Nanobeads Prove Useful for Making Optical Molecular Sensors

David L. Shenkenberg

Optical chemical sensors are employed for testing in numerous fields, from marine research to the aerospace and automotive industries to medicine and biotechnology. Such sensors often consist of indicator fluorophores in a polymeric matrix.

Researchers from University of Technology Graz in Austria have explored nanoscale poly(styrene-block-vinylpyrrolidone) beads as a novel matrix for developing optical chemical sensors. They used these nanobeads to develop sensors for oxygen, temperature, pH, chloride and copper ions, creating techniques for immobilizing the sensing chemistry to either the core or the surface of the particles.

The size of the beads varied from 100 to 500 nm, with a mean of 245 nm, as determined with a particle size analyzer from Malvern Instruments GmbH of Herrenberg, Germany. The researchers commented that the preparation was easy.

The spectra of novel sensors based on nanobeads were homogenous and did not overlap, making the particles useful for sensing multiple molecules simultaneously.

To characterize the optical properties of the nanobeads, the investigators used several devices. They excited some of the beads with a violet LED (405-nm FWHM) from Roithner LaserTechnik of Vienna, Austria, and detected the luminescence with a Hamamatsu photomultiplier tube. They measured luminescence phase shifts with a two-phase lock-in amplifier from Stanford Research Systems Inc. of Sunnyvale, Calif. They measured other beads with a Hitachi spectrometer or with a BMG Labtech microplate reader. The scientists monitored biological cell toxicity and penetration with a Carl Zeiss microscope equipped with a Jenoptik digital camera.

This microscopic image shows cultivation media containing E. coli (left) and Pichia pastoris (right) along with nanobeads stained with a temperature indicator. Reprinted with permission of Analytical Chemistry.

Overall, each nanobead exhibited relatively homogenous spectra with little spectral overlap with other beads, making them useful for measuring multiple analytes at once. In cases of spectral overlap, the fluorescence lifetime could be used to resolve the signals from the particles, the researchers noted. The nanobeads exhibited fast response times, indicating that measurement of rapid processes is possible, and they could be detected even in complex media. They also did not penetrate or induce toxicity in bacterial cells, evidence that they could be useful for biological imaging.

Analytical Chemistry, Feb. 1, 2008, pp. 573-582.