1 Introduction

The toxicity of uranium is widely accepted problem to be dealt with [1,2,3,4]. Generally, uranium exposure to human is from hazardous waste from nuclear plant, sewage or laboratory wastewater from nuclear plant, industrial waste as well as rock soil erosion [5,6,7,8,9]. Uranium is present in nature as 238U, 235U and 234U [10,11,12]. Among them, the analyte of interest in this work was 238U which is present in nature in aqueous ion form as U6+. The toxicity of uranium leads to several diseases including cancer [13,14,15], nephrotoxicity, genotoxicity, developmental defects [13, 16]. The maximum permissible limit for uranium in drinking water is 30 µg/L (~ 30 ppb) [17]. Recent reports verify that there is significant uranium contamination in ground water in various places of Indian subcontinent region [18,19,20,21,22,23,24,25]. It has been reported that the uranium contaminant in ground water has been exceeded the maximum permissible limit in several places in Punjab [26]. Therefore, it is highly necessary to fabricate a low-cost uranyl ion sensor. The conventional detection techniques available for uranyl ion are atomic absorption spectroscopy [27], inductively coupled plasma mass spectroscopy [28, 29], laser induced fluorescence [30], X-ray fluorescence spectrometry [31], laser Raman spectrometry [32], optical emission spectrometry [33, 34]. Although these provide accurate results, the main problems associated with these techniques are high operating and maintenance cost [33], some having lower sensitivity [35] and most importantly, they lack online monitoring capability [33]. Moreover, in few cases, interference is also an issue. Therefore, a low-cost suitable online monitoring system is necessary to detect uranyl ion qualitatively and quantitatively with high selectivity. Electrochemical detection techniques overcome all these issues [4, 9, 36,37,38,39,40,41]. One such electrochemical technique is square wave voltammetry (SWV).

Selectivity, sensitivity and low limit of detection (LOD) are the prime parameters to construct an electrochemical sensor. Polyaniline, being a conducting polymer, has been used widely to detect heavy metal ions [42,43,44,45]. In our earlier works [46,47,48] polyaniline-based electrode was used for detection of Cd2+ ions by SWV. A novel composite from aniline, N-phenylglycine and graphene oxide (GO) was synthesized to detect Cd2+ with lesser interference from several ions. Polyaniline-based material has a variety of applications but due to its incompatibility, infusibility and modest environmental stability due to its rigid chain structure, makes it un-processable. Therefore, incorporation of electron withdrawing group such as -COOH can improve its processability. N-phenylglycine has one electron withdrawing group -COOH and has similar structure to that of aniline. Incorporation of N-phenylglycine improves the reactivity by providing additional active sites[49]. It was observed that the reactivity, and electrochemical surface area of the composite synthesized from aniline, N-phenylglycine and GO was higher than those from the composite synthesized from aniline and GO, as shown in our earlier work [48]. For interference analysis, conventional peak reduction (PR) technique was used and a novel barrier width (BW) technique employing electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), was introduced for the first time in case of aqueous ion sensor. PR technique requires a series of experiments to predict the interference while BW technique overcomes this issue by requiring a handful number of experiments. Subsequently this BW technique was benchmarked with PR technique for interference analysis. In this work, another composite based on aniline, N-phenylglycine and GO and a ligand was used to detect U6+ and both the PR and BW techniques were used to study the interference of a series of test ions towards U6+ ions. To enhance the sensitivity and selectivity a number of suitable ligands or components are reported in literature used as complexing agents for U6+. These are L-DOPA [50], 8-hydroxyquinoline [51], chloranilic acid [52], arsenazo (II) [53], cephradine [54], TriBAP, TetraBAP [55], cupeferron [56,57,58], dipicolinic acid [59], PAA [60]; or as complexing agent at the electrode surface, e.g. 4sulfonic-calix[6]arene [61], 4-carboxyphenyl [62], carbamoylphosphonic acid [63], DNAzyme [64], phosphorothioated oligonucleotides [65]. p-tertbutyl calix[4]arene is one such ligand which can be used to detect U6+. There are several reports where composite made from polyaniline or GO and p-tertbutyl calix-[4]-arene have been used for water purification or in catalysis [66,67,68,69]. Guo et al., have synthesized chemically a composite of polyaniline and p-tertbutylcalix-[4]-arene as adsorbent for water purification [66]. Similarly Mohammadi et al., have synthesized an adsorbent from Polyurathane foam and p-tert-butyl thiacalix [4] arene, by chemical synthesis route, used for water purification [67]. Again Mohammadi et al., have used a composite synthesized by chemical means from Alginate, GO, p-tert-butyl calix[4]arene, sodium p-sulfonatocalix[4]arene and used it for water purification [68]. Zhang et al., have chemically synthesized a composite from GO, p-tertbutyl calix-[6]-arene and used as adsorbent [69]. Gutsche et al., have used P-tert butyl calix-[6]-arene—GO composite, synthesized by chemical route, and have used for catalysis [70]. Hyder et al., synthesized para-dimethylamine calix[4]arene modified self-assembled GO and used as adsorbent [71]. C-undecylcalix[4]resorcinarene and reduced GO based composite was synthesized by Wu et al., for detection of tryptophan [72]. There is no report till date where a composite from aniline, N-phenylglycine, GO and p-tertbutyl calix[4]arene was used to detect U6+ and here lies the novelty of this work.

In this work a novel composite from aniline, N-phenylglycine, GO and p-tertbutylcalix[4]arene, was synthesized for the first time by electrochemical route and used to detect U6+ by SWV. The surface morphology of the sensing film was achieved by FESEM and HRTEM and the material characterization was carried out by FTIR. Sensitivity of the system was evaluated, and adsorption isotherm was constructed. Interference of various test ions (Ru3+, Cs+, As5+, Cr6+, Cu2+, Na+, Mg2+, Al3+, K+, Sr2+, Mn2+, Fe3+, Fe2+, Zn2+) were examined by conventional peak reduction (PR) technique. The mechanism of interference was explained by barrier width (BW) technique.

2 Materials and methods

2.1 Materials

Cadmium chloride, lead acetate, strontium nitrate, copper nitrate were procured from Rankem (India); aniline, chromium oxide, sodium arsenate salts were obtained from Loba Chemicals (Mumbai, India); zinc nitrate, sodium acetate, potassium ferrocyanide, magnesium chloride, aluminium chloride, potassium ferricyanide, potassium chloride were purchased from Fischer Scientific (USA); N-phenylglycine, ruthenium (III) chloride hydrate and cesium chloride were obtained from Sigma Aldrich (USA); sulfuric acid (H2SO4. 6H2O) was procured from Qualigens (USA); diethylamine, di-sodium hydrogen phosphate and hydrochloric acid were obtained from Merck (Germany). The Uranium (VI) stock solution was purchased from S.P.Analytical, Navi Mumbai, India. Analytical grade chemicals were used in this work.

Double distilled water with conductivity lesser than 0.2 µS cm−1 was used to prepare all the solutions. Auto-lab PGSTAT 302N, Eco chemie, The Netherlands, connected to a three-electrode cell with platinum wire as counter electrode, saturated Ag/AgCl as reference electrode was used to perform all the electrochemical experiments. The working electrode comprised a stainless steel (SS, 304) plate (1.5 cm × 1 cm) of thickness 1 mm onto which the novel composite was electrodeposited. The SS 304 electrode was dipped to 0.5 cm into the electrolyte to maintain 1.5 cm2 (i.e. both surfaces of 0.5 cm × 1.5 cm) as the effective surface area for all experiments. For all the electrochemical experiments the volume of the electrolytes was 50 ml.

2.2 Experimental methods

2.2.1 Synthesis and characterization of the novel composite film

The synthesis procedure for the novel composite film is similar to the synthesis of composite ANGO described in our previous works [48]. In this work, the novel composite was synthesized by chronopotentiometry at constant current of 2 mA cm−2. For this purpose, 0.02 M aniline, 0.1 g N-phenylglycine, 0.1 g of GO and 0.1 g of p-tertbutylcalix[4]arene was taken in 1 M sulphuric acid solution. The amount of p-tertbutylcalix[4]arene was optimized by screening experiments. Since GO and p-terbutylcalix[4]arene were not soluble in 1 M sulphuric acid, therefore, the electrolyte 1 M sulphuric acid containing aniline, N-phenylglycine, GO and p-tertbutylcalix[4]arene was ultrasonicated for one hour at room temperature for uniform dispersion. After deposition of the composite film, the electrodes were dried at 100 °C for 24 h in a vacuum oven. The dried sensing material was scraped out from the stainless-steel electrode surface for FTIR, FESEM and HRTEM analysis.

2.2.2 Material characterization

To verify the presence of the functional groups in the composite material, FTIR spectroscopy was performed at room temperature by using a Bruker (Vertex-70) FTIR spectroscope with thin pellets of KBr. The range of 4000–400 cm−1 was employed in the analysis with a resolution of 8 cm−1. The surface morphology study of the composite films was conducted using a field emission scanning electron microscopy (FESEM) (Model MIRA3, TESCAN, USA). High resolution transmission electron microscopy (HRTEM) (Model FEI Titan G2 60–300 TEM, USA) was used to investigate the material structure.

2.2.3 Metal ion sensing

SWV experiments were performed to detect metal ions. For desired concentrations of uranyl ion (U6+), uranium stock solution was used in electrolyte. To make desired ion concentrations of various test ions, ruthenium chloride hydrate (for Ru3+), cesium chloride (for Cs+), cadmium chloride (for Cd2+), lead chloride (for Pb2+), sodium arsenate (for As5+), strontium nitrate (for Sr+), copper nitrate (for Cu2+), chromium oxide (for Cr6+), sodium acetate (for Na+), potassium chloride (for K+), aluminium chloride (for Al3+), potassium ferricyanide (for Fe3+), zinc nitrate (for Zn2+), potassium ferrocyanide (for Fe2+), magnesium chloride (for Mg2+) were used. All the metal ion solutions were prepared with an electrolyte of pH 3.3. Based on screening experiments, electrolyte pH 3.3 was decided. It was observed that the film can show better response at pH 2.5. The signal reduces as pH increases and shows minimum at pH 4.2. However, at pH 2–2.8, strong Pb2+ interference was observed which was possible to avoid at pH 3.3 or above. That is why pH 3.3 was chosen for metal ion sensing. To obtain the electrolyte solution, 0.1 M di-sodium-hydrogen phosphate was used as the base solution and a pH value of 3.3 was achieved after a gradual addition of 0.1 M hydrochloric acid.

To detect metal ions, the modified electrodes were immersed in the metal ion containing solution for 30 min which is regarded as the preconcentration step at open circuit potential. Next, the analyte was allowed to deposit on the electrode at a constant potential of 0.1 V for 200 s. SWV experiments were conducted in the potential window from 0.1 V to − 1.5 V. The various SWV parameters were, 0.1 V for amplitude, 0.075 V for the step potential and the frequency was maintained at 25 Hz. The LOD and sensitivity values were extracted, and adsorption isotherms were plotted from which − ∆Gads values were calculated.

2.2.4 Experiments to calculate barrier width

To calculate the barrier width, it is necessary to obtain the barrier height and charge transport resistance values [46, 73, 74]. For this purpose, CV experiments were performed to obtain the lowest unoccupied molecular orbital (LUMO) from which barrier height values were calculated (not shown here). The electrolyte was the same as in SWV experiment. 10 ppm concentration for U6+ was chosen as CV peaks were discernable at lower concentration of U6+. To obtain the barrier heights of U6+ in presence of other test ions, a bicomponent salt solutions with 10 ppm of U6+ and a test ion at concentration of 20 ppm was used. For better interference analysis it is necessary to maintain higher concentration of the test ions than U6+. The CV experiments were carried out at a potential window from − 1.5 V to 0.1 V with a scan rate of 100 mV s−1.

Electrochemical impedance spectroscopy (EIS) experiments were performed for the film to obtain the charge transport resistances. A constant potential of 0.4 V was maintained, and a frequency range from 100 kHz to 1 Hz was used. The constant potential of 0.4 V was optimized by screening experiments. From the Nyquist plots obtained from EIS experiments, equivalent circuits were constructed from which charge transport resistances were evaluated. The electrolyte was maintained the same as in SWV experiments. For interference analysis bicomponent salt solution of U6+ and a test ion were chosen. The concentrations of U6+ and the test ions are mentioned in Sect. 3.3.2. BW technique. From the charge transport resistances and LUMO values, BW values were calculated.

2.2.5 CV for electroactive surface area

The electroactive surface area for the four different films were calculated from CV measurements using the Randles–Sevcik equation [75] (not shown here). These CV experiments were carried out with various sensing materials deposited on SS 304 electrode by chronopotentiometry (working) electrode, platinum counter electrode and Ag/AgCl reference electrodes at a potential window − 1.5 V to 1 V with the electrolyte being 0.1 M KCl containing 5 mM [Fe(CN)6]−3/−4. The various sensing materials were (i) polyaniline, (ii) copolymer of aniline and N-phenylglycine, (iii) composite of aniline and GO, composite from aniline, N-phenylglycine and GO, (iv) aniline and GO and (v) the novel composite from aniline, N-phenylglycine, GO and p-tertbutylcalix-[4]-arene. The synthesis procedures of all these materials except the novel composite are described in our previous works [46, 48].

3 Results and discussion

3.1 Film synthesis and characterization

The film was deposited by chronopotentiometry at constant current of 2 mA cm−2 for 120 s onto the SS 304 electrode. The constant current deposition plot is shown in Fig. 1. In our one of the previous work [46], a novel composite from aniline and N-phenylglycine was synthesized by chronoamperometry, chronopotentiometry and cyclic voltammetry (CV). The film synthesized by chronopotentiometry had higher BET surface area as compared to the films synthesized by other two methods. It showed better response with lower LOD, higher sensitivity, higher Gibb’s free energy of adsorption (− ΔGad) than the films synthesized by chronoamperometry and CV. Therefore, in a subsequent work [48], another novel composite was synthesized by chronopotentiometry from aniline, N-phenylglycine and GO. In this work too, the same technique was employed due to its better performance. The constant current deposition of the films showed first an increasing trend and reached to the maximum at 16 s and then showed decreasing trend and then remained constant. This showed similar trend as synthesis of polyaniline, copolymer of aniline and N-phenylglycine [46] and composite named ANGO synthesized from aniline, N-phenylglycine and GO [48]. As mentioned in our previous work, for polyaniline, the peak was observed at 18 s, for copolymer the plot showed a peak at 52 s [46] and for the composite ANGO the peak was at 20 s [48]. The plot was compared with chronopotentiometric deposition of polyaniline reported in literature [76] and it showed similar behaviour.

Fig. 1
figure 1

A constant current deposition of the composite film synthesized from aniline, N-phenylglycine, GO and p-tertbutyl calix[4]arene on SS 304 electrode

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The FTIR characterization of the film is shown in Fig. 2a. For FTIR analysis, FTIR spectra of commercially available p-terbutyl calix-[4]-arene was compared with the novel composite. For the FTIR spectra of p-tertbutyl calix-[4]-arene, the bands at 3235 cm−1 was due to OH and CH stretching vibrations. The bands around 3162 cm−1 was due to OH-stretching. At around 2937 cm−1 the bands were due to asymmetric C–H stretch of methylene groups. At around 2910 and 2865 cm−1 the bands were due to symmetric C–H vibrations of methyl and methylene groups respectively. Around 1610, 1306 and 1258 cm−1 the bands were due to aromatic ring CC stretch and CCH bending. 1125, 1029 and 977 cm−1, the bands were due to aromatic C–H in plane bend. At 854, 740 cm−1 the bands were due to aromatic C–H and CC out of plane. CCC in plane bends were observed at 570 cm−1 [77, 78]. For the novel composite, the bands at around 1590 and 1496 cm−1 were due to stretching mode of quinoid and benzenoid rings of polymer. The peak at 1481 cm−1 and 1566 cm−1 were the qualitative measure of the oxidation process. The aromatic C-N vibration from quinoid and benzenoid ring was indicated by the peak at around 1310 cm−1 [79]. The spectra at 1666 cm−1 was due to the stretching mode of carbonyl in carboxyl groups [80]. These peaks indicate the presence of aniline and N-phenylglycine as mentioned in our previous work [46]. A broader band at 3339 cm−1 was due to stretching of C = O from GO. Peaks at 1230 and 1430 cm−1 were due to the presence of C-O from GO [81, 82]. The presence of p-tert butyl calix-[4]-arene was indicated by the presence of peak at 780 cm−1 which was due to aromatic C-H and CC out of plane band. At 550 cm−1 the peak was due to CCC in plane bend. Band at 1310 cm−1 was observed due to the aromatic ring CC stretch and CCH bending. Aromatic C-H in plane bend was observed at 1103 cm−1. At 2950 and 2930 cm−1 the peaks are due to asymmetric C–H stretch of methyl group and asymmetric C–H stretch of methylene group respectively. A small peak for symmetric C–H vibration of methyl and methylene group were observed at 2897 and 2856 cm−1 respectively [78].

Fig. 2
figure 2

FTIR of a the sensing material and b p-tertbutylcalix-[4]-arene; c FESEM and d HRTEM of the sensing material

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The FESEM image of composite film as shown in Fig. 2b shows tubular like structure embedded into GO matrix with an average diameter of ~ 92 nm. The structure is similar to the FESEM image of composite synthesized from aniline, N-phenylglycine and GO as reported in our previous work [48]. Since the sensing material is a novel material therefore it was not possible to find any literature for comparison with our FESEM results. However, the structure resembled the composites synthesized from aniline and GO as reported in the literature [83, 84].

The HRTEM image of composite film in Fig. 2c showed sheet like structure where p-tertbutyl calix-[4] arene was tethered in GO sheet along with aniline and N-phenylglycine. The composite matrix contained GO which showed sheet like structure [48, 83, 85]. The dark patches indicated the presence of conducting polymers [48, 83, 85]. It showed similar structure as shown by composite synthesized from aniline, N-phenylglycine and GO as mentioned in our previous work [48]. We could not find any report where this composite is synthesized, therefore, we could not provide any comparison to support our HRTEM results.

3.2 Metal ion detection and thermodynamic analysis

SWV experiments were conducted to detect U6+ ions. A schematic of the detection procedure is shown in Fig. 3. The sensing material was deposited on stainless steel electrode as described earlier. The ligand to capture uranyl ion, p-tertbutylcalix-[4]-arene was embedded into the sensing matrix as well as into the sensing surface. The entire detection procedure can be described in three parts—preconcentration at open circuit potential, preconcentration at 0.1 V followed by stripping of U6+ ion. As the preconcentration step at open circuit potential, the sensing electrodes were immersed in the aqueous ion containing solution for 30 min without applying any external voltage. Then a potential of 0.1 V was applied to deposit the ions into the electrode surface for 200 s. Following this, a square wave pulse was applied and the voltage was gradually decreased to − 1.5 V. At around − 0.6 V, U6+ ions were stripped out and a peak for U6+ was observed. There are reports where stripping peak for U6+ was observed in the range − 0.5 V to − 0.6 V range and similar SWV profile was obtained. [86, 87]

Fig. 3
figure 3

Schematic of U6+ detection by square wave voltammetry by the composite film

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Figure 4 shows the SWV plot for U6+ detection using the composite synthesized from aniline, N-phenylglycine and GO. The sensing material without p-tertbutyl calix[4] arene was able to detect U6+ till 10 ppm as there is no functional group or ligand attached to the surface to capture U6+ ion. That is why it is important to add a ligand in the sensing material such as p-tert butyl calix[4]arene so that it can capture U6+ ion at low concentration.

Fig. 4
figure 4

SWV plot for different concentrations of U6+ using the composite synthesized from aniline, N-phenylglycine and GO

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Using the composite synthesized from aniline, GO and p-tertbutyl calix[4]arene, LOD was 50 ppb. Using the novel sensing composite synthesized from aniline, N-phenylglycine, GO and p-tert butyl calix[4]arene, an LOD of 900 ppt (~ 2.27 nM) was achieved using this film electrode. The calibration plot was constructed, and the sensitivity of the system was found to be 0.201 mA [p [U6+]]−1. Figure 5a, b show the SWV plots and calibration plot for this system.

Fig. 5
figure 5

a SWV plot for different concentrations of U6+ using the sensing material synthesized from aniline, N-phenylglycine, GO and p-tert buty calix[4]arene b Calibration plot of the system

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The adsorption isotherm was constructed to understand the interaction of the analyte to the electrode surface. The details regarding the construction of adsorption isotherms are explained in our previous works [46, 47], therefore not explained here. The adsorption isotherm for the film is shown in Fig. 6 from which -∆Gads value for the system was evaluated. For U6+, the − ∆Gads value was 60.750 kJ mol−1 showing strong chemisorption. If the − ∆Gads value for a process is higher than 40 kJ mol−1, then chemisorption takes place due to the chemical reaction involved in between the electrode surface and the analyte of interest [88, 89].

Fig. 6
figure 6

Adsorption isotherm for the sensing film

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3.3 Interference study by peak reduction (PR) technique and barrier width (BW) technique

To conduct the interference study, both the techniques—PR technique as well as BW technique were employed with a number of test ions (Ru3+, Cs+, As5+, Cr6+, Cu2+, Na+, Mg2+, Al3+, K+, Sr2+, Mn2+, Fe3+, Fe2+, Zn2+). In each case a bicomponent solution with U6+ and one test ion in electrolyte solution of pH 3.3 was considered.

3.3.1 PR technique

For all the tests 4 ppb of U6+ was chosen. No interference was observed from 4 ppm of As5+, Na+, Mg2+, Al3+, K+, Sr2+, Mn2+, Fe3+, Fe2+, Zn2+ to 4 ppb of U6+. It was observed that 10 ppb of Cu2+, Cr6+ reduces 4 ppb U6+ peak by 25–30% while 5–10% reduction in peak for 4 ppb of U6+ was observed in presence of 15 ppb Ru3+ and 25 ppb Cs+. In presence of 10 ppb of Cu2+ and Cr6+ each, the LOD was 4 ppb while in presence of 15 ppb of Ru3+ and 25 ppb of Cs+, the LOD was 3 ppb. To understand the mechanism of interference, BW technique was employed. The details of the PR technique are mentioned in Table 1.

Table 1 Interference results obtained from PR technique
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p-tertbutyl calix-[4]-arene has been successfully used as macromolecular ligand to capture U6+ ion from water [90,91,92]. Duncan and Cockayne, have used a derivative of this ligand in PVC based ion selective electrodes for detection of U6+ from wastewater comprising a various number of anions and cations. The limit of detection was 1 ppm. The limit of detection rises to 50 ppm with 10% waste background [90]. Gupta et al., have used commercially available p-tertbutylcalix-[4]-arene in PVC membrane for U6+ detection. They have checked the interference by fixed interference method and could detect U6+ till 3.9 × 10–5 M [91]. Ramirez et al., have used p-tertbutylcalix-[4]-arene in U6+ containing solution for its detection by optical means. They have checked the ligands ability if it can form complex with U6+. Quantitative analysis for U6+ detection and interference analysis were not performed in that work [92]. We could not find any literature where polyaniline-based composite along with p-tert butylcalix[4]arene was used as sensing material for U6+ ion capture. However some comparisons were made where polyaniline or GO or p-tert butyl calix[4]arene based sensing material were used to detect U6+ ion as shown in Table 2.

Table 2 Comparison of different polyaniline or GO or calixarene based electrochemical sensor to detect U6+
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From existing literature, it can be observed that only GO modified H4L ion imprinted sensor can detect U6+ till 523 ppt level (Li et al., 2021 [96]). However, based on the existing literature, it can be concluded that the novel composite was able overcome interference to better extent than several sensors reported in the literature, and it could detect U6+ ion in drinking water with a limit of detection of 4 ppb in presence of highly interfering species.

The sensor was used to detect U6+ in three commercially available packaged drinking water and tap water. It was successful in detecting U6+ with no significant interference.

3.3.2 BW technique

BW technique was employed to understand the underlying mechanism of the interference analysis conducted by PR technique. A detailed description of the methodology of the BW technique based on Simmon’s model is illustrated in our previous works [46, 47], not explained here. To calculate the barrier width for charge transport, following equation was used.

$$R_{ct} = R_{ct0} \exp \left( {\beta d} \right)$$
(1)

[46, 73, 74]

$$\beta = \sqrt {\frac{{2m_{e} \phi }}{{\hbar^{2} }}}$$
(2)

[46, 73, 74]

Where, Rct denotes the charge transport resistance of U6+, Rct0 is the charge transport resistance in blank electrolyte solution, d is the barrier width, β is the tunnelling constant calculated from Eq. (2), \({m}_{e}\) is the mass of electron, \(\phi\) is the Barrier height (extracted from LUMO) and \(\hslash\) is the Planck’s constant. Using CV and EIS, barrier width values were calculated. To calculate the barrier height, first the LUMO values were calculated as described in our previous work [46] and from these values barrier height values were evaluated [97,98,99,100].

To perform EIS experiments the concentrations of U6+ and the interfering ions were same as mentioned in Table 1. From the EIS experiments Nyquist plots were obtained for each of the bicomponent solution case from which equivalent circuit was constructed. Figure 7 shows the experimental Nyquist plot obtained from EIS experiment for 4 ppb of U6+. The equivalent circuit fitted to the plot is shown in inset of Fig. 7. The equivalent circuit consists of four elements – solution resistance RS, charge transport resistance Rct, double layer capacitance CDL and Warburg impedance W. From the LUMO values obtained from CV and Rct values from EIS, the barrier width values for different cases were calculated. The barrier width for U6+ was 0.244 Å. Table 3 shows the barrier width values for U6+ in presence of interference species. No change in d values were observed to 4 ppb concentration of U6+ in presence of 4 ppm of As5+, Na+, Mg2+, Al3+, K+, Sr2+, Mn2+, Fe3+, Fe2+, Zn2+.

Fig. 7
figure 7

Nyquist plot obtained for the film in presence of 4 ppb of U.6+ in electrolyte solution of pH 3.3 –experimental (solid line) and fitted to the equivalent circuit (symbol) (inset—equivalent circuit)

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Table 3 Parameters obtained from BW technique
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From Table 2 it can be observed that presence of 10 ppb of Cr6+ and Cu2+, each, increases the d value to almost 5 times raising hindrance to charge transfer and decreasing the sensing signal by 25–30%. Comparative to them, presence of Ru and Cs increases it to almost 1.5 times leading to 5–10% reduction in sensing signal. Therefore, the barrier width value supports the analysis made by PR technique.

3.4 Calculation of the electro-active surface area of the novel composite film

Electroactive surface area is a key factor which has a strong influence in selectivity and sensing. Therefore, to investigate the reason behind the good sensitivity and selectivity of the novel composite, the electroactive surface area of the film was calculated and compared with the films synthesized from (i) aniline, (ii) aniline-N-phenylglyine, (iii) aniline-GO and (iv) aniline-N-phenylglycine –GO as described in our previous works [46, 47]. All the films were synthesized by chronopotentiometry. The film thicknesses were maintained at 12 µm. CV was performed with electrolyte containing 5 mM [Fe(CN)6]−3/−4 in 0.1 M KCl. To calculate the electroactive surface area, the Randles–Sevcik equation [75],

$$I_{p} = 2.69 \times 10^{5} A \times D^{1/2} n^{2/3} C\upsilon^{1/2}$$
(3)

was used where, \({I}_{p}\) denotes the maximum current in Amp, A is the electrode area (1.5 cm2), \(D\) is the diffusion coefficient of Fe(CN)6–3/−4 solution (7.6 × 10–6 cm2 s−1) [101] in cm2 s−1 (in our case similar value was obtained), \(n\) is the number of electron transferred (here \(n=1\)), \(C\) is the concentration in mol cm−3 (5 mM) and \(\upsilon\) is the scan rate in mV s−1. The electroactive surface area of all the films are mentioned in Table 4.

Table 4 Comparison of the different electrodes based on their electro-active surface area
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From Table 3 it is clear that the electroactive surface area of the novel composite electrode was the highest as compared with other electrodes which makes it enable for maximum U6+ ion intake. The advantage of the novel composite lies in its synthesis procedure and can be used as it does not require any surface modification for U6+ detection and it can overcome interference to a great extent.

4 Conclusion

In this work, a novel composite was synthesized from aniline, N-phenylglycine, GO and p-tertbutylcalix[4]arene. SWV experiments to detect U6+ were performed and LOD of 900 ppt (~ 2.273 nM) was obtained. The sensitivity of the film was 0.201 mA [p[U6+]]−1. To understand the interaction of the analyte to the electrode surface, thermodynamic analysis was carried out. Adsorption isotherm was constructed and − ∆Gads value was found to be 60.750 kJ mol−1 indicating a strong chemisorption process. Interference study was carried out with a large number of test ions (Ru3+, Cs+, As5+, Cr6+, Cu2+, Na+, Mg2+, Al3+, K+, Sr2+, Mn2+, Fe3+, Fe2+, Zn2+) by both PR technique and BW technique. The PR technique showed that only Ru3+, Cs+, Cr6+ and Cu2+ interferes U6+ detection in both the cases. 10 ppb concentration of Cr6+, Cu2+ each shows 25–30% reduction in sensing signal while a 5–10% decrease in sensing signal was observed in presence of 15 ppb Ru3+ and 25 ppb Cs+ each. To understand the mechanism of interference BW technique was used. It was observed that the barrier width for only U6+ was 0.244 Å. In presence of 10 ppb of Cu2+ and Cr6+ each it increases to 1.126 and 1.221 Å respectively. In presence of 15 ppb Ru3+ and 25 ppb Cs+, the BW value of U6+ changes to 0.387 and 0.394 Å respectively. Therefore, BW technique results support the results obtained by PR technique. The sensing material synthesized in this work has been used for the first time for uranyl ion detection with less interference has the advantage in ease in synthesis procedure requiring no surface modification and leading to a LOD of 4 ppb for U6+ in presence of interfering ions, which is far below the permissible limit for U6+ in drinking water.