Citation: Li, M.; Kuang, Y.; Fan, Z.;
Qin, X.; Hu, S.; Liang, Z.; Liu, Q.;
Zhang, W.; Wang, B.; Su, Z.
Simultaneous Electrochemical
Sensing of Indole-3-Acetic Acid and
Salicylic Acid on Poly(L-Proline)
Nanoparticles–Carbon Dots–
Multiwalled Carbon Nanotubes
Composite-Modified Electrode.
Sensors 2022, 22, 2222. https://
doi.org/10.3390/s22062222
Academic Editors: Ki-Hyun Kim and
Deepak Kukkar
Received: 14 February 2022
Accepted: 10 March 2022
Published: 13 March 2022
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Article
Simultaneous Electrochemical Sensing of Indole-3-Acetic Acid
and Salicylic Acid on Poly(L-Proline) Nanoparticles–Carbon
Dots–Multiwalled Carbon Nanotubes
Composite-Modified Electrode
Mengxue Li
1
, Yiwen Kuang
2
, Ziyan Fan
1
, Xiaoli Qin
1
, Shiyu Hu
1
, Zhanning Liang
1
, Qilin Liu
1
,
Weizhong Zhang
1
, Birui Wang
1
and Zhaohong Su
1,
*
1
College of Chemistry and Materials Science, Hunan Agricultural University, Changsha 410128, China;
limengxue@stu.hunau.edu.cn (M.L.); ziyanfan@stu.hunau.edu.cn (Z.F.); qinxl@hunau.edu.cn (X.Q.);
hushiyu@stu.hunau.edu.cn (S.H.); liangzhanning@stu.hunau.edu.cn (Z.L.);
1525518484@stu.hunau.edu.cn (Q.L.); zhangweizhong@stu.hunau.edu.cn (W.Z.);
biruiwang@stu.hunau.edu.cn (B.W.)
2
College of Bioscience and Biotechnology, Hunan Agricultural University, Changsha 410128, China;
kuangyiwen@stu.hunau.edu.cn
* Correspondence: zhaohongsu@hunau.edu.cn
Abstract:
Sensitive simultaneous electrochemical sensing of phytohormones indole-3-acetic acid
and salicylic acid based on a novel poly(L-Proline) nanoparticles–carbon dots composite consisting
of multiwalled carbon nanotubes was reported in this study. The poly(L-Proline) nanoparticles–
carbon dots composite was facilely prepared by the hydrothermal method, and L-Proline was
used as a monomer and carbon source for the preparation of poly(L-Proline) nanoparticles and
carbon dots, respectively. Then, the poly(L-Proline) nanoparticles–carbon dots–multiwalled carbon
nanotubes composite was prepared by ultrasonic mixing of poly(L-Proline) nanoparticles–carbon dots
composite dispersion and multiwalled carbon nanotubes. Scanning electron microscope, transmission
electron microscope, Fourier transform infrared spectroscopy, ultraviolet visible spectroscopy, energy
dispersive spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and linear
sweep voltammetry were used to characterize the properties of the composite. poly(L-Proline)
nanoparticles were found to significantly enhance the conductivity and sensing performance of the
composite. Under optimal conditions, the composite-modified electrode exhibited a wide linear
range from 0.05 to 25
µ
M for indole-3-acetic acid and from 0.2 to 60
µ
M for salicylic acid with
detection limits of 0.007
µ
M and 0.1
µ
M (S/N = 3), respectively. In addition, the proposed sensor was
also applied to simultaneously test indole-3-acetic acid and salicylic acid in real leaf samples with
satisfactory recovery.
Keywords:
poly(L-Proline) nanoparticles; carbon dots; multiwalled carbon nanotubes; indole-3-acetic
acid; salicylic acid; simultaneous detection
1. Introduction
A phytohormone is a kind of small signal molecule produced in plants, which has
a great impact on the growth and development of the plant [
1
]. Plant physiological pro-
cesses are usually the result of network regulation of a variety of phytohormones [
2
]; thus,
it is necessary to develop an effective method for simultaneous detection of phytohor-
mones. At present, the established methods for phytohormone detection mainly include
chromatography [
3
,
4
], liquid chromatography tandem mass spectrometry [
5
], capillary elec-
trophoresis [
6
], fluorescence method [
7
], and electrochemical method [
8
–
10
], among others.
Among the many detection methods, the electrochemical method is favored because of its
convenient operation, inexpensive equipment, rapid response, and high sensitivity [11].
Sensors 2022, 22, 2222. https://doi.org/10.3390/s22062222 https://www.mdpi.com/journal/sensors
