Tank Circuit for Ultrafast Single-Particle Detection in Micropores

We present an ultrafast single submicron particle detection method based on a half-bowtie coplanar waveguide. The method is capable of resolving the translocation of these particles at a bandwidth greater than 30 MHz. We compare experimentally the simultaneous use of our radio-frequency technique with conventional dc based resistive pulse recordings and find that our method has a throughput that is enhanced by 2 orders of magnitude. The technique incorporates a microfluidic circuit and has the potential to be employed for screening microparticles and biological cells at frequencies in excess of 1 GHz.

Figure 1

(a) Schematic of the chip design. The gold CPW is patterned on a microscope glass slide and the micropore is embedded within the open end of the CPW signal line. Test particles are introduced in suspension and a suction drives them through the pore. (b) Scanning electron microscopy images of the fluidic reservoir of the sensing region and zoomed-in view of a micropore embedded between the metal electrodes. (c) Electric fields in the sensing regions of the half bowtie CPW open circuits. The half bowtie has a tapering signal line and a small protrusion from the open circuit ground conductor. (d) Power spectral density of the reflected voltage signal.

Figure 2

Schematic of the measurement setup. The rf signal is generated with a signal generator and fed into the sensor chip via an rf circulator. The sensor is connected to an impedance matching network comprised of a varactor diode, whose capacitance $C_{\mathrm{var}}$ can be tuned using an external dc source. A mixer is used to mix the reflected rf signal with a local oscillator with a fixed frequency difference for down-conversion of the signal. The response is measured with an oscilloscope (DSO) and the dc measurements are performed using a conventional amplifier.

Figure 3

(a) Direct measurement of $S_{11}$ vs the frequency and varactor voltage to determine the point of minimal reflection, for a dry device and a device with electrolyte solution. The voltage and frequency step size is 10 mV and 150 kHz, respectively. (b) Sections to the measurement seen in (a) for the best frequency and voltage response. (c) Measured translocation events of a suspension of polystyrene beads with diameters of 500 nm and $1\mu \mathrm{m}$, respectively. The red trace shows the dc measurement with a cutoff frequency of 3.4 kHz. The blue trace shows the corresponding rf trace that is acquired with an IFBW of 1 kHz. Alignment of the traces is exercised by determining the covariance for every time delay until a maximized covariance is found. (d) Enlarged views into single events in the data partially shown in (c). The dc measurement (red trace) was inverted to show the correlation between the two measurements.

Figure 4

(a) Simultaneous rf (blue) and dc (red) measurement of $1\mu \mathrm{m}$ sized beads translocating through the half bowtie CPW sensor. The bead suspension used for those experiments were monodispersed in 145 mM NaCl, 5 mM KCl, 1.8 mM ${\text{CaCl}}_2$, 10 mM HEPES in DI water resulting in a $p\mathrm{H}=7.4$. (b) An enlarged view of the reflected rf signals obtained from the beads passing through the sensor pore.