesized through a one-pot hydrothermal method by utilizing cellulolytic enzyme lignin as the carbon source and ammonia as the solvent and nitrogen source. The obtained N-CQDs had good water solubility and stable optical properties. The introduction of nitrogen increased fluorescence quantum yield (QY) to 8.23%, which was almost four times as high as that before nitrogen doping. The N-CQDs were fabricated as a label-free biosensor to detect Cyt c and trypsin. The fluorescence of N-CQDs was quenched with positively charged Cyt c due to electrostatic induction aggregation and static quenching. However, Cyt c tended to be hydrolyzed into small peptides in the presence of trypsin, which caused fluorescence recovery of the N-CQDs/Cyt c complex. A wide linear response range was achieved for Cyt c within 1-50 μM and the developed N-CQDs/Cyt c complex displayed a linear response for trypsin within 0.09-5.4 U/mL. The detection limits were 0.29 μM for Cyt c and 0.013 U/mL for trypsin, respectively. Furthermore, this assay had been applied to Cyt c and trypsin detection in serum samples with the recoveries in the range of 94.6-98.5% and 95.5-102.0%, respectively. The established method was sensitive, selective, easy to operate, and low cost, which proved its potential application in clinical diagnosis. The synthesis and fluorescence mechanism of N-CQDs and the strategy for Cyt c and trypsin detection.

An ultrasensitive colorimetric and fluorescence dual-readout assay for glutathione with a carbon dot-MnO2 nanosheet platform based on the inner filter effect

An ultrasensitive colorimetric and fluorescence dual-readout assay based on the inner filter effect (IFE) was developed for glutathione (GSH) determination, in which carbon dots (C-dots) were used as a fluorophore and MnO2 nanosheets as an absorber. Due to the excellent optical absorption properties of MnO2 nanosheets and the good spectral overlap between the fluorophore and absorber, MnO2 nanosheets could effectively quench the fluorescence of C-dots via the IFE. As the target, GSH could reduce MnO2 nanosheets to Mn2+ ions, which inhibited the IFE and resulted in the fading of solution color and the recovery of the fluorescence signal. And these two kinds of signals were respectively used for qualitative and quantitative detection of GSH. The results showed that this proposed assay could distinguish 10 μM GSH with the naked eye and quantitatively detect GSH within the concentration range of 0.1-400 μM. The limit of detection was 6.6 nM. Moreover, this assay showed sensitive responses in human serum and urine samples, which indicated that this IFE-based assay has great potential in GSH-related clinical and bioanalytical applications.

Natural source of carbon dots from part of a plant and its applications: a review

Carbon dots (CDs) are carbon nanoparticles with a size of less than 10 nm, and are synthesized from various sources; they have been of great interest to scientists worldwide due to their unique optical, electrical, and chemical properties. Sources of carbon are inexpensive and can be classified as a renewable natural resources. Many researchers use CDs because of their low toxicity, better water solubility, high biocompatibility, and stable photoluminescence. The simple methods for producing CDs are hydrothermal and use inexpensive equipment, have low energy consumption, simple manipulation, and one-step preparation. Since the discovery of CDs, researchers have used them in various applications such as sensing, bioimaging, drug delivery, and catalysis. In this review, CDs synthesized from natural resources such as samples from herbs, roots, leaves, flowers, and fruit and some applications are described. This review provides a summary of carbon dots that is expected to provide further information for development of new CDs.

A "switch-on" fluorescence assay based on silicon quantum dots for determination of ascorbic acid

Water dispersible silicon quantum dots (SiQDs) showing blue fluorescence were synthesized with 3-aminopropyltriethoxysilane (APTES) as silicon source. Based on the synthesized SiQDs as the photoluminescence unit, MnO₂ nanosheets (NS) as the quencher, a "switch-on" fluorescence assay for the determination of ascorbic acid (AA) was designed. The fluorescence of SiQDs can be effectively quenched by MnO₂ NS because of the internal filtration effect. In the presence of AA, MnO₂ is reduced to Mn²⁺, so that the fluorescence of SiQDs is partially recovered. The recovered fluorescence intensity was related to the concentration of AA. Under the optimal experimental conditions, the linear response range of the assay to AA is 1-80 µM, and the detection limit is 0.48 µM. The method for the determination of AA has the advantages of simple, low cost, good selectivity and sensitivity. The assay has been successfully applied to the quantification of AA in beverage (mizone) samples, which proves the practicability of the assay.

Biocompatibility nanoprobe of MXene N-Ti3C2 quantum dot/Fe3+ for detection and fluorescence imaging of glutathione in living cells

MXene quantum dots have attracted much attention due to their great optical performance and excellent water solubility. Glutathione (GSH) plays a key role in living cells. In this study, a biocompatibility nanoprobe was prepared for detecting intracellular GSH based on MXene N-Ti₃C₂ quantum dots (N-Ti₃C₂ QDs). The N-Ti₃C₂ QDs act as the fluorescence reporters and the ferric iron (Fe³⁺) as the quenchers based on nonradiative electron-hole annihilation. When Fe³⁺ encounters the amino group of N-Ti₃C₂ QDs, the electrons of N-Ti₃C₂ QDs in the excited state will transfer to the half-filled 3d orbitals of Fe³⁺, leading to the fluorescence quenching of N-Ti₃C₂ QDs. When the N-Ti₃C₂ QDs/Fe³⁺ nanoprobe acts on the cancer cell MCF-7, the abundant GSH in the cancer cells can reduce Fe³⁺ to Fe²⁺, which will restore the fluorescence of N-Ti₃C₂ QDs. The N-Ti₃C₂ QDs/Fe³⁺ nanoprobe displays a high sensitivity for GSH with a detection limit of 0.17 μM in range of 0.5-100 μM. It becomes a promising probe for detecting and showing cellular imaging of GSH in MCF-7 cells. The N-Ti₃C₂ QDs/Fe³⁺ nanoprobe might provide a new way for imaging-guided precision cancer diagnosis.

Recent advances in the construction and analytical applications of carbon dots-based optical nanoassembly

Recently, more and more attention has been focused on the construction and analytical applications of optical nanoassembly through combining carbon dots (CDs) with various other functional nanomaterials. The rational design and manufacture of CDs-based optical nanoassembly will be critical to meeting the needs of analytical science. The last decade has witnessed the immense potential of CDs-based optical nanoassembly in multiple sensing applications owing to their controlled optical properties, adjustable surface chemistry and microscopic morphology. This feature article collects the recent advances in the research and development of CDs-based optical nanoassembly and their applications in analytical sensors, aiming to provide vital insights and suggestions to inspire their broad sensing applications.

Sensitive Fluorescence Assay for the Detection of Alkaline Phosphatase Based on a Cu2+-Thiamine System

The authors describe a novel, facile, and sensitive fluorometric strategy based on a Cu²⁺-thiamine (Cu²⁺-TH) system for the detection of alkaline phosphatase (ALP) activity and inhibition. The principle of the method is as follows. Under a basic conditions, TH, which does not exhibit a fluorescence signal, is oxidized into fluorescent thiochrome (TC) by Cu²⁺. Ascorbic acid 2-phosphate (AAP), which is the enzyme substrate, is hydrolyzed to produce ascorbic acid (AA) by ALP. The newly formed AA then reduces Cu²⁺ to Cu⁺, which prevents the oxidation of TH by Cu²⁺; as a result, the fluorescent signal becomes weaker. On the contrary, in the absence of ALP, AAP cannot reduce Cu²⁺; additions of Cu²⁺ and TH result in a dramatic increase of the fluorescent signal. The sensing strategy displays brilliant sensitivity with a detection limit of 0.08 U/L, and the detection is linear in the concentration range of 0.1 to 100 U/L. This approach was successfully applied to ALP activity in human serum samples, indicating that it is reliable and may be applied to the clinical diagnosis of ALP-related diseases.

A dual-mode strategy for sensing and bio-imaging of endogenous alkaline phosphatase based on the combination of photoinduced electron transfer and hyperchromic effect

Benefit from the additional correction of the output signal in dual-mode detection, traditional dual signal readout strategies are performed by constructing the ratiometric fluorescent probe through excitation energy transfer (EET) or fluorescence resonance energy transfer (FRET). To avoid the complicated modification process and obtain the results rapidly, a simple dual-mode sensing strategy based on the electronic effects of p-nitrophenol (PNP) is described to monitor the activities of alkaline phosphatase (ALP). In the sensing platform, p-nitrophenylphosphate was used as a substrate to produce the PNP using ALP as the catalyst. Due to the PNP possesses negative effect of induction and conjugation, photoinduced electron transfer and hyperchromic effect have been achieved between PNP and polyethyleneimine-protected copper nanoclusters (PEI-Cu NCs), which caused the changes of the fluorescence intensity and UV-visible absorption. The dual-mode signal sensing system showed the satisfactory linear results of ALP from 1 to 100 U/L for fluorescent sensing strategy and 1-70 U/L for the absorption method with a competitive LOD of 0.27 and 0.87 U/L (signal-to-noise ratio of 3). This strategy detected biological ALP in human serum and bio-imaging of endogenous ALP in A549 cells successfully, which verifies a certain potential of the strategy for the practical applications.

Applications of fluorescent materials in the detection of alkaline phosphatase activity

Alkaline phosphatase (ALP) is important in the diagnosis of many diseases. Because ALP is used to detect biomarkers for many diseases, many researchers conduct investigations to develop ALP detection strategies. The use of fluorescent material has attracted attention because of the technique's high sensitivity and the low sample volume required. Herein, we review and discuss the working mechanisms and advantages of four main categories:DNA fluorescent probes, molecular fluorescent probes, chemical coordination-based probes, and nanoparticle probes. Development prospects and trends are also discussed.

Visual bio-detection and versatile bio-imaging of zinc-ion-coordinated black phosphorus quantum dots with improved stability and bright fluorescence

Zero-dimensional black phosphorus quantum dots (BPQDs) have unique structural characteristics and excellent properties for promising applications over other BP structures. With the decrease of BP stacked layers, BP becomes much unstable and is easy to be oxidized and degraded in air and water. To prevent BP oxidation and degradation is crucial during the preparation process of highly stable BPQDs with strong fluorescence (FL). Herein, we explored a zinc-ion-coordinated strategy to achieve the emerging Zn@BPQDs with improved colloidal and FL stabilities. Zn ions can be adsorbed on BP surface via cation-π interactions, which passivates lone pair electrons of phosphorus and makes BP much stable upon exposure to air and water. Zn@BPQDs were prepared through sonication-assisted liquid-phase exfoliation of bulk BP crystals in the presence of Zn ions and solvothermal reaction of exfoliated few-layer Zn@BP nanosheets. Experimental results confirm the preparation of Zn@BPQDs with improved stability and high FL. Zn@BPQDs were used for both FL spectral detection and naked-eye visual FL detection of glutathione in practical samples. As emerging FL reagents, biocompatible Zn@BPQDs were further used for efficient in-vitro cell imaging and in-vivo imaging in natural plants and living aquatic animals.

Detection of B-type natriuretic peptide by establishing a low-cost and replicable fluorescence resonance energy transfer platform

Aiming at the establishment of a sensitive and specific diagnostic method for early heart failure (HF), we developed a cost-effective fluorescence resonance energy transfer (FRET) platform for the detection of B-type natriuretic peptide (BNP), a characteristic biomarker of HF. Graphene oxide (GO) was selected as the FRET receptor in view of its advantages including commercial availability, low-cost and chemical stability, and dye-modified aptamer was used as the energy donor of FRET as well as in charge of the specific recognition of BNP. Based on the ON (strong emission) and OFF (quenching) states of FRET in the presence and absence of BNP, respectively, specific detection of BNP was achieved in the range 0.074-0.56 pg/mL with a limit of detection as low as 45 fg/mL (3σ). This FRET platform was applied to detect BNP in 45 blood samples to demonstrate its practicability in clinical diagnosis. Compared to the commonly used Siemens method (chemiluminescence immunoassay, CLIA) in hospital, our approach is more accurate and specific for HF diagnosis with areas under the receiver operating characteristic curves of 0.869 (95% CI 0.733-1.00, P < 0.05) vs 0.850 (95% CI 0.703-0.997, P < 0.05) and specificity of 68.8% vs 65.6%. This platform is promising in early diagnosis of HF through ultrasensitive and specific detection of BNP. Graphical abstract To solve the clinical diagnostic problem for early heart failure (HF) which lacks sensitivity and specificity, we established a cost-effective and rapid fluorescence analysis method based on fluorescence resonance energy transfer (FRET) platform for the detection of B-type natriuretic peptide (BNP), a characteristic biomarker of HF.

Biomedical application of manganese dioxide nanomaterials

Manganese dioxide nanomaterial is a new type of inorganic nanomaterial offering numerous advantages: simple preparation, low cost, and environmental friendliness. This review summarizes the traditional and novel synthetic methods for manganese dioxide nanomaterials and mainly discusses their potential in biomedical applications. Manganese dioxide nanomaterials are mainly used as drug carriers in tumor therapy. In recent years, the construction of multifunctional nano-platforms using manganese dioxide has gradually improved. The main mechanism is that manganese dioxide nanomaterials can combine with reactive oxygen species in the tumor microenvironment to alleviate tumor hypoxia. Manganese dioxide has also been used to quench fluorescent carbon dots in fluorescent probes. Based on the oxidation ability and catalytic activity of MnO₂, MnO₂ nanosheets are widely used to build biosensors. New research shows that manganese dioxide nanomaterials also have great potential in gene therapy and nuclear magnetic imaging.

Synthesis of water soluble CuGaS2/ZnS quantum dots for ultrasensitive fluorescent detection of alkaline phosphatase based on inner filter effect

Developing monitoring technique for alkaline phosphatase (ALP) is crucial due to the important role it plays in living cells. Here, a kind of biocompatible glutathione-modified CuGaS₂/ZnS quantum dots (GSH-CGS/ZnS QDs) was used as a fluorescent substance and then fabricated "turn-off" fluorescent biosensor for detection of ALP by help of inner filter effect (IFE). Firstly, we prepared CuGaS₂/ZnS (CGS/ZnS) QDs using solvothermal method and explored the efficient ligand (GSH) exchanges strategy for transferring oil-soluble CGS/ZnS QDs to aqueous phase. More importantly, we also explored the potential biological applications of the nanohybrid QDs. The obtained GSH-CGS/ZnS QDs emitted strong yellow fluorescence with the maximum excitation (400 nm) and emission (601 nm). Then, GSH-CGS/ZnS QDs were mixed with p-nitrophenylphosphate (PNPP) and ALP. PNPP could be hydrolyzed to p-nitrophenol (PNP) by help of catalysis of ALP, and the excitation spectrum of the GSH-CGS/ZnS QDs overlapped well with the absorption spectrum of PNP, so the fluorescence of GSH-CGS/ZnS QDs was initially quenched via the so-called "IFE". Finally, a novel "turn-off" biosensor for sensitive detection of ALP in the range of 0.05-10 U L ^-1(R² = 0.98) with a detection limit of 0.01 U L^-1 was successfully obtained. Results indicated that I-III-VI₂ nanocrystals have great potential for their promising biomedical application.

A novel ratiometric fluorescent sensing method based on MnO2 nanosheet for sensitive detection of alkaline phosphatase in serum

Alkaline phosphatase (ALP) is an important biomarker for clinical diagnosis. Abnormal levels of ALP are closely related to many diseases. In this contribution, a ratiometric fluorescent sensing method based on the competition between two oxidation-reduction reactions related to MnO₂ nanosheets was developed for ALP detection. Moreover, an advanced model was derived for the quantitative analysis of the fluorescence measurements obtained by the proposed ratiometric fluorescent sensing method. With the aid of the advanced model, the proposed method achieved satisfactory quantitative results for ALP in real-world serum samples, with accuracy comparable to the corresponding results obtained by an automatic biochemical analyzer. Its recovery rates for the spiked serum samples were in the range of 98.4-115.0%, which were quite satisfactory considering the complexity of the matrices of the samples. The limit of detection and limit of quantification were estimated to be 0.09 and 0.30 U L^-1, respectively. Therefore, it is reasonable to expect that the proposed ratiometric fluorescent sensing method can be further developed to be a competitive alternative for ALP detection.

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.

A nopinone based multi-functional probe for colorimetric detection of Cu2+ and ratiometric detection of Ag. Photochem Photobiol Sci 2020;19:49-55. [PMID: 31793618 DOI: 10.1039/c9pp00297a]

A dual-signal probe PPN based on the natural β-pinene derivative nopinone was synthesized for the colorimetric detection of Cu2+ and ratiometric detection of Ag+. Upon the addition of Ag+, a significant fluorescence change from blue to green was observed with a low detection limit (0.86 μM). However, upon the addition of Cu2+, a significant color change from colorless to yellow was observed with a low detection limit (0.56 μM). The novel probe PPN was applied as a probe for the colorimetric detection of Cu2+ and ratiometric detection of Ag+ with a high selectivity, good sensitivity and fast response time. The detection mechanisms of probe PPN for Cu2+/Ag+ were confirmed by 1H NMR and HRMS-ESI. Besides, probe PPN could sense Cu2+/Ag+ on test strips. Additionally, probe PPN could be applied to quantitatively detect the concentration of Ag+ in water samples and image Ag+ in living cells.