Gregory P. Lopinski

Folded cavity SOI microring sensors for high sensitivity and real time measurement of biomolecular binding

We demonstrate folded waveguide ring resonators for biomolecular sensing. We show that extending the ring cavity length increases the resonator quality factor, and thereby enhances the sensor resolution and minimum level of detection, while at the same time relaxing the tolerance on the coupling conditions to provide stable and large resonance contrast. The folded spiral path geometry allows a 1.2 mm long ring waveguide to be enclosed in a 150 microm diameter sensor area. The spiral cavity resonator is used to monitor the streptavidin protein binding with a detection limit of approximately 3 pg/mm(2), or a total mass of approximately 5 fg. The real time measurements are used to analyze the kinetics of biotin-streptavidin binding.

Silicon photonic wire biosensor array for multiplexed real-time and label-free molecular detection

We demonstrate a silicon photonic wire waveguide biosensor array chip for the simultaneous monitoring of different molecular binding reactions. The chip is compatible with automated commercial spotting tools and contains a monolithically integrated microfluidic channel for sample delivery. Each array sensor element is a 1.8-mm-long spiral waveguide folded within a 130 microm diameter spot and is incorporated in a balanced Mach-Zehnder interferometer with a near temperature independent response. The sensors are arranged in a 400 microm spacing grid pattern and are addressed through cascaded 1x2 optical power splitters using light from a single input fiber. We demonstrate the real-time monitoring of antibody-antigen reactions using complementary and mismatched immunoglobulin G receptor-analyte pairs and bovine serum albumin. The measured level of detection for each sensor element corresponds to a surface coverage of less than 0.3 pg/mm(2).

Determination of the absolute chirality of individual adsorbed molecules using the scanning tunnelling microscope

The adsorption of organic molecules on solid substrates is a fundamental step in many important heterogeneous catalytic processes, and is also becoming an increasingly significant aspect of surface modification in microelectronics and sensing technology. The conformation of adsorbed molecules not only influences the outcome of surface-catalysed reactions but also becomes important for recognition processes involved in chemical sensors. The scanning tunnelling microscope (STM) is uniquely able to monitor surface structures at the individual-molecule level, and has been shown previously to be capable of distinguishing between different molecular conformations on a surface. Here we show that the geometric configuration (cis or trans) of several simple alkenes chemisorbed on the silicon (100) surface can be determined using the STM, through its ability to identify individual methyl groups. Because both the position and the orientation of these groups can be seen, we can determine the absolute configuration (R or S) for each of the chiral centres formed on chemisorption. Thus the STM can probe enantioselective processes at surfaces at the single-molecule level, and may assist in the development of structured chiral surfaces capable of complex recognition tasks.

Spiral-path high-sensitivity silicon photonic wire molecular sensor with temperature-independent response

We demonstrate a new silicon photonic wire waveguide evanescent field (PWEF) sensor that exploits the strong evanescent field of the transverse magnetic mode of this high-index-contrast, submicrometer-dimension waveguide. High sensitivity is achieved by using a 2 mm long double-spiral waveguide structure that fits within a compact circular area of 150 microm diameter, facilitating compatibility with commercial spotting apparatus and the fabrication of densely spaced sensor arrays. By incorporating the PWEF sensor element into a balanced waveguide Mach-Zehnder interferometer circuit, a minimum detectable mass of approximately 10 fg of streptavidin protein is demonstrated with near temperature-independent response.

Label-free biosensor array based on silicon-on-insulator ring resonators addressed using a WDM approach

We report a silicon-on-insulator ring resonator biosensor array with one output port, using wavelength division multiplexing as the addressing scheme. With the use of on-chip referencing for environmental drift cancellation, simultaneous monitoring of multiplexed molecular bindings is demonstrated, with a resolution of 0.3 pg/mm(2) (40 ag of total mass) for protein concentrations over 4 orders of magnitude down to 20 pM. Reactions are measured over time periods as long as 3 h with high stability.

BENZENE/SI(100) : METASTABLE CHEMISORPTION AND BINDING STATE CONVERSION

Abstract Scanning tunneling microscopy (STM) has been used to investigate the adsorption of benzene on the Si(100)(2×1) surface. At room temperature two types of bonding configurations are observed; on-top of a single Si dimer and bridging two dimers. Upon dosing the single dimer state is populated preferentially but is seen to be metastable with respect to the bridging configuration, in agreement with semi-empirical quantum cluster calculations. The single dimer to bridge conversion is activated, with a barrier of 0.95 eV. The single dimer state can be re-populated via a process assisted by the STM tip.

Resolving organic molecule–silicon scanning tunneling microscopy features with molecular orbital methods

Abstract Organic molecular adsorption on Si(100) is modelled using a combination of semi-empirical, Hartree–Fock and density functional theory molecular orbital methods together with a cluster model of a surface. Various adsorption configurations for benzene on Si(100) are considered. For each, fully optimized structures and absolute adsorption energies are calculated. Valence region occupied state current density iso-surfaces are calculated to simulate constant current STM images. Detailed comparison with experimental energetics and STM images allows unambiguous identification of three room temperature stable structures.

Multifunctional single-walled carbon nanotube–cellulose composite paper

Single-walled carbon nanotubes (SWCNTs) have been used as fillers to produce electrically conductive composite papers. While conductive composite papers have been made using other fillers, they either suffer from instability or low conductivity. Using simple papermaking techniques, we have made SWCNT–cellulose composite paper which possesses a conductivity of 3 × 10−2 S cm−1 and is comparable to or exceeds other reports of carbon nanotube–cellulose papers made by layer-by-layer assembly. These composite papers are multifunctional, having both improved electrical conductivity and enhanced flame retardant properties over the control paper.

Silicon-on-insulator guided mode resonant grating for evanescent field molecular sensing

We present experimental and theoretical results of label-free molecular sensing using the transverse magnetic mode of a 0.22 mum thick silicon slab waveguide with a surface grating implemented in a guided mode resonance configuration. Due to the strong overlap of the evanescent field of the waveguide mode with a molecular layer attached to the surface, these sensors exhibit high sensitivity, while their fabrication and packaging requirements are modest. Experimentally, we demonstrate a resonance wavelength shift of approximately 1 nm when a monolayer of the protein streptavidin is attached to the surface, in good agreement with calculations based on rigorous coupled wave analysis. In our current optical setup this shift corresponds to an estimated limit of detection of 0.2% of a monolayer of streptavidin.

Current-induced organic molecule-silicon bond breaking : consequences for molecular devices

Abstract The current carrying capacity of individual organic molecules covalently bound to silicon has been studied. Adsorbates consisting exclusively of saturated CC bonds were found to be entirely stable whereas adsorbates containing π bonds could be controllably dislodged under modest conditions. The π bonds act as a chromophore, taking energy from a scattered electron, energy that can be selectively channeled into SiC bond breaking. The class of adsorbates that are dislodged is closely related to unsaturated molecules widely investigated for enhanced molecular wire character. It is predicted that molecular devices containing such molecules will fail when operated. Measures to avoid current-induced bond breaking are described. New processes based upon the controlled bond breaking phenomenon are suggested.