Jennifer I. L. Chen

Plasmonic Nanoparticle Dimers for Optical Sensing of DNA in Complex Media

We introduce a new sensing modality based on the actuation of discrete gold nanoparticle dimers. Binding of the target DNA leads to a geometrical extension of the dimer, thereby yielding a spectral blue shift in the hybridized plasmon mode as detected by single nanostructure scattering spectroscopy. The magnitude and opposite direction of this shift enabled us to spectroscopically distinguish the target from nonspecific binding and to detect the target in complex media like serum.

Synergy of Slow Photon and Chemically Amplified Photochemistry in Platinum Nanocluster-Loaded Inverse Titania Opals

We show that the photodegradation efficiency of TiO2 has been amplified 4-fold via the cooperativity of slow photons in photonic crystal and the incorporation of Pt nanoclusters. Various loadings of Pt nanoparticles were photodeposited on the surface of TiO2 inverse opals and the photodegradation of adsorbed acid orange was investigated. While slow photons increased the effective path length of light, Pt nanoparticles extended the lifetimes of the UV-excited electrons and holes. With 2-4 wt % of Pt on the TiO2 inverse opal, we demonstrate a synergistic optical and chemical enhancement for the first time.

Slow photons in the fast lane in chemistry

A driving force in the rapidly developing field of photonic crystals has been the photonic bandgap, a range of energies where the propagation of light is completely forbidden. The photonic bandgap allows the design of photonic lattices that localize, guide and bend light at sub-micron length scales, providing opportunities for the creation of miniature optical devices and integrated optical circuits to help drive the revolution in photonics. A less well known attribute of photonic crystals is their theoretical ability to slow light to a velocity of zero. This phenomenon can be achieved at the high and low energy edges of photonic stopgaps where the photonic bands are flat and light exists as a standing wave commensurate with the photonic lattice and travels at a group velocity of zero, referred to as “slow photons” herein. It has been shown theoretically that the probability of harvesting slow photons scales inversely with their group velocity. This means that a number of well known photon driven processes and devices in chemistry and physics can be enhanced by capturing this unique property of slow photons. In this paper we will look at slow photons mainly through the eye of chemistry and highlight some recent developments in this exciting and emerging field that demonstrate the potential of slow photons in materials chemistry and nanochemistry.

Electron accumulation on metal nanoparticles in plasmon-enhanced organic solar cells.

Plasmonic metal nanoparticles have been used to enhance the performance of thin-film devices such as organic photovoltaics based on polymer/fullerene blends. We show that silver nanoprisms accumulate long-lived negative charges when they are in contact with a photoexcited bulk heterojunction blend composed of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM). We report both the charge modulation and electroabsorption spectra of silver nanoprisms in solid-state devices and compare these spectra with the photoinduced absorption spectra of P3HT/PCBM blends containing silver nanoprisms. We assign a previously unidentified peak in the photoinduced absorption spectra to the presence of photoinduced electrons on the silver nanoprisms. We show that coating the nanoprisms with a 2.5 nm thick insulating layer can completely inhibit this charging. These results may inform methods for limiting metal-mediated losses in plasmonic solar cells.

Heterogeneous photocatalysis with inverse titania opals: probing structural and photonic effects

Investigation of solution-phase photocatalysis using inverse TiO2 opals reveals their structural benefits, such as high surface area and facilitated diffusion of molecules, while the photonic effect arising from slow photons is only present when the TiO2 filling fraction is increased.

Photoswitchable Oligonucleotide-Modified Gold Nanoparticles: Controlling Hybridization Stringency with Photon Dose

We describe a new class of stimulus-responsive DNA-functionalized gold nanoparticles that incorporate azobenzene-modified oligonucleotides. Beyond the classic directed assembly and sensing behaviors associated with oligonucleotide-modified nanoparticles, these particles also exhibit reversible photoswitching of their assembly behavior. Exposure to UV light induces a trans-cis isomerization of the azobenzene which destabilizes the DNA duplex, resulting in dissociation of the nanoparticle assemblies. The isomerization is reversible upon exposure to blue light, resulting in rehybridization and reassembly of the DNA-linked nanoparticle clusters. We show that perfectly complementary and partially mismatched strands exhibit clearly distinguishable photoinduced melting properties, and we demonstrate that photon dose can thus be used in place of temperature or ionic strength to control hybridization stringency with the ability to discriminate single-base mismatches.

Tailoring the Electrical Properties of Inverse Silicon Opals ‐ A Step Towards Optically Amplified Silicon Solar Cells

electrical conductivity of i-Si-o, and how these material properties correlate with the optical properties of i-Si-o. To reach our objectives, we fabricated i-Si-o by infiltrating the voids of a silica-opal template with Si using chemical vapor deposition (CVD), and subsequently etching the silica spheres with hydrofluoric acid (HF). [19] Accurate measurements of the dc electrical dark conductivities (sd) require electrodes and electrical contacts on the i-Si-o films. Therefore, instead of using glass substrates, which are most commonly employed, we prepared all samples on sapphire substrates due to their high stability against HF etching. Note that the i-Si-o fabricated on glass substrates usually gets detached and becomes free-standing after HF etching, while high- quality intact i-Si-o films can be achieved on sapphire substrates.

Photoisomerization Quantum Yield of Azobenzene-Modified DNA Depends on Local Sequence

Photoswitch-modified DNA is being studied for applications including light-harvesting molecular motors, photocontrolled drug delivery, gene regulation, and optically mediated assembly of plasmonic metal nanoparticles in DNA-hybridization assays. We study the sequence and hybridization dependence of the photoisomerization quantum yield of azobenzene attached to DNA via the popular d-threoninol linkage. Compared to free azobenzene we find that the quantum yield for photoisomerization from trans to cis form is decreased 3-fold (from 0.094 ± 0.004 to 0.036 ± 0.002) when the azobenzene is incorporated into ssDNA, and is further reduced 15-fold (to 0.0056 ± 0.0008) for azobenzene incorporated into dsDNA. In addition, we find that the quantum yield is sensitive to the local sequence including both specific mismatches and the overall sequence-dependent melting temperature (Tm). These results serve as design rules for efficient photoswitchable DNA sequences tailored for sensing, drug delivery, and energy-harvesting applications, while also providing a foundation for understanding phenomena such as photonically controlled hybridization stringency.

Morphology-based plasmonic nanoparticle sensors: controlling etching kinetics with target-responsive permeability gate.

We present a sensing platform based on the morphological changes of plasmonic nanoparticles. Detection is achieved by using a stimulus-responsive polyelectrolyte-aptamer thin film to control the rate of diffusion of etchants that alter the shape and size of the nanoparticles. We show that the extent of morphological change and the colorimetric response depends on the amount of analyte bound. Contrary to conventional plasmonic sensors, our detection scheme does not rely on any interparticle interaction and is completely label-free, both in terms of the analyte and the capture probe. It presents new opportunities for designing facile, low-cost, and portable chip-based sensors for biodiagnostic and field analysis.

Electrical properties of p-type and n-type doped inverse silicon opals - towards optically amplified silicon solar cells

While the silicon photonic crystals have promised revolutionary developments in the field of optical telecommunications and optical computing, it has only recently been realized that their prowess to trap and slow photons could potentially and significantly improve the efficiency of silicon solar cells. In this work n-doped and p-doped inverse silicon opals are synthesized and processed to optimize their electrical charge transport properties, which are shown to be of semiconductor device quality. Moreover a prototype p-i-n junction solar cell based on the inverse silicon opal is reduced to practice and its optoelectronic behavior is evaluated.