Manufacturing Bits: Sept. 9

Whispering sensors
Inspired by a whispering technology from a famous cathedral, Washington University in St. Louis and Tsinghua University in China have developed a new sensor that can detect and count nanoparticles down to 10nm and perhaps below.

Researchers have devised a Raman microlaser sensor in a silicon dioxide chip. The microsensor is called a whispering gallery mode resonator (WGMR). The device works much like the whispering gallery in London’s St. Paul’s Cathedral. In this structure, a whisper against the wall is audible on the opposite side.

Arrays of self-referenced and self-heterodyned Whispering-Gallery Raman microlasers for single nanoparticle detection. (Source: Washington University)

The microsensor does much the same thing with light frequencies. The WGMR is a mini-laser that supports “frequency degenerate modes,” according to researchers. The sensor does not require rare-earth ions to provide optical gain for the microlaser.

Applications include the electronics, acoustics, biomedical, plasmonics, security and metamaterials fields. “Our new sensor differs from the earlier whispering gallery sensors in that it relies on Raman gain, which is inherent in silica, thereby eliminating the need for doping the microcavity with gain media, such as rare-earth ions or optical dyes, to boost detection capability,” said Sahin Kaya Ozdemir, a research scientist, on the university’s Web site. “This new sensor retains the biocompatibility of silica and could find widespread use for sensing in biological media.”

Lan Yang, the Das Family Career Development Associate Professor in Electrical & Systems Engineering at Washington University, added: “It doesn’t matter what kind of wavelength is used, once you have the Raman laser circulating inside and there is a molecule sitting on the circle, when the beam sees the particle it will scatter in all kinds of directions. Initially you have a counterclockwise mode, then a clockwise mode, and by analyzing the characterization of the two split modes, we confirm the detection of nanoparticles.”

2D biosensors
The University of California at Santa Barbara (UCSB) has demonstrated a thin and ultrasensitive biosensor based on a molybdenum disulfide (MoS2) field-effect transistor (FET) technology.

FET-based biosensors offer fast, cheap and label-free detection. The industry has devised FET-based biosensors using nanotubes and nanowires, but there are various fabrication challenges with these materials, according to researchers.

Concept art of a molybdenum disulfide field-effect transistor based biosensor (Source: UCSB)

Emerging 2D materials, including MoS2, provide high sensitivities and are easier to fabricate, according to researchers. Graphene is also a 2D material. The MoS2-based FET biosensor is 74 times more sensitive than graphene, according to researchers.

In fact, researchers demonstrated a MoS2-based pH sensor, which achieved sensitivities as high as 713 for a pH change by 1 unit along with efficient operation over a wide pH range. Ultrasensitive and specific protein sensing is also achieved, according to researchers.

“Monolayer or few-layer MoS2 have a key advantage over graphene for designing an FET biosensor: They have a relatively large and uniform band gap (1.2-1.8 eV, depending on the number of layers) that significantly reduces the leakage current and increases the abruptness of the turn-on behavior of the FETs, thereby increasing the sensitivity of the biosensor,” said UCSB professor of electrical and computer engineering Kaustav Banerjee, on the university’s Web site.

Deblina Sarkar, a PhD student, added: “While one-dimensional materials such as carbon nanotubes and nanowires also allow excellent electrostatics and at the same time possess band gap, they are not suitable for low-cost mass production due to their process complexities. Moreover, the channel length of MoS2 FET biosensor can be scaled down to the dimensions similar to those of small biomolecules such as DNA or small proteins, still maintaining good electrostatics, which can lead to high sensitivity even for detection of single quanta of these biomolecular species.”

Water monitoring with 3D printed sensors
The University of Bath has developed a low-cost sensor using 3D printing technology. The sensor could be used in developing countries to monitor the quality of drinking water without expensive lab equipment.

In the lab, a small-scale, single chamber air-cathode microbial fuel cell was fabricated by 3D printing. The sensor contains bacteria, which produces an electric current. The current drops when the bacteria is disturbed by toxins in the water. This, in turn, alerts that pollutants are in the water.

The linear detection range was 3–164 ppm. It had a sensitivity of 0.05 μA mM−1 cm−2,according to researchers. The response time was as fast as 2.8 minutes. Meanwhile, at saturating acetate concentrations, the miniature fuel cell could rapidly detect the presence of cadmium in water with high sensitivity. When the cell was with fresh wastewater and no pollutant, the initial steady-state current was recovered within 12 minutes.