A selective off–on fluorescent aptasensor for alpha-fetoprotein determination based on N-carbon quantum dots and oxidized nanocellulose

Slide-type waveflex biosensor based on signal enhancement technology for alpha-fetoprotein detection

The development of signal enhancement technology in optical fiber biosensors is beneficial for the accurate measurement of low-concentration samples. Here, a localized surface plasmon resonance (LSPR)-based fiber biosensor combining a slide-type fiber structure (thus named WaveFlex Biosensor) and low-dimensional materials is proposed for alpha-fetoprotein (AFP) detection. A symmetric transverse offset splicing technology was used to fabricate the multi-mode fiber (MMF-multi-core fiber (MCF)-MMF structure. Furthermore, the MMF on one side was prepared into an S-taper, forming a slide-type fiber structure to generate more energy leakage. The LSPR signal generated by gold nanoparticles (AuNPs) was enhanced by the CeO2 NPs and C3N quantum dots functionalized on the fiber probe. The excellent performance of NPs was conducive to improving the sensitivity of the WaveFlex biosensor and enabling the rapid detection of samples. An AFP antibody was used to identify AFP micro-biomolecules in a specific manner. Based on the combination of the above two methods, the developed fiber probe was applied to detect AFP, and the sensitivity and limit of detection were 32 pm/(ng/mL) and 6.65 ng/mL, respectively. The experimental results demonstrate that the signal-enhanced AFP WaveFlex biosensor has great potential for the rapid and accurate detection of AFP.

Synthesis of molecularly imprinted polymer as a nanosorbent for dispersive magnetic micro solid-phase extraction and determination of valsartan in biological samples by UV-Vis Spectrophotometry: Isotherm, kinetics, and thermodynamic studies

A magnetic molecularly imprinted polymer (MIP) was synthesized by precipitation polymerization utilizing Fe₃O₄@SiO₂-MPS as a magnetic core, itaconic acid as a functional monomer, azobisisobutyronitrile as an initiator, and ethylene glycol dimethacrylate as a cross linker. It was then applied as a nanosorbent for dispersive magnetic micro solid phase extraction (DM-µ-SPE) and determination of valsartan in biological fluids. The morphology and structure of magnetic MIP were characterized by Fourier-transform infrared spectroscopy, Field Emission Scanning electron microscopy, Vibrating sample magnetometer, Energy dispersive x-ray analysis, and Thermogravimetric analysis. The influence of operation conditions on sorption, such as pH (4-10), contact time (10-25 min), initial concentration (1-30 mg L^-1), and temperature (25-40 °C) was investigated. After the extraction step, the valsartan concentration was determined by UV-Vis spectrophotometer at 253 nm. The isotherm and kinetic of valsartan sorption were best fitted by the Langmuir model (R² = 0.987) and the Pseudo second-order kinetic model (R² = 0.971), respectively. The maximum monolayer sorption capacity for magnetic MIP was obtained to be 4.56 mg g^-1. The analytical approach demonstrated favorable figures of merit, with a linear dynamic range of 10-100 µg L^-1, a low detection limit of 0.56 µg L^-1, and an acceptable preconcentration factor of 5 acquired in optimum conditions. The recoveries of the suggested technique at three spiked levels of analysis were in the range of 101 %-102 %. Valsartan was extracted from various real samples (urine and human blood plasma samples) utilizing the proposed magnetic nanosorbent, and the results exhibited that magnetic MIP was favorable for extraction and measurement of trace amounts of valsartan in biological samples.

Synthesis of magnetic molecular imprinted polymer with a new functional monomer for dispersive micro solid phase extraction of captopril from wastewater and biological samples and determination by UV-Vis spectrophotometry

A magnetic molecularly imprinted polymer (MMIP) was fabricated for captopril by surface polymerization of Fe₃O₄@SiO₂ nanoparticles using a new functional monomer of N-(allylcarbamothioyl)-2-chlorobenzamide. It was then employed as a selective nanosorbent for dispersive magnetic micro solid phase extraction (DM-μ-SPE) of captopril from biological and wastewater samples. To characterize the physicochemical properties of the MMIP, different analytical methods such as the vibrating sample magnetometer, field emission scanning electron microscopy, Brunauer-Emmett-Teller, and Fourier transform infrared spectroscopy were utilized. To gain the maximum extraction recovery of captopril, the influence of various operating conditions was investigated and experimental settings optimized. After the extraction step, the concentration of captopril was measured by UV-Vis spectrophotometer at 245 nm. The assessments demonstrated that the MMIP provides higher extraction efficiency in comparison to magnetic non-imprinted polymer, suggesting the establishment of selective recognition binding sites at the MMIP surface. The method depicted desirable figures of merit of a low detection limit of 0.16 μg L^-1, a limit of quantification of 0.50 μg L^-1, a linear dynamic range of 0.50-22.0 μg L^-1, and an acceptable preconcentration factor of 333. The magnetic MIP was successfully employed for preconcentration and extraction of trace amounts of captopril in real samples, such as human blood serum, urine, and wastewater samples, with recoveries in the range 95.7 to 102.6% and relative standard deviations < 5%.

A Selective Fluorescent Nanoprobe Based on Graphene Quantum Dots and Hg2+ for the Determination of Tetracycline in Biological Samples

A simple, sensitive, and selective fluorometric method based on graphene quantum dots and Hg²⁺ is presented for the determination of tetracycline. The fluorescence emission of graphene quantum dots at 463 nm decreased in the presence of Hg²⁺ ions due to its electrostatic interaction with the negatively charged surface of quantum dots at pH = 8.0. The addition of tetracycline to this system resulted in the retrieval of the fluorescence emission of the graphene quantum dots proportional to the tetracycline concentration. This is because of the interaction between tetracycline and Hg²⁺ that results in the release of the quantum dots' surface. Under the optimized conditions, the calibration curve indicated good linearity in the range of 2.0-44.0 nmol L^-1 with a detection limit of 0.52 nmol L^-1 for tetracycline. The designed nanoprobe was capable of the determination of tetracycline in serum and urine samples.

Fluorescence Labeling of Cellulose Nanocrystals—A Facile and Green Synthesis Route

Efficient chemical modification of cellulose nanocrystals (CNCs) by grafting commonly involves aprotic solvents, toxic reactants, harsh reaction conditions, or catalysts, which have negative effects on the particle character, reduced dispersibility and requires further purification, if products are intended for biomedical applications. This work, in contrast, presents a robust, facile, and green synthesis protocol for the grafting of an amino-reactive fluorophore like fluorescein isothiocyanate (FITC) on aqueous CNCs, combining and modifying existent approaches in a two-step procedure. Comparably high grafting yields were achieved, which were confirmed by thermogravimetry, FTIR, and photometry. The dispersive properties were confirmed by DLS, AF4-MALS, and TEM studies. The presented route is highly suitable for the introduction of silane-bound organic groups and offers a versatile platform for further modification routes of cellulose-based substrates.

Etching reaction of carbon quantum dot-functionalized MnO2 nanosheets with an enzymatic product for photoelectrochemical immunoassay of alpha-fetoprotein

An etching reaction-based photoelectrochemical (PEC) immunoassay was developed to monitor alpha-fetoprotein (AFP) by coupling with the enzymatic product toward the dissolution of MnO2 nanosheets.