|
This zoom-in Scanning Electron
Microscope image shows a five-nozzle M3 emitter, where each nozzle
measures 10x12 microns.
Image by Daojing Wang, Lawrence
Berkeley National Laboratory |
"Proteomics has become an indispensable tool
in biological research, be it diagnostics, therapeutics, bioenergy or
stem cell research, and mass spectrometry is proteomics’ enabling
technology," said Daojing Wang, a scientist with Berkeley Lab’s Life
Sciences Division who leads the proteomics research group and was the
principal investigator behind the development of the multinozzle
nanoelectrospray emitter.
"Lab-on-a-chip technology has enormous potential
for proteomics research," Wang said, "but for this potential to be
fully realized, a major advance in interfacing microfluidics with mass
spectrometry is needed. Our device provides that interface."
Wang and Peidong Yang, a leading nanoscience
authority with Berkeley Lab’s Molecular Foundry and Materials Sciences
Division, and also a chemistry professor with the University of
California’s Berkeley campus, co-authored a paper on this work which
is being published by the American Chemical Society (ACS). The paper,
which is now available in the on-line version. is entitled: "Microfabricated
Monolithic Multinozzle Emitters for Nanoelectrospray Mass Spectrometry."
Other authors of the ACS paper were Woong Kim, a
postdoctoral fellow in the Molecular Foundry, and Mingquan Guo, a
postdoctoral fellow in the Life Sciences Division.
When the Human Genome Project was completed in
2003, giving scientists a complete catalogue of human DNA, the next
big effort focused on genomics, identifying DNA sequences that code
for proteins, aka, genes. With the identification of each and every
new gene, the emphasis shifts to determining the biochemical function
of its associated proteins.
All biological cells are constructed from
aggregations of proteins that interact with other protein aggregations
like an elaborate, finely choreographed network of interdependent
machines. This biomolecular machinery also controls nearly every
chemical process inside a cell, and forms much of the connectivity
that enable cells to come together into tissues and organs. One of the
first steps in proteomics research is to determine the identity and
modifications of individual proteins that make up a cell or tissue
sample. The principal means of doing this is through mass spectrometry.
Mass spectrometers use a combination of ionization
and magnets to separate a protein’s constituent peptides. Detection
and analysis of this mass spectrum can then be used to identify the
protein and quantify its presence in a sample. The most popular
technique today for ionizing a protein’s constituents for mass
spectrometry is to liquefy the protein and send it through
electrically charged capillaries – a technique known as electrospray
ionization. One of the best candidates for high throughput integration
of the detection and analysis processes is to interface the mass
spectrometers with lab-on-a-chip technology, where biological fluids
are introduced onto a microprocessor chip. However, microfluidic
analysis of proteins has been a separate process from mass
spectrometry - until now.
"Ours is the first report of a silicon/silica
microfluidic channel that is integrated monolithically with a
multinozzle nanoelectrospray emitter," said Wang. "This paves the way
for the large scale integration of mass spectrometry and lab-on-a-chip
analysis in proteomics research."
Each emitter consists of a parallel array of silica
nozzles protruding out from a hollow silicon sliver with a conduit
size of 100 x 10 microns. Multiple nozzles (100 nozzles per millimeter
was a typical density) were used rather than single nozzles in order
to reduce the pressure and clogging problems that arise as the
microfluidic channels on a chip downsize to a nanometer scale. The
emitters and their nozzles were produced from a silicon wafer, with
the dimension and number of nozzles systematically and precisely
controlled during the fabrication process. Fabrication required the
use of only a single mask and involved photolithographic patterning
and various etching processes.
Said Peidong Yang, "Once integrated with a mass
spectrometer, our microfabricated monolithic multinozzle emitters
achieved a sensitivity and stability in peptide and protein detection
comparable to commercial silica-based capillary nanoelectrospray tips.
This indicates that our emitters could serve as a critical component
in a fully integrated silicon/silica-based micro total analysis system
for proteomics."
Added Daojing Wang, "This is also the first report
of a multinozzle emitter that can be fabricated through standard
microfabrication processes. In addition to having lower back pressure
and higher sensitivity, multinozzle emitters also provide a means to
systematically study the electrospray ionization processes because the
size of each nozzle and density of nozzles on the emitters can be
adjusted."
According to Wang and Yang, the fabrication and
application of the microfabricated monolithic multinozzle emitters,
called "M3 emitters" for short, could be commercialized immediately
and should be highly competitive with current silica capillary
emitters in terms of cost and mass production.
"We are now in the process of creating a chip that
integrates sample processing and preparation as well as detection and
analysis," said Wang. "The ability to perform the full process on a
single chip has enormous commercial potential." |
