Lateral Flow Aptasensor for Small Molecule Targets Exploiting Adsorption and Desorption Interactions on Gold Nanoparticles

A lateral flow assay (LFA) can provide a rapid and cost-effective means to detect targets in situ; however, existing LFA formats (predominantly sandwich assays) are not suitable for small molecule targets. We present a new LFA design that probes the dissociation of aptamers from the surface of gold nanoparticles upon recognition of small targets. The target-induced removal of aptamer molecules from the surface of the colored particles results in the particles being captured on a test line comprised of the protein bovine serum albumin immobilized on nitrocellulose. On the other hand, in the absence of target, aptamer coated particles are protected from capture on the test line and are instead captured at a control line comprised of the protein lysozyme. This protein is strongly positively charged under measurement conditions and therefore captures all gold nanoparticles regardless of the presence of aptamers. The effectiveness and operation mechanism of this simply fabricated sensor was demonstrated by using a previously reported 35-mer aptamer for a small molecule, 17β-estradiol. The sensor exhibited nanomolar level of detection, excellent selectivity against potential interfering molecules, and robust operation in natural river water samples. The simplicity and performance of the sensor platform renders it applicable to a wide range of other aptamers targeting small molecules, as we demonstrated with a novel bisphenol A aptamer. Additionally, we show that our LFA design is not confined to the specific proteins used as test and control lines, provided that their charge is appropriate to modulate the interaction with aptamer-coated or bare nanoparticles

Distance Distributions of Photogenerated Charge Pairs in Organic Photovoltaic Cells

Strong Coulomb interactions in organic photovoltaic cells dictate that charges must separate over relatively long distances in order to circumvent geminate recombination and produce photocurrent. In this article, we measure the distance distributions of thermalized charge pairs by accessing a regime at low temperature where charge pairs are frozen out following the primary charge separation step and recombine monomolecularly via tunneling. The exponential attenuation of tunneling rate with distance provides a sensitive probe of the distance distribution of primary charge pairs, reminiscent of electron transfer studies in proteins. By fitting recombination dynamics to distributions of recombination rates, we identified populations of charge-transfer states and well-separated charge pairs. For the wide range of materials we studied, the yield of separated charges in the tunneling regime is strongly correlated with the yield of free charges measured via their intensity-dependent bimolecular recombination dynamics at room temperature. We therefore conclude that populations of free charges are established via long-range charge separation within the thermalization time scale, thus invoking early branching between free and bound charges across an energetic barrier. Subject to assumed values of the electron tunneling attenuation constant, we estimate critical charge separation distances of ∼3–4 nm in all materials. In some blends, large fullerene crystals can enhance charge separation yields; however, the important role of the polymers is also highlighted in blends that achieved significant charge separation with minimal fullerene concentration. We expect that our approach of isolating the intrinsic properties of primary charge pairs will be of considerable value in guiding new material development and testing the validity of proposed mechanisms for long-range charge separation

Transient Grating Photoluminescence Spectroscopy: An Ultrafast Method of Gating Broadband Spectra

Ultrafast photoluminescence (PL) spectroscopy can cleanly resolve excited-state dynamics and coupling to the environment, however, there is a demand for new methods that combine broadband detection and low backgrounds. We present a new method, transient grating photoluminescence spectroscopy (TGPLS), that addresses this challenge by exploiting a focusing geometry where ultrafast broadband spectra are transiently diffracted away from the background PL. We show that TGPLS can resolve the complex spectral relaxation observed in conjugated polymer and oligomer solutions, with an essentially flat spectral response throughout the visible region and potentially beyond. The benefits we demonstrate using TGPLS could expand access to spectral information, particularly for other multichromophoric and heterogeneous materials where complex spectral relaxation is expected

Transient Grating Photoluminescence Spectroscopy: An Ultrafast Method of Gating Broadband Spectra

Ultrafast photoluminescence (PL) spectroscopy can cleanly resolve excited-state dynamics and coupling to the environment, however, there is a demand for new methods that combine broadband detection and low backgrounds. We present a new method, transient grating photoluminescence spectroscopy (TGPLS), that addresses this challenge by exploiting a focusing geometry where ultrafast broadband spectra are transiently diffracted away from the background PL. We show that TGPLS can resolve the complex spectral relaxation observed in conjugated polymer and oligomer solutions, with an essentially flat spectral response throughout the visible region and potentially beyond. The benefits we demonstrate using TGPLS could expand access to spectral information, particularly for other multichromophoric and heterogeneous materials where complex spectral relaxation is expected

Broadband Ultrafast Photoluminescence Spectroscopy Resolves Charge Photogeneration via Delocalized Hot Excitons in Polymer:Fullerene Photovoltaic Blends

Conventional descriptions of excitons in semiconducting polymers do not account for several important observations in polymer:fullerene photovoltaic blends, including the ultrafast time scale of charge photogeneration in phase separated blends and the intermediate role of delocalized charge transfer states. We investigate the nature of excitons in thin films of polymers and polymer:fullerene blends by using broadband ultrafast photoluminescence spectroscopy. Our technique enables us to resolve energetic relaxation, as well as the volume of excitons and population dynamics on ultrafast time scales. We resolve substantial high-energy emission from hot excitons prior to energetic relaxation, which occurs predominantly on a subpicosecond time scale. Consistent with quantum chemical calculations, ultrafast annihilation measurements show that excitons initially extend along a substantial chain length prior to localization induced by structural relaxation. Moreover, we see that hot excitons are initially highly mobile and the subsequent rapid decay in mobility is correlated with energetic relaxation. The relevance of these measurements to charge photogeneration is confirmed by our measurements in blends. We find that charge photogeneration occurs predominately via these delocalized hot exciton states in competition with relaxation and independently of temperature. As well as accounting for the ultrafast time scale of charge generation across large polymer phases, delocalized hot excitons may also account for the crucial requirement that primary charge pairs are well separated in efficient organic photovoltaic blends

Solution Synthesis and Optical Properties of Transition-Metal-Doped Silicon Nanocrystals

A new synthetic method was developed to produce a range of transition-metal (Mn, Ni, and Cu) doped silicon nanocrystals (Si NCs). The synthesis produces monodisperse undoped and doped Si NCs with comparable average sizes as shown by transmission electron microscopy (TEM). Dopant composition was confirmed by EDX (energy dispersive X-ray spectroscopy). The optical properties of undoped and doped were compared and contrasted using absorption (steady-state and transient) and photoluminescence spectroscopy. Doped Si NCs demonstrated unique dopant-dependent optical properties compared to undoped Si NCs such as enhanced subgap absorption, and 40 nm shifts in the emission. Transient absorption (TA) measurements showed that photoexcitations in doped Si NCs relaxed via dopant states not present in undoped Si NCs

Solution Synthesis and Optical Properties of Transition-Metal-Doped Silicon Nanocrystals

A new synthetic method was developed to produce a range of transition-metal (Mn, Ni, and Cu) doped silicon nanocrystals (Si NCs). The synthesis produces monodisperse undoped and doped Si NCs with comparable average sizes as shown by transmission electron microscopy (TEM). Dopant composition was confirmed by EDX (energy dispersive X-ray spectroscopy). The optical properties of undoped and doped were compared and contrasted using absorption (steady-state and transient) and photoluminescence spectroscopy. Doped Si NCs demonstrated unique dopant-dependent optical properties compared to undoped Si NCs such as enhanced subgap absorption, and 40 nm shifts in the emission. Transient absorption (TA) measurements showed that photoexcitations in doped Si NCs relaxed via dopant states not present in undoped Si NCs

Effect of Carrier Thermalization Dynamics on Light Emission and Amplification in Organometal Halide Perovskites

The remarkable rise of organometal halide perovskites as solar photovoltaic materials has been followed by promising developments in light-emitting devices, including lasers. Here we present unique insights into the processes leading to photon emission in these materials. We employ ultrafast broadband photoluminescence (PL) and transient absorption spectroscopies to directly link density dependent ultrafast charge dynamics to PL. We find that exceptionally strong PL at the band edge is preceded by thermalization of free charge carriers. Short-lived PL above the band gap is clear evidence of nonexcitonic emission from hot carriers, and ultrafast PL depolarization confirms that uncorrelated charge pairs are precursors to photon emission. Carrier thermalization has a profound effect on amplified stimulated emission at high fluence; the delayed onset of optical gain we resolve within the first 10 ps and the unusual oscillatory behavior are both consequences of the kinetic interplay between carrier thermalization and optical gain

Ultrasensitive Colorimetric Detection of 17β-Estradiol: The Effect of Shortening DNA Aptamer Sequences

We report a strategy enabling ultrasensitive colorimetric detection of 17β-estradiol (E2) in water and urine samples using DNA aptamer-coated gold nanoparticles (AuNPs). Starting from an established sensor format where aggregation is triggered when target-bound aptamers dissociate from AuNP surfaces, we demonstrated that step-change improvements are easily accessible through deletion of excess flanking nucleotides from aptamer sequences. After evaluating the lowest energy two-dimensional configuration of the previously isolated E2 binding 75-mer aptamer (<i>K</i>_D ∼25 nM), new 35-mer and 22-mer aptamers were generated with <i>K</i>_D’s of 14 and 11 nM by simply removing flanking nucleotides on either side of the inner core. The shorter aptamers were found to improve discrimination against other steroidal molecules and to improve colorimetric sensitivity for E2 detection by 25-fold compared with the 75-mer to 200 pM. In comparing the response of all sequences, we find that the excess flanking nucleotides suppress signal transduction by causing target-bound aptamers to remain adhered to AuNPs, which we confirm via surface sensitive electrochemical measurements. However, comparison between the 22-mer and 35-mer systems show that retaining a small number of excess bases is optimal. The performance advances we achieved by specifically considering the signal transduction mechanism ultimately resulted in facile detection of E2 in urine, as well as enabling environmental detection of E2 at levels approaching biological relevance

Shape‑, Size‑, and Composition-Controlled Thallium Lead Halide Perovskite Nanowires and Nanocrystals with Tunable Band Gaps

Perovskite nanocrystals have shown themselves to be useful for both absorption- and emission-based applications, including solar cells, photodetectors, and LEDs. Here we present a new class of size-, composition-, and shape-tunable nanocrystals made from Tl₃PbX₅ (X= Cl, Br, I). These can be synthesized via colloidal methods to produce faceted spheroidal nanocrystals, and perovskite TlPbI₃ nanowires. Crystal structures for the orthorhombic and tetragonal phase materials, for both pure and mixed halide species, are compared to the literature and also calculated from first-principles in VASP. We show the ability to tune the band gap by halide substitution to create materials that can absorb strongly between 250 and 450 nm. In addition, we show evidence of the confinement effect in pure halide Tl₃PbBr₅ nanocrystals suggesting size-tuning is possible as well. By tuning the band gap we can create materials with specific absorption spectra suitable for photodetection that display conduction and photoresponse properties similar to previously observed perovskite nanocrystals. We also observe weak emission consistent with indirect band-gap materials. Finally, we are able to demonstrate shape control in these materials, to give some insight into observable phase changes with varying reaction conditions, and to demonstrate the utility of the TlPbI₃ perovskite nanowires as wide-band-gap photoconductors. These novel perovskite nanocrystalline materials can be expected to find applications in photodetectors, X-ray detectors, and piezoelectrics