Detection of sulfadimethoxine using optical images of liquid crystals

Ratiometric fluorescent determination of sulfadimethoxine in foods based on a dual-emission metal-organic framework

Ratiometric fluorescence detection is endowed with higher accuracy than single fluorescence signal assay. In this work, we construct a ratiometric fluorescence probe for the facile quantification of sulfadimethoxine (SDM) in foods. By wrapping N-doped carbon dots (N-CDs) and gold nanoclusters (AuNCs) into zeolitic imidazolate framework-8 (ZIF-8), the nanocomposite of N-CDs/AuNCs@ZIF-8 is facilely prepared and emits two fluorescence including 475 nm from N-CDs and 650 nm from AuNCs. Since bovine serum albumin (BSA) is the stabilizer of AuNCs, SDM can form a complex with BSA, resulting in the fluorescence quenching of AuNCs at 650 nm by a static quenching mechanism. In contrast, SDM has a rare influence on the fluorescence of N-CDs (475 nm). As a result, the use of the probe of N-CDs/AuNCs@ZIF-8 for SDM detection enables simultaneous measurement of response signal and reference signal. Under the optimal condition, the SDM assay based on the probe has a good linear relationship within 10 to 2 × 106 ng/mL and the limit of detection (LOD) is low to 1.064 ng/mL. In addition, the fluorescent probe shows good reliability for the detection of SDM in practical food samples.

Electrochemical biosensor for rapid and sensitive monitoring of sulfadimethoxine based on nanoporous carbon and aptamer system

In this study, a straightforward electrochemical aptasensor was developed to detect sulfadimethoxine (SDM). It included a glassy carbon electrode decorated by boron nitride quantum dots (BNQDs) and aptamer-functionalized nanoporous carbon (APT/CZ). CZ was first synthesized by calcinating a zeolitic imidazolate framework (ZIF-8). Then, the electroactive dye methylene blue (MB) was entrapped inside its pores. By attaching aptamer to the CZ surface, APT/CZ acted as a bioguard, which prevented the MB release. Therefore, the electrochemical signal of the entrapped MB was high in the absence of SDM. Introducing SDM caused the conformation of aptamers to change, and a large number of MB was released, which was removed by washing. Therefore, the detection strategy was done based on the change in the electrochemical signal intensity of MB. The aptasensor was applied to detect SDM at a concentration range of 10-17 to 10-7 M with a detection limit of 3.6 × 10-18 M.

An aptamer-based colorimetric/SERS dual-mode sensing strategy for the detection of sulfadimethoxine residues in animal-derived foods

Residues of sulfadimethoxine (SDM) in animal-derived foods have attracted widespread public concern. Herein, we propose an aptamer-based colorimetric/SERS dual-mode sensing strategy for the determination of SDM based on hexadecyl trimethyl ammonium bromide (CTAB) induced aggregation of nanoparticles. In the absence of SDM, the SDM aptamer formed a supramolecular composite with CTAB, and the 4-mercaptopyrimidine functionalized gold nanoparticles (AuNPs@4-MPY) remained dispersed due to the lack of CTAB. Upon the addition of SDM, the SDM aptamer preferentially combined with SDM, resulting in the release of CTAB and subsequent aggregation of AuNPs@4-MPY, and the solution color changed from red to blue and presented a dynamic UV-absorbance curve based on different aggregation states. On the other hand, when, gold-silver core-shell nanoparticles (Au@AgNPs) were added additionally, the released CTAB narrowed the nanogap between AuNPs@4-MPY and Au@AgNPs, thus exhibiting enhanced SERS intensity of 4-MPY. This strategy achieved colorimetric detection of SDM with a linear range of 4.00-200.00 ng mL^-1 and a detection limit of 2.41 ng mL^-1, while SERS had a detection range of 1.20-120.00 ng mL^-1 with a detection limit of 0.89 ng mL^-1. This strategy is simple and cost-effective for the rapid detection of SDM within 20 minutes. It was further applied for the detection of SDM in spiked milk and honey samples with satisfactory recoveries. Therefore, it exhibits great potential for fast and on-site SDM detection.

Interactions of a biological macromolecule with thermotropic liquid crystals: Applications of liquid crystals in biosensing platform

Liquid crystal biosensor was developed based on a 4'-octyl-4-biphenylcarbonitrile (8CB) by adsorption of biological macromolecule bovine serum albumin (BSA) at the 8CB interface. BSA was detected by examining the changes in the director configurations of 8CB molecules under a polarizing optical microscope. The transitions in the director configuration were due to the non-covalent bonds. This technique demonstrated high sensitivity at a concentration of 100 µM of BSA. The binding events between the 8CB and BSA were investigated through molecular docking studies that confirmed the protein-ligand interaction. The most probable binding location of 8CB to dock with BSA were determined at a subdomain IB of Sudlow's site I. The active residues on analyzing were found to stabilize the 8CB molecules through different interactions. These active residues that were involved in the protein-ligand interaction were further confirmed with Raman spectroscopy. This study provided the vibrational properties and structural changes that occurred due to the various interactions between the 8CB and BSA. The results presented in this work lead to a potential biosensing tool for detecting and sensing proteins using LCs.

Automated Calculation of Liquid Crystal Sensing Images Based on Deep Learning

Liquid crystal (LC)-based sensors have been extensively applied in the detection of chemical and biological events. However, the calculation of the optical images of the LC-based sensors is usually time-consuming and also might bring some errors due to the use of different judgment criteria by different users. In the present study, an automated calculation method for LC sensing images based on deep learning is provided. A convolutional network is trained with the prepared LC sensing images and their corresponding segmentation annotations to predict the positive responses. The ratio is calculated from the area of positive response to the total area selected by our image processing method. The robustness of the proposed algorithm is validated on both the test set and the label-free Cd²⁺ detection. The results show that the method based on deep learning can detect the positive response area in real time and the speed is much faster than the manual processing method. In addition, deep learning method can be directly applied to other label-free molecular detection assays.

Liquid crystal-amplified optofluidic biosensor for ultra-highly sensitive and stable protein assay

Protein assays show great importance in medical research and disease diagnoses. Liquid crystals (LCs), as a branch of sensitive materials, offer promising applicability in the field of biosensing. Herein, we developed an ultrasensitive biosensor for the detection of low-concentration protein molecules, employing LC-amplified optofluidic resonators. In this design, the orientation of LCs was disturbed by immobilized protein molecules through the reduction of the vertical anchoring force from the alignment layer. A biosensing platform based on the whispering-gallery mode (WGM) from the LC-amplified optofluidic resonator was developed and explored, in which the spectral wavelength shift was monitored as the sensing parameter. The microbubble structure provided a stable and reliable WGM resonator with a high Q factor for LCs. It is demonstrated that the wall thickness of the microbubble played a key role in enhancing the sensitivity of the LC-amplified WGM microcavity. It is also found that protein molecules coated on the internal surface of microbubble led to their interactions with laser beams and the orientation transition of LCs. Both effects amplified the target information and triggered a sensitive wavelength shift in WGM spectra. A detection limit of 1 fM for bovine serum albumin (BSA) was achieved to demonstrate the high-sensitivity of our sensing platform in protein assays. Compared to the detection using a conventional polarized optical microscope (POM), the sensitivity was improved by seven orders of magnitude. Furthermore, multiple types of proteins and specific biosensing were also investigated to verify the potential of LC-amplified optofluidic resonators in the biomolecular detection. Our studies indicate that LC-amplified optofluidic resonators offer a new solution for the ultrasensitive real-time biosensing and the characterization of biomolecular interactions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s43074-021-00041-1.

Applications of liquid crystals in biosensing

Liquid crystals (LCs), as a promising branch of highly-sensitive, quick-response, and low-cost materials, are widely applied to the detection of weak external stimuli and have attracted significant attention. Over the past decade, many research groups have been devoted to developing LC-based biosensors due to their self-assembly potential and functional diversity. In this paper, recent investigations on the design and application of LC-based biosensors are reviewed, based on the phenomenon that the orientation of LCs can be directly influenced by the interactions between biomolecules and LC molecules. The sensing principle of LC-based biosensors, as well as their signal detection by probing interfacial interactions, is described to convert, amplify, and quantify the information from targets into optical and electrical parameters. Furthermore, commonly-used LC biosensing targets are introduced, including glucose, proteins, enzymes, nucleic acids, cells, microorganisms, ions, and other micromolecules that are critical to human health. Due to their self-assembly potential, chemical diversity, and high sensitivity, it has been reported that tunable stimuli-responsive LC biosensors show bright perspectives and high superiorities in biological applications. Finally, challenges and future prospects are discussed for the fabrication and application of LC biosensors to both enhance their performance and to realize their promise in the biosensing industry.

Aptamer Laden Liquid Crystals Biosensing Platform for the Detection of HIV-1 Glycoprotein-120

We report a label-free and simple approach for the detection of glycoprotein-120 (gp-120) using an aptamer-based liquid crystals (LCs) biosensing platform. The LCs are supported on the surface of a modified glass slide with a suitable amount of B40t77 aptamer, allowing the LCs to be homeotropically aligned. A pronounced topological change was observed on the surface due to a specific interaction between B40t77 and gp-120, which led to the disruption of the homeotropic alignment of LCs. This results in a dark-to-bright transition observed under a polarized optical microscope. With the developed biosensing platform, it was possible to not only identify gp-120, but obtained results were analyzed quantitatively through image analysis. The detection limit of the proposed biosensing platform was investigated to be 0.2 µg/mL of gp-120. Regarding selectivity of the developed platform, no response could be detected when gp-120 was replaced by other proteins, such as bovine serum albumin (BSA), hepatitis A virus capsid protein 1 (Hep A VP1) and immunoglobulin G protein (IgG). Due to attributes such as label-free, high specificity and no need for instrumental read-out, the presented biosensing platform provides the potential to develop a working device for the quick detection of HIV-1 gp-120.

A Label-Free Liquid Crystal Biosensor Based on Specific DNA Aptamer Probes for Sensitive Detection of Amoxicillin Antibiotic

We developed a liquid crystal (LC) aptamer biosensor for the sensitive detection of amoxicillin (AMX). The AMX aptamer was immobilized onto the surface of a glass slide modified with a mixed self-assembled layer of dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DMOAP) and (3-aminopropyl) triethoxysilane (APTES). The long alkyl chains of DMOAP maintained the LC molecules in a homeotropic orientation and induced a dark optical appearance under a polarized light microscope (POM). In the presence of AMX, the specific binding of the aptamer and AMX molecules induced a conformational change in the aptamers, leading to the disruption of the homeotropic orientation of LCs, resulting in a bright optical appearance. The developed aptasensor showed high specificity and a low detection limit of 3.5 nM. Moreover, the potential application of the developed aptasensor for the detection of AMX in environmental samples was also demonstrated. Therefore, the proposed aptasensor is a promising platform for simple, rapid, and label-free monitoring of AMX in an actual water environment with high selectivity and sensitivity.

Advances in Gold Nanoparticles-Based Colorimetric Aptasensors for the Detection of Antibiotics: An Overview of the Past Decade

Misuse of antibiotics has recently been considered a global issue because of its harmful effects on human health. Since conventional methods have numerous limitations, it is necessary to develop fast, simple, sensitive, and reproducible methods for the detection of antibiotics. Among numerous recently developed methods, aptasensors are fascinating because of their good specificity, sensitivity and selectivity. These kinds of biosensors combining aptamer with colorimetric applications of gold nanoparticles to recognize small molecules are becoming more popular owing to their advantageous features, for example, low cost, ease of use, on-site analysis ability using naked eye and no prerequisite for modern equipment. In this review, we have highlighted the recent advances and working principle of gold nanoparticles based colorimetric aptasensors as promising methods for antibiotics detection in different food and environmental samples (2011–2020). Furthermore, possible advantages and disadvantages have also been summarized for these methods. Finally, the recent challenges, outlook, and promising future perspectives for developing novel aptasensors are also considered.

An ultrasensitive platform for PCB77 detection: New strategy for liquid crystal-based aptasensor fabrication

Polychlorinated biphenyls (PCBs) are considered persistent bio-accumulative toxicants which threats global food safety and environmental health. Traditional analytical techniques for detection of PCBs are time-consuming and they do not satisfy urgent need for rapid and accurate monitoring of these persistent pollutants. Biosensor technology may be promising in this respect. Here we demonstrate a novel liquid crystal (LC)-based aptasensing platform as a promising label-free and rapid biosensor for PCB77 detection. This novel molecular strategy utilize triple-helix molecular conformational switch which is mediated formation of duplex on sensing platform in presence of target. Duplex forming leads to optical change from dark to bright in a liquid crystal based aptasensor. The limit of quantification of the LC-aptasensor to PCB77 is 1.5 × 10^-5 μg/L with comparable selectivity. Besides, we also demonstrated that this system is able to detect PCB77 in tap water, environmental water and milk. This strategy has potential for label-free and portable detection of different targets without any aptamer sequence length restrictions.

Determination of sulfadimethoxine in milk with aptamer-functionalized Fe3 O4 /graphene oxide as magnetic solid-phase extraction adsorbent prior to HPLC

An aptamer (Apt) functionalized magnetic material was prepared by covalently link Apt to Fe₃ O₄ /graphene oxide (Fe₃ O₄ /GO) composite by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride and N-hydroxysuccinimide, and then characterized by FTIR spectroscopy, X-ray diffraction, and vibration sample magnetometry. The obtained composite of Fe₃ O₄ /GO/Apt was employed as magnetic solid-phase extraction adsorbent for the selective preconcentration of sulfadimethoxine prior to analysis by high-performance liquid chromatography. Under the optimal conditions (sample pH of 4.0, sorbent dosage of 20 mg, extraction time of 3 h, and methanol-5% acetic acid solution as eluent), a good linear relationship was obtained between the peak area and concentration of sulfadimethoxine in the range of 5.0 to 1500.0 µg/L with correlation coefficient of 0.9997. The limit of detection (S/N = 3) was 3.3 µg/L. The developed method was successfully applied to the analysis of sulfadimethoxine in milk with recoveries in the range of 75.9-92.3% and relative standard deviations less than 8.1%. The adsorption mechanism of Fe₃ O₄ /GO/Apt toward sulfadimethoxine was studied through the adsorption kinetics and adsorption isotherms, and the results show that the adsorption process fits well with the pseudo-second-order kinetic model and the adsorbate on Fe₃ O₄ /GO/Apt is multilayer and heterogeneous.

Computer-Aided aptamer design for sulfadimethoxine antibiotic: step by step mutation based on MD simulation approach

This study introduces a computational method to design a new aptamer with higher binding affinity to a special target in comparison with the experimentally available aptamers. The method is called step by step mutation based on MD simulation, which includes some steps. First, MD simulation is performed for the SELEX-introduced (native) aptamer in the presence of the target. Afterwards, conformational factor (P_i) is calculated for the simulated system, which obtains the affinity of the aptamer residues to the target. A nucleotide exchange is done for the residue with the least P_i parameter to the nucleotide with the highest P_i value that results in a mutant aptamer. MD simulation is performed for the target-mutant complex, and P_i values are calculated again. The nucleotide exchange is performed similarly, and the designing process is proceeded repeatedly that results in a mutant with the improved specificity to the target. The aptamer affinity to the target is also determined in each step through calculating the binding Gibbs energy (ΔG_Bind) as a reliable parameter. The introduced strategy is utilized efficiently to design a mutant aptamer with improved specificity toward sulfadimethoxine (SDM) antibiotic as a case study. The great difference in the ΔG_Bind values about 579.856 kJ mol^-1 highlights that the M5 mutant possesses the improved specificity toward SDM in comparison with the native aptamer. Besides, the selectivity of the M5 aptamer toward SDM is examined among some conventional interfering compounds by using MD simulation that confirms the applicability of the designed aptamer for further experimental studies.

The Growing Interest in Development of Innovative Optical Aptasensors for the Detection of Antimicrobial Residues in Food Products

The presence of antimicrobial residues in food-producing animals can lead to harmful effects on the consumer (e.g., allergies, antimicrobial resistance, toxicological effects) and cause issues in food transformation (i.e., cheese, yogurts production). Therefore, to control antimicrobial residues in food products of animal origin, screening methods are of utmost importance. Microbiological and immunological methods (e.g., ELISA, dipsticks) are conventional screening methods. Biosensors are an innovative solution for the development of more performant screening methods. Among the different kinds of biosensing elements (e.g., antibodies, aptamers, molecularly imprinted polymers (MIP), enzymes), aptamers for targeting antimicrobial residues are in continuous development since 2000. Therefore, this review has highlighted recent advances in the development of aptasensors, which present multiple advantages over immunosensors. Most of the aptasensors described in the literature for the detection of antimicrobial residues in animal-derived food products are either optical or electrochemical sensors. In this review, I have focused on optical aptasensors and showed how nanotechnologies (nanomaterials, micro/nanofluidics, and signal amplification techniques) largely contribute to the improvement of their performance (sensitivity, specificity, miniaturization, portability). Finally, I have explored different techniques to develop multiplex screening methods. Multiplex screening methods are necessary for the wide spectrum detection of antimicrobials authorized for animal treatment (i.e., having maximum residue limits).

Quantitative and sensitive detection of lipase using a liquid crystal microfiber biosensor based on the whispering-gallery mode

We demonstrate a quantitative and sensitive strategy for monitoring the lipase concentration using a liquid crystal microfiber biosensor based on the whispering-gallery mode.

A liquid-crystal-based immunosensor for the detection of cardiac troponin I

Cardiac troponin I (cTnI) is one of the most sensitive and specific markers of myocardial cell injury. In this study, a label-free biosensor that utilizes the birefringence property of liquid crystal (LC) for the detection of cTnI is demonstrated.

Design of a liquid crystal-based aptasensing platform for ultrasensitive detection of tetracycline

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 modified 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.