Ultrasensitive detection of trypsin activity and inhibitor screening based on the electron transfer between phosphorescence copper nanocluster and cytochrome c

Dual-mode upconversion sensors for detecting differently charged biotargets based on the oxidase-mimicking activity of Ce4+ and electrostatic control

Heparin is a highly negatively charged sulfated linear polymer glycosaminoglycan that has been widely used as an anticoagulant in medicine. Protamine is a cationic protein rich in arginine that is used to treat the blood-brain barrier during excess heparin surgery. Trypsin is the most important digestive enzyme-encoding generated by the pancreas and can specifically cleave the carboxyl ends of arginine and lysine residues. Heparin, protamine, and trypsin interact and constrain each other, and their fluctuations reflect the body's dysfunction. Therefore, it is necessary to develop a fast, sensitive, and highly selective assay for regularly monitoring the levels of heparin, protamine, and trypsin in serum. Herein, a fluorescent and colorimetric dual-mode upconversion nanoparticle (UCNP) biosensor was used for the determination of heparin, protamine, and trypsin based on the oxidase-mimicking activity of Ce4+ and electrostatic control. The biosensor exhibited sensitive detection of heparin, protamine, and trypsin with low limits of detection (LODs) of 16 ng/mL, 87 ng/mL and 31 ng/mL, respectively. Furthermore, the designed biosensor could eliminate autofluorescence, which not only effectively increased the accuracy of the sensor but also provided a new sensing pathway for the detection of differently charged biotargets.

Fluorescent metal nanoclusters: From luminescence mechanism to applications in enzyme activity assays

Metal nanoclusters (MNCs) have outstanding fluorescence property and biocompatibility, which show widespread applications in biological analysis. Particularly, evaluation of enzyme activity with the fluorescent MNCs has been developed rapidly within the past several years. In this review, we first introduced the fluorescent mechanism of mono- and bi-metallic nanoclusters, respectively, whose interesting luminescence properties are mainly resulted from electron transfer between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Meanwhile, the charge migration within the structure occurs through ligand-metal charge transfer (LMCT) or ligand-metal-metal charge transfer (LMMCT). On such foundation, diverse enzyme activities were rigorously evaluated, including three transferases and nine hydrolases, in turn harvesting rapid research progresses within past 5 years. Finally, we summarized the design strategies for evaluating enzyme activity with the MNCs, presented the major issues and challenges remained in the relevant research, coupled by showing some improvement measures. This review will attract researchers dedicated to the studies of the MNCs and provide some constructive insights for their further applications in enzyme analysis.

Impact of luminescent MoSe2 quantum dots on activity of trypsin under different pH environment

It is vital that a straightforward detection approach for trypsin should be developed as it is important diagnostic tool for a number of diseases. Herein, the impact of luminescent MoSe2 quantum dots on trypsin activity under different pH environment has been studied. Addition of trypsin to MoSe2 quantum dots enhanced the fluorescence of quantum dots whereas quantum dots resulted in quenching of fluorescence of trypsin. The quenching behavior at various pH and temperature was examined and revealed that the MoSe2-trypsin complex stabilized through the electrostatic interactions. The obtained negative values of zeta potential of the complex -0.11 mV, -0.30 mV and -0.59 mV for pH 6.0,7.6 and 9.0 respectively confirmed the stability of the complex. The separation between the donor and acceptor atoms in energy transfer mechanism was found to decrease (1.48 nm to 1.44 nm to 1.30 nm) with increasing value of pH. It was also evident that trypsin retained its enzyme activity in the trypsin-MoSe2 complex and under different pH environment. The Vant Hoff plot from quenching revealed 1 binding site for quantum dots by trypsin for all pH of buffer solution. The complex formation of trypsin-MoSe2 quantum dots was verified for the first time using fluorescence spectroscopy and it revealed that tryspin form complex with MoSe2 quantum dots through electrostatic interactions. Our results revealed that the MoSe2 quantum dots stabilized and sheltered the active sites of trypsin, which was likely the cause of the increased bioavailability of MoSe2 quantum dots in enzymes.

Label-Free, High-Throughput, Sensitive, and Logical Analysis Using Biomimetic Array Based on Stable Luminescent Copper Nanoclusters and Entropy-Driven Nanomachine

The development of an array for high-throughput and logical analysis of biomarkers is significant for disease diagnosis. DNA-templated copper nanoclusters (CuNCs) have a strong potential to serve as a label-free photoluminescence source in array platforms, but their luminescent stability and sensitivity need to be improved. Herein, we report a facile, sensitive, and robust biomimetic array assay by integrating with stable luminescent CuNCs and entropy-driven nanomachine (EDN). In this strategy, the luminescent stability of CuNCs was improved by adding fructose in CuNCs synthesis to offer a reliable label-free signal. Meanwhile, the DNA template for CuNCs synthesis was introduced into EDN with excellent signal amplification ability, in which the reaction triggered by target miRNA would cause the blunt/protruding conformation change of 3'-terminus accompanied by the production or loss of luminescence. In addition, a biomimetic array fabricated by photonic crystals (PCs) physically enhanced the emitted luminescent signal of CuNCs and achieved high-throughput signal readout by a microplate reader. The proposed assay can isothermally detect as low as 4.5 pM of miR-21. Moreover, the logical EDN was constructed to achieve logical analysis of multiple miRNAs by "AND" or "OR" logic gate operation. Therefore, the proposed assay has the advantages of label-free property, high sensitivity, flexible design, and high-throughput analysis, which provides ideas for developing a new generation of facile and smart platforms in the fields of biological analysis and clinical application.

Label-free Fluorescence Turn on Trypsin Assay Based on Gemini Surfactant/heparin/Nile Red Supramolecular Assembly

In this research, we designed a label-free fluorometric turn-on assay for trypsin and inhibitor screening, based on a spherical cationic gemini surfactant ethylene-bis (dodecyl dimethyl ammonium bromide) (EDAB)/heparin/Nile red (NR) supramolecular assembly system. The introduction of gemini surfactant EDAB as template greatly enhanced its salt resistance and resulted in the supramolecular assemblies with diameters ranging from 20 to 100 nm. The fluorometric assay for trypsin was performed by firstly disassembling with protamine (a heparin-binding protein) and then re-assembling through hydrolysis of protamine. The disassembly and reassembly of the system resulted in a turn-off first and then a turn-on behavior of the corresponding fluorescence. The overall processes were characterized by fluorescence spectra, TEM measurements and zeta potential tests. The detection level of this assembly system for trypsin was as low as 4.2 ng mL-1. Also, the EDAB/heparin/NR assembly could be used to screen the trypsin inhibitors. The assembly system was easily-fabricated and cost-effective, but also exhibited good salt tolerance in NaCl solution at the concentration of 0-500 mM. At last, the supramolecular assembly was successfully applied to detect trypsin in human urine, demonstrating its great potential on clinical diagnosis applications.

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.

Fluorometry detection for trypsin via inner filter effect between cytochrome C and in-situ formed fluorescent thiochrome

A fluorometry assay for trypsin sensitive determination has been presented. The fluorescence of the system at 370/445 nm is derived from thiochrome obtained by in-situ oxidation of thiamine. Based on the inner filter effect, cytochrome C (Cyt C) can quench the fluorescence at 445 nm effectively. Cyt C is specifically hydrolyzed by trypsin through an enzymatic reaction, giving rise to the enhancement of the fluorescence intensity. The change value of fluorescence intensity is proportional to trypsin concentration, which is successfully used for trypsin quantitative detection. This method exhibits good repeatability and selectivity with a detection limit of 0.15 μg mL^-1 and a quantification limit of 0.50 μg mL^-1 for trypsin sensing. Moreover, it is applied to detect trypsin in practical serum and urine samples with accurate results. The proposed assay is not only a promising candidate for trypsin determination in practical application but also a potentially valuable tool in urine comprehensive analysis and disease diagnosis.

Copper nanoclusters: designed synthesis, structural diversity, and multiplatform applications

Atomically precise metal nanoclusters (MNCs) have gained tremendous research interest in recent years due to their extraordinary properties. The molecular-like properties that originate from the quantized electronic states provide novel opportunities for the construction of unique nanomaterials possessing rich molecular-like absorption, luminescence, and magnetic properties. The field of monolayer-protected metal nanoclusters, especially copper, with well-defined molecular structures and compositions, is relatively new, about two to three decades old. Nevertheless, the massive progress in the field illustrates the importance of such nanoobjects as promising materials for various applications. In this respect, nanocluster-based catalysts have become very popular, showing high efficiencies and activities for the catalytic conversion of chemical compounds. Biomedical applications of clusters are an active research field aimed at finding better fluorescent contrast agents, therapeutic pharmaceuticals for the treatment and prevention of diseases, the early diagnosis of cancers and other potent diseases, especially at early stages. A huge library of structures and the compositions of copper nanoclusters (CuNCs) with atomic precisions have already been discovered during last few decades; however, there are many concerns to be addressed and questions to be answered. Hopefully, in future, with the combined efforts of material scientists, inorganic chemists, and computational scientists, a thorough understanding of the unique molecular-like properties of metal nanoclusters will be achieved. This, on the other hand, will allow the interdisciplinary researchers to design novel catalysts, biosensors, or therapeutic agents using highly structured, atomically precise, and stable CuNCs. Thus, we hope this review will guide the reader through the field of CuNCs, while discussing the main achievements and improvements, along with challenges and drawbacks that one needs to face and overcome.

A carbon nanoparticle-peptide fluorescent sensor custom-made for simple and sensitive detection of trypsin

Herein, we report a novel sensor to detect trypsin using a purpose-designed fluorescein-labelled peptide with negatively charged carbon nanoparticles (CNPs) modified by acid oxidation. The fluorescence of the fluorescein-labelled peptide was quenched by CNPs. The sensor reacted with trypsin to cleave the peptide, resulting in the release of the dye moiety and a substantial increase in fluorescence intensity, which was dose- and time-dependent, and trypsin could be quantified accordingly. Correspondingly, the biosensor has led to the development of a convenient and efficient fluorescent method to measure trypsin activity, with a detection limit of 0.7 μg/mL. The method allows rapid determination of trypsin activity in the normal and acute pancreatitis range, suitable for point-of-care testing. Furthermore, the applicability of the method has been demonstrated by detecting trypsin in spiked urine samples.

Application of high-resolution ultrasonic spectroscopy for real-time monitoring of trypsin activity in β-casein solution

High-resolution ultrasonic spectroscopy (HR-US) was applied for real-time monitoring of β-casein hydrolysis by trypsin at various conditions for the first time. The technique is based on the precision measurement of hydration changes proportional to the number of peptide bond hydrolyzed. As HR-US exhibits ultrasonic transparency for most solution, the analysis did not require optical transparency like for 2,4,6-trinitrobenzenesulfonic acid (TNBS) assay. Appropriate enzymatic models were fitted with degree of hydrolysis (d_h) profiles to provide kinetic and mechanistic description of proteolysis in terms of initial hydrolysis rate, r⁰, and rate constant of hydrolysis, k_h, and enzyme inactivation, k_d. Maximal r⁰ and d_h were obtained at 45 °C and pH 8. The exponential dependence of kinetic parameters allowed determination of the activation (E_A = 50.3 ± 7 kJ/mol) and deactivation (E_D = 62.23 ± 3 kJ/mol) energies of hydrolysis. The ultrasonic assay provided rapid detection of trypsin activity even at sub-nanomolar concentration.

Label-free analytical performances of a peptide-based QCM biosensor for trypsin

A label-free biosensor was fabricated for the detection of trypsin by using a peptide-functionalized quartz crystal microbalance gold electrode. The synthetized peptide chains were immobilized tightly on the QCM electrode via a self-assembly method, which formed a thin and approximate rigid layer of peptides. The detection signal was achieved by calculating the mass changes on the QCM electrode because the peptide chains could be specifically cleaved in the carboxyl terminuses of arginine and lysine by trypsin. When gold nanoparticles were coupled to the peptide chains, the sensing signal would be amplified 10.9 times. Furthermore, the sensor interface shows a lower resonance resistance change when the peptide chain is immobilized horizontally. Independent detections in parallel on different electrodes have a wide linear range. Under the optimum conditions, the signal-amplified biosensor allowed the measurement of trypsin over the range of 0-750 ng mL-1 with a detection limit of 8.6 ng mL-1. Moreover, for screening the inhibitor of trypsin, the IC50 values were obtained to be 1.85 μg mL-1 for benzamidine hydrochloride and 20.5 ng mL-1 for the inhibitor from soybean.

Low-concentration trypsin detection from a metal-enhanced fluorescence (MEF) platform: Towards the development of ultra-sensitive and rapid detection of proteolytic enzymes

Proteolytic enzymes, which serve to degrade proteins to their amino acid building blocks, provide a distinct challenge for both diagnostics and biological research fields. Due to their ubiquitous presence in a wide variety of organisms and their involvement in disease, proteases have been identified as biomarkers for various conditions. Additionally, low-levels of proteases may interfere with biological investigation, as contamination with these enzymes can physically alter the protein of interest to researchers, resulting in protein concentration loss or subtler polypeptide clipping that leads to a loss of functionality. Low levels of proteolytic degradation also reduce the shelf-life of commercially important proteins. Many detection platforms have been developed to achieve low-concentration or low-activity detection of proteases, yet many suffer from limitations in analysis time, label stability, and ultimately sensitivity. Herein we demonstrate the potential utility of fluorescein derivatives as fluorescent labels in a new, turn-off enzymatic assay based on the principles of metal-enhanced fluorescence (MEF). For fluorescein sodium salt alone on nano-slivered 96-well plates, or Quanta Plates™, we report up to 11,000x enhancement for fluorophores within the effective coupling or enhancement volume region, defined as ~100 nm from the silver surface. We also report a 9% coefficient of variation, and detection on the picomolar concentration scale. Further, we demonstrate the use of fluorescein isothiocyanate-labeled YebF protein as a coating layer for a MEF-based, Quanta Plate™ enzymatic activity assay using trypsin as the model enzyme. From this MEF assay we achieve a detection limit of ~1.89 ng of enzyme (2.8 mBAEE activity units) which corresponds to a minimum fluorescence signal decrease of 10%. The relative success of this MEF assay sets the foundation for further development and the tuning of MEF platforms for proteolytic enzyme sensing not just for trypsin, but other proteases as well. In addition, we discuss the future development of ultra-fast detection of proteases via microwave-accelerated MEF (MAMEF) detection technologies.

Electrochemiluminescence resonance energy transfer between luminol and black phosphorus nanosheets for the detection of trypsin via the “off–on–off” switch mode

In this work, the electrochemiluminescence behavior of a luminol–H₂O₂ system was studied on a black phosphorus nanosheet modified electrode.