Sensitive detection of 4-nitrophenol based on pyridine diketopyrrolopyrrole-functionalized graphene oxide direct electrochemical sensor

Abstract

For highly sensitive detection of 4-nitrophenol (4-NP) in the environment, a novel pyridine diketopyrrolopyrrole-functionalized graphene oxide (PDPP–GO) composite was constructed for the first time by an improved Hummers' method. Herein, PDPP was completely dissolved in sulfuric acid (6 mol L⁻¹) and reacted with GO, promoting PDPP evenly adhering to the GO surface. Moreover, the specific surface area increased from 15.51 to 22.033 m² g⁻¹. Infrared spectroscopy and X-ray photoelectron spectroscopy simultaneously demonstrated that PDPP was bound to GO by the strong intermolecular hydrogen bonding and π–π stacking conjugation. During the cyclic voltammetry test, the PDPP–GO coated glassy carbon electrode (PDPP–GO/GCE) direct electrochemical sensor gave expression to the best electrocatalytic activity for 4-nitrophenol detection than GO/GCE and bare GCE. Under optimization conditions, the as-prepared PDPP–GO/GCE sensor brought out remarkable sensitivities of 18.54 (0.5–50 μM) and 6.61 μA μM⁻¹ cm⁻² (50–163 μM) in the linear detection of 4-NP. Besides, a low detection limit of 0.10 μM, reliable long-term stability, excellent selectivity, and reproducibility were obtained. In the real sample test, the PDPP–GO/GCE demonstrated sensitive and reliable determination.

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1. Introduction

4-Nitrophenol (4-NP) as an important raw material is widely used to synthesize medicines, pesticides, and dyes.^1,2 However, typically, the toxicity of 4-NP can cause cancer, deformity and is mutation-carcinogenic for humans and animals.^3–5 Therefore, it is very important to monitor 4-NP at low concentrations during environmental analysis. Besides, the Soviet Union stipulates that the initial safe level of surface water is 0.02 mg L⁻¹. To date, many efficient methods have been developed to supervise 4-NP, such as high-performance liquid chromatography,⁶ fluorescence measurement,⁷ gas chromatography-mass spectrometry, gas chromatography,^8,9 capillary electrophoresis, photoelectrochemical,¹⁰ and electrochemical methods.^11,12 Among these methods, the electrochemical methods are often preferred by researchers for merits of sensitivity, selectivity, rapid response time, simple operation, and being environment-friendly.

For the electrochemical method, the comprehensive performance of the electrochemical sensor depends on the surface property and composition of modified materials, and thus endless new materials are being synthesized and used to detect objects.^13,14 Using 4-NP as the target, various composite materials based on metals, metals oxides, carbon nanomaterials, metal–organic frameworks, and organics have been studied with satisfactory results.^15–17 However, the fabrication of a simple, highly sensitive, stable electrochemical sensor is always a great challenge at low cost. Thus far, graphene-based materials with novel physical and chemical properties have become a preferred partner of modified electrodes.¹⁸ Graphene oxide (GO) possesses abundant polar functional groups with hydroxyl, carboxyl, and epoxy bonds, which offer opportunities for modification and hybridization with other materials,¹⁹ but it is well known that the non-conductive GO is against the electrocatalytic reaction.

Pyridine diketopyrrolopyrrole (PDPP) belongs to diketopyrrolopyrrole (DPP) conjugated polymers with central symmetry. As an electron-deficient system, DPP has a conjugated planar backbone, which results in strong π–π stacking interactions. Moreover, DPP conjugated polymers show ultra-high corrosion resistance, good photoelectric properties,²⁰ high-charge carrier mobility,²¹ excellent crystallinity²² and are environment-friendly and exhibit promising potential in organic field-effect transistors and polymer solar cells.^23–25 In organic systems, PDPP and other DPP-conjugated polymers have been extensively investigated.^26–28 However, because of the limitation of non-modification PDPP that can be dissolved in sulfuric acid (C ≥ 6 mol L⁻¹) only, relevant studies in aqueous solution have not been reported. Compared with the DPP ring, the planar conjugated bicyclic structure and secondary amino of PDPP can also prompt the formation of strong π–π interaction, chemical bond, and intermolecular hydrogen bonding with others;²⁹ thus, it is expected to combine with two-dimensional nanomaterials and develop into a new field.

In order to fabricate a simple rapid electrochemical sensor with highly sensitive and long-stable based on GO, the organic semiconductor material PDPP was used to change the electronic structure and surface properties of GO. During the preparation process of GO with the Hummers' method, the whole system involves a concentrated sulfuric acid system, which provides a good solvent condition for PDPP. Therefore, we attempted to synthesize PDPP–GO composite material in the above suitable solvent environment. After a survey on the molecular structure of PDPP and GO, the amine bond of PDPP may react with the hydroxyl and carboxyl bonds of GO by chemical bonding, hydrogen bonding, or intermolecular forces.³⁰ Furthermore, the introduction of PDPP could bring new opportunities for the development of GO or PDPP-based composite materials.

In this work, PDPP–GO composite material was first synthesized in an aqueous solution and PDPP–GO modified glassy carbon electrode (PDPP–GO/GCE) showed excellent electrocatalytic activity for 4-NP detection. The interactions between the PDPP and GO molecules were fully analysed by infrared spectroscopy and X-ray photoelectron spectroscopy, and PDPP tightly combined with the GO by strong hydrogen bonding and π–π stacking conjugation. The comprehensive sensing performances of PDPP–GO/GCE with excellent sensitivity, selectivity, stability, and reproducibility perfectly meet the requirements for modern electrochemical sensors.

2. Materials and methods

2.1 Materials and instrumentation

Graphite powder was purchased from Xiya Chemical Technology (Shandong) Co., Ltd. Sodium nitrate (NaNO₃), potassium permanganate (KMnO₄), dipotassium hydrogen phosphate (K₂HPO₄), and potassium dihydrogen phosphate (KH₂PO₄) were received by Chengdu joint chemical reagent research institute (Sichuan). Concentrated sulfuric acid (H₂SO₄) and hydrogen peroxide (30% H₂O₂) were obtained from Chengdu Kelong Chemical Reagent Factory (Sichuan). Pyridine-diketopyrrolopyrrole (PDPP) was obtained from the laboratory of Chengdu Zhongjin Pharmaceutical Technology Co., Ltd. Other reagents used for the interference determination were all analytically pure. All the aqueous solutions were prepared with deionized water.

The surface morphology was observed using a field-emission scanning electron microscope (FE-SEM) (Carl Zeiss SMT Pte Ltd., Germany). X-ray diffraction (XRD) (D/max-r A type Cu Kα, 3–80°) and X-ray photoelectron spectroscopy (XPS) were used for studying the crystalline structures and chemical environment of elements, respectively. Fourier transformation infrared spectroscopy (FT-IR) data were collected using an FT-IR spectrometer (Spectrum one, America). A specific surface area and pore size analyzer (Autosorb-IQ, N2, Quantachrome Instruments) was used for measuring the special surface area of samples. All electrochemical experiments were carried out on the CHI 630E workstation (CH Instrument, USA) with a three-electrode system, in which PDPP–GO-modified glassy carbon electrode (GCE) acted as the working electrode, standard calomel electrode Hg/HgCl₂ and platinum wire played roles of the reference and the auxiliary electrode, respectively.

2.2 Synthesis of PDPP–GO composite materials

PDPP–GO composite material was synthesized by the improved Hummers' method.³¹ First, 0.0500 g PDPP was dissolved in 30.0 mL concentrated H₂SO₄ in a beaker. Then, 0.5000 g graphite, 0.2500 g NaNO₃, and 1.5000 g KMnO₄ were added to the above solution and stirred for 12 h at room temperature. Thereafter, the solution was diluted with 50.0 mL of distilled water by keeping the temperature below 20 °C, and then 2.5 mL H₂O₂ (30%) and 100.0 mL of distilled water were added to the suspension, constantly stirred and sonicated for 6 h at 60 °C. In order to remove the uncompounded PDPP, the obtained solid was washed with sulfuric acid (6 mol L⁻¹), and then dealt with deionized water until the pH value reached 7. Finally, the obtained PDPP–GO composite was dried at 60 °C for 12 h. The other mass ratios of PDPP–GO (wt%: 15%, 20%, 25%, 30%) and GO were also obtained by the same method. The 25% PDPP–GO sample was denoted as PDPP–GO.

2.3 Preparation of PDPP–GO/GCE

Before use, GCE was polished carefully with 0.3 mm alumina and washed thoroughly with deionized water and ethanol. Then 10.0 μL different ratios of PDPP–GO (1.0 mg mL⁻¹) dispersion in deionized water were dropped on the surface of GCE, and the prepared electrode was labelled as PDPP–GO/GCE. Before the electrochemical test, the electrolytic solution was fed with nitrogen for 10 minutes to remove oxygen. The synthetic processes of PDPP–GO/GCE are shown in Scheme 1.

Scheme 1 The synthetic processes of PDPP–GO/GCE.

The electrode surface area of PDPP–GO/GCE was analyzed by cyclic voltammetric technique with 1.0 mM K₃Fe(CN)₆ as a probe at various scan rates in 0.1 M KCl. For a reversible electrode process (T = 298 K), the Randles–Sevcik equation is as follows:

i_pa = 0.4463(F³/RT)^1/2n^3/2A₀D₀^1/2v^1/2C₀ (1)

where i_pa is the anodic peak current, and the n, A₀, D₀, C₀, and v, respectively, represent the number of electrons transferred, surface area, diffusion coefficient, concentration, and scan rate of K₃Fe(CN)₆. Meanwhile, for 1.0 mM K₃Fe(CN)₆ in 0.1 M KCl electrolyte, F = 96 485 C mol⁻¹, R = 8.314 J K⁻¹ mol⁻¹, T = 298 K, n = 1, and D₀ = 7.6 × 10⁻⁶ cm² s⁻¹. Hence, the A₀ can be obtained from the slope of the plot of i_pa vs. v^1/2.^32,33 At last, the A₀ of GCE and PDPP–GO/GCE were found to be 0.0406 and 0.1749 cm², respectively.

3. Results and discussion

3.1 Characterization of PDPP–GO composite material

FT-IR spectroscopy was chosen to probe the reaction mechanism and structure of PDPP–GO composite material. Fig. 1 shows the FT-IR spectra of PDPP, GO, and PDPP–GO. These peaks appeared at 3130–3430, 2700–3050, 1730, and 1651 cm⁻¹, corresponding to the stretching vibrations of –NH, –CH=CH, C=O, and C=C, respectively.²⁶ In the curve of GO, the stretching vibration peaks of –OH, –CH=CH–, C=O, and C=C are displayed at 3430, 2820–3000, 1729, and 1625 cm⁻¹, respectively.³⁴ By analysing the obtained curves shown in the figure, what is notable is that the –CH=CH at 2830–2930 cm⁻¹ of PDPP–GO has a red-shift and blue-shift compared with GO and PDPP, respectively, and the C=O (1725 cm⁻¹) characteristic peak remains unchanged compared with GO. These results indicate that π–π stacking interactions or hydrogen bond exit in the PDPP–GO, which push the electron cloud of PDPP towards GO.

Fig. 1 FT-IR spectra of PDPP, GO, and PDPP–GO.

Scanning electron microscopy was utilized to reveal the morphologies of the target materials. The rice-like morphology with the length of 300 nm belongs to PDPP (Fig. 2a). Noticeably, the PDPP–GO composite material retains the lamellar structure of GO with the thickness of the clay flake increased (Fig. 2b), which is because PDPP evenly attached to the surface of GO from the enlarged view (Fig. 2c). Besides, it is worth noting that the size of PDPP in the composite has gone down compared with a pure substance. This is caused by the solvent effect of the PDPP crystalline compound. The chemical composition of the PDPP–GO was analysed by an energy dispersive spectrometer (Fig. S1†), and the spectrum clearly indicates the presence of C, N, and O elements. Additionally, the samples were tested using XRD to confirm the crystalline structure (Fig. 2d). For GO, a strong peak at 6.8° is ascribed to the characteristic peak of GO,³⁵ and a very tiny peak at 26° belongs to the characteristic peak of graphite.^36,37 In the curve of the composite material PDPP–GO, a strong and sharp peak can be observed at 10.8° of GO, illustrating the lamellar spacing of GO decreases. Meanwhile, the curve of PDPP–GO shows a new peak at 21.7°, which reveals that a few GO have been reduced to reduced graphene oxide (rGO). Moreover, the peak intensity was enhanced and the peak width became smaller, demonstrating that the PDPP molecules are spread over GO sheets. It also shows that the crystal structure of PDPP has changed and is completely consistent with the SEM. Besides, the surface area of PDPP–GO (22.033 m² g⁻¹, Fig. 2e) is 1.42 times that of GO (15.51 m² g⁻¹, Fig. 2f), which can profitably improve the electrocatalytic property. Raman spectroscopy also provides the structural information of the samples, as shown in Fig. S2,† the results show that the D band (1349 cm⁻¹) and G band (1602 cm⁻¹) of GO have no change in the peak position.

Fig. 2 SEM images of PDPP (a), PDPP–GO (b), and its enlarged view (c). (d) XRD pattern of samples. The special surface area of GO (e) and PDPP–GO (f).

XPS analysis was used to further investigate the PDPP–GO composite material. As shown in Fig. 3a, the C 1s characteristic peaks of GO, respectively, are located at 284.8 eV (C=C/C–C), 286.6 eV (C–O), 287.3 eV (C=O), 288.6 eV (O–C=O),³⁸ and these peaks at 284.2 eV (C=C/C–C), 286.1 eV (C–O), 286.7 eV (C=O), 288.1 eV (O–C=O) belong to PDPP–GO. Remarkably, it must be added that all the binding energies of the PDPP–GO decrease by 0.5–0.6 eV with the binding form of carbon unchanged compared with the pure GO. Fig. 3b shows the high resolution of N 1s. Two obvious peaks at 398.8 and 400.1 eV correspond to C=N and NH of PDPP,^22,26 respectively. When PDPP combines with GO, the binding energies of the nitrogen move to higher energies 399.6 eV (C=N) and 401.3 eV (NH). The above results amply prove that π–π conjugation is generated between PDPP and GO molecules, which facilitated the electron cloud of PDPP transfer towards GO. Furthermore, what is particularly striking is that the binding energy of NH has a larger 1.2 eV difference value, demonstrating that the NH of PDPP and the OH of GO formed a hydrogen bond. These analysis results were further proved by the FT-IR spectra analysis.

Fig. 3 XPS spectra of samples: high resolution of C 1s (a) and N 1s (b).

3.2 Electrochemical behaviour of 4-NP at PDPP–GO/GCE

The direct electrocatalytic ability of the different modified electrodes towards 4-NP was investigated by cyclic voltammetry (CV) at a scan rate of 50 mV s⁻¹. Fig. 4a shows electrocatalytic activities of the obtained three electrodes (bare GCE, GO/GCE, and PDPP–GO/GCE) without 4-NP in 0.2 M PBS (pH = 6.4), and there were no characteristic peaks between 0.25 V and −1.13 V, which confirmed that these sensors are electro-inactive in the potential region and electrochemically stable. After adding 0.1 mM 4-NP (Fig. 4b), two reduction peaks R₁ (−0.87 V), R₂ (−0.02 V), and an oxidation peak O₁ (0.01 V) were observed in the CV curve of GCE, but all the peak current values are small. After GO and PDPP–GO modification of GCE, the reduction peak current (I_pc) of R₁ increases dramatically, and the peak current of R₂ and O₁ peaks remain virtually unchanged, hence the R₁ peak was used for sensitive detection of 4-NP. In the three above-mentioned electrodes, the PDPP–GO/GCE possesses the largest I_pc (−272 μA) at a more positive reduction potential of −0.84 V compared to others. These results demonstrated that PDPP enhanced the catalytic activity of GO to 4-NP, which is because the introduction of PDPP changes the electronic structure and morphology of the GO surface. The reduction peak R₁ at PDPP–GO/GCE revealed a four-electron transfer process to form 4-hydroxyaminophenol,^39,40 as shown in Fig. 5. The redox peaks at R₂ and O₁ correspond to the transformation between 4-hydroxyaminophenol and 4-nitroasophenol by two electrons.⁴¹ Different mass ratios of PDPP–GO composite materials were also investigated by CV, as displayed in Fig. 4c. As the proportion of PDPP evolves from 0% to 30%, the reduction peak potential hardly moved, while 25% PDPP–GO/GCE exhibited the greatest I_pc (Fig. 4d). Hence, 25% PDPP–GO composite was used for subsequent studies.

Fig. 4 CVs of bare GCE, GO/GCE, and PDPP–GO/GCE without (a) and with (b) 0.1 mM 4-NP in 0.2 M PBS (pH = 6.4), (c) CVs with different ratios of PDPP–GO composite material (wt%: 0%, 10%, 15%, 20%, 25%, 30%) with 0.1 mM 4-NP in 0.2 M PBS. (d) The plots of cathodic peak currents and potentials vs. mass ratio obtained from CVs. Scan rate: 50 mV s⁻¹. Potential window: 0.25 to −1.13 V.

Fig. 5 Structure diagram and catalytic mechanism of PDPP–GO.

3.3 Effect of the scan rate and mechanism of the electrode reaction of 4-NP oxidation

The phenolic hydroxyl group of 4-NP is always involved in the electrochemical reaction, so it is quite necessary to investigate the pH value of the electrolyte. Fig. 6a shows the electrochemical redox behaviours of 0.1 mM 4-NP at the PDPP–GO/GCE at various pH (5.8, 6.0, 6.2, 6.4, 6.6, 6.8, 7.0) of 0.2 M PBS. It is evident that the reduction peak potential (E_pc) shifts negatively with pH increasing linearly, indicating that the proton participates in the electrochemical reaction of 4-NP. The linear relationship of E_pc vs. pH is clearly displayed by the linear regression eqn (2):

E_pc (V) = −0.0593pH − 0.490 (R² = 0.9926) (2)

Fig. 6 (a) CVs of 0.1 mM 4-NP at PDPP–GO/GCE at various pH from 5.8 to 7.0, the scan rate is 50 mV s⁻¹; (b) linear relationship of E_pc versus pH value and the plot of I_pc versus pH value. (c) CVs of the PDPP–GO/GCE for 0.1 mM 4-NP in 0.2 M PBS at various scan rates: 10–250 mV s⁻¹, (d) the plots of cathodic peak currents vs. ν^1/2. Potential window: 0.25–1.13 V.

According to the Nernst equation, the obtained slope value (0.0593 V pH⁻¹) is almost equal to the theoretical value (0.0591 V pH⁻¹), demonstrating that an equal number of proton and electron transfer were involved in the electrochemical reduction process of 4-NP. Moreover, the reduction catalytic current increases gradually from pH 5.8 to 6.4 and reach max at pH 6.4. When the pH value exceeded 6.4, the I_pc goes down (Fig. 6b). The results indicate that the optimal pH value for the 4-NP electrochemical reduction at PDPP–GO/GCE is 6.4.

The effect of the scan rate (ν) on the peak current of 4-NP was also studied using CV with the scan rate changing from 10 to 250 mV s⁻¹. Fig. 6c shows the I_pc augments, stage by stage, and the E_pc moves to more negative. Applying the method of mathematical regression analysis of I_pc and scan rate, an excellent linear relationship is given on the basis of I_pc and the square root of the scan rate with regression eqn (3), as displayed in Fig. 6d.

I_pc (μA) = −26.64ν^1/2 (mV s⁻¹)^1/2 + 60.69 (R² = 0.9966) (3)

The obtained results clearly deduce that the electrochemical action of 4-NP at PDPP–GO/GCE belongs to the diffusion-controlled process.

3.4 Determination of 4-NP at PDPP–GO/GCE

In order to evaluate the electrochemical sensing capability of PDPP–GO/GCE, differential pulse voltammetry (DPV) was performed to measure I_pc under optimal conditions with a potential window of −0.6 to −1.05 V. The scan rate is 50 mV s⁻¹. It can be seen that the I_pc gradually becomes bigger with the 4-NP concentration ranging from 0.1 to 180 μM (Fig. 7a), and two linear regression eqn (4) and (5) can be obtained from Fig. 7b.

I_pc = 1.310C_4-NP + 108.5 (C_4-NP: 0.5–50 μM, R² = 0.992) (4)

I_pc = 0.467C_4-NP + 154.1 (C_4-NP: 50–163 μM, R² = 0.993) (5)

Fig. 7 (a) DPV curves of 4-NP with increasing concentration (0.1–180 μM) in 0.2 M PBS (pH = 6.4); (b) calibration plot of 4-NP versus concentration. Potential window: −0.6 to −1.05 V. Scan rate: 50 mV s⁻¹.

The sensitivities of the PDPP–GO/GCE are 18.54 and 6.61 μA μM⁻¹ cm⁻², respectively, by calculation, illustrating that the PDPP–GO/GCE executes more sensitively at low concentrations. When the PDPP–GO/GCE is exposed to a low concentration of 4-NP, the number of active sites is more than those required for 4-NP molecules; thus, 4-NP reacted completely and quickly. However, with the continuous increase of the concentration, the decreasing active sites compared to the total number of 4-NP molecules or absorbing oxidation product on the electrode surface led to the decrease of the second slope. The detection limit of 0.10 μM was calculated using the formula 3S/k, in which k and S are the slope of the curve and the relative standard deviation of the blank solution. The results show that PDPP–GO/GCE could serve as an efficient 4-NP sensor with a wide detection range, low detection limit, and superior sensitivity. Table 1 displays the comparable 4-NP sensor, and the higher sensitivities of 18.54 and 6.61 μA μM⁻¹ cm⁻² emerge as focal points.

Table 1 Comparison of the electrocatalytic performance of different modified electrodes towards 4-nitrophenol detection

Electrode	Analytical method	Linear range (μM)	LOD (μM)	Sensitivity (μA μM^−1 cm^−2)	Ref.
SnO2@ZIF-8/gC3N4	DPV	10–100	0.565	2.63	42
PSF/GCE	DPV	0.1–120	0.01		43
rGO/GCE	DPV	50–800	4.2	—	44
MMIPs/Au-NPs/AuE	DPV	0.1–1400	0.10	—	45
Fcc/hcp-Ni/GCE	CV	20–150	0.66	0.404	46
CTS/NPC/ITO	CV	50–400	27.55	4.85	47
rGO-HNT-AgNP/SPCE	DPV	0.1–364	0.048	35.25	17
PDPP–GO/GCE	DPV	0.5–163	0.10	18.54, 6.61	This work

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3.5 Interference, stability, and reproducibility

Interference, stability, and reproducibility are important static indicators for the electrochemical sensor. The interferences were performed in 0.2 M PBS containing 0.01 mM 4-NP in the presence of 0.25 mM of various possible interfering species, such as o-cresol, p-chlorophenol, 3-aminophenol, hydrogen peroxide, vitamin C, hydroquinone and 1,3-dihydroxybenzene. Fig. 8a shows the relative error of the I_pc after adding interferents and all data are less than ±5.5%, indicating these interferents have no significant effect on 4-NP detection. The PDPP–GO/GCE could be used for highly selective detection of 4-NP in the real sample. The stability of the PDPP–GO/GCE was estimated by intermittently measuring the current response of 0.1 mM 4-NP, and the current response remains 90% of its initial value over a storage period of 16 days (Fig. 8b), demonstrating the long-term stability of PDPP–GO/GCE. As for reproducibility, comparative analysis of the I_pc of five PDPP–GO/GCE, showed an excellent reproducibility with the relative standard deviation (RSD) of 2.52%, revealing the outstanding comprehensive performance of PDPP–GO/GCE to 4-NP.

Fig. 8 (a) Selectivity of PDPP–GO/GCE containing 0.01 mM 4-NP in the presence of possible interfering compounds by DPV. The concentration of interferents was 0.25 mM. (b) Long-term stability of PDPP–GO/GCE toward 0.1 mM 4-NP at time intervals of four days at ambient conditions by DPV.

3.6 Real sample analysis

The application of the PDPP–GO/GCE sensor for the detection of 4-NP was researched in tap water by the standard addition method, as shown in Table 2. Before testing, the tap water must be filtered to remove suspended materials. The test of the real water consisted of 2.00 mL filtered water and 8.00 mL 0.2 M PBS (pH = 6.4), and the DPV test showed that there was no signal for 4-NP. Therefore, 4-NP with different concentrations (5, 20, 50, 110 μM) were further analysed in the above solution. It is worth noting that the recoveries were between 98.8% and 104.5% with RSDs of 0.6–2.2%, exhibiting a sensitive and reliable determination for 4-NP using PDPP–GO/GCE in real samples.

Table 2 Recovery tests of 4-NP in tap water samples at PDPP–GO/GCE

Analyte	Samples	Detected^a (μM)	Added (μM)	Found^a (μM)	Recovery (%)	RSD (%)
a Mean value ± standard deviation for n = 5.
4-NP	1	—	5	5.1 ± 0.11	102.0	2.2
	2	—	20	20.9 ± 0.13	104.5	0.6
	3	—	50	50.6 ± 0.45	101.2	0.9
	4	—	110	108.7 ± 1.23	98.8	1.1

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4. Conclusions

In this work, a PDPP–GO composite material was synthesized by a simple method in an aqueous solution. Physical and chemical characterization results show that PDPP was tightly combined with GO by π–π stacking conjugation and strong hydrogen bonding. The advocated synthesis method simultaneously solved the electrical conductivity of GO and the solubility of PDPP, which opens up a new field for further construction with other materials in an aqueous solution. Notably, PDPP–GO/GCE shows excellent electrocatalytic activity for 4-NP detection with higher sensitivities of 18.54 and 6.61 μA μM⁻¹ cm⁻² compared with other GO-based sensors. The PDPP–GO/GCE was proved for the detection of 4-NP in real samples and the comprehensive sensing performance of PDPP–GO/GCE perfectly meets the requirements for modern electrochemical sensors.

Author contributions

Lingpu Jia: formal analysis, conceptualization, methodology, investigation, validation, visualizations, writing – original draft, writing – review and editing. Shuangshuang Wang: software, investigation, formal analysis, methodology, resources. Long yang: writing – review and editing, methodology, investigation. Kunping Liu: writing – review and editing, supervision, project administration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (No. 2019QZKK0201), the National Natural Science Foundation of China (No. 22102134), The National Key Research and Development Program of Region innovation of Science and Technology Department of Sichuan Province (No. 2022YFQ0050) and the Introduction of Talent Research Start-up Fund of Chengdu University (No. 2081922036).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ra07239d

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