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Special functionalization of semiconducting polymer dots (Pdots) is highly desired to expand their applications in chemo/biosening. Herein, carboxyl-functionalized poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}-thiadiazole)] dots covalently tagged with aminated β-cyclodextrin (NH2-CD) have been designed to construct a ratiometric sensor for cholesterol (Cho). Using CD-Pdots as energy donors with rhodamine B (RB) as energy acceptors, a fluorescence resonance energy transfer (FRET) pair has been built because the host-guest interaction between RB and CD attached to Pdots brings donors and acceptors into close proximity. In the presence of Cho, the acceptors will depart from the donors because of the competitive inclusion interaction between Cho and RB with CD, resulting in the hindering of the FRET process between CD-Pdots and RB. On the basis of the turn-on fluorescence of CD-Pdots and turn-off fluorescence of RB, a sensitive ratiometric method for the determination of Cho in the concentration range from 25 to 350 nM with a detection limit of 4.9 nM was achieved. The proposed method was validated to determine free Cho in human serum samples with satisfactory results. [ABSTRACT FROM AUTHOR]

We report a platform for the ratiometric fluorescent sensing of endogenously generated gaseous transmitter H2S in its aqueous form (bisulfide or hydrogen sulfide anion) based on the alteration of Forster resonance energy transfer from an emissive semiconductor quantum dot (QD) donor to a dithiol-linked organic dye acceptor. The disulfide bridge between the two chromophores is cleaved upon exposure to bisulfide, resulting in termination of FRET as the dye diffuses away from the QD. This results in enhanced QD emission and dye quenching. The resulting ratiometric response can be correlated quantitatively to the concentration of bisulfide and was found to have a detection limit as low as 1.36 ± 0.03 µM. The potential for use in biological applications was demonstrated by measuring the response of the QD-based FRET sensor microinjected into live HeLa cells upon extracellular exposure to bisulfide. The methodology used here is built upon a highly multifunctional platform that offers numerous advantages, such as low detection limit, enhanced photochemical stability, and sensing ability within a biological milieu. [ABSTRACT FROM AUTHOR]

In this work, we developed a sensitive and efficient ratiometric electrochemical method for lipopolysaccharide (LPS) detection using Cu-based metal-organic frameworks (Cu-MOFs) as a catalyst and target-triggered quadratic cycles for signal amplification. First, in the presence of target LPS, the conformation change of the specifically designed hairpin probes 1 (HP1) triggered the target cyclic-induced polymerization to produce the output DNA with the aid of phi29 DNA polymerase (phi29). Then, the obtained output DNA hybridized with ferrocene-labeled hairpin probes 2 (Fc-HP2, which were immobilized on the electrode) to generate a nicking endonuclease (N.BstNBI) cleavage site. Thus, with N.BstNBI, the original signal molecules of Fc left from the electrode, and the single-stranded capture-probe-modified sensing interface was obtained. At this time, signal probes conducted by Au-nanoparticles-functionalized Cu-MOFs and labeled hairpin probes 3 (HP3/AuNPs/Cu-MOFs) were hybridized with capture probes for hairpin assembly. Herein, AuNPs/Cu-MOFs were not only used as nanocarriers for immobilizing HP3 but also acted as electroactive materials for signal reporting. With the proposed target-triggered quadratic cycles, the cleavage sites of Fc-HP2 were cut, and capture probes were obtained to hybridize with HP3/AuNPs/Cu-MOFs, which caused the signal decrease of Fc. Then Cu-MOFs were closed to the electrode for the signal increase of Cu-MOFs. Furthermore, when glucose was present in the detection solution, AuNPs/Cu-MOFs catalyzed the oxidation of glucose to realize the enzyme-free signal amplification. By measuring the peak currents ratio of the Cu-MOFs and Fc, the proposed aptasenor for LPS detection showed a low detection limit (0.33 fg/mL) and a wide linear range from 1.0 fg/mL to 100 ng/mL with high accuracy and sensitivity. This ratiometric electrochemical approach is expected to be a valuable strategy for detection of other analytes. [ABSTRACT FROM AUTHOR]

Since its introduction in 1998, genetically encoded pH-sensitive sensor ratiometric pHluorin proved to be a valuable tool for cell physiology studies. Here, we show how the sensitivity of pHluorin-based monitoring of intracellular pH changes performed with cell suspensions can be enhanced by using synchronously scanned fluorescence spectroscopy. In the suspensions of S. cerevisiae cells subjected to varying extracellular pH values, we have been able to measure statistically significant changes in intracellular pH of less than 0.1 unit, which were not detectable using a standard ratiometric approach. [ABSTRACT FROM AUTHOR]

Developments of ratiometric bioprobes are highly appealing due to the superiority of their self-calibration capability for the quantitative biotracking. In this work, we designed and synthesized a novel lanthanide complex-based ratiometric luminescence probe, [4'-(2,4-dinitrophenyloxy)-2,2':6',2"-terpyridine-6,6"-diyl]bis(methylenenitrilo) tetrakis(acetate)-Eu3+/Tb3+ (NPTTA-Eu3+/Tb3+), for the specific recognition and quantitative time-gated luminescence detection of hydrogen sulfide (H2S) in aqueous and living cell samples. Due to the presence of the photoinduced electron transfer (PET) process from the terpyridine-Eu3+/Tb3+ moiety to 2,4-dinitrophenyl (DNP), the probe itself is weakly luminescent. In physiological pH aqueous media, the reaction of NPTTA-Eu3+/Tb3+ with H2S leads to the cleavage of DNP moiety from the probe molecule, which affords the deprotonated (4'-hydroxy-2,2':6',2"-terpyridine-6,6"-diyl)bis(methylenenitrilo) tetrakis(acetate)-Eu3+/Tb3+ and terminates the PET process. Meanwhile, the intensity of Tb3+ emission at 540 nm is remarkably increased, while that of the Eu3+ emission at 610 nm is slightly decreased. After the reaction, the intensity ratio of Tb3+ emission to Eu3+ emission, I540/I610, was ~220-fold increased, and the dose-dependent enhancement of I540/I610 showed a good linearity upon the increase of H2S concentration with a detection limit of 3.5 nM. This unique luminescence response allowed NPTTA-Eu3+/Tb3+ to be conveniently used as a ratiometric probe for the time-gated luminescence detection of H2S with I540/I610 as a signal. In addition, the applicability of the probe for the quantitative time-gated luminescence imaging of intracellular H2S in living cells was investigated. The results demonstrated the efficacy and advantage of the new probe for the time-gated luminescence cell imaging application. [ABSTRACT FROM AUTHOR]

An organic salt as a fluorescent probe based on intramolecular charge transfer for Ca2+ determination is developed. Ca2+ can be detected by ratiometric emission at 490 and 594 nm with an excitation wavelength of 405 nm. This probe is highly selective for Ca2+ over other divalent metal cations and displays a large Stokes shift of 189 nm that can avoid interference of the excitation light beam and autofluorescence of biological samples. The dissociation constant for Ca2+ is 2.25 ± 0.47 μM and pertinent to Ca2+ detection in cellular resting and dynamic states. The probe demonstrates its application in monitoring Ca2+ in living cells under confocal microscopic imaging. [ABSTRACT FROM AUTHOR]

A lanthanide complex-based ratiometric luminescence probe, [4'-(2,4-dinitrobenzenesulfonyloxy)-2,2':6',2''-terpyridine-6,6''-diyl] bis(methylenenitrilo) tetrakis(acetate)-Eu3+/Tb3+ (NSTTA-Eu3+/Tb3+), has been designed and synthesized for the specific recognition and time-gated luminescence detection of biothiols in physiological pH aqueous media. The probe itself is almost nonluminescent due to the presence of photoinduced electron transfer (PET) from the terpyridine-Ln3+ moiety to the 2,4-dinitrobenzenesulfonyl (DNBS) moiety. In the presence of biothiols, the reaction of NSTTA-Eu3+/Tb3+ with biothiols results in the cleavage of DNBS to afford the deprotonated (4'-hydroxy-2,2':6',2''-terpyridine-6,6''-diyl) bis(methylenenitrilo) tetrakis(acetate)-Eu3+/Tb3+ (HTTA-Eu3+/Tb3+), which terminates the PET process. After the reaction, the intensity of Eu3+ emission at 610 nm is unchanged, while that of Tb3+ emission at 540 nm is remarkably increased, which provides a 36-fold enhanced intensity ratio of Tb3+ emission to Eu3+ emission (I540/I610). This unique luminescence response allows NSTTA-Eu3+/Tb3+ to be used as a ratiometric probe for the time-gated luminescence detection of biothiols, using the intensity ratio of I540/I610 as a signal. Thus, based on the probe NSTTA-Eu3+/Tb3+, a ratiometric time-gated luminescence detection method for biothiols was established and successfully used for the quantitative detection of the total biothiols in several living cell samples. [ABSTRACT FROM AUTHOR]

A probe based on 2-(2′-hydroxyphenyl) benzothiazole (HBT) has been synthesized and used for the ratiometric detection of hydrazine. The probe is designed in such a way that the excited state intramolecular proton transfer (ESIPT) of the HBT moiety gets blocked. The chemodosimetric approach of hydrazine to the probe results in the recovery of the ESIPT by removal of a free HBT moiety through subsequent substitution, cyclization, and elimination processes. The probe is successfully demonstrated to enable the detection of hydrazine in live cells. [ABSTRACT FROM AUTHOR]