Supplement material:
Simulation model information:
A good understanding of the affinity immune binding process requires an extensive numerical model, which
couples the physics of the electrostatics, the energy equation, incompressible flow, and the adsorption
kinetics. Simulation is performed via Comsol Multiphysics 3.5a. The model settings and simulation results
are provided in the supplement material.
Fig. S1 shows geometry and boundary condition settings of the rapid in situ capacitive immunoassay. The
affinity binding occurs on the channel bottom, thus two geometries (the 2D model is for the channel, and the
1D model is for the reaction surface) are used to simulate the experimental work. The numerical model of
SAW array is shown in Fig. S1(a). Here the proposed 2D model consists of a micro chamber with two pairs
of symmetric electrodes on its bottom. Because the SAW electrode has a large slenderness ratio, the specific
binding occurs on the electrode surface, the substrate, as well as the side wall of the electrodes. On the other
hand, the 5/5/25/25µm electrode array has a small slenderness ratio, thus the specific binding surface comes
to be the channel bottom, as shown in Fig. S1(b).
In the 2D geometry model, the electrostatics, convection and conduction, incompressible Navier–Stokes,
convection and diffusion equations are used to solve the static electric potential V in the channel, the
temperature distribution T, ACET flow field u = (u, v), pressure p, concentration c, the concentration of
target antibody in the micro channel[S1, S2]. While the surface concentration, B, the concentration of the
bound antibody on the reaction surface, is solved in the 1D surface geometry.
(b)
-V0
V0
V0
-V0
V0
insulate, silicon heat flux, no slip
B
insulate, PDMS heat flux, no slip
symmetric, outlet,
convective flux
V, u, v, p,T, c, B
symmetric, outlet,
convective flux
symmetric, inlet,
concentration c0
insulate, PDMS heat flux, no slip
symmetric, inlet,
concentration c0
(a)
V, u, v, p,T, c, B
specific binding reaction, flux
-V0
V0
-V0
insulate, silicon heat flux, no slip
Specific binding surface
Specific binding surface
Fig. S1
B
Geometry and Boundary condition settings of the immunoassay
Simulation results:
By comparing the cases with and without AC electric field, significant increase in bounded molecules is
observed when AC fields are applied, much higher than the case without AC field. Simulation of ACET
and DEP velocities yielded vortices around electrodes for convection of macromolecules.
Fig. S2 provides the simulation results of the target concentration at different times (t=0s, 20s, 40s, and 60s).
Here the AC voltage is 1V as a numerical example, and the initial target antibody concentration is set as
1e-12M. The value of the association rate and the disassociation rate in the simulation[S3, S4] are
kon=1e8[mol/(mol•s)] and koff=0.02[1/s], respectively. Fig. S2(a) shows the condition of no voltage when
the target antibody movement is driven by nature diffusion only. When ACET flow is taken into
consideration, the antibody is transported by the flow convection, thus the concentration distribution is
changed, as shown in Fig. S2(b). If the DEP effect is included, as shown in Fig. S2(c) and Fig. S2(d), the
target antibody is strongly affected by the particle trapping near the electrode corners, so the amount of
targets decreases rapidly due to strong affinity binding.
(a) nature diffusion
(b)
ACET enhanced diffusion
(c) effects of DEP forces on target concentration
(d) effects of DEP forces on target concentration
(linear form)
(logarithmic form)
Fig. S2
Simulation results of target concentration at different times[S5]
For SAW electrodes, the electric fields are concentrated around the electrode corners and between the
electrodes, which is caused by an electrode thickness of ~150 nm and a separation of 1.1 µm. In comparison,
5/5/25/25 electrodes have a thickness of 100 nm and a separation of 5 µm. Therefore, with 5/5/25/25
electrodes electric field reaches further into the fluids and produce much larger ACET vortices than SAW
electrodes. Simulation also shows that bounded molecule numbers on 5/5/25/25 electrode is about 1.6 times
of that on SAW electrodes at 100mV/µm, agreeing with the data in Fig. 3 in the main text.
Although both geometries can include ACET flows, there are also differences between the flows generated
by symmetric and asymmetric electrodes. Two electrode geometries differ significantly in electric field,
temperature distribution and flow field patterns, leading to a remarkable difference in their ability to induce
ACET net flow. As mentioned above, while both electrode array are able to generate local ACET flow
vortex, there is no net flow for the SAW geometry electrodes due to the symmetric distribution of electric
and flow fields. As a result, there is no continuous supply of fresh serum at the binding sites. So the
concentration of target antibody decreases with time rapidly, as shown in Fig.S3 (a).
In contrast, the 5/5/25/25 electrode array breaks the symmetry of electric and flow fields, and there is a net
flow on top of the electrodes. The fluid flows from the smaller electrode to the bigger one[S6, S7]. Thus the
target antibody will be transported to the binding sites continuously in the whole process, and keep the target
concentration at a higher level than that in the SAW electrode array, as shown in Fig.S3(b). This is the basic
reason to obtain the experimental data in Fig.3(a).
(a) symmetric SAW electrode array
(b) asymmetric 5/5/25/25 electrode array
Fig. S3 Simulation results of target concentration distribution
Specificity of the method:
Fig. S4 shows the capacitance change rate of several control tests compared with the experimental group.
‘no antigen’ stands for the data of the JD positive serum loaded on to the array coated without JD antigen.
‘JD+’ stands for the data of the JD positive serum loaded on to the array well coated with JD antigen. ‘JD-’
stands for the data of the JD negative serum loaded on to the array well coated with JD antigen. ‘TB+, JD-’
stands for the data of the JD negative serum but turbulence positive loaded on to the array well coated with
JD antigen. Here this serum contains antibody similar as the JD antibody, but does not bind to the JD antigen.
The last one, ‘0.1×Buffer B’ , stands for the blank surrounding buffer without any antibodies.
dC/C per minute (‰)
-50
-40
-30
-20
-10
0
no antigen
JD +
Fig. S4
JD -
TB+, JD- 0.1xBuffer B
Specificity of the method
Limit of Detection and sensitivity:
The limit of detection (LOD) were also conducted for 5/5/25/25 electrodes, using goat anti-bovine IgG (H+L)
antibody with immobilized bovine IgG whole molecules. The sensitivity of this method is found by linear
curve fitting to be -6.059 ‰/(ng/mL) with a Pearson correlation coefficient R2 of 0.9889. The linear range of
this method is 0.2ng/mL to 10ng/mL. As the concentration goes higher, the capacitance change rate linear
fitting curve shows smaller slope.
References:
[S1] Yuan, Q., Wu, J., Biomedical microdevices 2013, 15, 125-133.
[S2] Yuan, Q., Yang, K., Wu, J., Microfluidics and Nanofluidics 2014, 16, 167-178.
[S3] Yang, K., Wu, J., Biomicrofluidics 2010, 4, 034106.
[S4] Myszka, D. G., Journal of Molecular Recognition 1999, 12, 390-408.
[S5] Li, S., Ren, Y., Liu, X., Hou, Z., Jiang, H., SCIENCE CHINA Technological Sciences 2014, 2,
219-228(in Chinses).
[S6] Hong, F., Bai, F., Cheng, P., Microfluidics and nanofluidics 2012, 13, 411-420.
[S7] Wu, J., Lian, M., Yang, K., Applied physics letters 2007, 90, 234103-234103-234103.