Electrooxidation and amperometric determination of vorinostat on hierarchical leaf-like gold nanolayers

Talanta 178 (2018) 704–709

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Electrooxidation and amperometric determination of vorinostat on hierarchical leaf-like gold nanolayers R. Dehdari Vaisa, K. Karimianb, H. Helia, a b

MARK

⁎

Nanomedicine and Nanobiology Research Center, Shiraz University of Medical Sciences, Shiraz, Iran Arasto Pharmaceutical Chemicals Inc., Yousefabad, Jahanarar Avenue, Tehran, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Gold Electrodeposition Green synthesis Vorinostat Electroanalysis

Hierarchical leaf-like gold nanolayers were electrodeposited using choline chloride as a shape directing agent and characterized using field emission scanning electron microscopy. The electrooxidation behavior of vorinostat was then studied on the nanolayers and the kinetic parameters of the electrodic process were obtained by voltammetric measurements in a phosphate buffer solution at pH 7.40. Vorinostat was electrooxidized on the nanolayers’ surface at a lower potential and with a higher rate, compared to a polycrystalline smooth gold surface, through an irreversible process. Based on the results, an amperometric sensor was designed using the hierarchical leaf-like gold nanolayers for the determination of vorinostat. A linear dynamic range of 4.0–52 μmol L−1 with a calibration sensitivity of 7.7 mA mol−1 L, and a detection limit of 1.40 μmol L−1 were obtained. The amperometry method was also applied to the analysis of vorinostat capsules.

1. Introduction Synthesis of new nanostructured materials with novel properties and diversities in shape and size is one of the most important recent developments in nanotechnology [1]. Nanomaterials have been extensively utilized in various field of nanomedicine including imaging [2], drug delivery [3], diagnosis [4], therapy [5], antibacterial materials [6] and biosensing [7,8]. These materials cause to enhancements in sensitivity and selectivity of the sensing and biosensing methods [9–13] arising from their unique physicochemical properties related to the size, shape, atomic arrangement and the electronic and local dielectric specifications [14–17]. Therefore, the exact controlling size and shape are key parameters to control the physicochemical properties of nanostructures [16,18]. Study of the redox properties of biologically active compounds can give insight into their pharmacological activities, metabolic fate, their mechanism of action in vivo, and quantitation in pharmaceutical dosage forms and biological fluids [19,20]. In this regard, nanomaterials have attracted considerable attentions [21–23]. These materials have been successfully applied to the electroanalysis of drugs, pharmaceuticals and biologically important compounds [9,10,19,24,25]. Vorinostat (N-Hydroxy-N′-phenyloctanediamide, VOR, Scheme 1) is a synthetic hydroxamic acid derivative with antineoplastic activity. It binds to the catalytic domain of the histone deacetylases (HDACs) resulting in the hydroxamic moiety to chelate zinc ion in the active site of

⁎

HDAC. This causes to inhibition of enzyme activity and hyperacetylation of histones. Hyperacetylation of histone proteins results in the upregulation of the cyclin-dependant kinase p21, followed by G1 arrest. This drug also sensitizes tumor cells to apoptosis. Vorinostat is administrated for the treatment of cutaneous T cell lymphoma, a type of skin cancer, to be used when the disease persists, gets worse, or comes back during or after treatment with other medicines. There are a few reports on the determination of VOR [26–30] based on liquid chromatography-mass spectrometry which is generally complicated, tedious, expensive and time-consuming. In addition, no electrochemical study has been performed on VOR. In the present study, hierarchical leaf-like gold nanolayers were electrodeposited using choline chloride as the shape directing agent, and then applied to the electrocatalytic oxidation and determination of VOR. The methodology provides accuracy, precision and excellent limits of detection and quantitation, which are critical parameters in the analysis of a drug. 2. Experimental section 2.1. Materials All chemicals were of analytical grade purchased form Merck (Germany) or Sigma (USA) and were used without further purification. VOR was received from Arasto Pharmaceutical Chemicals Inc., Tehran, Iran.

Corresponding author. E-mail addresses: [email protected], [email protected] (H. Heli).

http://dx.doi.org/10.1016/j.talanta.2017.10.001 Received 26 August 2017; Received in revised form 29 September 2017; Accepted 2 October 2017 Available online 05 October 2017 0039-9140/ © 2017 Elsevier B.V. All rights reserved.

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O

H N

N H

O

In order to obtain information about the morphology and size of the synthesized gold nanostructure, field emission scanning electron microscopy (FESEM) was performed using a Zeiss, Sigma-IGMA/VP instrument (Germany) equipped with energy dispersive X-ray spectroscopy (EDS) capability.

OH

Scheme 1. Chemical structure of VOR.

2.3. Modification of the Au electrode with the leaf-like gold nanolayers

2.2. Apparatus

Au/LN-Au electrode was prepared by a potentiostatic electrodeposition method. Firstly, the Au electrode was polished by sand papers and then on a polishing microcloth with 0.05 μm-alumina powder lubricated with water to attain a mirror-like surface. The electrode was then cleaned by immersion in a 1:3 water/ethanol mixture and ultrasonication for 5 min in an ultrasonic bath. The electrode was then electropolished by immersion in a 500 mmol L−1 H2SO4 solution and applying potential in the range of cathodic to anodic edges of the

Electrochemical experiments were performed in a conventional three-electrode cell powered by a µ-Autolab type III potentiostat/galvanostat (The Netherlands). An Ag/AgCl-saturated solution of KCl, a glassy carbon rod, and a bare (Au) or modified gold disk electrode with the hierarchical leaf-like gold nanolayers (Au/LN-Au) were used as the reference, counter and working electrodes, respectively. The system was run on a PC through the GPES 4.9 software.

Fig. 1. FESEM images (A-D) with different magnifications and an EDS (E) of the Au/LN-Au electrode surface.

705

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3. Results and discussion Au/LN-Au

3

Au/LN-Au/VOR

2.5

Au/VOR

2 I ( A)

Fig. 1A-D shows FESEM images of the Au/LN-Au electrode surface with different magnifications. The images recorded at low magnifications show that the Au surface was covered by leaf-like layers. The thickness of the layers was estimated to be less than 100 nm. FESEM images recorded at high magnifications reveal that the leaf-like gold nanolayers are comprised from ensembles of nanoparticles of 120 ± 22 nm (n = 20). Fig. 1E shows an EDS of the Au/LN-Au electrode surface confirming the purity of the gold nanolayers. Fig. 2 shows cyclic voltammograms of the Au and Au/LN-Au electrodes recorded in PBS in the absence and presence of 0.4 mmol L−1 VOR. VOR was electrooxidized as one anodic peak in the voltammograms using both the electrodes and no cathodic counterpart appeared in the backward sweep. The electrooxidation of VOR was irreversible on both the electrode surfaces. VOR was oxidized through an ill-defined peak on the Au electrode surface with a peak potential of ∼655 mV and a peak current of ∼1.4 μA, while, the Au/LN-Au electrode provided a well define peak with a peak potential of 455 mV and a peak current of ∼2.0 μA. These results indicated that VOR was oxidized on the Au/LNAu electrode surface at a lower potential of ∼200 mV. Therefore, hierarchical leaf-like gold nanolayers enhanced the electrooxidation process from the thermodynamic point of view. On the other hand, the anodic peak current for VOR oxidation using the Au/LN-Au electrode was ∼1.4 times higher than that of the Au electrode. This rate enhancement was in accordance with the increase in the real surface area of the Au/LN-Au electrode, and therefore, the Au/LN-Au electrode can detect VOR with an enhanced sensitivity. It has been reported that the chemical activity and electrochemical properties of gold are shape depend [33], and the gold atoms located at the edges of the nanostructures have a higher activity. VOR bears oxidizable functional groups, namely the hydroxylamine, and is therefore amiable to electrooxidation on the Au/LN-Au electrode surface. The N-substituted hydroxylamine part of the VOR structure is oxidizable according to the reaction presented in Scheme 2 [34]. It should be mentioned here that we did not study the effect of pH on the electrooxidation reaction; our study was limited to physiological pH due to instability of VOR at acidic and basic pHs. VOR is decomposed by acid or base according to the reaction presented in Scheme 3. Fig. 3A represents a pseudo-steady-state current-potential curve recorded for the electrooxidation of VOR on the Au/LN-Au electrode. Typical S-shaped plot was obtained and the transfer coefficient (α) was found by Tafel plot, as shown in Fig. 3B. The charge transfer coefficient for the electrooxidation of VOR on the Au/LN-Au electrode surface was obtained as α = 0.39. This indicates that the barrier for potential energy change during the electrooxidation process had a curvature to the reaction product(s). Cyclic voltammograms of a 0.4 mmol L−1 VOR solution recorded at different potential sweep rates from 5 to 500 mV s−1 using the Au/LNAu electrode are shown in Fig. 4A. The peak current increased, and the peak potential shifted to more positive potentials with increase in the potential sweep rate. The latter further confirmed the irreversible nature of the electrooxidation process. The electron transfer coefficient of the oxidation reaction can be obtained using the dependency of the anodic peak potential on the natural logarithm of the potential sweep rate using the following equation [35]:

Au

1.5 1 0.5 0 -0.5 -1 -50

50

150

250

350

450

550

650

750

850

E (mV)

Fig. 2. Cyclic voltammograms of Au and Au/LN-Au electrodes recorded in PBS in the absence and presence of 0.4 mmol L−1 VOR. The potential sweep rate was 50 mV s−1.

electrolyte stability in a regime of cyclic voltammetry for 25 consecutive cycles. Upon this pretreatment, a clean and stable surface was obtained. The Au electrode was then placed in the cell containing 20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 choline chloride, and a potential of 0.0 mV for 300 s was applied. The obtained Au/LN-Au electrode was then rinsed thoroughly with distilled water.

2.4. General procedures The real surface areas of the Au and Au/LN-Au electrodes were determined electrochemically using K4[Fe(CN)6] (0.5 mmol L−1) as a redox probe. For a reversible redox process, the dependency of the peak current on the potential sweep rate is given by [31]:

Ip = (20.69 × 105)n3/2AC*D1/2v1/2

(1)

where Ip is the peak current, n is the number of exchanged electrons, A is the surface area, C* is the bulk concentration, D is the diffusion coefficient, and ν is the potential sweep rate. For the redox transition of [Fe(CN)6]4-, D = 7.60 × 10−6 cm s−1 [32]. Cyclic voltammograms using both Au and Au/LN-Au electrodes were recorded and the real surface areas were obtained. The Au/LN-Au electrode was found to have a surface area of ∼1.4 times of the Au electrode surface. Standard solutions of VOR were prepared by dissolving the drug in a small volume of dimethyl sulfoxide, and stored in the dark at 4 °C. Dimethyl sulfoxide is not electroreactive on the working electrodes in the potential range of electroreactivity of VOR. Additional dilutions were performed daily just before use with 100 mmol L−1 sodium phosphate buffer solution at pH 7.40 (PBS). PBS was used as the supporting electrolyte throughout the electrochemical measurements on VOR. The calibration curves for the drug in PBS were recorded by amperometry. A working potential of 480 mV was employed, in which the transient currents were allowed to decay to steady-state values. In order to analyze the drug capsules, the average mass of 10 capsules was measured and the capsules were then finely powdered and homogenized in a mortar and an accurately weighed amount of this sample was dissolved in an appropriate volume of dimethyl sulfoxide. The mixture was then sonicated in an ultrasonic bath for 5 min. The supernatant was removed and the clear solution was employed as a stock solution. Appropriate volumes of this solution were diluted with BPS and directly analyzed. O

H N O

N H

OH

- 2H+ - 2e

E pa = (RT/2αF ) ln ν

(2)

where Epa, α and ν are the anodic peak potential, electron transfer coefficient and the potential sweep rate, and the other variables have O

H N O

706

N H

OH

Scheme 2. Oxidation xylamine part of VOR.

of

N-substituted

hydro-

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Scheme 3. VOR decomposition in acid or base.

their usual meanings. Fig. 4B shows the linear dependency of Epa on ln ν, and the electron transfer coefficient was obtained as 0.41. In addition, the heterogeneous electron transfer rate constant and the electron transfer coefficient can also be obtained using the dependency of the natural logarithm of the anodic peak current on (Epa-E0′) value using the following equation [31]:

Ipa = 0.227 FAC*k 0 exp[−(αF /RT)(Epa−E 0 ′)]

(3)

where Ipa, C*, k° and E°' are the anodic peak current, the standard rate constant, and the formal potential. The formal potential was obtained as the extrapolated peak potential at potential sweep rate of zero. Fig. 4C shows the linear dependency of ln Ipa on (Epa-E°'), and based on this plot, the electron transfer coefficient and the standard rate constant were obtained as 0.39 and 0.31 cm s−1, respectively. In order to develop a simple and time-saving procedure for the analysis of VOR in pure form and a pharmaceutical formulation, amperometry technique was employed. Typical amperometric signals obtained during successive increments of VOR to PBS using the Au/LN-Au electrode are depicted in Fig. 5. Gentle stirring for a few seconds was needed to promote solution homogenization after each injection. The electrode response was quite rapid and proportional to the VOR concentration. The corresponding calibration curve for the amperometric signals is shown in the inset of Fig. 5. The plot is linear with a regression equation of y = (0.0077 ± 9.41 × 10−5)x + (0.0136 ± 0.0030) in a range of 4.0–52 μmol L−1 of VOR. The limits of detection (LOD) and quantitation (LOQ) of the procedure were calculated according to the 3 SD/m and 10 SD/m criteria, respectively, where SD is the standard deviation of the intercept and m is the slope of the calibration curve [36]. The determined parameters for calibration curve of the drug, accuracy and precision, LOD and LOQ and the slope of calibration curve are reported in Table 1. A comparison between the determined parameters for the drug determination was also made in Table 2. Although the present method had a higher LOD, it is more simple, faster and lowcost, compared to liquid chromatography-electrospray ionization tandem mass spectrometry. As for inspect the repeatability and reproducibility of the amperometry method, three concentrations of VOR were determined by three independent measurements over one day (intra-day assay) and over three days (inter-day assay). The obtained results are presented in Table 3. In addition, a VOR solution of 28 µmol L−1 was analyzed three times using the same Au/LN-Au electrode, and a relative standard deviation (RSD) value of 4.52% was obtained. Similar determinations were performed using three fabricated Au/LN-Au electrodes, and a RSD value of 6.03% was obtained. Based on these results the repeatability and reproducibility of the amperometry method were good. The applicability of the amperometry method for the assay of a sample dosage form was examined by analyzing the VOR capsules. The recoveries of the drug were calculated using the corresponding regression equation of previously plotted calibration plot. The results of recovery experiments using the developed assay procedure are also presented in Table 4. The results indicated the absence of interference from commonly encountered pharmaceutical excipients used in the selected formulation. Therefore, the method can be applied for determination of the drug in capsules without any interference from inactive ingredients. The selectivity of the amperometry method for the ingredients of magnesium stearate, methyl cellulose, sodium croscarmelose and glucose were also checked (Supplementary material S1) and no interfering effect was observed for these compounds.

Fig. 3. A) A steady-state current-potential curve recorded for the electrooxidation of 0.4 mmol L−1 VOR on the Au/LN-Au electrode. B) The corresponding Tafel plot.

Fig. 4. A) Cyclic voltammograms of 0.4 mmol L−1 VOR solution recorded at different potential sweep rates of 5, 7, 10, 15, 20, 40, 50, 75, 100, 150, 200, 250, 300, 350, 400, 450 and 500 mV s−1 using the Au/LN-Au electrode. B) Dependency of the anodic peak potential on the natural logarithm of the potential sweep rate. C) Dependency of the natural logarithm of the anodic peak current on (Epa-E°').

707

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Fig. 5. Typical amperometric signals obtained during successive increments of VOR to PBS using the Au/LN-Au electrode. Inset: The corresponding calibration curve.

100 nA 10 s

0.5 I ( A)

0.4 0.3 0.2

y = 0.0077x + 0.0136 R2 = 0.9983

0.1 0 0

20

40

60

C ( M)

Table 1 The determined parameters for the calibration curve of VOR and accuracy and precision using the Au/LN-Au electrode. Linear range (μmol L−1) Sensitivity (Slope, mA mol−1 L) Intercept (μA) R2 Standard error of slope (P = 0.005) Standard error of intercept (P = 0.005) LOD (μmol L−1) LOQ (μmol L−1)

Table 3 Precision (n = 3) for assay three concentrations of VOR by the amperometry method.

4.0–52 7.7 0.0136 0.9983 9.41 × 10−5 0.0030 1.40 4.60

Liquid chromatographyelectrospray ionization tandem mass spectrometry Liquid chromatographytandem mass spectrometry Liquid chromatography-mass spectrometry Ultra-performance liquid chromatography tandem mass spectrometry Liquid chromatographytandem mass spectrometry Amperometry

RSD% (intra-day assay)

RSD (inter-day assay)

16 28 40

4.82 4.52 4.71

6.01 5.36 4.88

Table 4 Determination of VOR in capsules.

Table 2 A comparison between the analytical parameters of some methods for the determination of VOR. Method

C (µmol L−1)

Capsule

Amount labeled (mg)

Amount found (mg)

Bias%

A B C

100 100 100

104.5 99.2 98.3

+4.5 −0.8 −1.7

Linear range (μmol L−1)

LOD (μmol L−1)

Reference

–

0.01 (LOQ)

[26]

quantitation, which are critical parameters in the analysis of drugs. The gold nanolayers can therefore be employed for the analysis of the drug, having the potential to be used for successful determination of VOR in capsules as well as in biological fluids.

to 3.78

0.0005

[27]

Acknowledgments

0.019–1.89

0.02 (LOQ)

[28]

We would like to thank the Research Council of Shiraz University of Medical Sciences (12314) for supporting this research.

0.008–7.57

–

[29]

0.008–1.89

–

[30]

1.40

This work

Appendix A. Supplementary material

4.0–52

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4. Conclusion Hierarchical leaf-like gold nanolayers were synthesized by an electrodeposition method on a gold electrode surface, and then employed to fabricate a VOR amperometric sensor. Other gold nanostructures can be synthesized by varying the electrodeposition conditions (e.g. deposition potential values, mode of potential applying, galvanostatic deposition, and concentration of gold ions). Clearly, the additives to the synthesis solution play a key role to determine the shape and size of the resultant gold nanostructures. The gold nanolayers showed an efficient activity toward the electrooxidation of VOR in physiological pH arising from the special shape and structure of the nanolayers. The methodology used provided accuracy, precision and excellent limits of detection and 708

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