Mass is a fundamental physical property common to all substances. Accurate measurements of mass are critical for a broad range of investigations, but at the scale of single molecules, such sensitive detections are notoriously tricky, laborious and expensive.
Nongjian (NJ) Tao and his colleagues at Arizona State University's Biodesign Institute are developing a clever way to perform useful analyses of single molecules, including precise measurements of mass as well as of the binding affinity of these particles. The technique will find diverse applications in such areas as pharmaceutical drug screening, analysis of single cells, food safety, environmental science, biomedical research and clinical diagnosis.
A recent 4-year, $1 million grant from the W. M. Keck Foundation will enable Tao's group to build, test and further refine the mass detection method, which makes use of molecular oscillators. Oscillators are essential components found in everyday devices such as clocks, computers and radios. They also play a vital role in biological systems, involved in regulatory networks and rhythmic patterns of activity.
According to Tao, "the Keck grant provides us with a great opportunity to address how we can identify a single molecule based on precise measurement of its mass, and study its affinity properties without using labels. This capability will overcome the limitations of the current mass spectroscopy and immunoassay technologies for detection and functional analysis of molecules."
Until now, measurements of mass at the molecular scale have often been performed using elaborate and costly devices known as mass spectrometers, in which the mass to charge ratio of a molecule is detected after it has been ionized. The technique has been an enormous boon to science, allowing for detailed analysis of proteins, the identification of biomarkers for disease diagnosis, monitoring of environmental toxins, and chemical agent screening.
Nevertheless, conventional mass spectrometry has limits. The technique is slow, bulky and expensive. Further, elaborate sample preparation is involved and the method is not capable of providing real-time information about molecular binding affinity. Affinity information is often critical for understanding biological activities such as antigen-antibody and drug-target interactions as well as a variety of molecular functions.
The new approach provides a simple, fast and miniaturized means of conducting molecular mass measurements. Unlike conventional mass spectrometry, Tao's approach aims at detecting the binding of molecules in their native environment, including under aqueous conditions, allowing not only mass measurement but also functional analysis. (more)
The mechanical oscillators are self-assembled on a chip surface. This bottom-up approach permits rapid, accurate fabrication of these tiny structures, measuring just a few nanometers in size. The method also permits precise control over the oscillators' resonant frequencies, at the same time allowing simple detection of the oscillations with high throughput.
Tao's technique makes use of two quantities, known in advance with precision: each oscillator's mass and the spring constant--a measure of the connecting molecule's elasticity. The mass and spring constant of the oscillators are precisely defined, but they also can be tuned. This enables the resonant frequency of the oscillator to be adjusted from gigahertz to terahertz frequencies.
This frequency range is important, because it spans a region of the electromagnetic spectrum falling between infrared and microwave. As Tao notes, such frequencies are too low for conventional optical measurements and too high for typical electronic detections. To overcome these detection limits, the group will use specialized techniques.
A property known as surface plasmon resonance will be used for optical detection of the molecular oscillators and has been shown in preliminary experiments to be capable of detecting gold nanoparticles a few nm in size. The technique relies on the fact that coherent electron oscillations on a metal surface--known as plasmons-- can be optically excited, yielding a detectable signal.
In describing the new method, Tao says: "We use a molecular self-assembly method to fabricate mechanical oscillators, and a plasmonic method to read the oscillations."
Two gifted young researchers in Tao's laboratory in Biodesign's Center for Bioelectronics and Biosensors are critical to the endeavor. Shaopeng Wang heads up a team responsible for the fabrication and assembly of the oscillators, while Josh Hihath leads another research group focusing on the detection of the oscillators.
The project paves the way for versatile, low-cost mass spectrometry measurements that do not require ionization. If successful, the method will provide a powerful analytical tool for both basic and applied research, permitting close examination of single-molecule binding events, as well as evaluation of chemicals of environmental, security and biomedical relevance.
Written by: Richard Harth
Science Writer: The Biodesign Institute