Self-assembly of cucurbit[7]uril on the surface of graphene/gold modified electrode

Abstract

Using supramolecular recognition capability of cucurbit[7]uril to create a novel electrochemical sensing platform was accomplished by self-assembly of cucurbit[7]uril on the surface of graphene–gold composites modified glassy carbon electrode. The graphene–gold composites consisted of electrochemically reduced graphene oxide–nafion matrix and anchored electrodeposited gold nanoparticles and the graphene–gold composites modified glassy carbon electrode (labelled as graphene–gold) were characterized by scanning electron microscopy and Raman spectrometer. Self-assembly of cucurbit[7]uril was achieved by immersing the graphene–gold electrode into cucurbit[7]uril solution and confirmed by Fourier transform infrared spectra, cyclic voltammetry and electrochemical impedance spectroscopy. Ferrocene and its derivatives served as the substrates, and the electrochemical responses of fabricated graphene–gold/cucurbit[7]uril electrode were evaluated. The results showed that the presence of cucurbit[7]uril increased the peak current, improved the reversibility of reaction and extended the recognition ability of the modified electrode.

Introduction

Graphene (Gr), whose structure consists of a single layer of graphite in which sp ²-hybridized carbon atoms are packed in a hexagonal lattice, has attracted tremendous scientific and technological attention because of its unique properties, including superhigh specific surface area, excellent thermal and electrical conductivity, good mechanical properties and biocompatibility and ease of functionalization for the catalysis application. ^{1 –6} Especially, Gr and its derivations, such as N-doped Gr, Gr oxide (GO), chemically reduced GO and electrochemically reduced GO (ERGO), have been widely adopted as electrode modification materials for non-enzymatic electrochemical sensors. ^{7 –9}

Meanwhile, many other inorganic nanomaterials, including electroactive noble metals (e.g. gold (Au), platinum (Pt) and silver (Ag)), transition metals (e.g. copper, nickel and carbon monoxide), transition metal oxides (e.g. copper oxide, copper(I) oxide and cerium(IV) oxide), transition metal hydroxides (e.g. nickel(II) hydroxide and copper(II) hydroxide) and alloys (e.g. Au–Pt), have been extensively utilized for the fabrication of non-enzymatic electrochemical sensors, ^{10 –13} owing to the advantages of simplicity, reproducibility and stability in aggressive environment. Among them, gold nanoparticles (AuNPs) have attracted much interest because of their good electrical conductivity, biological compatibility and stability. ¹⁰ Moreover, it was found that electrocatalytic synergy of Gr and AuNPs can extremely improve the performance of the resultant electrochemical sensor. ^{14 –19} The Gr–Au composites commonly inherit the integrated features of fast charge transport kinetics and good catalytic activity, which are favourable for the applications in electrochemical sensors. Previous investigations have demonstrated that the Gr–Au hybrid materials could offer impressive sensitivity in the detection of certain electroactive chemicals. However, because of the lack of special interactions between Gr/AuNPs with target electroactive chemicals, non-enzymatic electrochemical sensors may have an intrinsic limitation on selectivity that restricts their further development in practical applications.

Introducing novel elements into Gr–Au composites may overcome the problem. Macrocyclic receptors, such as cyclodextrins, crown ethers, calixarenes and their special host–guest interactions, have been extensively explored in the last two decades. ^{20 –22} They can improve the selectivity of electrochemical sensor efficiently through the special host–guest interactions between macrocyclic compound and target molecule. Cucurbit[n] urils (CB[n]) are a new group of macrocyclic compounds with polar carbonyl groups surrounding the portals and pumpkin-shaped molecules. ^23,24 Owing to the interaction with biomolecules, CB[n] are found to be used very popular in electrochemical biosensing recently. ^{25 –28} It is expected that immobilization of CB[n] on the surface of Gr–Au composites modified electrode will improve the selectivity of the electrochemical sensor because of the host–guest recognitions.

It has been reported that ferrocenyl derivatives can strongly and reversibly bind to CB[7]-coated Au surfaces. ²⁹ To prove our concept, the ferrocenyl derivatives in CB[7]-based host–guest systems were selected. In this work, we have made use of the supramolecular recognition capability of CB[7] to create a novel electrochemical sensing platform by combining with Gr–Au composites modified glassy carbon electrode (GCE; Figure 1). The Gr–Au composites modified GCE was constructed with ERGO–nafion nanocomposites and electrodeposited AuNPs. The self-assembly of CB[7] was confirmed by Fourier transform infrared (FTIR) and electrochemical measurements. Ferrocene (Fc) and its derivatives serve as the substrates, and the electrochemical properties with the response of cyclic voltammetry (CV) of Gr–Au composites modified GCE were explored.

Figure 1.

An illustration of self-assembly of CB[7] on the surface of Gr/AuNPs modified electrode. CB[7]: cucurbit[7]uril; Gr: graphene; AuNP: gold nanoparticle.

Experimental

Material

The GO was prepared and characterized as described in our previous works. ^30,31 Nafion (5% ethanol (EtOH) solution) was purchased from Alfa Aesar Chemical, United Kingdom. Chloroauric acid hydrate (HAuCl₄·4H₂O) was purchased from Sinopharm Chemical, Beijing. Fc, ferrocenecarboxylic acid (Fc–COOH) and ferrocenecarboxaldehyde (Fc–CHO) were purchased from J&K Chemical, Beijing. CB[7] was supplied by Prof. Guangtao Li, Tsinghua University. Ultra-pure water were produced using a Millipore-Q system, Merckmillipore, Germany (>18 MΩ cm).

Fabrication of GCE/ERGO–nafion/AuNPs electrode

GCE/ERGO–nafion/AuNPs electrode (‘/’ represents which is used to modify electrodes in successive steps and ‘-’ means which is mixed first and then used to modify electrodes) was constructed as our previous report. ¹⁴ In brief, 6 μL of the suspension of GO–nafion mixture were cast onto the surface of the GCE and air-dried. Then, GO was electrochemically reduced to form ERGO–nafion electrode. After that, AuNPs were electrodeposited on the surface using potential scanning from –0.55 V to −0.95 V for 15 cycles by immersing the electrode into 0.5 M sulphuric acid aqueous solution containing 0.4 mg/mL HAuCl₄ to achieve the GCE/ERGO–nafion/AuNPs electrode (labelled as Gr–Au).

Self-assembly of CB[7] on the surface of electrode

GCE/ERGO–nafion/AuNPs electrode was immersed into the solution of CB[7] (10 mM) for 12 h, then washed with pure water for three times and dried under nitrogen gas. The obtained modified electrode was labelled as Gr–Au/CB[7].

Electrochemical study of Fc and its derivatives

The as-prepared Gr–Au/CB[7] electrode was then used for the electrochemical study of Fc and its derivatives. Typically, the Gr–Au/CB[7] electrode was immersed in the acetonitrile solution of Fc (10 mM), Fc–COOH (1 mM) or Fc–CHO (10 mM) for 10 min and then washed with acetonitrile, EtOH and water to remove surface-adsorbed Fc or its derivatives. Then, CV was used to test the electrochemical response of Fc or its derivatives in 1 mM phosphate buffered saline solution at a scan rate of 0.1 V/s.

Characterization

Scanning electron microscopy (SEM) observations were carried out with LEO-1503 (German). The Raman spectra were recorded from 1000 cm⁻¹ to 2000 cm⁻¹ using a Raman spectrometer (Senterra&Veate X70; Bruker, Germany) with an argon ion laser excitation of 532 nm. FTIR spectra were obtained using a Spectra Two (PerkinElmer, USA) spectrophotometer within the spectral range of 4000–400 cm⁻¹ with attenuated total reflection (ATR) accessory. CHI660a electrochemical analyser (Chen Hua Instruments, China) was used to perform electrochemical experiments using a three-electrode system, where a modified (Φ = 3 mm) GCE, a Ag/silver chloride (3.5 M potassium chloride (KCl) solution) electrode and a Pt wire used as the working electrode, the reference electrode and the auxiliary electrode, respectively. The CV experiments were performed in a quiescent solution of 10 mM KCl and 10 mM Fe(CN)₆ ^3−/4−(1:1). Electrochemical impedance spectroscopy (EIS) was performed in 10 mM KCl solution containing 10 mM Fe(CN)₆ ^3−/4−(1:1) at frequencies ranging from 0.01 Hz to 100 kHz at open circle potential.

Results and discussion

The Gr–Au electrode was fabricated according to our previous report. ¹⁴ The GO–nafion nanocomposites were first assembled on GCE surface and then reduced electrochemically. The SEM image of the obtained ERGO–nafion was shown in Figure 2(a); the ERGO sheets look folded and wrinkled and buried in polymer matrix. The folded and wrinkled structure of GO could supply more surface area for the successive Au deposition. Raman spectroscopy was used to confirm reduction. As shown in Figure 2(b), two distinct peaks appear at 1342 and 1578 cm⁻¹, which correspond to the D-and G-bands, respectively. The D-band is attributed to the in-plane carbon-ring breathing (A _1g) mode, and the G-band corresponds to the E _2g vibrational and out-of-plane modes within aromatic carbon rings. In general, the intensity ratio of D and G peaks (I _D/I _G) is used as a measure for the size of sp ² domains according to the Tuinstra–Koenig relation. ³² The I _D/I _G of the GO–nafion is only 1.03; after reduction, the value increases to 1.33. It suggests that the average size of the sp ² domains in GO decreases after reduction, which is caused by more numerous in number and smaller in size of the new created sp ² domains in ERGO than that exist in the GO. In addition, the overtone 2D-band of GO–nafion is invisible, but after electrochemical reduction, the intensity of the overtone 2D-band of ERGO–nafion turns more evident. The 2D-band peak is centred at 2680 cm⁻¹, which is in agreement with that of reduced GO in previous report, ³³ indicating the success of electrochemical reduction of GO. The AuNPs were electrodeposited on the surface of GCE/ERGO–nafion electrode. As shown in Figure 2(c), numerous spherical AuNPs locate on the surface of the electrodes. The average diameter of the AuNPs is about 76 nm (Figure 2(d)). These small AuNPs provided a large accessible surface area for the subsequent self-assembly of CB[7].

Figure 2.

(a) SEM image of GCE/ERGO–nafion electrode. (b) Raman spectra of GO–nafion and ERGO–nafion. (c) SEM image of AuNPs on the surface of GCE/ERGO–nafion electrode. (d) Statistical analysis of sizes of AuNPs in (c). SEM: scanning electron microscopy; GCE: glassy carbon electrode; ERGO: electrochemically reduced graphene oxide; GO: graphene oxide; AuNP: gold nanoparticle.

It was first reported by An et al. ³⁴ and Tao et al. ³⁵ that CB[n] can easily self-assemble on the surface of Au by immersing Au electrode in CB[n] solution. Here, the Gr–Au electrode was immersed into the solution of CB[7] for 12 h to construct Gr–Au/CB[7] electrode. The formation of Gr–Au/CB[7] was detected by ATR-FTIR (Figure 3). In the ATR FTIR spectrum of CB[7], two characteristic peaks of CB[7] at 1751 and 1474 cm⁻¹ were, respectively, ascribed to C=O and C–N stretching vibrations. ³⁶ While in the spectrum of Gr–Au, there was no obvious band in that position. After self-assembly of CB[7], the two characteristic peaks of CB[7] were clearly detected in the spectrum of Gr–Au/CB[7], indicating the existence of CB[7] molecules on the surface of Gr–Au/CB[7] electrode.

Figure 3.

ATR-FTIR spectra of Gr–Au, CB[7] and Gr–Au/CB[7]. ATR: attenuated total reflection; FTIR: Fourier transform infrared; Gr: graphene; Au: gold; CB[7]: cucurbit[7]uril.

CV and EIS techniques have been employed to further support the efficient formation of Gr–Au/CB[7] electrode. Figure 4(a) shows CV curves of Gr–Au and Gr–Au/CB[7] electrodes in 10-mM KCl aqueous solution containing 10-mM Fe(CN)₆ ^3−/4− (1:1). A pair of redox peaks of Fe(CN)₆ ^3−/4− are observed at 0.302 V (oxidation peak potential (E _Pa)) and 0.123 V (reduction potential (E _Pc)), respectively (Figure 4(a)), on Gr–Au electrode. The volt response of E _Pa shifts negatively to 0.266 V, while the volt response of E _Pc shifts positively to 0.172 V on the Gr–Au/CB[7] electrode. The calculated ΔE_{a –c} values decreased from 179 mV to 94 mV, indicating that the presence of CB[7] leads to more reversible electrochemical behaviour when working with the Gr–Au/CB[7] electrode. In addition, both the oxidation peak current (I _Pa) and reduction current (I _Pc) have a little increase. The influence of a varying scan rate on the peak current was also investigated, as shown in Figure 4(b). As the scan rate increased from 30 mV/s to 150 mV/s, the redox peak currents linearly increased with a scan rate (Figure 4(c)), which revealed that the electrochemical kinetics was a surface-controlled process. Figure 4(d) and its inset displayed the EIS profiles of Gr–Au and Gr–Au/CB[7] electrodes, in which the Nyquist plots were shown with the real part (Z′) on the X-axis and the imaginary part (−Z″) on the Y-axis. The data were fitted in a typical equivalent circuit, and the R _et value of Gr–Au was 176.2 ± 8.3 Ω, while the value of Gr–Au/CB[7] slightly decreased to 155.2 ± 9.2 Ω. It suggested that when CB[7] was immobilized on the electrode, CB[7] promoted the electron transfer from the probe molecules (Fe(CN)₆ ^3−/4−) to the surface of electrode. In addition, the change of electrochemical capacitance was used to determine the CB[7] assembly. After the deposition of CB[7], the capacitance decreased along with the soak time (Online Supplementary Figure S1). The trends were very similar at the different scan rates, which were consistent with the results reported by An et al. ³⁴

Figure 4.

(a) CV curves of (a′) Gr–Au and (b′) Gr–Au/CB[7] electrodes in 10-mM KCl aqueous solution containing 10-mM Fe(CN)₆ ^3−/4−(1:1) at a scan rate of 0.1 V/s. (b) CV curves of Gr–Au/CB[7] electrode at scan rates of 0.03, 0.05, 0.07, 0.1 and 0.15 V/s. (c) Plot of redox peak current against scan rate. (d) Nyquist plots of the EIS diagrams of (a′) Gr–Au and (b′) Gr–Au/CB[7] electrodes. The frequency range is from 100 MHz to 100 kHz. Inset: Enlarged Nyquist plots of the EIS diagrams (a″) Gr–Au and (b″) Gr–Au/CB[7] electrodes. CV: cyclic voltammetry; Gr: graphene; Au: gold; CB[7]: cucurbit[7]uril; KCl: potassium chloride; EIS: electrochemical impedance spectroscopy.

To explore the potential application of Gr–Au/CB[7] as a sensing platform, the ability of Gr–Au/CB[7] to envelop Fc and its derivatives was investigated by recording their CV responses (Figure 5(a)). Before the electrode was immersed in any substrate solution, Gr–Au/CB[7] exhibited no redox peaks. After interacting with Fc, Fc–COOH or Fc–CHO for 12 h, respectively, Gr–Au/CB[7], a pair of nearly reversible redox peaks were obtained on Gr–Au/CB[7]. This result indicates that the formed Gr–Au/CB[7] electrode can detect Fc and its derivatives by capturing them with host–guest recognitions. The CV curves of Gr–Au electrode responding to above three substrates were also recorded, as shown in Figure 5(b). There were small redox current peaks in the CV responses of Fc and Fc–CHO, but no peaks in the CV responses of Fc–COOH were observed even from −1.0 V to 1.0 V. The presence of CB[7] increased the peak current, improved the reversibility of reaction and extended the recognition ability through effective formation of inclusion complex with Fc and its derivatives. The different positions of the redox couple peaks for Fc and its derivatives make the sensor be able to detect them in a single run. The prepared Gr–Au/CB[7] electrode exhibited good repeatability. The response current of the second time detection maintained 101.8% of its initial electrochemical response (Online Supplementary Figure S2). Furthermore, the proposed sensing platform has wide potential applications in many fields, such as detection of the amino acids (Online Supplementary Figure S3), and to distinguish o-, m- or p-hydroxybenzyl alcohol isomers (Online Supplementary Figure S4).

Figure 5.

CV curves of (a) Gr–Au/CB[7] and (b) Gr–Au before and after adsorbing Fc, Fc-COOH and Fc-CHO. CV: cyclic voltammetry; Gr: graphene; Au: gold; CB[7]: cucurbit[7]uril; Fc: ferrocene; Fc-COOH: ferrocenecarboxylic acid; Fc-CHO: ferrocenecarboxaldehyde.

Since An et al. ³⁴ reported that CB[n] can easily self-assemble on the surface of Au by immersing Au electrode in CB[n] solution, several works about CB[n] on Au surface have been published. Taylor et al. ³⁷ and Tao et al. ³⁵ almost simultaneously reported the fabrication of Au aggregates with uniform 1-nm nanogaps through the assembly of CB[7]s with Au colloids. Jones et al. ³⁸ further used to align Au nanorods with subnanometer separation using CB[n] for surface-enhanced Raman scattering (SERS) applications. Shi et al. ³⁹ developed a new SERS-based sensor (assembled from AuNPs and Gr using CB[7] as a precise molecular glue) for the selective trace measurement of Pb²⁺. Blanco et al. ³⁶ presented the comparative study of the spontaneous adsorption of cucurbit[6]uril and CB[7] on Au by means of atomic force microscopy. The CB[7]-coated Au surfaces were used to immobilize cells by integrin binding arginine-glycine-aspartate (RGD) peptide–Fc conjugates ⁴⁰ and used for the site-selective immobilization of fluorescent proteins that were monovalently labelled with ferrocenylamine. ²⁹ However, self-assembly of CB[7] on the surface of Gr/Au modified electrode as a novel electrochemical sensing platform described in this work was the first example. The CB[n] (n = 5–8) have several members, which present a cavity diameter of 0.44, 0.58, 0.73 and 0.88 nm for n = 5, 6, 7 and 8, respectively. ³⁶ Their general inclusion properties are high affinity and highly selective, constrictive binding interactions with lots of specific guest species, driven by a combination of ion–dipole interactions, hydrogen bonds and hydrophobic effect. Benefiting from the diversity of the CB[n], the guest molecules and the supramolecular interactions, the strategy of self-assembly of CB[7] on the surface of Gr/Au modified electrode can open a new way for the construction of electrochemical sensors with high selectivity.

Conclusion

To the best of our knowledge, this is the first example of CB[n] modified Gr/Au composite-based electrode being used as an electrochemical sensing platform. Self-assembly of CB[7] can easily achieved by just immersing the Gr/Au composite-based electrode into CB[7] solution. Self-assembly of CB[7] leads to more reversible electrochemical behaviour and larger redox peak currents when working with the Gr–Au/CB[7] electrode and also improves electron transfer. Fc and its derivatives serve as the model substrates, and the presence of CB[7] can increase the peak current, improve the reversibility of reaction and extend the recognition ability through effective formation of inclusion complex with substrates. This first attempt of self-assembly of CB[7] on the surface of Gr/Au modified electrode as a novel electrochemical sensing platform may open a new way for the construction of high selective electrochemical sensors.