Kinetic and Thermodynamic Characterization of the Cobalt and Manganese Oxyhydroxide Cores Formed in Horse Spleen Ferritin

How stable are the collagen and ferritin proteins for application in bioelectronics?

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.

Magnetoferritin: Process, Prospects, and Their Biomedical Applications

Ferritin is a spherical iron storage protein composed of 24 subunits and an iron core. Using biomimetic mineralization, magnetic iron oxide can be synthesized in the cavity of ferritin to form magnetoferritin (MFt). MFt, also known as a superparamagnetic protein, is a novel magnetic nanomaterial with good biocompatibility and flexibility for biomedical applications. Recently, it has been demonstrated that MFt had tumor targetability and a peroxidase-like catalytic activity. Thus, MFt, with its many unique properties, provides a powerful platform for tumor diagnosis and therapy. In this review, we discuss the biomimetic synthesis and biomedical applications of MFt.

Negative Differential Resistance Behavior of the Iron Storage Protein Ferritin

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.

Permanganate-based synthesis of manganese oxide nanoparticles in ferritin

This paper investigates the comproportionation reaction of Mn^II with [Formula: see text] as a route for manganese oxide nanoparticle synthesis in the protein ferritin. We report that [Formula: see text] serves as the electron acceptor and reacts with Mn^II in the presence of apoferritin to form manganese oxide cores inside the protein shell. Manganese loading into ferritin was studied under acidic, neutral, and basic conditions and the ratios of Mn^II and permanganate were varied at each pH. The manganese-containing ferritin samples were characterized by transmission electron microscopy, UV/Vis absorption, and by measuring the band gap energies for each sample. Manganese cores were deposited inside ferritin under both the acidic and basic conditions. All resulting manganese ferritin samples were found to be indirect band gap materials with band gap energies ranging from 1.01 to 1.34 eV. An increased UV/Vis absorption around 370 nm was observed for samples formed under acidic conditions, suggestive of MnO₂ formation inside ferritin.

Nanoscale On-Silico Electron Transport via Ferritins

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).

A Protein-Based Ferritin Bio-Nanobattery

Nanostructured materials are increasingly important for the construction of electrochemical energy storage devices that will meet the needs of portable nanodevices. Here we describe the development of a nanoenergy storage system based on inorganic mineral phases contained in ferritin proteins. The electrochemical cell consists of an anode containing~2000 iron atoms as Fe(OH)2in the hollow protein interior of ferritin and a cathode containing~2000 of Co(OH)3in a separate ferritin molecule. The achieved initial voltage output from a combination of Fe2+- and Co3+-ferritins adsorbed on gold electrodes was~500 mV, while a combination of Fe2+- and Co3+-ferritins immobilized on gold produced a voltage of 350–405 mV. When fully discharged, Fe(OH)3and Co(OH)2are the products of a single electron transfer per metal atom from anode to cathode. The spent components can be regenerated by chemical or electrochemical methods restoring battery function. The properties of ferritins are presented and their unique characteristics are described, which have led to the development of a functional bio-nanobattery.

Tuning band gap of holoferritin by metal core reconstitution with Cu, Co, and Mn

Utility of ferritin in molecular electronics, especially in single molecule electronics based devices, has recently been proposed, since the iron core of holoferritin is semiconducting in nature. However, the practical aspects, e.g., how its electronic properties can be varied/tuned, need to be better addressed. In this direction, we have performed direct tunneling experiments using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) on several metal core reconstituted ferritins, where the reconstitution has been carried out using biocompatible metals like copper, cobalt, and manganese that are found naturally in the human body. We show, for the first time, that, by metal core reconstitution of the ferritin protein, the band gap of the protein can be tuned to different values (here, within the range 1.17-0.00 eV, considering iron-containing holoferritin and apoferritin as well). From the respective current-voltage curves and the well-defined band gaps, clear distinction can be made among the five different ferritins indicating that the metal core has direct contribution in the observed electrical conductivities of ferritins. It is further revealed that the electrical conductivities of the reconstituted ferritins are of the same order as that for the free metal conductivities, meaning that the relative changes in the free metal conductivities are reflected in the contributions of the metals in protein shell-confinement (i.e., the ∼8 nm core of ferritin). This finding could lead to a strategy for fine-tuning ferritin band gap by preselecting a metal on the basis of the free metal conductivity values.

Biological synthesis of platinum nanoparticles with apoferritin

A novel biological method for the synthesis of platinum nanoparticles using the horse spleen apoferritin (HSAF) is reported. When HSAF was incubated with K(2)PtCl(6) at 23 degrees C) for 48 h followed by subsequent reduction with NaBH(4) it resulted in the formation of spherical platinum nanoparticles, size 4.7 +/- 0.9 nm, with narrow particle size distribution confirmed by transmission electron microscopy and energy dispersive X-ray analysis. As the initial platinum concentration increased through 0.155, 0.31, 0.465 to 0.62 mM the efficiency of its removal from solution by the apoferritin was 99, 99, 84 and 71% respectively. The maximum uptake of platinum salt per mM apoferritin was estimated at 12.7 mmol l(-1) h(-1). These results clearly indicate that the HSAF protein cage can successfully serve as a suitable size-constrained support matrix for the biological synthesis of platinum nanoparticles.

Efficient self-assembly of Archaeoglobus fulgidus ferritin around metallic cores

Interfacing biological systems with inorganic nanoparticles is of great interest, as it offers means of particle stabilization and spatial control in electronic or biomedical applications. We report on the particle-directed assembly of hyperthermophile Archaeoglobus fulgidus ferritin subunits around negatively charged colloidal gold. An annealing process allows rapid assembly of the protein in near-native stoichiometry. Transmission electron microscopy suggests that greater than 95% of nanoparticles are encapsulated while the self-assembly process ensures that almost 100% of the assembled ferritin cavities are occupied.

Structure and activity of apoferritin-stabilized gold nanoparticles

A simple method for synthesizing gold nanoparticles stabilized by horse spleen apoferritin (HSAF) is reported using NaBH(4) or 3-(N-morpholino)propanesulfonic acid (MOPS) as the reducing agent. AuCl(4)(-) reduction by NaBH(4) was complete within a few seconds, whereas reduction by MOPS was much slower; in all cases, protein was required during reduction to keep the gold particles in aqueous solution. Transmission electron microscopy (TEM) showed that the gold nanoparticles were associated with the outer surface of the protein. The average particle diameters were 3.6 and 15.4 nm for NaBH(4)-reduced and MOPS-reduced Au-HSAF, respectively. A 5-nm difference in the UV-Vis absorption maximum was observed for NaBH(4)-reduced (530 nm) and MOPS-reduced Au-HSAF (535 nm), which was attributed to the greater size and aggregation of the MOPS-reduced gold sample. NaBH(4)-reduced Au-HSAF was much more effective than MOPS-reduced Au-HSAF in catalyzing the reduction of 4-nitrophenol by NaBH(4), based on the greater accessibility of the NaBH(4)-reduced gold particle to the substrate. Rapid reduction of AuCl(4)(-) by NaBH(4) was determined to result in less surface passivation by the protein. Methods for studying ferritin-gold nanoparticle assemblies may be readily applied to other protein-metal colloid systems.

Cobalt oxide hollow nanoparticles derived by bio-templating

We present here the first fabrication of hollow cobalt oxide nanoparticles produced by a protein-regulated site-specific reconstitution process in aqueous solution and describe the metal growth mechanism in the ferritin interior.