Biosensing platform for the detection of uric acid based on graphene quantum dots and G-quadruplex/hemin DNAzyme

Bioconjugation of nanozyme and natural enzyme to enable a one-step cascade reaction for the detection of metabolites

Nanozyme, with enzyme-mimicking activity and excellent stability, has attracted extensive attention. However, some inherent disadvantages, including poor dispersion, low selectivity, and insufficient peroxidase-like activity, still limit its further development. Therefore, an innovative bioconjugation of a nanozyme and natural enzyme was conducted. In the presence of graphene oxide (GO), histidine magnetic nanoparticles (H-Fe₃O₄) were first synthesized by a solvothermal method. The GO-supported H-Fe₃O₄ (GO@H-Fe₃O₄) exhibited superior dispersity and biocompatibility because GO was the carrier and possessed outstanding peroxidase-like activity because of the introduction of histidine. Furthermore, the mechanism of the peroxidase-like activity of GO@H-Fe₃O₄ was the generation of •OH. Uric acid oxidase (UAO) was selected as the model natural enzyme and covalently linked to GO@H-Fe₃O₄ with hydrophilic poly(ethylene glycol) as a linker. UAO could specifically catalyze the oxidation of uric acid (UA) to generate H₂O₂, and subsequently, the newly produced H₂O₂ oxidized the colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue ox-TMB under the catalysis of GO@H-Fe₃O₄. Based on the above cascade reaction, the GO@H-Fe₃O₄-linked UAO (GHFU) and GO@H-Fe₃O₄-linked ChOx (GHFC) were used for the detection of UA in serum samples and cholesterol (CS) in milk, respectively. The method based on GHFU exhibited a wide detection range (5-800 μM) and a low detection limit (1.5 μM) for UA, and the method based on GHFC exhibited a wide detection range (4-400 μM) and a low detection limit (1.13 μM) for CS. These results demonstrated that the proposed strategy had great potential in the field of clinical detection and food safety.

A new application of papain: As a peroxidase-like catalyst for fluorometric detection of uric acid

Papain, as a classical cysteine protease, has been widely used in the food, pharmaceutical, chemical, and cosmetic fields. However, there are few information about the peroxidase-like activity of papain catalyzed substrate to produce fluorescence. In this study, we found that papain can catalyze H₂O₂ to convert o-phenylenediamine (OPD), and generate fluorescence emission at 550 nm under 430 nm excitation. Based on this foundation, we report a papain/OPD/H₂O₂ system for fluorescence detection of uric acid. The method exhibits a wide linear range of 10-1000 μM with a limit-of-detection of 4.6 μM, and has been successfully used to detect uric acid in human serum. This study paves the way for the application of papain as catalyst for fluorescence detection of different target biomolecules, such as cholesterol, glucose, lactate, for which H₂O₂ is a product of oxidoreductase enzymes.

Enhanced Luminol Chemiluminescence with Oxidase-like Properties of FeOOH Nanorods for the Sensitive Detection of Uric Acid

FeOOH nanorods, as one-dimensional nanomaterials, have been widely used in many fields due to their stable properties, low cost, and easy synthesis, but their application in the field of chemiluminescence (CL) is rarely reported. In this work, FeOOH nanorods were synthesized by a simple and environmentally friendly one-pot hydrothermal method and used for the first time as a catalyst for generating strong CL with luminol without additional oxidant. Remarkably, luminol-FeOOH exhibits about 250 times stronger CL than the luminol-H₂O₂ system. Its CL intensity was significantly quenched by uric acid. We established a simple, rapid, sensitive, and selective CL method for the detection of uric acid with a linear range of 20-1000 nM and a detection limit of 6.3 nM (S/N = 3). In addition, we successfully applied this method to the detection of uric acid in human serum, and the standard recoveries were 95.6-106.4%.

Graphene quantum dots: A review on the effect of synthesis parameters and theranostic applications

The rising demand for early-stage diagnosis of diseases such as cancer, diabetes, neurodegenerative can be met with the development of materials offering high sensitivity and specificity. Graphene quantum dots (GQDs) have been investigated extensively for theranostic applications owing to their superior photostability and high aqueous dispersibility. These are attractive for a range of biomedical applications as their physicochemical and optoelectronic properties can be tuned precisely. However, many aspects of these properties remain to be explored. In the present review, we have discussed the effect of synthetic parameters upon their physicochemical characteristics relevant to bioimaging. We have highlighted the effect of particle properties upon sensing of biological molecules through 'turn-on' and 'turn-off' fluorescence and generation of electrochemical signals. After describing the effect of surface chemistry and solution pH on optical properties, an inclusive view on application of GQDs in drug delivery and radiation therapy has been given. Finally, a brief overview on their application in gene therapy has also been included.

Recent advances in flexible and wearable sensors for monitoring chemical molecules

In recent years, real-time health management has received increasing attention, benefiting from the rapid development of flexible and wearable devices. Conventionally, flexible and wearable devices are used for collecting health data such as electrophysiological signals, blood pressure, heart rate, etc. The monitoring of chemical factors has shown growing significance, providing the basis for the screening, diagnosis, and treatment of many diseases. Nowadays, in order to understand the health status of the human body more comprehensively and accurately, researchers in the community have started putting effort into developing wearable devices for monitoring chemical factors. Progressively, more flexible chemical sensors with wearable real-time health-monitoring functionality have been developed thanks to advances relating to wireless communications and flexible electronics. In this review, we describe the variety of chemical molecules and information that can currently be monitored, including pH levels, glucose, lactate, uric acid, ion levels, cytokines, nutrients, and other biomarkers. This review analyzes the pros and cons of the most advanced wearable chemical sensors in terms of wearability. At the end of this review, we discuss the current challenges and development trends relating to flexible and wearable chemical sensors from the aspects of materials, electrode designs, and soft-hard interface connections.

A novel 3D coordination polymer constructed by dual-ligand for highly sensitive detection of purine metabolite uric acid

Uric acid (UA), as the final product of purine metabolism, exists in urine and serum, which plays an important role in human metabolism, immunity and other functions. The sensitive, efficient, and rapid detection of UA has far-reaching significance in clinical diagnosis and disease prevention. Herein, a novel coordination polymer constructed by dual-ligand was successfully prepared, which exhibited excellent thermal and water stability. The polymer was interlaced by coordination bonds and hydrogen bonds to form an infinitely extended three-dimensional framework, which showed a rare and novel topological structure. The complex selectively recognized UA through significant fluorescence quenching response in the presence of various interferences. The excellent detection sensitivity (the limited detection of 1.2 μM), outstanding anti-interference ability and remarkable recyclability marked the complex to be a promising sensor material towards UA. In addition, the detection mechanism of UA by the complex was investigated in detail by combining density functional theory (DFT) and a variety of other analytical methods.

A novel metal organic gel with superior oxidase-like activity for efficient and sensitive chemiluminescence detection of uric acid

It is found that MIL-100(Fe) gels, as a kind of metal-organic gels (MOGs), constitutting of iron (Fe³⁺) and trimesic acid (H₃BTC), has been regarded as the efficient catalyst of luminol chemiluminescence (CL) system without the presence of extra oxidants in the present work. MIL-100(Fe) gels that have possessed mimicking oxidase-like activity can excellently enhanced luminol CL intensity by accelerating the generation of reactive oxygen species. Furthermore, with the addition of uric acid (UA), the CL signal has been dramatically inhibited under alkaline condition. Hence, the CL intensity inhibiting ratio (I₀/I_S) was proportional to the increasing concentration of UA in the rang from 10 nM to 4000 nM with the detection limit of 5.9 nM. This method has been successfully applied for analysis of UA with acceptable recoveries ranging from 97.0% to 107.9% in urine sample. These results indicates that this study open up a novel, sensitive and convenient method to detect UA in biological samples.

Recent advances in heteroatom-doped graphene quantum dots for sensing applications

Graphene quantum dots (GQDs) are carbon-based fluorescent nanomaterials having various applications due to attractive properties. But the low photoluminescence (PL) yield and monochromatic PL behavior of GQDs put limitations on their real-time applications. Therefore, heteroatom doping of GQDs is recognized as the best approach to modify the optical as well as electronic properties of GQDs by modifying their chemical composition and electronic structure. In this review, the new strategies for preparing the heteroatom (N, B, S, P) doped GQDs by using different precursors and methods are discussed in detail. The particle size, emission wavelength, PL emissive color, and quantum yield of recently developed heteroatom doped GQDs are reported in this article. The investigation of structure, crystalline nature, and composition of heteroatom doped GQDs by various characterization techniques such as high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) are also described. The recent progress on the impact of mono or co-doping of heteroatoms on PL behavior, and optical, electrochemiluminescence (ECL), and electrochemical properties of GQDs is also surveyed. Further, heteroatom doped GQDs with attractive properties used in sensing of various metal ions, biomolecules, small organic molecules, etc. by using various techniques with different limits of detection are also summarized. This review provides progressive trends in the development of heteroatom doped GQDs and their various applications.

Nucleic Acid Tests for Clinical Translation

Nucleic acids, including deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are natural biopolymers composed of nucleotides that store, transmit, and express genetic information. Overexpressed or underexpressed as well as mutated nucleic acids have been implicated in many diseases. Therefore, nucleic acid tests (NATs) are extremely important. Inspired by intracellular DNA replication and RNA transcription, in vitro NATs have been extensively developed to improve the detection specificity, sensitivity, and simplicity. The principles of NATs can be in general classified into three categories: nucleic acid hybridization, thermal-cycle or isothermal amplification, and signal amplification. Driven by pressing needs in clinical diagnosis and prevention of infectious diseases, NATs have evolved to be a rapidly advancing field. During the past ten years, an explosive increase of research interest in both basic research and clinical translation has been witnessed. In this review, we aim to provide comprehensive coverage of the progress to analyze nucleic acids, use nucleic acids as recognition probes, construct detection devices based on nucleic acids, and utilize nucleic acids in clinical diagnosis and other important fields. We also discuss the new frontiers in the field and the challenges to be addressed.

Investigation and improvement of catalytic activity of G-quadruplex/hemin DNAzymes using designed terminal G-tetrads with deoxyadenosine caps

It is generally acknowledged that G-quadruplexes (G4s) acquire peroxidase activity upon interaction with hemin. Hemin has been demonstrated to bind selectively to the 3′-terminal G-tetrad of parallel G4s via end-stacking; however, the relationships between different terminal G-tetrads and the catalytic functions of G4/hemin DNAzymes are not fully understood. Herein, the oligonucleotide d(AGGGGA) and its three analogues, d(AG^BrG^BrGGA), d(AG^BrGGG^BrA) and d(AG^BrGG^BrGA) (G^Br indicates 8-bromo-2′-deoxyguanosine), were designed. These oligonucleotides form three parallel G4s and one antiparallel G4 without loop regions. The scaffolds had terminal G-tetrads that were either anti-deoxyguanosines (anti-dGs) or syn-deoxyguanosines (syn-dGs) at different proportions. The results showed that the parallel G4 DNAzymes exhibited 2 to 5-fold higher peroxidase activities than the antiparallel G4 DNAzyme, which is due to the absence of the 3′-terminal G-tetrad in the antiparallel G4. Furthermore, the 3′-terminal G-tetrad consisting of four anti-dGs in parallel G4s was more energetically favorable and thus more preferable for hemin stacking compared with that consisting of four syn-dGs. We further investigated the influence of 3′ and 5′ deoxyadenosine (dA) caps on the enzymatic performance by adding 3′-3′ or 5′-5′ phosphodiester bonds to AG₄A. Our data demonstrated that 3′ dA caps are versatile residues in promoting the interaction of G4s with hemin. Thus, by increasing the number of 3′ dA caps, the DNAzyme of 3′A5′-5′GG3′-3′GG5′-5′A3′ with two 5′-terminal G-tetrads can exhibit significantly high catalytic activity, which is comparable to that of 5′A3′-3′GG5′-5′GG3′-3′A5′ with two 3′-terminal G-tetrads. This study may provide insights into the catalytic mechanism of G4-based DNAzymes and strategies for promoting their catalytic activities.

Investigation of the peroxidase activities of G4/hemin DNAzymes using designed terminal G-tetrads by eliminating the steric effect of loop regions.

Applications of Graphene Quantum Dots in Biomedical Sensors

Due to the proliferative cancer rates, cardiovascular diseases, neurodegenerative disorders, autoimmune diseases and a plethora of infections across the globe, it is essential to introduce strategies that can rapidly and specifically detect the ultralow concentrations of relevant biomarkers, pathogens, toxins and pharmaceuticals in biological matrices. Considering these pathophysiologies, various research works have become necessary to fabricate biosensors for their early diagnosis and treatment, using nanomaterials like quantum dots (QDs). These nanomaterials effectively ameliorate the sensor performance with respect to their reproducibility, selectivity as well as sensitivity. In particular, graphene quantum dots (GQDs), which are ideally graphene fragments of nanometer size, constitute discrete features such as acting as attractive fluorophores and excellent electro-catalysts owing to their photo-stability, water-solubility, biocompatibility, non-toxicity and lucrativeness that make them favorable candidates for a wide range of novel biomedical applications. Herein, we reviewed about 300 biomedical studies reported over the last five years which entail the state of art as well as some pioneering ideas with respect to the prominent role of GQDs, especially in the development of optical, electrochemical and photoelectrochemical biosensors. Additionally, we outline the ideal properties of GQDs, their eclectic methods of synthesis, and the general principle behind several biosensing techniques.

A novel and sensitive fluorescent assay for artemisinin with graphene quantum dots based on inner filter effect

The authors describe a novel method for the determination of artemisinin (ART) by using graphene quantum dots (GQDs) as the fluorescent probes. This method is based on the fact that ART can react with p-aminophenylboronic acid (p-ABA) to produce p-aminophenol (p-AP). While in the presence of tyrosinase (TYR), p-AP can be oxidized into 4-amino-1,2-benzoquinone, which effectively quenched the fluorescence of GQDs due to the inner filter effect (IFE). By making use of these reactions, a novel and sensitive fluorescent assay for ART has been developed. The calibration curve for the determination of ART is linear in the range of 0.1-5 μM and 5-55 μM with the detection limit of 33 nM, which is more sensitive than most of other methods. Some common coexisting substances including Ca²⁺, Na⁺, Mg²⁺, K⁺, PO₄^3-, starch, lactose, dextrin, and magnesium stearat have negligible effects on the fluorescence intensity of GQDs-TYR-p-ABA system. Finally, the sensing system was successfully applied to the detection of the compound naphthoquine phosphate tablet samples with satisfactory recoveries. This IFE-based GQDs fluorescence sensing strategy is facile and sensitive for the determination of ART because neither the surface modification nor the linking between the receptor and the fluorophore is required.

Modification of nanocrystalline TiO2 coatings with molecularly imprinted TiO2 for uric acid recognition

Combining the surface modification and molecular imprinting technique, a novel piezoelectric sensing platform with excellent molecular recognition capability was established for the detection of uric acid (UA) based on the immobilization of TiO₂ nanoparticles onto quartz crystal microbalance (QCM) electrode and modification of molecularly imprinted TiO₂ (MIT) layer on TiO₂ nanoparticles. The performance of the fabricated biosensor was evaluated, and the results indicated that the biosensor exhibited high sensitivity in UA detection, with a linear range from 0.04 to 45 μM and a limit of detection of 0.01 μM. Moreover, the biosensor presented high selectivity towards UA in comparison with other interferents. The analytical application of the UA biosensor confirmed the feasibility of UA detection in urine sample.

Harnessing a previously unidentified capability of bacterial allosteric transcription factors for sensing diverse small molecules in vitro

A plethora of bacterial allosteric transcription factors (aTFs) have been identified to sense a variety of small molecules. Introduction of a novel aTF-based approach to sense diverse small molecules in vitro will signify a broad series of detection applications. Here, we found that aTFs could interact with their nicked DNA binding sites. Building from this new finding, we designed and implemented a novel aTF-based nicked DNA template-assisted signal transduction system (aTF-NAST) by using the competition between aTFs and T4 DNA ligase to bind to the nicked DNA. This aTF-NAST could reliably and modularly transduce the signal of small molecules recognized by aTFs to the ligated DNA signal, thus enabling the small molecules to be measured via various mature and robust DNA detection methods. Coupling this aTF-NAST with three DNA detection methods, we demonstrated nine novel biosensors for the detection of an antiseptic 4-hydroxybenzoic acid, a disease marker uric acid and an antibiotic tetracycline. These biosensors show impressive sensitivity and robustness in real-life analysis, highlighting the great potential of our aTF-NAST for biosensing applications.

Enzymatic determination of uric acid using water-soluble CuInS/ZnS quantum dots as a fluorescent probe

Glutathione-capped water-soluble CuInS/ZnS quantum dots (QDs) were prepared by a microwave-assisted method. Their fluorescence, with excitation/emission peaks at 380/570 nm, is found to be quenched by hydrogen peroxide (H₂O₂) that is produced by the uricase catalyzed oxidation of uric acid (UA) and oxygen. The findings are used in a quenchometric method for the determination of UA. The effects of different ligands on the QDs, of pH value, buffers, enzyme ratio and reaction time were optimized. The detection limit for UA is 50 nM which is lower than other QD-based method, and the detection ranges extends from 0.25-4.0 μM. The assay is simple and sensitive, and no further modification of the QDs is required. Graphical abstract ᅟ.

Graphene Quantum Dots and Enzyme-Coupled Biosensor for Highly Sensitive Determination of Hydrogen Peroxide and Glucose

In this paper, a simple and specific graphene quantum dots (GQDs)-based fluorescent biosensor adopted for the determination of glucose based on the combination of the enzyme-coupled method and fluorescence quenching mechanism is demonstrated. Glucose was oxidized by the enzyme glucose oxidase (GOx), forming hydrogen peroxide (H 2 O 2 ) via the catalysis by horseradish peroxidase (HRP). H 2 O 2 was then employed to oxidize phenol to quinone, which led to effective quenching effect in the GQDs⁻GOx⁻HRP⁻phenol system. By optimizing the reaction conditions of the GQDs-enzyme system, a linear relationship between the concentration of glucose and the fluorescence intensity over a range of 0.2⁻10 μ mol/L was obtained. The limit of detection for glucose is 0.08 μ mol/L. The present biosensor for the determination of glucose showed satisfactory reproducibility and accuracy in human serum samples. Since the enzymes have high specificity and unique affinity to the certain substance, the enzyme-coupled system promises a sensitive way for further detection of those chemicals which could be oxidized by enzymes and generated H 2 O 2 or glucose. GQDs and other fluorescent materials coupled with several enzymes can be applied to extensive sensing field.

Anomalous fluorescence enhancement and fluorescence quenching of graphene quantum dots by single walled carbon nanotubes

We explore the mechanism of the fluorescence enhancement and fluorescence quenching effect of single walled carbon nanotubes (SWCNTs) on highly fluorescent graphene quantum dots (GQDs) over a wide range of concentrations of SWCNTs. At very low concentrations of SWCNTs, the fluorescence intensity of the GQDs is enhanced, while at higher concentrations, systematic quenching of fluorescence is observed. The nature of the Stern-Volmer plot for the latter case was found to be non-linear indicating a combined effect of dynamic and static quenching. The contribution of the dynamic quenching component was assessed through the fluorescence lifetime measurements. The contribution of static quenching is confirmed from the red shift of the fluorescence spectra of the GQDs after addition of SWCNTs. The fluorescence intensity is first enhanced at very low concentration due to improved dispersion and higher absorption by GQDs, while at higher concentration, the fluorescence of GQDs is quenched due to the complex formation and associated reduction of the radiative sites of the GQDs, which is confirmed from time-resolved fluorescence measurements. Laser confocal microscopy imaging provides direct evidence of the enhancement and quenching of fluorescence at low and high concentrations of SWCNTs, respectively. This study provides an important insight into tuning the fluorescence of GQDs and understanding the interaction between GQDs and different CNTs, which is important for bio-imaging and drug delivery applications.

A bimetallic nanoparticle/graphene oxide/thionine composite-modified glassy carbon electrode used as a facile ratiometric electrochemical sensor for sensitive uric acid determination

A novel and facile ratiometric electrochemical sensor was developed for sensitive determination of uric acid.

Ultrasensitive MicroRNA Assay via Surface Plasmon Resonance Responses of Au@Ag Nanorods Etching

Quantification of trace serum circulate microRNAs is extremely important in clinical diagnosis but remains a great challenge. Herein we developed an ultrasensitive platform for microRNA 141 (miR-141) detection based on a silver coated gold nanorods (Au@Ag NRs) etching process accompanied by surface plasmon resonance (SPR) shift. Both SPR absorption and scattering responses were monitored. Combined amplification cascades of catalyzed hairpin assembly (CHA) and hybridization chain reaction (HCR) with the sensitive SPR responses of plasmonic Au@Ag NRs, the proposed bioassay exhibited ultrahigh sensitivity toward miRNA-141 with dynamic range from 5.0 × 10^-17 M to 1.0 × 10^-11 M. With target concentration higher than 1.0 × 10^-13 M, the color of the solution changed obviously that could be observed with naked eyes. Under dark-field microscopy observation of individual particle, a limit of detection down to 50 aM could be achieved. Owing to the superior sensitivity and selectivity, the proposed method was applied to detecting trace microRNA in serum. Similar SPR assays could be developed simply by redesigning the switching aptamer for the detections of other microRNAs or targets such as small molecule, DNA, or protein. Considering the convenient operation, good performance and simple observation modes of this method, it may have great potential in trace bioanalysis for clinical applications.