Surface charge, electroosmotic flow and DNA extension in chemically modified thermoplastic nanoslits and nanochannels

Abstract

Thermoplastics have become attractive alternatives to glass/quartz for microfluidics, but the realization of thermoplastic nanofluidic devices has been slow in spite of the rather simple fabrication techniques that can be used to produce these devices. This slow transition has in part been attributed to insufficient understanding of surface charge effects on the transport properties of single molecules through thermoplastic nanochannels. We report the surface modification of thermoplastic nanochannels and an assessment of the associated surface charge density, zeta potential and electroosmotic flow (EOF). Mixed-scale fluidic networks were fabricated in poly(methylmethacrylate), PMMA. Oxygen plasma was used to generate surface-confined carboxylic acids with devices assembled using low temperature fusion bonding. Amination of the carboxylated surfaces using ethylenediamine (EDA) was accomplished via EDC coupling. XPS and ATR-FTIR revealed the presence of carboxyl and amine groups on the appropriately prepared surfaces. A modified conductance equation for nanochannels was developed to determine their surface conductance and was found to be in good agreement with our experimental results. The measured surface charge density and zeta potential of these devices were lower than glass nanofluidic devices and dependent on the surface modification adopted, as well as the size of the channel. This property, coupled to an apparent increase in fluid viscosity due to nanoconfinement, contributed to the suppression of the EOF in PMMA nanofluidic devices by an order of magnitude compared to the micro-scale devices. Carboxylated PMMA nanochannels were efficient for the transport and elongation of λ-DNA while these same DNA molecules were unable to translocate through aminated nanochannels.

High process yield rates of thermoplastic nanofluidic devices using a hybrid thermal assembly technique

“…Device 5 was bonded using a pressure of 370 kN/m 2 at 80°C for ~7 min as previously reported by our group. 34 Device 1 (U-COC/(PL-COC)) produced a bond strength that was too low for performing fluidic experiments because we experienced difficulty in filling the assembled device; filling caused delamination of the cover plate from the substrate. Using PMMA as the substrate produced devices with bond strengths of 66 ±7 m J/cm 2 and 68 ±7 mJ/cm 2 for U-PMMA/(PL-COC) and PL-PMMA/(PL-COC), respectively, that easily filled by capillary action.…”

“…As shown in Table 1, in both cases, the measured bond strengths were lower than that of the hybrid devices. Though PL-PMMA/(PL-PMMA) devices have been used for DNA transport studies, 15, 16, 34 the process yield rate for both devices was relatively low (<50% for PL-PMMA/(PL-PMMA) and <10% for U-PMMA/(U-PMMA)) due primarily to deformation and collapse of the nanochannels following thermal fusion bonding and possible delamination of the cover plate during an experiment. 15 As a comparison, using the optimized thermal fusion bonding conditions noted above for U-PMMA/(PL-COC) devices, the process yield rate was >90% with a similar value noted for devices consisting of U-COC/(PL-COC).…”

“…34 Herein, we evaluated the surface charge density of hybrid U-PMMA/(PL-COC) devices assembled with the optimum conditions reported above.…”

“…Conductance plots were generated using KCl solutions in the concentration range of 10 −6 to 0.1 M KCl following procedures previously reported. 34, 39 For UV/O 3 activation of the nanochannels, assembled devices were placed in a 265 nm UV chamber with the cover plate facing the light source and exposed to a 350 m J/cm 2 of UV/O 3 light through the COC cover plate. In all cases, fluidic devices were initially flushed with methanol/ultrapure water (50% v/v) followed by rinsing with deionized water.…”

“…All measurements were performed using the Axopatch 200B amplifier coupled to a Digidata 1440A digitizer with signal acquisition and analysis performed with the pClamp10 software. The average conductance generated from five trials was plotted against the electrolyte concentration in a log-log plot and the surface charge (σ s ) determined by fitting the graphs with the conductance equation; 34 $$G_T={10}^{3}true({\mu }_{{\mathrm{normalK}}^{+}}+{\mu }_{\mathrm{normalC}{\mathrm{l}}^{-}}true)c{N}_{A}e·\frac{nwh}{L}+2{\mu }_{\mathit{\text{opp}}}{\mathrm{\sigma }}_{\mathrm{s}}n\frac{false(w+hfalse)}{L}$$ where G T is the total measured conductance in the nanochannel, w, L and h are the nanochannel width, length and height, respectively, N A is Avogadro’s number, e is the electron charge (1.602 × 10 −19 C), c is the electrolyte concentration in mol/L, n is the number of nanochannels in the device and μ K+ and μ Cl − are the ion mobilities of K + and Cl − ions, respectively (μ K+ = 7.619 × 10 −8 m 2 /V s and μ Cl − = 7.912 × 10 −8 m 2 /V s) and μ opp ≈ μ K+ for the deprotonated carboxyl surface. Finally, we assessed the effects of electrolyte pH on the surface conductance using KCl solutions prepared over a pH range of 5 – 9 adjusted using HCl or KOH solutions.…”