Selective aptamer conjugation to silver-coated magnetite nanoparticles for magnetic solid-phase extraction of trace amounts of Pb2+ ions

A review of aptamer-conjugated nanomaterials for analytical sample preparation: Classification according to the utilized nanomaterials

BACKGROUND

Sample extraction before detection is a critical step in analysis. Since targets of interest are often found in complex matrices, the sample can not be directly introduced to the analytical instrument. Nanomaterials with unique physical-chemical properties are excellent supports for use in sorbent-based extraction. However, they lack selectivity and thus need to be functionalized with target-capturing molecules. Antibodies and molecularly imprinted polymers (MIPs) can be used for this purpose, but they have some problems that limit their practical applications. Hence, functionalization of nanomaterials for selectivity remains a problem.

RESULTS

Nucleic acid aptamers are affinity reagents that can provide superiority to antibodies since they can be selected in vitro and at a lower cost. Moreover, aptamers can be chemically synthesized and easily modified with different functional groups. Hence, aptamers are good candidates to impart selectivity to the nanomaterials. Recent studies focus on the integration of aptamers with magnetic nanoparticles, carbon-based nanomaterials, metal-organic frameworks, gold nanoparticles, gold nanorods, silica nanomaterials, and nanofibers. The unique properties of nanomaterials and aptamers make the aptamer-conjugated nanomaterials attractive for use in sample preparation. Aptamer-functionalized nanomaterials have been successfully used for selective extraction of proteins, small molecules, and cells from different types of complex samples such as serum, urine, and milk. In particular, magnetic nanoparticles have a wider use due to the rapid extraction of the sample under magnetic field.

SIGNIFICANCE

In this review, we aim to emphasize how beneficial features of nanomaterials and aptamers could be combined for extraction or enrichment of the analytes from complex samples. We aim to highlight that the benefits are twofold in terms of selectivity and efficiency when employing nanomaterials and aptamers together as a single platform.

Development of silver nanoparticles and aptamer conjugated biosensor for rapid detection of E. coli in a water sample

A simple, rapid, and sensitive electrochemical biosensor based on a screen-printed carbon electrode (SPCE) was developed for onsite detection of E. coli in real time. This work analyzed the effect of aptamer conjugation and PBS buffer solution on the colloidal stability of the silver nanoparticles (AgNPs). Aggregations of the AgNPs after aptamer conjugation in PBS buffer were observed from the particle size distribution analysis. The AgNP-aptamer conjugation and its affinity towards E. coli (DH5α) were confirmed by UV-visible spectrophotometry, which showed a linear increment in the absorption with increasing E.coli concentration. The screen-printed carbon electrodes were modified by drop-casting of AgNPs, which were used as an effective immobilization platform for E. coli-specific aptamers. The modified electrode's surface modification and redox behavior were characterized using cyclic voltammetry. Finally, E. coli was detected using differential pulse voltammetry with an optimized incubation time of 15 min. The developed biosensors showed a linear decrease in current intensity with an increase in the concentration of E. coli. The biosensor had a relative standard deviation (RSD) of 6.91% (n = 3), which showed good reproducibility. The developed biosensors are highly sensitive and have a limit of detection (LOD) as low as 150 CFU/ml. The biosensor showed good selectivity for E.coli coli when comparing the signal response obtained for bacteria other than E.coli. Also, the biosensor was found stable for four weeks at room temperature and showed high recoveries from 95.27% to 107% during the tap water sensitivity validation.

Organic solvent-free solid-phase extraction of acetamiprid in food samples using Strep-Tag system integrated apta-magnetic sorbents

A novel magnetic sorbent, named ABA-(Strep-tag II)-STMB, was prepared by modifying the acetamiprid-binding aptamer (ABA) onto Strep-Tactin-coated magnetic beads (STMBs) via Strep-tag II. The integration of the Strep-tag system allows the elution of the target by the addition of D-biotin, which can compete with Strep-tag II for the Strep-Tactin sites on the magnetic beads, instead of organic solvents. The sorbent showed good selectivity and reusability, and the extraction efficiency could still reach 90.5 % after 8 reuses. Under the optimized conditions, the developed magnetic solid-phase extraction (MSPE) method exhibited good linearity in the range of 0.1-100 μM, with the limits of detection (LOD) of 0.017-0.019 μM, and the limits of quantification (LOQ) of 0.057-0.066 μM. The relative standard deviations (RSDs) were below 5.51 %. The spiked recoveries were 84.4 %-96.0 %. The analysis results were in good agreement with those of the QuEChERS method.

Magnetite azolla impedimetric nanobiosensor for phthalic acid esters quantification

Phthalic acid esters (PAEs) are a group of organic compounds that show vulnerability effects in different stages of human development. In this work, two sensitive and efficient impedimetric biosensors (IBs) were introduced and their interactions with four PAEs, namely dibutyl phthalate (DBP), dimethyl phthalate (DMP), di(2-ethylhexyl) phthalate (DEHP), and dicyclohexyl phthalate (DCHP), in aqueous solutions with these biosensors were separately investigated via electrochemical impedance spectroscopy (EIS). The surface of a copper electrode was modified by azolla fern dried powder (AZ) and magnetite-modified azolla nanocomposites (MAZ NCs) to form an azolla-based impedimetric biosensor (AZIB) and magnetite azolla nanocomposite-based impedimetric nanobiosensor (MAZIB), respectively. Determinations of PAEs with the designed biosensors were conducted based on their blocking effect on the biosensor surface to ferrous ions oxidation. After each impedimetric measurement, the electrode surface was covered again with the modifier. Nyquist plots were obtained and indicated that the charge-transfer resistance (R_CT) values of the bare electrode, AZIB, and MAZIB without injection of PAEs were 468.8, 438.7, and 285.1 kΩ, respectively. After the separate injection of DBP, DMP, DEHP, and DCHP (3 μg L^-1) on the surface of AZIB and MAZIB, R_CT values were obtained as 563.9, 588.5, 548.7, and 570.1 kΩ for AZIB and 878.2, 1219.2, 754.3, and 814.7 kΩ for MAZIB, respectively. It was observed that the PAE blockers with a smaller structure provided better point-by-point coverage of the surface, which led to a bigger shift in R_CT. The linear relationship between the EIS responses and each PAE concentration was investigated in the range of 0.1-1000 μg L^-1. The limit of detection (LOD) and limit of quantification (LOQ) values were obtained in the ranges of 0.003-0.005 μg L^-1 and 0.010-0.016 μg L^-1 for AZIB and 0.008-0.009 μg L^-1 and 0.027-0.031 μg L^-1 for MAZIB, respectively. The results showed that these biosensors can be used to determine PAEs in real aqueous samples with good relative recoveries ranging from 93.0-97.7% (RSD < 2.58%) for AZIB and 93.3-99.3% (RSD < 2.45%) for MAZIB. The results confirmed that these impedimetric biosensors offer high sensitivity and performance for the determination of trace PAEs in aqueous samples.

Applications of Smartphone-Based Aptasensor for Diverse Targets Detection

Aptamers are a particular class of functional recognition ligands with high specificity and affinity to their targets. As the candidate recognition layer of biosensors, aptamers can be used to sense biomolecules. Aptasensors, aptamer-based biosensors, have been demonstrated to be specific, sensitive, and cost-effective. Furthermore, smartphone-based devices have shown their advantages in binding to aptasensors for point-of-care testing (POCT), which offers an immediate or spontaneous responding time for biological testing. This review describes smartphone-based aptasensors to detect various targets such as metal ions, nucleic acids, proteins, and cells. Additionally, the focus is also on aptasensors-related technologies and configurations.

One-Pot Strategy as a Green and Rapid Method to Fabricate Magnetic Molecularly Imprinted Nanoparticles for Selective Capture of Sulfonylurea Herbicides

Magnetic solid-phase extraction (MSPE) based on molecularly imprinted nanoparticles (MINs) has attracted wide attention in sample pretreatment because it combines the merits of high selectivity and quick extraction procedures. However, laborious, time and solvent-consuming steps were involved in the synthesis of magnetic imprinted particles in existing approaches. To circumvent this dilemma, a green and rapid "one-pot" strategy was proposed to prepare MINs. Halosulfuron-methyl (HSM) was selected as a template molecule, and Gaussian 09 simulation software was employed to screen the 2,4,6-trivinylboroxin pyridine complex (TBP) as a functional monomer. Subsequently, the fabrication was simply conducted using a hydrothermal approach by mixing self-assembly solution of TBP-HSM, Fe³⁺, Fe²⁺, dimethyl sulfoxide, and azobisisobutyronitrile in one-pot with a total reaction time of 3.0 h. Various characterized results well evidenced the successful imprint of HSM and the resultant HSM-MINs presented satisfying superparamagnetism and saturation magnetism. Under the optimized parameters, the obtained HSM-MINs displayed good recognition capability and selectivity toward HSM (recognition coefficient was 2.60), as well as a satisfactory saturation adsorption capacity (1781 μg/g). The quantification of sulfonylurea herbicides at trace levels in environmental water and soil samples was selected as a paradigm to demonstrate the practicality and reliability of HSM-MINs/MSPE. The present study provides a convenient, reliable, and green approach for fabricating a magnetic molecular-imprinting adsorbent for MSPE.

Fabrication of a novel core-shell-shell temperature-sensitive magnetic composite with excellent performance for papain adsorption

Herein, a novel temperature-sensitive magnetic composite (Fe3O4@SiO2@P(NIPAM-co-VI)/Cu2+) with a uniform core-shell-shell structure was successfully prepared via a layer-by-layer method. The resulting magnetic composite revealed good magnetic properties and remarkable affinity to papain with a maximum adsorption capacity of 199.17 mg g-1. The adsorption equilibrium data fitted the pseudo-second-order kinetic and Freundlich models well, and the major thermodynamics parameters indicated that adsorption was an endothermic and spontaneous process. Fe3O4@SiO2@P(NIPAM-co-VI)/Cu2+ could thermally protect papain, which is attributed to the reversible hydrophilic-hydrophobic transition of the composite at temperatures below and above the lower critical solution temperature. More importantly, the magnetic composite could be recycled at least six times without a remarkable loss in its adsorption capacity, and the process of adsorption and elution had no significant effect on the activity and structure of papain. This work could provide a novel separation method for papain without loss of its activity.