**1. Introduction**

Dopamine (DA) and uric acid (UA) usually coexist in the serum and extracellular fluids of the central nervous system and play a significant role in regulating human metabolism activity [1]. As an indispensable catecholamine neurotransmitter, DA plays critical roles in regulating the function of the cardiovascular and central nervous systems, as well as maintaining emotional control and hormonal balance [2]. Abnormal levels of DA can lead to various neurological disorders such as schizophrenia, Parkinson's and Alzheimer's disease [3–5]. Therefore, the precise detection of the DA level in physiological fluids is essential for the early diagnosis of these neurological disorders. However, the rapid and reliable detection of DA in physiological samples remains critical and challenging due to the low DA concentration in the extracellular matrix (usually in the range of 0.01–1 μM) and its susceptibility to interference from endogenous substances such as UA and ascorbic acid (AA). Uric acid is another crucial biomolecule in physiological fluids and is often regarded as the end-product of purine metabolism in the human body [6]. For a heathy individual, the UA concentration is generally in the range of 4.1 ± 8.8 mg/100 mL [7]. The dysfunction of UA in bodily fluids likely causes several diseases, including gout, pneumonia and hyperuricemia [8]. As stated, both DA and UA are regarded as important biomolecules for the regulation of human metabolic activity. Thus, determining the concentration of DA and UA in biological matrices (i.e., human urine, serum) can provide valuable clues for healthcare and disease diagnosis. Since DA and UA usually coexist in physiological fluids, it is of the utmost importance to propose a highly e fficient technique for the simultaneous determination of DA and UA.

To date, several analytical techniques have been proposed for the detection of DA and UA, including, but not limited to, high performance liquid chromatography [9,10], fluorescent [11], spectrophotometry [12], electrogenerated chemiluminescent [13] and surface plasmon resonance [14] etc. Although quite reliable, these techniques usually involve tedious and time-consuming analytical procedures that require expensive equipment, well-trained technical personnel or a large quantity of toxic solvents [15]. Compared with other techniques, the electroanalytical techniques are more suitable for sensing DA and UA because of their low price, fast response, facile operation and excellent anti-interfering ability [16–18]. Owing to the considerable superiorities including cost-e ffectiveness, rapidness, convenience and high e fficiency, electroanalytical methods have drawn increasing attention for the detection of small biomolecules, food additives and contaminants [19–24]. DA and UA are electroactive species whose redox processes can be quantitatively detected by electroanalytical techniques. However, it becomes a grea<sup>t</sup> challenge to simultaneously detect DA and UA on bare electrodes due to the fouling e ffect that occurs during the oxidation [7] and cross-interferences as a result of similar oxidation potentials [25]. To resolve this problem, nanostructured materials were employed to achieve high sensitivity and prevent overlapping of the oxidation peaks [25]. In recent years, much effort has been devoted to developing promising alternatives as sensing materials for the simultaneous detection of DA and UA, including noble metal nanoparticles [25], metal oxides nanocomposites [26], alloyed nanoparticles [27], polymer films [28,29], and nanocarbon materials [30–32] etc.

Iron oxide (Fe2O3) has become one of the most versatile transition metal oxides not only due to low cost, more abundant, good biocompatibility and excellent electrochemical performances, but also for its widespread applications [33–38]. In particular, α-Fe2O3 nanoparticles ( α-Fe2O3 NPs) have been considered as the most promising modifying material, because of the variable valence state of iron oxides that can be recovered in situ via electrochemical reducing or oxidizing during the sensing process, thus triggering the heterogeneous redox of the target analysts [39]. As far as we know, various morphologies of α-Fe2O3 NPs have been made available in previous reports, including wire [40], rod [41,42], tube [41], sphere [43,44], flower [45], spindle [44], cubic [44,46–49], thorhombic [46–48], discal [47], and shuttle [50]. Many studies demonstrate that the morphologies of nanostructured α-Fe2O3 have a significant impact on optical, magnetic, photocatalytic and electrochemical properties [46,47,51,52]. However, the morphology-dependent electrochemical sensing performances with respect to small biomolecules have rarely been investigated. Hence, it is essential to explore the morphology–dependent sensing properties of di fferent α-Fe2O3 NPs. Unfortunately, the electrochemical sensing performances of pure α-Fe2O3 NPs modified electrodes are relatively poor probably because of their poor electrical conductivity and dispersibility [37,53,54]. To address this issue, iron oxides were often used in a composite with graphene for the detection of DA. For example, nitrogen and sulfur dual doped graphene-supported Fe2O3 (NSG-Fe2O3) has been utilized for the electrochemical detection of DA in the presence of AA, with a wide linear response range (0.3–210 μM) and low LOD (0.035 μM) [55]. However, the procedure for the doping of nitrogen and sulfur is very complicated. Moreover, the simultaneous detection of DA and UA is not clear for this nanocomposite. As an important derivant of graphene, graphene oxide (GO) usually works as a conductive component to enhance the electron transfer between electrode surface and target analysts [18,19]. Indeed, there are abundant hydrophilic oxygen-containing functional groups (OxFGs) presented on the hydrophobic basal planes of GO, which can behave like an amphiphilic surfactant to improve the dispersion of α-Fe2O3 NPs [56]. As far as

we know, α-Fe2O3/GO nanocomposites have rarely reported for the simultaneous detection of DA and UA.

Herein, α-Fe2O3 NPs with various morphologies including cubic, thorhombic and discal shapes were synthesized by a facile meta-ion mediated hydrothermal route then composite with GO nanosheets. The electrocatalytic activities of DA and UA at Fe2O3/GO nanohybrids decorated glassy carbon electrodes (Fe2O3/GO/GCE) were measured in this work. The electrochemical measurements exhibited that the discal Fe2O3 NPs had the most remarkable electrochemical response toward the simultaneous detection of DA and UA, attributing to the more surface defects and rougher surface. After further coupled with GO, the discal α-Fe2O3 NPs/GO nanohybrids (d-Fe2O3/GO) showed superior electrochemical sensing performances toward DA and UA, due to the notable synergistic effect from d-Fe2O3 and GO. Therefore, a novel and ultrasensitive electrochemical sensor based on d-Fe2O3/GO nanohybrids was proposed for the simultaneous detection of DA and UA.

#### **2. Materials and Methods**

#### *2.1. Chemicas and Solutions*

Dopamine (DA), uric acid (UA), Iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O), cupric acetate anhydrous (Cu(Ac)2), Zinc acetate (Zn(Ac)2·2H2O), aluminum acetate (Al(Ac)3), disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O) and Sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O) were purchased from Aladdin Reagents Co., Ltd. (Shanghai, China). Graphite powder, potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), sodium nitrate (NaNO3), potassium ferricyanide(K3[Fe(CN)6]), potassium ferrocyanide (K4[Fe(CN)6]), ammonia (NH3·H2O), sodium hydrate (NaOH), concentrated hydrochloric acid (HCl, 37%), concentrated sulfuric acid (H2SO4, 98%) and absolute ethanol (CH3CH2OH) were supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were analytically pure and directly used as received. Human serum samples were provided by Zhuzhou People's Hospital (Zhuzhou, China). The stock solutions of DA and UA (1 mM) were prepared by dissolving appropriate amount of DA and UA in 500 mL 0.1 M PBS. Then lower concentration series of the standard solution were obtained by appropriately diluting the stock solution with 0.1 M PBS. Deionized water (DI water, 18.2 MΩ) was used in all the experiments.
