editorial
Seeking Valuable pH Paper
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ccasionally I have offered guidance to authors about topics or emphases that are good, or not, for publication in Analytical Chemistry. The last “not” that I recall was entitled “Cadmium Horses” and Glucose, which was published in the May 1, 2004, issue (p 149 A). This time, the Editorial’s guidance is about visual color sensing, a venerable form of analytical chemistry that includes titration indicators and pH paper (for weekend gardeners like myself to check soil acidity) and that is encountering critical reviews. An innovative form of color sensing was published in 1996 with the “observation that oligonucleotide-modified nanoparticles and sequence-specific particle assembly events, induced by target DNA, could be used to generate materials with unusual optical and melting properties. Specifically, when 13 nm gold particles were used in the assay, the color of the solution changed from red to blue upon the analytedirected aggregation of gold nanoparticles, a consequence of interacting particle surface plasmons and aggregate scattering properties” (Chem. Rev. 2005, 105, 1547–1562). In simpler words, find an analyte-specific way to aggregate gold nanoparticles of small dimensions, and you will see a color change (and spectral ones as well, of course). The DNA hybridization-based specificity of this scheme provoked wide interest and a consequent stream of valuable research. This journal has welcomed many subsequent submissions pointing to visual color changes as a mode of analytical detection, and it also has turned many away. This Editorial explains the difference in receptiveness. First, reviewers and readers expect papers offering innovation in the principles, methodology, and instrumentation of our discipline. Second, papers on applications of known methodology to practical problems should be truly innovative as to analyte, analytical context, and improvement over past practice. Our instructions to authors (http://pubs.acs. org/paragonplus/submission/ancham/ancham_authguide. pdf) say as much. Since the 1996 paper, there have been innovations—by many authors—in the chemistry of making gold (and other) nanoparticles aggregate in an analyte-specific manner that evokes spectral and color changes. Color change via nanoparticle aggregation is consequently no longer innovative; rather,
© 2008 American Chemical Societ y
the innovation is in the analyte-specific chemistry provoking aggregation—is it genuinely new or an incremental advance over past work? Is it selective and immune to matrix effects? The innovation also may be in the analyte, if it is an important one that is not readily accessible by other analytical routes, or perhaps the color change has value as a yes/no response bracketing some known, important threshold level. In other cases, the color change may be objectively readable (i.e., even by a person with some degree of color blindness), or perhaps the color change is more usefully read by an elementary photometer, rather than the human eye. I could cite further qualifications, but these have been important factors in reviewers qualifying a paper for publication in Analytical Chemistry in the color-test area. I should add that these remarks also apply to other schemes of making color changes that do not involve nanoparticles. My previous Editorials show that I strongly appreciate the modern drive for simplified analysis, but in the end, one must always consider its context. Metal-ion detection is a highly problematic example. An analysis for mercury that is based on nanoparticle aggregation (color A/B) is ideally centered on an accepted concentration threshold (e.g., safe/unsafe), but in fact, the reality threshold is fuzzy (e.g., OK for how long?). One can say the same for most metal ions, for which numerous alternative quantitative methodologies are available. In cases where a yes/no threshold is known to be indecisive, color-change methods must demonstrate a very strong level of analyte significance to merit publication. Finally, I would remark that modern measurement science is driven toward observations that are quantitative as opposed to semiquantitative (which color changes are). It seems retrograde to believe that major advances come by being less quantitative. Less quantitative schemes demand a sharply defined target to justify the research investment.
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