Pain, despite its association with discomfort and malaise, is one of nature's mechanisms to warn living creatures of dangerous circumstances in their bodies and prevent them from committing further harm to themselves. However, it becomes a purposeless agony when surgeries and medical procedures are performed, due to which attempts to vanquish the feeling of pain have been widely explored from time immemorial. The earliest known shreds of evidence on the usage of anesthetics date back to around 800 AD when herbal mixtures of opium, hemlock, and mandrake were formulated into anesthetic vapors to sedate patients during surgical procedures [1], [2]. Since then, countless milestones have been made in the medical field to conquer pain and eliminate the fear of surgeries among patients. One such historic landmark was accomplished in the 1900 s when Procaine Hydrochloride (P.HCl) was introduced to the medical sector for the first time by a German chemist Alfred Einhorn, and thereupon P.HCl became the first injectable local anesthetic drug used by humankind [3]. Unlike general anesthetic drugs, local anesthetic drugs numb only a small portion of the body during surgical procedures like deep-cut suturing, repairing a broken bone, skin biopsies, and breast biopsies. Prior to the discovery of P.HCl, cocaine was widely used for local anesthesia because of its ability to act as a vasoconstrictor [4], [5]. However, prolonged use of cocaine resulted in severe local and systemic toxicities, because of which the introduction of P.HCl into the field of anesthesia soon became widely accepted owing to its less hazardous nature than cocaine. In contrast to cocaine, P.HCl is a vasodilator and is usually administered with a vasoconstrictor like epinephrine so that low drug concentrations can be used with a longer duration of action. Additionally, it is advantageous over other anesthetic drugs, as it is simple to prepare and has a shorter recovery time [6].

P.HCl is usually administered as an infiltration anesthetic rather than a surface anesthetic, where the anesthetic solution is directly injected into the area of terminal nerve endings as it is ineffective when applied directly onto the skin [7]. The dosage is determined primarily based on the type of procedure, the number of nerves to be numbed, the size of the application region, and the patient's underlying conditions [8]. According to the formulations recommended by the United States Food and Drug Administration (FDA), the single safe dose of P.HCl must strictly not exceed 1–5 ml of 1% P.HCl [8]. If administered in appropriate amounts, P.HCl provides a reversible loss of sensation with instant pain relief. It manages to have such an effect as it is a benzoate ester derivative that releases free alkalies upon hydrolysis, and these alkalies bind to the voltage-gated sodium channels of the nervous system, obstructing the conduction of nerve impulses. This provides a speedy repose to the patients it is being administered, facilitating certain serious and delicate clinical procedures. Concomitantly, an excessive or an inadequate dose of the drug has shown adverse side effects, including cardiological and neurological toxicity like dizziness, nausea, chills, coma, allergic reactions, convulsions, arrhythmia, and even cardiopulmonary arrest in some cases [9]. Real-time monitoring of the drug after its administration is highly recommended to track whether the patient is showing hypersensitivity to the drug or is developing signs of local and systemic toxicity. This helps reduce the risks associated with the drug and assists in alerting the professionals to take further action if any atypical symptoms emerge. These aforementioned reasons motivated us to develop a simple yet sensitive and selective analytical method for determining P.HCl as such an analysis may not just help in real-time monitoring but also aids in ensuring optimum therapeutic concentration required during treatments and assist in the quality assurance of the drug during pharmaceutical preparations.

Various analytical methods have been reported to determine P.HCl, including the following: spectrophotometry [10], [11], [12], [13], [14], [15], high-performance liquid chromatography [16], [17], [18], fluorimetry [19], [20], [21], gas chromatography [22], [23], liquid chromatography-mass spectrometry [24], ion-pairing flow injection analysis [25], dead-stop titration [26], and chemiluminescence [27], [28]. Despite these methods having high accuracy, they are usually not cost-effective, sophisticated, time-consuming, require pretreatments, and most often demand skilled operators trained in handling certain procedures of these techniques. Electrochemical methods are preferred over other analytical techniques for pharmaceutical determinations owing to their low-cost instrumentation, experimental simplicity, quick surface renewability, high sensitivity and selectivity, rapid response time, and exceptional ability to sense continuously. These advantages of electrochemical sensors make them superior methods in pharmaceutical analysis, providing rapid yet reliable information about the drug [29], [30], [31], [32].

The presence of an aromatic amine moiety makes P.HCl an electroactive molecule and enables the drug to generate good analytical responses in electrochemical studies. However, one of the substantial hurdles confronted in the bare electrodes is the high overpotential required for detection. This leads to lower selectivity, stability, and reproducibility due to electrode fouling, ultimately reducing the sensor's performance [33]. In the current study, the failure of unmodified CPE to detect P.HCl was a serious limitation. An effective way to overcome this limitation is by modifying electrodes, based on which several procedures for P.HCl determination have been investigated. Kataky and Palmer investigated the use of ion-selective electrodes (ISEs) for P.HCl detection and reported detection limits in the range of 10 µM. [34] Chemically modified electrodes such as metal-oxide dispersed glassy carbon electrodes (GCEs) were reported by Zhou et al. for amperometric detection of P.HCl with a detection limit of 3.2 µM [35]. Similarly, Bergamini et al. developed screen-printed carbon electrodes (SPCEs) for amperometric determination of P.HCl with a wide linear range of 9–100 µM and a detection limit of up to 6 µM [36]. However, these reported methods involved sophisticated procedures, expensive modifiers, and acidic pH values, resulting in further complications in the sensor development, which will be addressed in Section 4.2. Moreover, there has been a growing demand for the development of rapid, simple, and inexpensive methods for determination for various analytes in the recent past, which inspired us to develop an electrochemical sensor based on carbon paste electrodes (CPE) [37]. CPE are excellent candidates for the detection of various agricultural, industrial, and pharmaceutical products as they proffer the possibility of numerous surface modifications and biocompatibility and are cost-efficient [38], [39], [40], [41]. Modifying CPE with nanomaterials like carbon nanotubes, graphene sheets, metal nanoparticles, and others [38], [39], [40], [41], [42], [43], [44], [45] allows one to strategically tailor the electrocatalytic behavior of the sensor towards the target analyte P.HCl. In addition, it is important to mention that there have been no previous reports of amperometric detection of P.HCl based on modified CPEs which further motivated us to explore this method.

For the development of the sensor, we attempted bulk modification of CPE with hybrid multiwalled carbon nanotubes (MWCNTs). MWCNTs are regarded as the chemical genius of carbon, owing to their exceptional electric conductivities, structural flexibility, high surface areas, exorbitant tensile strength, and excellent chemical stability [46]. The functionalization of MWCNTs relieves them of their chemical inertness and water incompatibility, which adds to their ability to act as outstanding electrocatalysts in the field of sensors [47], [48]. Initially, we attempted to modify the bare CPE with various other functionalized nanocomposite materials such as ZnO-CNT, Fe2O3-CNT, MgO-CNT, MWCNT-COOH, Bi2O3-CNT and CuO-MWCNT which were found to give no resolvable signals for P.HCl detection. Upon incorporating BaO-MWCNT onto the CPE matrix, there was a staggering current response for P.HCl oxidation, in contrast to the bare CPE, which gave no response towards P.HCl. Compared to other alkaline oxides, barium oxide (BaO) is conceived to possess superior catalytic activity due to its strong basic sites [49]. Thus, the hybridization of MWCNTs with BaO emanated a synergistic advantage of both electrical and mechanical characteristics, enhancing the detection properties of the sensor at a reduced overpotential.

In the present study, we have synthesized, characterized, and applied BaO-MWCNT nanocomposites for the detection of P.HCl. The BaO-MWCNT/CPE was optimized using cyclic voltammetry (CV), after which P.HCl was quantified employing amperometry (AMP). The novelty of the sensor is its ability to operate at a beneficially lower over-potential than other previously published papers on P.HCl, as in Table 1. A lower over-potential not just sustains the electrode stability and prevents it from fouling but indicates the requirement of lower activation energy, making the electrode energy-efficient. Our observations from the surface morphology images and EIS studies demonstrate the efficient electrocatalytic behavior of the electrode upon modification, owing to the extraordinary 66-fold increase in current that we have achieved. The fabricated sensor was also competent in detecting P.HCl in the presence of various interferents like ascorbic acid (AA), dopamine (DA), epinephrine (EP), and uric acid (UA) and a few metal ions, including iron, magnesium, manganese, and copper, manifesting the desirable selectivity. Finally, our ultimate initial objective of investigating the sensor's analytical application was successfully validated by determining P.HCl in human urine and blood serum samples.