A Protein‐Based Free‐Standing Proton‐Conducting Transparent Elastomer for Large‐Scale Sensing Applications

Abstract

A most important endeavor in modern materials’ research is the current shift toward green environmental and sustainable materials. Natural resources are one of the attractive building blocks for making environmentally friendly materials. In most cases, however, the performance of nature‐derived materials is inferior to the performance of carefully designed synthetic materials. This is especially true for conductive polymers, which is the topic here. Inspired by the natural role of proteins in mediating protons, their utilization in the creation of a free‐standing transparent polymer with a highly elastic nature and proton conductivity comparable to that of synthetic polymers, is demonstrated. Importantly, the polymerization process relies on natural protein crosslinkers and is spontaneous and energy‐efficient. The protein used, bovine serum albumin, is one of the most affordable proteins, resulting in the ability to create large‐scale materials at a low cost. Due to the inherent biodegradability and biocompatibility of the elastomer, it is promising for biomedical applications. Here, its immediate utilization as a solid‐state interface for sensing of electrophysiological signals, is shown.

Shaping Macromolecules for Sensing Applications—From Polymer Hydrogels to Foldamers

“…Polymeric materials used for sensors are divided into natural hydrogels, derived from natural polymers, e.g., proteins (collagen [ 76 , 80 , 83 ], elastin [ 84 , 85 ], gelatin [ 86 , 87 , 88 ], albumin [ 29 , 89 ]), polysaccharides (hyaluronic acid [ 90 ], alginate [ 91 , 92 , 93 , 94 , 95 ], chitosan [ 82 , 96 , 97 , 98 , 99 , 100 , 101 ], cellulose [ 74 ]), and synthetic hydrogels [ 39 , 102 , 103 , 104 , 105 , 106 ], based on laboratory-made polymers. Hydrogels in sensing applications are often used as a hybrid material, blend of polymers or in composition with inorganic materials, e.g., graphene, graphene oxide, or silica [ 31 ].…”

“…If needed, the properties of natural hydrogels can be adjusted by chemical modification [ 28 ] or the preparation of composite polymers with synthetic hydrogels [ 100 ]. Natural hydrogels have been used for the sensing of glucose [ 80 , 93 , 97 , 98 , 100 ], dopamine [ 83 ], antioxidants [ 82 ], pH [ 74 , 88 , 89 ], explosives [ 39 , 85 , 86 , 87 , 96 ], biomarkers [ 76 , 84 , 90 , 91 , 92 ], and body signals [ 29 , 30 ] using electrochemical and optical detection methods ( Table 1 ).…”

“… Electrochemical Methods Polyacrylic acid-lignosulfonate-alginate-Ca 2+ Resistance Strain Resistance changes vs. time when monitoring different body joints motions, responsive performance up to 500 cycles. Compression stress—835 kPa Tensile fracture stress—357 kPa Stretching strain—1144% [ 28 ] BSA crosslinked by cysteine disulfide bridges Amperometry Physiological signals Electrocardiography (ECG) for heart activity, electroencephalography (EEG) for brain activity, and electrooculography (EOG) for eye activity, conductivity = 5.3 mS cm −1 [ 29 ] Chitosan/cationic guar gum Amperometry Human body motions 0.296 kPa pressure sensitivity when pressure was lower than 1.25 kP [ 30 ] Methacrylated-collagen, polypyrrole and glucose oxidase Amperometry Glucose LOD = 2 mM, PBS buffer (pH = 7.4) LOD ≅ 200 mM in porcine meat Linear range: 0–4 mM High selectivity in vivo [ 80 ] Collagen from grass carp skin, graphene oxide and aptamer Linear Sweep Voltammetry (LSV) Dopamine LOD = 0.75 nM, PBS buffer (pH = 7) Linear range: 1–1000 nM [ 83 ] Alginate copper oxide with glucose oxidase Amperometry Glucose LOD = 1.6 µM in human serum Linear ranges: 0.04–3 mM and 4–35 mM …”

“…Gels can be formed by covalent or non-covalent bonds, e.g., ionic and hydrogen bonds [ 27 ]. Gel materials in sensing can be generally classified into hydrogels formed by natural [ 28 , 29 , 30 , 31 ] or synthetic [ 16 , 32 , 33 ] polymers [ 22 , 25 ]. Gels coupled with other types of materials, e.g., gold nanoparticles [ 34 , 35 , 36 , 37 , 38 ] and quantum dots [ 39 , 40 ], can be distinguished as a hybrid group of hydrogel materials [ 31 ].…”

Light‐Modulated Cationic and Anionic Transport across Protein Biopolymers**

“…[29,[34][35] Our choice of the BSA mat is not solely based on the proteinsn atural functionality,b ut mainly on its sustainable nature,b eing one of the waste products of the bovine industry,r esulting in its large availability and low price,a swell as its ability to form free-standing proton-conducting biopolymers due to the high abundance of oxo-amino-acids. [29,30,33,36] We use here the BSA to form electrospun mats (a detailed analysis of the mats microstructure can be found in our recent work [33] ), followed by covalent binding of either ap hotoacid or ap hotobase to the protein functional groups.Wefollow both the steady-state and time-resolved fluorescence properties of the polymers as well as their electrical properties to explore the dynamicity of the ESPT and ESPC and their influence on the electrical properties while discussing different modes of ionic transport. Since our work here is the first report of covalently binding both photoacids and photobases to am aterial, and thus manipulate both cationic and anionic charge carrier concentrations with light, we believe we lay the groundwork for any application using excited-state modulation of ionic current.…”