B American Society for Mass Spectrometry, 2018 J. Am. Soc. Mass Spectrom. (2018)
DOI: 10.1007/s13361-018-1983-1
SHORT COMMUNICATION
Investigation of the Ionization Mechanism
of NAD
+
/NADH-Modified Gold Electrodes in ToF-SIMS
Analysis
Xin Hua, Li-Jun Zhao, Yi-Tao Long
Key Laboratory for Advanced Materials and Department of Chemistry, East China University of Science and Technology, 130
Meilong Road, Shanghai, 200237, People’sRepublicofChina
Abstract. Analysis of nicotinamide adenine dinu-
cleotide (NAD
+
/NADH)-modified electrodes is im-
portant for in vitro monitoring of key biological
processes. In this work, time-of-flight secondary
ion mass spectrometry (ToF-SIMS) was used to
analyze NAD
+
/NADH-modified gold electrodes.
Interestingly, no obvious characteristic peaks of
nicotinamide fragment could be observed in the
mass spectra of NAD
+
/NADH in their neutral so-
dium pyrophosphate form. However, after acidifi-
cation, the characteristic peaks for both NAD
+
and NADH were detected. This was due to the suppression effect
of inner pyrophosphoric salts in both neutral molecules. Besides, it was proved that the suppression by inner salt
was intramolecular. No obvious suppression was found between neighboring molecules. These results demon-
strated the suppression effect of inner salts in ToF-SIMS analysis, providing useful evidence for the study of ToF-
SIMS ionization mechanism of organic molecule-modified electrodes.
Keywords: Secondary ion mass spectrometry, Ionization mechanism, Nicotinamide adenine dinucleotide,
Modified electrode, Inner salt, Suppression effect
Received: 27 November 2017/Revised: 27 April 2018/Accepted: 27 April 2018
Introduction
N
icotinamide adenine dinucleotide (NAD
+
)anditsreduced
form (NADH) are important coenzymes that are found in
all living cells [1]. NAD
+
/NADH are involved in most redox-
related cellular processes, such as glycolysis, tricarboxylic acid
cycle and cell respiration [2]. NAD
+
is responsible for the oxida-
tion of cellular enzymes, while NADH plays an important role in
the reduction of cellular enzymes. Recently, lots of biomimetic
model systems have been developed to monitor the electron
transfer processes of NAD
+
/NADH-related biological reactions
[3–5]. Real-time measurement of current changes was realized by
modification of electrodes with biomolecules, such as NAD
+
/
NADH and enzymes. Comparing with in vivo studies, these
biomimetic systems show advantages of low cost, easy fabrication
and the capability of real-time measurement of electron transfer
processes. By modifying NAD
+
/NADH on gold electrode, the
pathways of various biological redox processes were revealed. For
example, electron transfer between NAD
+
/NADH and ubiqui-
none in the initial stages of respiration was studied by constructing
a biomimetic membrane containing ubiquinone and
NADH/NAD
+
redox couple [6]. Besides, it was shown that
NAD
+
/NADH-modified electrodes are useful for the development
of various high-performance biosensors or fuel cells [7, 8]. Real-
time recording of electrochemical parameters (i.e., current, poten-
tial, resistance, etc.) is the most direct way for the study of electron
transfer processes. However, chemical information is limited in
these parameters, especially for complex electrochemical reac-
tions. It is of significant importance to perform surface analysis
on modified electrodes for a better understanding of the chemical
changes as well as electron transfer processes.
Several surface analysis techniques have been used for the
analysis of electrode surfaces, such as electron microscopy (EM)
[9, 10], atomic force microscopy (AFM) [11, 12], energy-
Xin Hua and Li-Jun Zhao contributed equally to this work.
Correspondence to: Yi–Tao Long; e-mail: ytlong@ecust.edu.cn
@Phil_Robichaud
@deepthiw
@JoseServera