PDMS microchip coated with polydopamine/gold nanoparticles hybrid for efficient electrophoresis separation of amino acids

Multilayer coatings based on gold nanoparticles and polymers with bovine serum albumin as a functional layer for the chiral separation in capillary electrochromatography

In this study, we developed physically adsorbed multi-layer coatings using poly-l-lysine or poly(diallyldimethylammonium chloride) and gold nanoparticles, which were functionalized with bovine serum albumin for the chiral separation in electrochromatography. The approach involves sequentially depositing positively charged polymers and negatively charged citrate-stabilized gold nanoparticles. By repeating this modification cycle, we created two- and four-layer coatings, which were sequentially functionalized with albumin forming three- and five-layer coatings that were finally applied for the separation of enantiomers of dl-tryptophan. The formed coatings exhibit stability across a pH range of 2-10 and feature a dense, uniform surface, as confirmed by scanning electron microscope images. The number of layers impacted nanoparticle deposition density, with five-layer coatings being denser than three-layer ones. Five-layer coatings enable baseline separation of dl-tryptophan enantiomers, whereas three-layer coatings require the presence of albumin in the background electrolyte for separation. Therefore, increasing the number of layers and gold nanoparticles density enhances albumin active center concentration on capillary walls, improving the separation of dl-tryptophan enantiomers. The five-layer coatings can be easily fabricated and possess good repeatability of analytes migration time.

Use of metal nanoparticles for preconcentration and analysis of biological thiols

Metal nanoparticles (NPs) exhibit several unique physicochemical properties, including redox activity, surface plasmon resonance, ability to quench fluorescence, biocompatibility, or a high surface-to-volume ratio. They are being increasingly used in analysis and preconcentration of thiol containing compounds, because they are able to spontaneously form a stable Au/Ag/Cu-S dative bond. They thus find wide application in environmental and particularly in medical science, especially in the analysis of biological thiols, the endogenous compounds that play a significant role in many biological systems. In this review article, we provide an overview of various types of NPs that have been applied in analysis and preconcentration of biological thiols, mainly in human biological fluids. We first discuss shortly the types of NPs and their synthesis, properties, and their ability to interact with thiol compounds. Then we outline the sample preconcentration and analysis methods that were used for this purpose with special emphasis on optical, electrochemical, and separation techniques.

Mechanistic studies of droplet electrophoresis: A review

Electrophoresis (EP) of droplets is an intriguing phenomenon that has applications in biological systems, separation strategies, and reactor engineering. Droplet EP is significantly different from the classic particle EP because of droplet characteristics such as a mobile surface charge and the nonrigidity of the interface. Also, the liquid-liquid system, where there is an interplay between the hydrodynamic and electrokinetic forces in both phases, adds to the complexity of electrophoretic motion. Due to the vast amount of potential applications of droplet EP, a mechanistic understanding of the droplet motion in the presence of an external electric field is crucial. This review provides a background on the mechanism of droplet EP and summarizes the intrinsic interplay between the different relevant forces in these systems. The review also describes the key differences between droplet EP and particle EP, and the impact of these differences on droplet mobility. Additionally, we schematically summarize the effects of key parameters on droplet EP mobility, such as electric double layer polarization, the development of internal flow inside a droplet and boundary effects.

Rapid detection of carbamate pesticide residues using microchip electrophoresis combining amperometric detection

The long-term consumption of food with pesticide residues has harmful effects on human health and the demand for pesticide detection technology tends to be miniaturized and instant. To this end, we demonstrated the first application of indirectly detecting two carbamate pesticides, metolcarb and carbaryl, by gold nanoparticle-modified indium tin oxide electrode in dual-channel microchip electrophoresis and amperometric detection (ME-AD) system. m-Cresol and α-naphthol were obtained after pesticide hydrolysis in alkaline solution, and then separated and detected by ME-AD. Parameters including the detection potential and running buffer concentration and pH were optimized to improve the detection sensitivity and separation efficiency. Under the optimal conditions, the two analytes were completely separated within 80 s. m-Cresol and α-naphthol presented a wide linear range from 1 to 100 μM, with limits of detection of 0.16 μM and 0.34 μM, respectively (S/N = 3). Moreover, the reliability of this system was demonstrated by analyzing metolcarb and carbaryl in spiked vegetable samples.

Enhanced Biocompatibility and Differentiation Capacity of Mesenchymal Stem Cells on Poly(dimethylsiloxane) by Topographically Patterned Dopamine

Controlling the behavior of mesenchymal stem cells (MSCs) through topographic patterns is an effective approach for stem cell studies. We, herein, reported a facile method to create a dopamine (DA) pattern on poly(dimethylsiloxane) (PDMS). The topography of micropatterned DA was produced on PDMS after plasma treatment. The grid-topographic-patterned surface of PDMS-DA (PDMS-DA-P) was measured for adhesion force and Young's modulus by atomic force microscopy. The surface of PDMS-DA-P demonstrated less stiff and more elastic characteristics compared to either nonpatterned PDMS-DA or PDMS. The PDMS-DA-P evidently enhanced the differentiation of MSCs into various tissue cells, including nerve, vessel, bone, and fat. We further designed comprehensive experiments to investigate adhesion, proliferation, and differentiation of MSCs in response to PDMS-DA-P and showed that the DA-patterned surface had good biocompatibility and did not activate macrophages or platelets in vitro and had low foreign body reaction in vivo. Besides, it protected MSCs from apoptosis as well as excessive reactive oxygen species (ROS) generation. Particularly, the patterned surface enhanced the differentiation capacity of MSCs toward neural and endothelial cells. The stromal cell-derived factor-1α/CXantiCR4 pathway may be involved in mediating the self-recruitment and promoting the differentiation of MSCs. These findings support the potential application of PDMS-DA-P in either cell treatment or tissue repair.

One-Pot Decoration of Cupric Oxide on Activated Carbon Fibers Mediated by Polydopamine for Bacterial Growth Inhibition

Despite the widespread application of activated carbon fiber (ACF) filters in air cleaning owing to their high surface area and low price, they have certain limitations in that they facilitate bacterial growth upon prolonged use as ACF filters can provide favorable conditions for bacterial survival. The deposition of cupric oxide (CuO) on ACFs can be an effective way of resolving this problem because CuO can inhibit the proliferation of bacteria owing to its antimicrobial properties. However, finding a new method that allows the simple and uniform coating of CuO on ACF filters is challenging. Here, we demonstrate one-pot CuO deposition mediated by polydopamine (PD) to realize an ACF filter with antimicrobial activity. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) analyses reveal that CuO and PD are uniformly deposited on the ACF surface. The amount of CuO formed on the ACFs is measured by thermogravimetric analysis (TGA). Finally, the changes in surface area, pressure drop, and antimicrobial activity after coating PD-CuO on the ACFs are evaluated. The use of PD-CuO on the ACFs effectively suppresses the growth of bacteria and enhances the mechanical properties without significantly sacrificing the original characteristics of the ACF filter.

Magnetic ferroferric oxide and polydopamine molecularly imprinted polymer nanocomposites based electrochemical impedance sensor for the selective separation and sensitive determination of dichlorodiphenyltrichloroethane (DDT)

Dichlorodiphenyltrichloroethane (DDT) is a kind of broad-spectrum insecticides, which is potentially toxic and persistently threatens the safety of environment and food, due to their stability in nature and difficulty to degrade. For the first time, a novel impedance chemical sensor based on magnetic Fe₃O₄ and polydopamine molecularly imprinted polymer magnetic nanoparticles (PDA@Fe₃O₄ MIP MNPs) was designed. Bisphenol A (BPA) and dopamine were used as virtual template molecules and functional monomer for MIP synthesis, respectively. Recognition cavities formed in PDA layers could specifically recognize and effectively adsorb DDT molecules, with the help of virtual templates that had similar molecular structure to DDTs. The as-prepared PDA@Fe₃O₄-MIP MNPs could be used for specific adsorption and efficient extraction of target molecules 4,4'-DDT from food samples. The electrochemical impedance of the PDA@Fe₃O₄-MIP MNPs increased sensitively with the adsorption of 4,4'-DDT, the correlationship between of the electrochemical impedance response and the concentration of 4,4'-DDT were applied in the construction of electrochemical impedance sensors for the determination of 4,4'-DDT. The sensor showed a good linear relationship between the charge transfer resistance (R_ct) and the 4,4'-DDT concentration over a range from 1 × 10^-11 to 1 × 10^-3 mol L^-1 with a detection limit of 6 × 10^-12 mol L^-1. The sensor also exhibited excellent sensitivity and selectivity as well as high stability for the detection of pesticide residues and other environmentally harmful chemicals in various food samples.

A bioinspired polydopamine approach toward the preparation of gold-modified magnetic nanoparticles for the magnetic solid-phase extraction of steroids in multiple samples

In this work, a simple, facile, and sensitive magnetic solid-phase extraction method was developed for the extraction and enrichment of three representative steroid hormones before high-performance liquid chromatography analysis. Gold-modified Fe₃ O₄ nanoparticles, as novel magnetic adsorbents, were prepared by a rapid and environmentally friendly procedure in which polydopamine served as the reductant as well as the stabilizer for the gold nanoparticles, thus successfully avoiding the use of some toxic reagents. To obtain maximum extraction efficiency, several significant factors affecting the preconcentration steps, including the amount of adsorbent, extraction time, pH of the sample solution, and the desorption conditions, were optimized, and the enrichment factors for three steroids were all higher than 90. The validity of the established method was evaluated and good analytical characteristics were obtained. A wide linearity range (0.8-500 μg/L for all the analytes) was attained with good correlation (R²  ≥ 0.991). The low limits of detection were 0.20-0.25 μg/L, and the relative standard deviations ranged from 0.83 to 4.63%, demonstrating a good precision. The proposed method was also successfully applied to the extraction and analysis of steroids in urine, milk, and water samples with satisfactory results, which showed its reliability and feasibility in real sample analysis.

Polydopamine Surface Chemistry: A Decade of Discovery

Polydopamine is one of the simplest and most versatile approaches to functionalizing material surfaces, having been inspired by the adhesive nature of catechols and amines in mussel adhesive proteins. Since its first report in 2007, a decade of studies on polydopamine molecular structure, deposition conditions, and physicochemical properties have ensued. During this time, potential uses of polydopamine coatings have expanded in many unforeseen directions, seemingly only limited by the creativity of researchers seeking simple solutions to manipulating surface chemistry. In this review, we describe the current state of the art in polydopamine coating methods, describe efforts underway to uncover and tailor the complex structure and chemical properties of polydopamine, and identify emerging trends and needs in polydopamine research, including the use of dopamine analogs, nitrogen-free polyphenolic precursors, and improvement of coating mechanical properties.

Determination of fermentable sugars in beer wort by gold nanoparticles@polydopamine: A layer-by-layer approach for Localized Surface Plasmon Resonance measurements at fixed wavelength

Polydopamine decorated in-situ with Localized Surface Plasmon Resonance (LSPR)-active gold nanoparticles (AuNPs) may extend the applicability of nanoplasmonic materials to original and innovative applications in several fields. Here we report the modification of disposable UV-Vis polystyrene cuvettes with AuNPs@PDA for refractive index LSPR-based measurements. An original layer-by-layer deposition method of PDA followed by AuNPs growth is here developed, showing linear correlation between PDA thickness and optical properties. In particular, the modulation from wavelength sensitivity toward absorbance sensitivity is obtained, allowing measurements at fixed wavelength (578 nm). As applicative example of the photonic cuvettes, the measurement of fermentable sugars in beer wort is here reported. The analytical performance of our approach has been directly compared to portable refractometer of reference, displaying excellent results in terms of the precise estimation of sugars in beer wort (expressed in degrees Brix), reproducibility and sensitivity. The approach may be extended to other materials of interest in LSPR based optical sensors, e.g. optical fibers.

Injectable Mussel-Inspired Immobilization of Platelet-Rich Plasma on Microspheres Bridging Adipose Micro-Tissues to Improve Autologous Fat Transplantation by Controlling Release of PDGF and VEGF, Angiogenesis, Stem Cell Migration

Platelets-rich plasma (PRP) can produce growth factors (GFs) to improve angiogenesis. However, direct injection of PRP does not lead to highly localized GFs. The current study employs a mussel-inspired polydopamine to immobilize PRP on gelatin microspheres (GMs) with the purpose of bridging adipose micro-tissues to help implanted fat survive (GM-pDA-PRP). Enhanced PRP adhesion leads to a prolonged and localized production of GFs, which is verified by platelet counting and by ELISA of vascular endothelial growth factors (VEGFs) and of platelet derived growth factors (PDGFs). The GM-pDA-PRP "hatches" a microenvironment for the proliferation of adipose-derived stem cells. After the adipose micro-tissue has bridged with GM-pDA-PRP after 16 weeks, triple-fluorescence staining reveals that the mature adipocytes, blood vessels, and capillaries are arranged like in normal adipose tissue. The survival fat increases significantly compared to that in control, PRP, and GM-PRP groups (84.8 ± 11.4% versus 47.8 ± 8.9%, 56.9 ± 9.7%, and 60.2 ± 10.5%, respectively). Both histological assessments and CD31 immunofluorescence indicate that the improvement of angiogenesis in GM-pDA-PRP is higher than in the fat graft group (6.4-fold in quantitative CD31 positive cells). The CD34 positive cells in the GM-pDA-PRP group are around 3.5-fold the amount in the fat graft group, which suggests that more stem cells migrate to the implant area. Cell proliferation staining shows that the number of Ki67 positive cells is around five times as high as that in the fat graft group.

Recent trends in analytical methods for the determination of amino acids in biological samples

Amino acids are widely distributed in biological fluids and involved in many biological processes, such as the synthesis of proteins, fatty acids, and ketone bodies. The altered levels of amino acids in biological fluids have been found to be closely related to several diseases, such as type 2 diabetes, kidney disease, liver disease, and cancer. Therefore, the development of analytical methods to measure amino acid concentrations in biological samples can contribute to research on the physiological actions of amino acids and the prediction, diagnosis and understanding of diseases. This review describes the analytical methods reported in 2012-2016 that utilized liquid chromatography and capillary electrophoresis coupled with ultraviolet, fluorescence, mass spectrometry, and electrochemical detection. Additionally, the relationship between amino acid concentrations and several diseases is also summarized.

Hierarchical bioceramic scaffolds with 3D-plotted macropores and mussel-inspired surface nanolayers for stimulating osteogenesis

The hierarchical structure of biomaterials plays an important role in the process of tissue reconstruction and regeneration. 3D-plotted scaffolds have been widely used for bone tissue engineering due to their controlled macropore structure and mechanical properties. However, the lack of micro- or nano-structures on the strut surface of 3D-plotted scaffolds, especially for bioceramic scaffolds, limits their biological activity. Inspired by the adhesive versatility of mussels and the active ion-chelating capacity of polydopamine, we set out to prepare a hierarchical bioceramic scaffold with controlled macropores and mussel-inspired surface nanolayers by combining the 3D-plotting technique with the polydopamine/apatite hybrid strategy in order to synergistically accelerate the osteogenesis and angiogenesis. β-Tricalcium phosphate (TCP) scaffolds were firstly 3D-plotted and then treated in dopamine-Tris/HCl and dopamine-SBF solutions to obtain TCP-DOPA-Tris and TCP-DOPA-SBF scaffolds, respectively. It was found that polydopamine/apatite hybrid nanolayers were formed on the surface of both TCP-DOPA-Tris and TCP-DOPA-SBF scaffolds and TCP-DOPA-SBF scaffolds induced apatite mineralization for the second time during the cell culture. As compared to TCP scaffolds, both TCP-DOPA-Tris and TCP-DOPA-SBF scaffolds significantly promoted the osteogenesis of bone marrow stromal cells (BMSCs) as well as the angiogenesis of human umbilical vein endothelial cells (HUVECs), and the TCP-DOPA-SBF group presented the highest in vitro osteogenic/angiogenic activity among the three groups. Furthermore, both TCP-DOPA-Tris and TCP-DOPA-SBF scaffolds significantly improved the formation of new bone in vivo as compared to TCP scaffolds without a nanostructured surface. Our results suggest that the utilization of a mussel-inspired Ca, P-chelated polydopamine nanolayer on 3D-plotted bioceramic scaffolds is a viable and effective strategy to construct a hierarchical structure for synergistically accelerating osteogenesis.

One-step preparation and application of mussel-inspired poly(norepinephrine)-coated polydimethylsiloxane microchip for separation of chiral compounds

In this paper, using the self-polymerization of norepinephrine (NE) and its favorable film-forming property, a simple and green preparation approach was developed to modify a PDMS channel for enantioseparation of chiral compounds. After the PDMS microchip was filled with NE solution, poly(norepinephrine) (PNE) film was gradually formed and deposited on the inner wall of microchannel as permanent coating via the oxidation of NE by the oxygen dissolved in the solution. Due to possessing plentiful catechol and amine functional groups, the PNE-coated PDMS microchip exhibited much better wettability, more stable and suppressed EOF, and less nonspecific adsorption. The water contact angle and EOF of PNE-coated PDMS substrate were measured to be 13° and 1.68 × 10(-4) cm(2) V(-1) s(-1) , compared to those of 108° and 2.24 × 10(-4) cm(2) V(-1) s(-1) from the untreated one, respectively. Different kinds of chiral compounds, such as amino acid enantiomer, drug enantiomer, and peptide enantiomer were efficiently separated utilizing a separation length of 37 mm coupled with in-column amperometric detection on the PNE-coated PDMS microchips. This facile mussel-inspired PNE-based microchip system exhibited strong recognition ability, high-performance, admirable reproducibility, and stability, which may have potential use in the complex biological analysis.

Layer-by-layer self-assembly of polydopamine/gold nanoparticle/thiol coating as the stationary phase for open tubular capillary electrochromatography

Much attention has been paid to utilizing polydopamine (PDA) as the stationary phase in open-tubular capillary electrochromatography (OT-CEC) owing to its diverse properties, such as strong adhesion to various surfaces, latent reactivity toward amine and thiol groups and metal ion chelating/redox activities. In this study, a novel open-tubular capillary column coated with polydopamine/gold nanoparticles/thiols (PDA/Au NPs/thiols) has been fabricated based on the multiple properties of PDA for the first time. The capillary inner surface was firstly functionalized with a layer of PDA/Au NPs using the strong adhesive and metal ion redox properties of PDA. Thiols were then introduced and covalently reacted with the hybrid coating based on the Michael addition reaction of PDA and thiols and also Au-S bonds. Moreover, benefitting from the porosity of PDA, layer-by-layer (LBL) self-assembly was further applied to increase the amounts of stationary phase (Au NPs and thiols), which can significantly enhance the separation effectiveness and stability of the coated column. The formation of the PDA/Au NP/thiol coating in the capillary was confirmed and characterized by scanning electron microscopy (SEM), Energy Dispersive Spectrometry (EDS) and AFM (Atomic Force Microscopy). Then the separation effectiveness of the PDA/Au NP/thiol@capillary was verified by the separation of alkylbenzenes, which can achieve baseline separation easily with high column efficiency. In addition, the column showed long lifetime and good stability. The relative standard deviations (RSDs) for intra-day and inter-day repeatability of the PDA/Au NP/thiol@capillary were lower than 5%. Therefore, the layer-by-layer self-assembly of PDA/Au NPs/thiols on the capillary inner-surface could be an effective capillary modification strategy.

An overview of the use of microchips in electrophoretic separation techniques: fabrication, separation modes, sample preparation opportunities, and on-chip detection

This chapter is intended as a basic introduction to microchip-based capillary electrophoresis to set the scene for newcomers and give pointers to reference material. An outline of some commonly used setups and key concepts is given, many of which are explored in greater depth in later chapters.

Present state of microchip electrophoresis: state of the art and routine applications

Microchip electrophoresis (MCE) was one of the earliest applications of the micro-total analysis system (μ-TAS) concept, whose aim is to reduce analysis time and reagent and sample consumption while increasing throughput and portability by miniaturizing analytical laboratory procedures onto a microfluidic chip. More than two decades on, electrophoresis remains the most common separation technique used in microfluidic applications. MCE-based instruments have had some commercial success and have found application in many disciplines. This review will consider the present state of MCE including recent advances in technology and both novel and routine applications in the laboratory. We will also attempt to assess the impact of MCE in the scientific community and its prospects for the future.

Facile preparation of novel Au–polydopamine nanoparticles modified by 4-mercaptophenylboronic acid for use in a glucose sensor

MPBA–Au–PDA composite particles were prepared for the ultrasensitive non-enzymatic electrochemical immunoassay for glucose.

Ferrocene-functionalized polydopamine as a novel redox matrix for H2O2 oxidation

Simple synthesis of a ferrocene-functionalized polydopamine [poly(DA-Fc)] is described. This redox film displays excellent electrocatalytic oxidation of H₂O₂ in neutral buffer solutions in a Fenton-type reaction, and is found to be selective to H₂O₂ with negligible interference from other small molecules.

PDMS bonding to a bio-friendly photoresist via self-polymerized poly(dopamine) adhesive for complex protein micropatterning inside microfluidic channels

Protein micropatterned surfaces integrated with microfluidics are useful in numerous bioanalytical and biological applications. In this study, we demonstrated the fabrication of complex protein micropatterned surfaces within poly(dimethylsiloxane) (PDMS) microfluidic channels by attaching the PDMS channels to bio-friendly photoresist films and subsequently performing microscope projection photolithography (MPP). A muscle-inspired poly(dopamine) (PDA) coating was employed to mediate the bonding between the PDMS and the bio-friendly photoresist poly(2,2-dimethoxy nitrobenzyl methacrylate-r-methyl methacrylate-r-poly(ethylene glycol) methacrylate) (PDMP). By adjusting the dip-coating time for the PDA coating, we could successfully introduce sufficient amounts of functional groups on the PDMP surfaces to mediate strong bonding between the PDMS channels and the PDA-coated PDMP thin films with minimal alteration of the surface properties of the PDMP thin films that are critical for protein micropatterning. Using this novel bonding strategy, we successfully fabricated multiple protein micropatterns and gradient micropatterns of proteins within microfluidic channels. The technique developed in this study will be useful for the fabrication of complex biochips for multiplex bioassays and fundamental cell biological studies.