Magnetic particle biosensors 4

Chapter Optical Tweezers Chapter OPTICAL TWEEZERS 4.1 INTRODUCTION In the early 1970s, Authur Ashkin of Bell Telephone Laboratories first reported the observation of micron sized particles being accelerated and trapped in stable optical potential wells by utilizing only the radiation pressure created by a continuous laser [1, 2]. This revolutionary method led to the development of a single-beam, gradient force optical trap, commonly known as Optical Tweezers (OT) [3]. Optical tweezers have since been used in a vast variety of fields, ranging from that of fundamental physical sciences to biology, with new applications constantly being introduced [4-7]. Optical tweezers has the ability of applying pico-newton forces to micron sized particles making them a versatile tool for the study of colloidal systems. In such systems, transparent particles such as glass or polystyrene particles are commonly used as they have a higher index of refraction than their surrounding medium (typically aqueous), thus attracting them toward the region of maximum laser intensity [8]. More recently, superparamagnetic particles though not totally transparent, have also been added to the list of entities which can be manipulated by optical tweezers [9, 10]. It has been demonstrated that superparamagnetic particles can be manipulated by electromagnetic field induced forces from current flow in microfabricated wires as well as spin valve (SV) trapping elements [11-13]. Current carrying wires are able to move 69 Chapter Optical Tweezers single superparamagnetic particles (~ µm) to and from the active region of a sensor. However, this method is unable to selectively position a single particle over the sensor as the forces induced on the particle would cause it to move across the sensor surface. The other limitation is that current lines are unable to move single magnetic particles of small dimensions (~ 400 nm) across the sensor as they readily cluster in an external field, making it difficult for single particle detection [14]. In this chapter, a new combinatory approach will be presented where optical tweezers was used to trap and position a single superparamagnetic particle over a SV sensor, with the particle then detected by the sensor (Fig. 4.1.1). This approach is demonstrated using superparamagnetic particles of µm diameter (Micromer-M, Micromod) together with a SV sensor of dimensions x µm2. The observations will be explained by a model whereby the particle is taken as a pure dipole [15]. As mentioned in earlier chapters, there has been increasing interest in using MR sensing for the detection of single magnetic particles with the eventual goal of achieving a chip-based immunoassay [16, 17]. Our new approach not only allows very fine x, y, z adjustment of a single bead over a sensor in aqueous solution, but also uses a more sensitive MR detection method, the SV, compared to previous studies. 70 Chapter Optical Tweezers Focus lens Cover glass Bead Water electrode SV Al2O3 Passivation layer (30 nm) electrode substrate Fig. 4.1.1 Schematic detail showing the optical trapping of a magnetic particle. This chapter will start by describing the method of optical trapping. The fabrication methods adopted in order to use the SV sensors in a liquid medium will be discussed in section 2, while section will describe the experiments done to calibrate the laser power and the trapping forces on a single superparamagnetic particle. Sections and will cover the detailed experimental setup and results respectively, with section describing our model. 4.2 OPTICAL TRAPPING An optical trap uses the forces exerted by a highly focused laser beam in order to trap and manipulate microscopic dielectric objects. The beam is typically focused through a microscopic objective in order to achieve a narrow beam waist. Dielectric particles will be attracted to the centre of the beam waist due to the transfer of momentum from the 71 Chapter Optical Tweezers scattering of incident photons [18]. The particles are suspended in a surrounding liquid medium. The theories governing optical trapping can be broken down into two cases. The first is for dielectric particles of radius a much larger than the wavelength of the laser (i.e. a >> λ), and theory based on Mie’s approach is adopted. The second case is when the particles are much smaller than the wavelength of the laser (i.e. a > λ), the object behaves like a lens and photonic forces can be computed from ray optics [19]. The incident laser light undergoes refraction in the particle which corresponds to a change in the momentum carried by the stream of incident photons. The rate of change of momentum produces a resultant force on the sphere, which is proportional to the light intensity of the incident laser. With a dielectric sphere of refractive index larger than that of the medium, the refracted light will induce a force in the direction of the intensity gradient, causing the sphere to move towards the focal point. Conversely, with a dielectric sphere of refractive index smaller than that of the medium, the refracted light will induce a force directing the sphere away from the focal point (Fig. 4.2.1). 72 Chapter (a) Optical Tweezers b a (b) b a Fb Fa F F sphere Fb Fa b b water a air a water Fig. 4.2.1 Ray optics diagram indicating path of laser light passing through dielectric spheres. (a) Sphere with refractive index larger than surrounding media. (b) Sphere with refractive index smaller than surrounding media. Focal point is where the green and red rays intersect. In the Rayleigh regime, when the laser light is incident upon a small dielectric sphere (a [...]... Optical Tweezers 240 235 (a) (b) (c) (d) (e) (f) SV voltage signal (µV) 230 225 220 215 210 205 200 195 Magnetic particle 190 Laser beam 185 138 140 142 144 146 148 150 152 1 54 156 158 160 162 Time (s) Fig 4. 7.2 Real time response (without gain) of a SV due to the laser and a single 2 µm magnetic particle scanning across the device The sensor response is given by (V1-V0), where V1 is the measured voltage... figure 4. 4.2 From figure 4. 4.2, we estimate the critical velocity of the liquid v to be ~10.5 µms-1 for the 2 µm particles By using Eq 4. 5 and taking r =1 µm and the viscosity of water to be 1.002 x 10-3 Nsm-2, Fvis was calculated to be ~0.20 pN This is the trapping force experienced by a single 2 µm particle Cumulative particle number 9 100 95 70 40 10 1 0 11 12 13 10 11 12 13 (a) 35 10 (b) Particle. .. position with respect to SV sensor -12 (a) -10 -8 (b) -6 -4 -2 0 2 Laser y position ( ) (c) (d) 4 6 8 (e) 100 80 60 SV voltage signal (µV) 40 20 s1 0 s2 -20 -40 -60 -80 (j) -100 -12 -10 -8 (i) -6 -4 (h) -2 0 (g) 2 (f) 4 6 Laser y position (µm) Fig 4. 7.1 Response of a SV sensor during particle manipulation (a)-(e) Position of the laser spot and magnetic particle with respect to the SV sensor when the laser...Chapter 4 Optical Tweezers It was observed that the laser power fluctuated wildly for current values above 1.2 A (Fig 4. 4.1(a)), while laser power in the current range of 0.89 A to 1.2 A was relatively stable and could be fitted in this range to a polynomial y = 35.036 − 88 .41 4 x + 55.0 14 x 2 (4. 4) where y is the laser power and x is the laser current (Fig 4. 4.1(b)) In our experiments,... scan the particle moves with the laser spot i.e is trapped In the reverse scan (red data points), the magnetic particle was lost by the optical tweezers during one instant when the laser light was blocked Figures 4. 7.1(f)-(j) show the positions of the laser beam relative to the magnetic particle At positions 4. 7.1(g) and (h), the laser was already scanning across the sensor while the magnetic particle. .. trapping of a magnetic particle [15] In our experiments, low concentrations of 2 µm superparamagnetic particles (Micromer-M, Micromod) in DI water were sealed between a cover glass slide and SV chip using the PDMS window concept (see Fig 4. 5.3(a)) Permanent magnets were used to apply a magnetic field in the transverse direction (y) of the SV in order to induce a dipole moment in the magnetic particles... (μm/s) Fig 4. 4.2 Distribution curve of 1 04 measurements of the critical velocity (at which the optical tweezers can no longer trap the particle) (a) Cumulative counts of particle number vs critical velocity (b) Histogram of velocity measurements, fitted to a Normal distribution 4. 5 EXPERIMENTAL SETUP: COMBINED OT AND SV We used the optical tweezers to selectively manipulate single superparamagnetic particles... as in Section 4. 6 whereby single 2 µm magnetic particles were manipulated using an infrared laser (λ = 10 64 nm) IR lasers will be useful when working with biological samples 95 Chapter 4 Optical Tweezers REFERENCES [1] A Ashkin, Phys Rev Lett 24( 4), 156, (1970) [2] A Ashkin, Appl Phys Lett 19(8), 283, (1971) [3] A Ashkin, J.M Dziedzic, J.E Bjorkholm and S Chu, Opt Lett 11, 288 (1986) [4] A Ashkin, Science... single particle was positioned over the SV The laser was only blocked to measure the voltage drop across the SV from the magnetic particle once the particle was in position This can be done because the SV recovers from the laser interaction in ~ 100 ms (Section 4. 6(b)) Figure 4. 7.2 shows the typical real time voltage changes across a SV sensor during an experiment The laser is used to position a single magnetic. .. single magnetic particle at the center 91 Chapter 4 Optical Tweezers of the SV, (Fig 4. 7.2(a)), resulting in a peak due to the interaction between the laser and the sensor When the laser is blocked (Fig 4. 7.2(b)), the signal immediately returned to a stable value which was entirely due to the field generated by the single particle The laser is next used to capture the particle (Fig 4. 7.2(c)) and move . could be fitted in this range to a polynomial 2 0 14. 5 541 4.88036.35 xxy +−= (4. 4) where y is the laser power and x is the laser current (Fig. 4. 4.1(b)). In our experiments, the laser current used. 1.6 A (Fig. 4. 4.1). 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 0 1 2 3 4 5 6 7 8 9 Y = A + B1*X + B2*X^2 Parameter Value Error A 35.03626 5.365 34 B1 -88 .41 371 10.17539 B2 55.01365 4. 79123 . of 1 04 such velocities were obtained and the results are shown in figure 4. 4.2. From figure 4. 4.2, we estimate the critical velocity of the liquid v to be ~10.5 µms -1 for the 2 µm particles.