# Focus: Weighing DNA Down to the Zeptogram

Phys. Rev. Focus 27, 25
A proposed system for measuring tiny masses could be several times more sensitive than others because it avoids electrical connections, using laser techniques instead.

Researchers have proposed a technique for measuring the masses of tiny objects to within a zeptogram $(10-21$ grams), which would be several times more sensitive than previous techniques. Described 24 June in Physical Review B, the technique combines a collection of nanoscale tools and techniques that have already been demonstrated separately. A tiny vibrating bar will change its vibration frequency if connected to the even tinier object to be measured–such as a strand of DNA. The researchers propose using excitations of metallic electrons called plasmons to allow a laser to precisely measure such a bar’s altered vibration frequency. Using lasers, rather than wires, for the detection is the key to the high sensitivity, the authors say.

In recent years, many researchers have been exploring nanotechnologies to create more sensitive measuring instruments. For measuring masses, researchers have used tiny vibrating objects such as carbon nanotubes or silicon nanobeams, for example. Some of these have been sensitive to as little as 7-zeptogram mass changes, but they have all relied on electrical circuitry to communicate with the sample. Electric wires can soak up energy by heating up, and they don’t work well at the highest frequencies, where measurements often have the best sensitivity to small changes.

For an all-optical technique, Jin-Jin Li and Ka-Di Zhu of the Shanghai Jiao Tong University propose to combine components that have been demonstrated to work individually but never combined before. Their concept starts with a so-called nanomechanical resonator, a semiconductor bar tens of nanometers thick that acts like a plank across a trench, clamped at both ends. This nanobar will flex up and down with some natural frequency that will change slightly if a small mass is placed on it. The rest of the system is for measuring that tiny change in resonant frequency, without wires.

Built into the center of the nanobar is a quantum dot, a region of a different semiconductor material that confines any excited electrons. A quantum dot has a series of electron energy levels similar to those in an atom, and these levels have slightly different values in a vibrating nanobar, compared with a stationary one, because the dot size oscillates.

A standard laser technique could be used to measure the dot’s energies, but for high precision, the team proposes adding another piece to the system–a metal nanoparticle, perhaps a gold sphere a few nanometers across. Putting the nanoparticle above the dot allows electrons on the nanoparticle’s surface to interact with those in the dot.

The next step is for a pump laser to excite electrons in the dot/nanoparticle complex and a probe laser to measure its light absorption spectrum. The dot/nanoparticle complex generates electron waves called plasmons at a single frequency that show up as an extremely narrow line in the spectrum. This line’s frequency depends on the dot’s energy levels, which in turn depend on the mass of the nanobar. The mass of the added object can then be determined solely by the frequency shift of the sharp spectral line.

For an indium arsenide quantum dot, a gold nanoparticle, and a gallium arsenide nanobar, Li and Zhu calculated the effect of placing a 5081-zeptogram E. coli DNA molecule on the nanobar. The spectral peak would shift from 1.2 GHz for the bar alone to 1.200575 GHz. This shift would not be hard detect, and its position could be measured with high precision, thanks to the extremely narrow spectral line. The team says that the sensitivity would be several times higher than previous electrical methods using nanoresonators.

“This work proposes a very interesting and original idea of using optical techniques to measure masses down to the zeptogram range, says Hui-Chun Liu of the National Research Council of Canada in Ottawa. “The work may lead to new measurement techniques for the booming fields of nanoscience and technology.”

–David Harris

David Harris is a freelance science writer in Palo Alto, California.

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