Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Design of a liquid crystal-based aptasensing platform for ultrasensitive detection of tetracycline Zeinab Rouhbakhsh a , Asma Verdian a, ⁎ , Ghadir Rajabzadeh b a Department of Food Safety and Quality Control, Research Institute of Food Science and Technology (RIFST), Mashhad, Iran b Department of Food Nanotechnology, Research Institute of Food Science and Technology (RIFST), Mashhad, Iran ARTICLEINFO Keywords: Liquid crystals Biosensor Aptamer Tetracycline Polarized light microscopy ABSTRACT We develop a novel label-free liquid crystal (LC) aptasensor based on intrinsic properties of nematic LCs for ultra-sensitive detection of tetracycline. The aptasensor is assembled by immobilizing aptamers onto the glass slide modifed with both homeotropic alignment and silane coupling agents. Designed aptasensor makes use of the target-induced aptamer conformational switching and disruption of the orientation of LCs which lead to an obvious change of the optical appearance from a dark to a bright response. We describe the optimized condition for maintaining the homeotropic orientation of LCs, which are suitable for the tetracycline detection. The average gray-scale intensities of polarizing optical microscopy images were calculated to quantitatively detect tetracycline concentrations. The aptasensor works especially at trace level of tetracycline as low as 0.5 pM. Moreover, the LC aptasensor was successfully used to detect tetracycline in the real milk sample. According to the results, the proposed LC aptasensor for tetracycline detection is simple, ultra-sensitive, label free and ease of preparation. 1. Introduction Tetracycline is a broad-spectrum antibiotic used in the therapy of human and animal. The overuse of tetracycline, an antibiotics and growth stimulants, in veterinary medicine can lead to their accumula- tion in animal-based food [1,2]. The residue of tetracycline in food can lead to unpleasant side efects in human and animal health, including the emergence of antibiotic resistance, allergic reactions, liver and dental damage [3,4]. Furthermore, antibiotics in milk cause the elim- ination of benefcial bacteria used in the production of cheese, yogurt and other fermentation products [5,6]. Therefore, in addition to creating a health problem, the presence of antibiotics in milk also re- duces the quality and quantity of products produced. Diferent analytical and bioanalytical methods such as spectro- photometry, chromatography, electrophoresis, microbiology and im- munochemical techniques are employed to determine the tetracycline residues in foodstufs, which are unusable in the dairy industry due to their expensive and time-consuming [7–11]. Recently, commercial kits are used in the dairy industry to detect tetracycline. In such kits specifc antibodies are usually used to detect the residue of antibiotics in raw milk [12,13]. Although antibodies are very sensitive and selective, their stability and expiration date are very short and their production and screening process are hard and expensive especially for small targets [14–16]. To resolve the limita- tions, aptamer-based biosensors have been developed as the promising alternatives, in which aptamers are used as the intelligent recognition elements [17]. Aptamer or synthetic antibodies are oligonucleotides which can recognize a wide range of contaminants, such as antibiotics, toxins, heavy metal ions, drug residues, microbial cells, and so on, owing to their special three-dimensional structure [18,19]. Compared to antibody-based biosensors, aptasensor have outstanding features, such as higher stability, reusability, low immunogenicity and more af- fordable. These features make the aptamers into unobtrusive recogni- tion elements for the design of biosensors [20]. The label-free techniques has been greatly advanced by the com- bination of sensors with nanomaterial sciences [21,22]. These rapidly advancing sensors aim to provide data without the intervention of label molecules. The liquid crystal (LC) aptasensors have received substantial attention in the last few decades as new candidates of label-free optical biosensors due to low cost, rapid detection, wide response range, and facile operation [23,24]. LCs are soft matters with fuidity and long- range order in orientation of the molecules; and very low anchoring energy which make them extremely sensitive to external stimuli [25,26]. These responsive materials based on change of orientation LC molecules, amplify and transduce the binding of aptamer and target into optical signal visible by the polarized light microscope. This https://doi.org/10.1016/j.talanta.2019.120246 Received 2 June 2019; Received in revised form 4 August 2019; Accepted 8 August 2019 ⁎ Corresponding author. E-mail address: a.verdian@rifst.ac.ir (A. Verdian). Talanta 206 (2020) 120246 Available online 12 August 2019 0039-9140/ © 2019 Elsevier B.V. All rights reserved. T