Jinxia Xu

The quenching of the fluorescence of carbon dots: A review on mechanisms and applications

AbstractCarbon dots (CDs) possess unique optical properties such as tunable photoluminescence (PL) and excitation dependent multicolor emission. The quenching and recovery of the fluorescence of CDs can be utilized for detecting analytes. The PL mechanisms of CDs have been discussed in previous articles, but the quenching mechanisms of CDs have not been summarized so far. Quenching mechanisms include static quenching, dynamic quenching, Förster resonance energy transfer (FRET), photoinduced electron transfer (PET), surface energy transfer (SET), Dexter energy transfer (DET) and inner filter effect (IFE). Following an introduction, the review (with 88 refs.) first summarizes the various kinds of quenching mechanisms of CDs (including static quenching, dynamic quenching, FRET, PET and IFE), the principles of these quenching mechanisms, and the methods of distinguishing these quenching mechanisms. This is followed by an overview on applications of the various quenching mechanisms in detection and imaging. Graphical abstractSchematic representation of the quenching mechanisms of carbon dots (CDs) which include static quenching, dynamic quenching, Förster resonance energy transfer (FRET), photoinduced electron transfer(PET), surface energy transfer (SET), Dexter energy transfer (DET) and inner filter effect (IFE). All these effects can be used to detect and image analytes.

Rhodamine-based ratiometric fluorescent probes based on excitation energy transfer mechanisms: construction and applications in ratiometric sensing

Ratiometric fluorescent probes allow the simultaneous measurement of two fluorescence signals at different wavelengths followed by calculation of their intensity ratio, which can provide more precise measurement results than intensity-based fluorescent probes. Excitation energy transfer is widely used in the design of ratiometric fluorescent probes. Rhodamine is a convenient platform for the construction of “OFF–ON” ratiometric chemosensors. Rhodamine-based ratiometric fluorescent probes based on the excitation energy transfer mechanism can be constructed by conjugated or non-conjugated connections with other chromophores. In this review, we summarized the recent advances regarding rhodamine-based ratiometric fluorescent probes based on excitation energy transfer. We reviewed these probes according to the classification of “through-space” and “through-bond” probes; we focused on the contributions of different donor fluorophores and the types of connections between the energy donors and acceptors.

Cobalt(II) ions detection using carbon dots as an sensitive and selective fluorescent probe

A simple method was designed for detecting cobalt ions (Co2+) based on the analyte-induced fluorescence quenching of carbon dots (CDs). CDs with a quantum yield of 38.7% were synthesized by hydrothermal treatment of Carbopol 934 and diethylenetriamine. Through the metal–ligand interaction, the prepared CDs can allow highly sensitive and selective detection of Co2+. The color change (from transparent to brown) of the solution can be clearly seen with the naked eye. This effective sensing platform shows high sensitivity and selectivity towards Co2+. Moreover, the CDs are also successfully utilized for monitoring the Co2+ content of natural water.

The FRET performance and aggregation-induced emission of two-dimensional organic-inorganic perovskite, and its application to the determination of Hg(II)

AbstractThe authors describe the synthesis of a two-dimensional organic-inorganic perovskite of the type NH3(CH2)10NH3)PbBr4 by a fast precipitation method. The resulting powder under photoecitation at 270 nm exhibits yellow luminescence (peaking at 550 nm) in DMSO/water mixtures if the ratio is bigger than 4/6. On addition of Hg(II) and a rhodamine spirolactam, ring opening occurs and colorimetric and fluorescent responses can be seen. Specifically, the emission at 550 nm gradually decreases and a new emission peak appears at 585 nm (corresponding to the emission peak of RBEA-Hg(II) complex) while the color of the solution gradually turns to pink under visible light. The emission peak of the modified perovskite overlaps the 585 nm absorption of rhodamine B, and this results in a FRET effect whose Förster radius (donor-acceptor distance) is calculated from decay time data to be 10.4 Å. Based on these findings, a FRET-based assay for Hg(II) was worked that has a 2.4 μM detection limit. Graphical abstractIn DMSO/H2O = 1/9 mixed solution, energy transfer from NH3(CH2)10NH3)PbBr4 (C10PbBrPE) to rhodamine B occurred. Fluorescence resonance energy transfer (FRET) process is available by rhodamine B ethylenediamine and C10PbBrPE for detection of Hg(II).

Silica-based optical chemosensors for detection and removal of metal ions

Because some metal ions are highly toxic even at trace level, a constant demand of developing methods for monitoring and removing these metal ions is extremely urgent. Silica-based optical chemosensors are supposed as good alternatives to classical instrumental methods for detecting and adsorbing metal ions, due to their effect and lower price. Silica nanoparticles, silica gel and mesoporous silica are used as supporting platforms to fabricate optical chemosensors. They have certain properties containing high porosity and expectant adsorption capacity. Chromogenic-type and fluorogenic-type optical probes, such as azobenzene, naphthalimide and rhodamine, are grafted to the surface of silica-based materials by sol–gel reaction, the limit of detection, response time and selective properties of optical sensors are improved sequentially. In this paper, the articles of silica-based optical chemosensors are retrospected since 2008, describing silica-based optical sensors used for sensing metal ions. The sensing mechanism, optical phenomenon, detection limit, adsorption capacity and application are also reviewed.

A dual spectroscopic fluorescence probe based on carbon dots for detection of 2,4,6-trinitrophenol/Fe (III) ion by fluorescence and frequency doubling scattering spectra and its analytical applications

A convenient, highly sensitive and reliable assay for 2,4,6‑trinitrophenol (TNP) and Fe (III) ion (Fe3+) in the dual spectroscopic manner is developed based on novel carbon dots (CDs). The CDs with highly blue emitting fluorescent were easily prepared via the one-step potassium hydroxide-assisted reflux method from dextrin. The as-synthesized CDs exhibited the high crystalline quality, the excellent fluorescence characteristics with a high quantum yield of ~13.1%, and the narrow size distribution with an average diameter of 6.3±0.5nm. Fluorescence and frequency doubling scattering (FDS) spectra of CDs show the unique changes in the presence of TNP/Fe3+ by different mechanism. The fluorescence of CDs decreased apparently in the presence of TNP via electron-transfer. Thus, after the experimental conditions were optimized, the linear range for detection TNP is 0-50μM, the detection limit was 19.1nM. With the addition of Fe3+, the FDS of CDs appeared to be highly sensitive with a quick response to Fe3+ as a result of the change concentration of the scattering particle. The emission peak for FDS at 450nm was enhanced under the excitation wavelength at 900nm. The fluorescence response changes linearly with Fe3+ concentration in the range of 8-40μM, the detection limits were determined to be 44.1nM. The applications of CDs were extended for the detection of TNP, Fe3+ in real water samples with a high recovery. The results reported here may become the potential tools for the fast response of TNP and Fe3+ in the analysis of environmental pollutants.

Dual-channel fluorescence detection of mercuric (II) and glutathione by down- and up-conversion fluorescence carbon dots

The fluorescent carbon dots (CDs) with high fluorescent quantum yield (φf = 62%) and down- and up-conversion fluorescence properties were synthesized by one-pot hydrothermal treatment of citric acid and tris(hydroxymethyl)methyl aminomethane. The CDs displayed the capability to absorb excitation wavelength at 660 nm and 330 nm with fluorescence emission wavelength at 398 nm and 399 nm, respectively. The CDs showed high selectivity towards Hg2+ against various metal ions. Around 70% fluorescence was quenched by 40 μM Hg2+ through dynamic and static quenching mechanisms. Because of stronger affinity between the thiol and Hg2+, over 90% fluorescence was recovered by adding 40 μM glutathione to CDs-Hg2+ system. The calibration curves exhibited wide linear region for Hg2+ (0-4 μM) and glutathione (0-30 μM). The limits of detection with down- and up-conversion for Hg2+ were calculated to be 0.23 μM and 0.25 μM, and for glutathione were 0.28 μM and 0.29 μM, respectively. Inspired by the sensing results, logic gates with Hg2+ and glutathione as inputs were also established. Most importantly, this method was applied to detect Hg2+ and glutathione in tap water and lake water, and the recovery values were obtained to be 96.2%-110.4% and 93.4%-96.9%.