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

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

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.

Recent Progress in Single-Nucleotide Polymorphism Biosensors

Single-nucleotide polymorphisms (SNPs), the most common form of genetic variation in the human genome, are the main cause of individual differences. Furthermore, such attractive genetic markers are emerging as important hallmarks in clinical diagnosis and treatment. A variety of destructive abnormalities, such as malignancy, cardiovascular disease, inherited metabolic disease, and autoimmune disease, are associated with single-nucleotide variants. Therefore, identification of SNPs is necessary for better understanding of the gene function and health of an individual. SNP detection with simple preparation and operational procedures, high affinity and specificity, and cost-effectiveness have been the key challenge for years. Although biosensing methods offer high specificity and sensitivity, as well, they suffer drawbacks, such as complicated designs, complicated optimization procedures, and the use of complicated chemistry designs and expensive reagents, as well as toxic chemical compounds, for signal detection and amplifications. This review aims to provide an overview on improvements for SNP biosensing based on fluorescent and electrochemical methods. Very recently, novel designs in each category have been presented in detail. Furthermore, detection limitations, advantages and disadvantages, and challenges have also been presented for each type.

Deep eutectic solvent-based manganese dioxide nanosheets composites for determination of DNA by a colorimetric method

BACKGROUND

Nucleic acid is the carrier of genetic information and the keymolecule in life science. It is important to establish a simple and feasible method for nucleic acid quantification in complex biological samples.

METHODS

Four kinds of hydrogen bond acceptors (choline chloride (ChCl), L-carnitine, tetrabutylammonium chloride (TBAC) and cetyltrimethylammonium bromide (CTAB)) were used to synthesize deep eutectic solvents (DESs) with hexafluoroisopropanol (HFIP). DESs based manganese dioxide (MnO2) nanosheets composites was synthesized and characterized. DNA concentration was determined by a UVVis spectrometer. The mechanism of DNA-DES/MnO2 colorimetric system was further discussed.

RESULTS

The composite composed of DES/MnO2 exhibited excellent oxidase-like activity and could oxidize 3,3',5,5' -tetramethylbenzidine (TMB) to produce a clear blue change with an absorbance maximum at 652 nm. When DNA is introduced, the DNA can interact with the DES by hydrogen bonding and electrostatic interactions, thereby inhibiting the color reaction of DES/MnO2 with TMB. After condition optimization, ChCl/HFIP DES in 1:3 molar ratio was used for the colorimetric method of DNA determination. The linear range of DNA was 10-130 µg/mL and exhibited good selectivity.

CONCLUSION

A colorimetric method based on DES/MnO2 was developed to quantify the DNA concentration. The proposed method can be successfully used to quantify DNA in bovine serum samples.

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.

Recent progress of microfluidic chips in immunoassay

Microfluidic chip technology is a technology platform that integrates basic operation units such as processing, separation, reaction and detection into microchannel chip to realize low consumption, fast and efficient analysis of samples. It has the characteristics of small volume need of samples and reagents, fast analysis, low cost, automation, portability, high throughout, and good compatibility with other techniques. In this review, the concept, preparation materials and fabrication technology of microfluidic chip are described. The applications of microfluidic chip in immunoassay, including fluorescent, chemiluminescent, surface-enhanced Raman spectroscopy (SERS), and electrochemical immunoassay are reviewed. Look into the future, the development of microfluidic chips lies in point-of-care testing and high throughput equipment, and there are still some challenges in the design and the integration of microfluidic chips, as well as the analysis of actual sample by microfluidic chips.

CDs-MnO2-TPPS Ternary System for Ratiometric Fluorescence Detection of Ascorbic Acid and Alkaline Phosphatase

Manganese dioxide (MnO₂) nanosheet-based fluorescence sensors often use oxidase-like activity or wide absorption spectrum for detection of antioxidants. In those strategies, MnO₂ nanosheets were reduced to Mn²⁺ by antioxidants. However, few strategies emphasize the role of Mn²⁺ obtained from MnO₂ reduction in the design of the fluorescence sensor. Herein, we expanded the application of a MnO₂ nanosheet-based fluorescence sensor by involving Mn²⁺ in the detection process using ascorbic acid (AA) as a model target. In this strategy, carbon dots (CDs), MnO₂ nanosheets, and tetraphenylporphyrin tetrasulfonic acid (TPPS) comprise a ternary system for ratiometric fluorescence detection of AA. Initially, CDs were quenched by MnO₂ nanosheets based on the inner filter effect, while TPPS maintained its fluorescence intensity. After the addition of AA, MnO₂ nanosheets were reduced to Mn²⁺ so that the fluorescence intensity of CDs was recovered and TTPS was quenched by coordination with Mn²⁺. Overall, AA triggered an emission intensity increase at 440 nm for CDs and a decrease at 640 nm for TPPS. The ratio intensity of CDs to TPPS (F ₄₄₀/F ₆₄₀) showed a good linear relationship from 0.5 to 40 μM, with a low detection limit of 0.13 μM for AA detection. By means of the alkaline phosphatase (ALP)-triggered generation of AA, this strategy can be applied for the detection of ALP in the range of 0.1-100 mU/mL, with a detection limit of 0.04 mU/mL. Furthermore, this sensor was applied to detect AA and ALP in real, complex samples with ideal recovery. This novel platform extended the application of MnO₂ nanosheet-based fluorescence sensors.

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Various nanoparticles have been developed for bio-applications. However, nanocomposites particles, (NCPs) with effective and ability to sensing cholesterol, are still needed. Herein, we present new cholesterol and pH-responsive NCPs as sensor particles to promote sensitivity. The traditional method of mixing and grinding was used to fabricate the oxides of magnesium and manganese MgO-MnO2 NCPs with different mixing ratios. Structural properties, detection of cholesterol, and pH sensitivity were examined. Results propped the high efficiency of MgO-MnO2 NCPs compared with individual oxides (MgO and MnO2), low response time, while the analytical results confirmed the homogeneous structure of MgO-MnO2 NCPs. Particle size distribution results for NCPs were within 16.4 to 100 nm, which makes it promising in medical and bio-applications.

A ratiometric fluorescence sensor based on enzymatically activatable micellization of TPE derivatives for quantitative detection of alkaline phosphatase activity in serum

A novel ratiometric fluorescence assay via enzymatically activatable micellization in aqueous solution was devised for quantitative detection of alkaline phosphatase (ALP) activity. We demonstrated that the dephosphorylation of the water-soluble, phosphate-functionalized, fluorophore monomer P-TPE-TG, induced by an enzymatic reaction of ALP, leads to micelle formation in aqueous solution because its water-soluble functionality is reduced. The dephosphorylation-induced micellization of P-TPE-TG exhibited a ratiometric sensing response for various ALP concentrations (10–200 mU mL⁻¹) and provided a suitable sensing platform for naked eye detection with increased fluorescence quantum yield (Φ = 3.2%), even compared to a typical TPE-based sensor (Φ = 1.0%), where ALP can be sensed with a detection limit of 0.034 mU mL⁻¹. In addition, P-TPE-TG displayed excellent sensing performance at concentrations from 0 to 50 mU mL⁻¹ in diluted human serum (10%), which offers the capability to exploit ratiometric responses for bioactive substances under practical conditions.

A novel ratiometric fluorescence assay via enzymatically activatable micellization in aqueous solution was devised for quantitative detection of alkaline phosphatase (ALP) activity.