Opto -thermoelectric trapping of Fluorescent Nanodiamonds on Plasmonic Nanostructures ASHUTOSH SHUKLA1 SUNNY TIWARI12 AYAN MAJUMDER3 KASTURI SAHA3 AND G.V.P AVAN

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Opto-thermoelectric trapping of Fluorescent

Nanodiamonds on Plasmonic Nanostructures

ASHUTOSH SHUKLA1, SUNNY TIWARI1,2, AYAN MAJUMDER3, KASTURI SAHA3, AND G.V.PAVAN

KUMAR1,∗,

1Department of Physics, Indian Institute of Science Education and Research, Pune, India

2(present address) Department of Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom

3Department of Electrical Engineering, Indian Institute of Technology, Bombay, India

*Corresponding author: pavan@iiserpune.ac.in

Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX

Deterministic optical manipulation of fluorescent

nanodiamonds (FNDs) in fluids has emerged as an

experimental challenge in multimodal biological imaging.

Designing and developing nano-optical trapping

strategies to serve this purpose is an important task. In

this letter, we show how chemically-prepared gold

nanoparticles and silver nanowires can facilitate Opto-

thermoelectric force to trap individual entities of FNDs

using a long working distance lens, low power-density

illumination (532 nm laser, 12 𝝁𝑾/𝝁𝒎𝟐). Our trapping

configuration combines the thermoplasmonic fields

generated by individual plasmonic nanoparticles and the

opto-thermoelectric effect facilitated by the surfactant to

realise a nano-optical trap down to a single FND 120 nm in

diameter. We utilise the same trapping excitation source

to capture the spectral signatures of single FNDs and track

their position. By tracking the FND, we observe the

differences in the dynamics of FND around different

plasmonic structures. We envisage that our drop-casting

platform can be extrapolated to perform targeted, low-

power trapping, manipulation, and multimodal imaging of

FNDs inside biological systems such as cells.

Fluorescent nanodiamonds have garnered significant

attention for their applications in sensing, biomedical imaging

and quantum optics [1–4]. As a sensing material, FNDs have

several unique advantages over other sensors as they are

extremely stable, biocompatible, and can be engineered to

respond to specific stimuli. They can also be used in harsh

environments and detect a wide range of physical and

chemical properties. In biomedical imaging, fluorescent

nanodiamonds are used as labels to track the movement of

cells and other biological structures. Their biocompatibility,

small size, and bright fluorescence make them ideal imaging

probes. They can also be functionalised with targeting

molecules to selectively bind to specific cells or tissues,

providing an even more specific imaging contrast. Overall,

the unique properties of fluorescent nanodiamonds make

them a promising material for sensing and biomedical

imaging applications. But in order to expand their potential

use for single spin imaging and optically detected magnetic

imaging, their precise control in solutions is crucial[5–7].

While current studies on FNDs use separate laser beams for

trapping and spectroscopy, this design can be limiting due to

charge state perturbations and potential damage to sensitive

environments such as living cells[8]. Thus, there is a need to

develop new nano-optical and opto-thermophoretic trapping

methods that facilitate trapping, spectroscopic probing and

imaging using a single low-power laser[1,9–17].

Opto-thermoelectric trapping [18] has emerged as a promising

technique for manipulating small particles, including nanoparticles

and cells. A surfactant is added to the solution, which dissociates in

anions and positively charged micelles. The particles to be trapped

also get coated in a surfactant bilayer and become positively

charged. A localised laser heating of a plasmonic structure (usually

thin gold films) generates a thermal gradient leading to the

separation of charges, as chlorine anions have higher mobility than

the cationic micelles. This charge separation creates an electric field

towards the heat source, creating an opto-thermoelectric force and

trapping the positively charged particle. This technique has shown

great potential for applications in biophysics, nanotechnology, and

microfluidics, as it enables non-invasive and precise manipulation

of particles in solution. In recent years, much research has been

focused on exploring the potential of opto-thermoelectric trapping

using various nanostructures, such as gold films, plasmonic nano-

antennas, and metallic nanoparticles[18–20].

However, conventional opto-thermoelectric trapping using gold

films has limitations, including low trapping efficiency, poor spatial

control, and thermal damage to the trapped particles. The

localisation is often improved by making the laser focus tighter,

which leads to a restriction of having a short working distance from

the objective. We previously showed low-power trapping of single

gold nanoparticles using a plasmonic-nanostructure-based

trapping platform[21]. This paper presents an approach for long

working distance, low power opto-thermoelectric trapping of

fluorescent nanodiamonds using plasmonic nanostructures instead

of gold films or lithographically prepared structures.

Fig. 1. a) Schematic of plasmon-assisted Opto-thermoelectric trapping of

fluorescent nanodiamonds on Gold Nanoparticles. The surfactant (CTAC)

facilitates an electric field which attracts the FND to AuNP. b) & c)

Simulated temperature and temperature gradient distributions for a 400

nm AuNP placed on a glass substrate and surrounded by water. d)

Comparison of opto-thermoelectric (OTE) and optical force in our

geometry showing OTE force dominates the trapping.

We demonstrate that chemically synthesised plasmonic

nanostructures can be used to generate a highly localised thermal

gradient that can trap and manipulate fluorescent nanodiamonds

with high efficiency and spatial resolution. To this effect, we used

drop-casted plasmonic nanostructures, such as gold nanospheres

(AuNPs) and silver nanowires (AgNWs). Figure 1a shows the

schematic of our trap. The AuNPs could, in principle, be drop-casted

on a desired substrate, and FNDs can be trapped there.

We investigated the capability of such anchor-particle-based

trapping by calculating the OTE force on the FND. The temperature

distribution around the nanoparticle was calculated using

numerical calculations based on the finite element method using

COMSOL Multiphysics (version 5.5) as a solver. The Wave optics

module was coupled with the Heat transfer module to simulate the

temperature increment and temperature gradient upon excitation

with a light source. The simulation model consists of a 400 nm

AuNP placed on a glass substrate and immersed in an aqueous

solution. A focused Gaussian laser source of wavelength 532 nm,

which is near the absorption maxima of the AuNP and FND, was

used to excite the nanoparticle. This creates a temperature gradient

around the AuNP due to the thermoplasmonic heating of the

nanoparticle[22]. The size of the excitation spot (3.34 𝜇𝑚) was

taken from the experimental measurements. The resulting

temperature gradient has been plotted in figure 1b. It can be

observed that the temperature gradient is sharp in the vicinity of the

nanoparticle and was found to be around 30 K/µm. From this

thermal gradient, the thermoelectric field in our geometry was

calculated as [20]

𝑬𝑻=𝒌𝑩𝑻𝜵𝑻

𝒆(∑𝑿𝒊𝒄𝒊𝑺𝑻,𝒊

∑𝑿𝒊

𝟐𝒄𝒊)

Here i denotes the ionic species, 𝑘𝐵 is the Boltzmann constant, T is

the surrounding temperature, e is the elementary charge, and 𝑐𝑖,

and 𝑆𝑇,𝑖 are the concentration and Soret diffusion coefficient of the

ionic species i, respectively, and 𝑋𝑖=± 1 for positive and negative

ions, respectively. See supplementary information for details on the

Fig. 2. Optical schematic of the trapping setup including lenses (L), mirrors

(M), beam splitters (BS), white light sources (WL), 50x objective lens of 0.5

numerical aperture, a dark field (DF) condenser of numerical aperture 1.4, a

charge-coupled device (CCD) camera, an electron multiplying CCD

(EMCCD) camera, and a spectrometer. This scheme integrates dark-field

microscopy, optical spectroscopy, tracking, and fluorescent imaging.

calculation. The thermoelectric force, 𝐹𝑇,𝐸, is then calculated as

𝑭𝑻,𝑬 =∫𝝈𝑬𝑻𝒅𝑨, where 𝜎 is the effective surface charge density of

the FNDs measured indirectly using zeta-potential measurements.

The thermoelectric force on the FND is calculated at the height of 10

nm above the AuNP. The optical force on the same particle is

calculated using the Maxwell stress tensor method (refer

supplementary information fig S2-4.) Figure 1c compares the OTE

force with the optical force obtained for the same geometry. It is

evident that the OTE force is dominant compared to the optical

force, and therefore the resultant trapping potential is mainly

facilitated by the OTE process.

Experiments. AuNPs and FNDs were purchased from Sigma Aldrich.

The reported mean diameters of AuNP and FND are 150 nm ± 15

nm and 120 nm, respectively. AuNPs were redispersed in ethanol

solution. The FNDs contain 3 ppm NV centres. FNDs were dispersed

in a solution containing a specific concentration of

cetyltrimethylammonium chloride (CTAC) molecules. AgNWs of an

average diameter of 350 nm were synthesised using a polyol

process[23] and immersed in an ethanol solution.

An assembly similar to the schematic of the AuNP driven opto-

thermoelectric trap of FNDs in a microchamber shown in figure 1a

was made. AuNPs dispersed in ethanol were drop-casted on a glass

substrate, and the solvent was evaporated. These AuNPs remain

firmly attached to the surface and do not re-diffuse back in the

solution. A dilute solution of FNDs and surfactant (CTAC) is added

to the drop-casted layer, and the chamber is sealed with an adhesive

spacer and another glass coverslip. The chamber is placed on the

stage of our custom-built optical trapping microscope, whose

optical schematic is shown in figure 2. The L1 and L2 are lenses used

to expand the beam to overfill the back aperture of the lens. The

WL1 and WL2 are white light sources for top and dark-field

illumination, respectively. The L3 and L4 lenses illuminate the

sample plane with WL1, whereas a 1.4 NA darkfield condenser

focuses white light from below. BS1 is a beam splitter combining

white light and laser light in the input path of the upright

microscope. A 50×, 0.5 NA microscope objective focuses the

incoming beam on the anchored AuNP. The scattered light is

collected by the top objective and is passed through a 532 nm notch

filter to remove the Rayleigh scattered light. Then the emission is

Fig. 3. a) Trajectories of trapped FND using 150 nm and 400 nm diameter

AuNPs. The corresponding radial probability distribution shown in c)

shows that the trapped FNDs radial distribution varies with the diameter of

the anchor particle. Snapshots of single FND trapping are shown for c) 150

nm AuNP (See Visualization 1 and 2), d) 400 nm AuNP (See Visualization

3). The times series fluorescent images show the trap and release ability of a

single FND.

sent to a CCD camera, an EMCCD, or the spectrometer using mirrors

M2 and M3. Lenses L5 and L6 focus the light to form the image on

cameras and the spectrometer. This upright microscope has the

following capabilities: darkfield imaging to visualise the drop-

casted AuNPs (see supplementary figure, S6); 532 nm laser

illumination to trap and excite the fluorescence in FND. The

objective lens used for this purpose was a low numerical aperture

(NA=0.5) air objective lens, ensuring that the power density

delivered at the sample is low. The emitted fluorescence from a

trapped FND was either captured by the spectrometer or directly

imaged using an EM-CCD or a conventional CCD camera after

rejecting the Rayleigh scattered light at 532 nm. This dual camera

capability was harnessed to identify individual AuNP, trap, and

perform single FND fluorescence tracking. The images were

captured in the CCD camera at a frame rate of 35 frames per second

for full frame and on the EMCCD at a frame rate of 60 fps. The

position of the FND is tracked using the TrackMate[24] plugin

available through Fiji[25].

We next discuss the FND trapping in our AuNPs trap. We executed

the trap using two different-sized AuNPs (150 nm and 400 nm in

diameter). The sample trajectories for the trapped FND are plotted

in figure 3a. In figure 3c, d, we show the fluorescence image of a

reversible trap of a single FND achieved by controlling the

illumination of AuNPs. Upon illumination of a single AuNP, we

observed directed diffusion of FND towards the illuminated AuNP.

When the laser is off, the FND diffuses back to the solution. The

observed switching of the FND trap was relatively quick (about 10

seconds) and could be achieved repeatedly over multiple cycles.

With the 150 nm AuNP based trap, the minimum laser power which

could be used to trap the FND was 1.2 mW in the sample plane,

corresponding to a power density of 34 𝜇𝑊/𝜇𝑚2. The absorption

cross-section of a 400 nm particle is four times that of a 150 nm

particle at 532 nm wavelength (see fig S1). Consequently, the

minimum laser power with which trapping is achieved is 0.43 mW

Fig. 4. a) Schematic showing the trapping on Silver Nanowires (AgNW). b)

Position distribution of FND is shown along the wire and in the direction

perpendicular to the wire. c) Snapshots of single FND trapping on 400 nm

diameter AgNW. The times series fluorescent images show the trap and

release ability of a single FND (See visualization 4).

which implies a power density of 12.2 𝜇𝑊/𝜇𝑚2. This is much

lower power than is required for trapping using conventional

optical trapping (see supplemental figures S7,8). The plotted

trajectory in figure 3a shows that the position of the trapped particle

is not gaussian distributed as it is for optical trapping. The trap gets

affected by the presence of AuNP in the centre. The particles diffuse

around the AuNP, showing interesting dynamics as power and

surfactant concentration are varied. The radial probability

distribution of the trajectories plotted in figure 3b also shows the

same. To plot the radial position distribution, the radial distance of

the FND in the trap for all positions in the trajectory (shown in figure

3a) was calculated and then plotted as a histogram as a function of

radial distance. Figure 3b shows the most probable radial position

variation for the two different AuNPs, 130 nm and 210 nm,

respectively, for 150 nm and 400 nm AuNP. Thus, we can

manipulate the localisation of the trapped FND. The concentration

dependence of the surfactant on the trajectory of the trapped

particle is also studied and shown in the supplementary

information, figure S5.

We also performed trapping experiments with 350 nm average

diameter AgNWs used as the heat source. We have previously

shown that these nanowires can be used as nanophotonic

waveguides[26–28]. The schematic for the experiments is shown in

figure 4a. The AgNWs were drop-casted on the glass substrate, and

ethanol was evaporated. The aqueous solution of FND and CTAC

was placed on top of it and sealed using an adhesive spacer and a

coverslip. The trapping with AgNW is similar to the procedure

described for the AuNP. Again, we used the laser polarized along the

nanowire to excite the plasmons (and FND fluorescence) and create

a thermal gradient. The snapshots of the trap are shown in figure 4c,

where subsequent images show the trapping and release ability of

the trap. The power required to trap the particle is, in this case, 3

mW (85 𝜇𝑊/𝜇𝑚2). The position distribution of FND is shown

parallel and perpendicular to the wire. It is observed that the

particle has a broader position distribution parallel to the wire than

in the perpendicular direction. This can be understood as the

temperature distribution is much steeper in the perpendicular

direction to the nanowire than in the parallel direction, as we have

previously shown[28].

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Opto-thermoelectrictrappingofFluorescentNanodiamondsonPlasmonicNanostructuresASHUTOSHSHUKLA1,SUNNYTIWARI1,2,AYANMAJUMDER3,KASTURISAHA3,ANDG.V.PAVANKUMAR1,∗,1DepartmentofPhysics,IndianInstituteofScienceEducationandResearch,Pune,India2(presentaddress)DepartmentofPhysics,ClarendonLaboratory,Universityo...

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Opto -thermoelectric trapping of Fluorescent Nanodiamonds on Plasmonic Nanostructures ASHUTOSH SHUKLA1 SUNNY TIWARI12 AYAN MAJUMDER3 KASTURI SAHA3 AND G.V.P AVAN

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