Simultaneous deposition of cerium oxide and gold nanostructures-characterization and analytical properties toward glucose electro-oxidation and sensing

Abstract

Here, we present for the first time an Au–CeO₂ nanocomposite synthesized in the presence of a mild pressure of oxygen (1.33 Pa) by the cross-beam laser deposition technique. The deposit was grown on a conductive (current collector) substrate composed of a porous network of carbon microfibers. The structure and morphology of the deposit were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XPS analysis demonstrated that the interaction of Au with CeO₂ during deposition induced charge redistribution in the ceria nanoparticles (Ce^3+,4+), resulting in positively charged Au species (Au³⁺). TEM analysis revealed that the basal planes of the carbon fibers are coated with a monolayer of interconnected nanoparticles, whereas their outer walls are covered with an ultra-thin film (∼40 nm thickness) formed of nanoparticles gathering into columns. The direct electrocatalytic oxidation of glucose in a neutral medium, such as phosphate buffered saline (pH = 7.2) solution, has been investigated with cyclic voltammetry, chronoamperometry and square-wave voltammetry. Sensitivity as high as 57.5 μA cm⁻² mM⁻¹ up to 10 mM of glucose and a low detection limit of 10 μM were obtained with square-wave voltammetry. Such electrocatalytic activity is because of the presence of cationic gold (electronic effect) and the size reduction of gold particles (size effect). This interesting analytical performance in addition to the low loading of gold (100 μg cm⁻²) makes the laser fabricated Au–CeO₂ nanocomposite electrode potentially promising for glucose fuel cells, assessment of glucose in biological fluids, as well as in the analysis of food and beverages.

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1. Introduction

The glucose electro-oxidation reaction (GER) is very important in implantable fuel cells,¹ and in heterogeneous catalysis.² The determination of glucose concentration is vital in clinical applications such as biosensors for diabetes requiring regular measurements of blood glucose levels,^3–5 as well as in food processing and fermentation.⁶

Gold is an attractive catalyst for the GER because of its excellent biological compatibility and oxidation potential in neutral and alkaline media that is more negative compared to the other metals,⁷ and thus has been extensively examined.^4,8–10

Numerous metal oxides are inexpensive, largely available, and environmentally benign, and thus are developing as a distinctive class of electrode materials for a broad range of applications such as photoelectrochemical cells and energy storage and conversion devices.^11–13 Among metal oxides, ceria is one of the cheapest and most abundant rare earth oxides. Hence, nanostructured CeO₂ is one of the extensively studied in materials applications due to improvements in redox properties, oxygen transport properties and surface to volume ratio in comparison to the bulk materials.¹⁴

Combinatorial utilization of Au and CeO₂ materials as a composite catalyst toward glucose GER is rather new. To the best of our knowledge, we were the first to consider this system for this electrochemical reaction. In our recent publication, we reported a single-beam pulsed laser deposition (PLD) of a layer onto layered (lol) CeO₂–Au nanostructures.¹⁵ Various morphologies of CeO₂ were synthesized under three different background atmospheres, i.e., vacuum, inert (He gas) and oxygen gas. The effect of the obtained morphologies (regular segments composed of clusters of particles, fluffy-like texture, lamellae-like texture) of the CeO₂ layer on the electroactivity of gold (itself deposited by PLD) towards GER was presented. Among the fabricated electrodes, CeO₂ synthesized in the presence of 1.33 Pa of oxygen exhibited the best electrocatalytic supporting properties to Au towards GER with a sensitivity of 44 μA cm⁻² mM⁻¹, and a detection limit of 10 μM within 0.01 and 10 mM of glucose concentration range.

In the present study, we present the synthesis and characterization of simultaneously deposited CeO₂ and Au materials by cross-beam pulsed laser deposition (CBPLD) method in the presence of 1.33 Pa pressure of oxygen. The objective here is twofold: from a fundamental viewpoint it is to investigate the microstructural interactions of the co-deposited CeO₂ and Au as compared with the previously developed lol architecture. From a practical viewpoint it is to seek further enhancement in the electrocatalytic activity and sensing of glucose for gold.

XRD, XPS, SEM and TEM were employed to investigate the structural properties of the Au–CeO₂ composite. Cyclic voltammetry (CV), chronoamperometry and square-wave voltammetry (SWV) electrochemical were employed to study the activity towards GER conducted in phosphate buffered saline (PBS, pH = 7.2) solution containing glucose concentrations varying from 0.01 mM to 100 mM.

2. Experimental

2.1. Synthesis

Gold and CeO₂ were deposited onto carbon paper (CP, Toray) by CBPLD at room temperature using a pulsed KrF excimer laser (λ = 248 nm, pulse width = 17 ns, and repetition rate = 50 Hz). A pure polycrystalline Au target (99.99%, Kurt J. Lesker Co.) and a pure polycrystalline CeO₂ target (99.99%, Kurt J. Lesker Co.) were used as raw materials for the co-deposition. Prior to laser ablation, the PLD chamber was evacuated with a turbo pump (∼0.0026 Pa). The fluence was fixed to 4 J cm⁻² and the target-to-substrate distance was set to 50 mm. The composite Au–CeO₂ was deposited under 1.33 Pa of oxygen with 20 000 laser pulses. A gold loading of about 100 μg cm⁻² was estimated by weighing the substrate before and after the deposition of gold with 20 000 laser pulses using a single-beam PLD. During deposition, the Au and CeO₂ targets were continuously rotated and translated to obtain a uniform ablation over the entire surface of the targets. A detailed description of the CBPLD setup has been provided elsewhere.¹⁶

2.2. Material characterization

The morphologies of the Au–CeO₂ structures were observed with SEM (JEOL JSM-6300F operating at 20 kV) and TEM (JEOL JEM-2100F operating at 200 kV). The samples were subjected as-produced to SEM and TEM analyses. XPS measurements were carried out via a VG Escalab 220i-XL equipped with an Al Kα source (1486.6 eV). The anode was operated at 10 kV and 20 mA and the pass energy of the analyzer was fixed at 20 eV. The samples were analyzed with a spot size of 250 × 1000 μm located approximately in the center of the sample. A survey spectrum ranging from 0 to 1300 eV was first acquired, and then higher resolution multiplex scan spectra (Au 4f, Ce 3d, O 1s, and C 1s core levels) were obtained. Quantification of the elements was performed with CasaXPS software version 2.3.15 (Casa Software Ltd.) by fitting the core level spectra to several Pseudo-Voigt functions (sum of Gaussian–Lorentzian function) after a Shirley background removal. The C 1s core level peak at 284.6 eV, resulting from hydrocarbon contaminants at the surface, was used as an internal reference. All the spectra were recalibrated with respect to the C 1s core level peak of adventitious carbon contamination.

The crystalline structure of the samples was determined by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer equipped with a Cu Kα source. The diffractometer was operated at 40 kV and 40 mA. Diffractograms were carried out in the Grazing Incidence Diffraction (GID) scan mode with an incident angle of 2° and a 2θ angular step size of 0.05° with an acquisition time of 5 s per step.

2.3. Electrochemical characterization

Prior to the electrochemical measurements, the surface of the working electrode was cleaned electrochemically by potential cycling in 0.5 M H₂SO₄ (96% purity from Acros Organics). The electrochemical properties of the Au–CeO₂ electrodes were investigated using CV, chronoamperometry and SWV in a de-aerated PBS solution (pH = 7.2, Sigma-Aldrich) with glucose (D-(+)-glucose, ACS reagent grade, Sigma-Aldrich). All the electrochemical measurements were carried out at room temperature using a three-compartment electrochemical cell using an Ag/AgCl, 3 M KCl reference electrode and a platinum coil as a counter electrode. Measurements and data acquisition were conducted with an Autolab potentiostat/galvanostat obtained from Eco Chemie.

3. Results and discussion

3.1. Structural characterization

Fig. 1a is an SEM image at nominal magnification 200× showing the general features of Au–CeO₂ deposited in the presence of 1.33 Pa of oxygen on carbon paper substrate. Images 1b and 1c are at nominal magnifications from 5000 to 20 000× of a selected area from the entire sample. The images show an Au–CeO₂ layer of rough morphology fully covering the carbon paper substrate.

Fig. 1 (a)–(c) SEM images at increasing magnifications of Au–CeO₂ deposited onto carbon paper by CBPLD under 1.33 Pa of oxygen partial pressure. (d) Corresponding XRD pattern.

The corresponding XRD profile of the Au–CeO₂ composite is shown in Fig. 1d. The diagram shows peaks corresponding to the (111), (200), (220), and (311) crystallographic planes of the face centered cubic (fcc) gold (JCPDF 89-3697). In addition, the XRD profile shows the presence of diffraction peaks (111), (200), and (220) corresponding to the fcc fluorite structure of CeO₂ (JCPDS file 13-1002). By using the Debye-Scherrer equation and the most intense (111) diffraction peak for Au and CeO₂, the average crystallite size was estimated to be 36.9 nm and 6.2 nm for CeO₂ and Au, respectively. The lattice constant of CeO₂ is equal to 5.416 Å (close to theoretical calculations (5.411 Å)), whereas that of gold is 4.077 Å (close to theoretical calculations (4.080 Å)).

Fig. 2a shows the TEM image taken on one of the microfibers composing the carbon paper substrate. The high resolution TEM images show that the morphology of the Au–CeO₂ deposit is a mixture of an ultra-thin layer composed of strongly interconnected nanoparticles along the basal plane of the carbon fiber (Fig. 2b) and a thicker film of about 40 nm thickness on the outer walls of the carbon fiber (Fig. 2c). A closer look shows that the thicker film assembles into the columns of nanoparticles. The HR-TEM image (Fig. 2d) shows that it is difficult to distinguish between Au and CeO₂ particles. The particle average diameter estimated from Fig. 2d was 5.5 ±0.8 nm.

Fig. 2 TEM and HR-TEM analyses of Au–CeO₂ deposited onto carbon paper by CBPLD under 1.33 Pa of oxygen partial pressure.

Afterward, the XPS technique was employed to examine the surface constituent phases and to obtain further information on the interaction between CeO₂ and Au. The XPS survey (not shown) presented peaks only for C, O, Ce and Au with a surface composition of 31.04, 38.08, 12.16 and 18.71 at%, respectively. Fig. 3a shows the high-resolution XPS Au 4f core level spectrum, which reveals that Au is present in more than one oxidation state. The Au 4f spectrum is composed of two doublets at energies 83.9/87.7 eV and 86/89.6 eV. The first doublet corresponds to metallic Au⁰ while the second oxidation state can be linked with the oxidized Au³⁺ species. Fitting the spectrum with two doublets gave an Au³⁺/Au⁰ ratio of 0.26. Au–CeO₂ composites exhibiting ionized gold have been observed by some researchers,^17–19 and this is the first time it has been observed for Au–CeO₂ films prepared by CBPLD technique.

Fig. 3 XPS analysis of Au–CeO₂ deposit. (a) High-resolution XPS spectrum in the Au 4f region and (b) High-resolution XPS spectrum in the Ce 3d region.

The Ce 3d core level XPS spectrum of the Au–CeO₂ composite reported in Fig. 3b consists of three 3d_5/2–3d_3/2 doublets characteristic of Ce⁴⁺ (CeO₂) and two doublets of Ce³⁺.²⁰ The multiplets are because of the spin–orbit coupling energies due to different 4f electron configuration in the final state. The results of Fig. 3 show that cerium oxide was partially reduced. The Ce³⁺/Ce⁴⁺ ratio obtained by calculating the sums of the areas of the Ce 3d doublets was 0.07.

3.2. Electrochemical and glucose determination studies

The synthesized Au–CeO₂ nanocomposite was first characterised by CV to confirm the typical electrochemical response for Au. Fig. 4a shows the CV profile for Au–CeO₂ in a de-aerated solution of 0.5 M H₂SO₄ at a scan rate of 50 mV s⁻¹. The voltammetric profile shows the characteristic redox processes due to the formation of Au oxide within 1.1 V to 1.47 V potential range and a reduction peak centered at 0.94 V vs. Ag/AgCl, which confirm the cleanliness of Au in the synthesized Au–CeO₂ nanocomposite. The electroactive surface area (ESA), estimated by integrating the charge consumed for reducing the Au oxides formed in the positive scan by assuming that the reduction of a monolayer of Au oxides requires 400 μC cm⁻²,²¹ was found to be 6.03 cm², which is higher than the ESA obtained with the lol electrode structure CeO₂–Au (4.30 cm²).¹⁵

Fig. 4 CVs recorded at the Au–CeO₂ electrode in an argon-saturated solution of: (a) 0.5 M H₂SO₄ recorded with 50 mV s⁻¹ potential scan rate, and (b) 0.01 M PBS (pH = 7.2) containing 100 mM glucose recorded with 2 mV s⁻¹. Arrows indicate the scan direction.

The glucose electro-oxidation was first investigated at the Au–CeO₂ nanocomposite electrode in a pH 7.2 PBS solution containing 100 mM glucose. Fig. 4b shows the CV obtained at a slow scan rate of 2 mV s⁻¹ (quasi-steady state). The electrode displayed the typical response of glucose oxidation at the Au electrode.²² The peak current in the forward scan is 674 μA, which is 1.45 times larger than that delivered by the lol CeO₂–Au electrode (450.45 μA).¹⁵

Afterward, slow scan CV with 2 mV s⁻¹ was conducted in argon-saturated PBS solution containing various concentrations of glucose (0.01 mM to 100 mM) at the Au–CeO₂ nanocomposite electrode (Fig. 5a). The maximum current (peak current) extracted from Fig. 5a was then plotted against glucose concentration, and the corresponding curve is shown in Fig. 5b. It can be seen that the peak current increases as the concentration of glucose increases. The current response varied linearly with glucose concentration from 0.01 mM to 10 mM with a regression equation of y = 26.7x + 4.21 (x: concentration/mM, y: current/μA cm⁻²) and a correlation coefficient of R² = 0.9982. The sensitivity and the low detection limit were found to be 26.7 μA cm⁻² mM⁻¹ and 10 μM, respectively. For glucose concentrations higher than 10 mM, a non-linear dependence was observed because of the saturation of active adsorption sites with glucose.

Fig. 5 (a) Voltammetry measurements recorded with 2 mV s⁻¹ at the Au–CeO₂ electrode in argon-saturated 0.01 M PBS solution at pH 7.2 containing glucose concentrations varying from 0.01 mM to 100 mM. (b) Dependence of the peak current over the entire range of glucose concentration. Arrows indicate the scan direction.

Chronoamperometry was also employed as a detection method. Fig. 6a shows the chronoamperometric measurements of glucose using the Au–CeO₂ nanocomposite electrode. This figure represents the current-time profiles obtained by setting the working electrode at +0.25 V vs. Ag/AgCl for various concentrations of glucose. The stationary current signals recorded after 60 s depended linearly on glucose concentration up to 10 mM of glucose (Fig. 6b) with a regression equation of y = 25.2x + 2.86 (x: concentration/mM, y: current/μA cm⁻²) and a correlation coefficient of R² = 0.9973. The sensitivity and the low detection limit were found to be 25.2 μA cm⁻² mM⁻¹ and 10 μM, respectively. These results are very similar to those obtained with voltammetry.

Fig. 6 (a) Chronoamperograms recorded at an applied potential of 0.25 V vs. Ag/AgCl at the Au–CeO₂ electrode in argon-saturated 0.01 M PBS solution at pH 7.2 containing glucose concentration varying from 0 mM to 100 mM. (b) Dependence of the steady-state current density over the entire range of glucose concentration.

Square-wave voltammetric techniques are widely used to improve the electroanalytical parameters for the direct measurement of concentration, e.g., the lowest limit of detection and sensitivities. Fig. 7a shows SWV responses recorded with the Au–CeO₂ nanocomposite electrode for various concentrations of glucose under optimized conditions, i.e., frequency of 20 Hz, square-wave amplitude of 20 mV, and step potential of 1 mV. Fig. 7b shows the analytical curve, i.e., the dependence of peak current density on glucose concentration for the interval between 0.01 and 100 mM glucose. The current response varied linearly with glucose concentration from 0.01 mM to 10 mM with a regression equation of y = 57.5x + 8.86 (x: concentration/mM, y: current/μA cm⁻²) and a correlation coefficient of R² = 0.9971. The sensitivity was found to be 57.5 μA cm⁻² mM⁻¹ and a low detection limit 10 μM was obtained. The sensitivity obtained with the SWV technique is twice that of those obtained by CV and chronoamperometry, and the acquisition time is also considerably shorter with the former technique. The linearity from 0 to 10 mM is far beyond the physiological level (3–8 mM). Further work will be performed to investigate the interference species, such as uric acid and ascorbic acid, in real samples envisaging practical glucose analysis.

Fig. 7 (a) SWV recorded with an amplitude of 20 mV, a step potential of 1 mV and a frequency of 20 Hz at the Au–CeO₂ electrode in argon-saturated 0.01 M PBS solution at pH 7.2 containing glucose concentration varying from 10 μM to 100 mM. (b) Dependence of the peak current over the entire range of glucose concentration. Arrows indicate the scan direction.

3.3. Comparison with other laser synthesized Au based electrodes

The PLD technique is one of the most suitable techniques for the synthesis of model materials for fundamental studies because the technique allows for robust control of the morphology and the dimensions as well as the retaining of stoichiometry of the target in the deposited film.²³ In addition, ultra-low loading together with planar deposition are two other essential characteristics that could make PLD promising as a future-manufacturing route for on-chip miniaturized biomedical devices including analytical sensors.

It is also well-known that the method of synthesis enormously affects the performance of a material. Therefore, it would be more appropriate to compare the performance of gold-based electrodes produced by the same method of synthesis at least from a fundamental standpoint. Hence, Table 1 compares sensitivity performances obtained by the SWV of gold toward glucose with different catalytic supports (carbon paper (CP), carbon nanotubes (CNTs) and CeO₂). In all the cases, gold and CeO₂ were synthesized by the PLD technique. In light of the results shown in Table 1, it is clear that CeO₂ brings an improved effect to gold vis-à-vis the GER as compared to CP or CNTs supports. Table 1 further shows that within the two architectures of the CeO₂ and Au system, the lol architecture delivered an electroactivity of gold toward oxidative sensing of glucose lower than the simultaneously deposited Au–CeO₂ nanocomposite (this work). This shows the importance of the designing of the Au–CeO₂ architecture to fully benefit from the promoting properties of CeO₂.

Table 1 Comparison of the sensitivity and crystallite size of various gold-based electrode structures

Electrode structure	Sensitivity (μA mM^−1 cm^−2)	Crystallite size (nm)	References
CP/Au	3.6 (Fig. S3a)	34.8 (Fig. S3b)	—
CNT/Au	25	20–35	24
CNT/AuOx	43.6	12–20	25
CeO2–Au (lol)	44	37.86	15
Au–CeO2	57.5	6.2	This work

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XPS analysis of the lol CeO₂–Au structure did not show a change in the electronic structure of Au deposited onto the CeO₂ layer in the presence of 1.33 Pa of oxygen.¹⁵ On the contrary, XPS shown in this work revealed that when CeO₂ is simultaneously deposited with Au, the latter is partially oxidized to Au³⁺ and Ce⁴⁺ is moderately reduced to Ce³⁺. It should be noted that we conducted the co-deposition of Au and CeO₂ by CBPLD under vacuum and under an inert atmosphere of helium gas; no cationic gold was detected by XPS (Fig. S1†). Thus, it can be expected that PLD deposition in an atmosphere of oxygen would result in the dissociation of O₂ molecules and the production of ionized oxygen, which would react with gold to form an oxide.²⁶ However, such mechanism was observed only for oxygen pressures higher than 13.33 Pa.^25,27 Similarly, in the case of CeO₂ and Au co-deposited under 1.33 Pa of oxygen partial pressure, a part of this oxygen dissociates and ionizes due to the laser during deposition and there is added ionized oxygen obtained from the CeO₂ target. Consequently, the proportion of oxygen is larger during the deposition, which enables the occurrence of gold oxide. Therefore, despite the concentration of Au³⁺ being low, its presence in the co-deposited Au–CeO₂ sample appears to have contributed to the enhancement of electroactivity towards GER as compared to the CeO₂–Au lol structure.

However, the presence of gold ions cannot be solely responsible for the good electrochemical performance of the co-deposited CeO₂–Au nanocomposites. Indeed, the size of the crystallites of gold, which is another obvious parameter with a more positive impact on the activity toward GER. This can be understood by Table 1, which summarizes the crystallite sizes of gold determined by XRD with different catalyst supports and electrode structures. As can be seen, the co-deposition of CeO₂ with Au yielded crystallite sizes of the latter considerably smaller than the other electrode structures. This indicates that CeO₂ affects the dispersion quality of Au nanoparticles. We conducted control syntheses by co-depositing Au and CeO₂ by CBPLD under vacuum or inert gas (He), and obtained an average crystallite size for Au of 8 nm and 6 nm, respectively (Fig. S2†). Therefore, small crystallite sizes of Au are only obtained when CeO₂ and Au are simultaneously deposited irrespective of the background atmosphere in the deposition chamber. It is known that as the grain size reduces, the reactive sites increase and exhibit enhanced catalytic activity.²⁸

A further improvement of the Au–CeO₂ electrode is that it contains a smaller amount of Au (100 μg cm⁻²) as compared to the CeO₂–Au lol architecture (265 μg cm⁻²), which is appealing for commercial applications considering the cost of gold.

4. Conclusion

The CBPLD method is employed for the first time to synthesize a Au–CeO₂ nanocomposite on a substrate composed of a porous network of carbon microfibers. The synthesis was performed in the presence of a mild pressure of oxygen (1.33 Pa) in the deposition chamber. TEM analysis showed that the basal planes of the carbon fibers were coated with a monolayer of strongly interconnected nanoparticles, whereas their outer walls were coated with an ultra-thin film (∼40 nm) formed of nanoparticles assembling into columns. Ceria particles possessed Ce both in the 3+ and 4+ oxidation states as detected by XPS with very low concentrations of the former. The gold deposit exhibited a polycrystalline structure and revealed a low concentration of Au³⁺ ions. These results suggest that the interaction of Au with CeO₂ in the presence of oxygen involves charge redistribution in the ceria nanoparticles, resulting in positively charged Au species.

The analytical application of such composites towards the oxidation and detection of glucose was investigated using CV, chronoamperometry and SWV techniques. A sensitivity as high as 57.5 μA cm⁻² mM⁻¹ was obtained with SWV up to 10 mM of glucose and a low detection limit of 10 μM. This performance constitutes a very good enhancement as compared with the layer onto layer Au–CeO₂ architecture and carbon support-based gold electrodes due to the presence of cationic gold (electronic effect) and to the size reduction of gold particles (size effect).

This interesting analytical performance makes the laser fabricated Au–CeO₂ nanocomposite electrode potentially promising for glucose fuel cells, biomedical analysis (assessment of glucose in biological fluids, such as blood, saliva, sweat, urine, and serum), as well as for food and beverage analysis.

Acknowledgements

This work was financially supported by the Natural Sciences Engineering Research Council of Canada (NSERC), and the Fonds Québécois pour la Recherche en Nature et Technologie (FQRNT).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05374e

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