Doped zinc sulfide quantum dots based phosphorescence turn-off/on probe for detecting histidine in biological fluid

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.

Dual-emission fluorescence detection of histidine using carbon dots and calcein/Ni2+ complexes

Histidine (His) is a natural amino acid that plays very important roles in biota. However, the low concentrations of His in biological fluids and the similar structures and properties of other amino acids mean it is difficult to selectively determine His concentrations in biological fluids with a high degree of sensitivity. A novel ratiometric fluorescence probe for detecting His in aqueous solutions is described here. The method involves carbon dots (CDs) and calcein/Ni²⁺ complexes. At an excitation wavelength of 480 nm, the CD/calcein system emits green fluorescence (maximum emission from calcein at 512 nm) and red fluorescence (maximum emission from CDs at 617 nm). The presence of Ni²⁺ decreases the calcein fluorescence intensity because of static quenching caused by the formation of calcein/Ni²⁺ complexes but the CD fluorescence intensity remains almost unchanged. Fluorescence of calcein/Ni²⁺ complexes provides the response, and the presence of His binds to Ni²⁺ via cooperative chelation and produces free calcein causing fluorescence to be recovered. CDs provide a self-calibration fluorescence signal, the intensity of which remains almost unchanged in the presence of His. The ratio of the fluorescence intensities at 512 and 617 nm (I₅₁₂/I₆₁₇) was strongly related to the His concentration in the range 0.5-22 μM, and the detection limit was 0.16 μM. The specificity of Ni²⁺/His interactions allows His to be determined without interference from other species. The method was successfully used to determine His in diluted human urine. The recovery was acceptable, suggesting that the biosensor can be used to determine His in real samples.

A neutral mononuclear rhenium(I) complex with a rare in situ-generated triazolyl ligand for the luminescence "turn-on" detection of histidine

A rare in situ-generated mononuclear rhenium complex [Re(bpt)(CO)₃(NH₃)] (1, bpt = 3,5-bis(2-pyridyl)-1,2,4-triazolate) can be used as a "turn-on" luminescent probe for selectively sensing L-histidine against other amino acids. Compound 1 was prepared by reacting Re₂(CO)₁₀, 2-cyanopyridine and hydrazine with an in situ formed bpt ligand through cyclization via C-N and N-N couplings with its single-side chelating mode arrayed with respect to the Re center. Compound 1 was highly stable and showed a green light MLCT emission in DMF solution at 507 nm upon excitation at 360 nm. Interestingly, the emission from 1 could be quenched by the addition of metal ions such as Ni²⁺ and Cu²⁺ but the emission efficiently recovered with the introduction of histidine. However, histidine could only be selectively detected when a combination of compound 1 and Ni²⁺ was used. Therefore, the luminescence response of the Ni²⁺-modified compound 1 could be utilized as a "turn-on" probe for the selective detection of histidine. This work provides a simple method for developing new sensing platforms of a discrete metal complex based on rare in situ generation.

Design and Evaluation of a Competitive Phosphorescent Immunosensor for Aflatoxin M1 Quantification in Milk Samples Using Mn:ZnS Quantum Dots as Antibody Tags

Aflatoxin M1 (AFM1) is one of the most widespread aflatoxins that can be present in the milk of lactating mammals. It can cause carcinogenicity, mutagenesis, teratogenesis, genotoxicity and immunosuppression. The WHO recommends reducing the AFM1 concentration in food products, so the European Commission has set a maximum allowable limit of 0.05 µg L−1 in milk and its products. Thus, there is a need to develop new methodologies to satisfy the demand for reliable, cost-effective, robust and sensitive AFM1 routine controls. In the present work, a competitive phosphorescent immunosensor for AFM1 quantification in milk, based on antibody–antigen recognition and Mn:ZnS quantum dots (d-QDs) as photoluminescent labels, has been developed. Two different assay strategies based on the use of d-QDs as labels of secondary antibodies (direct assay), or of a derivative species of the antigen AFM1-Bovine Serum Albumin (indirect assay) were compared in terms of analytical performance for AFM1 quantification. The best analytical results were obtained with the immunoassay format that uses d-QDs as tags of secondary antibodies (direct assay), and said design was finally selected. The selected immunosensor provided a detection limit for AFM1 quantification of only 0.002 µg L−1, which greatly satisfied the maximum tolerable limit of AFM1 in milk of 0.05 µg L−1. The accuracy, calculated as recovery of AFM1 in fortified skimmed milk samples, ranged from 81 to 90%, with relative standard deviations from 3% to 14%. These results bring to light the good performance of such phosphorescent biosensors as simple and fast alternatives to conventional chromatographic analytical methods.

A novel electrochemical sensor for the determination of histidine based on a molecularly imprinted copolymer

The present study aimed to report a novel electrochemical sensor through electropolymerization of o-aminophenol (o-AP) and m-dihydroxy benzene (m-DB) as monomers on the surface of the glassy carbon electrode (GCE) for the determination of histidine (His) as a template molecule. The developed sensor exhibited satisfactory sensitivity and high selectivity, and also offered a linear range between 0.005 and 10.0 μM with a detection limit of 0.9 nM. Finally, it is worth mentioning that we also aimed at employing the proposed sensor for the detection of His in blood serum samples.

Vitamin B6 cofactors guided highly selective fluorescent turn-on sensing of histamine using beta-cyclodextrin stabilized ZnO quantum dots

Histamine, one of the most important biogenic amines (BAs) is considered as food hazard and therefore various agencies have fixed threshold in different food and beverages. In this manuscript, two novel fluorescent turn-on probes were developed for the instantaneous detection of histamine. The β-cyclodextrin (β-CD) capped ZnO quantum dots (QDs) were decorated with the vitamin B₆ cofactors like pyridoxal 5'-phosphate (PLP) and pyridoxal (Py) by forming host-guest inclusion complexation between the capped β-CD and PLP/Py. The cofactors decorated QDs (ZnO@PLP and ZnO@Py) were applied for the sensing of BAs. Addition of histamine to the ZnO@PLP and ZnO@Py solution resulted selective fluorescence enhancement at 473 nm and 460 nm, respectively. Without any interference from the other tested BAs, the fluorescence response of the probes ZnO@PLP and ZnO@Py showed good linearity to histidine concentration from 2.49 to 24.4 μM and 7.44 to 47.6 μM with the detection limit down to 0.59 μM and 0.97 μM, respectively.

Histidine-triggered turning-on of gold/copper nanocluster fluorescence for the sensitive and selective detection of histidine

Gold/copper nanoclusters with histidine-triggered turn-on fluorescence for sensing histidine sensitively and selectively.

An "off-on" phosphorescent aptasensor for the detection of thrombin based on PRET

Thrombin plays an important role in the blood coagulation cascade and it stimulates the process of platelet aggregation. Herein, we developed a highly efficient and sensitive phosphorescent aptasensor system for the quantitative analysis of thrombin. The phosphorescence of 3-mercaptopropionic acid capped Mn-doped ZnS quantum dots (MPA-Mn:ZnS QDs) was gradually quenched with the addition of thrombin binding aptamers-BHQ2 (TBA-BHQ2) based on phosphorescence resonance energy transfer (PRET). With the addition of the target analyte thrombin into the system, TBA-BHQ2 could change its spatial structure from a random coil to an antiparallel G-quadruplex which resulted from the combination of thrombin and TBA-BHQ2, leading to the phosphorescence recovery. Finally, the concentration of thrombin could be accurately determined by means of measuring the phosphorescence intensity change value (ΔP). The limit of detection (LOD) was obtained as low as 15.26 pM with wide linear ranges both from 60 to 2000 pM and from 2 to 900 nM. The proposed strategy was also successfully applied for thrombin detection in human serum samples and plasma samples with satisfactory recoveries from 96% to 99% and 95% to 104%, respectively. The long lifetime of phosphorescent QDs possessed a suitable time delay to eliminate autofluorescence and scattered light interference from biological matrices effectively. Thus, the signal to noise ratio of the phosphorescent aptasensor was improved visibly for the analysis of target analytes.

Bright Yellow Fluorescent Carbon Dots as a Multifunctional Sensing Platform for the Label-Free Detection of Fluoroquinolones and Histidine

Owing to their diverse properties, fluorescent carbon dots (CDs) have attracted more attention and present enormous potential in development of sensors, bioimaging, drug delivery, microfluidics, photodynamic therapy, light emitting diode, and so forth. Herein, a multifunctional sensing platform based on bright yellow fluorescent CDs (Y-CDs) was designed for the label-free detection of fluoroquinolones (FQs) and histidine (His). The Y-CDs with superior optical and biological merits including high chemical stability, good biocompatibility, and low cytotoxicity were simply synthesized via one-step hydrothermal treatment of o-phenylenediamine ( o-PD) and 4-aminobutyric acid (GABA). The Y-CDs can be utilized to directly monitor the amount of FQs based on fluorescence static quenching owing to the specific interaction between FQs and Y-CDs. Then, the fluorescence of this system can be effectively recovered upon addition of His. The multifunctional sensing platform exhibited high sensitivity and selectivity toward three kinds of FQs and His with low detection limits of 17-67 and 35 nM, respectively. Benefiting from these outstanding characters, the Y-CDs were successfully employed for trace detection of FQs in real samples such as antibiotic tablets and milk products. Furthermore, the probe was also extended to cellular imaging. All of the above prove that this multifunctional sensing platform presents great prospect in multiple applications such as biosensing, biomedicine, disease diagnosis, and environmental monitoring.

An ''off-on'' phosphorescent aptasensor switch for the detection of ATP

An "off-on" phosphorescent aptasensor based on the 3-mercaptopropionic acid (MPA) capped Mn-doped ZnS quantum dots (MPA-Mn:ZnS QDs)/aptamer hybrid system was developed to detect adenosine triphosphate (ATP) in biological fluids. The phosphorescence of MPA-Mn:ZnS QDs was obviously quenched when ATP aptamer was added due to the aggregation induced effect. ATP aptamer, adsorbed on the surface of the phosphorescent MPA-Mn:ZnS QDs, has a high affinity for ATP. And then, with the addition of ATP, phosphorescence was gradually recovered because of the stronger special binding interaction between ATP and ATP aptamer than that between QDs and ATP aptamer. In this case, a high sensitivity and selectivity of phosphorescent aptasensor for the detection of ATP has constructed with a low detection limit of 0.9 nM and a wide linear range from 2 nM to 9 µM. What's more, the phosphorescent aptasensor does not require complex pretreatments and can effectively eliminate the interference from auto fluorescence and scattering light.

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.

Sensitive prostate specific antigen quantification using dihydrolipoic acid surface-functionalized phosphorescent quantum dots

Herein, high-quality Mn-doped ZnS quantum dots (QDs) have been synthesized using a facile approach directly in aqueous media. The surface of the obtained QDs was further modified by cap-exchange of the native cysteine shell with dihydrolipoic acid (DHLA) ligands resulting in nanocrystals with high water-stability having an intense phosphorescent signal. Covalent bioconjugation of the DHLA-coated nanoparticles with an anti-IgG antibody was then carried out. Interestingly the QD immunoprobe (QD-labelled antibodies) maintained an intense phosphorescence emission, without any significant spectral-shift (as compared to the free QDs). Coupling of an asymmetric flow field flow fractionation technique to an elemental mass spectrometry detection enabled the accurate determination of the efficiency of the bioconjugation reaction. The obtained nanoparticle-antibody bioconjugate was then applied to develop a quantitative sandwich-type phosphorescent immunoassay for Prostate Specific Antigen (PSA), and a limit of detection (LOD) of 17 pg mL^-1 of PSA was achieved and allow to quantify such biomarker in samples within clinically relevant levels. Finally, the assay was validated for the quantification of PSA in the cellular media of prostate cancer cells. Obtained results proved the robustness of the proposed immunoassay based on long-lived phosphorescence measurements against eventual photoluminescent interferences significantly affecting the conventional short-lived fluorescence detection.

Phosphorescence detection of 2-mercaptobenzothiazole in environmental water samples by Mn-doped ZnS quantum dots

A room-temperature phosphorescence sensor was constructed based on MPA-capped Mn-doped ZnS QDs for the detection of MBT in water samples.

A label-free phosphorescence sensing platform for trypsin based on Mn-ZnS QDs

A label-free phosphorescence strategy to determine trypsin was proposed using Mn-ZnS QDs as the phosphorescence probe with a better validity.

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.

Highly selective and sensitive recognition of histidine based on the oxidase-like activity of Cu2+ions

A simple, highly sensitive and selective PL sensing platform for histidine has been developed, based on the oxidase-like activity of Cu²⁺ions.