An Ultrasensitive and Fast Moisture Sensor Based on Self‐Assembled Dopamine–Melanin Thin Films

Abstract

biomacromolecule remains a challenge. [ 11 ] Diffi culties associated with the extraction of natural melanin have inspired the development of synthetic melanin, [ 12 ] which can mimic the properties of natural melanin, including its hydrationdependent conducting behavior. For example, the conductivity of 1 mm thick 3,4-dihydroxyphenylalanine (DOPA) melanin pellets was found to be 10 −13 S cm −1 at relative humidity (RH) of 0% and 10 −5 S cm −1 at 100% RH, an eight order of magnitude difference. [ 13 ] Thin fi lms are considered ideal building blocks for constructing miniatured sensing devices due to high surface to volume ratio. However, the preparation of homogeneous melanin thin fi lms at a large scale are extremely diffi cult using conventional solution processing methods due to insolubility of natural and synthetic melanin. [ 6 ] Several attempts have been made to prepare DOPA melanin thin fi lms. For example, ready-made DOPA melanin powders have been used to prepare ammonia-aided melanin aqueous dispersions, which was used to produce DOPA melanin fi lms at a micrometer scale by spin casting. [ 6 ] Synthesized DOPA melanin, soluble in organic solvents such as dimethyl sulfoxide (DMSO) and N , N -dimethyl formamide was also used to produce nanofi lms using spin coating. [ 14 ] A 30 nm thick DOPA melanin nanofi lm spin coated on silicon from DMSO suspension showed about a three order of magnitude change in conductivity on going from 50% RH to 100% RH. [ 15 ] The adhesive proteins secreted by mussels have inspired the design of dopamine-melanin nanofi lms dozens of nanometers thick. [ 16 ] Previous studies have focused mainly on hydration dependence of melanin conductivity (humidity sensitivity), without assessing other parameters critical for practical applications of humidity sensors including response and recovery times, and humidity detection range.This study describes a highly sensitive humidity sensor with rapid response fabricated of self-assembled dopamine-melanin thin fi lms that formed at air/solution interface. These thin fi lms could be transferred to a variety of substrates. Film thickness could be controlled in the nanoscale range by adjusting the reaction times between dopamine monomers and melanin. Examination of the fi lm surface morphology using atomic force microscopy (AFM) showed that the surface was compact and smooth at the nanoscale level. These dopamine-melanin thin fi lm sensor exhibited ultrasensitive and rapid responses to moisture, relatively good reproducibility, and its linear response to RH, allowing their calibration.Dopamine undergoes oxidative polymerization in aqueous solutions in the presence of dissolved oxygen, resulting insoluble melanin aggregates. [ 16 ] In practice, the synthesis of dopamine-melanin starts with deprotonating protonated dopamine (e.g., dopamine hydrochloride), which is chemically stable in air. High-quality melanin thin fi lms were formed at

Decoupling Ionic and Electronic Currents in Melanin

“…The reaction in Figure is a simple manifestation of Le Chatelier's principle with the addition of water driving hydronium and SQ formation, and long range proton transport occurring above a matrix percolation threshold. This process leads to orders of magnitude change in the conductivity of melanin as a function of water content . It is interesting to speculate whether the mechanism is generic amongst other biomacromolecules that support electrical currents such as proteins or polysaccharides .…”

Versatile Surface Modification Using Polydopamine and Related Polycatecholamines: Chemistry, Structure, and Applications

“…Other nucleophilic buffer components such as ammonia and primary amines have similar effects on polymerization as Tris, leading to generally similar material properties and the small variations between nucleophilic buffers have been used to tune the ultimate aggregate properties. For example, ammonia is used to control particle size in solution and aniline has been used to suppress particle formation in the formation of polydopamine thin films at the air–water interface . Further, the changing polymerization processes in nucleophilic and nonnucleophilic buffers have specifically been exploited, with dopamine deposited from phosphate buffer and then from Tris to increase surface roughness of the coating for a superhydrophobic surface …”

“…Early work with natural melanins suggested that they had semiconductor properties . However, based on the hydration dependence of those properties, more recent research shows that the melanin structure is best described as a hybrid ionic–electronic conductor in which both electrons and protons released by comproportionation are involved in electrical conduction ( Figure ) . Polydopamine exhibits the same hybrid ionic–electronic conductivity as natural eumelanins, which has already been exploited variously and provides great potential for use in bioelectronic devices that are capable of integration with living tissue …”

“…A number of polycatecholamine films have electrical properties that vary with hydration . This has been exploited with polydopamine coatings mounted onto gold electrodes and incorporated with reduced graphene oxide into polyvinylalcohol films for use as humidity sensors. Polydopamine is also able to bind variety of chemical species including metal ions and organic molecules.…”

Free-standing polydopamine films generated in the presence of different metallic ions: the comparison of reaction process and film properties

“…The formation of free-standing polydopamine (PDA) lms has attracted great attention recently. [1][2][3][4][5][6][7][8][9] PDA lms show different colours modulated by the polymerization conditions, 3 and they can be transferred to various substrates to form functional coatings. A promising humidity sensor with quick response and recovery time was developed based on PDA lms.…”