TiO2@GO composite coated GC electrode for

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Fabrication of SnS/TiO2@GO composite coated GC electrode for concomitant determination of paracetamol, tryptophan and caffeine in pharmaceutical formulations Eagambaram Murugan, and kalpana kumar Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05531 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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Analytical Chemistry

Fabrication of SnS/TiO2@GO composite coated GC electrode for concomitant determination of paracetamol, tryptophan and caffeine in pharmaceutical formulations Eagambaram Murugan* and Kalpana Kumar Department of Physical Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai, 600025, Tamil Nadu, India Email: [email protected] Abstract Designing an electrochemical sensor which is simple, cheap, sensitive, fast and accurate is inevitable, as it is important in drug quality control, point-of-care diagnosis and other clinical studies. Sensors for simultaneous determination of paracetamol, tryptophan and caffeine has not been reported so far, and we report a electrochemical sensor via incorporating tin sulphide(SnS) and titanium dioxide(TiO2) on graphene oxide sheets(GO)(SnS/TiO2@GO ternary composite) for separate and their simultaneous determination through cyclic voltammetry and differential pulse voltammetry techniques. The surface morphology and structural properties of the composite were characterized by analytical techniques. The electrochemical study of SnS/TiO2@GO composite modified glassy carbon electrode(GC-SnS/TiO2@GO) showed high activity towards the oxidation of paracetamol, tryptophan and caffeine with significant decrease in over potential due to large surface, high carrier mobility. The peak currents during separate determination of paracetamol, tryptophan and caffeine increased linearly with the increase in concentration from 9.8 nM to 280 µM for paracetamol, 13.3 nM to 157 µM for tryptophan and 16.6 nM to 333 µM for caffeine. The detection limit (3σ/S) was 7.5, 7.8 and 4.4 nM for paracetamol, tryptophan and caffeine, respectively. The electron transfer coefficient(α), surface coverage concentration(Г), number of the electrons transferred(n) and diffusion coefficient(D) were calculated and discussed. The fabricated electrode showed low detection limit, wide linear range, excellent reproducibility, selectivity and stability. The study was also extended to the analysis of commercial tablets and different beverages. Therefore, the present electrode can hold a great promise for identification and quantification of drugs in combination. Key words: SnS/TiO2@GO Ternary Composite, Paracetamol, Tryptophan, Caffeine, Differential Pulse Voltammetry. It is very much essential to develop simple, sensitive, and accurate methods for detecting active ingredients in drug formulations, since drug monitoring plays an important role in drug quality control, and it has a great impact on public health. Paracetamol (Para), one of the most commonly used antipyretic and analgesic drugs is also known as acetaminophen. It relieves pain, headache and fever, and is usually combined with other active ingredients in medicines for treatment of cold, flu, allergy, sleeplessness and severe pain associated with arthritis etc.[1-6]. However, overdoses of para cause severe liver damage and 1 ACS Paragon Plus Environment

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high accumulation of toxic metabolites, and can be fatal hepatotoxicity

[7,8]

and nephrotoxicity

[9].

Caffeine (Caf) is a natural alkaloid stimulant to the central nervous system commonly used in energy drinks, beverages (coffee, tea), food supplements and pharmaceutical formulations

[10].

It is also used

therapeutically in the treatment of migraine, in combination with nonsteroidal anti-inflammatory drugs and often analgesic pharmaceutical formulations because of its diuretic activity

[11].

In spite of that, on

consuming the excessive amount it may cause several mutation effects namely inhibition of DNA repairing, cyclic AMP phosphodiesterase activity, it can also induce cancer, heart diseases and complications for pregnant women

[12,13].

Tryptophan (Trp) is an essential amino acid for human and a

vital element for indispensable growth of human for positive nitrogen balance. It cannot be synthesized directly from the human body, but it can be obtained from food supplementations and pharmaceutical formulations. The Trp concentration in plasma plays a crucial role in the synthesis of the neurotransmitter serotonin. But it should not be metabolized by Trp-2,3-dioxygenase. Administration of Para can enhance the availability of Trp by inhibiting the action of Trp-2,3-dioxygenase[14]. It was also reported that Caf does similar function as that of Para in enhancing the availability of Trp, which in turn generates serotonin and 5-hydroxyindole acetic acid in brain

[15].

If dolopar-500 mg is administered, both Para and

Caf enter human body and Trp enters the body through food, and hence all the three components are to co-exist. So, if there is an analytical technique to quantitatively estimate them simultaneously it can very well be applied for the determination of the same in a human body by which it could be monitored their availability and activity. Currently, a variety of analytical methods have been developed to detect these drugs separately, which include titrimetry, UV–vis spectrophotometry, capillary electrophoresis, high performance liquid chromatography, and electrochemical techniques

[16-21].

All these methods have some

disadvantages including tedious extraction process before detection, time consuming, high cost and requirement of pretreatment. In contrast, the electrochemical technique shows selectivity, high sensitivity, simple instrumentation, low cost, facile miniaturization and quick response. Since, all the above drugs are electroactive, electro analytical techniques could be strong alternatives for their determination

[22,23].

Moreover, detection of these compounds in the coexisting status is rather difficult and surprisingly, no electrochemical procedure exists for the simultaneous determination of Para, Caf and Trp. Graphene with high surface area and high conductivity has established tremendous application in different arena

[24-28],

and also facilitated electron transfer reaction between an electrode and electrolyte

interface. Graphene oxide (GO) is the hydrophilic oxidative derivative of graphene, and owing to its oxygen functionalities and direct electron transfer properties, it acts as a sensitive electrodic surface as well as a two dimensional nanocarbon matrix for anchoring active materials

[29].

Murugan and Kalpana

reported oxidation of formic acid through fabricating GC-r-GO-PAMAM(G3)-Pd electrode which results in less negative oxidation potential

[30].

Meanwhile, over the past few years, a vast variety of 2

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nanostructured active materials including metals

[31-32],

metal oxides, metal hydroxides and metal

chalcogenides have been used for electrochemical sensors due to their size dependent properties, nontoxicity and long-term stability. Among these active materials, metal oxide such as TiO2 has emerged popular due to its large specific surface area, excellent biocompatibility and high uniformity

[33].

Metal

chalcogenides such as tin based chalcogenides have received intensive research attention in recent years in which SnS has great advantage over SnS2 due to its hole concentration adjustment, layer dependent physical properties and smaller band gap (~1.43 eV)

[34].

So, in the present work, it was aimed to

incorporate both SnS and TiO2 on GO to form a ternary composite, so as to prompt the electrochemical activity. Moreover, synthesis of the same ternary composite has not been attempted by any researchers for any application. Ali Babaei et al reported a sensor based on MCM-41-Ni(OH)2 nanoparticle MWCNT modified GC electrode for sensitive simultaneous determination of levodopa, paracetamol and tryptophan[14]. In continuation of this, we have fabricated novel, simple, cheap, fast, sensitive GCSnS/TiO2@GO electrode for the first time using SnS/TiO2@GO ternary composite for the simultaneous determination of Para, Trp and Caf by Differential Pulse Voltammetry technique. To the best of our knowledge, no electrochemical sensor has been reported for the simultaneous determination of Para, Trp and Caf to the best of our knowledge. The practical applicability of the prepared electrode was also demonstrated for the determination of the same drugs using commercial tablet and different beverages. 1. Experimental Section Powdered Graphite (particle size: 48mm, 99.95% purity), hydrazine hydrate, titanium isopropoxide, paracetamol, caffeine and tryptophan were purchased from Sigma Aldrich, and used as received. Thiourea and ethylene glycol were obtained from Sisco Research Laboratories PVT Ltd and used. Tin(II) chloride dihydrate was purchased from Merck. Phosphate buffer solution (PBS) was prepared by mixing 0.1 M NaH2PO4 and 0.1 M Na2HPO4. HPLC water used for HPLC analysis is purchased from Sigma Aldrich. 1.1. Synthesis of SnS microspheres The hydrothermal synthesis of SnS microspheres was carried out using SnCl2.2H2O and thiourea as tin and sulfur precursors, respectively

[35].

Initially, 0.07g of SnCl2.2H2O and 0.280g of thiourea were

dissolved in 30 mL of ethylene glycol separately. Then, SnCl2 solution was added to thiourea solution dropwise under magnetic stirring. The suspension become dark brown with the addition of SnCl2 solution. Then, the mixture was transferred to a 100 mL Teflon-lined autoclave, and heated at 170 ºC for 24 hrs. The resulting grey colour product was centrifuged, washed with double distilled water and ethanol, and finally dried at 80 ºC for 3 hrs in a vacuum oven. 1.2. Synthesis of SnS/TiO2@GO ternary composite

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SnS/TiO2@GO ternary composite was prepared by hydrothermal method. First, graphene oxide (GO) was prepared from graphite powder using modified Hummer's method

[36].

0.05 g of GO was taken and

dispersed in 20 mL of distilled water by sonication. Then, 0.014 g of SnS and 0.2 mL of titanium isopropoxide were added under stirring to get homogeneous solution. After 30 min, the suspension was transferred to a 100 mL Teflon lined autoclave, and heated to 120 ºC for 8 hrs. The resulting black colour product was then repeatedly washed with double distilled water for five times and the precipitate was then dried in a vacuum oven at 80 ºC. Finally, the dried sample was calcined in a furnace at 530 ºC for 3 hrs. 1.3. Fabrication of SnS/TiO2@GO ternary composite modified GC electrode (GCE) Prior to the fabrication, GCE was polished to a mirror like surface and sequentially with finer grade alumina powders having a grain size of 0.05 micron. Then, the electrode was rinsed with ethanol and distilled water, and dried at room temperature. Then, 10 mg of the SnS/TiO2@GO ternary composite was dispersed in 10 mL water, and it was used as a stock solution. Prior to the surface coating, the stock was sonicated for 5 min and from that stock solution, 5 μL of the suspension was withdrawn and drop-coated on the pre-treated GCE, and allowed it to dry in ambient temperature to form the GC-SnS/TiO2@GO electrode. Similarly, control electrodes were also prepared following the same procedure, and obtained GC-GO, GC-SnS and GC-TiO2 electrodes. 1.4. Real sample preparation and measurement procedures A human blood sample obtained from normal human was solidified at 3°C and centrifuged at 4000 rpm for 15 min at room temperature. From the serum thus obtained, 1 mL was pipetted out and added into a volumetric cell containing 25 mL of 0.1M PBS without any further pretreatment and the resulting solution labeled as sample 1. Further, the stock solutions were prepared using 5 µM of Dolopar-500 tablet suspension for both Para and Caf content, 5 µM of L-tryptophan-500mg tablet suspension for Trp content and labeled as sample 2, respectively. The simultaneous determination of paracetamol, tryptophan and caffeine was carried out through the addition of sample 2 to the above serum solution. Similarly, an another set of analysis was also carried out using 10 µM of Dolo-650 tablet suspension for Para content, 10 µM of L-tryptophan-500 tablet suspension for Trp content and 5 µM of Red Bull beverage for Caf content and labeled as sample 3. The HPLC analysis of the above solutions was also carried out using C18 column with the mobile phase containing acetonitrile and phosphate buffer (60:40 (v/v)) at ambient temperature and the elution was monitored with the UV detector at 254 nm. The concentration of serum, Para, Trp and Caf used in this method was same as in voltammetric method. 2.Results and discussion 2.1. Morphology studies of SnS/TiO2@GO ternary composite The morphological features of the SnS/TiO2@GO ternary composite and its controls GO and SnS were studied with SEM, and the images are shown in Fig.1. The image showed no evidence of multi-layer 4 ACS Paragon Plus Environment

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stacks of graphite, thus confirming the formation of layered GO (Fig.1(a)). Fig.1(b) shows the magnified SEM image of synthesized SnS. It showed a spherical micro flower structure constructed by curved nanoflakes

[37,38].

Fig. 1(c) demonstrates a general view of the SnS/TiO2@GO ternary composite, it

showed a spherical micro flower embedded GO sheets and Fig.1(d) is the magnified view of the composite. The EDAX spectrum of ternary composite is shown in Fig.2, and its result are shown in Table. S1. It confirms the presence of elements such as C, O, S, Sn and Ti that constitute the elements of the electrode

[39].

Similarly, the EDAX spectrum and results obtained for SnS

[40]

showed 0.645 mole of Sn

and 0.718 mole of S, it nearly matches the stoichiometry of SnS (Fig.S1. and Table. S2).

Fig.1. SEM images of (a) GO, (b) SnS microsphere (c) & (d) SnS/TiO2@GO ternary composite

Fig. 2. EDAX of SnS/TiO2@GO ternary composite 2.2. XRD, FT-IR and Raman analyses SnS/TiO2@GO ternary composite The XRD patterns of GO, SnS microspheres and SnS/TiO2@GO ternary composite are shown in Fig.3(a)(c). The diffraction peak at 2θ = 11.13 is due to the (002) of GO (Fig.3(a))[41]. Fig.3(b) shows peak positions and intensities of SnS at 2θ = 22.26° (111), 31.65° (110), 39.07° (120), 42.88° (021), 45.47° (101), 51.30° (112) and 56.43° (131), and the observed peaks were indexed to orthorhombic phase of SnS, which is in good agreement with JCPDS no. 39-0354

[42].

In addition, the intense and sharp

diffraction peaks indicate high crystallinity of the microspheres, and the absence of SnS2 and SnO2 peaks confirms the formation of pure SnS phase. On comparing with the XRD pattern of GO, the SnS/TiO2@GO ternary composite displayed additional peaks at 2θ = 25.42°, 38.00°, 48.28° ascribed to 5 ACS Paragon Plus Environment


TiO2

[43,44]

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and the major peaks of SnS (Fig.3(c)). The interaction between SnS and TiO2 in

SnS/TiO2@GO is clearly evident by the shift in the 2θ values of its SnS reflections to lower values than that of free SnS in their XRD patterns: for example, the reflections at 22.2599°, 31.5797° and 42.8757° of SnS were shifted to 22.1931°, 29.9228° and 42.3115° of SnS in SnS/TiO2@GO, respectively. In addition, the corresponding d spacing values were also shifted: for example, the d spacing values 3.9973, 2.8332 and 2.1112 A° of the above reflections of free SnS were shifted to 11.8206, 8.2474 and 2.9861 A° in SnS of SnS/TiO2@GO, respectively. The average crystal size of TiO2 was calculated using the Scherrer equation by choosing the 2θ values of intense reflections at 25.42° and 48.28°. The full width at half maximum for peaks at 25.42° and 48.28° was equal to 0.5353 and the average crystal size was equal to 16.35 nm. Similarly, the average crystal size of SnS was calculated using the intense reflections at 31.65° and 39.07°. Their full width at half maximum was equal to 0.4015 and the average crystal size equal to 21.3 nm. So the microsphere observed in the SEM image shown above, is verified as nanoaggregates. All these results strongly confirms the incorporation of SnS and TiO2 on GO which is also matching well with Raman results.

Fig. 3. XRD patterns of (a) GO, (b) SnS microsphere and (c) SnS/TiO2@GO ternary composite Fig.4. shows the FT-IR spectra of GO, SnS, TiO2 and SnS/TiO2@GO ternary composite. The broad band observed at 3428 cm-1 is due to the stretching vibration of O-H group present in GO (Fig.4(a)). Subsequently, the peaks corresponding to C-O and C=C groups were observed at 1042 and 1630 cm-1, respectively. Similarly, the SnS showed its Sn-S stretching vibrations at 574, 1029, and 2359 cm−1 indicating the formation of orthogonal SnS [45,46]. A new broad band at 632 cm-1 in Fig.4(c) is due to Ti-OTi bond vibration in SnS/TiO2@GO ternary composite

[47,48]

and it showed the combined characteristic

peaks of their controls, thus verifying the formation of ternary composite which strongly agrees with the previous reports.


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Fig. 4. FT-IR spectra of (a) GO, (b) SnS microsphere and (c) SnS/TiO2@GO ternary composite To further explore the structural property of the SnS/TiO2@GO ternary composite, it was analysed by Raman spectroscopy. Fig.5 and inset shows the comparative Raman spectra of GO, SnS, TiO2 and SnS/TiO2@GO ternary composite. The Raman spectra of GO (Fig.5 inset) showed two bands at 1361 and 1590 cm−1 corresponding to the D and G bands, respectively. The peaks observed at 70, 109, 154 and 223 cm−1 in Fig.5 (inset) matched with the orthorhombic structure of SnS, and they are assigned to the Ag, B2g and 2Ag modes

[49].

As seen in Fig. 5, the SnS/TiO2@GO ternary composite showed the combined

characteristic peaks of GO, SnS, and a new set of peaks appeared at 142, 193, 227, 276, 398, 445 and 669 cm−1 corresponding to TiO2 [50], thus confirming coexistence of these compounds, and these results are in accordance with the XRD results.

Fig. 5. Raman spectra of GO and SnS microspheres (inset) and SnS/TiO2@GO ternary composite 2.3. Electrochemical behavior of SnS/TiO2@GO ternary composite As discussed in the experimental section, four different electrodes were fabricated, and Fig.6. shows the CV response of (a) bare GC, (b) GC-GO, (c) GC-SnS, (d) GC-TiO2 and (e) GC- SnS/TiO2@GO 7 ACS Paragon Plus Environment


electrodes with (Fig.6) and without (Fig.S2) paracetamol containing 0.1 M PBS at a scan rate of 50 mVs1.

It can be seen that in the absence of paracetamol, no peak potential was observed with (a) bare GC, (b)

GC-GO, (c) GC-SnS, (d) GC-TiO2 and (e) GC-SnS/TiO2@GO electrodes, whereas in the presence of paracetamol (16.5 µM) bare GC and GC-GO showed a redox behavior with high over potential. In contrast GC-SnS and GC-TiO2 showed significant decrease in over potential with peak potentials of 0.42 & 0.33V and 0.49 & 0.37V, respectively. The enhanced behavior of GC-SnS and GC-TiO2 electrodes is ascribed to the large surface to volume ratio. In the case of GC-SnS/TiO2@GO electrode, a distinct well defined redox peaks appeared at 0.40 & 0.31V with enhanced peak currents. The observed oxidation over potential of paracetamol in GC-SnS/TiO2@GO electrode was 0.18 V more negative than that of bare GC (0.58V), 0.11V than GC-GO (0.51V), 0.02 V than GC-SnS (0.42V), 0.09V than GC-TiO2 (0.49V). The oxidation of Para on GC-SnS/TiO2@GO electrode occurred at lower potential than GC-SnS and GC-TiO2 electrodes due to high electrical conductivity, availability of active absorption sites in SnS microspheres, defective sheets of GO transfer

[53].

[51,52]

and effective interface and microenvironment provided by TiO2 for electron

It also confirms electrical contact between both the semiconductors and GC, and uniform

distribution of the applied potential at GC over the outer most surface of the electrode. If SnS or TiO2 at the surface independently takes part in oxidation, then the oxidation of the drug might have occurred at two different potentials, but it occurred at a potential lower than that of both, so, a new species formed by interaction of both might be taking part in oxidation. As oxidation or reduction occurs at the electrode surface, direct contact of analyte on the electrode surface is a necessity. This condition requires better adsorption of the analyte on the surface of the composite electrode than the other two electrodes, as the oxidation potential is lowered. Therefore, there might be significant interaction, particularly of Lewis acid base type, between SnS and TiO2 in the matrix (i.e., bonding of sulphide sites with Ti sites). Since strong adsorption of analyte on the electrode surface leads to weakening of its bonds, it is oxidized at lower potential. The results demonstrate that GC-SnS/TiO2@GO electrode possess enhanced electrocatalytic activity towards paracetamol oxidation, which is in good agreement with DPV results (Fig.10.). Moreover, the detailed electrocatalytic activity of GC-SnS/TiO2@GO electrode was studied through CVs with different paracetamol concentrations from 0.093 to 200.51 µM (Fig.7(a)). The peak currents increased linearly against the paracetamol concentration. Fig.7(b), shows two linear relationships in the range from 0.093 to 10.07 µM and 20.35 to 200.51 µM, as Ipa [10-6] = -2.2565 [Para/µM] - 1.0908 (R2=0.9882) & Ipc [10-6] = 1.0531 [Para/µM] + 0.8411 (R2=0.9914), and Ipa [10-6] = -0.0172 [Para/µM] 3.2658 (R2=0.9876) & Ipc [10-6] = 0.0062 [Para/µM] + 1.9085 (R2=0.9746), respectively.


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Fig.6. Cyclic voltammograms of (a) bare GC, (b) GC-GO, (c) GC-SnS, (d) GC-TiO2 and (e) GCSnS/TiO2@GO electrodes in the presence of 16.5 µM paracetamol in 0.1M PBS at a scan rate of 50mVs-1

Fig.7. Cyclic voltammograms of effect of [Para] in 0.1M PBS at a scan rate of 50mVs-1 at (a) GCSnS/TiO2@GO electrode at different concentrations of paracetamol and (b) Plot of [Para] vs redox peak currents. 2.4. Effect of pH and scan rate The pH of the electrolyte can influence the peak shape, peak potential and peak current of the modified electrode and also it is more useful in the estimation of proton to electron ratio involved in the electrode reaction. As shown in Fig.8, the effect of pH was studied from 3 to 11 at a scan rate of 50 mVs-1. The observed peak potentials were shifted towards negative with the increase in pH thus confirming direct involvement of proton in the rate determining step

[54].

On comparing the peak currents of paracetamol,

the maximum response was observed at pH 7, which is a physiological pH, hence the same pH was selected for further determination. Fig. 8 (inset) shows the linear relationship between the anodic peak potential and pH with a linear regression equation of Epa(V) = -0.04521(pH) + 0.77634 (R=0.9943). The slope was close to Nernst equation, and this indicates the involvement of equal number of proton and 9 ACS Paragon Plus Environment


electrons in the Para oxidation at GC-SnS/TiO2@GO electrode. The number of electrons transferred in the oxidation reaction is calculated as 2 using equation (1) [55] through substituting the slope value of Epa vs. pH (Fig. 8 inset). E=E0-0.0591pH/n .…………………………. (1) Where E is the intercept, E0 the initial potential and n the number of electrons transferred during oxidation. These results are in good agreement with findings of the earlier studies and proposed mechanism as shown in scheme S1. To establish the transport characteristics of the GC-SnS/TiO2@GO electrode, the influence of scan rate at 100 µM paracetamol was studied in the range 10-100 mVs-1. As shown in Fig.9(a), the anodic and cathodic currents increased linearly, and the redox peak potentials shifting towards more positive direction

[56].

On plotting log Ip of anodic and cathodic vs log scan rate separately (Fig.9(b)), two linear

lines were observed from 10 to 30 and 40 to 100 mVs-1 and their slopes were equal to 0.35 and 0.93 for cathodic 0.37 and 0.91 for anodic, respectively. These results suggest that the electrode involves mixed diffusion and surface confined process

[57,58].

Degree of surface coverage also affects high over potential,

fouling and interference of other electroactive species, and based on the Laviron's model, the surface coverage concentration of GC-SnS/TiO2@GO electrode was calculated as 3.29 nMcm-2 from the following equation 2

[59].

This result also supports improved conductivity of the GC-SnS/TiO2@GO

electrode. Ip=n2F2AГυ/4RT……………………………. (2) Where Ip is the oxidation peak current, n the number of electrons transferred, F the Faraday constant, Г the surface coverage concentration, A the surface area, υ the scan rate, R the gas constant and T the temperature (K). To know more about electron transfer kinetics, diffusion coefficient was also calculated using Randles–Sevcik equation (equation 3)

[55]

and it was equal to 2.13 x 10-5 cm2 S-1. The observed diffusion

coefficient is comparatively higher than previous reports indicating that this oxidation of para involved fast electron transport [60]. Ip=2.69×105n3/2AD1/2Cν1/2……………………….(3) Where Ip is the oxidation peak current, n the number of electrons transferred, D the diffusion coefficient, C the concentration of para, A the surface area and υ the scan rate.


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Fig.8. Cyclic voltammograms of the effect of pH in 0.1 M PBS at a scan rate of 50mVs-1 at GCSnS/TiO2@GO electrode

Fig.9. Cyclic voltammograms of the effect of scan rates in 0.1M PBS at (a) GC-SnS/TiO2@GO electrode and (b) plot of log cathodic/anodic current vs log ν. 2.5. Voltammetric determination of paracetamol The differential pulse voltammetry (DPV) has advantages of increase in sensitivity and high resolution in quantitative analysis compared to conventional CVs. Hence, DPV was employed to detect trace amount of paracetamol in 0.1 M PBS, and the results are shown in Fig.10. The oxidation peak current increased linearly with the increasing concentration of paracetamol and two linearity were observed (Fig.S3). The first linearity was observed from 9.8 nM to 10.07 µM with the linear regression equation of Ip(10-6A) = 9.926 [Para/µM] - 2.0332 (R2=0.9861), and the second linearity from 20.35 µM to 405.07 µM with the linear regression equation of Ip(10-6) = -0.0883 [Para/µM] - 9.1812 (R2=0.9919). The detection limit was equal to 6.7 nM. The detailed analytical performance was compared with the previously reported sensors in Table.1.



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Fig.10. Differential pulse voltammograms of the effect of [Para] in 0.1 M PBS at GCSnS/TiO2@GO electrode One of the main features of the present investigation is the fabricated electrode is able to oxidize paracetamol, tryptophan and caffeine simultaneously with large peak-to-peak potential separation through DPV experiment. To establish the sensitivity and selectivity of the electrode, three separate experiments were carried out under optimum conditions. In the first case, the concentration of Para was increased from 0.0098 to 280.07 µM while the concentration of Trp and Caf was kept constant at 13.3 nM and 16.6 nM, respectively. The results of DPV are illustrated in Fig.11. The observed results reveal a slight change in peak current for Trp and Caf while increasing the concentration of Para, and thus confirms some intermolecular effects between the analytes. The oxidation peak current of Para increased linearly with the increase in concentration. Fig.11 (inset) shows two linear relationships in the range of 9.8 nM to 10.07 µM and 20.34 µM to 280.07 µM with the linear regression equation of Ip(10-6) = -10.6489[Para/µM] 4.1299 (R2=0.9929) and Ip(10-6) = -0.0557[Para/µM] - 14.7830 (R2=0.9831), respectively. The detection limit was equal to 7.5 nM. In the second case, the concentration of Trp was varied, while the concentration of Para and Caf was kept constant. Fig.12, shows increase in peak current with increasing concentration of Trp with the linear relationship of 13.3 nM to 157.03 µM as Ip(10-6) = -0.0519 [Trp/µM] - 4.2599, R= 0.9766. The detection limit was equal to 7.8 nM. It is important to mention here that, the oxidation of tryptophan occurred at 0.63 V at pH 7 through loss of one proton and an electron as shown in the following reaction scheme S1, and similar observation was also reported in the literature

[19].

On

increasing the concentration of tryptophan, two peaks were obtained with the one matching with the peak at 0.63 V and an another new one close to 1.0 V. The high potential peak deserves comment, as its origin was not discussed in the literature

[61].

With the increase in the addition of tryptophan, the pH must be

gradually decreased as the oxidation involves loss of a proton and an electron. Due to decrease in pH, the aromatic ring of Trp might form charge transfer complex with the released proton. Formation of such complex enhances resonance delocalization of nitrogen lone pair. It results in the shift of oxidation of Trp 12 ACS Paragon Plus Environment

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close to 1.0 V. So, in the DPV, the current due to the oxidation at 0.63 V gradually increased with the increase in concentration. Similarly, the current due to oxidation of proton complexed Trp at 1.0 V showed an increase in current with the increase in concentration. The Trp ring nitrogen site might not be protonated, as its lone pair electrons are already delocalized by resonance, and it is enhanced when a charge transfer complex formed. Similarly, in the third case, the concentration of caffeine varied, while that of Para and Trp was kept constant. Fig.13 (inset) shows two linear relationships in the range of 16.6 nM to 9.76 µM and 14.73 to 333.07 µM to with the linear regression equation of Ip(10-6) = 1.6417[Caf/µM] - 14.2078, R= 0.9913 and Ip(10-6) = -0.0911 [Caf/µM] - 23.7931, R=0.9702 and the detection limit is 4.4 nM. In addition, the simultaneous determination of paracetamol, tryptophan and caffeine was carried out and the observed DPV for electrocatalytic oxidation of Para, Trp and Caf in different concentrations at GC-SnS/TiO2@GO electrode is shown Fig.14. The oxidation peak current of Para increased linearly with the increase in concentration from 9.8 nM - 124.06 µM with two linear relationship in the range of 9.8 nM to 10.07 µM and 20.34 µM to 124.06 µM with the linear regression equation of Ip(10-6) = -0.9020[Para/µM]-1.9068 (R2=0.9893) and Ip(10-6) = -5.1309[Para/µM] -0.0426 (R2=0.9940), respectively. Similarly, peak current of Trp increased linearly with the increase in concentration with two linear relationship in the range of 13.3 nM to 13.18 µM and 23.03 to 115.03 µM with the linear regression equation of Ip(10-6) = -6.6395[Para/µM] - 2.0906 (R2=0.9903) and Ip(10-6) = 8.4565 [Para/µM] - 0.0377 (R2=0.9851), and for Caf, 16.6 nM to 10.94 µM and 21.06 to 103.07 µM with the linear regression equation of Ip(10-6) = -12.5089 [Para/µM] - 7.0019 (R2=0.9906) and Ip(10-6) = 0.1602[Para/µM] – 21.9448 (R2=0.9986). The results showed well-defined three oxidation peak potentials of 0.33, 0.63, and 1.3 V, corresponding to paracetamol, tryptophan and caffeine, respectively. The peak potential divergence is more than 0.3V and thus it allows simultaneous determination of each of them without ambiguity. Hence, the simultaneous determination of these analytes together is feasible without any interference using this ternary composite electrode.



Fig.11. Differential pulse voltammograms of the effect of [Para] at GC-SnS/TiO2@GO electrode with a fixed concentration of Trp and Caf, and its plot of concentration vs peak current (inset)

Fig.12. Differential pulse voltammograms of the effect of [Trp] at GC-SnS/TiO2@GO electrode with a fixed concentration of Para and Caf, and its plot of concentration vs current (inset)

Fig.13. Differential pulse voltammograms of the effect the of [Caf] at GC-SnS/TiO2@GO electrode with fixed concentration of Para and Trp, and its plot of concentration vs current (inset) 2.6. Interference Study In order to establish the selectivity of GC-SnS/TiO2@GO electrode, the oxidation of Para, Trp and Caf was examined in the presence of inorganic salts such as NaCl, sodium acetate and KCl, and organic substances such as uric acid and cysteine. The results are present in the Fig.S5 and no change in the oxidation potential was observed irrespective of inorganic salts and organic substances. Hence, the oxidation potentials of the analytes proved to have no interference due to such added components. But, a slight increase in peak current of Caf was observed due to added species. It is due to increase in the ionic 14 ACS Paragon Plus Environment

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strength of the medium. Hence, the electrodes are verified as very selective towards oxidation of Para, Trp and Caf. 2.7. Stability, repeatability and reproducibility The long term stability of the GC-SnS/TiO2@GO electrode was evaluated through CV. The electrode was stored in 0.1 M PBS solution containing 100 μL of 0.01mM Para at room temperature. Even after ten days, no obvious decrease of current density was observed, for 100 µL of 0.01mM Para thus confirming the electrode stability (Fig.S5.). The repeatability of the electrode was also studied for 5 consecutive experiments. It showed RSD of 3.21% suggesting acceptable repeatability (Fig.S6). In addition, the assay reproducibility of the GC-SnS/TiO2@GO electrode was studied through fabricating the same electrode four times and carrying out measurements, it showed 1.9 % RSD indicating a good reproducibility. All these results conclude that the electrode can be employed for practical applications of determining the content of the chosen drugs. 2.8. Analysis of paracetamol, tryptophan and caffeine in human blood serum To establish the applicability of the GC-SnS/TiO2@GO electrode for real sample analysis, a human blood serum was collected by solidifying the human blood at 3°C and centrifuged at 4000 rpm. As discussed in the experimental section, serum was diluted in 25 mL of 0.1 M PBS and the respective stock solution of Para, Trp and Caf was added. Then the simultaneous electrochemical analysis of sample 1, 2 and 3 were carried between 0.2 to 1.5V at a scan rate of 50 mV/s, separately and the same procedure was repeated three times. The obtained differential pulse voltammogram is presented in Fig.S7. The oxidation of Para, Trp and Caf occurred at their respective potentials and the measured volumes of respective components are given in Table.2. In addition, a HPLC analysis was additionally carried out in order to validate the accuracy of the present method with a standard procedure, and the results obtained were compared with that of the voltammetric method (Table.2). The results of both the analytical techniques, shown in Table.2. are comparable, thus confirming the sensitivity, reliability and applicability of GCSnS/TiO2@GO electrode for the simultaneous determination of Para, Trp and Caf in real samples like serum without any interference from one over the other.



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Fig.14. DPVs for simultaneous determination of paracetamol, tryptophan and caffeine at GCSnS/TiO2@GO electrode and its plot of concentration vs current. Table.1 Comparison of analytical performance of GC-SnS/TiO2@GO electrode with other reported electrodes Electrode Pt/CeO2@Cu2OCPE rGO-PEDOT Imprinted PANI rGO/SnO2/GCE AgNPs/MIL-101/GCE Boron-doped diamond Lignin/GCE MIS/MWCNTs–VTMS/GCE Nafion/GO-GCE GC-SnS/TiO2@GO electrode

Analyte Para Para Para Trp Trp Trp Caf Caf Caf Para Trp Caf

LOD (µM) 0.091 0.40 0.05 0.04 0.14 0.5 0.837 0.22 0.2 0.0075 0.0078 0.0044

Linear Range (µM) 0.5-160 1-35 0.4-1000 1-100 1-150 2-20 6 -100 0.75-40 0.4-80 0.0098-280 0.013-157 0.016-333


Ref. [62] [63] [64] [65] [66] [67] [68] [69] [70]

This work

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Table.2 Analysis of paracetamol, tryptophan and caffeine in real samples with GC-SnS/TiO2@GO electrode Sample

Sample 1

Sample 2

Sample 3

a-Mean

Compositi on

Added/µM

Founda/µM

Recovery (%)

Para

Trp

Caf

Para

Trp

Caf

Blood Serum alone

-

-

-

-

1.5 (±0.02)

-

Dolopar

5

-

0.2 2

4.95 (±0.03)

Ltryptophan 500mg

-

5

-

-

Dolo-650

10

-

-

Ltryptophan 500mg

-

10

-

-

Red Bull

-

-

5

-

-

0.226 (±0.07)

Par a

Trp

Caf

-

99

-

10 2

4.8 (±0. 03)

-

0.2 (±0. 07)

-

(±0. 05)

-

96

-

-

(±0. 02)

-

98

-

-

-

-

97

-

-

(±0.02)

-

-

-

4.96

1.32 (±0. 03)

-

-

-

-

-

98

(±0.05)

Caf

(±0. 03)

-

11.22

Trp

-

-

(±0.03)

Para

6.1 (±0. 01)

6.4 (±0.01)

9.81

HPLC methoda/ µM

-

9.75

6.3

4.8

value for n=3

3. Conclusions In this work, a novel electrode was fabricated using a new SnS/TiO2@GO ternary composite for separate and concomitant determination of paracetamol, tryptophan and caffeine. The electrochemical results reveal enhanced electrocatalytic activity towards the analytes. On comparing GC-SnS/TiO2@GO electrode with the control electrodes viz., (a) bare GC, (b) GC-GO, (c) GC-SnS, and (d) GC-TiO2, the oxidation current of paracetamol increased significantly and oxidation peak potential also shifted to less positive value for the former. The peak current for separate and simultaneous determination of paracetamol, tryptophan and caffeine increased with the increasing concentration and the detection limit was 7.5, 7.8 and 4.4 nM for paracetamol, tryptophan and caffeine, respectively. The GC-SnS/TiO2@GO electrode showed excellent sensing ability, wide linear range, low detection limit and decrease in over potential due to its higher electrical conductivity and surface area. In addition, the Lewis acid base type interaction between SnS and TiO2 in the electrode matrix must be the dominant factor for efficient sensing ability. The analytes with their Lewis basic sites could easily get adsorbed and provide favorable path for easy electron transfer to the electrode with the subsequent reduction of over potential. So, this study forecasts chances for fabrication many composite electrodes in the future for the determination of drugs and other organics at reduce potentials. The analytical application of the fabricated electrode was examined with human blood serum for quantification of contents and which yielded acceptable results. Therefore, this study provides a good platform for simultaneous determination of biological samples 17 ACS Paragon Plus Environment


containing more than one drug using SnS/TiO2@GO ternary composite electrode, and hence, it has wide scope in fabrication of commercial sensor for drug detection at low concentration. Acknowledgments The authors acknowledge DST-SERB-EMEQ and DST-PURSE-PHASE II, New Delhi, Government of India, for providing ﬁnancial assistance. References 1. Chen, X.; Zhu, J.; Xi, Q.; Yang, W. A high performance electrochemical sensor for acetaminophen based on single-walled carbon nanotube–graphene nanosheet hybrid films. Sens. Actuators, B 2012, 161, 648–654. 2. Wang, S.; Xie, F.; Hu, R. Carbon-coated nickel magnetic nanoparticles modified electrode as a sensor for determination of acetaminophen. Sens. Actuators, B 2007, 123, 495–500. 3. Samanta, S.; Srivastava, R. Simultaneous determination of epinephrene and paracetamol at copper cobalt oxide spinel decorated nanocrystalline zeolite modified electrodes. J. Colloid Interface Sci. 2016, 475, 126–135. 4. Tefera, M.; Geto, A.; Tessema, M.; Admassie, Sh. Simultaneous determination of caffeine and paracetamol by square wave voltammetry at poly(4-amino-3-hydroxynaphthalene sulfonic acid) modified glassy carbon electrode. Food Chem. 2016, 210, 156–162. 5. Glavanovic, S.; Glavanovic, M.; Tomisic, V. Simultaneous quantitative determination of paracetamol and tramadol in tablet formulation using UV spectrophotometry and chemometric methods. Spectrochim. Acta, part A 2016, 157, 258–264. 6. Ardakani, M.M.; Mohseni, M.A.S.; Alibeik, M.A.; Benvidi, A. Electrochemical sensor for simultaneous determination of norepinephrine, paracetamol and folic acid by a nanostructured mesoporous material. Sens. Actuators, B 2012, 171, 380–386. 7. Mahmoud, B. G.; Khairy, M.; Rashwan, F. A.; Banks, C.E. Simultaneous Voltammetric Determination of Acetaminophen and Isoniazid (Hepatotoxicity-Related Drugs) Utilizing Bismuth Oxide Nanorod Modified Screen-Printed Electrochemical Sensing Platforms. Anal. Chem. 2017, 89, 2170−2178. 8. Olaleye, M.T.; Rocha, B.T.J. Acetaminophen-induced liver damage in mice: Effects of some medicinal plants on the oxidative defense system. Exp. Toxicol. Pathol. 2008, 59, 319–327. 9. Mazer, M.; Perrone, J. Acetaminophen-Induced Nephrotoxicity: Pathophysiology, Clinical Manifestations, and Management. J. Med. Toxicol. 2008, 4, 2–6. 10. Kong, S.; Yang, J. C.; Parka, J. Y. Caffeine-Imprinted Conducting Polymeric Films with 2D Hierarchical Pore Arrays Prepared via Colloidal Mask-Assisted Electrochemical Polymerization. Sens. Actuators, B 2018, 260, 587–592. 18 ACS Paragon Plus Environment

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TiO2@GO composite coated GC electrode for

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