A novel ratiometric fluorescence probe based on calix[4]arene functionalized polymer dots for bisphenol A detection in real water samples

Bisphenol A (BPA) is one of key raw materials used in the production of epoxy resins and plastics, which has toxicological effects on humans by disrupting cell functions through a variety of cell signaling pathways. Therefore, it is of great significance to develop a simple, rapid, and accurate BPA detection method in real water samples. In this study, a ratiometric fluorescence method based on yellow-emitting surface-functionalized polymer dots (PFBT@L Pdots) and blue-emitting carbon dots (Cdots) was described for the detection of BPA. Pdots as the detecting part were synthesized by using highly fluorescent hydrophobic Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-(2,1',3)-thiadiazole)] (PFBT) polymer and (R)-5,11,17,23-Tetra-tert-butyl-25,27-bis[(diphenylphosphinoyl)methoxy]-26-(3-oxabutyloxy)-28-[(1-phenylethyl)- carbamoylmethoxy]calix [4]arene (L) functionalizing ligand, and Cdots as internal reference were prepared by hydrothermal treatment of citric acid and urea. In the presence of BPA, chemical binding of the phosphorus atoms of nearby PFBT@L Pdots with BPA hydroxyl functional groups led to the aggregation of the PFBT@L Pdots aggregation and quenching their yellow emission, but the blue emission of Cdots, on the other hand, remained stable. The proposed PFBT@L Pdots probe was successfully applied for the detection of BPA in real water samples, and the results were in good agreement with those obtained by HPLC-FLD. To the best of our knowledge, this is the first report that the calixarene has been utilized to modify Pdots.

Aggregation-caused dual-signal response of gold nanoclusters for ratiometric optical detection of cysteine

Designing ratiometric sensors for cysteine (Cys) monitoring with high accuracy is of great significance for disease diagnosis and biomedical studies. The current ratiometric methods mainly rely on multiplex probes, which not only complicates the operation but also increases the cost, making it difficult for quantitative Cys detection in resource-limited areas. Herein, one-pot prepared gold nanoclusters (Au NCs) that glow red fluorescent were synthesized by employing glutathione as the stabilizer and reducing agent. When Fe3+ is present with Au NCs, the fluorescence is quenched and the scattering is strong because of the aggregation of Au NCs. With introduction of Cys, Cys can efficiently compete with glutathione-modified Au NCs for Fe3+, which leads to increase of fluorescence and decrease of scattering. The ratiometric determination of Cys can be thereby realized by collecting the fluorescence and SRS spectrum simultaneously. The linear range for Cys was 5-30 µM with a detection limit of 1.5 µM. In addition, the sensing system exhibits good selectivity for Cys and shows potential application in biological samples.

Application of green-synthesized carbon dots for imaging of cancerous cell lines and detection of anthraquinone drugs using silica-coated CdTe quantum dots-based ratiometric fluorescence sensor

Chemotherapy drugs of daunorubicin and doxorubicin treat cancers with many side effects. So, detection of them in the biological system for regulation and controlling of usage is essential. In this study, a ratiometric fluorescent method was introduced for detection of daunorubicin and doxorubicin using bell pepper-based carbon dots, as the variable signal, and silica-coated CdTe quantum dots, as the constant signal. The detection was done based on variations of carbon dots intensity in the presence of drugs in comparison with the constant intensity of silica-coated CdTe quantum dots. The proposed ratiometric fluorescent method was successfully used for detection of daunorubicin and doxorubicin range of 54.37-13594.34 nmolL^-1 and 86.2-17242 nmolL^-1, with a detection limit of 18.53 nmolL^-1 and 29 nmolL^-1, respectively. Also, this method was used for detection of drugs in serum samples with recovery ranges of 86.14-99.62 (RSD 3-1.47%) and 86.32-97.53 (3.38-1.48%), respectively. Finally, after evaluation of carbon dots toxicity by MTT test, carbon dots was applied for imaging of prostate cancer cell lines (PC-3) and breast cancer cell lines (MCF7). The results demonstrated that despite improvement of the repeatability and interferences reduction by ratiometric method, also carbon dots were successfully applied for imaging of cell lines.

Metal-induced G-quadruplex polymorphism for ratiometric and label-free detection of lead pollution in tea

Lead pollution are critical concerns for food safety and human health. Herein, a ratiometric metal-induced G-quadruplex polymorphism was introduced to construct aptamer probes, enabling label-free and ratiometric detection of lead in tea, thus is promising for on-site detection of lead pollution. The key feature of the aptamer probe is the synergistic utilization of the dual-wavelength fluorescent signal outputs from a G-quadruplex specific dye and a DNA intercalation dye under a single-wavelength excitation, leading to a more stable and reliable recognition of Pb²⁺ than that of analyses based on single fluorescent reporter. The aptamer probe allowed to a mix-and-read, rapid, cost-effective detection of Pb²⁺ with high specificity and accuracy. Pb²⁺ analysis in tap water and tea exhibited good performance with recovery rates of 92.3%-109.0%. The adoption of ratiometric metal-induced G-quadruplex polymorphism would be a compelling design strategy for constructing robust aptasensor, facilitating the translation of aptamer for food safety control.

Error correction due to background subtraction in ratiometric calcium measurements with CCD camera

Background

Ca²⁺ plays an important role in many physiological processes and an accurate study of these signals is important. In modern fluorescence microscopy, a charge-coupled device (CCD) camera is widely deployed for calcium imaging. The ratiometric method is used for the fluorescence dye Fura-2 and Grynkiewitz's formula (Grynkiewicz et al., 1985) is commonly used to convert fluorescence to free Ca²⁺ concentration ([Ca²⁺]). But the need to subtract the background signal can lead to a big error in ratiometric calcium measurements. When the error due to background subtraction occurs, the fluorescence ratio of 340 nm divided by 380 nm lights may be twice as large as the actual value. Under conditions when the excitation intensity is not adjusted to ensure the same throughput of the objective lens for ultraviolet dye illumination, the indicator does not gradually bleach out for channels with a wavelength of 340 nm and 380 nm light, which lead to an additional error in determining the concentration of Ca²⁺.

New method

Here we present a new approach for calculating [Ca²⁺] from the ratiometric fluorescence of Fura-2 dye imaged by a CCD camera. It is designed to optimize [Ca²⁺] measurements with photobleaching correction without background subtraction error. A mathematical method is also provided for removing the existing underestimated value of fluorescence at an excitation wavelength of 340 nm and compensating for the bleaching rate for both channels with wavelengths of 340 nm and 380 nm using a power function.

Results

In cultured neurons, the calculations of the free Ca²⁺ concentration during Ca²⁺ transients estimated by the old and new methods, determine it to the same extent. This comparison was made under conditions without errors through background subtraction. If there is this error, the old method calculates [Ca²⁺] with a much higher, rather than the actual value.

Conclusions

We present a modified Grynkiewitz's formula for calculation [Ca²⁺] for ratiometric dye, such as Fura-2 imaged by a CCD camera, with photobleaching correction without background subtraction error.

Ratiometric fluorescent immunoassay for the cardiac troponin-I using carbon dots and palladium-iridium nanocubes with peroxidase-mimicking activity

A nanozyme-linked immunosorbent assay is described for cardiac troponin I which is a biomarker for myocardial infarction. The method is based on the use of Pd-Ir nanocubes with excellent peroxidase-like activity. The nanocubes catalyze the oxidization of nonfluorescent o-phenylenediamine (OPD) by H₂O₂ to form a yellow fluorescent product (oxOPD) with excitation/emission maxima at 400/570 nm. Carbon dots are added as a reference fluorophore. Under the same excitation wavelength, they display blue fluorescence (450 nm). The ELISA uses the Pd-Ir nanocubes as a label for the secondary antibody and OPD as substrate. The ratio of fluorescence intensities at 570 and 450 nm increases in the 1 pg·mL^-1 to 1 ng·mL^-1 cardiac troponin I concentration range, and the detection limit is 0.31 pg·mL^-1. The method was applied to analyze spiked serum samples, and the results compared well with those obtained by a commercial chemiluminescence assay. Graphical abstract Schematic presentation of the ratiometric fluorescence immunoassay for cardiac troponin-I. Pd-Ir nanocubes were employed to fabricate nanozyme-based signal labels for its excellent peroxidase-mimicking activity.

A ratiometric fluorometric heparin assay based on the use of CdTe and polyethyleneimine-coated carbon quantum dots

CdTe quantum dots (QDs) were integrated with polyethyleneimine-coated carbon dots (PEI-CDs) to form a dually emitting probe for heparin. The red fluorescence of the CdTe QDs is quenched by the PEI-CDs due to electrostatic interactions. In the presence of heparin, the blue fluorescence of PEI-CDs remains unaffected, while its quenching effect on the fluorescence of CdTe QDs is strongly reduced. A ratiometric fluorometric assay was worked out. The ratio of the fluorescences at 595 and 436 nm serves as the analytical signal. Response is linear in the concentration range of 50-600 ng·mL^-1 (0.1-1.2 U·mL^-1) of heparin. The limit of detection is 20 ng·mL^-1 (0.04 U·mL^-1). This makes the method a valuable tool for heparin monitoring during postoperative and long-term care. This assay is relatively free from the interference by other analogues which commonly co-exist with heparin in samples, and it is more robust than single-wavelength based assays. Graphical abstract In the presence of heparin, the fluorescence of polyethyleneimine-coated carbon dots (PEI-CDs) at 436 nm remains unaffected, while its quenching effect on the fluorescence of CdTe at 595 nm is strongly reduced.

Colorimetric determination of Hg(II) via the gold amalgam induced deaggregation of gold nanoparticles

A method is described for the colorimetric determination of mercury(II). In the absence of Hg(II), aminopropyltriethoxysilane (APTES) which is positively charged at pH 7 is electrostatically absorbed on the surface of gold nanoparticles (AuNPs). This neutralizes the negative charges of the AuNPs and leads to NP aggregation and a color change from red to blue-purple. However, in the presence of Hg(II), reduced Hg (formed through the reaction between Hg(II) and citrate on the AuNP surface) will replace the APTES on the AuNPs. Hence, the formation of aggregates is suppressed and the color of the solution does not change. The assay is performed by measuring the ratio of absorbances at 650 and 520 nm and can detect Hg(II) at nanomolar levels with a 10 nM limit of detection. The specific affinity between mercury and gold warrants the excellent selectivity for Hg(II) over other environmentally relevant metal ions. Graphical Abstract Schematic of the method for determination of Hg²⁺ based on the gold amalgam-induced deaggregation of gold nanoparticles in the presence of APTES with the LOD of 10.1 nM.

Application of Fluorescence Lifetime Imaging (FLIM) to Measure Intracellular Environments in a Single Cell

Fluorescence lifetime imaging (FLIM) has now been used in many bioscience fields, which comes from the quantification of fluorescence lifetime. The procedure for obtaining lifetime images is very similar to that used in fluorescence microscopy. However, obtaining reliable lifetime images requires an understanding of the theory of fluorescence lifetime, principle of FLIM systems, and evaluation procedure of intracellular environments. In this chapter, the materials, methods, and notes on FLIM measurements have been described, in conjunction with a brief explanation of the background of FLIM.