One-Step Synthesis of Rox-DNA Functionalized CdZnTeS Quantum Dots for the Visual Detection of Hydrogen Peroxide and Blood Glucose

Fe-codoped carbon dots serving as a peroxidase mimic to generate in situ hydrogen peroxide for the visual detection of glucose

Nanozyme technology has gained significant regard and been successfully implemented in various applications including chemical sensing, bio-medicine, and environmental monitoring. Fe-CDs were synthesized and characterized well in this study. As compared to HRP (3.7 mM), the Fe-CDs exhibited a higher affinity towards H2O2 (0.2 mM) using the steady-state kinetic assay and stronger catalytic capability by changing the color of TMB to the blue color of the oxidized state, oxTMB. Additionally, an efficient peroxidase mimic, Fe-CDs/GOx, based on the hybrid cascade system to produce in situ H2O2 for the visual detection of glucose (color change: colorless to blue, and then to green), has been developed in detail, with limits of detection (LODs) for H2O2 and glucose of 0.33 μM and 1.17 μM, respectively. The changes further demonstrate a linear relationship between absorbance and H2O2 concentration, ranging from 10 to 60 μM, and for glucose (1 to 60 μM). To assess the accuracy and detection capability of the Fe-CDs/GOx system, we evaluated a real human serum sample obtained from adult males in a local hospital. In conclusion, Fe-CDs serving as a peroxidase mimic have the potential for various applications in the fields of biomedicine and nanozymes.

Disubstituted thiourea as a suitable sulfur source in the gram-scale synthesis of yellow- and red-emitting CdTeS/Cd x Zn1-x S core/shell quantum dots

The key parameters of semiconductor quantum dots (QDs) that determine the suitability and efficiency for the design of most optoelectronic devices are the spectral positions of absorbance (ABS) and photoluminescence (PL) maxima, Stokes shift, photoluminescence quantum yield (PL QY) and photoluminescence lifetime (PL LT). All these parameters have been considered in the design of new ternary core CdTeS and core/shell CdTeS/Cd x Zn1-x S QDs. One-pot synthesis conducted in an organic medium at 160 °C using substituted thioureas as new, highly reactive sulfur sources allowed for the formation of a series of size- and emission-tunable CdTe0.05S0.95 QDs. Gram-scale synthesis of yellow-red emitting CdTe0.06S0.94 and CdTe0.12S0.88 cores was performed through the manipulation of their precursor ratio for the controllable formation of CdTeS/Cd x Zn1-x S (x = 0.1, 0.2, and 0.3) core/shell QDs. The development of the designed nanomaterials was carried out with a special emphasis on their optical properties, in particular a high PL QY up to 87% and extremely large Stokes shift, reaching ≈280 nm for core/shell QDs. Promisingly, for biolabeling and diagnostics, the synthesized core/shell QDs were transferred into water via surface ligand modification with the expected loss of photoluminescence efficiency. The results indicated that the availability of initial components, high yield of the desired product, stability in the organic phase, and high optical characteristics can scale up the synthesis of the developed nanomaterials from the laboratory level to industrial production.

Target-triggered enzyme-free amplification for highly efficient AND-gated bioimaging in living cells

Rapid, simultaneous, and sensitive detection of biomolecules has important application prospects in disease diagnosis and biomedical research. However, because the content of intracellular endogenous target biomolecules is usually very low, traditional detection methods can't be used for effective detection and imaging, and to enhance the detection sensitivity, signal amplification strategies are frequently required. The hybridization chain reaction (HCR) has been used to detect many disease biomarkers because of its simple operation, good reproducibility, and no enzyme involvement. Although HCR signal amplification methods have been employed to detect and image intracellular biomolecules, there are still false positive signals. Therefore, a target-triggered enzyme-free amplification system (GHCR system) was developed, as a fluorescent AND-gated sensing platform for intracellular target probing. The false positive signals can be well avoided and the accuracy of detection and imaging can be improved by using the design of the AND gate. Two cancer markers, GSH and miR-1246, were used as two orthogonal inputs for the AND gated probe. The AND-gated probe only works when GSH and miR-1246 are the inputs at the same time, and FRET signals can be the output. In addition to the use of AND-gated imaging, FRET-based high-precision ratiometric fluorescence imaging was employed. FRET-based ratiometric fluorescent probes have a higher ability to resist interference from the intracellular environment, they can avoid false positive signals well, and they are expected to have good specificity. Due to the advantages of HCR, AND-gated, and FRET fluorescent probes, the GHCR system exhibited highly efficient AND-gated FRET bioimaging for intracellular endogenous miRNAs with a lower detection limit of 18 pM, which benefits the applications of ratiometric intracellular biosensing and bioimaging and offers a novel concept for advancing the diagnosis and therapeutic strategies in the field of cancer.

Target-triggered 'colorimetric-fluorescence' dual-signal sensing system based on the versatility of MnO2 nanosheets for rapid detection of uric acid

A simple dual-signal assay that combined colorimetric and fluorometric strategy for uric acid (UA) rapid detection was designed based on the versatility of facile synthesized MnO2 nanosheet. The oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) and the fluorescence quenching of quantum dots (QDs) occurred simultaneously in the presence of MnO2 nanosheet. UA could decompose MnO2 nanosheet into Mn2+, resulting in the fluorescence recovery of QDs, along with the fading of the blue color of ox TMB. Based on the principles above, the detection of UA could be realized by the change of the dual signals (colorimetric and fluorometric). The linear range of the colorimetric mode was 5-60 μmol L-1, and the limit of detection (LOD) was 2.65 μmol L-1; the linear range of the fluorescence mode was wide at 5-120 μmol L-1, and the LOD could be as low as 1.33 μmol L-1. The method was successfully used for analyzing UA levels in human serum samples, indicating that this new dual-signal method could be applied in clinical diagnosis.

Non-invasive detection of haemoglobin, platelets, and total bilirubin using hyperspectral cameras

Hyperspectral imaging has emerged as a promising high-resolution and real-time imaging technology with potential applications in medical diagnostics and surgical guidance. In this study, we developed a high-speed hyperspectral camera by integrating a Fabry-Perot cavity filter on each CMOS pixel. We used it to non-invasively detect three blood components (haemoglobin, platelet, and total bilirubin). Specifically, we acquired transmission images of the subject's fingers, extracted spectral signals at each wavelength, and used dynamic spectroscopy to obtain non-invasive blood absorption spectra. The prediction models were established using the PLSR method and were modelled and validated based on the standard clinical-biochemical test values. The experimental results demonstrated excellent performance. The best predictions were obtained for haemoglobin, with a high related coefficient (R) of 0.85 or more in both the calibration and prediction sets and a mean absolute percentage error (MAPE) of only 5.7%. The results for total bilirubin were also ideal, with R values exceeding 0.8 in both sets and a MAPE of 10.6%. Although the prediction results for platelets were slightly less satisfactory, the error was still less than 15%, indicating that the results were also acceptable. Overall, our study highlights the potential of hyperspectral imaging technology for the development of portable and affordable devices for blood analysis, which can be used in various settings.

Recent Advances in Silicon Quantum Dot-Based Fluorescent Biosensors

With the development of nanotechnology, fluorescent silicon nanomaterials have been synthesized and applied in various areas. Among them, silicon quantum dots (SiQDs) are a new class of zero-dimensional nanomaterials with outstanding optical properties, benign biocompatibility, and ultra-small size. In recent years, SiQDs have been gradually utilized for constructing high-performance fluorescent sensors for chemical or biological analytes. Herein, we focus on reviewing recent advances in SiQD-based fluorescent biosensors from a broad perspective and discussing possible future trends. First, the representative progress for synthesizing water-soluble SiQDs in the past decade is systematically summarized. Then, the latest achievement of the design and fabrication of SiQD-based fluorescent biosensors is introduced, with a particular focus on analyte-induced photoluminescence (fluorescence) changes, hybrids of SiQDs with other materials or molecules, and biological ligand-modification methods. Finally, the current challenges and prospects of this field are highlighted.

Noninvasive and simultaneous quantitative analysis of multiple human blood components based on the grey analysis system

Noninvasive detection of human blood components is the dream of human beings and the goal of clinical detection. From the perspective of mathematical analysis, based on the grey analysis system, the principle of spectral chemical quantitative analysis and the solution method of multivariate linear equation, this paper pioneers the spectrum elimination method, and obtains a complete, high-precision, synchronous and noninvasive detection system for a variety of human blood components. The spectral elimination method applies the principle of elimination method in mathematics to the noninvasive quantitative analysis of human blood components by spectral method, reduces the influence of non-target components on the detection of target components, and improves the accuracy of noninvasive quantitative analysis of human blood components. To demonstrate the effectiveness of the method, taking the analysis of the contents of seven blood components (hemoglobin, red blood cell count, neutrophils, lymphocytes, monocytes, eosinophils and basophils) in blood as an example, fourteen models were established by two different methods. From the comparison of modeling results, it can be concluded that when the seven models established by spectral elimination method predict the corresponding seven components of all samples, the predicted correlation coefficients are more than 0.9500. The experimental results show that the spectral elimination method and non-invasive detection system proposed can predict the content of human blood components with high accuracy. This paper studies a high-precision, simultaneous and noninvasive quantitative analysis system of multiple human blood components for the first time, which not only makes great progress in the non-invasive chemical quantitative analysis of human blood components by spectroscopy, but also has great application value for clinical medical treatment and disease diagnosis.

Ratiometric fluorescence immunoassay of SARS-CoV-2 nucleocapsid protein via Si-FITC nanoprobe-based inner filter effect

The global pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus has necessitated rapid, easy-to-use, and accurate diagnostic methods to monitor the virus infection. Herein, a ratiometric fluorescence enzyme-linked immunosorbent assay (ELISA) was developed using Si-fluorescein isothiocyanate nanoparticles (FITC NPs) for detecting SARS-CoV-2 nucleocapsid (N) protein. Si-FITC NPs were prepared by a one-pot hydrothermal method using 3-aminopropyl triethoxysilane (APTES)-FITC as the Si source. This method did not need post-modification and avoided the reduction in quantum yield and stability. The p-nitrophenyl (pNP) produced by the alkaline phosphatase (ALP)-mediated hydrolysis of p-nitrophenyl phosphate (pNPP) could quench Si fluorescence in Si-FITC NPs via the inner filter effect. In ELISA, an immunocomplex was formed by the recognition of capture antibody/N protein/reporter antibody. ALP-linked secondary antibody bound to the reporter antibody and induced pNPP hydrolysis to specifically quench Si fluorescence in Si-FITC NPs. The change in fluorescence intensity ratio could be used for detecting N protein, with a wide linearity range (0.01-10.0 and 50-300 ng/mL) and low detection limit (0.002 ng/mL). The concentration of spiked SARS-CoV-2 N protein could be determined accurately in human serum. Moreover, this proposed method can accurately distinguish coronavirus disease 2019 (COVID-19) and non-COVID-19 patient samples. Therefore, this simple, sensitive, and accurate method can be applied for the early diagnosis of SARS-CoV-2 virus infection.

Electronic Supplementary Material

Supplementary material (characterization of Si-FITC NPs (FTIR spectrum, XRD spectra, and synchronous fluorescence spectra); condition optimization of ALP response (fluorescence intensity ratio change); mechanism investigation of ALP response (fluorescence lifetime decay curves and UV-vis absorption spectra); detection of N protein using commercial ELISA Kit; analytical performance of assays for ALP detection or SARS-CoV-2 N protein detection; and determination results of SARS-CoV-2 N protein in human serum) is available in the online version of this article at 10.1007/s12274-022-4740-5.

A novel ratiometric nanoprobe based on copper nanoclusters and graphitic carbon nitride nanosheets using Ce(III) as crosslinking agent and aggregation-induced effect initiator for sensitive detection of hydrogen peroxide and glucose

Herein, glutathione-capped copper nanoclusters (CuNCs) and graphitic carbon nitride nanosheets (g-C₃N₄ NSs) were synthesized by a facile one-pot chemical reduction and directly thermal pyrolysis following ultrasonic exfoliation approaches, respectively. The introduction of Ce(III) (Ce³⁺) played dual functions in constructing a fluorescence-enhanced ratiometric nanoprobe (g-C₃N₄ NSs-Ce³⁺-CuNCs), i.e., triggering aggregation-induced emission of CuNCs and conjugating g-C₃N₄ NSs with CuNCs by virtue of electrostatic and coordination interactions. The as-fabricated nanohybrid displayed 460 and 625 nm dual-emitting peaks, attributing to the emission of g-C₃N₄ NSs and CuNCs, respectively. Upon addition of H₂O₂, the 625 nm emission was dramatically quenched, whereas the 460 nm emission remained nearly unchanged, thereby causing obvious color changes from purple to blue under a 365-nm UV lamp. A ratiometric fluorescent assay, based on g-C₃N₄ NSs-Ce³⁺-CuNCs, was devised for sensitive and visual detection of H₂O₂, which spanned the linear range of 2-100 μM with a detection limit of 0.6 μM. In the presence of glucose oxidase, the ratiometric nanoprobe could be simultaneously employed to detect glucose across the linear range of 1.6-320 μM with a detection limit of 0.48 μM. In milk and human serum samples, the fortified recoveries for H₂O₂ and glucose by the nanoprobe were in the range of 95.5-103.6% with RSDs <3.8%. The real detection levels for glucose are consistent with those by a standard glucometer. As such, the ratiometric nanoprobe offers a promising methodology for several practical applications, such as point-of-care diagnosis and workplace health evaluations.

A Novel Enzyme-Free Ratiometric Fluorescence Immunoassay Based on Silver Nanoparticles for the Detection of Dibutyl Phthalate from Environmental Waters

A novel ratiometric fluorescent immunoassay was developed based on silver nanoparticles (AgNPs) for the sensitive determination of dibutyl phthalate (DBP). In the detection system, AgNPs were labeled on the secondary antibody (AgNPs@Ab₂) for signal amplification, which aimed to regulate the H₂O₂ concentrations. When AgNPs-Ab₂ and antigen-primary antibody (Ab₁) were linked by specific recognition, the blue fluorescence of Scopoletin (SC) could be effectively quenched by the H₂O₂ added while the red fluorescence of Amplex Red (AR) was generated. Under the optimized conditions, the calculated detection of limit (LOD, 90% inhibition) reached 0.86 ng/mL with a wide linear range of 2.31-66.84 ng/mL, which was approximately eleven times lower than that by HRP-based traditional ELISA with the same antibody. Meanwhile, it could improve the inherent built-in rectification to the environment by the combination of the dual-output ratiometric fluorescence assays with ELISA, which also enhanced the accuracy and precision (recoveries, 87.20-106.62%; CV, 2.57-6.54%), indicating it can be applied to investigate the concentration of DBP in water samples.

Applications of hybridization chain reaction optical detection incorporating nanomaterials: A review

The development of powerful, simple and cost-effective signal amplifiers has significant implications for biological research and analysis. Hybridization chain reaction (HCR) has attracted increasing attention because of its enzyme-free, simple, and efficient amplification. In the HCR process, an initiator probe triggered a pair of metastable hairpins through a cross-opening process to propagate a chain reaction of hybridization events, yielding a long-nicked double-stranded nucleic acid structure. To achieve more noticeable signal amplification, nanomaterials, including graphene oxide, quantum dots, gold, silver, magnetic, and other nanoparticles, were integrated with HCR. Various types of colorimetric, fluorescence, plasmonic analyses or chemiluminescence optical sensing strategies incorporating nanomaterials have been developed to analyze various targets, such as nucleic acids, small biomolecules, proteins, and metal ions. This review summarized the recent advances of HCR technology pairing diverse nanomaterials in optical detection and discussed their challenges.

Ratiometric fluorescent probe: a sensitive and reliable reporter for the CRISPR/Cas12a-based biosensing platform

Using a ratiometric probe as the reporter for the CRISPR-Cas12a based biosensing system, the change of two fluorescence intensities can be monitored, while the TaqMan probe appears only one signal.

Biocatalytic CsPbX3 Perovskite Nanocrystals: A Self-Reporting Nanoprobe for Metabolism Analysis

CsPbX₃ perovskite nanocrystals (NCs), with excellent optical properties, have drawn considerable attention in recent years. However, they also suffer from inherent vulnerability and hydrolysis, causing the new understanding or new applications to be difficultly explored. Herein, for the first time, it is discovered that the phospholipid membrane (PM)-coated CsPbX₃ NCs have intrinsic biocatalytic activity. Different from other peroxidase-like nanozymes relying on extra chromogenic reagents, the PM-CsPbX₃ NCs can be used as a self-reporting nanoprobe, allowing an "add-to-answer" detection model. Notably, the fluorescence of PM-CsPbX₃ NCs can be rapidly quenched by adding H₂ O₂ and then be restored by removing excess H₂ O₂ . Initiated from this unexpected observation, the PM-CsPbX₃ NCs can be explored to prepare multi-color bioinks and metabolite-responsive paper analytical devices, demonstrating the great potential of CsPbX₃ NCs in bioanalysis. This is the first report on the discovery of nanozyme-like property of all-inorganic CsPbX₃ perovskite NCs, which adds another piece to the nanozyme puzzle and opens new avenues for in vitro disease diagnostics.

A fluorescence color card for point-of-care testing (POCT) and its application in simultaneous detection

Diabetes mellitus has received much attention because its complications include liver, kidney, eye, heart and cerebrovascular diseases. Thus, it would be highly significant to develop a rapid and efficient method for glucose detection in biological samples. In this work, a point-of-care testing (POCT) method of glucose detection was proposed using a standard colorimetric card for semi-quantitative determination patterns. In the prepared fluorescence color card for glucose, a good linear relationship was acquired by plotting the ratio of the grayscale value (I/I0) versus the logarithm of glucose concentration within 100.0 to 1000.0 μmol L-1, and the LOD of glucose detection was 1.1 μmol L-1. A large number of actual samples (30 serum and 7 urine) were analyzed and the results demonstrated that this method had good potential to be applied in the primary screening of diabetic patients. In addition, this method is universal and can be applied in the simultaneous detection of multiple small molecules. It provides a new strategy for the primary screening of multiple diseases simultaneously, which presents excellent application potential.

Optimizing the dynamic and thermodynamic properties of hybridization in DNA-mediated nanoparticle self-assembly

DNA-directed nanoparticle (DNA-NP) systems provide various applications in sensing, medical diagnosis, data storage, plasmonics and photovoltaics. Bonding probability and melting properties are helpful to evaluate the selectivity, thermostability and thermosensitivity of these applications. We investigated the influence of temperature, nanoparticle size, DNA chain length and surface grafting density of DNA on one nanoparticle on the DNA dynamic hybridization percentage and melting properties of DNA-NP assembly systems by molecular dynamics simulation. The high degree of consistency of free energy estimations for DNA hybridization via our theoretical deduction and the nearest-neighbor rule generally used in experiments validates reasonably our DNA model. The melting temperature and thermosensitivity parameter are determined by the sigmoidal melting curves based on hybridization percentage versus temperature. The results indicated that the hybridization percentage presents a downward trend with increasing temperature and nanoparticle size. Applications based on DNA-NP systems with bigger nanoparticle size, such as DNA probes, have better selectivity, thermostability and thermosensitivity. There exist optimal DNA chain length and surface grafting density where the hybridization percentage reaches the maximal value. The melting temperature reaches a maximum at the point of optimal grafting density, while the thermosensitivity parameter presents an upward trend with the increase of grafting density. Several physical quantities consisting of the radial density function, root mean square end-to-end distance, contact distance parameter and effective volume fraction are used to analyse DNA chain conformations and DNA-NP packing in the assembly process. Our findings provide the theoretical basis for the improvement and optimization of applications based on DNA-NP systems.

Nanomaterial-Based Dual-Emission Ratiometric Fluorescent Sensors for Biosensing and Cell Imaging

Owing to the unique optophysical properties of nanomaterials and their self-calibration characteristics, nanomaterial-based (e.g., polymer dots (Pdots) quantum dots (QDs), silicon nanorods (SiNRs), and gold nanoparticle (AuNPs), etc.) ratiometric fluorescent sensors play an essential role in numerous biosensing and cell imaging applications. The dual-emission ratiometric fluorescence technique has the function of effective internal referencing, thereby avoiding the influence of various analyte-independent confounding factors. The sensitivity and precision of the detection can therefore be greatly improved. In this review, the recent progress in nanomaterial-based dual-emission ratiometric fluorescent biosensors is systematically summarized. First, we introduce two general design approaches for dual-emission ratiometric fluorescent sensors, involving ratiometric fluorescence with changes of one response signal and two reversible signals. Then, some recent typical examples of nanomaterial-based dual-emission ratiometric fluorescent biosensors are illustrated in detail. Finally, probable challenges and future outlooks for dual-emission ratiometric fluorescent nanosensors for biosensing and cell imaging are rationally discussed.

Ex vivo and in vivo fluorescence detection and imaging of adenosine triphosphate

Background

Ex vivo and in vivo detection and imaging of adenosine triphosphate (ATP) is critically important for the diagnosis and treatment of diseases, which still remains challenges up to present.

Results

We herein demonstrate that ATP could be fluorescently detected and imaged ex vivo and in vivo. In particular, we fabricate a kind of fluorescent ATP probes, which are made of titanium carbide (TC) nanosheets modified with the ROX-tagged ATP-aptamer (TC/Apt). In the constructed TC/Apt, TC shows superior quenching efficiency against ROX (e.g., ~ 97%). While in the presence of ATP, ROX-tagged aptamer is released from TC surface, leading to the recovery of fluorescence of ROX under the 545-nm excitation. Consequently, a wide dynamic range from 1 μM to 1.5 mM ATP and a high sensitivity with a limit of detection (LOD) down to 0.2 μM ATP can be readily achieved by the prepared TC/Apt. We further demonstrate that the as-prepared TC/Apt probe is feasible for accurate discrimination of ATP in different samples including living cells, body fluids (e.g., mouse serum, mouse urine and human serum) and mouse tumor models.

Conclusions

Fluorescence detection and imaging of ATP could be readily achieved in living cells, body fluids (e.g., urine and serum), as well as mouse tumor model through a new kind of fluorescent ATP nanoprobes, offering new powerful tools for the treatment of diseases related to abnormal fluctuation of ATP concentration.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-00930-4.

Limitation-induced fluorescence enhancement of carbon nanoparticles and their application for glucose detection

Rational design of detection strategy is the key to high-performance fluorescence analysis. In this article, we found that the glucose-induced limitations can greatly enhance the fluorescence of functionalized carbon nanoparticles (CNPs), which are synthesized through one-step thermal pyrolysis method using phenylboronic acid derivative as the precursors. The glucose can assembly onto the surface of the CNPs to form a "shell", limiting the surfaces' intramolecular rotation and reducing non-radiative decay, which hence resulted in enhanced fluorescence of the CNPs. Under optimal conditions, the fluorescence intensity of the CNPs is nearly 70-fold enhanced, and the method has low detection limit (10 μM) and linear response in the concentration range from 50 μM to 2000 μM. Based on this interesting "target-triggered limitation-induced fluorescence enhancement" phenomenon, a simple and effective non-enzymatic fluorescence enhancement method was developed and successfully applied to the determination of glucose in spiked serum samples. This work provides new insight into the design of fluorescence-enhanced detection strategies based on the limitation-induced property.

Point-of-care testing (POCT) of patients with a high concentration of uric acid by using alginate hydrogel microspheres embedded with CdZnTeS QDs and urate oxidase (Alg@QDs-UOx MSs)

A more convenient method for POCT of patients with a high concentration of uric acid by using Alg@QDs-UOx MSs is developed.

Novel Method of Clickable Quantum Dot Construction for Bioorthogonal Labeling

Bioorthogonal chemistry has been considered as a powerful tool for biomolecule labeling due to its site specificity, moderate reaction conditions, high yield, and simple post-treatment. Covalent coupling is commonly used to modify quantum dots (QDs) with bioorthogonal functional group (azide or cycloalkyne), but it has a negative effect in the decrease of QDs' quantum yield and stability and increase of QDs' hydrodynamic diameter. To overcome these disadvantages, we propose a novel method for the preparation of two kinds of clickable QDs by the strong interaction of -SH with metal ions. One system involves azide-DNA-functionalized QDs, which are used for bioconjugation with dibenzocyclooctyne (DBCO)-modified glucose oxidase (GOx) to form a GOx-QDs complex. After bioconjugation, the stability of QDs was improved, and the activity of GOx was also enhanced. The GOx-QDs complex was used for rapid detection of blood glucose by spectroscopy, naked eye, and paper-based analytical devices. The second system involves DBCO-DNA-functionalized QDs, which are used for an in situ bioorthogonal labeling of HeLa cells through metabolic oligosaccharide engineering. Therefore, these clickable QDs based on DNA functionalization can be applied for rapid and effective labeling of biomolecules of interest.

Catalase active metal-organic framework synthesized by ligand regulation for the dual detection of glucose and cysteine

Mimic enzymes greatly improve the inherent insufficiencies of natural enzymes. Therefore, mimic enzyme sensors attract increasing research interest. Metal-organic framework (MOF) is emerging in the field of mimic enzyme catalysis due to its remarkable structural properties. In this paper, a colorimetric method is designed for rapid and sensitive detection of glucose and cysteine levels. The MOF Eu-pydc (pydc-2,5-pyridinedicarboxylic acid) is synthesized by a new strategy which is regulated by ligands at room temperature and found to have peroxidase activity. Then, the MOF is used as a mimic enzyme to catalyze chromogenic substrate (3,3',5,5'-tetramethylbenzidine, TMB) for colorimetric sensing of glucose. The developed method can accurately detect glucose in the range of 10 μM-1 mM (R² = 0.9958) with a relatively low detection limit about 6.9 μM. Moreover, a cysteine sensor with a detection limit of 0.28 μM is also established based on the disappearance of the color of oxTMB. Additionally, the proposed glucose sensor exhibits excellent selectivity and is successfully applied to blood glucose detection. At the same time, the detection of cysteine is also highly sensitive. In short, the dual sensor is fast, low cost, and convenient, and has great application potential in the diagnosis of disease.

Glucose oxidase@Cu-hemin metal-organic framework for colorimetric analysis of glucose

The work presents a novel glucose oxidase@Cu-hemin metal-organic frameworks (GOD@ Cu-hemin MOFs) with a ball-flower structure as bienzymatic catalysts for detection of glucose. The GOD@Cu-hemin MOFs exhibits great stability as compared with free horseradish peroxidase and GOD toward harsh conditions because the ball-flower-like shell of Cu-hemin MOF effectively protects from GOD. Thus, the GOD@Cu-Hemin MOFs can be used in external harsh conditions such as high temperature and acid/base. The GOD@Cu-hemin MOFs is capable of sensitive and selective detection of glucose via peroxidase-like of Cu-hemin MOFs and GOD by using 3,3',5,5'-tetramethylbenzidine (TMB) as a substrate. Under the existence of glucose, O₂ is reduced into H₂O₂ via GOD@Cu-hemin MOFs. The produced H₂O₂ as well as Cu-hemin MOFs oxidize TMB into blue oxTMB which shows UV-Vis absorbance at 652. The absorption intensity of oxTMB linearly increases with the increasing concentration of glucose from 0.01 to 1.0 mM with detection limit of 2.8 μM. An integrated agarose hydrogel film (Aga/GOD@Cu-hemin MOF/TMB) sensor is rationally designed for colorimetric detection of glucose. The sensor displays a response range of 30 μM-0.8 mM with a detection limit of 0.01 mM. The result indicates that the Cu-hemin MOFs are an ideal carrier for the encapsulation of enzymes.

DNA-scaffold copper nanoclusters integrated into a cerium(III)-triggered Fenton-like reaction for the fluorometric and colorimetric enzymatic determination of glucose

A fluorometric and colorimetric method are described for the determination of hydrogen peroxide and glucose by integrating copper nanoclusters (CuNCs) into a Fenton-like reaction. The mechanism mainly depends on the fast formation of long-strand DNA-templated CuNCs with strong red fluorescence (with excitation/emission maxima at 340/640 nm) in the absence of H₂O₂. The DNA can be cleaved into short-oligonucleotide fragments by hydroxy radicals as formed in the Ce(III)-triggered Fenton-like reaction in the presence of H₂O₂. As a result, short-strand DNA loses the ability as a template for the formation of CuNCs. This leads to a decrease of fluorescence. The colorimetric assay, in turn, is based on the oxidation of colorless Ce(III) ions to the distinctly yellow Ce(IV) ions (with an absorption maximum at 400 nm) by H₂O₂. Compared with those assays based on the use of enzyme mimics, this method does not require any chromogenic substrates such as ABTS and TMB. Based on the dual-signal readout platform, we successfully achieved the detection of H₂O₂ and glucose. LODs are as low as 0.266 μM and 2.92 μM. The methods were applied to the sensitive determination of glucose by using glucose oxidase (GOx) which catalyzes the oxidization of glucose to produce H₂O₂. The practical application was demonstrated by determination of glucose in human serum, with apparent recoveries of 98.4-101.9% and 99.1-105.6%, respectively. The concentration of glucose ranges from 1 to 500 μM and 50 to 600 μM based on the dual-signal readout platform, respectively. This fluorometric and colorimetric dual-mode strategy will pave a new avenue for constructing effective assays for H₂O₂-related analytes in biochemical and clinical applications. Graphical abstractSchematic representation of a fluorometric and colorimetric dual-readout strategy for the sensitive determination of hydrogen peroxide and glucose. The assay has been designed by integrating copper nanoclusters into a Ce(III)-triggered Fenton-like reaction.

DNA-templated quantum dots and their applications in biosensors, bioimaging, and therapy

Over the past 10 years, DNA functionalized quantum dots (QDs) have attracted considerable attention in sensing and imaging of disease-relevant biological targets, as well as cancer therapy. Considerable efforts have been devoted to obtaining DNA functionalized QDs with enhanced stability and quantum yield. Here, we focus on a one-pot method, in which phosphorothioate-modified DNA is used as the co-ligand on the basis of the strong binding of sulfur and Cd²⁺. After a short summary of the preparation of DNA-templated QDs, versatile bioapplications based on the constructed ratiometric fluorescent probes, nanobeacons and multiple bottom-up assemblies will be discussed. A substantial part of the review will focus on these applications, ranging from small molecule, biological macromolecule, cancer cell and pathogen sensing to in vitro and in vivo imaging. Besides, drug or siRNA delivery based on DNA-templated QD assemblies will also be briefly discussed here.

A facile SERS strategy for quantitative analysis of trace glucose coupling glucose oxidase and nanosilver catalytic oxidation of tetramethylbenzidine

Highly stable, SERS active and catalytic nanosilver sol (AgNP) was synthesized under the exposure of light wave, using AgNO₃ as precursor and sodium citrate as reducer. Under the conditions of pH 7.0 NaH₂PO₄-Na₂HPO₄ buffer solution (PBS), the glucose can be catalyzed by glucose oxidase to produce H₂O₂ specifically. Based on the nanocatalyst and SERS substrate of AgNP, H₂O₂ can oxidize the 3,3',5,5'-tetramethylbenzidine (TMB) quickly to form a blue oxidation product (TMB_ox) that induced the AgNPs aggregation, which exhibited a strong SERS signal at 1606 cm^-1. As the concentration of glucose increases, the TMB_ox molecular probes and AgNPs aggregation increase, and the intensity of SERS peak at 1606 cm^-1 increase linearly. Thus, a new SERS strategy for quantitative analysis of 0.33-6.67 μmol/L glucose was developed, with a detection limit of 0.035 μmol/L, coupled the catalysis of nanosilver with glucose oxidase, and label-free molecular probe of TMB_ox.

Highly sensitive colorimetric detection of glucose through glucose oxidase and Cu2+-catalyzed 3,3',5,5'-tetramethylbenzidine oxidation

We develop a glucose oxidase (GOx)-mediated strategy for detecting glucose based on oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB), which is generated from Cu²⁺-catalyzed 3,3',5,5'-tetramethylbenzidine (TMB)-H₂O₂ reaction, as colorimetric readout. The sensing system involves two processes: generation of H₂O₂ from GOx-catalyzed oxidation of glucose, and H₂O₂-induced the oxidization of TMB via the catalysis of Cu²⁺. The H₂O₂ formed by GOx-catalyzed oxidation of glucose oxidizes colorless TMB to blue oxTMB, thus enhancing the absorbance intensity at 670 nm. Therefore, we draw a conclusion that the enhancement in colorimetric signal relies directly on H₂O₂ concentration, which, in turn, depends on glucose concentration. This color change can be used not only for visual detection of glucose by naked eyes but for reliable glucose quantification in the range from 1 to 100 nM with a detection limit of 0.21 nM. The method possesses the following advantages: simple design, low experimental cost, and no any additional experimental equipment for heating, illuminating, or bubbling.

A fluorometric turn-on aptasensor for mucin 1 based on signal amplification via a hybridization chain reaction and the interaction between a luminescent ruthenium(II) complex and CdZnTeS quantum dots

A fluorometric method is described for the determination of the tumor biomarker mucin 1 (MUC1). It is based on signal amplification of the hybridization chain reaction (HCR), and the interaction between a luminescent ruthenium(II) complex and CdZnTeS quantum dots (QDs). If MUC1 bind to the biotin-labeled aptamer, it will initiate HCR with hairpins H₁ and H₂ to form a long-range dsDNA. The long nucleic acid chains are then linked on the surface of streptavidin-modified magnetic microparticles (MMPs) through streptavidin-biotin interaction. The luminescent ruthenium(II) complex is then embedded in the long dsDNA linked to the MMPs. Hence, there is little Ru complex in the supernatant after magnetic separation, and the fluorescence of the CdZnTeS QDs (best measured at excitation/emission wavelengths of 350/530 nm) is only slightly quenched. In the absence of target, the fluorescence of the CdZnTeS QDs is strongly quenched. Fluorescence increases linearly in the 0.2-100 ng·mL^-1 MUC1 concentration range, and the LOD is 0.13 ng·mL^-1 (at S/N = 3). The method was applied to the determination of MUC1 in spiked human serum samples. Graphical abstract A fluorometric turn-on aptasensor for mucin 1 is described that is based on the interaction between a Ru(II) complex and quantum dots (QDs). The detection system includes biotin-labeled aptamer-H₀, hairpins H₁ and H₂, streptavidin-modified magnetic microparticles (MMPs), Ru(bpy)₂(dppx)²⁺ and CdZnTeS QDs.

In situ synthesis of photoluminescence-quenching nanopaper for rapid and robust detection of pathogens and proteins

A green and facile method is presented for in situ synthesis of a novel photoluminescence-quenching nanopaper with a highly-efficient quenching ability, rapid reaction time and long-term storage. The as-prepared nanopaper is further used to construct an aptasensor platform with high performance, rapidness and robustness.

Chemistries for DNA Nanotechnology

The predictable nature of DNA interactions enables the programmable assembly of highly advanced 2D and 3D DNA structures of nanoscale dimensions. The access to ever larger and more complex structures has been achieved through decades of work on developing structural design principles. Concurrently, an increased focus has emerged on the applications of DNA nanostructures. In its nature, DNA is chemically inert and nanostructures based on unmodified DNA mostly lack function. However, functionality can be obtained through chemical modification of DNA nanostructures and the opportunities are endless. In this review, we discuss methodology for chemical functionalization of DNA nanostructures and provide examples of how this is being used to create functional nanodevices and make DNA nanostructures more applicable. We aim to encourage researchers to adopt chemical modifications as part of their work in DNA nanotechnology and inspire chemists to address current challenges and opportunities within the field.

A novel dual response ratiometric fluorescent probe for the determination of H2O2 and glucose via etching of silver nanoparticles

Based on the inner filter effect and charge transfer, a dual response ratiometric fluorescence assay for sensing H₂O₂ and glucose was proposed based on etching of Ag NPs.

An enzyme cascade-based electrochemical immunoassay using a polydopamine-carbon nanotube nanocomposite for signal amplification

By coupling tyrosinase (Tyr) and β-galactosidase (Gal) into one redox-cycling scheme, an enzyme cascade-based electrochemical immunosensor with boosted selectivity and sensitivity was constructed using polydopamine-functionalized multiwalled carbon nanotube (MWCNTs-PDA) nanohybrid modified electrodes. The MWCNTs-PDA nanohybrid presented a 5 times enhanced capability for antibody conjugation, which was responsible for signal amplification. In the proposed enzyme cascade scheme, Gal was captured on the immunosensor surface by a sandwiched immunoreaction, which catalyzed phenyl β-d-galactopyranoside (P-GP) into phenol based on a hydrolysis reaction. The resulting phenol was used as a substrate of Tyr, which was catalyzed to catechol and subsequently to o-quinone. The o-quinone was then electrochemically reduced to catechol, forming a redox cycle between catechol and o-quinone. The enzyme cascade-based immunoassay not only significantly amplified the electrochemical signal, but also led to a high selectivity. Taking the detection of CEA as an example, the enzyme cascade-based electrochemical immunosensor showed a detectable range of 10 pg mL^-1 to 10 ng mL^-1 and a low detection limit of 8.39 pg mL^-1 (S/N = 3), which was superior/comparable to those using other methodologies in previous reports. The selectivity of the enzyme cascade-based immunosensor was 44-80% higher than that of a single enzyme-based immunosensor. This work shows great potential of the coupling enzyme cascade in immunosensing for clinical diagnosis with boosted selectivity and sensitivity.