Selective fluorescence quenching of papain-Au nanoclusters by self-polymerization of dopamine

A ratiometric fluorescence sensor based on the inner filtration effect of gold nanoparticles on quantum dots for monitoring dopamine

In this study, a smart phone assisted ratiometric fluorescence sensor was designed for detecting dopamine (DA). The ratiometric fluorescence sensor was prepared by simple physical mixing green quantum dots (GQDS) and red quantum dots (RQDS). DA could induce gold nanoparticles (AuNPs) aggregate via hydrogen-bonding interactions, and further changed the absorption spectrum of gold nanoparticles to overlap with a certain emission spectrum of ratiometric fluorescence sensor. AuNPs had inner filtration effect (IFE) on the ratiometric fluorescence sensor. Due to the IFE, the dispersive AuNPs could quench GQDS, whereas the clustered AuNPs could quench RQDS. With the addition of DA, the color of ratiometric fluorescence changed from orange red to green. To simplify the detection process, a smartphone was employed to detecting DA in human urine by measuring RGB value of fluorescence color changes with a detection limit of 86 nM. This proposed method has the advantages of low cost, easy prevalence and simple operation, thus provides a great promise for rapid detection of biomarker in biological samples.

Perspectives of different colour-emissive nanomaterials in fluorescent ink, LEDs, cell imaging, and sensing of various analytes

In the past 2 decades, multicolour light-emissive nanomaterials have gained significant interest in chemical and biological sciences because of their unique optical properties. These materials have drawn much attention due to their unique characteristics towards various application fields. The development of novel nanomaterials has become the pinpoint for different application areas. In this review, the recent progress in the area of multicolour-emissive nanomaterials is summarized. The different emissions (white, orange, green, red, blue, and multicolour) of nanostructure materials (metal nanoclusters, quantum dots, carbon dots, and rare earth-based nanomaterials) are briefly discussed. The potential applications of different colour-emissive nanomaterials in the development of fluorescent inks, light-emitting diodes, cell imaging, and sensing devices are briefly summarized. Finally, the future perspectives of multicolour-emissive nanomaterials are discussed.

A new application of papain: As a peroxidase-like catalyst for fluorometric detection of uric acid

Papain, as a classical cysteine protease, has been widely used in the food, pharmaceutical, chemical, and cosmetic fields. However, there are few information about the peroxidase-like activity of papain catalyzed substrate to produce fluorescence. In this study, we found that papain can catalyze H₂O₂ to convert o-phenylenediamine (OPD), and generate fluorescence emission at 550 nm under 430 nm excitation. Based on this foundation, we report a papain/OPD/H₂O₂ system for fluorescence detection of uric acid. The method exhibits a wide linear range of 10-1000 μM with a limit-of-detection of 4.6 μM, and has been successfully used to detect uric acid in human serum. This study paves the way for the application of papain as catalyst for fluorescence detection of different target biomolecules, such as cholesterol, glucose, lactate, for which H₂O₂ is a product of oxidoreductase enzymes.

Facile one-pot method of AuNPs/PEDOT/CNT composites for simultaneous detection of dopamine with a high concentration of ascorbic acid and uric acid

In this research, a facile one-pot method was used to synthesize gold/poly-3,4-ethylene-dioxythiophene/carbon nanotube (AuNPs/PEDOT/CNTs) composite material. The composite material was investigated by Fourier Transform Infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Then the synthesized nanocomposite material was dropped on a bare glassy carbon electrode (GCE) to improve the detection performance of dopamine with a high concentration of ascorbic acid and uric acid. The electrochemical behavior of AuNPs/PEDOT/CNTs/GCE was studied by Cyclic Voltammetry (CV), Differential Pulse Voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). Under optimum conditions, AuNPs/PEDOT/CNTs/GCE showed a good linear response in the concentration range from 9.14 to 29.704 μM with a detection limit (LOD) and sensitivity of 0.283 μM and 1.557 μA μM-1, respectively. This sensor was applied to detect practical samples with good average recovery. It also exhibited good reproducibility and stability.

Determination of dopamine based on its enhancement of gold-silver nanocluster fluorescence

Dopamine (DA) is one of the most important neurotransmitters in human bodies and its sensitive detection remains a challenge. Herein, protein stabilized gold-silver nanoclusters (Au-AgNCs) were synthesized at first. It was found that the introduction of dopamine lead to a significant enhancement of the fluorescence from the nanoclusters, together with a red-shift of the peak. Through related spectroscopic and electrochemical studies, the fluorescence enhancement was attributed to the reduction of the nanoclusters by dopamine. This enhancement was then adopted for quantitative measurements, and linear responses toward dopamine in the ranges 0.01-1.7 μM and 1.7-10 μM were constructed. A limit of detection was obtained at 6.9 nM. The present study provided a facile and efficient method for the determination of dopamine, and the method was successfully applied for related measurements in serum samples.

Recent progress in plant-gold nanoparticles fabrication methods and bio-applications

The preparation of gold nanoparticles via green routes applying plant extracts as the reducing agents and stabilizers has received broad interest in the last decades. Plant-gold nanoparticles have been well-developed and applied in biochemical and medical research, but there are still challenges that must be overcome. The main challenges include the construction of chemically-robust plant-gold nanoparticles, the precise design of biomimetic surfaces to fabricate nanozymes with high catalytic activities, and the development of approaches to construct biosensors with high selectivities and sensitivities. The cores and surfaces of plant-gold nanoparticles must be considered, as well as their catalytic activities and biosensing mechanisms. This review highlights the latest achievements in plant-gold nanoparticle preparation, heterogeneous nucleation, and surface functionalization, while also focusing on their optical properties and various biological and catalytic activities. Moreover, their antioxidant and cell apoptosis mechanisms, and biological activities are described. Plant-gold nanoparticles have shown great potential in high-performance analytical assays, high-activity catalysts, effective intracellular imaging, and clinical treatment.

Dopamine-Modified AuCu Bimetallic Nanoclusters as Charge Transfer-Based Biosensors for Highly Sensitive Glycine Detection

Glycine is the simplest amino acid in living organisms and plays important roles in biology and medicine. However, few biosensors for glycine sensing have been reported. Herein, we present a facile strategy to construct dopamine-modified AuCu bimetallic nanoclusters (denoted as AuCu NC-DA) as charge transfer-based biosensors for highly sensitive glycine sensing. The AuCu NCs stabilized by bovine serum albumin (BSA) exhibited a fluorescence maximum at 400 nm. Because of the high affinity of BSA for dopamine (DA), the surface of the AuCu NCs was modified with DA without any complicated chemical reactions, resulting in fluorescence quenching through a charge transfer process. Among 20 amino acids, AuCu NC-DA exhibited an off/on fluorescence switching response specifically toward glycine through the formation of hydrogen bonds with oxidized DA, which inhibited the charge transfer process, leading to the emergence of a new emission peak at 475 nm. Spectroscopic and thermodynamic results combined with molecular docking analyses provided comprehensive understanding of the sensing mechanism. Furthermore, we showed that AuCu NC-DA was able to sense glycine in cells by imaging. Finally, the practicability of AuCu NC-DA for glycine detection was validated in milk drink samples. This study presents a promising type of a charge transfer-based sensor.

Ultrafine Bi-Sn nanoparticles decorated on carbon aerogels for electrochemical simultaneous determination of dopamine (neurotransmitter) and clozapine (antipsychotic drug)

This present study describes the synthesis of ultrafine Bi-Sn nanoparticles decorated on carbon aerogels (Bi-Sn NP/CAG) as a nanocomposite for the electrochemical simultaneous determination of dopamine (DA) and clozapine (CLZ). The typical characterization techniques, such as XRD, Raman, BET, FT-IR, TGA, XPS, and FE-SEM/TEM, showed useful insights into the crystal phase and morphology of Bi-Sn NP/CAG. Integrated Bi-Sn NP/CAG built into a cost-effective screen printed carbon electrode (SPCE) offers a high electrochemical surface area (ECSA) compared to unmodified, Bi-Sn, and CAG/SPCEs, such that it favourably allowed the binding of DA and CLZ molecules onto the surface at the Bi-Sn/CAG, which was demonstrated by cyclic and differential pulse voltammetry techniques. As a result, the DA and CLZ sensing exhibited low detection limits (DL, 4.6 and 97.6 nM (S/N = 3)), and sensitivity (3.402 and 0.4 μA μM-1 cm-2) over a wide linear range (0.02-97.59 and 0.5-2092 μM), respectively. To go a step further, the Bi-Sn NP/CAG/SPCE was applied for the simultaneous determination of DA and CLZ which featured lower DL (23.1 and 31.3 nM (S/N = 3)), and sensitivity (0.4979 and 0.04 μA μM-1 cm-2) over a wide linear range (2-182 and 10-910 μM), respectively. The selectivity for DA and CLZ in the presence of a 10-fold concentration of their potentially interfering active species was demonstrated. Finally, this sensing methodology enables the rapid electrochemical determination of the amount of DA and CLZ in a rat brain region serum sample with successful recovery outcomes.

Advances in biomedical and pharmaceutical applications of protein-stabilized gold nanoclusters

The interest in using gold nanoclusters (AuNCs) as imaging probes is growing, covering wide ranges of applications. The stabilization of AuNCs with protein ligands enhances their biomedical and pharmaceutical applications. This is due to the biocompatibility, water solubility and bioactivity of proteins. Different factors can control the optical properties of AuNCs such as protein size, amino acids content and conformational structure. Controlling the synthesis conditions can result in tuning the AuNCs excitation, emission, fluorescence intensity and physicochemical properties to fulfill different applications. NIR-emitting protein-stabilized AuNCs are promising as imaging agents for targeting and visualization of cancer in vitro and in vivo. They are promising to be included as an important part of multifunctional theranostic nanosystems, due to their potential dual functions as imaging and photosensitizing agent for photodynamic therapy. Additionally, the protein around AuNCs represents a rich environment of active functional groups that are susceptible for conjugation with various biomolecules. Protein-AuNCs can act as fluorescent probes for rapid and selective analysis of different analytes in solution, cells or biological fluids. In conclusion, the variability of protein-AuNC applications can advance research in different biomedical and pharmaceutical fields.

Solid-Contact Potentiometric Sensors Based on Stimulus-Responsive Imprinted Polymers for Reversible Detection of Neutral Dopamine

Herein, we present for the first time a novel potentiometric sensor based on the stimulus-responsive molecularly imprinted polymer (MIP) as a selective receptor for neutral dopamine determination. This smart receptor can change its capabilities to recognize according to external environmental stimuli. Therefore, MIP-binding sites can be regenerated in the polymeric membrane by stimulating with stimulus after each measurement. Based on this effect, reversible detection of the analyte via potentiometric transduction can be achieved. MIPs based on 4-vinylphenylboronic acid as the functional monomer were prepared as the selective receptor. This monomer can successfully bind to dopamine via covalent binding and forming a five- or six-membered cyclic ester in a weakly alkaline aqueous solution. In acidic medium, the produced ester dissociates and regenerates new binding sites in the polymeric membrane. The proposed smart sensor exhibited fast response and good sensitivity towards dopamine with a limit of detection 0.15 µM over the linear range 0.2–10 µM. The selectivity pattern of the proposed ISEs was also evaluated and revealed an enhanced selectivity towards dopamine over several phenolic compounds. Constant-current chronopotentiometry is used for evaluating the short-term potential stability of the proposed ISEs. The obtained results confirm that the stimulus-responsive MIPs provide an attractive way towards reversible MIP-based electrochemical sensors designation.

Protamine-stabilized gold nanoclusters as a fluorescent nanoprobe for lead(II) via Pb(II)-Au(I) interaction

The authors report on a one-pot approach for synthesizing highly fluorescent protamine-stabilized gold nanoclusters. These are shown to be a viable nanoprobe for selective and sensitive fluorometric determination of lead(II) via quenching of fluorescence via Pb(II)-Au(I) interaction. Under optimized conditions, fluorescence measured at excitation/emission peaks of 300/599 nm drops in the 80 nM-15 μM lead(II) concentration range. The detection limit is 24 nM, and relative standard deviations (for n = 11) at concentrations of 0.10, 4.0 and 15 μM are 1.6, 2.5 and 1.9%, respectively. The relative recoveries of added lead(II) in the water samples ranged from 97.9 ± 2.29% to 101.2 ± 1.83%. Graphical abstract Lead(II) ions are found to be able to selectively and sensitively quench the fluorescence of the protamine-gold nanoclusters (PRT-AuNCs). Thereby, an inexpensive, selective and sensitive lead(II) assay was established.

Papain Loaded Poly(ε-Caprolactone) Nanoparticles: In-silico and In-Vitro Studies

Papain is a protease enzyme with therapeutic properties that are very valuable for medical applications. Poly(ε-caprolactone) (PCL) is an ideal polymeric carrier for controlled drug delivery systems due to its low biodegradability and its high biocompatibility. In this study, the three-dimensional structure and action mechanism of papain were investigated by in vitro and in silico experiments using molecular dynamics (MD) and molecular docking methods to elucidate biological functions. The results showed that the size of papain-loaded PCL nanoparticles (NPs) and the polydispersity index (PDI) of the NPs were 242.9 nm and 0.074, respectively. The encapsulation efficiency and loading efficiency were 80.4 and 27.2%, respectively. Human embryonic kidney cells (HEK-293) were used for determining the cytotoxicity of papain-loaded PCL and PCL nanoparticles. The in vitro cell culture showed that nanoparticles are not toxic at low concentrations, while toxicity slightly increases at high concentrations. In silico studies, which were carried out with MD simulations and ADME analysis showed that the strong hydrogen bonds between the ligand and the papain provide stability and indicate the regions in which the interactions occur.

Gold nanocluster-based ratiometric fluorescent probes for hydrogen peroxide and enzymatic sensing of uric acid

A method is described for ratiometric fluorometric assays of H₂O₂ by using two probes that have distinct response profiles. Under the catalytic action of ferrous ion, the 615 nm emission of protein-stabilized gold nanoclusters (under 365 nm photoexcitation) is quenched by H₂O₂, while an increased signal is generated with a peak at 450 nm by oxidizing coumarin with the H₂O₂/Fe(II) system to form a blue emitting fluorophore. These decrease/increase responses give a ratiometric signal. The ratio of the fluorescences at the two peaks are linearly related to the concentration of H₂O₂ in the range from 0.05 to 10 μM, with a 7.7 nM limit of detection. The detection scheme was further coupled to the urate oxidase catalyzed oxidation of uric acid which proceeds under the formation of H₂O₂. This method provides an simple and effective means for the construction of ratiometric fluorometric (enzymatic) assays that involve the detection of H₂O₂. Graphical abstract Under catalysis by ferrous ion, hydrogen peroxide quenches the luminescence of gold nanoclusters (AuNCs) and oxidizes coumarin into a fluorescent derivative, which rendered fluorescence ON and OFF at two distinct wavelengths for ratiometric measurements.