Leak-free integrated microfluidic channel fabrication for surface plasmon resonance applications

Abstract

In this paper, we describe a novel fabrication method of a microfluidic integrated surface plasmon resonance (SPR) gold chip based on a (3-mercaptopropyl) trimethoxy silane (MPTMS) self-assembled monolayer. This monolayer was formed at the surface of a microfluidic chip made of polydimethylsiloxane (PDMS). Its presence was confirmed by contact angle and Fourier transform infrared spectroscopy measurements on the modified PDMS surface. A basic, but nevertheless appropriate, 4-channel microfluidic system was made on PDMS and reported on a gold SPR sensor. Sealing tests were carried-out by injecting continuous flows of solutions under gradient pressure up to 1.8 bar. Bonding strength of chemical and corona binding were measured and compared. The test of the integrated microfluidic SPR sensor on an SPR bench validated its functionality and proved as well that no leakage is observed between the different microfluidic channels.

PDMS Bonding Technologies for Microfluidic Applications: A Review

“…An alternative approach could be using a tensile testing machine in shear mode, as illustrated in Figure 1f. Although in some works the results of these tests are referred to as tensile strength [50], due to the parallel forces, it is more appropriate to refer to them as lap shear strength, which is defined as the failure load divided by the bond area [51]. There are two ways to perform this test: either two PDMS blocks can be bonded to one substrate (as in [50]), or two substrates can be bonded to one PDMS block to avoid the difficulty of clamping PDMS.…”

“…Although the discharge can be expanded to cover larger areas (with appropriate electrodes), the sampleelectrode distance, movement speed, treatment time are all factors that need to be precisely controlled [19]. It has to be noted that the discharge could be harmful to metallic structures, e.g., it has been reported to have damaged a 35 nm thick plasmonic gold thin film on glass beyond applicability [50]. Corona surface activation can also be used combined with chemical gluing [43,47].…”

“…The formation of a glassy, brittle SiO x layer [85] might not be desirable for some applications that rely on the flexibility of PDMS and a matching substrate. UV/ozone is also used for surface activation in chemical gluing [50].…”

“…For this purpose, surface activation + MPTMS functionalization of the PDMS could be used, relying on the terminal thiol groups to bond covalently to the gold surface. Bakouche et al successfully demonstrated this method with an SPR (surface plasmon resonance) chip and even obtained higher lap shear strength values than for PDMS-glass bonding with simple corona surface activation [50]. Parylene, an organic polymer with a para-xylylene backbone, has many favorable properties considering microfabrication or microfluidics [91].…”

“…Although metals cannot be considered as typical substrates in microfluidic fabrication, it has to be noted that by simple chemical gluing, PDMS can be conveniently bonded with various metals. The thiol-metal bonds, which were discussed in accordance with gold-coated glass [50], were successfully used to bond PDMS with iron, copper, aluminum, or brass by using a simple surface activation + MPTMS treatment protocol [43].…”