Although such label-based methods are widely used and usually very sensitive, they inevitably require the involvement of a label and a chemical conjugation step to link the label to the biorecognition event. applications with novel innovations to overcome the challenges related to small molecule detection. Optical label-free methods with different transduction schemes, including evanescent wave and optical fiber sensors, surface plasmon resonance, surface-enhanced Raman spectroscopy, and interferometry, using various biorecognition elements, such as antibodies, aptamers, enzymes, and bioinspired molecularly imprinted polymers, are reviewed. Keywords: label-free, optical biosensor, small molecule, surface plasmon resonance, surface-enhanced Raman spectroscopy, interferometry, evanescent wave, optical fiber 1. Origin and Occurrence of Small Molecules Small molecules can be defined as low molecular weight organic molecules which are typically less than 1000 Da in size. This category includes 20(S)-Hydroxycholesterol a wide variety of different chemical compounds, of either natural or pharmaceutical origin, many of which are biological, pharmacologically, or environmentally relevant, which makes detection and quantification of these 20(S)-Hydroxycholesterol molecules important in many disciplines. Naturally, nearly every cell contains a collection of 100 to 200 different low molecular weight 20(S)-Hydroxycholesterol organic molecules, including the common amino acids, nucleotides, sugars, and their phosphorylated derivatives. Additionally, there exists a wide variety of small biomolecules which are specific to certain types of cells or organisms, for example, many plants contain so-called secondary metabolites which include compounds that give plants their characteristic scents, and compounds such as morphine, nicotine, and caffeine that are valued for their physiological effects on humans [1]. On the other hand, small synthetic molecules, man-made or produced by synthetic biology [2], have been extensively applied in a broad variety of fields including pharmaceutical, clinical, environmental, or food analysis, to name a few. Many small molecules are well-known contaminants in food, feed, and other agricultural products. For example, mycotoxins, which are produced as secondary metabolites by filamentous fungi, include more than 500 different small molecules with different physicochemical properties and various effects ranging from cancer to acute toxicity and developmental defects. Agriculturally, the most 20(S)-Hydroxycholesterol critical mycotoxins are aflatoxins, fumonisins, trichothecenes, ochratoxins, and zearalenone family which are common contaminants in crops worldwide [3,4]. Algal toxins, phycotoxins, and cyanotoxins, in turn, are produced by toxicogenic microalgae and cyanobacteria, and they can enter the marine food chain via phytoplankton and subsequently to humans by contaminated seafood. Several small molecule toxins originating from seafood are known to cause severe illnesses, such 20(S)-Hydroxycholesterol as paralytic shellfish poisoning, puffer fish poisoning, and neurotoxic shellfish poisoning, and it has been reported that seafood poisonings are increasing in frequency and new intoxications are emerging [5,6]. All the aforementioned toxins are known to cause, besides a threat to human and animal health, but also substantial economic losses in aqua- and agriculture, and several of them are categorized as international and national regulations. Combined with the created impurities normally, many man made low molecular fat materials are and pharmacologically significant environmentally. For example, natural and chemical substance warfare agents which may be lethal also at low amounts are some of the most feared weapons of mass devastation. Nerve agents, which are organophosphates usually, have speedy and severe results on individual and animal wellness because of their ability to stop acetylcholinesterase (AChE) that’s essential for the standard functioning from the anxious systems. Also, many insecticides and pesticides participate in the same chemical substance course of organophosphates, and they contain the same setting of actions as nerve realtors but are much less hazardous [7]. For instance, chlorpyrifos, a wide range pesticide which is one of the band of organophosphates and can be used worldwide in an array of crops, presents a significant concern for potential environmental overuse and contaminants, improper removal or storage space of the substances [8,9]. Pharmaceuticals, either of artificial or organic origins, are ubiquitous chemicals of interest not merely in clinical medication but also in medication screening process and environmental basic safety. Small molecules, specifically, are of great curiosity about pharmaceutical research, for instance, because they are able to combination the blood-brain hurdle. Detection of several drugs is normally diagnostically and medically relevant but Vegfb can be appealing in pharmaceutical sciences for medication screening and advancement of brand-new biotherapeutics, aswell as for learning the experience, kinetics, and balance of pharmaceuticals [10]. Healing medication monitoring, which goals to optimize the pharmacological response of the drug while staying away from adverse effects, methods the medication concentrations within a natural matrix and with suitable interpretation impacts the prescribing techniques [11]. Alternatively, after administration, many medications are excreted with the patients into.