White- and blue-light-emitting dysprosium(III) and terbium(III)-doped gadolinium titanate phosphors.

Here we report the synthesis and structural, morphological, and photoluminescence analysis of white- and blue-light-emitting Dy3+ - and Tm3+ -doped Gd2 Ti2 O7 nanophosphors. Single-phase cubic Gd2 Ti2 O7 nanopowders consist of compact, dense aggregates of nanoparticles with an average size of ~25 nm for Dy3+ -doped and ~50 nm for Tm3+ -doped samples. The photoluminescence results indicated that ultraviolet (UV) light excitation of the Dy3+ -doped sample resulted in direct generation of white light, while a dominant yellow emission was obtained under blue-light excitation. Intense blue light was obtained for Tm3+ -doped Gd2 Ti2 O7 under UV excitation suggesting that this material could be used as a blue phosphor.

Femtogram-scale photothermal spectroscopy of explosive molecules on nanostrings.

We demonstrate detection of femtogram-scale quantities of the explosive molecule 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) via combined nanomechanical photothermal spectroscopy and mass desorption. Photothermal spectroscopy provides a spectroscopic fingerprint of the molecule, which is unavailable using mass adsorption/desorption alone. Our measurement, based on thermomechanical measurement of silicon nitride nanostrings, represents the highest mass resolution ever demonstrated via nanomechanical photothermal spectroscopy. This detection scheme is quick, label-free, and is compatible with parallelized molecular analysis of multicomponent targets.

Plasmon assisted thermal modulation in nanoparticles.

Single-particle interactions hold the promise of nanometer-scale devices in areas such as data communications and storage, nanolithography, waveguides, renewable energy and therapeutics. We propose that the collective electronic properties possessed by noble metal nanoparticles may be exploited for device actuation via the unapparent mechanism of plasmon-assisted heat generation and flux. The temperature dependence of the dielectric function and the thermal transport properties of the particles play the central role in the feasibility of the thermally-actuated system, however the behavior of these thermoplasmonic processes is unclear. We experimentally and computationally analyzed modulation via thermoplasmonic processes on a test system of gold (Au) nano-islands. Modulation and energy transport in discontinuous domains exhibited quantitatively different characteristics compared to thin films. The results have implications for all surface plasmon based nano-devices where inevitable small-scale thermal processes are present.

Spectroscopy and imaging of arrays of nanorods toward nanopolarimetry.

The polarization dependence of the optical scattering properties of two-dimensional arrays of metal nanostructures with sub-wavelength dimensions (nanoantennas) has been investigated. Arrays of 500 nm × 100 nm gold nanorods covering a 100 × 100 µm(2) area were fabricated with varying orientations on an electrically conductive substrate. The experimental and computational analysis of the angularly organized nanorods suggest potential use toward the development of an integrated polarimeter. Using the gold nanorods on a transparent substrate as a preliminary system, we show that in the proper spectral range the scattering properties of the structures may be tuned for such an application.

Optomechanical spectroscopy with broadband interferometric and quantum cascade laser sources.

The spectral tunability of semiconductor-metal multilayer structures can provide a channel for the conversion of light into useful mechanical actuation. Responses of suspended silicon, silicon nitride, chromium, gold, and aluminum microstructures are shown to be utilized as a detector for visible and IR spectroscopy. Both dispersive and interferometric approaches are investigated to delineate the potential use of the structures in spatially resolved spectroscopy and spectrally resolved microscopy. The thermoplasmonic, spectral absorption, interference effects, and the associated energy deposition that contributes to the mechanical response are discussed to describe the potential of optomechanical detection in future integrated spectrometers.

Virtual resonance and frequency difference generation by van der Waals interaction.

The ability to explore the interior of materials for the presence of inhomogeneities was recently demonstrated by mode synthesizing atomic force microscopy [L. Tetard, A. Passian, and T. Thundat, Nature Nanotech. 5, 105 (2009).]. Proposing a semiempirical nonlinear force, we show that difference frequency ω_ generation, regarded as the simplest synthesized mode, occurs optimally when the force is tuned to van der Waals form. From a parametric study of the probe-sample excitation, we show that the predicted ω_ oscillation agrees well with experiments. We then introduce the concept of virtual resonance to show that probe oscillations at ω_ can efficiently be enhanced.

Nanometrology of delignified Populus using mode synthesizing atomic force microscopy.

The study of the spatially resolved physical and compositional properties of materials at the nanoscale is increasingly challenging due to the level of complexity of biological specimens such as those of interest in bioenergy production. Mode synthesizing atomic force microscopy (MSAFM) has emerged as a promising metrology tool for such studies. It is shown that, by tuning the mechanical excitation of the probe-sample system, MSAFM can be used to dynamically investigate the multifaceted complexity of plant cells. The results are argued to be of importance both for the characteristics of the invoked synthesized modes and for accessing new features of the samples. As a specific system to investigate, we present images of Populus, before and after a holopulping treatment, a crucial step in the biomass delignification process.

Standoff spectroscopy of surface adsorbed chemicals.

Despite its immediate applications, selective detection of trace quantities of surface adsorbed chemicals, such as explosives, without physically collecting the sample molecules is a challenging task. Standoff spectroscopic techniques offer an ideal method of detecting chemicals without using a sample collection step. Though standoff spectroscopic techniques are capable of providing high selectivity, their demonstrated sensitivities are poor. Here we describe standoff detection of trace quantities of surface adsorbed chemicals using two quantum cascade lasers operated simultaneously, with tunable wavelength windows that match with absorption peaks of the analytes. This standoff method is a variation of photoacoustic spectroscopy, where scattered light from the sample surface is used for exciting acoustic resonance of the detector. We demonstrate a sensitivity of 100 ng/cm(2) and a standoff detection distance of 20 m for surface adsorbed analytes such as explosives and tributyl phosphate.

Laser reflectometry of submegahertz liquid meniscus ringing.

Optical techniques that permit nondestructive probing of interfacial dynamics of various media are of key importance in numerous applications such as ellipsometry, mirage effect, and all-optical switching. Characterization of the various phases of microjet droplet formation yields important information for volume control, uniformity, velocity, and rate. The ringing of the meniscus and the associated relaxation time that occurs after droplet breakoff affect subsequent drop formation and is an indicator of the physical properties of the fluid. Using laser reflectometry, we present an analysis of the meniscus oscillations in an orifice of a piezoelectric microjet.

Images of the DNA double helix in water.

The scanning tunneling microscope can image uncoated DNA submerged in water. The grooves of the double helix were clearly resolved in images of the 146-base pair fragment extracted from calf thymus nucleosome. In contrast to images obtained with dry DNA, the helix pitch varied only a small amount (36 +/- 5 angstroms). The path of the helix shows considerable variation. It is quite straight when the molecules are densely packed, but it curves and bends in isolated molecules.

Crack-like sources of dislocation nucleation and multiplication in thin films.

With the combination of the height sensitivity of atomic force microscopy and the strain sensitivity of transmission electron microscopy, it is shown that near singular stress concentrations can develop naturally in strained epitaxial films. These crack-like instabilities are identified as the sources of dislocation nucleation and multiplication in films of high misfit. This link between morphological instability and dislocation nucleation provides a method for studying the basic micromechanisms that determine the strength and mechanical properties of materials.

An experimental investigation of analog delay generation for dynamic control of microsensors and atomic force microscopy.

We present an implementation of pure-time-delay generation in analog signals located in the kilo-Hertz frequency band. The controlled constant delays that are produced engage in a feedback system to investigate the dynamic response of microcantilevers. Delayed systems offer a vast richness of eigenvalues resulting in the possibility of excitations at frequencies other than that of the fundamental mode. Different cantilever actuation and delay generation approaches are investigated and compared, and detailed experimental observation of the dynamic response of the system is presented. Based on our results, an acoustic excitation is devised that may be used as an efficient sensor.

Bioassay of prostate-specific antigen (PSA) using microcantilevers.

Diagnosis and monitoring of complex diseases such as cancer require quantitative detection of multiple proteins. Recent work has shown that when specific biomolecular binding occurs on one surface of a microcantilever beam, intermolecular nanomechanics bend the cantilever, which can be optically detected. Although this label-free technique readily lends itself to formation of microcantilever arrays, what has remained unclear is the technologically critical issue of whether it is sufficiently specific and sensitive to detect disease-related proteins at clinically relevant conditions and concentrations. As an example, we report here that microcantilevers of different geometries have been used to detect two forms of prostate-specific antigen (PSA) over a wide range of concentrations from 0.2 ng/ml to 60 microg/ml in a background of human serum albumin (HSA) and human plasminogen (HP) at 1 mg/ml, making this a clinically relevant diagnostic technique for prostate cancer. Because cantilever motion originates from the free-energy change induced by specific biomolecular binding, this technique may offer a common platform for high-throughput label-free analysis of protein-protein binding, DNA hybridization, and DNA-protein interactions, as well as drug discovery.

Microscale Marangoni actuation: all-optical and all-electrical methods.

We present experimental results from an all-optical microfluidic platform that may be complimented by a thin film all-electrical network. Using these configurations we have studied the microfluidic convective flow systems of silicone oil, glycerol, and 1,3,5-trinitrotoluene on open surfaces through the production of surface tension gradients derived from thermal gradients. We show that sufficient localized thermal variation can be created utilizing surface plasmons and/or engaging individually addressable resistive thermal elements. Both studies manipulate fluids via Marangoni forces, each having their unique exploitable advantages. Surface plasmon excitation in metal foils are the driving engine of many physical-, chemical-, and bio-sensing applications. Incorporating, for the first time, the plasmon concept in microfluidics, our results thus demonstrate great potential for simultaneous fluid actuation and sensing.

Bioelectromechanical imaging by scanning probe microscopy: Galvani's experiment at the nanoscale.

Since the discovery in the late 18th century of electrically induced mechanical response in muscle tissue, coupling between electrical and mechanical phenomena has been shown to be a near-universal feature of biological systems. Here, we employ scanning probe microscopy (SPM) to measure the sub-Angstrom mechanical response of a biological system induced by an electric bias applied to a conductive SPM tip. Visualization of the spiral shape and orientation of protein fibrils with 5 nm spatial resolution in a human tooth and chitin molecular bundle orientation in a butterfly wing is demonstrated. In particular, the applicability of SPM-based techniques for the determination of molecular orientation is discussed.

Nonradiative surface plasmon assisted microscale Marangoni forces.

When a liquid droplet experiences a temperature inhomogeneity along its bounding surface, a surface energy gradient is engendered, which when, in a continuous sense, exceeding a threshold, results in a convective flow dissipating the energy. If the associated temperature gradients are sustained by the interface between the liquid and a supporting substrate, the induced flow can result in the lateral motion of the droplet overcoming the viscosity and inertia. Recently, pico-liter adsorbed and applied droplets were shown experimentally to be transported, and divided by the decay of optically excited surface plasmons into phonons in a thin gold foil. The decaying events locally modify the temperature of the liquid-solid interface, establishing microscale thermal gradients of sufficient magnitude for the droplet to undergo thermocapillary flow. We present experimental evidence of such gradients resulting in local surface modification associated with the excitation of surface plasmons. We show theoretically that the observed effect is due to Marangoni forces, and computationally visualize the flow characteristics for the experimental parameters. As an application based on our results, we propose a method for an all-optical modulation of light by light mediated by the droplet oscillations. Furthermore, the results have important consequences for microfluidics, droplet actuation, and simultaneous surface plasmon resonance sensing and spectroscopy.

Nanomechanical sandwich assay for multiple cancer biomarkers in breast cancer cell-derived exosomes.

The use of exosomes as cancer diagnostic biomarkers is technically limited by their size, heterogeneity and the need for extensive purification and labelling. We report the use of cantilever arrays for simultaneous detection of multiple exosomal surface-antigens with high sensitivity and selectivity. Exosomes from breast cancer were selectively identified by detecting over-expressed membrane-proteins CD24, CD63, and EGFR. Excellent selectivity however, was achieved when targeting the cell-surface proteoglycan, Glypican-1 at extraordinary limits (∼200 exosomes per mL, ∼0.1 pg mL(-1)).

Clustering mechanism of ethanol-water mixtures investigated with photothermal microfluidic cantilever deflection spectroscopy.

The infrared-active (IR) vibrational mode of ethanol (EtOH) associated with the asymmetrical stretching of the C-C-O bond in pico-liter volumes of EtOH-water binary mixtures is calorimetrically measured using photothermal microfluidic cantilever deflection spectroscopy (PMCDS). IR absorption by the confined liquid results in wavelength dependent cantilever deflections, thus providing a complementary response to IR absorption revealing a complex dipole moment dependence on mixture concentration. Solvent-induced blue shifts of the C-C-O asymmetric vibrational stretch for both anti and gauche conformers of EtOH were precisely monitored for EtOH concentrations ranging from 20-100% w/w. Variations in IR absorption peak maxima show an inverse dependence on induced EtOH dipole moment (µ) and is attributed to the complex clustering mechanism of EtOH-water mixtures.

Opto-nanomechanical spectroscopic material characterization.

The non-destructive, simultaneous chemical and physical characterization of materials at the nanoscale is an essential and highly sought-after capability. However, a combination of limitations imposed by Abbe diffraction, diffuse scattering, unknown subsurface, electromagnetic fluctuations and Brownian noise, for example, have made achieving this goal challenging. Here, we report a hybrid approach for nanoscale material characterization based on generalized nanomechanical force microscopy in conjunction with infrared photoacoustic spectroscopy. As an application, we tackle the outstanding problem of spatially and spectrally resolving plant cell walls. Nanoscale characterization of plant cell walls and the effect of complex phenotype treatments on biomass are challenging but necessary in the search for sustainable and renewable bioenergy. We present results that reveal both the morphological and compositional substructures of the cell walls. The measured biomolecular traits are in agreement with the lower-resolution chemical maps obtained with infrared and confocal Raman micro-spectroscopies of the same samples. These results should prove relevant in other fields such as cancer research, nanotoxicity, and energy storage and production, where morphological, chemical and subsurface studies of nanocomposites, nanoparticle uptake by cells and nanoscale quality control are in demand.