Dual core quantum dots for highly quantitative ratiometric detection of trypsin activity in cystic fibrosis patients

Single-atom catalysts with peroxidase-like activity boost gel-sol transition-based biosensing

Gel-sol transition-based biosensors are a promising and popular alternative for portable, cost-effective, and user-friendly point-of-care testing (POCT). However, the improvement of sensitivity and practicability is highly demanded. In this work, a Fe-NC single-atom catalyst (SAC) is successfully synthesized and used as a signal amplification element for highly sensitive gel-sol transition-based biosensing. The Fe-NC SAC owns excellent peroxidase-like activity of 188 U/mg due to its definite atomically active centers and maximum atomic utilization of active metal atoms. As a proof-of-concept, the Fe-NC SAC is uniformly encapsulated in gelatin hydrogel to obtain a hydrogel sensor that allows colorimetric detection of trypsin based on gel-sol transition. The gelatin hydrogel network collapses derived from the hydrolysis by trypsin, and thereby the released Fe-NC SAC leads to the colorimetric sensing process. The designed hydrogel sensor offers a low detection limit of 1 ng/mL with a range from 1 to 100 ng/mL toward trypsin detection, exhibiting excellent selectivity and sensitivity, and well-performed practical detection in human serum. This work offers a successful paradigm for designing a promising SACs-related detection strategy and paves a new way to develop high-performance gel-sol transition-based sensors and various POCT applications.

Determination of trypsin using protamine mediated fluorescent enhancement of DNA templated Au nanoclusters

A fluorescent method is described for trypsin determination through the strong electrostatic interactions between cationic polyelectrolytes and single-stranded DNA (ssDNA) templated Au nanoclusters (AuNCs). The ssDNA-AuNCs display improved fluorescence emission with excitation/emission maxima at 280/475 nm after being incorporated with poly(diallyldimethylammonium chloride) (PDDA). Fluorescent enhancement is mainly attributed to the electrostatic interactions occurring  between PDDA and ssDNA templates. This can make the conformation of the ssDNA templates to change. Thus, it offers a better microenvironment for stabilizing and protecting ssDNA-AuNCs, and results in fluorescence emission enhancement. By using protamine as a model, the method is employed for the determination of trypsin. The assay enables trypsin to be determined with good sensitivity and a linear response ranging from 5 ng⋅mL^-1 to 60 ng⋅mL^-1 with a 1.5 ng⋅mL^-1 limit of detection. It is also extended to determine  the trypsin contents in human's serum samples with recoveries between 98.7% and 103.5% with relative standard deviations (RSDs) between 3.5% and 4.8%. A novel fluorescent strategy has been developed for of trypsin determination by using protamine mediated fluorescent enhancement of DNA templated Au nanoclusters.

Dual-reverse-signal ratiometric fluorescence method for malachite green detection based on multi-mechanism synergistic effect

The proposition of ratiometric detection mode has demonstrated great superiority in improving analysis accuracy by forming self-calibration. Herein, the novel dual-reverse-signal ratiometric fluorescence detection for malachite green (MG) was first achieved based on synergistic effect of fluorescence resonance energy transfer (FRET) and inner filter effect (IFE). The ratiometric fluorescence probe (B-RCDs) was self-assembled via electrostatic attraction between blue-emission carbon dots (BCDs) and red-emission carbon dots (RCDs), followed with FRET effect from BCDs to RCDs and exhibited dual-emission at 450 nm and 627 nm. In the presence of MG, the IFE effect between MG and RCDs quenched the fluorescence at 627 nm and restored the fluorescence at 450 nm, sending out two reverse signals along with an obvious color change from pink to purple (302 nm UV lamp). This ratiometric method not only simplified the preparation process, but also improved the detection sensitivity, showing a low limit of detection (LOD) of 41.8 nM, which exhibited superiority than that of single-signal RCDs (157.3 nM). This method held a rapid response of 10 min and represented satisfactory recoveries (99.14%-109.08%) in real water samples, revealing it was a promising candidate in the fast, sensitive and practical detection of MG. Moreover, the design of synergistic effect supplied a new perspective for the development of ratiometric sensing in the future.

Fluorescent microsphere probe for rapid qualitative and quantitative detection of trypsin activity

Current technologies still face a big challenge to achieve simultaneous rapid qualitative and quantitative detection of trypsin. In our present study, we developed a simple and effective strategy to sensitively, qualitatively and quantitatively analyze the activity of trypsin using a fluorescent polystyrene (PS) microsphere probe. PS spheres were first functionalized by the surface coating of polyethylene glycol (PEG), which could significantly decrease the possibility of nonspecific physical adsorption of the fluorescein isothiocyanate isomer-modified peptide (peptide-FITC). Then, the obtained PS-PEG spheres were chemically interacted with peptide-FITC, which were then employed to monitor the real-time activity of trypsin. The peptide used in our work contained rich lysine and arginine residues, which were the recognition sites of trypsin. When trypsin interacted with the PS-FITC-peptide microspheres, the peptide-FITC rapidly decomposed into free small fragments in solution, resulting in a gradual decrease in the fluorescence of the PS spheres. By taking advantage of the fluorescence changes using confocal microscopy imaging and fluorescence spectrum intensity, it is easy to achieve the qualitative and quantitative detection of trypsin, with a highly sensitive detection limit as low as 0.5 ng mL^-1 and high selectivity. Thus, the designed fluorescent PS microsphere probe would be very promising in various applications such as food safety inspection, personal healthcare and on-site environmental monitoring.

Recent developments in protease activity assays and sensors

Proteases play a pivotal role in regulating important physiological processes from food digestion to blood clotting. They are also important biomarkers for many diseases such as cancers. The importance of proteases has led to extensive efforts in the screening of proteases and their inhibitors as potential drug molecules. For example, human immunodeficiency virus (HIV) patients have been treated with HIV-1 protease inhibitors to prolong the life expectancy of patients. Such a close relationship between diseases and proteases provides a strong motivation for developing sensitive, selective, and robust protease assays and sensors, which can be exploited to discover new proteases and inhibitors. In this aspect, protease assays based on levels of proteolytic activities are more relevant than protease affinity assays such as immunoassays. In this review, recent developments of protease activity assays based on different detection principles are discussed and compared. For homogenous assays, fluorescence-based techniques are the most popular due to their high sensitivity and quantitative results. However, homogeneous assays have limited multiplex sensing capabilities. In contrast, heterogeneous assays can be employed to detect multiple proteases simultaneously, given the microarray technology that is already available. Among them, electrochemical methods, surface spectroscopy techniques, and enzyme-linked peptide protease assays are commonly used. Finally, recent developments in liquid crystal (LC)-based protease assays and their applications for detecting proteases and their inhibitors are discussed.

Aggregation-induced emission enhancement of gold nanoclusters triggered by silicon nanoparticles for ratiometric detection of protamine and trypsin

Metal nanoclusters protected by glutathione (GSH) have attracted a wide attention due to the unique aggregation-induced emission (AIE) feature. However, the "trigger" effects of ethanol, temperature, pH values, and metal ions may restrict the application of these particles. In this work, the amino modified silicon nanoparticles (SiNPs) and GSH-capped gold nanoclusters (GSH-AuNCs) can self-assemble into well-defined spherical particles due to the electrostatic interaction. As a result, the unique aggregation-induced emission enhancement (AIEE) of GSH-AuNCs arises at 570 nm, and the SiNPs keep their own blue fluorescence at 450 nm, so a novel nanohybrid probe (SiNPs@GSH-AuNCs) with dual-emission property has been constructed. When protamine is added to SiNPs@GSH-AuNCs, the cationic protamine can compete with SiNPs and absorb onto the surface of GSH-AuNCs, which inhibits the self-assembly and leads to the fluorescence quenching of GSH-AuNCs; while trypsin can catalyze the hydrolysis of protamine, the self-assembly starts again, producing the AIEE recovery. In the whole process, the SiNPs act as an internal standard and their emission stays constant. By means of the fluorescence intensity ratios I₅₇₀/I₄₅₀, the linear range of protamine is from 0.15 to 3.00 μg mL^-1 with the limit of detection (LOD) of 0.07 μg mL^-1, and trypsin shows a linear response in the range from 10 to 100 ng mL^-1 with LOD of 4.50 ng mL^-1. Furthermore, this strategy exhibits good sensitivity and selectivity, and has been further validated by applying it for the determination of protamine and trypsin in serum samples.

A simple fluorescent probe based on a pyrene derivative for rapid detection of protamine and monitoring of trypsin activity

We report the synthesis of a simple pyrene derivative and its application in protamine detection and monitoring of trypsin activity. This assay can be conducted in aqueous solution and features rapid response, visual detection, high sensitivity and selectivity. The limit of detection of protamine was 0.5 μg mL(-1). The IC50 value of a soybean trypsin inhibitor was estimated to be 0.51 U mL(-1).