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In recent times, collagen, which is one of the most abundant proteins in animals, has appeared to be an attractive candidate for biomaterial applications, for example, in medical implants and wearable electronics. This is because collagen is water-insoluble, biocompatible, and nontoxic. In addition, films of different sizes and shapes can be made using this protein as it is malleable and elastic in nature. However, its electron transport capacity or its absence has remained largely untested so far. Therefore, in this work, the electron transport behavior of collagen has been studied in both film and single-fiber states in a local probe configuration using current-sensing atomic force spectroscopy (CSAFS). From the CSAFS analyses, the electronic (transport) band gap of collagen has been estimated. It has been found that collagen behaves as a wide band gap semiconductor (near-insulating) in a variety of experimental conditions. The transition to a semiconducting material with a low electronic band gap and a nearly 1000-fold enhancement of current (picoampere to nanoampere level) occurs by metal ion treatment (here, Fe

We have developed a multilayered protein-based material using iron-storage protein ferritin, where only proteins were used as components, having no other chemical/biochemical moieties interspersed in between the layers. A simple method of layer-by-layer (LbL) electrostatic adsorption of successive layers of cationized HoSF (with positive surface charge) and native HoSF (with negative surface charge) onto substrates like heavily doped silicon and optically transparent quartz was employed for multilayer (up to 12 layers) fabrication. This technique is economical and time-effective, and is advantageous over other related methods, for example, spin-coating, in several aspects. From optical absorption analysis, it is elicited that the holoferritin multilayer is an indirect band gap material, while the iron-free apo form is a direct band gap material. Importantly, the ferritin multilayer is found to be capable of long-range electron transport – the maximum range being 40 nm for ‘across the multilayer’ (or vertical) transport as observed in a C-AFS study, and 40 μm for ‘along the multilayer’ (or lateral) transport as observed in an interdigitated electrode-based measurement. We found that the substrate influence minimizes as the layer thickness increases. It is also observed that the multilayer is by and large electronically homogeneous, probably because no intermediate junctions of other materials are present between the protein layers. The ferritin multilayer can be considered as a bulk electron transport system beyond 45 nm layer thickness, meaning an unusually long exciton radius of 45 nm, which is one of the longest exciton radii reported so far. This is also the first report of electronic confinement inside any biomaterial.

Silicon is a solid-state semiconducting material that has long been recognized as a technologically useful one, especially in electronics industry. However, its application in the next-generation metalloprotein-based electronics approaches has been limited. In this work, the applicability of silicon as a solid support for anchoring the iron-storage protein ferritin, which has a semiconducting iron nanocore, and probing electron transport via the ferritin molecules trapped between silicon substrate and a conductive scanning probe has been investigated. Ferritin protein is an attractive bioelectronic material because its size (X-ray crystallographic diameter ∼12 nm) should allow it to fit well in the larger tunnel gaps (5 nm), fabrication of which is relatively more established, than the smaller ones. The electron transport events occurring through the ferritin molecules that are covalently anchored onto the MPTMS-modified silicon surface could be detected at the molecular level by current-sensing atomic force spectroscopy (CSAFS). Importantly, the distinct electronic signatures of the metal types (i.e., Fe, Mn, Ni, and Au) within the ferritin nanocore could be distinguished from each other using the transport band gap analyses. The CSAFS measurements on holoferritin, apoferritin, and the metal core reconstituted ferritins reveal that some of these ferritins behave like n-type semiconductors, while the others behave as p-type semiconductors. The band gaps for the different ferritins are found to be within 0.8 to 2.6 eV, a range that is valid for the standard semiconductor technology (e.g., diodes based on p-n junction). The present work indicates effective on-silico integration of the ferritin protein, as it remains functionally viable after silicon binding and its electron transport activities can be detected. Potential use of the ferritin-silicon nanohybrids may therefore be envisaged in applications other than bioelectronics, too, as ferritin is a versatile nanocore-containing biomaterial (for storage/transport of metals and drugs) and silicon can be a versatile nanoscale solid support (for its biocompatible nature).

One major obstacle in development of biomolecular electronics is the loss of function of biomolecules upon their surface-integration and storage. Although a number of reports on solid-state electron transport capacity of proteins have been made, no study on whether their functional integrity is preserved upon surface-confinement and storage over a long period of time (few months) has been reported. We have investigated two specific cases—collagen and ferritin proteins, since these proteins exhibit considerable potential as bioelectronic materials as we reported earlier. Since one of the major factors for protein degradation is the proteolytic action of protease, such studies were made under the action of protease, which was either added deliberately or perceived to have entered in the reaction vial from ambient environment. Since no significant change in the structural characteristics of these proteins took place, as observed in the circular dichroism and UV-visible spectrophotometry experiments, and the electron transport capacity was largely retained even upon direct protease exposure as revealed from the current sensing atomic force spectroscopy experiments, we propose that stable films can be formed using the collagen and ferritin proteins. The observed protease-resistance and robust nature of these two proteins support their potential application in bioelectronics.

Realization of useful nanometer length scale devices in which metalloproteins are junction-confined in a distinct molecular arrangement for generating practical electronic signals (e.g., in bioelectronic switch configuration) is elusive till date. This is mostly due to difficulties in observing an electronically appropriate signal (i.e., reproducible and controllable), when studied under junction-assembled condition. A useful "ON"-"OFF" behavior, based on the negative differential resistance (NDR) peak characteristics in the current-voltage response curves, acquired using metal-insulator-metal (MIM) configuration, has been observed only in the case of a few proteins, namely, azurin, cytochrome c, bacteriorhodopsin, so far. The case of NDR in ferritin, an iron storage protein having a semiconducting iron core consisting of few thousands of iron atoms connected in an oxide network, has not been studied in the MIM configuration where single (or a few) molecule(s) are junction-trapped, for example, as in the case of local probe configuration of scanning probe microscopy. The present study by scanning tunneling microscopy (STM), using the naturally occurring iron-containing ferritin (human liver), as well as different iron-loaded ferritins, provides clear indication of the capability of ferritins to be NDR capable, at varying sweep conditions. As ferritin can be tailor-made in a structurally conserved manner, metal core-reconstituted ferritins, that is, Mn(III)-ferritin, Cu(II)-ferritin, and Ag-ferritin, were prepared. A correlation between the NDR peak signatures, as observed in the respective current-voltage response curves of these reconstituted ferritins, and the nature of the metal core is demonstrated. In support of our earlier proposition, here, we affirm that the ferritin protein behaves as a conductor-insulator (metal core-polypeptide shell) composite, where the overall electronic structure of the material can alter as a function of the nature of the conducting filler placed inside the insulated matrix.