2D hematene, a bioresorbable electrocatalytic support for glucose oxidation

Towards the aim of developing implantable and fully biodegradable sensors and biofuel cells, 2D nanosheets of hematite have been exfoliated and processed into electrode materials for glucose sensing. Gold, (Au) nanoparticles were electrodeposited onto the 2D substrate to develop a sensitive non-enzymatic glucose sensor. Despite a low loading of a catalyst, the composite achieved a sensitivity of 10 μA mM−1 cm−2, good linearity (0–3.2 mM) with a detection limit of 0.4 mM, a response time of less than 10 s, and long-term performance stability. These results make Au/Fe2O3 hematene nanosheet, a promising catalytic material not only for glucose monitoring but also from which to construct biofuel cells using glucose as fuel.


Introduction
Glucose plays a pivotal role in several physiological processes and thus being able to measure its concentration is essential for monitoring specific medical conditions such as diabetes or use it as a fuel in biofuel cell applications. Whether to be converted in electrical energy or measured for diagnosis, glucose is electro-oxidized at the electrode surface. The importance of glucose oxidation has led to a steady increase in demand for more electroactive, stable and lowcost glucose catalysts. Implantable sensors or biofiuel cells have been extensively studied over the past few decades wit high sensitivity and low cost enzyme, used as electrocatalyst [1][2][3]. Enzymatic catalysts have been gradually replaced with non-enzymatic glucose electrocatalyst s to overcome drawbacks associated with the enzyme, i.e. complex immobilization processes, oxygen concentration dependence and poor thermal and chemical stability. Furthermore, while efforts have been focusing on performances (activity, durability and cost) and miniaturization to imply less tissue damage, further improvements of existing sensor and biofuel cell technologyies with the use of biocompatible and resorbable materials for minimizing the need for a second surgery, reducing the risk for hemorrhage and infection is the next step for improving the quality of medical diagnosis and treatment.
Several metals such as Cu, Fe, Pt, and Au have been studied for glucose electrocatalytic activity resulting in different by-products depending on the pH and the metal used. Among them, Au has been particularly used for the electrochemical oxidation of glucose. Although bulk Au metal is poorly catalytically active, its properties can be enhanced when size reaches the nanometre scale because of the particle phase transitions. Although considerable efforts have been made on the synthesis of nanoporous Au [4][5][6] or nanoparticles [7,8], films, nanowires [9], films [10], nanoclusters [11], nanoplates [12] etc research on the substrate has been neglected and is currently limited to the biocompatible carbon nanostructures such as graphene, carbon nanotubes. Among oxides, iron oxide is one of the most abundant materials readily available, and has been widely used for bio-applications [13][14][15]. Additionally, iron oxides possess intrinsic enzyme-like activity, and are now regarded as novel enzyme mimetics [16]. Hematite (α-Fe 2 O 3 ), the most thermodynamically stable iron oxide has shown to be biologically and electrochemically inert [17]. But by modifying its morphology, size, and crystallinity, its properties can be affected; for instance, hematite nanowire has been established to be an ideal electrode material for glucose oxidation due to the intrinsic peroxidase-like catalytic activity [18]. Additionally, hematite is biodegradable so using this electrode material enables development of sensors with the ability to entirely dissolve in the body after the implantation, which can prevent the need for transcutaneous removable sensors or retrieval surgery. In a recent work, we reported the mechanical exfoliation of 2D α-Fe 2 O 3 sheet from core hematite [19]. 2D materials have excellent properties such as high mobility, high surface area and high Young's modulus and strength, despite their low weight in comparisons to their bulk counterparts [19,20]. The high mobility and electrical conductivity facilitate the electron transfer process during the glucose oxidation with reduced ohmic losses whereas its high surface area provides more accessible sites for gold nanoparticle deposition. The high mechanical strength ensures the stability of the electrodes during the use and the biodegradable properties can ensure the electrodes would be cleared from bodies after the designed time period. Furthermore, hematene can be prepared in a cost-effective and scalable method simply via sonication compared with other 2D materials. In the present work, we report the fabrication of a fully biodegradable glucose sensor made on 2D hematene for the electrocatalytic oxidation of glucose.

Au-hematene electrocatalyst preparation
Two-dimensional (2D) hematene was mechanically exfoliated mechanical in dimethylformamide (DMF) solution [19,21]. Briefly, 15 mg of natural hematite (α-Fe 2 O 3 ) was washed three times in water, acetone and then dispersed in 20 ml DMF solution. The solution was sonicated using an unmodified benchtop ultrasonic bath (Branson UltraSonic Model 1510 MT, 42 kHz, 70 W) for 24 h. The resulting reddish suspension (1 ml) was then transferred in a high-speed amalgamator (Ultramat 2, 4600 oscillations per min) and vigorously agitated for a 10 min with a high-speed amalgamator (Ultramat 2, 4600 oscillations per min). To prevent from potential overheating, the 10 min agitation is performed in two times. Finally, the solution was centrifuged with a mini-centrifuge for 1 min, and then the supernatant that contained the exfoliated hematene was pipetted for further tests.
Chronoamperometric electrodeposition technique was adopted for gold nanoparticle deposition in a nitrogen-purged electrolyte containing 1.5 mM HAuCl 4 ·3H 2 O and 0.5 M ethylene glycol (EG). Prior to the Au deposition, 40 µl of hematene suspension was drop-casted on well-polished glassy carbon electrodes (4.92 mm φ) followed by a constant applied potential at −0.2 V (vs. RHE) for 60, 120 and 180 s. The mass of deposited gold was obtained by integration of electrical charge consumed during the deposition process Q Au (C cm −2 ), according to equation (1.1): Where M and F are the atomic weight of Au (196.97 g mol −1 ) and the Faraday constant (96 485.309 C mol −1 ). The calculation is based on the assumption that the current efficiency was 100% for Au reduction (equation (1.2)).

Characterization of Au-hematene electrocatalyst
The morphology of the sample was investigated by transmission electron microscopy (TEM, FEI Tecnai F20, 200 kV) and scanning electron microscopy (SEM). The surface composition and atomic bonding information were studied by x-ray photoelectron spectroscopy (Thermo Scientific K-Alpha). X-ray diffraction (XRD) patterns were obtained on Bruker D8 Discovery instrument operating at 40 kV and 20 mA, using CuKα radiation. Electrochemical measurements were tested in a cell with three electrode system; Au/hematene deposited on glassy carbon electrode as working electrode, Pt wire as counter electrode and a saturated calomel electrode (SCE) as a reference electrode. The cyclic voltammetry was carried out in 0.1 M KOH solution with and without 10 mM glucose at a scan rate of 10 mV s −1 . The chronoamperometric response of Au/hematene in 0.1 M KOH was tested in a constant potential of 0.9 V (vs. RHE) under stirring condition. The glucose concentrations in the solution was increased gradually from 0.4 mM to 3.2 mM.

Results and discussion
Here, we reported a mechanical exfoliation method involving two steps to isolate mono-layer hematene: sonication for 24 h followed by agitation with high-speed amalgamator for 10 mins in DMF. Bright-field TEM at low and higher magnifications confirm the exfoliation of single-and bi-layer hematene (figure 1). A high-resolution TEM image (figure S1) was showed bi-layer hematene with orientation in the [001] direction, corresponding to the hexagonal symmetry of hematene with lattice parameters α = 0.50356 nm, β = 1.37489 nm [21]. After drop-casting hematene suspension in DMF on glassy carbon electrodes, gold nanoparticles were electrodeposited on hematene using chronoamperometry method at an applied potential of −0.2 V (vs. RHE) for 120 s [11]. A gold loading of 28.5, 76.5 and 81.2 µg cm −2 was calculated based on  the electrical charge consumed during a deposition process of 60 s, 120 s and 180 s respectively (figures S2 (stacks.iop.org/TDM/7/025044/mmedia), and S3). These electrodes were first screened electrochemically towards the electroxidation of glucose to select the best candidate for further characterization. The results show that the electrode with 120 s deposition time exhibiting the best catalytic activity ( figure  S3). SEM and TEM results showed some Au clusters with the size of 30~50 nm deposited homogeneously on hematene supports (figures 2(a) and (b)). At higher magnification, TEM images showed that the Au clusters are made from distinct Au nanoparticles of~5 nm (figure 2(c)), which was further confirmed with EDS ( figure 2(d)). X-ray diffraction (XRD) was directly performed on GC and Au/hematene-GC samples, respectively (figure S4). The peaks associated with hematene have been detected and indexed to hematite crystal structure (JCPDS card 33-664, a = 5.035 Å and c = 13.74 Å), and the 2θ values of 38.1 • and 64.5 • were identified for the gold nanoparticles [19,22].
The chemical composition was investigated with x-ray photoelectron spectroscopy (XPS) (figures 2(e)-(g)). A higher resolution of Fe 2p spectrum reveals that the oxidation state of Fe was + 3 with Fe 2p 3/2 and Fe 2p 1/2 peaks at 710.8 eV and 725.4 eV [8]. Au exhibits two main peaks at approximately 87.2 eV and 83.5 eV for Au 4f 5/2 and Au 4f 7/2 respectively; this result indicates that the majority of Au is metallic. Previous reports have demonstrated the biocompatibility and biodegradability of Au without toxicity [23]. Above 50 nm, Au nanoparticles are considered not biodegradable, and even show some long term toxicity concerns [24] whereas nanoparticles with sizes less than~6 nm have shown to achieve total clearance from the body [25,26]. Given that we electrochemically deposited nanoclusters consisting of 5 nm Au nanoparticles adsorbed on 2D hematene, this composite catalyst is considered as nontoxic and biodegradable.
The performance of Au/hematene towards glucose oxidation was studied in 0.1 M KOH solution (figure 3). Cyclic voltammetry (CV) was performed with and without glucose (10 mM). The results clearly show that the oxidation of glucose peaked at~0.6 V after glucose incorporation (10 mM) on Au/hematene electrode whereas no glucose oxidation occurred on the glassy carbon (GC) electrode. The mechanism upon which Au oxidizes glucose involves the creation of a thin layer of gold oxide (Au n O m ) at high potentials followed by the reaction: Finally, it is thought that a rapid electrochemical regeneration of the surface oxide takes place followed by equation (1.3) [3,4]; The electrocatalytic capacity of Au/hematene catalyst towards glucose oxidation was further studied at different scan rates ( figure 3(a)). The cyclic voltammetry responses at different scan rates clearly shows that the oxidation current is proportional to the square root of the scan rate, which indicates that the electrooxidation reaction is not limited by the materials itself but by a diffusion process ( figure 3(b)) [8]. The durability of the Au/hematene catalyst was investigated by multiple of oxidation cycle tests in 10 mM glucose solution ( figure 3(c)). At 100 cycles, CV did not show any obvious decreased in current density compared to the first scan, confirming the excellent stability of this new electrocatalyst. Chronoamperometry was employed to evaluate the steady-state response of Au/hematene electrodes ( figure 3(d)). The chronoamperometric response of Au/hematene in 0.1 M KOH was tested at a constant potential of 0.9 V (vs. RHE) under constant stirring to prevent mass diffusion limitation when adding glucose in the electrochemical cell.
The glucose concentrations in the electrochemical cell was steadily increased by 0.4 mM as soon as the current stabilised starting at 0 mM up to 3.2 mM. The response time of the electrode was less than 10 sec, indicating a fast response of the sensor towards glucose detection. The linear response of the glucose sensor to glucose concentration was linear with a detection limit (LOD) of 0.4 mM with the sensor sensitivity ca. 10 µA mM −1 cm −2 . Some promising reports of glucose detection have been reported for Au NP decorated electrodes, such as Zeng et al reported a gold NPs/polyaniline/CNTs composite modified GC electrode with a high sensitivity of 29.17 µA mM −1 cm −2 [27], and Shan et al Polyimide-carbon-AuNP-GOx No PBS (pH 7.4) 0.1~100 nA 0.026 [47] described graphene/Au NPs/chitosan composite film with a sensitivity of 0.55 µA mM −1 [28]. Instead of Au NPs, Wan et al employed gold electrode further modified by chitosan and CNTs, achieving a high sensitivity of 21 µA mM −1 cm −2 [29]. In addition to complex electrode fabrication process, they usually used enzymes (e.g. glucose oxidase), which in turns increase the costs of the system and require low-temperature storage conditions for long-term stability of these sensors. Compared to reported results, with a low gold loading of 76.5 ± 10.2 µg cm −2 , the Au-hematene electrode was able to achieve comparable sensitivity without the assistance of enzymes.
With respect to non-enzymatic composite materials, such as Au deposited on carbon nanomaterials [30][31][32] (e.g. graphene or carbon nanotube) or Cu based substrate, the limit of detection of the Auhematene electrocatalyst is fairly high (table 1). The low LOD of carbon-based substrate is probably due to its higher electrical conductivity compared to iron oxide. Indeed Au on ITO [33] exhibited an extremely poor LOD of 2 mM compared to Auhematene (0.4 mM). In spite of being 10 and even 100 times higher than the biocompatible Au-carbon based support, Au-hematene must be appreciated as a fully resorbable electrocatalytic materials. This is the first time that an electrocatalytic materials can be implanted to electrooxidize glucose and dissolved over time without harming the body. The electrochemical results indicated that Au-hematene based electrode is a fairly sensitive, easy method made of biodegradable materials to quickly oxidise glucose for detection or energy supply purposes.

Conclusion
In summary, we successfully synthesized the first fully resorbable sensor for glucose oxidation prepared from hematene and gold nanoparticles, two biodegradable materials. The electrochemical investigation of glucose oxidation in 0.1 M KOH solution was performed on Au nanoparticle-decorated hematene supported on glassy carbon electrodes. This innovative electrocatalyst exhibited excellent glucose oxidation in alkaline condition in contrast to the absence of any oxidative current on control unmodified glassy carbon electrodes. This Au-hematene electrode not only demonstrated a good sensitivity, linearity, short response time (less than 10 s), but also a high durability in KOH solution. In the future, we believe that the use of this new electrocatalyst can be extended to the development of entirely biodegradable fuel cells.