Over the past two decades, enormous progress has been made in designing fluorescent sensors or probes for divalent metal ions. In contrast, the development of fluorescent sensors for monovalent metal ions, such as sodium (Na + ), has remained underdeveloped, even though Na + is one the most abundant metal ions in biological systems and plays a critical role in many biological processes. Here, we report the in vitro selection of the first (to our knowledge) Na + -specific, RNA-cleaving deoxyribozyme (DNAzyme) with a fast catalytic rate [observed rate constant (k obs ) ∼0.1 min −1 ], and the transformation of this DNAzyme into a fluorescent sensor for Na + by labeling the enzyme strand with a quencher at the 3′ end, and the DNA substrate strand with a fluorophore and a quencher at the 5′ and 3′ ends, respectively. The presence of Na + catalyzed cleavage of the substrate strand at an internal ribonucleotide adenosine (rA) site, resulting in release of the fluorophore from its quenchers and thus a significant increase in fluorescence signal. The sensor displays a remarkable selectivity (>10,000-fold) for Na + over competing metal ions and has a detection limit of 135 μM (3.1 ppm). Furthermore, we demonstrate that this DNAzyme-based sensor can readily enter cells with the aid of α-helical cationic polypeptides. Finally, by protecting the cleavage site of the Na + -specific DNAzyme with a photolabile o-nitrobenzyl group, we achieved controlled activation of the sensor after DNAzyme delivery into cells. Together, these results demonstrate that such a DNAzyme-based sensor provides a promising platform for detection and quantification of Na + in living cells.M etal ions play crucial roles in a variety of biochemical processes. As a result, the concentrations of cellular metal ions have to be highly regulated in different parts of cells, as both deficiency and surplus of metal ions can disrupt normal functions (1-4). To better understand the functions of metal ions in biology, it is important to detect metal ions selectively in living cells; such an endeavor will not only result in better understanding of cellular processes but also novel ways to reprogram these processes to achieve novel functions for biotechnological applications.Among the metal ions in cells, sodium (Na + ) serves particularly important functions, as changes in its concentrations influence the cellular processes of numerous living organisms and cells (5-8), such as epithelial and other excitable cells (9). As one of the most abundant metal ions in intracellular fluid (10), Na + affects cellular processes by triggering the activation of many signal transduction pathways, as well as influencing the actions of hormones (11). Therefore, it is important to carefully monitor the concentrations of Na + in cells. Toward this goal, instrumental analyses by atomic absorption spectroscopy (12), Xray fluorescence microscopy (13), and 23 Na NMR (14) have been used to detect the concentration of intracellular Na + . However, it is difficult to use these methods to o...

DNAzymes have shown great promise as a general platform for detecting metal ions, as many metal-specific DNAzymes can be obtained using in vitro selection. While DNAzyme-based metal sensors have found many applications in the extracellular environment, no intracellular application of DNAzyme sensors has yet been reported. Here we demonstrate a novel type of metal ion sensor for intracellular metal ion detection. The probe consists of a 13 nm gold nanoparticle (AuNP) core functionalized with a shell consisting of a uranyl-specific 39E DNAzyme whose enzyme strand contains a thiol at the 3′ end for conjugation to the AuNP, and whose substrate strand is modified with a Cy3 fluorophore at the 5′ end and a molecular quencher at the 3′ end. In the absence of uranyl, the fluorescence of the Cy3 is quenched by both AuNP and the molecular quencher. In the presence of uranyl, the DNAzyme cleaves the fluorophore-labeled substrate strand, resulting in release of the shorter product strand containing the Cy3 and increased fluorescence. We demonstrate that this DNAzyme-AuNP probe can readily enter cells and can serve as a metal ion sensor within a cellular environment, making it the first demonstration of DNAzymes as intracellular metal ion sensors. Such a method can be generally applied to the detection of other metal ions using other DNAzymes selected through in vitro selection.

Lanthanide-doped upconversion nanoparticles (UCNPs) have shown promise in biomedical applications. However, as the UCNPs are normally capped with hydrophobic ligands, it remains challenging to prepare biocompatible UCNPs with specific molecular recognition capabilities. We herein report an exceptionally simple strategy to prepare uniform DNA-modified upconversion nanoparticles (DNA-UCNPs) as versatile bioprobes. The approach can directly convert as-prepared hydrophobic UCNPs into water-soluble DNA-UCNPs without any chemical modification of UCNPs or oligonucleotides. Furthermore, DNA molecules on the DNA-UCNPs retain their biorecognition ability, allowing programmable assembly of hybrid nanostructures. More importantly, we show that these DNA-UCNPs are capable of crossing cell membranes without the need of transfection agents, and their use as agents for bioimaging and DNA delivery are also demonstrated. Finally, DNA aptamer-conjugated UCNPs can be readily used for targeted imaging of cancer cells.

DNAzymes, sequences of DNA with catalytic activity, have been demonstrated as a potential platform for sensing a wide range of metal ions. Despite significant promise, cellular sensing using DNAzymes has been difficult, mainly due to the ‘always-on’ nature of first generation DNAzyme sensors. To overcome this limitation, we demonstrate herein the design and synthesis of a photoactivatable or photocaged DNAzyme, and its application in sensing Zn(II) in living cells. In this design the adenosine ribonucleotide at the scissile position of the 8–17 DNAzyme is replaced by 2′-O-nitrobenzyl adenosine, rendering the DNAzyme inactive and thus allowing its delivery into cells intact, protected from non-specific degradation within cells. Irradiation at 365 nm restores DNAzyme activity, allowing for temporal control over the DNAzyme’s sensing activity for metal ions. The same strategy has also been applied towards the GR-5 DNAzyme for detection of Pb(II), demonstrating its broad generalizability.

Taming interfacial electronic effects on Pt nanoparticles modulated by their concomitants has emerged as an intriguing approach to optimize Pt catalytic performance. Here, we report Pt nanoparticles assembled on vacancy-abundant hexagonal boron nitride nanosheets and their use as a model catalyst to embrace an interfacial electronic effect on Pt induced by the nanosheets with N-vacancies and B-vacancies for superior CO oxidation catalysis. Experimental results indicate that strong interaction exists between Pt and the vacancies. Bader charge analysis shows that with Pt on B-vacancies, the nanosheets serve as a Lewis acid to accept electrons from Pt, and on the contrary, when Pt sits on N-vacancies, the nanosheets act as a Lewis base for donating electrons to Pt. The overall-electronic effect demonstrates an electron-rich feature of Pt after assembling on hexagonal boron nitride nanosheets. Such an interfacial electronic effect makes Pt favour the adsorption of O2, alleviating CO poisoning and promoting the catalysis.