A new colorimetric assay for amylase based on starch-supported Cu/Au nanocluster peroxidase-like activity

Enzyme-integrated metal-organic framework platform for cascade detection of α-amylase

As one of the most important industrial enzymes, α-amylase is widely used in food processing, such as starch sugar and fermentation, bringing high added value to industry of more than a trillion dollars. We developed a multi-enzyme system (Glu&Gox@Cu-MOF-74) prepared by embedding α-glucosidase (Glu) and glucose oxidase (Gox) into the biomimetic metal-organic framework Cu-MOF-74 using in situ encapsulation within 15 min at room temperature for efficient and sensitive detection of α-amylase activity. Benefitting from the remarkable peroxidase-mimicking property and rigid skeleton of Cu-MOF-74, the biocatalytic platform exhibited excellent cascade activity and tolerance in various extremely harsh environments compared to natural enzymes. On this basis, a cascade biocatalytic platform was constructed for the detection of α-amylase activity with wide linear range (5-100 U/L) and low limit of detection (1.45 U/L). The colorimetric cascade scheme is important for the sensitive and selective determination of α-amylase in complex fermentation samples, and the detection time is short (∼0.5 h). This work provides new ideas for the detection of α-amylase based on the cascade amplification method.

Challenges and Opportunities of Using Fluorescent Metal Nanocluster-Based Colorimetric Assays in Medicine

Development of rapid colorimetric methods based on novel optical-active metal nanomaterials has provided methods for the detection of ions, biomarkers, cancers, etc. Fluorescent metal nanoclusters (FMNCs) have gained a lot of attention due to their unique physical, chemical, and optical properties providing numerous applications from rapid and sensitive detection to cellular imaging. However, because of very small color changes, their colorimetric applications for developing rapid tests based on the naked eye or simple UV-vis absorption spectrophotometry are still limited. FMNCs with peroxidase-like activity have significant potential in a wide variety of applications, especially for point-of-care diagnostics. In this review, the effect of using various capping agents and metals for the preparation of nanoclusters in their colorimetric sensing properties is explored, and the synthesis and detection mechanisms and the recent advances in their application for ultrasensitive chemical and biological analysis regarding human health are highlighted. Finally, the challenges that remain as well as the future perspectives are briefly discussed. Overcoming these limitations will allow us to expand the nanocluster's application for colorimetric diagnostic purposes in medical practice.

Cu/Au nanoclusters with peroxidase-like activity for chemiluminescence detection of α-amylase

Herein, a novel chemiluminescence method was developed for efficient and sensitive detection of α-amylase activity. α-Amylase is closely related to our life, and α-amylase concentration is a marker for the diagnosis of acute pancreatitis. In this paper, Cu/Au nanoclusters with peroxidase-like activity were prepared using starch as a stabilizer. Cu/Au nanoclusters can catalyze H₂O₂ to generate reactive oxygen species and increase the CL signal. The addition of α-amylase makes the starch decompose and causes the nanoclusters to aggregate. The aggregation of the nanoclusters caused them to increase in size and decrease in the peroxidase-like activity, resulting in a decrease in the CL signal. α-Amylase was detected by the CL method of signal changes caused by dispersion-aggregation in the range of 0.05-8 U mL^-1 with a low detection limit of 0.006 U mL^-1. The chemiluminescence scheme based on the luminol-H₂O₂-Cu/Au NC system is of great significance for the sensitive and selective determination of α-amylase in real samples, and the detection time is short. This work provides new ideas for the detection of α-amylase based on the chemiluminescence method and the signal lasts for a long time, which can realize timely detection.

A fluorescent assay for sensitive detection of kanamycin by split aptamers and DNA-based copper/silver nanoclusters

Kanamycin was a group of essential antibiotics generally served in treating infections of animals which leached into the environment residual in food, causing health concerns. Thus, selective and sensitive monitoring of kanamycin was significant for food safety. In this work, split aptamers were used as templates to prepare fluorescent Cu/Ag NCs for detection of kanamycin. According to the impressive affinity of the aptamer to kanamycin, two different detection modes were designed using kanamycin aptamer as a recognition molecule, in which one was to combine split aptamer Apt-1 with Apt-2 to form an entangled DNA as a Cu/Ag NCs template, the other was to associate the normal aptamer after encirclement to form Cu/Ag NCs templates. After the addition of kanamycin, the fluorescence signals of the Cu/Ag NCs synthesized in the two modes were both enhanced, but the approach with split aptamer exhibited a superior observable sensitivity than that of the normal type. The detection range showed a well linear relationship between 80 nM and 10 μM when the emission wavelength was 560 nm, and the detection limit was 13.3 nM. In addition, when streptomycin, oxytetracycline, chloramphenicol and chlortetracycline were involved in the selective interference experiment under the same conditions, the fluorescence intensity of the system performed no significant changes. The results demonstrated that this method possessed favorable specificity and selectivity for the assay of kanamycin, proficiently achieving efficient, rapid and sensitive evaluation of kanamycin in the milk samples.

An Oligopeptide-Protected Ultrasmall Gold Nanocluster with Peroxidase-Mimicking and Cellular-Imaging Capacities

Recent decades have witnessed the rapid progress of nanozymes and their high promising applications in catalysis and bioclinics. However, the comprehensive synthetic procedures and harsh synthetic conditions represent significant challenges for nanozymes. In this study, monodisperse, ultrasmall gold clusters with peroxidase-like activity were prepared via a simple and robust one-pot method. The reaction of clusters with H₂O₂ and 3,3',5,5'-tetramethylbenzidine (TMB) followed the Michaelis-Menton kinetics. In addition, in vitro experiments showed that the prepared clusters had good biocompatibility and cell imaging ability, indicating their future potential as multi-functional materials.

A new colorimetric assay for sensitive detection of glucose-6-phosphate dehydrogenase deficiency based on silver nanoparticles

Glucose-6-phosphate dehydrogenase (G6PD) is the principal enzyme in the pentose phosphate pathway that plays a fundamental role in the production of nicotinamide adenine dinucleotide phosphate, which is very important in preventing the oxidation of cells, especially red blood cells. This enzyme deficiency was associated with many disorders, the most common of which were hemolysis episodes. In the last decade, nanoparticles have been used to design optical and electronic sensors due to their unique properties. This report presents a new colorimetric method that used silver nanoparticles to detect glucose 6-phosphate dehydrogenase activity directly. The glucose-6-phosphate dehydrogenase detection mechanism was based on an aggregation of silver nanoparticles, leading to increased nanoparticle size, which causes discoloration. In the presence of the enzyme, the color of the solution was yellow, and when the enzyme was not present, the color of the solution was grayish. Utilizing this method, colorimetric sensing of glucose 6-phosphate dehydrogenase was gained with a detection limit of 0.009 U ml^-1and a linear range of 0-16.0 U ml^-1. In this way, the presence or absence of the enzyme can be easily detected with the naked eye during one step.

A fluorescent aptasensor based on copper nanoclusters for optical detection of CD44 exon v10, an important isoform in metastatic breast cancer

Recent studies suggest that breast cancer cells express various CD44 isoforms. CD44 is an integral transmembrane protein encoded by a single 20-exon gene. Exon v10 of CD44 plays a critical role in promoting cancer metastasis, so sensitive detection of this isoform helps in early diagnosis of metastatic breast cancer and facilitates the treatment process. This study aimed to use v10-specific aptamers to set up an optical aptasensor based on fluorescent metal nanoclusters. For this purpose, nanoclusters of silver, gold, and copper were prepared by different CD44 v10 DNA aptamers as molecular templates. UV-vis, TEM, and fluorescence spectrometer results confirmed the accuracy and quality of the synthesized aptamer-templated nanoclusters (Apt-NCs). Finally, we compared the performance of the as-prepared Apt-NCs in response to different cultured cell lines. According to the results, the optical response of M-Apt4-CuNCs was more efficient and correlated well with the concentrations of CD44 v10-enriched cells. The detection limit of the aptasensor was 40 ± 5 cells per mL.

Glutathione Disulfide as a Reducing, Capping, and Mass-Separating Agent for the Synthesis and Enrichment of Gold Nanoclusters

Water-soluble nanoclusters, which are facilely enrichable without changes in the original properties, are highly demanded in many disciplines. In this contribution, a new class of gold nanoclusters (AuNCs) was synthesized using glutathione disulfide (GSSG) as a reducing and capping agent under intermittent heating mode. The as-prepared GSSG–AuNCs had a higher quantum yield (4.1%) compared to the conventional glutathione-protected AuNCs (1.8%). Moreover, by simply introducing the GSSG–AuNC solution to acetonitrile at a volume ratio of 1:7, a new bottom phase was formed, in which GSSG–AuNCs could be 400-fold enriched without changes in properties, with a percentage recovery higher than 99%. The enrichment approach did not need additional instruments and was potentially suitable for large-scale enrichment of nanoclusters. Further, density functional theory calculations indicated that the hydrogen bonding between GSSG and acetonitrile plays a key role for the bottom phase formation. Our work suggests that the highly emissive GSSG–AuNCs possess great potential not only in fluorescent measurements but also in other scenarios in which high-concentration AuNCs may be needed, such as catalysis, drug delivery, and electronic and optical industries.

Sensitive detection of S. Aureus using aptamer- and vancomycin -copper nanoclusters as dual recognition strategy

The proposed aptamer- and antibiotic-based dual detection sensor, combines copper nanoclusters (CuNCs) as an effective approach for the recognition and quantification of Staphylococcus aureus (S. aureus) as a pathogenic bacteria. A facile method for CuNCs based on vancomycin as the template using a fluorescence platform was proposed for the recognition of the S. aureus whole cells via antibiotic and aptamer. Using dual receptor functionalized CuNCs linked to vancomycin and a specific aptamer and during aggregation induce emission process enhanced fluorescence signal linearly with S. aureus concentrations between 10²-10⁸ CFU/mL, and the detection limit was 80 CFU/mL after 45 min as the optimum incubation time. Non-target bacteria generated negative results, proving the high specificity of the presented sensor. This strategy showed recoveries ranging 86%-98% in milk as real sample and can be used for the development of universal detection platforms for efficient and specific S. aureus detection with great potential applications for monitoring pathogenic bacteria.

Colorimetric determination of cysteine based on inhibition of GSH-Au/Pt NCs as peroxidase mimic

In this work, we reported a facile and highly sensitive strategy for colorimetric detection of cysteine (Cys) based on the inhibition of catalytic activity of bimetallic nanoclusters induced by Cys. Glutathione-modified gold-platinum nanoclusters (GSH-Au/Pt NCs) with different Au/Pt molar ratios were prepared via one-pot approach and utilized as peroxidase mimics to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂. It has been found that Cys could inhibit the peroxidase-like activity of GSH-Au/Pt NCs efficiently, which leads to a decrease of the absorption intensity of the system at 652 nm with a fading of the blue color. These findings provide a worthy method for visualization and quantitative detection of Cys with different concentrations in the range from 0.5 to 30 μM, and the detection limit is 0.154 μM. Moreover, this method displays a promising application in colorimetric analysis of Cys in urine samples.

Norfloxacin detection based on the peroxidase-like activity enhancement of gold nanoclusters

The use of nanomaterials as mimic enzymes provides a promising way to implement bio-molecule detection in living systems. However, to achieve highly efficient catalytic processes with gold nanocluster-based nanozymes is still challenging. In this study, a facile reduction method was utilized to synthesize gold nanoclusters with 1-methyl-D-tryptophan as the reducing and capping agent. The obtained gold nanoclusters exhibited a peroxidase-mimicking property in the redox reaction of 3,3',5,5'-tetramethylbenzidine to blue oxidized 3,3',5,5'-tetramethylbenzidine in the presence of H₂O₂. The addition of norfloxacin endowed the nanozymes with a 10-fold enhancement in catalytic efficiency due to the surface charge-controlled electron transfer modulation. The colorimetric sensing system presented a high selectivity toward norfloxacin. The good linear relationship of norfloxacin monitoring was gained in the range of 1.25~8.0 μM (R² = 0.996), with a detection limit of 0.2 μM. The practical application of the proposed protocol for the measurement of norfloxacin in capsules was realized. This demonstrates that on account of their significant catalytic efficiency enhancement, the gold nanocluster-based nanozymes hold great promise in realizing the selective detection of drugs. Graphical Abstract.

Detection of Escherichia coli O157:H7 Using Automated Immunomagnetic Separation and Enzyme-Based Colorimetric Assay

The food industry requires rapid and simple detection methods for preventing harm from pathogenic bacteria. Until now, various technologies used to detect foodborne bacteria were time-consuming and laborious. Therefore, we have developed an automated immunomagnetic separation combined with a colorimetric assay for the rapid detection of E. coli O157:H7 in food samples. The colorimetric detection method using enzymatic reaction is fascinating because of its simplicity and rapidity and does not need sophisticated devices. Moreover, the proposed procedures for the detection of bacteria in food take less than 3 h including pre-enrichment, separation and detection steps. First, target-specific immunomagnetic beads were introduced to contaminated milk in a pre-enrichment step. Second, the pre-enriched sample solution containing target bacteria bound on immunomagnetic beads was injected into an automated pretreatment system. Subsequently, the immunomagnetic beads along with target bacteria were separated and concentrated into a recovery tube. Finally, released β-galactosidase from E. coli O157:H7 after lysis was reacted with chlorophenol red β-galactopyranoside (CPRG) used as a substrate and the colorimetric change of CPRG was determined by absorbance measuring or the naked eye. By the proposed approach in this study, we could detect 3 × 10² CFU/mL of E. coli O157:H7 from a milk sample within 3 h.