Enantioselective Detection of (D)- and (L)-Limonene from Essential Oil Samples Using a Molecularly Imprinted Polypyrrole Electrochemical Sensor

Abstract

Limonene is a monoterpene compound that has two enantiomers, (D)- and (L)-limonene, in different quantities and proportions in several essential oils, and it is used in the chemical, pharmaceutical, and food industries. While the (D)-limonene is widely found in citrus, the (L)-limonene is found mainly in oils of Pinus and Mentha. The enantioselective detection holds the fact that the two isomers are found together in different quantities, in which each has its properties. The preferential use of one depends on the purpose, (D)-limonene is used in the pharmaceutical and chemical industry, while (L)-limonene has organoleptic properties and is used in the food industry, cosmetic, and personal hygiene industries. The methods existing in the literature involve the detection of the enantiomers by gas chromatography-mass spectrometry and high-performance liquid chromatography. Since the two enantiomers can be found together in different samples, is very important to have a simple and faster method to determine both. This work aims to develop an electrochemical sensor using a “dual template” technique, with both limonene enantiomers and a monomer (pyrrole), forming a molecularly imprinted polymer (MIP) film and determining the analyte “one-by-one”, by saturating the cavities of one specific enantiomer to determine the other one, using differential pulse voltammetry (DPV). This is the first electrochemical sensor for the enantioselective determination of (D) and (L)-limonene. Initially, a suspension of 0.4 mg mL-1 of graphene oxide in 0.1 mol L-1 of Na2SO4 solution was directly reduced on a glassy carbon electrode surface through chronoamperometry, applying a potential of -1.4 V for 500s. Then, AuNPs were electrodeposited from a 5.0×10-4 mol L-1 HAuCL solution in 0.5 mol L-1 H2SO4 onto the GCE/rGO electrode, also through chronoamperometry, in which a potential of 0.4 V was applied until the charge of 1.0x10-3 C was reached. Next, for the electropolymerization process, a solution containing 3.0x10-2 mol L-1 pyrrole and both (D)- and (L)-limonene enantiomers in a phosphate buffer solution (pH 7.0), was used in 5 consecutive voltammetric cycles on the GCE/rGO/AuNPS electrode, applying a potential range from 0.0 to 1.0 V in a scan rate of 0.02 V s-1, forming the MIP film. The extraction of the template molecules involved the application of 5 consecutive voltammetric cycles in NaOH 0.1 mol L-1 in a potential range from 0.0 to 1.5 V (0.05 V s-1). For the rebinding process, the electrode was placed for five minutes in different concentrations of one specific enantiomer after saturation of the other one, in PB solution pH 7.0. All the experimental parameters that influence the analytical performance of the device were optimized, such as pyrrole concentration, electropolymerization and extraction cycles, rebinding times, electropolymerization and rebinding pH, and nanoparticle charge. The electrode was characterized by scanning electron microscopy, electrochemical impedance spectroscopy, and cyclic voltammetry. The analytical methodology was developed, with a limit of detection (LOD) for (D)-limonene of 1.8x10-12 mol L-1, a limit of quantification (LOQ) of 5.5x10-12 mol L-1, and an amperometric sensibility (AS) of 5.8x105 A L mol-1. For the (L)- enantiomer, the LOD, LOQ, and AS are 1.4x10-12 mol L-1, 4.3x10-12 mol L-1, and 6.2 A L mol-1, respectively. The linear range is from 2.0x10-12 to 1.0x10-11 mol L-1 for both enantiomers. It was analyzed the stability and repeatability inter and intra-day of the sensor. The selectivity study of both analytes was evaluated with the imprinted factor (α) and the selectivity factor (β) with different molecules present in the sample, showing that the sensor is capable of selectively recognizing limonene. Furthermore, this method was applied to real samples of the essential oil of orange (Citrus aurantium), the essential oil of peppermint (Mentha piperita), and the essential oil of Siberian pine (Abies sibirica). The quantification of (D)- and (L)-limonene was successfully performed using the standard addition method and the results obtained were validated by recovery assays. All relative standard deviations were below 5%. These results indicate that the proposed sensor exhibits good analytical performance, being suitable for the determination of both limonene enantiomers in essential oils.Acknowledgements:São Paulo Research Foundation (FAPESP) (Grants: 2023/00817-9 and 2017/22401-8)