Abstract
An all-integrated on-chip electrochemical microcell (10 × 11 mm2) is developed using silicon technology. The potentiometric nitrate ion detection is based on the functionalization of the working microelectrode array with a polymer membrane in fluoropolysiloxane (FPSX) containing ionophore tetradodecylammoniumnitrate (TDDAN) and ionic additive potassium tetrakis[3,5-bis(trifuoromethyl)phenyl]borate (KTFPB) to form an all-solid-state ion selective electrode (ISE). The addition of an ion-to-electron transducing layer between the platinum working electrode and the polymer membrane helped to improve the sensor performances, especially the response time, the sensitivity, and the stability. Composites formed with two conductive polymers were compared: Polyethylenedioxythiophène (PEDOT) and Polypyrrole (PPy), doped with Poly(styrene sulfonate) or double-walled carbon nanotubes (DWCNTs).
1. Introduction
With the development of industries and intensive agriculture, the use of fertilizers is responsible for the rejection of increasingly large amounts of nitrates in surface water. Although nitrates have an essential role in the nitrogen cycle, in too large of an amount nitrate can be a strong contaminant as it can disturb ecosystems and be harmful to human health. This work aims to develop a low-cost, easy-to-produce, and miniaturized sensor to directly detect in situ nitrate concentrations.
2. Materials and Methods
2.1. Microsensor Geometry
The microsensor was fabricated using a photolithography process. It is composed of a platinum working microelectrode array (5 × 5 microelectrodes of 10 um2 diameter that are each interconnected), a silver/silver chloride quasi-reference electrode, and a platinum counter electrode (Figure 1) [1].
Figure 1.
Ultra-microelectrode array integrated on silicon wafer.
2.2. Working Microelectrode Array Functionalization
Transducing layer (Figure 2): The platinum microelectrode array was functionalized by electro-polymerization of an aqueous solution containing the conductive monomer (EDOT or Py) and the dopant (NaPSS or oxidized DWCNTs) [2]. The compositions are given in Table 1. The electro-polymerizations were all carried out through chronopotentiometry at a current density of 1 mA/cm2 across 360 s (PEDOT:PSS), 180 s (PEDOT:DWCNTs), and 75 s (PPy:DWCNTs).
Figure 2.
SEM images respectively of PEDOT:PSS, PEDOT:DWCNT, and PPy:CNT deposits on an ultramicroelectrode of the array.
Table 1.
Electropolymerization solution compositions.
Ion-sensitive membrane. The fabrication process of the nitrate-ion-sensitive membrane involved 134 mg FPSX, 6.00 mg TDDAN, and 4.16 mg KFTPB dissolved in 1 mL tetrahydrofuran (THF). The solution was drop-casted on the microelectrode array (2 µL). The deposit was left to dry for 72 h before use.
3. Discussion
The transducing layer improved the response time and the stability of the sensor because it helped to remove the water layer between the electrode and the membrane [3]. The DWCNTs greatly increased the sensitivity, similarly to a Nernstian response (Figure 3, Table 2). The sensors with a conductive polymer doped with carbon nanotubes showed good selectivity performances against principal surface water interfering ions (Table 3) and largely acceptable stability for rapid in situ measurement. Better performances were obtained with a PEDOT:DWCNT deposit as the transducing layer.
Figure 3.
Open circuit potential value of the different nitrate ion sensors with pNO3.
Table 2.
Sensitivity, limit of detection, and stability of the sensors.
Table 3.
Selectivity coefficient obtained by FIM method with 1.10−2 M interfering ion concentration.
Author Contributions
C.B.: conceptualization, methodology, validation, investigation, formal analysis, writing—original draft preparation; M.L. and E.F.: resources, writing—review and editing; J.L.: conceptualization, supervision, writing—review and editing, project administration, funding acquisition; P.T.-B.: conceptualization, supervision, writing—review and editing. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by ANR, grant number ANR-18-CE04-0007.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The data presented in this study are available at https://sites.laas.fr/projects/BELUGA/ accessed on 6 March 2024.
Acknowledgments
The technological realizations and associated research works were partly supported by the French RENATECH network.
Conflicts of Interest
The authors declare no conflicts of interest.
References
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