III-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

The electron transfer channel in the sugar recognition systemassembled on gold nano particles
T. Goto,∗1 T. Hashimoto,∗1 D. P. Sari,∗2 and I. Watanabe∗3

∗1
∗2
∗3

Faculty of Technology and Science, Sophia University
SIT Research Laboratories, Shibaura Institute of Technology
RIKEN Nishina Center

0.060

GNP-B12 (d =12nm)

0.055

λLF (µsec−1)

The recently reported electrochemical sugar recognition system consisting of a gold nano particle (GNP)
with a diameter of 12 nm, a ruthenium complex and aphenylboronic acids, attracts much interest because of
its high sensitivity for various sugars such as D-glucose
or D-fructose. When sugar molecules are “recognized” by
the phenylboronic site, the response of electrochemical
voltammetry of the ruthenium complex site drasticallychanges, enabling the system to work as a highly sensitive sugar-sensor.1,2) In this recognition process, thechange in the electronic state at the phenylboronic acidssite caused by sugar must be transferred to the ruthenium complex site. The purpose of this study is to ﬁnd
out by LF-µSR technique a channel of the electron transfer from the phenylboronic acid site to the ruthenium
complex via gold nano particle. By its ﬁnding, we will
be given a better understanding of the sugar-recognition
mechanism, and also, a possibility to develop a sensor
with still higher sensitivity and more functions.
As the present system includes alkyl-chains in theruthenium complex and the phenylboronic acid site, theinjected muons pick up one electron to form a neutral
atomic state, which is soon thermalized and bonded to a
relatively-reactive site on the chain. The brought-in electrons may show one-dimensional motion along the chain,
if the electron transfer channel between the ruthenium
complex and the phenylboronic site exists.
This behavior can be detected sensitively by measuring the muon-spin relaxation process, which is causedby the magnetic interaction between the muon spin andthe moving electron produced by muon itself. In otherwords, in place of the attachment of sugar molecules tophenylboronic acid site, one utilizes muon as a trigger
to induce a moving electron and also as an observer ofit. This method is known as the muon-labelling measurement.
The characteristic dimensionality of the electron motion can be readily studied by measuring LF dependenceof µSR relaxation rate λ;3) for one-dimensional motion,
λ is expected to be proportional to (HLF )−1/2 , whereHLF is the externally applied ﬁeld along the muon spinpolarization. This method was ﬁrst applied to the polaron motion in oganic polymer chains,4) and also successfully to the typical one dimensional system, DNA.5)
Zero (ZF) and longitudinal (LF) ﬁeld-µSR measurements at room temperature were performed on a powder sample at RIKEN-RAL Muon Facility using a spinpolarized pulsed surface-muon (µ+ ) beam with a momentum of 27 MeV/c. The muon spin depolarization

0.38T

190.050
180.20T

300K
170.045024
6Time ( µsec)0.2

H −0.580.3

0.4

Magnetic Field HLF (T)

0.5

Fig. 1. Temperature dependence of dynamical component ofrelaxation rate λ under various longitudinal fields HLF .
The inset shows the typical depolarization curves of theasymmetry (%) with fitted functions. Each curve is vertically shifted for clarity.

data above 0.1 T were analyzed with the function eλτ ,where λ is the depolarization rate due to the dynamicalspin ﬂuctuation. Note that in this ﬁeld region, the effect of quasi static nuclear spins or GKT , Kubo-Toyabe
function is neglected.
The ﬁeld dependence of λ is shown in Fig. 1. Withdecreasing HLF from the highest ﬁeld 0.38 T, λ showeda slight but ﬁnite increase until HLF ∼= 0.2 T, below
which λ saturates. An appreciably high depolarization in the totally non-magnetic compound indicatesthat some muon-triggered moving electron process exists. The observed increase is weaker than expected forthe one dimensional diﬀusion case. This indicates thatthe contribution from the diﬀusion process has the lower
boundary in the ﬂuctuation spectrum, as is evident fromthe functional form of λHLF dependence, and in fact isreported for previous report.4)
In order to conﬁrm that the observed behavior of λcomes from the one-dimensional movement along alkylchain, investigation on wider ﬁeld region or equivalently
on wider ﬂuctuation spectrum, including NMR is inevitable, and is now on the progress.
References

1) A. Endo et al., Anal. Methods 6, 8874 (2014).
2) T. Goto et al., IEEE Trans. Mag. 55, 2300404 (2019).
3) Risch and Kehr, Phys. Rev. B 46, 5246 (1992).
4) F. Pratt et al., Phys. Rev. Lett. 79, 2855 (1997).
5) E. Torikai et al., Hyperfine. Int. 138, 509 (2001).

- 135 -1