Fabrication of a cyclic olefin copolymer planar waveguide embedded in a multi-channel poly(methyl methacrylate) fluidic chip for evanescence excitation

Abstract

The fabrication and characterization of a novel cyclic olefin copolymer (COC) waveguide embedded in a poly(methyl methacrylate), PMMA, fluidic chip configured in a multi-channel format with an integrated monolithic prism for evanescent fluorescence excitation are reported. The fabrication approach allowed the embedded waveguide to be situated orthogonal to a series of fluidic channels within the PMMA wafer to sample fluorescent solutions in these channels using the evanescence properties of the waveguide. Construction of the device was achieved using several fabrication techniques including high precision micromilling, hot embossing and stenciling of a polymer melt to form the waveguide and coupling prism. A waveguide channel was fabricated in the fluidic chip's cover plate, also made from PMMA, and was loaded with a COC solution using a pre-cast poly(dimethylsiloxane), PDMS, stencil containing a prism-shaped recess. The PMMA substrate contained multiple channels (100 microm wide x 30 microm deep with a pitch of 100 microm) that were situated orthogonal to the waveguide to allow penetration of the evanescent field into the sampling solution. The optical properties of the waveguide in terms of its transmission properties and penetration depth of the evanescent field in the adjacent solution were evaluated. Finally, the device was used for laser-induced fluorescence evanescent excitation of a dye solution hydrodynamically flowing through multiple microfluidic channels in the chip and processed using a microscope equipped with a charge-coupled device (CCD) for parallel readout. The device and optical system were able to image 11 channels simultaneously with a limit-of-detection of 7.1 x 10(-20) mol at a signal-to-noise ratio of 2. The waveguide was simple to manufacture and could be scaled to illuminate much higher channel numbers making it appropriate for high-throughput measurements using evanescent excitation.

Fig. 1

(a) Molecular structure of Topas COC; x and y represent the monomer units, which are polymerized by metallocene catalyzed polymerization. The T_g and the refractive index, n, can be modified by increasing or decreasing the amount of norbornene (y) units in the monomer mixture during polymerization. (b) Schematic representation of the fluidic device with embedded COC planar waveguide with a monolithic coupling prism: (i) diagonal view, (ii) frontal view, and (iii) cross-sectional view. (c) Schematic representation of a portion of the device showing the multi-channel fluidic architecture and interconnected waveguide.

Fig. 2

(a) Schematic representation of the stepwise process for the fabrication of the embedded COC orthogonal waveguide in a PMMA chip. A relief was used for casting a PDMS pre-polymer (PDMS + curing agent at 10 : 1 ratio) to form the stencil, which contained the recess for molding the COC prism and an access reservoir to allow filling of the COC melt (i). The PDMS stencil was peeled from the relief after curing at 70 °C for 90 min (ii) and placed on the surface of a PMMA sheet, which would serve as the device cover plate, containing a pre-fabricated waveguide channel (waveguide channel was embossed from a mold master fabricated using high precision micromilling) (iii) and a COC melt (prepared using toluene as the solvent) was introduced into the assembly to form the waveguide and coupling prism (iv). The PDMS stencil was then peeled off from the PMMA cover plate, which created the embedded waveguide with the monolithic coupling prism (v). Finally, the PMMA cover plate with waveguide assembly was thermal fusion bonded to a PMMA substrate containing multiple fluidic channels (vi) that were prepared using hot embossing. The fluidic substrate and the PMMA cover plate were thermally fusion bonded at ~105 °C, near the T_g of both polymeric materials. (b) Photographs of the PMMA sheet showing the embedded waveguide with the integrated monolithic prism (to the right is the SEM of a section of the prism). (c) Optical micrograph of the embedded waveguide integrated to the fluidic channels.

Fig. 3

(a) Left: AFM image of the surface of the cured COC planar polymer waveguide embedded in sheet PMMA; z-scale is 4104 nm per div., and x-scale is 20 μm per div. Right: section analysis of the waveguide surface; top panel shows the surface roughness with RMS = 61.248 ± 0.112 nm. (b) Optical transmission spectra (600 nm–900 nm) of COC (black), cured COC waveguide (red), PMMA (blue), PC (polycarbonate) (green) and PDMS (purple). (c) Moisture resistance of COC compared to other polymers.

Fig. 4

The top panel shows a typical fluorescence image acquired with the CCD when light was launched into the waveguide with a fluorescent solution sandwiched between a cover slip and the COC waveguide surface. The bottom panel shows the resultant fluorescence intensity at different launch angles. The solid blue line represents the penetration depth plotted as a function of the launch angle using eqn (2).

Fig. 5

(a) Fluorescence image acquired from multiple fluidic channels (11 microchannels shown) filled with 100 nM AlexaFluor 647 when light was launched into the COC planar waveguide through the monolithic prism; there was a clear distinction between channels (with sample) showing fluorescence signal with fairly uniform intensity (bottom panel) and the inter-channel area showing dark background. The image was acquired with a 10 microscope objective (NA = 0.5). (b) Fluorescence image from the same device acquired with a 2× microscope objective (NA = 0.1) to clearly show the waveguide geometry.