Supporting Information
for
Conductance exhibiting stochastic resonance
in Mn12 redox network without tuning
Yoshiaki Hirano1,4, Yuji Segawa2, Takayoshi Kuroda-Sowa3, Tomoji Kawai2,
and Takuya Matsumoto*, 1
1
Department
of
Chemistry,
School
of
Science,
Osaka
University,
1-1
Machikaneyama-cho, Toyonaka, Osaka 560-0043 Japan
Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka,
Ibaraki, Osaka 567-0047, Japan
3
Department of Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1
Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
4
Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui, 910-8507,
Japan
2
1. Sample preparation
1.1.
Synthesis
and
Characterization
of
Mn12
{[Mn12O12(O2CC6H5)12(O2CC6H4NH2)4(H2O)4]·2(CH2Cl2)=3091.1 (g/mol)}.
The
substitution method of four aniline groups on the exterior of Mn12 molecule, and the
characterization results were the same as that reported previously.20
1.2. Preparation of Mn12 solution, Mn12/DNA Complex and Fixing method. The
preparation method of Mn12 aqueous solution was the identical with that reported
previously, and the concentration was 0.128 mM. -DNA solution with the
concentration of 360 (μg/mL) was purchased from Takara Bio Inc. (Otsu, Japan). The
length of the DNA was approximately 16 m.
The Mn12 solution was added to the
-DNA solution, where the concentration ratio was set to [Mn12]:[phosphate groups on
-DNA]=1:6. The Mn12/DNA complex was fixed on the substrate as follows,
referring to our previous paper.15 A 15 μL portion of the mixed solution was dropped
on a SiO2(100) substrate pretreated with an ozone-UV cleaner for 60 min, and was then
held under 100% humidity for 30 min. Any excess solution was finally removed from
the substrate in one direction using air flow.
1.3. Fabrication of Nanogap Electrodes. Gold (Au) nanogap electrodes with the
average gap width of order of several tens nm were formed on the Mn12/λ-DNA
complex fixed on the SiO2 substrate using an angled incidence deposition method, as
reported by Otsuka et al.22 The cross-sectional geometric view of the device is shown in
Figure S1.
Figure S1. Cross-sectional geometric illustration of the device consisting of Mn12/DNA
network and Au nano-gap electrodes on an SiO2/Si substrate.
2. Apparatus and set up
2.1. AFM Measurements. Atomic force microscopy (AFM) measurements were
carried out using a Jeol SPM 4200 instrument. AFM images of Mn12/λ-DNA
complexes on SiO2 substrates were obtained in tapping mode using a silicon cantilever.
2.2. Current-Voltage (I-V) Measurements. Current-voltage (I-V) measurements
were carried out from 10 up to 300 K using a Keithley 4200-SCS semiconductor
parameter analyzer under a vacuum of 5 × 10−2 Pa or less.
2.3. Complex impedance Measurements. Complex impedance measurements from 5
×107 down to 10-2 Hz were conducted at 10 and 300 K using a Solartron SI 1260
Impedance/Gain-phase analyzer and a Keithley 428 under a vacuum of 5 × 10−2 Pa or
less. Since there is no dc current within the threshold voltage at 10 K, the frequency
response shows only the capacitance between nanogap electrodes (See supporting
information). For this reason, Cole-Cole plot at 10 K was taken with bias voltage which
is larger than threshold voltage.
2.7. Stochastic Resonance (SR) Experiments. A square wave with a frequency of
0.1 Hz and an amplitude of 200 mVpp was used, and white noise with a frequency from
10 to 100 Hz
was gradually injected using a noise generator into the Mn12 redox network array device
at 10 K. While the voltage of the square wave remained constant, that of the white
noise was systematically increased. The I-V converter had a sensitivity in the order of
10−8 or 10-7 A/V, a time constant of 1 s, and a 1 Hz low-pass filter; therefore, output
signals were detected using an oscilloscope. Moreover, SR experiments were
performed with the above square wave and thermal noise from 10 up to 300 K as noise
source.
3. Analysis
3.1. Evaluation of correlation coefficients. The correlation coefficient (C.C.) between
the input reference and output waveforms was estimated using
S in S out  S in
C.C. 
S in
2
 S in
2
S out
S out
2
 S out
2
(1)
where the angle brackets < > denote an ensemble average, and Sin and Sout are the input
reference and output signals, respectively.
3.2. Evaluation of signal-to-noise ratio.
SNR 
P0.1 
SN
SNR is defined as
(2)
where P(0.1) and SN are the integration values of the peak around 0.1 Hz and the noise
in the FFT power spectra of the output signals, respectively.16 For the SNR estimation,
the integration ranges of P(0.1) and SN are normalized, and the SNR value for the
output signal with no noise at 10 K is then postulated to be zero because of the
accidental and slight spike signals based on the transient response.