What are biosensors? 

Biosensors are innovative and multifaceted devices designed to detect the presence or concentration of a chemical or biological analyte. These amazing tools are used to analyze biological or non-biological systems by employing essential biomolecules such as enzymes, antibodies, proteins, or small metabolites. The core functionality of biosensors lies in their ability to integrate these biomolecules for specific and sensitive detection tasks.

Biosensors have found widespread applications across various fields, including environmental monitoring, agriculture, and biomedicine. Periodically, advancements in technology lead to the creation of innovative biosensors. These advancements enable the monitoring, capturing, quantification, optimization, and provision of detailed information about various systems. They serve as the basis for developing various applications that can be seamlessly integrated into everyday activities, enhancing our ability to interact with and understand our environment and health.

A typical biosensor consists of three main components: a recognition element for the analyte and signal production, a signal transducer, and a reader device. The recognition element is responsible for interacting specifically with the analyte of interest, while the signal transducer converts this interaction into a measurable signal, which is then read and interpreted by the reader device.

Biosensors can be classified based on the type of material being analyzed or the mode of physicochemical transduction. This includes optical, thermal, electrochemical, or piezoelectric methods. Each method offers unique advantages in terms of sensitivity, selectivity, and application context.

The research and development of biosensors have become a widely studied discipline. This is largely due to their potential for easy, rapid, low-cost, highly sensitive, and highly selective detection. Such features are crucial for advancements in the next generation of ultrasensitive point-of-care detection for various targets. As technology continues to evolve, biosensors are poised to play an increasingly significant role in various aspects of our lives, from healthcare to environmental conservation.

liquid crystal (LC) aptasensors

In recent decades, liquid crystal (LC) aptasensors have emerged as promising label-free optical biosensors, garnering significant attention for their cost-effectiveness, rapid detection capabilities, wide detection range, and user-friendly operation. Liquid crystals exhibit unique characteristics, being soft materials possessing both fluidity and long-range molecular orientation order, coupled with very low anchoring energy, rendering them highly responsive to external stimuli. By capitalizing on the responsiveness of LC molecules to changes in orientation, these aptasensors amplify and convert the binding interactions between aptamer and target molecules into observable optical signals, discernible through polarized light microscopy. Below, we delve into the various components involved in the design and functioning of liquid crystal aptasensors. Watch this YouTube video below: 

https://youtu.be/LF4kztQeMqk

What are aptamers?

Aptamers, serving as bio-probes within biosensor structures, have garnered significant attention. There are two primary categories of aptamers based on their chemical structure: oligonucleotide aptamers and peptide aptamers. However, when the term "aptamer" is mentioned, it typically refers to oligonucleotide aptamers.

Aptamers meaning

Oligonucleotide aptamers are short-chain RNA or single-stranded DNA molecules, typically ranging from 40 to 100 nucleotides. They were simultaneously introduced by three groups of scientists in 1990: Tuerk and Gold, Robertson and Joyce, and Ellington and Szostak. The name "aptamer" originates from the Latin "aptus," meaning fit, attached, or joined, and the Greek "meros," meaning part. Aptamers stand out due to their remarkable affinity, a result of their flexibility and ability to fold upon binding to a target.

These functional nucleic acids are crafted using an in vitro Darwinian evolution method known as SELEX (Systematic Evolution of Ligands by EXponential enrichment). Aptamers have been designed for a wide array of targets, encompassing small molecules, peptides, amino acids, proteins, and even cell membrane proteins. They exhibit high affinity for their targets, boasting Kd values in the low nanomolar to picomolar range, often surpassing the selectivity of antibodies.

Aptamers vs antibodies

Aptamers offer a compelling alternative to antibodies, primarily due to their stability and resistance to thermal degradation. Unlike proteins, which are prone to denaturation at high temperatures, aptamers can regain their original shape and bind to targets after heating. They can be produced in large quantities with precision and consistency through chemical synthesis, making them more cost-effective than antibodies, which require time-consuming and expensive monoclonal antibody production methods. 

Furthermore, aptamers are obtained through chemical synthesis, eliminating the need for laboratory animals, and they maintain their properties during storage and in various environmental conditions. Thanks to these unique characteristics, aptamers have found successful applications in analytical and biotechnology fields, including purification processes, target validation, drug discovery, diagnostics, and therapy.

What are the principles of aptosensors?

Aptasensors represent a remarkable evolution in biosensor technology, far beyond the early days of enzyme electrodes for glucose detection pioneered by Clark and Lyons in the 1960s. These advanced biosensors utilize aptamers as bioreceptors capable of binding to a wide range of targets, spanning from tiny micromolecules like metals to substantial macromolecules such as proteins. Aptamers, often referred to as chemical antibodies, exhibit a remarkable ability to specifically bind to diverse analyte molecules with a high degree of affinity.

In the realm of aptasensors, the binding event gives rise to detectable and quantifiable signals through a transducer, typically consisting of two key components: a signal amplifier and a detector/reporter. A variety of techniques can be employed to enhance the signal within the transducer.

One of the significant advantages of aptasensors lies in their adaptability to various platforms and transducers. These elements effectively convert the signals generated by aptamer-analyte interactions into measurable outputs, which can take the form of fluorescence, colorimetry, or electrochemical signals, all contingent upon the concentration of the sample analyte.

While much of the focus in aptamer development revolves around medical applications, particularly for measuring substances in blood or human serum, notable progress has been made in the environmental domain as well. The amalgamation of biological and electronic components enables rapid, sensitive, and precise detection, not only of trace chemical compounds but also of potentially hazardous microorganisms and toxins. This versatility positions aptasensors as invaluable tools with a broad spectrum of applications.

What is a liquid crystal?

Liquid crystals (LCs), with their intriguing intermediate characteristics bridging the realms of solids and liquids, have unlocked a plethora of research opportunities across diverse scientific fields. The aromatic structure of LC molecules, comprising side chains and terminal groups, imparts distinctive physical and optical attributes to these substances. Their intermediary nature, situated between low-viscosity liquids and optically anisotropic crystalline solids, grants LCs access to a wide spectrum of phases, each possessing unique electrical and optical properties. This exceptional versatility has significantly contributed to the advancement of the optoelectronics industry over recent decades.

The most common liquid crystals

LCs can be categorized in various ways based on their properties. Geometrically, LCs fall into two primary categories: calamitics, composed of rod-shaped molecules, and discotics, comprising disc-like molecules. Their classification as thermotropics or lyotropics depends on the synthesis method, involving thermal procedures or solvent influence, respectively. Within the thermotropic category, three common LC groups emerge: nematics, cholestrics, and smectics. The presence or absence of a central linkage group in LCs plays a pivotal role in determining their chemical stability.

The advantages and disadvantages of liquid crystal

Among the key applications of liquid crystals, LC display technologies stand out prominently. These technologies leverage the ability of LCs to alter their orientation when subjected to an electric field, thereby altering their optical appearance. LCs also find utility in elastomeric materials as mechanical actuators, generating responses through changes in shape. Additionally, LCs are employed in various contexts, such as photoconductors, electro-optical devices, medical devices, and in analytical chemistry.

On the flip side, LCs are sensitive to external stimuli, including chemical species, electric fields, or magnetic fields, which can disrupt the alignment and orientation of LC molecules. This property forms the foundation for designing and fabricating LC sensors, showcasing the multifaceted potential of liquid crystals across a broad spectrum of scientific and technological domains.

Liquid crystal-based aptasensors have emerged as compelling candidates for label-free optical biosensors, garnering significant attention over recent decades. Their appeal lies in their cost-effectiveness, rapid detection capabilities, wide response range, and straightforward operation. The orientational order of liquid crystals (LCs) proves highly sensitive to surface properties, influenced by various chemical species and physical fields, such as electric fields in displays. This inherent sensitivity serves as a foundation for sensor design and construction.

How do liquid crystals work?

In LC aptasensors, the binding of aptamers to their respective targets induces changes in the orientation of LC molecules, effectively amplifying and transducing this binding event into an optical signal visible under polarized light microscopy. These responsive biosensors excel at detecting nanoscale events, offering ease of observation without necessitating complex or costly laboratory equipment. Their portability, coupled with the absence of labeling requirements, renders them ideal for optically visualizing and detecting a wide array of analytes.

Notably, these biosensors have shown promise in being adapted for use with smartphones, paving the way for practical applications in both industrial settings and everyday life. The future holds the potential for widespread adoption of these biosensors. Currently, only one LC-based sensor for toxic gases has successfully reached commercialization. Furthermore, there is a growing trend toward employing LC-based aptasensors for the detection of specific targets. The convergence of LC-based sensor technology and aptamer science heralds a new era marked by high sensitivity, label-free, real-time, point-of-care (POC) biosensors.