Metal ion sensors are an important yet challenging field in analytical chemistry. Despite much effort, only a limited number of metal ion sensors are available for practical use because sensor design is often a trial-and-error-dependent process. DNAzyme-based sensors, in contrast, can be developed through a systematic selection that is generalizable for a wide range of metal ions. Here, we summarize recent progress in the design of DNAzyme-based fluorescent, colorimetric, and electrochemical sensors for metal ions, such as Pb2+, Cu2+, Hg2+, and UO22+ In addition, we also describe metal ion sensors based on related DNA molecules, including T-T or C-C mismatches and G-quadruplexes.

Catalytic beacon has emerged as a general platform for sensing metal ions and organic molecules. However, few reports have taken advantage of the true potential of catalytic beacons in signal amplification through multiple enzymatic turnovers, as existing designs require either equal concentrations of substrate and DNAzyme or an excess of DNAzyme in order to maintain efficient quenching, eliminating the excess of substrate necessary for multiple turnovers. Based on the large difference in the melting temperatures between the intramolecular molecular beacon stem and intermolecular products of identical sequences, we here report a general strategy of catalytic and molecular beacon (CAMB) that combines the advantages of the molecular beacon for highly efficient quenching with the catalytic beacon for amplified sensing through enzymatic turnovers. Such a CAMB design allows detection of metal ions such as Pb 2+ with a high sensitivity (LOD=600 pM). Furthermore, aptamer sequence has been introduced into DNAzyme to use the modified CAMB for amplified sensing of adenosine with similar high sensitivity. These results together demonstrate that CAMB provides a general platform for amplified detection of a wide range of targets.A current focus of research is the development of sensors for metal ions and small organic molecules,1 -5 as they play either beneficial or deleterious roles in biology or in the environment. Unlike protein and nucleic acid detections, there are few methods that can be generally applied to sensing metal ions and small organic molecules. To fill this technology gap, catalytic DNA/RNA, aptamers and aptazymes, collectively called functional nucleic acids, have been developed into sensors. As a result, catalytic DNA (called DNAzymes in this work) that are highly specific for a number of metal ions such as Pb 2+,6,7 Mg 2+,8 Zn 2+,9 Co 2+, 10,11 UO 2 2+,12 have been isolated using in vitro selection. This high metal ion specificity makes DNAzymes an attractive and general platform to sense metal ions. Therefore, we and others have converted DNAzymes into highly sensitive and selective fluorescent, 7,12,13-19 colorimetric, 15,20-25, electrochemical26 , 27 and electrochemiluminescent 28 sensors. While most of these sensors have detection limits that are below the maximum contamination levels (MCL) in water as defined by the U. S. Environmental Protection Agency (EPA), an even higher sensitivity is desired if these sensors are to be used in complex cellular or environmental samples to ensure large signal to noise ratios.* To whom correspondence should be addressed. xbzhang@hnu.cn, yi-lu@illinois.edu. SUPPORTING INFORMATION AVAILABLE.Experimental details and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org. One way to improve the sensitivity is through amplified detection. An excellent example is the use of protein enzymes such as horseradish peroxidase for amplified electrochemical and colorimetric detection. [29][30][31] Recently, a DNAzym...

T hymic stromal lymphopoietin (TSLP) stimulates in vitro proliferation of human fetal B-cell precursors. However, its in vivo role during normal human B lymphopoiesis is unknown. Genetic alterations that cause overexpression of its receptor component, cytokine receptor-like factor 2 (CRLF2), lead to high-risk B-cell acute lymphoblastic leukemia implicating this signaling pathway in leukemogenesis. We show that mouse thymic stromal lymphopoietin does not stimulate the downstream pathways (JAK/STAT5 and PI3K/AKT/mTOR) activated by the human cytokine in primary high-risk leukemia with overexpression of the receptor component. Thus, the utility of classic patient-derived xenografts for in vivo studies of this pathway is limited. We engineered xenograft mice to produce human thymic stromal lymphopoietin (+T mice) by injection with stromal cells transduced to express the cytokine. Control (-T) mice were produced using stroma transduced with control vector. Normal levels of human thymic stromal lymphopoietin were achieved in sera of +T mice, but were undetectable in -T mice. Patient-derived xenografts generated from +T as compared to -T mice showed a 3-6-fold increase in normal human B-cell precursors that was maintained through later stages of B-cell development. Gene expression profiles in high-risk B-cell acute lymphoblastic leukemia expanded in +T mice indicate increased mTOR pathway activation and are more similar to the original patient sample than those from -T mice. +T/-T xenografts provide a novel pre-clinical model for understanding this pathway in B lymphopoiesis and identifying treatments for high-risk B-cell acute lymphoblastic leukemia with overexpression of cytokine-like factor receptor 2.

Genetically encoded fluorescent proteins (FPs) have been used for metal ion detection. However, their applications are restricted to a limited number of metal ions owing to the lack of available metal‐binding proteins or peptides that can be fused to FPs and the difficulty in transforming the binding of metal ions into a change of fluorescent signal. We report herein the use of Mg2+‐specific 10–23 or Zn2+‐specific 8–17 RNA‐cleaving DNAzymes to regulate the expression of FPs as a new class of ratiometric fluorescent sensors for metal ions. Specifically, we demonstrate the use of DNAzymes to suppress the expression of Clover2, a variant of the green FP (GFP), by cleaving the mRNA of Clover2, while the expression of Ruby2, a mutant of the red FP (RFP), is not affected. The Mg2+ or Zn2+ in HeLa cells can be detected using both confocal imaging and flow cytometry. Since a wide variety of metal‐specific DNAzymes can be obtained, this method can likely be applied to imaging many other metal ions, expanding the range of the current genetically encoded fluorescent protein‐based sensors.