Portable Smartphone Platform Based on Aggregation-Induced Enhanced Emission Carbon Dots for Ratiometric Quantitative Sensing of Fluoride Ions.

The development of an instrument-free, on-site, real-time, sensitive, and visualized fluoride-ion (F-) content rapid detection strategy is crucial to ensuring the health of the population. Smart microdevices that are portable, directly read, and easy to operate have recently attracted much attention. Herein, a ratiometric fluorescent probe (AA-CDs@[Ru(bpy)3]2+)-based smartphone sensing platform was developed for the detection of F-. The red fluorescent ruthenium bipyridine [Ru(bpy)3]2+ molecule was chosen as the reference signal, and the carbon dots (AA-CDs) with Al3+ aggregation-induced enhanced emission (AIE) were designed as the response signal. The ratiometric probe fluorescence changed continuously from red to cyan in response to different concentrations of F-, and the red-green-blue (RGB) channel values of the fluorescence image were extracted through the smartphone color recognition application (APP). There was a linear relationship between the blue-red (B/R) ratio and the F- concentration, with a limit of detection (LOD) of 1.53 µM, far below the allowable content of F- in drinking water prescribed by the World Health Organization. The F- content was rapidly detected on-site with satisfactory repeatability and relative standard deviation using several water and toothpaste samples as the real sample. The platform features low cost, portability, easy operation, and good stability, selectivity, and repeatability, which provides a powerful tool for the visual quantitative detection of smartphone-based microsensing platforms possibly in the fields of environmental protection, diagnosis, and food safety assessment.

A Ratiometric Fluorescence Probe for Selective Detection of ex vivo Methylglyoxal in Diabetic Mice.

Accurate monitoring of methylglyoxal (MGO) at cell and living level was crucial to reveal its role in the pathogenesis of diabetes since MGO was closely related to diabetes. Herein, a ratiometric fluorescence strategy was constructed based on the capture probe 2,3-diaminonaphthalene (DAN) for the specific detection of MGO. Compared to the fluorescent probes with a single emission wavelength, the ratiometric mode by monitoring two emissions can effectively avoid the interference from the biological background, and provided additional self-calibration ability, which can realize accurate detection of MGO. The proposed method showed a good linear relationship in the range of 0-75 µm for MGO detection, and the limit of detection was 0.33 µm. DAN responded to MGO with good specificity and was successfully applied for detecting the ex vivo MGO level in plasma of KK-Ay mice as a type II diabetes model. Besides, the prepared DAN test strip can be visualized for rapid semi-quantitative analysis of MGO using the naked eye. Furthermore, human skin fibroblasts and HeLa cells were utilized for exogenous MGO imaging, and ex vivo MGO imaging was performed on tissues of KK-Ay mice. All results indicated that the DAN-based ratiometric fluorescence probe can be used as a potential method to detect the level of MGO, thus enabling indications for the occurrence of diabetes and its complications.

Multicolor hydrogen sulfide sensor for meat freshness assessment based on Cu2+-modified boron nitride nanosheets-supported subnanometer gold nanoparticles.

Hydrogen sulfide (H2S) has emerged as an important indicator in the spoilage process of meat. In this study, a mimetic enzyme based on Cu2+-modified boron nitride nanosheets-supported gold nanoparticles (AuNPs/Cu2+-BNNS) was synthesized, which can be used to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). The H2S gas can inhibit the activity of AuNPs/Cu2+-BNNS toward catalytic oxidation of TMB. Meanwhile, the usage of headspace method could avoid most interferences in the rotten sample. Various concentrations of TMB+ could change the aspect ratio of the gold nanoroads (AuNRs), which results in vivid color changing and UV-vis spectra shifting. The sensor had a good linear relationship with H2S concentration ranging from 10.0 to 90.0 µmol/L, and the detection limit is 7.8 µmol/L. The AuNPs/Cu2+-BNNS sensors were successfully applied to detect H2S produced by meat spoilage with satisfying results.

1,2,4-Triaminobenzene as a Fluorescent Probe for Intracellular pH Imaging and Point-of-Care Ammonia Sensing.

As one of the health indicators, intracellular pH plays important roles in many processes of cell functions. Abnormal pH changes would result in the occurrence of inflammation, cancer, and other diseases. Thus, it is of significant importance to develop effective techniques for sensitive detection of pH changes for the clinical diagnosis of various diseases related to cells. In this paper, 1,2,4-triaminobenzene hydrochloride was explored as an organic molecular fluorescent probe for sensitive and selective detection of intracellular pH changes for the first time. Due to the protonation and deprotonation of amino groups of the probe, its fluorescent intensity at 599 nm or the ratio of absorbance at 505 and 442 nm has a good linear relationship with pH values in the range of 5.0-7.0. Benefiting from the excellent physical and chemical properties of 1,2,4-triaminobenzene hydrochloride, the fluorescent probe has good water solubility, low toxicity, high photostability, great reversibility, good cell penetration, fast response speed, and so on. As a proof-of-concept demonstration, the proposed probe is employed for the fluorescence imaging of cells and mouse tissue sections with satisfactory performance in pH differentiation. Additionally, the probe was successfully employed to prepare test strips as a kind of point-of-care testing device to detect ammonia, which showed great potential in practical applications.

Emission Wavelength Switchable Carbon Dots Combined with Biomimetic Inorganic Nanozymes for a Two-Photon Fluorescence Immunoassay.

In this work, o-phenylenediamine is utilized as a precursor to synthesize the fluorescent emission wavelength switchable carbon dots (o-CDs). Our investigation reveals that ferrous ions (Fe2+) can effectively induce fluorescence quenching of o-CDs by chelation and aggregation. After the addition of hydrogen peroxide (H2O2), the fluorescence of o-CDs recovers and the fluorescent color changes from yellow to green. As far as we know, o-CDs are the first reported CDs with switchable fluorescence emission wavelength. In order to fabricate an enzyme-free immunosensor, an amino-functionalized dendritic mesoporous silica nanoparticle (DMSN)-gold nanoparticle (Au NP) nanostructure was fabricated as a glucose oxidase mimetic nanoenzyme by in situ coating of the Au NPs on the surface of the DMSNs. Then, the functionalized DMSN-Au NPs were modified on the detection antibody and hydrolyzed with glucose to produce H2O2. This immune induced recognition strategy combines with the o-CDs+Fe2+ signal generation system to achieve specific and sensitive detection of the target. The replacement of glucose oxidase by DMSN-Au NPs not only reduces the cost but also provides significantly amplified signals due to DMSNs haing a high specific surface area. We show the detection of carcinoembryonic antigen (CEA) as an example target to evaluate the analytical figure of merits of the proposed strategy. Under the optimal conditions, two-photon-based o-CDs displayed excellent performances for CEA and the limit of detection as low as 74.5 pg/mL with a linear range from 0.1 to 80 ng/mL. The proposed fluorescent immunosensor provides an optional and potential scheme for low cost, high sensitivity, and versatile discovery of clinical biomarkers.

Cu2+-Modified Boron Nitride Nanosheets-Supported Subnanometer Gold Nanoparticles: An Oxidase-Mimicking Nanoenzyme with Unexpected Oxidation Properties.

In recent years, inorganic biomimetic nanozymes that mimic the activity of natural biological enzymes have attracted extensive research interest, and some mimic enzymes have been successfully applied in the fields of biosensing, catalysis, and oncotherapy. Herein, we report the preparation and mechanism study of a novel nanocomposite, Cu2+-modified hexagonal boron nitride nanosheets-supported subnanometer gold nanoparticles (Au NPs/Cu2+-BNNS). Interestingly, our investigation reveals that Cu2+-BNNS exhibits strong peroxidase mimetic nanoenzyme activity, while Au NPs/Cu2+-BNNS exhibits excellent oxidase-like activity, that is, it can catalyze the oxidation reaction of the substrate in the absence of an oxidant such as H2O2. For example, Au NPs/Cu2+-BNNS can efficiently and selectively oxidize 3,3',5,5'-tetramethylbenzidine (TMB) and 3,3'-dimethylbiphenyl-4,4'-diamine (OT) coloration without the presence of horseradish peroxidase (HRP) and H2O2. It is worthy to note that AuNPs/Cu2+-BNNS-induced TMB coloration only takes 4 min to reach the platform, while the conventional HRP-H2O2 system takes more than 30 min to reach the platform. Further mechanism study shows that the zeta potential, oxidation potential, and steric hindrance of the oxidative chromogenic substrate determine the selectivity of oxidation coloration, while the oxidase-like properties of Au NPs/Cu2+-BNNS are derived from reactive oxygen species generated by the adsorbed oxygen, and Cu2+ ion can synergistically promote the oxidation process. Compared with conventional biological enzymes, Au NPs/Cu2+-BNNS has the advantages of being HRP free and H2O2 free, having high efficiency, low cost, and good stability, and is successfully demonstrated for the detection of carcinoembryonic antigen (a universal cancer biomarker) and H2S (the third gaseous signal molecule).

Ratiometric Fluorescent Hydrogel Test Kit for On-Spot Visual Detection of Nitrite.

In this work, we proposed a new method based on carbon dots (named m-CDs) for selective and efficient detection of nitrite (NO2-), which was based on the interaction between the amine group of m-CDs and NO2- via a diazo reaction that produced diazonium salts and induced the fluorescence quenching of m-CDs. The concentration of NO2- shows a good linear relationship with a quenched fluorescence intensity from 0.063 to 2.0 µM ( R2 = 0.996) with a detection limit of 0.018 µM. In addition, a ratiometric fluorescence probe ( m-CDs@[Ru(bpy)3]2+) was constructed via electrostatic interaction by introducing Ru(bpy)3Cl2·6H2O as an internal reference fluorescent reagent. Interestingly, a transition of the fluorescent color of the ratiometric probe from cyan to red could be visually observed upon increasing the concentration of NO2-. Based on these findings, a ratiometric fluorescent-based portable agarose hydrogel test kit was fabricated and applied for on-spot assessment of NO2- content within 10 min. As far as we know, this is the first ratiometric fluorescent sensor for visual detection of NO2-. It has broad application prospects in environmental monitoring and food safety assessment.

Preparation of an Efficient Ratiometric Fluorescent Nanoprobe (m-CDs@[Ru(bpy)3]2+) for Visual and Specific Detection of Hypochlorite on Site and in Living Cells.

Hypochlorite (ClO-) is one of the most important reactive oxygen species (ROS), which plays an important role in sustaining human innate immunity during microbial invasion. Moreover, ClO- is a powerful oxidizer for water treatment. The safety of drinking water is closely related to its content. Herein, m-phenylenediamine (mPD) is used as a precursor to prepare carbon dots (named m-CDs) with highly fluorescent quantum yield (31.58% in water), and our investigation shows that the strong fluorescent emission of m-CDs can be effectively quenched by ClO-. Based on these findings, we developed a novel fluorescent nanoprobe (m-CDs) for highly selective detection of ClO-. The linear range was from 0.05 to 7 µM (R2 = 0.998), and the limit of detection (S/N = 3) was as low as 0.012 µM. Moreover, a portable agarose hydrogel solid matrix-based ratiometric fluorescent nanoprobe (m-CDs@[Ru(bpy)3]2+) sensor was subsequently developed for visual on-site detection of ClO- with the naked eyes under a UV lamp, suggesting its potential in practical application with low cost and excellent performance in water quality monitoring. Additionally, intracellular detection of exogenous ClO- was demonstrated via ratiometric imaging microscopy.

Boron nitride nanosheets as a platform for fluorescence sensing.

Hexagonal boron nitride nanosheets (BNNS), one kind of inorganic graphene analogue, exhibit high fluorescence quenching ability via the photo-induced electron transfer (PET) and different affinity toward single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). As a proof of concept, the BNNS was first used as a sensing platform for the rapid detection of DNA and small molecules with high sensitivity and selectivity in this study. This strategy is versatile and quick fluorescence sensing of DNA and extensive DNA related analytes such as metal cations and small molecules. Moreover, this strategy might be available for the practical application in future. This work provides an avenue for understanding the interaction between two-dimensional nanomaterials and biomolecules and designing novel sensing strategies for extending the applications of nanomaterials in bioanalysis.