material world
Capturing anions from complex samples Investigators have developed a colorimetric hydrogel-based sensor array that detects multiple anions simultaneously.
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PC2 (28.7%)
mall, inorganic anions just don’t Anzenbacher explains the rationale quantitation of a trace amount of an anlend themselves well to detection. of having an array of nonselective senalyte in a jumble of others in a complex They have lower charge-to-radius ratios sors; “You could see it as a coarse and environmental sample, they say the work than the corresponding cations, making fine tuning. The coarse tuning would has merit. “The area of anion detection it harder for them to effectively bind to show the prevalent anion. But the cross- and the development of chemoresponreceptors. But being able to sense and reactivity, as a fine-tuning tool, would sive receptors and reporting systems that quantitate anions is critical for probalso give you a fingerprint-like trace work on anions has indeed been a challems, such as environmental analysis or signature generated by the other anions lenge for the chemistry community,” says medical diagnostics. So John McDevitt at the Pavel Anzenbacher, Jr., University of Texas AusSensors 1 2 3 4 5 6 7 8 and colleagues at Bowltin. “This is a nice piece Blank 4 ing Green State Uniof work that pushes the versity have designed a science forward.” AcO– – colorimetric-array-based “The interesting part 2 BzO Aquafresh assay to detect several is they have successfully Br – Colgate Crest anions simultaneously demonstrated sensors Cl– 0 Fluoridex – (J. Am. Chem. Soc. that work well for highly F NaF 2007, 129, 7538–7544). hydrophilic anions, like NO3– –2 Electrochemical, optifluoride. That’s not comH2PO4– cal, colorimetric, and mon, and it isn’t easy HP2O73– –4 chromatographic techto do,” says Purnendu HSO4– –6 –4 –2 0 2 4 niques to detect anions Dasgupta at the UniverHS– PC1 (52.4%) already exist. But, despite sity of Texas Arlington. being accurate, most of “It’s much easier to get the techniques are labor- (Left) Array of eight sensors that detected 10 anions. (Right) Principal composensors for highly hydrointensive, and expensive, nents analysis of repeated trials shows that Aquafresh, Crest, and Colgate form phobic ions like perchloand require a high level one cluster, whereas Fluoridex and a NaF solution form another. rate.” Anzenbacher says of expertise. For those the work also proves a reasons, Anzenbacher says, he and his present in the sample that the sensor different point: “We were keen to show colleagues wanted to develop a simple also sees.” The investigators demonthat supramolecular chemistry, which sensor that could be read by a scanner strated that the array could operate in is oftentimes perceived as a fairly acaand operated by nontechnical personnel. concentrations across 4 orders of magdemic discipline, can really help real-life The investigators incorporated eight nitude (0.20–360.0 ppm). applications.” anion sensors they had previously develThe investigators tested the senThe investigators are focused on oped into a polyurethane hydrogel. Each sor on several brands of toothpaste. working with blood plasma and urine. sensor changed color when a particular Toothpaste is a complex, cumbersome “You could see a lot of metabolites of anion bonded to it. The hydrogel helped gel with high concentrations of several various pharmaceuticals and drugs in to extract the anions from the bulk solu- anions and other unknown, proprietary urine, because many of those are antion and strip away the associated solvent analytes. The investigators discovered ions,” says Anzenbacher. And as for molecules so that the anions could easily that Aquafresh, Colgate, and Crest apblood, medical evidence suggests anion access the sensing element. pear to be similar to one another in monitoring can be critical. Anzenbacher Anzenbacher and colleagues opted terms of fluoride, pyrophosphate, and says in a recently published study (Arch. to have sensors that were cross-reactive other anion concentrations but differ Intern. Med. 2007, 167, 879–885), and not strictly selective for a single anfrom Fluoridex (a specialty toothpaste “Phosphate levels in blood were linked ion. The eight-sensor array was capable for use before and after tooth whitening with cardiovascular disease. If you have of detecting fluoride and pyrophosphate treatments) and a NaF solution. a disease, it’s probably linked to some but was also reactive toward phosphate, Although some experts note that the molecular entity, and there’s a fair carboxylates, and chloride. Overall, the array is yet to be tested on more difficult chance anions are involved.” a array could sense 10 anions. problems, such as the identification and —Rajendrani Mukhopadhyay 7238
A n a ly t i c a l C h e m i s t r y / o c t o b e r 1 , 2 0 0 7
© 2007 American Chemical Societ y
