Enabling Subcellular pH Sensing

Daniel S. Burgess

A team of scientists at Rice University in Houston has reported that gold nanoshells — structures on the order of 100 nm in diameter comprising a nonconducting core and an outer layer of gold — can be functionalized to yield surface-enhanced Raman scattering spectra that indicate the pH level at that site. The investigators propose that the nanoparticles may enable applications in the biosciences based on the real-time sensing of chemical changes at the subcellular level, including the noninvasive identification of cancerous tumors and the monitoring of transplanted tissues for early signs of rejection.

Naomi J. Halas, director of the university’s Laboratory for Nanophotonics, characterized the development as an evolution in the research into such nanoscale structures from the theoretical to the practical.

“Most work in the nanoparticle field has been on how to make nanoparticles and studies of their properties,” she explained. “What we have done instead is to make nanoparticles that are essentially light in/light out nanodevices that report back specific information with high accuracy — in our case, (about) pH — at an accuracy similar to what could be measured by standard equipment at the laboratory bench.”

To do so, Halas and her colleagues exploited the fact that nanoshells enhance local electromagnetic fields through the action of plasmons, optical excitations coupled with oscillations of their conduction electrons. The optical resonance of the particles is a function of the relative size of their core and shell, and the investigators thus designed structures that would amplify the near-IR spectroscopic response of a pH-sensitive monolayer of paramercaptobenzoic acid adsorbed to the surface of the nanoshells. To optimize the performance for biological sensing, they further chose dimensions that also would yield a far-field absorbance maximum in the near-IR, ensuring high transmission of the excitation signal for interrogating the nanoparticles through blood and tissue.

In characterizing the performance of the functionalized nanoshells, the investigators employed a Raman microscope from Renishaw plc of Wotton-under-Edge, UK, to excite particles deposited on a silicon substrate and exposed to phosphate-buffered saline of various known pH values. They collected the backscattered signal with a 63× water-immersion lens from Leica Microsystems GmbH of Wetzlar, Germany, using an integration time of 30 s to yield high-resolution Raman spectra from spots on the sample.

They discovered several reproducible changes in the spectra that could be used to assess the pH level in the region immediately around a nanoparticle. In a proof-of-principle demonstration of the application of the structures for pH sensing, they applied a locally linear manifold approximation algorithm to multispectral data sets collected at pH intervals of 0.2 for solutions with pH values of 5.80 to 7.60, which they selected for its potential importance in distinguishing more acidic cancerous cells from healthy ones. Using this information, the researchers obtained estimates of 6.47 and 7.01, respectively, for the pH values of solutions known to be 6.10 and 7.15.

Halas said that the nanoshell-based approach to sensing pH is unique in that it is repeatable, highly site-specific, nontoxic and suitable for use over a relatively wide range of pH values.

“Cells are a highly complex and compact chemical environment,” Halas said. “Measuring changes in pH would indicate changes in what proteins are being manufactured in the cell, which could indicate other conditions, such as the onset of disease, a change in disease pathology or response to stress or a stimulus — chemical or physical.”

The researchers are collaborating with interested parties in hopes of developing nanoparticle-based sensors for assessing cell viability under various conditions.

Nano Letters, August 2006, pp. 1687-1692.