RESEARCH PAPER

Optical delivery of nanospheres using arbitrarybending nanofibers

Ying Li • Linlin Xu • Baojun Li

Received: 27 September 2011 / Accepted: 25 February 2012

� Springer Science+Business Media B.V. 2012

Abstract This work reports an optical delivery of

about 700-nm diameter polystyrene spheres along

arbitrary bending nanofibers (600 nm in diameter)

including complete loop structure. Dependence of

bending loss on bending radii and central angles of the

nanofiber has also been investigated. The results show

that, for a specific input optical power, there is a

corresponding minimum bending radius for optical

trapping and delivery. In other words, for a specific

input optical power, when the bending radius of the

nanofiber is larger than the minimum bending radius,

the 700-nm diameter nanospheres can be trapped and

delivered along the bending nanofiber. Vice versa, the

nanospheres will be escaped from the bending nano-

fiber during the delivery process because of relatively

large bending loss.

Keywords Optical delivery � Nanosphere �Arbitrary bending � Nanofiber � Loop structure

Introduction

Since the first report of optical trapping (Ashkin 1970;

Ashkin and Dziedzic 1987), it has been widely

developed to manipulate micro- or nano-particles in

research of physical, chemical, and biological sciences

(Block et al. 1990; Grier 2003; Ozkan et al. 2003;

Wang et al. 2004; MacDonald et al. 2003; Yu et al.

2004; Grigorenko et al. 2008). Traditionally, optical

force can be exerted on particles using laser beams

focused by high numerical aperture lens, which

enables noninvasive trapping with high precision.

But diffraction limit is the key obstacle for this

technique. Recently, there is a growing trend toward

using optical near field through the use of optical

gradient force and scattering force to trap and transport

particles (Tanaka and Yamamoto 2000; Gaugiran et al.

2005; Yang et al. 2009; Brambilla et al. 2007; Xin and

Li 2011). For example, Yang et al. (Yang et al. 2009)

exploited sub-wavelength slot waveguides for optical

manipulation of nanoparticles and DNA molecules.

Sub-wavelength optical wires were also used for

micro-sized polystyrene spheres manipulation

(Brambilla et al. 2007). Moreover, by using resona-

tors, particles can be transported along bending

waveguides, such as silicon-based planar microring

resonators (Lin et al. 2010; Cai and Poon 2010).

Electronic supplementary material The online version ofthis article (doi:10.1007/s11051-012-0799-3) containssupplementary material, which is available to authorized users.

Y. Li (&) � L. Xu � B. Li

State Key Laboratory of Optoelectronic Materials

and Technologies, School of Physics and Engineering,

Sun Yat-Sen University, Guangzhou 510275, China

e-mail: xingziliying@126.com

B. Li

e-mail: stslbj@mail.sysu.edu.cn

Y. Li

School of Information and Engineering, Guangdong

Medical College, Dongguan 523808, China

123

J Nanopart Res (2012) 14:799

DOI 10.1007/s11051-012-0799-3

However, optical delivery along arbitrary routes has

not been addressed yet. Here, optical delivery of

polystyrene nanospheres along arbitrary bending

nanofibers has been demonstrated by injecting

650-nm red light. Compared with planar waveguides,

nanofibers require no substrate, which maximizes the

fraction of the mode outside the nanofibers. Further-

more, they have the excellent flexibility in a three-

dimensional geometry, this makes the nanofibers

well-suitable for arbitrary bending. In addition, bio-

logical tissues have a window of high transmission

spectrum ranging from 650 to 900 nm, and polymer

can be used as carriers for drug delivery in tissue

engineering (Sokolsky-Papkov et al. 2007), therefore,

the use of 650-nm red light and this nondestructive

technique could find some potential applications in

delivering drug particles to desired position along

arbitrary bending nanofibers.

Theoretical analysis

Because optical loss is in connection with diameter

and bending radius of nanofibers, to gain an appropri-

ate diameter for optical delivery along arbitrary routes,

theoretical analysis was performed by a three-dimen-

sional finite difference time domain method. First,

electric (E)-field distribution of the 650-nm red light

at the interface between straight nanofibers (refractive

index n1 = 1.45) and surrounding water (refractive

index n2 = 1.33) has been simulated, as shown in

Fig. 1a. Figure 1b shows the simulated relation of the

normalized E-field and the nanofiber diameter for

straight nanofibers (i.e., bending radius R = ?).

It shows that, the maximum E-field at the interface

between the nanofiber and water occurs for diameter

of 450 nm. The diameters of the nanofibers for high-

efficiency delivery of the nanospheres by 650-nm red

light along a straight nanofiber are 400–600 nm. For

curved nanofiber, the bending loss is a critical factor

affecting the performance of optical propagation.

To find more suitable diameter nanofibers for optical

delivery along arbitrary bending nanofibers, the

bending loss of semicircular nanofibers with a bending

radius of R was simulated, as shown in Fig. 1c, which

was obtained by the ratio of output power of the

curving nanofiber and straight nanofiber with the same

length. Figure 1d shows the calculated bending loss

for different bending radius and fiber diameters

(D) ranging from 400 to 600 nm at the same input

power of 10 mW. It shows that the bending loss for the

same diameter nanofiber exhibits an exponential

attenuation trend with increasing the bending radius.

For the same bending radius, with an increase of

the nanofiber diameter, the bending loss decreases,

this means relatively better optical confinement.

Fig. 1 a Schematic

representation of a straight

nanofiber in water

environment. b Normalized

electric (E)-field distribution

of the 650-nm red light

versus nanofiber diameter at

the interface between

straight nanofibers and

surrounding water.

c Schematic representation

of a semicircular bent

nanofiber with a bending

radius of R. d Bending loss

versus bending radius with

diameters ranging from 400

to 600 nm

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Therefore, in this work, the diameter of nanofiber was

chosen to be 600 nm.

To further understand the bending property of the

600 nm nanofiber, the dependence of bending loss on

bending radii and central angles (hc = 90�, 180�, 270�and 360�) has also been investigated, as shown in

Fig. 2. It can be seen that the bending loss decreases

monotonically with the increasing bending radius for

each central angle. For instance, compared to the

circular bending nanofiber (i.e., hc = 360�), the

bending loss is as high as 38 dB for 5 lm bending

radius while it decreases to 2.2 dB for 80 lm bending

radius. For the same bending radius, with the increas-

ing central angle of the nanofiber, the bending loss

increases. It can be concluded that, for a specific input

optical power, there is a corresponding minimum

bending radius for optical trapping and delivery. In

other words, for a specific input optical power, when

the bending radius of nanofiber is larger than the

minimum bending radius, the nanospheres can be

trapped and delivered along the bending nanofiber.

Vice versa, the nanospheres will be escaped from the

bending nanofiber during the delivery process because

of relatively large bending loss.

Experimental setup

Figure 3 schematically shows the experimental setup,

nanofibers with a diameter of 600 nm were draw

from standard single mode optical fibers by means of

‘‘flame brushing’’ technology. In the experiment, a

bending nanofiber was fixed by two tunable micro-

stages, one end of which was connected to a 650 nm

laser source, which showed extremely low insertion

loss, the coupling loss between the source output and

the nanofiber input was estimated to be 0.4 dB,

which is much lower than that in Refs. (Lin et al.

2010; Cai and Poon 2010). The bending region of

nanofiber was immersed into a solution which was

formed by diluting 700-nm polystyrene spheres in

deionized water (volume ratio of spheres to water is

1:1,000). A computer connected CCD camera and

microscope were used for real-time monitoring. The

bending radius of the nanofiber could be changed

through adjusting the two microstages under an

optical microscope.

Results and discussion

Delivery of nanospheres was first demonstrated by

injecting the 650-nm red light into a 600-nm straight

nanofiber. The injected power was measured at the

output of the laser source using an optical power

meter. By increasing the injected optical power, it was

found that when the optical power was increased to

12 mW, the nanospheres (700 nm in diameter) near

the nanofiber can be captured by optical gradient force

and then delivered along the direction of light

propagation due to the scattering force induced by

the evanescent optical field (Neuman and Block 2004).

As an example, Fig. 4 shows three sequential images

of a single nanosphere which was delivered by a power

of 32 mW. The images were taken by the CCD with an

interval of 2.5 s (see Supplementary Movie 1). The

nanosphere exhibits as a bright spot due to scattering

of strong evanescent field. It can be seen that, within an

interval of 2.5 s, the nanosphere moved a distance of

Fig. 2 Bending loss versus bending radius at different central

angles of 90�, 180�, 270�, and 360� Fig. 3 Schematic of the experimental setup

J Nanopart Res (2012) 14:799 Page 3 of 7

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49 lm. The calculated delivery velocity is 19.6 lm/s.

Figure 5 shows the dependence of the measured

delivery velocities on injected optical power. The

results show that, average velocities of particles as

high as 35 lm/s were achieved at an input power of

46 mW. Fitted result shows that the delivery velocity

is linear with the input power. This is because the

optical force is proportional to optical power, which

was calculated by the integral of the Maxwell stress

tensor along the total external surface of the sphere

(Grier 2003). The velocity (v) of the sphere was

calculated by v = F/6prg, where F is the viscous force

which equals to the optical force in the propagation

direction of laser, r is the radius of polystyrene

spheres, and g is the room temperature dynamic

viscosity of water. Therefore, the velocity of the

sphere increases linearly with the input power.

Figure 6 shows the delivery of 700-nm nanospheres

along a bent nanofiber by injecting an optical power of

46 mW. The diameter of the nanofiber is 600 nm and

the bending radius is about 80 lm. Figure 6a shows

that three nanospheres A–C are trapped. Figure 6b

shows that after 2 s, the nanospheres B and C were

delivered to new locations along the fiber with

estimated distances of 48.4 and 51.8 lm, respectively.

The nanosphere A was trapped but not delivered. This

is possibly due to surface roughness of the nanofiber,

and the binding strength of the nanosphere A is larger

than the optical propulsion strength. By further

observation, it has been known that other nanospheres

can be delivered passing the fixed nanosphere A (see

Supplementary Movie 2). Compared Fig. 6a with b,

estimated delivery velocities of the B and C are 24.23

and 25.92 lm/s, respectively.

At the same input power of 46 mW, compared with

straight nanofiber, bending loss of 80 lm bending

radius nanofiber results in a slower nanosphere

velocity. In the experiment, nanofiber has also been

bended to an annular structure. Figure 7a shows

sketch of the annular configuration with an average

bending radius of about 52 lm while Fig. 7b shows its

optical image. For clarity, the structure is denoted as

regions I–III. J1 and J2 are coupling junction of the

nanofiber. Figure 7c–f shows four consecutive optical

images taken by the CCD with an interval of 5 s by

injecting a 40-mW power into the nanofiber, as

indicated by the white arrow in Fig. 7c. Figure 7c

shows that nanospheres A and B are trapped. After

t = 15 s, nanosphere A was delivered to region I

(Fig. 7f) from region III (Fig. 7c) via coupling junc-

tion J1 with an estimated average velocity of 5 lm/s.

Nanosphere B was delivered along II-J1-I-J2-II at a

velocity of about 7.34 lm/s. Figure 7e shows that, at

t = 10 s, nanosphere C near the nanofiber was trapped

and delivered in region I. At t = 15 s, nanosphere C

Fig. 4 The sequential

images of a 700 nm

polystyrene sphere

delivered along a straight

nanofiber at an injected

optical power of 32 mW

Fig. 5 Measured delivery velocity versus input optical power.

Error bars deviation of measurements, the delivery velocity is

linearly fitted with the input power

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moved to region III via the coupling junction J2 with

the average velocity of about 5.02 lm/s. Detailed

delivery process can be found in Supplementary

Movie 3.

Figure 8 (see also Supplementary Movie 4) shows

optical images of delivery process along a loop with a

radius of 32 lm at an optical power of 20 mW. Based

on the above analysis, nanospheres can be stably

transported along a straight nanofiber with a power as

low as 12 mW, according to simulated results, nano-

spheres should be trapped and propelled along the loop

with the bending radius up to 70 lm. With increasing

the input power, the required radius decreased. Con-

versely, decreasing power required larger bending

radius. Figure 8a shows that nanosphere A was trapped

and propelled. After t = 2 s, the nanosphere A was

delivered to a new location of 13 lm along the loop.

Estimated delivery velocity is 6.5 lm/s. The inset of

Fig. 8b shows the simulated electric field distribution.

Due to low photon density caused by bending loss,

after t = 13 s, the nanosphere A was escaped from the

trapping and suspended in water (Fig. 8c).

Figure 9 shows the dependence of the angular

velocity (x) on input power at bending radii of 32, 52,

Fig. 6 Optical images for

delivery of nanospheres

along a bending nanofiber

Fig. 7 a Sketch of an annular configuration. b Optical image of

the annular configuration, the insets indicate the simulated

optical field distribution at coupling region J1 and J2. c–f Four

sequential images taken by the CCD with an interval of 5 s for

delivery process along the annular configuration

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and 80 lm, which was calculated by x = v/R, where

v is the measured velocity and R is the bending radius

of nanofiber. It can be seen that, the angular velocity

increases with the increasing input power for each

bending radius. For the same input power, the angular

velocity decreases with the increasing bending radius

of the nanofiber.

Conclusions

Optical delivery of polystyrene nanospheres along

arbitrary bending nanofibers has been demonstrated by

injecting 650-nm red light. For a specific injected

optical power, there is a corresponding minimum

bending radius for optical trapping and delivery. When

the bending radius of nanofiber is larger than the

minimum bending radius, the nanospheres can be

trapped and delivered along the arbitrary bending

nanofiber. Otherwise, the nanospheres will be escaped

from the nanofiber during delivery process because of

relatively large bending loss. Since biological tissues

have a high transmission in 650-nm wavelength,

polymer can be used as carriers for drug delivery in

tissue engineering, therefore, this nondestructive

technique could find some potential applications in

delivering drug particles to desired position along

different routes.

Acknowledgments This work was supported by the National

Natural Science Foundation of China (Grants 60625404 and

10974261).

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