A practical and highly sensitive C3N4-TYR fluorescent probe for convenient detection of dopamine

The potential of a novel enzyme-based surface plasmon resonance biosensor for direct detection of dopamine

Dopamine is one of the significant neurotransmitters and its monitoring in biological fluids is a critical issue in healthcare and modern biomedical technology. Here, we have developed a dopamine biosensor based on surface plasmon resonance (SPR). For this purpose, the carboxymethyl dextran SPR chip was used as a surface to immobilize laccase as a bioaffinity recognition element. Data analysis exhibited that the acidic pH value is the optimal condition for dopamine interaction. Calculated kinetic affinity (KD) (48,545 nM), obtained from a molecular docking study, showed strong association of dopamine with the active site of laccase. The biosensor exhibited a linearity from 0.01 to 189 μg/ml and a lower detection limit of 0.1 ng/ml (signal-to-noise ratio (S/N) = 3) that is significantly higher than the most direct dopamine detecting sensors reported so far. Experiments for specificity in the presence of compounds that can co-exist with dopamine detection such as ascorbic acid, urea and L-dopa showed no significant interference. The current dopamine biosensor with high sensitivity and specificity, represent a novel detection tool that offers a label-free, simple procedure and cost effective monitoring system.

Fluorescent-Based Neurotransmitter Sensors: Present and Future Perspectives

Neurotransmitters (NTs) are endogenous low-molecular-weight chemical compounds that transmit synaptic signals in the central nervous system. These NTs play a crucial role in facilitating signal communication, motor control, and processes related to memory and learning. Abnormalities in the levels of NTs lead to chronic mental health disorders and heart diseases. Therefore, detecting imbalances in the levels of NTs is important for diagnosing early stages of diseases associated with NTs. Sensing technologies detect NTs rapidly, specifically, and selectively, overcoming the limitations of conventional diagnostic methods. In this review, we focus on the fluorescence-based biosensors that use nanomaterials such as metal clusters, carbon dots, and quantum dots. Additionally, we review biomaterial-based, including aptamer- and enzyme-based, and genetically encoded biosensors. Furthermore, we elaborate on the fluorescence mechanisms, including fluorescence resonance energy transfer, photon-induced electron transfer, intramolecular charge transfer, and excited-state intramolecular proton transfer, in the context of their applications for the detection of NTs. We also discuss the significance of NTs in human physiological functions, address the current challenges in designing fluorescence-based biosensors for the detection of NTs, and explore their future development.

From Enzymatic Dopamine Biosensors to OECT Biosensors of Dopamine

Neurotransmitters are an important category of substances used inside the nervous system, whose detection with biosensors has been seriously addressed in the last decades. Dopamine, a neurotransmitter from the catecholamine family, was recently discovered to have implications for cardiac arrest or muscle contractions. In addition to having many other neuro-psychiatric implications, dopamine can be detected in blood, urine, and sweat. This review highlights the importance of biosensors as influential tools for dopamine recognition. The first part of this article is related to an introduction to biosensors for neurotransmitters, with a focus on dopamine. The regular methods in their detection are expensive and require high expertise personnel. A major direction of evolution of these biosensors has expanded with the integration of active biological materials suitable for molecular recognition near electronic devices. Secondly, for dopamine in particular, the miniaturized biosensors offer excellent sensitivity and specificity and offer cheaper detection than conventional spectrometry, while their linear detection ranges from the last years fall exactly on the clinical intervals. Thirdly, the applications of novel nanomaterials and biomaterials to these biosensors are discussed. Older generations, metabolism-based or enzymatic biosensors, could not detect concentrations below the micro-molar range. But new generations of biosensors combine aptamer receptors and organic electrochemical transistors, OECTs, as transducers. They have pushed the detection limit to the pico-molar and even femto-molar ranges, which fully correspond to the usual ranges of clinical detection of human dopamine in body humors that cover 0.1 ÷ 10 nM. In addition, if ten years ago the use of natural dopamine receptors on cell membranes seemed impossible for biosensors, the actual technology allows co-integrate transistors and vesicles with natural receptors of dopamine, like G protein-coupled receptors. The technology is still complicated, but the uni-molecular detection selectivity is promising.

Framing of Poly(arylene-ethynylene) around Carbon Nanotubes and Iodine Doping for the Electrochemical Detection of Dopamine

Dopamine (DA), an organic biomolecule that acts as both a hormone and a neurotransmitter, is essential in regulating emotions and metabolism in living organisms. The accurate determination of DA is important because it indicates early signs of serious neurological disorders. Covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) have received considerable attention in recent years as promising porous materials with an unrivaled degree of tunability for electrochemical biosensing applications. This study adopted a solvothermal strategy for the synthesis of a conjugated microporous poly(arylene ethynylene)-4 (CMP-4) network using the Sonagashira-Hagihara cross-coupling reaction. To increase the crystallinity and electrical conductivity of the material, CMP-4 was enveloped around carbon nanotubes (CNTs), followed by iodine doping. When used as an electrochemical probe, the as-synthesized material (I₂-CMP-CNT-4) exhibited excellent selectivity and sensitivity to dopamine in the phosphate-buffered solution. The detection limits of the electrochemical sensor were 1 and 1.7 μM based on cyclic voltammetry (CV) and differential pulse voltammetry (DPV).

Investigation of the one-step electrochemical deposition of graphene oxide-doped poly(3,4-ethylenedioxythiophene)-polyphenol oxidase as a dopamine sensor

In this paper, we fabricated poly(3,4-ethylenedioxythiophene) (PEDOT)-graphene oxide-polyphenol oxidase (PEDOT-GO-PPO) as a dopamine sensor. The morphology of PEDOT-GO-PPO was observed using scanning electron microscopy. Cyclic voltammetry was conducted to study the oxidation-reduction characteristics of dopamine. To optimize the pH, potential and limit of detection of dopamine, the amperometric technique was employed. The found limit of detection was 8 × 10-9 M, and the linear range was from 5 × 10-8 to 8.5 × 10-5 M. The Michaelis-Menten constant (K m) was calculated to be 70.34 μM, and the activation energy of the prepared electrode was 32.75 kJ mol-1. The electrode shows no significant change in the interference study. The modified electrode retains up to 80% of its original activity after 2 months. In the future, the biosensor can be used for the quantification of dopamine in human urine samples. The present modified electrode constitutes a tool for the electrochemical analysis of dopamine.

Dopamine Sensing Based on Ultrathin Fluorescent Metal-Organic Nanosheets

The importance of dopamine (DA) detection as a biomarker for several diseases, especially Parkinson''s disease, has persuaded scientists to develop new nanomaterials for efficient sensing of DA in clinical samples. Ultrathin metal-organic nanosheets due to their exceptional thickness, large surface area, and flexibility are endowed with many accessible active sites and optimal surface interaction with the target analyte molecules. In this regard, a novel layered fluorescent metal-organic nanomaterial with a honeycomb topology based on europium, [Eu(pzdc)(Hpzdc)(H₂O)]_n (ECP) (H₂pzdc = 2,3-pyrazine dicarboxylic acid), was synthesized. X-ray crystallography revealed that the 3D supramolecular architecture of ECP is constructed from noncovalent interactions of coordinated water molecules between the 2D layers along the b axis. These layers that are only ∼4 nm thick were conveniently separated through ultrasound-induced liquid phase exfoliation. Optical studies show that the reduction of ECP thickness enhances the fluorescence intensity and serves as an efficient optical marker for DA detection. ECP nanoflakes exhibited fast response and high selectivity for DA detection in clinical samples. Good linearity for DA detection in the range of 0.1-10 μM with a detection limit of 21 nM proves the potential of ECP nanoflakes in DA sensing applications.

Colorimetric Immunosensor Based on Au@g-C3N4-Doped Spongelike 3D Network Cellulose Hydrogels for Detecting α-Fetoprotein

A colorimetric immunoassay is a powerful tool for detecting tumor markers, with outstanding advantages of visualization and convenience. This study designed a colorimetric immunoassay using the antibody/antigen to control the catalytic activity to be "switched on/off". This system, where Au NPs (18.5 ± 3.9 nm) were loaded on the g-C₃N₄ nanosheets that were fixed in a three-dimensional porous cellulose hydrogel, was used as a binding site for the antibody/antigen. After being incubated with an antibody of a cancer marker, the turned-off catalytic sites on Au NPs in Au@g-C₃N₄/microcrystalline cellulose hydrogels would not be "turned on" until the corresponding antigen was added. The number of the recovered Au active sites was related to the amount of the antigen added. The Fourier transform infrared and X-ray photoelectron spectroscopy measurements did not detect the existence of Au-S bonds. Catalyzed by the turned-on Au NPs, 4-nitrophenol was reduced to 4-aminophenol accompanied by a color fading. The color and the absorption spectrum changes in the process were used as the colorimetric quantitative basis for immunoassays. The colorimetric immunoassay showed a linear relationship with the liver cancer marker (α-fetoprotein, AFP) in the range of 0.1-10 000 ng/mL with the detection limit of 0.46 ng/mL. In addition, 4-nitrophenol had a significant color fading when the AFP concentration exceeded the healthy human threshold. The clinical patient's serum test results obtained from the developed colorimetric immunosensor were consistent with those obtained from the commercial enzyme-linked immunosorbent assay. Furthermore, the immunosensor exhibited a good selectivity, repeatability, and stability, which demonstrated its potential for practical diagnostic application.

Transition metal-coordinated graphitic carbon nitride dots as a sensitive and facile fluorescent probe for β-amyloid peptide detection

Herein, we developed a sensitive graphitic carbon nitride quantum dot (gCNQD)-based fluorescent strategy for β-amyloid peptide monomer (Aβ) determination down to the ng mL-1 level for the first time. To realize this goal, the nanostructured gCNQDs were firstly coordinated with four transition metal ions (Cu2+, Cu+, Fe3+, Zn2+). Our findings showed that the fluorescence (FL) intensity of gCNQDs was quenched in the presence of these metal ions possibly due to the effective chelation with the nitrogen element in gCNQDs and subsequent photoinduced electron transfer (PET) of gCNQDs. The degree of fluorescence quenching was found to be the most intense with the addition of Cu2+ and therefore, we selected Cu2+ as the quencher for the following Aβ determination. Through binding to Cu2+, the introduction of Aβ unexpectedly induced a further decline of FL intensity. Importantly, on account of different peptide sequences coexisting in the same cerebral system, including Aβ1-11, Aβ1-16, Aβ1-38, Aβ1-40 and Aβ1-42, their affinities to Cu2+ could be reflected by the distinguished declining extent of FL intensity. The possible mechanism of Aβ sensing by the probe was clarified by TEM characterization. The developed fluorescent biosensor was demonstrated to give a wide linear range from 1 to 700 ng mL-1 and a low detection limit of 0.18 ng mL-1 for Aβ1-42. In the end, the proposed fluorescence approach was successfully applied to monitoring of Aβ1-42 variations in the cortex and hippocampus of AD rats.

A high-performance fluorescent probe for dopamine detection based on g-C3N4 nanofibers

A novel fluorescent sensor based on g-C₃N₄ nanofibers for the sensitive detection of dopamine (DA) has been proposed. We synthesized g-C₃N₄ nanofibers by directly hydrolyzing bulk g-C₃N₄ in the alkaline atmosphere (3 M NaOH). The obtained ultrathin g-C₃N₄ nanofibers were verified by characterizations of Transmission electronic microscope (TEM), X-ray diffractometer (XRD), Fourier transformation-infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). It was found that the fluorescence intensity of g-C₃N₄ nanofibers was obviously quenched by DA. Fluorescence resonance energy transfer (FRET) between DA and g-C₃N₄ nanofibers led to the fluorescence reduction of g-C₃N₄ nanofibers. The fluorescent probe based on g-C₃N₄ nanofibers exhibits linear responses to the concentration of DA in the range from 0 to 4 μM and 4 to 20 μM, the limit of detection is 17 nM. The fluorescent probe shows excellent stability, good selectivity with its application in serums.

Facile one-step economical methodology of metal free g-C3N4 synthesis with remarkable photocatalytic performance under visible light to degrade trans-resveratrol

Nanoropes crafted graphitic carbon nitride (NRCN-2), prepared by a novel, cost effective and easy to scale up method. We used naphthalene to induce nanoropes like structure inside of the graphitic carbon nitride; on industrial scale, it is easy to recycle naphthalene. The Naphthalene decomposition temperature (>700 °C) is much higher than the polymerization temperature ( = 520 °C) of NRCN-2. At higher temperature, naphthalene molecular vapors moved vigorously through graphitic carbon nitride sheets and caused the sheets to adopt nanoropes like morphology. Photocatalyst (NRCN-2) presented extraordinary specific surface area (351.08 m² g^-1), stimulated the parting effectiveness of photo-generated electrons and holes, by extending the visible light harvesting ability to 600 nm. The nanoropes adopted photocatalysts can easily degrade MO in 90 min and trans-resveratrol in 120 min. The buoyant energetic reactive species (h⁺, OH and O₂-), created during photocatalysis caused mineralization of pollutants. The apparent rate constant, half-life time period and regression coefficients for MO and trans-resveratrol calculated with the help of pseudo first order kinetics. For MO the value of K_app is 2.968 × 10^-2 min^-1, for trans-resveratrol, K_app is 2.490 × 10^-2 min^-1, corresponding half-life values are 23.354 min and 27.837 min respectively.

A conjugated carbon-dot–tyrosinase bioprobe for highly selective and sensitive detection of dopamine

A conjugated carbon-dot–tyrosinase bioprobe was first designed and applied for sensitive and selective dopamine detection in human serum.

Enhanced fluorescent effect of graphitic C3N4@ZIF-8 nanocomposite contribute to its improved sensing capabilities

A novel graphitic carbon nitride (g-C₃N₄)@ZIF-8 nanocomposite was synthesized by a facile approach and applied as a fluorescent sensor.

Dual-Signal Luminescent Detection of Dopamine by a Single Type of Lanthanide-Doped Nanoparticles

Detection of dopamine, an important neurotransmitter, is vital for understanding its roles in mammals and disease diagnosis. However, commonly available methods for dopamine detection typically rely on a single signal readout, which can be susceptible to interference by internal or external factors. Here, we report a dual-signal detection of dopamine based on label-free luminescent NaGdF₄:Tb nanoparticles. In the presence of dopamine, the NaGdF₄:Tb nanoparticles exhibit luminescence quenching under the excitation of 272 nm, while they give enhanced luminescence under 297 nm excitation, realizing both turn off and turn on detection of dopamine. The nanoparticle-based dual-signal sensors exhibit high sensitivity, with a detection limit of ∼30 nM, and good selectivity, which offers the possibility to identify potential interferents in the samples. We further demonstrate that the dual-signal response results from different energy-transfer processes within the nanoparticles under the excitation of different light. The new strategy demonstrated here should pave the way for the development of multiresponse nanosensors based on lanthanide-doped luminescent nanomaterials.

In-situ deposition of gold nanoparticles onto polydopamine-decorated g-C3N4 for highly efficient reduction of nitroaromatics in environmental water purification

A green synthesized gold-catalyst (PDA-g-C₃N₄/Au) for highly efficient reduction of nitroaromatics by NaBH₄ was proposed. Polydopamine (PDA) served as the reductant and stabilizer for AuNPs reduction, avoiding the use of chemical reductant and stabilizer that may result in secondary contamination. g-C₃N₄ not only acted as the support but also provided compatibility for AuNPs deposition, enhancing the stability and deposition of AuNPs, which improved the catalytic activity. Different experimental parameters including the amount of Au loading, concentration of NaBH₄, and dosage of catalyst were studied. Results showed that PDA-g-C₃N₄/Au₍₃₎ revealed higher catalytic activity with a rate constant of 0.0514 s^-1 and TOF of 545.60 h^-1 for 4-NP reduction. In addition, the catalyst was highly efficient in reduction of other nitroaromatics and the reduction rates of these compounds were found as the sequence: methyl orange > 2-nitrophenol > 2, 4-dinitrophenol > Erichrome Black T > Congo red. Moreover, the PDA-g-C₃N₄/Au₍₃₎ catalyst kept high stability and excellent conversion efficiency over ten reduction cycles. The practical application on different real water samples suggests that this Au catalyst has promising application in environmental water purification. The simple and green synthetic Au catalyst expands the range of application and provides potential application on environmental remediation.

Highly specific and sensitive determination of propyl gallate in food by a novel fluorescence sensor

Propyl gallate (PG), one of the most widely used synthetic phenolic antioxidants in edible oil, cookies and fried food, has received extensive concern due to its possible toxic effects on human health. Herein, a novel fluorescence analytical method is firstly proposed to sensitively and selectively determine propyl gallate (PG) by utilizing the unique fluorescence quenching property of organic molybdate complex (OMC) formed by the specific reaction between MoO₄^2- and PG to g-C₃N₄ nanosheets. Under the optimum conditions, the developed fluorescence sensor allows highly sensitive detection of PG in a wide range from 0.5 to 200 μg mL^-1 with a detection limit of 0.11 μg mL^-1, and possesses excellent specificity and good recoveries. All the analytical results indicate the present method provides an effective approach for rapid detection of PG in common products, which is beneficial for monitoring and reducing the risk of overuse of PG.

Regioselective plasmonic nano-assemblies for bimodal sub-femtomolar dopamine detection

Reliable and ultrasensitive quantification of dopamine (DA) is essential in the precise diagnotherapy of neurological diseases. In this study, dual mode counterpropagating-responsive gold@silver nanoparticle-gold nanorod (Au@AgNP-AuNR) nano-assemblies were fabricated for the precise quantification of DA. The plasmonic Au@AgNP-AuNR assemblies possessed high surface-enhanced Raman scattering (SERS) activity and strong fluorescence quenching, due to the prominent electromagnetic enhancement between the hotspots of the assemblies. In the presence of a DA target, the Au@AgNP-AuNR assemblies dissociated, leading to an increase in fluorescence intensity and a decrease in SERS intensity. The limit of detection (LOD) of DA was 0.04 fM and 0.02 fM by the fluorescence method and SERS method, respectively. The linear range was 0.1-10 fM. These findings demonstrated that the simultaneous positive fluorescence response and negative SERS response ensured the accurate detection of DA, which opens up a new avenue for the early diagnotherapy of neurological diseases.

Fabrication of a CuBi2O4/g-C3N4 p–n heterojunction with enhanced visible light photocatalytic efficiency toward tetracycline degradation

CuBi₂O₄/g-C₃N₄ p–n heterojunction photocatalysts with enhanced photocatalytic activity were prepared through a facile calcining method.

Synthesizing a nano-composite of BSA-capped Au nanoclusters/graphitic carbon nitride nanosheets as a new fluorescent probe for dopamine detection

A strong red fluorescent nanocomposite, consisting of graphite-like carbon nitride nanosheets (g-C₃N₄ NSs) and serum albumin-capped Au nanoclusters (AuNCs), was synthesized. Dopamine (DA) can quench the red fluorescence of the nanocomposite, based on the Forster resonance energy transfer (FRET) mechanism. In this quenching process, the energy is transferred from the fluorescent g-C₃N₄ NSs-AuNCs to the oxidized DA quinine molecules (DA is easily oxidated to form DA quinine in air). The red fluorescence emission at 420 nm decreases dramatically and the quenching ratio (F₀ - F)/F₀ is linearly related to the concentration of DA in the range of 0.05-8.0 μmol L^-1 with a detection limit of 0.018 μmol L^-1 (S/N = 3). Additionally, this sensor has a potential of application to assay the DA in the real samples, such as human serum and human urine.

A fluorescent graphitic carbon nitride nanosheet biosensor for highly sensitive, label-free detection of alkaline phosphatase

Graphitic C3N4 (g-C3N4) nanosheets provide an attractive option for bioprobes and bioimaging applications. Utilizing highly fluorescent and water-dispersible ultrathin g-C3N4 nanosheets, a highly sensitive, selective and label-free biosensor has been developed for ALP detection for the first time. The developed approach utilizes a natural substrate of ALP in biological systems and thus affords very high catalytic efficiency. This novel biosensor is demonstrated to enable quantitative analysis of ALP in a wide range from 0.1 to 1000 U L(-1) with a low detection limit of 0.08 U L(-1), which is among the most sensitive assays for ALP. It is expected that the developed method may provide a low-cost, convenient, rapid and highly sensitive platform for ALP-based clinical diagnostics and biomedical applications.

Quadruplex Integrated DNA (QuID) Nanosensors for Monitoring Dopamine

Dopamine is widely innervated throughout the brain and critical for many cognitive and motor functions. Imbalances or loss in dopamine transmission underlie various psychiatric disorders and degenerative diseases. Research involving cellular studies and disease states would benefit from a tool for measuring dopamine transmission. Here we show a Quadruplex Integrated DNA (QuID) nanosensor platform for selective and dynamic detection of dopamine. This nanosensor exploits DNA technology and enzyme recognition systems to optically image dopamine levels. The DNA quadruplex architecture is designed to be compatible in physically constrained environments (110 nm) with high flexibility, homogeneity, and a lower detection limit of 110 µM.