Optical trapping of optical nanoparticles: Fundamentals and applications

(a) Time evolutions of the longitudinal surface plasmon resonance (LSPR) peak position (blue points) and the rotation frequency (red) as the laser power (green) is swept in sawtooth ramps with linearly increasing maxima. The dashed blue line is fit to the low laser power spectral peak positions, illustrating the continuous reshaping behavior. The grey area indicates a change in the slope of the linear relation between the laser power and λ_LSPR, where nucleation of vapor is suggested to occur. (b) Temperature determination based on LSPR spectroscopy (red data points, based on water’s refractive index (RI) temperature variation and subsequent λ_LSPR variation) and rotational Brownian dynamics (blue data points, based on water’s viscosity temperature variation and subsequent rotation frequency change) obtained from the data in (a). Figure reproduced with permission from ref.⁴⁹, Copyright 2017 American Chemical Society.

Plasmonic nanoparticles for investigation of nanoparticle-nanoparticle interaction. (a) Time evolution of the darkfield spectrum of optically trapped silver nanoparticles. Observing intensity steps at the silver plasmon resonance (~450 nm) allowed to count the number of nanoparticles entering inside the trap. (b) Experimental darkfield spectra of one (black), two noncoupled (blue), and three (two of them coupled) (red) trapped silver nanoparticles. (c) Theoretical plane wave scattering cross section spectra of one (black), two noncoupled (blue), and two coupled (distance = 2 nm) plus one noncoupled (red) silver nanoparticles, assuming randomly changing orientation of nanoparticles in space. Evolution of scattering color at the optical trap (true color images) with time when two silver nanoparticles are simultaneously trapped: (d) initially, (e) 10 s after (d), (f) 60 ms after (e). No other particles were observed entering the trap during the color evolution process. (g) Darkfield images at different times (left and center) and time-lapse image (right) illustrating the optical transport of several 60 nm diameter silver nanospheres around a 3 μm radius ring-shaped optical trap. Darker blue for monomers, lighter blue for close-standing nanospheres, light green for dimers. (h) Illustration of the thermo-responsive behavior of the core (purple)-satellite (orange) plasmonic nano-assemblies incorporating thermo-responsive polymers. Figure reproduced with permission from: (a–f) ref.⁴⁸, Copyright 2011 American Chemical Society; (g) ref.⁶², Copyright 2021, Chinese Laser Press.

Lanthanide-doped nanoparticles for thermal sensing. (a) Energy scheme of Er³⁺-Yb³⁺ co-doped UCNP. Black continuous arrows indicate ⁴F_7/2 level population by an energy transfer process. Grey discontinuous arrows indicate energy transfer processes between Er³⁺ and Yb³⁺ ions. Dark green (²H_11/2→⁴I_15/2), light green (⁴S_3/2→⁴I_15/2), and red (⁴F_9/2→⁴I_15/2) continuous arrows indicate radiative emissions centered at 520 nm, 540 nm, and 660 nm, respectively. (b) Schematic representation of the experimental setup used for thermal scanning in the surroundings of a HeLa cell subjected to a plasmonic photothermal treatment. The arrow indicates the scanning direction. (c) On the top, the temperature decay measured from cell surface for three different distances from the substrate. The symbols are the experimental data, and the lines are a guide for the eyes. On the bottom, the control experiment in absence of the 800 nm heating laser. (d) Functional scheme of the active dual-wavelength optical multitrap setup. The 980 nm beams (orange cones) for trapping of the cell and trapping/reading of the optical microthermometers, while the 1064 nm optical trap (green cone) for optical trapping/heating. (e) Microscope image of the cell, silica microparticles and optical microthermometers being trapped, activated, and read. Red rectangles represent 980 nm trapping laser, while green rectangles represent 1064 nm trapping laser. (f) Time lapse images of the heated cells at 10, 30, and 65 s after switching on the heating. On the bottom-right, graphs of relative Trypan Blue (TB) accumulation in the dying cell and the corresponding temperature (right axis). Figure reproduced with permission from: (b, c) ref.¹⁴, Copyright 2016 John Wiley and Sons; (d–f) ref.⁸⁰, Copyright 2017 American Chemical Society.

Lanthanide-doped particles for intracellular viscosity sensing. (a) Diagram of the three excitation configurations. σ-polarization state, defined by propagating light along the optical axis (z-axis) with a polarization perpendicular to it (parallel to x- or y-axes) and α- and π-polarization states, where light propagates perpendicularly to the optical axis of the UCNP with a polarization parallel (π) or perpendicular (α) to it. (b) Emission spectra along the three polarization states. (c) Schematic representation of the rotation of a UCNP inside a HeLa cell. (d) Measured viscosity as a function of the rotation frequency. Inset is the log−log representation. Blue data correspond to the values measured by the active method while red data is the averaged value obtained from passive method experiments. The rotation frequency for passive method is estimated by dividing the accumulated rotated angle by the elapsed drag. Black dashed lines correspond to the best fitting. Figure reproduced with permission from ref.¹⁵, Copyright 2016 American Chemical Society.

Lanthanide-doped particles for chemical sensing. (a) Red to green emission ratio depending on pH value for a single UCMP in a buffer solution. Blue dots are experimental data. Grey line is the linear fitting. (b) Red to green emission ratio of the optically trapped UCMP vs. pH in various buffer solutions. Figure reproduced with permission from ref.¹⁷, under a Creative Commons Attribution 3.0 International License.

(a) Jablonsky diagram of the electronic states involved in the optical resonance effect. (b) Theoretical calculations of the population of each state vs the 488 nm excitation power. (c) Changes in the trapping stiffness vs the excitation wavelength, there is no fluorescence or resonance effect without 488 nm excitation. (d) Changes in the trapping stiffness vs the 488 nm laser power. Figure reproduced with permission from ref.¹⁰¹. Copyright 2021 American Chemical Society.

Fluorescent polymeric particles for investigation of single particle dynamics. (a) Left: Schematic of multiplane microscope with optical tweezer. Right: Time frame of fluorescent images from different planes. (b) 3D incorporation trajectories of 200 nm fluorescent polystyrene particles in an optical trap. Blue and red lines represent the incorporation trajectories going through the inner and outer cones, respectively. (c) Schematic of mesoscopic mechanical motions induced by the photochromic reaction of a single PMMA particle. (d) Time course of the positional change along the Z axis (red line) and the fluorescence intensity (blue line) of a trapped particle with the continuous wave 532 nm laser. (e) Z displacement of a fluorescent particle under trapping. (f) Profile of radial flow component u_r for various particle numbers (N) and diameters (d_p). (g–h) Snapshots of the fluorescence images of different numbers of 1 μm polystyrene particles around the optical trap. Figure reproduced with permission from: (a, b) ref.¹⁰², under a Creative Commons Attribution 4.0 International License; (c–e) ref.¹⁰³, Copyright 2023 American Chemical Society; (g–h) ref.¹⁰⁴, Copyright 2020 American Chemical Society.

Fluorescent polymeric particles for biological applications. (a) Scheme of the scanning procedure: a latex bead is trapped by the focused laser beam (red arrows). The fluorophore in the latex beads is excited by laser light via a two-photon absorption process. When the trapped bead is moved across an obstacle such as a cell (grey arrow), it is pushed out of the laser focus and the fluorescence intensity decreases. (b) A 500 nm polystyrene bead is tethered to cover glass surface by a 1010 base-pair DNA molecule, consisting of a long strand (black) joined to a shorter 15- base-pair duplex region (red). (c) Simultaneous records of force (red trace) and fluorescence (blue trace). Rupture occurred at 2 s at an unzipping force of 9 pN. The dye unquenched at the point of rupture, and later bleached at 9 s. Figure reproduced with permission from: (b, c) ref.¹⁰⁷, under a Creative Commons Attribution 4.0 International License.

Fluorescent nano-semiconductors for single particle spectroscopy. (a) Time series fluorescence snapshots of trapping and releasing events of 35 nm QDs on graphene oxide. (b) Plot of luminescence intensity of trapped QD with time. The decrease in the emission of the QD on graphene oxide with time can be clearly observed. Evolution of emission peak intensity for trapped (c) QD@SiO₂ with only SiO₂ shell and (d) pQD@SiO₂ with sulfur and SiO₂ shells. Insets: diagram of trapped (c) QD@SiO₂ and (d) pQD@SiO₂. Figure reproduced with permission from: (a, b) ref.¹²¹, Copyright 2020 American Chemical Society; (c, d) ref.¹²², Copyright 2017 American Chemical Society.

Fluorescent nano-semiconductors for cell imaging. (a) Sketch of the indirect excitation of a dye labeled cell. An optically trapped QD₅₄₀@SiO₂ cluster is positioned in the vicinity of a labeled Jurkat cell with Alexa Fluor 546 to induce its photoluminescence. Bright field microscopy images and collected spectra (black curves) from a QD₅₄₀@SiO₂ cluster trapped (b) ~10 μm away from the Jurkat cell, (c) close (a few microns) to the labeled Jurkat cell membrane, (d) in the vicinity (within 1 μm) of the Jurkat cell membrane, and (e) above the whole Jurkat cell structure. The optical trap position roughly matches the reticle center. Red curves represent in all cases the emission of the original QD₅₄₀@SiO₂. Figure reproduced with permission from ref.¹²⁵, Copyright 2019 American Chemical Society.

Fluorescent nanodiamonds for cell imaging. (a) Confocal microscope image of a human brain microvascular endothelial cell (HMEC-1). (b) Fluorescence image of the cell with mitochondria stained by Mito-Tracker Green FM (excitation: 490 nm, emission: 516 nm). (c) Fluorescence image of the nanodiamonds microaggregates at different locations in the HMEC-1. Blue and pink arrows pointed to the microspheres near the nuclear membrane and cell membrane, respectively. Figure reproduced from ref.¹³⁵, under a Creative Commons Attribution License.