Label-free colorimetric sensor for Pb2+ determination using catalytic activity of MnO2 nanoflowers and elongated aptamer

Aggregation-induced emission-based aptasensors for the detection of various targets: Recent progress

The advancement of aptasensors utilizing aggregation-induced emission (AIE) has progressed remarkably in recent years, owing to various unique benefits provided by aggregation-induced emission luminogens (AIEgens) as a novel category of fluorescent substances and aptamers as exceptional recognition components. AIE refers to a photophysical phenomenon identified in certain luminogens that show minimal or absent emission in dilute solutions, yet display considerable emission when in aggregate or solid states. Fluorescent sensing is an effective technique for the detection of various targets; however, many traditional dyes frequently demonstrate an aggregation-caused quenching (ACQ) effect in solid form, which limits their applicability on a larger scale. In contrast, fluorescent probes that leverage AIE characteristics have garnered considerable interest, owing to their elevated fluorescence quantum yields and ease of fabrication. This review discusses the application of various AIEgens in the design of diverse sensitive and selective AIE-based aptasensors for monitoring various targets, with a particular focus on recent advances. The AIE-based aptasensors exploit the supreme affinity of the aptamers to their targets and the remarkable properties of AIEgen, including its photostability and high quantum yield, and the interaction between AIEgen and DNA. The objective is to acquaint researchers with the various categories of materials exhibiting AIE characteristics and their potential applications in the creation of different aptasensors, enabling them to introduce novel kinds of innovative AIEgens and AIE-integrated aptasensors.

A FRET-based aptasensor exploiting the multiple functions of zirconium-based bimetallic organic frameworks for the quantitative determination of sulfadimethoxine

A novel fluorescent aptasensor was introduced for the assessment of sulfadimethoxine (SDM) using multiple functions of ZrFe metal-organic framework (MOF). This sensing strategy exploits the ZrFe MOF features including the peroxidase-mimicking feature, fluorescence characteristics, single-stranded DNA adsorption on its surface, and the change in its catalytic activity. In the detection mechanism, ferrofluid was used as separation agent and the peroxidized product quenched the fluorescence intensity of the ZrFe MOF through the fluorescence resonance energy transfer (FRET) effect. In this FRET-based approach, the existence or absence of SDM affects the attendance or non-attendance of complementary strand of aptamer (CS) in supernatant solution, which subsequently alters the peroxidase-like behavior of ZrFe MOF. Thus, the discrepancy in fluorescence intensity is caused by the variation in the amount of produced peroxidation product. Employing this sensing platform, a detection limit of 3 pM was achieved. Furthermore, the suggested sensing platform, leveraging the specificity of the aptamer for its target, demonstrated a significant potential to mitigate the influence of other interfering substances. The aptasensor has the capability to quantitatively detect SDM content in human serum samples. Therefore, it is well-suited for use in intricate matrices.

Colorimetry/fluorescence dual-mode detection of gentamicin based on curcumin nanoparticles and manganese dioxide nanosheets

This study used curcumin nanoparticles (CNPs) in phosphate buffer solution (pH 9) [CNPs (pH 9)] as a new oxidative substrate to detect gentamicin (GEN). Manganese dioxide nanosheets (MnO2-BSA NS) with excellent oxidizing properties were applied to catalytic oxidize CNPs (pH 9), which resulted in visual color changed from red to orange-yellow under sunlight. Moreover, the fluorescence of CNPs (pH 9) changed from orange to yellow. And we introduced carbon dots (CDs) as a reference signal, the fluorescence of CNPs quenched the fluorescence of CDs to generate a ratiometric fluorescence response. Therefore, we semi-qualitatively determined GEN with the naked eye and quantitatively evaluated by ratiometric colorimetry (I510 nm/I436 nm) and ratiometric fluorescence (F560 nm/F436 nm). The dual-mode immunoassay linearly correlated with GEN ranged from 10-4 μg/mL to 100 μg/mL with a detection limit (LOD) of 4.44 × 10-5 μg/mL and 3.16 × 10-5 μg/mL. Meanwhile, a smartphone-assisted system was designed for sensing of GEN by the distinct color change, providing a dependable, convenient and on-site assay of detecting GEN.

A turn-on fluorescent aptasensor for Pb2+ detection based on rhodamine B dye leakage from the internal cavity of hollow gold nanoparticles

In this project, a sensitive fluorescent aptasensor was fabricated to detect lead ions (Pb2+) by applying hollow gold nanoparticles (HGNPs) as a nano-carrier and rhodamine B (RDB) fluorescent dye as the signal agent. In the aptasensor that was created, the specific attachment of the aptamers to Pb2+ ions led to the release of aptamer from the chitosan (CTS) coated-HGNPs loaded with RDB, causing an increase in fluorescence intensity due to the leakage of RDB. The method demonstrated specific detection of the target analyte, achieving a detection limit (LOD) of 1 ppb and a broad linear dynamic range spanning from 2 to 1000 ppb. The aptasensor was able to accurately measure the concentration of Pb2+ in human serum, low-fat milk, and mineral water samples. The suggested biosensor, which offers the benefits of simplicity, user-friendliness, affordability, and high sensitivity, is well-suited for use with complex samples such as environmental and clinical samples.

Perylene Diimide-based Fluorescent Aptasensor for Quantitative Analysis of Pb2+ Based on Terminal Deoxynucleotidyl Transferase-assisted Formation of Elongated Aptamer and Gold Nanoparticles

Due to the exceedingly poisonous properties of Pb2+, it is imperative to conduct a thorough assessment of its quantity in both biological and environmental samples, as this is crucial for safeguarding public health. This study describes an economic turn-off fluorescent aptasensor for the quantitative analysis of Pb2+ employing 3,4,9,10-perylenetetracarboxylic acid diimide (PTCDI) as a cost-effective fluorophore, gold nanoparticles (AuNPs) as separating agent and an elongated aptamer as both targeting agent and PTCDI loading site. The fundamental principle of the suggested fluorescent aptasensor, which is based on PTCDI, relies on detecting variations in the fluorescence intensity of PTCDI when an elongated aptamer (as single-stranded DNA) is present or absent. The advanced aptasensor is advantageous due to the elongation of the lead aptamer sequence length induced by terminal deoxynucleotidyl transferase (TdT), resulting in enhanced sensitivity. The presence of Pb2+ and the centrifugation process causes the separation of the poly A-modified aptamer/Pb2+ conjugate from the poly T sequence. Hence, the interaction of PTCDI with the poly A moiety in the modified aptamer leads to a decrease in its fluorescence emission. The findings showcased that the fluorescent aptasensor exhibited exceptional specificity towards Pb2+ ions, while the biosensing platform accomplished an impressive detection limit of 3.7 pM. Moreover, the suggested aptasensor utilizing PTCDI exhibits a commendable capability in quantitatively analyzing Pb2+ within human serum samples and mineral water.

A sensitive aptasensor mediated by gold nanoparticles/metal organic framework lattice for detection of Pb2+ ion in marine products

Herein, an enzyme-free fluorescent aptasensor was introduced for the ultrasensitive quantification of lead (Pb2+) ion as a hazardous pollutant of the environment and foodstuffs. A nanocomposite of zeolitic imidazolate frameworks-8 and gold nanoparticles (ZIF-8@AuNPs) was utilized as an efficient quencher of the fluorescence intensity of carboxyfluorescein (FAM) signal reporter. The establishment of a hybrid structure between attached aptamer on ZIF-8@AuNPs nanocomposite, and its FAM-tagged complementary (CP) strand decreased the fluorescence response. The preferential binding between the aptamer and Pb2+ released CP strands, which retrieved the fluorescence signal. The aptasensor could assess Pb2+ in the linear concentration range of 1 pM-1 nM with a detection limit (LOD) of 0.24 pM. Besides, it could quantify Pb2+ in various samples, including fish, shrimp, tap water, milk, and serum samples. The developed aptasensor with the superiorities of easiness, cost-effectiveness, easy-to-operate, and rapidness is promising for controlling marine foodstuff safety.

Colorimetric aptasensor utilizing MOF-235 with exceptional peroxidase-like activity for the detection of oxytetracycline residues in raw milk

In this work, a simple, convenient and cost-effective colorimetric aptasensor was successfully constructed for the detection of antibiotic residues in raw milk based on the property that aptamer (Apt) synergistically enhances the catalase-like activity of MOF-235. Under optimised conditions, the proposed colorimetric aptasensor exhibited a wide detection range (15-1500 nM) with a low detection limit (6.92 nM). Furthermore, the proposed aptasensor demonstrated high selectivity, good resistance to interference and storage stability. The proposed aptasensor was validated by spiking recovery in camel milk, cow milk and goat milk with satisfactory recoveries, which demonstrated the great potential of the aptasensor for further application in real food samples, and also suggested that MOF-235 can be used as a potential universal platform to build a sensitive detection platform for other targets.

A portable solid sampling visualization nano-sensor for soil Cd based on "three-phase transforming" technique

Direct analysis of solid samples is always challenging for ionic sensors due to solidified elemental presence and matrix interference. In this work, a "three-phase transforming" technique was first established to make solid sampling elemental sensors and visual detection possible in the future. For Cd transforming from soil samples, a metal ceramic heater (MCH) electrothermal vaporizer (ETV) coupled with a dielectric barrier discharge quartz trap (DBD-QT) was first utilized to fulfill the solid sampling and preconcentration of Cd in soil; for on-site analysis, a colorimetric sensor based on the trithiocyanuric acid (TMT) functionalized gold nanoparticles (AuNPs) was chosen as a chromogenic analysis model. The portable and miniature ETV-DBD apparatus directly introduced Cd from soil and then captured Cd, consuming only <130 W and 4.5 kg weight; finally, only 200 μL water was injected as eluent to dissolve Cd for the following colorimetric detection. Herein, the Cd analyte underwent a "three-phase transforming" from solid (Cd compounds in soil), to aerosol (vaporization and transportation), to solid (Cd oxides trapped on quartz surface) and to liquid (Cd2+ in eluate). Under optimized conditions, the method limit of detection (LOD) reached 0.04 mg/kg Cd (50 mg sample), fulfilling fast monitoring of Cd contamination in soil, with <20 % relative standard deviations (RSDs). The analysis time was <10 min excluding sample digestion and acid application, as well as the interference of Pb2+ on the AuNPs sensor can be eliminated via the "three-phase transforming" process, proving an excellent anti-interference for solid analysis. This "three-phase transforming" processing technique coupled with colorimetric sensor holds a great potential for direct and on-site analysis in solid samples without complicated handling, providing a fantastic methodology for the application of ionic sensors and making solid sampling elemental sensor and visual detection possible.

CRISPR-Cas12a-based colorimetric aptasensor for aflatoxin M1 detection based on oxidase-mimicking activity of flower-like MnO2 nanozymes

Given the highly mutagenic and carcinogenic nature of Aflatoxin M1 (AFM1), the quantity assessment of AFM1 residues in milk and dairy products is necessary to maintain consumer health and food safety. Herein, CRISPR-Cas12a-based colorimetric aptasensor was developed using the catalytic activity of flower-like nanozymes of MnO2 and trans-cleavage property of CRISPR-Cas12a system to quantitatively detect AFM1. The basis of the developed colorimetric aptasensor relies on whether or not the CRISPR-Cas12a system is activated, as well as the contrast in oxidase-mimicking capability exhibited by flower-like MnO2 nanozymes when AFM1 is absent or present. When AFM1 is not present in the sample, single-stranded DNA (ssDNA) is degraded by the activated CRISPR-Cas12a, and the solution turns into yellow due to the catalytic activity of the nanozymes. While, in the attendance of AFM1, ssDNA degradation does not occur due to the inactivation of the CRISPR-Cas12a. Therefore, with the adsorption of the ssDNA on the MnO2 nanozymes, their catalytic activity decreases, and the solution color becomes pale yellow due to less oxidation of the chromogenic substrate. In this aptasensor, the relative absorbance changes increased linearly from 6 to 160 ng L-1, and the detection limit was 2.1 ng L-1. The developed aptasensor displays a selective detection performance and a practical application for quantitative analysis of AFM1 in milk samples. The results of the introduced aptasensor open up the way to design other selective and sensitive aptasensors for the detection of other mycotoxins by substitution of the used sequences.

Signal-off nanozyme-based colorimetric aptasensor for sensitive detection of ampicillin using MnO2 nanoflowers and gold nanoparticles

The combination of nanomaterials possessing distinct characteristics and the precision of aptamers facilitates the creation of biosensors that exhibit exceptional selectivity and sensitivity. In this manuscript, we present a highly sensitive aptasensor that utilizes the distinctive characteristics of MnO2 nanoflowers and gold nanoparticles to selectively detect ampicillin (AMP). In this aptasensor, the mechanism of signal change is attributed to the difference in the oxidase-mimicking activity of MnO2 nanoflowers in the presence of a free sequence. The inclusion of AMP hindered the creation of a double-stranded DNA configuration through its binding to the aptamer, resulting in an observable alteration in absorbance. The relative absorbance varied linearly with the concentration of AMP in the range of 70 pM to 10 nM with a detection limit of 21.7 pM. In general, the colorimetric aptasensor that has been developed exhibits exceptional selectivity and remarkable stability. It also demonstrates favorable performance in human serum, making it a highly reliable diagnostic tool. Additionally, its versatility is noteworthy as it holds great potential for detecting various antibiotics present in complex samples by merely replacing the utilized sequences with new ones.