Highly Selective Electrochemical Strategy for Monitoring of Cerebral Cu2+ Based on a Carbon Dot-TPEA Hybridized Surface

Profuse color-evolution-based fluorescent test paper sensor for rapid and visual monitoring of endogenous Cu2+ in human urine

The fluorescent paper for colorimetric detection of metal ions has been widely fabricated using various sensing probes, but it still remains an elusive task to design a test paper with multicolor variation with target dosages for accurate determination. Herein, we report a profuse color-evolution-based fluorescent test paper sensor for rapid and visual monitoring of Cu²⁺ in human urine by printing tricolor probe onto filter paper. The tricolor probe consists of blue-emission carbon dots (bCDs), green-emission quantum dots (gQDs) and red-emission quantum dots (rQDs), which is based on the principle that the fluorescence of gQDs and rQDs are simultaneously quenched by Cu²⁺, whereas the bCDs as the photostable internal standard is insensitive to Cu²⁺. Upon the addition of different amounts of Cu²⁺, the ratiometric fluorescence intensity of the tricolor probe continuously varied, leading to color changes from shallow pink to blue with a detection limit of 1.3nM. When the tricolor probe solution was printed onto a sheet of filter paper, as-obtained test paper displayed a more profuse color evolution from shallow pink to light salmon to dark orange to olive drab to dark olive green to slate blue to royal blue and to final dark blue with the increase of Cu²⁺ concentration compared with dual-color probe-based test paper, and dosage scale as low as 6.0nM was clearly discriminated. The sensing test paper is simple, rapid and inexpensive, and serves as a visual platform for ultrasensitive monitoring of endogenous Cu²⁺ in human urine.

An "on-off-on" fluorescent nanoprobe for recognition of chromium(VI) and ascorbic acid based on phosphorus/nitrogen dual-doped carbon quantum dot

Chromium (VI) [Cr(VI)] is a harsh environmental contaminates and has been proved to be highly toxic, carcinogenic and mutagenic. Therefore, developing an inexpensive, good selective and highly sensitive nanoprobe for the detection of Cr(VI) is in urgent demand. Recently, the highly fluorescent carbon quantum dots (CQDs) have been successfully utilized as efficient fluorescent nanoprobes for the detection of ions, pH and molecular substances. In this work, an "on-off" fluorescence phosphorus/nitrogen dual-doped CQDs (PNCQDs) probe was developed for the determination of Cr(VI) based on inner filter effect (IFE). The proposed PNCQDs nanoprobe shows its distinct merits of simplicity, convenience, fast implementation, good selectivity and high sensitivity towards Cr(VI), allowing its potential application in the determination of Cr(VI) in environment and biosystem. In addition, the chelation effect of the functional groups in reductant and Cr(VI), and the easy-conversion of Cr(VI) to reduced states (i.e. Cr(III) and Cr(0)) by reductants makes the minimization of IFE with a concomitant recovery of PNCQDs fluorescence possible. Hence, the PNCQDs/Cr(VI) hybrid was used as an "off-on" fluorescence probe for sensing ascorbic acid (AA), which is a model reductant. For the detection of Cr(VI), the linear range and the limit of detection achieved were 1.5-30 μmol/L and 23 nmol/L, respectively. For the detection of AA, the linear range and the limit of detection obtained were 5.0-200 μmol/L and 1.35 μmol/L, respectively. The as-constructed "on-off-on" PNCQDs fluorescent nanoprobe was successfully applied for detecting Cr(VI) and AA in biosystem. Furthermore, the as-constructed fluorescent sensing system was successfully applied to the analyses of AA in fresh fruits and in commercial fruit juices with satisfactory results.

A simplistic approach to green future with eco-friendly luminescent carbon dots and their application to fluorescent nano-sensor 'turn-off' probe for selective sensing of copper ions

Zero-dimensional fluorescent nanoparticles having specificity as molecular probe appears to be strategically balanced fluorescent nano-probes. In this work, purified lemon extract and l-arginine have been thermally coupled for the extremely acute detection of Cu²⁺ in aqueous medium. The Cu²⁺ ions may be captured by the amino groups on the surface of the nano-sensor to form cupric ammine complex resulting in quenched fluorescence via an inner filter effect. Our proposed nano-probe is N-doped carbon dots (NCDs) which are efficiently selective as fluorescent chemosensor due to enormous binding affinity towards Cu²⁺ in a wide range of concentration (0.05-300μM) within a few minutes.

Effects of copper on viability and functional properties of hippocampal neurons in vitro

Copper (Cu²⁺) is an essential metal presented in the mammalian brain and released from synaptic vesicles following neuronal depolarization. However, the disturbance of Cu²⁺ homeostasis results in neurotoxicity. In our study we performed for the first time a combined functional investigation of cultured hippocampal neurons under Cu²⁺ exposure, its effect on spontaneous spike activity of hippocampal neuronal network cultured on multielectrode array (MEA), and development of long-term potentiation (LTP) in acute hippocampal slices in the presence of Cu²⁺. Application of 0.2mM CuCl₂ for 24h reduced viability of cultured neurons to 40±6%, whereas 0.01mM CuCl₂ did not influence significantly on the neuronal survival. However, exposure to the action of 0.01mM Cu²⁺ resulted in pronounced reduction of network spike activity and abolished LTP induced by high-frequency stimulation of Schaffer's collaterals in CA1 pyramidal neurons of hippocampal slices. Antioxidant Trolox, the hydrosoluble vitamin E analogue, prevented neurotoxic effect and alterations of network activity under Cu²⁺ exposure, but didn't change the impairment of LTP in Cu²⁺-exposured hippocampal slices. We hypothesized that spontaneous network neuronal activity probably is one of the potential targets of Cu²⁺-induced neurotoxicity, in which free radicals can be involved. At the same time, it may be suggested that Cu²⁺-induced alterations of long-lasting trace processes (like LTP) are not mediated by oxidative damage.

Hydrophobic-carbon-dot-based dual-emission micelle for ratiometric fluorescence biosensing and imaging of Cu2+ in liver cells

Copper is closely related to liver damage, therefore, it is essential to develop a simple and sensitive strategy to detect copper ions (Cu²⁺) in liver cells. A hydrophobic carbon dots (HCDs)-based dual-emission fluorescent probe for Cu²⁺ was prepared by encapsulating HCDs in micelles formed by self-assembly of amphiphilic polymer DSPE-PEG and tetrakis (4-carboxyphenyl) porphyrin (TCPP)-modified DSPE-PEG. The obtained probe showed characteristic fluorescence emissions of HCDs and TCPP with large emission shift of 170nm with single-wavelength excitation. In the presence of Cu²⁺, the fluorescence of TCPP was quenched and that of HCDs remained unchanged, displaying ratiometric fluorescence response to Cu²⁺. The developed probe exhibited high sensitivity (detection limit down to 36nM) and selectivity to Cu²⁺ over other substances, and the probe was used to image the changes of Cu²⁺ level in liver cells successfully.

Advances, challenges and promises of carbon dots

Carbon-dots with unique physical and chemical properties have versatile applications in environmental and energy fields.

Electrochemical Methods to Study Photoluminescent Carbon Nanodots: Preparation, Photoluminescence Mechanism and Sensing

With unique and tunable photoluminescence (PL) properties, carbon nanodots (CNDs) as a new class of optical tags have been extensively studied. Because of their merits of controllability and sensitivity to the surface of nanomaterials, electrochemical methods have already been adopted to study the intrinsic electronic structures of CNDs. In this review, we mainly deal with the electrochemical researches of CNDs, including preparation, PL mechanism, and biosensing.

Nitrogen and Phosphorus Co-Doped Carbon Nanodots as a Novel Fluorescent Probe for Highly Sensitive Detection of Fe(3+) in Human Serum and Living Cells

Chemical doping with heteroatoms can effectively modulate physicochemical and photochemical properties of carbon dots (CDs). However, the development of multi heteroatoms codoped carbon nanodots is still in its early stage. In this work, a facile hydrothermal synthesis strategy was applied to synthesize multi heteroatoms (nitrogen and phosphorus) codoped carbon nanodots (N,P-CDs) using glucose as carbon source, and ammonia, phosphoric acid as dopant, respectively. Compared with CDs, the multi heteroatoms doped CDs resulted in dramatic improvement in the electronic characteristics and surface chemical activities. Therefore, the N,P-CDs prepared as described above exhibited a strong blue emission and a sensitive response to Fe(3+). The N,P-CDs based fluorescent sensor was then applied to sensitively determine Fe(3+) with a detection limit of 1.8 nM. Notably, the prepared N,P-CDs possessed negligible cytotoxicity, excellent biocompatibility, and high photostability. It was also applied for label-free detection of Fe(3+) in complex biological samples and the fluorescence imaging of intracellular Fe(3+), which indicated its potential applications in clinical diagnosis and other biologically related study.

One-step synthesis of amikacin modified fluorescent carbon dots for the detection of Gram-negative bacteria like Escherichia coli

In this paper, we report a one-step strategy to synthesize amikacin modified fluorescent carbon dots (CDs@amikacin) for assaying pathogenic bacteria, Escherichia coli.

Simple and signal-off electrochemiluminescence immunosensor for alpha fetoprotein based on gold nanoparticle-modified graphite-like carbon nitride nanosheet nanohybrids

A simple, signal-off electrochemiluminescence immunosensor for sensitive and selective detection of alpha fetoprotein was developed based on gold nanoparticle modified graphite-like carbon nitride nanosheet nanohybrids.

Tunable Fluorescent Silica-Coated Carbon Dots: A Synergistic Effect for Enhancing the Fluorescence Sensing of Extracellular Cu²⁺ in Rat Brain

Carbon quantum dots (CDs) combined with self-assembly strategy have created an innovative way to fabricate novel hybrids for biological analysis. This study demonstrates a new fluorescence platform with enhanced selectivity for copper ion sensing in the striatum of the rat brain following the cerebral calm/sepsis process. Here, the fabrication of silica-coated CDs probes is based on the efficient hybridization of APTES which act as a precursor of organosilane self-assembly, with CDs to form silica-coated CDs probes. The fluorescent properties including intensity, fluorescence quantum yield, excitation-independent region, and red/blue shift of the emission wavelength of the probe are tunable through reliable regulation of the ratio of CDs and APTES, realizing selectivity and sensitivity-oriented Cu(2+) sensing. The as-prepared probes (i.e., 3.33% APTES-0.9 mg mL(-1) CDs probe) show a synergistic amplification effect of CDs and APTES on enhancing the fluorescence signal of Cu(2+) detection through fluorescent self-quenching. The underlying mechanism can be ascribed to the stronger interaction including chelation and electrostatic attraction between Cu(2+) and N and O atoms-containing as well as negatively charged silica-coated CDs than other interference. Interestingly, colorimetric assay and Tyndall effect can be observed and applied to directly distinguish the concentration of Cu(2+) by the naked eye. The proposed fluorescent platform here has been successfully applied to monitor the alteration of striatum Cu(2+) in rat brain during the cerebral calm/sepsis process. The versatile properties of the probe provide a new and effective fluorescent platform for the sensing method in vivo sampled from the rat brain.

A two-photon "turn-on" fluorescent probe based on carbon nanodots for imaging and selective biosensing of hydrogen sulfide in live cells and tissues

Determination of hydrogen sulfide (H2S) in live cells and tissues is still a challenge for evaluating the key roles that H2S plays in physiological and pathological processes. In this work, a "turn-on" two-photon fluorescent (TPF) sensor for H2S is developed, in which carbon nanodot (C-Dot) was employed as a two-photon fluorophore due to its large two-photon absorption cross-section (σ), and AE-TPEA-Cu(2+) complex [AE-TPEA = N-(2-aminoethyl)-N,N,N'-tris(pyridin-2-ylmethyl)ethane-1,2-diamine] was first designed as a specific receptor for H2S. The fluorescence of C-Dot conjugated with AE-TPEA (C-Dot-TPEA) was quenched upon the addition of Cu(2+). Then, the fluorescence was restored after the addition of H2S, because Cu(2+) could be released from TPEA binding site when H2S interacted with the Cu(2+) ion. The designed C-Dot-TPEA-Cu(2+) fluorescent sensor exhibited high specificity for H2S over biothiols, sulfur-containing compounds, reactive oxygen species (ROS), and other biological interferences. Meanwhile, a broad linear range from 5 μM to 100 μM was obtained and the detection limit was achieved to 0.7 μM. In addition, the C-Dot-based TPF probe exhibited bright two-photon fluorescence, favourable photostability against light illumination and pH change, and low cytotoxicity. Accordingly, the nanohybridized TPF sensor with high selectivity and sensitivity, as well as the fascinating properties of C-Dot themselves, successfully provided a new way for TPF imaging and biosensing of H2S in live cells and tissues. We believe this is the first report of TPF imaging and biosensing of H2S in live cells and tissues using a specially engineered C-Dot-based nanosystem.

Synthesis of highly fluorescent nitrogen and phosphorus doped carbon dots for the detection of Fe(3+) ions in cancer cells

Highly fluorescent nitrogen and phosphorus-doped carbon dots with a quantum yield 59% have been successfully synthesized from citric acid and di-ammonium hydrogen phosphate by single step hydrothermal method. The synthesized carbon dots have high solubility as well as stability in aqueous medium. The as-obtained carbon dots are well monodispersed with particle sizes 1.5-4 nm. Owing to a good tunable fluorescence property and biocompatibility, the carbon dots were applied for intercellular sensing of Fe(3+) ions as well as cancer cell imaging.

Single Probe for Imaging and Biosensing of pH, Cu(2+) Ions, and pH/Cu(2+) in Live Cells with Ratiometric Fluorescence Signals

It is very essential to disentangle the complicated inter-relationship between pH and Cu in the signal transduction and homeostasis. To this end, reporters that can display distinct signals to pH and Cu are highly valuable. Unfortunately, there is still no report on the development of biosensors that can simultaneously respond to pH and Cu(2+), to the best of our knowledge. In this work, we developed a single fluorescent probe, AuNC@FITC@DEAC (AuNC, gold cluster; FITC, fluorescein isothiocyanate; DEAC, 7-diethylaminocoumarin-3-carboxylic acid), for biosensing of pH, Cu(2+), and pH/Cu(2+) with different ratiometric fluorescent signals. First, 2,2',2″-(2,2',2″-nitrilotris(ethane-2,1-diyl)tris((pyridin-2-yl-methyl)azanediyl))triethanethiol (TPAASH) was designed for specific recognition of Cu(2+), as well as for organic ligand to synthesize fluorescent AuNCs. Then, pH-sensitive molecule, FITC emitting at 518 nm, and inner reference molecule, DEAC with emission peak at 472 nm, were simultaneously conjugated on the surface of AuNCs emitting at 722 nm, thus, constructing a single fluorescent probe, AuNC@FITC@DEAC, to sensing pH, Cu(2+), and pH/Cu(2+) excited by 405 nm light. The developed probe exhibited high selectivity and accuracy for independent determination of pH and Cu(2+) against reactive oxygen species (ROS), other metal ions, amino acids, and even copper-containing proteins. The AuNC-based inorganic-organic probe with good cell-permeability and high biocompatibility was eventually applied in monitoring both pH and Cu(2+) and in understanding the interplaying roles of Cu(2+) and pH in live cells by ratiometric multicolor fluorescent imaging.

Carbon nanomaterial-based electrochemical biosensors: an overview

Carbon materials on the nanoscale exhibit diverse outstanding properties, rendering them extremely suitable for the fabrication of electrochemical biosensors. Over the past two decades, advances in this area have continuously emerged. In this review, we attempt to survey the recent developments of electrochemical biosensors based on six types of carbon nanomaterials (CNs), i.e., graphene, carbon nanotubes, carbon dots, carbon nanofibers, nanodiamonds and buckminsterfullerene. For each material, representative samples are introduced to expound the different roles of the CNs in electrochemical bioanalytical strategies. In addition, remaining challenges and perspectives for future developments are also briefly discussed.

Glowing graphene quantum dots and carbon dots: properties, syntheses, and biological applications

The emerging graphene quantum dots (GQDs) and carbon dots (C-dots) have gained tremendous attention for their enormous potentials for biomedical applications, owing to their unique and tunable photoluminescence properties, exceptional physicochemical properties, high photostability, biocompatibility, and small size. This article aims to update the latest results in this rapidly evolving field and to provide critical insights to inspire more exciting developments. We comparatively review the properties and synthesis methods of these carbon nanodots and place emphasis on their biological (both fundamental and theranostic) applications.

Synthesis of polyethyleneimine capped carbon dots for preconcentration and slurry sampling analysis of trace chromium in environmental water samples

Carbon dots capped with polyethyleneimine (CD-PEI) were synthesized and applied in selective separation and preconcentration of trace Cr(VI). Dispersed particle extraction (DPE) slurry sampling with flame atomic absorption spectrometry (FAAS) was used to selectively and sensitively determine Cr(VI) in water samples. The as-synthesized CD-PEI was confirmed by Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, elemental analysis, fluorescence and zeta potential measurement. The adsorption of Cr(VI) on CD-PEI was evaluated. Its isothermal adsorption was studied and fitted in the Langmuir model. Nearly 85% of Cr(VI) was adsorbed within 10 min showed that the CD-PEI exhibited fairly fast kinetics for the sorption of Cr(VI). Experimental conditions, including the content and size of CD-PEI, sample pH, adsorption time, sample volume, slurry volume and interfering ions, were further optimized to obtain efficient preconcentration and high-precision determination of Cr(VI). CD-PEI with small size turned to be a good candidate for the preparation of slurry. CD-PEI served not only as a promising adsorbent for separation and preconcentration of Cr, but also a signal-enhancing agent in FAAS. The method achieved an enhancement factor of 30 and a detection limit (S/N=3) of 0.21 µg L(-1) Cr(VI) with a consumption of 14.0 mL sample and an adsorption time of 5 min, which provided two times of signal enhancement. The RSD for 11 replicate measurements of 5.0 µg L(-1) Cr(VI) was 2.8%. The possible signal enhancement mechanism was proposed. The developed method has been applied to determine trace Cr(VI) in a variety of water samples.