Polydopamine Thin Films Research Articles

Electrosynthesis of polydopamine films - tailored matrices for laccase-based biosensors

This work explores the electrosynthesis of bio-inspired polydopamine thin films (ePDA), a route that is still overlooked in polycatecholamine coatings formation, aiming at the construction of prime matrices for biosensor applications. A combination of surface characterization techniques attests the electrochemical and optical properties, wettability, morphology, thickness and chemical composition of the ePDA films. ePDA coatings exhibit structural differences from the chemically synthesized polymers unveiling distinct polymerization pathways. We propose that fast electropolymerization forms a co-polymer enriched by chains of dopamine in the open form, in detriment of indole moieties, yielding more organized and conducting polymeric matrices. For thin coatings (ca. 3 nm), ePDA presents high electroactivity of pendent quinone groups, appropriate for covalent biomolecule interaction. The suitability of the functional ePDA films towards electrochemical biosensing was demonstrated through the catalytic activity of immobilized laccase. A clear improvement of transducing performances towards 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) detection was achieved when using the more electroactive ePDA thin film as supporting matrix, reaching a sensitivity of 342 mA M−1 cm−2. The new structural insights presented in this work, undoubtedly prove the advantage of electrosynthesis regarding molecular oxygen-driven polymerization to prepare more reproducible and organized polycatecholamines for amperometric sensors.

In situ electrodeposition of cholesterol oxidase-modified polydopamine thin film on nanostructured screen printed electrodes for free cholesterol determination

The in situ electrodeposition of cholesterol oxidase (ChOx)-modified polydopamine (PDA) thin film on nanostructured-screen printed electrodes (SPEs) is described here. Prussian Blue (PB) and multi-wall carbon nanotubes (MWCNTs) were used for their outstanding properties in biosensing applications. Cyclic Voltammetry (CV), Constant Potential Amperometry (CPA) and Electrochemical Impedance Analysis (EIS) were used to confirm the correct electrodeposition of the enzyme-modified PDA thin film on the SPE. In addition, the assembly protocol was fully revised to optimize the biosensor response. The proposed cholesterol biosensor showed Michaelis-Menten behavior with KappM, Vmax and Hill parameter values of 1.97mM, 7.33μA and 1.06, respectively. Biosensor sensitivity, stability, reproducibility, repeatability and anti-interference properties were obtained and confirmed the feasibility of using this biosensor for cholesterol determination. In order to show the applicability of such an approach, the biosensor was incorporated in a Flow Injection Analysis (FIA) device and had a high sensitivity of 32.14mAM−1cm−2 (R2=0.999), a linear range over two orders of magnitude and a detection limit (S/N=3) of 1.5μM. The results confirmed the success of the biosensor for determining cholesterol in future applications in diagnostic laboratories.

Structural evolution and electrical properties of metal ion-containing polydopamine

Polydopamine (PDA) is a biopolymer that can be synthesized under mild conditions. Thermal annealing can convert PDA into a conductive phase, the so-called carbonized PDA (cPDA). This work studied the effect of three metal ions, i.e., Cu2+, Mg2+, and Na+, on the synthesis of PDA and its conversion to cPDA. Both Cu2+ and Mg2+ could interact with PDA, which in turn influenced (1) the growth of PDA thin film, (2) morphology change of PDA particles upon thermal annealing, and (3) the electrical properties of heat-treated thin films. In contrast, the presence of Na+ ion during the synthesis of PDA did not show any effect. In this study, the morphology of PDA thin films and powder particles was examined using SEM and TEM; their chemical compositions were studied by EDS and ICP-MS; the structure was investigated using electron diffraction and Raman spectroscopy; and the properties were evaluated with respect to the electrical conductivity and thermoelectric Seebeck coefficient. The results from this work provide a potential approach to control the structure and properties of PDA and cPDA materials through metal ion doping.

A Facile and Sensitive Colorimetric Approach to Confirming the Presence of Polydopamine Thin Films on (Bio)Material Surfaces

Mussel foot adhesive proteins are one of nature's remarkable triumphs; they guarantee a secure and robust bond between the mollusc's byssal threads and the surface to which they settle ensuring that they are securely fastened. We now know that some of these adhesive proteins are especially rich in lysine and 3,4-dihydroxy-L-phenylalanine (DOPA) residues.

Patterning Hydrophobic Surfaces by Negative Microcontact Printing and Its Applications.

Here, a negative microcontact printing method is developed to form hydrophilic polydopamine (PDA) patterns with micrometer resolution on hydrophobic including perfluorinated surfaces. In the process of the negative microcontact printing, a uniform PDA thin film is first formed on the hydrophobic surface. An activated polydimethylsiloxane (PDMS) stamp is then placed in contact with the PDA-coated hydrophobic surface. Taking advantage of the difference in the surface energy between the hydrophobic surface and the stamp, PDA is removed from the contact area after the stamp release. As a result, a PDA pattern complementary to the stamp is obtained on the hydrophobic surface. By using the negative microcontact printing, arrays of liquid droplets and single cells are reliably formed on perfluorinated surfaces. Microlens array with tunable focal length for imaging studies is further created based on the droplet array. The negative microcontact printing method is expected to be widely applicable in high-throughput chemical and biological screening and analysis.

Rapid synthesis of MoS2-PDA-Ag nanocomposites as heterogeneous catalysts and antimicrobial agents via microwave irradiation

Two dimensional nanomaterials have attracted great interest for diverse applications due to their unique structure feature, small size and high specific surface areas. Among them, the two-dimensional metal dichalcogenides have recently emerged as the outstanding ones for various applications ranged from biosensing, energy conversion and storage, and cancer photothermal treatment. In this work, a fast, facile and environmentally benign microwave irradiation method was developed to fabricate MoS2-PDA-Ag nanocomposites. To obtain MoS2-PDA-Ag nanocomposites, MoS2 nanosheets were first coated with polydopamine (PDA) films via self-polymerization of dopamine in alkaline aqueous solution. Then Ag nanoparticles were facilely deposed onto the surface of MoS2-PDA via in situ reduction by using microwave irradiation. Here, the PDA thin films were as the ad-layers for reduction and stabilizing the Ag nanoparticles simultaneously. The successful preparation of MoS2-PDA-Ag nanocomposites was characterized by means of transmission electron microscope, thermogravimetry analysis and X-ray photoelectron spectroscopy. We demonstrated that MoS2-PDA-Ag nanocomposites can be facilely fabricated via combination of mussel inspired chemistry and microwave irradiation. The resultant composites exhibited high catalytic activity towards 4-nitrophenol reduction and great antibacterial activity compared with pure MoS2 and MoS2-PDA.

Single-layer graphene-coated gold chip for enhanced SPR imaging immunoassay

Surface plasmon resonance imaging (SPRi) is a powerful tool for label-free, real-time and high-throughput immunoassay, but its sensitivity needs essential improvement. It has been recently reported that the sensitivity of angular-interrogation surface plasmon resonance (SPR) could be efficiently enhanced on single-layer graphene-coated gold chip. The usefulness of this method however remains unclear for reflectivity-based SPRi platform. Herein for the first time we demonstrate that the graphene coating-enabled intrinsic sensitivity enhancement is applicable to high-throughput SPRi and the enhancement factor is further quantitatively evaluated. High-quality single-layer graphene is transferred in a site-controllable manner to cover half of spots on the SPRi gold array chip. With anti-zearalenone (ZEN) antibody as a model target and polydopamine (PDA) thin film as a reactive layer for probe immobilization, the SPRi detection signals on two types of sensing spots are quantitatively compared on a single chip and the enhancement factor is evaluated. It is unambiguously disclosed that on the reflectivity-based SPRi platform, single-layer graphene coating results in a ca. 40% enhancement on the detection signal (reflectivity increase, ΔR). This work unveils the compelling SPRi sensitivity enhancement of graphene coating and may facilitate its broad practical applications.

(Invited) Conductive Biopolymers for Selective Metal-Devoid Electrocatalysis

Electrocatalysis can be useful in energy conversion, in particular for the production of synthetic fuels from renewable energy. However, this requires catalytic conductors, which sustain the reaction conditions that foremost occur in aqueous solution. Conducting polymers represent a chemically robust but conductive materials class that could be harnessed for aqueous electrocatalytic processes in order to facilitate the important reactions steps to electrosynthesize fuels. In terms of CO2 reduction this has been demonstrated by using a conducting form of polydopamine.(1–3) This system no than other incorporates a richly functionalized backbone able to attach CO2 and electro-reduce to formate efficiently. We shed light on the relevant electro-activation of the incorporated functional themes in the biopolymer and compare this mechanisms to similarly efficient inorganic catalysts. We find that it is the actual nature of the incorporated theme that significantly influences its final selectivity. Such tunability enables the tailoring of selective and highly active catalytic surfaces from organic-functional matter. The functionalzed organic catalysts rely on different reaction mechanisms than state-of-the-art metal-based systems and possibly can be optimized as future sustainable contenders for metal-based electrocatalysts. H. Coskun et al., Chemical vapor deposition - based synthesis of conductive polydopamine thin-films. Thin Solid Films. 645, 320–325 (2018).X. Zheng et al., Sulfur-Modulated Tin Sites Enable Highly Selective Electrochemical Reduction of CO2 to Formate. Joule (2017), doi:10.1016/j.joule.2017.09.014.H. Coskun et al., Biofunctionalized conductive polymers enable efficient CO2 electroreduction. Sci. Adv. 3, e1700686 (2017).

Mussel-Inspired Polydopamine Coating for Flexible Ternary Resistive Memory.

In recent years, numerous organic molecules and polymers carrying various functional groups were synthesized and used in fabrication of wearable electronic devices. Compared to previous materials that suffer from poisonousness, stiffness and complex film fabrication, we circumvent above matters by taking advantage of mussel-inspired polydopamine as our active material to realize resistive random access memories (RRAMs). Polydopamine thin films were grown on indium tin oxide glass catalyzed by Cu2 SO4 /H2 O2 and characterized by Fourier infrared spectroscopy (FT-IR), UV/Vis spectroscopy and scanning electron microscopy. The Al/Polydopamine film/ITO devices possess ternary memory behavior with good ternary device yield with two threshold voltages around 1.50 V and 3.50 V, long data retention over 104 s of continuous reading or 104 pulse reading. The two resistance switchings are attributed to defects functioning as charge traps and the formation of conductive filaments. A flexible device based on Al/polydopamine film/ITO/polyethylene terephthalate retains its ternary memory behavior after being bent with a bending radius of 1.54 cm and bending cycles up to 5000, demonstrating good compatibility and flexibility of polydopamine.

Copper-polydopamine composite derived from bioinspired polymer coating

Metal matrix composites with nanocarbon phases, such carbon nanotube (CNT) and graphene, have shown potentials to achieve improved mechanical, thermal, and electrical properties. However, incorporation of these nanocarbons into the metal matrix usually involves complicated processes. This study explored a new processing method to fabricate copper (Cu) matrix composite by coating Cu powder particles with nanometer-thick polydopamine (PDA) thin films and sintering of the powder compacts. For sintering temperatures between 300 °C and 750 °C, the Cu-PDA composite samples showed higher electrical conductivity and thermal conductivity than the uncoated Cu samples, which is likely related to the higher mass densities of the composite samples. After being sintered at 950 °C, the thermal conductivity of the Cu-PDA sample was approximately 12% higher than the Cu sample, while the electrical conductivity did not show significant difference. On the other hand, Knoop micro-hardness values were comparable between the Cu-PDA and Cu samples sintered at the same temperatures.

Electrochemically enhanced antibody immobilization on polydopamine thin film for sensitive surface plasmon resonance immunoassay

For sensitive immunoassay, it is essentially important to immobilize antibody on a surface with high density and full retention of their recognition activity. Bio-inspired polydopamine (PDA) thin film has been widely utilized as a reactive coating to immobilize antibody on various surfaces. We herein report that the antibody immobilization capacity of PDA thin film is electrochemically enhanced by applying an oxidative potential to convert the surface catechol group to reactive quinone group. Quantitative surface plasmon resonance (SPR) investigation unveils that upon proper electrochemical oxidization, the antibody loading capacity of PDA film is significantly improved (up to 27%) and is very close to the theoretically maximal capacity of a planar surface if concentrated antibody solution is used. Using prostate-specific antigen (PSA) as a model target, it is further demonstrated that the SPR immunoassay sensitivity is greatly enhanced due to the improved antibody immobilization. This work offers an efficient strategy to enhance the reactivity of PDA film towards nucleophiles, and may also facilitate its immunoassay application among others.

Electrochemical and optical characterization of thin polydopamine films on carbon surfaces for enzymatic sensors

The electrochemistry, optical properties, wettability and morphology of polydopamine (PDA) thin films were extensively studied on carbon electrodes, in order to evaluate their potential to be used as electrochemical biosensor interfaces. To fulfil this purpose, several PDA films with different thicknesses were allowed to grow spontaneously and their electrochemical stability and coverage of active quinone/hydroquinone groups were assessed by cyclic voltammetry. Ellipsometry was used to evaluate the dielectric properties of PDA, and film thickness increase with the deposition time on glassy carbon electrodes. Atomic Force Microscopy corroborates film thickness and revealed the formation of uniform coatings for all dopamine incubation times. PDA wettability validates the presence of hydrophilic quinone/hydroquinone. Electrochemical and optical characterization of the modified electrodes points to the formation of a poorly conducting polymeric matrix, yet allowing the charge transfer process of electroactive probes, for 3 ± 1 nm thick films, proving their applicability as electrochemical transducers. The catalytic activity of immobilized Glucose Oxidase or Laccase on graphite/PDA electrodes were validated against glucose and 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), respectively. The analytical parameters are particularly promising for Laccase modified electrodes towards ABTS detection, undoubtedly proving the biocompatibility and excellent electrochemical transduction of thin polydopamine films.

Thickness dependence of polydopamine thin films on detection sensitivity of surface plasmon-enhanced fluorescence biosensors

A bioinspired polydopamine (PDA) coating is a good candidate for the rapid and cheap chemical modification of biosensor surfaces. Herein, we report the effect of PDA thickness on the detection sensitivity of a fluorescence biosensor utilizing surface plasmon-enhanced fluorescence. The thickness of PDA films was tuned by the incubation time of the dopamine solution and varied from 1 to 17 nm. The detection sensitivity was evaluated as the limit of detection (LOD) of a fluorescently labelled target analyte by a model immunoassay. The LOD was determined to be 1.6 pM for the thickest PDA film and was improved to 1.0 pM by reducing the thickness to the range from 1 to 5 nm, corresponding to the incubation time of 10 to 60 min. The experimental results indicate that the PDA coating is suitable for the surface functionalization of biosensors in mass production as it does not require precise control of the incubation time.

Chemical vapor deposition - based synthesis of conductive polydopamine thin-films

Polydopamine (PDA) represents a family of synthetic bio-inspired pigments offering high functional activity combined with semiconducting properties. To date, it represents one of the main synthetic biopolymers used mainly because of its simple synthesis in aqueous solutions. Thereby dopamine polymerizes in the presence of ambient oxygen to polydopamine. However, its structure renders a sophisticated backbone relating to variations of ambient growth parameters such as temperature, local pH and partial oxygen pressure; preponderant repeating units found in aqueous polydopamine are 5,6-hydroxyl-indole derivatives. However, hydrogen-bonded aggregation competes with the polymerization leading to complex systems. In order to reduce the aggregation we hypothesized that acidic oxidants will direct the polymerization towards CC coupling and hence create synthetic biopolymers.In this work we demonstrate oxidative chemical vapor deposition (o-CVD) for PDA, where we obtain the desired consistent biopolymer thin-films as shown by structural analysis. Furthermore, as-gained polydopamine is conductive and renders fingerprint signatures of free charge carriers. Concomitantly it preserves its functionality – imperative for potential applications in catalysis or as bio-linker.

Functionalized Non-vascular Nitinol Stent via Electropolymerized Polydopamine Thin Film Coating Loaded with Bortezomib Adjunct to Hyperthermia Therapy

Gastrointestinal malignancies have been a tremendous problem in the medical field and cover a wide variety of parts of the system, (i.e. esophagus, duodenum, intestines, and rectum). Usually, these malignancies are treated with palliation with the use of non-vascular nitinol stents. However, stenting is not a perfect solution for these problems. While it can enhance the quality of life of the patient, in time the device will encounter problems such as re-occlusion due to the rapid growth of the tumor. In this study, we propose a functionalization technique using electropolymerization of polydopamine directly onto the nitinol stent struts for the combined application of hyperthermia and chemotherapy. The coating was characterized using FESEM, XPS, and FT-IR. Drug release studies show that facile release of the anticancer drug BTZ from the surface of the polydopamine-coated stent could be achieved by the dissociation between catechol groups of polydopamine and the boronic acid functionality of BTZ in a pH-dependent manner. The anti-cancer property was also evaluated, and cytotoxicity on ESO26 and SNU-5 cancer cell lines were observed. Our results suggest that the introduced approach can be considered as a potential method for therapeutic stent application.

Bifunctional polydopamine thin film coated zinc oxide nanorods for label-free photoelectrochemical immunoassay

Photoelectrochemical (PEC) detection is a promising method for label-free immunoassay by reporting the specific biological recognition events with electrical signals. However, it is challenging to rationally incorporate immunosensing components with a photocurrent conversion interface, which generally necessitates multistep fabrication and careful tailoring of various components such as photoactive material and biological probe. For high detection reliability and reproducibility, it is highly desirable to rationally construct an efficient PEC interface with architecture as simple as possible. In this work, a novel yet simple PEC immunosensor based on bio-inspired polydopamine (PDA) thin film-coated zinc oxide (ZnO) nanorods was reported. In this PEC immunosensor, the PDA thin film serves simultaneously as a unique sensitizer for charge separation as well as a functional layer for probe antibody attachment. The photocurrent on this electrode under illumination decreases upon the immunoreaction on the surface, possibly due to the blocking effect of formed immunocomplexes on the access of reducing reagent to the photoelectrode, thus offering a simple and reliable platform for PEC label-free immunoassay. By using an antibody-antigen pair as a model, successful label-free immunoassay was achieved with a detection limit of 10pgmL−1 and a dynamic range from 100pgmL−1 to 500ngmL−1. This work demonstrates intriguing electro-optical property and bioconjugation activity of PDA film and may pave the way toward advanced PEC immunoassays.

Facile preparation of carbon nanotubes based carboxymethyl chitosan nanocomposites through combination of mussel inspired chemistry and Michael addition reaction: Characterization and improved Cu2+ removal capability

The adsorption removal of heavy metal ions from aqueous solution using polymer nanocomposites has attracted great interest because of their advantages from both nanomaterials and polymers. In this work, a facile and novel method has been developed for surface modification of multiwalled carbon nanotubes (CNT) with carboxymethyl chitosan via combination of mussel inspired chemistry and Michael addition reaction. To obtain resultant CNT based chitosan nanocomposites (CNT-PDA-CS), pristine CNT were first coated with polydopamine (PDA) thin films through self polymerization of dopamine under alkaline aqueous solution. Then PDA modified CNT were further functionalized with carboxymethyl chitosan (CS) via Michael addition reaction. The successful preparation of CNT-PDA-CS nanocomposites was ascertained by a series of characterization techniques including transmission electron microscopy, Fourier transform infrared spectroscopy, thermal gravimetric analysis and X-ray photoelectron spectrometry. The utilization of the resultant CNT-PDA-CS for removal of Cu2+ from aqueous solution was also examined in this work. The results showed that CNT-PDA-CS exhibited obviously enhanced Cu2+ removal capability as compared with the unmodified CNT. Considered the versatility of mussel inspired chemistry, the method described in this work should be a general route for fabricating highly efficient adsorbents, which should be of great interest for environmental applications.

Polydopamine Thin Films as Protein Linker Layer for Sensitive Detection of Interleukin-6 by Surface Plasmon Enhanced Fluorescence Spectroscopy.

Polydopamine (PDA) thin films are introduced to the surface modification of biosensor surfaces utilizing surface plasmon enhanced fluorescence spectroscopy (SPFS) as the linker layer of capture antibody on to the sensor surfaces. The capture antibody can be directly attached to the sensor surface without using any coupling agent by functionalizing the gold sensor surface with PDA thin films. The PDA coating is performed by a single-step preparation process by applying the dopamine solution on the sensor surface, which requires an extremely short incubation time (10 min). The real-time in situ measurement of the adsorption kinetics of the capture antibody onto the PDA-coated sensor surface is studied by surface plasmon resonance (SPR) spectroscopy. It reveals that the immobilization of capture antibody immediately occurs after introduction of a solution containing capture antibody, and the sensor surface is fully covered with the capture antibody. The sensitive detection of the cytokine marker interleukin-6 (IL-6) is performed by SPFS using a sandwich assay format with fluorescently labeled detection antibody. The sensor chips functionalized by PDA chemistry exhibited sensitive sensor responses with low nonspecific adsorption of the detection antibody onto the sensor surface. The detection limit of IL-6 with the developed SPFS biosensor is determined to be 2 pg/mL (100 fM), which is within the range of the diagnostic criteria. Our observation elucidates the remarkable utility of PDA coatings for chemical modification of the metallic sensor surfaces by a simple, brief, and inexpensive manner.

Mussel inspired preparation of functional silica nanocomposites for environmental adsorption applications

Surface modification of nanomaterials with polymers is an effective route to render new functions and improve the performance of the final nanocomposites. Here, a facile method was developed to fabricate polyacrylic acid (PAA)-grafted monodisperse SiO2 nanoparticles (SiO2-PDA-PAA) through a combination of mussel inspired chemistry and Michael addition reaction. To obtain the products, the SiO2 nanoparticles were first coated with polydopamine (PDA) through self-polymerization of dopamine under rather mild conditions. The PDA thin films can then be further conjugated with amino-terminated PAA, which was synthesized by chain transfer free radical polymerization using cysteamine hydrochloride as a chain transfer agent and acrylic acid as a monomer. The SiO2-PDA-PAA nanocomposites were characterized via transmission electron microscopy, Fourier transform infrared spectroscopy, thermal gravimetric analysis, and X-ray photoelectron spectroscopy. The effects of contact time, solution pH, temperature and methylene blue (MB) concentration on the removal of MB were investigated. The results demonstrated that SiO2-PDA-PAA showed significant improvement in adsorption efficiency towards MB. The kinetics and isotherm studies showed that pseudo-second-order and Langmuir isotherm models were well fitted the experimental data. The values of thermodynamics parameters such as entropy change (ΔS0), enthalpy change (ΔH0) and Gibbs free energy (ΔG0) were calculated based on the Van’t Hoff equation. The negative values of thermodynamic parameters indicated that the adsorption of MB was a feasible, spontaneous and exothermic process. In summary, we developed a facile method to fabricate SiO2-based polymer nanocomposites, which showed obviously enhanced adsorption capability towards MB.

Lipase immobilized on polydopamine-coated magnetite nanoparticles for biodiesel production from soybean oil

Lipase from Pseudomonas cepacia was covalently attached to magnetite nanoparticles coated with a thin polydopamine film, and employed in the enzymatic conversion of soybean oil into biodiesel, in the presence of methanol. The proposed strategy explored the direct immobilization of the enzyme via Michael addition and aldolic condensation reactions at the catechol rings, with no need of using specific coupling agents. In addition, a larger amount of enzymes could be bound to the magnetic nanoparticles, allowing their efficient recycling with the use of an external magnet. In the biodiesel conversion, the transesterification reaction was carried out directly in soybean oil by the stepwise addition of methanol, in order to circumvent its inactivation effect on the enzyme. A better yield was obtained in relation to the free enzyme, achieving 90% yield at 37 oC. However, the catalysis became gradually less effective after the third cycle, due to its prolonged exposition to the denaturating methanol medium.