Iwamoto M., Kwon Y.-S., Lee T. (Eds.) Nanoscale Interface for Organic Electronics

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Nanoscale Bioelectronic Device Consisting of Biomolecules 359

been developed to emulate the electron transport function of biological

photosynthesis. Isoda et al. proposed a biophotodiode consisted of

flavin-porphyrin hetero LB films and investigated its electrical

properties.

They use respectively flavin and porphyrin as a sensitizer (S) and an

electron acceptor (A). Fujihira et al. also developed an electrochemical

photodiode which was composed of the LB films of three functional

biomolecules.

Monitoring the electron transfer between the electrode

and the excited dye molecules were also performed, in which ferrocene;

pyrene and viologen were used respectively as the electron donor (D), S

and A units. The metal/insulator/metal (MIM) structured device was

fabricated composed of hetero-type LB films of D, S and A. Also, the

photoinduced electron transfer observed. Currently, a biomolecular

photodiode consisted of electron D/S/Relay (R)/A type 4 component

MIM devices has been developed by Choi et al.

Figure 7 depicts scheme of MIM device, and photoswitching

characteristics of biomolecular photodiode. A biophotodiode which is

composed of LB films of ferrocene, flavin, viologen and TCNQ as the D,

S, R and A units, respectively, was proposed based on the photoinduced

electron transport in a biological system.

By introducing two acceptor

molecules (R and A), the time for the separated charge state (A-/R/S/D+)

can be retained longer than those of the A-/S+ hetero system. Charge

recombination from R and A, to the ground state S, can be decreased

Fig. 7. Schematic diagram of MIM biomolecular photodiode and photoswitching property

of biomolecular device.

J.-W. Choi, T. Lee and J. Min 360

because of the rapid electron transport from S* to A, via R, and the

extended distance between S and A in the existence of R. The

biophotodiode consisted of D/S/R/A hetero LB films is predicted to

display better diode and switching properties than those of S/A and

D/S/A hetero LB films based on these effects. The backward electron

transport of excited S can be reduced by adding the D molecules and the

charge recombination from A to ground state S can be reduced by the

addition of the R molecules. Choi et al. developed some biophotodiodes

consisting of green fluorescent protein (GFP) and cytochrome c.

81,82

Recently, researchers have studied the nanoscale molecular diode using

scanning probe microscopy (SPM).

Khomutov et al. measured the

single molecular conductivity of cytochrome c LB layer based on I-V

measurement by STS.

The biomolecular diode which is contained a

chlorophyll a and ferredoxin heterolayer was investigated by STS based

I-V characteristics. Figure 8 shows experimental set-up for STS based

I-V measurement of biodevice, and rectifying property of biomolecular

diode.

3.2. Nanoscale Biomemory Devices

3.2.1. A Basic Concept of Protein-Based Biomemory Device

The basic principle of metalloprotein based biomemory device is shown

in Fig. 9. The redox property of metalloprotein is controlled upon

Fig. 8. Scheme of Experimental set-up for I-V measurement of bioelectronic device and

rectifying property of biomolecular diode.

Nanoscale Bioelectronic Device Consisting of Biomolecules 361

application of an external potential to possess two electrically distinct

positions.

When an oxidation potential is applied to metalloprotein, the

metalloprotein loses an electron to the inorganic base, and thereby

entrapment of positive charge occurs in the protein biofilm, as shown in

Fig. 10(A). This trap process of positive charge inside the metalloprotein

layer corresponds to the function of storage (writing) of information in a

conventional silicon-based memory device. The trapped charge (written

information) in the protein biofilm is measured (read) when an open-

circuit potential (OCP) is applied to the fabricated electrode.

When a reduction potential is applied to the fabricated electrode after

the initial step of oxidation the inorganic base gives back the electron to

the metalloprotein monolayer, as shown in Fig. 10(B). Thus, the initial

charge trapped in the protein biofilm during the time of oxidation, is

neutralized (erasing). The sequential application of oxidation potential,

OCP and reduction potential makes the developed protein-based

biomemory device write, read and erase the information as a

conventional inorganic memory device.

However, efficient combination techniques of recombinant protein in

conventional electronic devices have not yet been reported; nevertheless,

it is a key process to sustain and control the electrochemical properties

of the recombinant protein to mimic the biological memory system.

Fig. 9. Schematic representation of cyclic voltammogram of azurin assembled layer on

gold working electrode, which depicts the memory function upon application of proper

bias potentials. Reprinted with permission from Ref. 35 S.-U. Kim et al., Charge storage

investigation in self-assembled monolayer of redox-active recombinant azurin, Curr.

Appl. Phys. 9, e71 (2009), Copyright Elsevier (2009).

J.-W. Choi, T. Lee and J. Min 362

Fig. 10. Schematic diagram showing the electron transfer mechanism of recombinant

azurin on Au electrode. (A) Application of oxidation voltage causes the electron to move

from protein to Au electrode. (B) Application of reduction voltage sends the electron

back to the protein. Reprinted with permission from Ref. 35 S.-U. Kim et al., Charge

storage investigation in self-assembled monolayer of redox-active recombinant azurin,

Curr. Appl. Phys. 9, e71 (2009), Copyright Elsevier (2009).

Fig. 11. Write-read-erase cycles of a protein-based biomemory device. The top curve

shows the applied sequence of pulses for “write” “read” and “erase” functions and the

bottom curve is corresponding current response. Reprinted with permission from Ref. 34

J.-W. Choi et al., Protein-based biomemory device consisting of the cysteine-modified

azurin, Appl. Phys. Lett. 91, 263902 (2007), Copyright American Institute of Physics

(2009).

Nanoscale Bioelectronic Device Consisting of Biomolecules 363

Information about the reduction and oxidation of protein molecules can

be obtained from the measurement of OCPA (open circuit potential

amperometry) with time. The potential decay in solution in open circuit

conditions with time has been recorded. This potential was used to read

the stored charge. OCPA is qualitatively similar to conventional

chronoamperometry. However, the method differs by the measurement

of the charge associated with the oxidized protein. The OCPA method is

an electrochemical technique that is used in pulse mode to write-read-

erase cycles, as shown in Fig. 11. In each memory cycle, an oxidation

voltage of -500 mV was applied to store the charge in the protein layer;

reduction voltage of -260 mV was applied to erase all the stored charge

in the device as described in our previous report.

35,38

3.2.2. Write-Once-Read-Many-Times (WORM) Biomemory Device

To read the stored charge multiple times we applied an OCP of -200 mV

at very small disconnecting time and the observed current reveals that the

Fig. 12. (A) Multiple times reading test. (B) Current response validating the memory

switching cycles. Reprinted with permission from Ref. 36. A. K. Yagati et al., Write-

Once–Read-Many-Times (WORM) biomemory device consisting of cysteine modified

ferredoxin, Electrochem. Commun., 11, 854 (2009), Copyright Elsevier (2009).

J.-W. Choi, T. Lee and J. Min 364

stored charge can be read multiple times. Oxidation voltage of -260 mV

(20 ms) can reproducibly charges on the device. A set of three voltage

pulses of -200 mV (OCP) of 20 ms duration with small disconnecting

times showed the current switchings thereby maintaining the oxidized

state of the memory device. Finally, a voltage pulse of -500 mV erases

all the stored charge. The detailed change of current I as a function of

time for two memory cycles were shown in Fig. 12. Further studies will

be required to determine lower limit for the switching time.

3.2.3. Multi-Bit Biomemory Device

Protein based multi-bit biomemory device consisting of recombinant

azurin with a cysteine residue modified by site directed mutagenesis has

been developed.

The recombinant azurin was directly immobilized on

four different gold (Au) electrodes patterned on a single silicon substrate.

Figure 13 represents an experimental system of a protein based device to

store a 4-bit data. Input bias potentials of oxidation and open circuit

potential were applied to the four Au electrodes for duration of 25 msec

consisting azurin SAM and corresponding charging currents were

Fig. 13. Schematics of the experimental system for measuring charging currents from

azurin adsorbed on gold working electrodes; 1, PC with data acquisition system; 2,

multichannel potentiostat; 3, fabricated gold working electrodes (active area 1 mm ×

1 mm) on a Si surface (24 mm × 13 mm); 4, signals obtained from the measuring device.

Reprinted with permission from Ref. 37. A. K. Yagati et al., Multi-bit biomemory

consisting of recombinant protein variants, azurin, Biosens. Bioelectron. 24, 1503,

(2009), Copyright Elsevier (2009).

Nanoscale Bioelectronic Device Consisting of Biomolecules 365

measured as shown in Fig. 14(a). In experiment 1, when the electrodes

no. 1 and 3 were applied with oxidizing potential, which produces a high

faradaic current nearly 7 µA whereas the electrodes no. 2 and 4 were

connected with OCP produces practically a low current of 2 µA. In the

Fig. 14. Faradaic currents observed form a four bit memory device (a) application of

oxidation and open circuit potentials in tandem to the four electrodes (high current

represents storage of “1” and low current represents “0”) represents a 1010 logic system

(b) table represents the results obtained from 4 experiments for storing bits by these

combinations similarly this can be extended for all 16 combinations. Reprinted with

permission from Ref. 37. A. K. Yagati et al., Multi-bit biomemory consisting of

recombinant protein variants, azurin, Biosens. Bioelectron. 24, 1503, (2009), Copyright

Elsevier (2009).

J.-W. Choi, T. Lee and J. Min 366

proposed device, the low faradaic current indicates storage of bit “0” and

high faradaic currents indicates storage of bit “1” as shown in the truth

table. Hence a logic pattern of 1010 can be stored in the biodevice as

shown in Fig. 14(b). Similarly, we preformed experiments for four

different combinations of input bias potentials applied to the electrodes

and corresponding faradaic currents were measured as shown in the

table. In this manner, input potentials with 16 different combinations can

be applied and each provides a combination of logical patterns. Finally, it

is concluded that the proposed bioelectric device composed of azurin

layer can be used as multi-bit molecular storage system by controlling

the redox potentials of azurin.

3.2.4. Multi-Level Biomemory Device

Since blue protein azurin contains copper, which is a key element in the

electron transfer mechanisms, it can be used as an electron acceptor in

the development of molecular electronic devices. Due to its charge

transfer and trap function as well as thermal and chemical stability,

azurin was directly applied in the development of a novel biomemory

device and cytochrome c is the iron storage proteins of the organisms.

Its primary structure consists of a chain of approximately 100 amino

acids (about 12 kDa). If the redox property of the well-organized layer

consisting of cytochrome c on the target electrode can be controlled, it

also would be possible to fabricate a molecular memory device with

these biomolecules.

Recombinant azurin containing cysteine residues was immobilized

directly on Au surface and cytochrome c was adsorbed onto the

immobilized azurin layer by electrostatic bonding. This heterolayer

composed of recombinant azurin and cytochrome c was capable of

storing the two pairs of information, which is referred to as the

multi-state memory. Figure 15(A) depicts schematic diagram for the

electron transfer mechanism of a cytochrome c/recombinant azurin

layer on Au surface, and This multi-state memory displayed two

pairs non-volatile write, read and erase function and it is described

in Fig. 15(B).

Nanoscale Bioelectronic Device Consisting of Biomolecules 367

Figures 16(A) and 16(B) show the faradaic currents obtained upon

applying proper bias potentials to the protein-based molecular memory

device. Here, applying the oxidation voltage is the writing step, whereas

applying the open circuit voltage is the reading step with respect to the

Fig. 15. (A) The electron transfer mechanism of a cytochrome c/recombinant azurin layer

on Au surface. (B) Schematic representation for cyclic voltammogram of hetero layers

consisting of recombinant azurin/cytochrome c on gold surface which depicts the

memory function upon each applications of proper bias potentials.

Fig. 16. The memory function characteristics of multi-state protein based biomemory

device. (A) Two pairs of input pulse potentials of oxidation (charge write 1, 2), open

circuit (charge read 1, 2) and reduction (charge erase 1, 2) potentials were applied to gold

electrodes simultaneously, having a pulse width of 20 msec. (B) the corresponding

charging currents were measured for a total duration of 260 msec.

J.-W. Choi, T. Lee and J. Min 368

measurement of currents. Finally, the use of the reduction potential

erased all the stored charge. This result showed that when two pairs

of redox potentials were applied to the protein film for a duration of

280 msec clear transient currents for the two pairs of the charge to write,

read, and erase functions were monitored, which are a prerequisite for

any molecular memory storage device.

3.3. Biomolecular Electroluminescence Device

The electroluminescent device (ED) consisting of organic molecule is a

light emitting diode (LED) which is based on carbon-based molecules

instead of current inorganic semiconductors. It is totally different

from the conventional inorganic semiconductors. According to recently

research, the organic EL device is better than the current normal liquid

crystal LCD such as its brightness, thickness, fastness and light intensity.

This EL device also need less power, higher contrast, looks better just

as bright from all viewing angles, and low fabrication cost to produce

than inorganic LCD. Also, in accordance with research, EL device which

was composed of biomaterial has more advantage than organic EL

device, for example, its efficiency and driving voltage aspect.

In these reasons, bio EL device could be powerful alternative to

the next generation display which replaces the organic and inorganic

based on EL device. Tajima et al. first proposed a biomolecular

electroluminescence (EL) device that contained cytochrome c.

However, this EL device showed relatively low light intensity, also, it

has just emitted light near the red end of the spectrum. Moreover, the bio

EL device consisted of cytochrome c could not be applied commercially

because of its poor stability in ambient condition.

The bio EL device composed of a biological pigment based

heterolayer aligned as the emitting layer, and reported its EL

performance. The bio EL performance heterolayer that emits a blue light

sustains to be investigated. In our research, the external quantum

efficiency (η ext) of the bio EL devices is higher than current organic

based EL device. Figure 17 describes a schematic diagram of bio EL

device. A bio EL device composed of chlorophyll a was developed

by the application of photo-excited properties. The bilayered bio-EL