June 29, 2006 -- Using unique nanoparticles that
convert laser light into useful information, Rice
University scientists have created the world's first
nano-sized pH meter.
The discovery, which appears online this week in
the journal Nano Letters, presents biologists with
the first potential means of measuring accurate pH
changes over a wide pH range in real-time inside
living tissue and cells.
"Almost every biologist I speak with comes up with
one or two things they'd like to measure with this," said
lead researcher Naomi Halas, the Stanley C. Moore
Professor of Electrical and Computer Engineering,
professor of chemistry and director of Rice's Laboratory
for Nanophotonics (LANP).
example, pH may be useful in determining whether
or not some cancer tumors are malignant. With current
methods, a piece of the tumor would need to be
physically removed via biopsy – a painful and invasive procedure – and
visually evaluated under a microscope. Halas said
LANP's new nano-pH meter could be used instead as
an "optical biopsy" to measure the pH inside the
tumor with nothing more invasive than an injection.
Halas's LANP team created the pH sensor using nanoshells,
optically tuned nanoparticles invented by Halas.
Each nanoshell contains a tiny core of non-conducting
silica that's covered by a thin shell of metal, usually
gold. Many times smaller than living cells, nanoshells
can be produced with great precision and the metal
shells can be tuned to absorb or scatter specific
wavelengths of light.
form the pH sensor, Halas' team coated the nanoshells
with pH-sensitive molecules called paramercaptobenzoic
acid, or pMBA. When placed in solutions of varying
acidity and illuminated, the nanoshell-molecule device
provides small but easily detectable changes in the
properties of the scattered light that, when "decoded," can
be used to determine the pH of the nanodevice's local
environment to remarkably high accuracy. Inspired
by techniques normally applied to image recognition,
the team formulated an efficient statistical learning
procedure to produce the device output, achieving
an average accuracy of 0.1 pH units.
term "pH" was coined by the Danish chemist Søren
Sørensen in 1909 as a convenient way of expressing
a solution's acidity. pH ranges from one – the most
acidic – to 14 – the most alkaline.
Co-authors on the paper include postdoctoral researchers
Sandra Bishnoi, now an assistant professor at the
Illinois Institute of Technology, and Muhammed Gheith;
graduate students Christopher Rozell and Carly Levin;
Bruce Johnson, distinguished faculty fellow of chemistry
and executive director of the Rice Quantum Institute;
and Don Johnson, J.S. Abercrombie Professor of Electrical
and Computer Engineering and Statistics.
The research was supported by the Department of
Defense's Congressionally Directed Medical Research
Program, the Air Force Office of Scientific Research,
the Keck Foundation, the Robert A. Welch Foundation
and by Texas Instruments.