Improvement of amperometric transducer selectivity using nanosized phenylenediamine films
© The Author(s). 2017
Received: 29 December 2016
Accepted: 25 October 2017
Published: 14 November 2017
In this work, we studied the conditions of deposition of a semipermeable polyphenylenediamine (PPD)-based membrane on amperometric disk platinum electrodes. Restricting an access of interfering substances to the electrode surface, the membrane prevents their impact on the sensor operation. Two methods of membrane deposition by electropolymerization were compared—at varying potential (cyclic voltammetry) and at constant potential. The cyclic voltammetry was shown to be easier in performing and providing better properties of the membrane. The dependence of PPD membrane effectiveness on the number of cyclic voltammograms and phenylenediamine concentration was analyzed. It was shown that the impact of interfering substances (ascorbic acid, dopamine, cysteine, uric acid) on sensor operation could be completely avoided using three cyclic voltammograms in 30 mM phenylenediamine. On the other hand, when working with diluted samples, i.e., at lower concentrations of electroactive substances, it is reasonable to decrease the phenylenediamine concentration to 5 mM, which would result in a higher sensitivity of transducers to hydrogen peroxide due to a thinner PPD layer. The PPD membrane was tested during continuous operation and at 8-day storage and turned out to be efficient in sensor and biosensors.
Biosensors are novel analytical devices; the usage of which is an alternative to chromatography, spectroscopy, and colorimetry. Biosensors are much cheaper and easier to use than these traditional methods, however, often inferior to them by the analytical characteristics. At present, the research in the field of biosensorics is in active progress .
According to the classical definition of the International association of researchers in fundamental and applied chemistry, a biosensor is an integrated device based on receptor and transducer, which is able to provide quantitative or semi-quantitative analysis using biological recognition element . By the type of transducer, biosensors are classified into several groups (electrochemical, optical, piezoelectric, etc.), among which electrochemical biosensors are one of the largest groups and in turn are divided into amperometric, potentiometric, conductometric, and impedimetric .
One of important analytical characteristics of biosensors is their selectivity, i.e., an ability to identify the target compound only. The biosensor selectivity is determined by selectivity of biological material and selectivity of transducer. Basically, enzymes and antibodies used in electrochemical biosensors as biomaterial are very selective, whereas electrodes, which serve as transducers, are rather nonselective. The biosensor selectivity is of particular importance when working with real biological fluids or other complex samples; therefore, its investigation is a necessary stage in the development of biosensors.
Concentration of electroactive substances in human blood serum
Concentration in serum
0.4–2.0 mg/dL = 23–114 μM
1.8–17.9 mg/L = 10.2–101 μM
208–428 μM (males)
155–357 μM (females)
1 nM (healthy people)
0.9–6 μM (patients with Parkinson’s disease treated with l-dopa)
5.8 ± 0.9 μM
1.6 ± 0.3 μM
There are two main approaches to prevent oxidation of interfering substances on the electrode surface—a decrease of working potential by introduction of additional substances into the bioselective membrane or deposition of additional semipermeable membranes, which allows selective access of the target substance to the electrode surface . The deposition of semipermeable membranes is methodologically simpler and slightly affects the biosensor functioning.
In biosensors, hydrogen peroxide oxidizes or reduces on the electrode and thus the biosensor signal is generated. Therefore, an actual issue is the development of nanoporous films, which are permeable for hydrogen peroxide and prevent the penetration of other substances. Among these membranes, the polymer films based on phenylenediamine (PD) attract considerable attention . Polyphenylenediamine (PPD)-based membrane has nanopores; their size is sufficient for penetrating low molecular weight compounds, including hydrogen peroxide, through membrane to the electrode surface. On the other hand, the membrane does not allow passing through or oxidation of larger substances like ascorbic acid or dopamine. Thus, the membrane improves selectivity to hydrogen peroxide, which, in turn, increases the biosensor accuracy. In several works, different PD isomers and methods for PD polymerization were studied. Particularly, PPD membranes were formed on ruthenium-coated carbon fiber microelectrodes by electrodeposition at a constant potential (+ 0.7 V) when creating biosensors based on glucose oxidase, lactate oxidase, and glutamate oxidase . Three PD isomers were tested; the results with meta-isomer were the best. Because some sensitivity to ascorbic acid still remained, ascorbate oxidase was added to eliminate it completely. In , the authors studied PPD membranes deposited onto Pt–Ir cylinders by CV or constant potential amperometry. Sensitivity to ascorbic acid notably decreased with the membranes based on meta- and ortho-isomers oxidized at a constant potential whereas sensitivity to hydrogen peroxide decreased by 10% only. The results obtained with PPD membranes deposited onto palladium disk electrodes were quite different . Electrodeposition of m-PD by CV caused the formation of films with threefold higher hydrogen peroxide permeability compared with m-PD oxidation at a constant potential. Thus, m-PD was shown to be preferable among all isomers. The recently reported hydrogen peroxide sensor using CV-deposited o-PD film with Au nanoparticles  demonstrated good avoidance of interfering effects. Generally, it can be concluded that m-PD is superior to others for all electrodes, whereas the procedure of PD polymerization should be optimized in each particular case. Additionally, PD-based membranes can be also used in sensors without the biological element. As was recently shown, bovine serum albumin could be detected by a sensor based on conjugated copolymers of PD and other aromatic compounds (quenching of the protein fluorescence after binding with the copolymer was observed) .
Thus, the aim of the present work was to compare different methods of m-phenylenediamine deposition and select an optimal procedure of PPD formation on the platinum disk electrodes.
Ascorbic acid, cysteine, uric acid, dopamine, hydrogen peroxide, m-phenylenediamine, and HEPES were purchased from Sigma-Aldrich Chemie (USA). All other chemicals were of p.a. grade.
The samples of human blood serum were obtained from Kyiv Municipal Scientific and Practical Center of Nephrology and Hemodialysis (Ukraine).
Design of Amperometric Transducers
In this work, self-made platinum disk electrodes served as amperometric transducers. Platinum wire 0.4 mm in diameter and 3 mm long was sealed at one end of a glass capillary with an outer diameter of 3.5 mm. An open end of the wire was the working surface of transducer. An inner end of the platinum wire was soldered by Wood’s alloy to one end of a silver wire inside the capillary; its another end was connected to a potentiostat. The electrodes were used repeatedly; before usage, their working surface was treated with HCl for 30 s, washed with ethanol, and grinded by abrasive paper P1500 PS 8A.
Methods of Measurement
The UV-vis absorption spectra of the samples were measured on Thermo Evolution 600 spectrometer in the 200–900-nm wavelengths range in diffuse-reflectance mode using an integration sphere. The Spectralon diffuse reflectance standard and platinum disk were used as blank samples for m-phenylenediamine powder and PPD layer on the surface of Pt electrode respectively.
For electrochemical measurements, working electrodes were placed in a classical electrochemical cell with an auxiliary (platinum wire) and a reference (Ag/AgCl in saturated KCl) electrodes connected to the PalmSens potentiostat (Palm Instruments BV, Netherlands). Usage of the eight-channel multiplexer (from the same producer), connected to the potentiostat, allowed simultaneous monitoring of signals from eight electrodes; however, in our work, we usually used three electrodes because of small size of the working cell.
The chronoamperometric measurements (“amperometric detection” technique) were carried out at room temperature in an open 3-mL glass cell with permanent stirring by a magnetic stirrer and at constant potential of 0.6 V versus Ag/AgCl reference electrode. Ten millimolars of HEPES, pH 7.4, was used as a working buffer in all experiments. The substrate concentrations in the working cell were obtained by addition of aliquots of stock solutions (50 mM hydrogen peroxide, 20 mM ascorbic acid, 3 mM cysteine, 4.5 mM uric acid, 2.1 mM dopamine). New solutions were prepared just before the experiment. All electroactive substances except uric acid were dissolved in the working buffer; uric acid due to its small solubility was dissolved in distilled water with 5 mM NaBrO3. Phenylenediamine was dissolved in 40 mM phosphate buffer, pH 7.4.
Cyclic voltammetry was carried out in the same measuring cell without stirring. Start potential was 0 V, final potential +0.9 V, scan rate (rate of potential change) 20 mV/s, and step of potential change 5 mV.
All experiments were carried out in three repetitions. Data in the tables and figures represent a mean value of experiments ± standard deviation, computated by OriginLab OriginPro 8.5 program.
Results and Discussion
To confirm the reasons of deposition of additional membrane on amperometric transducer for improvement of its selectivity to hydrogen peroxide, it was necessary to verify sensitivity and selectivity of this transducer regarding possible interferents.
Biosensors can be employed for measurements in both undiluted and diluted samples. The option of dilution depends on the concentration of substance to be analyzed and the biosensor sensitivity: if the biosensor can identify the target substance in concentrations, which are dozens of times lower than those actual in the samples, the latter should be diluted to decrease the content of interferents and thus improve the array accuracy. Additionally, it allows dozens-fold decrease of the substrate volume required for measurements.
Responses and sensitivity of platinum disk transducer to electroactive materials. Responses are received in 10 mM HEPES buffer, pH 7.4
20-fold dilution of the sample
100-fold dilution of the sample
Mean sensitivity of transducer, nA/mM
Transducer response, nA
Substance concentration in working cell, μM
Transducer response, nA
Substance concentration in working cell, μM
Transducer response, nA
4.5 ± 1.5
4.6 ± 1.4
4.2 ± 1.6
90 ± 30
52.5 ± 25.1
2.7 ± 1.1
0.4 ± 0.2
438 ± 209
9.0 ± 5.7
0.5 ± 0.3
0.1 ± 0.1
30 ± 19
62.4 ± 31.2
3.0 ± 1.4
0.6 ± 0.2
139 ± 70
2.7 ± 0.5
0.1 ± 0.1
450 ± 83
Currently, there is only fragmented information regarding the methods of PPD membrane deposition on transducers. Therefore, at the next stage of work, it was evaluated which method of two most common and promising is more feasible.
Comparison of standard methods of deposition of PPD membrane
Analyzed substance and its concentration
10 CVAs in 5 mM phenylenediamine
40 min at a constant potential of 0.7 V in 100 mM phenylenediamine
% of response without membrane
% of response without membrane
H2O2, 50 μM
12.0 ± 1.4
267.3 ± 26.0
4.5 ± 1.3
99.3 ± 29.7
Ascorbic acid, 120 μM
Cysteine, 300 μM
0.2 ± 0.2
1.9 ± 1.9
0.6 ± 0.4
6.3 ± 3.8
Uric acid, 450 μM
Dopamine, 6 μM
However, cyclic voltammetry has one disadvantage—voltammograms may be simultaneously obtained on one electrode only (even using multiplexer), whereas membrane deposition at a constant potential allows simultaneous connection of 8–16 working electrodes (depending on the multiplexer type). Therefore, further work should be focused on optimization of the conditions of cyclic voltammograms to decrease the time of transducer pretreatment.
PD electropolymerization by CV and constant potential amperometry is supposed to occur by different pathways through quite complicated mechanism . Thus, CV involves high applied potential, which leads to the generation of less conjugated oligomers of PD. For this reason, it is supposed that at PD polymerization by CV, the pores are larger, and the PPD layer permeability is higher if compared to the polymerization at a constant potential [8, 13]. However, as noted in the “Background” section, different authors came to contradictory conclusions about the preferential method of PD deposition, and in many cases, CV gave good results. In our opinion, both CV and constant potential amperometry can provide the generation of PPD membranes with good permselective properties, and optimization is necessary for each particular case.
For better interpretation of the obtained results, it would be useful to estimate the size of pores in the PPD membranes produced by different methods. However, direct determination of the pore size in PPD membrane is almost impossible, since the membrane consists of multiple layers of PD and pores in bottom layers can have different size. Killoran and O’Neill determined that the thickness of effective membrane from m-PD was 15 nm, and cross-sectional area of one oligomeric polymer strand estimated by del Valle et al. was 1 nm [7, 15]. Thus, the PPD membrane contains roughly 15 layers of polymer. Since the PPD membrane has hydrophobic and isolating properties, the membrane should have perforating nanopores that stretch to the electrode surface and allow by-pass of hydrogen peroxide molecules, otherwise H2O2 cannot be oxidized and generate amperometric signal. The pores are definitely not uniform and the minimal diameter of pores should be less than 1 nm in order to reject electroactive molecules, and thus it is quite difficult to analyze the pores even with electron or atomic force microscopes. For these technical reasons, it is much easier to estimate effectiveness of the PPD membrane by evaluating the membrane permeability for different molecules. Such indirect approach is widespread and allows comparison of practical characteristics of different membranes.
Notably, the use of 5 mM phenylenediamine was sufficient to eliminate the responses to interferents of small concentrations remaining after sample dilution, but deficient to work with undiluted samples. An increase of phenylenediamine concentration to 20 mM and three CVAs turned out to be sufficient for complete elimination of the cysteine impact and a decrease of responses to ascorbic acid to the lowest level (0.1% of the response to ascorbic acid without PPD membrane). The use of higher (up to 100 mM) phenylenediamine concentration resulted in twofold decrease of the transducer sensitivity to hydrogen peroxide, probably, because of too thick PPD layer. Thus, the deposition of PPD membrane using three CVAs in 30 mM phenylenediamine is an optimal procedure. As one voltammogram lasted about 2 min, the membrane deposition on one sensor took 6 min.
Finally, the PPD membrane effectiveness was validated at analysis of real biological samples. The transducers with no membranes demonstrated weak signals after addition of blood serum to the working cell due to the presence of electroactive compounds. However, after deposition of the membrane, no responses were obtained. Similar results were obtained with the lysate of neurons. These experiments demonstrate that the developed method of PPD deposition on platinum disk electrodes is effective and the modified transducers can be used for work with complex biological samples.
Comparison of the developed method of deposition of PPD membrane with previously reported methods
Optimal method of PD polymerization
Type of electrode
Optimal isomer of PD
Duration of the electrode preparation
Effectiveness of blocking properties
CV (from − 0.3 V to +0.9 V vs. Ag/AgCl)
Full blocking of AA, cys, acetaminophen.
Small interference from UA
Constant potential (+0.7 V vs. Ag/AgCl)
Ru-covered carbon fiber ME
Full blocking of UA, cys, acetaminophen, DA.
Small interference from AA—addition of ascorbate oxidase to remove it
> 2 days
Glucose, lactate and glutamate biosensors, in vivo analysis in rat brain
Constant potential (+0.7 V vs. SCE)
Pt–Ir cylinders ME
Full blocking of AA
> 7 days
(from −0.25 V to +0.9 V vs. SCE)
Glassy carbon electrode
Full blocking of AA, UA and DA
> 15 days
N/A, H2O2 detection in human serum samples
Constant potential (+0.7 V vs. Ag/AgCl)
Pt cylinders ME
Full blocking of AA
(from 0 V to +0.9 V vs. Ag/AgCl)
Full blocking of AA, cys, UA, DA
> 8 days
Human serum samples and cell lysate
As seen, the presented method is the quickest one and blocking properties of the obtained membrane are better or at least not worse than those of other PPD membranes.
We investigated the conditions of deposition of a semipermeable polyphenylenediamine-based membrane aimed at decreasing an impact of interfering substances on the biosensor operation. It was shown that phenylenediamine electropolymerization by cyclic voltammetry was easier and provided better properties of the membrane if compared to electropolymerization at constant potential. The dependence of PPD membrane effectiveness on the number of cyclic voltammograms and phenylenediamine concentration was investigated. It was shown that the impact of interfering substances on the sensor operation can be completely eliminated by using three cyclic voltammograms in 30 mM phenylenediamine. On the other hand, when working with diluted samples, i.e., lower concentrations of electroactive substances, it is reasonable to decrease the phenylenediamine concentration to 5 mM, which would result in higher transducer sensitivity to hydrogen peroxide due to a thinner PPD layer. The PPD membrane can be used with no significant loss in its selectivity to hydrogen peroxide during at least 2 h of continuous operation and can be stored at least 8 days. It was shown that the transducer with the PPD membrane is not sensitive to the electroactive substances present in biological samples and can be used for the biosensor creation.
This study was supported by the National Academy of Sciences of Ukraine in the frame of Scientific and Technical Government Program “Sensor systems for medico-ecological and industrial-technological requirement: metrological support and experimental operation”.
OVS performed all experiments. ISK helped OVS with experiments and wrote the manuscript. VMP and OOS planned and supervised the experiments. SAA performed experiments with UV-vis spectroscopy. SVD is the supervisor of the whole work, the results of which are presented in this article. All the authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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