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Electrochemical impedance spectroscopy says “cheese!”
MAMANTOS PRODROMIDIS
net activity. Prodromidis explains that thiol self-assembled monolayers affected Cheese is a gastronomic icon in many the sensor’s performance. The neutral countries such as Switzerland, Italy, and of they chose to use EIS because there is the potential to exploit the low cost of monolayer was the most advantageous course, France, where Charles de Gaulle electrode manufacturing and the capabecause it was hydrophilic and chemically once quipped, “How can you govern a reactive towards the amino country in which there are 246 groups on the micelles, bypasskinds of cheese?” The producing the need for coupling agents tion of many cheeses relies on or activators to attach the mirennet, a natural enzymatic mixcelles. Prodromidis and colture that coagulates milk. But leagues found that the sensor’s monitoring rennet activity can performance was also strongly be difficult. In a recent Analytiaffected by the size of the artifical Chemistry paper (DOI cial casein micelles and the con10.1021/ac1017925), Mamancentration and ionic strength of tos Prodromidis and colleagues the immobilization solution. at the University of Ioannina Experts expressed enthusi(Greece) describe a sensor based When rennet cleaves κ-casein in the artificial casein micelles, asm for the work. “This paper on electrochemical impedance [Fe(CN)6]-3/-4 can better access the gold electrode, causing a drop shows that clever programming spectroscopy (EIS) that uses a in the charge-transfer resistance. GMP is the soluble glycomacof modified gold electrodes can different approach to analyze ropeptide that is produced from the cleavage of κ-casein. be used in new and interesting rennet. biosensing applications based Milk is a colloidal suspenbilities of miniaturization and multiplex- on EIS,” says Roland De Marco of Curtin sion. The colloids, called micelles, are ing in future generations of the sensor. University (Australia). He adds the sensor festooned on the surface with a -casein The investigators attached artificial “can be easily used for spot analyses durprotein layer, the top of which looks like casein micelles onto a gold electrode ing production for the analytical monitorwild, curly hair. The micelles are negamodified with a thiol self-assembled ing of cheese clotting.” Heinz-Bernhard tively charged, and repulsive forces keep Kraatz at the University of Western Onthem in suspension. Rennet contains the monolayer. They measured the resistance to an AC potential at the tario (Canada) says the work’s “novelty protease chymosin, which chews off electrode⫺electrolyte interface before lies in the complexity of attaching micelles -casein, decreasing the repulsion beand after the sensor was immersed in a to a surface and using an enzymaticallytween the micelles. The micelles can rennet sample. As rennet degraded the driven reaction to alter the properties of then aggregate to form curds. the sensing surface.” Sources of commercial rennet include artificial casein micelles on the electrode surface, it produced neutral The investigators tested the sensor on cows, goats, camels, and geneticallystructures without the hair-like layer various commercial powdered or liquid engineered microbes. Because the that could cluster. The clustering furrennet samples and measured values that amount of chymosin in these sources ther exposed the electrode surface, matched those calculated with a refervaries, scientists have been developing allowing a redox probe in bulk soluence method. They observed reliable ways to evaluate rennet’s clotting capation, [Fe(CN)6]⫺3/⫺4, more access to signal changes within 5 min of incubabilities since the end of the 19th cention, suggesting that the sensor would tury. Most methods rely on visual obser- the electrode. The increased interacbe useful in routine analysis. vation of milk as it coagulates, which can tion with [Fe(CN)6]⫺3/⫺4 decreased the charge-transfer resistance at the The investigators are now busy further be subjective. A commercial bench inelectrode⫺electrolyte interface. developing the sensor. “So far, experistrument, called the Optigraph, is also Changes in the charge-transfer resisments have been conducted in the lab in available to more quantitatively measure tance before and after sampling, excombination with a bench-type electrothe NIR attenuation as milk clots. pressed as a relative decrease, gave a chemical analyzer,” says Prodromidis. But the investigators wanted to dequantitative number for the rennet’s “However, in combination with portable velop a cheaper and simpler approach milk-clotting ability. The more dramatic instrumentation, the sensors in their curthat doesn’t involve milk because, as the drop in charge-transfer resistance, rent formOor even better, as flat, interProdromidis points out, milk may differ the more active the enzyme. digitated electrodesOcan be used as disby source, age, season, location, and The investigators tested how negatively posable sensors for on-site measurements. other factors. Thus they designed a —Rajendrani Mukhopadhyay charged, positively charged, and neutral single-use, EIS-based sensor to test ren8756
ANALYTICAL CHEMISTRY /
NOVEMBER 1, 2010
10.1021/AC102467A 2010 AMERICAN CHEMICAL SOCIETY
Published on Web 09/27/2010
