A label-free G-quadruplex-based fluorescence assay for sensitive detection of alkaline phosphatase with the assistance of Cu2

Stimuli-responsive aggregation-induced emission of molecular probes by electrostatic and hydrophobic interactions: Effect of organic solvent content and application for probing of alkaline phosphatase activity

We suggest that aggregation-induced emission (AIE) molecular probes with single charged/reactive group can exist in the formation of nanostructures but not monomers at extremely low organic solvent content. The nanoaggregates show good dispersivity and emit week emission. Stimuli-responsive assembly of nanoaggregates by electrostatic interactions can turn on the fluorescence, facilitating the design of biosensors with single-charged molecular probes as the AIE fluorogens. To prove the concept, tetraphenylethene-substituted pyridinium salt (TPE-Py) was used as the AIE fluorogen for probing of alkaline phosphatase (ALP) activity with pyrophosphate ion (PPi) as the enzyme substrate. The dynamic light scattering and transmission electron microscope experiments demonstrated that TPE-Py probes existed in aqueous solution at nanometer size and morphology. Stimuli such as the negatively charged PPi, citrate, ATP, ADP, NADP and DNA could trigger the aggregation of the positively charged TPE-Py nanoparticles, thus enhancing the fluorescence via AIE effect. ALP-enzymatic hydrolysis of PPi into two phosphate ions (Pi) limited the aggregation of TPE-Py nanoparticles. The strategy was used for the assay of ALP with a low detection limit (1 U/L) and wide linear range (1-200 U/L). We also investigated the effect of organic solvent content on the AIE process and found that high concentration of organic solvent can prevent the hydrophobic interaction between AIE molecules but show no essential influence on the electrostatic interaction-mediated assembly. The work should be evaluable for understanding AIE phenomenon and developing novel, simple and sensitive biosensors using a molecular probe with single charged/reactive group as the signal reporter.

A colorimetric and ratiometric photometric sequential assay for ascorbic acid and alkaline phosphatase in serum based on valence states modulation

The photometric method is widely used in real clinical tests due to its simple operation, low cost and convenient. Many of the reported colorimetric ALP assays so far are non- ratiometric because the detection was based on changes in absorbance at a single wavelength. The development of novel colorimetric and ratiometric assay is of importance for quantitatively measuring target with high accuracy. The challenge in the design of ratiometric photometric assay is that the chromophore must have a significant spectral shift before and after binding to the target. Here, we report a colorimetric and ratiometric photometric sequential assay of AA and ALP based on the complexation between ARS and Cu²⁺ and redox reaction between AA and Cu²⁺. The absorption band of ARS centered at 425 nm (yellow color), which could be shifted to 510 nm (red color) upon Cu²⁺ binding. However, as far as we know, this classic color reaction has not been used to develop a ratiometric photometric method to sequentially detect AA and ALP, although photometric methods based on the regulation of other color reagents with oxidizing metal ions have been reported. The proposed sensing system shows a limit of detection for ALP at 0.24 U L^-1 and could be applied for detecting ALP in newborn calf serum. The established sensing system makes a useful contribution to the detection of ALP in complex clinical samples.

Highly Sensitive Alkaline Phosphatase Biosensor Based on Internal Filtration Effect between G-Quadruplex/N-methylmesoporphyrin IX and p-Nitrophenol

In this work, an alkaline phosphatase (ALP) biosensor was established based on G-quadruplex/N-methylmesoporphyrin IX (G4/NMM) and p-nitrophenol (PNP). Because the absorption of PNP was close to the excitation wavelength of G4/NMM, PNP could reduce the fluorescence of G4/NMM. Meanwhile, PNP was the hydrolysis product of p-nitrophenylphosphate (PNPP) by ALP. Therefore, ALP could be detected. This ALP biosensor had a linear analytical range from 2.5 to 25 U/L a the detection limit of 0.81 U/L. Moreover, it showed a satisfactory selectivity and recovery rates.

N-alkylation briefly constructs tunable multifunctional sensor materials: Multianalyte detection and reversible adsorption

A series of N-alkyl-substituted polybenzimidazoles (SPBIs), synthesized by simple condensation and N-alkylation, act as functional materials with tunable microstructures and sensing performance. For their controllable morphologies, the formation of nano-/microspheres is observed at the n(RBr)/n(PBI) feed ratio of 5:1. Products with different degrees of alkylation can recognize metal ions and nitroaromatic compounds (NACs). For example, SPBI-c, obtained at the feed ratio of 1:1, can selectively detect Cu2+, Fe3+, and NACs. By contrast, SPBI-a, obtained at the feed ratio of 0.1:1, can exclusively detect Cu2+ with high sensitivity. Their sensing mechanisms have been studied by FT-IR spectroscopy, SEM, XPS, and DFT calculations. Interestingly, the SPBIs can adsorb Cu2+ in solution and show good recyclability. These results demonstrate that polymeric materials with both sensing and adsorption applications can be realized by regulating the alkylation extent of the main chain, thus providing a new approach for the facile synthesis of multifunctional materials.

Band-pass filter-assisted ratiometric fluorescent nanoprobe composed of N-(2-aminoethyl-1,8-naphthalimide)-functionalized gold nanoclusters for the determination of alkaline phosphatase using digital image analysis

A smartphone-based dual-wavelength digital imaging platform containing red (539-695 nm) and blue (389-511 nm) band-pass filters was developed for point-of-care (POC) testing of alkaline phosphatase (ALP) activity. The platform was based on dual-emitting fluorescent nanohybrids (AuNC@NAN), the ratiometric probe, which had a fluorescence "on-off-on-off" response. The probe comprised red-emitting gold nanoclusters (AuNCs) acting as the signal report units and blue-emitting N-(2-aminoethyl-1,8-naphthalimide) (NAN) acting as an internal reference. The different responses of the ratiometric probes resulted in a continuous color-multiplexing change from pink-red to dark-purple upon exposure to ALP. The dual-wavelength digital imaging platform was employed to acquire images of AuNC or NAN fluorescence signals without the influence of background light. Unlike the classical one-time digital imaging mode, the accurate red (R) and blue (B) channel values of the generated images can help to directly judge or eliminate the disturbance from unavoidable interfering factors. The R/B values were successfully employed for determining the ALP activity at a range 2.0 to 35.0 mU·mL^-1 with the detection limit of 1.04 mU·mL^-1. Such sensing imaging platform is also successful in determining ALP activity in human serum with 94.9-105% recoveries and relative standard deviation in the range 4.2-5.6%. A novel dual-wavelength smartphone-based digital imaging platform was proposed for simultaneous readout of the reporting and internal reference signals from dual-emitting ratiometric fluorescence probes, which allowed us to the accurate, reliable, and highly sensitive assay of ALP activity in complex samples.