Design and synthesis of a task-specific ionic liquid as a transducer in potentiometric sensors

Article abstract of DOI:10.1039/c3ra23367g

A task-specific ionic liquid called N,N,N-trimethylaniline hexafluorophosphate was synthesized and further employed as an ion-to-electron transducer in potentiometric sensors with excellent performance. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra23367g

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Design and synthesis of a task-specific ionic liquid as
a transducer in potentiometric sensors†
Cite this: RSC Adv., 2013, 3, 19782
*
Jianfeng Ping, Yixian Wang, Yibin Ying and Jian Wu
Received 17th December 2012
Accepted 14th August 2013
DOI: 10.1039/c3ra23367g
www.rsc.org/advances
A
task-specific ionic liquid called N,N,N-trimethylaniline hexa- selective membrane.6 However, the redox side reaction with
fluorophosphate was synthesized and further employed as an ion- dissolved oxygen and the unavoidable water layer at the inter-
to-electron transducer in potentiometric sensors with excellent face between ion-selective membrane and conducting polymers
performance.
has limited practical applications of these SC-ISEs.7 Recently,
carbon nanomaterials like fullerene, carbon nanotubes, and
graphene, are employed as the solid-contact layer to develop
high-performance SC-ISEs.8 These carbon nanomaterials
possess excellent conductivity and large double-layer capaci-
tance that are able to convert effectively the ionic signal through
the ion-selective membrane into an electronic signal between
the metal contact and the sensing member.9 However, due
to the insolubility of these nanomaterials in common solvents,
the fabrication of unique nanomaterials-based lm is hard and
time-consuming. Therefore, new functional materials are still
highly desirable for the fabrication of robust SC-ISEs.
In this communication, we rstly synthesized a novel type of
aniline-functionalized TSIL called N,N,N-trimethylaniline hex-
auorophosphate. This novel TSIL possesses redox, ion
conductive and high hydrophobic properties due to the aniline-
functionalized cation and hexauorophosphate anion. On the
basis of these excellent properties, this TSIL was employed as a
solid-contact layer to construct a novel solid-contact potassium
ion-selective electrode (K-ISE) with excellent performance.
Synthesis of TSIL is illustrated in Scheme 1. Firstly, the N,N-
dimethylaniline was dissolved in ethyl acetate (EtOAc). Then,
Ionic liquids have attracted extensive attention in the eld of
chemistry due to their outstanding properties, such as negli-
gible vapor pressure, good thermal stability, wide electro-
chemical window, excellent designability and intrinsic
conductivity.1 Especially, the designability in which the struc-
tures and properties of ionic liquids can be easily tuned by
selecting the appropriate combination of organic cations and
anions, provides a facile and promising way to prepare multi-
functional materials called task-specic ionic liquids (TSILs).2
During the past decade, various types of TSILs have been
designed and synthesized for specic purposes, such as catal-
ysis, synthesis, gas adsorption, analysis, and construction of
nanomaterials.3
In the eld of chemical sensors, potentiometric sensors or
ion-selective electrodes (ISEs) are the most frequently used ones
due to their intrinsic advantages such as portability, low-energy
consumption, and low cost.4 The development of solid-contact
ion-selective electrodes (SC-ISEs) is regarded as the important
improvement of ISEs, since these could be truly miniaturized.5
However, the initial form of SC-ISEs, i.e. coated wire electrode
(CWE), suffers from the high charge transfer resistance at the
interface induced the low potential stability. The introduction
of conducting polymers as internal ion-to-electron transducers
has signicantly improved the potential stability of SC-ISEs in
view of they could couple the electronic conductivity of the
electrode substrate and the ionic conductivity of the ion-
College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou
310058, PR China. E-mail: wujian69@zju.edu.cn; Fax: +86-571-88982180; Tel: +86-
571-88982180
† Electronic supplementary information (ESI) available: Details of synthetic
procedure; fabrication of K-ISM; results of water layer test and interference test;
selectivity data. See DOI: 10.1039/c3ra23367g
Scheme 1 Synthesis of TSIL. 1 represents N,N-dimethylaniline; 2 represents
N,N,N-trimethylaniline
hexafluorophosphate.
iodide;
3
represents
N,N,N-trimethylaniline
19782 | RSC Adv., 2013, 3, 19782–19784
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methyl iodide (CH3I) was added into the solution. Aer keeping (GCE/TSIL/K-ISM, the fabrication process can be seen in ESI†).
the solution at 65 ꢀC for 12 h, a viscous yellow liquid was It can be seen that the resistance, which is equal to the bulk
obtained. The resulted liquid was dried in a vacuum oven at membrane resistance coupled with the contact resistance
60 ꢀC for 24 h to obtain the N,N,N-trimethylaniline iodide. between the electrode substrate and the polymeric membrane,
Secondly, the N,N,N-trimethylaniline iodide was added into the decreases from 13.12 MU (CWE) to 7.86 MU (SC-ISE), indicating
solution containing potassium hexauorophosphate (KPF6), the presence of TSIL lm between the GCE and polymeric
and the solution was stirred for 2 h at 25 ꢀC. Aer that, the membrane makes the charge transport process much easier. In
mixed solution was divided into two parts. The upper part of the the region of low frequencies, a large semicircle is observed in
solution was removed. While the underlying part was collected the impedance plot of GCE/K-ISM, suggesting the large charge-
and washed with deionized water for several times. Finally, the transfer resistance with a small capacitance at the “blocked”
ꢀ
liquid was dried in a vacuum oven at 60 C for 24 h to obtain interface (GCE|K-ISM). The negligible low-frequency semicircle
the aniline-functionalized TSIL, i.e. N,N,N-trimethylaniline in the case of SC-ISE shows that the aniline-functionalized TSIL
hexauorophosphate.
lm signicantly increases low-frequency redox capacitance
The redox properties of the synthesized TSIL were studied and thus effectively facilitates the ion-to-electron transduction
using cyclic voltammetry (Fig. 1A). It is clear that the aniline- process between the electronically conducting electrode
functionalized TSIL shows a complex electrochemistry, which substrate and ionically conducting ion-selective membrane.5,8
contains three oxidative states, that is very similar to the poly-
Reversed chronopotentiometry is used to evaluate the elec-
aniline modied electrode.10 In addition, the background trical capacity of the solid-contact and the potential stability of
current of the bare GCE in 0.1 M KCl solution is quite low, while the developed electrode. The typical chronopotentiograms
the distinct capacitive current is obtained at the TSIL lm recorded for the GCE/K-ISM and GCE/TSIL/K-ISM are illustrated
modied GCE (GCE/TSIL), indicating the large redox capaci- in Fig. S1 (ESI†). The potential jump in the response is used to
tance at the TSIL lm. Since the solid-contact layer with suitable calculate the total resistance of the electrode. The estimated
redox capacitance is one of the conditions required for the total resistance for the TSIL-based SC-ISE is 7.91 MU that is in
potential stability of the solid-contact potentiometric ion good agreement with the resistance of impedance measure-
sensors,5 aniline-functionalized TSIL could be used as an ment. Furthermore, the potential stability can be derived from
effective solid-contact transducer.
the ratio DE/Dt. The calculated potential dri value of the GCE/
To conrm the excellence of TSIL-based transducer, elec- TSIL/K-ISM is about 27.6 mV sꢁ1, which is much lower than the
trochemical impedance spectroscopy, chronopotentiometry, value of the CWE (1593 mV sꢁ1), and comparable with those
and water layer test were studied in details. Fig. 1B compares obtained at carbon nanomaterials or conductive polymers-
the impedance behaviors of the CWE (GCE/K-ISM) and SC-ISE based SC-ISEs.6,8 The low-frequency capacitance (CLF) of the
aniline-functionalized TSIL in the GCE/TSIL/K-ISM was calcu-
lated using the equation DE/Dt ¼ i/CLF. The estimated value is
65.8 mF, which is similar with those of conductive polymers
based solid-contact transducers.6
In the long-term measurements, the formation of aqueous
layer inside the electrodes has been proved to be the main
reason of instability because its composition changes due to
permeability of the membrane to ions and CO2, leading to
changes in pH. For this purpose, a water layer test was con-
ducted at the GCE/K-ISM and GCE/TSIL/K-ISM (Fig. S2, ESI†). As
expected, the response of TSIL-based SC-ISE shows no potential
dri, while a substantial potential dri was found at the
response of the CWE. This demonstrates that the high hydro-
phobicity of TSIL lm resulted from the hexauorophosphate
anion could signicantly eliminate the accumulative water layer
inside the electrodes. In addition, the intermediate term
stability is calculated with the signal recorded in the third step
of the test. The obtained potential dri is 34.6 mV hꢁ1 in the 20 h
continuous monitoring, revealing that the TSIL-based SC-ISE
can be reliably applied in the long term online analysis.
The last outstanding property of the TSIL-based SC-ISE is its
insensitivity towards environmental factors, such as light and
gas. As illustrated in Fig. S3 (ESI†), no signicant potential dri
is found when the measurement cell was exposed to different
Fig. 1 (A) Cyclic voltammograms of GCE (a) and GCE/TSIL (b) in 0.1 M KCl
lights, suggesting the aniline-functionalized TSIL transducer
has no light sensitivity. The result of gas sensitivity test is shown
in Fig. S4 (ESI†). It can be seen that the TSIL-based SC-ISE is
solution. Scan rate: 100 mV sꢁ1; (B) impedance plots of GCE/K-ISM (a) and GCE/
TSIL/K-ISM (b) in 0.1 M KCl solution. Frequency range, 100 kHz to 0.3 Hz; Edc
0.2 V; DEac, 10 mV.
,
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The authors acknowledge the nancial support from the
National Natural Science Foundation of China (no. 31271617)
and China Postdoctoral Science Foundation (no. 2012M521177).
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hydrophobic character of TSIL materials, no water lm was
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19784 | RSC Adv., 2013, 3, 19782–19784
This journal is ª The Royal Society of Chemistry 2013