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Heavy metal ions, such as thallium(I) and lead(II) are environmental toxicants known to cause a severe threat to human and ecosystem health. This work investigates aptamers and intercalating chromophore-based complexes for the detection of these toxic species. In one method, a selective label-free "turn-on" biosensor was developed using a G-quadruplex-intercalating agent, berberine. Fluorescence, melting temperature (T m ), and circular dichroism analysis confirmed the affinity and selectivity results, illustrating the potential of these aptasensor methods for improving detection limits. These fluorescence assays were found to perform with a detection limit of 3.4 μM for Tl(I) and 0.84 nM for Pb(II). Furthermore, the assays were challenged successfully with Tl(I) and Pb(II) spiked into river water samples. We next developed paper-based fluorescent assays for Tl(I) and Pb(II), where the aptamer/berberine complex was spotted onto the paper test zone. When Tl(I) or Pb(II) ions solutions were spotted onto the top of the test zone and the spot was illuminated with a portable UV light (365 nm), a strong green fluorescence could be easily visualized with the naked eye. The lowest detection limits achieved with these fluorescent paper-based assays for Tl(I) and Pb(II) were 1.1 nM and 1.6 nM, respectively. The two fluorescent approaches presented here have the potential to be the basis of rapid, fast, and cost-efficient screening assays for these toxic species., Competing Interests: Declarations. Competing interests: The authors declare no competing interests., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature.)

Regrettably, before online publication the figure of Scheme 2 has been pasted twice as Scheme 1.

Two fluorescent aptasensor methods were developed for the detection of ATP in biochemical systems. The first method consisted of a label-free fluorescent "turn-on" approach using a guanine-rich ATP aptamer sequence and the DNA-binding agent berberine complex. In the presence of ATP, the ATP preferentially binds with its aptamer and conformationally changes into a G-quadruplex structure. The association of berberine with the G-quadruplex results in the enhancement of the fluorescence signal of the former. The detection limit of ATP was found to be 3.5 μM. Fluorescence, circular dichroism and melting temperature (T m ) experiments were carried out to confirm the binding specificity and structural changes. The second method employs the ratiometric fluorescent approach based on the Forster resonance energy transfer (FRET) for the detection of ATP using berberine along with a quencher (AuNRs, AgNPs) and a fluorophore (red quantum dots (RQDs), carbon dots (CDs)) labeled at 5' and 3' termini of the ATP-binding aptamer sequence. Upon addition of ATP and berberine, ATP specifically binds with its aptamer leading to the formation of G-quadruplex, and similarly, berberine also binds to the G-quadruplex. This leads to an enhancement of fluorescence of berberine while that of RQD and CDs were significantly quenched via FRET. The respective detection limits calculated were 3.6 μM and 3.8 μM, indicating these fluorescent aptasensor methods may be used for a wide variety of small molecules. Graphical abstract.

Nucleic acid aptamers are novel molecular recognition tools that offer many advantages compared to their antibody and peptide-based counterparts. However, challenges associated with in vitro selection, characterization, and validation have limited their wide-spread use in the fields of diagnostics and therapeutics. Here, we extracted detailed information about aptamer selection experiments housed in the Aptamer Base, spanning over two decades, to perform the first parameter analysis of conditions used to identify and isolate aptamers de novo. We used information from 492 published SELEX experiments and studied the relationships between the nucleic acid library, target choice, selection methods, experimental conditions, and the affinity of the resulting aptamer candidates. Our findings highlight that the choice of target and selection template made the largest and most significant impact on the success of a de novo aptamer selection. Our results further emphasize the need for improved documentation and more thorough experimentation of SELEX criteria to determine their correlation with SELEX success.