For the chemist, small is not only beautiful but also potentially cheap, quick, efficient and portable. The laboratory, with its racks of test tubes and glassware, spectrometers and balances, is being shrunk to the size of a single silicon chip. It should soon be possible to conduct a detailed chemical analysis of very small samples -- for medical and environmental monitoring or forensic science, for example -- directly at the scene, rather than in some remote lab.
Now a device that measures the acidity or pH of fluids at a scale of ten thousandths of a millimetre, reported in the journal
The new pH meter, developed by Scott Manalis of the Massachusetts Institute of Technology and colleagues, is a chimera of two existing devices. The measurement of acidity is conducted by a 'light-addressable potentiometric sensor' or LAPS, a solid-state device that can be made out of silicon just as microelectronic circuits are made on chips. This is attached to the roving probe tip of an atomic force microscope (AFM), a device for mapping the shape of a surface at the molecular scale.
The pH-sensing region of the LAPS consists of a silicon protrusion coated with a thin layer of silicon nitride. The silicon structure is wired into an electrical circuit, and when it is illuminated with a laser beam, it generates an electrical current in much the same way as a silicon solar cell produces electricity. The light beam comes from a miniaturized, solid-state laser device. The crucial feature of the LAPS is that the current generated in the silicon depends on the electrical charge on the surface of the thin silicon nitride layer that sits on top.
This layer contains chemical groups that can form bonds with hydrogen ions (positively charged hydrogen atoms). The greater the concentration of hydrogen ions in a solution, the more acid it is and the lower its pH. An alkaline solution has a high pH. The pH of neutral water is 7; that of lemon juice is around 2.3.
In an acid, more hydrogen ions bind to the silicon nitride film, giving it a greater positive charge. This is registered by a change in the light-induced current from the silicon element of the LAPS. It can detect differences in acidity of just 0.01 pH units.
The AFM consists of a tiny, stiff, flat cantilevered arm with a fine needle-like tip, rather like a miniaturized record-player head. This scans the surface of a sample, the tip rising and falling with any bumps and pits. Motion of the tip can be detected by bouncing a laser beam off its back. Most AFM arms are made of silicon nitride, and Manalis and colleagues fashioned a tiny LAPS device directly onto the end of such a cantilever, and used the accurate positioning mechanism of the AFM to produce pH scans over a surface. An added advantage is that the device, operated as an AFM, can simultaneously take snapshots of the sample.
The researchers used their new 'scanning probe potentiometer', to map the pH in a so-called 'microfluidic system'. This consists of microscopic channels etched into a silicon wafer, which carry fluids here and there across the surface like a diminutive irrigation system. Microfluidic networks are an essential part of a laboratory-on-a-chip, carrying chemicals from one place to another for analysis or to take part in reactions. The researchers showed pH rising and falling across fluid streams just a tenth of a millimetre wide.
Ultimately they hope to be able to monitor the gradients in pH produced around single living cells. They say such instruments could map out the distribution of electrical charge in chip-bound arrays of biomolecules such as DNA and proteins, which are increasingly used in biotechnological analysis.