Electrical conductivity of ferritin proteins by conductive AFM

Precise and rapid point-of-care quantification of albumin levels in unspiked blood using organic field-effect transistors

Nanowire-based field-effect transistors (FETs) are widely used to detect biomolecules precisely. However, the fabrication of such devices involves complex integration procedures of nanowires into the device and most are not easily scalable. In this work, we report a straightforward fabrication approach that utilizes the grain boundaries of the semiconducting film of organic FETs to fabricate biosensors for the detection of human serum albumin (HSA) with an enhanced sensitivity and detection range. We used trichromophoric pentapeptide (TPyAlaDo-Leu-ArTAA-Leu-TPyAlaDo, TPP) as a receptor molecule to precisely estimate the concentration of HSA protein in human blood. Bi-layer semiconductors (pentacene and TPP) were used to fabricate the OFET, where the pentacene molecule acted as a conducting channel and TPP acted as a receptor molecule. This approach of engineering the diffusion of receptor molecules into the grain boundaries is crucial in developing OFET-based HSA protein sensors, which cover a considerable detection range from 1 pM to 1 mM in a single device. The point-of-care detection in unspiked blood samples was confirmed at 4.2 g dL-1, which is similar to 4.1 g dL-1 measured using a pathological procedure.

Albumin Protein Impact on Early-Stage In Vitro Biodegradation of Magnesium Alloy (WE43)

Mg and its alloys are promising biodegradable materials for orthopedic implants and cardiovascular stents. The first interactions of protein molecules with Mg alloy surfaces have a substantial impact on their biocompatibility and biodegradation. We investigate the early-stage electrochemical, chemical, morphological, and electrical surface potential changes of alloy WE43 in either 154 mM NaCl or Hanks' simulated physiological solutions in the absence or presence of bovine serum albumin (BSA) protein. WE43 had the lowest electrochemical current noise (ECN) fluctuations, the highest noise resistance (Zn = 1774 Ω·cm2), and the highest total impedance (|Z| = 332 Ω·cm2) when immersed for 30 min in Hanks' solution. The highest ECN, lowest Zn (1430 Ω·cm2), and |Z| (49 Ω·cm2) were observed in the NaCl solution. In the solutions containing BSA, a unique dual-mode biodegradation was observed. Adding BSA to a NaCl solution increased |Z| from 49 to 97 Ω·cm2 and decreased the ECN signal of the alloy, i.e., the BSA inhibited corrosion. On the other hand, the presence of BSA in Hanks' solution increased the rate of biodegradation by decreasing both Zn and |Z| while increasing ECN. Finally, using scanning Kelvin probe force microscopy (SKPFM), we observed an adsorbed nanolayer of BSA with aggregated and fibrillar morphology only in Hanks' solution, where the electrical surface potential was 52 mV lower than that of the Mg oxide layer.

The Role of Structural Order in the Mechanism of Charge Transport across Tunnel Junctions with Various Iron-Storing Proteins

In biomolecular electronics, the role of structural order in charge transport (CT) is poorly understood. It has been reported that the metal oxide cores of protein cages (e.g., iron oxide and ferrihydrite nanoparticles (NPs) present in ferritin and E2-LFtn, which is E2 protein engineered with an iron-binding sequence) play an important role in the mechanism of CT. At the same time, the NP core also plays a major role in the structural integrity of the proteins. This paper describes the role of structural order in CT across tunnel junctions by comparing three iron-storing proteins. They are (1) DNA binding protein from starved cells (Dps, diameter (∅) = 9 nm); (2) engineered archaeal ferritin (AfFtn-AA, ∅ = 12 nm); and (3) engineered E2 of pyruvate dehydrogenase enzyme complex (E2-LFtn, ∅ = 25 nm). Both holo-Dps and apo-Dps proteins undergo CT by coherent tunneling because their globular architecture and relative structural stability provide a coherent conduction pathway. In contrast, apo-AfFtn-AA forms a disordered structure across which charges have to tunnel incoherently, but holo-AfFtn-AA retains its globular structure and supports coherent tunneling. The large E2-LFtn always forms disordered structures across which charges incoherently tunnel regardless of the presence of the NP core. These findings highlight the importance of structural order in the mechanism of CT across biomolecular tunnel junctions.

Morphological and Surface Potential Characterization of Protein Nanobiofilm Formation on Magnesium Alloy Oxide: Their Role in Biodegradation

The formation of a protein nanobiofilm on the surface of degradable biomaterials such as magnesium (Mg) and its alloys influences metal ion release, cell adhesion/spreading, and biocompatibility. During the early stage of human body implantation, competition and interaction between inorganic species and protein molecules result in a complex film containing Mg oxide and a protein layer. This film affects the electrochemical properties of the metal surface, the protein conformational arrangement, and the electronic properties of the protein/Mg oxide interface. In this study, we discuss the impact of various simulated body fluids, including sodium chloride (NaCl), phosphate-buffered saline (PBS), and Hanks' solutions on protein adsorption, electrochemical interactions, and electrical surface potential (ESP) distribution at the adsorbed protein/Mg oxide interface. After 10 min of immersion in NaCl, atomic force microscopy (AFM) and scanning Kelvin probe force microscopy (SKPFM) showed a higher surface roughness related to enhanced degradation and lower ESP distribution on a Mg-based alloy than those in other solutions. Furthermore, adding bovine serum albumin (BSA) to all solutions caused a decline in the total surface roughness and ESP magnitude on the Mg alloy surface, particularly in the NaCl electrolyte. Using SKPFM surface analysis, we detected a protein nanobiofilm (∼10-20 nm) with an aggregated and/or fibrillary morphology only on the Mg surface exposed in Hanks' and PBS solutions; these surfaces had a lower ESP value than the oxide layer.

Ferritin-Based Single-Electron Devices

We report on the fabrication of single-electron devices based on horse-spleen ferritin particles. At low temperatures the current vs. voltage characteristics are stable, enabling the acquisition of reproducible data that establishes the Coulomb blockade as the main transport mechanism through them. Excellent agreement between the experimental data and the Coulomb blockade theory is demonstrated. Single-electron charge transport in ferritin, thus, establishes a route for further characterization of their, e.g., magnetic, properties down to the single-particle level, with prospects for electronic and medical applications.

Nanocage Ferritin Reinforced Polyacrylamide Hydrogel for Wearable Flexible Strain Sensors

Biocomposite hydrogels are promising for applications in wearable flexible strain sensors. Nevertheless, the existing biocomposite hydrogels are still hard to meet all requirements, which limits the practical application. Here, inspired by the structure and composition of natural ferritin, we design a PAAm-Ferritin hybrid hydrogel through a facile method. Ferritin is uniformly distributed in the cross-linking networks and acts as a nanocage spring model, leading to the enhanced tensile strength of the hydrogel. The fracture stress is 99 kPa at 1400% maximum elongation. As fabricated PAAm-Ferritin hybrid hydrogels exhibit high toughness and low elastic modulus (21 kPa). The PAAm-Ferritin hybrid hydrogels present excellent biocompatibility and increased conductivity compared with PAAm hydrogel. Impressively, as a wearable flexible strain sensor, the PAAm-Ferritin hybrid hydrogels have high sensitivity (gauge factor = 2.06), excellent reliability, and cycling stability. This study indicates the feasibility of utilizing ferritin to synthesize functional materials, which is conducive to expanding the use of protein synthesis of materials technology and application fields.

Electronic properties and surface potential evaluations at the protein nano-biofilm/oxide interface: Impact on corrosion and biodegradation

The formation of a protein nano-biofilm, which exhibits a special electronic behavior, on the surface of metals or oxide biomaterials considerably influences the crucial subsequent interactions, particularly the corrosion and biodegradation processes. This study discusses the impact of electrical surface potential (ESP) of a single or nano-biofilm of albumin protein on the electrochemical interactions and electronic property evolutions (e.g., charge carriers, space charge capacitance (SCC), and band bending) occurring on the surface oxide of CoCrMo implants. Scanning Kelvin probe force microscopy (SKPFM) results indicated that ESP or surface charge distribution on a single or nano-biofilm of the albumin protein is lower than that of a CoCrMo complex oxide layer, which hinders the charge transfer at the protein/electrolyte interface. Using a complementary approach, which involved performing Mott-Schottky analysis at the electrolyte/protein/oxide interface, it was revealed that the albumin protein significantly increases the SCC magnitude and number of n-type charge carrier owing to increased band bending at the SCC/protein interface; this facilitated the acceleration of metal ion release and metal-protein complex formation. The nanoscale SKPFM and electrochemical analyses performed in this study provide a better understanding of the role of protein molecules in corrosion/biodegradation of metallic biomaterials at the protein nano-biofilm/oxide interface.

Indication of Strongly Correlated Electron Transport and Mott Insulator in Disordered Multilayer Ferritin Structures (DMFS)

Electron tunneling in ferritin and between ferritin cores (a transition metal (iron) oxide storage protein) in disordered arrays has been extensively documented, but the electrical behavior of those structures in circuits with more than two electrodes has not been studied. Tests of devices using a layer-by-layer deposition process for forming multilayer arrays of ferritin that have been previously reported indicate that strongly correlated electron transport is occurring, consistent with models of electron transport in quantum dots. Strongly correlated electrons (electrons that engage in strong electron-electron interactions) have been observed in transition metal oxides and quantum dots and can create unusual material behavior that is difficult to model, such as switching between a low resistance metal state and a high resistance Mott insulator state. This paper reports the results of the effect of various degrees of structural homogeneity on the electrical characteristics of these ferritin arrays. These results demonstrate for the first time that these structures can provide a switching function associated with the circuit that they are contained within, consistent with the observed behavior of strongly correlated electrons and Mott insulators.

Use of a Ferritin L134P Mutant for the Facile Conjugation of Prussian Blue in the Apoferritin Cavity

Since the bullfrog H-ferritin L134P mutant in which leucine 134 is replaced with proline was found to exhibit a flexible conformation in the C₃ axis channel, homologous ferritins with the corresponding mutation have often been studied in terms of a mechanism of iron release from the mineral core within the protein cavity. Meanwhile, a ferritin mutant with the flexible channel is an attractive material in developing a method to encapsulate functional molecules larger than mononuclear ions into the protein cavity. This study describes the clathrate with a horse spleen L-ferritin L134P mutant containing Prussian blue (PB) without a frequently used technique, disassembly and reassembly of the protein subunits. The spherical shell of ferritin was confirmed in a TEM image of the clathrate. The produced clathrate (PB@L134P) was soluble in water and reproduced the spectroscopic and electrochemical properties of PB prepared using the conventional method. The catalytic activity for an oxidoreductive reaction with H₂O₂, one of the major applications of conventional PB, was also observed for the clathrate. The instability of PB in alkaline solutions, limiting its wide applications in aqueous media, was significantly improved in PB@L134P, showing the protective effect of the protein shell. The method developed here shows that horse spleen L-ferritin L134P is a useful scaffold to produce clathrates of three-dimensional complexes with 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.

Electronics of peptide- and protein-based biomaterials

Biologically inspired peptide- and protein-based materials are at the forefront of organic bioelectronics research due to their inherent conduction properties and excellent biocompatibility. Peptides have the advantages of structural simplicity and ease of synthesis providing credible prospects for mass production, whereas naturally expressed proteins offer inspiration with many examples of high performance evolutionary optimised bioelectronics properties. We review recent advances in the fundamental conduction mechanisms, experimental techniques and exemplar applications for the bioelectronics of self-assembling peptides and proteins. Diverse charge transfer processes, such as tunnelling, hopping and coupled transfer, are found in naturally occurring biological systems with peptides and proteins as the predominant building blocks to enable conduction in biology. Both theory and experiments allow detailed investigation of bioelectronic properties in order to design functionalized peptide- and protein-based biomaterials, e.g. to create biocompatible aqueous electrodes. We also highlight the design of bioelectronics devices based on peptides/proteins including field-effect transistors, piezoelectric energy harvesters and optoelectronics.

Applications of chitosan (CHI)-reduced graphene oxide (rGO)-polyaniline (PAni) conducting composite electrode for energy generation in glucose biofuel cell

A glassy carbon electrode (GC) immobilized with chitosan (CHI)@reduced graphene (rGO)-polyaniline (PAni)/ferritin (Frt)/glucose oxidase (GOx) bioelectrode was prepared. The prepared electrode was characterized by using cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) techniques. The morphological characterization was made by scanning electron microsopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. This bioelectrode provided a stable current response of 3.5 ± 0.02 mAcm-2 in 20 mM glucose. The coverage of enzyme on 0.07 cm2 area of electrode modified with CHI@rGO-PAni/Frt was calculated to be 3.80 × 10-8 mol cm-2.

Transition in nanoscale electrical conductivity in the Langmuir-Blodgett film of a novel liquid crystalline oligomer

We have studied the nanoscale electrical conductivity of a monolayer film of a novel star shaped liquid crystalline molecule, hexatriphenylene substituted anthraquinone (AQD6). The molecule has a central core of electron deficient anthraquinone moiety connected to six electron rich triphenylene moieties by flexible alkyl chains. The monolayer formed at air-water interface was transferred onto the solid substrates by Langmuir-Blodgett (LB) technique and its surface topography was imaged using an atomic force microscope (AFM). The limiting area obtained from the surface pressure-area per molecule isotherm and the topography of the AFM images suggest that the anthraquinone moiety of the AQD6 molecule is organized in face-on configuration on the substrate and the triphenylene moieties are in edge-on configuration extended away from the substrate. We have studied the electrical conductivity of the AQD6 monolayer deposited on gold coated silicon substrate using a current sensing AFM. Analysis of current (I) − voltage (V) characteristics of the metal-monolayer film-metal junction showed a transition from direct tunneling to an injection tunneling. Further, we have estimated the barrier height and the effective mass of electron in the metal-monolayer film-metal junction.

Lab on a tip: Applications of functional atomic force microscopy for the study of electrical properties in biology

Electrical properties, such as charge propagation, dielectrics, surface potentials, conductivity, and piezoelectricity, play crucial roles in biomolecules, biomembranes, cells, tissues, and other biological samples. However, characterizing these electrical properties in delicate biosamples is challenging. Atomic Force Microscopy (AFM), the so called "Lab on a Tip" is a powerful and multifunctional approach to quantitatively study the electrical properties of biological samples at the nanometer level. Herein, the principles, theories, and achievements of various modes of AFM in this area have been reviewed and summarized. STATEMENT OF SIGNIFICANCE: Electrical properties such as dielectric and piezoelectric forces, charge propagation behaviors play important structural and functional roles in biosystems from the single molecule level, to cells and tissues. Atomic force microscopy (AFM) has emerged as an ideal toolkit to study electrical property of biology. Herein, the basic principles of AFM are described. We then discuss the multiple modes of AFM to study the electrical properties of biological samples, including Electrostatic Force Microscopy (EFM), Kelvin Probe Force Microscopy (KPFM), Conductive Atomic Force Microscopy (CAFM), Piezoresponse Force Microscopy (PFM) and Scanning ElectroChemical Microscopy (SECM). Finally, the outlook, prospects, and challenges of the various AFM modes when studying the electrical behaviour of the samples are discussed.

Electron Transport in Muscle Protein Collagen

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, Fe³⁺) of the native collagen. To the best of our knowledge, this is the first report of a molecular level study of the electron transport behavior of collagen proteins and estimation of transport band gap values of collagen and metalated collagen.

Arylic versus Alkylic-Hydrophobic Linkers Determine the Supramolecular Structure and Optoelectronic Properties of Tripodal Amphiphilic Push-Pull Thiazoles

The supramolecular structures and their constituents essentially determine the optoelectronic properties of thin films. The introduction of amphiphilicity to the constituents and interface assembly is one established technique to control supramolecular structures and resulting material properties. To yield amphiphilicity, rather hydrophobic chromophores are linked to hydrophilic head groups via flexible alkyl chains. In the present work, we investigate whether replacement of the alkyl linkers by a phenylene linker, that is, replacing an electrically isolating moiety with a potentially semiconducting one, increases the conductivity through the resulting layers. After investigating the influence of the linker on molecular properties of the 2-(4- N, N-dimethylaminophenyl)-4-hydroxy-5-nitrophenyl-1,3 thiazoles exemplarily used in this work, we produce supramolecular structures by means of the Langmuir-Blodgett (LB) technique. Atomic force microscopy (AFM) and UV-vis absorption spectroscopy reveal that thin films made from the more rigid thiazole bearing the arylic linker feature a more homogeneous and stable supramolecular structure as compared to those made from the thiazole dye containing the flexible alkylic linker. Finally, conductive AFM (cAFM) results disclose that the LB films made from the thiazole bearing the π-conjugated arylic linker are less conductive than their counterparts based on the alkylic linkers. In the latter layers, the alkylic linkers provide sufficient motional degrees of freedom to allow for supramolecular rearrangement upon electrical operation during cAFM measurements, hence yielding supramolecular structures featuring increased conductivity with successive cAFM measurements. This work highlights the importance of supramolecular structures for optoelectronic properties by presenting a case where supramolecular effects excel the property changes introduced by molecular modifications.

Piezoelectric Peptide and Metabolite Materials

Piezoelectric materials are important for many physical and electronic devices. Although many piezoelectric ceramics exhibit good piezoelectricity, they often show poor compatibility with biological systems that limits their biomedical applications. Piezoelectric peptide and metabolite materials benefit from their intrinsic biocompatibility, degradability, and convenient biofunctionalization and are promising candidates for biological and medical applications. Herein, we provide an account of the recent progress of research works on piezoelectric peptide and metabolite materials. This review focuses on the growth mechanism of peptide and metabolite micro- and nanomaterials. The influence of self-assembly processes on their piezoelectricity is discussed. Peptide and metabolite materials demonstrate not only outstanding piezoelectric properties but also unique electronic, optical, and physical properties, enabling their applications in nanogenerators, sensors, and optical waveguiding devices.

Photoconductivity enhancement and charge transport properties in ruthenium-containing block copolymer/carbon nanotube hybrids

Functional polymer/carbon nanotube (CNT) hybrid materials can serve as a good model for light harvesting systems based on CNTs. This paper presents the synthesis of block copolymer/CNT hybrids and the characterization of their photocurrent responses by both experimental and computational approaches. A series of functional diblock copolymers was synthesized by reversible addition-fragmentation chain transfer polymerizations for the dispersion and functionalization of CNTs. The block copolymers contain photosensitizing ruthenium complexes and modified pyrene-based anchoring units. The photocurrent responses of the polymer/CNT hybrids were measured by photoconductive atomic force microscopy (PCAFM), from which the experimental data were analyzed by vigorous statistical models. The difference in photocurrent response among different hybrids was correlated to the conformations of the hybrids, which were elucidated by molecular dynamics simulations, and the electronic properties of polymers. The photoresponse of the block copolymer/CNT hybrids can be enhanced by introducing an electron-accepting block between the photosensitizing block and the CNT. We have demonstrated that the application of a rigorous statistical methodology can unravel the charge transport properties of these hybrid materials and provide general guidelines for the design of molecular light harvesting systems.

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.

Solid-state electrical applications of protein and peptide based nanomaterials

This review summarizes recent advancements in electrical properties and applications of natural proteins and mutated variants, synthetic oligopeptides and peptide–π conjugates.

Current Technical Approaches for the Early Detection of Foodborne Pathogens: Challenges and Opportunities

The development of novel and high-tech solutions for rapid, accurate, and non-laborious microbial detection methods is imperative to improve the global food supply. Such solutions have begun to address the need for microbial detection that is faster and more sensitive than existing methodologies (e.g., classic culture enrichment methods). Multiple reviews report the technical functions and structures of conventional microbial detection tools. These tools, used to detect pathogens in food and food homogenates, were designed via qualitative analysis methods. The inherent disadvantage of these analytical methods is the necessity for specimen preparation, which is a time-consuming process. While some literature describes the challenges and opportunities to overcome the technical issues related to food industry legal guidelines, there is a lack of reviews of the current trials to overcome technological limitations related to sample preparation and microbial detection via nano and micro technologies. In this review, we primarily explore current analytical technologies, including metallic and magnetic nanomaterials, optics, electrochemistry, and spectroscopy. These techniques rely on the early detection of pathogens via enhanced analytical sensitivity and specificity. In order to introduce the potential combination and comparative analysis of various advanced methods, we also reference a novel sample preparation protocol that uses microbial concentration and recovery technologies. This technology has the potential to expedite the pre-enrichment step that precedes the detection process.

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).

Electrical Transport Properties of Au Nanoparticles and Thin Films on Ge Probed Using a Conducting Atomic Force Microscope

In this work, gold nanoparticles (Au NPs) were distributed on an n-Ge substrate using the colloidal NP deposition method to form Au NP/Ge Schottky diodes (SDs), and the current transport properties of these nano-SDs were studied. The current density-voltage (J-V) characteristics were measured on each nanometer-sized Au particle using a conducting atomic force microscope (C-AFM). These Au NP/Ge diodes showed a rectifying behavior. According to the thermionic emission (TE) model, the effective Schottky barrier height (SBH) and ideality factors n were obtained. The SBH for the Au NP/Ge diodes ranges from 0.22 to 0.30 eV and the ideality factor ranges from 3.8 to 8.6. The current density and the barrier height increase while the ideality factor decreases with increasing Au NP diameters. This indicates that the tunneling effect is enhanced because of the narrowed depletion width and decreased size of the Au NP/Ge SDs. To compare the electrical behavior with Au NP/Ge diodes, the Au thin film/Ge diodes were also prepared and their SBHs were much larger because of the image-charge lowering effect and the tunneling effect in Au NP/Ge diodes.

Charge transport in a liquid crystalline triphenylene polymer monolayer at air-solid interface

We have prepared a monolayer of a novel liquid crystalline polymer derived from 2,6-dihydroxy-3,7,10,11-tetraalkoxy-triphenylene (PHAT) at an air-water interface and transferred it onto freshly cleaved mica as well as gold coated mica substrates by the Langmuir-Blodgett (L-B) technique. The atomic force microscope (AFM) images of these L-B films show a uniform coverage with a thickness of 1.5 nm. Electrical conductivity measurements were carried out on the PHAT monolayer deposited on the gold coated mica substrate using a current sensing AFM (CSAFM). The gold substrate-PHAT monolayer-cantilever tip of CSAFM forms a metal-insulator-metal (M-I-M) junction. The CSAFM yields a non-linear current-voltage (I-V) curve for the M-I-M junction. The analysis of the I-V characteristics of the M-I-M junction indicated that the charge transport in the liquid crystalline polymer monolayer is by the direct tunneling mechanism. The barrier height for the PHAT monolayer was estimated to be 1.22 ± 0.02 eV.

Long-Range Tunneling Processes across Ferritin-Based Junctions

The mechanism of long-range charge transport across tunneling junctions with monolayers of ferritin is investigated. It is shown that the mechanism can be switched between coherent tunneling, sequential tunneling, and hopping by changing the iron content inside the ferritin. This study shows that ferritins are an interesting class of biomolecules to control charge transport.

Versatile Polymer-Free Graphene Transfer Method and Applications

A new method for transferring chemical vapor deposition (CVD)-grown monolayer graphene to a variety of substrates is described. The method makes use of an organic/aqueous biphasic configuration, avoiding the use of any polymeric materials that can cause severe contamination problems. The graphene-coated copper foil sample (on which graphene was grown) sits at the interface between hexane and an aqueous etching solution of ammonium persulfate to remove the copper. With the aid of an Si/SiO2 substrate, the graphene layer is then transferred to a second hexane/water interface to remove etching products. From this new location, CVD graphene is readily transferred to arbitrary substrates, including three-dimensional architectures as represented by atomic force microscopy (AFM) tips and transmission electron microscopy (TEM) grids. Graphene produces a conformal layer on AFM tips, to the very end, allowing easy production of tips for conductive AFM imaging. Graphene transferred to copper TEM grids provides large-area, highly electron-transparent substrates for TEM imaging. These substrates can also be used as working electrodes for electrochemistry and high-resolution wetting studies. By using scanning electrochemical cell microscopy, it is possible to make electrochemical and wetting measurements at either a freestanding graphene film or a copper-supported graphene area and readily determine any differences in behavior.

Creating Reversible p-n Junction on Graphene through Ferritin Adsorption

An alternative way to construct a stable p-n junction on graphene-based field effect transistor (G-FET) through physical adsorption of ferritin (spherical protein shell) is presented. The produced p-n junction on G-FET could also operate through water-gate. Native ferritins are known to be negatively charged in wet condition; however, we found that native negatively charged ferritins became positively charged after performing electron beam (EB)-irradiation. We utilized this property to construct p-n junction on G-FET. We found also that EB-irradiation could remove the effect of charged impurity adsorbed on graphene layer, thus the Dirac point was adjusted to gate voltage Vg = 0.

Layer number dependence of MoS2 photoconductivity using photocurrent spectral atomic force microscopic imaging

Atomically thin MoS2 is of great interest for electronic and optoelectronic applications because of its unique two-dimensional (2D) quantum confinement; however, the scaling of optoelectronic properties of MoS2 and its junctions with metals as a function of layer number as well the spatial variation of these properties remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy (PCS-AFM) to image the current (in the dark) and photocurrent (under illumination) generated between a biased PtIr tip and MoS2 nanosheets with thickness ranging between n = 1 to 20 layers. Dark current measurements in both forward and reverse bias reveal characteristic diode behavior well-described by Fowler-Nordheim tunneling with a monolayer barrier energy of 0.61 eV and an effective barrier scaling linearly with layer number. Under illumination at 600 nm, the photocurrent response shows a marked decrease for layers up to n = 4 but increasing thereafter, which we describe using a model that accounts for the linear barrier increase at low n, but increased light absorption at larger n creating a minimum at n = 4. Comparative 2D Fourier analysis of physical height and photocurrent images shows high spatial frequency spatial variations in substrate/MoS2 contact that exceed the frequencies imposed by the underlying substrates. These results should aid in the design and understanding of optoelectronic devices based on quantum confined atomically thin MoS2.

Non-native Co-, Mn-, and Ti-oxyhydroxide nanocrystals in ferritin for high efficiency solar energy conversion

Quantum dot solar cells seek to surpass the solar energy conversion efficiencies achieved by bulk semiconductors. This new field requires a broad selection of materials to achieve its full potential. The 12 nm spherical protein ferritin can be used as a template for uniform and controlled nanocrystal growth, and to then house the nanocrystals for use in solar energy conversion. In this study, precise band gaps of titanium, cobalt, and manganese oxyhydroxide nanocrystals within ferritin were measured, and a change in band gap due to quantum confinement effects was observed. The range of band gaps obtainable from these three types of nanocrystals is 2.19-2.29 eV, 1.93-2.15 eV, and 1.60-1.65 eV respectively. From these measured band gaps, theoretical efficiency limits for a multi-junction solar cell using these ferritin-enclosed nanocrystals are calculated and found to be 38.0% for unconcentrated sunlight and 44.9% for maximally concentrated sunlight. If a ferritin-based nanocrystal with a band gap similar to silicon can be found (i.e. 1.12 eV), the theoretical efficiency limits are raised to 51.3% and 63.1%, respectively. For a current matched cell, these latter efficiencies become 41.6% (with an operating voltage of 5.49 V), and 50.0% (with an operating voltage of 6.59 V), for unconcentrated and maximally concentrated sunlight respectively.

Electronic transport via proteins

A central vision in molecular electronics is the creation of devices with functional molecular components that may provide unique properties. Proteins are attractive candidates for this purpose, as they have specific physical (optical, electrical) and chemical (selective binding, self-assembly) functions and offer a myriad of possibilities for (bio-)chemical modification. This Progress Report focuses on proteins as potential building components for future bioelectronic devices as they are quite efficient electronic conductors, compared with saturated organic molecules. The report addresses several questions: how general is this behavior; how does protein conduction compare with that of saturated and conjugated molecules; and what mechanisms enable efficient conduction across these large molecules? To answer these questions results of nanometer-scale and macroscopic electronic transport measurements across a range of organic molecules and proteins are compiled and analyzed, from single/few molecules to large molecular ensembles, and the influence of measurement methods on the results is considered. Generalizing, it is found that proteins conduct better than saturated molecules, and somewhat poorer than conjugated molecules. Significantly, the presence of cofactors (redox-active or conjugated) in the protein enhances their conduction, but without an obvious advantage for natural electron transfer proteins. Most likely, the conduction mechanisms are hopping (at higher temperatures) and tunneling (below ca. 150-200 K).

Current transport mechanism at metal-semiconductor nanoscale interfaces based on ultrahigh density arrays of p-type NiO nano-pillars

The present work focuses on a qualitative analysis of localised I-V characteristics based on the nanostructure morphology of highly dense arrays of p-type NiO nano-pillars (NiO-NPs). Vertically aligned NiO-NPs have been grown on different substrates by using a glancing angle deposition (GLAD) technique. The preferred orientation of as grown NiO-NPs was controlled by the deposition pressure. The NiO-NPs displayed a polar surface with a microscopic dipole moment along the (111) plane (Tasker's type III). Consequently, the crystal plane dependent surface electron accumulation layer and the lattice disorder at the grain boundary interface showed a non-uniform current distribution throughout the sample surface, demonstrated by a conducting AFM technique (c-AFM). The variation in I-V for different points in a single current distribution grain (CD-grain) has been attributed to the variation of Schottky barrier height (SBH) at the metal-semiconductor (M-S) interface. Furthermore, we observed that the strain produced during the NiO-NPs growth can modulate the SBH. Inbound strain acts as an external field to influence the local electric field at the M-S interface causing a variation in SBH with the NPs orientation. This paper shows that vertical arrays of NiO-NPs are potential candidates for nanoscale devices because they have a great impact on the local current transport mechanism due to its nanostructure morphology.

Organic field-effect transistor memory devices using discrete ferritin nanoparticle-based gate dielectrics

Organic field-effect transistor (OFET) memory devices made using highly stable iron-storage protein nanoparticle (NP) multilayers and pentacene semiconductor materials are introduced. These transistor memory devices have nonvolatile memory properties that cause reversible shifts in the threshold voltage (Vth ) as a result of charge trapping and detrapping in the protein NP (i.e., the ferritin NP with a ferrihydrite phosphate core) gate dielectric layers rather than the metallic NP layers employed in conventional OFET memory devices. The protein NP-based OFET memory devices exhibit good programmable memory properties, namely, large memory window ΔVth (greater than 20 V), a fast switching speed (10 μs), high ON/OFF current ratio (above 10(4)), and good electrical reliability. The memory performance of the devices is significantly enhanced by molecular-level manipulation of the protein NP layers, and various biomaterials with heme Fe(III) /Fe(II) redox couples similar to a ferrihydrite phosphate core are also employed as charge storage dielectrics. Furthermore, when these protein NP multilayers are deposited onto poly(ethylene naphthalate) substrates coated with an indium tin oxide gate electrode and a 50-nm-thick high-k Al2 O3 gate dielectric layer, the approach is effectively extended to flexible protein transistor memory devices that have good electrical performance within a range of low operating voltages (<10 V) and reliable mechanical bending stability.

Nanoscale biomemory composed of recombinant azurin on a nanogap electrode

We fabricate a nanoscale biomemory device composed of recombinant azurin on nanogap electrodes. For this, size-controllable nanogap electrodes are fabricated by photolithography, electron beam lithography, and surface catalyzed chemical deposition. Moreover, we investigate the effect of gap distance to optimize the size of electrodes for a biomemory device and explore the mechanism of electron transfer from immobilized protein to a nanogap counter-electrode. As the distance of the nanogap electrode is decreased in the nanoscale, the absolute current intensity decreases according to the distance decrement between the electrodes due to direct electron transfer, in contrast with the diffusion phenomenon of a micro-electrode. The biomemory function is achieved on the optimized nanogap electrode. These results demonstrate that the fabricated nanodevice composed of a nanogap electrode and biomaterials provides various advantages such as quantitative control of signals and exclusion of environmental effects such as noise. The proposed bioelectronics device, which could be mass-produced easily, could be applied to construct a nanoscale bioelectronics system composed of a single biomolecule.

Fast, exact, and non-destructive diagnoses of contact failures in nano-scale semiconductor device using conductive AFM

We fabricated a novel in-line conductive atomic force microscopy (C-AFM), which can analyze the resistive failures and examine process variance with an exact-positioning capability across the whole wafer scale in in-line DRAM fabrication process. Using this in-line C-AFM, we introduced a new, non-destructive diagnosis for resistive failure in mobile DRAM structures. Specially, we focused on the self-aligned contact (SAC) process, because the failure of the SAC process is one of the dominant factors that induces the degradation of yield performance, and is a physically invisible defect. We successfully suggested the accurate pass mark for resistive-failure screening in the fabrication of SAC structures and established that the cause of SAC failures is the bottom silicon oxide layer. Through the accurate pass mark for the SAC process configured by the in-line C-AFM analyses, we secured a good potential method for preventing the yield loss caused by failures in DRAM fabrication.

Temperature and force dependence of nanoscale electron transport via the Cu protein azurin

Solid-state electron transport (ETp) via a monolayer of immobilized azurin (Az) was examined by conducting probe atomic force microscopy (CP-AFM), as a function of both temperature (248-373K) and applied tip force (6-15 nN). At low forces, ETp via holo-Az (with Cu(2+)) is temperature-independent, but thermally activated via the Cu-depleted form of Az, apo-Az. While this observation agrees with those of macroscopic-scale measurements, we find that for holo-Az the mechanism of ETp at high temperatures changes upon an increase in the force applied by the tip to the proteins; namely, above 310 K and forces >6 nN ETp becomes thermally activated. This is in contrast to apo-Az, where increasing applied force causes only small monotonic increases in currents due to decreased electrode separation. The distinct ETp temperature dependence of holo- and apo-Az is assigned to a difference in structural response to pressure between the two protein forms. An important implication of these CP-AFM results (of measurements over a significant temperature range) is that for reliable ETp measurements on flexible macromolecules, such as proteins, the pressure applied during the measurements should be controlled or at least monitored.

Electron Transfer Mechanism in Helical Peptides

Electrochemical studies of a set of ferrocene-labeled helical peptides of increasing length were carried out by forming self-assembled monolayers (SAMs) on gold electrodes. Electron transfer (ET) rates showed a very weakly distance dependent nature that has been interpreted as a result of a dynamically controlled tunneling mechanism. Specifically, the slow equilibrium between the α- and the 310 helical conformers in a SAM has been invoked, and the rate of formation of the more conductive 310 conformer has been proposed to be related to the ET rates observed.

A protein transistor made of an antibody molecule and two gold nanoparticles

A major challenge in molecular electronics is to attach electrodes to single molecules in a reproducible manner to make molecular junctions that can be operated as transistors. Several attempts have been made to attach electrodes to proteins, but these devices have been unstable. Here, we show that self-assembly can be used to fabricate, in a highly reproducible manner, molecular junctions in which an antibody molecule (immunoglobulin G) binds to two gold nanoparticles, which in turn are connected to source and drain electrodes. We also demonstrate effective gating of the devices with an applied voltage, and show that the charge transport characteristics of these protein transistors are caused by conformational changes in the antibody. Moreover, by attaching CdSe quantum dots to the antibody, we show that the protein transistor can also be gated by an applied optical field. This approach offers a versatile platform for investigations of single-molecule-based biological functions and might also lead to the large-scale manufacture of integrated bioelectronic circuits.

Surface differentiation of ferritin and apoferritin with atomic force microscopic techniques

In the study reported herein, we differentiated the structure of ferritin from that of its demetalated counterpart, apoferritin, using field-effect-based atomic force microscopic (AFM) techniques. When ferritin was subjected to conductive-mode AFM analysis, the protein resembled a pancake with a diameter of 10 nm adsorbed on the indium-doped tin-oxide substrate with its fourfold channel perpendicular to the substrate, whereas a flat, empty cavity was revealed for apoferritin. We also attempted to verify the conformational difference with magnetic-mode AFM. However, the resulting phase images failed to differentiate the proteins due to interference from the fringe effect. Despite this, the ferritin analysis revealed a sound correlation between the surface conductivity profiles and the phase profiles. In contrast, apoferritin showed a chaotic relationship in this respect. These results not only suggest that the magnetic domain of ferritin is limited to the iron aggregate in the core, but also demonstrate that AFM is a useful tool for protein conformation analysis.

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.