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Surface enhanced Raman scattering (SERS) is an ultrasensitive analytic technique. However, the application of SERS in quantitative analysis usually suffers from poor reliability due to the limitations of currently developed SERS substrates. In the present work, aggregated gold nanoparticles (a-AuNPs) fabricated by Ca2+-mediated assembly are dispersed in polyvinyl alcohol solution to prepare a novel hydrogel SERS chip through a physical crosslinking method. Taking advantage of the uniform distribution of SERS active a-AuNPs in the three-dimension hydrogel and the excellent barrier effect of hydrogel towards oxygen and macromolecules, the obtained hydrogel SERS chips show many outstanding advantages including high sensitivity, good repeatability, long-term stability, and a robust anti-interference ability. These advantages enable hydrogel SERS chips to be used to quantitatively analyse some complex samples without complex sample preprocessing. As a model, the hydrogel SERS chips are used for the detection of triazophos and phosmet in orange samples. The good recoveries suggest good applicability of the hydrogel SERS chips in food safety detection. This work provides a reliable and convenient platform for the quick detection and on-site monitoring of chemical contaminants and would promote greatly the performance of SERS techniques in quantitative analysis. [ABSTRACT FROM AUTHOR]

In this study, a turn-on two-photon fluorescent probe (Lyso-TP-NO) for nitric oxide (NO) was developed. It was synthesized using 4-ethylamino-1,8-naphthalimide as the two-photon fluorophore and N-methylaniline moiety as the reaction site. The probe and fluorophore were tested under one- and two-photon modes. The fluorescence intensity of the system was enhanced 23.1-fold after reacting with NO in the one-photon mode. However, the maximal two-photon action cross-section value of 200 GM was obtained under excitation at 840 nm. The probe exhibits high selectivity and sensitivity over other reactive oxygen species (ROS) and reactive nitrogen species (RNS), with a detection limit as low as 3.3 nM. The two-photon fluorescence imaging of living cells and mouse brain tissues can capture inflammation-induced endogenous NO production in lysosomes during stroke occurrence. [ABSTRACT FROM AUTHOR]

In this paper, a paper-supported analytical device based on a sandwich immunoreaction and luminescence resonance energy transfer (LRET) was reported for the visual and quantitative determination of a cancer biomarker, in which upconversion nanoparticles (UCNPs) were located on the surface of the paper as energy donors and gold nanoparticles (AuNPs) were used as energy acceptors. Upon the recognition of the cancer biomarker by two rationally selected antibodies, the upconversion luminescence was quenched by the AuNPs in a biomarker concentration-dependent manner. As a model target, CEA was detected using this immunosensor, and a linear relationship within 0.5–30 ng mL−1 was obtained in buffer solution, with a detection limit of 0.21 ng mL−1. The immunosensor was also applicable in 20-fold diluted human serum with a linear range of 0.5–30 ng mL−1 and a detection limit of 0.36 ng mL−1. This technique also realized the qualitative judgment of the critical concentration of CEA in serum samples by the naked eye. This approach displays great application potential for point-of-care testing in clinical applications, as well as the potentiality to be extended to other kinds of disease-related biomolecules. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide (H2O2) plays crucial roles in immunological processes and has been revealed to be closely linked to the hypoxic–ischemic process. Thus, it is important to develop a reliable method for monitoring H2O2 in hypoxic–ischemic brain injury (HIBI). To achieve this, we report on a rationally designed fluorene-based ratiometric two-photon fluorescent probe for H2O2, i.e., FH2O2. The probe and fluorophore were tested under either one- or two-photon modes, respectively. After reacting with H2O2, the relative emission intensity ratio at wavelengths of 390–465 (Fblue) and 500–550 nm (Fyellow) of FH2O2 had a 6.5-fold increase (Fyellow/Fblue) within 40 min, and the maximal two-photon active cross-section value was detected as 66 GM at a wavelength of 750 nm. The probe exhibited high selectivity, low cytotoxicity, high sensitivity with a detection limit of 0.57 μM, and adequate photostability. After confirming satisfactory sensing performance of the probe in vitro, we were able to monitor H2O2in situ in mice with HIBI via two-photon microscopy, which could provide a potential method for clinical diagnosis during the neonatal HIBI process. [ABSTRACT FROM AUTHOR]

A turn-on two-photon fluorescent probe HCA-Green for hypochlorous acid (HOCl) was synthesized using 4-methylamino-1,8-naphthalimide (MNA) as a two-photon fluorophore and p-hydroxyaniline as a leaving-recognition domain. Both the probe and the fluorophore were investigated under one- and two-photon excitation modes. The fluorescence intensity of the probe was enhanced by ∼229-fold and ∼193-fold under one-photon and two-photon excitation, respectively, after reacting with HOCl. A maximal two-photon action cross-section of 50 GM was obtained under excitation at 810 nm. The probe exhibited high sensitivity with a detection limit of 42.3 nM, as well as high selectivity, low cytotoxicity, and good photostability. Two-photon microscopy (TPM) was conducted to visualize HOCl levels in living cells and tissues. The production of endogenous HOCl induced by lipopolysaccharide-mediated inflammation was successfully monitored with this probe. [ABSTRACT FROM AUTHOR]

In this paper, we constructed a label-free ratiometric fluorescence nanoprobe for ascorbic acid (AA) on the basis of the change in dual-emission signals by the specific reaction between cobalt oxyhydroxide (CoOOH) and AA. CoOOH nanoflakes were used as the recognition unit while polymer dots (PFO dots) and gold nanoclusters (AuNCs) served as luminescent units of the nanoprobe. After electrostatic assembly of PFO dots and AuNCs on the surface of CoOOH nanoflakes, the energy transfer between the PFO dots and CoOOH could occur and thereby quench the fluorescence of the PFO dots. When AA was present, CoOOH nanoflakes were reduced to Co2+, resulting in a recovery of the fluorescence of the PFO dots. In the meantime, the generated Co2+ could combine with the surface ligands of AuNCs, accompanied by a reduction in the luminescence intensity of the AuNCs. Therefore, the fluorescence intensity ratio of PFO dots to AuNCs would be related to the concentration of AA. The nanoprobe enabled highly sensitive analysis of AA with a detection limit of 1.9 μM and showed great performance in human serum samples and living cells. This report may not only endow a CoOOH-based nanoprobe with improved ability but also offer a novel strategy for the construction of ratiometric nanoprobes for AA and have potential applications in research into and clinical diagnosis of AA-related diseases. [ABSTRACT FROM AUTHOR]

A two-photon fluorescent probe FNTR for nitroreductase was synthesized by using 9,9-dimethyl-2-acetyl-fluoren-7-methylamino (1) as a two-photon fluorophore and a p-nitrobenzyl carbamate group as a recognition domain for nitroreductase (NTR). The probe and the fluorophore were tested under one- and two-photon modes respectively. After reacting with nitroreductase, FNTR had a 130-fold fluorescence enhancement at 563 nm in 10 min and the maximal two-photon action cross-section value was detected as 66 GM at 750 nm. The probe showed a high sensitivity with a detection limit as low as 23.67 ng ml−1, high selectivity, low cytotoxicity and good photostability. In the presence of reduced nicotinamide adenine dinucleotide (NADH), endogenous NTR was detected in living cells, tissues and zebrafish. Cobalt chloride was used to induce chemical hypoxia to produce NTR, which generated enhanced fluorescence in cells and tumor tissues. Finally, two-photon fluorescence imaging of NTR was achieved in zebrafish at a penetration depth of up to 200 μm. [ABSTRACT FROM AUTHOR]

Using 6-hydroxyl-quinoline-2-benzothiazole (HQB) as a two-photon fluorophore and tert-butyldiphenylsilyl as a recognition domain for F−, a ratiometric two-photon fluorescent fluoride probe, QF, was synthesized and fully characterized. QF displays both one- and two-photon ratiometric responses towards fluoride ions in aqueous solution. QF was enabled to detect exogenous fluoride ions in living cells by a ratiometric method. Two-photon microscopic imaging of fluoride ions in living HeLa cells and zebrafish has also been achieved. QF has been demonstrated to be an excellent fluorescent probe with high selectivity, low cytotoxicity and good photostability. [ABSTRACT FROM AUTHOR]

We herein report a sensitive and selective immunosensor for carcinoembryonic antigen (CEA) based on the joint use of upconversion phosphors (UCPs) and magnetic beads (MBs). UCPs as the signal probe were designed with a core–shell structure which provided a 40-fold enhancement of the luminescence intensity. Poly(acrylic acid) (PAA)-modified UCPs were covalently conjugated with the anti-CEA antibody (coating), and streptavidin functionalized magnetic beads were combined with another biotin-tagged anti-CEA antibody. With the assistance of a magnet, the as-formed immune sandwich in the presence of CEA can be readily separated from the assay matrix. The immunosensor showed a linear dynamic range for CEA within 0.05–20 ng mL−1 in a buffered aqueous solution, and 0.1–20 ng mL−1 in a human serum sample. The sensor was highly specific to CEA. Our results have suggested the potential application of the UCP-MB based immunoassay for CEA in clinical analysis. [ABSTRACT FROM AUTHOR]

A new strategy for quantitatively detecting micrococcal nuclease (MNase) is proposed using electrostatic interaction-based fluorescence resonance energy transfer (FRET) between positively charged QDs and negatively charged dye-labeled single-stranded DNA (dye-ssDNA). Herein, we have made our attempt to develop a strategy where the variation of FRET efficiency is due to the change of the electrostatic interaction between QDs and the ssDNA that result from the cleavage of dye-ssDNA by a single-strand-specific nuclease. To demonstrate the feasibility of this design, positively charged QDs (lysozyme modified QDs, Lyz-QDs) are prepared as the energy donor, with the fluorescent dye 6-carboxy-X-rhodamine (ROX) that is labeled to ssDNA serving as the energy acceptor. The ROX-labeled probe ssDNA (ROX-ssDNA) is absorbed to the surface of the QDs through electrostatic interaction, which results in resonance energy transfer between the QDs and the dye. In the presence of MNase which cleaves the ROX-ssDNA into small fragments, the weakened interaction between QDs and the shortened ssDNA causes the decrease of FRET efficiency. At given amounts of donor and acceptor, the ratio of fluorescence intensity of QDs to ROX changes in a MNase concentration-dependent manner. Under optimized conditions, the ratio is linear with MNase concentration over the range of 8 × 10−3to 9.0 × 10−2unit mL−1, with a limit of detection of 1.6 × 10−3unit mL−1. This new detection strategy features straightforward design and easy operation, which is capable of expanding the application of the positively charged QDs-based FRET in DNA-related bioassays. [ABSTRACT FROM AUTHOR]

Correction for 'A paper-supported sandwich immunosensor based on upconversion luminescence resonance energy transfer for the visual and quantitative determination of a cancer biomarker in human serum' by Mengyuan He et al., Analyst, 2020, 145, 4181–4187, DOI: 10.1039/c9an02307k. [ABSTRACT FROM AUTHOR]

A novel two-photon excited fluorescent probe for cadmium (named as TPCd) was designed and synthesized utilizing a prodan (6-acetyl-2-methoxynaphthalene) derivative as the two-photon fluorophore and an o-phenylenediamine derivative as the Cd(2+) chelator, which possessed favorable photophysical properties and good water-solubility. The probe was designed with a photoinduced electron transfer (PET) mechanism and thus was weakly fluorescent itself. After binding with Cd(2+) which blocked the PET process, the fluorescence intensity of the probe was enhanced by up to 15-fold under one-photon excitation (OPE) and 27-fold under two-photon excitation (TPE), respectively. The two-photon action cross-section (Φδ) of the TPCd-Cd complex at 740 nm reached 109 GM compared to 3.6 GM for free TPCd, indicating the promising prospect of the probe in two-photon application. TPCd chelated Cd(2+) with 1 : 1 stoichiometry, and the apparent dissociation constant (K(d)) was 6.1 × 10(-5) M for the one-photon mode and 7.2 × 10(-5) M for the two-photon mode. The probe responded to Cd(2+) over a wide linear range from 0.1 to 30 μM with a detection limit of 0.04 μM. High selectivity of the probe towards Cd(2+) was acquired in Tris-HCl/sodium phosphate buffer. The probe was pH-independent in the biologically relevant pH range and non-toxic to living cells at reasonable concentration levels, warranting its in vivo applications. Through two-photon microscopy imaging, the probe was successfully applied to detect Cd(2+) uptake in living HepG2 cells.

In the present work, a two-photon excited fluorescent chemosensor for Cu(2+) was prepared. The probe was constructed on the basis of internal charge transfer (ICT) principle with macrocyclic dioxotetraamine as the Cu(2+) receptor. The good water-solubility of the molecule enabled recognition and assay of Cu(2+) ions in biological media. The photophysical properties of the chemosensor were investigated in detail, exhibiting favorable fluorescence quantum yield and moderate two-photon absorption cross-section. The studies on binding thermodynamics demonstrated the formation of 1 : 1 complex between the chemosensor and Cu(2+) and an association constant of ca. 1.04 × 10(5) M(-1). Due to the rational design of the molecular structure, the sensor was highly specific to Cu(2+), which ensured high selectivity in Cu(2+) determination. Upon Cu(2+) binding, the intramolecular charge-transfer extent within the chromophore was weakened resulting in a remarkable quenching of fluorescence, based on which quantitative determination of Cu(2+) was performed. Good linearity was obtained between the fluorescence quenching value and Cu(2+) concentration ranging from 0.04 to 2.0 μM in aqueous solution. Benefiting from the merits of two-photon excitation, the chemosensor was free of interference from background luminescence in serum. A homogeneous quantitative determination of Cu(2+) was achieved in the serum medium with a linear range of 0.04 to 2.0 μM. Considering the structural flexibility of the sensor, this work also opens up the possibility to construct other two-photon excited chemosensors for direct homogeneous assay of various molecules/ions in complicated biological sample matrices.