CeFlowBot: A Biomimetic Flow-Driven Microrobot that Navigates under Magneto-Acoustic Fields.

Aquatic organisms within the Cephalopoda family (e.g., octopuses, squids, cuttlefish) exist that draw the surrounding fluid inside their bodies and expel it in a single jet thrust to swim forward. Like cephalopods, several acoustically powered microsystems share a similar process of fluid expulsion which makes them useful as microfluidic pumps in lab-on-a-chip devices. Herein, an array of acoustically resonant bubbles are employed to mimic this pumping phenomenon inside an untethered microrobot called CeFlowBot. CeFlowBot contains an array of vibrating bubbles that pump fluid through its inner body thereby boosting its propulsion. CeFlowBots are later functionalized with magnetic layers and steered under combined influence of magnetic and acoustic fields. Moreover, acoustic power modulation of CeFlowBots is used to grasp nearby objects and release it in the surrounding workspace. The ability of CeFlowBots to navigate remote environments under magneto-acoustic fields and perform targeted manipulation makes such microrobots useful for clinical applications such as targeted drug delivery. Lastly, an ultrasound imaging system is employed to visualize the motion of CeFlowBots which provides means to deploy such microrobots in hard-to-reach environments inaccessible to optical cameras.

Dye-assisted gain of strongly confined surface plasmon polaritons in silver nanowires.

Noble metal nanowires are excellent candidates as subwavelength optical components in miniaturized devices due to their ability to support the propagation of surface plasmon polaritons (SPPs). Nanoscale data transfer based on SPP propagation at optical frequencies has the advantage of larger bandwidths but also suffers from larger losses due to strong mode confinement. To overcome losses, SPP gain has been realized, but so far only for weakly confined SPPs in metal films and stripes. Here we report the demonstration of gain for subwavelength SPPs that were strongly confined in chemically prepared silver nanowires (mode area = λ(2)/40) using a dye-doped polymer film as the optical gain medium. Under continuous wave excitation at 514 nm, we measured a gain coefficient of 270 cm(-1) for SPPs at 633 nm, resulting in partial SPP loss compensation of 14%. This achievement for strongly confined SPPs represents a major step forward toward the realization of nanoscale plasmonic amplifiers and lasers.

Influence of cross sectional geometry on surface plasmon polariton propagation in gold nanowires.

We investigated the effects of cross sectional geometry on surface plasmon polariton propagation in gold nanowires (NWs) using bleach-imaged plasmon propagation and electromagnetic simulations. Chemically synthesized NWs have pentagonally twinned crystal structures, but recent advances in synthesis have made it possible to amplify this pentagonal shape to yield NWs with a five-pointed-star cross section and sharp end tips. We found experimentally that NWs with a five-pointed-star cross section, referred to as SNWs, had a shorter propagation length for surface plasmon polaritons at 785 nm, but a higher effective incoupling efficiency compared to smooth NWs with a pentagonal cross section, labeled as PNWs. Electromagnetic simulations revealed that the electric fields were localized at the sharp ridges of the SNWs, leading to higher absorptive losses and hence shorter propagation lengths compared to PNWs. On the other hand, scattering losses were found to be relatively uncorrelated with cross sectional geometry, but were strongly dependent on the plasmon mode excited. Our results provide insight into the shape-dependent waveguiding properties of chemically synthesized metal NWs and the mode-dependent loss mechanisms that govern surface plasmon polariton propagation.

Turning the corner: efficient energy transfer in bent plasmonic nanoparticle chain waveguides.

For integrating and multiplexing of subwavelength plasmonic waveguides with other optical and electric components, complex architectures such as junctions with sharp turns are necessary. However, in addition to intrinsic losses, bending losses severely limit plasmon propagation. In the current work, we demonstrate that propagation of surface plasmon polaritons around 90° turns in silver nanoparticle chains occurs without bending losses. Using a far-field fluorescence method, bleach-imaged plasmon propagation (BlIPP), which creates a permanent map of the plasmonic near-field through bleaching of a fluorophore coated on top of a plasmonic waveguide, we measured propagation lengths at 633 nm for straight and bent silver nanoparticle chains of 8.0 ± 0.5 and 7.8 ± 0.4 µm, respectively. These propagation lengths were independent of the input polarization. We furthermore show that subradiant plasmon modes yield a longer propagation length compared to energy transport via excitation of super-radiant modes.

Mechanistic study of bleach-imaged plasmon propagation (BlIPP).

Bleach-imaged plasmon propagation, BlIPP, is a far-field microscopy technique developed to characterize the propagation length of surface plasmon polaritons in metallic waveguides. To correctly extract the propagation length from the measured photobleach intensity, it is necessary to understand the mechanism by which dye photobleaching occurs. In particular, 1- vs 2-photon bleaching reactions yield different propagation lengths based on a kinetic model for BlIPP. Because a number of studies have reported on the importance of 2-photon processes for dye photobleaching, we investigate here the nature of the photobleaching step in BlIPP. We are able to demonstrate that only 1-photon absorption is relevant for typical BlIPP conditions as tested here for a thin film of indocyanine green fluorescent dye molecules coated over gold nanowires and excited at a wavelength of 785 nm. These results are obtained by directly measuring the excitation intensity dependence of the photobleaching rate constant of the dye in the presence of the metallic waveguide.

Identification of higher order long-propagation-length surface plasmon polariton modes in chemically prepared gold nanowires.

A comprehensive understanding of the type of modes and their propagation length for surface plasmon polaritons (SPPs) in gold nanowires is essential for potential applications of these materials as nanoscale optical waveguides. We have studied chemically synthesized single gold nanowires by a novel technique called bleach-imaged plasmon propagation (BlIPP), which relies on the plasmonic near-field induced photobleaching of a dye to report the SPP propagation in nanowires. We observed a much longer propagation length of 7.5 ± 2.0 µm at 785 nm compared to earlier reports, which found propagation lengths of ~2.5 µm. Finite difference time domain simulations revealed that the bleach-imaged SPP is a higher order m = 1 mode and that the lowest order m = 0 mode is strongly quenched due to the loss to the dye layer and cannot be resolved by BlIPP. A comparative assessment of BlIPP with direct fluorescence imaging furthermore showed that the significant difference in propagation lengths obtained by these two techniques can be attributed to the difference in their experimental conditions, especially to the difference in thickness of the dye layer coating on the nanowire. In addition to identifying a higher order SPP mode with long propagation length, our study infers that caution must be taken in selecting indirect measurement techniques for probing SPP propagation in nanoscale metallic waveguides.

Excitation polarization sensitivity of plasmon-mediated silver nanotriangle growth on a surface.

In this contribution, we report an effective and relatively simple route to grow triangular flat-top silver nanoparticles (NPs) directly on a solid substrate from smaller NPs through a wet photochemical synthesis. The method consists of fixing small, preformed nanotriangles (NTs) on a substrate and subsequently irradiating them with light in a silver seed solution. Furthermore, the use of linearly polarized light allows for exerting control on the growth direction of the silver nanotriangles on the substrate. Evidence for the role of surface plasmon resonances in governing the growth of the NTs is obtained by employing linear polarized light. Thus, this study demonstrates that light-induced, directional synthesis of nanoparticles on solid substrates is in reach, which is of utmost importance for plasmonic applications.

Electromagnetic energy transport in nanoparticle chains via dark plasmon modes.

Using light to exchange information offers large bandwidths and high speeds, but the miniaturization of optical components is limited by diffraction. Converting light into electron waves in metals allows one to overcome this problem. However, metals are lossy at optical frequencies and large-area fabrication of nanometer-sized structures by conventional top-down methods can be cost-prohibitive. We show electromagnetic energy transport with gold nanoparticles that were assembled into close-packed linear chains. The small interparticle distances enabled strong electromagnetic coupling causing the formation of low-loss subradiant plasmons, which facilitated energy propagation over many micrometers. Electrodynamic calculations confirmed the dark nature of the propagating mode and showed that disorder in the nanoparticle arrangement enhances energy transport, demonstrating the viability of using bottom-up nanoparticle assemblies for ultracompact opto-electronic devices.

Free volume dependence of the internal rotation of a molecular rotor probe in room temperature ionic liquids.

The fluorescence efficiency of a well-known microviscosity probe, 9-(dicyanovinyl)julolidine (DCVJ), which is highly sensitive to the viscosity of the medium, has been studied in seven imidazolium ionic liquids (ILs) of varying viscosities over a temperature range of 10-60 degrees C. The microviscosities around the probe in different ILs have been estimated from the linear dependence of the logarithm of fluorescence quantum yield (log phi(f)) on the logarithm of the bulk viscosity (log eta) in various conventional solvents of different viscosities at room temperature. These microviscosities, which represent the local environments around the probe, are found to be significantly different from the directly measured bulk viscosities of these ILs. The log phi(f) vs log (eta/T) plots, which are also expected to be linear, interestingly show a bilinear behavior in more viscous ILs with a break around 28-30 degrees C. The observation of a similar break in the Arrhenius plots of the rate constant of the internal rotation in DCVJ and absence of any such break in the temperature dependence of the mobility of the ILs allow us to determine the important role of the free volume around the probe in dictating the nonradiative deactivation rate or the fluorescence efficiency of DCVJ. The break in the plots, which implies a change in the available free volume around the probe at approximately 28-30 degrees C, presumably arises from the repositioning of the probe from one environment to a different one of these microheterogeneous ILs with change of temperature.

Interaction of the major protein from bovine seminal plasma, PDC-109 with phospholipid membranes and soluble ligands investigated by fluorescence approaches.

The major protein from bovine seminal plasma, PDC-109 binds selectively to choline phospholipids on the sperm plasma membrane and plays a crucial role in priming spermatozoa for fertilization. The microenvironment and accessibility of tryptophans of PDC-109 in the native state, in the presence of phosphorylcholine (PrC) and phospholipid membranes as well as upon denaturation have been investigated by fluorescence approaches. Quenching of the protein intrinsic fluorescence by different quenchers decreased in the order: acrylamide>succinimide>>Cs(+)>I(-). Ligand binding afforded considerable protection from quenching, with shielding efficiencies following the order: dimyristoylphosphatidylcholine (DMPC)>lysophosphatidylcholine (Lyso-PC)>PrC. This has been attributed to a partial penetration of the protein into the DMPC membranes and Lyso-PC micelles, as well as a further stabilization of the binding due to the interaction of PDC-109 with lipid acyl chains and the resulting tightening of the protein structure, leading to a decreased accessibility of the tryptophan residues. Red-edge excitation shift (REES) studies yielded REES values of 4 nm for both native and denatured PDC-109, whereas reduced and denatured protein gave a REES of only 0.5 nm, clearly indicating that the structural and dynamic features of the microenvironment around the tryptophan residues are retained even after denaturation, presumably due to the constraints imposed on the protein structure by disulfide bonds. Upon binding of PDC-109 to DMPC membranes and Lyso-PC micelles the REES values were reduced to 2.5 and 1.0 nm, respectively, which could be due to the penetration of some parts of the protein, especially the segment containing Trp-90 into the membrane interior, where the red-edge effects are considerably reduced.

Effect of nonpolar solvents on the solute rotation and solvation dynamics in an imidazolium ionic liquid.

Recognizing the potential of the mixed solvent systems comprising ionic liquid as one of the constituents in real applications, the steady-state and time-resolved fluorescence behavior of C153 has been studied in neat 1-butyl-3-methylimidazolium hexafluorophosphate and its mixtures with nonpolar solvents, namely, toluene and 1,4-dioxane. No significant effect of the cosolvent on the steady-state absorption or fluorescence spectra of C153 in ionic liquid has been observed. Time-resolved fluorescence anisotropy measurements show a decrease of the rotational correlation time of C153 with gradual addition of the cosolvent. Solvation dynamics in ionic liquid-cosolvent mixtures is found to be biphasic, and a decrease of the average solvation time is observed with increasing amount of the cosolvent in solution. The time-zero spectrum of C153 is found to shift toward higher energy with gradual addition of the nonpolar solvent, suggesting that the probe molecule experiences a more nonpolar environment at the early stage of the dynamics in mixed solvents. The blue shift of the time-zero spectrum caused by the addition of the nonpolar solvent results in a larger Stokes shift of the time-dependent spectra due to solvent relaxation in mixed solvents. A comparison of the time-dependent spectral data of the ionic liquid-toluene and ionic liquid-dioxane systems shows that, while a small amount of toluene can significantly affect the dynamics, comparatively, a larger amount of dioxane is required to bring about the same effect. This is explained in terms of favorable interactions between toluene and the imidazolium ring system leading to a more effective solubilization of toluene in the cybotactic region of the probe.

Solute rotation and solvation dynamics in an alcohol-functionalized room temperature ionic liquid.

Steady-state and time-resolved fluorescence behaviors of two dipolar solutes, coumarin 153 and 4-aminophthalimide, have been studied in an alcohol-functionalized room-temperature ionic liquid, 1-(hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The steady-state fluorescence parameters have been exploited for the estimation of the polarity of this ionic liquid and to obtain information on the hydrogen bonding interaction between the ionic liquid and the probe molecules. The time-resolved measurements have been focused on the dynamics of solvation by studying the dynamic Stokes shift in the ps-ns time scale and solute rotation by measuring the time dependence of the fluorescence anisotropy. The time-resolved anisotropy studies reveal a significant slow down of the rotational motion of one of the probe molecules. The time-dependent fluorescence Stokes shift measurements suggest that the time-resolvable part of the dynamics is biphasic in nature, highly dependent on the probe molecule and the ultrafast component is comparatively less than that in other ionic liquids. The influence of the hydrogen bonding interaction between the probe molecules and the ionic liquids on the solute rotation and the various components of the solvation dynamics is carefully analyzed in an attempt to obtain further insight into the mechanism of solvation in these novel media.

Photoinduced electron transfer reaction in room temperature ionic liquids: a combined laser flash photolysis and fluorescence study.

A detailed study of the photoinduced electron transfer (PET) reaction between pyrene and N,N-dimethylaniline has been made in four different room temperature ionic liquids (ILs) using steady state and time-resolved fluorescence and laser flash photolysis techniques. Unlike that in the conventional media, no exciplex emission for this well-known system could be observed in ILs. The rate constants for the PET induced quenching of the fluorescent state of pyrene, which lie between 6.9 and 37 x 107 M-1 s-1 depending on the viscosity, are found to be 2-4 times higher than the diffusion-controlled rates in ILs. The primary photoproducts of the PET process have been characterized by transient absorption spectroscopy, and the yields of the solvent-separated PET products have been determined. Even in the least viscous IL, [emim][Tf2N], the yield of the solvent-separated radical ion is estimated to be only 0.015 +/- 0.005. In more viscous ILs such as [bmim][PF6], the yield is found to be so low that absorption due to these species could not be observed. The rate constant for the escape of the ionic products from the geminate ion pair in ILs has been estimated to be nearly 2-3 orders of magnitude lower than the back electron transfer rate. However, the small fraction of the PET products, which manage to escape geminate recombination, have been found to survive much longer compared to those in less viscous conventional solvents.

On the optical properties of the imidazolium ionic liquids.

Room-temperature ionic liquids, particularly those based on substituted imidazolium cations, are currently being extensively studied for a variety of applications. Herein, we explore the suitability of several imidazolium salts in optical applications by carefully examining the electronic absorption and fluorescence behavior of these substances, generally believed to be transparent in most of the UV region and fully transparent in the visible region. It is shown that all imidazolium ionic liquids are characterized by significant absorption in the entire UV region and a long absorption tail that extends into the visible region. These absorption characteristics are attributed to the imidazolium moiety and its various associated structures. When excited in the UV or early part of the visible region, these liquids exhibit fluorescence, which covers a large part of the visible region and shows dramatic excitation wavelength dependence. The excitation wavelength dependent shift of the fluorescence maximum has been rationalized taking into consideration the existence of the various associated structures of the ionic liquids and the inefficiency of the excitation energy-transfer process between them. The results imply that these liquids may have serious drawbacks in some of the optical studies.