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東京大学 大学院理学系研究科山野井 慶徳

シアノバクテリアの光化学系Iを活用した電極の作製と物性評価

研究概要本研究では、光合成タンパク質複合体の高い光電変換能を最大限に利用するた

め、これらのタンパク質と微小電極を組み合わせた光センサを構築した。具体的には、グラフェンFET上に金ナノ粒子と光化学系Iを固定化し、グラフェンのI-V特性変化から光を検出する手法について研究を行った。まずグラフェン上にナノ粒子と光化学系Iを固定化する最適条件を見出した。続いてグラフェンFET上にこれらを固定化し、励起光照射下でのI-V特性を調査した。

今後の展望研究結果をもとに、最終的には微小電極上に1つの光化学系Iを固定化し、電圧

測定によって単電子移動を観測することを目指している。電気化学測定によって単電子移動を捉えられれば、生体分子を用いた単光子検出システムとして、学術上、応用上ともに重要な成果となる。現在は、グラフェンFET上の金ナノ粒子と光化学系Iを用いて微弱光の検出を目

指している。本研究を通して、グラフェンを用いた光検知という新たな研究領域を開拓できる。

Photosystems - Key proteins in photosynthesis

Chloroplast

CO2H2O

GlucoseO2

Photosystems are promising materials for new light sensing devices.

Photo-electric conversion Φ ≈ 100 %

Q

PC

Photosystem II(PSII)

Cyt. b6/f

H2O½ O2 + 2H+

FdFNR

Thylakoid membrane

ATP Synthaze

Photosystem I(PSI)

e-

Photosystem I

Gold nanoparticle

Gate of FET

PhotonInput (light signal)

Output (electric signals)

“Bio-photosensor” invented by our research group

e-

Terasaki, N. Hiraga, T. Inoue, Y. Nishihara, H. Minakata,M. Fujii, M. et al. Biochimica et Biophysica Acta,Bioenergetics 2007, 1767, 653-659.

Previous research on PSI

PSI = New material for light sensors

Development of the previous research

Use a small electrode to fabricate the light sensor

The electrode is small. One single photon ➝ One signal

Previous system Ideal system

The electrode was large. A large number of photons ➝ One signal

One signal

Light

PSI particlesPotentiostat

➝ Convert every single photon into an electric signal

Signals

Photons

PSI

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Graphene field effect transistors for sensors

Highly sensitive detection of biological and chemical molecules.

Structure of graphene FETs Change in the drain current induced by biomolecules

Zaifuddin, N. M. et al. J. Appl. Phys. 2013, 52, 06GK04-1/06GK04-4.

Ohno, Y. et al. Biosens. Bioelectron. 2010, 26, 1727-1730.

My research objective

Drain

Small-sized graphene

PSIGold nanoparticles (AuNPs)

Linker molecule

Top gate

SiliconSubstrate

Source

hν

ΔIsd

A light sensor based on PSI, AuNPs and graphene FET for the single photon detection

Electron transfer ➝ Change in the source-drain current (Isd)

e-

Synthesis of gold nanoparticles

HAuCl4aq

hexanethiol

Au

tetraoctylamm-onium bromide

toluene[TBA]+ AuCl4

-

NaBH4

05

1015202530354045

Coun

ts

Diameter (nm)

Size distribution

Average diameter 2.3±0.6 nm

Transmission electron microscopy (TEM) analysis

20 nm

PSI and graphene FET

PSI : Tokyo University of Science (Prof. Tomo’s lab)T. elongatus (BP-1)

Isolation and purification

Graphene FET: Tokyo University of Agriculture and Technology (Prof. Maehashi’s lab)

Source Drain

Graphene

12 µm

5 µm

PSI in buffer solution

52 graphene samples

2 µm

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Functionalization of the electrode

DMF1 h

AuNPs

CH2Cl224 h

Au

Graphene

2. Coupling reagent(EDC / NHS)

PSI

Phosphate buffer2 h

1.

PSI

Au

AuNP deposition on graphene

PSI immobilization

Au Au

Immobilized AuNPs or PSI on graphene

0

5

10

0 0.5 1 1.5 2

Hei

ght (

nm)

Position (µm)

0

5

10

0 0.5 1 1.5 2Position (µm)

AuNPs (1 µM, 24 h) PSI (100 nM, 2 h)

AuNPs : 4-6 nm(including protecting ligands) PSI : 8-10 nm

Graphene on a Si substrate

Deposition of AuNP or PSI

Atomic force microscopy (AFM) analysis

400 nm 400 nm

0

5

10

15

20

0 0.2 0.4 0.6 0.8 1Position (µm)

Immobilized AuNPs and PSI on a graphene FET

PSI + AuNP≈ 14 nm

AFM image of PSI and AuNP on graphene

Deposition of AuNP and PSI

AuNPs in CH2Cl2

PSI in buffer

Scanning electron microscopy (SEM) analysis

1 µm

Gold electrode

Graphene

Height

AuNPs 4-6 nm

PSI 8-10 nm

200 nm

FET measurements - Experimental setup

Excitationlight

Rubber pool

DrainSource

Reference electrode Electrolyte

Electrode

Electrolyte:100 mM NaClO4, 250 mMNaAsc, 2.5 mM DCIP, 20 mMMES (pH 6.4)

Tran, T.-T.; Mulchandani, A. Trends in Analytical Chemistry 2016, 79, 222-232.

Illumination of excitation light➝ Charge transfer (PSI/AuNPs)➝ Shifts in CNPs

Charge neutrality point (CNP)

VG

≈

≈

Source-drain current (Isd) against Gate voltage (VG)

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6.4

6.8

7.2

7.6

8

140 160 180 200

I sd(µ

A)

VG (mV)

1

2

3

4

5

6

8

10

12

0 50 100 150 200

I sd(µ

A)

VG (mV)

1

2

3

4

5

FET measurements - Results

CNP1 CNP2

1 + 1７2 mV + 146 mV

2 + 172 mV + 146 mV

3 + 174 mV + 150 mV

4 + 175 mV + 150 mV

5 + 174 mV + 152 mV

CNP2 CNP1 1: dark2: 680 nm Intensity = 20%3: 440 nm Intensity = 20%4: 440 nm Intensity = 40%5: dark

Blue light irradiation ➝ h+ doping of graphene

Source-drain current under illumination

+2 mV +4 mV

Mechanisms of the hole doping effect

0

－ －

PSI

e-

Light

++ +Isd

h+

Reference electrode AscAsc+

0

－ －

Light

++ +Isd

h+

Reference electrode O2

ions

Source Drain DrainSource

Mechanism 1 : Gating effect Mechanism 2 : Electron transfer

e.g. Charged proteins adsorbed on the graphene FET

Kim, J. E.; No, Y. H.; Kim, J. N.; Shin, Y. S.; Kang, W.T.; Kim. Y. R.; Kim, K. N.; Kim, Y. H., Yu, Q. J. Appl.Phys. Lett. 2017, 110, 203702.

e-

e.g. Redox reactions on the surface of graphene FET

Improvement : Increase the Debye length

𝝀𝑫 = 𝟎. 𝟑𝟓 𝑰 𝟏/𝟐lD

𝑰 : inonic strength

Maehashi, K.; Ohno, Y.; Matsumoto, K. Nanobiosensors in Disease Diagnosis 2016, 5, 1-13.

Weak ionic strengthDetection of large molecules

Debye length [lD] = Thickness of the electric double layer

Biomolecules larger than lDcannot be detected.

Introduce hexylamine to increase the Debye length or

Hexylamine

80

82

84

86

160 200 240 280I sd

(µA)

VG (mV)

No shift in the CNPs

Positive CNP (+227 mV)

Results – Light irradiation and CNP shifts

85

86

87

88

89

-300 -250 -200 -150

I sd(µ

A)

VG (mV)

Without hexylamine With hexylamine

Dark680 nm

Dark680 nm

Unstable I-V curves

Negative CNP (-203 mV)

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Effect of hexylamine

Adsorbed anions➝ Positive CNPs

Without hexylamine With hexylamine

○

Hydrophobic environment was introduced by hexylamine.

The I-V curves were unstable under the negative VG.

Hexylamine

+ + + +Adsorbed Asc-

-

graphene

Si

- -

--

Adsorbed hexylamine➝ Negative CNPs

Two possible solutions

1. Use cationic and anionic detergents

Cationic detergent

+ +- -

Anionic detergent

Asc-

--

2. Decrease the concentration of the electrolyte

Increase in the Debye length & Neutral surface charge

➝ Improvement in the sensitivity toward the light

Conclusions

Gold nanoparticles and photosystem I were immobilized on graphene field effect transistors to fabricate a light sensor.

Light irradiation induced slightly positive shifts in the charge neutrality points of graphene.

Further improvements would be possible by optimizing the electrolyte composition.