2019/11/9

Photosystems - Key proteins in photosynthesis

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

CO2
H2O

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。

“Bio-photosensor” invented by our research group
Input (light signal)

Photosystem II Photosystem I(PSII)
(PSI)
Cyt.
b6/f

Chloroplast

ATPSynthaze

H2O

Thylakoid
membraneQ½ O2 + 2H+e-PC

Photo-electric conversionΦ ≈ 100 %Photosystems are promising materials for new lightsensing devices.

Goldnanoparticle
Gate of FET

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

PSI = New material for light sensors

Output (electric signals)

Ideal system

Previous system

Photons

Light
PSI particles

Photosystem Ie-FNR
Fd

Development of the previous researchPrevious research on PSI

PhotonGlucoseO2

Potentiostat

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

PSI

Signals The electrode is small.
 One single photon➝ One signal

Use a small electrode to fabricate the light sensor➝ Convert every single photon into an electric signal

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Graphene field effect transistors for sensors
Change in the drain current
induced by biomolecules

Structure of graphene FETs

My research objective
A light sensor based on PSI, AuNPs and graphene FET
for the single photon detection
Goldnanoparticles(AuNPs)hνTop gate

PSI

LinkermoleculeeSource

Drain

ΔIsd

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

Highly sensitive detectionof biological and chemicalmolecules.

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

Synthesis of gold nanoparticles
hexanethiol

toluene

NaBH4[TBA]+ AuCl4-

CountsTransmission electron
microscopy (TEM) analysis

454035302520151050

Electron transfer ➝ Change in the source-drain current (Isd)PSI and graphene FET

tetraoctylammonium bromide

HAuCl4aq

SiliconSubstrate

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

Au

Size distribution
Average diameter2.3±0.6 nm

2 µm

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

52 graphenesamples

20 nm
Diameter (nm)

PSI in buffer solution
Isolation andpurification

SourceDrain

12 µm
5 µm22019/11/9

Functionalization of the electrodeImmobilized AuNPs or PSI on graphene

AuNP deposition on graphene

Au

Graphene

Atomic force microscopy (AFM) analysis
AuNPs (1 µM, 24 h)
PSI (100 nM, 2 h)Graphene ona Si substrate

400 nmAuNPs
CH2Cl224 h

PSI immobilization
1.
2. Coupling
reagent(EDC / NHS)Height (nm)

PSI

PSI
Au

Phosphatebuffer2 hAu

PSI inbuffer

Referenceelectrode

Graphene

1 µm

AuNPs in CH2Cl2Gold
electrode

PSI : 8-10 nm

101055000.5

11.5

Position (µm)

200.511.5Position (µm)

2

FET measurements - Experimental setup

Scanning electron
microscopy (SEM) analysis

Deposition of AuNP and PSI

AuNPs : 4-6 nm(including protecting ligands)

0Immobilized AuNPs and PSI on a graphene FET

400 nmExcitation
light

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

Electrolyte

Charge neutrality
point (CNP)

VG
Source≈DMF
1hAu

Deposition ofAuNP or PSI

Drain≈

AFM image of PSI and AuNP on graphene
2015

PSI + AuNP
≈ 14 nm

105200 nm

000.2

0.40.60.8Position (µm)

1Height

Electrode Rubber pool

AuNPs

4-6 nm

PSI

8-10 nmElectrolyte:
100 mM NaClO4, 250 mM
NaAsc, 2.5 mM DCIP, 20 mM
MES (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

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Mechanisms of the hole doping effect

FET measurements - Results
Source-drain current under illumination
CNP2 CNP1210Isd (µA)

138460508100

VG (mV)150200517.6Isd (µA)

27.2

36.846.4
140160180200VG (mV)

1: dark
2: 680 nm Intensity = 20%
3: 440 nm Intensity = 20%
4: 440 nm Intensity = 40%
5: dark

5CNP1CNP21+ 1７2 mV+ 146 mV

2+ 172 mV+ 146 mV

3+ 174 mV+ 150 mV

4+ 175 mV+ 150 mV

5+ 174 mV+ 152 mV

+2 mV+4 mV

Mechanism 1 : Gating effect
Referenceelectrode

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

Introduce hexylamine toincrease the Debye length－Sourcee-+h+－Drain++0Hexylamine
orO2+ h +0Isd

Light+－Drain+Isde.g. Redox reactions on thesurface of graphene FETKim, 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.

Results – Light irradiation and CNP shifts
Without hexylamine

With hexylamine
89

86

88

Isd (µA)

84

Weak ionic strength
Detection of large molecules－Sourcee.g. Charged proteins adsorbedon the graphene FET

Biomolecules larger than lD
cannot be detected.
𝝀𝑫 = 𝟎. 𝟑𝟓 𝑰 𝟏/𝟐
𝑰 : inonic strength

Referenceelectrode

Lighte-ions

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

Asc

PSI

Blue light irradiation➝ h+ doping of graphene

Improvement : Increase the Debye length

Asc+Mechanism 2 : Electron transfer

Isd (µA)

1282

Dark
680 nm87Dark
680 nm

8685

80
160

200

240

280-300-250-200-150

VG (mV)VG (mV)No shift in the CNPs

Unstable I-V curves

Positive CNP (+227 mV)Negative CNP (-203 mV)

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Two possible solutions

Effect of hexylamine
With hexylamine

Without hexylamine

AdsorbedAsc--graphene-

SiHexylamine++

1. Use cationic and anionicdetergents++○Adsorbed anions➝ Positive CNPs

Adsorbed hexylamine➝ Negative CNPs

Hydrophobic environment was introduced by hexylamine.

Cationic
detergent+

Anionic
detergent-+-

2. Decrease the concentrationof the electrolyte

Asc---

Increase in the Debye length & Neutral surface charge➝ Improvement in the sensitivity toward the light

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

Conclusions
 Gold nanoparticles and photosystem I wereimmobilized on graphene field effect transistors tofabricate a light sensor.
 Light irradiation induced slightly positive shifts in thecharge neutrality points of graphene.
 Further improvements would be possible by
optimizing the electrolyte composition.

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