A split aptamer sensing platform for highly sensitive detection of theophylline based on dual-color fluorescence colocalization and single molecule photobleaching

Recent Advances in Food Safety Detection: Split Aptamer-Based Biosensors Development and Potential Applications

Ensuring food safety is a shared responsibility across the entire food supply chain, encompassing manufacturers, processors, retailers, consumers, and regulatory bodies. However, traditional detection methods have several limitations, including slow processing times, high costs, limited sensitivity, and susceptibility to false positives or negatives. These shortcomings underscore the urgent need for faster, more accurate, and cost-effective detection technologies. Aptamers and aptasensors have emerged as promising alternatives. Aptamers offer advantages over traditional recognition probes due to their high affinity and specificity for diverse targets. The aptasensors enable rapid detection, cost reduction, shelf life extension, and minimal batch-to-batch variability, making them highly suitable for food safety applications. Detecting small molecules such as toxins, antibiotics, pesticides, contaminants, and heavy metals remains challenging due to steric hindrance, nonspecific binding, and reduced accuracy. Recent advancements in aptamer technology have focused on pre- and postmodifications to enhance detection performance. One of the most promising innovations is the development of split aptamers. These engineered aptamers, designed to operate in segments known as split aptamers, offer improved flexibility and binding specificity, effectively addressing the challenges of detecting small-sized targets. This review examines the evolution of aptamers and aptasensors, focusing on their application in detecting small molecules that are essential to food safety. It reported the strategies for modifying and optimizing selected aptamers, providing details on developing split aptamers as a promising approach to address the unique challenges of small-molecule detection. Additionally, recent advancements in split aptamer technology and its integration into aptasensor development are highlighted, showcasing how these innovations are revolutionizing the detection of food safety hazards by overcoming the limitations of traditional detection methods.

An ultrasensitive and specific fluorescence split-aptasensor for VEGF165 detection based on nicking enzyme-assisted 3D DNA walker coupling with CRISPR-Cas12a

The overexpression of vascular endothelial growth factor 165 (VEGF165) in cancer cells plays a pivotal role in promoting tumor metastasis by facilitating their excessively rapid proliferation and division. Hence, the development of analytical methods possessing high sensitivity and resistance to interference is imperative for the detection of VEGF165. Various types of aptasensors have been devised for VEGF165 detection; however, the performance of these biosensors can be influenced by non-target signals caused by conformational changes in unbound aptamers. The paper shows the creation of a precise and sensitive fluorescence biosensor designed to detect VEGF165 by using a VEGF165-specific split aptamer. Additionally, this biosensor employs nicking enzyme-assisted DNA walker coupling with CRISPR-Cas12a to achieve dual-signal amplification. The VEGF165 calibration curve shows a detection limit of 268 fM and has a broad linear range from 5 to 4000 nM. The fluorometric biosensor was utilized to detect VEGF165 in human serum and cellular homogenate samples, yielding good outcomes. The innovative design serves as proof of concept and demonstrates significant potential in detecting various targets.

An RNA aptamer photoelectrochemical biosensor based on the exciton energy transfer constructed for theophylline detection

A novel photoelectrochemical (PEC) biosensor was developed incorporating a specifically designed RNA aptamer for the detection of theophylline (TP). This involved utilizing two nucleotide base aptamers with tailored sequences designed to target TP. The 3' end of a single-stranded RNA sequence (5'-GGAUACCA-(CH2)6-SH-3') and the 5' end of a complementary stranded RNA sequence (5'-HS-(CH2)6-CCUUGGAAGCC-3') were linked to gold nanoparticles (AuNPs) and CdS quantum dots (QDs), respectively. These two single-stranded RNAs (ssRNA) formed a double-stranded RNA (dsRNA) capable of recognizing TP. This major structural change altered the spacing between QDs and NPs, which signaled the presence and concentration of TP. TP was photoelectrochemical catalytic oxidation by the hole of CdS QDs under illumination, then anode photocurrent was generated. Due to the increase in surface impedance and the effect of exciton energy transfer (EET) between QDs and AuNPs, the photocurrent would undergo varying degrees of change. TP was detected by changes in photocurrent. PEC detection of TP was achieved in the range of 0.1 μM-200 μM. The detection limit was 0.033 μM. The method exhibited commendable reproducibility and remarkable selectivity. The biosensor was used to measure TP content in tea, beverages and blood samples, resulting in satisfactory recovery rates.

Ratiometric electrochemical aptasensor based on split aptamer and Au-rGO for detection of aflatoxin M1

A novel ratiometric electrochemical aptasensor based on split aptamer and Au-reduced graphene oxide (Au-rGO) nanomaterials was proposed to detect aflatoxin M1 (AFM1). In this work, Au-rGO nanomaterials were coated on the electrode through the electrodeposition method to increase the aptamer enrichment. We split the aptamer of AFM1 into 2 sequences (S1 and S2), where S1 was immobilized on the electrode due to the Au-S bond, and S2 was tagged with methylene blue (MB) and acted as a response signal. A complementary strand to S1 (CS1) labeled with ferrocene (Fc) was introduced as another reporter. In the presence of AFM1, CS1 was released from the electrode surface due to the formation of the S1-AFM1-S2 complex, leading to a decrease in Fc and an increase in the MB signal. The developed ratiometric aptasensor exhibited a linear range of 0.03 μg L-1 to 2.00 μg L-1, with a detection limit of 0.015 μg L-1 for AFM1 detection. The ratiometric aptasensor also showed a linear relationship from 0.2 μg L-1 to 1.00 μg L-1, with a detection limit of 0.05 μg L-1 in natural milk after sample pretreatment, indicating the successful application of the developed ratiometric aptasensor. Our proposed strategy provides a new way to construct aptasensors with high sensitivity and selectivity.

Fluorescence Thermometers Involving Two Ranges of Temperature: Coordination Polymer and DMSP Embedding

The measurement of temperature is indispensable in the fields of life, science, and industry. Fluorescence thermometers are attractive to researchers because of their advantages such as noncontact, high sensitivity, fast response, and excellent anti-interference. Here, a new coordination polymer (HNU-76) was synthesized by assembling Zn2+ with the H3TCA ligand, a fluorescent molecule with an AIE behavior, which can be used as a fluorescence thermometer. At 100-210 K, the fluorescence intensity ratio of HNU-76 versus temperature conforms to an Arrhenius-type decay relationship (R2 = 0.997), which can be the candidate for low-temperature sensing. In order to increase the sensing range, 4-[4-(dimethylamino)styryl] pyndine (DMSP) was successfully embedded on HNU-76, obtaining HNU-76⊃DMSP. The fluorescence intensity of HNU-76⊃DMSP conforms to an Arrhenius-type decay relationship (R2 = 0.997) at 270-360 K versus temperature. HNU-76 can be used for fluorescence detection at low temperatures, due to the DMSP loading, and HNU-76⊃DMSP can serve as the temperature thermometer in a range of temperatures common. Both materials show good cyclability and have the potential to be used in fluorescence thermometers.

Urine-derived stem cells-extracellular vesicles ameliorate diabetic osteoporosis through HDAC4/HIF-1α/VEGFA axis by delivering microRNA-26a-5p

Critical roles of stem cell-extracellular vesicles (EVs) in the management of osteoporosis have been documented. Here, this study was designed to enlarge the research of the specific effects and underlying mechanism of urine-derived stem cells-EVs (USCs-EVs) on osteoporosis in diabetes rats. Firstly, miR-26a-5p and histone deacetylase 4 (HDAC4) expression in USCs of rats after diabetic osteoporosis (DOP) modeling induced by streptozotocin injection was determined, followed by study of their interaction. Then, USCs-EVs were co-cultured with osteogenic precursor cells, the effects of miRNA-26a-5p (miR-26a-5p) on osteoblasts, osteoclasts, bone mineralization deposition rate were evaluated. Meanwhile, the effect of USCs-EVs carrying miR-26a-5p on DOP rats was assessed. Elevated miR-26a-5p was seen in USCs-EVs which limited HDAC4 expression. Moreover, USCs-EVs delivered miR-26a-5p to osteogenic precursor cells, thereby promoting their differentiation, enhancing the activity of osteoblasts, and inhibiting the activity of osteoclasts, thereby preventing DOP through the activation of hypoxia inducible factor 1 subunit alpha (HIF-1α)/vascular endothelial growth factor A (VEGFA) pathway by repressing HDAC4. In a word, USCs-EVs-miR-26a-5p is a promising therapy for DOP by activating HIF-1α/VEGFA pathway through HDAC4 inhibition. 1. USCs-EVs-miR-26a-5p targeted HDAC4 and limited HDAC4 expression. 2. miR-26a-5p was delivered by USCs-EVs into osteoblast precursor cells. 3. USCs-EVs-miR-26a-5p promoted the differentiation of osteoblast precursor cells into osteoblasts. 4. miR-26a-5p delivered by USCs-EVs could inhibit HDAC4. 5. USCs-EVs-miR-26a-5p could prevent the pathogenesis of DOP via HIF-1α/VEGFA aix.

An indirect competitive assay-based method for the sensitive determination of tetracycline residue using a real-time fluorescence-based quantitative polymerase chain reaction

Tetracycline (TC) is an effective antibiotic used to treat humans and livestock, but its inappropriate use imposes toxic effects, including pollution, on environmental ecology and food. Currently, sensitive, accurate, and cost-effective methods that can detect lower concentrations of TC residues in environmental and food samples are needed. In this study, a novel indirect competitive assay-based aptamer method was developed for detecting TC residues through signal amplification by real-time fluorescence-based quantitative polymerase chain reaction. The response surface methodology was introduced to optimize the optimal concentrations (influencing factors) of the three types of single-stranded DNA in the competitive assay process. The optimal conditions for the three types of ssDNA were 112 nM for the specific aptamer of TC (Apt40), 115 nM for the signal DNA, and 83 nM for the DNA catcher. As expected, under optimal conditions, the Ct value was linearly related to the logarithm of TC concentration. The calibration curve equation was Ct = -0.34516 log[TC] + 9.9345 (R2 = 0.998) in the range of 10-3-103 ng mL-1, and the limit of detection was 7.02 × 10-5 ng mL-1. The new method was effectively applied to detect TC residues in wastewater, honey, and milk samples. It achieved an average recovery rate of 101.19% with a small variation of 5.16%. The validation was carried out using an enzyme-linked immunosorbent assay. This approach demonstrates high sensitivity and selectivity, making it well suited for detecting leftover antibiotics in food when using suitable aptamers.

Color-encoded Escherichia coli assay via enzyme-induced etching of Au@MnO2 nanoparticles

Au@MnO₂ nanoparticles (NPs), as core-shell nanostructures, have been widely used in ions, molecules and enzyme activities detection due to their stable properties and easy preparation, but their application in bacterial pathogens detection is rarely reported. In this work, Au@MnO₂ NPs is employed for Escherichia coli (E. coli) detection through monitoring and measuring β-galactosidase (β-gal) activity based enzyme-induced color-code single particle enumeration (SPE) method. In the existence of E. coli, p-aminophenylβ-D-galactopyranoside (PAPG) can be hydrolyzed to generate p-aminophenol (AP) by the endogenous β-gal of E. coli. MnO₂ shell reacts with AP and produces Mn²⁺, causing the blue shift of the localized surface plasmon resonance (LSPR) peak and color change of the probe from bright yellow to green. With the SPE method, the amount of E. coli can be quantified readily. The detection limit reaches 15 CFU/mL with dynamic range from 100 to 2900 CFU/mL. Besides, this assay is effectively employed to monitor E. coli in river water sample. The designed sensing strategy provides an ultrasensitive and low cost way for E. coli detection and has the possibility to detect other bacteria in environmental monitoring and food quality analysis.

Dual-binding domain electrochemiluminescence biosensing platform with self-checking function for sensitive detection of synthetic cathinone in e-cigarettes

Current single signal electrochemiluminescence (ECL) sensors are susceptible to false positive or false negative phenomena due to experimental conditions. Therefore, sensors with "self-checking" function are attracting democratic attention. In quick succession, a highly sensitive single-cathode dual ECL signal aptasensor with self-checking function to improve the shortcomings mentioned above was designed. This aptasensor used In-based metal-organic framework (MIL-68) as load and stabilizer to effectively attenuate the aggregation-induced quenching (ACQ) effect of porphyrin derivatives (Sn-TCPP) while improve ECL stability. The introduction of cooperative-binding split-aptamers" (CBSAs) aptamers increased the specificity of the aptasensor and its unique double-binding domains detection accelerated the detection efficiency. When analyzing 3,4-methylenedioxypyrovalerone (MDPV), we could calculate two concentrations based on the strength of ECL 1 and ECL 2. If the concentrations are the same, the result would be obtained; if not, it should be retested. Depending on the above operation, the results achieve self-check. It was found that the designed aptasensor could quantify the concentration of MDPV between 1.0 × 10^-12 g/L and 1.0 × 10^-6 g/L with the limit of detection (LOD) of 1.4 × 10^-13 g/L and 2.0 × 10^-13 g/L, respectively (3 σ/slope). This study not only improves the detection technology of MDPV, but also explores the dual-signal detection of porphyrin for the first time and enriches the definition of self-checking sensor.

A sensing platform based on zinc-porphyrin derinative in hexadecyl trimethyl ammonium bromide (CTAB) microemulsion for highly sensitive detection of theophylline

A new porphyrin-based sensing platform in hexadecyl trimethyl ammonium bromide (CTAB) microemulsion is developed for highly sensitive detection of theophylline. In this sensing system, the zinc-porphyrin-cinnamic acid conjugate (Zn-TPPCA) works as fluorescence probe while theophylline can decrease fluorescence intensity of the probe. Further studies indicate the linear relationship between the fluorescence quenching value and the concentration of theophylline within a given range. And the introduction of CTAB microemulsion can greatly enhance sensibility and stability of this detecting system and facilitate the detection of theophylline. On the basis above, a highly sensitive sensing platform for theophylline is created with a low limit of detection (LOD) of 0.0083 μg mL^-1 under the optimal detection conditions. And further application of this method in determination of commercially available theophylline preparation shows excellent results. Subsequent studies on quenching mechanism indicate that static quenching appears between Zn-TPPCA and theophylline. Therefore, this work provides not only a highly sensitive method for determination of theophylline but also further evidence for creation of biosensors for drugs with porphyrin derivatives.

Split aptamer regulated CRISPR/Cas12a biosensor for 17β-estradiol through a gap-enhanced Raman tags based lateral flow strategy

It is significant to exploit the full potential of CRISPR/Cas based biosensor for non-nucleic-acid targets. Here, we developed a split aptamer regulated CRISPR/Cas12a and gap-enhanced Raman tags based lateral flow biosensor for small-molecule target, 17β-estradiol. In this assay, one split aptamer of 17β-estradiol was designed to complement with crRNA of Cas12a so that the trans-cleavage ability of CRISPR/Cas12a can be regulated by the competitive binding of 17β-estradiol and split aptamers. Through integration of the signal amplification ability of CRISPR/Cas12a and the ultra-sensitive gap-enhanced Raman tags based lateral flow assay, a visible-SERS dual mode determination of 17β-estradiol can be established. 17β-estradiol can be visibly recognized as low as 10 pM and accurately quantified with a detection limit of 180 fM by SERS signals, which is at least 10³-fold lower than that of the previous immunoassay lateral flow strategies. Our assay provides a novel perspective to develop split aptamer regulated CRISPR/Cas12a coupling with SERS lateral flow strips for ultrasensitive and easy-to-use non-nucleic-acid targets detection.

Single molecule detection; from microscopy to sensors

Single molecule detection is necessary to find out physical, chemical properties and their mechanism involved in the normal functioning of body cells. In this way, they can provide a new direction to the healthcare system. Various techniques have been developed and employed for their successful detection. Herein, we have emphasized various traditional methods as well as biosensing technology which offer single molecule sensitivity. The various methods including plasmonic resonance, nanopores, whispering gallery mode, Simoa assay and recognition tunneling are discussed in the initial part which has been followed by a discussion about biosensor-based detection. Plasmonic, SERS, CRISPR/Cas, and other types of biosensors are focused in this review and found to be highly sensitive for single molecule detection. This review provides an overview of progression in different techniques employed for single molecule detection.

Recent Progress on Single-Molecule Detection Technologies for Food Safety

Rapid and sensitive detection technologies for food contaminants play vital roles in food safety. Due to the complexity of the food matrix and the trace amount distribution, traditional methods often suffer from unsatisfying accuracy, sensitivity, or specificity. In past decades, single-molecule detection (SMD) has emerged as a way to realize the rapid and ultrasensitive measurement with low sample consumption, showing a great potential in food contaminants detection. For instance, based on the nanopore technique, simple and effective methods for single-molecule analysis of food contaminants have been developed. To our knowledge, there has been a rare review that focuses on SMD techniques for food safety. The present review attempts to cover some typical SMD methods in food safety, including electrochemistry, optical spectrum, and atom force microscopy. Then, recent applications of these techniques for detecting food contaminants such as biotoxins, pesticides, heavy metals, and illegal additives are reviewed. Finally, existing research challenges and future trends of SMD in food safety are also tentatively proposed.

Polypyrrole and enzyme free cholesterol flakes based composite: Selective determination of theophylline

In the present work, we demonstrated the non-enzymatic theophylline (TP) detection using nanostructured Polypyrrole (Ppy) and Cholesterol (Ch) flakes coordinated nanocomposite coated on Glassy Carbon Electrode. The higher active surface area and good mechanical stability of the Ppy integrated Ch composite exhibited better oxidation towards TP. The electron transfer across hydrogen bonding is of fundamental importance in enhancing the biochemical process in this biosensor system. Further, the scan rate analysis confirmed the quasi-reversible diffusion controlled electrochemical process. The differential pulse voltammetry report of the electrocatalyst shows a linear response of TP in the concentration range 1μM - 1 mM with detection limit of 720 nM (S/N = 3σ/b). The proposed electrochemical sensor exhibited intriguing sensing properties with good selectivity, reproducibility towards TP detection without any surface fouling. The overall analysis confirmed that the prepared nanocomposite modified electrode has quantified better accuracy towards target molecule without the aid of any bio receptors and binders in tea leaves and human urine samples.

Efficient preparation of dual-emission ratiometric fluorescence sensor system based on aptamer-composite and detection of bis(2-ethylhexyl) phthalate in pork

A ratiometric fluorescence sensor system is proposed for detecting bis(2-ethylhexyl) phthalate (DEHP) in pork, which is based on aptamer recognition with molybdenum disulfide quantum dots and cadmium telluride quantum dots (MoS₂ QDs/CdTe-Apta). Two signals exist in the system, among which the response signal is transmitted by CdTe-Apta. The amide condensation between aptamers and CdTe QDs shortens the distance between CdTe QDs and DEHP, thus quenching the fluorescence of CdTe QDs, possibly through a photoinduced electron transfer mechanism. The MoS₂ QDs deliver the self-calibration signal, and the fluorescence of MoS₂ QDs remains almost constant when co-existing with DEHP. Linearity (R² = 0.9536) was established for the DEHP concentration range 0.005-3.0 mg·L^-1, with a limit of detection of 0.21 μg·L^-1. The system was successfully applied in the determination of DEHP in pork. The system has potential for the quantitative determination of DEHP in practical applications.

The method for integrating dual-color fluorescence colocalization and single molecule photobleaching technology on the theophylline sensing platform

• Smart usage of single molecule photobleaching technology and dual-color fluorescence colocalization is of critical importance for exploiting the sensing platform. Here, we provide the detailed protocols related to the article “A split aptamer sensing platform for highly sensitive detection of theophylline based on dual-color fluorescence colocalization and single molecule photobleaching” (published online by Biosensors and Bioelectronics) (Liu et al., 2020). The protocols contain: (1) how to clean the slides; (2) how to prepare the probe and detection sample; (3) Single molecule imaging; 4) Data processing by using the Image J. Finally, we used a simple model to confirm the feasibility of the method for integrating dual-color fluorescence colocalization and single molecule photobleaching technology on the theophylline sensing platform.

• A simple, ultrasensitive method for the detection of theophylline.

• The method is easily comprehensible.

• Both strategy formulation and data processing are simple, learnability, and highly reproducible.