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

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.

Rapid detection of Salmonella in food matrices by photonic PCR based on the photothermal effect of Fe3O4

Salmonella causes most deaths from diarrheal disease worldwide. Therefore, Salmonella must be accurately and quickly detected, even in complex food matrices, which is difficult to achieve using conventional culture methods. Here we propose a novel photonic polymerase chain reaction (PCR) method based on ferroferric oxide (Fe3O4) for the detection of Salmonella typhimurium in complex samples. Owing to the great photothermal conversion performance of Fe3O4, rapid thermal cycling could be accomplished. Our optimized photonic PCR system specifically detected Salmonella typhimurium in complex food matrices within 50 min. Quantitative data showed a limit of detection up to 102 CFU/mL in food samples. This method is suitable for the detection of all pathogenic microorganisms and is universal.

A Cascade Signal Amplification Strategy for the Ultrasensitive Fluorescence Detection of Cu2+ via λ-Exonuclease-Assisted Target Recycling with Mismatched Catalytic Hairpin Assembly

Herein, an ultrasensitive DNAzyme-based fluorescence biosensor for detecting Cu2+ was designed using the cascade signal amplification strategy, coupling λ-exonuclease-assisted target recycling and mismatched catalytic hairpin assembly (MCHA). In the designed detection system, the target, Cu2+, can activate the Cu2+-dependent DNAzyme to cause a cleavage reaction, releasing ssDNA (tDNA). Then, tDNA binds to hairpin DNA (H0) with an overhanging 5'-phosphorylated terminus to form dsDNA with a blunt 5'-phosphorylated terminus, which activates the dsDNA to be digested by λ-Exo and releases tDNA along with another ssDNA (iDNA). Subsequently, the iDNA initiates MCHA, which can restore the fluorescence of carboxyfluorescein (FAM) previously quenched by tetramethylrhodamine (TAMRA), resulting in a strong fluorescent signal. Furthermore, MCHA efficiently improves the signal-to-noise ratio of the detection system. More importantly, tDNA recycling can be achieved with the λ-Exo digestion reaction to release more iDNA, efficiently amplifying the fluorescent signal and further improving the sensitivity to Cu2+ with a detection limit of 60 fM. The practical application of the developed biosensor was also demonstrated by detecting Cu2+ in real samples, proving it to be an excellent analytical strategy for the ultrasensitive quantification of heavy metal ions in environmental water sources.

Detection of Streptavidin Based on Terminal Protection and Cationic Conjugated Polymer-Mediated Fluorescence Resonance Energy Transfer

In this paper, a fast and simple strategy for sensitive detection of streptavidin (SA) was proposed based on terminal protection of small molecule-linked DNA and cationic conjugated polymer-mediated fluorescence resonance energy transfer (FRET). In principle, we designed a biotin-labelled DNA probe (P1) as the recognitive probe of SA, along with a complementary DNA probe (P2) to form double-stranded DNA (dsDNA) with P1. SYBR Green I (SG I) as a fluorescent dye was further used to specifically bind to dsDNA to emit stronger fluorescence. The cationic poly[(9,9-bis(6′-N,N,N-triethy-lammonium)hexyl) fluorenylene phenylene dibromide] (PFP) acted as the donor to participate in the FRET and transfer energy to the recipient SG I. In the absence of SA, P1 could not hybridize with P2 to form dsDNA and was digested by exonuclease I (Exo I); thus, only a weak FRET signal would be observed. In the presence of SA, biotin could specifically bind to SA, which protected P1 from Exo I cleavage. Then, P1 and P2 were hybridized into dsDNA. Therefore, the addition of SG I and PFP led to obvious FRET signal due to strong electrostatic interactions. Then, SA can be quantitatively detected by monitoring FRET changes. As the whole reagent reaction was carried out in 1.5 mL EP and detected in the colorimetric dish, the operation process of the detection system was relatively simple. The response time for each step was also relatively short. In this detection system, the linear equation was obtained for SA from 0.1 to 20 nM with a low detection limit of 0.068 nM (S/N = 3). In addition, this strategy has also achieved satisfactory results in the application of biological samples, which reveals the application prospect of this method in the future.

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.

Detection of coralyne and heparin by polymerase extension reaction using SYBR Green I

Polydeoxyadenosine (poly (dA)) has been extensively applied for detecting many drug molecules. Herein, we developed a sensitive method for detecting coralyne and heparin using a modified DNA probe with poly (dA) at one end. In the absence of coralyne, the DNA probe was digested by the Exonuclease I (Exo I), and therefore the SYBR Green I (SG I) emitted an extremely low fluorescent signal. While coralyne specifically binding to poly (dA) with strong propensity could remarkably restrain the disintegration of the DNA probe, through which as a template the second strand of DNA sequence was formed with the introduction of DNA polymerase. Therefore, the fluorescent signal of SG I was intensified to quantify coralyne. Based on this method, heparin can be determined due to its strong affinity towards coralyne. This method showed a linear range from 2 to 500 nM for coralyne with a low detection limit of 0.98 nM, and the linear range of heparin was from 1 to 100 nM when 1.25 nm was the detection limit. The proposed method was also implemented successfully in biological samples and showed a potential application for screening potential therapeutic molecules.