Orotic acid-capped Tb(III)-doped calcium sulphate nanorods for the selective detection of tryptophan

Lanthanide-based luminescent materials have gained huge attention due to their applications in optoelectronic devices, sensing, bio-imaging, anti-counterfeiting, and more. In this work, we report a luminescence-based sensor for the detection of tryptophan using orotic acid-capped Tb3+-doped CaSO4 nanorods (NRs). Orotic acid (OA) was found to play a dual role as a capping agent to control the growth of the nanorods and as a sensitizer for Tb3+ ions. The resulting nanorods exhibited excellent dispersibility and strong photoluminescence signals characteristic of Tb3+ ions in the visible region. Nearly 10-fold enhancement in the emission intensity was noted through OA sensitization compared to direct excitation of Tb3+ ions (acceptors). Interestingly, the strong emission intensity of the NRs reduced significantly with the addition of tryptophan. In contrast, hardly any change was noted with the addition of other amino acids and metal ions, suggesting greater selectivity for tryptophan. Moreover, there is barely any notable interference from other amino acids toward the detection of tryptophan. The limit of detection is found to be ∼0.61 μM. Finally, the sensing study was extended to biological samples to detect tryptophan present in blood plasma, urine, and saliva samples. The nanorods demonstrated high detection abilities, indicating the potential of the developed materials for biomedical applications.

A Multifunctional Hydrogel for Simultaneous Visible H2 O2 Monitoring And Accelerating Diabetic Wound Healing

Diabetic wound is one of the chronic wounds that is difficult to heal, and effective treatment of it still confronts a great challenge. Monitoring the variation of diabetic wound microenvironment (such as hydrogen peroxide (H2 O2 )) can understand the wound state and guide the wound management. Herein, a multifunctional hydrogel with the abilities of monitoring the H2 O2 concentration, alleviating oxidative stress and promoting wound healing is developed, which is prepared by encapsulating manganese-containing bioactive glass (MnBG) and CePO4 :Tb in biocompatible gelatin methacryloyl (GelMA) hydrogel (CPT-MnBG-Gel). On the one hand, the H2 O2 -dependent fluorescence quenching effect of the CePO4 :Tb contributes to visible monitoring of the H2 O2 concentration of wounds via smartphone imaging, and the CPT-MnBG-Gel hydrogel can effectively monitor the H2 O2 level of 10.35-200 µmol L-1 . On the other hand, MnBG can alleviate oxidative stress and promote the proliferation, migration and differentiation of fibroblasts and endothelial cells in vitro owing to the bioactive Mn and Si ions, and in vivo evaluation also demonstrates that the CPT-MnBG-Gel hydrogels can effectively accelerate wound healing. Hence, such multifunctional hydrogel is promising for diabetic wound management and accelerating wound healing.

The use of a chemiluminescence in the assessment of the nanomaterials antioxidant activity

Nanomaterials are one of the most promising classes of advanced materials with fine-tuned biological activities. This is evidenced by the presence of redox activity of a number of nanoparticles aimed at inhibiting free radicals and/or mimicking the functions of enzymes. At the same time, it is impossible to study the expression of these biological properties without the use of well-standardized, representative techniques that provide availability, high precision, sensitivity, and selectivity of the measured characteristics. A method that satisfies these requirements is chemiluminescence analysis, which is widely used both in clinical analysis and to characterize the antioxidant activity of substances of natural or synthetic origin. Recently, a trend of using chemiluminescence analysis to study the biological activity of nanomaterials has appeared as a suitable alternative to spectroscopic and electrochemical techniques. This review briefly describes the examples of successful applications of chemiluminescence methods to study radical-binding and enzyme-like activities of nanomaterials. We discuss the data about the effect of the used reagents (radical-generating systems, chemiluminescence activators) and experimental conditions on the obtained values characterizing the nanomaterials activity.

Graphical Abstract

Auto-catalytic redox polymerisation using nanoceria and glucose oxidase for double network hydrogels

A novel auto-catalytic reaction that utilizes both the redox properties of nanoceria and oxidoreductase properties of glucose oxidase to graft polymers on the surface of nanoceria in an open vessel to form double network hydrogel nanocomposites.

Design of Lanthanide-Doped Colloidal Nanocrystals: Applications as Phosphors, Sensors, and Photocatalysts

The unique optical characteristics of lanthanides (Ln³⁺) such as high color purity, long excited-state lifetimes, less perturbation of excited states by the crystal field environment, and the easy spectral conversion of wavelengths through upconversion and downconversion processes have caught the attention of many scientists in the recent past. To broaden the scope of using these properties, it is important to make suitable Ln³⁺-doped materials, particularly in colloidal forms. In this feature article, we discuss the different synthesis strategies for making Ln³⁺-doped nanoparticles in colloidal forms, particularly ways of functionalizing hydrophobic surfaces to hydrophilic surfaces to enhance their dispersibility and luminescence in aqueous media. We have enumerated the various strategies and sensitizers utilized to increase the luminescence of the nanoparticles. Furthermore, the use of these colloidal nanoparticle systems in sensing application by the appropriate selection of capping ligands has been discussed. In addition, we have shown how the energy transfer efficiency from Ce³⁺ to Ln³⁺ ions can be utilized for the detection of toxic metal ions and small molecules. Finally, we discuss examples where the spectral conversion ability of these materials has been used in photocatalysis and solar cell applications.

Luminescent CeO2:Eu3+ nanocrystals for robust in situ H2O2 real-time detection in bacterial cell cultures

Hydrogen peroxide (H2O2) quantification in biomedicine is valuable as inflammation biomarker but also in assays employing enzymes that generate or consume H2O2 linked to a specific biomarker. Optical H2O2 detection is typically performed through peroxidase-coupled reactions utilizing organic dyes that suffer, however, from poor stability/reproducibility and also cannot be employed in situ in dynamic complex cell cultures to monitor H2O2 levels in real-time. Here, we utilize enzyme-mimetic CeO2 nanocrystals that are sensitive to H2O2 and study the effect of H2O2 presence on their electronic and luminescent properties. We produce and dope with Eu3+ these particles in a single-step by flame synthesis and directly deposit them on Si and glass substrates to fabricate nanoparticle layers to monitor in real-time and in situ the H2O2 concentrations generated by Streptococcus pneumoniae clinical isolates. Furthermore, the small CeO2:Eu3+ nanocrystals are combined in a single-step with larger, non-responsive Y2O3:Tb3+ nanoparticles during their double-nozzle flame synthesis to engineer hybrid luminescent nanoaggregates as ratiometric robust biosensors. We demonstrate the functionality of these biosensors by monitoring their response in the presence of a broad range of H2O2 concentrations in vitro from S. pneumoniae, highlighting their potential for facile real-time H2O2 detection in vitro in cell cultures.

Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications

Because of the high catalytic activities and substrate specificity, natural enzymes have been widely used in industrial, medical, and biological fields, etc. Although promising, they often suffer from intrinsic shortcomings such as high cost, low operational stability, and difficulties of recycling. To overcome these shortcomings, researchers have been devoted to the exploration of artificial enzyme mimics for a long time. Since the discovery of ferromagnetic nanoparticles with intrinsic horseradish peroxidase-like activity in 2007, a large amount of studies on nanozymes have been constantly emerging in the next decade. Nanozymes are one kind of nanomaterials with enzymatic catalytic properties. Compared with natural enzymes, nanozymes have the advantages such as low cost, high stability and durability, which have been widely used in industrial, medical, and biological fields. A thorough understanding of the possible catalytic mechanisms will contribute to the development of novel and high-efficient nanozymes, and the rational regulations of the activities of nanozymes are of great significance. In this review, we systematically introduce the classification, catalytic mechanism, activity regulation as well as recent research progress of nanozymes in the field of biosensing, environmental protection, and disease treatments, etc. in the past years. We also propose the current challenges of nanozymes as well as their future research focus. We anticipate this review may be of significance for the field to understand the properties of nanozymes and the development of novel nanomaterials with enzyme mimicking activities.

A MnO2–[Ru(dpp)3]Cl2 system for colorimetric and fluorimetric dual-readout detection of H2O2

Two-dimensional (2D) MnO₂ nanosheets were synthesized by a template-free and one-step route, and the dye [Ru(dpp)₃]Cl₂ was linked onto the MnO₂ nanosheet surface via electrostatic interaction. The formed MnO₂–[Ru(dpp)₃]Cl₂ hybrid was used for a dual optical detection for H₂O₂, an important reactive oxygen species (ROS). Upon addition of H₂O₂, the reaction of MnO₂ with H₂O₂ results in the dissolution of MnO₂ nanosheets and simultaneous generation of O₂. The fading of the solution and simultaneous fluorescence change of [Ru(dpp)₃]Cl₂, sensitive to O₂, enables colorimetric and fluorimetric dual-mode detection of H₂O₂. The dual-output assay in a single probe provides a good sensitivity with a detection limit of 0.18 μM H₂O₂. The dual-signal strategy can efficiently overcome the shortcoming of the single detection mode, and improve the detection accuracy by an additional correction of output signals from each other. Moreover, the successful determination of H₂O₂ in the serum samples demonstrates the potential applicability of the MnO₂–[Ru(dpp)₃]Cl₂ based probe in biosensing and bioanalysis.

The MnO₂ nanosheets with anchored [Ru(dpp)₃]Cl₂ were prepared for colorimetric and fluorimetric dual-mode detection of H₂O₂.

Human serum albumin templated MnO2 nanosheets are oxidase mimics for colorimetric determination of hydrogen peroxide and for enzymatic determination of glucose

This paper reports on a colorimetric assay for H₂O₂ and glucose. It is based on the use of human serum albumin-templated MnO₂ nanosheets that possess oxidase-like activity. They are capable of oxidizing 3,3',5,5'-tetramethylbenzidine (TMB) with oxygen to give a blue product (oxTMB) with an absorbance maximum at 652 nm. When H₂O₂ is introduced, the MnO₂ nanosheets are reduced to Mn(II) ions, and this inhibits the formation of oxTMB. Based on these findings, a colorimetric assay was established for H₂O₂ that has a 0.56 μM detection limit. If glucose is oxidized by glucose oxidase under formation of H₂O₂, the nanosheets can be used to quantify H₂O₂ and thereby to sense glucose. Response is linear in the 0.5 μM to 50 μM glucose concentration range, and the detection limit is 0.32 μM. The method was applied to the determination of glucose in spiked serum samples and gave satisficatory results. Graphical abstract Human serum albumin (HSA) is used as a template for the synthesis of MnO₂ nanosheet. These possess oxidase mimicking activity. H₂O₂ can reduce the nanosheets. The effect is exploited in colorimetric assays for H₂O₂ and glucose using tetramethylbenzidine (TMB) as a chromogenic substrate.

An enzymatic reaction mediated glucose sensor activated by MnO2 nanosheets acting as an oxidant and catalyst

A self-regulated smart system would be highly desirable for analyte detection, in which a specific environment for detection could be self-modulated and the required reagents could also be in situ generated without further addition.

Enzyme-Mimetic Antioxidant Luminescent Nanoparticles for Highly Sensitive Hydrogen Peroxide Biosensing

Hydrogen peroxide (H₂O₂) is an abundant molecule associated with biological functions and reacts with natural enzymes, such as catalase. Even though direct H₂O₂ measurement can be used to diagnose pathological conditions, such as infection and inflammation, H₂O₂ quantification further enables the detection of disease biomarkers in enzyme-linked assays (e.g., ELISA) in which enzymatic reactions may generate or consume H₂O₂. Such a quantification is often measured optically with organic dyes in biological media that suffer, however, from poor stability. Currently, the optical H₂O₂ biosensing without organic-dyes in biological media and at low, submicromolar, concentrations has yet to be achieved. Herein, we rationally design biomimetic artificial enzymes based on antioxidant CeO₂ nanoparticles that become luminescent upon their Eu³⁺ doping. We vary systematically their diameter from 4 to 16 nm and study their catalase-mimetic antioxidant activity, manifested as catalytic H₂O₂ decomposition in aqueous solutions, revealing a strong nanoparticle surface area dependency. The interaction with H₂O₂ influences distinctly the particle luminescence rendering them highly sensitive H₂O₂ biosensors down to 0.15 μM (5.2 ppb) in solutions for biological assays. Our results link two, so far, unrelated research domains, the CeO₂ nanoparticle antioxidant activity and luminescence by rare-earth doping. When these enzyme-mimetic nanoparticles are coupled with alcohol oxidase, biosensing can be extended to ethanol exemplifying how their detection potential can be broadened to additional biologically relevant metabolites. The enzyme-mimetic nanomaterial developed here could serve as a starting point of sophisticated in vitro assays toward the highly sensitive detection of disease biomarkers.

Electrostatic Assembly of Platinum Nanoparticles along Electrospun Polymeric Nanofibers for High Performance Electrochemical Sensors

A novel polyacrylonitrile (PAN) nanofibrous membrane conjugated with platinum nanoparticles (PtNPs) was fabricated by electrospinning and electrostatic assembly techniques. In this procedure, PAN was electrospun with 3-aminopropyltriethoxysilane (APS) together as precursor materials. First, amine groups were introduced onto PAN nanofibers, and then the as-prepared negative-charged platinum nanoparticles (PtNPs) were conjugated onto the surface of the amino-modified PAN nanofibers uniformly by the electrostatic interaction-mediated assembly. The fabricated PAN-PtNPs hybrid nanofibrous membrane was further utilized to modify the glassy carbon electrodes and was used for the fabrication of a non-enzymatic amperometric sensor to detect hydrogen peroxide (H₂O₂). The electrochemical results indicated that, due to the uniform dispersion of PtNPs and the electrostatic interaction between amine groups and PtNPs, the fabricated PAN-PtNPs nanofibrous membrane-based electrochemical sensor showed excellent electrocatalytic activity toward H₂O₂, and the chronoamperometry measurements illustrated that the fabricated sensor had a high sensitivity for detecting H₂O₂. It is anticipated that the strategies used in this work will not only guide the design and fabrication of functional polymeric nanofiber-based biomaterials and nanodevices, but also extend their potential applications in energy storage, cytology, and tissue engineering.

Fabrication of Ag-Cu2O/Reduced Graphene Oxide Nanocomposites as Surface-Enhanced Raman Scattering Substrates for in Situ Monitoring of Peroxidase-Like Catalytic Reaction and Biosensing

Highly sensitive biosensors are essential in medical diagnostics, especially for monitoring the state of an individual's disease. An ideal way to achieve this objective is to analyze human sweat secretions by noninvasive monitoring. Due to low concentrations of target analytes in human secretions, fabrication of ultrasensitive detection devices is a great challenge. In this work, Ag-Cu₂O/reduced graphene oxide (rGO) nanocomposites were prepared by a facile two-step in situ reduction procedure at room temperature. Ag-Cu₂O/rGO nanocomposites possess intrinsic peroxidase-like activity and rapidly catalyze oxidation of the peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂. On the basis of the excellent SERS properties and high peroxidase-like activity of the Ag-Cu₂O/rGO nanocomposites, the catalytic oxidation of TMB can be monitored by SERS. This approach can detect H₂O₂ and glucose with high sensitivity and distinguish between diabetic and normal individuals using glucose levels in fingerprints. Our work provides direction for designing other SERS substrates with high catalytic activity and the potential for application in biosensing, forensic investigation, and medical diagnostics.

Ag@Au nanoprism-metal organic framework-based paper for extending the glucose sensing range in human serum and urine

Ag@Au nanoprism-MOFs-based paper for enhancing the glucose sensing range in human serum and in urine.

Nitrogen-Doped Carbon Dots as A New Substrate for Sensitive Glucose Determination

Nitrogen-doped carbon dots are introduced as a novel substrate suitable for enzyme immobilization in electrochemical detection metods. Nitrogen-doped carbon dots are easily synthesised from polyacrylamide in just one step. With the help of the amino group on chitosan, glucose oxidase is immobilized on nitrogen-doped carbon dots-modified carbon glassy electrodes by amino-carboxyl reactions. The nitrogen-induced charge delocalization at nitrogen-doped carbon dots can enhance the electrocatalytic activity toward the reduction of O₂. The specific amino-carboxyl reaction provides strong and stable immobilization of GOx on electrodes. The developed biosensor responds efficiently to the presence of glucose in serum samples over the concentration range from 1 to 12 mM with a detection limit of 0.25 mM. This novel biosensor has good reproducibility and stability, and is highly selective for glucose determination under physiological conditions. These results indicate that N-doped quantum dots represent a novel candidate material for the construction of electrochemical biosensors.

Synthesis of EDTA-assisted CeVO4 nanorods as robust peroxidase mimics towards colorimetric detection of H2O2

CeVO₄ nanorods were developed as novel, simple and highly sensitive biomimetic catalysts and used for colorimetric detection of H₂O₂.

Cu2O/CuO@rGO heterostructure derived from metal–organic-frameworks as an advanced electrocatalyst for non-enzymatic electrochemical H2O2 sensor

A hybrid heterostructure comprising well-dispersed Cu₂O/CuO particles and reduced graphene oxide (rGO) is synthesized by calcinating a mixture of MOFs-118 and GO in nitrogen atmosphere to improve the sensitivity and selectivity of H₂O₂ sensors.

Nanoporous Carbon Nanofibers Decorated with Platinum Nanoparticles for Non-Enzymatic Electrochemical Sensing of H₂O₂

We describe the preparation of nanoporous carbon nanofibers (CNFs) decorated with platinum nanoparticles (PtNPs) in this work by electrospining polyacrylonitrile (PAN) nanofibers and subsequent carbonization and binding of PtNPs. The fabricated nanoporous CNF-PtNP hybrids were further utilized to modify glass carbon electrodes and used for the non-enzymatic amperometric biosensor for the highly sensitive detection of hydrogen peroxide (H₂O₂). The morphologies of the fabricated nanoporous CNF-PtNP hybrids were observed by scanning electron microscopy, transmission electron microscopy, and their structure was further investigated with Brunauer-Emmett-Teller (BET) surface area analysis, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectrum. The cyclic voltammetry experiments indicate that CNF-PtNP modified electrodes have high electrocatalytic activity toward H₂O₂ and the chronoamperometry measurements illustrate that the fabricated biosensor has a high sensitivity for detecting H₂O₂. We anticipate that the strategies utilized in this work will not only guide the further design and fabrication of functional nanofiber-based biomaterials and nanodevices, but also extend the potential applications in energy storage, cytology, and tissue engineering.

Surface engineering on CeO₂ nanorods by chemical redox etching and their enhanced catalytic activity for CO oxidation

Controllable surface properties of nanocerias are desired for various catalytic processes. There is a lack of efficient approaches to adjust the surface properties of ceria to date. Herein, a redox chemical etching method was developed to controllably engineer the surface properties of ceria nanorods. Ascorbic acid and hydrogen peroxide were used to perform the redox chemical etching process, resulting in a rough surface and/or pores on the surface of ceria nanorods. Increasing the etching cycles induced a steady increase of the specific surface area, oxygen vacancies and surface Ce(3+) fractions. As a result, the etched nanorods delivered enhanced catalytic activity for CO oxidation, compared to the non-etched ceria nanorods. Our method provides a novel and facile approach to continuously adjust the surface properties of ceria for practical applications.

Fast synthesis of fluorescent SiO2@CdTe nanoparticles with reusability in detection of H2O2

In this study, highly fluorescent core/shell SiO₂@CdTe nanoparticles (NPs) were synthesized conveniently and efficiently via a hydrothermal method. The as-prepared SiO₂@CdTe NPs were uniform with good fluorescence preservation. The SiO₂@CdTe NPs could be used for the rapid detection of H₂O₂ with good sensitivity within several minutes. Excellent linear relationships existed between the quenching degrees of the SiO₂@CdTe NPs and the concentration of H₂O₂ in the range of 0.005 mM to 0.1 mM. The limit of detection (LOD) for H₂O₂ was 10 nM. Furthermore, it was proved that SiO₂@CdTe NPs could be used repeatedly for H₂O₂ detection due to their easy separation, which is an important feature. The excellent performance of SiO₂@CdTe NPs should facilitate their applications in chemistry or biology for detection of H₂O₂.

Time-resolved luminescent biosensing based on inorganic lanthanide-doped nanoprobes

In this feature article, we review the latest advancements in lanthanide-doped luminescent nanocrystals as time-resolved luminescent nano-bioprobes, from their fundamental optical properties to their potential applications for ultrasensitive biodetection and high-resolution bioimaging.

Functionalized lanthanide coordination polymer nanoparticles for selective sensing of hydrogen peroxide in biological fluids

The fluorescence of Phe/Tb CPNPs functionalized with CPBA (Phe/Tb-CPBA CPNPs) was selectively quenched upon the addition of H₂O₂.