Effect of RGO-Y2O3 and RGO-Y2O3:Cr3+ nanocomposite sensor for dopamine

The RGO-Y2O3 and RGO-Y2O3: Cr3+ (5 mol %) nanocomposite (NC) synthesized by hydrothermal technique. The structure and morphology of the synthesized NCs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Y2O3:Cr3+ displays spherical-shaped particles. Conversely, the surface of the RGO displays a wrinkly texture connecting with the existence of flexible and ultrathin graphene sheets. The photoluminescence (PL) emission spectra showed series of sharp peaks at 490, 591, and 687 nm which corresponding to 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, and 4F9/2 → 6H11/2 transitions and lies in the blue, orange, and red region. The prepared NCs were used for the preparation of modified carbon paste electrodes (MCPE) in the electrochemical detection of dopamine (DA) at pH 7.4. Both modified electrodes provide a good current response towards voltammetric detection of DA. Doping is an effective method to improve the conductivity of Y2O3:Cr3+ and developed a method for the sensor used in analytical applications.

www.nature.com/scientificreports/ well as energy storage devices. Reduction of GO results in the partial restoration of graphitic network conventionally attained using chemical, thermal, and electrochemical pathways 12 . Generally, for chemical reduction of GO, reducing agents (RA) such as hydrazine (N 2 H 4 ), hydrazine hydrate, and sodium borohydride (NaBH 4 ) were used. Conversely, these RA are unsafe to human health and the environment. Further, topological defects and vacancies were created upon thermal reduction of the GO sheet. The presence of these defects which affects the electronic properties of the RGO, resulting in the decrease of ballistic transport path length and introduce scattering sites. Therefore, the execution of a greener reduction route can offer a viable substitute methodology for large production of RGO 13 . Other than the utilization of GO and RGO, these days a lot of consideration for the integration of inorganic phosphor with GO to manufacture composites or hybrids has become a hotly debated issue of exploration because of their upgraded functionalities that cannot be accomplished by either part alone. It is well known that the attachment of inorganic NPs onto GO may inhibit the aggregation and improve the significant persuasive effect on electrochemical properties through attaching them onto GO sheets. Amongst the metal oxide NPs decorating RGO, much attention has been given to doped and undoped NPs especially the stable Y 2 O 3 host 14,15 .
Till date, many approaches have been utilized for the fabrication of MO/RGO composites namely microwave, hydrothermal, pyrolysis method, etc. [16][17][18][19][20] when the addition of MO NPs to the GO matrix, an upsurge in porosity happens and the GO-MO attains the properties that are dissimilar from those exhibited by each distinct component. On the other hand, Y 2 O 3 NPs are chemically steady and have a narrow bandgap that enables electron transfer and offers excellent electrochemical sensitivity.
The present work RGO-Y 2 O 3 and RGO-Y 2 O 3 : Cr 3+ NCs synthesized by hydrothermal technique. The prepared NCs were well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). NCs were used as the electrochemical sensor for voltametric determination of DA. The developed sensors show good current sensitivity towards DA. The prepared RGO-Y 2 O 3 and RGO-Y 2 O 3 : Cr +3 MCPEs provide good selectivity, high sensitivity, and excellent stability and reproducibility over several days. In addition to comparably doping is an effective method to improve the conductivity of Y 2 O 3 :Cr 3+ and doping shows good linearity, sensitivity, lower detection limit compare to undoped NCs towards DA. Finally, the modified electrodes were utilized for the detection DA in biological samples.

Experimental section
Reagents & investigation techniques. The sodium hydroxide, Dopamine hydrochloride, Uric acid, Na 2 HPO 4 & NaH 2 PO 4 was from nice chemicals. The graphite powder (Loba Chemie), silicon oil (Himedia) & all other required solutions were prepared from distilled water. The electrochemical experiments were performed on a voltammetric instrument of model CHI-660c (CH Instrument-660 electrochemical workstation). The Shimadzu-made diffractometer provided with CuKα radiation was utilized for structural characterization. Surface morphology and particle size were studied with the help of a field emission scanning electron microscope (FESEM, TESCON) and transmission electron microscope (TEM, H-600, Hitachi, Japan) respectively. The Horiba made spectrofluorimeter (Jobin Yvon) was used for Photoluminescence (PL) with a 450 W Xenon lamp as an excitation source.
Preparation of RGO-Y 2 O 3 composite using hydrothermal synthesis. The chemicals procured in the present work are analytical grade and used without further purification. The modified Hummers method has been used for the synthesis of GO 21 . The obtained GO solution was sonicated for about 25-30 min, centrifuged to wipe off the unreacted GO.
A single-step hydrothermal route was utilized for the fabrication of pure Y 2 O 3 and Cr doped Y 2 O 3 in RGO. In a typical synthesis, stoichiometric quantities of yttrium chloride (YCl 3 ·6H 2 O) and chromium chloride (CrCl 3 ·6H 2 O) were dissolved to 120 mL of mixed liquid, encompassing 60 mL of C 2 H 5 OH and 60 mL of GO aqueous dispersion before stirred for 25-30 min to get a clear solution. Then the resulting solution was shifted to a 180 mL Teflon-lined stainless-steel autoclave and treated thermally at 175 °C for 12 h. Afterward, the autoclave was naturally cooling down to room temperature. The final yield was washed away several times with distilled water and then ethanol, separately, and then centrifugation. The as-obtained products were dried at 80 °C for 12 h in air, and annealed at 500 °C for 2 h in Ar and subsequently 200 °C for 12 h in the air to form the RGO/ Y 2 O 3 :Cr 3+ NCs. Pure Y 2 O 3 and a series of RGO-Y 2 O 3 :Cr 3+ NCs with the GO contents of 2, 4, 6, 8, and 10 mg were synthesized by changing the concentration of GO aqueous dispersion.
Preparation of BCPE and MCPEs. The BCPE was prepared by hand mixing of graphite powder and silicon oil at the ratio of 70:30 (w/w) in an agate mortar for about 30 min until a homogenous paste was obtained. The prepared carbon paste was tightly packed into a PVC tube (3 mm internal diameter) and the electrical contact was provided by a copper wire connected to the paste at the end of the tube and polished using smooth paper 22,23 . The CPE was modified by taking different weights of RGO-Y 2 O 3 and RGO-Y 2 O 3 Cr 3+ NCs (2, 4, 6, 8, and 10 mg) in silicon oil and graphite powder. Then this mixture was thoroughly mixed in an agate mortar for about 30 min and packed into a homemade Teflon cavity current collector and polished using soft paper (Scheme 1) 24 .

Results and discussion
Characterization of RGO-Y 2 O 3 and RGO-Y 2 O 3 Cr 3+ NCs. Figure 1 shows the powder X-ray diffraction (PXRD) patterns of GO, rGO, Y 2 O 3 :Cr 3+ and RGO/Y 2 O 3 :Cr 3+ NCs. The GO exhibited a distinctive (001) peak at 2θ = 11° (JCPDS No. 89-8490) with an interlayer distance of 0.8 nm, which is larger than the interlayer distance of graphite (0.34 nm), revealing that many different oxygen-containing groups were intercalated within the  Figure 2a shows the typical FESEM image of the rGO. As can be seen from the figure, the ultrathin, crumpled nanosheets are found to be transparent and wrinkled like wavy silk veils with randomly arranged and overlapped with each other. Figure 2b,c shows the TEM images of Y 2 O 3 :Cr 3+ nanopowders and rGO/Y 2 O 3 :Cr 3+ NCs respectively. It is clearly evident that the nanopowders exhibit almost spherical shaped particles. However, in the case of   Photoluminescence analysis. Figure 3a shows the excitation spectrum of Y 2 O 3 : Cr 3+ (5 mol% Fig. 3b. The emission spectra displays sharp peaks at ~ 490, 591, and 689 nm corresponding to 4 F 9/2 → 6 H 15/2 , 4 F 9/2 → 6 H 13/2, and 4 F 9/2 → 6 H 11/2 transitions which lie in the blue, orange and red region. As can be seen from the figure, it is apparent that the red emission was dominating when compared to blue and orange emissions. The transition corresponding to orange emission is magnetically allowed and hardly differs with the crystal field strength around the Cr 3+ ion. Whereas, the transition corresponding to blue emission belongs to the hypersensitive electric field (forced electric dipole) transition with the selection rule ΔJ = 2, which is strongly influenced by the outside surrounding environment. When Cr 3+ is located at a low-symmetry local site (without inversion center), red emission is often dominant.
In the present study, since the red emission is dominant, the Cr 3+ ions occupy lower symmetry local site in the Y 2 O 3 host matrix.    Figure 6a &b shows the Ip increment with a slight positive shift in the peak potential when the υ was a hike in the range from 50 to 500 mVs -1 . The kinetics of the electrode was evaluated by plotting of υ v/s Ipa presents marvelous linearity (Fig. 6c). The υ 1/2 v/s Ipa (Fig. 6d) also shows good linearity. This suggests the adsorption-controlled phenomena on electrodes 26 . The heterogeneous rate constant (k 0 ) (  Figure 7a Fig. 7a,b). The limit of detection (LOD) and limit of quantification (LOQ) was calculated by using the equations 27 :

Success of concentration.
To determine, the LOD for DA was 6.01 and 3.26 µM. & LOQ was 20.04 and 10.88 µM for DA at the RGO-Y 2 O 3 and RGO-Y 2 O 3 Cr 3+ MCPEs respectively. The comparative analytical performance of electrode for DA is designed in Table 2 28-37 . Influence of pH. The effect of pH on the electrochemical response of the dopamine at the RGO-Y 2 O 3 and RGO-Y 2 O 3 Cr +3 MCPEs was carefully examined in the pH series of 6.2-7.8 shown in Fig. 8a,b respectively. The peak potential shifts to a negative side with hike pH in the MCPEs. The anodic peak potential (Epa) versus pH    Fig. 8a,b) figure (a,b) shows the graph of Ipa versus concentration of DA.   Interference study. Lastly, to evaluate the feasibility of the proposed method, the interference of possible chemicals in the determination of DA was conducted, the interference study was performed in the mixture of samples of DA and UA at the RGO-Y 2 O 3, and RGO-Y 2 O 3 Cr 3+ MCPEs by differential pulse voltammetry (DPV). The RGO-Y 2 O 3 MCPE (Fig. 10a) shows the Ipa of UA was increased with increased concentration from 10 to 60 µM by keeping the constant concentration of 10 µM DA, Similarly, DA concentration was varied and its Ipa is increased with the concentration (Inset Fig. 10a). The same procedure also adopted for RGO-Y 2 O 3 Cr 3+ MCPE and varying the concentration of UA and DA shows in Fig. 10b (Inset Fig. 10b) respectively. This result shows higher current sensitivity and absence of background current and this result helps in the accurate and precise determination of DA and UA at RGO-Y 2 O 3 and RGO-Y 2 O 3 Cr 3+ MCPEs.
The analytical application of the proposed sensor. To verify the success of the proposed sensor in the dopamine hydrochloride injection. The injection was procured from VHB Medi Sciences Ltd with a specified content of DA 40.0 mg/mL (suitable dilution in 0.2 M PBS). The samples were analyzed by the standard addition method. The results have been shown in Table 3. Therefore, the proposed modified electrode could be applied for the real sample analysis with satisfactory results.