A sensitive assay for trypsin using poly(thymine)-templated copper nanoparticles as fluorescent probes

Expanding the scope of self-assembled supramolecular biosensors: a highly selective and sensitive enzyme-responsive AIE-based fluorescent biosensor for trypsin detection and inhibitor screening

Trypsin, a pancreatic enzyme associated with diseases like pancreatic cancer and cystic fibrosis, requires effective diagnostic tools. Current detection systems seldom utilize macrocyclic molecules and tetraphenyl ethylene (TPE) derivative-based supramolecular assemblies, known for their biocompatibility and aggregation-induced emission (AIE) properties, for trypsin detection. This study presents an enzyme-responsive, AIE-based fluorescence 'Turn-On' sensing platform for trypsin detection, employing sulfated-β-cyclodextrin (S-βCD), an imidazolium derivative of TPE (TPE-IM), and protamine sulfate (PrS). The anionic S-βCD and cationic TPE-IM formed a strongly fluorescent supramolecular aggregation complex in an aqueous buffer. However, PrS suppresses fluorescence because of its strong binding affinity with S-βCD. The non-fluorescent TPE-IM/S-βCD/PrS supramolecular assembly system exhibits trypsin-responsive properties, as PrS is a known trypsin substrate. Trypsin restores fluorescence in the TPE-IM/S-βCD system through the enzymatic cleavage of PrS, correlating linearly with trypsin catalytic activity in the 0-10 nM concentration range. The limit of detection is 10 pM. This work contributes to the development of self-assembled supramolecular biosensors using charged TPE derivatives and β-cyclodextrin-based host-guest chemistry, offering an innovative fluorescence 'Turn-On' trypsin sensing platform. The sensing system is highly stable under various conditions, selective for trypsin, and demonstrates potential for biological analysis and disease diagnosis in human serum. Additionally, it shows promise for the screening of trypsin inhibitors.

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.

Heparin-enhanced peroxidase-like activity of iron-cobalt oxide nanosheets for sensitive colorimetric detection of trypsin

Iron-cobalt oxide nanosheets (FeCo-ONSs) were proved to have intrinsic peroxidase-like activity. Additionally, the peroxidase-like activity of FeCo-ONSs toward the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) was dramatically enhanced after heparin addition due to the stronger affinity toward TMB. Protamine combines with heparin, so the promotion of peroxidase-like activity of FeCo-ONSs with heparin was suppressed. With the addition of trypsin, protamine was hydrolyzed and the enhancement effect of catalytic activity of FeCo-ONSs was recovered. Based on above process, a sensitive colorimetric platform for trypsin activity determination was constructed through measuring the absorbance of produced oxTMB at 652 nm, providing a linear detection range of 5 to 500 ng/mL and a low detection limit of 2.8 ng/mL. The method was applied to trypsin determination in real samples (human urine sample and multienzyme tablet sample) with satisfactory results, illustrating the potential application of this biosensor.

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.

Optical assay of trypsin using a one-dimensional plasmonic grating of gelatin-modified poly(methacrylic acid)

The geometry of resonant absorbers (RA) is varied by tryptic digestion to design a probe platform. The process includes fabrication of a line array of poly(methacrylic acid) (PMAA) brush as an RA, tailed by the immobilization of gelatin. The gelatin-modified PMAA RA is a kind of one-dimensional plasmonic grating, possessing an optical feature with a characteristic absorption peak. The growth of gelatin on PMAA RA resulted in a blue shift of the absorption peak from 465 to 263 nm. Trypsin catalyzes the hydrolysis of peptide bonds, breaking down gelatin into smaller peptides causing the change in geometry of RA. The gelatin of RA was digested in a wide linear range of activity of trypsin from 34 to 1088 U mL^-1 resulting in a red shift of the absorption peak of RA from 263 to 474 nm within 10 min. The limit of detection achieved is 11 U mL^-1 with ca. 1.9% standard deviation and 101.4% recovery of spiked serum samples. The chemical selectivity of the trypsin assay is evidenced by motoring the changes in a shift of the absorption peak of gelatin-modified PMAA RA using chymotrypsin and horseradish peroxidase. Graphical abstract Schematic representation of synthesis route of 1D gelatin grating on silicon surface for trypsin probing.

Recent progress in copper nanocluster-based fluorescent probing: a review

Copper nanoclusters (CuNCs) are an attractive alternative to other metal nanoclusters. The synthesis of CuNCs is highly efficient and fast, with low-cost and without any complicated manipulation. Because of their tunable fluorescence and low toxicity, CuNCs have been highly exploited for biochemical sensing. This review (with 172 refs.) summarizes the progress that has been made in the field in the past years. Following an introduction into the fundamentals of CuNCs, the review first focuses on synthetic methods and the fluorescence properties of CuNCs (with subsections on the use of proteins, peptides, DNA and other molecules as templates). This is followed by a section on the use of CuNCs in fluorometric assays, with subsections on the detection of small molecules, proteins, nucleic acids, various other biomolecules including drugs, and of pH values. A further large chapter summarizes the work related to environmental analyses, specifically on determination of metal ions, anions and pollutants. Graphical abstract Schematic representation of the synthesis and potential applications of copper nanocluster (CuNCs) in biochemical analysis, emphatically reflected in some vital areas such as small molecule analysis, biomacromolecule monitoring, cell imaging, ions detection, toxic pollutant, etc.

Protamine-Induced Supramolecular Self-Assembly of Red-Emissive Alkynylplatinum(II) 2,6-Bis(benzimidazol-2'-yl)pyridine Complex for Selective Label-Free Sensing of Heparin and Real-Time Monitoring of Trypsin Activity

A label-free detection assay is developed based on the design and synthesis of a new anionic alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine complex with water-soluble pendants. With the aid of electrostatic interaction and noncovalent metal-metal and π-π stacking interactions, protamine is shown to induce supramolecular self-assembly of platinum(II) complexes with drastic UV-vis absorption and red emission changes. On the basis of the strong binding affinity of protamine and heparin, the ensemble has been further employed to probe heparin by monitoring the spectroscopic changes. Other than heparin, this ensemble can also detect the activity of trypsin, which can hydrolyze protamine into fragments, leading to the deaggregation of platinum(II) complexes. By modulation of the self-assembly properties of platinum(II) complexes via real-time UV-vis absorption and emission studies, the reported assay has been demonstrated to be a sensitive and selective detection method for trypsin, as well as trypsin inhibitor screening, which is essential for drug discovery.

Alkaline phosphatase determination via regulation of enzymatically generated poly(thymine) as a template for fluorescent copper nanoparticle formation

We propose a new fluorometric method for alkaline phosphatase (ALP) determination. This method is based on the regulation of enzymatically generated poly(thymine) for the preparation of copper nanoparticles (CuNPs). 2'-Deoxythymidine 5'-triphosphate (dTTP) serves as the source for polymerization mediated by terminal deoxynucleotidyl transferase (TdT). This process generates poly(thymine), which acts as the template for synthesis of fluorescent CuNPs. However, if ALP catalyzes the hydrolysis of dTTP, the TdT-mediated polymerization will be disabled. This prevents the formation of CuNPs and causes a drop in fluorescence. The findings were used to design a sensitive and selective fluorometric method for ALP determination. A linear response in the activity range from 0.1 to 20 U L^-1 and a limit of quantification of 0.3 U L^-1 were obtained. The results indicate that the proposed method can be successfully applied to ALP assay in spiked diluted serum. This demonstrates the method's reliability and practicability. Graphical abstract A fluoromoetric method for alkaline phosphatase assay has been developed based on regulation of enzymatically generated poly(thymine) as template for the formation of fluorescent CuNPs.

Progress in biosensor based on DNA-templated copper nanoparticles

During the last decades, by virtue of their unique physicochemical properties and potential application in microelectronics, biosensing and biomedicine, metal nanomaterials (MNs) have attracted great research interest and been highly developed. Deoxyribonucleic acid (DNA) is a particularly interesting ligand for templating bottom-up nanopreparation, by virtue of its excellent properties including nanosized geometry structure, programmable and artificial synthesis, DNA-metal ion interaction and powerful molecular recognition. DNA-templated copper nanoparticles (DNA-CuNPs) has been developed in recent years. Because of its advantages including simple and rapid preparation, high efficiency, MegaStokes shifting and low biological toxicity, DNA-CuNPs has been highly exploited for biochemical sensing from 2010, especially as a label-free detection manner, holding advantages in multiple analytical technologies including fluorescence, electrochemistry, surface plasmon resonance, inductively coupled plasma mass spectrometry and surface enhanced Raman spectroscopy. This review comprehensively tracks the preparation of DNA-CuNPs and its application in biosensing, and highlights the potential development and challenges regarding this field, aiming to promote the advance of this fertile research area.

A fast, highly sensitive and selective assay of iodide ions with single-stranded DNA-templated copper nanoparticles as a fluorescent probe for its application in Kunming mice samples

The development of fast, sensitive, selective and flexible methods for the detection of iodide is highly demanded and is of great significance. In this work, single-stranded DNA-templated copper nanoparticles (ssDNA-CuNPs) generated by sodium ascorbate reduction of Cu²⁺ along the single-stranded DNA of poly-T were utilized as a fluorescent probe for the determination of iodide ions (I^-). The detection scheme is based on the instant quenching of the fluorescence of ssDNA-CuNPs by iodide ions. I^- can be quantified in the concentration range from 0.050 to 40 μM and from 40 to 80 μM, and the limit of detection is as low as 15 nM. This method provides a simple and convenient strategy for the biochemical assay of I^-, which is also helpful for early diagnosis of related diseases. The establishment of a low cost and fast detection method would be particularly important in developing countries where medical supplies are lacking.

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.

An impedimetric determination of alkaline phosphatase activity based on the oxidation reaction mediated by Cu2+ bound to poly-thymine DNA

A novel impedimetric assay for the accurate determination of alkaline phosphatase (ALP) activity is developed based on the Cu²⁺-mediated oxidation of ascorbic acid on a poly-thymine DNA-modified electrode.

Hairpin loop-enhanced fluorescent copper nanoclusters and application in S1 nuclease detection

A novel hairpin DNA template with an AT24 double strand stem and a six-base loop was demonstrated for the first time to prepare CuNCs with dramatically enhanced fluorescence and applied for the sensitive detection of S1 nuclease.

Silver triangular nanoplates as an high efficiently FRET donor-acceptor of upconversion nanoparticles for ultrasensitive "Turn on-off" protamine and trypsin sensor

Silver triangular nanoplates (STNPs) as a high efficient fluorescence quenching reagent of upconversion nanoparticles (UCNPs) was used to constract a novel label-free fluorescence nanosensor for ultrasensitive detection of protamine and trypsin based on fluorescence resonance energy transfer (FRET) between STNPs and UCNPs. In this assay, the negatively charged STNPs can bind with positively charged UCNPs through electrostatic interaction, and then quenched the fluorescence of UCNPs. When protamine was added to the mixture of UCNPs-STNPs, the STNPs interacted with protamine and then detached from the surface of UCNPs and aggregated, which result in the recovery of the fluorescence of UCNPs. Trypsin could catalyze the hydrolysis of protamine and effectively quench the fluorescence recovered by protamine. By measuring the changes of the fluorescence of UCNPs, the concentrations of protamine and trypsin were determined. Under the optimized conditions, the linear response range was obtained from 10 to 500ng/mL, 5-80ng/mL and with the low detection limit of 3.1ng/mL and 1.8ng/mL for protamine and trypsin, respectively. Meanwhile, the nanosensor shows good selectivity, sensitivity and can be successfully applied to detection of protamine and trypsin in serum samples.

TTE DNA–Cu NPs: enhanced fluorescence and application in a target DNA triggered dual-cycle amplification biosensor

A crowded TTE DNA structure for the preparation of Cu NPs with enhanced fluorescence was prepared and applied for the ultrasensitive detection of target DNA.

Label-Free Fluorescent Detection of Trypsin Activity Based on DNA-Stabilized Silver Nanocluster-Peptide Conjugates

Trypsin is important during the regulation of pancreatic exocrine function. The detection of trypsin activity is currently limited because of the need for the substrate to be labeled with a fluorescent tag. A label-free fluorescent method has been developed to monitor trypsin activity. The designed peptide probe consists of six arginine molecules and a cysteine terminus and can be conjugated to DNA-stabilized silver nanoclusters (DNA-AgNCs) by Ag-S bonding to enhance fluorescence. The peptide probe can also be adsorbed to the surface of graphene oxide (GO), thus resulting in the fluorescence quenching of DNA-AgNCs-peptide conjugate because of Förster resonance energy transfer. Once trypsin had degraded the peptide probe into amino acid residues, the DNA-AgNCs were released from the surface of GO, and the enhanced fluorescence of DNA-AgNCs was restored. Trypsin can be determined with a linear range of 0.0-50.0 ng/mL with a concentration as low as 1 ng/mL. This label-free method is simple and sensitive and has been successfully used for the determination of trypsin in serum. The method can also be modified to detect other proteases.

Rapid and ultrasensitive detection of microRNA by target-assisted isothermal exponential amplification coupled with poly (thymine)-templated fluorescent copper nanoparticles

We devised a novel method for rapid and ultrasensitive detection of target microRNA (miRNA) by employing target-assisted isothermal exponential amplification (TAIEA) combined with poly (thymine)-templated fluorescent copper nanoparticles (CuNPs) as signaling probes. The target miRNA hybridizes to the unimolecular template DNA and works as a primer for the extension reaction to form double-stranded product, which consequently generates two nicking endonuclease recognition sites. By simultaneous nicking and displacement reactions, exponential amplification generates many poly (thymine) strands as final products, which are employed for the synthesis of fluorescent CuNPs. Based on the fluorescent signal from CuNPs, target miRNA is detected as low as 0.27 fM around 1 h of total analysis time. The diagnostic capability of this system has been successfully demonstrated by reliably detecting target miRNA from different cell lysates, showing its great potential towards real clinical applications.

A label-free and enzyme-free system for operating various logic devices using poly(thymine)-templated CuNPs and SYBR Green I as signal transducers

For the first time by integrating fluorescent polyT-templated CuNPs and SYBR Green I, a basic INHIBIT gate and four advanced logic circuits (2-to-1 encoder, 4-to-2 encoder, 1-to-2 decoder and 1-to-2 demultiplexer) have been conceptually realized under label-free and enzyme-free conditions. Taking advantage of the selective formation of CuNPs on ss-DNA, the implementation of these advanced logic devices were achieved without any usage of dye quenching groups or other nanomaterials like graphene oxide or AuNPs since polyA strands not only worked as an input but also acted as effective inhibitors towards polyT templates, meeting the aim of developing bio-computing with cost-effective and operationally simple methods. In short, polyT-templated CuNPs, as promising fluorescent signal reporters, are successfully applied to fabricate advanced logic devices, which may present a potential path for future development of molecular computations.

Cascade Signal Amplification Based on Copper Nanoparticle-Reported Rolling Circle Amplification for Ultrasensitive Electrochemical Detection of the Prostate Cancer Biomarker

An ultrasensitive and highly selective electrochemical assay was first attempted by combining the rolling circle amplification (RCA) reaction with poly(thymine)-templated copper nanoparticles (CuNPs) for cascade signal amplification. As proof of concept, prostate specific antigen (PSA) was selected as a model target. Using a gold nanoparticle (AuNP) as a carrier, we synthesized the primer-AuNP-aptamer bioconjugate for signal amplification by increasing the primer/aptamer ratio. The specific construction of primer-AuNP-aptamer/PSA/anti-PSA sandwich structure triggered the effective RCA reaction, in which thousands of tandem poly(thymine) repeats were generated and directly served as the specific templates for the subsequent CuNP formation. The signal readout was easily achieved by dissolving the RCA product-templated CuNPs and detecting the released copper ions with differential pulse stripping voltammetry. Because of the designed cascade signal amplification strategy, the newly developed method achieved a linear range of 0.05-500 fg/mL, with a remarkable detection limit of 0.020 ± 0.001 fg/mL PSA. Finally, the feasibility of the developed method for practical application was investigated by analyzing PSA in the real clinical human serum samples. The ultrasensitivity, specificity, convenience, and capability for analyzing the clinical samples demonstrate that this method has great potential for practical disease diagnosis applications.

A sensitive and label-free trypsin colorimetric sensor with cytochrome c as a substrate

The development of simple and sensitive methods for protease sensing plays important roles in clinical diagnostics and drug development. Here a simple, rapid, label-free, and sensitive trypsin colorimetric sensor was developed by employing cytochrome c (cyt c) as an enzyme substrate and 3,3´,5,5´-tetramethylbenzidine (TMB) as a chromogenic reagent. It was found that cyt c hardly catalyzes H2O2-mediated TMB oxidation to produce a blue solution. But the hydrolysate of cyt c by trypsin displays an intense catalytic effect on the aforementioned reaction, resulting in the formation of a blue solution immediately. The detection process allows visually perceiving as low as 50 ng/mL trypsin with the naked eyes. With the aid of a spectrophotometer, the absorbance at 652 nm was proportional to the concentration of trypsin in the range from 5.0 ng/mL to 2.0 μg/mL with a detection limit of 4.5 ng/mL. The sensor showed better precision with relative standard deviation of 2.5% and 1.7% for eleven repetitive measurements of 50.0 ng/mL and 1.0 μg/mL trypsin solution, respectively. The procedure has been successfully applied to the determination of trypsin in human urines and for inhibitor screening, demonstrating its potential application in clinic diagnosis and drug development.

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).

Label-Free real-time quantification of enzyme levels by interferometric spectroscopy combined with gelatin-modified nanoporous anodic alumina photonic films

Herein, we present an interferometric sensor based on the combination of chemically functionalized nanoporous anodic alumina photonic films (NAA-PFs) and reflectometric interference spectroscopy (RIfS) aimed to detect trace levels of enzymes by selective digestion of gelatin. The fabrication and sensing performance of the proposed sensor were characterized in real-time by estimating the changes in effective optical thickness (i.e., sensing principle) of gelatin-modified NAA-PFs (i.e., sensing element) during enzymatic digestion. The working range (WR), sensitivity (S), low limit of detection (LLoD), and linearity (R(2)) of this enzymatic sensor were established by a series of experiments with different concentrations of gelatin (i.e., specific chemical sensing element) and trypsin (i.e., analyte), a model protease enzyme with relevant implications as a biomarker in the diagnosis of several diseases. The chemical selectivity of the sensor was demonstrated by comparison of gelatin digestion by other nonspecific enzyme models such as chymotrypsin and horseradish peroxidase. Furthermore, the role of the chemical sensing element (i.e., gelatin) was assessed by using hemoglobin instead of gelatin. Finally, we demonstrated that this sensor can be readily used to establish the kinetic parameters of enzymatic reactions. The obtained results revealed that the presented sensor has a promising potential to be used as a point-of-care system for fast detection of gastrointestinal diseases at early stages.

A rapid, sensitive and label-free sensor for Hg(ii) ion detection based on blocking of cysteine-quenching of fluorescent poly(thymine)-templated copper nanoparticles

A rapid fluorescence sensor was developed for Hg²⁺ detection based on blocking of cysteine-quenching of poly T templated Cu NPs.

H2O2-mediated fluorescence quenching of double-stranded DNA templated copper nanoparticles for label-free and sensitive detection of glucose

A label-free fluorescent sensor has been developed for glucose detection based on H₂O₂-mediated fluorescence quenching of ds-DNA templated Cu NPs.

Inhibition of double-stranded DNA templated copper nanoparticles as label-free fluorescent sensors for l-histidine detection

A label-free fluorescent sensing strategy was reported for l-histidine detection by the inhibition of double-stranded DNA templated copper nanoparticles.