Citation: Munteanu, I.G.; Gr ˘ adinaru, V.R.; Apetrei, C. Sensitive Detection of Rosmarinic Acid Using Peptide-Modified Graphene Oxide Screen-Printed Carbon Electrode. Nanomaterials 2022, 12, 3292. https://doi.org/10.3390/ nano12193292 Academic Editors: Mohammad Abdul Wahab and Mircea Dragoman Received: 3 September 2022 Accepted: 19 September 2022 Published: 22 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Sensitive Detection of Rosmarinic Acid Using Peptide-Modified Graphene Oxide Screen-Printed Carbon Electrode Irina Georgiana Munteanu 1 , Vasile Robert Grădinaru 2 and Constantin Apetrei 1, * 1 Department of Chemistry, Physics and Environment, Faculty of Sciences and Environment, “Dunărea de Jos” University of Gala¸ ti, 47 Domneasca Street, 800008 Gala¸ ti, Romania 2 Faculty of Chemistry, Alexandru Ioan Cuza University, 11 Carol I Bd., 700506 Iasi, Romania * Correspondence: apetreic@ugal.ro; Tel.: +40-727-580-914 Abstract: Peptides have been used as components in biological analysis and fabrication of novel sensors due to several reasons, including well-known synthesis protocols, diverse structures, and acting as highly selective substrates for enzymes. Bio-conjugation strategies can provide a simple and efficient way to convert peptide-analyte interaction information into a measurable signal, which can be further used for the manufacture of new peptide-based biosensors. This paper describes the sensitive properties of a peptide-modified graphene oxide screen-printed carbon electrode for accurate and sensitive detection of a natural polyphenol antioxidant compound, namely rosmarinic acid. Glutaraldehyde was chosen as the cross-linking agent because it is able to bind nonspecifically to the peptide. We demonstrated that the strong interaction between the immobilized peptide on the surface of the sensor and rosmarinic acid favors the addition of rosmarinic acid on the surface of the electrode, leading to an efficient preconcentration that determines a high sensitivity of the sensor for the detection of rosmarinic acid. The experimental conditions were optimized using different pH values and different amounts of peptide to modify the sensor surface, so that its analytical performances were optimal for rosmarinic acid detection. By using cyclic voltammetry (CV) as a detection method, a very low detection limit (0.0966 μM) and a vast linearity domain, ranging from 0.1 μM to 3.20 μM, were obtained. The novelty of this work is the development of a novel peptide- based sensor with improved performance characteristics for the quantification of rosmarinic acid in cosmetic products of complex composition. The FTIR method was used to validate the voltammetric method results. Keywords: rosmarinic acid; sensor; peptide; graphene oxide; cyclic voltammetry 1. Introduction In recent years, a remarkable area of study using nanotechnology has been the devel- opment and characterization of new functional and nutraceutical products from natural sources, in order to improve the quality of food without the need for the use of synthetic additives, and the design of new functionalities, especially related to health promotion, such as antioxidant, antimicrobial, and antitumor properties of food [1]. There is a growing concern for and continuous and advanced research on the use of polyphenolic compounds in both the food and pharmaceutical industries, as these compounds are considered sup- plements with preventive and curative properties for certain diseases [2]. Among polyphenolic compounds, rosmarinic acid has attracted considerable interest due to its important therapeutic properties and health benefits. The compound was first isolated by Scarpati and Oriente in 1958 [3] and is found in the composition of several medicinal plants of the Laminaceae family, including rosemary (Rosmarinus officinalis), mint (Mentha spicata), sage (Salvia officinalis), Spanish sage (Salvia lavandulifolia), marjoram (Origanum majorana), lemon balm (Melissa officinalis), basil (Ocimum tenuiflorum), oregano (Origanum vulgare), and thyme (Thymus vulgaris)[4–6]. Chemically, this compound is an Nanomaterials 2022, 12, 3292. https://doi.org/10.3390/nano12193292 https://www.mdpi.com/journal/nanomaterials