Independent optimization of capillary electrophoresis separation and native fluorescence detection conditions for indolamine and catecholamine measurements

Fabrication of 3D Ni/NiO/MoS2/rGO foam for enhancing sensing performance

The accurate electrochemical detection of dopamine (DA) is hard to achieve due to the serious interference of a substance with similar redox properties.

Colorimetric Detection of Dopamine with J-Aggregate Nanotube-Integrated Hydrogel Thin Films

The deficiency of dopamine (DA) is clinically linked to several neurological diseases. The detection of urinary DA provides a noninvasive method for diagnosing these diseases and monitoring therapies. In this paper, we report the coassembly of lithocholic acid (LCA) and 3,3'-diethythiadicarbocyanine iodide (DiSC₂(5)) at the equimolar ratio in ammonia solution into J-aggregate nanotubes. By integrating the J-aggregate nanotubes into transparent agarose hydrogel films formed on the wall of quartz cuvettes, we fabricate a portable and reproducible sensor platform for the optical detection of DA in synthetic urine. The J-band intensity of the integrated J-aggregate nanotubes is found to linearly decrease with the increase of DA concentrations from 10 to 80 nM, giving the limit of detection of ∼7 nM. The detection mechanism is based on the photoinduced electron transfer (PET) from the excited J-aggregate nanotubes to adsorbed DA-quinone. The PET process used in the sensor platform can reduce the interference of ascorbic acid and uric acid in the detection of DA in synthetic urine. The high sensitivity of the sensor platform is contributed by the delocalized exciton of J-aggregate nanotubes.

Gold nanoparticle-based detection of dopamine based on fluorescence resonance energy transfer between a 4-(4-dialkylaminostyryl)pyridinium derived fluorophore and citrate-capped gold nanoparticles

A colorimetric/fluorometric dual-signal assay is described for the determination of dopamine (DA). A nanoprobe was obtained by linking a 4-(4-dialkylaminostyryl)pyridinium derived fluorophore to citrate-capped gold nanoparticles (AuNPs). The fluorescence of the fluorophore is quenched by the AuNPs via fluorescence resonance energy transfe. In the presence of DA, the catechol group of DA can absorb on the surface of AuNPs to induce aggregation, which is accompanied by a color change from red to blue. The yellow fluorescence of the fluorophore with excitation/emission maximum at 365/570 nm is recovered. The dual-signal detection allows the quantitative analysis of DA within 300 μM by the colorimetric method and 80 μM by the fluorometric method. The detection limits for the colorimetric/fluorometric methods are 1.85 μM and 0.29 μM, respectively. Quantitative determination of DA in spiked urine samples was successfully demonstrated, with recoveries ranging from 98.2 to 106.0%. Graphical abstract A colorimetric/fluorometric dual-signal assay is described for the determination of dopamine by linking a fluorophore to gold nanoparticles. The dopamine causes aggregation of the nanoparticles to induce color change, which is followed by the recovery of the fluorescence.

The role of pramipexole functionalized MWCNTs to the fabrication of Pd nanoparticles modified GCE for electrochemical detection of dopamine

BACKGROUND

Interest in functionalized carbon nanotubes for many applications arises from a variety on the kind of modification atoms or molecules that are attached to it. Dopamine, the feel-good hormone, release by neurons and playing an important role in body systems. Abnormal dopamine levels cause nerve disorders such as Parkinson's disease and schizophrenia.

OBJECTIVES

The aim of this study was the design and fabrication of electrochemical sensor based on MWCNTs and Pd nanoparticles for detection and determination of dopamine in biological samples.

METHODS

For this purpose, we report the synthesis of pramipexole-functionalized MWCNTs (pp-MWCNTs) for efficient capture of palladium nanoparticles and fabrication of Pd/pp-MWCNTs nanocomposite. Morphological and structural characteristics of the nanocomposites were characterized using various techniques including field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FT-IR).

RESULTS

This newly synthesized nanocomposite may have numerous applications in nanotechnology and sensing. We show that the synthesized nanocomposite reported here will be applicable for modifications of bare glassy carbon electrode (Pd/pp-MWCNTs/GCE) to sense of dopamine electrochemically. Two linear calibrations for dopamine are obtained over ranges of 0.01 to 10 μM and 10 to 200 μM with a detection limit of 1.4 nM. The Pd/pp-MWCNTs/GCE shows high stability and sensitivity, and an acceptable decrease of over-potential for the electrooxidation of dopamine that decreases interference in the analysis. The proposed Pd/pp-MWCNTs nanocomposite can be used as a voltammetric detector for dopamine monitoring in routine real sample analysis.

CONCLUSIONS

The proposed sensor showed high sensitivity and selectivity in sensing dopamine in biological samples. Graphical abstract Preparation of Pd/pp-MWCNTs/GCE for detection of dopamine.

A Colorimetric Sensor for Dopamine Detection Based on Peroxidase-like Activity of Ce2(MoO4)3 Nanoplates

Introduction:

Artificial enzyme mimics are materials with similar catalytic function of natural enzymes. Among several types of artificial enzymes, nanomaterial-based products or nanozymes have been of particular interest to researchers.

Materials and Methods:

In this work, Ce2(MoO4)3 nanoplates were synthesized via a one-pot hydrothermal approach. SEM and EDS characterizations show a plated-like architecture with high purity. These nanoplates are shown to have an intrinsic peroxidase-mimetic activity. In the presence of H2O2, Ce2(MoO4)3 nanoplates could catalyse the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with high performance to produce a blue dye (with an absorbance maximum at 652 nm). Dopamine (DA) has some reducibility due to the phenol hydroxyl group, which results in using H2O2 and causing the blue shallowing of the reaction solution by inhibiting the reaction between H2O2 and TMB. Based on that, a visual, sensitive and simple colorimetric method using Ce2(MoO4)3 nanoplates as peroxidase mimics was developed for detecting DA.

Results and Conclusions:

Suitable linear relationship for DA was obtained from 0.1 to 10 µM. The limit of detection (LOD) of the proposed method was calculated as 0.05 µM and the relative standard deviation (RSD) was less than 4.0%. The proposed method was successfully applied to DA detection in human serum sample.

Direct electrochemical reduction of hematite decorated graphene oxide (α-Fe2O3@erGO) nanocomposite for selective detection of Parkinson's disease biomarker

An unusual approach is reported herein to fabricate magnetic hematite (α-Fe₂O₃) decorated electrochemically reduced graphene oxide (α-Fe₂O₃@erGO) nanocomposite. The method utilizes direct electrochemical reduction of self-assembled, ex-situ synthesized α-Fe₂O₃ anchored GO to erGO (α-Fe₂O₃@erGO) on glassy carbon electrode (GCE) for selective detection dopamine (DA), an important biomarker of Parkinson's disease. The formation of α-Fe₂O₃@erGO/GCE has been confirmed by XPS and Raman spectroscopy. α-Fe₂O₃@erGO modified GCE exhibits synergistic catalytic activity nearly 2.2 and 5 fold higher than α-Fe₂O₃@GO and other modified electrodes, respectively towards oxidation of DA. The fabricated sensor exhibited linear dynamic ranges over 0.25 - 100 µM in response to DA with a LOD of 0.024 µM (S/N = 3), LOQ of 0.08 µM (S/N = 10), and a sensitivity of 12.56 µA µM^-1 cm^-2. Finally, the practical analytical application of the proposed α-Fe₂O₃@erGO/GCE was investigated for the determination of DA in commercially available pharmaceutical formulation and human serum samples, and showed satisfactory recovery results towards DA.

Highly selective and sensitive biosensing of dopamine based on glutathione coated silver nanoclusters enhanced fluorescence

The emission intensity of red emissive GSH-AgNCs is notably enhanced after the addition of dopamine. The increasing emission intensity is attributed to the hydrogen bonding interaction between the carboxyl groups of GSH-AgNCs and amino groups of dopamine.

Selective electrochemical detection of dopamine in the presence of uric acid and ascorbic acid based on a composite film modified electrode

An electrochemical dopamine sensor based on vanadium-substituted polyoxometalates, copper oxide and chitosan–palladium was fabricated by the LBL technique.

Synergetic catalysis based on the proline tailed metalloporphyrin with graphene sheet as efficient mimetic enzyme for ultrasensitive electrochemical detection of dopamine

In this paper, linking with the butoxycarbonyl (BOC) protection of proline, a new tailed metalloporphyrin with many useful active functions, nickel (II) 5-[4-N-(tert-Butoxycarbonyl)-l-prolinecoxylpropyloxy]phenyl-10,15,20-triphenylporphyrin (NiTBLPyP), was designed and synthesized. And the NiTBLPyP polymer (poly(NiTBLPyP)) was successfully obtained via a low-cost electrochemical method and exploited as an efficient mimic enzyme. Subsequently, a noncovalent nanohybrid of poly(NiTBLPyP) with graphene (rGO) sheet (rGO-poly(NiTBLPyP)) was prepared through π-π stacking interaction for the ultrasensitive and selective detection of DA. The nanohybrid was characterized by UV-vis spectroscopy, Fourier transform infrared spectra, Raman spectroscopy, scanning electron microscopy and electrochemical impedance spectroscopy. Due to the excellent electrocatalytic ability of poly(NiTBLPyP) film and aromatic π-π stacking interaction between poly(NiTBLPyP and rGO sheet, the obtained rGO-poly(NiTBLPyP) film exhibited a great synergistic amplification effect toward dopamine oxidation. Under optimum experimental conditions, the logarithm of catalytic currents showed a good linear relationship with that of the dopamine concentration in the range of 0.01-200 μM with a low detection limit of 1.40 nM. With good sensitivity and selectivity, the present method was applied to the determination of DA in real sample and the results was satisfactory. Thus, the rGO-poly(NiTBLPyP) film is one of the promising mimetic enzyme for electrocatalysis and relevant fields.

Dual Reaction-Based Multimodal Assay for Dopamine with High Sensitivity and Selectivity Using Functionalized Gold Nanoparticles

A simple and dual chemical reaction-based multimodal assay for dopamine with high sensitivity and selectivity using two types of functionalized gold nanoparticles (FB-AuNPs/NsNHS-AuNPs), i.e. fluorescein modified gold nanoparticles (FB-AuNPs) and Nile blue modified gold nanoparticles (NsNHS-AuNPs), was successfully fabricated. This assay for dopamine presents colorimetric visualization and double channel fluorescence enhancement at 515 and 665 nm. The absorbance and fluorescence changes were linearly proportional to the amounts of dopamine in the range of nanomolar scale (5-100 nM). The detection limits for absorbance and fluorescence were as low as 1.2 nM and 2.9 nM (S/N = 3), respectively. Furthermore, the extent application of this multimodal assay has been successfully demonstrated in human urine samples with high reliability and applicability, showing remarkable promise in diagnostic purposes.

An efficient optical-electrochemical dual probe for highly sensitive recognition of dopamine based on terbium complex functionalized reduced graphene oxide

A novel organic-inorganic hybrid sensor based on diethylenetriaminepentaacetic acid (DTPA) modified reduced graphene oxide (RGO-DTPA) chelated with terbium ions allows detection of dopamine (DA) through an emission enhancement effect. Its luminescence, peaking at 545 nm, has been improved by a factor of 25 in the presence of DA (detection limit = 80 nM). In addition, this covalently bonded terbium complex functionalized reduced graphene oxide (RGO-DTPA-Tb) can be successfully assembled on a glassy carbon electrode. The assay performed through differential pulse voltammetry (DPV) yielded obvious peak separation between DA and excessive amounts of the interfering ascorbic acid (AA).

Selective dopamine chemosensing using silver-enhanced fluorescence

Condensation product of salicylaldehyde and 1,3 propylenediamine becomes a diiminic Schiff base, which is oxidized by AgNO3 in alkaline solution, and in turn, stable Ag(0) is produced at room temperature. Under this condition, the solution exhibits intense silver nanoparticle enhanced fluorescence (SEF) with the λ(em) at 412 nm. Dopamine is selectively detected down to the nanomolar level via exclusive fluorescence quenching of the SEF. Dopamine-infested solution regains the fluorescence [i.e., SEF in the presence of Hg(II) ions]. Thus dopamine and Hg(II) in succession demonstrate "turn off/on" fluorescence due to the change in the scattering cross section of Ag(0) and gives a quantitative measure of dopamine in real samples. The proposed method is free from interferences of common biocompetitors.

Self-assembly of J-aggregate nanotubes and their applications for sensing dopamine

J-aggregates are an attractive supramolecular structure with interesting excitation properties found in the light-harvesting antenna of green sulfur bacteria. To structurally mimic the light-harvesting antenna, we synthesize J-aggregate nanotubes with a sharp and intense absorption band (J-band) by the coassembly of lithocholic acid (LCA) and 3,3'-dipropylthiadicarbocyanine iodide (DiSC3(5)) in aqueous solution. We show that the J-aggregate nanotubes can be used as a supramolecular probe for the sensitive and selective detection of dopamine (DA) in phosphate buffered saline (PBS) solution with the detection limit of ∼0.4 nM by simply observing the intensity change of the J-band due to the efficient photoinduced electron transfer from the J-aggregate nanotubes to the adsorbed DA.

Label-free fluorescence detection of aromatic compounds in chip electrophoresis applying two-photon excitation and time-correlated single-photon counting

In this study, we introduce time-resolved fluorescence detection with two-photon excitation at 532 nm for label-free analyte determination in microchip electrophoresis. In the developed method, information about analyte fluorescence lifetimes is collected by time-correlated single-photon counting, improving reliable peak assignment in electrophoretic separations. The determined limits of detection for serotonin, propranolol, and tryptophan were 51, 37, and 280 nM, respectively, using microfluidic chips made of fused silica. Applying two-photon excitation microchip separations and label-free detection could also be performed in borosilicate glass chips demonstrating the potential for label-free fluorescence detection in non-UV-transparent devices. Microchip electrophoresis with two-photon excited fluorescence detection was then applied for analyses of active compounds in plant extracts. Harmala alkaloids present in methanolic plant extracts from Peganum harmala could be separated within seconds and detected with on-the-fly determination of fluorescence lifetimes.

Automated method for analysis of tryptophan and tyrosine metabolites using capillary electrophoresis with native fluorescence detection

Capillary electrophoresis (CE) with laser-induced native fluorescence (LINF) detection offers the ability to characterize low levels of selected analyte classes, depending on the excitation and emission wavelengths used. Here a new automated CE-LINF system that provides deep ultraviolet (DUV) excitation (224 nm) and variable emission wavelength detection was evaluated for the analysis of small molecule tryptophan- and tyrosine-related metabolites. The optimized instrument design includes several features that increase throughput, lower instrument cost and maintenance, and decrease complexity when compared with earlier systems using DUV excitation. Sensitivity is enhanced by using an ellipsoid detection cell to increase the fluorescence collection efficiency. The limits of detection ranged from 4 to 30 nmol/L for serotonin and tyrosine, respectively. The system demonstrated excellent linearity over several orders of magnitude of concentration and intraday precision from 1-11 % relative standard deviation (RSD). The instrument's performance was validated via tryptophan and serotonin characterization using tissue extracts from the mammalian brain stem, with RSDs of less than 10 % for both metabolites. The flexibility and sensitivity offered by DUV laser excitation and tunable emission enables a broad range of small-volume measurements.

Quantitation of dopamine, serotonin and adenosine content in a tissue punch from a brain slice using capillary electrophoresis with fast-scan cyclic voltammetry detection

Methods to determine neurochemical concentrations in small samples of tissue are needed to map interactions among neurotransmitters. In particular, correlating physiological measurements of neurotransmitter release and the tissue content in a small region would be valuable. HPLC is the standard method for tissue content analysis but it requires microliter samples and the detector often varies by the class of compound being quantified; thus detecting molecules from different classes can be difficult. In this paper, we develop capillary electrophoresis with fast-scan cyclic voltammetry detection (CE-FSCV) for analysis of dopamine, serotonin, and adenosine content in tissue punches from rat brain slices. Using field-amplified sample stacking, the limit of detection was 5 nM for dopamine, 10 nM for serotonin, and 50 nM for adenosine. Neurotransmitters could be measured from a tissue punch as small as 7 µg (7 nL) of tissue, three orders of magnitude smaller than a typical HPLC sample. Tissue content analysis of punches in successive slices through the striatum revealed higher dopamine but lower adenosine content in the anterior striatum. Stimulated dopamine release was measured in a brain slice, then a tissue punch collected from the recording region. Dopamine content and release had a correlation coefficient of 0.71, which indicates much of the variance in stimulated release is due to variance in tissue content. CE-FSCV should facilitate measurements of tissue content in nanoliter samples, leading to a better understanding of how diseases or drugs affect dopamine, serotonin, and adenosine content.

A hyphenated optical trap capillary electrophoresis laser induced native fluorescence system for single-cell chemical analysis

Single-cell measurements allow a unique glimpse into cell-to-cell heterogeneity; even small changes in selected cells can have a profound impact on an organism's physiology. Here an integrated approach to single-cell chemical sampling and assay are described. Capillary electrophoresis (CE) with laser-induced native fluorescence (LINF) has the sensitivity to characterize natively fluorescent indoles and catechols within individual cells. While the separation and detection approaches are well established, the sampling and injection of individually selected cells requires new approaches. We describe an optimized system that interfaces a single-beam optical trap with CE and multichannel LINF detection. A cell is localized within the trap and then the capillary inlet is positioned near the cell using a computer-controlled micromanipulator. Hydrodynamic injection allows cell lysis to occur within the capillary inlet, followed by the CE separation and LINF detection. The use of multiple emission wavelengths allows improved analyte identification based on differences in analyte fluorescence emission profiles and migration time. The system enables injections of individual rat pinealocytes and quantification of their endogenous indoles, including serotonin, N-acetyl-serotonin, 5-hydroxyindole-3-acetic acid, tryptophol and others. The amounts detected in individual cells incubated in 5-hydroxytryptophan ranged from 10(-14) mol to 10(-16) mol, an order of magnitude higher than observed in untreated pinealocytes.

Label-free analysis in chip electrophoresis applying deep UV fluorescence lifetime detection

Herein we introduce deep UV fluorescence lifetime detection in microfluidics applied for label-free detection and identification of various aromatic analytes in chip electrophoresis. For this purpose, a frequency quadrupled Nd:YAG (neodymium-doped yttrium aluminum garnet) picosecond laser at 266  nm was incorporated into an inverse fluorescence microscope setup with time-correlated single photon counting detection. This allowed recording of photon timing with sub-nanosecond precision. Thereby fluorescence decay curves are gathered on-the-fly and average lifetimes can be determined for each substance in the electropherogram. The aromatic compounds serotonin, propranolol, 3-phenoxy-1,2-propanediol and tryptophan were electrophoretically separated using a fused-silica microchip. Average lifetimes were independently determined for each compound via bi-exponential tail fitting. Time-correlated single photon counting also allows the discrimination of background fluorescence in the time domain. This results in improved signal-to-noise-ratios as demonstrated for the above model analytes. Microchip electrophoretic separations with fluorescence lifetime detection were also performed with a protein mixture containing lysozyme, trypsinogen and chymotrypsinogen emphasizing the potential for biopolymer analysis.

Electrochemical detection of dopamine using porphyrin-functionalized graphene

A new type of porphyrin-functionalized graphene was synthesized and used for highly selective and sensitive detection of dopamine (DA). The aromatic π-π stacking and electrostatic attraction between positively-charged dopamine and negatively-charged porphyrin-modified graphene can accelerate the electron transfer whereas weakening ascorbic acid (AA) and uric acid (UA) oxidation on the porphyrin-functionalized graphene-modified electrode. Differential pulse voltammetry was used for electrochemical detection, the separation of the oxidation peak potentials for AA-DA, DA-UA and UA-AA is about 188 mV, 144 mV and 332 mV, which allows selectively determining DA. The detection limit of DA can be as low as 0.01 μM. More importantly, the sensor we presented can detect DA in the presence of large excess of ascorbic acid and uric acid. With good sensitivity and selectivity, the present method was applied to the determination of DA in real hydrochloride injection sample, human urine and serum samples, respectively, and the results was satisfactory.

Serotonin and its metabolism in basal deuterostomes: insights from Strongylocentrotus purpuratus and Xenoturbella bocki

Serotonin (5-HT), an important molecule in metazoans, is involved in a range of biological processes including neurotransmission and neuromodulation. Both its creation and release are tightly regulated, as is its removal. Multiple neurochemical pathways are responsible for the catabolism of 5-HT and are phyla specific; therefore, by elucidating these catabolic pathways we glean greater understanding of the relationships and origins of various transmitter systems. Here, 5-HT catabolic pathways were studied in Strongylocentrotus purpuratus and Xenoturbella bocki, two organisms occupying distinct positions in deuterostomes. The 5-HT-related compounds detected in these organisms were compared with those reported in other phyla. In S. purpuratus, 5-HT-related metabolites include N-acetyl serotonin, gamma-glutamyl-serotonin and 5-hydroxyindole acetic acid; the quantity and type were found to vary based on the specific tissues analyzed. In addition to these compounds, varying levels of tryptamine were also seen. Upon addition of a 5-HT precursor and a monoamine oxidase inhibitor, 5-HT itself was detected. In similar experiments using X. bocki tissues, the 5-HT-related compounds found included 5-HT sulfate, gamma-glutamyl-serotonin and 5-hydroxyindole acetic acid, as well as 5-HT and tryptamine. The sea urchin metabolizes 5-HT in a manner similar to both gastropod mollusks, as evidenced by the detection of gamma-glutamyl-serotonin, and vertebrates, as indicated by the presence of 5-hydroxyindole acetic acid and N-acetyl serotonin. In contrast, 5-HT metabolism in X. bocki appears more similar to common protostome 5-HT catabolic pathways.

Bioanalytical applications of capillary electrophoresis with laser-induced native fluorescence detection

In this article we describe recent developments and applications of capillary electrophoresis (CE) coupled with laser-induced native fluorescence (LINF) detection in the analysis of biological, pharmaceutical and environmental samples. Compared with traditional UV absorbance detection used in CE, the LINF technique can greatly improve the concentration sensitivity of CE without the need for derivatization; the only requirement being that the analyte must have native fluorescence. Instrumentation and laser sources used in CE–LINF are summarized and specific applications of CE–LINF to small-biomolecule analysis, profiling of human biofluids, detection of native fluorescent peptides and proteins, single-cell analysis and the use of online sample preconcentration methods are also reviewed in detail.