This article describes the design and synthesis of quinoxaline-based semiconducting polymer dots (Pdots) that exhibit near-infrared fluorescence, ultrahigh brightness, large Stokes shifts, and excellent cellular targeting capability. We also introduced fluorine atoms and long alkyl chains into polymer backbones and systematically investigated their effect on the fluorescence quantum yields of Pdots. These new series of quinoxaline-based Pdots have a fluorescence quantum yield as high as 47% with a Stokes shift larger than 150 nm. Single-particle analysis reveals that the average per-particle brightness of the Pdots is at least 6 times higher than that of the commercially available quantum dots. We further demonstrated the use of this new class of quinoxaline-based Pdots for effective and specific cellular and subcellular labeling without any noticeable nonspecific binding. Moreover, the cytotoxicity of Pdots were evaluated on HeLa cells and zebrafish embryos to demonstrate their great biocompatibility. By taking advantage of their extreme brightness and minimal cytotoxicity, we performed, for the first time, in vivo microangiography imaging on living zebrafish embryos using Pdots. These quinoxaline-based NIR-fluorescent Pdots are anticipated to find broad use in a variety of in vitro and in vivo biological research.

Semiconducting polymer dots (P-dots) recently have emerged as a new type of ultrabright fluorescent probe with promising applications in biological imaging and detection. With the increasing desire for near-infrared (NIR) fluorescing probes for in vivo biological measurements, the currently available NIR-emitting P-dots are very limited and the leaching of the encapsulated dyes/polymers has usually been a concern. To address this challenge, we first embedded the NIR dyes into the matrix of poly[(9,9-dioctylfluorene)-co-2,1,3-benzothiadiazole-co-4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole] (PF-BT-DBT) polymer and then enclosed the doped P-dots with polydiacetylenes (PDAs) to avoid potential leakage of the entrapped NIR dyes from the P-dot matrix. These PDA-enclosed NIR-emitting P-dots not only emitted much stronger NIR fluorescence than conventional organic molecules but also exhibited enhanced photostability over CdTe quantum dots, free NIR dyes, and gold nanoclusters. We next conjugated biomolecules onto the surface of the resulting P-dots and demonstrated their capability for specific cellular labeling without any noticeable nonspecific binding. To employ this new class of material as a facile sensing platform, an easy-to-prepare test paper, obtained by soaking the paper into the PDA-enclosed NIR-emitting P-dot solution, was used to sense external stimuli such as ions, temperature, or pH, depending on the surface functionalization of PDAs. We believe these PDA-coated NIR-fluorescing P-dots will be very useful in a variety of bioimaging and analytical applications.

In recent years, semiconducting polymer dots (Pdots) have emerged as a novel class of extraordinarily bright fluorescent probes with burgeoning applications in bioimaging and sensing. While the desire for near-infrared (NIR)-emitting agents for in vivo biological applications increases drastically, the direct synthesis of semiconducting polymers that can form Pdots with ultrahigh fluorescence brightness is extremely lacking due to the severe aggregation-caused quenching of the NIR chromophores in Pdots. Here we describe the synthesis of dithienylbenzoselenadiazole (DBS)-based NIR-fluorescing Pdots with ultrahigh brightness and excellent photostability. More importantly, the fluorescence quantum yields of these Pdots could be effectively increased by the introduction of long alkyl chains into the thiophene rings of DBS to significantly inhibit the aggregation-caused emission quenching. Additionally, these new series of DBS-based Pdots can be excited by a commonly used 488 nm laser and show a fluorescence quantum yield as high as 36% with a Stokes shift larger than 200 nm. Single-particle analysis indicates that the per-particle brightness of the Pdots is at least 2 times higher than that of the commercial quantum dot (Qdot705) under identical laser excitation and acquisition conditions. We also functionalized the Pdots with carboxylic acid groups and then linked biomolecules to Pdot surfaces to demonstrate their capability for specific cellular labeling without any noticeable nonspecific binding. Our results suggest that these DBS-based NIR-fluorescing Pdots will be very practical in various biological imaging and analytical applications.

Recently, semiconducting polymer dots (Pdots) have become a novel type of ultrabright fluorescent probes which hold great promise in biological imaging and analytical detection. Here we developed a visual sensor based on Pdots for Pb(2+) detection. We first embedded near-infrared (NIR) dyes into the matrix of poly[(9,9-dioctylfluorene)-co-2,1,3-benzothiadiazole-co-4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole] (PFBT-DBT) polymer and then capped the Pdots with polydiacetylenes (PDAs), in which parts of the PDAs were prefunctionalized with 15-crown-5 moieties to form Pdots. The high selectivity of these Pdots for lead ions is attributed to the formation of 2:1 15-crown-5-Pb(2+)-carboxylate sandwich complex on the Pdot surface. After Pb(2+) chelation, the conjugation system of the PDA was perturbed and strained, causing a chromatic change of the PDA from blue to red. At the same time, the encapsulated NIR dyes were liable to leach out that resulted in an emission variation of the Pdots. Accordingly, lead ions can be recognized by either color change or emission variation of the Pdots. We also loaded these nanoprobes into live HeLa cells through endocytosis, and then monitored changes in Pb(2+) levels within cells, demonstrating their utility for use in cellular and bioimaging applications. In addition, we fabricated easy-to-prepare test strips impregnated with Pdot-poly(vinyl alcohol) films to identify Pb(2+) in real samples, which proved their applicability for in situ on-site detection. Our results suggest that this Pdot-based visual sensor shows promising potential for advanced environmental and biological applications.

This communication demonstrates a simple platform for using coumarin-encapsulated Pdots as a sensor for ratiometric determination of fluoride ions.