Differential Plasmonic Sensors Mustafa FIRAT Department of Physics, Bilkent University, Ankara, 06800, Turkey Abstract – In this project we are aiming to fabricate a surface plasmon resonance sensor inside a microfluidic channel. Microfluidic systems provide simple means to control and manipulate fluids at micrometer scales. Integration of microfluidic systems and plasmon resonance sensors could provide very sensitive refractive index sensors with very small detection volume. In this project we will use a grating surface coated with a 50nm Ag layer will be used to excite surface plasmons. Furthermore we would like to improve detection sensitivity by performing a phase sensitive measurement. Introduction use gold, because of its dielectric properties. Also, gold does not react with The aim of this project is to generate a oxygen and its surface is not oxidized in sensor that can measure physical, chemical time. Since we need its surface in order to and biological properties of fluids. By generate a surface plasmon, the surface combining the special characteristics of should not be oxidized for a good sensing. surface plasmons and microfluidic These are basic advantages of using a gold channels, we can get information about the layer to generate a surface plasmon. fluids that flow inside the microchannels. First of all, we inject two fluids into the Indeed, there is a distinction between microchannels. For the first tries, we will surface plasmons and surface plasmon use water and 10% Ethylene Glycol polaritons. At the boundary of the metal, because they are both transparent and there there are a huge number of electrons due to is a sensible difference between their index the free electrons of the metal. Since these of refraction, so in their dielectric are not binded to nucleus, they can be constants. Dielectric constants of the oscillated easily with an external electric medium determine the resonance condition field. If we can apply correct frequency of the surface plasmon sensor. We will and give the correct energy with the correct take advantage of the laminar flow inside momentum to these free electrons, they the microchannels to change the medium start oscillating with an exact frequency. on the surface plasmon sensor. In order to Actually they generate a plane wave in this do so, we should know Reynolds number. electron sea. This is called surface plasmon. And if the surface plasmons are Surface Plasmon Resonance coupled with a photon, the resulting hybridized excitation is called a surface Surface plasmons, also known as surface plasmon polariton (SPP). And we will plasmon polaritons, are the surface generate SPPs by sending photons from a electromagnetic waves that propagate in a Helium-Neon laser. And the technique of direction parallel to the metal/dielectric (or excitation of surface plasmons by light is metal/vacuum) interface. [1] In our setup, denoted as a surface plasmon resonance we use gold as the metal surface and we (SPR) for planar surfaces. use water-10% Ethylene Glycol laminar flow as the dielectric surface. Since all the The electric field of a propagating metals have free electrons, actually surface electromagnetic wave, here our light beam plasmon can be generated with almost all coming from Helium-Neon laser, can be metals. The condition is; dielectric expressed as constant of the metal must be negative and most of the metals have this property. We $ʚ&ī3ͮ&ĭ5ͯh/ʛ (1) ̿ Ɣ ͙̿ͤ where k is the wave number and ω is the the coming photon, from eq (4). If we frequency of the wave. By solving equate k x and k SPP we get Maxwell's equations for the ͥ⁄ ͦ electromagnetic wave at an interface h TuTv (9) ͤͧ͟͢͝ Ɣ ʠ ʡ between two materials with relative TuͮTv h dielectric constants ε1 and ε2 (see figure 1) Here we can see that k 0 and cancels we get each other according to (7). Since this is the horizontal component of the total & & ĭu ƍ ĭv Ɣ 0 (2) momentum at the same time, we can add g Tu Tv factor of the grating to this equation and according to (5). And finally we get ͦ ͦ ͦ h ͟ ƍ ͟ Ɣ ʠ ʡ ͝ Ɣ 1,2 (3) T T ͥ⁄ ͦ Z 3 5$ $ ͧ͢͝ Ɣ ʠ u v ʡ ƍ ͡ (10) where c is the speed of light in the vacuum. TuͮTv Solving these two equations, the dispersion This angle is the plasmon angle for a relationship for a wave propagating on the plasmon that that will be take place surface is between gold - 10% Ethylene Glycol interface. The only thing that we should do ͥ⁄ ͦ is to obtain the correct angle that gives the h TuTv (4) correct momentum in x direction. ͟3 Ɣ ʠ ʡ TuͮTv From the property of the grating, this momentum is shifted by g factor of the grated surface. (5) ͟ Ɣ ͟3 ƍ ͛͡ Here, m is a positive integer that describes first, second, third... order shifting to the momentum (comes from the grating) and g is the g factor of the grating, given by Z (6) ͛ Ɣ Figure 1 : Coordinate system for two material where, λ is the wavelength of the coming interfaces and drawing of the vectors that were used photon and is the periodicity of the in equations 1, 2, 3, 4 and 8. grating. We used a grating that has a periodicity = 600 nm and we used Helium-Neon laser that has a wavelength 632,8 nm. By knowing the wavelength of Reynolds Number and Laminar Flow the Helium-Neon laser, we can get the momentum of the photons as Laminar flow occurs when a fluid flows in h parallel layers, with no disruption between (7) the layers. In fluid dynamics, laminar flow ͤ͟ Ɣ And one can get the horizontal component is a flow regime characterized by high of the momentum of the coming photon momentum diffusion and low momentum [2] from k 0. If k 0 is the total momentum of the convection. There can be small mixing coming photon, k x is the x component of k 0 at interference layer of the fluids due to that is horizontal to the propagation of the diffusion but there is no mixing due to SPP. We can find k x from k 0 easily by turbulence in laminar flow. Turbulent flow (8) is just opposite the laminar flow. To ͟3 Ɣ ͤͧ͟͢͝ And we know the momentum of the understand laminar flow in microchannels, surface plasmon that will be generated by we should examine Reynolds number at first. Reynolds number is an important parameter in the equations that describe photolithography technique. The gratings whether the flow will be laminar or that we use were fabricated by a graduate turbulent. It is a dimensionless number. student in the lab. Fabricating a grating is Reynolds number is about flow geometry. not a part of this project. Furthermore, The fluid properties; density and viscosity, there are several companies fabricating plus flow velocity and a characteristic such gratings. One can design the shape length or characteristic dimension and sizes of the gratings on the computer determine the Reynolds number. By and get them prepared to those companies. characteristic length or characteristic It is the easy way. dimension, we mean diameter, width, height... etc. For cylindrical shapes diameter is the characteristic dimension but since we have rectangular channels, we will use the height of the microchanells that is in the order of 100 µm. Having one dimension in µm is enough to see laminar flow easily. That’s why laminar flow can occur even with water in microchannels but it is very hard to observe a laminar flow in pipes in daily life. According to the definition, Reynolds number is given by ` (11) Figure 2 : Silicon masters used to mold elastomeric ͙͌ Ɣ [ Ɣ \ Ɣ \ microchannels. We have used pattered SU8 where: photoresist as a master. These masters were fabricated in the clean room by a graduate student in the lab. • V is the mean fluid velocity (m/s) • L is a characteristic length, (traveled length of fluid) (m) • µ is the dynamic viscosity of the Design of Microfluidic Channel fluid (Pa·s or N·s/m² or kg/m·s) • ν is the kinematic viscosity ( ν = µ / For design of microchannels, again we use ρ) (m²/s) silicon masters. These masters are prepared • ρ is the density of the fluid (kg/m³) with the same procedure as grating • Q is the volumetric flow rate (m³/s) masters. Photolithography technique is the • A is the pipe cross-sectional area most commonly used method. As shown in (m²) [3] figure 2, we get a silicon master that has a raise with height 100 µm. When we pour In our microchannels, if the Reynolds liquid Polydimethylsiloxane (PDMS) onto number is less than 2300, we get a laminar our master, and keep it on the hot plate at flow. 70 oC for one hour, PDMS becomes amorphous solid. And we get our microchannels at the bottom of this elastic PDMS structure. This is the upper part of Grating Fabrication the channels. We need a cover to the bottom of the channels so that water and We used silicon grating masters to get a ethanol will pass through. Also, this part is grating on our PDMS. These gratings are the layer where we will generate the SPs. periodic on the silicon master and the So, grating and the golden layer will be in periodicity of the grating is 600 nm. These this part. We coated this second layer of silicon grating masters are prepared by PDMS by gold with a thickness 50nm.