FeCo nanoparticles-embedded carbon nanofibers as robust peroxidase mimics for sensitive colorimetric detection of l-cysteine

Synergistically Enhanced Oxidase-like Property of Core-Shell MOF Nanozymes by Decorating Au and Ag/AgCl Nanoparticles for l-Cysteine Colorimetric Sensing

Monitoring l-cysteine (l-Cys) is of importance for human health and food safety. Herein, we designed a novel strategy for bimetallic Au and Ag/AgCl anchoring on Ni-doped ZIF-67 to form core-shell nanocubes (Ni-ZIF-67/AuAg/AgCl) using the galvanic replacement processes. The unique properties of ZIF-67 nanocubes were conducive to generating strong synergistic catalytic effects with Au and Ag/AgCl, particularly when Ni-ZIF-67/AuAg/AgCl composites were employed as oxidase mimics for catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). The Ni-ZIF-67/AuAg/AgCl composites displayed strong affinity toward TMB, displaying a lower Michaelis constant Km value of 0.25 mM and a higher maximum initial rate Vmax of 9 × 10-8 M s-1. By virtue of the nanozyme, the colorimetric sensor was constructed for l-Cys detection with a relatively low detection limit of 0.051 μM. The superior catalytic performance of the as-prepared Ni-ZIF-67/AuAg/AgCl composites can be ascribed to the core-shell structure, large specific surface area, and strong synergistic catalytic effects, which are beneficial for exposing more active sites and enhancing the conductivity to further boost their catalytic activity.

An off-on fluorescent nanoprobe for L-cysteine sensing based on the FRET effect

A self-assembled fluorescent nanosensor for the determination of L-cysteine (Cys) was constructed based on the mechanism of fluorescence resonance energy transfer (FRET). In this system, CdTe/ZnS QDs serve as the energy donor while AuNPs serve as the receptor, resulting in the occurrence of FRET with dramatic fluorescence quenching of the QDs (turn off). Once Cys is added, AuNPs can adsorb Cys, leading to the release of the QDs. The process would inhibit the FRET, which contributed to the recovery of fluorescence (turn on) and an off-on fluorescence aptasensor for Cys detection was constructed accordingly. The linear response range of the fluorescence sensor is from 0.8 to 50 μM, and the detection limit is 0.24 μM. The sensor demonstrates great sensitivity and selectivity to Cys. More importantly, the QD-based sensing platform was successfully used for the detection of Cys in milk samples with high precision and accuracy, indicating the potential of the probe in practical applications.

A novel nanocomposite (g-C3N4/Fe3O4@P2W15V3) with dual function in organic dyes degradation and cysteine sensing

Among the important needs of human societies is the elimination of environmental pollution and also the construction of high-performance and inexpensive biosensors. In this regard, the construction of multi-functional composites has been considered. A novel magnetic graphite carbon nitride decorated by tri-vanadium substituted Dawson-type heteropolytungstate nanocomposite (C₃N₄/Fe₃O₄@P₂W₁₅V₃) effectively synthesized and characterized by prevalent functional analysis. The prepared nano-catalyst presents bi-functional usage involving photocatalytic removal of dyes (methylene blue, congo red and phenyl red) (around 98%) under visible light radiation and greatly sensitive colorimetric sensing of cysteine in an aqueous media. Moreover, synthesized nano-catalyst successfully recovered five times without any considerable deficiency on its photocatalytic ability. Further, Moreover, we propose a novel method for cysteine detection based on the C₃N₄/Fe₃O₄@P₂W₁₅V₃ nanocomposite. This nanocomposite displayed a privileged catalytic feature for cysteine oxidation to extend a clock reaction of methylene blue as an indicator in the presence of NaBH₄ in acidic solution. More importantly, this colorimetric sensing method of cysteine presents an easy, low-cost, selective, and rapid colorimetric assay with a detection limit value of 7.2 μM in the acceptable linear range of 5-600 μM.

The synthesis of Pt doped WO3 nanosheets and application on colorimetric detection of cysteine by naked eye using response surface methodology for optimization

We present a simple, sensitive, and specific colorimetric using the peroxidase properties method based on Pt doped WO₃ nanosheets to detect the cysteine. Pt@WO₃NSs were synthesized by hydrothermal method and characterized by Fourier transform infrared (FTIR), Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction patterns (XRD) methods. The response surface methodology (RSM) method based on the central composite design (CCD) was used to optimize test parameters such as pH, nanosheet concentration, and temperature. When cysteine is present in the environment due to its competition with 3,3', 5,5'-Tetramethylbenzidine (TMB) in the use of hydrogen peroxide, the blue discoloration is reduced compared to the absence of cysteine and leads to its detection. We have favorably created a peculiar approach for sensing cysteine based on the colorimetric method in solution and paper with linear range 0.01-15 μM, 0.005-14 μM and R² = 0.9887 and R² = 0.9871 respectively. The detection limit for solution-based is 1.2 nM and for paper-based is 1 nM.

Versatile Carbon Nanofiber-Based Sensors

Carbon nanofibers (CNFs) display colossal potential in different fields like energy, catalysis, biomedicine, sensing, and environmental science. CNFs have revealed extensive uses in various sensing platforms due to their distinctive structure, properties, function, and accessible surface functionalization capabilities. This review presents insight into various fabrication methods for CNFs like electrospinning, chemical vapor deposition, and template methods with merits and demerits of each technique. Also, we give a brief overview of CNF functionalization. Their unique physical and chemical properties make them promising candidates for the sensor applications. This review offers detailed discussion of sensing applications (strain sensor, biosensor, small molecule detection, food preservative detection, toxicity biomarker detection, and gas sensor). Various sensing applications of CNF like human motion monitoring and energy storage and conversion are discussed in brief. The challenges and obstacles associated with CNFs for futuristic applications are discussed. This review will be helpful for readers to understand the different fabrication methods and explore various applications of the versatile CNFs.

Graphdiyne Oxide Quantum Dots: The Enhancement of Peroxidase-like Activity and Their Applications in Sensing H2O2 and Cysteine

As one of the typical carbon nanomaterials, graphdiyne (GDY) with unique chemical, physical, and electronic properties has a great potential in various fields. Although it is an important member of carbon nanozymes, the research on its intrinsic enzyme mimetic properties and applications is still limited. Herein, graphdiyne oxide quantum dots (GDYO QDs) have been synthesized through oxidative cleavage, which exhibit enhanced peroxidase-like activity with lower K_m and higher V_max than those of most carbon-based nanozymes. The catalytic mechanism is explored, showing that the enhanced catalytic performance is attributed to the good conjugated structure, large number of oxygen-containing groups, and small-sized nanosheets with few layers. As a kind of peroxidase mimetic, the GDY-based nanozyme has excellent potential in sensing H₂O₂ and biological antioxidants through the colorimetric assay, with a linear range from 5 to 500 μM and detection limit of 1.5 μM for H₂O₂ and a linear range from 0 to 90 μM and detection limit of 0.48 μM for l-cysteine. Our work will be beneficial to develop high-performance artificial enzymes and to understand their mechanism for better applications.

Pd-Fe3O4 Janus nanozyme with rational design for ultrasensitive colorimetric detection of biothiols

Although nanozyme-based colorimetric assays have been broadly used for biosensing, some limitations such as low catalytic activity of nanozyme, poor sensitivity to analytes and lack of understanding the structure-activity relationship remain unsolved. In this work, we developed an ultrasensitive colorimetric method for biothiols detection based on density functional theory-assisted design of janus Pd-Fe₃O₄ nanozyme. The Pd-Fe₃O₄ dumbbell-like nanoparticles (DBNPs) prepared by seed-mediated approach shows a uniform heterodimeric nanostructure. Ultrasensitive biothiols detection is achieved from two aspects. On one hand, due to the synergistic effect between Pd and Fe₃O₄ in the dumbbell structure, Pd-Fe₃O₄ DBNPs show enhanced peroxidase-mimic activity compared to the individual components. On the other hand, when the target biothiols molecule is present, its inhibition effect on the janus Pd-Fe₃O₄ nanozyme is also significantly enhanced. The above results are confirmed both in experiment and theoretical calculation. Based on the rational design, a simple, highly selective and urtrasensitive colorimetric and quantitative assay for biothiols is developed. The limit of detection (LOD) can reach as low as 3.1 nM in aqueous solution. This assay is also successfully applied to the detection of biothiols in real urine samples. Moreover, the Pd-Fe₃O₄ nanozyme is used to discriminate biothiols levels in normal and cancer cells with high sensitivity at the cell density of 15,000/mL, which demonstrates its great potential in biological and clinical analysis. This work not only shows the great promise of janus bimetallic nanozymes' excellent functionalities but also provides rational guidelines to design high-performance nanozymes for biosensing and biomedical applications.

A novel colorimetric sensor for naked-eye detection of cysteine and Hg2+ based on "on-off" strategy using Co/Zn-grafted mesoporous silica nanoparticles

In an attempt to explore the significance of inorganic mimetic enzymes as sensors, this study introduces a naked-eye analytical sensing platform for the detection of L-cysteine (cys), mercury ions (Hg²⁺) based on (turn off/turn-on) catalytic activity of zinc and cobalt grafted mesoporous silica nanoparticles (MSNs). To this end, Zn-MSN and Co/Zn-MSN catalysts were synthesized and characterized using XRD, FT-IR, FESEM, TEM, and nitrogen adsorption-desorption methods. Then, using the intrinsic peroxidase-like activity of as-synthesized samples, the oxidation reactions of the chromogenic substrate (2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS)) was designed using H₂O₂, which produced green colored cation radical of ABTS. Considering the high peroxidase-like activity of Co/Zn-MSN in comparison to Zn-MSN, it was employed to detect cys and then Hg²⁺. The results indicated that the strong interaction between cys and Co/Zn-MSN was proved by a limit of detection (LOD) down to 0.24 nM and the linear relationship from 0.8-50 nM (turn off). Given the fact that Hg²⁺ has a high-affinity tendency to combine with cys, we were suggested a novel colorimetric path for sensing of Hg²⁺ in the presence of cys (turn on). Based on this method, LOD was found 0.17 nM with the linear range of 0.57-50 nM. Taken together, results showed that the as-prepared catalysts are superior to other nanoparticles as a sensor to measure the target molecules in biological monitoring and clinical diagnostics.

Hollow POM@MOF-derived Porous NiMo6 @Co3 O4 for Biothiol Colorimetric Detection

Developing highly active and sensitive peroxidase mimics for _L -cysteine (_L -Cys) colorimetric detection is very important for biotechnology and medical diagnosis. Herein, polyoxometalate-doped porous Co₃ O₄ composite (NiMo₆ @Co₃ O₄ ) was designed and prepared for the first time. Compared with pure and commercial Co₃ O₄ , NiMo₆ @Co₃ O₄ (n) composites exhibit the enhanced peroxidase-mimicking activities and stabilities due to the strong synergistic effect between porous Co₃ O₄ and multi-electron NiMo₆ clusters. Moreover, the peroxidase-mimicking activities of NiMo₆ @Co₃ O₄ (n) composites are heavily dependent on the doping mass of NiMo₆ , and the optimized NiMo₆ @Co₃ O₄ (2) exhibits the superlative peroxidase-mimicking activity. More importantly, a sensitive _L -Cys colorimetric detection is developed with the sensitivity of 0.023 μM^-1 and the detection limit at least 0.018 μM in the linear range of 1-20 μM, which is by far the best enzyme-mimetic performances, to the best our knowledge.

L-Cysteine as an Irreversible Inhibitor of the Peroxidase-Mimic Catalytic Activity of 2-Dimensional Ni-Based Nanozymes

The ability to modulate the catalytic activity of inorganic nanozymes is of high interest. In particular, understanding the interactions of inhibitor molecules with nanozymes can bring them one step closer to the natural enzymes and has thus started to attract intense interest. To date, a few reversible inhibitors of the nanozyme activity have been reported. However, there are no reports of irreversible inhibitor molecules that can permanently inhibit the activity of nanozymes. In the current work, we show the ability of L-cysteine to act as an irreversible inhibitor to permanently block the nanozyme activity of 2-dimensional (2D) NiO nanosheets. Determination of the steady state kinetic parameters allowed us to obtain mechanistic insights into the catalytic inhibition process. Further, based on the irreversible catalytic inhibition capability of L-cysteine, we demonstrate a highly specific sensor for the detection of this biologically important molecule.

A green and facile approach to a graphene-based peroxidase-like nanozyme and its application in sensitive colorimetric detection of L-cysteine

A facile and green approach to the preparation of peroxidase-like nanozymes by reducing and functionalizing graphene oxide (rGO) with Ganoderma polysaccharide (GP) has been achieved in this work. Our results showed that the as-fabricated nanozyme, namely rGO-GP, possessed the excellent property of simulating peroxidase with higher catalytic activity compared with GO or rGO obtained by using chitosan, which may be due to the better dispersion of rGO-GP in the solution. Steady-state kinetics studies further showed that the catalytic process conformed to Michaelis-Menten equation and ping-pong mechanism. Benefiting from the excellent peroxidase property of rGO-GP, we have also successfully established a highly sensitive and selective colorimetric detection approach to trace detection of L-cysteine (L-Cys). The limit of detection (LOD) of L-cysteine is 0.1 μM and the linear detection range is 2-30 μM. Furthermore, the nanozyme was successfully applied for detecting L-cysteine in serum. This work therefore demonstrates the advantages of rGO-GP as an effective nanozyme in both its green synthesis and detecting application.

Tailoring cysteine detection in colorimetric techniques using Co/Fe-functionalized mesoporous silica nanoparticles

Over the past decade, there has been a dramatic increase in the number of studies focused on sensors for cysteine (Cys) as a crucial factor in physiological function and disease diagnosis. Among those sensors, nanomaterial-based peroxidase mimetics have received particular attention from researchers. This study introduces a new series of mesoporous silica nanoparticles (MSNs) incorporated with iron and cobalt (Co/Fe-MSN) with a molar ratio of Si/Fe = 10 and cobalt species at 1, 3, and 5 wt% that have great potential in the sensing application. These nanomaterial characterization was investigated by FTIR spectroscopy, SEM, TEM, XRD, and nitrogen adsorption-desorption. The peroxidase activity of these nanomaterials was studied through kinetic analysis. The findings revealed that Co/Fe-MSN (1%) showed higher peroxidatic activity than the others towards the common chromogenic substrate 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) diammonium salt. Based on the enzymatic activity of Co/Fe-MSN (1%), a colorimetric sensing platform was designed to detect H2O2 and Cys. The limit of detection (LOD) for H2O2 and Cys was determined to be 1.1 μM and 0.89 nM, respectively. The results indicated that the proposed enzyme mimic exhibited excellent potential as a sensor in medical diagnostics and biological systems.

Nanozyme based on CoFe2O4 modified with MoS2 for colorimetric determination of cysteine and glutathione

A nanozyme based on CoFe₂O₄ modified with MoS₂ was constructed for colorimetric determination of cysteine (Cys) and glutathione (GSH). Firstly, ferrite CoFe₂O₄ is synthesized, and it is then modified by MoS₂ to form a flower-like polymer (MoS₂@CoFe₂O₄). In the presence of H₂O₂, a redox interaction takes place, and the resulting hydroxyl promoted a colorimetric conversion from colorless to blue in the presence of 3,3',5,5'-tetramethylbenzidine (TMB). However, once Cys or GSH is added, they are capable to compete with the interaction of the hydroxyl with TMB, resulting in an inhibition of the colorimetric conversion. The colorimetric distinction is sensitive to the amount of target. The results obtained proved that the catalytic efficiency of MoS₂@CoFe₂O₄ is 4.4-fold and 1.8-fold to that of MoS₂ and CoFe₂O₄. Meanwhile, the K_m values to TMB and H₂O₂ are 0.067 and 0.048 mM, respectively, which are 6.5-fold and 77-fold, respectively smaller than those of natural peroxidase such as HPR. This indicates that the MoS₂@CoFe₂O₄ possesses a favorable interaction affinity. Additionally, the colorimetric distinction caused by the competition between TMB and cysteine or glutathione is obvious. The signal responses to cysteine and glutathione are linear in the range 0.5~15 μM and 0.5~35 μM, and the LODs are 0.10 and 0.21 μM, respectively. In practical assay of Cys in serum, the RSD of the sample tests is 4.6%, and the recoveries for the spiked assays are 95.3% and 96.0% with the RSD of 2.1% and 4.2%, respectively.

Development of a cysteine sensor based on the peroxidase-like activity of AgNPs@ Fe3O4 core-shell nanostructures

In this study, a facile one step solvo-thermal procedure has been employed in generating magnetite-silver core-shell nanocomposites (AgNPs@ Fe₃O₄) with superior peroxidase-like catalytic property than bare magnetic nanoparticles (Fe₃O₄). The composites were characterized using different techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and surface-enhanced infrared absorption spectroscopy (SEIRA). In the presence of hydrogen peroxide, the synthesized composites were able to oxidize the colorless o-phenylenediamine (OPD) to a yellow colour 2, 3-diaminophenazine (DAP) with a better peroxidase-like activity than Fe₃O₄ alone. The obtained K_m value of AgNPs@ Fe₃O₄ with H₂O₂ and OPD substrates are 28.0 mM and 2.91 mM respectively. These are substantially lower than previously reported values and indicate the strong binding affinity of the substrates towards AgNPs@ Fe₃O₄ nanocomposites. Based on the obstruction activity of cysteine on the peroxidase-like catalytic property of the nanocomposites, a sensor was developed for detection of cystein with a limit of detection as low as 87 nM and a wider range of linearity. The sensor also exhibited excellent selectivity against potentially interfering molecules.

An Au@NH2-MIL-125(Ti)-based multifunctional platform for colorimetric detections of biomolecules and Hg2+

Au@NH₂-MIL-125(Ti) was fabricated and explored as a multifunctional platform for sensitive colorimetric detections of biomolecules and Hg²⁺.

Fe/C magnetic nanocubes with enhanced peroxidase mimetic activity for colorimetric determination of hydrogen peroxide and glucose

Iron-carbon (Fe/C) magnetic nanocubes were synthesized by direct pyrolysis of Prussian blue nanocubes under an inert gas atmosphere. They are shown to possess intrinsic peroxidase mimicking activity to catalyze the oxidation of peroxidase substrate N,N-diethyl-p-phenylenediamine sulfate salt to form a purple colored product in the presence of H₂O₂. The values for K_m and V_max are 74 μM and 46 nmol s^-1, respectively. Steady-state kinetic analysis also indicates that the catalysis reaction follows a ping-pong mechanism. Based on these findings, an ultrasensitive colorimetric H₂O₂ assay was worked. Absorbance (best value measured at 550 nm) increases linearly in the 10 nM to 0.2 mM H₂O₂ concentration range, and the limit of detection is 1.5 nM. The method was also applied to the quantification of glucose, which is oxidized by glucose oxidase in the coexistence of H₂O₂. The response covers the 0.1 to 500 μM glucose concentration range, and the limit of detection is 16 nM. The method was applied to the determination of H₂O₂ in rainwater samples. The glucose assay was used to analyze serum samples, and satisfactory results were obtained. Other attractive features include good chemical activity, low cost, easy storage, and high catalytic efficiency. Graphical abstract Schematic presentation of converting Prussian blue nanoparticles into Fe/C magnetic nanocubes by a pyrolysis technique and the use of glucose oxidase and Fe/C magnetic nanocubes to establish a one-step spectrophotometric method for the determination of glucose and hydrogen peroxide.

POMOF/SWNT Nanocomposites with Prominent Peroxidase-Mimicking Activity for l-Cysteine "On-Off Switch" Colorimetric Biosensing

In order to explore novel colorimetric biosensors with high sensibility and selectivity, two new Keggin polyoxometalates (POMs)-based Cu-trz (1,2,4-triazole) metal-organic frameworks (MOFs) with suitable specific surface areas and multiple active sites were favorably fabricated; then single-walled carbon nanotubes (SWNTs) were merged with new POMOFs to construct POMOF/SWNT nanocomposites. Herein, POMOF/SWNT nanocomposites as peroxidase mimics were explored for the first time, and the peroxidase-mimicking activity of the prepared POMOF/SWNT nanocomposites is heavily dependent on the mass ratio of POMOFs and SWNTs, in which the maximum activity is achieved at the mass ratio of 2.5:1 (named PMNT-2). More importantly, PMNT-2 exhibits the lowest limit of detection (0.103 μM) among all reported materials to date and the assumable selectivity toward l-cysteine (l-Cys) detection. With these findings, a convenient, sensitive, and effective "on-off switch" colorimetric platform for l-Cys detection has been successfully developed, providing a promising prospect in the biosensors and clinical diagnosis fields.

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes (II)

An updated comprehensive review to help researchers understand nanozymes better and in turn to advance the field.

Functional nanomaterials with unique enzyme-like characteristics for sensing applications

We highlight the recent developments in functional nanomaterials with unique enzyme-like characteristics for sensing applications.

Umbelliferone as a Small Molecular Peroxidase Mimic towards Sensitive Detection of H2O2 and Glucose

In this work, umbelliferone, a kind of coumarin derivative, was proved to exhibit peroxidase-like activity that could catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide to generate a blue-colored oxide (oxTMB). The catalytic mechanism is similar to that of native enzymes (e.g. horseradish peroxidase, HRP) and nanozymes, which follow the Michaelis-Menten kinetics behavior. Meanwhile, the 7-hydroxyl group of umbelliferone plays a significant role in the peroxidase-like activity. Compared with enzymes and nanozymes, this small molecular mimic enzyme possesses the advantages of low cost, simple molecular structures, small molecular weight and high stability against harsh conditions. Based on the favorable peroxidase mimetic activity of umbelliferone, a convenient, practical and sensitive H₂O₂ and glucose detection method was successfully established. This work not only opens some new inspirations into seeking for novel molecular enzyme mimetics with excellent catalytic activities, but also provides promising assays for clinical diagnosis.

Oxidase-mimicking activity of perovskite LaMnO3+δ nanofibers and their application for colorimetric sensing

Excellent oxidase-like mimics based on the ABX₃-type perovskite structure and the corresponding sensitive colorimetric detection of l-cysteine have been developed.