Graphene Oxide Decorated Tin Sulphide Quantum Dots for Electrochemical Detection of Dopamine and Tyrosine

The current study highlights the design and construction of a sensitive and selective sensor for detection of dopamine and tyrosine using a GO-SnS2 quantum dots by a drop casting method on glassy carbon electrode. Highly porous nano-crystalline GO-SnS2 quantum dots were synthesized by using ultrasonication followed by hydrothermal method in a facile manner. XRD, SEM, XPS, TEM, and pore size distribution techniques were used to characterize the quantum dots that were produced. The newly fabricated electrode was evaluated for EIS (Electrochemical impedance spectroscopy), CV (cyclic voltammetry) and chronoamperometric methods. The observed limit of detection of dopamine was observed to be 26 nM. High selectivity and sensitivity were observed for electrochemical detection of dopamine and tyrosine.

SnS, the electrochemical performance of tin sulphide has recently been improved by grafting nanosized tin sulphide into various types of carbon matrices (e.g., carbon spheres, amorphous carbon, macroporous carbon, carbon nanotubes, or graphene) [17][18][19][20][21][22][23]. Despite considerable gains in gravimetric capacity and cycle performance, the nanostructure of these composites, in combination with the low tap density of carbon matrix, can restrict volumetric capacity [24,25]. Furthermore, the creation of these composites typically involves severe conditions or sophisticated synthesis, both of which are costly to industry. As a result, achieving a simple, scalable synthesis of tin sulphide-based graphene materials with superior volumetric storage remains a major challenge.
We used a facile hydrothermal method to create a novel graphene Oxide/ SnS 2 (GO-SnS 2 ) composite. SnS 2 quantum dots are tightly supported on porous graphene oxide (PGO) in the composite, forming a primary microstructure and then assembling into a secondary nanostructure. The tap density of the nanostructured SnS 2 and PGO hybrid is very high. The combination of SnS 2 quantum dots and PGO nanosheets inside nanosized building blocks can not only improve overall electron/ion transport, but also efficiently insert SnS 2 volume change and provide strong structural stability to the composite. As a result, the tightly compacted GO-SnS 2 quantum dots show high, fast, and stable dopamine electrochemical detection. Thus, the prepared GO-SnS 2 quantum dots were found to be exhibiting superior electrochemical performance, combined with its simple scalable synthesis, makes it a promising material for practical application.

Reagents and chemicals
Stannous dichloride (SnCl 2 .4H 2 O), Sodium Sulphide flakes (make: SD fine chemicals, INDIA), Dopamine (make: Aldrich), Tyrosine from Fischer Scientific Ltd., Potassium ferrocyanide and Potassium Chloride from SD Fine. Chemicals used in this work are of AR grade or analytical grade and used as received.

Synthesis of SnS 2 nanoparticles using plant extract
The synthesis of SnS 2 quantum dots was carried out as per our earlier work in slightly modified two step manner [35] Fresh Syzigium cumini (S. cumini) leaves (100 g) were collected and rinsed several times with distilled water to remove foreign particles before being ground using a mortar and (e.g., uric acid (UA), ascorbic acid (AA), and catecholamine molecules). Signal interference might greatly limit the sensitivity of dopamine detection since the reduction and oxidation potentials of these biological substances allegedly coincide with those of dopamine [15][16][17]. Furthermore, the electrochemical sensitivity of dopamine is still lower than that of other traditional techniques like HPLC and ELISA, which is a substantial hurdle to overcome before this approach can be utilized to detect accurate levels of dopamine [18]. By functionalizing electrode surfaces or introducing other types of conductive materials, several attempts have been made to overcome the issues of selectivity and sensitivity.
Graphene, a two-dimensional (2D) honeycomb structure made up of pure carbon molecules, has been widely exploited in different scientific fields, including batteries, display panels, solar cells, and even biological applications [19][20][21][22]. Furthermore, graphene derivatives have been shown to exhibit notable dopamine-detection properties [23], which are principally owing to π-π and electrostatic interactions between the graphene oxides' surfaces. Various graphene-derivative-modified electrodes have been created to increase the performance of dopamine biosensors, including graphene/glassy carbon electrode (GCE), graphenegold nanoparticles/GCE, TiO 2 -graphene/GCE, and GO/ GCE electrodes [24][25][26][27]. One of the most intriguing carbonaceous compounds is graphene, a one-layer thick sheet with exceptional optical, thermal, and electrical characteristics. The discovery of porous graphene oxide (PGO), a type of graphene-oxide sheet with numerous hydroxyl groups and a porous surface [28], has the potential to improve the electrostatic interaction between the PGO and the analytes while also facilitating electron transfer between the molecules and the underlying electrode substrates [29].
Generally graphene oxide functionalized metals oxides or sulphides shows high affinity towards sensing of the bio molecules [30][31][32]. Richard et al. reported ZnO-ZnFe 2 O 4 /Fe 3 O 4 /carbon nanocomposite with ultrasensitive and selective electrode for detection of dopamine [33]. SnO 2 nano wires were used for selective detection of riboflavin [31]. Apart from this many of the sensors were fabricated for real time monitoring [34].
Among the various binary compounds of tin chalcogenides, tin sulphides are well studied or explored owing to adaptable chemical nature and can be fabricated into hybrids, composites, non-toxic nature etc. hence they are widely used in energy storage devices, solar cells and optoelectronic devices. Despite this, the electrochemical procedure produces substantial capacity fading in tin sulphides due to the high-volume change. [14][15][16]. Because these matrices can greatly promote electron/ion transfer and effectively accommodate cycle-induced stress/strain of

Fabrication of electrode
Before being utilised to form the working electrode, the glassy carbon electrode (GCE) was thoroughly washed with deionized water and polished with an alumina polishing pad. The material was sonicated for 15 min after being distributed in 1 mL DMF. To make a thin layer, the resultant mixture was drop casted over the surface of GCE with a micropipette and air dried overnight at room temperature. The modified electrode after casting was analysed for SEM analysis in order to check the uniformity of the material on the surface of the electrode (Fig. 4e). The SEM images clearly show the uniformity of the material that is coated.

Material characterization
The produced GO-SnS 2 quantum dots were examined using a various of characterization methods. The size of GO-SnS 2 quantum dot phase purity and crystalline nature were examined using the X'pert Pro X-ray diffractometer with Ni filtered Cu Kα radiation (λ = 1.5406Å, 2θ = 0-60). SEM (ZEISS EVO 18 model) was used to record the morphology of the GO-SnS 2 quantum dots. The GO-SnS 2 quantum dots were photographed and their selected area electron diffraction (SAED) patterns were obtained using an FEI TECHNAI G2 transmission electron microscope (TEM). A UV-1800 pc Shimadzu spectrophotometer was used to detect colloidal dispersions of GO-SnS 2 quantum dots in 200 to 1100 nm range. X-ray photoemission spectra were obtained on a KRATOS AXIS 165 with Mg Kα radiation (1253.6 eV) at 75 W. The C 1s line at 284.6 eV was utilized pestle. The combination is placed in a beaker with distilled water, agitated for half an hour to ensure equal dispersion throughout the medium, and then filtered using Whatman filter paper to get pure S. cumini extract. The crude extract was diluted with distilled water before being kept in airtight bottles in the refrigerator. SnS 2 quantum dot composites were made by dissolving a 0.1 M solution of SnCl 2 .4H 2 O in 250 ml of heated S. cumini leaf extract and shaking it. After 5 min, the Na 2 S solution was added drop by drop, and the solution became yellow after 30 min of stirring. To get the SnS 2 -nanoparticles, the resultant solid is centrifuged and washed with water, ethanol, then kept in an oven for drying at 60 °C for about 8 h.

Synthesis of Graphene oxide/SnS 2 nanocomposites (NCs)
Ultra-sonication followed by the hydrothermal method is simple and fast for synthesis of graphene oxide/ SnS 2 nanocomposite materials as reported in our earlier work [35]. In a beaker about 500 mg of graphene oxide in 100 mL water are homogenized by using an ultrasonic bath. The homogenous graphene solution in the beaker were mixed with preformed SnS 2 synthesized, followed by hydrothermal treatment for 24 h at 100° C without adding any precipitating agent. The resultant colloidal solution was rinsed with ethanol and water, and then aged for roughly 12 h in beaker. Finally, the resultant combination solutions were dried in an oven at 65 °C for 24 h, yield GO-SnS 2 quantum dots.

Material characterization
as an internal reference. Asymmetric gaussian forms were adopted in each situation. Binding energies of similar samples were typically constant within 0.1 eV.  (111), 59.14(200) and its corresponding planes is attributed to the hexagonal phase of SnS 2 [36]. The diffraction patterns agree well with the JCPDS card No. 23-0677. The obtained peaks are sharp with no impurities. The particle size calculated based on Scherrer's equation at 2θ = 26.0, 34.09 and

XRD
The crystal structure of the as synthesized GO-SnS 2 quantum dots was given in Fig. 1.
The diffraction peaks at 2θ = 26.0 and also a hump around 25 shows the sp 2 graphene carbon. The peaks observed   Fig. 3c, the adsorption isotherms are similar to that of type (IV) isotherms with prominent hysteresis loop in the P/ P0 range of 0.5-1 reflecting the presence of mesopores. The synthesized GO-SnS 2 quantum dots showing the mesoporosity and is further confirmed by corresponding pore size distribution [38]. The SnS 2 -GO quantum dots have a surface area of 13.865 m 2 /g, an average pore size of 15.24 nm, and a measured pore volume of 0.066 cc/g. The findings are consistent with previous reported results [38].

XPS
The oxidation states and surface chemical functionalities were examined by using XPS spectra. XPS patterns of C1s shows a prominent peak at binding energy of 284.78 eV which refer to sp 2 graphitic carbon and shown in Fig. 2c. As given in Fig. 2b, showing the high-resolution Sn 3d spectra with two peaks at Sn 3d 5/2 (487.98 eV) and Sn 3d 3/2 (496.28 eV) with a 9.3 eV energy gap, suggesting the presence of Sn 4+ as the main phase present in the material. The peaks at 161.7 eV and 284.78 eV corresponding to S 2p and C 1s. Moreover the SnS phase was not detected in the SnS 2 -GO XRD patterns, as previously stated. [37].

EIS
Electrochemical impedance experiments in 0.1 M KCl at its formal potential in the frequency range 100 kHz to 100 mHz with a 10mV amplitude were performed to examine the electrical characteristics of the prepared electrodes. A typical EIS response of bare GCE and GO-SnS 2 quantum dots/GCE are shown in Fig. 5. At the bare GCE, a partial semicircle with a virtually straight tail indicates electron transport resistance to the redox probe. On the GO-SnS 2 quantum dot/GCE, the semicircle does not appear, suggesting a lower barrier to electron transmission. This is due to the high conductivity of the graphene oxide-SnS 2 formed on the surface. As indicated by the enhanced electrode's impedance behaviour, GO-SnS 2 has been effectively adsorbed on the GCE surface. The modified electrode's resistance is lower than the bare graphite electrodes, which could be due to improved conductivity of the modified electrode [40,41]. The impedance charts match the behaviour of the CV. morphology of the graphene on which the SnS 2 particles are decorated and the sheets are attached to each other due to tiny dimensions and large surface energy. Each nano-cluster size ranges from, 31.40-57.83 nm, as indicated from SEM images. The SnS 2 particles were seem to be embedded in the grapheme sheets [39]. The EDAX patterns are given in Fig. 4(c) and the surface composition of C, O, Sn and S are in a specific area are 82.05, 15.78, 1.84 and 0.32 (atomic weight%) respectively.
The TEM images of GO-SnS 2 in Fig. 4b, demonstrates the quantum dots of size around 3 nm. The quantum dots are showing good crystallinity and the SAED pattern (Fig. 4d) infringes with hexagonal phase of tin sulphide and SnS 2 phases were decorated on graphene oxide, which is well in accordance with that of XRD.

Electrochemical performance of dopamine and tyrosine on the GO-SnS 2 nanocomposite modified electrode
Dopamine and tyrosine coexist in blood and many biological fluids and interfere with each other in the detection and moreover the concentration of tyrosine is generally low. High concentration of DA may interfere in determining the tyrosine. Hence, simultaneous determination of DA and Tyrosine is highly essential in electrochemical analytical research.
Three electrode voltammetry was carried out for electrochemical characterization and sensing. The primary event that reveals the existence of dopamine is the oxidation of The electrochemical areas calculated by using the equation are 0.112 cm 2 and 0.226 cm 2 for base GCE and GO-SnS 2 /GCE respectively. As seen from the values the electroactive surface area increases nearly by 50% compared to that of bare GCE.

Effect of scan rate on peak current of dopamine and tyrosine
Using cyclic voltammetry, the impact of changing the sweep rate for 100 M dopamine in 0.1 M PBS at pH 7 was examined (Fig. 7 A). Different scan speeds ranging from 50 to 400 mV/s were used to record the CV profiles. Peak current rose with a minor positive shift in peak potential in the region of 50 to 400 mV/s, as seen in the graph. Ipa vs. potential and Ipa vs. square root of scan rate demonstrate a linear relationship with zero intercept as seen in the figure inset. The regression equation is expressed as Ipa = 2.9852 x + 0.03638 (R 2 = 0.99418). The Ipa increased linearly with scan rate and the corresponding regression equation is obtained as Ipa = 0.082 x + 0.01431(R 2 = 0.99172). All these results confirm the diffusion-controlled process controlling the overall kinetics.
The number of electrons 'n' was estimated using Laviron's equation, which is expressed as below [43].
In the above equation, Ip represents the anodic peak current (A), Q represents the charge associated with oxidation (C), υ is the scan rate (V s − 1 ), R represents the gas constant

Simultaneous detection of dopamine and tyrosine on GO-SnS 2 modified electrode
Simultaneous detection studies of dopamine and tyrosine were given in Fig. 6. There are no peaks observed in bare glassy carbon electrode as seen in Fig. 6(a) and moreover in case of GO-SnS 2 /GCE with analyte only tyrosine is showing Ipa (current) 0.0187mA at Epa (voltage) 0.782 V which is given in Fig. 6(f). And in the third case simultaneous detection of tyrosine and dopamine was carried out using 100 µM Dopamine and 500µM of tyrosine in PBS buffer solution at pH 7 and is given in Fig. 6(e). The oxidation peaks of dopamine and tyrosine are very well separated and peaks appeared at 0.202 V with current 0.0257962 mA, and the peak at 0.7952 V with current 0.02579 mA correspond to dopamine and tyrosine respectively. Thus, the CV studies clearly showing the modified GO-SnS 2 /GCE was successful in separating and distinguishing the analytes dopamine and tyrosine.
With precise redox behaviour of dopamine, GO-SnS 2 /GCE demonstrated a three-fold increase in anodic peak current of 0.0362 mA. (Fig. 6d). These GO-SnS 2 dots, which boosted conductivity and surface area, are responsible for the better electrochemical current responsiveness.
To prove the surface area of GCE increases with modification with GO-SnS 2 , the electroactive area of bare GCE, GO-SnS 2 /GCE were determined and compared using CV technique as per the Randles-Sevik equation [42].
i p = (2.69 × 10 5 ) n 3/2 D 1/2 v 1/2 Ac. reduction peak was observed at -0.445 mV potential. Apart from the oxidation and reduction peaks of dopamine, the third peak due to leucodopaminechrome observed due to ring closure of dopamine-o-quinone [44,45]. Effect of varying concentration of tyrosine at modified GO-GO-SnS 2 /GCE is shown in Fig. 8(B) and a similar trend as seen in dopamine were observed. As prominent oxidation peak at 500µM is seen in tyrosine studies, this concentration was chosen for all the comparative studies.

Effect of pH on dopamine and tyrosine studies
CV tests were performed to assess the influence of pH on oxidation of dopamine at PBS solutions with varying pH ranging from 4 to 12 at a scan rate of 50 mV/s to enhance the electrochemical responsiveness of the GO-SnS 2 /GCE towards the electrochemical oxidation of DA ( Fig. 9 A). The peak potential shifts to the negative side when pH rises from 4 to 11, owing to enhanced reversibility of the oxidation which involves deprotonation at elevated pH ranges ( Fig. 9 A). Furthermore, pH = 7 PBS had a superior electrochemical response in sensor applications. As a result, pH = 7 PBS was discovered to be optimal electrolyte for electrochemical research. Effect of pH on tyrosine oxidation in PBS solutions with varying pH from 4 to 12 at a scan rate of 50 mV/s was given (8.314 J K − 1 mol − 1 ), and T is the temperature (K). The value of n was calculated as 1.94, which equals 2, indicating that dopamine oxidation is a two-electron transfer process (scheme 1).
Cyclic voltammetry profiles were recorded to investigate the influence of scan rate on the electroactive surface of GO-SnS 2 /GCE, on tyrosine (500 µM) are shown in Fig. 7. (B). Using Randles -Sevcil equation, a plot of Ipa versus square root of scan rate (50 to 400 mV/s) shows excellent linearity with zero intercept, Ipa = 0.00403 x -0.0059 with R 2 = 0.9945 suggesting the diffusion controlled process for the oxidation of tyrosine. The charge transfer coefficient for GO-SnS 2 /GCE was found to be 0.492 and the theoretical value is 0.5, indicating that the adsorption of reactants of intermediates onto the modified sensor is diffusion controlled, sluggish and irreversible.  of GO-SnS 2 /GCE electrode. In the concentration range of 2.5 × 10 − 6 to 250 × 10 − 6 M, a linear connection between peak current and DA concentration was observed, with the lowest detection limit being 26 nM. In order to understand improvement of the modified sensor the calculated limit of detection and sensitivity were compared with earlier reported dopamine sensors and is given in Table 1.

Interference studies
The interference studies on GO-SnS 2 /GCE was carried out by taking the common interfering biomolecules like 50 µM uric acid (UA), 50 µM ascorbic acid (AA) in phosphate buffer at pH 7 by using the chronoamperometry and the results are given in Fig. 10(b).The synthesized sensor system shows insignificant current intensity changes with respect to dopamine as demonstrated in Fig. 11.

Stability and repeatability of GO-SnS 2 / GCE electrode
The electrode's stability was tested by immersing it for three weeks in a phosphate buffer solution with a pH of 7.0. Every week, CVs were collected and compared to the ones received on the first day. The oxidation peak current was found to be somewhat lower than anticipated. The current reduction was just 10% after three weeks as demonstrated in Fig. 11(a) indicating that the modified electrode is highly stable.
The modified electrode's repeatability was tested ten times with 100 µM DA. After each measurement, the modified electrode was rinsed with buffer solution and evaluated in Fig. 9(B). With increase in concentration there is increase in anodic peak current but the relative response was low when compared to dopamine. There is shift in negative peak potentials due to increase in reversibility of the oxidation at elevated pH of 7 and concentration of 500 µM was found to be optimum for tyrosine studies.

Chronoamperometric studies
In order to understand the response character of GO-SnS 2 /GCE to dopamine chronoamperometry studies were carried out by successive addition of 50 µM dopamine at 25 s time intervals in PBS solution containing 0.1 M KCl is shown in Fig. 10. The response current was measured at fixed potential of + 0.25 V under stirring, for each addition of dopamine almost equal current steps were observed which indicates the efficient and catalytic activity  for the same concentration as illustrated in Fig. 11(b). The enhanced electrode has an RSD (relative standard deviation) of 3.8%, showing that it is not vulnerable to surface fouling.

Conclusions
Finally, using a facile ultrasonication and hydrothermal method, the quantum dots of SnS 2 -carbon composites were synthesized. The as synthesized materials were characterized by using various techniques like XRD, SEM, TEM, EDX and elemental mapping, XPS, and pore size distribution. These dots were used to construct a modified glassy carbon electrode for dopamine and tyrosine detection. For EIS, CV and chronoamperometric studies, the electrocatalytic activity of modified electrodes is investigated. Intriguingly, chronoamperometric studies discloses a LOD of 26 nM for dopamine detection. Compared to dopamine the relative response of tyrosine is less. The modified electrode has excellent stability, selectivity, sensitivity, and reproducibility, according to our research.