Article pubs.acs.org/Langmuir
Nanoscale On-Silico Electron Transport via Ferritins Sudipta Bera, Jayeeta Kolay, Siddhartha Banerjee,† and Rupa Mukhopadhyay* Department of Biological Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India S Supporting Information *
ABSTRACT: 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 nextgeneration 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).
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INTRODUCTION Miniaturization of electronic devices to nanoscale dimensions has recently been considered to be essential for the future of the electronics industry, as it offers benefits of reduced power consumption and improved device performance in terms of speed and functionality.1 In this regard, attempts for integration of natural/tailor-made biomolecular components onto industrystandardized substrates and assessment of bioelectronic properties of such nanoscale components are increasingly being made. Amongst all the biomolecular types, metalloproteins appear to be suitable candidates for applications in metal−insulator− metal (MIM) diode configuration because they can be entrapped between the two metal electrodes in controlled manner. Their natural capacity for fast, directional, long-range (∼10−20 Å) electron-transfer properties2 and solid-state electron transport abilities3 can potentially be exploited in multifarious electronics applications. The prospects of ferritin proteins in molecular electronics as an effective bioelectronic component are particularly strong, since ferritin contains a semiconducting iron core,4 and has been shown to be electronically communicative under widely different conditions.4−8 The comparatively large size of ferritin (X-ray crystallographic diameter ∼12 nm) compared with most of the metalloproteins means that it can fit more effectively in the relatively easy-to-fabricate wide tunnel gaps (≥5 nm). © 2017 American Chemical Society
Ferritin, which is a universal intracellular protein, is found in almost all living organisms, from algae and bacteria to the higher plants and animals. In human, it is present predominantly in liver and spleen and acts as an iron-storage protein that plays a crucial role in maintaining the physiologically relevant iron balance.9 Its structure is unique since a unique ordered arrangement of 24 subunits leads to the formation of a hollow sphere that can contain a few thousands of iron atoms from ∼200010,11 to ∼450012 in the form of ferrihydrite phosphate [(FeOOH)8(FeOPO3H2)]. Ferritin is nature’s own nanoparticle because its external diameter is 12 nm and its core can be 7 to 8 nm in size.12 The choice of ferritin for on-surface electron transport studies/applications can be further justified as it is a structurally robust protein that can largely withstand adsorption-induced structural damages, if any. The influence of molecular orientations on the solid-state electron transport characteristics could also be minimal in case of ferritins as the protein is spherical in shape. The advantages like its ability to stay in functional form in an aqueous environment within a wide pH range (4.0−9.0) and well up to 80 °C temperature13 are the added boons for the ferritin-based Received: November 14, 2016 Revised: January 11, 2017 Published: February 1, 2017 1951
DOI: 10.1021/acs.langmuir.6b04120 Langmuir 2017, 33, 1951−1958
Article
Langmuir Scheme 1. Schematic Presentation of Thiol-Assisted Ferritin Immobilization on Linker-Modified Silicon Substrate
previously reported. All metal-core reconstituted ferritins were characterized by high-resolution TEM (model no. JEM-2010) with an operating voltage 200 keV and spectrophotometrically by using Varian Cary 50 Bio UV−visible spectrophotometer at 25 °C temperature using a 1 cm cuvette (see Figures S1 and S2 in the Supporting Information). All of the reconstituted ferritin solutions were stored at 4 °C. Preparation of Ferritin Films onto Silicon Substrate. A doped (resistivity
