1 Introduction

HCQ is utmost commonly consumed as a first-line medication for auto immune based diseases in patients like systemic lupus erythematosus and rheumatoid arthritis and has increased wide media attention as a promising antiviral drug for usage versus severe acute respiratory syndrome (which causes COVID-19) [1,2,3,4,5,6,7]. Unfortunately, the origination of research on HCQ has preceded to misperception in the rheumatological community concerning the protection consequences of HCQ in its conventional customs. Medical examinations on COVID-19 and broadcasting courtesy have directed to communal awareness based on HCQ [7,8,9]. Due to the extensive usage of this medication, numerous analytical methods have been operated for the analysis of HCQ involving extraction ensued through nonaqueous titrimetric approach [10], assay-based methods, HPLC [11], fluorescence [12], ultraviolet spectroscopy [13], spectrophotometry [14], and tandem mass spectrometry [15]. Nonetheless, these analytical approaches show a variety of difficulties like costly and time-consuming instrumentation, complicated sample pre-treatment processes, necessity of skilled analyst, more consumption of compounds and high detection limit. Electrochemical approaches frequently have numerous benefits over these conventional methods like, easy and lesser price instrumentation, less consumption of analyte, high detection ability and rapid investigation time [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34]. Nanotechnology has become very popular in the sensor field in recent times due to the variety of profiles, dimensions and conformations. Nowadays, carbon based nano-materials like carbon-based nanoparticles, carbon nanotubes, carbon nano-composites, carbon dots and carbon nano-wires are too significant in various fields like sensors, supercapacitors, pharmaceutical and biological samples analysis and so on. In that, carbon nanotubes show very important applications such as larger surface area, more active sites, good electrical conductivity, high stability, and fine analyzing ability for the examination of medicinal samples and which might afford a significant and achievable platform for the electroanalysis and particularly in the formulation of electroanalytical sensor materials [35,36,37,38,39,40,41,42,43,44,45]. Corresponding to the overhead points, the application of carbon nanotubes is important to develop an electrochemical sensor with high sensitivity and the reduction of over potential based on modifying agents. In the present scrutiny, we developed the SDSMCNTPE for the analysis of HCQ in phosphate buffer solution (PBS) of pH 6.5 in presence of PC and AA molecules based on CV methodology. The high electro-catalytic capacity of SDSMCNTPE for the HCQ oxidation was verified in both real and commercial HCQ samples. A deep literature review guided that no effort has been completed on HCQ analysis at SDSMCNTPE. Thus, the purpose of this analysis was focused on the study and optimization of experimental circumstances for the HCQ detection in tablet sample by means of SDSMCNTPE as a fast, simple and sensitive tool.

2 Experimental

2.1 Materials and Reagents

SDS, PC, AA and sodium salts of monobasic (NaH2PO4) and dibasic phosphate (Na2HPO4) were got from HiMedia chemicals. CNTs are obtained from Sisco Research Laboratory. K4[Fe(CN)6] was bought from Sigma-Aldrich. All the used chemicals are of analytical reagent grade with 95–99.5% of purity. Distilled water was utilized during the preparation of all the chemical solutions. The required solutions of HCQ (0.1 mM), PC (1.0 mM) and AA (1.0 mM) are prepared by suspending the essential measure of corresponding solute in distilled water.

2.2 Instruments

CV and EIS are carried out with an electroanalytical instrument of model CHI-6038E (CH Instruments, USA). The electrochemical cell was constituted to this work attached by three electrodes, SDSMCNTPE and BCNTPE as the working, calomel electrode (Hg2Cl2 saturated with KCl) as reference and a Pt wire as counter electrode. The pH meter (Equiptronics) was used for adjusting pH values. This overall analysis was accomplished at the lab temperature of 25(± 2) ℃.

2.3 Preparation of Surfactant Modified Carbon Electrodes

The BCNTPE was prepared according to the procedure described by our previous works [46, 47]. We describe a trouble-free process to get SDS surfactant immobilized CNTPE. The scheme is constructed on the accumulation of surfactant molecules on the surface of CNTPE. Initially, the electrochemical response of HCQ at SDSMCNTPE is measured. SDS was immobilized on the surface of CNTPE by surface modification method using micropipette and the electrode was rinsed with distilled water.

3 Results and Discussion

3.1 Material Characterization

The response of the electrochemical sensor was related to its physical morphology. Typical FE-SEM imageries of distinctive electrode materials were revealed in Fig. 1. The field emission-scanning electron microscopy (FE-SEM) image of BCNTPE surface (Fig. 1a) was categorized through a dense tube-like shape which are detached and erratically arranged. Although, the FE-SEM picture of SDSMCNTPE showed vastly unlike morphology associated to BCNTPE, after the SDS immobilization on BCNTPE. Here, SDS was dispersed on the exterior of the CNTPE surface with consistent formation (Fig. 1b), representing that the SDS was efficaciously accumulated on CNTPE and promotes the electron transfer in HCQ oxidation.

Fig. 1
figure 1

FE-SEM pictures of a BCNTPE and b SDSMCNTPE

Full size image

3.2 Effect of Surfactant Concentration

The impact of boosted concentration of SDS in the range from 5.0 to 25.0 μL at CNTPE surface for the HCQ oxidation was examined by recording CVs and the obtained responses are shown in Fig. 2a. Here, the oxidation current of HCQ molecule in PBS at different concentration of SDS on CNTPE is varied due the molecular interactions among SDS and HCQ. Also, from Fig. 2b, the oxidation current of HCQ was higher at 10.0 μL of SDS than other SDS concentrations like 5.0, 15.0, 20.0 and 25.0 μL. This effect is most possibly due to the result of critical accumulation concentration of SDS on CNTPE surface. Then, 10.0 μL of SDS concentration was operated as finest concentration for highly sensitive HCQ oxidation throughout the experiment.

Fig. 2
figure 2

a CVs of 0.1 mM HCQ in 0.2 M PBS at the surface of CNTPE with different SDS concentrations in the range from 5.0 to 25.0 µL. b Plot of concentration of SDS vs. peak current of HCQ

Full size image

3.3 Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is the significant tool for quantifying the conductance and resistance of the used working electrode materials at the interface of PBS and electrode surface. The EIS outcomes for BCNPE (a) and SDSMCNTPE (b) are explicated through the utilization of Nyquist plots shown in Fig. 3. The EIS was accomplished for K4[Fe(CN)6] (1.0 mM) in KCl (0.10 M) having the potential of 0.10 V. From the Nyquist plots (a, b), the BCNTPE semicircle radius was higher than SDSMCNTPE, which specified that the charge transfer resistance (Rct) of SDSMCNTPE is lesser as compared to BCNTPE. This obtained data shows that, the conductance of SDSMCNTPE is more with more active cites and this is probably due to the strong interaction of SDS and CNTPE surface [46].

Fig. 3
figure 3

EIS plots of BCNTPE (a) and SDSMCNTPE (b)

Full size image

3.4 The pH Effect

The impact of pH is the important feature to get the expectant solution pH shows superior oxidation peak current for HCQ. The solution pH impacts the oxidation nature of HCQ at SDSMCNTPE with respect to peak current and potential. The influence of solution pH was analyzed for the oxidation of HCQ in different pHs in the range from 6.0 to 8.0 on SDSMCNTPE via CV technique at 0.1 V/s scan rate. The CVs for 1.0 mM HCQ at SDSMCNTPE show the movement of oxidation peak potential towards the negative direction as the intensification of pH of PBS (Fig. 4a). The pH vs. Epa relation parades a good linearity (Fig. 5b) with the linear regression equation is Epa(V) = 1.2847–0.0720 pH (V/pH) (R = 0.9931). Here, the slope value (-0.070 V/pH) is nearer to the hypothetical value which describes that the oxidation nature of HCQ is proceeds over the transport of alike number of protons and electrons [48,49,50]. Moreover, Fig. 5b reveals that, the 6.5 pH imparts higher oxidation current of HCQ, hence this was selected as optimal pH for this study.

Fig. 4
figure 4

a CVs of 0.1 mM HCQ at SDSMCNTPE in 0.2 M PBS having altered pHs (6.0–8.0) at the potential gap of 0.5 V–1.0 V with the scan rate of 0.1 V/s. b Plot of pH vs peak potential and pH vs Ipa

Full size image
Fig. 5
figure 5

a CVs of 0.1 mM HCQ on SDSMCNTPE in 0.2 M PBS (pH 6.5) based on different scan rate ranging from 0.1 V/s–0.3 V/s with the potential gap of 0.5 V–1.0 V. b Plot of log scan rate vs. log peak current

Full size image

3.5 Effect of Scan Rate on HCQ at SDSMCNTPE

The scan rate influence on the oxidation reaction of 0.1 mM HCQ (with 0.2 M PBS having pH of 6.5) at SDSMCNTPE was studied via CV method by varying the scan rate from 0.10 to 0.30 V/s having the potential difference from 0.5 to 1.0 V. The CVs of HCQ in Fig. 5a, the oxidation current of each CV sequence is enhanced with small alteration in the potential towards the positive way as the rise of scan rate. Figure 5b specifies the graph log oxidation current (log Ipa) vs. log scan rate (log υ) influencing a good linear agreement with the linear regression equation is log (Ipa, μA) = 1.5141 + 0.8247 log (υ, V/s) (R = 0.9902). The obtained slope value suggests that, the electro-catalytic action of SDSMCNTPE for the oxidation response of HCQ was completed by the adsorption-controlled pathway [51,52,53].

3.6 Electrochemical Response of HCQ

The electrochemical oxidation response of 0.1 mM HCQ (presence and absence) in 0.20 M PBS with pH 6.5 on SDSMCNTPE and BCNTPE was evaluated by utilizing CV method at the potential interval of 0.5–1.0 V at the scan rate of 0.1 V/s. The CVs in Fig. 6 reveals that, the HCQ absence (blank) in 0.2 M PBS having pH 6.5 at SDSMCNTPE display no electrochemical reply. Nevertheless, the CVs for HCQ at SDSMCNTPE and BCNTPE presents the electrochemical reaction with comparable oxidation peak current and potentials for HCQ. The BCNTPE discloses a very low electrochemical oxidation response for HCQ with smaller peak current compared to the SDSMCNTPE. This data explains that, the SDSMCNTPE shows higher sensitivity with enhanced electro-catalytic action for the oxidation of HCQ with quicker rate of proton and electron movement than at BCNTPE. Also, the electrochemical response of HCQ at SDSMCNTPE was observed to be enhanced by five fold compared to that noticed at BCNTPE. This could be due to the presence of higher surface area. The possible oxidation reaction mechanism of HCQ on SDSMCNTPE is shown in Fig. 6b [54].

Fig. 6
figure 6

a CVs for the presence and absence (blank) of 0.1 mM HCQ on SDSMCNTPE and BCNTPE in 0.2 M PBS of 6.5 pH at the potential gap of 0.5–1.0 V with the scan rate of 0.1 V/s. b Possible oxidation reaction mechanism of HCQ on SDSMCNTPE

Full size image

3.7 Electroactive Surface Area

CV was implemented for the analysis of active surface area of BCNTPE and SDSMCNTPE based on the electrochemical response of 1 mM K4[Fe(CN)6] in 0.10 M KCl with the scan rate of 0.1 V/s within the potential range of − 0.20 to 0.60 V (Fig. 7). The redox peak response of standard analyte at BCNTPE and SDSMCNTPE is displayed in Fig. 7, here SDSMCNTPE shows less redox potential with raised redox current as compared to BCNTPE. This outcome is due to the more active sites, decrease of overpotential and high surface area of the SDSMCNTPE. The active surface area of the operated electrochemical sensors was verified using the Randles–Sevcik equation [33, 55],

$${\text{I}}_{{\text{p}}} \, = \,{2}.{69}\, \times \,{1}0^{{5}} {\text{n}}^{{{3}/{2}}} {\text{A D}}^{{{1}/{2}}} {\text{C }}\upsilon^{{{1}/{2}}}$$
(1)
Fig. 7
figure 7

CVs for 1.0 mM K4[Fe(CN)6] in 0.1 M KCl on SDSMCNTPE and BCNTPE at the potential gap of 0.5 V–1.0 V with the scan rate of 0.1 V/s

Full size image

here, A is the electrochemical active surface area (cm2), Ip is the analyte peak current (A), D is the analyte diffusion coefficient (cm2/s), n is the number of electrons, υ is the scan rate (V/s) and C is the analyte concentration (M). The premeditated electro-active surface area of SDSMCNTPE and BCNTPE are obtained to be 0.044 cm2 and 0.014 cm2, correspondingly.

3.8 Simultaneous Determination of HCQ, PC and AA

The instant examination of HCQ with PC and AA in 0.20 M PBS (pH = 6.5) at BCNTPE and SDSMCNTPE was completed through the operation of CV at 0.1 V/s scan rate (Fig. 8). At BCNTPE, HCQ, PC and AA moieties displays feeble electrochemical action with lesser sensitive peak currents at higher potentials. Nevertheless, at SDSMCNTPE, HCQ, PC and AA parades preeminent electrochemical oxidation with greater peak currents and decreased potentials at 0.854, 0.358 and 0.0491 V, respectively. This evidence reveals that, SDSMCNTPE is highly active for the analysis of HCQ in the occurrence of PC and AA with elevated electrocatalytic achievement and rapid electron transport rate than BCNTPE.

Fig. 8
figure 8

CVs for HCQ, PC, and AA in 0.2 M PBS at SDSMCNTPE and BCNTPE at the potential gap of − 0.2 to 1.0 V with the scan rate of 0.1 V/s

Full size image

3.9 Calibration Curve

The CVs at SDSMCNTPE (Fig. 9) attained with consecutive addition of different concentration of HCQ at the optimized conditions. The analytical curve shown in Fig. 9a, b, displayed an acceptable linear assortment ranging from 10 to 40 µM and the regression equation obtained was Ipa = 1.4245×10−5 + 0.2929 [HCQ] (M), (R = 0.9995). The detection limit and quantification limit are calculated using the formulations [56],

$${\text{Detection limit}}\, = \,{3}.0\left( {\text{standard deviation of the blank}} \right)/\left( {\text{slop of analytical curve}} \right)$$
(2)
$${\text{Quantification limit}}\, = \,{1}0.0\left( {\text{standard deviation of the blank}} \right)/\left( {\text{slop of analytical curve}} \right)$$
(3)
Fig. 9
figure 9

a CVs for different concentrations of HCQ on SDSMCNTPE in 0.2 M PBS of pH 6.5 at the potential gap of 0.5 V–1.0 V. b Plot of concentration of HCQ vs. peak current

Full size image

The premeditated detection limit and quantification limit values were obtained to be 0.85 and 28.08 µM, which are decent values as associated to some reported HCQ sensors shown in Table 1 [17, 57]. Also, the electrode sensitivity was calculated based on surface area and slope of the calibration plot and the value was found to be 6.65 M/A/cm2.

Table 1 Comparison of previously reported HCQ sensors with the present HCQ sensor
Full size table

3.10 Application to Pharmaceutical Formulation Analysis

The adaptability of SDSMCNTPE was confirmed through the standard addition scheme towards the detection of HCQ in pharmaceutical tablet sample. The HCQ tablet sample was diluted with 0.2 M PBS (pH = 6.5) and subjected to the CV method separately having various concentration on SDSMCNTPE at optimized conditions. The HCQ tablet sample at SDSMCNTPE displays a good recovery. The premeditated results of percentage recovery of HCQ in tablet are tabulated in Table 2. The achieved results proposed that the SDSMCNTPE is a good substitute for HCQ detection in tablet sample in the occurrence of other interferents.

Table 2 Premeditated results of percentage recovery of HCQ in tablet sample
Full size table

3.11 Reproducibility and Stability of SDSMCNTPE

The reproducibility of the SDSMCNTPE was assessed by cycling five CV cycles for HCQ (fixed) in PBS at five distinctly prepared SDSMCNTP electrodes. Here, the prepared SDSMCNTPE delivers the relative standard deviation (RSD) of 1.3%, signifying an adequate reproducibility. The stability of SDSMCNTPE for HCQ in PBS was examined via CV technique by keeping the electrode for 2 days in a closed vessel. The retainance of 97.28% of the preliminary current presents the adequate stability of SDSMCNTPE.

4 Conclusion

The current effort suggests a receptive and modest SDSMCNTPE for the examination of HCQ through CV method, which shows the heightened electrode conductivity, sensitivity and selectivity. The investigative process has been completely justified by fine linearity, accurateness, sensitivity and reproducibility owing to the improvement of the rate of electron transfer progression with a greater fundamental electroactive surface area and active sites of SDSMCNTPE. This analysis devotes a modest, robust and precise analysis of HCQ and its significance in tablet sample analysis. Additionally, this SDSMCNTPE delivers an exceptionally appropriate and applicable technique for the concurrent detection of HCQ, PC and AA.