Materials whose dielectric constant varies spatially with submicrometer periodicity exhibit diffractive optical properties which are potentially valuable in a number of existing and emerging applications. Here, such systems are fabricated by exploiting the spontaneous crystallization of monodisperse silica spheres into close-packed arrays. By reliance on a vertical deposition technique to pack the spherical colloids into close-packed silica-air arrays, high quality samples can be prepared with thicknesses up to 50 µm. These samples are planar and thus suitable for optical characterization. Scanning electron microscopy (SEM) of these materials illustrates the close-packed ordering of the spherical colloids in planes parallel to the substrate; cross-sectional SEM micrographs of the arrays as well as optical methods are used to measure sample thickness and uniformity. Normal-incidence transmission spectra in the visible and near-infrared regions show distinct peaks due to diffraction from the colloidal layers. While these basic optical characteristics are similar to thicker and polycrystalline gravity-sedimented colloidal crystals, the systematic control over the number of colloidal layers allows the effect of sample thickness on the optical spectrum to be studied for the first time.

The fabrication of polymeric materials with ordered submicron-sized void structures is potentially valuable for many separation technologies as well as for emerging optical applications. This paper reports the preparation of macroporous polymer membranes with regular voids and the characterization of their diffractive optical properties. These materials are made using a colloidal crystal template of silica microspheres; the air between the spheres can be replaced by monomers that can be subsequently polymerized. The use of silica microspheres as templates makes it possible to employ chemical rather than thermal methods for template removal. For this reason, polymers as diverse as polyurethane and polystyrene can be used to create free-standing macroporous films, with thickness ranging from 0.5 to 50 μm. Scanning electron microscopy of these samples indicates a well-formed porous structure consisting of voids ranging in diameter from 200 to 400 nm. These large cavities are not isolated, but rather interconnected by a network of monodisperse smaller pores (d = 50−130 nm) whose size can be controlled by varying the polymerization temperature. These membranes exhibit striking optical properties due to the periodic arrangement of air spheres in the polymer medium. Normal-incidence transmission measurements of these samples are compared to a theoretical model based on a scalar wave approximation. This model assumes an ordered structure of close-packed, three-dimensional air spheres. The good agreement between theory and experiment provides additional evidence of the long-range order of these samples.

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.

We report observations of the optical stop band of periodic planar arrays of submicron silica spheres, and of macroporous polymers grown from these silica templates. The stop-band width and peak attenuation depend on the number of layers and on the dielectric contrast between the spheres and the interstitial regions, both of which are experimentally controlled. The results are compared to the predictions of the scalar wave approximation. This is the first systematic study of the thickness dependence of the stop band in colloidal photonic band gap structures.The optical properties of photonic band gap materials have been the subject of many experimental and theoretical studies in recent years [1]. In these systems the dielectric function is spatially periodic in one or more dimensions, and as a result their optical properties are dominated by strong diffraction effects. The propagation of light is strongly inhibited over a narrow range of frequencies; this produces a dip in the transmission spectrum known as a stop band. In order to construct structures whose stop bands are centered in the visible region, the length scale of the periodicity must shrink accordingly. As a result, lithographic fabrication of materials with visible stop bands is quite challenging.One class of materials which offers a unique solution to this problem is colloidal crystals [2][3][4][5]. Here, one relies on the tendency of submicron dielectric spheres to spontaneously self-assemble into ordered arrays. This approach has several advantages over the conventional "top-down" fabrication techniques, including a wider versatility with respect to the choice of materials, the relative ease of casting high quality planar films as optical coatings, and the low cost of implementation. Further, colloids as small as 50 nm have been crystallized [6]; such systems could extend the applicability of photonic band gap materials into the soft-x-ray range. Finally, unlike lithographic and machining techniques, which generally limit sample thickness to only a few repeating layers, colloidal crystals can be fabricated with controlled thickness, up to hundreds of layers [7]. This may prove to be a significant advantage if control over sample thickness provides an avenue for tuning the optical properties.Thus, an understanding of the evolution of the optical properties of these materials with an increasing number of layers is an important ingredient in the study of these strongly diffracting systems. Yet, a systematic investigation of this thickness dependence has been lacking. This is due to the difficulty in controlling the thickness of colloidal samples, and also in producing samples with sufficient uniformity and optical quality for such measurements. Although the thickness dependence of the optical properties of periodic dielectric media has been studied in the microwave and submillimeter range [8,9], these systems do not mimic the spherical close-packed arrangement common to most colloidal crystals.In this work, we address this issue using colloidal crysta...

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.

Spatial and temporal distributions of metal ions in vitro and in vivo are crucial in our understanding of the roles of metal ions in biological systems, and yet there is a very limited number of methods to probe metal ions with high space and time resolution, especially in vivo. To overcome this limitation, we report a Zn2+-specific near infrared (NIR) DNAzyme nanoprobe for real-time metal ion tracking with spatiotemporal control in early embryos and larvae of zebrafish. By conjugating photocaged DNAzymes onto lanthanide-doped upconversion nanoparticles (UCNPs), we have achieved upconversion of a deep tissue penetrating NIR 980 nm light into 365 nm emission. The UV photon then efficiently photo-decages a substrate strand containing a nitrobenzyl group at the 2’-OH of adenosine ribonucleotide, allowing enzymatic cleavage by a complementary DNA strand containing a Zn2+-selective DNAzyme. The product containing a visible FAM fluorophore that is initially quenched by BHQ1 and Dabcyl quenchers, is released after cleavage, resulting in higher fluorescent signals. The DNAzyme-UCNP probe enables Zn2+ sensing by exciting in the NIR biological imaging window in both living cells and zebrafish embryos, and detecting in the visible region. This report introduces a platform that can be used to understand the Zn2+ distribution with spatiotemporal control, thereby giving insights into the dynamical Zn2+ ion distribution in intracellular and in vivo models.

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.