Bio-Nanobattery Development and Characterization
Glen C. King
1
, Sang H. Choi
1
, Sang-Hyon Chu
2
, Jae-Woo Kim
3
, Gerald D. Watt
4
, Peter
T. Lillehei
1
, Yeonjoon Park
3
, and James R. Elliott
1
1
NASA Langley Research Center, Hampton, VA 23681
2
National Institute of Aerospace, Hampton, VA 23666
3
Science and Technology Corp., Hampton, VA 23666
4
Brigham Young University, Provo, UT 84601
Abstract
A bio-nanobattery is an electrical energy storage device that utilizes organic materials
and processes on an atomic, or nanometer-scale. The bio-nanobattery under development
at NASA’s Langley Research Center provides new capabilities for electrical power
generation, storage, and distribution as compared to conventional power storage systems.
Most currently available electronic systems and devices rely on a single, centralized
power source to supply electrical power to a specified location in the circuit. As
electronic devices and associated components continue to shrink in size towards the
nanometer-scale, a single centralized power source becomes impractical. Small systems,
such as these, will require distributed power elements to reduce Joule heating, to
minimize wiring quantities, and to allow autonomous operation of the various functions
performed by the circuit. Our research involves the development and characterization of
a bio-nanobattery using ferritins reconstituted with both an iron core (Fe-ferritin) and a
cobalt core (Co-ferritin). Synthesis and characterization of the Co-ferritin and Fe-ferritin
electrodes were performed, including reducing capability and the half-cell electrical
potentials. Electrical output of nearly 0.5 V for the battery cell was measured. Ferritin
utilizing other metallic cores were also considered to increase the overall electrical
output. Two dimensional ferritin arrays were produced on various substrates to
demonstrate the feasibility of a thin-film nano-scaled power storage system for
distributed power storage applications. The bio-nanobattery will be ideal for nanometer-
scaled electronic applications, due to the small size, high energy density, and flexible
thin-film structure. A five-cell demonstration article was produced for concept
verification and bio-nanobattery characterization. Challenges to be addressed include the
development of a multi-layered thin-film, increasing the energy density, dry-cell bio-
nanobattery development, and selection of ferritin core materials to allow the broadest
range of applications. The potential applications for the distributed power system include
autonomously-operating intelligent chips, flexible thin-film electronic circuits,
nanoelectromechanical systems (NEMS), ultra-high density data storage devices,
nanoelectromagnetics, quantum electronic devices, biochips, nanorobots for medical
applications and mechanical nano-fabrication, etc.
Introduction
Bio-nanobatteries will function as distributed power sources within an electrical circuit,
unlike currently utilized central power storage systems. The bio-nanobattery can be
made as a flexible thin film and incorporated into a fabric or made to conform to various
applications, even incorporating power harvesting devices for recharging the bio-
nanobattery. The ferritin-based bio-nanobattery may also be biocompatible, depending
on the core materials. They are lightweight, have a high energy density, and because of
their size, can function as a chip scale power source. This characteristic will make
possible a smart chip, able to operate autonomously. Bio-nanobatteries will have
numerous applications, including flexible thin-film electronic circuits, ultra-high density
data storage devices,
4
nanoelectromagnetics,
5
quantum electronic devices,
6
biochips,
nanorobots for medical applications and mechanical nano-fabrication, nanomechanical
devices, etc.
Ferritins, naturally occurring iron storage proteins, are necessary for the biological
mechanisms of humans, animals, and even bacteria, and may contain up to 4,500 Fe
+3
atoms. Ferritins consist of 24 monomer subunits arranged in a spherical shell with an
outer diameter of about 12.5 nanometers and an inner diameter of around 7.5 nanometers
(Figure 1).
7
They form a stable and robust structure able to withstand biologically
extremes of high temperature (up to 80 °C) and pH variations (2.0-10.0).
8
Both 3-fold
and 4-fold channels in the organic shell allow for the transport of ions and molecules,
making electron conduction through the ferritin shell possible. Research conducted at
Brigham Young University indicates that naturally occurring HEME groups of
bacterioferritins should facilitate electron transport through shell.
Using the reconstitution process of site-specific biomineralization, ferritins may be
loaded with different core materials within the protein shell.
8
Core materials are
incorporated into the ferritin shell with the addition of an oxidant (see Figure 2).
Replacing the naturally occurring iron core using this reversible reaction, ferritins for the
bio-nanobattery can be tailored according to redox capability. Ferritins with cobalt and
manganese cores have already been made for the bio-nanobattery, as well as ferritins with
other core materials for extended applications.
Fe-ferritins and Co-ferritins were used for a unit cell of the bio-nanobattery. In the
absence of chelators at pH = 7.0, the Fe(OH)
3
iron core of ferritins undergoes reversible
reduction to produce a stable Fe(OH)
2
core, while all 4500 iron atoms remain within the
ferritin interior. Redox reactions between ferritin with different core materials involve the
transfer of an electron from a donor to an acceptor ferritin (Figure 3). We have found
that the half-cell potential of Fe-cored ferritins, -400 mV, and the Co-cored ferritins, 1000
mV, indicates that a cell having a 1.4V potential is possible. The charge density per gram
exceeds that of both the “D cell” and “button battery.”
2
Spin coating, dipping, and LangmuirBlodgett deposition were utilized in producing
ferritin arrays for incorporating into the bio-nanobattery. Spin coating, a quick and
simple process for producing a flat and uniform layer, relies on air drag and centrifugal
force, the layer thickness controlled by viscosity and spinning speed. The dipping
method also produces a thin ferritin layer by physical adsorption on substrates, the
thickness controlled by process time and solution concentration. LangmuirBlodgett
deposition is accomplished through monolayer adsorption at the air/water interface.
9,10
With proper surfactant selection, it can form highly ordered ferritin monolayers. The
surface pressure of the protein layer controls the film thickness. Figure 4 shows
integration of the thin-film bio-nanobattery with a photovoltaic component for high
capacity, high efficiency, compact size, and flexible applications.
Experimental and Result
Ferritins were purified through size exclusion chromatography and de-mineralized
through a reduction process to make apo-ferritin, ferritins without a core material. Co
ferritin was synthesized by adding Co
2+
to apo-ferritin in the presence of H
2
O
2
.
11
Similarly, ferritins can be reconstituted with other metallic cores. Ferritin arrays were
fabricated using cationized Ferritin, enabling a strong electrostatic attraction to the
negatively charged Si substrate.
The spin self-assembly (SSA) deposition method
12
was used to produce Ferritin arrays on
various substrates. Depositing successive layers of ferritins on silicon substrates formed
the arrays, redox charge transfer chains. The total output current and voltage can be
tailored by selecting either serial or parallel connection of the pairs. Examination of the
layer structure was accomplished using scanning probe microscopy (SPM), while the
magnetic properties of the ferritin with metallic cores allowed a magnetic force
microscopy (MFM) tip to be used. SPM images show the 2-D ferritin arrays to be
smooth and uniform, suggesting that the SSA deposition method will produce fast,
reliable arrays for the bio-nanobattery.
Stability tests of Co-ferritin indicate that most of the cobalt(II) remains bound to Co-
ferritin (>90%) for extended time periods, demonstrating the stability of the Co(OH)
2
mineral phase within the ferritin interior. Figure 5 shows the reduction of Co
3+
to Co
2+
in
Co-ferritin at 350 nm. The absorbance decreases with the reduction of the cobalt core as
ascorbic acid (a 2 electron donor) is added, until it becomes stable when all Co
3+
-ferritin
is reduced to Co
2+
-ferritin. This result shows that 1.85 Co
3+
are reduced per ascorbic acid
added. An electrochemical cell was also used for coulometric reduction measurements,
showing that 1.10 e/Co was taken up during the electrolysis of Co
3+
-ferritin. The
reduction equilibrium and the reduction kinetics of both Co-ferritin and Mn-ferritin were
also examined. Preliminary results showed that an equilibrium condition between the M
II
and M
III
core material was achieved. The reduction kinetics show a reduction of core
material with time. Results indicate that the manganese reduces much faster than the
cobalt.
3
The mechanism of electron conduction through the protein shell is a key factor to
determine power density, maximum discharge rate, and duty cycle life of bio-nanobattery
cell units. Figure 6 illustrates the results of a dynamic redox reaction between Fe
2+
-
ferritin and Co
3+
-ferritin. A chelating agent was added to the solution containing ferrous
ions. The remaining Fe
2+
is indicated by the decrease in absorbance at 511 nm as the
redox reaction takes place, corresponding to the remaining Fe
2+
in the solution. The
reaction reached an equilibrium state after 4 min, with the absorbance around 0.07. The
initial reaction increased significantly when a piece of gold foil was added to Fe-Co
solution, with a corresponding decrease in the absorbance. This result indicates that the
presence of gold expedites electron transport from Fe
2+
to Co
3+
, and that the mechanism
of electron conduction through the protein shell is a key factor to determine power
density, maximum discharge rate, and the duty cycle life of bio-nanobattery cell units.
Two methods may be employed to determine electron transport in ferritins. The first
involves conductivity measurement of a single ferritin cell using an AFM tip (Figure 7).
A single ferritin cell element is isolated on a positively charged gold substrate. Electrical
current applied through the ferritin cell with the AFM tip is then measured. The
mechanism of electron transport may be either electrons hopping over the cell surface
(electron avalanche?) or electron tunneling through the ferritin cell. The current-voltage
(I-V) curves for holo-ferritin (with core material) and apo-ferritin (without core material)
indicate that Fowler Nordheim tunneling may be the mechanism of electron transport in
the ferritin cell unit. As Figure 8 depicts, the tunneling barrier height is larger for apo
than holo ferritin. Note that the apo-ferritin has no current flow when 0.5 volts is applied,
while the holo-ferritin begins to conduct electrons. The electron conduction is influenced
by the presence of a core material, suggesting that electron tunneling may be the
mechanism of electron transport. Another possible method for conductivity measurement
involves measuring a DC current through a nonconductive substrate, both with and
without a 2-D ferritin array. The number of ferritins sandwiched between electrodes can
be approximated to estimate the conductivity of a single ferritin.
Summary
The bio-nanobattery will enable distributed power storage systems, making more
flexibility in circuit design. Characterization of Fe-ferritin and Co-ferritin indicate that
they would be good candidates for the bio-nanobattery half cell units. Reconstituting
ferritins with other metallic core materials having a higher redox potential may improve
the power density of the bio-nanobattery. Two-dimensional arrays of ferritins were
successfully fabricated on silicon substrates using the spin self-assembly deposition
method. Improving the electron transport and using multilayered ferritin arrays and
ferritins with other core materials may improve the bio-nanobattery performance.
Acknowledgement
This work was partially supported under NASA grants NCC-1-02005 and NCC-1-02043
to the National Institute of Aerospace and Brigham Young University. Related research
4
performed under contract to Science and Technology Corporation was also utilized in this
work.
Figures
Figure 2. Ferritin reconstitution
Fi
g
ure 3. Bio-nanobatter
y
unit cell
Figure 1. Ferritin protein shell
Figure 4. Integration of bio-nanobattery cell
Time (min)
024681012141618
Absorbance (at 511 nm)
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
With gold
Without gold
Figure 6. Absorbance change of Fe
2+
-
chelates as a function of reaction time.
Figure 5. Reducing power of Co-ferritin
5
