Matthew L. Johnston

Debye Screening in Single-Molecule Carbon Nanotube Field-Effect Sensors

Point-functionalized carbon nanotube field-effect transistors can serve as highly sensitive detectors for biomolecules. With a probe molecule covalently bound to a defect in the nanotube sidewall, two-level random telegraph noise (RTN) in the conductance of the device is observed as a result of a charged target biomolecule binding and unbinding at the defect site. Charge in proximity to the defect modulates the potential (and transmission) of the conductance-limiting barrier created by the defect. In this Letter, we study how these single-molecule electronic sensors are affected by ionic screening. Both charge in proximity to the defect site and buffer concentration are found to affect RTN amplitude in a manner that follows from simple Debye length considerations. RTN amplitude is also dependent on the potential of the electrolyte gate as applied to the reference electrode; at high enough gate potentials, the target DNA is completely repelled and RTN is suppressed.

FBAR-CMOS Oscillator Array for Mass-Sensing Applications

Thin-film bulk acoustic resonators (FBAR) are an effective platform for sensitive biological and chemical detection, where their high operating frequencies make them many times more sensitive than a quartz crystal microbalance. Here, we present a monolithic, solidly mounted FBAR oscillator array on CMOS for mass-sensing applications. Through monolithic integration with CMOS drive circuitry, we aim to overcome the spatial and parasitic load limitations of externally coupled resonators to build dense sensor arrays without specialized fabrication techniques. The sensors in this work are constructed in a 6 × 4 array atop a 0.18 ¿m CMOS active substrate, and mass sensitivity comparable to off-chip FBAR sensors is demonstrated.

Polydimethylsiloxane based microfluidic diode

In this paper, we present a novel elastomer-based microfluidic device for rectifying flow. The device is analogous to an electronic diode in function since it allows flow in one direction and stops flow in the opposing direction. The device is planar, in-line and can be replica molded via standard soft lithography techniques. The fabrication process is outlined in detail and follows a simple procedure that requires only photolithography and one replica molding step. Several geometries of devices are presented along with their flow versus pressure characteristics. A brief discussion of the device behavior is presented along with possible uses for the device.

Thermal management in microfluidics using micro-Peltier junctions

We report refrigeration and heating of nanoliter fluid volumes with micro-Peltier junctions. The temperature of small liquid reservoirs can be rapidly changed and controlled within a range between -3 degrees C to over 120 degrees C with good long-term stability. These thermal management systems enable the fabrication of complex chip-based chemical and biochemical reaction systems in which the temperature of many processes can be controlled independently.

Exploring the limits of ultrafast polymerase chain reaction using liquid for thermal heat exchange: A proof of principle

Thermal ramp rate is a major limiting factor in using real-time polymerase chain reaction (PCR) for routine diagnostics. We explored the limits of speed by using liquid for thermal exchange rather than metal as in traditional devices, and by testing different polymerases. In a clinical setting, our system equaled or surpassed state-of-the-art devices for accuracy in amplifying DNA∕RNA of avian influenza, cytomegalovirus, and human immunodeficiency virus. Using Thermococcus kodakaraensis polymerase and optimizing both electrical and chemical systems, we obtained an accurate, 35 cycle amplification of an 85-base pair fragment of E. coli O157:H7 Shiga toxin gene in as little as 94.1 s, a significant improvement over a typical 1 h PCR amplification.

Integrated VOC vapor sensing on FBAR-CMOS array

This paper reports first results of volatile organic compound (VOC) detection on a monolithically integrated film bulk acoustic resonator (FBAR) array on a silicon integrated circuit substrate. The combined sensor platform uses thin polymer layers as gas absorbers for individual FBAR functionalization, and frequency shifts are measured on-chip in response to changing VOC concentration. Integrating sensors, drive, and readout functionality on a single CMOS die enables a robust, multiplex sensor platform and obviates external measurement equipment.

An array of monolithic FBAR-CMOS oscillators for mass-sensing applictions

We present a monolithic, solidly-mounted CMOS-FBAR oscillator array for mass sensing applications. Thin-film bulk acoustic resonators (FBAR) are an affective platform for sensitive biological and chemical detection, where their high operating frequencies make them many times more sensitive than a quartz crystal microbalance. By monolithic integration with CMOS drive circuitry, we aim to overcome the spatial limiations of externally-coupled resonators to build dense sensor arrays without specialized fabrication techniques. The sensors in this work are constructed in a 6 × 4 array atop a 0.18µm CMOS active substrate, and mass sensitivity comparable to off-chip FBAR sensors is demonstrated.

On-chip high-voltage SPAD bias generation using a dual-mode, closed-loop charge pump

Single-photon avalanche diodes (SPAD) are high sensitivity photon detectors used in time-resolved imaging and very low-light applications. Operation as an avalanche detector requires reverse bias beyond the breakdown voltage, which is typically much higher than supported voltages in a standard CMOS process. In order to realize a fully integrated photon detection system using SPAD devices, this high-voltage bias needs to be generated on chip. Generated voltages beyond the breakdown threshold of the process pose particular design challenges, especially for output sampling and closed-loop regulation. This paper presents design and architectural techniques used to implement a closed-loop DC-DC converter that can be fully integrated on-chip and produces a regulated output voltage exceeding 15 V. Particular design constraints imposed by high on-chip voltages are addressed, and presented simulation results are based on an implemented design in a standard 130 nm CMOS process. The charge pump generates an output voltage of 15 V in 2 μs from start, with a modeled SPAD load of 10 pF. Following an avalanche current pulse, the recovery transient time is <150 ns to settle within 140 mV of the desired output voltage.

Portable real-time PCR system using tablet-based fluorescence imaging

The quantitative polymerase chain reaction (qPCR) is a key medical tool for diagnosing and monitoring viral infections. Due to the high cost and large size of existing qPCR machines, it is rarely viable for remote and resource-limited areas. A portable and affordable instrument for qPCR could make a significant difference in the accessibility of this important diagnostic technique across the world. In this work, a solution is proposed that uses widely available technology found in mobile phones and tablet computers, integrated with an affordable battery-powered thermal cycler, to cheaply and effectively run real-time PCR reactions. The demonstrated prototype performs 2-step and 3-step PCR reactions, and fluorescence is measured in real time using a tablet-integrated camera. These results serve as a proof-of-concept for the use of smartphones and tablets as quantitative image processing devices to enable portable, battery-powered qPCR instrumentation.