Controllable and robust dual-emissive quantum dot nanohybrids as inner filter-based ratiometric probes for visualizable melamine detection

Ratiometric fluorescence aptasensor for the detection of patulin in apple juice based on the octahedral UiO-66-TCPP metal-organic framework and aptamer systems

Patulin (PAT) is a potentially harmful mycotoxin to human health and is known to contaminate apple juice. In this work, we developed a ratiometric fluorescence aptasensor using tetrakis(4-carboxyphenyl)porphyrin (H2TCPP)-treated octahedral UiO-66-NH2 (defined as UiO-66-TCPP) to detect PAT. This 2-aminoterephthalic acid and H2TCPP functionalized metal-organic framework showed multiple adsorption effects (hydrogen bonding and π-π stacking) on the aptamer (Apt) and served as a quenching material. When the target PAT bound specifically to the Apt, the fluorescence of the 6-carboxyfluorescein-labeled Apt would recover, and the fluorescence of the H2TCPP ligand remained unchanged. This ratiometric fluorescence property improved the accuracy of PAT detection. Moreover, the introduction of the H2TCPP ligand enhanced the quenching efficiency of UiO-66-NH2, thus improving the sensitivity of the fluorescent aptasensor (UiO-66-TCPP vs. UiO-66-NH2: 0.0162 ng/mL vs. 1.8 ng/mL). In addition, we used UiO-66-TCPP to detect PAT in apple juice samples. This work provides a good paradigm for the construction of ratiometric fluorescence aptasensors with high sensitivity and accuracy.

A test strip constructed by molecular imprinting for ratiometric fluorescence with ultra-low limit of detection for selective monitoring of Sudan I in chili powder

An up-conversion molecularly imprinted ratiometric fluorescent probe with a monodisperse nuclear-satellite structure and its test strip are designed which can avoid fluorescent background interference to detect Sudan I in chili powder highly selective and sensitive. The detection mechanism is based on the selective recognition of Sudan I by imprinted cavities on the surface of ratiometric fluorescent probe and the inner filter effect between Sudan I molecules and the emission of up-conversion materials (NaYF₄:Yb,Tm). Under optimized experimental conditions, the response of fluorescent ratio signals (F₄₇₅/F₆₄₅) of this test strip show a good linear relationship in the range 0.02-50 μM Sudan I. The limits of detection and quantitation are as low as 6 nM and 20 nM, respectively. Sudan I is selectively detected in the presence of fivefold higher concentrations of interfering substances (imprinting factor up to 4.4). Detection of Sudan I in chili powder samples show ultra-low LOD (44.7 ng/g), satisfactory recoveries (94.99-105.5%) and low relative standard deviation (≤ 2.0%). This research offers a reliable strategy and promising scheme for highly selective and sensitive detection of illegal additives in complex food matrix via an up-conversion molecularly imprinted ratiometric fluorescent test strip.

A smartphone-based ratiometric fluoroprobe based on blue-red dual-emission signals of thiochrome and copper nanoclusters for sensitive assay of metam-sodium in cucumbers

It is of great importance to develop the highly efficient fluorescence strategy for rapid/sensitive detection of metam-sodium (MES) in evaluating its residual safety, especially in fresh vegetables. Herein, we prepared an organic fluorophore (thiochrome, TC) and glutathione-capped copper nanoclusters (GSH-CuNCs), and their combination (TC/GSH-CuNCs) was sucessfully employed as a ratiometric fluoroprobe by means of the blue-red dual emission. The fluorescence intensities (FIs) of TC decreased upon the addition of GSH-CuNCs via the fluorescence resonance energy transfer (FRET) process. When fortified at the constant levels of GSH-CuNCs and TC, MES substantially reduced the FIs of GSH-CuNCs, while this was not the case in the FIs of TC except for the prominent red-shift of ∼30 nm. Compared to the previous fluoroprobes, the TC/GSH-CuNCs based fluoroprobe supplied wider linear range of 0.2-500 μM, lower detection limit (60 nM), and satisfactory fortification recoveries (80-107%) for MES in the cucumber samples. Based on the fluorescence quenching phenomenon, a smartphone application was used to output RGB values of the captured images for the colored solution. The smartphone-based ratiometric sensor could be utilized for the visual fluorescent quantitation of MES by virtue of the R/B values in cucumbers, which gave linear range (1-200 μM) and LOD (0.3 μM). By means of blue-red dual-emission fluorescence, the smartphone-based fluoroprobe provides a cost-effective, portable and reliable avenue for the on-site, rapid and sensitive assay of MES's residues in complex vegetable samples.

Semiconductor/Carbon Quantum Dot-based Hue Recognition Strategy for Point of Need Testing: A Review

The requirement to establish novel methods for visual detection is attracting attention in many application fields of analytical chemistry, such as, healthcare, environment, agriculture, and food. The research around subjects like "point-of-need", "hue recognition", "paper-based sensor", "fluorescent sensor", etc. has been always aimed at the opportunity to manufacture convenient and fast-response devices to be used by non-specialists. It is possible to achieve economic rationality and technical simplicity for optical sensing toward target analytes through introduction of fluorescent semiconductor/carbon quantum dot (QD) and paper-based substrates. In this Review, the mechanisms of anthropic visual recognition and fluorescent visual assays, characteristics of semiconductor/carbon QDs and ratiometric fluorescence test paper, and strategies of semiconductor/carbon QD-based hue recognition are described. We cover latest progress in the development and application of point-of-need sensors for visual detection, which is based on a semiconductor/carbon quantum dot-based hue recognition strategy generated by ratiometric fluorescence technology.

Ratiometric fluorescent Si-FITC nanoprobe for immunoassay of SARS-CoV-2 nucleocapsid protein

Coronavirus disease 2019 (COVID-19) highlights the importance of rapid and reliable diagnostic assays for the management of virus transmission. Here, we developed a one-pot hydrothermal method to prepare Si-FITC nanoparticles (NPs) for the fluorescent immunoassay of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid protein (N protein). The synthesis of Si-FITC NPs did not need post-modification, which addressed the issue of quantum yield reduction during the coupling reaction. Si-FITC NPs showed two distinct peaks, Si fluorescence at λ em = 385 nm and FITC fluorescence at λ em = 490 nm. In the presence of KMnO4, Si fluorescence was decreased and FITC fluorescence was enhanced. Briefly, in the presence of N protein, catalase (CAT)-linked secondary antibody/reporter antibody/N protein/capture antibody immunocomplexes were formed on microplates. Subsequently, hydrogen peroxide (H2O2) and Si-FITC NPs/KMnO4 were injected into the microplate together. The decomposition of H2O2 by CAT resulted in remaining of KMnO4, which changed the fluorescence intensity ratio of Si-FITC NPs. The fluorescence intensity ratio correlated significantly with the N protein concentration ranging from 0.02 to 50.00 ng/mL, and the detection limit was 0.003 ng/mL, which was more sensitive than the commercial ELISA kit with a detection limit of 0.057 ng/mL. The N protein concentration can be accurately determined in human serum. Furthermore, the COVID-19 and non-COVID-19 patients were distinguishable by this method. Therefore, the ratiometric fluorescent immunoassay can be used for SARS-CoV-2 infection diagnosis with a high sensitivity and selectivity.

Electronic Supplementary Material

Supplementary material (characterization of Si-FITC NPs (FTIR, HRXPS); stability investigation of Si-FITC NPs (photostability, pH stability, anti-interference ability); stability investigation of free FITC (pH value, KMnO4); quenching mechanism of KMnO4 (UV-vis absorption spectra, fluorescence lifetime decay curves); reaction condition optimization of biotin-CAT with H2O2 (pH value, temperature, time); detection of N protein using commercial ELISA Kit; selectivity investigation of assays for SARS-CoV-2 N protein detection; determination results of SARS-CoV-2 N protein in human serum) is available in the online version of this article at 10.1007/s12274-022-5005-z.

Ratiometric fluorescence sensing with logical operation: Theory, design and applications

The construction of ratiometric fluorescence sensing logic systems has gradually become a hot topic in fluorescence analysis, due to the multi-target analysis potential of logic operations and the high specificity and selectivity of ratiometric fluorescence sensing. In this paper, the basic principles of various logic functions implemented in ratiometric fluorescence detection are discussed in the context of sensing mechanisms, and the strategies for constructing logic systems in different ratiometric fluorescence sensing application areas are summarized. Although there are limitations such as cumbersome operations and complicated experiments, ratiometric fluorescence sensing logic circuits that combine the visualization of logic operations and the accuracy of ratiometric fluorescence are still worthy of in-depth study. This review may be useful for researchers interested in the construction of logic operations based on ratiometric fluorescence sensing applications.

Ultrafast Ratiometric Detection of Aflatoxin B1 Based on Fluorescent β-CD@Cu Nanoparticles and Pt2+ Ions

Rapid detection of aflatoxin B1 (AFB1) is a very important task in food safety monitoring. However, it is still challenging to achieve highly sensitive detection without antibody or aptamer biomolecules. In this work, a rapid detection of aflatoxin B1 was achieved using a ratiometric fluorescence probe without antibody or aptamer for the first time. In the ratiometric fluorescence system, the fluorescence emission of AFB1 at 433 nm was significantly enhanced due to the β-cyclodextrin-AFB1 host-guest interaction and the complexation of AFB1 and Pt²⁺. Meanwhile, the inclusion of aflatoxin B1 also quenched the fluorescence emission of β-CD@Cu nanoparticles (NPs) at 650 nm based on inner filter effect mechanism. On the basis of the above effects, the ratiometric detection of aflatoxin B1 was achieved in the range of 0.03-10 ng/mL with a low detection limit of 0.012 ng/mL (3σ/s). In addition, the β-CD@Cu NPs based nanoprobe could achieve stable response within 1 min to AFB1. The above ratiometric detection also demonstrated excellent application potential in the rapid on-site detection of AFB1 in food due to the advantages of convenience, rapidness, and high accuracy.

Fluorescent carbon dots embedded in mesoporous silica nanospheres: A simple platform for Cr(VI) detection in environmental water

In this work, amino-functionalized mesoporous silica nanospheres (NH₂-mSiO₂) anchored with carbon dots (CDs) have been designed to construct an outstanding fluorescent sensor for heavy metal detection. Uniform mSiO₂ was chosen to provide an optically transparent scaffold for immobilizing CDs. With the help of amino group modification on the surface of silica, benzene-1,4-diboronic acid (BA) was used as raw material to load CDs in the pores of mSiO₂ by one-step solvothermal method. The proposed nanohybrid can solve the problem of aggregation-induced fluorescence quenching, leading to bright blue emission at 450 nm. Meanwhile, the fluorescence of NH₂-mSiO₂@CDs showed high sensitivity to Cr(VI) in acetic acid buffer solution (pH = 4) with detection limit as low as 5 nM by inner filter effect (IFE) and electrostatic interaction (EI). The proposed method can also be extended to other CDs-based detection systems for chemical/biological sensors.