Aggregation-induced phosphorescence enhancement of Mn-doped ZnS quantum dots: the role of dot-to-dot distance

Portable and Rapid Fluorescence Turn-On Detection of Total Pepsin in Saliva Based on Strong Electrostatic Interactions

Salivary pepsin has been proposed as a promising diagnostic marker for gastroesophageal reflux disease (GERD). However, the activity of human pepsin is strongly influenced by pH, and the acidic condition (pH ∼ 2) is optimal, which is a contradiction for the pepsin detection kit based on its catalytic activity. Thus, its accurate quantification in saliva (neutral pH) by readily rapid tools with simplicity and low cost is still challenging. Herein, a convenient fluorescence assay has been developed for the rapid detection of pepsin at neutral pH based on its electrostatic interaction with SYBR Green (SG) rather than the bioactivity. At neutral pH, the positively charged SG fluorophore can be effectively adsorbed by the negatively charged pepsin due to the low isoelectric point (pI) and large molecular size of pepsin. Thus, the molecular rotation of SG is limited, and its fluorescence intensity is significantly increased. The strategy was further confirmed to have the same fluorescence response as that of normally active and inactivated pepsin. Due to the unique pI of pepsin, the fluorescence strategy is highly selective for pepsin compared to other proteins. On the basis of this strategy, a smartphone-based fluorescence capture device integrated with a programmed Python program was fabricated for both color recognition and the accurate detection of pepsin within 3 min. Under the optimal conditions, this turn-on sensor allowed for the on-site analysis of pepsin with a detection limit of 0.2 μg/mL. Moreover, this strategy was successfully used to assess salivary pepsin to aid in the noninvasive diagnosis of GERD.

Recent Progresses in Development of Biosensors for Thrombin Detection

Thrombin is a serine protease with an essential role in homeostasis and blood coagulation. During vascular injuries, thrombin is generated from prothrombin, a plasma protein, to polymerize fibrinogen molecules into fibrin filaments. Moreover, thrombin is a potent stimulant for platelet activation, which causes blood clots to prevent bleeding. The rapid and sensitive detection of thrombin is important in biological analysis and clinical diagnosis. Hence, various biosensors for thrombin measurement have been developed. Biosensors are devices that produce a quantifiable signal from biological interactions in proportion to the concentration of a target analyte. An aptasensor is a biosensor in which a DNA or RNA aptamer has been used as a biological recognition element and can identify target molecules with a high degree of sensitivity and affinity. Designed biosensors could provide effective methods for the highly selective and specific detection of thrombin. This review has attempted to provide an update of the various biosensors proposed in the literature, which have been designed for thrombin detection. According to their various transducers, the constructions and compositions, the performance, benefits, and restrictions of each are summarized and compared.

Highly sensitive detection for alkaline phosphatase using doped ZnS quantum dots with room temperature phosphorescence and its logic gate function

This paper presents a highly sensitive sensing system for alkaline phosphatase by room temperature phosphorescence of Mn doped ZnS quantum dots and pyrophosphate. The sensing system has intense room temperature phosphorescence emission in the absence of alkaline phosphatase. The phosphorescence is quenched gradually with the addition of alkaline phosphatase. The emission "on" without alkaline phosphatase may be attributed to the increased probability of charge transfer from one of surface traps to the dopant bands of another resulted from the shortened dot-to-dot distance by the strong chelation of pyrophosphate and Zn²⁺ ion and the hydrogen bonding between pyrophosphate and β-cyclodextrin. The addition of alkaline phosphatase causes pyrophosphate hydrolyzed to orthophosphate and the dot-to-dot distance of quantum dots back to the normal, and then the phosphorescence "off". The factors affecting the sensing system performance were also optimized. Under the optimal experimental conditions, the linear range for alkaline phosphatase is determined as 0.2-10 U/L with a LOD at 0.045 U/L. The recovery of human serum was determined from 93.75%-103.03%, indicating a potential application in biomedical diagnosis. Furthermore, an RTP-based "INHIBIT" logic gate using the doped ZnS quantum dots was also presented.

Aggregation-Induced Room-Temperature Phosphorescence Obtained from Water-Dispersible Carbon Dot-Based Composite Materials

Room-temperature phosphorescence (RTP) materials are desirable in chemical sensing because of their long emission lifetime and they are free from background autofluorescence. Nevertheless, the achievement of RTP in aqueous solution is still a highly challenging task. Herein, a molten salt method to prepare carbon dot (CD)-based RTP materials is presented by direct calcination of carbon sources in the presence of inorganic salts. The resultant CD composites (CDs@MP) exhibit bright RTP with a quantum yield of 26.4% and a lifetime of 1.28 s, which lasts for about 6 s to the naked eye. Importantly, their aqueous dispersion also has good RTP characteristics. This is the first time that the long-lived CDs@MP with RTP are achieved in aqueous solution owing to the synergistic effect of crystalline confinement and aggregation-induced phosphorescence. Further investigations reveal that three key processes may be responsible for the observed RTP of the composite materials: (1) The rigid crystalline salt shell can preserve the triplet states of CDs@MP in water and suppress the nonradiative deactivation; (2) The addition of high-charge-density metal ions Mg(II) and phosphorus element in the composite facilitates the singlet-to-triplet intersystem crossing process and enhances the RTP emission; (3) The aggregation of CDs@MP nanocomposites enables the matrix shell to self-assemble into a network, which further improves the rigidity of the shell and prevents the intermolecular motions, hence prolonging the RTP lifetime. The unique RTP feature and good water dispersibility allow the CD-based composite materials to be applicable in detection of temperature and pH in the aqueous phase. Our approach for producing long-lived RTP CDs@MP is effective, simple, and low-cost, which opens a new route to develop RTP materials that are applicable in aqueous solution.

Fluorometric sensing of oxygen using manganese(II)-doped zinc sulfide nanocrystals

Manganese(II)-doped zinc sulfide nanocrystals (Mn:ZnS NCs) with dual-emission fluorescence (peaks at 445 nm and 590 nm under 330 nm excitation), good water stability and low toxicity were synthesized by hot injection. The fluorescence intensity of both emission bands of the nanocrystals can change rapidly by the content of gaseous and dissolved oxygen. The process is fully reversible. Compared with the maximum intensity of Mn:ZnS sensing film in 100% nitrogen, the emission of the blue emission decreases by 72% in the presence of 100% oxygen, and the yellow emission by 32%. Response is linear in the presence of 3% to 12% of oxygen percentage in gas. For water-dissolved oxygen, the linear response occurs between 0.54 and 11.4 mg·L^-1. Graphical abstractMn-doped ZnS NCs with dual-emission fluorescence were synthesized by hot-injection method. The reversible and rapid sensing characteristics of Mn-doped ZnS NCs to oxygen were studied, and the possible sensing mechanism was investigated.

An "off-on" phosphorescent aptasensor for the detection of thrombin based on PRET

Thrombin plays an important role in the blood coagulation cascade and it stimulates the process of platelet aggregation. Herein, we developed a highly efficient and sensitive phosphorescent aptasensor system for the quantitative analysis of thrombin. The phosphorescence of 3-mercaptopropionic acid capped Mn-doped ZnS quantum dots (MPA-Mn:ZnS QDs) was gradually quenched with the addition of thrombin binding aptamers-BHQ2 (TBA-BHQ2) based on phosphorescence resonance energy transfer (PRET). With the addition of the target analyte thrombin into the system, TBA-BHQ2 could change its spatial structure from a random coil to an antiparallel G-quadruplex which resulted from the combination of thrombin and TBA-BHQ2, leading to the phosphorescence recovery. Finally, the concentration of thrombin could be accurately determined by means of measuring the phosphorescence intensity change value (ΔP). The limit of detection (LOD) was obtained as low as 15.26 pM with wide linear ranges both from 60 to 2000 pM and from 2 to 900 nM. The proposed strategy was also successfully applied for thrombin detection in human serum samples and plasma samples with satisfactory recoveries from 96% to 99% and 95% to 104%, respectively. The long lifetime of phosphorescent QDs possessed a suitable time delay to eliminate autofluorescence and scattered light interference from biological matrices effectively. Thus, the signal to noise ratio of the phosphorescent aptasensor was improved visibly for the analysis of target analytes.