Henriques04 - i-SECTION HIGHLIGHTS www.rsc.org/analyst The...

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Analyst, 129 , 2004 This journal is © The Royal Society of Chemistry 2004 478 www.rsc.org/analyst i-SECTION HIGHLIGHTS THE ANALYST Introduction This Highlight discusses new developments in the science and technology of Coulter counting, also known as resistive pulse sensing, that have extended the applicability of this simple and reliable method to the analysis of nanoscale objects, including polymeric beads, DNA and other polymers, viruses, and metal ions. Coulter counters consist of two chambers divided by an insulating membrane that contains a single channel (Fig. 1a). Electrodes immersed in an electrolyte solution present in each chamber are used to drive an ionic current through the channel. If, in addition to the electrolyte, particles having a size on the order of the channel diameter are present in the solution, then they may enter the channel and thereby reduce the magnitude of the ionic current. The output of a Coulter counter is a plot of current vs. time (Fig. 1b) that contains a string of current pulses. Under favorable conditions these current pulses can be correlated to the size, mobility, and concentration of the particles. The reason this fifty-year-old method is once again attracting attention is because of its simplicity, single-particle sensitivity, nanometer-scale particle-size resolution, and because the data it provides is rich in information. 1 The purpose of this article is to highlight the current state-of-the-art of the Coulter counting methodology, provide some illustrative examples of how it has been used to solve analytical problems, and then discuss how it might evolve in the future. Background The Coulter counter was patented in 1953 by W. H. Coulter, and throughout the last fifty years has been widely used in medical laboratories to determine biological cell concentrations. 1,2 For example, in the early years, Coulter counters were used mainly to provide accurate complete blood counts (CBCs). These first-generation devices relied on pressure-driven flow to move blood cells into the channels, which were made from glass and had channel diameters ranging from ~20 m m to 2 mm. Prior to development of the Coulter counter, CBCs were determined by counting blood cells immobilized on glass slides with the aid of a microscope. This process was time consuming and often inaccurate; the Coulter counter substantially improved throughput and accuracy. In the 1970s, DeBlois and Bean demonstrated that submicron-sized analytes could be detected when passing through the channel of a Coulter counter. 3 Specifically, they showed that polystyrene beads as small as ~90 nm in diameter could be detected and that their size could be accurately measured utilizing track- etched tapered pores in polycarbonate having diameters ranging from 0.4–0.5 m m. They also demonstrated that it was possible to detect and measure the sizes of viruses. 4
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Henriques04 - i-SECTION HIGHLIGHTS www.rsc.org/analyst The...

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