1 Introduction

Organic semiconducting or electronic materials remain of vital interest in the field of electrochemistry for their flexibility in chemical and electrochemical behavior. Electrochemical sensors are becoming popular, owing to their good detection performances, ease of operation, less expensive, fast, and selective analytic technique [1]. The sensing performance is highly influenced by the electrode materials for enhancing the catalytic effects and elimination of interferers; thus, it is of great importance to design new electrode materials. For developing electrode materials, we can hardly ignore conjugated organic semiconductors due to their high electrical conductivity, highly efficient, tailorable charge transport characteristics, and rapid signal transduction [2,3,4,5,6]. Due to associate π-conjugation in the backbone of conjugated organic semiconductors [7, 8], they exhibit a direct interaction with the analyte in a redox process and thus, help in increasing the selectivity and sensitivity. Park et al. fabricated chromium (VI) sensor based on catalytic reduction using the nanoporous layer of poly (aminopyrimidyl-terthiophene)/AuNi composite and reported the detection limit of 0.25 ± 0.05 ppb [9]. Shim and co-workers developed chronocoulometric sensor for heavy metal ions using a diaminoterthiophene modified electrode doped with graphene oxide [10]. In another report, Lee and his research group compared enzymatic and non-enzymatic glucose sensors based on hierarchical Au-Ni alloy with conductive polymer. The linear range of enzymatic sensor was attained from 1 μM–30 mM with a detection limit of 0.29 μM [11]. In search of important findings related to conjugated organic semiconductors, this work reports the synthesis of indandione oligomer as electrode material. Our synthetic strategy is to design a small molecule semiconductor, 2-(5″-hexyl-[2,2′:5′2″ terthiophen]-5-yl) methylene)-1H- indene-1,3(2H) dione, HTD, comprised of 1, 3 indandione and three thiophene (3 T) unit containing one hexyl side chain. In synthesized HTD, hexyl side chain not only enhances the solubility in common organic solvents but also helpful for molecular packing, improves the charge transport properties, and supports the formation of excellent thin films [12].

Rapid industrialization has led to the release of heavy metals ions such as Cd2+, Pb2+, and Cu2+ ions in wastewater. These highly toxic and non-degradable heavy metals are not only imposing threats to the environment but also destructing the health of living beings [13,14,15,16]. The intake of heavy metal by the human body, even at trace concentrations, might cause irreversible damage to human health [17,18,19,20]. Averting the harm of heavy metal ions to the environment and humans, it is quite demanding to develop a direct and highly sensitive determination method in monitoring the contamination. Among the detection methods, conventional analytical techniques such as atomic absorption spectroscopy (AAS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS), and hyper-Rayleigh scattering, fluorescence detection [21, 22] are largely explored but despite of exhibiting high sensitivity and reproducibility; the implementation of these techniques is expensive which restricts their usage [23]. Moreover, most of the determination methods are focused on single ion detection, and thus, simultaneous detection of heavy metal ions is challenging due to mutual interference between heavy metal ions, the formation of intermediate alloys, and competitive adsorption on active sites [24,25,26]. Currently, square wave anodic stripping voltammetry (SWASV), one of the electrochemical methods, displays impressive ability of simultaneous determination of trace heavy metal ions, shows excellent sensitivity, low detection limits, portable, and of cheap cost [27, 28]. Armstrong et al. developed bismuth-based electrode for the stripping analysis of heavy metal ions and obtained a high sensitivity with low detection limits of ~93 ng L−1 and ~54 ng L−1 for Pb (II) and Cd (II), respectively [29]. Wang and co-workers established simultaneous detection of three main heavy metal ion pollutants (Cd2+, Pb2+, and Cu2+ ions) with improved reproducibility and reliability using SWASV method [30]. Simultaneous detection of Cd2+, Cu2+, Hg2+, and Pb2+ ions with low detection limits and wide dynamic ranges was performed by differential pulse voltammetry (DPV) measurements [31]. Suvina et al. studied the formation of novel hydrogels of conjugate polymer polypyrrole (PPy) and reduced graphene oxide (rGO) for the simultaneous detection of four different heavy metals ions such as Cd2+, Pb2+, Cu2+, and Hg2+ [32]. A variety of electrode surface modifications have been explored for increasing the sensing performances. One common approach of modifying the electrode surface is using graphene oxide (GO) due to the high specific surface area, chemical stability, high π-conjugation, and hydrophilic properties [33, 34]. The aim of this research work is the modification of GCE through the functionalization of indandione oligomer (HTD) with GO. The presence of π-bond in HTD oligomer supports the surface adsorption of GO due to π–π* stacking interactions, hydrogen bond, and Vander Waals interactions [35]. Herein, the incorporation of carbonaceous material to HTD oligomer can improve the electroanalytical qualities of the system through an increase in the surface area and can improve the mass transport of the analyte. Moreover, the properties like electrical conductivity and dispensability are also improved due to the synergistic effect of HTD oligomer and GO. To the best of our knowledge, it is the first report of sensitive and selective simultaneous detection of Cr2+ and Cu2+ ions with HTD@GO/GCE sensor probe by a simple electrochemical method at room conditions.

2 Experimental section

2.1 Synthesis of 2-(5″-hexyl-[2,2′:5′2″ terthiophen]-5-yl) methylene)-1H- indene-1,3(2H) dione (HTD)

HTD oligomer was synthesized by the simple coupling reaction using 1, 3 Indandione (1) and 2-(5-brmothiophen-2-yl)methylene)-1H-indene-1,3(2H)-dione (2) [36], as illustrated in scheme 1 (supporting information). The coupling reaction for HTD oligomer was obtained by mixing (2) (0.35 g, 1.09 mmol), 2-(5′-hexyl-[2, 2′-bithiophen]-5-yl)-4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolane (0.53 g, 1.42 mmol), and 5 mol % Pd(PPh3)4 in anhydrous toluene (C7H8, 20 ml, Sigma Aldrich) and stirred in a round bottom flask. After that, potassium carbonate (K2CO3, 2 M, Sigma Aldrich) in 5 ml of deionized (DI) water was introduced into the reaction mixture, and refluxed at 110 °C for 24 h under argon atmosphere. After completion, the cooled reaction mixture was quenched with DI water, and sequential washing with brine and DI was performed. An organic phase was then extracted with dichloromethane (DCM) and dried over magnesium sulfate (MgSO4, Sigma Aldrich). The resultant solution was filtered, evaporated in vacuum under reduced pressure and the obtained residue was further purified by flash column using (dichloromethane (DCM): heaxane (C6H14), 1:4 v/v) and recrystallized (DCM: EtOH, 1: 5 v/v) (yield 67.62% 0.36 g). 1H NMR (500 MHz, CDCl3, ppm) δ: 7.91 (m, 2H), 7.85 (s, 1H), 7.80 (d, J = 4.1 Hz, 2H), 7.73 (m, 2H), 7.32 (d, J = 3.9 Hz, 1H), 7.20 (d, J = 4.0 Hz, 1H), 7.04–7.09 (dd, J = 4.1 Hz, 3.7 Hz 2H), 6.68 (d, J = 3.6 Hz, 1H), 2.78 (t, J = 7.5 Hz, 2H), 1.65 (m, 4H), 1.34 (m, 4H), 0.89 (t, J = 6.9 Hz, 3H). 13C NMR (100 MHz, CDCl3, ppm) δ: 190.42, 189.82, 150.36, 146.92, 143.81, 142.04, 140.54, 140.41, 135.93, 135.83, 135.01, 134.80, 134.24, 133.98, 127.35, 125.22, 124.52, 124.40, 124.16, 123.46, 122.99, 122.82, 31.65, 31.60, 30.32, 28.85, 22.67, 14.18. MS: m/z calc. for [C28H24O2S3 + H]+: 489.68; found: 489.56.

2.2 Fabrication of sensor probe

GO was prepared according to modified Hummer’s method using the graphite powder [37]. Optimized solution of HTD oligomer (10 mg) with different amounts of GO (0.2 mg, 0.5 mg and 1 mg) were dispersed in 1 ml chlorobenzene solvent and drop casted on cleaned GCE, followed by drying in air oven at 70 °C. Herein, prior to the deposition, GCE was cleaned by using wet soft polishing cloth with alumina powder (0.5 and 0.03 mm, successively), followed by cleaning with DI water and ethanol. Synthesized HTD@GO functionalized nanocomposite was used as sensor probe for the simultaneous detection of Cr2+ and Cu2+ ions.

2.3 Characterizations of HTD oligomer and HTD@GO functionalized nanocomposite

The configuration and structure of HTD oligomer were confirmed by nuclear magnetic resonance spectroscopy with (1HNMR, JEOL FT-NMR) in deuterated chloroform (CDCl3) solvent at 600 MHz for 1H and 100 MHz for 13C13. High-resolution mass spectrometry was used to support the synthesis of HTD oligomer. The presence of bond structures was examined by Fourier transform infra-red spectroscopy by using JASCO FTIR-4100 Japan. The morphological observations of as-synthesized HTD@GO functionalized nanocomposite were observed by field emission scanning electron microscope (FESEM, Hitachi S-4700). The absorption and photoluminescence (PL) spectrum of HTD@GO functionalized nanocomposite were recorded by UV–Visible spectrophotometer (JASCO, V-670) and FP-6500 (JASCO) respectively. The structural modifications of HTD@GO functionalized nanocomposite were studied by Raman scattering (Raman microscope, Renishaw). X-rays photoelectron spectroscopy (XPS) was performed by using AXIS-NOVA CJ109, Kratos Inc., ranges 0 − 800 eV for surface composition and the surface interaction of HTD@GO functionalized nanocomposite. The cyclic voltammetry (CV) experiments were conducted in 0.1 M phosphate buffer solution (PBS, pH = 7) at the scan rate of 50 V/s, using WPG100 electrochemical measurement system which was carried out using 10 ml electrochemical cell comprising of three-electrode system including GCE working electrode, Pt wire as counter electrode and Ag/AgCl reference electrode.

3 Results and discussion

3.1 Formation and morphological studies of HTD@GO functionalized nanocomposite

The formation of HTD oligomer is confirmed by various spectroscopic characterizations such as 1HNMR, 13CNMR, FTIR, and high resolution mass spectrometry (HRMS), as given in supporting information (SI). The morphologies of HTD oligomer and HTD@GO functionalized nanocomposite are investigated by FESEM analysis. Figure 1a shows irregular small rods like morphology of HTD oligomer which is the usual morphology of organic small molecule [38]. From Fig. 1b, c, HTD@GO functionalized nanocomposites show uniformly mixed morphology of HTD oligomer and GO. Moreover, a high amount of GO (1 mg) in HTD@GO functionalized nanocomposite, as depicted in Fig. 1d, causes an agglomeration of rods and GO which results in large aggregates. Thus, GO (0.5 mg) in HTD@GO nanocomposite is the optimized ratio for obtaining the uniform and well-mixed thin film. Herein, HTD oligomer might form the bond with GO, which will be later confirmed by XPS studies.

Fig. 1
figure 1

FESEM images of a HTD oligomer and HTD@GO functionalized nanocomposite with b 0.2 mg GO, c 0.5 mg GO, and d 1 mg GO

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3.2 Optical properties of HTD@GO functionalized nanocomposite

UV–Vis absorption was recorded quantitatively in chloroform solution for HTD oligomer and HTD@GO functionalized nanocomposites. From Fig. 2a, the synthesized HTD oligomer presents an absorption band at ~355 nm, referring to π-π* transition, whereas the dominant absorption band at ~515 nm indicates n-π* transitions of HTD oligomer [39].

Fig. 2
figure 2

UV–vis absorption spectra of a HTD oligomer, HTD@GO functionalized nanocomposite; b HTD@GO functionalized nanocomposite with various GO concentrations; c photoluminescence (PL) spectra of HTD oligomer, HTD@GO functionalized nanocomposite; and d HTD@GO functionalized nanocomposite with various GO concentrations

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However, HTD@GO functionalized nanocomposite displays three absorption bands at ∼251 nm, ∼355 nm and ∼525 nm, corresponding to the characteristics bands of GO and HTD oligomer, respectively [40]. Significantly for HTD@GO functionalized nanocomposite, a slight shift of absorption band towards a higher absorption edge indicates π–π* interaction between HTD oligomer and GO [41]. From Fig. 2b, GO (0.5 mg) in HTD@GO functionalized nanocomposite shows the highest absorption intensity as compared to other ratio of HTD@GO functionalized nanocomposites, which again suggests a good mixing of GO and HTD oligomer. Furthermore, PL spectra of HTD oligomer and HTD@GO functionalized nanocomposites are examined using an excitation wavelength of ~350 nm, as shown in Fig. 2c, d. HTD oligomer displays a prominent emission PL peak centered at ~635 nm in the green region due to the benzenoid groups of HTD oligomer [36]. Referring to HTD@GO functionalized nanocomposite, a considerable shift from 635 to ~656 nm with prominent PL quenching is observed. This quenching describes the generation of singlet excitons [42]. The quenching is intensified after an interaction of GO with HTD oligomer. HTD@GO functionalized nanocomposite with GO (0.5 mg) displays the maximum quenching, suggesting the emission process of conjugated organic semiconductors like HTD oligomer.

3.3 Structural characterization of HTD@GO functionalized nanocomposite

HTD oligomer and HTD@GO functionalized nanocomposite are further analyzed by FTIR spectra, as shown in Fig. 3a. Herein, HTD oligomer shows C-H stretching (aliphatic) at ~2922 and ~2861 cm−1, C = O stretching at ~1716 and ~1672 cm−1, C = C stretching is displayed at ~1576 cm−1, and C = C (aromatic) stretching appears at ~1432 cm−1, and H-C-H bonding are shown by the peaks at ~ 1380 and ~1337 cm−1 [43]. The other peak for C-S-C bending is visible at ~1196 cm−1, and C-S stretching appears at ~97 and ~726 cm−1 [44]. However, in HTD@GO functionalized nanocomposite, IR bands are shifted negatively to ~1708 cm−1, ~1568 cm−1, and ~1231 cm−1 indicating the linkage between GO and HTD oligomer. Considerably, the marginal shifting in IR band at ~1568 cm−1 and ~1708 cm−1 again supports the interaction between GO and HTD oligomer via π-π* interactions and partial hydrogen bonding [45]. The structural behavior of HTD@GO functionalized nanocomposite is further characterized by Raman spectroscopy, as illustrated in Fig. 3b. Typically, GO Raman bands are appeared at ~1356 cm−1 and ~1595 cm−1 which normally refer to D band or K-point phonons of A1g symmetry and Raman active G band of sp2 hybridized carbon (the E2g phonons), respectively [46, 47]. However in Fig. 3b, Raman bands for D and G in HTD@GO functionalized nanocomposites are visibly shifted from ~1356 cm−1 to ~1347 cm−1 and ~1595 cm−1 to ~1540 cm−1, favoring the interaction between HTD oligomer and GO, in particular specifying the interaction between C-S+.of HTD oligomer and carboxylate group of GO [50]. These results are in excellent agreement with the FTIR results.

Fig. 3
figure 3

a FTIR spectra of HTD oligomer, HTD@GO functionalized nanocomposite, and b Raman spectrum of HTD@GO functionalized nanocomposite with 0.5 mg GO

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XPS measurements are performed for HTD@GO functionalized nanocomposite to investigate the chemical states of O 1 s, S 2p, and C 1 s. The survey XPS spectrum (Fig. 4a exhibits three binding energies peaks at ~ 284.5 eV, ~ 164 eV, and ~ 520.2 eV, representing to C 1 s, S 2p, and O 1 s peaks, respectively. To define the detailed structural interaction and bonding between HTD oligomer and GO, a high-resolution C 1 s, S 2p, and O 1 s XPS are deconvoluted. Figure 4b shows C 1 s XPS spectra of HTD@GO functionalized nanocomposite with binding energies positioned at deconvoluted peaks at ~289.1, ~286.3, ~283.4, and ~284.8 eV, referring to −O–C = O, C–O, C–S, C–C, and C–H bonds [48, 49]. In Fig. 4b, the binding energy at ~284.8 eV represents sp2 carbon skeleton of C–C bond in GO and C–C of the benzonoid ring in HTD oligomer, showing the bonding of GO–S (thiophene unit in HTD oligomer) which confirms the intermixing of C–C aromatic bond of GO and HTD oligomer. The deconvoluted O1s XPS of HTD@GO functionalized nanocomposite exhibits three binding energy peaks with one center binding energy peak, as shown in Fig. 4c. The binding energy positioned at ~530.9 eV is normally assigned to C = O or C–OH group in carbon-based materials [50]. The binding energy at ~531.8 eV could be attributed to C = O and O = C–O− functional groups in HTD@GO functionalized nanocomposite, while a higher binding energy peak at ~ 532.4 eV corresponds to the bound water molecules [51]. Figure 4d displays S 2p XPS spectrum of HTD@GO functionalized nanocomposite which is consisted of two binding energies at ~165.1 eV (S 2p1/2) and ~163.8 eV (S 2p3/2), resulted from C–S bond [52]. The appearance of binding energy at ~162.5 eV might be originated from thiophene (S) of HTD oligomer. Importantly, the position of binding energy is slightly shifted towards higher binding energy as compared to S 2p XPS of GO–S bond [53], suggesting the interaction of sulfur in HTD oligomer to carboxylate group of GO, which partially forms the hydrogen bonding.

Fig. 4
figure 4

a Survey, b C 1 s, c O 1 s, and d S 2p XPS spectra of HTD@GO functionalized nanocomposite

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3.4 Electrochemical characterization of HTD@GO/GCE sensor probe

3.4.1 Selectivity of metal ions detection through CV measurements

The selectivity of HTD@GO/GCE sensor probe towards heavy metal ion detection is determined by CV measurements at scan rate of 50 mV/s. Herein, the supporting electrolyte of 0.1 M PBS solution is used in different metal ion solutions of K+, Al3+, Mn2+, Zn2+, Cu2+, and Cr2+ ions. Firstly, CV plots of bare GCE, GO/GCE, and HTD@GO/GCE in 0.1 M PBS electrolyte with 0.05 M KCl are measured to explain the sensing behavior of HTD@GO functionalized nanocomposite, as shown in Fig. 5a. A weak current response is recorded with bare GCE whereas, a slightly higher response is observed for GO/GCE. Herein, a low current response in bare GCE shows the poor conductivity of GCE electrode and the slightly improved current response with GO/GCE is related to the presence of GO which has considerably increased the number of active sites over the electrode surface. As seen in Fig. 5a, a high current response for HTD@GO/GCE sensor probe is observed with the existence of significant oxidation current peak. Herein, a large specific surface area of GO, well dispersion of HTD oligomer into GO (as confirmed from FESEM results), and a good bonding between HTD oligomer and GO (corroborated from XPS results) collectively promoted the electron transfer rate and improved the surface properties of HTD@GO/GCE sensor probe. In general, the modification of GO/GCE surface with conjugated organic semiconductors is a proven strategy to improve the current response characteristics of the fabricated sensor by improving the conductivity of the electrode. In our case, the improvement of HTD@GO/GCE sensor probe is realized via a charge hopping transport mechanism of HTD oligomer through the depletion of the charge injection barrier. Figure 5b shows the CV responses using HTD@GO/GCE sensor probe towards the determination of 6 metal ions viz K+, Al3+, Mn2+, Zn2+, Cu2+, and Cr2+ ions at the concentration of 1 μM. Among of all ions, a strong anodic current response for Cr+2 ions at − 0.31 V is observed, whereas Cu+2 ion also shows a strong anodic current response at 0.21 V.

Fig. 5
figure 5

a CV plots of bare GCE, GO/GCE, and HTD@GO/GCE sensor probe in 0.1 M PBS electrolyte; b CV responses using HTD@GO/GCE sensor probe towards the determination of 6 metal ions viz K+, Al3+, Mn2+, Zn2+, Cu2+, and Cr2+ ions at the uniform concentration of 1 μM; c simultaneous detection of Cr2+ and Cu2+ ions by SWASV using same electrode in 0.1 M PBS; and d CV plots of Cr2+ and Cu2+ ions measured with HTD@GO/GCE sensor probe

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These responses indicate that HTD@GO/GCE sensor probe is promising for the detection of Cr+2 and Cu+2 ions in 0.1 M PBS electrolyte. The enhancement of the sensing signal for Cr+2 and Cu+2 ions can be ascribed to the factors of (i) HTD oligomer owns negative charges due C–S− and C = O−, causing HTD@GO/GCE sensor probe to adsorb more target ions and receive stronger signal secondly, and (ii) GO moieties provide more effective active area for the nucleation due to its large surface-to-volume ratio [54]. To further verify the feasibility of an electrochemical method for the simultaneous detection of metal ions, the presence of Cr+2 and Cu+ ions is detected by stripping voltammetry using same electrode in 0.1 M PBS (pH = 7), as depicted in Fig. 5c. The peak potentials for Cr+2 and Cu+ ions are observed at ~ −0.29 V and ~0.24 V, respectively. Moreover, the peak intensity of Cr2+ ion is lower than that of Cu2+ peak detection intensity in the supporting electrolyte. This reduction of stripping signal might due to the formation of intermetallic compounds and the existence of competition between the analytes and the interfering ions for the active sites of HTD@GO/GCE sensor probe [55]. These results indicate that the fabricated HTD@GO/GCE sensor probe is capable of simultaneous detection of multiple heavy metal ions (such as Cr+2 and Cu+ ions) with a high sensitivity.

The suggested mechanism for the determination of Cr+2 and Cu+ ions is shown in Fig. 6. As described above, the combination of HTD oligomer with advantageous large surface area of GO considerably provides a large amount of active sites as well as improves the electrical conductivity of electrode, which are necessitated for high adsorption of heavy metal ions (in our case, Cr+2 and Cu+ ions) and enhances the electron transfer properties of the electrode. Thus, Cr+2 and Cu+ ions easily interact with the surface of HTD@GO/GCE sensor probe, resulting in increasing the current and conductance.

Fig. 6
figure 6

Proposed mechanism for Cr+2 and Cu+ ions detection over HTD@GO/GCE sensor probe

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3.4.2 Sensitivity, reproducibility, and stability of HTD@GO/GCE sensor probe

The details of sensing properties of HTD@GO/GCE sensor probe for Cr+2 and Cu+ ions have been further analyzed by measuring the series of CV plots using different concentrations of ions (1–100 μM) in PBS electrolyte. Figure 7a shows the CV plots of HTD@GO/GCE sensor probe for different concentrations of Cr2+ ions in 0.1 M PBS electrolyte. The anodic current at −0.31 V gradually increases with an increase of Cr2+ ions concentration, indicating an elevation in the ionic strength of an electrolyte. From Fig. 7b, a similar behavior of HTD@GO/GCE sensor probe is recorded for the detection of Cu2+ ions, in which an increase of Cu2+ ion concentration leads to the continuous enhancement in an anodic current. Thus, an anodic current in CV measurements with different ion concentration prominently explains the sensing characteristics of HTD@GO/GCE sensor probe for the determination of Cr+2 and Cu+2 ions.

Fig. 7
figure 7

CV plots of HTD@GO/GCE sensor probe for different concentrations of a Cr2+ ions and b Cu2+ ions in 0.1 M PBS electrolyte, and the calibration plot of anodic current responses from CV measurements for the determination of c Cr2+ and d Cu2+ ions

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The calibration plots of anodic current responses from CV measurements for the determination of Cr+2 and Cu+2 ions are plotted in Fig. 7c, d. The sensing parameters such as sensitivity, limit of detection (LOD), linear range, and regression coefficient for the detection of Cr+2 and Cu+2 ions are summarized in Table 1. Calibration plots for Cr+2 and Cu+2 ions present a linearity of current with the increase of ion concentrations. Linear dynamic for Cr+2 and Cu+2 ions are observed as ~1–100 μM, whereas reasonably good LODs of ~ 3.65 μM and ~ 2.25 μM are estimated for Cr+2 and Cu+2 ions, respectively. The results obtained by HTD@GO/GCE sensor probe are compared with different electrochemical systems for the determination of metal ions, as discussed in Table 2.

Table 1 Sensing parameters of HTD@GO/GCE sensing probe for the determination of Cr2+ and Cu2+ ions
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Table 2 Comparison of sensing performances with other reported chemical sensors
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The reproducibility of HTD@GO/GCE sensor probe is studied by obtaining the sensing responses from six electrodes prepared under similar conditions, as shown in Fig. 8a, b. The relative standard deviations (RSDs) of Cr+2 and Cu+2 ions are calculated to be ~ 10%, ~ 6.40%, respectively. The obtained result indicates that HTD@GO/GCE sensor probes vary less from standard values, and therefore, this is highly reproducible and reliable sensor electrode for the detection of Cr+2 and Cu+2 ions. To explore its stability, the sensing behavior of fabricated HTD@GO/GCE sensor probe was tested for 30 days. Figure 8c presents that sensitivity of the sensor maintains the values of ~ 92.5%, and ~ 91.4% for Cr+2 and Cu+2 ions, respectively, after 30 days of checking, indicating a good stability of the developed electrochemical sensor.

Fig. 8
figure 8

Reproducibility test of HTD@GO/GCE from six electrodes for a Cr2+, b Cu2+ ions, and c stability test of HTD@GO/GCE sensor probe for 30 days

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3.4.3 Real samples analysis of HTD@GO/GCE sensor probe

To investigate the practical applicability of the fabricated sensor with HTD@GO/GCE sensor probe, the electrochemical performance for the determination of Cr+2 and Cu+2 ions (0.1 M PBS) is checked in real sample using tap water as supporting electrolyte, as shown in Fig. 9a, b. The anodic currents of Cr+2 and Cu+2 ions are weak in bare tap water sample, indicating that the concentrations of Cr+2 and Cu+2 ions are lower at the potentials of ~0.31 V, and ~0.21 V. Moreover, as shown in Fig. 5c, the stripping peaks exhibit well-defined and well-separated signals for Cr2+ and Cu2+ ions, indicating that the HTD@GO/GCE sensor probe can simultaneously detect the target ions at such a low concentration as 1 µM.

Fig. 9
figure 9

Real sample applications for the determination of a Cr2+ and b Cu2+ ions

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4 Conclusions

In this work, the planar 2-(5″-hexyl-[2,2′:5′2″ terthiophen]-5-yl) methylene)-1H- indene-1,3(2H) dione oligomer (HTD) is synthesized by Suzuki-coupling reaction. Functionalized nanocomposite of HTD oligomer with graphene oxide (GO), HTD@GO, exhibits the synergic effects of HTD and GO as electrode material for the fabrication of a simple, reliable, and sensitive sensor to simultaneously detect the trace Cr2+ and Cu2+ ions. The covalently linked HTD oligomer with GO not only improves the conductivity, dispersion, and surface area of HTD@GO functionalized nanocomposite, but also supports the diffusion and promotion of the electron transfer rate. Due to unique structural characteristics and good electrochemical performance of HTD@GO functionalized nanocomposite, simultaneous detection of Cr2+ and Cu2+ ions with high sensitivity and reliability has been realized. The practicality of our fabricated sensor is validated by the recovery test of target metal ions in tap water.