Voltammetric determination of phenylephrine hydrochloride using a multi-walled carbon nanotube-modified carbon paste electrode

A chemically modified carbon paste electrode (CPE) was designed by mixing graphite and multi-walled carbon nanotubes (MWCNT). The electrochemical behaviour was studied, and the determination method of phenylephrine hydrochloride (PHE) on this sensor was established. According to the results, the optimal ratio of MWCNTs was approximately 12.5% (w/w). MWCNT-modified carbon paste electrodes (MWCNT-CPEs) showed high electrochemical activity for PHE, producing a sharp oxidation peak current (Ip) at approximately +0.816 V versus a saturated calomel electrode (SCE) reference electrode in phosphate buffer solution (PBS, pH 6.45), and the Ip increased by approximately two times compared to that of the bare CPE. The anodic Ip was linearly related with 5.0 × 10−6–7.5 × 10−4 mol l−1 PHE, with a detection limit of 3.7 × 10−7 mol l−1. Furthermore, MWCNT-CPEs were successfully applied to the determination of PHE in injection, eye drop and nasal spray liquid samples as a simple, rapid and low-cost method.


Introduction
Phenylephrine hydrochloride (PHE) (scheme 1) is an adrenergic receptor agonist that acts on a receptors in skin, mucosa, viscera and other tissues, to contract blood vessel and increase blood pressure. Clinically, it is mainly used to maintain a stable blood pressure during shock and under anaesthesia, and it can also be  At present, determination methods of PHE include HPLC [1,2], UPLC-MS [3], spectrophotometry [4,5] and capillary electrophoresis [6,7]. The pretreatment processes of these methods are relatively complex, requiring large-scale instruments. Thus, they are not convenient for on-site quick detection. The sample pretreatment process of electrochemical determination is relatively simple, i.e. it is a convenient and fast measuring process using inexpensive instruments. Meanwhile, a chemically modified carbon paste electrode (CM-CPE) can improve selectivity and sensitivity of the CPE and integrate separation, enrichment and selective determination into a single step; therefore, it is one research focus of the analysis and detection field [8][9][10]. Our research group has made some achievements in the research of modified CPE [11][12][13].
It has also been reported that a modified glassy carbon electrode (GCE) can be used to detect PHE [14,15], while reports on using modified CPE to detect PHE are relatively rare [16,17]. Because of the high sensitivity and convenience in making and updating multi-walled carbon nanotube carbon paste electrodes (MWCNT-CPEs), this paper used MWCNT-CPEs as the working electrode, researched the electrochemical behaviour of PHE on the electrode and established the method of direct determination of PHE. Furthermore, different dosage forms were detected, such as injection, eye drops and nasal spray liquid, with satisfactory results. The method does not require complex pretreatment of samples and combined with a small portable electrochemical workstation, samples can be detected on site, i.e. there is no need to send them to professional laboratories.

Apparatus and chemicals
Cyclic voltammetry (CV) was performed on an Ingsens-1010 series hand-held electrochemical workstation (Ingsens Instruments, Guangzhou, China). A conventional three-electrode system with an MWCNT-CPE (home-made) as the workstation, a saturated calomel electrode (SCE; Chenhua Instruments, Shanghai, China) as the reference electrode and a platinum electrode (Chenhua Instruments, Shanghai, China) as the indicating electrode was used. A pH meter ( pHS-25, Leici Instrument, Shanghai, China) with a double junction glass electrode was used to check the pH of the solutions. PHE (99%), MWCNTs (SP, 3 -5 nm in diameter), graphite powder (SP, 40 nm) and liquid paraffin (HPLC) were purchased from Shanghai Aladdin Reagent Co., Ltd. Other chemicals were analytically pure, and the water used in the experiment was deionized water.

Preparation of solutions
A 1.0 Â 10 23 mol l 21 PHE solution was prepared by dissolving 20.37 mg of PHE standard substance in 100 ml of deionized water, and the solution was kept in a refrigerator at 48C. Phosphate buffer solution (PBS) with different pH values was prepared by mixing stock solutions of NaH 2 PO 4 and KH 2 PO 4 , and the pH was adjusted with H 3 PO 4 and NaOH.
The product instruction manual showed that the main component of PHE injection is deoxidation PHE, and the auxiliary materials are sodium metabisulfite, sodium chloride and edetate disodium. The main components of compound tropicamide eye drops are tropicamide and PHE. The main components of compound PHE nasal spray liquid are PHE, dexamethasone sodium phosphate and lincomycin hydrochloride. The above samples did not need pretreatment. An appropriate amount of sample can be directly measured and was diluted with PBS ( pH ¼ 6.45) to the required concentration.

Preparation of MWCNT-CPE
MWCNT-CPE was made by mixing graphite powder, MWCNTs and liquid paraffin evenly with a certain ratio. The mixture was placed in a glass tube with a radius of 2.0 mm and compacted. Copper wire was used as a guide line, and the electrode was fixed after drawing off the guide line. Then, the surface of the electrode was smoothed and polished on a piece of smooth white paper.
Based on the above methods, we also obtained bare CPE with no MWCNTs added.

Electrochemical analysis procedure
An appropriate amount of PHE solution was placed in a small beaker, and the three-electrode system was inserted. The enrichment time was adjusted to 10 s and the scan speed was 400 mV s 21 . The initial and final potentials were from 0.2 to 1.0 V, and the peak voltage (E p ), peak current (I p ) and other information were recorded. After the detection was completed, the working electrode was placed into a 0.1 mol l 21 NaOH solution and cyclically scanned 10 times to clear PHE adsorbed on the electrode.

Characteristics of the MWCNT-CPE using cyclic voltammetry
The electrochemical behaviour of PHE was researched using MWCNT-CPE and CPE, i.e. two different kinds of electrodes, to, respectively, scan in blank or 5.0 Â 10 25 mol l 21 PHE solution (figure 1). The anodic E p is approximately 0.82 V, and no cathode peak is observed, which shows that PHE oxidation is irreversible on the electrode. The I p of PHE on CPE (b) is relatively less; and the I p is increased by approximately two times on MWCNT-CPE (a).

Effect of amount of MWCNTs
To obtain the optimal conditions to improve electrochemical performance, the ratio of MWCNTs added was studied. MWCNTs were mixed with graphite powder with a certain ratio, and an appropriate amount of liquid paraffin was added to obtain five MWCNT-CPEs. Scanning was conducted in 5.0 Â 10 25 mol l 21 PHE solution (figure 2), and it shows that when the ratio of MWCNTs and graphite powder is 12.5% (figure 2a), the I p is relatively higher, and the peak shape is better.

Effect of buffer solution
Britton-Robinson (BR), KH 2 PO 4 -H 3 PO 4 , Na 2 HPO 4 -citric acid, NaAc-HAc, NH 3 -NH 4 Cl, PBS and other buffer solutions were used to dilute PHE solutions to 5.0 Â 10 25 mol l 21 , and cyclic scanning was conducted on the above solutions. We found anodic peaks for BR, KH 2 PO 4 -H 3 PO 4 , NH 3 -NH 4 Cl and PBS, while the peak shape in BR is not obvious and is unstable, and the E p is approximately 0.60 V; E p s in NH 3 -NH 4 Cl and PBS are approximately 0.82 V. The peak shape in PBS is better, and the I p is higher. Furthermore, various experiments show that reproducibility in PBS is also better.

Effect of pH
The influence of pH on I p (5.0 Â 10 25 mol l 21 PHE) by CV in PBS with various degrees of acidity was examined, and according to figure 3, the I p increases to its peak and then decreases (figure 3b). When the pH is 6.16 and 6.45, the I p is larger and the peak pattern is better ( figure 3a); therefore, PBS at pH of 6.45 is chosen in the following experiment. Furthermore, it is also found that PHE E p decreases with the increase in pH, and there is also a linear relationship between them (figure 3c), which indicates that protons are involved in the PHE oxidation

Effect of accumulation time
The influence of accumulation time on the I p for two different PHE concentrations was investigated, and when the concentration is 1.0 Â 10 24 mol l 21 , the I p reaches the maximum in 10 s. When the concentration is 5.0Â10 25 mol l 21 , with increasing accumulation time, there is no significant change in I p ; therefore, the accumulation time in the following experiment is 10 s.

Effect of scan rate
The effect of scan rate on the electrochemical behaviour of PHE was investigated at the MWCNT-CPE by CV. In PBS-buffered solution with pH 6.45, accumulation time of 10 s, and PHE concentration of 5.0Â10 25 mol l 21 , the scan rate was changed from 50 to 800 mV S 21 , and the changes in I p and E p of PHE were recorded (figure 4a); with an increasing scan rate, the I p , as well as E p , gradually increases. In the experiment, it is also found that there is a good linear relationship (figure 4b) between the I p and scan rate. The linear equation of I and v is I ¼ 0.0099v þ 0.6147 (r ¼ 0.9990), which illustrates that the redox reaction of PHE on MWCNT-CPE is mainly adsorption controlled.
In addition, there is also a good linear relationship (figure 4c) between E and the lnv, and the linear equation is:  According to reversible electrode reactions, the following relationship exists between the E p and scan rate [18]: where a is the charge transfer coefficient, n is the number of transferred electrons, R is the gas constant (8.314 J K 21 ), T is the temperature (K, 298) and F is the Faraday constant (96487 C mol 21 ). Combining equation (3.1) with (3.2), 1/2 Â RT/anF ¼ 0.0223, and an is calculated to be 0.58. The value of a is assumed to be 0.5 for an irreversible electrochemical process [19], and the electron participating in the reaction is calculated as 1.16 (close to 1). Combining the conclusions from before, the reaction mechanism of PHE on MWCT-CPE is shown in scheme 2.

Interference studies
Some common substances or constituents of pharmaceutical formulations were added to a 5 Â 10 25 mol l 21 PHE solution, and the changes in I p and E p were measured (table 1). The result shows that these substances have no interference on PHE detection, and this sensor is highly selective for PHE.

Electrode precision and repeatability
After obtaining a stable scan in the blank solution, the freshly prepared modified electrode was evaluated by continuously measuring the PHE standard solution 10 times. As the relative standard deviation (RSD) was 0.7%, the modified electrode was highly precise. Seven electrodes were prepared with the same condition at the same time. RSD was 2.9% when measuring the same PHE standard solution, which indicated good repeatability.

Calibration curve and limit of detection
Under the optimized conditions, CV was selected to determine PHE at MWCNT-CPEs. When the PHE concentration is within 5.0 Â 10 26 -7.5 Â 10 24 mol l 21 , there is a good linear relationship (figure 5) between PHE concentration and I p . The linear equation is I ¼ 5.8764c (Â10 24 mol l 21 ) þ 5.737 (r ¼ 0.9995), and based on the signal-to-noise ratio of 3, the detection limit is obtained as 3.7 Â 10 27 mol l 21 . Scheme 2. Oxidation mechanism of PHE at MWCNT-CPE.

Conclusion
A sensitive method of determining PHE with a selective MWCNT-CPE was established. The optimal conditions were as follows: the ratio of MWCNTs was approximately 12.5% (w/w) in the working electrode, PBS at pH 6.45 was chosen as the buffer and the accumulation time was 10 s. With a scan rate of 400 mV s 21 , PHE exhibited an oxidation peak at 0.816 V. When the concentrations were 5.0 Â 10 26 -7.5 Â 10 24 mol l 21 , there was a good linear relationship between PHE concentration and I p , and the detection limit was found to be 3.7 Â 10 27 mol l 21 (S/N ¼ 3