Rhodium Nanoparticle-mesoporous Silicon Nanowire Nanohybrids for Hydrogen Peroxide Detection with High Selectivity

Abstract

Developing nanostructured electrocatalysts, with low overpotential, high selectivity and activity has fundamental and technical importance in many fields. We report here rhodium nanoparticle and mesoporous silicon nanowire (RhNP@mSiNW) hybrids for hydrogen peroxide (H₂O₂) detection with high electrocatalytic activity and selectivity. By employing electrodes that loaded with RhNP@mSiNW nanohybrids, interference caused from both many electroactive substances and dissolved oxygen were eliminated by electrochemical assaying at an optimal potential of +75 mV. Furthermore, the electrodes exhibited a high detection sensitivity of 0.53 μA/mM and fast response (< 5 s). This high-performance nanohybrid electrocatalyst has great potential for future practical application in various oxidase-base biosensors.

Developing electrocatalysts with high activity and selectivity for H₂O₂ detection is of both academic and industrial importance because H₂O₂ is the product of most oxidase catalyzed bioreactions and is commonly used for the determination of various clinical important substrates¹. Electrochemical assay is a simple, rapid and effective approach². Electrochemical detection of H₂O₂ can be achieved via either anodic oxidation or cathodic reduction. Despite many advantages, anodic H₂O₂ measurement suffers from interference caused from endogenous species, such as ascorbic, urea and uric acids that can be electro-oxidized at a similar potential³,⁴. Consequently, the detection selectivity and accuracy are affected. In order to minimize such interference, one common route is to use electrocatalysts that allow electro-oxidation of H₂O₂ at lower potential. Transition metals⁵,⁶,⁷,⁸, metal porphyrins⁹,¹⁰, redox polymers¹¹,¹², graphene¹³,¹⁴ and carbon nanotubes¹⁵,¹⁶,¹⁷ have thus been employed. Although significant advances have been made, detection error still exists because some easily oxidized substances, such as ascorbic acid (AA) is electroactive even at potential of as low as +150 mV¹⁸. These interferences accompanying H₂O₂ electro-oxidation can be eliminated if H₂O₂ is measured via electrochemical reduction¹⁹,²⁰. But cathodic detection of H₂O₂ is normally interfered by dissolved oxygen that can be reduced at a similar potential²¹,²²,²³. Therefore, it is highly desirable but yet a challenge to fabricate electrode materials that can inhibit the interferences caused from both endogenous species and dissolved oxygen. Here, we report rhodium nanoparticle-mesoporous silicon nanowire (RhNP@mpSiNW) nanohybrids for H₂O₂ detection with high selectivity and sensitivity via cathodic reduction. At an operating potential of +75 mV, interferences caused from the oxidation of endogenous substrates and the reduction of dissolved oxygen were both eliminated.

Results

RhNP@mpSiNW nanohybrids were prepared via a two-step approach. Firstly, mpSiNWs were prepared via a metal-assisted chemical etching approach²⁴,²⁵. Figure 1a is a typical field emission scanning electron microscopy (FE-SEM) cross-sectional view of the as-synthesized mpSiNW arrays. The NWs have homogenously distributed nano-pores with an average size of about 15 nm as revealed from the transmission electron microscopy (TEM) shown in Supplementary Fig. S1a. The continuous crystal lattices across these nano-pores shown in Supplementary Fig. S1b indicate a single crystal feature of the as-prepared mpSiNWs. After the as-prepared NWs were scratched off from the silicon substrate they were loaded with RhNPs via chemical reduction approach using NaBH₄ as a reducing agent. As revealed from Figure 1b, a typical SEM image of RhNP@mpSiNW nanohybrids, small size of RhNPs are distributed on the side surface of mpSiNW. TEM image shown in Figure 1c indicates that the large RhNP is aggregation of small particles with size of 4–6 nm. Each of these small particles is single crystal as can be seen from the high resolution TEM (HR-TEM) image shown in Figure 1d.

Figure 1.

FE-SEM images of the mpSiNWs (a) before and (b) after RhNPs loading. (c) TEM image of the RhNP@mpSiNW nanohybrid. (d) HR-TEM image of the RhNP@mpSiNW nanohybrid, indicating that each of the Rh NPs is highly crystallized.

The as-prepared nanohybrids were loaded onto glassy carbon (GC) electrode for H₂O₂ measurement. Figure 2a shows the linear sweep voltammetry (LSV) scan of the RhNP@mpSiNW/GC electrode in an oxygen-free buffer (solid line), air saturated buffer (dash line) and oxygen-free buffer containing 1 mM H₂O₂ (dash dot line). In comparison to electrode response in oxygen-free buffer, only a slightly higher cathodic current was observed in air saturated buffer which can be attributed to reduction of dissolved oxygen. The cathodic reduction starts at the potential of around + 60 mV. In marked contrast, the electrode gives a significantly higher cathodic reduction current in oxygen free buffer containing 1 mM H₂O₂. This could be attributed to the high electrocatalytic activity of the RhNPs towards H₂O₂ reduction. From these results, it can be seen that the dissolved oxygen will have little effect on the cathodic reduction of H₂O₂ if electrode potential of more positive than + 60 mV is used. Figure 2b depicts two almost identical calibrated lines of the RhNP@mpSiNW/GC electrodes at + 75 mV for the reduction of H₂O₂ with different concentrations in the presence and absence of dissolved oxygen (their original electrode LSV responses in oxygen-free solutions to various concentrations of H₂O₂ are shown in Supplementary Fig. S2). The sensitivity can be calculated to be 0.53 μA/mM from the linear part of the calibration curve. Inset in Figure 2b is electrode responses in low concentration ranges. These results show that almost no influence caused from oxygen was observed over a broad range of H₂O₂ concentrations, which can be attributed to the high electro activity of nanohybrids to H₂O₂ reduction and the considerably suppressed cathodic reduction activity to oxygen.

Figure 2.

(a) LSV response of the RhNP@mpSiNW/GC electrode with RhNP loading of 7.4 μg/mm² (loading percentage of Rh on mpSiNW is 22.7 wt%) in oxygen-free buffer, air-saturated buffer and oxygen-free buffer containing 1 mM H₂O₂ solution. (b) Plots of the current against H₂O₂ concentration in oxygen-free buffer and air-saturated buffer. Inset in (b) shows the magnified part marked with dash square. The experiments were performed at +75 mV.

Figure 3 shows a dynamic amperometric current-time response of RhNP@mpSiNW/GC electrode at +75 mV with a successive addition of H₂O₂ from 1 μM to 0.5 mM. The currents increase immediately and reached their equilibrium states rapidly after the addition of H₂O₂. The average rising time is less than 5 s, indicating a fast amperometric response towards H₂O₂ cathodic reduction. From the inset in Figure 3 we can see that the detection limit can be down to micro-mole range even in the presence of soluble oxygen. These results further reveal that the as-prepared RhNP@mpSiNW nanohybrids have good electrocatalytic activity towards H₂O₂ reduction and high selectivity against dissolved oxygen.

Figure 3. Amperometric responses of RhNP@mpSiNW/GC electrode in buffered solution after successive addition of H₂O₂ with different concentrations.

Inset: the magnified part of the curve marked with dash ellipse. The experiments were performed at +75 mV.

Figure 4a shows the current response of electroactive compounds of AA, urea and uric acid on RhNP@mpSiNW/GC electrode with different amount of RhNPs loading at potential of +75 mV during the measurement of 1 mM H₂O₂. The electrode amperometric response to H₂O₂ is not influenced by the addition of urea and uric acid in all three cases. The relative current responses of AA to that of H₂O₂ were as low as 4.5%, 2.2% and 1.0% with RhNP loading of 3.7, 7.4 and 11.1 μg/mm², respectively. Such behavior indicates that the activity towards H₂O₂ reduction increases with the increase of RhNP loading, while the response of AA is not sensitive to the RhNP loading variation. As a result, the relative current response of AA to that of H₂O₂ decreases with the increase of RhNP loading. This indicates that the interference caused from AA, which is particularly challenging in oxidase-based biosensors, can be eliminated by using RhNP@mpSiNW nanohybrids based electrode with suitable amount of RhNP loading.

Figure 4. Amperometric response of (a) RhNP@mpSiNW/GC and (b) RhNP loaded GC electrode with RhNP loading of (1) 3.7, (2) 7.4 and (3) 11.1 μg/mm² to 1 mM H₂O₂, 0.1 mM ascorbic acid (AA), 0.1 mM urea (U) and 0.1 mM uric acid (UA), respectively.

The experiments were performed at +75 mV.

It is worth to note out that electrodes based on single-component RhNPs have been widely studied because of the relatively low operating anodic potential required for H₂O₂ detection, where current contributions of endogenous substances might be minimized. Figure 4b shows the current response of electroactive compounds of AA, urea and uric acid on RhNP loaded GC electrodes without mSiNWs at a potential of +75 mV. Significantly high anodic currents that resulted from the addition of AA were observed. The electrode output increases with the increase of RhNP loading upon the addition of both H₂O₂ and AA. The ratio of current response of AA to that of H₂O₂ can be calculated to be, respectively, as high as 39.6%, 29.6% and 32.0% for GC electrodes with RhNP loading of 3.7, 7.4 and 11.1 μg/mm², indicating that AA can be easily electro-oxidized at single-component RhNPs based electrode and bring current interference even at such a low potential. In marked contrast, as shown in Figure 4a, the interference caused from AA was significantly suppressed on nanohybrids modified electrode.

The stability of the RhNP@mpSiNW/GC electrode was further evaluated by continuous chronoamperometric measurements of 1 mM H₂O₂ at +75 mV. As shown in Supplementary Fig. S3, the electrode sensitivity is almost the same even after 50 time continuous measurements, indicating a good operational stability.

Discussion

In recent years, a large range of electrocatalysts have been developed in order to lower the H₂O₂ electro-oxidation potential so that interferences caused from electroactive species could be eliminated. Despite these efforts, detection error still exists because substances, such as AA are electroactive even at low anodic oxidation potential. Cathodic reduction is a promising approach that can avoid such interference during H₂O₂ detection. But, cathodic reduction lacks of reliability because the oxygen, whose concentrations are fluctuant in solution, can be reduced at similar potential. Mesoporous silicon nanowires are attractive for developing high-performance biosensors owing to their excellent semiconducting²⁶, mechanical²⁴, optical properties²⁷,²⁸,²⁹ and favorable biocompatibility³⁰,³¹. In this work, the RhNP-mpSiNWs composite structure resulted in excellent selectivity against both many electroactive substrates and dissolved oxygen at an optimal potential of +75 mV. The remarkable selectivity against electroactive species of RhNP@mpSiNW can be attributed to the unfavorable electrostatic interaction between the mesoporous silicon and the probe molecular³². In a buffered solution with pH value of 7.2, the SiNW matrix covered with a large density of hydroxyl may bear a negatively charged surface and work as a permselective barrier that likely to prevent the negatively charged AA from reaching the electrode surface easily.

Conclusion

In this paper, RhNP@mpSiNW nanohybrids were prepared and their electro-catalytic properties were demonstrated. The amperometric detection of H₂O₂ was achieved by cathodic reduction at low potential of +75 mV, where acceptable low interferences caused from both dissolved oxygen and regular electroactive substances were achieved. Therefore, such nontoxic RhNP@mpSiNW nanohybrids are promising and have great potential for future practical artificial mediator free biosensor applications.

Methods

Chemicals

Hydrofluoric acid (HF, ≥40%), silver nitrate (AgNO₃, ≥99.8%), H₂O₂ (≥30%), sulfuric acid (H₂SO₄, 98%), nitric acid (HNO₃, 65% to 68%), sodium hydroxide (NaOH, ≥96%), sodium borohydride (NaBH₄, ≥99%), acetone, ethanol, potassium chloride (KCl, ≥99.8%), urea, uric acid and AA were purchased from Sinopharm Chemical Reagent Co., Ltd. Silicon (100) wafers (p-type, boron-doped, <1 Ω·cm) were purchased from Kaihua Lijing Silicon Materials Co., Ltd. The resistivity of the silicon wafers is 18.5 mΩ·cm according to square resistance measurement. Nafion and rhodium chloride (RhCl₃, 98%) were purchased from Sigma and used as received. All solutions were prepared with Milli-Q water.

Instrumentation and characterization

All electrochemical experiments were carried out with a CHI 660E (CHI Instruments Inc., Austin, USA). A conventional three-electrode system was used in this work. GC electrode, platinum wire and Ag/AgCl (3 M KCl) electrode were used as working, counter and reference electrode, respectively. Phosphate buffer with a concentration of 0.2 M (pH 7.2) was employed as the supporting electrolyte. Electrolyte solutions were bubbled with argon for at least 40 min and argon atmosphere was maintained over the solution during the deoxygenated electrochemical measurements. The microstructures of the samples were characterized using FE-SEM (S4800, Hitachi, Tokyo, Japan) and TEM Tecnai F20 (FEI, Hillsboro, OR, USA) electronic microscope at an accelerating voltage of 200 kV.

Preparation of mpSiNWs

Silicon (100) wafers (1.2 cm × 1.2 cm) were washed in a mixed solution of ethanol, acetone and water for 15 min and then immersed in a mixed solution of H₂SO₄ and H₂O₂ (v/v 3:1) for 10 min to remove surface organic materials. The wafers were then immersed in 8% HF solution for 10 min to form H-terminated surface and then dipped into a solution containing 8% HF and 8.3 mM AgNO₃ to coat a uniform layer of Ag nanoparticles. Subsequently the wafers were transferred to an etchant solution composed of 8% HF and 0.27 M H₂O₂. After being etched for 30 min at room temperature, the wafers were washed using 1 M diluted HNO₃ to remove the Ag catalyst and then washed with 0.1 M NaOH solution for 10 min to remove the oxide sheath. After being thoroughly washed with water, the as-prepared mpSiNWs were detached from the Si substrate by sonicating treatment, collected and re-dispersed in 5 mL water.

Preparation of RhNP@mpSiNW hybrid

90 μL of 1 wt% RhCl₃ was added to the as-prepared mpSiNWs solution (adding 45, 90 and 135 μL of the RhCl₃ solution resulted in RhNP@mpSiNW with RhNP loading of 3.7, 7.4 and 11.1 μg/mm², respectively). Then excess amount of 0.1 wt% NaBH₄ solution was added to the solution slowly at 4°C during stirring in order to prepare RhNP@mpSiNW nanohybrid. The resultant solution was stirred for another 30 min to ensure full loading of the RhNPs. The obtained RhNP@mpSiNW nanohybrid was collected, washed and re-dispersed in 1 mL water.

Preparation of RhNP@mpSiNW/GC electrode

GC electrodes were mechanically polished with 1, 0.3 and 0.05 mm alumina slurry and then washed in water, ethanol and water for 5 min sequentially. GC electrodes were then dried with nitrogen stream. The RhNP@mpSiNW solution was thoroughly mixed with Nafion solution (0.5 wt%) at 1:1 ratio (v/v). The RhNP@mpSiNW hybrid modified GC electrodes were obtained by drop costing 3 μL of the mixed solution onto GC electrode.

Preparation of the RhNP loaded GC electrode

Various amounts (45, 90 and 135 μL) of 1 wt% of RhCl₃ were added to 5 mL water. Then excess amount of 0.1 wt% NaBH₄ solution was added slowly at 4°C under stirring to form RhNPs. The resultant solution was stirred for another 30 min, washed and re-dispersed in 1 mL water. RhNP loading is similar to the process of RhNP@mpSiNW hybrid loading.

Author Contributions

X.F. developed the concept and coordinated the project. Z.S. and H.C. performed the research, analyzed the data and wrote the paper. C.X. and W.Z. contributed to the SEM and TEM characterizations.

Supplementary Material

Supplementary Information

Supporting information