No abstract

Current super-resolution techniques offer unprecedented spatial resolution, but quantitative counting of spatially unresolvable molecules remains challenging. Here, we use the programmable and specific binding of dye-labeled DNA probes to count integer numbers of targets. This method, called quantitative Points Accumulation In Nanoscale Topography (qPAINT), avoids the challenging task of analyzing the environmentally sensitive hard-to-predict photophysics of dyes, and enables robust counting by analyzing the predictable binding kinetics of dye-labeled DNA probes. We benchmarked qPAINT in vitro and in situ by counting strands on DNA nanostructures, Nup98 protein clusters in the nuclear pore complex, Bruchpilot proteins in Drosophila, and finally the number of fluorescence in situ hybridization probes on single mRNA targets in fixed cells. We achieved high accuracy (~98–99 %), high precision (~80–95 %), and multiplexed detection over a large dynamic range.

A key attribute of drug delivery systems (DDSs) is their ability to regulate drug release, minimizing side effects and improving therapeutic efficacy of conventional pharmaceuticals. 1 Two approaches can be used to regulate the release of the therapeutic payload from the carrier: endogenous and exogenous activation. Endogenous activation strategies 2 exploit specific physiochemical characteristics of the pathological microenvironment, providing biologicallycontrolled release. Exogenous activation 3 provides a complementary approach, employing orthogonal external stimuli to effect drug release.Light provides a highly orthogonal external stimulus, allowing spatiotemporal control of payload release. In a recent applications of this strategy, drug encapsulated carriers of 100-500nm size (i.e. mesoporous silica, self-assembled molecular aggregates) containing a photo switch for cargo release has been developed. 4 In an alternative approach, caged drugs have been developed where the activity of the drug is suppressed by attaching it to a blocking element through a photoremovable protecting group. 5Monolayer protected gold nanoparticls (AuNPs) provide an appealing synthetic scaffold for the creation of DDSs due to their functional versatility, better biocompatibility, and low toxicity. 6 Moreover, through appropriate choice of particle size (2-10 nm), enhanced biodistribution (i.e. passive targeting) can be obtained through the enhanced permeation and retention (EPR) effect. 7 The EPR effect arises from the increased permeability of the tumor tissue vasculature, which allows nanocarriers to extravasate into the interstitial space, 1, 7 resulting in an enrichment of the carriers within the tumor tissue. We describe here the use of AuNPs for photocontrolled release of a caged anticancer drug (5-fluorouracil, 5-FU) by conjugating the drug to the particle surface through a photoresponsive o-nitrobenzyl (ONB) linkage. In this approach, the particle serves both to cage and transport the therapeutic.The fluorouracil conjugated gold nanoparticles (Au_PCFU) synthesized for this study possess a gold core diameter of ~2-nm and feature a surface functionality comprising of a mixed selfassembled monolayer of photocleavable and zwitterionic thiol ligands. The two ligands feature a common basic structure, where an alkyl segment is used to confer stability on the particle, while the tetra(ethylene glycol) component provides water solubility and superior biocompatibility. 8, 6d The zwitterionic ligand serves to enhance solubility and prevent cellular uptake, 9 while the photocleavable ligand integrates fluorouracil (5-FU) moieties to the nanoparticle surface through a terminally anchored orthonitrobenzyl (ONB) group. The ONB group has long term stability under ambient light in biological environments. However, it E-mail: rotello@chem.umass.edu. Supporting Information Available: Synthesis of ligands, preparation of the nanoparticle, protocols for in vitro and cellular studies, and other experimental procedures. This information is avail...