Optically Biased Diffusion of Single Molecules Studied by Confocal Fluorescence Microscopy

Gaining control on optical force by the stimulated-emission resonance effect

The resonance between an electronic transition of a micro/nanoscale object and an incident photon flux can modify the radiation force exerted on that object, especially at an interface. It has been theoretically proposed that a non-linear stimulated emission process can also induce an optical force, however its direction will be opposite to conventional photon scattering/absorption processes. In this work, we experimentally and theoretically demonstrate that a stimulated emission process can induce a repulsive pulling optical force on a single trapped dye-doped particle. Moreover, we successfully integrate both attractive pushing (excited state absorption) and repulsive pulling (stimulated emission) resonance forces to control the overall exerted optical force on an object, validating the proposed non-linear optical resonance theory. Indeed, the results presented here will enable the optical manipulation of the exerted optical force with exquisite control and ultimately enable single particle manipulation.

Light-Induced Selective Hydrogenation over PdAg Nanocages in Hollow MOF Microenvironment

Selective hydrogenation with high efficiency under ambient conditions remains a long-standing challenge. Here, a yolk-shell nanostructured catalyst, PdAg@ZIF-8, featuring plasmonic PdAg nanocages encompassed by a metal-organic framework (MOF, namely, ZIF-8) shell, has been rationally fabricated. PdAg@ZIF-8 achieves selective (97.5%) hydrogenation of nitrostyrene to vinylaniline with complete conversion at ambient temperature under visible light irradiation. The photothermal effect of Ag, together with the substrate enrichment effect of the catalyst, improves the Pd activity. The near-field enhancement effect from plasmonic Ag and optimized Pd electronic state by Ag alloying promote selective adsorption of the -NO₂ group and therefore catalytic selectivity. Remarkably, the unique yolk-shell nanostructure not only facilitates access to PdAg cores and protects them from aggregation but also benefits substrate enrichment and preferential -NO₂ adsorption under light irradiation, the latter two of which surpass the core-shell counterpart, giving rise to enhanced activity, selectivity, and recyclability.

Optical force spectroscopy for measurement of nonlinear optical coefficient of single nanoparticles through optical manipulation

Compared with manipulation of microparticles with optical tweezers and control of atomic motion with atom cooling, the manipulation of nanoscale objects is challenging because light exerts a significantly weaker force on nanoparticles than on microparticles. The complex interaction of nanoparticles with the environmental solvent media adds to this challenge. In recent years, optical manipulation using electronic resonance effects has garnered interest because it has enabled researchers to enhance the force as well as sort nanoparticles by their quantum mechanical properties. Especially, a precise observation of the motion of nanoparticles irradiated by resonant light enables the precise measurement of the material parameters of single nanoparticles. Conventional spectroscopic methods of measurement are based on indirect processes involving energy dissipation, such as thermal dissipation and light scattering. This study proposes a theoretical method to measure the nonlinear optical constant based on the optical force. The nonlinear susceptibility of single nanoparticles can be directly measured by evaluating the transportation distance of particles through pure momentum exchange. We extrapolate an experimentally verified method of measuring the linear absorption coefficient of single nanoparticles by the optical force to determine the nonlinear absorption coefficient. To this end, we simulate the third-order nonlinear susceptibility of the target particles with the kinetic analysis of nanoparticles at the solid-liquid interface incorporating the Brownian motion. The results show that optical manipulation can be used as nonlinear optical spectroscopy utilizing direct exchange of momentum. To the best of our knowledge, this is currently the only way to measure the nonlinear coefficient of individual single nanoparticles.

Room-Temperature Molecular Manipulation via Plasmonic Trapping at Electrified Interfaces

For the motion control of individual molecules at room temperature, optical tweezers could be one of the best approaches to realize desirable selectivity with high resolution in time and space. Because of physical limitations due to the thermal fluctuation, optical manipulation of small molecules at room temperature is still a challenging subject. The difficulty of the manipulation also emerged from the variation of molecular polarizability depending on the choice of molecules as well as the molecular orientation to the optical field. In this article, we have demonstrated plasmonic optical trapping of small size molecules with less than 1 nm at the gap of a single metal nanodimer immersed in an electrolyte solution. In situ electrochemical surface-enhanced Raman scattering measurements prove that a plasmonic structure under electrochemical potential control realizes not only the selective molecular condensation but also the formation of unique mixed molecular phases which is distinct from those under a thermodynamic equilibrium. Through detailed analyses of optical trapping behavior, we established the methodology of plasmonic optical trapping to create the novel adsorption isotherm under applying an optical force at electrified interfaces.

Structural region essential for amyloid fibril formation in cytochrome c elucidated by optical trapping

Amyloid fibril formation of cytochrome c is spatially and temporally controlled by the optical trapping method, identifying that the structural change in the region containing Ala83 is essential for the amyloid fibril formation.

More Than 50-Fold Enhanced Nonlinear Optical Response of Porphyrin Molecules in Aqueous Solution Induced by Mixing Base and Organic Solvent

The difference absorption spectrum (DAS) of porphyrin molecules (tetraphenyl-porphyhrin sulfonic acid, TPPS) in aqueous solution induced by continuous wave laser irradiation has been reported previously. It was interpreted that the DAS was caused by the formation of TPPS aggregates induced by laser irradiation. However, transient spectra similar in their shape have already been reported and are attributed to the excited-state absorption and saturable absorption (SA) effects due to the triplet state formation in TPPS. In the present study, we investigated the triplet quenching effect by O2 on the DAS of TPPS aqueous solution and revealed that it originated from the triplet state formation. We also found that mixing the appropriate amount of MeOH and NaOH in TPPS aqueous solution increased its absorbance change by more than 50 times. This may be due to the decrease in dissolved oxygen concentration by mixing them. This result suggests the possibility of controlling the performance of NLO materials by adjusting the solvent mixture ratio and base/acid concentration.

Optical Force-Induced Chemistry at Solution Surfaces

When an intense 1,064-nm continuous-wave laser is tightly focused at solution surfaces, it exerts an optical force on molecules, polymers, and nanoparticles (NPs). Initially, molecules and NPs are gathered into a single assembly inside the focus, and the laser is scattered and propagated through the assembly. The expanded laser further traps them at the edge of the assembly, producing a single assembly much larger than the focus along the surface. Amino acids and inorganic ionic compounds undergo crystallization and crystal growth, polystyrene NPs form periodic arrays and disklike structures with concentric circles or hexagonal packing, and Au NPs demonstrate assembling and swarming, in which the NPs fluctuate like a group of bees. These phenomena that depend on laser polarization are called optically evolved assembling at solution surfaces, and their dynamics and mechanisms are elucidated in this review. As a promising application in materials science, the optical trapping assembly of lead halide perovskites, supramolecules, and aggregation-induced emission enhancement-active molecules is demonstrated and future directions for fundamental study are discussed.

Solvent Effects in Highly Efficient Light-Induced Molecular Aggregation

It has been reported that when irradiated with laser light non-resonant with the main absorption peaks, porphyrin molecules (4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid, TPPS) in an aqueous solution become 10,000 to 100,000 times more efficient in light-induced molecular aggregation than expected from the ratio of gradient force potential to the thermal energy of molecules at room temperature. To determine the mechanism of this phenomenon, experiments on the light-induced aggregation of TPPS in alcohol solutions (methanol, ethanol, and butanol) were performed. In these alcohol solutions, the absorbance change was orders of magnitude smaller than in the aqueous solution. Furthermore, it was found that the absorbance change in the aqueous solution tended to be saturated with the increase of the irradiation intensity, but in the ethanol solution, the absorbance change increased linearly. These results can be qualitatively explained by the model in which intermolecular light-induced interactions between molecules within a close distance among randomly distributed molecules in the laser irradiation volume are highly relevant to the signal intensity. However, conventional dipole–dipole interactions, such as the Keesom interaction, are not quantitatively consistent with the results.

Using optical tweezing to control phase separation and nucleation near a liquid-liquid critical point

About 20 years ago, it was shown that lasers can nucleate crystals in super-saturated solutions and might even be able to select the polymorph that crystallises. However, no theoretical model was found explaining the results and progress was slowed down. Here we show that laser-induced nucleation may be understood in terms of the harnessing of concentration fluctuations near a liquid-liquid critical point using optical tweezing in a process called laser-induced phase separation (LIPS) and LIPS and nucleation (LIPSaN). A theoretical model is presented based on the regular solution model with an added term representing optical tweezing while the dynamics are modelled using a Kramers diffusion equation, and the roles of heat diffusion and thermophoresis are evaluated. LIPS and LIPSaN experiments were carried out on a range of liquid mixtures and the results compared to theory.

Modern Laser Scanning Confocal Microscopy

Since its commercialization in the late 1980's, confocal laser scanning microscopy (CLSM) has since become one of the most prevalent fluorescence microscopy techniques for three-dimensional structural studies of biological cells and tissues. The flexibility of the approach has enabled its application in a diverse array of studies, from the fast imaging of dynamic processes in living cells, to meticulous morphological analyses of tissues, and co-localization of protein expression patterns. In this chapter, we introduce the principles of confocal microscopy and discuss how the approach has become a mainstay in the biological sciences. We describe the components of a CLSM system and assess how modern implementations of the approach have further expanded the use of the technique. Finally, we briefly outline some practical considerations to take into account when acquiring data using a CLSM system. © 2018 by John Wiley & Sons, Inc.

Resonance optical trapping of individual dye-doped polystyrene particles with blue- and red-detuned lasers

We demonstrate resonance optical trapping of individual dye-doped polystyrene particles with blue- and red-detuned lasers whose energy are higher and lower compared to electronic transition of the dye molecules, respectively. Through the measurement on how long individual particles are trapped at the focus, we here show that immobilization time of dye-doped particles becomes longer than that of bare ones. We directly confirm that the immobilization time of dye-doped particles trapped by the blue-detuned laser becomes longer than that by the red-detuned one. These findings are well interpreted by our previous theoretical proposal based on nonlinear optical response under intense laser field. It is discussed that the present result is an important step toward efficient and selective manipulation of molecules, quantum dots, nanoparticles, and various nanomaterials based on their quantum mechanical properties.

Aggregation kinetics and colloidal stability of functionalized nanoparticles

The functionalization of nanoparticles has primarily been used as a means to impart stability in nanoparticle suspensions. In most cases even the most advanced nanomaterials lose their function should suspensions aggregate and settle, but with the capping agents designed for specific solution chemistries, functionalized nanomaterials generally remain monodisperse in order to maintain their function. The importance of this cannot be underestimated in light of the growing use of functionalized nanomaterials for wide range of applications. Advanced functionalization schemes seek to exert fine control over suspension stability with small adjustments to a single, controllable variable. This review is specific to functionalized nanoparticles and highlights the synthesis and attachment of novel functionalization schemes whose design is meant to affect controllable aggregation. Some examples of these materials include stimulus responsive polymers for functionalization which rely on a bulk solution physicochemical threshold (temperature or pH) to transition from a stable (monodisperse) to aggregated state. Also discussed herein are the primary methods for measuring the kinetics of particle aggregation and theoretical descriptions of conventional and novel models which have demonstrated the most promise for the appropriate reduction of experimental data. Also highlighted are the additional factors that control nanoparticle stability such as the core composition, surface chemistry and solution condition. For completeness, a case study of gold nanoparticles functionalized using homologous block copolymers is discussed to demonstrate fine control over the aggregation state of this type of material.

Decoding genomes

The primary sequence of DNA can be decoded a million times faster and cheaper than it could 20 years ago. This capability is transforming our understanding of biology and has stimulated efforts to influence modern medicine through routine sequencing of human genomes. I describe how Solexa-Illumina sequencing originated from our laboratory and was developed into widely used commercial sequencing platforms. I also discuss examples of how this approach is being employed to exploit genome sequencing for medicine.

Plasmonic Optical Tweezers toward Molecular Manipulation: Tailoring Plasmonic Nanostructure, Light Source, and Resonant Trapping

This Perspective describes recent progress in optical trappings of nanoparticles based on localized surface plasmon. This plasmonic optical trapping has great advantages over the conventional optical tweezers, being potentially applicable for a molecular manipulation technique. We review this novel trapping technique from the viewpoints of (i) plasmonic nanostructure, (ii) the light source for plasmon excitation, and (iii) the polarizability of the trapping target. These findings give us future outlook for plasmonic optical trapping. In addition to a brief review, recent developments on plasmonic optical trapping of soft nanomaterials such as proteins, polymer chains, and DNA will be discussed to point out the important issue for further development on this trapping method. Finally, we explore new directions of plasmonic optical trapping.

A surface-bound molecule that undergoes optically biased Brownian rotation

Developing molecular systems with functions analogous to those of macroscopic machine components, such as rotors, gyroscopes and valves, is a long-standing goal of nanotechnology. However, macroscopic analogies go only so far in predicting function in nanoscale environments, where friction dominates over inertia. In some instances, ratchet mechanisms have been used to bias the ever-present random, thermally driven (Brownian) motion and drive molecular diffusion in desired directions. Here, we visualize the motions of surface-bound molecular rotors using defocused fluorescence imaging, and observe the transition from hindered to free Brownian rotation by tuning medium viscosity. We show that the otherwise random rotations can be biased by the polarization of the excitation light field, even though the associated optical torque is insufficient to overcome thermal fluctuations. The biased rotation is attributed instead to a fluctuating-friction mechanism in which photoexcitation of the rotor strongly inhibits its diffusion rate.

Enumerating virus-like particles in an optically concentrated suspension by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is one of the most sensitive methods for enumerating low concentration nanoparticles in a suspension. However, biological nanoparticles such as viruses often exist at a concentration much lower than the FCS detection limit. While optically generated trapping potentials are shown to effectively enhance the concentration of nanoparticles, feasibility of FCS for enumerating field-enriched nanoparticles requires understanding of the nanoparticle behavior in the external field. This paper reports an experimental study that combines optical trapping and FCS to examine existing theoretical predictions of particle concentration. Colloidal suspensions of polystyrene (PS) nanospheres and HIV-1 virus-like particles are used as model systems. Optical trapping energies and statistical analysis are used to discuss the applicability of FCS for enumerating nanoparticles in a potential well produced by a force field.

Optical trapping of nanoparticles by ultrashort laser pulses

Optical trapping with continuous-wave lasers has been a fascinating field in the optical manipulation. It has become a powerful tool for manipulating micrometer-sized objects, and has been widely applied in physics, chemistry, biology, material, and colloidal science. Replacing the continuous-wave- with pulsed-mode laser in optical trapping has already revealed some novel phenomena, including the stable trap, modifiable trapping positions, and controllable directional optical ejections of particles in nanometer scales. Due to two distinctive features; impulsive peak powers and relaxation time between consecutive pulses, the optical trapping with the laser pulses has been demonstrated to have some advantages over conventional continuous-wave lasers, particularly when the particles are within Rayleigh approximation. This would open unprecedented opportunities in both fundamental science and application. This Review summarizes recent advances in the optical trapping with laser pulses and discusses the electromagnetic formulations and physical interpretations of the new phenomena. Its aim is rather to show how beautiful and promising this field will be, and to encourage the in-depth study of this field.

Proposed nonlinear resonance laser technique for manipulating nanoparticles

We propose nonlinear resonant laser manipulation, a technique that drastically enhances the number of degrees of freedom when manipulating nano-objects. Considering the high laser intensity required to trap single molecules, we calculate the radiation force exerted on a molecule in a focused laser beam by solving the density matrix equations using the nonperturbative method. The results coherently elucidate certain recently reported puzzling phenomena that contradict the conventional understanding of laser trapping. Further, we demonstrate unconventional forms of laser manipulations using "stimulated recoil force" and "subwavelength laser manipulation."

Enhancing single molecule imaging in optofluidics and microfluidics

Microfluidics and optofluidics have revolutionized high-throughput analysis and chemical synthesis over the past decade. Single molecule imaging has witnessed similar growth, due to its capacity to reveal heterogeneities at high spatial and temporal resolutions. However, both resolution types are dependent on the signal to noise ratio (SNR) of the image. In this paper, we review how the SNR can be enhanced in optofluidics and microfluidics. Starting with optofluidics, we outline integrated photonic structures that increase the signal emitted by single chromophores and minimize the excitation volume. Turning then to microfluidics, we review the compatible functionalization strategies that reduce noise stemming from non-specific interactions and architectures that minimize bleaching and blinking.

Sequencing nucleic acids: from chemistry to medicine

Chemistry has played a vital role in making routine, affordable sequencing of human genomes a reality. This article focuses on the genesis and development of Solexa sequencing that originated in Cambridge, UK. This sequencing approach is helping transform science and offers intriguing prospects for the future of medicine.

Evaluation of radiation force acting on macromolecules by combination of Brownian dynamics simulation with fluorescence correlation spectroscopy

The effect of optical gradient force from a focused laser beam on the fluorescence correlation spectroscopy (FCS) was investigated by a computing method based on Brownian dynamics simulation. A series of calculations revealed that, in relatively shallow optical force potential up to 1.0kTR (TR=298.15 K), the conventional theoretical model of FCS without consideration of the optical gradient force could evaluate the increase in the average number of molecules and the diffusion time in the potential. On the other hand, large deviation between the simulated fluorescence correlation curve and the theoretical model was observed under the potential depth >1.0kTR. In addition, by integrating the optical force potential with the temperature elevation under optical trapping condition, it was deduced that the temperature rise does not seriously affect the average number of particles in the sampling area, but the average residence time is more sensitively affected by the temperature elevation. The present study using the simulation also provides a method to experimentally estimate molecular polarizabilities from FCS measurements.

Dye diffusion at surfaces: charge matters

Fluorescence correlation spectroscopy and single molecule burst analysis were used to measure the effects of glass surface interactions on the diffusion of two common fluorescent dyes, one cationic and one anionic. The effects of dye-surface interactions on measured diffusion rates as a function of distance from the surface were investigated. Use of a three-axis piezo stage, combined with reference calibration measurements, enabled the accurate acquisition of surface-distance-dependent transport data. This analysis reveals attractive interactions between the cationic dye and the surface, which significantly alter the extracted diffusion values and persist in the measurements up to 1.0 microm from the surface. The Coulomb attraction between the cationic dye and the surface also results in rare, long-lived association events that lead to irreproducibility in extracted diffusion values. In addition to an assignment of the association lifetime for these events, this paper demonstrates that, if experiments must be performed with cationic probes near a glass surface, the use of solution electrolytes can eliminate deleterious dye-surface interactions, as the dyes were tested in a variety of environments. Finally, our data demonstrate that a better dye choice is an anionic probe, which exhibits no depth dependence of diffusion characteristics above a glass surface.

Characterizing manufactured nanoparticles in the environment: multimethod determination of particle sizes

Sizes of stabilized (24 h) nanoparticle suspensions were determined using several state-of-the-art analytical techniques (transmission electron microscopy; atomic force microscopy; dynamic light scattering; fluorescence correlation spectroscopy; nanoparticle tracking analysis; flow field flow fractionation). Theoretical and analytical considerations were evaluated, results were compared, and the advantages and limitations of the techniques were discussed. No "ideal" technique was found for characterizing manufactured nanoparticles in an environmental context as each technique had its own advantages and limitations.

Light tolerance of R-phycoerythrin and a tandem conjugate observed by single molecule recrossing events

The recrossing of single molecules in a probe volume was used to investigate light harvesting and energy transfer between R-phycoerythrin (R-PE) and a tandem conjugate dye. The normalized recrossing ratio, Nr/Nt, was defined as the number of molecules that reenter the probe volume (Nr) to the total number of molecules detected (Nt). The energy transfer process in phycobiliproteins was studied as a function of excitation irradiation and irradiation time. This was achieved by investigating the average baseline-subtracted fluorescence intensity, normalized molecular recrossing ratio (Nr/Nt), and the number of molecules detected per second. The photon saturation irradiance of the R-PE and the tandem conjugate were compared with each other, showing that energy transfer to the tandem dye significantly improves photostability and light tolerance of the phycobiliprotein. The Nr/Nt ratio was used to study the photophysical properties of R-phycoerythrin and the tandem conjugate Streptavidin R-Phycoerythrin-AlexaFluor-647 (PE-647). Normalized molecular recrossings showed that energy transfer to a tandem conjugate could reduce the formation of triplet states in R-phycoerythrin and extend the light tolerance of certain phycobiliproteins.

Sizing subcellular organelles and nanoparticles confined within aqueous droplets

This article describes two complementary techniques, single-particle tracking and correlation spectroscopy, for accurately sizing nanoparticles confined within picoliter volume aqueous droplets. Single-particle tracking works well with bright particles that can be continuously illuminated and imaged, and we demonstrated this approach for sizing single fluorescent beads. Fluorescence correlation spectroscopy detects small intensity bursts from particles or molecules diffusing through the confocal probe volume, which works well with dim and rapidly diffusing particles or molecules; we demonstrated FCS for sizing synaptic vesicles confined in aqueous droplets. In combination with recent advances in droplet manipulations and analysis, we anticipate this capability to size single nanoparticles and molecules in free solution will complement existing tools for probing cellular systems, subcellular organelles, and nanoparticles.

Coupling confocal fluorescence detection and recirculating microfluidic control for single particle analysis in discrete nanoliter volumes

The recent proliferation of platforms designed to handle arrays of nano- and picolitre volumes is in response to the need to perform biological assays on discrete entities, such as single cells. However, a critical challenge associated with this trend for in vitro compartmentalization is the need for highly sensitive, yet low-volume detection platforms. In this paper, we coupled confocal fluorescence detection with recirculating microfluidic control to perform single particle DNA assays within five nL chambers. The performance of this low-volume assay was shown to match that of traditional single molecule detection platforms. However, volume requirements per measurement were nearly 3 orders of magnitude less than conventional systems, enabling future integration with lab-on-a-chip systems that require discrete or digitalized sample processing.

Single-molecule level analysis of the subunit composition of the T cell receptor on live T cells

The T cell receptor (TCR) expressed on most T cells is a protein complex consisting of TCRalphabeta heterodimers that bind antigen and cluster of differentiation (CD) 3epsilondelta, epsilongamma, and zetazeta dimers that initiate signaling. A long-standing controversy concerns whether there is one, or more than one, alphabeta heterodimer per complex. We used a form of single-molecule spectroscopy to investigate this question on live T cell hybridomas. The method relies on detecting coincident fluorescence from single molecules labeled with two different fluorophores, as the molecules diffuse through a confocal volume. The fraction of events that are coincident above the statistical background is defined as the "association quotient," Q. In control experiments, Q was significantly higher for cells incubated with wheat germ agglutinin dual-labeled with Alexa488 and Alexa647 than for cells incubated with singly labeled wheat germ agglutinin. Similarly, cells expressing the homodimer, CD28, gave larger values of Q than cells expressing the monomer, CD86, when incubated with mixtures of Alexa488- and Alexa647-labeled antibody Fab fragments. T cell hybridomas incubated with mixtures of anti-TCRbeta Fab fragments labeled with each fluorophore gave a Q value indistinguishable from the Q value for CD86, indicating that the dominant form of the TCR comprises single alphabeta heterodimers. The values of Q obtained for CD86 and the TCR were low but nonzero, suggesting that there is transient or nonrandom confinement, or diffuse clustering of molecules at the T cell surface. This general method for analyzing the subunit composition of protein complexes could be extended to other cell surface or intracellular complexes, and other living cells.

Plasmon resonance-based optical trapping of single and multiple Au nanoparticles

The plasmon resonance-based optical trapping (PREBOT) method is used to achieve stable trapping of metallic nanoparticles of different shapes and composition, including Au bipyramids and Au/Ag core/shell nanorods. In all cases the longitudinal plasmon mode of these anisotropic particles is used to enhance the gradient force of an optical trap, thereby increasing the strength of the trap potential. Specifically, the trapping laser is slightly detuned to the long-wavelength side of the longitudinal plasmon resonance where the sign of the real component of the polarizability leads to an attractive gradient force. A second (femtosecond pulsed) laser is used to excite two-photon fluorescence for detection of the trapped nanoparticles. Two-photon fluorescence time trajectories are recorded for up to 20 minutes for single and multiple particles in the trap. In the latter case, a stepwise increase reflects sequential loading of single Au bipyramids. The nonlinearity of the amplitude and noise with step number are interpreted as arising from interactions or enhanced local fields amongst the trapped particles and fluctuations in the arrangements thereof.

Investigation of photobleaching and saturation of single molecules by fluorophore recrossing events

A method for investigation of photobleaching and saturation of single molecules by fluorophore recrossing events in a laser beam is described. The diffraction-limited probe volumes encountered in single-molecule detection (SMD) produce high excitation irradiance, which can decrease available signal. The single molecules of several dyes were detected and the data was used to extract interpeak times above a defined threshold value. The interpeak times revealed the number of fluorophore recrossing events. The number of molecules detected that were within 2 ms of each other represented a molecular recrossing for this work. Calcein, fluorescein and R-phycoerythrin were analyzed and the saturation irradiance and photobleaching effects were determined as a function of irradiance. This approach is simple and it serves as a method of optimizing experimental conditions for single-molecule detection.

Electromagnetic and chemical interaction between Ag nanoparticles and adsorbed rhodamine molecules in surface-enhanced Raman scattering

The critical importance of the junction between touching or closely adjacent Ag nanoparticles associated with single-molecule sensitivity (SMS) in surface-enhanced Raman scattering (SERS) was confirmed via the following observations: (1) an additional peak is observed in elastic scattering only for the SERS-active state, which originated from absorption of adsorbates, (2) local- and far-field evaluation using a finite difference time domain method could reproduce this extra peak and anticipate the significantly enhanced field even inside the adsorbates sitting at the junction through an increased coupling of the localized surface plasmons, and (3) in addition to enhanced fluorescence of adsorbed dye, an inelastic scattering peak was observed and attributed to the metal surface electron. Concerning the chemical enhancement in SERS, Cl- anions activate the Ag-Cl-R6G (rhodamine) samples by inducing intrinsic electronic interaction between Ag and R6G molecules. This electronic interaction is irreversibly quenched by the addition of thiosulfate anions which dissolve Ag+ cations while the electromagnetic (EM) effect remains intact.

Accurate sizing of nanoparticles using confocal correlation spectroscopy

The ability to accurately size low concentrations of nanoscale particles in small volumes is useful for a broad range of disciplines. Here, we characterize confocal correlation spectroscopy (CCS), which is capable of measuring the sizes of both fluorescent and nonfluorescent particles, such as quantum dots, gold colloids, latex spheres, and fluorescent beads. We accurately measured particles ranging in diameter from 11 to 300 nm, a size range that had been difficult to probe, owing to a phenomenon coined biased diffusion that causes diffusion times, or particle size, to deviate as a function of laser power. At low powers, artifacts mimicking biased diffusion are caused by saturation of the detector, which is especially problematic when probing highly fluorescent or highly scattering nanoparticles. However, at higher powers (>1 mW), autocorrelation curves in both resonant and nonresonant conditions show a structure indicative of an increased contribution from longer correlation times coupled with a decrease in shorter correlation times. We propose that this change in the autocorrelation curve is due to the partial trapping of the particles as they transit the probe volume. Furthermore, we found only a slight difference in the effect of biased diffusion when comparing resonant and nonresonant conditions. Simulations suggest the depth of trapping potential necessary for biased diffusion is > 1 k(B)T. Overcoming artifacts from detector saturation and biased diffusion, CCS is particularly advantageous due to its ability to size particles in the small volumes characteristic of microfluidic channels and aqueous microdroplets. We believe the method will find increasing use in a wide range of applications in measuring nanoparticles and macromolecular systems.

Geometry Dependent Features of Optically Induced Forces between Silver Nanoparticles

A recently devised, discrete-dipole approximation (DDA) based method for computing optical forces is used to explore geometry dependent aspects of the light induced interactions between pairs of silver nanoparticles, including the influence of particle shape, relative positioning of the particles, and incident field orientation. The interactions are observed to have a large degree of generic character, independent of the details of the particle shape. The size of the optical forces is also compared to estimates for the van der Waals forces, and the results are used to assess the potential importance of radiation forces on recent experiments demonstrating photoinduced self-assembly of triangular silver nanoprisms.