As an investigative tool, live-cell imaging requires superior probe design to guarantee imaging quality and data validity. The ability to simultaneously address the robustness, sensitivity, and consistency issues in a single-assay system is highly desired, but it remains a largely unsolved challenge. We describe herein a probe-design strategy called a nanoamplicon comparator (NAC) and demonstrate its proofof-concept utility in intracellular microRNA (miRNA) imaging. This novel designer architecture builds upon spherical nucleic acids (SNAs) for robustness, catalytic hairpin assembly (CHA) for sensitivity, and upconversion nanoparticles (UNPs) for consistency. A catalytic circuit comprising a UNP−hairpin-DNA (UNP-HDNA) conjugate and a hairpin-DNA−organic-fluorophore (HDNA-F) conjugate as probe responds to target miRNA and generates the UNP-HDNA−HDNA-F complex as an NAC for quantitative UNP-to-organic-fluorophoreluminescence-resonance-energy-transfer (LRET) imaging against a native UNP-emission reference channel. An imaging application with miR21 shows the ability to monitor miRNA-expression levels across different cell lines and under an external stimulus.

A target-induced cyclic strategy for DNAzyme formation was proposed to achieve simple, sensitive and universal detection of protein biomarkers with convenient colorimetric or chemiluminescence imaging readout. In the assay, the target protein was recognized by a pair of DNA-labeled antibodies (Ab1-DNA1 and Ab2-DNA2) to form a proximate complex, which could hybridize with the conjugate DNA3/DNA4 to release the guanine-rich DNA4 and thus formed G-quadruplex/hemin horseradish peroxidase-mimicking DNAzyme. The process could be further recycled with Exonuclease III by cleaving DNA3 to free the proximate complex, resulting in the cyclic formation of DNAzyme. The G-quadruplex/hemin DNAzyme could catalyze the HO-mediated oxidation of 3,3,5,5-tetramethylbenzidine to produce the color change from colorless to blue or enhance the chemiluminescence of a luminol-HO system. Thus the signal could be read out with the naked eye, and by colorimetry and chemiluminescence imaging. Using a carcinoembryonic antigen as a model target, the proposed assay showed a detection range of 4 orders of magnitude along with detection limits of 170 and 16 pg mL for colorimetric and chemiluminescence imaging assays respectively. This assay had the advantages of easy operation, sensitive detection, target flexibility and diversified signal readout, providing a great opportunity for commercial application.