Tyrosine-functionalized CuInS2 quantum dots as a fluorescence probe for the determination of biothiols, histidine and threonine

Construction of a Dual-Mode Biosensor with Ferrocene as Both a Signal Enhancer and a Signal Tracer for Electrochemiluminescent and Electrochemical Enantioselective Recognition

We demonstrate for the first time the construction of a dual-mode biosensor for electrochemiluminescent (ECL) and electrochemical chiral recognition of l- and d-isomers of amino acids, with ferrocene (Fc) as both a signal enhancer and a signal tracer. With the dissolved oxygen as a coreactant, ZnIn2S4 acts as the ECL emitter to generate a weak cathodic ECL signal. Fc can enter into the β-cyclodextrin (β-CD) cavity on ZnIn2S4-modified electrode as a result of host-guest interaction. Since Fc can promote H2O and O2 to produce abundant reactive oxygen species (ROS) (e.g., O2·- and ·OH), the ECL signal of ZnIn2S4 can be further amplified with Fc as a coreaction accelerator. Meanwhile, Fc molecules on the β-CD/ZnIn2S4-modified electrode can be electrochemically oxidized to Fc+ to produce a remarkable oxidation peak current. When l-histidine (l-His) is present, the matching of the l-His configuration with the β-CD cavity leads to the entrance of more l-His into the cavity of β-CD than d-histidine (d-His), and the subsequent competence of l-His with Fc on the Fc/β-CD/ZnIn2S4-modified electrode induces the decrease in both Fc peak current and ZnIn2S4-induced ECL intensity. This dual-mode biosensor can efficiently discriminate l-His from d-His, and it can sensitively monitor l-His with a detection limit of 7.60 pM for ECL mode and 3.70 pM for electrochemical mode. Moreover, this dual-mode biosensor can selectively discriminate l-His from other l- and d-isomers (e.g., threonine, phenylalanine, and glutamic acid), with potential applications in the chiral recognition of nonelectroactive chiral compounds, bioanalysis, and disease diagnosis.

Detection of mercury(II) and glutathione using a carbon dots-based "off-on" fluorescent sensor and the construction of a logic gate

In this paper, we proposed an efficient method for mercury(II) and glutathione detection using a fluorescent nanoprobe as a sensor. Carbon dots were synthesized from polyethyleneimine and ammonium citrate via a one-step hydrothermal method. The fluorescence of carbon dots was quenched since electron transfer occurred due to the interaction between mercury(II) and functional groups on the surface of carbon dots. Adding glutathione to the carbon dots-mercury(II) system, the fluorescence was recovered due to the stronger binding ability of glutathione to mercury(II). Based on the above-mentioned principle, this "off-on" fluorescent sensor can easily achieve the detection of mercury(II) and glutathione, which provided limits of detection of 22.45 nM and 61.89 nM, respectively. In this paper, the proposed method has been applied to detect mercury(II) and glutathione in real lake water and serum, respectively, and a logic gate for sensing glutathione was presented. The developed "off-on" fluorescent sensor with high sensitivity and selectivity has shown great potential for mercury(II) and glutathione detection in environmental and biosensing fields.

Fluorescence assay for acrylamide determination in fried products based on AgInS2/ZnS quantum dots

AgInS₂/ZnS quantum dots were synthesized via solvothermal aqueous phase method using 3-mercaptopropionic acid as the stabilizer. AgInS₂/ZnS quantum dots were employed for acrylamide sensing under two strategies: (1) quenching of the fluorescence signal by the synthesis of polyacrylamide under UV light and (2) use of 2-naphthalenethiol for quenching of the fluorescence signal of quantum dots followed by a recovery of the signal after the addition of acrylamide. Both methodologies display adequate limits of detection, 15.6 and 4.8 μg L^-1, respectively. However, the 2-napthalenethiol based method exhibited better precision and selectivity compared to the other methodology. Both methodologies were applied for acrylamide detection in fried snack products and acceptable accuracy was obtained using 2-napthalenethiol method.

Ternary Quantum Dots in Chemical Analysis. Synthesis and Detection Mechanisms

Ternary quantum dots (QDs) are novel nanomaterials that can be used in chemical analysis due their unique physicochemical and spectroscopic properties. These properties are size-dependent and can be adjusted in the synthetic protocol modifying the reaction medium, time, source of heat, and the ligand used for stabilization. In the last decade, several spectroscopic methods have been developed for the analysis of organic and inorganic analytes in biological, drug, environmental, and food samples, in which different sensing schemes have been applied using ternary quantum dots. This review addresses the different synthetic approaches of ternary quantum dots, the sensing mechanisms involved in the analyte detection, and the predominant areas in which these nanomaterials are used.

A turn-on fluorescence strategy for biothiols determination by blocking Hg(II)-mediated fluorescence quenching of adenine-rich DNA-templated gold nanoclusters

Fluorescent adenine (A)-rich DNA-templated gold nanoclusters were demonstrated to be a novel probe for determination of biothiols (including cysteine, glutathione, and homocysteine). Fluorescence intensity of adenine-rich DNA-templated gold nanoclusters could be greatly quenched by Hg(II) ions through the formation of a gold nanoclusters-Hg(II) system. When biothiols (cysteine as the model) were introduced into the system, the fluorescence intensity recovered due to the formation of a more stable Hg(II)-thiol coordination complex using Hg-S metal-ligand bonds, which inhibited the Hg(II)-mediated fluorescence quenching of adenine-rich DNA-templated gold nanoclusters. Based on this fluorescence phenomenon, an on-off-on fluorescence strategy was designed for the sensitive determination of biothiols. The method allowed sensitive detection of cysteine with a linear detection range from 100 nM to 5 μM and a limit of detection of 30 nM. Additionally, the assay can be applied for detection of biothiol levels in human plasma samples. Therefore, it can provide a simple and rapid fluorescent platform for biothiol detection.

The Role of Amino Acids in Neurotransmission and Fluorescent Tools for Their Detection

Neurotransmission between neurons, which can occur over the span of a few milliseconds, relies on the controlled release of small molecule neurotransmitters, many of which are amino acids. Fluorescence imaging provides the necessary speed to follow these events and has emerged as a powerful technique for investigating neurotransmission. In this review, we highlight some of the roles of the 20 canonical amino acids, GABA and β-alanine in neurotransmission. We also discuss available fluorescence-based probes for amino acids that have been shown to be compatible for live cell imaging, namely those based on synthetic dyes, nanostructures (quantum dots and nanotubes), and genetically encoded components. We aim to provide tool developers with information that may guide future engineering efforts and tool users with information regarding existing indicators to facilitate studies of amino acid dynamics.

Fluorescence "Off-On" Probe for L-Cysteine Detection Based on Nitrogen Doped Carbon Dots

Herein, a simple and efficient fluorescence analysis method for L-Cysteine (L-Cys) was established. The method was based on the fluorescent "off-on" mode of nitrogen doped carbon dots (NCDs). The NCDs were prepared via a facile one-step solvothermal method. In the process of exploring the bio-functional application of these newly synthesized NCDs, we found these NCDs with rich functional groups exhibited excellent optical properties. In addition, these newly synthesized NCDs showed an excitation-dependent emissions photolumine-scent (PL) property and exhibited good performance in the detection of Fe³⁺ ions by quenching the blue emission fluorescence. Interestingly, the quenched fluorescence of NCDs was recovered with the addition of L-Cys, which provided a novel approach for L-Cys detection. The NCDs-based fluorescent "off-on" sensor has a wide linear detection range (0-100 μM), and a relatively low detection limits (0.35 μM) for L-Cys. This simple fluorescent "off-on" approach is, very sensitive and selective for L-Cys detection, which also provides a new insight on NCDs biosensor application.

A novel label-free colorimetric detection of l-histidine using Cu2+-modulated G-quadruplex-based DNAzymes

We proposed a colorimetric method for l-histidine detection based on Cu²⁺-mediated DNAzyme and G-quadruplex-hemin complex catalyzed oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS). In this system, after the addition of l-histidine, the formation of G-quadruplex-hemin complex will be disturbed, thus the colorimetric signal intensity conversely corresponds to the concentration of histidine. In this assay, a lower detection limit of l-histidine (50 nM) is addressed comparing to previously reported colorimetric methods. The cost is extremely low as the proposed design is both label-free and enzyme-free. All the more vitally, the colorimetric detection procedure is substantially straightforward with no further modification processes. By and large, the sensor can provide a promising plan for the detection of l-histidine.

Electrochemically prepared oxygen and sulfur co-doped graphitic carbon nitride quantum dots for fluorescence determination of copper and silver ions and biothiols

Although great advances have been achieved in synthesis of fluorescent graphitic carbon nitride quantum dots (g-C₃N₄-dots), it is still challenging to develop g-C₃N₄-dots with high fluorescence quantum yield (FLQY) and multiple sensing functionalities. Herein, the oxygen and sulfur co-doped graphitic carbon nitride quantum dots (OS-g-C₃N₄-dots) with high FLQY of 33.9% were firstly synthesized by a simple electrochemical "tailoring" process. It was found that OS-g-C₃N₄-dots could specifically bind copper ions (Cu²⁺) and silver ions (Ag⁺), accompanied with a dramatic "turn-off" fluorescence response. With the help of different masking agents, OS-g-C₃N₄-dots are able to selectively detect Cu²⁺ and Ag⁺. Furthermore, the generated OS-g-C₃N₄-dots/Ag⁺ displayed a "turn-on" fluorescent response specific to biothiols (HCy, Cys and GSH). Therefore, the multiple functional sensing platforms based on "ON-OFF-ON" fluorescence response of OS-g-C₃N₄-dots for the detection of Cu²⁺, Ag⁺ and biothiols were constructed. Under the optimal conditions, the detection limits of Cu²⁺, Ag⁺, HCy, Cys and GSH were as low as 7.0 × 10^-10 M, 2.0 × 10^-9 M, 1.0 × 10^-8 M, 1.0 × 10^-8 M and 8.4 × 10^-9 M, respectively. Moreover, the prepared platforms could be successfully applied to the determination of Cu²⁺, Ag⁺ and biothiols in practical samples and exhibited excellent sensitivity and selectivity.

Optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals

Optically-active nanocrystals (such as quantum dots and plasmonic noble metal nanoparticles) have received great attention due to their size-tunable optical properties. The indicator displacement assay (IDA) with optically-active nanocrystals has become a common practice for optical sensor development, since no sophisticated surface functionalization of nanoparticles is required. Among the IDA-based optical sensors, the use of metal ions as receptors seems to be attractive. Therefore, in this review, the research progress of optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals has been summarized. In particular, metal ion-mediated selective recognition has been summarized here based on the classical Hard-Soft-Acid-Base (HSAB) principle, which has been seldom mentioned before. Most of the references were therefore categorized according to their located place based on the HSAB theory. Besides, several metal ion modulation strategies that were not related to the HSAB theory (e.g., redox modulation) were also included. Finally, due to the cross-talk of metal ions in selective recognition, we have also summarized sensor array development based on multiple metal ion receptors in IDA sensing with optically-active nanocrystals. Several interesting applications of the IDA sensing with metal ions as receptors and optically-active nanocrystals as indicators are presented, with specific emphasis on the design principles and photophysical mechanisms of these probes.

Cu2+ modulated nitrogen-doped grapheme quantum dots as a turn-off/on fluorescence sensor for the selective detection of histidine in biological fluid

A highly sensitive sensor for detection of histidine (His) based on the nitrogen-doped graphene quantum dots (N-GQDs)-Cu²⁺ system has been designed. The N-GQDs were synthesized by one-step hydrothermal approach according to previous report. The fluorescence of N-GQDs can be effectively quenched by Cu²⁺ due to the binding between Cu²⁺ and functional groups on the surface of N-GQDs. The high affinity of His to Cu²⁺ enables Cu²⁺ to be dissociated from the surface of N-GQDs and recovering the fluorescence. The sensor displayed a sensitive response to His in the concentration range of 0-35μmolL^-1, with a detection limit of 72.2nmolL^-1. The proposed method is successfully applied to detect His in samples with a recovery range of 96-102%.

A label-free fluorimetric detection of biothiols based on the oxidase-like activity of Ag+ ions

In this work, a label-free and sensitive fluorimetric method has been developed for the detections of biothiols including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), based on the specific biothiol-induced inhibition of the oxidase-like activity of silver ions (Ag⁺). It is well established that o-phenylenediamine (OPD) can be oxidized by Ag⁺ ions to generate fluorescent 2,3-diaminophenazine (OPDox). The introduction of biothiols would inhibit the oxidation of OPD by Ag⁺ due to the strong coordination between biothiols and Ag⁺. The changes of fluorescence intensities obtained in the Ag⁺-OPD system exhibited good linear correlations in the ranges of 0.50-30.0μM for Cys, 1.0-45.0μM for Hcy and 0.50-40.0μM for GSH. The detection limits (S/N=3) of Cys, Hcy and GSH were 110nM, 200nM and 150nM, respectively. Subsequently, the developed fluorimetric method was successfully applied for the detection of biothiols in human serum.

Highly sensitive fluorescent detection of glutathione and histidine based on the Cu(ii)-thiamine system

A novel fluorescence method for the simultaneous detection of glutathione and histidine based on their inhibitory effects on the oxidation of thiamine by Cu(ii) is proposed.

An integrated logic system for time-resolved fluorescent "turn-on" detection of cysteine and histidine base on terbium (III) coordination polymer-copper (II) ensemble

Cysteine (Cys) and histidine (His) both play indispensable roles in many important biological activities. An enhanced Cys level can result in Alzheimer's and cardiovascular diseases. Likewise, His plays a significant role in the growth and repair of tissues as well as in controlling the transmission of metal elements in biological bases. Therefore, it is meaningful to detect Cys and His simultaneously. In this work, a novel terbium (III) coordination polymer-Cu (II) ensemble (Tb(3+)/GMP-Cu(2+)) was proposed. Guanosine monophosphate (GMP) can self-assemble with Tb(3+) to form a supramolecular Tb(3+) coordination polymer (Tb(3+)/GMP), which can be suited as a time-resolved probe. The fluorescence of Tb(3+)/GMP would be quenched upon the addition of Cu(2+), and then the fluorescence of the as-prepared Tb(3+)/GMP-Cu(2+) ensemble would be restored again in the presence of Cys or His. By incorporating N-Ethylmaleimide and Ni(2+) as masking agents, Tb(3+)/GMP-Cu(2+) was further exploited as an integrated logic system and a specific time-resolved fluorescent "turn-on" assay for simultaneously sensing His and Cys was designed. Meanwhile it can also be used in plasma samples, showing great potential to meet the need of practical application.

Graphitic carbon nitride nanodots: As reductant for the synthesis of silver nanoparticles and its biothiols biosensing application

The graphitic carbon nitride nanodots (g-C₃N₄-dots) were synthesized by a simple electrochemical "tailoring" process from bulk graphitic carbon nitride (g-C₃N₄) under alkaline solution for the first time. Compared with the bulk g-C₃N₄, the novel g-C₃N₄-dots not only exhibit enhanced fluorescence and excellent dispersion stability in water but also show the reducibility for the reduction of Ag⁺ to AgNPs at 60°C. The biothiols can bound with Ag⁺ through formation of biothiol-Ag⁺ complex to consume the Ag⁺ and act as capping agent to prevent the growth of AgNPs, which cause the decrease of the absorption peak of the AgNPs. Therefore, an optical sensor was developed for the detection of biothiols based on the change of the plasmon resonance absorption peak of the AgNPs. The proposed method exhibits excellent sensitivity and selectivity to biothiols with low detection limit for cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) with 11.5, 16.1, and 15.5nM, respectively. This method also has been successfully applied for the detection of biothiols in human serum with satisfactory results.

Nitrogen-doped carbon nanoparticle modulated turn-on fluorescent probes for histidine detection and its imaging in living cells

In this work, nitrogen-doped carbon nanoparticle (N-CNP) modulated turn-on fluorescent probes were developed for rapid and selective detection of histidine. The as synthesized N-CNPs exhibited high fluorescence quantum yield and excellent biocompatibility. The fluorescence of N-CNPs can be quenched selectively by Cu(II) ions with high efficiency, and restored by the addition of histidine owing to the competitive binding of Cu(II) ions and histidine that removes Cu(II) ions from the surface of the N-CNPs. Under the optimal conditions, a linear relationship between the increased fluorescence intensity of N-CNP/Cu(II) ion conjugates and the concentration of histidine was established in the range from 0.5 to 60 μM. The detection limit was as low as 150 nM (signal-to-noise ratio of 3). In addition, the as-prepared N-CNP/Cu(II) ion nanoprobes showed excellent biocompatibility and were applied for a histidine imaging assay in living cells, which presented great potential in the bio-labeling assay and clinical diagnostic applications.

Integrated logic gate for fluorescence turn-on detection of histidine and cysteine based on Ag/Au bimetallic nanoclusters-Cu²⁺ ensemble

By means of employing 11-mercaptoundecanoic acid (11-MUA) as a reducing agent and protecting ligand, we present straightforward one-pot preparation of fluorescent Ag/Au bimetallic nanoclusters (namely AgAuNCs@11-MUA) from AgNO3 and HAuCl4 in alkaline aqueous solution at room temperature. It is found that the fluorescence of AgAuNCs@11-MUA has been selectively quenched by Cu(2+) ions, and the nonfluorescence off-state of the as-prepared AgAuNCs@11-MUA-Cu(2+) ensemble can be effectively switched on upon the addition of histidine and cysteine. By incorporating Ni(2+) ions and N-ethylmaleimide, this phenomenon is further exploited as an integrated logic gate and a specific fluorescence turn-on assay for selectively and sensitively sensing histidine and cysteine has been designed and established based on the original noncovalent AgAuNCs@11-MUA-Cu(2+) ensemble. Under the optimal conditions, histidine and cysteine can be detected in the concentration ranges of 0.25-9 and 0.25-7 μM; besides, the detection limits are found to be 87 and 111 nM (S/N = 3), respectively. Furthermore, we demonstrate that the proposed AgAuNCs@11-MUA-based fluorescent assay can be successfully utilized for biological fluids sample analysis.

A novel glutathione-stabilized silver–gold nano-alloy/Cu2+ combination as a fluorescent switch probe for l-histidine

This paper reports the synthesis of glutathione-stabilized silver–gold nano-alloys and their use as a fluorescent switch probe for the detection of l-histidine.

Nitrogen-doped graphene quantum dots-based fluorescent probe for the sensitive turn-on detection of glutathione and its cellular imaging

Fluorescence turn-on sensor based on nitrogen-doped graphene quantum dots can be used for glutathione detection in living cells.