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

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.

A colorimetric sensor for the sensitive and rapid detection of ampicillin based on CS-Cu,Fe/HS nanozyme

A novel copper and iron doped containing chitosan and heparin sodium carbon dots (CS-Cu,Fe/HS) nanozyme was formulated through a single-step microwave digestion method. CS-Cu,Fe/HS exhibits excellent peroxidase (POD)-like activity and positive charge characteristics, and it can oxidize the negatively charged 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in the presence of H2O2 to produce a green compound (ox-ABTS). Furthermore, CS-Cu,Fe/HS enhances electron transfer and provides additional active sites through the valence state transformations of Fe2+/Fe3+ and Cu2+/Cu+. Interestingly, the POD-like activity of CS-Cu,Fe/HS is inhibited with the introduction of ampicillin (AMP), which may be because the Cu and Fe ions in CS-Cu,Fe/HS form complexes with AMP, leading to changes in the structure or surface properties of the nanozyme, thereby reducing the number of active sites on the nanozyme. Drawing from this, a straightforward and reliable colorimetric sensor was constructed for AMP detection, featuring a linear range of 0.033 to 110 μg/mL and a detection limit as low as 11.6 ng/mL. The proposed detection method for AMP performed well in real samples, with recoveries ranging from 94.8% to 110.2%.

Manganese-arginine nanozymes as oxidase mimics for smartphone-based intelligent detection of 4'-O-methylpyridoxine

By self-assembly of MnCl2 and arginine under alkaline conditions, ultra-small MnArg nanoparticles were successfully constructed as oxidase (OXD) mimics for intelligent detection of the Ginkgo toxin 4'-O-methylpyridoxal (MPN). The obtained MnArg nanozymes possessed excellent OXD-like activity and thermal stability. Based on the inhibitory effect of MPN for the catalytic activity of MnArg, this system was utilized for the colorimetric sensing of MPN with a low limit of detection (LOD) of 2.16 µg mL-1. The detection  system exhibited good selectivity against other potential interferents. FTIR data showed that the presence of MPN binds with MnArg and shields the active sites, thereby interfering with the oxidase-like activity. Combined with a smartphone and the ColorMax software, this nanozyme-based intelligent detection platform could effectively detect MPN with a LOD of 2.1 µg mL-1. Our MnArg nanozyme-based system was applied to detect real ginkgo nut samples with recoveries of 92.4-108.7%, and the relative standard deviations were less than 0.7%. This work may promote the development of novel nanozymes and expand their applications in the field of food safety detection.

Development of a highly sensitive ampicillin sensor utilizing functionalized aptamers

To develop a sensitive and simple ampicillin (AMP) sensor for trace antibiotic residue detection, the influencing factors of the modification effect of nanogold-functionalized nucleic acid sequences (Adenine: A, Thymine: T) were comprehensively analyzed in this study, including the modification method, base length and type. It was found that under the same base concentration, longer chains are more likely to reach saturation than shorter chains; and when the base concentration and length are both the same, A exhibits a higher saturation modification level compared to T. Based on these research findings, a highly sensitive fluorescence aptamer sensor for detecting ampicillin was constructed using the optimized functionalized sequence (ployA6-aptamer) and experimental conditions (6 hours binding time between nucleic acid aptamer and complementary strand, pH 7 working solution, 20 minutes detection time) based on the principle of fluorescence resonance energy transfer. The sensor has a detection range of 0.18 ng ml-1 to 3.11 ng ml-1 for ampicillin, with a detection limit of 0.04 ng ml-1. It exhibits significant selectivity and achieves an average recovery rate of 98.71% in tap water and 91.83% in milk. This method can be used not only for residual ampicillin detection, but also for highly sensitive detection of various antibiotics and small biological molecules by replacing the aptamer type. It provides a research basis for the design of highly sensitive fluorescence aptamer sensors and further applications of nanogold@DNA composite structures.

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.